Biocompatible Croconaine Absorbers As NIR-Ⅱ Probes For Deep-tissue Optical Imaging

Optical imaging in the second near-infrared window(NIR-Ⅱ,1000-1700 nm)can achieve deeper imaging depth,better contrast and imaging quality than the first near-infrared window(NIR-Ⅰ,650-900 nm),since biological tissue has less light scattering and light absorption,lower auto-fluorescence interference in this region.It is critical to develop probes with absorption or emission in NIR-Ⅱ for achieving NIR-II optical imaging.As a fast-developing non-invasive hybrid imaging technology,photoacoustic imaging has been widely used in biomedical imaging combining the advantages of high-sensitivity optical imaging and high spatial resolution ultrasound imaging.At present,most of the NIR-Ⅱ photoacoustic imaging contrast agents are inorganic nanoparticles,which have strong and stable signal intensity but are difficult to be metabolized in vivo.Photoacoustic imaging contrast agents based on organic small molecules or conjugated polymers have good biocompatibility and are promising for clinical transformationThe croconaine dyes belong to the cyanine dye family,which has better stability than the traditional cyanine dye due to the plane-rigid five-membered ring in the center of the structure.The unique mesoionic form of the large conjugated structure greatly promotes electronic resonance and energy transfer,resulting in strong and sharp near-infrared absorption.A1094 has the smallest molecular weight among the few small organic molecules with absorption wavelength>1000 nm,which also has a high molar extinction coefficient,strong stability,and the tendency to form aggregates.Herein,A1094 was synthesized by a two-step method,characterized and examined for photothermal stability and photoacoustic properties in vitro.In order to prepare biocompatible A1094 NIR-Ⅱ aggregate probes for in vivo imaging,we used amphiphilic DSPE-PEG2000 liposomes to encapsulate A1094 molecules.The aggregation of the small molecules within the hydrophobic liposome produces a～150 nm absorption bathochromic-shift,and the size of aggregate particles can be regulated by controlling the ratio of A1094 and DSPE-PEG2000.The aggregate probe had no obvious toxicity on U87 MG cells and major organs,it can accumulate in the subcutaneous tumor and realize fluorescence and photoacoustic imaging of the tumor.With strong absorption in NIR-Ⅱ,the probe can achieve deep photoacoustic imaging of inguinal lymph nodes,gliomas(-4.5 mm)and its evaluation after partial surgical resection.In summary,this paper prepared a kind of carrier-wrapped A1094 aggregates,rationally utilized the J-aggregation of A1094 to carry out NIR-Ⅱ photoacoustic imaging of deep tissue in living organisms,expanded the biomedical application of small organic molecules in NIR-Ⅱ.