Multielemental speciation of As, Se, Sb and Te by HPLC-ICP-MS

An anion exchange HPLC-ICP-MS procedure allowing the simultaneous multielemental speciation analysis of arsenic, selenium, antimony and tellurium has been developed. Four arsenic species (As^III, As^V, monomethylarsonic acid and dimethylarsinic acid), two selenium species (Se^IV and Se^VI) may be determined in a single run as well as one antimony (Sb^V) and one tellurium species (Te^VI). Alternatively Sb and/or Te may be used as internal standards for As and Se speciation studies. Optimisation of ICP-MS conditions led to satisfactory relative (0.01 (Sb^V) to 1.8 (Se^VI) ng ml⁻¹) and absolute detection limits (1–180 pg). Reproducibility ranged from 3.1 to 5.6% and the linearity was verified in the 0–200 ng ml⁻¹ range.     

Fluorescent photoionic devices with two receptors and two switching mechanisms: applications to pH sensors and implications for metal ion detection

Two leading designs of fluorescent sensors are combined to yield the novel hybrid system of the ‘Fluorophore-Receptor₁-Spacer-Receptor₂’ format. We use 4-(dialkylaminoalkylamino)-7-nitrobenzo-2-oxa-1,3-diazoles as examples. The emission from internal charge transfer excited states in the present instances are highly responsive to N-H deprotonation as well as being quenched by intramolecular tertiary amine groups via photoinduced electron transfer (PET). When applied to pH sensing, this leads in favourable cases to two steps in the fluorescence-pH profile which can be viewed as a multi-stable photoionic device, even though single steps are more usual. The former situation is favoured when the two proton-associated equilibria are sufficiently separated on the pH scale and when the PET process is of moderate efficiency. These systems have the added feature of excitation/emission wavelengths in the visible region. As a secondary theme, we point out that caution is required when designing sensors for transition metal ions from systems with intrinsically proton-sensitive fluorescence due to receptors either integrated with or spaced from the fluorophore.     

Polymerisation stereoelective en phase homogene du methyl-2 thiiranne amorcee par des semences optiquement actives

Optically active seeds have been prepared by polymerization of the laevorotatory enantiomer of 2-methylthiirane with cadmium thiolate; they have been used for the polymerization of the racemic monomer, which is stereoelective. These results are explained by the formation of two types of chiral sites in unequal numbers, each type of site choosing only one enantiomer.     

Synthesis of Graft Bacterial Polyesters for Nanoparticles Preparation

Graft copolymers of bacterial polyesters were prepared by direct condensation of poly(3‐hydroxyoctanoate‐co‐9‐carboxy‐3‐hydroxydecanoate) (PHOD) and poly(ethylene glycol) (PEG) or poly(lactic acid) (PLA). Nanoparticles from PHO, PHOD, PHOD‐g‐PEG, and PHOD‐g‐PLA were obtained by solvent displacement without stabilizer, and their stability in different aqueous media with different salt concentrations were studied. The results showed that the presence of hydrophilic PEG on the particle surface prevents the aggregation promotion by salts in aqueous solution. PHOD‐g‐PEG appears to be a promising candidate for site‐specific drug delivery systems.

¹H NMR spectrum of PHOD‐g‐PLA in CDCl₃.     

Electrocatalytic sensor devices: (I) cyclopentadienylnickel(II) thiolato Schiff base monolayer self-assembled on gold

The fabrication of a self-assembled monolayer (SAM) of a cyclopentadienylnickel(II) thiolato Schiff base compound, [Ni(SC₆H₄NC(H)C₆H₄OCH₂CH₂SMe)(η⁵-C₅H₅)]₂ on a gold electrode is described. Effective electronic communication between the Ni(II) centres and the gold surface was established by electrochemically cycling the Schiff base-doped Au electrode in 0.1 M NaOH from −200 mV to +600 mV. The SAM-modified electrode exhibited quasi-reversible electrochemistry. The integrity of this electrocatalytic SAM, with respect to its ability to block and electro-catalyse certain Faradaic processes, was interrogated using cyclic voltammetric experiments. The formal potential, E°′, varied with pH to give a slope of about −30 mV pH⁻¹. The surface concentration, G, of the nickel redox centres was found to be 1.548×10⁻¹¹ mol cm⁻². By electrostatically doping the SAM using an applied potential of +700 mV versus Ag/AgCl, in the presence of horseradish peroxidase (HRP), it was fine-tuned for amperometric determination of H₂O₂. The electrocatalytic-type biosensor displayed typical Michaelis-Menten kinetics and the limit of detection was found to be 6.25 mM.     

Speciation of arsenic and selenium compounds by HPLC hyphenated to specific detectors: a review of the main separation techniques

This review deals with liquid phase separation of major arsenic and selenium species followed by element specific detection. It concerns papers published since 1980 and describing only currently used methods that were or could be applied to As and Se speciation in environmental matrices. Methods performances are compared on the basis of efficiency, rapidity, absolute and concentration detection limits and applicability to real world environmental samples.     

Highly luminescent lead sulfide nanocrystals in organic solvents and water through ligand exchange with poly(acrylic acid)

Hydrophobic lead sulfide quantum dots (PbS/OA) synthesized in the presence of oleic acid were transferred from nonpolar organic solvents to polar solvents such as alcohols and water by a simple ligand exchange with poly(acrylic acid) (PAA). Ligand exchange took place rapidly at room temperature When a colloidal solution of PbS/OA in tetrahydrofuran (THF) was treated with excess PAA, the PbS/PAA nanocrystals that formed were insoluble in hexane and toluene but could be dissolved in methanol or water, where they formed colloidal solutions that were stable for months. Ligand exchange was accompanied by a small blue shift in the band-edge absorption, consistent with a small reduction in particle size. While there was a decrease in quantum yield associated with ligand exchange and transfer to polar solvents, as is commonly found for colloidal quantum dots, the quantum yields determined were impressively high: PbS/OA in toluene (82%) and in THF (58%); PbS/PAA in THF (42%) and in water (24%). The quantum yields for the PbS/PAA solutions decreased over time as the solutions were allowed to age in the presence of air.     

Optically active polymers of 3-alkylmalic acids: Contribution of the bioconversion for diversifying the chiral precursors

The need for new optically active monomers and polymers is conducive to the setting up of stereospecific synthesis routes starting from chiral precursors. The biomass can be considered as a major source for extracting such biomolecules aimed at chemoenzymatic transformation and further polymerization. Due to its versatility, ß-methylaspartate ammonia-lyase, from cell-free extracts of Clostridium tetanomorphum, has been used in the bioconversion of alkylfumarates into optically active pure 3-alkylaspartic acids with alkyl=methyl, ethyl, isopropyl. These amino acids have been transformed in several steps into optically active benzyl 3-alkylmalolactonates leading to semi-crystalline polyesters. 3-Methylaspartic acid includes two chiral centers and the racemic compound containing the four stereoisomers can be prepared by a multiple step synthesis. The ability of ß-methylaspartase to catalyse both syn- and anti-elimination of ammonia from natural 3-methylaspartic acid has been expressed to retain one stereoisomer and this bioconversion is a preparative method for obtaining unnatural stereoisomers. Moreover, the catalytic hydrogenolysis of the benzyl α,ß-substituted ß-lactone yields stable 3-alkylmalolactonic acid which can be coupled with functional alcohols and copolymerized. At last the introduction of (2S)-3,3-dimethyl-2-butanol, using Rhodotorula glutinis as microorganism in a biological synthesis step, as chiral ester pendant group, has conducted to optically active polyesters with very high melting transition temperatures. The combination of bioconversion and chemical synthesis is a very useful tool for building hydrolyzable functionalized polyesters required for temporary applications.     

Characterisation and Classification of Foodborne Bacteria Using Reflectance FTIR Microscopic Imaging

This work investigates the application of reflectance Fourier transform infrared (FTIR) microscopic imaging for rapid, and non-invasive detection and classification between Bacillus subtilis and Escherichia coli cell suspensions dried onto metallic substrates (stainless steel (STS) and aluminium (Al) slides) in the optical density (OD) concentration range of 0.001 to 10. Results showed that reflectance FTIR of samples with OD lower than 0.1 did not present an acceptable spectral signal to enable classification. Two modelling strategies were devised to evaluate model performance, transferability and consistency among concentration levels. Modelling strategy 1 involves training the model with half of the sample set, consisting of all concentrations, and applying it to the remaining half. Using this approach, for the STS substrate, the best model was achieved using support vector machine (SVM) classification, providing an accuracy of 96% and Matthews correlation coefficient (MCC) of 0.93 for the independent test set. For the Al substrate, the best SVM model produced an accuracy and MCC of 91% and 0.82, respectively. Furthermore, the aforementioned best model built from one substrate was transferred to predict the bacterial samples deposited on the other substrate. Results revealed an acceptable predictive ability when transferring the STS model to samples on Al (accuracy = 82%). However, the Al model could not be adapted to bacterial samples deposited on STS (accuracy = 57%). For modelling strategy 2, models were developed using one concentration level and tested on the other concentrations for each substrate. Results proved that models built from samples with moderate (1 OD) concentration can be adapted to other concentrations with good model generalization. Prediction maps revealed the heterogeneous distribution of biomolecules due to the coffee ring effect. This work demonstrated the feasibility of applying FTIR to characterise spectroscopic fingerprints of dry bacterial cells on substrates of relevance for food processing.