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1 May 2012 Realization of hybrid systems coupling molecules and gold nanoparticles towards fluorescence enhancement
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Hybrid systems coupling gold nanoparticles to fluorophores have been realized, aiming to investigate the conditions to get two-photon fluorescence (TPF) enhancement effects through nanoantenna or Purcell effects. The use of gold nanorods (NR) was chosen : due to their anisotropic form they indeed exhibit two localized surface plasmon resonance (SPR) modes: one in the visible (associated to the transverse size of the NR) and another in the infrared (associated to the NR longitudinal size), one key point being the possibility to adjust these two resonances to the optical properties of the two-photon fluorophores to be further coupled to the NRs (emission λem and excitation λexc wavelengths). Detailed investigation of the intrinsic NR TPF signal dependence was first considered. Experiments were performed in aqueous solutions using a Ti-Sapphire laser source emitting 100 fs pulses in the 750-950 nm wavelength range. We observe that the maximum TPF signal is located at the NR surface plasmon resonance wavelength, pointing the role of field enhancement effects in the observation of the increased NR TPF. As a next step, the nanoparticles were immobilized onto previously treated indium tin oxide (ITO) coated glass substrates and a method to couple fluorescent molecules (a polyphenylene vinylene (PPV) derivative) to the previously immobilized NRs was then studied: the so-called layer-by-layer technique was more particularly investigated in order to control the realization of hybrid systems coupling the fluorescent PPV polymer and particles at varying distances. In order to perform joint optical and topographic characterizations, a stand-alone atomic force microscopy (AFM) platform was integrated to our TPF microscopy set-up. The influence of the number of spacing layers on the TPF of such hybrid systems was studied. First results seem to indicate the existence of a specific distance allowing TPF enhancement. A more detailed study considering the intensity and lifetime of such hybrid system is currently under way in order to fully quantify the signal enhancement origin.
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Yara El Harfouch, Kang Liu, Fabrice Charra, Sylvie Marguet, Henri Perez, and Céline Fiorini-Debuisschert "Realization of hybrid systems coupling molecules and gold nanoparticles towards fluorescence enhancement", Proc. SPIE 8424, Nanophotonics IV, 842418 (1 May 2012);

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