*5.3. Parameters Specific to Bioelectrocatalysis Stabilization*

Bioelectrocatalysis implies not only the immobilization of the enzyme on a conductive support but also the application of a potential that the enzyme will sense. Hence, the stability of a bioelectrode will include the stability of the enzyme in the immobilized state and the stability of the electrocatalytic response under an electric field which is reflected in most cases by a measure of a current, a voltage or the impedance of the system. However, the variation of the electrochemical signal may have different origins. Progressive leaching of the enzymes from the electrochemical interface can explain a decrease in the catalytic current, while changes in the conformation or in the reorientation of the enzyme can explain modification in the catalytic response due to lower electron transfer rates. Methods probing the relationship between electroactivity and amount of proteins on the electrochemical interface, as well as the relationship between electroactivity and changes in enzyme conformation, are not readily available. Actually, many bioelectrocatalysis-related studies report a decrease of the catalytic current with time. This decrease has long been named as "film loss", suggesting that enzyme leaching from the electrode would be responsible for the current evolution. However, coupling SPR and quartz crystal microbalance with dissipation (QCM-D) to electrochemistry, it was shown that other phenomena than simple enzyme desorption must account for bioelectrode instability [271,272]. In the following, we would like to emphasize some critical parameters that will affect the overall electrocatalytic response stability and the methods developed to get more insights in the overall process.
