| The work presented here demonstrates the fabrication, characterization, and scanning electrochemical microscopy (SECM) applications of two different types of ultramicroelectrodes (UMEs), or tips, namely amperometric poly(oxyphenylene)-coated carbon fiber (POCF) tips and potentiometric iridium oxide (IrOx)-pH sensitive tips, These UMEs were prepared using high yield, simple, reliable, relatively inexpensive, and reproducible methods.; POCF-UMEs were constructed by sealing a single 7-mum-diameter carbon fiber into pulled-glass capillaries followed by electrochemical etching of the protruding fiber. The protruding fiber was then insulated (except at the tip) with a layer of poly(oxyphenylene) formed by electropolymerization.; The voltammetric characteristics and the integrity of the insulation were evaluated. Optical micrographs of the coated electrodes show that the thickness of the coating is in the range 1--3 mum depending on the deposition potential and deposition time.; These POCF UMEs were used as SECM tips for imaging several substrates (e.g., Au, IDA, Au/Kel-F, Al/SiCp metal matrix composite) with a reasonable resolution.; IrOx-pH UMEs were prepared by electrochemical oxidation of Ir microparticles via cyclic voltammetry in 0.5 M H2SO4. The IrOx-pH electrodes prepared this way are chemically stable, uniform, and exhibit rapid, and linear response to H+ over the pH range 2--12 with a slightly super-Nernstian slope (64 +/- 7 mV/pH) at room temperature. The pH response is reproducible and remains stable over a period of two months. The application of these electrodes in SECM imaging is demonstrated in imaging the pH changes produced due to electrochemical reactions at the surface of different substrates (e.g., Au, Au/Kel-F, Ni, Fe) in aqueous solutions with different pH values at several substrate potentials.; Both types of UMEs were integrated in SECM measurements to image cast iron samples. In this work, POCF tips and the reaction-rate mode of the SECM were used to study the variation of the rate of oxidation of ferrocenyl methanol (ferrocenemethanol) as well as the rate of O2 reduction on graphite nodules and ferrous matrix in the cast iron samples thus reflecting the variation in the reactivity on the cast iron surface. The results showed that the O 2 reduction occurs preferentially at the graphite sites. The effect of substrate potential on the O2 reduction was studied at several negative potentials down to the open circuit potential (OCP). Finally, IrOx-pH UMEs were used to image expected local pH changes on the cast iron surface due to O2 reduction. |
