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 (Δ t ≈ 15-40 s) likely due to sedimentation. We assume cells travel a threshold distance ( T ) to generate collision signals of duration Δ t . 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 /Δ t , 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 Δ t recorded here for yeast cells, which was undetectable for the fast microbeads used in early pioneering work. Because Δ t depends on transport, it provides another analytical parameter besides current for characterizing slow-moving cells like yeast.