Preparation,characterization and electrocatalytic properties of polynuclear mixed-valent ruthenium oxide/hexacyanoruthenate film modified electrodes

Polynuclear mixed-valent ruthenium oxide/ruthenocyanide (ruthenium oxide/hexacyanoruthenate or mvRuO/RuCN) films were prepared using consecutive cyclic voltammetry directly from the mixing of Ru³⁺ and Ru(CN)₆^4? ions from solutions of two divalent cations (Ba²⁺ and Ca²⁺), and seven monovalent cations (H⁺, Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, and Ga⁺). The films exhibited three redox couples with Ba(NO₃)₂ or BaCl₂ aqueous solutions, and the formal potentials of the redox couples showed a cation and pH effect. An electrochemical quartz crystal microbalance (EQCM), cyclic voltammetry, UV–visible spectroscopy, and the stopped-flow method (SFM) were used to study the growth mechanism of the mvRuO/RuCN films. The results indicated that the redox process was confined to the immobilized ruthenium oxide/ruthenocyanide. The EQCM results showed a Ba²⁺ ion exchange reaction for the two most negative redox couples. The electrocatalytic reduction properties of SO₅^2?, and S₂O₈^2? by the ruthenium oxide/ruthenocyanide films were determined. The electrocatalytic oxidation of NADH and dopamine were also determined, and revealed two different types of properties. The electrocatalytic oxidations of SO₃^2?, S₂O₃^2?, and N₂H₄ were also investigated. The electrocatalytic reactions of the ruthenium oxide/ruthenocyanide films were investigated using the rotating ring-disk electrode method.     

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.     

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.     

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.     

The electrochemical properties of dopamine,epinephrine, norepinephrine,and their electrocatalytic reactions on cobalt(II) hexacyanoferrate films

The electrochemical properties of epinephrine, dopamine, and norepinephrine in aqueous solutions with various pH values have been investigated. The second redox couple of epinephrine shows an obvious reversible redox, with the formal potentials being pH dependent in the range 3≤pH≤13. The electrochemical properties of dopamine, epinephrine, and norepinephrine were pH dependent, and they exhibited different cyclic voltammograms. Cobalt(II) hexacyanoferrate films were electrocatalytically oxidation active for dopamine, epinephrine, and norepinephrine in aqueous solutions, with the electrocatalytic oxidation current developing through the Co(II)CNFe(III) film. Cobalt(II) hexacyanoferrate also shows reversible electrocatalytic properties. Such films electrocatalytically reduce those oxidation products of dopamine, epinephrine, and norepinephrine that are produced from the electrocatalytic oxidation of these compounds by a cobalt(II) hexacyanoferrate film. Cobalt(II) hexacyanoferrate modified films are also electrocatalytically oxidation active for dopamine, epinephrine, and norepinephrine, and their electrochemical properties were investigated using the rotating ring-disk electrode method.     

Microgravimetric monitoring of transport of cations during redox reactions of indium(III) hexacyanoferrate(III,II): Radiotracer evidence for the flux of anions in the film

We demonstrate here that, primarily, electrolyte cations but also, to some extent, anions are capable of penetrating indium hexacyanoferrate films during redox reactions. We find from electrochemical quartz crystal microbalance measurements that the electrolyte cation (K⁺) undergoes sorption and desorption during the system's reduction and oxidation, respectively. The formal potential, which has been determined from the system's well-defined voltammetric peaks recorded with the use of an ultramicroelectrode, decreases ～40 mV per decade of decreasing K⁺ concentration. The latter value is lower than the 60 mV change expected for the involvement of a cation in the reaction mechanism according to the ideal Nernstian dependence. We also demonstrate, using 35-S labelled sulfate, that anion penetrates the reduced film and its concentration markedly increases during oxidation. Careful examination of cyclic voltammetric responses of the system shows that, in addition to the well-defined peaks, capacitance-like currents appear during oxidation. During reduction anion is largely expelled from the film. This complex ionic penetration and transport in indium hexacynoferrate may be explained in terms of formation of two forms: ‘soluble’, KIn^III[Fe^II(CN)₆], and ‘insoluble’ (‘normal’), In^III₄[Fe^II(CN)₆]₃, during the electrochemical growth or potential cycling of the films. These forms would require cations and anions, respectively, to provide charge balance during reactions. Regardless of the actual mechanism, penetration of anions cannot be neglected completely in the discussion of charging of indium hexacyanoferrate.     

Hybrid nickel hexacyanoferrate/polypyrrole composite as mediator for hydrogen peroxide detection and its application in oxidase-based biosensors

The synthesis and electrochemical characterization of a new organic/inorganic hybrid material was performed by combining polypyrrole and a hexacyano metalate (nickel hexacyanoferrate (NiHCNFe)) aiming to obtain an electrocatalyst for H₂O₂ reduction in the presence of either Na⁺ or K⁺ ions. The use of this material as a redox mediator in an oxalate biosensor based on the immobilization of oxalate oxidase enzyme was also discussed. The electrochemical properties of the hybrid material were investigated by using impedance measurements and compared with those of the nickel hexacyanoferrate film alone. The electrocatalytic properties of the hybrid for reducing H₂O₂, in the presence of both Na⁺ and K⁺ ions, are higher than those of the NiHCNFe film due to the presence of polypyrrole chains that enhances the electronic conductivity of the material.     

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.     

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.     

Electrochemical preparation and characterization of electrodes modified with mixed hexacyanoferrates of nickel and palladium

Mixed nickel/palladium hexacyanoferrates have been prepared both as thin films and bulk precipitates (powders) attached to electrode surfaces. The mixed material does not seem to be a simple mixture of hexacyanoferrates of nickel and palladium, and it shows unique voltammetric and electrochromic characteristics when compared with the respective single-metal hexacyanoferrates. Electrodeposition of a mixed film is achieved by potential cycling in the solution for modification containing nickel(II), palladium(II) and hexacyanoferrate(III). It comes from elemental analysis that, in general, the stoichiometric ratios of nickel to palladium in mixed metal hexacyanoferrate films reflect relative concentrations of Pd(II) and Ni(II) in the solutions for modification. In the case of the films that have been electrodeposited from the solutions containing palladium ions in amounts lower or comparable with those of nickel ions, the mechanism of film growth seems to involve formation of nickel hexacyanoferrate during negative potential scans followed by simultaneous insertion of palladium ions as countercations into the system. In such cases, palladium ions tend to substitute potassium countercations at interstitial positions in the electrodeposited nickel hexacyanoferrate microstructures. We have determined the following stoichiometric formula, K_1.74?2yPd^II_yNi^II_1.13[Fe^II(CN)₆] (where y<0.72) for such films. At higher molar fractions of palladium in solutions for modification, the formation of a mixed phase of nickel/palladium hexacyanoferrate (in which both nickel(II) and palladium(II) are nitrogen-coordinated within the cyanometallate lattice) is expected. This seems to be more probable than simple codeposition of separate palladium hexacyanoferrate and nickel hexacyanoferrate microstructures during the film growth. Mixed (composite) nickel/palladium hexacyanoferrate films show long-term stability as well as promising charge storage and transport capabilities during voltammetric potential cycling. Well-defined and reversible cyclic voltammetric responses have been obtained in lithium, sodium and potassium electrolytes.     

Monomolecular films of a nickel(II) pentaazamacrocyclic complex for the electrocatalytic oxidation of hydrogen peroxide at gold electrodes

Self-assembled monolayers (SAMs) of a redox active nickel(II) pentaazamacrocyclic complex 1 and mixed monolayers of 1 with ethyl disulfide have been fabricated on a gold electrode, and their electrochemical behavior has been studied by cyclic voltammetry in aqueous solutions of Na₂SO₄ and NaNO₃. The results demonstrate that the redox behavior as well as the electrocatalytic activity towards the oxidation of H₂O₂ of the SAM and mixed monolayers of 1 largely depend on the electrolyte anions, i.e. SO₄^2? and NO₃^?: the formal potential in the SO₄^2? electrolyte is about 220 mV less positive than that in the NO₃^? electrolyte, and the SAM and mixed monolayers of 1 possess an efficient electrocatalysis for the oxidation of H₂O₂ in the NO₃^? electrolyte, but not in the SO₄^2? electrolyte. In addition, a unique cyclic voltammogram with a sharp peak of inverted ‘V’ shape has been observed in the cathodic scan for the electrocatalytic oxidation of H₂O₂, largely depending on the concentration of the NO₃^? electrolyte anion and the solution pH. These electrolyte anion-dependent redox behaviors have been discussed on the basis of different coordinating tendencies of SO₄^2? and NO₃^? to the nickel(III) centre of the complex and a possible reaction mechanism for the observed electrocatalytic reaction.     

Effects of sodium sulfamate on electrodeposition of Cu(In,Ga)Se2 thin film

Cu(In,Ga)Se₂ thin films have been prepared by electrodeposition from chloride electrolytes using sodium sulfamate as a complexing agent. Cyclic voltammograms indicate that sodium sulfamate can inhibit the reduction of Cu²⁺, Cu⁺ and H₂SeO₃, and accordingly hinder the formation of copper selenides. EDS analysis reveals that with the increase of sodium sulfamate concentration in the bath, atom ratio (Cu + Se)/(In + Ga) decreases while gallium content increases, and the film composition transforms from Cu-rich to Cu-poor. SEM and Raman spectra also show that copper selenides phases in electrodeposited films diminish with the increase of sodium sulfamate concentration.     

Access in clay-modified electrodes: effects of pH and film thickness on the electrochemical activity of gallery exchange ions and surface ion pairs

The effects of pH and clay film loading (thickness) on the electrochemistry of Fe(bpy)²⁺₃, A^n? ion pairs (A^n? = CH₃CO^?₂, CCl₃CO^?₂, SO^2?₄) and Ru(NH₃)³⁺₆ exchange cations in montmorillonite- and laponite-modified graphite electrodes are investigated. Included in the study is the effect of pH on the activity of Fe(CN)^3?₆ anions in a clay film. We find that the number of surface-bound Fe(bpy)²⁺₃, A^n? ion pairs capable of accessing the electrode surface decreases with increasing acidity, with the more basic CH₃CO^?₂ counter-anions exhibiting the greater pH dependence. In contrast, the electroactivity of Ru(NH₃)³⁺₆ exchange ions intercalated in the gallery regions of the clay film is pH independent. Also, the fraction of electroactive Fe(bpy)²⁺₃, A^n? ion pairs is essentially independent of clay film loading, indicating that the electroactive sites are confined to a narrow region of the film close to the electrode surface. However, the electroactivity of the more mobile Ru(NH₃)³⁺₆ exchange ions increases with film loading. Fe(CN)^3?₆ anions are easily displaced from a clay film by washing, but at pH 3.0–4.0 the complex anion decomposes to give a cyclic voltamogram indicative of Prussian blue (Fe₃[Fe(CN)₆]₂) deposited on the clay surface. These differences in electrochemical behavior for Fe(bpy)²⁺₃, A^n? ion pairs, Ru(NH₃)³⁺₆ exchange cations and Fe(CN)^3?₆ anions are inconsistent with the clay-layer protonation mechanism proposed previously to explain the decrease in electrochemical activity with decreasing pH for Ru(bpy)²⁺₃, SO^2?₄ and K⁺,Fe(CN)^3?₆ salts at clay-modified electrodes. Instead, the pH dependence for M(bpy)²⁺₃ species is most likely due to changes in the degree of ion-pair formation caused by the protonation of the A^n? counter-ions. These results, together with those of earlier studies, provide insights regarding the electrochemical accessibility of redox active species in clay-modified electrodes.     

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.     

Solvent effects on the solid-state electrochemistry of samarium (III) hexacyanoferrate (II)

An electroactive polynuclear inorganic compound of a rare earth metal hexacyanoferrate, samarium (III) hexacyanoferrate (II) (SmHCF), was prepared chemically. The electrochemical characteristics of SmHCF mechanically attached to the surface of a graphite electrode were studied in organic solvents, such as tetrahydrofuran, acetone and N,N-dimethylformamide, etc., using cyclic voltammetry and ac impedance. The cyclic voltammograms of SmHCF in organic solvent were well defined and exhibited a pair of redox peaks, which corresponded to the redox reaction of the Fe(II)/Fe(III) couple in SmHCF, and the redox reaction was accompanied by insertion of a counter-cation into the SmHCF during the reduction and its exclusion upon oxidation. The redox potentials increased linearly with the increase of the dielectric constant of solvents. The transport behavior of K⁺, Na⁺ and Li⁺ counter-cations through the ion channel of SmHCF were also studied by voltammetry.     

Electrochemical behavior of iodide at a nickel pentacyanonitrosylferrate film modified aluminum electrode prepared by an electroless procedure

The surface of an aluminum disk electrode was modified by a thin film of nickel pentacyanonitrosylferrate and used for electrocatalytic oxidation of iodide. The cyclic voltammogram of the modified Al electrode showed surface redox behavior due to the [Ni^IIFe^III/II(CN)₅NO]^0/1? redox couple. The modifying layer shows excellent catalytic activity toward the oxidation of iodide. Different supporting electrolytes containing different alkali metal cations affected the apparent formal potential of the redox films and thus, changed the thermodynamic tendency and kinetics of the modifying film toward the catalytic oxidation of iodide. This was explained by including the concept of a surface coverage normalized-catalytic current. The kinetics of the catalytic reaction were investigated by cyclic voltammetry and rotating disk electrode voltammetry in a suitable supporting electrolyte. The results were explained using the theory of electrocatalytic reactions at chemically modified electrodes. The heterogeneous rate constant for the catalytic reaction, k, diffusion coefficient of iodide in solution, D, and transfer coefficient, α, were found to be 5.8 × 10² M^?1 s^?1, 1.3 × 10^?5 cm² s^?1 and 0.66, respectively. In addition the effect of electrode surface coverage on the dynamic range of a calibration curve was investigated. Under optimum conditions a linear calibration graph was obtained over an iodide concentration range of 2–100 mM.     

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.     

Modification of carbon electrodes with zinc hexacyanoferrate

Electrodes modified with metal hexacyanoferrates find a unique place in the contemporary electrochemistry in view of their multifarious applications. The modification of carbon substrates with thin films of zinc hexacyanoferrate (ZnHCF) is reported for the first time. Stable surface modification is favoured when the Zn²⁺/Fe(CN)₆^3? ratio is 1:1, while deviation from this ratio leads to interesting electrocrystallisation phenomena. K⁺ ion has a facile entry and exit into the lattice of ZnHCF compared to other cations such as Na⁺, Li⁺, NH₄⁺ etc.     

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.     

Electroactive endohedral metallofullerene film electrodes in aqueous solutions

The aqueous electrochemistry of Dy@C₈₂ films and membrane films of didodecyldimethylammonium bromide (DDAB) encapsulated with Dy@C₈₂ has been studied systematically. The redox responses of the Dy@C₈₂ films depend strongly on the supporting electrolytes. In the aqueous solution of tetrabutylammonium bromide, we obtained four cathodic waves at 0.040, ?0.40, ?1.04, and ?1.35 V, and one oxidation wave at +0.31 V. For the membrane films of Dy@C₈₂–DDAB film, however, the well-defined redox responses in water are relatively independent of the supporting electrolytes because of the encapsulation and fine dispersion of Dy@C₈₂ in the DDAB membrane. Reversible reduction and oxidation waves were obtained even in the pH 7.0 buffer solution containing KCl, which is associated with the charge balance of the DDAB ions with the ions of Dy@C₈₂. It was found that Dy@C₈₂ in the DDAB membrane promotes the encapsulation of hemoglobin (Hb) and Hb can catalyze the reduction of Dy@C₈₂. Significantly enhanced electrocatalytic reduction of O₂ by Hb in the presence of Dy@C₈₂ has also been demonstrated. Here Dy@C₈₂ acts an electron-transfer mediator, which points to the potential application of metallofullerenes in biochemical sensing.