Test Strips for Lead(II) Based on a Unique Color Change of PVC-film Containing O-Donor Macrocycles and an Anionic Dye

Abstract

Glassy test strips partially coated with PVC-film including O-donor macrocyclic receptors (L), tetrabromophenolphthalein ethyl ester (TBPE^-), and a plasticizer sensed Pb²⁺ in aqueous solutions by a unique color change. Yellow films successively changed color to green, dark-blue and purple with increases of the Pb²⁺ concentration. In contrast with the ordinary “optode”, a characteristic absorption band at 525 nm was newly appeared independently of the protonation and deprotonation of HTBPE (yellow to blue). The unique color change occurred only when asymmetric receptors with respect to the basal plane were coupled with Pb²⁺. This optical-structural correlation is likely to be induced by the H aggregate of two sets of TBPE^- in the 1:2 ion-pair, [Pb-L]²⁺·(TBPE^-)₂. The color change, based on metachromasy, was exclusive for Pb²⁺ among common metal cations (Ca²⁺, Al³⁺, Cd²⁺, Zn²⁺, Fe³⁺, Co²⁺, Hg²⁺) and anions (Cl^-, SO₄^2-, PO₄^3-, S₂O₃^2-).