In stochastic blocking electrochemistry, microparticles generate individual current steps when they adsorb on a microelectrode and decrease the current and flux of a redox mediator reacting at the surface. The amplitude of the current step informs on particle size and landing locus, while step frequency correlates with particle transport. Here, we report a new method to estimate the average arrival velocities of single rod-shaped bacteria (bacilli). The method relies on simulating the nearby threshold distance from the surface where the bacillus no longer perturbs mediator flux and the current step approaches zero. We estimated the average velocities of bacillus arrival by dividing the threshold distance over the current step duration, a parameter that here we detect for the first time and increases with bacillus length. By comparing diffusional fluctuations to bacillus average velocity, we estimated diffusion and migration contributions as a function of bacterium size. Average arrival velocities increase with bacillus length at the same time as migration intensifies and diffusion weakens. Our analysis is universal and more effective in determining transport mode contributions than the present approach of comparing theoretical and experimental step frequencies. Uncertainty in landing locus is inconsequential because the step duration used to calculate the average arrival speed already contains such information and knowing bacillus electrophoretic mobility or ζ-potential is not needed. Additionally, by simulating and assigning edge landings to the most repeated values of current steps in a recording, we obtain bacilli lengths and widths similar to scanning electron microscopy, from which we infer landing orientation.

Current–time recordings of emulsified toluene microdroplets containing 20 mM Ferrocene (Fc), show electrochemical oxidation peaks from individual adsorption events on disk microelectrodes (5 μm diameter). The average droplet diameter (∼0.7 μm) determined from peak area integration was close to Dynamic Light Scattering measurements (∼1 μm). Random walk simulations were performed deriving equations for droplet electrolysis using the diffusion and thermal velocity expressions from Einstein. The simulations show that multiple droplet-electrode collisions, lasting ∼0.11 μs each, occur before a droplet wanders away. Updating the Fc-concentration at every collision shows that a droplet only oxidizes ∼0.58% of its content in one collisional journey. In fact, it would take ∼5.45 × 106 collisions and ∼1.26 h to electrolyze the Fc in one droplet with the collision frequency derived from the thermal velocity (∼0.52 cm/s) of a 1 μm-droplet. To simulate adsorption, the droplet was immobilized at first contact with the electrode while the electrolysis current was computed. This approach along with modeling of instrumental filtering, produced the best match of experimental peaks, which were attributed to electrolysis from single adsorption events instead of multiple consecutive collisions. These results point to a heightened sensitivity and speed when relying on adsorption instead of collisions. The electrochemical current for the former is limited by the probability of adsorption per collision, whereas for the latter, the current depends on the collision frequency and the probability of electron transfer per collision (J. Am. Chem. Soc. 2017, 139, 16923–16931).