Probing self-assembly of silane molecules in monolayers at active solid-liquid interfaces

Self-assembly is a particularly appealing phenomenon for bottom-up approaches to nanotechnology involving spontaneous ordering of molecules. When self-assembly of molecules occurs at active solid-liquid interfaces, the superficial chemistry of substrates can be altered. In the present study, self-assembly of trimethoxysilanes solely differing for their head group chemistry is investigated. In particular, glass substrates were pre-activated by hydroxylation and then silane anchoring was achieved by means of a wet chemistry-based route. Synergistic investigation in terms of morphology and chemistry turned out to be an appropriate evaluation tool for the quality attributes of the silane layering on glass. Preliminary assessment of effective functionalization was carried out in terms of contact angle and surface zeta potential analyses. Then, chemistry of functionalized glass was probed by means of X-Ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF SIMS). Lastly, Atomic Force Microscopy was adopted for the characterization of surface topography. Keeping the same optimized synthesis protocol for all the samples, we were able to highlight the effect of varying head group chemistries of silanizing agents on the final functionalization features, such as spatial homogeneity and number of layers. High-quality monolayers carrying thiol, methacrylate and glycidyloxy exposed groups were successfully synthesized. Surface roughness resulted to be around 1 nm and presence of characteristic elements confirmed effective functionalization. Conversely, as regards amino-terminated silanes, a multi-layered structure was obtained. Surface roughness dramatically increased and correlation between ideal and experimental elemental ratios characterizing the monolayer was lost. We conclude that the polarity of the functionalizing agent turned out to have a pivotal role in directing self-assembly at active interfaces and leading to the effective formation of monolayers on amorphous substrates. High-quality and reproducibility make such surfaces ideal candidates to be used as heteronucleants for pharmaceutical crystallization studies.