Direct Electrochemical Sensing of Phosphate in Aqueous Solutions Based on Phase Transition of Calcium Phosphate

Electrochemical determination of phosphate in aqueous solutions attracts considerable interests in both biological and environmental fields. Because of the electrochemically inactive nature of phosphate, direct electrochemical detection of phosphate is still a highly challenging task. Herein, we reported a direct electrochemical approach for the determination of phosphate based on the oxidation of coordinated OH during the phase transition of calcium phosphates (CaPs). The mixture of amorphous CaPs and octacalcium phosphate (Ca₈(HPO₄)₂(PO₄)₄·5H₂O), which acts as the starting material for hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂), was self-assembled on a Nafion-modified glassy carbon electrode. The as-prepared electrode (CaPs/Nafion) showed a distinct oxidation peak at 1.0 V versus Ag/AgCl in phosphate solution. The peak heights were directly proportional to the concentration of phosphate from 0.1 to 10 μM in the presence of 1 mM Ca²⁺. After comprehensive characterization of the CaPs/Nafion electrode, it was understood that phosphate ions as a proton acceptor could stimulate the generation of coordinated OH from coordinated water (H₂O) in CaP. The addition of Ca²⁺ could magnify the coordinated H₂O source because of its hydration to H₂O. The CaPs/Nafion electrode also displayed good selectivity as the electrochemical oxidization response was not affected by up to 10 μM of potentially competitive species like CO₃^2-, NO₃^-, CH₃COO^-, SO₄^2-, and Cl^-. The results obtained in this work not only provided a new method for direct detection of phosphate in aqueous solution but also suggested that Ca²⁺ could be a promoter for electrochemical oxygen generation.