Nanoparticle biosynthesis, using living organisms or biological material, offers cost-effective and environmentally friendly methods for nanoparticle synthesis. Bacteria, fungi, and plant extracts have been successfully employed to produce silver and gold nanoparticles with various applications, including catalysis, drug delivery, and monitoring of biochemicals.
Nanoparticles gained significant prominence in the early 21st century due to the expansion of the nanotechnology industry, and much research has been done to find cost-effective, convenient, and safe manufacturing methods. Nanoparticle biosynthesis – production of nanoparticles by living organisms or material of biological origin – is a path that shows much promise. There are numerous types of biosynthesis that can be employed: for example, nanoparticles can be synthesized using living bacteria or fungi or using plant extracts. These techniques can provide advantages over more traditional methods of nanoparticle synthesis because they are environmentally friendly, can occur at or below room temperature, and require little intervention or energy input. The organisms involved are usually easily grown in simple organic soil, are a renewable resource, and can usually just be left to do their job.
It has long been known that various organisms could synthesize inorganic particles, including silica and calcium carbonate or gypsum. Many microorganisms are capable of reducing metal ions to metal. Some bacteria can produce magnetic material by reducing iron compounds, by embedding magnetic nanoparticles in bodies known as magnetosomes within their cells. Interest in these microbial activities has led to the development of technologies designed to enable 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 nanoparticle biosynthesis research has been on these metals. While metals in their most familiar forms aren’t 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, anticancer treatments, and in the monitoring of various biochemicals.
Numerous types of bacteria have been successfully employed in nanoparticle biosynthesis. This can occur both intracellularly, inside living cells, and extracellularly, outside cells. A readily available strain of the 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 were synthesized by bacteria from water-soluble gold-chlorine compounds known as chloroaurates, which feature an AuCl4 anion. Different bacteria have been successfully used for this purpose and nanoparticles can be produced inside and outside bacterial cells. In some cases, the shape of the gold nanoparticles produced can be controlled by adjusting the pH of the medium.
Mushrooms and flowering plants have also been used experimentally to synthesize nanoparticles. Preparations of several species of Aspergillus and other molds, as well as at least one species of edible fungi, have been found to produce extracellular nanoparticles of 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 when mixed with suitable soluble compounds of these metals.
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