Quantitative analysis of the surface quenching effect of lanthanide-doped upconversion nanoparticles in solvents

Laser-induced upconversion luminescence of lanthanide-doped nanoparticles has attracted great interest from researchers for many years due to its unique optical properties. The influence of solvents on the surfaces of these nanoparticles is a common problem in practical applications of these materials. However, traditional analysis methods are incapable of quantifying the influences of solvents. In response to this difficulty, we used a Monte Carlo simulation to reconstruct macroscopic upconversion luminescence at the microscopic level of ion-ion interaction. Then, we succeeded in obtaining quantified analysis results of the surface effects from four different aqueous solvents, which were water, methanol, ethanol and N, N-dimethylformamide(DMF). Both steady-state and dynamic spectra results show that the surface quenching rate of the upconversion nanoparticles in the highest to the lowest order of the four solvents are water, methanol, ethanol and DMF, which is attributed to the hydroxyl group and its activity. The computational simulation results show that the surface quenching rates of the Yb³⁺ excited state(²F_5/2) in NaYF₄:20%Yb, 2%Er upconversion nanoparticles in the four solvents are 2.5×10⁴ s^-1(DMF), 1×10⁵ s^-1(methanol and ethanol) and 5×10⁵ s^-1(water), which confirms our hypothesis.