Comparative studies of upconversion luminescence characteristics and cell bioimaging based on one-step synthesized upconversion nanoparticles capped with different functional groups

Herein, three types of upconverting NaGdF₄:Yb/Er nanoparticles (UCNPs) have been synthesized via one-step hydrothermal synthesis with polyethylene glycol (PEG), polyethylenimine (PEI) and 6-aminocapronic acid (6AA) functionalization. To evident the presence of these groups, FTIR spectra and ζ-potentials were measured to support the successful capping of PEG, PEI and 6AA on the UCNPs. The regular morphology and cubic phase of these functionalized UCNPs were attributed to the capping effect of the surfactants. Tunable upconversion luminescence (UCL) from red to green were observed under 980 nm laser excitation and the UCL tuning was attributed to the presence of various surface ligands. Moreover, surface group dependent UCL bioimaging was performed in HeLa cells. The enhanced UCL bioimaging demonstrated by PEI functionalized UCNPs evident high cell uptake. The significant cell uptake is explained by the electrostatic attraction between the amino groups (-NH₂) and the cell membrane. Moreover, the functionalized UCNPs demonstrated low cytotoxicity in MTT assay. Additional, paramagnetic property was presented by these UCNPs under magnetic field.