Microwave-assisted synthesis is a promising method for synthesis of silver NPs. Microwave heating is better than a conventional oil bath when it comes to consistently yielding nanostructures with smaller sizes, narrower size distributions, and a higher degree of crystallization (57). Microwave heating has shorter reaction times, reduced energy consumption, and better product yields which prevents the agglomeration of the particles formed (57). Moreover, other than the elimination of the oil bath, microwave-assisted synthesis, in conjunction with benign reaction media, can also drastically reduce chemical wastes and reaction times in several organic syntheses and chemical transformations (58).
It was reported that silver NPs could be synthesized by microwave-assisted synthesis method employing carboxymethyl cellulose sodium as reducing and stabilizing agent. The size was depended on concentration of sodium carboxymethyl cellulose and silver nitrate. The produced NPs were uniform and stable, and were stable at room temperature for 2 months without any visible changes (59). Production of silver NPs in the presence of Pt seeds, polyvinyl pyrrolidine and ethylene glycol was also reported (60).
Furthermore, starch has been employed as a template and reducing agent for synthesis of silver NPs with an average size of 12 nm, using microwave-assisted synthetic method. Starch functions as a template, preventing the aggregation of produced silver NPs (61). Microwaves in combination with polyol process were applied for synthesis of silver nanospheroids using ethylene glycol and poly N-vinylpyrrolidone as reducing and stabilizing agents, respectively (62). In a typical polyol process inorganic salt is reduced by the polyol (e.g., ethylene glycol which serves as both solvent and reducing agent) at a high temperature. Yin and coworkers (63) reported that large-scale and size-controlled silver NPs could be rapidly synthesized under microwave irradiation from an aqueous solution of silver nitrate and trisodium citrate in the presence of formaldehyde as a reducing agent. Size and size distribution of produced silver NPs are strongly dependent on the states of silver cations in the initial reaction solution. Silver NPs with different shapes can be synthesized by microwave irradiation of a silver nitrate-ethylene-glycol-H2[PtCl6]-poly(vinylpyrrolidone) solution within 3 min (64). Moreover, the use of microwave irradiation to produce monodispersed silver NPs using basic amino acids (as reducing agents) and soluble starch (protecting agent) has been reported (65). Radiolysis of silver ions in ethylene glycol, in order to synthesize silver NPs, was also reported (66). Moreover, silver NPs supported on silica aero-gel were produced using gamma radiolysis. The produced silver clusters were stable in 2-9 pH range and started agglomeration at pH>9 (67). Oligochitosan as a stabilizer can be used in preparation of silver NPs by gamma radiation. It was reported that stable silver NPs (5-15 nm) were synthesized in a 1.8-9.0 pH range by this method (68). Silver NPs (4-5 nm) were also synthesized by γ-ray irradiation of acetic water solutions containing silver nitrate and chitosan (69).
Silver nanospheroids (1-4 nm) have been produced by γ-ray irradiation of silver solution in optically transparent inorganic mesoporous silica. Reduction of silver ions within the matrix is brought about by hydrated electrons and hydroalkyl radicals generated during radiolysis of 2-propanol solution. The produced NPs within the silica matrix were stable in the presence of oxygen for at least several months (70). Moreover, silver NPs have been produced by irradiating a solution, prepared by mixing silver nitrate and poly-vinyl-alcohol, with 6-MeV electrons (71). Pulse radiolysis technique has been applied to study reactions of inorganic and organic species in silver nanoparticle synthesis, to understand the factors controlling the shape and size of the NPs synthesized by a common reduction method using citrate ions (as reducing and stabilizing agents) (72), and to demonstrate the role of phenol derivatives in formation of silver NPs by the reduction of silver ions with dihydroxy benzene (73). Dihydroxy benzene could be used to reduce silver ions to synthesize stable silver NPs (with an average size of 30 nm) in air-saturated aqueous solutions (73).
Silver, gold, platinum, and gold-palladium nanostructures have been prepared using microwave-assisted synthetic approach. Morphologies and sizes of NPs can be controlled by altering some experimental parameters such as the concentration of metallic precursors, surfactant polymers, solvents, and temperature. Moreover, monodisperse silver NPs can be synthesized in large quantities using microwave-assisted chemistry method in an aqueous system. In this method, amino acids act as reducing agents and soluble starch acts as a protecting agent.
Not only silver, but silver doped lanthanum chromites can also be synthesized with microwave energy (74). Microwave energy and thermal reduction can be coupled to synthesize silver NPs that can be deposited on oxidized carbon paper electrodes. The silver NPs that are synthesized through this method maintain a uniform size between particles and are well-dispersed over the carbon paper substrate. The microwave-assisted synthesis of silver NPs is made possible by depositing the silver catalysts on carbon paper electrodes. This method can potentially be used in alkaline fuel cells because the synthesis occurs quickly, there is high activity, and the process is very simple (75).
Nanosized calcium deficient hydroxy-apatites can be used to generate nanosized calcium deficient hydroxyapatite with a silver substitution in three different concentrations by microwave-assisted synthesis. This study showed that controlling the parameters of the microwave process could influence the size of the crystals produced. It was shown that the microwave power had more of an impact on the size of the particles than the length of time of the treatment. The ensuing powder product could be used in the field of medicine and biomedical engineering to make grafts and coating metal implants in addition to work against bacterial infections without the use of antibiotics. This method can reduce medicinal costs and time of hospitalization (76).
Polymer based silver composites were produced using microwave energy on the basis of interfacial polymerization. A water/chloroform interface was used under microwave irradiation with no oxidizing agent. The produced silver NPs (about 20 nm in size) were spherical and well-dispersed (77). Silver nitrate provided silver ions for the thermal polymerization of pyrrole. The ions were converted to silver/polypyyrole nano-composites. Transmission electron microscopy (TEM) images proved that the particles were about 5−10 nm in size. The silver/polypyyrole had a thick film, which could sense ammonia, hydrogen sulfide, and carbon dioxide at 100, 250, and 350°C, respectively (78).
Microwave radiation and ethylene glycol can be used to synthesize silver powders from silver nitrate at temperatures of 100-200°C. It was reported that when polyvinyl pyrrolidone was used in the mixture of silver nitrate, the NPs ranged from 62 to 78 nm in diameter (79). Moreover, Fe-Ag bimetallic NPs could be synthesized by using microwave heating and an oil-soluble silver salt (80). The produced silver NPs were characterized through freeze-etching replication TEM which revealed the nanoparticle diameter and distribution. The produced silver NPs (30 nm) were spherical in shape (81).
Hydrolysis of alkoxysilanes along with the silver salt, in the presence of microwave irradiation, can produce silver/SiO2 composite sols, which displayed antimicrobial properties (82). Meng and coworkers (83) discussed the utilization of various water-based synthesis routes toward the shape-controlled synthesis of silver NPs and microstructures. Several one-pot methods employing commercial microwave ovens, inexpensive/low power ultrasound cleaners, or two-electrode electro-chemistry were described. Synthesis of silver nanostructures with various shapes in solution and their doping on unmodified silica and on/inside carbon spheres were investigated.
Microwave-assisted synthesis was used to prepare different kinds of nanosilver colloids. Silver nitrate was mixed with sodium citrate and then split into five groups. Each group was heated for varying durations of time at different temperatures. It was determined that the nanosilver colloids had a negatively charged surface when heated for a long period of time and a positively charged surface when heated for a short period of time (84). Moreover, silica-alumina can be used to synthesize silver NPs with precursors like Ag2O or AgNO3. The particles were as small as 3 nm in diameter or as big as 50 nm. They were not oxidized, and the particles were well spread out (85). In another study, nanosilver/polyvinylpyrrolidone composite materials were synthesized using the microwave approach. The produced NPs ranged from 15-25 nm and were evenly spread out in the polyvinylpyrrolidone matrix (86).
Polymers and polysaccharides
Silver NPs were prepared using water as an environmentally friendly solvent and polysaccharides as capping/reducing agents. For instance, synthesis of starch-silver NPs was carried out with starch (capping agent) and β-D-glucose (reducing agent) in a gently heated system (87).
The binding interactions between starch and produced silver NPs were weak and could be reversible at higher temperatures, allowing separation of the synthesized NPs. In dual polysaccharide function, silver NPs were synthesized by reduction of silver ions inside of nanoscopic starch templates (87,88). The extensive network of hydrogen bands in templates provided surface passivation or protection against nanoparticle aggregation. Green synthesis of silver NPs using negatively charged heparin (reducing/stabilizing agent and nucleation controller) was also reported by heating a solution of silver nitrate and heparin to 70 °C for about 8 h (89). TEM micrographs demonstrated an increase in particle size of silver NPs with increased concentrations of silver nitrate (substrate) and heparin. Moreover, changes in heparin concentration varied the morphology and size of silver NPs. The synthesized silver NPs were highly stable, and showed no signs of aggregation after two months (89). In another study, stable silver NPs (10-34 nm) were synthesized by autoclaving a solution of silver nitrate (substrate) and starch (capping/reducing agent) at 15 psi and 121 °C for 5 min (90). These NPs were stable in solution for three months at about 25 °C. Smaller silver NPs (≤ 10 nm) were synthesized by mixing two solutions of silver nitrate containing starch (capping agent), and NaOH solutions containing glucose (reducing agent) in a spinning disk reactor with a reaction time of less than 10 min (91).
Silver nitrate, glucose, sodium hydroxide, and starch can be used, respectively, to serve as precursor, reducing agent, accelerator, and stabilizer for the reduction synthesis of silver nitrate. Polyethylene glycol (reducing agent and stabilizing agent) was used to prepare stable monodisperse silver colloids (~10 nm) (92). Biodegradable starch worked as a stabilizing agent to synthesize silver NPs (5-20 nm). The analyses showed that the NPs were coated with a layer of starch (93).
Silver NPs (~13 ± 3 nm) can be synthesized using sulfated polysaccharide which can be obtained from marine red algae; Porphyra vietnamensis. It was reported that sulfate moiety from the polysaccharides was involved in silver nitrate reduction. Zeta potential measurements of −35.05 mV showed that the anionic polysaccharide had indeed capped the nanoparticles’ surfaces and contributed to the electrostatic stability. The NPs were stable at a very broad pH range, from 2 to 10, and electrolyte concentration of 10−2 M (94).
Polymers that have ion-exchangeable capacity can be used in many fields of science. The polymer often used contained phosphonic acid groups and had a low molecular weight. For instance, silver NPs were stabilized in the presence of an ion-exchange polymer. The surface morphology indicated that cubes and rectangular prism structures were formed (95). Co-polymers like cyclodextrin, grafted with poly acrylic acid, can be used to produce silver NPs where potassium per sulfate was used as the initiator. The co-polymer reduces and stabilizes the silver ions that yielded silver NPs. The concentration of the alkali, silver nitrate, the co-polymer, and the method of heating all played an important role in determining the size of the produced NPs (96).
Poly (methyl vinyl etherco- maleic anhydride) could be used as a reducing and stabilizing agent as well. The produced NPs were stable at room temperature for up to a month and had a 5-8 nm coat of poly (methyl vinyl etherco-maleic anhydride) surrounding them (97). It was reported that the NPs (10.2-13.7 nm) were face-centered cubic (FCC) structures, not aggregating, and very spherical in shape (98). Sarkar and colleagues (99) examined the synthesis of silver nanowires and NPs. Through a polypol process, with the help of a polymer, silver nanowires and NPs were formed. It was reported that the NPs were 60-200 nm in size and held prismatic and hexagonal shapes while the nanowires had diameters from 50 to 190 nm and lengths between 40 and 1000 μm. The reaction occurred at 210°C when ethylene glycol was used as the solvent. The different photoluminescence emission from the nano-clusters spread out through the methanol and the ethylene glycol at room temperature. The excitation wavelengths were measured between 300 and 414 nm (99). By changing the reducing and capping agents that are used to synthesize silver NPs, one can change the morphologies of the NPs, as well. The synthesis yielded NPs which were spherical in shape and around 15-43 nm in size after being heated at 70°C for 30 min; while at room temperature, the particles were only 8-24 nm. Sodium hydroxide reduced salt in ethylene glycol and cubes were formed upon some aggregation. By adding 5 wt % poly-vinylpyrrolidone to 1 wt % of starch solution (aq), mixtures of spherical and anisotropic structures were produced. The reaction took place at 70°C for 1 h (100).