Size-controlled preparation of fluorescent gold nanoparticles using pamoic acid

We previously showed that pamoic acid (PA) could be utilized as a reducing and capping reagent for preparing monodispersed gold nanoparticles (AuNPs) with diameters of 10.8 nm. Here, we show that the size of these PA-capped AuNPs can be varied in a controlled manner by changing the pH used in the preparation process. By changing the added amount of NaOH in the PA solution before mixing with HAuCl₄, several types of AuNPs were prepared at different pH values. The absorption spectra showed a red shift of the characteristic peak of the AuNPs with decreasing pH, indicating the formation of larger AuNPs. The TEM results confirmed this relationship between AuNP size and pH, and the zeta potentials indicated that the AuNPs capped with PA were more stable than the AuNPs prepared by citrate reduction method. Furthermore, the PA-capped AuNPs showed much higher fluorescence intensities than those of citrate-capped AuNPs, which is due to the fluorescence of PA. The sizes of PA-capped spherical AuNPs could also be controlled by a seed-mediated growth approach. However, the original PA-capped AuNPs, with diameters of 10.8 nm, exhibited the highest fluorescence intensities among the various types of PA-capped AuNPs that were grown. In addition to conferring fluorescence properties to the AuNPs, the capping by PA also provided the nanoparticles with carboxylate groups.