Linear diffusion impedance. General expression and applications

Linear diffusion in thin film electrodes can be interpreted in terms of a matrix relationship. A general expression for the mass-transfer function and the diffusion impedance can be written, irrespective of the boundary conditions at the film surfaces. This expression applies, for example, to bounded or restricted diffusion in single-layer or multilayer electrodes and also to boundary conditions leading to resistive, capacitive or CPE interfacial behaviour.     

Determination of elastic modulus of demineralized resin-infiltrated dentin by self-etch adhesives

The purpose of this study was to determine ultrasonically the changes in elastic modulus of demineralized adhesive-infiltrated dentin. Dentin disks were obtained from bovine incisors and shaped into a rectangular form. The specimens were immersed in single-step self-etch adhesives, then stored in distilled water and run through thermal cycles between 5 and 60 degrees C. The longitudinal and shear wave sound velocities and the elastic modulus were determined using ultrasonic equipment composed of a pulser-receiver, transducers, and an oscilloscope. After 24 h of storage, the elastic modulus of mineralized dentin was 16.9 GPa and that of demineralized dentin was 2.1 GPa. The immersion of demineralized dentin in adhesives significantly increased the elastic modulus to 3.3-5.9 GPa. After 30,000 thermal cycles, the elastic modulus of dentin was 32.4 GPa, whereas that of demineralized adhesive infiltrated dentin was 3.1-4.1 GPa. Thermal stresses did not cause adhesive-infiltrated demineralized dentin to deteriorate, as measured by elastic modulus.     

Surface protection of copper by azoles in borate buffers-voltammetric and impedance analysis

The inhibitory action of five azoles namely benzotriazole (BTA), mercaptobenzothiazole (MBT), benzimidazole (BIMD), mercaptobenzimidazole (MBIMD) and thiadiazole (TDA) on copper was examined in sodium borate solution of pH 6.4, 8.4 and 10.4 using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). A considerable drop off in the corrosion rate of copper was detected in the presence of the investigated compounds. CV showed that the inhibitors were of mixed type that they affect both the anodic and cathodic currents. Impedance spectra were characterized in terms of the charge transfer resistance and transport of copper ions through the oxide layer. The results indicate that the surface layer is of dielectric nature, and its protection increases with increasing inhibitor concentration and solution pH. On the basis of calculated adsorption data the process was concluded as chemisorption. Some quantum chemical parameters were also calculated using ab initio Hartree–Fock 6-31G*basis set and were used to complement the experimental results. The relative inhibition efficiencies of the azoles in controlling the copper corrosion was found to be in order of MBT > TDA > MBIMD≈BIMD > BTA.     

Influence of elastic modulus mismatch between dentin and post-and-core on sequential bonding failure

Purpose

Clinical failures of teeth restored with post-and-core are critical issues for the survival of teeth and maintenance of oral functions. A tooth with post-and-core restoration is a complex structure. Cement adhesion is believed to be the weakest component, and breakage in this component leads to changes in stress distribution in the complex structure. The tested hypothesis was that cement breaking processes of prosthetic treated teeth were affected by elastic properties of post-and-cores.

A study by impedance spectroscopy of the reactivity of freshly polished galena electrodes: oxidation and interaction with ethylxanthate

The electrochemical behaviour of natural galena (PbS) is studied in relation to mineral flotation. Following a previous paper mainly devoted to the study of the semiconducting properties of such electrodes, the present one is focused on two important problems related to mineral processing; the sample oxidation at high positive potentials and the reactivity with two collection reagents, ethylxanthate (EtX^?), a collector, and Na₂S, a depressant. By electrochemical impedance spectroscopy, it is found that the surface oxidation begins beyond +0.14 V (SCE) in the absence of these reactants. The kinetic law for the growth of the oxidised films is established. However, the competition between the adsorbabilities of OH^?, collector and depressant, seems to play a determining role in the recovery or depression of galena.     

Protonation and deprotonation of cysteine and cystine monolayers probed by impedance spectroscopy

The desorption mechanism for electrochemically-formed self-assembled monolayers from cysteine and cystine on a Au(1 1 1) electrode was investigated. The monolayer desorption in 0.1 M KClO₄ is found to occur in two steps with differing potentials. At potentials more positive than the desorption potential (circa −0.65 V vs. SCE), a quasi-reversible process, assigned to protonation/deprotonation, is observed. This process is found to occur at very discreet potentials when cysteine monolayers are probed by impedance spectroscopy. Monolayers derived from the disulfide (cystine) are found to protonate/deprotonate over a wider potential range. This suggests that electrochemically-formed monolayers obtained from disulfides do not afford the same level of organization as those derived from thiol species. The value of the double layer capacitance increases from 15 to 30 μF cm⁻² after deprotonation. This suggests that the adsorption of cysteine weakens following the deprotonation. As expected, the protonation/deprotonation step is pH dependent and it is not observed in strong basic medium (0.1 M NaOH).     

Analysis of the impedance spectra of Pt|poly(o-phenylenediamine) electrodes – hydrogen adsorption and the brush model of the polymer films

Electrochemical impedance spectra obtained for poly(o-phenylenediamine) films on platinum in contact with 0.5 M H₂SO₄ solution were analyzed on the basis of the brush model elaborated for conducting polymer films on electrodes. It is assumed that the polymer film consists of short and long polymer chains forming bundles in such a way that only a small part of the metal substrate is covered by the polymeric material. The analysis of the impedance spectra taken at several potentials in the region of hydrogen adsorption on platinum supports the validity of the model. The model has been tested also by varying the film thickness and the roughness of the platinum substrate.     

Detection and evaluation of measurement noise and stochastic non-linear distortions in electrochemical impedance measurements by a model based on a broadband periodic excitation

A measurement model based on a broadband periodic excitation and applicable to electrochemical impedance spectroscopy was implemented. The model was able to detect, quantify, and separate the present measurement noise and stochastic non-linear contributions. By means of it, the impedance response of copper in neutral chloride solutions containing millimole additions of imidazole and pyrazole was measured. The results obtained showed that the different electronic structures and molecular conformations of the structural isomers studied significantly affected both the impedance spectra and the magnitude of the non-linear distortions.     

Hydrogen insertion in metals: the mechanism and effect of recombination of ad-atoms

The model for hydrogen evolution reaction via a Volmer–Tafel mechanism has been considered, in which a recombination stage was represented as a set of successive and contemporary reactions. The solution of the respective kinetic equation has been obtained for the steady-state regime, which enables us to analyze the balance of the surface processes affecting the metal hydrogenation. It is demonstrated that the diffusing hydrogen flux into the metal bulk is weaker in the case of surfaces with greater concentration of adsorption centres and weaker bond energy between the active centres and hydrogen atoms.     

Intra and intermolecular hydrogen bonding effects in the electrochemical reduction of α-phenolic-naphthoquinones

The electrochemical reduction of 1,4-naphthoquinone (NQ), 5-hydroxy-1,4-naphthoquinone (HNQ) and 5,8-dihydroxy-1,4-naphthoquinone (H₂NQ) has been studied in dimethylsulfoxide and acetonitrile, both in the absence and in the presence of proton donors such as methanol and acetic acid. It was established that the intervention of intramolecular hydrogen bonds in the α-phenolic-naphthoquinones (β-hydroxy-naphthoquinones) contributes significantly to the stabilization of the quinone reduced species. This effect is particularly important as a factor determining the strength of intermolecular interactions between the quinone dianion and proton donors. The intramolecular hydrogen bond diminishes the association capacity of the quinones and of their reduction products with molecules of methanol, and avoids the protonation of the semiquinone with acetic acid, favoring only association processes. The low reactivity of the reduced intermediates towards the protonation by acetic acid suggests that analogues structures of HNQ and H₂NQ will not be protonated.     

Theoretical and experimental study of the catalytic hydrogen evolution reaction in the presence of an adsorbed catalyst by means of square-wave voltammetry

Surface catalytic electrode mechanism based on the hydrogen evolution reaction is analyzed both theoretically and experimentally under conditions of square-wave voltammetry (SWV). The electrode mechanism involves preceding chemical reaction in which the adsorbed catalyst (Cat_(ads)) is undergoing protonation at the electrode surface, i.e., ${\mathrm{Cat}}_{(\mathrm{ads})}+{\mathrm{H}}_{(\mathrm{aq})}^{+}?{\mathrm{CatH}}_{(\mathrm{ads})}^{+}$. The protonated form of the catalyst $({\mathrm{CatH}}_{(\mathrm{ads})}^{+})$ is irreversibly reduced yielding the initial form of the catalyst and atomic hydrogen, i.e., ${\mathrm{CatH}}_{(\mathrm{ads})}^{+}+\mathrm{e}\to {\mathrm{Cat}}_{(\mathrm{ads})}+{\mathrm{H}}_{(\mathrm{aq})}$. The concentration of protons is assumed to be constant in the course of the voltammetric experiment due to the buffered solution, whereas the current is controlled by the variation of the surface concentration of both unprotonated and protonated forms of the catalyst. The overall voltammetric behavior of the system is a specific combination of a simple surface irreversible electrode reaction, surface catalytic reaction and surface reaction preceded by a chemical reaction (CE mechanism). The effect of the thermodynamics and kinetics of the preceding chemical reaction, as well as the kinetics of the irreversible electrode reaction, to the SW voltammetric response is examined in detail. The overall catalytic effect is predominantly controlled by the ratio of the protonation rate constant and the time window of the voltammetric experiment that is represented by the frequency of the potential modulation. Theoretical predictions are illustrated by experiments with the hydrogen evolution reaction at the hanging mercury drop electrode catalyzed by adsorbed famotidine.     

H2S在模拟正畸力作用下对人牙周膜细胞内ALP表达的影响

目的 研究模拟临床正畸力的张力作用下,人牙周膜细胞内碱性磷酸酶(alkaline phosphatase,ALP)在不同浓度硫化氢(H2S)的刺激下,mRNA及蛋白表达的变化.从而探索气体信号分子H2S调控牙周组织改建的机理,为进一步的临床应用研究打下基础.方法 组织块法原代培养人牙周膜细胞(hPDLCs),传代培养并进行细胞来源鉴定.以不同浓度H2S对hPDLCs处理24 h及采用基底形变加载方式对hPDLCs施加张力1、3、6h,采用qRT-PCR和Western blotting检测hPDLCs内ALP的表达变化.结果 hPDLCs内ALP的mRNA水平和蛋白水平在H2S处理24 h后及张力处理1、3、6h后均升高,且在张力作用下更为明显(P＜0.05).结论 在相对较短的时间段内,H2S可上调ALP的mRNA和蛋白水平的表达.     

Electrode reactions and accumulation of hydrogen at carbon paste electrodes in the presence of tetrachloropalladate

The electrochemical behaviour of the unmodified carbon paste electrode (CPE) in the presence of [PdCl₄]^2? in chloride solutions of different pH (in the range from 3 to 7) is studied by cyclic voltammetry (CV). During cathodic treatment or cycling, Pd(0) is deposited in situ on the electrode surface. Results obtained at negative potentials as a function of the concentrations of Pd(II) and protons in the solution are presented. It is concluded that hydrogen ions are reduced in different ways: first, the reduction at deposited Pd(0) or the simultaneous reduction of H⁺ and [PdCl₄]^2?, respectively, which occurs during the whole cathodic cycle; second, the reduction in the potential range from about ?0.5 to ?1.0 V versus SCE, where surface groups are reduced at the unmodified CPE. The second pathway is the prevailing reduction at lower pH, which occurs at the carbon surface probably via protonated chloropalladate complexes adsorbed at the electrode surface. Potentiostatic pretreatment in the voltammetric range between 0 and +0.2 V leads in solutions of pH 5 to a pronounced increase of the H⁺ reduction current caused by an increase of the sorption capacity of the electrode or by the accumulation of protons at the electrode surface. Protons could be accumulated at the electrode surface by formation of protonated chloropalladate complexes at the CPE.     

Combined effects of hydrogen sulfide and lipopolysaccharide on osteoclast differentiation in rats

Background: Lipopolysaccharide (LPS) stimulates osteoclast differentiation through toll‐like receptors (TLRs) 2 and 4, and hydrogen sulfide (H₂S) induces osteoclast differentiation. If H₂S activates TLRs, H₂S may enhance the effects of LPS on osteoclast differentiation. The purpose of the present study is to examine the combined effects of sodium hydrogen sulfide (NaHS, an H₂S donor drug) and LPS on osteoclast differentiation and TLR expression in rat periodontal tissue. Methods: Twenty‐eight male Wistar rats (8 weeks old) were divided into four groups (n = 7 per group): a control (no treatment) group and three experimental groups (NaHS group, LPS group, and a combination [NaHS + LPS] group). At 1 day after topical application of NaHS and/or Porphyromonas gingivalis LPS into the gingival sulcus of first molars, the number of tartrate‐resistant acid phosphate (TRAP)‐positive osteoclasts in the periodontal tissue was counted. Expression of TLR2 and TLR4 mRNAs and proteins in the gingival was also assessed. Results: The number of TRAP‐positive osteoclasts was significantly higher in the combination group than in any other group (P <0.01). The combination group had 11.0‐fold higher TLR4 mRNA levels than the control group. TLR4 protein levels were also higher in the combination group than in the NaHS or LPS group. However, the TLR2 mRNA and protein levels were not significantly different in the combination group and the LPS group. Conclusion: In rat periodontal tissue, NaHS and LPS had an additive effect on osteoclast differentiation through activation of the TLR4 pathway but not the TLR2 pathway.     

Surface copper immobilization by chelation of alizarin complexone and electrodeposition on graphite electrodes,and related hydrogen sulfide electrochemistry; matrix isolation of atomic copper and molecular copper sulfides on a graphite electrode

The irreversibly adsorbed alizarin complexone (AC) was employed to immobilize and maintain Cu^II ions on the graphite electrode. The coordination chemistry between the adsorbed alizarin complexone ligand and the Cu^II ion on the surface was examined by surface cyclic voltammetry. Upon reduction of the Cu^II center to a Cu⁰ atom, a submonolayer of individual atoms of Cu⁰ rather than a continuous layer is formed on the electrode surface. The immobilized surface displays electrocatalytic activity towards the oxidation of sulfide ion from [S^2?] ion to S⁰. The electrocatalytic activity for the sulfide oxidation on a [Cu^II(AC)]^? adsorbed electrode is shown to be essentially identical with that of a electrode that contains an electrodeposited submonolayer of Cu⁰. The active catalyst in both cases is identified to be a submonolayer of cupric sulfide. The electrochemistry of the Cu⁰ submonolayer-coated electrode in aqueous solution containing hydrogen sulfide was also examined. When the modified electrode was polarized from ?1.1 V to 0.2 V, three electrode processes were observed. The first, near ?0.7 V, is a surface reaction between surface Cu⁰ and adsorbed [S^2?] ion to form a submonolayer of cuprous sulfide. The second appeared near ?0.23 V and is another surface process between Cu₂S and adsorbed sulfide ion to form a submonolayer of cupric sulfide. The third reaction is the electrochemical oxidation of sulfide ion catalyzed by CuS to form sulphur which deposits on the electrode surface when the potential is positive of ?0.2 V.     

Determination of mineral changes in human dental enamel by longitudinal microradiography and scanning optical monitoring and their correlation with chemical analysis

Both longitudinal microradiography (LMR) and scanning optical monitoring (OM) are non-destructive methods for measuring mineral changes in dental tooth enamel slices with time at 169 locations on the slice. Average calcium losses from four human tooth enamel slices (300-400 micron thickness), etched in HClO4, were determined by LMR and chemical analysis (C). As predicted from theory, LMR and C correlate very well (r = 0.99), but the appearance of a systematic error of unknown source of 30% made with LMR, C, or both could not be avoided. Another, more complex, experiment concerned six human tooth enamel slices of the same thickness which were demineralized in an aqueous buffered acid solution containing Ca and PO4. From this experiment it was found that average calcium loss as measured by C and LMR correlated well with the optical scattering as measured with scanning OM (Spearman rank correlation rs approximately equal to 0.79). It was also found that three-dimensional plots of local calcium loss by LMR and scanning OM as a function of tooth slice surface position show a well-defined tooth-dependent increase due to local demineralization and rather similar behaviour with time. From the experiments it follows that LMR and scanning OM are reliable methods to determine the mineral change in a tooth tissue as a function of local tooth slice surface position and of time and that with LMR and scanning OM time- and position-dependent measurements with an oral device become feasible.     

Activity of jaw-opening and jaw-closing muscles and their influence on dentofacial morphological features in normal adults

The purpose of this study was to determine the relationship between the dentofacial morphology and the function of masticatory muscles estimated by kinesiological measurements during isotonic jaw-opening (O) and closing (C) muscle contractions. We examined 26 male adult subjects who had no history of masticatory dysfunction. Three kinesiological parameters, the maximum force (F(max)), maximum velocity (V(max)) and maximum power (P(max)) were measured in both O and C muscles. Correlation analysis was performed between these parameters and dentofacial morphological features determined on lateral roentgenographic cephalograms. The F(max) (C), V(max) (O), log O/C of V(max), and P(max) (O) significantly correlated with anterior overbite. P(max) (C) significantly and negatively correlated with SN-ArG. Furthermore, log O/C of P(max) significantly and negatively correlated with SNA, SNB and ArG/GMe, and positively with SN-ArG. These results, combined with our previous findings that electromyographic muscle activity during isometric contraction correlated significantly with mandibular morphology, indicate that the dentofacial morphology is influenced by the function of both O and C muscles.     

Studies on the redox behaviour of some polythiophene derivatives by impedance spectroscopy in symmetrical and asymmetrical configurations

The generation and transport of ionic and electronic charges in three polythiophene derivatives, namely poly(3-methyl thiophene), poly(thiophene-3-methanol) and poly(3-thienyl methacrylate), have been studied using impedance spectroscopy in both asymmetrical (Pt ∣ polymer film ∣ electrolyte) and symmetrical (Pt ∣ polymer film ∣ metal contact) configurations. The symmetrical measurements are to our knowledge reported for the first time in the literature for these polymer films. The homogeneous film model proposed by Vorotyntsev was found to describe adequately the impedance spectra for poly(3-methyl thiophene) in the asymmetrical configuration. For the two other polymers, the use of the ordinary Nernst–Planck equations to describe the charge transport seems to be an oversimplification. The impedance spectra of the systems studied in the symmetrical configuration were found to be described adequately by an extension of the homogeneous film model for the metal ∣ film ∣ metal contact. On the basis of the calculated impedance parameters, estimates for the diffusion coefficient and concentration of ionic and electronic charge carriers in the film as a function of potential are obtained. The results are discussed in connection with previous dc resistance and FTIR/UV–vis spectroscopic measurements of the same materials.     

An in vitro study of the antimicrobial activity of some endodontic medicaments and their bases using an agar well diffusion assay

Background:  The aim of this study was to determine the in vitro antimicrobial activities of various endodontic medicaments and their bases against selected organisms using an agar diffusion assay.
Methods:  An agar well diffusion assay was used to test the antimicrobial action of some commonly used endodontic medicaments (Ledermix paste, Pulpdent paste, Ultracal paste, and a 50:50 mix of Ledermix and Pulpdent pastes) and their bases. Three bacterial species ( E. faecalis, P. micros, P. intermedia ) and one yeast ( C. albicans ) were selected. The diameters of growth inhibition zones and pH were assessed.
Results:  P. micros demonstrated the highest level of in vitro resistance. Pulpdent and Ultracal pastes had the highest pH (12.64 and 12.53, respectively). The addition of Pulpdent to Ledermix did not increase the zone sizes significantly.
Conclusions:  All the commercial products showed some in vitro antimicrobial activity. Ledermix paste and the 50:50 Ledermix/Pulpdent mixture being the most effective in this model. The known anti-inflammatory/analgesic properties of Ledermix and the results from this agar model suggest that the 50:50 Ledermix/Pulpdent combination would be the preferred medicament for clinical use in symptomatic cases, even though the addition of calcium hydroxide to Ledermix did not appear to be synergistic in terms of enhancing the antimicrobial action.