Tunability and stability of gold nanoparticles obtained from chloroauric acid and sodium thiosulfate reaction
© Zhang et al.; licensee Springer. 2012
Received: 13 April 2012
Accepted: 22 June 2012
Published: 22 June 2012
In the quest for producing an effective, clinically relevant therapeutic agent, scalability, repeatability, and stability are paramount. In this paper, gold nanoparticles (GNPs) with precisely controlled near-infrared (NIR) absorption are synthesized by a single-step reaction of HAuCl4 and Na2S2O3 without assistance of additional templates, capping reagents, or seeds. The anisotropy in the shape of gold nanoparticles offers high NIR absorption, making it therapeutically relevant. The synthesized products consist of GNPs with different shapes and sizes, including small spherical colloid gold particles and non-spherical gold crystals. The NIR absorption wavelengths and particle size increase with increasing molar ratio of HAuCl4/Na2S2O3. Non-spherical gold particles can be further purified and separated by centrifugation to improve the NIR-absorbing fraction of particles. In-depth studies reveal that GNPs with good structural and optical stability only form in a certain range of the HAuCl4/Na2S2O3 molar ratio, whereas higher molar ratios result in unstable GNPs, which lose their NIR absorption peak due to decomposition and reassembly via Ostwald ripening. Tuning the optical absorption of the gold nanoparticles in the NIR regime via a robust and repeatable method will improve many applications requiring large quantities of desired NIR-absorbing nanoparticles.
Keywordsgold nanoparticles gold colloid gold nanoplates near-infrared absorption surface plasmon resonance sodium thiosulfate core-shell structure
Metal nanoparticles are one of the basic building blocks of nanotechnology. Gold nanoparticles (GNPs) have attracted enormous attention in chemistry, biomedicine, and electronics due to their very small size, oxide-free surfaces, bio-conjugation properties, good biocompatibility, and unique optical properties. Specifically, because of their optical activity in the near infrared (NIR), GNPs are extensively utilized in immunoassays [1, 2], drug delivery systems  as well as imaging, detection, and thermal therapy of cancer [4–6]. These applications have sparked great interest in the development of synthetic methods for preparing different gold-based nanostructures. The anisotropy in nanoparticle shape offers high near-infrared absorption and improved Raman scattering . Based on Mie scattering theory, shifts in the surface plasmon resonance (SPR)  occur when the particles deviate from spherical geometry. Non-spherical gold nanoparticles present multiple absorption bands correlating with their multiple axes, and they can support both propagating and localized surface plasmon resonances . The number of SPR peaks usually increases as the symmetry of nanoparticles decreases; spherical nanoparticles exhibit only one peak, whereas two and three peaks are often observed in nanorods, nanodisks, and triangular nanoplates, respectively. Many anisotropic gold nanostructures like gold nanotubes [9, 10], nanocages , gold nanoshells , gold nanorods , and gold triangular nanoprisms [13, 14] have been developed and demonstrate enhanced and adjustable absorption in the NIR region. However, most of these gold nanostructures require a complicated multistep and time-consuming synthesis process, which includes shaping the particles by use of templates, kinetically controlling the facet growth rates of seeds with assistance of capping reagents, and assembly of preformed spherical colloid nanoparticles .
In this work, GNPs with controllable NIR absorption were synthesized by the reaction of chloroauric acid and sodium thiosulfate. This reaction was derived from the reaction of chloroauric acid and sodium sulfide that Zhou et al. first reported, whereby a proposed core-shell-type Au2S nanoparticle structure was produced via a two-step reduction of HAuCl4 by Na2S [15, 16]. Later, Norman et.al proposed that the resulting optical properties are simply from aggregation of gold nanoparticles [15, 17]. In molecular sensor studies based on scattering spectroscopy, Raschke et al. reported that gold products from Na2S reaction showed great improvement in scattering compared to solid GNPs, and those particles have a dielectric nanocrystal property and behave like gold nanoshells . Subsequent investigations revealed that this reaction lacks reproducibility because the Na2S solution requires an aging process, and the aging time and reaction conditions required for this process were not well defined . Schwartzberg et al. mentioned that the Na2S solution is not chemically stable during the aging process. Na2S may convert to different compounds, and sodium thiosulfate is one of the final compounds producing GNPs . These findings encouraged us to attempt to reveal the key factor that dominates the nanostructure formation and the stability of the GNPs in the HAuCl4/Na2S2O3 reaction. We found that the NIR absorption of the gold products from this reaction can be well controlled and show good reproducibility when the molar ratio of HAuCl4/Na2S2O3 is in a suitable range. The instability of the GNPs is affected by the reaction conditions, resulting in the diversification of the nanostructures.
GNPs were prepared by mixing 1.71 mM HAuCl4 (Au 49.50%; Alfa Aesar, Ward Hill, MA, USA) with 3 mM Na2S2O3 (99.999%; Aldrich, St. Louis, MO, USA) solution. The Na2S2O3 solution is quickly added into the HAuCl4 solution with the desired volume ratio and vortexed for 20 s for uniform mixing. The water used in the experiments was purified by a Thermo Scientific Easypure II system (18.2 MΩ cm; Thermo Scientific Corp., Logan, UT, USA). GNPs were purified and separated by an Allegra® X-12 Series Centrifuge (Beckman Coulter Inc., Brea, CA, USA). The as-synthesized GNP suspensions were centrifuged at 1,000 × g for 20 min, and then, the pellets were dispersed in deionized (DI) water for further study. The optical absorbance and intensity of nanoparticles were measured by a UV-visible-IR spectrophotometer (Cary-50Bio, Varian, Palo Alto, CA, USA). The hydrodynamic size of the nanoparticles was measured by a Zetasizer (Nano-ZS90, Malvern Instruments Ltd., Worcestershire, UK). An FEI Tecnai F20 transmission electron microscope (TEM; FEI Company, Hillsboro, OR, USA) operated at 200 KV was used to determine the shape and size of the GNPs.
Results and discussion
NIR absorption of the gold nanoparticles from sodium thiosulfate reaction
Crystal structure of the gold nanoparticles
Unstable gold nanoparticles and mechanism
In summary, we report on a convenient synthesis process to precisely control the optical absorption within the NIR region and established the suitable range of concentrations to allow stable nanoparticle formation. In this procedure, a single-step reaction of HAuCl4 and Na2S2O3 was examined in details to analyze the products of self-assembly. The nanoparticles produced from this reaction include small spherical colloidal gold particles with resonance at 530 nm and anisotropic gold nanostructures with NIR resonance. We found that the placement of the peak resonance into the NIR is controllable and repeatable with increasing molar ratios of HAuCl4/Na2S2O3. From this, it was found that in order to achieve a peak resonance above 950 nm, a molar ratio of HAuCl4/Na2S2O3 > 2.0 was required and resulted in unstable nanoparticles. The instability appears to be due to Ostwald ripening behavior based on TEM analysis over time for reactants with molar ratio greater than 2.0. Our study outlines an easy way to produce GNPs with tunable NIR absorption on a large scale in a short time and serves as the basis for additional studies to improve the efficiency of the synthesis system. This work will benefit many applications in the physical, chemical, and biomedical fields where strong NIR-absorbing nanoparticles may be used for energy transfer to create heat.
GZ, JLH, DPS, DP, and AMG are from the Department of Bioengineering, J.B. Speed School of Engineering, University of Louisville. AMG is an assistant professor. GZ is a senior research associate. JLH and DPS are bachelor degree students. DP is a master degree student. JBJ is a research scientist at the Conn Center for Renewable Energy Research, J.B. Speed School of Engineering, University of Louisville.
dynamic light scattering
selected area electron diffraction
surface plasmon resonance
scanning transmission electron microscopy
transmission electron microscope.
We acknowledge Dr. Andrea Gobin and Dr. Bo Xu for their help with the experiments and sample characterization. The Early Career Phase 1 Award from Wallace Coulter Foundation and the School of Medicine Summer Research Scholar Program supported this research.
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