Ultrabright BODIPY-Tagged Polystyrene Nanoparticles: Study of Concentration Effect on Photophysical Properties

Fluorescent nanomaterials are invaluable tools for bioimaging. Polymeric nanoparticles labeled with organic dyes are very promising for this purpose. It is thus very important to fully understand their photophysical properties. New fluorescent core-shell nanoparticles have been prepared. The outer part is a poly(ethylene glycol)-block-poly(acrylic acid) copolymer, and the core is a copolymer of styrene and methacrylic BODIPY fluorophore. The hydrophilic and hydrophobic parts are covalently linked, ensuring both stability and biocompatibility. We prepared nanoparticles with increasing amounts of BODIPY, from 500 to 5000 fluorophores per particles. Increasing the concentration of BODIPY lowers both the fluorescence quantum yield and the lifetime. However, the brightness of the individual particles increases up to 8 X 10(7). To understand the loss of fluorescence efficiency, fluorescence decays have been recorded and fitted with a mathematical model using a stretched exponential function. This result gives an insight into the fluorophore arrangement within the hydrophobic core.