Excitation Energy Transfer from Semi-Conducting Polymer Nanoparticles to Surface-Bound Fluorescent Dyes

Excitation energy transfer from semi-conducting polymers to fluorescent dyes (or dye-labeled analytes) is commonly used in sensor applications, e.g., for DNA and protein sensing. Semi-conducting polymers show rather high absorption cross-sections and a high intra-chain mobility of the delocalized excitations leading to a high sensitivity for the excitation light. However, sensing of biological molecules needs an aqueous environment. One possibility for realizing this is the use of water-soluble semi-conducting polymers. Water solubility can be achieved by the introduction of ionic side groups (e.g., ammonia or sulfonate groups) into semiconducting polymers. However, the synthesis of the polyelectrolytes is often not simple and the products are difficult to characterize. A suitable alternative is the use of an aqueous dispersion of semi-conducting polymer nanoparticles. We have published a simple way toward such nanoparticles via a mini-emulsion protocol. Hereby, a small amount of a cationic, anionic, or nonionic surfactant is used in the preparation of the (precursor) mini-emulsions and (final) dispersions after removal of the organic solvent. The surfactant is finally embedded in the outer shell of the nanoparticles and stabilizes the primarily formed aqueous mini-emulsion and the final nanoparticle dispersion. Such semi-conducting polymer nanoparticle dispersions are stable over time periods of several months. If an ionic surfactant is used, the nanoparticles carry positive or negative Summary: The first examples of the dye-coated semiconducting polymer nanoparticles as well as experiments to demonstrate the excitation energy transfer from the excited chromophor of the nanoparticle to the fluorescent dye are described. We have demonstrated that the blue fluorescence of the dye-coated polyfluorene nanoparticles is only slightly quenched after dye deposition. However, a new emission band of the surface-bound dye (Rhodamine 6G or Rhodamine TM) appears in the wavelength region of 530–600 nm. These results clearly indicate an effective excitation energy transfer from the excited PF chromophores to the fluorescent dye.