**Xiaomei Yan, Jing Tang, David Tanner, Jens Ulstrup and Xinxin Xiao \***

Department of Chemistry, Technical University of Denmark, 2800 Kongens Lyngby, Denmark; xiyan@kemi.dtu.dk (X.Y.); tangjing\_12@163.com (J.T.); dt@kemi.dtu.dk (D.T.); ju@kemi.dtu.dk (J.U.)

**\*** Correspondence: xixiao@kemi.dtu.dk

Received: 8 November 2020; Accepted: 10 December 2020; Published: 14 December 2020

**Abstract:** Self-assembled molecular monolayers (SAMs) have long been recognized as crucial "bridges" between redox enzymes and solid electrode surfaces, on which the enzymes undergo direct electron transfer (DET)—for example, in enzymatic biofuel cells (EBFCs) and biosensors. SAMs possess a wide range of terminal groups that enable productive enzyme adsorption and fine-tuning in favorable orientations on the electrode. The tunneling distance and SAM chain length, and the contacting terminal SAM groups, are the most significant controlling factors in DET-type bioelectrocatalysis. In particular, SAM-modified nanostructured electrode materials have recently been extensively explored to improve the catalytic activity and stability of redox proteins immobilized on electrochemical surfaces. In this report, we present an overview of recent investigations of electrochemical enzyme DET processes on SAMs with a focus on single-crystal and nanoporous gold electrodes. Specifically, we consider the preparation and characterization methods of SAMs, as well as SAM applications in promoting interfacial electrochemical electron transfer of redox proteins and enzymes. The strategic selection of SAMs to accord with the properties of the core redox protein/enzymes is also highlighted.

**Keywords:** self-assembled molecular monolayers; electron transfer; direct electron transfer; bioelectrocatalysis; oxidoreductase; gold electrode; metallic nanostructures
