PEG/Folic Acid Coloaded Er:Y ₂ O ₃ Upconversion Nanoparticles: Enhanced Internalization and Potential for High-Contrast Bioimaging in HCT-116 Cells

Erbium-doped yttrium oxide nanoparticles (Er:Y₂O₃ NPs) with a log-normal size distribution peaking at 43 nm were synthesized and coloaded with PEG and folic acid (FA) to achieve tumor cell targeting while maintaining good water dispersibility. Structural and optical analyses (TEM, FTIR, PL, upconversion) confirmed successful functionalization without significant alterations in the fluorescence signal. In colorectal cancer cells (HCT-116), MTT assays showed >80% viability for concentrations of NPs between 0.1 and 1 μg/mL, indicating low cytotoxicity. Confocal microscopy revealed fluorescence signals consistent with potential nanoparticle internalization, although contrast was limited by cellular autofluorescence. ICP-OES quantification supported greater internalization of PEG-FA nanoparticles compared to PEG nanoparticles, confirming the role of FA in enhancing internalization. Moreover, NIR excitation (980 nm) suppressed cellular autofluorescence, suggesting the potential of these nanoparticles for high-contrast bioimaging applications.