Building quantum ion sensors based on solid-state defects in nanodiamond

1. Introduction Alkali metal ions such as sodium and potassium ions play fundamental roles in biology. Therefore, developing highly sensitive and selective methods to both detect and quantify these ions for biological applications including medical diagnostics and imaging is of considerable importance. In recent years, quantum sensors have emerged as powerful tools to detect chemical and biological signals. In particular, nitrogen-vacancy (NV) centers in diamond act as stable fluorescence markers and magnetic field sensors, and have been investigated as quantum sensors for applications ranging from material science to chem-istry and biology. A promising avenue to detect biological signals is to transduce them into the signals that can be readout by NV centers. For instance, the NV's charge state is closely related to its surrounding electrostatic environment [1, 2]. Meanwhile, NV centers in nanodiamonds (NDs) have already demonstrated their ability to detect biological processes with high spatial resolution [3, 4], and are the object of mant intense study due to their favorable properties, ranging from very high photo- and thermal-stability to biocompatibility, in contrast to many florescent biomarkers that are used to detect ions. In this work, with surface engineering of ND, we study a quantum sensor that is capable of detecting specific ions such as sodium ions. We will show that the presence of metal ions will change the charge state of NV centers inside ND, which can be read out by measuring the photoluminescence (PL) spectrum.