Chiroptical Signals Research Articles

Chiroptical spectrophotometer and analytical method for optically anisotropic samples.

A new dual-polarization-modulated comprehensive chiroptical spectrophotometer (D-CCS) was successfully developed for recording artifact-free chiroptical circular dichroism (CD) and circularly polarized luminescence (CPL) spectra of optically anisotropic samples on the basis of the Stokes-Mueller matrix approach. The D-CCS includes an additional second photoelastic modulator (PEM) set in front of the sample and a second PEM controller being driven independently of the first PEM driver, which are essential for obtaining artifact-free chiroptical signals of optically anisotropic samples containing non-chiral signals, i.e., linear birefringence, linear dichroism, and linearly polarized luminescence. The theory and implementation of the method are described, and examples of its application to CD and CPL samples with macroscopic anisotropies are provided. An important advantage of the developed D-CCS is that it reduces the essential step of sample rotation in the plane, including the gravitational axis, to obtain artifact-free CD signals. Consequently, this dual-polarization method shortens the time required to obtain artifact-free CD signals by half and could be very effective for samples, such as high fluidity samples, gels, liquid crystals, and supramolecules in solution, for which artifact-free chiroptical signals are difficult to obtain using conventional methods.

Chiroptical spectroscopy of a freely diffusing single nanoparticle

Chiral plasmonic nanoparticles can exhibit strong chiroptical signals compared to the corresponding molecular response. Observations are, however, generally restricted to measurements on stationary single particles with a fixed orientation, which complicates the spectral analysis. Here, we report the spectroscopic observation of a freely diffusing single chiral nanoparticle in solution. By acquiring time-resolved circular differential scattering signals we show that the spectral interpretation is significantly simplified. We experimentally demonstrate the equivalence between time-averaged chiral spectra observed for an individual nanostructure and the corresponding ensemble spectra, and thereby demonstrate the ergodic principle for chiroptical spectroscopy. We also show how it is possible for an achiral particle to yield an instantaneous chiroptical response, whereas the time-averaged signals are an unequivocal measure of chirality. Time-resolved chiroptical spectroscopy on a freely moving chiral nanoparticle advances the field of single-particle spectroscopy, and is a means to obtain the true signature of the nanoparticle’s chirality.

Chiral sensing with achiral isotropic metasurfaces

Metasurfaces, the two-dimensional analogues of metamaterials, are ideal platforms for sensing molecular chirality at the nanoscale, e.g. of inclusions of natural optically active molecules, as they offer large accessible areas (they are essentially surfaces) and can accommodate the necessary strong resonances for coupling the probing radiation with the chiral inclusions. Here, we examine theoretically achiral isotropic metasurfaces, and we treat them as polarizable surfaces that support resonant electric and magnetic currents, which are coupled via the chiral inclusions. We derive analytically, and verify numerically, expressions that provide insight to the enhancement mechanism of the magneto-electric coupling and explain why circular dichroism signals (difference in absorption between left- and right- circularly polarized waves) can arise from both the real and the imaginary part of the chirality parameter $\kappa$. Our analysis demonstrates distinct chiroptical signals where the contributions from both the real and imaginary part of $\kappa$ can be independently observed and, based on such measurements, we propose a scheme for the unambiguous determination of an unknown chirality.

Absolute Chiral Sensing in Dielectric Metasurfaces Using Signal Reversals.

Sensing molecular chirality at the nanoscale has been a long-standing challenge due to the inherently weak nature of chiroptical signals, and nanophotonic approaches have proven fruitful in accessing these signals. However, in most cases, complete sensing of the chiral part of the molecule's refractive index (magnitude and sign of both its real and imaginary part) has not been possible, while the strong inherent signals from the nanostructures themselves obscure the weak chiroptical signals. Here, we propose a dielectric metamaterial system that overcomes these limitations and allows for complete measurements of the total chirality and discrimination of the effects of its real and imaginary part, possible also in an absolute manner via the application of a crucial signal reversal (excitation with reversed polarization) that enables chirality measurements without the need for sample removal. As proof of principle, we demonstrate signal enhancements by a factor of 200 for ultrathin, subwavelength, chiral samples over a uniform and accessible area.

DNA‐Based Adaptive Plasmonic Logic Gates

AbstractSelf‐assembled plasmonic logic gates that read DNA molecules as input and return plasmonic chiroptical signals as outputs are reported. Such logic gates are achieved on a DNA‐based platform that logically regulate the conformation of a chiral plasmonic nanostructure, upon specific input DNA strands and internal computing units. With systematical designs, a complete set of Boolean logical gates are realized. Intriguingly, the logic gates could be endowed with adaptiveness, so they can autonomously alter their logics when the environment changes. As a demonstration, a logic gate that performs AND function at body temperature while OR function at cold storage temperature is constructed. In addition, the plasmonic chiroptical output has three distinctive states, which makes a three‐state molecular logic gate readily achievable on this platform. Such DNA‐based plasmonic logic gates are envisioned to execute more complex tasks giving these unique characteristics.

DNA-Based Adaptive Plasmonic Logic Gates.

Self-assembled plasmonic logic gates that read DNA molecules as input and return plasmonic chiroptical signals as outputs are reported. Such logic gates are achieved on a DNA-based platform that logically regulate the conformation of a chiral plasmonic nanostructure, upon specific input DNA strands and internal computing units. With systematical designs, a complete set of Boolean logical gates are realized. Intriguingly, the logic gates could be endowed with adaptiveness, so they can autonomously alter their logics when the environment changes. As a demonstration, a logic gate that performs AND function at body temperature while OR function at cold storage temperature is constructed. In addition, the plasmonic chiroptical output has three distinctive states, which makes a three-state molecular logic gate readily achievable on this platform. Such DNA-based plasmonic logic gates are envisioned to execute more complex tasks giving these unique characteristics.

Enhanced Circularly Polarized Luminescence Activity in Chiral Platinum(II) Complexes With Bis- or Triphenylphosphine Ligands.

Distinct circularly polarized luminescence (CPL) activity was observed in chiral (C∧N∧N)Pt(II) [(C∧N∧N) = 4,5-pinene-6′-phenyl-2,2′-bipyridine] complexes with bis- or triphenylphosphine ligands. Compared to the pseudo-square-planar geometry of chiral (C∧N∧N)Pt(II) complexes with chloride, phenylacetylene (PPV) and 2,6-dimethylphenyl isocyanide (Dmpi) ligands, the coordination configuration around the Pt(II) nucleus of chiral (C∧N∧N)Pt(II) complexes with bulk phosphine ligands is far more distorted. The geometry is straightforwardly confirmed by X-ray crystallography. The phosphines' participation enhanced the CPL signal of Pt(II) complexes profoundly, with the dissymmetry factor (glum) up to 10−3. The distorted structures and enhanced chiroptical signals were further confirmed by time-dependent density functional theory (TD-DFT) calculations.

Chiral auxiliary and induced chiroptical sensing with 5d/4f lanthanide-organic macrocycles.

A stereocontrolled self-assembly of 5d/4f heterometal-organic macrocycles has been realized by a post-assembly chiral auxiliary approach. Interligand π-π stacking interactions promoted chiral transfer from point-chirality of the auxiliary ligand to metal centers and then the helicates to produced CD and CPL active europium assemblies, facilitating a feasible enantiomeric excess determination by induced chiroptical signals.

Design of Chiral Supramolecular Polymers Exhibiting a Negative Nonlinear Response.

Many synthetic and supramolecular chiral polymeric systems are known to exhibit the "majority rules effect" (MRE), a positive nonlinear response in which a small enantiomeric excess (ee) of the chiral building blocks leads to unproportionally large chiroptical signals near zero ee. In contrast, the opposite "racemate rules effect" (RRE), a negative nonlinear response in which the chiroptical signals are flat near zero ee, while giving large nonlinear chiroptical responses to ee at high values, has only been occasionally observed. The origin of this unusual ee dependence remains elusive largely because few systems have been established that exhibit this effect. Herein, we present a design approach that enables the development of chiral supramolecular polymers with a pronounced negative nonlinear response akin to RRE. This is achieved by in situ generating a bidentate inducer for supramolecular polymerization that exists in both meso- and homochiral forms upon reacting with chiral guests. The presence of the meso-inducer creates an aggregate structure that has a little response in the circular dichroism (CD) spectra as a function of ee at a particular wavelength, but a homochiral inducer gives large changes in response to ee at this wavelength. This allowed for an RRE-like response to be observed when the CD intensity of the supramolecular polymers was plotted against the ee of the chiral guests that generate the meso- and homochiral inducers without the necessity of the racemic guest preferentially being incorporated into the polymer.

Importance of higher-order multipole transitions on chiral nearfield interactions

Abstract Surface-enhanced circular dichroism (SECD) of chiral molecules adsorbed on plasmonic nanostructures can substantially enhance chiroptical molecular signals by several orders, which is otherwise very weak to be directly measured. Several mechanisms were proposed to explain this extreme enhancement, but the exact mechanism is still controversial. We investigate strong higher-order multipole contribution to SECD near plasmonic nanostructures using the superposition T-matrix method and discuss how 3-dimensional full-field simulations implementing a homogeneous chiral medium have succeeded in the reconstruction of the extreme enhancement. We also discuss how theoretical studies modeling chiral molecules based on dipole approximation have failed to reconstruct the extreme enhancement and show that SECD enhancement of such chiral dipoles is directly governed by optical chirality enhancement. In addition, strong multipolar transitions in subwavelength chiral plasmonic nanoparticles are discussed based on the T-matrix. This work reviews theoretical frameworks describing chiral molecules, demonstrates significant contribution of a multipolar transition on the extreme SECD enhancement near plasmonic nanostructures, and emphasizes the importance of a multipolar transition in chiral nearfield interaction.

Detection of the Enzymatic Cleavage of DNA through Supramolecular Chiral Induction to a Cationic Polythiophene.

Water-soluble π-conjugated polymers are increasingly envisioned in biosensors, in which their unique optical and electronic properties permit a highly sensitive detection of biomolecular targets. In particular, cationic π-conjugated polymers are attractive for DNA sensing technologies, through the use of the fluorescence signals either in physiological solutions or in thin films. However, in the context of enzymatic activity assays, fluorescence-based methods require covalently labeling DNA with a dye or an antibody and are limited to short time scale due to dye photobleaching. In this frame, we report here a novel possible approach to probe the cleavage of DNA by a restriction enzyme, in continuous and without covalently labeled DNA substrate. This is achieved by exploiting unique chiroptical signals arising from the chiral induction of DNA to a poly[3-(6'-(trimethylphosphonium)hexyl)thiophene-2,5-diyl] upon interaction. The cleavage of DNA by HpaI, an endonuclease enzyme, is monitored through circular dichroism (CD) signals in the spectral range where the polymer absorbs light, i.e., far away from the spectral ranges of both DNA and the enzyme. We compare the results to a conventional noncontinuous assay by polyacrylamide gel electrophoresis, and we demonstrate that induced CD signals are effective in probing the enzymatic activity. By means of molecular dynamics simulations and calculations of CD spectra, we bring molecular insights into the structure of DNA/polymer supramolecular complexes before and after the cleavage of DNA. We show that the cleavage of DNA modifies the dynamics and the organization of the polymer backbone induced by the DNA helix. Altogether, our results provide detailed spectroscopic and structural insights into the enzymatic cleavage of DNA in interaction with a π-conjugated polymer, which could be helpful for developing chiroptical detection tools to monitor the catalytic activity in real time.

Supramolecular Self‐Assembly of DNA with a Cationic Polythiophene: From Polyplexes to Fibers

AbstractCationic polythiophenes constitute an interesting class of polymers for prospective applications in imaging, gene delivery and biosensing, as they combine solubility in aqueous media and have sensitive optical properties for the detection of biomolecules such as DNA. Here, we study the supramolecular self‐assembly of poly[3‐(6’‐(trimethylphosphonium)hexyl)thiophene‐2,5‐diyl] (P3HT‐PMe3) with different types of DNA such as single‐stranded oligonucleotides or long genomic DNA. Self‐assembly in buffered aqueous solution yields polyplexes (polymer/DNA complexes) with hydrodynamic radii ranging from 7 nm to around 25 nm, as observed by Taylor Dispersion Analysis. In these polyplexes, the achiral polymer presents chiroptical signals induced by supramolecular organization along chiral DNA. When the solution of long DNA/P3HT‐PMe3 is deposited on surfaces, tens of μm‐long fibers are formed by condensation of DNA and compaction of the polyplexes, as evidenced by microscopy techniques.

Enantioselective sensing of carboxylic acids with a bis(urea)oligo(phenylene)ethynylene foldamer

The development of molecular probes for optical sensing of chiral compounds has received increasing attention in recent years, in particular because of the potential to accelerate asymmetric reaction analysis. In this study, we prepared conformationally flexible oligo(phenylene)ethynylene foldamers carrying peripheral bis(trifluoromethyl)phenylurea units that undergo hydrogen bonding with chiral carboxylic acids. This interaction results in a chiral amplification process across the stereodynamic sensor scaffold which coincides with characteristic circular dichroism signals at high wavelengths. The induced chiroptical signals allow quantitative determination of the enantiomeric excess of the substrate which was demonstrated with nonracemic samples of tartaric acid. The chirality sensing assay is fast, sufficiently accurate for high-throughput screening purposes and adaptable to parallel analysis with multiwell plate readers.

Metamaterials and chiral sensing: a review of fundamentals and applications

Abstract Chirality, a property of broken mirror symmetry, prevails in nature. Chiral molecules show different biochemical behaviors to their mirror molecules. For left or right circularly polarized lights, the fundamental chiral states of electromagnetic fields interact differently with chiral matter, and this effect has been used as a powerful tool for the detection of chiral molecules. This optical sensing, also termed chiral sensing, is not only easy to implement but also non-invasive to the analytes. However, the measurements made by the optical sensing of chiral molecules are challenging, as chiroptical signals are extremely weak. Recent years have seen active research efforts into metamaterial and plasmonic platforms for manipulating local fields to enhance chiroptical signals. This metamaterial approach offers new possibilities of chiral sensing with high sensitivity. Here, we review the recent advances in chiral sensing using metamaterial and plasmonic platforms. In addition, we explain the underlying principles behind the enhancement of chiroptical signals and highlight practically efficient chiral sensing platforms. We also provide perspectives that shed light on design considerations for chiral sensing metamaterials and discuss the possibility of other types of chiral sensing based on resonant metamaterials.

Optical Chirality Sensing with an Auxiliary-Free Earth-Abundant Cobalt Probe.

Broadly useful chiroptical enantiomeric excess (ee) sensing remains challenging and typically involves carefully designed molecular receptors or supramolecular assemblies. Herein, we report on the enantioselective sensing of 35 amino acids, amino phosphonic acids, hydroxy acids, amino alcohols, and diamines with an auxiliary-free cobalt probe. Chiroptical analysis of the enantiomeric composition and concentration of minute sample amounts was achieved with high accuracy by using earth-abundant cobalt salts and hydrogen peroxide as the oxidant. Despite the absence of an auxiliary ligand, the cobalt assay is applicable to aromatic and aliphatic compounds and yields strong CD signals at high wavelengths. This method obviates the general prerequisite for chromophoric metal ligands to generate chiroptical signals through ECCD (exciton-coupled circular dichroism) effects or through analyte-to-ligand chirality induction, and it offers operational simplicity, cost efficiency, waste reduction, and speed.

Chiral ZnO nanoparticles for detection of dopamine

Dopamine (DA) is an important catecholamine neurotransmitter in the mammalian central nervous system, and is involved in many behavioral responses and brain functions, and thus the detection of DA is much important. Chiral nanoparticles (NPs) have aroused much attention and showed many potential applications. In this work, Chiral ZnO NPs was prepared, and the nanoparticles can disperse well without aggregation. The Circular dichroism (CD) test showed strong chiroptical signals, demonstrating it is an effect method to prepare Chiral ZnO with cysteine as ligand. The photoluminescent properties of Chiral ZnO was investigated and showed quenching effect to dopamine, which make it possible to be used as probes to detect dopamine in biological samples with high selectivity and sensitivity.

Application of a polarized modulation technique in supramolecular science: chiroptical measurements of optically anisotropic systems

The chirality of a supramolecular assembly provides particularly valuable information because the bonding nature of noncovalent interactions, such as electrostatic interactions, π effects, van der Waals forces, and hydrophobic effects, makes the development of supramolecular assemblies an attractive and useful approach in chiral induction, chiral amplification, and chirality transfer. However, chiroptical measurements of optically anisotropic samples cannot be generally achieved with modern chiroptical spectrophotometric methods such as circular dichroism (CD) or circular birefringence (CB) and circularly polarized luminescence (CPL) that are based on polarization modulation techniques because of the coupling effect of the nonideal optics and the electronics with strong macroscopic anisotropies, that is, nonchiral signals related to linearly polarized phenomena. These artifact signals are often much stronger than the chiroptical signals. Only CD and CPL spectrophotometers developed in 2001 and 2016, respectively, and integrated into a Stokes–Mueller matrix analysis for optically anisotropic samples are capable of obtaining accurate chirality measurements of samples with macroscopic anisotropies. Therefore, these spectrophotometers enable chiral investigations of optically anisotropic samples, for example, single crystals, supramolecular assemblies, gels, films, membranes, polymers, and liquid crystals. This focus review presents a short and elementary discussion of the chiroptical measurement techniques for optically anisotropic samples in supramolecular science and signal interpretation in polarization spectroscopy.

Self-assembly and chiroptical properties in supramolecular complexes of adenosine phosphates and guanidinium-bispyrene.

Supramolecular systems that respond to the hydrolysis of adenosine phosphates (APs) are attractive for biosensing and to fabricate bioinspired self-assembled materials. Here, we report on the formation of supramolecular complexes between an achiral guanidinium derivative bearing two pyrene moieties, with each of the three adenosine phosphates: AMP, ADP, and ATP. By combining results from circular dichroism spectroscopy and molecular modeling simulations, we explore the induced chirality, the dynamics of the complexes, and the interactions at play, which altogether provide insights into the supramolecular self-assembly between APs and the guanidinium-bispyrene. Finally, we identify the chiroptical signals of interest in mixtures of the guanidinium derivative with the three APs in different proportions. This study constitutes a basis to evolve toward a chiroptical detection of the hydrolysis of APs based on organic supramolecular probes.

Interferometric control of the ellipticity of a femtosecond extreme ultraviolet source

A method for ultrafast ellipticity modulation of femtosecond intense lasers is introduced and demonstrated. The method is based on the coherent superimposition of two linearly polarized visible/infrared (Vis-IR) laser beams with orthogonal polarizations. Tuning their delay by a quarter of the wavelength, i.e., a few hundred nanometers, achieves the same function as the rotation of a quarter-wave plate by 45°, switching the polarization from linear to circular. We demonstrate the portability of this method to high-intensity processes by upconverting a femtosecond Vis-IR laser beam to the extreme ultraviolet (EUV) spectral range through high-harmonic generation. These results open the way to lock-in detection of small absorption chiroptical signals in the EUV spectral range, including pump probe signals.

Reversible Quadruple Switching with Optical, Chiroptical, Helicity, and Macropattern in Self‐Assembled Spiropyran Gels

Enantiomeric glutamate gelators containing a spiropyran moiety are designed and found to self‐assemble into a nanohelix through gelation. Upon alternating UV and visible light irradiation, the spiropyran experiences a reversible change between a blue zwitterionic merocyanine state and a colorless closed ring state spiropyran in supramolecular gels. This photochromic switch causes a series of subsequent changes in the optical, chiroptical, morphological properties from supramolecular to macroscopic levels. While the solution of the gelator molecules does not show any circular dichroism (CD) signal in the region of 250–700 nm due to the fact that the chromophore is far from the chiral center, the gel shows chiroptical signals such as CD and circularly polarized luminescence (CPL) because of the chirality transfer by the self‐assembly. These signals are reversible upon alternating UV/vis irradiation. Therefore, a quadruple optical and chiroptical switch is developed successfully. During such process, the self‐assembled nanostructures from the enantiomeric supramolecular gels also undergo a reversible change between helices and fibers under the alternating UV and visible light trigger. Furthermore, a rewritable material fabricated from their xerogels on a glass is developed. Such rewritable material can be efficiently printed over 30 cycles without significant loss in contrast and resolution using UV and visible light.