Surface Modification of ZrO₂:Er³⁺ Nanoparticles to Attenuate Aggregation and Enhance Upconversion Fluorescence

Abstract

ZrO2:Er3+ nanoparticles are synthesized and further modified via a ligand-capped/ligand-exchanging method with TEOS, APTES, and SA. Antiaggregation investigations using TEM and FT-IR indicate that severe aggregation can be reduced by adhering ammonic or carboxylic functional groups to the nanoparticle surfaces. The upconversion fluorescence spectra of nonmodified and the modified nanoparticles with the same peak splitting and positions show that local crystalline environments in which the Er3+ ions are embedded are identical before and after modifying the surfaces. The remarkable upconversion fluorescence enhancements of 4.7 and 1.5 times for amine- and carboxyl-modified nanoparticles are observed under the same excitation power densities, respectively. An enhancement mechanism of upconversion luminescence, in which an asymmetric association crystalline field from both the degenerated crystalline field of the host interface and a complementary crystal field of the SiO2 shell can make the ‘dormant' rare earth ions on nanoparticle surfaces be activated, is presented. In addition, the improved spontaneous emission rate of Er3+ ions due to the enhanced local classical density of states and organic ligands with high vibrational energy on the nanoparticle surfaces are also considered. Thus, intense upconversion fluorescence and hydrophilicity via ammonic or carboxylic functional groups will provide the doped core−shell nanoparticles great potential as biolabels in the future.