Rotating ring-disk voltammetry: Diagnosis of catalytic activity of metallic copper catalysts toward CO<Subscript>2</Subscript> electroreduction

Using the rotating ring (platinum)—disk (glassy carbon) electrode methodology, electrocatalytic activity of the microstructured copper centers (imbedded within the polyvinylpyrrolidone polymer matrix and deposited onto the glassy carbon disk electrode) has been monitored during electroreduction of carbon dioxide both in acid (HClO₄) and neutral (KHCO₃) media as well as diagnosed (at Pt ring) with respect to formation of the electroactive products. Combination of the stripping-type and rotating ring-disk voltammetric approaches has led to the observation that, regardless the overlapping reduction phenomena, the reduction of carbon dioxide at copper catalyst is, indeed, operative and coexists with hydrogen evolution reaction. Using the fundamental concepts of surface electrochemistry and analytical voltammetry, the reaction products (thrown onto the platinum ring electrode) could be considered and identified as adsorbates (on Pt) under conditions of the stripping-type oxidation experiment. Judging from the potentials at which the stripping voltammetric peaks appear in neutral CO₂-saturated KHCO₃ (pH 6.8), formic acid or carbon monoxide seem to be the most likely reaction products or intermediates. The proposed methodology also permits correlation between the CO₂ electroreduction products and the potentials applied to the disk electrode. By performing the comparative stripping-type voltammetric experiments in acid medium (HClO₄ at pH 1) with the adsorbates of formic acid, ethanol and acetaldehyde (on Pt ring), it can be rationalized that, although C₂H₅OH or CH₃CHO are very likely CO₂-reduction electroactive products, formation of some HCOOH, CH₃OH or even CO cannot be excluded.     

Label-free molecular imaging of immunological synapses between dendritic and T cells by Raman micro-spectroscopy

Confocal Raman micro-spectroscopy (CRMS) was used to measure spectral images of immunological synapse formation between dendritic and T cells without using molecular labels or other invasive procedures. The purpose-built inverted CRMS instrument integrated an environmental enclosure and a near-infrared laser to allow measurements on live cells maintained under physiological conditions. The integration of the wide-field fluorescence also enabled viability assays and direct comparison between Raman spectral images and gold-standard immuno-fluorescence images for specific molecules. Raman spectral images of nucleus and proteins were built by fuzzy c-mean clustering method. The Raman images were found to be in good correspondence with the immuno-fluorescence images of DNA and actin. These results indicate that actin is a main contributor to the Raman spectrum of the cytoplasm of dendritic and T cells. While for control cells the Raman spectral images of proteins indicated a more homogeneous distribution of proteins in the cytoplasm of dendritic cells, they indicated a higher accumulation of proteins at the immunological synapses when dendritic cells were pre-treated with laminin. These conclusions were also supported by confocal immuno-fluorescence imaging after cell fixation and labelling. This study demonstrates the potential of CRMS for label-free non-invasive imaging of junctions between live cells. Therefore, this technique may become a useful tool for studying cellular processes in live cells and where non-invasive molecular specific imaging is desirable, such as cell-cell interactions.     

Algebraic invariant curves for the Liénard equation

Odani has shown that if then after deleting some trivial cases the polynomial system does not have any algebraic invariant curve. Here we almost completely solve the problem of algebraic invariant curves and algebraic limit cycles of this system for all values of and . We give also a simple presentation of Yablonsky's example of a quartic limit cycle in a quadratic system.

     

Electrocatalytic properties of platinum nanocenters electrogenerated at ultra-trace levels within zeolitic phosphododecatungstate cesium salt matrices

A unique preparation method of obtaining stable composite film (with ultra-low platinum content) highly active towards oxygen reduction and hydrogen oxidation is presented here. The matrix for platinum centers consists of high-surface-area zeolite-type acidic salt of cesium phosphododecatungstate (Cs_2.5H_0.5PW₁₂O₄₀) admixed with carbon (Vulcan XC-72) carriers. Platinum nanoparticles were deposited on the working electrode modified with matrix via corrosion of platinum counter electrode during cyclic voltammetry experiment conducted in acid electrolyte containing chloride ions. The results obtained from rotating disk voltammetry revealed that the composite film containing Pt nanoparticles at very low loadings (on the level of 2–5 μg cm^?2) demonstrated remarkable electrocatalytic activity towards both oxygen reduction and hydrogen oxidation, particularly, when compared to the performance of the Cs_2.5H_0.5PW₁₂O₄₀-free system (i.e., containing only Vulcan support) prepared and examined under analogous conditions. The phenomenon should be primarily ascribed to the mesoporous nature of the matrix enabling immobilization and stabilization of small catalytic nanoparticles (1–2 nm diameters) inside the pores as well as to high surface acidity of the polyoxometalate-based salt providing proton-rich environment at the electrocatalytic interface.     

Electrochemical characterization of Prussian blue type nickel hexacyanoferrate redox mediator for potential application as charge relay in dye-sensitized solar cells

A polynuclear electronically/ionically (redox) conducting mixed-valent inorganic material such as nickel(II) hexacyanoferrate(II,III), NiHCF, was considered for potential application as a redox mediator (charge relay) in dye-sensitized solar cell (DSSC). The NiHCF redox reactions were found fast and reversible not only when the system was studied as thin film exposed to an aqueous supporting electrolyte but also as bulk material (pasted powder) in solid state, i.e., in the absence of contact with external liquid electrolyte phase. Usefulness of NiHCF material was diagnosed using conventional electroanalytical approaches, solid-state voltammetric methodology, as well as the dynamic electrochemical impedance spectroscopy technique that permitted monitoring of impedance spectra under potentiodynamic conditions. The material was utilized in a mixed-valent state, i.e., as a mixture of K₄Ni^II[Fe^II(CN)₆] and K₃Ni^II[Fe^III(CN)₆] in which iron(II) and iron(III) sites were at the 1:1 ratio. Under such conditions, dynamics of electron-hopping between mixed-valent iron sites was maximized. Our DSSC utilized cis–dithiocyanoatobis(4,4′-dicarboxylic acid-2,2′-bipyridine) ruthenium(II) dye (N3) adsorbed onto TiO₂ semiconductor and NiHCF as redox mediator. Although performance of our DSSC was not optimized in terms of the NiHCF film thickness and morphology, as well as lower photocurrents in comparison to those characteristic of the iodine/iodide based DSSC were obtained, our system yielded readily fairly high open-circuit photovoltages on the level of 800 mV. An important issue was that the formal potential of NiHCF was more positive relative to the potential of the iodide/triiodide couple while being still more negative than that equivalent to the ground state of the N3 dye. Thus, NiHCF mediator was able to regenerate the dye.

     

Non‐invasive time‐course imaging of apoptotic cells by confocal Raman micro‐spectroscopy

Confocal Raman micro‐spectroscopy (CRMS) was used to measure time‐course spectral images of live cells undergoing apoptosis without using molecular labels or other invasive procedures. Human breast cancer cells (MDA‐MB‐231) were exposed to 300 µM etoposide to induce apoptosis, and Raman spectral images were acquired from the same cells at 2‐h intervals over a period of 6 h. The purpose‐built inverted confocal Raman micro‐spectrometer integrated an environmental enclosure and wide‐field fluorescence imaging. These key instrumental elements allowed the cells to be maintained under sterile physiological conditions (37 °C, 5% CO₂) and enabled viability and apoptosis assays to be carried out on the cells at the end of CRMS measurements. The time‐course spectral images corresponding to DNA Raman bands indicated an increase in signal intensity in apoptotic cells, which was attributed to DNA condensation. The Raman spectral images of lipids indicated a high accumulation of membrane phospholipids and highly unsaturated non‐membrane lipids in apoptotic cells. This study demonstrates the potential of CRMS for label‐free time‐course imaging of individual live cells. This technique may become a useful tool for in vitro toxicological studies and testing of new pharmaceuticals, as well as other time‐dependent cellular processes, such as cell differentiation, cell cycle and cell–cell interactions. Copyright © 2010 John Wiley & Sons, Ltd.     

On the behaviour of Au plasmonic nanoparticles during hydrogen evolution at p-Si

Brewster Angle Reflectometry (BAR) and spectral photocurrent analysis show that Au nanoparticles are in a resonant state during light-induced hydrogen evolution with unpolarized light near-normal incidence. Keggin-type phosphododecamolybdate (PMo₁₂O₄₀^3?) anions were used as reducing and stabilizing agents to form novel Au nanoparticles (Au-NPs). BAR was employed to analyze the resonant optical response after adsorption to the surface of hydrogen terminated p-Si(111):H (1 × 1) and SiO₂. Proton reduction in 1 M H₂SO₄ occurs at asymmetric Au-NPs where also multipole field effects influence the plasmonic behaviour. In a comparative discussion, the contribution of surface plasmon polariton excitation to the observed pronounced photocurrent enhancement is outlined.     

Enhanced Water Splitting at Thin Film Tungsten Trioxide Photoanodes Bearing Plasmonic Gold–Polyoxometalate Particles

Tungsten trioxide (WO₃) is one of a few stable semiconductor materials liable to produce solar fuel by photoelectrochemical water splitting. To enhance its visible light conversion efficiency, we incorporated plasmonic gold nanoparticles (Au NPs) derivatized with polyoxometalate (H₃PMo₁₂O₄₀) species into WO₃. The combined plasmonic and catalytic effect of Au NPs anchored to the WO₃ surface resulted in a large increase of water photooxidation currents. Shielding the Au NPs with polyoxometalates appears to be an effective means to avoid formation of recombination centers at the photoanode surface.     

Electrochemical characterization of Prussian blue type nickel hexacyanoferrate redox mediator for potential application as charge relay in dye-sensitized solar cells

A polynuclear electronically/ionically (redox) conducting mixed-valent inorganic material such as nickel(II) hexacyanoferrate(II,III), NiHCF, was considered for potential application as a redox mediator (charge relay) in dye-sensitized solar cell (DSSC). The NiHCF redox reactions were found fast and reversible not only when the system was studied as thin film exposed to an aqueous supporting electrolyte but also as bulk material (pasted powder) in solid state, i.e., in the absence of contact with external liquid electrolyte phase. Usefulness of NiHCF material was diagnosed using conventional electroanalytical approaches, solid-state voltammetric methodology, as well as the dynamic electrochemical impedance spectroscopy technique that permitted monitoring of impedance spectra under potentiodynamic conditions. The material was utilized in a mixed-valent state, i.e., as a mixture of K₄Ni^II[Fe^II(CN)₆] and K₃Ni^II[Fe^III(CN)₆] in which iron(II) and iron(III) sites were at the 1:1 ratio. Under such conditions, dynamics of electron-hopping between mixed-valent iron sites was maximized. Our DSSC utilized cis–dithiocyanoatobis(4,4??dicarboxylic acid-2,2??bipyridine) ruthenium(II) dye (N3) adsorbed onto TiO₂ semiconductor and NiHCF as redox mediator. Although performance of our DSSC was not optimized in terms of the NiHCF film thickness and morphology, as well as lower photocurrents in comparison to those characteristic of the iodine/iodide based DSSC were obtained, our system yielded readily fairly high open-circuit photovoltages on the level of 800?mV. An important issue was that the formal potential of NiHCF was more positive relative to the potential of the iodide/triiodide couple while being still more negative than that equivalent to the ground state of the N3 dye. Thus, NiHCF mediator was able to regenerate the dye.     

Effective charge propagation and storage in hybrid films of tungsten oxide and poly(3,4-ethylenedioxythiophene)

Hybrid (composite) electroactive films consisting of such an organic conducting polymer as poly(3,4-ethylenedioxythiophene), PEDOT, and such a polynuclear inorganic compound as amorphous tungsten oxide, WO₃/H _x WO₃ were fabricated on carbon electrodes through electrodeposition by voltammetric potential in acid solution containing EDOT monomer and sodium tungstate. Electrostatic interactions between the negatively charged tungstic units (existing within WO₃) and the oxidized positively charged conductive polymer (oxidized PEDOT) sites create a robust hybrid structure which cannot be considered as a simple mixture of the organic and inorganic components. It is apparent from scanning electron microscopy that hybrid structures are granular but fairly dense. Because PEDOT and mixed-valence tungsten oxides are electronically conducting, the resulting hybrid films are capable of fast propagation. The reversible and fast redox reactions of tungsten oxide component lie in the potential range where PEDOT matrix is conductive. Furthermore, the hybrid films exhibit good mediating capabilities towards electron transfers between model redox couples such as cationic iron(III,II) and anionic hexacyanoferrate(III,II). Since the films accumulate effectively charge and show high current densities at electrochemical interfaces, they could be of importance to electrocatalysis and to construction of redox capacitors.