Studies On The Real-time Electrochemical Multi-sensor Techniques

To execute systematic identification of multi-component gas mixtures, the currently successful detection modes are mostly that several specific gas sensors are chosen to construct a multi-sensor array, and pattern recognition techniques are adopted to process sensor-array signals. This approach is feasible, nevertheless, there are many possibilities affecting system performance and a precise mathematical model is to be determined. Therefore, it is rather difficult to put the method into practice.In this work, a novel electrochemical multi-sensor technique for fast and parallel monitoring of multi-component gases, using a single gas-sensing microelectrode and by fusing a computer-controlled fast potential modulation method as well as functions of data acquisition, conversion and processing, has been proposed and established.The fast and parallel monitoring of oxygen and carbon dioxide in respiratory gases is chosen as a study example. On the basis of studying the kinetics of cross reaction for the coexisting gases O₂ and CO₂ in non-aqueous media, fast nucleophilic addition reaction between superoxide ion O₂^-, one-electron reduction product of oxygen, and molecular CO₂:2 O₂^- + 2CO₂ â†' C₂O₆²- + O₂ (1)a special modulated pulse potential-time waveforms was designed as the dynamic excitation signal upon the Pt gas-sensing microelectrode (diameter 60 um) by means of the computer software program, so as to control the modulated polarization program of the working electrode and to execute acquisition and processing of the electrochemical transient response signals. The dynamic detection was carried automatically out at room temperature and the real-time acquired data could be stored. During initial cathode potential pulse, the O₂ is fully reduced to superoxide ion O₂ in dimethylsulfoxide solution containing 0.1 mol/L tetraethylammonium perchlorate. The portion of O₂^- reacts immediately with the CO2 present in the non-aqueous media according to the reaction Eq.(l), and the excess amount of O₂^- is backward oxidized to O2 during the subsequent application of anode pulse potential. In view of the above, by acquiring the corresponding transient response current signals at a certain instant, it is possible to detect rapidly and simultaneously the gas concentration of both O₂ and CO2, that is to say, the same gas-sensing element (electrode) exhibits different gas-sensing response characteristics under applying cathode and anode potential pulse, which means integrating two gas sensors of O₂ and CO₂ on a single gas-sensingelectrode and possessing functions of a two-component gas sensor. In this approach, the detection range of O2 is unlimited. The detection range of CO2 is approximately 0—12 %v/v, and the sensitivity of detection increase at lower CO2 concentrations. The total time required for measuring once is in the order of a few decamilliseconds. The stability of continuous operation is rather good.As for fast and parallel monitoring of multi-component gases whose component number is larger than two, the above-mentioned principle of electrochemical multi-sensor technique is fully applicable. The key and difficulty for establishing this multi-sensor technique are to discover and master the specific conditions and kinetic characteristics of cross reactions occurred to detected multi-component gases, from which to convert the fatal defect of cross-sensitivity to multi-component gases into the feasible kinetic conditions and to seek out the dynamic relationship and control conditions of fast and parallel monitoring of multi-component gases with a single gas-sensing microelectrode. This is also the theoretical basis for further making clear electrochemical transient response mechanism and optimizing performance of the multi-component gas sensor.