Ce ⁴⁺ ‐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 Ce 4+ 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 Ce 4+ . The sensing mechanism exploits Ce 4+ 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 Ce 4+ 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 Ce 4+ 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.