High-Performance Chemiresistive Humidity Sensing and Quantum Isotopic Differentiation with Single Crystals of a Conductive MOF Hosting Electron Spin Qubits

Conductive metal–organic frameworks (MOFs) offer a versatile platform for chemiresistive humidity sensing, a capability vital for industrial and biomedical applications. However, grain boundaries and uncontrolled crystallite orientations in commonly used polycrystalline devices limit sensing performance, while electrical responses alone lack specificity for distinguishing H2O from D2O. Here, we report high-performance chemiresistive humidity sensing and quantum isotopic differentiation using single crystals of a representative conductive MOF, Cd2(TTFTB) (TTFTB4− = tetrathiafulvalene tetrabenzoate). Leveraging intrinsic charge transport modulated by a solvent screening effect, the single-crystal devices significantly outperform polycrystalline counterparts that exhibit negligible sensing capability. Specifically, they achieve an ultralow detection limit of 100 ppm, a broad dynamic range up to 100% relative humidity, and a high average on/off response exceeding 3 × 106. Comparative measurements across solvent vapors reveal selectivity for water vapor and relate current modulation to solvent dielectric constant, hydrogen-bonding ability, and molecular volume. Furthermore, we harnessed the electron spin coherence of embedded TTF⋅+ radical qubits in a collection of Cd2(TTFTB) crystals to unambiguously distinguish H2O from D2O via hyperfine spectroscopy, complementing high-performance single-crystal chemiresistive detection with quantum-enabled isotopic identification. Taken together, these results demonstrate the feasibility and considerable promise of a dual-mode sensing approach that integrates chemiresistive and quantum sensing.