Optimizing the immobilization of gold nanoparticles on functionalized silicon surfaces: amine- vs thiol-terminated silane

Immobilization of gold nanoparticles on planar surfaces is of great interest to many scientific communities; chemists, physicists, biologists, and the various communities working at the interfaces between these disciplines. Controlling the immobilization step, especially nanoparticles dispersion and coverage, is an important issue for all of these communities. We studied the parameters that can influence this interaction, starting with the nature of the terminal chemical function. Thus, we have carefully grafted silanes terminated by either amine or thiol groups starting from aminopropyltriethoxysilane (APTES) or mercaptopropyltriethoxysilane. We also changed the chain length for thiol-terminated layers through covalent grafting of mercaptoundecanoic acid (MUA) on APTES-modified layers, and the protocol of nanoparticles deposition to evaluate whether other factors must be taken into consideration to rationalize this interaction. The formed layers were characterized by X-ray photoelectron spectroscopy and gold nanoparticles deposition was monitored by scanning electron microscopy and surface-enhanced Raman scattering. We observed significant differences in terms of nanoparticles dispersion and density depending on the nature of the chemical layer on silicon. The use of ultrasounds during the deposition process was very efficient to limit aggregates formation. The optimal deposition procedures were obtained through the use of APTES and APTES/MUA functionalization. They were compared in terms of coverage, dispersion, and densities of isolated nanoparticles. The APTES/MUA surfaces clearly showed better results that may arise from both the longer chain and the dilution of thiol end groups.