Tracking the spin properties of single nitrogen-vacancy centers in nanodiamonds in aqueous buffer solutions
We report on the sensing stability of quantum nanosensors in aqueous buffer solutions for the two detection schemes of quantum decoherence spectroscopy and nanoscale thermometry. The electron spin properties of single nitrogen-vacancy (NV) centers in 25-nm-sized nanodiamonds have been characterized by tracking individual nanodiamonds during a continuous change in pH from 4 to 11. We have determined the stability of the NV quantum sensors during the pH change, which provides the fluctuations of $\pm$13\% and $\pm$0.3 MHz for $T_2$ and $\omega_0$ of their mean values. The observed fluctuations are significant when performing quantum decoherence spectroscopy and thermometry in various biological contexts. We discuss how the present observation affects the measurement scheme of nanodiamond-based NV quantum sensing.