Electrochemical biosensors electronics

J.H.T. Luong, S. Hrapovic, D. Wang, F. Bensebaa, and B. Simard, Solubilization of multiwall carbon nanotubes by 3-aminopropyltriethoxysilane towards the fabrication of electrochemical biosensors with promoted electron transfer. Electroanalysis 16, 132-139 (2004). 

Mainly, three approaches have been used to immobilize the enzyme on transducer or electrode surface, single layer, bilayer, and sandwich configurations [69, 98], In some studies enzymes are covalently linked with sol-gel thin films [99], Sol-gel thin films are highly convenient for fast, large, and homogeneous electron transfer [17]. With an increase in gel thickness the signal decays and diffusion of analytes to biomolecule active site becomes difficult eventually these factors lead to poor response. By employing thin films various biosensors such as optical and electrochemical biosensors have been reported. 

The goal of this book is to summarize the recent advances in carbon nanotubes as a new material for electrochemical sensors. Since their discoveiy in 1991, carbon nanotubes have received considerable attention in different fields. Their speeial geometry and unique electronic, mechanical, chemical and thermal properties make them a very attractive material for the design of electrochemical biosensors. 

Novel biointerfaces of dendrimers with heme proteins have been reported. Such studies have helped to accomplish direct electron transfer between the biomolecule and electrode. Several attempts have been made to widen the scope of the dendrimer-based electrochemical biosensors for environmental monitoring. Analytes like glutamate, ethanol and several pesticides have been detected. Exclusive studies to detect hydrogen peroxide at dendrimer-modified electrodes have also been made. 

Freire, R.S., Pessoa, C.A., Mello, L.D., and Kubota, L.T. (2003) Direct electron transfer an approach for electrochemical biosensors with higher selectivity and sensitivity. Journal of Brazilian Chemical Society, 14 (2), 230-243. 

Nowadays, the construction of electrochemical biosensors based on the use of gold nanoparticles constitutes an intensive research area because of the unique advantages that this nanomaterial lends to biosensing devices. So, gold nanoparticles provide a stable surface for immobilization of biomolecules with no loss of their biological activity. Moreover, they facilitate direct electron transfer between redox proteins and electrode materials, and constitute useful interfaces for the electrocatalysis of redox processes of molecules such as H202 or NADH involved in many biochemical reactions (1, 2). 

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. 

NAD was covalently linked via a PQQ spacer to a gold electrode. This modified electrode is capable of binding LDH over the exposed nicotinamide groups. Upon oxidation of lactate to pyruvate the excess electrons tunnel from NADH in the active site to PQQ and eventually to the anode. Thus, a kind of electrical linkage between the enzyme and the electrode is established. The enzymes were crosslinked, as LDH is a homotetramer and might dissociate during the reaction. This approach is not only useful for electrochemical biosensors but might be transferred to other oxidoreductase reactions. 

Figure 9.1 A general scheme of ligand-receptor contact interaction. The operation of an electrochemical biosensor which is based on the properties of a modified BLM or s-BLM. L, ligand (substrate, antigen, hormone, ion or electron, donor or acceptor) R, receptor (enzyme, antibody, receptor site, carrier or channel, acceptor or donor).

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