Sensor Clark oxygen electrode

A major advance in the performance of amperometric oxygen sensors has been achieved by placing both the cathode and the anode behind the oxygen-permeable membrane (Fig. 7.4). This sensor is known as the Clark oxygen electrode. 

Acetylcholineesterase and choline oxidase Sensors were developed consisting of a Clarke oxygen electrode modified by superposition (over the polypropylene membrane of the electrode) of a dialysis membrane on which AChE and ChO were immobilized. Both formats are based on the conversion of the substrate to choline which is oxidized in the presence of choline oxidase causing a reduction in p02 which is detected by the electrode. Free choline is measured with a similar electrode with only choline oxidase immobilized on the membrane. Calibration graphs were rectilinear upto 90 pM choline and upto 120 pM acetylcholine. Results were presented for the analysis of tissue extracts and pharmaceutical formulations. [115]... 

Enzyme electrodes play a vital role in the operation of - biosensor and -> biofuel cells. The term enzyme electrode was coined by Clark and Lyons after their first demonstration of a -> glucose sensor in which glucose oxidase was entrapped at a - Clark oxygen electrode using a dialysis membrane. The decrease in measured oxygen concentration was proportional to the glucose concentration. 

Probably the most important developments in amperometric sensors were the Clark oxygen electrode and the amperometric glucose sensor. The latter is the most successful example of an enzyme-based sensor. 

We note here that the widely employed Clark oxygen electrode differs fundamentally from these devices (18, 63). The Clark device is similar in construction to the apparatus of Figure 2.4.5, in that a polymer membrane traps an electrolyte against a sensing surface. However, the sensor is a platinum electrode, and the analytical signal is the steady-state current flow due to the faradaic reduction of molecular oxygen. 

In this section the basic design and operation of these types of sensors are described. Examples of such sensors in common use are the Clark oxygen electrode [10] and the Severinghaus carbon dioxide electrode [11]. 

The principal advantages of the Clark oxygen electrode are that proteins and other materials which can foul solid eletrode do not contact the platinum and reference dectrodes. The entire sensor assembly can be immersed in the test solution as a single unit without an external reference electrode. Hence the oxygen electrode is well suited to construct a flow-through enzyme immunosensor with such a porous antibody-bound membrane using GOD or catalase as a label. Since the enzyme immunosensor has an excellent response time, the enzyme activity on the sandwich structure membrane can be determined with flow injection mode, which makes the detecting procedure simple and convenient. 

A milestone was the invention of carbon paste by Ralph Buzz Adams in 1958, which favoured, due to its heterogeneous paste-like composition, simple and multiple modification of the electrode material even with labile biological components. The first amperometric sensor, the famous Clark oxygen electrode was described in by Feland Clark 1954. Currently, new substances have a primary position in electrochemical literature, such as boron-doped diamond or nanostruc-tured materials. 

Oxygen amperometric sensors (Clark-type electrodes) have been also used as basic transducers for the construction of choline electrodes. The signal in this case is based on the reduction of molecular oxygen which is the co-reactant for the oxidation of choline. Free acetylcholinesterase was more sensitive to paraoxon than immobilized acetylcholinesterase, depending on the nature and concentration of solvent (500-fold in the presence of 5 % cyclohexane). Under these conditions, as low as 10 M paraoxon could be detected. The sensitivity of paraoxon detection with free acetylcholinesterase in the presence of 5 % cyclohexane corresponded to 0.2 ppb paraoxon, tenfold below the limit admitted in the European Economic Community. 

The first enzyme biosensor was a glucose sensor reported by Clark in 1962 [194], This biosensor measured the product of glucose oxidation by GOD using an electrode which was a remarkable achievement even though the enzyme was not immobilized on the electrode. Updark and Hicks have developed an improved enzyme sensor using enzyme immobilization [194], The sensor combined the membrane-immobilized GOD with an oxygen electrode, and oxygen measurements were carried out before and after the enzyme reaction. Their report showed the importance of biomaterial immobilization to enhance the stability of a biosensor. 

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).

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... 

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.

Figure 11. Electrodes in microcalorimetric vessels. A Schematic diagram of a section through a titration-perfusion microcalorimetric vessel equipped with a polarographic oxygen electrode and a pH electrode, a, sample compartment, volume 3 ml b, hollow stirrer shaft c, steel tube d, turbine stirrer e, O-rings f, combination pH electrode protected by a steel tube g, polarographic oxygen sensor (Clark electrode). B Record from a growth experiment with T-lymphoma cells. The vessel was completely filled with medium. Once the baseline had been established, the experiment was started (as indicated by the arrow) by the injection of 100 pi concentrated cell suspension.

Amperometric gas sensors are - electrochemical cells that produce a - current signal directly related to the concentration of the - analyte by - Faraday s law and the laws of - mass transport. The schematic structure of an amperometric gas sensor is shown in Fig. 1. The earliest example of this kind of sensor is the - Clark sensor for oxygen. Since that time, many different geometries, membranes, and electrodes have been proposed for the quantification of a broad range of analytes, such as CO, nitrogen oxides, H2S, O2, hydrazine, and other vapors. 

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