Indium Phosphide Quantum Dots for Near-Infrared Luminescence-Based Bioimaging and Theranostics: From Photophysical...

Indium phosphide (InP) quantum dots (QDs) have emerged as promising cadmium-free luminescent nanomaterials for near-infrared (NIR) bioimaging and theranostic applications owing to their tunable optical properties, photostability, and improved biocompatibility. This review highlights recent advances in the photophysical engineering of InP QDs, focusing on the mechanisms that govern NIR luminescence, including quantum confinement, excitonic relaxation, defect-state modulation, and band-edge fine-structure effects. The influence of molecular-level surface engineering, such as ligand optimization, core/shell heterostructure design, interfacial alloying, and oxidative passivation, is critically discussed in relation to emission efficiency, colloidal stability, and environmental resistance. Recent studies demonstrating extended NIR emission, multiplexed bioimaging, resonance energy transfer-based sensing, and image-guided therapeutic applications underline the growing biomedical potential of InP QDs. In addition, current knowledge regarding biodistribution, biotransformation, and biological interactions is assessed to better understand their in vivo performance and safety. Despite significant progress, challenges related to long-term stability, degradation pathways, scalable manufacturing, regulatory approval, and clinical implementation remain. Overall, this review provides an integrated perspective on the relationships between photophysical properties, surface chemistry, and biological behavior, offering strategic directions for the development of efficient luminescent InP QDs for next-generation biomedical applications.