Statistical Characterization of Temperature and Pressure Vertical Profiles for the Analysis of Laser Heterodyne Radiometry Data.

The statistical analysis of historic pressure and temperature profiles from radiosonde launches for use in the fitting of molecular oxygen line shapes is presented. As the O2 mixing ratio is nearly constant throughout the lower atmosphere, only variations in pressure and temperature profiles will affect the fit of observed O2 features in Laser Heterodyne Radiometry (LHR) spectra. Radiosonde temperature and pressure data are extracted from the Integrated Global Radiosonde Archive (IGRA) for a given station, date, and launch time. Data may be extracted for a single launch, for the same date over several years, and/or within a window centered on a target date. The temperature and pressure profiles are further characterized by the statistical variation in coefficients of polynomial fits in altitude. The properties of the probability distributions for each coefficient are used to constrain fits of O2 line shapes through Nelder-Mead optimization. The refined temperature and pressure profiles are then used in the retrieval of vertically resolved mixing ratios for greenhouse gases (GHGs) measured in the same instrument. In continuous collections, each vertical profile determination may be treated as a Bayesian prior to inform subsequent measurements and provide an estimate of uncertainties.

Precision heterodyne oxygen-corrected spectrometry: vertical profiling of water and carbon dioxide in the troposphere and lower stratosphere.

We describe the development of a near-infrared laser heterodyne radiometer: the precision heterodyne oxygen-corrected spectrometer (PHOCS). The prototype instrument is equipped with two heterodyne receivers for oxygen and water (measured near 1278 nanometers) and carbon dioxide (near 1572 nanometers) concentration profiles, respectively. The latter may be substituted by a heterodyne receiver module equipped with a laser to monitor atmospheric methane near 1651 nanometers. Oxygen measurements are intended to provide dry gas corrections and-more importantly-determine accurate temperature and pressure profiles that, in turn, improve the precision of the ${{\rm CO}_2}$CO2 and ${{\rm H}_2}{\rm O}$H2O column retrievals. Vertical profiling is made feasible by interrogating the very low-noise absorption lines shapes collected at $ \approx {0.0067}\;{{\rm cm}^{ - 1}}$≈0.0067cm-1 resolution. PHOCS complements the results from the Orbiting Carbon Observatory (OCO-2), Active Sensing of ${{\rm CO}_2}$CO2 Emissions over Nights, Days, and Seasons (ASCENDS), and ground-based Fourier transform spectrometers. In this paper, we describe the development of the instrument by Mesa Photonics and present the results of initial tests in the vicinity of Washington, DC.

The Molecular Composition of Soot.

Soot (sometimes referred to as black carbon) is produced when hydrocarbon fuels are burned. Our hypothesis is that polynuclear aromatic hydrocarbon (PAH) molecules are the dominant component of soot, with individual PAH molecules forming ordered stacks that agglomerate into primary particles (PP). Here we show that the PAH composition of soot can be exactly determined and spatially resolved by low-fluence laser desorption ionization, coupled with high-resolution mass spectrometry imaging. This analysis revealed that PAHs of 239-838 Da, containing few oxygenated species, comprise the soot observed in an ethylene diffusion flame. As informed by chemical graph theory (CGT), the vast majority of species observed in the sampled particulate matter may be described as benzenoids, consisting of only fused 6-membered rings. Within that limit, there is clear evidence for the presence of radical PAH in the particulate samples. Further, for benzenoid structures the observed empirical formulae limit the observed isomers to those which are nearly circular with high aromatic conjugation lengths for a given aromatic ring count. These results stand in contrast to recent reports that suggest higher aliphatic composition of primary particles.

Towards a taxonomy of topology for polynuclear aromatic hydrocarbons: linking electronic and molecular structure.

Trends linking the topological characteristics of polynuclear aromatic hydrocarbons (PAH) to their electronic properties are reported. TD-DFT electronic spectra computations, using the 6-31G* basis set and B3LYP exchange correlation functional, were calculated for a series of PAH, allowing for the HOMO-LUMO gaps to be reported. Clar structures provide an avenue to link the physical structure and the aromaticity of the molecule; which, when extended by bond length and harmonic oscillator model of aromaticity analysis, provide powerful tools to understand the link between electronic and physical structure. These results lead to the conclusion that all PAH structures show a decrease in HOMO-LUMO gap as a function of size, but the rate of that decrease is directly related to the topology of the molecules. A PAH taxonomy was developed that categorizes PAH into categories with similar topological properties, which allows for modelling of changes in the HOMO-LUMO gap with PAH size. An atom-pair minimization algorithm was used to calculate the binding energy (BE) of homogeneous dimers of the studied PAH. The BE per carbon atom increases with the overall size of the structure to an asymptotic limit, but as with the HOMO-LUMO gap, topology plays a critical secondary factor. Previously published, experimentally determined optical band gaps (OBG) from Tauc/Davis-Mott analysis of extinction spectra in various laminar, non-premixed flames produced a correlation between the HOMO-LUMO gaps of high-symmetry, nearly circular D2h symmetry molecules to molecular size. The work presented here provides a much more nuanced and predictive evaluation of how OBG depends on structure and size.

Extinction measurements for optical band gap determination of soot in a series of nitrogen-diluted ethylene/air non-premixed flames.

Visible light extinction was measured in a series of nitrogen-diluted, ethylene/air, non-premixed flames and this data was used to determine the optical band gap, OBG, as a function of flame position. Collimated light from a supercontinuum source is telescopically expanded and refocused to match the f- number of a dispersing monochromator. The dispersed light is split into a power metering channel and a channel that is periscoped and focused into the flame. The transmitted light is then recollimated and focussed onto a silicon photodiode detector. After tomographic reconstruction of the radial extinction field, the OBG was derived from the near-edge absorption feature using Tauc/Davis-Mott analysis. A slight evolution in OBG was observed throughout all flame systems with a consistent range of OBG observed between approximately 1.85 eV and 2.35 eV. Averaging over all positions the mean OBG was approximately 2.09 eV for all flame systems. Comparing these results to previously published computational results relating calculated HOMO-LUMO gaps for a variety of D2h PAH molecules to the number of aromatic rings in the structure, showed that the observed optical band gap is consistent with a PAH of about 14 rings or a conjugation length of 0.97 nm. This work provides experimental support to the model of soot formation where the transition from chemical to physical growth starts at a modest molecular size; about the size of circumpyrene.

Detection of trace hydrocarbons in flames using direct sampling mass spectrometry coupled with multilinear regression analysis.

A technique for the determination of species' concentrations from the molecular growth regions of flames is presented. Samples are obtained by microprobe extraction from a nitrogen-diluted, methane/air, nonpremixed laminar flame supported on a coannular burner. Quantification of measurements is accomplished by doping the flame's fuel flow with argon at a level to match that in the laboratory's air. A library of 70 eV fragmentation patterns for several flame species is used in conjunction with a simplex algorithm to analyze mass spectra obtained at each flame location. Each fragmentation pattern is normalized for its integrated intensity and its ionization cross-section relative to argon. This technique provided sub-part-per-million sensitivity of a large range of major and minor carbon-containing species ranging in size from C(2) to C(12) hydrocarbons. This flame can be acoustically forced to oscillate at a frequency emulating natural flame flickering behavior. Time-resolved measurements are obtained using a modified quartz microprobe synchronized to open and close with the flame oscillations. The near real-time sampling and analysis time and the relatively high sensitivity make this technique preferable to other extraction-based flame measurements.

Combinatorial targeting and nanotechnology applications.

The development of improved methods for targeted cell detection is of general interest in many fields of research and drug development. There are a number of well-established techniques for the study and detection of biomarkers expressed in living cells and tissues. Many of them rely on multi-step procedures that might not meet ideal assay requirements for speed, cost, sensitivity, and specificity. Here we report and further validate an approach that enables spontaneous molecular assembly to generate biologically active networks of bacteriophage (phage) assembled with gold (Au) nanoparticles (termed Au-phage nanoshuttles). Here, the nanoshuttles preserve the cell binding and internalization attributes mediated by a displayed peptide targeted to a cell surface receptor. The organization of such targeted assemblies can be further manipulated to be used as a multimodal detection assembly, and they can be characterized as fractal nanostructures by angle-dependent light scattering fractal dimension analysis. Targeted Au-phage nanoshuttles offer multiple functionalities for nanotechnology-based sensing and reporting, including enhanced fluorescence and improved contrast for darkfield microscopy.

A Review of Terminology Used to Describe Soot Formation and Evolution under Combustion and Pyrolytic Conditions.

This review presents a glossary and review of terminology used to describe the chemical and physical processes involved in soot formation and evolution and is intended to aid in communication within the field and across disciplines. There are large gaps in our understanding of soot formation and evolution and inconsistencies in the language used to describe the associated mechanisms. These inconsistencies lead to confusion within the field and hinder progress in addressing the gaps in our understanding. This review provides a list of definitions of terms and presents a description of their historical usage. It also addresses the inconsistencies in the use of terminology in order to dispel confusion and facilitate the advancement of our understanding of soot chemistry and particle characteristics. The intended audience includes senior and junior members of the soot, black carbon, brown carbon, and carbon black scientific communities, researchers new to the field, and scientists and engineers in associated fields with an interest in carbonaceous material production via high-temperature hydrocarbon chemistry.

Development of a multiple gas analyzer using cavity ringdown spectroscopy for use in advanced fire detection.

A portable cavity ringdown spectroscopy (CRDS) apparatus was used to detect effluents from small test fires in the Fire Emulator/Detector Evaluator (FE/DE) and a small room in the Building Fire and Research Laboratory at the National Institute of Standards and Technology (NIST). The output from two lasers is combined to detect four combustion gases, CO, CO(2), HCN, and C(2)H(2), near simultaneously using CRDS. The goal of this work was to demonstrate the feasibility of using a CRDS sensor as a fire detector. Fire effluents were extracted from several test facilities and measurements of CO, CO(2), HCN, and C(2)H(2) were obtained every 25-30 s. In the FE/DE test, peak concentrations of the gases from smoldering paper were 420 parts in 10(6) (ppm) CO, 1600 ppm CO(2), 530 parts in 10(9) (ppb) HCN, and 440 ppb C(2)H(2). Peak gas concentrations from the small room were 270 ppm CO, 2100 ppm CO(2), and 310 ppb C(2)H(2).

Intermolecular potential calculations for polynuclear aromatic hydrocarbon clusters.

Calculations of intermolecular potentials are presented for homo-molecular and hetero-molecular clusters of 24 peri-condensed PAH spanning monomer masses ranging from 78 to 1830 Da. Binding energies of homo-molecular dimers rise rapidly with molecular size and asymptotically approach the experimentally established exfoliation energy for graphite of 5.0 kJ mol(-1) (carbon atom)(-1). Binding energies of hetero-molecular dimers correlate well with the reduced mass of the pair. From calculations of homo-molecular stacks, binding energies were observed to increase with each added molecule and rise asymptotically, approaching a limit which scales linearly with monomer molecular mass. These results are reviewed in the context of molecular growth in flames and in the context of astrophysical observations.

Bottom-up assembly of hydrogels from bacteriophage and Au nanoparticles: the effect of cis- and trans-acting factors.

Hydrogels have become a promising research focus because of their potential for biomedical application. Here we explore the long-range, electrostatic interactions by following the effect of trans-acting (pH) and cis-acting factors (peptide mutation) on the formation of Au-phage hydrogels. These bioinorganic hydrogels can be generated from the bottom-up assembly of Au nanoparticles (Au NP) with either native or mutant bacteriophage (phage) through electrostatic interaction of the phage pVIII major capsid proteins (pVIII). The cis-acting factor consists of a peptide extension displayed on the pVIII that mutates the phage. Our results show that pH can dictate the direct-assembly and stability of Au-phage hydrogels in spite of the differences between the native and the mutant pVIII. The first step in characterizing the interactions of Au NP with phage was to generate a molecular model that identified the charge distribution and structure of the native and mutant pVIII. This model indicated that the mutant peptide extension carried a higher positive charge relative to the native pVIII at all pHs. Next, by monitoring the Au-phage interaction by means of optical microscopy, elastic light scattering, fractal dimension analysis as well as Uv-vis and surface plasmon resonance spectroscopy, we show that the positive charge of the mutant peptide extension favors the opposite charge affinity between the phage and Au NP as the pH is decreased. These results show the versatility of this assembly method, where the stability of these hydrogels can be achieved by either adjusting the pH or by changing the composition of the phage pVIII without the need of phage display libraries.

In vivo detection of gold-imidazole self-assembly complexes: NIR-SERS signal reporters.

Here we report in vitro and in vivo detection of self-assembled Au-imidazole by using near-infrared surface-enhanced Raman scattering (NIR-SERS). In vivo, the Au-imidazole structures were administered into tumor-bearing mice and detected noninvasively. The self-assembled Au-imidazole complexes were generated by the adsorption of imidazole molecules onto Au nanoparticles (NP) and were then characterized as aqueous suspensions by using NIR-SERS, angle-dependent light scattering with fractal dimension analysis, and visible extinction spectroscopy. The structure and optical activity was sensitive to imidazole concentration and Au NP size. Specifically, the Au-imidazole assemblies formed at lower imidazole concentrations had the lowest fractal dimension (D(f) = 1.2) and the largest Raman enhancement factors for the dominant NIR-SERS feature, a ring-breathing vibrational mode at 954 cm(-1). Changes in elastic scattering intensity, fractal dimension, and surface plasmon absorption were observed with increasing imidazole concentration. The Raman enhancement factor was also found to range between 10(6) and 10(9) with different primary Au nanoparticle sizes. For the higher enhancement factor systems, NIR-SERS detection of Au-imidazole was performed with data acquisitions time of only 5 s. The largest enhancement was observed for the 954-cm(-1) feature at an imidazole concentration of 1.9 microM when coupled to 54-nm-diameter Au NPs (the largest NP tested). Finally, we show the first demonstration of in vivo, noninvasive, and real-time SERS detection.

Networks of gold nanoparticles and bacteriophage as biological sensors and cell-targeting agents.

Biological molecular assemblies are excellent models for the development of nanoengineered systems with desirable biomedical properties. Here we report an approach for fabrication of spontaneous, biologically active molecular networks consisting of bacteriophage (phage) directly assembled with gold (Au) nanoparticles (termed Au-phage). We show that when the phage are engineered so that each phage particle displays a peptide, such networks preserve the cell surface receptor binding and internalization attributes of the displayed peptide. The spontaneous organization of these targeted networks can be manipulated further by incorporation of imidazole (Au-phage-imid), which induces changes in fractal structure and near-infrared optical properties. The networks can be used as labels for enhanced fluorescence and dark-field microscopy, surface-enhanced Raman scattering detection, and near-infrared photon-to-heat conversion. Together, the physical and biological features within these targeted networks offer convenient multifunctional integration within a single entity with potential for nanotechnology-based biomedical applications.