Anisotropic Growth of Gold Nanorods Using Custom-Designed Quaternary Ammonium Salts: Implications for Photothermal Therapy

The conventional synthesis of gold nanorods (AuNRs) has been extensively done with cetyltrimethylammonium bromide (CTAB)-based facet-directing surfactants. However, small changes in the surfactants and the high cytotoxicity of CTAB result in limitations in photothermal therapy. To address these questions, a series of surfactants based on imidazolium quaternary salts and cyclic quaternary ammonium salts have been rationally designed, synthesized, and characterized. The interactions of headgroups of surfactants and AuNRs, the steric structure, and the distance of bilayers affected by the headgroups of surfactants were discussed. It revealed that AuNRs grown in the imidazolium quaternary salts with C16-carbon chains had similar aspect ratios of about 3.6. However, cyclic quaternary ammonium salts gave a wide range of aspect ratios from 2.2 to 6.3. AuNRs grown with 1-cetyl-3-methyl imidazolium bromide, cetylmethylpiperidinium bromide, and cetyl(cyclohexyl) dimethylammonium bromide (DCMH) showed lower cytotoxicity in high Au concentration than CTAB-passivated AuNRs. Furthermore, AuNRs grown with DCMH showed a good photothermal conversion property. This finding gives a new route to prepare AuNRs with tunable dimensions and good biocompatibility with the use of a variety of unconventional surfactants. This is a promising strategy for designing practically useful AuNRs for photothermal therapy in biochemical applications.