  Quantum sensing with spin defects in diamond, such as the nitrogen-vacancy
(NV) center, enables the detection of various chemical species on the
nanoscale. Molecules or ions with unpaired electronic spins are typically
probed by their influence on the NV center's spin relaxation. Whereas it is
well-known that paramagnetic ions reduce the NV center's relaxation time
($T_1$), here we report on the opposite effect for diamagnetic ions. We
demonstrate that millimolar concentrations of aqueous diamagnetic electrolyte
solutions increase the $T_1$ time of near-surface NV center ensembles compared
to pure water. To elucidate the underlying mechanism of this surprising effect,
single and double quantum NV experiments are performed, which indicate a
reduction of magnetic and electric noise in the presence of diamagnetic
electrolytes. In combination with ab initio simulations, we propose that a
change in the interfacial band bending due to the formation of an electric
double layer leads to a stabilization of fluctuating charges at the interface
of an oxidized diamond. This work not only helps to understand noise sources in
quantum systems but could also broaden the application space of quantum sensors
towards electrolyte sensing in cell biology, neuroscience and electrochemistry.
