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 Cu2+, Ni2+ and Fe2+ as models. The limit of detection was as low as 0.02 mg/L (colorimetry of Ni2+), 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.