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 × 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.
■ INTRODUCTIONOptical imaging is becoming increasingly attractive in medicine, biology, and biochemistry because it can achieve high spatial and temporal resolution and is noninvasive. 1 In addition, fluorescence imaging is a very sensitive technique which can be miniaturized into high sensitivity and cost-effective devices. 2 Usually, organic fluorophores are hydrophobic compounds and as such are not soluble in biological media. They can be modified with water-solubilizing groups; however, most of the time this results in a dramatic drop of the fluorescence quantum yield. 3 An appealing alternative is to incorporate them in organic or inorganic nanoparticles that can be dispersed in water. 1,4 The advantages are numerous because high local concentration of dye can be achieved, leading to lower total amount of material needed for detection of the fluorescence signal. Additionally, multiple fluorophores can be used to achieve bar-coding or multiplex analysis using energy transfer. 5 Furthermore, the nanocarrier itself can bring complementary properties such as protection toward photodegradation, water solubility, and, more importantly, biocompatibility while being small enough to penetrate cells.To this end, polymeric nanoparticles have received widespread interest because they benefit from a great variety of synthetic mode, available monomers, and compatibility with organic fluorophores. Two main doping techniques have been developed: physical trapping of dyes in polymeric particles 6 and covalent attachment. 7 One of the main problems of the first approach is that the fluorophores can leak out of the particles with time. Covalent linking of the fluorophore to the polymer backbone can be achieved either by postmodifying the polymer 8 with reactive fluorophores or by copolymerizing fluorescent monomers with a comonomer. As such, rhodamine, 9 fluorescein, 10 or BODIPY-derived monomers 11,12 have been successfully used to prepare fluorescent nan...