The substrate-generation/tip-collection mode of scanning electrochemical microscopy was used to detect hydrogen peroxide formed as an intermediate during oxygen reduction at various electrodes. The experiment is conceptually similar to rotating ring-disk experiments but does not require the production of a ring-disk assembly for the specific electrode material in question. In order to limit the extension of the diffusion layer above the sample, the sample electrode potential is pulsed while the Pt ultramicroelectrode probe (UME) is held at a constant potential for oxidative amperometric detection of hydrogen peroxide. The signal at UME is influenced by the sample region within the diffusion length of hydrogen peroxide during the pulse of 2.5 s. The method is tested with three model electrodes showing different behavior with respect to the oxygen reduction reaction (ORR) in acidic solution. Simple analytical models were used to extract effective rate constants for the most important reaction paths of ORR at gold and palladium-cobalt samples from the chronoamperometric response of the UME to a reduction pulse at the sample electrode.

A soft stylus microelectrode probe has been developed to carry out scanning electrochemical microscopy (SECM) of rough, tilted, and large substrates in contact mode. It is fabricated by first ablating a microchannel in a polyethylene terephthalate thin film and filling it with a conductive carbon ink. After curing the carbon track and lamination with a polymer film, the V-shaped stylus was cut thereby forming a probe, with the cross section of the carbon track at the tip being exposed either by UVphotoablation machining or by blade cutting followed by polishing to produce a crescent moon-shaped carbon microelectrode. The probe properties have been assessed by cyclic voltammetry, approach curves, and line scans over electrochemically active and inactive substrates of different roughness. The influence of probe bending on contact mode imaging was then characterized using simple patterns. Boundary element method simulations were employed to rationalize the distance-dependent electrochemical response of the soft stylus probes.Scanning electrochemical microscopy (SECM) is a scanning probe technique that provides spatially resolved detection of electrochemical and chemical surface reactivity.1,2 Typically, the probe is an amperometric disk-shaped ultramicroelectrode (UME) enclosed in an insulating sheath, e.g. glass. Due to the hemispherical diffusion occurring at the microdisc electrode, a steadystate current can be monitored as a function of the horizontal (x, y) and vertical (z) probe positions. SECM has found many applications ranging from surface reactivity imaging 3-13 to the study of the kinetics of homogeneous and heterogeneous reactions at solid/liquid, gas/liquid, and liquid/liquid interfaces. 14-20 Because the response of a SECM probe can be described by continuum models of mass transport coupled to electrochemical kinetics, quantitative kinetic information can be extracted by comparing experimental and simulated curves in many practically relevant situations. 21-25The response of a SECM probe depends on the surface reactivity of the sample and the distance d between the surface and the active area of the probe electrode. In order to obtain a reactivity image that is not influenced by topography, d must be kept constant. In conventional SECM operations, this is achieved by working on samples with a roughness that is considerably smaller than the radius r T of the UME and by leveling the plane of the sample surface with respect to the x,y-scanning plane of the positioning system. However, this approach becomes inappropriate on rough surfaces of large aspect ratios or when large scan areas (in the mm 2 range) have to be investigated so that leveling becomes a major obstacle. Similar limitations apply when imaging curved samples. This drawback has been recognized for a long time. In the case of a small r T , a constant d can be maintained by combining

Patterned cell cultures obtained by microcontact printing have been modified in situ by a microelectrochemical technique. It relies on lifting cell-repellent properties of oligo(ethylene glycol)-terminated self-assembled monolayers (SAMs) by Br2, which is produced locally by an ultramicroelectrode of a scanning electrochemical microscope (SECM). After Br2 treatment the SAM shows increased permeability and terminal hydrophobicity as characterized by SECM approach curves and contact angle measurements, respectively. Polarization-modulation Fourier transform infrared reflection-absorption spectroscopic (PM FTIRRAS) studies on macroscopic samples show that the Br2 treatment removes the oligo(ethelyene glycol) part of the monolayer within a second time scale while the alkyl part of the SAM degrades with a much slower rate. The lateral extension of the modification can be limited because heterogeneous electron transfer from the gold support destroys part of the electrogenerated Br2 once the monolayer is locally damaged in a SECM feedback configuration. This effect has been reproduced and analyzed by exposing SAM-modified samples to Br2 in the galvanic cell Au|SAM|5 microM Br2 + 0.1 M Na2SO4||10 microM KBr + 0.1 M Na2SO4|Au followed by an PM FTIRRAS characterization of the changes in the monolayer system.

The BEM algorithm developed earlier for steady-state experiments in the scanning electrochemical microscopy (SECM) feedback mode has been expanded to allow for the treatment of more than one independently diffusing species. This allows the treatment of substrate-generation/tip-collection SECM experiments. The simulations revealed the interrelation of sample layout, local kinetics, imaging conditions, and the quality of the obtained SECM images. Resolution in the SECM SG/TC images has been evaluated, and it depends on several factors. For most practical situations, the resolution is limited by the diffusion profiles of the sample. When a dissolved compound is converted at the sample (e.g., oxygen reduction or enzymatic reaction at the sample), the working distance should be significantly larger than in SECM feedback experiments (ca. 3 r(T) for RG = 5) in order to avoid diffusional shielding of the active regions on the sample by the UME body. The resolution ability also depends on the kinetics of the active regions. The best resolution can be expected if all the active regions cause the same flux. In one simulated example, which might mimic a possible scenario of a low-density protein array, considerable compromises in the resolving power, were noted when the flux from two neighboring spots differs by more than a factor of 2.