Quantum Sensing of Paramagnetic Spins in Liquids with Spin Qubits in Hexagonal Boron Nitride

Paramagnetic ions and radicals play essential roles in biology and medicine, but detecting them requires highly sensitive and ambient-operable sensors. Optically addressable spin color centers in 3D semiconductors are useful for detecting paramagnetic spins due to their sensitivity to spin magnetic noise. However, creating high-quality spin defects near the surface of 3D materials is challenging. Here, we show that spin qubits in hexagonal boron nitride (hBN), a layered van der Waals (vdW) material, can efficiently detect paramagnetic spins in liquids at nanoscales. We create shallow spin defects near the hBN surface, which maintain high-contrast optically detected magnetic resonance (ODMR) in liquids. Then, we detect paramagnetic ions in water using spin relaxation measurements, with a sensitivity of about 10–18 mol/ for Gd3+ ions. Finally, we show that paramagnetic ions reduce the contrast of spin-dependent fluorescence, enabling efficient detection by continuous wave ODMR. Our results demonstrate the potential of ultrathin hBN quantum sensors for chemical and biological applications.