Surface Modification of Citrate-Capped Gold Nanoparticles Using CTAB Micelles

The surface of nanoparticles governs the overall properties of the nanoparticles because nanoparticles have a large surface-to-volume ratio. Charge, crystallinity, and chemical properties (such as hydrophilicity) of surfaces determine the stability, structure, and reactivity of nanoparticles. In this respect, it is essential to have a capability to control and modify the surfaces of nanoparticles. Among many nanoparticles, gold nanoparticles (AuNPs) are fascinating because of their unique plasmonic properties. Surface plasmons, collective oscillation of conduction electrons, permit a wide variety of applications of AuNPs in spectroscopy, imaging, energy, and biomedicine. AuNPs are typically synthesized using citrate-reduction of Au ions in an aqueous solution, which results in the AuNP surfaces covered with citrate anions. The citrate anions on the AuNP surface are weakly bound and thus easy to replace with more strongly binding thiol ligands. Changing the surface functionality of AuNPs from citrate anions (negative charge) to carboxylate (negative charge) or hydroxyl group (hydrophilic group) has been achieved using the corresponding organofunctional thiol compounds. More challenging in modifying the surface of citratecapped AuNPs is to impart positive charges to the surface. Simple ligand exchange using thiol with cationic functional groups (e.g., NH3 ) generally causes irreversible agglomeration of AuNPs due to multiple electrostatic crosslinking. Another formidable task is to conjugate hydrophobic materials to the AuNPs prepared in an aqueous phase. In this study, we aim to resolve these two issues, which will help us expand our ability to control and modify the surface of citratecapped AuNPs for further advanced applications. The molecule of choice for converting the surface charge from negative to positive was cetyltrimethylammonium bromide (CTAB), a cationic surfactant with a positive ammonium head group and a long hydrocarbon tail. CTAB is a commonly used surface ligand in nanochemistry, particularly for the synthesis of gold nanorods or nanocubes. CTAB stabilizes Au nanorods by forming a stable bilayer on the surfaces with the cationic head groups exposed to the outside. When we added a CTAB solution to a solution of citratecapped AuNPs, we observed an anomalous concentrationdependent response of the AuNPs. Addition of a small amount of CTAB expectedly resulted in aggregation of AuNPs. The cationic CTAB head group binds to the negatively charged citrate-capped AuNPs via electrostatic interaction. Consequently, either surface charge neutralization or hydrophobic interaction causes the AuNPs to aggregate in an aqueous solution. When we increased the concentration of added CTAB, we obtained surprisingly unexpected results. Addition of a large amount of CTAB stabilized the AuNPs rather