Biosensor redox-active molecules

The use of electrodes modified with immobilized redox-active molecules provides a simple methodology by which to study the ultrathin film electrochemistry of water-insoluble redox-active molecules, encouraging the application of such techniques to biomimetic membranes in aqueous media. It is of interest to use monolayer and LB films of enzymes, proteins, and antibodies as biosensors or biomolecular switches because of their high sensitivity for their substrates and antigens, respectively. The formation of thin fullerene films including fullerene-lipid hybrid and fullerene-lipid composite bilayer membranes is of interest both from a fundamental and practical application point of view. Multiwalled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs) are novel nanomaterials that have remarkable electronic, mechanical, and thermal properties, and specific functions. Soluble carbon nanotubes in aqueous and organic systems are of interest since their study allows the... 

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. 

Aptamer based biosensors, for example for recombinant human erythropoietin (as model analyte), can be made more sensitive by amplification with a boronic acid tethered gold nanoparticle that is then associated with an alkaline phosphatase to produce a redox active probe molecule. A similar re-usable bio-immuno-sensor has been suggested for carcinoembryonic antigen. A phenylboronic acid is assembled on gold to (reversibly) bind the antibody horseradish peroxidase conjugate. Interaction with the antigen slows down the hydrogen peroxide reduction. An HIV-1 immunoassay based on electroluminescence has been proposed by Zhou etaV In this process the... 

On the other hand, organic dyes in combination with CNTs are also used in electrode fabrication and hybridization techniques for developing an ultrasensitive, selective, and miniaturized electrochemical DNA biosensor for quick and reliable DNA sequence analysis. CNTs have good biocompatibility which allows them to increase the attached DNA amount on the substrate surface, thanks to their high surface area, an enhanced electronic conductivity, and a high mechanical resistance in electroanalytical chemistry can accelerate the electron-transfer rate between the redox active ssDNA molecule and electrode. 

While the variety of NPs used in catalytic and sensor applications is extensive, this chapter will primarily focus on metallic and semiconductor NPs. The term functional nanoparticle will refer to a nanoparticle that interacts with a complementary molecule and facilitate an electrochemical process, integrating supramolecular and redox function. The chapter will first concentrate on the role of exo-active surfaces and core-based materials within sensor applications. Exo-active surfaces will be evaluated based upon their types of molecular receptors, ability to incorporate multiple chemical functionalities, selectivity toward distinct analytes, versatility as nanoscale receptors, and ability to modify electrodes via nanocomposite assemblies. Core-based materials will focus on electrochemical labeling and tagging methods for biosensor applications, as well as biological processes that generate an electrochemical response at their core. Finally, this chapter will shift its focus toward the catalytic nature of NPs, discussing electrochemical reactions and enhancement in electron transfer. 

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