A voltammetrically potentionmetric pH sensor based on multiply hydroxylated porphyrin

Using voltammetry to readout potentiometrically the solution acidity requires easy pH tunability of the probe's redox properties. In this work, tri-hydroxyphenyl porphyrin (POH₃) was synthesized as the electrochemically active probe to develop a voltammetrically potentiometric sensor (VPS) with a pH-sensitive response. By comparing POH₃ with control di- and mono-hydroxyphenyl porphyrins (POH₂ and POH₁), we found that protonation of the pyrrole macrocycle, electron communication of the para-hydroxyl groups with the pyrrole nitrogens during structure tautomerization, and the presence of multiple ionizable hydroxyl groups are the critical factors to report reversibly the pH-sensitive VPS response at the appropriate potential range. Common metal ions have negligible effects on the sensor performance. In comparison to the conventional potentiometric sensor (CPS), our VPS strategy benefits from the advantage that the electrode modification is not required. Thus, the complex phase boundary models necessary for CPS are unnecessarily involved in explaining the potential response.