Differentiation of heavy metal ions by fluorescent quantum dot sensor array in complicated samples

The interference of co-existing substances often causes trouble in the analysis of trace metal ions in complex samples, even for highly selective sensors. For this reason, an instant and inexpensive fluorescence sensor array for discrimination of heavy metal ions in complex samples was developed. The sensor array was constructed using four kinds of Mn doped ZnS quantum dots (Mn: ZnS QDs), which were facilely modified with N-Acetyl-cysteine, citric acid, mercaptopropionic acid and triammonium-N-dithiocarboxyiminodiacetate, respectively. For each metal ion, quantitative calibration curves were obtained with the concentration level ranging from 10 to 100 pg/mL (R-2 > 0.96). Due to the different quenching effect for Cu, Hg, Ag and Cd on various QDs, the sensor array exhibited a unique pattern of fluorescence variations at a low concentration of 30 pg/ml and can be discriminated successfully by principal component analysis (PCA). The contribution of individual sensors within the array was demonstrated and the obtained information was used to design sensor arrays with two and three sensor elements. The sensor array was also applied to identify the metal ions in unknown mixtures, containing single, binary, ternary and quaternary constitutions. Linear discriminant analysis (LDA) showed that the samples could be well recognized and distinguished. The sensor array was finally applied in water samples with three different concentrations, indicating the co-existing substances rarely influenced discriminatory capacity of the sensor array. Compared to instrumental analysis, this fluorescence sensor array-based method has proven to be more convenient since the nanoparticles can be prepared flexibly according to the property of the target.