Biosensors response measurements

As the term suggests, the use of biosensors to measure transmitter release rests on exploiting a biological response which is proportional to the amount of transmitter in... 

A flow injection optical fibre biosensor for choline was also developed55. Choline oxidase (ChOX) was immobilized by physical entrapment in a photo-cross-linkable poly(vinyl alcohol) polymer (PVA-SbQ) after adsorption on weak anion-exchanger beads (DEAE-Sepharose). In this way, the sensing layer was directly created at the surface of the working glassy carbon electrode. The optimization of the reaction conditions and of the physicochemical parameters influencing the FIA biosensor response allows the measurement of choline concentration with a detection limit of 10 pmol. The DEAE-based system also exhibited a good operational stability since 160 repeated measurements of 3 nmol of choline could be performed with a variation coefficient of 4.5%. 

BOD was determined in wastewater using a microbial sensor based on an organic-inorganic hybrid material for immobilization of the biofilm. The biosensor response to the sample exhibited good reproducibility, long-term stability, and required only 10 min for each measurement.130 Near-infrared spectroscopy was used for the rapid determination of COD and BOD in wastewater.131... 

A bioelectrode functioning optimally has a short response time, which is often controlled by the thickness of the immmobilized enzyme layer rather than by the sensor as well as many other factors (see Table 7). The biosensor response time depends on (1) how rapidly the substrate diffuses through the solution to the membrane surface, (2) how rapidly the substrate diffuses through the membrane cmd reacts with the biocatalyst at the active site, and (3) how rapidly the products formed diffuse to the electrode surface where they are measured. Mathematical models describing this effea are thoroughly presented in the biosensor literature (5, 68). 

Fig. 10.13. Schematic representation of the different methodologies for estimation of the enzyme inhibition, (a) Indirect determination the biosensor response is measured before and after its incubation with the inhibitor for a given time, (b) Direct determination the inhibitor is added during the steady-state reaction.

SPR biosensor experiments measure only relative changes in the molecular mass attached to the sensor surface from the beginning of the interaction being studied. The response is then directly proportional to the concentration of the boimd analyte (conditions that guarantee a linear sensor response are assumed throughout the chapter). In the case of a single type of analyte binding to the receptors in a 1 1 stoichiometric ratio, the response is proportional... 

SPR-based biosensors can measure the interactions of biomolecules directly without the need for labeling. This feature has allowed these analytical instruments to become essential tools for characterizing molecular interactions. The ability to directly measure interactions in real time allows us to quantitatively determine kinetic parameters, thermodynamics, and concentration, or qualitatively characterize relationships between ligands and analytes. Due to the fast response and high sensitivity of SPR-based biosensors compared... 

Simulation of the biosensor action usually involves calculation of the concentrations of the compounds as well as the response for the time interval from the beginning of the action up to the moment called the biosensor response time. The moment of the measurement depends on the type of the device. Devices operating in the stationary mode usually use the time when the absolute response, e.g., anodic or cathodic current, slope value falls below a given small value. Since the biosensor responses usually vary even in orders of magnitude, the response is normalized. In other words, the time Tneeded to achieve a given dimensionless decay rate e is accepted as the response time ... 

Figure 8.3 Biosensor response in batch measurement and flow injection analysis.

In Vivo Biosensing. In vivo biosensing involves the use of a sensitive probe to make chemical and physical measurements in living, functioning systems (60—62). Thus it is no longer necessary to decapitate an animal in order to study its brain. Rather, an electrochemical biosensor is employed to monitor interceUular or intraceUular events. These probes must be small, fast, sensitive, selective, stable, mgged, and have a linear response. 

Entrapment of biochemically reactive molecules into conductive polymer substrates is being used to develop electrochemical biosensors (212). This has proven especially useful for the incorporation of enzymes that retain their specific chemical reactivity. Electropolymerization of pyrrole in an aqueous solution containing glucose oxidase (GO) leads to a polypyrrole in which the GO enzyme is co-deposited with the polymer. These polymer-entrapped GO electrodes have been used as glucose sensors. A direct relationship is seen between the electrode response and the glucose concentration in the solution which was analyzed with a typical measurement taking between 20 to 40 s. 

NADH. Immobilized redox mediators, such as the phenoxazine Meldola Blue or phenothiazine compoimds, have been particularly useful for this purpose (20) (see also Figure 4-12). Such mediation should be useful for many other dehydrogenase-based biosensors. High sensitivity and speed are indicated from the flow-injection response of Figure 3-21. The challenges of NADH detection and the development of dehydrogenase biosensors have been reviewed (21). Alcohol biosensing can also be accomplished in the presence of alcohol oxidase, based on measurements of the liberated peroxide product. 

Direct detection is usually preferred in applications, where direct binding of analyte of concentrations of interest produces a sufficient response. If necessary, the lowest detection limits of the direct biosensors can be improved by using a sandwich assay. Smaller analytes (molecular weight < 10,000) are usually measured using inhibition assay, Figure 14. 

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