Colloidal Gold Nanostructures for Plasmonics

International audienc

Photophysical properties of 5-methylcytosine

International audienceCytosine methylation, which determines the hot spots for DNA photo-damage, is shown to induce a red-shift of the nucleoside absorption spectrum, making the chromophore more vulnerable to solar radiation, and a tenfold increase of the fluorescence lifetime, making excited state reactions more probable. A femtosecond investigation of the excited state deactivation reveals a quite complex mechanism

Colloidal Gold Nanostructures for Plasmonics

International audienceGold nanoparticles (Au-NPs) of high crystalline quality can now be readily synthesized in large quantities thanks to significant advances in colloidal chemistry over the past decade. Assembly of such NPs leads to the appearance of new properties, still relatively poorly explored, and is therefore particularly promising for production of cost-effective devices. Our research activities concentrate on the synthesis and assembly of gold NPs and aims at providing appropriate materials for researchers working in the various fields of plasmonics (1,2,3). Over the past years, we have been developed an expertise in the synthesis of NPs with tunable sizes and shapes, such as spheres, rods, octahedra, cubes, triangles and also micro-particles. Some of them are very promising and produced only in very few laboratories worl-wide, especially in Asia. For example, thick triangles are of particular interest for ultrasensi-tive sensing and can assemble in the edge-to-edge fashion or through face to face interactions (fig.). Microplatelets (hexagonal or triangular) are very attractive due to their large atomically flat facets and can form ordered 1D-columnar aggregates. In the literature, 3D-arrays of NPs are currently related to SERS applications. Here, we report the spontaneous self-assembly of gold NPs by simply evaporating concentrated solutions on non-patterned sub-strates. The degree of reproducibility of this method, the maximum size of the perfectly organized area and the key parameters related to a controlled-deposition are presented. We are looking for collaborations to study the properties of these unusual assemblies

Gold Nanoparticles for Plasmonics and Medicine

International audienceGold-bioconjugates are studied worldwide and promising for new technologies for health. In the long term, biodegradable gold na-noparticles (NPs) are expected to have a large impact on diagnosis through the development of new contrast agents for imaging or new ultra-sensitive sensors. Gold-NPs have a high potential as contrast agents for several bioimaging modalities such as computed tomography, pho-toacoustic imaging, dark field scattering, multi-photon luminescence, high frequency ultra-sound, quantitative phase contrast. New therapies and surgeries will develop because these gold-nano are also able to generate Reactive Oxygen Species (ROS) and heat. Our research activities concentrate on the synthesis and assembly of gold nanoparticles of high quality to provide original materials for plasmonics since 2008. The irradiation of gold NPs by short laser pulses sets off a cascade of complex transient phenomena. Following this irradiation, one observe exaltation and confinement of the incident light at the surface of the NP but also singlet oxygen generation and ROS through the injection of hot electrons (or hot holes) to a nearby molecule. In short, gold na-noparticles are nanosources of light, heat and hot carriers and the morphology of the NP is a key point for these three characteristics because competitive relaxation processes depend on the size, shape and aspect ratio of NP. We use colloidal chemistry to synthesize gold nanoparticles of controlled shape and size. The figure illustrates some of the gold NPs we propose. Contrary to spherical and rod-shaped NPs that are commercially-available, other shapes such as cubes, triangles .. and plates with tunable sizes are only produced in our lab and in few laboratories worldwide. In literature, triangular-gold nanoplates are already used for ultrasensitive sensing, and the construction of original plasmon-based optical devices. The recent discovery of a third and fourth biological transparency windows centered respectively at 1.8µm and 2.2µm and the recent commercialization of new NIR-lasers make gold-nanoplates attractive for biomedicine in this still unexplored spectral domain. The synthesis of highly uniform shapes relies on trial and error procedure because it is very hard to predict the right recipe and additional purification steps are also often required. In the future, we are eager to widen the application range of these NPs to medicine through new collaborations either with biochemists for appropriate biofunctionalization, biophysicists for imaging and physicians for therapy. As a first step towards this objective, we are developing recipes to produce Au@SiO 2 core-shell NP while waiting for precise specifications. Figure 1: illustration of some of the monodis-perse gold particles (nano-and micro-) and gold hybrids (Au@SiO2) that have been synthesized , with tunable sizes and thicknesses

Synthesis of Monodisperse Gold Nanoparticles for Plasmonics

International audienceOur research activities concentrate on the synthesis and assembly of gold nanoparticles for plasmonics that is a rapidly growing discipline at the interface of physics, chemistry and biology with promising applications. We use a seed-mediated growth method of colloidal chemistry to synthesize gold nano-and micro-particles of controlled shape and size. This morphology control provides a fine tuning of the plasmon resonance wavelength and of the local field enhancement factor. Compared to top-down materials, these gold particles of high crystalline quality offer better surface-confinement of the electromagnetic field. The figure illustrates some of the monodis-perse gold NPs and gold nanohybrids we have been synthesized. Contrary to spherical and rod-shaped NPs that are commercially-available, other shapes such as nanocubes, na-notriangles and micro-plates with tunable sizes are only produced in our lab and in few laboratories worlwide, mainly in Asia. Triangles are of particular interest for ultrasensitive sensing, and plates (hexagonal or triangular) are very attractive for the construction of original plasmon-based optical devices due to their large atomically flat facets. At the present time, the synthesis of highly uniform triangles and plates still required purification steps that render them difficult to produce. Our research is made in collaboration with various research teams expert in the different field of plasmonics and aims at providing appropriate materials to study enhanced-photochemistry, enhanced-spectroscopy and nanosources of light. In the future, we are eager to widen the application range of these NPs to sensors and metamaterials through new expected collaborations

Two-photon luminescence of single colloidal gold nanorods: revealing the origin of plasmon relaxation in small nanocrystals

The two-photon luminescence (TPL) of small 10 nm x 40 nm colloidal gold nanorods (GNR) is investigated at the single object level, combining polarization resolved TPL and simultaneously acquired topography. A very high dependence of the TPL signal with both the nanorods longitudinal axis and the incident wavelength is observed confirming the plasmonic origin of the signal and pointing the limit of the analogy between GNRs and molecules. The spectral analysis of the TPL evidences two emission bands peaks: in the visible (in direct connection with the gold band structure), and in the infrared. Both bands are observed to vary quadradically with the incident excitation beam but exhibit different polarization properties. The maximum two-photon brightness of a single GNR is measured to be a few millions higher than the two-photon brightness of fluorescein molecules. We show that the important TPL observed in these small gold nanorods results from resonance effects both at the excitation and emission level : local field enhancement at the longitudinal surface plasmon resonances (LSPR) first results in an increase of the electron-hole generation. Further relaxation of electron-hole pairs then mostly leads to the excitation of the GNR transverse plasmon mode and its subsequent radiative relaxation

Mapping the Electromagnetic Near-Field Enhancements of Gold Nanocubes

International audienceWe imaged and quantitatively characterized electromagnetic hot spots near the surfaces of plasmon resonant gold nanocubes. The strongest fields are localized at the nanocube corners as compared to those on the sides. The near-field enhancement on the surface of the cube was imaged as a function of incident polarization, leading to information on the localization of fields on specific regions on the surface. We found that the field intensity drops dramatically when the nanocube corner is slightly tilted with respect to the incident laser polarization. This dramatic dependence on angle was verified by electrodynamics simulations. These results will enable the use of gold nanocubes in field enhancement applications and refractive-index sensing

Molecular spectroscopy: Complexity of excited-state dynamics in DNA

Absorption of ultraviolet light by DNA is known to lead to carcinogenic mutations, but the processes between photon absorption and the photochemical reactions are poorly understood. In their study of the excited-stated dynamics of model DNA helices using femtosecond transient absorption spectroscopy1, Crespo-Hernández et al. observe that the picosecond component of the transient signals recorded for the adenine–thymine oligonucleotide (dA)18(dT)18 is close to that for (dA)18, but quite different from that for (dAdT)9(dAdT)9; from this observation, they conclude that excimer formation limits excitation energy to one strand at a time. Here we use time-resolved fluorescence spectroscopy to probe the excited-state dynamics, which reveals the complexity of these systems and indicates that the interpretation of Crespo-Hernández et al. is an oversimplification. We also comment on the pertinence of separating base stacking and base pairing in excited-state dynamics of double helices and question the authors' assignment of the long-lived signal component found for (dA)18(dT)18 to adenine excimers

Etude theorique et experimentale de la relaxation des etats excites du colorant laser DCM. Transfert d'electron intramoleculaire et photoisomerisation. Effets de solvant

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