Plasmon-induced Circular Dichroism Research Articles

Chiral plasmonic Au–Ag core shell nanobipyramid for SERS enantiomeric discrimination of biologically relevant small molecules

The identification of enantiomers is of great importance in chiral separations and medicinal chemistry. While Surface-enhanced Raman spectroscopy (SERS) is a technique that provides vibrational fingerprints of analytes. The enantiomers identification relies on the SERS difference between left and right-handed circularly polarized light or additional selectors for indirect distinction. In this work, Au–Ag core shell nanobipyramid (L/D-Au@Ag BPs) were synthesized guiding by chiral encoder of L/D-cysteine. L/D-Au@Ag BPs produced plasmon-induced circular dichroism signals in the plasmon resonance absorption band, which can be tuned by modulation the amount of cysteine. Moreover, the chiral anisotropy factor of L/D-Au@Ag BPs at 532 nm can reach 5.11 × 10−3. Due to the selective resonance coupling between L/D-Au@Ag BPs and different enantiomers, L/D-Au@Ag BPs were further used as SERS substrates for efficient discrimination of biologically relevant small molecules. Chiral Au@Ag BPs display the potential for chiral drug identification.

Unraveling the Complex Chirality Evolution in DNA‐Assembled High‐Order, Hybrid Chiroplasmonic Superstructures from Multi‐Scale Chirality Mechanisms

AbstractHierarchical, chiral hybrid superstructures of chromophores and nanoparticles are expected to give rise to intriguing unveiled chiroptical responses originating from the complex chiral interactions among the components. Herein, DNA origami cavity that could self‐assemble into one‐dimensional (1D) DNA tubes was employed as a scaffold to accurately organize metal nanoparticles and chromophores. The chiral interactions were studied at the level of individual hybrid particles and their 1D hybrid superstructures. Complex chirality mechanisms involving global structural chirality, plasmon‐induced circular dichroism (PICD) and exciton‐coupled circular dichroism (ECCD) were disentangled. The multiplexed CD spectrum superposition revealed the chirality evolution at different length scales. These results can offer a model for boosting the theoretical understanding of classical‐quantum hybrid systems, and would inspire the future design of optically‐active substances across length scales.

Unraveling the Complex Chirality Evolution in DNA-Assembled High-Order, Hybrid Chiroplasmonic Superstructures from Multi-Scale Chirality Mechanisms.

Hierarchical, chiral hybrid superstructures of chromophores and nanoparticles are expected to give rise to intriguing unveiled chiroptical responses originating from the complex chiral interactions among the components. Herein, DNA origami cavity that could self-assemble into one-dimensional (1D) DNA tubes was employed as a scaffold to accurately organize metal nanoparticles and chromophores. The chiral interactions were studied at the level of individual hybrid particles and their 1D hybrid superstructures. Complex chirality mechanisms involving global structural chirality, plasmon-induced circular dichroism (PICD) and exciton-coupled circular dichroism (ECCD) were disentangled. The multiplexed CD spectrum superposition revealed the chirality evolution at different length scales. These results can offer a model for boosting the theoretical understanding of classical-quantum hybrid systems, and would inspire the future design of optically-active substances across length scales.

Plasmon-induced Circular Dichroism and Asymmetric Hot-electrons Generation Triggered by a Chiral Molecule for Polarization-dependent Chemical Reactions

Performing chiral photodetection, photocatalysis or photochemical reactions at the molecular level has always been a nearly impossible task, due to the very low efficiency of the generated optical circular dichroism signals. On the contrary, chiral colloidal nanocrystals have been shown recently to offer a very large differential response to circularly polarized light. Such a response is able to generate hot-electrons with a very strong asymmetry, thus potentially able to perform the aforementioned tasks. In this paper, an intermediate picture is chosen, for which an achiral small assembly of identical particles triggered by a chiral molecule is able to generate large plasmon-induced circular dichroism (PICD), in turn able to generate the required asymmetry in the generation rates of hot-electrons. By performing Finite Difference Time Domain simulations based on the combination of a classical model of PICD generation and a quantum-based model of hot-electrons generation, the simple design of an achiral gold NPs’ dimer triggered by a chiral molecule located in the center and oriented with its transition electric dipole moment parallel to the dimer axis is shown to be able to generate a strong asymmetry in its HEs’ generation response. The PICDs and related hot-electrons generation rates increase as a function of volume, surface, respectively, of the considered systems, thereby providing a way to trigger chemical reactions.

Photothermal Circular Dichroism Induced by Plasmon Resonances in Chiral Metamaterial Absorbers and Bolometers.

Chiral photochemistry remains a challenge because of the very small asymmetry in the chiro-optical absorption of molecular species. However, we think that the rapidly developing fields of plasmonic chirality and plasmon-induced circular dichroism demonstrate very strong chiro-optical effects and have the potential to facilitate the development of chiral photochemistry and other related applications such as chiral separation and sensing. In this study, we propose a new type of chiral spectroscopy-photothermal circular dichroism. It is already known that the planar plasmonic superabsorbers can be designed to exhibit giant circular dichroism signals in the reflection. Therefore, upon illumination with chiral light, such planar metastructures should be able to generate a prominent asymmetry in their local temperatures. Indeed, we demonstrate this chiral photothermal effect using a chiral plasmonic absorber. Calculated temperature maps show very strong photothermal circular dichroism. One of the structures computed in this Letter could serve as a chiral bolometer sensitive to circularly polarized light. Overall, this chiro-optical effect in plasmonic metamaterials is much greater than the equivalent effect in any chiral molecular system or plasmonic bioassembly. Potential applications of this effect are in polarization-sensitive surface photochemistry and chiral bolometers.

Induced chirality in micron wave through electromagnetic coupling between chiral molecules and graphene nanostructures

The intricate mechanisms of plasmon-induced circular dichroism (CD) in a visible region are considered based on chemical and physical influences, in which a simple model and formulations are required. Here, we demonstrate theoretically that plasmon-induced CD is approximately contributed by the cross-interaction between equivalent electric and magnetic dipole moments for chiral molecules and plasmon nanostructures. To prove electromagnetic couplings, we introduce graphene into plasmon nanostructures and design asymmetrically inscribed graphene dual-rings arrays (IGDAs) with high-order hybrid modes. Results show that ultrahigh-order plasmon-induced CD signals are achieved in micron wave, which is easily detected by mature microwave technology. The maximum enhancement factor of induced CD could reach up to four orders of magnitude. In addition, an induced CD signal could be tuned only by varying the Fermi energy of IGDAs rather than by varying geometric dimensions. For different molecules, the electromagnetic couplings still hold. The results could be used to dynamically design chiral sensors in biology and chemistry.

(Invited) Surface Plasmon Resonance Sensors for Biomolecular Chirality

Surface Plasmon Resonance Sensors for Biomolecular Chirality Mingzhao Liu, Fang Lu, Ye Tian, Dong Su, and Oleg Gang A chiral object, i.e., one that is distinguishable from its mirror image, interacts differently with photons of left-hand (+) and right-hand (-) circular polarizations. The difference in the absorption part is known as circular dichroism (CD), which is usually observed for biomolecules with one or more chiral centers and is widely used to probe their molecular stereometry. Such probing has an increasingly broad importance for biomedical and pharmacological fields due to synthesis/separation/detection of homochiral species, biological role of chiral organization, and structural response to environmental conditions and enantiomeric drugs. Recent theoretical and experimental works demonstrated that the CD signal from chiral organic molecules could appear in the plasmonic band when they are coupled with plasmonic particles. This talk will cover an intriguing case in which non-chiral plasmonic nanoparticles are surrounded by a monolayer of DNA molecules that have chirality arising from its individual nucleobases and helix structure. It is found that the surface plasmon-induced CD is very sensitive to the nonchiral plasmonic nanoparticles’ shape and chemical composition. We identify gold/silver core/shell nanocubes as most effective by providing a giant, 2 orders of magnitude CD enhancement in a near-visible region, while DNA’s native CD signal appears at much higher energy. The discovered phenomenon opens novel opportunities in ultrasensitive probing of biomolecules and for novel optical nanomaterials based on the chiral elements.

A giant chiroptical effect caused by the electric quadrupole.

Recently, there has been great interest in studying ultrasensitive detection and characterization of biomolecules using plasmonic particles, because they are of considerable importance in biomedical science and pharmaceutics. So far, all the theories on plasmon-induced circular dichroism (CD) have been based on the dipole approximation; the electric quadrupolar contribution is generally considered to be relatively small and neglected. Here we demonstrate that the electric quadrupolar contribution not only cannot be ignored, but it also plays a key role in many cases. Particularly, for the chiral medium that possesses preferential molecular orientations and is located at the hotspot of plasmonic nanostructures, the plasmonic CD strength contributed by molecular electric quadrupoles (EQs) can be two orders of magnitude higher than that contributed by molecular electric/magnetic dipoles. Unlike the case of the dipole approximation, molecular EQ associated plasmonic CD activity appears mainly at the plasmonic resonance absorptions that facilitate the optically enhanced near-field with steep electric field gradients, and is correlated with the boosted emission rate of a molecular EQ. Based on such physical understandings, we can design nanostructures to realize a giant chiroptical effect using the EQ contribution according to the requirements, which provide a new strategy for ultrasensitive detection and quantification of molecular chirality.

Plasmonically Engineered Nanoprobes for Biomedical Applications.

The localized surface plasmon resonance of metal nanoparticles is the collective oscillation of electrons on particle surface, induced by incident light, and is a particle composition-, morphology-, and coupling-dependent property. Plasmonic engineering deals with highly precise formation of the targeted nanostructures with targeted plasmonic properties (e.g., electromagnetic field distribution and enhancement) via controlled synthetic, assembling, and atomic/molecular tuning strategies. These plasmonically engineered nanoprobes (PENs) have a variety of unique and beneficial physical, chemical, and biological properties, including optical signal enhancement, catalytic, and local temperature-tuning photothermal properties. In particular, for biomedical applications, there are many useful properties from PENs including LSPR-based sensing, surface-enhanced Raman scattering, metal-enhanced fluorescence, dark-field light-scattering, metal-mediated fluorescence resonance energy transfer, photothermal effect, photodynamic effect, photoacoustic effect, and plasmon-induced circular dichroism. These properties can be utilized for the development of new biotechnologies and biosensing, bioimaging, therapeutic, and theranostic applications in medicine. This Perspective introduces the concept of plasmonic engineering in designing and synthesizing PENs for biomedical applications, gives recent examples of biomedically functional PENs, and discusses the issues and future prospects of PENs for practical applications in bioscience, biotechnology, and medicine.

Plasmonic Circular Dichroism of the Helical Nanosphere Assemblies and the Helical Nanoellipsoid Assemblies

We investigate the circular dichroism (CD) of both the helical gold nanosphere assemblies and the helical gold nanoellipsoid assemblies by means of the discrete dipole approximation (DDA). For the helical gold nanosphere assemblies, we find that, with increasing the number of the nanospheres within a helical pitch, the CD spectra of the assemblies are red-shifted and the intensity of the CD signals is strengthened. We also investigate the E-field distributions of the helical nanosphere assemblies. The results show that the strong CD response of the nanosphere assemblies can be ascribed to the collective dipole and multipole interactions between nanoparticles. The CD signals of the solid helical structure exhibit periodical behaviors in half pitch, which is quite different from that of the nanosphere assemblies. The helical nanoellipsoid assemblies exhibit complicated plasmon-induced CD signals at the localized surface plasmon resonance (LSPR) frequency. The strength and wavelength of the CD signals can be manipulated in the range of 525 to 688 nm via tuning the aspect ratio of the nanoellipsoids. These results are important for designing the chiral plasmonic nanostructures with strong and tailorable optical properties.

Experimental Observation of Giant Chiroptical Amplification of Small Chiral Molecules by Gold Nanosphere Clusters

We have experimentally observed around 2 orders of magnitude circular dichroism (CD) enhancement in the visible region for cysteine molecules located in the hotspots of gold nanosphere clusters. The observed plasmon-induced CD responses show a significant correlation with the chiral nature of molecules at the hotspots. These results provide a concrete experimental demonstration on the predicted chiroptical transfer and amplification effect that arises from hotspot-mediated exciton–plasmon interactions in a strongly coupled metallic nanostructure, even though the exciton–plasmon coupling works at a far off-resonant regime. Our findings suggest here that plasmonic hotspot-based CD amplifier may provide a new strategy for ultrasensitive detection and quantification of molecular chirality—a key aspect for various bioscience and biomedicine applications.

Helical silver(I)-glutathione biocoordination polymer nanofibres

Helical nanofibres of silver(I)-glutathione (Ag-GSH) biocoordination polymer (BCP) are fabricated by introducing dimethyl sulfoxide into the mixture solution of Ag⁺ ions and L-GSH molecules. The prepared BCP nanofibres show hierarchical helical structures, which are constructed via twisting of small fibres. Water-soluble helices could be further cross-linked with Ca²⁺ ions to form a well-dispersed aqueous suspension. When gold nanorods are adsorbed onto these helical nanofibres, the unique plasmon-induced circular dichroism characteristic is observed in the region of the local surface plasmon resonance of gold nanorods. This type of chiroptical metamaterials may have promising applications in nonlinear optics, negative refraction and biosensing.

Chiral guanosine 5′-monophosphate-capped gold nanoflowers: Controllable synthesis, characterization, surface-enhanced Raman scattering activity, cellular imaging and photothermal therapy

Plasmonics and chirality in metal nanomaterials are intriguing and inspiring phenomena. Nanoscale chirality of metal nanomaterials has emerged as a hot topic in the past several years. Generally, most plasmon-induced circular dichroism (CD) responses of nanomaterials (> 10 nm) have been artificially created by modifying pre-made achiral nanomaterials with chiral agents, because the in situ generation of plasmon-induced CD responses of nanomaterials with larger size (> 10 nm) is not easy. Herein, we report a simple one-pot green synthesis of chiral gold nanoflowers (GNFs) with abundant petal-shaped tips in the chiral reduction environment arising from the presence of chiral guanosine 5′-monophosphate (5′-GMP) and the chiral reducing agent L-ascorbic acid (L-AA). Different reducing agents can impact the shape and chirality of the products. In addition, the size and chirality of the GNFs can be controlled by adjusting the reaction time. The as-synthesized GNFs have good biocompatibility and can be used for surface-enhanced Raman scattering (SERS) enhancement, cellular dark-field imaging and photothermal therapy. Open image in new window

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.

Plasmon-Induced Circular Dichroism of a Chiral Molecule in the Vicinity of Metal Nanocrystals. Application to Various Geometries

The Coulombic interaction between a chiral molecule and a nonchiral metal nanocrystal creates new plasmonic lines in a circular dichroism (CD) spectrum. In many cases, optical lines in CD spectra of chiral molecules and plasmon absorption bands in metal nanocrystals are not in resonance. Typically, CD lines of chiral molecules are in the UV spectral range, whereas plasmon bands of metal nanocrystals are in the visible range. Nevertheless, plasmon excitations can strongly interact with dipoles of chiral molecules and become chiral. We demonstrate this effect theoretically and derive a general equation for the CD spectrum of a complex composed of metal nanocrystals and a single chiral molecule. In our theory, new plasmon peaks in the CD spectra come from an effect of interference between external and induced fields inside a hybrid complex. Our models involve single spherical nanoparticles, nanoparticle pairs, and nanoshells. The results obtained here can be used to design new optical materials and hybrid na...

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.

Theory of Circular Dichroism of Nanomaterials Comprising Chiral Molecules and Nanocrystals: Plasmon Enhancement, Dipole Interactions, and Dielectric Effects

Our calculations show that a nonchiral nanocrystal is able to dramatically change the circular dichroism (CD) of a chiral molecule when the nanocrystal and molecule form a complex and couple via dipole and multipole Coulomb interactions. Plasmon resonances of metal nanocrystals in the nanocrystal-molecule complex result in both the resonant enhancement of CD signals of molecules and the appearance of new spectral structures. Two mechanisms, in which a nanocrystal can influence the CD effect, have been identified. The first mechanism is the plasmon-induced change in the electromagnetic field inside the chiral molecule. The second is the optical absorption of the nanocrystal-molecule complex due to the chiral currents inside the metal nanocrystal induced by the dipole of the chiral molecule. The first mechanism creates a change in the angle between the effective electric and magnetic dipoles of the molecule. This mechanism can lead to symmetry breaking and to a plasmon-induced CD signal of the nonchiral molecule. Both mechanisms create interesting Fano-like shapes in the CD spectra. Importantly, the second mechanism gives the main contribution to the CD signal at the plasmon frequency when the absorption band of the chiral molecule is far from the plasmon resonance. This may happen in many cases since many biomolecules are optically active in the UV range, whereas plasmon resonances in commonly used nanometals are found at longer wavelengths. As concrete examples, the paper describes alpha-helix and calixarene ligand molecules coupled with metal nanocrystals. The above results are also applied to complexes incorporating semiconductor nanocrystals. The results obtained here can be used to design a variety of hybrid nanostructures with enhanced and tailored optical chirality in the visible wavelength range.