Electrochemical Impedance Spectroscopic Analysis of Diffusion-Layer Thickness Distribution Associated with Flagellar Motion of Volvox carteri (Supporting Information)

Although electrochemical methods have been used to relate microbial motility with ionic or redox signals, the evaluation of diffusion-layer variations induced by such motility using electrochemical impedance spectroscopy (EIS) remains limited. Thus, in this study, EIS is used to investigate how the flagellar activity of Volvox carteri modulates the diffusion layer above an electrode. Finite-element simulations and distributed equivalent-circuit modeling are performed for both uniform and non-uniform diffusion-layer thicknesses. Simulations predict that phototactic flagellar convection results in impedance spectra with a slope lower than 45° in the mid-frequency region and a pronounced finite-diffusion bend at low frequencies. These features are validated through experiments involving Volvox-immobilized electrodes under dark and illuminated conditions. Light irradiation reduces the effective diffusion-layer thickness from 2.4 × 10−2 cm to 6.6 × 10−3 cm and introduces a distributed thickness ranging from 4.5 × 10−4 to 9.0 × 10−3 cm, as extracted by transmission-line fitting. These results provide quantitative insights into bio-induced mass-transport modulation and demonstrate the applicability of distributed diffusion models in bioelectrochemical systems.尽管电化学方法已被用于关联微生物运动与离子或氧化还原信号，但利用电化学阻抗谱（electrochemical impedance spectroscopy, EIS）评估此类运动引发的扩散层变化的研究仍较为有限。据此，本研究采用电化学阻抗谱，探究卡特团藻（Volvox carteri）的鞭毛活动如何调控电极表面上方的扩散层。本研究针对均匀与非均匀扩散层厚度，分别开展了有限元模拟与分布式等效电路建模。模拟结果预测，趋光性鞭毛对流会使阻抗谱在中频区域呈现低于45°的斜率，并在低频区域出现显著的有限扩散弯折特征。上述特征通过在黑暗与光照条件下的卡特团藻固定电极实验得到了验证。通过传输线拟合提取的结果显示，光照可将有效扩散层厚度从2.4×10⁻² cm降至6.6×10⁻³ cm，并引入了4.5×10⁻⁴ cm至9.0×10⁻³ cm范围的分布式厚度。本研究结果为生物诱导的物质传输调控提供了定量认识，并证实了分布式扩散模型在生物电化学系统中的适用性。