Investigation of Lattice Surface Layers by Scanning Probe Microscopy
Scanning probe microscopy (SPM) is emerging as an important complementary technique to conventional microscopy for high-resolution surface investigation (Wickramasinghe, 1990). In SPM, no lenses are used and the image is formed by a finely tipped probe that scans the specimen surface (see Fig. 1). Using this technique, direct quantitative analysis of a specimen’s surface topology, electronic structure, as well as other physical properties are possible in vacuum, gaseous, or liquid environments. Two types of SPM, scanning tunneling microscopy (STM; Binning et al., 1982), and atomic force microscopy, (AFM; Binning et al., 1986) image topographical details on biological surfaces to nanometer resolution. In STM, conductivity is an important factor in image formation since a tunneling current is established between the microscope tip and the surface under study. However, conduction through biological surfaces is a complex process (Spong et al., 1989; Travaglini et al., 1988), so these surfaces are often prepared for STM imaging by overlaying them with a conductive film Such films tend to obscure structural detail and further progress is needed in the development of conductive fine-grained films suitable for high resolution work (Wepf et al., 1991; Amrein et al., 1991). The overall design of the AFM is based on that of the STM but the probe used is a force sensor which is mounted on a cantilever to measure interatomic attractive/repulsive forces (Burnham et al., 1991). This important operational difference enables the native topology of uncoated surfaces to be imaged. Biological applications of STM/AFM have been largely devoted to topographical analysis, but reports of chemical differentiation by tunneling spectroscopy (Spong et al., 1989), controlled ‘nanodissection’ of biological deposits by AFM (Hoh et al., 1991; Henderson, 1992), and AFM imaging of polymerizing macromolecules under water (Drake et al.,1989) have also been published. For recent reviews of biological applications of SPM see Engel, 1991; Edstrom et al., 1990; Blackford et al., 1991a; and Hansma et al., 1988.
KeywordsAtomic Force Microscopy Scanning Tunneling Microscopy Scan Probe Microscopy Outer Face Purple Membrane
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