First hyperpolarizability of the natural aromatic amino acids tryptophan, tyrosine, and phenylalanine and the tripeptide lysine-tryptophan-lysine determined by hyper-Rayleigh scattering

Multistep Micellization of Standard Surfactants Evidenced by Second Harmonic Scattering

Processes involving in solution a reduced number of molecules are difficult to identify and characterize. Here, we show that micellization of standard surfactants, namely sodium dodecyl sulfate and trimethyl tetradecyl ammonium bromide, two nonefficient compounds for quadratic nonlinear optics, can be investigated by second harmonic scattering (SHS). In particular, the formation of aggregates at concentrations smaller than the critical micellar concentration is evidenced through a nonmonotonic behavior of the SHS intensity as a function of the surfactant concentration. A simple model based on chemical equilibria between monomers and micelles is proposed to account for the experimental observations. Signature of long-range molecular orientation correlation is revealed by polarization resolved experiments and is discussed regarding micellization and charge-induced effects.

Gold-seeded Lithium Niobate Nanoparticles: Influence of Gold Surface Coverage on Second Harmonic Properties

Hybrid nanoparticles composed of an efficient nonlinear optical core and a gold shell can enhance and tune the nonlinear optical emission thanks to the plasmonic effect. However the influence of an incomplete gold shell, i.e., isolated gold nano-islands, is still not well studied. Here LiNbO₃ (LN) core nanoparticles of 45 nm were coated with various densities of gold nano-seeds (AuSeeds). As both LN and AuSeeds bear negative surface charge, a positively-charged polymer was first coated onto LN. The number of polymer chains per LN was evaluated at 1210 by XPS and confirmed by fluorescence titration. Then, the surface coverage percentage of AuSeeds onto LN was estimated to a maximum of 30% using ICP-AES. The addition of AuSeeds was also accompanied with surface charge reversal, the negative charge increasing with the higher amount of AuSeeds. Finally, the first hyperpolarizability decreased with the increase of AuSeeds density while depolarization values for Au-seeded LN were close to the one of bare LN, showing a predominance of the second harmonic volumic contribution.

Theoretical Diagnostics of Second and Third-order Hyperpolarizabilities of Several Acid Derivatives

The density functional theory (DFT) at B3LYP/6-31G(d) level has been utilized to achieve the electric dipole moment $\left( \mu \right),$static dipole polarizability $\left( \alpha \right)$and first hyperpolarizability $\left( \beta \right)$values for ferulic acid (1) and chenodeoxycholic acid (2). The time-dependent Hartree-Fock (TDHF) technique as a powerful quantum chemical method has been implemented to reveal the dynamic $\alpha ,\,\beta $and third-order hyperpolarizabilities $\left( \gamma \right)$of the examined compounds. Our computational conclusions have been compared with the results of similar materials in the literature. The first and second frontier molecular orbitals (MOs) and their band gaps have also been investigated by means of DFT.

Sub-100 nanometer silver doped gold–cysteine supramolecular assemblies with enhanced nonlinear optical properties

The ability of gold(i) thiolates to self-assemble into supramolecular architectures opens the route for a new class of nanomaterials with a unique structure–optical property relationship.

Non-linear optical properties of gold quantum clusters. The smaller the better

By developing a new method for synthesizing atomically monodisperse Au15 nanoclusters stabilized with glutathione molecules and using the current state-of-the-art methods for synthesizing monodisperse protected Au25 nanoclusters, we investigated their nonlinear optical (NLO) properties after two-photon absorption. The two-photon emission spectra and the first hyperpolarizabilities of these particles were obtained using, in particular, a hyper-Rayleigh scattering technique. The influence on NLO of the excitation wavelength, the size as well as the nature of the ligands is also explored and discussed. Au15, the smallest stable thiolated gold nanocluster, presents remarkable nonlinear properties with respect to two-photon processes. The two-photon absorption cross-section at 780 nm for Au15 is ∼65,700 GM. This experimental cross-section value points to a quantum yield for two-photon emission of about 3 × 10(-7) at 475 nm for Au15. The first hyperpolarizability β for Au15 clusters (509 × 10(-30) esu), as compared to Au25 clusters (128 × 10(-30) esu), is larger considering the difference in the number of gold atoms. Also, 10(30) β per atom values reported for Au15 and Au25 clusters are more than two orders of magnitude larger than the values reported for Au NPs in the size range 10-50 nm, outlining the quantum cluster regime.

Second harmonic generation from tryptophan-rich short peptides: W(n)K(m) and gramicidin A

We report the first hyperpolarizability of a series of tryptophan-rich short peptides with the respective sequence KWK, KWWK, KWWWK, KWWKWWK, where W and K stand for tryptophan and lysine. The measurements were performed with the technique of hyper-Rayleigh scattering in the bulk of an aqueous Tris buffer solution at a pH of 8.5 and a salt concentration of 150 mM at the non-resonant fundamental wavelength of 784 nm. The first hyperpolarizability of the different peptides follows a simple additive model scaling with the number of tryptophan residues contained in the peptide. However, it appears that the first hyperpolarizability response of a single tryptophan residue in the peptide strongly differs from that of an isolated tryptophan. Hence, it is therefore demonstrated that the local environment of the tryptophan residues within the peptide strongly influences its nonlinear optical response. A comparison with the first hyperpolarizability of the natural peptide gramicidin A measured in trifluoroethanol (TFE) further confirms the key role of the local environment on the first hyperpolarizability of tryptophan residues in peptides.

Reorientation of the helix of the tryptophan-rich gp41W peptide from HIV-1 at interfaces

The glycoprotein gp41 from the Human Immunodeficiency Virus type 1 (HIV-1) has an amino acid sequence enriched in tryptophan residues, the so-called gp41W peptide (i.e., KWASLWNWFNITNWLWYIK) and plays a crucial role in HIV-1 host cell infection. Using the coupling of Second Harmonic Generation targeting the tryptophan residues with lateral surface tension measurements, we investigate the interaction of gp41W with a neat air∕water and a lipid∕water interfaces. At the air∕water interface, gp41W presents a well-defined orientation and this orientation is strongly modified at the lipid∕water interface, depending on the surface pressure. These results show that this strategy is well suited to monitor tryptophan containing α-helices orientation at lipid∕water interfaces.

A bottom-up approach to build the hyperpolarizability of peptides and proteins from their amino acids

We experimentally demonstrate that some peptides and proteins lend themselves to an elementary analysis where their first hyperpolarizability can be decomposed into the coherent superposition of the first hyperpolarizability of their elementary units. We then show that those elementary units can be associated with the amino acids themselves in the case of nonaromatic amino acids and nonresonant second harmonic generation. As a case study, this work investigates the experimentally determined first hyperpolarizability of rat tail Type I collagen and compares it to that of the shorter peptide [(PPG)10]3, where P and G are the one-letter code for Proline and Glycine, respectively, and that of the triamino acid peptides PPG and GGG. An absolute value of (0.16 ± 0.01) × 10(-30) esu for the first hyperpolarizability of nonaromatic amino acids is then obtained by using the newly defined 0.087 × 10(-30) esu reference value for water. By using a collagen like model, the microscopic hyperpolarizability along the peptide bond can be evaluated at (0.7 ± 0.1) × 10(-30) esu.

A THEORETICAL STUDY ON SECOND HARMONIC GENERATION HYPERPOLARIZABILITIES OF PHENYLALANINE POLYPEPTIDES

The second harmonic generation (SHG) hyperpolarizabilities of phenylalanine and homopolypeptides are investigated by configuration interaction among singly excited configurations (CIS) technique combined with the sum-over-states (SOS) method. The geometries of peptides containing phenylalanine ( Phe )n(n = 1–8) are optimized by B3LYP/6-31g(d) method, and they form the special structures like β-sheet (a common protein secondary structure). It is found that the energy gaps of various peptides are reduced and the hyperpolarizabilities are increased with the peptide chains lengthened. We discuss the origin of the second-order nonlinear optical response in phenylalanine homopolypeptides and confirm that the π → π* transitions in the aromatic residue of phenylalanine make the most important contributions to the second-order polarizability. Our results strongly suggest that the hyperpolarizabilities are dominated from the propagation direction of peptide chains.

Effect of a thioalkane capping layer on the first hyperpolarizabilities of gold and silver nanoparticles

We have measured the first hyperpolarizabilities of thioalkane capped silver and gold metallic nanoparticles. The values found are β(AgC 12-10 nm) = (2.10 ± 0.23) × 10(-26) esu for 10 nm diameter silver nanoparticles and β(AuC 18-18 nm) = (3.37 ± 0.08) × 10(-26) esu for 18 nm diameter gold nanoparticles at the fundamental wavelength of 784 nm. By comparison to the corresponding values reported for citrate capped silver and gold metallic nanoparticles, after size corrections, decreases by factors of 4.3 and 6.5 respectively are observed. These decreases are tentatively attributed to the bonds formed between the gold and silver surface atoms and the sulfur atoms of the capping layer.