A self polishing electronic tongue

doi:10.1016/j.snb.2006.04.042

Sensors and Actuators B: Chemical

Volume 118, Issues 1–2, 25 October 2006, Pages 461-465

https://doi.org/10.1016/j.snb.2006.04.042 Get rights and content

Abstract

An investigation to obtain reproducible measurements with a pulse voltammetric electronic tongue has lead to the development of self polishing device. A grit paper covered bar rotating over the working electrodes is performing the polishing, to avoid measurements while the polishing bar covers the electrodes an angular decoder is fitted. Measurements in buffer, 2 mM K₃[Fe(CN)₆] and a buffered tea samples shows that polishing reduces drift, sensitivity decreases with electrode fouling, pre-treatment or conditioning of electrodes post polishing must be optimised concerning the analyte. Also found was that drift due to electrode fouling is a repeatable mechanism which pattern can be used to increase information about the analyte.

Introduction

Different types of electronic tongues or taste sensors have been described, such as potentiometric using ion selective electrodes or lipid/lipid membranes. Recently an electronic tongue based on voltammetry was described [1]. As been pointed out in many applications, drift is an unwanted problem. Sensor drift cause a problem such as losses in detection limits, low reproducibility, and drift in electronic tongues is by far means no exception. There are two obvious ways to overcome this; firstly a mathematical transform for drift adjustment can be used, using a reference solution of known concentration. Examples include normalization [2] or component correction [3]. Secondly, the sensor itself can be treated, for the voltammetric electrodes electrochemical cleaning steps are used [4], and in difficult cases it is necessary to use mechanical polishing or cutting off the outer most electrode layer to regain the electrode surfaces [5]. Electrode surfaces: electrode material, its adsorption and chemical sorption properties, homogeneity and smoothness has extensive influence on its behaviour. However these properties change with use. Applied potentials give rise to building oxidised or reduced layers on the active surface, species adsorb to the surface, even electrochemical induced reconstruction of electrode surfaces has been reported [6]. To eradicate these problems and to get a renewable electrode surface mechanical polishing is suggested.

Section snippets

Equipment

A self polishing electronic tongue, based on pulsed voltammetry has now been developed. It consists of four working electrodes and a counter electrode, no reference electrode is used. The working electrodes are of gold, iridium, platinum and rhodium with diameters of 1 mm. The working electrodes are imbedded in dental material inside a stainless steel tube that also works as counter electrode. The polishing mechanism is carried out by a bar covered with grit paper that is pressed onto the

Results and discussion

The tea samples were chosen to get a complex analyte with known adsorption induced drift [7], buffer and ferro cyanide was included as references to previous investigations. As shown in Fig. 2, repeated measurements with polishing, observations of tea spreads more than the less complex samples of buffer and ferro cyanide, as expected. When not conducting mechanical polishing, drift is apparent to all samples even to the reference samples. Nevertheless the direction of tea induced drift is

Conclusions

Mechanically regenerating the electrodes in a voltammetric electronic tongue removes the redox reaction products that accumulate during some experiments. Conditioning of recently polished electrodes surfaces must be optimised with respect to the analyte. The rate of fouling can be seen in the corresponding decrease in sensitivity. The fouling process appears to be exclusive regarding the analyte and potentials used. The electrode fouling pattern contains information concerning the analyte,

John Olsson received the MS degree in chemistry from Umeå University, Umeå, Sweden in 2002. He is currently working towards the PhD degree at Linköping University. His present research interests are sensor system, such as the voltammetric electronic tongue, and pattern recognition.

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The study of interfacial activity in metal-solution systems is of great interest due to its potential applications in science and technology. Some examples include the optimization of the capacitive deionization process of water [1], improvement in the design of electrolytic capacitors for storing energy, improvement in techniques employed in the electropolishing of metal surfaces (electronic tongue treatment) [2,3], and environmental applications such as electrocatalytic carbon dioxide reduction [4]. However, despite the practical importance of these fields of application and all the effort dedicated to them, advances in the last 50 years have uncovered theoretical and practical difficulties [5].
Potential step voltammetry is used to characterize the resistive and capacitive properties of electrochemical systems. Within those systems, electrochemical processes are modeled by means of equivalent circuits (ECs). In this study, we established and experimentally validated algebraic expressions describing the electrical response of those ECs. We then analyzed the features that allow significant differentiation between the ECs. Finally, we have proposed a method to graphically visualize and analyze the electrical response of the circuit to a potential step sequence. The application of this method to real electrochemical systems will allow us to not only identify the electrochemical processes taking place in an experiment but to also assess parameters such as the double layer capacitance, the solution resistance, and the polarization resistance of the redox process.
Monitoring honey adulteration with sugar syrups using an automatic pulse voltammetric electronic tongue
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The new tendency to detect adulterated honey is the development of affordable analytical equipment that is in-line and manageable, enabling rapid on site screening. Therefore, the aim of this work was to apply an electronic tongue based on potential multistep pulse voltammetry, in combination with multivariate statistical techniques to detect and quantify syrup in honey. Pure monofloral honey (heather, orange blossom and sunflower), syrup (rice, barley and corn), and samples simulating adulterated honey with different percentages of syrup (2.5, 5, 10, 20 and 40) were evaluated. An automatic, electrochemical system for cleaning and polishing the electronic tongue sensors (Ir, Rh, Pt, Au) significantly improved the repeatability and accuracy of the measurements. PCA analysis showed that the proposed methodology is able to distinguish between types of pure honey and syrup, and their different levels of adulterants. A subsequent PLS analysis successfully predicted the level of the adulterants in each honey, achieving good correlations considering the adjusting parameters. The best results being for sunflower honey adulterated with corn syrup and heather honey with barley syrup (r² = 0.997), and heather with corn (r² = 0.994) whereas the weakest was found for heather honey adulterated with brown rice syrup (r² = 0.763) and orange blossom honey with corn syrup (r² = 0.879). The measurement system here proposed could be a very quick and effective option for the honey packaging sector with the finality of providing information about a characteristic as important as the adulteration of honey.
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In the interval of experiment, the electrodes are always polished by filter paper (Rodríguez-Méndez et al., 2009), sand cloth (Oliveri et al., 2009), and alumina paper (Manzoli et al., 2014), than rinsed with distilled water, and those procedures are manipulated manually. For polishing the electrodes efficiently, Olsson and co-workers developed a self polishing mechanism for the VE-tongue (Olsson et al., 2006). The polishing mechanism is carried out by a bar covered with grit paper that is pressed onto the surface of the working electrodes and turned by an electric motor.
Voltammetric electronic tongue (VE-tongue) is a promising technology for advanced sensing and measurement applications. The review examines the measurement principles, working parameters and configurations of different types of voltammetric electronic tongues (VE-tongue): the VE-tongue based on bare electrodes, modified electrodes and biosensors. The working parameters of these VE-tongues (electrode cleaning method, pulsed voltammetry and pre-processing of responses data) are described, and the development of VE-tongue based on miniaturized sensors arrays and automatic flow techniques is also presented. The applications of foodstuff analysis commented include recognition of basic tastes substances, tracing origins/samples recognition, process monitoring, and quality investigation. Finally, the unsolved problems and significant efforts of VE-tongues are depicted.
A self-polishing platinum ring voltammetric sensor and its application to complex media
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There are different strategies to avoid or overcome drift problems [11,12]: (i) using mathematical tools for drift correction, such as normalization or component correction; (ii) recovering the surface with a mechanical polishing step; (iii) applying electrochemical cleaning steps; (iv) using a mechanical self-polishing device which renews the electrode surface. For the last strategy different self-polishing devices have been developed and applied to VETs with disc electrodes, to avoid fouling of the electrode [13]. After polishing the electronic tongue needs some time for equilibration of the electrodes and during this time drift appears in the measurements.
A self-polishing voltammetric sensor was recently developed and has been applied to samples of urea, milk and sewage water. The polishing device continuously grinds a platinum ring electrode, offering a reproducible and clean electrode surface. Principal component analysis (PCA) and partial least squares (PLS) techniques were applied to interpret the data and to build prediction models. In an evaluation of samples with different urea concentrations, the grinding step allows for repeatable measurements, similar to those after electrochemical cleaning. Furthermore, for the determination of sewage water concentrations in drinking water and for the evaluation of different fat contents in milk samples, the polishing eliminates sensor drift produced by electrode fouling. The results show that the application of a self-polishing unit offers a promising tool for electrochemical studies of difficult analytes and complex media.
Recent developments of green analytical techniques in analysis of tea's quality and nutrition
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However, their application is limited to redox-active substances, and temperature dependence and large surface alteration will cause drifts in sensor response. In order to solve this problem, a shelf-polishing equipment has been put forward recently (Olsson, Winquist, & Lundström, 2006). An E-tongue based on impedance technique was first described by (Riul, Malmegrim, Fonseca, & Mattoso, 2003).
Public attention in tea quality and nutrition has increased significantly in recent decades, due in part to changes in consumer behavior and the gradually increasing consumption of tea. Demand for high quality tea products obviously requires high standards of quality assurance and process control; satisfying this demand in turn requires appropriate analytical tools for analysis of tea quality and nutrition. Green analytical tool, as an alternative to conventional analysis methods, has the desirable features in terms of operating speed, ease-of-use, minimal or no sample preparation, and avoidance of sample destruction. This paper reviews recent developed technologies, such as near infrared (NIR) spectroscopy, electronic tongue (E-tongue), electronic nose (E-nose), computer vision, integration of multiple sensors, and latest research efforts to assess tea quality and nutrition. Particularly, we have reviewed some relevant data processing algorithms involved in each green analytical tool. Finally, we provided the technical challenges and outlook for the application of these green analytical technologies in analysis of tea quality and nutrition.
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The use of a voltammetric electronic tongue as a tool to monitor grape ripeness is proposed herein. The electronic tongue consists of eight metallic electrodes housed inside a stainless steel cylinder. The study was carried out over a period of ca. 1 month (August 2012) on different grape varieties (Macabeo, Chardonnay, Pinot Noir, Cabernet Sauvignon, Shyrah, Merlot and Bobal) from various vineyards near Requena and Utiel (Valencia, Spain). Apart from the electrochemical studies, the physico-chemical parameters, such as, Total Acidity, pH and °Brix, were also determined in grapes. The PCA models, obtained using the physico-chemical or electrochemical data, showed variation of ripeness with time. Moreover the study was completed by using partial least squares (PLS) regression in an attempt to establish a correlation between the data collected from the electronic tongue and Total Acidity, pH and °Brix values. A good predictive model was obtained for the prediction of Total Acidity and °Brix. These results suggest the possibility of employing electronic tongues to monitor grape ripeness and of, therefore, evaluating the right time for harvesting.

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Fredrik Winquist has more than 20 years of experience from analytical research, and he has authored and co-authored several scientific papers. His research activities cover a broad span from biosensor activities to semiconductor physics. Further development of sensory system eventually led to the concept of electronic noses and tongues which is a combination of sensor signals with pattern recognition routines. This line of development has led to the creation of olfactory and taste pictures. His current interests concern sensor system, signal fusion, multivariate data analysis, imaging ellipsometry and biosensors.

Ingemar Lundström is professor in and head of the Division of Applied Physics at Linköping University, where he directs a multidisciplinary research group working on new concepts for bio- and chemical sensing. His achievement include the development of catalytic gate field-effect for chemical detection and new methods for biospecific interactions, and the use of computer screen as a versatile light source for bio- and chemical analysis.

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A self polishing electronic tongue

Abstract

Introduction

Section snippets

Equipment

Results and discussion

Conclusions

Anal. Chim. Acta

Anal. Chim. Acta

Anal. Chim. Acta

Electrochim. Acta