Surfactant-free gold nanoparticles were prepared by laser ablation of a gold metal plate in water. The nanoparticles were characterized by absorption spectroscopy, ζ-potential measurements, and XPS spectroscopy. The nanoparticles are negatively charged because the surface atoms are partially oxidized to Au-O -, according to the literature by Sacher and co-workers (J. Phys. Chem. B, 2004, 108, 16864). We further examined electrostatic interactions between nanoparticle and cationic surfactants. It was found that the surfactant cations attach to the particle surface, neutralizing the particle charge. Taking advantage of the electrostatic interactions, we estimated that 3.3-6.6% of the surface gold atoms was oxidized in water.

Gold nanoparticles with an average diameter of approximately 8 nm (Au approximately 15,000) were irradiated with a tightly focused pulse laser at 355 nm in an aqueous solution of sodium dodecyl sulfate (SDS). Transient absorption spectra of the solution were measured at 25-100 ns after the laser irradiation. The observed transient absorption around 720 nm is assignable to the 2p <-- 1s transition of solvated electrons produced via multiple ionization of the gold nanoparticles. The nascent charge state of the gold nanoparticles was estimated from the transient absorbance. The dependence of the charge state on the SDS concentration shows a gradual increase from approximately +60 to approximately +70 in the 2 x 10(-4) to 3 x 10(-4) M range and an abrupt increase up to approximately +710 at the critical micelle concentration (CMC) of SDS, 8 x 10(-3) M. TEM measurements after laser irradiation reveal that the gold nanoparticles fragment into Au(approximately 1000) at a SDS concentration of 3 x 10(-4) M, whereas they are significantly dissociated into Au(approximately 100) above the CMC. The observed correlation between the nascent charge states and the extent of size reduction of the gold nanoparticles after the laser treatment indicates that the size reduction is caused by the Coulomb explosion of the highly charged gold nanoparticles. The mechanism of laser-induced size reduction is quantitatively discussed based on the liquid drop model.

Gold nanoparticles exhibit optical absorption bands assignable to a broad interband in the UV region and a narrow intraband (surface plasmon band) in the 520 nm wavelength region. The gold nanoparticles were multiply ionized in an aqueous solution of sodium dodecyl sulfate when excited by a tightly focused nanosecond pulsed laser at 532 nm resonant to the intraband or at 355 nm resonant to the interband. Although the absorbance at 355 nm is almost comparable with the absorbance at 532 nm, gold nanoparticles with higher charge states were formed when the interband at 355 nm was excited. Our experimental findings and the theoretical prediction by Grua et al. (Phys. ReV. B 2003, 68, 035424) lead to the conclusion that electrons are predominantly released from the gold nanoparticles through thermionic emission by the excitation of the interband, whereas excitation of the surface plasmon band hardly contributes to the ionization processes.

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