Long-Lived Mechanically Detected Molecular Spins for Quantum Sensing

Quantum sensors based on individual spins provide unprecedented access to local magnetic fields in condensed matter, chemistry, and biology, with solid-state defect spins emerging as the leading platform. However, their molecular-sensing capabilities are limited by confinement to a host lattice, which prevents placement in close proximity to a target molecule. Molecular spins offer an alternative, enabling chemical tunability and flexible positioning relative to the target system. Here we present a nanoscale sensing platform that combines molecular electron spins, ultrasensitive mechanical readout, and Hamiltonian engineering. Using a modified XYXY dipolar decoupling sequence, we suppress electron-electron dipolar interactions across a broad distribution of control fields, extending coherence times to ∼ 400 μ s in an attoliter-scale droplet containing ∼ 100 trityl-OX063 radicals. Leveraging this sequence, we demonstrate frequency-selective detection of nanotesla-scale ac fields and perform sensing and spectroscopy of small, local nuclear-spin ensembles. Collectively, these results establish spin-based quantum integrated nanomechanical transduction as a framework for quantum sensing that affords molecular-level control over sensor properties and enables direct integration into complex molecular targets.