Electrocatalysis with a Dirhodium-Substituted Polyoxometalate Anchored in a Xerogel-Based Composite: Application to the Oxidation of Methionine and Cystine at Physiological pH

Replacing a tungsten oxide group of phosphotungstic acid with a Rh(II) acetate dimer yielded a species, Rh2POM, that catalyzed the electrochemical oxidation of L-methionine (L-Met), L-cystine, and As(III). The electrocatalytic activity of Rh2POM was greater than that of the simple Rh(II) acetate dimer in homogeneous solution. A conducting carbon-composite electrode (CCE) comprising Rh2POM and carbon powder in a silica matrix prepared by sol-gel chemistry was catalytically active over the pH range 2–10. The CCEs produced voltammetric responses toward L-Met and L-cystine that were pH independent over the above range and were stable for at least 4 months under dry storage. If passivated, the catalytic activity was reproducibly generated by polishing. For example, a relative standard deviation of 2.9 % (5 points) was obtained when the surface was polished between cyclic voltammetric experiments on 5.0 mM L-Met at pH 7.4. A least squares fit of flow injection amperometric data obtained over the range 5–156 μM L-Met (7 points) yielded the following: slope, 0.85 μA mM−1; intercept, 0.05 μA; and r / 0.995. The detection limit was 2.3 μM.