Scientific and technological innovation is increasingly playing a role for promoting the transition towards a circular economy and sustainable development. Thanks to its dual function of harvesting energy from waste and cleaning up waste from organic pollutants, microbial fuel cells (MFCs) provide a revolutionary answer to the global environmental challenges. Yet, one key factor that limits the implementation of larger scale MFCs is the high cost and low durability of current electrode materials, owing to the use of platinum at the cathode side. To address this issue, the scientific community has devoted its research efforts for identifying innovative and low cost materials and components to assemble lab-scale MFC prototypes, fed with wastewaters of different nature. This review work summarizes the state-of the-art of developing platinum group metal-free (PGM-free) catalysts for applications at the cathode side of MFCs. We address how different catalyst families boost oxygen reduction reaction (ORR) in neutral pH, as result of an interplay between surface chemistry and morphology on the efficiency of ORR active sites. We particularly review the properties, performance, and applicability of metal-free carbon-based materials, molecular catalysts based on metal macrocycles supported on carbon nanostructures, M-N-C catalysts activated via pyrolysis, metal oxide-based catalysts, and enzyme catalysts. We finally discuss recent progress on MFC cathode design, providing a guidance for improving cathode activity and stability under MFC operating conditions.

Wireless biosensor systems were developed in our lab for monitoring blood glucose concentrations in fish as an indicator of fish stress. However, uniform immobilization of the enzyme on the surface of the electrode is difficult, so the sensor response is typically reduced at a range of high glucose concentrations during the stress monitoring. In this study, we attempted to enhance sensor response by using a self-assembled monolayer-immobilized enzyme. Glucose oxidase was immobilized on a working electrode modified with a self-assembled monolayer. The proposed biosensor showed a good correlation between the output current and the glucose concentration range of 10–3500 mg dL⁻¹ under an optimized working condition. The dynamic measurement range of this newly developed sensor is significantly improved, especially over a high concentration range, which helps the sensor to achieve better performance in dramatic changes in the stress response of fish. In addition, we used biological samples from test fish and obtained a good correlation coefficient between the sensor output current and the glucose concentration using a conventional method. The proposed wireless biosensor system was also applied to monitor fish stress responses in real time through different stressors and to obtain some precise data that reflect real fish stress responses.

Blood cortisol concentration change in fish is widely used as an indicator of stress. In this study, we developed a flow system which equipped by an electrode replacement unit which can replace sensor conveniently. Multiple sensors sequentially disposed vertically in the holder, and the conductive portion of the sensor to be used was bonded with which set in the bottom of the holder. After measurement, next sensor was inserted into the holder and the replacement would be completed by sequentially moving. The flow system was constructed by replaceable working electrodes which sequence placed in the electrode replacement unit and count electrode into a reaction loop which operated by micro-pump. During the measurement, the potassium ferricyanide was transported into the flow loop and applied a voltage (+250 mV). Output current values were measured both before and after the immune-reaction and the change between them was calculated to determine the cortisol levels. As a result, replacement of the immunosensor performed in only several seconds and a good linear relationship was observed between the sensor response and cortisol in the range of 0–40 ng ml−1. We also performed a measurement of cortisol levels in plasma of tilapia (Oreochromis niloticus) both by proposed biosensor system and ELISA which got a good correlation. We believe that the proposed system can provide a simple and rapid detection of cortisol levels for fish and its principle could be a foundation of a continuous in vivo monitoring of fish cortisol change in fish in the future