Quantum Sensing of Lithium Ions with Defect-Engineered Recognition Pockets in a TEMPO-Modified Covalent Organic Framework

Nuclear-spin quantum sensing requires coherent molecular electron spin qubits embedded in analyte-recognizing local environments that bring target nuclei into their short-range hyperfine detection volume. Here, we introduce TA-TEMPO, a covalent organic framework in which dilute nitroxide radicals and Li+-binding triazine moieties form defect-engineered recognition pockets for room-temperature 7Li sensing. Optimizing radical density preserves spin coherence while maximizing the 7Li/1H response acquired by hyperfine spectroscopy. TA-TEMPO detects Li+ in a tetrahydrofuran solution of LiBr with a sensitivity of 1.0 × 10–4 mol·L–1 at room temperature, a 50-fold improvement over the previous benchmark, and enables quantum sensing across several lithium-based electrolytes. Control experiments and density functional theory calculations show that triazine sites preferentially bind Li+ over Na+ and position it near nitroxide radicals, accounting for selective detection in a large excess of Na+. These results establish chemically programmed host–guest recognition as a strategy to integrate sensitivity, selectivity, and coherent spin readout in porous molecular quantum sensors.