Sensors diffusion controlled

Under reaction-controlled conditions, the total rate of catalytic oxidation is governed by the rate of surface reaction and is independent of the gas transfer rate from the gas phase to the catalyst surface, so that the CTL intensity is also independent of the flow velocity of sample gas around the sensor. Under diffusion-controlled conditions, the rate of catalytic oxidation is independent of the catalytic activity, but depends on the transfer rate of combustible gas in the gas phase, so that the CTL intensity depends on the flow rate of the gas... 

In the laminar flow type of sensor cell shown in Fig. 13a, we obtain the reaction rate rx under diffusion-controlled conditions as follows, applying an approximation of kv [Pg.120]

There are a variety of different polymer systems available to deliver a predictable and reproducible release of chemical compounds (13,14). Two of the most versatile systems, adaptable to sensor design, release incorporated reagent by chemically-controlled or diffusion-controlled processes. Figure 1 shows examples of these two systems. 

Mechanistic aspects of the action of tyrosinase and the usual transduction schemes have been summarized on several occasions [166,170-173]. In short, this copper enzyme possesses two activities, mono- and di-phenolase. Due to the predominant presence of the mono phenolase inactive form (met-form), the enzyme is inherently inefficient for the catalysis of these monophenol derivatives. However, in the presence of a diphenol, the catalytic cycle is activated to produce quinones and the scheme results in an efficient biorecognition cascade. This activation is achieved more efficiently when combined with electrochemical detection through the reduction of the produced quinones [166], as illustrated in Fig. 10.5. Consequently, a change in the rate-hmiting step can be observed through kinetic to diffusion controlled sensors with a concomitant increase in stability and sensitivity, as depicted in Fig. 10.6. 

At /e > 25 internal diffusion control is reached. Any substrate molecule diffusing into the enzyme layer is converted therein only part of the enzyme is acting catalytically. Diffusion controlled sensors exhibit the following characteristics ... 

At low substrate concentration the sensitivity of kinetically controlled sensors increases linearly with Umax. Consequently, the application of several identical enzyme layers one over the other enhances the measuring signal. When the amount of enzyme becomes sufficiently high as to provide complete substrate conversion the system passes over to diffusion control. Under these conditions a decrease of the diffusion resistance by decreasing the layer thickness results in an increased sensitivity. Nevertheless, a membrane-covered enzyme electrode is only 10 to 50% as sensitive as a bare electrode for an analogous electrode-active substance. 

The variation of the enzyme loading is a means of determining the minimum amount of enzyme required for maximum sensitivity. Furthermore, this test reveals the magnitude of the enzyme reserve of diffusion controlled sensors. 

Weigelt et al. (1987a) optimized an LMO-gelatin membrane for application in the Glukometer analyzer (ZWG, GDR). The enzyme loading test (Fig. 56) shows that above 1 U/cm2 the sensor is diffusion controlled,... 

It is evident from the equation that potentiometric CO2 electrodes as well as amperometric O2 or H2O2 electrodes can be used as transducers. Both potentiometric and amperometric sensors have been covered by a layer of oxalate oxidase protected by a dialysis membrane (Bradley and Rechnitz, 1986 Rahni et al.f 1986a). The sensors had a pH optimum at pH 3.5-4. Diffusion control was reached at 1 U oxalate oxidase per electrode. Oxalate determination was not affected by ascorbic acid or amino acids. The hydrogen peroxide-detecting sensor (Rahni et al., 1986a) has been used to measure oxalate in urine diluted 1 40. 

Enzyme loading test. The minimum amount of enzyme required for maximum sensitivity is determined by varying the enzyme loading (figure 17.2). This test also, reveals the enzyme limit of diffusion-controlled sensors. Owing to differences in /Ka/ values and the layer thickness, the transition from the kinetic to diffusion control of different enzyme electrodes takes place at rather... 

Diffusion controlled sensors exhibit the following characteristics ... 

The enzyme loading to a major extent determines the stability of a biosensor. An enzyme reserve is built up by employing more enzyme activity in front of the electrochemical probe than is minimally required to achieve diffusion control. As long as this reserve lasts, the sensitivity will remain essentially constant. This is only significant, however, for sensors for substrate determinations. If effectors of the biocatalytic sensing reactions are to be measured, kinetic control is desired, which permits the enzyme loading to be varied only in a relatively narrow range. 

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