Silver nanomaterials as future colorants and potential antimicrobial agents for natural and synthetic textile materials

Abstract

Over the past few years, antimicrobial textiles have gained considerable interest for use in different application fields. Because of these reasons, a wide range of antimicrobial agents with diverse chemical structures such as quaternary ammonium compounds, N-halamine siloxanes, heterocyclic compounds with anionic groups, polybiguanides, triclosan, metal salts, and synthetic colorants have been used to impart antimicrobial properties to different textile materials. However, most of these antimicrobial agents suffer from many disadvantages such as action on non-target microorganisms, toxicity to the environment and low durability of antimicrobial finish. To overcome these problems, silver nanoparticles with strong cytotoxicity towards a broad range of microorganisms, low toxicity to human cells, high selectivity, long term durability, increased dyeability and biocompatibility are drawing a tremendous level of attention from both academic research and industry. Silver nanomaterials, due to their unique properties are particularly attractive for production of textiles surfaces with novel properties such UV protection, water resistant, self cleaning and antimicrobial activity. The present review is intended to describe recent advances about the use of silver nanomaterials as novel colorants and antimicrobial agents for different textiles materials. Finally it also highlights the current challenges and provides scope for future studies.

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Shahid-ul-Islam

Dr Shahid-ul-Islam received the M.S degree in chemistry from Jamia Hamdard, New Delhi in 2010 and his PhD. degree in Organic Chemistry from Jamia Millia Islamia (Central University), New Delhi in 2016 under the supervision of Dr Faqeer Mohammad. His current research interests include natural product chemistry, thermodynamics and kinetics of colorants, nanomaterials, radiation modifications, fluorescent, UV-blocking and antibacterial textile finishing. He has numerous academic publications in International journals of high repute to his credit and has also contributed to several internationally recognized books published by John Wiley & Sons, Springer, and Studium Press LLC.

B. S. Butola

Dr B. S. Butola obtained his B. Tech. (1990) and PhD. degrees (2005) in textile technology from IIT Delhi. Currently he is an associate professor at the department of textile technology, IIT Delhi. His research interests include functionalization of textiles with metal oxides, use of shear thickening fluids for improving the impact performance of ballistic textiles, polymeric nanocomposites and smart colorants.

Faqeer Mohammad

Dr Faqeer Mohammad is a Senior Assistant Professor in the Department of Chemistry, at Jamia Millia Islamia, New Delhi, India. He received his M.Sc., M. Phil., and PhD. in 1975, 1979, and 1982, respectively, from Aligarh Muslim University, Aligarh, UP, India. During his PhD. he was awarded JRF and SRF Research Fellowships from UGC and CSIR. He has published numerous research articles, reviews, and book chapters in journals of international repute. To date he has supervised 23 graduate M.Sc. and 5 PhD. thesis students. His research interests include natural dyeing and functional finishing of textiles.

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1. Introduction

Textile materials such as cotton, wool, silk etc. provide ideal conditions such as moisture, temperature, oxygen, and nutrients for hosting many drug resistant pathogens.^1–3 Contamination by deadly microorganisms is of great concern in a variety of areas. They may result in emission of unpleasant odors, discoloration, cross-transmission of infectious diseases, allergic responses and deterioration or undesirable change in aesthetic value of textile materials.^4–6 To overcome these problems use of antimicrobial textiles in healthcare facilities is a new concept. Therefore, research involving antimicrobial-modifications of textiles to improve performances or to create unprecedented functions is flourishing.⁷ Up to now various kinds of antimicrobial agents with diverse structures such as organometallics, heavy metal ions, phenols, formaldehydes, quaternary ammonium salts, chitosan, synthetic dyes, organosilicones and natural dyes have been used to impart antimicrobial properties to textile materials.⁸ However, most of these antimicrobial agents suffer from many disadvantages such as action on non-target microorganisms, toxicity to the environment and low durability of antimicrobial finish.⁹ To overcome these problems intense research efforts are being made to investigate the possible effects of nano science and nanotechnology in textile industry. Nanotechnology has attracted attention of textile and polymer scientists and has been playing extraordinary role over the past few decades in the functional finishing of different textile materials.¹⁰ Nanoparticles due to their diverse functions have been utilized to impart flame retardant, UV-blocking, water repellent, self-cleaning, and antimicrobial properties to the textile fibres.^11,12 Among several inorganic nanomaterials, silver is one of the most important and most studied due to their remarkable antibacterial, antifungal activity, less toxicity and low cost.¹³

Silver has been known since antiquity as a popular agent to fight against infections and prevent spoilage. But due to emergence of antibiotic resistant microorganisms over the last decade use of silver compounds has been declined remarkably.¹⁴ At present nanoscale materials and particularly silver in nano form in view of their large surface area to volume ratio has been highlighted as a potential candidate to fight antibiotic resistant microorganisms due to a broad spectrum of antimicrobial activity.^15,16 Researchers have devoted special attention to various preparation techniques including gamma irradiation, photo-catalytic reduction, chemical reduction, microwave processing, photo-chemical method, metallic wire explosion, sonochemical, polyols, electron irradiation, and biological methods for the synthesis of silver nanoparticles.¹⁷ Various theories have been proposed to explain its antimicrobial activity; however it's mode of action is yet not fully elucidated. The most accepted mechanism is on the basis of their interaction with cell membrane causing extensive cell surface alternations and permeability, leading to intracellular leakage of cytoplasm and finally death of microorganism. Interaction with thiol group compounds found in the respiratory or vital enzymes of bacterial cells is another route described to explain their action-mechanism. Silver binds to the bacterial cell wall and cell membrane and inhibits the respiration process causes micro-organism structural changes, and then inhibits the metabolic pathway through producing reactive oxygen species.¹⁸ Other mechanism suggests the antimicrobial activity due to interaction of silver nanoparticles with the sulphur and phosphorus of the DNA, leading to or inducing problem in its replication ability and eventual cell death.¹⁹ The antimicrobial mechanism of silver nanoparticles is shown in Fig. 1. A more detailed discussion about the mechanism of action of silver nanoparticles is described in a recent review article by Duran et al.²⁰

Fig. 1 Possible interaction pathways of silver-containing compounds with bacterial cells: direct interaction with (i) the cell wall, (ii) DNA, (iii) membrane proteins, and (iv) formation of reactive oxygen species. Reprinted with permission from ref. 19 Copy right 2013 American Chemical Society.

The application of silver nanoparticles on different textile materials has emerged as a burgeoning area of research in the field of textile and polymer sciences. Silver nanomaterials offer unique properties such as large surface area, surface plasma resonance, excellent antimicrobial activity. With these properties, novel shades of elegant hue and tune, and excellent antimicrobial activities on different textile materials such as cotton, wool, silk, nylon, polyester, and polyamide have been achieved. In view of these facts, this review outlines recent advances in the use of silver nanomaterials as coloring and antimicrobial finishing agents for different natural and synthetic textile materials.

2. Coloration and antimicrobial finishing of natural textile materials

Silver nanoparticles are endowed with unique localized surface plasmon resonance property (LSPR), by which they exhibit brilliant colors.²¹ The remarkable relationship between the morphology of silver nanoparticles and their color is proving to be very helpful for their use in different textile finishing applications.²² It is worthy to note that control on the morphology of AgNps is a promising way to tailor the LSPR band and effectively tune the color of silver nanoparticles. This has recently motivated research activities to directly employ silver nanoparticles in the dyeing of cotton; silk, and wool.^23,24 Furthermore natural fibres as already mentioned can be easily colonized by high numbers of microbial pathogens, including both moulds and bacteria therefore, are potentially responsible for the nosocomial infections.²⁵ Silver nanoparticles are nowadays considered as next generation antimicrobial agents for functional finishing of different textile materials. Various finishing technologies such as padding, rinsing, sol–gel, sputtering, and printing have been developed to incorporate silver nanoparticles onto different natural fibre/fabric surfaces.²⁶

2.1 Cotton

Considering the application of silver nanoparticles to different textile materials cotton is one of the most extensively studied because of its breathable, soft, comfort and other outstanding attributes. Some researchers recently reported the possibility of utilizing localized surface plasmon resonance property of silver nanoparticles for enhancement of the color strength of cotton based fabrics and their antimicrobial properties. Chattopadhyay et al.²⁷ analysed the dyeing property of natural fabrics including cotton with direct dyes pre-treated with silver nano colloids synthesised by the reduction of silver nitrate, and found improvement in color strength and fastness properties. Tang et al.²⁸ used this property to study the coloration of cotton using anisotropic silver nanoparticles with different colors. At low temperature, they assembled silver nanoparticles on cotton by linking poly(diallyldimethylammonium chloride) to produce colorful shades with acceptable color and fastness properties. Solution dipping method was applied by Wu et al.²⁹ to produce colorful shades with satisfactory fastness and durable antibacterial properties on cotton. They reported that the treatment of cotton with branched poly(ethylenimine) (PEI), silver nanoparticles with different colors, and fluorinated-decyl polyhedral oligomeric silsesquioxane led to the production of cotton with durable antibacterial activity against E. coli and B. subtilis and self healing property. The deposition of silver nanoparticles on PEI-doped cotton indicated that primary, secondary, and tertiary amino groups of PEI and functional groups of the silver nanoparticles were involved in electrostatic and hydrogen bonding interactions. The TEM images, UV spectra and different beautiful and elegant hues on cotton fabrics produced with silver nanoparticles are shown in Fig. 2.

Fig. 2 (a–c) TEM images of AgNPs with different shapes and colors: (a) yellow, (b) red, and (c) blue. (d) UV-vis absorption spectra and (e) aqueous solutions of AgNPs with different colors: (1) yellow, (2) orange, (3) red, (4) blue, (5) green, and (6) violet. (f) Cotton fabrics dyed with AgNPs of the same colors as in (e). Reprinted with permission from ref. 29 Copyright 2015 Wiley-VCH.

Likewise Emam and colleagues³⁰ reported two solvent less techniques namely sorption and padding to deposit silver nanoparticles onto cotton in order to produce colored and multifunctional cotton. It was shown that sorption technique was more efficient to deposit 69.3–6094.8 mg kg⁻¹ of silver than padding which showed only 33.8–609.3 mg kg⁻¹ silver content on the cotton. Increase in the concentration of AgNO₃ resulted in the color change of cotton from gray-to reddish yellow and improvement in the color strength values. By the presence of 5912.3 mg kg⁻¹ of silver content in cotton, they observed excellent antibacterial activity against E. coli and S. aureus. Ghosh et al.³¹ prepared antibacterial cotton by applying two types of silver nanoparticles namely mesosilver and silpure. SEM images of the treated samples indicated good dispersion of nano silver particles on the cotton surface. Vigneshwaran et al.³² reported the formation of silver nanoparticles with average size of 20.9 ± 13.7 nm on cotton fabrics using aldehyde terminal of starch as reducing as well as stabilization agent. They found that the cotton fabrics impregnated with silver nanoparticles were highly active against S. aureus and K. pneumonia. Also, the deposition of silver nanoparticles using ultrasound irradiation on different textile materials including cotton was investigated by Perelshtein et al.³³ In their research work, various techniques including X-ray diffraction, high resolution scanning electron microscopy, and Raman spectroscopy were used to characterize nanosilver treated fabrics, and it was found that 6 (wt%) is the effective concentration to completely eradicate E. coli and S. aureus within 1 h. Tarimala et al.³⁴ evaluated the antibacterial property of dodecanethiol-capped AgNPs prepared by sol–gel method. They found that cotton fabrics treated with these silver nanoparticles show excellent antibacterial activity against E. coli. Ilic et al.³⁵ applied colloidal silver nanoparticles fabricated without adding any stabilizer on cotton to study the color change and antimicrobial activity of AgNps against E. coli, S. aureus and fungus Candida albicans. They observed that silver nanoparticles from 50 ppm colloid solution had more laundering durability than 10 ppm solution. Thanh et al.³⁶ used the polyol method involving microwave heating to fabricate nano-sized silver colloid for application on cotton. They found that the presence of 758 mg kg⁻¹ of silver nanoparticles on cotton is highly effective in killing of bacterial pathogens. Raza et al.³⁷ introduced an enzymatic pretreatment with aquzagym SDL (an amylase), scourzyme L (a pectinase) and celluloft L (a cellulase) enzymes in order to fabricate silver nanoparticles via one spot synthetic route on the surface of cotton. They exploited the reducing and stabilizing power of starch and discovered that the treated cotton displays durable antibacterial activity against E. coli and S. aureus. In another research work carried by Montazer and co-workers,³⁸ silver nanoparticles were synthesised and applied on cotton using 1,2,3,4-butanetetracarboxylic acid as a stabilizer. They concluded that cotton fabrics treated with butanetetracarboxylic acid resulted in enhancement in functional properties. Khalil-Abad et al.³⁹ examined the application of silver nanoparticles for antibacterial activity on cotton previously cationized with 3-chloro-2-hydroxypropyl trimethyl ammonium chloride. Chen and Chiang et al.⁴⁰ grafted a chelating agent namely glycidyl methacrylate–iminodiacetic acid (GMA–IDA) onto cotton fabric and claimed the formation of cotton fibre-graft-GMA–IDA/Ag⁺ complex after irradiation by ultraviolet lamp. Their study proved that treatment of grafted cotton with silver nanoparticles (75 nm) resulted in good antibacterial activity of the cotton fabrics against E. coli. Hebeish et al.²³ used silver nanoparticles (AgNPs 6–8 nm) to impart antimicrobial property to cotton fabrics against E. coli (Gram −ve) and S. aureus (Gram +ve) bacteria. They synthesized the silver nanoparticles using hydroxypropyl starch (HPS) as both reducing and stabilizing agent and proposed that the silver nanoparticles bind to the functional groups of cotton, using Printofix Binder MTB EG liquid (based on acrylate) as the binder. The results revealed that the AgNP treated cotton samples in presence of binder retained antibacterial properties even after 20 washing cycles, confirming the positive role of binder in improving the antimicrobial finishing of cotton fabrics. El-Rafie et al.,⁴¹ likewise, reported deposition of nanosilver particles (54 ppm) synthesised by making use of fungus Fusarium solani onto cotton fabrics in the presence and absence of same binder to assess antibacterial activity of treated cotton fabrics. These authors found that the use of Printofix Binder MTB EG binder could be successfully employed to retain significant amount of antimicrobial activity in cotton (94% reduction for S. aureus and 85% for E. coli) after 20 washing cycles.

Several research studies have been carried out over the last few years dealing with the use of natural products for the synthesis of silver nanoparticles. To decrease the cost involved in synthesis of AgNps, Satishkumar et al.⁴² investigated reduction of silver ions to nano-sized particles using chemical compounds present in Curcuma longa tuber powder and extract for their application to cotton fabrics. They found that cotton fabrics treated with silver nanoparticles in the presence of polyvinylidene fluoride (PVDF) resulted in good wash durability of the claimed antimicrobial activity on cotton. Likewise, antimicrobial activity of silver nanoparticles (20 nm) prepared by using Eucalyptus citriodora and Ficus bengalensis extracts against E. coli on cotton fabrics was investigated by Ravindra et al.⁴³ It was interesting to observe that 2% leaf extract demonstrated excellent antimicrobial results. Aqueous extract of lemon leaves (Citrus limon) have been investigated as prominent reducing and stabilizing agents for the synthesis of extracellular stable silver nanoparticles by Vankar and Shukla.⁴⁴ The reaction between silver salt and plant extract was monitored by a number of techniques such as FT-IR, UV-visible spectroscopy, TEM, SEM, and AFM. They noticed that the silver nanoparticles synthesised by extract of lemon leaves after application on cotton exhibited excellent antifungal activity against Fusarium oxysporum and Alternaria brassicicola. Thomas and colleagues⁴⁵ modified cotton with chitosan to enhance its antibacterial properties. They stated that these improvements are due to in situ synthesis of nano silver within the chitosan modified cotton. Thus, through modification with chitosan nano silver imparts desirable antibacterial properties to cotton. Very recently, in 2016, Balakumaran et al.⁴⁶ also introduced the role of Aspergillus terreus Bios PTK 6 in the synthesis of highly stable 8–20 nm silver nanoparticles for their use in antimicrobial finishing of cotton fabrics. They observed that treated cotton fabrics are highly effective against both bacterial and fungal strains with excellent wash durability maintaining antimicrobial activity up to 15 washings.

Zhang and co-workers⁴⁷ reported that the synthesis of a novel nanosilver solution by mixing of AgNO₃ aqueous solution and an amino terminated hyperbranched polymer is an effective way to introduce an antimicrobial finishing to cotton fabrics. Several techniques including dynamic light scattering, TEM and UV/visible absorption spectrophotometry were employed to confirm the formation of antibacterial silver nanoparticles of 10–30 nm size. They observed that the cotton fabrics treated with nano-silver colloidal solution showed 99.01% reduction of S. aureus and 99.26% of E. coli. Hebeish et al.⁴⁸ fabricated nano silver for application on cotton through a newly prepared copolymer β-cyclodextrin grafted with poly acrylic acid which acts as both reductant and stabilizing agent. Potassium persulphate was used as an initiator. They observed that cotton treated with these silver nanoparticles is highly active against bacteria and exhibits durable antibacterial properties. Zhang et al.⁴⁹ found that applying antibacterial silver nanoparticles on cotton fabricated by the reaction between silver nitrate and amino-terminated hyperbranched polymer increased the laundering durability of antimicrobial activity against S. aureus and E. coli up to 50 home washings. Further, multifunctional cotton having excellent antibacterial activity against Gram-positive and Gram-negative bacteria was produced by Abdel-Mohsen et al.⁵⁰ using core–shell (silver nanoparticles (Ag) as core and chitosan-O-methoxy polyethylene glycol (CTS-O-MPEG) as shell) nanoparticles.

In another investigation which was conducted by Perera et al.,⁵¹ the role of ex situ chemical and in situ photo reduction methods in producing AgNP for functional treatment of cotton were investigated. It was observed that in situ produced nanoparticles were more effective compared to ex situ synthesised silver nanoparticles in inducing strong antimicrobial property and laundering durability even with low concentrations of AgNO₃. Yazdanshenas and Shateri-Khalilabad⁵² synthesised silver nanoparticles within the cotton fabrics in the presence of sodium hydroxide. In their approach, sodium hydroxide treated cotton was dipped into the silver nitrate solution followed by chemical reduction of silver metal. Cotton fabric treated with high concentration of NaOH had higher concentration of silver and exhibited high antibacterial activity against E. coli and S. aureus which was retained up to 10 washing cycles. Xue et al.⁵³ fabricated nano silver on cotton by the reduction of [Ag(NH₃)₂]⁺ complex with glucose, producing antimicrobial activity against Gram negative E. coli. Bacciarelli-Ulacha et al. also applied silver nanoparticles fabricated through in situ reaction between silver nitrate and L-ascorbic acid on cotton to study antimicrobial potential of nano silver against E. coli and Bacillus subtilis. They observed that silver nanoparticles formed on cotton by screen printing method were very effective antibacterial agents and were able to retain activity against both the strains up to 50 laundering cycles. In situ deposition of AgNPs through a novel method for the development of antibacterial cotton was studied by Jiang et al.,⁵⁴ and it was found that amount of nano silver on cotton fabrics increased from 0.6890 to 1.3561 mg per gram of cotton by immersing the fabric in 160 mM silver nitrate solution maintained at 90 °C. The nano silver treated cotton fabrics displayed excellent wash resistant antimicrobial activity with reduction rate of 98.5 and 94.3% against E. coli and S. aureus, respectively after 20 laundering cycles. Likewise, El-Shishtawy et al.⁵⁵ reported the in situ formation of AgNp and studied its antibacterial properties on cotton against a number of disease causing microorganisms by growth inhibition zone test method. They observed brownish yellow color on cotton finished with silver nanoparticles, and used several techniques such as SEM, energy-dispersive spectroscopy (EDS) and FT-IR to characterize silver coated cotton fabric. The change in peak intensity as revealed by their FT-IR spectrum confirmed the binding of silver nanoparticles with cotton functional groups, which displayed good antibacterial activity. Montazer et al.⁵⁶ also fabricated silver nanoparticles on cotton through a novel green method using Tollens reagent. Silver nitrate (AgNO₃) was reduced to nano silver by utilizing reducing and stabilizing power of cellulose. They found that color coordinates (CIEL a*b* color values) for the nano silver treated cotton showed yellow color which displayed high antibacterial activity against S. aureus and E. coli maintained up to 30 washing cycles.

2.2 Wool

Wool is complex in structure and essentially composed of three tissues, cuticle, cortex and the medulla.⁵⁷ It contains free carboxylic acid and amino groups which have been employed to cross link via covalent bonds or through secondary interactions such as hydrogen bonds, van der Waals forces, and dipole–dipole interactions with different textile finishing agents currently used in textile industries.⁵⁸ Scientists have in recent years directed intense research towards silver nanoparticles as new functional agents for wool based fabrics.⁵⁹ The keratin proteins of wool have been employed to fabricate silver nanoparticles for their application as antibacterial agents. Lu and Cui et al.⁶⁰ explored the role of keratin extracted from wool in the formation of stable silver nanoparticles. They used UV-vis and Fourier transform infrared spectroscopy methods to study the effect of keratin concentration, and possible modes of interaction between silver core and capping agent, respectively. They found that this method is highly facile and cost-effective to produce silver nanoparticles which are stable in aqueous medium up to three months. Cell membrane complex in wool and other low sulphur regions are identified as main regions of entry for inorganic and polymer based nanoparticles.⁶¹ Osorio and co-workers⁶² reported that in situ deposition of silver nanoparticles on wool can be more efficiently done in an aqueous system compared to ethanolic system. Perumalraj⁶³ reported the dyeing and fastness properties of wool fibres in the presence and absence of silver nanocolloids with acid and direct dyes. Their results showed that silver nanocolloids can be suitably applied on wool fibres to produce shades having good color strength and improved fastness towards light and washing. Raja et al.⁶⁴ reported the synthesis of silver nanoparticle-polyvinyl pyrrolidone composite in powder form containing silver particles of size 50–60 nm using sono-chemical method, and studied their antimicrobial activity on textile substrates including wool. The treated wool displayed good antibacterial activity S. aureus, E. coli and P. aeruginosa with a reduction percentage of 100%, 97% and 99% respectively. Ki and co-workers²⁴ synthesised sulphur nano-silver colloidal solution with an average size of 4.2 nm and applied it to wool based textiles for antibacterial activity against Gram-positive S. aureus and Gram-negative K. pneumonia. In addition to antibacterial activity, they claimed that sulphur nano-silver colloid solution also imparts mothproofing, antibiotic, and antistatic property to wool. Recently silver in nano form has also been utilized as a coloring agent to develop novel and beautiful hues on wool materials. For example, Hadad et al.⁶⁵ studied the deposition of silver nanoparticles with average size of 5–10 nm on the surface of wool using power ultrasound. Under argon atmosphere, silver–wool nanocomposite was produced by irradiating slurry of wool, AgNO₃, and NH₃ in an aqueous medium for 120 min with sonochemical radiation. Tang et al.⁶⁶ reported that anisotropic silver nanoparticles assembled at 40 °C in a solution having pH 4 can produce beautiful colors on wool fabrics. They found that this method of coloring wool is interesting because the color of wool fabrics can be altered depending upon the morphology of silver nanoparticles. The treated wool fabrics also indicated antibacterial activity against E. coli. The SEM images of wool fabrics treated with silver nanoparticles of different morphologies such as nano prism and nano disc are shown in Fig. 3. Likewise, in another research investigation Tang et al.⁶⁷ applied silica nanoparticles followed by deposition of silver nanoparticles on wool and reported enhancement in color as other functional properties including antibacterial activity.

Fig. 3 SEM images of wool fibres treated with silver NPs: (a) and (b) nanoprism I, (c) nanodisk I, and (d) nanodisk II. Reprinted with permission from ref. 66 Copy right 2011 Elsevier.

Rad et al. applied acid dye and different concentration of antimicrobial silver nanoparticles on wool yarns using the exhaustion method. The antibacterial activity of treated wool yarns was carried against E. coli and S. aureus, and it was found that acid dye along with 25 nm of silver nanoparticles showed high antimicrobial activity retained after ten washing cycles. Kelly and Johnston⁶⁸ used silver nanoparticles with LSPR properties to color merino wool fibres and fabrics. They synthesized silver nanoparticles through the reduction of silver ions in solution by trisodium citrate in the presence of merino wool fibres or fabrics and proposed that the silver nanoparticles simultaneously bind to the amino acids of the keratin protein in the wool fibres, using trisodium citrate as the linker. Falletta et al.⁶⁹ prepared the few-nanometre-sized silver nanoparticles by the reduction of silver nitrate in the presence of poly(acrylates) of different molecular weights through sodium borohydride reducing agent or by irradiation with UV light for the antibacterial finishing of wool and other textile substrates. They observed that the wool fabrics treated with synthesized silver nanosols exhibited good antimicrobial inhibition against S. aureus, S. epidermidis, P. aeruginosa, and C. albicans pathogens.

Barani et al.⁷⁰ applied silver nanoparticles on wool fabricated through in situ synthesis method as depicted in eqn (1)–(3). To enhance the diffusion of AgNps into the wool, they employed lecithin which is a biological lipid and reported that increasing the concentration of lecithin decreased the release of nano silver thereby reducing cytotoxicity. The enhancement in diffusion of silver ions into wool was ascribed to electrostatic interaction between positively charged silver ions and negatively charged carboxyl groups. Further, the treated wool with nanosilver/lecithin combination displayed good antibacterial activity.

NH₃⁺–wool–COO⁻ + Ag⁺ → NH₃⁺–wool–COO⁻Ag⁺ (1)

NH₃⁺–wool–COO⁻ + Ag⁺ + NaBH₄ → NH₃⁺–wool–COO⁻Ag (2)

n(NH₃⁺–wool–COO⁻Ag_m) → (NH₃⁺–wool–COO⁻Ag_m)n (3)

Montazer et al.⁷¹ reported antibacterial activity of wool against S. aureus and E. coli applied with Ag/TiO₂ nanocomposite fabricated through UV irradiation in an ultrasonic bath, and stabilized with citric acid as a cross-linker. XRD patterns, SEM photographs, and EDS analysis of the finished wool samples confirmed the presence of Ag/TiO₂ nanocomposite. Response surface methodology was used to assess the impact of independent variables such as Ag/TiO₂ nanocomposite, citric acid and sodium hypophosphite concentrations on antimicrobial activity, and it was found that increasing the content of citric acid led to more content of Ag/TiO₂ nanocomposite onto wool resulting in more reduction of bacterial pathogens. The response surface for antibacterial activity as a function of Ag/TiO₂ and citric acid (CA) are shown Fig. 4.

Fig. 4 Response surface for antimicrobial activity as a function of Ag/TiO₂ and citric acid for treated samples against: (a): S. aureus and (b) E. coli. Reprinted with permission from ref. 71 Copy right 2011 Elsevier.

Hosseinkhani et al.⁷² indicated that antibacterial silver nanoparticles can be in situ immobilized on wool fibres using sodium dithionite and sodium bisulfite as reducing agents. SEM, EDX, AAS and XRD analysis revealed the formation and deposition of silver nanoparticles within the protein chains of wool. Further, it was interesting to note that the tensile strength increased in most of the silver treated wool samples which was attributed to ionic-cross linking of silver nanoparticles within the wool fibres. They also found that synthesized silver nanoparticles on wool have good antibacterial activity against S. aureus and E. coli. Boroumand et al.,⁷³ in a more recent study have investigated antibacterial property of wool obtained by a novel method using natural dye from pomegranate peel extract as a reducing as well as stabilizing agent for the synthesis of antibacterial silver nanoparticles. It was observed that wool loaded with silver nanoparticles demonstrated good laundering durable antibacterial activity against E. coli.

2.3 Silk

Silk structure is simpler and similar to wool but contain amino acids having smaller pendant groups than those found in wool. Owing to their excellent mechanical properties, biodegradability, softness, smoothness, luster, comfortableness, and hygroscopicity, silk fibres are extensively used in a variety of application fields.⁷⁴ Nanomaterials and particularly silver is used by researchers nowadays to obtain multifunctional effects on silk protein. Numerous research investigations have been carried over the past few decades to synthesize silver nanoparticles of diverse morphologies for their use in coloring and antimicrobial finishing of silk fibres and fabrics.²⁷ Chemical reduction of silver nitrate using hydrazine and glucose as reducing agents was examined by Gulrajani et al.⁷⁵ In their research work, synthesised silver nanoparticles having average size of 10 and 35 nm were applied on the silk fabrics to obtain antibacterial activity against Gram positive S. aureus. It was observed that the silk fabrics treated with 40 ppm and 60 ppm silver hydrosols produced at 5 °C and 60 °C had 100% activity against tested microorganism. Vankar and Shukla⁴⁴ reported the use of Citrus limon extract as reducing and encapsulating cage for the reduction of Ag⁺ ion to Ag⁰. The synthesised silver nanoparticles were used for the antibacterial modification of silk against Fusarium oxysporum and Alternaria brassicicola. Tang et al.⁷⁶ carried out the coloration of silk using different morphologies of anisotropic silver nanoparticles and obtained colorful shades with good fastness properties. The silver colloid solutions with different colors, their extension spectra, and the silk fabrics colored by corresponding silver nanoparticles along with their respective color strength values are shown in Fig. 5. The silk fabrics coated with silver nanoparticles were also assessed for antibacterial activity against E. coli. They concluded that the treatment with silver nanoparticles does not cause any damage to silk fabric and could be a promising way to achieve a range of colorful shades as well as antibacterial finishing of silk. Zhang et al.⁷⁷ in a research investigation claimed that in situ formation of silver nanoparticles produced by their method involving the use of multi-amino compound which acts as reducing agent results in the development of antibacterial silver fabrics. They observed that the antimicrobial activity of treated silk fabric against S. aureus and E. coli was retained for more than 50 washings.

Fig. 5 (Top) Photograph of synthesized AgNP solutions with different colors. (Bottom) Extinction spectra of the AgNP corresponding to the above solutions (A) photograph of silk fibres colored by corresponding AgNP solutions (B) K/S curves of corresponding silk fibres colored by AgNPs. Reprinted with permission from ref. 76. Copy right 2013 American Chemical Society.

Several latest approaches such as UV and ultrasound assisted, gamma irradiation, steam, layer-by-layer assembly and electro less silver plasting methods have been studied so far to realize in situ fabrication of silver nanoparticles on silk. Potiyaraj et al.⁷⁸ synthesised silver chloride nanocrystals by dipping of silk fibres in alternate solutions of AgNO₃ and NaCl, and revealed that 100 nm AgCl nanocrystals are formed after 20 alternate dipping steps. They proposed that the treated silk fibre could be used as an antibacterial agent. Abbasi and Morsali et al.⁷⁹ synthesised silver iodide nanoparticles on silk fibre in the presence of potassium iodide and silver nitrate under ultrasound irradiation with sequential dipping method. In a subsequent research study, Abbasi and Morsali⁸⁰ studied the role of ethylene glycol as a reduction and protecting reagent in the formation of crystalline silver nanoparticles on silk yarn, and discovered that using power ultrasound technique yields smaller sized nano particles. Lu et al.⁸¹ used ultraviolet light (UV)-assisted method to fabricate silver nanoparticles on degummed silk fibres and investigated antibacterial properties of treated silk using growth curve, zone of inhibition and FITC/PI dual staining assays against S. aureus and E. coli. XRD patterns revealed that UV irradiation and AgNPs immobilization on degummed silk fibres did not caused any damage to the structure of silk fibre. As could be predicted, in situ produced crystalline silver nanoparticles were found highly active against both Gram-positive and Gram-negative pathogens. Yu et al.⁸² utilized polydopamine as a reducing agent to produce in situ silver nanoparticles with face-centered cubic crystalline structures for antibacterial finishing of silk. The silk fibres coated with silver nanoparticles exhibited high antibacterial activity against E. coli and S. aureus. Wang et al.⁸³ assessed the in situ Ag nanocluster formation on silk fibre using ultraviolet light-induced reduction method and realized the production of luminescent fibre with emission band at about 550 nm as well as good antibacterial activity of the treated fibre against E. coli and S. aureus. Zhang et al.⁸⁴ synthesised in situ nano silver particles using polyamide polymer by steam method and then applied on silk to obtain the antibacterial properties. The deposition of silver nanoparticles on silk had shown high activity against S. aureus and E. coli. Durability to the antimicrobial activity against both the bacteria was maintained up to 30 consecutive home washing cycles. Chang et al.⁸⁵ have described strong antibacterial property of silk fibres obtained by gamma ray irradiation method, based on in situ formation of 20 nm silver nanoparticles on the fibre surface. The mechanism for silver nanoparticles formation was ascribed to the electrostatic and coordination interactions arising between silver ions and amino acid present on silk fibre as depicted in Fig. 6. It was shown that 1 mM AgNO₃ solution and 10 kGy γ-radiations are the optimum concentration and radiation dose at which 96% activity against S. aureus was attained by treated silk. Li et al.⁸⁶ studied the multifunctional properties including antibacterial activity of silk fibroin fabrics after being treated for the first time with nanocomposite containing TiO₂ and TiO₂@Ag nanoparticles, and discovered that surface of silk treated with synthesised composite results in high antibacterial activity against E. coli, S. aureus, and P. aeruginosa.

Fig. 6 Schematic presentation of the formation of silver nanoparticles on the surface of silk fibres via γ-radiation. Reprinted with permission from ref. 85 Copy right 2009 Wiley.

Dubas et al.⁸⁷ used layer-by-layer deposition method for the immobilization of silver nanoparticles on nylon and silk fibres. Colored thin films which were synthesised by dipping of nylon or silk fibres in dilute solutions of poly(diallyldimethylammonium chloride) (PDADMAC) and silver nanoparticles capped with poly(methacrylic acid) (PMA) resulted in 80% bacterial reduction for the silk fibre and 50% for the nylon fibre. Likewise, Meng et al.⁸⁸ synthesised silver nanoparticles of 10 nm average size on silk fibres with the layer-by-layer deposition method. In their approach, silk fibres were treated with poly(acrylic acid) (PAA)/poly(dimethyldiallylammonium chloride) (PDDA) which served as 3-D matrix to produce AgNPs with good crystalline structures for antibacterial application. It was observed that synthesised nanoparticles by this method rendered silk fibres with durable antibacterial activity against S. aureus and E. coli retaining activity up to 120 h. Further it has been revealed that AgNPs were held on silk via ionic bonds prolong the release of silver ions from modified silk.

3. Synthetic textile materials

3.1 Nylon

The application of silver nanoparticles to synthetic fibres for functional finishing is most extensively studied on nylon. Perkas et al.⁸⁹ studied the application of silver nanoparticles to nylon 6, 6 for the preparation of antibacterial nylon. They utilized ammonia and ethylene glycol as reducing agents and ultrasound irradiation as efficient method for fabricating silver nanoparticles and found that nylon 6, 6 yarns treated with nano-crystalline 50–100 nm silver nanoparticles exhibited good antibacterial activity against S. aureus and P. aeruginosa. Montazer and colleagues⁹⁰ applied the silver nanoparticles on nylon producing antibacterial fabrics. Dilute solutions of silver nanoparticles (100 ppm and 200 pm) were stabilized on nylon surface in the presence of 1,2,3,4-butanetetracarboxylic acid as cross-linking agent and sodium hypophosphite as catalyst through padding method. They found that the nylon fabrics treated with 200 ppm nanosilver retained about 96% of antibacterial activity against S. aureus and E. coli up to 20 consecutive washings. Montazer et al.⁹¹ also employed Tollen's reagent [Ag (NH₃)₂]⁺ as a reducing agent and polyvinyl pyrrolidone (PVP) as a stabilizing agent to produce silver nanoparticles through ultraviolet radiations and studied their use in antimicrobial fining of nylon. It was observed that 200 ppm nano silver colloidal solution effectively inhibited both S. aureus and E. coli and the resultant fabric displayed a reduction percentage 99.2% against both pathogens even after 20 washings.

Rangari et al.⁹² employed ultrasonic irradiation method to produce Ag/carbon nano tube hybrid nanoparticles, and studied their infusion into the nylon-6 polymer through extrusion method. They concluded that Ag/CNT-infused nylon 6 composite has excellent antimicrobial activity against S. aureus, S. pyogenes, E. coli and S. enteric than commercially available silver nanoparticles, pristine carbon nanotubes and neat nylon-6. Many efforts have also been made by textile and polymer chemists to incorporate silver nanoparticles into various electrospun polymer fibres such as polypropylene, cellulose acetate and nylon in order to enhance antimicrobial properties of these composite fibres. Recently Dong et al.⁹³ reported uniform assembling of various antibacterial nanoparticles including silver on the surface of electrospun nylon 6 nanofibres, and claimed that a large quantity of silver nanoparticles were assembled when the pH values of precursor solution was in the range 3–6. They proposed that citrate covered nanoparticles, containing carboxylic acid groups; interact via hydrogen bonding with the amine groups of nylon fibres, leading to more assembling of silver nanoparticles on the surface. Fig. 7 shows the brief mechanism of pH induced assembly of nanoparticles on the surface of nylon 6 nanofibres. The results showed that assembled silver nanoparticles impart strong antibacterial activity against E. coli to nylon 6 fibres.

Fig. 7 Mechanism of pH-induced assembly of metal nanoparticles on the surface of nylon 6 nanofibres by Dong et al.⁹³

Francis and co-workers,⁹⁴ likewise, fabricated silver nanoparticles using AgNO₃ as precursor in the presence of ethylene glycol as solvent and poly(N-vinylpyrrolidone) (PVP) as a capping agent for incorporation into nylon matrix. Such nanocomposite nylon–silver system resulted in improved mechanical strength as compared to pure nylon-6 fibre membrane. Park et al.⁹⁵ prepared nylon 6 nanofibers containing silver nanoparticles with the electrospinning method and reported their excellent antibacterial activity against S. aureus and K. pneumonia. Prior to electrospinning Montazer and Malekzadeh⁹⁶ added silver nitrate as a metal precursor and sodium borohydride as reducing agent to polymer solution for the formation of nylon nanofibers/AgNPs and reported their antimicrobial activity against E. coli and S. aureus. Shi et al.⁹⁷ synthesised silver nanoparticle-filled nylon 6 nanofibres with the electrospinning method. They found that the electrospinning solvent used in their method acts as a reducing agent and polymer matrix works as stabilizing agents for the in situ production of silver nanoparticle-filled nylon 6 hybrid nanofibres which yield strong antibacterial reduction percentage of 99.9% against B. cereus and almost 99.9999% against E. coli.

3.2 Polyester and polypropylene

Various deposition techniques for application of silver nanoparticles are currently in operation for antimicrobial and functional finishing of polyester based textile fabrics. Radetic et al.⁹⁸ employed corona treatment (electrical discharge at atmospheric pressure) to modify polyester and polyamide textile materials prior to deposition of silver nanoparticles for enhancing their binding interaction. It was observed that both of these fabrics when exposed to corona treatment prior to nano silver deposition had high laundering durable antibacterial activity. Gorensek et al.⁹⁹ found that silver nanoparticles deposited on raw polyester fabrics previously modified by Ar/N₂ (50%:50%) plasma results in increased dyeability and enhancement in antimicrobial activity of treated fabrics against P. aeruginosa, E. coli and S. faecalis. More research work on polyester has been done by Jiang et al.¹⁰⁰ to introduce multifunctional properties such as ultraviolet radiation protection, hydrophobicity and excellent antibacterial properties to polyester fabrics by the deposition of silver nanoparticles with sputtering method. Likewise, in order to obtain wash permanent antibacterial activity Malhtig and Textor¹⁰¹ studied the application of sol–gel coatings onto polyamide by treating polyamide fabrics with modified silica sols containing silver components. The treated polyamide fabrics displayed excellent antibacterial activity against E. coli which was maintained even up to 40 washing cycles. Textor et al.¹⁰² modified polyamide surface with glutaraldehyde before the treatment with Tollens' reagent. They observed that their method produced yellow/brownish color in addition to excellent and durable antimicrobial activity on polyamide fabrics. Xu et al.¹⁰³ used dopamine-modified polyester formed by the sequential dipping of polyester fabrics in aqueous solution of dopamine for in situ generation of silver nanoparticles, and discovered that the dopamine–polyester loaded silver nano particles results in durable antibacterial activity. Interestingly, likewise, silver bromine nanoparticles have been deposited on polyester by Moosavi et al.¹⁰⁴ under UV irradiation by sequential dipping of polyester in alternate potassium bromide and AgNO₃ solutions. IIic et al.¹⁰⁵ modified polyester fabric surface by low temperature radio frequency plasma prior to application of colloidal silver nanoparticles to enhance their binding efficiency for effective and durable antibacterial effects. It was shown that plasma treated and nano silver loaded fabrics had good antibacterial activity against S. aureus and E. coli. In another research work carried out by Ali et al.,¹⁰⁶ silver nanoparticles were loaded with chitosan nanoparticles formed by ionic gelation method for antibacterial finishing of polyester fabrics. Tripolyphosaphate was used as a crosslinking agent and it was found that the antibacterial activity of finished polyester against Gram positive S. aureus gets enhanced due to the synergetic effect of silver and chitosan nanoparticles. Table 1 summarizes some of the important works on antimicrobial finishing of synthetic textile materials using silver nanoparticles.

Table 1 AgNPs application on different synthetic textile materials

Authors	Substrate	Deposition method	Microorganism tested	Ref.
Sadu et al.	Polyester	Dip coating method	E. coli, S. epidermidis	2
Dubas et al.	Polyamide	Layer-by-layer method	S. aureus	87
Dastjerdi et al.	Polyester	Embedding of Ag NPs in crosslinkable polysiloxane layer	S. aureus, K. pneumonia	107
Radetic et al.	Polyester and polyamide	Pretreatment with corona	S. aureus and E. coli	98
Perelshtein et al.	Nylon, cotton and polyester	Ultrasound irradiation	S. aureus and E. coli	33
Majumdar et al.	Polyester	Blended polyester–silver nanocomposite with polyester	S. aureus	108
Ali et al.	Polyester	Ionic gelation method	S. aureus	104
Jeong et al.	Polypropylene	Dipping–pad–dry method	S. aureus, K. pneumonia and E. coli	109
Yeo et al.	Polypropylene	Conjugate spinning	S. aureus, K. pneumonia	110

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Polypropylene in view of its low cost and other superior properties including mechanical characteristics and chemical inertness than other synthetic fibres, finds extensive applications in textile products.¹¹¹ A great deal of research has indicated that introduction of functional agents during the spinning of polypropylene or through deposition of nanomaterials at the time of finishing endows antimicrobial properties in polypropylene fibres for hygienic and medical applications.¹¹² The application of silver nanoparticles for antibacterial finishing of polypropylene fabrics has become an interesting subject in the textile and polymer sciences. Yeo and Jeong¹¹⁰ have reported the formation of polypropylene (PP)/Ag nanocomposite fibres using conjugate spinning method. They observed that the sheath core–shell fibres produced in their method with PP/silver in shell and PP in core section and opposite had permanent antimicrobial activity against S. aureus and K. pneumonia. Jeong et al.¹⁰⁷ in a subsequent research investigation applied different types of nano-sized silver colloids fabricated through a normal dipping–pad–dry method on PE/PP nonwovens to study the antibacterial activity of finished fabric against S. aureus, K. pneumoniae, and E. coli. They found that nano-sized silver colloids dispersed in ethanol and ethanol-based silver/sulphur composite had more efficient and durable antibacterial activity than that of silver nanoparticles dispersed in water media. Ultraviolet grafting method was employed by Li et al.¹¹³ to modify polypropylene with N-isopropylacrylamide (NIPAAm) and N-(3-aminopropyl) methacrylamide hydrochloride (APMA) prior to loading of AgNPs synthesised by carboxylic D-α-tocopheryl polyethylene glycol 1000 succinate. Modified polypropylene was found to exhibit antioxidative and excellent antibacterial properties. Likewise, Goli et al.¹¹⁴ modified polypropylene nonwoven surface by denatured lysozyme proteins prior to application of AgNPs using protein surface modification technique in order to produce antibacterial activity in polypropylene. It was shown that lysozyme protein treatment introduces amino acid functional groups into the structure of PP for more deposition of silver nanoparticles resulting in more efficient activity against E. coli. An introduction of amino functional groups by dipping PP with varying fractional protein coverage in Ag (2.0) nano particle sols at pH 7.4 clearly enhanced its binding efficiency with charged silver nanoparticles through electrostatic interactions. Color appearing over the PP surface as well as SEM analysis as shown in Fig. 8 also confirmed the deposition of silver nanoparticles.

Fig. 8 Control and protein modified PP nonwovens coated with silver nanoparticles. The sample dimensions are ∼3 × 3 cm² (top). Respective SEM images of the corresponding PP nonwovens after treatment with silver nanoparticles (Ag(2.0)NP). Reprinted with permission from ref. 114 Copy right 2013 American Chemical Society.

4. Release of silver nanoparticles from textile materials

Despite the widespread use of silver nanoparticles in textile materials, little research work has been done so far on their release into waters. The content and form of AgNPs in the textile goods and their release into environment is a hot topic in scientific community. Although, toxicological studies on silver nanoparticles in relation to their potential human health risks has been very limited, however many researchers have reported that silver nanoparticles may induce adverse reactions or toxicity to humans.¹¹⁵ The toxicity imposed by silver depends upon the form in which it is released during washing. Release of silver during washing from nine textile substrates was investigated by Geranio et al.¹¹⁶ The authors found that little dissolution of silver nanoparticles occurred under relevant to washing conditions (pH 10); however surfactants and bleaching agents such as hydrogen peroxide or peracetic acid had significant effect on leaching of silver into the environment. The effect of bleach on the release of silver was also reported by Impellitteri et al.¹¹⁷ They found that in the presence of bleach Ag was transformed to AgCl. Benn and Westerhoff¹¹⁸ have studied the release of Ag from six types of commercially available nanosilver treated socks into distilled water. They have shown that large fraction of silver (10–500 nm) in the form of colloidal and ionic form was released in waste waters. Benn et al.¹¹⁹ studied different commercial products including textile shirt, a medical mask and cloth functionalized with silver nanoparticles and investigated the release of Ag from these substrates into wastewater. They observed that up to 45 μg Ag g per product was released into wastewater and size fractions of the released nano silver were both larger and smaller than 100 nm. Farkas et al.¹²⁰ investigated the release of silver in the effluent from a commercially available silver nanowashing machine. Single particle ICP-MS, ion selective electrode measurements and filtration techniques pointed out the presence of nanosilver (size 10 nm) in the discharged effluent. The released or leeched silver nanoparticles reduced the abundance of natural bacterial community. To quantify the release of silver from silver textiles, Lorentz et al.¹²¹ studied eight different textile substrates finished with silver to observe the extend of release during a washing and rinsing cycle. They followed the effect of washing with detergent, and rinsing of textile materials with tap water on silver leeching. It was shown that different forms of silver such as Ti/Si–AgCl nanocomposites, AgCl nanoparticles, large AgCl particles, nanosilver sulfide and metallic nano-Ag are present in washing solutions. Considering the growing role of nanomaterials in different textile modification applications, further efforts may be focused on the use of innovative cross-linking agents that can strongly bind silver nanoparticles in order to reduce their release into the environment. Bioremediation of silver nanoparticles from waste waters with bacteria is another approach which offers full potential to address the current challenges. For instance, there has been some evidence that the bacterium Chromobacterium violaceum may be efficiently utilized for the elimination of silver nanoparticles remaining in the wash water obtained after several washing of cotton textiles impregnated with silver nanoparticles. The silver nanoparticles recovered by this method can be reused once again to avoid any pollution to the environment.¹²² Therefore, exploitation of bacteria in bioremediation or use of other biological options to recover leeched out silver nanomaterials requires more investigations in order to open new windows for scientists working in this area of research.

5. Conclusions and future prospects

We have discussed in the previous sections the results related to the interaction of silver nanomaterials with different textile materials. According to literature, the work on silver nanoparticles for functional finishing of natural and synthetic textile materials is much more than any other currently available nanomaterials. Silver in nano form has broad spectrum of antimicrobial properties towards bacteria and fungi and has become more prevalent in textile materials as antimicrobial agents in response to the rising need for safer and effective antimicrobials. A lot of more facts about silver nanoparticles are also confirmed, the surface plasma resonance property of silver nanoparticles has recently been utilized to develop colorful shades on cotton, wool, and silk. The way from hypothesis to a full mechanism of action of the silver nanomaterials after application on textiles is a hot topic of debate among scientific community.

It is worth noting that silver nanoparticles compared to other metals are considered safer antimicrobial agents due to their low toxicity for humans. Up to now, most of the studies conducted in this realm have focussed on the ex situ and in situ methods for production of silver nanoparticles and their application as potent antimicrobial agents on different textile materials such as cotton, wool, silk, nylon, polyester and others. Scientists have also employed over the past few years some innovative crosslinking agents and pre-treatment technologies in order to enhance the binding efficiency of deposited silver nanoparticles for long lasting antimicrobial activity.¹²³ Despite a plenty of research devoted to the application of silver nanoparticles for functional finishing, there are only few studies that have demonstrated the effects of nano silver content released in waste water bath, or its contact or effects on humans and environment.^116,121,124 The washed out silver nanoparticles from different textile substrates may have lethal effects on both flora and fauna. To address this issue, more and more research should be carried out to highlight the side effects of silver nanoparticles on human health and ecology. This will lead to a better understanding and facilitate use of silver nanomaterials on a commercial large scale as novel colorants and antimicrobial agents for application onto different textile materials.

Acknowledgements

The author Shahid-ul-Islam is highly grateful to University Grants Commission, Government of India, for financial support provided through BSR Research Fellowship in Science for Meritorious Students.

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