Influence of Electroosmotic Flow on Stochastic Collisions at Ultramicroelectrodes

The collision events of single Lactococcus lactis bacteria at Pt disk ultramicroelectrodes (UMEs) were characterized using electrochemistry with correlated microscopy. A finite element model was developed which applied coupled simulations of concentration and solution velocity to elucidate the influence of electroosmotic flow on transport of bacteria near the electrode. The model established that, in stochastic collision experiments with steady-state oxidation at disk UMEs in low ionic strength solutions, electroosmotic flow occurring at the glass insulation of the electrode produces significant convection in the vicinity of the electrode disk (velocities >50 mu m/s). For L. lactis, the particle velocity due to convection driven by electroosmotic flow exceeded that of electrophoresis at locations radial to the electrode disk, leading to transport away from the electrode. Correlated microscopy of collision experiments of L. lactis using a 5 mu m radius Pt disk UME in 2 mM ferrocenemethanol (FcM) with either 0.035 or 0.1 mM KCl confirmed that L. lactis experienced transport by convection due to electroosmotic flow. Velocities of L. lactis extracted from correlated microscopy movies collected at the two KCl concentrations agreed with the finite elements model. Current-time (i-t) curves recorded during the collision experiments showed transients that occurred when colliding L. lactis reduced transport of FcM to the electrode. The current transients had step shapes when L. lactis collided and adsorbed and spike shapes when they collided and then moved away from the electrode. By comparing the microscopy to simulations, we concluded that the driving mechanism for the spike-shaped transients was convection due to electroosmotic flow. Moreover, these findings suggest that electroosmotic flow is significant for particle transport in stochastic collision experiments in solutions of low ionic strength, regardless of the analyte.