Surfactant-assisted phase selective sonochemical synthesis of sodium yttrium fluoride nanoparticles

The potential of rare earth-doped sodium yttrium fluoride (NaYF₄) as upconverting nanoparticles extends to various applications, from electronics to biology. The hexagonal (β) phase of NaYF₄ nanoparticles is known to exhibit a photoluminescence quantum yield higher than the cubic (α) phase. The typical synthetic methods for β-NaYF₄ present limitations, such as low colloidal stability in aqueous medium and the need for ligand exchange. Herein, we employed high-throughput robotics, automated sonication, and high-throughput X-ray diffraction (HT-XRD) to synthesize and characterize hundreds of samples under various conditions and obtain NaYF₄ nanoparticles dispersible in an aqueous medium. We characterized the samples through XRD for phase determination, SEM and TEM for morphology assessment, UV-Vis spectroscopy and zeta potential for colloidal stability evaluation, and photoluminescence spectroscopy and time-resolved photoluminescence spectroscopy to investigate their upconverting properties. We found that only sodium dodecyl sulfate (SDS) promotes β-NaYF₄ formation. The order of addition is also important: adding SDS before the precursor salts leads to pure β-NaYF₄ formation. Our proposed mechanism for β-NaYF₄ formation involves the initial formation of smaller α-NaYF₄ crystals in the presence of SDS, which were more easily converted to β-NaYF₄ during sonication compared to the larger α-NaYF₄ crystals formed in the absence of SDS.