Shape-tailoring and catalytic function of anisotropic gold nanostructures
© Premkumar et al; licensee Springer. 2011
Received: 8 April 2011
Accepted: 5 October 2011
Published: 5 October 2011
We report a facile, one-pot, shape-selective synthesis of gold nanoparticles in high yield by the reaction of an aqueous potassium tetrachloroaurate(III) solution with a commercially available detergent. We prove that a commercial detergent can act as a reducing as well as stabilizing agent for the synthesis of differently shaped gold nanoparticles in an aqueous solution at an ambient condition. It is noteworthy that the gold nanoparticles with different shapes can be prepared by simply changing the reaction conditions. It is considered that a slow reduction of the gold ions along with shape-directed effects of the components of the detergent plays a vital function in the formation of the gold nanostructures. Further, the as-prepared gold nanoparticles showed the catalytic activity for the reduction reaction of 4-nitrophenol in the presence of sodium borohydride at room temperature.
Keywordsgold nanoparticles detergent catalytic activity one-pot synthesis
Nanosized metal particles are of great interest and important for their applications in an ample diversity of areas such as catalysis, optical devices, nanotechnology, and biological sciences [1–4]. Among the nanoparticles of many metals, gold (Au) nanoparticles have gained much attention, because they have shown to be a technologically important material that can be potentially used in application such as catalysis [5, 6], chemical sensors , in biological and medical areas [4, 8], and for the miniaturization of electronic devices due to their unique optical and electrical properties [4, 9–12]. Interestingly, these characteristic properties often depend critically not only on the particle size, but also on the particle shape [13–15]. Specially, the formation of aqueous dispersible nanoparticles has concerted many efforts in view of prospective biomedical applications and environmental effects connected with the use of organic solvents. Depending on the synthesis techniques and the kind of stabilizing and reducing agents, particles with various properties can be generated. While the size-controlled synthesis of gold nanoparticles (AuNPs) has been actively pursued, little has been done to shape manipulations of AuNPs in an aqueous medium, which could facilitate the various potential applications in the fields of physics, chemistry, biology, medicine, and materials science as well as their different interdisciplinary fields.
Though a number of preparative protocols have been attempted and introduced to control the shape of silver nanoparticles [16–20], analogous reports for AuNPs are comparatively few and are more recent. These include liquid crystal , templates , solution-based techniques [23, 24], and others  guide to the fabrication of planar Au nanostructures with reasonable control over their optical properties. Further, it has been reported that AuNPs with controlled shapes were synthesized by introducing acetylacetone and several related ligands . For example, hexadecylaniline was used for the synthesis of organically dispersible AuNPs, where the spontaneous reduction of aqueous HAuCl4 solution results in variable shapes of nanoparticles . Recently, biosynthetic methods, an alternative to chemical synthetic procedures and physical processes, have been introduced to the formation of AuNPs by using plant extracts [28, 29]. Also, an excellent shape-selective formation of single crystalline triangular AuNPs by using the extract of a lemongrass plant (Cymbopogon flexuosus) was reported [30, 31]. In a break from tradition, which has hitherto relied on the use of either reducing agents such as NaBH4, N2H4, and/or external energies such as photochemical, microwave irradiation, and radiolysis in the synthesis of AuNPs in an aqueous solution, we have recently shown that neutral surfactants such as polysorbate 80 may be used to synthesize spherically shaped AuNPs of different sizes in an aqueous solution at different experimental conditions without utilizing any additional reducing agents and energies .
In this paper, we report the facile, one-pot, shape-selective synthesis of AuNPs in high yield by the reaction of an aqueous KAuCl4 solution with a commercially available detergent. We demonstrate that by altering the reaction conditions such as concentration of the reactants and temperatures, the percentage of Au nanostructures can be manipulated. It is considered that a slow reduction of the Au ions along with the shape-directed effects of the components of the detergent plays a vital function in the formation of the Au nanostructures. The approach introduced here does not need any harsh and toxic reducing agents and it requires no manipulative skills. Further, the as-prepared AuNPs showed the catalytic activity for the reduction reaction of 4-nitrophenol in the presence of sodium borohydride. An important aspect of nanotechnology concerns the development of experimental processes for the synthesis of nanoparticles of different chemical compositions, sizes, shapes, and controlled dispersity with facile approach and low cost. This method is facile and employs gentle reaction conditions in contrast to the conventional techniques using polymers or surfactants and harsh reductants.
Results and discussion
Major shape distribution of AuNPs at different experimental conditions
Concentration of gold salt (mM)/detergent (wt.%)
Shape distribution (%)
In one particular experiment, the Au precursor concentration was increased to 1 from 0.4 mM, which was used for the synthesis of triangle and spherical nanoparticles, and the final concentration between the Au precursor and detergent was maintained at 1 mM and 1 wt.%, respectively. Interestingly, the observation by TEM showed (Figure 3d) that approximately 47% of the particles had a projected hexagonal shape and a size range of 20 ± 5 nm. In addition to the hexagonal shapes, which formed the majority of the product, a small portion (approximately 18%) of pentagonal (sizes of 19 ± 5 nm), triangular, and spherical (each approximately 14%) particles were also commonly observed in the final products (Table 1).
Even though mechanisms for the formation of metal plate-like structures or anisotropic particles have been proposed according to the nature of the reaction conditions, it still remains a subject of controversy. However, it is generally accepted that the shape of an fcc nanocrystal is mostly decided by the ratio of the growth rate along the  versus the  direction . It has been reported that the triangular and hexagonal nanoparticles bound by the stable  planes and the perfect cube bounded by less stable  planes . Further, some authors proposed that the formation of the thin plate-like structures is due to the preferential adsorption of protecting agents such as some specific surfactants or polymers onto favored crystalline planes. It is obvious that the detergent, which contains complex functional compounds and (amine oxide, hydroxyl, and carboxylic) groups, plays the key role in the present case for the formation of differently shaped nanoparticles at ambient condition. We believe that the amine oxide and hydroxyl functional groups present in the detergent may facilitate the reduction process and the carboxylic groups may have an interaction with the surface of the AuNPs and in turn stabilize the AuNPs. The chances of influencing the shapes of AuNPs by other foreign materials are ruled out, since the detergent acts as both reducing and protecting agent and no additional agents or materials are added in the present system. Hence, we believe that the specific interaction between the detergent and the different surface planes of the AuNPs at ambient conditions could significantly increase the growth rate along the  direction and, in turn, reduce the growth rate along  plane, thus, favoring the formation of nanoparticles of triangular and hexagonal shape .
An easy and inexpensive, one-pot, shape-selective synthesis of AuNPs through the reaction of aqueous KAuCl4 with a commercially available detergent was developed. We proved that a commercial detergent can act as a reducing as well as the stabilizing agent for the synthesis of differently shaped AuNPs in an aqueous solution at an ambient condition. Hence, this route permits well-dispersed AuNPs to be obtained at room temperature, without employing reducing agents and external energy. Therefore, this approach is facile and inexpensive method for the shape-selective synthesis of AuNPs, even without any additional reagents and sophisticated equipment or facilities. The approach introduced here does not need any harsh and toxic reducing agents that might enhance the local concentration of any reagent in solution during addition. Thus, the synthetic process is thought viable to be readily integrated into a variety of systems, especially those that are relevant to biomedical applications, as it uses water as the solvent. It is noteworthy that the AuNPs with different shapes can be prepared by simply changing the reaction conditions. It is considered that a slow reduction of the Au ions along with the shape-directed effects of the components of the detergent plays a vital function in the formation of the Au nanostructures. Further, the as-prepared AuNPs showed catalytic activity for the reduction reaction of 4-nitrophenol in the presence of NaBH4, which has been established by UV-vis spectroscopy. This method is facile and employs gentle reaction conditions in contrast to the conventional techniques using polymers or surfactants and harsh reductants. The morphology and dimensions of the product were found to strongly depend on the reaction conditions such as concentration of the Au precursor, detergent, and temperature. This synthetic strategy has the potential to be a generalized process that can be extended readily to the synthesis of different kinds of metal plate-like structures. Studies in this direction are underway.
KAuCl4 as the precursor for the formation of AuNPs was obtained from Aldrich (St. Louis, MO, USA). Commercial detergent (Jayeonpong, brand name) from LG Household and Health Care, Seoul, South Korea and was used as both reducing and protecting agents. It consists of approximately 23% surfactants such as alcoholic (anionic), olefinic (anionic), and aminic (nonionic) and approximately 77% pine needles extracts, etc. 4-Nitrophenol (Junsei, Tokyo, Japan) and NaBH4 (Aldrich) were purchased and used without further purification.
The UV-visible absorption spectra were recorded on a Varian Cary 500 spectrophotometer (Varian, Inc., Palo Alto, CA, USA). The TEM was performed with a Philips T20ST instrument (Philips, Amsterdam, Netherlands). The TEM specimens were prepared by placing a few drops of sample solution on a copper mesh covered with a carbon film and allowing the solvent to evaporate at room temperature for overnight. The particle shape distributions were calculated by image analysis, always over more than 100 counts. The UV-vis-NIR spectroscopic measurements of the AuNPs prepared were analyzed on a JASCO model V-570 dualbeam spectrophotometer (JASCO, Easton, MD, USA). The AFM images of AuNPs were obtained in the contact mode on a Dimension™ 3100 Atomic Force Microscope (Digital Instruments, Veeco Metrology Group, Santa Barbara, CA, USA).
Synthesis of gold nanoparticles
The AuNPs were prepared by the reduction of Au3+ ions in an aqueous solution containing the detergent at room temperature. The procedure was quite easy and straightforward. In a typical experiment, 0.4 mM KAuCl4 was added to 10 mL of 1 wt.% aqueous solution of the detergent at room temperature, leading to the slow formation of AuNPs, as manifested by a pinkish coloration of the solution. No stirring was necessary after the solution was gently hand shaken (approximately 1 min) for homogenization. Therefore, the samples were left standing for the reaction to proceed at room temperature. The extent of the reaction depended on the concentration of detergent, while the time needed for its completion mainly depended on the concentration of the reactants and on the reaction temperature.
Catalytic reduction of 4-nitrophenol
An aqueous solution of NaBH4 (1 mL, 15 mM) was mixed with 4-nitrophenol (1.7 mL, 0.2 mM) in a standard quartz cuvette. The light yellow color of the 4-nitrophenol was turned to yellowish green due to the formation of 4-nitrophenolate ion. An aliquot of AuNPs prepared at 4°C (0.3 mL) was added to the resulting solution, and the time-dependent absorbance spectra were recorded with a time interval of 1 min in the scanning range of 200 to 800 nm at room temperature.
transmission electron microscopy
atomic force microscopy.
This work was supported by the World-Class University (WCU) program through a grant provided by the Ministry of Education, Science and Technology (MEST) of Korea (Project no. R31-10026).
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