An Introduction to AFM
Atomic force microscopy (AFM) is part of a microscopy group called scanning probe microscopy. Atomic force microscopy has roots in scanning tunneling microscopy (STM) which measures topography of surface electronic states using the tunneling current which is dependent on the separation between the probe tip and a highly conductive sample surface. STM research includes viewing charge density waves.
AFM differs from other types of non-optical microscopy in that it can image samples under natural conditions - in air or water - without the samples being placed under destructive artificial conditions, such as drying, coating with metal, vacuum or freezing. It is therefore especially useful for biological applications, and as AFM performance and range of time and spatial resolution improves, increasingly smaller biological samples (such as molecules and molecular assemblies) can be imaged at increasingly faster speeds, producing videos of biological processes at very sharp resolutions (down to several nanometers). The importance of the AFM being developed to allow researchers to track biological processes in real time cannot be overstated - such a capability will allow researchers to actually see, at both spatial (down to the atomic scale) and time resolutions, how disease develops, and how the healthy body functions. One question that AFM can help answer is how proteins pathologically misfold in the human body, producing diseases such as Alzheimer's and Parkinson's Diseases. (Depending on the stage of conformation, proteins fold and misfold at various rates - from nanoseconds to several seconds). Please visit our page on AFM development to read about exciting recent developments in producing high-speed AFMs.
With AFM, a sample is analyzed by probing the surface with a tip, and the interaction between tip and sample is measured. Physical topography, charge density, magnetic field, and other surface properties can be discerned and measured. The Atomic Force Microscope (also AFM) can also determine the mechanical properties of molecules by pulling on them - a technique called single molecule force spectroscopy, which was developed by Hermann Gaub and Julio Fernandez with the AFM. Figure 1 is a schematic showing the basic operation of the AFM [Oberhauser et al. PNAS (January 2001), Vol. 98 (2): 468-472]. Figure 1 is also a particular example of the single-molecule force spectroscopy technique, as it shows the AFM pulling on a protein with three folded domains (the bunchy structures linked with the protein backbone).
Figure 1. Basic operation of the AFM. As the AFM cantilever probes the surface by moving its tip along its contours, or when it pulls on a protein, the movement of the cantilever is detected with a laser beam that is focused to beam to the head of the cantilever and refracts into a photodetecter. The movement, or deflection, of the cantilever deflects the laser correspondingly, and this creates an image or produces other data about the surface or the sample (which may be a molecule). In single molecule force spectroscopy, the cantilever is pressed against a layer of proteins attached to a substrate, and the tip adsorbs a single protein molecule, which is then extended. Extension of the molecule by retraction of the piezoelectric positioner results in deflection of the cantilever. [Oberhauser et al. PNAS (January 2001), Vol. 98 (2): 468-472]
Figure 2. A small cantilever about to image red blood cells. The rounded bump on the top of the cantilever refracts a laser that beams up to a photodiode. As the shapes and topography of the surface determine the movement of the cantilever, the laser's path along the photodiode moves correspondingly, recording the properties of the surface and thus producing an image. A small tip can be seen at the end of the underside of the cantilever.
Figure 3. One of our very early prototype AFMs.
Figure 4. A high-speed AFM prototype in 2005.