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In environmental analysis, cyanide and heavy metals play an important role because both components are toxic for human. Due to their good chemical and physical properties cyanide as well as heavy metals are frequently used for industrial purposes. Thus, via industrial effluents and exhausts they are released into the environment. Furthermore, both components exist naturally.
In this work, silicon-based chemical sensors and biosensors for a qualitative and quantitative detection of cyanide and heavy metals in solutions have been developed. In the case of cyanide detection, capacitive electrolyte-insulator-semiconductor (EIS) structures with immobilised cyanide selective enzyme, Cyanidase, as well as Ag-based ion-selective electrodes (ISEs) have been designed, fabricated by means of thin-/thick-film technology, characterised and optimised towards their intrinsic sensor properties. The cyanide biosensor uses the local pH-shift, which is caused by the selective catalysis of cyanide to ammonia and formate by Cyanidase. The cyanide detection with the help of Ag-halogenide and Ag-chalcogenide-glass ISE is based on a complex reaction between CN-ions from the analyte solution and Ag-ions in the sensor membrane, which results in a change of the surface potential of the ISE. Both, the enzyme-based biosensor and Ag-based ISE have shown a cyanide depended signal, which correlates well with theoretically expected values and experimental data from the literature. Due to the low enzyme activity and stability of the immobilised Cyanidase and based on theoretical model calculation it could be shown, that the developed cyanide biosensor delivers reliable and reproducible results only under optimal laboratory conditions.
For the detection of heavy metals in liquids, various chip-based chalcogenide-glass ISEs have been realised by means of the pulsed laser deposition technique and characterised in Ag-, Cu-, Pb- and Cd-ion containing analyte solutions. The measurement principle is based on a concentration depended, dynamic ion exchange between particular heavy metal ions from the solution with heavy metal ions in the sensor membrane, which leads to a concentration depended change of the surface potential. The chalcogenide-glass ISEs have shown a heavy metal depended signal, which correlates well with the theoretically expected Nernst-sensitivity.
For the analysis of mixed heavy metal solutions, in addition to the high sensitivity of chalcogenide-glass ISEs (e.g., Cu-ISE to Cu-ions) the cross-sensitivity of these ISEs have been used advantageously in a multi-sensor device. Therefore, in close cooperation with the university Bonn-Rhein-Sieg (Prof. W. Heiden) a primary “electronic tongue” which exists of three chalcogenide-glass ISEs (for Ag-, Cu- and Pb-ion detection) have been developed. In the first step, it could be shown that with the help of a fuzzy-logic software heavy metal ions of two-component solutions could be quantitatively and qualitatively determined.
For the simultaneous detection of cyanide and heavy metals in liquids, a portable hybrid measuring cell and a measurement protocol have been presented for the first time. Therefore, in a first step cyanide and heavy metals have been separated by means of acidification of the analyte solution and applied temperature to the system. In a second step both toxic components have been simultaneously detected with the help of particular chalcogenide-glass ISEs. This hybrid measuring cell could be successfully applied for the simultaneous detection of Cu-, Cd- or Pb-ions and CN-ions.