Rafal Jurczakowski

Media Effects on the Mechanism of Antioxidant Action of Silybin and 2,3-Dehydrosilybin: Role of the Enol Group

Silybin (SIL) and 2,3-dehydrosilybin (DHS) are constituents of milk thistle extract (silymarin) applied in the treatment of cirrhosis, hepatitis, and alcohol-induced liver disease. The molecular mechanism of their action is usually connected with antioxidant action. However, despite experimental and theoretical evidence for the antioxidant activity of SIL and DHS, the mechanism of their antiradical action still remains unclear. We studied the kinetics of SIL/DHS reactions with 2,2-diphenyl-1-picrylhydrazyl radical in organic solutions of different polarity and with peroxyl radicals in a micellar system mimicking the amphiphilic environment of lipid membranes. Kinetic studies together with determination of acidity and electrochemical measurements allowed us to discuss the structure-activity relationship in detail. In nonpolar solvents the reaction with free radicals proceeds via a one-step hydrogen atom transfer (HAT) mechanism, while significant acceleration of the reaction rates in methanol and water/methanol solutions suggests the dominating contribution of a sequential proton-loss electron-transfer (SPLET) mechanism with participation of the most acidic hydroxyl groups: 7-OH in SIL and 7-OH and 3-OH in DHS. In a heterogeneous water/lipid system, both mechanisms operate; however, the reaction kinetics and the antioxidant efficacy depend on the partition between lipid and water phases.

Construction of DNA biosensor at glassy carbon surface modified with 4-aminoethylbenzenediazonium salt

A simple, label-free electrochemical impedance-spectroscopy method for sequence-specific detection of DNA using a 4-aminoethylbenzenediazonium (AEBD) salt as a binder for amino-modified probe DNA is reported. This novel method simplifies the anchoring of DNA at the GC surface and opens new ways for the detection of hybridization. The hybridization of target DNA, without and with mismatches, with the probe DNA anchored at the GC surface modified with AEBD, greatly increases the interfacial electron transfer resistance at the double-stranded DNA modified electrodes for the redox couple Fe(CN)(6)(3-/4-). The resistance was measured using electrochemical impedance spectroscopy. The sensor response increased linearly with logarithm of concentration of target DNA in the range 2×10(-12)÷2×10(-6) M. The obtained quantification limit was circa 6.5×10(-17) mole in a 7 μL droplet and corresponded to a concentration of 9.2×10(-12) M of target DNA in the sample. This limit is equivalent to the detection of circa 4×10(7) copies of DNA in a 7 μL droplet or circa 5.7×10(12) DNA copies in one litre of sample.

CO2 Electroreduction at Bare and Cu-Decorated Pd Pseudomorphic Layers: Catalyst Tuning by Controlled and Indirect Supporting onto Au(111)

We report here the results of electrochemical studies on CO2 electroreduction at multilayered catalyst composed of the monatomic layer of copper covering palladium overlayers (0.8-10 monolayers) deposited on the well-defined Au(111) surface. These multilayered systems were obtained by successive underpotential deposition steps: Pd on Au(111) as well as Cu on Pd/Au(111). Low index orientation of Au substrate was chosen to compare Pd overlayers with bulk Pd(111), which is known to reduce CO2 to CO adsorbates in acidic solutions. The process of CO2 electroreduction was studied by using classical transient electrochemical methods. Catalytic activity of bare Pd layers was investigated in acidic and neutral solutions. In the latter case, much higher activity of Pd overlayers was observed. The results showed that the palladium layer thickness significantly changed the catalytic activities of both bare Pd overlayers and the one Cu monolayer covered electrodes toward CO2 electroreduction. Results show that catalytic activity can be finely tuned by using the multilayered near-surface-alloy approach.

The oscillatory course of the electroreduction of thiocyanate complexes of nickel(II) at mercury electrodes — experiment and simulation

Abstract Following our previous studies confirming the role of adsorbed electrocatalytic SCN− ions in the formation of negative polarisation resistance in the electroreduction of the Ni(II)SCN− complexes at the mercury electrode, we present further experimental and theoretical investigations of the oscillatory behaviour of this process. Examples of current oscillations collected for various experimental conditions are shown. A numerical model involving partial differential equations of diffusion transport is constructed for simulations of reported oscillations. Comparison of experimental and theoretical oscillations confirms both the electrochemical source of oscillations and the validity of the numerical approach proposed. With this approach it was possible to explain the effect of a side formation of the inhibitory layer of NiS on the shapes of oscillations.

Monitoring of spatiotemporal patterns in the oscillatory chemical reactions with the infrared camera: experiments and model interpretation.

An infrared camera was used for the first time to monitor the progress of traveling fronts in oscillatory chemical reactions, taking the Belousov-Zhabotinsky (BZ) reaction as the test system. The experiments involved comparative visual imaging and infrared thermography measurements for the thin-layer of the BZ solution in the Petri dish, including both aqueous and gel media, the latter one hindering the convection. Infrared thermography experiments that supply information on the temperature distribution at the solution surface were compared with the bulk temperature variations of the stirred solution with BZ reaction, measured simultaneously with the oscillatory variations of the Pt electrode potential. The experimentally observed correlation between the ferroin catalyst concentration and the temperature distribution was compared with the results of numerical modeling of these distributions in 2-D reactor space, based on the classical Oregonator. Analogous experiments were performed for the oscillatory oxidation of thiocyanates with hydrogen peroxide, catalyzed with Cu(2+) ions, in search of factors causing the development of traveling fronts, previously reported. The inhomogeneous distribution of the free surface temperature that could contribute to surface instabilities was found. Also, periodical increase and decrease in temperature of solution surface was reported. This was interpreted as periodically predominating cooling of the surface in contact with the surroundings because for the bulk, thermally isolated stirred solution, the temperature monotonically increases. In terms of our nine-variable kinetic model of this system, it was possible to identify the reaction steps responsible for the experimentally observed temperature dynamics and ascribe the appropriate heat effects to them. Our results constitute the first contribution to the thermochemical characteristics of the H(2)O(2)-SCN(-)-OH(-)-Cu(2+) oscillator.

Outstanding Catalytic Activity of Ultra-Pure Platinum Nanoparticles

Small (4 nm) nanoparticles with a narrow size distribution, exceptional surface purity, and increased surface order, which exhibits itself as an increased presence of basal crystallographic planes, can be obtained without the use of any surfactant. These nanoparticles can be used in many applications in an as-received state and are threefold more active towards a model catalytic reaction (oxidation of ethylene glycol). Furthermore, the superior properties of this material are interesting not only due to the increase in their intrinsic catalytic activity, but also due to the exceptional surface purity itself. The nanoparticles can be used directly (i.e., as-received, without any cleaning steps) in biomedical applications (i.e., as more efficient drug carriers due to an increased number of adsorption sites) and in energy-harvesting/data-storage devices.

Electrochemical oscillations and bistability during anodic dissolution of vanadium electrode in acidic media—part I. Experiment

Dynamic instabilities, current oscillations and bistability observed during anodic dissolution of both stationary and rotating disk vanadium electrode in acidic phosphoric media, were reported using both dc and ac techniques. The effect of various experimental conditions, concentration of H3PO4, temperature, disk rotation rate, and external resistance, was analyzed. Systematic studies allowed the construction of bifurcation diagrams, showing the regions of oscillations and bistability, including the complex behaviors like coexistence of both dynamic regimes. Analogous comparative measurements and analyses were performed for other media—sulfuric, nitric, perchloric, and trifluoroacetic acids—also indicating complex dynamic behaviors. These complexities arise not only due to the difficulties with the precise identification of the composition of the passive layer in every acid but are also due to relatively fast dissolution of V electrode, even in the presence of passive layer, which causes permanent drift of the system’s characteristics. Due to these experimental difficulties, theoretical modeling seems to be an appropriate method to analyze the essential nonlinear dynamic properties of the studied system.

Ultrasensitive 4-methylumbelliferone fluorimetric determination of water contents in aprotic solvents.

A novel approach to the quantification of relatively small amounts of water present in low polarity, aprotic solvents is proposed. This method takes advantage of protolitic reaction of 4-methylumbelliferone dissolved in the solvent with water, acting as a base. The low emission intensity neutral 4-methylumbelliferone is transformed in reaction with water to a highly fluorescent anionic form. Thus the increase in emission intensity is observed for increasing water contents in aprotic solvents. For low water contents and highly lipophilic solvents, this method yields (in practical conditions) higher sensitivity compared to biamperometric Karl Fischer titration method in volumetric mode. It is also shown that organic compounds of protolitic character (amines, acids) not only interfere with water contents determination but increase the sensitivity of emission vs. water contents dependence. Introduction of aqueous solution of strong acid or base (HCl or NaOH) has similar effect on the system as introduction of pure water.

On the source of oscillatory instabilities in the electroreduction of thiocyanate complexes of nickel(II) at mercury electrodes

Abstract The mechanistic aspects of the electroreduction of thiocyanate complexes of nickel(II) at mercury electrodes, responsible for the formation of the negative polarization resistance and oscillatory instabilities, were studied. Of two possible sources of negative resistance–adsorption of an inhibitory layer of NiS and the desorption of catalytic SCN− from mercury surface at negative potentials, the latter was found to predominate under the conditions studied. Nickel(II) sulphide was suggested as one of the causes for the limited lifetime of oscillations only, and not as a source of actually observed instabilities. An inner-sphere bridge mechanism of the electroreduction of thiocyanate complexes of nickel(II) was proposed, as this pathway is considerably faster than the irreversible outer-sphere reduction, occurring at more negative potentials. The proposed mechanism was confirmed quantitatively by its digital simulation and compared with that suggested earlier for In(III)SCN− complexes. An example of current oscillations is presented.

Studies on the formation and kinetics of self-decomposition of Ni(IV)-N-3 complexes in aqueous media

Both electrochemical (at Pt electrode) and homogeneous (with Cl2(aq)) oxidation of the aqueous solution of Ni(II)-N3− yields the solution of a brown product that was found to be the azide complex of Ni(IV). Its UV/Vis spectrum, consisting of a single band of high intensity (ε = 26500 cm−1 dm3 mol−1 at λmax = 471 nm), has been ascribed to the ligand-to-metal charge-transfer mechanism (LMCT). The Ni(IV)-N3− complex undergoes spontaneous decomposition in aqueous solution, with Ni(IV) reduced back to Ni(II) and N3− oxidized to molecular nitrogen. The first-order kinetics of this process was studied spectrophotometrically and the following parameters were found: rate constant k = 1.96×10−2 s−1, Arrhenius activation energy Ea = 82±7 kJ mol−1, standard activation enthalpy ΔH≠0 = 79±7 kJ mol−1, and small standard activation entropy ΔS≠0 = 10±20 J mol−1 K−1. The outline mechanism for the intramolecular redox Ni(IV)-N3− decomposition has been suggested. Nickel ions at various oxidation states were found to be powerful catalysts for the redox processes of azide ions.