Silver nanoparticles (NPs) have been the subjects of researchers because of their unique properties (e.g., size and shape depending optical, antimicrobial, and electrical properties). A variety of preparation techniques have been reported for the synthesis of silver NPs; notable examples include, laser ablation, gamma irradiation, electron irradiation, chemical reduction, photochemical methods, microwave processing, and biological synthetic methods. This review presents an overview of silver nanoparticle preparation by physical, chemical, and biological synthesis. The aim of this review article is, therefore, to reflect on the current state and future prospects, especially the potentials and limitations of the above mentioned techniques for industries.
The most common approach for synthesis of silver NPs is chemical reduction by organic and inorganic reducing agents. In general, different reducing agents such as sodium citrate, ascorbate, sodium borohydride (NaBH4), elemental hydrogen, polyol process, Tollens reagent, N, N-dimethylformamide (DMF), and poly (ethylene glycol)-block copolymers are used for reduction of silver ions (Ag+) in aqueous or non-aqueous solutions. These reducing agents reduce Ag+ and lead to the formation of metallic silver (Ag0), which is followed by agglomeration into oligomeric clusters. These clusters eventually lead to the formation of metallic colloidal silver particles (,,). It is important to use protective agents to stabilize dispersive NPs during the course of metal nanoparticle preparation, and protect the NPs that can be absorbed on or bind onto nanoparticle surfaces, avoiding their agglomeration (). The presence of surfactants comprising functionalities (e.g., thiols, amines, acids, and alcohols) for interactions with particle surfaces can stabilize particle growth, and protect particles from sedimentation, agglomeration, or losing their surface properties.
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 (). Moreover, silver NPs have been produced by irradiating a solution, prepared by mixing silver nitrate and poly-vinyl-alcohol, with 6-MeV electrons (). 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) (), and to demonstrate the role of phenol derivatives in formation of silver NPs by the reduction of silver ions with dihydroxy benzene (). 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 ().
The properties (chemical, electronic, optical, etc.) of nanoparticles depend not only on their size, but also on their shape and arrangement. In general, solution phase synthesis methods offer greater flexibility and control of particle shape than is possible in gas phase methods. In solution, approaches including "seeding" with another material, templating with an existing structure, or modifying the energies of different crystal surfaces using mixtures of surfactants can lead to growth of anisotropic (non-spherical) nanocrystals. Different morphologies can often result from relatively subtle changes in the reaction conditions used. The "seeding" approach, in which a second material nucleates heterogenously on a pre-existing nanoparticle, can allow production of anisotropic nanoparticles, when the growing material eventually falls off the seed particle, or can lead to hybrid nanoparticles in which the "seed" particle and growing material remain joined, with a quasi-epitaxial interface between them. This can even be repeated to form three-component (ternary) hybrid nanoparticles. This allows us to combine different classes of inorganic materials (semiconductors, metals, oxides) each with tunable optical, electronic, magnetic, or chemical (catalytic) properties in unique ways. This has potential for producing new materials with applications ranging from catalysis to photovoltaics to nanomedicine.
Silver NPs (5-50 nm) could be synthesized extracellularly using Fusarium oxysporum, with no evidence of flocculation of the particles even a month after the reaction (). The long-term stability of the nanoparticle solution might be due to the stabilization of the silver particles by proteins. The morphology of NPs was highly variable, with generally spherical and occasionally triangular shapes observed in the micrographs. Silver NPs have been reported to interact strongly with proteins including cytochrome c (Cc). This protein could be self-assembled on citrate-reduced silver colloid surface (). Interestingly, adsorption of (Cc)-coated colloidal gold NPs onto aggregated colloidal Ag resulted Ag: Cc: Au nanoparticle conjugate (). In UV-vis spectra from the reaction mixture after 72 h, the presence of an absorption band at ca. 270 nm might be due to electronic excitations in tryptophan and tyrosine residues in the proteins. In F. oxysporum, the bioreduction of silver ions was attributed to an enzymatic process involving NADH-dependent reductase (). The exposure of silver ions to F. oxysporum, resulted in the release of nitrate reductase and subsequent formation of highly stable silver NPs in solution (). The secreted enzyme was found to be dependent on NADH cofactor. They mentioned high stability of NPs in solution was due to capping of particles by release of capping proteins by F. oxysporum. Stability of the capping protein was found to be pH dependent. At higher pH values (>12), the NPs in solution remained stable, while they aggregated at lower pH values () have demonstrated enzymatic synthesis of silver NPs with different chemical compositions, sizes and morphologies, using α-NADPH-dependent nitrate reductase purified from F. oxysporum and phytochelatin, in vitro. Silver ions were reduced in the presence of nitrate reductase, leading to formation of a stable silver hydrosol 10-25 nm in diameter and stabilized by the capping peptide. Use of a specific enzyme in vitro synthesis of NPs showed interesting advantages. This would eliminate the downstream processing required for the use of these NPs in homogeneous catalysis and other applications such as non-linear optics. The biggest advantage of this protocol based on purified enzyme was the development of a new approach for green synthesis of nanomaterials over a range of chemical compositions and shapes without possible aggregation. Korbekandi and colleagues () demonstrated the bioreductive synthesis of silver NPs using F. oxysporum. Previous researchers reported qualitative production of silver NPs by F. oxysporum, but they did not optimize the reaction mixture. In SEM micrographs, silver NPs were almost spherical, single (25-50 nm) or in aggregates (100 nm), attached to the surface of biomass. The reduction of metal ions and stabilization of the silver NPs was confirmed to occur by an enzymatic process. It seems that the first step involves trapping of the Ag+ ions by F. oxysporum cells. More details of the location of NPs production by this fungus were revealed, and the previous theories were corrected. In contrast with the previous studies, it is claimed that the nanoparticle production in F. oxysporum is intracellular by engulfing the NPs in vesicles, transporting, and excreting of them through exocytosis outside of the cells (). Ingle and coworkers () demonstrated the potential ability of Fusarium acuminatum Ell. and Ev. (USM-3793) cell extracts in biosynthesis of silver NPs. The NPs produced within 15-20 min and were spherical with a broad size distribution in the range of 5-40 nm with the average diameter of 13 nm. A nitrate-dependent reductase enzyme might act as the reducing agent. The white rot fungus, Phanerochaete chrysosporium, also reduced silver ions to form nano-silver particles (). The most dominant morphology was pyramidal shape, in different sizes, but hexagonal structures were also observed.
Silicon nanocrystals, like other semiconductor nanocrystals, exhibit size-dependent optical and electronic properties. Despite the fact that bulk silicon is a very poor light-emitter, silicon nanocrystals show efficient emission at wavelengths from the near-infrared to green as their size decreases from about 5 nm to 1.5 nm in diameter. As a result, they have tremendous potential for applications in bioimaging, biomedical assays, solid-state lighting, displays, and photovoltaics. Compared to other semiconductor nanocrystals, they offer potentially important advantages in terms of cost and, especially, low toxicity. However, they are much less amenable to solution phase synthesis than CdSe and other compound semiconductor nanocrystals. Thus, developing improved methods of preparing these nanocrystals and modifying their surfaces is an ongoing research challenge. We have developed a unique, three-step process, illustrated below, that allows us to prepare macroscopic quantities of photoluminescent silicon nanocrystals and modify their surface for subsequent processing and applications.
Pandey, S., Goswami, G.K. & Nanda, K.K., Green Synthesis of Biopolymer-Silver Nanoparticle Nanocomposite: An Optical Sensor for Ammonia Detection, Int. J. Biol. Macromol., 51(4), pp. 583-589, 2012.