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) acet ate dimer yielded a species, Rh2POM, that catalyzed the electrochemical oxi dation of L-methionine (L-Met), L-cystine, and As(III). The electrocatalyti c activity of Rh2POM was greater than that of the simple Rh(II) acetate dim er in homogeneous solution. A conducting carbon-composite electrode (CCE) c omprising Rh2POM and carbon powder in a silica matrix prepared by sol-gel c hemistry 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 stora ge. If passivated, the catalytic activity was reproducibly generated by pol ishing. For example, a relative standard deviation of 2.9 % (5 points) was obtained when the surface was polished between cyclic voltammetric experime nts on 5.0 mM L-Met at pH 7.4. A least squares fit of flow injection ampero metric data obtained over the range 5-156 mu M L-Met (7 points) yielded the following: slope, 0.85 mu A mM(-l); intercept, 0.05 mu A; and r = 0.995. T he detection limit was 2.3 mu M.