The Effect of Particle Size in Second Harmonic Generation from the Surface of Spherical Colloidal Particles. II: The Nonlinear Rayleigh−Gans−Debye Model

The Rayleigh−Gans−Debye model, adapted for nonlinear optical phenomena, is used to describe the second harmonic scattering from the surface of spherical particles in colloids. Specifically, the effect of the size of the particle on the efficiency of second harmonic generation from Malachite Green (MG) molecules adsorbed on polystyrene particles is examined. The model is found to be adequate for describing scattering patterns from smaller particles with diameters ≤200 nm, but less so for larger particles with diameters approaching 1 μm. From the model fit of experimentally measured data (Part I of this series, J. Phys. Chem. A 2009, 113, 4758) it is determined that χ⊥||||S is the dominant susceptibility element. This result suggests that the MG molecules with a dominant βz′′x′′x′′ component adsorb on the surface of the spherical nanoparticles with the C2 axis nearly perpendicular to the surface

Chemically Induced Changes to Membrane Permeability in Living Cells Probed with Nonlinear Light Scattering

Second-harmonic light scattering (SHS) permits characterization of membrane-specific molecular transport in living cells. Herein, we demonstrate the use of time-resolved SHS for quantifying chemically induced enhancements in membrane permeability. As proof of concept, we examine the enhanced permeability of the cytoplasmic membrane in living <i>Escherichia coli</i> following addition of extracellular adenosine triphosphate (ATPe). The transport rate of the hydrophobic cation, malachite green, increases nearly an order of magnitude following addition of 0.1 mM ATPe. The absence of an ATPe-enhanced permeability in liposomes strongly suggests the induced effect is protein-mediated. The utility of SHS for elucidating the mechanism of action of antimicrobials is discussed

Label-Free Optical Method for Quantifying Molecular Transport Across Cellular Membranes In Vitro

We demonstrate a nonlinear optical method for the label-free quantification of membrane transport rates of small/medium size molecules in living cells. Specifically, second-harmonic generation (SHG) laser scattering permits surface-specific characterization of transport across membranes. Unfortunately, most biologically relevant molecules are SHG-inactive. In the interest of extending this methodology for characterizing transport of any molecule, we monitor the SHG produced from an SHG-active reference molecule, in the presence of an SHG-inactive target molecule-of-interest as both molecules compete to cross a membrane. Of significance, the SHG-inactive target transport rate can be deduced as a perturbation in the measured transport rate of the reference. As proof-of-principle, we examine competitive transport of the strongly SHG-active cation, malachite green (MG), in the presence of a weakly SHG-active dication, propidium (Pro), across the outer-membrane protein channels in living bacteria. Comparison of the extracted and directly measured Pro transport rates validates the effectiveness of the method

Azithromycin-Induced Changes to Bacterial Membrane Properties Monitored <i>in Vitro</i> by Second-Harmonic Light Scattering

We present a nonlinear light scattering method for monitoring, with real-time resolution and membrane specificity, changes in molecular adsorption, and transport at bacterial membranes induced by an antimicrobial compound. Specifically, time-resolved second-harmonic light scattering (SHS) is used to quantify azithromycin-induced changes to bacterial membrane permeability in colloidal suspensions of living <i>Escherichia coli</i>. Variations in membrane properties are monitored through changes in the adsorption and transport rates of malachite green, a hydrophobic cation that gives SHS signal. Regardless of concentration, instantaneous treatment with azithromycin showed no significant changes in membrane permeability. However, 1 h pretreatment with subminimum inhibitory concentrations of azithromycin induced an order-of-magnitude enhancement in the permeability of both the outer membrane and, through facilitation of a new transport mechanism, the cytoplasmic membrane of the bacteria as well. This study illustrates SHS as a novel tool for monitoring antimicrobial-induced changes to membrane properties in living bacteria

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

Control of Chemical Reactions through Coherent Excitation of Eigenlevels: A Demonstration via Vibronic Coupling in SO₂

Through coherent excitation of a pair of vibronically coupled eigenlevels, an oscillation of 130 kcal/mol in energy excitation between electronic and vibrational motions (on a time scale of 10–8 s) is created for the triatomic molecule, sulfur dioxide (SO2). The reactivity of the molecule can be influenced depending upon whether the molecule is vibrationally or electronically excited with this substantial amount of energy. The effect of excitation on reactivity is demonstrated through SO2 photodissociation as a function of time following coherent excitation, monitored by multiphoton ionization of the SO product

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

Collisional Relaxation of Highly Vibrationally Excited Acetylene Mediated by the Vinylidene Isomer

Collisional relaxation of highly vibrationally excited acetylene, generated from the 193 nm photolysis of vinyl bromide with roughly 23,000 cm–1 of nascent vibrational energy, is studied via submicrosecond time-resolved Fourier transform infrared (FTIR) emission spectroscopy. IR emission from vibrationally hot acetylene during collisional relaxation by helium, neon, argon, and krypton rare-gas colliders is recorded and analyzed to deduce the acetylene energy content as a function of time. The average energy lost per collision, ⟨ΔE⟩, is computed using the Lennard-Jones collision frequency. Two distinct vibrational-to-translational (V–T) energy transfer regimes in terms of the acetylene energy are identified. At vibrational energies below 10,000–14,000 cm–1, energy transfer efficiency increases linearly with molecular energy content and is in line with typical V–T behavior in quantity. In contrast, above 10,000–14,000 cm–1, the V–T energy transfer efficiency displays a dramatic and rapid increase. This increase is nearly coincident with the acetylene-vinylidene isomerization limit, which occurs nearly 15,000 cm–1 above the acetylene zero-point energy. Combined quasi-classical trajectory calculations and Schwartz-Slawsky-Herzfeld-Tanczos theory point to a vinylidene contribution being responsible for the large enhancement. This observation illustrates the influence of energetically accessible structural isomers to greatly enhance the energy transfer rates of highly vibrationally excited molecules