*3.2. Preparation of β-NaYF4:Yb,Er,Gd (60, 18, 2, 20 mol%) Fluorescent Nanorods*

The preparation of β-NaYF4:Yb,Er,Gd (60, 18, 2, 20 mol%) highly fluorescent nanorods (UCNPs) is summarized as follows [21,39]: Firstly, a mixed solution with 10 mL of ethanol, 3 mL of deionized water, and 0.6 g of NaOH was obtained. A total of 10 mL of oleic acid was then dropped in the above mixture under vigorous stirring. The stirring was continued for 20 min; the obtained mixture was named A solution. Moreover, 145.6 mg of YCl3·6H2O, 59.47 mg of GdCl3·6H2O, 4.38 mg of ErCl3, and 55.8 mg of YbCl3·6H2O were dissolved in 4 mL of deionized water; this mixture was named B solution. Then, B solution was slowly dropped into the A solution under vigorous stirring. After 10 min, 2.0 mL of NH4F (2 M) solution was added to the above mixture dropwise. Finally, a milky colloidal solution was produced. The obtained solution was encapsulated in a 50 mL autoclave flask and heated to 180 ◦C at a heating rate of 3 ◦C/min, then it was kept at 200 ◦C for 2 h. The prepared UCNPs were collected and washed with ethanol and ultrapure water, respectively.
