Article pubs.acs.org/JPCC
Gold Nanoparticle Synthesis Using Spatially and Temporally Shaped Femtosecond Laser Pulses: Post-Irradiation Auto-Reduction of Aqueous [AuCl4]− Behzad Tangeysh, Katharine Moore Tibbetts, Johanan H. Odhner, Bradford B. Wayland,* and Robert J. Levis* Department of Chemistry and the Center for Advanced Photonics Research, Temple University, Philadelphia, Pennsylvania 19122, United States S Supporting Information *
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.
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INTRODUCTION Expanding applications of gold nanoparticles (Au NPs) in medicine,1−4 optics,5,6 and catalysis7,8 have stimulated continuing development of convenient methods that attain size and shape selectivity.9 Nanoparticles of gold have the longest history10−12 and are the most thoroughly investigated among any class of nanoparticles and thus serve as a benchmark for comparing approaches to generate, stabilize, and process nanoparticles.13 Gold and other metal nanoparticles are most often formed by chemical reduction in the presence of a surfactant that gives size control and stabilization by coating the particle surface.14−18 Different irradiation methods, including photochemical,19,20 glow discharge,21,22 sonochemical,23,24 and pulsed laser,25−27 have been used effectively to produce Au NPs without addition of chemical reductants. Femtosecond laser irradiation has not yet been widely applied for condensed phase chemical transformations and offers new regimes of energy deposition for controlling the generation and transformation of nanomaterials. Femtosecond (fs) laser irradiation of aqueous [AuCl4]− in the presence of a variety of surfactants in solution has been employed to generate and stabilize Au NPs.28−32 Reduction of aqueous [AuCl4]− using high-energy, near-infrared fs laser © 2013 American Chemical Society
irradiation has been attributed to reactions of radicals (H•) and solvated free electrons (eaq−) produced from homolytic dissociation of water and ionization in the plasma that occurs in the region of the laser focus.31,33,34 The mechanism for reduction of aqueous [AuCl4]− is based on the production of the reducing agents from water, in distinct contrast with UV photolysis where reduction of [AuCl4]− is accomplished by direct homolysis of Au−Cl bonds.19,35 This article reports on two phenomena observed during near-infrared (790 nm) femtosecond laser-induced reduction of aqueous [AuCl4]− to form gold nanoparticles. First, aqueous H2O2 produced during laser irradiation is found to react efficiently with [AuCl4]− to grow and transform Au NPs after the laser irradiation is completed, even when only a small fraction of the initial [AuCl4]− is reduced during laser irradiation. Second, the addition of polyethylene glycol (PEG45) is found to significantly accelerate the laser-induced reduction processes, as well as control the Au NP diameter and size distribution in strong laser fields. Received: June 7, 2013 Revised: August 12, 2013 Published: August 14, 2013 18719
dx.doi.org/10.1021/jp4056494 | J. Phys. Chem. C 2013, 117, 18719−18727
The Journal of Physical Chemistry C
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Article
EXPERIMENTAL SECTION Materials. Potassium tetrachloroaurate(III) was used as obtained from Strem Chemicals. Methoxy poly (ethylene glycol) (PEG, Mn = 2000, PDI = 1.06, Sigma Aldrich) was purified by precipitation from petroleum ether prior to use. Stock solutions of [AuCl4]− were prepared from weighed samples of K[AuCl4] diluted with HPLC-grade deionized water (Fisher). All samples of aqueous [AuCl4]− used in the irradiation studies were prepared to a concentration of 0.50 mM and a final pH of 4.4−4.7. Instrumentation. A titanium−sapphire-based chirpedpulse amplifier delivers 35 fs pulses with bandwidth centered at 790 nm at a 1 kHz repetition rate. Pulses were spectrally dispersed using a grating pair (1200 l/mm), then focused with a f = 50 mm aspheric lens into a 10 × 10 × 40 mm quartz cuvette containing 3.0 mL of the liquid sample (Scheme 1A). The
nonlinear propagation effects that elongate the focus and clamp the laser intensity, it is possible to deliver higher laser intensities to a well-defined focal region and also to control the intensity distribution to a much higher degree than without the SSTF apparatus. The negative frequency chirp introduced by the grating pair results in a pulse duration of 36 ps at the focal point. In all experiments presented, the pulse energy after the focusing lens was fixed at 2.5 mJ, which corresponds to ∼1.65 × 10−8 moles of photons per pulse at 790 nm. The beam waist in the focus was measured to be 20 ± 2 μm, giving a calculated fluence of ∼200 J/cm2. A JEOL JEM-1400 TEM operating at an accelerating voltage of 120 kV was used for evaluating the diameters and shape of gold nanoparticles. Aqueous dispersions of particles resulting from the experiments were typically diluted by a factor of 10 with deionized water. One drop of each solution was deposited on the Formvar side of an ultrathin carbon type-A 400 mesh copper grid (Ted Pella Inc., Redding, CA), and the droplet was then blotted and allowed to evaporate under ambient conditions overnight. Statistical analysis and histograms were obtained by using Origin lab 7.5 on a minimum of 150 particle counts. Electronic spectra of all samples were recorded on a Shimadzu UV-1800 spectrophotometer in the same cuvette used for laser irradiation. Measurements of pH were performed using a Fisher Scientific Excel XL 15 pH meter equipped with a Thermo Scientific Orion micro electrode. Zeta potentials of the samples were measured using a Zetasizer Nano ZS (Malvern Instruments, Westborough, MA) and disposable capillary cells (DTS1061). All the measurements were performed after 2 min of equilibration time at 25 °C. The reported zeta potential value of each sample results from averaging 12 runs by the instrument DTS software. Mass spectra of poly (ethylene glycol) at 1 × 10−5 M concentration in water were measured using a Bruker micrOTOF-II electrospray mass spectrometer operating in the positive ion mode.
Scheme 1. Femtosecond Laser Irradiation Experimental Set upa
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RESULTS AND DISCUSSION I. Laser Irradiation of Aqueous [AuCl4]− Solutions. Aqueous solutions of [AuCl4]− with an initial concentration of 0.50 mM were irradiated for times ranging between 1 and 20 min. Electronic spectra of the aqueous solution and dispersed species recorded immediately after irradiation are shown in Figure 1. Irradiation for 1 min resulted in a small decrease in the intensity of the [AuCl4]− ligand-to-metal charge transfer (LMCT) band at 290 nm, which is attributed to conversion of a small amount of [AuCl4]− to the Au(0) species (Figure 1A). The broad feature observed as a displacement of the baseline from 700 to 450 nm shown in the inset to Figure 1A, which actually extends throughout the entire UV−vis spectrum and is consistent with the presence of gold particles. The absence of an observed surface plasmon resonance peak in the inset indicates that the gold particles must be quite small (
