Glucomannan-mediated facile synthesis of gold nanoparticles for catalytic reduction of 4-nitrophenol
© Gao et al.; licensee Springer. 2014
Received: 16 July 2014
Accepted: 9 August 2014
Published: 20 August 2014
A facile one-pot approach for synthesis of gold nanoparticles with narrow size distribution and good stability was presented by reducing chloroauric acid with a polysaccharide, konjac glucomannan (KGM) in alkaline solution, which is green and economically viable. Here, KGM served both as reducing agent and stabilizer. The effects of KGM on the formation and stabilization of as-synthesized gold nanoparticles were studied systematically by a combination of UV-visible (UV-vis) absorption spectroscopy, transmission electron microscopy, X-ray diffraction, dynamic light scattering, and Fourier transform infrared spectroscopy. Furthermore, the gold nanoparticles exhibited a notable catalytic activity toward the reduction of 4-nitrophenol to 4-aminophenol.
In the recent years, noble metal nanoparticles, especially gold nanoparticles (AuNPs), have attracted great interest and wide attention. AuNPs have proven to be a versatile platform in many areas such as catalysis, biosensing, optoelectronics, biological imaging, and therapeutic techniques[1–3]. Recently, the preparation and potential applications of AuNPs are becoming increasingly popular among researchers due to their distinctive optical properties, particularly tuneable surface plasmon resonance. Up to now, a number of chemical and physical methods for synthesis of metal nanoparticles have been reported, such as chemical reduction, electro-reduction, photo-reduction, and heat evaporation[4–6]. In most cases, the synthetic processes either involve the use of borohydride, hydrazine, citrate, etc. or require rather complex procedures or rigorous conditions, followed by surface modification with some protecting ligands like thiols and oleic acid. Thus, both toxicity and high cost make these materials less promising in industrial and biological applications.
To address these problems, biosynthesis of biological materials has received considerable attention. Compared to traditional methods, biosynthesis has many advantages by decreasing the use of toxic chemicals in the process and eliminating risks in industrial, pharmaceutical, and biomedical applications. To date, a broad range of biological materials has been introduced for the biosynthesis of metal nanoparticles including phytochemicals (polyphenol extract, catechin, lemongrass leaf extract, aloe extract, and fruit extracts)[7–13], microorganisms (bacteria and yeast)[14–16], protein[17, 18], peptide[19, 20], and polysaccharide[21–24].
Among the various biological materials, polysaccharides are emerging as an important natural resource for the synthesis of metal nanoparticles. In such processes, polysaccharides usually act as a reducing agent or stabilizer because of their special structure and properties. Since Raveendran et al. proposed a completely green method for preparation of silver nanoparticles with starch, many researchers have investigated the effects and mechanism of various polysaccharides on the formation of metal nanoparticles, such as cellulose, chitosan, alginic acid, hyaluronic acid, and agarose[21–25]. Konjac glucomannan (KGM), a kind of natural polysaccharide, has been widely used for its several valuable functions in healthcare and pharmacology, such as obesity suppression, tumor suppression, and treatment of cough, hernia, and skin disorders[26, 27]. The studies on the applications of konjac glucomannan have been extended greatly from food and food additives to various fields[28, 29]. Herein, we explore the use of KGM in the preparation of nanosized materials and thus further promote its application in nanotechnology.
Chloroauric acid (HAuCl4 · 4H2O, 99.9%) was purchased from Aladdin (Shanghai, China). Purified konjac glucomannan was obtained from Shengtemeng Konjac Powder Co. (Sichuan, China). All solutions were prepared in double-distilled water, and all glassware used was rinsed with aqua regia solution (HCl/HNO3, 3:1) and then washed with double-distilled water before use. All other common reagents and solvents used in this study were of analytical grade.
Synthesis of AuNPs in aqueous solution with KGM
KGM powders (0.25 g) were dispersed in double-distilled water (100 mL) by stirring for 1 h at room temperature, and then the solution was held at 80°C for 1 h. The preparation of gold nanoparticles is quite straightforward. In a typical preparation, sodium hydroxide solution (0.4 mL, 1 M) was added to KGM solution (20 mL, 0.25 wt%) under stirring, and then aqueous HAuCl4 (2 mL, 10 mM) solution was introduced. The mixture was incubated at 50°C for 3 h. The obtained gold nanoparticles were collected by centrifugation and washed thoroughly with DI water.
All UV-visible (UV-vis) spectra were recorded on a Pgeneral TU-1810 spectrophotometer (Purkinje Inc., Beijing, China) with 1-cm quartz cells. At different time intervals, aliquots of the solution were taken out and the samples were cooled to ambient temperature and then tested immediately. The morphology of the prepared gold nanoparticles in KGM solutions was examined with a JEOL JEM-2100 F transmission electron microscope (TEM, JEOL Inc., Tokyo, Japan) operated at an acceleration voltage of 200 kV. After ultrasonication for approximately 10 min in a bath sonicator, samples were prepared by placing a drop of aqueous gold nanoparticles onto a 300-mesh carbon-coated copper grid, and the grids were left to air dry at ambient temperature. Dynamic light scattering measurements were performed using a Brookhaven ZetaPlus Nanoparticle Size Analyzer instrument (Brookhaven Instruments Corporation, Holtsville, NY, USA) equipped with a 633-nm laser. The intensity of light scattered was monitored at a 90° angle. The XRD data was collected on a D/MAX 2500 diffractometer (Cu Kα radiation, λ = 1.5406 Å; Rigaku Co., Tokyo, Japan) at 100 mA and 40 kV. The sample was scanned over a 2θ range of 10° to 90° with a step size of 0.02° 2θ and a scan rate of 1 step/s. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet-560 FTIR spectrometer (Nicolet Co., Madison, WI, USA) with 20 scans and a resolution of 2 cm-1 in the range of 400 to 4,000 cm-1. Freeze drying under vacuum was applied overnight to get the very dry gold nanoparticles, and then the samples were deposited on the surface of a KBr plate.
Catalytic activity of gold nanoparticles
The catalytic activity of AuNPs was studied using sodium borohydride reduction of 4-NP as a model system. The reaction was completed in a quartz cell with a 1-cm path length. In a typical catalysis reaction, 15 μL of 10 mM 4-NP solution was mixed with 3 mL of 10 mM NaBH4 solution while stirring. Immediately after 15 μL of the prepared AuNP solution was added to the mixture, the reaction was monitored by a UV-vis spectrophotometer.
Results and discussion
Synthesis of AuNPs in aqueous KGM solution
As shown in Figure 2, all spectra exhibit an absorption peak around 522 nm with no significant peak shift, which is attributed to the surface plasmon resonance (SPR) band of the AuNPs, indicating the formation of gold nanoparticles. During the formation of AuNPs, the color of the reaction mixture changed from colorless to light pink within approximately 0.5 h and finally to wine red after 3 h. The phenomenon of color changes revealed a nucleation-growth mechanism (which is different from the typical synthesis of gold nanoparticles in citrate reduction) according to Ji et al.'s work, which would be discussed later.
Analysis of morphologies and crystalline structure of AuNPs
Mechanism analysis by FTIR study and DLS
Besides, the stabilizing effect was also confirmed by FTIR spectra. As shown in Figure 5, the absorption peak in the area of 3,421 cm-1 arose due to O-H stretching vibrations of the hydrogen-bonded hydroxyl (OH) group. A remarkable difference between the curves for pure KGM and KGM-protected AuNPs was the narrowing at 3,421 cm-1 (Figure 6, curve b). The narrowing of this peak was due to the damage of hydrogen bonding of the hydration between the KGM molecular chain and the water molecule in alkaline solutions[31, 34]. Thus, the formation of free -OH group facilitates the coordination interaction with gold ions by the breaking of hydrogen bonding. Taken together, the FTIR results demonstrate that initially gold ions bind to the surface of the KGM molecules and are subsequently reduced by hydroxyl groups, leading to the generation of nucleation sites for further reduction and ultimately to the formation of gold nanoparticles. The in situ reduction process prevents the aggregation of AuNPs.
Formation mechanism of gold nanoparticles in aqueous KGM solution
In our work, KGM was employed both as reducing agent and stabilizer for the synthesis of gold nanoparticles (Figure 1). Here, abundant hydroxyl groups of KGM act as the reducing groups for the reduction of Au3+ ions to Au0. It is worth noting that the deacetylation and cross-linking of KGM following alkali addition play an important role. The alkali damaged the hydrogen bonding of the hydration between the molecular chain and water molecules, resulting in the formation of free -OH group along the KGM chains which play the role of reduction and stabilization. Due to deacetylation and cross-linking behavior, KGM macromolecules contain size-confined molecular level capsules, which can act as templates for nanoparticle growth. Raveendran et al. reported a similar situation where starch served as a good template or dispersant for preparing well-dispersed nanoscale Ag particles in aqueous media without agglomeration. Thus, our results showed that it may be possible to achieve better size distribution control of the nanoparticles and good dispersity by selecting the appropriate reductant and stabilizer from various biological materials. In conclusion, the AuNPs formed in the KGM solution could be stabilized by a combination of gold-hydroxyl interaction and the steric stabilization owing to the molecular-scale entanglement of the polysaccharide.
Transition metal nanoparticles are attractive to use as catalysts due to their high surface-to-volume ratio compared to bulk catalytic materials. To date, the use of metal nanoparticles synthesized with polysaccharide is very limited. Here, our TEM images above showed that the gold nanoparticles are nearly spherical in shape and are composed of numerous (100) and (111) planes with corners and edges at the interfaces of these facets. Hence, the as-prepared gold nanoparticles are expected to be catalytically active. To investigate their catalytic activity, the reduction of 4-NP to 4-AP by NaBH4 was selected as a model system. It is well known that the absorption spectrum of a mixture of 4-NP and NaBH4 shows an absorption peak at 400 nm corresponding to the formation of an intermediate 4-nitrophenolate ion. Thus, the reaction process can be monitored by monitoring the changes in the absorption spectra of the 4-nitrophenolate ion at 400 nm. In a control experiment without AuNP addition, the absorbance at 400 nm did not change with time, indicating that no reduction of 4-NP occurred in the absence of AuNPs. Immediately after addition of nanoparticles, there was a remarkable decrease in the intensity of the absorption peak at 400 nm, and at the same time, a new peak at 298 nm appeared indicating the formation of reduction product, 4-AP.
Recent studies on the reduction of 4-NP with biologically synthesized AuNPs
Rate constant (s-1)
11 to 26
2.98 to 4.65 × 10-3
Au-calcium alginate composite
291 to 306
5 ± 2
0.23 to 0.33 × 10-3
AuNPs synthesized with fruit extract (Prunus domestica)
4 to 38
1.9 to 5.1× 10-3
AuNPs synthesized with protein extract (Rhizopus oryzae)
5 to 65
2.81 to 4.13× 10-3
KGM-synthesized AuNPs (this work)
12 to 31
6.03 × 10-3
In this study, we describe a facile and economically viable route for the synthesis of well-dispersed spherical gold nanoparticles using konjac glucomannan. The synthesized nanoparticles exhibit uniform spherical shape, a narrow size distribution with a mean diameter of 21.1 ± 3.2 nm, and excellent stability after 3 months of storage. The morphology and crystalline structure were characterized by TEM and XRD. Furthermore, the formation mechanism of AuNPs and the role of KGM both as reducing agent and stabilizer were analyzed by the results of UV-vis, TEM, DLS, and FTIR. Finally, the as-prepared gold nanoparticles were found to serve as effective catalysts for the reduction of 4-nitrophenol in the presence of NaBH4. Our work promotes the use of natural polysaccharide for the biosynthesis of nanomaterials, and more efforts should be made to extend their applications in biologically relevant systems.
This work was supported by the Ministry of Science and Technology of China (Nos. 2012YQ090194 and 2012AA06A303), the Natural Science Foundation of China (Nos. 51473115 and 21276192), and the Ministry of Education (No. NCET- 11–0372).
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