Thermodynamic insights into ligand exchange and surface defect passivation of indium phosphide quantum dots for biomedical interface engineering

Indium phosphide (InP) quantum dots (QDs) are promising cadmium-free nanomaterials for biomedical and optoelectronic applications because of their tunable optical properties and comparatively favorable biocompatibility. Nevertheless, interfacial instability, surface oxidation, and defect-mediated nonradiative recombination continue to limit their performance and biological translation. This review critically evaluates published evidence concerning the thermodynamic and surface-chemical principles governing ligand exchange and defect passivation in InP QDs, with particular attention to colloidal stability, photoluminescence efficiency, and biomedical functionality. Experimental studies employing isothermal titration calorimetry, nuclear magnetic resonance spectroscopy, and complementary spectroscopic techniques are reviewed and compared to clarify ligand-binding energetics, cooperative exchange mechanisms, surface coverage, and interligand interactions. Relevant computational investigations are additionally assessed to connect adsorption, desorption, surface, and defect-formation energies with experimentally observed interfacial behavior. Inorganic and atomistic passivation strategies, polymer coatings, and charged-ligand functionalization are comparatively examined for their ability to suppress surface trap states and improve interfacial robustness. Evidence from the reviewed literature indicates that thermodynamically favorable ligand binding and effective defect passivation can enhance optical stability, dispersion behavior, and biofunctionalization for biosensing, imaging, and drug-delivery applications. The review establishes a characterization-oriented framework linking ligand thermodynamics, surface structure, and functional performance, while providing practical guidelines for designing stable InP QDs for biomedical interfaces.