The Effect of Composition, Morphology, and Susceptibility on Nonlinear Light Scattering from Metallic and Dielectric Nanoparticles

To facilitate second-harmonic light scattering as an effective tool for sensing and imaging nanoparticles, a fundamental understanding of how particle properties affect the nonlinear light scattering process is necessary. The angle-resolved second harmonic scattering patterns, measured in various polarization combinations, from spheroidal Ag particles (80 nm in diameter) are presented for the first time and compared with those from similarly sized spherical polystyrene particles adsorbed with nonlinear-optically active malachite green molecules. Comparison of the data with theoretical models is used to determine how optical constants (related to the particle composition), nonlinear susceptibility tensor elements, and shape may affect second-harmonic scattering from nanoparticles

Adsorption of Anionic Thiols on Silver Nanoparticles

The adsorption of negatively charged 3-mercaptopropanesulfonate (MPS) on the surface of citrate-stabilized Ag nanoparticles in water is investigated using colloidal particle surface sensitive techniques. The adsorption of this negatively charged thiol appears to be qualitatively different from that of neutral thiols and highlights the importance of repulsive interactions of electrostatic and steric origins pertaining to charged thiols. For the charged MPS thiol, the adsorption process occurs in two phases. At low surface coverage, where the intermolecular repulsion is negligible and the adsorption is dominated by the formation of the S–Ag bond, MPS molecules need to overcome an activation energy barrier <i>E</i>_<b>a</b> = (7.5 ± 0.9) kcal/mol with an associated free energy change Δ<i>G</i>_ads = −(14.3 ± 0.3) kcal/mol and behave similar to neutral thiols. On the other hand, at high surface coverage where the repulsive interactions among MPS molecules cannot be neglected, the adsorption is characterized by a higher <i>E</i>_a = (12.4 ± 0.5) kcal/mol and lower Δ<i>G</i>_ads = −(7.4 ± 0.1) kcal/mol

Observation of Organic Molecules at the Aerosol Surface

Organic molecules at the gas-particle interface of atmospheric aerosols influence the heterogeneous chemistry of the aerosol and impact climate properties. The ability to probe the molecules at the aerosol particle surface in situ therefore is important but has been proven challenging. We report the first successful observations of molecules at the surface of laboratory-generated aerosols suspended in air using the surface-sensitive technique second harmonic light scattering (SHS). As a demonstration, we detect trans-4-[4-(dibutylamino)­styryl]-1-methylpyridinium iodide and determine its population and adsorption free energy at the surface of submicron aerosol particles. This work illustrates a new and versatile experimental approach for studying how aerosol composition may affect the atmospheric properties

Gram’s Stain Does Not Cross the Bacterial Cytoplasmic Membrane

For well over a century, Hans Christian Gram’s famous staining protocol has been the standard go-to diagnostic for characterizing unknown bacteria. Despite continuous and ubiquitous use, we now demonstrate that the current understanding of the molecular mechanism for this differential stain is largely incorrect. Using the fully complementary time-resolved methods: second-harmonic light-scattering and bright-field transmission microscopy, we present a real-time and membrane specific quantitative characterization of the bacterial uptake of crystal-violet (CV), the dye used in Gram’s protocol. Our observations contradict the currently accepted mechanism which depicts that, for both Gram-negative and Gram-positive bacteria, CV readily traverses the peptidoglycan mesh (PM) and cytoplasmic membrane (CM) before equilibrating within the cytosol. We find that not only is CV unable to traverse the CM but, on the time-scale of the Gram-stain procedure, CV is kinetically trapped within the PM. Our results indicate that CV, rather than dyes which rapidly traverse the PM, is uniquely suited as the Gram stain

Polarized Absorption in Crystalline Pentacene: Theory vs Experiment

The polarized absorption spectra of crystalline pentacene are obtained for excitation normal to the <i>ab</i> herringbone plane by measuring transmitted light in ultrathin crystals. The spectral line shapes for excitation polarized along <i>b</i> and orthogonal to <i>b</i> are analyzed theoretically using a Holstein-like Hamiltonian which includes both Frenkel and charge transfer (CT) excitons represented in a multiparticle basis set. The model agrees with prior estimates regarding the strong CT contribution (≈45%) of the exciton responsible for the <i>b</i>-polarized lower Davydov component. The polarization resolution allows one to also establish the nature of the upper Davydov component, which is found to contain far less CT content (≈15%), as well as the natures of the higher-energy vibronic excitons, which are found to consist of a complex mixture of Frenkel one- and two-particle states and CT excitons. Generally, the spectrum polarized along <i>b</i> displays J-aggregate-like vibronic signatures while the spectrum polarized orthogonal to <i>b</i> displays H-aggregate-like vibronic signatures. The assignment is entirely consistent with the calculated exciton band dispersions which agree well with the measured ones

Photoactivated Production of Secondary Organic Species from Isoprene in Aqueous Systems

Photoactivated reactions of organic species in atmospheric aerosol particles are a potentially significant source of secondary organic aerosol material (SOA). Despite recent progress, the dominant chemical mechanisms and rates of these reactions remain largely unknown. In this work, we characterize the photophysical properties and photochemical reaction mechanisms of imidazole-2-carboxaldehyde (IC) in aqueous solution, alone and in the presence of isoprene. IC has been shown previously in laboratory studies to participate in photoactivated chemistry in aerosols, and it is a known in-particle reaction product of glyoxal. Our experiments confirmed that the triplet excited state of IC is an efficient triplet photosensitizer, leading to photosensitization of isoprene in aqueous solution and promoting its photochemical processing in aqueous solution. Phosphorescence and transient absorption studies showed that the energy level of the triplet excited state of IC (³IC*) was approximately 289 kJ/mol, and the lifetime of ³IC* in water under ambient temperature is 7.9 μs, consistent with IC acting as an efficient triplet photosensitizer. Laser flash photolysis experiments displayed fast quenching of ³IC* by isoprene, with a rate constant of (2.7 ± 0.3) × 10⁹ M^–1 s^–1, which is close to the diffusion-limited rate in water. Mass spectrometry analysis showed that the products formed include IC–isoprene adducts, and chemical mechanisms are discussed. Additionally, oxygen quenches ³IC* with a rate constant of (3.1 ± 0.1) × 10⁹ M^–1 s^–1

Excitonic and Confinement Effects of 2D Layered (C₁₀H₂₁NH₃)₂PbBr₄ Single Crystals

Recognition of unusual optoelectronic properties for two-dimensional (2D) layered organic–inorganic lead­(II) halide materials (C_<i>n</i>H_2<i>n</i>+1NH₃)₂PbX₄ (X = I, Br, and Cl) has attracted intense renewed interest in this class of materials. Single crystals of the 2D layered materials (C₁₀H₂₁NH₃)₂PbBr₄ and pseudo-alloy (C₁₀H₂₁NH₃)₂PbI₂Br₂ were grown for photophysical evaluation. A 10-carbon alkylammonium cation was selected for investigation to provide strong dielectric screening in order to highlight quantum confinement effects of the anionic (PbX₄^2–) semiconductor layer. Single crystals of the 2D layered (C₁₀H₂₁NH₃)₂PbBr₄ compound display a characteristic free exciton with a binding energy of ca. 280 meV. Observation of a short photoluminescence lifetime of 2.8 ± 0.2 ns suggests that this electronic transition for the PbBr₄-based layered material has only singlet character. Sheets of (C₁₀H₂₁NH₃)₂PbBr₄ with thicknesses of a few layers were fabricated, and the dimensions were verified by AFM experiments. Excitonic emissions from (C₁₀H₂₁NH₃)₂PbBr₄ and (C₁₀H₂₁NH₃)₂PbI₄ exhibit relatively small spectral shifts from the bulk down to a thickness of five layers indicative of the strong confinement effect of the 10-carbon alkylammonium spacers. Single crystals of the pseudo-alloy (C₁₀H₂₁NH₃)₂PbBr₂I₂ give an excitonic absorption peak close to that of the tetrabromide (C₁₀H₂₁NH₃)₂PbBr₄ and an emission peak with a large Stokes shift to a position similar to that of the tetraiodide (C₁₀H₂₁NH₃)₂PbI₄