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Theranostics 2020, Vol. 10, Issue 20 nanoparticles must be adequately considered in the application. Based on these two approaches, frequently used methods for synthesizing AgNPs, including physical, chemical and biological methods are discussed herein. Physical Method The physical synthesis of AgNPs involves mechanical processes and vapor-based processes. Energies are used to reduce particle size, including mechanical energy (ball milling method) [60], electrical energy (electrical arc-discharge method) [61], light energy (laser ablation method) [62], and thermal energy (physical vapor deposition) [6] (Table 1). During the ball milling progress, high-speed collisions between rigid balls, such as ceramics, flint pebbles, and stainless steels, can produce localized high pressures, which grind the metal into very fine powders [60]. The electrical arc-discharge method can obtain NPs via arc discharge device under direct current (DC) power [63]. The device uses the powder reagent layer as the anode and the electrodes are immersed in dielectric liquids such as hydrocarbons, liquid inert gas, and deionized water. Laser ablation method refers to the ablation of a metal plate by a high-power laser, the metal target absorbs the laser beam energy and photoions, followed by nucleation and growth of metal particles during the plasma plume cooling process and eventually synthesize NPs [62, 64]. Sputtering and evaporation are two processes in physical vapor deposition. Sputtering refers to bombarding a target coating material with a high-energy electrical charge to sputter off atom or molecule that can be deposited on the substrate. While evaporation refers to heating the coating material to the boiling point in a vacuum environment and evaporating, and the evaporated material rises in the vacuum chamber and condenses on the substrate. Although physical synthesis can produce AgNPs on a large scale, AgNPs may aggregate and form large-sized particles which will affect subsequent applications. In order to avoid the re-aggregation of Table 1. Synthesis of Silver Nanoparticles by Physical Methods 8999 AgNPs, some stabilizers are used to obtain stable colloids AgNPs. For example, polyvinyl pyrrolidone (PVP) may be used as both the electrolyte and stabilizer in the synthesis of AgNPs by laser ablation method [65]. Our team prepared Ångstrom silver particles, capped with fructose as stabilizer, can be stable for a long time [21]. In summary, the physical method can quickly produce NPs with uniform size distribution and high purity, but complex equipment and external energy are required. Ball Milling Method Mechanical ball milling technique is to put milling balls and metal materials with a specific mass ratio as well as gas (air or inert gas) in a container rotated at a high speed. The milling time, rotating speed and the atmospheric medium in the process of ball milling are playing essential roles in the morphology of metal materials. A suitable milling time is closely related to the production of particles with a satisfactory size. The smaller size of particles, the higher surface energy, therefore particles prefer to aggregate. The temperature of the powder in the ball milling process influences the diffusivity and phase of nanoparticles [60]. Generally speaking, a higher temperature of powder tends to synthesize intermetallic compounds, while lower temperature tends to obtain amorphous and nanocrystalline phases [52]. Electrical Arc-Discharge Method The electrical arc-discharge apparatus consists of DC power between two silver rods, which are immersed in dielectric liquids [61, 66]. During the process of arc discharge, the silver electrode is etched in the dielectric medium, and the surface of the silver electrode is vaporized because of the high temperature near the electrode. Subsequently, the silver vapor is condensed into AgNPs and suspended in the dielectric liquid. This apparatus can obtain pure AgNPs with a simple and low-cost device. Method Ball milling method Electrical arc-discharge method Laser ablation method Physical vapour condensation Silver precursor Silver powder Silver wire Silver wire Silver wire Silver wire Silver wire Silver plate Silver plate Silver plate Silver wire Stabilizer/Surfactant/Dispersant – – – – – – – PVP – Fructose Operating conditions Dry, under protective Ar gas atmosphere, below –160 ± 10°C Multi-walled carbon nanotubes-aqueous nanofluids, 15–40°C, DC power 25°C, current, voltage, deionized water DC arc-discharge system, 70°C, stirring DC arc-discharge system, room temp. DC arc-discharge system, deionized water, stirring Laser pulses, organic solvent Laser pulse, stirring Laser pulse, solution of chlorobenzene, stirring High voltage power, rapid cooling Size (nm) 4–8 About 100 – 72 19 20–30 4–10 20–50 25–40 19.2±3.8 Ång Shape Reference Spherical [85] Spherical [86] – [66] Spherical [63] Cubic [87] Spherical [61] Spherical [88] Spherical [65] Spherical [89] Spherical [21] http://www.thno.orgPDF Image | Silver nanoparticles Synthesis medical applications safety
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