Carbon Electrodes Modified with Ruthenium Metallodendrimer Multilayers for the Mediated Oxidation of Methionine and Insulin at Physiological pH

Abstract

A pentaerythritol-based metallodendrimer with ruthenium(II) terpyridine units, Ru^IIDen, catalyzed the oxidation of l-methionine and insulin at pH 7.0. The Ru^IIDen was immobilized on a carbon surface through layer-by-layer electrostatic deposition; the negatively charged polymer, poly(styrene sulfonate), was its counterpart. These bilayers were assembled on a glassy carbon electrode that was first modified by deposition of a layer of the conjugate base of sulfanilic acid and then with quaternized poly(4-vinylpyridine). Reversible voltammetry for the Ru^II/III redox couple was observed, the current for which increased linearly with layer number, n, of Ru^IIDen up to n = 12. Cyclic voltammetry was used to demonstrate the mediation of l-methionine oxidation by a Ru^IIDen-containing multilayer assembly. Flow injection amperometric determination of insulin at pH 7.0 at this modified electrode yielded a calibration curve with the following characteristics: linear dynamic range, 6 nM−0.4 μM; sensitivity, 225 nA μM^-1; detection limit (k = 3 criterion), 2 nM. Of particular importance was that the sensitivity was proportional to the number of Ru^IIDen layers.