Chiral Signals Research Articles

Finding the chiral gravitational wave background of an axion- SU(2) inflationary model using CMB observations and laser interferometers

A detection of B-mode polarization of the Cosmic Microwave Background (CMB) anisotropies would confirm the presence of a primordial gravitational wave background (GWB). In the inflation paradigm this would be an unprecedented probe of the energy scale of inflation as it is directly proportional to the power spectrum of the GWB. However, similar tensor perturbations can be produced by the matter fields present during inflation, breaking this simple relationship. It is therefore important to be able to distinguish between different generation mechanisms of the GWB. In this paper, we analyse the detectability of a new axion-SU(2) gauge field model using its chiral, scale-dependent tensor spectrum. We forecast the detectability of the resulting CMB TB and EB cross-correlations by the LiteBIRD satellite, considering the effects of residual foregrounds, gravitational lensing, and for the first time assess the ability of such an experiment to jointly detect primordial TB and EB spectra and self-calibrate its polarimeter. We find that LiteBIRD will be able to detect the chiral signal for $r_*>0.03$ with $r_*$ denoting the tensor-to-scalar ratio at the peak scale, and that the maximum signal-to-noise for $r_*<0.07$ is $\sim 2$. We go on to consider an advanced stage of a LISA-like mission, and find that such experiments would complement CMB observations by providing sensitivity to GWB chirality on scales inaccessible to the CMB. We conclude that in order to use the CMB to distinguish this model from a conventional vacuum fluctuation model two-point statistics provide some power, but to achieve high statistical significance we would require higher order statistics which take advantage of the model's non-Gaussianity. On the other hand, in the case of a spectrum peaked at very small scales, inaccessible to the CMB, a highly significant detection could be made using space-based laser interferometers.

Induction of Chirality in Two-Dimensional Nanomaterials: Chiral 2D MoS2 Nanostructures.

Two-dimensional (2D) nanomaterials have been intensively investigated due to their interesting properties and range of potential applications. Although most research has focused on graphene, atomic layered transition metal dichalcogenides (TMDs) and particularly MoS2 have gathered much deserved attention recently. Here, we report the induction of chirality into 2D chiral nanomaterials by carrying out liquid exfoliation of MoS2 in the presence of chiral ligands (cysteine and penicillamine) in water. This processing resulted in exfoliated chiral 2D MoS2 nanosheets showing strong circular dichroism signals, which were far past the onset of the original chiral ligand signals. Using theoretical modeling, we demonstrated that the chiral nature of MoS2 nanosheets is related to the presence of chiral ligands causing preferential folding of the MoS2 sheets. There was an excellent match between the theoretically calculated and experimental spectra. We believe that, due to their high aspect ratio planar morphology, chiral 2D nanomaterials could offer great opportunities for the development of chiroptical sensors, materials, and devices for valleytronics and other potential applications. In addition, chirality plays a key role in many chemical and biological systems, with chiral molecules and materials critical for the further development of biopharmaceuticals and fine chemicals, and this research therefore should have a strong impact on relevant areas of science and technology such as nanobiotechnology, nanomedicine, and nanotoxicology.

Chiral near-fields around chiral dolmen nanostructure

Discriminating the handedness of the chiral molecule is of great importance in the field of pharmacology and biomedicine. Enhancing the chiral near-field is one way to increase the chiral signal of chiral molecules. In this paper, the chiral dolmen nanostructure (CDN) is proposed to enhance the chiral near-field. Numerical results show that the CDN can increase the optical chirality of the near-field by almost two orders of magnitude compared to that of a circularly polarized incident wave. In addition, the optical chirality of the near-field of the bonding mode is enhanced more than that of the antibonding mode. These results provide an effective method for tailoring the chiral near-field for biophotonics sensors.

Stimulus-Responsive Plasmonic Chiral Signals of Gold Nanorods Organized on DNA Origami.

In response to environmental variations, living cells need to arrange the conformational changes of macromolecules to achieve the specific biofunctions. Inspired by natural molecular machines, artificial macromolecular assemblies with controllable nanostructures and environmentally responsive functions can be designed. By assembling macromolecular nanostructures with noble metal nanoparticles, environmental information could be significantly amplified and modulated. However, manufacturing dynamic plasmonic nanostructures that are efficiently responsive to different stimuli is still a challenging task. Here we demonstrate a stimulus-responsive plasmonic nanosystem based on DNA origami-organized gold nanorods (GNRs). L-shaped GNR dimers were assembled on rhombus-shaped DNA origami templates. The geometry and chiral signals of the GNR nanoarchitectures respond to multiple stimuli, including glutathione reduction, restriction enzyme action, pH change, or photoirradiation. While the glutathione reduction or restriction enzyme caused irreversible changes in the plasmonic circular dichroism (CD) signals, both pH and light irradiation triggered reversible changes in the plasmonic CD. Our system transduces external stimuli into conformational changes and circular dichroism responses in near-infrared (NIR) wavelengths. By this approach, programmable optical reporters for essential biological signals can be fabricated.

The Construction of Homochiral Lanthanide Quadruple-Stranded Helicates with Multiresponsive Sensing Properties toward Fluoride Anions.

A series of unique homochiral lanthanide tetranuclear quadruple-stranded helicates have been self-assembled controllably by using the intrinsic advantages of chiral bridging ligands, (S)-H2 L and (R)-H2 L, and lanthanide ions with high coordination numbers. The self-assembly process of these chiral helicates not only ensures the structural stability and quadruple-stranded feature of lanthanide cluster in the solid state and solution, but also achieves effective transfer and amplification of the chirality code from the ligand to a higher supramolecular level. Moreover, through using optical rotation, circular dichroism spectra analysis, and luminescence measurements, we demonstrate that these chiral lanthanide helicates could serve as sensitive and multi-responsive sensors to recognize and detect F- anions based on the change of chiral signal and NIR luminescence simultaneously, which represents a meaningful exploration for developing functional lanthanide-based polynuclear clusters.

Highly selective and sensitive method for Cu2 + detection based on chiroptical activity of L-Cysteine mediated Au nanorod assemblies

Herein, we demonstrated a simple and efficient method to detect Cu2+ based on amplified optical activity in the chiral nanoassemblies of gold nanorods (Au NRs). L-Cysteine can induce side-by-side or end-to-end assembly of Au NRs with an evident plasmonic circular dichroism (PCD) response due to coupling between surface plasmon resonances (SPR) of Au NRs and the chiral signal of L-Cys. Because of the obvious stronger plasmonic circular dichrosim (CD) response of the side-by-side assembly compared with the end-to-end assemblies, SS assembled Au NRs was selected as a sensitive platform and used for Cu2+ detection. In the presence of Cu2+, Cu2+ can catalyze O2 oxidation of cysteine to cystine. With an increase in Cu2+ concentration, the L-Cysteine-mediated assembly of Au NRs decreased because of decrease in the free cysteine thiol groups, and the PCD signal decreased. Taking advantage of this method, Cu2+ could be detected in the concentration range of 20pM–5nM. Under optimal conditions, the calculated detection limit was found to be 7pM.

Symmetric Raman Tensor Contributes to Chiral Vibrational Sum-Frequency Generation from Binaphthyl Amphiphile Monolayers on Water: Study of Electronic Resonance Amplitude and Phase Profiles

Heterodyne-detected vibrationally electronically doubly resonant chiral sum-frequency generation (HD-DR chiral SFG) spectra were observed for the first time on monolayers. Langmuir monolayers of R- and S-binaphthyl amphiphiles on water exhibited complex chiral vibrational sum frequency generation (VSFG) spectra whose amplitudes were approximately the same, but whose phases were different by π. By comparing the electronic resonance profiles of the chiral VSFG signal amplitude and phase with simulations based on a nonadiabatic resonance Raman theory, we concluded that the chiral VSFG signals from the monolayers originated not from the antisymmetric Raman tensor but from the symmetric one. This is the first experimental evidence that the symmetric Raman tensor indeed contributes to the chiral VSFG signals from oriented monolayer systems.

An in-plane magnetic chiral dichroism approach for measurement of intrinsic magnetic signals using transmitted electrons

Electron energy-loss magnetic chiral dichroism is a powerful technique that allows the local magnetic properties of materials to be measured quantitatively with close-to-atomic spatial resolution and element specificity in the transmission electron microscope. Until now, the technique has been restricted to measurements of the magnetic circular dichroism signal in the electron beam direction. However, the intrinsic magnetization directions of thin samples are often oriented in the specimen plane, especially when they are examined in magnetic-field-free conditions in the transmission electron microscope. Here, we introduce an approach that allows in-plane magnetic signals to be measured using electron magnetic chiral dichroism by selecting a specific diffraction geometry. We compare experimental results recorded from a cobalt nanoplate with simulations to demonstrate that an electron magnetic chiral dichroism signal originating from in-plane magnetization can be detected successfully.

Isolating the chiral contribution in optical two-dimensional chiral spectroscopy using linearly polarized light.

The full development of mono- or multi-dimensional time-resolved spectroscopy techniques incorporating optical activity signals has been strongly hampered by the challenge of identifying the small chiral signals over the large achiral background. Here we propose a new methodology to isolate chiral signals removing the achiral background from two commonly used configurations for performing two-dimensional optical spectroscopy, known as BOXCARS and gradient assisted photon echo spectroscopy (GRAPES). It is found that in both cases an achiral signal from an isotropic system can be completely eliminated by small manipulations of the relative angles between the linear polarizations of the four input laser pulses. Starting from the formulation of a perturbative expansion of the signal in the angle between the beams and the propagation axis, we derive analytic expressions that can be used to estimate how to change the polarization angles of the four pulses to minimize achiral contributions in the studied configurations. The generalization to any other possible experimental configurations has also been discussed.

Wettability Switching of Electrode for Signal Amplification: Conversion of Conformational Change of Stimuli-Responsive Polymer into Enhanced Electrochemical Chiral Analysis.

Signal amplification of chiral interaction is a much needed task for sensing of enantiomers due to nearly identical chemical and physical properties of the chiral isomers. In this article, we established an electrochemical chiral sensing method with high sensitivity and selectivity for monosacharrides based on the stimuli-responsive copolymer/graphene hybrid-modified screen-printed carbon electrodes. The hybrid synthesized by the "grafting from" atom transfer radical polymerization (ATRP) process not only acted as a chiral recognition element but also provided a chiral signal amplification strategy. This occurs due to high sensitivity of conformational transition of copolymer on graphene to the weak chiral interactions that greatly facilitating the diffusion of electroactive probes and monosaccharides to the electrode surface. The described method can quantify monosaccharides, even the concentration of one enantiomer is as low as 1 nM. Apart from the demonstrated chiral distinguish ability, good selectivity toward monosaccharides in comparison to potential interference molecules was also observed. The electrodes with significant analytical performance were successfully applied for discriminating glucose enantiomers in live cells and studying their different transport mechanism. Together, the results show that the coupling of amplification-by-wettability switching concept with electrochemical method offers great promises in providing a sensitive, facile, and cost-effective solution for chiral recognition of molecules in biological process.

Axion inflation with an SU(2) gauge field: detectable chiral gravity waves

We study a single field axion inflation model in the presence of an SU(2) gauge field with a small vev. In order to make the analysis as model-independent as possible, we consider an arbitrary potential for the axion that is able to support the slow-roll inflation. The gauge field is coupled to the axion with a Chern-Simons interaction $$ \frac{\lambda }{f}{F}_{\mu \nu}^a{\tilde{F}}_a^{\mu \nu } $$ where $$ \frac{\lambda }{f}\sim \frac{\mathcal{O}(10)}{M_{\mathrm{pl}}} $$ . It has a negligible effect on the background evolution, $$ \frac{\rho \mathrm{Y}\mathrm{M}}{M_{\mathrm{pl}}^2{H}^2}\lesssim {\upepsilon}^2 $$ . However, its quantum fluctuations make a significant contribution to the cosmic perturbation. In particular, the gauge field has a spin-2 fluctuation which explicitly breaks the parity between the left- and right-handed polarization states. The chiral tensor modes are linearly coupled to the gravitational waves and lead to a circularly polarized tensor power spectrum comparable to the unpolarized vacuum power spectrum. Moreover, the scalar sector is modified by the linear scalar fluctuations of the gauge field. Since the spin-0 and spin-2 fluctuations of the SU(2) gauge field are independent, the gauge field can, at the same time, generate a detectable chiral gravitational wave signal and have a negligible contribution to the scalar fluctuations, in agreement with the current CMB observations.

Dynamic Chiral Nanoparticle Assemblies and Specific Chiroplasmonic Analysis of Cancer Cells.

Fabricated Ag@Au core-shell nanoparticle (CS NP) assemblies exhibit pronounced and reverse chiral bisignate plasmonic signals spanning 400 to 580 nm, in comparison to Ag NP assemblies. The time-dependent chiro-optical response of assemblies that shift with shell deposition is systematically recorded. Chiral Ag@Au CS NP assemblies first achieve the special discrimination of circulating tumor cells with HER2 overexpression.

Chiral and deconfining phase transitions from holographic QCD study

A preliminary quantitative study to match the lattice QCD simulation on the chiral and deconfining phase transitions of QCD in the bottom-up holographic framework is given. We constrain the relation between dilaton field ϕ and metric warp factor Ae and get several reasonable models in the Einstein-Dilaton system. Using the potential reconstruction approach, we solve the corresponding gravity background. Then we fit the background-related parameters by comparing the equation of state with the two-flavor lattice QCD results. After that we study the temperature dependent behavior of Polyakov loop and chiral condensate under those background solutions. We find that the results are in good agreement with the two-flavor lattice results. All the studies about the equation of state, the Polyakov loop and the chiral condensate signal crossover behavior of the phase transitions, which are consistent with the current understanding on the QCD phase transitions with physical quark mass. Furthermore, the extracted transition temperatures are comparable with the two-flavor lattice QCD results.

Non-local real-space analysis of chiral optical signals.

A spatially non-local response tensor description of linear chiral signals such as circular dichroism is developed. By working directly with the vector potential rather that the electric and magnetic fields, we recast the signals in terms of correlation functions of charge and current densities and avoid the tedious expansion in multipoles.

Biomimetic Taste Receptors with Chiral Recognition by Photoluminescent Metal-Organic Frameworks Chelated with Polyaniline Helices.

The adsorption of phenylaniline (Phe) enantiomers on (+)-polyaniline (PAN)-chelated [In(OH)(bdc)]n microcrystals was carefully designed and studied by using the Job titration, circular dichroism, X-ray photoelectron spectroscopy, and photoluminescence to mimic heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors in selective, but not specific, ligand binding with chiral recognition and signal transduction. Six essential working principles across different length scales are unraveled: 1) a chiral (+)-PAN (host), 2) specific sites for Phe-(+)/PAN (guest-host) binding, 3) a conformational change of (+)-PAN after binding with Phe enantiomers, 4) different degrees of packing for (+)-PAN, 5) interactions between (+)-PAN and the underlying signal-generating framework (i.e., [In(OH)(bdc)]n microcrystals), and 6) a systematic photoluminescent signal combination by using principal-component analysis from the other three polymer-chelated metal-organic frameworkds (MOFs), such as poly(acrylic acid) (PAA), sodium alginate (SA), and polyvinylpyrrolidone (PVP) to enhance the selectivity and discrimination capabilities.

Formation of Enhanced Uniform Chiral Fields in Symmetric Dimer Nanostructures.

Chiral fields with large optical chirality are very important in chiral molecules analysis, sensing and other measurements. Plasmonic nanostructures have been proposed to realize such super chiral fields for enhancing weak chiral signals. However, most of them cannot provide uniform chiral near-fields close to the structures, which makes these nanostructures not so efficient for applications. Plasmonic helical nanostructures and blocked squares have been proved to provide uniform chiral near-fields, but structure fabrication is a challenge. In this paper, we show that very simple plasmonic dimer structures can provide uniform chiral fields in the gaps with large enhancement of both near electric fields and chiral fields under linearly polarized light illumination with polarization off the dimer axis at dipole resonance. An analytical dipole model is utilized to explain this behavior theoretically. 30 times of volume averaged chiral field enhancement is gotten in the whole gap. Chiral fields with opposite handedness can be obtained simply by changing the polarization to the other side of the dimer axis. It is especially useful in Raman optical activity measurement and chiral sensing of small quantity of chiral molecule.

Synthesis and optical properties of enantiomeric cinchonidine/cinchonine-terpyridine derivatives and their metal complexes

We report here the synthesis and spectroscopic properties of two enantiomeric chiral tridentate ligands (TPyCCD and TPyCCN) composed of terpyridine (TPy) and cinchonidine (CCD) or cinchonine (CCN) substituent, as well as their metal complexes with Zn2+, Fe2+, Eu3+, and Tb3+ ions. For the pure ligands, the fluorescent emission intensity increased as their concentrations increased in the dilute solutions, but they decreased when the concentrations were greater than 6.9×10−6mol/l due to concentration quenching. No significant influence on ligand luminescence was observed for the Zn2+/Fe2+–TPyCCD and –TPyCCN metal complexes. Their lanthanide (Eu3+ and Tb3+) complexes displayed both ligand and lanthanide ion emissions, suggesting that the excited energy of both ligands could be transferred to the central Eu3+ and Tb3+ ions. Mirror-image circular dichroism spectra were recorded, with several signals centered at about 230, 250–270, 296, and 320nm; the first ones corresponded to the chiral CCD and CCN substituents, while the latter ones corresponded to the TPy substituent and the metal–TPy (M-TPy) coordination units. It is suggested that the chirality of TPy and the metal complexes was induced by the coexisting chiral CCD and CCN substituents. In addition, due to formation of the metal complexes, the relative intensity of the chiral signals of M–TPy coordination units was enhanced at about 320nm.

Sensitive and quantitative probe of molecular chirality with heterodyne-detected doubly resonant sum frequency generation spectroscopy.

Heterodyne-detected vibrationally electronically doubly resonant chiral sum frequency generation (HD-DR chiral SFG) spectroscopy has been developed for the study of chiral molecules with chromophores. The method enables us to detect and distinguish chiral molecules with high sensitivity and to obtain information on molecular vibrations. Strong enhancement due to the electronic resonance improves the sensitivity, and heterodyne detection ensures that the signal intensity is linear to the sample concentration. Detection of HD-DR chiral SFG signal from a dilute solution of binaphthol with 20 mM concentration and tens of nanometers thickness was demonstrated. Taking advantage of the enantiomer-dependent sign and linearity of the signal to the concentration, molecular concentrations and enantiomeric excesses were accurately evaluated. HD-DR chiral SFG is expected to have widespread application in the study of molecular chirality of thin films or samples of a very small quantity.

Broad-Bandwidth Chiral Sum Frequency Generation Spectroscopy for Probing the Kinetics of Proteins at Interfaces.

The kinetics of proteins at interfaces plays an important role in biological functions and inspires solutions to fundamental problems in biomedical sciences and engineering. Nonetheless, due to the lack of surface-specific and structural-sensitive biophysical techniques, it still remains challenging to probe protein kinetics in situ and in real time without the use of spectroscopic labels at interfaces. Broad-bandwidth chiral sum frequency generation (SFG) spectroscopy has been recently developed for protein kinetic studies at interfaces by tracking the chiral vibrational signals of proteins. In this article, we review our recent progress in kinetic studies of proteins at interfaces using broad-bandwidth chiral SFG spectroscopy. We illustrate the use of chiral SFG signals of protein side chains in the C–H stretch region to monitor self-assembly processes of proteins at interfaces. We also present the use of chiral SFG signals from the protein backbone in the N–H stretch region to probe the real-time kinetics of proton exchange between protein and water at interfaces. In addition, we demonstrate the applications of spectral features of chiral SFG that are typical of protein secondary structures in both the amide I and the N–H stretch regions for monitoring the kinetics of aggregation of amyloid proteins at membrane surfaces. These studies exhibit the power of broad-bandwidth chiral SFG to study protein kinetics at interfaces and the promise of this technique in research areas of surface science to address fundamental problems in biomedical and material sciences.

Significant Chiral Signal Amplification of Langmuir Monolayers Probed by Second Harmonic Generation.

With the development of the nonlinear optical technique such as SHG (second harmonic generation), the in situ measurements of the chirality in the monolayers at the air/water interface have become possible. However, when performing the SHG measurement of the chirality in a monolayer, it is still a great challenge to obtain the chiral signals with a good S/N (signal-to-noise) ratio. In this Letter, interfacial assemblies with induced supramolecular chirality were used to amplify the weak chiral SHG signals from the monolayers at the air/water interface. Tetrakis(4-sulfonatophenyl) porphyrin (TPPS) J aggregates were used as the subphase, and when chiral amphiphilic molecules were spread on it, chiral domains of the amphiphile/TPPS J aggregates were formed and then significantly amplified chiral signals that otherwise could not be detected. Moreover, the sign of the DCE (degree of chiral excess) changed with the chirality of the amphiphilic molecules, thus providing a possible way to obtain the absolute chiral information in situ in the monolayers.