G. Lévi

A Scheme for Detecting Every Single Target Molecule with Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is now a well-established technique for the detection, under appropriate conditions, of single molecules (SM) adsorbed on metallic nanostructures. However, because of the large variations of the SERS enhancement factor on the surface, only molecules located at the positions of highest enhancement, so-called hot-spots, can be detected at the single-molecule level. As a result, in all SM-SERS studies so far only a small fraction, typically less than 1%, of molecules are actually observed. This complicates the analysis of such experiments and means that trace detection via SERS can in principle still be vastly improved. Here we propose a simple scheme, based on selective adsorption of the target analyte at the SERS hot-spots only, that allows in principle detection of every single target molecule in solution. We moreover provide a general experimental methodology, based on the comparison between average and maximum (single molecule) SERS enhancement factors, to verify the efficiency of our approach. The concepts and tools introduced in this work can readily be applied to other SERS systems aiming for detection of every single target molecule.

Gold particle interaction in regular arrays probed by surface enhanced Raman scattering

Lithographically designed two-dimensional arrays consisting of gold nanoparticles deposited on a smooth gold film are used as substrate to examine the SERS effect of the trans-1,2-bis (4-pyridyl) ethylene molecule. These arrays display two plasmon bands instead of the single one observed for the same arrays of particles but deposited on indium tin oxide coated glass. Laser excitation within the short wavelength band does not bring about any SERS spectrum, while excitation within the long wavelength band yields SERS spectra with a gain per molecule rising up to 10(8). The simultaneous investigation of extinction and Raman spectra of arrays exhibiting various topography parameters enables us to suggest an interpretation for both the occurrence of the two plasmon resonances and for the high Raman enhancement. We suggest to assign the short wavelength band to a plasmon wave propagating at the gold glass interface and the long wavelength one to an air/gold surface plasmon mode modified by particle-particle interaction.

Linear Self-Assembly of Nanoparticles Within Liquid Crystal Defect Arrays

In the presence of oriented smectic liquid crystal defects, hybrid systems of nanoparticles/liquid crystals form straight chains of nanoparticles of length longer than tens of micrometers and width equal to one single nanoparticle. The interparticle distance in a chain can be varied between a few micrometers and 1.5 nm, highlighting the control of optical absorption by light polarization monitored by gold nanoparticle concentration.

Grating-induced plasmon mode in gold nanoparticle arrays

We study the dipolar coupling of gold nanoparticles arranged in regular two-dimensional arrays by extinction micro-spectroscopy. When the interparticle spacing approaches the plasmon resonance wavelength of the individual particles, an additional band of very narrow width emerges in the extinction spectrum. By systematically changing the particles dielectric environment, the particles shape, the grating constant and angle of incidence, we show how this band associated to a grating induced-resonance can be influenced in strength and spectral position. The spectral position can be qualitatively understood by considering the conditions for grazing grating orders whereas the strength can be related to the strength of dipolar scattering from the individual particles.

Surface enhanced Raman scattering arising from multipolar plasmon excitation

Visible and near infrared extinction spectra of gold nanorod regular arrays exhibit several bands assigned to high multipolar order plasmon resonances. These up to ninth order multipolar resonances generate surface enhanced Raman scattering spectra with typically 5 x 10(4) enhancement which is of similar magnitude as those obtained for dipolar excitations.

Multipolar surface plasmon peaks on gold nanotriangles

In this paper, we report on the observation of multipolar surface plasmon excitation in lithographically designed gold nanotriangles, investigated by means of far-field extinction microspectroscopy in the wavelength range of 400-1000 nm. Several bands are observed in the visible and near infrared regions when increasing the side length of the triangles. The assignment of these peaks to successive in-plane multipolar plasmon modes is supported by calculations using the discrete dipole approximation method. We show that the lowest three multipolar excitations are clearly resolved in the visible and near infrared range. These new spectral features could be very promising in nanooptics or for chemosensing and biosensing applications.

Design and Optical Properties of Active Polymer-Coated Plasmonic Nanostructures

The grafting of stimuli-responsive polymer brushes on plasmonic structures provides a perfectly controlled two-dimensional active device with optical properties that can be modified through external stimuli. Herein, we demonstrate thermally induced modifications of the plasmonic response of lithographic gold nanoparticles functionalized by thermosensitive polymer brushes of (poly(N-isopropylacrylamide), PNIPAM). Optical modifications result from refractive local index changes due to a phase transition from a hydrophilic state (swollen regime) to a hydrophobic state (collapsed regime) of the polymer chains occurring in a very small range of temperatures. The refractive index of the polymer in aqueous solution is estimated in both states, deduced from the discrete dipole approximation (DDA) method. The combination of lithographic gold NPs and thermoresponsive polymer chains leads to a new generation of perfectly calibrated and dynamically controlled hybrid gold/polymer system for real-time nanosensors.

Molecular reorientation of carbon monoxide dissolved in dense simple fluids

Extensive infrared measurements of the carbon monoxide fundamental vibration band for CO−Ne, CO−Ar, CO−Kr, CO−Xe, CO−N2, and CO−O2 systems have been carried out in the dense fluid region. We have investigated both the whole liquid range from the triple to the critical point along the saturation line and the continuous change from gas to liquid along an isobar well above the critical pressure. In the case of O2 and N2, we have also studied the liquid−solid transition. The rotational correlation functions Φu(t) and the orientational correlation time τu have been determined from band profile. An order of magnitude of the intermolecular mean square torques has been obtained from band moments analysis. We discuss the ability of ’’m’’ and ’’j’’ diffusion models to describe experimental dipolar momentum correlation functions when the solvent density increases continuously from gas to liquid. These models appear to be unsatisfactory in the liquid phase except for N2 solutions where the ’’j’’ diffusion model gives...

Revisiting Surface-Enhanced Raman Scattering on Realistic Lithographic Gold Nanostripes

In this article, we investigate the Surface-Enhanced Raman Scattering (SERS) efficiency of methylene blue (MB) molecules deposited on gold nanostripes which, due to their fabrication by electron beam lithography and thermal evaporation, present various degrees of crystallinity and nanoscale surface roughness (NSR). By comparing gold nanostructures with different degrees of roughness and crystallinity, we show that the NSR has a strong effect on the SERS intensity of MB probe molecules. In particular, the NSR features of the lithographic structures significantly enhance the Raman signal of MB molecules, even when the excitation wavelength lies far from the localized surface plasmon resonance (LSPR) of the stripes. These results are in very good agreement with numerical calculations of the SERS gain obtained using the discrete dipole approximation (DDA). The influence of NSR on the optical near-field response of lithographic structures thus appears crucial since they are widely used in the context of nano-optics or/and molecular sensing.

The Fokker–Planck–Langevin model for rotational Brownian motion. II. Comparison with the extended rotational diffusion model and with observed infrared and Raman band shapes of linear and spherical molecules in fluids

A detailed comparison of two extensions of the Debye model for molecular reorientation in liquids, Gordon’s extended J‐diffusion model and the rotational Fokker–Planck–Langevin model, is presented. It is shown that the two models, although they represent very different physical pictures of rotational dynamics in fluids, lead to remarkably similar reorientational correlation functions, memory functions, correlation times and spectral densities. Only for values of the angular momentum correlation time appropriate to low density fluids do the two models give significantly different results, but the validity of the FPL model is questionable in this region. The results of an infrared study of the ν3 band of N2O in N2O/O2 along the coexistence curve from 105–153 K, and in N2O/N2 along the 80 bar isobaric line from 100–300 K are presented. The reorientational correlation functions from these measurements, and from earlier infrared studies of CO/N2 and Raman studies of CF4 and N2O liquids are compared with correl...