Oscillating probe atomic force microscopy in fluids is simplified when an oscillating force is applied directly to a magnetized cantilever using a solenoid. The response of the detector is simpler than that obtained with acoustic excitation. Reliable operation over a broad frequency range is achieved with excitation amplitudes of a few nm. This lower amplitude appears to facilitate imaging by means of small asperities on the tip. Images of a DNA plasmid bound weakly to mica in water have a full width of 5 nm when scanned with tips of a nominal macroscopic radius of curvature of 50 nm.

We have developed a method that combines dynamic force microscopy with the simultaneous molecular recognition of an antigen by an antibody, during imaging. A magnetically oscillated atomic force microscopy tip carrying a tethered antibody was scanned over a surface to which lysozyme was bound. By oscillating the probe at an amplitude of only a few nanometers, the antibody was kept in close proximity to the surface, allowing fast and efficient antigen recognition and gentle interaction between tip and sample. Antigenic sites were evident from reduction of the oscillation amplitude, as a result of antibody-antigen recognition during the lateral scan. Lysozyme molecules bound to the surface were recognized by the antibody on the scanning tip with a few nanometers lateral resolution. In principle, any ligand can be tethered to the tip; thus, this technique could potentially be used for nanometer-scale epitope mapping of biomolecules and localizing receptor sites during biological processes.

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We have used a series of metalloporphyrin compounds to test for a relationship between the contrast of STM images and the electrochemical properties of the molecules. Molecules were tethered to a gold (111) surface by means of an isothiocyano linkage and both images and current−voltage (I−V) curves were obtained with the sample submerged in oxygen-free mesitylene. The contrast of the reducible molecules changed strongly with bias, and the corresponding I−V curves were highly asymmetric. The derivative of these curves (dI/dV) had a Gaussian-shaped peak at a voltage characteristic of the compound, although local measurements showed that there was considerable variation in this value from molecule to molecule of a given compound. These bias-dependent features were not observed in the less electroactive molecules, so the STM is capable of distinguishing electroactive molecules from non-electroactive molecules as we demonstrate with images of mixed films. We discuss one- and two-step electron-transfer mechanisms which are consistent with these observations.