Other Methods of Electrode Modification

In addition to studies on adsorbed monomers, other methods of electrode modifications for the detection of thiols have appeared in the literature. Zagal and 

Ballarin et al. used sonogel-carbon composite electrodes containing CoPc for the oxidation of cysteine The catalytic process is mediated by the generation of the Co Pc species. Oxidation of sulfide ion was catalyzed by microcrystals of CoPc immobilized at a surface of paraffin impregnated graphite electrode. Filanovsky reported the use of a carbon electrode prepared by hand pressing carbon powder mixed with CoPc and a polymer solution (poly(methyl methacrylate)) in dichloromethane, followed by evaporating the dichloro-methane, for the analysis of mercaptoethanol in petroleum and cysteine in human urine with detection limits of 8 x 10 M for the former and 2 x 10 M for the latter . 

MPc modified CPFs have been employed by several researchers for the analysis of thiols. Carbon paste electrodes incorporating OMo (OH)Pc catalyzed the oxidation of cysteine with a considerable reduction in overpotential Table 7.2. The oxidation of cysteine was mediated by Mo Pc species. CoPc modified CPMF was used as a sensor for analysis of cysteine and glutathionine in urine using electrophoresis , with detection limits of 3.1 x 10 and 3.0x10 M, respectively. CoPc modified CPE was employed for the potentiometric oxidation of 2-mercaptobenzimidazole and 2-mercaptobenzothiazole . Low peak potentials of 0.45 V for the former and 0.27 V for the latter were obtained . A detection limit of 5 x 10 M was obtained. CPFs modified with CoPc showed catalytic activity for the determination of thiocarbonyl compounds (thiourea, thioac-etamide, thiobenzamide, and dithiooxamide) and the oxidation of thioglyconic acid . The latter occurred via a two step process, which leads to the formation of the dimer of the thiol . A carbon paste electrode constructed from NiTSPc immobilized on silica gel modified with 1102 catalyzed the oxidation of cysteine . The immobilization caused the increase in the monomeric form of the catalysts. 

Since pyrrole and its derivatives form very stable conducting surfaces, Za-gal, Bedioui, and coworkers used ultramicro-carbon-fiber electrode modified with polypyrrole-dopped cobalt-tetrasulfonated phthalocyanine for the electrocatalytic oxidation of 2-ME The resulting electrode was stable and showed a detection limit of 8 10 M. Pyrolle substituted phthalocyanines have been used for the analysis of cysteine. Electrocatalytic behavior of MPc complexes has also been examined under homogeneous conditions. Water soluble OMo (OH)TSPc catalyzed the oxidation of cysteine under homogeneous conditions.  

Sensors based on CoTM-3,4pyPz or CoTAPc coated HOPG electrode and further coated with Nafion ion exchange membranes or FePc modified carbon paste electrode were employed for potentiometric analysis of sulfide ion and 2-mercaptoethanol and 2-pyridinethiol. The sensors showed a faster response time than commercial potentiometric sulfide indicators . The overpotential of the analytes were lowered by more than 400 mV in the case of FePc modified carbon paste electrodes .  

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