Fig. 5. (a) Room-temperature upconversion fluorescence spectra of NaGdF₄:18% Yb³⁺,2%Er³⁺@NaYF₄ core/shell particles with different concentrations of doxorubicin (0 μg mL⁻¹, 20 μg mL⁻¹, 50 μg mL⁻¹, 70 μg mL⁻¹, and 100 μg mL⁻¹) under the 980 nm excitation with power intensity of 0.08 W cm⁻² simultaneously. The inset shows the absorption spectrum of doxorubicin. (b) The ratio of simultaneous green and red emission intensity.