Voltammetric Studies of Cadmium‐ and Zinc‐Containing Metallothioneins at Nafion‐Coated Mercury Thin Film Electrodes

Voltammetric studies of rabbit liver metallothioneins (MTs, containing both Zn and Cd ions) and Zn₇‐MT were carried out at Nafion‐coated mercury film electrodes (NCMFEs). The accumulation of MT molecules into the NCMFEs enhances the voltammetric signals and the electrostatic interaction between the Nafion membrane and MT facilitates facile electron transfer reactions. Two well‐defined redox waves, with reduction potential (E_pc) values at ?0.740 and ?1.173 V, respectively, were observed. The peak at E_pc =?0.740 V is attributable to the reduction of the Cd‐MT complex, whereas that at E_pc=?1.173 V was assigned to the reduction of the Zn‐MT complex. Zn₇‐MT exhibits only one redox wave with E_pc=?1.198 V. The NCMFE was found to be more advantageous than thin mercury film electrode (MFE), because the pristine metal ions in MTs (e.g., Cd²⁺ and/or Zn²⁺) are not significantly replaced by Hg²⁺. The NCMFE is also complementary to Nafion‐coated bismuth film electrode in that it has a greater hydrogen overpotential, which allows the reduction of the Zn‐MT complex to be clearly observed. Moreover, intermetallic compound formation between Cd and Zn appears to be less serious at NCMFEs. Consequently, the amounts of Cd and Zn deposited into the electrode upon the reduction reactions can be quantified more accurately.     

Voltammetry of immobilized cytochrome c on novel binary self-assembled monolayers of thioctic acid and thioctic amide modified gold electrodes

Horse heart cytochrome c (cyt c) was adsorbed on the binary self-assembled monolayers (SAMs) composed of thioctic acid (T-COOH) and thioctic amide (T-NH₂) at gold electrodes via electrostatic interaction. The cyt c adsorbed on the modified gold electrode exhibited well-defined reversible electrochemical behavior in 10 mM phosphate buffer solution (PBS, pH 7.0). The surface concentration (Γ) of electroactive species, cyt c, on the binary SAMs was higher than that in single-component SAMs of T-COOH, and reached a maximum value of 9.2 × 10⁻¹² mol cm⁻² when the ratio of T-COOH to T-NH₂ in adsorption solution was of 3:2, and the formal potential (E^0′=(E_pa+E_pc)/2) of cyt c was −0.032 V (vs. Ag|AgCl (3 M NaCl)) in a 10 mM PBS. The interaction between cyt c and the binary SAMs made the E^0′ shift negatively when compared with that of cyt c in solution (+0.258 V vs. NHE, i.e., +0.058 V vs. Ag|AgCl (3 M NaCl)). The fractional coverage of bound cyt c was a 0.64 theoretical monolayer. The standard electron transfer rate constant of cyt c immobilized on the binary SAMs was also higher than that on single-component SAMs of T-COOH, and the maximum value of 15.8 ± 0.6 s⁻¹ was obtained when the ratio of T-COOH to T-NH₂ in adsorption solution was at 3:2. The results suggest that the electrode modified with the binary SAMs functions better than the electrode modified with single-component SAMs of T-COOH.     

Effect of Thallium Ions on Anodic Dissolution of Gold in Alkali–Cyanide Electrolytes at Potentials More Positive Than the NHE Potential

The gold dissolution rate iin solutions containing 0.1 M KOH, 0.1 M KCN, and 2.5 × 10^–7to 1.5 × 10^–5M TlNO₃is studied as a function of potential Eof the electrode whose surface is renewed prior to each experiment, the TlNO₃concentration c, and the time tof the electrode contact with solution. At cexceeding 0.5 × 10^–5M and t 0, the rate is 1.5–2 times that at c= 0. Initial portions of ivs. tcurves in the absence and presence of TlNO₃coincide only at cbelow 10^–6M. Potentiostatic and potentiodynamic measurements show that, at positive E, only small coverages of the electrode surface with thallium are obtained, which make no impact on iat E< 0 and heavily increase it at 0 < E< 0.3 V. The discovered effects are attributed to the formation, during the adsorption of oxidized thallium forms, of dipoles comprising thallium adions and gold atoms. Presumably, the dipoles face the gold with their negative ends and make the potential of zero free charge more negative.     

Anodic Dissolution of Gold in Cyanide Solutions Containing Lead Ions: Effect of the Solution pH

The effect of the electrode potential on the gold dissolution rate in alkali–cyanide solutions with and without 10^–5 M of hydroxy compounds of lead is studied. With the compounds, the process rate passes through a maximum, whose potential E _m shifts in the negative direction and whose height drops with increasing pH. The pH dependence of E _m is linear, with the slope dE _m/dpH = –71 ± 5 mV, and correlates with that of the potential at which lead adatoms start to undergo desorption from the gold surface in alkali solutions. Without the compounds, the gold dissolution rate in alkali–cyanide solutions is independent of the solution pH at E < 0. Thus, the effect of the solution pH in this potential range is connected not with a direct participation of hydroxide ions in the anodic process but is of a secondary nature caused by the dependence of the region of adsorption of catalytically active lead adatoms on the hydroxide ion content in solution.     

Anodic Dissolution of Gold in Alkali–Cyanide Solutions Containing Microquantities of Mercury Ions

An admixture of mercury ions accelerates dissolution of gold at negative (in the hydrogen scale) potentials and hinders it at positive potentials. In contradistinction to a similar effect exerted by admixtures of thallium, bismuth, and lead ions, the influence of mercury ions, all other conditions being equal, manifests itself at much longer times of contact between gold and solution. This difference is due to a low rate of the act of adsorption (discharge) of mercury ions. The rate increases at more negative potentials, and at E –1.2 V (NHE) the act accelerates to such an extent that looses the limiting role, which passes to the stage of supply of mercury ions to the electrode, as with solutions containing thallium, bismuth, and lead. Comparing these results with earlier data on the adsorption of cyanide ions on gold shows that the discharge of the Hg(CN)^2- ₄ anions stops limiting the formation of a layer of mercury atoms when the adsorption of cyanide ions turns insignificant.     

Adsorption of -histidine on copper surface as evidenced by surface-enhanced Raman scattering spectroscopy

The adsorption of -histidine on a copper electrode from H₂O- and D₂O-based solutions is studied by means of surface-enhanced Raman scattering (SERS) spectroscopy. Different adsorption states of histidine are observed depending upon pH, potential, and the presence of the SO²⁻₄ and Cl⁻ ions. In acidic solutions of pH 1.2 the imidazole ring of the adsorbed histidine remains protonated and is not involved in the chemical coordination with the surface. The SO²⁻₄ and Cl⁻ ions compete with histidine for the adsorption sites. In solutions of pH 3.1 three different adsorption states of histidine are observed depending on the potential. Histidine adsorbs with the protonated imidazole ring oriented mainly perpendicularly to the surface at potentials more positive than −0.2 V. Transformation of that adsorption state occurs at more negative potentials. As this takes place, histidine adsorbs through the α-NH₂ group and the neutral imidazole ring. The Cl⁻ ions cause the protonation and detachment of the α-NH₂ group from the surface and the formation of the ion pair NH⁺₃ … Cl⁻ can be observed. In the neutral solution of pH 7.0 histidine adsorbs through the deprotonated nitrogen atom of the imidazole ring and the α-COO⁻ group at E ≥ −0.2 V. However, this adsorption state is transformed into the adsorption state in which the α-NH₂ group and/or neutral imidazole ring participate in the anchoring of histidine to the surface, once the potential becomes more negative. In alkaline solutions of pH 11.9 histidine is adsorbed on the copper surface through the neutral imidazole ring.     

EQCM Study of Iridium Anodic Oxidation in H2SO4 and KOH Solutions

In the present study anodic oxidation of iridium layer formed thermally on a gold‐sputtered quartz crystal electrode has been investigated by electrochemical quartz crystal microgravimetry (EQCM) in the solutions of 0.5 M H₂SO₄ and 0.1 M KOH. The emphasis here has been put on the microgravimetric behavior of iridium as a metal, because a few previous EQCM studies reported in literature have been devoted to iridium oxide films (IROFs). The objective pursued here has been to elucidate the nature of the main voltammetric peaks, which occur at different ranges of potential in the solutions investigated. It has been found that anodic oxidation of iridium electrode in 0.5 M H₂SO₄ and 0.1 M KOH solutions is accompanied by irregular fluctuations of the electrode mass at 0.4 V<E<0.8 V followed by regular increase in mass at 0.8 V<E<1.2 V. The cathodic process initially, at 1.2 V>E>0.9 V, proceeds without any or with slight increase in electrode mass, whereas at E<0.8 V a regular decrease in mass is observed. It has been found that mass to charge ratio characterizing the processes of interest is 2 to 3 g F^?1in acidic medium, whereas in the case of alkaline one it is 4 to 6 g F^?1. The main pair of peaks seen in the voltammograms of Ir electrode in alkaline medium at E<0.8 V is attributable to redox transition Ir(0)→Ir(III), whereas those observed in the case of acidic medium at E>0.8 V should be related to the redox process Ir(0)→Ir(IV) going via intermediate stage of Ir(III) formation. As a consequence of these redox transitions, the gel‐like surface layer consisting of Ir(III) or Ir(IV) hydrous oxides forms on the electrode surface.     

Surface poisoning during electrocatalytic monosaccharide oxidation reactions at gold electrodes in alkaline medium

In the present study, the surface poisoning of electrocatalytic monosaccharide oxidation reactions at gold electrodes were investigated. In the cyclic voltammetric studies, the electrocatalytic oxidation of aldohexose and aldopentose type monosaccharides, aminosugars, acetyl-glucosamine and glucronamide were observed at gold plate electrodes in alkaline medium. However, in controlled-potential electrolytic studies ranging −0.3 to −0.2 V in reaction solutions, current flows during electrolyses decreased quickly with time, except when glucosamine was used as a substrate.Results from surface enhanced infrared adsorption (SEIRA) spectroscopic measurements at an evaporated gold electrode for the electrocatalytic oxidation of glucose in 0.1 mol dm⁻³ NaOH at −0.3 V and Gaussian simulated spectra indicated that the gluconic acid as a 2-electron oxidation product and/or its analogs adsorbed onto the gold surface. Electrochemical quartz crystal microbalance (EQCM) measurement results, along with surface adsorption results from surface poisoning at the gold electrode during electrolytic reactions, suggested that gluconic acid and/or its analogs adsorbed vertically onto electrode surfaces in a full monolayer packing-like conformation. In the case of the electro oxidation of glucosamine in 0.1 mol dm⁻³ NaOH at −0.2 V, the obtained SEIRA spectra and EQCM results, clearly indicated that the glucosaminic acid as a 2-oxidation glucosamine product did not strongly bind onto the gold electrode surface.     

Contact electroresistance of metals in aqueous solutions during the anion adsorption involving charge transfer

Notions about charge transfer during adsorption of anions on metals in aqueous solutions are rendered. The role played by the electron tunneling on macrocontacts during the signal formation in the method of contact electroresistance (CER) is considered. It is shown that CER depends on the metal surface coverage by adsorbed species and their effective charge. Bell-like CER vs.E curves are obtained for copper, silver, and gold in solutions containing halide ions. Potentials of maximums in the curves,E _max, correspond to the charge transfer onset and depend on the nature of the metal and anion and on the anion concentration. AtE belowE _max, halides adsorb in the form of ions, involving no substantial charge transfer. At potentials exceedingE _max by 0.1 to 0.2 V, practically complete charge transfer occurs. With changing anion nature,E _max for a given metal rises in the series I^- < Br^- ≪ Cl^-. For a given anion (say, I^-),E _max increases with the metal nature in the series Cu ≤Ag ≪ Au. The link between the charge transfer during adsorption of anions and the surface reconstruction in single-crystal electrodes is discussed.     

Simulating Unstable States during the Coumarin Adsorption on a Mercury Electrode

Dependences of differential capacitance C on potential E of a stationary electrode (hanging mercury drop) in aqueous 0.1 M NaF solutions containing 4.6 × 10^–4 to 3 × 10^–3 M C₉H₆O₂ are obtained using an automatic impedancemeter. At coumarin concentrations below 0.001 M and potential slowly scanned near –1.1 V (SCE) the capacitance is unstable, which results in differently-shaped C vs. E curves in this potential range. The obtained results are attributed to nonequilibrium phase transitions in the adsorption layer, during which the orientation of coumarin molecules at the electrode surface alters. These phenomena are explained semiquantitatively on the basis of a developed theory.     

Direct electrochemical reduction of hemin in imidazolium-based ionic liquids

The direct electrochemical reduction of hemin, protoporphyrin(IX) iron(III) chloride, ligated with strong or weak heterocyclic bases, was investigated in the ionic liquids (IL), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) and 1-octyl-3-methylimidazolium hexafluorophosphate ([omim][PF₆]), using cyclic voltammetry and chronocoulometry. Hemin complexed with N-methylimidazole (NMI) or with pyridine had E_1/2 values slightly (4–59 mV) more positive in IL (without electrolyte) than in methanol (1.0 M electrolyte) using a gold electrode. NMI-ligated hemin had a lower E_1/2 than pyridine-ligated hemin in either IL, consistent with the stronger electron donor characteristic of NMI. [Bmim][PF₆] solutions consistently yielded E_1/2 values 30 mV more negative than [omim][PF₆] solutions. The diffusion coefficients D_o of hemin in the IL ranged between 1.50 and 2.80×10⁻⁷ cm² s⁻¹, while the heterogeneous electron-transfer rate constants k_s ranged between 3.7 and 14.3×10⁻³ cm s⁻¹. Cyclic voltammetry of hemin adsorbed to a gold surface through 4,4′-bispyridyl disulfide (AT4) linkages showed a large positive shift in the oxidation wave, indicating that adsorption stabilizes the reduced hemin state. The surface concentration Γ_o of the adsorbed hemin was determined to be 1.21×10⁻¹⁰ mol cm⁻², indicating the presence of one or more complete monolayers of hemin. These findings suggest that while hemin is electrochemically active in IL, its behavior is modified by the ligand field strength and surface adsorption phenomena.     

Adsorption of pyridine on a polycrystalline gold electrode surface studied by infrared reflection absorption spectroscopy

The adsorption behavior of pyridine on a smooth polycrystalline gold electrode surface was investigated over a wide wavenumber region (2000–500 cm⁻¹) by in situ infrared reflection absorption spectroscopy (IRAS). The reversible adsorption/desorption of pyridine was observed upon the change in applied electrode potential, and the adsorption state at positive potentials was found to depend strongly on the kind of halide ion used as a supporting electrolyte. Symmetry analysis of absorption bands observed revealed that pyridine molecules adsorb with the molecular axis (C₂ axis) perpendicular to the electrode surface (vertical configuration) at positive potentials in 0.5 M KF, KCl and KBr solutions. A band due to the out-of-plane bending mode of the adsorbed pyridine molecule was observed at potentials more negative than ca. 0 V for 0.5 M KF solution containing 100 mM pyridine. We concluded that even in the 100 mM pyridine solution, adsorbed pyridine forms a monolayer and that the molecules reorient from a flat (parallel) to the vertical configuration as the potential becomes less negative. No bands due to adsorbed pyridine were detected for 0.5 M KI solution. The amount of adsorbed pyridine was found to depend strongly on the strength of specific adsorption of halide ions.     

Electrochemical and FTIR studies of -phenylalanine adsorption at the Au(111) electrode

The adsorption of -phenylalanine (Phe) at the Au(111) electrode surface has been studied using electrochemical techniques and subtractively normalized interfacial Fourier transform infrared (SNIFTIR) techniques. The electrochemical measurements of cyclic voltammetry, differential capacity and chronocoulometry were used to determine Gibbs energies of adsorption and the reference (E₁) and sample (E₂) potentials to be used in the spectroscopic measurements. The vibrational spectra have been used to determine: (i) the orientation of the molecule at the surface as a function of potential; (ii) the dependence of the band intensity on the surface coverage; (iii) the character of surface coordination, and (iv) the oxidation of adsorbed Phe molecules at positive potentials. The adsorption of Phe is characterized by ΔG values ranging from −18 to −37 kJ mol⁻¹ that are characteristic for a weak chemisorption of small aromatic molecules. The electrochemical and SNIFTIR measurements indicated that adsorbed Phe molecules change orientation as a function of applied potential. At the negatively charged surface Phe is predominantly adsorbed in the neutral form of the amino acid. At potentials positive to the pzc, adsorption occurs predominantly in the zwitterionic form with the ---COO⁻ group directed towards the surface and the ammonium group towards the solution. At more positive potentials electrocatalytic oxidation of Phe occurs and is marked by the appearance of the CO₂ asymmetric stretch band in the FTIR spectrum. Thus, relative to pzc, Phe is weakly chemisorbed at negative potentials, changes orientation at potentials close to the pzc and is oxidized at positive potentials.     

Electrochemical Characterization of Pyrocatechol Sulfonephthalein-Modified Glassy Carbon Electrode and Separation of Electrocatalytic Responses for Ascorbic Acid and Dopamine Oxidation

A pyrocatechol sulfonephthalein- (PS-) modified glassy carbon (PS/GC) electrode has been prepared by adsorption of PS on a glassy carbon electrode surface. Cyclic voltammograms of the PS/GC electrode indicate the presence of a couple of well-defined redox peaks, and the formal potential shifts in the negative direction with increasing solution pH. The relation between formal potential,E^0′, and solution pH can be fit to the equationE^0′(mV) = −51.4 pH + 538.7. The PS/GC electrode shows high electrocatalytic activity toward ascorbic acid oxidation, with an overpotential ca. 380 mV less than that of the bare electrode and a drastic enhancement of the anodic currents. The electrocatalytic reaction rate constant (k), which was decreased with increasing concentration of H₂A, was determined using rotating disk electrode measurements. The values ofkwas also affected by the solution pH. The electrode can also separate the electrochemical responses of ascorbic acid and dopamine. The separation between the anodic peak potentials of ascorbic acid and dopamine is more than 50 mV by the differential pulse voltammetry.     

Study of Adsorption, Condensation, Orientation, and Reduction of Quinoline Molecules on a Pure Mercury Electrode Using Raman Microprobe Spectroscopy

Quinoline is known to adsorb on a mercury electrode surface with several differentorientations and it sometimes blocks other electrochemical reactions. The Ramanmicroprobe technique has been applied successfully to observe reorientations ofquinoline adsorbed on the mercury surface from neutral and basic aqueoussolutions. The orientation-distance profile from the mercury surface was also studied.A Raman band intensity of quinoline (1373 cm^–1) relative to the intensity ofperchlorate ion (931 cm^–1) was measured. The peak positions did not shift evenwhen the applied potential was altered, but the relative peak intensity changed.It was concluded that the adsorbed quinoline changes its orientation from a flatat –0.1 > E > –0.3V, to a standing at E < –0.5 V, passing through a mixtureof the two orientations when –0.3 > E > –0.5 V.     

Ion adsorption behaviour of hydroxyapatite with different crystallinities

This study aimed to correlate crystallinity of hydroxyapatite (HA) with the ion adsorption behaviour of the material. Hydroxyapatite powders of various crystallinities (X_c) and specific surface area (SSA) were prepared by precipitation following heat treatment. Adsorption experiments were carried out by using (i) multi-component ion solutions containing a broad range of light and heavy ions to study competitive adsorption and (ii) lead and zinc solutions with concentrations up to 250 ppm to determine the adsorption isotherms of the material. While as-prepared HA powders of low crystallinity (X_c = 0%) and a high SSA of 170 m²/g showed quantitative removal for divalent Pb, Zn, Be, U, Bi, V, Al, Cu and Ga ions, calcined powders with higher crystallinity (X_c = 65–95%) and lower SSA between 5 and 30 m²/g led to a quantitative removal only for a few elements (Pb, Bi, Ga). The time and concentration dependant ion removal capacity for Pb²⁺ and Zn²⁺ single element solutions showed quantitative removal even after short immersion times of less than 10 min for as-prepared HA powders. XRD analysis of the powders after ion adsorption revealed the presence of pyromorphite (Pb₅(PO₄)₃OH) and hopeite (Zn₃(PO₄)₂) phases, respectively.     

The influence of cathodic pretreatment on the kinetics of hydroxide ion oxidation on polycrystalline gold electrode

The electrochemical oxidation of the hydroxide ion was studied on a gold rotating disc electrode (RDE), in aqueous NaOH solutions in the presence of lithium perchlorate as a supporting electrolyte. By potentiodynamic polarization within the limits −1.6 V and +1.6 V vs. SCE, it was demonstrated that the overvoltage of the OH⁻ ion oxidation reaction may be significantly reduced with a 5 min long delay at the vertex cathodic potential of −1.6 V. This finding was explained in terms of the type of gold oxide formed on the gold surface under different experimental conditions.     

Adsorption phenomena on polycrystalline gold electrode modified by Zn adatoms

Summary In this paper, experimental results obtained by the in-situ radiotracer and voltammetric studies of the competitive adsorption of the HSO₄^-/SO₄^2- (labeled with ³⁵S) and Cl^- (labeled with ³⁶Cl) anions on bare Au_poly substrate and Au_poly surfaces modified by the Zn adatoms in the lower pH region (pH≤4.5) are presented and discussed. In addition, some data on the formation of Zn adlayers (labeled with ⁶⁵Zn) are reported to demonstrate the complex features of the underpotential deposition (UPD) phenomenon. It has been revealed that (1) the relative adsorption strength of Cl^- ions on bare Au_poly is higher than that of H₂PO₄^-/PO₄^3- and HSO₄^-/SO₄^2- ions in the entire pH region studied; (2) anomalous tendency of the specific adsorption of anions on polycrystalline gold modified by Zn adatoms occurs at pH 4.5 as follows: PO₄^3->SO₄^2->Cl^->>ClO₄^- and (3) the UPD of Zn²⁺ ions on polycrystalline gold is measurable even from solutions containing 5 ^. 10^-8M Zn²⁺ over the entire potential range where the hydrogen evolution takes place.It is, however, plausible to assume that the coverage of the gold surface by Zn adatoms measured even at solution concentrations of c≥5 ^. 10^-4M does not exceed one monolayer.     

XPS studies on the gold oxide surface layer formation

The initial stage of gold oxide layer formation on the gold electrode surface was investigated in 0.5 M H₂SO₄. X-ray photoelectron spectroscopy (XPS) spectra of pure gold and the anodically polarized gold electrode surface were compared quantitatively. It was found that gold anodic polarization in the E range from ∼1.3 to 2.1 V causes increase in intensity of the XPS spectra at an electron binding energy ε_b=85.9 eV for gold and at ε_b=530 eV for oxygen. These ε_b values correspond to Au³⁺ and O²⁻ oxidation states in hydrous or anhydrous gold oxide. The larger the amount of the anodically formed surface substance the higher is the intensity of the spectrum at the ε_b values mentioned above. It was concluded that gold anodic oxidation, yielding most likely an Au(III) hydroxide surface layer, takes place in the E range of the anodic current wave beginning at E≈1.3 V. At E_B=1.7 V (the potential of the Burshtein minimum) the stationary surface layer consists of 2.5 to 3 molecular layers of Au(OH)₃. The theoretical amount of charge required for the reduction of one molecular layer of Au(OH)₃ is ∼0.15 mC cm⁻², since the Au(OH)₃ molecule is planar and occupies about four atomic sites on the electrode surface.     

Voltammetric determination of sorbite at a mechanically renewed copper electrode

Potentials and currents of D-sorbitol oxidation peaks as a function of polarization conditions for a copper electrode in situ renewed by mechanically cutting a 0.5-μm surface layer are studied by direct-current cyclic voltammetry. Oxidation peaks of sorbite emerge in cyclic voltammograms recorded in alkaline supporting electrolytes (0.05–0.10 M KOH and NaOH solutions) upon scanning the potential to the anodic region (E _p = 0.50–0.58 V) and in the reverse direction (E _p = 0.60–0.62 V). The shape and parameters of these peaks depend on the concentration of KOH, because of the different copper oxides involved in the oxidation of sorbite formed at the electrode surface. The regeneration of the electrode surface is the necessary condition for good reproducibility of the peak parameters. The signals obtained on the surface of the unrenewed electrodes are almost halved and less reproducible. The calibration graph of the current of the sorbite oxidation peak as a function of its concentration is linear in the range from 5 × 10⁻⁴ to 1 × 10⁻² M.