Author: Kaylie Young
Institution: Brown University
Date: May 2007
Abstract
Silver nanocubes 30-50 nm in diameter have been synthesized using a polyol process in which silver nitrate is reduced by ethylene glycol in the presence of a capping agent poly(vinylpyrrolidone) (PVP). A ligand exchange reaction was used to replace the PVP with another capping agent, allowing the nanocubes to be soluble in chloroform. Oleylamine, oleic acid, and decane-thiol were among the ligands investigated. The silver cubes were then used as sacrificial templates to generate hollow gold nanocages using a galvanic replacement reaction during which the silver cubes were titrated with chloroauric acid. The use of different capping agents allows us to further understand the role of the ligand in the galvanic replacement reaction.
Introduction
The properties and applications of metallic nanostructures are mainly determined by their size, shape, composition, crystallinity, and structure (solid versus hollow). Optical properties of nanomaterials are of importance for the detection and treatment of cancer, particularly a phenomenon known as surface plasmon resonance (SPR), which refers to the characteristic wavelength at which free electrons in a nanomaterial collectively oscillate and scatter/absorb an incident electromagnetic wave (1). The ability to tune the SPR of gold nanocages, which are hollow nanostructures with porous walls, into the infrared region promises uses as both contrast agents for optical imaging in early stage tumor detection and as therapeutic agents for photothermal cancer treatment (2). The Xia group developed a process to synthesize gold nanocages with smaller dimensions than previously reported using a galvanic replacement reaction between silver templates and chloroauric acid (3). This galvanic replacement reaction had previously been done in aqueous conditions with polyvinylpyrrolidone (PVP). Here we extended the replacement reaction to an organic medium, chloroform, for Ag nanocubes passivated with various capping ligands.
Methods and Materials
Silver Cube Synthesis
Silver nanocubes were prepared using the polyol process, during which ethylene glycol serves as both the reducing agent and the solvent. After heating ethylene glycol in a vial in an oil bath for one hour, the silver precursor, the capping ligand (PVP), and an etching agent were added. The reaction was stopped when the solution turned a grayish-silver color and appeared to be opalescent, which took about 15-25 minutes. The vial was submerged in cold water and the samples were washed once with acetone and twice with water to remove the excess PVP and ethylene glycol. The nanocubes were stored in water.
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Ligand Exchange Reaction
Before the ligand exhange reaction was performed, a small amount of PVP was added to the silver nanocubes to ensure their stability in chloroform. The silver cubes were then transferred from water into chloroform by evaporating off the water using a vacuum chamber and adding chloroform. The desired ligand was added in excess and the solution was briefly stirred and then allowed to sit for one hour.
Galvanic replacement reaction
A dilute solution of chloroauric acid (HAuCl4) in chloroform was added slowly to the newly-capped silver nanocubes using a syringe pump. The reaction, which was done at an elevated temperature, proceeded through a number of color changes and was stopped at a grayish color. The resulting gold nanocages were washed with ethanol and stored in chloroform.
Results
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A galvanic replacement reaction has been demonstrated as a means for preparing gold nanocages using silver nanocubes as sacrificial templates. The template silver nanocubes, with edge lengths ranging from 30-50 nm, are shown in Figure 1. The product hollow gold nanocages, with oleylamine used as the capping agent during the galvanic replacement reaction, are shown in Figures 2 and 3. Since the standard reduction potential of AuCl4-/Au (0.99 V vs standard hydrogen electrode, SHE) is larger than that of Ag+/Ag (0.80 V vs SHE), silver is oxidized to Ag+when gold ions (HAuCl4) are added to the silver nanocubes:
3Ag(s) + HAuCl4 (aq) → Au(s) + 3 AgCl(aq) +HCl(aq)
[3,4]. The SPR peak was significantly shifted toward the infrared region for the gold nanocages versus the silver nanocubes. This is evident in the absorption spectra of the silver nanocubes and the gold nanocages in Figure 4.
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The morphology of the resulting gold nanocages was found to depend strongly on the capping ligand of the silver nanocubes used in the galvanic replacement reaction. The capping agent oleylamine gave the best nanocages with the highest yield. PVP-capped silver nanocubes resulted in some gold nanocages, although they were often found in large clumps and in low yields. In the cases of oleic acid and 1-decane thiol, no nanocages were formed. After the addition of HAuCl4 to the nanocubes capped with oleic acid, only tiny spheres and amorphous clumps were observable. The nanocubes no longer existed. In comparison, after the addition of HAuCl4 to the nanocubes capped with 1-decane thiol, only nanocubes were observable. The results of adding HAuCl4 to silver nanocubes passivated with each ligand are summarized in Figure 5.
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Discussion
Galvanic replacement reactions have been attempted using four common ligands for noble metal nanomaterials as capping agents for the starting silver nanocube templates. As previously mentioned, the Xia group developed a method to synthesize hollow gold nanostructures with much smaller dimensions than previously reported while maintaining a strong SPR peak in the near- IR region (800-1200 nm) [2]. However, this reaction was limited to aqueous conditions and one capping agent, polyvinylpyrrolidone (PVP). This study has demonstrated the use of oleylamine as a capping agent for the silver nanocubes in chloroform to synthesize gold nanocages using a similar galvanic replacement reaction. Oleic acid and 1-decane thiol were found to be unsuitable ligands for the galvanic replacement reaction as they do not produce gold nanocages. The thiol was thought to have bound so strongly to the silver nanocubes that it prevented the galvanic replacement reaction from occurring, leaving the silver nanocubes as they were before the reaction. The oleic acid was thought to have strongly etched the silver nanocubes before the galvanic replacement reaction took place, resulting in tiny amorphous clumps.
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The galvanic replacement reaction is thought to start at the sites on the nanocubes with the highest surface energy, dissolving small holes at these active sites once the chloroauric acid is added. The reaction is analogous to a corrosion process, with silver being oxidized at the anode. The free electrons then migrate to the surface of the silver nanocube where they are used to reduce AuCl4- to Au atoms which deposit on the surface of the gold epitaxially. Epitaxial deposition is favored due to similar crystalline structures (face-centered cubic) and lattice constants of silver and gold. This process will continue until a thin layer of gold coats the silver nanocube, forming an Ag-Au alloy. As more HAuCl4 is added to the system, a dealloying process occurs during which the silver atoms are selectively removed and the morphology of the nanocubes is reconstructed to form the product hollow, porous gold nanocages (3). Evidence of the formation of the gold nanocages from the silver nanocubes include SEM and TEM images as well as absorption spectra. As shown in Figure 4, the SPR peak shifts from 450 nm for the silver nanocubes to 1000 nm for the gold nanocages. Interestingly some batches of gold nanocages formed with oleylamine had the {100} faces of the cube etched while other batches had the {111} faces etched. This is illustrated by Figures 2 and 3. The only difference between the batches shown in Figures 2 and 3 was the amount of time that passed between the ligand exchange reaction and the galvanic replacement reaction. This suggests possible control over the face of the nanocube that etches simply by changing the amount of time that lapses before the galvanic replacement reaction is conducted after the ligand exchange reaction. Control over the final morphology of the gold nanocages could potentially expand their uses in the biomedical field to include such modalities as drug delivery. The possibility of selective etching of nanocubes needs to be further investigated in future work.
Acknowledgements
This work was supported by the NSF and the 2006 NNIN REU program. Special thanks to my mentor, Xianmao Lu, and the rest of the Xia group for their encouragement and advice during the summer of 2006.
References
1 Chen et al. "Gold Nanocages: Engineering Their Structure for Biomedical Applications"; Advanced Materials, 17, 2005 (2255- 2261).
2 Chen et al. "Gold Nanocages: Bioconjugation and Their Potential Use as Optical Imaging Contrast Agents"; Nano Letters, 5, 2005 (473- 477).
3 Sun and Xia. "Mechanistic Study on the Replacement Reaction between Silver Nanostructures and Chloroauric Acid in Aqueous Medium"; JACS, 126, 2004 (3892- 3901).
4 Sun and Xia. "Shape-Controlled Synthesis of Gold and Silver Nanoparticles."; Science, 298, 2002 (2176-2179).