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 - 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 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. 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)