Effect of Branching on the Optical Properties of Poly(p-phenylene ethynylene) Conjugated Polymer Nanoparticles for Bioimaging.

Fluorescent conjugated polymers formulated in nanoparticles show attractive properties to be used as bioimaging probes. However, their fluorescence brightness is generally limited by quenching phenomena due to interchain aggregation in the confined nanoparticle space. In this work, branched conjugated polymer networks are investigated as a way to enhance the photoluminescence quantum yield of the resulting conjugated polymer nanoparticles (CPNs). 1,3,5-Tribromobenzene and 2,2',7,7'-tetrabromo-9,9'-spirobifluorene are chosen as branching moieties and are added in 3 or 5 mol % to the poly(p-phenylene ethynylene) (PPE) conjugated polymer synthesis. Nanoparticles of all samples are prepared via the combined miniemulsion/solvent evaporation technique. The optical properties of the branched polymers in solution and in nanoparticle form are then compared to those of the linear PPE counterpart. The fluorescence quantum yield of the CPNs increases from 5 to 11% for the samples containing 1,3,5-tribromobenzene. Furthermore, when 5 mol % of either branching molecule is used, the one-photon fluorescence brightness doubles. The nanoparticles show low cytotoxicity in A549 human lung carcinoma cells up to a concentration of 100 µg/mL for 24 h. They also exhibit good particle uptake into cells and compatibility with two-photon imaging.