Preparation of phase diagram of gold nanorods in mixture solvent of DMSO and water and its application for efficient surface-modification

Abstract Gold nanorods (AuNRs) have been widely studied in various biological and biomedical fields. The complete removal of a cationic binary surfactant cetyltrimethyl ammonium bromide (CTAB), which is strongly bound to prepared AuNRs, is necessary due to their cytotoxicity as well as further functionalization of the surface. Here, we report an appropriate procedure which can remove CTAB and modify the surface by ω-mercaptocarboxylic acid efficiently. The detachment mechanism of CTAB from AuNR surface varying with the volume ratio of dimethylsulfoxide (DMSO)/water ( V DMSO : V AuNRs ) and the concentration of CTAB was also studied. For samples after twice-centrifugation (C CTAB  = 15 μM), CTAB thoroughly detached from AuNRs under the condition of V DMSO : V AuNRs  = 0.3–0.5, while AuNRs attained colloidal stability at either V DMSO : V AuNRs  = 0–0.2 or 0.6–0.9. Especially, the complete removal of interdigitated CTAB bilayer was confirmed at V DMSO : V AuNRs  = 0.5 by X-ray photoelectron spectroscopy surface composition analysis. Based on the colloidal stability of AuNRs related with the volume ratios of DMSO/water and CTAB concentration, the phase diagram of AuNRs was further drawn. Both the minimal and maximum DMSO-fraction thresholds for efficient removal of CTAB are generally increased with the increase of CTAB concentration. After the removal of CTAB, mercaptohexadecanoic acid (16-MHDA) was utilized for the further modification of AuNRs. In a wide range of CTAB concentration and with high DMSO fraction, there is no aggregation after the modification. This technique may be used to functionalize the surface of AuNRs for use in a diverse range of applications.