Conjugated polymer nanoparticles for photoacoustic vascular imaging

Abstract

Photoacoustic (PA) imaging is an emerging technique in biological imaging, which is able to provide high resolution and deep penetration depth. Exogenous contrast agents with strong near infrared (NIR) absorbance are highly desirable for PA imaging. In this work, we synthesized a conjugated polymer, poly[9,9-bis(4-(2-ethylhexyl)phenyl)fluorene-alt-co-6,7-bis(4-(hexyloxy)phenyl)-4,9-di(thiophen-2-yl)-thiadiazoloquinoxaline] (PFTTQ) by Suzuki polymerization, which shows strong absorption in the NIR range. The processability of PFTTQ allowed for its encapsulation into a 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] (DSPE-PEG₂₀₀₀) matrix to yield PFTTQ loaded DSPE-PEG₂₀₀₀ NPs (PFTTQ NPs) that are dispersible in water. As a result of their strong NIR absorbance and high non-radiative quantum yield (1 − fluorescence quantum yield), PFTTQ NPs show a strong ability to generate a PA signal. In addition, PFTTQ NPs show much higher photostability in comparison with Au NRs upon high energy pulse laser illumination at a fixed laser fluence of 15 mJ cm⁻². The potential of PFTTQ NPs as a PA probe was further demonstrated using brain vascular imaging as an example.