Single molecule tracking scheme using a near-field scanning optical microscope

The use of a near-field scanning optical microscope (NSOM) to measure the diffusion of fluorescently tagged molecules on a quasi-two-dimensional system is described. The heart of the apparatus lies in the circular motion of the NSOM probe, allowing the detection of small displacements on single molecules. Using a 100 nm tip, and a simulated diffusion coefficient of Dsimilar to1x10(-12) m(2)/s, we found that molecules can be tracked within similar to40 nm over several seconds. We foresee that D as large as similar to5x10(-12) m(2)/s can be measured with minor modifications to the experimental setup. These values of D encompass those found in many liquid crystalline systems, particularly lipids in bilayer membranes. (C) 2002 American Institute of Physics.