Luminescent upconverting nanoparticles (UCNPs) are lanthanide doped nanocrystals and have found widespread use in biological applications enabled by their unique properties. These include Anti-Stokes shifts, multiple narrow emission bands, low scattering/autofluorescence, low toxicity and lack of blinking/photobleaching. Their diverse applications include targeted drug delivery, gene therapy, photodynamic therapy, bioimaging and bioassays. Despite success, UCNPs have consistently faced practical challenges that arise from the limited stability of surface coatings in the presence of water and inorganic phosphate leading to poor colloidal stability, particle dissolution and reduced luminescence. Multidentate phosphonate ligands can protect UCNPs from these deleterious effects, however, these systems require improved ligand exchange procedures, stronger binding affinities, higher ligand densities and simpler synthetic routes. They must also facilitate subsequent surface chemistry modifications such as bioconjugation and PEGylation. To address these issues a multidentate ligand library was created to investigate surface coatings that will protect UCNPs from these deleterious effects. The ligands have a variable number of phosphonate groups mapped to match the spacing of cations on the surface of UCNPs based on the crystal structure of β-NaYF<sub>4</sub>. Colloidal stability of phosphonate coated UCNPs was evaluated in phosphate buffer solutions using dynamic light scattering and transmission electron microscopy. Lastly, atomic emission spectroscopy was used to determine the influence of ligand dimensions on surface density.

Rational design of phosphonate ligands for upconverting nanoparticles in biological applications

ACS Spring 2022

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