Zhange Feng

Adsorption of Cd2+ on Carboxyl-Terminated Superparamagnetic Iron Oxide Nanoparticles

The affinity of Cd(2+) toward carboxyl-terminated species covalently bound to monodisperse superparamagnetic iron oxide nanoparticles, Fe(3)O(4)(np)-COOH, was investigated in situ in aqueous electrolytes using rotating disk electrode techniques. Strong evidence that the presence of dispersed Fe(3)O(4)(np)-COOH does not affect the diffusion limiting currents was obtained using negatively and positively charged redox active species in buffered aqueous media (pH = 7) devoid of Cd(2+). This finding made it possible to determine the concentration of unbound Cd(2+) in solutions containing dispersed Fe(3)O(4)(np)-COOH, 8 and 17 nm in diameter, directly from the Levich equation. The results obtained yielded Cd(2+) adsorption efficiencies of ~20 μg of Cd/mg of Fe(3)O(4)(np)-COOH, which are among the highest reported in the literature employing ex situ methods. Desorption of Cd(2+) from Fe(3)O(4)(np)-COOH, as monitored by the same forced convection method, could be accomplished by lowering the pH, a process found to be highly reversible.

Rotating Ring-Disk Electrode Method for the Detection of Solution Phase Superoxide as a Reaction Intermediate of Oxygen Reduction in Neutral Aqueous Solutions

A method is herein described that allows for solution phase superoxide generated via the reduction of dioxygen in neutral aqueous solutions at a rotating disk electrode to be oxidized at a concentric Au ring electrode bearing a covalently linked monolayer of 3-mercapto-1-propanol, a modified surface that blocks the oxidation of solution phase of hydrogen peroxide. Experiments were performed in which the potential of a glassy carbon disk electrode was linearly scanned in the oxygen reduction region and the ring voltage was poised at a value at which superoxide oxidation ensued yielded bell-shaped ring currents. This behavior is consistent with changes in the relative rates constant for the processes involved in the mechanism of oxygen reduction on this carbonaceous material induced by the applied potential which so far had remained undetected using other techniques.

Quantitative Aspects of Ohmic Microscopy

The potential difference between two microreference electrodes, Δφ(sol), immersed in an aqueous sulfuric acid solution was monitored while performing conventional cyclic voltammetric experiments with a Pt disk electrode embedded in an insulating surface in an axisymmetric cell configuration. The resulting Δφ(sol) vs E curves, where E is the potential applied to the Pt disk electrode were remarkably similar to the voltammograms regardless of the position of the microreference probes. Most importantly, the actual values of Δφ(sol) were in very good agreement with those predicted by the primary current distribution using Newmans formalism (Newman, J. J. Electrochem. Soc. 1966, 113, 501-502). These findings afford a solid basis for the development of ohmic microscopy as a quantitative tool for obtaining spatially resolved images of electrodes displaying nonhomogenous surfaces.

New advances in ohmic microscopy

A series of electrochemical measurements involving metal disks, ring-disks and facetted single crystals in aqueous 0.1 M H2SO4 have been performed to monitor differences in the electrostatic potential in the electrolyte, Δϕsol, induced by the passage of current, and thus assess the prospects of ohmic microscopy as an in situ imaging tool of electrodes in solution. Excellent quantitative agreement was found between the experimental values of Δϕsol and those predicted by theory for current pulses several milliamperes in magnitude and tens of microseconds in duration, applied to a Pt disk electrode embedded in a coplanar insulating surface, assuming a strict primary current distribution. Cyclic voltammetry measurements involving a gapless Pt–Ir ring|Au disk electrode yielded Δϕsol versus potential, E, curves, consistent with contributions derived from each of the two electrodes assuming a uniform current distribution. Also explored were extensions of ohmic microscopy to the study of facetted Pt single crystals using microreference electrodes housed in a double barrel capillary. Data collected in voltammetric experiments in which the tip of the capillary was placed directly above and at very close distance from one of the (111) facets recorded with the entire single crystal immersed in the electrolyte, yielded Δϕsol vs. E curves displaying pronounced features believed to be characteristic of that surface. Possible strategies toward improving the spatial resolution of this emerging technique are also discussed.