Average collision velocity of single yeast cells during electrochemically induced impacts.

We recorded current-time (-) profiles for oxidizing ferrocyanide (FCN) while spherical yeast cells of radius ( ≈ 2 μm) collided with disk ultramicroelectrodes (UMEs) of increasing radius ( ≈ 12-45 μm). Collision signals appear as minority steps and majority blips of decreased current overlayed on the - baseline when cells block ferrocyanide flux (). We assigned steps to adsorption events and blips to bouncing collisions or contactless passages. Yeast cells exhibit impact signals of long duration (Δ ≈ 15-40 s) likely due to sedimentation. We assume cells travel a threshold distance () to generate collision signals of duration Δ. Thus, represents a distance from the UME surface, at which cell perturbations on blend in with the UME noise level. To determine , we simulated the UME current, while placing the cell at increasing distal points from the UME surface until matching the bare UME current. -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 /Δ, 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 - signals only when cells move inside the sensitive T-region, because outside, perturbations of fall within the noise level set by and / (experimentally established). If true, this notion will enable selecting conditions to maximize sensitivity in stochastic blocking electrochemistry. We also exploited the long Δ recorded here for yeast cells, which was undetectable for the fast microbeads used in early pioneering work. Because Δ depends on transport, it provides another analytical parameter besides current for characterizing slow-moving cells like yeast.