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In this contribution, a nanoarchitectural approach was employed to produce a nanolayer of polyoxometalate (POM) on the surface of a glassy carbon electrode (GCE) to achieve a higher surface area with higher electrocatalytic activity toward the electrochemical hydrogen evolution reaction (HER). To accomplish this, the well-known layer-by-layer (LbL) technique was employed, which involved the alternate adsorption of the POM, Na

The Dawson-type sulfate polyoxometalate (POM) [S2W18O62]4- has successfully been entrapped in polypyrrole (PPy) films on glassy carbon electrode (GCE) surfaces through pyrrole electropolymerization. Films of varying POM loadings (i.e., thickness) were grown by chronocoulometry. Film-coated electrodes were then characterized using voltammetry, revealing POM surface coverages ranging from 1.9 to 11.7 x 10-9 mol center dot cm-2, and were stable over 100 redox cycles. Typical film morphology and composition were revealed to be porous using atomic force microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy, and the effects of this porosity on POM redox activity were probed using AC impedance. The hybrid organic- inorganic films exhibited a good electrocatalytic response toward the reduction of iodate with a sensitivity of 0.769 mu A center dot cm-2 center dot mu M-1. Funding Agencies|Dundalk Institute of Technology

The Dawson-type sulfate polyoxometalate (POM) [S

Reaction of the mixed-valent Mn12-acetato complex [MnIII8MnIV4O12(CH3COO)16(H2O)4] with the trilacunary Wells-Dawson-type heteropolytungstate [P2W15O56]12- in acidic acetate solution (pH 1.1) resulted in the tetra-MnIII-containing polyanion [MnIII4(H2O)2(P2W15O56)2]12- (1). Single-crystal XRD on Na12[MnIII4(H2O)2(P2W15O56)2]·84H2O (1a) revealed that four MnIII ions form a rhombic Mn4O16 core encapsulated by two [P2W15O56]12- units. X-ray photoelectron spectroscopy (XPS) data confirm the +3 oxidation state of the four manganese ions in 1. Magnetic measurements from 1.8-300 K in a 100 Oe magnetic field allowed for the extraction of full fitting parameters from the susceptibility data for 1. The negative Ja value (Ja = -2.16 ± 0.08 K, Jb = 3.24 ± 1.73 K, g = 2.35 ± 0.040, and ρ = 0.34 ± 0.03) suggests a dominant antiferromagnetic spin exchange interaction between the four MnIII ions, with the positive Jb being an accompanying result of Ja. Electrochemical studies revealed a reversible MnIV/MnIII redox couple in 1 at the +0.80 to +1.1 V potential region with E1/2 = +0.907 V.

Ruthenium (II) metallodendrimer (RuDen) and the wheel-shaped Cu20-Tungstophosphate [Cu20Cl(OH)24(H2O)12(P8W48O184)]25− heteropolyanion (HPA) were successfully immobilised on a glassy carbon electrode through employment of the Layer-by-Layer (LBL) deposition method. The fabricated (Cu20P8W48/RuDen) multilayer films have been investigated with cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and surface based techniques. The layers exhibited well behaved redox activity for both the HPA and RuDen moieties and exhibited good stability towards redox cycling and associated thin layer behaviour. The charge transfer resistance (Rct) and Warburg Impedance (Zw) obtained values for the layers by EIS indicated an increase the compactness of the layers upon construction. The (Cu20P8W48/RuDen) multilayer films exhibited electrocatalytic ability towards the reduction of iodate.

peer-reviewed The nickel substituted (Ni4[(P8W48O184)(WO2)]28–, crown type polyoxometalate was electrochemically polymerised with conducting polymer pyrrole for the electrocatalytic reduction of bromate in water. The immobilized films of different thickness were characterised by electrochemical and surface based techniques. The resulting films were found to be extremely stable towards redox switching between the various redox states associated with the incorporated POM. This system was found to effectively electrocatalyse the reduction of bromate in water without the interference of other common anions. The detection limit was found to be 0.2 µm, with a linear region from 0.1 mm up to 2 mm bromate. The resulting POM doped polypyrrole films were found to be highly conductive by AC impedance. Surface characterization of the polymer films was carried out by using X‐ray photoelectron spectroscopy, atomic force microscopy, and scanning electron microscopy.

Reaction of the mixed-valent Mn

Graphene oxide-based nanocomposite multilayer films with polyethylene imine (PEI) and H4[SiMo12O40]4- POM interface have been investigated for their electrocatalytic reduction of chloroform in water. Resulting thin films were characterized by both electrochemical and surface based techniques. Cyclic voltammetry was employed to show the layer growth and redox behaviour of the modified electrode system with redox chemistry of the incorporated POM being present. AC impedance gave insights into the porosity and conductivity of the constructed multilayer POM system. Surface characterisation by XPS, AFM and SEM of the modified electrode was also performed to gain insights into the surface morphology and elemental composition of the modified electrode. The constructed multilayer system exhibited an electrocatalytic response to chloroform in water.

Synthesis and functionalization of magnetite nanoparticles (Fe3O4) was achieved with the view to covalently bind both cholesterol oxidase and cholesterol esterase biorecognition agents for the development of free and total cholesterol biosensors. Prior to enzyme attachment, Fe3O4 was functionalized with 3-aminopropyltriethoxysilane (APTES) and polyamidoamine (PAMAM) dendrimer. Characterization of the material was performed by FT-IR and UV spectroscopy, SEM/EDX surface analysis and electrochemical investigations. The response to cholesterol and its palmitate ester was examined using cyclic voltammetry. Optimum analytical performance for the free cholesterol biosensor was obtained using APTES-functionalized magnetite with a sensitivity of 101.9 μA mM−1 cm−2, linear range 0.1–1 mM and LOD of 80 μM when operated at 37 °C. In the case of the total cholesterol biosensor, the best analytical performance was obtained using PAMAM dendrimer-modified magnetite with sensitivity of 73.88 μA mM−1 cm−2 and linear range 0.1–1.5 mM, with LOD of 90 μM. A stability study indicated that the free cholesterol biosensors retained average activity of 98% after 25 days while the total cholesterol biosensors retained 85% activity upon storage over the same period.

A series of multilayer electrode assemblies have been fabricated through the layer by layer (LBL) technique composed of the water soluble Cu-phthalocyanine (Alcian Blue 8GX) cationic species and a range of Dawson type polyoxometalates (POMs) on glassy carbon electrodes in pH 7.0. Electrochemistry for both the cationic phthalocyanine and the incorporated POMs is clearly visible. The layers were characterized by cyclic voltammetry, AC impedance, X-ray photoelectron spectroscopy (XPS), Field Emission-Scanning Electron Microscopy (FE-SEM) and Atomic Force Microscopy (AFM). The layers were found to electrocatalytically reduce phosphate at pH 7.

Dawson-type 18-molybdodisulphate anion [S2Mo18O62](4-) has been immobilized within polypyrrole film by means of electrochemical polymerization. The polymer modified electrodes have been characteriz ...

The transition metal-substituted Krebs-type polyoxometalates (POMs) [Sb2W20M2O70(H2O)6]n−, M = Fe(III), Co(II), or Cu(II), were surface immobilized within the conducting polymer 3,4-ethylenedioxythiophene (PEDOT) on glassy carbon electrode surfaces. The immobilized films of different thicknesses were characterized by electrochemical and surface-based techniques. The inherent redox activity for the Krebs-type POMs, [Sb2W20M2O70(H2O)6]n−, M = Fe(III), Co(II), or Cu(II), that were observed in the solution phase were maintained in the polymeric PEDOT matrix. The resulting films were found to be extremely stable toward redox switching between the various POM-based redox states. The films exhibited pH-dependent redox activity and thin layer behavior up to 100 mV s–1. The films were found to be highly conductive through the employment of electrochemical impedance spectroscopy. Surface characterization of the films was carried out by X-ray photoelectron spectroscopy, atomic force microscopy, and scanning electron...

Herein we report the use of scanning electrochemical microscopy (SECM) together with electrochemical and spectroscopic techniques to develop and characterise a stable and uniformly reactive chemically modified platinum electrode for NADH electrocatalysis. In order to achieve this, a range of different approaches for thionine entrapment within an electropolymerised poly (3,4-ethylendioxythiophene) (PEDOT) film were evaluated using SECM imaging in the presence of NADH, demonstrating the uniformity of the reactive layer towards NADH oxidation. The effect of electrolyte type and time scale employed during PEDOT electropolymerisation was examined with respect to thionine loading and the resulting charge transport diffusion coefficient (DCT) estimated via chronoamperometry. These studies indicated a decrease in DCT as thionine loading increased within the PEDOT film, suggesting that charge transport was diffusion limited within the film. Additionally, thionine functionalised nanotubes were formed, providing a stable support for lactate dehydrogenase entrapment while lowering the rate of thionine leaching, determined via SECM imaging. This enabled lactate determination at Eapp = 0.0 V vs Ag/AgCl over the range 0.25–5 mM in the presence of 1 mM NAD+.

Hierarchically structured materials (HSMs) are becoming increasingly important in catalysis, separation and energy applications due to their advantageous diffusion and flux properties. Here, we introduce a facile modular approach to fabricate HSMs with tailored functional conducting polypyrrole microparticles (PPyMP). The PPyMPs were fabricated with a calcium carbonate (CaCO3) template-assisted polymerization technique in aqueous media at room temperature, thus providing a new green chemistry for producing water-processable functional polymers. The sacrificial CaCO3 template guided the polymerization process to yield homogenous PPyMPs with a narrow size distribution. The porous nature of the CaCO3 further allows the incorporation of various organic and inorganic dopants such as an electrocatalyst and redox mediator for the fabrication of functional PPyMPs. Dawson-type polyoxometalate (POM) and methylene blue (MB) were chosen as the model electrocatalyst and electron mediator dopant, respectively. Hierarchically structured electrochemical interfaces were created simply by self-assembly of the functional PPyMPs. We demonstrate the versatility of this technique by creating two different hierarchical structured electrochemical interfaces: POM-PPyMPs for hydrogen peroxide electrocatalysis and MB-PPyMPs for mediated bioelectrocatalysis. We envision that the presented design concept could be extended to different conducting polymers doped with other functional organic and inorganic dopants to develop advanced electrochemical interfaces and to create high surface area electrodes for energy storage.

Multilayer assemblies composed of the crown type polyoxometalate, Ni4[(P8W48O148)(WO2)]28−, and silver nanoparticles, have been immobilised onto glassy carbon electrode surfaces through the layer-by-layer (LBL) technique. The resulting thin films have been characterised by electrochemical and surface based techniques with the multilayers showing electrocatalytic ability towards the reduction of chlorate in solution. The layers exhibited redox activity for both the POM and silver nanoparticle moieties with the layers showing good stability towards redox cycling and thin layer behaviour. The layers were found to be highly conductive due to the presence of the nanoparticles as shown by AC impedance and resulting charge transfer resistance values (Rct) obtained for the layers. A clear and significant catalytic ability of Ni4[(P8W48O148)(WO2)]28/AgNP's assembly towards the chlorate reduction was observed with a measured sensitivity of 32.3mAcm−2/(μM).

The surface immobilization of the parent Dawson polyoxometalate (POM) as a counter-ion for the electropolymerization of polypyrrole (PPy) or as an electrode-adhered solid was utilized for voltammetric studies of the surface adhered POM in room temperature ionic liquids (RTIL). Illustrating the efficiency of intermediate stabilization, voltammetry at POM-modified electrodes in a PF6-based RTIL revealed richer redox behaviour and higher stabilization in comparison with aqueous electrolytes and with BF4-based RTIL, respectively. High stability of the POM-doped PPy towards continuous charge-discharge voltammetric redox cycles was confirmed by minor changes in film morphology observed after the cycling in RTILs. (c) 2017 Elsevier Ltd. All rights reserved.

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 POM's 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 functionalised Fe-substituted Keggin Na14[Fe4(C2O4)4(H2O)2(SbW9O33)2]·60H2O type POM termed “Fe4Ox4” has been successfully immobilised onto carbon electrode surfaces through the employment of conducting polypyrrole films and the layer-by-layer (LBL) technique. For the POM doped polypyrrole films the redox systems associated with the POM's tungsten-oxo framework was not apparent upon redox cycling, however a reversible redox couple associated with the FeIII/II redox system was clearly seen within the pH range 2–7. Organised multilayer assemblies were constructed by the employment of the layer by layer (LBL) technique through alternating anionic Fe4Ox4 layers and cationic RuII metallodendrimers with poly(diallyldimethylammonium chloride) (PDDA) employed as an initial base layer. Stable redox couples associated with both the FeIII/II and tungsten-oxo framework, for the Fe4Ox4 POM, and the RuIII/II for the metallodendrimer, were clearly observed upon layer construction and redox switching within the pH domain of 2–7. The resulting multilayer assembly showed good stability towards redox cycling. Further investigations into the multilayer assembly were undertaken by determining it is charge transfer resistance using AC-impedance voltammetry. The layer also showed catalytic ability towards the reduction of H2O2 at pH 6.5.

A series of transition metal (i.e. Cu2+ and Co2+) substituted Krebs type polyoxometalates (POMs), of the general formula [X2W20M2O70(H2O)6]n−, X = Sb or Bi, M = Co(II) or Cu(II), have been successfully immobilised onto carbon electrode surfaces through the employment of the layer-by-layer (LBL) technique. This involved the construction of alternating anionic POM, [X2W20M2O70(H2O)6]n−, layers and the cationic metallodendrimer, Ru(II)-metallodendrimer as the cationic layers, in addition to a [poly(diallyldimethylammonium chloride)] PDDA base layer. Stable multielectron redox couples associated with the W–O framework, for the Krebs type POMs, and the Ru(III/II) for the metallodendrimer, were clearly observed upon layer construction and redox switching within the pH domain of 2–6.5. The constructed multilayer assemblies exhibited pH dependent redox activity and thin layer behaviour up to 100 mV s−1. The porosity and permeability of the individual multilayer assemblies towards an anionic probe were determined by AC impedance and cyclic voltammetry. The surface morphology of each multilayer was also determined by Atomic Force Microscopy (AFM).

Novel Co-Ni based catalysts on activated carbon support were prepared using NaBH4 as a reducing agent in aqueous conditions and examined with respect to direct amperometric uric acid detection. The surface morphology and composition of the synthesised materials were examined using transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry (CV) was employed in order to confirm the typical metallic electrochemical response of the Co, Ni and Co-Ni based materials. The combination of metal hydroxides/oxides and nanoparticles in the carbon supported Co-Ni based materials were found to play a key role in uric acid determination. Upon surface confinement of the Co1Ni1/C material, uric acid sensitivity 248.2 µA mM−1 cm−2 and limit of detection 0.08 µM at Eapp=+0.4 V vs. Ag/AgCl was found by hydrodynamic amperometry over the range 0–250 µM (r2=0.9992). The sensor provided linear and reproducible behaviour over a wide range (25–575 µM) of uric acid. The composite materials showed excellent selectivity with respect to commonly found interferences with no response even at 10-fold concentration of urea, glucose and oxalate, and minimal influence of ascorbic acid (2 fold concentration). Overall, these materials are excellent candidates for direct uric acid detection in a stable, sensitive and very specific fashion over relevant physiological ranges, eliminating the pH, temperature sensitivity and lifetime issues associated with enzyme based systems. The materials are very promising for a range of applications including wound care/management and as non-enzymatic disposable uric acid test strips.

Nanocomposites of Pt:Au (ratios 1:1, 1:2, 1:3 and 1:4) on activated carbon were synthesized using a novel synthetic route, leading to materials with surface enrichment of Pt(0) with high catalytic performance for the direct electro-oxidation of glucose. The overall metallic content (Pt + Au) was 20% (w/w) and the bimetallic nanoparticles were found to be uniformly distributed in the activated carbon (80%) matrix. The surface morphology and composition of the synthesized materials were characterised using scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDX), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry was employed in order to confirm the typical metallic electrochemical response of Pt, Au and Pt–Au nanomaterials. The optimum nanocomposite material (Pt 1 Au 3 /C) for glucose electro-oxidation exhibited a sensitive (4.7 μA mM −1 cm −2 , limit of detection = 2.0 μM) response to glucose over the range 0–10 mM ( r 2 = 0.99) at E app = 0.35 V vs Ag/AgCl under neutral conditions (phosphate buffer solution, pH 7.4). The electrocatalysts showed an excellent response to glucose in comparison to other structurally related sugars (no/negligible response by xylose, maltose, galactose and fructose) and normally co-existing electroactive species (minimal response for ascorbic acid, acetaminophen, uric acid and dopamine). Overall, the materials examined demonstrated excellent glucose sensing capability, while eliminating the pH, temperature sensitivity and lifetime issues associated with enzyme based glucose systems and demonstrate great promise in the quantification of glucose in real clinical samples.

The electrochemical properties of a poly(3,4-ethylenedioxythiophene) film doped with the organic dye, thionine are investigated. It is proposed that an interaction is occurring between the Thionine and PEDOT film due to the positive shift of the thionine couple, improved film retention, and the lack of reactive amino groups. The electrocatalytic oxidation of NADH by this modified electrode was also investigated, together with glutamate sensing.

Four new organic-inorganic hybrid compounds based on dmso-coordinated heteropolytungstates as [Ru(bpy) 3 ] 2+ (Rubpy) salts ([Ru(bpy) 3 ] 2 Na 2 [Sb 2 W 22 (dmso) 4 O 72 ]·4dmso. 4H 2 0 (1a), [Ru(bpy) 3 ] 2 Na 2 [Bi 2 W 22 (dmso) 4 O 72 ]·4dmso-4H 2 O (2a), [Ru(bpy) 3 ] 2 [Sb 2 W 20 Fe 2 (dmso) 8 O 68] ·9dmso·12H 2 O (3a), [Ru(bpy) 3 ] 2 [Bi 2 W 20 Fe 2 (dmso) 8 O 68 ]·9dmso·12H 2 O (4a)} have been synthesized and characterized by IR spectroscopy, elemental analysis, thermogravimetry, cyclic voltammetry, X-ray powder diffraction and single-crystal X-ray diffraction. The structural analyses indicate that the dmso molecules are coordinated onto the polyanion frameworks through four W-O-S(CH 3 ) 2 bonds for 1a and 2a, six Fe-O-S(CH 3 ) 2 and two W-O-S(CH 3 ) 2 bonds for 3a and 4a. The compounds 1a-4a represent novel members of dmso-coordinated tungstoantimonates and tungstobismuthates prepared by routine synthetic reactions in the mixed solutions with dmso/H 2 0 (1:1, v/v). The cyclic voltammetry studies of 1a-4a in dmso/H 2 SO 4 medium using the glassy carbon electrode as a working electrode show the respective electrochemical behaviors of the W and Ru centers within 1a and 2a, and the W, Fe and Ru centers within 3a and 4a.

The redox mediator Meldola blue (MB) was entrapped into two polymers, poly-1,2-diaminobenzene (p-DAB) and poly-3,4-ethylenedioxythiophene (p-EDOT) by potential cycling and films were applied to NADH oxidation with subsequent glutamate detection using immobilized glutamate dehydrogenase. Both polymer films were tested for electrocatalysis of NADH using amperometry at Eapp=0.1 V vs. Ag/AgCl and similar response characteristics were obtained with sensitivity values of 6.1 nA μM−1, linear range up to 0.5 mM (R2=0.9972) and LOD of 50 μM. Subsequent amperometric determination of glutamate resulted in sensitivity 0.7 nA μM−1, linearity 0–100 μM and detection limit of 2 μM glutamate.

The electrochemical transformation of a Cu2+/0-system on a mercury monolayer carbon fiber electrode by means of fast cyclic stripping voltammetry (v ⩾ 1 kV s−1) was investigated. The analytical signal was increased by several orders as a result of fast catalytic regeneration of the analyte species by highly chemically active hydrogen intermediates. The addition of micromolar quantities of protolytic active and specifically adsorbing thiocyanate ions supplemented the surface concentration of the catalytic substrate. As a result, the Cu2+ detection limit decreased up to 0.10 pM for a subsecond accumulation period, corresponding to ca. 0.5 zeptomole of Cu on a cylinder microelectrode (A = 0.16 mm2).

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).

The conducting polymer, polypyrrole was successfully employed to surface immobilise the Krebs type polyoxometallate, K6NaH[Sb2W20Fe2O70(H2O)6]·29H2O, through cyclic voltammetry. The resulting K6NaH[Sb2W20Fe2O70(H2O)6]·29H2O doped polypyrrole films were found to exhibit the redox activity associated with both the Fe(III) and W–O redox centres within the POM. The former was found to be situated at potentials within the conducting part of the polymer whilst the latter redox process was in the insulating domain of the polypyrrole. The Fe(III/II) POM based redox process was found to be pH dependent. Upon redox switching of the polymer through this Fe(III/II) redox system, a process of cation insertion and expulsion into the polypyrrole matrix was observed with both the nature and concentration of the supporting electrolyte having a substantial effect upon the potential values at which this process occurred. This cation insertion–expulsion process was investigated through the application of the Electrochemical Quartz Crystal Microbalance (EQCM) technique. The result of which indicated that it was both the passage of alkali metal cations and protons from the background electrolyte into the polymer film which maintained electroneutrality within the POM polypyrrole films upon redox switching through the Fe(III/II) redox system. Finally the POM doped polypyrrole films exhibited a clear catalytic property towards the reduction of hydrogen peroxide (H2O2) with a sensitivity of 131.8 (±3.5) μA cm−2/mM with a LOD of 16.6 μM at neutral pH.

The electrochemical characterisation of films formed by co-polymerisation of [Os(2,2′-bipyridine)2XCl] (Os–PMP) where X is 3-(pyrrole-1yl-methyl)pyridine with 3-methyl thiophene or 1,2-diaminobenzene was carried out. The use of the membrane template method, which allowed growth of tubules of redox co-polymer was employed in the case of 1,2-diaminobenzene. Cyclic voltammetry allowed formation of the most stable 3-methylthiophene/Os–PMP films while chronocoulometry was used to co-polymerise Os–PMP with 1,2-diaminobenzene, resulting in stable films of co-polymer micro-tubules with thin-film behaviour up to 0.5 V s−1 (r2 = 0.9997). Surface analysis was carried out using SEM.

The iron-substituted sandwich-type polyoxometalate (POM), comprised of the main group element Sb(III) as the central heteroatom, [Sb2W20Fe2O70(H2O)6]8− (Sb2W20Fe2), is one of the Krebs-type derivatives. For the first time, the POMs' redox electrochemistry has been elucidated under acidic conditions employing cyclic voltammetry. It exhibited what is believed to be a bielectronic redox couple associated with the two Fe(III) centers followed by four-electron and two-electron redox processes, respectively, with these being attributed to redox processes of the tungsten-oxo framework. The oxidized form of this POM was found to be stable from pH 1.5 to pH 6. Release of the iron centers from the complex, namely demetallization, was observed upon reduction of the Fe(III) sites at room temperature, with an influence of the solution pH being observed. Through the technique of layer-by-layer (LBL) assembly, the POM was successfully immobilized on both quartz and glassy carbon electrode (GCE) surfaces by alternate deposition with the polyelectrolyte poly(diallydimethylammonium chloride) (PDDA). Thus-prepared multilayer films have been characterized by cyclic voltammetry (CV), UV-vis spectroscopy (UV-vis) and XPS. The electrocatalytic activities of the multilayer films containing Sb2W20Fe2 have been investigated towards the reduction of NO3− and IO3−. With an increase in the number of Sb2W20Fe2 monolayers within the assembly, the catalytic current towards the reduction of IO3− was enhanced.

A ruthenium(II) functionalized pyrrole, Ruthenium-bis-N,N′-(2,2′-bipyridyl)-N-(pyridine-4-ylmethyl-(8-pyrrole-1-yl-octyl)amine)chloride, 1, has been synthesized and characterized by spectroscopic (UV/vis, 1H NMR) techniques and cyclic voltammetry. In solution 1 gave a redox couple associated with the Ru3+/2+ redox system and an irreversible wave associated with the oxidation of the covalently linked pyrrole moiety. What is believed to be homopolymerization, redox active surface films of 1 have been prepared by electrooxidation of the monomeric solution. The resulting polymeric film exhibited clear redox activity associated with the incorporated Ru2+ redox centre, which is covalently linked ruthenium centre to the pyrrole moiety in 1. The effect of surface coverage upon the electrochemical behavior of the deposited films has been undertaken. Preliminary investigations into the homogeneous charge transport dynamics of the polymeric film deposited onto platinum microelectrodes has been undertaken. Copolymerization with 3-methylthiophene was carried out and a clear ruthenium response was seen. Films were formed by both cyclic voltammetry and chronocoulometry and studied. Various ratios were attempted but the ideal was found to be 52 : 5 mmol (3-methylthiophene: 1).

The successful formation of organized multilayer assemblies, formed from the Dawson type mixed addenda heteropolyanion, [P2W17 VIVO62]8−, and a Ru2+ pentaerythritol based metallodendrimer, [RuDen]8+, has been achieved on carbon electrode surfaces. Cyclic voltammetric studies of the assembly across a wide pH domain, namely, 2 to 7 was possible. Upon redox switching, the redox couples associated with the RuIII/II redox system, of the cationic metallodendrimer, and both the VIV/V and the tungsten-oxo framework of the heteropolyanion, were clearly evident. The multilayer assembly exhibited good stability towards both redox cycling and soaking over extended periods of time in aqueous buffer solutions. In addition, the constructed multilayer films were found to be quite compact in nature. The films were further characterized by X-ray photoelectron spectroscopy (XPS) to determine their elemental composition. Surface morphology of the multilayer films was determined by atomic force microscopy (AFM). The electrocatalytic reduction of iodate by the multilayer film was also investigated.

Multilayer assemblies of two crown-type type heteropolyanions (HPA), [Cu20Cl(OH)(24)(H2O)(12)(P8W48O184)](25-) and Ni-4(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. Funding Agencies|PRDSP Program (Institute of Technology Tallaght, Dublin, Ireland); Council of Directors TSR Program Strand 3; Dundalk Institute of Technology; CeNano grant (Linkoping University)

A series of Dawson-type heteropolyanions (HPAs) mono-substituted with transitional metal ions ((alpha 2)- [P2W17O61FeIII](8-), (alpha 2)-[P2W17O61CuII](8-) and (alpha 2)-[P2W17O61NiII](8-)) have exhibited electrocatalytic properties towards nitrate and nitrite reduction in slightly acidic media (pH 4.5). The immobilization of these HPAs into water-processable films developed via layer-by layer assembly with polymer-stabilized silver nanoparticles led to the fabrication of the electrocatalytic interfaces for both nitrate and nitrite reduction. The LBL assembly as well as the changes in the HPA properties by immobilization has been characterized by electrochemical methods. The effects of the substituent ions, outer layers and the cationic moieties utilized for the films assembly of the developed film on the performances of nitrate electrocatalysis has been elucidated. (C) 2015 Elsevier Ltd. All rights reserved. Funding Agencies|CeNano grant (Linkoping University)

The difference in electrochemical properties of three crown-type polyoxometalates multi-substituted by Fe3+, Ni2+ or Co2+ ions and their precursor has been rationalized with respect to their electrocatalytic performances studied in solution and in the immobilized state within the layer-by-layer film formed with a positively charged pentaerythritol-based Ru(II)-metallodendrimer. The film assembly was monitored with electrochemical methods and characterized by surface analysis techniques. An influence of the terminal layer on the electrode reaction and on film porosity has been observed. The electrocatalytic performance of the compounds on nitrite reduction was assessed in solution and in the immobilized state. (C) 2015 Elsevier Ltd. All rights reserved.

Copolymerisation of an Osmium (II) functionalised pyrrole moiety, Osmium -bis-N,N’-(2,2’-bipyridyl)-N-(pyridine-4-ylmethyl-(8-pyrrole-1yl-octyl)-amine)chloride (1) with 3- methylthiophene was carried out. The resulting conducting polymer film exhibited a clear redox couple associated with the Os3+/2+ response and the familiar conducting polymer backbone signature. The effect of film thickness upon the redox properties of the copolymer was investigated in organic electrolyte solutions. Scanning electron micrographs (SEM) along with energy dispersive X-ray (EDX) spectra of the copolymerised films were undertaken, both after formation and redox cycling in neutral buffer solution. These clearly show that electrolyte is incorporated into the polymer film upon redox cycling through the Os3+/2+ redox system. The Os3+/2+ response associated with the copolymer was seen to be significantly altered in the presence of ascorbic acid both in acidic and neutral pH buffer solutions. This pointed to an electrocatalytic reaction between the ascorbic acid and the Os3+ form of the copolymer. Under acidic conditions the copolymer film exhibited a sensitivity of 1.76 (± 0.05) uA / mM with a limit of detection (LOD) of 1.45 μM for ascorbic acid. Under neutral pH conditions the copolymer exhibited a sensitivity of 19.26 (±1.05) uA / mM with a limit of detection (LOD) of 1.28 μM for ascorbic acid.

The electrochemical behaviour of an Os (II) complex of the structurally well-defined conjugated polymer alternating regioregularly alkylated thiophene and 2,2’-bipyridine units (P4Os) has been elucidated in aqueous solution. In typical aqueous electrolyte systems the cyclic voltammogram of the resulting P4Os film exhibits a one electron reversible process corresponding to the Os3+/2+ redox system. However the observance of this reversible couple did depend upon the concentration of the supporting electrolyte. It was found that the Os3+ form did form ion-pairs with an anion from the electrolyte solution. Preliminary investigations into the homogeneous charge transport dynamics associated with this redox couple have been undertaken. The technique of cyclic voltammetry, and hence the Randles-Sevick expression, in conjunction with platinum microelectrodes have been employed to determine the DCT c value.

[Os(bpy)3]3[P2W18O62] has been synthesised and characterised by elemental analysis, spectroscopic (UV–vis, IR spectroscopy) and electrochemical techniques. In 0.1 M Bu4NPF6 DMSO the complex shows a series of redox couples associated with the Os3+/2+ and bipyridine ligands of the cationic [Os(bpy)3]2+ moiety and the tungsten-oxo framework of the associated Dawson parent heteropolyanion, [P2W18O62]6−. At this electrolyte concentration, the Os3+ redox form of the complex was seen to adsorb onto the electrode surface. When the electrolyte concentration is lowered to 0.01 M Bu4NPF6 in addition to the Os3+/2+ redox couple, the redox process associated with the [P2W18O62]8−/7− couple also exhibited properties indicating surface based processes were present. Electroactive films of the complex were formed on carbon macroelectrodes by the redox switching of the transition metal within the complex. Voltammetric investigations into the film's behaviour in a range of buffer solutions (pH 2.0, 4.5 and 7.0) were performed. The films were found to possess better stability in acidic pH and the same pH dependence for the tungsten-oxo framework of the heteropolyanions as in solution. Solid state electrochemical measurements on mechanically attached microparticles of the complex were performed, with the effect of both the nature and concentration of the aqueous electrolyte on this behaviour being investigated. Upon redox switching between the Os2+/3+ redox states, there is an associated insertion/expulsion of anions from/to the solution phase. Scanning electron micrographs of these solid state films were attained before and after redox cycling.

The metal complexes [Os(byp) 2 (pic)] + and [Ru(byp) 2 (pic)] + where byp is 2,2′-bipyridine and HPic is o -picolinic acid were synthesised and characterised using spectroscopic and electrochemical techniques. These complexes were then evaluated as mediators for a glucose oxidase (GOx)-based biosensor. Results demonstrate the electrocatalytic behaviour of both metal couples towards regeneration of the flavoprotein GOx (FADH 2 ) group, when co-immobilised with glucose oxidase. Surface immobilisation was achieved by potential cycling in aqueous solutions of the metal complexes at a glucose oxidase (GOx)/Nafion modified electrode. This proved successful in terms of catalytic efficiency and stability of redox sites. Kinetic parameters associated with both enzymatic and mediator reactions were estimated and the stability/performance properties of the sensor were tested.

Two functionalised calix[4]arenes, 5,11,17,23-tetra-tert-butyl-25,27-bis(2-pyridylmethoxy)calix[4]arene (L1) and 5,11,17,23-tetra-tert-butyl-25,27-bis(2-pyridylmethoxy)-26,28-dibutoxycalix[4]arene (L3), were prepared and characterised. The copper(I) complexes of both calix[4]arenes were synthesised and their reactivities were analysed and compared. The presence of the metal induced a radical in the case of L1 whereas no such radical was observed in the metal complex of ligand L3.

The Os2+ functionalised pyrrole monomer, osmium-bis-N,N′-(2,2′-bipyridyl)-N-(pyridine-4-yl-methyl- (8-pyrrole-1-yl-octyl)-amine)chloride, 1, has been synthesised and characterised by spectroscopic (UV–Vis, 1H NMR, IR spectroscopy) techniques and cyclic voltammetry. Solution phase studies of 1 gave a redox couple associated with the Os3+/2+ and an irreversible wave associated with the oxidation of the pyrrole moiety. Attempts to form stable polymeric films of 1 on vitreous carbon electrodes, by homopolymerisation, proved unsuccessful. However electroactive films of 1 were obtained by the electrooxidation of 1 through the Os3+/2+ couple in low dielectric media such as toluene + acetonitrile mixtures. The efficiency of film deposition and properties depend upon the supporting electrolyte and the solvent employed. Films formed in mixtures of acetonitrile and low dielectric constant solvents (dichloromethane and toluene) are electrochemically active and exhibit a redox couple associated with the Os3+/2+ system. The voltammetric behaviour of these solid-state films was investigated in both aqueous and non-aqueous solvents with a variety of supporting electrolytes. Upon redox switching between the Os3+/2+ redox states the solid-state charge-transfer processes are coupled to the insertion/expulsion of anions from/to the solution phase. Scanning electron microscopy reveals that films formed in the presence of acetonitrile exhibit different morphology from films formed in acetonitrile-free solutions. Slow and fast linear sweep voltammograms have been employed to provide an absolute determination of the fixed site concentration as 1.5 M and an apparent diffusion coefficient of 5.2 × 10−12 cm2 s−1 in 0.3 M KCl. Electroactive films of 1 were then investigated for their electrocatalytic ability towards the oxidation of ascorbic acid in acidic medium. The anodic oxidation peak current was linearly dependent on the ascorbic acid concentration and a linear calibration curve was obtained in the range 0.1–2 × 10−3 M of ascorbic acid with a correlation coefficient of 0.9993. The detection limit (3σ) was found to be 5.5 × 10−5 M.

The electrocatalytic behavior of a group of metal substituted heteropolyanions towards the oxidation of adrenaline was investigated in solution studies using both UV/vis and electrochemical techniques. Of the metal (M′) substituted Dawson phosphotungstates ([P2W17O61M′]n−) examined, iron, copper and vanadium substituted compounds accelerated the oxidation of adrenaline to adrenochrome. Both UV and electrochemical data support this conclusion, though further studies are necessary in order to deduce the exact role the metal plays in this process. The results presented here will form the basis for more thorough investigations into heteropolyanions as catalysts for catecholamines in the immobilized phase with the ultimate development of a sensor for adrenaline.

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.

Surface anchoring of an organic functionalized POM, TBA3K[SiW10O36(PhPO)2] was carried out by two methods, the layer-by-layer (LBL) assembly technique by employing a pentaerythritol-based ruthenium(II) metallodendrimer as a cationic moiety and also by entrapping the POM in a conducting polypyrrole film. The redox behavior of the constructed films was studied by using cyclic voltammetry and electrochemical impedance spectroscopy. The surface morphologies of the constructed multilayers were examined by scanning electron microscopy and atomic force microscopy. X-ray photoelectron spectroscopy was conducted to confirm the elements present within the fabricated films. The multilayer assembly was also investigated for its catalytic efficiency towards the reduction of nitrite.

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.

The electrocatalytic ability of the iron-substituted crown-type polyoxometalate (POM) Li4K16[P8W48O184Fe16(OH)(28)(H2O)(4)]center dot 66H(2)O center dot 2KCl (P8W48Fe16) towards the reduction of both nitrite and hydrogen peroxide reduction has been studied in both the solution and immobilized states for the POM. P8W48Fe16 was surface immobilised onto carbon electrode surfaces through employment of the layer-by-layer technique (LBL) using pentaerythritol-based Ru(II)-metallodendrimer [RuD](PF6)(8) as the cationic layer within the resulting films. The constructed multilayer films have been extensively studied by various electrochemical techniques and surface based techniques. Cyclic voltammetry and impedance spectroscopy have been utilized to monitor the construction of the LBL film after the deposition of each monolayer. The electrochemical behaviour of both a cationic and anionic redox probes at the LBL films has been undertaken to give indications as to the films porosity. The elemental composition and the surface morphology of the LBL films was conifmrde through the employment of AFM, XPS and SEM.

The charge transfer (CT) complexes between tetracyanoethylene (TCNE) and various pyrrole moieties (pyrrole, N-methylpyrrole and N-phenylpyrrole) have been characterised by UV-vis spectroscopy with values for their extinction coefficients (e), equilibrium constants (K), λ max values, heats of formation (ΔH), along with thermodynamic and kinetic information regarding their instability in solution being obtained. The tricyanovinylated products of the reaction between TCNE and the pyrrole moieties have been isolated and fully characterised. The three tri-cyanovinylated pyrrole species namely, 2-tricyanovinyl-pyrrole (C 4 H 4 N-C 5 N 3 ), 2-tricyanovinyl-N-methylpyrrole (C 5 H 6 N-C 5 N 3 ) and 2-tricyanovinyl-N-phenylpyrrole (C 10 H 8 N-C 5 N 3 ), have been characterised electrochemically by cyclic and normal pulse voltammetries. Each compound exhibits an irreversible anodic wave at more positive potentials than the unsubstituted monomer species along with a reversible reduction based one-electron redox couple. The calculated diffusion coefficients are similar to solution phase species, C 4 H 4 N-C 5 N 3 (3.77 × 10 -5 cm 2 s -1 ), C 5 H 6 N-C 5 N 3 (1.099 x 10 -5 cm 2 s -1 ) and C 10 H 8 N-C 5 N 3 (2.382 x 10 -5 cm 2 s -1 ). The reduction based redox processes for all synthesised compounds exhibited facile kinetics with heterogeneous rate constants, k°, of 2.65 × 10 -2 cm s-' (C 4 H 4 N-C 5 N 3 ), 1.35 × 10 -2 cm s -1 (C 5 H 6 N-C 5 N 3 ) and 7.85 × 10 -3 cm s -1 (C 10 H 8 N-C 5 N 3 ). Conducting polypyrrole and poly(N-methylpyrrole) films were grown electrochemically, by both chronocoulometry and cyclic voltammetry from reaction solutions of TCNE and the various pyrrole moieties. However, it was found that it was possible to form conducting polymer films from acetonitrile solutions containing the monomer species (0.1 M) in the absence of background electrolyte and TCNE. Upon electrochemical switching in various electrolyte solutions, between oxidised and reduced states the polypyrrole films exhibited cation movement as the predominant process that maintains charge neutrality. The latter films were unstable towards electrochemical cycling with a loss of global electroactivity after ten scans. On the other hand the poly(N-methylpyrrole) films exhibited anion movement as the predominant process upon redox switching with the films exhibiting a high level of stability towards electrochemical cycling.

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.