Microenvironment effects on the electrochemical and photoelectrochemical properties of thionine loaded Nafion® films

The polarity of the microenvironments within a Nafion^® (Nf) film was studied by electrochemical and photoelectrochemical techniques using a phenothiazine dye, thionine (TH) as a probe. When tetrabutylammonium ion-exchanged Nf film (TBA-Nf) is immersed in TH solution, TH partly displaces TBA ions into hydrophobic regions and binds to the less polar ionic cluster region of the Nf film. Although TH is able to enter the TBA-Nf film, the amount of TH incorporated in the TBA-Nf film is lower than the amount of TH incorporated in the Nf film. This is because Nf shows higher selectivity for TBA ions over TH. TH loaded into TBA-Nf film (TBA-Nf/TH) shows a 50 mV negative shift in the E_1/2 value and most of the exchanged TH becomes electroinactive. The observed electrochemical properties of the TBA-Nf/TH film could be attributed to the less polar environment experienced by TH molecules in the dehydrated TBA-Nf film. In the dehydrated TBA-Nf/TH film, the movement of protons is hampered by the lack of water molecules. However, when the TBA-Nf/TH film is immersed in supporting electrolyte solution for long times, the dehydrated TBA-Nf/TH film becomes hydrated due to the entry of ions and water molecules into the TBA-Nf/TH film, which leads to a positive shift in the E_1/2 value with increased electroactive TH species. The less polar environment experienced by TH in TBA-Nf film was also reflected in the photogalvanic cell measurements. In the photogalvanic cell, when the Nf/TH film was exposed to visible light, a cathodic photocurrent was observed whereas an anodic photocurrent was observed for the TBA-Nf/TH film. The observed difference in the polarity of the photocurrent again reiterates the polar and less polar environments experienced by TH in Nf and TBA-Nf films, respectively. The polarity of the photocurrent changes from anodic to cathodic when the TBA-Nf/TH film is soaked in the photogalvanic cell solution for longer times.     

On the impedance of porous electrodes – double-layer charging and charge transfer on an inhomogeneous inside electrode surface

A model of the impedance of porous electrodes was derived. The model generalized the de Levie model in so far that an inhomogeneous inside electrode surface is assumed. Then the charge transfer resistance and the double-layer capacity were assumed to be distributed. The width of the distribution is characterized by a distribution parameter. The inhomogeneities led to characteristic deviations of the Nyquist impedance plots compared to those for homogeneous inside electrode surfaces: the slope angle of the straight line for higher frequencies was less than π/4, the slope angle of π/2 of the straight line obtained for double-layer charging only and lower frequencies decreased and the semicircle behaviour obtained for double-layer charging and a Faradaic reaction in parallel were replaced by a depressed semicircle. The model allows the determination of the following parameters: the total effective double-layer capacity, the total effective charge transfer resistance, the distribution parameter and the total resistance of the electrolyte inside the pores. The impedance of porous zinc electrodes in weakly alkaline electrolyte can be theoretically well described with the new model. A scanning force microscopy 2-D-image of the morphology of the zinc electrode was performed and discussed.     

Kinetics of the electrodeposition of PbSn alloys: Part I. At glassy carbon electrodes

The electrodeposition of lead, tin and lead–tin alloys on glassy carbon has been studied by electrochemical techniques. Potentiostatic I–t transients were recorded to obtain the nucleation mechanism, while cyclic voltammetry was used to characterize the system. The alloy composition was determined by differential pulse anodic stripping voltammetry. Since the redox potentials for lead and tin are similar in the non-complexing electrolyte used, HBF₄, a peak-fitting program was used. In the fitting procedure the half-peak width obtained in the single metal systems were retained for the alloy, while the height and the position of the peaks were allowed to change. Structural information on the electrodeposited layers was obtained by X-ray diffraction, and scanning electron microscopy was used to determine the surface morphology. The experimental results clearly show that the deposition of lead, tin and lead–tin alloys on a glassy carbon substrate is a diffusion-controlled process with a three-dimensional (3D) growth mechanism. The nucleation is instantaneous, and the number of nucleation sites increases with increasing overpotentials, dE/d log(N₀)?60 mV decade^?1. The deposition of tin results in well-defined crystals with tetragonal shape and large areas of free glassy carbon surface. The crystallites have different sizes, which indicates fast surface diffusion of small units. In the presence of lead the microstructure of the electrodeposited tin changes drastically, resembling the microstructure of pure lead. Even small amounts of lead inhibit the deposition of tin on tin and prevent the formation of dendrites. From the stripping analysis it can be concluded that although tin and lead seem to be deposited side by side in an eutectic type alloy, some influence on the stripping potential is observed.     

The oxidation of β-nicotinamide adenine dinucleotide (NADH) at poly(aniline)-coated electrodes: Part II. Kinetics of reaction at poly(aniline)–poly(styrenesulfonate) composites

Poly(aniline)–poly(styrenesulfonate) composite-coated glassy carbon electrodes are shown to produce a stable, reproducible amperometric response to NADH in citrate–phosphate buffer at pH 7. These responses have been studied as a function of electrode potential, film thickness and both NADH and NAD⁺ concentration. The results show that the oxidation of NADH occurs throughout the whole of the film and that NAD⁺, the reaction product, reversibly inhibits the reaction. Rate constants for the different processes have been obtained by kinetic modelling and compared with those previously determined for poly(aniline)–poly(vinylsulfonate) films. Preliminary comparisons imply that diffusion within ‘pores’ in these poly(aniline) composite matrices is important in determining the magnitude of the amperometric responses.     

Oxygen reduction reaction kinetics and mechanism on platinum nanoparticles inside Nafion®

The aim of this work is to study kinetic parameters and the mechanism of the oxygen reduction reaction (orr) on platinum nanoparticles supported on carbon, inside Nafion^® (i.e. in PEMFC cathode conditions). Stationary and electrochemical impedance spectroscopy techniques were used to measure exchange current densities, Tafel slopes, and reaction orders with respect to O₂ pressure and H⁺ activity. The platinum nanoparticle size effect was confirmed. A specific low frequency inductive behaviour of the cathode impedance was observed. The latter demonstrates the presence of (at least) two electrochemical steps in the orr mechanism. Secondly, dc and ac modelling of the reaction in a gas diffusion electrode is proposed in order to simulate current–potential curves and impedance spectra. This paper reveals that an ECE mechanism for oxygen reduction proposed by Damjanovic and coworkers on bulk platinum in acidic medium is also valid for that on Pt nanoparticles.     

Effects of chemical environment on diffusivities within thin Nafion® films as monitored from chronoamperometric responses of generator–collector double microband assemblies

The possibility of monitoring in situ the modifications of a polymeric thin film in relation to its chemical environment by means of measurement of diffusive properties and assessment of the volume available to diffusion is investigated. These measurements are performed electrochemically through the monitoring of the currents at paired-microband electrodes operated in a generator–collector mode. The experimental results reported here were primarily focused to assess the validity of the principle. This involved the evaluation of the effects of the chemical environment of the polymer on the diffusional cross-talk of Fe^III redox species incorporated into a Nafion^® micrometric film coating the electrode assembly. Modification of the chemical environment was performed by adding different amounts of organic compound such as methanol, ethanol, ethyleneglycol, propanol and dimethylformamide (DMF) (from 0 to 20% v/v) into the 0.1 mol l⁻¹ H₂SO₄ electrolyte in which the assembly is placed. An procedure that could be automated based on a theoretical diffusional model developed previously was used to analyze the generator–collector chronoamperometric responses. This allowed the determination of the concentration c^o and the diffusion coefficients of Fe^III and Fe^II species within Nafion^® films as well as the film thickness h. This showed a concentration dependence of the Fe^III diffusion coefficient suggesting that the rate of physical diffusion of iron centers governs mainly the charge transport within Nafion^® thin films under the conditions investigated. The presence of an organic compound in the surrounding electrolyte caused a decrease of the steady-state generator–collector current, the effect being the most significant for DMF. The magnitude of this decrease was observed to depend on the nature and the concentration of the organic compound added to the electrolyte. Detailed chronoamperometric analyses established that the current decrease was related principally to the decrease in diffusion coefficient and to a moderate change of the amount of iron species available to diffusional transport, without significant change of the film thickness. This suggested that the presence of organic compound modified the plasticization of the polymer matrix.     

Airway Centric® TMJ philosophy/Airway Centric® orthodontics ushers in the post-retraction world of orthodontics

Objective: Airway Centric® Dentistry/Orthodontics was defined in a previous article (Gelb M. Airway centric TMJ philosophy. CDA Journal. 2014) that also suggested airway considerations were more important than condylar position issues in determining patient health and welfare. Indeed, that article called for a new paradigm in the profession, but specific treatment techniques to achieve optimal airways and avoid reducing the airway were not discussed. The present article amplifies on that article and identifies specific orthodontic treatment methods, which are or are not, congruent with this new paradigm.

Method: The basis of traditional orthodontic diagnosis is outlined with references from the literature that show the scientific foundation for treatment is weak. A new approach to diagnosis and treatment with the goal of airway optimization is discussed.

Discussion: Six keys for optimal orthodontic outcomes are presented as new goals, and none involve the teeth. Ten specific treatment goals are outlined, and some are the diametric opposite of the current standard of care in the profession.

Conclusion: We recommend that optimizing the airway for every patient and never doing any treatment which will diminish the airway, even minutely, needs to become the standard of care in Airway Centric® Dentistry.     

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.     

FTIR characterization of the setting reaction of biodentine™

Objective

To provide insight of the setting reactions of Biodentine?, a hydraulic calcium silicate cement, based upon observations using Fourier Transform Infra-Red (FTIR) spectroscopy.

Application of ionic liquid–dsDNA biocomposite film for the direct electrochemistry of myglobin on carbon ionic liquid electrode

An ionic liquid (IL) and double-stranded DNA (dsDNA) composite material was used to investigate the direct electron transfer of myglobin (Mb) on carbon ionic liquid electrode (CILE). The presence of the IL–dsDNA biocomposite film on the electrode surface provided great improvement to the direct electron transfer rate of Mb with the CILE, which was due to the synergistic contributions of specific characteristics of dsDNA, IL and their interaction. The electrochemical parameters of Mb in the IL–dsDNA composite film modified electrode were carefully investigated with the charge transfer coefficient (α) and the electron transfer rate constant (k_s) calculated as 0.42 and 0.84 s⁻¹, respectively. The fabricated Mb modified electrode exhibited good electrocatalytic ability to the reduction of trichloroacetic acid and H₂O₂, which showed the potential applications in the third-generation electrochemical biosensor.     

Langmuir–Blodgett films of azocrown ethers on electrodes — voltammetric recognition of isomers

Monolayers of amphiphilic derivatives of crown ethers bearing electroactive azo groups in the macrocycle were transferred from the air ∣ water interface onto indium–tin oxide (ITO) or thin mercury film electrodes (TMFE) using the Langmuir–Blodgett technique. Differences in the electrochemical reversibility of the systems observed on these two electrodes were explained by different orientation of the azocrown molecules on the hydrophobic (TMFE) and hydrophilic (ITO) electrode surfaces. The electrochemical studies of the monolayer modified electrodes proved that the organization of the monolayer and the redox properties of the azocrown molecules depend on the geometry around the NN moiety. Voltammetry allowed Z- and E-isomers of the azocrowns to be recognized and their isomerization processes in monolayers to be followed. The effects of pH and of the alkali metal cations on the stability of the isomers were studied. Isomerization to the more stable E-isomer was hindered in alkaline solutions and under these conditions well separated reduction peaks of the E- and Z-isomers were obtained.     

Locator® versus ceramic/electroplated double-crown attachments: a prospective study on the intraindividual comparison of implant-supported mandibular prostheses

Clinical Oral Investigations - Implant-supported overdentures are an established dental treatment mode. The aim of this prospective study was and interindividual comparison of patient satisfaction...     

Abnormal electrochemical behavior of copper–zinc superoxide dismutase on mercury electrodes

The electrochemical behavior of porcine superoxide dismutase (PESOD) on mercury electrodes was investigated by cyclic voltammetry and direct current polarography in the absence of mediators for the first time. It is interesting and abnormal that PESOD displays two pairs of redox adsorbed peaks in the CV diagram, which are located in the potential regions of the disulfide bonds of many proteins reported by some authors, but we attributed them to the redox peaks of copper(II) and zinc(II) by reconstitution of PESOD. The apparent numbers of electrons in the reduction process were obtained by polarography and the electrode process was discussed.     

A monomer–dimer equilibrium in self-assembled monolayers of N-ethyl-N′-octadecylviologen on the electrode surface. The influence of water content and hexafluorophosphate ion

This paper describes the dimerization of self-assembled monolayers (SAMs) of N-ethyl-N′-octadecylviologen (1) on GC and Au electrode surfaces in the presence of 0.1 M NH₄PF₆ aqueous solutions. The ‘wet’ and ‘dry’ SAMs of 1 showed multiple redox peaks for the first reduction of 1 in the presence of NH₄PF₆, in contrast to the case of other supporting electrolytes (typically KCl, NaNO₃, Na₂SO₄ and NaClO₄) where both wet and dry SAMs of 1 exhibited a single redox wave for the first reduction. The dry SAM showed a well defined reduction peak at ?0.57 V along with a shoulder reduction peak at ?0.50 V and two oxidation peaks at ?0.50 and ?0.42 V. On the contrary, the wet SAM gave a very sharp reduction peak at ?0.50 V and a small shoulder peak at ?0.57 V in addition to two oxidation peaks like those observed for the dry SAM. The reduction peak of ?0.50 V was ascribed to the reduction of strongly hydrated dications of 1, while the reduction peak at more negative potential (?0.57 V) was attributed to the reduction of the dehydrated dications of 1. The two oxidation peaks at ?0.50 and ?0.42 V were ascribable to the oxidation of the usual radical cation monomer and the radical cation dimer, respectively. In the case of the wet SAM, upon continuous potential cycling, the sharp reduction peak of ?0.50 V clearly decreased, whereas the more negative reduction peak of ?0.57 V was highly stable. In this case, in the oxidation process, the monomer peak of ?0.50 V increased, while the dimer peak of ?0.42 V decreased. Thus it is reasonably assumed that in the wet SAM, initially the radical cations of 1 feel an aqueous environment in the monolayer where the dimerization is highly favored and at subsequent potential cycles, due to the entry of hydrophobic anions of PF₆^? into the monolayer, the pre-existent water molecules are expelled from the monolayer and under this circumstance the radical cations of 1 may feel the environment very similar to non-aqueous media where the dimerization is totally suppressed. The adsorption tendency of 1 on the electrode surface was also studied using the SAMs prepared by dissolving 1 in water+ethanol mixtures of different ratios. The appearance of multiple peaks was found to depend significantly on the alkyl chain length of asymmetric viologen. The inclusion/expulsion of solvents and anions into/from the SAM during the redox reaction were studied by the electrochemical quartz crystal microbalance (EQCM). It was found that in the presence of SO₄^2? ions ca. 17 water molecules per one SO₄^2? ion were transported to the SAM of 1 during the oxidation, whereas ca. five water molecules were transported in the presence of PF₆^? ions.     

Influence of the microbiological component of Cariogram® for evaluating the risk of caries in children

Objective: To compare the risk for caries in children as determined by Cariogram^® software (CS; Stockholm, Sweden) with and without its microbiological component and by a form based on Cariogram^® (FBC).

Methods: Children (n?=?28) aged 3–9 years were included. Data were collected clinically and from anamnesis. The salivary levels of Streptococcus mutans (SM) were evaluated. A linear regression model was used to determine which variables were predictive for each type of risk analysis. Caries risk was the dependent variable and the independent variables were caries experience, related disease, plaque amount, diet frequency, salivary levels of SM, fluoride sources and clinical judgment. A paired Student t-test was used for the following comparisons: (a) CS with and without SM; (b) CS without SM and FBC; (c) CS with SM and FBC.

Results: The mean dmft/DMFT was 5.56?±?2.51. There was no difference between the methods (p?p?=?.889). Caries experience, plaque amount, diet frequency and fluoride sources were predictors of caries risk in all assessment methods. Clinical judgment was a significant predictor in CS.

Conclusions: Caries experience, plaque amount, diet frequency and fluoride sources are valuable predictors of caries risk; microbiological tests are not necessary for evaluating caries risk in children, which can be assessed similarly by CS without SM and FBC.