Thiol–Ene Click Chemistry for Functionalizing Silica-Overcoated Gold Nanorods

Gold nanorods (GNRs) coated with SiO2 are functionalized through thermally initiated free-radical thiol–ene click reactions, which couple vinyl groups on the SiO2 surface with thiols to form thioethers. This method of functionalization is developed as an alternative approach to thiolate functionalization of the gold surface. GNRs are synthesized using cetyltrimethylammonium bromide (CTAB), which is challenging to displace with thiols in high yield. In this work, a shell of SiO2 is instead deposited on the outer surface of the GNRs, which also maintains colloidal stability. A reaction with a vinyl silane then prepares the outer surface of the SiO2 shells for subsequent thiol–ene click reactions with five thiols that are selected to represent variations in structure and functional groups, including aliphatic and aromatic structures and acids and bases, demonstrating the versatility of the reaction. The SiO2 shell is initially 17 nm thick and further grows to 20 nm when functionalized with vinyl groups. Deposition of vinyl groups and the formation of thioethers are confirmed by Fourier-transform infrared (FTIR) spectroscopy and time-of-flight secondary ion mass spectrometry (ToF-SIMS). While FTIR spectroscopy is well-known for characterizing the surface of nanoparticles, ToF-SIMS has been applied less for this purpose and strongly complements analysis by FTIR spectroscopy.