Surface bioengineering of lanthanide nanoparticles for theranostic applications: From hydrophilic modification to multimodal imaging and therapy.

Lanthanide nanoparticles (LnNPs) are recognized as next-generation luminescent bioprobes due to their tunable emission spectra, long luminescence lifetimes, and high photostability. Although extensively studied, these nanomaterials have recently garnered renewed interest across biodetection, bioimaging, and therapeutics. To serve as suitable optical labels, LnNPs need to overcome inherent limitations such as hydrophobic surfaces, insufficient luminescence intensity, and limited environmental responsiveness. Accordingly, the integration of optical modulation, hydrophilic surface modification, and biofunctionality has become essential for advancing their biomedical applications. Recent surface engineering strategies have systematically addressed challenges related to hydrophilicity, colloidal stability, and biocompatibility, with substantial progress achieved. This review outlines mainstream approaches for hydrophilic surface modification and bio-functionalization of LnNPs, including ligand engineering, surface coating, and biomimetic modification. The design rationale and implementation of these methods are discussed, alongside representative applications in multimodal imaging and targeted therapy. Current challenges and future prospects in this rapidly evolving field are also summarized.