Functionalization of Emissive Conjugated Polymer Nanoparticles by Coprecipitation: Consequences for Particle Photophysics and Colloidal Properties

Recent developments in materials synthesis, bio-conjugation methods and luminescence techniques have led to a rapid proliferation of novel fluorescence-based approaches to imaging in the life sciences. However, for... [ view full abstract ]

Recent developments in materials synthesis, bio-conjugation methods and luminescence techniques have led to a rapid proliferation of novel fluorescence-based approaches to imaging in the life sciences. However, for high-resolution or long-time duration imaging applications, molecular dyes suffer from limitations of low brightness, poor photo-stability and fluorescence intermittency (blinking). Consequently, we are exploring a potentially promising alternative based on highly fluorescent conjugated polymer materials. Here, the functionalization of polyfluorene (PFO) nanoparticles by coprecipitation of the conjugated polymer with an amphiphilic comb polymer, consisting of a hydrophobic polystyrene backbone with hydrophilic, carboxylic acid-terminated polyethylene oxide side-chains (PS-PEG-COOH), is investigated. The comb polymer affects the properties of the formed hybrid nanoparticles. Non-functionalized particles are typically larger (28 nm) than functionalized ones (20 nm); this size difference impacts peak molar extinction coefficients accordingly. Zeta potentials are negative, consistent with negative surface charge on PFO particles due to chemical defect formation, with additional charge on functionalized particles due to the pendant carboxylic acid groups. Emission quantum yields of functionalized particles are typically larger, consistent with lower efficiency of energy transfer to quenchers in smaller particles and weaker PFO interchain interactions due to chain dilution. The trend in per-particle fluorescence brightness values, as confirmed by single particle fluorescence imaging, reflects the size-dependent nanoparticle absorption cross sections. Photostability studies on aqueous dispersions of hybrid particles indicate mild photobrightening under continuous illumination while PFO particles exhibit slow exponential emission decay. Functionalized particles also tend to be more resistant to aggregation during exposure to adenocarcinoma cells; see Figure (polyfluorene nanoparticles (blue) in MCF-7 cells (red / phalloidin-TRITC)). Generally, the hybrid particles exhibit more favourable time-, pH- and medium-dependent stabilities, likely due to steric and electrostatic stabilization by PEG- carboxylic acid functionalities. Overall, the functionalized particles exhibit attractive properties: Reasonably small size, tight size distribution, high absorption cross section, radiative rate and emission quantum yield, excellent brightness and photostability, and good colloidal stability.