Synthesis of Gold Nanosheets with Controlled Morphology by Combining a Natural Amino Acid with High-Frequency Ultrasound

In this study, we report on the formation of gold nanosheets with different morphologies using a naturally occurring amino acid, viz, tryptophan, as a reducing and capping agent at ambient conditions. The importance of in situ-oxidized tryptophan for promoting anisotropic growth of gold (Au) nuclei to form two-dimensional gold nanosheets is demonstrated. Furthermore, we investigated the effect of high frequency ultrasound (490 kHz) and tryptophan to fabricate ultrathin gold nanosheets with controlled morphologies (triangular and polygonal shapes), without using any other reagents/stabilizers. The size of gold sheets (200 nm to micrometer range) can be controlled by tuning the experimental conditions. A possible mechanism for the formation of well-defined gold nanostructures with different morphologies is provided. High frequency sonication leads to the simultaneous dimerization of tryptophan and Au reduction, and we show that the interaction of tryptophan dimers with Au nuclei plays an important role in the formation of gold sheets. The sonochemically synthesized AuNSs displayed superior catalytic activity in the reduction of 4-nitrophenol to 4-aminophenol with a specific rate constant of 1.96 × 103 s-1 per unit gram of active catalyst and with excellent recyclability.