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There have been significant advancements in the application of nanotechnology in various fields such as electronics, medicine, cosmetics, packaging optics, and mining. The conventional processes for the production of nanoparticles often affect the environment adversely, and chemicals used also have harmful effects on human health. This has created a constant demand for novel methods for the synthesis of nanoparticles and to study its applications.
This article discusses the recent developments of the green synthesis of nanoparticles (for example, quantum dots (QDs), graphene, carbon nanotubes (CNTs), metal nanoparticles), which is an exciting area of research in nanoscience and technology.
To protect the environment and combat the harmful effects of existing methods, a group of scientists has envisioned “green technology” as the answer to the problems. They believe that promoting green technology will have a less negative impact on the environment and would subsequently reduce the consumption of energy and other toxic chemicals required in the case of conventional processes.
The eco-friendly approach is rapidly gaining popularity, and many scientists are conducting extensive research to come up with novel, simplified methods utilizing plants and microbes as the chief reducing agents. Cyanobacteria and microalgae have also been used for the production of metallic nanoparticles.
Green Nanotechnology
Green nanotechnology is a technology that has been developed by exploiting the concepts of green chemistry and green engineering. This technology focusses on the reduction of the use of energy by maximum usage of renewable sources and minimal utilization of other expensive materials.
In green nanotechnology, the use of phytoformulations and microbial biomolecules for the biosynthesis of various nanomaterials and nanoproducts has significantly contributed to the development of accessible, safe, and ecofriendly methods.
Scientists have reported that in the synthesis of gold nanoparticles using plant extracts, the size, shape, and surface properties of the nanoparticles could be controlled. Other nanoparticles, such as copper and copper oxide, titanium dioxide, and zinc oxide, have also been synthesized from several plant extracts.
Essential Factors for the Green Synthesis of Nanoparticles
Microorganisms employ intracellular or extracellular pathways for the biosynthesis of nanoparticles. Plant extracts contain a plethora of natural compounds of industrial and pharmaceutical benefits, such as polyphenols, aldehydes, flavones, ketones, carboxylic acids, amides, terpenoids, and ascorbic acids, that initiate the production of nanoparticles by reducing metal salts to metal nanoparticles. Other factors on which the green synthesis of nanoparticles or nanoproducts depend on are solvent systems, temperature, pressure, and pH conditions.
Many of the microorganisms and plant extracts not only facilitate synthesis but also provide stability to the nanoparticles. Among the established green methods of synthesis for biogenic nanoparticles, for the large-scale production of nanoparticles, the use of plant extracts is mostly preferred over bacteria and/or fungi.
Therefore, the main aim of green nanotechnology is the development of techniques with the following objectives:
- Prevention and minimization of waste
- Reduction of derivatives or pollution
- Utilization of safer or non-toxic chemicals (solvent/auxiliaries)
- Maximum utilization of renewable resources
In the green synthesis of nanoparticles, water is an ideal and the most suitable solvent system for bio-synthesis processes. Water is also the safest, cheapest and most commonly accessible solvent on earth.
Scientists have been successful in the production of gold and silver nanoparticles at room temperature in the presence of gallic acid molecules, using water as its solvent system.
Biological Components for the Synthesis of Nanoparticles
Bacteria
Several prokaryotic bacteria and actinomycetes are utilized for the synthesis of nanoparticles as they possess the ability to reduce metal ions. Some of the common bacteria that aids in the synthesis of nanoparticles are Lactobacillus casei, Bacillus cereus, Escherichia coli, Aeromonas sp, and Pseudomonas sp.
Fungi and Yeast
Many fungal species, for example, Fusarium sp, Aspergillus sp, are popularly utilized in the synthesis of metal/metal oxide nanoparticles. The possible mechanism for the formation of the metallic nanoparticles is an enzymatic reduction (reductase) in the cell wall or inside the fungal cell. An efficient fungus can produce larger amounts of nanoparticles compared to bacteria. The fungi-mediated biosynthesis of nanoparticles results in the production of monodispersed nanoparticles with prominent morphologies. Similarly, Saccharomyces cerevisiae can bio-synthesize gold and silver nanoparticles.
Plants
Several commonly available plants such as Aloe barbadensis (aloe), Coriandrum sativum (Coriander), Citrus limon (Lemon), Brassica juncea (Mustard), Medicago sativa (alfalfa), Ocimum sanctum (Tulsi), and Cymbopogon flexuosus (lemongrass) have been widely used in the synthesis of nanoparticles, especially silver and gold nanoparticles.
In a recent Nature publication, Deverra tortuosa extract of the aerial parts acts as an efficient reducing and capping agent for zinc oxide nanoparticles.
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Assessment and Management of Risks Related to Green Synthesis of Nanoparticles
The assessment and management of risks associated with green synthesis nanomaterials on the health and safety of workers have been an urgent concern for the scientific community and government officials. Owing to the lack of toxicological data for a variety of nanomaterials, the process of risk assessment suffers greatly. More clarification over long-term and low-dose exposure of nanomaterials and other developmental chemicals on vital health effects (genotoxicity, pulmonary toxicity, or carcinogenicity) is required. Risk assessments help to generate computable predictions that could effectively manage the potential risks related to green nanotechnology.
Another important reason for the identification and assessment of the potential problems is the elimination or substitution of harmful elements with a non-hazard component. These measures would lead to the establishment of advanced green technology. Risk management also includes the use of personal protective equipment (PPE), respiratory and eye protection, gloves, and lab coats, to minimize harmful exposures.
Scientists believe that in the coming years, green nanotechnology will gain popularity and be extensively used commercially. The use of plant extracts for producing metallic nanoparticles is recognized as a simple, less energy-intensive, inexpensive, and eco-friendly process. The green process is specifically advantageous for making nanoparticles required for biomedical and therapeutic applications.
References and Further Reading
Selim, Y.A., et al. (2020) Green Synthesis of Zinc Oxide Nanoparticles Using Aqueous Extract of Deverra tortuosa and their Cytotoxic Activities. Science Report. 10, 3445 https://doi.org/10.1038/s41598-020-60541-1
Singh, J., et al. (2018) ‘Green’ synthesis of metals and their oxide nanoparticles: applications for environmental remediation. Journal of Nanobiotechnology. 16, 84. https://doi.org/10.1186/s12951-018-0408-4
Verma, A., et al. (2019) Green Nanotechnology: Advancement in Phytoformulation Research. Medicines. 6, 39, https://dx.doi.org/10.3390%2Fmedicines6010039
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