LED-compatible copper(II)-selective optrode membrane based on lipophilized Zincon

Abstract

An optical single shot test for copper(II) ion is presented. The membrane consists of a polyester support and an active layer composed of hydrogel, a hydroxylic plasticizer, and Zincon. The latter has been made lipophilic by ion-pairing with tetraoctyl-ammonium bromide. The membrane responds to copper ions in giving a color change from pink to blue. Because the layer irreversibly extracts copper from the solution, a kinetic approach has been used for quantitation, in that the membrane has been exposed to the sample for 2 min and the initial slope of the response curve has been determined. A linear calibration graph has been obtained for the 1 to 100 μmol/l (63.5 μg/kg to 6.35 mg/kg) copper concentration range. The complexed form of the dye exhibits an absorption maximum of 620 nm which matches the emission band of the orange light-emitting diode (LED). Measurements have been performed at pH 6, because no complexation of zinc occurs in this range. The long term stability of the membrane exceeds 6 months when stored dry and in the dark. The strong binding of copper ions along with the fairly high selectivity of the membrane also suggests its use as a preconcentration phase.