| Electronic tongue is a kind of modern instrument that can be used to measure and compare food samples automatically. Over the past years, our group developed this new type pulse voltammetric electronic tongue and used it for the detections of food systems. The metal electrode array sensor of electronic tongue mainly composed of 6 metal working electrodes including platinum electrode (Pt-E), gold electrode (Au-E), palladium electrode (Pd-E), tungsten electrode (W-E), titanium electrode (Ti-E), silver electrode (Ag-E), one metal counter electrode (Pt-E) and another metal reference electrode (Ag/AgCl). To prevent the deactivation caused by the chemical and physical adsorptions on the surface of the metal electrode array sensor in the practical detections and applications, the polishing method was often used to achieve the activation. However, this method is time-consuming, laborious, operationally discontinuous, and difficult to achieve automation. Therefore, the establishment of qualitative and quantitative characterization method for the metal electrodes, and the development of in-situ electrode array cleaning systems to achieve time-saving, labor-saving, as well as continuously automatic operation are of great theoretical significance and application value.On the basis of the DI value and basic theory of electrochemistry, a series of qualitative and quantitative methods for the metal electrode characterization were established for systematic studies on the deactivation mechanisms; Combining with the structure and application features of electronic tongue,3 kinds of activated systems for the electrode array sensor including the system of in-situ cleaning of chemical solvents, the system of electrochemical in-situ cleaning and the system of plasma in-situ cleaning were developed and used to evaluate the practical process of sample detections.The main contents and conclusions of this thesis are as follow:(1) Based on the testing condition, operational parameters and data processing methods, combing with the characteristics of the metal electrodes, a series of qualitative and quantitative characterization methods of deactivating and activating the metal electrode array sensor including RC and MD characterization, sensitivity characterization, SEM characterization, wet ability characterization, CV characterization, CA characterization, RS analysis, GC-MS qualitative analysis and complex (loss) motility quantitative evaluation method were established.(2) The adsorptive deactivation mechanisms of the metal electrodes for three typical samples including liquor, tea drinks and pure milk were well studied. The results showed the signal value, the resolution capability, the sensitivity and the static contact angle decreased as the electrode was deactivated. The anti-deactivation ability of platinum and gold electrodes was weak, but the anti-deactivation ability of titanium and silver electrodes was strong, and the anti-deactivation ability of palladium and tungsten electrodes was varied with different samples. The sequence of the deactivation of three samples from high to low is milk, liquor and tea. The results of adsorption experiments showed that the adsorbents in liquor were distributed non-uniformly and massively on the surface of the metal electrodes by physical and chemical adsorptions. The adsorbents in tea were uniformly punctuated on the surface of the electrode. The adsorbents in milk were ascribed to the physical adsorption with chunks of sticky substance. In addition, the results of GC-MS detection showed that the adsorbents of milk were the most complex, and then followed by the liquor, while the tea was relatively simple. The adsorbents of the three samples on the surface of the electrode were these compounds containing carbonyl functional group, where the lone pair electrons could form coordination bonds with metal. The deactivation mechanisms of the metal electrode array sensor for the electric tongue could be described as follows:Step 1, a proportional voltage was applied across the electrodes; Step 2, the redox reactions took place on the surface of electrodes; Step 3, chemical or physical adsorption occurred; Step 4, the effective surface of the electrodes was reduced; Step 5, the response current was decreased; Step 6, the sensitivity was decreased; Step 6, the dispersion became bigger; Step 7, the resolution capacity appeared poorly.(3) On the basis of the formation of hardware, the development of circuit and the re-design of the software, an electronic in-situ chemical solvent cleaning system of electronic tongue was developed. The system consists of two work stations of cleaning and drying. The functions of the system include ultrasound, stirring, clearing with combined multiple solutions, and air drying dehumidification etc. The optimal operating parameters of the system were obtained:90 s of cleaning time; 3.0% of hydrogen peroxide cleaning solution, the cleaning liquid composited of ultra-pure water, hydrogen peroxide and ultra-pure water, the drying with assistance of hot air, the ultrasonic and stirring. Using the three samples of yellow rice wine, icy black tea and juice as real test samples, the sample distinguishment ability for the electronic tongue was effectively enhanced through comparatively analyzing two current and PCA diagrams of cleaning systems and exhibited high practical application value.(4) The optimal parameters were obtained as follows:1.0 V～7.0 V and 10 s～90 s for the platinum, gold and tungsten electrodes; 1.0 V～5.0 V and 5 s～45 s for the palladium, titanium and silver electrodes through the optimization of the interval of electrochemical cleaning voltage operation, the impact of bubbles, and the interval of stirring effect and cleaning time, the appropriate interval of electrochemical cleaning voltage and optimal time. The 6 regression equations and the best response voltage were obtained as follows:3.1 V and 90 s for Pt,6.6 V and 22.1 s for Au,1.1 V and 27.9 s for Pd,6.1 V and 90 s for W,3.0 V and 27.1 s for Ti,5.0 V and 39.2 s for Ag by the optimal analysis of experimental by Minitab using the response curved surface regression analysis and the optimization of cleaning voltage and time. Based on the above results, an electrochemical in-situ cleaning system of electronic tongue was set up through the formation of hardware, the development of circuit and the re-design for the software. This system was further used to detect the three samples of liquor, tea, and milk etc by the effectiveness evaluation indexed of resurrection rate (A) and PCA diagrams. The results showed that the electrochemical cleaning system designed was effective, and the diction of metal electrode array sensor showed high stability.(5) Based on the structural features of metal electrode array sensor for electronic tongue, an electrode of disc-needle structure was designed independently, which could simultaneously satisfied the need of uniform spark discharge of 6 working electrodes with the enhanced cleaning efficiency and the increasing service life of metal electrode array sensor. Through the formation of hardware, the development of circuit and the re-design of the software, an in-situ cleaning system of pressure-air-plasma spark discharge for electronic tongue was established, and the simultaneously multiple functions including the rinsing, dried discharge cleaning and drying could be achieved. Based on the optimized operational conditions, such as the manner of plasma discharge, discharge voltage, discharge time and the discharge distance etc., the best optimum parameters conditions were obtained as follows:a discharge voltage of 9.0 kV, a discharge frequency of 200.0 Hz, a discharge (purge) time of 30 s, and a space of discharge electrode of 1.0 mm. Finally, the qualitative analysis results by GC-MS, the resolution capability (DI), and the detection of three samples from yellow rice wine, juice and icy black tea, showed that plasma cleaning can effectively remove the contaminants on the surface of the electrodes, and also activate the electrode of array. |
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