Inverted chemical force microscopy

Fig. 8 Schematic drawing of inverted chemical force microscopy for the reaction between NHS-esters and n-butylamine in aqueous medium. In iCFM the pull-off forces between an AFM tip covered with a SAM of NHS - CIO and an inert octadecanethiol SAM are measured in situ during the conversion of the reactive groups attached to the tip. The interaction between tip and inert surface varies systematically with the extent of the reaction and hence it allows one to quantitatively investigate the reaction kinetics. (Reprinted from reference [139], copyright (2004), with permission from Elsevier)...

Kinetics and Domain Formation in Surface Reactions by Inverted Chemical Force Microscopy and FTIR Spectroscopy... 

As described here, these problems can be circumvented by "inverted" chemical force microscopy (CFM) [14], In inverted CFM, pull-off forces [15] between the tip coated with the reactant and an inert surface are monitored as a function of time (Figure 1). The contact area of the tip at pull-off in such experiments using non-reactive SAMs (as inert samples) deposited on Au(lll) varies between approximately 10-100 effectively interacting molecular pairs [16,17],... 

The origin of the differences in reactivity mentioned above must lie in the structure of the surface of the SAMs. In situ information on the composition of the reacting monolayers was obtained by the "inverted chemical force microscopy approach described in the introduction. In these experiments, the force required to pull the AFM tip coated with 1, 2, or 3 away from contact with an inert octadecanethiol SAM on flat Au(l 11) was followed in real time in situ during the hydrolysis. The Au(lll) substrates are atomically smooth over distances of several hundred nanometers with only occasional steps and depressions present (Figure 4). With highly ordered octadecanethiol SAMs on top of these substrates it is ensured that the interaction between tip and surface takes place between exactly the same functional groups at the tip apex and a homogeneous inert substrate. 

We have demonstrated for the first time that average macroscopic kinetics of reactions in SAMs can be correlated and explained by nanometer level force (adhesion) measurements in the confined environment of the monolayer studied. Using a modified CFM approach, which we termed inverted chemical force microscopy , reactions of as few as 10 to 100 molecules could be followed in situ. Structure-reactivity differences, arising from differences in monolayer structure, as observed on the nanometer scale, on average agree well with macroscopic behavior observed by FT-IR. The studies show that reagents penetrate functionalized monolayers at specific defect sites or at domain boundaries. 

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