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.     

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.     

The kinetics of neutral methyl viologen in acidic H2O+DMF mixed solutions studied by cyclic voltammetry

The chemistry of the two-electron reduction product of viologen (1,1′-dialkyl-4,4′-bipyridinium, V²⁺) neutral species, is important in understanding the electrochemical behavior of viologens and their utilization. The kinetics for the reactions of neutral methyl viologen (V⁰) in the presence of H⁺ (from HCl), CH₃COOH (pK_a=4.75), ClCH₂CH₂COOH (pK_a=4.00), HCOOH (pK_a=3.75) in aqueous media was examined by cyclic voltammetry according to the EEC_i mechanism. To avoid the electrodeposition of V⁰, we used a 9:1 (v/v%) H₂O+DMF mixture as the solvent medium. To evaluate the rate constants for the chemical reaction followed by the second electron transfer step of V²⁺, the ratio of the anodic and cathodic peak current (I_pa2/I_pc2) corresponding to V⁰–e^??V⁺ was plotted against log τ, where τ is the time between E_1/2 and the switching potential, at various scan rates of 0.02–3.5 V s^?1. The chemical reaction was found to be a parallel reaction consisting of H⁺-catalyzed and general-acid (HA) catalyzed reactions. The second-order rate constants are determined as k_H⁺=3.5×10³ M^?1 s^?1, k_CH3COOH=5.7 M^?1 s^?1, k_HCOOH=4.6×10¹ M^?1 s^?1, and k_ClCH2CH2COOH=3.2×10¹ M^?1 s^?1 using the Nicholson–Shain method and k_H2O was estimated as <3×10^?6 M^?1 s^?1. The CVs were digitally simulated under the assumption of a two-step reaction of V⁰ following the two-step electrode reactions of V²⁺ to V⁰. The simulated CVs show good agreement with those obtained experimentally, when the first-step reaction of V⁰ is a relatively fast reversible reaction and the second-step reaction is a slow irreversible one. Based on these results, we propose that V⁰ is in pseudo-equilibrium with H⁺ or HA to produce VH⁺ which undergoes a reaction with H₂O.     

Electrochemical reduction of 2-fluorenecarboxaldehyde: A mechanism with a grandparent–grandchild reaction

The electrochemical reduction of 2-fluorenecarboxaldehyde, 1, has been investigated, principally in N,N-dimethylformamide. The initially formed anion radical undergoes an irreversible dimerization reaction with a rate constant of 2600 M^?1 s^?1. The very basic dimer dianion is protonated by the starting material. This “grandparent–grandchild” reaction well accounts for the amount of conjugate base of 1 that is formed. The effect of adding various OH, NH and CH acids was investigated. The enhancement of the dimerization rate observed with the OH and NH acids is interpreted in terms of the formation of hydrogen-bonded complexes with the anion radical which in turn undergo more rapid dimerization reactions than does the uncomplexed anion radical.     

Heterocyclic quinones as core units for redox switches: UV–vis/NIR,FTIR spectroelectrochemistry and DFT calculations on the vibrational and electronic structure of the radical anions

Spectroscopic and quantum chemical studies on benzodithiophene quinone (2) and benzodifuran quinone (3) and their radical anions are reported. Investigations by UV–vis and FTIR spectroelectrochemistry were undertaken. The electrochemical and spectroscopic properties of the neutral and charged heterocyclic systems are compared with those of anthraquinone (4). On reduction to the radical anions the ν(CO) stretching frequency of 2 diminishes from 1660 to 1502 cm^?1 and of 3 from 1684 to 1536 cm^?1. Molecular orbital calculations at the density functional (DF) level were carried out. In their electrochemical and optical properties, 2 and 3 are alike functionalized 1,4-benzoquinones. The design of an electrochemical cell for FTIR spectroelectrochemistry with exceptional mechanical stability for experiments under strictly anaerobic conditions is presented.     

Effect of pH and the extent of micellization on the redox behavior of non-ionic surfactants containing an anthraquinone group

Cyclic voltammetry has been employed for the study of aqueous electrochemistry of the surfactants, α-(anthraquinonyloxyhexyl)-ω-hydroxy-oligo(ethylene oxide) (ACPEG) and α-anthraquinonyl-ω-hydroxy-oilgo(ethylene oxide) (APEG), which have wide differences in surface activity. Potential–pH diagrams have been constructed and the various features of the diagrams have been analyzed in the light of the change in solution equilibria and the difference in the extent of micellization. The redox potentials of the surfactants have been found to exhibit strong pH dependence. The electrode reaction involves two-electron reduction of anthraquinone (AQ) to its dianion (AQ^2?), which is highly sensitive to the pH of the solution. At controlled pH, potential–pH plots allow the establishment of the values of the ionization constants for dihydroanthraquinone (AQH₂) and its monoanion (AQH^?) as pK_a¹=7.83 and pK_a²=11.38, respectively. Under unbuffered conditions, the effective pH close to the electrode surface controls the potential of the electrode process. The changeover from the H⁺-available to the H⁺-depleted electrode process gives rise to a sudden jump in potential. In highly alkaline solutions, AQ forms an adduct with hydroxyl ion, which causes a linear decrease in the potentials with increase in pH. The different extent of micellization results in a difference in the peak current and the half wave potentials (E_1/2) for ACPEG and APEG but causes no significant change in the shapes of the E_1/2–pH diagrams. This has been explained in terms of the disruption reaction of the micelles, preceding the electrochemical reaction.     

CBCT-based bone quality assessment in decompression of large odontogenic cystic lesions

Objectives

Bone quality comprises bone mineral density and trabecular microstructure. The aim of this study was to explore the effectiveness of cone-beam computed tomography (CBCT) in evaluating bone quality of large odontogenic cystic lesions after decompression using CBCT and BoneJ software, and to determine whether secondary definitive surgery can be guided using CBCT data.

Methods

Twenty-seven patients with large odontogenic cystic lesions treated by decompression were evaluated by CBCT. Medical history and perioperative details were analyzed.

Results

The \(\Delta\)CT values for all patients with cystic lesions decreased after decompression, with no differences for age, sex, and histology (p?>?0.05). Bone volume fraction and trabecular number of new cancellous bone (0.012%, 0.17/mm³) were lower than those of normal cancellous bone (0.189%, 0.47/mm³) (p?<?0.05), while new cancellous bone trabecular separation (11.344?±?2.556 mm) was stronger than normal cancellous bone trabecular separation (4.833?±?2.232 mm) (p?<?0.05). There were no differences in trabecular thickness between new cancellous bone (3.812?±?1.593 mm) and normal cancellous bone (4.598?±?3.573 mm) (p?=?0.746). The \(\Delta\)CT values of five patients with favorable osteogenesis were ??72, ?86, ??86, ?47, and ??55, those of three patients with moderate osteogenesis were ??107, ?120, and ??71, and those of two patients with poor osteogenesis were ??165 and ??127 during secondary definitive surgery.

Conclusions

CBCT is considered beneficial for evaluating bone quality of large odontogenic cystic lesions after decompression, while providing potentially useful information for referral to secondary definitive surgery.

     

Access in clay-modified electrodes: effects of pH and film thickness on the electrochemical activity of gallery exchange ions and surface ion pairs

The effects of pH and clay film loading (thickness) on the electrochemistry of Fe(bpy)²⁺₃, A^n? ion pairs (A^n? = CH₃CO^?₂, CCl₃CO^?₂, SO^2?₄) and Ru(NH₃)³⁺₆ exchange cations in montmorillonite- and laponite-modified graphite electrodes are investigated. Included in the study is the effect of pH on the activity of Fe(CN)^3?₆ anions in a clay film. We find that the number of surface-bound Fe(bpy)²⁺₃, A^n? ion pairs capable of accessing the electrode surface decreases with increasing acidity, with the more basic CH₃CO^?₂ counter-anions exhibiting the greater pH dependence. In contrast, the electroactivity of Ru(NH₃)³⁺₆ exchange ions intercalated in the gallery regions of the clay film is pH independent. Also, the fraction of electroactive Fe(bpy)²⁺₃, A^n? ion pairs is essentially independent of clay film loading, indicating that the electroactive sites are confined to a narrow region of the film close to the electrode surface. However, the electroactivity of the more mobile Ru(NH₃)³⁺₆ exchange ions increases with film loading. Fe(CN)^3?₆ anions are easily displaced from a clay film by washing, but at pH 3.0–4.0 the complex anion decomposes to give a cyclic voltamogram indicative of Prussian blue (Fe₃[Fe(CN)₆]₂) deposited on the clay surface. These differences in electrochemical behavior for Fe(bpy)²⁺₃, A^n? ion pairs, Ru(NH₃)³⁺₆ exchange cations and Fe(CN)^3?₆ anions are inconsistent with the clay-layer protonation mechanism proposed previously to explain the decrease in electrochemical activity with decreasing pH for Ru(bpy)²⁺₃, SO^2?₄ and K⁺,Fe(CN)^3?₆ salts at clay-modified electrodes. Instead, the pH dependence for M(bpy)²⁺₃ species is most likely due to changes in the degree of ion-pair formation caused by the protonation of the A^n? counter-ions. These results, together with those of earlier studies, provide insights regarding the electrochemical accessibility of redox active species in clay-modified electrodes.     

Comparative analysis of alkali and alkaline-earth cation transfer assisted by monensin across the water ∣ 1,2-dichloroethane interface

In the first part of this paper the electrochemical transfer of alkali cations (M⁺) assisted by monensin (HX) across the water ∣ 1,2-dichloroethane (DCE) interface at pH<5, combined with a chemical exchange reaction at 5<pH<9, is proposed as the only mechanism responsible for the transfer of these cations. At pH>9 the current is voltammetrically negligible. An equation for the dependence of Δ_o^wφ_1/2 on pH and Na⁺ concentration is developed. In the second part of the paper the transfer of alkaline earth cations assisted by the same ionophore is studied. The electrochemical reactions (Me_(w)²⁺+HX_(o)?MeHX²⁺_(o)) and (Me_(w)²⁺+X^?_(o)?MeX⁺_(o)) are responsible for the peaks observed at pH<5.0 and at pH>9.0, respectively. As expected, in both cases ΔE_p=0.030 V while at intermediate pH the electrochemical exchange reaction (Me²⁺_(w)+HX_(o)?MeX⁺_(o)+H⁺_(w)) is proposed. The net charge transfer of +1 at the interface accounts for ΔE_p=0.060 V for the peak observed ΔE_p in agreement with the hyper-Nernstian slope of 60 mV found for Ca²⁺ and Ba²⁺ ISE based on antibiotics with carboxylic groups at intermediate pH values. The higher selectivity for Ba²⁺ and the tendency in selectivity at alkaline pH found in the ISEs are also observed and thus explained according to the mechanism proposed.     

Solid-state electrochemical investigation of poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene]

The solid-state electrochemical behavior of the luminescent conjugated polymer, poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and its blend with polyethylene oxide (PEO) was investigated by cyclic voltammetry and potential step experiments. It was found that the nature of the ions affects the ionic conductivity and the electrochemical redox behavior of the polymer(particularly on electrochemical n-doping). An electrochemical loop, which is similar to that observed in the electrochemical crystallization, was observed in the cyclic voltammetry of MEH-PPV/PEO(Li⁺) blend. The diffusion of the electrolyte ions is slow, but it was improved by the addition of PEO. The diffusion coefficient of the counterion in the blend is up to 10^?8 cm² s^?1. These solid-state electrochemical results help to shed light on the mechanism of light-emitting electrochemical cells.     

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.     

Characterization of lapachol in artificial organic-film membrane: Application for the trans-membrane transport of Mg

Species transfer across a liquid|liquid interface is studied by means of a thin film-modified electrode using cyclic voltammetry and square-wave voltammetry. The thin film-modified electrode consists of an edge plane pyrolytic graphite electrode (EPG) covered with a thin film of a water-immiscible electro-inactive organic solvent (nitrobenzene) containing a neutral redox probe and/or a suitable electrolyte. For this study we used, as redox probe in the organic phase, 2-hydroxy-3-isopropenyl-1,4-naphthoquinone also known as lapachol (Q) and an appropriate electrolyte. The redox transformations of Q at the graphite electrode/organic (EG|NB) interface was coupled to an ion-transfer reaction from aqueous to organic phase. The proton transfer at the nitrobenzene/water (NB|W) interface is essential for the electrochemical conversion of Q within the membrane. The voltammograms obtained are influenced by the pH of the aqueous phase. Q has two reduction systems due to the redox transformation of its two tautomeric forms resulting from the migration of a proton between the hydroxyl group in position 2 and the carbonyl group in position 4. The electrochemical mechanism consist of 2e⁻/2H⁺ exchange to form the separate redox compounds H₂Q. The experiments conducted reveal the ability of both tautomers to form 1:1 complexes with Mg²⁺ when this cation is present in the aqueous phase.     

Electrocatalytic oxidation of dopamine at aluminum electrode modified with nickel pentacyanonitrosylferrate films,synthesized by electroless procedure

The electrocatalytic oxidation of dopamine (DA) at a home-made aluminum electrode modified with nickel pentacyanonitrosylferrate (NiPCNF) film, has been studied by electrochemical approaches. The immobilization of NiPCNF film was performed by a simple dip-coating procedure. The cyclic voltammogram of the resulting modified Al electrode prepared under optimum conditions, shows a well-behaved redox couple due to the [Ni^IIFe^III/II(CN)₅NO]^0/?1 system. The NiPCNF films, formed on the Al electrode show excellent electrocatalytic activity toward the oxidation of DA. The effect of the solution pH on the voltammetric response of DA was examined using phosphate buffer solution of different pHs. Under optimum conditions a linear calibration graph was obtained over the DA concentration range 2–33 mM. The kinetics of the catalytic reaction were investigated by cyclic voltammetry and rotating disk electrode voltammetry. The results were explained using the theory of electrocatalytic reactions at chemically modified electrodes. The rate constant for the catalytic reaction k, the diffusion coefficient of DA in the solution D, the electron diffusion coefficient in the film D_e and transfer coefficient α, were found to be 3.1×10² M^?1 s^?1, 3.4×10^?6 cm² s^?1, 2.2×10^?11 cm² s^?1 and 0.67, respectively. The interference of ascorbic acid was investigated and greatly reduced using a thin film of Nafion^® on the surface-modified electrode. Further examination of the modified electrode shows that the modifying layers (NiPCNF) on the aluminum substrate show reproducible behavior and a high level of stability during electrochemical experiments, making it interesting for analytical applications.     

Apparent diffusion coefficient-based differentiation of cystic lesions of the mandible

Objective

To evaluate the apparent diffusion coefficient (ADC) for differentiation of cystic lesions of the mandible.

Methods

We retrospectively evaluated 27 cystic lesions of the mandible. We determined the overall ADCs and the pixel-based ADC profiles (ADC mapping) of the fluid areas using two b values (500 and 1000?s/mm²).

Results

The overall ADCs of the fluid areas of ameloblastomas (2.45?±?0.27?×?10^?3?mm²/s) and simple bone cysts (2.52?±?0.14?×?10^?3?mm²/s) were significantly larger than those of dentigerous cysts (1.50?±?0.49?×?10^?3?mm²/s; p?=?0.047 and p?=?0.041, respectively), radicular cysts (0.90?±?0.21?×?10^?3?mm²/s; p?=?0.048 and p?=?0.042, respectively), and keratocystic odontogenic tumors (0.87?±?0.13?×?10^?3?mm²/s; p?=?0.032 and p?=?0.021, respectively). However, the overall ADCs were similar between ameloblastomas and simple bone cysts and among dentigerous cysts, radicular cysts, and keratocystic odontogenic tumors. ADC mapping demonstrated that ameloblastomas and simple bone cysts had 80?C99?% fluid areas with high ADC levels, whereas radicular cysts and keratocystic odontogenic tumors had 66?C100?% fluid areas with low or extremely low ADC levels. However, the ADC mapping profiles of dentigerous cysts differed substantially among the cases. The ADC-based criteria did not effectively differentiate between ameloblastomas and simple bone cysts or between radicular cysts and keratocystic odontogenic tumors.

Conclusions

The ADC-based differentiation was effective for discriminating some types of cystic lesion of the mandible.     

Kinetic and mechanistic study of hydroxyl ion electrosorption at the Pt(111) surface in alkaline media

The adsorption of OH^? ions on the Pt(111) plane has been studied by fast cyclic voltammetry in sodium hydroxide solutions (0.03 to 1 M), under quasi-equilibrium and Tafel approximation conditions. It was shown that the OH^? ion adsorption is an electrosorption process with one electron exchanged between an OH^? ion and the platinum surface. The electrosorption process follows the Frumkin adsorption isotherm with low intensity repulsive interactions of the adsorbed species (f=2–3). The estimated values of the standard electrochemical rate constant (k°=5.6×10^?4 cm s^?1) and the standard exchange current density (j^o_o=5.45×10^?2 A cm^?2) indicate a rather fast electrochemical process.     

Clinical and biological indicators of dental caries and periodontal disease in adolescents with or without obesity

Objective

This study aims to assess clinical, microbiological and inflammatory parameters as indicators for caries and periodontal disease in adolescents with obesity.

Material and methods

Twenty-seven adolescents with obesity [body mass index (BMI) 37?±?4 kg/m²] and 28 controls (BMI 20?±?2 kg/m²) answered questionnaires and were investigated regarding salivary parameters, plaque pH drop after a 1-min glucose rinse, oral clinical parameters, inflammatory markers in gingival crevicular fluid (GCF) and sub-gingival mirobiota.

Results

Compared with controls, adolescents with obesity had a lower stimulated salivary secretion rate (1.55?±?0.63 vs. 2.05?±?1.05 mL/min, p?<?0.05), higher concentrations of secretory immunoglobulin A (sIgA) (p?<?0.001), more decayed tooth surfaces (3.4?±?6.6 vs. 0.8?±?1.1, p?<?0.05) and more gingivitis (p?<?0.01) after controlling for possible confounders. Overall, similar snacking habits, plaque amounts and numbers of deep periodontal pockets were observed. Following the glucose rinse, a slightly more pronounced drop in plaque pH was observed in the obesity group (p?>?0.05). No differences in sub-gingival inflammatory or microbial indicators were detected (p?>?0.01).

Conclusions

More caries and gingival inflammation were observed in adolescents with obesity. Of the indicators tested, salivary secretion rate was lower and sIgA levels were higher in the obesity group. We are unable to confirm whether differences in caries and gingival inflammation are due to systemic changes that are associated with obesity or due to possible irregular dietary/oral hygiene habits.

Clinical relevance

Customised oral health preventive programmes and appropriate collaboration with medical personnel in selecting the best diet, medication and psychological support can help improve the general well-being, including oral health, of children with obesity. This may even reduce the risk of oral diseases.     

Effects of dentifrices differing in fluoride compounds on artificial enamel caries lesions in vitro

The aim of this study was to compare the caries-preventive effect of a stabilized stannous fluoride/sodium fluoride dentifrice containing sodium hexametaphosphate with those of a regular, solely sodium fluoride-containing and amine fluoride-containing dentifrice on pre-demineralized bovine enamel specimens using a pH-cycling model. Bovine enamel specimens with two artificial lesions each were prepared. Baseline mineral loss of both lesions was analyzed using transversal microradiography (TMR). Eighty-five specimens with a mean (SD) baseline mineral loss of 3393 (683) vol% × µm were selected and randomly allocated to five groups (n = 13/15). Treatments during pH-cycling (28 days and 2 × 20 min demineralization/day) were: brushing twice daily with slurries of AmF (1400 ppm F^?), NaF (1450 ppm F^?), SnF₂/NaF (1100 ppm F^?/350 ppm F^?), and fluoride-free (FF) dentifrices or they were immersed in distilled water and remained unbrushed (NB). Subsequently, from each specimen one lesion was covered with acid-resistant varnish, while the remaining lesion was demineralized for another 14 days. Differences in integrated mineral loss (??Z) were calculated between values before and after pH-cycling (??Z _E1) as well as before pH-cycling and after second demineralization (??Z _E2) using TMR. Treatments AmF and NaF induced a significantly higher mineral gain (??Z _E1/??Z _E2) compared to treatments FF and NB (p < 0.05; ANOVA test). Except for treatments AmF and NaF no significant differences in mineral loss between before and after pH-cycling could be observed (p < 0.05; t test) [??Z _E1: AmF:1563 (767); NaF:1222 (1246); SnF₂/NaF:258 (1259); FF:?52 (1223); NB:?151 (834)]. Both dentifrices with either AmF or NaF promoted remineralization, whereas SnF₂/NaF dentifrice did not promote remineralization in a biofilm-free pH-cycling model.     

Effect of a chitosan additive to a Sn2+-containing toothpaste on its anti-erosive/anti-abrasive efficacy—a controlled randomised in situ trial

Objectives

It is well known that Sn²⁺ is a notable anti-erosive agent. There are indications that biopolymers such as chitosan can enhance the effect of Sn²⁺, at least in vitro. However, little information exists about their anti-erosive/anti-abrasive in situ effects. In the present in situ study, the efficacy of Sn²⁺-containing toothpastes in the presence or absence of chitosan was tested.

Methods

Ten subjects participated in the randomised crossover study, wearing mandibular appliances with human enamel specimens. Specimens were extraorally demineralised (7 days, 0.5 % citric acid, pH?2.6; 6?×?2 min/day) and intraorally exposed to toothpaste suspensions (2?×?2 min/day). Within the suspension immersion time, one half of the specimens were additionally brushed intraorally with a powered toothbrush (5 s, 2.5 N). Tested preparations were a placebo toothpaste (negative control), two experimental toothpastes (F/Sn = 1,400 ppm?F^?, 3,500 ppm Sn²⁺; F/Sn/chitosan = 1,400 ppm?F^?, 3,500 ppm Sn²⁺, 0.5 % chitosan) and an SnF₂-containing gel (positive control, GelKam = 3,000 ppm Sn²⁺, 1,000 ppm?F^?). Substance loss was quantified profilometrically (μm).

Results

In the placebo group, tissue loss was 11.2?±?4.6 (immersion in suspension) and 17.7?±?4.7 (immersion in suspension?+?brushing). Immersion in each Sn²⁺-containing suspension significantly reduced tissue loss (p?≤?0.01); after immersion in suspension?+?brushing, only the treatments with GelKam (5.4?±?5.5) and with F/Sn/chitosan (9.6?±?5.6) significantly reduced loss [both p?≤?0.05 compared to placebo; F/Sn 12.8?±?6.4 (not significant)]

Conclusion

Chitosan enhanced the efficacy of the Sn²⁺-containing toothpaste as an anti-erosive/anti-abrasive agent.

Clinical relevance

The use of Sn²⁺- and chitosan-containing toothpaste is a good option for symptomatic therapy in patients with regular acid impacts.     

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^?.     

Effectiveness and mode of action of whitening dentifrices on enamel extrinsic stains

Objectives

This study was conducted in order to investigate the mode of action and the whitening effect of whitening dentifrices.

Material and methods

Two hundred fifty-six bovine enamel specimens (10?×?10 mm²) were prepared, partially stained, and assigned into eight groups (n?=?32): six whitening dentifrices, one nonwhitening and deionized water (negative control), and further divided in two subgroups (n?=?16), according to the test model: chemical (dentifrice slurry treatment only) or chemo-mechanical (slurry?+?toothbrushing). Specimens were treated with dentifrice slurries 2×/day for 1 min and toothbrushed or not, according to each model. In between dentifrice treatments, specimens were artificially stained for 5 h. This protocol was repeated for 5 days and enamel color changes (?E) were measured after each day (days 1–5). The abrasive level of the dentifrices was determined following the ISO11609 guidelines.

Results

In the chemo-mechanical model, the whitening action of all dentifrices was observed after day 1, being higher than the negative control group (p?<?0.05). In days 2–5, nonsignificant changes in color were observed for all groups (p?>?0.05). Differences on ?E among dentifrices were observed, and they seemed to correlate well with their abrasive level (r ²?=?0.80). In the chemical model, no significant differences were observed among groups (p?>?0.05), with ?E remaining constant throughout the study. Higher ?E values were observed in the chemo-mechanical model compared to the chemical (p?<?0.05).

Conclusions

All tested dentifrices were effective in whitening stained enamel and their mode of action showed to be mainly mechanical (toothbrushing abrasion).

Clinical relevance

The abrasive level of dentifrices seems to determine its whitening effectiveness.