Preparation,characterization, and electrocatalytic properties of copper hexacyanoferrate film and bilayer film modified electrodes

Copper(II)hexacyanoferrate films have been prepared from various electrolyte aqueous solutions using consecutive cyclic voltammetry. The cyclic voltammograms recorded the direct deposition of copper(II)hexacyanoferrate films from the mixing of Cu²⁺ and Fe(CN)₆^3? ions from solutions of ten cations: Li⁺, Na⁺, K⁺, Rb⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, H⁺ and Al³⁺. An electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry were used to study the in situ growth of the copper(II)hexacyanoferrate films. The copper(II)hexacyanoferrate film showed a single redox couple that exhibited a cation effect (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺) and an anion effect (F^?, Cl^? and Br^?) in the cyclic voltammograms and formal potential of the redox couple. The electrochemical and EQCM properties of the film indicate that the redox process was confined to the immobilized copper(II)hexacyanoferrate, and the interaction between the copper(II)hexacyanoferrate film with K⁺ (monovalent cation) and Ca²⁺ (divalent cation). The electrocatalytic oxidation properties of NADH, NH₂OH, N₂H₄, SO₃^2? and S₂O₃^2? were also determined. The electrocatalytic reduction properties of SO₅^2? and S₂O₈^2? by monolayered iron, nickel, and cobalt hexacyanoferrate films, and by bilayered metal–copper hexacyanoferrate films were determined. Two-layered modified electrodes and hybrid films composed of a copper(II)hexacyanoferrate film with iron(II)hexacyanoferrate, cobalt(II)hexacyanoferrate, or nickel(II)hexacyanoferrate film were prepared, and these films caused the electrocatalytic reduction of SO₅^2? and S₂O₈^2?.     

Characterization of copper hexabromoplatinate films and their electrocatalytic properties with dopamine,NADH, and S2O32−

Copper(II) hexabromoplatinate (CuPtBr₆) films have been prepared by mixing Cu²⁺ and PtBr₆^2? ions in an aqueous KBr solution. An electrochemical quartz crystal microbalance (EQCM), a rotating ring-disk electrode, UV–visible absorption spectroscopy and cyclic voltammetry were used to study the deposition and growth mechanism of the copper(II) hexabromoplatinate films. The electrochemical and EQCM properties of the films indicate that a single redox process was confined to the immobilized copper(II) hexabromoplatinate films. The deposition of a copper hexabromoplatinate film occurs when Cu²⁺ is reduced to Cu⁺. In the aqueous KBr solution, Pt^IVBr₆^2? is reduced electrochemically to Pt^IIBr₆^4?, and the Cu⁺ reacts with the Pt^IIBr₆^4? and Pt^IVBr₆^2?species. The electrocatalytic oxidation properties of dopamine, NADH, and S₂O₃^2? were determined using the copper(II) hexabromoplatinate films. The electrocatalytic reaction of dopamine with a copper(II) hexabromoplatinate film was investigated using the rotating ring-disk electrode method.     

Comparison of the direct electrochemistry of myoglobin and hemoglobin films and their bioelectrocatalytic properties

The electrocatalytic properties of two related proteins, myoglobin (Mb) and hemoglobin (Hb), immobilized in a surfactant film (didodecyldimethylammonium bromide, DDAB) on an electrode were investigated. The direct electrochemistry of the myoglobin/DDAB and hemoglobin/DDAB films was compared and showed one redox couple, two redox couples, and three redox couples, when transferred to strong acidic, weak acidic and basic, and pH 13 aqueous solutions, respectively. The redox couples and their formal potentials are pH dependent. An electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry were used to study the in situ growth of both DDAB deposition on gold disk electrodes and myoglobin deposition on DDAB film-modified electrodes. The electrocatalytic properties were investigated and showed that the myoglobin/DDAB and hemoglobin/DDAB film modified electrodes are both electrocatalytically active for oxygen reduction, and more H₂O₂ was produced during electrocatalytic reduction using a myoglobin/DDAB film than when using a hemoglobin/DDAB film. The electrocatalysis that was active for l-cystine, N₂O, and 2,2′-dithiosalicylic acid reductions showed an electrocatalytic current developed from the cathodic peak of the redox couple at about ?0.9 V (Fe(II)/Fe(I) redox couple) in neutral and weak basic aqueous solutions. The electrocatalysis that was active for l-cysteine oxidation, showed an electrocatalytic current developed from the cathodic peak of the redox couple at about +0.2 V (Fe(III)/Fe(IV) redox couple). Mb/DDAB and Hb/DDAB film modified electrodes are electrocatalytically active for trichloroacetic acid reduction in weak acidic and basic buffered aqueous solutions through the Fe(II)/Fe(I) redox couple. However, the electrocatalytic current developed from the cathodic peak of the redox couple at a potential of about ?0.1 V (from the Fe(III)/Fe(II) redox couple) in strong acidic aqueous solutions occurs only with higher concentrations of reactant.     

Clay-modified electrodes as studied by the quartz crystal microbalance: Redox processes of ruthenium and iron complexes

The electrochemical quartz crystal microbalance (EQCM) has been employed to investigate the mass transport processes on a clay-modified electrode. The systems investigated were the redox couples of [Ru(bpy)₃]²?₆ (bpy = 2,2′-bipyridine), [Ru(NH₃)]₆^2+/3+ and [Fe(CN)₆]^{4 ?/3?}. A clay-modified electrode was prepared by depositing synthetic saponite onto a gold coated quartz crystal. An electrode was allowed to swell for more than 50 h in 0.01 M Na₂SO₄ or NaCl or NaClO₄ prior to the adsorption and electrochemical measurements. The EQCM results revealed that the charge balancing during a redox reaction was accomplished by leaching or incorporating mobile ions in the clay film. For the [Ru(bpy)₃]^2+/3 couple, one [Ru(bpy)₃]^3? molecule was eliminated from the clay film when three [Ru(bpy)₃]²⁺ ions were oxidized. For the [Ru(NH₃)₆]^{2+ 3+} couple, one SO₄² ion, which was co-adsorbed with the ruthenium complex, was removed from the clay film when two [Ru(NH₃)₆]³⁺ molecules were reduced. For the [Fe(CN)₆]^4?/3? couple, a part of the excess charge generated by the initial oxidation of [Fe(CN)₄]^4? was canceled out by the elimination of a sodium ion bound by a clay layer. No mass transfer was detected during the oxidation at the later stage. This was probably because sodium ions in the aqueous medium within a clay film carried excess charge. The results are discussed in relation to the reported electrochemical behavior of these metal complexes.     

Assembly,electrochemical characterisation and electrocatalytic ability of multilayer films based on [Fe(bpy)3]2+, and the Dawson heteropolyanion, [P2W18O62]6−

Successive adsorption onto a glassy carbon electrode of the Dawson heteropolyanion, [P₂W₁₈O₆₂]^6?, and the multiply charged cation [Fe(bpy)₃]²⁺, resulted in the formation of stable multilayer assemblies on the electrode surface. Surface coverages were found to be typical of monolayer coverages for multilayer systems. Cyclic voltammetric studies of the assembly in aqueous 0.5 M NaHSO₄, gave a range of redox couples associated with the Fe^3+/2+ redox system, of the cationic [Fe(bpy)₃]²⁺ moiety and the tungsten-oxo framework of the Dawson parent heteropolyanion, [P₂W₁₈O₆₂]^6?. It was possible to immobilise up to thirty monolayers, with the system exhibiting well-behaved redox behaviour. The stability of the assembly towards redox switching was investigated, with it being found to be extremely stable once the outer layer is anionic in nature. The immobilised film was also tested for electrocatalytic activity for the reduction of nitrite, hydrogen peroxide and bromate, and was shown to be an efficient electrocatalyst.     

Electrochemistry of alumina pillared clay modified electrodes

A simple method for preparing metal oxide pillared clay films on graphite electrodes, based on the use of preformed pillared structures, is described. The films exhibited very good electroactivity when immersed in solutions containing redox active cationic species, e.g. Fe(bpy)₃²⁺, Os(bpy)₃²⁺, Ru(NH₃)₆³⁺ and methyl viologen. Aging a pillared clay film, loaded with electroactive cations, in de-ionized water or varying the concentration of the supporting electrolyte have pronounced effects on the voltammetric behavior of the pillared clay films. These effects demonstrate clearly the key role of ion-paired electroactive cations for the electroactivity of pillared clay films. The pH dependence of the observed electroactivity for an alumina pillared clay film arises in part from the binding of the electroactive cations at the surface hydroxyl groups of the alumina pillars. This binding favors further formation of electroactive ion pairs. The voltammetric response from anionic redox couples, observed in acidic solutions, is attributed to the amphoteric nature of the alumina pillars. Discrepancies in the voltammetric behavior of silica and alumina pillared montmorillonite are explained in the light of different degrees of attachment of the redox active cations to the surface hydroxyl groups of the two metal oxide pillars. A model describing the charge transport in metal oxide pillared clays is also presented.     

Polynuclear nickel hexacyanoferrates: monitoring of film growth and hydrated counter-cation flux/storage during redox reactions

An electrochemical quartz crystal microbalance (EQCM) was employed to monitor directly the growth of nickel(II) hexacyanoferrate(III) (NiHCNFe) films on gold substrates during electrodeposition as well as a result of sol-gel aggregation in colloidal nickel ferricyanide solutions used for modification. Frequency changes due to mass changes of the gold/crystal working electrode were correlated with cyclic voltammetric (CV) data. Evidence is also provided for the sorption of counter-cations (Li⁺, Na⁺ and K⁺), and associated water molecules, during redox reactions of the film. There is a strict relationship between the amount of alkali metal ions incorporated into the film during reduction, or excluded from the film during oxidation, and the frequency changes during EQCM measurements. The amount of solvent (H₂O) transferred and sorbed in the NiHCNFe film reflects the degree of hydration of the investigated counter-ions. Anions also seem to participate in NiHCNFe electrochemistry, but their role is much less pronounced.     

Characterization and electrocatalytic properties of cobalt hexacyanoferrate films immobilized on Au-colloid modified gold electrodes

An electroactive cobalt hexacyanoferrate (CoHCF) film was electrodeposited from a solution containing Co²⁺ and Fe(CN)₆^3? ions on the bare gold or the Au-colloid modified electrode. The cation (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) and the anion (F^?, Cl^? and Br^?) effects on the redox peak of the CoHCF film were investigated in detail. On the other hand, the electrocatalytic oxidations of thiosulfate at the CoHCF/gold and CoHCF/Au-colloid/gold electrodes were compared. At the CoHCF/Au-colloid/gold electrode, we obtained a response current larger by a factor of 2 and a three times lower detection limit than those at a CoHCF/gold electrode. The linear ranges were 1.0 × 10^?4 to 2.8 × 10^?3 M for the CoHCF/gold electrode and 7.5 × 10^?5 to 4.8 × 10^?3 M for the CoHCF/Au-colloid/gold electrode. These results showed that the immobilized CoHCF at the Au-colloid modified electrode exhibited a higher catalytic activity and a wider linear range toward thiosulfate. Additionally, the effects of the applied potential and the solution pH were studied.     

EQCM study of electropolymerization and redox cycling of 3,4-polyethylenedioxythiophene

Polyethylenedioxythiophene films were electropolymerized potentiostatically (E=1.1–1.3 V/SHE) at a gold electrode covering an EQCM from solutions containing monomer, lithium perchlorate and non-ionic surfactant (polyoxyethylene-10-laurylether). The ion exchange was studied by EQCM during redox cycling at a scan rate of 20–50 mV s^?1 in 0.5 M LiClO₄+0.5 M LiCl+0.5 M Na toluenesulfonate solutions in the potential range ?0.5<E<0.8 V (SHE). Mass versus charge curves display a hysteresis. During the reduction scan the ion content of the film is in equilibrium and is defined by the potential, whereas during oxidation the mass lags behind the charge due to slow water exchange. The reduction scan can be described by equilibrium theory supposing that about 40% of counter-ions remain bound in the film. During polymerization at E≥1.2 V, the mass gain calculated from the frequency change slows down more rapidly than is estimated from the current. This may be attributed to the viscosity of the growing film with acoustic decay length of 2.45 μm.     

Study of the electrocatalytic reduction of nitrite with silicotungstic heteropolyanion

Three couples of reversible redox waves of the SiW₁₂O^4?₄₀ anion which are composed of two one-electron and one two-electron processes occur in the potential range +0.1 to ?0.7 V in aqueous solutions. The first (most positive) and third (most negative) redox waves exhibit good electrocatalytic activities for nitrite reduction in acid solutions with pH < 2 for the former and with $\text{pH}\text{? 4}$ for the latter. The behavior of the third catalytic wave, which is quite unusual, was studied in detail. The rate constant governing the reduction of NO^?₂ by the first wave of SiW₁₂O^4?₄₀ was measured by an ultramicrodisk electrode as 3.73 × 10³ M^?1 s^?1.     

PbTe electrodeposition studied by combined electrochemical quartz crystal microbalance and cyclic voltammetry

Electrodeposition of PbTe thin films from alkaline solutions, and the electrochemical behavior of the related precursors TeO₃^2? and PbEDTA^2?, were studied for the first time by means of an electrochemical quartz crystal microbalance (EQCM) combined with cyclic voltammetry. PbTe was found to form by an induced codeposition mechanism via six electron reduction. The reduction mechanism of Te was complicated and sensitive to the substrate surface. On the other hand, the reduction of the PbEDTA^2? complex to Pb⁰ was a simple two electron reaction.     

Ion-channel-mimetic sensor for trivalent cations based on self-assembled monolayers of thiol-derivatized 4-acyl-5-pyrazolones on gold

Self-assembled monolayers (SAMs) of thiol-derivatized 4-acyl-5-pyrazolone on gold electrodes were used for ion-channel-mimetic sensing of inorganic cations. The SAMs on gold electrodes were characterized with reductive-desorption and capacitance measurements. The pH dependence of the cyclic voltammograms (CVs) of [Fe(CN)₆]^3? and [Ru(NH₃)₆]³⁺ as electroactive markers suggested the protonation not only of the β-diketone part but also of the nitrogen moiety in the pyrazolone. With regard to the voltammetric responses to metal cations, an increase and a decrease in redox current of [Fe(CN)₆]^3?/4? and [Ru(NH₃)₆]^3+/2+, respectively, were observed with increasing concentrations of the trivalent cations examined (La³⁺, Gd³⁺, Yb³⁺ or Al³⁺) from 10^?6–10^?4 up to 10^?2 M at pH 5.5, at which the chelating group is present as its deprotonated form. In contrast, such responses were negligible in the presence of up to 10^?2 M divalent cation (Mg²⁺, Ca²⁺, Sr²⁺ or Ba²⁺), Li⁺ or Na⁺. The order of the magnitudes of the responses was Al³⁺>Yb³⁺≈Gd³⁺>La³⁺?divalent cations, which is quite similar to that of the stability of 1:1 and 1:2 complexes between a β-diketonate-type chelate and the metal ions. The highly selective responses to trivalent cations seem to reflect the selectivity of the chelating group as well as the large change in the surface charge induced by the complexation.     

Preparation and characterization of ruthenium oxide/hexacyanoferrate and ruthenium hexacyanoferrate mixed films and their electrocatalytic properties

Polynuclear mixed-valent films of ruthenium oxide/hexacyanoferrate $(\mathrm{RuO}/\mathrm{Fe}(\mathrm{CN}{)}_6^{3-})$ and ruthenium hexacyanoferrate (RuHCF) have been prepared using repetitive cyclic voltammetry. The deposition process and the films’ electrocatalytic properties in electrolytes containing various cations have been investigated. The cyclic voltammograms recorded the deposition of the mixed ruthenium oxide/hexacyanoferrate and ruthenium hexacyanoferrate films directly from the mixing of Ru³⁺ and $\mathrm{Fe}(\mathrm{CN}{)}_6^{3-}$ ions from the acidic aqueous solutions containing various anions. The electrochemical quartz crystal microbalance, cyclic voltammetry, and UV–Vis spectroscopy were used to study the growth mechanism of the mixed ruthenium oxide/hexacyanoferrate and ruthenium hexacyanoferrate films. The mixed ruthenium oxide/hexacyanoferrate and ruthenium hexacyanoferrate films exhibited four redox couples with formal potentials that demonstrated a proton effect in acidic aqueous solution and an anion effect. The electrochemical quartz crystal microbalance results indicated that the redox process was confined to the immobilized mixed ruthenium oxide/hexacyanoferrate and ruthenium hexacyanoferrate film. Electrocatalytic reactions of dopamine, epinephrine, norepinephrine, ${\mathrm{S}}_2{\mathrm{O}}_8^{2-}$, and ${\mathrm{SO}}_5^{2-}$ were carried out by the mixed ruthenium oxide/hexacyanoferrate and ruthenium hexacyanoferrate films. The electrocatalytic oxidation of ethyl alcohol, isopropyl alcohol, l-cysteine, and ascorbic acid by the mixed ruthenium oxide/hexacyanoferrate and ruthenium hexacyanoferrate films too was investigated.     

Novel voltammetric responses of a 1,1′-ferrocene bis(methylene aza-18-crown-6) receptor to group 1 and 2 metal cations in acetonitrile

A 1,1′-ferrocene bis(methylene aza-18-crown-6) ligand (L) and its inclusion complexes with Na⁺, K⁺, Ba²⁺ and Mg²⁺ are studied in acetonitrile by cyclic and square wave voltammetry. The ligand-metal complex stoichiometry is found to vary from 1:1 to 1:2 with increasing concentration of the metal cation. There is also a binding selectivity for dications over monocations. Simultaneous addition of Mg²⁺ and Ba²⁺ gives rise to a novel voltammetric feature which is rationalised by the formation of three different species; the symmetrical homobimetallic complexes, L—2Ba²⁺ and L—2Mg²⁺, and the asymmetrical heterobimetallic complex, L—Ba²⁺—Mg²⁺. L is therefore a redox active receptor capable of the simultaneous electrochemical recognition of different metal cations. An electrostatic model is proposed to account for the observed changes in the mean potential of L upon cation complexation.     

Electrochemical characteristics of a cobalt pentacyanonitrosylferrate film on a modified glassy carbon electrode and its catalytic effect on the electrooxidation of hydrazine

Electrochemically prepared thin films of cobalt pentacyanonitrosylferrate (CoPCNF) were used as surface modifiers for glassy carbon electrodes. The electrochemical behavior of a CoPCNF-modified glassy carbon electrode was studied by cyclic voltammetry; the modified electrode shows one pair of peaks with a surface-confined characteristic in 0.5 M KNO₃ as supporting electrolyte. The effect of different alkali metal cations in the supporting electrolyte on the behavior of the modified electrode was studied and the transfer coefficient (α) and charge transfer rate constant (k_s) for the electron transfer between the electrode and modifier layer were calculated. The experimental results show that the peak potential and peak current vary with different alkali metal cations, but anions such as Cl^?, NO₃^?, CH₃COO^?, H₂PO₄^?/HPO₄^2? and SO₄^2? at 0.5 M concentration have no effect on the peak potential and peak current. An extensive study showed that the response of the modified electrode is not affected within a pH range of 2–8. The CoPCNF films on glassy carbon electrodes show excellent electrocatalytic activity toward the oxidation of hydrazine in 0.5 M KNO₃. The kinetics of the catalytic reaction were investigated by using cyclic voltammetry, rotating disk electrode (RDE) voltammetry and chronoamperometry. The average value of the rate constant for the catalytic reaction and the diffusion coefficient were evaluated by different approaches for hydrazine.     

An old workhorse of oxide investigations: new features of Co3O4

The electrochemical behaviour of Co₃O₄ from exhaustive cobalt nitrate decomposition (T=260–850°C) is investigated on graphite-supported electrodes, mainly relying on quasi-reversible results. Two well defined 1?e^? redox systems are observed related to Co²⁺/Co³⁺ and Co³⁺/Co⁴⁺ reactions of non-equivalent oxide surface sites. Oxide stoichiometry is directly estimated by voltammetry in ‘wet’ conditions and depends on preparation temperature as in ex situ solid state data. Adsorption of surface intermediates involved in charge transfer is examined. Langmuir and Temkin-type adsorption isotherms are obeyed for the Co²⁺/Co³⁺ and Co³⁺/Co⁴⁺ redox systems, respectively. Unusually negative (attractive) lateral interaction parameters are calculated for the latter system. Separation of oxide and graphite currents is achieved to permit real electrode surface area determination.     

Alkaline-earth cation transfer assisted by monensin across the water ∣ 1,2-dichloroethane interface in the presence of alkali cations

The electrochemical transfer of alkaline-earth cations (Me²⁺) assisted by monensin (HX) across the water ∣ 1,2-dichloroethane (DCE) in the presence of alkali cations (M⁺) is reported. A shift in the peak potential for Ba²⁺ transfer and an increase in ΔE_p from 0.030 to 0.060 V are the principal effects produced by the presence of M⁺. These findings depend on pH and the relative concentrations of both cations. The following chemical exchange reaction M_(w)⁺+HX_(c)+H₂O?MX_(o)+H₃O⁺ coupled to the electrochemical transfer of Ba²⁺ accounts for ΔE_p=0.060 V of the peak observed as a consequence of the net charge transfer of +1 at the interface. These results are in agreement with the hyper-Nernstian slope of 60 mV found for Ca²⁺ and Ba²⁺ ion selective electrodes based on antibiotics with carboxylic groups at intermediate pH values. A theoretical equation for the dependence of Δ_o^wφ_1/2 for Me²⁺ with pH and M⁺ activity was developed.     

Electrosynthesis of polyfluorene in an ionic liquid and characterization of its stable electrocatalytic activity for formic acid oxidation

An air and moisture stable ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF₆) was utilized as an electrolyte for the direct anodic oxidation electropolymerization of fluorene. The corresponding electroactive polyfluorene films showed good redox activity and structural stability. ¹H NMR and FTIR spectra together with density functional theory calculations indicated that the polymerization of fluorene occurred at the C₂ and C₇ positions. Scanning electron microscopy results demonstrated that smooth and compact polyfluorene films were obtained, due to the use of bmimPF₆ as both the growth medium and the electrolyte. Doped polyfluorene films were examined to oxidize formic acid in 0.5 mol/L sulfuric acid by cyclic voltammetry and demonstrated stable electrocatalytic activity, showing their potential use in fuel oxidation.     

Study of the electrochemical deposition and properties of cobalt oxide species in citrate alkaline solutions

Glassy carbon electrode substrates coated with thin films of cobalt oxyhydroxide were investigated in moderately alkaline medium by cyclic voltammetry. The cobalt oxide formation process was accomplished in alkaline solution (30 mM NaOH) containing millimolar concentrations of CoCl₂ and ligand species such as sodium citrate. The cobalt oxide/hydroxide films were obtained either by voltage cycling between 0.4 and 1.1 V versus SCE or under potentiostatic conditions at moderate positive potentials (i.e. 0.5 V). The resulting electrodeposited film was characterized electrochemically in alkaline solutions by cyclic voltammetry and, independently of the electrodeposition procedure adopted, reveals three well-defined sets of redox waves related to the Co(OH)₂/Co₂O₃, Co₂O₃/CoOOH and CoOOH/CoO₂ electrochemical transitions. The charge/discharge processes and the oxygen evolution reaction during continuous potential cycling were severely inhibited in the presence of increasing concentrations of electroinactive electrolytes (i.e. NaNO₃, K₂SO₄, Na₃PO₄, etc.). The electrochemical behaviour of the electrodeposited cobalt oxide/hydroxide film in alkaline solution and in the presence of various organic molecules was also investigated.