In situ imaging of amorphous intermediates during brucite carbonation in supercritical CO2.

Progress in understanding crystallization pathways depends on the ability to unravel relationships between intermediates and final crystalline products at the nanoscale, which is a particular challenge at elevated pressure and temperature. Here we exploit a high-pressure atomic force microscope to directly visualize brucite carbonation in water-bearing supercritical carbon dioxide (scCO2) at 90 bar and 50 °C. On introduction of water-saturated scCO2, in situ visualization revealed initial dissolution followed by nanoparticle nucleation consistent with amorphous magnesium carbonate (AMC) on the surface. This is followed by growth of nesquehonite (MgCO3·3H2O) crystallites. In situ imaging provided direct evidence that the AMC intermediate acts as a seed for crystallization of nesquehonite. In situ infrared and thermogravimetric-mass spectrometry indicate that the stoichiometry of AMC is MgCO3·xH2O (x = 0.5-1.0), while its structure is indicated to be hydromagnesite-like according to density functional theory and X-ray pair distribution function analysis. Our findings thus provide insight for understanding the stability, lifetime and role of amorphous intermediates in natural and synthetic systems.

Molecular depth profiling of sucrose films: a comparative study of C60(n+) ions and traditional Cs(+) and O2(+) ions.

Depth profiling of sucrose thin films was investigated with time-of-flight secondary ion mass spectrometry (TOF-SIMS) using 10 keV C(60)(+), 20 keV C(60)(2+), and 30 keV C(60)(3+), and 250, 500, and 1000 eV Cs(+) and O(2)(+) as sputtering ions. With C(60)(n+) ions, the molecular ion signal initially decreases and reaches a steady state that is about 38-51% of its original intensity, depending on the energy of the C(60)(n+) ions. In contrast, with Cs(+) and O(2)(+) sputtering, molecular ion signals decrease quickly to the noise level, even using very low-energy (250 eV) ions. In addition, the measured width of the sucrose/Si interface is much narrower using C(60)(+) ions than that using Cs(+) or O(2)(+) ions. To understand the mechanisms of sputtering-induced damage by these ions, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used to characterize the bottoms of these sputter craters. XPS data show very little chemical change in the C(60)(+) sputter crater, while considerable amorphous carbon was found in the O(2)(+) and Cs(+) sputter craters, indicating extensive decomposition of the sucrose molecules. AFM images show a very flat bottom in the C(60)(+) sputter crater, while the bottoms of the Cs(+) and O(2)(+) sputter craters are significantly rougher. We used the sputtering model developed by Wucher and co-workers to quantitatively analyze our C(60)(1-3+) data. The results show that low energy C(60)(+) ions generate a relatively thin damage layer with a high molecular ion signal, suggesting that low energy C(60)(+) may be the optimal choice for molecular depth profiling of sucrose films.

Spectroscopic characterization of extracellular polymeric substances from Escherichia coli and Serratia marcescens: suppression using sub-inhibitory concentrations of bismuth thiols.

Free and bound (or capsular) EPS produced by suspended cultures of Escherichia coli and Serratia marcescens were characterized in detail using colorimetric analysis of total proteins and polysaccharides, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) in the presence and absence of bismuth-based antifouling agents. Subtle differences in the chemical composition of free and bound EPS were observed for both bacteria in the absence of bismuth. Total polysaccharides and proteins in free and bound EPS decreased upon treatment with subinhibitory concentrations of lipophilic bismuth thiols (bismuth dimercaptopropanol, BisBAL; bismuth ethanedithiol, BisEDT; and bismuth pyrithione, BisPYR), with BisBAL being most effective. Bismuth thiols also influenced acetylation and carboxylation of polysaccharides in EPS from S. marcescens. Extensive homology between EPS samples in the presence and absence of bismuth was observed with proteins, polysaccharides, and nucleic acids varying predominantly only in the total amount produced. Second derivative analysis of the amide I region of FTIR spectra revealed decreases in protein secondary structures in the presence of bismuth thiols. Hence, antifouling properties of bismuth thiols appear to originate in their ability to suppress O-acetylation and protein secondary structure formation in addition to free and bound EPS secretion.

Role of extracellular polymeric substances in bioflocculation of activated sludge microorganisms under glucose-controlled conditions.

Extracellular polymeric substances (EPS) secreted by suspended cultures of microorganisms from an activated sludge plant in the presence of glucose were characterized in detail using colorimetry, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy. EPS produced by the multi-species community were similar to literature reports of pure cultures in terms of functionalities with respect to C and O but differed subtly in terms of N and P. Hence, it appears that EPS produced by different microorganisms maybe homologous in major chemical constituents but may differ in minor components such as lipids and phosphodiesters. The role of specific EPS constituents on microbial aggregation was also determined. The weak tendency of microorganisms to bioflocculate during the exponential growth phase was attributed to electrostatic repulsion when EPS concentration was low and acidic in nature (higher fraction of uronic acids to total EPS) as well as reduced polymer bridging. However, during the stationary phase, polymeric interactions overwhelmed electrostatic interactions (lower fraction of uronic acids to total EPS) resulting in improved bioflocculation. More specifically, microorganisms appeared to aggregate in the presence of protein secondary structures including aggregated strands, beta-sheets, alpha- and 3-turn helical structures. Bioflocculation was also favored by increasing O-acetylated carbohydrates and overall C-(O,N) and O=C-OH+O=C-OR functionalities.