Researches on Heavy Metal Electrochemical Sensors and Their Application in Seawater Analysis
|Keywords||Electrochemical sensor Determination of heavy metals Microelectrode array (MEA) Light-addressable potentiometric sensor (LAPS) Microfabrication technology Analysis device Seawater detection|
Heavy metals are one class of contaminants that can lead to undesirable effects even if they are present in extremely minute quantities. They are the most insidious pollutants because of their nonbiodegradable nature and ability to persist for long periods. All forms of ecological systems are affected to various extents by heavy metals. Through the food chain, heavy metals may deposit into the human body, cause serious health problems, specifically cancer, and even transfer to the next generation on the genetic level. Therefore, the quantitative analysis of trace heavy metals is extremely important in medical, food, clinical and environmental monitoring fields.In recent years, water pollution has been more and more aggravating in Chinese sea area. It is in great requirement to control marine pollution and protect marine ecological environment. So the project to establish national ship-board marine ecological environmental monitoring system was proposed during the Tenth Five-Year Program of China. Compared with other heavy metal detection methods, electrochemical stripping voltammetry(SV) exhibits the merits of its fast multi-element capabilities, simple operation, high sensitivity, relative small volume and low cost of equipment, as well as its suitability for on-line, in-situ and automatic application. Modern sensors and their analysis instruments tend to micromation, integration and automatization. And miniature electrochemical sensors allow low cost in batch production, have good uniformity, demand only a small amount of sample solution, enable relatively rapid and sensitive analysis.This thesis concentrates reseaches on heavy metal ion electrochemical sensors and their application in seawater analysis. The challenges are to develop miniature, integrate sensors and automatic, portable analysis instrument. The research was financed by the National High Technology Research and Development Program of China (863 Program), China-Russia Special Fundation for International Cooperation in National Natural Science Fundation of China (NSFC) and International Cooperation Major Fundation of Zhejiang Province.The major contents of this thesis are given in the following.1) Heavy metal elements, including zinc, cadmium, lead, copper, manganese and arsenic are determined by electrochemical stripping voltammetry. A series ofexperiments have been carried out to obtain optimized parameters. And the experimental results are analyzed and discussed. Based on mercury-film electrode, Zn(II), Cd(II), Pb(II) and Cu(II) are simultaneously detected. Their linear ranges are respectively 10~1000//g/L, l~50Qag/L? 2~80Qag/L and 5~50Qag/L. Their detection limits are respectively 2.8l?g/L, 0.2/ig/L, 0.8//g/L and l.lfigfL. Mn(II) are determinated using Hg-film electrode. Good results are obtained in a linear range of 5~200/ug/L and with a detection limit of 1/Jg/L. As(III) measurements with gold electrode yield good results in a linear range of 10~50Qug/L, and its detection limit is 3.2fig/L.2) Based on microfabrication technology, a miniature electrochemical sensor has been developed. With microfabrication processes, such as oxidation, sputtering, photolithography and etching, a mircoelectrode array(MEA) is fabricated on a 5.5x5.5mm2 silicon substrate, containing 30x30 individual lOjum diameter Au disk microelectrodes, spaced 150//m with each other. Experiments and studies of Hg-plated AuMEA were performed to examine four heavy metals of Zn(II), Cd(II), Pb(II) and Cu(II). By means of differential pulse stripping voltammetry, measurements show good results with their linear ranges separately in 5~ 4O0fig/L, 0.3~20QMg/L, l~~2DQ/ig/L and l~300y?g/L. And their detection limits respectively are 0.8/ig/L, 0.1//g/L, 0.5/igfL and 0.3/^g/L. In comparison with the traditional ordinary electrode, MEA holds many merits. It needs little sample without stirring and supporting electrolyte, provides fast scanning velocity, high signal to noise ratio, high resolution and low detection limit.3) An integrated miniature sensor has been developed for multielement detection, such as Zn(II), Cd(II), Pb(II), Cu(II), Mn(II), As(III), Fe(III) and Cr(VI). Based on microfabrication processes, two MEAs and two light-addressable potentiometric sensors(LAPS) are integrated on one silicon chip, only 6x6mm2 in size. Each MEA consists of 8x8 Au microelectrodes, with 10/im diameter and separated by 8Qwm from each other. Electroplating silver and platinum on the chip is to serve for miniature reference electrodes and counter electrodes. Iron(III) and chromium(VI) ion selective thin films are deposited on the relative two 2 X 2mm2 surfaces of the chip and form LAPS. The determinations of Zn(II), Cd(II), Pb(II) and Cu(II) with stripping voltammetry show the linear ranges of 10~200/ug/L, 1 ~ lOO^ug/L, 5~100^g/L and 10~200/*g/Lrespectively. The Detection limits of them respectively are 3[ig[L, O.SfigfL, 1/ugfL and 2/xgTL. For Mn(II) and As(III) detection, linear ranges of 1~ 100//g/L and S^lSQag/L are obtained separately. And experiments result in the limits of detection of 0.3/^g/L and 1/Jg/L separately for Mn(II) and As(III). The detection limits of 3.6,wg/L and O.^g/L are obtained by Fe(III)-LAPS and Cr(VI)-LAPS, separately.4) An automatic analyzer for heavy metals has been developed based on the integrated miniature sensors. With SV and LAPS methods, the device has realized simultaneous automatic determination of eight heavy metal ions including Zn(II), Cd(II), Pb(II), Cu(II), Mn(II), As(III), Fe(III) and Cr(VI) in pretreated seawater sample. In the system, computer multithreadingly controls the sensors and their detection circuits. Computerised sampling, detection, drain and cleaning were realized automatically coupled with pumps and valves. One measurement lasts 15min. After the measurement, the computer automatically analyzes results and outputs data. The instrument can provide continuous ship-board on-line detection.