Ruthenium polypyridyl complexes are widely studied due to their strong light absorption, tunable emission, and rich redox chemistry. In this study, we investigate the photophysical properties of [Ru(bqp)(2)]Cl-2 (bqp = 2,6-di(quinolin-8-yl)pyridine) in the presence of the crown-shaped polyoxometalate (POM) K28Li5H7[P8W48O184].92H(2)0, shortly [P8W48O184](40-), in acidic solution (0.1 M HCl). UV-visible spectroscopy reveals a broad MLCT absorption for [Ru(bqp)(2)](2+) centred at 491 nm, while difference electronic spectroscopy upon titration with [P8W48O184](40-) (0.2-2 & times; 10(-6) M) reveals a weak new charge-transfer band at similar to 568 nm, indicating the subtle electronic communication between the two species. Steady-state luminescence measurements show efficient quenching of the (MLCT)-M-3 [Ru(bqp)(2)](2+) emission at 695 nm by [P8W48O184](40-) with near-complete extinction of emission observed at a molar ratio of 1:20 POM to Ru. Time-resolved fluorescence studies reveal a modest decrease in the excited-state lifetime, indicating a mixed static and dynamic quenching regime and a dynamic quenching constant of 2.9 & times; 10(5) M-1. The quenching mechanism is dominated by static quenching attributed to the formation of a ground-state Ru-POM complex. Upon visible-light irradiation (lambda > 390 nm), spectroscopic changes confirm photoreduction of the POM, accompanied by a progressive loss of ligand-centred (280 nm) and MLCT (similar to 495 nm) absorption features of the Ru complex, indicating oxidative changes to the sensitiser. Our results demonstrate that [Ru(bqp)(2)](2+) may serve as a visible-light sensitiser for POM photoreduction, highlighting how Ru-based photosensitisers can extend the redox chemistry of POM into the visible region, but that this activity can be accompanied by chemical modification to the sensitiser.

The systematic electrochemical investigation of Na12[(alpha-A-SiW9O34)Ni14(AleH)5(Ale)2(H2O)11(OH)7]& sdot;75H2O, (Ale = alendronate), abbreviated as (Ni14SiW9Ale7), in the solution was conducted using cyclic voltammetry (CV). Further, the Na-SiW9Ni14Ale7 polyoxometalate (POM) was immobilised within a conductive poly (3,4-ethylenedioxythiophene) (PEDOT) matrix on a glassy carbon electrode (GCE) via the chronocoulometry technique. Films of varying thicknesses were constructed, and their electrochemical properties were systematically analysed using CV, electrochemical impedance spectroscopy (EIS) and electrochemical quartz crystal microbalance (EQCM). The scan rate dependence study indicated a surface-confined electron transfer behaviour up to a scan rate of 500 mV/s. The POM-doped film exhibited pH-dependent redox activity due to proton-coupled electron transfer reactions. The Ni14SiW9Ale7-Doped PEDOT hybrid film demonstrated excellent electrochemical stability upon repeated redox cycling. EIS was employed to investigate the electrical properties of the Ni14SiW9Ale7Doped PEDOT hybrid films during the reduction and oxidation of the POM. Furthermore, In-situ EQCM analysis was conducted in a pH 2 buffer solution, demonstrating the solvation of ions associated with POM redox activity.

This study demonstrates the Layer-by-Layer approach to assemble the [M 4 (H 2 O) 2 (P 2 W 15 O 56 ) 2 ] n − (M = Mn II and Mn III ) polyoxometalates with gold nanoparticles (AuNps) decorated multi-walled carbon nanotubes (MWCNT) on carbon electrode surfaces.

Abstract Keggin‐type polyoxometalates (POMs), (n‐Bu4N)4[AsVMo11O40] (AsVMo11) and (n‐Bu4N)4H[AsV2Mo10O40] (AsV2Mo10), have been immobilised with the conducting polymer poly (3,4‐ethylenedioxythiophene) (PEDOT) on the surface of the glassy carbon electrode via chronocoulometry technique. The voltammetric behaviour of these modified electrodes was investigated with cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM). The resulting films showed the surface adsorbed behaviour and dependence on the pH of the electrolyte solution. By using AC impedance, it was found that the resulting POM‐doped PEDOT films were highly conductive. The redox switching stability of the polymer films was investigated, and it was found that the AsVMo11/PEDOT hybrid film is significantly more stable for continuous potential sweep than the AsV2Mo10/PEDOT hybrid film. Additionally, the EQCM measurements confirm that during the redox potential cycling, the anions in the supporting electrolyte replace the POMs anion doped onto the film.

Abstract In this work, Preyssler‐type POM (NH4)14[NaP5W30O110].44H2O (NH4P5W30), has been synthesised and its electrochemical behaviour in solution was examined at the surface of glassy carbon (GC) and gold electrodes. Furthermore, multilayer assemblies of NH4P5W30 POM were constructed onto the surfaces of GCE, gold electrode, and gold quartz electrode via the electrostatic Layer‐by‐Layer (LBL) technique employing polyethyleneimine as the cationic layer and POM as an anionic layer. Cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and electrochemical quartz crystal microbalance measurements (EQCM) were used to monitor the LBL assembly as the NH4P5W30 POM layer was being built. These techniques revealed significant differences in film growth. The multilayer film exhibited well‐defined redox couples associated with POM's tungsten‐oxo framework and showed surface‐confined behaviour up to 100 mVs−1 on both the GC and gold electrodes. The pH dependency and stability of the film were investigated. EIS demonstrated that when the POM layer was the outer layer, the layers were less conductive, and resistance increased as the number of layers increased. In addition, the charge transfer resistance values (Rct) for the layers were calculated. The solvation of ions into the film associated with POM redox activity was studied employing an in‐situ EQCM.

Dawson-type polyoxometalate (POM), [(n-C3H7)4N]4S2W18O62 [S2W18], was co-assembled with silver nanoparticles (AgNps) into a multilayer film on the surface of a glassy carbon electrode by using a layer-by-layer approach. The systematic growth of the multilayer film was monitored by cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The [S2W18]- AgNps multilayers exhibited excellent redox activity and good stability towards multiple redox cycling and thin layer behaviour up to 100 mV/s. CV of the multilayer film showed two redox peaks corresponding to the W units present in the POM structure. The slope values of the pH plot suggesting ca. 0.2 and 0.6 protons were related to processes I (one electron transfer) and II (two electron transfer), respectively. The multilayers were found to be highly conductive due to the presence of the nanoparticles as inferred by EIS and resulting charge transfer resistance values obtained for the layers.

Poly(3,4-ethylenedioxythiophene) (PEDOT) films were electrochemically synthesised with sodium dodecylbenzenesulfonate (DBS) and chloride acting as dopant anions within the polymer matrix. Upon redox switching of the PEDOT/DBS film between conducting and non-conducting states, the DBS anion remained within the polymer and cation insertion and expulsion occurred, as confirmed by Electrochemical Quartz Crystal Microbalance (EQCM) measurements. Electrolytes composed of alkali metal cations of varying masses (Li+, Na+, K+) were employed to investigate the cation insertion/expulsion processes, thereby resulting in varying mass changes being observed upon film redox switching. The charging and discharging of bulky anion doped polymer films presented higher capacitance upon charging and lower capacitance when discharging, which is expected during doping and de-doping as confirmed by AC impedance. In this work, the main results obtained by chemical-physical characterisation are presented and critically discussed, with regard to the possible use of a viable conducting polymer as a drug delivery vehicle.

Keggin-type polyoxometalate (POM) containing anilinium cation, (ANIH)5[PCu(H2O)W11O39](ANI).8H2O, has been synthesised and analysed using FT-IR and UV-vis spectroscopy. A thin film of (ANIH)5[PCu(H2O)W11O39] (ANI).8H2O, abbreviated as PANI/[PCu(H2O)W11O39], has been formed onto glassy carbon surfaces by the electro polymerisation technique. The electrochemical behaviour of the modified electrode was investigated using various electrochemical methods, such as cyclic voltammetry and electrochemical impedance spectroscopy. Amperometric detection of iodate studies revealed that the PANI/[PCu(H2O)W11O39] modified electrode has two linear ranges of 2-20 mu M and 40-2500 mu M, as well as a limit of detection (LOD) and sensitivity of 0.8 mu M (S/N = 3) and 0.347 mu Acm  2/mu M for the lower range and 1.7 mu M (S/N = 3) and 0.014 mu Acm  2/mu M for the higher range respectively. In the presence of typical interfering analytes such as KH2PO4, KCl, NaClO3, NaNO3, Na2SO4, and NaClO4, the suggested electrochemical sensor exhibits significant catalytic activity. The modified electrode showed excellent stability, rapid amperometric response (within 3 s), and the possibility of rapid electrode preparation. Chemico-physical characterisation of the target systems was performed scanning electron micro-scopy, Fourier transform infrared spectroscopy and UV-visible spectroscopy.

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, Na0.3[N(C4H9)4]7.7 [(Mo3O8)4(O3PC(O)(C3H6NH2CH2C4H3S)PO3)4], abbreviated as [(TBA)Mo12(AleThio)4], and polyethyleneimine (PEI) polymer. This nanolayered electrode exhibited catalytic properties toward the HER in 0.5 M H2SO4 with the resulting polarization curves indicating an increase in the HER activity with the increasing number of POM layers, and the overpotential required for this reaction was lowered by 0.83 V when compared with a bare GCE. The eighth PEI/[(TBA)Mo12(AleThio)4] bilayer exhibited a significantly lower HER overpotential of -0.077 V at a current density of 10 mA cm-2. Surface characterization of the LbL-assembled nanolayers was carried out using X-ray photoelectron spectroscopy, atomic force microscopy, and scanning electron microscopy. We believe that the synergetic effect of the positively charged PEI polymer and the catalytically active molybdate POM is the cause for the successful response to the electrochemical HER.

A Mn-III-substituted Wells-Dawson-type polyoxoanion, [Mn-4(III) (H2O)(2)(P2W15O56)(2)](12-) (Mn-4(P2W15)(2)), was surface immobilised within the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) on a glassy carbon electrode (GCE) surface. The resulting immobilised films were characterised by electrochemical techniques and subjected to a compositional and morphological characterization by means of X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM). The redox activity of the entrapped Mn-4(P2W15)(2) was observed within the polymer phase. The conducting polymer films exhibited a pH-dependant redox activity associated with the polyanion Mn-4(P2W15)(2) and thin layer behaviour up to 100 mVs(-1) with enhanced stability towards redox cycling.

The investigation focused on integrating heteropolyanions (HPAs) into a polymeric matrix of polypyrrole (PPy), aiming to achieve effective immobilisation for potential electrocatalytic applications. Specifically, the successful immobilization of a Dawson-type mixed addenda HPA, alpha 2 - K8[P2W17V4+O62].10H2O, within conductive PPy films was achieved. These [P2W17V4+O62]-doped PPy films exhibited robust stability at both pH 2 and pH 7 during continuous potential cycling involving the V5+/4+ redox process. However, it was noted that in the negative potential range leading to polymer backbone reduction, the HPA tended to leach out from the film. In terms of catalytic assessments, the [P2W17V4+O62]-dopped PPy films displayed noteworthy electrocatalytic performance in the oxidation of ascorbic acid. Notably, they demonstrated a sensitivity of 8.18 (+/-0.15) mu A mM-1 and a limit of detection (LOD) of 1.65 mM at pH 2.0. Furthermore, under neutral conditions, the films exhibited a sensitivity of 6.97 (+/-0.30) mu A mM-1 and a LOD of 4.24 mM. Further, endeavours were made to immobilise alternative variations of HPAs, specifically, the crown HPA, K28Li5H7P8W48O184 & sdot;92H2O, and Cu2+ substituted Krebs HPA, KxNa(10-x)Bi2W20Cu2O70(H2O)6 & sdot;30H2O (with x ranging from 1 to 10), into PPy films. Initial findings from these attempts are also briefly discussed in this study.

Wells-Dawson type polyoxometalate (POM) [(n-C3H7)4N]4S2W18O62 (S2W18) was successfully immobilised with a conducting polymer, poly(3,4-ethylenedioxythiophene) (PEDOT), by chronocoulometry technique. The immobilised films with different surface thicknesses were characterised by various electrochemical techniques. The S2W18-doped PEDOT film gave four couples of redox waves, the first one corresponds to the redox process of the polymer itself and the other three couples are electron transfer at tungstate in the framework. The films exhibited inherent pH-dependent redox activity and stability of S2W18 in the thin layer up to 100 mV/s. Only slight changes were observed in the magnitude of peak currents after continuous redox cycling, indicating the relative stability of the S2W18 in the conducting PEDOT. The conductive behaviour of the film was investigated with electrochemical impedance spectroscopy. The study of electrochemical bromate ion sensing based on S2W18-doped PEDOT film employing chronoamperometric technique revealed that at an applied potential of -0.1 V, the S2W18-doped PEDOT film can detect bromate at concentrations between 100 μmol L−1 and 2000 μmol L−1. Moreover, the S2W18-doped PEDOT film shows a detection limit of 4 μmol L−1 without interference from other common ions present in the water. Hybrid films exhibit significant catalytic activity with high selectivity at a low reduction potential of -0.1 V. Chemico-physical characterization of the target systems was performed using atomic force microscopy, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy and Raman spectroscopy.

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 × 10-9 mol·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 μA·cm-2·μM-1.

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.

Synthesis and functionalization of magnetite nanoparticles (Fe 3 O 4 ) 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, Fe 3 O 4 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. Graphical abstract Schematic representation of cholesterol esterase and oxidase loaded magnetic nanoparticles (Fe 3 O 4 @APTES or Fe 3 O 4 @APTES-PAMAM) generating hydrogen peroxide from cholesterol palmitate.

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.

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.

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 (D-CT) estimated via chronoamperometry. These studies indicated a decrease in D-CT 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 E-app = 0.0 V vs Ag/AgCl over the range 0.25-5 mM in the presence of 1 mM NAD(+).

Modified electrodes composed of the anionic Kreb type polyoxometalates, K-10[Bi2W20CO2O70(H2O)(6)]nH(2)O, K-10[Sb2W20Co2O70(H2O)(6)]nH(2)O and K-8(Sb2W20Cu2O70(H2O)(6)]nH(2)O, and cationic Poly(ethyleneimine) (PEI) capped silver nanoparticles have been constructed through the layer-by-layer (LBL) technique onto carbon electrode surfaces. The cyclic voltammograms recorded during film construction exhibited redox activity associated with the Ag nanoparticles and each POM's tungsten-oxo four electron redox process. The modified electrodes were then investigated for their stability towards redox cycling at pH 2.5, thin layer behaviour and pH dependent redox chemistry. Electrochemical impedance spectroscopy was employed for the modified electrode system showing the relatively low charge transfer resistance (R-CT) of the POM based films.