Observation of a single particle diffusive motion on the membrane by 3-dimensional particle tracking microscope.

Three-dimensional motion of particles on the surface of giant vesicle membranes has been tracked with nanometer scale resolution using a quadrant photodiode, which is used as a position sensitive detector (PSD), placed in the diffraction pattern formed behind a 100x microscopic objective. Two He-Ne beams were focused to trap a single particle in solution between cover slips with 100 micro-meter gap. The single particle is brought very close to the surface of a membrane system by optical tweezer. By reducing the optical power, the particle sits on the membrane surface and moves diffusively. The diffusing motion is measured using a fast-feedback controller designed to respond to the axial and the lateral position of the particle simultaneously in less than 250 micro-seconds. This work has better positional and temporal accuracy of 3-dimensional particle tracking than conventional video-tracking methods.