Selective enzymes at the core of advanced electroanalytical tools: The bloom of biosensors

Enzymes are biological catalysts whose mission is to accelerate biochemical reactions in living organisms. Once extracted from cells they can be used in a broad range of applications for the benefit of humankind. One of the best examples is the so-called biosensor, i.e., a bioanalytical device where enzymes play a key role in the selective recognition of the analyte. Either as point-of-care tests, bench instruments, or continuous analysis systems, they have been useful in clinical diagnostics, environmental and food control, forensic sciences, and industrial processing, often replacing bulky and expensive equipment that require trained operators. Enzymes are by far the most commonly used biological elements in biosensors, being typically associated to electrochemical transducers, as in the case of the biggest commercial success, the blood glucose meter. In this context, redox enzymes are clearly dominant, since electrons exchange during the biochemical reaction facilitates their interaction with electrodes. However, hydrolases can also be used, provided that an electroactive species participates in the reaction. In this review, we address the topic of enzyme-based biosensors, with special emphasis on the electrochemical ones. The manuscript will cover four representative classes of enzymes, namely oxidases, dehydrogenases, reductases, and hydrolases.