Molecular recognition between biotinylated bovine serum albumin and polyclonal, biotin-directed IG antibodies has been measured directly under various buffer conditions using an atomic force microscope (AFM). It was found that even highly structured molecules such as IgG antibodies preserve their specific affinity to their antigens when probed with an AFM in the force mode. We could measure the rupture force between individual antibody-antigen complexes. The potential and limitations of this new approach for the measurement of individual antigen/antibody interactions and some possible applications are discussed.

We calculate the eigenfrequencies of a rectangular cantilever of an atomic force microscope immersed in a fluid or a gas. To do so, the problem of combined elastomechanical and hydrodynamical equations is solved approximatively. The results are compared with experimentally obtained frequencies. For water the difference between experiment and theory is less than 4% if the ratio of the height of the cantilever to the width is less than 1/20 and if the corresponding eigenmode has at least four nodes.

We have applied the non-contact dynamic force microscopy method to investigate soft biological materials such as hexagonally-packed intermediate layers, DNA, and tobacco mosaic virus under ambient conditions. This method, where a stiff cantilever is oscillated close to its resonance frequency with an amplitude of 0.3-1.5 nm above the sample, allows highly reliable investigation of soft organic matter with minimized normal and lateral forces between tip and sample. The vertical and lateral resolution are determined to be c 1 A and 1-2 nm, respectively, comparing favorably to established results from repulsive-mode scanning force microscopy experiments on adsorbate covered surfaces in liquids. The interaction forces are found to be attractive, dominated by damping mechanisms and attractive force gradients of capillary and van der Waals interactions.

A simple model is presented for rectangular cantilevers when vibrating in various media. The mass of the surrounding medium affected by the motion of the lever is calculated. It depends on the dimensions of the lever, on the excited mode, and on the density of the medium. Although the viscosity of the media is not taken into account, the resulting predictions for the resonance frequencies agree well with experimental data obtained for levers in air and water up to the seventh harmonic.

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