Ce4+-Triggered Stable Pyrene Radical Cation Formation in a Hydrogen-Bonded Organic Framework Enabling on-Site Dual-Mode Sensing.

The rapid, on-site detection of environmentally relevant Ce4+ ions remains challenging due to limitations in conventional analytical methods. Herein, we report a pyrene-based hydrogen-bonded organic framework (Py-HOF) that enables highly sensitive and selective dual-mode (colorimetric and fluorometric) visual detection of Ce4+. The sensing mechanism exploits Ce4+ as a potent one-electron oxidant to trigger the formation of stable pyrene cation radicals (Py•+) within the structurally ordered HOF. Crucially, the rigid, columnar π-stacked architecture of Py-HOF effectively stabilizes the Py•+ species. The synergistic interplay between naked-eye-visible color change and fluorescence quenching enables a gradual transition from yellow to cyan under irradiation as the concentration of Ce4+ increases (response time < 8 s), significantly enhancing visual resolution. The process is chemically reversible with reductants like ascorbic acid (AA). Leveraging this mechanism, we fabricate a 3D-printed portable sensor for smartphone-assisted rapid quantification of Ce4+ in real water samples with a linear range of 1-35 µM and a limit of detection (LOD) of 0.3 µM, and AA in commercial juices with a linear range of 2-22 µM and LOD of 0.6 µM. This work demonstrates the feasibility of radical-based optoelectronic HOF materials as a dual-mode platform for metal ion sensing.