The antibacterial activity of silver has been known since ancient times. The king of Persia, during war, used silver containers for boiled water storage [1,2] . The toxicity of silver at low concentrations to human cells is quite lower than to bacteria. Silver is still used as a disinfectant for swimming pools.
Nano silver as an antimicrobial agent
In case of nano silver, the antimicrobial agent contains nano silver particles that get rid of bacteria, repressing them in warm and humid conditions. The silver largely depends on killing the bacteria by denaturation or oxidisation, and this is the reason the bacteria cannot get immune to the silver. The durability of the nano silver antimicrobial finish on textile lasts longer than other antimicrobial finishes. This finish leads to improved utility and imparts special properties to the textile, thus leading to decreased final cost.The textile treated with nano silver agent will have antibacterial, antifungal and antiviral properties. The nano silver finish is effectively used in clothing for special purposes like the sports garments, which can be ridden with bacteria and fungi after getting drenched in sweat. With nano silver antimicrobial finish, the garments smell fresh and deter the growth of bacteria responsible for the unpleasant odour. This technology is used in military clothing as well as in several developed countries. The durability of the antimicrobial finish on silver-based textile lasts longer than other antimicrobial finishes .With development of nano latest generation in silver treatment by a Canada based company, the textile manufacturing companies can add superior odour to the clothing.
This new generation of silver treatment also provides resistance to textiles against fibre degradation, as it uses metallic silver, which is a dominant antimicrobial material. The anti-static properties of nano silver also maintain the skin’s biological balance during sports activities. Following these properties, the nano silver finishes are also being used in shoe soles. The medical industry is also greatly relying on the nano silver finishes these days. The hygiene products are also treated with the nano silver finishes. Baby products, undergarments, regular and special clothing, and footwear often go through this treatment.
As a consequence, nano silver is being consistently used in several products in the US market like athletic clothing, cosmetics, bed and bath linens, electric shavers, baby bottles, stuffed animals, keyboards, paints, and food containers. It’s also used in hospital equipment including catheters, stents, bandages and wound dressings, as well as on surfaces including wheelchair seats and door handles. In Southeast Asia, the usage is more common and, in Hong Kong, nano silver is sprayed in subways and touted on Korean toothpaste tubes.
The nano silver antimicrobial finish is especially useful in case of natural fibres like silk, cotton etc as the natural fibres are more at risk to bacterial attack than synthetic fibres. The porous and hydrophilic nature of the natural fibres retains water and oxygen along with nutrients, thus encouraging rapid growth of microorganisms.
More developed countries like the US, Korea, Australia etc are more inclined to use nano silver finishing on regular garments, as awareness regarding cleanliness and hygiene among the consumers is more in comparison to the consumers of developing countries. Nevertheless, the nano silver finishes have also raised several eyebrows, as several experts believe that the antimicrobial finish wipes off after regular wash. It is not a surprise that the nano silver finish carries a grievance, and reacts like any other antimicrobial finish would to oust the bacteria, fungi and other infection causing microbes. The reason, as per a Danish research and a Swiss research, is that there are several textile manufacturers in Denmark and Switzerland who falsely claim that the garment has been treated with nano silver antimicrobial finish. There are also some companies asserting that the clothing has not been treated with nano silver, but they use nano silver finishing in their garments.
Silver (Ag) and Ag-based compounds have become the most widely represented and studied inorganic antimicrobial agents for use in textiles 4-6 . Ag is a leaching antimicrobial agent, whose efficiency depends directly on the concentration of Ag cations (Ag + ) and/or nanoparticles (Ag NPs) releases from the textile fibres in which they reside. After being released into the surrounding environment, these species act as a poison to a wide range of microorganisms, such as gram-negative and gram-positive bacteria, fungi, molds, viruses, yeasts, and algae. In addition to its antimicrobial properties, Ag is not harmful to human health, especially in low concentrations 7 . Ag is suggested for use in various areas, such as medicine, pharmaceutics, agriculture, food packaging and water disinfection. The use of Ag-based antimicrobials has increased significantly with the development of modern methods for the preparation of Ag NPs that possess a large surface area, resulting in greater activity at lower metal concentrations. The synthesis of such Ag NPs has enabled new applications of Ag not only in the field of textiles but also in the realms of medicine, pharmacy, biology, biochemistry and food technology. Due to its chemical stability at high temperatures and under UV illumination, Ag can be used in textile manufacturing as an additive introduced during the conventional spinning or electro-spinning processes used to make fibres 8 . It can also be utilized as a finishing agent in the chemical finishing of fibres, yarns, fabrics, and nonwovens 
Antimicrobial activity of silver
The antimicrobial activity of Ag has been attributed to the controlled release of both Ag + and Ag NPs, which are nano-scale clusters of metallic silver atoms, Ago(10-12) . The release of Ag + occurs during the dissociation of Ag salts dissolved in water. Ag + also occurs as a result of the oxidation of Ag NPs in the presence of water and oxygen. For controlled release mechanisms, Ag + and Ag NPs should be leached from the solid surface to preserve the level of biocidal activity at the concentrations at which the metal is lethal to microbes. According to the literature, the mechanisms of the antimicrobial activity of Ag + and Ag NPs are very similar to each other. Both Ag + and Ag NPs can participate in intermolecular interactions with the cell membrane of bacteria. Furthermore, Ag particles smaller than 10 nm have been reported to penetrate into the interior of microorganism cells, where they bind to
the thiol groups of enzymes and nucleic acids (10-12) .
Silver compounds for antimicrobial textiles as colloidal silver
Colloidal Ag is a stable dispersion consisting of highly dispersed Ag NPs. It is prepared using the bottom-up approach, which in general means that atoms or molecules are assembled into
their molecular structure in the nanometer range. Two different routes for the incorporation of colloidal Ag into textile fibres are most commonly employed: the application of previously
prepared colloidal Ag and the in situ synthesis of Ag NPs within textile substrates. Furthermore, commercial colloidal Ag products can be applied directly to textile fibres  .
Preparation of colloidal Ag
Different chemical methods for preparing colloidal Ag have been developed, among which chemical reduction is most commonly used. Chemical reduction involves dissolving Ag salt in a solvent and subsequently reducing with a suitable reducing agent  . During the reduction step, stabilizers are used to stabilize the Ag NPs in the solution, control their growth through agglomeration and prevent their precipitation.
Stabilizers are an important additive in the preparation of colloidal Ag NPs because they prevent aggregation of Ag NPs and control the size and shape of the Ag NPs, which directly influence the antimicrobial activity of the NPs. These additives react with the surface of the Ag NPs, as well as with Ag + and create a barrier that prevents intermolecular interactions. Because of their propensity to interact with metals, compounds with carboxylic, amino, hydroxyl and thiol groups are often used as stabilizers. Among these compounds, polyvinylpyrrolidone (PVP) is most commonly used in experiments reported in the literature  .
Antimicrobial activity of silver on textile fibres
The antimicrobial activity of Ag functionalized fibres is directly affected by the concentration, as well as the particle size and shape of Ag present on the fibres. The concentration of Ag adsorbed onto the fibres increases with increasing concentration of Ag in the finishing solution, irrespective of the application method. The efficiency of Ag NPs is believed to strongly increase
with a decrease in particle size because the specific surface area of a particle increases as size decreases  . This effect allows small particles to interact with microorganisms while simultaneously enabling a significant increase in the concentration of Ag + released. Accordingly, to produce an equivalent level of antimicrobial activity, a lower concentration of smaller Ag NPs is needed compared to that of larger Ag NPs. Furthermore, the particles with the three-sided prism shape exhibit better antimicrobial activity than particles with a spherical or rod shape  .
Influence of fibre properties and binding mode of Ag
The amount of Ag adsorbed directly depends on the chemical structure, as well as on the morphological and topological properties of the fibres in question  . The characteristics of the
fibres directly influence the mode of Ag binding and its adhesion ability, the absorption capacity of the fibre and the moisture content necessary for Ag release. As the amorphous chemical structure of the fibres and the amount of functional groups available as binding sites for Ag + increase, both the uptake of Ag solution and the concentration of the absorbed Ag increase. In general, hydrophilic fibres can absorb a larger amount of Ag particles than hydro- phobic fibres. Ag can penetrate into the amorphous regions of fibres through the fibre pores, cavities and inter-fibre spaces, where they physically bind to the substrate  . Particles have been observed to easily bind to the rougher surface of natural fibres than to the smooth surface of synthetic fibres. In other words, the increase in the surface area of the fibres results in an increase in the amount of deposited Ag NPs.
Antimicrobial activity is also directly influenced by the mode through which Ag binds to the functional groups of the fibres [20 ]. Specifically, the continuous release of Ag + and Ag o is only possible if Ag is bonded to the fibres through physical sorption. Chemical binding of Ag to the fibres may significantly reduce its effectiveness as an antimicrobial agent. This phenomenon has been observed in the case of wool fibres, where the chemical binding of Ag to the thiol groups on the wool and the subsequent formation of Ag mercaptides hindered the release of Ag + from the fibres into the surroundings,as reflected by the insufficient antibacterial activity. Accordingly, an increase in the concentration of absorbed Ag was required to achieve a biocidal effect. The application of Ag colloid in the presence of 1,2,3,4- butanetetracarboxylic acid in acidic solution improves the adsorption of Ag NPs, contributes to former physical entrapment in the fibres and controls the release of Ag NPs from the covered surfaces  .
Influence of the fibre pre-treatment
The amount of Ag adsorbed can be enhanced by the application of various types of pre-treatment to the textile substrates. Among these processes, plasma etching is used to induce fibre surface activation through the introduction of new functional groups and an
increase of the surface area, resulting in an enhancement of the loading of Ag NPs from colloids. Simultaneously, the plasma treatment contributes to higher uniformity of the Ag NP distribution on the fibre surface. Another important pre-treatment process includes the
grafting of different acids on the fibre surface, such as tannic acid and ethylene-diamine tetraacetic dianhydride, butane tetracarboxylic acid, mercapto- acetic acid, acrylic and polyacrylic acid, deoxyribonucleic acid, and glycidyl methacrylate iminodiacetic acid  .
The role of the acids as the grafting agents on the fibre surface is to increase the negative charge through the creation of carbonyl and carboxyl acid groups on the surface, which
enhances the loading efficiency of Ag + via electrostatic interactions, as well as to act as capping and stabilizing agents for Ag NPs. In addition, through the application of mercapto acetic acid to the cellulose fibres, thiol modified fibres are created on which Ag NPs can be attached by a strong Ag-S bond. Furthermore, in the case of cellulose fibres, alkali pretreatment is used to activate surface hydroxyl groups to enhance the deposition efficiency of Ag NPs. Mercerization, as well as the application of reactive polymer β-cyclodextrin as the host compound for Ag NPs, is applied to the cellulose macromolecules in the pre-treatment process. It has also been found that cationization of cellulose fibres with 3-chloro-2-hydroxypropyl methyl ammonium chloride increases the roughness and decreases the zeta potential of the fibre surface, which in turn increases the degree of Ag NP adsorption.
To achieve a more tenacious interaction between Ag and the fibre structure, to prolong the of Ag + and Ag o and to improve the washing fastness of the coating, amino functional silver nanoparticles were prepared with amino-terminated hyper branched polymer and grafted on the oxidized cotton fabric subsequently. Furthermore, Ag has been embedded into polymer matrices created by hydrogels, chitosan, poly-acrylonitrile, polyvinyl pyridine, and silicon alkoxides of different structures, i.e. tetra-ethoxysilane, amino-functionalized siloxane, water glass, mercapto-functionalized triethoxysilane and 3-glycidyloxypropyl-trimethoxysilane. As most silicon alkoxides are soluble in alcohols, which require a closed circuit for application, studies have focused on the development of precursors that can be mixed into water. The results of these studies have shown that the presence of a polymer matrix enhances the adsorption capacity of the fibres and consequently increases the concentration of bound Ag, contributes to the uniformity of the particle distribution, prevents particle agglomeration and extends the release time of Ag + and Ag o . However, studies have also shown that the antimicrobial
efficiency and the washing fastness of the coating are almost eradicated. Namely, if the polymer matrix contains in its structure functional groups that allow chemical binding of the silver particles, it enables a stronger binding interaction of Ag with the polymer matrix, which simultaneously results in a reduced concentration of released particles. In this manner, only the
biostatic activity of silver can be obtained due to a slower release of the silver cations in the presence of moisture. To eliminate this shortcoming, Ag was embedded into a polymer matrix created by active bio-barrier-forming antimicrobial agents  .
Wash fastness of Ag modified fibres
The binding of Ag through physical interactions does not allow for permanent textile chemical modification, which is an important disadvantage of the Ag coating processes. Physically adsorbed Ag NPs gradually release from the fibres during repeated washings  . Accordingly, the antimicrobial efficiency of Ag NPs remaining on the fibres after the repeated washings could be ensured by a sufficiently high concentration of Ag NPs applied to the fibres. The results also show that the release of the Ag NPs from the thiol-modified cellulose fibres during the washing is very low, indicating that the covalent linkage between Ag and cellulose has much higher washing durability compared to Ag modified cellulose without covalent linkage. The post treatment of Ag-modified fibres with cross-linkable polysiloxane also importantly improved
the antimicrobial durability.
In this work, the cotton fabric has been made antibacterial by treating it with different concentrations of silver nano particle colloidal solution by pad-dry method. The preliminary studies indicated that a minimum 12 gm/l of silver colloidal solution is required to get durable antibacterial effect which can withstand 20 washes and above. Therefore, the entire experiments carried out in this work were by applying 12 gm/l of silver colloidal solution on bleached cotton fabric.
Materials and methods
The fabric used for experimentation was 100% bleached cotton plain weave fabric having weight 145 gm/m 2 .
The antibacterial chemical used was a colloidal solution made up of silver nanoparticles (Ag NPs).
The percentage pickup of 100% cotton bleached fabric, at various pressure readings (2, 3, 4 & 5 bar) and at constant speed of 2.62 m/min, was measured using two bowls vertical padding mangle, made by Mathis, Switzerland. The fabric was weighed before and after padding in water. The difference in the weight of wet and dry fabric expressed as percentage of weight of
dry fabric gave the percentage pickup as per following formula:
[(W – D )/D ]x 100 = % Pickup,
where W is the weight of wet fabric and D is the weight of dry fabric.
Application of antibacterial
100% cotton bleached fabric was padded with 12 gm/l silver colloidal solution at pH 5 to 6 maintained using 0.5 gm/l acetic acid. The following padding conditions were used to ensure 80% pickup:
Mangle pressure: 4.5 bar
Speed: 2.62 m/min
% Pickup = 80%
After padding, the treated fabric samples were dried in SDL Mini dryer at 135 o C for 3 min.
The quantitative evaluation of antibacterial activity in four samples treated with 12 gm/l silver colloidal solution was carried out by the JIS L 1902:2008 test method.
Permanency of antibacterial effect
In order to measure the permanency of antibacterial, the cotton fabric treated with 12 gm/l, silver colloidal solution was subjected to washing cycles 10 and 20 times respectively, employing DIN EN ISO 6330 standard for domestic laundry using launder-o-meter made by Mesdan Lab, Italy under following laundry conditions:
Test organisms: Staphylococcus aureus and Klebseilla Pneumoniae
Dilution medium used: Tryptic soya broth (TSB)
Eluting medium used: Sterile normal saline (0.85% NaCl)
Method of sterilization (pre-cleaning): Autoclave, 121 o C, 15 lbs, 15 min
Size of swatch per sample: 0.4 gm
Untreated control (incubation time): 18 hr contact time
Treated samples (incubation time) : 18 hr contact time
Washing temperature: 40 C
Washing time: 15 min
Detergent used: EEC non phosphate without optical brightening agent
Detergent concentration: 1.25 gm/l
Drying temperature: 70 o C
Time: 10 min.
After each washing cycle, the fabric was rinsed thoroughly with tap water, squeezed and dried at 70 o C for 10 min. After completing 10 and 20 washing cycles, the antibacterial
efficacy test was done using JIS L 1902:2008 test method.
Particle size of silver in colloidal solution
The particle size of silver nanoclusters was measured using Malvern Zetasizer Nano ZS instrument available in SMITA lab in the Textile Department at Indian Institute of Technology, New Delhi. The instrument works on the principle of Dynamic Light Scattering (DLS) which measures particle size in the submicron region. DLS measures Brownian motion which is the random movement of particles due to the bombardment of solvent molecules.
Zeta potential of silver colloidal solution
The zeta potential of silver colloidal solution was measured using Malvern Zetasizer Nano ZS instrument available in SMITA lab in Textile Department at Indian Institute of Technology, New
Delhi. Zeta potential determines the colloidal solution stability. If zeta potential becomes zero, the colloidal particles will attract each other because of Van der Waals forces and
agglomeration of particles into bigger particle size will take place. It will result in settling down of bigger size particles.
Silver content in treated cotton fabric after 10 & 20 washes
The silver content in treated cotton fabric; treated but washed 10 times; and treated but washed 20 times was measured using Environmental Scanning Electron Microscope model FEI Quanta 200F with Oxford – EDX system IE 250 X Max 80 instrument available in SMITA lab in Textile Department at Indian Institute of Technology, New Delhi. It is a type of electron microscope that images the sample surface by scanning it with a high energy beam of electrons. The EDX consists of the latest 80 mm 2 silicon drift detector (SDD) which enables detection of elements under high resolution.
Article was originally published in Asian Dyer Magazine (Issue Jun-July 2020)
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