Effects of matrix variations on pH and Cu2+ sensing properties of sol–gel entrapped Eriochrome Cyanine R

Abstract

Eriochrome Cyanine R has been entrapped in a range of acid- and base-catalysed silica sol–gel matrices using various values of the water ∶ alkoxide ratio (R) and total precursor volume and incorporating varying amounts of the organic modifier methyltriethoxysilane (MTES). Diffuse reflectance spectra of thin films of polyisobutylene-bound finely-ground samples of these carefully aged materials have been measured as a function of time in solutions of various pH and Cu²⁺ concentration. Effects of pore size distribution and organic modification on these sensing properties have been interpreted in terms of data from nitrogen adsorption porosimetry and EXAFS. Optimum sensitivity to low Cu²⁺concentrations is achieved for gels incorporating 10% MTES, and the use of the polymer-bound ground samples increases response speed by a factor of 300 relative to bulk monolith sol–gel samples with elimination of cracking effects.