Electrostatic control of NV charge states for quantitative pH sensing at diamond–electrolyte interfaces

Nanoscale pH sensing is important for studying local biochemical and electrochemical processes, but conventional fluorescent indicators can be nonreusable and unstable under harsh conditions. Nitrogen-vacancy (NV) centers in diamond provide a robust sensing platform scalable from nanodiamonds to bulk crystals. We present a pH readout approach for electrolyte solutions in contact with diamond based on the NV-/NV0 fluorescence ratio. By applying and scanning an external voltage, we electrostatically tune the NV charge-state response and support pH discrimination over an extended operating range. The results support voltage-assisted, diamond-based pH sensing for applications in electrochemistry and environmental monitoring.