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.