Environmentally friendly synthesis of gold nanoparticles

Noble metal (i.e., metals resistant to corrosion and oxidation) nanoparticles have received considerable attention because of size- and shape-dependent physicochemical properties that are not observed in their bulk counterparts. The use of these structures for various applications (e.g., electronics, optics, and medicine) has led to intensified efforts in the development of the chemical and physical methods required for their synthesis. With such techniques, metal nanoparticles with well-defined chemical compositions, size, and morphology can be produced.¹ These methodologies, however, are often associated with high production costs or with the application of hazardous chemicals.

Bio-mediated synthesis methods have been suggested as alternative ways to produce noble metal nanoparticles. These simple techniques have low costs and can be conducted in eco-friendly ways.² Indeed, various approaches to the biosynthesis of gold nanoparticles (GNPs) have been demonstrated. These include the use of micro-organisms,³ fungi,⁴ or plants.⁵ In the bio-mediated techniques, the organisms can be used to achieve active uptake of gold ions from a solution, reduce them to neutral gold atoms, and form gold nanoparticles. The gold nanoparticles are subsequently accumulated inside, or excreted outside, the organism. These approaches, however, all involve working with complex living organisms.

In our work, to avoid the elaborate process of culturing living cells, we therefore use plant extracts to achieve bioreduction of gold ions and stabilization of the nanostructures that are formed.⁶ In particular, we have chosen mint (Mentha piperita) extract as the source of the reducing and stabilizing agents.⁷ In addition, we apply an established protocol⁸ for the synthesis of GNPs. With this method we can thus produce GNPs under mild conditions, without the use of any harmful agents. Furthermore, the nanoparticles we obtain are coated with biological agents that occur naturally in the plant, which ensures low cytotoxicity. This is an especially important aspect of our work, given its potential biomedical, diagnostic, and pharmaceutical applications.

We have used a so-called green chemistry approach for our nanoparticle synthesis method.⁷ To synthesize the GNPs, we optimized the conditions of the process so that the reaction yield was maximized. To do this, we first prepared mint extract from dried, powdered leaves of the plant. We then added the mint extract to aqueous chloroauric acid solution and thus produced GNPs. This reaction was conducted in the dark, at 28°C, for 24 hours. During this time the color of the solution changed from yellow to ruby red, which indicated the presence of the GNPs.

We have also characterized our obtained nanoparticles with the use of UV-visible light absorption spectroscopy and transmission electron microscopy techniques. In an absorption spectrum (see Figure 1), two peaks that are related to the surface plasmon resonance of GNPs (i.e., an indication of adsorption) are revealed. We also used the transmission electron microscope to investigate the morphology of the GNPs. Our results (see Figure 2) indicate that the use of mint extract for the bioreduction of chloroauric acid caused the formation of nanoparticles with a variety of shapes (e.g., spheres, triangles, and hexagons) and sizes (10–300nm).

Figure 1. UV-visible (UV-Vis) light absorption spectra from gold nanoparticles (GNPs) that were synthesized using mint extract.⁷ The two peaks in the final spectrum are associated with the surface plasmon resonance of the GNPs.

Figure 2. Transmission electron microscope image of the obtained GNPs.⁷These nanoparticles exhibit a variety of shapes and sizes (ranging from 10 to 300nm).

To study the cytotoxicity of our GNPs, we used an MTT assay. The MTT assay is a colorimetric, enzyme-based test that is widely used to assess cell viability after chemical treatment.⁹ We were thus able to compare the cytotoxic effect of our biologically obtained GNPs with gold nanorods (GNRs) that were chemically synthesized through seed-mediated growth.¹⁰ From this comparison (see Figure 3), we observe that our biosynthesized nanoparticles are significantly less toxic than the chemical nanoparticles. The high cytotoxicity of the GNRs is caused by the stabilizing agent that is used for their production. This agent—cetyltrimethylammonium bromide (CTAB)—is widely used for the chemical synthesis of GNPs. In contrast, by applying the mint extract for the synthesis of the GNPs, we obtained nanoparticles that are coated with biological agents and thus ensured low cytotoxicity levels.

Figure 3. Comparison of the cytotoxic effect of different types of nanoparticles, measured with an MTT assay.⁷

In this work we have reported the results from a biological procedure for the synthesis of gold nanoparticles. In our methodology, we use a plant extract as the stabilizing agent. The most important advantages of our technique are its simplicity, cost-effectiveness, and compatibility with biomedical and pharmaceutical applications. There remain, however, considerable challenges in understanding the synthesis mechanism and determining the biocompounds that are involved with the process. In our future work we will focus on the examination of these molecules, which may play a role in the synthesis. We will study the composition of the plant extract as well as the molecules that cover the surface of the obtained nanoparticles. By investigating these issues, we may be able to predict and control the size and shape of the nanoparticles that are produced. Moreover, understanding the capping agents present at the surface of the GNPs is essential for designing intelligent structures that can be used for diagnostic and therapeutic purposes, i.e., to recognize specific targets.

We acknowledge financial support from the National Science Centre, through Harmonia grant DEC-2012/04/M/ST5/0034 and Opus grant DEC-2013/09/B/ST5/03417. This work was financed by a statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Faculty of Chemistry of Wroclaw University of Technology.