Towards the Green Synthesis of Gold Nanoparticles

Tiger Zhang
Thomas Jefferson High School for Science and Technology

This article was originally included in the 2016 print publication of the Teknos Science Journal

Imagine taking a gold coin and dividing it into a thousand little pieces. Now, take each piece and cut it into a thousand more pieces — and slice each minuscule piece into a thousand more pieces. The resulting particles, not pieces, are so small that scientists have coined a new term to describe them — nanoparticles. In fact, over 1000 gold nanoparticles could fit across the diameter of a human hair [6].

Gold nanoparticles are not only extremely small, but have unique properties not found in bulk gold. For example, while the gold in gold jewelry is remarkably unreactive, gold nanoparticles are excellent catalysts, speeding up a wide variety of chemical reactions [3]. As a result of their ability to selectively target cancer cells, gold nanoparticles are being researched for their potential in cancer treatment. In addition, gold nanoparticles have applications in drug delivery, optics, and even chemical sensors [4]. In fact, gold nanoparticles have been used to develop a 15-minute diagnostic test for HIV and syphilis using a simple wallet-sized device that attaches to one’s iPhone [7]. However, the most striking property of gold nanoparticles is their color. By varying their size and shape, the nanoparticles are able to adopt a wide variety of hues, from the cool cerulean of the open ocean to the electrifying pink of a plastic flamingo [2].

Given the numerous applications, it is necessary to develop efficient and safe synthesis methods for gold nanoparticles. Standard synthesis methods involve first converting gold ions to gold nanoparticles and then coating the particles with a chemical that prevents them from agglomerating together. However, these methods pose several problems. Standard synthesis methods requires heating and take several hours, making them difficult to implement in a factory setting. These synthesis routes also use toxic and environmentally harmful chemicals, such as sodium borohydride [5].

This year, as part of my school’s chemistry analysis class, I am working with a group of students researching the applications of plant extracts for synthesizing gold nanoparticles. The use of plant extracts has several advantages over standard synthesis methods. Unlike toxic reducing and capping agents, plant extracts are environmentally-friendly and benign. Researchers have also found that synthesizing the nanoparticles with plant extracts not only occurs at room temperature, but does so within several minutes without deterioration in nanoparticle quality. The synthesis of gold nanoparticles with plant extracts is also cheap and easily scalable, which could help bring gold nanoparticles from research labs into the mainstream marketplace [5]. Finally, plant extracts are biocompatible, meaning they can be used in protocols involving the human body, such as cancer treatments and drug delivery methods [6].

Due to the growing shift towards green chemistry — chemistry that emphasizes the use of nontoxic and environmentally-benign materials — many scientists are researching how gold nanoparticles can be synthesized with plant extracts. For example, Yu et al. (2016) used Citrus maxima, or pomelo plant, to successfully synthesize gold nanoparticles. The resulting nanoparticles also demonstrated a catalytic ability to help convert 4-nitrophenol, an industrial waste product, to 4-aminophenol, a chemical with applications in cosmetics and drug synthesis. Other scientists have also successfully synthesized gold nanoparticles using germanium, lemongrass, aloe vera, tea leaves, and a variety of other plants [5].

Although many researchers have demonstrated the successful synthesis of gold nanoparticles using a wide variety of plant extracts, the specific mechanisms of synthesis are not well-understood. Additional research is needed to determine what phytochemical or combination of phytochemicals in plant extracts are responsible for the reducing and capping of gold nanoparticles. In addition, the quality of plant extract-synthesized gold nanoparticles has not been extensively compared to nanoparticles synthesized using standard methods. As a result, more research is needed to confirm the quality and effectiveness of plant extract-synthesis gold nanoparticles in applications such as biosensors, optics, and catalysis.

References

[1] Choi, Y., Choi, M.-J., Cha, S.-H., Kim, Y. S., & Cho, S. (2014). Catechin-capped gold nanoparticles: Green synthesis, characterization, and catalytic activity toward 4-nitrophenol reduction. Nanoscale Research Letters. http://dx.doi.org/10.1186/1556-276X-9-103

[2] Gold nanoparticles: Properties and applications. (n.d.). Retrieved January 14, 2016, from Sigma-Aldrich website: http://www.sigmaaldrich.com/materials-science/nanomaterials/gold-nanoparticles.html

[3] Jiang, K. (2015). Chapter two - noble metal nanomaterials: Synthetic routes, fundamental properties, and promising applications. In K. Jiang & A. Pinchuk (Authors), Solid state physics (Vol. 66, pp. 131-211). http://dx.doi.org/10.1016/bs.ssp.2015.02.001

[4] Llevot, A., & Astruc, D. (2012). Applications of vectorized gold nanoparticles to the diagnosis and therapy of cancer. Chemical Society Reviews, (1). http://dx.doi.org/10.1039/C1CS15080D

[5] Mittal, A., Chisti, Y., & Banerjee, U. (2013). Synthesis of metallic nanoparticles using plant extracts. Biotechnology Advances, 31(2). http://dx.doi.org/10.1016/j.biotechadv.2013.01.003

[6] Rajan, A., MeenaKumari, M., & Philip, D. (2014). Shape tailored green synthesis and catalytic properties of gold nanocrystals. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 118. http://dx.doi.org/10.1016/j.saa.2013.09.086

[7] Phillip, A. (2015, February 6). A 15-minute HIV and syphilis test — from your iPhone. The Washington Post. Retrieved from https://www.washingtonpost.com/news/to-your-health/wp/2015/02/06/a-15-minute-hiv-and-syphilis-test-from-your-iphone/

[8] Seoudi, R., & Said, D. (2011). Studies on the effect of the capping materials on the spherical gold nanoparticles catalytic activity. World Journal of Nano Science and Engineering, 1(2). http://dx.doi.org/10.4236/wjnse.2011.12008

[9] Yu, J., Xu, D., Guan, H. N., Wang, C., Huang, L. K., & Chi, D. F. (2016). Facile one-step green synthesis of gold nanoparticles using Citris maxima aqueous extracts and its catalytic activity. Materials Letters, 166. http://dx.doi.org/10.1016/j.matlet.2015.12.031