The atomic force microscope (AFM) is used for high resolution imaging of atomic scale structures. The use of AFM for imaging biological samples such as cells and tissues poses certain challenges as these samples have to be kept alive in liquids to replicate their actual physiological conditions.
Troll AFM
Researchers at the University of Illinois have developed a technique they refer to as “trolling AFM” to address the challenges in conventional imaging of biological samples. The new method has improved the quality factor of AFM imaging by a magnitude of two orders.
Standard AFM has a probe attached to the end of a cantilever arm submerged in the liquid holding the sample to facilitate the probe to scan the sample surface for electrical, mechanical and chemical properties. The probe cannot be dragged across the sample as it could damage the soft cells and tissues. The AFM is thus operated in oscillation mode with the probe tip tapping the sample at places for detection of resistance. The oscillation however gives rise to other challenges. The cantilever is comparatively large. The oscillations of the lever affect the liquid holding the sample and leads to hydrodynamic drag. The resulting noise drowns out the required data from the sample. To overcome the high noise, the probe tapping has to be much harder. The pressure from the probe tip not only causes deformation of cells, but also cannot image the contours and structure of the cell membrane.
The team at the University of Illinois devised a technique in which the cantilever oscillates on top of the liquid and the probe is extended to touch the sample by means of a long nanoneedle. The use of the nanoneedle reduces drag drastically as liquid displacement is very minimal and the responsive nanoneedle necessitates only small amplitudes of cantilever oscillation. Labeled “trolling AFM” in reference to the fishing technique which has part of the fishing line above water and a part immersed in water, the minimal disturbance offered facilitates high frequency operation to image cellular structures that were difficult to detect using existing methods.