Probing Intermolecular Forces and Potentials with Magnetic Feedback Chemical Force Microscopy

Magnetic feedback chemical force microscopy (MF-CFM) was used to map the complete force profile between hydroxyl- and carboxyl-terminated self-assembled monolayers (SAMs) in aqueous solution. The snap-to-contact and snap-out instabilities intrinsic to force-displacement measurements made with weak spring constant cantilevers were eliminated by using a cantilever with an attached magnetic particle and a solenoid in a servo loop to balance the tip−sample interactions. The interaction between hydroxyl-terminated surfaces in deionized water was well fit by a van der Waals model to short distances. The Hamaker constant that was determined from experiment, 1.0 × 10-19 J, is similar to that expected for a gold−gold interaction and shows that the underlying gold support dominates the attractive interaction over a large range of separations. The interaction between a carboxyl-terminated tip and a sample in 0.010 M phosphate buffer at pH 7.0 was fit with a model that includes both van der Waals and electrostatic terms. The Hamaker constant, 1.2 × 10-19 J, which is similar to that obtained for hydroxyl-terminated surfaces, confirms that the gold−gold interaction dominates the attractive part of the interaction. In addition, the Debye length, 2.9 nm, surface potential, −1.5 × 102 mV, and charge regulation parameter, −0.71, obtained from analysis of the data are consistent with previous work (Vezenov et al. J. Am. Chem. Soc. 1997, 119, 2006−2015. Hu; Bard. Langmuir 1997, 13, 5114−5119). The implications of these results and applications of MF-CFM are discussed.