Experimental study on the surface modification of Y₂O₃:Tm³⁺/Yb³⁺ nanoparticles to enhance upconversion fluorescence and weaken aggregation

In order to improve the solubility of doped nanoparticles in solutions, Y₂O₃:Tm³⁺/Yb³⁺ nanoparticles were synthesized using the Pechini-type sol–gel method, and their surfaces were modified with amino or carboxylic functional groups using ligand-capped and ligand-exchanging methods. The nanoparticles with modified surfaces were characterized by transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy and zeta potential (ζ), and their photoluminescence was studied by fluorescence spectrophotometry. The results indicate that the upconversion fluorescence of amine- and carboxyl-modified nanoparticles was enhanced after the surfaces of nanoparticles were modified. Compared to the upconversion fluorescence intensity of non-modified nanoparticles, the upconversion fluorescence intensities of amine- and carboxyl-modified nanoparticles were enhanced by 9.4 and 1.4 times, respectively. These results are attributed to the formation of the chemical bonds between Y₂O₃:Tm³⁺/Yb³⁺ core and non-crystalline SiO₂ shell via Y–O–Si bridges, which activate the 'dormant' Tm³⁺/Yb³⁺ ions on the surfaces of nanoparticles. The results of the solubility investigations for amine- and carboxyl-modified nanoparticles indicate that severe aggregation can be weakened by adhering amino or carboxylic functional groups to the surfaces of nanoparticles. It is therefore concluded that the good hydrophilicity resulting from active functional groups in solutions and more intense upconversion fluorescence enable the doped core–shell nanoparticles to have great potential to be used as fluorescence biolabels in the future.