The oxygen evolution reaction (OER) is the limiting step in splitting water into its constituents, hydrogen and oxygen. Hence, research on potential OER catalysts has become the focus of many ...studies. In this work, we investigate capable OER catalysts but focus on catalyst stability, which is, especially in this case, at least equally as important as catalyst activity. We propose a specialized setup for monitoring the corrosion profiles of metal oxide catalysts during a stability testing protocol, which is specifically designed to standardize the investigation of OER catalysts by means of differentiating between catalyst corrosion and deactivation, oxygen evolution efficiency, and catalyst activity. For this purpose, we combined an electrochemical flow cell (EFC) with an oxygen sensor and an inductively coupled plasma–optical emission spectrometry (ICP-OES) system for the simultaneous investigation of catalyst deactivation, activity, and faradaic efficiency of catalysts. We tested various catalysts, with IrO2 and NiCoO2 used as benchmark materials in acidic and alkaline environment, respectively. The scalability of our setup will allow the user to investigate catalytic materials with supports of higher surface area than those which are typical for microelectrochemical flow cells (thus, under conditions more similar to those of commercial electrolyzers).
Photosystem 2 (PS2) that catalyses light driven water splitting in photosynthesis was ‘wired’ to electrode surfaces via osmium‐containing redox polymers based on poly(vinyl)imidazol. The redox ...polymer hydrogel worked as both immobilization matrix and electron acceptor for the enzyme. Upon illumination, the enzymatic reaction could be switched on and a catalytic current was observed at the electrode. The catalytic current is directly dependent on the intensity of light used for the excitation of PS2. A typical current density of 45 μA cm−2 at a light intensity of 2.65 mW cm−2 could be demonstrated with a significantly improved operational stability.
The purpose of this study was to develop a scanning electrochemical microscopy (SECM) and scanning electrogenerated chemiluminescence (SECL) setup to visualize the localized enzymatic activity using ...glucose oxidase as a model. Combination of SECM and electrogenerated chemiluminescence (ECL) was made possible by integrating a photomultiplier tube (PMT) within a SECM setup which is mounted on top of an inverted microscope. An enzyme−polymer spot formed on a glass slide and placed on top of the entrance window of the PMT was used as a model sample to evaluate the potential of the combined SECM/ECL setup. Hydrogen peroxide, which was locally generated by the glucose oxidase (GOx)-catalyzed reaction, reacted with oxidized luminol which was simultaneously electrochemically generated at the positioned SECM electrode tip. By using the phase-sensitive lock-in amplifier, the potential applied to the SECM tip was sinusoidally swept to invoke an associated oscillation of the ECL. Thus, sensitivity of SECL could be substantially enhanced. Images of the local immobilized enzyme activity obtained both by ECL and generator/collector (GC) mode of SECM were compared to elucidate the pathway in which the SECM and SECL signals are generated.
Sensible DNA: An electrochemical DNA assay based on specific Salmonella spp. capture probes and enzyme labeling with alkaline phosphatase was optimized by using a 48-electrode microarray and scanning ...electrochemical microscopy (SECM). SECM was further used to evaluate potential amplification strategies due to redox cycling.Due to insufficient detection limits and selectivity, electrochemical DNA sensors are not yet used as everyday tools in diagnostics. Here, we present an electrochemical DNA assay that is based on specific Salmonella spp. capture probes. Our optimization strategies and the specific features of related electrochemical DNA sensor arrays, which are comprised of a chip with 48 gold electrodes, are also described. A ssDNA monolayer is formed by chemisorption of the thiol-modified capture strand on the different gold electrodes of the array after spotting with a needle spotter. The assay parameters were optimized for the use of minimum amounts of sample and reagents and short assay times. Scanning electrochemical microscopy (SECM) has been used to visualize the local activity of an enzyme label used for amplified hybridization detection at high lateral resolution. The potential of SECM to further amplify the sensor signal by means of redox cycling is demonstrated by using single-stranded DNA capture probe modified gold microelectrodes as SECM tips. The detection limit of the proposed DNA sensor is shown to be in the femtomolar range without redox cycling amplification.
The direct electrochemical detection of synthetic DNA and native 16S rRNA fragments isolated from Escherichia coli is described. Oligonucleotides are detected via selective post-labeling of double ...stranded DNA and DNA–RNA duplexes with a biotinylated intercalator that enables high-specific binding of a streptavidin/alkaline phosphatase conjugate. The alkaline phosphatase catalyzes formation of p-aminophenol that is subsequently oxidized at the underlying gold electrode and hence enables the detection of complementary hybridization of the DNA capture strands due to the enzymatic signal amplification. The hybridization assay was performed on microarrays consisting of 32 individually addressable gold microelectrodes. Synthetic DNA strands with sequences representing six different pathogens which are important for the diagnosis of urinary tract infections could be detected at concentrations of 60nM. Native 16S rRNA isolated from the different pathogens could be detected at a concentration of 30fM. Optimization of the sensing surface is described and influences on the assay performance are discussed.
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•Label-free electrochemical detection of synthetic DNA and native RNA fragments.•Proflavine-based selective DNA intercalator carrying a biotin modification.•Array of 32 microelectrodes bonded to a dipstick suitable for automation.•Detection of synthetic DNA of pathogens associated with urinary tract infections.•Limit of detection of 60nM for DNA and 30fM for native RNA fragments.
A new approach is reported for analysis of ultrasmall volumes. It takes advantage of the versatile positioning of a dispenser to shoot ∼150-pL droplets of liquid onto a specific location of a ...substrate where analysis is performed rapidly, in a fraction of the time that it takes for the droplet to evaporate. In this report, the site where the liquid is dispensed carries out fast-scan cyclic voltammetry (FSCV), although the detection method does not need to be restricted to electrochemistry. The FSCV is performed at a microcavity having individually addressable gold electrodes, where one serves as working electrode and another as counter/pseudoreference electrode. Five or six droplets of 10 mM Ru(NH3)6Cl3 in 0.1 M KCl were dispensed and allowed to dry, followed by redissolution of the redox species and electrolyte with one or five droplets of water and immediate FSCV, demonstrating the ability to easily concentrate a sample and the reproducibility of redissolution, respectively. Because this approach does not integrate detection with microfluidics on the same chip, it simplifies fabrication of devices for analysis of ultrasmall volumes. It may be useful for single-step and multistep sample preparation, analyses, and bioassays in microarray formats if dispensing and changing of solutions are automated. However, care must be taken to avoid factors that affect the aim of the dispenser, such as drafts and clogging of the nozzle.
Actinomycin D or proflavine which are known to intercalate within the helix of double‐stranded DNA (dsDNA) are used as label‐free control to unequivocally prove complementary DNA hybridization by ...means of electrochemical impedance spectroscopy (EIS). Based on a carefully designed interface comprising a thiol‐tethered (20mer) oligonucleotide capture probe which forms a self‐assembled monolayer on a gold electrode together with a short chain hydroxyl‐terminated alkylthiol, formation of dsDNA can be monitored by an increase of the charge‐transfer resistance of a free‐diffusing negatively charged redox species (Fe(CN)63−/4−). The increase of the charge transfer resistance due to complementary hybridization was about 10 times from the unmodified Au surface to the dsDNA modified electrode. Specific interaction of intercalators with dsDNA leads to a decrease in charge transfer resistance due to the conformational changes in the dsDNA monolayer and partial charge compensation caused by the positively charged intercalators. No shift in the charge transfer resistance was observed in case of incubation of a ssDNA surface with intercalators or when hybridization was invoked using a noncomplementary DNA sequence. Thus, hybridization can be unambiguously detected using EIS by first recording the increase in charge‐transfer resistance due to hybridization with the matching target strand followed by recording a decrease in charge transfer resistance caused by intercalation. Nonspecific adsorption can hence be doubtlessly excluded as a reason for the observed changes in the impedance spectrum.
Ring-disk microelectrodes are proposed to be applied in a double constant-current mode using simultaneously an irreversible and a reversible reaction at the SECM tip. This allows an independent ...determination of the tip-to-sample distance concomitantly with the visualization of the lateral electrochemical reactivity of the investigated sample surface. The principle feasibility is demonstrated using a large and topographically as well as electrochemically challenging model sample.
► Constant-current mode for the investigation of topography and electrochemical activity. ► Utilization of a reversible and irreversible redox species. ► Ring-disk electrodes for the independent detection of two different redox mediators. ► Constant-distance mode for large surface areas.
A measurement protocol for the evaluation of catalyst inks for the oxygen evolution reaction
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rotating disc electrode measurements was conceived and applied in a multi-partner project. It was ...found that the electrochemical performances determined for a standardized electrode based on nickel-cobalt-oxide in 1 M KOH show a spread in the range of 50 mV at 10 mA cm
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, when comparing results from different groups. The variation of results obtained within individual groups, on the other hand, were found to be significantly lower. From this finding, we argue that the characterization of catalyst inks
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rotating disk electrode measurements is strongly affected by individual sample preparation and handling, leading to an additional uncertainty that depends on the individual experimenter. Consequently, the use of this technique for the evaluation and comparison of catalysts for the oxygen evolution reaction needs to be discussed and potentially revisited.
Elaborated and uniform experimental protocols are a prerequisite for OER catalyst evaluation
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RDE. Still, individual sample preparation and variations in microenvironment can lead to a significant spread of results between different laboratories.
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