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The growing popularity of nanotechnology around the world has led the global community to pressure researchers to develop synthesis techniques that are both economically advantageous and environmentally friendly. To this end, recent advancements in nanoparticle synthesis methods have looked to biological systems, such as microorganisms, plants, and animals as a source of metal nanoparticles.
Environmental Challenges by Nanoparticles
Green nanotechnology emerged as a way to improve the environmental safety of nanoparticles. Unfortunately, many researchers struggle to attain this goal as a result of the inherent limitations of most nano-designed formulations, such as poor degradability. While nanoparticles exhibit various advantages in technical capabilities when compared to their bulk materials, they are also associated with negative impacts when released into the environment during their synthesis and use. Furthermore, many of the physical and chemical techniques used to produce nanoparticles are highly expensive and require the use of toxic chemicals.
To address these challenges, recent advancements have led to the development of ‘green synthesis,’ which is the synthesis of nanomaterials from natural sources including phytochemicals, animal-derived biomaterials and microbial organisms. Although green synthesis is still in its early stages of development, researchers are hopeful that further investigation into this area will lead to widespread global acceptance of this discipline.
Plant Sources of Metallic Nanoparticles
Several different plant species have been evaluated for their potential to be used for the green synthesis of metallic nanoparticles. For example, angiosperms are the most widely studied plant species that have been studied for this purpose. Angiosperms, which have been used throughout history for a variety of natural remedies for both human and animal diseases, are particularly attractive due to their safety when ingested.
Gold nanoparticles have been synthesized using various angiosperms including Azadirachta indica, Camellia sinesis, Lemongrass plant extract, Avena sativa, Cinnamomum camphora and Cintella asiatica. It is important to note that gold cations have large standard reduction potentials, thereby allowing plant molecules with lower ionization potentials to more easily reduce these particles. Future work in this area should aim to identify angiospermic plant species that exhibit a higher electron donating capacity, which will thereby expand the type of metallic nanoparticles they are capable of synthesizing.
Synthesizing Metallic Nanoparticles from Microbial Organisms
The microbial synthesis of nanoparticles can be achieved by simple microbial cultivation under cellular, biochemical and molecular mechanisms. Depending on whether the microbial organism is bacteria, fungi or viruses, intracellular or extracellular reproduction methods can be used to synthesize nanomaterials.
In terms of intracellular mechanisms, positive metal ions can be incorporated into the cell wall of microorganisms by interacting with the negative ions that naturally exist within the cell wall. Once the metal ions arrive at this location in the cell, enzymes will reduce the metal ions into nanoparticles that can eventually diffuse across the cell wall of the bacteria. Some microorganisms that have already demonstrated the successful synthesis of different metallic nanoparticles through intracellular methods include Lactobacillus, Aquaspirillum magnetotacticum and P. boryanum UTEX 485.
Animal-Derived Nanomaterial Sources
One of the biggest advantages associated with synthesizing nanoparticles from biological sources, particularly those within the animal world, is to improve tissue distribution of these materials once they are absorbed by the body. Currently, metallic nanoparticles are used for a wide variety of biomedical purposes including cancer treatment, catheters, dental material, medical devices and much more.
While the data on animal-derived synthesis of metallic nanoparticles is limited when compared to that which exists for plant and microbial sources, research has indicated that the silk fibroin synthesized by various insects and spiders shows exceptional promise in clinical applications. Silk fibroin, which is a natural semi-crystalline biopolymer, has already been used in medicine as a clinical suture and material for the tissue engineering of skin, bone and nerve tissue. A recent study has utilized silk fibroin to create crystalline nanoparticles that measured at a length of 100 nm, thereby indicating the potential use of this biological material for the production of nanoparticles.
Sources and Further Reading
Das, R. K., Pachapur, V. L., Lonappan, L., Naghdi, M., Pulicharla, R., Maiti, S., et al. (2017). Biological synthesis of metallic nanoparticles: plants, animals and microbial aspects. Nanotechnology for Environmental Engineering 2(18). DOI: 10.1007/s41204-017-0029-4.