Surface-enhanced hyper-Raman Scattering Research Articles

Exploring Excited State Landscapes with Surface Enhanced Hyper-Raman Spectroscopy.

In this Perspective, we provide a historical overview of the surface-enhanced hyper-Raman scattering (SEHRS) effect, describe its essential components, highlight the close connection between theory and experiment in several vignettes, and discuss recent analytical applications. SEHRS, the two-photon analog of surface-enhanced Raman scattering (SERS), is a spontaneous nonlinear scattering exhibited by molecules in a plasmonic field. Hyper Raman provides distinctive information on the molecular vibrations and electronic excited states of analytes. A 40-year old mystery surrounding the SEHRS spectra of R6G is used to illustrate the power of SEHRS to explore excited electronic states, revealing how non-Condon effects can influence the two-photon molecular properties. The exceptionally large enhancement factors (>1013) obtained from SEHRS enable the analysis of single molecules and molecules at very low concentrations. This high sensitivity is further augmented by an increased sensitivity to chemical effects, allowing SEHRS to probe changes in the local environment and the orientation of surface ligands. As most SEHRS experiments employ near-infrared (NIR) and short-wave infrared (SWIR) light, it also holds promise for bioimaging studies. Before concluding, we discuss future directions and challenges for the field as it moves forward.

Surface Enhanced Nonlinear Raman Processes for Advanced Vibrational Probing.

Surface enhanced Raman scattering (SERS) is not restricted to the well-known one-photon excited spontaneous Raman process that gives information on molecular composition, structure, and interaction through vibrational probing with high sensitivity. The enhancement mainly originates in high local fields, specifically those provided by localized surface plasmon resonances of metal nanostructures. High local fields can particularly support nonlinear Raman scattering, as it depends on the fields to higher powers. By revealing plasmon-molecule interactions, nonlinear Raman processes provide a very sensitive access to the properties of metal nanomaterials and their interfaces with molecules and other materials. This Perspective discusses plasmon-enhanced spontaneous and coherent nonlinear Raman scattering with the aim of identifying advantages that lead to an advanced vibrational characterization of such systems. The discussion will highlight the aspects of vibrational information that can be gained based on specific advantages of different incoherent and coherent Raman scattering and their surface enhancement. While the incoherent process of surface enhanced hyper Raman scattering (SEHRS) gives highly selective and spectral information complementary to SERS, the incoherent process of surface enhanced pumped anti-Stokes Raman scattering (SEPARS) can help to infer effective nonresonant SERS cross sections and allows to see "hot" vibrational transitions. Surface enhanced coherent anti-Stokes Raman scattering (SECARS) and surface enhanced stimulated Raman scattering (SESRS) combine the advantages of high local fields and coherence, which gives rise to high detection sensitivity and offers possibilities to explore molecule-plasmon interactions for a comprehensive characterization of composite and hybrid structures in materials research, catalysis, and nanobiophotonics.

Multivariate Imaging for Fast Evaluation of In Situ Dark Field Microscopy Hyperspectral Data.

Dark field scattering microscopy can create large hyperspectral data sets that contain a wealth of information on the properties and the molecular environment of noble metal nanoparticles. For a quick screening of samples of microscopic dimensions that contain many different types of plasmonic nanostructures, we propose a multivariate analysis of data sets of thousands to several hundreds of thousands of scattering spectra. By using non-negative matrix factorization for decomposing the spectra, components are identified that represent individual plasmon resonances and relative contributions of these resonances to particular microscopic focal volumes in the mapping data sets. Using data from silver and gold nanoparticles in the presence of different molecules, including gold nanoparticle-protein agglomerates or silver nanoparticles forming aggregates in the presence of acrylamide, plasmonic properties are observed that differ from those of the original nanoparticles. For the case of acrylamide, we show that the plasmon resonances of the silver nanoparticles are ideally suited to support surface enhanced Raman scattering (SERS) and the two-photon excited process of surface enhanced hyper Raman scattering (SEHRS). Both vibrational tools give complementary information on the in situ formed polyacrylamide and the molecular composition at the nanoparticle surface.

Excitation Conditions for Surface-Enhanced Hyper Raman Scattering With Biocompatible Gold Nanosubstrates.

Surface enhanced hyper Raman scattering (SEHRS) can provide many advantages to probing of biological samples due to unique surface sensitivity and vibrational information complementary to surface-enhanced Raman scattering (SERS). To explore the conditions for an optimum electromagnetic enhancement of SEHRS by dimers of biocompatible gold nanospheres and gold nanorods, finite-difference time-domain (FDTD) simulations were carried out for a broad range of excitation wavelengths from the visible through the short-wave infrared (SWIR). The results confirm an important contribution by the enhancement of the intensity of the laser field, due to the two-photon, non-linear excitation of the effect. For excitation laser wavelengths above 1,000 nm, the hyper Raman scattering (HRS) field determines the enhancement in SEHRS significantly, despite its linear contribution, due to resonances of the HRS light with plasmon modes of the gold nanodimers. The high robustness of the SEHRS enhancement across the SWIR wavelength range can compensate for variations in the optical properties of gold nanostructures in real biological environments.

Surface-enhanced hyper-Raman scattering of Rhodamine 6G isotopologues: Assignment of lower vibrational frequencies.

We report a comprehensive experimental and theoretical study of the lower-wavenumber vibrational modes in the surface-enhanced hyper-Raman scattering (SEHRS) of Rhodamine 6G (R6G) and its isotopologue R6G-d4. Measurements acquired on-resonance with two different electronic states, S1 and S2, are compared to the time-dependent density functional theory computations of the resonance hyper-Raman spectra and electrodynamics-quantum mechanical computations of the SEHRS spectra on-resonance with S1 and S2. After accounting for surface orientation, we find excellent agreement between experiment and theory for both R6G and its isotopologue. We then present a detailed analysis of the complex vibronic coupling effects in R6G and the importance of surface orientation for characterizing the system. This combination of theory and experiment allows, for the first time, an unambiguous assignment of lower-wavenumber vibrational modes of R6G and its isotopologue R6G-d4.

Bio‐probing with nonresonant surface‐enhanced hyper‐Raman scattering excited at 1,550 nm

AbstractThe two‐photon excited process of surface‐enhanced hyper‐Raman scattering (SEHRS) provides advantages for studies of complex biological samples, yet suitable SEHRS nanoprobes and labels, as well as experimental conditions, must be established. Here, SEHRS spectra of the four reporter molecules (2‐naphthalenethiol [2‐NAT], para‐aminothiophenol (pATP), para‐nitrothiophenol (pNTP), and crystal violet), as well as the two antidepressant drugs (desipramine and imipramine) were obtained at an excitation wavelength of 1,550 nm using different citrate‐stabilized gold nanoparticles and silver nanoparticles, and under conditions that permit experiments with living cultured cells. As the results suggest, the short‐wave infrared laser excitation and the hyper‐Raman scattered light (corresponding to wavelengths > 775 nm) match the plasmonic properties of the employed gold and silver nanoaggregates, as well as the requirements regarding the viability of the cells. The two‐photon excited spectra of three types of SEHRS labels containing 2‐NAT as reporter inside macrophage cells show that molecules in the cellular environment can also be observed. The possibility to use short‐wave infrared excitation with gold nanostructures has important implications for the utilization of SEHRS in bio‐probing.

Surface-Enhanced Hyper Raman Spectra of Aromatic Thiols on Gold and Silver Nanoparticles.

We report the two-photon excited nonresonant surface-enhanced hyper Raman scattering (SEHRS) spectra of six aromatic thiol molecules during their interaction with gold and silver nanostructures. SEHRS spectra were obtained from thiophenol, benzyl mercaptan, and phenylethyl mercaptan and from the three isomers 2-aminothiophenol (2-ATP), 3-aminothiophenol (3-ATP), and 4-aminothiophenol (4-ATP). All SEHRS spectra were excited off-resonance at a wavelength of 1064 nm and compared to surface-enhanced Raman scattering (SERS) spectra excited at 785 nm or at 633 nm. The SEHRS spectra show a different interaction of thiophenol, benzyl mercaptan, and phenylethyl mercaptan with silver and gold nanostructures. Density functional theory calculations were used to support band assignments, in particular, for the unknown SERS spectrum of 3-ATP, and identify a band of phenylethyl mercaptan as a vibrational mode unique to the SEHRS spectrum and very weak in the Raman and infrared spectra. 2-ATP, 3-ATP, and 4-ATP show a different interaction with gold nanostructures that was found to depend on pH. Bands in the SEHRS spectrum of 2-ATP could be assigned to 2,2′-dimercaptoazobenzene, suggested to be obtained in a plasmon-assisted reaction that occurred during the SEHRS experiment. The results provide the basis for a better characterization of organic thiols at surfaces in a variety of fields, including surface functionalization and plasmonic catalysis.

Gold‐ and Silver‐Coated Barium Titanate Nanocomposites as Probes for Two‐Photon Multimodal Microspectroscopy

AbstractImproved multiphoton‐excited imaging and microspectroscopy require nanoprobes that can give different nonlinear optical signals. Here, composite nanostructures with a barium titanate core and a plasmonic moiety at their surface are synthesized and characterized. It is found that the core provides a high second‐order nonlinear susceptibility for sensitive second harmonic generation (SHG) imaging in living cells. As a second function in the two‐photon regime, the plasmonic part yields high local fields for resonant and nonresonant surface enhanced hyper Raman scattering (SEHRS). SEHRS complements the one‐photon surface enhanced Raman scattering (SERS) spectra that are also enhanced by the plasmonic shells. Barium titanate silver core–shell (Ag@BaTiO3) composites are specifically suited for SEHRS and SHG excited at 1064 nm, while gold at barium titanate (Au@BaTiO3) nanoparticles can be useful in a combination of SHG and SERS at lower wavelengths, here at 785 nm and 850 nm. The theoretical models show that the optical properties of the BaTiO3 dielectric core depend on probing frequency, shape, size, and plasmonic properties of the surrounding gold nanoparticles so that they can be optimized for a particular type of experiment. These versatile, tunable probes give new opportunities for combined multiphoton probing of morphological structure and chemical properties of biosystems.

Plasmon Enhanced Two‐Photon Probing with Gold and Silver Nanovoid Structures

AbstractNonlinear optical signals benefit greatly from the enhanced local optical fields in the vicinity of plasmonic nanostructures. Gold and silver nanovoid arrays of varying size and thickness, fabricated by electrochemical deposition are shown here to act as stable plasmonic nanostructures and to enhance the weak, incoherent two‐photon excited process of surface‐enhanced hyper Raman scattering (SEHRS) with high microscopic homogeneity and reproducibility that typical SEHRS experiments have not been addressing so far. Silver nanovoids yield stronger enhancement than gold voids, but gold nanovoid arrays show improved stability at high laser excitation intensities. Combined screening experiments using SEHRS and second‐harmonic generation (SHG) reveal a dependence of the enhancement of both signals on void structural parameters and similar optimum geometries for both two‐photon processes. The results confirm the suggested important role for the enhancement of the near‐infrared excitation field in SEHRS and suggest SHG as a fast screening tool to identify nanostructures that can support high SEHRS enhancement.

Non-Condon Effects in the Resonance Hyper-Raman Scattering of Chalcogen-Substituted Rhodamine Derivatives

Molecules with a strong two-photon response have gained significant interest because of their applications in two-photon imaging, all-optical switching, and energy up-conversion. The surface-enhanced hyper-Raman scattering (SEHRS) of tetramethyl chalcogenorosamines are taken on- and off-resonance and compared to time-dependent density functional theory calculations. Specifically, changes in the SEHRS spectra are tracked as a function of structural geometry because of perturbations induced by larger chalcogen atom substitutions to the xanthene ring. Here we show that the spectral changes and non-Condon effects can be understood by tracking the vibrational normal modes and the vibrationally induced changes to the highest occupied molecular orbitals. With a system in place to explore the relationship between structural geometry and nonlinear properties, various rhodamine derivatives can be characterized as a means to optimally design a series of two-photon bright compounds.

Utilizing Molecular Hyperpolarizability for Trace Analysis: A Surface-Enhanced Hyper-Raman Scattering Study of Uranyl Ion.

Surface-enhanced hyper-Raman scattering (SEHRS), the nonlinear analog of surface-enhanced Raman scattering (SERS), provides unique spectral signatures arising from the molecular hyperpolarizability. In this work, we explore the differences between SERS and SEHRS spectra obtained from surface-bound uranyl ion. Exploiting the distinctive SEHRS bands for trace detection of the uranyl ion, we obtain excellent sensitivity (limit of detection = 90 ppb) despite the extreme weakness of the hyper-Raman effect. We observe that binding the uranyl ion to the carboxylate group of 4-mercaptobenzoic acid (4-MBA) leads to significant changes in the SEHRS spectrum, whereas the surface-enhanced Raman scattering (SERS) spectrum of the same complex is little changed. The SERS and SEHRS spectra are also examined as a function of both substituent position, using 2-MBA, 3-MBA, and 4-MBA, and the carbon chain length, using 4-mercaptophenylacetic acid and 4-mercaptophenylpropionic acid. These results illustrate that the unique features of SEHRS can yield more information than SERS in certain cases and represent the first application of SEHRS for trace analysis of nonresonant molecules.

Gold Nanostructures for Plasmonic Enhancement of Hyper-Raman Scattering

Surface-enhanced hyper-Raman scattering (SEHRS) is very useful for the vibrational characterization of organic and biological molecules and their interaction with noble metal nanostructures. Many potential applications should ideally make use of gold nanostructures in order to enhance both the excitation and the weak hyper-Raman light, rather than silver nanostructures. Here, we report high SEHRS enhancement from spherical gold nanoparticles with different particle diameters ranging from 30 to 70 nm and from gold nanorods. SEHRS data of the two molecules crystal violet and rhodamine 6G obtained at an excitation wavelength of 1064 nm, absorbance spectra, and finite-difference time domain simulations of the electromagnetic field enhancement provide evidence that the SEHRS enhancement relies on the formation of nanoaggregates, with higher SEHRS signals yielded with increasing size of the nanoparticles in the aggregates. Gold nanorods and their aggregates are shown to provide optical properties that are speci...

Specific Interaction of Tricyclic Antidepressants with Gold and Silver Nanostructures as Revealed by Combined One- and Two-Photon Vibrational Spectroscopy

We report two-photon excited nonresonant surface-enhanced hyper Raman scattering (SEHRS) spectra of tricyclic antidepressant (TCA) molecules during their interaction with biocompatible gold nanostructures and with silver nanostructures. The SEHRS spectra of amitriptyline, desipramine, and imipramine are compared with surface-enhanced Raman scattering (SERS) spectra on both kinds of nanoparticles, obtained with excitation at 532 and 785 nm. The SEHRS spectra of the TCA molecules show several intense contributions by infrared-active vibrations. Combining SEHRS with SERS therefore enables a comprehensive vibrational characterization of the interaction of the molecules with the nanostructures. SEHRS and SERS data indicate that the molecules interact with the silver nanostructures mainly via their ring moiety. In contrast, in the interaction with gold, the methylaminopropyl side chain plays a very important role, along with parts of the ring system. It is possible to obtain the spectra of the molecules with ne...

Manifestation of a strong quadrupole interaction and peculiarities in the SERS and SEHRS spectra of 4,4'-bipyridine

The paper analyzes Surface Enhanced Raman Scattering (SERS) and Surface Enhanced Hyper Raman Scattering (SEHRS) spectra of 4,4'-bypiridine molecule for two possible geometries, which are described by D2 and D2h symmetry groups. It is pointed out on appearance of sufficiently strong lines, caused by vibrations with the unit irreducible representation for both possible configurations. Appearance of these lines in the SEHRS spectrum points out the existence of a strong quadrupole light-molecule interaction. In addition one observes the lines, caused by vibrations both with the unit irreducible representations A or Ag and the irreducible representation B1 or B1u. The last ones describe transformational properties of the dz component of the dipole moment, which is perpendicular to the surface. This property of the spectrum is caused by peculiarity of the geometry of the molecule, which consists of two benzene rings, which are weakly connected with each other. The linear combinations of the vibrations of the rings create two nearly degenerated symmetric and anti symmetrical states, which cannot be identified in the experimental spectra. The result is in a full agreement with the dipole-quadrupole theory of SERS and SEHRS.

Surface-Enhanced Resonance Hyper-Raman Scattering Elucidates the Molecular Orientation of Rhodamine 6G on Silver Colloids.

Herein, we utilize surface-enhanced hyper-Raman scattering (SEHRS) under resonance conditions to probe the adsorbate geometry of rhodamine 6G (R6G) on silver colloids. Our results show resonance SEHRS is highly sensitive to molecular orientation due to non-Condon effects, which do not appear in its linear counterpart surface-enhanced Raman scattering. Comparisons between simulated and measured SEHRS spectra reveal R6G adsorbs mostly perpendicular to the nanoparticle surface along the ethylamine groups with the xanthene ring oriented edgewise. Our results expand upon previous studies that rely on indirect, qualitative probes of R6G's orientation on plasmonic substrates. More importantly, this work represents the first determination of adsorbate geometry by SEHRS and opens up the possibility to study the orientation of single molecules in complex, plasmonic environments.

Surface Enhanced Hyper-Raman Scattering of the Amino Acids Tryptophan, Histidine, Phenylalanine, and Tyrosine

In this work, we report nonresonant surface-enhanced hyper-Raman (SEHRS) spectra of the amino acids tryptophan, histidine, phenylalanine, and tyrosine using silver nanoparticles. The spectra are obtained at an excitation wavelength of 1064 nm and compared to the corresponding surface-enhanced Raman scattering (SERS) spectra measured at 532 nm excitation. The majority of the bands in the SEHRS spectra are assigned. Important hallmarks of the spectra include strongly diminished or absent bands from the ring breathing modes. SEHRS and SERS spectra obtained from histidine and tyrosine indicate changes at slightly varied amino acid concentration. Small changes in the SEHRS spectra were more pronounced than variation in the corresponding SERS data, supporting the high sensitivity of the SEHRS spectra with respect to structural changes due to small variations in surface environment. The possibility to measure nonresonant SEHRS spectra of amino acids in solution and the complementary information obtained from the...

Surface enhanced hyper Raman scattering (SEHRS) and its applications.

Surface enhanced hyper Raman scattering (SEHRS) is the spontaneous, two-photon excited Raman scattering that occurs for molecules residing in high local optical fields of plasmonic nanostructures. Being regarded as a non-linear analogue of surface enhanced Raman scattering (SERS), SEHRS shares most of its properties, but also has additional characteristics. They include complementary spectroscopic information resulting from different selection rules and a stronger enhancement due to the non-linearity in excitation. In practical spectroscopy, this can translate to advantages, which include a high selectivity when probing molecule-surface interactions, the possibility of probing molecules at low concentrations due to the strong enhancement, and the advantages that come with excitation in the near-infrared. In this review, we give examples of the wealth of vibrational spectroscopic information that can be obtained by SEHRS and discuss work that has contributed to understanding the effect and that therefore provides directions for SEHRS spectroscopy. Future applications could range from biophotonics to materials research.

Measurement of pH-dependent surface-enhanced hyper-Raman scattering at desired positions on yeast cells via optical trapping.

Surface-enhanced hyper-Raman scattering (SEHRS) spectra were obtained at desired positions on yeast by focusing a continuous wave near-infrared laser beam while silver nanoparticles (AgNPs) were simultaneously optically trapped. However, the optically trapped colloidal AgNP suspension bubbled up at the focusing point, preventing spectral measurement. In the case of optically trapped AgNPs functionalized with 4-mercaptobenzoic acid (p-MBA), surface-enhanced hyper-Rayleigh scattering was considerably strong, indicating the suppression of the photothermal conversion to form the bubble. Interestingly, the SEHRS peaks that are attributed not only to p-MBA, but also to other species, were very occasionally observed. They may be partly assigned to the β1,3 glucan and protein amide II band. The SEHRS peak at 1366 cm-1 was barely visible in the measurements of conventional baker's yeast even in the suspension (pH 9) despite the effects of high pH on p-MBA. In contrast, the SEHRS peak in the measurements of yeast for biological applications was occasionally observed at 1366 cm-1. This suggests that acidity is correlated with fermentation efficiency. At different positions on single yeast cells, the intensity of the SEHRS peak at 1366 cm-1 varied. This result represents the pH distribution on yeast.

Simulating Surface-Enhanced Hyper-Raman Scattering Using Atomistic Electrodynamics-Quantum Mechanical Models.

Surface-enhanced hyper-Raman scattering (SEHRS) is the two-photon analogue of surface-enhanced Raman scattering (SERS), which has proven to be a powerful tool to study molecular structures and surface enhancements. However, few theoretical approaches to SEHRS exist and most neglect the atomistic descriptions of the metal surface and molecular resonance effects. In this work, we present two atomistic electrodynamics-quantum mechanical models to simulate SEHRS. The first is the discrete interaction model/quantum mechanical (DIM/QM) model, which combines an atomistic electrodynamics model of the nanoparticle with a time-dependent density functional theory description of the molecule. The second model is a dressed-tensors method that describes the molecule as a point-dipole and point-quadrupole object interacting with the enhanced local field and field-gradients (FG) from the nanoparticle. In both of these models, the resonance effects are treated efficiently by means of damped quadratic response theory. Using these methods, we simulate SEHRS spectra for benzene and pyridine. Our results show that the FG effects in SEHRS play an important role in determining both the surface selection rules and the enhancements. We find that FG effects are more important in SEHRS than in SERS. We also show that the spectral features of small molecules can be accurately described by accounting for the interactions between the molecule and the local field and FG of the nanoparticle. However, at short distances between the metal and molecule, we find significant differences in the SEHRS enhancements predicted using the DIM/QM and the dressed-tensors methods.

Surface-Enhanced Hyper-Raman Spectra of Adenine, Guanine, Cytosine, Thymine, and Uracil.

Using picosecond excitation at 1064 nm, surface-enhanced hyper-Raman scattering (SEHRS) spectra of the nucleobases adenine, guanine, cytosine, thymine, and uracil with two different types of silver nanoparticles were obtained. Comparing the SEHRS spectra with SERS data from the identical samples excited at 532 nm and with known infrared spectra, the major bands in the spectra are assigned. Due to the different selection rules for the one- and two-photon excited Raman scattering, we observe strong variation in relative signal strengths of many molecular vibrations obtained in SEHRS and SERS spectra. The two-photon excited spectra of the nucleobases are found to be very sensitive with respect to molecule–nanoparticle interactions. Using both the SEHRS and SERS data, a comprehensive vibrational characterization of the interaction of nucleobases with silver nanostructures can be achieved.