Electrochemical sensor definition

Because water is ubiquitous both in the sensing environment and inside many sensors (especially electrochemical sensors), the hydrophobic or hydrophilic nature of a polymer used in a sensor is often crucial. For example, a polymer that is to be used as a hydrogel is by definition hydrophilic. On the other hand, gas-permeable membranes are often made of hydrophobic polymers to prevent passage of water through the membrane. These conventions are not always the case, however. An electrolyte for a sensor operating with non-aqueous electrochemistry may be less hydrophilic. Similarly, an in situ sensor to analyse polar degradation products in motor oil may use a hydrophilic membrane to allow passage of the analyte into the aqueous electrolyte from the non-polar hydrocarbon sample [14]. 

In the field of electrochemical sensors for liquids, there is the recent approach known as electronic tongue, which is inspired on the sense of taste. A widely accepted definition of electronic tongue (Holmberg et al. 2004) entails an analytical instrument comprising an array of non-specific, poorly selective, chemical sensors with crosssensitivity to different compounds in a solution, and an appropriate chemometric tool for data processing. 

Chemistry (lUPAC) titled "Classification and Nomenclature of Elec-troanal)d icar Techniques" [1], "Recommended Terms, Symbols, and Definitions for Electroanal Aical Chemistry" [2], and "Recommended Terms, Symbols, and Definitions for Electroanal d ical Chemistry (Recommendations 1985)" [3] and in Compendium of Analytical Nomenclature The Orange Book [4]. Some special articles characterize electrochemical sensors [5]. A special lUPAC technical report, "Electrochemical Biosensors Recommended Definitions and Classification" [6], deals with techniques and terms of electrochemical biosensors. 

Inspired by the success of PB-modified electrodes, different HCFs [54] have been more recently proposed for electroanalytical applications. Amongst others, Cu, Co, and Ni HCF have been the most frequently studied. In analogy to PB-modified electrodes, they have been proposed for the development of electrochemical sensors for H2O2 detection [55-57]. However, the performance of these materials was found to be definitely lower than that of PB or, as very recently demonstrated [58], only attributable to the presence of PB impurities in the structure of HCF films. On the other hand, non-iron HCF coatings, quite interestingly, demonstrate electrocatalytic properties in oxidation reactions involving, for instance, hydrazine [59], dopamine [60], and ascorbic acid [61, 62]. 

Unfortunately and somewhat confusing the term electrode has a variety of different meanings in electrochemistry. Unlike the commonly and also above-used definition, where it refers to the electronically conducting component of the cell only, it is also applied to the combination of an electronically conducting material in contact with an ionicaUy conducting phase [1]. Thus, the half-cell described in the next chapter is often colloquially referred to as electrode. Furthermore, the term electrode is even used to denote complete electrochemical sensors such as glass electrode measuring chains. 

Conductometric sensors, measuring the variatimi of the solution conductance due to electrical charge concentrations changes. The method is simple but not selective, since the conductance depends on the ionic concentration of all of the present species. In view of the fact that no electrochemical processes take place, conductometric sensors are not strictly electrochemical ones. They are considered, according to lUPAC chemical sensors definitions and classification [1], as a subclass of electrical devices in which the signal results from the change of electrical properties caused by the interaction of the analyte. Nevertheless, electrical devices are frequently put into one category with the electrochemical devices [1]. 

Development of ionic conductors based on stabilized zirconia has reached a level of maturity, where most of the research on such materials concentrates mainly on obtaining incremental empirical improvements in conductivity by better processing control and refinement of the microstructure of the solid electrolyte and SE. Further increases in the conductivity are important in terms of enhancing the efficiency of systems such as O2 sensors, zirconia-based mixed-potential gas sensors, electrochemical oxygen pumps, heating elements, and fuel cells [4-7]. The systematic errors, as have been considered before, are errors with a known determined functional connection with the source of their cause, and the conformity of their appearance can be definitely described. 

In the electrochemical community at that period the research on ion selective electrodes (ISE) was very active and the idea to extend the range of sensors to non electrochemical active compounds, and even to non ionic compounds, like glucose, has been very well accepted. We saw at that time the possibility to extend much more the research activity. The groups active in ISE development have been definitively the first to shift to the development of electroanalytical biosensors. 

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