Polypyrrole-ferrocene-biofilm electrode for rapid BOD biosensing and energy recovery from wastewater: Bayesian...

Developing high-performance bioelectrodes for environmental sensing and energy recovery remains challenging. We present a ternary platform integrating polypyrrole (PPy), ferrocene, and an electroactive Rhodococcus fascian biofilm on graphite paste electrodes. Optimal PPy loading (50 μg per 0.2 cm²) was determined by impedance spectroscopy, giving a charge-transfer resistance of 28 ± 6 Ω·cm² for the PPy-ferrocene-biofilm system - more than 1200 times lower than that of the PPy-biofilm electrode. The bioelectrode served both as a BOD biosensor and as an anode in a 3D-printed dual-chamber microbial fuel cell (MFC). The biosensor exhibited a wide linear range of 4-600mg O₂/dm³, a ∼5min response time, and 6% RSD. For the first time in a whole-cell BOD sensor, Bayesian calibration reduced the required standards from 15 to 7 while preserving accuracy and providing uncertainty quantification for the Hill equation parameters. Validation against the standard BOD₅ method on real surface waters yielded R²=0.9828. In the MFC, the ternary anode delivered a power density of 2.0mW·m^-2 and removed 89% of COD and 92% of BOD from domestic wastewater. These results confirm the synergistic effect of the conductive polymer, mediator, and biofilm, offering a versatile platform for rapid water-quality monitoring and wastewater treatment with concomitant electricity generation.