Enzyme biosensors biosensor calibration

Figure 10. Ethanol calibration curve from the internal enzyme biosensor with alcohol dehydrogenase. (Reproduced with permission from ref. 13. Copyright 1988 Pergamon.)...

Inhibitors reduce the signal of a biosensor by reducing the activity of the immobilized enzyme on the transducer. This modifies a biosensor calibration curve that is plotted as a function of substrate concentration. As previously described (see 4.2.4.b), the curve is displaced according to the inhibition mode. The inhibitor is determined by fixing the substrate concentration and varying the inhibitor concentration. A new calibration curve is then plotted as a function of inhibitor... 

The stability of enzyme electrodes is difficult to define because an enzyme can lose some of its activity. Deterioration of immobilized enzyme in the potentiometric electrodes can be seen by three changes in the response characteristics (a) with age the upper limit will decrease (e.g., from 10-2 to 10 3 moll-1), (b) the slope of the analytical (calibration) curve of potential vs. log [analyte] decrease from 59.2 mV per decade (Nernstian response) to lower value, and (c) the response time of the biosensor will become longer as the enzyme ages [59]. The overall lifetime of the biosensor depends on the frequency with which the biosensor is used and the stability depends on the type of entrapment used, the concentration of enzyme in the tissue or crude extract, the optimum conditions of enzyme, the leaching out of loosely bound cofactor from the active site, a cofactor that is needed for the enzymatic activity and the stability of the base sensor. 

Calibration profiles of the sensor based on the final nylon-enzyme net (III) were disappointing oonpared with the analogous sensor based on nylon net type II. The lower detection limit was only 0.1 mM glucose and currents produced were about 80% smaller. However, this alternative immobilization scheme serves to illustrate the synthetic versatility of nylon-6,6 in the biosensor field. 

Long Term Performance of Sensors. The sensors showed excellent long term stability stored in buffer at 4 °C. Table 1 shows the long term performance of four typical biosensors. The usual current response to 6 mM glucose for a new biosensor is about 2 fiA. The precision of the absolute current response to 6 mM glucose remains within 15 % for up to four months. This level of precision is unusual for amperometric biosensors. Normally, variations in current are compensated by calibration. The sensitivity of the biosensors, as indicated by the slope of the response, is also stable. This stability is due to the film which maintains the ferrocene within the sensing layer. Biosensors with adsorbed mediator and immobilized enzyme but without film are not stable for any period of... 

The process depicted for phenol in equations 5 consists of an enzyme-catalyzed oxidation to a quinone, and a reduction process taking place at the electrode these reactions may serve for electrode calibration. The development of AMD biosensors for detection of phenols in environmental waters has been described for phenoloxidases such as tyrosinases and laccases and less specific oxidases such as peroxidases. Such biosensors may be part of a FIA system for direct determination of phenols or may serve as detectors for LC °. 

Fitting into the trend towards improvement of the availability and simplification the preparation of biocatalytic layers for biosensors, the use of crude materials has been explored. Arnold and coworkers investigated the feasibility of employing Jack bean meal in a urea sensor (Arnold and Glaizer, 1984) and rabbit muscle acetone powder in a sensor for adenosine monophosphate (Fiocchi and Arnold, 1984). Both sensors turned out to be serious contenders with the appropriate enzyme electrodes with respect to lifetime and slope of the calibration curves. Other parameters, such as response time and linear range, were quite similar. 

Abstract. The biosensors described in this work, for the monitoring of pesticides, are based on acetylcholinesterase immobilized on the surface of screen-printed electrodes. The principle of the biosensor is that the degree of inhibition of an enzyme sensor by a pesticide is dependent on the concentration of that pesticide. The DPV technique was used as a detection method and methyl-paraoxon as a reference pesticide for sensor calibration. 

Conventional enzyme electrodes employ disorete-maorosoopio membranes to overcome problems associated with interferences, enzyme immobilization, and electrode fouling. While these types of enzyme electrodes have been commercially developed, there are some limitations with this approach. Some sensors use three relatively thick membranes, resulting in a slow smd complex diffusion path for reactants reaching the enzyme and hydrogen peroxide reaching the electrode. Slow diffusion in this type of system adversely affects the response and recovery time, decreasing sampling rate. Each sensor must be individually constructed, and this construction technique is limited to two-dimensional surfaces. In addition, for sensors that have complex and slow diffusion paths, rates of diffusion must remain constant, otherwise calibration of the biosensor, and more important the maintenance of calibration, are difficult. A variety of factors can influence rates of diffusion, and consequently the performance of the enzyme layer and the performance of the sensor. These complicated, and most often uncharacterizable, properties have made the development of roost biosensors difficult. 

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