Clark type sensor

Figure 13.10 shows the calibration curve of the LED-based optical oxygen sensor compared with the calibration curve of a commercially available Clark-type sensor (Ingold Electrodes, Wilmington, Massachusetts). While the Clark-type shows a linear calibration, the optical sensor allows a hyperbolic response as predicted by the Stem-Volmer-type equation 72 ... 

Performance comparisons with a Clark-type sensor demonstrated the applicability of the optical sensor in monitoring dissolved oxygen (DO) levels in a bioreactor.<73) Figure 13.11 shows the response profiles of the optical sensor and Clark-type electrode... 

Sensing of chlorine is possible with a phthalocyanine-based optode that is elec-trochemically reset [101]. Also a direct electrochemical Clark-type sensor employing carbon electrodes has been investigated [102]. For this type of sensor, the various types of carbon gave different responses and the edge-plane sites of graphitic electrodes were identified as electrochemically active. Both chlorine reduction and chlorine evolution were studied and the effects of the trichloride anion, Ch", were highlighted. 

It is sometimes desirable to make a sensor whose response is independent of the membranes properties. This would improve the reproducibility of measurements which may differ due to the intrinsic differences in individual membranes. Such differences may arise during the manufacturing process. This problem may be solved by considering the response of a Clark type sensor to the application of a potential pulse.When such a pulse is applied to the working electrode it takes a finite amount of time for the diffusion gradient to develop across the electrolyte layer and the membrane. At short times the diffusion gradient does not... 

The Clark Type Sensor. The working and reference electrodes, e d, are part of the main body, c, which is screwed into the external body a. It is held tightly against the membrane g which is held in place by o-ring f. A thin layer of the electrolyte, b, is held between electrode and membrane. 

The measurement of oxygen concentration or partial pressure is usually effected using Clark type sensors (Figure 3). 

A sensor based on this reaction scheme has been described by Hahn [37]. This is a Clark type sensor which operates using a double pulse method. The first pulse is to a potential at which the reduction of oxygen occurs at its diffusion limited rate. The integrated current is used as a measure of oxygen concentration. The second pulse is to a potential where the superoxide ion is oxidised back to oxygen. In the absence of COj the integrated current should match that of the forward pulse. When CO2 is present the reverse pulse is diminished. The difference between the two pulses is therefore a measure of the amount of carbon dioxide present. The interested reader is referred to the review by Hahn for further details of this system. 

The sensor designed by this group is also of interest. Instead of the Clark type sensor they have modified a channel electrode employing a rectangular flow cell. This is shown schematically in Figure 11. 

Figure 13.10. Calibration curves for optical sensor and Clark-type oxygen electrode. The phase response of optical sensor and the voltage response of the Clark-type electrode is plotted against percent oxygen in the gas mixture (oxygen and nitrogen) sparged. While the Clark-type electrode shows a linear calibration the optical sensor shows a Siem-Volmer lype relationship (see Section 13.10.2).

Figure 13.11. Dissolved oxygen profile during a typical Escherichia coli fermentation. Using a cubic spline interpolation of the data shown in Figure 13.3, the responses of the two sensors have been converted into percent oxygen and plotted as shown. The optical sensor closely tracks the response of the Clark-type electrode throughout the fermentation.

Microbial Cell-containing Membranes for Molecular Recognition. Suzuki et al. (87) have proposed a miocrobial sensor which consists of membrane-bound microbial cells and an electrochemical device. The assemblies of microbial sensors are similar to enzyme sensors. Two types of microbial sensors have been developed as presented In Figure 9. The first monitors the respiration activity of membrane-bound microbial cells with a Clark-type oxygen electrode. The... 

Amperometric sensors for hydrogen, nitrous oxides, and carbon dioxide have been developed by modification of the Clark-type electrode (Hanus et al., 1980 Albery and Barron, 1982). 

The development of enzyme membrane sensors for glucose began with the first enzyme electrode constructed by Clark. The sensors developed since differ with respect to the biological component and, even more, with the transducer type used. A common element to all of them is that the biocomponent is fixed in front of the transducer by a semi-permeable membrane. 

The above measuring principle has been modified for the measurement of HRP substrates which are themselves electrochemically active. To avoid electrochemical interference a Clark-type oxygen probe was used together with catalase coimmobilized with GOD. The hydrogen peroxide not consumed by HRP is cleaved by catalase to oxygen which is indicated at the electrode. Consequently, the biochemical basis of the sensor is the competition of HRP and catalase for the common substrate, H202. 

With a constant of K = 2.7640-5 mol/1 (pH 7.0, 25°C) the equilibrium of the LDH-catalyzed reaction lies far to the lactate side. This means that whereas for lactate sensors based on LDH the forward reaction has to be forced by alkaline buffer and pyruvate- or NADH-trapping agents, the reduction of pyruvate proceeds spontaneously under normal conditions. This direction of the reaction has been used in a sequence electrode for pyruvate assay (Weigelt et al., 1987b). In the presence of lactate monooxygenase (LMO) lactate formed from pyruvate by LDH is oxidized by molecular oxygen, the consumption of which was indicated at a Clark-type electrode. The enzymes were immobilized in a gelatin membrane. Of course such a sensor measures the concentration of lactate in the sample, too. Therefore it is suited to the determination of the lactate/pyruvate ratio, which is a clinically important parameter. Pro-... 

Sidwell and Rechnitz (1985) placed a slice of banana pulp tissue on the gas-permeable membrane of a Clark-type oxygen electrode. The banana tissue contains polyphenol oxidase which catalyzes the oxidation of dopamine to dopamine quinone and further to melanin at the expense of oxygen. Wang and Lin (1988) integrated this biocatalytic phase in the electrode body of a membrane-free carbon paste electrode and measured the formation of dopamine quinone at a potential of -0.2 V. This arrangement permitted selective determination of dopamine in the presence of ascorbic acid with a response time of only 12 s. It seems likely that this improved performance of a tissue-containing sensor could be extended to other analytes. 

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