A Self-Powered Biosensor System: Integrating Upcycled MFC Transducers With Low-Voltage Interface Circuits

Developing autonomous, self-powered instrumentation systems is critical for distributed Environmental Internet of Things (E-IoT) wastewater monitoring. This study presents a low-cost, self-powered measurement framework featuring a proton exchange membrane (PEM)-free, single-chamber microbial fuel cell (MFC) as a bioelectrochemical transducer. Utilizing a "strain-independent" mixed-culture biofilm within an upcycled PET bottle structure, the system converts organic analytes into electrical signals. A five-point calibration demonstrated a linear range of 50–250 mg/L chemical oxygen demand (COD) (R² = 0.98, sensitivity = 0.69 mV/(mg/L), limit of detection = 35 mg/L). Under continuous-flow operation, a single transducer maintained a stable output of 0.14 V for 21 days (coefficient of variation = 2.8%). The MFC units exhibited a peak power density of 8.6 mW/m² (inter-unit CV = 6.8%, n = 12), with a 12-unit array delivering approximately 51 μW of regulated power. A dedicated sensor interface circuit converts the weak ~140 mV bio-signal into a binary digital alarm output (0/3.3 V) with a total active consumption of only 9.78 μW (interface circuit: 9.24 μW; autonomous reference voltage generation: 0.54 μW). The resulting positive energy margin of 41.22 μW (5.2:1 surplus ratio) validates the thermodynamic feasibility of fully autonomous operation. This work demonstrates a complete "transducer-to-digital" pathway, providing an experimentally verified foundation for battery-free water quality early-warning networks.