Chiral Spectroscopy of Nanostructures

Deep tissue sensing of chiral molecules using polarization-enhanced photoacoustics

Chiral molecule sensing is typically performed using techniques like chromatography, electrophoresis, enzymatic assays, mass spectrometry, and chiroptical methods. While polarimetry allows for in vivo sensing up to 1 mm depth using ultraviolet-visible light, it is limited by dominant light scattering beyond this depth. We propose that photoacoustic sensing in the near-infrared II (NIR-II) window can enable deep tissue sensing as acoustic waves scatter less than light. To achieve this, we developed a photoacoustic polarization-enhanced optical rotation sensing (PAPEORS) system, capable of estimating optical rotation from photoacoustic signals and correlating it with chiral molecular concentration for depths up to 3.5 mm. Experiments were conducted using aqueous glucose solutions, naproxen, serum-based glucose samples, and ex vivo chicken tissue. PAPEORS achieved a detection limit of 80 mg/dl while using circularly polarized light with serum samples, demonstrating the potential for deep-tissue chiral molecular sensing. PAPEORS holds promise for in vivo sensing and easy miniaturization using single wavelength.

Chiral plasmonic superlattices from template-assisted assembly of achiral nanoparticles

The creation of chiral plasmonic architectures combining templates with achiral plasmonic particles leads to strong chiroptical responses that can be finely tuned via the characteristics of the colloidal building blocks. Here we show how elastomeric molds, pre-patterned with a hexagonal array of triskelia motifs, can guide the assembly of ordinary noble metal colloids into chiral plasmonic architectures with strong dichroism values. Under normal incidence, the chiral arrays made with gold and silver colloids showed g-factors of 0.18 and 0.4, respectively. In all cases, increasing the size of the colloid allows tuning the optical properties of the structure in the VIS-NIR range. When a superstrate layer is deposited onto the structures, the extrinsic chirality response of the 2D superlattice is revealed and strongly amplified by the chiral motifs under oblique inspection, leading to g-factors of ± 1.2 at ± 14°. Finally, these chiral plasmonic resonances sustained by the triskelion array are used to produce circularly polarized photoluminescence from achiral organic dyes placed on top with up to 20% of dissymmetry.

White circularly polarized luminescence from a dual-component emitter induced by FRET between tetraphenylene and PDI derivatives

A chiral agent, TPE-ASP, incorporating aspartic acid as the chiral source and tetraphenylene derivatives as chromophores, was designed and synthesized. The chiral agent was self-assembled into regular spherical nanoparticles with a maximum luminescence asymmetry factor of |2.41 × 10-2| at 460 nm which is attributed to TPE-ASP. These nanoparticles can be co-assembled with a perylenediimide (PDI) derivative through electrostatic interactions, enabling the successful construction of a chiral light-harvesting system (C-LHS). The maximum Förster resonance energy transfer (FRET) efficiency (ΦET) of 94.7% was achieved at the optimal molar ratio of TPE-ASP to PDI. Fortunately, multicolour circularly polarized luminescence (CPL), spanning from blue to red, was successfully achieved with a two-component co-assembly system, and bright white CPL with CIE coordinates of (0.33, 0.32) was also obtained. Meanwhile, the average glum is |7.1 × 10-3| in the wavelength range of 400-700 nm. This discovery demonstrates the potential for spectral regulation through the two-component co-assembly strategy. It is significant for developing CPL devices with white light emission via the FRET process and also expands the functional range of chiral light-harvesting systems.

Circularly Polarized Organic Light-Emitting Diode Based on Device Functional Layer Materials

Circularly polarized light-emitting devices have found extensive application prospects in 3D displays and optoelectronic information. Among them, circularly polarized organic light-emitting diodes (CP-OLED), as a rising star of circularly polarized light-emitting devices, achieved good research results. However, the preparation of CP-OLED with a high electroluminescence asymmetry factor and high external quantum efficiency is a hot and difficult research topic. At present, the approaches for achieving circularly polarized electroluminescence via CP-OLED are: 1) Using chiral materials as luminescent materials, 2) Utilizing chiral functional materials. This review summarizes recent methodologies used for manufacturing CP-OLED. It focuses on the construction strategies and applications of chiral functional materials (chiral host materials, chiral hole transport materials, and chiral electron transport materials) in CP-OLED. While challenges such as complex chiral design and material interactions persist, advancements in material design and device architecture propel CP-OLED forward. These developments promise to elevate CP-OLED as a focal point in optoelectronic research, facilitating high-performance, circularly polarized luminescence (CPL)-capable devices for practical applications.

Patternable chiral Au nanocrystal-doped composite films for information encryption: the role of optical rotation

Optical information encryption technology has garnered significant attention in currency security, information protection, and personal identification. While optical metasurfaces are considered ideal platforms for information encryption, their high cost and time-intensive fabrication processes have limited their widespread applications. To address this, emergent chiroptical nanomaterials offer new opportunities for information encryption through their polarization capabilities. In this study, composite films consisting of chiral Au nanocrystals embedded in curable polymers are utilized as a patternable platform for information encryption. Theoretical simulations demonstrate that chiral Au nanocrystals can rotate linearly polarized light of different wavelengths in various directions. Notably, Au nanocrystals with opposite chirality show reversed optical rotation effects for linearly polarized light while exhibiting the same extinction properties for non-polarized light. Based on these investigations, patternable composite films with embedded chiral Au nanocrystals are fabricated, showcasing their potential to encode information via optical rotation. This work establishes the feasibility of chiral Au nanocrystals as a patternable platform for information encryption and presents a simple, convenient, and cost-effective approach for optical information security.

Nanoscale chirality generated in zinc(II) orthophosphate clusters: evidence by vibrational circular dichroism

Layered zinc(II) hydroxides (LZH) intercalating the deprotonated forms of R-(-) or S-(+)-1,1'-binaphthyl-2,2'-diyl hydrogenphosphate (denoted as R- or S-BNDHPH, respectively) were prepared from Zn(NO3)2 at pH 5 and 60 °C by the mixing method. The obtained hybrid compounds (denoted as R- or S-BNDHP-/LZH, respectively) were heated from room temperature up to 800 °C under nitrogen atmosphere. According to the thermal gravimetric/differential thermal analysis measurements, hydroxyl groups were dehydrated at 270-400 °C, followed by the decomposition of organic components at 420-600 °C. X-ray diffraction patterns and scanning electron microscopy images indicated that the final products were a mixture of α-Zn3(PO4)2, ZnO crystals and non-crystalline zinc(II) orthophosphates. Vibrational circular dichroism (VCD) spectra were recorded before and after calcination. Before calcination, R- or S-BNDHP-/LZH exhibited VCD peaks assigned to intercalated R- or S-BNDHP-. The calcined products exhibited several VCD peaks in the range of 900-1200 cm-1, maintaining the mirror-image relationship between R-BNDHP-/LZH and S-BNDHP-/LZH used as starting materials. The observed peaks were assigned to the PO (symmetric), -POO-, and PO (asymmetric) stretching vibrations of the PO43- groups. According to theoretical simulations, the observed VCD activity can be rationalised in terms of vibrational coupling between two PO43- groups in a generated chiral zinc(II) orthophosphate cluster.

Fluorescence and Circular Dichroism Dual-Mode Probe for Chiral Recognition of Tyrosine and Its Applications in Bioimaging

Chiral amino acids (AAs) are essential in metabolism and understanding physiological processes, and they could be used as biomarkers for the diagnosis of different diseases. In this study, chiral Cdots@Van were prepared by postmodifying an achiral Cdots core with vancomycin for recognizing and determining the enantiomeric excess (ee) of tyrosine (Tyr) enantiomers. The fluorescence response of Cdots@Van is based on an "on-off" strategy, with different quenching percentages for d- and l-tyrosine. Interestingly, the circular dichroism (CD) spectrum of Cdots@Van responded to only one form of Tyr enantiomer, specifically d-Tyr, and remained nearly unchanged upon the addition of l-Tyr. Quantum mechanical (QM) calculations were in excellent agreement with the experimental results, confirming the stronger binding affinity of Cdots@Van for d-Tyr compared to l-Tyr. We further investigated the chiral recognition ability of the interconnected vancomycin particles, which was synthesized using the EDC/NHS coupling reaction between vancomycin molecules without a Cdots core. Surprisingly, unlike free vancomycin molecules, interconnected vancomycin displayed an enantiomeric recognition ability by CD spectroscopy, similar to what was observed for Cdots@Van. Crucially, this chiral probe has been successfully utilized for cell imaging applications.

Strong NIR-II Magneto-Optical Activity of a Chiral Sm15Cu54 Cage

The magneto-optical response of chiral materials holds significant potential for applications in physics, chemistry, and biology. However, exploration of the near-infrared (NIR) magneto-optical response remains limited. Herein, we report the synthesis and strong NIR-II magneto-optical activity of three pairs of chiral 3d-4f clusters of R/S-Ln15Cu54 (Ln = Sm, Gd, and Dy). Structural analysis reveals that R/S-Ln15Cu54 features a triangular prism cage with C3 symmetry. Interestingly, magnetic circular dichroism (MCD) spectra exhibit remarkable magneto-optical response in the NIR-II region, driven by the f-f transition. The maximum g-factor of R/S-Sm15Cu54 reaches 5.5 × 10-3 T-1 around 1300-1450 nm, surpassing values associated with DyIII and CuII ions. This remarkable NIR-II magneto-optical activity may be attributed to strong magnetic-dipole-allowed f-f transitions and helix chirality of the structure. This work not only presents the largest Ln-Cu clusters to date but also demonstrate the key role of magnetic-dipole-allowed transitions on magneto-optical activity.

Strong coupling of an epsilon-near-zero mode to a chiral plasmon

The reconfigurable chiroptical effect is highly desirable for spin photonics, chiral spectroscopy, and photocatalysis due to its merits for dynamic and broadband applications. The coupling of an epsilon-near-zero (ENZ) mode to a chiral plasmon is expected to enable active and effective manipulation of the chiroptical effect but remains unexplored. Here we, for the first time to our knowledge, propose and demonstrate the strong coupling of an ENZ mode to a chiral plasmon by using a hybrid system composed of two identical vertically placed gold nanorods and an in-between ENZ film. An analytical three-oscillator model combined with numerical simulations is established to study the coupling mechanism, which predicts a Rabi splitting up to 240 meV with an ENZ film thickness of 60 nm in circular dichroism.

Chiral 3D structures through multi-dimensional transfer printing of multilayer quantum dot patterns

Three-dimensional optical nanostructures have garnered significant interest in photonics due to their extraordinary capabilities to manipulate the amplitude, phase, and polarization states of light. However, achieving complex three-dimensional optical nanostructures with bottom-up fabrication has remained challenging, despite its nanoscale precision and cost-effectiveness, mainly due to inherent limitations in structural controllability. Here, we report the optical characteristics of intricate two- and three-dimensional nanoarchitectures made of colloidal quantum dots fabricated with multi-dimensional transfer printing. Our customizable fabrication platform, directed by tailored interface polarity, enables flexible geometric control over a variety of one-, two-, and three-dimensional quantum dot architectures, achieving tunable and advanced optical features. For example, we demonstrate a two-dimensional quantum dot nanomesh with tuned subwavelength square perforations designed by finite-difference time-domain calculations, achieving an 8-fold enhanced photoluminescence due to the maximized optical resonance. Furthermore, a three-dimensional quantum dot chiral structure is also created via asymmetric stacking of one-dimensional quantum dot layers, realizing a pronounced circular dichroism intensity exceeding 20°.

Resolving plasmon-mediated high-order multiphoton excitation pathways in dolmen nanostructures using ultrafast nonlinear optical interferometry

The multiphoton excitation pathways of plasmonic nanorod assemblies are described. By using dolmen structures formed from the directed assembly of three gold nanorods, plasmon-mediated three-photon excitation is resolved. These high-order multiphoton excitation channels were accessed by resonantly exciting a hybrid mode of the dolmen structure that was resonant with the 800-nm carrier wavelength of an ultrafast laser system. Rotation of the exciting field polarization to a non-resonant configuration did not generate third-order responses. Hence, the multiphoton excitation and resultant non-equilibrium electron distributions were generated by structure- and mode-selective excitation. Correlation between high-order and resonant plasmon excitation was achieved through sub-cycle time-resolved interferometric detection of incoherent nonlinear emission signals. The results illustrate the advantages of nonlinear optical interferometry and Fourier analysis for distinguishing plasmon-mediated processes from those that do not require plasmon excitation.

Nanoparticle-Empowered Core-Shell Microcapsules: From Architecture Design to Fabrication and Functions

Compartmentalization is a powerful concept to integrate multiscale components with diverse functionalities into miniature architectures. Inspired by evolution-optimized cell compartments, synthetic core-shell capsules enable storage of actives and on-demand delivery of programmed functions, driving scientific progress across various fields including adaptive materials, sustainable electronics, soft robotics, and precision medicine. To simultaneously maximize structural stability and environmental sensitivity, which are the two most critical characteristics dictating performance, diverse nanoparticles are incorporated into microcapsules with a dense shell and a liquid core. Recent studies have revealed that these nano-additives not only enhance the intrinsic properties of capsules including mechanical robustness, optical behaviors, and thermal conductivity, but also empower dynamic features such as triggered release, deformable structures, and fueled mobility. In this review, the physicochemical principles that govern nanoparticle assembly during microencapsulation are examined in detail and the architecture-controlled functionalities are outlined. Through the analysis of how each primary method implants nanoparticles into microcapsules, their distinct spatial organizations within the core-shell structures are highlighted. Following a detailed discussion of the specialized functions enabled by specific nanoparticles, the vision of the required fundamental insights and experimental studies for this class of microcarriers to fulfill its potential are sketched.

Circularly Polarized Room Temperature Phosphorescence through Twisting-Induced Helical Structures from Polyvinyl Alcohol-Based Fibers Containing Hydrogen-Bonded Dyes

Room temperature phosphorescence (RTP) materials have garnered significant attention owing to its distinctive optical characteristics and broad range of potential applications. However, the challenge remains in producing RTP materials with more simplicity, versatility, and practicality on a large scale, particularly in achieving chiral signals within a single system. Herein, we show that a straightforward and effective combination of wet spinning and twisting technique enables continuously fabricating RTP fibers with twisting-induced helical chirality. By leveraging the hydrogen bonding interactions between polyvinyl alcohol (PVA) and quinoline derivatives, along with the rigid microenvironment provided by PVA chains, typically, Q-NH2@PVA fiber demonstrates outstanding phosphorescent characteristics with RTP lifetime of 1.08 s and phosphorescence quantum yield of 24.6 %, and the improved tensile strength being 1.7 times than pure PVA fiber (172±5.82 vs 100±5.65 MPa). Impressively, the transformation from RTP to circularly polarized room temperature phosphorescence (CP-RTP) is readily achieved by imparting left- or right-hand helical structure through simply twisting, enabling large-scale production of chiral Q-NH2@PVA fiber with dissymmetry factor of 10-2. Besides, an array of displays and encryption patterns are crafted by weaving or seaming to exemplify the promising applications of these PVA-based fibers with outstanding adaptivity in cutting-edge anti-counterfeiting technology.

Ultrasensitive Solvatochirochromism of Single Benzene Chromophores

Solvents influence the structure, aggregation and folding behaviors of solvatochromic compounds. Ultrasensitive solvent mediated chiroptical response is conducive to the fabrication of molecular platform for sensing and recognition, which however, remains great challenges in conceptual or applicable design. Here we report a cysteine-based single benzene chromophore system that shows ultrasensitivity to solvents. Compared to the ratiometrically responsive systems, the chiroptical activities could be triggered or inverted depending on the substituents of chiral entities with an ultralow solvent volume fraction (<1 vol %). One drop of dipolar solvents shall significantly induce the emergence or inversion of chiroptical signals in bulky phases. Based on the experimental and computational studies, the ultrasensitivity is contributed to the intimate interplay between solvents and chiral compounds that anchors the specific chiral conformation. It illustrates that structurally simple organic compounds without aggregation or folding behaviors possess pronounced solvatochiroptical properties, which sheds light on the next-generation of chiroptical sensors and switches.

Automatically Aligned and Environment-Friendly Twisted Stacking Terahertz Chiral Metasurface with Giant Circular Dichroism for Rapid Biosensing

Chiral metasurfaces are capable of generating a huge superchiral field, which has great potential in optoelectronics and biosensing. However, the conventional fabrication process suffers greatly from time consumption, high cost, and difficult multilayer alignment, which hinder its commercial application. Herein, we propose a twisted stacking carbon-based terahertz (THz) chiral metasurface (TCM) based on laser-induced graphene (LIG) technology. By repeating a two-step process of sticking a polyimide film, followed by laser direct writing, the two layers of the TCM are aligned automatically in the fabrication. Laser manufacturing also brings such high processing speed that a TCM with a size of 15 × 15 mm can be prepared in 60 s. In addition, due to the greater dissipation of LIG than that of metals in the THz band, a giant circular dichroism (CD) of +99.5 to -99.6% is experimentally realized. The THz biosensing of bovine serum albumin enhanced by the proposed TCMs is then demonstrated. A wide sensing range (0.5-50 mg mL-1) and a good sensitivity [ΔCD: 2.09% (mg mL-1)-1, Δf: 0.0034 THz (mg mL-1)-1] are proved. This LIG-based TCM provides an environment-friendly platform for chiral research and has great application potential in rapid and low-cost commercial biosensing.

Chirality in Atomically Thin CdSe Nanoplatelets Capped with Thiol-Free Amino Acid Ligands: Circular Dichroism vs. Carboxylate Group Coordination

Chiral semiconductor nanostructures and nanoparticles are promising materials for applications in biological sensing, enantioselective separation, photonics, and spin-polarized devices. Here, we studied the induction of chirality in atomically thin only two-monolayer-thick CdSe nanoplatelets (NPLs) grown using a colloidal method and exchanged with L-alanine and L-phenylalanine as model thiol-free chiral ligands. We have developed a novel two-step approach to completely exchange the native oleic acid ligands for chiral amino acids at the basal planes of NPLs. We performed an analysis of the optical and chiroptical properties of the chiral CdSe nanoplatelets with amino acids, which was supplemented by an analysis of the composition and coordination of ligands. After the exchange, the nanoplatelets retained heavy-hole, light-hole, and spin-orbit split-off exciton absorbance and bright heavy-hole exciton luminescence. Capping with thiol-free enantiomer amino acid ligands induced the pronounced chirality of excitons in the nanoplatelets, as proven by circular dichroism spectroscopy, with a high dissymmetry g-factor of up to 3.4 × 10-3 achieved for heavy-hole excitons in the case of L-phenylalanine.

Chiral Plasmonic Hybrid Nanostructures: A Gateway to Advanced Chiroptical Materials

Chirality introduces a new dimension of functionality to materials, unlocking new possibilities across various fields. When integrated with plasmonic hybrid nanostructures, this attribute synergizes with plasmonic and other functionalities, resulting in unprecedented chiroptical materials that push the boundaries of the system's capabilities. Recent advancements have illuminated the remarkable chiral light-matter interactions within chiral plasmonic hybrid nanomaterials, allowing for the harnessing of their tunable optical activity and hybrid components. These advancements have led to applications in areas such as chiral sensing, catalysis, and spin optics. Despite these promising developments, there remains a need for a comprehensive synthesis of the current state-of-the-art knowledge, as well as a thorough understanding of the construction techniques and practical applications in this field. This review begins with an exploration of the origins of plasmonic chirality and an overview of the latest advancements in the synthesis of chiral plasmonic hybrid nanostructures. Furthermore, representative emerging categories of hybrid nanomaterials are classified and summarized, elucidating their versatile applications. Finally, the review engages with the fundamental challenges associated with chiral plasmonic hybrid nanostructures and offer insights into the future prospects of this advanced field.

Nature-Inspired Chiral Structures: Fabrication Methods and Multifaceted Applications

Diverse chiral structures observed in nature find applications across various domains, including engineering, chemistry, and medicine. Particularly notable is the optical activity inherent in chiral structures, which has emerged prominently in the field of optics. This phenomenon has led to a wide range of applications, encompassing optical components, catalysts, sensors, and therapeutic interventions. This review summarizes the imitations and applications of naturally occurring chiral structures. Methods for replicating chiral architectures found in nature have evolved with specific research goals. This review primarily focuses on a top-down approach and provides a summary of recent research advancements. In the latter part of this review, we will engage in discussions regarding the diverse array of applications resulting from imitating chiral structures, from the optical activity in photonic crystals to applications spanning light-emitting devices. Furthermore, we will delve into the applications of biorecognition and therapeutic methodologies, comprehensively examining and deliberating upon the multifaceted utility of chiral structures.

Geometric Control and Optical Properties of Intrinsically Chiral Plasmonic Nanomaterials

Intrinsically chiral plasmonic nanomaterials exhibit intriguing geometry-dependent chiroptical properties, which is due to the combination of plasmonic features with geometric chirality. Thus, chiral plasmonic nanomaterials have become promising candidates for applications in biosensing, asymmetric catalysis, biomedicine, photonics, etc. Recent advances in geometric control and optical tuning of intrinsically chiral plasmonic nanomaterials have further opened up a unique opportunity for their widespread applications in many emerging technological areas. Here, the recent developments in the geometric control of chiral plasmonic nanomaterials are reviewed with special attention given to the quantitative understanding of the chiroptical structure-property relationship. Several important optical spectroscopic tools for characterizing the optical chirality of plasmonic nanomaterials at both ensemble and single-particle levels are also discussed. Three emerging applications of chiral plasmonic nanomaterials, including enantioselective sensing, enantioselective catalysis, and biomedicine, are further highlighted. It is envisioned that these advanced studies in chiral plasmonic nanomaterials will pave the way toward the rational design of chiral nanomaterials with desired optical properties for diverse emerging technological applications.