**3.4 Effects of oxygen fluctuations on electrochemical performances**

Since the O2 level in biofluids may vary, first-generation biosensors, employing O2-dependent mechanism, are subject to inaccuracy. This issue can be addressed by using fluorocarbon pasting liquids to supply internal O2 [65]. Using redox mediator as a second-generation sensor is another way to eliminate this error. Furthermore, FAD-dependent glucose dehydrogenase is an option to address O2-dependent problems due to its O2-insensitive nature, compared with GOx [49]. In addition, because of the high rate of homogeneous electron transfer rate between GOx and oxygen, GOx prefers to transfer electrons to oxygen rather than to the electrode, causing undesirable O2 competition effect [66]. Moreover, for BFCs and selfpowered sensors, the commonly used ORR cathode may cause the error under anaerobic conditions. The use of Ag2O/Ag redox cathode, which does not depend on ORR, can be used to operate BFCs, mitigating the possible O2 errors [30, 67]. Note that the reduction potential of Ag2O/Ag (0.342 V vs. SHE) is close to that of O2/ OH<sup>−</sup> (0.401 V vs. SHE) at pH 7. Moreover, using O2-rich cathode is another possible option to mitigate O2-deficit effects [68].
