Catalytic reduction of 4-nitrophenol with gold nanoparticles synthesized by caffeic acid
Seo et al. Nanoscale Research Letters
Catalytic reduction of 4-nitrophenol with gold nanoparticles synthesized by caffeic acid
Yu Seon Seo 0
Eun-Young Ahn 0
Jisu Park 0
Tae Yoon Kim 0
Jee Eun Hong 0
Kyeongsoon Kim 1
Yohan Park 0
Youmie Park 0
0 College of Pharmacy and Inje Institute of Pharmaceutical Sciences and Research, Inje University , 197 Inje-ro, Gimhae, Gyeongnam 50834 , Republic of Korea
1 Department of Pharmaceutical Engineering, Inje University , 197 Inje-ro, Gimhae, Gyeongnam 50834 , Republic of Korea
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.
Gold nanoparticles; Caffeic acid; Catalytic activity; 4-Nitrophenol reduction reaction; Centrifugation
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Background
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 [1].
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-tovolume ratio of AuNPs leads to increase in chemical
reactivity [5]. 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 [5].
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 [6]. 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
© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
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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 [17]. 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, (...truncated)