Discrimination of Trace Heavy‐Metal Ions by Filtration on Sol–Gel Membrane Arrays

Abstract

The increasing concern for environmental pollution, especially the impact of heavy metals (e.g., Hg, Pb, As, Cd, and Ag), has stimulated active investigations.[1] These heavy metals arise mainly from industrial wastewater and exist in cationic forms. They are absorbed by fish or shellfish and accumulated millions of times even at very low concentrations. Once these heavy-metal ions enter the human food chain they cannot be degraded and are extremely harmful to our health. They can cause serious damage (e.g., in Japan in the 1950s hundreds of people died from the Minamata disease caused by the release of methylmercury from industrial wastewater) and affect the central nervous system, kidneys, liver, bones, or teeth. Some heavy metals (e.g., Cu, Zn, and Ni) are, on the one side, essential for life, but on the other side, toxic in excessive amounts. [2] There are numerous conventional methods for the detection of heavy-metal ions, [3] including atomic absorption spectrometry (AAS), inductively coupled plasma atomic emission spectrometry (ICP-AES), inductively coupled plasma mass spectrometry (ICP-MS), anodic stripping voltammetry, and X-ray fluorescence spectrometry. Most such technologies offer a good limit of detection, but do, however, suffer from other severe limitations; they are expensive and generally not suitable for portable on-site applications. Some heavy-metal ions may change form after processing, for example, from inorganic to organic (mercuric salts to methylmercury), or may be oxidized or reduced to a different oxidation state. Therefore, there is a clear need for the rapid, sensitive, and highly portable on-site identification of these heavy-metal ions, not only for monitoring industrial wastewater discharge, but also from the human health perspective.In recent years, optical sensors have been widely investigated.[4] A number of colorimetric sensors for the detection of heavy-metal ions have been proposed [5] (e.g., modified or functionalized nanoparticles, DNAzyme, and nanosized dyebased test strips). These methods show good sensitivity and selectivity, but lack the capability of distinguishing multiple heavy-metal ions. Lately, sensor-array technology has shown a unique potential for this discrimination. In combination with sensor-array technologies and pattern-recognition methods, optical sensor arrays have been extensively applied for determination of toxic industrial gases, [6] nucleic acid and biomolecules, [7] diols, [8] and even for diagnostics. [9] Recently, cross-reactive fluorescence sensor arrays for metal ions were developed.[10] These arrays show tremendous potential for identification of multiple metal ions, however, they either need complicated synthetic procedures for fluorescence molecules or use external fluorescence excitation sources. Moreover, most of these arrays monitor fluorescence changes of analyte solutions based on the interactions between the fluorescent reagents and the analytes. Without any enrichment, the sensitivity of these sensor arrays entirely depends on the ...

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