Unexpected complexation behaviour of a sol-gel immobilised dye: the development of an optical copper(II) sensor

The metal-complexing dye Eriochrome Cyanine R (ECR) has been entrapped in sol-gel composites of good optical quality. The immobilisation of ECR had a dramatic effect on the complexation of metal ions. The strong colour change of aqueous ECR solutions, associated with the complexation of aluminium(III) ions, was not observed with ECR-containing composites. Instead a selectivity for copper(II) ions was found and a sensing system for these ions was developed. Thickness and pore structure were optimised to enhance sensitivity, lowering the detection limit for copper(II) ions to 0.1 ppm. The altered complexation behaviour is discussed in terms of effects of entrapment on entropy and enthalpy changes of complexation.