Single Molecule Tracking of a Semiflexible Polyelectrolyte Chain in Solvent Under Uniform Electroosmotic Flows

Direct single molecule tracking of a polyelectrolyte chain by using fluorescence microscopy has allowed both the assumptions and the predictions of relevant theories to be tested. The center-of-mass displacement is determined as a function of the time that elapses between images, where the radius of gyration can be estimated from a first moment of the image distribution. The translational self-diffusion for the molecule is an ensemble property of the mean square displacement (MSD) with lag time in each trajectory. Experimentally viable two-dimensional imaging of semiflexible polyelectrolyte was performed on a fluorescein-labeled xanthan chain in electroosmosis-driven uniform flow fields. The radius of gyration was almost constant under variations of the electroosmotic flow velocity determined by an externally applied electric field. We try to develop a correction of the MSD in flow field, taking into account the velocity fluctuations. Its advantage allows acquiring the linear fit for the MSD vs lag time, for a good estimate of the translational diffusion. Increasing behavior of the diffusion with increasing fluid velocity ensures a quadratic equation fit, which should connect with the convective effect. Our results exhibit a screening effect such that strong screening caused by a high ionic concentration leads to higher diffusion due to the compact chain conformation. Considering the uniform flow serves as a basis for understanding the behavior of individual polyelectrolyte chains under controlled fluidic flow in confined spaces.