Rational design of a NIR-II fluorescent nanosystem with maximized fluorescence performance and applications

Near-infrared (NIR) fluorescence imaging (FI) has become a research hotspot in the field of in vivo imaging. Here, we intend to synthesize a NIR-II fluorescent nanosystem with excellent fluorescence performance to achieve high-resolution and long circulating vascular imaging performance in vivo. In this work, fluorescence liposomes with differently charged phospholipids and various amounts of IR-1061 were designed and synthesized for analysing the encapsulation effect and conformational changes of IR-1061 within liposomes. Furthermore, an IR-1061 liposome (IR1061-ALP-N3) with optimal fluorescence intensity was prepared to study its in vivo FI effect as well as the long circulation angiographic effect. The experimental results demonstrated that cationic liposomes showed the worst encapsulation effect on IR-1061 and anionic liposomes showed the best, which was consistent with the electrostatic interactions between phospholipids and IR-1061. Next, we found that IR-1061 possessed two states as a function of its concentration in liposomes: the free state and aggregate state. The aggregate state caused by the increase of IR-1061 content will decrease the fluorescence properties of the IR-1061 liposome. In vivo FI showed that IR1061-ALP-N3 exhibited excellent fluorescence performance in mice and successfully achieved clear systemic angiography with excellent spatial resolution. Moreover, long-term imaging also proved that IR1061-ALP-N3 can maintain high fluorescence intensity in vessels for more than 16 hours, indicating its angiographic function in the long circulation. The encapsulation effect of IR-1061 in liposomes was proved to be tightly related to the charge of phospholipids, and the limited space within liposomes would significantly affect the conformational changes of IR-1061. These results indicate that the rational design of IR-1061 liposomes with appropriate electrostatic effects and drug concentrations is an essential prerequisite to achieving high fluorescence performance. The excellent imaging effect and long circulation angiography contributed by this nanosystem also provide more favorable application prospects of NIR-II fluorophores for intravital imaging.