Semiconducting Polymer Nanoparticles for Optical Imaging: Self-luminescence Imaging and Photoacoustic Imaging

Optical imaging plays an important role in the biomedical field due to its noninvasiveness, high spatiotemporal resolution, and high sensitivity capabilities, which allows for real-time visualization of numerous cellular and molecular processes in living organisms to investigate their biological functions. As one of the most widely used optical imaging techniques, fluorescence imaging suffers the relatively poor signal-to-background ratio (SBR) and low tissue penetration depth due to light scattering and tissue autofluorescence induced by real-time light excitation. To address these issues, self-luminescence and photoacoustic (PA) imaging have recently developed, which eliminate concurrent light excitation and detect acoustic signals with minimized acoustic scattering, respectively, leading to higher SBR and deeper imaging depth relative to fluorescence imaging. This review focuses on the recent development of semiconducting polymer nanoparticles (SPNs) for self-luminescence and PA imaging. The molecular engineering design approaches to amplifying the self-luminescence efficiency and PA brightness of SPNs are highlighted. The design strategies based on the chemically initiated electron exchange luminescence (CIEEL) to amplify the chemiluminescence intensity of SPNs and the afterglow luminescence imaging of phenylenevinylene-based SPNs are introduced. The design strategies including the fluorescence quenching or accelerated heat dissipation to amplify the PA brightness of SPNs are also introduced. Then, the SPN-based smart activatable probes for in vivo PA imaging are discussed. The facile modification and intraparticle engineering allow SPNs responsive to various biological and pathological indexes, including pH and reactive oxygen species (ROS), and the designs and PA imaging applications of these SPN-based activatable probes are introduced in detail. Finally, current challenges and perspectives of SPNs in the biomedical field are proposed.