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International audience; The soot nucleation process, defined as the transition from molecular precursors to condensed matter, is the less understood step in the whole soot formation process. The possibility that polycyclic aromatic hydrocarbon (PAH) dimers, especially those containing moderate-sized PAHs, can play a major role in soot nucleation is a very controversial issue. Although PAH dimers have often been considered as potential soot precursors, their formation is not thermodynamically favored at a typical flame temperature, their binding energies being considered too weak to allow them to survive in this environment. Hereby, we report experimental evidence supporting the existence of PAH dimers in the proximity of the soot nucleation region of a methane laminar diffusion flame that gives strong evidence for the nucleation process to be kinetically rather than thermodynamically controlled.

Chlorophyll fluorescence measurements have been widely applied as non-destructive methods to study the photosynthetic efficiency of plants, under control or stress conditions. Compared to most protocols of pulse amplitude modulation (PAM) fluorometry, laser induced chlorophyll fluorescence (LIF) has the advantages of not requiring pulses to be delivered at close range, allowing the remote analysis of a great number of individual plants in a short period of time. Such analyses are extremely useful, for instance, when doing large phenotyping screens of Arabidopsis thaliana mutants or ecotypes. Water deficit is a major abiotic stress compromising plant growth and productivity. Arabidopsis has been adopted as the main model organism in plant sciences and it has been widely used in plant stress studies. However, reports on the applications of LIF techniques to this model plant are scarce. Here we report the use of LIF to investigate changes in chlorophyll a (Chl a ) fluorescence signature under progressive drought of potted Arabidopsis plants (Slow Stress) and under fast dehydration of detached leaves (Fast Stress). Results show that the two dehydration methods cause distinct modifications on the red/far-red Chl a fluorescence ratio ( F 690 / F 730 ) and on the wavelengths of Chl a fluorescence maxima. These differences are likely related to distinct changes in water content, photosynthetic pigments, anthocyanins, fatty acid composition and other metabolic adaptations, which are differently regulated in Slow and Fast Stress. Of particular interest are Chl a emission fluorescence changes, which take place under progressive drought, before a substantial decrease in leaf water content. Additionally, no differences were found on LIF emission spectra between fully expanded and young leaves. However, the choice of leaf surface influenced fluorescence emission, with the abaxial surface displaying lower fluorescence and higher F 690 / F 730 ratios. Results suggest that LIF is a fast and non-destructive tool suitable for high-throughput phenotyping of Arabidopsis under water deficit.

This study describes the development of a novel fluorescent tag, O-2-[aminoethyl]fluorescein, for the separation of sugars by capillary electrophoresis with fluorescence detection using an argon ion laser. The tag was synthesised using three consecutive steps namely: esterification, alkylation and hydrolysis, specifically designed to offer a flexible way in which to make an assortment of fluorescent tags from cheap and readily available starting reagents (typically less than $1 per g of fluorescent tag). Via this flexible synthetic pathway, O-2-[aminoethyl]fluorescein was designed and synthesised with a spacer group to lower steric effects between the fluorescein backbone and the reducing end of the carbohydrate which were anticipated to improve the reactivity of the tag. The newly synthesised tag, O-2-[aminoethyl]fluorescein was evaluated against structurally similar commercial fluorescent motifs namely fluorescent 5-aminomethylfluorescein and non-fluorescent 5-aminofluorescein. Kinetic studies indicated that O-2-[aminoethyl]fluorescein showed similar labeling efficiencies as 5-aminomethylfluorescein, but were achieved in only 30 min, supporting the notion of improved reactivity of the spacer group. The sensitivity of O-2-[aminoethyl]fluorescein was evaluated using maltoheptaose with a detection limit of 1 nM obtained, which was slightly higher than that of 0.3 nM obtained with 5-aminomethylfluorescein, and was due to its lower quantum yield (0.24) when conjugated to the sugar. The separation performance of the tag was also benchmarked with the two commercial reagents using a range of corn syrup oligosaccharides, from 4 to 10 glucose units, typically found in rice starch. Separations were performed using an electrolyte containing 100 mM boric acid, tris at pH 8.65 as background electrolyte, 30 kV applied voltage, 50 μm I.D. × 40 cm (30 cm effective length) capillary. The novel tag showed better resolution of small oligosaccharides, G3 and G4, than the other two reagents, but slightly worse resolution for the longer oligosaccharides, most likely due to the monovalent charge state of the O-2-[aminoethyl]fluorescein compared to the divalent charge of the other two tags. Refereed/Peer-reviewed

International audience; The formation of OH radicals and its diffusion into the gas phase during the excitation of TiO2 in the presence of H2O has been studied using the very sensitive and selective detection method of laser induced fluorescence LIF. The time-resolved evolution of the OH radical concentration has been observed at pressures between 4 and 600 Torr and at varying distances between the photocatalytic surface and the detection volume. OH radicals have been detected even at the highest pressures, opening the assumption that gas phase reactions with OH radicals may very well be involved in the photocatalytic degradation of VOCs, even at atmospheric pressure. Interestingly, a second fluorescence signal peak has been observed at longer delays with respect to the excitation pulse. The use of different fluorescence laser energies leads us to interpret this second peak as H2O2, released from the surface into the gas phase and detected by a 2-photon process: in fact, we use fluorescence laser fluencies high enough to photolyse H2O2 molecules at the fluorescence excitation wavelength (282nm) and excite the generated OH radicals within the same laser pulse.

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