Flow-through electrochemical biosensors

The essential difference between the biosensors described in Section 3.2.1.1 and those dealt with in this Section is that, while the former must have a minimal flow-cell void volume in order to provide adequate sensitivity because measurements are made on the solution held in the cell, electro-analytical sensors rely on measurements during the process that takes place at the electrode/solution interface — the solution must be in contact with the sensing surface, so the cell volume is not a limiting factor. 

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Figure 5.11 — Diagram of a generic electrochemical flow-through biosensor using a permselective film coating the surface of the en2yme electrode. RE reference electrode AE auxiliary electrode W waste.

The development of biosensors is parallelled by that of analyzers using enzymes as dissolved reagents or immobilized in flow-through reactors coupled with electrochemical or spectrophotometric indication. 

Coupling between a biologically catalyzed reaction and an electrochemical reaction, referred to as bioelectrocatalysis, is the constructional principle for enzyme-based electrochemical biosensors. This means that the flow of electrons from a donor through the enzyme to an acceptor must reach the electrode in order for the corresponding current to be detected. In case a direct electron transfer between the active site of an enzjane and an electrode is not possible, a small molecular redox active species, e.g. hydrophobic ferrocene, meldola blue and menadione as well as hydrophilic ferricyanide, can be used as an electron transfer mediator. This means that the electrons from the active site of the enzyme reduce the mediator molecule, which, in turn, can diffuse to the electrode, where it donates the electrons upon oxidation. When these mediator molecules are employed for coupling of an enzymatic redox reaction to an electrode at a constant potential, the resulting application can be referred to as mediated amperometry or mediated bioelectrocatalysis. 

Simkova D, Labuda J (2011) Electrochemical DNA biosensors and flow-through analysis. A review. Curr Anal Chem 7(l) 2-7. doi 10.2174/157341111793797662... 

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