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International audience; We present a general formalism to model and calculate linear and nonlinear optical processes in composite systems,based on a graphical representation of light-matter interactions by loop diagrams associated with Feynmanrules. Through this formalism, we recover the usual second-order response of a simple system by drawing fourtimes fewer loop diagrams than doubled-sided ones. For composite systems, we introduce coupling Hamiltoniansbetween subsystems (for example, a molecule and a substrate), graphically represented by virtual bosons. In thisway, we enumerate all the diagrams describing the second-order response of the system and show how to selectthose relevant for the calculation of the molecular second-order hyperpolarizabilities under the influence of thesubstrate, including effective second-order contributions from the molecular third-order response. As it appliesto all nonlinear processes and an arbitrary number of interacting partners, this representation provides a generalframe for the calculation of the nonlinear response of arbitrarily complex systems.

The plasmonic amplification of non-linear vibrational sum frequency spectroscopy (SFG) at the surfaces of gold nanoparticles is systematically investigated by tuning the incident visible wavelength. The SFG spectra of dodecanethiol-coated gold nanoparticles chemically deposited on silicon are recorded for twenty visible wavelengths. The vibrational intensities of thiol methyl stretches extracted from the experimental measurements vary with the visible color of the SFG process and show amplification by coupling to plasmonics. Since the enhancement is maximal in the orange-red region rather than in the green, as expected from the dipolar model for surface plasmon resonances, it is attributed mostly to hotspots created in particle multimers, in spite of their low surface densities., Comment: 17 pages, 3 figures, includes 12 pages SI

International audience

Colloidal semiconductor quantum dots (QDs) constitute zero-dimension excitonic materials characterized by quantized states and able to emit fluorescence. QDs are promising materials for catalysis, molecular recognition and biosensing. In this context, our work consists in the design of a new class of biochemical sensors based on QD-grafted chips to benefit from their high opto-electronic activity in the visible range. In order to achieve this, we functionalize silica and CaF2 susbtrates with 4 nm-diameter CdTe QDs and organic molecules (e.g. phenylamine). The originality of our work then lies in two-colour sum-frequency generation nonlinear optical spectroscopy, mixing Raman and IR spectroscopies. This technique enables to probe and to quantify the coupling between the excitonic properties of the QDs and the vibrational response of their molecular environment: two tunable visible and IR laser beams are mixed on the QD-grafted chips to electronically excite the QDs while the vibrational spectroscopy of the surrounding organic species is performed. Through this approach, we clearly demonstrated a correlation between QDs and molecules. Especially, the vibrational response of the molecules is maximized when the first excitonic state of the CdTe QDs is pumped by the visible beam, which means it is possible to enhance the detection of given biomolecules thanks to QDs. Considering that confined excitons transfer their energy to molecules through dipolar interaction, the model we developed accounts indeed for such a behaviour.

Given the tunability of their optical properties over the UV–Visible–Near IR spectral range, ligand-capped quantum dots (QDs) are employed for the design of optical biosensors with low detection threshold. Thanks to non-linear optical spectroscopies, the absorption properties of QDs are indeed used to selectively enhance the local vibrational response of molecules located in their vicinity. Previous studies led to assume the existence of a vibroelectronic QD–molecule coupling based on dipolar interaction. However, no systematic study on the strength of this coupling has been performed to date. In order to address this issue, we use non-linear optical Two-Color Sum-Frequency Generation (2C-SFG) spectroscopy to probe thick QD layers deposited on calcium fluoride (CaF2) prisms previously functionalized by a self-assembled monolayer of phenyltriethoxysilane (PhTES) molecules. Here, 2C-SFG is performed in Attenuated Total Reflection (ATR) configuration. By comparing the molecular vibrational enhancement measured for QD–ligand coupling and QD–PhTES coupling, we show that the spatial dependence of the QD–molecule interactions (∼1/r3, with r the QD–molecule distance) is in agreement with the hypothesis of a dipole–dipole interaction.

International audience; To take advantage of the singular properties of matter, as well as to characterize it, we need to interact with it. The role of optical spectroscopies is to enable us to demonstrate the existence of physical objects by observing their response to light excitation. The ability of spectroscopy to reveal the structure and properties of matter then relies on mathematical functions called optical (or dielectric) response functions. Technically, these are tensor Green’s functions, and not scalar functions. The complexity of this tensor formalism sometimes leads to confusion within some articles and books. Here, we do clarify this formalism by introducing the physical foundations of linear and non-linear spectroscopies as simple and rigorous as possible. We dwell on both the mathematical and experimental aspects, examining extinction, infrared, Raman and sum-frequency generation spectroscopies. In this review, we thus give a personal presentation with the aim of offering the reader a coherent vision of linear and non-linear optics, and to remove the ambiguities that we have encountered in reference books and articles.

In this paper, we report on the study of a novel type of substrate based on a highly crystalline ZnO film photo-irradiated using UV for enhancing the Raman signal. This effect is called photo-induced enhanced Raman spectroscopy (PIERS). This PIERS substrate is composed of a photo-irradiated thin ZnO film on which gold nanoparticles are deposited and allows large photo-induced SERS enhancement to be obtained for the chemical detection of small molecules compared to normal SERS signals. This photo-induced SERS enhancement is due to increasing electron density of the gold nanoparticles and charge transfer mechanisms. Here, we achieve a high quality PIERS substrate, the signal of which exhibits weaker fluctuations and a similar or greater gain (up to 7.52) than those reported in the current literature. Henceforth, these PIERS substrates can be of great potential for industrial applications.

International audience; As luminescent quantum dots (QDs) are known to aggregate themselves through their chemical activation by carbodiimide chemistry and their functionalization with biotin molecules, we investigate both effects on the fluorescence properties of CdTe QDs and their impact on Förster Resonant Energy Transfer (FRET) occurring with fluorescent streptavidin molecules (FA). First, the QDs fluorescence spectrum undergoes significant changes during the activation step which are explained thanks to an original analytical model based on QDs intra-aggregate screening and inter-QDs FRET. We also highlight the strong influence of biotin in solution on FRET efficiency, and define the experimental conditions maximizing the FRET. Finally, a free-QD-based system and an aggregated-QD-based system are studied in order to compare their detection threshold. The results show a minimum concentration limit of 80 nM in FA for the former while it is equal to 5 nM for the latter, favouring monitored aggregation in the design of QDs-based biosensors.

International audience; We report on the recent scientific research contribution of non-linear optics based on Sum-Frequency Generation (SFG) spectroscopy as a surface probe of the plasmonic properties of materials. In this review, we present a general introduction to the fundamentals of SFG spectroscopy, a well-established optical surface probe used in various domains of physical chemistry, when applied to plasmonic materials. The interest of using SFG spectroscopy as a complementary tool to surface-enhanced Raman spectroscopy in order to probe the surface chemistry of metallic nanoparticles is illustrated by taking advantage of the optical amplification induced by the coupling to the localized surface plasmon resonance. A short review of the first developments of SFG applications in nanomaterials is presented to span the previous emergent literature on the subject. Afterwards, the emphasis is put on the recent developments and applications of the technique over the five last years in order to illustrate that SFG spectroscopy coupled to plasmonic nanomaterials is now mature enough to be considered a promising research field of non-linear plasmonics.

International audience; We establish a new procedure to study the steady state adsorption of carbon monoxide (CO) at the surface of platinum under electrochemical control over a wide potential range in aqueous acidic solution. Using highly concentrated solutions of CO in perchloric acid medium enables the formation of a tightly packed and very stable CO monolayer at the surface of the metal, which allows cyclic voltammetry in the [−1200;+1500 mV] potential range (vs Ag/AgCl). We use sum-frequency generation (SFG) spectroscopy to show the unusual stability of the monolayer: going toward negative potentials, hydrogen evolution is first blocked then strongly quenched; toward positive potentials, CO oxidation is prevented as a consequence of its dense packing as a "frozen" monolayer. Diluting CO concentration by a factor of 10 is enough to recover most of the usual properties of the CO/Pt(110) interface, still maintaining the high stability of the CO adlayer. Such a protocol can be applied to any problem for which a CO poisoning mechanism has to be scrutinized or as a model of very stable electrochemical interfaces for fundamental studies.

International audience; This work describes an electrochemical study of 4-nitrobenzene diazonium (4-NBD) reduction onto glassy carbon (GC) electrode coupled to in situ sum-frequency generation (SFG) spectroscopy. After 4-NBD grafting at 0.3 V vs. saturated calomel electrode (SCE) onto GC, SFG allowed a clear signal assigned to the symmetrical vibration mode of the nitro (NO2) groups to be observed at 1349 cm-1 or 1353 cm-1 depending on whether the spectrum was recorded in air or inside the solution. This result proved that 4-NBD grafting actually occurs at a potential as high as 0.3 V vs. SCE. The combination of SFG data and cyclic voltammetry (CV) also indicated that at such a potential, NO2 groups did not experience reduction process into hydroxylamine (NHOH) or amine (NH2) groups. The electrolysis of grafted NO2 moieties at -0.1 V was followed by CV and in situ by SFG. The exponential decay of the NO2 signal located at 1353 cm-1 vs. electrolysis time was in accordance with a charge transfer-limited reaction rate for a species immobilized at the electrode surface, and allowed a first order kinetic rate constant for NO2 reduction to be estimated k = 0.006 s-1. The integration of the peaks observed on the corresponding cyclic voltammograms (CVs) which were attributed to the NO/NHOH reversible system showed that the NO2 reduction produced both hydroxylamine and amine groups and was not quantitative. The fact that SFG spectroscopy was silent for long electrolysis time values suggested the remaining nitro groups to be located far from the electrode surface, as a consequence of an electron tunneling efficiency which decreased throughout the film thickness. Further electrolysis at -0.8 V allowed the remaining nitro groups to be reduced into NH2 with almost quantitative yields. All these results suggest the existence of a stratified layer during the electrolysis process, in which there is no limitation due to H+ diffusion in the organic film.

Gold nanotriangles structured as honeycombs and fabricated by nanosphere lithography on a gold film are functionalised by thiophenol molecules in order to be used as plasmonic sensors in nonlinear optical sum frequency generation (SFG) spectroscopy. The monitoring and the characterisation of the surface optical properties are performed by UV–visible differential reflectance spectroscopy showing an absorbance maximum located at 540 nm for p- and s-polarisation beams. SFG spectroscopy proves to be effective for thiophenol detection in ssp-polarisation scheme, while the molecular SFG signal disappears in ppp-configuration due to the strong s–d interband contribution of gold. However, in ssp-configuration, the vibration modes of thiophenol molecules at 3050 and 3071 $${\hbox {cm}}^{-1}$$ are yet observed thanks to the excitation of a transversal plasmon mode by the incident visible laser beam, whereas they are usually very difficult to distinguish by surface-enhanced Raman scattering and other vibrational optical probes.

International audience; We develop an innovative manufacturing process, based on radio-frequency magnetron sputtering (RFMS), to prepare neat CdSe quantum dots (QDs) on glass and silicon substrates and further chemically functionalize them. In order to validate the fabrication protocol, their optical properties are compared with those of QDs obtained from commercial solutions and deposited by wet chemistry on the substrates. Firstly, AFM measurements attest that nano-objects with a mean diameter around 13 nm are located on the substrate after RFMS treatment. Secondly, the UV-Vis absorption study of this deposited layer shows a specific optical absorption band, located at 550 nm, which is related to a discrete energy level of QDs. Thirdly, by using two-color sum-frequency generation (2C-SFG) nonlinear optical spectroscopy, we show experimentally the functionalization efficiency of the RFMS CdSe QDs layer with thiol derived molecules, which is not possible on the QDs layer prepared by wet chemistry due to the surfactant molecules from the native solution. Finally, 2C-SFG spectroscopy, performed at different visible wavelengths, highlights modifications of the vibration mode shape whatever the QDs deposition method, which is correlated to the discrete energy level of the QDs.

International audience; In this paper we report an in situ 2D visible-infrared sum frequency generation (VIS-IR SFG) study of the potentiostatic electrodeposition of gold from a solution containing [Au(CN) 2 ] À in 1-butyl-1-methyl-pyr-rolidinium bis(trifluoromethylsulfonyl) amide ([BMP][TFSA]), a room-temperature ionic-liquid (RTIL). We followed the potential-dependent co-adsorption of BMP + and CN À , released by reduction of the Au(I) complex, in the range-0.8 to À3.0 V vs. Au QRE. We found that the pyrrolidinium ring of BMP + reorients as a function of potential value and interaction mode with coadsorbed CN À. The Stark tuning of CN À shows that this pseudohalide can be adsorbed in two different ways, that can be correlated with the presence of Au(I) reduction intermediates at the dynamic electrodeposition interface.

Within semiconductor quantum dots (QDs), exciton recombination processes are noteworthy for depending on the nature of surface coordination and nanocrystal/ligand bonding. The influence of the molecular surroundings on QDs optoelectronic properties is therefore intensively studied. Here, from the converse point of view, we analyse and model the influence of QDs optoelectronic properties on their ligands. As revealed by sum-frequency generation spectroscopy, the vibrational structure of ligands is critically correlated to QDs electronic structure when these are pumped into their excitonic states. Given the different hypotheses commonly put forward, such a correlation is expected to derive from either a direct overlap between the electronic wavefunctions, a charge transfer, or an energy transfer. Assuming that the polarizability of ligands is subordinate to the local electric field induced by excitons through dipolar interaction, our classical model based on nonlinear optics unambiguously supports the latter hypothesis. Exciton/recombination events in semiconductor quantum dots are highly dependent on surface coordination environments and these processes are well studied. Here, conversely, the authors use sum-frequency generation spectroscopy to probe the effect of the quantum dot on the vibrational structure of the ligands.

In this paper we report an SFG/DFG investigation of the adsorption of CN− – used as a probe molecule to study the electrochemical double-layer structure – at a polycrystalline Au electrode in 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl) amide ([BMP][TFSA]) room-temperature ionic liquid (RTIL). The adsorption of CN− yielded single SFG and DFG bands in the range from ca. 2125 to 2135 cm−1, exhibiting a Stark tuning of ca. 3 cm−1 V−1. Vibrational resonances – corresponding to modes of the RTIL coadsorbed with CN−, were found in the range from ca. 1200 to 1500 cm−1. The study of the double-layer structure was complemented by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements, from which the differential double-layer capacity (CDL) was estimated. Keywords: Ionic liquid, [BMP][TFSA], Spectroelectrochemistry, SFG, DFG, Cyanide

In view of improved durability, the study of electrocatalyst supports for PEMFCs alternative to carbon is raising notable research interest. In this paper we investigated the oxidation of ethanol on WC-supported electrodeposited Pt, in pristine and aged conditions by: (i) electrochemical measurements, (ii) in situ spectroscopy (FTIR and sum-frequency generation spectroscopy (SFG)) carried out during the electro-oxidation of ethanol and (iii) ex situ synchrotron-based space-resolved photoelectron spectroscopy (SPEM). The complementarity of FTIR and SFG allows an insightful understanding of the electrochemical interface as a function of applied potential and catalyst ageing. In situ FTIR and SFG revealed adsorbed acetate and ethanol, in addition to linearly and bridge-bonded CO and solution-phase CO2. The Stark tuning of CO indicates that it is less strongly bonded to aged electrodes. Quantitative analysis of potential-dependent FTIR and SFG spectra reveals that the population of adsorption sites for CO, acetate and ethanol changes with ageing and correlates with loss of catalytic activity. Changes in catalyst morphology as well as Pt and WC chemical state have been pinpointed by SPEM: electrodeposited Pt nanoparticles tend to agglomerate and the distribution of oxidised and reduced forms of Pt and W correlates with the attack of the catalyst support.

International audience; Eight 1 cm 2 samples of self-organising zinc oxide (ZnO) nanopillar arrays with preferential vertical orientation were grown by pulsed laser deposition and then coated with 30 nm of Au using either thermal or electron-beam evaporation. Each sample had a different set of ZnO and Au growth conditions. The Au/ZnO nanoarrays were then tested for use in surface enhanced Raman scattering (SERS) detection of thiophenol molecules. The ratio of I SERS /I Raman was adopted as a measure of the SERS sensitivity and was found to vary from 1.7 to 23.7 within the 8 samples. The impact of the density, width, filling factor, orientation, homogeneity and shape of the nanostructures on the average SERS intensity and the within-wafer reproducibility of the SERS response were considered for 9 paired comparisons based on fixing all but one of the growth parameters for each pairing. Overall, smaller nanopillar width was found to correlate with stronger average SERS signal while more vertically aligned arrays with higher filling factors showed better within-wafer reproducibility.

International audience; A global method for handling fluorescence spectra at high concentration derived from the competition between emission and absorption of colloidal CdTe quantum dots This paper demonstrates that the concentration dependent attenuation and redshift of quantum dots' fluorescence is due to primary and secondary inner filter effects and not to collisional quenching as commonly stated. We present a global method to predict and process such altered fluorescence spectra that can be applied to any fluorophore. We investigate the effects of the concentration of CdTe quantum dots (QDs) on their fluorescence in water. The emission spectra, acquired in right angle geometry, exhibit highly variable shapes. The measurements evidence a critical value of the concentration beyond which the intensity and the spectral bandwidth decrease and the fluorescence maximum is redshifted. To account for these observations, we develop a model based on the primary and secondary inner filter effects. The accuracy of the model ensures that the concentration dependent behaviour of QD fluorescence is completely due to inner filter effects. This result is all the more interesting because it precludes the assumption of dynamic quenching. As a matter of fact, the decrease of the emission intensity is not a consequence of collisional quenching but an effect of competition between fluorescence and absorption. We even show that this phenomenon is linked not only to the QD concentration but also to the geometric configuration of the setup. Hence, our analytical model can be easily adapted to every fluorescence spectroscopy configuration to quantitatively predict or correct inner filter effects in the case of QDs or any fluorophore, and thus improve the handling of fluorescence spectroscopy for highly concentrated solutions.

International audience; In two previous papers (C. Mele, M. Catalano, A. Taurino, B. Bozzini. Electrochim. Acta 87 (2013) 918; B. Bozzini, A. Gianoncelli, C. Mele, M. Kiskinova. Electrochim. Acta 114 (2013) 889), we have: (i) fabricated high-capacitance materials consisting of nanoporous gold (NPG)-supported MnO 2 nanowires (NW), by electrochemical etching of single-phase Au-Mn alloys electrodeposited from a deep eutectic solvent (DES) electrolyte and (ii) investigated some aspects of the precursor Au-Mn alloy electrodeposition process by following it in situ with space-resolved soft X-ray fluorescence (XRF) and absorption (XAS) microspectroscopies: this study has allowed to single-out the peculiarities of elemental and chemical-state distribution that contribute to the nanostructure fabrication. The present paper completes the electrodeposition study by investigating the potential-dependent interfacial composition of the growing Au-Mn alloys by complementary in situ linear (Raman) and non-linear (Sum-Frequency Generation À SFG) vibrational spectroscopies. The results regarding alloy electrodeposition are compared to those obtained with a pure Au bath. In situ spectroscopy during electrodeposition reveals that both choline cation and urea are present at the growing metal/DES interface, coadsorbed with CN À resulting from the decomposition of the Au complex. Electrostatic adsorption controls the surface coverage scenario at the Au/DES interface, while Mn favours the relative surface coverage with urea. Moreover, the interaction of urea with the metal film is modified by the addition of Mn, switching from solid-like to liquid-like at the Mn-alloying potential threshold. Also the CN À adsorption scenario is sensitive to surface alloying: the Mn-containing interface shows two adsorption sites with lower degree of metal-adsorbate charge transfer. Finally, the degree of surface enhancement correlates well on the one hand with the applied potential and the interfacial chemistry, and on the other hand with the crystallite morphology induced by alloying Au with Mn. The correlation among the spectroelectrochemical scenario, the potential-dependent alloy composition and the crystallite shape expressed by this investigation fits within the framework set by recent modelling of dynamic electrodeposition morphochemistry and opens up novel opportunities for improving the control over the functional properties of net-shape electrodeposited materials. ã

International audience; We experimentally determine the effective nonlinear second-order susceptibility of gold over the visible spectral range. To reach that goal, we probe by vibrational two-color sum-frequency generation spectroscopy the methyl stretching region of a dodecanethiol self-assembled monolayer adsorbed on a gold film. The sum-frequency generation spectra show a remarkable shape reversal when the visible probe wavelength is tuned from 435 to 705 nm. After correcting from Fresnel effects, the methyl stretching vibrations serve as an internal reference, allowing to extract the dispersion of the absolute phase and relative amplitude of the effective nonlinear optical response of gold in the visible range. Published by AIP Publishing. https://doi.

International audience; We have investigated a metal-supported lipid film by using a sum frequency generation (SFG) spectrometer coupled to a free electron laser (FEL). DFT calculations on a molecular model have enabled to assign the vibrational signatures. The SFG vibrations detected in the backbone region, between 1375 cm À1 and 1000 cm À1 , originate from the vibrational motions of the 2,4-dinitrophenyl polar head of the lipid, which holds antigenic properties, and can therefore be used to selectively study the biological interactions with complementary antibodies. Theses results strengthen the role of SFG spectroscopy as a highly sensitive tool for biological detection.

Les étoiles...ces points lumineux qui illuminent en permanence notre ciel, lumières en apparence froides et immobiles qui nous posent sans cesse la question de nos origines et de notre destinée. Qu’est-ce qui les caractérisent le mieux que leur éclat, leurs couleurs quand on commence à les regarder de plus près avec des instruments optiques de plus en plus perfectionnés ? Notre étoile, le soleil, est la première source d’énergie qui nous permet d’appréhender notre environnement afin de mieux le comprendre et le maîtriser. Cette étoile nous permet d’observer et d’étudier déjà beaucoup de choses à l’échelle humaine mais ne suffit pas à expliquer ce qui se passe à plus petite échelle, celle des molécules et des atomes constitutifs de la matière qui nous compose ou nous entoure. Pour commencer à répondre à ces questions, des générations d’hommes et de femmes se sont succédé dansl’Histoire afin d’élaborer heure après heure les outils et les théories nécessaires à faire grandir l’arbre de la connaissance scientifique. En ce qui me concerne (et concerne tous ceux qui embrassent la carrière scientifique), il a d’abord fallu s’approprier et comprendre une partie plus ou moins importante de l’héritage qui nous est légué, en fonction de nos capacités. De là, j’ai pu commencer à élaborer pierre après pierre ma propre modeste contribution à cet héritage afin de la léguer aux générations futures. Mon domaine de recherche se situe dans le cadre de l’optique nonlinéaire : il fait donc appel aux propriétés de la lumière (sa couleur, son intensité) non pas d’une étoile mais de celle produite par la matière lorsqu’on la soumet à des champs électriques très importants (on peut faire l’analogie avec la force des champs électriques rencontrés dans les nuages et qui forment les éclairs). C’est ce que je fais en utilisant des lasers : l’envoi de flashes lumineux intenses mais contrôlés (contrairement aux éclairs d’orage) dans la matière crée une source de lumière dont je peux contrôler la couleur et l’intensité. Je récupère alors l’énergie contenue dans cette lumière en l’envoyant sur ce que je veux étudier au niveau moléculaire. Quand je change la couleur de cette lumière, le matériau qui la reçoit va réagir de manière plus ou moins importante en fonction de sa nature et fournir une signature unique caractéristique de ce qui le constitue et comment il est organisé au niveau moléculaire. Cela fonctionne efficacement quand il y a énormément de molécules identiques qui constituent l’objet étudié et sont alors facilement détectables sans avoir besoin d’utiliser l’énergie importante contenue dans un faisceau laser. Par contre, cela devient très compliqué lorsqu’il s’agit de discriminer des objets plus complexes composés de molécules différentes situés dans des endroits confinés, ce qui est le cas à la surface de n’importe quel matériau ou à l’interface entre deux matériaux. Dans le cas présent, il faut alors beaucoup d’énergie (au propre comme au figuré) pour arriver à distinguer la zone précise qui nous intéresse de l’environnement dans lequel elle est cachée. Les lasers ont cette capacité à dévoiler les mystères de ces milieux mais il faut encore arriver à voir des mécanismes particuliers qui émettent très peu de lumière grâce à des outils de détection sélectifs et performants. Cespropriétés optiques appartiennent à la classe des effets optiques dits nonlinéaires dont l’origine sera expliquée et illustrée concrètement dans ce manuscrit à travers de nombreux exemples. Les pages qui composent ce dernier détaillent mon parcours scientifique depuis le début de ma thèse (fin 1998) à ce jour. Il se développe en 4 grandes parties qui ont pour ambition de dévoiler de façon progressive et cohérente comment à partir de mécanismes de plus en plus élaborés, on peut mettre en évidence les propriétés diverses et subtiles qui gouvernent la vie moléculaire des interfaces.

La microscopie à force atomique (AFM) est utilisée pour décrire le processus de formation de complexes ADN/polymère. Les deux principaux objectifs de cette recherche sont présentés ci-àprès. Le premier consiste à appliquer l'AFM à l'analyse de molécules d'ADN et de différents complexes polycations/ADN afin d'évaluer leur degré de condensation (taille et forme). Le second objectif est de déterminer une relation entre l'état de condensation de l'ADN et son efficacité de transfection. Dans cette analyse, des polymères à base de méthacrylate linéaire et de polymères globulaires de SuperFect sont utilisés pour induire la condensation de l'ADN. Des complexes ternaires, composé de polymère à base de méthacrylate et de copolymère à base de polyéthylène glycol (PEG), sont étudiées. L'AFM nous a permis de confirmer les bonnes conditions de condensation et de les relier (ou pas) aux efficacités de transfection. Ces résultats AFM (obtenus après sèchage à l'air) sont comparés à des mesures déduites d'expériences de diffusion dynamique de lumière (DLS), réalisées en conditions liquide. Cette comparaison nous a permis d'identifier les modifications structurelles issues du dépôt à la surface du mica., Atomic Force Microscopy (AFM) is used to describe the formation process of polymer/DNA complexes. Two main objectives of this research are presented. The first one is to apply AFM as an effective tool to analyse DNA molecules and different polycation/DNA complexes in order to evaluate their degree of condensation (size and shape). The other one is to search for a relationship between the condensation state of DNA and its transfection efficiency. In this study, linear methacrylate based polymers and globular SuperFect polymers are used in order to induce DNA condensation. Ternary complexes, composed of methacrylate based polymers and polyethylene glycol (PEG)-based copolymers, are also investigated. AFM allows us to confirm good condensation conditions and relate them (or not) to transfection efficiencies. These AFM results (obtained after drying in air) are compared with measurements deduced from dynamic light scattering (DLS) experiments performed in water. This comparison allowed us to identify the structural modifications resulting from deposition on the mica surface.

We use infrared-visible sum-frequency generation (SFG) spectroscopy in order to investigate the adsorption properties on Pt(111) of molecules having CH3-C6H4-(O-CH2-CH2)(n)-O-(CH2)(m) SH as general chemical formula. We synthesized three molecules defined by the values m =5 n=4, m=11 n=4, m=11 n=8 and characterized them by Nuclear Magnetic Resonance spectroscopy. Thanks to spectroscopic measurements, we show that these molecules build self-assembled monolayers on Pt(111). First, the weak SFG signals arising from the ad-layer indicate low order and surface coverage of the substrate by these molecules. Next, the vibrational fingerprints of the aforementioned molecules are determined between 2825 and 3125 cm(-1) and the observed SFG spectral features are ascribed on the basis of the analysis of shorter and simpler molecules (1-dodecanethiol, 4-methylbenzenethiol and CH3-C6H4-O-(CH2)(11)-SH) also adsorbed on Pt(111). The occurrence of methylene vibration modes indicates a significant amount of chain defects whatever the n and m numbers are. Finally, the identification of a particular vibration mode, characteristic of the aromatic ring, enables us to qualitatively discuss the effect of the number of methylene and ethylene glycol entities on its orientation. More precisely, higher these numbers, more tilted (with respect to the substrate normal) the aromatic ring plane is. (c) 2005 Elsevier B.V. All rights reserved.

International audience; We use sum-frequency generation spectroscopy (SFG) in the infrared 2800-3000 cm −1 spectral range and UV-vis spectroscopy (trans-mission) in the 450-650 nm spectral range in order to characterize vibrational and electronic properties of various interfaces composed of organic monolayers adsorbed on gold nanoparticles (AuNPs) with 19 nm average diameter. SFG signal is observed for AuNPs films deposited on glass substrates using the following silane intermediates: 3-(aminopropyl) triethoxysilane and 3-(mercaptopropyl) trimethoxysilane. The density of AuNPs and their aggregates are measured with a scanning electron microscope. For the samples showing a strong well-defined surface plasmon resonance (SPR), we also observe an enhancement of their non-linear optical properties. Furthermore, the SFG measurements show that 1-dodecanethiol films are rather well ordered on specific AuNPs substrates. In this way, the presence of the SFG signal, which comes from both the bulk electronic s-d interband transition and the vibrational states of the adsorbed molecules, depends on a SPR process. This phenomenon is evidenced on the AuNPs by the incident visible beam located at 532 nm, i.e. near the SPR energy maximum of these interfaces. These results open the door to experiments involving macromolecular and biological materials networks deposited on ultrathin metal electrodes in a controlled electrochemical environment.

International audience; In this paper we report an in situ electrochemical Sum-/Difference Frequency Generation (SFG/DFG) spectroscopy investigation of the adsorption of nitrile and CN − from the ionic liquid 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl) amide ([BMP][TFSA]) containing 4-{2-[1-(2-cyanoethyl)-1,2,3,4-tetrahydroquinolin-6-yl]-diazenyl}benzonitrile (CTDB) at Au electrodes in the absence and in the presence of the Au-electrodeposition process from K[Au(CN) 2 ]. The adsorption of nitrile and its coadsorption with CN − resulting either from the cathodic decomposition of the dye or from ligand release from the Au(I) cyanocomplex yield potential-dependent single or double SFG bands in the range 2,125-2,140 cm −1 , exhibiting Stark tuning values of ca. 3 and 1 cm −1 V −1 in the absence and presence of electrodeposition, respectively. The low Stark tuning found during electrodeposition correlates with the cathodic inhibiting effect of CTDB, giving rise to its levelling properties. The essential insensitivity of the other DFG parameters to the electrodeposition process is due to the growth of smooth Au.

The molecular orientation in self-assembled monolayers of thiophenol (TP) on Ag(1 1 1) and of para-nitroanilino-dodecane-thiol (NAT) on Au was investigated by infrared-visible sum-frequency generation between 500 and 1500 cm−1. Ab initio calculations were performed to determine the interface nonlinear response. The combined experimental and theoretical results reveal that the S–C bond of TP is tilted 3 7 ∘ from the Ag(1 1 1) surface normal, with the aromatic ring plane perpendicular to the surface. On Au, the tilt angle of the NAT molecule stabilizes below 6 0 ∘ at the end of film growth.

International audience

The orientation of thiophenol adsorbed on silver is determined by comparing ab initio calculated and experimental sum-frequency generation (SFG) spectra measured between 9 and 20 μm using a tandem KTP/ CdSe optical parametric oscillator. The molecule S-C bond is tilted ∼37° from the surface normal with the aromatic ring plane perpendicular to the surface.

International audience; The methanol dissociative adsorption on Pt(1 0 0) at low potentials in the hydrogen region is studied using nonlinear vibrational sum-frequency generation (SFG) spectroscopy. It is shown that adsorbed hydrogen atoms effectively block atop sites at the surface. In the hydrogen potential region, CO produced by the methanol dissociation adsorbs at step-edges and in bridge sites on terraces as shown by the characteristic stretching frequencies observed for adsorbed CO.

We demonstrate that a new emerging technique, two-colour sum-frequency generation (SFG) spectroscopy, can be used to probe the molecular electronic properties of self-assembled monolayers (SAMs). In the CH spectral range (2800–3200 cm−1), we show that the sum-frequency generation signal of a porphyrin alkanethiol derivative adsorbed on Pt(1 1 1) reaches a maximum intensity at ∼435 nm SFG wavelength. This wavelength corresponds to the porphyrin moiety specific π–π∗ molecular electronic transition which is called the Soret or B band. This resonant behaviour is not observed for 1-dodecanethiol SAMs, which are devoid of molecular electronic transition in the investigated visible spectral range.

We measured IR–visible sum-frequency generation spectra of CH3–(C6H4)2–(CH2)3–S–H (Biphenyl-3) self-assembled monolayers on a silver and a gold substrate. For the latter substrate, we observed different interference patterns between the resonant signal of the CH vibration and the non-resonant contribution of the substrate as a function of the visible beam wavelength. The non-linear response of the gold substrate is enhanced around 480 nm corresponding to the s–d interband transition. Such effect is not observed for the silver substrate the interband transition of which is located out of the investigated visible spectral range of 450–700 nm. 2002 Elsevier Science B.V. All rights reserved.

The vibrational nonlinear activity of films of 2,4-dinitrophenyl phospholipid (DNP) at the solid interface is measured by sum-frequency generation spectroscopy (SFG). Hybrid bilayers are formed by a Langmuir-Schaefer approach in which the lipid layer is physisorbed on top of a self-assembled monolayer of dodecanethiol on Pt with the polar heads pointing out from the surface. The SFG response is investigated in two vibrational frequency domains, namely, 3050-2750 and 1375-1240 cm. The first region probes the CH stretching modes of DNP films, and the latter explores the vibrational nonlinear activity of the 2,4-dinitroaniline moiety of the polar head of the lipid. Analysis of the CH stretching vibrations suggests substantial conformational order of the aliphatic chains with only a few gauche defects. To reliably assign the detected SFG signals to specific molecular vibrations, DFT calculations of the IR and Raman activities of molecular models are performed and compared to experimental solid-state spectra. This allows unambiguous assignment of the observed SFG vibrations to molecular modes localized on the 2,4-dinitroaniline moiety of the polar head of DNP. Then, SFG spectra of DNP in the 1375-1240 cm frequency range are simulated and compared with experimental ones, and thus the 1,4-axis of the 2,4-dinitrophenyl head is estimated to have tilt and rotation angles of 45±5° and 0±30°, respectively. Nonlinear vibrational response of a hybrid bilayer of 2,4-dinitrophenyl phospholipid (DNP) supported on a Pt surface is interpreted qualitatively and quantitatively by sum-frequency generation spectroscopy and density functional theory calculations. Spectral simulations also reveal the orientation of the 2,4-dinitrophenyl polar head of DNP, which is estimated to have tilt and rotation angles of 45±5° and 0±30°, respectively (see picture).

International audience; Layers of thiophenol functionalized spherical gold nanoparticles grafted on Si(100) are probed by linear UV-vis, Fourier transform infrared and nonlinear infrared-visible vi-brational sum/difference-frequency generation spectroscopies as a function of the nanoparticles surface coverage. Depending on the dipping time (5 min, 20 min, 1 h, and 24 h) in the colloidal solution, AFM imaging corroborates that the silicon surface coverage with gold nanoparticles increases, while the distance between neighbouring nanoparticles decreases, leading to their aggregation which dramatically impacts their optical properties. In the UV-vis reflectance spectra after the appearance of the 525-nm individual plasmonic band, a second broad band located at 660 nm and related to the gold nanoparticles aggregation on silicon rapidly dominates in intensity. Nonlinear vibrational spectroscopy is able to detect the specific vibration of the thiophenol molecules (3, 055 cm −1) whatever the immersion time and at least down to 1 % of the substrate filling factor by the gold nanoparticles, overtaking the molecular sensitivity threshold of surface in-frared and Raman spectroscopies on small gold nanostructures (17 nm) adsorbed on a semiconductor. Moreover, a quantitative analysis of the nonlinear vibrational fingerprint from 5 min to 24 h in the framework of the effective medium models of Maxwell-Garnett and Bruggeman illustrates the role played by the interband and the plasmonic properties of gold modulated by the silicon optical response. In this case, the sample reflectivity affects the molecular oscillator strength measured by nonlinear optical vibrational spectroscopy. For this latter technique, no coupling with the optical properties of aggregated AuNps is evidenced while the localized surface plasmon resonance excitation amplifies the molecular response.

International audience; We report the study of methanol electro-oxidation on ordered and defective Pt(1 1 0) surfaces using infrared-visible sum-frequency generation vibrational spectroscopy in the COads internal stretch spectral region (1700-2150 cm(-1)). Comparison of the spectra as a function of the structural quality of the electrodes and of the applied potential shows that: (i) dissociative chemisorption occurs primarily at low potentials on defect sites such as steps and (ii) the signal of CO adsorbed on terraces decreases more sharply at higher potentials than the signal of CO adsorbed on defects. This latter behavior appears to contradict the fact that dense CO adlayers on terraces block the oxidation process. This apparent paradox is solved by considering the interplay of preferential oxidation of CO at defects sites and transfer of adsorbed CO from terraces reservoir to defects. (C) 2012 Elsevier B.V. All rights reserved.

International audience; Nonlinear optical Sum and Difference-Frequency spectroscopies are used to probe and model the surface of thiophenol-functionalised gold nanoparticles grafted on a Si(100) substrate through two different silanization procedures. By scanning the [980-1100 cm À1 ] infrared spectral range with the CLIO Free Electron Laser, ring deformation vibrations of adsorbed thiophenol are investigated. Quantitative data analysis addresses three levels of organization: microscopic, nanoscopic and molecular. Grafting with p-aminophenyl-trimethoxysilane shows an increase of around 40% in surface density of nanoparticles (N s) as compared to 3-aminopropyl-triethoxysilane. The relative amplitudes of the resonant and nonresonant contributions to the SFG and DFG spectra are discussed in terms of N s , Fresnel reflectivity factors and local amplification of the nonlinear signals by coupling to the surface plasmon of the particles. They are shown to quantitatively scale with N s , as measured by atomic force microscopy. Vibration mode assignment is performed through a critical analysis of literature data on IR and Raman spectroscopies coupled to DFT calculations, for which a methodology specific to molecules adsorbed on gold atoms is discussed.

International audience; Infrared Àvisible sum-frequency generation (SFG) vibrational spectroscopy is applied to evidence the extramolecular PtÀC chemical bond and deduce an experimental value of its Raman activity at the CO/Pt(110) interface. This measurement, performed on a flat metal surface, without surface roughening, does not involve any enhancement of the electromagnetic fields. We evidence that the experimental Raman polarizability of the extramolecular PtÀC stretching vibration mode is around 1.49 times smaller than that of the intramolecular CO stretching vibration mode. Furthermore, SFG data show that the infrared dynamic dipole moments differ in sign for the two vibrational modes, as suggested by density functional theory (DFT) calculations. The weak SFG intensity experimentally measured from the PtÀC chemical bond is accounted for by its low infrared activity. For surface chemistry, these results open the door to systematic investigation of extramolecular vibrational activity at the submonolayer level.

Publisher Summary This chapter discusses different aspects of infrared-visible sum-frequency generation (SFG) spectroscopy. It emphasizes that this noninvasive optical probe could be considered as a versatile technique with various applications to biointerfaces. It underlines that SFG is more than a complementary probe to classical IR and Raman spectroscopies. It possesses specific assets, which make it a fully original tool: sensitivity to orientation, order, and symmetry; absolute intensity measurements; ultrashort temporal resolution; vibrational microscopy below the Rayleigh limit; and the possibility to study couplings between molecular levels at identical or different energy scales. However, SFG remains a spectroscopic tool among others. While IR-visible SFG spectroscopy is a rather young optical probe of matter, it has proved mature enough to study complex interfaces and tackle delicate questions mainly related to in vitro biology. Considering the speed at which this technique spreads worldwide and technically develops, it seems reasonable to believe that it should continuously evidence in the future new details on various mechanisms involving biological matter, while still experiencing new developments to improve its power of investigation. Scientific content described in this chapter should therefore be considered as a beginning of a long relationship between SFG spectroscopy and biological materials.

International audience; In this paper we report in situ sum-frequency generation (SFG) spectroscopy experiments, carried out during the electro-oxidation of ethanol on Pt-black supported on tungsten carbide (WC). Well-defined SFG resonances were measured in the 1320-1350 cm −1 spectral range with the VIS beam set at 523.5 nm. The assignment of SFG bands was carried out with the aid of vibrational spectra calculated in the framework of Density Functional Theory (DFT). The spectra recorded in situ showed various kinds of changes in terms of spectral patterns and peak intensities, as a function of the applied potential and of the oxidation time, yielding information on the modification of the catalyst-adsorbate structure. To the best of the authors' knowledge, this is the first in situ electrochemical SFG work carried out in this particular wavelength range-specially diagnostic for electrocatalysis and made accessible by use of the CLIO IR free-electron laser-and with an electrode functionalised with a real catalyst, in powder form.

We have studied the apatite growth dynamics on tantalum oxide surfaces. This nucleation is obtained via an organosilane intermediate layer between the apatite and the substrate surface. Four organosilane layers (differing by their terminal functionality) were investigated. Their characterization with atomic force microscopy and other techniques such as X-ray photoelectron spectroscopy (XPS) and wetting measurements highlighted the influence of the organosilane terminal groups on the apatite growth rates. Results revealed that apatite is indeed growing faster on phosphate terminal groups than on the three other groups studied (vinyl, hydroxyl and carboxyl). © 2008 Elsevier B.V. All rights reserved.

International audience; In this paper we report the first spectroelectrochemical in situ doubly resonant sum frequency generation (DR-SFG) spectroscopy experiment carried out with a free-electron laser tunable IR source and IR and VIS Optical Parametric Oscillators. We studied the potential-dependent adsorption and reaction of the quinoline dye 4-{2-[1-(2-cyanoethyl)-1,2,3,4-tetrahydroquinolin-6-yl]diazenyl} benzonitrile (CTDB) onto a Cu(100) electrode. SFG band assignments were based on density functional theory calculations. Strong SFG enhancement was obtained in the IR range 1100-1300 cm-1 , corresponding to ArsCtN stretching, with the VIS beam set at 441.6 nm and in the IR range 2000-2300 cm-1 , corresponding to nitrile stretching, with the VIS beam set at 532 nm and the pH adjusted in order to match the bathocromic shift of the adsorbed CTDB chromophore with the input VIS laser frequency. Doubly resonant conditions were achieved via resonance of the VIS input with the electronic structure of the adsorbate orbitals and of the adsorption bond.

International audience; Sum frequency generation (SFG) and difference frequency generation (DFG) are applied to study vibrational resonance of the thiophenol molecule adsorbed on two different gold samples. One sample is made of 17 nm gold nanoparticles (AuNPs) fixed on a silicon substrate that has been previously functionalized with a silane monolayer (aminopropyltriethoxysilane, APTES). This sample is fully characterized through visible reflection spectroscopy and AFM. The second sample is a gold monocrystal also covered with thiophenol molecules. From their comparison, an enhancement factor of 21 is deduced for the SFG signal on AuNPs with respect to the Au(111), related to the surface plasmon resonance (SPR). From a combined analysis of the SFG and DFG spectra, we demonstrate that SFG/DFG spectroscopy is able to identify the nature of the substrate where the molecules are adsorbed. This opens new perspectives for this nonlinear spectroscopy by adding to its well-known intrinsic surface specificity, the ability to selectively probe the chemical layer capping the AuNPs.

International audience; Standard vibrational sum-frequency generation (SFG) spectroscopy is performed to probe the electrochemical cyanoacetylene/ Au(1 1 1) interface, enlightening a 20 cm À1 /V Stark shift of the free CN vibration mode of this cyanopolyynes-class molecule. SFG data suggest an orientation for the adsorbed molecules with the CN moiety pointing out in a direction perpendicular to the metal surface. A newly developed two-colour SFG (2C-SFG) setup based on the CLIO free electron laser synchronized with a tuneable visible laser source is used to probe simultaneously the vibrational and electronic fingerprint of a thiophenol/Ag(1 1 1) interface. Structural information on the adsorbed self-assembled monolayer is put in evidence as a function of the immersion time of the silver substrate in the thio-phenol solution (24 h or 5 days). With a longer immersion time, the molecular packing is of better quality and more compact, with the carbon rings less tilted with respect to the surface normal. This new experimental setup combines the advantages to have a higher spectral and temporal resolution with higher power energies at far infrared wavelengths than these presently available from tabletop infrared optical parametric oscillators.

International audience; A two-colour sum-frequency generation (2C-SFG) spectroscopy experiment was carried out on an assembly of Ag nanoparticles embedded in a Si3N4 matrix. The visible beam was fixed either at ωvis = 441.6 nm in the surface plasmon resonance (SPR) of the Ag nanoparticles or at ωvis = 632.8 nm, out of the SPR. The SFG signal was detected while the infrared beam was scanned around 1100 cm −1 , a spectral range where an absorption signal has been detected in infrared (IR) reflection absorption spectroscopy. At ωvis = 441.6 nm, the SFG signal put in evidence an electronic resonance contrary to ωvis = 632.8 nm. This second-order nonlinear process is related to the nanostructured system. An interpretation is given in terms of an electronic state arising at the interface of the metallic cluster and the insulating matrix.

International audience; Recently, thiocyanate groups were successfully used as precursors for thiolate assemblies. Indeed, they provide a useful alternative to thiol groups for self-assembly on metallic substrates. In order to check the adsorption properties and the quality of the thiocyanate-based self-assembled monolayers (SAMs), we use sum-frequency generation spectroscopy (SFG) and apply it to characterize the ad-layers of two similar molecules: decanethiol (DT) and decyl thiocyanate (DTCN) adsorbed on platinum surfaces. By comparing the SFG signals of the methyl and methylene vibration modes, we show that DTCN SAMs are less ordered than DT ones. These effects are related to the SAMs quality which depends on the molecular packing as highlighted by scanning tunnelling microscopy and X-ray photoelectron spectroscopy.

In a recent paper Ciszek et al. [J.W. Ciszek, M.P. Stewart, J.M. Tour, J. Am. Chem. Soc. 126 (2004) 13172] showed that organic thiocyanates may be an interesting alternative to the use of thiols for thiolate assemblies. We use scanning tunnelling microscopy (STM), infrared reflection absorption and sum-frequency generation spectroscopies (IRRAS and SFG) in order to study the adsorption properties of decyl thiocyanates (DTCN) and compare them to the decanethiol (DT) ones. Firstly, IRRAS measurements show that DTCN molecules form self-assembled monolayers (SAMs) on gold via a thiolate link with the metallic substrate. Secondly, the DTCN SAM on gold is less ordered than the DT one as highlighted by SFG spectroscopy. Indeed, the intensities of the methyl vibration modes vanish while the methylene ones increase when DTCN molecules are adsorbed on the substrate instead of DT. We explain the differences in SAMs quality on the basis of STM measurements which reveal differences in molecular order and packing. (c) 2006 Elsevier B.V. All rights reserved.

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Overlayers of a fatty acid (palmitic and lauric acid) formed at the interface between a solution of this molecule in phenyloctane and the basal plane of graphite are studied by in situ scanning tunnelling microscopy. The layers organize into lamellae, which are formed by a close packing arrangement of molecules parallel to the graphite surface. Chemical modification of the STM tips used allowed identification of the functional group. Indeed, the gold tips used are functionalized with 4-mercaptobenzoic acid (4-MBA) and 4-mercaptotoluene (4-MT). The same functional group on a sample is then 'seen' as a dark and a bright spot when imaged with 4-MBA and 4-MT modified tips. respectively. This contrast distinction is related to interactions (or a lack of them) between the carboxyl group on the sample and Molecules on the tip, which can facilitate (or hinder) the electron tunnelling.

The influence of chemical modification of scanning tunnelling microscopy tips on image contrast is studied. This technique is applied to the identification of an ester functional group, hardly visible otherwise. Self-assembled overlayers of wax esters [CH3-(CH2)14-CO-O-(CH2)15-CH3], adsorbed at the interface between highly oriented pyrolitic graphite and a solution of phenyloctane, are imaged. The gold tips used are chemically modified by 4-mercaptobenzoic acid and 4-mercaptotoluene. The stability of the ordered overlayers formed facilitates the reproducible set of images with submolecular resolution. This allows the identification of the layer regular structure and of other features within molecules, which can be unambiguously related to the fingerprints of the COO bond. Moreover, we are interested in finding evidence of molecular motions observed at domain boundaries.