Carbon electrodes modified with ruthenium metallodendrimer multilayers for the mediated oxidation of methionine and insulin at physiological pH

A pentaerythritol-based metallodendrimer with ruthenium(II) terpyridine units, Ru(II)Den, catalyzed the oxidation of L-methionine and insulin at pH 7.0. The Ru(II)Den 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-II-Den up to n = 12. Cyclic voltammetry was used to demonstrate the mediation of L-methionine oxidation by a Ru(II)Den-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 nM0.4 muM; sensitivity, 225 nA muM(-1); detection limit (k = 3 criterion), 2 nM. Of particular importance was that the sensitivity was proportional to the number of Ru(II)Den layers.