An in situ UV-vis and IR spectroelectrochemical study of the deposition of a hydroquinone anion salt on platinum electrodes from dichloromethane solutions

Cyclic voltammetry with a platinum electrode of hydroquinone (BQH₂) solutions in dichloromethane, containing tetrabutylammonium perchlorate supporting electrolyte, shows a sharp asymmetric irreversible oxidation peak at about ?0.3 V (SCE). This feature is seen, in addition to the expected features in this system, when the cycle is extended to potentials more negative than ?0.6 V (SCE). Cyclic voltammetry, in situ UV-vis and infrared spectroelectrochemistry have shown that hydroquinone anion (BQH^?) is formed at negative potentials and this appears to arise via surface decomposition of hydroquinone to p-benzosemiquinone (BQH) followed by reaction of BQH with the p-benzoquinone radical anion (BQ^?). The sharp asymmetric peak in the cyclic voltammograms is due to oxidation of the hydroquinone anion in the insoluble tetrabutylammonium salt on the electrode surface. The oxidation of BQH^? appears to occur via disproportionation of (BQH) and leads to BQH₂ and p-benzoquinone (BQ) as the products.     

Reduction of p-benzoquinone on lipid-modified electrodes: effect of the alkyl chain length of lipids on the electron transfer reactions

Using a phosphatidylcholine (PC) modified electrode, an unusual stabilization of p-benzoquinone anion radical (BQ^?) is observed even in a buffered solution of pH 7.0. To clarify this incorporation reaction of BQ^? in a PC layer, the effect of the molecular structure of the lipid was studied by constructing the PC modified electrodes with four PC derivatives; dicaproyl-, dilauroyl-, dipalmitoyl- and diarachidoyl-PC. At 25°C, dilauroyl-PC with a moderate alkyl chain length stabilized BQ^? and allowed the approach of benzoquinol onto the surface of the modified electrode. From the pH dependence of the electrochemical responses, the incorporation mechanism of BQ^? with P(O)OH was clarified. In addition to dilauroyl-PC, because dipalmitoyl-PC was found to incorporate BQ^? over 40°C, the relationship between its incorporation and the phase transition temperature of each lipid has become clear. The correlation and the incorporation profiles were examined by measuring the oxidation of benzoquinol as well as that of BQ^?. Also, the diffusion coefficients were determined for the lipids incorporating BQ^?.     

Assessments of salivary antioxidant activity using electron spin resonance spectroscopy

ObjectiveIn recent years, the function of saliva has been focused on evaluation of general status. The relationship between salivary antioxidant activity and periodontal disease progression is unclear. The aim of this study is to assess the relationship between periodontal disease and salivary antioxidant activity towards various reactive oxygen species (ROS) using electron spin resonance (ESR) technique.MethodsWe demonstrated that whole saliva derived rats or human subjects scavenged ROS such as superoxide (O₂^?) and hydroxyl radical (HO) using ESR spectroscopy with spin trapping agent. In addition, we assessed the relationship between antioxidants activity towards ROS and periodontal index with superoxide dismutase (SOD) activity in human subject saliva.ResultsAntioxidant activity towards O₂^? was increased by Porphyromonas gingivalis (P. gingivalis) infection in rat, although antioxidant activity towards HO was not changed. In human, a strong correlation (r = 0.88, p < 0.01) recognized between salivary antioxidant activity towards O₂^? and probing pocket depth (PPD). In addition, the intensity of salivary antioxidant activity depended on SOD activity level. SOD activity was also correlated with PPD.ConclusionsRat salivary antioxidant activity towards O₂^? was up-regulated by the inflammatory response caused by P. gingivalis infection. Similar response was recognized in human saliva with periodontal index. Additionally, a linear correlation between antioxidant activity towards O₂^? and SOD activity was verified by ESR technique. Therefore, evaluation of the salivary antioxidant activity towards O₂^? might be an effective parameter for the objective assessment of periodontal disease progression.     

The effect of triphenylmethyl cations on the electroreduction of O2 in nitrobenzene or acetonitrile

The electroreduction of O₂ in two aprotic solvents (nitrobenzene and acetonitrile) was examined in the absence and in the presence of the very strong Lewis acid, triphenylmethyl cation, φ₃C⁺ (φ=phenyl). The addition of φ₃C⁺ causes the electroreduction of O₂ to change from the one-electron reduction to O₂^? into the two-electron reduction to φ₃CO₂Cφ₃ at a potential that is 1 V more positive than that where O₂ is reduced to O₂^?. This large positive shift in potential is used to estimate a value of the equilibrium constant for the association between O₂^2? and two φ₃C⁺ cations (K=3×10²⁶ M^?2). The mechanism of the reduction of O₂ to φ₃CO₂Cφ₃ can be regarded as an inner-sphere electron-transfer reaction between φ₃C and O₂ or, equivalently, as a radical addition to O₂.     

Indirect anodic fluorodesulfurization of S-aryl thiobenzoates using a triarylamine mediator

The catalytic effects of a triarylamine [(2,4-Br₂C₆H₃)₃N] (2) for the fluorodesulfurization of S-aryl thiobenzoates were investigated by cyclic voltammetry. It was found that the electron transfer from S-p-methoxyphenyl thiobenzoate to 2⁺ was fast while that from S-phenyl and S-p-chlorophenyl derivatives to 2⁺ was slow. The indirect macro-electrolysis of S-aryl thiobenzoates gave benzoyl fluoride in good yield at a less positive potential compared with direct electrolysis. A possible mechanism for triarylamine mediated fluorodesulfurization was proposed.     

Electrogenerated chemiluminescence of sterically hindered porphyrins in aqueous media

A sterically hindered water-soluble porphyrin, tetrakis(3-sulfonatomesityl)porphyrin (H₂TSMP), could form stable radical cation in aqueous media after electrochemical one electron oxidation. The anodic oxidation of H₂TSMP in the presence of tripropylamine or C₂O₄^2? as a coreactant in aqueous solution produces electrogenerated chemiluminescence (ECL) with maxima at 640 and 700 nm. The same emission spectrum of ECL and fluorescence indicates that the ECL emission is from the singlet state of H₂TSMP. The annihilation reactions of ZnTSMP⁺and ZnTSMP^?, which are generated electrochemically, in CH₃CN+H₂O (1:1) mixed solution results in an emission which is identical to the photoluminescence. Protection of the active sites against the nucleophilic attack of water or OH^? by sterical hinderance is a successful strategy in designing new ECL-active compounds in aqueous media. Both ECL reaction mechanisms are proposed.     

Electrogenerated chemiluminescence of lucigenin enhanced by the modifications of electrodes with self-assembled monolayers and of solutions with surfactants

Electrogenerated chemiluminescence (ECL) of lucigenin (Luc²⁺·2NO^?₃, N,N′-dimethyl-9,9′-biacridinium dinitrate) in dioxygen-saturated alkaline aqueous solutions of pH 10 has been examined utilizing modifications of electrodes (i.e. self-assembled monolayer (SAM)-modified gold electrodes) and of solutions (i.e. micellar solutions containing a nonionic surfactant, Triton X-100) for the first time. In both cases of the modifications, enhanced ECL was observed, and the ECL was considered to be derived from the decomposition of a dioxetane-type intermediate formed by the radical–radical coupling reaction between an electrogenerated superoxide ion (O₂^?) and a one-electron reduced form of Luc²⁺ (i.e. a radical cation, Luc⁺). The surface modification of gold electrodes was achieved with dimercaptosuccinic acid (DMSA) and dl-thioctic acid (dl-TA) having carboxyl end groups. The amount of ECL at dl-TA-SAM-modified electrodes was about five times as great as that at the bare electrode. The enhancement of ECL at the SAM-modified electrodes would be due to the concentration effect of positively charged Luc²⁺ ions, the prevention of adsorption of the electrogenerated chemiluminescent product (i.e. N-methylacridone (NMA)) and the two-electron reduced form of Luc²⁺ (Luc⁰) on the electrode surfaces, and the effective generation of O₂^?. On the other hand, the amount of ECL in the micellar solution increased by about six times in comparison with that in the solution containing no surfactant. The enhanced ECL in micellar solutions would result mainly from the inhibition of adsorption of NMA and Luc⁰ insoluble in water on electrode surfaces.     

Electrocatalytic oxidation of styrene by a high valent ruthenium porphyrin cation radical

A sterically-hindered carbonylruthenium(II) porphyrin Ru^II(CO)(TMP) (where TMP=meso-tetramesitylporphyrinato dianion) has been synthesized. Chemical oxidation of Ru^II(CO)(TMP) by m-chloroperbenzoic acid (m-CPBA) gives the dioxoruthenium(VI) porphyrin (Ru^VI(O)₂TMP). Cyclic voltammograms show that Ru^VI(O)₂TMP is reversibly oxidized at E_1/2=+1.24 V in CH₂Cl₂. Thin-layer absorption spectra for oxidation of Ru^VI(O)₂TMP at +1.32 V indicates that the product is a porphyrin cation radical (Ru^VI(O)₂TMP⁺). Electrogenerated Ru^VI(O)₂TMP⁺ reacts selectively with styrene to give phenylacetaldehyde (96%) and benzaldehyde (4%). We report the first case of styrene oxidation by high valent ruthenium porphyrin under electrochemical conditions. An electrocatalytic oxidation reaction scheme is proposed.     

Fumarodinitrile electrohydrodimerization and acrylonitrile electroreduction on liquid gallium: a comparison with mercury

Electrohydrodimerization (EHD) of fumarodinitrile (R) from dilute aqueous solutions of the strong surfactant Triton X-100 takes place both on liquid gallium and on mercury, via a rate-determining homogeneous radical–radical coupling step. However, the potential range over which EHD takes place is much narrower on gallium than on mercury. This behaviour is due to the fact that adsorbed water molecules act as proton donors towards the radical anion R^?, giving rise to the neutral radical RH, which is immediately reduced to the saturated monomer RH₂. Therefore, EHD requires the complete displacement of water molecules from the adsorbed state by Triton X-100; this is more difficult on the hydrophilic gallium than on the hydrophobic mercury. This also explains why, even though fumarodinitrile is much more strongly adsorbed on gallium than on mercury, no appreciable radical–radical coupling takes place in the adsorbed state on gallium. Acrylonitrile is reduced to the saturated monomer on gallium under the same experimental conditions in which it undergoes EHD on mercury. This is also due to the high hydrophilicity of gallium.     

Mechanism of the electrochemical reduction of benzyl chlorides catalysed by Co(salen)

The electrochemical reduction of benzyl and 4-(trifluoromethyl)benzyl chlorides catalysed by Co(salen) (H₂salen, N,N′-bis(salicylidene)-ethane-1,2-diamine) was studied in acetonitrile. Electrogenerated (Co¹(salen))^? reacts with the halide to give an organocobalt(III) complex. Further one-electron reduction of the latter yields an unstable intermediate that undergoes rapid decomposition by cleavage of the CoC bond. The mechanism of bond breaking in the one-electron-reduced organocobalt(II) complex was investigated. The results of preparative-scale electrolysis on solutions containing Co(salen) and benzyl chloride, performed under different experimental conditions, in particular in the presence of radical or carbanion scavengers, indicate homolytic cleavage of the CoC bond.     

Three-dimensional accuracy of mandibular reconstruction by patient-specific pre-bent reconstruction plates using an “in-house” 3D-printer

Purpose

The purpose of this study was to compare the three-dimensional accuracy of mandibular reconstruction following mandible continuity resection in patients treated with patient-specific, pre-bent reconstruction plates, using an ‘in-house’ 3D printer, with that in patients treated with conventional, intraoperatively bent plates.

Functions of transcription factors NF-κB and Nrf2 in the inhibition of LPS-stimulated cytokine release by the resin monomer HEMA

Objective

Resin monomers like 2-hydroxyethyl methacrylate (HEMA) interfere with effects induced by stressors such as lipopolysaccharide (LPS) released from cariogenic microorganisms. In this study, mechanisms underlying monomer-induced inhibition of the LPS-stimulated secretion of inflammatory cytokines from immunocompetent cells were investigated.

Electroreduction of methyl viologen in the presence of nitrite. Its influence on enzymatic electrodes

The electroreduction of methyl viologen (MV) in the presence of nitrite was studied by cyclic voltammetry. A catalytic wave for the reduction of MV²⁺ was observed at ?0.740 V for which an EC catalytic mechanism is proposed. The rate constant for this chemical reaction under pseudo-first-order conditions, evaluated using working curves, was employed in the simulation of the voltammetric response. The second-order rate constant was also evaluated. Influences of the reaction at ?0.800 V on enzymatic electrodes employing nitrate reductase (NR) and MV⁺ as mediator were also analysed by chronoamperometry.     

Potential-dependent electrochemiluminescence of luminol in alkaline solution at a gold electrode

The behavior of luminol electrochemiluminescence (ECL) at a polycrystalline gold electrode was studied under conventional cyclic voltammetric (CV) conditions. At least six ECL peaks were observed at 0.28 (ECL-1), 0.56 (ECL-2), 0.95 (ECL-3), 1.37 (ECL-4), ?0.43 (ECL-5) and 1.00 (ECL-6, a broad wave after the reverse scan from +1.66) V (vs. SCE), respectively, on the curve of ECL intensity versus the potential. These ECL peaks were found to depend on the presence of O₂ and N₂, the pH of the solution, KCl concentration, scan rate, and potential scan ranges. The emitter of all ECL peaks was identified as 3-aminophthalate by analyzing the CL spectra. It is believed that ECL-1 at 0.28 V was correlated to luminol radicals produced by the electro-oxidation of luminol anion and ECL-2 at 0.56 V was caused by the reaction of luminol radical anions with gold oxide formed on the electrode surface. ECL-1 and ECL-2 could be strongly enhanced by O₂ and O₂^?. ECL-3 at 0.95 V was likely to be due to the reaction of luminol radical anions with O₂ oxidized by OH^?. ECL-4 at 1.37 V suggested that OH^? was electro-oxidized to HO₂^? at this potential and then to O₂^?, which reacted with luminol radical anions to produce light emission. ECL-5 at ?0.43 V seems to be due to the reaction of luminol with ClO^? electrogenerated at higher positive potential and HO₂^? electrogenerated at negative potential. ECL-6 was attributed to the reaction of luminol radical anions and ClO^? electrogenerated at higher positive potential. The results indicated that luminol ECL can be readily initiated by various oxygen-containing species electrogenerated at different potentials, leading to multi-channel light emissions. Furthermore, the present work also reveals that ECL-2 is a predominant ECL reaction route at a gold electrode with higher potential scan rates under CV conditions.     

Dependence of dimerization of electrogenerated methylalkylviologen radical cations on the length of alkyl chain in the presence of γ-cyclodextrin

The effect of γ-cyclodextrin (γ-CD) on the monomer-dimer equilibria of 1-methyl-1′-alkylviologen radical cations (C₁C_nV⁺: n = 1?10) has been studied by electrochemical and spectroelectrochemical techniques. The apparent dimerization is enhanced upon addition of γ-CD for n = 1?8 and the effect of γ-CD is a maximum for n = 4. This is attributed to the stabilization of dimers by the inclusion of the dimer in the cavity of γ-CD. However, the dimerization of C₁C₉V^?+ and C₁C₁₀V^?+, which exhibits strong tendencies of dimerization in the absence of γ-CD due to electrodeposition of the radical cations, is suppressed upon addition of γ-CD. From the dependence of the apparent dimerization constants on the concentration of γ-CD, the association constants of the dimer (K_C2) and monomer (K_C1) with γ-CD were determined for n = 1?8. Comparing the present results with our previous report with dialkylviologen radical cations (C_nCV^?+: n = 1?5), we show that the dimers with the longer alkyl substituents in the same and opposite sides have little difference in their stability in the absence of γ-CD, but the former type of dimer is included preferentially in the cavity of γ-CD.     

Electrochemical reduction of 1,4-benzoquinone. Interaction with alkylated thymine and adenine nucleobases

The electrochemical reduction of 1,4-benzoquinone (Q) in the presence of 1-octylthymine and 9-octyladenine has been performed in dimethylsulfoxide on glassy carbon electrodes. The electrochemical behaviour shows that the semiquinone (Q^?) and the quinone dianion (Q^2?) interact with 1-octylthymine (TH) following a mechanism which involves association and protonation reactions. It is demonstrated by cyclic voltammetry and NMR experiments, that the protonated dianion (QH^?) is stabilised by an excess of TH, alternatively it disproportionates into an association complex of dianion-hydroquinone [Q,QH₂]^2?. The 1-octylthymine anion (T^?) produced in the proton transfer reactions activates a homogeneous chain mechanism allowing the consumption of benzoquinone. For the experiments carried out in the presence of 9-octyladenine (AH), it was observed that only a strong hydrogen bonding association takes place between the nucleobase and Q^2?.     

Photoelectrochemical reduction of CO2 mediated with methylviologen at roughened silver electrodes

The photoelectrocatalytic effect for the reduction of CO₂ mediated with methylviologen (MV) was studied at mercury, polished silver and roughened silver electrodes using electrochemical and surface-enhanced Raman scattering (SERS) techniques. A large photoelectrocatalytic effect for the reduction of CO₂ in the presence of MV was observed at the roughened silver electrode, whereas there was only a very small photoelectrocatalytic current at a more negative potential on mercury and polished silver electrodes. The SERS spectra of MV in the presence and absence of CO₂, along with the electrochemical results, demonstrate that the surface adsorbed complexes, MV⁺–Ag and MV⁰–Ag, played a role as the mediator for photoinduced electron transfer to CO₂ in the solution. The results also suggest that the surface plasmon resonance of the nanoscale silver particle contributes to the overall photoelectrocatalytic effect on a roughened silver electrode.     

Post-surgical effects on the maxillary segments of children with oral clefts: New three-dimensional anthropometric analysis

This study aimed to use new three-dimensional (3D) anthropometric analyses to verify the post-surgical effects on the maxillary segments of children with unilateral cleft lip and palate. The sample was composed by digitized dental models of 60 children with unilateral complete cleft lip and alveolus (UCLA) and complete unilateral cleft lip and palate (UCLP). The impressions were taken before cheiloplasty (T1), after cheiloplasty (T2), and after palatoplasty (T3). The 3D anthropometric analyses of digitized dental casts were obtained through a specific software. Intragroup changes were applied paired t test and Wilcoxon test (UCLA group) and for the UCLP group, repeated-measures analyses of variance followed by the Tukey test. For intergroup analyses, an independent t test and Mann–Whitney test were used. The palatal dimensional changes of UCLA group showed that the distances IC, I?T′, and I–T significantly increased after cheiloplasty (p = 0.0002, p = 0.0007 and p < 0.0001, respectively). In the UCLP group, the IC′ distance statistically decreased in the post-surgical periods (p < 0.0001), while the I–T distance increased (p < 0.0001). The IC distance increased after cheiloplasty (p < 0.0001). The I?T′ distance increased between T2 and T3 with statistically significant differences (p = 0.0037). The intergroup analysis of palatal development (T2-T1) showed that the distances IC′ and I–T′ demonstrated a reduction of the dental arches growth of UCLP group compared with the UCLA group, with statistically significant differences. The new 3D anthropometric analysis showed that the development of the maxillary segments changed after surgical repair. The UCLP group demonstrated a reduction of the dental arches growth compared with the UCLA group.     

Reply to the “Comments on the paper by M.-S. Zheng and S.-G. Sun entitled ‘In situ FTIR spectroscopic studies of CO adsorption on electrodes with nanometer-scale thin films of ruthenium in sulfuric acid solutions’ by C. Pecharromán,A. Cuesta and C. Gutiérrez”

An absorption spectral and electrochemical study for the zinc(II) complexes of meso-tetraphenylporphyrin dianion (TPP), meso-tetramesitylporphyrin dianion (TMP) and meso-tetra(2,6-dichlorophenyl)porphyrin dianion (TDCPP) in the presence of nitrogeneous bases in CH₂Cl₂ solution is reported. The Soret and Q bands of the zinc porphyrins were red-shifted in the presence of imidazole bases, however, the formation constants (K_f) with imidazole or 2-methylimidazole titration were found to be of a similar magnitude. The K_f and the formal electrode potentials of zinc porphyrins are in agreement with the electron-donating–withdrawing properties of the substituents in the phenyl groups. K_f for complexation of the imidazole to the oxidized and reduced zinc porphyrins was obtained from the electrochemical study. The one-electron oxidation of zinc porphyrins showed greater affinity toward imidazole ligation than the other oxidation states of zinc porphyrins. ZnTDCPP⁺ was found to have the greatest affinity with K_f up to 1.35×10⁸. Spectroelectrochemical methods were used to obtain absorption spectra of the complexation of zinc porphyrin cation radical with imidazole. The results showed a slight spectral difference between the complexed and uncomplexed zinc porphyrin cation radical.     

Spectroelectrochemical investigations on the reduction of thin films of hexadecafluorophthalocyaninatozinc (F16PcZn)

Redox potentials were determined for solutions and thin films of hexadecafluorophthalocyaninatozinc (F₁₆PcZn). A value of ?0.6 V versus SCE was determined for the first reduction to the radical anion of F₁₆PcZn^? in N,N′-dimethylformamide (DMF) and ?0.9 V versus SCE for the second reduction to F₁₆PcZn^2?. Both potentials were shifted about 0.4 V towards positive potentials when compared to the unsubstituted phthalocyaninatozinc (PcZn) caused by the stabilization of the π-system by the electron-withdrawing fluorine atoms in the ligand. Vapor-deposited thin films of F₁₆PcZn on indiumtinoxide (ITO) were reduced in contact with aqueous electrolytes of pH 5.5 to avoid H⁺-reduction in acidic regimes. A chemically reversible reduction accompanied by cation intercalation was found. The kinetics of the reaction were studied in detail by cyclic voltammetry under variation of the intercalating ionic species, the film thickness and sweep rate. From the charge uptake as well as from the dependence of peak current densities on the square root of the sweep rate it was found that the reaction rate is limited by the diffusion of intercalating cations. Optical absorption spectra were collected in situ. An irreversible structural change was observed during initial reduction, also in accordance with a strongly irreversible initial CV before the reversible behavior was obtained. The presence of the dianionic F₁₆PcZn^2? was detected even under conditions of an average charge uptake of less than 1 electron/molecule. From the analysis of peak current densities according to the Randles–Sev?ik equation as well as the observed charge flow dependent on film thickness and chronoamperometric characterization of the reaction a diffusion constant D_i for K⁺ in F₁₆PcZn in the range from 1.6×10^?12 to 8.0×10^?12 cm² V^?1 s^?1 was calculated.