Localized surface plasmon resonance spectroscopy of single silver triangular nanoprisms

The plasmonic properties of single silver triangular nanoprisms are investigated using dark-field optical microscopy and spectroscopy. Two distinct localized surface plasmon resonances (LSPR) are observed. These are assigned as in-plane dipolar and quadrupolar plasmon excitations using electrodynamic modeling based on the discrete dipole approximation (DDA). The dipole resonance is found to be very intense, and its peak wavelength is extremely sensitive to the height, edge length, and tip sharpness of the triangular nanoprism. In contrast, the intensity of the quadrupole resonance is much weaker relative to the dipole resonance in the single particle spectra than in the ensemble averaged spectrum. Several parameters relevant to the chemical sensing properties of these nanoprisms have been measured. The dependence of the dipole plasmon resonance on the refractive index of the external medium is found to be as high as 205 nm RIU(-1) and the plasmon line width as narrow as approximately 0.17 eV. These data lead to a sensing figure of merit (FOM), the slope of refractive index sensitivity in eV RIU(-1)/line width (eV), as high as 3.3. In addition, the LSPR shift response to alkanethiol chain length was found to be linear with a slope of 4.4 nm per CH2 unit. This is the highest short-range refractive index sensitivity yet measured for a nanoparticle.     

Gold catalysed selective oxidation of alcohols in supercritical carbon dioxide

The oxidation of benzyl alcohol to benzaldehyde over different supported gold catalysts in supercritical carbon dioxide has been investigated in a high-pressure batch reactor. Only molecular oxygen was used as oxidant and no base was needed. Different supports and preparation methods for the catalysts were tested and parameters like reaction temperature, pressure and molar ratios of the components were varied to study the catalytic behaviour. Gold colloids deposited on a titania support (1%Au/TiO₂) yielded a conversion of 16.0% after 3 h and a high selectivity to benzaldehyde of 99% under single-phase conditions. The reaction rate was significantly higher than in a corresponding “solvent-free” reaction without CO₂. Even higher rates were found when a CO₂-expanded phase was present. Monitoring of the oxidation in a high-pressure view cell via infrared transmission spectroscopy unravelled a slowdown of the reaction rate above 15% conversion. In addition, 1-octanol and geraniol were oxidised as well under similar conditions, yielding conversions of 4% and 10%, respectively, with selectivities towards octanal and geranial of 90% and 30%. Thus, the combined application of gold-based catalysts and supercritical CO₂ offers an interesting alternative to the known methods of alcohol oxidation.     

Ozone as a Sink for Atmospheric Carbon Aerosols Today and Following Nuclear War

In recent years the expected environmental consequences of nuclear war have been expanded to include the global impact of injection of huge quantities of smoke and dust into the atmosphere. Solar heating could cause this smoke cloud to rise to stratospheric heights, increasing local temperatures as much as 100 K. The lofting of carbonaceous soot particles into the stratosphere could cause the total ozone column to be reduced due to: (1) increases in catalytic destruction of ozone; (2) transport of ozone-rich, stratospheric air to regions where it is more quickly photolyzed as the ozone-poor, tropospheric air associated with the smoke cloud pushes into the stratosphere; and, (3) direct reactions of ozone with the carbon aerosols. Ozone reacts with carbon black surfaces to produce an oxygen molecule for every ozone lost. Removal of surface oxides formed in the reaction is the rate-limiting step for further reactions of ozone with the surface. Soot surfaces also react with ozone. This reaction is found to have an activation energy of ～10.6 kcal/mol and a suggested fractional order with respect to ozone.     

A Chemical Ionization High-Resolution Time-of-Flight Mass Spectrometer Coupled to a Micro Orifice Volatilization Impactor (MOVI-HRToF-CIMS) for Analysis of Gas and Particle-Phase Organic Species

We describe a new instrument, chemical ionization (CI) high-resolution time-of-flight mass spectrometer (ToFMS) coupled to a micro-orifice volatilization impactor (MOVI-HRToF-CIMS). The MOVI-HRToF-CIMS instrument is unique in that, within a compact field-deployable package, it provides (1) quantifiable molecular-level information for both gas and particle-phase organic species on timescales ranging from ≤1 s for gases to 10–60 min for particle-phase compounds that can be used to efficiently probe oxidation and secondary organic aerosol (SOA) formation mechanisms, and (2) relative volatility information of the detected compounds simultaneously estimated using the programmed thermal desorption information obtained from the MOVI. We demonstrate the capabilities of a prototype instrument using known test compounds and complex mixtures generated from the oxidation of biogenic and anthropogenic hydrocarbons. We present spectra obtained using both negative and positive ion CI with acetate (CH₃C(O)O^?) and protonated water clusters (H₃O⁺·(H₂O) _n ), respectively, as reagent ions. The instrument has high mass resolving power (R = 5000 above m/Q 250 Th) and mass accuracy (±20 ppm) enabling estimation of compound elemental composition. Instrument sensitivity in negative ion mode was tested using formic acid as a representative gas-phase compound, and that for particle-phase compounds was tested using palmitic, azelaic, and tricarballylic acids. With a heated MOVI inlet, an ion count rate of ～15 Hz is achieved when sampling 1 pptv (= 1 pmol/mol) of formic acid (or other monocarboxylic acids) under typical operating conditions. This sensitivity translates to detection limits less than 1 ng/m³ for carboxylic acids in the particle-phase. We also discuss the remaining challenges with this instrument to broadly characterizing gaseous and particulate oxygenated organic compounds in situ.

Copyright 2012 American Association for Aerosol Research     

Analysis of Phospholipids in Rat Brain Using Liquid Chromatography–Mass Spectrometry

The brain is a lipid-rich organ containing complex polar lipids including phospholipids (PLs) and sphingolipids. These lipids are involved in the structure and function of cell membranes in the brain. We developed a fast and efficient liquid chromatography–tandem mass spectrometry (LC–MS/MS) method to quantify five different classes of PLs [Choline glycerophospholipid (consists of phosphatidyl choline and plasmenyl choline in these samples), ethanolamine glycerophospholipid (consist of phosphatidyl ethanolamine and plasmenyl ethanolamine in these samples), phosphatidyl serine, phosphatidyl inositol, and sphingomyelin] in the brain tissues of 80-day-old Wistar rats. The PLs were extracted from rat brain using chloroform/methanol/water. After separation using a hydrophilic high performance liquid chromatography column, PL-class-specific fragmentation (head group identification) with a tandem mass spectrometer in positive ion mode was utilized to measure changes in the relative concentration of the five PL classes. The advantage of this approach was its improved specificity over previously reported LC–MS methods. The method had good repeatability (coefficient of variation 3–9%, excluding phosphatidyl inositol) and recovery (92–103%) and compared well with more laborious traditional methods.