*2.5. Perl's Prussian Blue Staining*

The cellular uptake of the nanoparticles was visualized by Perl's Prussian blue staining. The cells were cultured in 24-well plates and transfected as indicated. After incubation for 12 h, the cells were fixed with 4% Paraformaldehyde Fix Solution (P0099, Beyotime) for 15 min. The cells were washed with PBS and stained with 5% potassium ferrocyanide in 10% hydrochloric acid for 30 min at room temperature. The working solution was made by mixing equal volumes of 10% potassium ferrocyanide and 20% hydrochloric acid solution just before use. The cells were observed under a microscope (Olympus CKX41, Tokyo, Japan).
