Nanoarchitecture of Plasma Membrane Visualized with Atomic Force Microscopy
The atomic force microscope (AFM) was developed in 1986 (1). Like the scanning-tunnelling microscope (STM) that preceded it, the AFM is a scanning-probe microscope. These instruments differ from “conventional” optical and electron microscopes because they work by moving a probe back and forth across a surface and recording features as the probe encounters them. Scanning-probe microscopes thus produce images that are not compromised by the limitations of the wavelengths of the various types of electromagnetic radiation. This means that very high resolution can be obtained. In some cases, studying physically hard samples down to atomic resolution, but with softer samples, the maximum resolution is at present in the order of 1 and 0.1 nm in the lateral and vertical directions, respectively. The probe of the STM is made from a conductive material and the instrument works by measuring a current — the “tunnelling current” between the probe and the sample. Thus, the STM is suitable for use only to study electrically conductive materials. The AFM, on the other hand, can be used on nonconducting specimens. As a result of the high resolution and the fact that it can be used on samples under fluid, the AFM (which was originally designed with applications in physical sciences in mind) soon attracted interest among biological scientists (2) as a tool for imaging cellular and subcellular structures under physiological or near-physiological conditions and to study dynamic features of molecular behavior at high resolution in “real time” under these conditions. Biological work using the AFM has shown considerable growth throughout the 1990s (3). As a result, partly owing to technical innovations, the realization that useful images could be obtained under fluid, together with the ready availability of microscopes from a number of manufacturers, the field has moved on and expanded far beyond that envisaged when the instrument was developed.
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