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-

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

Evidence for a hopping mechanism in metal|single molecule|metal junctions involving conjugated metal–terpyridyl complexes; potential-dependent conductances of complexes [M(pyterpy)₂] ²⁺ (M = Co and Fe; pyterpy = 4′-(pyridin-4-yl)-2,2′:6′,2′′-terpyridine) in ionic liquid

Extensive studies of various families of conjugated molecules in metal|molecule|metal junctions suggest that the mechanism of conductance is usually tunnelling for molecular lengths < ca. 4 nm, and that for longer molecules, coherence is lost as a hopping element becomes more significant. In this work we present evidence that, for a family of conjugated, redox-active metal complexes, hopping may be a significant factor for even the shortest molecule studied (ca. 1 nm between contact atoms). The length dependence of conductance for two series of such complexes which differ essentially in the number of conjugated 1,4-C₆H₄- rings in the structures has been studied, and it is found that the junction conductances vary linearly with molecular length, consistent with a hopping mechanism, whereas there is significant deviation from linearity in plots of log(conductance) vs. length that would be characteristic of tunnelling, and the slopes of the log(conductance)–length plots are much smaller than expected for an oligophenyl system. Moreover, the conductances of molecular junctions involving the redox–active molecules, [M(pyterpy)₂] ²⁺/³⁺ (M = Co, Fe) have been studied as a function of electrochemical potential in ionic liquid electrolyte, and the conductance–overpotential relationship is found to fit well with the Kuznetsov–Ulstrup relationship, which is essentially a hopping description

Development of a microfluidic confocal fluorescence detection system for the hyphenation of nano-LC to on-line biochemical assays

One way to profile complex mixtures for receptor affinity is to couple liquid chromatography (LC) on-line to biochemical detection (BCD). A drawback of this hyphenated screening approach is the relatively high consumption of sample, receptor protein and (fluorescently labeled) tracer ligand. Here, we worked toward minimization of sample and reagent consumption, by coupling nano-LC on-line to a light-emitting diode (LED) based capillary confocal fluorescence detection system capable of on-line BCD with low-flow rates. In this fluorescence detection system, a capillary with an extended light path (bubble cell) was used as a detection cell in order to enhance sensitivity. The technology was applied to a fluorescent enhancement bioassay for the acetylcholine binding protein, a structural analog of the extracellular ligand-binding domain of neuronal nicotinic acetylcholine receptors. In the miniaturized setup, the sensitive and low void volume LED-induced confocal fluorescence detection system operated in flow injection analysis mode allowing the measurement of IC(50) values, which were comparable with those measured by a conventional plate reader bioassay. The current setup uses 50 nL as injection volume with a carrier flow rate of 400 nL/min. Finally, coupling of the detection system to gradient reversed-phase nano-LC allowed analysis of mixtures in order to identify the bioactive compounds present by injecting 10 nL of each mixture

CE of tricyclic antidepressant clomipramine and metabolites: electromigration and wall adsorption

CE of tricyclic antidepressants clomipramine and its metabolites demethylclomipramine, didemethylclomipramine and 8-hydroxyclomipramine resulted in partly extremely tailing peaks in bare fused-silica capillaries. Especially at high pH of the BGE this behavior was not unexpected as adsorption of the cationic analytes onto the negatively charged wall due to electrostatic attraction can be supposed. Less expected was the observation that peak tailing could not be overcome neither by using a capillary with dynamic coating with cationic CTAB added to the BGE, nor by the usage of a capillary permanently coated with polyvinyl alcohol (PVA), both operated at acidic pH. As this tailing was even more pronounced than with bare fused silica, and was suppressed upon addition ofMeCN to the BGE, nother source of adsorption than pure ion\u2013ion interaction seems plausible. In the bare silica apillary the mobility, m, of the analytes followed roughly the pHdependence of a monoacidic base, but two deviations from the sigmoid theoretical curve were evident: (i) even at low pH the mobilities were not constant; they decreased in contrary withpHover the entire range; (ii) the apparent pKa values of two analytes, derived at the pH with halve the mobility at low pH, are significantly smaller than the thermodynamic pKa. Upon modifying the expression for m = f(pH), and considering thepHdependence of the negative charge density at the wall by an additional term which takes chromatographic retention into account, an equation was derived which enables the description of the observed electromigration of the analytes as function of pH, pKa of analytes and surface silanol groups, actual mobility of analytes, distribution coefficient (or retention factor) due to adsorption including itspHdependence. The interplay of electrophoretic movement and residual adsorptive retention allowed to resolve the analytes finally in an uncoated capillary, namely at pH 7.65 (30 mM ionic strength), whereas at the cost of the robustness of the separation system

The effects of 4,7-di(pyrrolidin-1-yl) substituents on the reduction and oxidation mechanisms of 1,10-phenanthrolines: New perspectives in tailoring of phenantroline derivatives

The oxidation and reduction mechanisms of substituted 4,7-di(pyrrolidin-1-yl)-1,10-phenanthrolines were investigated in non-aqueous environment by means of cyclic voltammetry. Reduction of these derivatives leads to the formation of radical anion and subsequent reductive cleavage of pyrrolidine-1-yl moiety in overall ECE reduction processes. The regenerating formation of 1,10-phenanthroline was observed. IR spectroelectrochemistry and HPLC-MS/MS analysis were applied to support this result. The presence of pyrrolidine-1-yl moiety significantly thermodynamically facilitates the reduction of the 1,10-phenanthroline structure. Concerning oxidation, the primarily formed cation radical on nitrogen of the pyrrolidine moiety is further oxidized and undergoes a coupled nucleophilic addition of water and hydroxylated compound is formed as the oxidation product. Additionally, further opening of pyrrolidine ring and oxidative cleavage of alkyl are other proposed reactions. The results presented in this work pave the way for novel chelating agents with electrochemically controlled polarity. Furthermore, reductive regeneration of individual components of the molecular assemblies described in this work opens new directions in electrochemical technologies for the environmental protection

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

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

Electric conductivity measurements employing 3D printed electrodes and cells

The electric conductivity is one the most routinely inspected characteristics of aqueous media, being employed in monitoring drinking water quality and determining thermodynamic properties of electrolytes. In this work, we utilize fused deposition modelling 3D printing (3DP) to manufacture a platform composed of supporting cells and sensing electrodes for electric conductivity measurements of aqueous solutions. For the first time, the electric conductivity of liquid electrolytes is sensed by 3DP electrodes, employing a direct electrode/electrolyte contact. Conductivity measurements performed in the presented 3DP platform are controlled by an electronic circuitry developed employing a programmable system on chip prototyping kit. The entire conductometric setup was validated employing commercial conductance standards as well as in-lab prepared aqueous solutions of potassium chloride and formic and acetic acid as representatives of strong and weak electrolytes. Conductivity measurements enabled the correct determination of limiting molar conductivity (for potassium chloride) and dissociation constants (for the two weak acids). Finally, the functionality of the presented platform was confirmed by measuring conductivity of various bottled water samples. Results obtained in this work pave the wave for further development and applications of conductometers based on 3DP electrodes and cells