Hydrophilic Small-Size NaYF₄:Yb,Er Upconversion Nanoparticles with High Photoluminescent Red-to-Green Ratio and Quantum Yield

Near-infrared (NIR)-excitable hydrophilic NaYF₄:Yb,Er upconversion nanoparticles (UCNPs) with high photoluminescent (PL) performance are promising for luminescence-driven applications like photodynamic therapy (PDT). However, synthesizing hydrophilic UCNPs that simultaneously achieve a red-to-green (R/G) ratio > 5, a quantum yield (QY) > 1% (500-700 nm), and size < 100 nm remains challenging. Here, we report a solvothermal approach to water-soluble UCNPs with high colloidal stability using NaCl + NH₄F + RECl₃ (RE = Y + Yb + Er). A positive correlation between [Na]/[RE] feed molar ratio (0.50-2.00) and PL properties was identified. Upon 980 nm excitation, the UCNPs display an R/G ratio of ∼5.8 and a PLQY of ∼8.0% (500-700 nm). Conjugation with chlorin e6 (Ce6) enabled efficient Förster resonance energy transfer (FRET), generating singlet oxygen (¹O₂) that eradicated <i>Staphylococcus aureus</i> within 10 min. This study advances the design and synthesis of small hydrophilic UCNPs with high performance for luminescence-driven applications.