Recent advances in conjugated polymer lanthanide-doped upconversion nanoparticles and their biological applications

This review article provides a comprehensive overview of the synthesis, functionalization, and biomedical applications of lanthanide-doped upconversion nanoparticles (LN-UCNPs) modified with polymers. UCNPs, which exhibit anti-Stokes luminescence, have gained significant attention due to their unique optical properties, including high photostability, deep tissue penetration, and low autofluorescence. These characteristics make them ideal candidates for applications in bioimaging, photodynamic therapy (PDT), photothermal therapy (PTT), and drug delivery systems. However, the inherent hydrophobicity and potential toxicity of UCNPs require surface modifications to enhance their biocompatibility and functionality. This review initially discusses the fundamental mechanisms of upconversion luminescence (UCL), the importance of lanthanide doping in improving the optical performance of UCNPs, and the most common methodologies for synthesis, highlighting their advantages and limitations. This analysis examines how polymer coatings can improve the colloidal stability, biocompatibility, and functional versatility of UCNPs, and provides an overview of recent studies that have employed this combination. Different strategies for polymer modification, such as ligand exchange, encapsulation, and layer-by-layer (LbL) assembly, are explored in detail. In the context of biomedical applications, recent studies on the use of polymer-modified UCNPs in bioimaging are discussed and the integration with photosensitizers and photothermal agents for their roles in PDT and PTT that offer targeted cancer therapies with potentially reduced side effects are examined. The evaluation also covers the development of UCNP-based drug delivery systems, where polymers facilitate controlled and stimuli-responsive release of therapeutic agents, enhancing treatment efficacy. Despite the advances in the area, challenges such as tissue overheating during NIR irradiation and the lack of standardized protocols for cytotoxicity assessment remain.