uniformly sized nanoparticles is being done. ion implantation ion implantation has been used to create silver nanoparticles embedded in glass, polyurethane, silicone, polyethylene, and poly(methyl methacrylate). particles are embedded in the substrate by means of bombardment at high accelerating voltages. at a fixed current density of the ion beam up to a certain value, the size of the embedded silver nanoparticles has been found to be monodisperse within the population, after which only an increase in the ion concentration is observed. a further increase in the ion beam dose has been found to reduce both the nanoparticle size and density in the target substrate, whereas an ion beam operating at a high accelerating voltage with a gradually increasing current density has been found to result in a gradual increase in the nanoparticle size. there are a few competing mechanisms which may result in the decrease in nanoparticle size; destruction of nps upon collision, sputtering of the sample surface, particle fusion upon heating and dissociation. the formation of embedded nanoparticles is complex, and all of the controlling parameters and factors have not yet been investigated. computer simulation is still difficult as it involves processes of diffusion and clustering, however it can be broken down into a few different sub-processes such as implantation, diffusion, and growth. upon implantation, silver ions will reach different depths within the substrate which approaches a gaussian distribution with the mean centered at x depth. high temperature conditions during the initial stages of implantation will increase the impurity diffusion in the substrate and as a result limit the impinging ion saturation, which is required for nanoparticle nucleation. both the implant temperature and ion beam current density are crucial to control in order to obtain a monodisperse nanoparticle size and depth distribution. a low current density may be used to counter the thermal agitation from the ion beam and a buildup of surface charge. after implantation on the surface, the beam currents may be raised as the surface conductivity will increase. the rate at which impurities diffuse drops quickly after the formation of the nanoparticles, which act as a mobile ion trap. this suggests that the beginning of the implantation process is critical for control of the spacing and depth of the resulting nanoparticles, as well as control of the substrate temperature and ion beam density. the presence and nature of these particles can be analyzed using numerous spectroscopy and microscopy instruments. nanoparticles synthesized in the substrate exhibit surface plasmon resonances as evidenced by characteristic absorption bands; these features undergo spectral shifts depending on the nanoparticle size and surface asperities, however the optical properties also strongly depend on the substrate material of the composite. biological synthesis the biological synthesis of nanoparticles has provided a means for improved techniques compared to the traditional methods that call for the use of harmful reducing agents like sodium borohydride. many of these methods could improve their environmental footprint by replacing these relatively strong reducing agents. the commonly used biological methods are using plant or fruit extracts, fungi, and even animal parts like insect wing extract. the problems with the chemical production of silver nanoparticles is usually involves high cost and the longevity of the particles is short lived due to aggregation. the harshness of standard chemical methods has sparked the use of using biological organisms to reduce silver ions in solution into colloidal nanoparticles. in addition, precise control over shape and size is vital during nanoparticle synthesis since the nps therapeutic properties are intimately dependent on such factors. hence, the primary focus of research in biogenic synthesis is in developing methods that consistently reproduce nps with precise properties. fungi and bacteria bacterial and fungal synthesis of nanoparticles is practical because bacteria and fungi are easy to handle and can be modified genetically with ease. this provides a means to develop biomolecules that can synthesize agnps of varying shapes and sizes in high yield, which is at the forefront of current challenges in nanoparticle synthesis. fungal strains such as verticillium and bacterial strains such as klebsiella pneumoniae can be used in the synthesis of silver nanoparticles. when the fungus/bacteria is added to solution, protein biomass is released into the solution. electron donating residues such as tryptophan and tyrosine reduce silver ions in solution contributed by silver nitrate. these methods have been found to effectively create stable monodisperse nanoparticles without the use of harmful reducing agents. a method has been found of reducing silver ions by the introduction of the fungus fusarium oxysporum. the nanoparticles formed in this method have a size range between 5 and 15 nm and consist of silver hydrosol. the reduction of the silver nanoparticles is thought to come from an enzymatic process and silver nanoparticles produced are extremely stable due to interactions with proteins that are excreted by the fungi. bacterium found in silver mines, pseudomonas stutzeri ag259, were able to construct silver particles in the shapes of triangles and hexagons. the size of these nanoparticles had a large range in size and some of them reached sizes larger than the usual nanoscale with a size of 200 nm. the silver nanoparticles were found in the organic matrix of the bacteria. lactic acid producing bacteria have been used to produce silver nanoparticles. the bacteria lactobacillus spp., pediococcus pentosaceus, enteroccus faeciumi, and lactococcus garvieae have been found to be able to reduce silver ions into silver nanoparticles. the production of the nanoparticles takes place in the cell from the interactions between the silver ions and the organic compounds of the cell. it was found that the bacterium lactobacillus fermentum created the smallest silver nanoparticles with an average size of 11.2 nm. it was also found that this bacterium produced the nanoparticles with the smallest size distribution and the nanoparticles were found mostly on the outside of the cells. it was also found that there was an increase in the ph increased the rate of which the nanoparticles were produced and the amount of particles produced. plants the reduction of silver ions into silver nanoparticles has also been achieved using geranium leaves. it has been found that adding geranium leaf extract to silver nitrate solutions causes their silver ions to be quickly reduced and that the nanoparticles produced are particularly stable. the silver nanoparticles produced in solution had a size range between 16 and 40 nm. in another study different plant leaf extracts were used to reduce silver ions. it was found that out of camellia sinensis (green tea), pine, persimmon, ginko, magnolia, and platanus that the magnolia leaf extract was the best at creating silver nanoparticles. this method created particles with a disperse size range of 15 to 500 nm, but it was also found that the particle size could be controlled by varying the reaction temperature. the speed at which the ions were reduced by the magnolia leaf extract was comparable to those of using chemicals to reduce. the use of plants, microbes, and fungi in the production of silver nanoparticles is leading the way to more environmentally sound production of silver nanoparticles. a green method is available for synthesizing silver nanoparticles using amaranthus gangeticus linn leaf extract. products and functionalization synthetic protocols for silver nanoparticle production can be modified to produce silver nanoparticles with non-spherical geometries and also to functionalize nanoparticles with different materials, such as silica. creating silver nanoparticles of different shapes and surface coatings allows for greater control over their size-specific properties. anisotropic structures silver nanoparticles can be synthesized in a variety of non-spherical (anisotropic) shapes. because silver, like other noble metals, exhibits a size and shape dependent optical effect known as localized surface plasmon resonance (lspr) at the nanoscale, the ability to synthesize ag nanoparticles in different shapes vastly increases the ability to tune their optical behavior. for example, the wavelength at which lspr occurs for a nanoparticle of one morphology (e.g. a sphere) will be different if that sphere is changed into a different shape. this shape dependence allows a silver nanoparticle to experience optical enhancement at a range of different wavelengths, even by keeping the size relatively constant, just by changing its shape. this aspect can be exploited in synthesis to promote change in shape of nanoparticles through light interaction. the applications of this shape-exploited expansion of optical behavior range from developing more sensitive biosensors to increasing the longevity of textiles. triangular nanoprisms triangular-shaped nanoparticles are a canonical type of anisotropic morphology studied for both gold and silver. though many different techniques for silver nanoprism synthesis exist, several methods employ a seed-mediated approach, which involves first synthesizing small (3-5 nm diameter) silver nanoparticles that offer a template for shape-directed growth into triangular nanostructures. the silver seeds are synthesized by mixing silver nitrate and sodium citrate in aqueous solution and then rapidly adding sodium borohydride. additional silver nitrate is added to the seed solution at low temperature, and the prisms are grown by slowly reducing the excess silver nitrate using ascorbic acid. with the seed-mediated approach to silver nanoprism synthesis, selectivity of one shape over another can in part be controlled by the capping ligand. using essentially the same procedure above but changing citrate to poly (vinyl pyrrolidone) (pvp) yields cube and rod-shaped nanostructures instead of triangular nanoprisms. in addition to the seed mediated technique, silver nanoprisms can also be synthesized using a photo-mediated approach, in which preexisting spherical silver nanoparticles are transformed into triangular nanoprisms simply by exposing the reaction mixture to high intensities of light. nanocubes silver nanocubes can be synthesized using ethylene glycol as a reducing agent and pvp as a capping agent, in a polyol synthesis reaction (vide supra). a typical synthesis using these reagents involves adding fresh silver nitrate and pvp to a solution of ethylene glycol heated at 140 °c. this procedure can actually be modified to produce another anisotropic silver nanostructure, nanowires, by just allowing the silver nitrate solution to age before using it in the synthesis. by allowing the silver nitrate solution to age, the initial nanostructure formed during the synthesis is slightly different than that obtained with fresh silver nitrate, which influences the growth process, and therefore, the morphology of the final product. coating with silica electron micrograph of core-shell nanoparticles, which comprise dark silver cores and light silica shells in this method, polyvinylpyrrolidone (pvp) is dissolved in water by sonication and mixed with silver colloid particles. active stirring ensures the pvp has adsorbed to the nanoparticle surface. centrifuging separates the pvp coated nanoparticles which are then transferred to a solution of ethanol to be centrifuged further and placed in a solution of ammonia, ethanol and si(oet4) (tes). stirring for twelve hours results in the silica shell being formed consisting of a surrounding layer of silicon oxide with an ether linkage available to add functionality. varying the amount of tes allows for different thicknesses of shells formed. this technique is popular due to the ability to add a variety of functionality to the exposed silica surface. metrology a number of reference materials are available for silver nanoparticles. nist rm 8017 contains 75 nm silver nanoparticles embedded in a cake of the polymer polyvinylpyrrolidone to stabilize them against oxidation for a long shelf life. they have reference values for mean particle size using dynamic light scattering, ultra-small-angle x-ray scattering, atomic force microscopy, and transmission electron microscopy; and size distribution reference values for the latter two methods. the bam-n001 certified reference material contains silver nanoparticles with a specified size distribution with a number-weighted median size of 12.6 nm measured by small-angle x-ray scattering and transmission electron microscopy. use catalysis using silver nanoparticles for catalysis has been gaining attention in recent years. although the most common applications are for medicinal or antibacterial purposes, silver nanoparticles have been demonstrated to show catalytic redox properties for dyes, benzene, and carbon monoxide. other untested compounds may use silver nanoparticles for catalysis, but the field is not fully explored. note: this paragraph is a general description of nanoparticle properties for catalysis; it is not exclusive to silver nanoparticles. the size of a nanoparticle greatly determines the properties that it exhibits due to various quantum effects. additionally, the chemical environment of the nanoparticle plays a large role on the catalytic properties. with this in mind, it is important to note that heterogeneous catalysis takes place by adsorption of the reactant species to