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

Abstract

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 (PEG₄₅) accelerates the rate of the [AuCl₄]⁻ reduction during laser irradiation and directs the exclusive formation of spherical nanoparticles. Varying the concentration of PEG₄₅ tunes the diameter and size distribution of the Au nanoparticles formed by laser processing from 3.9 ± 0.7 to 11 ± 2.4 nm