Cyclic voltammetric,EQCM and impedance study of the 1-octanol-β-cyclodextrin-electrodeposited platinum system in perchloric medium

The system of 1-octanol-β-cyclodextrin in 0.5 M HClO₄ at Pt electrodeposited on an Au/quartz crystal (Pt/Au/Q) has been studied by cyclic voltammetry (CV), the electrochemical quartz crystal microbalance (EQCM) and impedance measurements. β-Cyclodextrin (β-CD), a cyclic oligomer of seven α-d-glucopyranose units, was chosen for this study because it forms an inclusion complex with primary alcohols and because of its hydrophilic exterior. Previous cycling in 1-octanol profoundly affected (“aged”) the electrodeposited Pt, as is readily seen in subsequent CVs and mass curves in 0.5 M HClO₄. Since adsorbed 1-octanol and β-CD would tend to render the Pt surface hydrophobic and hydrophilic, respectively, these opposite tendencies should be detectable by the EQCM. Effectively, in a positive potential scan, 1-octanol and β-CD added at ?0.22 V produced a mass decrease and increase, respectively, of aged Pt/Au/Q, owing to a decrease and increase of the amount of adsorbed water and/or ions, which was attributed to physisorption of these compounds. With freshly deposited Pt/Au/Q, 1-octanol added at open circuit produced a mass decrease over the whole potential cycle, again evidence of adsorption of a hydrophobic compound.The potential at which the aged Pt/Au/Q electrode immersed in an 1-octanol solution was held while β-CD was added to the electrolyte crucially affected the subsequent voltammogram and mass curve. If β-CD and 1-octanol were added together at ?0.22 V, the current in a subsequent positive scan was the same as in 1-octanol added at ?0.22 V, but the mass was higher, both at ?0.22 V and over the whole positive scan. This mass increase was probably due to physisorption (since the current was unaffected) of the inclusion complex, since β-CD alone did not affect the mass at ?0.22 V. On the contrary, if β-CD was added at open circuit to an electrolyte already containing 1-octanol, also added at open circuit, both the H desorption and Pt oxidation currents were lower than in 1-octanol, indicating a strong interaction of β-CD with the Pt surface, and the mass was also lower over most of the positive scan. Most probably this strong interaction of β-CD involves adsorbed residues formed in the dissociative chemisorption of 1-octanol at open circuit.     

The adsorption and orientation of pyrazine on silver electrodes: a surface enhanced Raman scattering study

Surface enhanced Raman scattering (SERS) spectra of pyrazine (pz) adsorbed on a silver electrode from aqueous solutions containing either 1.0 M KCl or 1.0 M KBr are presented. The SER spectra display bands which are usually forbidden in the normal Raman spectrum of pz. The presence of these forbidden bands and the orientation of the molecules on the electrode surface are discussed under the two most accepted surface enhancement theories: the charge transfer and the electromagnetic model. The dependence of SERS intensity of several pz vibrational modes on the applied potential (potential profile) is also presented for different pz concentrations and excitation wavelengths. SER spectra, obtained at potentials more negative than ?900 mV (vs. SCE), contain new bands due to reduction of the pz molecule. It is clear that some of these products of decomposed pz remain trapped on the electrode surface, and this can lead to misunderstandings of the interpretation of the adsorbed pz spectrum. Several controversial aspects presented in the literature about the surface Raman spectrum of pz are clarified and discussed.     

Thiourea adsorption on a Pt surface as detected by electrochemical methods and surface-enhanced Raman spectroscopy

The adsorption behavior of thiourea (TU) on a roughened Pt electrode was investigated by combining electrochemical methods and surface-enhanced Raman spectroscopy. It was found that TU could be dissociated on a roughened Pt surface at the open circuit potential to form a surface sulfide. The dissociation process can be effectively suppressed if the electrode is immersed into the solution with its potential controlled at a negative value. Using these tactics, we are able to perform a surface-enhanced Raman spectroscopy (SERS) study of the adsorption behavior of TU on Pt. The SERS results reveal that TU is adsorbed through its sulfur atom in a tilted way and the orientation changes with the applied potential. The study points out the importance of controlling the electrode potential properly upon immersion of the electrode into the solution.     

Surface enhanced Raman scattering (SERS) of 1-hydroxybenzotriazole adsorbed on a copper electrode surface

The adsorption of 1-hydroxybenzotriazole (BTAOH) on the copper electrode was studied by surface-enhanced Raman scattering spectroscopy (SERS) as a function of electrode potential and pH value of the solution. A striking similarity between the SERS spectra of BTAOH and the respective spectra of benzotriazole (BTAH) was observed in spite of significant differences in the normal Raman spectra of these compounds. A hypothesis assuming transformation of the BTAOH molecule into BTAH at the copper surface has been verified by H/D isotopic exchange experiments. The unique sensitivity of the SERS spectrum to isotopic exchange found in acidic medium (pH 2) at considerably negative electrode potentials together with some differences between the SERS spectra of BTAOH and BTAH indicated that BTAOH is mainly adsorbed in its molecular form. However, the transformation from the adsorbed BTAOH molecule to the surface complex of BTA^? anion with Cu(I) was suggested for alkaline solutions and potentials more positive than ?0.2 V.     

EIS study of the redox reaction of Fe(CN)63−/4− at poly(3,4-ethylenedioxythiophene) electrodes: influence of dc potential and cOx:cRed ratio

The electron transfer between the ferri/ferrocyanide redox couple and poly(3,4-ethylenedioxythiophene) (PEDOT) electrodes has been investigated by electrochemical impedance spectroscopy (EIS). Different thicknesses of the conducting polymer films were investigated (i) at a constant concentration of ferrocyanide in the solution at different applied dc potentials and (ii) at the open circuit potential with different ratios of Fe(CN)₆^3?:Fe(CN)₆^4? in the solution. PEDOT was prepared by galvanostatic electropolymerization on platinum electrodes from aqueous solutions containing 0.01 M 3,4-ethylenedioxythiophene (EDOT) and 0.1 M poly(sodium 4-styrenesulfonate) (NaPSS) as the supporting electrolyte. All impedance spectra were obtained in aqueous solutions with 0.1 M KCl as the supporting electrolyte at dc potentials, where the polymer is in the oxidized state. The EIS data were fitted to an equivalent electrical circuit resembling the Randles' circuit, where the double layer capacitance is replaced by the bulk redox capacitance and the associated transport impedance of the conducting polymer. The same potential and thickness dependence of the charge transfer resistance (R_ct) were obtained both (i) by varying the concentration ratio of the redox species in the solution and (ii) by applying different potentials at a constant concentration of ferrocyanide. The potential dependence of k₀ calculated from R_ct indicates that the conducting polymer influences the rate of electron transfer for the ferri/ferrocyanide redox couple.     

Electrochemical behaviour of amino acids on Pt(h,k, l). A voltammetric and in situ FTIR study. Part II. Serine and alanine on Pt(111)

A spectroelectrochemical study of the adsorption and oxidation of serine and alanine on Pt(111) electrodes has been carried out in 0.1 M HClO₄ solutions. From the voltammetric and FTIR spectroscopic results, we concluded that adsorbed cyanide is formed at the Pt(111) surface as a product of serine and alanine oxidation. In the case of serine, adsorbed cyanide is detected at potentials above 0.4 V, whereas a higher potential is needed in the case of alanine. Linear and multibonded CO have also been detected as poisoning species during serine oxidation. These adsorbed species have been isolated by irreversible adsorption experiments of amino acids. Dissolved CO₂ has been found by FTIR spectroscopy as an oxidation product of both amino acids. The role played by the R group in the electrochemical behaviour of these molecules is significant. Reversibly adsorbed serinate and alaninate anions have been also detected. As in the case of the anions present in glycine solutions, the serinate and alaninate anions are two-fold coordinated to the Pt(111) surface through the car?ylate group.     

The orientation of 2,2′-bipyridine adsorbed at a SERS-active Au(1 1 1) electrode surface

Surface-enhanced Raman scattering (SERS) spectra from 2,2′-bipyridine (22BPY) adsorbed on a SERS-active Au(1 1 1) electrode at several applied potentials were obtained. The SERS-active Au(1 1 1) surface was prepared following an electrochemical cleaning procedure. This procedure involves the application of continuous oxidation–reduction cycles (orcs) within a potential region where no significant surface structural changes are expected to occur. The adsorbed 22BPY may assume several conformations, including the cis- and trans-configurations. Normal Raman spectra of aqueous 22BPY at several different acidities are presented. These spectra are compared to the Raman features of both the ‘free’ 22BPY and its Zn complex in the solid state. The normal Raman experiments showed that unique spectral characteristics were distinguishable for both the cis- and trans-configurations. Based on these results, the potential-dependent orientation (conformation) of 22BPY adsorbed on a SERS-active Au(1 1 1) has been established. The SERS results suggested that, at a positively-charged surface, 22BPY adsorbs end-on, using both nitrogens (cis-configuration). Although no strong spectroscopic evidence suggesting potential-induced reorientation was found, the pyridine rings may no longer be coplanar at the negatively-charged surface. However, the molecules keep the upright position with both nitrogens pointing towards the surface even in these negative limits.     

Cyanate and thiocyanate adsorption at copper and gold electrodes as probed by in situ infrared and surface-enhanced Raman spectroscopy

Vibrational spectroelectrochemical techniques have been used to investigate the structure and dynamics of the electrochemical double layer using pseudohalides as specifically adsorbed probe molecules. Infrared and surface-enhanced Raman spectra of thiocyanate and infrared spectra of the structurally related cyanate at the copper|water and gold|water interface are reported. In neutral solutions, N- and S-bound adsorbed thiocyanate was found at the gold electrode, whereas at copper only S-bound thiocyanate exists. In both cases, thiocyanate is adsorbed in the whole polarizable potential range. At both copper and gold electrodes, cyanate coordinates via the nitrogen atom, not being adsorbed at negative potentials and occupying bridged adsorption sites at lower coverages. In alkaline solution, no adsorption of cyanate occurs, whereas the thiocyanate adsorption is not affected by the coadsorption of hydroxyl ions. These results lead to the assumption that the binding of thiocyanate to the surface is stronger and more covalent, whereas the coordination of cyanate is more ionic. In addition, at negative sample potentials cyanate was found to adsorb as cyanide, which is formed from cyanate by reduction. This process seems to lead to an activation of the surface in the case of gold.     

In situ FTIR spectroscopic studies of the oxidation of CO adsorbates on Ru(0 0 0 1) electrodes under open circuit conditions

This paper reports the first observation, using in situ FTIR spectroscopy, of the oxidation of CO adsorbates on the Ru(0 0 0 1) electrode to CO₂ under open circuit (oc) conditions in both perchloric acid and sulphuric acid solution at 20 and 55 °C. While the significant oc oxidation of the adsorbed CO on the Ru(0 0 0 1) electrode was observed in perchloric acid solution, much less oc oxidation took place in sulfuric acid solution due to the specific adsorption of bisulfate at the Ru surface which inhibits the surface oxidation and reduces the reactivity of the surface towards the oxidation of CO_ads. The oc oxidation of the CO_ads depends strongly on the oxygen concentration in the solution and the temperature. The data so obtained are compared to those observed at the gas|solid interface, as well as to those obtained from the electro-oxidation of CO_ads, and possible new catalytic oxidation reaction mechanisms are discussed. In addition, it is shown that the C–O frequency of the adsorbed CO may be used as an effective probe of the open circuit potential.     

Inverted correlations between rate constants and redox potential of the catalyst for the electrooxidation of 2-aminoethanethiol mediated by surface confined substituted cobalt-phthalocyanines

This study reports on the role of both electron donor and electron acceptor substituents on the periphery of phthalocyanine cycle on the electrocatalytic activity of cobalt phthalocyanines for the oxidation of 2-aminoethanethiol (2-AET). By using these groups it is possible to modulate the Co(II)/(I) formal potential of the catalysts. The reported results show that the electrocatalytic activity of the adsorbed phthalocyanines containing electron-withdrawing groups decreases as the Co(II)/(I) formal potential of the catalyst becomes more positive, while electron-donating groups on the periphery of the ligand increase the catalytic activity.     

In situ monitoring of the mercury electrode surface in HClO4 solutions by means of infrared reflection absorption spectroscopy

In situ IRAS at a Hg–Au electrode in HClO₄ solutions shows that the adsorption state of water molecules on mercury changes depending on the applied potential as in the case on gold rather than on platinum. At the negatively charged electrode surface, it is expected that the dipole moment of the adsorbed water molecules is oriented toward the surface to some extent. The mercury–water interaction is considered to be stronger than the gold–water interaction. As the sample potential changes in the positive direction, the adsorbed water molecules are expelled and/or reorient and perchlorate ions adsorb on the Hg–Au electrode surface.     

Electrochemical detection of silver ions and the study of metal–polymer interactions on a polybenzidine film electrode

Silver ions (Ag(I)) were preconcentrated efficiently at open circuit from aqueous silver nitrate solution into polybenzidine (poly-Bz) film electrodes. The poly-Bz films were electrodeposited by means of cyclic voltammetry from benzidine (Bz) in acidic aqueous solutions. It was found that preconcentration of Ag(I) ions into the film is highly dependent on the pH, being more favored at neutral pH, that is, when the nitrogen in imine–amine groups in the polymer chain are not protonated. Under these conditions the Ag(I) ions do not compete with the H⁺ ions for the same site within the film. A simple diffusion model is assumed to explain the incorporation of Ag(I) ion into the poly-Bz electrodes. The distribution of the Ag(I) ion between the solution and film was established and a probable analytical application given.     

Oxalic acid adsorption and oxidation at platinum single crystal electrodes

Oxalic acid adsorption and oxidation processes have been studied at platinum single crystal electrodes with basal orientations. Cyclic voltammetry and charge displacement experiments have been combined with in situ external reflection infrared experiments for the study of oxalic acid adsorption at potentials below 0.70 V. Whereas reversible anion-like adsorption is observed for Pt(100) and Pt(111) electrodes, oxalic acid is reduced at Pt(110) leading to the formation of an irreversibly bonded adsorbate which has been identified as adsorbed CO from the infrared spectra. Spectroscopic data confirm the potential dependent behaviour of oxalic acid adsorption on Pt(111) and Pt(100) electrodes derived from electrochemical data. At the same time, the infrared spectra show distinct C–O stretching bands for adsorbates on Pt(100) and P(111). In acidic solutions (pH = 1), for which oxalic acid and bioxalate anions predominate as solution species, oxalate anions are adsorbed at the Pt(100) electrode surface whereas bioxalate anions seem to be the adsorbed species at Pt(111). In neutral solutions, oxalate anions are adsorbed on both Pt(111) and Pt(100) surfaces. No intermediates coming from oxalic acid other than adsorbed CO for Pt(110) and adsorbed (bi)oxalate anions for Pt(111) and Pt(100) are detected during the oxidation of oxalic acid. Adsorbate bands are observed between 1400 and 1600 cm^?1 for the Pt(111) electrode for potentials between 0.85 and 1.0 V. These bands are related to adsorbed carbonate anions formed in the presence of carbon dioxide molecules generated from oxalic acid oxidation.     

Electrochemical behaviour of oxocarbons on single crystal platinum electrodes Part 3. Croconic acid oxidation on Pt(111) surfaces in acid medium

The electrochemical behaviour of croconic acid (CRA) on Pt(111) electrodes has been studied in acid medium by cyclic voltammetry. Its oxidation proceeds via a dual path mechanism. The direct oxidation path takes place at potentials higher than 0.9 V, probably involving incipient surface oxide formation.The formation of strongly adsorbed species upon CRA adsorption is influenced by the concentration of CRA and the adsorption potential. At open circuit, only CO_ads is formed, while at controlled potentials lower than 0.40 V, other adsorbates, probably with more than one carbon atom, exist. These are oxidized to CO in the range 0.4–0.6 V. The activity for poison formation at open circuit is much lower for concentrations higher than 1 mM. FTIR experiments clearly indicate that carbon monoxide is not formed at potentials lower than 0.40 V. Linearly- and bridge-bonded CO adspecies appear at potentials higher than 0.40 V, which are oxidized to CO₂ before the direct oxidation of CRA begins. A band at 1780 cm^?1 indicates the presence of leuconic acid at potentials between 0.95 and 1.22 V as a product of the latter process. At higher potentials, CRA is completely oxidized to CO₂.     

The role of lipopolysaccharide on the electrochemical behavior of titanium

Lipopolysaccharide (LPS) may induce peri-implantitis and implant failure. However, the role of LPS in titanium (Ti) electrochemical behavior remains unknown. We hypothesized that LPS in saliva with different pHs affects Ti corrosion properties. Thirty-six Ti discs (15 mm × 3 mm) were divided into 12 groups according to saliva pH (3, 6.5, and 9) and Escherichia coli LPS concentration (0, 0.15, 15, and 150 μg/mL). Electrochemical tests, such as open circuit potential, potentiodynamic, and electrochemical impedance spectroscopy, were conducted in a controlled environment. Data were evaluated by Pearson correlation and regression analysis (α = 0.05). LPS and pH affected Ti corrosive behavior. In general, lower pH and higher LPS concentration accelerated Ti corrosion. In the control group, the increase of pH significantly reduced the corrosion rate and increased the capacitance of the double layer. In LPS groups, the decrease of pH significantly increased the corrosion rate of Ti. LPS negatively influenced Ti corrosion behavior. Abbreviations: C(dl), capacitance of double layer; E(corr), corrosion potential; EIS, electrochemical impedance spectroscopy; I(corr), corrosion current density; I(pass), passivation current density; LPS, lipopolysaccharide; OCP, open circuit potential; R(p), polarization resistance; Ti, titanium.     

An electrochemical impedance study on the interfacial behaviour of KC8 electrodes in LiClO4 containing electrolytes

The evolution of the impedance spectra of KC₈ electrodes in 1 M LiClO₄+ethylene carbonate+dimethylcarbonate solution have been studied under open circuit voltage as a function of time. Additional studies have been performed before and after potassium deintercalation and in cycled electrodes. It has been found that KC₈ slowly decomposes giving rise to a layer of insoluble KClO₄ on the electrode surface that protects the compound from further decomposition. The interface may be simulated using two passivating layers in addition to the charge transfer and diffusion impedance. The same type of equivalent circuit, with the eventual inclusion of a third layer, may be used to simulate the spectra after K deintercalation and during lithium intercalation.     

Platelet-derived growth factor enhancement of two alloplastic bone matrices

BACKGROUND: The use of alloplastic matrices that mimic the mineral phase of bone has become a viable alternative to current mainstream therapies in dentistry such as allografts and autogenous grafts. Because alloplastic bone substitutes generally have relatively poor osteogenic properties, analyzing their potential as vehicles to deliver growth factors is an important step in assessing methods to enhance their clinical efficacy. The aim of these studies was to treat beta-tricalcium phosphate (beta-TCP) and calcium sulfate (CaSO(4)) with platelet-derived growth factor (PDGF)-BB to enhance the osteogenic capabilities of these materials. METHODS: In the beta-TCP studies, PDGF-BB adsorption and release were accomplished using (125)I radiolabeled growth factor and non-radioactive human recombinant PDGF at a ratio of 1:300 M. For the adsorption studies, the radiolabeled PDGF-BB/ non-radioactive PDGF solutions with resultant PDGF concentrations of 10(7) and 10(8) M were incubated with beta-TCP from 1 to 120 minutes, and the amount of adsorbed (125)I-PDGF-BB was measured using a gamma counter. Similar adsorption studies were conducted with a 30-minute incubation of beta-TCP with various PDGF concentrations. In vitro release studies were conducted with beta-TCP to which radiolabeled PDGF had been adsorbed as above. Release studies were also conducted with CaSO(4) that was hydrated with the radioactive PDGF solution described above for the TCP studies. In vivo PDGF-BB release from beta-TCP and CaSO(4) was evaluated in a mouse model, where the radioactive PDGF/non-radioactive PDGF-BB treated beta-TCP or CaSO(4) sample was inserted subcutaneously and later removed for radioactive measurement. Proliferation of human osteoblastic cells in the presence of PDGF- treated beta-TCP or CaSO(4) was assessed by (3)H thymidine incorporation. RESULTS: The absorption studies revealed that PDGF-BB was absorbed in a concentration and time-dependent manner to beta-TCP. In the in vitro release studies, approximately 45% of the adsorbed PDGF-BB was released after 10 days. In vivo release from both materials occurred faster than in vitro release. Osteoblastic cells incubated with PDGF-BB-treated matrices showed significantly (P <0.05, ANOVA) greater proliferation than with control matrices alone. CONCLUSION: These experiments demonstrate the feasibility of using PDGF-BB in combination with alloplastic materials such as beta-TCP or CaSO(4) to serve as more effective bone graft materials with enhanced osteogenic properties.     

过氧化氢对银汞合金电化学腐蚀性能的实验研究

目的研究不同浓度过氧化氢对银汞合金电化学腐蚀性能的影响。方法采用电化学工作站的开路电位、电化学阻抗谱及动电位极化曲线法对不同浓度过氧化氢（0%、3.6%、10%及30%）浸泡24h后的牙科银汞合金在人工唾液中的腐蚀行为进行比较。结果 10%和30%过氧化氢浸泡后的银汞合金在人工唾液中开路电位较快地达到了一个稳定的值,且开路电位较短时间变化范围大小顺序为0%组〈3.6%组〈10%组〈30%组;不同浓度过氧化氢浸泡后的银汞合金在人工唾液中的等效电路极化电阻大小顺序为：0%组〉3.6%组〉10%组〉30%组,与动电位极化曲线腐蚀电位（Ecorr）结果一致;而自腐蚀电流密度（Icorr）结果为：0%组〈3.6%组〈10%组〈30%组。结论随着过氧化氢浓度的升高,银汞合金在人工唾液中的耐腐蚀性能越来越差。     

Surface Raman scattering studies on the adsorption of p-hydroxybenzoic acid at Au electrodes with ultraviolet excitation

The potential-dependent surface Raman spectrum of adsorbed p-hydroxybenzoic acid (PHBA) on a roughened Au electrode has been obtained with ultraviolet excitation at 325 nm. The surface Raman spectra of PHBA are sensitive to the negative shift of potential and the electrode potential at which the resonance (potential of maximum intensity) occurs varies with the vibrational mode, indicating that the PHBA molecules are chemisorbed on the Au surface. A non-radiative charge transfer mechanism could probably explain the experiment results in the present work. The probable reasons are given.     

Corrosion behavior of dental alloys used for retention elements in prosthodontics

The purpose of this study was to investigate the corrosion behavior of 10 different high noble gold‐based dental alloys, used for prosthodontic retention elements, according to ISO 10271. Samples of 10 high‐noble and noble gold‐based dental alloys were subjected to: (i) static immersion tests with subsequent analysis of ion release for eight different elements using mass spectrometry; (ii) electrochemical tests, including open‐circuit potential and potentiodynamic scans; and (iii) scanning electron microscopy, followed by energy‐dispersive X‐ray microscopy. The results were analyzed using one‐way ANOVA and Sidak multiple‐comparisons post‐hoc test at a level of significance of α = 0.05. Significant differences were found among the 10 alloys studied for all ions (P < 0.001). The potentiodynamic analysis showed values from ?82.5 to 102.8 mV for the open‐circuit potential and from 566.7 to 1367.5 mV for the breakdown potential. Both the open‐circuit and the breakdown potential varied considerably among these alloys. Scanning electron microscopy analysis confirmed the existence of typically small‐diameter corrosion defects, whilst the energy‐dispersive X‐ray analysis found no significant alteration in the elemental composition of the alloys. The results of this study reveal the variability in the corrosive resistance among the materials used for retention elements in prosthodontics.