Nanoparticles have gained considerable importance in the early 21st century due to the expansion of the nanotechnology industry, and much research has gone into finding cheap, convenient and safe methods of production. The biosynthesis of nanoparticles — production of nanoparticles by living organisms or material of biological origin — is one route that shows a lot of promise. There are a number of types of biosynthesis that can be employed — for example, nanoparticles may be synthesized using living bacteria or fungi, or using plant extracts. These techniques may provide advantages over more traditional methods of synthesizing nanoparticles because they are environmentally friendly, can take place around room temperature or lower, and require little intervention or input of energy. The organisms involved are generally easily cultured in simple organic media, are a renewable resource, and can usually simply be left to do their work.
It has long been known that various organisms could synthesize inorganic particles, including silica and calcium carbonate, or chalk. Many microorganisms are able to reduce metal ions to metal. Some bacteria can produce magnetic material by the reduction of iron compounds, incorporating magnetic nanoparticles into bodies known as magnetosomes within their cells. Interest in these microbial activities has led to the development of technologies designed to enable the biosynthesis of nanoparticles.
Silver and gold nanoparticles are of particular interest, as they have a wide range of possible applications, and the main focus of research into the biosynthesis of nanoparticles has been on these metals. Although the metals in their more familiar forms are not very reactive, they are — like many substances — much more reactive in nanoparticle form. This is largely due to the much higher surface area to volume ratio. Silver and gold nanoparticles can be used as catalysts, antibacterial agents, drug delivery systems, anti-cancer treatments and in the monitoring of various biochemicals.
A number of types of bacteria have been successfully employed in the biosynthesis of nanoparticles. This can take place both intracellularly — inside living cells — and extracellularly — outside the cells. One strain of the readily available bacterium Escherichia coli has been found to produce intracellular and extracellular silver nanoparticles when a solution of silver nitrate (AgNO3) is added to its growth medium. A number of other bacteria, including cyanobacteria, can also produce silver nanoparticles from silver nitrate. It is thought that the bacteria use the nitrate anion (NO3-) as a source of nitrogen, leaving metallic silver.
Gold nanoparticles have been synthesized by bacteria from water-soluble gold-chlorine compounds known as chloroaurates, which feature an AuCl4- anion. A number of different bacteria have been used successfully for this purpose and nanoparticles can be produced inside and outside the bacterial cells. In some cases, the shape of the gold nanoparticles produced can be controlled by adjusting the pH of the medium.
Fungi and flowering plants have also been used experimentally to synthesize nanoparticles. Preparations from several species of Aspergillus and other molds, as well as at least one species of edible mushroom have been found to produce extracellular nanoparticles of both silver and gold. Extracts from a number of flowering plants, including Aloe vera and Pelargonium graveolens, a type of geranium, have been observed to form silver and gold nanoparticles on mixing with suitable soluble compounds of these metals.