Catalytic reduction of 4-nitrophenol with gold nanoparticles synthesized by caffeic acid
© The Author(s). 2017
Received: 7 October 2016
Accepted: 7 December 2016
Published: 5 January 2017
In this study, various concentrations of caffeic acid (CA) were used to synthesize gold nanoparticles (CA-AuNPs) in order to evaluate their catalytic activity in the 4-nitrophenol reduction reaction. To facilitate catalytic activity, caffeic acid was removed by centrifugation after synthesizing CA-AuNPs. The catalytic activity of CA-AuNPs was compared with that of centrifuged CA-AuNPs (cf-CA-AuNPs). Notably, cf-CA-AuNPs exhibited up to 6.41-fold higher catalytic activity compared with CA-AuNPs. The catalytic activity was dependent on the caffeic acid concentration, and the lowest concentration (0.08 mM) produced CA-AuNPs with the highest catalytic activity. The catalytic activities of both CA-AuNPs and cf-CA-AuNPs decreased with increasing caffeic acid concentration. Furthermore, a conversion yield of 4-nitrophenol to 4-aminophenol in the reaction mixture was determined to be 99.8% using reverse-phase high-performance liquid chromatography. The product, 4-aminophenol, was purified from the reaction mixture, and its structure was confirmed by 1H-NMR. It can be concluded that the removal of the reducing agent, caffeic acid in the present study, significantly enhanced the catalytic activity of CA-AuNPs in the 4-nitrophenol reduction reaction.
For many years, gold had been considered as an inert metal. The first discovery of gold nanoparticles (AuNPs) in the catalytic field was an oxidation of carbon monoxide by AuNPs supported on the transition metal oxide . Acting as catalysts in organic reactions, AuNPs have attracted considerable attention due to their unique physical and chemical properties [2–4]. One of its merits in catalysis is that many organic reactions can be achieved under mild conditions, and the high surface-area-to-volume ratio of AuNPs leads to increase in chemical reactivity . Examples of organic reactions that use AuNPs include (i) hydrogenation reactions of unsaturated carbonyls and reduction of nitro groups, (ii) alkyne activation, (iii) coupling reactions, and (iv) oxidation reactions of cyclohexane, toluene, alcohols, and alkenes .
To assess the catalytic activity of AuNPs, the reduction reaction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with excess NaBH4 is generally employed as a model reaction . 4-NP and its derivatives are used to manufacture herbicides, insecticides and synthetic dyestuffs, and they can substantially damage the ecosystem with common organic pollutants of wastewater [7, 8]. The reaction product, 4-AP, is a useful compound used as an intermediate for manufacturing analgesics and antipyretics.
Recently, many researchers have actively studied green synthetic methods using biological entities as reducing agents to convert Au ions to AuNPs. Such methods eliminate the use of toxic chemicals and increase the biocompatibility of the resulting AuNPs. Moreover, these methods also have the benefits of using aqueous solvents, conducting reactions in one-pot and being eco-friendly. In this study, caffeic acid, one of phenolic compounds in plants, was used as a reducing agent to synthesize AuNPs (referred to hereafter as CA-AuNPs). Caffeic acid is abundant in honey, olive oil, coffee beans and medicinal plants. Caffeic acid and its derivatives have a variety of biological activities, including anti-atherosclerotic, anti-bacterial, anti-cancer, anti-inflammatory, anti-oxidative, anti-viral, immunostimulatory, and neuroprotective properties [9–16].
There are research reports regarding the enhancement of catalytic activity of colloidal AuNPs [17–19]. Kim and coworkers designed anisotropic/partially aggregated AuNPs possessing a strong and wide absorbance in visible and near-infrared light to enhance reaction rates of 4-NP to 4-AP under light irradiation . Upon light irradiation, the anisotropic/partially aggregated ones efficiently convert photon to heat, thus, the reaction rate of 4-NP to 4-AP increased notably, whereas the monodispersed ones showed only moderate increase in reaction rates . Recently, You and coworkers have reported the surface modification of metallic nanoparticles by changing capping ligands for the enhancement of catalytic activity . They modified the surface of metallic nanoparticles from citrate with a cationic polymer, poly(diallyldimethylammonium chloride) (PDDA) capping. The PDDA produced net positive charges on the surface of metallic nanoparticles which afforded strong electrostatic attraction between the surface and negatively charged ions (nitrophenolate and borohydride ions) and finally enhanced catalytic activity in the 4-NP reduction reaction . Most recently, we green-synthesized AuNPs with Artemisia capillaris extract and their catalytic activity was assessed in the 4-NP reduction reaction . The AuNPs were centrifuged and re-dispersed with water to remove extract on the surface and the catalytic activity of the initial AuNPs and the centrifuged AuNPs were compared. Remarkably, the centrifuged AuNPs exhibited an enhancement of catalytic activity up to 50.4% . In addition to the size and shapes of metallic nanoparticles, the surface modification by removal of capping agents is another crucial factor to control catalytic activity. This can be explained by Langmuir-Hinshelwood mechanism in the following.
Langmuir-Hinshelwood mechanism was proposed by Wunder and co-workers as a mechanistic model for the reduction of 4-NP by NaBH4 in the presence of metallic nanoparticles [20, 21]. According to their proposed model, the surface of metallic nanoparticles serves as the location where the catalytic reduction process occurs. Borohydride ions bind to the surface, and concomitantly, 4-NP also adsorbs on the surface. Subsequently, 4-NP is reduced by borohydride ions to 4-AP, which is a rate-determining step. Ciganda and co-workers have reported “restructuration” on the surface during the catalytic process, where ligands are displaced by substrates on the surface . Restructuration on the surface is a dominant aspect of the Langmuir-Hinshelwood mechanism. Generally, no induction time is observed when a very facile ligand displacement occurs by substrates. In contrast, stronger binding of ligands on the surface is responsible for their difficult displacement by substrates, leading to longer induction times. Based on previous reports [20–22], we hypothesized that removal of the ligand (caffeic acid in the present study) will considerably affect the “restructuration” process on the surface and facilitate the displacement process between ligands and substrates. This will lead to an increase in rate constants and finally enhance the catalytic activity.
Therefore, the present report focuses on (i) evaluating the catalytic activities of CA-AuNPs synthesized under various concentrations of caffeic acid, (ii) removing caffeic acid by centrifugation (referred to hereafter as cf-CA-AuNPs) and comparing the catalytic activity of CA-AuNPs with that of cf-CA-AuNPs, (iii) obtaining a conversion yield by reverse-phase high performance liquid chromatography (RP-HPLC), and finally (iv) purifying 4-AP by silica gel column chromatography and characterizing it by 1H-NMR. The reduction reaction of 4-NP to 4-AP in the presence of NaBH4 was selected as the model reaction.
Hydrochloroauric acid trihydrate (HAuCl4∙3H2O), caffeic acid, 4-nitrophenol, 4-aminophenol, ninhydrin, acetic acid and NaBH4 were purchased from Sigma-Aldrich (St. Louis, MO, USA). DMSO-d6 was obtained from Armar Chemicals (Dottingen, Switzerland). All other reagents were of analytical grade. TLC analyses were performed using Merck pre-coated TLC plates (silica gel 60 GF254, 0.25 mm, Germany). Syringe filtration was conducted using Minisart RC syringe filters (hydrophilic, 0.2 μm, Sartorius Stedim Biotech GmbH, Goettingen, Germany). All solutions were prepared in deionized water.
UV-visible spectra were acquired on a Shimadzu UV-2600 using a quartz cuvette (Shimadzu Corporation, Kyoto, Japan). Hydrodynamic size measurements by dynamic light scattering were performed using a NanoBrook 90Plus Zeta (Brookhaven Instruments Corporation, Holtsville, NY, USA). A JEOL JEM-3010 TEM operating at an accelerating voltage of 300 kV was used to acquire high-resolution transmission electron microscopy (HR-TEM) images (JEOL Ltd., Tokyo, Japan). The nanoparticle solution was pipetted 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 12 h at room temperature prior to HR-TEM analysis. The crystalline nature of the AuNPs was analyzed using high-resolution X-ray diffraction (HR-XRD) with a Bruker D8 Discover high-resolution X-ray diffractometer in the range of 20° to 90° (2θ scale). HR-XRD was equipped with a Cu-Kα radiation source (λ = 0.154056 nm) (Bruker, Germany). The powdered sample was prepared using a FD8518 freeze-dryer (IlShinBioBase Co. Ltd., Gyeonggi-do, Republic of Korea). A Varian 500 MHz spectrometer was used to acquire 1H-NMR spectra (Palo Alto, CA, USA).
Preparation of CA-AuNPs and cf-CA-AuNPs
Catalytic activity in the 4-NP reduction reaction
A schematic representation of the 4-NP reduction reaction is presented in Fig. 1b. 4-NP (0.4 mM, 1 mL) was mixed with deionized water (1.8 mL) in a quartz cuvette, then added with an aqueous solution of NaBH4 (200 mM, 200 μL). To this mixture, either CA-AuNPs (1 mL) or cf-CA-AuNPs (1 mL) was added as a catalyst. Final concentrations of 4-NP, NaBH4, and Au atoms in 4 mL water were 0.1, 10, and 0.05 mM, respectively. The molarity of Au atoms was calculated based on a final concentration of hydrochloroauric acid trihydrate (0.2 mM). The reaction progress was monitored using a UV-visible spectrophotometer.
Calculation of conversion yield in the 4-NP reduction reaction
A conversion yield from 4-NP to 4-AP was determined using RP-HPLC. Shimadzu RP-HPLC system (CBM-20A) was composed of an autosampler (SIL-20 AC), a pump (LC-20AT), a column oven (CTO-20A), a UV detector (SPD-M20A), and a degassing unit (DGU-20A5R). A Thermo AQUASIL C18 column (150 mm length × 4.6 mm i.d., 5 μm particle size) was used with an isocratic elution. Mobile phase was consisted of 10% acetonitrile in ammonium bicarbonate buffer (10 mM, pH 8.11). The injection volume was 10 μL with a flow rate of 0.8 mL/min. Column oven temperature was set at 30 °C, and UV detection wavelength was at 254 nm. The 4-AP standard was dissolved in deionized water to produce a standard stock solution (50 mM). A calibration curve was established based on six concentrations of 4-AP standard solutions in the range of 0.0125 to 0.2 mM by serial dilution of the standard stock solution with deionized water. A linearity was observed in a concentration range of 0.0125 to 0.2 mM with a regression equation of y = 840160 × + 14095 (r 2 = 0.999). After completion of the reduction reaction of 4-NP to 4-AP by cf-CA-AuNPs, the reaction mixture was filtered using Minisart RC syringe filters (hydrophilic, 0.2 μm) prior to RP-HPLC injection. To calculate the conversion yield, cf-CA-AuNPs that were synthesized using the lowest caffeic acid concentration (0.08 mM) were employed as a catalyst.
Purification and characterization of 4-AP
The mixture was prepared by mixing 4-NP (10 mM, 4.5 mL), an aqueous solution of NaBH4 (300 mM, 7.5 mL) and deionized water (8 mL). To this reaction mixture, CA-AuNPs synthesized with 0.08 mM caffeic acid (2.5 mL) were added, and the mixture was stirred for 20 min. The final concentrations of reagents in deionized water (22.5 mL) were as follows: 4-NP (2 mM), NaBH4 (100 mM), Au atoms (0.022 mM), and caffeic acid (3.56 μM). After completion of the reaction, HCl solution (2 M, 3 mL) was added to quench the reaction. Then, the reaction mixture was neutralized by adding NaOH (1 M, 4.5 mL). To this solution, ethyl ether (30 mL) was added, and a liquid-liquid extraction was conducted. The extraction step was repeated three times. Ethyl ether fractions containing 4-AP were pooled, and moisture was removed using Na2SO4. Then, ethyl ether was evaporated under reduced pressure. A silica gel column chromatography (6.5 cm diameter × 23 cm length) was performed by eluting with solvents composed of hexane:ethyl acetate (2:1→1:2). Each eluent was loaded on a TLC plate together with 4-AP standard and developed with a ninhydrin reagent. The fractions containing 4-AP were pooled, and solvents were evaporated under reduced pressure. Both authentic 4-AP from a commercial source and purified 4-AP from the above procedure were characterized by 1H-NMR. Authentic 4-AP: 1H-NMR (500 MHz, DMSO-d6) δ 8.34 (s, 1H), 6.47–6.40 (dd, 4H, J 1 = 30.5 Hz, J 2 = 8.5 Hz), 4.37 (s, 2H); Purified 4-AP: 1H-NMR (500 MHz, DMSO-d6) δ 8.34 (s, 1H), 6.47–6.34 (dd, 4H, J 1 = 30.5 Hz, J 2 = 8.5 Hz), 4.42 (s, 2H).
Results and discussion
Green synthesis of CA-AuNPs with various concentrations of caffeic acid
CA-AuNPs and cf-CA-AuNPs synthesized using various concentrations of caffeic acid
Caffeic acid final concentrations
Particle size (nm) from HR-TEM images
32.61 ± 6.13
31.37 ± 6.06
29.33 ± 5.41
29.99 ± 7.43b
23.73 ± 3.20
25.79 ± 4.73
26.51 ± 5.27
27.46 ± 5.82
29.55 ± 7.43
Number of particles in HR-TEM taken for size measurement
Hydrodynamic size (nm)d
Hydrodynamic size (nm)
Absorbance ratio [cf-CA-AuNPs/CA-AuNPs] × 100 (%)
Centrifugation of CA-AuNPs to generate cf-CA-AuNPs
Catalytic activity in the 4-NP reduction reaction
Induction time and rate constant of the 4-NP reduction reaction in the presence of either CA-AuNPs or cf-CA-AuNPs
Caffeic acid final concentrations
Induction time (s)
Rate constant (s−1)
2.63 × 10-3
1.53 × 10-3
1.26 × 10-3
0.86 × 10-3a
0.49 × 10-3
0.72 × 10-3
0.51 × 10-3
0.44 × 10-3
0.33 × 10-3
5.73 × 10-3
3.40 × 10-3
3.32 × 10-3
3.24 × 10-3
3.14 × 10-3
3.25 × 10-3
3.17 × 10-3
2.55 × 10-3
0.80 × 10-3
Fold of increase
Rate constants revealed that the catalytic activity of cf-CA-AuNPs was higher than that of CA-AuNPs (Table 2 and Fig. 5). The rate constants of CA-AuNPs were in the range of 0.33 × 10-3 to 2.63 × 10-3∙s-1, and there was a decreasing tendency with increasing caffeic acid concentration. The same tendency was also observed in case of cf-CA-AuNPs (0.80 × 10−3–5.73 × 10−3∙s−1). According to these results, removing caffeic acid by centrifugation enhanced the catalytic activities by 2.18–6.41-fold. Interestingly, the rate constant was the highest when the lowest caffeic acid concentration (0.08 mM) was used for the synthesis, i.e., 2.63 × 10−3∙s−1 for CA-AuNPs and 5.73 × 10−3∙s−1 for cf-CA-AuNPs.
As mentioned previously, the induction time was affected by displacement of ligands with substrates on the surface of metallic nanoparticles. As expected, with increasing caffeic acid concentrations, longer induction times were observed for both CA-AuNPs and cf-CA-AuNPs (Table 2). This is because more time was required to displace a higher concentration of caffeic acid with 4-NP on the surface. No induction time (0 s) was observed at low concentrations: 0.08–0.20 mM for CA-AuNPs and 0.08–0.28 mM for cf-CA-AuNPs. At high concentrations, the induction time increased from 180 s (at 0.24 mM) to 1380 s (0.40 mM) for CA-AuNPs. In case of cf-CA-AuNPs, the induction time was remarkably shorter than that of CA-AuNPs. Consequently, the removal of caffeic acid by centrifugation resulted in a fast restructuration on the surface and reduced the induction time, which directly increased the rate constant.
Why did the lowest concentration of caffeic acid show the highest rate constant for both CA-AuNPs and cf-CA-AuNPs? For CA-AuNPs, the rate constant gradually decreased with increasing caffeic acid concentration (Fig. 5). Thus, we are able to state that a major factor affecting the catalytic activity of CA-AuNPs was the caffeic acid concentration. In case of cf-CA-AuNPs, the lowest concentration (0.08 mM) showed the highest rate constant. For intermediate concentrations (0.12–0.32 mM), the rate constants remained almost constant without significant changes. The rate constant began to decrease at 0.32 mM, and the lowest rate constant was found at 0.40 mM. We assumed that caffeic acid was completely removed after centrifugation; thus, the major factor affecting the catalytic activity of cf-CA-AuNPs was most likely the shapes. With the lowest concentration of 0.08 mM, the shapes were spherical and amorphous, and it exhibited the highest rate constant (5.73 × 10−3∙s−1). Regardless of size, the rate constants of spherical shapes were nearly constant (3.17 × 10−3∙s-1–3.40 × 10−3∙s−1). The lowest rate constant was found in sea-urchin-like shapes. The larger the size in the sea-urchin-like shape was, the lower the catalytic activity was; i.e. 0.32 mM (3.17 × 10−3∙s−1, induction time of 300 s), 0.36 mM (2.55 × 10−3∙s−1, induction time of 300 s) and 0.40 mM (0.80 × 10−3∙s−1, induction time of 780 s). Furthermore, the longer induction time was observed with larger sea-urchin-like shapes.
HR-XRD of CA-AuNPs (0.08 mM)
Conversion yield and purification/characterization of 4-AP
Comparison of catalytic activities of green-synthesized AuNPs in 4-NP reduction reaction
Comparison of the catalytic activities of green-synthesized AuNPs in the 4-NP reduction reaction
Reducing agent used for synthesis of AuNPs
Particle size (nm)
Rate constant (s-1)
4-NP (mM) a
NaBH4 (mM) a
Au atoms (mM) a,b
Caffeic acid (cf-CA-AuNPs)
38.61 ± 6.21
5.73 × 10-3
10 (100 equiv.)
0.05 (0.5 equiv.)
In the present study
Punica granatum juice
3.67 × 10-3
15 (300 equiv.)
0.0135 (0.27 equiv.)
Saraca indica bark extract
4.83 × 10-3
15 (300 equiv.)
0.021 (0.42 equiv.)
Aerva lanata leaf extract
2.78 × 10-3
10 (189 equiv.)
0.1383 (2.61 equiv.)
Prunus domestica extract
20 ± 6
1.90 × 10-3
11.268 (120 equiv.)
0.019 (0.2 equiv.)
1.52 × 10-3
2.269 (67 equiv.)
0.075 (2.22 equiv.)
Bupleurum falcatum root extract
10.5 ± 2.3
0.82 × 10-3
7 (100 equiv.)
0.1 (1.43 equiv.)
In conclusion, the removal of caffeic acid by simple centrifugation significantly enhanced the catalytic activity of CA-AuNPs in 4-NP reduction reaction by up to 6.41-fold. The removal of reducing agents facilitates a fast restructuration process on the surface, reduces induction times, and increases rate constants, enhancing the catalytic activity. In this respect, the current approach can be applied or extended to other metallic nanoparticles to enhance their catalytic activity. Furthermore, we purified 4-AP and confirmed its structure by 1H-NMR. The conversion yield from 4-NP to 4-AP was measured by RP-HPLC with an excellent yield of 99.8%.
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).
YSS performed the green synthesis of CA-AuNPs and cf-CA-AuNPs. EYA and KK acquired 1H-NMR spectra. JP and TYK performed RP-HPLC analyses. JEH had a contribution to purify 4-AP by silica gel column chromatography. YP and 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.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
- Haruta M, Yamada N, Kobayashi T, Iijima S (1989) Gold catalysts prepared by coprecipitation for low-temperature oxidation of hydrogen and of carbon monoxide. J Catal 115:301–9View ArticleGoogle Scholar
- Haruta M (1997) Size-and support-dependency in the catalysis of gold. Catal Today 36:153–66View ArticleGoogle Scholar
- Daniel M-C, Astruc D (2004) Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology. Chem Rev 104:293–346View ArticleGoogle Scholar
- Stratakis M, Garcia H (2012) Catalysis by supported gold nanoparticles: beyond aerobic oxidative processes. Chem Rev 112:4469–506.View ArticleGoogle Scholar
- Takale BS, Bao M, Yamamoto Y (2014) Gold nanoparticle (AuNPs) and gold nanopore (AuNPore) catalysts in organic synthesis. Org Biomol Chem 12:2005–27View ArticleGoogle Scholar
- Li M, Chen G (2013) Revisiting catalytic model reaction p-nitrophenol/NaBH4 using metallic nanoparticles coated on polymeric spheres. Nanoscale 5:11919–27View ArticleGoogle Scholar
- Chiou J-R, Lai B-H, Hsu K-C, Chen D-H (2013) One-pot green synthesis of silver/iron oxide composite nanoparticles for 4-nitrophenol reduction. J Hazard Mater 248–249:394–400View ArticleGoogle Scholar
- Zhao B, Mele G, Pio I, Li J, Palmisano L, Vasapollo G (2010) Degradation of 4-nitrophenol (4-NP) using Fe–TiO2 as a heterogeneous photo-Fenton catalyst. J Hazard Mater 176:569–74View ArticleGoogle Scholar
- Nardini M, D’Aquino M, Tomassi G, Gentili V, Di Felice M, Scaccini C (1995) Inhibition of human low-density lipoprotein oxidation by caffeic acid and other hydroxycinnamic acid derivatives. Free Radic Biol Med 19:541–52View ArticleGoogle Scholar
- Setzer WN, Setzer MC, Bates RB, Nakkiew P, Jackes BR, Chen L et al (1999) Antibacterial hydroxycinnamic esters from Piper caninum from Paluma, north Queensland, Australia. The crystal and molecular structure of (+)-bornyl coumarate. Planta Med 65:747–9View ArticleGoogle Scholar
- Fiuza SM, Gomes C, Teixeira LJ, Girao Da Cruz MT, Cordeiro MN, Milhazes N et al. (2004) Phenolic acid derivatives with potential anticancer properties––a structure–activity relationship study. Part 1: Methyl, propyl and octyl esters of caffeic and gallic acids. Bioorg Med Chem 12:3581–9View ArticleGoogle Scholar
- Sud’Ina GF, Mirzoeva OK, Pushkareva MA, Korshunova GA, Sumbatyan NV, Varfolomeev SD (1993) Caffeic acid phenethyl ester as a lipoxygenase inhibitor with antioxidant properties. FEBS Lett 329:21–4View ArticleGoogle Scholar
- Chen JH, Ho C-T (1997) Antioxidant activities of caffeic acid and its related hydroxycinnamic acid compounds. J Agric Food Chem 45:2374–8View ArticleGoogle Scholar
- King PJ, Ma G, Miao W, Jia Q, McDougall BR, Reinecke MG et al (1999) Structure-activity relationships: analogues of the dicaffeoylquinic and dicaffeoyltartaric acids as potent inhibitors of human immunodeficiency virus type 1 integrase and replication. J Med Chem 42:497–509View ArticleGoogle Scholar
- Lin L-C, Kuo Y-C, Chou C-J (1999) Immunomodulatory principles of Dichrocephala bicolor. J Nat Prod 62:405–8View ArticleGoogle Scholar
- Kim SR, Kim YC (2000) Neuroprotective phenylpropanoid esters of rhamnose isolated from roots of Scrophularia buergeriana. Phytochemistry 54:503–9View ArticleGoogle Scholar
- Kim JH, Lavin BW, Boote BW, Pham JA (2012) Photothermally enhanced catalytic activity of partially aggregated gold nanoparticles. J Nanopart Res 14:995 (10pp)View ArticleGoogle Scholar
- You JG, Shanmugam C, Liu YW, Yu CJ, Tseng WL (2017) Boosting catalytic activity of metal nanoparticles for 4-nitrophenol reduction: modification of metal nanoparticles with poly(diallyldimethylammonium chloride. J Hazard Mater 324(Pt B):420–7Google Scholar
- Lim SH, Ahn EY, Park Y (2016) Green synthesis and catalytic activity of gold nanoparticles synthesized by Artemisia capillaris water extract. Nanoscale Res Lett 11:474 (11pp)View ArticleGoogle Scholar
- Wunder S, Polzer F, Lu Y, Mei Y, Ballauff M (2010) Kinetic analysis of catalytic reduction of 4-nitrophenol by metallic nanoparticles immobilized in spherical polyelectrolyte brushes. J Phys Chem C 114:8814–20View ArticleGoogle Scholar
- Wunder S, Lu Y, Albrecht M, Ballauff M (2011) Catalytic activity of faceted gold nanoparticles studied by a model reaction: evidence for substrate-induced surface restructuring. ACS Catal 1:908–16View ArticleGoogle Scholar
- Ciganda R, Li N, Deraedt C, Gatard S, Zhao P, Salmon L et al (2014) Gold nanoparticles as electron reservoir redox catalysts for 4-nitrophenol reduction: a strong stereoelectronic ligand influence. Chem Commun 50:10126–9Google Scholar
- Kim H-S, Seo YS, Kim K, Han JW, Park Y, Cho S (2016) Concentration effect of reducing agents on green synthesis of gold nanoparticles: size, morphology, and growth mechanism. Nanoscale Res Lett 11:230 (9pp)Google Scholar
- Dash SS, Bag BG (2014) Synthesis of gold nanoparticles using renewable Punica granatum juice and study of its catalytic activity. Appl Nanosci 4:55–9View ArticleGoogle Scholar
- Dash SS, Majumdar R, Sikder AK, Bag BG, Patra BK (2014) Saraca indica bark extract mediated green synthesis of polyshaped gold nanoparticles and its application in catalytic reduction. Appl Nanosci 4:485–90View ArticleGoogle Scholar
- Joseph S, Mathew B (2015) Microwave assisted facile green synthesis of silver and gold nanocatalysts using the leaf extract of Aerva lanata. Spectrochim Acta Mol Biomol Spectrosc 136:1371–9View ArticleGoogle Scholar
- Dauthal P, Mukhopadhyay M (2012) Prunus domestica fruit extract-mediated synthesis of gold nanoparticles and its catalytic activity for 4-nitrophenol reduction. Ind Eng Chem Res 51:13014–20View ArticleGoogle Scholar
- Choi Y, Choi M-J, Cha S-H, Kim YS, Cho S, Park Y (2014) Catechin-capped gold nanoparticles: green synthesis, characterization, and catalytic activity toward 4-nitrophenol reduction. Nanoscale Res Lett 9:103 (8pp)View ArticleGoogle Scholar
- Lee YJ, Cha SH, Lee KJ, Kim YS, Park Y (2015) Plant extract (Bupleurum falcatum) as a green factory for biofabrication of gold nanoparticles. Nat Prod Commun 10:1593–6Google Scholar
- Seo YS, Cha SH, Cho S, Yoon HR, Kang YH, Park Y (2015) Caffeic acid: potential applications in nanotechnology as a green reducing agent for sustainable synthesis of gold nanoparticles. Nat Prod Commun 10:627–30Google Scholar