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 Staphylococcus aureus within 10 min. This study advances the design and synthesis of small hydrophilic UCNPs with high performance for luminescence-driven applications.