Indeed, pH can affect the functional groups at the ND oxidized surface (carboxylic acids, ketones, alcohols, or esters), thus changing the properties and charge stability of shallow NV centers. At low pH, e.g., carboxylates will be protonated to a much higher extent than under physiological conditions at pH≈7.2, with potential effects on charge state of nearby NV centers. While plausible, this hypothesis requires further investigation.