Fig. 2. Electrostatic adsorption technique.

a Schematic diagram of the synthetic process of B/USCs-PEG-DOX²⁴. b Schematic illustration of the preparation of NaYF₄:Yb/Tm-PLL@g-C₃N₄²⁵. c1 Schematic of the synthetic process of UCNPs and MOF nanocomposites. c2 SEM images of UiO-66-NH₂@NaYF₄:Yb/Er, UiO-66@NaYF₄:Yb/Er, MOF-801@NaYF₄:Yb/Er, PCN-223@NaYF₄:Yb/Er nanocomposites. c3 TEM images of MOF@UCNPs@MOF sandwiched nanocomposites²⁶. a Reprinted with permission from ref. ²⁴ Copyright 2020, Elsevier. b Reprinted with permission from ref. ²⁵ Copyright 2016, American Chemical Society. c1–c3 Reprinted with permission from ref. ²⁶ Copyright 2018, American Chemical Society