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Theranostics 2020, Vol. 10, Issue 20 Electrochemical Method Electrochemical method can form an electric potential in the electrolyte and reduce Ag+ to Ag0 [80]. The nucleation and growth of AgNPs occur almost simultaneously under the external electric field. Electrochemical method can synthesize AgNPs with different sizes by adjusting the current density. Besides, electrode types, electrolytes, and solvents are also important in the synthesis of size-controlled AgNPs. In the synthesis process, increased precursor concentration and enhanced current intensity, as well as prolonged implementation time, will obtain more AgNPs with smaller size [81]. To obtain dispersed and stable AgNPs, stabilizers and capping agents are optional additives. The steric hindrance formed by these additives will prevent the aggregation of AgNPs. The electrochemical method has the advantages of easy reaction control, mild reaction conditions, and less environmental pollution. Microwave-assisted Method Microwave-assisted method refers to rapid heating the silver precursor by microwave irradiation, which may promote the generation of nuclei in site [82]. Several factors may influence the microwave-assisted synthesis of AgNPs, including the concentration of precursor and the type of stabilizer,powerinputandirradiationtimeofthe microwave, dielectric constant, refractive index of the medium and chirality of reducing agents [55]. Water and alcohol are ideal media for microwave heating stabilizer because of their high dielectric losses [83]. For example, polar molecules such as H2O attempt to orient the electric field in the microwave. When dipolar molecules attempt to reorient relative to an alternating electric field, they lose energy in the form of heat which may contribute to the reduction of Ag+. Microwave-assisted method has the advantages of high energy conversion efficiency, time-saving, cleanliness, and convenience, most importantly, it can be used to obtain large-scale production of high dispersive AgNPs. Sonochemical Method Sonochemical method refers to the cavitation effect generated by ultrasonic irradiation, which produces a local hot spot and promotes the synthesis ofAgNPs[84].Theinstantaneoushighpressureand microjet generated by ultrasonic irradiation can uniformly mix the solution and generate bubbles, which may suddenly collapse when the bubbles grow. The adiabatic compression of the gas phase in the bubble creates a local hot spot, which accelerates the contact of Ag+ with the reducing agent and rapidly reduces it to AgNPs. Ultrasound prevents the 9003 agglomeration of nanoparticles in the aqueous solution to decrease the size of AgNPs. Besides the high temperature, other factors such as pressure, pH, high-speed microjet, and high cooling rate may also contribute to the synthesis process. In summary, the sonochemical method is a simple, economical, and environment-friendly technique for preparing colloidal silver nanoparticles. Biological Method In recent decades, a variety of microorganism- and plant-mediated biological syntheses of AgNPs are developed. The microorganisms can evolve metal tolerance genes and metal bioconcentration capability to survive in an extreme silver-rich environment [105, 106]. These adaptive evolutionary mechanisms include altering and decreasing the cytotoxicity of metal and resulting formation of AgNPs. AgNPs can be regarded as “by-product” of the resistance mechanism of microorganisms against free Ag+. Plant mediated synthesis can reduce Ag+ to Ag0 using functional groups such as O-H and =C-H in organic components and their derivatives contained in the extract of plant parts [107]. Commonly used plant parts include bark, peel, callus, leaves, flower, fruit, stem, seed, and rhizome. In the process of biosynthesis, various biological components act as reducing agents, such as exopolysaccharide, peptides, nitrate reductase, reducing cofactors, c-type cytochromes, separated from microorganisms, and starch, cellulose, chitin, dextran, alginates, separated from plants. However, the organic components in the biosynthesis of AgNPs require to be further studied due to their complex interaction with AgNPs and the diversity of plants. Compared with physical or chemical methods, biological method can be carried out at normal temperature and pressure and avoid the use of toxic or hazardous additives. In this part, we will introduce several microbial and plant synthesis approaches of AgNPs, as well as the mechanisms involved in these processes. Bacteria-Mediated Synthesis Since Tanja Klaus et al. firstly reported the phenomenon of aggregation of AgNPs in Pseudomonas stutzeri AG259 in 1999 [105], series of bacteria, both Gram-negative and Gram-positive, are been screened for the synthesis of AgNPs (Table 3). The property of bacteria to survive in an extreme silver-rich environmentmightcontributetotheaccumulationof AgNPs [105, 108]. Depending upon the location of the nanoparticles distribution, AgNPs may be synthesized intracellularly or extracellularly using biomass, supernatant, cell-free extracts, and derived components of the bacteria. Among these two modes, extracellular method is advantageous over http://www.thno.orgPDF Image | Silver nanoparticles Synthesis medical applications safety
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