Gold Nanoparticle Synthesis Using Spatially and Temporally Shaped Femtosecond Laser Pulses: Post-Irradiation Auto-Reduction of Aqueous [AuCl4]-

Simultaneous spatiotemporal focusing (SSTF) of femtosecond laser radiation is used to produce gold nanoparticles from aqueous [AuCl4](-) solutions. Multiphoton ionization and dissociation of water produces electrons and hydrogen atoms for the reduction of [AuCl4](-) to Au(0) during irradiation with the temporally chirped (36 ps) pulse and produces hydrogen peroxide (H2O2) as a long-lived reducing agent which persists after irradiation is terminated. Aqueous H2O2 is found to reduce [AuCl4](-), remaining in solution after the laser irradiation is terminated, leading to growth and transformation of the existing Au(0) species. The highly efficient postirradiation reduction of [AuCl4](-) to Au(0) by H2O2 is ascribed to reactions occurring on gold nanoparticle surfaces. In the absence of added surfactant, the negatively charged gold particles formed during irradiation are a complex mixture of irregularly shaped and spherical morphologies that are only metastable as aqueous dispersions. These particles become transformed into more perfectly shaped gold crystals, as the remaining [AuCl4](-) is reduced in the postirradiation period. The addition of polyethylene glycol (PEG(45)) accelerates the rate of the [AuCl4](-) reduction during laser irradiation and directs the exclusive formation of spherical nanoparticles. Varying the concentration of PEG(45) tunes the diameter and size distribution of the Au nanoparticles formed by laser processing from 3.9 +/- 0.7 to 11 +/- 2.4 nm.