We prepared thin films of gold nanoflowers (AuNF) according to a previously reported wetchemical method using a HEPES buffer solution as a structure-directing agent. Initially, 200 µL of 100 mM HEPES buffer solution (pH 7.4 ± 0.5) was thoroughly mixed with 1.8 mL water for 5 min. Subsequently, 40 µL of 25 mM HAuCl 4 (aq) solution was quickly added, and the mixture was aged undisturbed for 2 h. The color change from pale yellow via colorless to dark blue confirmed the growth process. The final AuNFs were centrifuged and washed three times by re-dispersion in 500 µL water. Transmission electron microscopy (TEM) showed that the resulting particles resemble spheres of (60 ± 5) nm diameter with an irregular surface (see Figure ).