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Nanotechnology is a rapidly developing science and a number of methods are now available for producing nanoparticles. However, some of these methods employ high energy requirements, low material conversions and the use of hazardous chemicals. Hence, there is a growing need to develop eco-friendly nanoparticle synthesis methods.
Biosynthetic methods such as those that employ plant extracts or microorganisms have emerged as viable alternatives to physical and chemical synthetic procedures.
Nanoparticle Synthesis Using Fungi
Fungi are ideal for synthesizing metal nanoparticles compared with other organisms due to the proteins on their cell surface.
The most common method of intracellular biosynthesis is the production of metal nanoparticles by reducing reductase enzymes present in the cell wall of the fungal cells.
Synthesis of gold nanoparticles has been investigated using Aspergillus, Neurospora, Fusarium and Verticillium species.
Fungus Trichoderma viride has been used to synthesize polydispersed silver nanoparticles of sizes ranging from 5 to 40 nm at near room temperatures. An important finding in this research was that the anti-bacterial activities of substances produced by these fungi were significantly enhanced with the production of silver nanoparticles.
Nanoparticle Synthesis Using Bacteria
Actinomycetes and prokaryotic bacteria have been widely used for the synthesis of metal nanoparticles. One of the main reasons for using bacteria is their relative ease of manipulation.
Researchers have demonstrated an extracellular synthesis of well-dispersed gold nanoparticles using prokaryotic microorganisms such as Thermomonospora and Brevibacterium species. It was also reported that TiO2 nanoparticles can also be synthesized using Sachharomyces and Lactobacillus species at room temperature.
Recently researchers carried out vivo biosynthesis of metal nanoparticles using recombinant Escherichia coli (E. coli). The co-expression of metal binding proteins in recombinant E. coli cells resulted in the generation of fine-ordered nanoparticles through an enhanced attachment of diverse metal elements.
Nanoparticle Synthesis Using Yeasts
Yeasts are eukaryotic microorganisms that belong to the family of fungi, with more than 1500 species.
Several studies indicate the efficiency of yeasts in nanoparticle preparation. For instance, silver-tolerant yeast strain MKY3 was used to demonstrate the extra-cellular formation of silver nanoparticles. Yarrowia lipolytica was reported to be an effective reducing agent to produce gold nanoparticles and nanoplates by changing the concentration of chloroauric acid.
Nanoparticle Synthesis Using Algae
There are only few reports of algae being used for the biosynthesis of metallic nanoparticles. Spirulina platensis has been reported in the preparation of gold and silver nanoparticles. In addition, Tetraselmis suecica, marine microalgae was also used to manufacture gold nanoparticles using chlorauric solution.
Researchers have demonstrated the synthesis of single-crystalline hexagonal and triangular gold nanoparticles by treating aqueous chloroauric solution with the extract of Chlorella vulgaris at room temperature. This research revealed that proteins play a vital role in the reduction and capping of gold nanoparticles.
Nanoparticle Synthesis Using Plants
Plants have shown great potential in the detoxification and accumulation of heavy metals over the years. One of the first reports on effective synthesis of gold and silver nanoparticles used alfalfa plants grown in a target metal-salt-rich environment. Nanoparticle synthesis was achieved within the living plants through an uptake of gold and silver ions from solid media.
In another research project, biosynthesis of silver nanoparticles was carried out by treating Cycas leaf extract with silver nitrate solution.
In addition, silver nanoparticles with high antimicrobial properties were reported to be synthesized using commercially available plant powders, such as Citrus sinensis, Centella asiatic, Syzygium cumini and Solanum tricobatum.
Conclusion
Nanotechnology is currently one of the most significant research areas in material science. For years, scientists have continuously explored various biosynthetic methods to produce nanoparticles without releasing any hazardous or toxic substances.
The use of various organisms such as bacteria, fungi, yeasts, plants in the preparation of metal nanoparticles under mild reaction conditions has drawn attention from a wider scientific community due to its rapid, economical, eco-friendly, single-step protocol.
However, scientists feel that more research needs to be focused on tailoring the electronic, electrical and optical properties of nanoparticles synthesized using eco-friendly methods, by controlling their shape and size.
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