*3.4. Sol–Gel Method*

The sol–gel method uses metal organics or inorganic salts as the matrix, generates gelatin through the process of hydrolysis and polycondensation, and then is dried or sintered to obtain UCNP. It has been successfully applied to the preparation of thin film coatings and glass materials for upconversion luminescent materials [115,116], but usually requires high-temperature post-processing to improve its crystallinity in order to obtain better luminescence effects. Prasad and colleagues first prepared erbium-doped ZrO2 nanoparticles using sol-latex-gel technology [115]. However, the size of UCNP is difficult

to control, and the agglomeration after high temperature calcination brings some difficulties to the surface modification of the material and limits its biomedical applications [116–121].
