Seedless One-Pot Synthesis of Gold Nanotriangles by Non-Thermal Liquid Phase Reduction Method

Gold nanotriangles (GNTs) with edge lengths of hundreds nanometer exhibits a strong absorption peak at the wavelength of approximately 600 nm due to localized surface plasmon resonance of nanoparticles. Since the peak wavelength of GNTs lied in long wavelength in comparison with spherical GNPs and the transmitted color of solution is blue, GNTs shows an improved detection on immunochromatography.<br/>GNTs can be synthesized by the seed-growth method [1] and non-seed method [2]. In the seed method, GNPs with a few nanometer particle sizes are pre-synthesized as a seed material and the seed are grown by adding gold ion solution, capping agent, and reducing agent. In the non-seed method, GNTs can be synthesized by one pot using thermal liquid-phase reduction with citrate acid. However, since thermal reduction requires precise temperature control of the solution and the reduction rate of gold ions affects to the edge lengths of GNTs, the non-seed method is difficult to realizing a stable synthesis of GNTs with edge lengths of hundreds of nanometer. In order to stably synthesize GNTs by non-seed and non-thermal liquid-phase reduction method, we demonstrated the synthesis method using reducing agent with composition of citrate acid and tannin acid.<br/>Solution-A was a mixture of 0.21 mL hydrogen tetrachloroaurate tetrahydrate aqueous solution (29.7 mM) and 4.8 mL cetyltrimethyl ammonium bromide (CTAB) aqueous solution (9.15 mM). Solution-B was a mixture of 128 μL sodium citrate acid solution (100 mM), 0.1 to 0.5 mL tannin acid solution (5.88 mM), and 7.5 mL distilled water. Solution-A and solution-B were mixed for 10 min in the 100 mL glass beaker with the mixing speed 700 rpm. In this experiment, the volume of sodium citrate acid solution was fixed, and the volume of tannin acid solution changed 0.1 to 0.5 mL. Following 3 days of stasis post synthesis at room temperature (23°C), the visible light absorption spectrum of the as-synthesized GNTs was recorded to analyze the peak wavelength. The edge lengths and yield of triangles (number of GNTs/total number of particles) of the as-synthesized solution after 3 days was evaluated by TEM observation.<br/>From UV-visible absorbance spectra of solution, the solution with 0.1 mL of tannin acid solution exhibited purple color and the absorption peak at the wavelength of approximately 584 nm. The solution with 0.2 mL of tannin acid solution showed blue color and the strong absorption peak at the wavelength of approximately 619 nm. The solution with 0.3 to 0.5 mL of tannin acid solution indicated dark brown color and weak absorption peak. Moreover, the same color solution was obtained even if it was synthesized three times under the same conditions. From TEM observation, the synthesized solution confirmed that GNTs were observed in all solutions. The average edge length depends on volume of tannin acid solution and showed 127 nm with coefficient of variation of 0.74 for the solution with 0.2 mL of tannin acid solution. The yield of the triangles was 40.1 % for 0.1 mL tannin acid, 41.7 % for 0.2 mL tannin acid, 48.5 % for 0.3 mL tannin acid, and 24.8 % for 0.5 mL tannin acid. In thermal reduction method without tannin acid solution, the success rate of synthesis GNTs was 5 % and yield of the triangle was 30 %. These results confirmed that GNTs can be stably synthesized by seedless and non-thermal liquid-phase reduction method using tannin acid solution.<br/>[1] X. Fan, Z. R. Guo, J. M. Hong, Y. Zhang, J. N. Zhang, and N.Gu, “Size-controlled growth of colloidal gold nanoplates and their high-purity acquisition”, Nanotechnology, 21, 105602 (2010).<br/>[2] D. V. R. Kumar, A. A. Kulkarni, and B. L. V. Prasad, “Synthesis of triangular gold nanoplates: Role of bromide ion and temperature”, Colloids and Surfaces A: Physicochem. Eng. Aspects, 422, pp. 181-190 (2013).