Figure 4

UV–vis spectra of gelatin-AuNPs synthesized at different pH, respectively ( C _gelatin : 0.4 wt%, T : 80°C). (1) pH = 2.4, (2) pH = 4.7 (the isoelectric point of gelatin, pI), (3) pH = 7.2, (4) pure gelatin solution (0.4 wt%). The reaction time was monitored by UV–vis spectra. When the typical absorption band of gold nanoparticle was stable, the reaction is considered to be over. The reaction time of curve 1, 2, 3 was 12, 4, 48 h, respectively. The inset is digital images of gelatin-AuNPs corresponding to curve 1, 2, 3, respectively. The pH of gelatin was adjusted by 0.1 M NaOH and HCl.