Enzyme based amperometric wide field biosensors: Is single-molecule detection possible?

This review discloses the technological advances involving enzyme-based amperometric biosensors engaging challenging limits of detection as low as a single molecule. At first, we summarise the most recent findings concerning electrode modification toward the enhancement of the enzyme loading accomplished mainly through the deposition of nanomaterials. The increase of the electron transfer (ET) rate is mostly based on the enzyme site-specific immobilization through the analysis of the enzyme structure/sequence and protein bioengineering is overviewed. However, both approaches are not appropriate to develop enzyme-based amperometric biosensors able to reach reliable analytical detections below micro-/nano-molar. The last part is devoted to single-molecule electrochemistry that has been widely exploited as a near-field approach in the last decades as a proof-of-concept for the detection of single ET events. Organic electrochemical transistors operated as Faradaic current amplifiers do not detect below micro-/nano-molar. We here propose an alternative approach based on the combination of an electrochemical cell with a bipolar junction transistor in the extended base configuration, drawing some conclusions and future perspectives on the detection of single ET events at a large electrode for the development of Point-of-Care devices.