Weak Optical Activity Research Articles

Ultrasensitive Raman Detection of Biomolecular Conformation at the Attomole Scale using Chiral Nanophotonics.

Understanding the function of a biomolecule hinges on its 3D conformation or secondary structure. Chirally sensitive, optically active techniques based on the differential absorption of UV-vis circularly polarized light excel at rapid characterisation of secondary structures. However, Raman spectroscopy, a powerful method for determining the structure of simple molecules, has limited capacity for structural analysis of biomolecules because of intrinsically weak optical activity, necessitating millimolar (mM) sample quantities. A breakthrough is presented for utilising Raman spectroscopy in ultrasensitive biomolecular conformation detection, surpassing conventional Raman optical activity by 15 orders of magnitude. This strategy combines chiral plasmonic metasurfaces with achiral molecular Raman reporters and enables the detection of different conformations (α-helix and random coil) of a model peptide (poly-L/D-lysine) at the ≤attomole level (monolayer). This exceptional sensitivity stems from the ability to detect local, molecular-scale changes in the electromagnetic (EM) environment of a chiral nanocavity induced by the presence of biomolecules using molecular Raman reporters. Further signal enhancement is achieved by incorporating achiral Au nanoparticles. The introduction of the nanoparticles creates highly localized regions of extreme optical chirality. This approach, which exploits Raman, a generic phenomenon, paves the way for next-generation technologies for the ultrasensitive detection of diverse biomolecular structures.

Perovskite versus Nonperovskite: Modulating the Nature and Optical Properties of One-Dimensional Chiral Lead-Bromide Networks.

In the last 5 years, low-dimensional chiral metal-halide semiconductors have attracted great interest in the generation of chiroptical activity. Among this new family of materials, one-dimensional (1D) networks have appeared as the best candidates for strong circular dichroism (CD) and circularly polarized luminescence (CPL). Here, we present a new family of 1D chiral lead-bromide materials prepared from organic S/R/rac-1-hydroxypropyl-2-ammonium (S/R/rac-HP2A) cations. The presence or absence of polarity in the crystal structure as well as the perovskite or nonperovskite nature of the inorganic network depends on the initial stoichiometry of metal-halide salt and chiral amine during the crystallization. The perovskite-type networks exhibit strong CD and second harmonic generation (SHG) responses, while the nonperovskite compounds show the presence of polymorphism in the crystal phase and weak natural optical activity in the final material. These results underline the impact of synthetic conditions and thin film morphology on the structural and optical properties of metal-halide hybrid networks.

Helical polymers for dissymmetric circularly polarized light imaging.

Control of the spin angular momentum (SAM) carried in a photon provides a technologically attractive element for next-generation quantum networks and spintronics1-5. However, the weak optical activity and inhomogeneity of thin films from chiral molecular crystals result in high noise and uncertainty in SAM detection. Brittleness of thin molecular crystals represents a further problem for device integration and practical realization of chiroptical quantum devices6-10. Despite considerable successes with highly dissymmetric optical materials based on chiral nanostructures11-13, the problem of integration of nanochiral materials with optical device platforms remains acute14-16. Here we report a simple yet powerful method to fabricate chiroptical flexible layers via supramolecular helical ordering of conjugated polymer chains. Their multiscale chirality and optical activity can be varied across the broad spectral range by chiral templating with volatile enantiomers. After template removal, chromophores remain stacked in one-dimensional helical nanofibrils producing a homogeneous chiroptical layer with drastically enhanced polarization-dependent absorbance, leading to well-resolved detection and visualization of SAM. This study provides a direct path to scalable realization of on-chip detection of the spin degree of freedom of photons necessary for encoded quantum information processing and high-resolution polarization imaging.

Chiral nanoparticle‐induced amplification in optical activity of molecules with chiral centers

AbstractSensitive differentiation of an enantiomer from its mirror image (ie, enantiodifferentiation), a perennial challenge for pharmaceutical production and disease diagnosis, is technically limited by the weak optical activity (OA) of enantiomers, mainly due to their dimensional mismatch with light wavelengths in the ultraviolet (UV)‐visible region. Here we use silver chiral nanoparticles (Ag CNPs) with nominally sub‐5 nm helical pitch (P) to amplify the OA of (2′R, 3′R, 4′S)‐riboflavin‐5′‐phosphate sodium salts (RP), which have been found to indirectly affect metabolic processes, through the formation of an RP thin film (TF) covering a close‐packed array of Ag CNPs. The OA of the RP in the deep‐UV region can be amplified up to 80‐fold, ascribed to the aggregation of RP in the TFs and the interactions between RP and the atomically chiral lattices at the CNPs' surfaces. The former contribution, not associated with the chiral Ag topographies, plays a dominant role by thickening the RP TFs, so that the observed amplification has no enantioselective dependence on the chirality of the Ag CNPs. This study extends progress in the sensitive detection of bio‐enantiomers, which is highly desired for advanced bio‐detection in disease diagnosis and production of single‐enantiomer pharmaceuticals.image

Binary Chiral Nanoparticles Exhibit Amplified Optical Activity and Enhanced Refractive Index Sensitivity

Metallic chiral nanoparticles (CNPs) with a nominal helical pitch (P) of sub-10 nm contain inherent chirality and are promisingly applied to diverse prominent enantiomer-related applications. However, the sub-wavelength P physically results in weak optical activity (OA) to prohibit the development of these applications. Herein, a facile method to amplify the CNPs' OA by alloying the host CNPs with metals through a three-step layer-by-layer glancing angle deposition (GLAD) method is devised. Promoted by the GLAD-induced heating effect, the solute metallic atoms diffuse into the host CNPs to create binary alloy CNPs. Chiral alloying not only induces the plasmonic OA of the diffused solute and the created alloys but also amplifies that of the host CNPs, generally occurring for alloying Ag CNPs with diverse metals (including Cu, Au, Al, and Fe) and alloying Cu CNPs with Ag. Furthermore, the chiral alloying leads to an enhancement of refractive index sensitivity of the CNPs. The alloy CNPs with amplified plasmonic OA pave the way for potentially developing important chirality-related applications in the fields of heterogeneous asymmetric catalysis, enantiodifferentiation, enantioseparation, biosensing, and bioimaging.

Amplified measurement of weak optical activity using a spin-phase-gradient beam.

Development of alternate techniques to polarimetry for the measurement of weak optical rotation, with improved sensitivity, is becoming increasingly important as one understands the role of chirality in drug design and synthesis and the fundamentals of chiral light-matter interaction. We demonstrate here an optical amplification scheme using a spin-phase-gradient beam to measure ultra-small optical rotation angle (4mdeg), with a sensitivity of 220μdeg/μm, due to dilute (mg/mL) dextro-rotatory sugar solution. A Soleil-Babinet compensator is used to generate a tunable spin-phase-gradient beam which enables us to achieve high measurement sensitivities. Theoretical formalism of the technique leads us to the possibility to realize much higher measurement sensitivity of up to 10μdeg/μm by tuning-in the experimental parameters.

Alkali metal ions regulate the supramolecular chirality of interfacial assembly of achiral phthalocyanine

Because of their unique optical, electrical, and magnetic properties, fabrication of novel phthalocyanines supramolecular assemblies is becoming extremely important. However, developing phthalocyanines assemblies with tunable supramolecular chirality is still difficult due to the good planarity of phthalocyanine molecules, which could induce strong π-π interaction and tight aggregation. Herein, the supramolecular chirality of interfacial assembly of achiral phthalocyanine was thoroughly regulated by simply using different alkali metal ions. Achiral phthalocyanine with crown ether substituents (H2Pc(15C5)4) could form chiral assemblies with helical nanostructures upon interaction with potassium ions at the air-water interface. It is found that the formation of phthalocyanine dimers plays very important role. However, smaller lithium ions cannot induce this achiral phthalocyanine form chiral assemblies. Although sodium ions can form good host-guest complex with crown ether substituents, only weak optical activity can be detected from H2Pc(15C5)4/Na+ assemblies.

Ultraviolet and X-ray irradiance and flares from low-mass exoplanet host stars

AbstractThe spectral and temporal behavior of exoplanet host stars is a critical input to models of the chemistry and evolution of planetary atmospheres. High-energy photons (X-ray to NUV) from these stars regulate the atmospheric temperature profiles and photochemistry on orbiting planets, influencing the production of potential “biomarker” gases. We report first results from the MUSCLES Treasury Survey, a study of time-resolved UV and X-ray spectroscopy of nearby M and K dwarf exoplanet host stars. This program uses contemporaneous Hubble Space Telescope and Chandra (or XMM) observations to characterize the time variability of the energetic radiation field incident on the habitable zones planetary systems at d ≲ 20 pc. We find that all exoplanet host stars observed to date exhibit significant levels of chromospheric and transition region UV emission. M dwarf exoplanet host stars display 30–7000% UV emission line amplitude variations on timescales of minutes-to-hours. The relative flare/quiescent UV flux amplitudes on weakly active planet-hosting M dwarfs are comparable to active flare stars (e.g., AD Leo), despite their weak optical activity indices (e.g., Ca II H and K equivalent widths). We also detect similar UV flare behavior on a subset of our K dwarf exoplanet host stars. We conclude that strong flares and stochastic variability are common, even on “optically inactive” M dwarfs hosting planetary systems. These results argue that the traditional assumption of weak UV fields and low flare rates on older low-mass stars needs to be revised.

Ultrasmall polarization rotation measurements via weak value amplification

• We present a class of weak measurements where the measurer is an angular variable of the system. • Photon-energy qubits are required, which seems to be the first application of this kind of light. • Both weak optical activity and reflection-induced polarization rotation can be amplified. • This protocol can amplify the optical activity signal in nanostructures and biological tissues. We propose a framework to analyze weak measurements based on an angular version of the von Neumann measurement scheme, where the coupling between the system and the meter causes rotation of the measuring variable. We also discuss an experimental application of this theory in which measurements of weak optical activity and reflection-induced polarization rotation could be amplified in nearly two orders of magnitude. It can shed a new light on a great variety of physical chemistry, molecular biology and nanotechnology studies.

Fabrication of multifunctional ferromagnetic Au3Pd–CoSe nanoparticles

Multifunctional Au3Pd–CoSe nanoparticles synthesized through catalyst-aided chemical vapour deposition exhibits weak ferromagnetism and optical activity making them ideal for magneto-optics.

Differential and integral extended Jones matrices for oblique light propagation through a deformed crystal

An integral Jones matrix for oblique light propagation through a deformed crystal is derived using the differential Jones matrix approach. We show that generally at oblique light propagation a crystal monotonically deformed along its normal should show both linear and circular birefringence, even without twist deformation. The capability of the approach is illustrated by the calculation of the integral extended Jones matrices for oblique light propagation through several deformed liquid crystal nematic cells. In particular, we predict that at oblique light propagation in certain directions the nontwisted hybrid nematic cell (in which the tilt angle of the optic axis with respect to the crystal plate normal smoothly varies through the thickness) shows optical activity, a phenomenon of spatial light dispersion akin to so-called weak optical activity in nonchiral polar solid crystals.

Polarisation characteristics of optical waves in crystals in the presence of weak optical activity

Basing on the analysis of parameters of the optical waves that propagate in optically uniaxial crystals possessing a weak and ordinary optical activity we have shown that the polarisation characteristics of the outgoing wave should differ for the azimuth of the incoming wave which correspond to the orthogonal eigen compo- nents. This fact can be used while measuring the weak optical activity with po- larimetric methods.

Optical Coupling Between Chiral Biomolecules and Semiconductor Nanoparticles: Size‐Dependent Circular Dichroism Absorption

Sizing up QDs: CdTe or CdSe quantum dots (QDs) stabilized by chiral biomolecules show size-dependent circular dichroism (CD) characteristics in the visible light region. Theoretical calculations reveal that the origin of this CD is the combination of the weak optical activity of the biomolecules and the large enhancement effects from the strong absorption of QDs.

On the behaviour of Poynting vector in material media with weak optical activity

We have obtained phenomenological relation for the Poynting vector of electromagnetic wave propagating in crystals that possess a so-called weak optical activity. The appearance of transverse component of the Poynting vector and a transverse shift of the optical beam due to spin-orbit interaction are discussed.

Chirality and the origin of atmospheric humic-like substances

Abstract. Aerosol water extracts and atmospheric humic-like substances (HULIS) obtained from PM2.5-fraction aerosol samples collected in a rural/continental background environment and in an urban environment in spring and summer, and at a tropical site that was heavily impacted by biomass burning were studied. HULIS was obtained as the water-soluble, methanol-elutable material isolated from a solid-phase extraction procedure. The mean organic matter-to-organic carbon mass conversion factor and the standard deviation of 2.04±0.06 were derived for HULIS from biomass burning. Mean atmospheric concentrations of HULIS for the rural and urban environments and for the biomass burning during daylight periods and nights, were 1.65, 2.2, 43, and 60 μg m−3, respectively. This and other abundances indicate that intense emission sources and/or formation mechanisms of HULIS operate in biomass burning. Mean contributions of C in HULIS (HULIS-C) to water-soluble organic carbon (WSOC) were 35, 48, 63, and 76%, respectively, for the sample set listed. HULIS-C is the major component of the WSOC in tropical biomass burning. The data also suggest that HULIS most likely do not share common origin in the three environments studied. Differentiation among the possible formation processes was attempted by investigating the optical activity of HULIS through their (electronic and vibrational) circular dichroism properties. The urban HULIS did not show optical activity, which is in line with the concept of their major airborne formation from anthropogenic aromatics. The rural HULIS revealed weak optical activity, which may be associated with one of their important formation pathways by photo-oxidation and oligomerisation, i.e., with the formation from chiral biogenic precursors with one of the enantiomers slightly enriched. The The biomass burning of HULIS exhibited a strong effect in the vibrational circular dichroism as a clear distinction from the other two types. This was related to the contribution of the thermal degradation products of lignins and cellulose. The biomass burning of HULIS resemble Suwannee River Fulvic Acid standard more closely in some aspects than the urban and rural types of HULIS, which may be related to their common origin from plant material.

Coexistence of ordinary gyration and weak optical activity in crystals: eigen waves of a new type

Abstract We present the results of analysis for the refractive indices and the ellipticity of eigen waves under the conditions of coexisting gyration and weak optical activ-ity. Quantitative relations describing the appropriate parameters are derived. It is shown that the ellipticities of eigen waves become different and one of that waves acquires a complicated (longitudinally-transverse) elliptical polarisation state. This represents a new type of electromagnetic eigen waves in crystals. Keywords: weak optical activity, gyration, spatial dispersion, refraction, polarisation PACS: 78.20.Ek UDC: 535.56 1. Introduction Optical activity is described with taking into consideration an inhomogeneity of the elec-tric displacement D j of an optical wave propagating through a medium: 0 jiijj ijkk DEBD ix γ∂=+∂, (1) where E i denotes electric field, 0 B ij the optical-frequency impermeability tensor without accounting for spatial dispersion effects, γ ijk the third-rank antisymmetric polar tensor (γγ

On the light refraction and polarisation in the presence of weak optical activity

We show that the weak optical activity should manifest itself in changing refractive indices and optical birefringence of a medium. Increments of both the refractive indices and the birefringence which appear due to the weak optical activity are quite small and depend on the signs of neither the optical activity vector nor the light wave vector.

Charging Quenches Multiple Exciton Generation in Semiconductor Nanocrystals: First-Principles Calculations on Small PbSe Clusters

We demonstrate using symmetry adapted cluster theory with configuration interaction (SAC-CI) that charging of small PbSe nanocrystals (NCs) greatly modifies their electronic states and optical excitations. Conduction and valence band transitions that are not available in neutral NCs dominate low energy electronic excitations and show weak optical activity. At higher energies these transitions mix with both single excitons (SEs) and multiple excitons (MEs) associated with transitions across the band-gap. As a result, both SEs and MEs are significantly blue-shifted, and ME generation is drastically hampered. The overall contribution of MEs to the electronic excitations of the charged NCs is small even at very high energies. The calculations support the recent view that the observed strong dependence of the ME yields on the experimental conditions is likely due to the effects of NC charging.

Ab Initio Time-Domain Study of Phonon-Assisted Relaxation of Charge Carriers in a PbSe Quantum Dot

The phonon-induced relaxation dynamics of charge carriers in a PbSe quantum dot is studied for the first time by ab initio density functional theory in the time-domain. The picosecond time scale of the relaxation and the absence of the phonon bottleneck are rationalized by relatively high electron and hole state densities. While many of these states show only weak optical activity, most of them participate in the electron-vibrational relaxation. Our simulations demonstrate that the slight asymmetry in the electron and hole band structure is sufficient to allow symmetry-forbidden S-P, P-S, etc. transitions, which are seen in the experimental absorption spectra. The relaxation is nonexponential, in agreement with the strongly non-Lorentzian spectral line shapes observed in experiments. The energy exchanged during individual transitions is typically greater than the characteristic phonon energy, indicating that the transitions are multiphonon. Both electrons and holes interact better with low-frequency acoustic phonons, rather than higher frequency optical modes. Holes decay only slightly faster than electrons, rendering the hole-assisted Auger relaxation pathways inefficient. This relatively symmetric vibrational relaxation of electrons and holes proceeds on a picosecond time scale, much slower than the ultrafast highly efficient carrier multiplication that was reported recently in relation to improved solar power conversion.

Induced Chirality of the Light-Harvesting Carotenoid Salinixanthin and Its Interaction with the Retinal of Xanthorhodopsin

In xanthorhodopsin, a retinal protein-carotenoid complex of Salinibacter ruber, the carotenoid salinixanthin functions as a light-harvesting antenna in supplying additional excitation energy for retinal isomerization and proton transport. Another retinal protein, archaerhodopsin, has been shown to contain a carotenoid, bacterioruberin, but without an antenna function. We report here that the binding site confers a chiral geometry on salinixanthin in xanthorhodopsin and confirm that the same is true for bacterioruberin in archaerhodopsin. Cell membranes containing these rhodopsins exhibit CD spectra with sharp positive bands in the visible region where the carotenoids absorb, and in the case of xanthorhodopsin a negative band at 536 nm, as well as bands in the UV region. The carotenoid in ethanol has very weak optical activity in the visible region of the spectrum. Denaturation of the opsin upon deprotonation of the Schiff base at pH 12.5 eliminates the induced CD bands in both proteins. In one of these proteins, but not in the other, the carotenoid binding site depends entirely on the retinal. Hydrolysis of the retinal Schiff base of xanthorhodopsin with hydroxylamine eliminates the induced CD bands of salinixanthin. In contrast, hydrolysis of the Schiff base in archaerhodopsin does not abolish the CD bands of bacterioruberin. Thus, consistent with its antenna function, the carotenoid binding site interacts closely with the retinal only in xanthorhodopsin, and this interaction is the major source of the CD bands. In this protein, protonation of the counterion with a decrease in pH from 8 to 5 causes significant changes in the CD spectrum. The observed spectral features suggest that binding of salinixanthin in xanthorhodopsin involves the cyclohexenone ring of the carotenoid and its conformational heterogeneity is restricted.