Influence of the TGA Modification on Upconversion Luminescence of Hexagonal-Phase NaYF4:Yb3+, Er3+ Nanoparticles

Abstract

Fluorescent nanoparticles used for biomedical applications should be small with narrow size distribution, and water-soluble with high luminescent efficiency. In this paper, uniform hexagonal-phase NaYF4:20%Yb, 2%Er nanocrystals (NCs) were synthesized in high boiling solvent oleylamine at 320 °C. The upconversion luminescence (UCL) intensity of the hexagonal-phase NaYF4:Yb, Er NCs in this work is much higher than that of other cubic-phase NCs. These hydrophobic NCs were further rendered hydrophilic using the ligand thioglycollic acid (TGA). The UCL properties of the surface-modified NCs were studied in contrast to the unmodified NCs under 980 nm laser diode excitation. After surface modification, the UCL revealed not only an enhancement of the red (4F9/2−4I15/2) emission with respect to the green (2H11/2, 4S3/2−4I15/2) one, but also an increase of the blue (2H9/2−4I15/2) due to the involving of surface vibration bonds −COOH, while the overall intensity had rarely changed. UCL dynamics were also studied, which indicate that the decay time constants of the green and the red emissions had only a little decrease, while the rising time constants of them decreased considerably. The rate equations for the red and green emissions were set up and solved to analyze the luminescent dynamics. The present results demonstrate that the modification of TGA can both make the NaYF4 NCs realize biofunctionality and efficient UCL.