Colorimetric sensing of H2PO4− using a Pb2+ complex of a pyridylazo dye in aqueous solution

This study develops a sensitive and selective colorimetric probe for the detection of dihydrogen phosphate (H₂PO₄⁻) based on a lead ion (Pb²⁺) complex of a commercially available pyridylazo dye (PAPS) in aqueous media under physiological conditions. The complex was made by adding Pb²⁺ to PAPS in a 1:1 stoichiometric ratio (PAPS-Pb²⁺), which was accompanied by the remarkable solution change from yellow to purple. The colorimetric sensing of H₂PO₄⁻ over other anions with PAPS-Pb²⁺ occurred owing to the regeneration of original PAPS by the interaction of H₂PO₄⁻ with Pb²⁺ in a 1:2 molar ratio. Moreover, the H₂PO₄⁻ detection was less affected by the presence of other coexisting competitive anions. The colorimetric naked-eye behaviour of PAPS-Pb²⁺ rendered a useful and convenient probe for the detection of H₂PO₄⁻ via the indicator displacement assay.