Nonlinear superchiral meta-surfaces: tuning chirality and disentangling non-reciprocity at the nanoscale.

Circularly polarized light is incident on a nanostructured chiral meta-surface. In the nanostructured unit cells whose chirality matches that of light, superchiral light is forming and strong optical second harmonic generation can be observed.

Poly(3-alkylthiophene)s show unexpected second-order nonlinear optical response.

Regioregular poly(3-hexylthiophene)s with chain lengths varying from 5 to 100 monomers are synthesized. Poly(3-hexylthiophene)s show in solution an unexpectedly significant second-order nonlinear optical response. The increase in transition dipole moment upon oligomerisation causes the significant second-order nonlinear optical response.

Polyphenols can inhibit furin in vitro as a result of the reactivity of their auto-oxidation products to proteins.

UNLABELLED: Methods using fluorogenic peptide substrates have been proposed for screening of proprotein convertase (PC) inhibitors and they are attractive since they offer the advantage of being sensitive, cost-effective and susceptible to miniaturization. Several polyphenols, including epigallocatechin gallate ((-)EGCG), the main component of green tea, and quercetin, widely distributed in fruit and vegetables, however, led to false positive results when fluorogenic peptide substrates were used. Processing of genuine furin substrates was not inhibited by these polyphenols. In the present study, these discordant effects of (-)EGCG on the PC furin were studied. While quercetin can form aggregates in solution, aggregate-based promiscuous inhibition could be ruled out as underlying mechanism for (-)EGCG. Hydrogen peroxide production, from auto-oxidation, was too low to be a major factor but appeared associated to furin inhibition, suggesting a role for other auto-oxidation products. Since the instability of catechins is related to their electrophilic character, we tested the nucleophilic substance glutathione for stabilization. Indeed glutathione reduced furin inhibition and (-)EGCG binding to furin and serum albumin as shown by redox-cycling staining. Catechins, therefore, seem to form reactive compounds and this should be taken into account in screening assays. Adding glutathione to the detergent-based assay, as used in these studies to measure furin processing activity, strongly reduced inhibition by a number of polyphenols (catechins, gallic acid and quercetin), while the effect on the genuine inhibitor nona-D-arginine remained unchanged. IN CONCLUSION: the combined use of detergent and glutathione in the screening assay for furin inhibitors improves the predictive value.

Chirality in nonlinear-optical response of planar G-shaped nanostructures.

Chirality effects in optical second harmonic generation (SHG) are studied in periodic planar arrays of gold G-shaped nanostructures. We show that G-shaped structures of different handedness demonstrate different SHG efficiency for the left and right circular polarizations, as well as the opposite directions of the SHG polarization plane rotation. The observed effects are interpreted as the appearance of chirality in the SHG response which allows clear distinguishing of two enantiomers.

The role of chiral local field enhancements below the resolution limit of Second Harmonic Generation microscopy.

While it has been demonstrated that, above its resolution limit, Second Harmonic Generation (SHG) microscopy can map chiral local field enhancements, below that limit, structural defects were found to play a major role. Here we show that, even below the resolution limit, the contributions from chiral local field enhancements to the SHG signal can dominate over those by structural defects. We report highly homogeneous SHG micrographs of star-shaped gold nanostructures, where the SHG circular dichroism effect is clearly visible from virtually every single nanostructure. Most likely, size and geometry determine the dominant contributions to the SHG signal in nanostructured systems.

Coherent and incoherent second harmonic generation in planar G-shaped nanostructures.

Azimuthal anisotropy of Stokes parameters of the second harmonic generation (SHG) generated and observed in reflection from a periodic planar area of G-shaped gold nanostructures is studied. A strong anisotropy of both coherent and incoherent SHG components is observed. Finite-difference time-domain calculations prove that the observed effects are due to the anisotropic enhancement of the fundamental radiation within the G-shaped structures.

Hotspot decorations map plasmonic patterns with the resolution of scanning probe techniques.

In high definition mapping of the plasmonic patterns on the surfaces of nanostructures, the diffraction limit of light remains an important obstacle. Here we demonstrate that this diffraction limit can be completely circumvented. We show that upon illuminating nanostructures made of nickel and palladium, the resulting surface-plasmon pattern is imprinted on the structures themselves; the hotspots (regions of local field enhancement) are decorated with overgrowths, allowing for their subsequent imaging with scanning-probe techniques. The resulting resolution of plasmon pattern imaging is correspondingly improved.

Fast and accurate peanut allergen detection with nanobead enhanced optical fiber SPR biosensor.

This paper is the first report of a fiber optic SPR biosensor with nanobead signal enhancement. We evaluated the system with a bioassay for the fast and accurate detection of peanut allergens in complex food matrices. Three approaches of an immunoassay to detect Ara h1 peanut allergens in chocolate candy bars were compared; a label-free assay, a secondary antibody sandwich assay and a nanobead enhanced assay. Although label-free detection is the most convenient, our results illustrate that functionalized nanobeads can offer a refined solution to improve the fiber SPR detection limit. By applying magnetite nanoparticles as a secondary label, the detection limit of the SPR bioassay for Ara h1 was improved by two orders of magnitude from 9 to 0.09 µg/mL. The super paramagnetic character of the nanoparticles ensured easy handling. The SPR fibers could be regenerated easily and one fiber could be reused for up to 35 times without loss of sensitivity. The results were benchmarked against a commercially available polyclonal ELISA kit. An excellent correlation was found between the Ara h1 concentrations obtained with the ELISA and the concentrations measured with the SPR fiber assay. In addition, with the SPR fiber we could measure the samples twice as fast as compared to the fastest ELISA protocol. Since the dipstick fiber has no need for microchannels that can become clogged, time consuming rinsing step could be avoided. The linear dynamic range of the presented sensor was between 0.1 and 2 µg/mL, which is considerably larger than the ELISA benchmark.

Linearly polarized second harmonic generation microscopy reveals chirality.

In optics, chirality is typically associated with circularly polarized light. Here we present a novel way to detect the handedness of chiral materials with linearly polarized light. We performed Second Harmonic Generation (SHG) microscopy on G-shaped planar chiral nanostructures made of gold. The SHG response originates in distinctive hotspots, whose arrangement is dependent of the handedness. These results uncover new directions for studying chirality in artificial materials.

Asymmetric optical second-harmonic generation from chiral G-shaped gold nanostructures.

We present a new electromagnetic phenomenon-the asymmetric second-harmonic generation from planar chiral structures. The effect consists in distinguishing the handedness of a chiral material by rotating the sample in an experiment involving solely linearly polarized light. This phenomenon originates in the surface plasmon resonance of chiral gold nanostructures, where homodyne interference of anisotropic and chiral electric and/or magnetic multipoles appears to play an important role.

Plasmonic ratchet wheels: switching circular dichroism by arranging chiral nanostructures.

We demonstrate circular dichroism (CD) in the second harmonic generation (SHG) signal from chiral assemblies of G-shaped nanostructures made of gold. The arrangement of the G shapes is crucial since upon reordering them the SHG-CD effect disappears. Microscopy reveals SHG "hotspots" assemblies, which originate in enantiomerically sensitive plasmon modes, having the novel property of exhibiting a chiral geometry themselves in relation with the handedness of the material. These results open new frontiers in studying chirality.

Strong enhancement of nonlinear optical properties through supramolecular chirality

A new approach to second-order nonlinear optical (NLO) materials is reported, in which chirality and supramolecular organization play key roles. Langmuir-Blodgett films of a chiral helicene are composed of supramolecular arrays of the molecules. The chiral supramolecular organization makes the second-order NLO susceptibility about 30 times larger for the nonracemic material than for the racemic material with the same chemical structure. The susceptibility of the nonracemic films is a respectable 50 picometers per volt, even though the helicene structure lacks features commonly associated with high nonlinearity. Susceptibility components that are allowed only by chirality dominate the second-order NLO response.

Exceptionally thermally stable polyimides for second-order nonlinear optical applications.

The thermal stability of the electric field induced poled order in a new class of second-order optically nonlinear polymers, "donor-imbedded" side-chain polyimides containing no flexible connectors or tethers to the nonlinear optical (NLO) chromophore, is investigated. In these polymers, the electron-donor part of the chromophore is a diaryl-substituted amine that is incorporated as a part of the polymer backbone. The donorimbedded systems used in this study have exceptional chemical stabilities at elevated temperatures (350 degrees C) and impressive poled order stability at extremely high temperatures (300 degrees C). In both respects, they were significantly more stable than a true side-chain polyimide with a similar NLO-active chromophore covalently linked to the polymer backbone by a flexible tether group.

Nonlinear optical properties of proteins measured by hyper-rayleigh scattering in solution.

Hyper-Rayleigh scattering has been used to determine the nonlinear optical properties of a chromophore-containing protein in solution. Because the technique of hyper-Rayleigh scattering allows the measurement of hyperpolarizabilities in an isotropic solution without the application of an electric field, this method is ideally suited for the study of proteins that carry a net charge. The observed orientational correlation between the nonlinear optical chromophores in incompletely solubilized protein molecules suggests that guidelines from protein structures can be used for the engineering of supramolecular structures with high optical nonlinearity.

Determination of the hyperpolarizability of an octopolar molecular ion by hyper-Rayleigh scattering.

The first observation to our knowledge of a second-order nonlinear optical effect in a nonpolar molecular ion is reported. Hyper-Rayleigh scattering was observed from aqueous and organic solutions of the tricyanomethanide ion [C(CN)(3)](-). Because of the absence of a dipole moment and the presence of a charge in this planar, nonpolar, but highly polarizable molecular anion with D(3h) symmetry, no electric-field-induced second-harmonic-generation experiments are possible. The experimental values for the first hyperpolarizability beta(xxx) [(7 +/- 1.5) x 10(-30) esu, independent of the solvent] are in good agreement with the results from quantum-chemical calculations.