Formation process of self-assembled monolayer on gold nanosphere probed by second harmonic generation

The formation process of a hemicyanine-terminated alkanethiolate self-assembled monolayer (SAM) on gold nanospheres immobilized on a glass substrate was studied by absorption spectroscopy, optical second harmonic generation, and Monte Carlo simulation. It was found that hemicyanine thiolate SAMs mainly form in the upper hemisphere region of the gold nanospheres in the early stage, followed by the additional SAM formation in the lower region of gold nanospheres. The hemicyanine SAM does not homogeneously form over the nanosphere surface and does not fully cover the nanospheres even after long exposure to the hemicyanine solution. This is because of the narrow space under the nanosphere, where the binding of the alkane disulfide to the gold surface is considered to be a diffusion-controlled reaction. The orientation of the hemicyanine molecules on gold nanospheres is similar to that of hemicyanine on a flat gold substrate. These results are important to understand the surface chemistry at nanostructure surfaces as well as the localized surface plasmon biosensing using metallic nanostructures.