- Nano Express
- Open Access
Green Synthesis and Catalytic Activity of Gold Nanoparticles Synthesized by Artemisia capillaris Water Extract
© The Author(s). 2016
- Received: 4 August 2016
- Accepted: 19 October 2016
- Published: 26 October 2016
Gold nanoparticles were synthesized using a water extract of Artemisia capillaris (AC-AuNPs) under different extract concentrations, and their catalytic activity was evaluated in a 4-nitrophenol reduction reaction in the presence of sodium borohydride. The AC-AuNPs showed violet or wine colors with characteristic surface plasmon resonance bands at 534~543 nm that were dependent on the extract concentration. Spherical nanoparticles with an average size of 16.88 ± 5.47~29.93 ± 9.80 nm were observed by transmission electron microscopy. A blue shift in the maximum surface plasmon resonance was observed with increasing extract concentration. The face-centered cubic structure of AC-AuNPs was confirmed by high-resolution X-ray diffraction analysis. Based on phytochemical screening and Fourier transform infrared spectra, flavonoids, phenolic compounds, and amino acids present in the extract contributed to the reduction of Au ions to AC-AuNPs. The average size of the AC-AuNPs decreased as the extract concentration during the synthesis was increased. Higher 4-nitrophenol reduction reaction rate constants were observed for smaller sizes. The extract in the AC-AuNPs was removed by centrifugation to investigate the effect of the extract in the reduction reaction. Interestingly, the removal of extracts greatly enhanced their catalytic activity by up to 50.4 %. The proposed experimental method, which uses simple centrifugation, can be applied to other metallic nanoparticles that are green synthesized with plant extracts to enhance their catalytic activity.
- Gold nanoparticles
- Catalytic activity
- 4-Nitrophenol reduction reaction
- Artemisia capillaris extract
With the development of nanotechnology, metallic nanoparticles (MNPs) have attracted considerable attention due to their wide range of applications. Among MNPs, gold nanoparticles (AuNPs) have been considered an interesting area of research for useful applications in drug/gene delivery, catalysis, sensing, and imaging [1, 2]. Studies of AuNPs regarding synthesis, electrochemistry, and optical properties have been well reviewed by Sardar and co-workers . AuNPs were first used in catalysis for the oxidation of carbon monoxide, in which AuNPs were supported on a transition metal oxide . Examples of organic reactions catalyzed by AuNPs include the following : (i) hydrogenation reactions: hydrogenation of unsaturated carbonyls and reduction of nitro groups; (ii) alkyne activation; (iii) coupling reactions; (iv) oxidation reactions: oxidation of cyclohexane, oxidation of toluene, oxidation of alcohols, and oxidation of alkenes; and (v) miscellaneous reactions. The use of AuNPs in catalysis is advantageous. One of the merits of AuNPs is their high surface-area-to-volume ratio, which allows for the enhancement of chemical reactivity. The reduction of 4-nitrophenol (4-NP) in the presence of excess sodium borohydride is the most frequently used model reaction for evaluating the catalytic activity of AuNPs. The reaction product, 4-aminophenol (4-AP), is an important intermediate for analgesics and antipyretics.
The most common method for synthesizing AuNPs involves chemical reducing agents for the reduction of Au ions to AuNPs. However, these chemical methods employ toxic reagents, and the resulting AuNPs are most likely unsuitable for in vitro and in vivo applications. These applications require processes that are green, environmentally benign, and eco-friendly. Recent sustainability initiatives have proposed the use of plant extracts as reducing agents in the synthesis of AuNPs. The use of plant extracts possesses several advantages. (i) The reaction is facile and can be performed in a single pot; (ii) the reaction is easy to scale up; (iii) the resulting AuNPs are biocompatible; and (iv) synergistic activities are expected due to the combination of two materials (i.e., plant extracts and AuNPs). Many studies have reported the green synthesis of AuNPs using the following plant extracts as reducing agents and catalysts in the 4-NP reduction reaction: Bupleurum falcatum , Salicornia brachiata , Aerva lanata , Coleus forskohlii , Phoenix dactylifera , Garcinia combogia , Trigonella foenum-graecum , Gnidia glauca , and Breynia rhamnoides .
The aerial part of Artemisia capillaris (Compositae) has been used in traditional Chinese medicine and displays antiviral, antibacterial, and anti-inflammatory activities . Our laboratory has reported the synthesis of silver nanoparticles (AgNPs) using A. capillaris extract as a reducing agent [16, 17]. The synthesized AgNPs showed excellent antibacterial activity against Gram-negative bacteria . Furthermore, AgNPs synthesized using A. capillaris in the presence of cetyltrimethylammonium bromide exhibited antibacterial properties against methicillin-resistant Staphylococcus aureus .
In the present study, A. capillaris water extract was used as a reducing agent to synthesize AuNPs (hereafter referred to as AC-AuNPs). The catalytic activity of AC-AuNPs was then evaluated in the 4-NP reduction reaction in the presence of excess sodium borohydride. To investigate the effect of extract concentration on the catalytic activity, five different extract concentrations were used to synthesize AC-AuNPs. Additionally, the extract was removed by centrifugation (referred to hereafter as cf-AC-AuNPs), and the catalytic activities of AC-AuNPs and cf-AC-AuNPs were compared in the reduction reaction of 4-NP to 4-AP.
Hydrochloroauric acid trihydrate (HAuCl4·3H2O), 4-nitrophenol, and sodium borohydride were purchased from Sigma-Aldrich (St. Louis, MO, USA). A. capillaris was purchased from Ominherb (Uiseong-gun, Gyeongsangbuk-do, Republic of Korea). All other reagents were of analytical grade. Syringe filters (0.45 μm) were purchased from Sartorius Stedim Biotech (Goettingen, Germany). All solutions were prepared in deionized water.
A Shimadzu UV-2600 was used to acquire UV-visible spectra with a quartz cuvette (Shimadzu Corporation, Kyoto, Japan). A JEM-2100F microscope operated at 200 kV was used for transmission electron microscopy (TEM) imaging (JEOL Ltd., Tokyo, Japan). The nanoparticle solution was loaded onto a carbon-coated copper grid (carbon type B, 300 mesh, Ted Pella Inc., Redding, CA, USA), and the sample-loaded grid was dried for 24 h at ambient temperature prior to TEM analysis. Hydrodynamic size and zeta potential measurements were performed using a NanoBrook 90Plus Zeta (Brookhaven Instruments Corporation, Holtsville, NY, USA). Each sample was measured ten times to determine the hydrodynamic size and five times to determine the zeta potential; the measured values were then averaged to obtain mean values. High-resolution X-ray diffraction (HR-XRD) was performed at 2θ values ranging from 20° to 90° using a Bruker D8 Discover high-resolution X-ray diffractometer equipped with a Cu-Kα radiation source (λ = 0.154056 nm) (Bruker, Karlsruhe, Germany). FT-IR spectra were acquired using a Varian 640IR in the attenuated total reflectance mode (Agilent Technologies, Santa Clara, CA, USA). A FD8518 freeze dryer was used to prepare powdered samples (IlShinBioBase Co. Ltd., Gyeonggi-do, Republic of Korea). Centrifugation was performed using either a 5424R (Eppendorf AG, Hamburg, Germany) or UNION 55R (Hanil Science Industrial Co. Ltd., Incheon, Republic of Korea).
Preparation of A. capillaris Water Extract
Phytochemical screening of A. capillaris water extract
Tannic acid test
Ferric chloride test
Alkaline reagent test
Lead acetate test
Modified Borntrager’s test
Copper acetate test
Synthesis of AC-AuNPs
Two stock solutions with concentrations of 0.1 % (extract) and 1 mM (hydrochloroauric acid trihydrate) were prepared in deionized water. For the synthesis of AC-AuNPs, the final concentration of hydrochloroauric acid trihydrate was fixed at 0.25 mM. The extract concentration was varied at 0.015, 0.025, 0.035, 0.045, and 0.055 %. The final volume was adjusted to 4 mL by adding deionized water. The incubation was conducted at 80 °C in a dry oven for 1 h. UV-visible spectra were acquired between 400 and 700 nm, and the hydrodynamic size and zeta potentials were measured.
Preparation of cf-AC-AuNPs
Centrifugation was performed to remove the extract from AC-AuNPs (1 mL) using a 5424R centrifuge at 24 °C (18,000g, 20 min). After centrifugation, the supernatant containing the extract was removed, and the pellet was re-dispersed with deionized water (final volume 1 mL) to generate cf-AC-AuNPs. UV-visible spectra were acquired between 400 and 700 nm, and the hydrodynamic size and zeta potentials were measured.
Catalytic Activity of AuNP Catalysts in 4-NP Reduction Reaction
Stock solutions of sodium borohydride and 4-NP were freshly prepared prior to use. 4-NP (0.4 mM, 1 mL) and sodium borohydride (40 mM, 1 mL) were mixed in a 4 mL quartz cuvette. Then, 100 μL of either AC-AuNPs or cf-AC-AuNPs was added, and the final volume was adjusted to 4 mL with deionized water. Upon the addition of AuNP catalysts, the reaction progress was monitored every 5 min by UV-visible spectrophotometry between 200 and 500 nm range at 25 °C. In order to evaluate the reusability of catalysts, cf-AC-AuNPs was centrifuged at 24 °C (18,000g, 20 min), and the pellet was re-dispersed with deionized water (final volume 1 mL). Also, the catalytic activity was assessed in the presence of sodium borohydride with the same procedure described above.
Synthesis of AC-AuNPs
Preparation of cf-AC-AuNPs
TEM Images and Size Histograms
Hydrodynamic Size and Zeta Potential Measurements
Hydrodynamic size (n = 10) and zeta potential (n = 5) values of AC-AuNPs and cf-AC-AuNPs
A. capillaris final concentration (%)
Hydrodynamic size (nm) (polydispersity index)
Zeta potential (mV)
Hydrodynamic size (nm) (polydispersity index)
Zeta potential (mV)
To predict the colloidal stability, zeta potential measurements are generally performed. As shown in Table 2, negative zeta potentials were observed for the AC-AuNPs (−16.45~−20.63 mV). With an increase in extract concentration, the absolute value of the zeta potential increased. This result demonstrated that the colloidal stability of the AC-AuNPs was improved by increasing the extract concentration. Negative values were also observed for the cf-AC-AuNPs (−27.15~−30.27 mV); however, the zeta potential values of the cf-AC-AuNPs showed no general tendency.
Phytochemical Screening and FT-IR Spectra
A series of phytochemical screenings was conducted to verify the presence of primary and secondary metabolites in the A. capillaris extract. As shown in Table 1, saponins, amino acids, phenolic compounds, flavonoids, and diterpenes were present in the extract.
Catalytic Activity of AC-AuNPs and cf-AC-AuNPs in 4-NP Reduction Reaction
−kt = ln(C t /C 0) = ln(A t /A 0), where A t and A 0 are substituted for C t and C 0, respectively. A t and A 0 are the absorbance of 4-NP at 400 nm at time t and time 0, respectively.
Rate constants in 4-NP reduction reaction with AuNP catalysts in the presence of sodium borohydride
A. capillaris final concentration (%)
Rate constant (sec−1)
1.07 × 10−3
1.17 × 10−3
1.33 × 10−3
1.46 × 10−3
1.73 × 10−3
cf-AC-AuNPs (percentage of increase compared to AC-AuNPs)
1.44 × 10−3 (34.6 %)
1.76 × 10−3 (50.4 %)
1.84 × 10−3 (38.3 %)
1.87 × 10−3 (28.1 %)
2.21 × 10−3 (27.7 %)
1st recycle of cf-AC-AuNPs
0.038 × 10−3
0.038 × 10−3
0.39 × 10−3
1.75 × 10−3
0.75 × 10−3
Plant extracts are promising reducing agents for the synthesis of AuNPs in fulfilling global sustainability initiatives. In the current study, an A. capillaris extract was successfully utilized as a reducing agent for the synthesis of AC-AuNPs under different extract concentrations. Phytochemical screening indicated that saponins, amino acids, phenolic compounds, flavonoids, and diterpenes were present in the A. capillaris water extract. Among these compounds, flavonoids, phenolic compounds, and amino acids were involved in the synthesis, as verified by FT-IR spectroscopy. The average size of AC-AuNPs decreased with an increase in extract concentration, as measured by TEM imaging. The extract was then removed from AC-AuNPs to produce cf-AC-AuNPs, and the catalytic activities of both sets of nanoparticles in the 4-NP reduction reaction were compared. Interestingly, the rate constants increased with increasing extract concentration for both AC-AuNPs and cf-AC-AuNPs. These results imply that the metallic core size of AuNPs mostly likely affects their catalytic activities. Small nanoparticles possess a large surface-area-to-volume ratio, increasing their catalytic activities. Finally, the removal of the extract enhanced the particles’ catalytic activity by up to 50.4 %. Thus, the current method, which uses simple centrifugation, can be applied to other metallic nanoparticles that are green-synthesized with plant extracts to enhance their catalytic activity in the 4-NP reduction reaction.
This study was financially supported through grants from the National Research Foundation of Korea (NRF) funded by the Korean government (the Ministry of Education, NRF-2015R1D1A1A09059054).
SHL performed the green synthesis of AC-AuNPs and cf-AC-AuNPs. SHL also characterized both sets of AuNPs and evaluated the catalytic activity. EYA obtained the HR-TEM images. YP supervised the entire process and drafted the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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