Direct colorimetric detection of copper(II) ions in sampling using diffusive gradients in thin-films

A novel binding phase was developed for use in diffusive gradients in thin-film (DGT) sampling for Cu(II) by employing methylthymol blue as a chelating and chromogenic agent. Methylthymol blue was adsorbed onto beads of Dowex 1 × 8 resin (200–400 mesh) and the resin beads were then immobilised onto an adhesive disc. Analysis of exposed binding discs by either UV–vis spectrophotometry or computer imaging densitometry provided robust quantification of adsorbed Cu(II) in the 0.2–1 μg cm−2 range, allowing detection at μg L−1 concentrations in the test solution (ca. 17 μg L−1 for a 24 h deployment), and in good agreement with established DGT theory. The method was shown to be a potential replacement for binding phases based on Chelex 100 where a colorimetric response to a specific metal is desired.