Catechin-capped gold nanoparticles: green synthesis, characterization, and catalytic activity toward 4-nitrophenol reduction
© Choi et al.; licensee Springer. 2014
Received: 10 February 2014
Accepted: 24 February 2014
Published: 3 March 2014
An eco-friendly approach is described for the green synthesis of gold nanoparticles using catechin as a reducing and capping agent. The reaction occurred at room temperature within 1 h without the use of any external energy and an excellent yield (99%) was obtained, as determined by inductively coupled plasma mass spectrometry. Various shapes of gold nanoparticles with an estimated diameter of 16.6 nm were green-synthesized. Notably, the capping of freshly synthesized gold nanoparticles by catechin was clearly visualized with the aid of microscopic techniques, including high-resolution transmission electron microscopy, atomic force microscopy, and field emission scanning electron microscopy. Strong peaks in the X-ray diffraction pattern of the as-prepared gold nanoparticles confirmed their crystalline nature. The catalytic activity of the as-prepared gold nanoparticles was observed in the reduction of 4-nitrophenol to 4-aminophenol in the presence of NaBH4. The results suggest that the newly prepared gold nanoparticles have potential uses in catalysis.
Recent advances in nanotechnology have resulted in diverse applications of gold nanoparticles (AuNPs) in various research fields. AuNPs are the most stable NPs and are used in novel applications, including as vehicles for drug/gene delivery, catalysts, optical sensors, and imaging and visualization agents [1–3]. In addition, the catalytic properties of AuNPs have been explored, and the AuNPs have been found to exhibit improved catalytic performance compared with that of their bulk counterpart. The catalytic activity of AuNPs has been commonly evaluated using a well-known reaction: the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of NaBH4. 4-NP is an industrial waste and environmental hazard with a long degradation time. Thus, the removal of this component from water is important for public health. The product, 4-AP, is a useful intermediate in the manufacture of antipyretics and analgesics.
Recently, the green synthesis of AuNPs using biological entities as reducing agents has been rapidly replacing chemical methods in which toxic chemicals are utilized. This approach provides numerous benefits, including the high biocompatibility and good water solubility of the resultant AuNPs. Furthermore, the process is eco-friendly and time and cost effective. Plant extracts and pure compounds from plant sources have been demonstrated to be highly effective reducing agents for the synthesis of AuNPs .
Catechins are flavanol compounds that are abundant in tea. The biological activities of tea catechins have been extensively reviewed elsewhere [5–8]. Among tea catechins, catechin and epigallocatechin gallate have been used for the synthesis or modification of NPs [9–12]. Ointment of a combination of AuNPs with the antioxidant epigallocatechin gallate and α-lipoic acid accelerated cutaneous wound healing through anti-inflammatory and antioxidant effects . In particular, the topical application of this combined ointment promoted the proliferation and migration of dermal keratinocytes and fibroblasts, which enhanced the restoration of normal skin structures. The same research group has reported that the topical application of the ointment of AuNPs (3 to 5 nm in size) with epigallocatechin gallate and α-lipoic acid effectively promoted wound healing in diabetic mice . The attractive biological activity of epigallocatechin gallate-modified AuNPs is their anticancer activity, which includes efficacy in the treatment of prostate and bladder cancers [11, 12]. As an analytical application, catechin-modified TiO2-NPs were used as matrices for the analysis of steroid hormones using surface-assisted laser desorption/ionization mass spectrometry . When catechin was bound to the TiO2-NP surface, the absorption wavelength increased at 337 nm when compared with that of the unmodified TiO2-NPs, which led to an increase in the N2 laser absorption efficiencies . As another analytical application, catechin-synthesized AuNPs were used as a nanosensor for the fluorescent detection of lead in water and urine samples .
Herein, catechin was used as a reducing agent for the green synthesis of AuNPs at room temperature for 1 h, and the use of other toxic chemicals as reducing agents was avoided (referred to hereafter as catechin-AuNPs). The catechin-AuNPs were characterized using UV-visible spectrophotometry, high-resolution transmission electron microscopy (HR-TEM), atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), and high-resolution X-ray diffraction (HR-XRD). The reaction yield of the synthesis was measured using inductively coupled plasma mass spectrometry (ICP-MS). Furthermore, the catalytic activity of catechin-AuNPs was evaluated on the basis of the reduction of 4-NP to 4-AP in the presence of NaBH4.
4-Nitrophenol, hydrochloroauric acid trihydrate (HAuCl4 · 3H2O), sodium borohydride, and (+)-catechin hydrate were purchased from Sigma-Aldrich (St. Louis, MO, USA). Carbon-coated copper grids (carbon type-B, 300 mesh) were purchased from Ted Pella (Redding, CA, USA). The RTESP AFM probe (MPP-11100-10, premium high-resolution tapping mode silicon probe) was obtained from Bruker Nano (Santa Barbara, CA, USA). Mica (grade V-1, 25 mm × 25 mm length, 0.15 mm thick) was purchased from SPI Supplies Division of Structure Probe (West Chester, PA, USA). All the other reagents were of analytical grade. The UV-visible spectra were recorded using a Shimadzu UV-2600 with a quartz cuvette (Shimadzu Corporation, Kyoto, Japan). The HR-TEM images were acquired with a JEM-3010 (JEOL, Tokyo, Japan) operated at 300 kV. The AFM images were obtained using a Dimension® Icon® (Bruker Nano, Santa Barbara, CA, USA) operated under tapping mode. The sample-loaded mica and copper grids were dried in a 60°C oven overnight before the analyses. The FE-SEM images were collected in a JSM-7100 F SEM using an accelerating voltage of 15 kV (JEOL). ICP-MS analysis was performed in an ELAN 6100 (Perkin-Elmer SCIEX, Waltham, MA, USA). The ICP-MS samples were prepared using centrifugation. The centrifugation of catechin-AuNPs was performed at 12,300 × g for 40 min, and the supernatant containing the unreacted Au3+ was used for ICP-MS analysis. The total concentration of Au3+ of the catechin-AuNPs solution was also measured using ICP-MS. The average value of the three measurements was used to determine the yield. For HR-XRD analyses, the catechin-AuNP solution was centrifuged at 12,300 × g for 40 min to remove the supernatant. The pellet was pooled and freeze-dried. The freeze-dried samples were prepared with a FD5505 freeze dryer (Il Shin Bio, Seoul, Korea). A Bruker D8 Discover high-resolution X-ray diffractometer (Bruker, Karlsruhe, Germany) equipped with a CuKα radiation source (λ = 0.1541 nm) was used in the range of 20° to 90° (2θ scale).
The stock solutions of HAuCl4 · 3H2O (0.5 mM) and catechin (0.5 mM) were prepared using deionized water. Then, 600 μL of HAuCl4 · 3H2O (0.5 mM) was placed in a 5-mL glass vial with 200 μL of deionized water, and catechin (0.5 mM, 200 μL) was subsequently added to this solution. The reaction mixture was then further incubated under ambient temperature (26°C) for 1 h. The synthesis of gold nanoparticles was monitored through the acquisition of UV-visible spectra.
To evaluate the catalytic activity of the catechin-AuNPs, the reduction of 4-NP to 4-AP in the presence of NaBH4 was performed. The catalytic reduction of 4-NP was conducted in aqueous solution under ambient temperature (26°C), and UV-visible spectra were measured in a quartz cuvette. The 4-NP solution (899.9 μL, 0.15 mM) was mixed with deionized water (450.1 μL). Then, freshly prepared NaBH4 (1.65 mL, 5.5 mM) was added. To this reaction mixture, 1 mL of freshly synthesized catechin-AuNPs was added. UV-visible spectra were recorded at a time interval of 5 min in the range of 200 to 700 nm.
Results and discussion
Green synthesis and the yield of catechin-AuNPs
In general, the stability of tea catechins is affected by temperature and pH [15, 16]. The thermal degradation of catechins is noticeable upon with an increase in temperature. Furthermore, tea catechins are very stable at pH levels less than 4, whereas the stability of catechins decreases in alkaline solutions. In terms of the stability point, the reaction conditions that were used in the present research minimized the thermal and pH degradation of catechin, which may have facilitated the reaction. The pH of the HAuCl4 solution was less than 4, and no other reagents were added to adjust the pH. In addition, the reaction was performed under ambient temperature (26°C) without the input of any external energy.
We determined the yield of the reaction by measuring the concentration of unreacted Au3+ using ICP-MS. After the sample was subjected to centrifugation, the purple color disappeared in the supernatant, which indicated that the AuNPs were effectively separated from the unreacted Au3+. The yield was 99.1% indicating that the reaction occurred very efficient.
AFM and FE-SEM images
The Debye-Scherrer equation (D = 0.89λ/W cosθ) was employed to estimate the particle diameter from the (111) peak, and the estimated diameter was approximately 16.6 nm. The definition of each term in the equation is as follows: λ is the wavelength of CuKα radiation (0.1541 nm), W is the full-width at half-maximum of the (111) peak, θ is the diffraction angle, and D is the particle diameter.
Catalytic activity toward 4-nitrophenol reduction
The relationship between ln(C t /C0) and time (min) revealed a linear correlation (y = −0.091x + 0.071, r2 = 0.981), where C0 and C t are the 4-NP concentration at time 0 and time t, respectively (Figure 7B) . The ratio of absorbance, A t /A0, could be substituted for the ratio of concentration, C t /C0 (i.e., C t /C0 = A t /A0) because the concentration of 4-NP is proportional to its absorbance . On the basis of these results, we determined that the shell did not affect the catalytic activity of the catechin-AuNPs.
Catechin, which is a potent antioxidant, has been successfully utilized as a green reducing agent for the synthesis of AuNPs. No external energy was necessary during the 1 h reaction, which was simple, fast, energy-saving, and eco-friendly. Together with spherically shaped AuNPs, anisotropic AuNPs with diverse shapes were also observed. The crystalline nature of the AuNPs was confirmed by the resulting HR-XRD peaks and the lattice fringes in the HR-TEM images. Most notably, the capping of AuNPs with catechins was clearly visualized in the microscopic images. The width and height information of the shells was obtained from the HR-TEM and AFM images, respectively. The catechin shells were observed to disappear after the catechin-AuNPs were stored at ambient temperature, during which the aggregation of the AuNPs increased. Thus, catechin plays a role as a reducing agent and is also responsible for the capping of AuNPs. The catalytic activity of catechin-AuNPs for the reduction of 4-NP demonstrated that the newly-prepared AuNPs can be used as a catalyst that is prepared via a green synthesis route.
This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government: the Ministry of Education (NRF-2012R1A1A2042224) and the Ministry of Science, ICT & Future Planning (NRF-2010-18282). This financial support is gratefully acknowledged. The authors would like to thank Ms. Sang Hui Jun for assisting in the preparation of this manuscript.
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