Surface-enhanced hyper-Raman Scattering Research Articles

Templated green synthesis of plasmonic silver nanoparticles in onion epidermal cells suitable for surface-enhanced Raman and hyper-Raman scattering.

We report fast and simple green synthesis of plasmonic silver nanoparticles in the epidermal cells of onions after incubation with AgNO3 solution. The biological environment supports the generation of silver nanostructures in two ways. The plant tissue delivers reducing chemicals for the initial formation of small silver clusters and their following conversion to plasmonic particles. Additionally, the natural morphological structures of the onion layers, in particular the extracellular matrix provides a biological template for the growth of plasmonic nanostructures. This is indicated by red glowing images of extracellular spaces in dark field microscopy of onion layers a few hours after AgNO3 exposure due to the formation of silver nanoparticles. Silver nanostructures generated in the extracellular space of onion layers and within the epidermal cell walls can serve as enhancing plasmonic structures for one- and two-photon-excited spectroscopy such as surface enhanced Raman scattering (SERS) and surface enhanced hyper-Raman scattering (SEHRS). Our studies demonstrate a templated green preparation of enhancing plasmonic nanoparticles and suggest a new route to deliver silver nanoparticles as basic building blocks of plasmonic nanosensors to plants by the uptake of solutions of metal salts.

Probing Two-Photon Molecular Properties with Surface-Enhanced Hyper-Raman Scattering: A Combined Experimental and Theoretical Study of Crystal Violet

The surface-enhanced hyper-Raman scattering spectra of crystal violet are experimentally measured and theoretically calculated for excitation energies spanning the two lowest-lying electronic states (12,700–27,400 cm–1). The theory and experiment are in qualitative agreement over the measured energy range, indicating that first-principles calculations capture many of the complex resonance contributions in this prototypical octupolar system. The discrepancies between theory and experiment are investigated by comparing spectra obtained in different local environments as well as from higher-order surface-enhanced spectroscopies. A comparison between relative surface-enhanced hyper-Raman scattering band ratios plotted as a function of excitation wavelength and crystal violet’s absorption spectra elucidates correlations between groups of vibrations and the excited-electronic states. Our results suggest that the spectral features across the range of resonance excitation energies (∼15,500–27,400 cm–1) are domina...

Surface-Enhanced Raman and Surface-Enhanced Hyper-Raman Scattering of Thiol-Functionalized Carotene.

A thiol-modified carotene, 7′-apo-7′-(4-mercaptomethylphenyl)-β-carotene, was used to obtain nonresonant surface-enhanced Raman scattering (SERS) spectra of carotene at an excitation wavelength of 1064 nm, which were compared with resonant SERS spectra at an excitation wavelength of 532 nm. These spectra and surface-enhanced hyper-Raman scattering (SEHRS) spectra of the functionalized carotene were compared with the spectra of nonmodified β-carotene. Using SERS, normal Raman, and SEHRS spectra, all obtained for the resonant case, the interaction of the carotene molecules with silver nanoparticles, as well as the influence of the resonance enhancement and the SERS enhancement on the spectra, were investigated. The interaction with the silver surface occurs for both functionalized and nonfunctionalized β-carotene, but only the stronger functionalization-induced interaction enables the acquisition of nonresonant SERS spectra of β-carotene at low concentrations. The resonant SEHRS and SERS spectra are very similar. Nevertheless, the SEHRS spectra contain additional bands of infrared-active modes of carotene. Increased contributions from bands that experience low resonance enhancement point to a strong interaction between silver nanoparticles and electronic levels of the molecules, thereby giving rise to a decrease in the resonance enhancement in SERS and SEHRS.

The theory of Surface Enhanced Hyper Raman Scattering (A review)

The Dipole Quadrupole theory of Surface Enhanced Hyper Raman Scattering (SEHRS), created by the authors is expounded in detail. Peculiarities of behavior of electromagnetic field on rough metal surfaces are considered. It is demonstrated that there is an enhancement of the dipole and quadrupole light-molecule interaction near the places of the surface with a large curvature. The expression for the SEHRS crosssection of symmetrical molecules is obtained. Selection rules for the scattering contributions are derived and a qualitative classification of the contributions after the enhancement degree is performed. Analysis of experimental spectra of pyrazine and phenazine, and also some another molecules is performed too. It is demonstrated a full coincidence of experimental regularities in these spectra with the theory suggested.

Surface-enhanced hyper-Raman and Raman hyperspectral mapping.

We investigate distributions of crystal violet and malachite green on plasmonic surfaces by principal component analysis (PCA) imaging of surface-enhanced hyper-Raman scattering (SEHRS) data. As a two-photon excited Raman scattering process, SEHRS provides chemical structure information based on molecular vibrations, but follows different selection rules than the normal, one-photon excited surface-enhanced Raman scattering (SERS). Therefore, simultaneous hyperspectral mapping using SEHRS excited at 1064 nm and SERS excited at 532 nm improves spatially resolved multivariate discrimination based on complementary vibrational information. The possibility to map distributions of the structurally similar dyes crystal violet and malachite green demonstrates the potential of this approach for multiplex imaging of complex systems.

Surface-Enhanced Hyper-Raman Scattering: A New Road to the Observation of Low Energy Molecular Vibrations

The plasmon enhancement of molecular hyper-Raman scattering, the nonlinear counterpart of Raman scattering, which involves the absorption of two fundamental photons, is investigated with emphasis on low energy molecular vibrations. The two-photon excitation of the molecule is treated using its hyperpolarizability β, and the emission of the hyper-Raman photons is computed using a dipole emitter located at the molecule position. The electromagnetic response of the plasmonic systems is evaluated using a surface integral equation method, which makes possible considering both planewave and dipole excitations in a single formalism. Taking into account different geometries (including multiresonant antennas and silver heptamers supporting Fano resonances), the experimental parameters influencing the enhancement of the molecular hyper-Raman scattering are discussed in detail. In particular, it is shown that a good excitation at the fundamental stage is not sufficient for reaching a good enhancement factor and that...

Combined near-infrared excited SEHRS and SERS spectra of pH sensors using silver nanostructures.

Surface-enhanced hyper-Raman scattering (SEHRS) and surface-enhanced Raman scattering (SERS) of para-mercaptobenzoic acid (pMBA) were studied with an excitation wavelength of 1064 nm, using different silver nanostructures as substrates for both SEHRS and SERS. The spectra acquired for different pH values between pH 2 and pH 12 were compared with SERS data obtained from the identical samples at 532 nm excitation. Comparison of the ratios of the enhancement factors from SEHRS and SERS experiments with those from calculations using plasmonic absorbance spectra suggests that the difference between total surface-enhancement factors of SEHRS and SERS for pMBA is mainly explained by a difference between the electromagnetic contributions for linear and non-linear SERS. SERS and SEHRS spectra obtained at near-infrared (NIR) excitation indicate an overall reduction of enhancement by a factor of 2-3 at very low and very high pH, compared to neutral pH. Our data provide evidence that different molecular vibrations and/or different adsorption species are probed in SERS and SEHRS, and that SEHRS is very sensitive to slight changes in the pMBA-nanostructure interactions. We conclude that the combination of SEHRS and SERS using NIR excitation is more powerful for micro-environmental pH sensing than one-photon spectra excited in the visible range alone.

Electrodynamical forbiddance of strong quadrupole light–molecule interaction and peculiarities of the SEHR spectra in the methane molecule

It is demonstrated that the strong quadrupole light–molecule interaction, which is responsible for the most enhancement of Surface Enhanced Hyper Raman Scattering (SEHRS), experiences so-called electrodynamical forbiddance in the methane molecule and molecules with cubic symmetry groups and does not influence on formation of the SEHRS spectra. This forbiddance results in the fact that the lines, caused by the totally symmetric vibrations, which are observed in pyrazine and phenazine, must be slight, or absent at all. In this case, in methane, the most enhanced lines are those, caused by vibrations, transforming after the irreducible representations T1, or T2.

Investigation of Linear and Nonlinear Raman Scattering for Isotopologues of Ru(bpy)32+

Tris(2,2′-bipyridine)ruthenium(II), Rubpy, an important transition metal complex for its robust photochemistry, is studied using simulated resonance Raman scattering (RRS) and resonance hyper-Raman scattering (RHRS) in comparison to measured surface-enhanced Raman scattering (SERS) and surface-enhanced hyper-Raman scattering (SEHRS). Detailed examination of the simulated data shows that many of the observed features in the experiments are captured by the theory. For the metal-to-ligand charge transfer (MLCT) absorption band at 452 nm, it is demonstrated that the shoulder on the absorption band at 425 nm is not a vibronic feature and that the line shape should be considered as coming from two separate MLCT states. We find that good agreement can be obtained by comparing simulated spectra to the SERS and SEHRS spectra on resonance with the absorption band. However, the simulations do not capture the high sensitivity of relative peak intensities observed during wavelength scanned SERS and SEHRS experiments. This result is interpreted on the basis of discussion of the literature and the approximations made in the vibronic model, where it is concluded that the simulations underestimate interference effects. These results demonstrate the complexity of using theoretical methods for accurately describing the electronic structure of large molecules, and that commonly used exchange-correlation functionals like B3LYP and LC-PBE cannot completely describe all of the vibronic features in the Raman scattering of Rubpy.

Surface-Enhanced Hyper-Raman Scattering from Single Molecules

We report on the ability of surface-enhanced hyper-Raman scattering (SEHRS) to detect single molecules. Evidence for this conclusion is provided by measuring a large number of individual spectra from a mixture containing isotopically labeled dye molecules.

Cross‐section and selection rules in surface‐enhanced hyper Raman scattering

AbstractThe expression for the surface‐enhanced hyper Raman scattering (SEHRS) cross‐section of symmetrical molecules within the framework of the dipole–quadrupole theory is presented. It is formed by contributions that depend on various dipole and quadrupole moments. The enhancement coefficients for the quadrupole enhancement mechanism in some limited cases can reach a value 1030. It is demonstrated that the contributions follow some selection rules. Qualitative classification of the contributions based on the enhancement degree is given. It is demonstrated that the SEHR spectra of pyrazine and pyridine can be explained by the presented theory. Copyright © 2011 John Wiley & Sons, Ltd.

Strong quadrupole interaction of light with molecules and its manifestation in surface-enhanced hyper-Raman scattering spectra

The possibility of the giant enhancement of hyper-Raman scattering by molecules adsorbed on rough metal surfaces is demonstrated. The theory is based on the qualitative consideration of electromagnetic field enhancement near some model rough surfaces and individual irregularities, as well as on the quantum-mechanical features of dipole and quadrupole interactions of light with molecules (as in the theory of surface-enhanced Raman scattering), proposed by the author. A consideration of symmetric molecules makes it possible to obtain selection rules for surface-enhanced hyper-Raman scattering (SEHRS) spectra and establish such a regularity as the occurrence of strong forbidden lines (which are due to totally symmetric vibrations); these lines are transformed according to unitary irreducible representation in molecules with the symmetry groups Cnh, D, and higher. An analysis of the data in the literature for trans-1,2-bis (4-pyridyl)ethylene and pyridine molecules shows that their spectra can be explained in terms of the dipole-quadrupole theory of SEHRS. At the same time, the analysis of the SEHRS spectra of pyrazine revealed the presence of strong forbidden bands due to totally symmetric vibrations. This finding substantiated the proposed theory, which makes it possible to interpret the entire spectrum in detail. These results are in good agreement with the general mechanism of the optical effects enhanced by molecules adsorbed on metal surfaces, which was developed by the author.

Evaluation of electromagnetic enhancement of surface enhanced hyper Raman scattering using plasmonic properties of binary active sites in single Ag nanoaggregates

We found a binary active site of surface enhanced Raman scattering (SERS) and surface enhanced hyper Raman scattering (SEHRS) in single Ag nanoaggregates by single particle spectroscopy. The intensity fluctuation of SEHRS was canceled by dividing SEHRS intensity by SERS intensity on the basis of the binary active site analysis. Thanks to the identification of the plasmons common to both SERS and SEHRS we revealed that an enormous enhancement in SEHRS is ascribed to the electromagnetic (EM) enhancement entirely coupled with longitudinal plasmon modes excluding other kinds of enhancement factors. Our results indicate that EM enhancement factors of metal nanostructures are estimated from spectral information on the longitudinal plasmon resonance band obtained by the scattering or absorption spectra of the nanostructures.

Halide‐ion‐assisted increase of surface‐enhanced hyper‐Raman scattering: a clear observation of the chemical effect

AbstractWe show that the increase of surface‐enhanced hyper‐Raman scattering (SEHRS) intensity of organic dye molecules adsorbed on single silver (Ag) colloid aggregate in the presence of halide ions is a direct evidence of the chemical effect in the enhancement mechanism. Time‐dependent SEHRS measurements before and after adding halide ions enabled us to distinctly observe the chemical effect. The presence of the halide ions results to a more stable chemical interaction between metal and dye molecule, making it more resistant against photodegradation effects. This study can contribute in elucidating the chemical effect mechanism and aid in the development of SEHRS as a useful spectroscopic tool. Copyright © 2008 John Wiley & Sons, Ltd.

SERS—a single-molecule and nanoscale tool for bioanalytics

Surface enhanced Raman scattering (SERS) at extremely high enhancement level turns the weak inelastic scattering effect of photons on vibrational quantum states into a structurally sensitive single-molecule and nanoscale probe. The effect opens up exciting opportunities for applications of vibrational spectroscopy in biology. The concept of SERS can be extended to two-photon excitation by exploiting surface enhanced hyper-Raman scattering (SEHRS). This critical review introduces the physics behind single-molecule SERS and discusses the capabilities of the effect in bioanalytics (100 references).

One- and Two-Photon Excited Optical pH Probing for Cells Using Surface-Enhanced Raman and Hyper-Raman Nanosensors

We demonstrate spatially resolved probing and imaging of pH in live cells by mobile and biocompatible nanosensors using surface-enhanced Raman scattering (SERS) of 4-mercaptobenzoic acid (pMBA) on gold nanoaggregates. Moreover, we also show that this concept of pH nanosensors can be extended to two-photon excitation by using surface-enhanced hyper-Raman scattering (SEHRS). In addition to the advantages of two-photon excitation, the SEHRS sensor enables measurements over a wide pH range without the use of multiple probes.

Surface-enhanced hyper-Raman spectroscopy using optical trapping of silver nanoparticles for molecular detection in solution

Optical trapping of silver nanoparticles dispersed with rhodamine 6G (R6G) and NaCl in water gives surface-enhanced hyper-Raman scattering (SEHRS) of R6G. In this paper we have measured the SEHRS spectrum in the R6G concentration range between 10−7 and 10−11 M. SEHRS intensity and signal-to-noise ratio show clear differences at the lower R6G concentrations than ~10−9 M, indicating that the number of R6G in a trapped silver aggregate approaches the single-molecule level. This trapping SEHRS technique is applied for the detection of lysozyme molecules labelled with tetramethylrhodamine dye.

Two-photon vibrational spectroscopy for biosciences based on surface-enhanced hyper-Raman scattering.

Two-photon excitation is gaining rapidly in interest and significance in spectroscopy and microscopy. Here we introduce a new approach that suggests versatile optical labels suitable for both one- and two-photon excitation and also two-photon-excited ultrasensitive, nondestructive chemical probing. The underlying spectroscopic effect is the incoherent inelastic scattering of two photons on the vibrational quantum states called hyper-Raman scattering (HRS). The rather weak effect can be strengthened greatly if HRS takes place in the local optical fields of gold and silver nanostructures. This so-called surface-enhanced HRS (SEHRS) is the two-photon analogue to surface-enhanced Raman scattering (SERS). SEHRS provides structurally sensitive vibrational information complementary to those obtained by SERS. SEHRS combines the advantages of two-photon spectroscopy with the structural information of vibrational spectroscopy and the high-sensitivity and nanometer-scale local confinement of plasmonics-based spectroscopy. We infer effective two-photon cross-sections for SEHRS on the order of 10(-46) to 10(-45) cm4 x s, similar to or higher than the best "action" cross-sections (product of the two-photon absorption cross-section and fluorescence quantum yield) for two-photon fluorescence, and we demonstrate HRS on biological structures such as single cells after incubation with gold nanoparticles.

Surface-Enhanced Hyper-Raman Scattering (SEHRS) on Ag Film over Nanosphere (FON) Electrodes: Surface Symmetry of Centrosymmetric Adsorbates

Electrochemical surface-enhanced hyper-Raman scattering (SEHRS) and surface-enhanced Raman scattering (SERS) of centrosymmetric molecules on Ag film over nanosphere (AgFON) electrodes are presented. The SEHR spectra of trans-1,2-bis(4-pyridyl)ethylene (BPE) at different potentials (vs Ag/AgCl) are presented for the first time, and a reversible potential tuning of the SEHR spectra of BPE is demonstrated. The SEHRS and SERS techniques were used to determine to what extent either site symmetry reduction or field gradient effects dictate the origin of the observed vibrational spectra. It is found that the SEHR and SER spectra for the molecules studied were distinctly different at all frequency regions at a fixed voltage, suggesting that centrosymmetry is largely retained upon adsorption to the AgFON surface and that field gradient effects are negligible. This work also shows that the SEHR spectra clearly depend on potential, whereas the SER spectra are essentially independent of potenial. It is determined that the combination of changes in deltaGads and the presence of coadsorbed counterions are responsible for altering the local symmetry of the adsorbate and only SEHRS has the sensitivity to detect these changes in the surface environment. Thus, SEHRS is a uniquely useful spectroscopic tool that is much more sensitive to the local adsorption environment than is SERS.

Surface‐enhanced hyper Raman scattering in silver colloidal solutions

AbstractWe report surface‐enhanced hyper Raman scattering (SEHRS) from molecules in silver colloidal solutions using high repetition rate mode‐locked picosecond excitation at 1064 nm. SEHRS was is studied under non‐resonant and resonant conditions by choosing adenine and crystal violet as test molecules. Compared with its SERS spectrum, the SEHRS spectrum of adenine displays strong ‘new’ lines, which, in agreement with theory, in most cases also show up in the infrared absorption spectrum. We also explore the capability of SEHRS for trace detection and demonstrate detection and identification of crystal violet based on its SEHRS signals down to nanomolar concentrations. Copyright © 2005 John Wiley & Sons, Ltd.