Highly Selective Fluorescent Sensing of Phosphite through Recovery of Poisoned Nickel Oxide Nanozyme

Phosphorus is a key element responsible for eutrophication, and its measurement and speciation is a critical topic in analytical chemistry. Most research efforts have been devoted to detecting phosphate (P(V)), while few reports on phosphite (P(III)) are available, making it difficult for sensor-based understanding of the phosphorus cycle. This study presents a fluorescent "turn-on" sensor for quantitative and highly selective analysis of phosphite based on the different coordination strength of N and P lone-pair electrons toward nickel oxide (NiO). A few N-containing compounds (mainly Good's buffers) were screened as inhibitors for the oxidase-like activity of NiO nanoparticles for the oxidation of Amplex red (AR). HEPES was found to be most effective for inhibiting the formation of fluorescent resorufin, the oxidation product of AR. Among various phosphorus-, arsenic-, selenium-, and sulfur-containing species, along with various cations, phosphite was the only one that could restore the activity, likely due to its stronger affinity with the surface, and it is not an inhibitor. Under the optimum condition, the sensor detected P(III) up to 1 mM with a detection limit of 1.46 μM. The phosphite analysis with recovery rates ranged from 74.2 ± 2.6% to 107.5 ± 0.5% in real water and biological samples, suggesting the potential applicability of this sensor.