Acceptor Engineering of Small Molecule Fluorophores for NIR-II Fluorescent and NIR-I Photoacoustic Imaging

Fluorescence imaging in the second near-infrared window (NIR-II) has been an emerging technique in diverse in vivo applications with high sensitivity/resolution and deep tissue penetration. To date, the designing principle of reported NIR-II organic fluorophores has heavily relied on benzo [1,2-c:4,5-c’]bis([1,2,5]thiadiazole) (BBTD) as a strong electron acceptor. Here, we report rational design and synthesis of a NIR-II fluorescent molecule with rarely used [1,2,5]thiadiazolo[3,4-f]benzotriazole (TBZ) core to replace BBTD as the electron acceptor. Thanks to the weaker electron deficiency of TBZ core than BBTD, the newly yielded NIR-II molecule (BTB) based nanoparticle has higher mass extinction coefficient and quantum yield in water. On the contrary, the nanoparticle suspension of its counterpart with BBTD as the core is nearly nonemissive. The NIR-II BTB nanoparticles allow video-rate fluorescence imaging for vasculature imaging in mouse ear, hindlimb, and brain. Additionally, its large absorptivity in NIR-I region also promotes bioimaging using photoacoustic microscopy (PAM) and tomography (PAT). Upon surface conjugation with Arg-Gly-Asp (RGD) peptide, the functionalized nanoparticles ensured targeted detection of integrin-overexpressed tumors through both imaging modalities in two- and three-dimensional views. Thus, our approach on engineering acceptor of organic fluorophores offers a promising molecular design strategy to afford new NIR-II fluorophores for versatile biomedical imaging applications.