Silver colloidal silver is a well-known antibacterial and antifungal agent, used by mankind for centuries. However, its electrochemical production requires precise control of many factors such as current and temperature. Historically, those trying to produce colloidal silver failed due to their lack of these requirements, with the result that many unstable ionic solutions were produced.
Several methods of synthesis exist which can be modified to produce a variety of sizes and shapes. Synthetic protocols are also able to functionalize the nanoparticles with different materials, such as silica, which can be useful for controlling their size-specific properties.
One of the best ways to produce silver colloidal silver is by bacterial or fungal synthesis. When bacteria or fungi are added to solution, they produce protein biomass that can reduce silver ions in the water that then contributes to the formation of the nanoparticles.
The reduction of silver ions is facilitated by the presence of electron donating residues, such as tryptophan and tyrosine. These can be synthesized by a range of fungi and bacteria.
Another method of synthesis involves the use of reducing sugars such as glucose. These sugars are highly soluble in aqueous solution and have free aldehyde or ketone groups that allow them to act as reducing agents, reducing silver ions to silver atoms.
The interaction of the reducing sugars with the silver ions in the solution produces a variety of other compounds, including gluconate which can be used as an effective capping agent. The use of a wide array of these compounds allows for the production of a variety of silver nanoparticles, each with a specific geometry and surface coatings.