  Detecting magnetic noise from small quantities of paramagnetic spins is a
powerful capability for chemical, biochemical, and medical analysis. Quantum
sensors based on optically addressable spin defects in bulk semiconductors are
typically employed for such purposes, but the 3D crystal structure of the
sensor inhibits the sensitivity by limiting the proximity of the defects to the
target spins. Here we demonstrate the detection of paramagnetic spins using
spin defects hosted in hexagonal boron nitride (hBN), a van der Waals material
which can be exfoliated into the 2D regime. We first create negatively charged
boron vacancy (V$_{\rm B}^-$) defects in a powder of ultrathin hBN nanoflakes
($<10$~atomic monolayers thick on average) and measure the longitudinal spin
relaxation time ($T_1$) of this system. We then decorate the dry hBN nanopowder
with paramagnetic Gd$^{3+}$ ions and observe a clear $T_1$ quenching, under
ambient conditions, consistent with the added magnetic noise. Finally, we
demonstrate the possibility of performing spin measurements including $T_1$
relaxometry using solution-suspended hBN nanopowder. Our results highlight the
potential and versatility of the hBN quantum sensor for a range of sensing
applications, and pave the way towards the realisation of a truly 2D,
ultrasensitive quantum sensor.
