Monitoring surface reactions at an AFM tip:: An approach to follow reaction kinetics in self-assembled monolayers on the nanometer scale

The kinetics of alkaline hydrolysis of ester groups in self-assembled monolayers (SAMs) were monitored by a combination of atomic force microscopy (AFM) on the nanometer scale and FT-IR spectroscopy in the continuum limit. The main objective was to study surface reactions in situ with chemical specificity, from the nanometer perspective, using an atomic force microscope. This could not be achieved by conventional AFM friction or force measurements due to insufficient resolution, and instrumental or thermal drift, respectively. These problems were circumvented by a novel approach, which we termed "inverted" chemical force microscopy (ICFM). In ICFM, chemical reactions, which take place at the surface of the tip coated with reactants, are probed in situ by force-distance measurements on a scale of less than 100 molecules. The pull-off forces of different reactive SAMs were shown to vary with the extent of the reaction. Reactivity differences for these monolayers observed in this manner by AFM on the nanometer scale agree well with macroscopic behavior observed by FT-IR and can be related to differences in the SAM structure. These results, together with additional force microscopy data, support the conclusion that, for closely packed ester groups, the reaction spreads from defect sites, causing separation of the homogeneous surfaces into domains of reacted and unreacted molecules.