2.3. Synthesis of the Core NaYF4:Nd(15%),Yb(15%) and NaYF4:Yb(10%),Er(0.5%) NPs

The upconversion nanoparticles (UCNPs) were synthesized according to a literature reported method with minor modifications [35]. Core NaYF₄:Nd,Yb NPs, a total of 1 mmol Ln-triacetate in the Y:Nd:Yb = 70:15:15 mol% ratio (for NaYF₄:Yb,Tm NPs, in the Y:Yb:Tm = 89.5:10:0.5 mol% ratio) were added to a 10 mL oleic acid (OA) and 15 mL octadecene (ODE) solution. The temperature of solution was increased to 160 °C under argon flow for 60 min. After cooling, 4 mmol NH₄F and 2.5 mmol NaOH in 10 mL methanol solution containing was added to the solution and stirring for 1 h at about 65 °C. Afterward, the temperature of the solution was increased to 300 °C for 90 min. After cooling, the resulting NaYF₄:Nd³⁺(15%),Yb³⁺(15%) and NaYF₄:Yb³⁺ (10%),Tm³⁺(0.5%) NPs were centrifuged for collecting the NPs, the precipitate was washed by ethanol for several times, and redispersed in n-hexane for following reaction.