pH-dependent binding interaction between functionalized thiol monolayers and electroactive ions

The charge transfer kinetics at ionizable thiol monolayer-modified gold electrodes was investigated as a function of the redox-couple composition and the solution pH by potential scan voltammetry and electrochemical impedance spectroscopy. The adsorption of indigotetrasulfonate and chromate onto a 4-aminothiophenol monolayer, at low pH values, resulted primarily from an electrostatic attraction between the protonated terminal amino groups and the anions. An increase in the solution pH, however, raised the apparent heterogeneous rate constant of indigotetrasulfonate at a 4-aminothiophenol-modified electrode. This pH dependence, which was opposite to that observed at a 1-butanethiol-modified electrode, can be ascribed to a change in the potential difference between the gold electrode and monolayer surface. A large change in the heterogeneous rate constant with the pH implies that in addition to electrostatic interaction, a specific intermolecular force contributes to the interactions between the 4-aminothiophenol monolayer and the redox species.     

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.     

Direct electrochemistry of histamine dehydrogenase from Nocardioides simplex

Histamine dehydrogenase from Nocardioides simplex (HmDH) belongs to the family of soluble iron–sulfur flavoproteins having one [4Fe–4S] cluster and one 6-S-cysteinyl flavin mononucleotide per monomer. Direct electrochemistry of HmDH was studied using several carbon particle-modified glassy carbon electrodes (GCEs) and indium tin oxide (ITO) electrodes. HmDH gave a clear catalytic wave of the histamine oxidation without any mediator at a GCE modified with Ketjen Black (KB) with an average particle diameter of 39.5 nm, although redox signal of the cofactors itself was not clearly recognized. Experimental data supported the irreversible adsorption of HmDH on the KB particles and the importance of the size of the carbon particle as well as the surface area as factors determining the current density of the direct electron transfer (DET)-type bioelectrocatalysis. On the other hand, HmDH was adsorbed on ITO electrodes as a monolayer, as evidenced by quartz crystal microbalance measurement, and showed a clear redox wave ascribed to the [4Fe–4S] cluster. However, some mediators were required to observe catalytic wave of histamine oxidation at ITO electrodes, indicating that the interaction between the two redox cofactors seems to be disrupted in the monomer HmDH adsorbed on the planar and hydrophilic surfaces of ITO electrodes. Surface properties of electrodes favorable for DET-type bioelectrocatalysis of HmDH are discussed.     

Studies of structural disorder of self-assembled thiol monolayers on gold by cyclic voltammetry and ac impedance

Self-assembly of octadecyl mercaptan on gold has been investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CV experiments show that well-assembled thiol monolayers on gold are essentially free of pinhole defects. Support for the existence of ‘collapsed’ sites in thiol monolayers is obtained by comparing the apparent electron transfer rate constant, which is obtained from CV experiments at a film covered Au electrode, with the theoretical one, which is calculated by assuming electrons tunnel across the collapsed-site-free monolayer. The empirical relationship Z=R_s+1/[σ(jω)^α] is used to describe the film coated Au in inert electrolyte and the degree of disorder of the monolayer structure is estimated from the α value. It is demonstrated that the admittance plane plot of an ideally polarizable electrode with constant phase element behavior is a circular arc which can be used to determine α. Results of EIS experiments show that α increases sharply after the initial adsorption time and approaches its final value slowly. This is attributed to the rapid adsorption step followed by a slow crystallization process in the kinetics of alkanethiol adsorption onto the Au electrode. For a well-assembled thiol monolayer on gold, α can approach 0.99, which means that the number of collapsed sites is not large and the surface of the monolayer Au electrode is rather smooth.     

Characterization of a 1,4-dithiane gold self-assembled monolayer: an electrochemical sensor for the cyt-c redox process

Electrochemical desorption and spectroscopic investigations of the gold electrode surface modified with 1,4-dithiane (1,4-dt) organothiol species were performed. The wave observed at ?0.87 V versus Ag ∣ AgCl in the LSV (linear sweep voltammetry) reductive curve of the 1,4-dt compared to that for a similar 4-mercaptopyridine (pyS) system (?0.56 V) is indicative of a most effectively chemisorbed monolayer. The evaluation of the capability of the 1,4-dt self-assembled monolayer (SAM) in assessing the direct electron transfer (ET) of cytochrome c (cyt c) metalloprotein was investigated by cyclic voltammetry. The electrochemical response of the cyt c (E_1/2 ≈0.0 V vs. Ag ∣ AgCl, ΔE_p ≈50 mV) showed the characteristics of a reversible redox process. The cyt c voltammetric parameters acquired with the 24-h air exposure modified electrode, and after 100 cycles suggest a considerable improvement of the 1,4-dt electrode performance. The surface enhanced Raman spectroscopy (SERS) spectra revealed that 1,4-dt species is in a mixed gauche and trans orientation on the gold surface. The shift for higher wavenumbers observed for the C–S stretching modes in the SERS spectra, comparatively to the normal Raman spectrum, is assigned to the 1,4-dt coordination to surface gold atoms via a π interaction with the sulfur p-orbitals. The data collected suggest that this π interaction plays an important role on the stability of the 1,4-dt adlayer, improving the assessment of the cyt c heterogeneous electron transfer reaction.     

Electrochemical study of a metallothionein modified gold disk electrode and its action on Hg2+ cations

A novel protein monolayer modified electrode has been prepared by the self-assembly of metallothionein (MT) at a gold disk electrode. The properties of MT in Tris–HCl buffer and in the monolayer are studied by using cyclic voltammetry and differential pulse voltammetry with a gold disk electrode. In the negative sweep, the voltammogram of MT in buffer shows two small peaks and different electrochemical behaviour from that at a mercury electrode. Cd²⁺ complexed to the thionein can easily be replaced by Hg²⁺ ions, and Hg²⁺ ions can firmly adsorb in the MT monolayer with a saturation coverage of (2.78±0.29)×10^?10 mol cm^?2. This behaviour has been used to preconcentrate trace Hg²⁺ for its determination by cathodic stripping differential pulse voltammetry. The cathodic stripping peak current is proportional to Hg²⁺ concentration in the range of 0.15–3 μM and the detection limit is ca. 0.08 μM (16 ppb) with a 2 min open circuit accumulation step. The relative standard deviation is 7.2% at 0.4 μM Hg²⁺ concentration (n=4). At higher concentration the adsorption of Hg²⁺exhibits a response similar to that expected for a Langmuir adsorption isotherm with the stability constant of (4.0±0.2)×10⁵ M^?1.     

Electrogenerated chemiluminescence of lucigenin enhanced by the modifications of electrodes with self-assembled monolayers and of solutions with surfactants

Electrogenerated chemiluminescence (ECL) of lucigenin (Luc²⁺·2NO^?₃, N,N′-dimethyl-9,9′-biacridinium dinitrate) in dioxygen-saturated alkaline aqueous solutions of pH 10 has been examined utilizing modifications of electrodes (i.e. self-assembled monolayer (SAM)-modified gold electrodes) and of solutions (i.e. micellar solutions containing a nonionic surfactant, Triton X-100) for the first time. In both cases of the modifications, enhanced ECL was observed, and the ECL was considered to be derived from the decomposition of a dioxetane-type intermediate formed by the radical–radical coupling reaction between an electrogenerated superoxide ion (O₂^?) and a one-electron reduced form of Luc²⁺ (i.e. a radical cation, Luc⁺). The surface modification of gold electrodes was achieved with dimercaptosuccinic acid (DMSA) and dl-thioctic acid (dl-TA) having carboxyl end groups. The amount of ECL at dl-TA-SAM-modified electrodes was about five times as great as that at the bare electrode. The enhancement of ECL at the SAM-modified electrodes would be due to the concentration effect of positively charged Luc²⁺ ions, the prevention of adsorption of the electrogenerated chemiluminescent product (i.e. N-methylacridone (NMA)) and the two-electron reduced form of Luc²⁺ (Luc⁰) on the electrode surfaces, and the effective generation of O₂^?. On the other hand, the amount of ECL in the micellar solution increased by about six times in comparison with that in the solution containing no surfactant. The enhanced ECL in micellar solutions would result mainly from the inhibition of adsorption of NMA and Luc⁰ insoluble in water on electrode surfaces.     

Dissociative electron transfer and the principle of microscopic reversibility

In contrast with recent claims, the concept of dissociative electron transfer is not incompatible with the principle of microscopic reversibility. The reasons why it is so are discussed briefly. Interactions between product fragments may exist but are not a consequence of the principle of microscopic reversibility. They rather depend on the molecular structure and solvent.     

Effect of pH on direct electron transfer between graphite and horseradish peroxidase

The effect of pH on the kinetics of the electroreduction of H₂O₂ catalysed by horseradish peroxidase (HRP) has been studied with LSV in the potential range from 700 to ?50 mV versus SCE (under steady-state conditions and with an RDE system) and at ?50 mV versus Ag/AgCl on HRP-modified graphite electrodes placed in a wall-jet flow-through electrochemical cell. Increasing [H₃O⁺] was shown to enhance significantly the current of the bioelectroreduction of H₂O₂ due to direct electron transfer (ET) between graphite and the enzyme over the potential range involved. It is demonstrated that at high overvoltages (E<0.2 V) H₃O⁺ does not affect the rate of the enzymatic reduction of H₂O₂, but it increases the rate of direct ET between graphite and HRP. The values of the apparent rate constant of heterogeneous ET between HRP and graphite, k_s, changed from a value of 0.54±0.05 s^?1 in phosphate buffer solution (PBS) at pH 7.9, to a value of 11.0±1.7 s^?1 in PBS at pH 6.0. Analysing the pH rate profile and the variation of the k_s with increasing [H₃O⁺] made it possible to consider the reaction mechanism as implying the participation of a proton in the limiting step of charge transfer.     

The distribution of standard rate constants for electron transfer between thiol-modified gold electrodes and adsorbed cytochrome c

The magnitude of the total rate constant of an electrochemical reaction at small overpotentials is shown to depend on the exponentials of the distance between the electron donor and acceptor, the reorganization energy, and the formal potential of the reaction. When a population distribution of one or more of these three parameters exists, the effect on the measured standard rate constant can be illuminated by using small-amplitude a.c. electrochemical impedance spectroscopy. Assuming a Gaussian distribution of the logarithm of the standard rate constants, the results from the system of cytochrome c molecules adsorbed on thiol-modified gold electrodes yield a standard deviation of approximately 0.5 for log k°/k_avg°. The peak broadening of reversible voltammograms is related to a comparable distribution of formal potentials.     

Simultaneous determination of epinephrine and uric acid at a gold electrode modified by a 2-(2,3-dihydroxy phenyl)-1, 3-dithiane self-assembled monolayer

In the present paper, the use of a gold electrode modified by 2-(2,3-dihydroxy phenyl)-1,3-dithiane self-assembled monolayer (DPDSAM) for the determination of epinephrine (EP) and uric acid (UA) was described. Initially, cyclic voltammetry was used to investigate the redox properties of this modified electrode at various scan rates. The apparent charge transfer rate constant, k_s, and transfer coefficient, α, were calculated. Next, the mediated oxidation of EP at the modified electrode was described. At the optimum pH of 8.0, the oxidation of EP occurs at a potential about 155 mV less positive than that of an unmodified gold electrode. The values of electron transfer coefficients (α = 0.356), catalytic rate constant (k = 1.624 × 10⁴ M⁻¹ s⁻¹) and diffusion coefficient (D = 1.04 × 10⁻⁶ cm² s⁻¹) were calculated for EP, using electrochemical approaches. Based on differential pulse voltammetry, the oxidation of EP exhibited a dynamic range between 0.7 and 500.0 μM and a detection limit (3σ) of 0.51 μM. Furthermore, simultaneous determination of EP and UA at the modified electrode was described. Finally, this method was used for the determination of EP in EP ampoule.     

Reduction of p-benzoquinone on lipid-modified electrodes: effect of the alkyl chain length of lipids on the electron transfer reactions

Using a phosphatidylcholine (PC) modified electrode, an unusual stabilization of p-benzoquinone anion radical (BQ^?) is observed even in a buffered solution of pH 7.0. To clarify this incorporation reaction of BQ^? in a PC layer, the effect of the molecular structure of the lipid was studied by constructing the PC modified electrodes with four PC derivatives; dicaproyl-, dilauroyl-, dipalmitoyl- and diarachidoyl-PC. At 25°C, dilauroyl-PC with a moderate alkyl chain length stabilized BQ^? and allowed the approach of benzoquinol onto the surface of the modified electrode. From the pH dependence of the electrochemical responses, the incorporation mechanism of BQ^? with P(O)OH was clarified. In addition to dilauroyl-PC, because dipalmitoyl-PC was found to incorporate BQ^? over 40°C, the relationship between its incorporation and the phase transition temperature of each lipid has become clear. The correlation and the incorporation profiles were examined by measuring the oxidation of benzoquinol as well as that of BQ^?. Also, the diffusion coefficients were determined for the lipids incorporating BQ^?.     

Preparation and characterization of electrochemically modified electrodes containing the metal complexing moiety quinolin-8-ol anchored to the polymeric film

This work presents a new electrochemically modified electrode which is chemically and mechanically stable, and can be easily constructed. It is being proposed here for the first time, a synthetic route for the monomer 5-(4-allyloxi-fenilazo)-quinolin-8-ol which is electrochemically polymerized via the allyl ether group resulting on a film that coats the electrode’s surface, preserving the quinolinic group intact for metal complexation. The film was characterized by spectroscopic and voltammetric techniques. A complete passivation of the three electrodes tested (glassy carbon, carbon rod and stainless steel) was obtained from ?0.9 to +0.75 V, implying a possible quantification of transition metal species within this potential range. The successful complexation of copper in water media at open circuit and the resulting redox waves are reported for a home made stainless steel coated electrode, showing the potential use of this modified electrode for analytical applications.     

Effect of Triton X-100 on electron transfer kinetics at the interface between two immiscible electrolyte solutions: a scanning electrochemical microscopy study

Adsorption of the nonionic surfactant, Triton X-100, at the interface between two immiscible electrolyte solutions (ITIES) and its effect on the oxidation of decamethyl ferrocene in 1,2-dichloroethane by aqueous Ru(CN)₆^3? has been investigated using scanning electrochemical microscopy. As the concentration of Triton X-100 in the aqueous phase was increased from 0 to 2.5×10^?4 M, the rate constant for electron transfer (ET) across the ITIES decreased in a manner consistent with the surfactant blocking the area available for the reaction. The decrease in rate constant with increasing surfactant concentration was successfully analyzed in terms of Langmuirian adsorption of the surfactant, with an equilibrium constant of (2.72±0.06)×10⁴ M^?1. The overall behavior observed indicates that the ET reaction occurs primarily at the portion of the ITIES free from surfactant.     

Voltammetric study of the interfacial electron transfer between bis(butylcyclopentadienyl)iron in 1,2-dichloroethane and hexacyanoferrate in water

The electron transfer (ET) reaction between bis(butylcyclopentadienyl)iron(II) ([Fe^II(C₅H₄Bu)₂]) in 1,2-dichloroethane (1,2-DCE) and the hexacyanoferrate redox couple ([Fe^II/III(CN)₆]^4?/3?) in water (W) at the 1,2-DCE∣W interface has been studied by use of normal pulse voltammetry and cyclic voltammetry. In normal pulse voltammetry, S-shaped current vs. potential curves with well-defined limiting currents attributable to the interfacial ET reaction were observed. The voltammetric results can be explained well by the theoretical equations [M. Senda, Rev. Polarogr. (Jpn), 49 (2003) 219; 50 (2004) 60] according to a so-called IT-mechanism, that is, the [Fe^II(C₅H₄Bu)₂] molecule is transferred from the 1,2-DCE to the W phase across the interface, then the ET reaction takes place homogeneously in the W phase between [Fe^III(CN)₆]^3?(W) and [Fe^II(C₅H₄Bu)₂](W) to produce [Fe^II(CN)₆]^4?(W) and [Fe^III(C₅H₄Bu)₂]⁺(W), which is followed by the transfer of the [Fe^III(C₅H₄Bu)₂]⁺ ion from the W to the 1,2-DCE phase across the interface to give the voltammetric current. Experimental results from cyclic voltammetry are also explained well by the IT-mechanism.     

Electrochemical study of the redox dyes Nile Blue and Toluidine Blue adsorbed on graphite and zirconium phosphate modified graphite

Solid graphite electrodes were modified with the redox dyes Nile Blue (NB) and Toluidine Blue (TB), following irreversible adsorption from ethanol solution, and a droplet evaporation procedure. The shift of redox potential for both dyes to the negative direction upon adsorption on graphite was estimated from cyclic voltammetry, and from extrapolation of the dependence of E_p versus I_p to I_p=0, to vary between 60 and 110 mV. For electrodes containing a controlled amount of NB or TB, heterogeneous charge transfer rate constants of 31 and 11.6 s^?1, respectively, were estimated by extrapolation of the data to zero coverage of the electrode surface. The rate constants obtained are several times higher than those obtained for multilayer coatings with the dyes. The modification of solid graphite electrodes with zirconium phosphate (ZrP), followed by the adsorption of NB and TB was performed. For NB, two forms of the dye were found to be present, one of them presenting an usual adsorbate of NB on graphite, and another one having E_pc shifted by ca. 0.15–0.30 V to more positive potential values. The latter form is ascribed to NB, included in the ZrP host matrix, as follows by comparing its electrochemical behavior with that observed for NB, included in carbon paste containing ZrP.     

微弧氧化-碱热处理后钛铌锆锡合金种植体-骨界面的扫描电镜观察

目的 通过观察微弧氧化-碱热处理后钛铌锆锡合金(Ti-24Nb-4Zr-8Sn)表面成骨细胞的生长情况和种植体-骨界面的结合情况,探讨对钛铌锆锡合金表面应用微弧氧化-碱热处理进行改性的效果.方法 用钛铌锆锡合金制备圆片形试件和圆柱形种植体各12个,均分为3个组(空白对照组、微弧氧化组和微弧氧化-碱热处理组)进行相应表面处理.将MC3T3-E1成骨样细胞接种于圆片形试件表面,培养3 d后扫描电镜(SEM)比较各组试件表面的细胞形态.将圆柱形种植体植入犬胫骨内,3个月后SEM观察种植体-骨界面的微观形貌.结果 微弧氧化和微弧氧化-碱热处理均可使钛铌锆锡合金表面生成具有生成微孔结构的氧化膜,后者试件表面的细胞与微孔嵌合更紧密,种植体与骨组织形成良好的骨结合.结论 碱热处理可进一步完善微弧氧化方法,提高种植体与骨界面的结合.

Abstract：

Objective To observe the efficacy of micro-arc oxidation and alkali-heat treatment (MAH) on Ti-24Nb-4Zr-8Sn(Ti2448). Methods Disks( diameter of 14. 5 mm, thickness of 1 mm) and cylinders (diameter of 3 mm, height of 10 mm) were fabricated from Ti2448 alloy. Samples were divided into three groups: polished( Ti2448 ), micro-arc oxidation( MAO-Ti2448 ), micro-arc oxidation and alkali-heat treatment(MAH-Ti2448). MC3T3-E1 osteoblastic cells were cultured on the disks and cell morphology was observed with scanning electron microscopy(SEM) aftre 3 days. The cylinder samples were implanted in the tibia of dogs and implant-bone interface was observed with SEM after 3 months. Results A rough and porous structure was shown in both MAO and MAH group. The MC3T3-E1 cells on the MAH-Ti2448 discs spread fully in intimate contact with the underlying coarse surface through active cytoskeletal extentions.Osseointegration was formed in the implant-bone interface in MAH samples. Conclusions MAH treatment can provide a more advantageous Ti2448 surface to osteoblastic cells than MAO treatment does, and the former can improve the implant-bone integration.     

A correlation study between the conformation of the 1,4-dithiane SAM on gold and its performance to assess the heterogeneous electron-transfer reactions

A gold electrode spontaneously modified by 1,4-dithiane (1,4-dt) organosulfur species has been characterized by cyclic voltammetry of [Fe(CN)₆]^3?/4? and cytochrome c (cyt c) probe molecules, SERS (surface-enhanced Raman scattering), and EIS (electrochemical impedance spectroscopy), using [Fe(CN)₆]^3?/4? as the redox-active molecule in solution. The cyclic voltammograms showed a decrease in the faradaic current response with an increase of the immersion time of the gold substrate into the 1,4-dt modifier solution, suggesting that the heterogeneous electron-transfer (hET) process of the [Fe(CN)₆]^3?/4? probe molecule occurs through the surface pinholes. This observation is an indication that the 1,4-dt chemisorption mechanism must involve a slow step that is responsible for the final configuration of the modifier molecules on the surface. The SERS spectra acquired for the surface as a function of immersion time of the gold electrode in the modifier solution presented an intensity enhancement of the bands assigned to the trans and gauche conformation of the 1,4-dt on the surface. The apparent rate constant, k_app, estimated by the EIS results, decreases with increase of the immersion time indicating that the pinholes on the surface act as a microarray. This result is consistent with those observed by cyclic voltammetry of the probe molecules, i.e., the pinhole density decreases with increase of the immersion time of the gold electrode in the 1,4-dt modifier solution, and a strong attenuation of the probe molecules interfacial electron-transfer response is observed.     

Electron transfer kinetics and morphology of cytochrome c at the biomimetic phospholipid layers

Electrochemistry of cytochrome c (cyt c) at biomimetic phospholipid layers was studied in a phosphate buffer solution, which were formed with dilauroyl phosphatidic acid (DLPA, C12:0), dipalmitoyl phosphatidic acid (DPPA, C16:0), distearoyl phosphatidic acid (DSPA, C18:0), and palmitoyl–oleoyl phosphatidic acid (POPA, C16:0–18:1). The lipid-layers formed firstly at the air/water interface were immediately transferred onto the electrode surface using the Langmuir–Blodgett (LB) technique. The electrochemical properties of cyt c at the lipid covered electrodes depended on the orientation, number of layers of phospholipids, tail (or head) group down, and vice versa. Atomic force microscopy (AFM) images of cyt c adsorbed on the POPA monolayer (showing the head group diameter of POPA to be ca. 0.7 nm) formed on highly oriented pyrolytic graphite (HOPG) displayed uniform surface morphology of lipid layer and clumps of aggregated cyt c molecules with a minimum size corresponding to four cyt c molecules. The heterogeneous electron transfer rate constants, k⁰ values, of cyt c were determined to be 1.02 × 10⁻³, 0.98 × 10⁻³, and 0.67 × 10⁻³ cm/s for the lipid monolayer in the tail down orientation (X-type) of POPA, DLPA, and DPPA, and 0.67 × 10⁻³ and 0.50 × 10⁻³ cm/s for the head down orientation (Z-type) of POPA and DLPA monolayers, respectively.