Transducers thermometric

Even if nowadays, the MCT may be considered a primary thermometer only on a narrow temperature range, it is considered the best dissemination standard in the millikelvin range [56-59], In fact, the 3He melting pressure is a good thermometric property because of its sensitivity over three decades of temperature with a resolution A T/T up to 10 5 [56], The good repeatability, the insensitivity to magnetic fields up to 0.5 T [60] and the presence of temperature-fixed points allow for the control of possible shifts in the calibration curve of the pressure transducer. The usefulness of these fixed points is evident, considering that the ITS-90 is based just on the definition of fixed points. 

Thermometric titration curves usually represent both the entropy and the free energy involved. The titrant is added to the solution at a constant rate in order that the voltage output of the thermister-temperature-transducer changes linearly with time upto the equivalence point. 

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. 

After a short historical survey the fundamentals of signal transducers and the present state of thermometric, optoelectronic, and piezoelectric biosensors are presented. The most relevant electrochemical techniques are outlined in detail because electrochemical transducers are predominant. The aim of the second section is to provide information on the function of the biocomponents used in biosensors, primarily enzymes, but also antibodies and chemoreceptors. Special attention is paid to the methods of immobilization of the biomaterial and to the discussion and mathematical modeling of the interplay of biochemical reactions with mass transfer processes in immobilized enzyme electrodes. 

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. 

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. 

The combination of any bioreceptor with any transducer leads to a large number of biosensors. In reality, the two components have to be compatible to give rise to an electrical signal. It is impossible, for example, to use a thermometric transducer if the substrate transformation reaction does not give rise to a variation in enthalpy. Electrochemical transducers couple relatively easily with enzymes, and so such biosensors are already on the market. Other bioreceptor-... 

Considering that molecular recognition generally uses well-defined reaction types and that the deletion method may be extremely varied, it is logical that biosensors should be classified primarily as a function of the bioreceptor used. However, a laboratory that only ever worics with enzymes, for example, could use a classification according to the transducer employed (electrochemical, thermometric, etc). In what follows, we have opted to use the classification by bioreceptor because this component determines the primary action of the biosensor. 

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