Trace determination of rare earths by adsorption voltammetry at a carbon paste electrode

A novel adsorption voltammetry has been described for the determination of trace rare earths in the presence of alizarin complexon (ALC) at a carbon paste electrode (CPE). A sensitive adsorptive oxidation peak was found for the complexes of middle and heavy rare earths(III) with ALC in a supporting electrolyte of 0.12 M HAc + NaAc and 0.03 M potassium biphthalate (pH 5.0) for linear-scanning from −0.2 to 0.8 V at 100 mV s⁻¹. The sensitivity of the heavy rare earths (Dy, Ho, Er, Tm, Yb, and Lu) is higher than that of other rare earths. Under the optimized procedure, a detection limit of 5.0 × 10⁻¹⁰ M holmium(III) (S/N=3) was realized for a 120 s preconcentration and the linear current–concentration relationship was observed up to 2.0 × 10⁻⁷ M. The relative standard deviation is 3.8% for 20 successive determinations of 4.0 × 10⁻⁸ M Ho(III) on the same electrode surface. The electrode processes of the adsorbed complex were investigated. The method developed was applied for determination of the sum of rare earths (except La, Ce, Pr, Nd, and Sc) in rare earth nodular cast iron samples with satisfactory results.     

Attachment of gold nanoparticles to glassy carbon electrode and its application for the voltammetric resolution of ascorbic acid and dopamine

Gold nanoparticles have been attached on glassy carbon electrode surface through sulfhydryl-terminated monolayer and the gold nanoparticles-immobilized glassy carbon electrodes have been applied to the electrocatalytic oxidation of ascorbic acid, reducing the overpotential by about 200 mV with obviously increased current response. Due to its strong electrocatalytic activity towards ascorbic acid, the gold nanoparticles modified electrode can resolve the overlapped voltammetric waves of ascorbic acid and dopamine into two well-defined voltammetric peaks with peak-to-peak separation in potentials of about 300 mV. This can be used to allow the selective determination of ascorbic acid in the presence of dopamine. The catalytic current obtained from differential pulse voltammetry is linearly dependent on ascorbic acid concentration over the range of 6.5 × 10^?6 to 1.45 × 10^?4 M with correlation coefficient of 0.998 in the presence of dopamine. The detection limit (3σ) for AA was found to be 2.8 × 10^?6 M. The simultaneous determination of ascorbic acid and dopamine in their binary mixture has also been investigated. The modified electrode shows good selectivity, stability and anti-fouling properties. The proposed methods have been used for the selective determination of ascorbic acid in the presence of dopamine and for the simultaneous determination of both them in their mixtures with satisfactory results.     

Electrochemical behavior of quercetin: Experimental and theoretical studies

Electrochemical oxidation of quercetin, as important biological molecule, has been studied in 0.1 M phosphate buffer solution, using cyclic voltammetry, chronoamperometry, rotating disk electrode voltammetry as well as quantum mechanical calculations. The heterogeneous charge transfer rate constant, k′, transfer coefficient, α, and exchange current density, j₀, for oxidation of quercetin at the glassy carbon electrode are determined as 4.84 × 10^?2 cm s^?1, 0.65 ± 0.01 and (1.17 ± 0.39) × 10^?7 A cm^?2, respectively. The formal potential, E^0′, of quercetin is pH dependent with a slope of ?60.1 mV per unit of pH which is close to the anticipated Nernstian value of ?59 mV for a two electrons and two protons process. The standard formal potential, E⁰, of quercetin was found to be equal with 558 mV versus saturated calomel electrode (SCE). The mechanism of oxidation was deduced from voltammetric data in various pHs and also in different concentrations of quercetin. The diffusion coefficient of quercetin was calculated as 3.18 × 10^?6 cm² s^?1 for the experimental condition, using chronoamperometric results. The results of density functional theory (DFT) calculations for the oxidation of quercetin in aqueous solution, are also presented. The theoretical standard electrode potential of quercetin is obtained to be 568 mV versus SCE, which is in good agreement with the experimental value. The discrepancy between theoretical and experimental values is only 10 mV. The agreement verifies the accuracy of experimental method and the validity of mathematical model.     

Efficacy of a mouthrinse based on hydroxyapatite to reduce initial bacterial colonisation in situ

ObjectiveThe present in situ - investigation aimed to specify the impact of pure hydroxyapatite microclusters on initial bioadhesion and bacterial colonization at the tooth surface.DesignPellicle formation was carried out in situ on bovine enamel slabs (9 subjects). After 1 min of pellicle formation rinses with 8 ml of hydroxyapatite (HA) microclusters (5%) in bidestilled water or chlorhexidine 0.2% were performed. As negative control no rinse was adopted. In situ biofilm formation was promoted by the intraoral slab exposure for 8 h overnight. Afterwards initial bacterial adhesion was quantified by DAPI staining and bacterial viability was determined in vivo/in vitro by live/dead-staining (BacLight). SEM analysis evaluated the efficacy of the mouthrinse to accumulate hydroxyapatite microclusters at the specimens’ surface and spit-out samples of the testsolution were investigated by TEM.ResultsCompared to the control (2.36 × 10⁶ ± 2.01 × 10⁶ bacteria/cm²), significantly reduced amounts of adherent bacteria were detected on specimens rinsed with chlorhexidine 0.2% (8.73 × 10⁴ ± 1.37 × 10⁵ bacteria/cm²) and likewise after rinses with the hydroxyapatite testsolution (2.08 × 10⁵ ± 2.85 × 10⁵ bacteria/cm², p < 0.001). No demonstrable effect of HA-particles on Streptococcus mutans viability could be shown. SEM analysis confirmed the temporary adsorption of hydroxyapatite microclusters at the tooth surface. Adhesive interactions of HA-particles with oral bacteria were shown by TEM.ConclusionHydroxyapatite microclusters reduced initial bacterial adhesion to enamel in situ considerably and could therefore sensibly supplement current approaches in dental prophylaxis.     

Novel solid-state cadmium ion-selective electrodes based on its tetraiodo- and tetrabromo-ion pairs with cetylpyridinium

Two novel cadmium solid-state ion-selective electrodes have been prepared by coating the surface of a graphite rod electrode directly with tetrahydrofuran solution containing PVC, cetylpyridinium–tetraiodocadmate (I) or cetylpyridinium–tetrabromocadmate (II), dioctyl phthalate and sodium tetraphenyl borate. The two sensors exhibit near-Nernstian anionic slopes of ?29.8 and ?25.1 mV/concentration decade, independently of pH over a wide range, with very fast response times of 3 and 7 s, respectively. The tetraiodocadmate (TIC) and tetrabromocadmate (TBC) electrodes posses linear ranges of 1.5 × 10^?6–1 × 10^?1 and 1.0 × 10^?6–1 × 10^?1 M and lower detection limits (LDL) of 6 × 10^?7 and 8 × 10^?7 M, respectively. The effects of membrane composition, type of plasticizer and pH of the sample solution were investigated thoroughly. The TBC electrode is shown to be free of all interference that is common for most of the reported cadmium ISEs except for that of Hg²⁺ ion. The two electrodes were applied for the determination of cadmium in some alloys and polluted water.     

Effect of phase transition on the electrochemical behavior of ferredoxin embedded in an artificial lipid membrane film

The effect of the phase transition of a bilayer membrane on the electrochemical behavior of ferredoxin embedded in a cast film of artificial lipid, 2C₁₈N⁺Br⁻, was investigated. The redox potential of ferredoxin embedded in the lipid film showed a linear, positive shift from −510 (± 2) to −466 (± 3) mV (vs. Ag|AgCl|saturated KCl) with increasing temperature from 25 to 53 °C. The diffusion coefficient (D) and heterogeneous electron transfer rate constant (k°^′) were evaluated at various temperatures by means of analyzing cyclic voltammograms. The overall shape of the simulated voltammograms fitted well with the experimentally observed voltammograms at various potential sweep rates, when the estimated D and k°^′ values were used for the simulation. The results of the temperature dependence of the estimated D and k°^′ values indicated that D and k°^′ were enhanced near the phase transition temperature (T_c), 46 °C, of the lipid film. The D and k°^′ values at temperatures above the T_c were approximately two-orders and one-order of magnitude larger than those estimated at temperatures under the T_c, respectively. The estimated D and k°^′ values were 2.1–5.8 × 10⁻¹⁰ cm² s⁻¹ and 1.2–1.6 × 10⁻⁴ cm s⁻¹ at 25 °C, and 3.5–3.7 × 10⁻⁸ cm² s⁻¹ and 1.1–1.2 × 10⁻³ cm s⁻¹ at 50 °C, respectively. The electrochemical behavior of ferredoxin in the lipid film changed drastically near the T_c.     

Direct simultaneous determination of dihydroxybenzene isomers at C-nanotube-modified electrodes by derivative voltammetry

The voltammetric behavior of dihydroxybenzene isomers was studied with glassy carbon electrodes modified with multi-wall carbon nanotubes. In 0.1 mol L^?1 HAc + NaAc buffer solution (pH 5.5), the modified electrode showed a good electrocatalytic response towards dihydroxybenzenes. The peak currents increased significantly and their oxidation potentials shifted negatively. Through a derivative technique, the three oxidation peaks of dihydroxybenzene isomers can be separated, thus the method can be applied to direct simultaneous determination without previous separation. The linear calibration ranges were 2 × 10^?6–1 × 10^?4 mol L^?1 for hydroquinone and catechol, respectively, and 5 × 10^?6 to 8 × 10^?5 mol L^?1 for resorcinol, with detection limits of 6 × 10^?7, 6 × 10^?7 and 1 × 10^?6 mol L^?1, respectively. This method has been applied to the direct determination of dihydroxybenzene isomers in artificial wastewater, and the recovery was from 92% to 104%.     

Diffusion-weighted imaging with histogram analysis of the apparent diffusion coefficient maps in the diagnosis of parotid tumours

The aim of this study was to investigate the role of diffusion-weighted imaging (DWI) with histogram analysis of apparent diffusion coefficient (ADC) maps in the characterization of parotid tumours. This prospective study included 39 patients with parotid tumours. All patients underwent magnetic resonance imaging with DWI, and ADC maps were generated. The whole lesion was selected to obtain histogram-related parameters, including the mean (ADC_mean), minimum (ADC_min), maximum (ADC_max), skewness, and kurtosis of the ADC. The final diagnosis included pleomorphic adenoma (PA; n = 18), Warthin tumour (WT; n = 12), and salivary gland malignancy (SGM; n = 9). ADC_mean (×10^?3 mm²/s) was 1.93 ± 0.34 for PA, 1.01 ± 0.11 for WT, and 1.26 ± 0.54 for SGM. There was a significant difference in whole lesion ADC_mean among the three study groups. Skewness had the best diagnostic performance in differentiating PA from WT (P = 0.001; best detected cut-off 0.41, area under the curve (AUC) 0.990) and in discriminating WT from SGM (P = 0.03; best detected cut-off 0.74, AUC 0.806). The whole lesion ADC_mean value had best diagnostic performance in differentiating PA from SGM (P = 0.007; best detected cut-off 1.16 × 10^?3 mm²/s, AUC 0.948). In conclusion, histogram analysis of ADC maps may offer added value in the differentiation of parotid tumours.     

The electrochemistry and thin-layer luminescence spectroelectrochemistry of rhodamine 6G at a 4,4′-bipyridine-modified gold electrode

We report on the first direct electrochemistry and fluorescence spectroelectrochemistry of rhodamine 6G at a 4,4′-bipyridine-modified gold electrode. The value of n determined in spectropotentiostatic experiments at 1.87×10^?6 mol l^?1 of rhodamine 6G in 0.20 mol l^?1 KCl solution is 1.15, and the experimental value obtained for E⁰′ is ?0.787 V versus Ag ∣ AgCl ∣ KCl_sat, which agrees very well with the value (E⁰′=?0.791 V) obtained using cyclic voltammetry at a modified gold electrode. The values of the diffusion coefficients D_O and D_R for the oxidized and reduced forms of rhodamine 6G calculated from results of potential step and in situ fluorescence measurement experiments are 4.0×10^?6 cm² s^?1 and 4.2×10^?6 cm² s^?1, respectively. Cyclic voltammograms of rhodamine 6G show that the peak current I_p is proportional to the square root of the potential scan rate v^1/2, the ratio of the reduction to the oxidation peak height is about unity, and the separation of both reduction and reoxidation peak potentials ΔE_P is essentially constant at 135 mV at low scan rates. These results indicate that electrochemistry of rhodamine 6G at a 4,4′-bipyridine-modified gold electrode is a quasi-reversible one-electron electrode process.     

The electrochemical behavior of p-sulfonated calix[4]arene

The electrochemical behavior of p-sulfonated calix[4]arene was studied. In aqueous solution, p-sulfonated calix[4]arene can be oxidized when the potential is more than 0.7 V versus SCE. It is confirmed that the reaction is an irreversible two-electron transfer electrochemical reaction. The anodic peak potential, E_p, is affected by the acidity of the solution. E_p shifts in the negative direction when the pH increases. The electron transfer coefficient, α, is 0.65. At 25 °C, the diffusion coefficient of p-sulfonated calix[4]arene is 3.1 × 10^?5 cm² s^?1. The activation energy, E_a, for the electrochemical reaction is (18.8 ± 0.2) kJ mol^?1.     

Microtensile bond strength of resin-based composites to Ti–6Al–4V

ObjectiveThe purpose of this study was to determine the microtensile bond strength of various resin composite/adhesive systems to alumina particle abraded Ti–6Al–4V substrate after aging for 24 h, 10 days, and 30 days in distilled water at 37 °C.MethodsFour laboratory resin composite veneering systems (Gradia, GR; Solidex, SOL; Ceramage, CER; and Sinfony, SF) were bonded to 25 mm diameter machined disks of Ti–6Al–4V with their respective adhesive and methodology, according to the manufacturer's instructions. Microtensile bars of approximate dimensions 1 mm × 1 mm × 6 mm were prepared for each resin composite/adhesive system. After cutting, groups (n = 12) from each adhesive system were separated and either stored in water at 37 °C for 24 h (baseline) or aged for 10 or 30 days prior to loading to failure under tension at a cross head speed of 1.0 mm/min. Failure modes were determined by means of scanning electron microscopy (SEM). Statistical analysis was performed through one-way ANOVA and Tukey's test at 95% level of significance.ResultsSignificant variation in microtensile bond strength was observed for the different systems and aging times. SOL and GR showed the highest mean bond strength values followed by SF and CER at baseline. Aging specimens in water had an adverse effect on bond strength for SOL and CER but not for the SF and GR groups.SignificanceIn vitro bond strength of laboratory resin composites to Ti–6Al–4V suggests that strong bonds can be achieved and are stable for certain systems, making them useful as an alternative for esthetic fixed prosthetic restorations.     

Catechin antioxidant action at various pH studied by cyclic voltammetry and PM3 semi-empirical calculations

The mechanism of catechin electro-oxidation at various pH was studied using cyclic voltammetry (CV) on the glassy carbon (GC) electrode and PM3 semi-empirical calculations. The influence of activation of the surface of the GC electrode on CV results has been discussed. Mixed adsorption–diffusion control has been observed by applying mechanistic criteria of CV to the results obtained at the activated electrode. The calculated catechin diffusion coefficient D = 2.78 × 10^?6 cm² s^?1. A linear increase of the current peak has been observed with the increase of substrate concentration up to 40 μmol dm^?3 (surface coverage Γ ～ 10^?11 mol cm^?2). In the whole investigated pH range, the dE/dpH value is very close to the anticipated Nernstian dependence of ?59 mV/pH indicating that the slope is not affected by the different sequences of e^? and H⁺ transfer. Molecular modeling results show a decrease of ≈5 kcal mol^?1 in ΔHoF (between radical and parent molecule) and a decrease of ≈6 eV in IP (of the parent molecule) when the parent molecule is changed from neutral to monoanionic form of catechin showing that both processes – hydrogen and electron abstraction are facilitated by deprotonation. Electrochemical oxidation of catechin is known to proceed as a two step one-electron oxidation of the B-ring of o-phenolic groups. Upon an increase in the pH, the mechanistic pathway of catechin electro-oxidation in both oxidation steps changes from an eH to the He process. In the reaction with a free radical, this may induce the change from hydrogen to electron donation.     

Differences in the biomechanical behaviors of natural teeth and dental implants

ObjectiveThe lack of a PDL, which acts as an energy absorber, is a contributor to implants’ early failure; however, these discrepancies are not well understood because of limited in vivo research. This study investigated the discrepancy in biomechanical behaviors between natural teeth and dental implants by detecting micro-movements in vivo.MethodsWe designed a device that could measure precisely mechanical behaviors such as creep, stress relaxation, and hysteresis by using load–control displacement on teeth and implants. We also compared energy dissipation between natural teeth and dental implants by subtracting the area of the hysteresis loop of natural teeth from that of dental implants.ResultsBiphasic curves with an initial phase of rapid response and a subsequent phase of slow response were confirmed in creep and stress relaxation curves for the load–time relationship in natural teeth. By contrast, the behavior of creep or stress relaxation was less prominent when the dental implants were tested. We observed that the periodontal ligament under an axial intrusive load of 300 g in a loading rate 3 g/s could dissipate the energy of 7.35 ± 1.18 × 10^?2 mJ, approximately 50 times that of the dental implants (1.47 ± 1.22 × 10^?3) with statistically significant (p < 0.05).SignificanceWe confirmed natural teeth could achieve greater energy dissipation compared to dental implants, which owe to that natural teeth exhibited fluid and viscoelastic properties.     

Ruthenium purple-containing zeolite modified electrodes and their application for the detection of glucose

Zeolite-entrapped ruthenium purple particles (denoted NaY ∣ RP) were prepared by exchanging Fe³⁺ into zeolite Y (NaY) and reacting further with ruthenium hexacyanide (Ru(CN)₆^4?). Although XRD and diffuse-reflectance UV-vis absorption analyses suggested that the RP_(NaY) particles were basically amorphous, the resulting electrode prepared from these particles and glucose oxidase (GOx) displayed a remarkable sensitivity to glucose. The limiting current showed a linear relationship with the bulk activity of glucose up to 4 mM (pH 5.1, phosphate), covering a range from 10^?6 to 10^?3 M. Flow-injection analysis, in addition, showed the detection limit reaching a level as low as 0.1 μM. Data simulation showed that the electrode sensitivity followed an electrocatalytic (EC′) mechanism based on the reduction of H₂O₂ by the reduced ruthenium purple. Accordingly, the reaction rate constant (pseudo-first-order) and the effective activity of RP were determined to be 110 M^?1 s^?1 (pH 5) and 1.5×10^?9 mol cm^?2, respectively. Electrochemical impedance spectroscopic (EIS) analysis showed that the charge-transfer resistance of the zeolite-electrode decreased systematically with the stepwise addition of glucose into the system. The exchange rate constant (k_o) and the diffusion coefficient of electrons (D_e) in the zeolite film were estimated to be 5×10^?6 cm s^?1 and 6×10^?10 cm² s^?1, respectively. These experimental results suggested that the RP sites in the NaY ∣ RP particles were separated widely with the average distance between the adjacent RP sites estimated to be about 1×10^?4 cm or equivalent to 500 supercages, which agreed well with the results obtained from data simulation.     

Characterization and electrocatalytic properties of cobalt hexacyanoferrate films immobilized on Au-colloid modified gold electrodes

An electroactive cobalt hexacyanoferrate (CoHCF) film was electrodeposited from a solution containing Co²⁺ and Fe(CN)₆^3? ions on the bare gold or the Au-colloid modified electrode. The cation (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) and the anion (F^?, Cl^? and Br^?) effects on the redox peak of the CoHCF film were investigated in detail. On the other hand, the electrocatalytic oxidations of thiosulfate at the CoHCF/gold and CoHCF/Au-colloid/gold electrodes were compared. At the CoHCF/Au-colloid/gold electrode, we obtained a response current larger by a factor of 2 and a three times lower detection limit than those at a CoHCF/gold electrode. The linear ranges were 1.0 × 10^?4 to 2.8 × 10^?3 M for the CoHCF/gold electrode and 7.5 × 10^?5 to 4.8 × 10^?3 M for the CoHCF/Au-colloid/gold electrode. These results showed that the immobilized CoHCF at the Au-colloid modified electrode exhibited a higher catalytic activity and a wider linear range toward thiosulfate. Additionally, the effects of the applied potential and the solution pH were studied.     

Maxillary sensory nerve responses induced by different types of dentures

PurposeThe purpose of this study was to investigate whether different types of dentures induced different responses to stimulations in sensory nerve underlying the denture-supporting mucosa using current perception threshold (CPT).Materials and methodsThe study population comprised 45 complete denture wearers with a mean age of 69.7 years (CD), 30 partial denture wearers (PD) with a mean age of 67.1 years, and 40 dentulous participants with a mean age of 69.0 years (Dent). Current perception threshold (CPT) on the greater palatine nerve at 2000 Hz, 250 Hz, and 5 Hz, corresponding to A-beta, A-delta, and C fibers respectively, were measured by the Neurometer^® NS3000 device. The differences CPTs among CD, PD, and Dent groups were analyzed by Kruskal–Wallis test and Mann–Whitney U test with adjusting the multiple comparisons’ inflation of type 1 error rate by a Bonferroni correction.ResultsCPTs of CD, PD, and Dent group at 2000 Hz were 61.5 ± 45.8, 53.5 ± 25.3, 33.0 ± 11.4 (10^?2 mA) respectively. CPTs of CD, PD, and Dent group at 250 Hz were 29.2 ± 28.2, 20.1 ± 13.2, 14.3 ± 5.9 (10^?2 mA) respectively. CPTs of CD, PD, and Dent group at 5 Hz were 28.9 ± 23.4, 17.8 ± 12.2, 12.2 ± 5.6 (10^?2 mA) respectively. The CPTs at all frequencies increased in the following order: Dent < PD < CD wearer. The statistical analyses showed that the different types of dentures significantly affected CPTs at 2000 Hz (p < 0.0001), 250 Hz (p < 0.0001), and 5 Hz (p < 0.0001).ConclusionThe different types of dentures induce different responses to stimulations in the sensory nerve underlying the denture-supporting mucosa.     

Viscosity and stickiness of dental resin composites at elevated temperatures

ObjectiveTo investigate the influence of pre-heating different classes of dental resin composites on viscosity and stickiness at five different temperatures.MethodsSix flowable, five conventional packable, and one thermo-viscous bulk-fill resin composites were heated up to 54 °C in a plate-plate rheometer to determine their complex viscosity. Normal force measurements were carried out for the six packable materials to determine the unplugging force and unplugging work (stickiness) over the same temperature range. Data were analyzed using Kolmogorov–Smirnoff test, one-way ANOVA and Tukey Post Hoc test with α = 0.05 as level of significance.ResultsAt 23 °C packable composites showed viscosity between 6.75 and 19.14 kPa s, while flowable composites presented significantly lower viscosities between 1.31 and 2.20 kPa s. Pre-heating led to a drop of 30–82% in the viscosity of packable materials. The thermo-viscous material dropped to the level of flowables at 45 and 54 °C thus behaving as a packable composite at room temperature with flowable-like viscosity at higher temperatures. No statistically significant differences for viscosity were observed among flowable composites at any temperature.The unplugging force decreased for packable composites, while their unplugging work generally increased at elevated temperature. At 23 °C unplugging force was measured between 7.50 and 19.18 N, while pre-heating up to 54 °C led to values between 2.9 and 6.2 N. Regarding unplugging work at 23 °C the calculated values were between 3.0 and 8.9 × 10^?3 J and at 54 °C between 8.8 and 13.0 × 10^?3 J.SignificancePre-heating significantly reduced viscosity of highly viscous resin composites, while no influence was shown for flowable composites. In general stickiness, measured as unplugging work, increased at elevated temperatures. The thermo-viscous material showed low viscosity comparable to flowable composites at 45 and 54 °C, yet its stickiness did not increase significantly compared to the values at 23 °C.     

Square-wave voltammetry of an aqueous solution of indigo

Indigo is dissolved in water in the concentration of 5×10^?5 M and measured by highly sensitive adsorptive stripping square-wave voltammetry in order to determine the parameters of its electrochemical reduction. The formal potential of the indigo/leucoindigo redox couple dissolved in the aqueous 0.7 M KNO₃, pH 12 (NaOH) is ?0.566±0.003 V versus Ag ∣ AgCl ∣ 3 M KCl. Both indigo and leucoindigo are adsorbed on the surface of the mercury electrode, and the standard rate constant of this redox reaction is 920±130 s^?1. The ratio of adsorption constants of indigo and leucoindigo is 0.75.     

Electrocatalytic oxidation of nitrite on a carbon paste electrode modified with Co(II) porphyrin adsorbed on SiO2/SnO2/Phosphate prepared by the sol–gel method

This paper describes the immobilization of 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H,23H-porphyrin ion on SiO₂/SnO₂/Phosphate obtained by the sol–gel processing method. The porphyrin was adsorbed on the surface of the modified material and furthermore metallized in situ with Co(II) ion. The porphyrin metallation process was followed using UV–Vis spectroscopy by inspecting the Q bands of the free and metallated porphyrin. A carbon paste electrode modified with material containing metallated porphyrin was used to study the electro-catalytic oxidation of nitrite ions by means of cyclic voltammetry, chronoamperometry and RDE voltammetry. The modified electrode was very stable and exhibited the electro-catalytic oxidation of nitrite ions at 0.72 V vs. SCE by a two electron mechanism producing nitrate ions at pH 5.4. The kinetic parameters of the electrode reaction process were calculated; (1 ? α)n_a was 0.479, D was (5.3 ± 0.11) × 10^?5 cm s^?1, and k⁰ could be determined as (5.4 ± 0.14) × 10^?3 cm s^?1.     

Bis-GMA co-polymerizations: Influence on conversion,flexural properties,fracture toughness and susceptibility to ethanol degradation of experimental composites

ObjectivesThe aim of this study was to evaluate the influence of monomer content on fracture toughness (K_Ic) before and after ethanol solution storage, flexural properties and degree of conversion (DC) of bisphenol A glycidyl methacrylate (Bis-GMA) co-polymers.MethodsFive formulations were tested, containing Bis-GMA (B) combined with TEGDMA (T), UDMA (U) or Bis-EMA (E), as follows (in mol%): 30B:70T; 30B:35T:35U; 30B:70U; 30B:35T:35E; 30B:70E. Bimodal filler was introduced at 80 wt%. Single-edge notched beams for fracture toughness (FT, 25 mm × 5 mm × 2.5 mm, a/w = 0.5, n = 20) and 10 mm × 2 mm × 1 mm beams for flexural strength (FS) and modulus (FM) determination (10 mm × 2 mm × 1 mm, n = 10) were built and then stored in distilled water for 24 h at 37 °C. All FS/FM beams and half of the FT specimens were immediately submitted to three-point bending test. The remaining FT specimens were stored in a 75%ethanol/25%water (v/v) solution for 3 months prior to testing. DC was determined with FT-Raman spectroscopy in fragments of both FT and FS/FM specimens at 24 h. Data were submitted to one-way ANOVA/Tukey test (α = 5%).ResultsThe 30B:70T composite presented the highest K_Ic value (in MPa m^1/2) at 24 h (1.3 ± 0.4), statistically similar to 30B:35T:35U and 30B:70U, while 30B:70E presented the lowest value (0.5 ± 0.1). After ethanol storage, reductions in K_Ic ranged from 33 to 72%. The 30B:70E material presented the lowest reduction in FT and 30B:70U, the highest. DC was similar among groups (69–73%), except for 30B:70U (52 ± 4%, p < 0.001). 30B:70U and 30B:35T:35U presented the highest FS (125 ± 21 and 122 ± 14 MPa, respectively), statistically different from 30B:70T or 30B:70E (92 ± 20 and 94 ± 16 MPa, respectively). Composites containing UDMA or Bis-EMA associated with Bis-GMA presented similar FM, statistically lower than 30B:35T:35U.SignificanceComposites formulated with Bis-GMA:TEGDMA:UDMA presented the best compromise between conversion and mechanical properties.