  Paramagnetic ions and radicals play essential roles in biology and medicine,
but detecting these species requires a highly sensitive and ambient-operable
sensor. Optically addressable spin color centers in 3D semiconductors have been
used for detecting paramagnetic spins as they are sensitive to the spin
magnetic noise. However, the distance between spin color centers and target
spins is limited due to the difficulty of creating high-quality spin defects
near the surface of 3D materials. Here, we show that spin qubits in hexagonal
boron nitride (hBN), a layered van der Waals (vdW) material, can serve as a
promising sensor for nanoscale detection of paramagnetic spins in liquids. We
first create shallow spin defects in close proximity to the hBN surface, which
sustain high-contrast optically detected magnetic resonance (ODMR) in liquids.
Then we demonstrate sensing spin noise of paramagnetic ions in water based on
spin relaxation measurements. Finally, we show that paramagnetic ions can
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.
