Jean-Louis Bijeon, Pierre-Michel Adam, Davy Gérard, Rodolphe Jaffiol, Jérôme Plain, Pascal Royer
We are interested mainly by the phenomena of interaction between metallic NanoParticles (NPMs) and adsorbents with the applicative theme nanospectroscopy. This theme is divided into three research projects: interactions with phosphors in Near-Field Optics (NFO) fluorescence spectroscopy, NFO Raman spectroscopy and extinction spectroscopy for the development of nanosensors.
NFO Fluorescence spectroscopy
The first project focuses on the study of interactions between metallic nanoparticles and phosphors (organic fluorophores, Q-dots). In particular, we are studying the physical phenomena that govern these interactions. Indeed, there is a controversy in the international literature, between groups that measure enhancement and those that measure only the "quenching" fluorescence. To answer these complex questions, we are studying these interactions systematically controlling the size, geometry NPMs, and the distance that separates them from phosphors. Thus we can see in figure 1, the evolution factor modification of the fluorescence depending on the size of NPMs gold, when a single Q-Dots recovered (F> 1: enhancement and F <1: Quenching).
NFO Raman spectroscopy
In NFO Raman spectroscopy we have in the framework of the PhD thesis of Manuel Lopes conducted a quantitative study of the depolarization properties of metallic and dielectric tips used in apertureless NFO microscopy and TERS (Tip Enhance Raman Scattering). Our results show depolarization factors between 5 and 30% (as can be seen as an example in figure 2(a)) which vary depending on the polarization of the incident light and shape of the tip. The consequences of this phenomenon of depolarization can be underlined in TERS experiments on crystalline silicon . One shows that if one goes into a particular configuration, depolarization induced by the proximity of the tip on the surface of silicon can be used selectively to exalt the phonon mode at 520 cm-1 compared to harmonics bands at 300 and 980 cm-1 (see fig. 2 (b)). The depolarization must therefore be taken into account for a correct estimation of the enhancement induced by the tip.
Fig 2:(a) Polar plot of the scattered light intensity by a metallic tip in function of the polarization angle, for a P-polarized light (black dots) and S (blue line).(b) Raman Spectra of cristalline Si  in a configuration of excitation/detection SS with the tip retracted (red spectra) and with the tip in contact (blue spectra).
The nanotechnologies enable the interaction of different scientific fields (Physics, Chemistry, Biology) in particular in developing nanosensors. It is within this context that are positioning our work, we propose here to improve and better monitor the form of metallic nanoparticles supporting LSPR (Localized Surface Plasmon Resonances) using lithography by electron beam (LFE) as manufacturing technique. We highlighted the important aspect of the geometry of nanoparticles on the plasmon resonance. For this, we measured, for different geometries (spheres, ellipsoids and cylinders) and for different metals (Gold and Silver), the extinction spectra of the plasmon resonance. The many experimental results show unambiguously the importance of the form (linked to the ratio major axis / short axis) of nanoparticles on the shift of the wavelength of plasmon resonance, in accordance with the theoretical predictions calculated by the method of finite differences time domains (FDTD, collaboration with group 4) and the different results that can be found in the literature. We have characterized the effectiveness of our nanosensors made by LFE (see Figure 3(a)) on biological species depending on their concentration, size and mass by using two models: the couple Biotin / Anti-biotin and the couple Biotin/Streptavidine (see Figure 3 (b) and 3 (c) for examples of measurements). As two important parameters, characteristics of these sensors are highlighted: the amplitude of spectral shift of the plasmon resonance Δλ and the value of the constant liaison confined to the surface Ka, surf. These two parameters inform us on the one hand about the dynamics of sensor and the other hand about its detection limit: the higher the value of Δλ, the greater the dynamic response of the sensor is high, and the higher the value of Ka, surf, the greater the sensor is sensitive to the low concentrations of biological/chemical species. Thus the amplitude of spectral shift of the plasmon resonance is greater for the couple Biotin/Anti-biotin than for the couple Biotin/Streptavidine: the size and weight of the anti-biotin are more important than the streptadivine. The value of Ka, surf being higher for streptadivine, the results show greater sensitivity to this compound (hence detection for lower concentration) than for anti-biotin. These measurements were made both on particles of gold and silver and show greater sensitivity for silver. Our work demonstrates without ambiguity that the sensitivities of LSPR nanosensor and SPR sensor (thin film) are of the same magnitude, or about 10-11 Molar.
Fig. 3: (a) Gold Nanodisks of 100 nm diameter, (b) Shift of the plasmon resonance in function of the concentration of the streptavidin SA and antibiotin AB, (C) examples of plasmon shift after each step of the functionnalization. (From the PhD Thesis of Gregory Barbillon, 2007)
Former PhD students: Grégory Barbillon (2007), Pierre Viste (2007)