Oxygen, determination membranes, sensor

Figure 19.6— The Clark sensor for oxygen determination. The Teflon membrane, permeable to gas, must be very close to the cathode so that the double diffusion process through the membrane and the liquid film will lead to a stable signal within a few seconds.

Oxygen concentration is determined electrochemically using a membrane oxygen electrode (Clark sensor). Oxygen is consumed by micro-organisms, and the reduction in oxygen consumption with time is a measure of biological activity. 

The Clark electrodes described above are not suitable for oxygen determination in dry gaseous samples such as air because the thin layer of electrolyte solution contained behind the membrane is prone to rapid drying. A different arrangement is therefore used for such applications. Amperometric gas sensors for oxygen (and sensors for other electroactive species in the vapor phase) usually consist of a porous PTFE membrane that bears a precious metal electrode deposited, also in porous form, directly on the backside. This keeps the diffusion length short while... 

A reinforced double-membrane sensor has been fabricated to improve the protective feature of the membrane and to enlarge the linear current response of oxygen. The inner current-determining Teflon film (25 m thick) is covered by an outer silicone membrane reinforced by a thin stainless steel mesh. This arrangement obviously increases the mass transfer impedance of oxygen. Thus, the current signal (at the cathode of the effective surface S) decreases according to... 

Determination of nonstoichiometry in oxides is a key point in the search for new materials for electrochemical applications. In recent decades, owing to their current and potential applications (electrodes in fuel cells, insertion electrodes, membranes of oxygen separation, gas sensors, catalytic materials, etc.), various methods of precise characterization of MfECs have been proposed, either the measurement of the defect concentrations and the stoichiometric ratio as functions of the oxide composition, of the surroxmding oxygen pressure and of temperature, or the transport properties. There are different methods to determine the electrical properties of MIECs and, more specifically, the ionic and electronic contributions. The most appropriate method depends on different parameters, i.e., the total electrical conductivity of the studied oxides, the ionic and electronic transport numbers, the... 

Amperometric gas sensors are the second most important group of electrochemical gas sensors. The development of these sensors can be traced back to the introduction of the Clark-electrode in the mid-1950s, which is well known for the determination of dissolved oxygen. Amperometric gas sensors consist of a working electrode mostly covered by a membrane, a counter and a reference electrode which are in connection with a liquid electrolyte solution. These sensors have been designed in different forms and are significant also in commercial terms. The schematic setup is shown in Fig. 19.5. ... 

Schmid et al. used the same principle to develop sensors to be incorporated into FI systems for the determination of ascorbic acid in fruit juices [38] and that of lactic acid in dairy products [39]. The membrane used in both applications consisted of decacyclene dissolved in silicone rubber that was treated similarly as the membrane in glucose sensors (Fig. 3.4.B). The oxygen optrode was coated with a sheet of carbon black as optical insulation in order to protect it from ambient light or intrinsic sample fluorescence. Ascorbic acid oxidase or lactic acid oxidase was immobilized by adsorbing it onto carbon black and cross-linking it with glutaraldehyde. The FI system automatically buffered and diluted the food samples, thereby protecting the biosensor from a low pH and interferents. 

Alcohol Sensor. On-line measurements of ethyl alcohol concentration in culture broth are required in fermentation industries. A microbial electrode consisting of immobilized yeasts or bacteria, a gas permeable Teflon membrane, and an oxygen electrode was prepared for the determination of methyl and ethyl alcohols(7). 

A microbial sensor consisting of immobilized methyl alcohol-utilizing bacteria (AJ 3993), a gas permeable membrane, and an oxygen electrode was applied to the determination of methyl alcohol. A linear relaionship was also observed between the current decrease and the concentration of methyl alcohol. Therefore, the sensor can be also applied to the determination of methyl alcohol. 

Haga et al. developed another type of immunosensor by combining an enzyme membrane immunoassay and an enzyme sensor using oxygen electrodes (HI). In this assay antigen molecules (theophylline) are attached on the surface of the liposomes and an enzyme (horseradish peroxidase) is encapsulated in the sensitized liposome. When antibody (antitheophylline antibody) and complement are added, the enzyme is released by the liposome lysis. The enzyme activity with the NADH-NAD reaction can be determined by the oxygen electrode. When antigen is added, it competitively binds to antibodies, then liposome lysis and enzyme activity are decreased. The sensitivity of this method for theophylline determination was reported as 0.7 ng/ml. 

Suleiman and Xu [128] described a novel reusable amperometric immuno-sensor for the determination of cocaine. Horseradish peroxidase and benzoylecgonine-antibody were co-immobilized on a chemically activated affinity membrane, which was then mounted over the tip of an oxygen electrode. The enzymatic electrocatalytic current response to the substrate is inhibited by the association of the antigen to the co-immobilized antibody. The calibration plot for cocaine was linear in the concentration range of lx 10 -1x10 M. 

Extensive research and development of microbial sensors has been carried out by Suzuki et al. (89-94) and Rechnitz et al. (95-97) (see Table III). Microbial sensors consisting of membrane-bound whole cells and an oxygen electrode were constructed for the determination of substrates such as assimilable sugars, acetic acid, alcohols and ammonia, and for the estimation of biochemical oxygen demand (BOD) (98-104). Glutamic acid was determined with a microbial sensor which consists of membrane-bound whole cells containing glutamate decarboxylase and a carbon dioxide gas electrode. These microbial sensors have been applied and evaluated for on-line measurements in fermentation processes (105,106). 

---