Feedback controller design for tracking a single fluorescent molecule

In this paper, we revisit the problem of tracking a single fluorescent molecule in a laser-scanning confocal microscope. We utilize optimal control theory to design a feedback controller and use numerical simulation to analyze its ability to track a molecule in two dimensions. A major theme in this paper is the inclusion of all relevant experimental limitations including moderate signal-to-noise fluorescence detection and the finite-bandwidth response of an electromechanical translation stage. The results presented here demonstrate the experimental feasibility of tracking single fluorescent molecules with diffusion coefficients as large as 0.1 mum(2)/ms. We show that for molecules that are even moderately confined along a third dimension (as, for example, by microscope cover slides), the two-dimensional tracking algorithm appears to be robust and effective. We expect this technology to enable single-molecule experiments with long observation times and high time-resolution.