Plasmons in strongly coupled metallic nanostructures.

Data Reduction And Error Analysis For The Physical Sciences

Templated Techniques for the Synthesis and Assembly of Plasmonic Nanostructures

[1] The atmospheric chemistry of volatile organic compounds (VOCs) in urban areas results in the formation of ‘photochemical smog’, including secondary organic aerosol (SOA). State-of-the-art SOA models parameterize the results of simulation chamber experiments that bracket the conditions found in the polluted urban atmosphere. Here we show that in the real urban atmosphere reactive anthropogenic VOCs (AVOCs) produce much larger amounts of SOA than these models predict, even shortly after sunrise. Contrary to current belief, a significant fraction of the excess SOA is formed from first-generation AVOC oxidation products. Global models deem AVOCs a very minor contributor to SOA compared to biogenic VOCs (BVOCs). If our results are extrapolated to other urban areas, AVOCs could be responsible for additional 3–25 Tg yr−1 SOA production globally, and cause up to −0.1 W m−2 additional top-of-the-atmosphere radiative cooling.

Secondary Organic Aerosol Formation from Anthropogenic Air Pollution: Rapid and Higher than Expected

Photoacoustic imaging (PAI) is an emerging tool that bridges the traditional depth limits of ballistic optical imaging and the resolution limits of diffuse optical imaging. Using the acoustic waves generated in response to the absorption of pulsed laser light, it provides noninvasive images of absorbed optical energy density at depths of several centimeters with a resolution of ∼100 μm. This versatile and scalable imaging modality has now shown potential for molecular imaging, which enables visualization of biological processes with systemically introduced contrast agents. Understanding the relative merits of the vast range of contrast agents available, from small-molecule dyes to gold and carbon nanostructures to liposome encapsulations, is a considerable challenge. Here we critically review the physical, chemical and biochemical characteristics of the existing photoacoustic contrast agents, highlighting key applications and present challenges for molecular PAI.

/pdf/contrast-agents-for-molecular-photoacoustic-imaging-1qnxfpef28.pdf

Contrast agents for molecular photoacoustic imaging

We present an algorithm that can be used to simultaneously determine the radius and the refractive index (with dispersion) of a spherical, homogeneous particle. This is accomplished by fitting characteristic resonances calculated using Mie scattering coefficients to the measured whispering gallery mode resonances. The advantage of this algorithm over those that have been presented previously is that a large portion of the search can be reduced to two dimensions (a search that includes radius and refractive index with dispersion will always be at least three dimensions). Using this algorithm, we analyze two large sets of cavity-enhanced Raman spectra from optically trapped aerosol particles. The speed of the algorithm allows for best fits to be found in real time. Precision is found to be limited by the resolution of the spectrograph.

Accurate and efficient determination of the radius, refractive index, and dispersion of weakly absorbing spherical particle using whispering gallery modes

The growth and optical properties of incomplete gold layers on silica particles (229 nm) are studied using visible/near-infrared spectroscopy and transmission electron microscopy. The gold particles that eventually coalesce to form a continuous gold layer are found to have droplet-like shapes. The optical properties of these systems are different from those of complete gold nanoshells. Using the discrete dipole approximation, it is found that the plasmon modes of such systems should exhibit two bands: one from 500−600 nm (“high energy”) and the other from 600−800 nm (“low energy”). The calculations show that, for increasing coating density of the droplet-like particles, the lower energy band (i) becomes stronger relative to the higher energy band and (ii) is red-shifted. Both of these trends are found in the spectra of the prepared particles. Furthermore, the observed plasmon bands fall within the limits established by the model.

Growth and optical properties of gold nanoshells prior to the formation of a continuous metallic layer.

A new method for determining the radius and refractive index of microspheres using Mie resonances is presented. Previous methods have relied on searching multidimensional space to find the radius and refractive index that minimize the difference between observed and calculated Mie resonances. For anything but simple refractive index functions, this process can be very time consuming. Here, we demonstrate that once the mode assignment for the observed Mie resonances is known, no search is necessary, and the radius and refractive index of best-fit can be found immediately. This superior and faster way to characterize microspheres using Mie resonances should supplant previous fitting algorithms. The derivation and implementation of the equations that give the parameters of best-fit are shown and discussed. Testing is performed on systems of physical interest, and the effect of noise on measured peak positions is investigated.

/pdf/determining-the-size-and-refractive-index-of-microspheres-5c370otsxp.pdf

Determining the size and refractive index of microspheres using the mode assignments from Mie resonances.

Direct measurements of the phase separation relative humidity (RH) and morphology of aerosol particles consisting of liquid organic and aqueous inorganic domains are presented. Single droplets of mixed phase composition are captured in a gradient force optical trap, and the evolving size, refractive index (RI), and morphology are characterized by cavity-enhanced Raman spectroscopy. Starting at a RH above the phase separation RH, the trapped particle is dried to lower RH and the transition to a phase-separated structure is inferred from distinct changes in the spectroscopic fingerprint. In particular, the phase separation RHs of droplets composed of aqueous solutions of polyethylene glycol (PEG-400)/ammonium sulfate and a mixture of C6-diacids/ammonium sulfate are probed, inferring the RH from the RI of the droplet immediately prior to phase separation. The observed phase separation RHs occur at RH marginally higher (at most 4%) than reported in previous measurements made from studies of particles deposited on hydrophobic surfaces by brightfield imaging. Clear evidence for the formation of phase-separated droplets of core-shell morphology is observed, although partially engulfed structures can also be inferred to form. Transitions between the different spectroscopic signatures of phase separation suggest that fluctuations in morphology can occur. For droplets that are repeatedly cycled through the phase separation RH, the water activity at phase separation is found to be remarkably reproducible (within ±0.0013) and is the same for the 1-phase to 2-phase transition and the 2-phase to 1-phase transition. By contrast, larger variation between the water activities at phase separation is observed for different droplets (typically ±0.02).

/pdf/liquid-liquid-phase-separation-in-mixed-organic-inorganic-4tcpi1vmme.pdf

Liquid-liquid phase separation in mixed organic/inorganic single aqueous aerosol droplets.

The kinetic stability of aqueous gold colloids containing particles of 11 nm diameter and bearing solution-facing carboxylic acid groups has been analyzed by experiment and theory. Three different types of particles were made whose surfaces were modified by three different thiolate-tethered acids of varying chain-length and pKa: 6-mercaptohexanoic and 12-mercaptododecanoic acids (pKa 4.80 in H2O/EtOH, 80:20 by volume), and N-(2-(S)-methylacetic acid)-6-mercaptohexamide (pKa 3.85). As measured by a combination of UV−visible absorption spectroscopy and dynamic light scattering, the particles were reversibly aggregated and dispersed by cycling the pH of solution between low ( pKa), or the ionic strength between high and low values, respectively. Conditions of aggregation were satisfactorily and quantitatively explained using a model incorporating a superposition of a repulsive electric double layer and van der Waals attraction. That is, colloidal kinetic stabi...

Thomas C. Preston

Papers

Accurate and efficient determination of the radius, refractive index, and dispersion of weakly absorbing spherical particle using whispering gallery modes

Growth and optical properties of gold nanoshells prior to the formation of a continuous metallic layer.

Determining the size and refractive index of microspheres using the mode assignments from Mie resonances.

Liquid-liquid phase separation in mixed organic/inorganic single aqueous aerosol droplets.

Role of Hydrogen Bonding in the pH-Dependent Aggregation of Colloidal Gold Particles Bearing Solution-Facing Carboxylic Acid Groups