For the preparation of AHMT-AuNPs, the above colloidal AuNPs (10 nM; 300 mL) was simply mixed with a 5.0 mL AHMT aqueous solution (0.1 mM) under stirring for 2 h in dark, where AHMT molecule was adsorbed onto the AuNPs through ligand exchange reactions of citrate with –SH group in AHMT. The resultant solution was very stable without aggregation observed. The AHMT-AuNPs were purified by centrifugation for at least three times. In each cycle, the supernatant liquid was decanted without disturbing the precipitate for the removal of free AHMT molecules in the solution. The precipitate was collected and re-dispersed in water.