915 nm Laser Excited Semiconducting Polymer Nanoparticles for High-Resolution and Large-Depth NIR-IIb Bioimaging

The NIR-IIb window enables "deep-tissue-transparent" fluorescence imaging with minimized photon-scattering, water absorption, and diminished autofluorescence. Semiconducting polymer nanoparticles (SPNs) as NIR-II fluorescent contrast agents have been extensively studied, while their imaging abilities in the second near-infrared IIb window (NIR-IIb, 1500-1700 nm) with long excitation wavelengths have barely been reported. Herein, as a proof-of-concept, we demonstrate for the first time that the SPNs that we synthesized and named L1057 nanoparticles (NPs) exhibit intense NIR-IIb signal due to their ultrabrightness and broad emission spectrum, and more importantly, we optimized the excitation wavelength of L1057 NPs to 915 nm simultaneously due to a high mass extinction coefficient and low water absorption at 915 nm. Accordingly, we applied L1057 NPs to multidimensional bioimaging, achieving superior spatial resolution and large imaging depth in the NIR-IIb window. This study reveals that the manipulation of excitation wavelength and emission window is an efficient strategy to maximize the potential of L1057 NPs as NIR-IIb fluorophores.