Conductive AFM: Probing Nano-scale Electrical Properties of Model Cell Membranes
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Conductive AFM: Probing Nano-scale Electrical Properties of Model Cell Membranes Paul Farrar1, Del Atkinson2 and Andrew J. Gallant1 1 School of Engineering and Computing Sciences, Durham University, South Road, Durham, DH1 3LE, UK. 2 Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK.
ABSTRACT Biologically relevant lipid bilayers supported on highly ordered pyrolytic graphite (HOPG) were probed both mechanically and electrically with a Conductive Atomic Force Microscope (C-AFM) capable of measuring ultra-low currents. Results show that these membranes undergo an elastic response up to 26 nN on average when compressed with an AFM tip. Measuring the films with a low contact force demonstrates that contact mode AFM can be used repeatedly to image without damaging the film. Based on current-voltage measurements made with the C-AFM, it is shown that apparently high resistances seen for the films could be the result of variable electrical contact between the tip and surface. As a result, the paper proposes that the deflection of the cantilever should always be measured in order to ensure knowledge of the location of the tip during all electrical measurements.
INTRODUCTION Substrate-supported biological films have been widely studied both as a model for the cell membrane [1] and in biosensor applications [2]. However, these thin films commonly contain defects that make interpretation of both macro-scale and micro-scale electrical measurements uncertain. Conductive Atomic Force Microscopy (C-AFM) offers the ability to apply a bias between an AFM tip and a sample during normal AFM scans and measure electrical properties at the nano-scale, thus in principle allowing the determination of extremely localized electrical behavior. Previous studies using C-AFM for bio-relevant structures have concentrated primarily on electrical measurements made on thin films of protein molecules [3, 4]. However, much of the cell membrane is a lipid bilayer and, in this study, we focus on supported bilayers composed of biologically relevant phospholipids. Although AFM is a well-established technique, C-AFM measurements on biological films are not straightforward, or trivial. Being relatively soft and only weakly bound to the supporting substrate, such films are sensitive to the magnitude of the force applied during AFM scans. In addition to this, a voltage bias between the tip and sample results in an additional electrostatic force during measurements. Previous studies have used C-AFM operating in jumping mode, where the tip is lifted from the surface between point measurements to limit shear forces [5]. In this study, the film is deliberately stressed with the AFM tip to reveal its mechanical properties and it is shown that contact-mode C-AFM measurements made with a mechanically soft cantilever indicate that the technique is viable, providing the contact forces are low. Finally there is a discussion of localized current-voltage (IV) measurements and they are related to the cantilever deflection in order to provide infor
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