Tuning the Performance of Inorganic Nanosized Fluorophores in Near‐Infrared Region II by Surface Chemical Modification

Near-infrared region-II (NIR-II) emitters have been extensively explored and applied in biomedical imaging because the majority of biological tissues are relatively transparent and display limited autofluorescence in this region. Inorganic nanoparticles are the most widely studied NIR-II emitters, with increasing research interest recently focusing on the engineering of diverse nanosized NIR-II emitters, such as quantum dots, metal-based clusters, rare-earth-doped nanoparticles, and carbon-based nanoparticles, etc. The imaging performances of these luminescent nanoparticles can be tuned by controlling the particle composition, size, shape, crystallinity, and surface chemistry. Among these, surface functionalization has been demonstrated to be a facile and efficient approach for enhancing quantum yields, regulating excitation/emission wavelengths, and improving stability, targetability, and biocompatibility characteristics of NIR-II nano-emitters. A timely discussion regarding the impact of surface chemistry may therefore enable the rational engineering of surface-modifying ligands to enhance imaging performances and accelerate the clinical translation of NIR-II nano-emitters. The current review summarizes previously reported NIR-II nanosized fluorophores and their surface functionalities, in addition to discussing the effects of the surface ligands on their optical properties, stabilities, targetabilities, and biocompatibilities. Moreover, a few promising approaches are highlighted for regulating the surface chemistries of nanoparticles to improve their imaging performances.