Hydrogen bonding in the water stabilized structure of the modified unsymmetrically substituted viologen chromophore, N-ethyl-N'-(2-phosphonomethyl)-4,4'-bipyridylium dichloride

The crystal structure of the modified unsymmetrically N, N'-substituted viologen chromophore, N-ethyl- N'-(2-phosphonoethyl)-4, 4'-bipyridinium dichloride 0.75 hydrate. (1) has been determined. Crystals are triclinic, space group P-1 with Z = 2 in a cell with a = 7.2550(1), b = 13.2038(5), c = 18.5752(7) Å, α = 86.495(3), β = 83.527(2), γ = 88.921(2)o. The two independent but pseudo-symmetrically related cations in the asymmetric unit form one-dimensional hydrogen-bonded chains through short homomeric phosphonic acid O-H...O links [2.455(4), 2.464(4)A] while two of the chloride anions are similarly strongly linked to phosphonic acid groups [O-H…Cl, 2.889(4), 2.896(4)Å]. The other two chloride anions together with the two water molecules of solvation (one with partial occupancy) form unusual cyclic hydrogen-bonded bis(Cl...water) dianion units which lie between the layers of bipyridylium rings of the cation chain structures with which they are weakly associated.

Hydrogen peroxide biosensor based on horseradish peroxidase-Au nanoparticles at viologen grafted glassy carbon electrode

A novel hydrogen peroxide biosensor was fabricated that is based on horseradish peroxidase-Au nanoparticles immobilized on a viologen-modified glassy carbon electrode (GCE) by amino cation radical oxidation in basic solution. The immobilized BAPV acts as a mediator and a covalent linker between GCE and the Au nanoparticles. The biosensor exhibited fast response, good reproducibility, and long-term stability.

Electron self-exchange reaction of viologen and its derivatives in poly(ethylene glycol)

The electron self-exchange rates (k(ex)) of viologen and its derivatives are estimated by using microelectrode voltammetry in poly(ethylene glycol) films. The dependences of supporting electrolyte concentration and sizes of viologen and its derivatives on k(ex) and diffusion coefficients (D) are discussed. Results show that k(ex) increases with the decrease of supporting electrolyte concentration and sizes of reactants. (C) 2000 Elsevier Science S.A. All rights reserved.

The electrochemistry of cytochrome c at a viologen-thiol self-assembled monolayer

At the self-assembled monolayer (SAM) of a thiol-functionalized viologen modified gold electrode, cytochrome c (cyt c) exhibits a quasi-reversible electrochemical reaction. The heterogeneous electron transfer rate constant of cyt c in 0.1 mol/L phosphate buffer solution(pH 6.96) is 0.164 cm.s(-1) at 500 mV/s. The adsorbed cyt c on the viologen SAM forms a closely packed monolayer, whose average electron transfer rate is 4.85 s(-1) in the scan range of 50 to 500 mV/s. These results suggest that the SAM of viologen-thiol is a relatively stable, ordered and well-behaved monolayer from an electrochemical standpoint and it promotes the electron transfer process of biomolecules on electrode surface well.

Spontaneous assembly of the inclusion complexes of viologen-attached alkanethiols and alpha-, beta-cyclodestrin on gold electrode

Electroactive self-assembled monolayers (SAMs) with well-defined electrochemical responses were prepared by spontaneous assembly of the inclusion complexes (CD/C8VComegaSH) of viologen-attached alkanethiols (C8VComegaSH) and alpha- and beta-cyclodextrin (CD). They were characterized by X-ray photoelectron spectroscopy and cyclic voltammetry. The results demonstrate that the chemisorption process of CD/C8VComegaSH on gold substrate occurs through S-Au bonds, and that the redox sites in SAMs of CD/C8VComegaSH are in a much more uniform environment than those in SAMs of C8VComegaSH.

Electrochemical study of interactions between DNA and viologen-thiol self-assembled monolayers

We investigated the binding characteristics of double-stranded DNA to self-assembled monolayers (SAMs) containing viologen groups formed on the surface of gold electrodes via Au-S bonds. The positive charged and hydrophobic surfaces of the viologen SAMs modified gold electrodes are suitable to bind strongly dth DNA, whose interactions to solution DNA and adsorbed DNA both lead to positive shifts (22.5 mV and 65 mV, respectively) in the first redox potential ci viologen centers, indicating that the main interaction is from a hydrophobic interaction. Meanwhile, the binding of DNA strongly affects the kinetics of electron transfer of the viologen group so that the separation of anodic and cathodic peak potentials becomes larger and the heterogeneous electron transfer constant becomes smaller.

Electrochemical Study of the Self-exchange between Viologen-thiol Self assembled Monolayers and Alkanethiol Gold Electrodes

The preparation and cyclic voltammetric behaviors of self assembled monolayers (SAMs) containing electroactive viologen group have been investigated. Treatment of this viologen SAM with solutions of alkanethiols remits in replacing the electroactive third, shifting negatively its formal potentials and decreasing its heterogeneous elixtron transfer constants along with the immersion time. The aim of the work is to understand the exchange regularity of the mixed SANK on gold electrode surface.

Electrochemical study of the interfacial characteristics of redox-active viologen thiol self-assembled monolayers

The electrochemical behavior of the electroactive self-assembled monolayers (SAMs) of thiol-functionalized viologen, CH3(CH2)(9)V2+(CH2)(8)SH, where V2+ is a viologen group, on the gold electrodes is examined by cyclic voltammetry and electrochemical a.c. impedance. A monolayer of viologen is immobilized on the gold electrode surface via the Au-S bond and the normal potentials corresponding to the two successive one-electron transfer processes of the viologen active centers are -310 mV and -652 mV (vs. Ag/AgCl) in 0.1 mol l(-1) phosphate buffer solution (pH 6.96) respectively. These results suggest that the viologen SAMs are stable and well-behaved monolayers. The experimental impedance data corresponding to different forms of viologen group have been fitted to equivalent electrical circuits, and the surface capacitances and resistances have been given. The heterogenous electron transfer rates of the first and the second redox processes are 7.57 s(-1) and 1.49 s(-1) respectively through a.c. impedance.

Viologen-thiol self-assembled monolayers for immobilized horseradish peroxidase at gold electrode surface

A stable, well-behaved self-assembled monolayer (SAM) of viologen-functionalized thiol was used to immobilize and electrically connect horseradish peroxidase (HRP) at gold electrode. Viologen groups in SAMs facilitated the electron transfer from the electrode to the protein active site so that HRP exhibited a quasi-reversible redox behavior. HRP adsorbed in the SAMs is very stable, and close to a monolayer with the surface coverage of 6.5 x 10(-11) mol/cm(2). The normal potential of HRP is -580 mV vs Ag/AgCl corresponding to ferri/ferro active center and the standard electron transfer rate constant is 3.41 s(-1) in 0.1 M phosphate buffer solution (pH 7.1). This approach shows a great promise for designing enzyme electrodes with other redox proteins and practical use in tailoring a variety of amperometric biosensor devices. Copyright (C) 1997 Elsevier Science Ltd.

Ac voltammetric and square wave voltammetric study of the self-assembled monolayers of thiol-functionalized viologen derivative

Electroactive self-assembled monolayers (SAMs) containing viologen group are formed through the adsorption of thiol-functionalized viologen compound CH3(CH2)(9)V2+(CH2)(8)SH, where V2+ is N,N'-dialkylbipyridinium (i.e. a viologen group), onto gold electrodes from methanol/water solution and its electrochemical behavior is investigated ty Ac voltammetry and square wave voltammetry, which have the high sensitivity against background charging. The viologen SAM formed is a sub-monolayer and the normal potentials corresponding to the two successive one-electron transfer processes of the active centers (viologen) are -360 mV and -750 mV (vs. Ag/AgCl) in 0.1 mol/L phosphate buffer solutions (pH 6.96) respectively, and the standard electron transfer rate constant is 9.0 s(-1). The electrochemical behavior of this SAM in various solutions has been preliminarily discussed.

A viologen-based UV indicator and dosimeter

A UV indicator/dosimeter based on benzyl viologen (BV2+) encapsulated in polyvinyl alcohol (PVA) is described. Upon exposure to UV light, the BV2+/PVA film turns a striking purple colour due to the formation of the cation radical, BV center dot+. The usual oxygen sensitivity of BV center dot+ is significantly reduced due to the very low oxygen permeability of the encapsulating polymer, PVA. Exposure of a typical BV2+/PVA film, for a set amount of time, to UVB light with different UV indices produces different levels of BV center dot+, as measured by the absorbance of the film at 550 nm. A plot of the change in absorbance at this wavelength, Delta Abs(550), as a function of UV index, UVI, produces a linear calibration curve which allows the film to be used as a UVB indicator, and a similar procedure could be employed to allow it to be used as a solar UVI indicator. A typical BV2+/PVA film generates a significant, semi-permanent (stable for > 24 h) saturated purple colour (absorbance similar to 0.8-0.9) upon exposure to sunlight equivalent to a minimal erythemal dose associated with Caucasian skin, i.e. skin type II. The current drawbacks of the film and the possible future use of the BV2+/PVA film as a personal solar UV dosimeter for all skin types are briefly discussed.

MECHANISM OF THE REDUCTION OF WATER BY REDUCED METHYL VIOLOGEN MEDIATED BY PLATINIZED ALUMINA - H/D ISOTOPE EFFECTS

The rate of oxidation of reduced methyl viologen (MV+4) by water, catalyzed by colloidal Pt/Al2O3, is reduced by a factor of congruent-to 5 when D2O is used as a solvent rather than H2O in the presence of a pH 4.40 acetate buffer. In contrast, the rate measured in the presence of a pH 3.05 buffer is reduced only slightly when D2O replaces H2O. H/D isotope separation factors for the methyl viologen mediated reduction of water to hydrogen catalyzed by Pt/Al2O3 are 4.22 (+/- 0.15) at pH 4.40 and 5.99 (+/- 0.11) at pH 3.05, at 25-degrees-C. These data are interpreted in terms of the electrochemical model for metal-catalyzed redox reactions with a pH-dependent mechanism for the hydrogen-evolving reaction. It is proposed that hydrogen atom combination on the catalyst surface is the rate-limiting step at pH 4.40, whereas at pH 3.05 diffusion of MV2+4 is rate limiting and hydrogen evolution proceeds via the electrochemical reaction between a surface-adsorbed hydrogen atom and a solution-phase proton.