Mixed Self-Assembled Monolayers with Internal Urea Group on Silica Surface

In this study, we report the successful grafting on silica surfaces by the coadsorption method, of mixed nitrobenzyl ester-/methyl-terminated self-assembled monolayers (SAMs), with ratios of two coupling agents (30/70) and (10/90). We show that the preparation of mixed monolayers using two organosilanes with internal urea groups in the alkyl chain seems to be appropriate to obtain a monolayer composition similar to the molar ratio of the two silanes in the deposition solution. This behavior could be due to the presence of the polar urea groups in the molecular structure of both organosilanes which allows them to have the same ability of self-assembly by hydrogen-bonding and also the same affinity with the hydrophilic surface. To locate the ester functions at the surface of the mixed monolayers, we investigated the covalent immobilization of amino functionalized gold nanoparticles (GNPs) as nanoscale marker. Polarization modulation infrared reflection-adsorption spectroscopy (PM-IRRAS) and atomic force microscopy (AFM) were used to monitor the chemical modifications of the silica surfaces and to demonstrate the successful attachment of gold nanoparticles onto the modified surfaces. Finally, we show that the mixed SAMs were grafted with a phase separation.