Anna L. Barker

Scanning electrochemical microscopy: beyond the solid/liquid interface

Recent progress has seen scanning electrochemical microscopy (SECM) emerge as a powerful technique for probing physicochemical interfacial processes, beyond the solid/liquid and electrode/electrolyte interfaces that were originally of primary interest. This review (with 92 references) assesses recent developments in SECM as a methodology for investigating liquid/liquid and liquid/gas interfaces, along with processes of biochemical and biophysical significance.

Scanning electrochemical microscopy as a local probe of oxygen permeability in cartilage

The use of scanning electrochemical microscopy, a high-resolution chemical imaging technique, to probe the distribution and mobility of solutes in articular cartilage is described. In this application, a mobile ultramicroelectrode is positioned close ( approximately 1 microm) to the cartilage sample surface, which has been equilibrated in a bathing solution containing the solute of interest. The solute is electrolyzed at a diffusion-limited rate, and the current response measured as the ultramicroelectrode is scanned across the sample surface. The topography of the samples was determined using Ru(CN)(6)(4-), a solute to which the cartilage matrix was impermeable. This revealed a number of pit-like depressions corresponding to the distribution of chondrocytes, which were also observed by atomic force and light microscopy. Subsequent imaging of the same area of the cartilage sample for the diffusion-limited reduction of oxygen indicated enhanced, but heterogeneous, permeability of oxygen across the cartilage surface. In particular, areas of high permeability were observed in the cellular and pericellular regions. This is the first time that inhomogeneities in the permeability of cartilage toward simple solutes, such as oxygen, have been observed on a micrometer scale.

Microelectrochemical studies of charge transfer at the interface between two immiscible electrolyte solutions: electron transfer from decamethyl ferrocene to aqueous oxidants☆

Experimental studies of electron transfer (ET) reactions at the interface between two immiscible electrolyte solutions (ITIES) were carried out with either Fe(CN) 6 3- , Ru(CN) 6 3- or IrCl 6 2- as oxidants in water and decamethyl ferrocene (DMFc) in 1,2-dichloroethane (DCE), using both scanning electrochemical microscopy (SECM) and microelectrochemical measurements at expanding droplets (MEMED). Either tetrabutylammonium cation (TBA + ) or ClO 4 - were employed in each phase to control the interfacial potential drop. SECM double potential step chronoamperometry was used to show that DMFc + , generated in the ET process, does not cross the interface in the potential range of interest. The ET rate constants were found to depend strongly on the interfacial potential drop, with an apparent measured ET coefficient of 0.38 when TBA + was used and the aqueous ionic strength was ca. 0.1 mol dm -3 . However, the potential dependence of the ET rate was complicated when ClO 4 - was used to change the interfacial potential drop. Although the rate constant increased when the driving force was increased by changing the aqueous oxidant, the rate constant decreased for a particular oxidant when the potential of the organic phase was made more negative relative to the aqueous phase, by increasing the concentration of ClO 4 - in the aqueous phase. A similar effect was observed with Fe(CN) 6 3- as the aqueous oxidant and DMFc as the electron donor in a nitrobenzene phase. In contrast, the rate constant for ET was found to be apparently insensitive to the ClO 4 - concentration in the aqueous phase (and hence potential drop) when the aqueous electrolyte concentration was increased to the salting out levels employed in earlier studies with externally-polarised ITIES. Possible reasons for the behaviour observed and the implications for further studies are discussed.

Observation and characterisation of the glycocalyx of viable human endothelial cells using confocal laser scanning microscopy

This paper describes the use of confocal laser scanning microscopy (CLSM) to observe and characterise the fully hydrated glycocalyx of human umbilical vein endothelial cells (HUVECs). Viable HUVECs in primary culture were studied at room temperature in HEPES-buffered, phenol red- and serum-free CS-C cell culture medium. A fluorescein isothiocyanate-linked wheat germ agglutinin (WGA-FITC) (2 μg ml−1, 30 min) was used to detect N-acetylneuraminic (sialic) acid, which is a significant component of the endothelial glycocalyx. Single confocal sections, less than 1.3 μm thick, were collected at intervals of 0.5 μm, scanning through the entire z-axis of a series of cells. Cell-surface associated staining was observed, which enabled the glycocalyx thickness to be deduced as 2.5 ± 0.5 μm. This dimension is significantly greater than that measured by electron microscopy, for glutaraldehyde-fixed cells (0.10 ± 0.04 μm). The specificity of WGA-FITC staining was demonstrated by treatments with several enzymes, known to degrade glycocalyx (heparatinase, chondroitinase, hyaluronidase and neuraminidase), of which neuraminidase (1 U ml−1, 30–60 min) was the most effective, removing up to 78 ± 2% of WGA-FITC binding to HUVECs. Cell viability was assessed simultaneously with ethidium homodimer-1 staining and confirmed by standard colorimetric 3-[4,5]dimethylthiazol-2,5diphenyltetrazolium bromide (MTT) test. CLSM thus provides a useful approach for in situ visualisation and characterisation of the endothelial glycocalyx in viable preparations, revealing a thickness that is an order of magnitude greater than found in ex situ measurements on fixed cells.

Measurement of the forward and back rate constants for electron transfer at the interface between two immiscible electrolyte solutions using scanning electrochemical microscopy (SECM): Theory and experiment

Abstract A new numerical model is developed for the scanning electrochemical microscopy (SECM) feedback mode for reversible electron transfer (ET) processes at the interface between two immiscible electrolyte solutions (ITIES). Results from this model were compared with data obtained using an earlier SECM feedback model in which the back reaction was not considered, to identify when the latter will be important. The dependence of the ET rate constant for the oxidation of 7,7,8,8-tetracyanoquinodimethane radical anion (TCNQ − ) in 1,2-dichloroethane (DCE) by aqueous ferricyanide on the interfacial potential drop ( Δ w o φ ) was studied using SECM. The Δ w o φ value was varied by changing the concentration of NaClO 4 in the aqueous phase while a fixed concentration of organic electrolyte, tetra- n -hexylammonium perchlorate, was used in the DCE phase. The results obtained were compared to earlier published studies on the forward reaction between TCNQ in DCE and aqueous ferrocyanide. Both the forward and back ET rate constants were found to depend strongly on the interfacial potential drop, with measured ET coefficients in the region of 0.5–0.6. A similar ET rate constant was observed at zero driving force for both the forward and back reactions. These experimental results suggest that the Butler–Volmer model applies to ET at the ITIES, when the driving force for the reaction is low, and under conditions of relatively high ionic strength in both the aqueous and organic phases.

Direct observation of oxygen depletion and product formation during photocatalysis at a TiO2 surface using scanning electrochemical microscopy

Using scanning electrochemical microscopy (SECM) we have measured quantitatively the depletion of O2 during the photodegradation of 4-chlorophenol at supported TiO2 films for the first time and established the connection between Cl-formation and O2 depletion rates.

Local amperometric detection of K+ in aqueous solution using scanning electrochemical microscopy ion-transfer voltammetry

Abstract A robust ultramicroelectrode (UME) probe is described for the amperometric determination of K+ ions in aqueous solution. The approach is based on ion-transfer voltammetry at the interface between two immiscible electrolyte solutions (ITIES), with a liquid ¦ liquid interface formed between a 1,2-dichloroethane solution, containing dibenzo-18-crown-6, in a glass capillary, which is placed in an aqueous K+ salt solution of interest (KCl in this study). The ITIES is externally polarised by applying a potential between silver electrodes in each phase. The UME probe has an inlaid disk geometry, making conventional ultramicroelectrode and scanning electrochemical microscopy (SECM) mass transport models applicable. Limiting current measurements of K+ in aqueous solution show a linear dependence on KCl concentration between 1 × 104 and 2.5 × 103 mol dm3. The K+ microprobe is shown to be particularly suitable for use in SECM, for both approach curve and imaging applications.

Scanning electrochemical microscopy investigation of the photodegradation kinetics of 4-chlorophenol sensitised by TiO2 films

Scanning electrochemical microscopy (SECM) has been used to investigate quantitatively the photodegradation kinetics of 4-chlorophenol (4-CP), in aerated and oxygenated aqueous solutions, at back-illuminated TiO2 films deposited from a suspension of Degussa P25 TiO2 powder at the end of an optical fibre. A potentiometric Ag/AgCl ultramicroelectrode (UME), positioned at a known distance above the TiO2 film, was used to monitor directly the Cl− production from the photodegradation of 4-CP. The effects of both light intensity and 4-CP concentration were investigated and the associated kinetics were determined. Amperometric measurements with a Pt UME demonstrated that O2 was depleted significantly adjacent to the TiO2 surface under these conditions. A theoretical model, which employs a Langmuir–Hinshelwood type kinetic equation, has been developed to interpret the kinetics of the photodegradation process and determine the associated quantum efficiency.

Effect of phospholipids on the kinetics of dioxygen transfer across a 1,2-dichloroethane/water interface

The effect of a series of 1,2-diacyl-sn-glycero-3-phosphocholines (C14 : 0, C16 : 0, C18 : 0) adsorbed at the interface between a buffered aqueous phase and 1,2-dichloroethane on the transfer of molecular oxygen between the two phases has been investigated by scanning electrochemical microscopy (SECM) in the induced transfer mode. A monolayer of C18 : 0 forms a barrier for dioxygen transfer from the organic to the aqueous phase. An increase in surface concentration of C18 : 0 results in a decrease in the interfacial rate constant characterising the transfer. This behaviour is analysed in terms of a simple energy barrier model. In contrast, C14 : 0 and C16 : 0 have only a small effect on dioxygen transfer on the SECM timescale, although surface tension measurements show similar adsorption behaviour and excess surface concentrations for all three phospholipids. Reasons for these observations are discussed.