High efficiency at low concentrations: investigating novel surfactants for the synthesis of gold nanobipyramids

Abstract Gold nanobipyramids (GNBPs) have garnered significant attention for photothermal therapy owing to their excellent photothermal conversion efficiency. Although the traditional surfactant cetyltrimethylammonium bromide (CTAB) effectively controls the morphology of gold nanomaterials, the cytotoxicity associated with its high concentration severely limits its biomedical application. In this study, four quaternary ammonium salt surfactants with bulkier headgroups were innovatively designed to synthesize GNBPs. For the synthesis of GNBPs, these novel surfactants require only very low concentrations (0.008–0.015 M), which are 6.7 to 12.5 times lower than the typical CTAB concentration used (0.1 M). The effects of surfactant concentration and solution pH on GNBPs synthesis were systematically investigated using a seed-mediated growth method. The results indicated that the optimal concentration and pH for GNBPs synthesis differed among the four surfactants, and possible reasons for this were proposed. These novel surfactants enabled the synthesis of GNBPs with high aspect ratios and absorption maxima between 800–850 nm at low concentrations. Furthermore, the use of extremely low concentrations significantly reduced the cytotoxicity of the surfactant residue. Future research will involve a comprehensive investigation into the photothermal conversion efficiency (PTE) of GNBPs fabricated with our novel surfactants, with direct comparisons to CTAB-synthesized counterparts. This work provides a superior alternative for the application of GNBPs in biomedicine and opens new prospects for the use of other extended surfactants for the synthesis of Au nanomaterials.