Quantum Sensing of Lithium Ions with Defect-Engineered Magic Pockets in a Molecular Qubit 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 “magic 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 tetrahydrofuran solution of LiBr with a sensitivity of 1.0 × 10−4 mol·L−1 at room temperature, a 50-fold improvement over 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.