Average collision velocity of single yeast cells during electrochemically induced impacts

We recorded current-time (i-t) profiles for oxidizing ferrocyanide (FCN) while spherical yeast cells of radius (r_c ≈ 2 μm) collided with disk ultramicroelectrodes (UMEs) of increasing radius (r_e ≈ 12-45 μm). Collision signals appear as minority steps and majority blips of decreased current overlayed on the i-t baseline when cells block ferrocyanide flux (J_FCN). We assigned steps to adsorption events and blips to bouncing collisions or contactless passages. Yeast cells exhibit impact signals of long duration (Δt ≈ 15-40 s) likely due to sedimentation. We assume cells travel a threshold distance (T) to generate collision signals of duration Δt. Thus, T represents a distance from the UME surface, at which cell perturbations on J_FCN blend in with the UME noise level. To determine T, we simulated the UME current, while placing the cell at increasing distal points from the UME surface until matching the bare UME current. T-Values at 90°, 45°, and 0° from the UME edge and normal to the center were determined to map out T-regions in different experimental conditions. We estimated average collision velocities using the formula T/Δt, and mimicked cells entering and leaving T-regions at the same angle. Despite such oversimplification, our analysis yields average velocities compatible with rigorous transport models and matches experimental current steps and blips. We propose that single-cells encode collision dynamics into i-t signals only when cells move inside the sensitive T-region, because outside, perturbations of J_FCN fall within the noise level set by J_FCN and r_c/r_e (experimentally established). If true, this notion will enable selecting conditions to maximize sensitivity in stochastic blocking electrochemistry. We also exploited the long Δt recorded here for yeast cells, which was undetectable for the fast microbeads used in early pioneering work. Because Δt depends on transport, it provides another analytical parameter besides current for characterizing slow-moving cells like yeast.