Timothy McCormac

Part I. A comparative electrochemical study of transition metal substituted Dawson type heteropolyanions

Abstract The comparative electrochemical behaviour, in aqueous solutions, of the Dawson lacunary anion [P 2 W 17 O 61 ] 10− and of the Fe III , Cu II , Co II , Ni II , Mn III and Mn II substituted Dawson lacunary heteropolyanions is presented. The literature concerning the latter heteropolyanions has been reviewed and completely new data are presented. The electrochemical behaviour observed for the reduction of the tungsten oxo centres, for all the heteropolyanions investigated, was found to be pH dependent. Electrochemical evidence for the presence of the Fe III , Cu II , Co II and the Mn II and Mn III cations in the ejected tungsten position was obtained with the Ni II cation processing no electroactivity.

Part II. Role of pH and the transition metal for the electrocatalytic reduction of nitrite with transition metal substituted Dawson type heteropolyanions

Abstract The parent Dawson and lacunary anion were both seen previously to have a catalytic effect (redox mediation) upon the reduction of nitrite at a pH equal to 2.00. However, at a pH value of 4.50 no electrocatalytic effect was observed for low concentrations of nitrite. It was shown that at the latter pH value the presence of certain transition metals substituted into the lacunary anion is necessary to observe the electrocatalytic reduction (chemical catalysis) of low concentrations of nitrite. It was found that the Fe III and Cu II substituted Dawson heteropolyanions Fe III HPA and Cu II HPA respectively were the most efficient at pH 4.50 for the electroreduction of nitrite. In addition, despite the lack of direct electroactivity of the Ni II site in Ni II HPA, the Ni II site is demonstrated by the observation of a catalytic effect for nitrite reduction at pH 4.50.

Synthesis and electrochemical characterisation of [Ru(bpy)3]3[P2W18O62]

Abstract Addition of [Ru(bpy) 3 ][Cl 2 ] to K 6 P 2 W 18 O 62 ·15H 2 O in aqueous solution resulted in the formation of the ionic complex [Ru(bpy) 3 ] 3 [P 2 W 18 O 62 ]. The complex was characterised by elemental analysis, spectroscopic (UV–Vis, 31 P-NMR, IR spectroscopy) techniques and cyclic voltammetry. Solution phase studies gave a range of redox couples associated with the Ru 3+/2+ and bipyridine ligands of the cationic [Ru(bpy) 3 ] 2+ moiety and the tungsten-oxo framework of the associated Dawson parent heteropolyanion (HPA), [P 2 W 18 O 62 ] 6− . With 0.5 mM [Ru(bpy) 3 ] 3 [P 2 W 18 O 62 ] in dimethylsulphoxide (DMSO) (0.1 M Bu 4 NPF 6 ), precipitation onto the electrode surface occurs upon oxidation through the Ru 3+/2+ system. This results in the formation of the mixed anion salt, [Ru(bpy) 3 ] 3 [P 2 W 18 O 62 ][PF 6 ]. Enhanced levels of surface based processes are also observed in the voltammetry of [Ru(bpy) 3 ] 3 [P 2 W 18 O 62 ] in DMSO (0.01 M Bu 4 NPF 6 ) with the [P 2 W 18 O 62 ] 7−/8− and Ru 3+/2+ redox systems both exhibiting surface behaviour. Voltammetric studies of adsorbed films of [Ru(bpy) 3 ] 3 [P 2 W 18 O 62 ] were performed in pHs of 2.0, 4.5 and 7.0. The film was found to exhibit the same solution pH dependence for the tungsten-oxo framework of the HPA with better stability towards redox cycling in pH 2.00 as compared to pH 7.00. Mechanically attached solid state films of [Ru(bpy) 3 ] 3 [P 2 W 18 O 62 ] were formed on carbon macroelectrodes and the electrochemical behaviour of the microparticles was investigated in a variety of electrolytes. Upon redox switching between the Ru 2+/3+ redox states the solid-state charge-transfer processes are coupled to insertion/expulsion of anions from/to the aqueous solution phase. Scanning electron microscopy of the mechanically attached deposit, reveals that voltammetric cycling in 1.0 M HClO 4 through the redox processes associated with the tungsten-oxo framework converts the amorphous array of microscopically small particles into a plate-like semi-crystalline form.

Photophysics of ruthenium polypyridyl complexes formed with lacunary polyoxotungstates with iron addenda

The interactions between luminophore [Ru(bpy)3]2+, and the lacunary Dawson heteropolyanions, [P2W17O61(FeOH2)]7-, [P2W17O61(FeBr)]6- and [P2W17O61]10- were investigated using a combination of photophysics, optical and Raman spectroscopy. Extensive quenching of the excited state of [Ru(bpy)3]2+ was observed in each case. Quenching is attributed to the formation of association complexes between [Ru(bpy)(3)]2+ and the heteropolyanions in which the charge on the heteropolyanions is fully compensated for by the ruthenium polypyridyl species. The interaction appears to be principally electrostatic in nature producing [Ru(bpy)3]3.5[P2W17O61(FeOH2)], [Ru(bpy)3]3[P2W17O61(FeBr)] and [Ru(bpy)3]5[P2W17O61]10-. The association constants for formation of the clusters were obtained from photophysical studies and surprisingly, despite the electrostatic nature of the interaction, there was no correlation between the charge on the polyoxometallate and the association constant. In particular, the unsubstituted lacunary, [P2W17O61]10-, showed considerably weaker association compared to the transition metal substituted lacunaries, in spite of its 10- charge. Difference absorption spectroscopy revealed a new transition at ca. 480 nm for each of the cluster complexes. From resonance Raman spectroscopy the origin of this transition was found to involve the polyoxometallate. Unlike previously reported adducts, the cluster complexes formed were not luminescent. In all cases the cluster complexes exhibit remarkable photostability, with no photodecomposition or photo-induced ligand exchange reactions evident in acetonitrile, under conditions where [Ru(bpy)3]2+ alone exhibits considerable photolability.

Immobilization of a Series of Dawson Type Heteropolyanions

Attempts to immobilize a series of Dawson type heteropolyanions (HPAs) by anodization and the use of polymeric, polyvinylpyridine (PVP), and conducting pyrrole and N-methylpyrrole, films has been undertaken. A stable film of the Dawson Parent, [P2W18O62]6–, HPA was obtained on an anodized carbon electrode, the film exhibited thin layer behavior and was stable up to a pH value of 2.00. The same HPA was successfully immobilized in a protonated PVP film with the resultant modified electrode being stable between pH values of 1.00 and 8.00, with the redox activity of the immobilized HPA exhibiting the same pH dependence as shown by the solution phase species. It was shown that a stable polypyrrole film doped with the Dawson parent HPA was obtained when the cycling of the film was restricted to the first two tungsten-oxo based redox processes in buffer solutions more alkaline than 2.00. The CuII Dawson HPA was successfully immobilized in a polypyrrole matrix exhibiting good stability and clear redox activity for the CuII HPA. Poly(N-methylpyrrole) was found to be the most suitable conducting polymer film for the immobilization of the FeIII Dawson HPA. The film was found to be extremely stable to electrochemical cycling in a buffer solution of pH 4.50. The redox activity of the immobilized HPA was clearly observed and well behaved. In addition to this the electrocatalytic properties of the FeIII HPA towards the reduction of nitrite, as observed in solution, was preserved in the immobilized state.

Surface Immobilization of a Tetra-Ruthenium Substituted Polyoxometalate Water Oxidation Catalyst Through the Employment of Conducting Polypyrrole and the Layer-by-Layer (LBL) Technique

A tetra Ru-substituted polyoxometalate Na10[{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2] (Ru4POM) has been successfully immobilised onto glassy carbon electrodes and indium tin oxide (ITO) coated glass slides through the employment of a conducting polypyrrole matrix and the layer-by-layer (LBL) technique. The resulting Ru4POM doped polypyrrole films showed stable redox behavior associated with the Ru centres within the Ru4POM, whereas, the POMs tungsten-oxo redox centres were not accessible. The films showed pH dependent redox behavior within the pH range 2-5 whilst exhibiting excellent stability towards redox cycling. The layer-by-layer assembly was constructed onto poly(diallyldimethylammonium chloride) (PDDA) modified carbon electrodes by alternate depositions of Ru4POM and a Ru(II) metallodendrimer. The resulting Ru4POM assemblies showed stable redox behavior for the redox processes associated with Ru4POM in the pH range 2-5. The charge transfer resistance of the LBL films was calculated through AC-Impedance. Surface characterization of both the polymer and LBL Ru4POM films was carried out using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Initial investigations into the ability of the Ru4POM LBL films to electrocatalytically oxidise water at pH 7 have also been conducted.

A stable and selective electrochemical biosensor for the liver enzyme alanine aminotransferase (ALT).

An electrochemical method to determine alanine aminotransferase (ALT) activity over its normal and elevated physiological range was developed based upon detection of L-glutamate at a glutamate oxidase-modified platinum electrode. Measurements were carried out in the presence of ALT co-substrates L-alanine and alpha-ketoglutarate and current response from either the oxidation of hydrogen peroxide or the re-oxidation of the mediator ferrocene carboxylic acid was employed. The enzyme electrode was tested over a 6-month period and found to retain 79% of its original activity towards ALT detection with >200 measurements performed over this time. Signals associated with interfering electroactive species (ascorbic acid and uric acid) were eliminated using background subtraction at a denatured glutamate oxidase enzyme electrode. The sensitivity of the device was found to be 0.845 nA U(-1) L ALT with t(90)=180 s, linear range 10-1000 U L(-1) and LOD of 3.29 U L(-1) using amperometry at E(app)=0.4 V vs. Ag/AgCl at 308 K (35 degrees C).

Redox switching of polyoxometalate-methylene blue-based layer-by-layer films.

Iron-substituted crown-type polyoxometalate (POM) [P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)](20-) has been successfully immobilized onto glassy carbon electrode surfaces by means of the layer-by-layer (LBL) technique employing the cationic redox active dye, methylene blue (MB). The constructed multilayers exhibit pH-dependent redox activity for both the anionic POM and the cationic dye moieties, which is in good agreement with their solution behavior. The films have been characterized by alternating current impedance, atomic force microscopy, and X-ray photoelectron spectroscopy, whereby the nature of the outer layer within the assemblies was found to have an effect upon the films behavior. Preliminary investigations show that the POM dye-based films show electrocatalytic ability toward the reduction of hydrogen peroxide, however, only when there is an outer anionic POM layer.

Electron Transfer to Covalently Immobilized Keggin Polyoxotungstates on Gold

Spontaneously adsorbed monolayers have been formed on gold electrodes using a Keggin polyoxotungstate with covalently attached alkanethiol linkers of two different lengths. Films of both polyoxotungstates show two well-defined reduction processes associated with the polyoxotungstate centers where the ionic liquid, [BMIM][BF4], acts as supporting electrolyte. The surface coverages are both less than that expected for a close-packed monolayer. For the short and long linkers, the voltammetric response can be described in terms of the Butler-Volmer response involving a surface confined species using standard heterogeneous electron transfer rate constants of 170 and 140 s(-1) for the first reduction and 150 and 100 s(-1) for the second reduction processes, respectively. The rate of electron transfer to a solution phase redox probe, ferrocyanide, is significantly more sensitive to the length of the linker than the rate of electron transfer to the tungstate centers. This behavior probably arises due to potential-induced changes in the film structure.

Enhancement of nitrite and nitrate electrocatalytic reduction through the employment of self-assembled layers of nickel- and copper-substituted crown-type heteropolyanions.

Multilayer assemblies of two crown-type type heteropolyanions (HPA), [Cu20Cl(OH)24(H2O)12(P8W48O184)](25-) and Ni4(P8W48O148)(WO2)](28-), have been immobilized onto glassy carbon electrode surfaces via the layer-by-layer (LBL) technique employing polycathion-stabilized silver nanoparticles (AgNP) as the cationic layer within the resulting thin films characterized by electrochemical and physical methods. The redox behaviors of both HPA monitored during LBL assembly with cyclic voltammetry and impedance spectroscopy revealed significant changes by immobilization. The presence of AgNPs led to the retention of film porosity and electronic conductivity, which has been shown with impedance and voltammeric studies of film permeabilities toward reversible redox probes. The resulting films have been characterized by physical methods. Finally, the electrocatalytic performance of obtained films with respect to nitrite and nitrate electrocatalytic reduction has been comparatively studied for both catalysts. Nickel atoms trapped inside HPA exhibited a higher specific activity for reduction.