Interaction of cationic gold nanoparticles and carboxylate-terminated poly(amidoamine) dendrimers

Interaction between cationic gold nanoparticles ligated with 3-mercaptopropyltrimethylammonium chloride (MPTAC) and anionic poly(amidoamine) (PAMAM) dendrimers with carboxylate terminals (Generation 3.5 and 5.5-COONa) has been investigated in aqueous solution and on carbon film by UV–vis, TEM, DLS and zeta potential measurements. In the absence of PAMAM dendrimers, the MPTAC–Au nanoparticles in water exhibited a surface plasmon resonance (SPR) band at 520 nm. The gold particles of 4.2 nm in mean diameter were homogeneously dispersed on the carbon grids. Addition of PAMAM brought about an increase in the absorbance and redshift of the SPR band, which was attributed to aggregation of the gold nanoparticles by adsorption of the dendrimer to the MPTAC ligand layers owing to electrostatic attraction. In particular, chainlike aggregates were formed at low concentrations of the dendrimer (r = [PAMAM]/[MPTAC] ≤ 1). While a further increase in the dendrimer concentration (r = 2) led to a remarkable decrease in the size of aggregates, and λmax of the SPR band returned to approximately the same wavelength as that for r = 0.5.