*Review* **Enzyme-Based Biosensors: Tackling Electron Transfer Issues**

#### **Paolo Bollella \* and Evgeny Katz**

Department of Chemistry and Biomolecular Science, Clarkson University, Potsdam, New York, NY 13699-5810, USA; ekatz@clarkson.edu

**\*** Correspondence: pbollell@clarkson.edu

Received: 30 May 2020; Accepted: 19 June 2020; Published: 21 June 2020

**Abstract:** This review summarizes the fundamentals of the phenomenon of electron transfer (ET) reactions occurring in redox enzymes that were widely employed for the development of electroanalytical devices, like biosensors, and enzymatic fuel cells (EFCs). A brief introduction on the ET observed in proteins/enzymes and its paradigms (e.g., classification of ET mechanisms, maximal distance at which is observed direct electron transfer, etc.) are given. Moreover, the theoretical aspects related to direct electron transfer (DET) are resumed as a guideline for newcomers to the field. Snapshots on the ET theory formulated by Rudolph A. Marcus and on the mathematical model used to calculate the ET rate constant formulated by Laviron are provided. Particular attention is devoted to the case of glucose oxidase (GOx) that has been erroneously classified as an enzyme able to transfer electrons directly. Thereafter, all tools available to investigate ET issues are reported addressing the discussions toward the development of new methodology to tackle ET issues. In conclusion, the trends toward upcoming practical applications are suggested as well as some directions in fundamental studies of bioelectrochemistry.

**Keywords:** enzyme-based biosensors; direct electron transfer (DET); redox enzymes; nanostructured electrodes; protein film voltammetry (PFV)
