Electrocatalytic activity of thianthrene toward one-electron oxidation of guanosine and DNA in a non-aqueous medium

In this study, the electrochemical behavior of thianthrene (TH) and its application toward the electrocatalytic oxidation of guanosine (Gs) and DNA in a non-aqueous solution are investigated using different voltammetric techniques. Guanosine and DNA are adsorbed on the glassy carbon electrode (GCE) by applying a positive potential to the GCE. The rate constant of catalytic reaction between DNA and TH and also between Gs and TH were evaluated using chronoamperometry which gave rate constants of 2.41 × 10⁶ cm³ mol⁻¹ s⁻¹ and 2.68 (±0.19) × 10⁶ cm³ mol⁻¹ s⁻¹, respectively. Also the diffusion coefficient of TH was obtained using hydrodynamic voltammetry (3.17 × 10⁻⁵ cm² s⁻¹). Furthermore, using hydrodynamic voltammetry, a one-electron mechanism for oxidation of Gs is suggested.     

Electrochemistry of magnolol and interaction with DNA

The electrochemical behaviors of magnolol have been studied at glassy carbon electrode using cyclic voltammetry, linear sweep voltammetry and chronocoulometry. Moreover, its interaction with DNA was investigated in solution by electrochemical methods and ultraviolet–visible spectroscopy. The experiment results indicated that the electrochemical oxidation of magnolol was an irreversible process with one proton and one electron transfer. The electron transfer coefficient (α) was calculated to be 0.441 ± 0.001. At the scan rate from 100 mV/s to 450 mV/s, the electrode process was controlled by the adsorption step and at the range of 600–950 mV/s the electrochemical oxidation was diffusion controlled process. The corresponding electrochemical rate constant (k_s) was 0.0760 ± 0.0001 s⁻¹. Through chronocoulometry experiment, the diffusion coefficient (D) and the surface concentration (Γ) were obtained as (3.76 ± 0.01) × 10⁻⁷ cm²/s and (2.98 ± 0.01) × 10⁻¹⁰ mol/cm². In addition, the interaction of magnolol and DNA was ascribed to be electrostatic interaction and the calculated association constant (β) and Hill coefficient (m) were 1.14 × 10⁵ M⁻¹ and 0.973. At last a sensitive and convenient electrochemical method was proposed for the determination of magnolol.     

Sorption of water, ethanol or ethanol/water solutions by light-cured dental dimethacrylate resins

Objectives

This work is concerned with the study of the sorption and desorption process of water, ethanol or ethanol/water solution 50% (v/v) or 75% (v/v) by the dental resins prepared by light curing of Bis-GMA, Bis-EMA, UDMA, TEGDMA or D₃MA.

Methods

A thin resin disc is placed in a bath of time to obtain the sorption curve m_t = f(t). Then the liquid is desorbed until a constant mass for the disc is reached and the desorption curve is recorded. These experimental curves help in the determination of the sorbed/desorbed liquid amount at equilibrium, the percentage of the extracted mass of unreacted monomer known as “solubility”, and the sorption/desorption diffusion coefficient which expresses correspondingly the rate of the liquid sorption/desorption.

Results

The highest liquid uptake by dental resins was 13.3 wt% ethanol for Bis-GMA-resin, 12.0 wt% ethanol for UDMA-resin, 10.10 wt% ethanol/water solution for TEGDMA-resin, 7.34 wt% ethanol for D₃MA-resin and 6.61 wt% ethanol for Bis-EMA-resin. The diffusion coefficient for all resins was higher in water than in ethanol/water solution or ethanol. Bis-GMA-resin showed the highest diffusion coefficient (11.01 × 10⁻⁸ cm² s⁻¹) followed by Bis-EMA-resin (7.43 × 10⁻⁸ cm² s⁻¹), UDMA-resin (6.88 × 10⁻⁸ cm² s⁻¹), D₃MA-resin (6.22 × 10⁻⁸ cm² s⁻¹) and finally by TEGDMA-resin (1.52 × 10⁻⁸ cm² s⁻¹).

Significance

All studied dental resins, except TEGDMA-resin, absorbed higher amount of pure ethanol than water or ethanol water solution. TEGDMA-resin absorbed higher amount of ethanol/water solution (50/50 or 75/25 (v/v)) than water or ethanol. For all studied dental resins the diffusion coefficient was higher in water than in ethanol/water solution or ethanol.     

Electrochemical impedimetry of electrodeposited poly(propylene imine) dendrimer monolayer

Voltammetric and electrochemical impedance spectroscopic (EIS) studies of generation one poly(propylene imine) (G1 PPI) dendrimer as an electroactive and catalytic nanomaterials both in solution and as an electrode modifier based on a simple one step electrodeposition method is presented. The G1 PPI exhibited a reversible one electron redox behaviour at E^0′ ca 210 mV in phosphate buffer pH 7.2 with diffusion coefficient and Warburg coefficient of 7.5 × 10⁻¹⁰ cm² s⁻¹ and 8.87 × 10⁻⁴ Ω s^−1/2 respectively. Cyclic voltammetric electrodeposition of a monolayer of G1 PPI on glassy carbon electrode was carried out between −100 mV and 1100 mV for 10 cycles. The nanoelectrode was electroactive in PBS at E^0′ ca 220 mV. Kinetic profiles such as time constant (4.64 × 10⁻⁵ s rad⁻¹), exchange current (1.55 × 10⁻⁴ A) and heterogeneous rate constant (4.52 × 10⁻³ cm s⁻¹) obtained from EIS showed that the dendrimer layer catalysed the redox reaction of Fe^2+/3+ in [Fe(CN)₆]^3−/4− redox probe.     

EIS study of solid-state transformations in the passivation process of bismuth in sulfide solution

Anodic oxidation of polycrystalline bismuth in alkaline medium, containing sulfide ions was investigated in situ. Cyclic voltammetry was used to define the potential regions of formation and stability of anodic Bi₂S₃ and Bi₂O₃ semiconductor films that translate bismuth to the passive state. The mechanism of elementary steps of the passivation process has been investigated using electrochemical impedance spectroscopy (EIS). The electric and dielectric properties of anodic films and structural parameters of the interfaces Bi–growing film–electrolyte in a wide region of potentials and frequencies of six decades, were determined. The EIS data have been analyzed and discussed in the frame of the point defect model (PDM) of anodic film formation and growth. The growth of passive surface films on bismuth takes place via transport of anionic vacancies generated at the metal–film interface. The slow step of the process is the layer-limited diffusion of anionic vacancies (D≈ 10^?16 cm² s^?1).Solid-state transformation of sulfide to the oxide film is a consequence of OH^? ion capture into the anionic vacancies of the sulfide film, the generation and transport of cation vacancies from the film–solution interface, their annihilation and formation of a vacancy condensate of a critical size at the metal–film interface.     

Highly temperature dependent mass-transport parameters for ORR in Nafion® 211

Nafion® 211 differs from previous versions of Nafion in that the membrane is cast from a dispersion rather than being melt-extruded. As such, the water sorption properties are different, as is the rate of increase in water content with temperature. Kinetic and mass-transport parameters for dispersion-cast Nafion® 211 were determined using solid-state electrochemistry in the temperature range 30–70 °C, 100% relative humidity, and 30 psi oxygen pressure. Exchange current densities, Tafel slopes, and transfer coefficients for ORR in Nafion® 211, are similar to those observed in Nafion® 117; mass-transport parameters are not. At 30 °C and 100% RH oxygen solubility and the diffusion coefficient is determined to be 1.16 × 10⁻⁵ mol cm⁻³ and 1.13 × 10⁻⁶ cm² s⁻¹, respectively. Oxygen permeability at 30 °C (1.28 × 10⁻¹¹ mol cm⁻¹ s⁻¹) is lower than in Nafion® 117 (5.31 × 10⁻¹¹ mol cm⁻¹ s⁻¹) by factor of 4, while at T > 60 °C the permeability of Nafion® 211 increases significantly to values higher than Nafion® 117, and is correlated with the increase in water content and hydration number (λ) with temperature.     

Poly(N-methylaniline)/nickel modified carbon paste electrode as an efficient and cheep electrode for electrocatalytic oxidation of formaldehyde in alkaline medium

This research in finding a cheap and efficient catalyst for electrooxidation of formaldehyde give us an attempt to make and examine the behavior of poly(N-methylaniline)/nickel modified carbon paste electrode (Ni/P(NMA)/MCPE) in absence and presence of formaldehyde. This involves in situ electropolymerization of N-methylaniline at carbon paste electrode, which is following to the incorporation of Ni(II) to polymeric layer by immersion of modified electrode in 1.0 M nickel sulphate solution. The electrocatalytic oxidation of formaldehyde was studied by cyclic voltammetry and chronoamperometry methods. The effects of scan rate and formaldehyde concentration on the electrocatalytic oxidation of formaldehyde were also investigated at the surface of Ni/P(NMA)/MCPE. The diffusion coefficient (D = 14.1 × 10⁻⁵ cm² s⁻¹), and some kinetic parameters such as the transfer coefficient (α = 0.45) and also second-order rate constant (k = 8.96 × 10⁻⁴ cm³ mol⁻¹ s⁻¹) of formaldehyde were calculated.     

A gold nanoparticle-modified PVC/TTF-TCNQ composite amperometric biosensor for glucose determination

A comparison of the analytical performances of several enzyme biosensor designs, based on the use of different tailored gold nanoparticle-modified composite PVC/TTF-TCNQ electrodes, is discussed. The analytical characteristics of glucose calibration plots and kinetic parameters of the enzyme reaction were compared for the biosensors tested: Au_coll + GOx, GOx + Au_coll, Mixed GOx + Au_coll and PVC/TTF-TCNQ-Au_coll. The presence of this nanomaterial enhances the analytical performance with respect to the precursor biosensor without Au. The proposed biosensor can be applied in batch (response linear up to 2.0 mM, sensitivity of 45 ± 0.5 mA M⁻¹ with a limit of detection, s/n = 3, of 6.2 × 10⁻⁶ M) and in FIA systems (linear range between 0.1 and 8 mM, sensitivity of 3.67 ± 0.3 mA M⁻¹ with a limit of detection, s/n = 3, of 1.9 × 10⁻⁵ M).     

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

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

Preparation of poly N,N-dimethylaniline/ferrocyanide film modified carbon paste electrode: Application to electrocatalytic oxidation of l-cysteine

Functionalized poly N,N-dimethylaniline film was prepared by adsorption of ferrocyanide onto the polymer forming at the surface of carbon paste electrode (CPE) in aqueous solution. The electrocatalytic ability of poly N,N-dimethylaniline/ferrocyanide film modified carbon paste electrode (PDMA/FMCPE) was demonstrated by oxidation of l-cysteine. Cyclic voltammetry and chronoamperometry techniques were used to investigate this ability. In the optimum pH (6.00), the electrocatalytic ability about 480 mV and the catalytic reaction rate constant, (k_h), can be seen 3.08 × 10³ M⁻¹ s⁻¹. The catalytic oxidation peak current determined by cyclic voltammetry method was linearly dependent on the l-cysteine concentration and the linearity range obtained was 8.00 × 10⁻⁵ –2.25 × 10⁻³ M. Detection limit of this method was determined as 6.17 × 10⁻⁵ M (2σ). At a fixed potential under hydrodynamic conditions (stirred solution), the calibration plot was linear over the l-cysteine concentration range 7.40 × 10⁻⁶ M–1.38 × 10⁻⁴ M. The detection limit of the method was 6.38 × 10⁻⁶ M (2σ).     

Scanning electrochemical microscope studies of dye regeneration in indoline (D149)-sensitized ZnO photoelectrochemical cells

A scanning electrochemical microscope (SECM) in the feedback and generation-collection modes was used to investigate the regeneration of photoexcited dye cations at the semiconductor/electrolyte interface in a dye-sensitized solar cell (DSSC) based on ZnO/D149. An effective dye regeneration rate constant k_ox of 9.55 × 10⁷ cm^9/2 mol^−3/2 s⁻¹ was obtained from feedback mode experiments at different wavelengths and light intensities on ZnO/D149 electrodes. Illuminated regions of the dye-sensitized electrodes could be differentiated from non-illuminated regions by local imaging in SECM generation-collection experiments with I^- as redox mediator. We also report SECM feedback measurements on non-illuminated dye-sensitized electrodes to investigate the electron transfer kinetics of dissolved redox couples at the underlying FTO substrate.     

Diffusion and concurrent solubility of self-adhering and new resin-matrix composites during water sorption/desorption cycles

Objectives

To investigate the kinetic process of water diffusion and mass change in new resin-matrix composites during water sorption/desorption cycles.

Methods

Five new resin-matrix composites were investigated [Filtek^® Silorane (FS), GC Gradia Direct Anterior (GDA), GC Gradia Direct Posterior (GDP), GC Kalore (GCK), Vertise^® Flow (VF)]. Five disk-shaped specimens, per material (15.0 ± 0.1) mm diameter by (2.0 ± 0.1) mm, were prepared according to ISO 4049. Each disk was immersed separately in de-ionized water for 150 d and then reconditioned for 75 d; all at (37 ± 1) °C. Mass was measured at different time intervals. Perspex disks were used as control. Data analysis was done by repeated measures ANOVA, one-way ANOVA and Tukey's post hoc test (p < 0.05).

Results

The water sorption (μg/mm³) after 150 d immersion ranged from 13.51 μg/mm³ (±0.40) for FS to 71.96 μg/mm³ (±0.90) for VF. The solubility ranged up to 16.95 μg/mm³ (±0.79) for VF. A significant mass reduction occurred in VF after the peak value [73.63 μg/mm³ (±0.31)] of water sorption was reached at 42 d. VF had the highest diffusion-coefficient for sorption: 5.23 × 10⁻⁹ cm²/s (±0.38) and desorption: 11.72 × 10⁻⁹ cm²/s (±0.16). Percentage sorption differences were significant for all materials (p < 0.001), except between GCK and GDP. The early correlation between mass change and square root of time was linear.

Significance

Each resin-matrix composite varied in sorption/desorption cycles which may affect clinical service. A concurrent solubility process occurred during sorption of the self-adhering composite VF. The silorane composite FS exhibited minimal sorption.     

A differential pulse voltammetric method for simultaneous determination of ascorbic acid,dopamine, and uric acid using poly (3-(5-chloro-2-hydroxyphenylazo)-4,5-dihydroxynaphthalene-2,7-disulfonic acid) film modified glassy carbon electrode

A stable modified glassy carbon electrode based on the poly 3-(5-chloro-2-hydroxyphenylazo)-4,5-dihydroxynaphthalene-2,7-disulfonic acid (CDDA) film was prepared by electrochemical polymerization technique to investigate its electrochemical behavior by cyclic voltammetry. The properties of the electrodeposited films, during preparation under different conditions, and their stability were examined. The homogeneous rate constant, k_s, for the electron transfer between CDDA and glassy carbon electrode was calculated as 5.25(±0.20) × 10² cm s⁻¹. The modified electrode showed electrocatalytic activity toward ascorbic acid (AA), dopamine (DA), and uric acid (UA) oxidation in a buffer solution (pH 4.0) with a diminution of their overpotential of about 0.12, 0.35, and 0.50 V for AA, DA, and UA, respectively. An increase could also be observed in their peak currents. The modified glassy carbon electrode was applied to the electrocatalytic oxidation of DA, AA, and UA, which resolved the overlapping of the anodic peaks of DA, AA, and UA into three well-defined voltammetric peaks in differential pulse voltammetry (DPV). This modified electrode was quite effective not only for detecting DA, AA, and UA, but also for simultaneous determination of these species in a mixture. The separation of the oxidation peak potentials for ascorbic acid–dopamine and dopamine–uric acid were about 0.16 V and 0.17 V, respectively. The final DPV peaks potential of AA, DA and UA were 0.28, 0.44, and 0.61 V, respectively. The calibration curves for DA, AA, and UA were linear for a wide range of concentrations of each species including 5.0–240 μmol L⁻¹ AA, 5.0–280 μmol L⁻¹ DA, and 0.1–18.0 μmol L⁻¹ UA. Detection limits of 1.43 μmol L⁻¹ AA, 0.29 μmol L⁻¹ DA and 0.016 μmol L⁻¹ UA were observed at pH 4. Interference studies showed that the modified electrode exhibits excellent selectivity toward AA, DA, and UA.     

Reduced polymerization stress of MAPO-containing resin composites with increased curing speed,degree of conversion and mechanical properties

Objectives

The degree and rate of photopolymerization in resin-based dental composites will significantly affect polymer network formation and resultant material properties that may determine their clinical success. This study investigates the mechanical properties, the generation of stress from polymerization, tooth cusp deflection and marginal integrity of experimental resin composites that contain different photoinitiators.

Methods

Experimental light-activated resin composites (60 vol% particulate filled in 50/50 mass% bis-GMA/TEGDMA) were formulated using a monoacylphosphine oxide (MAPO) photoinitiator and compared with a conventional camphoroquinone (CQ)-based system. Similar radiant exposure was used (18 J cm⁻²) for polymerization of each material although the curing protocol was varied (400 mW cm⁻² for 45 s, 1500 mW cm⁻² for 12 s and 3000 mW cm⁻² for 6 s). Degree and rate of polymerization was calculated in real-time by near infrared spectroscopy and the generation of stress throughout polymerization measured using a cantilever beam method. Flexural strength and modulus were acquired by three-point bend tests. Standardized cavities in extract pre-molar teeth were restored with each material, the total cuspal deflection measured and post-placement marginal integrity between the tooth and restoration recorded.

Results

Generally, MAPO- exhibited a significantly higher degree of conversion (72 ± 0.8 to 82 ± 0.5%) compared with CQ-based materials (39 ± 0.7 to 65 ± 1.6%) regardless of curing protocol (p < 0.05) and MAPO-based materials exhibited less difference in conversion between curing protocols. CQ-based materials exhibited between ∼85 and 95% of the maximum rate of polymerization at <15% conversion, whereas MAPO-based RBCs did not approach the maximum rate until >50% conversion. Higher irradiance polymerization had a significant deleterious effect on the mechanical properties of CQ-based materials (p < 0.05) whereas MAPO-based materials exhibited increased strength and modulus and were less affected by the curing method. Total cuspal deflection in restored extracted teeth was higher for CQ- compared with MAPO-based materials cured at the lowest irradiance curing protocol (12.9 ± 4.0 and 8.3 ± 1.5 μm) and similar at 3000 mW cm⁻¹ for 6 s (10.1 ± 3.5 and 9.0 ± 1.5 μm). A significant decrease in marginal integrity was observed for CQ-based RBCs cured at high irradiance for short exposure time compared with that of the MAPO-based RBC cured using a similar protocol (p = 0.037).

Significance

Polymer network formation dictates the final properties of the set composite and the use MAPO photoinitiators may provide an effective restorative material that exhibits higher curing speeds, increased degree of conversion, strength and modulus without compromise in terms of polymerization stress and marginal integrity between tooth and restoration.     

Efficacy of the stannous ion and a biopolymer in toothpastes on enamel erosion/abrasion

Objectives

Sn²⁺ has promising erosion-inhibiting properties in solutions, but little is known about respective effects in toothpastes. In addition, biopolymers might have protecting potential. Aim of this study was to investigate the effects of Sn²⁺ in toothpastes and of a biopolymer (chitosan) added to a Sn²⁺ formulation on erosion/abrasion.

Methods

Enamel samples were subjected to cyclic erosion procedures (10 days; 0.50% citric acid, pH 2.5; 6× 2 min/day), and brushing (2× 15 s/day, load 200 g) during immersion in slurries (2 min). The toothpastes were NaF formulations (NaF/1, NaF/2, NaF/3) and Sn²⁺ formulations (NaF/SnCl₂, AmF/SnF₂, AmF/NaF/SnCl₂) and AmF/NaF/SnCl₂ + 0.5% chitosan.Declared concentrations of active ingredients in toothpastes were 1400–1450 μg/g F⁻ and 3280–3500 μg/g Sn²⁺. Negative controls were erosion only and placebo, positive control was a SnF₂ gel. Tissue loss was quantified profilometrically, Sn on enamel surfaces was measured by energy dispersive X-ray spectroscopy.

Results

Loss values (μm) for erosion only and placebo were 14.4 ± 4.5 and 20.2 ± 3.8, respectively, and 4.6 ± 1.9 for the positive control (p ≤ 0.001 each compared to erosion only). The other loss values were: NaF/1 16.5 ± 3.0, NaF/2 14.0 ± 2.7, NaF/3 12.6 ± 3.9, NaF/SnCl₂ 14.7 ± 5.1, AmF/SnF₂ 13.5 ± 4.8, AmF/NaF/SnCl₂ 12.4 ± 4.2, AmF/NaF/SnCl₂ + chitosan 6.6 ± 3.5 (except NaF/1 all p ≤ 0.01 compared to placebo). AmF/NaF/SnCl₂/chitosan was more effective than all other toothpastes (p ≤ 0.01 each). Sn on the enamel surface ranged between 1.3 ± 0.3 and 2.8 ± 0.04 wt.% with no obvious relationship with efficacy.

Conclusions

The NaF and Sn²⁺ toothpastes without chitosan exhibited similar anti-erosion and abrasion-prevention effects. The experimental Sn²⁺ formulation with chitosan revealed promising results similar to those of the positive control.

Clinical significance

NaF toothpastes offer a degree of protection against erosion/abrasion, which is likely sufficient for most subjects with average acid exposures. For patients with initial erosive lesions, however, more effective toothpaste is desirable. The combination of Sn²⁺ and a biopolymer appears promising in this context.     

Electroanalytical determination of sildenafil in Viagra tablets using screen-printed and conventional carbon paste electrodes

This work compares the electroactivity of a conventional carbon-paste electrodes and screen-printed carbon electrodes. Potentiometric sensors responsive to sildenafil citrate (SILC) drug (the active component of Viagra) are described, characterized, compared and used for drug assessment. The proposed carbon paste electrode is fully characterized in terms of plasticizer type, response time, life span, soaking time, titrant, pH and temperature. The electrodes exhibited linear response with a Nernstian slope of 58.20 ± 1 and 58.82 ± 0.5 mV decade⁻¹ for SILC in the concentration range from 1.0 × 10⁻⁷ to 1.0 × 10⁻² and 5.30 × 10⁻⁷ to 1.0 × 10⁻² mol L⁻¹ with good reproducibility for CPE and SPE, respectively. Both CPE and SPE could be used in the pH range 3.0–5.0 and the isothermal coefficient is found to be 0.98 and 0.85 mV/°C, respectively. The limit of detection was found to be 9.0 × 10⁻⁸ and 3.5 × 10⁻⁷ mol L⁻¹ for CPE and SPE, respectively. They were applied to potentiometric determination of SILC in pure state and pharmaceutical preparation under batch conditions. The CPE and SPE sensors display good selectivity for SILC drug over large number of inorganic cations, sugars and amino acids commonly used in drug formulations. The CPE and SPE show high selectivity for the drug under investigation. The results obtained using the fabricated CPE is compared with those obtained by SPE for spiked pharmaceutical samples.     

Surface characteristics and corrosion properties of selective laser melted Co–Cr dental alloy after porcelain firing

Objective

We examined the surface characteristics and corrosion properties of selective laser melted (SLM) cobalt–chromium (Co–Cr) dental alloys before and after porcelain-fused-to-metal (PFM) firing.

Methods

Samples were manufactured utilizing SLM techniques and control specimens were fabricated using traditional casting methods. The microstructure and surface composition were examined using metallographic microscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Corrosion properties were evaluated using electrochemical impedance spectroscopy. Student's t-test was used to evaluate differences in numerical results of electrochemical corrosion tests between SLM and cast specimens before or after PFM firing. The results of electrochemical corrosion tests of the SLM and cast samples before and after firing were analyzed using one-way ANOVA.

Results

Although PFM firing altered the microstructure of the SLM specimens, they still exhibited a compact and homogeneous structure, and XPS analysis indicated that there were no significant differences in the surface composition of the specimens after firing. In artificial saliva at pH 5, the R_p value of the SLM specimens was 6.21 MΩ cm⁻² before firing and 2.84 MΩ cm⁻² after firing, suggesting there was no significant difference in electrochemical corrosion properties (P > 0.05). In artificial saliva at pH 2.5, the R_p value of the SLM group was 4.80 MΩ cm⁻² before firing and 2.88 MΩ cm⁻² after firing, again indicating no significant difference in electrochemical corrosion properties (P > 0.05). At pH 2.5, there was a significant difference in corrosion behavior between the cast and SLM groups, with the R_p value of the cast group being 0.78 MΩ cm⁻² vs. 2.88 MΩ cm⁻² for the SLM group.

Significance

The improved post-firing corrosion resistance of SLM specimens provides further support for their use in prosthodontic applications, as the oral environment may become temporarily acidic following meals.     

Determination of trace vanadium(V) by adsorptive anodic stripping voltammetry on an acetylene black paste electrode in the presence of alizarin violet

A sensitive and simplified voltammetric method is developed for the determination of trace amounts of vanadium(V) by adsorptive anodic stripping voltammetry using an acetylene black (AB) paste electrode. The method is based on the preconcentration of the V(V)–alizarin violet (AV) complex at open circuit while stirring the solution for 90 s in 0.15 mol dm⁻³ hexamethylenetetraamine–hydrochloric acid buffer (pH 4.4), the adsorbed complex is then oxidized, producing a response with a peak potential of 564 mV when scanning linearly from 0 to 1000 mV. For voltammetric determination of V(V), the parameters influencing the peak current have been optimized. Under the selected conditions, the peak current and concentration of V(V) accorded with linear relationship in the range of 8.0 × 10⁻¹⁰ mol dm⁻³–1.0 × 10⁻⁷ mol dm⁻³ (c_AV = 2.0 × 10⁻⁶ mol dm⁻³) and 1.0 × 10⁻⁷ mol dm⁻³–8.0 × 10⁻⁶ mol dm⁻³ (c_AV = 2.0 × 10⁻⁵ mol dm⁻³), the detection limit (three times signal to noise) was estimated to be 6.0 × 10⁻¹⁰ mol dm⁻³ for 90 s accumulation. The relative standard deviation (RSD) is 1.9% and 2.3% for V(V) concentrations of 1.0 × 10⁻⁷ mol dm⁻³ and 1.0 × 10⁻⁸ mol dm⁻³ respectively. Finally, this proposed method was successfully applied to the determination of V(V) in natural water samples.     

Investigation of polarographic interference between Se(IV) and Cr(VI), its elimination and application to Gerede river water

Trace chromium(VI) determination plays an important role since it is carcinogenic agent and toxic pollutant. For this purpose a direct method is developed using differential pulse polarography, DPP. When selenite was added into solutions of some ions such as copper(II), lead(II), cadmium(II), zinc(II), and chromium(VI) their DP polarographic peak decreased. This kind of interference will cause large errors in the determinations, its elimination is very important. The interference between selenite and Cr(VI) ions could be eliminated in B-R (Britton-Robbinson) buffer at pH 8.5. In this work the effect of components present in buffer has been investigated and it was found that phosphate and borate eliminated the formation of Cr–Se intermetallic compound formation. While it was possible to determine 1 × 10⁻⁵ M Cr(VI) in the presence of 100 times more selenite as (1.0 ± 0.1) × 10⁻⁵ M, in borate medium, it was possible to determine (1.0 ± 0.05) × 10⁻⁵ M in phosphate medium. In the presence of selenite detection limit (S/N = 3) was 9.0 × 10⁻⁸ M Cr(VI) by using either phosphate or borate This method was applied to Gerede river water, after oxidation all Cr(III) present into Cr(VI).     

Application of surfactant modified zeolite membrane electrode towards potentiometric determination of perchlorate

Potentiometric electrodes based on the incorporation of surfactant-modified zeolite Y (SMZ) particles into poly vinyl chloride (PVC) membranes were described. The electrode characteristics were evaluated regarding the response towards perchlorate ions. PVC membranes plasticized with dioctyl phthalate and without lipophilic additives (co-exchanger) are used throughout this study. The influence of membrane composition on the electrode response was studied. The electrode exhibited a Nernstian response towards perchlorate in the concentration range of 7.9 × 10⁻⁶–8.0 × 10⁻² M with a slope of 59.7 ± 0.9 mV per decade of perchlorate concentration with a working pH range of 1.7–9.5 with a fast response time of ≤10 s. The lower and upper detection limits were 4.07 × 10⁻⁷ and 0.13 M, respectively. The electrode response to perchlorate remains constant in the temperature range of 20–40 °C and in the presence of 2.5 × 10⁻⁶–1 × 10⁻² M NaNO₃. The selectivity coefficients for perchlorate anion as test species with respect to other anions were determined. The proposed modified zeolite-PVC electrode can be used for at least 30 days without any considerable divergence in potential. It was applied as indicator electrode in water samples with satisfactory results. The results of this study and our previous work show HDTMA plays different roles according to the zeolite type and matrix, as HDTMA-zeolite Y in a carbon paste matrix showed a good Nernstian behavior towards phosphate anion.