Evidence for bilayer assembly of cationic surfactants on the surface of gold nanorods

The surface structure of gold nanorods (NRs) capped with cationic surfactan ts in water was studied by FTIR, thermogravimetric analysis (TGA), and tran smission electron microscopy (TEM). For gold nanorods, the FTIR results sho w the formation of new bands, which indicate binding of the surfactant head group to the surface of the NR. These bands are stable at temperatures as h igh as 350 degreesC. For a surfactant mixture (used as capping material), T GA shows a weak weight loss peak at 235 degreesC and a strong peak at 298 d egreesC assigned to the surfactant molecules in monomer and aggregated form s, respectively. For gold nanorods, three weight loss peaks at about 230, 2 73, and 344 degreesC are observed. For gold nanospheres (NSs), TGA shows a strong mass loss at 225 degreesC and two weak mass loss peaks at 255 and 28 8 degreesC. The released material after combustion in the TGA process was a nalyzed by FTIR spectroscopy and found to be CO2. Our results suggest the f ollowing for both NRs and NSs: (1) There are two different binding modes fo r the surfactant molecules capping these nanoparticles. (2) Surfactant mole cules form a bilayer structure around the gold nanoparticles in which the i nner layer is bound to the gold surface via the surfactant headgroups. (3) With increase of the temperature, the outer layer desorbs at lower temperat ure and consequently the inner layer leaves the surface at higher temperatu re. (4) The higher desorption temperature of the bilayer in the NRs compare d to NSs is explained in terms of the difference in packing of the surfacta nt molecules and their adsorption energy to the different facets present in these nanoparticles. (5) TEM results suggest that the shape transformation of NRs to NSs occurs as the inner layer is released from the surface. (6) The CH2 rocking mode at 720 cm(-1) suggests that the methylene chains have free rotation and surfactants are packed in a hexagonal structure.