Robust (hydrogen) phosphate sensing based on reversible redox of cobalt(II) hydroxide

Response mechanism of the electrode is elucidated in terms of (hydrogen) phosphate accelerating oxidation of Co^II to Co^III for the first time. Cyclic voltammetric techniques in conjunction with XRD, XPS and Raman characterizations have demonstrated unambiguously the response of cobalt (II) hydroxide electrode involves a phosphate and hydrogen ion dependent charge transfer process. Phosphate ions induce Co(OH)₂ transformed into CoOOH within interlayer adsorption and restored the initial state after reduction. Meanwhile, the in common structural between Co(OH)₂ and CoOOH prevents extensive structural convertibility upon cycling, result in the advantage of reversibility in phase transformation. Demonstrated sustainable technique offered the determination of phosphate with robust reproducibility (1000 cycles), long storage stability (6 months) and selectivity (potential interference: Cl^-, NO₃^-, SO₄^2- and HCO₃^-), achieving a detection limit of 5 × 10^-8 M over a wide linear range up to 1.28 mM. Presented work provided insights into the unique selectivity towards phosphate in cobalt based sensors, which may inspire the rational design of Co(OH)₂-based electrodes with superior electrochemical performance or extended applications.