Interfacial characteristics of the self-assembly system of poly-l-lysine 3-mercaptopropionic acid bgold electrode

The interfacial characteristics of poly-l-lysine (PL) attached on self-assembled monolayers (SAMs) of 3-mercaptopropionic acid (MPA) were studied by an electrochemical method. The results indicated that PLMPA layer inhibited partly the diffusion process of redox species in solution, and the electrode surface behaved like a microelectrode array. Its permeation effect was also strongly affected by Mg²⁺. The more Mg²⁺ ions were added into the electrolyte solution, the greater the difficulty with which the electron transfer of potassium ferricyanide took place. The three different conformations of PL on the electrode surface had different influences on the electron transfer processes of ferricyanide. PL in random coil state hindered most strongly the electron transfer behavior of ferricyanide, while the α-helical PL had nearly no effect and the effect of the β-sheet state PL was intermediate of these.     

Structure and electrochemical behavior of a flavin sulfide monolayer adsorbed on gold

The electrochemical behavior and structure of monolayers of the synthetic flavin 10-(3′-methylthiopropyl)-isoalloxazinyl-7-carboxylic acid adsorbed on gold is reported. The redox behavior of the compound is quasi-reversible. The surface concentration is estimated to be 1.5×10^?10 mol cm^?2. An electron transfer rate constant of 340 s^?1 is estimated for the cathodic process and a value of 540 s^?1 is estimated for the anodic process. The reduction potential of the monolayer is found to shift with pH as expected for a 2e^?, 2H⁺ process. The monolayers have also been characterized by X-ray photoelectron spectroscopy (XPS) and low voltage field emission secondary electron microscopy (LVFESEM). The most likely orientation would have the long axis parallel to the surface with the carboxyl group exposed to the solution. A comparison of the C1s XPS spectra at glancing and normal emission indicates that the carboxyl group is at the film surface. The LVFESEM images indicate that the molecules pack in domains that do not follow the topology of the gold grains. Semi-quantitative examination of the micrographs shows that 10–15% of the gold surface is uncovered. The bare gold substrate catalyzes the oxidation of NADH; the presence of the flavin film reduces the observed catalysis by the gold substrate.     

Electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH) at a chlorogenic acid modified glassy carbon electrode

The redox response of chlorogenic acid solution at an inactivated glassy carbon electrode was investigated and an ECE mechanism was proposed for the electrode process. It has been shown that the oxidation of chlorogenic acid at an activated glassy carbon electrode leads to the formation of a deposited layer of about 4.5×10^?10 mol cm^?2 at the surface of the electrode. Cyclic voltammetry was used for the deposition process and the resulting modified electrode retains the activity of the quinone/hydroquinone group anticipated for a surface-immobilized redox couple. The properties of the electrodeposited films, during preparation under different conditions, and the stability of the deposited film were also examined. The pH dependence of the redox activity of these films was found to be 57 mV per pH unit, which is very close to the anticipated Nernstian dependence of 59 mV per pH unit. The modified electrode exhibits potent and persistent electrocatalysis for NADH oxidation in phosphate buffer solution (pH 7.0) with a diminution of the overpotential of about 430 mV and an increase in peak current. The electrocatalytic current increases linearly with NADH concentration from 0.1 to 1.0 mM. The apparent electron transfer rate constant, k_s, and the heterogeneous rate constant for electrooxidation of NADH, k_h, were also determined using cyclic voltammetry and rotating disk electrode voltammetry, respectively.     

Electrocatalytic oxidation of dopamine at aluminum electrode modified with nickel pentacyanonitrosylferrate films,synthesized by electroless procedure

The electrocatalytic oxidation of dopamine (DA) at a home-made aluminum electrode modified with nickel pentacyanonitrosylferrate (NiPCNF) film, has been studied by electrochemical approaches. The immobilization of NiPCNF film was performed by a simple dip-coating procedure. The cyclic voltammogram of the resulting modified Al electrode prepared under optimum conditions, shows a well-behaved redox couple due to the [Ni^IIFe^III/II(CN)₅NO]^0/?1 system. The NiPCNF films, formed on the Al electrode show excellent electrocatalytic activity toward the oxidation of DA. The effect of the solution pH on the voltammetric response of DA was examined using phosphate buffer solution of different pHs. Under optimum conditions a linear calibration graph was obtained over the DA concentration range 2–33 mM. The kinetics of the catalytic reaction were investigated by cyclic voltammetry and rotating disk electrode voltammetry. The results were explained using the theory of electrocatalytic reactions at chemically modified electrodes. The rate constant for the catalytic reaction k, the diffusion coefficient of DA in the solution D, the electron diffusion coefficient in the film D_e and transfer coefficient α, were found to be 3.1×10² M^?1 s^?1, 3.4×10^?6 cm² s^?1, 2.2×10^?11 cm² s^?1 and 0.67, respectively. The interference of ascorbic acid was investigated and greatly reduced using a thin film of Nafion^® on the surface-modified electrode. Further examination of the modified electrode shows that the modifying layers (NiPCNF) on the aluminum substrate show reproducible behavior and a high level of stability during electrochemical experiments, making it interesting for analytical applications.     

Electrocatalytic oxidation of NADH at graphite electrodes modified with osmium phenanthrolinedione

We report here a detailed study concerning the electrochemical behavior of Os(4,4′-dimethyl, 2,2′-bipyridine)₂(1,10-phenanthroline 5,6-dione) complex, adsorbed on spectrographic graphite, and about its electrocatalytic activity for NADH oxidation. Cyclic voltammetric measurements, performed in aqueous phosphate buffer solutions, at different scan rates and pH values, allowed us: (i) to relate the redox response of the o-quinone ligand (phendione) to that of the Os(II) central ion; (ii) to confirm that, in aqueous solutions, the phendione based redox process globally involves two electrons and two protons; (iii) to estimate the rate constant for the heterogeneous electron transfer corresponding to the phendione redox couple (k_s≈20.1 s^?1). The second order rate constant for electrocatalytic oxidation of NADH (k_1,[NADH]=0=1.9×10³ M^?1 s^?1, at pH 6.1) as well as its pH dependence (from pH 5.5 to 8.1) were evaluated from RDE experiments, using both Koutecky–Levich and Lineweaver–Burk data interpretations.     

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.     

ESR/UV–Vis–NIR cyclovoltammetry of macrocyclic complex [CuI(bite)]BF4 at different temperatures

Using in situ ESR/UV–Vis–NIR cyclovoltammetry at different temperatures, model systems for blue copper proteins like [Cu^I(bite)]BF₄ and [Cu^II(bite)](BF₄)₂ (bite=biphenyldiimino dithioether) were studied with respect to their redox behavior. Chemically reversible processes in the electrode reaction were observed by ESR and UV–Vis–NIR spectroscopy during the repetitive redox cycling of [Cu^I(bite)]BF₄. The complex cyclovoltammetric feature of the compounds in acetonitrile can be explained by adsorption and a slow heterogeneous electron transfer with rate constant k_s=2×10^?4 cm s^?1. Spectroelectrochemical studies of the redox products at 77 K confirmed the formation of a metastable pseudo-tetrahedral [Cu^II(bite)]²⁺ intermediate. This is direct experimental proof of processes in redox reactions of the [Cu^I(bite)]⁺ within a dual-pathway square reaction scheme indicating preferable electron transfer from the pseudo-tetrahedral [Cu^I(bite)]⁺ as the initial step. The DFT optimized geometries of quasi-tetrahedral and quasi-planar [Cu^I(bite)]^+/2+ complexes are in reasonable agreement with experimental data.     

Kinetics of the ferric/ferrous electrode reaction on Nafion®-coated electrodes

Cyclic and convolution voltammetry is used to study the kinetics of the ferric/ferrous electrode reaction on Nafion® coated Au and Pt electrodes in a perchloric acid solution. The reaction is characterized by the standard rate constant k_s⁰≈10^?6 cm s^?1 of the electron transfer across the metal ∣ Nafion® interface and by the diffusion coefficient of the ferric ion in the Nafion film D^(m)=(6.7±1.4)×10^?8 cm² s^?1. The Nafion® film apparently prevents the trace anions present in the perchloric acid solution (e.g. chlorides and sulfates) reaching the electrode surface and, thereby, influencing the ferric/ferrous electron transfer reaction via the inner-sphere catalysis. In addition, the polymer phase introduces a steric factor which causes the rate of the electron transfer reaction to decrease compared to the uncoated electrode, and proportionally the ion diffusion flux toward the metal surface, so that the values of the ratio k_s⁰/D^1/2 for the coated and uncoated electrodes are comparable. It is proposed that the considerably higher rate of the electron transfer on the Nafion® coated electrode in the sulfuric acid solution is due to the catalytic effect of sulfate or hydrogensulfate anions, which are transported through the Nafion® film as the sulfate complexes of the ferric cation, i.e. FeSO₄⁺ and FeHSO₄²⁺. The catalytic effect of oxides on the Nafion® coated electrodes is demonstrated.     

The influence of surfactants on the electron-transfer reaction at self-assembled thiol monolayers modifying a gold electrode

Monolayer films of thiols with different terminal groups (CH₃, COOH and NH₂) were prepared on gold electrodes using a self-assembling procedure. Cyclic voltammetry was used to investigate the electron-transfer reaction of potassium ferricyanide at the thiol modified gold electrode in the presence of surfactants. It was found that surfactants show some effects on the electron-transfer reaction at the thiol modified electrode. Cationic surfactants can improve the reversibility of a redox reaction at the thiol modified gold electrodes, while anionic surfactants cause a little inhibition of electron transfer of the redox at both methyl and carboxyl terminal thiol modified gold electrodes. Surfactants can interact with thiol monolayers in different ways, changing the structure and properties of the monolayer film, and can further affect electron transfer at the modified gold electrode.     

Comparative study of anion-involvement in redox transformations of poly(o-phenylene-diamine) (PPD) films by a coupled electrochemical and radiometric technique

The sorption of H₂PO^?₄, HSO^?₄ and Cl^? ions in poly(o-phenylene-diamine) (PPD) film was studied and compared by the radiotracer technique using ³⁵S, ³²P and ³⁶Cl isotopes for labelling. It was found that the anion sorption in both oxidized and reduced films depends on pH, i.e. on the protonation of the film; however, the protonation and anion sorption is not an essential condition for the occurrence of the redox process.Results furnished by differential volt-radiometric measurements advocate in the favour of the assumption that the reduction of film results in the formation of three or four types of centre with different behaviour with respect to the protonation and anion sorption. It was demonstrated that PPD films are very selective for the sorption of HSO^?₄(SO^2?₄) ions in the presence of a great excess of other anions.     

Diffusionless electron transfer of microperoxidase-11 on gold electrodes

Microperoxidase-11, MP-11, is made by proteolytic digestion of cytochrome c, cyt. c. It consists of a polypeptide of 11 amino residues attached covalently to the heme. Given that MP-11 has a more exposed heme than the complete protein, it would seem that electron transfer, ET, between immobilized MP-11 and electrodes would be at least as fast as for intact cyt. c. However, while the maximal heterogeneous ET rate for immobilized cyt. c is around 1000 s^?1, that reported previously for immobilized MP-11 does not exceed 20 s^?1. This work attempts to understand this difference in measured ET rates. The MP-11 was immobilized on gold electrodes using several protocols: (electrode A) the immobilization was done following a previously published carbodiimide based recipe yielding ET rates of the order of 20 s^?1; (B) MP-11 was bound to gold electrodes by Lomant’s reagent and gave an ET rate close to 4000 s^?1; (C) physisorbed MP-11 on gold electrodes with a self assembled monolayer, SAM, of alkane thiols gave an ET rate approaching 2000 s^?1 for the shortest length alkane thiol. Inspection of the immobilization chemistries suggests that the procedure employed in producing electrodes B and C are likely to lead to a monolayer or less of immobilized MP-11 while the procedure employed for electrode A may lead to a film comprised of a multilayer of MP-11. The presence of such a film on electrode A complicates the ET process since the MP-11 in the layer adjacent to the electrode could have fast ET rates while the MP-11 in the outer layers may have significantly slower ET rates. The net result would be an apparent ET rate constant which is much smaller than the value for the first layer. The measurements and calculations are presented in support of such an interpretation.     

Electrochemical evaluation of the reaction rate between methyl viologen mediator and diaphorase enzyme for the electrocatalytic reduction of NAD+ and digital simulation for its voltammetric responses

The electrocatalytic reduction of NAD⁺ using diaphorase enzyme was studied. Methyl viologen was used as an electron transfer mediator between an electrode and the enzyme. A catalytic wave for the reduction of NAD⁺ when all the species were in the solution was measured with cyclic voltammetry at a gold-amalgam electrode which showed low background currents at negative potentials. Steady-state currents could be obtained under the conditions of slow scan rate, low methyl viologen concentration, and high NAD⁺ concentration as the electrode reaction was converted to an electrochemical-catalytic (EC′) reaction. The bimolecular rate constant for the reaction of the reduced methyl viologen with the oxidized diaphorase was estimated as 7.5×10³ M^?1 s^?1 from the slope of the current versus [MV²⁺] plot. Another slope of the current against the square root of the enzyme concentration also gave a close value of 6.7×10³ M^?1 s^?1. In the calculation of the rate constant, the stoichiometric factor when it is not one-to-one was considered. With the evaluated rate constant, digital simulation using the suggested reaction mechanism was compared with the experimentally obtained voltammograms. Satisfactory agreement indicates that the evaluation methods of the rate constant and the suggested mechanism are appropriate for the mediated enzyme-catalyzed electrochemical reactions.     

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.     

Scanning electrochemical microscope studies of dye regeneration in indoline (D149)-sensitized ZnO photoelectrochemical cells

A scanning electrochemical microscope (SECM) in the feedback and generation-collection modes was used to investigate the regeneration of photoexcited dye cations at the semiconductor/electrolyte interface in a dye-sensitized solar cell (DSSC) based on ZnO/D149. An effective dye regeneration rate constant k_ox of 9.55 × 10⁷ cm^9/2 mol^−3/2 s⁻¹ was obtained from feedback mode experiments at different wavelengths and light intensities on ZnO/D149 electrodes. Illuminated regions of the dye-sensitized electrodes could be differentiated from non-illuminated regions by local imaging in SECM generation-collection experiments with I^- as redox mediator. We also report SECM feedback measurements on non-illuminated dye-sensitized electrodes to investigate the electron transfer kinetics of dissolved redox couples at the underlying FTO substrate.     

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.     

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.     

Electrochemical behaviour of N-aminopyridinium,and the viologens N,N′-diamino-4,4′-bipyridilium and N,N′-bis(4-aminophenyl)-4,4′-bipyridilium in aqueous acid:: towards the formation of polymer hybrids of pyridine and aniline

A new polymer has been prepared by anodic polymerisation of N,N′-diamino-4,4′-bipyridilium as the tetrafluoroborate salt in aqueous acidic at a gold electrode. The polymer was electroactive and conductive, with $\text{D}$ for the inserting counter ion being 8.0×10^?10 cm^?2 s^?1 during reduction and 4.8×10^?10 cm^?2 s^?1 during oxidation. The polymer was not electrochromic, being blue–black at all impressed potentials. Yields of polymer were poor, being particularly so at low pH (0<pH<1). The best pH for electrodeposition was about 2.0. N,N′-diamino-4,4′-bipyridilium was not electroactive at negative potentials. Conversely, N-aminopyridinium yields a blue water-soluble radical on reduction, and electron transfer to N,N′-bis(4-aminophenyl)-4,4′-bipyridilium as the dichloride salt—which has a particularly stable radical cation—yields a thin film of red–black radical-cation salt. Neither of these two latter species are electroactive at positive potentials.     

Scanning electrochemical microscopy: theory and experiment for the positive feedback mode with unequal diffusion coefficients of the redox mediator couple1

Theory for the chronoamperometric positive feedback mode of the scanning electrochemical microscope (SECM) is extended to include the situation where the oxidised and reduced forms of the redox mediator couple have arbitrary diffusion coefficients. Under typical positive feedback conditions, the solution initially contains a redox-active species, R, along with excess supporting electrolyte. The potential of the tip ultramicroelectrode (UME), positioned close to an interface of interest, is adjusted to a value where R is electrolysed to produce species O at a diffusion-controlled rate. O diffuses away from the tip towards the interface, where the reverse redox reaction occurs leading to the production, and diffusional feedback of R for electrolysis at the tip electrode. When positive feedback measurements are carried out under diffusion-controlled chronoamperometric conditions, the form of the normalised current–time behaviour, at a particular tip to interface distance, is found to be sensitive to the ratio of the diffusion coefficients of the O/R couple. As a steady-state is established, the normalised current becomes independent of the diffusion coefficient ratio and depends only on the tip to interface distance. Experimental measurements on the chronoamperometric oxidation of ferrocene (Fc) in acetonitrile solution at a Pt tip UME positioned close to a Pt substrate electrode, provide support for the theoretical predictions and demonstrate that the diffusion coefficient ratio can readily be determined from SECM chronoamperometry. Although Fc and Fc⁺ are often assumed to have the same diffusion coefficients, steady-state and chronoamperometric measurements at a conventional UME yield a value of 2.0×10^?5 cm² s^?1 for the diffusion coefficient of Fc, while SECM chronoamperometry indicates that the diffusion coefficient of Fc⁺ is 1.6×10^?5 cm² s^?1.     

Application of ionic liquid–dsDNA biocomposite film for the direct electrochemistry of myglobin on carbon ionic liquid electrode

An ionic liquid (IL) and double-stranded DNA (dsDNA) composite material was used to investigate the direct electron transfer of myglobin (Mb) on carbon ionic liquid electrode (CILE). The presence of the IL–dsDNA biocomposite film on the electrode surface provided great improvement to the direct electron transfer rate of Mb with the CILE, which was due to the synergistic contributions of specific characteristics of dsDNA, IL and their interaction. The electrochemical parameters of Mb in the IL–dsDNA composite film modified electrode were carefully investigated with the charge transfer coefficient (α) and the electron transfer rate constant (k_s) calculated as 0.42 and 0.84 s⁻¹, respectively. The fabricated Mb modified electrode exhibited good electrocatalytic ability to the reduction of trichloroacetic acid and H₂O₂, which showed the potential applications in the third-generation electrochemical biosensor.