ABSTRACT
The AFM was fi rst introduced by Binnig, Quate and Geber in 1986 to provide high quality topographic images of a sample surface and to detect intermolecular forces with atomic scale resolution (Binnig et al. 1986). The main components of an AFM are the piezoelectric scanner, detector and a cantilever probe (Fig. 3A). AFM cantilevers are nanofabricated using either silicon or silicon nitride with typical tip radii of 10 nm or less. Most
AFM operate by refl ecting a laser beam off the back of the AFM cantilever and into a photodiode detector (Fig. 3A). A sample is mounted on the piezoelectric scanner and the distance between the AFM tip and sample is held constant by the detector and piezoelectric scanner via a closedloop feedback mechanism. During operation, the detector senses subtle changes in defl ection experienced by the tip as it moves across the sample and sends a signal to the piezoelectric scanner to adjust the tip-to-sample distance (z-axis) to prevent tip and sample damage (Fig. 3B). An AFM image is acquired by the movement of the scanner in the z-direction as the tip rasters in the x-and y-direction over the sample (Fig. 3B), thus creating a “topographic map” of the sample surface (Fig. 3C). Since AFM can operate under fl uid and physiological conditions while still provide image resolution that rivals electron microscopy, such as TEM (Nag et al. 1999), the AFM has been used to resolve intricate surface structures of individual molecules or molecular assemblies, including DNA (Lyubchenko et al. 1992), proteins (Browning-Kelley et al. 1997) and cells (Rossetto et al. 2007).
