(A) Bright field and fluorescence images of solutions containing citrate-AuNPs, free RBITC, Ab2-RBITC-AuNPs, and Ab2-RBITC-AuNPs treated with 1 mM of cysteamine. The absorption spectra (B) and fluorescence emission spectra (C) of solutions containing citrate-AuNPs, free RBITC, Ab2-RBITC-AuNPs, and Ab2-RBITC-AuNPs treated with 1 mM of cysteamine. (D) Fluorescence emission spectra of Ab2-RBITC-AuNPs solutions treated with various concentrations of cysteamine ranging from 0 to 1 mM. The spectra were collected 1 min after the addition of cysteamine.