Nanosilver particles (NSPs) generally present at 1 to 100 nm in size in at least one dimension.1–4 As particle size decreases, the surface area-to-volume ratio of NSPs increases dramatically, which leads to significant changes in their physical, chemical, and biological properties. NSPs have been among the most commonly used nano-materials in our health care system for hundreds of years. Recently, NSPs have become of intense interest in biomedical applications (Figure 1), because of their antibacterial, antifungal, antiviral, and anti-inflammatory activity.5,6
Different synthetic NSP routes lead to variable sizes, shapes, morphology, and even stability. Generally, these methods can be classified into three broad categories: physical, chemical, and biological (or green) synthesis.
Evaporation/condensation and laser ablation are the main physical techniques for deriving nanosilver from metal samples. The evaporation/condensation technique uses a furnace tube under atmospheric pressure to produce NSPs; however, conventional furnace tubes have several drawbacks, such as high energy consumption, and require a long time to achieve thermal stability. Jung et al used a small ceramic heater with a local heating area, thus the evaporated vapor could cool at a suitable rate and a high concentration of nanosilver could be obtained.14 Laser synthesis employs the laser ablation of metals in solution without chemical reagents, which leads to pure nanosilver colloids.15 The concentration and morphology of nanosilver are affected by laser fluence and the number of laser shots. Greater laser fluence and amount of time, lead to larger particle size and higher particle concentration.16 Recently, Tien et al reported a novel arc-discharge method of producing silver suspension in pure water without any surfactants or stabilizers.17In their research, silver wires were utilized as positive and negative electrodes and etched in pure water. During discharge, the surface layer of the silver wires was evaporated and condensed in the water, thus stable and well-dispersed NSPs of 20–30 nm in size were obtained.17
Chemical reduction is the most frequent method of nanosilver synthesis, and uses silver salt, reductants, and a stabilizer or capping agents as three main components to control NSP growth (Figure 2). Among these, silver nitrate is a silver salt that is often used for NSPs, due to its low cost and chemical stability compared to the other available silver salts.17 The reductants include borohydride,18 citrate,19ascorbate,20 and hydrogen gas.11
Borohydride is a strong reducing agent that can result in small particles with a faster reduction rate, because borohydride can also act as an NSP stabilizer and avoid aggregation of NSPs during its decomposition.11 It is hard to obtain high concentrations of NSPs because of their aggregative instability. Using a stabilizer in preparation is a common strategy. The stabilizers include surfactants and ligands or polymers that contain functional groups such as polyvinylpyrrolidone, poly(ethylene glycol), poly(methacrylic acid), poly(methyl methacrylate), and others. Furthermore, temperature-sensitive polymers such as poly(N-isopropylacrylamide) and collagen can also serve as stabilizers, and nanosilver capped by those chemicals allows for novel thermal switching applications.1
NSPs can also be synthesized in a two-phase water-organic system. This method produces uniform and controllable nanoparticles. In this system, metal precursor and reducing agent are separated in two phases, thus the rate of interaction can be controlled by the intensity of interphase transport between aqueous and oil phases; however, large amounts of surfactant and organic solvent may contaminate the surface of formed NSPs, and the removal of surfactant and organic solvent is also time-consuming and expensive.
Biosynthesis (green synthesis) of nanosilver has received extensive attention due to the growing need for environmentally friendly synthesis methods that use eco-friendly reducing and capping agents, such as protein;21 peptides;22 carbohydrate;23 various species of bacteria,24 fungi,25 and yeast;26 and algae and plants.27 For example, Naik et al synthesized NSPs of 60–150 nm in size using silver-binding peptides identified from a combinatorial phage-display peptide library. The peptides were placed in an aqueous solution of 0.1 mM silver nitrate for 24–48 hours at room temperature.21 Thomas et al developed an economical, fascicled, and in situ approach to prepare large-scale chitosan–nanosilver (400 nm) films using chitosan as a chelating and stabilizing agent; the films demonstrated excellent antibacterial action against Escherichia coli and Bacillus.28 Sintubin et al reviewed different biological synthesis methods using microorganisms or plants for nanosilver synthesis.26
In biological synthesis, as the reducing agents and stabilizers are molecules produced by protein, carbohydrate, bacteria, fungi, yeasts, algae, or plants, organic solvents and toxic reagents are avoided. The possible mechanism of biological synthesis includes enzymatic and non-enzymatic reduction (Figure 3). Nicotinamide adenine dinucleotide phosphate-dependent reductase can produce NSPs by enzymatic reduction; however, the enzymatic reduction rate is often slow (between 24 and 120 hours).29 The non-enzymatic reduction of silver is similar to chemical reduction, but the reducing and stabilizing agents are microorganisms or plants. Non-enzymatic reduction is usually fast, often completed within a few minutes, and can handle extreme parameters, such as high pH or high temperature, that accelerate the synthesis.24