Silver Nanomaterials and Their Polymer Nanocomposites

Silver Nanomaterials and Their Polymer Nanocomposites

CHAPTER 2 Silver Nanomaterials and Their Polymer Nanocomposites Niranjan Karak Advanced Polymer and Nanomaterial Laboratory, Center for Polymer Scien...

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CHAPTER 2

Silver Nanomaterials and Their Polymer Nanocomposites Niranjan Karak Advanced Polymer and Nanomaterial Laboratory, Center for Polymer Science and Technology, Department of Chemical Sciences, Tezpur University, Tezpur, India

2.1 INTRODUCTION Chapter 1 reveals the innumerable interests of the scientific as well as the industrial community on various nanomaterials due to their unprecedented attributes. Among others, silver nanoparticles (AgNPs) deserve special attention due to their utilities in biomedical fields especially against microbes. The antiinflammatory effects and wound-healing ability without toxic effects up to a certain dose level are also commendable. They also have other industry applications, for which uniform dispersed AgNPs are utilized for the production of inkjet inks, electronic circuits, catalyst, sensor, biomarker, photography, etc. All of these are due to their easy synthesis and unique optical, electrical, biological, and thermal properties. These properties of AgNPs are strongly influenced by their shape, aspect ratio, size, and distribution, which are controlled by the synthetic methods. AgNPs are synthesized by using various chemical, photochemical, physical, and biological routes (Natsuki et al., 2015). However, each of these techniques has its pros and cons. In general, physical and photochemical methods need very critical conditions and expensive equipment, though as such no toxic chemicals are used. On the other hand, chemical methods are simple and less expensive, though they have used many toxic chemicals. However, from industrial standpoints, it is mandatory to utilize simple and low-cost methods to produce mass quantities of AgNPs. In this milieu, recent well-informed green chemistry principles are inclined to the use of biogenic AgNPs using plant based nontoxic phytochemicals. These principles are inclined to use a safer chemical process with an efficient transformation of desired end products. Thus environmentally benign reducing agents such as phytochemicals and biocompatible stabilizing agents such as sustainable polymer-based synthetic methods for manufacturing AgNPs are the more preferred industrial approaches. However, a microorganism-based biological process also suffers from some shortcomings and is generally not preferred for the industrial production of AgNPs. Further, this nanomaterial is synthesized by such approaches having different shapes that include zero-dimensional nanospheres, nanocubes, nanodecahedrons, nanodisks, and nanoplates, as well as one-dimensional nanobelts, nanowire, and nanotubes. Nanomaterials and Polymer Nanocomposites https://doi.org/10.1016/B978-0-12-814615-6.00002-3

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In all such methods the primary considerations are size, shape, morphology, stability, environmentally friendliness, and cost for large-scale production of AgNPs. In the recent decade, >350 tons/year of AgNPs are produced and utilized for various applications worldwide (Nowack et al., 2011). From the earlier studies, it is also clear that AgNPs are quite different from bulk silver mainly due to their differences in size, shape, aspect ratio, and surface chemistry. Thus the incorporation of AgNPs in different polymer matrices may result in unique and unusual properties in the resultant nanocomposites. The polymer matrix is found to modify the surface chemistry of AgNPs and thereby preventing the aggregation of the particles. The uniform dispersion of the nanoparticles and strong interaction with the polymer matrix create a huge nanodimensional interface in the nanocomposite. Again, these interfaces are responsible for achieving unique properties and result in many advanced applications. These unique properties include biological, optical, structural, catalytic, etc. More interestingly, the use of polymers as support for the preparation of AgNPs is advantageous due to the fact that nanoparticles can form under mild and ecofriendly conditions, while many polymers themselves also act as the reductants for silver ions. Again an AgNPs-based polymer nanocomposite is used as a heterogeneous reusable catalyst and hence can be easily separated from the reaction medium by filtration (Das et al., 2016). Thus an elaborative discussion on AgNPs and their polymer nanocomposites from synthesis to applications including characterization and properties are presented in this chapter.

2.2 HISTORICAL BACKGROUND AND SIGNIFICANCE AgNPs in the form of colloidal particles have been routinely used since ancient times in the medical field. Further, in the United States, colloidal silver was registered as a biocidal material since 1954. In addition, without understanding the nomenclature of nanomaterial, AgNPs have been commercially used for over 100 years in different fields of applications in photographics, pigments, conductive/antistatic composites, wound treatment, catalysts, biocide, etc. However, unintentionally obtained nanoparticles are not considered under the technical definition of nanoscience and nanotechnology, as materials should be engineered at nanoscale. In this milieu, in 1889, Lea reported citrate-stabilized silver colloidal particles with sizes of 7–9 nm (Lea, 1889). Further, in 1902, the stabilization of nanoscale particles of silver by using proteins was reported (Paal, 1902). Most importantly, these were commercialized for medical use since 1897 with the trade name “Collargol.” Also of note is the particle size was found in nanoscale (10 nm) (Bogdanchikova et al., 1992). Similarly, other reports on nanoscale silver particles were also found in literature. Gelatin stabilized silver particles (2–20 nm) were patented by Moudry in 1953, while silver particle- (<25 nm) impregnated carbon was patented by Manes in 1968 (Moudry, 1953; Manes, 1968). Most significantly, silverbased first biocidal product was registered under the Federal Insecticide, Fungicide,

Silver Nanomaterials and Their Polymer Nanocomposites

and Rodenticide Act (FIFRA) of the United States in 1954 under the trade name of Algaedyn derived from above Moudry’s patent, which is still used today as an algicide in domestic swimming pools. Similarly, over the last two decides, AgNPs containing polymer nanocomposites find applications in several engineered products from consumer to biomedical, such as clothing, respirators, water filters, soaps, contraceptives, antibacterial sprays, and detergents as well as in numerous other household products.

2.3 MATERIALS AND PREPARATIVE TECHNIQUES FOR NANOMATERIALS AgNPs are synthesized from silver-based raw materials through a variety of processes either through the bottom-up or top-down technique. Basically, under supersaturated conditions silver nuclei are formed, which subsequently grow by a molecular assembly approach through the diffusion of growth species towards nuclei. They finally restrict the growth at the metastable nanodimension by help of stabilizer. The complete process is shown is Fig. 2.1. Further, the size, shape, stability, and ultimate properties of AgNPs are dependent on the synthetic approach and conditions, including stabilizing/functionalizing and reducing agents used (Sengupta et al., 2005).

2.3.1 Precursors and Reagents Generally, three different kinds of raw materials are required to produce AgNPs by the bottom-up process, while two kinds are essential for the top-down method. These are silver precursors to supply the silver element and capping or stabilizing agents to stabilize the metastable AgNPs, while a reducing agent is also necessary to transform silver ions to AgNPs in the case of the bottom-up approach. AgNPs are mainly synthesized from their ions or bulk precursors, therefore different inorganic and organometallic compounds such as silver nitrate, silver acetate, silver chloride, silver bromide, silver chlorate, silver carbonate, silver oxide, silver sulfate, etc. are used (Zielinska et al., 2009). Beside precursors of silver, reducing agents such as sodium borohydride, sodium citrate, ascorbate, hydrazine, elemental hydrogen, Tollen’s reagent, N,N-dimethyl formamide, ammonium formate, dimethylformamide, aniline, phytochemicals like flavones salvigenin, chrysoeriol, quercetin, luteolin, apigenin, flavanone eriodyctol, flavanol taxifolin, oleanolic acid,

Fig. 2.1 General steps for the formation of AgNPs from silver ions.

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triterpenic acids, 2,3-dihydroxyoleanolic acid, pomolic acid, ursolic acid, tormentic acid, etc. and stabilizing agents such as different polymers including poly(vinyl alcohol), polyacrylamide, poly(vinyl pyrrolidone), polyester, poly(ester amide), polyurethane, etc.; surfactants like sodium lauryl or dodecyl sulfate (SLS or SDS), Triton-X 100, linear alkylbenzene sulfonates, lignin sulfonates, fatty alcohol ethoxylates, cetyl triethyl ammonium bromide (CTAB), etc. oleic acid, amino silanes, polyelectrolytes, etc. are also required for synthesizing AgNPs (Zewde et al., 2016). Some of these stabilizing agents also act as a reducing agent to form AgNPs. Here, it is pertinent to mention that stabilization is very much essential for AgNPs, as they are thermodynamically unstable, and huge surface energy will cause the aggregation of such particles, known as Oswald ripening, unless they are stabilized. The common ways of such stabilization are electrostatic stabilization, where opposite charge ions are used, and steric stabilization, where suitable sterically hindered molecules or polymers are used. However, electrostatic stabilization is preferred in the case of high ionic strength containing biological media. Further, stabilization by bioactive molecules such as bovine serum albumin (BSA), peptides, etc. enjoys added advantages as such molecules stabilize the nanoparticles, which are then bioactive and biocompatible as well. Furthermore, they provide additional functionalities for different biological interactions. More importantly, such bionano-conjugated nanoparticles are found to be toxic to bacteria or other microorganisms, but not to mammalian cells. To fabricate the polymer/silver nanocomposites, AgNPs or their nanohybrids with almost all types of natural and synthetic polymers are used. Thus all the polymers mentioned in the first chapter are used here also and hence are not specified again.

2.3.2 Techniques As mentioned, the syntheses of AgNPs are performed through top-down and bottom-up approaches. In the top-down approach an appropriate bulk silver material smashes down into fine particles by size reduction with a variety of lithographic techniques involving grinding, milling, sputtering, and thermal/laser ablation, whereas in the bottom-up approach, AgNPs have been synthesized by using chemical, photochemical, and biological methods by the self-accumulation of atoms to new nuclei, which grow into a particle of nanoscale. The techniques included are discharge, physical vapor deposition, highenergy ball milling, laser ablation, etc. as physical; they also include electrochemical, chemical vapor deposition, sonochemistry, sol-gel, coprecipitation, inverse micelles, or microemulsion, etc. as chemical. Among these, wet chemical reductive techniques are very popular for AgNPs. The main merit of this approach is that a huge amount of nanoparticles can be prepared in a short span of time, but in terms of demerit, chemicals used here are toxic, which result in nonecofriendly byproducts and thus are unpromising methods. However, “green” chemistry-based clean, nontoxic, and environmentfriendly techniques, such as the biogenic route using different microorganisms and plant

Silver Nanomaterials and Their Polymer Nanocomposites

extracts as an alternative to the conventional synthetic methods, is considered most justifiable. They enjoy the advantages like mild reaction conditions, environmentally friendly reagents, and comparatively easy scale-up protocols. These greener syntheses of AgNPs using biological products like plant extracts and microorganisms afford significant advantages over the other methods. Thus biological-like plant mediated approach, the microorganisms-based approach, and the photochemical approach are included as greener alternative methods to the petrochemical-based synthetic approach for AgNPs. 2.3.2.1 Physical Approach In this technique, AgNPs are generally synthesized by a physical vapor deposition technique, where a metallic silver precursor taken in a boat is vaporized, followed by the condensation of the vapor as carried by a carrier inert gas. The vaporization is done in a tube furnace at atmospheric pressure at a high temperature. But it is rarely used in practice as it suffers from some drawbacks like high energy consumption, large space needs for the furnace, and the required length of time; hence it is expensive. On the other hand, another physical technique, namely laser ablation, produces colloidal AgNPs in a single-step process without any additional contamination, hence there is no need of any purification. In the electrochemical method, AgNPs are produced on a platinum cathode at room temperature by the application of an external electric field, while the silver ions are produced from silver anode (Kasprowicz et al., 2010). The particle size of AgNPs is controlled by controlling the applied current density. Overall, physical approach permits production of mass-scale AgNPs in a single-step process. 2.3.2.2 Chemical Approach AgNPs are mostly produced by this approach as it can keep production at a low cost with a high yield. Additionally, a large variety of chemical reducing agents as mentioned above can be used for this purpose. The silver ions obtained from their precursors are reduced to AgNPs by the appropriate reducing agent in the presence of capping or stabilizing agents. The fine size and uniform distribution of AgNPs are achieved by producing the nuclei at the same time from the supersaturated condition, followed by subsequent growth and stabilization. The controlling parameters are the concentration of silver ions, the concentration and efficacy of the reducing agent, the concentration and nature of the stabilizing agent, along with the reaction temperature, pH, pressure, etc. Different shapes of AgNPs are possible to obtain by using suitable conditions and a shape directing agent. Overall a great variety of chemical methods are available, but most of the chemicals are toxic and cause environmental pollution as well as the synthesized AgNPs became cytotoxic. Further, these are technically and economically unfavorable, thus alternative greener methods are preferred.

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2.3.2.3 Photochemical Approach This approach is considered as a relatively greener technique to produce AgNPs as light energy is utilized. Silver ions are reduced to AgNPs in the presence of photoactive chemicals such as acetone with isopropanol, carbon dots, modified clay suspension, etc. It may also act as stabilizer to the nanoparticles, under the exposure of UV or solar light irradiation. In this approach, it is possible to obtain fine AgNPs with a stable dispersion. 2.3.2.4 Biological Approach In recent times, alternative greener routes have emerged as a simple and viable facile approach to synthesize AgNPs by using naturally reducing agents such as polysaccharides, microorganisms like bacteria and fungus, phytochemicals from plants extract, etc. Thus in the biological synthetic process, proteins and enzymes play significant roles. The utilized proteins act as the capping or stabilizing agents. Different bacteria strains such as Bacillus licheniformis, Klebsiella pneumonia, etc.; fungi strains such as Verticillium spp., Fusarium oxysporum, and Aspergillus flavus, etc.; actinomycetes, yeasts, algae; and a variety of plant products are used for the synthesis of AgNPs. Many bacteria and fungi are found to be capable for synthesizing AgNPs in their intracellular or extracellular cells. Bacteria are acting as biofactories for the synthesis of such nanoparticles in their intra- or extracellular cells. For example, silver mine-inhabiting Pseudomonas stutzeri AG259 reduces silver ions to form AgNPs (Kumar et al., 2007). Different shapes and sizes of AgNPs are obtained by using fungus and actinomycete species. F. oxysporum contains nitrate reductase, which catalyzes the reduction of silver ions to AgNPs by utilizing NADPH as reducing agent. This nitrate reductase is an important enzyme for the reduction of silver ions to AgNPs (Marambio-Jones and Hoek, 2010). Similarly, Shewanella oneidensis is able to interact with silver nitrate solution and form nearly monodispersed AgNPs. The size of AgNPs synthesized by microorganisms depends on the temperature of the medium, while the concentration of silver ions and the pH of the medium control the amount of AgNPs formed. Higher temperature produces smaller size AgNPs, while alkaline pH (<10) produced more AgNPs than the same under acidic pH. Again a very high pH or high concentration of silver ions kill the microorganisms. However, this microbes-mediated synthesis is industrially not feasible because of the prerequisite of extremely aseptic conditions, maintenance, and other sophistications, as well as a high cost. On the other hand, plant-mediated synthesis of AgNPs is gaining tremendous impetus because of the easy availability and presence of many polyphenolic compounds. They have a high reactivity and hence can reduce silver ions in a faster rate than fungi or bacteria. The simplicity of this process in comparison to biohazards and the intricate process of maintaining cell cultures also promotes this approach. It provides an easy and safe green route in the scale-up and industrial production of AgNPs. Furthermore a variety of plant products from leaf to bark including fruits and flower of various plants are available for

Silver Nanomaterials and Their Polymer Nanocomposites

Fig. 2.2 Reduction of silver ions to AgNPs using phytoextract by polyphenolic mechanism.

this purpose. These products are not only naturally renewable, but they are also environmentally friendly. The phytochemicals obtained from the leaves of various plants such as tea, Mesua ferrea, Thuja, Alovera, Bel, Calcacia esculanta, etc. are utilize to produce AgNPs and their nanohybrids by polyphenolic mechanism (Fig. 2.2). In fact, all photochemicalbased reduction processes mainly follow the polyphenolic mechanism because they have such compounds in the extract. For example, caffeine and theophylline are present in tea extract. Quercetin and polysaccharides are also able to reduce silver ions to AgNPs. Overall, the biological method provides a wide range of natural resources for the synthesis of AgNPs, though mass scale production is not simple.

2.3.3 Reports on the Synthesis of AgNPs A high number of reports are available in the literature for the synthesis of AgNPs. As it is not possible to summarize all of them in a chapter, a few such reports are cited below. 2.3.3.1 Physical Route AgNPs with a particle size of <10 nm are produced by the thermal decomposition of the silver precursor and oleate at elevated temperature, while monodispersion AgNPs are obtained by continuous heating in ceramic heating system (Lee and Kang, 2004; Jung et al., 2006). Arc discharge technique produced AgNPs with size <10 nm by employing silver electrodes in deionized H2O (Tien et al., 2008). 2.3.3.2 Chemical Route The wet chemical reduction of silver ions to AgNPs by using different reducing agents such as sodium borohydride, poly(ethylene glycol), aromatic hyperbranched polyamine, dimethylformamide, etc. are well reported in literature (Radziuk et al., 2007; Luo et al., 2005; Mahapatra and Karak, 2008; Deka et al., 2010). The colloidal suspension of AgNPs is obtained by the chemical reduction of silver ions by formaldehyde at elevated temperature in presence of poly(vinyl pyrrolidone) stabilizer with organic bases as the promoter (Hsu and Wu, 2007). Different shape and size of AgNPs are also reported by the

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researchers. As an example, Jana et al. synthesized silver nanorods and nanowires by seed mediated synthetic approach, where silver seed of 4 nm, obtained by reduction of silver ions by NaBH4 in the presence of trisodium citrate in water, is used along with extra silver ions in an alkaline aqueous solution of ascorbic acid to produce silver nanorods, while silver ions and seed produce silver nanowires in the presence of CTAB in similar alkaline medium ( Jana et al., 2001). The cubes of AgNPs are also obtained by adjusting poly(vinyl pyrrolidone) as stabilizing agent in a polyol process at 160°C using ethylene glycol as both a reductant and a solvent (Sun and Xia, 2002). On the other hand, silver nanoprisms are synthesized by a seed-mediated technique using spherical AgNPs of about 8 nm as seed and a photo-induced irradiation of 40 W fluorescent light as the energy source ( Jin et al., 2001). Similarly, truncated triangular AgNPs are prepared by following seeding, growth, and aging protocol, where seeds are produced by a sodium borohydride reduction of silver ions with sodium citrate as stabilizing agent. These more silver ions are again reduced in the presence of these seeds with cetyl trimethly ammonium bromide (CTAB) as stabilizing agent, whereas in the final step, these grown AgNPs are allowed to undergo aging to form the expected truncated triangular AgNPs and purified by centrifuge (Chen and Carroll, 2002). However, spherical AgNPs that are synthesized in most of the reports may be due to easy process. Controlled particle size of 10–30 nm diameters are obtained in mass scale by reducing silver ions by ascorbic acid and stabilized with Daxad 19 for different times of reaction (Sondi et al., 2003). These spherical particles form excellent aqueous stability dispersion at different pH (210). The monodispersed AgNPs are synthesized by the reduction of silver ions by oleylamine in the presence of liquid paraffin, where a oleylamine-paraffin system controls the particle size (Chen et al., 2007a). 2.3.3.3 Irradiation Route Various irradiations are utilized to synthesize AgNPs. These include UV or visible (solar) light, laser light, microwave and γ-radiation. Highly stable (6 months) AgNPs with size <10 nm are synthesized by using UV radiation from aqueous alkaline solution of AgNO3 in the presence of carboxymethylated chitosan (Huang et al., 2008.). AgNPs are synthesized in the presence of laser light using benzophenone. Furthermore the application of low laser power with shorter irradiation time helps to form relative bigger size AgNPs (20–30 nm), while high laser power with longer exposure time yields smaller particle size (5–7 nm). Similarly, Gamma irradiation is also utilized for the synthesis of AgNPs. Various stabilizers such as poly(vinyl pyrrolidone), poly(vinyl alcohol), chitosan, etc. are used for stabilizing AgNPs in such cases (Chen et al., 2007b). On the other hand the microwave-based synthesis of AgNPs offers a faster rate of reduction and provides a higher yield by using the same temperature and exposure time compared to the conventional heating-based chemical method (Prabhu and Poulose, 2012). Poly(vinyl alcohol) stabilized AgNPs with varying sizes are synthesized by irradiating a 6 MeV electron beam

Silver Nanomaterials and Their Polymer Nanocomposites

in a solution of silver nitrate and poly(vinyl alcohol) (Bogle et al., 2006). However, in most cases, it is difficult to control the size and shape of AgNPs by such methods. 2.3.3.4 Biological Routes Various biomolecules including proteins, enzymes, amino acids, polysaccharides, alkaloids, alcoholic compounds, and vitamins are used in synthesis as both reducing and stabilizing agents of AgNPs. These biomolecules are obtained by extracting from various microorganisms such as Verticillium sp., Aspergillus fumigatus, Aeromonas sp., K. pneumonia, Lactobacillus species, Enterococcus faecium, Escherichia coli, Enterobacter cloacae (Enterobacteriacae), A. flavus, Bacillus subtilis, etc. (Shahverdi et al., 2007; Mukherjee et al., 2001). Thus both Gram-positive and Gram-negative bacteria can be used for this purpose. These microorganisms actually act as cost-efficient ecofriendly nanoreactors for the synthesis of AgNPs. For example, they are synthesized by the reduction of aqueous silver ions with the culture supernatant of B. licheniformis (Kalishwaralal et al., 2008.). Lactobacillus species produces AgNPs with high yield and fine particle size (10–20 nm). These nanoparticles are highly stable because they accumulated in the cell and on the cell wall. Again the strain of the lactobacillus sp. and pH of the medium controlled the diameter and rate of AgNPs formation (De Gusseme et al., 2009). Similarly, S. oneidensis is able to interact with the silver nitrate solution and form the nearly monodispersed AgNPs (Suresh et al., 2010). The biomolecules such as amino acids including lysine, arginine, cysteine, methionine, etc. execute dual functions for the formation and stabilization of AgNPs (Tran et al., 2013). Similarly, pseudomonas strutzeri, which is the silver mine isolated bacterial strain, can reduce silver ions to AgNPs (Klaus et al., 1999). Biomass of Brevibacterium casei can also produce AgNPs of 10–50 nm size at 37°C within 24 h (Kalishwaralal et al., 2010). An interesting hot springs-isolated Ureibacillus thermosphaericus strain produced AgNPs with a tunable particle size of 10–80 nm by monitoring the concentration of silver ions in the temperature range of 60–80°C ( Juibari et al., 2011). AgNPs are also synthesized by very common bacterial strain like E. coli, where the supernatant of nutrient broth incubated E. coli formed AgNPs upon adding 1 mM of AgNO3 (1% v/v) within 10 min (Saklani et al., 2012). AgNPs of 5–50 nm size are also synthesized from silver nitrate by exposure of F. oxysporum fungus for 72 h (Ahmad et al., 2003). Similarly, A. fumigatus and Fusarium semitectum fungi are able to produce AgNPs (Bfilainsa and D’Souza, 2006; Basavaraja et al., 2008). Similarly, Trichoderma viride fungus can produce AgNPs with size <50 nm from AgNO3 (Fayaz et al., 2010). Other microbes like yeast (Kowshik et al., 2003) and algae biomass are also used to obtain AgNPs (Govindaraju et al., 2008). The plant extracts mediated process eliminates the requirement of identification, isolation, and maintaining culture for the microorganisms and thus is more attractive than above greener processes. The use of different plant extracts such as Acalypha indica, mangrove, Arbutus unedo, Tribulus terrestris, Rumex hymenosepalus, Eucalyptus chapmaniana,

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Coleus aromaticus, Andrographis paniculata, Cathranthus roseus, Murraya koenigii leaf, Mangosteen leaf, Mangifera indica leaf, Tansy fruit, Jatropha curcas, Cinnamomum zeylanicum leaf, Camellia sinensis, Aloe vera, and Medicago sativa seed, etc. are used for synthesis of AgNPs. Various parts such as leaf, flower, bark, seed, fruit peel, gum, root, etc. of these plants are used, in general, for this purpose. Basically, different plant metabolites such as terpenoids, polyphenols, sugars, alkaloids, phenolic acids, proteins, etc. present in such parts of the plants help in the reduction of silver ions to AgNPs. Flavonoids are polyphenolic compounds including anthocyanins, flavonols, isoflavonoids, flavones, chalcones, flavanones, etc. and are capable to form chelate with silver ions and reduce them to AgNPs (Makarov et al., 2014; Fig. 2.2). In the polysaccharide-based green method, naturally occurring polysaccharides such as starch, chitosan, cellulose, and its derivatives are used both as reducing and stabilizing agents for AgNPs (Hassabo et al., 2015). The carbohydratesbased Tollens method reduced ammonical silver complex by aldehydes in presence of stabilizers like sodium dodecyl sulfate (SDS) and poly(oxyethylene sorbitane monooleate) (Sharma et al., 2009). The concentration of the ammonia solution and the nature of the stabilizer strongly influence the size of AgNPs. Researchers showed that different morphology of AgNPs with size <10 nm are possible to synthesize by controlling the concentration of a stabilizing agent like n-hexadecyltrimethyl ammonium bromide and the Tollens reagent (Yu and Yam, 2004). Thus sugar and monosaccharides like glucose can reduce silver ions due to the presence of the free aldehyde group. In an autoclaving technique, AgNPs are synthesized using starch as a capping and reducing agent under 0.1 MPa pressure and 121°C temperature for 5 min (Vigneshwaran et al., 2006). The reduction of silver ions by the plant-mediated process depends on the nature (biomolecules present) and concentrations of the plant extract, pH of the medium, reduction temperature, exposure time, silver ions to extract ratio, etc. Temperature influences the rate of nucleation of AgNPs, while pH strongly controls the binding and reduction ability of phytochemicals; they affect the size, shape, and yield of AgNPs. A variety of plants with their active components are reported to use for synthesis of AgNPs (Table 2.1). Negatively charge heparin is also used to synthesize AgNPs from silver nitrate by heating at 70°C for about 8 h (Huang and Yang, 2004). 2.3.3.5 Synthesis of Silver Nanohybrids Silver nanohybrids are the combination of AgNPs with other zero-, one-, or twodimensional nanomaterial(s), which are synthesized either simultaneously or as one in the presence of other nanomaterials. Generally, these nanohybrids enjoy the advantages of the components in a synergistic manner. AgNPs provide the antimicrobial and optical properties along with a high surface area and surface energy, while some other special characteristics are obtained from the rest nanomaterial(s). Thus they find applications in the field of active thin films, coatings and paints, sensors, multispectral filters, water treatment, catalytic materials, etc. A nanohybrid of Fe3O4/AgNPs showed

Table 2.1 List of plant extracts along with their active components used for the preparation of AgNPs with different sizes and shapes Plant and its part Main component Size (nm)/shape

Acalypha indica, leaf

Acorus calamus, rhizome Aloe vera, leaf Azadirachta indica, leaf Carica papaya, fruit Datura metel, leaf Eclipta prostrate, leaf

Jatropha curcas, seed, latex Kedrostis feotidissima, plant Lemon, peels Memecylon edule, leaf Morinda tinctoria and Michelia champaca, leaf Parthenium hysterophorus, leaf Psoralea corylifolia, seed Rhizophora mucronata, leaf bud Rosa indica, petals Syzygium cumini, leaf and seed Tanacetum vulgare, fruit Tea leaf

Tribulus terrestris, fruit Vitis vinifera, fruit

Kaempferol glycosides, Tannins, ß-sitosterol, acalyphamide, aurantiamide, succinimide, flindersin, etc. alkaloids, flavonoids, gums, lectins, mucilage, phenols, quinone, saponins, sugars, tannins, triterpenes, etc. Anthraquinones, catechin, sinapic acid, quercitrin, etc. Nimbandiol, nimbolide, ascorbic acid, n-hexacosanol, amino acid, nimbiol, etc. Ferulic acid, caffeic acid, rutin, etc. Flavonoids, phenols, tannins, saponins, aminoacids, sterols, etc. Daucosterol, stigmasterol-3-O-glucoside, triterpenoid, glucoside, etc. Flavonoids, protein, etc. Alkaloids, flavonoids, phenols, triterpenoids, steroids, tannins, cardiac glycosides, and saponins Phenolic acids, flavonoids, etc. Triterpenes, tannins, flavonoids, etc. Palmitic acid, fucoxanthin, broussin, isoliquiritigenin, cynarin, oleuropein, etc. Parthenin, caffeic acid, vanillic acid, anisic acid, panisic acid, chlorogenic acid, parahydroxy benzoic acid Coumarins, flavonoids, meroterpenes, etc.

20–30/spherical

30–40/spherical

20–50/spherical 50–100/spherical 25–50/cubical 16–40/quasilinear 35–60/triangles, pentagons, hexagon 15–50/spherical 20–25/irregular

15–60/spherical 20–50/square 10–50/cubic and hexagonal 40–50/irregular

50–200/nanorods

Saponins, flavonoids, alkaloids, tannins, triterpenoid, etc. Acyclic monoterpene alcohols, geraniol, citronellol, nerol, etc. Glycosides, flavaloids, tannins, alkaloids, etc.

5–25/face center cube (FCC) 25–60/spherical

Transthujone, camphor, transchrysanthenyl acetate, etc. Catechin, epicatechin, epicatechin gallate, gallocatechin, epigallocatechin, epigallocatechin gallate, etc. Saponins, flavonoids, glycosides, alkaloids, tannins, etc. Vitamin C, tannins, phenolic acids, anthocyanins, resveratrol, etc.

16/triangular

29–90/spherical

20–90/spherical

115–30/spherical 18–20/spherical

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superparamagnetic and antibacterial activity against E. coli, Staphylococcus epidermis, and B. subtilis with recyclability character (Lee et al., 2005). Similarly, AgNPs with TiO2 and CNTs nanohybrids are also synthesized to enhance their photocatalytic activity (Ziemkowska et al., 2014; Kim and Song, 2014).

2.4 PREPARATIVE PROTOCOLS FOR NANOCOMPOSITES Silver-polymer nanocomposites have been fabricated by two different approaches using either preformed AgNPs or by introducing silver precursors into polymer matrix followed by AgNPs preparation. Similarly, polymer is also prepared in the presence of either preformed AgNPs or silver precursor. In the later process, AgNPs are formed by the reduction of polymer itself, by the solvent, or by additional reducing agent. All the conventional techniques, as discussed in Chapter 1, are employed for the fabrication of such nanocomposites using AgNPs as the nanomaterial, so these are not discussed again. AgNPs-polymer nanocomposites are fabricated as silver impregnated nanofibers from the polymer-AgNPs homogeneous nanoscale dispersion in a polar solvent or solvent mixture (e.g., methanol, ethanol, acetonitrile, etc.) by electrospinning technique. It can also be performed by the electrospinning of a homogeneous solution of silver ions anchored with the polymer chains after the reduction of silver ions to AgNPs using solvent molecules or polymer molecules as the reducing agent in presence of heat or irradiation. Nylon 6/silver nanofibers are obtained by a similar approach using plasma treatment for reduction, while chemical reduction is used for polypropylene/silver nanofibers (Zhang et al., 2016). Similarly, nylon 6/silver nanofibers with nonwoven, aligned, and crossed patterns are reported by the coaxial electrospinning process (Zhang et al., 2009). The author’s group also fabricated silver/ polyurethane nanocomposites by using the in situ technique, where AgNPs are formed during polymerization process itself (Karak et al., 2010; Deka et al., 2010).

2.5 CHARACTERIZATION 2.5.1 Physical and Chemical Aspects The physical and chemical structures of AgNPs are characterized by various instrumental analyses, which include spectroscopic like ultraviolet-visible spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), etc. diffraction like X-ray diffraction (XRD), microscopic like scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), etc. and analytical including zeta-sizer, dynamic light scattering (DLS), inductively coupled plasma (ICP), energy dispersive X-rays spectroscopy (EDX), atomic absorption spectroscopy (AAS), Brunauer-Emmett-Teller (BET), etc. analyses as discussed in Chapter 1. They have characterized particle size and its distribution, shape, crystallinity, fractal dimensions, structure of functionalized agent, pores size, and surface area.

Silver Nanomaterials and Their Polymer Nanocomposites

2.5.1.1 UV-Vis Spectroscopy The electronic structures of AgNPs by exciting electrons from ground to excited states (absorption) and relaxing from the excited to ground states (emission) are used for determination in UV-vis spectroscopy. Depending on the size, distribution, and surface chemistry of AgNPs, they exhibited different characteristics absorbance wave length of light in UV-vis region (200–800 nm). The absorption of such electromagnetic radiation by these nanoparticles creates from the coherent oscillation of the valence band surface electrons prompted by the interaction with the electromagnetic field. Thus the resonance created is known as surface plasmon resonance (SPR), which is characteristic only for AgNPs, though it is not observed for bulk silver particles or ions. The position and nature of plasmonic peaks in the spectrum are influenced by particle size and distribution (Gmoshinski et al., 2013). The absorption in the wavelength ranging from 400 to 450 nm is generally used to characterize AgNPs. 2.5.1.2 Fourier Transform Infrared Spectroscopy FTIR spectroscopy deals with the vibration of chemical bonds present in the capping or stabilizing agent, functionalized molecule or oxide of AgNPs and their nanocomposites at various frequencies, in the range of 400–4000 cm1, as mentioned in Chapter 1. Thus this spectroscopic analysis provides information on the nature of molecules responsible for the reduction of silver ions, the nature of stabilizing agent, the functionalization or oxidation of AgNPs, the interactions present between AgNPs, and the polymer matrix in nanocomposites, etc. 2.5.1.3 Raman Spectroscopy Like FTIR spectroscopy, raman spectroscopy also detects the presence of various functional groups in the stabilizing agent, the oxide formation of AgNPs, and the local environmental chemical functionality in the nanocomposites, etc., as stated in Chapter 1. 2.5.1.4 X-Ray Photoelectron Spectroscopy The surface chemical composition and the states of AgNPs and their nanocomposites are estimated by XPS or electron spectroscopy for chemical analysis (ESCA), as mentioned in Chapter 1. It is also used to examine the valence of AgNPs. 2.5.1.5 X-Ray Diffraction XRD can be used for the identification of AgNPs by a fingerprinting approach using JCPDS data library along with the crystal structure, degree of crystallinity, orientation of crystals, defects, etc. of them and their nanocomposites, as mentioned in Chapter 1.

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2.5.1.6 Scanning Electron Microscopy Scanning electron microscopy (SEM) is used to obtain the morphology of such nanostuctural materials. The size and polydispersity, shape, purity, surface morphology, and distribution of AgNPs are characterized by SEM analysis. 2.5.1.7 Transmission Electron Microscopy The transmission electron microscope (TEM) is high resolution imaging of thin films of nanostructural AgNPs and their polymer nanocomposites. Fig. 2.3 represents TEM images of AgNPs in both bare and polymer nanocomposites. It helps to estimate the size distribution and shape of AgNPs in the bare state as well as in the polymer nanocomposites like others, as discussed in Chapter 1. 2.5.1.8 Atomic Force Microscopy AFM is used to estimate the size and its distribution, shape, and surface area of AgNPs, as mentioned Chapter 1. Aloe vera aqueous extract-mediated AgNPs showed spherical shape particles with size of 6 nm by an AFM analysis (Fig. 2.4A).

Fig. 2.3 TEM images of AgNPs in polyurethane matrix at different wt% (A)–(E), and (F) prepared by aqueous extract of Thuja occidentalis leaf and stabilized with poly(ethylene glycol). (Reproduced with permission from Karak, N., Konwarh, R., Voit, B., 2010. Catalytically active vegetable-oil based thermoplastic hyperbranched polyurethane/silver nanocomposites. Macromol. Mater. Eng. 295, 159–169 and Barua, S., Konwarh, R., Bhattacharya, S.S., Das, P., Devi, K.S.P., Maiti, T.K., Mandal, M., Karak, N., 2013. Non-hazardous anticancerous and antibacterial colloidal ’green’ silver nanoparticles. Colloids Surf. B: Biointerfaces 105, 37–42, [email protected] and @2013, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim and Elsevier, respectively.)

Silver Nanomaterials and Their Polymer Nanocomposites

Fig. 2.4 (A) AFM image with roughness and (B) DLS analysis for size distribution of AgNPs. (Reproduced with permission from Das, V.K., Harsha, S.N., Karak, N., 2016. Highly efficient and active silver nanoparticles catalyzed conversion of aldehydes into nitriles: a greener, convenient and versatile "NOSE" approach. Tetrahedron Lett. 57, 549–553, [email protected], Elsevier.)

2.5.1.9 Zeta Sizer Zeta sizer measured the surface charge of AgNPs as mentioned in Chapter 1. The high negative zeta potential of 65.7 mV for AgNPs, obtained by using the aqueous leaf extract of C. aromaticus, indicate their good stability with an average size of 48  5 nm (Kotakadi et al., 2015). However, the zeta potential value depends on the ionic strength, temperature of testing, and pH of the medium, where AgNPs are dispersed, etc. 2.5.1.10 Dynamic Light Scattering It is used for estimating the hydrodynamic diameter of AgNPs in solution or a colloidal suspension’s hydrated particle size. A typical DLS analysis of AgNPs prepared by aloe vera aqueous extract resulted a zeta average size of 159.5 nm with a polydispersity index (PDI) of 0.291 (Fig. 2.4B).

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2.5.1.11 Inductively Coupled Plasma The concentration of AgNPs present in a particular system can be estimated by ICP. The bulk composition of AgNPs is estimated by using ICP-optical emission spectrometry and ICP-mass spectrometry. ICP also provides the total amount of silver content in the stabilized AgNPs. The SEM and TEM instruments are also associated with an energy dispersive-emitted X-ray (EDX) spectroscopy and utilized to estimate the chemical composition of this nanostructural material.

2.5.2 Biological Assays Various techniques are used to assess different biological activities such as antimicrobial, cytotoxicity, etc. of AgNPs and their polymer nanocomposites. These include measurements of colony forming units (CFU) by plate counting, agar disc diffusion assay, imaging by SEM, imaging by confocal laser scanning microscope (CLSM), imaging by atomic force microscopy (AFM), optical density measurement, etc. (Campoccia et al., 2012). CFU plate counting is the basic method used for estimating the number of viable bacterial cells present in the system. CFU is estimated by culturing the bacterial suspension, followed by spreading it uniformly on the agar plate and then incubating it at a suitable temperature for a specified period of time; it is expressed as CFU per unit weight, volume, or area. In the agar diffusion method the zone of inhibition of the microbe is measured after a specified period of incubation in the presence of AgNPs or their nanocomposites. The minimum lethal concentration (MLC) is measured by calculating the minimum concentration of nanostructral material to achieve a 99.9% inhibition microbe colony after 24 h of incubation at 37°C for a specified microbe. On the other hand, minimum inhibitory concentration (MIC) is measured by determining the lowest concentration of AgNPs to inhibit two orders of magnitude more compared to the positive control after 24 h of incubation at 37°C for a specified microbe. The cytotoxicity assay is generally performed by studying the cell growth and proliferation of specified eukaryotic cells. The results are utilized to understand the organelle functions and cellular transportation activities. Various assay methods are used to estimate the cells viability in the incubating medium in presence of AgNPs and their nanocomposites. These are tetrazolium reduction methods include MTT, MTS, XTT, and WST-1 assays, resazurin reduction, protease markers, and flow cytometry (Mosmann, 1983). Among these MTT and MTS are most commonly used assays. These assays provide a direct measurement of cell viability by the impact of AgNPs, as they determine the intensity of formazan product formed upon the interaction of the dye with the viable cells. Here, it is pertinent to mention that it is very much essential to determine the MIC value at which AgNPs are toxic to microbial cells, but not to the higher animals. Table 2.2 summarizes the different techniques of biological assays of AgNPs with their pros and cons.

Silver Nanomaterials and Their Polymer Nanocomposites

Table 2.2 Pros and cons of various techniques for biological assays of AgNPs Technique Pros Cons

Optical density (OD) measurement for microbe count Cell counting by microplate reader Spread plate for colony count

Simple, UV-visible spectroscopy is sufficient

Unable to discriminate viable and nonviable cells, less accurate

Accurate and rapid determination of cell proliferation Accurate

Required special dye (tryptophan blue), good microscope

Crystal violet staining

Rapid analysis and quantifies biofilm formation

Live/dead vital fluorescent stain (Resazurin-Alamar Blue Assay) Protease viability assay

Highly sensitive, viable cells seen visually, both qualitative and quantitative assays are possible

MTT/MTS/XTT/ WST assays

Cellular viability can be measured Cell viability can be measured directly

Determine only viable CFU, cells removal require from the surface before measurement, sterile agar is required Required a spectrophotometer and not used for planktonic bacteria growth Required expensive materials, fluorescent plate reader, microscope etc. Required expensive materials, fluorescent plate reader Required vital dye, spectrophotometer, or plate reader

2.6 PROPERTIES Attractive properties of AgNPs both in bare state and in polymer nanocomposites demand special attention. In their nanodimensional state, they are not only transparent, but they also emit special light by plasma absorption. Thus they exhibit completely different electromagnetic or physicochemical properties from the bulk silver. Most of these properties are dependent on their size, shape, and aspect ratio.

2.6.1 Physical The melting point of AgNPs (embedded in a silica matrix varied between 350°C for 30 nm and 160°C for 8 nm diameter) is lower than the bulk silver metal (961.8°C) as the number of atoms located at the surface is significantly higher in the former rather than the later (Yeshchenko et al., 2010). The lattice parameter of AgNPs is also decreased due to the high surface tension of the particles that exerts a compressive force on the particles. However, the density and durameter hardness of AgNPs containing polymer nanocomposites showed only a marginal increase in the density and hardness from the pristine polymers (Karak et al., 2010). On the other hand, the mechanical properties like tensile strength, tensile modulus, and elongation at break values of the polymer nanocomposites are improved, though the increments are not so significant (Table 2.3).

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Table 2.3 Physical properties of biobased polyurethane/silver nanocomposites Density Tensile (g cc21) at strength Modulus Elongation at Amount of (Mpa) (MPa) break (%) 25°C AgNPs (wt%)

Hardness (Shore D)

0.0 1.0 2.5 5.0 7.5

32–36 33–37 37–38 38–40 40–41

6–8 8–10 9–11 11–14 8–9

80–100 90–112 90–115 95–120 80–90

400–430 450–500 460–520 550–600 500–520

1.05–1.15 1.10–1.16 1.12–1.18 1.18–1.20 1.20–1.22

2.6.2 Optical Among all the properties the optical properties are very interesting and useful for AgNPs and their nanocomposites. They exhibit a unique SPR due to the interaction of applied electromagnetic radiation and the electrons in the conduction band around the nanomaterial, as stated in Chapter 1. The size, distribution, and concentration of AgNPs strongly influence this SPR spectrum, and the color phenomena with impressive strength, saturation, and transparency are observed for AgNPs. This transparency is due to the fact that particle sizes are smaller than the wavelength of visible light, hence the light scattering by the particles becomes negligible (Karak et al., 2010).

2.6.3 Catalytic AgNPs-based catalysts are of great interest in organic synthesis because of their unique reactivity, stability, selectivity, and recyclability in such reactions, as well as facile and green synthetic protocol for mass production of such catalysts (Yan et al., 2010). They are highly efficient photocatalysts under ambient conditions with visible light illumination to degrade organic contaminants including dyes. Research shows that chitosansupported AgNPs can be used as an efficient catalyst in selective CdC coupling reactions of phenolic compounds in the presence of molecular iodine (Murugadoss et al., 2009). Sunflower oil-based polyurethane/silver nanocomposites can catalyze the reduction of 4-nitrophenol to 4-aminophenol in presence of NaBH4 (Karak et al., 2010). The polymer/carbon black supported bimetallic Rh/AgNPs-based catalyst is used for asymmetric conjugate addition of arylboronic acids to enones (Yasukawa et al., 2012). The hiral ligands strongly affect their catalytic activity and enantioselctivity of the catalyst. The coupling of aldehydes, alkynes, and amines (A3-coupling) resulted in many useful products. The mesoporous SBA-15 (silica) supported AgNPs with average size 8 nm is utilized for A3-coupling reaction with high yields (Safaei-Ghomi and Ghasemzadeh, 2013). Similarly, surface-modified ZnO immobilized AgNPs catalyzes A3-coupling to produce propargylamines through a one-pot, three-components reaction (Movahedi et al., 2014).

Silver Nanomaterials and Their Polymer Nanocomposites

2.6.4 Electrical The size, shape, composition, crystallinity, and structure dependent electrical conductivity is found for AgNPs. Thus they behave as conductive materials in electronically conductive adhesives in electronic device (Pothukuchi et al., 2004). In fact, AgNPs improve the current density and contact resistance of the electronically conductive adhesives. The Kubo gap of spherical AgNPs with size 3 nm is around 5–10 meV, so at a slight elevated temperature, the electrical conductivity is the same as in bulk silver metal (Rao et al., 2000).

2.6.5 Biological AgNPs are useful therapeutics due to their antimicrobial properties. Compared to bulk silver, AgNPs exhibit more efficient antibacterial and antifungal properties due to their large surface area, as well as better and easier interactions with biomolecules including bacteria and fungi. Chemically functionalized AgNPs are conjugated with different active agents like antibodies, ligands, and drugs of interest; hence they could be used as potential carriers in targeted drug delivery and as vehicles for gene and diagnostic imaging. AgNPs are now replacing silver sulfadiazine drug as an effective agent in wound treatment. They have a broad spectrum of antimicrobial activity against Gram-positive bacteria, Gramnegative bacteria, fungi, and certain viruses. However, even now the mechanism of their antimicrobial activity is in mist, though the formation of reactive oxygen species (ROS) is supposed to be the cause. The slow oxidation and liberation of silver ions as biocide and the damaging of bio-chemical processes by the penetration of bacterial cell membranes are also explained as the reasons for this activity. Researchers have also suggested that upon exposure of AgNPs, the particles enter into the bacterial cell by the process of diffusion as well as led to endocytosis via the cell wall into the cytoplasm, thereby causing toxicity to the bacterial cell including damaging of mitochondria and thus ATP is depleted, which produce ROS (Xiu et al., 2012). Acorus calamus mediated AgNPs showed antioxidant, antibacterial, and anticancer properties (Nakkala et al., 2014a). While Boerhaavia diffusa plant extract-based AgNPs exhibited antibacterial activity against Pseudomonas fluorescens, Aeromonas hydrophila, and Flavobacterium branchiophilum, excellent activity is observed towards the last one (Nakkala et al., 2014b). The dried fruit body extract of T. terrestris L. plant-mediated AgNPs showed an antibacterial property against multidrug resistant bacteria such as Pseudomonas aeruginosa, Streptococcus pyogens, E. coli, B. subtilis, and Staphylococcus aureus (Gopinatha et al., 2012). Further, different crystal facets of AgNPs exhibited a different extent of antimicrobial activity (Pal et al., 2007). It is pertinent to mention that the crystal facet (111) plane of AgNPs is likely to adhere more to the cell membrane, which may be due to higher atom density and reactivity compared to other facets. Thus among spheres, rods, and truncated triangle AgNPs the triangle particles showed the highest antimicrobial activity. Furthermore the net surface charge of stabilized AgNPs controlled by the surface charge of nanoparticles and stabilizing agents

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also strongly influence the antimicrobial performance. Thus the stabilizing or capping agent also indirectly influences the antimicrobial effect of AgNPs (El Badawy et al., 2011). The antibacterial properties of AgNPs against Salmonella, Shigella, and Proteus are also reported by Lukman et al. (2011). Fungi biomass-based AgNPs with a 20–40 nm size exhibited an antiviral effect, which depends on the size of the nanoparticles (Gaikwad et al., 2013). Again, Sun et al. (2005) reported the antiviral activity of human serum albumin stabilized AgNPs (5–20 nm) obtained in a HEPES buffer solution; they can inhibit the HIV-1 replication process. Poly(vinyl pyrrolidone)-coated protein-conjugated AgNPs can control the infection caused in HEp-2 cell by syncytial virus (Sun et al., 2008). Similarly, report said that AgNPs are very effective in inhibiting the normal activity of H1N1 influenza A virus (Xiang et al., 2013). More interestingly, AgNPs-loaded titania nanotubes showed the killing ability to all planktonic bacteria present in a suspension, and thus such nanohybrid might be used in orthopedics, dentistry, and other biomedical devices (Zhao et al., 2011). TitaniumAgNP nanohybrids showed microgalvanic effects which result not only in controlled antibacterial activity against S. aureus and E. coli, but also excellent compatibility with low surface toxicity and enhanced proliferation of osteoblast cells (Cao et al., 2011). Similar effects are also found for the silver-palladium nanohybrid, which provides a biofilm-inhibiting property to kill bacteria by generating microelectric fields along with electrochemical redox processes (Chiang et al., 2009). Further, iron oxide AgNPs-based nanohybrids exhibit very effective antibacterial and antifungal activities (Prucek et al., 2011). AgNPs exhibited good antifungal activity against Trichophyton mentagrophytes and Candida albanicans fungi by causing abnormalities in their cell wall (Kim et al., 2008). The dose-dependent antifungal activity is also reported for AgNPs against C. albicans and C. glabrata and Trichophyton rubrum (Monteiro et al., 2013). A study showed that AgNPs even can inhibit the growth of E. Coli and S. aureus bacteria more than gold nanoparticles (Amin et al., 2009). Results showed that about 20–60 ppm of AgNPs are capable to inhibit from 104 to 105 CFU of E. coli (Sondi and Salopek-Sondi, 2004). Further, antibacterial activity against a broad spectrum of microorganisms, namely S. aureus, E. faecalis, E. coli, P. aeruginosa, methicillin susceptible Staphylococcus epidermidis, methicillin-resistant S. epidermidis, methicillin-resistant S. aureus, vancomycin-resistant E. faecium and K. pneumoniae, clearly demonstrated that SDS-stabilized AgNPs superior among SDS, Tween 80 [poly(oxyethylenesorbitane monooleate)], and poly(vinyl pyrrolidone) (PVP) stabilized the same nanoparticles (Kvı´tek et al., 2008). The magnetic [email protected] nanohybrid (40–80 nm size) is recyclable and able to inhibit the growth of gram-negative E. coli, gram-positive S. epidermidis, and B. subtilis spore bacterial starins (Gong et al., 2007). Again, chitosan/silver nanocomposite is used for the modification of cotton fabric surface to provide antibacterial property. The result showed that the zone of inhibition

Silver Nanomaterials and Their Polymer Nanocomposites

diameter against E. coli bacteria of such fabric is 30 mm (Thomas et al., 2011). It is also observed that dodecylamine-reduced AgNPs-based chitosan/silver nanocomposite exhibited 20, 20, 27, and 20.2 mm zones of inhibition for bacteria strains like E. coli and S. aureus along with fungal species such as A. terres and A. flavus, respectively, as determined by the disc diffusion method. Similarly, sodium alginate-stabilized AgNPs with particle sizes of 5–21 nm containing chitosan nanocomposite film showed strong antibacterial activity with the zone of inhibitions of 1.9, 3.1, 1.5, 6.0, and 5.1 mm for E. coli, P. aeruginosa, E. aerogenes, B. cereus, and E. faecalis, respectively (Sharma et al., 2012). Carboxymethyl chitosan/silver/polyethylene oxide nanocomposites with 10–12 nm AgNPs, where chitosan act both as a reducing and stabilizing agent for AgNPs, showed very effective antimicrobial activity with a zone inhibition of 20, 15, 18, and 12 against S. aureus, E. coli, P. aeruginosa, and Candida albicans, respectively (Fouda et al., 2013). In the same way a number of reports have been published on the antimicrobial activity of chitosan/silver nanocomposites, which include the biocompatible scaffold of chitosan/ hydroxyapatite/silver, chitosan/starch/silver, chitosan/bioglass/silver, chitosan/montmorillonite/silver, chitosan/4-(ethoxycarbonyl) phenyl-1-amino-oxobutanoic acid/silver, genipin-crosslinked chitosan/poly(ethylene glycol)/zinc oxide/silver, etc. (Thomas et al., 2011). The electrospun nanofibers of poly(vinyl alcohol)/chitosan/AgNPs is effectively used as wound dressing material (Li et al., 2013b). It is interesting to notice that in such nanocomposites, where AgNPs are synthesized via a chemical reductive method, exhibited higher antimicrobial activity against Gram-negative bacterial strains, whereas thermal reduction showed more active against Gram-positive bacteria. Further, electrochemical or UV rays reduced silver based such nanocomposites showed good antimicrobial potency against both Gram-positive and Gram-negative bacteria strains. Further, nylon 6/silver nanofibers exhibited a strong antibacterial activity of 99.9% inhibition to E. coli bacterial strain at dose level of 0.5 wt% but almost 100% inhibition at loading of 1.25 wt%, although the pristine polymer does not show any such activity. In addition the copolymer of ethylene-vinyl alcohol containing Ag-TiO2 nanohybrid film has proven to act as antimicrobial material towards Gram-negative, Gram-positive bacteria/cocci and yeasts, as well as displaying resistance to biofilm formation (Kubacka et al., 2009). High-performing biocompatible silver nanocomposites with antimicrobial surfaces with multifaceted properties are utilized as antimicrobial implants, scaffolds, implant coatings, etc. Chitosan is found to be a prime matrix for the dispersion and stabilization of AgNPs, and the chitosan-silver nanocomposite exhibited very effective antimicrobial activity (Latif et al., 2015). Natural rubber matrix is also used for obtaining its silver nanocomposite, where photochemical reduction latex with silver ion-based film contains 4–10 nm size nanoparticles in the matrix. The incorporation of AgNPs in polyurethane foam, ceramic composite, filter paper, etc. resulted in antibacterial activity ( Jain and Pradeep, 2005; Lv et al., 2009; Dankovich and Gray, 2011). On the other hand, viral inactivation is reported by the incorporation of biogenic AgNPs in a poly(vinylidene

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Table 2.4 Plant parts with antimicrobial activity for AgNPs Antimicrobial Plant part type Microorganism

Mentha leaf

Antibacterial

Escherichia coli, Staphylococcus aureus

Aleo vera leaf

Antibacterial

Escherichia coli, Staphylococcus aureus

Alternanthera Dentate leaf

Antibacterial

Acorus calamus Rhizome Boerhaavia diffusa plant

Antibacterial

Tribulus terrestris L. fruit

Antibacterial

Cocous nucifera plant

Antimicrobial

Abutilon indicum Cymbopogan citratus stapf (lemongrass) Syzygium cumini leaf

Antimicrobial

Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumonia, and Enterococcus fecal pathogenic bacteria like Escherichia coli Pseudomonas fluorescens, Aeromonas hydrophila, and Flavobacterium branchiophilum Streptococcus pyogens, Pseudomonas aeruginosa, Bacillus subtilis, Escherichia coli, and Staphylococcus aureus Salmonella paratyphi, Klebsiella pneumoniae, Bacillus subtilis, and Pseudomonas aeruginosa S. typhi, E. coli, S. Aureus, and B. substilus P. aeruginosa, P. mirabilis, E. coli, Shigella flexaneri, S. Somenei, and Klebsiella pneumonia Staphylococcus aureus, Escherichia coli, Bacillus subtilis, Klebsiella pneumoniae, Pseudomonas aeruginosa, Mycobacterium smegmatis, Trichophyton rubrum, Aspergillussp, and Candida albicans

Antibacterial

Antibacterial

Antimicrobial

Reference

Banerjee et al. (2017) Chandran et al. (2006) Kumar et al. (2014)

Nakkala et al. (2014a) Nakkala et al. (2014b) Gopinatha et al. (2012)

Mariselvam et al. (2014) Kumar et al. (2015) Geetha et al. (2014)

Gupta et al. (2014)

fluoride) membrane (De Gusseme et al., 2011). Table 2.4 provides the antimicrobial activity of AgNPs obtained by different palnt extracts. 2.6.5.1 Basic Understanding of the Antimicrobial Effect of AgNPs The surface of AgNPs forms a layer of water molecules that helps to release silver or its ions. Further, phospholipid bilayers and protein molecules of the bacteria cell membrane make it negatively charged, and thereby the positive-charged silver ions can easily bind to

Silver Nanomaterials and Their Polymer Nanocomposites

bacteria cell membranes. This phenomenon helps to damage the structures of bacterial membranes. More interestingly, silver ions can also chemically bind to the sulfhydryl group (SH) of amino acid present in the bacterial respiratory enzymes, thereby making the enzymes inactive so that bacteria ultimately dies (Zhang et al., 2013). However, the antibacterial effect of AgNPs is not only the release of silver ions, as no toxicity is observed for such nanomaterials in the presence of humic substances (Fabrega et al., 2009). Thus the toxicity of AgNP is also due to some other factors. Again the antimicrobial activity of AgNPs depends strongly on type of microorganisms, pH, temperature, and nanoparticles concentration (Marambio-Jones and Hoek, 2010). Several researchers reported a higher antimicrobial activity of AgNPs against Gram-negative bacteria compared to gram positive bacteria. This may be due to their cell wall structural differences with respect to the peptidoglycan layer thickness and beta barrel proteins (porins) (Geoprincy et al., 2013). Simply the cell wall of gram-positive bacteria is thicker than that of gram-negative bacteria, hence the result. AgNPs are also active against several types of viruses such as hepatitis B virus and herpes simplex virus (Galdiero et al., 2011). 2.6.5.2 Mechanism on Antimicrobial Activity The bactericidal effect of silver ions is a well-known fact. This is supposed to be mainly due to their strong affinity to bind with sulfur containing amino acids. Much research is devoted to the study of antimicrobial activity, especially the antibacterial effect of AgNPs, but still now the exact mechanism of their activity is not clearly understood. Various mechanisms put forward for the interaction of AgNPs with bacteria cells. These include the accumulation of nanoparticles in the cell, cell membrane disruption by AgNPs through the generation of reactive radicals, the anchoring of AgNPs with cell membranes, the dephosporylation of tyrosine by silver ions released from AgNPs, the interactions or binding of nanoparticles with sulphur- and phosphorous-containing DNA of microbe (soft acid-soft base interaction), and the interaction of silver with respiratory enzyme, etc. (Prabhu and Poulose, 2012). Generally, AgNPs adhere and anchor to the cell membrane at first and then penetrate into the cell wall, causing structural changes of the membrane. They can easily diffuse into the microbial cell membrane due to their fine size. Thus the cell organelles are exposed to the extracellular environment and hence normal functions of cell are disrupted. In different opinions, ion channels present in the cell allow for the diffusion of oxidized products of AgNPs and promote to interact with enzyme, which disrupts normal cell functions and hence result in the lysis of the cell. They generally interacted with thiol groups of DNA, respiratory chain enzymes, nucleic acids, and cytoplasmic components (Kim et al., 2011). The reactive oxygen species also influences respiratory enzyme functions, disrupting the functions of cell and thereby damaging the cell (Matsumura et al., 2003). A systematic study on antibacterial activities of AgNPs clearly showed that three independent mechanisms, namely the release of silver ions, the generation of ROS, and direct contact of them are mainly responsible for such

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activities (Sintubin et al., 2011). It is also reported that AgNPs facilitate the proliferation and migration of keratinocytes, reduce the formation of collagen by fibroblasts, and modulate the number of cytokines produced in burn patients (Wong and Liu, 2010). Again, there are some basic differences between bacteria and viruses. The size of a virus is much smaller than bacteria. A virus lacks cellular structure, whereas a protein coating or lipid envelope surrounding the coating is present instead of lipopolysaccharides, peptidoglycan, protein, etc. as in bacteria, though viruses do not have metabolism unlike bacteria. Thus the mechanism antiviral may be different from antibacterial mechanism. However, this is not studied systematically in any literature. A study showed a noticeable damage of the capsid as well as of the gene of H3N2 influenza virus and adenovirus by the interaction with AgNPs (Xiang et al., 2013; Chen et al., 2013). TEM analysis further proved a breached and broken morphology of the viruses. Further, study also confirmed the anti-HIV effect of AgNPs at an early stage of viral entry and blocking HIV-1 proteins at postentry stages of the virus life cycle (Lara et al., 2010). 2.6.5.3 Factors Affecting Antimicrobial Activity The antibacterial action of AgNPs and nanostructured silver-containing materials is found to be enhanced by the application of an electric field, which may be due to the release of more silver into the culture medium (Bekkeri, 2014). Thus upon employing an electric current across silver electrodes enhances antibiotic action at the anode. Beside this, different metal ions have positive (Ca2+, Mg2+, and Cl1) impacts on the antimicrobial effects of AgNPs (Gupta et al., 1998). Furthermore the pH of the medium also has strong influence on such activity of AgNPs (e.g., 90% inhibition at pH 9 is observed) while only 10% inhibition results at pH 6 on exposure of 2 ppm AgNPs (Fabrega et al., 2009). As mentioned earlier the capping agent has a strong influence on the antimicrobial effect of AgNPs (Table 2.5).

2.7 TOXICITY AND SAFETY ISSUES As AgNPs are the most widely studied and explored nanomaterials, the toxicity and safety issues of them must be explored prior to their commercial applications. Their toxicity is not only associated with human beings or higher animals, but they are also intended to protect ecosystems at large. The study showed that biogenic AgNPs exhibited lesser toxicity compared to chemically synthesized AgNPs. The toxicity of AgNPs is supposed to be relatively low, particularly to higher animal including humans, but generally toxicity is observed at relatively high concentrations (e.g., acute oral LD50 for tested rats is >1600 mg kg1 day1) (Wijnhoven et al., 2009). Furthermore, it is pertinent to mention that under real-life environmental exposure conditions, AgNPs is converted to silver sulfide; this has not been able to provide a measurable impact on wastewater treatment plants and was found to be much less toxic than ionic silver (Choi et al., 2009). Again, AgNPs

Silver Nanomaterials and Their Polymer Nanocomposites

Table 2.5 Antimicrobial activity of AgNPs with different capping agents Concentration of Microorganism Capping agent AgNPs (mg L21)

Reference

0.01–1 12.5 1–500 1–500 10–100 10–100 10–100 0.002–1

Escherichia coli Escherichia coli Pseudomonas Aeruginosa Staphylococcus aureus Escherichia coli Bacillus subtilis Staphylococcus epidermidis Bacillus spp.

Pal et al. (2007) Sintubin et al. (2011)

50 50

Adenovirus 3 H3N2 influenza virus

Fungi biomass

10

Biomass

5.4

Herpes simplex virus type 1 and 2, Human parainfluenza virus type 3 UZ1 bacteriophage

Chen et al. (2013) and Xiang et al. (2013) Gaikwad et al. (2013)

Citrate/CTAB Biomass

Triton X-100

Citrate/poly(vinyl pyrrolidone)/ branched polyethyleneimine Tannic acid

Gong et al. (2007) El Badawy et al. (2011)

De Gusseme et al. (2009)

exhibited different toxicity than dissolved silver. It is observed that AgNPs of smaller size are more cytotoxic, which may be due to an easier uptake and dissolution, along with increase of surface area (Powers et al., 2011). In addition, considering the long period uses of silver-based products, where some of the particle sizes are found to nanoscale, the assumption of high toxicity should not be speculated before actual testing. The cytotoxicity test of AgNPs on Sprague-Dawley rats for 4 weeks did not exhibit any abnormal behavior or change in weight; the biochemical and haemotological parameters by inhalation of such nanoparticles is also reported ( Ji et al., 2008). However, slight pulmonary cytotoxicity or inflammation in mice is found by other researchers after the exposure of AgNPs for a prolonged period of time (Stebounova et al., 2011). The cytotoxicity effect of AgNPs is found to mainly depend on their concentration, size, and surface chemistry. In vitro cytotoxicity studies of osteoblast and osteoclast bone cells and L929 fibroblast cells clearly showed that smaller size AgNPs are more cytotoxic than the larger size, though all nanoparticles having greater cytotoxicity than bulk (micron size) silver (Albers et al., 2013; Liu et al., 2010). The cubical shape AgNPs of size ca. 6 nm showed strong antibacterial activity against S. aureus and P. aeruginosa at 4 μM concentration (0.4 ppm), but exhibited cytotoxicity towards UMR-106 cell at

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>50 μM (5.40 ppm) concentration (Flores et al., 2013). Furthermore the study suggested that the minimum inhibitory concentration (MIC) of AgNPs against S. epidermidis bacteria is much higher (2–4 times) than the concentration required to cause cytotoxicity; that is, the decrease in the viability and proliferation of primary osteoblasts and osteoclasts cells (Albers et al., 2013). Properly surface functionalized AgNPs containing an absorbable suture not only showed an improvement in anastomosis healing, but they can also cause significantly less inflammation (Zhang et al., 2014). Thus the type and concentration of AgNPs are not only the factors for cytotoxicity, but other factors like surface functionalization, nature of stabilizing or functionalizing agent, concentration of capping or functionalizing agent are also important for the same. Hence it is suggested that AgNPs should be functionalized or stabilized by biocompatible agents like citrate, chitosan, polysaccharides, carbon nanodots, starch, peptide, bovine serum albumin, etc. (Kim et al., 2007; Zewde et al., 2016). AgNPs exhibited antiviral activity against HIV-1 at noncytotoxic concentrations, though the mechanism of such activity is not clear (Lara et al., 2010). Again, AgNPs are also a very effective and fast-acting fungicide against a broad spectrum of common fungi including Aspergillus, Candida, and Saccharomyces (Yu et al., 2005).

2.8 APPLICATIONS The unique characteristics of AgNPs make them and their nanocomposites very useful in the fields of medicine, catalysis, textile, biotechnology, electronics, optics, water treatment, etc. Significant inhibitory effects against different microbial pathogens promote them as effective antimicrobial agents in various consumer products such as cosmetics, air sanitizer sprays, respirators, socks, slippers, pillows, wet wipes, toothpastes, detergents, shampoos, soaps, air and water filters, vacuum cleaners, coatings of refrigerators, washing machines, wound dressings, bone cement, surgical dressings, cell phones, and food storage packaging. AgNPs are claimed to contain in about 30% of products, commercialized with nanomaterials. Further the use of heterogeneous AgNPs-based catalysts is proven to be an effective and useful strategy in terms of efficiency and selectivity for different organic transformations in addition to their antimicrobial activity. Again the cost of such catalysts are much less compare to analogous gold or platinum (1/50) or even to palladium (1/25) catalysts. AgNPs are also used in the field of transparent electrodes for flexible devices, transparent conducting films, flexible thin film tandem solar cells, etc. in addition to utilizing their antimicrobial properties (Zeng et al., 2010).

2.8.1 Biomedical AgNPs are an important product in the field of bionanotechnology due to their boundless interests in the biomedical field. The infections caused by different microorganisms such

Silver Nanomaterials and Their Polymer Nanocomposites

as viruses, bacteria, rickettsiae, fungi, and parasites are the daunting challenge to the public health and pharmaceutical industry for their prevention and treatment. This is due to the fact that such microbial infections and contamination can be devastating to one’s health. There is a >15% mortality rates observed worldwide due to S. aureus-based infections only, and postsurgical or hospital-related infections due to the microbial contamination of medical implants and devices mostly result in fatalities. Again, existing conventional antibiotics are no longer able to inhibit bacterial growth due to the adoption of antibiotic resistance by such microbes. In this milieu, AgNPs exhibited broad inhibitory activity towards 650 species of microbes, and more significantly against antibiotic-resistant bacterial strains (Rai et al., 2012; Marambio-Jones and Hoek, 2010). On the other hand, silver is considered as a therapeutic agent for many diseases in the ancient Indian medical system called Ayurveda. The medical activity of silver is known across the globe for over 2000 years (Prabhu and Poulose, 2012). 2.8.1.1 Antimicrobial Devices AgNPs are widely used in various biomedical devices such as surgical instruments, biosensing devices, vascular prostheses, orthopedics, ventricular drainage catheters, wound dressings, heart valves, etc. In addition, AgNPs are used in dental hygiene and eye treatments as well as for other infections (Cao et al., 2010). Topical ointments and creams containing AgNPs are already in use to avoid infecting the burns and to close the wounds (Song and Kim, 2009). The surface functionalized AgNPs are now used as nanocarriers for a controlled drug delivery system. Specially designed plant extract-mediated AgNPs are proven to be anticancerous. Thuja occidentalis leaf extract mediated AgNPs with size 10–15 nm exhibited anticancer properties against human breast cancer (MCF 7, MDA MB 231), cervical cancer (HeLa), and mouth epidermoid carcinoma (KB) cell lines (6.25–50 μg/mL) and antimicrobial activity against B. subtilis, S. aureus, Listeria monocytogenes, Salmonella typhimurium, and P. aeruginosa (5–10 μg mL1) (Barua et al., 2017). Mentha arvensis plant-mediated AgNPs are also showed promising anticancer activity against breast cancer. They are found to be less toxic and nonmutagenic and they mediate caspase 9-dependent apoptosis in MCF7 and MDA-MB-231 cells (Banerjee et al., 2017). Acticoat is reported as the first commercial wound dressing containing AgNPs, where poly(amide ester) membrane was used as the matrix (Trop et al., 2006). It promotes the healing process by reducing the frequency of burn wound sepsis and secondary bacteremia. Further, dressing Acticoat also provides the fastest and broadest-spectrum fungicidal activity without any tissue irritation, which is even better than well-known silver sulfadiazine (Wright et al., 1999). Further, AgNPs-based wound dressings (bandages) are available commercially for curing various infections such as burn wounds, chronic ulcers, Steven-Johnson syndrome, toxic epidermal necrolysis, pemphigus, etc. (Huy et al., 2013). To prevent inflammation after surgery by microbial infections an artificial heart valve is now designed from silicone-(Silzone) coated AgNPs with controlled

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endocarditis. For the same reason for preventing microbial infections, poly(methyl methacrylate)-based bone cement contains AgNPs and is used in hip and knee joint implantation (Alt et al., 2004). Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/AgNPs based nanofibers are used as scaffolds for bone and skin tissue regeneration (Xing et al., 2010). 2.8.1.2 Antimicrobial Textile Dress materials are the media for the growth of microorganisms like bacteria and fungi, especially in hospital environments. Thus there is every possibility for the transmission of diseases in such environments from patient to patient or even from patient to healthy person. Further the textile materials in the form of monofilament, multifilament, woven, or nonwoven structures are routinely used as sutures, bandages, scaffolds, wound dressing, masks, surgical gowns and hospital linens, etc. in the medical field where they need to be antimicrobial; AgNPs can provide that attribute. Compared to synthetic antimicrobial agents such as triclosan, metal and their salts, organometallics, phenols, quaternary ammonium compounds and organosilicons, etc. that are used commonly for such textiles, biogenic AgNPs are more effective and safer to use. AgNPs are incorporated into fibers during spinning itself or are employed to the surface of the drawn fibers as surface coating. However, the antimicrobial effect and durability of the functionalization textiles depend on the fabrication technique as the release profile of AgNPs is influenced by it (Geranio et al., 2009). AgNPs are commonly used in textiles for production antimicrobial T-shirts, underwear, socks, sports clothing, and so on (Benn and Westerhoff, 2008). However, AgNPs-impregnated fabrics exhibited promising antibacterial activity in the outer part in contrary to the core part (Samberg et al., 2011). 2.8.1.3 Antimicrobial Water Filter As already discussed, AgNPs are very effective against pathogens, preventing infections and exhibited effective antimicrobial activity, so they can be used against infectious diseases caused by the microbes present in water. The studies suggested that AgNPs containing core shell magnetic nanohybrid is an effective disinfectant in the water purification system (Chudasama et al., 2009). Bacteriostatic water filters are generally produced on activated carbon or ceramics with impregnated silver of nanoscale particle size (<100 nm). However, antimicrobial activity of AgNPs is found to be much lower in water matrices compared to the absence of that. As an example, 2 mg L1 of AgNPs can cause about 80% inhibition of P. fluorescens; while no inhibition is found under same conditions, when humic acids are present, it may be due to film formation over AgNPs or an antioxidant nature of the such matrices (Fabrega et al., 2009). Nowack et al. also reported AgNPs impregnated water filters for domestic water purification (Nowack et al., 2011).

Silver Nanomaterials and Their Polymer Nanocomposites

2.8.1.4 Antimicrobial Packaging AgNPs are used in food storage containers. The AgNPs containing packaging containers ensure the safety of the food, and they help to preserve food for longer period of times by killing the microorganisms (Das et al., 2008). More interestingly, AgNPs-based packaging films and coatings are able to adsorb and decompose ethylene molecules, the natural plant-ripening hormone, and thereby help to keep fruits and vegetables fresher for longer periods of time; that is, they act as shelf-life enhancers for such products (Silvestre et al., 2011). Although AgNPs showed significant effects in food preservation and packaging, the Food Safety Authority allows only limited forms of such nanomaterials (Cushen et al., 2012). 2.8.1.5 Antimicrobial Consumer Appliances AgNPs are used in a wide spectrum of consumer products including household water filters, clothing, respirators, antibacterial sprays, detergent, cosmetics, cutting boards, dietary supplements, socks, shoes, laptop keyboards, cell phones, children’s toys, etc. ( Jeeva et al., 2014).

2.8.2 Catalytic As mentioned earlier, AgNPs-based catalysts are widely used for many important organic transformations. TiO2-supported AgNPs-based catalysts are utilized for the direct coupling of terminal alkynes to aromatic aldehydes (Yu et al., 2012). Again, [email protected] nanohybrid containing poly(methylhydrosiloxane)-based semiinterpenetrating networks can be used as efficient catalysts for the selective 1,2-alkynylation of various trifluoromethyl ketones and α,β-unsaturated trifluoromethyl ketones in water to produce fluorinated alcohols with up to 98% yield (Wang et al., 2014). Again a poly(N-heterocyclic carbene)-supported AgNPs-based reusable heterogeneous catalyst is used for the carboxylation of terminal alkynes with CO2 (Sherbow et al., 2014). The γ-alumina-supported silver nanocluster-based catalyst can effectively oxidative cross-coupled secondary alcohols with primary alcohols to produce the desired products with good yield (Shimizu et al., 2009b). 2.8.2.1 Alkylation Alkylation is recognized as one of the most important organic reactions to produce many industrially vital products over the decades. A silica-supported, AgNPs-based reusable catalyst is efficiently used in Friedel Crafts alkylation of arenes, including less reactive aliphatic alcohols, styrene, and indene with good yields (Shimizu et al., 2010a). However, a mixture of ortho- and parasubstituted aromatics is produced in different ratios. N-alkylation of nitrogen-containing compounds is possible to perform by using a AgNPs-based catalyst. Thus γ-alumina-supported silver nanocluster (5 wt% of AgNPs) based catalyst provide N-alkylated products of anilines by alcohols in the presence of

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hexahydrated FeCl3 (Shimizu et al., 2009a). The organic ligand-free AgNPs-based catalyst like the Mo-Ag nanohybrid is also used successfully in the N-alkylation of different compounds like aromatic amines, sulfonamides, and carboxamides by simple alcohols with high yields (above 90%, Cui et al., 2011). 2.8.2.2 Reduction of Nitroaromatic Reduced nitoaromatics are used in various applications including analgesic and antipyretic drugs, photographic develops, corrosion inhibitors, etc. The most common reaction used for such catalytic reactions is the reduction of 4-nitrophenol to 4-aminophenol in the presence of NaBH4. The functionalized AgNPs with polystyrene beads, brush-gel hybrid film, silica nanotubes, porous silicon, etc. are used as effective catalysts in such reactions. Further, Brawny silver-hydrogel-based nanocatalyst, metalorganic framework-based AgNPs, semiinterpenetrating-network hydrogel-based AgNPs, bimetallic gold/silver core-shell nanoparticles or other silver-based bimetallic nanoparticles, zeolite Y-dispersed AgNPs, silver/reduced graphene oxide or N-doped graphene hybrid nanocomposite, superparamagnetic silver/halloysite nanotube/Fe3O4 nanocomposites, AgNPs supported on polymeric materials, etc. are also reported for reduction of 4-nitrophenol to 4-aminophenol (Dong et al., 2015). Again, hydrogenation of 4-nitrostyrene by AgNPs-Al2O3 catalyst produces 4-aminostyrene with 100% conversion and 96% selectivity (Shimizu et al., 2010b). The groups such as alkene, nitrile, amide, or ketone in such nitroaromatics produced respective aminoaromatics without any noticeable byproduct formation. Similarly, 2-nitrophenol is reduced by nanoporous anodic alumina membranes supported AgNPs-based polydopamine coated catalyst. Hydrotalcite-supported AgNPs also showed good catalytic activity for the reduction of nitroaromatic compounds in the presence of CO/H2O with >99% selectivity (Mikami et al., 2010). The study also showed that nitrobenzene can be efficiently and selectively reduced to aniline (96% yield) in the presence of styrene by [email protected] catalyst (Mitsudome et al., 2012). Li et al. reported a one-pot transformation of nitroaromatics containing an electron-withdrawing or electron-donating group using a silver-based bimetallic nanocatalyst, where the reductive amination of nitroaromatics with aldehydes resulted in the desired secondary amine product with good to excellent yields and high selectivity under mild reaction conditions (Li et al., 2013a). The reductions of nitroaromatics are found to be size- and stabilizing or functionalizing agent or support-dependent AgNPs catalyze chemoselective reactions. More interestingly a core-shell [email protected] magnetic nanohybrid can be used as a recyclable-efficient catalyst for the reduction of unsubstituted nitroaromatics as well as those substituted with -OH, -CN, halogen, ketone, etc. On the other hand, graphite-grafted poly(amido amine) dendrimer-supported AgNPs can reduce nitroaromatics to yield moderate to excellent conversions to the desired products (Rajesh and Venkatesan, 2012).

Silver Nanomaterials and Their Polymer Nanocomposites

2.8.2.3 Reduction of Organic Compound The core-shell [email protected] dispersed on the CeO2 nanocatalyst can efficiently reduce unsaturated aldehydes and alkyl aldehydes containing olefin functionality such as terpenes and aromatic α,β-unsaturated aldehydes to the corresponding alcohols with high selectivity. Hydrotalcite-supported AgNPs can also catalyze the deoxygenation of epoxides into alkenes using alcohols as reductant (Mitsudome et al., 2010). However the reaction is effective for aromatic epoxides such as styrene oxide, stilbene oxide, 1-phenylpropylene oxide, though cis-stilbene oxide yields a mixture of Z/E-alkene isomers, though it is not suitable for the aliphatic epoxides. Again the deoxygenation of the aromatic epoxides can be efficiently catalyzed into alkenes by using the same AgNPs-based catalyst in the presence of CO/H2O as a reductant (Mikami et al., 2010). On the other hand, irrespective of the nature of epoxides i.e., aromatic, aliphatic, or alicyclic can be smoothly reduced into the corresponding alkenes with excellent yields (94%–98%) and selectivity (>99) by using [email protected] nanohybrid catalyst (Mitsudome et al., 2012). More interestingly the catalyst can be recycled without loss of activity. Again, alkylamine- and alkanethiolatestabilized Pd-AgNPs can also be used efficiently for hydrogenation reaction. The octahedral molecular sieve-supported silver-based nanocatalyst is also used for the hydrogenolysis of glycerol, which produced 1,2-propanediol at 200°C under 50 atm pressure for 8 h with a 65%–70% conversion and a 90% selectivity as an important industrial product for various applications (Yadav et al., 2012). 2.8.2.4 Oxidation of Alcohol The oxidation of allylic alcohols by using Pd-AgNPs heterogenous catalyst (Pd7Ag93) can produce α,β-unsaturated carbonyl compounds (Yamamura et al., 1989). Similarly, monodispersed Au-AgNPs alloy nanocluster- (silver 10%, size 1.6–2.2 nm) based catalyst can oxidize p-hydroxybenzyl alcohol to p-hydroxy benzaldehyde (Chaki et al., 2007). The study also shows that aliphatic long chain alcohol can be transferred to the corresponding aliphatic aldehyde by silicon nanowire array-supported AgNPs in a gas phase reaction at 300–400°C (Zhang et al., 2008). On the other hand, hydrotalcite-supported AgNPs-based catalyst can oxidize 1-phenylethanol into acetophenone in the presence of styrene with 99% yield (Mitsudome et al., 2008a). The authors also claimed an extraordinary total turnover number of >100,000 for this nanocatalyst, as it could be reused repeatedly (4th cycles) without loss of activity. Similarly a variety of supported AgNPs-based catalysts such as SiO2 (10 wt% of Ag), γ-alumina (5 wt% of Ag), titania, CuO nanowires, etc. are used for the oxidation of alcohols to aldehydes or ketones (Dong et al., 2015). 2.8.2.5 Oxidation of Silane An environmentally benign protocol to obtain silanols from hydrosilanes is reported by Kaneda and coworkers using a hydroxyapatite-supported AgNPs catalyst in the presence

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of water as the green oxidant (Mitsudome et al., 2008b). Furthermore the catalyst can be easily separable just by filtration, reusable for several times (four times) with no loss of catalytic activity and selectivity. The AgNPs-based catalyst can also be used to oxidize both aromatic and aliphatic silanes into the corresponding silanols or silyl ethers in the presence of water or alcohols as the oxidants (Li et al., 2014). 2.8.2.6 Oxidation of Olefin The olefinic compounds are highly valuable yet easily available organic compounds. It is also found that their oxidized products like epoxides or other oxygen-containing compounds are similarly valuable particularly in the fields of fine chemical and pharmaceutical industries. Methyltrimethoxysilane-supported AgNPs-based catalyst of just 1 wt% (0.91 wt% silver) can oxidize styrene into styrene oxide under microwave irradiation with excellent selectivity (>99%) and good conversion (60%) by heating at 96°C (Purcar et al., 2009). Similarly, acyclic and cyclic olefins are transformed to the corresponding epoxides with high selectivity and good yield by using layered doublehydroxide-supported AgNPs and hollow AgNPs cages assembled catalyst at a slightly elevated temperature of 65°C (Anandhakumar et al., 2014). Again, Al2O3 supported AgNPs- (5 wt% Ag) based catalyst is reported for the one-pot transformation of imines from alcohols by oxidative dehydrogenation of the alcohols to aldehyde followed by reaction with primary amines (Mielby et al., 2014). The γ[email protected] catalyst can chemoselective oxidize primary amines into N-monoalkylated hydroxyamines. Further, carbon/AgNPs (size 12.4 nm) can efficiently produce azo compounds via the oxidative coupling of anilines with high yields by activating molecular oxygen under ambient conditions (Cai et al., 2013). 2.8.2.7 Dehydrogenative Coupling of Amines AgNPs-based catalysts are also very useful for dehydrogenative coupling of various amines. The γ-alumina-supported silver nanocluster-based catalyst (Ag 5 wt% and size 0.84 nm) is used for the direct dehydrogenative amidation of 4-fluoro-phenylmethanol by morpholine to produce the desired amide product (Shimizu et al., 2009a). 2.8.2.8 Natural Product In general the use of nanocatalysts, particularly AgNPs-based catalysts, is limited in natural product synthesis compared to others. In this context, AgNPs-based nanocatlyst is emerging, and researchers are using such catalysts for complex reactions. As an example, silica-supported 0.01 mol% AgNPs-based catalyst is used for the total synthesis of natural product panduratin A via the Diels-Alder reaction as the key step with a nearly quantitative yield (Cong et al., 2010). Again, AgNPs-based catalyst is used for total synthesis of sorocenol B via Diels-Alder cycloaddition reaction (Cong and Porco, 2012). Further,

Silver Nanomaterials and Their Polymer Nanocomposites

silica supported-AgNPs is utilized to obtain a bicycle [3.3.1] framework of serocenol B, while sorbiterrin A is obtained by AgNPs-catalyzed bridged aldol/dehydration to access the [3,3,1] framework (Cong and Porco, 2012; Qi et al., 2014). 2.8.2.9 Photocatalytic Dye Degradation The untreated dye effluents of textile dyeing and printing industries contain dark color dyes along with alkali, soap, and other organic toxic chemicals. These various industrial effluents commonly contaminated the aquatic ecosystem and soil, ultimately creating environmental hazards. On the one hand, these contaminants are causing health hazards (particularly responsible for skin cancer), while on the other hand the conventional effluent treatment is highly expensive. In this milieu, solar light-induced biogenic AgNPsmediated photocatalytic degradation of such contaminants is found to be effective (Fig. 2.5). Thus this decolorizing of dyes by active superoxide and hydroxide radicals has recently been treated as an alternative green approach. AgNPs are used to reduce methylene blue and phenosafranine dye (Kundu et al., 2002; Mallick et al., 2006). The nanocomposite of reduced graphene oxide, silver, and reduced carbon dots can be used to degrade ethyl paraoxon organophosphate, rhodamine B, etc. upon exposure to sunlight (Duarah and Karak, 2018).

2.8.3 Miscellaneous In addition to the aforementioned, AgNPs are also used to produce nanoprisms by various techniques such as nanosphere lithography, photoinduced aggregation, etc. by exploring their long wavelength-based SPR (Haider and Kang, 2015). The research showed that high aspect ratio AgNPs nanoprism can be obtained by low-intensity light-emitting diodes (LEDs) with different emission wavelengths in combination with

Fig. 2.5 Photocatalytic degradation mechanism of methylene blue (MB).

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various color filters for illumination (Bastys et al., 2006.). This nanoprism can be used in fields of biosensing and bioimaging (Shahjamali et al., 2012). AgNPs with large absorption coefficients of 1.55 μm are used for coating communication fibers to enable them to communicate signals with reduced loss and contamination (Haider and Kang, 2015). Beside these, AgNPs and their nanocomposites are also utilized in other fields as well. For examples, AgNPs are used in sensing and imaging applications by utilizing their valuable optical properties, where a wide range of detection techniques including colorimetric, scattering, surface enhanced resonance spectroscopy (SERS), mouse embryonic fibroblasts (MEF), etc. can be employed, even at extremely low concentration (Caro et al., 2010). Again, aqueous inks of poly(N-vinylpyrrolidone) with 20 wt% AgNPs along with a combination of raganic solvent and cosolvents is used as inkjet ink in a commercial Dimatix printer (Dang et al., 2015).

2.9 CONCLUSIONS AND FUTURE TRENDS AgNPs and their polymer nanocomposites are wonderful materials in nanoscience and technology because they have wide spectrum of applications. From the discussion in this chapter, it can be concluded that among various synthetic routes, plant extract-mediated synthetic protocol is considered to be most efficient and acceptable because of its economical, energy-efficient, health issues, societal and environmental benefits. However, most of the synthetic routes commonly suffer from problems like scalability, costs, controlling particle sizes, stability, etc. for AgNPs. Thus mass scale production of AgNPs on the industrial level by a commercially viable, economical, and environment-friendly approach still needs to be explored. Significant variations in chemical compositions of plant extracts, even for the same species, with a variation of soil and environmental conditions across the globe resulted variation in properties and activities of AgNPs. The antimicrobial and antiinflammatory activity, as well as the inhibiting effect of microbial proliferation and infection of AgNPs attracted the attention of scientists and engineers for the use of silver-based polymer nanocomposites in various biomedical fields, especially as implanted materials in artificial organs. Such nanocomposites show a new avenue in the field of nanomedicine, particularly in wound dressing and artificial implantation, to prevent postsurgical contamination caused by microbes. AgNPs-based heterogeneous catalysts are extremely valuable for many important orgaic transformations.

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FURTHER READING Barua, S., Konwarh, R., Bhattacharya, S.S., Das, P., Devi, K.S.P., Maiti, T.K., Mandal, M., Karak, N., 2013. Non-hazardous anticancerous and antibacterial colloidal ’green’ silver nanoparticles. Colloids Surf. B: Biointerfaces 105, 37–42.

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