Yan Waguespack

Fluorescence quenching of CdS nanocrystallites in AOT water-in-oil microemulsions

Abstract The luminescence of CdS nanoparticles suspended in “dry” AOT reversed micelles is quite intense but is quenched by water and even more effectively by thiol-containing compounds. Stern-Volmer plots provide information about the quenching efficiencies of various compounds. Of particular interest is 4-hydroxythiophenol which interacts strongly with the surfactant headgroups and thereby partitions to the nanoparticle surface. The quenching efficiency of 4-hydroxythiophenol is more than an order of magnitude greater than that of thiophenol. At the very low concentrations of 4-hydroxythiophenol required to bring about quenching, a remarkable recovery of fluorescence is observed upon continued irradiation. The fluorescence recovery is attributed to photochemical oxidation which consumes the hydroxythiophenol presumably giving a disulfide product.

Heavy metal biomonitoring by seaweeds on the Delmarva Peninsula, east coast of the USA

Abstract Metal pollution in aquatic ecosystems can pose a major environmental concern impacting various life-forms. The usefulness of seaweeds as biomonitors or indicators of metals in the coastal bays of the Delmarva Peninsula is highlighted in this study, conducted in early spring 2006. Lack of any preexisting data on metal loads in Delmarva seaweeds and in the ambient medium (water and sediments) led to this pilot study for preliminary investigations on the ecological health of these coastal bays. The seaweed species collected from three different sites of the Delmarva Peninsula were: phaeophytes Fucus vesiculosus, and Fucus distichus, chlorophytes Ulva lactuca, and Enteromorpha intestinalis, and rhodophyte Gracilaria tikvahiae. The sampled seaweeds, seawater, and sediments were subsequently analyzed for chromium, manganese, cobalt, nickel, copper, lead, arsenic, and zinc. The results showed high concentrations of Mn, and low concentrations of Cu in almost all the species. Bioconcentration factors (BCFs) in Ulva lactuca and Fucus vesiculosus, across sites, were similar (though the orders of magnitude varied) for Cu, Zn, and Ni. Calculation of metal pollution index (MPI) and Tomlinson pollution load index (PLI) showed that all three sites had low contamination with the metals investigated.

A spontaneous phase transition from reverse micelles to organogels due to surfactant interactions with specific benzenediols

Abstract Hydrogen-bonding interactions between suitable phenols and the head group of the twin-tailed anionic surfactant sodium bis(2-ethylhexyl)sulfosuccinate (AOT), form the basis for a novel class of organogels. The gels are novel in that very small quantities of these low molecular weight solutes are sufficient to cause gelation. Previous work has shown that the gel-forming propensity of the phenol is determined by its acid strength and also the substitution pattern on the aromatic ring. Evidence suggests that the underlying molecular architecture of these gels consists of strands of stacked and motionally restricted phenol molecules. The surfactant appears to be linked through hydrogen bonding to the external surface of the stack and its motion is relatively unrestricted. Here, we report the finding that the gels also form with select benzenediols (resorcinol derivatives). Interestingly, these gels are much stronger than some of the strongest gels made with phenols under similar conditions. A combination of NMR and FTIR spectroscopic evidence suggests that these gels derive their strength from binding with both carbonyl groups of the surfactant, whereas in the gels made with phenols, only one carbonyl group is bound. The stacked phenol microstructure of the organogels proposed previously seems to be generally valid for the case of gels formed with the benzenediols. The additional possibility of bridging the adjacent phenol molecules in the stack by the adsorbed surfactant allows for a “scaffolding” around the aromatic stack.

Exciton migration in a «pseudo»-one-dimensional crystal: luminescence dynamics of doped CsMnBr3

The exciton migration in CsMnBr3 (CMB), a linear chain salt which acts as a ‘‘pseudo’’‐one‐dimensional antiferromagnet, has been probed by examining the time‐resolved luminescence of doped crystals. A number of metal ions such as Cr2+, Cr3+, Fe2+, Cu2+, Mo3+, Er3+ were observed to behave as exciton traps, while Cd2+ and Mg2+ act as exciton scatterers (antitraps). Samples doped with traps and traps along with scatterers, as well as ‘‘dilute’’ CsMnx[Mg2+ or Cd2+ ]1−xBr3 crystals were investigated. Analysis of the data indicates that exciton migration between chains is significant with a room temperature hopping frequency of 107–108 s−1 and a thermal barrier of 500 cm−1. The barrier for intrachain migration appears to be somewhat smaller, between 350 and 400 cm−1. Only at temperatures below 60 K does the exciton migration seem to be strictly confined to one dimension.

Phytoplankton Pigments in Maryland Coastal Bay Sediments as Biomarkers of Sources of Organic Matter to Benthic Community

ABSTRACT Chen, J.; Oseji, O.; Mitra, M.; Waguespack, Y., and Chen, N., 2016. Phytoplankton pigments in Maryland coastal bay sediments as biomarkers of sources of organic matter to benthic community. In this study, the sources of pelagic organic matter and their transport to the surface sediment in the Maryland Coastal Bays (MCB), part of the U.S. mid-Atlantic coastal lagoon system, were examined. Photosynthetic pigments (chlorophylls a and b), accessory pigments (peridinin, fucoxanthin, zeaxanthin, alloxanthin, etc.), and chlorophyll a decomposition products (chlorophyllide a, pheophorbide a, pheophytin a, and pyropheophytin a) from surface sediments collected at 13 sites in the MCB in March 2013 were analyzed using high performance liquid chromatography. The spatial distributions of diagnostic pigments in surface sediments indicated that organic matter was mainly derived from nano- and picophytoplankton (i.e. cyanobacteria, cryptophytes, and chlorophytes) at sites characterized by high nutrient input, although diatoms dominated the standing phytoplankton biomass throughout the MCB except at one site. We attribute this phenomenon to selective microzooplankton grazing on nano- and picophytoplankton in the cold season and low grazing pressure on large phytoplankton species of diatoms and dinoflagellates resulting in the diatom bloom in early spring. At sites characterized by strong tidal current and mixing of bay waters and ocean waters, information from pigment data supports the deposition of dead/senescent diatoms. Results from this study indicate that nano- and picophytoplankton may play a crucial role in supplying organic matter to the benthic community, although their standing stock is low in the cold season.

An assay to screen bacterial adhesion to mucus biomolecules

To develop an assay for rapid screening of bacterial adhesion to various groups of biomolecules present in fish mucus.