Multiphoton Fragmentation of PAMAM G5-Capped Gold Nanoparticles Induced by Picosecond Laser Irradiation at 532 nm

Fifth-generation ethylendiamine-core poly(amidoamine) (PAMAM G5)-capped gold nanoparticles were prepared by picosecond laser ablation in water, with the fundamental and second harmonic of a picosecond Nd:YAG laser. Although the visible wavelength exhibited a lower ablation threshold than that of the infrared one, the ablation process at 532 nm reached early saturation because of both linear and nonlinear absorption mechanisms, accompanied by fragmentation of existing nanoparticles. We demonstrate that the onset of the fragmentation can be monitored by simple UV−vis spectroscopy, thanks to the ability of PAMAM G5 to stabilize gold cations, which results in the growth of an intense band at 290 nm. We observed that, while with 532 nm (2.33 eV) irradiation a two-photon absorption mechanism induces fragmentation of the nanoparticles, the suspensions remain stable when irradiated with 1064 nm (1.17 eV) up to 60 GW/cm2.