Spectral Engineering and Morphological Tuning of Amino Acid Capped Hydrophilic Upconversion Nanophosphors

Upconversion, the process of generating a higher-energy photon from lower-energy photons, has captivated research interest over the years due to its unique applications in the field of biological imaging, light harvesting, optical devices, and anti-counterfeiting. High luminescence efficiency and emission color tuning are key requirements for the materials to find application in diverse fields. We have developed a simple and facile methodology for the precise emission color tuning in lanthanide-based upconversion nanophosphors. The use of varying concentrations of pyroglutamic acid, an amino acid derivative, as the capping agent led to a change in the dominant emitting state resulting in a visual color tuning from red to green. The variation in concentration of the surface ligand also led to a profound morphological transformation of the nanostructures from large irregular rods to small uniform particles. The change in morphology was accompanied by a transformation of the crystal structure from a hexagonal to a cubic phase. This method stands out as a one-step approach for the synthesis of hydrophilic nanophosphors with tailored upconversion luminescence. The presence of amino acid derivatives on the surface renders biocompatibility to the small and uniformly sized luminescent particles. Furthermore, due to the advantages of pyroglutamic acid based surfactants, the synthesized nanophosphors have the potential to be developed as efficient cell-penetrating multicolor labeling agents.