*2.5. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES)*

For uptake confirmation, 100,000 cells per well were seeded in a 12-well plate. After 24 h, nanostars (2.3 <sup>×</sup> 1010 particles in 1 mL) were added to the cells and incubation continued for different time periods (1, 3, 6, 12 and 24 h). Next, cells were washed with PBS and again incubated overnight with fresh medium. After trypsinization, 100,000 cells were acid-digested with Kingswater (HCl/HNO3 with a ratio of 3:1) and diluted with de-ionized water to a volume of 10 mL for inductively coupled plasma optical emission spectroscopy (ICP-OES) (3300 DV, Perkin-Elmer, Waltham, MA, USA). Reference standards were prepared by dissolving HAuCl4 to final concentrations between 0.1 and 2 ppm.
