Sensor thermometric

Enzyme sensors can measure analytes that are the substrates of enzymatic reactions. Thermometric sensors can measure the heat produced by the enzyme reaction [31], while optical or electrochemical transducers measure a product produced or cofactor consumed in the reaction. For example, several urea sensors are based on the hydrolysis of urea by urease producing ammonia, which can be detected by an ammonium ion-selective ISE or ISFET [48] or a conductometric device [49]. Amperometric enzyme sensors are based on the measurement of an electroactive product or cofactor [50] an example is the glucose oxidase-based sensor for glucose, the most commercially successful biosensor. Enzymes are incorporated in amperometric sensors in functionalised monolayers [51], entrapped in polymers [52], carbon pastes [53] or zeolites [54]. Other catalytic biological systems such as micro-organisms, abzymes, organelles and tissue slices have also been combined with electrochemical transducers. 

Thermometric sensors are based on the measurement of the heat effects of a specific chemical reaction or an adsorption process that involves the analyte. In this group of sensors the heat effects may be measured in various ways, for example in catalytic sensors the heat of a combustion reaction or an enzymatic reaction is measured by use of a thermistor. Calorimetric biosensors detect variations of heat during a biological reaction. 

The velocity of the air flow is measured most frequently by thermoanemometers. A sensitive semiconductor or resistance thermometric sensor is situated in the flow of the air sampled. The resistance of the sensor depends on the velocity of the air flow and the calibration is provided at a constant temperature with the help of a suitable flow meter. The disadvantage of the thermo-anemometers is the necessity for constant temperature for the period of particular sampling procedures and also the need for a time consuming calibration. Connection to a recorder is advantageous. 

Thermometric sensors based on the measurement of the heat effect of a specific chemical reaction or adsorption which involves the analyte... 

The choice of sensor material determines range, sensitivity, and stability. By considering the latter factors, it is found that inorganic insulating compounds, such as most lamp phosphors and many solid state laser materials, are the most suitable materials for thermometric applications. Indeed, these materials are most commonly used in the existing commercial fluorescence thermometer schemes. 

In the recent past, multianalyte determination has found increased applications, i.e. specific and multiple reactions favor a system that allows the specific determination of each reaction, using the same principal measurement methods, detectors and conditions. In keeping with this idea, a flow injection thermometric method based on an enzyme reaction and an integrated sensor device was proposed for the determination of multiple analytes. In principle the technique relies on the specificity of enzyme catalysis and the universality of... 

The sensitive part of an electrode is covered with a membrane on which the enzyme is immobilized in immunocomplexes. The enzyme-catalyzed reaction takes place near the sensor (Mattiasson and Nilsson, 1977). The method is as fast as the thermometric assay but less sensitive. Electrode-based EIA using urease conjugates have been reviewed by Meyerhoff and Rechnitz (1980). This method has reasonably low detection limits. These promising potentiometric EIA are discussed by Boiteux et al. (1981) and Gabauer and Rechnitz (1982). 

An excellent feature of the NQR thermometer is that the thermometric property involved is a fundamental property of a substance, a unique frequency-temperature relationship that must be established only once and is always thereafter applicable for that substance. Thus, once the frequency-temperature relationship has been determined for a suitable sensor material, such as KClOg, that calibration will apply to all other samples of that material provided that the material has been prepared with consistent purity. This, then, eliminates the need to calibrate each thermometer individually as is required for most practical thermometers. Another advantage is that frequency can be easily and accurately measured and the thermometer can be easily made a part of an automated system for temperature monitoring and control. Through the use of standard frequency broadcasts by NBS, the accuracy of the frequency counter used in making measurements can be easily checked. 

At present, no calibration procedure for black and white standard thermometer is available that includes all stress factors (air temperature, air velocity, and humidity). Today, calibration traceability is guaranteed by a contact thermometric procedure. It would be preferable to measure the temperature at the surface of the coated sensor because this is the temperature of interest. A contactless surface temperature measurement requires knowing the emission ratio of the material and a minimization of the reflected and scattered radiation. For a minor error contact surface temperature measurement, a known method is the multiprobe measurement with extrapolation to the surface temperature. 

Figure 20.7 The effect of sample volume on the linear range of a thermometric glucose sensor with a 0.6 mm x 10 mm CPG column with glucose oxidase/catalase. The flow rate was 50 /xl min . ... 

Mitsubishi Electric Fujitsu ThermoMetric Universal Sensors Seiko Instruments... 

Test strips, which are available for the determination of about ten low-molecular mass substances (metabolites, drugs, and electrolytes) and eight enzymes [356], can be considered as precursors of optoelectronic biosensors. Efficient optoelectronic sensors based on immobilized dyes have been devised for the determination of glucose, urea, penicillin, and human serum albumin [357]. Other approaches use immobilized luciferase or horseradish peroxidase to assay ATP or NADH or, when coupled with oxidases, to measure uric acid or cholesterol. These principles have not yet been generally accepted for use in routine analysis. Thermistor devices involving immobilized enzymes or antibodies for a number of clinically relevant substances have also been described. Thermometric enzyme linked immunosorbent assays are being routinely employed for monitoring the production of monoclonal antibodies. 

Electrochemical biosensors may be expected to maintain their leading position up to the end of the century. In this respect, the availability of transducers, eg, ion-selective field-effect transistors prepared by mass-production technology, will result in widespread application. In addition to one-shot use, multifunctional sensing in minute volumes will be realized. In addition to electrodes, optical, thermometric, and piezoelectric transducers are likely to become exploited in the next generation of sensor. Inexpensive equipment to be used in all areas where material has to be detected and quantified will be produced by integrating the Hxation of the biocomponent with the micromechanical fabrication of the analyzers. 

Early thermometric measurements were laborious and time consuming and not very sensitive, although many excellent thermometric studies were reported up to the early 1950s, when thermometric titration became suitably automated for routine analytical use. Around that time, the thermistor temperature-sensor, the constant-delivery pump and sophisticated thermostatic control were introduced in rapid succession. 

Temperature-Sensing System. Temperature sensing is the heart of the thermometric titration technique. The principal temperature-sensors used are thermistors. A thermistor is a temperature-sensitive semiconductor whose resistance obeys the equation... 

Thermoelectric-, pyroelectric-, and thermoconductivity-based devices are other representatives of thermometric gas sensors (Korotcenkov 2011). In particular the thermal conductivity technique for detecting gas is suitable for the measurement of high (vol. %) concentrations of binary gas mixes. The heated sensing element is exposed to the sample and the reference element is enclosed in a sealed compartment (see Fig. 1.14). If the thermal conductivity of the sample gas is higher than that of the reference, then the temperature of the sensing element decreases. The higher their thermal conductivity, the lower the concentration which can be measured (Table 1.14). Power loss of a single filament thermistor by heat conduction via the ambient gas can be expressed as... 

We recall that S is the thermometric sensitivity of the DSC sensor that converts the thermocouple voltage (AV) to the temperature difference AT. 

Figure 6.1. Thermometric (calorimetric) sensors. Left thermistor, right platinum Ceramic...

Thermometric and calorimetric sensors are discussed here only for use in gases, hi the Uquid phase, the conditions are much less advantageous, since... 

Nearly all chemical sensors useful for liquid samples can be utiUzed to indicate titrations. Besides the preferred potentiometric, other electrochemical probes are also used, mainly amperometric and conductometric sensors. The so-called biamperometric titration works with simple wire pairs. Photometric and thermometric indication techniques are less common than electrochemical methods. Miniaturization does not play an important role for titration probes. Classical arrangements predominate to this day. Commercial titration instruments are only slowly starting to make use of the achievements of modern sensor technology. As an example, optodes have achieved a certain popularity in recent years for titration applications. 

Chemical sensors can be of gas, liquid, and solid particulate sensors based on the phases of the analyte. Depending on the operating principle of transducer in a chemical sensor, it can be used as optical, electrochemical, thermometric, and gravimetric sensor. Chemical sensors also include a special branch referred to as biosensors for the recognition of biochemicals and bio-reactions. The use of biological elements such as organisms, enzymes, antibodies, tissues, and cells as receptors differentiates biosensors from conventional chemical sensors. 

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