Measurement methods, catalytic sensor

Because the spatial area with higher temperature on the catalyst surface of one of the samples of the library is very small the detection of catalytic activities through temperature measurement cannot be carried out by direct temperature measurements but only by non-contact methods such as pyrometry or IRT. The IR video camera used here measures the emission at every point of the library in parallel. The detector consists of a 256x256 pixel array of Pt-silicide-IR-sensors. Each pixel delivers a voltage-signal that depends on the infrared radiation and the sensitivity of that pixel (fixed pattern noise). 

A very important aspect of gas sensors in automotive exhaust-gas environments is aftertreatment of the electrodes to control a specific sensor behavior. For example, to measure nonequilibrium raw emissions, the sensor needs excellent catalytic ability. Various methods are known to improve electrodes in Zr02-based sensors. One well known method is to increase the active platinum surface area and the three-phase boundary area by partial reduction of zirconia close to the electrode. This occurs when the ceramic is exposed to a reducing atmosphere at high temperatures or when an electrical cathodic current is applied through the electrode and electrolyte. A similar effect can be achieved by chemical etching of the elec-... 

Electrochemical methods for NO determination offer several features that are not available with spectroscopic approaches. Perhaps the most important is the capability of microelectrodes to directly measure NO in single cells in situ, in close proximity to the source of NO generation. Figure 2 shows sensors that have been developed for the electrochemical measurement of NO. One is based on the electrochemical oxidation of NO on a platinum electrode (the classical Clark probe for detection of oxygen) and operates in the amperometric mode [17]. The other is based on the electrochemical oxidation of NO on conductive polymeric porphyrin (porphyrinic sensor) [24]. The Clark probe uses a platinum wire as a working electrode (anode) and a silver wire serves as the counterelectrode (cathode). The electrodes are mounted in a capillary tube filled with a sodium chlo-ride/hydrochloric acid solution separated from the analyte by a gas-permeable membrane. A constant potential of 0.9 V is applied, and direct current (analytical signal) is measured from the electrochemical oxidation of NO on the platinum anode. In the porphyrinic sensor, NO is catalytically oxidized on a polymeric metalloporphyrin... 

The potentiostatic multi-pulse potentiometry described here allows the dynamic measurement of potentials. The advantages of this method are the short time required for the analysis and the low noise of the signal. The "ancestor" of this technique, enzyme chronopotentiometry [7 ], posed problems of reproducibility when it was applied to the immobilized redox polymer. The excellent reproducibility of our method is clearly shown in fig. 3b. These techniques were the fundamental developments to conceive redox-FETs for the first time. After immobilization of NAD -dependent dehydrogenases covalently on the surface of the transducer the enzymatically produced NADH would be catalytically oxidized in situ by the polymeric mediator. To this very compact combination the substrate and NAD+ as cosubstrate have to be applied externally. The coimmobilization of the coenzyme NAD+ would lead to reagentless sensors. This is a subject of forthcoming investigations... 

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... 

The first system, devised in the University of Porto, Portugal, was a sensor array for the measurement of creatinine in urine [18]. It consisted of a creatinine iminohydrolase enzyme immobilized by entrapment using a chitosan membrane onto a nonactin ISE. Catalytic hydrolysis by the enzyme generated ammonium ion which was then directly detected. The system was completed with ISEs for ammonium, potassium, sodium and calcium which allowed to correct for any endogenous ammonium (by the first ISE) or for alkaline and alkaline-earth interference in the ammonium-based biosensor (the other three). Linear response ranges were between 0.1 and 10 mM, what permitted the resolution of the multicomponent determination by PLS method. Comparison with reference Jaffe method showed a satisfactory correlation, although the slope of obtained vs. reference values was rather low the obtained value was 0.87, whereas theoretical value should be 1.0. 

Although IL electrolytes provide partial selectivity, the primary selectivity of an IL-electrochemical sensor comes from the redox properties of the analyte observed using amperometric methods, wherein the electrical current generated by reaction of an analyte at an electrode at a fixed or variable potential is measured [22]. We have shown redox chemistry that occurs only in ILs and can be exploited to enhance sensor performance [202], As shown in Fig. 2.16, we discovered that at platinum electrode in [NTf2]-based ionic liquids (ILs), facile methane electro-oxidation is observed suggesting a unique catalytic Pt-INTfj] interface for electron-transfer reaction of methane at room temperature. Little methane electro-oxidation signals are observed in ILs with other anions. In this experiment, an oxygen reduction process... 

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