Eco-friendly chemical oxygen demand (COD) sensors are highly desired with respect to the importance of COD determination in environmental protection. In this work, a new FTO/TiO₂/PbO₂ (FTO = fluorine-doped tin oxide) electrode was fabricated with a two-step method and used as an eco-friendly electrochemical COD sensor. The interlayer TiO₂ was employed to strengthen the adhesion of PbO₂ on the FTO substrates by providing a large TiO₂/PbO₂ interface area. The effects of the factors including applied potential, supporting electrolyte concentration and stirring speed on the sensing performance were investigated. Under the optimized conditions, linear responses to the COD of water with different COD sources were achieved, and a linear range from 5 to 120 mg/L was obtained in the case of sucrose as the COD source. The relative standard deviations (RSD) were determined to be less than 9% for the glucose solutions with the COD of 7.5, 12.5 and 17.5 mg/L. For real sample analysis, the obtained results were comparable with those measured with the conventional dichromate method, with a relative error less than 11%.

Self-powered glucose biosensor (SPGB) is of great interest due to the advantages including single configuration, good stability and particularly no need of external power sources. Herein, a mediator-free SPGB with high sensitivity and good selectivity is constructed based on a hybrid enzymatic biofuel cell (EBFC) composed of a glucose oxidase/cobalt phthalocyanine/1-pyrenebutyric acid/ buckypaper (GOD/CoPc/PBA/BP) bioanode and a MnO₂/PBA/BP capacitive cathode. The efficient electron transfer from GOD to electrodes is achieved successfully through the anode oxidation of hydrogen peroxide (H₂O₂), one nature product of glucose oxidation catalyzed by GOD, thus avoiding the potential drawbacks posed by the use of redox mediators. CoPc servers as an efficient catalyst to lower the anode potential required by the reaction of H₂O₂ to 0.17 V. The MnO₂/PBA/BP capacitive cathode is utilized because it can not only provide a high discharge potential and adequate capacitance to match the bioanode well, but also exhibit no potential interference to the anodic reaction. The concentration of glucose can be detected simply by measuring the output of the SPGB and a wide linear detection range from 0.5 to 8 mM has been obtained with high sensitivities of 48.66 and 32.12 μA·cm^-2·mM^-1 with and without stirring, respectively. The recoveries of glucose in grape juice and human serum are in the range from 99.5% to 101.2% with the relative standard deviation (RSD) less than 8%, indicating the good promise of the SPGB in sensing glucose in real samples.