Probing chemical reactions on the nanometer scale: Inverted chemical force microscopy of reactive self-assembled monolayers

The hydrolysis and aminolysis of 11,11′-dithiobis(N-hydroxysuccinimidylundecanoate) (NHS-C10) adsorbed as self-assembled monolayer (SAM) on gold surfaces were monitored on the nanometer scale by a novel approach termed “inverted” chemical force microscopy (iCFM). In iCFM the reactants are immobilized on an atomic force microscopy tip rather than on the substrate and the chemical reactions that take place at the surface of the tip are probed by force–displacement measurements on an inert octadecanethiol-covered Au substrate. The information obtained is confined to the contact area at pull-off, which is of the order of only several nm2. Thus interactions and hence reaction kinetics can be quantitatively studied on the level of ∼10–100 molecules. In particular, iCFM data show that the aminolysis reaction on SAMs of NHS-C10 is a spatially heterogeneous reaction. In addition information about the defect density of reactive SAMs can be obtained.