A single-molecule fluorescent sensing platform for differential detection of Hg²⁺ and SO₃^2- with portable visual...

The development of portable sensing systems capable of differentiating multiple analytes with distinct outputs is of considerable interest for practical chemical analysis in complex matrices. However, most reported fluorescent probes are designed for single-analyte detection, while dual-analyte systems often suffer from complicated molecular architectures, insufficient signal differentiation, and limited applicability in portable analytical formats. Herein, we report XK725, a single-molecule fluorescent sensing platform for the differential detection of Hg²⁺ and SO₃^2- with smartphone-assisted and film-based visual readout. XK725 integrates a Hg²⁺-responsive thiocarbonate moiety and an α,β-unsaturated ketone unit reactive toward sulfite species within one compact molecular scaffold, thereby translating the distinct chemical reactivities of the two analytes into clearly differentiated fluorescence responses. Specifically, Hg²⁺ induces red fluorescence enhancement at 617 nm through thiocarbonate cleavage, whereas SO₃^2- triggers a green fluorescence turn-on at 500 nm via Michael addition that disrupts the original π-conjugation. The two analyte-dependent sensing pathways were systematically validated by HPLC, HRMS, NMR, and DFT calculations, providing clear mechanistic support for the differential response behavior. For practical application, XK725 was further extended to smartphone-assisted RGB quantification in solution, spray-based qualitative surface screening, and an XK725@PVA film platform for visual detection. The solution format enabled quantitative analysis of Hg²⁺ and SO₃^2- with good linearity, while the spray-based and film-based formats supported rapid visual discrimination, real-sample analysis, and portable on-site surface screening. This work provides an application-oriented strategy for integrating differential analyte recognition, mechanistic interpretability, and portable analytical formats within a single molecular sensing platform.