Role of Extracellular Electrical Signal Transfer on Sensitivity in Electrochemically Active Biofilm Sensors for Signaling Water Acute-Toxicity Exposure

Electrochemically active biofilm (EAB) sensors have been widely used for real-time monitoring of water biotoxicity. Although extracellular electron transfer (EET) drives the electrical signal output of EAB sensors, its relationship with sensitivity is poorly understood. This raises two critical questions: (1) Do toxicants affect EET performance? (2) What is the relationship between EET performance and sensitivity. Herein, we employed a double-electrode-controlled electrochemical gating method (EGM) to evaluate the effects of different toxicants on EET performance across multiple biological scales, ranging from mixed- and purified-species biofilms to isolated OmcA proteins. Results indicated that five representative toxicants (0.02% formaldehyde, 5 mg/L NO₂^-, 5 mg/L tobramycin, 5 mg/L Cu²⁺, and 5 mg/L SDS) rarely impacted EET performance directly. On this basis, we probed the link between EET performance and sensitivity using riboflavin (RF) and anthraquinone-2,7-disulfonate (AQDS). After a 30 min toxicity exposure, the inhibition ratios were ranked as follows: AQDS-EABs > Control > RF-EABs. Although RF observably reduced the resistance, the high capacitance weakened the sensitivity. It is suggested that reducing resistance alone could not result in a higher sensitivity, and capacitance effects cannot be overlooked through modeling and electrochemical analysis. This study, therefore, proposes the time constant as a suitable metric for evaluating the relationship between the EET performance and sensitivity.