Nano-Confined Squaraine Dye Assemblies: New Photoacoustic and Near-Infrared Fluorescence Dual-Modular Imaging Probes in Vivo

For the purpose of near-infrared (NIR) fluorescence and photoacoustic (PA) tomography dual-modular imaging, self-assembly of squaraine (SQ) dyes is constructed in the hydrophobic phospholipid bilayers of liposomes (SQCL) with variable mixing ratios of SQ and phospholipids from 1:500 to 1:10 (w/w). When doping minimal amounts of SQ, molecularly dispersed SQ in bilayers shows remarkable fluorescence. Interesting, the PA signal is enhanced with increase of SQ in the nanoconfined bilayer region, which is attributed to the formation of SQ-based H-aggregates and enhanced thermal conversion efficiency (a). SQCL shows satisfactory chemical and thermal stabilities and photobleaching resistance. SQCL is well-distributed in the cytoplasm of MCF-7 cells and its fluorescence signal remains for 7 days without dramatic quenching owing to the good stability of SQCL. Furthermore, SQCL is subjected to in vivo NIR fluorescence imaging to evaluate the whole-body biodistribution in organ level. Particularly, PA imaging with deeper tissue penetration capability is utilized to investigate heterogeneous distribution SQCL inside solid tumor. The majority of SQCL are enriched in the area where the blood vessels are generated, implying that the liposomal nanocarriers exhibit lower tumor tissue penetration capability after the vascular leakage. This result is validated by histological examination of tumor tissue in parallel.