Physical chemical effects of zinc on in vitro enamel demineralization

Objectives

Zinc salts are formulated into oral health products as antibacterial agents, yet their interaction with enamel is not clearly understood. The aim was to investigate the effect of zinc concentration [Zn²⁺] on the in vitro demineralization of enamel during exposure to caries-simulating conditions. Furthermore, the possible mechanism of zinc's action for reducing demineralization was determined.

Methods

Enamel blocks and synthetic hydroxyapatite (HAp) were demineralized in a range of zinc-containing acidic solutions (0–3565 ppm [Zn²⁺]) at pH 4.0 and 37 °C. Inductively coupled-plasma optical emission spectroscopy (ICP-OES) was used to measure ion release into solution. Enamel blocks were analysed by Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR), and HAp by X-ray diffraction (XRD) and neutron diffraction (ND).

Results

ICP-OES analysis of the acidic solutions showed a decrease in [Ca²⁺] and [PO₄³⁻] release with increasing [Zn²⁺]. FTIR revealed a α-hopeite (α-Zn₃(PO₄)₂.4H₂O)-like phase on the enamel surfaces at >107 ppm [Zn²⁺]. XRD and ND analysis confirmed a zinc-phosphate phase present alongside the HAp.

Conclusions

This study confirms that zinc reduces enamel demineralization. Under the conditions studied, zinc acts predominantly on enamel surfaces at PO₄³⁻ sites in the HAp lattice to possibly form an α-hopeite-like phase.

Clinical significance

These results have a significant implication on the understanding of the fundamental chemistry of zinc in toothpastes and demonstrate its therapeutic potential in preventing tooth mineral loss.     

Effects of fluoride concentration on enamel demineralization kinetics in vitro

Objectives

The aim of the present study was to measure the effects of fluoride concentration on the real-time in vitro demineralization of enamel during exposure to caries-simulating conditions using Scanning Microradiography (SMR).

Methods

Enamel blocks obtained from non-carious human molars were fixed in SMR environmental cells, through which acidic solutions (0.1 M acetic acid, pH 4.0) were circulated for periods of 48 h. SMR was used to quantitatively measure continuous mineral mass loss. Subsequently, the effects of sequentially increasing fluoride concentration (0.1–4500 mg/L [F⁻]) in the acidic solutions were measured on the rate of enamel demineralization.

Results

The data shows a log-linear relationship between [F⁻] and reduction in demineralization up to 135 mg/L [F⁻]. Above 135 mg/L, no further significant decrease in demineralization occurred.

Conclusion

The optimum range of local fluoride concentration for reducing enamel demineralization was in the range 0.1–135 mg/L [F⁻] under the conditions studied.

Clinical significance

Relatively low [F⁻] can exhibit near-optimum protection. Increasing the fluoride concentrations above 135 mg/L may not necessarily give an increased cariostatic benefit. Improving the means of delivery of relatively low fluoride concentrations to the oral fluids through slow releasing mechanisms, such as the oral fluoride reservoirs, is the more appropriate way forward for sustaining long-term clinical efficacy.     

In vitro demineralization of human enamel natural and abraded surfaces: A micromechanical and SEM investigation

Objectives

To compare the demineralization pattern that occurs in abraded samples and unabraded ones quantitatively and microscopically using nanoindentation and SEM.

Methods

Using 12 human third molars, one half of each tooth was abraded to a depth approximately 200 μm below the outer surface enamel, and the other half was left untreated. All specimens were demineralized for 3 d, 5 d, 7 d and 14 d respectively. The cross sections of all lesions were evaluated with nanoindentation and SEM.

Results

Unabraded samples had least loss of mechanical properties and lesion depth compared to abraded samples, with most mechanical property loss occurring within a shallower layer of the lesion. The variation of mechanical properties in unabraded samples was wider than abraded samples. SEM images showed a characteristic “keyhole” structure for both samples after demineralization, with the rod core extensively demineralized while the interrod remained intact. Acid attack initiated at the rod sheath space then penetrated into the rod core before extending into neighbouring rods through the rod tail.

Conclusion

Abraded samples exposed to in vitro demineralization form deeper lesion depths and greater loss of mechanical properties than unabraded samples subjected to the same demineralization. Unabraded samples manifested characteristic subsurface demineralization with a shallow surface layer that remained intact, whereas no intact surface was found in abraded samples. The demineralization pattern of unabraded samples more closely resembled the pattern of natural white spot lesions and displayed wide inter-sample variation. Consideration should be given to experimental design with unabraded teeth specimens for future demineralization studies.     

Effect of fluoride varnish supplemented with sodium trimetaphosphate on enamel erosion and abrasion: An in situ/ex vivo study

Objective

To evaluate the effect of fluoride (F) varnishes supplemented or not with sodium trimetaphosphate (TMP) on enamel erosive wear followed or not by abrasion in situ.

Methods

Ten volunteers were selected and randomly divided into four groups, according to the varnishes tested: placebo (no F or TMP), 5% NaF (positive control), 2.5% NaF and 2.5% NaF/5% TMP. Enamel blocks (n = 4) were mounted in palatal devices and received an application of each test varnish, following a double-blind, crossover protocol. After 6 h, varnishes were completely removed and the blocks were subjected to erosive challenges by ex vivo immersion in citric acid (5 min, 4×/dia, 5 days). Following, half of the blocks were subjected to abrasion by brushing with a placebo dentifrice slurry for 15 s. Enamel wear (μm), surface hardness (SH_f) and cross-sectional hardness (ΔKHN) were assessed after each experimental period. Results were analyzed by ANOVA, Student–Newman–Keuls's test and Pearson correlation coefficient (p < 0.05).

Results

The fluoride varnish supplemented with TMP promoted significantly lower wear and ΔKHN when compared to the other groups after erosive challenges, followed or not by abrasion (p < 0.05). As for (SH_f) the fluoride varnish supplemented with TMP promoted similar results to the 5% NaF product, being significantly higher than the remaining groups after erosive and erosive + abrasive challenges (p < 0.05).

Conclusion

TMP significantly enhanced the effects of F on enamel wear after erosive challenges, followed or not by abrasion.     

Effect of experimental mouthrinses containing the combination of NaF and TiF4 on enamel erosive wear in vitro

Objective

This study analysed the anti-erosive effect of experimental solutions containing TiF₄ and NaF.

Methods

Bovine enamel samples (n = 15) were treated with: (1) commercial solution with SnCl₂/NaF (Erosion Protection^®); (2) experimental solution with 0.0815% TiF₄; (3) 0.105% NaF; (4) 0.042% NaF + 0.049% TiF₄; (5) 0.063% NaF + 0.036% TiF₄ or (6) control. The samples were submitted to pH cycles and daily fluoride applications for seven days. The enamel wear was measured using a contact profilometer and analysed by ANOVA (p < 0.05).

Results

The best anti-erosive effect was found for experimental solution with 0.0815% TiF₄ (99% reduction in enamel wear), followed by SnCl₂/NaF (78%) and 0.049% TiF₄ + 0.042% NaF solution (41%).

Conclusions

The experimental solution containing a specific combination of TiF₄ + NaF has the ability to partially reduce enamel erosion.     

Effect of fluoride toothpaste with nano-sized trimetaphosphate on enamel demineralization: An in vitro study

ObjectiveThis study evaluated the effect of toothpastes containing 1100 ppm F associated or not with micrometric or nano-sized sodium trimetaphosphate (TMP) on enamel demineralization in vitro, using a pH cycling model.DesignBovine enamel blocks (4 mm × 4 mm, n = 96) were randomly allocated into eight groups (n = 12), according to the test toothpastes: Placebo (without fluoride or TMP); 1100 ppm F (1100F); 1100F plus micrometric TMP at concentrations of 1%, 3% or 6%; and 1100F plus nanosized TMP at 1%, 3% or 6%. Blocks were treated 2×/day with slurries of toothpastes and submitted to a pH cycling regimen for five days. Next, final surface hardness (SHf), integrated hardness loss (IHL), differential profile of integrated hardness loss (ΔIHL) and enamel fluoride (F) concentrations were determined. Data were analyzed by ANOVA and Student-Newman-Keuls’ test (p < 0.05).ResultsThe use of 1100F/3%TMPnano led to SHf 30% higher (p < 0.001) and IHL ∼ 80% lower (p < 0.001) when compared to 1100F. This toothpaste also resulted in ∼64% reduction of mineral loss (ΔIHL) when compared to 1100F. Moreover, the addition of nano-sized TMP promoted increases in enamel F uptake of 90%, 160% and 100%, respectively for the concentrations of 1%, 3% and 6%, when compared to 1100F (p < 0.001).ConclusionThe addition of nano-sized TMP at 3% to a conventional toothpaste significantly decreased enamel demineralization when compared to its counterparts without TMP or supplemented with micrometric TMP.     

不同乳制（饮）品对离体乳牙牙釉质脱矿作用的比较研究

目的：比较3种市售乳制（饮）品对离体乳牙牙釉质的脱矿作用,为儿童合理选择饮用乳制（饮）品提供依据。方法将离体乳牙浸泡在奶粉液（奶粉液组）、酸奶（酸奶组）、果汁型乳饮品（果汁型乳饮品组）、蒸馏水（蒸馏水组）等乳制（饮）品中1周,用扫描电镜观察并比较经不同乳制（饮）品浸泡后乳牙牙釉质表面超微结构的改变;用维氏显微硬度仪测定并比较乳牙牙釉质表面显微硬度的变化。结果3种乳制（饮）品均可导致离体乳牙牙釉质表面显微硬度下降（P＜0.05）。不同乳制（饮）品对离体乳牙牙釉质脱矿作用程度不同（F=20.14,P<0.05）,果汁型乳饮品浸泡后的乳牙（果汁型乳饮品组）牙釉质表面显微硬度最低,其次是奶粉液组和酸奶组。离体乳牙经3种不同乳制（饮）品浸泡1周后,其釉质表面均有不同程度脱矿现象,扫描电镜下观察可见釉质表面形态不规则,釉柱出现裂纹、空洞等溶解脱矿现象,局部可见蜂窝状或深坑样外观。结论各种乳制（饮）品均可导致乳牙牙釉质不同程度的脱矿,其中果汁型乳饮品脱矿较为严重。     

Effect of rinsing with water immediately after neutral gel and foam fluoride topical application on enamel remineralization: An in situ study

Objective

The aim of this study was to evaluate, in situ, the effect of rinsing with water immediately after neutral fluoride foam application (Foam) or fluoride gel application (Gel).

Design

Ten volunteers wore acrylic palatal appliances containing 4 enamel blocks selected by surface hardness with artificial caries. Five experimental regimes of 3 days each were set according to treatment: placebo; Gel and Foam followed by no rinsing or consuming of liquids or solids for the next 30 min; Gel and Foam followed by immediately washing with water jet. After each phase, surface hardness was again measured for analysis of mineral gain, evaluated through percentage of surface hardness recovery (%SHR) and integrated loss of subsurface hardness (ΔKHN). The concentration of loosely bound fluoride (CaF2) and firmly bound fluoride (FA-like) formed and retained were also determined.

Results

Fluoride treatments produced greater remineralization (%SHR and ΔKHN) compared to placebo group (p < 0.05). There was no difference in the ability to promote remineralization and in the concentration of fluoride formed and retained, in each analysis, between Gel and Foam (p > 0.05).

Conclusion

The data suggest that rinsing with water immediately after topical application does not seem to have an influence on the ability of fluoride to promote remineralization.     

The erosive potential of some alcopops using bovine enamel: An in vitro study

Objectives

Alcoholic soft drinks have become increasingly popular and have high concentrations of citric acid and alcohol so might have the potential to cause dental erosion. This study aimed to investigate the erosive potential of alcopops on bovine enamel in vitro.

Methods

Six bovine upper incisors were prepared and sectioned to give six slabs per tooth, 4 mm × 4 mm each. Each slab was covered with nail varnish, leaving an exposed window (2 mm × 2 mm). Samples were immersed in 20 ml of each of the test solutions for 20 min, 1 h, and 24 h under gentle agitation (100 rpm). Enamel surface loss was determined using Quantitative Laser Fluorescence (QLF), Non-contact Profilometry (NCP) and Transverse Microradiography (TMR).

Results

Enamel loss occurred with all test drinks and the positive control (p < 0.05), and the depth of lesion correlated with pH and time. No significant difference was observed between 20 min and 1 h exposure, although both times had significantly (p < 0.05) greater erosion when compared with baseline. Within each alcopops group significant erosion had occurred at 24 h exposure compared with the baseline and previous times.

Conclusion

All the tested alcopops resulted in significant enamel loss at 24 h (p < 0.001) with direct correlation between degree of enamel loss and both pH and increasing exposure time.     

Effect of open-sandwich vs. adhesive restorative techniques on enamel and dentine demineralization: An in situ study

Objectives

To assess in situ the effect of different restorative techniques used with fluoride-releasing materials on enamel and dentine demineralization in the presence of a cariogenic challenge.

Methods

Thirty human molars were prepared for 60 Class V cavities, with enamel and dentine margins. The teeth were divided into four groups (n = 15): L1 – open sandwich technique with a conventional glass ionomer cement (GIC), L2 – open sandwich technique with a resin-modified GIC, A1 – total-etch bonding with a fluoride-containing adhesive, A2 – total-etch bonding with a nonfluoride-containing adhesive. All the cavities were restored with a nanofilled composite. Fifteen volunteers used appliances containing one specimen from each group. The cariogenic challenge was carried out with a 20% sucrose solution 8×/day for 7 days. The specimens were sectioned for microhardness test and EDS analysis at different depths below the enamel and dentine margins (25, 50 and 100 μm) and distances from the tooth-restoration interfaces (25 and 75 μm). The data were analyzed by ANOVA and Games–Howell test (α = 5%).

Results

Both GICs produced higher hardness in enamel at all depth-and-distance combinations, but only L1 produced higher hardness in dentine (p < 0.05). L2 and A1 exhibited similar dentine hardness at 25 μm distance for all depths (p > 0.05). L1 demonstrated significantly higher amounts of calcium in dentine than the other groups, but had similar amounts in enamel to L2 and A1.

Conclusions

The open sandwich technique using conventional GIC proved more effective in reducing enamel and dentine demineralization at depths of up to 100 μm under a cariogenic challenge.

Clinical significance

Conventional GICs should be considered the materials of choice for lining of cavities not having all margins in enamel, particularly using the open sandwich technique.     

The effect of epigallocatechin-3-gallate (EGCG) on human alveolar bone cells both in vitro and in vivo

Objective

The effects of epigallocatechin-3-gallate (EGCG), a major catechin in green tea, on human and mouse osteoblasts remain controversial. This study investigated the direct effects of EGCG on human alveolar bone-derived cells (hABCs) both in vitro and in vivo.

Design

hABCs which were collected from eight children (aged 7–9 years, seven males and one female) were treated with EGCG at various concentrations (1, 5, 10, 25, and 50 μM), and a proliferation assay, flow cytometric analysis for apoptosis evaluation, migration assay, and in vitro osteogenic differentiation were performed. hABCs that were pretreated with 10 μM EGCG and mixed with calcium phosphate carrier combined with EGCG (0.1, 0.5, or 1.5 mg) in vivo were transplanted into immunodeficient mouse. Histological staining, quantitative gene expressions, and alkaline phosphatase activity were evaluated in the retrieved transplants.

Results

The proliferation and migration were decreased when EGCG was present at over 25 μM. The osteogenic differentiation increased slightly when EGCG was present at up to 10 μM, and clearly decreased for higher concentrations of EGCG. In vivo, the potential for hard-tissue formation was slightly higher for the group with 0.1 mg of EGCG than for the control group, and decreased sharply for higher concentrations of EGCG.

Conclusion

The present observations suggest that EGCG at a low concentration can slightly enhance the osteogenic effect in vivo, whereas at a higher concentration it can prevent the osteogenic differentiation of hABCs both in vitro and in vivo.     

Effect of fluoride gels supplemented with sodium trimetaphosphate on enamel erosion and abrasion: In vitro study

Objective

This in vitro study aims to evaluate the effect of low fluoride (F) gel associate sodium trimetaphosphate (TMP) on erosion with or without abrasion.

Design

Enamel blocks (4 mm × 4 mm) selected through surface hardness (SH) is divided into five groups (n = 12): gel without F and TMP (placebo), gel containing 4500 ppm F (4500), gel containing 4500 ppm F plus TMP5% (4500 TMP5%), gel containing 9000 ppm F (9000), and gel containing 12,300 ppm F (acid gel). Those groups were additionally subdivided into conditions of erosion (Ero) and of erosion plus abrasion (Ero/Abra). The blocks have undergone a single application of gel on the first day of the study. The erosion challenge was produced by Sprite Zero^® for five minutes four times a day and abrasion was carried out by machine brushing for 15 s. After the challenges, the surface hardness (%SH), wear and cross-sectional hardness (ΔKHN) were analyzed. The data were analyzed using a 2-way ANOVA test followed by a Student-Newman–Keuls (p < 0.05).

Results

Lower values of %SH, wear and ΔKHN were observed for erosion challenge (p < 0.001). The %SH was lower in groups treated with fluoride gels, differing in the placebo (p < 0.05). With addition of TMP to the gel 4500, enamel wear was lower when compared with another groups (p < 0.05).

Conclusion

In vitro conditions, the 4500 5%TMP gel showed greatest effect against erosion and erosion/abrasion.     

Effect of desensitising toothpastes on dentinal tubule occlusion: A dentine permeability measurement and SEM in vitro study

Objectives

The purpose of this study was to evaluate the in vitro effectiveness of a new bioglass-containing and two commercial desensitising toothpastes on dentinal tubule occlusion after citric acid challenge or artificial saliva (AS) immersion.

Methods

One hundred dentin discs from human third molars were used. Specimens were randomly divided into five groups (n = 20), Group 1: EDTA-treated dentin; Group 2: brushing with distilled water; Group 3: brushing with Novamin; Group 4: brushing with Sensodyne Freshmint; Group 5: brushing with Colgate Sensitive. In each group, samples were then equally split into two subgroups (n = 10) to test two post-treatments: 6% citric acid challenge or 24 h immersion in artificial saliva. Dentine permeability of each specimen was measured before and after each treatment using a hydrostatic device working at 20 cm H₂O pressure. Data were analysed by two-way repeated measures ANOVA to determine if there were any significant differences within or between groups. Dentine morphology and surface deposits were observed by SEM.

Results

All three desensitising toothpastes significantly reduced dentine permeability and created precipitates on the treated dentine surfaces. Moreover, the reductions in dentine permeability showed partial recovery after a citric acid and artificial saliva immersion. Sensodyne showed significant resistant to acid attack and Novamin exhibited the lowest permeability after artificial saliva immersion for 24 h.

Conclusions

The application of the three toothpastes resulted in effective dentinal tubule occlusion. However, the new bioglass-containing toothpaste (Novamin) represented excellent occlusion effects after brushing treatment and AS immersion, while Sensodyne demonstrated more reduction in permeability when citric acid challenged.     

Effect of a 10% carbamide peroxide on wear resistance of enamel and dentine: In situ study

Objectives

This triple-blind, 2 × 2 crossover in situ study, was undertaken to verify whether the wear resistance of enamel and root dentine would be affected by bleaching with a 10% carbamide peroxide agent and a placebo agent.

Methods

Thirty slabs of each substrate (2 mm × 3 mm × 2 mm) were selected for each phase, after flattening and polishing procedures and microhardness test. After a 7-day lead-in period, one specimen of each substrate was randomly bonded on the facial surface of each one of 30 subject's upper second premolars. The volunteers received instructions on how to perform toothbrushing and application of gel in the tray. Fifteen volunteers bleached their maxillary arch with a 10% carbamide peroxide bleaching agent for a 2-week period, while the remainders used a placebo agent. After a 1-week washout period, a new set of enamel and root dentine slabs were bonded to the premolars and volunteers were crossed over to the alternate agent for 14 days. The resistance of enamel and root dentine to wear following bleaching, toothbrushing and intraoral exposure was measured with a profilometer, using reference areas.

Results

For enamel, ANOVA did not demonstrate significant difference between wear provided by placebo and bleaching agent (p = 0.3713), but higher wear depth was observed for bleached root dentine (p = 0.0346).

Conclusions

While overnight bleaching caused no alteration in wear resistance of enamel, root dentine showed increased tissue loss.     

Synthesis and redox properties of LixNi2(MoO4)3: a new 3-V class positive electrode material for rechargeable lithium batteries

A new framework type Li_xNi₂(MoO₄)₃ [0?x?4] polyanion compound was synthesized via a glycine-nitrate soft-combustion process at low temperature. The annealed powders were characterized by XRD to confirm the phase formation of the stoichiometric product, Ni₂(MoO₄)₃ in its non-lithiated state. The morphology of the annealed product was found to be composed of soft agglomerates embedded by ultrafine spherical grains. Electrochemical redox properties of the synthesized product were confirmed by employing the new material as a cathode in lithium-containing test cells in an aprotic electrolyte environment (1 M LiPF₆ in EC + DMC). Slow scan cyclic voltammetry (SSCV) confirmed the redox behavior corresponding to the reduction/oxidation of the transition metals, Ni and Mo between the potential window of 3.5–1.5 V. The lithium insertion/extraction process was confirmed by galvanostatic measurements on the test cells and they exhibited well discernible discharge/charge profiles with a reversible capacity of 170 mAh/g over the potential window of 3.5–1.5 V after the first charge/discharge cycle. Nevertheless, the discharge capacity was found to deteriorate slowly upon repeated cycling, which might presumably be due to disproportion reaction of the host structure beyond the extent of insertion of two lithium ions.     

Kinetic study of oxygen reduction on nanoparticles of ruthenium synthesized by pyrolysis of Ru3(CO)12

Electrocatalytic activity for molecular oxygen reduction on nanoparticles of ruthenium mixed with Nafion^® in 0.5 M H₂SO₄ was studied. The electrocatalytic material was synthesized by pyrolysis of Ru₃(CO)₁₂ in sealed ampoules at 190 °C for 3 h. The product of the pyrolysis was characterized by DSC, FTIR, XRD, SEM (EDX), TEM and also electrochemically. X-ray diffraction and TEM studies indicated that the synthesized powder presents a nano-crystalline structure. The electrochemical results obtained by rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques show that the oxygen reduction reaction take place by a multi-electronic charge transfer process (n=4e^?) with the formation of ca. 4.5% of hydrogen peroxide. A Tafel slope of ca. ?100 mV dec^?1 and an exchange current density of 7.9 ×  10^?7 mA cm^?2 were deduced. The kinetics of the reduction reaction were evaluated in the potential range of 0.7–0.0 V (SHE) through a simple Damjanovic model. Further interpretation of the analysis of the kinetic results in the light of Anastasijevic's procedure is also presented.     

Evaluation of the effectiveness of Er: YAG laser and calcium sodium phosphosilicate (Novamine) prophylaxis paste on remineralization of incipient enamel lesions in primary teeth: An in-vitro study

AimThe aim of this study was to evaluate the potential remineralization effect of Novamine and Er:YAG laser on artificial enamel lesions in primary teeth.Materials and methodsIn this in vitro study, 30 sound buccal and lingual surfaces of human primary canines were randomly divided (6 per group) into 5 groups (Novamine, Laser, Novamine+ Laser, Positive control, and a negative control). Remineralization agents were applied for 1 min to teeth using a rubber cup after creating artificially demineralised lesions. EDX analysis was performed to evaluate the Ca/P ratio and take Scanning electron microscope images.ResultsData were analyzed using Kruskal Wallis analysis. Ca/P ratio was higher in all 4 groups than the negative control group, and higher in Novamine+ Laser group than the positive control group with a statistically significant difference between groups. Overall Ca/P ratio was found to be highest in the Novamine group followed by the laser then positive control group. There were no significant differences between groups regard to Ca/P Ratios.ConclusionBased on these result Er:YAG laser combined with Novamine was provided the greatest contribution to remineralization process of primary teeth.     

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.     

Formation of Bi2S3 thin films on Au(111) by electrochemical atomic layer epitaxy: kinetics of structural changes in the initial monolayers

The kinetics and growth mechanism of the first and second monolayers of Bi₂S₃ by electrochemical atomic layer epitaxy (ECALE) has been studied using voltammetry and chronoamperometry techniques. Scan rate dependent cyclic voltammetry experiments reveal that the peak current in the stripping of the S layer from a Bi modified Au(111) electrode is a linear function of a 2/3 power of the scan rate, ν^2/3. Similar behavior is observed for the deposition of Bi on to one atomic layer of an S modified Au(111) electrode. The dissolution of the Bi atomic layer from the S modified Au(111) electrode involves an initial Langmuir-type adsorption–desorption accompanied by a nucleation and two-dimensional (2-D) growth process. When Bi atomic layers are used as a first layer, the nucleation and 2-D growth process was observed for both deposition and stripping. Analysis of the reduced variables demonstrates that the stripping of S from Au(111)Bi and stripping of Bi from Au(111)S fit the progressive model. Our results suggest that the nucleation and 2-D growth mechanism is the dominant process in the formation of Bi₂S₃ atomic layers if Bi is used as the first layer in the ECALE method.     

Oxygen reduction in alkaline medium at thin MnxCo3−xO4 (0≤x≤1) spinel films prepared by spray pyrolysis. Effect of oxide cation composition on the reaction kinetics

We have studied the relationships between the cation distributions in bulk and on the surface and the electrocatalytical reactivity towards the oxygen reduction reaction (orr) on thin film electrodes of cobalt and manganese spinel type oxides Mn_xCo_3?xO₄ (0≤x≤1). The cationic distributions were computed from XRD, XPS, magnetism and oxidation power measurements showing the presence of the Co³⁺/Co²⁺ and Mn⁴⁺/Mn³⁺ solid state redox couples and also vacancies in octahedral sites. The electrocatalytic activity and kinetic parameters for the orr were determined using the RRDE technique in 1 M KOH solution at 25 °C. Two Tafel zones of slopes b_c1 and b_c2 around ?60 and ?120 mV dec^?1 were observed. The disk current/ring current ratio, (I_D/I_R), increases with x and decreases with the applied overpotential. At constant overpotential η the real electrocatalytical current densities j increase when x increases. The orr can be characterized by an intrinsic kinetic parameter J′ obtained from this current ratio at various electrode rotational frequencies. This J′ increases with the redox couple Co²⁺/Co³⁺ ratio, increasing more in the bulk than at the surface of the electrode.