The electrical conductivity of biomaterials on a molecular scale is of fundamental interest in the life sciences. We perform first principles electronic structure calculations, which clearly indicate that lambda-DNA chains should present large resistance values. We also present two direct procedures to measure electrical currents through DNA molecules adsorbed on mica. The lower limit for the resistivity is 10(6) Omega . cm, in agreement with our calculations. We also show that low energy electron bombardment induces a rapid contamination and dramatically affects the measured conductivity, thus providing an explanation to recent reports of high DNA conductivity.

No abstract

We study the physical origins of phase contrast in dynamic atomic force microscopy (dAFM) in liquids where low-stiffness microcantilever probes are often used for nanoscale imaging of soft biological samples with gentle forces. Under these conditions, we show that the phase contrast derives primarily from a unique energy flow channel that opens up in liquids due to the momentary excitation of higher eigenmodes. Contrary to the common assumption, phase-contrast images in liquids using soft microcantilevers are often maps of short-range conservative interactions, such as local elastic response, rather than tip-sample dissipation. The theory is used to demonstrate variations in local elasticity of purple membrane and bacteriophage φ29 virions in buffer solutions using the phase-contrast images.atomic force microscopy | liquid environments | energy dissipation | higher eigenmodes | momentary excitation D ynamic atomic force microscopy (dAFM) is an essential experimental tool for the study of conservative and dissipative forces on surfaces at nanometer length scales, which has major implications for the physics of biomolecular interactions, chemical bond kinetics, adhesion, wetting and capillary action, friction and elasticity on material surfaces (1, 2). dAFM techniques have been developed to distinguish between dissipative (friction, viscoelastic, bond breaking, surface hysteresis, capillary condensation) and conservative (elastic, magnetic, electrostatic) forces between a sharp oscillating tip and the surface. In amplitude-modulated AFM (AM-AFM) phase-contrast imaging, the variation in the phase of the oscillating probe tip with respect to the drive signal is mapped over the sample. For the past decade phase contrast has been intimately connected with variation in tip-sample dissipation over the sample (2-6). Phase-contrast imaging is widely recognized as perhaps the most important AM-AFM mode for the measurement of compositional contrast.The connection between phase contrast and tip-sample dissipation rests on the assumption that the cantilever dynamics can be modeled by a single eigenmode (point-mass) oscillator. With this assumption, the tip-sample dissipation can be equated to the difference between work input to the oscillator and energy dissipated into a surrounding viscous medium (4, 5). This theory forms the bedrock upon which phase-contrast imaging is currently based, at least under ambient and vacuum conditions. dAFM is now a well-known and broadly extended technique for nanoscale imaging and force spectroscopy in the biology community (7-10). Because the natural medium for the study of biological samples is liquid, it is of fundamental importance to develop a proper description of the different working modes of dAFM when the probe and sample are immersed in liquids. In particular, there is little work on understanding the origins of phase contrast in liquids (6) where soft cantilevers (stiffness 1 N/m) with low-quality factors (Q 5) are routinely used for the imaging of soft biological samples. It is im...

In this work fundamental properties of the electrical transport of single-walled carbon nanotubes as a function of their length are investigated. For this purpose, we have developed a new technique that allows us to characterize electronic transport properties of single-walled carbon nanotubes by probing them at different spots. This technique uses scanning force microscopy to make mechanical and electrical nanocontacts at any selected spot of a given image. We have applied this technique to molecules with high intrinsic resistance. The results show a nonlinear resistance vs distance behavior as the nanotube is probed along its length. This is an indication of elastic electronic transport in one-dimensional systems.

Dynamic atomic force microscopy is widely used for the imaging of soft biological materials in liquid environments; yet very little is known about the peak forces exerted by the oscillating probe tapping on the sample in liquid environments. In this article, we combine theory and experiments in liquid on virus capsids to propose scaling laws for peak interaction forces exerted on soft samples in liquid environments. We demonstrate how these laws can be used to choose probes and operating conditions to minimize imaging forces and thereby robustly image fragile biological samples.