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Mixed silane monolayers for controlling the surface areal density of click-reactive alkyne groups: a method to assess preferential surface adsorption on flat substrates and a method to verify compositional homogeneity on nanoparticles.

SAMs formed from mixtures of alkyne-silanes and alkane-silanes are used to control the areal density of click-reactive alkyne groups on the surface of flat germanium substrates, silicon wafers, and silica nanoparticles. Two new analytical tools are described for characterization of the mixed SAMs: a thermogravimetric analysis (TGA) technique for quantifying the compositional homogeneity of the mixed monolayers formed on nanoparticles, and an infrared spectroscopy (IR) technique to detect preferential surface adsorption. The TGA technique involves measurement of the change in weight when azide-terminated polymers react with surface alkyne groups on silica nanoparticles via a click reaction, while the IR technique is based on the use of attenuated total reflectance infrared spectroscopy (ATR-IR) to monitor click reactions between azide compounds with infrared "labels" and alkyne-functional mixed SAMs deposited on germanium ATR plates. Upon application of the new characterization techniques, we are able to prove that the mixed silane monolayers show neither phase separation nor preferential surface adsorption on any of the three substrates studied. When reacted with azide terminal polymers, the areal density at saturation, σ(sat) is found to scale with molecular weight according to σ(sat) ≈ N(-0.57). We conclude that mixed monolayers of alkyne-silanes and alkane-silanes are an effective means of controlling the surface areal density of click-reactive alkyne groups on both flat and nanoparticle substrates.