Enhancement of Anodic Response for DMSO at Ruthenium Oxide Film Electrodes as a Result of Doping with Iron(III)

The oxidation of dimethyl sulfoxide (DMSO) to dimethyl sulfone (DMSO2) is representative of numerous anodic oxygen-transfer reactions of organosulfur compounds that suffer from slow kinetics at noble metal electrodes. Anodic voltammetric data for DMSO are examined at various RuO2-film electrodes prepared by thermal deposition on titanium substrates. The response for DMSO is slightly larger at RuO2 films prepared in a flame as compared with films prepared in a furnace; however, temperature is more easily controlled in the furnace. Doping of the RuO2 films with Fe(III) further improves the sensitivity of anodic response for DMSO. Optimal response is obtained at an Fe(III)-doped RuO2-film electrode prepared using a deposition solution of 50 mM RuCl3 and 10 mM FeCl3 in a 1:1 mixture of isopropanol and 12 M HCl at an annealing temperature of 450degreesC. The Levich plot (i vs. omega(1/2)) and Koutecky-Levich plot (1/i vs. 1/omega(1/2)) of amperometric data for the oxidation of DMSO at an Fe(Ill)-doped RuO2-film electrode configured as a rotated disk are consistent with an anodic response controlled by mass-transport processes at low rotational velocities. Flow injection data demonstrate that Fe(III)-doped RuO2-film electrodes exhibit detection capability for methionine and cysteine in addition to DMSO. Detection limits for 100-muL injections of the three compounds are ca. 3.2 x 10(-4) mM, i.e., ca. 32 pmol.