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

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

Second Harmonic and Sum-Frequency Generation from Aqueous Interfaces Is Modulated by Interference

The interfacial region of aqueous systems also known as the electrical double layer can be characterized on the molecular level with second harmonic and sum-frequency generation (SHG/SFG). SHG and SFG are surface specific methods for isotropic liquids. Here, we model the SHG/SFG intensity in reflection, transmission, and scattering geometry taking into account the spatial variation of all fields. We show that, in the presence of a surface electrostatic field, interference effects, which originate from oriented water molecules on a length scale over which the potential decays, can strongly modify the probing depth as well as the expected intensity at ionic strengths –3 M. For reflection experiments this interference phenomenon leads to a significant reduction of the SHG/SFG intensity. Transmission mode experiments from aqueous interfaces are hardly influenced. For SHG/SFG scattering experiments the same interference leads to an increase in intensity and to modified scattering patterns. The predicted scattering patterns are verified experimentally

Anchoring of Aminophosphonates on Titanium Oxide for Biomolecular Coupling

Aminophosphonates were chosen for a first step functionalization of TiO2 grown on titanium, as they possess a phosphonate group on one end, that can be exploited for coupling with the oxide surface, and an amino group on the other end to enable further functionalization of the surface. The deposition of aminophosphonates with different chain lengths (6 and 12 methylenes) was investigated. Oxygen plasma treatment proved useful in increasing the number of −OH groups at the TiO2 surface, thus helping to anchor the aminophosphonates. By combining different surface-sensitive experimental techniques, we found the existence of a discontinuous monolayer where the molecules are covalently coupled to the TiO2 surface. For the molecules with longer chains, we find evidence of their covalent coupling to the surface through Ti–O–P bond formation, of the exposure of the amino groups at the outer surface, and of an increase in the order of the layer upon thermal annealing

Both Poly(ethylene glycol) and Poly(methyl ethylene phosphate) Guide Oriented Adsorption of Specific Proteins

Developing new functional biomaterials requires the ability to simultaneously repel unwanted and guide wanted protein adsorption. Here, we systematically interrogate the factors determining the protein adsorption by comparing the behaviors of different polymeric surfaces, poly­(ethylene glycol) and a poly­(phosphoester), and five different natural proteins. Interestingly we observe that, at densities comparable to those used in nanocarrier functionalization, the same proteins are either adsorbed (fibrinogen, human serum albumin, and transferrin) or repelled (immunoglobulin G and lysozyme) by both polymers. However, when adsorption takes place, the specific surface dictates the amount and orientation of each protein

Control of the Orientational Order and Nonlinear Optical Response of the “Push−Pull” Chromophore RuPZn via Specific Incorporation into Densely Packed Monolayer Ensembles of an Amphiphilic 4-Helix Bundle Peptide: Second Harmonic Generation at High Chromophore Densities

The macroscopic nonlinear optical response of the “push−pull” chromophore RuPZn incorporated into a single monolayer of the amphiphilic 4-helix bundle peptide (AP0) covalently attached to a solid substrate at high in-plane density has been measured. The second-order susceptibility, χzzz, was found to be in the range of ∼15 × 10−9 esu, consistent with a coherent sum of the nonlinear contributions from the individual chromophores (β⃡) as previously measured in isotropic solution through hyper-Rayleigh scattering as well as estimated from theoretical calculations. The microscopic hyperpolarizability of the RuPZn chromophore is preserved upon incorporation into the peptide monolayer, suggesting that the chromophore−chromophore interactions in the densely packed ensemble do not substantially affect the first-order molecular hyperpolarizability. The polarization angle dependence of the second harmonic signal reveals that the chromophore is vectorially oriented in the two-dimensional ensemble. Analysis of the order parameter together with information obtained from grazing incidence X-ray diffraction help in determining the chromophore orientation within the AP0−RuPZn monolayer. Taking into account an average pitch angle of ∼20° characterizing the coiled-coil structure of the peptide bundle, the width of the bundle’s tilt angle distribution should be σ ≤ 20°, resulting in a mean value of the tilt angle 23° ≤ θ0 ≤ 37°

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

Pushing the High-Energy Limit of Plasmonics

The localized surface plasmon resonance of metal nanoparticles allows confining the eletromagnetic field in nanosized volumes, creating high-field “hot spots”, most useful for enhanced nonlinear optical spectroscopies. The commonly employed metals, Au and Ag, yield plasmon resonances only spanning the visible/near-infrared range. Stretching upward, the useful energy range of plasmonics requires exploiting different materials. Deep-ultraviolet plasmon resonances happen to be achievable with one of the cheapest and most abundant materials available: aluminum indeed holds the promise of a broadly tunable plasmonic response, theoretically extending far into the deep-ultraviolet. Complex nanofabrication and the unavoidable Al oxidation have so far prevented the achievement of this ultimate high-energy response. A nanofabrication technique producing purely metallic Al nanoparticles has at last allowed to overcome these limits, pushing the plasmon resonance to 6.8 eV photon energy (≈180 nm) and thus significantly broadening the spectral range of plasmonics’ numerous applications