An anion sensor based on the displacement of 2,6-dichlorophenol-indo-o-cresol sodium salt from a water-soluble tetrasulfonated calix[4]arene

A colorimetric displacement assay system for F− and H2PO4− anions based on a 2,6-dichlorophenol-indo-o-cresol sodium salt:calix[4]arene-p-sulfonate complex was studied in 1:1 DMSO:water solution. When calix[4]arene-p-sulfonate formed the complex with the blue 2,6-dichlorophenol-indo-o-cresol sodium salt dye, the absorption peak of the dye at 625 nm disappeared and the DMSO:water solution of the complex was colourless. However, the initial blue colour of the dye returned with the addition of both F- and H2PO4− anions which bound to the calix[4]arene-p-sulfonate and, in doing so, displaced the 2,6-dichlorophenol-indo-o-cresol sodium salt from the 2,6-dichlorophenol-indo-o-cresol:calix[4]arene-p-sulfonate complex. The findings imply that this displacement assay could be used to develop potential optical signalling systems for the detection of anions such as F− and H2PO4−.