Chiral Poly(fluorene-alt-benzothiadiazole) (PFBT) and Nanocomposites with Gold Nanoparticles: Plasmonically and Structurally Enhanced Chirality

DNA induced intense plasmonic circular dichroism of highly purified gold nanobipyramids

We report a strong and reversible CD response through the assembly of helical DNA and Au nanobipyramids (Au NBPs). Compared with common spherical Au nanoparticles or anisotropic Au nanorods, highly purified Au NBPs possess a more intense electromagnetic field and improved surface plasmon resonance. Thus, the assembly of DNA and Au NBPs exhibits an obviously enhanced plasmonic CD response. When the plasmonic CD is used for DNA detection, it shows high sensitivity and good reproducibility.

Manipulating nanoscale interactions in a polymer nanocomposite for chiral control of linear and nonlinear optical functions

The design, synthesis, and supramolecular organization of a nanocomposite in which nanoscale excitonic interactions between quantum dots and the chiral polymer dramatically enhance the optical activity is reported. This material is highly suitable for application in the emerging field of chiral photonics.

Emerging chirality in nanoscience

Chirality in nanoscience may offer new opportunities for applications beyond the traditional fields of chirality, such as the asymmetric catalysts in the molecular world and the chiral propellers in the macroscopic world. In the last two decades, there has been an amazing array of chiral nanostructures reported in the literature. This review aims to explore and categorize the common mechanisms underlying these systems. We start by analyzing the origin of chirality in simple systems such as the helical spring and hair vortex. Then, the chiral nanostructures in the literature were categorized according to their material composition and underlying mechanism. Special attention is paid to highlight systems with original discoveries, exceptional structural characteristics, or unique mechanisms.

Wafer scale fabrication of porous three-dimensional plasmonic metamaterials for the visible region: chiral and beyond

We report on a wafer scale fabrication method of a three-dimensional plasmonic metamaterial with strong chiroptical response in the visible region of the electromagnetic spectrum. The system was comprised of metallic nanoparticles arranged in a helical fashion, with high degree of flexibility over the choice of the underlying material, as well as their geometrical parameters. This resulted in exquisite control over the chiroptical properties, most importantly the spectral signature of the circular dichroism. In spite of the large variability in the arrangement, as well as the size and shape of the constituent nanoparticles, the average chiro-optical response of the material remained uniform across the wafer, thus confirming the suitability of this system as a large area chiral metamaterial. By simply heating the substrate for a few minutes, the geometrical properties of the nanoparticles could be altered, thus providing an additional handle towards tailoring the spectral response of this novel material.

Plasmonic Core-Satellites Nanostructures with High Chirality and Bioproperty

In this Letter, gold nanorods (Au NRs) and gold nanoparticles (Au NPs) were assembled into core-satellites (Au NR-NPs) nanostructures using DNA as the linkers. Circular dichroism (CD) measurements of the nanoassemblies displayed two plasmonic CD (PCD) peaks in the vicinity of the surface plasmon resonance (SPR). Interestingly, the number of Au NPs in the assemblies had a significant influence on the shape and intensity of the CD line. The assemblies were enzymatically disassembled by deoxyribonuclease I (DNase I), and the CD responses were simultaneously reversed. With the proof-of-concept design, the PCD response was no change by addding enzyme inhibitor. These experiments suggested that the chirality depended upon the structure of core-satellites nanoassemblies, and the results clarify that the possible origin of the optical activity comes from chiral arrangement of building blocks and Coulomb dipole-dipole interactions. This research also illuminated that the assemblies can be used to develop a new sensor for the sensitive screening of the enzyme inhibitors.

Self-assembly of chiral nanoparticle pyramids with strong R/S optical activity

Chirality at the nanometer scale represents one of the most rapidly developing areas of research. Self-assembly of DNA-nanoparticle (NP) hybrids enables geometrically precise assembly of chiral isomers. The concept of a discrete chiral nanostructure of tetrahedral shape and topology fabricated from four different NPs located in the corners of the pyramid is fundamental to the field. While the first observations of optical activity of mixed pyramidal assemblies were made in 2009 (Chen, W.; Nano Lett. 2009, 9, 2153-2159), further studies are difficult without finely resolved optical data for precisely organized NP pyramidal enantiomers. Here we describe the preparation of a family of self-assembled chiral pyramids made from multiple metal and/or semiconductor NPs with a yield as high as 80%. Purposefully made R- and S-enantiomers of chiral pyramids with four different NPs from three different materials displayed strong chiroptical activity, with anisotropy g-factors as high as 1.9 × 10(-2) in the visible spectral range. Importantly, all NP constituents contribute to the chiroptical activity of the R/S pyramids. We were able to observe three different circular dichroism signals in the range of 350-550 nm simultaneously. They correspond to the plasmonic oscillations of gold, silver, and bandgap transitions of quantum dots. Tunability of chiroptical bands related to these transitions is essential from fundamental and practical points of view. The predictability of optical properties of pyramids, the simplicity of their self-assembly in comparison with lithography, and the possibility for polymerase chain reaction-based automation of their synthesis are expected to facilitate their future applications.

Antibonding plasmon modes in colloidal gold nanorod clusters

The optical response of nanoplasmonic colloids in disperse phase is strictly related to their shape. However, upon self-assembly, new optical features, for example, bonding or antibonding modes, emerge as a result of the mutual orientations of nanoparticles. The geometry of the final assemblies often determines which mode is dominating in the overall optical response. These new plasmon modes, however, are mostly observed in silico, as self-assembly in the liquid phase leads to cluster formation with a broad range of particle units. Here we show that low-symmetry clustering of gold nanorods (AuNRs) in solution can also reveal antibonding modes. We found that UV-light irradiation of colloidal dispersions of AuNRs in N-methyl-2-pyrrolidone (NMP), stabilized by poly(vinylpyrrolidone) (PVP) results in the creation of AuNRs clusters with ladderlike morphology, where antibonding modes can be identified. We propose that UV irradiation induces formation of radicals in solvent molecules, which then promote cross-linking of PVP chains on the surface of adjacent particles. This picture opens up a number of relevant questions in nanoscience and is expected to find application in light induced self-assembly of particles with various compositions and morphologies.

First enantioseparation and circular dichroism spectra of Au38 clusters protected by achiral ligands

Bestowing chirality to metals is central in fields such as heterogeneous catalysis and modern optics. Although the bulk phase of metals is symmetric, their surfaces can become chiral through adsorption of molecules. Interestingly, even achiral molecules can lead to locally chiral, though globally racemic, surfaces. A similar situation can be obtained for metal particles or clusters. Here we report the first separation of the enantiomers of a gold cluster protected by achiral thiolates, Au(38)(SCH(2)CH(2)Ph)(24), achieved by chiral high-performance liquid chromatography. The chirality of the nanocluster arises from the chiral arrangement of the thiolates on its surface, forming 'staple motifs'. The enantiomers show mirror-image circular dichroism responses and large anisotropy factors of up to 4×10(-3). Comparison with reported circular dichroism spectra of other Au(38) clusters reveals that the influence of the ligand on the chiroptical properties is minor.

Manipulation of collective optical activity in one-dimensional plasmonic assembly

The manipulation of the chirality and corresponding optical activity in the visible-near-infrared (NIR) light region is significant to realize applications in the fields of chemical sensing, enantioselective separation, chiral nanocatalysis, and optical devices. We studied the plasmon-induced circular dichroism (CD) response by one-dimensional (1D) assembly of cysteine (CYS) and gold nanorods (GNRs). Typically, GNRs can form end-to-end assembly through the electrostatic attraction of CYS molecules preferentially attached on the ends of different GNRs. CD responses are observed at both the UV and visible-NIR light region in the 1D assembly, which are assigned to the CYS molecules and the GNRs, respectively. In addition, the wavelength of the CD responses can be manipulated from 550 nm to more than 900 nm through altering the aspect ratios of GNRs in 1D assembly. Anisotropic enhancement of optical activity is discovered, suggesting that the enhancement of the longitudinal localized surface plasmon resonance (LSPR) peak of GNRs in the CD response is much more apparent than that of the transverse LSPR. The CD responses of individual CYS-attached GNRs and CYS-assembled gold nanoparticles (GNPs) substantiate that the form of assembly and the shape of building blocks are significant not only for the intensity but for the line shape of the CD signals.

What makes single-helical metamaterials generate "pure" circularly polarized light?

Circular polarizers with left-handed helical metamaterials can transmit right-handed circularly polarized (RCP) light with few losses. But a certain amount of left-handed circularly polarized (LCP) light will occur in the transmitted light, which is the noise of the circular polarizer. Therefore, we defined the ratio of the RCP light intensity to the LCP light intensity as the signal-to-noise (S/N) ratio. In our previous work, it's found that circular polarizers with multi-helical metamaterials have two orders higher S/N ratios than that of single-helical metamaterials. However, it has been a great challenge to fabricate such multi-helical structures with micron or sub-micron feature sizes. Is it possible for the single-helical metamaterials to obtain equally high S/N ratios as the multi-helical ones? To answer this question, we systematically investigated the influences of structure parameters of single-helical metamaterials on the S/N ratios using the finite-different time-domain (FDTD) method. It was found that the single-helical metamaterials can also reach about 30dB S/N ratios, which are equal to the multi-helical ones. Furthermore, we explained the phenomenon by the antenna theory and optimized the performances of the single-helical circular polarizers.

Superior optical limiting, stabilization, and spatio-temporal reshaping of ultrashort laser pulses in an opto-stable intrinsic polymer film

A chemically modified poly(fluorene-alt-benzothiadiazole) (PFBT) polymer film is reported to exhibit high two-photon absorbing capability and chemical/physical stability upon the action of high-power laser pulses of ~780 nm wavelength and ~160 fs duration. A nonlinear transmission measurement is conducted by varying the input intensity from ~20 to ~600 GW/cm2, the corresponding nonlinear transmission of a ~70 μm thick film is reduced from ~0.8 to 0.18, indicating a superior optical limiting behavior. In the meantime, intensity fluctuation of laser pulses can be significantly reduced after passing through the same film sample. Based on the intensity-dependent nonlinear attenuation mechanism, a straightforward optical reshaping effect on spatio-temporal profiles of the laser pulses has also been demonstrated.

Plasmon-induced CD response of oligonucleotide-conjugated metal nanoparticles

Non-chiral metal nanoparticles conjugated with chiral oligonucleotide molecules demonstrated a circular dichroism (CD) at the plasmonic wavelengths due to aggregation effects.

Higher extinction ratio circular polarizers with hetero-structured double-helical metamaterials

We have recently shown circular polarizers with the homo-structured double-helical metamaterials, which have broader operation bands than those of the single-helical structures [Opt. Lett. 35, 2588 (2010)]. However, trying to get more operation bands deteriorates the extinction ratio. In this paper, we proposed circular polarizers with hetero-structured double-helical metamaterials. The extinction ratios of these circular polarizers are two times higher than those with homo-structured double-helical metamaterials. Furthermore, we qualitatively explained the phenomenon of the higher extinction ratio from the viewpoint of the interaction between the two helix-wires in a double-helical unit.

Fingers Crossed: Optical Activity of a Chiral Dimer of Plasmonic Nanorods

We investigate theoretically the optical activity of a dimer of plasmonic nanoantennas, mimicking the geometry of a molecule with two isolated chromophores, a situation commonly described as exciton coupling in organic chemistry. As the scale of the system increases and approaches the wavelength of visible light, a rich variety of effects arise that are unique to the plasmonic case. Scattering of light by the particles, negligible in very small clusters, strongly perturbs, and eventually dominates, the optical activity. Additionally, retardation effects in dimers with an interparticle separation commensurate with the wavelength of the incident light affect the electromagnetic coupling between the particles and lead to an asymmetric circular dichroism spectrum. We identify conditions for efficient interaction and predict remarkably large anisotropy factors.