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Dive into the research topics where Gunther Wittstock is active.

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Featured researches published by Gunther Wittstock.


Journal of Electroanalytical Chemistry | 1995

Mediator-modified electrodes for electrocatalytic oxidation of NADH

Bernd Gründig; Gunther Wittstock; Ulrich Rüdel; Beate Strehlitz

Abstract The large class of NAD(P) + -dependent dehydrogenases can be utilized for the construction of amperometric enzyme sensors if a suitable redox mediator catalyzes the electron transfer from NAD(P)H to the electrode. Among several compounds tested, the quinonoid cationic redox dyes N -methyl-phenazinium, 1-methoxy- N -methyl-Phenazinium and Medola Blue exhibit the best performance for NADH oxidation. These ionic redox dyes can be immobilized as tetrarhodanato-diammine-chromates (Reineckates) in graphite-epoxy composite electrodes. The slow release of the cations has been monitored using scanning electrochemical microscopy (SECM). The results support strongly a mechanism of mediated electron transfer via dissolved mediator species. The kinetic properties of the composite electrodes regarding heterogeneous and homogeneous rate constants were studied by cyclic voltammetry and chronoamperometry. The remarkable stability of the composite electrode allowing more than 9500 measurements results from the large amount of incorporated Reineckates. The suitability of the composite electrodes for detection of NADH at concentrations between 0.5 μM and 3 mM at potentials close to 0 mV (vs. Ag/vbAgCl) has been demonstrated. Based on the composite electrode as transducer, an amperometric enzyme sensor for ethanol was constructed using a novel method for coimmobilizing dehydrogenase and coenzyme in a polyurethane hydrogel. The sensor shows responses to ethanol in the concentration range 0.03–9.5 mM within 1–2 min (90%) even in the absence of dissolved coenzyme.


Analytical Chemistry | 2008

Detection of hydrogen peroxide produced during electrochemical oxygen reduction using scanning electrochemical microscopy.

Yan Shen; Markus Träuble; Gunther Wittstock

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.


Electrochimica Acta | 1997

Imaging of microstructured biochemically active surfaces by means of scanning electrochemical microscopy

Christine Kranz; Gunther Wittstock; Heidi Wohlschläger; Wolfgang Schuhmann

Abstract Scanning electrochemical microscopy (SECM) has been used to create and to image thin film structures of functionalized polypyrrole, to which glucose oxidase has been covalently immobilized. In addition, the SECM was applied to optimize the quality of all steps of this microfabrication process. For this purpose, imaging of the obtained structures was performed in the conventional feedback mode of the SECM with ferrocene derivatives or [Os (bpy) 2 fpy] (bpy = 2,2′-bipyridine, fpy = 4-formylpyridine). The same working mode is also used for the enzyme-modified microstructures exploiting the ability of glucose oxidase to accept the oxidized forms of these inorganic transition metal complexes as artificial cofactors. Successive images in the absence and presence of the enzymess substrate glucose added to the mediator solution allowed us to distinguish between the mediator regeneration caused by a heterogeneous electrontransfer reaction at the underlying gold support and the enzyme-mediated feedback. Prospects for the development of miniaturized biosensors are discussed.


Analytical Chemistry | 2011

Microfluidic Push-Pull Probe for Scanning Electrochemical Microscopy

Dmitry Momotenko; Fernando Cortés-Salazar; Andreas Lesch; Gunther Wittstock; Hubert H. Girault

This paper presents a microfluidic push-pull probe for scanning electrochemical microscopy (SECM) consisting of a working microelectrode, an integrated counter/reference electrode and two microchannels for pushing and pulling an electrolyte solution to and away from a substrate. With such a configuration, a droplet of a permanently renewed redox mediator solution is maintained just at the probe tip to carry out SECM measurements on initially dry substrates or in microenvironments. For SECM imaging purposes, the probe fabricated in a soft polymer material is used in a contact regime. SECM images of various gold-on-glass samples demonstrate the proof-of-concept of a push-pull probe for local surface activity characterization with high spatial resolution even on vertically oriented substrates. Finite element computations were performed to guide the improvement of the probe sensitivity.


Analytical Chemistry | 2011

Seeing Big with Scanning Electrochemical Microscopy

Fernando Cortés-Salazar; Dmitry Momotenko; Hubert H. Girault; Andreas Lesch; Gunther Wittstock

Specialized microelectrode probes fabricated in a soft polymer film now make it possible to use scanning electrochemical microscopy to image the reactivity of large, corrugated, tilted, and dry surfaces. (To listen to a podcast about this Feature, please go to the Analytical Chemistry multimedia page at pubs.acs.org/page/ancham/audio/index.html.)


Zeitschrift für Physikalische Chemie | 2008

Investigation of Localized Catalytic and Electrocatalytic Processes and Corrosion Reactions with Scanning Electrochemical Microscopy (SECM)

Sascha E. Pust; Wiebke Maier; Gunther Wittstock

Abstract Scanning electrochemical microscopy (SECM) has developed into a very versatile tool for the investigation of solid-liquid, liquid-liquid and liquid-gas interfaces. The arrangement of an ultramicroelectrode (UME) in close proximity to the interface under study allows the application of a large variety of different experimental schemes. The most important have been named feedback mode, generation-collection mode, redox competition mode and direct mode. Quantitative descriptions are available for the UME signal, depending on different sample properties and experimental variables. Therefore, SECM has been established as an indispensible tool in many areas of fundamental electrochemical research. Currently, it also spreads as an important new method to solve more applied problems, in which inhomogeneous current distributions are typically observed on different length scales. Prominent examples include devices for electrochemical energy conversion such as fuel cells and batteries as well as localized corrosion phenomena. However, the direct local investigation of such systems is often impossible. Instead, suitable reaction schemes, sample environments, model samples and even new operation modes have to be introduced in order to obtain results that are relevant to the practical application. This review outlines and compares the theoretical basis of the different SECM working modes and reviews the application in the area of electrochemical energy conversion and localized corrosion with a special emphasis on the problems encountered when working with practical samples.


Electrochimica Acta | 2003

Combination of an electrochemical tunneling microscope (ECSTM) and a scanning electrochemical microscope (SECM): application for tip-induced modification of self-assembled monolayers

Thomas H. Treutler; Gunther Wittstock

Abstract A combined instrument for electrochemical scanning tunneling microscopy (ECSTM) and scanning electrochemical microscopy (SECM) was constructed. It brings a combined ECSTM–SECM probe into tunneling contact with a sample and retracts it by defined distances to perform electrochemical experiments outside the tunneling distance but within a separation of 5–30 nm from the sample. The capabilities have been illustrated by the investigation of self-assembled monolayers (SAM) of dodecanethiolate on flame-annealed gold. By establishing a tunneling contact between the tip and the SAM-covered gold surface the structure of the SAM is disturbed. This is evident from current–distance curves in the SECM feedback mode. Depending on the time allowed for relaxation, the signal changes from positive feedback to negative feedback as it would be expected above undisturbed SAM. Spatially correlated images of topography and reactivity could be recorded by acquiring an ECSTM line scan first and than using the topographic information to guide the probe parallel to the surface in a much larger distance after changing the probe potential to cause the diffusion-controlled oxidation of a mediator. This mode was used to image rough gold electrodes where an almost complete suppression of the topographic information in the SECM data could be achieved. A sample with an inhomogeneous distribution of electrochemical reactivity was created by tip-induced removal of two 100×100 nm 2 large regions of a dodecanethiolate SAM on a rough gold surface. The two areas with a midpoint–midpoint distance of 850 nm could clearly be resolved by SECM imaging in the feedback mode.


Electrochimica Acta | 2001

Patterns of functional proteins formed by local electrochemical desorption of self-assembled monolayers

Thomas Wilhelm; Gunther Wittstock

Abstract Patterned self-assembled monolayers (SAMs) were formed using the scanning electrochemical microscope (SECM). The procedures is based on the local electrochemical desorption of an alkanethiolate monolayer by applying a 5 kHz square-wave voltage of 2 V (peak-to-peak) to a two-electrode configuration consisting of an ultramicroelectrode (UME) of 10 μm diameter placed about 5 μm above a macroscopic SAM-covered gold electrode. Desorption occurs on well-defined regions with a diameter of (12.8±2.8) μm. These regions of bare gold are able to chemisorb a ω-functionalized thiol or disulfide such as cystamine to form patterns of amino-terminated surfaces. Functional proteins can be coupled to the amino groups present at the modified regions of the monolayer. This approach was demonstrated by imaging the activity of horseradish peroxidase bound to the patterned SAMs in the generation–collection mode of the SECM. A considerable improvement of the procedure could be achieved by performing the desorption in a solution containing a millimolar concentration of the ω-functionalized thiol/disulfide ensuring effective refilling of the monolayer by the desired molecules and hence high concentration of the immobilized proteins. The method is discussed with respect to prospective application in the field of chip-based bioanalytical assays.


Analytical Chemistry | 2012

Electrochemical Push–Pull Scanner with Mass Spectrometry Detection

Dmitry Momotenko; Liang Qiao; Fernando Cortés-Salazar; Andreas Lesch; Gunther Wittstock; Hubert H. Girault

This manuscript presents a push-pull electrochemical scanner able to image reactivity of initially dry surfaces by scanning electrochemical microscopy (SECM) and to probe molecules present or generated at the surface by mass spectrometry (MS). The proof-of-concept is demonstrated by coupling SECM with matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) for imaging latent human fingerprints, which had been in contact with picric acid used here as a model explosive. The push-pull electrochemical scanner has also been coupled with electrospray ionization mass spectrometry (ESI-MS) to assay the activity of surface spotted enzymes. These experimental studies are complemented by 3D finite element simulations solving Navier-Stokes and diffusion-convection differential equations to optimize the coupling between SECM imaging and mass spectrometry detection.


Electroanalysis | 2000

Electrochemical Immunoassay with Microscopic Immunomagnetic Bead Domains and Scanning Electrochemical Microscopy

C. Ajith Wijayawardhana; Gunther Wittstock; H. Brian Halsall; William R. Heineman

The formation of mound-like microscopic domains of biochemically active paramagnetic beads reported earlier is extended towards a miniaturized sandwich enzyme immunoassay for the model analyte mouse-IgG. After mouse-IgG is captured by the primary anti-mouse-IgG antibody (Ab) coated magnetic beads of 2.8 µm in diameter, a second anti-mouse-IgG Ab conjugated to an enzyme-label is bound to the analyte. The enzyme label alkaline phosphatase (ALP) is used to catalyze the hydrolysis of 4-aminophenyl phosphate (PAPP) to 4-aminophenol (PAP). The enzymatic activity, which depends on the concentration of captured mouse-IgG, is recorded by scanning electrochemical microscopy (SECM) where a 10 µm Pt electrode scanned over the bead domains detects PAP by oxidation. The anodic current correlates with the concentration of mouse-IgG.

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Hubert H. Girault

École Polytechnique Fédérale de Lausanne

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Julia Witt

University of Oldenburg

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Andreas Lesch

École Polytechnique Fédérale de Lausanne

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Chuan Zhao

University of New South Wales

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