Diffusion detected biosensor

In the previous sections, several examples were reviewed to show that the diffusion change is sensitive enough for the detection of the protein-protein interaction. This method could be called the diffusion detected biosensor method. Characteristic features of this method are discussed in the following. 

We believe that these prominent characteristics of the diffusion detected biosensor are important for studying intermolecular interaction of sensor proteins in the time domain and will be used for many cases to reveal their essential features. 

The advantage of a diffusion-based biosensor is that it avoids the need for a sandwich assay by directly measuring the reduction in the diffusion coefficient of antibody-functionalized submicron particles caused by the binding of complementary antigens. This technique should be useful for bioanalytical techniques for dmg discovery and for pathogen detection in complex media. 

Utilization of whole cells and tissues in biosensor has increasingly been used. Enzyme stability, availability of different enzymes and reaction systems, and characteristics of cell surface are the advantages of using cells and tissues in biosensor designs. Multi-step enzyme reactions in cells also provide mechanisms to amplify the reactions that result in an increase in the detectability of the analytes. The presence of cofactors such as NAD, NADP, and metals in the cells allows the cofactor-dependent reactions to occur in the absence of reagents. (34, 50, 69). However, the diffusion of analytes through cell wall or membrane imposes constraint to this type of biosensors and results in a longer response time compared to the enzyme biosensors. 

Enzymatic reactions coupled to optical detection of the product of the enzymatic reaction have been developed and successfully used as reversible optical biosensors. By definition, these are again steady-state sensors in which the information about the concentration of the analyte is derived from the measurement of the steady-state value of a product or a substrate involved in highly selective enzymatic reaction. Unlike the amperometric counterpart, the sensor itself does not consume or produce any of the species involved in the enzymatic reaction it is a zero-flux boundary sensor. In other words, it operates as, and suffers from, the same problems as the potentiometric enzyme sensor (Section 6.2.1) or the enzyme thermistor (Section 3.1). It is governed by the same diffusion-reaction mechanism (Chapter 2) and suffers from similar limitations. 

The enzyme is immobilized on a nylon mesh, which also acts as a diffuse reflector for the light. The dynamic range of this sensor is between 1(T5 and 10 3M. Although the primary process that determines the steady-state concentration of the p-nitrophenoxide ion is the diffusion-reaction mechanism (which is governed by concentrations of all participating species), the detection of its concentration is again subject to the limitations of optical sensing of ionic species (Section 9.4.1). There are many similar optical enzyme biosensor schemes that utilize detection of... 

Any type of biosensor can operate either stationary or kinetically. This circumstance adds more variety to biosensor appliances. Stationary operation of biosensors is stipulated by the existence of diffusion-limiting stages, at which the biochemical activity of the selector has no effect on the stability of signal decoding from the detected substance. 

Capacitive biosensors [390] detect changes in the capacitance of an electrode upon the occurrence of a binding event. The capacitive structure comprises a series of components such as the electrochemical double layer including the diffuse layer from ions in solution, the grafting layer, and the biorecognition layer. Since the contribution of the biorecognition layer to the overall capacitance is typi-... 

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. 

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