Enantiomeric Discrimination by Surface-Enhanced Raman Scattering–Chiral Anisotropy of Chiral Nanostructured Gold Films

A surface-enhanced Raman scattering-chiral anisotropy (SERS-ChA) effect is reported that combines chiral discrimination and surface Raman scattering enhancement on chiral nanostructured Au films (CNAFs) equipped in the normal Raman scattering Spectrometer. The CNAFs provided remarkably higher enhancement factors of Raman scattering (EFs) for particular enantiomers, and the SERS intensity was proportional to the enantiomeric excesses (ee) values. Except for molecules with mesomeric species, all of the tested enantiomers exhibited high SERS-ChA asymmetry factors (g), ranging between 1.34 and 1.99 regardless of polarities, sizes, chromophores, concentrations and ee. The effect might be attributed to selective resonance coupling between the induced electric and magnetic dipoles associated with enantiomers and chiral plasmonic modes of CNAFs.     

Chiral Carbonaceous Nanotubes Modified with Titania Nanocrystals: Plasmon‐Free and Recyclable SERS Sensitivity

Chiral carbonaceous nanotubes (CNT) were successfully used in plasmon‐free surface‐enhanced Raman scattering (SERS) for the first time. Further modification of TiO₂ nanocrystals on the chiral CNTs successfully realized the recycling of SERS substrate as chiral CNT/TiO₂ hybrids. The high SERS sensitivity of methylene blue (MB) over the chiral CNT/TiO₂ hybrids is ascribed to the laser‐driven birefringence induced by the helical structure, which provides much more opportunities for the occurrence of Raman scattering. The TiO₂ nanocrystals highly dispersed on the surface and inside the hollow cavity of chiral CNTs can completely degrade the MB under the solar light irradiation, leading to the self‐cleaning of SERS substrate. The present research opens a new way for the application of chiral inorganic materials in plasmon‐free SERS detection.     

Long‐Term Stable Silver Subsurface Ion‐Exchanged Glasses for SERS Applications

We report on the formation of silver subsurface ion‐exchanged metal oxide (silver SIMO) glasses and their surface‐enhanced Raman scattering (SERS) activity. The samples were prepared by a combined thermal and chemical three‐step methodology and characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), environmental electron scanning microscopy (ESEM), and UV/Vis spectroscopy. This unique method provides SERS substrates with protection against contamination and strong, reliable and reproducible SERS enhancement. The Raman enhancement factors of the long‐term stable SIMO glasses were estimated to approximately 10⁷.     

Axially Chiral TADF‐Active Enantiomers Designed for Efficient Blue Circularly Polarized Electroluminescence

The use of a chiral, emitting skeleton for axially chiral enantiomers showing activity in thermally activated delayed fluorescence (TADF) with circularly polarized electroluminescence (CPEL) is proposed. A pair of chiral stable enantiomers, (?)‐(S)‐Cz‐Ax‐CN and (+)‐(R)‐Cz‐Ax‐CN, was designed and synthesized. The enantiomers, both exhibiting intramolecular π‐conjugated charge transfer (CT) and spatial CT, show TADF activities with a small singlet–triplet energy difference (ΔE_ST) of 0.029 eV and mirror‐image circularly polarized luminescence (CPL) activities with large g_lum values. Notably, CP‐OLEDs based on the enantiomers feature blue electroluminescence centered at 468 nm with external quantum efficiencies (EQEs) of 12.5 and 12.7 %, and also show intense CPEL with g_EL values of ?1.2×10^?2 and +1.4×10^?2, respectively. These are the first CP‐OLEDs based on TADF‐active enantiomers with efficient blue CPEL.     

Electrodeposition of dendritic gold/silver nanaoparticles on disposable screen‐printed carbon electrode and its application of 4‐mercaptopyridine in in situ electrochemical surface‐enhanced Raman scattering

Column electrodes pretreated through oxidation–reduction cycles were traditionally used in electrochemical surface‐enhanced Raman scattering (SERS). In this study, a disposable screen‐printed carbon electrode was introduced into in situ electrochemical SERS through the electrodeposition of dendritic gold/silver nanoparticles (Au/AgNPs) onto the surface of the carbon working electrode to induce the SERS enhancement effect on the electrode. Scanning electron microscopy images showed that dendritic Au/AgNPs nanostructures could be fabricated under appropriate electrodeposition conditions and could present a minimum SERS factor of 4.25 × 10⁵. Furthermore, the absorbed behavior of 4‐mercaptopyridine was investigated under different potentials. The adsorption configuration was inferred to transform from ‘vertical’ to ‘lying‐flat’. The proposed new electrode combined with a portable Raman spectrometer could be useful in the identifying products or intermediates during electrochemical synthesis or electrochemical catalysis in in situ electrochemical SERS. Copyright © 2016 John Wiley & Sons, Ltd.     

Local and Remote Charge‐Transfer‐Enhanced Raman Scattering on One‐Dimensional Transition‐Metal Oxides

The one‐dimensional (1D) transition‐metal oxide MoO₃ belt is synthesized and characterized with X‐ray diffraction, scanning electron microscopy, and Raman spectroscopy. Charge‐transfer‐(CT) enhanced Raman scattering of 4‐mercaptobenzoic acid (4‐MBA) on a 1D MoO₃ belt was investigated experimentally and theoretically. The chemical enhancement of surface‐enhanced Raman scattering (SERS) of 4‐MBA on the MoO₃ belt by CT is in the order of 10³. The SERS of 4‐MBA was investigated theoretically by using a quantum chemical method. The remote SERS of 4‐MBA along the 1D MoO₃ belt (the light excitation to one side of the MoO₃ belt, and the SERS spectrum is collected on the other side of the MoO₃ belt) is also shown experimentally, which provides potential applications of SERS. The incident polarization dependence of remote SERS spectra has also been investigated experimentally.     

Environmental‐Confinement‐Induced Stability Enhancement of Chiral Molecules

We computationally study the transition process of a chiral difluorobenzo[c]phenanthrene (DFBcPh) molecule within non‐polar fullerene C₂₆₀ to explore the confinement effect. We find blue‐shifts in the infrared and Raman spectra of the molecule inside the fullerene relative to those of isolated systems. Six types of spectrum features of the molecule appear in the 0–60 cm^?1 band. Interestingly, the energy barrier of the chiral transformation of the molecule is elevated by 15.88 kcal mol^?1 upon the confinement by the fullerene, indicating improvement in the stability of the enantiomers. The protection by C₂₆₀ lowers the highest occupied molecular orbital energy level and lifts the lowest unoccupied molecular orbital energy level of the chiral molecule such that the chiral molecule is further chemically stabilized. We concluded that the confinement environment has an impact at the nanoscale on the enantiomer transformation process of the chiral molecule.     

Novel Silver/Poly(vinyl alcohol) Nanocomposites for Surface‐Enhanced Raman Scattering‐Active Substrates

Summary: Surface‐enhanced Raman scattering (SERS)‐active substrates with high enhancement were prepared by an in situ reduction method. Novel silver/poly(vinyl alcohol) (PVA) nanocomposite films were obtained, in which the silver nitrate, poly(γ‐glutamic acid) (PGA), and PVA acted as precursor, stabilizer, and polyol reducant, respectively. The UV‐visible spectra of the as‐fabricated films showed that the surface plasmon resonance (SPR) absorption band was narrow and of a stronger intensity, which indicates that the Ag nanoparticle size distribution on the substrate was highly uniform. This finding was further confirmed by X‐ray diffraction (XRD), transmission electron microscopy (TEM), and field‐emission scanning electron microscope (FE‐SEM) measurements. It was found that a PGA‐stabilized PVA nanocomposite film revealed the presence of well‐dispersed spherical silver nanoparticles with an average diameter of 90 nm. The new substrate presents high SERS enhancement and the enhanced factor is estimated to be 10⁶ for the detection of benzoic acid.

The Raman scattering enhancement factor for the Raman spectra of benzoic acid on the various nanocomposite films.     

Chiral discrimination in hydrogen‐bonded complexes of hydrogen peroxide with methyl hydroperoxide: Theoretical study

For the first time, the discrimination of different chiral forms of 1:1 complexes with hydrogen peroxide and methyl hydroperoxide have been investigated using density functional theory (DFT) and Møller–Plesset type 2 (MP2) methods at varied basis set levels from 6‐31+G(d,p) to 6‐31++G(2d,2p). Three pairs of chiral enantiomers were considered. The optimized geometric parameters, interaction energies, and chirodiastatic energies for various isomers at different levels are estimated. To take into account the water solvation effect, the polarized continuum model (PCM) method has been used to evaluate the ΔG_solv. The gas phase results show that the heterochiral six‐membered ring complex (structure I) and homochiral five‐membered ring complexes (structures IV and V) are preferred configurations for the three pairs of chiral enantiomers. The solvation effect on six‐membered ring complexes (structures I and II) shows nonsignificant changes in the configurations preferred, but on five‐membered ring complexes, the homo‐/heterochiral preference is found to be inverse in the polar solvent. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Distinguishing Enantiomers by Tip‐Enhanced Raman Scattering: Chemically Modified Silver Tip with an Asymmetric Atomic Arrangement

Discrimination between enantiomers is achieved by tip‐enhanced Raman scattering (TERS) using a silver tip that is chemically modified by an achiral para‐mercaptopyridine (pMPY) probe molecule. Differences in the relative intensities of the pMPY spectra were monitored for three pairs of enantiomers containing hydroxy (?OH) and/or amino (?NH₂) groups. The N: or N⁺?H functionality of the pMPY‐modified tip participates in hydrogen‐bond interactions with a particular molecular orientation of each chiral isomer. The asymmetric arrangement of silver atoms at the apex of the tip induces an asymmetric electric field, which causes the tip to become a chiral center. Differences in the charge‐transfer (CT) states of the metal‐achiral probe system in conjunction with the asymmetric electric field produce different enhancements in the Raman signals of the two enantiomers. The near‐field effect of the asymmetric electric field, which depends on the number of analyte functional groups capable of hydrogen‐bond formation, improves the degree of discrimination.     

Chiral sensing of Eu(III)‐containing achiral polymer complex from chiral amino acids coordination induction

The β‐diketonate‐based achiral polymer P‐1 could be synthesized by the polymerization of 3,7‐dibromo‐2,8‐dimethoxy‐5,5‐dioctyl‐5H‐dibenzo[b,d]silole ( M1 ) with (Z)?1,3‐bis(4‐ethynylphenyl)?3‐hydroxyprop‐en‐1‐one ( M2 ) via typical Sonogashira coupling reaction. The β‐diketonate unit in the main chain backbone of P‐1 can further coordinate with Eu(TTA)_x [TTA^? = 4,4,4‐trifluoro‐1‐(thiophen‐2‐yl)butane‐1,3‐dionate anion, X = 1, 2, 3] to afford corresponding Eu(III)‐containing polymer complexes. The resulting achiral polymer complex P‐2 (X = 2) can exhibit strong circular dichroism (CD) response toward both N‐Boc‐l and d‐ proline enantiomers. The CD signal was preliminarily attributed to coordination induction between chiral N‐Boc‐proline and the Eu(III) complex moiety. The linear regression analysis of CD sensing shows a good agreement between the magnitude of molar ellipticity and concentration of chiral N‐Boc‐l or d‐ proline, which indicates this kind Eu(III)‐containing achiral polymer complex can be used as a chiral probe for enantioselective recognition of N‐Boc‐l or d‐ proline enantiomers based on Cotton effect of CD spectra. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3080–3086     

Natural terpene derivatives as new structural task‐specific ionic liquids to enhance the enantiorecognition of acidic enantiomers on teicoplanin‐based stationary phase by high‐performance liquid chromatography

We present the specific cooperative effect of a semisynthetic glycopeptide antibiotic teicoplanin and chiral ionic liquids containing the (1R ,2S ,5R )‐(–)‐menthol moiety on the chiral recognition of enantiomers of mandelic acid, vanilmandelic acid, and phenyllactic acid. Experiments were performed chromatographically on an Astec Chirobiotic T chiral stationary phase applying the mobile phase with the addition of the chiral ionic liquids. The stereoselective binding of enantiomers to teicoplanin in presence of new chiral ionic liquids were evaluated applying thermodynamic measurements and the docking simulations. Both the experimental and theoretical methods revealed that the chiral recognition of enantiomers in the presence of new chiral ionic liquids was enthalpy driven. The changes of the teicoplanin conformation occurring upon binding of the chiral ionic liquids are responsible for the differences in the standard changes in Gibbs energy (ΔG ⁰) values obtained for complexes formed by the R and S enantiomers and teicoplanin. Docking simulations revealed the steric adjustment between the chiral ionic liquids cyclohexane ring (chair conformation) and the β‐d ‐glucosamine ring of teicoplanin and additionally hydrophobic interactions between the decanoic aliphatic chain of teicoplanin and the alkyl group of the tested salts. The obtained terpene derivatives can be considered as “structural task‐specific ionic liquids” responsible for enhancing the chiral resolution in synergistic systems with two chiral selectors.     

Excitation‐wavelength‐dependent charge transfer resonance of bipyridines on silver nanoparticles: surface‐enhanced Raman scattering study

Charge transfer (CT) resonance mechanisms of 2,2′‐bipyridine (2,2′‐BiPy), 2,4′‐bipyridine (2,4′‐BiPy), and 4,4′‐bipyridine (4,4′‐BiPy) on silver nanoparticle surfaces have been comparatively investigated by means of surface‐enhanced Raman scattering (SERS) at the excitation wavelengths of 457, 514, 633, and 785 nm. A combination of the electromagnetic (EM) and charge transfer (CT) contributions should affect the SERS intensities for the bipyridine compounds adsorbed on silver nanoparticle surfaces. The CT resonance is assumed to occur in dissimilar ways for the bipyridine compounds, as evidenced from their different excitation‐wavelength‐dependent SERS enhancement factors. Ab initio density functional theory (DFT) calculations at the level of B3LYP/LANL2DZ have been carried out for the bipyridine‐Ag complexes. Copyright © 2007 John Wiley & Sons, Ltd.     

Amino Acid Ionic Liquids as Chiral Ligands in Ligand‐Exchange Chiral Separations

Recently, amino acid ionic liquids (AAILs) have attracted much research interest. In this paper, we present the first application of AAILs in chiral separation based on the chiral ligand exchange principle. By using 1‐alkyl‐3‐methylimidazolium L ‐proline (L ‐Pro) as a chiral ligand coordinated with copper(II), four pairs of underivatized amino acid enantiomers—dl ‐phenylalanine (dl ‐Phe), dl ‐histidine (dl ‐His), dl ‐tryptophane (dl ‐Trp), and dl ‐tyrosine (dl ‐Tyr)—were successfully separated in two major chiral separation techniques, HPLC and capillary electrophoresis (CE), with higher enantioselectivity than conventionally used amino acid ligands (resolution (R_s)=3.26–10.81 for HPLC; R_s=1.34–4.27 for CE). Interestingly, increasing the alkyl chain length of the AAIL cation remarkably enhanced the enantioselectivity. It was inferred that the alkylmethylimidazolium cations and L ‐Pro form ion pairs on the surface of the stationary phase or on the inner surface of the capillary. The ternary copper complexes with L ‐Pro are consequently attached to the support surface, thus inducing an ion‐exchange type of retention for the dl ‐enantiomers. Therefore, the AAIL cation plays an essential role in the separation. This work demonstrates that AAILs are good alternatives to conventional amino acid ligands for ligand‐exchange‐based chiral separation. It also reveals the tremendous application potential of this new type of task‐specific ILs.     

Chemical mechanism of surface‐enhanced raman scattering spectrum of pyridine adsorbed on Ag cluster: Ab initio molecular dynamics approach

The surface‐enhanced Raman scattering (SERS) spectrum of pyridine adsorbed on Ag₂₀ cluster (pyridine‐Ag₂₀) at room temperature is calculated by performing ab initio molecular dynamics simulations in connection with a Fourier transform of the polarizability autocorrelation function to investigate the static chemical enhancement behind the SERS spectrum. The five enhanced vibrational modes of pyridine, namely, υ_6a, υ₁, υ₁₂, υ_9a, and υ_8a, can be assigned and identified by using a new analytical scheme, namely, single‐frequency‐pass filter, which is based on a Fourier transform filtering technique. To understand the factors evoking the enhancement in the SERS spectrum, the dynamic properties of molecular structures and charges for both of the free pyridine and adsorbed pyridine are analyzed. The calculated results indicate that the vibrational amplitudes of adsorbed pyridine are enhanced due to both of the electron transfer from pyridine to Ag₂₀ cluster and the softening of pyridine bond. In addition, the N‐Ag stretching within pyridine‐Ag₂₀ will couple with these five vibrational modes of pyridine. Consequently, the electron transfer between pyridine and Ag₂₀ cluster induced by different molecular vibrational modes prompts the redistribution of electron density of pyridine. These factors collectively cause the noticeable change in polarizability during molecular vibrations and hence result in the enhancement of Raman peaks. © 2013 Wiley Periodicals, Inc.     

Optically Biaxial,Re‐entrant and Frustrated Mesophases in Chiral,Non‐symmetric Liquid Crystal Dimers and Binary Mixtures

Sixteen optically active, non‐symmetric dimers, in which cyanobiphenyl and salicylaldimine mesogens are interlinked by a flexible spacer, were synthesized and characterized. While the terminal chiral tail, in the form of either (R)‐2‐octyloxy or (S)‐2‐octyloxy chain attached to salicylaldimine core, was held constant, the number of methylene units in the spacer was varied from 3 to 10 affording eight pairs of (R & S) enantiomers. They were probed for their thermal properties with the aid of orthoscopy, conoscopy, differential scanning calorimetry and X‐ray powder diffraction. In addition, the binary mixture study was carried out using chiral and achiral dimers with the intensions of stabilizing optically biaxial phase/s, re‐entrant phases and important phase sequences. Notably, one of the chiral dimers as well as some mixtures exhibited a biaxial smectic A (SmA_b) phase appearing between a uniaxial SmA and a re‐entrant uniaxial SmA phases. The mesophases such as chiral nematic (N*) and frustrated phases viz., blue phases (BPs) and twist grain boundary (TGB) phases, were also found to occur in most of the dimers and mixtures. X‐ray diffraction studies revealed that the dimers possessing oxybutoxy and oxypentoxy spacers show interdigitated (SmA_d) phase where smectic periodicity is over 1.4 times the molecular length; whereas in the intercalated SmA (SmA_c) phase formed by a dimer having oxydecoxy spacer the periodicity was found to be approximately half the molecular length. The handedness of the helical structure of the N* phases formed by two enantiomers was examined with the aid of CD measurements; as expected, these enantiomers showed optical activities of equal magnitudes but with opposite signs. Overall, it appears that the chiral dimers and mixtures presented herein may serve as model systems in design and developing novel materials exhibiting the apolar SmA_b phase possessing D_2h symmetry and nematic‐type biaxiality.     

Silver Nanoparticle Thin Films with Nanocavities for Surface‐Enhanced Raman Scattering

The formation of nanometer‐sized gaps between silver nanoparticles is critically important for optimal enhancement in surface‐enhanced Raman scattering (SERS). A simple approach is developed to generate nanometer‐sized cavities in a silver nanoparticle thin film for use as a SERS substrate with extremely high enhancement. In this method, a submicroliter volume of concentrated silver colloidal suspension stabilized with cetyltrimethylammonium bromide (CTAB) is spotted on hydrophobic glass surfaces prepared by the exposure of the glass to dichloromethysilane vapors. The use of a hydrophobic surface helps the formation of a more uniform silver nanoparticle thin film, and CTAB acts as a molecular spacer to keep the silver nanoparticles at a distance. A series of CTAB concentrations is investigated to optimize the interparticle distance and aggregation status. The silver nanoparticle thin films prepared on regular and hydrophobic surfaces are compared. Rhodamine 6G is used as a probe to characterize the thin films as SERS substrates. SERS enhancement without the contribution of the resonance of the thin film prepared on the hydrophobic surface is calculated as 2×10⁷ for rhodamine 6G, which is about one order of magnitude greater than that of the silver nanoparticle aggregates prepared with CTAB on regular glass surfaces and two orders of magnitude greater than that of the silver nanoparticle aggregates prepared without CTAB on regular glass surfaces. A hydrophobic surface and the presence of CTAB have an increased effect on the charge‐transfer component of the SERS enhancement mechanism. The limit of detection for rhodamine 6G is estimated as 1.0×10^?8 M . Scanning electron microscopy and atomic force microscopy are used for the characterization of the prepared substrate.     

Highly Reproducible Surface‐Enhanced Raman Spectra on Semiconductor SnO2 Octahedral Nanoparticles

Highly reproducible surface‐enhanced Raman scattering (SERS) spectra are obtained on the surface of SnO₂ octahedral nanoparticles. The spot‐to‐spot SERS signals show a relative standard deviation (RSD) consistently below 20 % in the intensity of the main Raman peaks of 4‐mercaptobenzoic acid (4‐MBA) and 4‐nitrobenzenethiol (4‐NBT), indicating good spatial uniformity and reproducibility. The SERS signals are believed to mainly originate from a charge‐transfer (CT) mechanism. Time‐dependent density functional theory (TD‐DFT) is used to simulate the SERS spectrum and interpret the chemical enhancement mechanism in the experiment. The research extends the application of SERS and also establishes a new uniform SERS substrate.     

Evaluation and Comparison of a 3,5‐Dimethylphenyl Isocyanate Teicoplanin with Phenyl Isocyanate Teicoplanin Chiral Stationary Phase Using RP‐HPLC

HPLC enantiomeric separations of 8 α‐amino acids were achieved using two self‐made chiral stationary phases (CSP)–phenyl isocyanate teicoplanin (Phe‐TE) and 3,5‐dimethylphenyl isocyanate teicoplanin (DMP‐TE), using reversed phase mobile phases. The Phe‐TE or the DMP‐TE CSP was prepared from the TE using derivative agents, phenyl isocyanate or 3,5‐dimethylphenyl isocyanate, respectively. The chromatographic results were given as the retention, selectivity, resolution factor and the enantioselective free energy difference corresponding to the separation of the two enantiomers. The effect of pH, organic modifier type and amount were discussed, and the stereoselectivities for two TE‐based CSPs were compared. The chiral selectivity factor for six α‐amino acids on DMP‐TE is somewhat bigger than that on Phe‐TE CSP under reversed phase (RP) mode. Comparison of the enantiomeric separations using self‐made Phe‐TE and DMP‐TE was conducted to gain a better understanding of the chiral recognition mechanism of the macrocyclic glycopeptide CSP.     

Enantioselective Partitioning of Racemic Ibuprofen in a Biphasic Recognition Chiral Extraction System

Enantioselective partitioning of ibuprofen enantiomers in a biphasic recognition chiral extraction system was studied. A combination of hydrophobic L‐isobutyl tartrate in organic phase and hydrophilic β‐cyclodextrin derivative in aqueous phase is necessary to establish a biphasic recognition chiral extraction system. The studies performed involve an enantioselective extraction in a biphasic system, where ibuprofen enantiomers form four complexes with the β‐cyclodextrin derivative in aqueous phase and the D(L)‐isobutyl tartrate in organic phase, respectively. In these biphasic resolutions, the types and the concentrations of the extractants, pH and temperature all exert a considerable influence on the biphasic recognition process. Good enantioselectivities for ibuprofen enantiomers were obtained at pH≦2.5 and a ratio of 2:1 of [L‐isobutyl tartrate] to [HP‐β‐CD]. Biphasic recognition chiral extraction is of strong chiral separation ability, and may be very helpful to optimize the extraction systems and realize the large‐scale production of enantiomers.