When we talk about sustainability in chemistry, one thing we have to think about is how the chemicals we use in our experiments may affect the environment. The traditional production of nanoparticles frequently involves the use of toxic materials such as harsh solvents and surfactants to synthesize a diverse array of nanoparticles that range from metals through metal oxides.¹ A big concern is that disposal of these toxic materials in the environment may directly affect all organisms living in it, and subsequently change the ecological equilibrium of a particular ecosystem.

At this point you may wonder if there are any sustainable ways to produce nanoparticles. I have some good news: a new way to prepare nanoparticles has slowly made its way into nanotechnology, and it uses plants!

This new eco-friendly approach is an example of “green chemistry.” This term has been applied in many areas of chemistry for quite a while, and recently scientists have started looking for ways to decrease the use of toxic ingredients specifically in nanoparticle synthesis.

In a recent major advancement, scientists have figured out how to synthesize nanoparticles using plants instead of some of the toxic chemicals that had been required before. How did they think of this idea? Long ago, scientists realized that plants were able to accumulate metals in their tissues and organs far from their roots,² removing the metals (including pollutants) from the soil. Bioremediation is a technique in which the natural functions of plants are used to extract or recover metals from contaminated soil. Later, the metals are extracted from plants through sintering or smelting. In the sintering process, plants containing metals are treated with heat, and this forms a compacted metal solid mass without melting the metal. In smelting, plants containing metals are treated with extreme heat that melts the metals until they form a solid mass.

The big moment for green chemistry in nanotechnology came when scientists discovered that plants not only accumulate metals, but that they store the metals in the form of nanoscale particles. One of the first reports on the use of green chemistry to synthesize silver nanoparticles employed alfalfa plants.³ For nine days alfalfa plants were treated with silver nitrate as a source of silver ions. Even though the silver nitrate did not include nanoparticles, analysis of the plant tissues revealed that silver nanoparticles ranging from 2 to 20 nm in diameter had extensively accumulated in the plants’ roots and shoots.

Since the alfalfa experiment, different strategies have been developed in the synthesis of nanoparticles using plants. Some of the strategies in green nanotechnology involve treating plants with a metal salt (e.g. silver nitrate) or metal oxide (e.g. titanium oxide), and subsequently scientists extract the nanoparticles by drying the plant material. Another way is to use an aqueous plant extract to directly synthesize nanoparticles. Aqueous plant extracts are water-based solutions that contain the one or more biologically active ingredients from a specific plant. The use of plant extracts to synthesize nanoparticles is probably the fast approach. For example, green tea extract added to aqueous solutions of gold and silver salts has been shown to produce gold and silver nanoparticles.⁴

Another way plants can be useful in nanotechnology is controlling the growth of the metals in a nanoscale dimension. Molecules that regulate growth in this way are called reducing reagents. Acidity, temperature, and other parameters also affect nanoparticle growth. Proponents of green chemistry suggest that plants can provide a huge number of natural reducing agents that may reasonably produce nanoparticles from metal salts and metal oxides.¹ Among these naturally occurring molecules are terpenoids, polyphenols, sugars, alkaloids, and proteins.⁵ I recommend reading the Sustainable Nano post “Fungus Amongus – the Nanoparticle Producers,” which very clearly explains how nanoparticles are synthesized using natural materials.

There is still a long way to go to convert the traditional synthetic methods employed in nanotechnology to green chemistry. Most of the results so far have been obtained at small scales. However, the quality of the “green” nanoparticles is comparable to those produced by the traditional method. As an example, silver nanoparticles synthesized after using the extract of the tridax daisy have the same strong antimicrobial activity against some microorganisms as the silver nanoparticles obtained by traditional methods.⁶ Reports about the use of plants (or parts of plants such as leaves, roots, or fruits) to make nanoparticles also demonstrate very reliable results so far.

It is super cool that scientists have harnessed the power to convert plants into biological reactors that are able to create customized nanoparticles. Although the use of naturally occurring molecules from plants has opened a very interesting and active area of research, it is still a work in progress. Perhaps someday we will manufacture all of our nanoparticles using plants, but there is still a lot of work to do to mitigate nanotechnology’s impact on the environment.

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REFERENCES (most need subscriptions for full access)

1. Kharissova, O., Rasika Dias, H., Kharisov, B., Olvera Pérez, B., & Jiménez Pérez, V. The greener synthesis of nanoparticles. Trends in Biotechnology, 2013. 31(4): p. 240-248. doi: 10.1016/j.tibtech.2013.01.003
2. Sheoran, V., Sheoran, A., & Poonia, P. Phytomining of gold: A review. Journal of Geochemical Exploration, 2013. 128(0): p. 42-50. doi: 10.1016/j.gexplo.2013.01.008
3. Gardea-Torresdey, J., Gomez, E. Peralta-Videa, J., Parsons, J., Troiani, H., & Jose-Yacaman, M. Alfalfa Sprouts:  A Natural Source for the Synthesis of Silver Nanoparticles. Langmuir, 2003. 19(4): p. 1357-1361. doi: 10.1021/la020835i
4. Vilchis-Nestor, A., Sánchez-Mendieta, V., Camacho-López, M., Gómez-Espinosa, R., Camacho-López, M., & Arenas-Alatorre, J. Solventless synthesis and optical properties of Au and Ag nanoparticles using Camellia sinensis extract. Materials Letters, 2008. 62(17–18): p. 3103-3105. doi: 10.1016/j.matlet.2008.01.138
5. Makarov, V., Love, A., Sinitsyna, O., Makarova, S., Yaminsky, I., Taliansky, M., & Kalinina N. “Green” Nanotechnologies: Synthesis of Metal Nanoparticles Using Plants. Acta Naturae, 2014. 6(1): p. 35-44. Available open access: PMC3999464
6. Dhanalakshmi, T. & Rajendran, S. Synthesis of silver nanoparticles using Tridax procumbens and its antimicrobial activity. Archives of Applied Science Research, 2012. 4(3): p. 1289-1293. Available open access: Scholars Research Library.