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.     

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?.     

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.     

Mass transport accompanied with electron transfer between the gold electrode modified with 11-ferrocenylundecanethiol monolayer and redox species in solution — an electrochemical quartz crystal microbalance study

An electrochemical quartz crystal microbalance (EQCM) was employed to investigate mass transport during the redox reaction of the ferrocenylundecanethiolate (FcC₁₁S(H)) monolayer modified gold electrode in solution containing other redox species. The FcC₁₁S-monolayer on gold acts as a barrier for the electron transfer between a gold electrode and Fe(CN)₆^4?/3? in solution and as a mediator for the reduction of Fe³⁺ in solution. In both cases, electrochemical current responses were complicated because the observed currents were due to the redox of both the ferrocenyl group immobilized on gold and others in electrolyte solutions. The frequency change, i.e. interfacial mass change on the gold electrode surface, was observed only during the redox of ferrocenyl groups. The complex current response was deconvoluted into the current components of the redox reaction of ferrocene and that of other redox species in solution by comparing cyclic voltammograms with the current calculated from frequency changes.     

A piezoelectric gene-sensor using actinomycin D-functionalized nano-microspheres as amplifying probes

A gene-sensing system has been developed using actinomycin D-functionalized magnetic nano-microspheres, which can interact with double-stranded DNAs (dsDNAs) anchored on the gold film electrode of an electrochemical quartz crystal microbalance (EQCM). Actinomycin D acts as a guide that leads heavy microspheres onto the dsDNAs at the EQCM film. A magnetic separation shelf could separate unreacted microspheres conveniently. The modification and DNA hybridization at EQCM electrodes were examined by microgravimetric and electrochemical methods. In this way, an outstanding change in frequency decrease has been monitored owing to the mass increase on the EQCM electrodes. The limit for the determination of target DNA could be improved from 6.2×10^?8 to 2.0×10^?12 mol l^?1 by the amplifying technique.     

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.     

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.     

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.     

Analyses of passivation films on lithium and lithium alloys by electrochemical quartz crystal microbalance

Electrochemical quartz crystal microbalance (EQCM) measurements were carried out during electrochemical deposition/dissolution of Li on the Ti and Pt substrate electrodes of AT-cut quartz crystals in a 1 M LiPF₆ in propylene carbonate (PC) and dimethoxyethane (DME) (1:1 in volume) and a 1 M LiPF₆ in ethylene carbonate (EC) and DME (1:1 in volume). The mass change during Li deposition on the Pt substrate was about half that on the Ti substrate because the alloy formation of Li–Pt suppressed passivation reactions of deposited Li. Furthermore, it was also suggested that the amount of organic compounds in passivation films formed in the PC-based electrolyte was larger than that in the EC-based electrolyte due to PC reacting more easily with Li than EC. The EQCM measurement of the passivation film formations during alloying Li with Pb electrodeposited on the Ti substrate was also conducted successfully. The Li–Pb alloy formation occurred under 0.5 V, in which the potential profile was divided into two regions associated with the phase diagram of Li–Pb alloy. The mass change due to passivation film formations increased in accordance with the changes of Li–Pb alloy compositions into Li rich phases, which were thought either to enhance the reactiveness of Li or decrease the diffusiveness of Li in Li–Pb alloy.     

Electrochemical quartz crystal microbalance study of mass transport during electrochemical oxidation of [Os(bpy)3]2+ in clay-modified electrodes

Mass transport in clay films containing [Os(bpy)₃]²⁺ cations was investigated by EQCM and crystal impedance spectroscopy. Admittance measurements on 10 μg clay films exchanged with [Os(bpy)₃]²⁺ show no change in the width or height of the conductance peaks before and after potential scans or potential steps. The [Os(bpy)₃]²⁺ exchanged films could be considered to be rigid with no change in their viscoelastic properties, and the shifts in resonant frequency in the EQCM measurements interpreted as mass changes. In an electrode coated with a 8.5 μg clay film, oxidation of the adsorbed cations resulted in a large increase in frequency, corresponding to a decrease in mass. The mass per mole of electron transferred (MPE) was ?205 g/mol, or about one third of the weight of one [Os(bpy)₃]²⁺ ion. This was consistent with the ejection of one [Os(bpy)₃]³⁺ from the film for each three [Os(bpy)₃]²⁺ cations oxidized. The ratio of cathodic charge to anodic charge of 0.7 was also consistent with the loss of one third of the oxidized [Os(bpy)₃]³⁺ ions. The mechanism of charge neutralization was dependent of the weight of the clay films. Oxidation of [Os(bpy)₃]²⁺ in a 33 μg clay film resulted in a small decrease in frequency, corresponding to an increase in mass. The MPE, +50 g/mol, was consistent with charge neutralization by adsorption of sulfate anions from the electrolyte.     

Simultaneous electrochemical and quartz crystal microbalance studies of non-conducting microcrystalline particles of trans-Cr(CO)2(dpe)2 and trans-[Cr(CO)2(dpe)2]+ (dpe = Ph2PCH2CH2PPh2) attached to gold electrodes

Simultaneous cyclic voltammetric double potential step and electrochemical quartz crystal microbalance (EQCM) experiments on water insoluble trans-Cr(CO)₂(dpe)₂ and trans-[Cr(CO)₂(dpe)₂]X (dpe = Ph₂PCH₂CH₂PPh₂, X = Cl^?, Br^? and I^?), attached as an array of microcrystals, have been employed to probe mechanistic aspects of the redox chemistry of the [trans-Cr(CO)₂(dpe)₂]^+/0 process at the electrode |solid| solvent (electrolyte) interface in a variety of aqueous electrolytes. EQCM experiments show that the oxidation of solid trans-Cr(CO)₂(dpe)₂ involves the slow incorporation of non-solvated anions from the electrolyte solution into the solid. Interestingly, on the reverse scan of cyclic voltammetric experiments, EQCM data reveal that some but not all the anions are rapidly expelled from the crystal lattice. Double potential step experiments with the neutral chromium compound confirm that the oxidation reaction is a relatively slow process. The conclusion reached from all experiments is that the reduction process predominantly expels the anions that are relatively close to the solid|solution interface. EQCM investigations of trans-[Cr(CO)₂(dpe)₂]X compounds in electrolytes containing a different anion to that in the compound show that the anion originally in the salt is rapidly replaced by the anion in the aqueous electrolyte at open circuit potential, presumably via a rapid ion exchange process. The anion from the electrolyte is then expelled and incorporated into the solid during the reduction and oxidation steps respectively.     

Voltammetric and XPS investigations of nickel hydroxide electrochemically dispersed on gold surface electrodes

A chemically modified electrode composed of mixed hydroxide and oxyhydroxide nickel film (6–8 nmol cm^?2) on the gold substrate (Au ∣ Ni) was characterized by cyclic voltammetry and XPS techniques. The gold substrate electrodes were firstly electrochemically conditioned in 0.2 M NaOH by cycling the potential between ?0.25 and 0.6 V versus SCE, then modified by cathodic electrodeposition of nickel hydroxide films. These nickel films were obtained either by voltage cycling (50 mV s^?1) between 0.0 and ?0.5 V (SCE) or at constant potential of ?0.3 or ?0.5 V using non-deaerated 50 mM Ni(NO₃)₂ solutions. X-ray photoelectron spectroscopy (XPS) characterisation and voltammetric behaviour of Au ∣ Ni electrodes in alkaline solutions are described. Continuous electrochemical cycling of the Au ∣ Ni electrodes induces significant changes of the nickel films in terms of crystallographic structures and chemical composition. Combination of XPS and electrochemical methodologies have demonstrated the ability to follow the morphological and chemical changes in alkaline solutions upon cycling potentials. Angular-dependent XPS measurements have demonstrated that electrochemical treatment induces the formation of a uniform film layer with the following chemical distribution: Au ∣ Ni(OH)₂ ∣ NiOOH. The electrocatalytic activity of the Au ∣ Ni electrodes is investigated in alkaline medium using glucose as a model compound. The favourable combination of active species such as gold and nickel leads to a sensing electrode with strong catalytic activity over a wide range of applied potentials.     

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.     

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.     

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.     

Self-assembly of (3-mercaptopropyl)trimethoxysilane on iodine coated gold electrodes

The adsorption of (3-mercaptopropyl)trimethoxysilane (MPS) has been studied on iodine coated gold electrodes. The MPS adsorption from alcoholic solution on Au(111) and iodine coated Au(111) was studied by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The electrochemical formation of MPS monolayers was studied by cyclic voltammetry on polycrystalline uncoated and coated gold electrodes with different MPS pre-treatment conditions. Lead electrochemical deposition was used to probe the defect sites of the surfaces created. The MPS-over-iodine coated gold surface produces a lower-density monolayer than the MPS over pure gold. The MPS monolayer formed electrochemically on the iodine coated gold is chemically equal to its counterpart after the iodine desorption. The MPS adsorption occurs via an AuS bond, after the partial reductive-desorption of the iodine monolayer from the iodine coated gold electrode, and produces an ordered composite monolayer of MPS/iodine. The size of the defects can be controlled by varying the electrochemical preparation conditions, using the following reaction: AuI_(ads)+MPSH_(ac)+e⁻↔AuMPS_(ads)+H_(ac)⁺+I_(ac)⁻.     

Studies on ion transport during potential cycling of a Prussian blue (inner) | polyaniline (outer) bilayer electrode by quartz crystal microbalance and Fourier transform infrared reflection spectroscopy

Ion transport during the redox switching of a Prussian blue (PB) | polyaniline (PAn) bilayer electrode has been investigated by means of an electrochemical quartz crystal microbalance (EQCM) and in situ Fourier transform infrared (FTIR) reflection spectroscopy. The movement of ions in the potential cycling of the bilayer electrode is affected considerably by the thickness ratio of PB to PAn and the electrode potential. It is at the bilayer electrode with the medium thickness ratio (2.5–3.25) of PB (165 nm) to PAn that cation (K⁺) and anion (Cl^?) move simultaneously in opposite directions. On the positive scan of such an electrode, K⁺ ions are first ejected from PB to the PAn layer into which Cl^? ions are concurrently taken from solution to maintain the charge balance. The oxidation of PAn at more positive potential leads to the expulsion of K⁺ ions to solution and the simultaneous incorporation of Cl^? ions.     

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.     

X-ray photoelectron spectroscopic investigation and electrochemistry of polynuclear indium(III) hexacyanoferrate films

An electrochemically deposited indium(III)-hexacyanoferrate (InHCF) film on glassy carbon and platinum electrodes has been investigated by X-ray photoelectron spectroscopy (XPS). Attempts were made to characterise such a polynuclear inorganic film upon its electrochemical conditioning in both reduced and oxidised forms. The indium ions were unaffected by the oxidation state of the film and were present mainly as In(III). In the oxidised form (i.e. conditioned at +0.90 V) the InHCF is unstable during XPS analysis, with Fe being reduced from Fe^III to Fe^II. Profound alterations were also observed in the chemical form of nitrogen. Although the InHCF film was grown in potassium containing solutions, a very low amount of K⁺ ions was incorporated into the inorganic lattice. This fact, along with the iron to indium ratio of 0.76±0.06 (n=6), is consistent with a potassium-free structure of `insoluble' In₄[Fe(CN)₆]₃ · xH₂O. Yet the stability of InHCF films is not very good, and the electrochemical formation of InFe(CN)₆ and KInFe(CN)₆ is probably the main cause of the film dissolution.