Chapter 32
Biosynthesis and Applications of Silver Nanoparticles

Bipinchandra K. Salunke and Beom Soo Kim

Concise Definition of Subject

Silver is a valuable metal and conversion of silver to nano form changes its physicochemical characteristics. The nano form of silver is an ideal and treasured compound for different fields of science and technology. As a result of diverse range of applications of silver nanoparticles (AgNPs), the research in the field of AgNP synthesis is getting increased attention. Physical, chemical, and biological methods can be used to produce AgNPs. However, biological methods for AgNP synthesis are becoming popular due to limitations associated with chemical and physical methods. Plant-mediated AgNP synthesis is more advantageous over microbial and other modes due to their green synthesis approach. Higher rate of nanoparticle synthesis with different shapes and sizes can be achieved due to flexibility in reaction parameters of plants. Biochemical diversity with pharmacological importance gives added advantage for plant-mediated AgNP synthesis approach. This chapter highlights the elementary features related to nanomaterials and methods of AgNP synthesis with specific emphasis on plant extract-mediated synthesis. Methods for characterization for AgNPs, parameters responsible for higher yield, mechanisms of plant-mediated synthesis of AgNPs, and applications including future promise and toxicity aspects of AgNPs are also deliberated.

32.1 Introduction

Nanoscience, a new and recently established multifaceted science, is devoted to study fundamental properties of nanomaterials [1, 2]. The prefix “nano” designates one billionth or 10−9 units. The clusters of atoms in the size range of 1–100 nm are broadly regarded as nanoparticles [3, 4]. As compared with their bulk materials, the nanometer-size metallic particles as a result of their high surface-to-volume ratio show unique and considerably changed physical, chemical, and biological properties [5]. Therefore, nanoparticles have attracted considerable scientific interest in recent years [6, 7]. The size- and shape-dependent unique properties are displayed by metallic nanoparticles that have been found useful for applications in medicine, healthcare, electronics, agriculture, and so on [4, 5, 8, 9]. The utilization of specific synthesis methods, reducing agents, and stabilizers is found to be valuable to synthesize nanoparticles with uniform size, shape, and size distribution [10–13]. The different types of properties or activities of nanoparticles vary with their size, structure, shape, size distribution, and chemical–physical environment. Therefore, there is increasing interest to obtain nanoparticles with uniform size, shape, and size distribution. For example, the uniform smaller-sized silver nanoparticles (AgNPs) exhibit better antimicrobial activity than the larger ones [14].

In general, two strategies are adopted for the synthesis of nanomaterials and fabrication of nanostructures, that is, top-down and bottom-up (Figure 32.1). Top-down strategy includes nanomaterial synthesis via photo-reduction, mechanical/ball milling, laser/thermal ablation, kinetic sputtering, thermal/chemical electro-explosion, lithography, and chemical etching. Bottom-up strategy utilizes chemical vapor deposition, atomic/molecular condensation, laser/spray/aerosol pyrolysis, sol/gel, supercritical fluid synthesis, template synthesis, spinning, chemical/electrochemical precipitation, and biological methods. In the top-down approach, the bulk materials are sliced or successively cut to get nanosized particles. Materials from the bottom, that is, atom by atom, molecule by molecule, or cluster by cluster, are built up in the bottom-up approach. Both approaches have advantages and disadvantages and are playing an important role in nanotechnology and modern industry. Top-down strategy is very good for bulk production of nanomaterials. Therefore, this approach will continue to play an essential role in the synthesis of nanomaterials. The concept of the bottom-up approach is based on long-standing principles. The growth of all the living things in nature occurs by employing this approach, which is practiced in industries for over a century, for example, producing salt and nitrate in chemical industry. The bottom-up approach has an important role in the fabrication and processing of nanomaterials.

Illustration of Top-down and bottom-up strategies for the synthesis of nanomaterials and fabrication of nanostructures.

Figure 32.1 Top-down and bottom-up strategies for the synthesis of nanomaterials and fabrication of nanostructures.

The metal nanoparticles can be synthesized by employing different physical, chemical, and biological methodologies. Biological mode is more advantageous than physical and chemical methods as a result of the simplicity of synthesis method with no requirement of high temperature, pressure, energy, and toxic chemicals besides eco-friendliness, cost-effectiveness, and scalability of the processes. Biological syntheses of nanoparticles have been reported using plants, bacteria, fungi, algae, yeast, actinomycetes, lichen, and so on [4, 15]. These biological agents or their active constituents have the ability to reduce metal precursors to their respective nanoparticles. The general scheme of biological synthesis of nanoparticles is exhibited in Figure 32.2. As the microbe-mediated processes require maintenance of aseptic conditions and pure cultures, industrial feasibility of the process becomes difficult. The additional cost of isolation of microorganisms, culture media, and specialized equipment and expert manpower can enhance the cost of the process. Due to the chances of biohazard, elaborate process of maintaining cell cultures and expertise is required to handle the cultures [16]. Among biological methods, plant-mediated synthesis method is more advantageous than the other modes, as the plant metabolites act as natural capping agents for the stabilization of AgNPs and the process of synthesis is eco-friendly, cost-effective, and suitable for diverse applications including medical applications [4].

Illustration of biological synthesis of nanoparticles.

Figure 32.2 General scheme of biological synthesis of nanoparticles.

32.2 Silver Nanoparticles

In recent years, AgNPs as a result of their unique properties have become one of the attractive fascinating products among all noble metal nanoparticles in the field of nanotechnology. AgNPs display characteristics such as chemical stability, good conductivity, and catalytic activities [17]. They can be incorporated into composite fibers, cryogenic superconducting materials, cosmetic products, food industry, and electronic components [18, 19]. They are useful in biosensors [20], textiles [21], agriculture [22], and pest management [23]. They show antibacterial, antiviral, antifungal, and anti-inflammatory activities. They are useful in biomedical applications such as drug delivery [24], chemotherapeutics [25], wound dressings, topical creams, antiseptic sprays and fabrics, antimicrobials [26], and anti-tuberculosis agents [27]. They serve as antiseptic and display a broad biocidal effect against microorganisms through the disruption of their unicellular membrane thus disturbing their enzymatic activities. AgNPs can be synthesized via different approaches.

32.3 Plants in Nanoparticle Synthesis

As a result of rapidness, eco-friendliness, nonpathogenicity, and cost-effectiveness, a single-step process, the phyto-synthesis approach, is drawing increasing attention. A brief AgNP synthesis scheme is presented in Figure 32.2. Different phyto-constituents/active metabolites such as proteins, amino acids, enzymes, polysaccharides, alkaloids, tannins, phenolics, saponins, terpenoids, vitamins, and so on play a role in the reduction and stabilization of silver ions. Most of these metabolites are eco-friendly and have medicinal properties [4].

The general process of synthesis using plant involves the collection of plant materials and preparation of plant extracts. The plant materials are properly washed first with tap water. The plant materials are used either fresh or dried. Drying of plant materials is commonly carried out in the shade for 5–10 days. The materials are ground in a grinder to make fine powder. Five to 10 g of fresh plant materials or dried powder is usually added in 100 ml deionized distilled water and boiled for 5–10 min. The resulting broth is then filtered through Whatman filter paper. Plant broth (5–10%) is added in 1 mM AgNO3 solution, and synthesis of AgNPs through reduction of pure Ag(I) ions to Ag(0) is monitored by observing color change and UV–Vis spectra of the solution at regular intervals [4, 14, 28]. The synthesized AgNPs are purified by centrifugation at 15 000 rpm for 15–20 min and repeatedly washed two to three times. The settled AgNPs are recovered and freeze-dried to obtain AgNPs in powder form.

A range of different techniques such as UV–Vis spectroscopy, dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffractometry (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) are used to characterize nanoparticles [4, 5, 29, 30]. These methods give ideas about the shape, size, fractal dimensions, pore size, surface area, and crystallinity besides dispersion, orientation, and intercalation of nanoparticles. UV–Vis spectroscopy provides indication about the sample formation by showing the surface plasmon resonance. DLS delivers clue of size distribution of particles. XRD delivers information about crystallinity of particles. TEM, SEM, and AFM analyses help to understand the morphology and size of particles. AFM analyses provide three-dimensional images that are useful to calculate particle height and volume.

32.4 Parameters Affecting Synthesis of AgNPs

32.4.1 Effect of pH

Shape, size, production rate, and stability of nanoparticles are found to vary with change of pH after use of plant and other sources for AgNP synthesis [31–36]. Alkaline condition favored AgNP synthesis, while acidic conditions did not produce AgNPs with the use of Azadirachta indica (neem) leaf extracts [37]. However, Gan and Li [38] reported that phyto-mediated synthesis can produce a large number of stable nanoparticles over wide pH range.

32.4.2 Reaction Time, Precursor to Plant Extract Ratio, and Reaction Rate

Nanoparticle synthesis was found to increase with increase in reaction time [35, 39]. Metal precursor to plant extract is significant to obtain AgNPs with varying shape, size, and stability [38]. AgNP synthesis increased with increasing garlic extract ratio to constant AgNO3 concentration. Twenty percent A. indica (neem) leaf extract synthesized more AgNPs than the 40% extract [39]. The ability of nanoparticle synthesis varies with plant varieties. The plants that can synthesize stable and uniform AgNPs with use at lower quantity of plant extract are good. Song and Kim [28] studied extracellular synthesis of metallic AgNPs using five plant leaf extracts (pine, persimmon, ginkgo, magnolia, and platanus). Magnolia leaf broth was the best reducing agent in terms of synthesis rate and conversion to AgNPs of 15–500 nm size.

32.4.3 Effect of Temperature

Increase in temperature was found to increase nanoparticle synthesis [14, 28, 40, 41]. Only 11 min was required for more than 90% conversion at the reaction temperature of 95 °C using magnolia leaf broth [28]. Temperature change was found to alter the shape of nanoparticles [42, 43]. Physicochemical environment may be changing due to temperature enhancing nucleation and controlling aggregation leading to synthesis of stable nanoparticles. Detection of the presence of biomolecules at higher temperature is not much studied. Presumption that proteins are thermolabile suggests that other biomolecules in plants with thermostability may have an active role at higher temperature.

32.5 Mechanism of AgNP Synthesis

Different researchers have correlated synthesis of AgNPs with the presence of bioactive metabolites in plant extracts. Plant metabolites such as terpenoids, flavonoids, polyphenols [44], amines [45], saponins [46], aldehydes, ketones [47], arabinose, galactose [48], and starch [49] have been reported to have an active role in the synthesis of AgNPs, which involves proteins and enzymes. Carbonyl groups of amino acid of peptide and protein have a strong affinity to bind metal. Curcacyclin A (octapeptide), curcacyclin B (nonapeptide), and curcain (enzyme) present in the latex of Jatropha curcas were responsible for the synthesis of AgNPs [50]. Using FTIR analyses, authors reported that decrease in intensity of band at 1537 cm−1 after reduction confirms role of amines and shift of band from 1618 to 1604 cm−1 attributed to binding of (NH)CO group with nanoparticles. Patil et al. [26] also predicted the role of proteins from the latex of Euphorbian plants in the formation of AgNPs. The actual mechanism of reduction and capping using isolated pure compounds has not been much investigated. The course of AgNP synthesis may be happening like this. The metal precursor AgNO3 may be dissociated into Ag+ and NO3−. The bioactive phytochemicals such as phenolic compounds have hydroxyl and ketonic groups that have the ability to bind to metals and reduce the metal salt and provide stability against agglomeration. Plant extract gives protein and enzyme to the AgNO3 solution in which Ag+ ions may combine with the enzyme to form enzyme–substrate complex. The enzyme released from the plant extract may be acting on the silver ions and AgNPs may be released from this enzyme. These released AgNPs may be combining with the proteins from the plant extract thereby leading to the production of protein-capped AgNPs.

32.6 Applications of AgNPs

AgNPs are promising for a number of beneficial applications due to their unique properties. Different accredited bodies such as the United States Food and Drug Administration (US FDA), US Environmental Protection Agency (EPA), the Society of International sustaining growth for Antimicrobial Articles (SIAA) of Japan, Korea's Testing and Research Institute for the chemical industry, and FITI Testing and Research Institute have approved some products made with AgNPs [51–55]. Textile, health industry, food storage, and environmental sectors make use of AgNPs for diverse applications. They are valuable as antimicrobial agents for disinfecting medical devices, home appliances, and water treatment [56–60]. AgNPs were integrated in fibers to produce silver nanocomposite fabric [10]. Their enhanced antibacterial activity against Escherichia coli was observed for the cotton fibers [10, 61, 62]. AgNPs show antiviral activity against HIV-1 at noncytotoxic concentrations. However, the mechanism of HIV-inhibitory activity is not exactly understood yet [63]. Special interest has been directed at providing enhanced biomolecular diagnostics, including single-nucleotide polymorphism detection gene expression profiles and biomarker characterization. These strategies have been focused on the development of nanoscale devices and platforms that can be used for single-molecule characterization of nucleic acid, DNA or RNA, and protein at an increased rate when compared with traditional techniques [64]. AgNPs are useful to be used in nanoscale sensors as they show faster response times and lower detection limits due to their electrochemical properties [65]. AgNPs enhanced the bleaching of the organic dyes by the application of potassium peroxodisulfate in aqueous solution at room temperature [66]. Chemiluminescence from the luminol–hydrogen peroxide system showed better catalytic activity in the presence of AgNPs than Au and Pt colloid [67]. AgNP-supported halloysite nanotubes with Ag content of about 11% were able to exhibit improved reduction of 4-nitrophenol with NaBH4 in alkaline aqueous solutions [68]. AgNPs and their composites display catalytic activities such as dye reduction. The reduction of methylene blue by arsine in the presence of AgNPs was successful [69]. AgNPs showed catalytic activity by the reduction of phenosafranine dye [70].

32.7 Conclusion

There is an increasing awareness toward the development of environmentally friendly techniques for the synthesis of AgNPs using green methods. The use of plants can be more advantageous than other biological entities as the time-consuming process of employing microbes and maintaining their culture is not needed. Plants have diverse chemical compounds like proteins, carbohydrates, alkaloids, tannins, phenolics, oils, and saponins that have medicinal value and can act as reducing and capping agents for AgNP synthesis. The approach of using plant extracts for the synthesis of AgNPs is economical, energy efficient, and cost-effective. This approach will also provide avenue for healthier workplaces and communities, protecting human health and environment and producing safer products with lesser waste. AgNPs have many applications in textile, health industry, food storage, and environmental sectors. Large-scale synthesis study, finding mechanism of nanoparticle formation, isolation, purification, and genetic engineering of plant for production of active major compounds catalyzing reduction and capping are promising areas of research.

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