Purposefully Designed Surfactants for Facile and Controllable Gold Colloidal Nanocrystal Synthesis

Three new cationic surfactants-N-cetyl-bis(2-dimethylaminoethyl)ether bromide (CBDEB), N-dodecyl-bis(2-dimethylaminoethyl)ether bromide (DBDEB), and N-hexyl-bis(2-dimethylaminoethyl)ether bromide (HBDEB)-have been designed herein using a simple and tailorable synthesis route. CBDEB and DBDEB, the 16- and 12-carbon chain surfactants, demonstrate facile, rapid, and controllable aqueous syntheses of gold nanoparticles (AuNPs) as dual-action reducing and capping agents. The synthesis strategy, using only surfactant and HAuCl₄ salt, and 4 min of heating at 80 °C, results in spherical AuNPs (average diameters of 13.4 ± 3.8 nm for CBDEB and 12.0 ± 3.8 nm for DBDEB). Microwave irradiation was also investigated as a heating method and produces AuNPs in as little as 30 s. Control over the size and shape of AuNPs was proven to be feasible (toward populations of Euclidean shapes) by appropriately tuning reaction parameters, such as the molar ratio of surfactant to Au³⁺, temperature, incorporation of a time delay before heating, or shape control agents, such as Cu²⁺. Frustratingly, the cytotoxicity of CBDEB is similar to that of cetyltrimethylammonium bromide (CTAB), a popular 16-carbon chain cationic surfactant. Notably, while the shorter HBDEB (6-carbon chain) does not produce AuNPs under the applied conditions, it does appear to improve cell viability upon cytotoxicity evaluation and may be favorable as a new biological surfactant.