Application of scanning electrochemical microscopy to biological samples

The scanning electrochemical microscope can be used in the feedback mode in two-dimensional scans over biological substrates to obtain topographic information at the micrometer level. In this mode, the effect of distance between a substrate (either conductive or insulating) and a scanning ultramicroelectrode tip on the electrolytic current flowing at the tip is recorded as a function of the tip x-y position. Scans of the upper surface of a grass leaf and the lower surface of a Ligustrum sinensis leaf (which show open stomata structures) immersed in aqueous solution are shown. Scans of the upper surface of an elodea leaf in the dark and under irradiation, where the tip reaction is the reduction of oxygen produced by photosynthesis, demonstrate the possibility of obtaining information about the distribution of reaction sites on the substrate surface

Monitoring the Ejection and Incorporation of Ferricyanide Fe(CN)_6^(3-) and Ferrocyanide Fe(CN)_6^(4-) Counterions at Protonated Poly(4-vinylpyridine) Coatings on Electrodes with the Scanning Electrochemical Microscope

The precise positioning of microtlp electrodes close to the surface of substrate electrodes, as practiced in scanning electrochemlcal microscopy, was exploited to monitor the concentrations of Fe(CN)_6^(-3) and Fe(CN)_6^(-4) anions at the surfaces of protonated poly(4-vinylpyridine) coatings on glassy-carbon electrodes. Positive feedback, which enhanced the magnitude of currents at the monitoring tip electrode, gave way to negative feedback when reactant concentrations were increased to the point that electron propagation through the polyectrolyte coatings became the curent-limiting step. The election of counterions when cathodic currents were passed through coatings which were saturated with Fe(CN)_6^(3-) was readily detected, especially when current steps were applied to the coated substrate electrode. Delayed arrival of counterions at the monitoring tip could be associated with the time required for the ions to traverse the coatings before they were ejected. Reincorporation of multiply-charged counterions inmediately following their ejection appeared to be favored over the incorporation of singly charged anions present at much higher concentrations

Monitoring the Ejection and Incorporation of Ferricyanide Fe(CN)_6^(3-) and Ferrocyanide Fe(CN)_6^(4-) Counterions at Protonated Poly(4-vinylpyridine) Coatings on Electrodes with the Scanning Electrochemical Microscope

The precise positioning of microtlp electrodes close to the surface of substrate electrodes, as practiced in scanning electrochemlcal microscopy, was exploited to monitor the concentrations of Fe(CN)_6^(-3) and Fe(CN)_6^(-4) anions at the surfaces of protonated poly(4-vinylpyridine) coatings on glassy-carbon electrodes. Positive feedback, which enhanced the magnitude of currents at the monitoring tip electrode, gave way to negative feedback when reactant concentrations were increased to the point that electron propagation through the polyectrolyte coatings became the curent-limiting step. The election of counterions when cathodic currents were passed through coatings which were saturated with Fe(CN)_6^(3-) was readily detected, especially when current steps were applied to the coated substrate electrode. Delayed arrival of counterions at the monitoring tip could be associated with the time required for the ions to traverse the coatings before they were ejected. Reincorporation of multiply-charged counterions inmediately following their ejection appeared to be favored over the incorporation of singly charged anions present at much higher concentrations

Electrocatalytic dioxygen reduction on underpotentially deposited Pb on Au(111) studied by an active site blocking strategy

Abstract Electrochemical measurements and in situ scanning tunneling microscopy (STM) are performed to establish a structure-reactivity correlation for peroxide or dioxygen reduction on underpotentially deposited (upd) Pb on Au(111) in 0.1 M HClO 4 . While STM imaging reveals the presence of Pb islands with height of 0.25 ± 0.05 nm at the potential of highest catalytic activity toward the O 2 and H 2 O 2 reduction, the full Pb monolayer formed at −0.03 V vs. NHE shows about half the activity of the Pb islands. Ethanethiol (EtSH) significantly but not completely inhibits the H 2 O 2 reduction activity of the Pb island structure. STM shows that EtSH introduction leads to the formation of a 0.13-nm-high terrace along the edges of the Pb islands, which is assigned to EtSH bound to the Au surface near the Pb islands with the alkyl chain oriented roughly perpendicular to the surface. These results show that edge sites around the Pb island are the active site of catalysis, though the sites atop the Pb islands may also take part in catalytic O 2 reduction by Pb upd on Au(111)