BSA-protected clusters were synthesizing as described by Xie et al. Briefly, 5 mL of HAuCl4 solution (10 mM) was mixed with 5 mL of BSA solution (50 mg/mL). The pH of the solution was then adjusted to 11 by adding 1 M NaOH solution and then left for 24 h at 37 °C. The clear brown solution of BSA-AuNCs was ultrafiltrated using a 50 kDa ultrafilter tube (GE Healthcare) against MilliQ water and was kept the same volume after purification. The final concentration of products is 23 mg/mL after purification by dry-weighting.