Electronic Tongues
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Electronic Tongues F.Winquist, C. Krantz-Rülcker, and I. Lundström Abstract The use of multivariate data analysis combined with sensors with partially overlapping selectivities has become a very powerful tool in measurement technology. These systems are often referred to as artificial senses, because they function in a way similar to the human senses. One such system is the electronic nose. This article focuses on similar concepts as the electronic nose, but for use in aqueous solutions. Because these systems are related to the human sense of taste in the same way the electronic nose is related to olfaction, they have been termed taste sensors, or “electronic tongues.” Various measurement principles that can be used in electronic tongues are described and discussed in this article. These include electrochemical techniques such as potentiometry, voltammetry, and conductometry. Also, optical techniques based on light absorption at specific wavelengths or the use of surface plasmon resonance are described. Mass-sensitive devices based on piezoelectric crystals have also been used and are described here. A special emphasis is given to the voltammetric electronic tongue. Keywords: biomimetic sensors, electronic tongues, sensor arrays, taste sensors, voltammetry.
Introduction There has been a change in philosophy on handling information in measurement technology. In many cases, it has become more desirable to get information on quality parameters such as sample condition, the state of a process, or expected human perception (e.g., of food), rather than quantitative chemical analysis of specific compounds. In this respect, multivariate data analysis (MVDA) has become a very powerful tool. Thus, instead of using specific sensors to measure single parameters, sensor arrays with partially overlapping selectivities are used, and the data obtained are evaluated with MVDA. These systems are often referred to as artificial senses, because they function in a way similar to the human senses. One such system is the electronic nose, which is based on the combination of a gas-sensor array with different selectivity patterns and MVDA.1–3 Recently, there has been interest in similar concepts for use in aqueous solutions. These systems are related to the human sense of taste in the same way that the electronic nose is to olfaction; thus, they have been termed taste sensors, or “electronic tongues.”4–6 In many applications, it is advantageous to measure in the aqueous phase rather than the gas phase. Many compounds, such as ions or those having a low vapor pressure, can only be measured in the aqueous phase. Also, for many ap-
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plications, only systems measuring directly in the solution may be feasible. The sense of taste has two possible aspects. One denotes the five basic tastes of the human tongue: sour, salt, bitter, sweet, and umami (glutamates). These originate from different, discrete regions on the tongue containing specific receptors called papillae. This aspect of taste is often referred to as the “basic taste.” The secon
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