Metal complexation-triggered chromogenic reaction and fluorescence response: A universal dual-mode sensing strategy for fast visualization of heavy metal ions.

Multi-mode/signal visualization of heavy metal ions (HMIs) is of great significance for environmental analysis and food safety, but it is still scarce. Herein, a universal dual-mode sensing strategy was initiated for fast visualization of HMIs. Paper sensors were prepared by the copolymerization of CdTe quantum dots-doped silane coupling agent onto filter papers and then anchoring of metal complexants (viz., chromogenic agents). Binary responsivity of the sensors toward HMIs was designed using complex products of HMIs-complexants as response units, in which colorimetry was realized by the complexation-based color developing while fluorometry was achieved in terms of the fluorescence response mediated by inner filter effect and dynamic quenching. By comparison with single-signal sensing, enhanced sensitivity, improved selectivity and widened working range were experimentally proved in such a combined sensing mode. As concept-to-proofs, in-situ dual-mode fast visualization of HMIs in real water samples such as industrial sewage, river water and tap water was testified using Cu, Ni and Fe as models. The limit of detection was as low as 0.02 mg/L (colorimetry of Ni), and the sensing process was as fast as 0.5 min, and the sensing results were validated by smartphone-based digital image analysis in RGB/gray-scale/Lab modes. Besides, fine consistence among the results of dual-mode visual sensing and those of commercial kits and instrumental technique such as fluorescence quantification and ICP-AES further reinforced the credibility of the strategy proposed herein.