Spectroelectrochemical properties of 1,10- phenanthroline substituted by phenothiazine and carbazole redox-active units

Complexes of 1,10-phenanthrolines with cations of transition metals have broad range of applications. This work aims at designing and investigating phenothiazine and carbazole substituted 1,10-phenanthrolines as ligands for future complexes with transient metal cations. The combined electrochemical, spectroelectrochemical and DFT studies were employed to demonstrate the effect of broken symmetry in substituted 4,7-di(phenothiazine)-1,10-phenanthrolines on their spectroelectrochemical properties. A reversible color change (new absorption band around 500 nm) due to phenothiazine radical cation was observed in the first oxidation step. Results further indicate that phenothiazine substituents behave as two equivalent but almost electronically isolated redox centres. The work additionally presents a comprehensive reaction mechanistic study of oxidation and reduction processes complemented by HPLC-MS/MS identification

Tuning the contact conductance of anchoring groups in single molecule junctions by molecular design

A tetraphenylmethane tripod functionalized with three thiol moieties in the para position can serve as a supporting platform for functional molecular electronic elements. A combined experimental scanning tunneling microscopy break junction technique with theoretical approaches based on density functional theory and non-equilibrium Green`s function formalism was used for detailed charge transport analysis to find configurations, geometries and charge transport pathways in the molecular junctions of single molecule oligo-

Katalytické vlastnosti různě imobilizované houbové tyrosinázy: Kinetická studie pro budoucí vývoj biomimetických amperometrických biosenzorů

Mushroom tyrosinase was immobilized by direct embedding into electrode material (modified carbon paste electrode), incorporation of cross-linked enzyme aggregates into a polymer membrane (glassy carbon electrode covered by thin layer of Nafion (R)), and covalent attachment using self-assembled monolayers (gold electrode with the chemically bound enzyme). Both, standard UV-Vis spectrophotometry and amperometry in a batch configuration are presented as complementary methods to study the tyrosinase enzyme kinetics, whose catecholase activity results in electroactive products (ortho-quinones). Due to higher sensitivity of amperometric detection, evident advantage in the enzyme consumption was obtained. Prepared amperometric tyrosinase biosensors were characterized using cyclic voltammetry and atomic force microscopy. The Michaelis constant values of immobilized and unbound tyrosinase (free enzyme solution) towards dopamine and catechol were compared. The apparent Michaelis constant values for immobilized tyrosinase are significantly lower than the declared value of 0.840 mmol L-1 dopamine for the unbound enzyme. The enzymetyrosinase arranged in self-assembledmonolayer serves as an efficient sensor due to lowapparent Michaelis constant of 0.061 mmol L-1 dopamine and high maximum reaction velocity of 0.458 mu A s(-1). This fact reflects the ideal arrangement of enzymemolecules causing high availability of the binding site. Tris-glycine sodiumdodecyl sulphate polyacrylamide gel electrophoresis and atomic force microscopy clarified that the protein of molecular weight 25 kDa is bound preferably on chemically modified gold electrode. A sensor prepared by the immobilization of tyrosinase on gold electrode results in higher catecholase activity towards dopamine than in case of CPE and GC electrodes, where enzyme is immobilized physically.Houbová tyrosináza byla imobilizována přímým zapuštěním do elektrodového materiálu (modifikovaná uhlíková pastová elektroda), zabudováním zesítěných enzymových agregátů do polymerní membrány (skelná uhlíková elektroda pokrytá tenkou vrstvou Nafionu (R) a kovalentní vazbou pomocí samovolně sestavené monovrstvy (zlatá elektroda s chemicky vázaným enzymem). Standardní UV-Vis spektrofotometrie a amperometrie v dávkové konfiguraci jsou prezentovány jako doplňkové metody ke studiu kinetiky enzymu tyrosinázy, jehož katecholázová aktivita vede k elektroaktivním produktům (ortho-chinony). Díky vyšší citlivosti amperometrické detekce byla získána evidentní výhoda ve spotřebě enzymu. Připravené amperometrické tyrozinázové biosenzory byly charakterizovány pomocí cyklické voltametrie a mikroskopie atomových sil. Byly porovnány hodnoty Michaelisovy konstant imobilizované a nenavázané tyrosinázy (roztok volného enzymu) vůči dopaminu a katecholu. Zdánlivé Michaelisovy hodnoty konstant pro imobilizovanou tyrosinázu jsou významně nižší než deklarovaná hodnota 0,840 mmol L-1 dopaminu pro nenavázaný enzym. Enzym tyrosináza uspořádaná v samostatně sestavené monovrstvě slouží jako účinný senzor díky nízké zjevné Michaelisově konstantě 0,061 mmol L-1 dopaminu a vysoké maximální reakční rychlosti 0,458 uAs (-1). Tato skutečnost odráží ideální uspořádání molekul enzymu způsobujících vysokou dostupnost vazebného místa. Gelová elektroforéza a mikroskopie atomových sil tris-glycin natriumdodecylsulfát polyakrylamid objasnily, že protein o molekulové hmotnosti 25 kDa je výhodně navázán na chemicky modifikovanou zlatou elektrodu. Senzor připravený imobilizací tyrosinázy na zlaté elektrodě má za následek vyšší aktivitu katecholázy vůči dopaminu než v případě elektrod CPE a GC, kde je enzym fyzicky imobilizován

On the difference in decomposition of taxifolin and luteolin vs. fisetin and quercetin in aqueous media

The decomposition of flavonols quercetin and fisetin, flavone luteolin and flavanone taxifolin was studied in slightly alkaline solution under ambient conditions. The study was based on spectrophotometry and high-pressure liquid chromatography. Products formed by atmospheric oxygen oxidation and hydrolysis were identified by HPLC–DAD and HPLC–ESI-MS/MS. Only small differences in the chemical structure of flavonoids resulted in extremely variable oxidation pathways and products. Oxidation of flavonols led to the formation of both a benzofuranone derivative and several open structures. On the contrary, the benzofuranone derivative was not found as a product of taxifolin and luteolin oxidative decomposition. These compounds were oxidized to their hydroxylated derivatives and typical open structures. Quercetin was not identified as a possible oxidation product of taxifolin

Visualisation and quantification of flooding phenomena in gas diffusion electrodes (GDEs) used for electrochemical CO2 reduction: A combined EDX/ICP–MS approach

The most promising strategy to up-scale the electrochemical CO2 reduction reaction (ec-CO2RR) is based on the use of gas diffusion electrodes (GDEs) that allow current densities close to the range of 1 A/cm2 to be reached. At such high current densities, however, the flooding of the GDE cathode is often observed in CO2 electrolysers. Flooding hinders the access of CO2 to the catalyst, and by thus leaving space for (unwanted) hydrogen evolution, it usually leads to a decrease of the observable Faradaic efficiency of CO2 reduction products. To avoid flooding as much as possible has thus become one of the most important aims of to-date ec-CO2RR engineering, and robust analytical methods that can quantitatively assess flooding are now in demand. As flooding is very closely related to the formation of carbonate salts within the GDE structure, in this paper we use alkali (in particular, potassium) carbonates as a tracer of flooding. We present a novel analytical approach —based on the combination of cross-sectional energy-dispersive X-ray (EDX) mapping and inductively coupled plasma mass spectrometry (ICP--MS) analysis— that can not only visualise, but can also quantitatively describe the electrolysis time dependent flooding in GDEs, leading to a better understanding of electrolyser malfunctions

Electron transport through catechol-functionalized molecular rods

The charge transport properties of a catechol-type dithiol-terminated oligo-phenylene-ethynylene was investigated by cyclic voltammetry (CV) and by the scanning tunnelling microscopy break junction technique (STM-BJ). Single molecule charge transport experiments demonstrated the existence of high and low conductance regions. The junction conductance is rather weakly dependent on the redox state of the bridging molecule. However, a distinct dependence of junction formation probability and of relative stretching distances of the catechol- and quinone-type molecular junctions is observed. Substitution of the central catechol ring with alkoxy-moieties and the combination with a topological analysis of possible π-electron pathways through the respective molecular skeletons lead to a working hypothesis, which could rationalize the experimentally observed conductance characteristics of the redox-active nanojunctions

Charge Transport in Photoswitchable Dimethyldihydropyrene-Type Single-Molecule Junctions

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