nanoparticles synthesis

• Open access
• Published: 30 October 2018

‘Green’ synthesis of metals and their oxide nanoparticles: applications for environmental remediation

• Jagpreet Singh 1 ,
• Tanushree Dutta 2 ,
• Ki-Hyun Kim 3 ,
• Mohit Rawat 1 ,
• Pallabi Samddar 3 &
• Pawan Kumar   ORCID: orcid.org/0000-0003-0712-8763 4  

Journal of Nanobiotechnology volume  16 , Article number:  84 ( 2018 ) Cite this article

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In materials science, “green” synthesis has gained extensive attention as a reliable, sustainable, and eco-friendly protocol for synthesizing a wide range of materials/nanomaterials including metal/metal oxides nanomaterials, hybrid materials, and bioinspired materials. As such, green synthesis is regarded as an important tool to reduce the destructive effects associated with the traditional methods of synthesis for nanoparticles commonly utilized in laboratory and industry. In this review, we summarized the fundamental processes and mechanisms of “green” synthesis approaches, especially for metal and metal oxide [e.g., gold (Au), silver (Ag), copper oxide (CuO), and zinc oxide (ZnO)] nanoparticles using natural extracts. Importantly, we explored the role of biological components, essential phytochemicals (e.g., flavonoids, alkaloids, terpenoids, amides, and aldehydes) as reducing agents and solvent systems. The stability/toxicity of nanoparticles and the associated surface engineering techniques for achieving biocompatibility are also discussed. Finally, we covered applications of such synthesized products to environmental remediation in terms of antimicrobial activity, catalytic activity, removal of pollutants dyes, and heavy metal ion sensing.

Introduction

Over the last decade, novel synthesis approaches/methods for nanomaterials (such as metal nanoparticles, quantum dots (QDs), carbon nanotubes (CNTs), graphene, and their composites) have been an interesting area in nanoscience and technology [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 ]. To obtain nanomaterials of desired sizes, shape, and functionalities, two different fundamental principles of synthesis (i.e., top down and bottom up methods) have been investigated in the existing literature (Fig.  1 ). In the former, nanomaterials/nanoparticles are prepared through diverse range of synthesis approaches like lithographic techniques, ball milling, etching, and sputtering [ 10 ]. The use of a bottom up approach (in which nanoparticles are grown from simpler molecules) also includes many methods like chemical vapor deposition, sol–gel processes, spray pyrolysis, laser pyrolysis, and atomic/molecular condensation.

Different synthesis approaches available for the preparation of metal nanoparticles

Interestingly, the morphological parameters of nanoparticles (e.g., size and shape) can be modulated by varying the concentrations of chemicals and reaction conditions (e.g., temperature and pH). Nevertheless, if these synthesized nanomaterials are subject to the actual/specific applications, then they can suffer from the following limitation or challenges: (i) stability in hostile environment, (ii) lack of understanding in fundamental mechanism and modeling factors, (iii) bioaccumulation/toxicity features, (iv) expansive analysis requirements, (v) need for skilled operators, (vi) problem in devices assembling and structures, and (vii) recycle/reuse/regeneration. In true world, it is desirable that the properties, behavior, and types of nanomaterials should be improved to meet the aforementioned points. On the other hand, these limitations are opening new and great opportunities in this emerging field of research.

To counter those limitations, a new era of ‘green synthesis’ approaches/methods is gaining great attention in current research and development on materials science and technology. Basically, green synthesis of materials/nanomaterials, produced through regulation, control, clean up, and remediation process, will directly help uplift their environmental friendliness. Some basic principles of “green synthesis” can thus be explained by several components like prevention/minimization of waste, reduction of derivatives/pollution, and the use of safer (or non-toxic) solvent/auxiliaries as well as renewable feedstock.

‘Green synthesis’ are required to avoid the production of unwanted or harmful by-products through the build-up of reliable, sustainable, and eco-friendly synthesis procedures. The use of ideal solvent systems and natural resources (such as organic systems) is essential to achieve this goal. Green synthesis of metallic nanoparticles has been adopted to accommodate various biological materials (e.g., bacteria, fungi, algae, and plant extracts). Among the available green methods of synthesis for metal/metal oxide nanoparticles, utilization of plant extracts is a rather simple and easy process to produce nanoparticles at large scale relative to bacteria and/or fungi mediated synthesis. These products are known collectively as biogenic nanoparticles (Fig.  2 ).

Key merits of green synthesis methods

Green synthesis methodologies based on biological precursors depend on various reaction parameters such as solvent, temperature, pressure, and pH conditions (acidic, basic, or neutral). For the synthesis of metal/metal oxide nanoparticles, plant biodiversity has been broadly considered due to the availability of effective phytochemicals in various plant extracts, especially in leaves such as ketones, aldehydes, flavones, amides, terpenoids, carboxylic acids, phenols, and ascorbic acids. These components are capable of reducing metal salts into metal nanoparticles [ 11 ]. The basic features of such nanomaterials have been investigated for use in biomedical diagnostics, antimicrobials, catalysis, molecular sensing, optical imaging, and labelling of biological systems [ 12 ].

Here, we summarized the current state of research on the green synthesis of metal/metal oxide nanoparticles with their advantages over chemical synthesis methods. In addition, we also discussed the role of solvent systems (synthetic materials), various biological (natural extracts) components (like bacteria, algae, fungi, and plant extracts) with their advantages over other conventional components/solvents. The main aim of this literature study is to provide detailed mechanisms for green synthesis and their real world environmental remediation applications. Overall, our goal is to systematically describe “green” synthesis procedures and their related components that will benefit researchers involved in this emerging field while serving as a useful guide for readers with a general interest in this topic.

Biological components for “green” synthesis

Innumerable physical and chemical synthesis approaches require high radiation, highly toxic reductants, and stabilizing agents, which can cause pernicious effects to both humans and marine life. In contrast, green synthesis of metallic nanoparticles is a one pot or single step eco-friendly bio-reduction method that requires relatively low energy to initiate the reaction. This reduction method is also cost efficient [ 13 , 14 , 15 , 16 , 17 , 18 , 19 ].

Bacterial species have been widely utilized for commercial biotechnological applications such as bioremediation, genetic engineering, and bioleaching [ 20 ]. Bacteria possess the ability to reduce metal ions and are momentous candidates in nanoparticles preparation [ 21 ]. For the preparation of metallic and other novel nanoparticles, a variety of bacterial species are utilized. Prokaryotic bacteria and actinomycetes have been broadly employed for synthesizing metal/metal oxide nanoparticles.

The bacterial synthesis of nanoparticles has been adopted due to the relative ease of manipulating the bacteria [ 22 ]. Some examples of bacterial strains that have been extensively exploited for the synthesis of bioreduced silver nanoparticles with distinct size/shape morphologies include: Escherichia coli , Lactobacillus casei , Bacillus cereus , Aeromonas sp. SH10 Phaeocystis antarctica , Pseudomonas proteolytica , Bacillus amyloliquefaciens , Bacillus indicus , Bacillus cecembensis , Enterobacter cloacae , Geobacter spp., Arthrobacter gangotriensis , Corynebacterium sp. SH09, and Shewanella oneidensis . Likewise, for the preparation of gold nanoparticles, several bacterial species (such as Bacillus megaterium D01, Desulfovibrio desulfuricans , E. coli DH5a, Bacillus subtilis 168, Shewanella alga , Rhodopseudomonas capsulate , and Plectonema boryanum UTEX 485) have been extensively used. Information on the size, morphology, and applications of various nanoparticles is summarized in Table  1 .

Fungi-mediated biosynthesis of metal/metal oxide nanoparticles is also a very efficient process for the generation of monodispersed nanoparticles with well-defined morphologies. They act as better biological agents for the preparation of metal and metal oxide nanoparticles, due to the presence of a variety of intracellular enzyme [ 23 ]. Competent fungi can synthesize larger amounts of nanoparticles compared to bacteria [ 24 ]. Moreover, fungi have many merits over other organisms due to the presence of enzymes/proteins/reducing components on their cell surfaces [ 25 ]. The probable mechanism for the formation of the metallic nanoparticles is enzymatic reduction (reductase) in the cell wall or inside the fungal cell. Many fungal species are used to synthesize metal/metal oxide nanoparticles like silver, gold, titanium dioxide and zinc oxide, as discussed in Table  1 .

Yeasts are single-celled microorganisms present in eukaryotic cells. A total of 1500 yeast species have been identified [ 26 ]. Successful synthesis of nanoparticles/nanomaterials via yeast has been reported by numerous research groups. The biosynthesis of silver and gold nanoparticles by a silver-tolerant yeast strain and Saccharomyces cerevisiae broth has been reported. Many diverse species are employed for the preparation of innumerable metallic nanoparticles, as discussed in Table  1 .

Plants have the potential to accumulate certain amounts of heavy metals in their diverse parts. Consequently, biosynthesis techniques employing plant extracts have gained increased consideration as a simple, efficient, cost effective and feasible methods as well as an excellent alternative means to conventional preparation methods for nanoparticle production. There are various plants that can be utilized to reduce and stabilize the metallic nanoparticles in “one-pot” synthesis process. Many researchers have employed green synthesis process for preparation of metal/metal oxide nanoparticles via plant leaf extracts to further explore their various applications.

Plants have biomolecules (like carbohydrates, proteins, and coenzyme) with exemplary potential to reduce metal salt into nanoparticles. Like other biosynthesis processes, gold and silver metal nanoparticles were first investigated in plant extract-assisted synthesis. Various plants [including aloe vera ( Aloe barbadensis Miller), Oat ( Avena sativa ), alfalfa ( Medicago sativa ), Tulsi ( Osimum sanctum ), Lemon ( Citrus limon ), Neem ( Azadirachta indica ), Coriander ( Coriandrum sativum ), Mustard ( Brassica juncea ) and lemon grass ( Cymbopogon flexuosus )] have been utilized to synthesize silver nanoparticles and gold nanoparticles, as listed in Table  2 . The major part of this type of research has explored the ex vivo synthesis of nanoparticles, while metallic nanoparticles can be formed in living plants (in vivo) by reducing metal salt ions absorbed as soluble salts. The in vivo synthesis of nanoparticles like zinc, nickel, cobalt, and copper was also observed in mustard ( Brassica juncea ), alfalfa ( Medicago sativa ), and sunflower ( Helianthus annuus ) [ 27 ]. Also, ZnO nanoparticles have been prepared with a great variety of plant leaf extracts such as coriander ( Coriandrum sativum ) [ 28 ], crown flower ( Calotropis gigantean ) [ 29 ], copper leaf ( Acalypha indica ) [ 30 ], China rose ( Hibiscus rosa - sinensis ) [ 31 ], Green Tea ( Camellia sinensis ) [ 32 ], and aloe leaf broth extract ( Aloe barbadensis Miller) [ 33 ]. Readers can refer to the work of Iravani [ 34 ] for a comprehensive overview of plant materials utilized for the biosynthesis of nanoparticles.

Solvent system-based “green” synthesis

Solvent systems are a fundamental component in the synthesis process, whether it is “green” synthesis or not. Water is always considered an ideal and suitable solvent system for synthesis processes. According to Sheldon, “the best solvent is no solvent, and if a solvent is desirable then water is ideal” [ 35 ]. Water is the cheapest and most commonly accessible solvent on earth. Since the advent of nanoscience and nanotechnology, the use of water as a solvent for the synthesis of various nanoparticles has been carried out. For instance, synthesized Au and Ag nanoparticles at room temperature using gallic acid, a bifunctional molecule, in an aqueous medium [ 36 ]. Gold nanoparticles were produced via a laser ablation technique in an aqueous solution. The oxygen present in the water leads to partial oxidation of the synthesized gold nanoparticles, which finally enhanced its chemical reactivity and had a great impact on its growth [ 37 ].

In the literature, “green” synthesis consists of two major routes:

Wherein water is used as a solvent system.

Wherein a natural source/extract is utilized as the main component.

Both of these routes have been covered in the coming section according to the present literature. Hopefully, our efforts will help researchers gain a better knowledge of ‘green’ synthesis methods, the role of toxic/non-toxic solvents (or components), and renewable resources derived from natural sources. Ionic and supercritical liquids are one of the best examples in this emerging area. Ionic liquids (ILs) are composed of ions that have melting points below 100 °C. Ionic liquids are also acknowledged as “room temperature ionic liquids.” Several metal nanoparticles (e.g., Au, Ag, Al, Te, Ru, Ir, and Pt) have been synthesized in ionic liquids [ 38 , 39 , 40 , 41 ]. The process of nanoparticle synthesis is simplified since the ionic liquid can serve as both a reductant and a protective agent.

ILs can be hydrophilic or hydrophobic depending on the nature of the cations and anions. For example, 1-butyl-3-methyl imidazolium (Bmim) hexafluorophosphate (PF6) is hydrophobic, whereas its tetrafluoroborate (BF4) analogue is hydrophilic. Since both species are ionic in nature, they can act as catalysts [ 40 , 42 , 43 , 44 , 45 ]. Bussamara et al. have performed a comparative study by controlling the synthesis of manganese oxide (Mn 3 O 4 ) nanoparticles using imidazolium ionic liquids and oleylamine (a conventional solvent). They found that smaller sized nanoparticles (9.9 ± 1.8 nm) were formed with better dispersity in ionic liquids than in the oleylamine solvent (12.1 ± 3.0 nm) [ 46 ]. Lazarus et al. synthesized silver nanoparticles in an ionic liquid (BmimBF4). The synthesized nanoparticles were in both smaller isotropic spherical and large-sized anisotropic hexagonal shaped forms [ 47 ]. An electrochemical method was employed for this purpose [ 48 ]. Ionic liquid was used in the electrolytic reaction as a substitute for water without mechanical stirring. For the first time, Kim et al. developed a one-phase preparation technique for gold (Au) and platinum (Pt) nanoparticles by means of thiol-functionalized ionic liquids (TFILs). TFILs acted as a stabilizing agent to produce crystalline structures with small sizes [ 49 ]. Dupont et al. used 1-n-butyl-3-methylimidazolium hexafluorophosphate (which is room temperature ionic liquid) for synthesizing Ir(0) nanoparticles by Ir(I) reduction. The average size of synthesized nanoparticles was ~ 2 nm. Interestingly, the ionic liquid medium is impeccable for the production of recyclable biphasic catalytic systems for hydrogenation reactions [ 50 ].

The benefits of using ionic liquids instead of other solvents include the following. (a) Many metal catalysts, polar organic compounds, and gases are easily dissolved in ILs to support biocatalysts. (b) ILs have constructive thermal stabilities to operate in a broad temperature range. Most of these melt below room temperature and begin to decompose above 300 or 400 °C. As such, they allow a broader synthesis temperature range (e.g., three to four times) than that of water. (c) The solubility properties of IL can be modulated by modifying the cations and anions associated with them. (d) Unlike other polar solvents or alcohols, ILs are non-coordinating. However, they have polarities comparable to alcohol. (e) ILs do not evaporate into the environment like volatile solvents because they have no vapor pressure. (f) ILs have dual functionality because they have both cations and anions. The problems associated with the biodegradability of ionic liquids make them not acceptable for synthesis of metallic nanoparticles. To diminish these non-biodegradability issues, many new potentially benign ionic liquids are being developed with maximum biodegradation efficiency [ 51 , 52 , 53 , 54 ]. The innumerable ILs are used to synthesize various metallic nanoparticles as listed in Table  3 .

Likewise, ordinary solvents can be converted into super critical fluids at temperatures and pressures above critical point. In the supercritical state, solvent properties such as density, thermal conductivity, and viscosity are significantly altered. Carbon dioxide is the most feasible super critical, non-hazardous, and inert fluid [ 55 , 56 ]. Also, supercritical water can serve as a good solvent system for several reactions. As, water has critical temperature of 646 K and pressure of 22.1 MPa [ 57 ]. Silver and copper NPs can be synthesized in supercritical carbon dioxide [ 58 ]. Sue et al. suggested that decreasing the solubility of metal oxides around the critical point can lead to super saturation and the ultimate formation of nanoparticles [ 59 ]. Kim et al. synthesized tungsten oxide (WO 3 ) and tungsten blue oxide nanoparticles by using sub- and supercritical water and methanol [ 60 ].

Stability and toxicity of the nanoparticles

The environmental distribution and transport of released nanoparticles depend on their ability to make metastable aqueous suspensions or aerosols in environmental fluids. The stability of the nanoparticles in the environment can therefore be evaluated by estimating their propensity to aggregate or interact with the surrounding media. Aggregation is a time-dependent phenomena associated with the rate of particle collision while the stability of the suspension is largely determined by the size of the particles and affinity toward other environmental constituents. The “green” synthesis of AgNPs from tea leaf extraction was found to be stable after entering the aquatic environment [ 61 ]. Likewise, the stability of AgNPs (in aqueous medium) manufactured using plant extracts and plant metabolites was confirmed from the resulting material [ 62 ]. Surface complexation is also reported to affect the intrinsic stability of nanoparticles by regulating its colloidal stability. The nature and stability of nanoparticles were theoretically predicted through a mechanistic understanding of the surface complexation processes [ 63 ]. The colloidal stability (or rate of dissolution) of nanoparticles can be regulated by controlling the particle size and surface capping or through functionalization techniques [ 64 , 65 ]).

Transformation of nanoparticles is an essential property to consider when assessing their environmental impact or toxicity. For instance, sulfurization of AgNPs greatly reduced their toxicity due to the lower solubility of silver sulfide [ 66 ]. For similar reasons, the use of biocompatible stabilizing agents (e.g., biodegradable polymers and copolymers) have opened up a “greener” avenue of nanomaterial surface engineering. Such techniques can impart remarkable stability, e.g., in situ synthesis of AuNPs capped with Korean red ginseng root [ 67 ]. Apart from surface chemistry, other key structural features determining the nanomaterial toxicity are the size, shape, and composition of the nanomaterials [ 68 ]. Toxicity analysis of AgNP synthesized using plant leaf extracts showed enhanced seed germination rates in the AgNP chemical treatment for activation than the corresponding control treatments [ 69 ]. However, the mechanism of such rate enhancement effects was not reported.

Mechanism of “green” synthesis for metals and their oxide nanoparticles

Microorganism-based mechanism.

There are different mechanisms for the formation of nanoparticles using different microorganisms. First, metallic ions are captured on the surface or inside the microbial cells, and then these arrested metal ions are reduced into metal nanoparticles by the action of enzymes. Sneha et al. [ 70 ] described the mechanism of microorganism-assisted silver and gold nanoparticles formed via Verticillium sp. or algal biomass based on the following hypothesis. (a) First, the silver or gold ions were captured on the surface of fungal cells via electrostatic interactions between ions and negatively charged cell wall enzymes. (b) Then, silver or gold ions were bioreduced into silver or gold nuclei, which subsequently grew. The two key aspects in the biosynthesis of nanoparticles are NADH (nicotinamide adenine dinucleotide) and NADH-dependent nitrate reductase. Kalishwaralal et al. [ 71 ] demonstrated that the nitrate reductase was responsible for the production of bioreduced silver nanoparticles by B. licheniformis . Nonetheless, the bioreduction mechanisms associated with the production of metal salt ions and the resulting metallic nanoparticles formed by microorganisms remain unexplored.

Plant leaf extract-based mechanism

For nanoparticle synthesis mediated by plant leaf extract, the extract is mixed with metal precursor solutions at different reaction conditions [ 72 ]. The parameters determining the conditions of the plant leaf extract (such as types of phytochemicals, phytochemical concentration, metal salt concentration, pH, and temperature) are admitted to control the rate of nanoparticle formation as well as their yield and stability [ 73 ]. The phytochemicals present in plant leaf extracts have uncanny potential to reduce metal ions in a much shorter time as compared to fungi and bacteria, which demands the longer incubation time [ 74 ]. Therefore, plant leaf extracts are considered to be an excellent and benign source for metal as well as metal oxide nanoparticle synthesis. Additionally, plant leaf extract play a dual role by acting as both reducing and stabilizing agents in nanoparticles synthesis process to facilitate nanoparticles synthesis [ 75 ]. The composition of the plant leaf extract is also an important factor in nanoparticle synthesis, for example different plants comprise varying concentration levels of phytochemicals [ 76 , 77 ]. The main phytochemicals present in plants are flavones, terpenoids, sugars, ketones, aldehydes, carboxylic acids, and amides, which are responsible for bioreduction of nanoparticles [ 78 ].

Flavonoids contain various functional groups, which have an enhanced ability to reduce metal ions. The reactive hydrogen atom is released due to tautomeric transformations in flavonoids by which enol-form is converted into the keto-form. This process is realized by the reduction of metal ions into metal nanoparticles. In sweet basil ( Ocimum basilicum ) extracts, enol- to keto-transformation is the key factor in the synthesis of biogenic silver nanoparticles [ 79 ]. Sugars such as glucose and fructose exist in plant extracts can also be responsible for the formation of metallic nanoparticles. Note that glucose was capable of participating in the formation of metallic nanoparticles with different size and shapes, whereas fructose-mediated gold and silver nanoparticles are monodisperse in nature [ 80 ].

An FTIR analysis of green synthesized nanoparticles via plant extracts confirmed that nascent nanoparticles were repeatedly found to be associated with proteins [ 81 ]. Also, amino acids have different ways of reducing the metal ions. Gruen et al. [ 82 ] observed that amino acids (viz cysteine, arginine, lysine, and methionine are proficient in binding with silver ions. Tan et al. [ 83 ] tested all of the 20 natural α-amino acids to establish their efficient potential behavior towards the reduction of Au 0 metal ions.

Plant extracts are made up of carbohydrates and proteins biomolecules, which act as a reducing agent to promote the formation of metallic nanoparticles [ 34 ]. Also, the proteins with functionalized amino groups (–NH 2 ) available in plant extracts can actively participate in the reduction of metal ions [ 84 ]. The functional groups (such as –C–O–C–, –C–O–, –C=C–, and –C=O–) present in phytochemicals such as flavones, alkaloids, phenols, and anthracenes can help to generate metallic nanoparticles. According to Huang et al. [ 85 ], the absorption peaks of FTIR spectra at (1) 1042 and 1077, (2) 1606 and 1622, and (3) 1700–1800 cm −1 imply the stretching of (1) –C–O–C– or –C–O–, (2) –C=C– and (3) –C=O, respectively. Based on FTIR analysis, they confirmed that functional groups like –C–O–C–, –C–O–, –C=C–, and –C=O, are the capping ligands of the nanoparticles [ 86 ]. The main role of the capping ligands is to stabilize the nanoparticles to prevent further growth and agglomeration. Kesharwani et al. [ 87 ] covered photographic films using an emulsion of silver bromide. When light hit the film, the silver bromide was sensitized; this exposed film was placed into a solution of hydroquinone, which was further oxidized to quinone by the action of sensitized silver ion. The silver ion was reduced to silver metal, which remained in the emulsion.

Based on the chemistry of photography, we assumed that hydroquinone or plastohydroquinone or quinol (alcoholic compound) serve as a main reducing agent for the reduction of silver ions to silver nanoparticles through non-cyclic photophosphorylation [ 87 ]. Thus, this experiment proves that the biomolecules and heterocyclic compounds exist in plant extract were accountable for the extracellular synthesis of metallic nanoparticles by plants. It has already been well established that numerous plant phytochemicals including alkaloids, terpenoids, phenolic acids, sugars, polyphenols, and proteins play a significant role in the bioreduction of metal salt into metallic nanoparticles. For instance, Shanakr et al. [ 88 ] confirmed that the terpenoids present in geranium leaf extract actively take part in the conversion of silver ions into nanoparticles. Eugenol is a main terpenoid component of Cinnamomum zeylanisum (cinnamon) extracts, and it plays a crucial role for the bioreduction of HAuCl 4 and AgNO 3 metal salts into their respective metal nanoparticles. FTIR data showed that –OH groups originating from eugenol disappear during the formation of Au and Ag nanoparticles. After the formation of Au nanoparticles, carbonyl, alkenes, and chloride functional groups appeared. Several other groups [e.g., R–CH and –OH (aqueous)] were also found both before and after the production of Au nanoparticles [ 89 ]. Thus, they proposed the possible chemical mechanism shown in Fig.  3 . Nonetheless, the exact fundamental mechanism for metal oxide nanoparticle preparation via plant extracts is still not fully tacit. In general, there are three phases of metallic nanoparticle synthesis from plant extracts: (1) the activation phase (bioreduction of metal ions/salts and nucleation process of the reduced metal ions), (2) the growth phase (spontaneous combination of tiny particles with greater ones) via a process acknowledged as Ostwald ripening, and (3) the last one is termination phase (defining the final shape of the nanoparticles) [ 90 , 91 ]. The process of nanoparticle formation by plant extract is depicted in Fig.  4 [ 92 ].

Schematic for the reduction of Au and Ag ions [ 89 ]

Mechanism of nanoparticle formation by plant leaf extract [ 228 ]

Environmental remediation applications

Antimicrobial activity.

Various studies have been carried out to ameliorate antimicrobial functions because of the growing microbial resistance towards common antiseptic and antibiotics. According to in vitro antimicrobial studies, the metallic nanoparticles effectively obstruct the several microbial species [ 93 ]. The antimicrobial effectiveness of the metallic nanoparticles depends upon two important parameters: (a) material employed for the synthesis of the nanoparticles and (b) their particle size. Over the time, microbial resistance to antimicrobial drugs has become gradually raised and is therefore a considerable threat to public health. For instance, antimicrobial drug resistant bacteria contain methicillin-resistant, sulfonamide-resistant, penicillin-resistant, and vancomycin-resistant properties [ 94 ]. Antibiotics face many current challenges such as combatting multidrug-resistant mutants and biofilms. The effectiveness of antibiotic is likely to decrease rapidly because of the drug resistance capabilities of microbes. Hence, even when bacteria are treated with large doses of antibiotics, diseases will persist in living beings. Biofilms are also an important way of providing multidrug resistance against heavy doses of antibiotics. Drug resistance occurs mainly in infectious diseases such as lung infection and gingivitis [ 95 ]. The most promising approach for abating or avoiding microbial drug resistance is the utilization of nanoparticles. Due to various mechanisms, metallic nanoparticles can preclude or overwhelm the multidrug-resistance and biofilm formation, as described in Figs.  5 and 6 .

Schematic for the multiple antimicrobial mechanisms in different metal nanoparticles against microbial cells [ 96 ]

Various mechanisms of antimicrobial activity of metal nanoparticles [ 93 ]

Various nanoparticles employ multiple mechanisms concurrently to fight microbes [e.g., metal-containing nanoparticles, NO-releasing nanoparticles (NO NPs), and chitosan-containing nanoparticles (chitosan NPs)]. Nanoparticles can fight drug resistance because they operate using multiple mechanisms. Therefore, microbes must simultaneously have multiple gene mutations in their cell to overcome the nanoparticle mechanisms. However, simultaneous multiple biological gene mutations in the same cell are unlikely [ 96 ].

Multiple mechanisms observed in nanoparticles are discussed in Table  4 . Silver nanoparticles are the most admired inorganic nanoparticles, and they are utilized as efficient antimicrobial, antifungal, antiviral, and anti-inflammatory agents [ 97 ]. According to a literature survey, the antimicrobial potential of silver nanoparticles can be described in the following ways: (1) denaturation of the bacterial outer membrane [ 98 ], (2) generation of pits/gaps in the bacterial cell membrane leading to fragmentation of the cell membrane [ 99 , 100 ], and (3) interactions between Ag NPs and disulfide or sulfhydryl groups of enzymes disrupt metabolic processes; this step leads to cell death [ 101 ]. The shape-dependent antimicrobial activity was also examined. According to Pal et al. [ 102 ], truncated triangular nanoparticles are highly reactive in nature because their high-atom-density surfaces have enhanced antimicrobial activity.

The synthesis of Au nanoparticles is highly useful in the advancement of effective antibacterial agents because of their non-toxic nature, queer ability to be functionalized, polyvalent effects, and photo-thermal activity [ 103 , 104 , 105 ]. However, the antimicrobial action of gold nanoparticles is not associated with the production of any reactive oxygen species-related process [ 106 ]. To investigate the antibacterial potential of the Au nanoparticles, researchers attempted to attach nanoparticles to the bacterial membrane followed by modifying the membrane potential, which lowered the ATP level. This attachment also inhibited tRNA binding with the ribosome [ 106 ]. Azam et al. [ 107 ] examined the antimicrobial potential of zinc oxide (ZnO), copper oxide (CuO), and iron oxide (Fe 2 O 3 ) nanoparticles toward gram-negative bacteria ( Escherichia coli , Pseudomonas aeruginosa ) and gram-positive bacteria ( Staphylococcus Aureus and Bacillus subtilis ). Accordingly, the most intense antibacterial activity was reported for the ZnO nanoparticles. In contrast, Fe 2 O 3 nanoparticles exhibited the weakest antibacterial effects. The order of antibacterial activities of nanoparticles was found to be as ZnO (19.89 ± 1.43 nm), CuO (29.11 ± 1.61 nm), and Fe 2 O 3 (35.16 ± 1.47 nm). These results clearly depicts that the size of the nanoparticles also play a momentous role in the antibacterial potential of each sample [ 107 ]. The anticipated mechanism of antimicrobial action of ZnO nanoparticles is: (1) ROS generation, (2) zinc ion release on the surface, (3) membrane dysfunction, and (4) entry into the cell. Also, the antimicrobial potential of ZnO nanoparticles is concentration and surface area dependent [ 108 ]. Mahapatra et al. [ 109 ] determined the antimicrobial action of copper oxide nanoparticles towards several bacterial species such as Klebsiella pneumoniae , P. aeruginosa , Shigella Salmonella paratyphi s. They found that CuO nanoparticles exhibited suitable antibacterial activity against those bacteria. It was assumed that nanoparticles should cross the bacterial cell membrane to damage the crucial enzymes of bacteria, which further induce cell death. For instance, green synthesized nanoparticles show enhanced antimicrobial activity compared to chemically synthesized or commercial nanoparticles. This is because the plants [such as Ocimum sanctum (Tulsi) and Azadirachta indica (neem)] employed for synthesis of nanoparticles have medicinal properties [ 110 , 111 ]. For example, green synthesized silver nanoparticles showed an efficient and large zone of clearance against various bacterial strains compared to commercial silver nanoparticles (Fig.  7 ) [ 112 ].

Schematic for the antimicrobial activity for the five bacterial strains: a Staphylococcus aureus , b Klebsiella pneumonia , c Pseudomonas aeruginosa , d Vibrio cholera , and e Proteus vulgaris . Numbers of 1 through 6 inside each strain denote: (1) nickel chloride, (2) control ciprofloxacin, (3) Desmodium gangeticum root extract, (4) negative control, (5) nickel NPs prepared by a green method, and (6) nickel NPs prepared by a chemical method [ 229 ]

Catalytic activity

4-Nitrophenol and its derivatives are used to manufacture herbicides, insecticides, and synthetic dyestuffs, and they can significantly damage the ecosystem as common organic pollutants of wastewater. Due to its toxic and inhibitory nature, 4-nitrophenol is a great environmental concern. Therefore, the reduction of these pollutants is crucial. The 4-nitrophenol reduction product, 4-aminophenol, has been applied in diverse fields as an intermediate for paracetamol, sulfur dyes, rubber antioxidants, preparation of black/white film developers, corrosion inhibitors, and precursors in antipyretic and analgesic drugs [ 113 , 114 ]. The simplest and most effective way to reduce 4-nitrophenol is to introduce NaBH 4 as a reductant and a metal catalyst such as Au NPs [ 115 ], Ag NPs [ 116 ], CuO NPs [ 117 ], and Pd NPs [ 118 ]. Metal NPs exhibit admirable catalytic potential because of the high rate of surface adsorption ability and high surface area to volume ratio. Nevertheless, the viability of the reaction declines as a consequence of the substantial potential difference between donor (H 3 BO 3 /NaBH 4 ) and acceptor molecules (nitrophenolate ion), which accounts for the higher activation energy barrier.

Metallic NPs can promote the rate of reaction by increasing the adsorption of reactants on their surface, thereby diminishing activation energy barriers [ 119 , 120 ] (Fig.  8 ). The UV–visible spectrum of 4-nitrophenol was characterized by a sharp band at 400 nm as a nitrophenolate ion was produced in the presence of NaOH. The addition of Ag NPs (synthesized by Chenopodium aristatum L. stem extract) to the reaction medium led to a fast decay in the absorption intensity at 400 nm, which was concurrently accompanied by the appearance of a comparatively wide band at 313 nm, demonstrating the formation of 4-aminophenol [ 121 ] (Fig.  9 ).

Schematic of the metallic NP-mediated catalytic reduction of 4-nitrophenol to 4-aminophenol [ 120 ]

UV-visible spectra illustrating Chenopodium aristatum L. stem extract synthesized Ag NP-mediated catalytic reduction of 4-NP to 4-AP at three different temperatures a 30 °C, b 50 °C, and c 70 °C. Reduction in the absorption intensity of the characteristic nitrophenolate band at 400 nm accompanied by concomitant appearance of a wider absorption band at 313 nm indicates the formation of 4-AP [ 121 ]

Removal of pollutant dyes

Cationic and anionic dyes are a main class of organic pollutants used in various applications [ 122 ]. Organic dyes play a very imperative role due to their gigantic demand in paper mills, textiles, plastic, leather, food, printing, and pharmaceuticals industries. In textile industries, about 60% of dyes are consumed in the manufacturing process of pigmentation for many fabrics [ 123 ]. After the fabric process, nearly 15% of dyes are wasted and are discharged into the hydrosphere, and they represent a significant source of pollution due to their recalcitrance nature [ 124 ]. The pollutants from these manufacturing units are the most important sources of ecological pollution. They produce undesirable turbidity in the water, which will reduce sunlight penetration, and this leads to the resistance of photochemical synthesis and biological attacks to aquatic and marine life [ 125 , 126 , 127 ]. Therefore, the management of effluents containing dyes is one of the daunting challenge in the field of environmental chemistry [ 128 ].

The need for hygienic and safe drinking water is increasing day by day. Considering this fact, the use of metal and metal oxide semiconductor nanomaterials for oxidizing toxic pollutants has become of great interest in recent material research fields [ 129 , 130 , 131 ]. In the nano regime, semiconductor nanomaterials have superior photocatalytic activity relative to the bulk materials. Metal oxide semiconductor nanoparticles (like ZnO, TiO 2 , SnO 2 , WO 3 , and CuO) have been applied preferentially for the photocatalytic activity of synthetic dyes [ 31 , 132 , 133 , 134 ]. The merits of these nanophotocatalysts (e.g., ZnO and TiO 2 nanoparticles) are ascribable to their high surface area to mass ratio to enhance the adsorption of organic pollutants. The surface energy of the nanoparticles increases due to the large number of surface reactive sites available on the nanoparticle surfaces. This leads to an increase in rate of contaminant removal at low concentrations. Consequently, a lower quantity of nanocatalyst will be required to treat polluted water relative to the bulk material [ 135 , 136 , 137 , 138 ]. Like metal oxide nanoparticles, metal nanoparticles also show enhanced photocatalytic degradation of various pollutant dyes; for example, silver nanoparticles synthesized from Z. armatum leaf extract were utilized for the degradation of various pollutant dyes [ 127 ] (Fig.  10 ).

Schematic for the reduction of a safranine O, b methyl red, c methyl orange, and d methylene blue dyes using silver NPs synthesized from Z. armatum leaf extract by metallic nanoparticles [ 136 ]

Heavy metal ion sensing

Heavy metals (like Ni, Cu, Fe, Cr, Zn, Co, Cd, Pb, Cr, Hg, and Mn) are well-known for being pollutants in air, soil, and water. There are innumerable sources of heavy metal pollution such as mining waste, vehicle emissions, natural gas, paper, plastic, coal, and dye industries [ 139 ]. Some metals (like lead, copper, cadmium, and mercury ions) shows enhanced toxicity potential even at trace ppm levels [ 140 , 141 ]. Therefore, the identification of toxic metals in the biological and aquatic environment has become a vital need for proper remedial processes [ 142 , 143 , 144 ]. Conventional techniques based on instrumental systems generally offer excellent sensitivity in multi-element analysis. However, experimental set ups to perform such analysis are highly expensive, time-consuming, skill-dependent, and non-portable.

Due to the tunable size and distance-dependent optical properties of metallic nanoparticles, they have been preferably employed for the detection of heavy metal ions in polluted water systems [ 145 , 146 ]. The advantages of using metal NPs as colorimetric sensors for heavy metal ions in environmental systems/samples include simplicity, cost effectiveness, and high sensitivity at sub ppm levels. Karthiga et al. [ 147 ] synthesized AgNPs using various plant extracts used as colorimetric sensors for heavy metal ions like cadmium, chromium, mercury, calcium, and zinc (Cd 2+ , Cr 3+ , Hg 2+ , Ca 2+ , and Zn 2+ ) in water. Their as-synthesized Ag nanoparticles showed colorimetric sensing of zinc and mercury ions (Zn 2+ and Hg 2+ ). Likewise, AgNPs synthesized using mango fresh leaves and dried leaves (fresh, MF-AgNPs and sun-dried, MD-AgNPs) exhibited selective sensing for mercury and lead ions (Hg 2+ and Pb 2+ ). Also, AgNPs prepared from pepper seed extract and green tea extract (GT-AgNPs) showed selective sensing properties for Hg 2+ , Pb 2+ , and Zn 2+ ions [ 147 ] (Fig.  11 ).

Schematic of metal removal using metal oxides prepared by green synthesis. Left— a digital images and b absorption spectra of neem bark extract-mediated silver NPs (NB-AgNPs) with different metal ions and concentration-dependent studies of c Hg 2+ and d Zn 2+ . Right— a digital images and b absorption spectra of fresh mango leaf extract-mediated silver NPs (MF-AgNPs) with different metal ions and c concentration-dependent studies of Pb 2+ removal [ 147 ]

Conclusion and future prospects

‘Green’ synthesis of metal and metal oxide nanoparticles has been a highly attractive research area over the last decade. Numerous kinds of natural extracts (i.e., biocomponents like plant, bacteria, fungi, yeast, and plant extract) have been employed as efficient resources for the synthesis and/or fabrication of materials. Among them, plant extract has been proven to possess high efficiency as stabilizing and reducing agents for the synthesis of controlled materials (i.e., controlled shapes, sizes, structures, and other specific features). This review article was organized to encompass the ‘state of the art’ research on the ‘green’ synthesis of metal/metal oxide nanoparticles and their use in environmental remediation applications. Detailed synthesis mechanisms and an updated literature study on the role of solvents in synthesis have been reviewed thoroughly based on the literature available to help encounter the existing problems in ‘green’ synthesis. In summary, future research and development of prospective ‘green’ materials/nanoparticle synthesis should be directed toward extending laboratory-based work to an industrial scale by considering traditional/present issues, especially health and environmental effects. Nevertheless, ‘green’ material/nanoparticle synthesis based on biocomponent-derived materials/nanoparticles is likely to be applied extensively both in the field of environmental remediation and in other important areas like pharmaceutical, food, and cosmetic industries. Biosynthesis of metals and their oxide materials/nanoparticles using marine algae and marine plants is an area that remains largely unexplored. Accordingly, ample possibilities remain for the exploration of new green preparatory strategies based on biogenic synthesis.

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JS, KHK and PK made substantial contributions to interpretation of literature; drafted the article and revised it critically. All made substantial contributions to draft the article and revised it critically for important intellectual content and gave approval to the submitted manuscript. All authors read and approved the final manuscript.

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The corresponding author (KHK) acknowledges a supporting Grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, & Future Planning (No. 2016R1E1A1A01940995). Dr. Pawan Kumar would like to thank SERB and UGC, New Delhi, for the ‘Empowerment and Equity Opportunities for Excellence in Science’ video file No. EEQ/2016/00484 and the UGC-BSR Start Up-Research Project.

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Singh, J., Dutta, T., Kim, KH. et al. ‘Green’ synthesis of metals and their oxide nanoparticles: applications for environmental remediation. J Nanobiotechnol 16 , 84 (2018). https://doi.org/10.1186/s12951-018-0408-4

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Nanoparticles: synthesis and applications

Nguyen hoang nam.

1 Faculty of Physics, Hanoi University of Science, Vietnam National University, Hanoi, Hanoi, Vietnam

2 Nano and Energy Center, Hanoi University of Science, Vietnam National University, Hanoi, Hanoi, Vietnam

Nguyen Hoang Luong

This chapter focuses on the synthesis, functionalization, and applications of metallic, semiconductor, magnetic, and multifunctional nanoparticles. Synthesis methods such as chemical reduction, coprecipitation, seeding, microemulsion, hydrothermal synthesis, and sonoelectrodeposition are outlined. Functionalized nanoparticles are suitable for numerous applications. Several applications of nanoparticles in life sciences and the enviromment are discussed. Finally, some future trends are pointed out.

7.1. Introduction

Nanoparticles are defined by the worldwide federation of national standards bodies, the International Organization for Standardization (ISO), as nanoobjects with all external dimensions in the nanoscale, where the lengths of the longest and shortest axes of nanoobjects do not differ significantly ( ISO/TS 80004-2:2015 ). Though nanoscale is basically ranged from 1 to 100 nm, nanoparticles can be categorized by three size ranges: larger than 500 nm, between 100 and 500 nm, and between 1 and 100 nm ( European Commission, 2010 ). With respect to the size and the size distribution, nanoparticles may exhibit size-related intensive properties. If they are small enough to confine their electrons, they produce quantum effects and exhibit unexpected properties, for example, gold nanoparticles appear red in solution (see, for instance, Eustis and El-Sayed, 2006 ), and melt at much lower temperatures than that in slab form ( Buffat and Borel, 1976 ). The high surface-area-to-volume ratio of nanoparticles provides the significant changes in properties related to contact/surface area, such as catalytic ( Astruc, 2008 ), surface-enhanced plasmon resonance ( Melaine et al., 2015 ), etc. Depending on the composition and structure, nanoparticles can be of single properties such as metallic, dielectric, semiconductor, magnetic, or multifunctional which include more than one feature from single-property nanoparticles. Their applications, or potential applications, are in many different fields ( Salata, 2004 , Mody et al., 2010 , Lu et al., 2007 , Zhang et al., 2008 , Nguyen et al., 2015 ; and references therein). Among those, the advantages of nanoparticles in applications in life sciences and the environment are due to the fact that their size is comparable with the dimensions of objects such as viruses (about 10–100 nm) or cells (about 1–10 µm). This gives nanoparticles an ability to attach to biological entities without changing their functions, while the high surface-area-to-volume ratio of nanoparticles permits strong bonds with surfactant molecules. In environmental applications, the specific features (small size, large surface area) of nanoparticles can provide a tool for very sensitive detection of a specific contaminant from the presence of which pollution often arises. The engineering of nanoparticles can also offer opportunities to treat environmental contamination.

In this chapter we focus on the synthesis, functionalization, and applications of metallic, semiconductor, magnetic, and multifunctional nanoparticles. Compiling all the literature would greatly exceed the scope of this work, instead, we present typical and representative examples for discussion on the synthesis, functionalization, and applications of those nanoparticles.

7.2. Synthesis of Nanoparticles

7.2.1. chemical reduction.

Chemical reduction is an effective wet-chemical method for making zero-valent nanoparticles based on chemical-reducing aqueous salts of metals, such as silver nitrate (AgNO 3 ) in the case of synthesis of silver nanoparticles, for instance. To reduce the precursor metal salt, at least one reducing agent is used to produce electrons for metal ions that reduce them to become zero-valent. Commonly used reductants are borohydride, citrate, and ascorbate. Reduced nanoparticles are stabilized by a stabilizing agent. An example of a stabilizing agent is cetyltrimethylammonium bromide [(C 16 H 33 )N(CH 3 ) 3 Br; CTAB], which is widely used in gold nanoparticle synthesis. The stabilizing agents can be reducing agents themselves, such as citrate of sodium in making silver nanoparticles ( Shenava Aashritha, 2013 ). For more details the reader is referred to the review paper by Alaqad and Saleh (2016) and references therein.

7.2.2. Coprecipitation

Precipitation is the carrying down by a precipitate of soluble substances under certain conditions ( Patnaik, 2004 ). Generally, when the concentration of substances reaches supersaturation, a nucleation suddenly appears in solution. The nucleation will be grown by the diffusion on to its surface which then becomes nanoparticles. During the growth, the nucleation needs to be slowed down in order to get uniform nanoparticles. Several methods can be listed as precipitation: coprecipitation, microemulsion/inverse microemulsion, polyol, etc. Coprecipitation is a convenient way to synthesize Fe 3 O 4 nanoparticles ( Lu et al., 2007 , Quy et al., 2013 , Dung et al., 2016 , Khalil, 2015 , Mascolo et al., 2013 ). The mixture of two chloride salts of FeCl 2 and FeCl 3 with 1:2 molar ratios of Fe 2+ /Fe 3+ was vigorously stirred and kept at 70°C before NH 4 OH was added resulting in the black color precipitation. The Fe 3 O 4 nanoparticles were collected after purifying through magnetic separation with ethanol and distilled water several times to decontaminate the residual chemicals. By modifying the pH and ion concentration in solution, the size of nanoparticles can be controlled.

7.2.3. Seeding

The seeding method was discovered by Frens, 1972 , Frens, 1973 , where nanoparticles are grown by the reduction of salt in aqueous solution which contains seed nanoparticles. In this method, the stabilizers are used to control the size and shape of growing nanoparticles. This method was developed over the years using various types of seeding, reductant agents, and stabilizers ( Xu et al., 2007 , Han et al., 2009 , Perrault and Chan, 2009 , Ziegler and Eychmuller, 2011 , Rioux and Meunier, 2015 ). For example, the method which was developed by Perrault and Chan in 2009, reduces HAuCl 4 in gold nanoparticles seed-contained aqueous solution using hydroquinone. The gold nanoparticle seed can act in conjunction with hydroquinone to catalyze the reduction of gold ions into their surface. If the stabilizer is citrate, typically the seed nanoparticles were prepared by the citrate method. Using this method, the size of nanoparticles can be grown at least 30–300 nm.

7.2.4. Microemulsion and Inverse Microemulsion

Microemulsion is a popular method to synthesize nanoparticles, where microemulsions are an isotropic and thermodynamically stable mixture of “oil,” water, and surfactant, or in combination with a cosurfactant. The basic types of microemulsions are direct (oil dispersed in water) and reverse (water dispersed in oil). The small drops of aqueous phase (micelles) may contain salts and/or other ingredients, and the “oil” may actually be the mixture of the surfactants. The reaction to form nanoparticles also can be realized when the micelles mix with each other and the growth of the nanoparticles is controlled by the surfactants in “oil” ( Lopez-Quintela and Rivas, 1993 ). Using this method, the Fe 3 O 4 nanoparticles can be synthesized ( Feltin and Pileni, 1997 ) and also can be functionalized with a silica layer. This method can also be used to prepare Fe 3 O 4 /Au core–shell nanoparticles to prevent the oxidation of magnetic nanoparticles (MNPs) as well as form the biocompatibility ( Boutonnet et al., 1982 ). Furthermore, the inverse microemulsion method is the simplest way to produce multifunctional nanoparticles by creating the silica matrix in aqueous phase, which will fix the single particles inside ( Dung et al., 2016 ).

7.2.5. Hydrothermal Method

In the hydrothermal method, the crystals of nanoparticles are grown by heterogenous reaction under conditions of high temperature and high pressure from substances which are insoluble at normal temperature and pressure ( Byrappa and Adschiri, 2007 ). The crystal growth is carried out in an apparatus consisting of a steel pressure vessel called an autoclave, in which nutrients are supplied along with water. Hydrothermal synthesis is usually carried out below 300°C. The critical condition gives a favorable reaction field for formation of nanoparticles, owing to the enhancement of the reaction rate and large supersaturation based on the nucleation theory. This method has been used to synthesize metal oxide nanoparticles in supercritical water ( Hayashi and Hakuta, 2010 ), metal nanoparticles ( Kim et al., 2014 ), and semiconductor nanoparticles ( Bui et al., 2014 , Hoa et al., 2011 , Williams et al., 2007 ).

7.2.6. Sonoelectrodeposition

Sonoelectrodeposition is a useful synthesis method for nanoparticles and has been successfully applied to prepare metallic nanoparticles such as FePt and CoPt ( Luong et al., 2011 , Nam et al., 2012 ; and references therein). Sonoelectrodeposition is a technique combining the advantages of electrodeposition and mechanical waves of ultrasound to produce metallic nanoparticles ( Zhu et al., 2000 ). In Section 7.4 we discuss silver nanoparticles. One of main disadvantages of the conventional synthesis methods for silver nanoparticles, including chemical reduction, is the presence of unexpected toxic ions in the final products. The toxic ions in the product are mostly the ions of the silver precursor, such as nitrate and thiolsulfate. A good silver precursor such as silver acetate can be used ( Irzh et al., 2007 ), however, this chemical is expensive and manipulation is difficult under ambient conditions. Tuan et al. (2011) reported a modified sonoelectrodeposition technique to obtain silver nanoparticles in a nontoxic solution. The modification is that a silver plate was used as the cathode instead of silver salts thus allowing the avoidance of unexpected ions from the salts.

7.3. Functionalization/Coating of Nanoparticles

7.3.1. functionalization of nanoparticles.

Functionalization of nanoparticles can be defined as the addition of a chemical functional group on their surface in order to achieve surface modification that enables their self-organization and renders them compatible ( Subbiah et al., 2010 ). The most widely used functional groups are amino, biotin, steptavidin, carboxyl, and thiol groups ( Bruce and Sen, 2005 ). The main purpose of functionalizing nanoparticles is to cover their surface with a molecule that possesses the appropriate functionality needed for the designed application. For many biomedicine applications, nanoparticles need to be functionalized in order to conjugate with biological entities such as DNA, antibodies, and enzymes. For more details on the functionalization of nanoparticles, its methods, and class, as well as its implications in biomedical sciences, the reader may be referred to, for instance, the review by Subbiah et al. (2010) .

We focus here on the functionalization of gold nanoparticles and MNPs discussed in Section 7.4 . For application in detecting breast cancer cells, gold nanoparticles synthesized by a chemical reduction were functionalized with 4-aminothiolphenol (4-ATP, sometimes called p -aminothiolphenol [PATP]). For basal cell carcinoma (BCC) detection, different amounts of 4-ATP solutions were added to gold nanoparticles coated by CTAB. CTAB on the surface of gold nanoparticles was replaced by 4-ATP to form gold nanoparticles functionalized with 4-ATP (Au-4ATP). Fe 3 O 4 nanoparticles were functionalized using 3-aminopropyl triethoxysilane (APTS). APTS is a bifunctional molecule, an anchor group by which the molecule can attach to free –OH surface groups. The head group functionality –NH 2 is for conjugating with biological objects. The amino-NP is ready to conjugate with the DNA of the herpes virus and with the antiCD4 antibody.

7.3.2. Silica Coating of Magnetic Nanoparticles

Maintaining the stability of MNPs for a long time without agglomeration or precipitation is an important issue (see, for instance, Lu et al., 2007 ). The protection of MNPs against oxidation by oxygen, or erosion by acid or base, is necessary. The common method is protection by a layer which is impenetrable, so that oxygen, for example, cannot reach the surface of the particles. It is noted that the stabilization and protection of particles are often closely linked with each other. One of the ways to protect MNPs is coating them with silica. A silica shell not only protects the magnetic cores, but can also prevent direct contact of the magnetic core with additional agents linked to the silica surface that can cause unwanted interactions. The coating thickness can be controlled by varying the concentration of ammonium and the ratio of tetraethylorthosilicate (TEOS) to H 2 O. The surfaces of silica-coated MNPs are hydrophilic, and are readily modified with other functional groups ( Ulman, 1996 ). Quy et al. (2013) and Hieu et al. (2017) have prepared Fe 3 O 4 /SiO 2 nanoparticles by coating MNPs with silica using TEOS.

7.3.3. Multifunctional Nanoparticles

Recently, multifunctional nanoparticles have gained wide attention due to their advantages in the goal of applications. Potentially, multifunctional nanoparticles which include individual physicochemical properties of nanoparticles, such as plasmonic metallic nanoparticles, photoluminesable semiconductor nanoparticles or quantum dots, and MNPs, can complement some of the limitations of conventional applications using single nanoparticles, particularly in biomedicine. For example, bifunctional nanoparticles which are composed of MNPs and metallic nanoparticles not only can be used as optical labels in bioimaging, diagnosis and therapy, but also allow some biomolecules to be tagged and separated, together with targeted drug delivery and magnetic resonance imaging under the induction of an external magnetic field ( Cai et al., 2014 , Sotiriou et al., 2011 , Ilovitsh et al., 2015 , Giani et al., 2012 , Sun et al., 2006 ). Multinanoparticles can be in core–shell structures or in the complex structures which are a combination of at least two types of single nanoparticles.

Core–shell structured nanoparticles can be classified into inorganic/inorganic, inorganic/organic, organic/inorganic, organic/organic, core/multishell, and movable core/hollow shell nanoparticles ( Chaudhuri and Paria, 2012 ). The synthesis approaches to nanoparticles can be divided into top-down and bottom-up methods. The top-down approaches often use externally controlled tools to cut, mill, and shape materials into the designed nanoscale structures, for example, lithography methods, laser beams, mechanical techniques. The bottom-up approaches exploit the chemical properties of the molecules to cause them to self-assemble to become nanoparticles, such as chemical synthesis discussed above. The bottom-up methods can produce much smaller nanoparticles and are cost-effective, compared to the top-down methods. Both methods are used in the synthesis of core–shell structured nanoparticles. However, since ultimate control is needed for achieving a uniform coating of the shell, the bottom-up approach has proven more suitable. A combination of the two methods can also be utilized, for example, core particles synthesized by the top-down method but then coated by the bottom-up approach in order to maintain precise shell thickness. In general, various methods were used to prepare core–shell nanoparticles. For example, to produce iron oxide@Ag core–shell nanoparticles, several methods were used including impregnation ( Liu et al., 2012 ), surface functionalization followed by deposition ( Liu et al., 2010 ), solvo-thermal reduction ( Liu and Li, 2009 ), and chemical reduction ( Hu et al., 2010 , Sun et al., 2012 ). Reducing agents such as glucose and sodium borohydride are used for the reduction of silver salts, and the surface functionalization of iron oxide nanoparticles by different surface-modifying agents is required. Hu et al. (2010) used glucose for the reduction of Ag(NH 3 ) 2 + to Ag, which is adsorbed onto the surface of silica-coated iron oxide which is prepared by the coprecipitation method. Liu et al. (2010) have reported the surface functionalization of Fe 3 O 4 surface by APTS followed by reduction of AgNO 3 using sodium citrate and sodium borohydride. Sun et al. (2012) have used sodium borohydride as the reducing agent for the reduction of Ag(NH 3 ) 2 + to obtain Fe 3 O 4 @Ag core–shell nanoparticles. Dung et al. (2016) used ultrasound to assist in the reduction of silver ions following this strategy. Liu and Li (2009) have used dimethylformamide as the reducing agent during solvo-thermal synthesis of γ-Fe 2 O 3 @Ag microspheres. In parallel, one-step synthesis using thermal decomposition of silver acetate in the presence of iron oxide microspheres is also applicable and does not require the addition of any external reducing agent or surface modification of iron oxide ( Sharma and Jeevanandam, 2013 ). To control the overall size and the shell thickness, a microemulsion method, where water droplets act as a template or nanoreactor, is preferable to a bulk medium.

Other types of multifunctional nanoparticles are the complex structures of materials, which is the combination of at least two types of single nanoparticles. Similar to a core–shell structure, the nanomaterials used in complex structures can be categorized by two types of nanoparticles: organic, which includes micelles, liposomes, nanogels, dendrimes, and inorganic, which includes magnetic, semiconductor, lanthanide, and metallic nanoparticles. The combination can be achieved in many ways, however it is needed to fulfill the requirements of the applications. For example, the multifunctional nanoparticles should have superparamagnetic properties in order to be applied in drug delivery and DNA separation, and they should have plasmonic properties in order to be applied as biolabeling agents, and they should also be biocompatible. The simplest combination which fulfills these requirements is the complex of Fe 3 O 4 nanoparticles with superparamagnetic properties and Ag nanoparticles with plasmonic properties in a matrix of SiO 2 which provide the biocompatibility and also improve the stability of Ag and Fe 3 O 4 nanoparticles. Surface activator polyvinylpyrrolidone (PVP) was used to control the size of silver nanoparticles, which were synthesized by a wet-chemical reduction method with NaBH 4 as reductant. The synthesized nanoparticles were coated with 4-ATP to form functionalized Ag-4ATP nanoparticles. These functionalized nanoparticles were combined with the above-prepared Fe 3 O 4 nanoparticles by an inverse microemulsion method to form multifunctional nanoparticles ( Dung et al., 2014 ). In this method, the microemulsion was created by mixing the hydrophilic phase of the mixture of Ag-4ATP and Fe 3 O 4 solution and the hydrophobic phase of toluene. The mixture of Ag-4ATP/Fe 3 O 4 with different mass rates was moderated under a sonic bath for 2 hours, then TEOS was added to react with water in solution as in reaction ( 7.1 ). The formed SiO 2 coating layer in amorphous conformation covers both initial particles.

The multifunctional composites were also successfully prepared in a complex form using an ultrasound-assisted chemical method ( Dung et al., 2017a ). MNPs were firstly prepared by the coprecipitation method, then coated by a silica layer. The silica layer, after that, was modified by APTS. Silver ions were then absorbed on the surface of APTS-functionalized silica-coated MNPs. Under the ultrasonic wave of 200 W acting for 60 minutes these silver ions were reduced by sodium borohydride. In XRD characterization after synthesis, the relative intensity of diffraction peaks of silver crystals increases when the atomic ratio of silver to iron increases from 0.208 to 0.455. In parallel, all nanoparticles showed superparamagnetic properties with the saturation magnetization decreased from 44.68 to 34.74 emu/g with increasing silver:ion atomic ratio. The coexistence of strong surface plasmon absorption at 420 nm and these superparamagnetic properties make these particles promising for biomedical applications.

In another way, MNPs can be directly functionalized with an amino group without coating by silica layer ( Dung et al., 2017b ). In this way, Fe 3 O 4 -ZnO multifunctional nanoparticles were successfully synthesized in aqueous solution by ultrasound-assisted thermolysis. The as-prepared Fe 3 O 4 MNPs were modified by APTS to have free amine (−NH 2 ) groups on their surface. Zn 2+ ions then were added and stirred to adsorb onto the surface of Fe 3 O 4 -NH 2 nanoparticles in alkaline solution at pH 11. The solution was decomposed through thermolysis in an ultrasound bath. The characterization shows that photoluminescence of Fe 3 O 4 -ZnO multifunctional nanoparticles was enhanced in visible light at a wavelength of 565 nm to allow detection, labeling, diagnosis, and therapy in biomedicine. Furthermore, they exhibit superparamagnetic properties of Fe 3 O 4 with high saturation magnetization, which can be used for separation applications in biomedicine under an external magnetic field.

7.4. Applications

7.4.1. application of gold nanoparticles for breast cancer cell detection.

Gold nanoparticles are promising candidates for cell imaging and tumor-targeted drug delivery ( Sokolov et al., 2003 , Paciotti et al., 2004 , Jain, 2005 ), breast cancer diagnosis, and targeted therapy ( Yezhelyev et al., 2006 ). Being a member of the epidermal growth factor receptor tyrosine kinase family, HER2 is found to be overexpressed in 20%–30% of human breast cancers ( Harries and Smith, 2002 ; and references therein). Therefore, HER2 is an interesting target for breast cancer therapies. Anti-HER2 (trastuzumab, trade name Herceptin) is a humanized monoclonal antibody (mAb) designed specifically for antagonizing the HER2 function. Quynh et al. (2011) and Nguyen et al. (2015) have synthesized gold nanoparticles by a chemical reduction then applied them for imaging KPL4 breast cancer cells after conjugating them with trastuzumab.

Fig. 7.1 shows the bright-field and dark-field microscopy images of breast cancer cells after being incubated with gold nanoparticles nonconjugated with trastuzumab as well as conjugated with trastuzumab ( Nguyen et al., 2015 ). As can be seen from Fig. 7.1 , when the gold nanoparticles were not conjugated with trastuzumab, the dark-field image showed no signal of the gold nanoparticles (A 2 ). When the gold nanoparticles were directly conjugated with trastuzumab, the gold nanoparticles were bound onto cancer cells and these cancer cells were clearly observed in the dark-field image (A 4 ) through the scattering light from the gold nanoparticles. When the 4-ATP functionalized gold nanoparticles (amino-gold nanoparticles) were covalently conjugated with trastuzumab through l-ethyl-3-(3-dimethylaminopropyl) ethylcarbodiimide (EDC) connection, the gold nanoparticles also concentrated on the cancer cells, but these cancer cells were observed with slightly lower intensity in the dark-field image (A 6 ) compared to those in image A 4 . Nguyen et al. (2015) pointed out, however, that the gold nanoparticles directly conjugated with trastuzumab could be stored in a freezer for only about 2 weeks before they lost their activity, while the gold nanoparticles covalently conjugated with trastuzumab were stable with storage for about two months.

Bright-field (A 1 , A 3 , A 5 ) and dark-field (A 2 , A 4 , A 6 ) microscopy images of breast cancer cells after being incubated with gold nanoparticles nonconjugated with trastuzumab (A 1 , A 2 ), the gold nanoparticles conjugated with trastuzumab (A 3 , A 4 ) and the amino-gold nanoparticles covalently conjugated with trastuzumab through EDC connection (A 5 , A 6 ).

7.4.2. Basal Cell Carcinoma Fingerprinted Detection

Skin cancer is the most common cancer in humans and its incidence is increasing ( Baxter et al., 2012 ). Worldwide, BCCs constitute about 74% of skin cancer cases ( NCIN, 2013 ). Among the treatments for “high-risk” BCCs (i.e., BCCs on the face and neck, or recurrent BCCs), Mohs micrographic surgery (MMS) is the most efficient ( Mohan and Chang, 2014 ). This procedure helps to maximize the removal of tumor cells, while spares as much healthy tissue as possible. However, the need for a pathologist or specialized surgeons to diagnose frozen sections during surgery has limited the wider use of MMS, which leads to cases of inappropriate inferior treatment. Frozen-section histopathology also requires arduous and time-consuming procedures, increasing the costs compared to standard methods of BCC excision.

For skin cancer diagnosis, Raman spectroscopic imaging is a promising technique, because of its high sensitivity to molecular and structural changes associated with cancer. However, raster scanning Raman mapping requires long times for data acquisition, typically days for tissue specimens of 1 cm×1 cm. Recently, multimodal spectral imaging based on Raman spectroscopy and tissue autofluorescence was used to reduce the BCC diagnosis time to only 30–60 minutes, which becomes suitable for use during MMS ( Kong et al., 2013 , Takamori et al., 2015 ).

An alternative method that could allow to reduce data acquisition and BCC diagnosis times during MMS is surface-enhanced Raman spectroscopy (SERS). It was discovered that strongly increased Raman scattering signals can be obtained in the very close vicinity of metal nanostructures, which are mainly due to resonances between optical fields and the collective oscillations of the free electrons in a metal. Thus SERS has attracted great interest in the biolabeling field because significant enhancement of the labeling signals of molecular vibrations on the metallic nanoparticles surface can be obtained. Quynh et al. (2016) studied surface-enhanced Raman (SER) signal of 4-ATP that was linked to the surface of gold nanoparticles conjugated with skin carcinoma cell antibody BerEP4. Gold nanoparticles with sizes ranged from 2 to 5 nm were prepared by a wet-chemical method using CTAB. The Au-4ATP-antibody solutions were dropped on the surface of the tissue sample and the SER scattering signals were collected and analyzed. Fig. 7.2 shows the fingerprinted landscape of SER signals of Au-4ATP-antibody on a BCC tissue. Fig. 7.2A shows the colored image of a Gram-stained tissue, where the cancer cell area may be the dark-colored regions, for example, region A1, A2. However, the result of diagnosis essentially depends on the subjective decision of the pathologists because this nonspecific method may lead to misinterpretation of noncancer regions as cancer ones. Fig. 7.2B shows a bright-field microscopy image of the tissue, where B1 region corresponds to a hair follicle position, and B2 does not, although B1 and B2 have the same position on the tissue as regions A1 and A2 in Fig. 7.2A . Fig. 7.2C shows the result of an SER signal obtained by the principal component analysis ( Quynh et al., 2016 ). Fig. 7.2D shows the result of the SER signal analyzed using only the intensity of SER peaks at 1075 cm −1 . The Au-antibody colloids are oriented close to the BCC surface by the antigen–antibody coupling. The carcinoma sections act as a dock where a high concentration of Au-4ATP-antibody particles is distributed, then the SER peak intensity at 1075 cm −1 will be higher in these areas. In Fig. 7.2C , the colored areas such as C1 and C2 can be considered as cancer regions. However, in Fig. 7.2D the area D1 does not show the high intensity of the peak at 1075 cm −1 , while the others, such as the D2 area, indicate very high intensity of the peak at 1075 cm −1 . From Fig. 7.2 , only A2, B2, C2, and D2 regions can be definitely considered as the cancer areas, while A1, B1, C1, and D1 may assigned as the position of hair follicles where the cell concentration is higher than in other parts. Quynh et al. (2016) pointed out that, while the whole SER map collecting time should be longer than 2 hours (the collecting time of each spectrum was nearly 5 seconds), the fingerprinted image using peak height at 1075 cm −1 can be observed in around 5 minutes. Hence, this method may represent a solution for quick diagnosis, even during operation.

Fingerprinted landscape of SER signals of Au-4ATP-antibody on BCC tissue. (A) Image of Gram-stained BCC tissue where A1 and A2 are the areas of suspected BCC; (B) bright-field microscopy image of tissue, where regions B1 and B2 are in the same position on the tissue as regions A1 and A2, respectively; (C) SER signal landscape obtained by principal component analysis, where regions C1 and C2 are in the same position on the tissue as regions A1 and A2, respectively; (D) the fingerprinted landscape of intensity of SER peaks at 1075 cm −1 , where regions D1 and D2 are in the same position on the tissue as regions A1 and A2, respectively. The difference between D1 and D2 shows that only red-colored D2 (and the similar-colored area) are the infected area, while D1 is not.

7.4.3. Antibacterial Test Using Silver Nanoparticles

Silver nanoparticles (AgNPs) are commonly utilized nanomaterials due to their antibacterial properties, high electrical conductivity, and unique optical properties that can be used in various applications ( Sondi and Salopek-Sondi, 2004 ). It is believed that the high affinity of Ag toward sulfur or phosphorus is the key element of its antibacterial property. As sulfur and phosphorus are found in abundance throughout cell membranes, AgNPs react with sulfur-containing proteins inside or outside the cell membrane, which in turn affects cell viability ( Pal et al., 2007 , Elechiguerra et al., 2005 ). Another theory proposed that Ag + ions released from AgNPs can interact with phosphorus moieties in DNA, resulting in inactivation of DNA replication, or can react with sulfur-containing proteins to inhibit enzyme functions ( Sharma et al., 2009 ). These properties allow the incorporation of AgNP into various matrices such as activated carbon (AC), polymer networks, textiles, and wound dressing materials ( Sedaghat and Nasseri, 2011 ).

Many approaches have been developed to obtain AgNP of various shapes and sizes, including chemical reduction, laser ablation, gamma irradiation, electron irradiation, chemical reduction by inorganic and organic reducing agents, photochemical method, microwave processing, thermal decomposition of Ag oxalate in water and in ethylene glycol, and sonoelectrochemical method (see references in Tuan et al., 2011 ). As pointed out in Section 7.2.6 , one of main disadvantages of those methods is the presence of unexpected toxic ions in the final products. Tuan et al. (2011) report a modified sonoelectrochemical technique to obtain AgNP in a nontoxic solution. The silver particles are then directly loaded on AC produced by thermal activation of coconut husk. Here we concentrate to their work on the antibacterial properties of AgAC examined by inhibition growth of Escherichia coli . Fig. 7.3 shows the quantitatively antibacterial study of AgNP in Luria-Bertani (LB) broth. It presents the dynamics of E. coli growth in only LB broth (negative control), TSC control [LB broth supplemented with 120 µL trisodium citrate (TSC) solution], and AgNP antibacterial tests (LB broth supplemented with AgNP of concentration from 2 to 200 µg/mL). In this figure, OD 595 represents optical density at 595 nm (1 optical density at 595 nm, OD 595 , equals the concentration of 1.7×10 9 cells/mL). The initial number of E. coli inoculated into 2 mL LB medium of the tested tube was 1.7×10 6 cells, giving the final bacterial concentration of 8.5×10 5 cells/mL. It is observed that E. coli grew normally in the negative control and the TSC control. After 30 hours in the TSC control, the concentration of E. coli (OD 595 =2.5) is higher than that in the negative control (OD 595 =1.5) which suggests that TSC was not toxic to E. coli and may be even enabled for the growth of the bacteria. With the presence of AgNP, the situation is different because of the well-known antibacterial property of AgNP ( Kim et al., 2007 ). When AgNP concentration was 2 µg/mL, the result is similar to that of the negative control because the low value of AgNP could not inhibit bacterial growth. With a higher AgNP concentration, the inhibitory effect appeared within 8 hours even at a low AgNP concentration of 4 µg/mL. Fig. 7.3 clearly shows that, with the concentration of AgNP>16 µg/mL, the E. coli growth was inhibited.

Bar chart of optical density at 595 nm, OD 595 , presenting Escherichia coli concentration in LB in the presence of different concentrations of AgNP (µg/mL) as a function of time (h). Each test was conducted after 4, 8, 24, and 30 h. It is clear that, with the concentration of AgNP ≥ 16 µg/mL, E. coli growth was inhibited.

7.4.4. Magnetic Nanoparticles

MNPs are of great interest in biomedicine applications. In the applications described below these nanoparticles were synthesized by the coprecipitation method.

7.4.4.1. Arsenic removal from water

MNPs Fe 3 O 4 were reported to adsorb arsenic ions from contaminated water ( Leslie-Pelecky et al., 2005 ). In this environmental application, compared to other techniques currently used to remove arsenic from contaminated water, such as centrifuges and filtration systems, this method using MNPs has the advantage of being simple, and, most importantly, not requiring electricity. This is very important, because arsenic-contaminated sites are often found in remote areas with limited access to power ( Filipponi and Sutherland, 2010 ).

The arsenic adsorption abilities of Fe 3 O 4 , Fe 1− x Co x ·Fe 2 O 3 (Co-ferrites) and Fe 1− y Ni y O·Fe 2 O 3 (Ni-ferrites) with x =0, 0.05, 0.1, 0.2, 0.5 and y =0.2, 0.4 were studied with different conditions of stirring time, concentration of nanoparticles, and pH ( Hai et al., 2008 ). The starting arsenic concentration of 0.1 mg/L was reduced about 10 times down to the maximum permissible concentration (MPC) of 0.01 mg/L after a few minutes of stirring. The removal process seemed not to depend considerably on the concentration of x in the Co-ferrites. Similar results were found for the Ni-ferrites, where the arsenic concentration was reduced to the MPC value after a stirring time of a few minutes and the removal did not change considerably with y . Studying also the effects of the weight of the nanoparticles on the removal process, Hai et al. (2008) showed that, after 3 minutes of stirring, the optimal weight to reduce arsenic concentration down to a value lower than the MPC was 0.25 g/L for Fe 3 O 4 and 0.5 g/L for Co- and Ni-ferrites. Studying the desorption process, Hai et al. (2008) showed that 90% of the arsenic ions was desorbed from nanoparticles. After desorption, the nanoparticles did not show any difference in arsenic readsorption ability. Repeating the adsorption–desorption process four times, Hai et al. (2008) proved that the nanoparticles could be reused for arsenic removal.

7.4.4.2. Herpes DNA separation

Herpes simplex virus, or herpes, causes extremely painful infections in humans ( Ryan and Ray, 2004 ). Thus, the determination of the presence of herpes is important. A simple and fast way to recognize the presence of the DNA of the virus is to use an electrochemical sensor. However, electrochemical sensors exhibit a sensitivity limit, so they cannot measure concentrations lower than a few tens of nM/L ( Tuan et al., 2005 ). Therefore, a virus DNA separation before the measurement by using the electrochemical sensor is needed in order to increase the concentration of the DNA. Hai et al. (2008) used a DNA sequence, which is representative of the herpes, as a probe to hybridize with the target DNA in the sample. After being activated with EDC and 1-methylimidazole (MIA), the probe DNA was mixed with the amino-NP to have nanoparticles with the probe DNA on the surface (DNA-NP). The herpes DNA separation was carried out as follows: 1 mL of the solution containing 2 wt.% of DNA-NP was mixed with 2–20 mL of a solution with 0.1 nM/L of the herpes DNA. The hybridization of the probe DNA and the target DNA appeared at 37°C for 1 hour. Then, the nanoparticles with hybridized DNA were collected and redispersed in 0.1 mL of water using magnetic decantation. The dehybridization of the nanoparticles with the probe and target DNA was obtained at 98°C. Hai et al. (2008) obtained a solution with a high concentration of the DNA of the herpes virus after removing the DNA-NP from the solution by using magnetic decantation. When all the target DNA were separated, the DNA concentration had increased from 20 to 200 times. Fig. 7.4 shows the dependence of the output signal on the initial volume of the solution containing 0.1 nM/L of the herpes DNA before and after the magnetic separation ( Hai et al., 2008 ). The initial solution contained 0.1 nM/L of the DNA, which was much smaller than the sensor sensitivity. Therefore, the output signals before magnetic enrichment were almost zero ( Fig. 7.4 , open squares ). After magnetic enrichment, the output signals linearly increased with increasing initial solution volume, depending on the initial volume of the solution. The higher the volume, the higher the concentration. The result is higher output signals were obtained. This means that the concentration of the herpes DNA was much higher after the enrichment. With the highest initial volume that Hai et al. (2008) used in their studies, the concentration after magnetic enrichment was 200 times higher than the initial concentration.

Dependence of the output signal on the initial volume of solution containing herpes DNA before and after magnetic separation.

7.4.4.3. CD4 + cell separation

In human immunodeficiency disease, such as HIV/AIDS, helper T cells (CD4 + T cells) are considerably destroyed by HIV. For mechanisms of CD4 + T cell depletion in HIV infection we refer the reader to the review of, for instance, Okoye and Picker (2013) . A dropping number of CD4 + T cells (which are often referred to as CD4 cells) in blood is an indicator of immunodeficiency as in the case of HIV/AIDS. The number of CD4 + T cells in the blood of HIV-infected patients is often reduced to less than 500 cells/μL. According to the World Health Organization (WHO) classification, a person is diagnosed with AIDS if the CD4 count is less than 200 cells/μL ( World Health Organization, 2007 ). Thus, the CD4 count is very important for doctors to adjust treatment strategies. The principle of counting the number of CD4 + T cells in blood is based on the specific linker between the antiCD4 monoclonal antibody and CD4 + T receptor on the lympho T surface ( Casset et al., 2003 ; and references therein). Fluorescent-labeled antiCD4 antibody has been commonly used to count CD4 + T cells of HIV/AIDS patients due to its binding specificity to the cells and fluorescent emission signals. However, the fluorescent signals of labeled CD4 + T cells are sometimes interfered with by autofluorescence of other dead white cells, such as killers (CD8 + ) T cells, B cells, macrophages, or neutrophils, which contribute to the background in detection. To minimize this background interference, CD4 + T cells can be magnetically sorted from other cells in the blood, followed by fluorescent signal detection.

Hai et al. (2008) and Khuat et al. (2008) used Fe 3 O 4 MNPs coated with fluorescent-labeled antiCD4 antibody (antiCD4-MNPs) to count the CD4 + T cells. The antiCD4-MNPs were prepared through covalent linking between the carboxyl group of the antiCD4 antibody and the amino group of amino-modified MNPs. The antiCD4-MNPs were then used as a material to conjugate with CD4 + T cells for magnetic separation. These authors observed a number of cells bound with magnetic clusters and particles. Fig. 7.5 shows the conventional microscope visualization of the blood cells after being coupled with the antiCD4 antibody and antiCD4-MNPs and separated using a magnet ( Hai et al., 2008 ). For observing the CD4 + T cells, using fluorescence isothiocyanate labeled antiCD4-MNPs, the fluorescent intensity was improved by about two times compared to when cells were only labeled with the antiCD4 antibody. This result indicates the role of the MNPs and can be used for the treatment of an HIV-infected patient with a simple fluorescent microscope.

Microscope visualization of the blood cells under white light (A, C) and under blue light excitation (B, D), after being coupled with the antiCD4 antibody and antiCD4-MNPs and separated by using a magnet.

7.4.4.4. Detection of pathogenic viruses

Purification of nucleic acids (DNA and RNA) from clinical samples is an important step in diagnostics, such as detection of pathogenic viruses and bacteria using the polymerase chain reaction (PCR), paternity testing, genetic research, DNA fingerprinting, and DNA sequencing. The nucleic acid purification method based on interaction with silica, developed by Boom et al. (1990) , is commonly used. It is known that DNA binds to silica, even though both DNA and the silica surface are negatively charged. Thus, to provide insight into important issues such as the mechanism behind DNA binding to silica is of great interest. Using molecular dynamics simulations, Shi et al. (2015) showed that the two major mechanisms for binding of DNA to silica are attractive interactions between DNA phosphate and surface silanol groups and hydrophobic bonding between DNA base and hydrophobic region of silica surface. These short-range attractions can be sufficiently strong to overcome the electrostatic repulsion between negatively charged DNA and negatively charged silica surface.

Micrometer-size silica-coated magnetic beads have been developed by different groups (see, for instance, Akutsu et al., 2004 ) and biotech companies such as Roche Diagnostics, Life Technologies, Promega, and Beckman Coulter to improve the efficiency of purification. Berensmeier (2006) described methods based on magnetic microparticles for nucleic acid purification. Recently, the investigation and application of silica-coated MNPs for separation and purification of nucleic acids has become an emerging area. Ashtari et al. (2005) reported a method for recovery of target ssDNA using amino-modified silica-coated MNPs and used them to recover trace concentrations of target ssDNA fragments of severe acute respiratory syndrome virus with high efficiency and good selectivity. Quy et al. (2013) presented a method for synthesis of the silica-coated Fe 3 O 4 MNPs and their application for isolation and enrichment of DNA of Epstein–Barr virus (EBV), which is associated with particular cancers and lymphoma, and hepatitis virus type B (HBV) which causes hepatitis. Quy et al. (2013) have shown that the purification efficiency of DNA of both EBV and HBV using synthesized silica-coated Fe 3 O 4 MNPs was superior to that obtained with commercialized silica-coated Fe 3 O 4 magnetic microparticles. Quy et al. (2013) reported also on time saving in detection of EBV and HBV, namely the time required for DNA purification using silica-coated Fe 3 O 4 nanoparticles was significantly reduced as the particles were attracted to magnets more quickly (15–20 seconds) than the commercialized silica-coated Fe 3 O 4 microparticles (about 2–3 minutes). These results were attributed to the fact that silica-coated Fe 3 O 4 nanoparticles have a larger total surface area compared to that of the commercialized silica-coated Fe 3 O 4 microparticles.

7.4.4.5. Specific and rapid tuberculosis detection

The worldwide effort to eradicate tuberculosis (TB), the highly infectious disease caused by Mycobacterium tuberculosis (MTB), has thus far led to significant decreases in the number of incidents and mortality rates. TB, however, remains the second leading cause of death from an infectious disease and a major global health problem ( World Health Organization, 2011 ). Unfortunately, in the absence of an effective screening method, there are many cases of TB and multidrug-resistant-TB which are not opportunely detected or treated. In the early 1990s, a diagnostic procedure based on the amplification of the insert sequence (IS) 6110 was developed and soon became prevalent. This method is displaying advantages regarding detection limit and specificity through the amplification of this signature sequence using the PCR technique ( Kolk et al., 1992 , Kolk et al., 1998 , Kox et al., 1994 , Sankar et al., 2011 , Shukla et al., 2011 ). However, this procedure requires the time-consuming extraction of DNA from each sample, including a cell lysis step which is usually inefficient on account of the highly complex bacterial cell wall ( Noordhoek et al., 1994 , Ellis and Zabrowarny, 1993 , Ogbaini-Emovon, 2009 ).

Recently, the collaboration between biologists and physicists has allowed the development of nanomaterials in DNA extraction from different organisms. More importantly, using MNPs, multiple samples could be processed simultaneously on a microtiter plate, which would enhance the testing rate and reduce the contamination risk for testing personnel, especially in the case of dangerous pathogens (e.g., TB). Furthermore, this material could be constructed to form bioconjugates containing specific antibodies which would enhance the specificity of the detection method ( Arruebo et al., 2009 ). Recently, Pham et al. (2015) reported for the first time the development of a specific and rapid TB detection using MNPs. The MNPs were functionalized with amino groups to facilitate coupling with anti-TB antibodies. The coupled nanoparticles were used to enrich Mycobacterium. In addition, preliminary assessment of this method in testing clinical samples (sputum and throat wash specimens) was also noted. The results of this study indicated potential for the establishment of a high-throughput semiautomated TB diagnostic procedure, which is currently being studied. Specificity, or the capability of improving signal-to-noise ratio, is a critical criterion in any diagnostic procedure. Samples collected from patients (sputum in most cases) normally contain other microorganisms which might be the contamination source. By pretreatment with N -acetyl l -cysteine-sodium hydroxide (NALC-NaOH), the decontamination could be done for sputum samples. This technique, however, could not eliminate nonspecific signal entirely. Besides the specifically designed primers for the amplification of the signature sequence IS6110, the coupled anti-MTB antibody served as a sieve which captured only the MTB antigens. The whole procedure was done in approximately one hour, which was half of the total time required for the traditional DNA extraction method.

7.4.4.6. Biological treatment targeting Mycobacterium tuberculosis in contaminated wastewater

Wastewater from hospitals and facilities receiving patients infected with contagious microorganisms has dense concentrations of these pathogens, which may represent a danger to public health. Therefore, proper wastewater treatment to remove these contaminants before discharging to the sewage system is a great societal concern. Most common wastewater treatment methods are divided into physical (heating, ultraviolet radiation, etc.), chemical (e.g., hypochlorite/chlorination, ozonation), and biological categories. The main disadvantages of the first two categories are complex implementation and high maintenance cost (e.g., due to corrosion of the pipe systems). With the additional advantage of being environmentally friendly, biological methods have become valuable alternatives. Most of these methods utilize live microorganisms in either fermentation processes to remove toxic chemicals or filtration applications through the formation of biofilm ( Sewage water treatment vat., 2016 ) and, thus, are without specific targets. Nguyen et al. (2016) reported biological treatment targeting MTB in contaminated wastewater using lysing enzymes coupled to Fe 3 O 4 MNPs. This study was a further development from previous results obtained with a complex comprised of magnetic amino nanoparticles and anti-TB antibody molecules ( Pham et al., 2015 ). The complex, referred to as NP-NH 2 -anti-TB, was shown to be capable of specifically capturing MTB ( Pham et al., 2015 ), which would be killed with the addition of lysing enzymes. The major role of the nanoparticles is to bring these molecules in close proximity so that the lysing enzymes can work on the bacteria captured through the conjugated antibody on the same surface area. This, ideally, would solve the limit of diffusion, thus enhancing the reaction rate. Nguyen et al. (2016) also reported an initial assessment of the developed method in a wastewater model by spiking the wastewater samples collected from a hospital and a facility receiving TB patients with Mycobacterium bovis from bacillus Calmette-Guérin vaccine. The results of this study indicated potential applications of the NP-NH 2 -anti-TB complex combined with enzymes to efficiently treat MTB-contaminated wastewater.

7.4.5. Applications of Multifunctional Nanoparticles

Multifunctional nanoparticles are gradually attracting more and more attention because of their ability to combine numerous properties, such as electronic, magnetic, optical, and catalytic. They are highly functional materials with modified properties which can be quite different from those of the individual materials. The properties of core–shell nanoparticles can be modified by changing either the constituting materials or the core-to-shell ratio ( Oldenburg et al., 1998 ). Multifunctional nanoparticles show distinctive properties of the different materials employed together to meet the diverse application requirements. The purpose of the functionalization is manyfold, such as surface modification, the ability to increase the functionality, dispersibility, and stability, controlled release of the single nanoparticles, reduction in consumption of materials, etc. Applications of different multifunctional nanoparticles are summarized in review articles by Karele et al. (2006) , Seleci et al. (2016) , Jia et al. (2013) , and Bao et al. (2013) . The multifunctional nanoparticles are widely used in various applications such as pharmaceutical applications ( Caruso, 2001 ), biomedical ( Balakrishnan et al., 2009 , Salgueirino-Maceira and Correa-Duarte, 2007 ), catalysis ( Daniel and Astruc, 2004 , Phadtare et al., 2003 ), electronics ( Kortan et al., 1990 , Qi et al., 1996 ), enhancing photoluminescence ( Mews et al., 1994 , Kamat and Shanghavi, 1997 ), creating photonic crystals ( Scodeller et al., 2008 ), etc. Especially in the biomedical field, these nanoparticles are used for bioimaging ( Laurent et al., 2008 , Babes et al., 1999 , Dresco et al., 1999 ), controlled drug release ( Dresco et al., 1999 ), targeted drug delivery ( Laurent et al., 2008 , Gupta and Gupta, 2005 , Dresco et al., 1999 , Yan et al., 2009 ), cell labeling ( Laurent et al., 2008 , Jaiswal et al., 2003 , Michalet et al., 2005 ), and tissue engineering applications ( De et al., 2008 ). For instance, core–shell nanoparticles have attracted considerable attention in clinical and therapeutic applications ( Hirsch et al., 2003 , Loo et al., 2004 ). Core–shell nanoparticles, which are strong absorbers, can be used in photothermal therapy, while those which are efficient scatterers can be used in imaging applications. Silica core–gold shell nanoshells, a novel core–shell nanostructure, which either absorb or scatter light effectively, can be designed by varying their core-to-shell ratio ( Loo et al., 2004 ). In imaging applications of core–shell nanoparticles, they can be conjugated with specific antibodies for diseased tissues or tumors. When conjugated nanoparticles are inserted in the body, they get attached to diseased cells and can be imaged. In parallel, when the tumor has been located, resonance wavelength absorption of the core–shell nanoparticles will lead to localized heating of the tumor and it is destroyed. In other words, the imaging and photothermal therapy can be carried out together with core–shell or multifunctional nanoparticles. In drug-delivery systems using core–shell nanoparticles, the drug can be encapsulated or adsorbed onto the nanoparticle surface ( Sparnacci et al., 2002 ) via a specific functional group or by an electrostatic stabilization technique. The nanoparticles will come into contact with the biological medium, and then direct the drug. For instance, the enzyme- and antibody-conjugated core–shell nanoparticles, which are strong absorbers with gold shells, can be embedded in a matrix of the polymer, such as nisopropylacrylamide (NIPAAm), and acrylamide (AAm) ( West and Halas, 2000 ). These polymers exhibit a melting temperature which is slightly above body temperature. If the core–shell nanoparticles absorb heat from the illumination with resonant wavelength, the heat will transfer to the local environment, then cause collapse of the polymer matrix and release of the drug. Furthermore, when the core is MNPs, the core–shell nanoparticles with the bifunction of magnetic and gold nanoparticles can drive the drug and kill the tumor under the external magnetic field as well as by the irradiation of resonance wavelength. The usual methods of tumor treatment, such as chemotherapy or radiotherapy, have various side effects such as substantial loss of hair, lack of appetite, diarrhea, etc. The process of attacking the tumor also leads to the loss of many healthy cells. Core–shell or multifunctional nanoparticles offer an effective and relatively safer strategy to cure these ailments by significantly reducing the amount of chemicals or radiation using local treatments.

Another strategy of using multifunctional nanoparticles is using multiple types of nanoparticles in one application, such as in fast DNA diagnosis ( Quynh et al., 2014 ). This fast DNA kit created from NH 2 -modified SiO 2 -coated Fe 3 O 4 nanoparticles and highly fluorescent Mn-doped ZnS nanoparticles in a sandwich structure, which can be seen in Fig. 7.6 . The sandwich configuration attached the fluorescent particles to the docking matrix of MNPs with the complementary hybridization of the detector probe–target–capture probe structure. In one side of this sandwich structure, SiO 2 -coated Fe 3 O 4 nanoparticles, which were modifed by the NH 2 group, were employed as a docking matrix. The docking matrix was linked with a capture probe oligonucleotide chain, which specifically identifies the target DNA. In the other side of the sandwich, the detector probe formed by the signaling semiconductor nanoparticles contacted with another oligonucleotide chain. This sandwich configuration was applied to detect DNA of EBV using separation function of MNPs and the signaling function of semmiconductor nanoparticles. The kit firstly was stored as three separated solutions containing magnetic probe nanoparticles, semiconductor detector nanoparticles, and target DNA molecules, respectively. In order to measure the target DNA concentration, those solutions were mixed to assemble the magnetic donor–target DNA–semiconductor detector complexes via the specific hybridization of catcher probe and detector probe with the target DNA. The complexes, then, are easily collected by an outside magnetic field. The other components, which do not contain MNPs will be washed out. The collected complexes were redistributed in solution and measured by photoluminescence. The luminescent intensity at 586 nm of Mn-doped ZnS nanoparticles changes with changing the initially added DNA target concentration. The detection limit of target DNA is around 2×106 copies/mL (~0.3 fM), showing the ability of using the named multifunctional magnetic–semiconductor sandwich structure for fast KIT DNA detection during scene investigation and viral DNA detection.

Schematic sandwich structure of DNA detection Fast kit using multifunctional magnetic–semiconductor nanoparticles.

Finally, in addition to the improved material properties, multifunctional materials are also important from an economic point of view. Multifunctional nanoparticle-based drug-delivery systems have been developed to improve the efficiency and reduce the systemic toxicity of a wide range of drugs, where additional capabilities like targeting and image contrast enhancement added to the nanoparticles. However, additional functionality means additional synthetic steps and costs, more convoluted behavior and effects in vivo, and also greater regulatory hurdles. The tradeoff between additional functionality and complexity is discussed by Cheng et al. (2012) .

7.5. Conclusion and Perspectives

This chapter reviews various methods of synthesis and functionalization of metallic, semiconductor, magnetic, and multifunctional nanoparticles. Some applications of the fabricated nanoparticles in life sciences and the environment are discussed.

In the synthesis domain, it is expected that new preparation approaches will be introduced allowing the use of less energy and less toxic materials (“green manufacturing”). In order to meet requirements for different applications, the functionality of nanoparticles becomes more complex. Thus the major trend in the further development of nanoparticles is to make them multifunctional, with the potential to integrate various functionalities. Smart multifunctional nanoparticles will be very promising for a variety of applications.

Acknowledgment

The authors would like to thank Prof. N.H. Hai, Prof. N.T.V. Anh, Prof. P.T. Nghia, Dr. P. Yen, Mr. L.M. Quynh, Dr. I. Notingher, and Prof. M. Henini for close collaboration.

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Home > Books > Green Chemistry for Environmental Sustainability - Prevention-Assurance-Sustainability (P-A-S) Approach

Green Synthesis of Nanoparticles: A Biological Approach

Submitted: 29 May 2023 Reviewed: 05 June 2023 Published: 11 August 2023

DOI: 10.5772/intechopen.1002203

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Nanoparticles are often associated with their small size and numerous applications. However, the synthesis process is equally important as it determines the size and properties of the nanoparticles. While traditional nanoparticle synthesis methods require the use of hazardous chemicals and high-energy consumption, green synthesis offers a sustainable, cost-effective, and environmentally friendly alternative. This approach utilizes natural resources and biologically active compounds that can act as reducing, stabilizing, or capping agents in the one-step synthesis of nanoparticles. Green synthesis offers numerous advantages, including the development of processes with minimal environmental impact and improved safety for nanoparticle synthesis. Overall, the synthesis of nanoparticles using green chemistry is a promising approach for sustainable and efficient production. This chapter provides a general overview of nanoparticles, their applications, and green synthesis, and highlights the various biological resources used in these processes and the factors affecting their synthesis.

• green synthesis
• nanoparticles
• plant extract
• microorganisms
• phytochemicals

Author Information

Rafael álvarez-chimal *.

• Laboratory 113 Synthesis of Magnetic Nanomaterials, Condensed Matter Department, Institute of Physic, National Autonomous University of Mexico, Ciudad Universitaria, Mexico City, Coyoacán, Mexico

Jesús Ángel Arenas-Alatorre

*Address all correspondence to: [email protected]

1. Introduction

Nanoparticles are small particles with sizes ranging from 1 to 100 nanometers. These materials have gained importance and interest in recent years owing to their large number of applications, because the matter at this scale presents a more compact arrangement of atoms and molecules, generating phenomena and acquiring or enhancing mechanical [ 1 ], electrical [ 2 ], magnetic [ 3 ], optical [ 4 ], catalytic [ 5 ], and antibacterial [ 6 , 7 ] properties that are completely different from those of their macroscopic counterparts [ 8 ]. They can be classified based on their composition, shape, and size. The most common types of nanoparticles are metals, metal oxides, carbon-based, and quantum dots. Owing to their unique sizes and properties, nanoparticles have attracted significant attention in various fields including medicine, electronics, energy, and environmental science [ 9 , 10 ]. By reducing their size, nanoparticles can have a higher surface-to-volume ratio, enabling a greater number of atoms or molecules per volume, which means that less material is needed to obtain the same activity and exhibit other properties ( Figure 1 ) [ 11 ].

Surface-to-volume ratio of nanoparticles compared with that of bulk materials.

Nanoparticles have many potential benefits for the environment. For example, nanoparticles can be used to improve the efficiency of water treatment, air filtration, and soil remediation; reduce pollution, and develop new types of renewable energy technologies [ 12 ]. In medicine, nanoparticles have shown potential for drug delivery, imaging, and cancer therapy. They can be functionalized with targeting moieties, making them capable of selectively targeting cancer cells, while sparing normal cells. Additionally, nanoparticles can enhance the efficacy of chemotherapy by improving drug delivery to the tumor site and reducing systemic toxicity [ 13 ]. In electronics, nanoparticles are used to fabricate high-performance devices such as sensors, transistors, and solar cells [ 14 ]. Nanoparticles have potential applications in fuel cells, hydrogen storage, and catalysis [ 15 ].

However, it is also important to address the environmental impact of the nanoparticles. Some studies have shown that nanoparticles can harm plants, animals, and humans, but it depends on many factors, such as concentration, size, and time of exposure [ 16 , 17 ]. Nanoparticles can easily be released into the environment through various sources, such as industrial emissions, consumer products, and medical procedures. Once released into the environment, nanoparticles can be difficult to control and monitor. There is potential for long-term accumulation. Nanoparticles can accumulate in the environment, and they may be able to persist for long periods. This raises concerns about the potential for nanoparticles to cause long-term harm to the environment and human health [ 17 , 18 ]. However, one of the alternatives for reducing their environmental impact is to control the synthesis process.

There are many methods for synthesizing nanoparticles, including physical, chemical, and biological processes [ 19 ]. Green synthesis, which refers to the eco-friendly and sustainable production of nanoparticles without the use of hazardous chemicals or toxic solvents, has gained attention in recent years within biological processes. Natural sources, such as plants and microorganisms, are popular green synthesis approaches [ 20 ]. This method has several advantages over traditional synthesis methods, including low cost, scalability, and reduction of hazardous waste. Moreover, green synthesis can produce nanoparticles with unique shapes, sizes, and surface properties tailored for specific applications [ 21 ]. The biological sources used for the green synthesis of nanoparticles contain biologically active compounds, such as enzymes, proteins, polyphenols, flavonoids, and terpenoids, which can act as catalyzing, reducing, stabilizing, or capping agents for one-step synthesis [ 20 , 21 ].

In summary, this chapter provides a general overview of nanoparticles, their properties and applications, and how green synthesis is used to synthesize them. This chapter also discusses the different biological resources used for green synthesis, the factors that participate, and the mechanisms involved in their production.

2. Traditional nanoparticle synthesis methods

Chemical reduction: This method involves the reduction of metal ions in solution using chemical reagents such as sodium borohydride or sodium hydroxide to form nanoparticles [ 22 ].

Coprecipitation: Synthesis involves mixing two or more solutions containing metal ions. When the solutions are mixed, metal ions precipitate out of the solution and form nanoparticles [ 23 ].

Sol-gel: The process requires mixing a metal salt with a solvent and gelling agent. The solvent is evaporated leaving behind the gel. The gel is then heated, causing it to solidify and form nanoparticles [ 24 ].

Microemulsion: This method needs surfactants, water-soluble compounds, and oil-soluble compounds. The mixture forms small droplets that contain the metal ions. When droplets are heated, metal ions precipitate out of the solution and form nanoparticles [ 25 ].

Solvothermal/hydrothermal synthesis: This reaction involves heating a solution of metal ions in water or an organic solvent under high pressure. High pressure and temperature cause metal ions to precipitate out of the solution and form nanoparticles [ 26 ].

Sonochemical/electrochemical synthesis: This process uses ultrasound or an electrical current to break down metal salts into nanoparticles [ 27 ].

Nanometric scale and different approaches to nanoparticle synthesis.

In addition, there are physical processes, such as laser ablation, milling, and sputtering, where the material is reduced to nanoparticles by the mechanical action of the equipment used [ 28 ].

The choice of method depends on the type of nanoparticles being synthesized, the desired size and shape, and the availability of equipment and reagents.

2.1 Environmental limitations in nanoparticle synthesis

Traditional methods for synthesizing nanoparticles have several limitations.

Using organic reagents can harm the environment, humans, and animals, causing illnesses, such as liver damage [ 18 ]. In addition, wastewater generated from nanoparticle synthesis can contain harmful chemicals [ 29 ].

The low yield is another disadvantage: only a small percentage of the starting materials is converted into nanoparticles, generating raw material waste. The high cost of the starting materials, equipment, labor required, long-time synthesis, and the inability to control the size and shape can limit their applications [ 30 , 31 ].

2.2 Strategies to overcome barriers to nanoparticle synthesis

Several strategies can be used to overcome the disadvantages of nanoparticle synthesis, such as the use of environmentally friendly solvents, reagents, and processes. Using water, ionic liquids, and supercritical fluids are examples of eco-friendly solvents [ 21 , 32 ] or we can even perform solvent-free synthesis, eliminating the need for hazardous chemicals and reducing the environmental impact of nanoparticle synthesis [ 33 ].

Many nanoparticle synthesis methods are not scalable, which limits their application. Therefore, it is necessary to develop cost-effective and efficient processes to obtain large quantities of nanoparticles [ 8 ].

Multipurpose nanoparticles can be used to improve their performance in a variety of applications and fields. For example, biocompatible nanoparticles are used in biomedicine or as stable nanoparticles for long-term applications [ 34 ].

The characterization of nanoparticles is important for understanding their size, shape, surface properties, and chemical composition. This information can be used to understand how nanoparticles interact with their environment and ensure they are safe [ 35 ].

Strategies to overcome these barriers in nanoparticle synthesis are still under study to develop more innovative, efficient, cost-effective, and environmentally friendly methods.

3. Green synthesis of nanoparticles: an overview

Green synthesis aims to promote innovative chemical technologies to reduce or eliminate the use and production of hazardous substances in the design, manufacture, and use of chemical products. This involves minimizing or, if possible, eliminating the pollution produced in the synthesis processes, avoiding the consumption and wastage of nonrenewable raw materials, using hazardous or polluting materials in product manufacturing, and reducing the synthesis time. Paul J. Anastas, considered the father of green chemistry, defined it as “a work philosophy that involves the use of alternative tools and pathways to prevent pollution,” referring to both the design of the synthetic strategy and the treatment of possible secondary products originating from that route [ 36 , 37 ].

Two approaches can be used to generate nanoparticles [ 37 , 38 ] ( Figure 2 ).

“Top-down” approach: In which nanoparticles are produced using physical techniques such as grinding or abrasion of a material.

Chemical synthesis: The method of producing molecules or particles by the reaction of substances used as raw materials.

Self-assembly: A technique in which atoms or molecules self-order through physical and/or chemical interactions.

Positional assembly: The atoms, molecules, and aggregates are deliberately manipulated and positioned individually. However, this method is extremely laborious and unsuitable for industrial applications.

The “bottom-up” approach is preferred over the “top-down” approach because specialized equipment is not required and the time to obtain nanoparticles is shorter. Green synthesis is gaining relevance in producing nanoparticles within the “bottom-up” approach [ 37 ].

The use of plant species, algae, or microorganisms such as bacteria or fungi is one of the most commonly used resources for this procedure. Various compounds from plants or microorganisms, including terpenes, polyphenols, alkaloids, carbohydrates, proteins, and genetic materials, play an important role in the synthesis of nanoparticles by acting together [ 39 , 40 ].

In addition to the biological resources used to perform the synthesis (plants, algae, or microorganisms), other factors influence the shape and size of nanoparticles, such as the concentration of the metal ion, pH, reaction time, and temperature [ 39 , 41 ].

Initial phase: Obtaining the reaction medium, which is the aqueous extract of one or several parts of the plant species or the culture media for the growth of microorganisms, in addition to the precursor salt, which is the source of metal ions.

Activation phase: Chemical reduction of metal ions and generation of nucleation centers occur where nanoparticles emerge and grow.

Growth phase: Small adjacent nanoparticles spontaneously fuse into larger particles, forming aggregates, which are influenced by factors such as temperature, concentration, and type of compounds, pH, and reaction time.

Termination phase: The final shape of the nanoparticles is determined, and the compounds that participate in the reaction help stabilize and enhance their properties.

Phases involved in the green synthesis of nanoparticles.

3.1 Biological resources for the green synthesis of nanoparticles

As stated previously, nanoparticles have attracted attention in the fields of biology, medicine, and electronics in recent years, owing to their remarkable applications ( Figure 4 ). Numerous nanoparticle synthesis techniques have been developed; however, these may involve the use of toxic compounds and high-energy physical processes. An alternative is the use of biological methods to circumvent these obstacles. Bacteria, fungi, algae, and plant species are some of the most commonly used biological resources for the green synthesis of nanoparticles ( Figure 4 ). This biological approach has provided a method that is reliable, straightforward, benign, and environmentally beneficial [ 40 , 42 ].

Biological resources and compounds used for the green synthesis of nanoparticles and some of their applications [ 9 ].

3.1.1 Bacteria

Nanoparticle synthesis using bacteria is performed both extracellularly and intracellularly [ 38 ].

Intracellular: The synthesis is carried out inside the living microorganism, using its growth conditions to favor synthesis, known as “nanoparticle micro-factories.” To recover nanoparticles, bacteria must be destroyed [ 43 ].

Extracellular: The components released by the bacteria after lysis are used. The synthesis is performed by adding a metal salt precursor to the medium in which these components are located. Extracellular synthesis has the advantage of being faster because it does not require additional steps to recover nanoparticles from microorganisms [ 43 , 44 ].

Enzymes, such as reductases, which catalyze the reduction of metal ions into nanoparticles, participate in the synthesis. Even components of the genetic material participate in this process [ 45 , 46 ].

3.1.2 Fungi

Fungi contain active biomolecules, such as proteins or enzymes, that participate in nanoparticle synthesis, improving their yields and stability [ 47 ].

Some fungal species can synthesize nanoparticles using extracellular amino acids. For example, glutamic and aspartic acids on the surface of yeast or the reductase enzyme in the cytosol of fungi reduce metal ions to form nanoparticles. This is facilitated by the presence of hydroxyl groups in the mycelium, which donate electrons to the metal ion and reduce it to form nanoparticles. Aliphatic and aromatic amines or some proteins act as coating agents to stabilize them [ 48 , 49 ].

3.1.3 Algae

Algae are used in nanotechnology because of their low toxicity and their ability to bioaccumulate and reduce metals [ 50 ].

Nanoparticle synthesis can be intracellular, with the metal ion entering the alga, or extracellular, and involves compounds such as polysaccharides, proteins, and pigments that direct the reduction of metal ions and coat the newly formed nanoparticles. These particles are subsequently released from the cell in the form of colloids [ 51 ].

3.1.4 Plant species

The use of plants in nanoparticle synthesis is one of the most widely used methods because of its environmentally friendly nature, as it avoids the use of toxic or harmful substances. It is also one of the fastest and most economical methods because it involves fewer steps [ 39 , 40 ]. This makes it highly efficient in the nanoparticle production process compared to synthesis using microorganisms.

Plants contain several compounds (terpenes, flavonoids, polyphenols, alkaloids, proteins, etc.) that reduce metal ions and stabilize the resulting nanoparticles [ 52 ].

This type of synthesis can be performed using intracellular, extracellular, and phytochemical-mediated methods [ 53 ].

Intracellular: The synthesis is carried out inside the plant cell, and the nanoparticles are recovered by breaking down the structure, which is very similar to the intracellular method using microorganisms. Control of the growth factors of plant species is required so that they do not interfere with synthesis [ 54 ].

Extracellular: This method is the most commonly used because of its ease and speed. The process begins by obtaining a plant extract, which is generally water-based, to which a metal salt precursor is added. Owing to the action of the different compounds present in the extract, nanoparticles are generated and stabilized in a single step [ 54 , 55 ].

Phytochemically mediated: This is based on the extracellular method, but with the difference that isolated phytochemical compounds are used and other substances are added to stabilize the nanoparticles. There is greater control over the synthesis, but more components and steps are involved [ 53 ].

3.2 Factors involved in the green synthesis of nanoparticles

As in any synthesis process, reaction conditions, such as temperature, pH, and reaction time, play an important role in the shape, size, and yield of the synthesized nanoparticles [ 39 , 40 , 41 ] ( Figure 3 ).

Temperature: This is one of the most influential factors, as the shape (spherical, prismatic, flakes, triangular, octahedral, etc.), size, and synthesis depend on temperature. As the temperature increases, the reaction rate and the formation of nucleation centers increase, resulting in higher yields. Different temperatures promote different interactions between the reactants, giving rise to various shapes; the larger the temperature increase, the larger the size of the nanoparticles [ 56 , 57 ].

pH: This influences the nucleation centers, generating more centers at higher pH values. Another important influence of pH is that some nanoparticles can only be synthesized in acidic or alkaline media. For example, magnetic nanoparticles are synthesized at an alkaline pH, and metal oxide nanoparticles are generally synthesized at an acidic or neutral pH [ 58 ].

Time: This parameter plays an important role in defining the size of the nanoparticles. It has been observed that longer reaction times favor an increase in the size of the nanoparticles and higher yields, owing to the prolonged interaction time between reactants [ 59 ].

3.3 The mechanism involved in the green synthesis of nanoparticles

The plant extract or organism used for the synthesis is an important factor that influences the morphology and size of nanoparticles because different concentrations of metabolites or cellular components give rise to differences in the nanoparticles [ 40 , 60 ] ( Figure 5 ).

Green-synthesized nanoparticles. (a) Spherical ZnO nanoparticles using the leaves of Dysphania ambrosioides (plant). (b) Prismatic ZnO nanoparticles using the stems and leaves of Dysphania ambrosioides (plant). (c) Quasi-spherical Fe 3 O 4 nanoparticles using the leaves of Datura innoxia (plant). (d) Quasi-spherical Ag nanoparticles using stems of Aloe vera (plant) [ 61 ]. (e) Spherical and triangular Au nanoparticles using Lentinula edodes (fungus) [ 43 ]. (f) Irregular Ag and triangular Au nanoparticles using Ganoderma lucidum (fungus) [ 43 ]. (g) Hexagonal MgO nanoparticles using the flowers of Saussurea costus (plant) [ 62 ]. (h) Irregular Cu nanoparticles using Salmonella typhimurium (bacterium) [ 63 ]. (i) Quasi-spherical Ag nanoparticles using Dunaliella salina (alga) [ 64 ].

Proteins and enzymes facilitate the formation of nanoparticles from metal ions. Because of their high reducing activity, proteins and enzymes can attract metal ions to specific regions of a molecule responsible for reduction, facilitating the formation of nanoparticles; however, their chelating activity is not excessive. The amino acids of a protein can greatly influence the size, morphology, and quantity of nanoparticles generated, thus playing a very important role in determining their shape and yield. Removing a proton from amino acids or other molecules results in the formation of resonant structures capable of further oxidation. This process is accompanied by the active reduction of metal ions followed by the formation of nanoparticles [ 39 ].

Flavonoids are a large group of polyphenolic compounds that can actively chelate and reduce metal ions because they contain multiple functional groups capable of forming these structures. Structural transformations of flavonoids also generate protons that reduce metal ions to form nanoparticles; therefore, they are involved in the nucleation stage, their formation, and further aggregation. Saccharides can also play a role in nanoparticle formation. Monosaccharides, such as glucose, can act as reducing agents, as the aldehyde group of the sugar is oxidized to a carboxyl group through the addition of hydroxyl groups, which in turn leads to the reduction of metal ions and the synthesis of nanoparticles [ 39 ].

The mechanism of green synthesis of nanoparticles has been associated with the action of polyphenols, which act as ligands. Metal ions form coordination compounds, in which the fundamental structural unit is the central metal ion surrounded by coordinated groups arranged spatially at the corners of a regular tetrahedron. The aromatic hydroxyl groups in polyphenols bind to metal ions and form stable coordinated complexes. This system undergoes direct decomposition at high temperatures, releasing nanoparticles from the complex system [ 65 ].

Flavonoids, amino acids, proteins, terpenoids, tannins, and reducing sugars have hydroxyl groups that surround the metal ions to form complexes. After this process, the hydroxyl ions are oxidized to carbonyl groups, which stabilize the nanoparticles. Synthesis is favored if the participating molecules have at least two hydroxyl groups at the ortho- and para-positions [ 52 , 65 ].

Amino acids influence the size, morphology, and yield of nanoparticles generated [ 23 ], depending on the specific amino acids present in the extract and their concentration, along with the reaction conditions that give rise to nanoparticles with different shapes [ 65 ].

4. Confirming that the biological approach of nanoparticle synthesis is a green chemistry method

To corroborate that the processes of nanoparticle synthesis using biological resources are “green synthesis methods,” the 12 principles mentioned above are revisited [ 66 , 67 , 68 ] ( Table 1 ).

The 12 principles of green synthesis are fulfilled with the biological approach to produce nanoparticles.

Considering the above, the 12 principles of green synthesis are fulfilled using biological resources, such as plants, bacteria, fungi, and algae, to synthesize nanoparticles [ 69 , 70 , 71 ].

Finally, green synthesis of nanoparticles is a sustainable and environmentally friendly alternative to traditional methods of nanoparticle synthesis. Traditional methods often take long periods of time, use toxic chemicals and solvents, or generate waste products that can pollute the environment and pose health risks to humans and animals. In contrast, the green synthesis method uses renewable natural resources, such as plant extracts and microorganisms, which are less damaging and can be replenished over time. In addition, these methods are often more cost-effective and faster than traditional procedures because they do not require expensive chemicals or equipment and are considered one-step syntheses, which contribute to energy savings [ 72 ].

In furtherance of these advantages, green synthesis methods are still being developed to improve their efficiency and scalability, leading to the potential benefits of green synthesis of nanoparticles or even their application to the synthesis of other molecules as drugs or nutraceuticals.

5. Conclusion

Nanoparticles have emerged as a versatile and promising class of materials with unique properties that can be harnessed for various applications. The use of green synthesis utilizing natural resources and biologically active compounds to produce nanoparticles is an area of continuous research to improve processes, reduce environmental damage, and meet the increasing demand for the application of these nanostructures. Utilizing biological resources, the synthesis of nanoparticles is inexpensive, faster, and considered a one-step synthesis while preserving or even improving the physical and chemical properties of the nanoparticles. With the great potential of this method and the sustainable and efficient production of nanoparticles, different sizes and shapes can be obtained, which makes it a very attractive option not only for the synthesis of nanostructures, but also for the application of this technique in the synthesis of other compounds.

Acknowledgments

The authors acknowledge Dr. Samuel Tehuacanero Cuapa, Physicist. Roberto Hernández Reyes, and Arq. Diego Quiterio Vargas for their technical support.

Thanks to the Consejo Nacional de Humanidades, Ciencias y Tecnologías (CONAHCYT) for the scholarship granted to Rafael Álvarez-Chimal with the CVU number: 579637.

Funding was provided by the UNAM-DGAPA- PAPIIT project IN112422.

Conflict of interest

The authors declare no conflicts of interest.

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Green synthesis of zinc oxide nanoparticles for the removal of phenol from textile wastewater

• Open access
• Published: 28 August 2024
• Volume 4 , article number  24 , ( 2024 )

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• Fatai Alade Aderibigbe 1 ,
• Sherif Ishola Mustapha 1 ,
• Ishaq Alhassan Mohammed 1 ,
• Tunmise Latifat Adewoye 1 ,
• Esther Olubunmi Babatunde 1 ,
• Habeebllah Ifeoluwa Aminullah 1 &
• Kabiru Bab Muritala 1  

This study investigated the use of zinc oxide nanoparticles (ZnO NPs) as an adsorbent for removing total phenols from textile wastewater. The ZnO NPs were synthesized by reducing Zn(NO 3 ) 2 ⋅6H 2 O using an extract from Neem leaves ( Azadirachta indica ). Characterization of the adsorbent was performed using Fourier Transform Infrared (FTIR) spectroscopy to identify functional group modifications, high-resolution scanning electron microscopy (HRSEM) for structural orientation, energy dispersive spectroscopy (EDS) for elemental analysis, and X-Ray diffraction analysis (XRD) for crystallinity, revealing particle crystallinity around 200 nm. Adsorption experiments were conducted over contact times of 20–60 min, with adsorbent loadings between 0.2 and 1 g/100 mL, and temperatures ranging from 30 to 50 °C. Optimal phenol removal, achieving 55.93% (0.67 mg/L), occurred at 43.40 min, 33.70 °C, and an adsorbent dosage of 0.69 g/L of textile wastewater. The phenol adsorption process using ZnO NPs was exothermic, spontaneous, and required low energy, fitting well with the Langmuir isotherm and following a pseudo-second-order kinetic model.

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1 Introduction

Industrial effluents, particularly from the textile sector, are a major source of water pollution, posing significant environmental challenges [ 1 , 2 ]. According to the World Bank, textile dyeing accounts for approximately 17 to 20% of global wastewater [ 3 ]. This wastewater is laden with various chemicals, including hydrogen peroxide, surfactants, metal soaps, alkalis, acids, dyes, starch, and aromatic compounds, all of which contribute to its high toxicity and ecological impact [ 4 ]. Phenol, a common contaminant in textile wastewater, has a permissible limit of 1 mg/L set by the EPA, USA, and must be treated to avoid damaging municipal wastewater treatment plants [ 5 ]. The release of such effluents introduces hazardous substances into the environment, necessitating stringent management practices [ 2 ]. Researchers have been exploring improved methods for pollutant separation from textile wastewater due to the high operational costs associated with existing techniques [ 6 ]. Traditional phenol removal methods, such as chemical oxidation and activated carbon adsorption, face challenges like high costs and the production of secondary pollutants. Adsorption techniques are widely studied for phenol removal because of their efficiency and potential for pollutant recovery [ 7 ]. However, the high cost of activated carbon has driven the search for alternative, low-cost adsorbents [ 8 ].

Over the last two decades, nanotechnology has advanced considerably, finding uses in many scientific and technological sectors. Nano-sized adsorbents have been developed for water pollutant removal, effective even at low concentrations (µg/L) under varied conditions [ 9 ]. Different nanomaterials, like metal nanoparticles, carbon nanotubes, and graphene oxide-based composites, have proven highly effective as photocatalysts for organic pollutant degradation [ 10 ]. Their large surface areas enhance catalytic activity, enabling efficient dye and nitrophenol removal through chemical reduction [ 11 ]. Additionally, nanotechnology supports simultaneous pollutant removal, including pathogens and inorganic compounds, offering a cost-effective, environmentally friendly alternative to traditional water treatment methods [ 12 ]. Techniques like nanocatalysis, nanofiltration, and nanoadsorption have successfully eliminated persistent organic pollutants (POPs) from wastewater, despite some cost challenges [ 13 ]. Therefore, developing more efficient, selective, economical, eco-friendly, and rapid water treatment technologies remains essential.

In recent years, nanoparticle synthesis for environmental remediation has gained considerable interest. Zinc oxide (ZnO) nanoparticles are particularly notable for their superior adsorptive properties and environmental compatibility. Traditional methods of ZnO nanoparticle synthesis, however, involve hazardous chemicals and energy-intensive processes, diminishing their environmental benefits. An eco-friendly alternative is the green synthesis of ZnO nanoparticles using plant-based materials. Extracts from pineapple peels [ 14 ], Ruta chalepensis leaves [ 15 ], and date pulp waste [ 16 ] have been effective as reducing and capping agents. These green-synthesized ZnO nanoparticles display uniform particle size distribution, hexagonal wurtzite structure, and enhanced photocatalytic activity compared to chemically synthesized nanoparticles. They efficiently degrade dyes such as Malachite Green, Methylene Blue, and Eosin Yellow in wastewater treatment and possess antibacterial properties against pathogenic bacteria [ 16 ]. The green synthesis process is simple, cost-effective, and environmentally friendly, avoiding the use of hazardous solvents and high-pressure conditions required in traditional methods [ 17 ]. Neem ( Azadirachta indica ) leaves, known for their rich phytochemical content, have been explored for the biosynthesis of ZnO nanoparticles [ 18 ]. These biogenic nanoparticles not only reduce the reliance on toxic chemicals but also enhance the adsorption efficiency due to their unique surface properties. Despite the growing interest in green-synthesized ZnO nanoparticles, comprehensive studies on their application for phenol removal from textile wastewater remain limited. There is a need for detailed research that integrates the synthesis, characterization, and practical application of these nanoparticles in real wastewater treatment scenarios.

This study aims to address the gap by synthesizing ZnO nanoparticles using Neem leaf extract and evaluating their effectiveness in removing phenol from textile wastewater. The research will focus on the synthesis and characterization of the nanoparticles, and analyzing their adsorption performance under various conditions. This integrated approach will provide valuable insights into the potential of green-synthesized ZnO nanoparticles for wastewater treatment applications, thereby contributing to the development of sustainable and efficient remediation technologies.

2 Materials and methods

2.1 materials and collection of samples.

The textile wastewater analyzed in this study was collected from the Kaykab Textile Industry treatment plant in Ilorin, Nigeria. The solution was stored in prewashed 5-L kegs and then refrigerated. Zinc nitrate (Zn(NO 3 ) 2 ⋅6H 2 O) was procured from Lab Trade in Ilorin, while fresh Neem ( Azadirachta indica ) leaves were obtained from Neem trees at the University of Ilorin, Ilorin, Kwara State, Nigeria.

2.2 Preparation of Azadirachta indica leaf extract

The neem leaves were carefully washed under running water and then thoroughly rinsed with distilled water to ensure they were completely clean. The cleaned leaves were air-dried at room temperature before being ground into a fine powder using an electric grinder. Approximately 20 g of the powdered leaves were added to 100 mL of distilled water and boiled for 20 min [ 19 ]. After boiling, the mixture was allowed to cool to room temperature and then filtered using Whatman filter paper. The resulting leaf extract was stored in a refrigerator at 4 °C until needed for use.

2.3 Determination of the total phenolic content

The phenol content of the textile wastewater was estimated using the Folin-Ciocalteu method [ 19 ]. Approximately 0.5 mL of the textile wastewater sample was measured into a clean test tube using a micropipette. Additionally, 2.5 mL of Folin C reagent was poured into the sample, followed by the addition of 2.0 mL of Na 2 CO 3 . The mixture was allowed to settle for 30 min. After this settling period, absorbance readings were taken using a UV spectrophotometer at a wavelength of 760 nm. These absorbance values were then plotted against the corresponding phenol concentrations to create a calibration curve. Using this calibration curve, the phenol levels were determined. The total phenol concentration in the raw textile wastewater was calculated to be 1.193 mg/L.

2.4 Synthesis of zinc oxide nanoparticles (ZnO NPs)

Approximately 414 mL of Azadirachta indica aqueous extract from the stock solution was transferred into a 1500 mL glass beaker. Then, 41.4 g of anhydrous zinc nitrate (Zn (NO 3 ) 2 .6H 2 O) was added and thoroughly mixed using a magnetic stirrer until the zinc salt was completely dissolved. Once dissolved, the solution was heated and continuously stirred at 70 °C using a magnetic stirrer until a brown-like paste formed. To sequester all the nitrates and moisture, the paste was then transferred into a crucible and calcined at 400 °C for 2 h [ 20 ].

2.5 Characterization of the adsorbent

X-ray diffraction (XRD) was utilized to assess the crystallinity of the developed zinc oxide nanoparticles (ZnO NPs). Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were employed to investigate the structural and compositional characteristics of the ZnO NPs. Fourier-transform infrared spectroscopy (FTIR) was used to identify the functional groups present in the adsorbent [ 19 ]. These analyses were conducted on the ZnO NPs both before and after their application in the degradation of phenol in waste water.

2.6 Batch adsorption process

Adsorption studies were conducted by mixing 100 mL of textile wastewater with the adsorbent in closed 250 mL conical flasks, followed by constant shaking in a thermostatic water bath shaker at 150 rpm. The experimental parameters for the adsorption investigation included contact time (20–60 min), adsorbent dosage (0.2–1.0 g), and temperature (30–50 °C). The percentage removal of the targeted pollutant were then estimated using Eq.  1 .

where; \({C}_{o}\) and \({C}_{f}\) (mg/L) are the concentrations of phenol in the aqueous solution at the initial stage and final respectively.

The equilibrium amount ( \({Q}_{e}\) ) of phenol removed per mass of adsorbent was evaluated using Eq. ( 2 ):

where; \({Q}_{e}\) (mg g −1 ) represents the quantity of phenol adsorbed per unit mass of adsorbent at equilibrium. V (L) represents the volume of the solution, \({C}_{e}\) (mg/L) is the concentration at equilibrium and W (g) denotes the mass of the ZnO NPs used. Each of the experiments was conducted three times, and the average values were taken into account for analysis.

2.7 Determination of the effect of contact time

Approximately 0.6 g of ZnO nanoparticles and 100 mL of textile wastewater were placed in a 250 mL conical flask, which was then placed in a shaker at 40 °C for intervals ranging from 30 to 60 min at 10-min increments. At the end of each specified contact time, the suspension was filtered, and the filtrate was analyzed for phenol concentration. This process was repeated three times, and the average phenol concentration was calculated. These values were used to compute the percentage removal of phenol.

2.8 Determination of the effect of adsorbent dosage

To determine the optimal dosage of adsorbent, the amount of ZnO NPs was varied from 0.2 g to 1.0 g in 100 mL of textile wastewater, maintaining a contact time of 40 min at 40 °C. The resulting mixtures were filtered, and the filtrate was analyzed for phenol concentration. This procedure was repeated three times, and the average phenol concentration was used to calculate the percentage removal of adsorbed phenol.

2.9 Determination of the effect of temperature

Using the optimal time and dosage obtained, the temperature was varied from 30 to 50 °C under these conditions. The resulting mixture was filtered, and the filtrate was analyzed for phenol concentration. This process was repeated three times to ensure accuracy, and the average phenol concentration was used to calculate the percentage removal of adsorbed phenol.

2.10 Optimization studies

Initial experiments indicated that contact time (min), adsorbent loading (g), and temperature (°C) are key experimental factors influencing the synthesis of ZnO NPs. The interactive effects of these variables were analyzed using the central composite design (CCD) method [ 21 ]. This approach helped capture the impact of contact time, adsorbent dosage, and temperature on the adsorption process, as detailed in Table  1 .

2.11 Adsorption isotherms, thermodynamics, and kinetics of the study

2.11.1 adsorption isotherms of the process.

The equilibrium relationship between the concentration of the adsorbate in the mixture and its presence on the adsorbent surface under specific conditions was elucidated using adsorption isotherm models. This analysis is particularly relevant for understanding the adsorption behavior of phenol onto ZnO NPs, determined at an initial phenol concentration of 1.193 mg/L in raw textile wastewater. The equilibrium characteristics of this study were described using the Langmuir and Freundlich isotherm models.

2.11.2 The Langmuir isotherm model

This model suggests that adsorption occurs in monolayers on uniform surfaces with a finite number of adsorption sites. The linearized version of this model, as proposed by Langmuir [ 22 ], is expressed as follows.

where; \({q}_{max}\) (mg/g): maximum monolayer adsorbent capacity and \({k}_{L}\) (L/mg) is the Langmuir constant related to the energy of adsorption.

2.11.3 The Freundlich isotherm model

This model postulates heterogeneous adsorption on the surface. The linearized version of the Freundlich model, as represented by ref. [ 23 ] is expressed as follows:

where; n and \({k}_{f}\) (mg/g) represent the model parameters reflecting the intensity and capacity of the adsorption, respectively. The adsorption process can be categorized as physical, chemical, or linear whenever n > 1, n < 1 or n = 1. Nevertheless, findings show that n > 1 is favorable for adsorption [ 24 ].

2.11.4 The Temkin isotherm model

This model assumes that the heat of adsorption of all the molecules in the layer decreases linearly with coverage of the adsorbent surface [ 25 ] and is given as

where; \(\beta =\frac{RT}{b}\) , R (8.314 J/mol K −1 ) refers to the universal gas constant, T (K) is the temperature, α (L/mol) is the Temkin constant or equilibrium binding energy connected with the maximum binding energy, and \(\beta\) refers to the Temkin constant connected to the adsorption heat.

2.11.5 Process thermodynamics

The thermodynamics of the process were studied and are given as follows:

Where; ∆G is Gibb’s Free energy, \({K}_{T}\) is the apparent equilibrium constant, and \(\Delta \text{H}\) is the enthalpy \(\text{and }\Delta \text{S}\) is the entropy.

2.11.6 Adsorption kinetics

The rate and mechanism of adsorption of phenol onto ZnO NPs were analyzed using the pseudo-first-order and pseudo-second-order kinetic models.

2.11.7 The pseudo-first-order kinetic model

The pseudo-first-order model, as proposed by Lagergren [ 26 ] is linearly expressed as follows:

where; \({K}_{1}\) (min −1 ) is the pseudo-first order rate constant, and \({q}_{e}\) (mg/g) and \({q}_{t}\) (mg/g) are the quantities of phenol adsorbed per unit mass of the ZnO NP adsorbent at equilibrium and at time t (min), respectively.

2.11.8 The pseudo-second-order kinetic model

The pseudo-second-order model, in its linearized form, is represented by the following expression, as postulated by Ho et al. [ 27 ]:

where; \({K}_{2}\) (g/mg min) refers to the model rate constant.

3 Results and discussion

3.1 determination of the properties of neem leaf extract (ne ).

FTIR measurements of the neem extract (NE), depicted in Fig.  1 , indicated the presence of various functional groups. The peaks observed at 775, 1020, 1264, 1727, and 2927 cm −1 correspond to –NH, –C≡N, C–OH, –C=O, and –CH groups, respectively. Additionally, the sharp peaks at 1500 and 3688 cm −1 are attributed to –C=C and –OH groups, respectively [ 28 ]. These findings are consistent with previous analyses of neem leaves [ 29 ]. Furthermore, the presence of the C=O carboxylic group suggests the occurrence of alkaloids [ 30 ].

FTIR spectrum of neem leaf extract

3.2 Characterization of textile wastewater (TWW)

The textile wastewater (TWW) was analyzed to determine the concentration of phenol, as shown in Table  2 . The total phenol concentration was assessed using the Folin-Ciocalteu (F–C) method, which yielded an estimated concentration of 1.193 mg/L. This level is notably higher than the Standard Organization of Nigeria's (SON) limit of 0.25 mg/L and slightly exceeds the World Health Organization's (WHO) standard of ≤ 1.0 mg/L. High phenol concentrations pose significant health risks to aquatic life if discharged untreated. Although the developed ZnO NPs did not reduce the phenol concentration to meet the Nigerian standard, they were effective enough to align with the WHO guidelines.

3.3 Fourier Transform Infrared (FTIR) Spectroscopy Analysis

Figure  2 show the FTIR of adsorbent before adsorption (ABA) and adsorbent after adsorption (AAA) respectively. They exhibited similar peaks at 1050 (–N=O), 1552 (–C–N), 2354(C≡C), and 2821 cm −1 (–C–H) except that the peaks were sharper for ABA [ 31 ]. The band at 3160 cm −1 is possibly attributed to the aliphatic hydroxyl (OH) [ 32 ] while the bands at 3651 and 3400 cm −1 may also conform to the OH vibrations of the hydroxyl layers. In addition, the characteristic set of stretching vibrations attributed to nitrate anions became visible at 1050 cm −1 [ 33 ].

FTIR spectrum of adsorbent before adsorption (ABA) and after adsorption (AAA)

3.4 X-ray diffraction (XRD) analysis

Figure  3 presents the XRD patterns of the adsorbent, revealing highly crystalline material, indicated by the presence of sharp peaks. A review conducted by Agarwal et al. [ 34 ] affirmed that Azadirachta indica , a member of the Meliaceae family, is frequently utilized for the synthesis of ZnO nanoparticles (ZnO NPs).

XRD patterns of the adsorbent

3.5 Scanning electron microscopy and energy dispersive X-ray spectroscopy analysis (SEM–EDS)

Scanning electron microscope (SEM) analysis was conducted to examine the surface morphology of the ZnO nanoparticles (ZnO NPs). Figure  4 (a)-(d) displays the SEM images of the ZnO NPs before (ABA) and after (AAA) adsorption at various magnifications. These images confirmed the structure and approximate spherical shape of the nanoparticles. The energy-dispersive spectroscopy (EDS) results before (ABA) and after (AAA) adsorption are shown in Fig.  5 , revealing the presence of elements such as zinc (Zn), sodium (Na), calcium (Ca), potassium (K), chlorine (Cl), aluminum (Al), magnesium (Mg), phosphorus (P), oxygen (O 2 ), silicon (Si), titanium (Ti), iron (Fe), nitrogen (N 2 ), sulfur (S), and carbon (C). The composition of these elements varied, with 77.77% Zn detected before adsorbent usage, decreasing to 58.45% after usage.

SEM images of the adsorbent before (ABA) at magnifications of a 200 µm b 100 µm and after (AAA) treatment at magnifications at magnifications of c 200 µm and d 100 µm respectively

EDS data for adsorbent before adsorption (ABA) and after adsorption (AAA)

3.6 Optimization studies

The central composite design (CCD) was utilized to optimize three variables related to textile wastewater treatment. Table 3 presents the percentage of phenol removal for each experimental run, with the highest removal efficiency recorded at 54.35%. Statistical analysis was peroformed for phenol adsorption process, and the results are summarized in the analysis of variance (ANOVA) in Table  4 . The phenol uptake by the ZnO NP model is described using a second-order polynomial, as shown in Eq. ( 9 ).

In the model, P, Q, and R represent the contact time (min), adsorbent loading (g), and temperature (°C), respectively. The model’s R 2 value of 0.90 indicates a good fit. The predicted R 2 value of 0.7034 is reasonably close to the adjusted R 2 value of 0.8136, with a difference of less than 0.2. The analysis of variance (ANOVA) for the model is shown in Table  4 .

The Model F-value of 10.21 suggests that the model is significant [ 35 ]. Additionally, P, Q, PQ, and Q 2 are significant model terms. Thus, individual factors such as contact time, and adsorbent dosage, as well as their interactions (contact time and adsorbent dosage, and adsorbent dosage with itself), play a significant role in phenol adsorption using ZnO NPs.

Figure  6 displays the plot between the predicted and actual values of the percentage phenol removal. The numerical optimization yielded an optimum adsorption capacity of 55.93% (0.67 mg/L) for phenol removal at a contact time of 43.397 min, adsorbent loading of 0.689 g, and a temperature of 33.701 °C, with a desirability of 1.000.

Actual and predicted values for phenol removal (%)

3.7 Batch adsorption studies

3.7.1 effect of contact time.

The removal of phenol using ZnO as an adsorbent is influenced by the contact time, as illustrated in Fig.  7 . The data reveals that over 50% of phenol is removed within the first 40 min of the adsorption process. This suggests that phenol adsorption on ZnO occurs rapidly during the initial stages, likely due to the high diffusion rate of phenol molecules from the aqueous solution to the active sites on the adsorbent [ 20 ]. However, after the 40-min mark, the percentage of phenol removed begins to decrease. This reduction could be attributed to the decreasing availability of active sites or pores on the ZnO adsorbent over time. These results are consistent with those found in previous studies [ 32 ].

Effect of contact time on phenol adsorption using ZnO NPs

3.7.2 Adsorbent dosage effect

The phenol removal efficiency from the solution increased with the adsorbent dose, rising from 0.2 to 0.6 g/100 mL (Fig.  8 ). This trend is due to the increased number of available phenol adsorption sites on the adsorbent surfaces as the dosage increases [ 19 ]. However, further increases in the adsorbent dosage resulted in a decrease in phenol removal, suggesting that the adsorbent sites were fully occupied and could no longer adsorb additional phenol. The optimal phenol removal was observed at a dosage of 0.6 g/100 mL. These findings align with those reported by Jabar, Owokotomo, Ayinde, Alafabusuyi, Olagunju and Mobolaji [ 28 ].

Effect of adsorbent dosage on phenol adsorption using ZnO NPs

3.7.3 Effect of the adsorption temperature

Temperature was also found to affect the adsorption process as shown in Fig.  9 . The maximum phenol removal was obtained at 40 °C, and a further increase in temperature caused a decrease in the efficiency of phenol removal. This inverse connection may reveal that the process is exothermic [ 19 ].

Effect of temperature on phenol adsorption using ZnO NPs

3.7.4 Adsorption isotherm studies

The removal of phenol by ZnO NPs was effectively described using Langmuir, Freundlich, and Temkin isotherms. Table 5 summarizes the data obtained from these isotherms. Among them, the Langmuir model provided the best fit for the adsorption process, as indicated by the R 2 value of 0.9944, which is closest to unity.

3.7.5 Adsorption thermodynamic studies

From the graph shown in Fig.  10 and Table  6 , the negative value for the change in enthalpy (ΔH) of this process suggests that the adsorption of phenol onto ZnO NPs is an exothermic process. Additionally, the negative entropy change (ΔS) indicates that this adsorption process is less random. Finally, the negative Gibb’s free energy (ΔG) value confirms that the reaction is spontaneous and feasible.

Thermodynamics study of phenol adsorption on ZnO NPs

3.7.6 Adsorption kinetic studies

The adsorption process of phenol was analyzed using kinetic models, and the results are presented in Table  7 , which lists the parameters for both the pseudo-first-order and pseudo-second-order models. The pseudo-second-order model demonstrated a better fit, as the calculated Qe values closely matched the experimental Qe values. Moreover, the correlation coefficient (R 2 ) for the pseudo-second-order model was significantly closer to 1.000 compared to that of the pseudo-first-order model. These findings confirm that the adsorption of phenol on ZnO NPs is best described by the pseudo-second-order kinetic model.

4 Conclusions

The Neem leaf extract demonstrated effective reducing activity, converting zinc nitrate salts to zinc oxide nanoparticles (ZnO NPs). These ZnO NPs proved to be effective in adsorbing phenols from textile industry wastewater. Specifically, the initial phenol concentration of 1.193 mg/L in the untreated wastewater was reduced by 55.93% after 43.40 min when using an adsorbent dosage of 0.69 g/L at 33.70 °C. The kinetic study was best described by the pseudo-second-order model, while the Langmuir isotherm provided the best fit for the adsorption process. Thermodynamic analysis indicated that the adsorption process was spontaneous and exothermic.

Data availability

Data availability statements The authors confirm that the data supporting the findings of this study are available within the article.

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Fatai Alade Aderibigbe, Sherif Ishola Mustapha, Ishaq Alhassan Mohammed, Tunmise Latifat Adewoye, Esther Olubunmi Babatunde, Habeebllah Ifeoluwa Aminullah & Kabiru Bab Muritala

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Conceptualization, methodology, editing, resources, and supervision were contributed by Fatai Alade Aderibigbe; Experimental design, validation, and writing of the original manuscript and resources by Sherif Ishola Mustapha; Supervision, and resources were done by Ishaq Alhassan Mohammed; Formal analysis and review by Tunmise Latifat Adewoye and Esther Olubunmi Babatunde; Investigation and writing of original manuscript draft by Habeebllah Ifeoluwa Aminullah; Editing and result validation by Kabiru Bab Muritala. All authors have read and agreed with the published version of the manuscript.

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Aderibigbe, F.A., Mustapha, S.I., Mohammed, I.A. et al. Green synthesis of zinc oxide nanoparticles for the removal of phenol from textile wastewater. Discov Chem Eng 4 , 24 (2024). https://doi.org/10.1007/s43938-024-00061-w

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Alginate nanoparticle synthesis using n -heptane and isopropyl myristate/aot reverse micelles: the impact of the non-polar solvent, water content, and ph on the particle size and cross-linking efficiency †.

* Corresponding authors

a Departamento de Química, Universidad Nacional de Rio Cuarto, Agencia Postal 3, C.P. X5804BYA, Ruta Nacional 36, km 601, Rio Cuarto, Córdoba, Argentina E-mail: [email protected]

b Instituto de Desarrollo Agroindustrial y de la Salud (IDAS), Universidad Nacional de Río Cuarto, Agencia Postal 3, C.P. X5804BYA, Ruta Nacional 36, km 601, Río Cuarto, Córdoba, Argentina

The synthesis of monodisperse and stable alginate nanoparticles (ALG-NPs) was achieved through the crosslinking of sodium alginate with Ca 2+ ions within sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles (RMs) as nano-templates. This study addresses the challenge of controlling the size and stability of nanoparticles, which is critical for their applications in drug delivery and tissue engineering. We explored the effects of varying the water content, the choice of non-polar solvent, and the pH of the resuspension medium on nanoparticle formation. Using both n -heptane and isopropyl myristate (IPM) to form AOT RMs, we found that nanoparticle size increased with water content in both solvents, attributed to differing degrees of crosslinking efficiency influenced by the proximity of alginate and calcium ions at lower water content. Notably, IPM produced smaller and more crosslinked ALG-NPs than n -heptane, likely due to its impact on interfacial interactions. Additionally, raising the pH of the resuspension medium resulted in smaller NPs due to enhanced alginate availability for cross-linking. These findings highlight the potential of AOT RMs as versatile templates for generating polymeric nanoparticles with precise control over their characteristics. The significant role of solvent choice and pH in tailoring nanoparticle properties is underscored, providing valuable insights for future applications. The controlled size and stability of these ALG-NPs make them excellent candidates for drug delivery systems and tissue engineering, given their biocompatibility and biodegradability.

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Alginate nanoparticle synthesis using n -heptane and isopropyl myristate/AOT reverse micelles: the impact of the non-polar solvent, water content, and pH on the particle size and cross-linking efficiency

F. M. Duque, N. Mariano Correa and R. Dario Falcone, New J. Chem. , 2024, Advance Article , DOI: 10.1039/D4NJ02981J

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• Published: 28 August 2024

Synthesis of silver nanoparticles embedded into melamine polyaminal networks as antibacterial and anticancer active agents

• Maha M. Alotaibi 1 ,
• Bodoor Almalki 1 ,
• Nada Tashkandi 1 ,
• Fatemah Basingab 2 , 3 ,
• Samaa Abdullah 4 , 5 &
• Nazeeha S. Alkayal 1  

Scientific Reports volume  14 , Article number:  20008 ( 2024 ) Cite this article

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Silver nanoparticles were successfully incorporated into a melamine-based polymer, resulting in the synthesis of (Ag NPs@Bipy-PAN) through a reverse double solvent approach. The synthesised Ag NPs@Bipy-PAN polymer underwent extensive characterisation through Powder X-ray Diffraction (PXRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy and Energy Dispersive X-ray (EDX) and Thermal Gravimetric Analysis. PXRD analysis confirmed the successful encapsulation of Ag nanoparticles and provided insights into the amorphous nature of the polymer following encapsulation. SEM and EDX analyses further corroborated the presence and distribution of Ag nanoparticles on the polymer surface. The biological efficacy of the Ag NPs@Bipy-PAN polymer was evaluated through antibacterial, anti-breast cancer, and biocompatibility assays. The results demonstrated notable antibacterial and anticancer activities, with significant efficacy against bacterial strains and breast cancer cells. Biocompatibility assessments indicated acceptable compatibility, particularly at a concentration of 2.5 mg/mL, compared to untreated control cells. These findings suggest that Ag NPs@Bipy-PAN has considerable potential as a candidate for cancer-targeted and antimicrobial drug delivery systems. The incorporation of silver nanoparticles into the melamine-based polymer enhances the safety profile of these systems in in vivo conditions, making them a viable option for advanced therapeutic applications.

Introduction

One of the most crucial modern day health concerns is the antibacterial resistance, as pathogens continually adapt to resist traditional antibiotic remedies, resulting in diminished treatment effectiveness. However, despite the imperative need to address this crisis, the discovery of new antimicrobial compounds has notably declined. Conversely, the prevalence of pathogen resistance to widely prescribed antibiotics has significantly increased. This alarming trend underscores the critical need for the exploration and development of novel therapeutic and preventive antimicrobial interventions 1 , 2 .

In response to this urgent challenge, the exploration of novel antimicrobial agents such as melamine-based polyaminal polymers (PANs) has gained considerable attention in recent years 3 . PANs possess distinctive properties, including a high nitrogen content and the presence of amino groups, which impart them with antimicrobial activity 4 . Melamine, a compound consisting of cyanamide and 1,3,5 triazine, is recognised for its nitrogen-rich structure, allowing it to impart excellent sorption capabilities and chemical stability when used in the creation of porous polymers 4 . These polymers are synthesised through a one-pot polycondensation reactions and microwave methods, resulting in various interconnected networks such as azolinked polymer 5 , polyamide 5 , 6 , polyimide 5 , 6 , and polyaminal networks 7 . PANs are particularly of a significant importance due to their nitrogen-rich composition from triazine rings and aminal linkages 8 , along with features like abundant micropores 9 , a large surface area 9 , and simplified synthesis via one-pot catalyst-free polycondensation of aldehyde monomers combined with varieties of amines 10 .

Empirical investigations have demonstrated the broad-spectrum inhibitory effects of PANs against diverse bacterial strains, encompassing both Gram-positive and Gram-negative species. The underlying mechanisms of PANs' antibacterial action primarily involve interactions with bacterial cell membranes and intracellular components 11 , 12 . Positively charged amino groups within PANs facilitate electrostatic interactions with negatively charged bacterial cell surfaces, leading to membrane disruption and subsequent cellular lysis 13 . Furthermore, the nitrogen-rich composition of PANs may disrupt bacterial metabolic processes, contributing to their antimicrobial efficacy 14 . These multifaceted mechanisms underscore the potential of PANs to serve as promising candidates for combating antibacterial resistance.

Several subclasses of polyaminal polymers (PANs) have emerged, and each with their own distinctive characteristics and potential applications or antibacterial applications due to their ability to facilitate interactions with bacterial membranes and intracellular components 15 . Polymers with intrinsic microporosity (PIMs) exhibit inherent microporosity owing to their rigid and contorted structures 16 , which may contribute to enhanced antibacterial activity through mechanisms such as membrane disruption and inhibition of bacterial metabolic processes 17 , 18 , 19 . Covalent organic frameworks (COFs) represent another subclass with potential antibacterial properties, characterized by crystalline porous materials formed through the self-assembly of organic building blocks, offering tuneable porosity and surface functionality advantageous in combating bacterial infections 20 , 21 . Covalent triazine-based frameworks (CTFs), featuring triazine units linked by covalent bonds, provide high surface areas and thermal stability 22 , which may contribute to their efficacy in antibacterial applications by facilitating interactions with bacterial surfaces and promoting antimicrobial activity. Hyper-cross-linked polymers (HCPs) 23 and porous aromatic frameworks (PAFs) also hold promise for antibacterial applications, offering high porosity and surface areas conducive to interactions with bacterial membranes and intracellular components, thus potentially enhancing their antibacterial efficacy 24 .

On the other hand, silver nanoparticles (AgNPs) are well-known for their antimicrobial properties, capitalize on their elevated surface area to volume ratio to outstrip bulk silver in antimicrobial efficacy 25 . Silver nanoparticles exhibit antibacterial effects through multiple mechanisms. Firstly, they release silver ions continuously, which adhere to bacterial cell walls and cytoplasmic membranes, enhancing permeability and disrupting the bacterial envelope 26 . Within cells, silver ions deactivate respiratory enzymes, generate reactive oxygen species, and interfere with DNA replication and protein synthesis. Additionally, silver nanoparticles themselves can kill bacteria by accumulating in cell wall pits, causing membrane denaturation, and disrupting organelles, leading to cell lysis. They can also penetrate bacterial cell walls and alter membrane structure, affecting signal transduction and promoting cell apoptosis 26 , 27 . The suspension of silver nanoparticles in exposure media impacts their antibacterial efficacy, with smaller nanoparticles and certain shapes releasing silver ions more efficiently 26 , 28 . Gram-negative bacteria possess thinner cells walls and are thus highly susceptible to silver nanoparticles. However, biofilms can hinder the effectiveness of silver nanoparticles by impeding their transport and reducing their diffusion coefficients, thereby allowing bacteria within the biofilm to remain tolerant to silver nanoparticle exposure 29 .

Functioning through multiple mechanisms such as membrane disruption, interference with cellular processes, and generation of reactive oxygen species, AgNPs have been effectively utilized in textiles, medical devices, and coatings as antimicrobial agents 30 , 31 , 32 . The incorporation of PANs and AgNPs may synergistically improve their antimicrobial properties 33 . Specifically, the positively charged amino groups inherent in PANs can facilitate the binding and subsequent release of AgNPs, fostering enhanced interaction with microorganisms. In this study, we demonstrate the antibacterial potential of one-pot polycondensation bipyridine-based Polyaminal encapsulated with Ag nanoparticles (Ag NPs@Bipy-PAN).

Material and methods

The chemicals used in this study did not undergo any further purification. [2,2′-Bipyridine]-5,5′-dicarbaldehyde was purchased from Shanghai Sunchem Inc., Shanghai, China. Melamine (97.5%), silver acetate (CH 3 COOAg 98.5%), and dimethyl sulfoxide (DMSO 99%) were supplied by BDH Laboratory Reagents, England, UK. Tetrahydrofuran (THF ≥ 99.5%), acetone (99.5%), dichloromethane (DCM ≥ 99.9%), and Pd(NO 3 ) 2 (99%) were purchased from Fisher Scientific, Chicago, USA. NaOH (98%), NaBH 4 (99%), and (CH 3 .COO) 2 Cd.2H 2 O (99%) were supplied by BDH Chemicals, England, UK. NiSO 4 .6H 2 O (98%) and (CH 3 COO) 2 Cu.H 2 O (≥ 99.8%) were obtained from BDH Laboratory Supplies, England, UK. Ba(NO 3 ) 2 (99%) was provided by Ward's Natural Science, Rochester, NY, USA, and finally, HCl (35%) was supplied by LOBA Chemie, Mumbai, India.

Synthesis of bipyridine-based polyaminal-linked porous organic polymer (Bipy- PAN )

Melamine-based porous polyaminal was prepared according to a previous study 34 . The synthesis involved utilizing a dry three-necked flask equipped with a magnetic stirrer, and the condenser was degassed through an evacuation-argon-backfill cycle. Initially, a vacuum was applied to evacuate the flask. Melamine (0.5 g, 3.96 mmol) and [2,2′-Bipyridine]-5,5′-dicarbaldehyde (0.7 g, 5.94 mmol) were dissolved in 30 mL of DMSO. The resulting mixture was then subjected to heating at 175 °C for 3d, with continuous stirring under an argon flow. The precipitate was obtained by filtration and subjected to washing with dimethylformamide (3 times), dichloromethane (3 times), and acetone (3 times). Subsequently, the white solid product was dried under vacuum at 70 °C for 2 h, resulting in a yield of 72%.

Synthesis of the Ag NPs@Bipy- PAN

To encapsulate Ag NPs by Bipy-PAN, 170 mg of Bipy-PAN powder was suspended in 40 mL of deionized water as a hydrophilic solvent, and the mixture was sonicated for 1 h until it became homogeneous. After stirring for 30 min, a solution of \({\text{CH}}_{3}\text{COOAg}\) (0.02 mmol) dissolved in \(\text{DCM}\) 0.04 mL) as the hydrophobic solvent was added dropwise for 10 min with constant vigorous stirring. The resulting solution was continuously stirred for 5 h. Then, the as-prepared mixture was reduced by adding a highly concentrated \({\text{NaBH}}_{4}\) aqueous solution (2.7 M, 1 mL). Finally, the mixture was filtered, washed with deionized water, and dried at 70 °C for 2 h to obtain the solid sample.

Instrumentation

Using a Maxima XRD-7000X Powder X-ray Diffraction (PXRD) and Dispersive X-ray Spectroscopy (EDX) equipments (Shimadzu, Kyoto, Japan), Ag@BiPy-PAN was investigated. Using nickel-filtered Cu-Kb reduction, X-rays were produced at 40 kV and 100 mA during the process. Ten degrees per minute was the scan speed, and the scan range (2θ) was 5 to 70 degrees. Ag@BiPy-PAN was investigated using Thermal Gravimetric analysis (TGA) on a TG-DTA6300 (Shimadzu, Kyoto, Japan) with a heating rate of 10 ̊C min -1 at intervals of 25–500 ̊ C in an N2 environment. The structure and particle distribution of Ag@BiPy-PAN in powder form were investigated using Scanning Electron Microscopy (SEM) (FEI Inspect F50, FEI, Tokyo, Japan). The dry substance was examined at a voltage of 30 kV. Additionally, the Ag@BiPy-PAN's size and morphology were examined using Transmission Electron Microscopy (TEM) (JEM-F200, Jeol, Tokyo, Japan). Following negative staining (phosphotungstic acid, 2%) with one drop of the substance applied to a copper grid at the proper dilution, TEM images were obtained 35 .

Antibacterial assay

Gram-negative Escherichia coli ( E. coli, ATCC 25,922) and gram-positive Staphylococcus aureus ( S. aureus, ATCC 25,923) were selected to examine the antibacterial effects of BiPy-PAN and Ag@BiPy-PAN using the agar well diffusion method. In this method, bacterial cells were sub-cultured in nutrient broth agar for 24 h before compound antibacterial testing to ensure bacterial cells were in the log phase. Mueller–Hinton agar (MH) media were prepared and inoculated by spreading bacterial inoculum over the whole agar surface. Then holes of 8 mm diameter were punched using a sterile Cork borer to add 100 µl of 0.25 mg, 0.5 mg, and 1 mg of the compound under investigation. Bacterial cells were incubated in a humified incubator under sterile conditions at 37 °C for 24 h in the presence, or absence, of the compounds. The inhibition zone around the hole was measured using a calibre 36 .

The cell viability assay

Breast cancer cells (MCF-7) and immobilized human embryonic kidney cells (HEK) were maintained, and cultures following the American Type Culture Collection (ATCC) guidelines and purchased from ATCC, Manassas, Virginia. The cultured cell lines were grown under 37 °C, 80–90% confluence, and 5% CO 2. For the cell viability, the MTT assay kit was procured from Invitrogen, USA. Cells were initially left for incubation at 37 °C for 24 h using a 96-well plate 37 . In each well, 2 × 10 3 of each cell type was placed and kept inside the humified carbon dioxide incubator, maintained at 5% humidity. The treatment group of of BiPy-PAN and Ag@BiPy-PAN was added to the well at different concentrations and left for 48 h. After this, all the samples were centrifuged, and precisely 100 µl of obtained supernatant was replaced with that of DMSO and again left for incubation at 5% carbon dioxide and 37 °C for 4 h. The absorbance was recorded at 570 nm in a triplicate using a microplate reader 35 , 36 .

Results and discussion

The Bipy-PAN polymer was utilized as an effective material for stabilizing metal ions by leveraging the binding sites provided by the nitrogen atoms in Bipyridine, triazine units, and aminal linkage. This indicates that the Bipy-PAN polymer has the potential to serve as an outstanding support for immobilizing metal nanoparticles. The reverse double solvent method (RDSM) was employed to investigate the immobilization of silver nanoparticles onto the polymer. The process involved dispersing the Bipy-PAN polymer in deionized water and adding a solution of CH 3 COOAg/CH 2 Cl 2 (0.5 M). The mixture was then subjected to sonication, and after 5 h of stirring, NaBH 4 was used as a reducing agent to convert the silver ions into atoms, resulting in the formation of a grey powder known as Ag NPs@Bipy-PAN (Scheme 1 ). Various characterization techniques, including PXRD, SEM, TEM, EDX, TGA, and N 2 adsorption–desorption methods, were employed to confirm the successful formation of the Ag NPs@Bipy-PAN adsorption.

Preparation of Ag NPs@Bipy-PAN.

PXRD analysis

The powder X-ray diffraction (PXRD) was used to determine the crystal type of polymer and the concentration ratio of silver nanoparticles within the polymer matrix. From (Fig.  1 ), it was observed that the Ag NPs@BiPy-PAN exhibited broad diffraction peaks at 2θ of 21°, which are characteristic of the amorphous structure of the BiPy-PAN polymer, as well as sharp diffraction peaks at 2θ of 38.2°, which correspond to the (111) diffraction planes of the face-centred cubic structure of silver nanoparticles (JCPDS No. 04–0783). These results suggest that the silver nanoparticles were able to distribute and encapsulate within the BiPy-PAN polymer without significantly altering its original structure 34 .

PXRD patterns of Ag NPs@Bipy-PAN.

Morphological analysis by SEM, TEM and EDX

The morphology, structure, and particle size distribution were thoroughly investigated using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) analysis. The SEM image of Ag NPs@BiPy-PAN (Fig.  2 a–c) showed a compact, irregular shape. However, the morphology of Ag@BiPy-PAN remained unchanged during AgNPs loading. The TEM images of Ag@BiPy-PAN displayed a spherical morphology of BiPy-PAN, with black dots representing Ag particles uniformly distributed on BiPy-PAN surface (Fig.  2 d–f). The average particle size of Ag nanoparticles was 9.2 nm, likely due to the coordination between AgNPs and nitrogen active sites in BiPy-PAN (Fig.  2 h). Energy dispersive X-ray (EDX) analysis indicated the presence of dispersed C, N, and Ag on the surface of Ag@BiPy-PAN, as shown in (Fig.  2 e–g). Overall, the results demonstrated the successful anchoring of Ag nanoparticles onto BiPy-PAN without significant changes in its morphology 34 .

( a , b , c ) SEM images with various magnifications of Ag NPs@Bipy-PAN. ( d , e , f ) TEM images with various magnifications of Ag NPs@Bipy-PAN. ( g ) Area EDX spectra of Ag NPs@Bipy-PAN. ( h ) Particle size distribution histogram.

TGA analysis

The thermogravimetric analysis (TGA) was used to assess the thermal stability of Ag NPs@BiPy-PAN. (Fig.  3 ) illustrates that the TGA curve of Ag NPs@BiPy-PAN showed an initial weight loss (about 10 wt%) in the temperature range of 35–80 °C, which can be attributed to the removal of absorbed water and solvent molecules. Furthermore, the material exhibits high thermal stability up to 360 °C, beyond which a significant weight loss (39.78 wt%) occurs in the temperature range of 360–460 °C, ascribed to the decomposition of the polymer network. The TGA analysis also indicates that Ag NPs@BiPy-PAN retains about 36.7% of its total weight at 500 °C. Overall, the results demonstrate that Ag NPs@BiPy-PAN exhibits outstanding thermal stability up to 360 °C 34 .

TGA analysis of Ag NPs@Bipy-PAN.

Antibacterial effects

The effects of Ag@BiPy-PAN were examined against gram-negative E. coli and gram-positive S. aureus bacteria, in which selected bacteria were incubated either in the presence or in the absence of 100 µl of 0.25, 0.5 and 1 mg of the compounds. Table 1 represents the anti-bacterial effects of Ag@BiPy-PAN on E. coli and S. aureus by observing a clear inhibition zone around the compounds. Results in both Table 1 and Fig.  4 demonstrate that the Ag@BiPy-PAN compound created clear inhibition zones impacting the growth of both bacteria in comparison to the BiPy-PAN, especially at 0.25 mg, that could add to the safety profile. Ag@BiPy-PAN inhibited the growth of E. coli by creating an inhibition zone of 9 mm when using 0.25 mg and 0.5 mg of the compound but the clear inhibition zone was not detected despite that the bacterial growth was reduced by 14 mm around the compound with the addition of 1 mg (light zone). Moreover, Ag@BiPy-PAN is effective on S. aureus creating a 10 mm clear inhibition zone around the compound regardless of increasing concentrations. Structural differences between both types of bacteria underlie the different effects Ag@BiPy-PAN exhibited against them.

Antibacterial effects of Ag@BiPy-PAN and BiPy-PAN of on E. coli and S. aureus .

Due to the electrostatic interaction between the positively charged resin and negatively charged bacteria cell membrane, the Ag@BiPy-PAN resin could interfere with the balance and integrity of the bacterial cell wall exhibiting anti-microbial activity towards microorganisms. In addition, the free radicals, derived from the resin surface, could disturb the membrane lipids and structure of microorganism cells, leading to a breakdown of membrane functions 36 .

Anticancer and biocompatibility polymer effects

The cytotoxic effects of Ag@BiPy-PAN and BiPy-PAN are apparent in Fig.  5 against the breast cancer cells. The 50%-inhibitory concentrations (IC50) were 1.15 ± 0.11 mg/mL and 0.68 ± 0.21 mg/mL of Ag@BiPy-PAN and BiPy-PAN, respectively. As a result, the potency of the BiPy-PAN was higher than the Ag@BiPy-PAN. The BiPy-PAN in physiological conditions could be positively charged on the secondary amine functionality, which could enhance the BiPy-PAN cellular adherence to the bilayer phosphate group. Afterwards, the BiPy-PAN would imbalance the cellular membrane integrity causing cell death 37 , 39 . On the other hand, Ag@BiPy-PAN could follow the same mechanism, but the silver nanoparticles might interfere with the cellular adherence of the polymer, especially at concentrations lower than 2 mg/mL.

The MTT assay against MCF-7 of the Ag@BiPy-PAN and BiPy-PAN polymers. One-way ANOVA was to determine the significance between the Ag@BiPy-PAN and BiPy-PAN polymers (*), or between the Ag@BiPy-PAN and control (#), which the P -value of less than 0.05 was considered significant.

Regarding the polymers’ effects on the immobilized kidney cells, the cytotoxic effects of Ag@BiPy-PAN and BiPy-PAN are apparent in Fig.  6 against the normal kidney cancer cells. The 50%-inhibitory concentrations (IC50) were 1.45 ± 0.11 mg/mL and 0.91 ± 0.21 mg/mL of Ag@BiPy-PAN and BiPy-PAN, respectively. As a result, the potency of the BiPy-PAN was higher than the Ag@BiPy-PAN. The mechanism of actions and behavior could be related to the above explanation. The mechanism of action could be confirmed to be un-specific to the cancer cells 37 , 39 . As a result, a cancer-targeted drug delivery systems of Ag@BiPy-PAN and BiPy-PAN could enhance their safety profile when used against cancer 38 , 39 .

The MTT assay against HEK of the Ag@BiPy-PAN and BiPy-PAN polymers. One-way ANOVA was to determine the significance between the Ag@BiPy-PAN and BiPy-PAN polymers (*), or between the Ag@BiPy-PAN and control (#), which the P-value of less than 0.05 was considered significant.

This study successfully synthesised and characterised silver nanoparticle stabilised by a bipyridine-based polymer (Ag NPs@Bipy-PAN). The Bipy-PAN polymer demonstrated significant potential as a support material for immobilising metal nanoparticles, utilising the binding sites provided by its nitrogen atoms. The reverse double solvent method (RDSM) facilitated the embedding of silver nanoparticles within the polymer matrix, as confirmed through various characterisation techniques including PXRD, SEM, TEM, EDX, TGA, and N2 adsorption–desorption methods.

PXRD analysis revealed that the silver nanoparticles were well-distributed within the amorphous structure of the Bipy-PAN polymer, maintaining its original integrity. SEM and TEM analyses showed consistent morphology and uniform distribution of Ag nanoparticles, with an average particle size of 9.2 nm. EDX confirmed the elemental composition, verifying the presence of Ag, C, and N on the polymer surface. TGA analysis demonstrated that Ag NPs@Bipy-PAN exhibited remarkable thermal stability up to 360 °C, indicating its suitability for high-temperature applications.

Functionally, Ag NPs@Bipy-PAN exhibited effective antibacterial activity against both gram-negative E. coli and gram-positive S. aureus, as evidenced by clear inhibition zones that increased with the compound concentration. This antibacterial effect is likely due to electrostatic interactions and free radicals derived from the resin surface, disrupting bacterial cell membranes. However, variations in nanoparticle size and polymer matrix can influence antibacterial efficacy.

The anticancer properties of Ag NPs@Bipy-PAN were assessed against breast cancer cells and normal kidney cells. The results indicated that while the BiPy-PAN polymer alone exhibited higher cytotoxicity, Ag NPs@Bipy-PAN also showed significant inhibitory effects. The data suggested a potential mechanism involving the interference of silver nanoparticles with cellular adherence, indicating a non-specific action on cancer cells.

Ag NPs@Bipy-PAN demonstrate considerable promise as a multifunctional material with applications in antibacterial treatments and cancer therapy. Future studies should focus on developing targeted drug delivery systems to enhance the specificity and safety profile of Ag NPs@Bipy-PAN and BiPy-PAN in clinical applications. The continued exploration of nanoparticle-polymer composites could lead to significant advancements in biomedical applications, reflecting a broader trend in the development of multifunctional nanomaterials for medical use.

Data availability

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This research work was funded by Institutional Fund Projects under grant no. G:376–247-1443 Therefore, the author gratefully acknowledges the technical and financial support from the Ministry of Education and Deanship of Scientific Research (DSR), King Abdulaziz University (KAU), Jeddah, Saudi Arabia.

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Conceptualization, N.S.A.; Methodology, M.M. A. and B.A.; Validation, N.T.; Investigation, N.S.A., F.B. and S.A.; Resources, M.M. A.; Data curation, N.S.A. and F.B.; Writing—original draft, M.M.A. .F.B. and S.A.; Writing—review & editing, N.S.A., N.T. and M.M.A.; Supervision, N.S.A. ; Funding acquisition, M.M.A. All authors have read and agreed to the published version of the manuscript.

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Alotaibi, M.M., Almalki, B., Tashkandi, N. et al. Synthesis of silver nanoparticles embedded into melamine polyaminal networks as antibacterial and anticancer active agents. Sci Rep 14 , 20008 (2024). https://doi.org/10.1038/s41598-024-70606-0

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Bacterial synthesis of metal nanoparticles as antimicrobials

Affiliation.

• 1 Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada.
• PMID: 39150434
• PMCID: PMC11328525
• DOI: 10.1111/1751-7915.14549

Nanoscience, a pivotal field spanning multiple industries, including healthcare, focuses on nanomaterials characterized by their dimensions. These materials are synthesized through conventional chemical and physical methods, often involving costly and energy-intensive processes. Alternatively, biogenic synthesis using bacteria, fungi, or plant extracts offers a potentially sustainable and non-toxic approach for producing metal-based nanoparticles (NP). This eco-friendly synthesis approach not only reduces environmental impact but also enhances features of NP production due to the unique biochemistry of the biological systems. Recent advancements have shown that along with chemically synthesized NPs, biogenic NPs possess significant antimicrobial properties. The inherent biochemistry of bacteria enables the efficient conversion of metal salts into NPs through reduction processes, which are further stabilized by biomolecular capping layers that improve biocompatibility and functional properties. This mini review explores the use of bacteria to produce NPs with antimicrobial activities. Microbial technologies to produce NP antimicrobials have considerable potential to help address the antimicrobial resistance crisis, thus addressing critical health issues aligned with the United Nations Sustainability Goal #3 of good health and well-being.

© 2024 The Author(s). Microbial Biotechnology published by John Wiley & Sons Ltd.

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The authors declare no conflict of interest.

Identified and potential mechanisms of…

Identified and potential mechanisms of nanoparticle synthesis by bacteria. (1) reduction from outward‐facing…

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Bio-Inspired Synthesis of Zno Nanoparticles Using Taraxacum Officinale for Antibacterial, Antifungal, and Anticancer Applications

14 Pages Posted: 28 Aug 2024

K. Kasthuri

affiliation not provided to SSRN

J. Kishor kumar

Saveetha School of Engineering, Saveetha Institute of Medical and Technical Sciences (SIMATS)

A. Amala Jeya Ranchani

An efficient, straightforward, and environmentally friendly approach by the eco-conscious method to synthesize the ZnO nanomaterials. Aqueous extract from Taraxacum officinale (Dandelion) leaves served as a biological reducing agent to fabricate ZnO nanomaterials from Zinc sulfate heptahydrate. The annealed synthesized powder samples underwent comprehensive characterization using different sophisticated methods, including XRD, EDAX, FTIR, FESEM, and UV–vis spectrophotometry. From the results, it is substantiated that the ZnO crystals are present exhibiting a wide range distribution, with noticeable enhancement in crystallinity observed after annealing. Furthermore, notably, ZnO nanomaterial exhibited enhanced antibacterial, antifungal, and anti-cancer activity against various pathogens. This novel green synthesis route for ZnO nanocrystals is designed to minimize the reliance on hazardous materials in the fabrication of operational oxide nanoparticles. Such nanoparticles hold vast potential applications in various fields, particularly in biomedical science

Keywords: Dandelion, FTIR, UV-Visible, XRD, FESEM

Suggested Citation: Suggested Citation

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Rajkumar p (contact author), saveetha school of engineering, saveetha institute of medical and technical sciences (simats) ( email ).

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