Elucidating Strong Field Photochemical Reduction Mechanisms of Aqueous [AuCl₄]⁻: Kinetics of Multiphoton Photolysis and Radical-Mediated Reduction

Direct, multiphoton photolysis of aqueous metal complexes is found to play an important role in the formation of nanoparticles in solution by ultrafast laser irradiation. <i>In situ</i> absorption spectroscopy of aqueous [AuCl₄]⁻ reveals two mechanisms of Au(0) nucleation: (1) direct multiphoton photolysis of [AuCl₄]⁻ and (2) radical-mediated reduction of [AuCl₄]⁻ upon multiphoton photolysis of water. Measurement of the reaction kinetics as a function of solution pH reveals zeroth-, first-, and second-order components. The radical-mediated process is found to be zeroth-order in [AuCl₄]⁻ under acidic conditions, where the reaction rate is limited by the production of reactive radical species from water during each laser shot. Multiphoton photolysis is found to be first order in [AuCl₄]⁻ at all pHs, whereas the autocatalytic reaction with H₂O₂, the photolytic reaction product of water, is second order

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 (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

Triangular Gold Nanoplate Growth by Oriented Attachment of Au Seeds Generated by Strong Field Laser Reduction

The synthesis of surfactant-free Au nanoplates is desirable for the development of biocompatible therapeutics/diagnostics. Rapid Δ-function energy deposition by irradiation of aqueous KAuCl₄ solution with a 5 s burst of intense shaped laser pulses, followed by slow addition of H₂O₂, results in selective formation of nanoplates with no additional reagents. The primary mechanism of nanoplate formation is found to be oriented attachment of the spherical seeds, which self-recrystallize to form crystalline Au nanoplates

Mechanism of Improved Au Nanoparticle Size Distributions Using Simultaneous Spatial and Temporal Focusing for Femtosecond Laser Irradiation of Aqueous KAuCl₄

The production of gold nanoparticles (AuNPs) by irradiation of aqueous [AuCl₄]⁻ with femtosecond laser pulses is investigated using simultaneous spatial and temporal focusing (SSTF) and compared to the results of conventional geometric focusing (GF). The effects of capping agent, laser power, reaction conditions in the cuvette, and laser chirp are studied, and we find that SSTF produces smaller particles with fewer irregular structures and fewer outlying large particles than GF in all cases except for one, in which the particle size distributions are the same. The difference is primarily ascribed to the intrinsic plasma properties of the two geometries: SSTF produces a plasma that is more homogeneous and spatially symmetric than that of GF, promoting efficient intrinsic mixing of the solution

Gold Nanotriangle Formation through Strong-Field Laser Processing of Aqueous KAuCl₄ and Postirradiation Reduction by Hydrogen Peroxide

Femtosecond laser irradiation of aqueous KAuCl₄ followed by postirradiation reduction with hydrogen peroxide (H₂O₂) is investigated as a new approach for the synthesis of gold nanotriangles (AuNTs) without any added surfactant molecules. Laser irradiation was applied for times ranging from 5 to 240 s, and postirradiation reduction of the solutions was monitored by UV–vis spectroscopy. Laser processing of aqueous KAuCl₄ for 240 s, where the full reduction of Au­(III) occurred during irradiation, produced spherical gold nanoparticles (AuNPs) with an average size of 11.4 ± 3.4 nm. Irradiation for shorter times (i.e., 15 s) resulted in the formation of laser-generated AuNP seeds (5.7 ± 1.8 nm) in equilibrium with unreacted KAuCl₄ after termination of laser irradiation. The postirradiation reduction of these solutions by H₂O₂ produced a mixture of spherical and triangular AuNPs. Decreasing the laser irradiation time from 45 to 5 s significantly reduced the number of laser-generated Au seeds, the amount of H₂O₂ produced, and the rate of postirradiation reduction, resulting in the formation of a large number of AuNTs with sizes increasing from 29.5 ± 10.2 to 125 ± 43.2 nm. Postirradiation reduction is kinetically inhibited in the absence of laser-generated AuNP seeds