Enhanced optical imaging properties of lipid nanocapsules as vehicles for fluorescent conjugated polymers

Conjugated polymer nanoparticles (CPNs) have emerged as highly photostable probes for optical and photoacoustic imaging. However, the aggregation of conjugated polymer (CP) molecules upon nanoparticle formation is associated with fluorescence quenching, poor yields and mutable particle sizes. This study investigated whether the CP encapsulation within the liquid midchain triglyceride (MCT) core of lipid nanocapsules (LNCs) may achieve reduced packing of CP chains leading to a stable system with enhanced optical features. The red- and near infrared-emitting CPs, CN-PPV and PCPDTBT, showed precipitation and aggregation-induced quenching with concentrations >~25 µg/mL in MCT alone. Despite this, CP encapsulation within LNCs abolished quenching at concentrations up to 1500 µg/mL. PCPDTBT-LNCs exhibited a quantum yield of 2.8% and a higher signal:background ratio in an optical imaging phantom compared to literature reports of PCPDTBT encapsulated in PEG-PLGA nanoparticles. In contrast, PCPDTBT-LNCs had slightly lower photoacoustic amplitudes than reported PEG-PLGA systems. CP-LNCs were also stable in size (32 ± 0.7 nm) and photoluminescence over 21 days at 4 °C, 25 °C and 37 °C. In summary, encapsulation of CP within the liquid core of lipid nanocapsules enhances the optical properties of fluorescent CP.