Electrocatalytic activity of a new nanostructured polymeric tetraruthenated porphyrin film for nitrite detection

The preparation of the monomer and thin films of a new polymeric tetraruthenated porphyrin material and their characterization by spectroscopic and electrochemical techniques, are described. This material is one of the most active electrocatalyst for the oxidation of nitrite ions to nitrate, exhibiting a heterogeneous cross-exchange rate constant (k_f=(6.2±0.1×10⁴) M^-1 s^-1) 30 times higher than that previously described for the electrostatic assembled porphyrin films. The polymeric films were obtained by electropolymerization of the corresponding molecular films, previously prepared by dip-coating. This strategy leads to an increase in the efficiency of the reticulation process while minimizing the amount of monomer necessary for the preparation of the modified electrodes. The conductivity of the thin films close to the E_1/2 of the Ru(III/II) redox pair is very good, decreasing rapidly as the applied potential departs from it, as expected for a redox polymer. The conductivity decreases when the surface concentration becomes higher than 1.2×10^-8 mol cm⁻² also, reflecting a higher impedance for electrolyte diffusion inside the polymeric material. The high electrocatalytic activity associated with the high conductivity make this new nanostructured material suitable for sensor applications.     

Electrochemical nanogravimetric study of the electropolymerization of 6-aminoindole and the redox transformations of the polymer formed in aqueous media

The electrochemical quartz crystal nanobalance was employed to study the electropolymerization of 6-aminoindole on gold electrodes in acidic media. Potentiostatic or potential cycling electrooxidation of 6-aminoindole below 0.5 V vs. saturated calomel electrode leads to the formation of multilayer polymeric films, while at higher positive potentials, further oxidation takes place resulting in a different material which remains attached to the metal surface but shows decreased or no redox activity. A mechanism of the redox transformations of poly(6-aminoindole) which involves protonation–deprotonation accompanying the electron transfer is suggested. In this potential range, a yellow-green reversible color change occurs. At higher positive potentials blue-purple, indigo-type compounds are formed, and bond-breaking leads to the decrease of the electroactivity.     

Microelectrochemical electronic effects in two-layer structures of distinct Prussian blue type metal hexacyanoferrates

The formation of a rigid bilayer structure from two metal hexacyanoferrates: Prussian blue (PB) and nickel hexacyanoferrate (NiHCNFe), as inner and outer films, respectively, has been demonstrated. To avoid intermingling of the granular cyanometallate microstructures, namely of the outer film (NiHCNFe) into the inner film (PB), the morphology of outer film material was changed by forming a polymeric hybrid (composite) of NiHCNFe with poly(N-methylpyrrole). The outer NiHCNFe film is physically separated from the electrode surface, and it undergoes redox reactions at potentials characteristic of the inner PB film. This arrangement leads to the reversible charge state trapping and bistable switching during voltammetric potential cycling. Under solid-state voltammetric conditions in the absence of contact with the liquid electrolyte phase, when the bilayer structure of PB and the oxidized NiHCNFe was formed between two sandwich-forming carbon electrodes, unidirectional rectifying current flow has been observed.Dedicated to Zbigniew Galus on the occasion of his 70th birthday     

Synthesis,Characterization and Electrochemical Polymerization of a Ru2+ Functionalized Pyrrole Monomer

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

The voltammetric behaviour of solid 2,2-diphenyl-1-picrylhydrazyl (DPPH) microparticles

The electrochemical behaviour of 2,2-diphenyl-1-picrylhydrazyl (DPPH) microparticles, attached to a graphite electrode and adjacent to an aqueous electrolyte solution, has been studied by cyclic voltammetry. DPPH exhibits one reversible redox couple with a formal potential of 0.340 V versus Ag|AgCl (pH=7.0). At more positive potentials, a redox couple appears with a formal potential E_f=0.733 V versus Ag|AgCl. The oxidation at this potential is followed by an irreversible chemical reaction generating a product which gives a redox couple with a formal potential at 0.177 V versus Ag|AgCl. The reduction process of this couple is followed by a slow chemical reaction in the course of which DPPH is reformed.     

Kinetics of electron transfer reactions of metal complexes at impermeable redox active polymeric films on electrode surfaces and charge transport within the polymer film

At rotated Pt disk electrodes coated with thin films of the redox polymer poly-[Ru(vbpy)₃]²⁺, ruthenium and iron bipyridine complexes dissolved in acetonitrile can become oxidized by two pathways. The first is diffusion of the solute complex through the polymeric film to react at its normal potential E_sub⁰, at the Pt/polymer interface. The second is a mediated electron transfer cross-reaction between the solute complex and poly-[Ru(vbpy) ₃]²⁺ sites generated in the film at adequately positive potentials. The mediated reaction, as judged from the lack of variation of its rate k_crsΓ with the quantity of polymer mediator sites present in the multimolecular layer film, and from other evidence, is confined to the outer few (one?) monolayers of ruthenium polymer film sites. The mediated reaction becomes the dominant pathway for films with Γ_T ～2×10^?9 mol/cm² of ruthenium polymer sites, owing to the low permeability (measured independently) of the solutecomplexes into the film. The rate k_crsΓ could be measured when the solute complex oxidation potential is more positive than that of the redox film, and is too fast to measure when E_sub⁰, is more negative than the redox film E_cal?⁰, New theory is presented and evaluated to describe the rising portions of the voitammetric waves for the nine solute complexes studied. The rate of charge transport through the poly-[Ru(vbpy)₃]²⁺ film becomes controlling under certain conditions and can be thereby measured as well.     

Redox Active Two‐Component Films of Palladium and Covalently Linked Zinc Porphyrin–Fullerene Dyad

Redox active films have been generated electrochemically by the reduction of dyads consisting of fullerene C₆₀ covalently linked to zinc meso‐tetraphenyloporphyrin, ZnP? C₆₀, and palladium acetate. The films are believed to consist of a polymeric network formed via covalent bonds between the palladium atoms and the fullerene moieties. In these films, the zinc porphyrin moiety is covalently linked to the polymeric chains through the pyrrolidine ring of the fullerene. The ZnP? C₆₀/Pt films are electrochemically active in both positive and negative potential excursions. At positive potentials, two oxidation steps for the zinc porphyrin are observed. In the negative potential range, electron transfer processes involving the zinc porphyrin and the fullerene entities are observed. Film formation is also accompanied by palladium deposition on the electrode surface. The presence of a metallic phase in the film influences its morphology, structure and electrochemical properties.     

Electrochemistry of methanofullerenes embedded in hydrophobic ammonium cation films

The electrochemical behavior of several methanofullerenes, including methyl-C₆₁-propionic acid esters (Ph-C₆₁-(CH₂)₂COOC _n H_{2n + 1}, n = 1–4, 8), in tetraoctyl- and didodecyldimethylammonium bromide films on conducting glass was studied. A cyclic voltammetry study showed that the introduction of methanofullerene derivatives in a cation matrix led to redox transformations of these compounds at potentials accessible for aqueous solutions. The shift of their formal potentials toward positive values is mainly caused by the electrostatic interactions of the reduction products (methanofullerene anions) with the matrix cations. The chemical stability of the reduction products is provided by their hydrophobic microenvironment in the matrix in all cases. This makes it possible to perform quasireversible redox transformations in neutral aqueous media. The duration of the adjustment of the structure of the coating depends on the solvent used for its preparation, the length of the alkyl radical in the ester group of methanofullerene, and the length of the alkyl chain between the cyclopropane fragment and the COOC _n H_{2n + 1} group. The redox potentials of methanofullerene Ph-C₆₁-(CH₂)₂COOC₂H₅ incorporated in the didodecyldimethylammonium bromide film lie in the range of higher negative values relative to the potentials of the compound in the tetraoctylammonium bromide film.     

Electrochemical stability of polyaniline

The kinetics of the electrochemical degradation of polyaniline (PANI) films has been investigated in 0.5 M sulfuric acid solution at different electrode potentials ranging from 0.3 to 1.0 V vs. Ag/AgCl. Two kinds of PANI films were used, one doped with poly(styrene sulfonate) (PSS), and the other—with indigotetrasulfonate (ITS). Within a range of relatively low electrode potential (0.3-0.6 V), the degradation was found to proceed at a first-order rate constant of 4×10⁻⁵ to 5×10⁻⁵ s⁻¹, corresponding to degradation half-period of 4-5 h. A sharp increase in the degradation rate proceed by extending the electrode potential to higher values, with a maximum rate constant of ≈2.5×10⁻³ s⁻¹ for PANI-PSS, and ≈1.2×10⁻³ s⁻¹ for PANI-ITS films, obtained at a higher potential of 0.9-1.0 V. The data obtained are interpreted by different degradation rate for two distinct redox forms of PANI—emeraldine (slow degradation), and pernigraniline (fast degradation).     

Substituent effects on the redox potentials of dihydroxybenzenes: theoretical and experimental study

The redox reactions of p-hydroquinone and pyrocatechol undergo a two-proton-two-electron process in aqueous solution. We calculated their redox potentials at the B3LYP/6-311+G(d,p) level, and verified the values by employing cyclic voltammetry experiments. Then we selected seven substituent groups (–F, –Cl, –OH, –COOH, –CN, –NH₂, and –NO₂ groups) to systematically investigate the substituent effect, including the sort, position, and number of the substituent, on the redox potentials of p-hydroquinone and pyrocatechol. The calculated results show that –NH₂ and –OH groups can decrease the redox potentials, while –F, –Cl, –COOH, –CN, and –NO₂ groups increase the potential values of p-hydroquinone and pyrocatechol. The calculations can accurately predict the substituent effects on the redox potentials of pyrocatechol and p-hydroquinone. We would expect that the accurate calculation results for the model systems could be applied in the prediction of electrode potentials of other molecules.     

Electrochemistry of conducting polypyrrole films containing cobalt porphyrin

The electrochemical polymerisation of pyrrole-substituted cobalt tetraphenylporphine complex on a vitreous carbon electrode has been performed in acetonitrile+tetrabutylammonium tetrafluoroborate solution. The redox properties of the film have been examined by cyclic voltammetry and compared to those of cobalt-porphyrin monomer in solution. Voltammograms of these films exhibit a reversible process for the Co(II)/Co(I) reaction at a formal potential of −0.87 V/SCE. The cobalt-porphyrin content of the films has been estimated by cyclic voltammetry, and the conductivity of the polymers has been assessed by studying well-known electrochemical processes in solution at these modified electrodes. Thus, it appears that thick polyporphyrin films act as insulators in low potential range E < −1 V/SCE. Copolymerisation of the pyrrole-substituted cobalt porphyrin with pyrrole and 3-(pyrrol-1-ylmethyl)pyridine has been achieved. No improvement of the electrochemical properties has been noted for the copolymers obtained. We have also proved that interchain complexation reaction of the cobalt(III) sites occurs by the pyridine moieties of the copolymer films.     

Das Redoxverhalten von Bisbiphenylchrom(I)-jodid in Wasser

Bisbiphenylchromiumiodide [BBCr(I)J] is widely used as a “reference ion” in comparing redox potentials in different solvents. The voltametric behaviour in aqueous solutions however, is complicated by adsorption phenomena of the oxidized as well as the reduced form. Classical polarography, A—C polarography, cyclic voltametry, large scale electrolysis and pulsevoltametric methods have been employed to obtain information about the nature of the electrode reaction, its reversibility and the solubility of the reactants. Based on these data it is shown that the formal standard redox potential corresponds to the reactionBBCr(I)_ads/BBCr(O)_ads and has to be regarded as a “surface redox potential”.     

Interfacial energetics of electrodeposited cadmium selenide in acetonitrile

The open circuit voltage, E_V, was used to estimate the barrier height of thin film CdSe electrodeposited from aqueous selenosulfite solution in contact with various redox couples in acetonitrile. Films that were heat treated in air or oxygen had E_V values that increased approximately linearly with the formal redox potential, E°, in the range −520 mV < E° < +100 mV (SCE) to a maximum of E_v≅ 600 mV. For +100 < E° < +1300 mV, E_v is constant due to Fermi level pinning, a behavior present in single crystal CdSe. Cyclic voltammetry of thin film CdSe in the dark and the light gave useful qualitative information, but E_V values determined from the difference in the anodic peak positions on illuminated CdSe and Pt were smaller than those determined from direct measurement of the open circuit voltage. For redox potentials more negative than the flatband potential (E_FB), all films formed ohmic junctions, which are characterized by reversible electrochemical behavior in the dark and the light. Films that were annealed in the absence of oxygen behaved electrochemically as degenerate semiconductors, since there was no rectification in the dark, even for redox potentials quite positive of E_FB.     

Redox potentials of dopamine and its supramolecular complex with aspartic acid

Dopamine (DA) can be oxidized to dopamine quinone (DAquinone) through a one-step, two-electron redox reaction. The electron transfer property of DA and its supramolecular complex with aspartic acid (Asp) has been investigated by the theoretical calculations. We calculated the standard redox potentials (E ^o) of DA/DAquinone at the MP2/6-31G(d,p)//B3LYP/6-31G(d,p), MP2/6-31+G(d,p)//B3LYP/6-31+G(d,p), MP2/6-31G(d,p)//B3LYP/6-311G(d,p), and MP2/6-311+G(d,p)//B3LYP/6-311+G(d,p) levels. Comparing the experimental value, the redox potentials of DA/DAquinone obtained at MP2//B3LYP/6-311G(d,p) and MP2//B3LYP/6-311+G(d,p) levels can be considered as the upper and lower estimates. DA can form supramolecular complex (DA-Asp) with Asp through hydrogen bond (H-bond). Therefore, the values of 0.631 and 0.628 V obtained at MP2//B3LYP/6-311G(d,p) and MP2//B3LYP/6-311+G(d,p) levels for DA-Asp/DAquinone-Asp can be proposed as the upper and lower estimates of a probable (about 0.630 V) value of the corresponding redox potential. The calculated E ^o values of DA-Asp/DAquinone-Asp at the four theoretical levels are upper than those of DA/DAquinone, which indicates that the formation of H-bonds weaken the electron-donating ability of DA.     

Polymer films on electrodes: Part III. Digital simulation model for cyclic voltammetry of electroactive polymer film and electrochemistry of poly(vinylferrocene) on platinum

The electrochemistry of electrodeposited poly(vinylferrocene) (PVF) and poly(vinylferrocene acrylonitrile) (PVFAN) films on platinum electrodes was studied in acetonitrile solutions using perchlorate or p-toluenesulfonate as the counter ion by cyclic voltammetry. A model is proposed for the cyclic voltammetric behavior of polymeric films on electrode surfaces. The model incorporates non-equivalent redox sites with interconversion between such sites, electron-transfer kinetics at substrate/film interface and diffusion within the film. Parameters are obtained which yield a good fit to the experimental results.     

Evaluation of Thin Film Titanium Nitride Electrodes for Electroanalytical Applications

Titanium nitride is a hard and inert conducting material that has yet not been widely used as electrode material for electroanalytical applications although there are highly developed protocols available to produce well adherent micro and nanostructured electrodes. In this paper the possibilities of using titanium nitride thin films for electroanalytical applications is investigated. Scanning electrochemical microscope (SECM) was used for analysis of the redox kinetics of a selected fast redox couple at thin films of titanium nitride (TiN) in different thicknesses. The investigation was carried out by approaching an amperometric ultramicroelectrode (UME) to the TiN film while the soluble redox couple (ferrocenemethanol/ferrociniummethanol) served as mediator in a SECM configuration. The substrate was biased at a potential so that it rereduces the species being produced at the UME, thus controlling the feedback effect. Normalized current–distance curves were fitted to the theoretical model in order to find the apparent heterogeneous standard rate constant (k°) at the sample. The data are further supported by structural investigation of the TiN films using scanning force microscopy and X‐ray photoelectron spectroscopy. It was found that the kinetics are little influenced by prolonged storage in air. The heterogeneous standard rate constants in 2 mM ferrocenemethanol were (0.73±0.05)×10^?3 cm s^?1 for 20 nm TiN thin layer, (1.5±0.2)×10^?3 cm s^?1 for 100 nm TiN thin layer and (1.3±0.2)×10^?3 cm s^?1 for 300 nm TiN thin layer after prolonged storage in air. Oxidative surface treatment (in order to remove organic adsorbates) decreased the kinetics in agreement with a thicker oxide layer on the material. The results suggest that their direct use for amperometric detection of reversible redox systems in particular at miniaturized configurations may be advantageous.     

Comparison of electrochemical properties of two-component C60-Pd polymers formed under electrochemical conditions and by chemical synthesis

The electrochemical behavior of C₆₀-Pd polymer formed under electrochemical conditions and by the chemical synthesis was examined. In these polymers, fullerene moieties are covalently bonded to palladium atoms to form a polymeric network. Both materials deposited at the electrode surface show electrochemical activity at negative potentials due to the reduction of fullerene cage. Electrochemically formed thin polymeric films exhibit much more reversible voltammetric response in comparison to chemically synthesized polymers. The morphology and electrochemical behavior of chemically synthesized C₆₀-Pd polymer depend on the composition of grown solution. Chemical polymerization results in formation of large, ca. 50 μm, crystallic superficial structures that are composed of regular spherical particles with a diameter of 150 nm. The capacitance properties of C₆₀-Pd films were investigated by cyclic voltammetry and faradaic impedance spectroscopy. Specific capacitance of chemically formed films depends on the conditions of film formation. The best capacitance properties was obtained for films containing 1:3 fullerene to Pd molar ratio. For these films, specific capacitance of 35 Fg^?1 was obtained in acetonitrile containing (n-C₄H₉)₄NClO₄. This value is much lower in comparison to the specific capacitance of electrochemically formed C₆₀-Pd film.     

Poly(3-octylthiophene) as solid contact for ion-selective electrodes: contradictions and possibilities

The hydrophobic conductive polymer, poly(3-octylthiophene) (POT), is considered as uniquely suited to be used as an ion-to-electron transducer in solid contact (SC) ion-selective electrodes (ISEs). However, the reports on the performance characteristics of POT-based SC ISEs are quite conflicting. In this study, the potential sources of the contradicting results on the ambiguous drift and poor potential reproducibility of POT-based ISEs are compiled, and different approaches to minimize the drift and the differences in the standard potentials of POT-based SC ISEs are shown. To set the potential of the POT film, it has been loaded with a 7,7,8,8-tetracyanoquinodimethane (TCNQ/TCNQ^·?) redox couple. An approximately 1:1 TCNQ/TCNQ^·?ratio in the POT film has been achieved through potentiostatic control of the potential of the redox couple-loaded conductive polymer. It is hypothesized that once the POT film has a stable, highly reproducible redox potential, it will provide similarly stable and reproducible interfacial potentials between the POT film and the electron-conducting substrate and result in SC ISEs with excellent reproducibility and potential stability. Towards this goal, the potentials of Au, GC, and Pt electrodes with drop-cast POT film coatings were recorded in KCl solutions as a function of time. Some of the POT films were loaded with TCNQ and coated with a K⁺-selective membrane. The improvement in the potential stabilities and sensor-to-sensor reproducibility as a consequence of the incorporation of TCNQ in the POT film and the potentiostatic control of the TCNQ/TCNQ^·?ratio is reported.     

Facile cation electro-insertion into layer-by-layer assembled iron phytate films

Molecular layer-by-layer assembly from pre-saturated aqueous solutions of Fe³⁺ and phytate is employed to build up iron phytate deposits on tin-doped indium oxide (ITO) electrodes. Globular films with approximately 1 nm growth per layer are observed by AFM imaging and sectioning. In electrochemical experiments the iron phytate films show well-defined voltammetric responses consistent with an immobilised Fe(III/II) redox system in aqueous (LiClO₄, NaClO₄, KClO₄, phosphate buffer) and in ethanolic (LiClO₄, NaClO₄, NBu₄ClO₄) electrolyte solutions. The Fe(III/II) redox system is reversible and cation insertion/expulsion occurs fast on the timescale of voltammetric experiments even for more bulky NBu₄⁺ cations and in ethanolic solution. Peak shape analysis and scan rate dependent midpoint potentials suggest structural changes accompanying the redox process and limiting propagation. Iron phytate is proposed as a versatile and essentially colourless cation electro-insertion material and as a potential energy storage material.     

Monomeric and polymeric structures of oxovanadium(IV) complexes with Schiff base ligands derived from (R,R)-2,4-pentanediamine: Synthesis and crystal structure

New tetradentate Schiff base–oxovanadium(IV) complexes which have electron donating or withdrawing groups at the 5-position of the salicylaldehyde moieties, [VO{Xsal-(R,R)-2,4-ptn}] (H₂{Xsal-(R,R)-2,4-ptn}: N,N′-di-Xsalicylidene-(R,R)-2,4-pentanediamine; X=5-MeO (methoxy), 5-Br, and 5-NO₂) were prepared. The structures and redox potentials for the V(V)/V(IV) couple of the complexes were compared with those of other [VO{Xsal-(R,R)-2,4-ptn}] (X=3-EtO (ethoxy), 3-MeO, and H). The 5-MeO substituted complex which has electron donating groups at the 5-position of the salicylaldehyde moieties forms a monomeric structure in the solid state. The 3-EtO substituted complex has both monomeric and polymeric structures. On the other hand, the other [VO{Xsal-(R,R)-2,4-ptn}] (X=H, 3-MeO, 5-Br, 5-NO₂) complexes have only polymeric structures. X-ray crystal structure analysis of [VO{5-MeOsal-(R,R)-2,4-ptn}]⋅CH₃OH (1) was carried out. Complex 1 has a monomeric five-coordinate square–pyramidal structure. The six-membered N–N chelate ring forms a distorted flattened boat form with two methyl groups in the axial positions.