  Biocompatible nanoscale probes for sensitive detection of paramagnetic
species and molecules associated with their (bio)chemical transformations would
provide a desirable tool for a better understanding of cellular redox
processes. Here, we describe an analytical tool based on quantum sensing
techniques. We magnetically coupled negatively charged nitrogen-vacancy (NV)
centers in nanodiamonds (NDs) with nitroxide radicals present in a bioinert
polymer coating of the NDs. We demonstrated that the T1 spin relaxation time of
NV centers is very sensitive to the number of nitroxide radicals, with a
resolution down to ~10 spins per ND (detection of approximately $ 10^-23 $ mol
in a localized volume). The detection is based on T1 shortening upon the
radical attachment and we propose a theoretical model describing this
phenomenon. We further show this colloidally stable, water-soluble system can
be used dynamically for spatiotemporal readout of a redox chemical process
(oxidation of ascorbic acid) occurring near the ND surface in an aqueous
environment under ambient conditions.
