Sensing Diamagnetic Electrolytes with Spin Defects in Diamond

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 T1, here we report on the opposite effect for diamagnetic ions. We demonstrate that millimolar concentrations of aqueous diamagnetic electrolyte solutions increase the T1 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 oxygen-terminated diamond. This work not only helps to understand noise sources in quantum systems but also broadens the application space of quantum sensors towards electrolyte sensing in cell biology, neuroscience and electrochemistry.