Probing Intermolecular Forces and Potentials with Magnetic Feedback Chemical Force Microscopy

Abstract

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.

Direct Measurement of Glycosaminoglycan Intermolecular Interactions via High-Resolution Force Spectroscopy

“…20 These results suggest that the gold substrate dispersion interaction dominates over the surface hydrocarbon layer (i.e., only at very short distances (D < 1 nm) will the less polarizable hydrocarbon layer contribute to the effective Hamaker constant), and hence, we do not resort to a more complicated multilayer model for the effective Hamaker constant. 23,30 It has also been postulated that smaller probe tip radii allow penetration through the SAM layer, which in this case is approximately 1.6 nm thick; such a displacement of the hydrocarbon chains upon contact would allow the van der Waals interaction between the underlying gold surfaces to dominate. 30 (iii) Electrostatic Double Layer Component.…”

“…The measurement of repulsive DLVO forces between nonpolymeric charged surfaces is well documented in the literature using both the surface force apparatus (and similar instruments) [46][47][48][49][50][51] and the atomic force microscope. [20][21][22][23]30,[52][53][54][55][56][57][58][59][60][61][62] All of the theoretical models presented in this paper employ constant surface charge boundary conditions, rather than constant potential, for two reasons. First, both the substrate and the tip were electrically isolated from each other and from the instrument ground and, therefore, not electrically connected to any source that would maintain them at a constant potential.…”

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…When determining the Hamaker constant of the system by fitting the attractive part of the approach curve, the main problem is the presence of the jumpto-contact. Ashby et al [330] have taken advantage of a magnetic feedback AFM to probe the complete force profile between OH-and COOH-terminated self-assembled monolayers on gold in water solution, as shown in Fig. 19.…”

Cantilever-Based Optical Interfacial Force Microscopy

“…A decade ago for the purpose of avoiding the mechanical instability in measuring intermolecular forces, magnetic force feedback was implemented in AFM systems by attaching a magnet to the end of a cantilever (Ashby et al, 2000;Jarvis et al, 1996;A.M. Parker & J.L.…”

“…Parker, 1992;Yamamoto et al, 1997). However, the magnetic force feedback requires a tedious process of attaching magnets to the backside of the cantilever using an inverted optical microscope equipped with micromanipulators (Ashby et al, 2000; www.intechopen.com…”