Optimization of the Surfactant Ratio in the Formation of Penta-Twinned Seeds for Precision Synthesis of Gold Nanobipyramids with Tunable Plasmon Resonances

The synthesis of high-purity gold nano bipyramids (Au NBPs) with a narrow size distribution and tunable plasmon resonances is of great significance for plasmon resonance-related applications. However, the synthesis Au NBP approach involves multiple steps with many parameters that can affect the purity of the final product. In this work, we were devoted to studying the effect of the molar ratio between hexadecyltrimethylammonium chloride (CTAC) and sodium citrate tribasic dihydrate (CiNa3) on the seed formation stage. The results showed that the yield of Au NBP product has dramatically increased with the seed solution made from the molar ratio of CTAC:CiNa3 at 21:1. Furthermore, using this optimal seed, we can efficiently synthesize Au NBPs with various sizes by adjusting the concentration of the seed but keeping the rest of the parameters constant. In this study, the longitudinal localized surface plasmon resonances (LSPRs) of Au NBPs exhibit tunability beyond 450 nm across the visible and near-infrared regions from 774 to 1224 nm. We were able to successfully fine-tune the LSPRs of Au NBPs in the spectral region to become resonant with the excitation wavelengths of an 808 nm near-infrared (NIR) laser. The photothermal activities of Au NBPs were studied under 808 nm laser irradiation at ambient conditions. The present work demonstrates a paradigm for the synthesis of Au NBPs with tunable LSPRs in a precise and controllable manner, achieved by examining the surfactant ratios in the formation of penta-twinned seeds.