A second harmonic generation analog of optical rotatory dispersion for the study of chiral monolayers

Nonlinear Chiral Metasurfaces Based on Structured van der Waals Materials

Nonlinear chiral photonics explores the nonlinear response of chiral structures, and it offers a pathway to novel optical functionalities not accessible through linear or achiral systems. Here we present the first application of nanostructured van der Waals materials to nonlinear chiral photonics. We demonstrate the 3 orders of magnitude enhancement of the third-harmonic generation from hBN metasurfaces driven by quasi-bound states in the continuum and accompanied by strong nonlinear circular dichroism at the resonances. This novel platform for chiral metaphotonics can be employed for achieving large circular dichroism combined with high-efficiency harmonic generation in a broad frequency range.

Diagrammatic theory of magnetic and quadrupolar contributions to sum-frequency generation in composite systems

Second-order nonlinear processes like Sum-Frequency Generation (SFG) are essentially defined in the electric dipolar approximation. However, when dealing with the SFG responses of bulk, big nanoparticles, highly symmetric objects, or chiral species, magnetic and quadrupolar contributions play a significant role in the process too. We extend the diagrammatic theory for linear and nonlinear optics to include these terms for single objects as well as for multipartite systems in interaction. Magnetic and quadrupolar quantities are introduced in the formalism as incoming fields, interaction intermediates, and sources of optical nonlinearity. New response functions and complex nonlinear processes are defined, and their symmetry properties are analyzed. This leads to a focus on several kinds of applications involving nanoscale coupled objects, symmetric molecular systems, and chiral materials, both in line with the existing literature and opening new possibilities for original complex systems.

Negative Nonlinear CD-ee Dependence in Polycrystalline BINOL Thin Films

Generally, the relationship between the observed circular dichroism and the enantiomeric excess in chiral systems (CD-ee dependence) is linear. While positive nonlinear behavior has often been reported in the past, examples of negative nonlinear (NN) behavior in CD-ee dependence are rare and not well understood. Here, we present a strong NN CD-ee dependence within polycrystalline thin films of BINOL by using second-harmonic-generation circular dichroism (SHG-CD) and commercial CD spectroscopy studies. Theoretical calculations, microscopy, and FTIR studies are employed to further clarify the underlying cause of this observation. This behavior is attributed to the changing supramolecular chirality of the system. Systems exhibiting NN CD-ee dependence hold promise for highly accurate enantiomeric excess characterization, which is essential for the refinement of enantio-separating and -purifying processes in pharmaceuticals, asymmetric catalysis, and chiral sensing. Our findings suggest that a whole class of single-species systems, i.e., racemate crystals, might possess NN CD-ee dependence and thus provide us a vast playground to better understand and exploit this phenomenon.

Second harmonic Rayleigh scattering optical activity of single Ag nanohelices in a liquid

Determining the chirality of molecules and nanoparticles often relies on circular dichroism and optical rotation: two chiral optical (chiroptical) effects in the linear optical regime. Although these linear effects are weak compared to nonlinear chiroptical effects, they have the advantage of being measured in isotropic liquids - free from the complications of anisotropy. Recently, a nonlinear effect: hyper-Rayleigh scattering optical activity (HRS OA) has been shown to reliably distinguish between the two chiral forms of Ag nanohelices, suspended in isotropic liquids. However, this first demonstration of HRS OA also opened new questions. For instance, at a fundamental level, it is not clear what the role of interactions between nanoparticles is. Moreover, the influence of the ultrafast pulse chirp is unknown. Here, we demonstrate HRS OA from well below two Ag nanohelices in the illumination volume, precluding any interactions. Additionally, we performed the first measurements of HRS depolarization ratios in this system and find a value of ≈1. We also show that HRS is highly robust against the chirp of the ultrafast pulses. An important reason for the strong (down to single nanohelix) sensitivity of our experiments is the large chiroptical interaction at the fundamental frequency; this point is illustrated with two sets of numerical simulations of the electromagnetic near-fields. Our results highlight HRS OA as a highly sensitive experimental method for characterization of chiral solutions/suspensions, in tiny illumination volumes.

Polarimetric Measurements of Surface Chirality Based on Linear and Nonlinear Light Scattering

A molecule, molecular aggregate, or protein that cannot be superimposed on its mirror image presents chirality. Most living systems are organized by chiral building blocks, such as amino acids, peptides, and carbohydrates, and any change in their molecular structure (i.e., handedness or helicity) alters the biochemical and pharmacological functions of the molecules, many of which take place at surfaces. Therefore, studying surface chirogenesis at the nanoscale is fundamentally important and derives various applications. For example, since proteins contain highly ordered secondary structures, the intrinsic chirality can be served as a signature to measure the dynamics of protein adsorption and protein conformational changes at biological surfaces. Furthermore, a better understanding of chiral recognition and separation at bio-nanointerfaces is helpful to standardize chiral drugs and monitor the synthesis of adsorbents with high precision. Thus, exploring the changes in surface chirality with polarized excitations would provide structural and biochemical information of the adsorbed molecules, which has led to the development of label-free and noninvasive measurement tools based on linear and nonlinear optical effects. In this review, the principles and selected applications of linear and nonlinear optical methods for quantifying surface chirality are introduced and compared, aiming to conceptualize new ideas to address critical issues in surface biochemistry.

One-Fold Anisotropy of Silver Chiral Nanoparticles Studied by Second-Harmonic Generation

Second-harmonic generation (SHG) integrated with diverse nonlinear optical activity characterization has high sensitivity to detect the symmetry of materials at an interface, but the study is in its infancy. Here, we employ SHG with linear dichroism (or SHG-LD) to study the chiroptical origin of silver (Ag) chiral nanoparticles (CNPs) deposited by glancing angle deposition (GLAD). It is found that Ag CNPs show the chiroptical activity ascribed to not only the structural chirality (i.e., atomically chiral lattices) but also one-fold anisotropy at an interface due to the substrate rotation during GLAD. Therefore, the SHG-LD shows great potential to provide valuable complementary information to study the chiroptical properties of chiral metamaterials.

Understanding laser desorption with circularly polarized light

We present aspects of emerging optical activity in thin racemic 1,1'-Bi-2-naphthol films upon irradiation with circularly polarized light and subsequent resonant two-photon absorption in the sample. Thorough analysis of the sample morphology is conducted by means of (polarization-resolved) optical microscopy and scanning electron microscopy (SEM). The influence of crystallization on the nonlinear probing technique (second harmonic generation circular dichroism [SHG-CD]) is investigated. Optical activity and crystallization are brought together by a systematic investigation in different crystallization regimes. We find crystallization to be responsible for two counter-acting effects, which arise for different states of crystallization. Measuring crystallized samples offers the best signal-to-noise ratio, but it limits generation of optical activity due to self-assembly effects. For suppression of crystallization on the other hand, there is a clear indication that enantiomeric selective desorption is responsible for the generation of optical activity in the sample. We reach the current resolution limit of probing with SHG-CD, as we suppress the crystallization in the racemic sample during desorption. In addition, intensity-dependent measurements on the induced optical activity reveal an onset threshold (≈0.7 TW cm-2), above which higher order nonlinear processes impair the generation of optical activity by desorption with CPL.

Metal Ion Mediation of Interfacial Chiral Supramolecular Formation of Amphiphilic Schiff Bases Studied by In Situ Second Harmonic Generation

The coordination effects between chiral Schiff bases and metal ions can provide an effective strategy for fabricating chiral supramolecular self-assemblies. We studied the supramolecular self-assembly of the amphiphilic Schiff base enantiomer, 2-hydroxy-1-naphthylmethylamino-N,N'-bis(octadecyl)-l/d-glutamic diamide (l/dGJ), at the air/water interfaces by using in situ second harmonic generation linear dichroism (SHG-LD) technique combined with UV-vis spectroscopy and atom force microscopy. lGJ and dGJ monolayers can form mirror-image structures of each other at different interfaces, which can be mediated by metal ions. When Mg²⁺ and Zn²⁺ ions were added into the subphases, the l/dGJ monolayers self-assembled into a nanofiber-like structure and showed significant chirality similar to that at the pure water interface. However, when Cu²⁺ ions were added into the subphase, the chirality of the l/dGJ monolayer was destroyed because of the Cu²⁺ coordination effect. The degree of the chirality excess (DCE) value decreased with the increment of the concentration of Cu²⁺ in the subphase. Furthermore, when the surface pressure increased, the DCE value of Cu²⁺-l/dGJ complexes increased gradually, which indicated that the Cu²⁺-l/dGJ complex aggregated into a chiral supramolecular structure through lateral molecular packing.

Chiroptical Properties in Thin Films of π-Conjugated Systems

Chiral π-conjugated molecules provide new materials with outstanding features for current and perspective applications, especially in the field of optoelectronic devices. In thin films, processes such as charge conduction, light absorption, and emission are governed not only by the structure of the individual molecules but also by their supramolecular structures and intermolecular interactions to a large extent. Electronic circular dichroism, ECD, and its emission counterpart, circularly polarized luminescence, CPL, provide tools for studying aggregated states and the key properties to be sought for designing innovative devices. In this review, we shall present a comprehensive coverage of chiroptical properties measured on thin films of organic π-conjugated molecules. In the first part, we shall discuss some general concepts of ECD, CPL, and other chiroptical spectroscopies, with a focus on their applications to thin film samples. In the following, we will overview the existing literature on chiral π-conjugated systems whose thin films have been characterized by ECD and/or CPL, as well other chiroptical spectroscopies. Special emphasis will be put on systems with large dissymmetry factors (g_abs and g_lum) and on the application of ECD and CPL to derive structural information on aggregated states.

Enantiospecific Desorption Triggered by Circularly Polarized Light

The interest in enantioseparation and enantiopurification of chiral molecules has been drastically increasing over the past decades, since these are important steps in various disciplines such as pharmaceutical industry, asymmetric catalysis, and chiral sensing. By exposing racemic samples of BINOL (1,1′‐bi‐2‐naphthol) coated onto achiral glass substrates to circularly polarized light, we unambiguously demonstrate that by controlling the handedness of circularly polarized light, preferential desorption of enantiomers can be achieved. There are currently no mechanisms known that would describe this phenomenon. Our observation together with a simplified phenomenological model suggests that the process of laser desorption needs to be further developed and the contribution of quantum mechanical processes should be revisited to account for these data. Asymmetric laser desorption provides us with a contamination‐free technique for the enantioenrichment of chiral compounds.

Applications of Surface Second Harmonic Generation in Biological Sensing

Surface second harmonic generation (SHG) is a coherent, nonlinear optical technique that is well suited for investigations of biomolecular interactions at interfaces. SHG is surface specific due to the intrinsic symmetry constraints on the nonlinear process, providing a distinct analytical advantage over linear spectroscopic methods, such as fluorescence and UV-Visible absorbance spectroscopies. SHG has the ability to detect low concentrations of analytes, such as proteins, peptides, and small molecules, due to its high sensitivity, and the second harmonic response can be enhanced through the use of target molecules that are resonant with the incident (ω) and/or second harmonic (2ω) frequencies. This review describes the theoretical background of SHG, and then it discusses its sensitivity, limit of detection, and the implementation of the method. It also encompasses the applications of surface SHG directed at the study of protein-surface, small-molecule-surface, and nanoparticle-membrane interactions, as well as molecular chirality, imaging, and immunoassays. The versatility, high sensitivity, and surface specificity of SHG show great potential for developments in biosensors and bioassays.

Strong Rotational Anisotropies Affect Nonlinear Chiral Metamaterials

Masked by rotational anisotropies, xthe nonlinear chiroptical response of a metamaterial is initially completely inaccessible. Upon rotating the sample the chiral information emerges. These results highlight the need for a general method to extract the true chiral contributions to the nonlinear optical signal, which would be hugely valuable in the present context of increasingly complex chiral meta/nanomaterials.

Giant Nonlinear Optical Activity of Achiral Origin in Planar Metasurfaces with Quadratic and Cubic Nonlinearities

3D chirality is shown to be unnecessary for introducing strong circular dichroism for harmonic generations. Specifically, near-unity circular dichroism for both second-harmonic generation and third-harmonic generations is demonstrated on suitably designed ultrathin plasmonic metasurfaces with only 2D planar chirality. The study opens up new routes for designing chip-type biosensing platform, which may allow for highly sensitive detection of bio- and chemical molecules with weak chirality.

Orientational changes of supported chiral 2,2'-dihydroxy-1,1'binaphthyl molecules

Well defined thin molecular films of 2,2'-dihydroxy-1,1'binaphthyl (binol) molecules at coverages between 5 × 10(15) molecules per cm(2) and 10(17) molecules per cm(2) on thin glass (BK7) substrates were investigated under ultra-high-vacuum (UHV) conditions. Second-Harmonic-Generation Optical-Rotatory-Dispersion measurements (SHG-ORD) were performed using a dedicated spectroscopic setup which allows for the determination of the rotation angle of the SH-signal of two enantiomers. Rotation angles of up to 38 degrees were measured. The chirality of the two enantiomers has been studied at 674 nm (337 nm resonance wavelength) in the transmission mode. Coverage dependent orientation evolution of binol molecular films was revealed by precise monitoring of the surface coverage while performing SHG-ORD experiments. We show that the molecules reach their final orientation at a surface coverage of 5 × 10(16) molecules per cm(2). From the obtained experimental data the ratio of chiral and achiral susceptibility components could be calculated and was observed to change with coverage.

Chirality Discriminated by Heterodyne-Detected Vibrational Sum Frequency Generation

We first demonstrated chiral vibrational sum frequency generation (VSFG) in the heterodyne detection, which enables us to uniquely determine chiral second-order nonlinear susceptibility consisting of phase and amplitude and distinguish molecular chirality with high sensitivity. Liquid limonene was measured to evaluate the heterodyne-detected chiral VSFG developed in this study. R-(+)- and S-(-)-limonene showed clearly opposite signs in the complex spectra of the second-order nonlinear susceptibility in the CH stretching region. This is the first report of the chiral distinction by VSFG without any a priori knowledge about chiral and achiral spectral response. Furthermore, from the phase of the chiral VSFG field measured in the heterodyne detection, the origin of the chiral signal was ascribed to the bulk limonene. The heterodyne detection also improves detection limits significantly, allowing us to observe weak chiral signals in reflection. The heterodyne-detected chiral VSFG can provide information on absolute molecular configuration.

Nonlinear superchiral meta-surfaces: tuning chirality and disentangling non-reciprocity at the nanoscale

Circularly polarized light is incident on a nanostructured chiral meta-surface. In the nanostructured unit cells whose chirality matches that of light, superchiral light is forming and strong optical second harmonic generation can be observed.

Chiral imaging of collagen by second-harmonic generation circular dichroism

We provide evidence that the chirality of collagen can give rise to strong second-harmonic generation circular dichroism (SHG-CD) responses in nonlinear microscopy. Although chirality is an intrinsic structural property of collagen, most of the previous studies ignore that property. We demonstrate chiral imaging of individual collagen fibers by using a laser scanning microscope and type-I collagen from pig ligaments. 100% contrast level of SHG-CD is achieved with sub-micrometer spatial resolution. As a new contrast mechanism for imaging chiral structures in bio-tissues, this technique provides information about collagen morphology and three-dimensional orientation of collagen molecules.

Characterization of nanostructured plasmonic surfaces with second harmonic generation

Because of its high surface and interface sensitivity, the nonlinear optical technique of second harmonic generation (SHG) is a designated method for investigating nanostructured metal surfaces. Indeed, the latter present a high surface-to-volume ratio, but even more importantly, they can exhibit strong near-field enhancements or "hot spots". Hot spots often appear as a result of geometric features on the nanoscale or surface plasmon resonances, which are collective electron oscillations on the surface that, on the nanoscale, can readily be excited by light. In the last 10 years, near-field hot spots have been responsible for dramatic developments in the field of nano-optics. In this Feature Article, the influence of hot spots on the SHG response of nanostructured metal surfaces is discussed on both the microscopic and macroscopic levels. On the microscopic level, the nanostructured metal surfaces were characterized by scanning SHG microscopy, complemented by rigorous numerical simulations of the near-field and of the local electric currents at the fundamental frequency. On the macroscopic level, SHG-circular dichroism and magnetization-induced SHG characterization techniques were employed.

Chiral vibrational structures of proteins at interfaces probed by sum frequency generation spectroscopy

We review the recent development of chiral sum frequency generation (SFG) spectroscopy and its applications to study chiral vibrational structures at interfaces. This review summarizes observations of chiral SFG signals from various molecular systems and describes the molecular origins of chiral SFG response. It focuses on the chiral vibrational structures of proteins and presents the chiral SFG spectra of proteins at interfaces in the C-H stretch, amide I, and N-H stretch regions. In particular, a combination of chiral amide I and N-H stretches of the peptide backbone provides highly characteristic vibrational signatures, unique to various secondary structures, which demonstrate the capacity of chiral SFG spectroscopy to distinguish protein secondary structures at interfaces. On the basis of these recent developments, we further discuss the advantages of chiral SFG spectroscopy and its potential application in various fields of science and technology. We conclude that chiral SFG spectroscopy can be a new approach to probe chiral vibrational structures of protein at interfaces, providing structural and dynamic information to study in situ and in real time protein structures and dynamics at interfaces.

Enantioselective adsorption of surfactants monitored by ATR-FTIR

The selectivity of adsorption of chiral surfactants to a chiral monolayer at the solid-liquid interface was studied using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). One enantiomer of the chiral surfactant was deuterated, which causes a change in the IR absorption frequency, and allows independent measurement of the adsorption of each molecule. Both the surfactant, N-lauroyl phenylalanine (NLP), and the chiral monolayer, N-L-phenylalaninoyl, 11-undecyl-silicon, were amino acid derivatives. An enantiomeric excess of 56 +/- 22% of the L over D was observed for adsorption to the interface between a carbon tetrachloride solution containing a quasi-racemate of N-lauroyl phenylalanine and the N-L-phenylalaninoyl, 11-undecyl monolayer film on silicon. In contrast, equimolar adsorption occurred from an equimolar mixture of hydrogenated and deuterated forms of the L surfactant. The measured enantiomeric excess strongly depended on the density of chiral surface groups: the higher the density of chiral groups on the surface, the better the enantiodiscrimination, even though the total adsorption was roughly constant. This nonlinear behavior indicates that more than one chiral surface group is required for significant selectivity.

Linearly polarized second harmonic generation microscopy reveals chirality

In optics, chirality is typically associated with circularly polarized light. Here we present a novel way to detect the handedness of chiral materials with linearly polarized light. We performed Second Harmonic Generation (SHG) microscopy on G-shaped planar chiral nanostructures made of gold. The SHG response originates in distinctive hotspots, whose arrangement is dependent of the handedness. These results uncover new directions for studying chirality in artificial materials.

Rotationally Resolved Optical Rotation and Circular Dichroism Effects for Symmetric Top Molecules Induced by a Resonant Circularly Polarized Pumping Optical Field

The rotationally resolved laser-induced optical activity including the laser-induced optical rotation (LIOR) and laser-induced circular dichroism (LICD) effects of an IR probing light pumped by a collinear intense resonant circularly polarized light dependent on the third-order susceptibility due to the pure electric dipole interaction for achiral symmetric top molecules in the gas phase is discussed theoretically. The laser-induced optical activity contains four distinct contributions named A, B, C, and D terms: the B term of the LIOR and LICD arising from the rotational wave function perturbed by the pumping light is deduced using the semiclassical perturbation theory, and the expressions for A, C, and D terms, respectively, due to the ac Stark effect, the Boltzmann statistical redistribution, and the alteration of occupation polability, are obtained from previous results [Zheng, R.-H.; Chen, D.-M.; Wei, W.-M.; He, T.-J.; Liu, F.-C. J. Chem. Phys. 2004, 121, 6835]. The microwave-IR double resonant spectrum is proposed to detect the LIOR and LICD effects. As an example, the LIOR and LICD for the HCF(3) molecules in the conditions of 298.15 K and 0.3 Torr when the IR probing light sweeps over the rotational-vibrational transition of the v(5) and v(1) modes and the right circularly polarized microwave pumping light with the intensity of 1 kW cm(-2) at the resonant frequency 40.84 GHz are calculated on the basis of the B3LYP/6-311++G* computations. The theoretical results indicate that the B term can be of the same order of magnitude as the A and D terms, and the LIOR and LICD can be measurable in comparison to the rotationally resolved MVCD. The laser-induced optical activity may provide useful new information and form a basis for a different kind of optical activity spectroscopy.

Toward Chiral Sum-Frequency Spectroscopy

Chiral sum-frequency (SF) spectroscopy that measures both the real and the imaginary components of the SF spectral response was demonstrated for the first time. It was based on interference of the SF signal with a dispersionless SF reference. Solutions of 1,1'-bi-2-naphthol (BN) were used as model systems, and their chiral SF spectra over the first exciton-split transitions were obtained. Chiral spectra are useful for determination of absolute configuration and conformation of chiral molecules.

Nonlinear optical spectroscopy of chiral molecules

We review nonlinear optical processes that are specific to chiral molecules in solution and on surfaces. In contrast to conventional natural optical activity phenomena, which depend linearly on the electric field strength of the optical field, we discuss how optical processes that are nonlinear (quadratic, cubic, and quartic) functions of the electromagnetic field strength may probe optically active centers and chiral vibrations. We show that nonlinear techniques open entirely new ways of exploring chirality in chemical and biological systems: The cubic processes give rise to nonlinear circular dichroism and nonlinear optical rotation and make it possible to observe dynamic chiral processes at ultrafast time scales. The quadratic second-harmonic and sum-frequency-generation phenomena and the quartic processes may arise entirely in the electric-dipole approximation and do not require the use of circularly polarized light to detect chirality. They provide surface selectivity and their observables can be relatively much larger than in linear optical activity. These processes also give rise to the generation of light at a new color, and in liquids this frequency conversion only occurs if the solution is optically active. We survey recent chiral nonlinear optical experiments and give examples of their application to problems of biophysical interest.

Analysis of surface second-harmonic generation by orientational distribution function in a chiral polymer film

By adopting classical models of molecular chirality, contributions of the coupled-oscillator and helix natures to the chiral surface second-order susceptibilities are identified through introduction of a molecular orientational distribution. Experimentally, surface orientational distribution functions at interfaces of an isotropic chiral chitosan polymer film are determined from second harmonic generation measurement. The largest chiral component of surface nonlinear optical susceptibility is from the electric-magnetic coupling with dominant contribution from the helix nature of chitosan.

Chiroptical Effects in the Second Harmonic Signal of Collagens I and IV

We performed polarization-resolved surface second harmonic generation (SHG) experiments on thin films of collagen I and IV molecules, as well as conventional CD measurements. We found that collagen IV presents little CD and no SHG optical activity, whereas collagen I exhibits large chiroptical effects involving both one-electron and excitonic coupling mechanisms. We estimated that these chiral components enhance the SHG signal from fibrillar collagen in biological tissues by typically a factor of 2. By comparing the distinct behaviors of collagens I and IV in SHG microscopy and in surface SHG experiments, we concluded that SHG microscopy is a sensitive probe of the micrometer-scale structural organization of collagen in biological tissues.

Mixed electric-magnetic second-order nonlinear optical response of helicenes

The mixed electric-magnetic second-order nonlinear optical responses of oriented films of helicenes have been computed ab initio using the random phase approximation method and compared to the pure electric-dipole counterpart. It turns out that the mixed electric-magnetic responses can be of the same order of magnitude as the pure electric-dipole counterpart when there is no donor/acceptor (D/A) substituent or these D/A pairs are weak, i.e., when the pure electric-dipole response is small. When adding strong D/A substituents, the pure electric-dipole response increases substantially and much more than its mixed electric-magnetic counterpart. Consequently, the ratio between the mixed electric-magnetic and pure electric responses decreases. Although there is no general rule, the mixed responses evolve as a function of substitution quasi similarly to the pure electric contribution. This study confirms therefore the possibility of tuning the mixed electric-magnetic response by employing appropriate chiral molecules.

Using the intrinsic chirality of a molecule as a label-free probe to detect molecular adsorption to a surface by second harmonic generation

Chiral second harmonic generation (C-SHG) has been used for the label-free detection of (R)-(+)-1,1'-bi-2-naphthol (RBN) and (S)-(+)-1,1'-bi-2-naphthol (SBN) binding to planar-supported lipid bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphotidylcholine (POPC) based on the intrinsic chirality of the molecules. C-SHG adsorption isotherms of RBN and SBN reveal Langmuir adsorption behavior with binding constants of 2.7 +/- 0.2 x 10(5) M(-1) and 3.0 +/- 0.1 x 10(5) M(-1), respectively. The kinetics of RBN binding to a POPC bilayer was also measured. It was determined that the adsorption rate for RBN was 5.7 +/- 0.4 x 10(3) s(-1)M(-1) and the desorption rate was 2.1 +/- 0.8 x 10(-2) s(-1). From the kinetic data a binding constant of 2.7 +/- 1.0 x 10(5) M(-1) was calculated, which agrees well with the thermodynamic measurement. The C-SHG technique was correlated with surface tension measurements in order to determine the RBN surface excess within the POPC membrane. The maximum surface excess of RBN in a monolayer of POPC was 4.3 +/- 0.5 x 10(-11) mol cm2. Using the maximum surface excess in conjunction with the C-SHG binding data a lower limit of detection of 1.5 +/- 0.1 x 10(-13) mols cm(-2) was calculated. The results of these studies show that C-SHG is a powerful tool for the study of chiral molecular interactions at surfaces.

Imaging chirality with surface second harmonic generation microscopy

Chirality is a fundamental construct in nature which arises from an antisymmetric arrangement of atoms, molecules, or larger structures, resulting in the formation of nonsuperimposable mirror images. Bulk chiral effects can easily be measured using circular dichroism (CD) or optical rotary dispersion (ORD). However, the imaging of chirality originating from molecular surface films cannot be obtained with these linear optical methods. By using chiral second harmonic generation (C-SHG), with its inherent surface sensitivity and ability to discriminate between the symmetry of surface adsorbed species in combination with a counter-propagating optical geometry, we have developed the first nonlinear chiral microscope. In the study presented here, the intrinsic chirality of R- and S-(+)-1,1'-bi-2-naphthol (RBN, SBN) has been used to image a patterned planar supported lipid bilayer (PSLB) of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) using C-SHG. Spatial resolution of the patterned PSLB is visible when either RBN or SBN is intercalated into the membrane. No image is observed when a racemic mixture of RBN and SBN is present. The C-SHG images are compared with those obtained from fluorescence microscopy to verify the C-SHG imaging technique. The results presented here demonstrate that C-SHG possesses the requisite surface selectivity and sensitivity to detect interfacial chirality and provides a direct route for the visualization of chirality originating from molecular surface films.

Detection of chiral sum frequency generation vibrational spectra of proteins and peptides at interfaces in situ

In this work, we demonstrate the feasibility to collect off-electronic resonance chiral sum frequency generation (SFG) vibrational spectra from interfacial proteins and peptides at the solid/liquid interface in situ. It is difficult to directly detect a chiral SFG vibrational spectrum from interfacial fibrinogen molecules. By adopting an interference enhancement method, such a chiral SFG vibrational spectrum can be deduced from interference spectra between the normal achiral spectrum and the chiral spectrum. We found that the chiral SFG vibrational spectrum of interfacial fibrinogen was mainly contributed by the beta-sheet structure. For a beta-sheet peptide tachyplesin I, which may be quite ordered at the solid/liquid interface, chiral SFG vibrational spectra can be collected directly. We believe that these chiral signals are mainly contributed by electric dipole contributions, which can dominate the chiroptical responses of uniaxial systems. For the first time, to our knowledge, this work indicates that the off-electronic resonance SFG technique is sensitive enough to collect chiral SFG vibrational spectra of interfacial proteins and peptides, providing more structural information to elucidate interfacial protein and peptide structures.

Sum-frequency vibrational spectroscopy of a helically structured conjugated polymer

Sum-frequency vibrational spectroscopy was used as a novel technique to probe the molecular chirality of thin polymer films. Chiral vibrational spectra of poly(bithienylene-phenylene) were obtained, and the two enantiomers were distinguished by an interference method. Vibration-electronic double resonance was responsible for the observation of unusually strong chiral spectra of the phenylene vibration modes.

Ellipsometric Approach for the Real-Time Detection of Label-Free Protein Adsorption by Second Harmonic Generation

Second harmonic generation (SHG) was performed using a novel ellipsometric detection approach to selectively probe the real-time surface binding kinetics of an unlabeled protein. The coherence of nonlinear optical processes introduces new possibilities for exploiting polarization that are unavailable with incoherent methods, such as absorbance and fluorescence. Adsorption of bovine serum albumin (BSA) at silica/aqueous solution interfaces resulted in changes in the polarization state of the frequency-doubled light through weak, dynamic interactions with a coadsorbed nonlinear optical probe molecule (rhodamine 6G). Using a remarkably simple instrumental approach, signals arising exclusively from surface interactions with BSA were spatially isolated and selectively detected with high signal-to-noise. The relative intensities acquired during the kinetics experiments using both circularly and linearly polarized incident beams were in excellent agreement with the responses predicted from SHG ellipsometry polarization measurements. Analysis of the polarization-dependent SHG generated during BSA adsorption at glass/aqueous solution interfaces provided direct evidence for slow conformational changes within the protein layer after adsorption, consistent with protein denaturation. This polarization selection approach is sufficiently general to be easily extended to virtually all coherent nonlinear optical processes and a variety of different surface interactions and architectures.

Polarization Mapping: A Method To Improve Sum Frequency Generation Spectral Analysis

A "polarization mapping" method has been applied to improve the fitting quality of sum frequency generation (SFG) vibrational spectra of complicated systems and validate the data analysis of simple SFG spectra. Using such a method, two-dimensional SFG spectra can be constructed, from which more reliable spectral information can be obtained from a surface/interface. Model calculations as well as experiments have been employed to illustrate the power of the polarization mapping method for spectral analysis of SFG spectra. By using a deuterated polystyrene surface and interfacial protein molecules as examples, we demonstrate that this method is especially important for complicated molecules, such as polymers and proteins.

Experimental Confirmation of the Importance of Orientation in the Anomalous Chiral Sensitivity of Second Harmonic Generation

Macromolecular interactions were demonstrated to yield large chiroptical effects in second harmonic generation measurements of ultrathin surface films. Second harmonic generation (SHG) has recently shown to be several orders of magnitude more sensitive to chirality in oriented systems than common linear methods, including absorbance circular dichroism (CD) and optical rotary dispersion (ORD). Numerous mechanisms have been developed to explain this anomalous sensitivity, with a general emphasis on understanding the molecular origins of the chromophore chirality. In this work, orientational effects alone are shown to be the dominant factor for generating large SHG chiral dichroic ratios in many surface systems. Three distinct uniaxial surface films of SHG-active achiral chromophores oriented at chiral templated surfaces were observed to yield chiral dichroic ratios as great as 40% in magnitude.