Colloidal stabilization of nanoparticles in concentrated suspensions

The stabilization of nanoparticles in concentrated aqueous suspensions is required in many manufacturing technologies and industrial products. Nanoparticles are commonly stabilized through the adsorption of a dispersant layer around the particle surface. The formation of a dispersant layer (adlayer) of appropriate thickness is crucial for the stabilization of suspensions containing high nanoparticle concentrations. Thick adlayers result in an excessive excluded volume around the particles, whereas thin adlayers lead to particle agglomeration. Both effects reduce the maximum concentration of nanoparticles in the suspension. However, conventional dispersants do not allow for a systematic control of the adlayer thickness on the particle surface. In this study, we synthesized dispersants with a molecular architecture that enables better control over the particle adlayer thickness. By tailoring the chemistry and length of these novel dispersants, we were able to prepare fluid suspensions (viscosity < 1 Pa.s at 100 s-1) with more than 40 vol % of 65-nm alumina particles in water, as opposed to the 30 vol % achieved with a state-of-the-art dispersing agent. This remarkably high concentration facilitates the fabrication of a wide range of products and intermediates in materials technology, cosmetics, pharmacy, and in all other areas where concentrated nanoparticle suspensions are required. On the basis of the proposed molecular architecture, one can also envisage other similar molecules that could be successfully applied for the functionalization of surfaces for biosensing, chromatography, medical imaging, drug delivery, and aqueous lubrication, among others.     

Reactivity of conjugated and unconjugated pterins with singlet oxygen (O2(1Deltag)): physical quenching and chemical reaction

Pterins (PTs) belong to a class of heterocyclic compounds present in a wide range of living systems. They participate in relevant biological functions and are involved in different photobiological processes. We have investigated the reactivity of conjugated PTs (folic acid [FA], 10-methylfolic acid [MFA], pteroic acid [PA]) and unconjugated PTs (PT, 6-hydroxymethylpterin [HPT], 6-methylpterin [MPT], 6,7-dimethylpterin [DPT], rhamnopterin [RPT]) with singlet oxygen (1O2) in aqueous solutions, and compared the efficiencies of chemical reaction and physical quenching. The chemical reactions between 1O2, produced by photosensitization, and PT derivatives were followed by UV-visible spectrophotometry and high-performance liquid chromatography, and corresponding rate constants (k(r)) were evaluated. Whenever possible, products were identified and quantified. Rate constants of 1O2 total quenching by the PT derivatives investigated were obtained from steady-state 1O2 luminescence measurements. Results show that the behavior of conjugated PTs differs considerably from that of unconjugated derivatives, and the mechanisms of 1O2 physical quenching by these compounds and of their chemical reaction with 1O2 are discussed in relation to their structural features.     

As, Bi, Sb and Sn determination in atmospheric particulate matter by direct solid sampling-hydride generation-electrothermal atomic absorption spectrometry

A novel, rapid and simple method by hydride generation-electrothermal atomic absorption spectrometry (HG-ETAAS) after direct As, Bi, Sb and Sn hydrides generation from untreated filters of atmospheric particulate matter (PM₁₀ and PM_2.5) was optimised. PM₁₀ and PM_2.5 were not subjected to any pre-treatment: circular portions between 0.28 and 6.28 cm² were directly placed into the reaction vessel of a batch mode generation system. A 2⁸ × 3/64 Plackett–Burman design was used as a multivariate strategy for the evaluation of the effects of several variables affecting the hydride generation, trapping and atomisation efficiencies. Trapping temperature was the most statistically significant variable for As, Bi and Sn. Atomisation temperature was also statistically significant for Sb determination. Optimum values of significant variables were selected by using univariate optimisation approaches. An aqueous calibration method was used throughout. The developed method has been found to be precise with relative standard deviations of 6.2, 5.3, 9.1 and 7.5% for 11 determinations in a filter sample containing 0.7, 1.0, 1.4 and 1.7 μg l⁻¹ for As, Bi, Sb and Sn, respectively. Results obtained by direct solid sampling-HG-ETAAS have been found statistically comparable with those obtained after conventional method based on an acid digestion followed to ICP-MS. Absolute detection limits were 37, 15, 30, and 41 ng l⁻¹ for As, Bi, Sb and Sn, respectively. Detection limits referred to the air volume sampled (in the range of 0.020–0.050 ng m⁻³) were low enough for the determination of several hydride-forming elements from PM₁₀ and PM_2.5 samples collected in a non-polluted suburban area of A Coruña (NW Spain).     

Simultaneous determination of gasoline oxygenates and benzene, toluene, ethylbenzene and xylene in water samples using headspace-programmed temperature vaporization-fast gas chromatography-mass spectrometry

A sensitive method is presented for the fast analysis of seven fuel oxygenates (methanol, ethanol, tert-butyl alcohol (TBA), methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), tert-amyl methyl ether (TAME) and diisopropyl ether (DIPE)) and benzene, toluene, ethylbenzene and p-xylene (BTEX) in water samples. The applicability of a headspace (HS) autosampler in combination with a GC device equipped with a programmable temperature vaporizer (PTV) and a MS detector is explored. The proposed method achieves a clear improvement in sensitivity with respect to conventional headspace methods due to the use of the PTV. Two different packed liners with materials of different trapping strengths (glass wool and Tenax-TA) were compared. The benefits of using Tenax-TA instead of glass wool as packed material for the measurement of the 11 compounds emerged as better signal-to-noise ratios and hence better detection limits. The proposed method is extremely sensitive. The limits of detection are of the order of ng/L for six of the compounds studied and of the order of microg/L for the rest, with the exception of the most polar and volatile compound: methanol. Precision (measured as the relative standard deviation for a level with an S/N ratio close to 3) was equal to or lower than 15% in all cases. The method was applied to the determination of the analytes in natural matrixes (tap, river and sea water) and the results obtained can be considered highly satisfactory. The methodology has much lower detection limits than the concentration limits proposed in drinking water by the US Environmental Protection Agency (EPA) and the European Union for compounds under regulation.     

Photosensitization of 2'-deoxyadenosine-5'-monophosphate by pterin

UV-A radiation (320-400 nm) induces damages to the DNA molecule and its components through photosensitized reactions. Pterins, heterocyclic compounds widespread in biological systems, participate in relevant biological processes and are able to act as photosensitizers. We have investigated the photosensitization of 2'-deoxyadenosine-5'-monophosphate (dAMP) by pterin (PT) in aqueous solution under UV-A radiation. The effect of pH was evaluated, the participation of oxygen was investigated and the products analyzed. Kinetic studies revealed that the reactivity of dAMP towards singlet oxygen (1O2) is very low and that this reactive oxygen species does not participate in the mechanism of photosensitization, although it is produced by PT upon UV-A excitation. In contrast, analysis of irradiated solutions by means of electrospray ionization mass spectrometry strongly suggested that 8-oxo-7,8-dihydro-2'-deoxyadenosine-5'-monophosphate (8-oxo-dAMP) was produced, indicating that the photosensitized oxidation takes place via a type I mechanism (electron transfer).     

Enhanced photocatalytic activity of zeolite-encapsulated TiO2 clusters by complexation with organic additives and N-doping.

In the literature it was found that titanium oxide clusters of a few metal atoms encapsulated inside the micropores of zeolite Y exhibit large blue shifts in the Ti-O ligand-to-metal charge-transfer band as compared to non-encapsulated bulk titanium dioxide particles. This blue shift of the Ti-O absorption band is believed to have a negative effect on the photocatalytic activity of zeolite-encapsulated TiO2. We report here on circumventing this problem and increasing visible-light absorption by means of a red shift of the absorption band caused by addition of some organic molecular modifiers containing acidic OH groups that can strongly bind with titanol groups TiOH. In the studied series of zeolite-encapsulated TiO2 samples, the red shift of the optical spectrum follows the order: catechol > 4-aminobenzoic acid > benzoic acid. Also N-doping of zeolite-encapsulated TiO2 clusters by thermal treatment with urea leads to a red shift of the TiO2 absorption band that depends on the annealing and hydration conditions. By comparison to the degradation of phenol in aqueous solution, we have demonstrated that these changes in the absorption spectrum on addition of the organic modifier are also reflected in the photocatalytic activity of the samples; a greater increase in photocatalytic activity (about 30%) was observed for the additive catechol.     

The mechanism of proton transfer between adjacent sites exposed to water

The surface of a protein, or a membrane, is spotted with a multitude of proton-binding sites, some of which are only a few angstroms apart. When a proton is released from one site, it propagates through the water by a random walk under the bias of the local electrostatic potential determined by the distribution of the charges on the protein. Some of the released protons disperse into the bulk, but during the first few nanoseconds, the released protons can be trapped by encounter with nearby acceptor sites. This process resembles a scenario which corresponds with the time-dependent Debye-Smoluchowski equation. In the present study, we investigated the mechanism of proton transfer between sites that are only a few angstroms apart, using as a model the proton exchange between sites on a small molecule, fluorescein, having two, spectrally distinguishable, proton-binding sites. The first site is the oxyanion on the chromophore ring structure. The second site is the carboxylate moiety on the benzene ring of the molecule. Through our experiments, we were able to reconstruct the state of protonation at each site and the velocity of proton transfer between them. The fluorescein was protonated by a few nanosecond long proton pulse under specific conditions that ensured that the dye molecules would be protonated only by a single proton. The dynamics of the protonation of the chromophore were measured under varying initial conditions (temperature, ionic strength, and different solvents (H(2)O or D(2)O)), and the velocity of the proton transfer between the two sites was extracted from the overall global analysis of the signals. The dynamics of the proton transfer between the two proton-binding sites of the fluorescein indicated that the efficiency of the site-to-site proton transfer is very sensitive to the presence of the screening electrolyte and has a very high kinetic isotope effect (KIE = 55). These two parameters clearly distinguish the mechanism from proton diffusion in bulk water. The activation energy of the reaction (E(a) = 11 kcal mol(-1)) is also significantly higher than the activation energy for proton dissociation in bulk water (E(a) approximately 2.5 kcal mol(-1)). These observations are discussed with respect to the effect of the solute on the water molecules located within the solvation layer.     

Substituent effects on the photophysical properties of pterin derivatives in acidic and alkaline aqueous solutions

Pterins are heterocyclic compounds with important biological functions, and most of them may exist in two acid-base forms in the pH range between 3 and 13 in aqueous solution. In this work, the photophysical properties of acid and basic forms of six compounds of the pterin family (6-hydroxymethylpterin [HPT], 6-methylpterin [MPT], 6,7-dimethylpterin [DPT], rhamnopterin [RPT], N-methylfolic acid [MFA], and pteroic acid [PA]) have been studied. The effects of the chemical nature of the substituents at position 6 of the pterin moiety and the effects of the pH on the absorption and emission properties are analyzed. The fluorescence characteristics (spectra, quantum yields, lifetimes) of these compounds have been investigated using the single-photon-counting technique. Results obtained for pterin derivatives containing small substituents with 1 carbon atom (HPT, MPT, DPT) and short hydrocarbon chain (4 carbon atoms) (RPT) are different from those found for pterin derivatives containing a p-aminobenzoic acid (PABA) moiety in the substituent (MFA and PA). Fluorescence quantum yields (Phi(F)) of the first group of compounds are relatively high (>/=0.4), whereas MFA and PA exhibit very small Phi(F) values (相似文献