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Effect of Phenolic Compounds on the Synthesis of Gold Nanoparticles and Its Catalytic Activity in the Reduction of Nitro Compounds Elisabete C. B. A. Alegria 1,2, * ID , Ana P. C. Ribeiro 1, * ID , Marta Mendes 1,2 , Ana M. Ferraria 3 Ana M. Botelho do Rego 3 ID and Armando J. L. Pombeiro 1, * 1 2 3

*

ID

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Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal; [email protected] Chemical Engineering Departament, ISEL-Instituto Superior de Engenharia de Lisboa, Instituto Politécnico de Lisboa, 1959-007 Lisboa, Portugal CQFM-Centro de Química-Física Molecular and IN-Institute for Nanosciences and Nanotechnologies and IBB-Institute for Bioengineering and Biosciences, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal; [email protected] (A.M.F.); [email protected] (A.M.B.d.R.) Correspondence: [email protected] (E.C.B.A.A.); [email protected] (A.P.C.R.); [email protected] (A.J.L.P.)  

Received: 17 April 2018; Accepted: 8 May 2018; Published: 10 May 2018

Abstract: Gold nanoparticles (AuNPs) were prepared using an eco-friendly approach in a single step by reduction of HAuCl4 with polyphenols from tea extracts, which act as both reducing and capping agents. The obtained AuNPs were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet–visible spectroscopy (UV–vis), and X-ray photoelectron spectroscopy (XPS). They act as highly efficient catalysts in the reduction of various aromatic nitro compounds in aqueous solution. The effects of a variety of factors (e.g., reaction time, type and amount of reducing agent, shape, size, or amount of AuNPs) were studied towards the optimization of the processes. The total polyphenol content (TPC) was determined before and after the catalytic reaction and the results are discussed in terms of the tea extract percentage, the size of the AuNPs, and their catalytic activity. The reusability of the AuNP catalyst in the reduction of 4-nitrophenol was also tested. The reactions follow pseudo first-order kinetics. Keywords: gold nanoparticles; phytochemicals; catalysis; reduction; nitro compounds

1. Introduction Metallic nanoparticles (NPs) are often synthesized using chemical methodologies involving organic and inorganic reducing agents, which can also act as stabilizing agents to avoid the coalescence of the NPs, such as hydrazine, sodium borohydride (NaBH4 ), or N,N-dimethylformamide [1]. The environmental risks and toxicity associated with these chemicals have led to a growing interest in environmentally friendly and sustainable methods for the synthesis of a variety of metallic NPs with specific sizes and aggregations [2]. Some of these new “green” synthetic processes concern microorganisms, such as bacteria [3,4] or fungi [5], and plant extracts [6–12]. The use of phytochemicals in the synthesis of NPs, which include leaves, fruits, seeds, and stems, is starting to be developed and presents some advantages over the application of microorganisms in view of the high diversity of plant extracts and the simplicity and low cost of the method [2,6–10]. Gold nanoparticles (AuNPs) are recently gaining a great interest due to their potential application in many fields, such as catalysis, biosensing, photonics, drug-delivery systems, and antimicrobial agents [13–19]. They also have been used to catalyze electron-transfer and oxidation reactions, Nanomaterials 2018, 8, 320; doi:10.3390/nano8050320

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specifically glucose oxidation [20], aerobic alcohol oxidation [21], reduction of nitroarenes in aqueous media [22], or CO oxidation and propylene epoxidation [23]. AuNPs produced from vegetable oil in a green reaction medium have also been used in antimicrobial paints [24]. Due to its health benefits and antioxidant properties, the use of black tea [7] for the production of biocompatible AuNPs is being tested in diverse areas such as health and energy. The phytochemicals present in tea show a dual role: as reducing agents to reduce gold and as stabilizers to provide a robust coating on the AuNPs in a one-pot process. In this work, we used a black tea extract to act as a low-cost reducing and stabilizing agent for AuNPs synthesis, determined the variation of the total polyphenol content (TPC) and the atomic concentration of each element detected by XPS after the AuNPs were produced, and explored the catalytic properties of the AuNPs synthesized via tea extract solutions in simple model reactions; that is, the reduction of various nitro compounds (2- and 4-nitrophenol; 2-, 3-, and 4-nitroaniline, or nitrobenzene) in aqueous solution. In particular, the reduction of 4-nitrophenol has become the model reaction for the evaluation of the catalytic activity of metallic NPs, namely with Ag [25,26], Au [27–32], or Pd [33] metals. The existence of a well-defined absorption band characteristic of this substrate allows to easy screening of the reaction by UV–vis spectroscopy [34–36]. 2. Materials and Methods 2.1. Reagents and Instrumentation All synthetic work was performed in air. The reagents and solvents were obtained from commercial sources and used without further purification or drying. Black tea from Tetley (Yorkshire, UK); 4-nitrophenol (4-NP) (Acros Organics, Morris Plains, NJ, USA); 2-nitrophenol (2-NP) (BDH, St. Louis, MO, USA); nitrobenzene (NB) (Acros); 4-aminophenol (4-AP) (Aldrich, St. Louis, MO, USA); 2-aminophenol (2-AP) (BDH); 2-, 3-, and 4-nitroaniline (2-, 3-, and 4-NA) (Fluka, Bucharest, Romania); 2-, 3-, and 4-phenylenediamine (2-, 3-, and 4-PD) (Fluka, Bucharest, Romania); NaBH4 (Acros Organics, Morris Plains, NJ, USA); ascorbic acid (Merck, Darmstadt, Germany); D-glucose (Aldrich, St. Louis, MO, USA); NH4 Cl (Panreac, Barcelona, Spain); N2 H4 (Aldrich, Missouri, USA); and tetrachloroauric(III) acid (HAuCl4 ·3H2 O) (99.99%, Aldrich, St. Louis, MO, USA) were used as received. The synthesized AuNPs using tea extracts were characterized using UV–vis, XPS, XRD, SEM, EDS, and TEM techniques. UV–visible spectroscopic measurements of the synthesized AuNPs were carried out on a PerkinElmer (Massachusetts, EUA) Lambda 750 UV–visible spectrophotometer. XPS analysis was performed by a XSAM800 spectrometer from KRATOS, Manchester, UK. Details on operation conditions, data acquisition, and treatment were published elsewhere [37], except for the energy reference used to correct the charge shift which was the binding energy of aromatic carbons centered at 284.7 eV. TEM measurements were performed on a transmission electron microscope Hitachi 8100 (Tokyo, Japan) with ThermoNoran light elements EDS detector and digital image acquisition. Morphology and distribution of AuNPs were characterized using a scanning electron microscope (SEM) (JEOL 7001F with Oxford light elements EDS detector and EBSD detector). FTIR (Fourier transform) measurements were carried out on a Bruker Vertex 40 Raman/IR spectrometer (Massachusetts, EUA) in a range from 4000 to 100 cm−1 . 2.2. Preparation of Au Nanoparticles Using Tea Extracts The preparation of AuNPs using tea extracts followed the method described previously [15]. The tea solutions were prepared by mixing a weighted amount of dried black tea leaves (2 g) with 100 mL of distilled water followed by vigorous stirring for 15 min at room temperature in order to obtain three solutions with different percentages of tea (1, 5, or 10% w/v). After filtration, 6 mL of each tea solution was transferred to a glass vial and to each of them 0.1 mL of a tetrachloroauric

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acid (HAuCl4 ·3H2 O) solution (0.1 M) was added, with stirring for 20 min at room temperature. The solution changed from pale yellow to red color almost instantaneously, indicating the formation of Au nanoparticles. UV–vis spectroscopy was used to confirm the successful formation of the AuNPs on account of the occurrence of the characteristic surface plasmon resonance (SPR) band of AuNPs (538 nm) in the visible region [22,38–40]. X-ray photoelectron spectroscopy analysis confirms the existence of gold and its metallic state in the AuNPs for the 1% tea extract. 2.3. Determination of Total Polyphenol Content in Tea by the Folin–Ciocalteu Method The total polyphenol content (TPC) was determined colorimetrically using the Folin–Ciocalteu phenol reagent and gallic acid as the calibration standard following the reported method [41]. The absorbance of the various reducing media was measured by a UV–vis spectrophotometer at a wavelength of 765 nm, and a calibration curve was constructed with 10 gallic acid standard solutions (12.5–500 mg/L; R2 = 0.9996). 2.4. Reduction of Nitro Compounds in Aqueous Solution All catalytic reactions were performed in a standard 3 mL quartz cuvette with a 1 cm path length. Stock aqueous solutions of 4-nitrophenol (0.012 M) and NaBH4 (0.1 M) were freshly prepared for each experiment to minimize hydrolysis. Stock aqueous solutions of 2.5 mL of water and 0.5 mL AuNP tea solution (1, 5, or 10%) were prepared. In standard conditions, 0.5 mL of the diluted AuNP solution is placed in the cuvette and 2.5 mL of distilled water is added. To this solution, 100 µL of the nitro compound (2- or 4-nitrophenol; 2-, 3-, or 4-nitroaniline; or nitrobenzene) and finally 50 µL NaBH4 solution (0.1 M) was added. The UV–vis absorbance spectra were recorded in the 200–700 nm range at 2 min intervals. The catalytic activity was quantified in terms of conversion of the substrate and turnover frequency (TOF), considering that 2- or 4-aminophenol; 2-, 3-, or 4-phenylenediamine; and aniline are the only reduction products. 3. Results and Discussion 3.1. Au Nanoparticle Identification The addition of HAuCl4 ·3H2 O to a black tea extract resulted in an almost instantaneous change of color from light yellow to red, which indicates [22,38–40] the effectiveness of the Au reduction. As expected, the obtained Au colloid solution exhibits in the UV–vis spectrum the typical [22,38–40] absorption band due to the presence of Au nanoparticles (AuNPs), without any significant shift or change in the intensity over time. The effect of different concentrations of tea extract (1, 5, and 10% w/v) solutions on the size and dispersity of the AuNPs was analyzed (onwards, the AuNPs solutions will be named as AuNPs 1% tea, AuNPs 5% tea, and AuNPs 10% tea, respectively). Using different concentrations of tea extract (1, 5, and 10%) enables the variation of the size of the produced nanoparticles and as a result, the colors of their dispersion (Figure 1 and Figure S1). The surface plasmon resonance (SPR) bandwidth for AuNPs 1, 5, and 10% tea follows the predicted behavior, as it increases with the increasing size of gold nanoparticles [42–44]. In fact, the shorter bandwidth for the AuNPs 1% tea solution at 538 nm confirms the smaller size of these NPs (8–24 nm diameter range, see below), relative to the absorption band at 566 nm observed for the AuNPs 5% tea solution or that at ca. 602 nm for the larger-sized AuNPs 10% tea (57–113 nm, see below) (Figure 1).

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Figure 1. 1. Visible Visible absorbance absorbance spectra spectra for for AuNPs AuNPs produced produced using 1, 5, and 10% tea solutions. Figure 1. Visible absorbance spectra for AuNPs produced using using 1, 1, 5, 5, and and 10% 10% tea tea solutions. solutions. Figure

X-ray photoelectron photoelectron the Au 4f 4f region, a single doublet with the the Au photoelectronspectroscopy spectroscopy(XPS) (XPS)exhibits, exhibits,inin in the Au 4f region, single doublet with the X-ray spectroscopy (XPS) exhibits, the Au region, aa single doublet with 4f component centered at at 84.4 ±±±0.3 Au 4f7/2 7/2 component component centered at 84.4 0.3eV eV(Figure (Figure2). 2).This Thisdoublet doubletshows shows that that AuNPs AuNPs in in the the 1% 1% tea tea Au 4f centered 84.4 0.3 eV (Figure 2). This doublet shows 7/2 3+ 00 and/or Au+++ and that the precursor no longer 3+ reduction to Au 3+ 0 extract result from the Au in from the Au Au and/or Au and that the precursor no longer exists extract Au reduction and/or Au and that the precursor no longer exists in 0 0 0 the solution [45].It worthto tonotice noticethat thatalthough althoughthe thebinding binding energy for bulky Auis is established solution [45]. [45]. ItItis isisworth worth to notice that although the binding energy for bulky Au is established established to the energy for bulky Au to to centered 83.95 [46], drifts largeror lowerbinding bindingenergies energieshave havebeen beenreported reported for be be centered at at 83.95 eVeV [46], drifts totolarger larger ororlower lower binding energies have been reported for very very be centered at 83.95 eV [46], drifts to small gold gold nanoparticles nanoparticles [47]. [47]. Besides (0.1%), the the surface surface also shows potassium potassium (1.7%), (1.7%), chlorine Besides Au Au (0.1%), small (2.6%), nitrogen nitrogen (2.7%), (2.7%), oxygen oxygen (29.4%), (29.4%), and and carbonaceous carbonaceous species species (63.5%), (63.5%), probably probably due due to to the the tea tea (2.6%), extract. It that in in XPS, thethe atomic percentage depends on the state. It is isworth worthtoto toemphasize emphasize that in XPS, the atomic percentage depends onaggregation the aggregation aggregation extract. is worth emphasize that XPS, atomic percentage depends on the Therefore, for anfor element in thein form of nanoparticles, the atomic percentage is much lowerlower than state. Therefore, Therefore, for an element element in the form form of nanoparticles, nanoparticles, the atomic atomic percentage is much much lower state. an the of the percentage is what it would be if the atomsatoms were were dispersed on the This explains why why the than what what would be if ifcomponent the component component atoms were dispersed onsurface. the surface. surface. This explains explains whygold the than itit would be the dispersed on the This the percentage is the lowest one. gold percentage is the lowest one. gold percentage is the lowest one.

Intensity Intensity (c.p.s) (c.p.s)

Au 4f Au 4f 4f5/2 Au 5/2

94 94

92 92

90 90

88 88

Au 4f 4f7/2 Au 7/2

86 86

84 84

82 82

80 80

Binding Energy Energy (eV) (eV) Binding Figure 2. X-ray photoelectron spectroscopy (XPS) Au Au 4f region for for the the AuNPs AuNPs 1% 1% tea. tea. Figure Figure 2. 2. X-ray X-ray photoelectron photoelectron spectroscopy spectroscopy (XPS) (XPS) Au 4f 4f region region for the AuNPs 1% tea.

3.2. Characterization Characterization of AuNPs 3.2. 3.2. Characterization of of AuNPs AuNPs Transmission electron electron microscopy (TEM) wastoused used to determine determine thethedetails details of the the Transmission electron microscopy (TEM) was to the of Transmission microscopy (TEM) was used determine the details of microstructure microstructure in the samples (Figure 3). The AuNPs produced using a 1% (Figure 3a) or a 10% microstructure in the samples (Figure 3). The AuNPs produced using a 1% (Figure 3a) or a 10% in the samples (Figure 3). The AuNPs produced using a 1% (Figure 3a) or a 10% (Figure 3b) tea extract (Figure 3b) tea tea extract are spherical spherical andno dispersed with nearly nearly no aggregations, which was mainly mainly (Figure 3b) extract are and dispersed with aggregations, was are spherical and dispersed with nearly aggregations, whichno was mainly due which to the stabilization due to toofthe the stabilization effect ofdistribution the tea tea extract. extract. The size size distribution histograms of the the AuNPs due effect the The distribution histograms of AuNPs effect thestabilization tea extract. The sizeof histograms of the AuNPs (Figure 3c,d) were calculated (Figure 3c,d) were calculated from the corresponding TEM image in Figure 3a,b by measuring the (Figure 3c,d) were calculated from the corresponding TEM image in Figure 3a,b by measuring the from the corresponding TEM image in Figure 3a,b by measuring the particles in the image; the average particles in the image; the average particle sizes being in the 8–24 nm and 57–113 nm ranges, particles in the image; average being respectively. in the 8–24 nm and 57–113 nm ranges, particle sizes being in thethe 8–24 nm andparticle 57–113 sizes nm ranges, respectively. respectively. The analysis of the AuNPs by energy dispersive X-ray spectroscopy (EDS) confirmed the The analysis analysis of the the characteristic AuNPs by by energy energy dispersive X-ray spectroscopy spectroscopy (EDS) confirmed the the The of AuNPs dispersive X-ray (EDS) confirmed presence of the signals of gold, demonstrating the successful immobilization of presence of the signals characteristic of gold, demonstrating the successful immobilization of AuNPs presence of the signals characteristic of gold, demonstrating the successful immobilization AuNPs AuNPs (Electronic Supplementary Information, ESI, Figure S2). Figure 4 shows the ofscanning (Electronic Supplementary Information, ESI, Figure Figure S2). S2). Figure 44 Moreover, shows the thethe scanning electron (Electronic Supplementary ESI, Figure shows scanning electron electron microscopy (SEM) Information, images of typical samples of AuNPs. combination of microscopy (SEM) images of typical samples of AuNPs. Moreover, the combination of SEM and microscopy (SEM) images of typical samples of AuNPs. Moreover, the combination of SEM and TEM images images allows allows us us to to conclude conclude that that the the use use of of different different concentrations concentrations of of tea tea extract extract produced produced TEM

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SEM and TEMwith images allows us to conclude theare usesmaller of different concentrations extract nanoparticles different sizes. The AuNPs that 1% tea than the AuNPs 10% of tea,tea although nanoparticles with different sizes. The AuNPs 1% tea are smaller than thethan AuNPs 10% tea, although produced nanoparticles with different sizes. The AuNPs 1% tea are smaller the AuNPs 10% tea, both are well dispersed throughout the tea extract. The small white points in Figure 4b are more both are well dispersed throughout the tea extract. The small white points in Figure 4b are more although are well dispersed The small white points Figure4a), 4b the are clear thanboth in Figure 4a due to the throughout diference inthe thetea sizeextract. of AuNPs. For AuNPs 1% tea in (Figure clear than in Figure 4a due to the diference in the size of AuNPs. For AuNPs 1% tea (Figure 4a), the more clear than in Figure to the diference size of 1% tea (Figure average size of the AuNPs4aisdue smaller than for thatinofthe th 10% teaAuNPs. extract.For ThisAuNPs is in accordance with 4a), the average size of the AuNPs is smaller than for that of th 10% tea extract. This is in accordance with the the size of theproduced AuNPs isfrom smaller that ofThe th 10% teatea extract. is in accordance with sizeaverage distribution plot thethan TEMfor results. dried extractThis appears as large chunks size distribution plot produced from the TEM results. The dried tea extract appears as large chunks the sizethe distribution plot produced from the TEM results. The dried tea extract appears as large chunks where AuNPs are embedded. where AuNPs embedded. where thethe AuNPs areare embedded.

Figure 3. TEM images and size distribution of AuNPs 1% tea (a,c) and AuNPs 10% tea (b,d). Figure 3. TEM images andand sizesize distribution of AuNPs 1% 1% tea tea (a,c)(a,c) andand AuNPs 10%10% tea tea (b,d). Figure 3. TEM images distribution of AuNPs AuNPs (b,d).

Figure 4. 4. Scanning Scanning electron electron microscopy microscopy (SEM) (SEM) images images of Figure of (a) (a) AuNPs AuNPs 1% 1% tea tea and and (b) (b) AuNPs AuNPs 10% 10% tea. tea. Figure 4. Scanning electron microscopy (SEM) images of (a) AuNPs 1% tea and (b) AuNPs 10% tea.

FTIR spectra (ESI, Figure S3) were run to identify functional groups groups on on the the synthesized synthesized AuNPs’ AuNPs’ FTIR spectra (ESI, Figure S3) were run to identify functional groups on the synthesized AuNPs’ recorded for for the the dried dried tea tea extract extract (before (beforereaction reactionwith withHAuCl HAuCl44·∙3H surface. They were recorded 3H22O) and for the surface. They were recorded for the dried tea extract (before reaction with HAuCl4∙3H2O) and for the analogy. synthetized AuNPs, and showed a great analogy. synthetized AuNPs, and showed a great analogy. Both FTIR spectra displayed a strong and broad signal at 3432 cm−1 (hydroxyl group of alcohols Both FTIR spectra displayed a strong and broad signal at 3432 cm−1 (hydroxyl group of alcohols or phenols) and a much weaker one at 2926 cm−1 (aliphatic CH 2 asymmetric stretching). The band at or phenols) and a much weaker one at 2926 cm−1 (aliphatic CH2 asymmetric stretching). The band at 1700–1637 cm−1 is−1due to ν(C=O) of conjugated ketones, aldehydes, quinines, and esters, whereas that 1700–1637 cm is due to ν(C=O) of conjugated ketones, aldehydes, quinines, and esters, whereas that

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Nanomaterials 2018, 8, xdisplayed FOR PEER REVIEW Both FTIR spectra a strong and broad signal at 3432 cm−1 (hydroxyl group of alcohols6 of 18 or phenols) and a much weaker one at 2926 cm−1 (aliphatic CH2 asymmetric stretching). The band at at ca. 1406 cm−1 is assigned to the deformation of –CH3 of alkyl groups from the tea constituents. This 1700–1637 cm−1 is due to ν(C=O) of conjugated ketones, aldehydes, quinines, and esters, whereas that could be due to the existence of bioactive molecules in the tea in the absence and in the presence of at ca. 1406 cm−1 is assigned to the deformation of –CH3 of alkyl groups from the tea constituents. the AuNPs’ surface. This could be due to the existence of bioactive molecules in the tea in the absence and in the presence of the AuNPs’ surface. 3.3. Determination of Total Polyphenol Content

3.3. Determination of Totaltea Polyphenol Content The different extracts (1, 5, and 10%) as well as the produced AuNPs solutions (AuNPs 1% tea, different AuNPs 5% and AuNPs tea, before and aftersolutions the catalytic reduction The teatea, extracts (1, 5, and10% 10%) asrespectively), well as the produced AuNPs (AuNPs 1% of 4-nitrophenol, were analyzed for their total polyphenol content (TPC) (Figure 5). The tea, AuNPs 5% tea, and AuNPs 10% tea, respectively), before and after the catalytic reduction of total polyphenol concentration the total prepared tea stock solutions 1.18 g/L w/v solution), 3.28 4-nitrophenol, were analyzed forintheir polyphenol content (TPC)is(Figure 5).(for The1% total polyphenol g/L (for in 5%the w/v solution), g/L (for 10%g/L w/v(for solution), respectively, concentration prepared tea and stock3.55 solutions is 1.18 1% w/v solution), 3.28and g/L as (forexpected, 5% proportional to the amount of tea leaves used to prepare those aqueous solutions (Figure 1, w/v solution), and 3.55 g/L (for 10% w/v solution), respectively, and as expected, proportional 5, toentries the 4, and 7). leaves used to prepare those aqueous solutions (Figure 5, entries 1, 4, and 7). amount of tea The polyphenol levels present in the extract containing the producedAuNPs AuNPs(0.46, (0.46,2.14, 2.14, and The polyphenol levels present in the tea tea extract containing the produced 2.84 g/L for AuNPs 1% tea, AuNPs 5% tea, and AuNPs 10% tea, respectively) was determined and 2.84 g/L for AuNPs 1% tea, AuNPs 5% tea, and AuNPs 10% tea, respectively) was determinedand a relative toto the initial content (Figure 5, entries 2, 5, and areduction reductionofof61, 61,35, 35,and and20% 20%was wasobserved observed relative the initial content (Figure 5, entries 2, and 5, 8). As shown, there is a correlation between the percentage of polyphenols consumed and the size and 8). As shown, there is a correlation between the percentage of polyphenols consumed and the sizeof the As the thesize sizeofofthe theAuNPs AuNPs decreased, amount of polyphenol of theproduced produced nanoparticles. nanoparticles. As decreased, thethe amount of polyphenol usedused for for their formation increased (Figure 5). After the catalytic reduction of 4-NP in the presence of AuNPs, their formation increased (Figure 5). After the catalytic reduction of 4-NP in the presence of AuNPs, the resulting solutions were analyzed and a slight decrease of polyphenol amount was detected the resulting solutions were analyzed and a slight decrease of polyphenol amount was detected for the for the5% AuNPs 5% and10% AuNPs 10% solutions (Figure 9 in comparison with entries AuNPs and AuNPs solutions (Figure 5, entries5,6entries and 9 6inand comparison with entries 5 and 8,5 and 8, respectively). respectively).

Figure 5. Concentration of total polyphenols in solution (g/L) (4-NP = 4-Nitrophenol).

Figure 5. Concentration of total polyphenols in solution (g/L) (4-NP = 4-Nitrophenol).

Surprisingly, for the AuNPs 1% tea solution, an important increase of polyphenols was detected Surprisingly, for the 1% tea5,solution, important increase of polyphenols after the catalytic reduction ofAuNPs 4-NP (Figure entry 3 an in comparison with entry 2). This factwas candetected be after the catalytic reduction of 4-NP of (Figure 5, entryunder 3 in comparison with entry 2). Thisand fact(ii) can be attributed to (i) conceivable regeneration polyphenols catalytic reaction conditions attributed to (i) conceivable regeneration of polyphenols under catalytic reaction conditions and (ii) possible interference of the small AuNPs with the polyphenol measurements [48]. possible interference of the small AuNPs with the polyphenol measurements [48]. The surface plasmon resonance wavelength of AuNPs is strongly dependent on the size of individual AuNPs. The enhanced surface plasmon resonance effect of AuNPs is a function of the

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The surface plasmon resonance wavelength of AuNPs is strongly dependent on the size of average size and size distribution. The increasing content of polyphenols after the reaction can be individual AuNPs. The enhanced surface plasmon resonance effect of AuNPs is a function of the attributed to organic compounds present in the extract and not involved in the characteristic Folin– average size and size distribution. The increasing content of polyphenols after the reaction can Ciocalteu reaction, resulting in an overestimation. be attributed to organic compounds present in the extract and not involved in the characteristic Folin–Ciocalteu reaction, resulting in an overestimation. 3.4. Catalytic Studies 3.4. Catalytic Studies activity of the prepared AuNPs was investigated for the reduction of various The catalytic aromatic nitro compounds, 2- and AuNPs 4-nitrophenol (2- and 4-NP); 3-,reduction and 4-nitroaniline (2-, The catalytic activity of namely the prepared was investigated for2-, the of various 3-, and 4-NA); and nitrobenzene (NB) in aqueous medium. aromatic nitro compounds, namely 2- and 4-nitrophenol (2- and 4-NP); 2-, 3-, and 4-nitroaniline (2-, 3-, and 4-NA); and nitrobenzene (NB) in aqueous medium. 3.4.1. Reduction of 4-Nitrophenol in Aqueous Solution 3.4.1. Reduction of 4-Nitrophenol in were Aqueous Solutionin a standard 3 mL quartz cuvette with a 1 cm Catalytic reduction reactions performed path length.reduction The UV–vis absorbance spectra were recorded3 mL in the 200–700 nmwith range at 2path min Catalytic reactions were performed in a standard quartz cuvette a 1 cm intervals. A shift of the corresponding absorbance peak at 315 nm to 400 nm occurred immediately length. The UV–vis absorbance spectra were recorded in the 200–700 nm range at 2 min intervals. addition of NaBH4.absorbance This new peak is attributed [27–32] immediately to the 4-nitrophenolate Aafter shiftthe of the corresponding peak(at at 400 315 nm) nm to 400 nm occurred after the ion formed in the presence of NaBH 4 . addition of NaBH4 . This new peak (at 400 nm) is attributed [27–32] to the 4-nitrophenolate ion formed The decrease of the intensity of the latter peak readily started in the presence of AuNPs solution in the presence of NaBH 4. andThe wasdecrease monitored by intensity UV–vis spectroscopy recording the absorbance spectra 2 min of the of the latter upon peak readily started in the presence of at AuNPs intervals (Figure 6 for 4-NP reduction in the presence of AuNPs 1% tea). The catalytic reduction solution and was monitored by UV–vis spectroscopy upon recording the absorbance spectra at 2 minof 4-nitrophenol to 4-aminophenol widely applied as an industrial intervals (Figure(4-NP) 6 for 4-NP reduction in the(4-AP), presence of AuNPs 1% tea). The catalytic chemical reduction and of metabolite of common household analgesics such as paracetamol [49], was originally applied by Pal 4-nitrophenol (4-NP) to 4-aminophenol (4-AP), widely applied as an industrial chemical and metabolite et al. [35] and Esumi et al. [36] to test the catalytic activity of free and immobilized NPs. More of common household analgesics such as paracetamol [49], was originally applied by Pal et al. [35] and recently, established as free a model reaction to evaluate the catalytic Esumi et al.this [36]system to test has the been catalytic activity of and immobilized NPs. More recently, performance this system of different monoand bimetallic nanoparticles [35,50]. has been established as a model reaction to evaluate the catalytic performance of different mono- and bimetallic nanoparticles [35,50].

Figure 6. UV–vis spectra over the course of the reduction of 4-nitrophenol (4-NP) by AuNPs 1% tea. Figure 6. UV–vis spectra over the course of the reduction of 4-nitrophenol (4-NP) by AuNPs 1% tea. Reaction conditions: [AuNPs] = 3.2 × 10−5−5 M; [4-NP] = 3.8 ×−510−5 M; [NaBH4 ] = 1.6−3× 10−3 M. Reaction conditions: [AuNPs] = 3.2 × 10 M; [4-NP] = 3.8 × 10 M; [NaBH4] = 1.6 × 10 M. Starting Starting AuNPs tea extract aqueous solution (dark solid line); upon addition of 4-NP (dark dashed AuNPs tea extract aqueous solution (dark solid line); upon addition of 4-NP (dark dashed line); line); formation of 4-nitrophenolate (green solid line). Reduction reaction after 2 min (red solid line), formation of 4-nitrophenolate (green solid line). Reduction reaction after 2 min (red solid line), after 4 after 4 min (blue solid line), and after 6 min (yellow solid line). Inset: a) ln(c/c0 ) versus time (c = min (blue solid line), and after 6 min (yellow solid line). Inset: a) ln(c/c0) versus time (c = [4-nitrophenolate]). [4-nitrophenolate]).

Our study concerned the catalyzed reduction of 4-NP (Scheme 1) in the presence of the synthesized AuNPs, using sodium borohydride (NaBH4) as reducing agent. The three types of

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Our study theREVIEW catalyzed reduction of 4-NP (Scheme 1) in the presence of the synthesized Nanomaterials 2018,concerned 8, x FOR PEER 8 of 18 AuNPs, using sodium borohydride (NaBH4 ) as reducing agent. The three types of AuNPs (1, 5, and 10%) initially and thetested, best results (seebest below) were obtained AuNPs 1% tea. AuNPs (1,were 5, and 10%) tested, were initially and the results (see below) for were obtained for Thus, we1% chose for these further studies. is the only reduction product AuNPs tea. these Thus,AuNPs we chose AuNPs for4-Aminophenol further studies.(4-AP) 4-Aminophenol (4-AP) is the only obtained, and the high selectivity washigh confirmed [27–32] UV–vis spectroscopy. reduction product obtained, and the selectivity was by confirmed [27–32] by UV–vis spectroscopy. of different different reducing reducing agents agentsother otherthan thanNaBH NaBH also studied. Although in The effect of 4 was also studied. Although in the 4 was presence of hydrazine, the formation of theof intermediate 4-nitrophenolate ion wasion observed, further the presence of hydrazine, the formation the intermediate 4-nitrophenolate was observed, further reduction to 4-aminophenol verywhereas slight, whereas using other reducing agents such as reduction to 4-aminophenol was verywas slight, using other reducing agents such as ascorbic ascorbic acid, D-glucose, oreven NH4the Cl, even the formation of the intermediate ion detected. was not detected. acid, D-glucose, or NH4Cl, formation of the intermediate ion was not OH

O

-

OH

AuNPs

NaBH4, aq., room temp

NaBH4, aq., room temp NO2

4-NP

NO 2

NH2

4-AP

Scheme 1.1.Reduction Reductionofof 4-nitrophenol (4-NP) to 4-aminophenol by 4NaBH 4 catalyzed by 4-nitrophenol (4-NP) to 4-aminophenol (4-AP)(4-AP) by NaBH catalyzed by AuNPs. AuNPs.

In this method, the limitation resides in that the concentration of the 4-nitrophenolate ion, and thus In this method, the limitation resides in that the concentration of the 4-nitrophenolate ion, and the absorbance at 400 nm, is also dependent on other water chemistry parameters, namely the pH thus the absorbance at 400 nm, is also dependent on other water chemistry parameters, namely the of the solution. The pKa of 4-nitrophenol is 7.2 at room temperature, with the 4-nitrophenolate ion pH of the solution. The pKa of 4-nitrophenol is 7.2 at room temperature, with the 4-nitrophenolate as the dominant species at pH > 7.2. The introduction of the reducing agent (sodium borohydride) ion as the dominant species at pH > 7.2. The introduction of the reducing agent (sodium into the reaction mixture increases the pH and changes the acid–base speciation of 4-nitrophenol borohydride) into the reaction mixture increases the pH and changes the acid–base speciation of (as Scheme 1 and Figure 6 show). To determine if the tea extract has pH-buffering capacity that 4-nitrophenol (as Scheme 1 and Figure 6 show). To determine if the tea extract has pH-buffering may gradually decrease the pH of the reaction mixture over time and thus lead to a lowering of the capacity that may gradually decrease the pH of the reaction mixture over time and thus lead to a 4-nitrophenolate ion concentration, we monitored the pH of the reaction mixtures over time under the lowering of the 4-nitrophenolate ion concentration, we monitored the pH of the reaction mixtures same conditions, and realized that in the case of the reaction in water, the pH increased in the case over time under the same conditions, and realized that in the case of the reaction in water, the pH of using NaBH4 ; however, under the same conditions, but without adding NaBH4 , the pH of the tea increased in the case of using NaBH4; however, under the same conditions, but without adding extract solution remains constant (pH ≈ 7). There is a slight increase immediately after the addition of NaBH4, the pH of the tea extract solution remains constant (pH ≈ 7). There is a slight increase NaBH4 , but rapidly decreases to the original pH value. From this effect, the decrease in the absorbance immediately after the addition of NaBH4, but rapidly decreases to the original pH value. From this at 400 nm is due to the combined effects of the AuNP-catalyzed reduction and changes in pH. effect, the decrease in the absorbance at 400 nm is due to the combined effects of the AuNP-catalyzed Upon addition of the AuNPs, the electron donor (BH4 − ) and electron acceptor (4-nitrophenolate) reduction and changes in pH. are both adsorbed on the NP surface and the catalytic reduction of nitrophenolate by BH4 − starts. Upon addition of the AuNPs, the electron donor (BH4−) and electron acceptor (4-nitrophenolate) In the absence of any catalyst, the formation of the 4-nitrophenolate ion is observed, but no further are both adsorbed on the NP surface and the catalytic reduction of nitrophenolate by BH4− starts. In conversion to 4-aminophenol occurs, and the peak at 400 nm remains unaltered for a long period of time the absence of any catalyst, the formation of the 4-nitrophenolate ion is observed, but no further (several days). Thus, AuNPs promote the reduction of 4-nitrophenol by lowering the activation energy conversion to 4-aminophenol occurs, and the peak at 400 nm remains unaltered for a long period of of this reaction [51]. In the absence of NaBH4 , the AuNPs 1% tea catalyst showed no catalytic activity. time (several days). Thus, AuNPs promote the reduction of 4-nitrophenol by lowering the activation The peak due to 4-aminophenol expected at ca. 290 nm (observed for a pure 4-AP solution) could energy of this reaction [51]. In the absence of NaBH4, the AuNPs 1% tea catalyst showed no catalytic not be clearly observed in our case, since it is masked underneath the absorption band of the tea extract. activity. This absorption band is due to the presence of polyphenols, confirming the strong UV absorbance of The peak due to 4-aminophenol expected at ca. 290 nm (observed for a pure 4-AP solution) the black tea extract. could not be clearly observed in our case, since it is masked underneath the absorption band of the The size of metal NPs plays an important role in catalytic systems [52], and thus the application tea extract. This absorption band is due to the presence of polyphenols, confirming the strong UV of different sizes of AuNPs in the reduction of 4-nitrophenol was studied (Figure 7). Although the absorbance of the black tea extract. catalytic activity of the AuNPs usually increases when the size of the particles decreases [52–54], The size of metal NPs plays an important role in catalytic systems [52], and thus the application an optimal particle size for a particular catalytic system can occur [55,56]. of different sizes of AuNPs in the reduction of 4-nitrophenol was studied (Figure 7). Although the catalytic activity of the AuNPs usually increases when the size of the particles decreases [52–54], an optimal particle size for a particular catalytic system can occur [55,56].

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The size of the AuNPs influences the facility with which the substrate (4-NP) and the reducing agent (BH4−) are adsorbed on the NP surface. Thus, larger nanoparticles show a lower specific The area size of thehinder AuNPsthe influences theand facility with which the reactants, substrate (4-NP) andtheir the reducing surface and approach adsorption of the lowering catalytic − ) are adsorbed on the NP surface. Thus, larger nanoparticles show a lower specific surface agent (BH 4 performance [51]. For instance, for the larger-sized nanoparticles, that is, 75 nm or AuNPs 10% tea, area and hinder of the4-NP approach and adsorption of the reactants, lowering performance [51]. the conversion achieved after the first 6 min of reaction time their is ca.catalytic 60%, whereas the smaller For instance, for the larger-sized nanoparticles, that is, 75 nm or AuNPs 10% tea, the conversion of AuNPs (AuNPs 1% tea), prepared using the less concentrated tea solution, converts 94% of 4-NP 4-NP achieved after the first 6 min of reaction time is ca. 60%, whereas the smaller AuNPs (AuNPs 1% (Figure 7). tea), prepared using the less concentrated tea solution, converts 94% of 4-NP (Figure 7).

100 Yield, %

80 60 40 20 0 12

40

75

AuNPs size (nm) Figure 7. Conversion of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) as a function of the size of the AuNPs, 5 M; [4-NP] = 3.8 × 10−5 M; [NaBH ] = 1.6 × 10−3 M. in the first min of reaction. [AuNPs] = 3.2 × 10−to 4 Figure 7. 6Conversion of 4-nitrophenol (4-NP) 4-aminophenol (4-AP) as a function of the size of the

AuNPs, in the first 6 min of reaction. [AuNPs] = 3.2 × 10−5 M; [4-NP] = 3.8 × 10−5 M; [NaBH4] = 1.6 × 10−3 M.

The effect of catalyst (AuNPs) concentration on the 4-NP reduction reaction was also studied effect 8ofand catalyst (AuNPs) the 4-NP reaction also studied (TableThe 1, Figure Figure S4). Theconcentration conversion is on estimated byreduction the decrease of thewas band at 400 nm, (Tableis1,characteristic Figures 8 andofS4). conversion is estimated by the decrease of the band at 400 nm, which which theThe 4-nitrophenolate ion. is characteristic of the 4-nitrophenolate For a very low concentration (3 × ion. 10−7 M) of AuNPs, almost no conversion of 4-NP to 4-AP For a very low40concentration × 10−71,M) of AuNPs, almostthe no catalyst conversion of 4-NP to results 4-AP was was detected after min reaction (3 (entry Table 1). Increasing concentration in − 6 40 min reaction (entry 1,for Table theconcentration, catalyst concentration results in a adetected clear rateafter enhancement. For example, a 8 ×1).10Increasing M catalyst a conversion of 72% −5 M catalyst, aa conversion clear rate enhancement. For example, 8 × 10−6 for M catalyst conversion of of 94% 72% is is is observed after 18 min (entry 3, Tablefor 1), awhereas 3.2 × 10concentration, −5 M catalyst, observedafter afteronly 18 min (entry 3, 5, Table for (in 3.2 the × 10absence conversion of 94% achieved 6 min (entry Table1),1).whereas Blank tests of any aNP catalyst) were alsois achieved after only 6 min (entry 5, Table 1). Blank (in the absence of any NP catalyst) were also performed under common reaction conditions, andtests no conversion was observed. performed under common reaction and no conversion was observed. Since the reducing agent NaBHconditions, is in a high excess relatively to 4-NP, the reaction is expected 4 the reducing agent NaBH 4 is in a high excess relatively to 4-NP, the reaction is expected to to beSince independent of NaBH concentration [33,34] and to follow pseudo first-order kinetics 4 be independent of(2)), NaBH 4 concentration [33,34] and to follow pseudo first-order kinetics (Equations (Equations (1) and where c and c0 are the concentrations of 4-nitrophenolate at time t and at (2)), where and 0c0) =are the concentrations of 4-nitrophenolate at time t and at t =the 0, t(1) = 0,and respectively, andc ln(c/c ln(A/A ), where A and A are the corresponding absorbances of 0 0 respectively, and ln(c/c 0 ) = ln(A/A 0 ), where A and A 0 are the corresponding absorbances of the typical typical band at 400 nm [33,34,57]. dc band at 400 nm [33,34,57]. − = k0app · c (1) dt 𝑑𝑐 (1) =− 𝑘k0app. ·𝑐t ln− (2) (c/c0 ) =

𝑑𝑡

The value of kapp is the slope of the plot of ln(A/A ) versus time (see Figure 6a) for the case of ln 𝑐 𝑐 = −𝑘0 ∙ 𝑡 (2) AuNPs 1% tea. A plot of the constant concentration of the(see catalyst is shown Figure 8, slope of theversus plot ofthe ln(A/A 0) versus time Figure 6a) for in the case of The value ofapparent kapp is therate allowing thetea. rate constant (k) to be obtained as the slope (1.50 ± 0.05 M−1 ·min−1 ). AuNPs 1% Furthermore, the particle size distribution indicatedofthat NPs formed were A plot of the since apparent rate constant versus thestudies concentration thethe catalyst is shown in nearly Figure −1 −1 uniform in size we have used method Pikeramenou et al. [58] to 8, allowing theand ratespherical constant in (k)shape, to be obtained as thethe slope (1.50adopted ± 0.05 Mbymin ). calculate the numbersince of particles present (ESI, Table S1) instudies each amount of composite usedformed for catalysis. Furthermore, the particle size distribution indicated that the NPs were nearly uniform in size and spherical in shape, we have used the method adopted by Pikeramenou et

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al. [58] to calculate the number of particles present (ESI, Table S1) in each amount of composite used for catalysis. Nanomaterials 2018, 8, 320

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Table 1. Apparent rate constant, conversion, and TOF for the reduction of 4-NP to 4-AP with variable a. concentrations of AuNPs Table 1. Apparent rate constant, conversion, and TOF for the reduction of 4-NP to 4-AP with variable a. concentrations 3 [AuNPs]of AuNPs kapp × 10 Conversion b TOF b,c Time d Conversion TOF c Entry −1 −1 (M) (min ) (%) (h ) (min) (%) (h−1) 3 c b,c d k × 10 Conversion TOF Conversion [AuNPs] TOF Time app 1 Entry3.0 × 10−7 0.1 0.0 0.0 40 0.4 5.1 b − 1 − 1 − 1 (%) (%) (h ) (M) (h ) (min) (min ) 2 2.8 × 10−6 22.2 0.0 0.0 20 9.3 25.8 − 7 −6 10 1 8.0 ×3.0 0.0 40 0.4 5.177.1 3 10× 82.8 0.1 22.70.0 72.8 18 72.1 −6 2 2.3 ×2.8 0.0 20 9.3 25.8 −5 10 4 10× 325 22.2 64.30.0 71.8 14 93.1 45.2 −6 3 82.8 22.7 72.8 18 72.1 77.1 8.0 × 10 5 3.2 × 10−5 −5 464 94.0 75.4 6 94.0 75.4 4 325 64.3 71.8 14 93.1 45.2 2.3 × 10 a Reaction conditions: −5 −3 b M, [NaBH4] =75.4 1.6 × 10 M, catalyst = AuNPs 6 min 5 464= 3.8 × 10 94.0 6 94.0 1% tea.75.4 3.2 × 10−5 [4-NP] c TOF = TON per hour (estimated from Equation (3), see ESI, Table S1); TON = number reaction a Reactiontime. − 5 − 3 b conditions: [4-NP] = 3.8 × 10 M, [NaBH4 ] = 1.6 × 10 M, catalyst = AuNPs 1% tea. 6 min reaction d Time beyond which no further conversion was c TOFof of moles product per (estimated mol of metal time. = TON per hour from catalyst. Equation (3), see ESI, Table S1); TON = number of moles of product d Time beyond which no further conversion was significant. k per mol of metal catalyst. app = apparent rate constant. significant. kapp = apparent rate constant.

This method method was wasapplied appliedtotocalculate calculatethe the number surface atoms present in NPs, the NPs, which number of of surface atoms present in the which was was further forcalculations the calculations the turnover frequency (Equation further usedused for the of theofturnover frequency (TOF)(TOF) (Equation (3)). (3)).

= TOF =

M NtS ·∙t

(3) (3)

where M is is the the number number of of molecules molecules (4-NP) where M (4-NP) which which reacted reacted in in the the presence presence of of catalyst catalyst in in time time tt to to produce the product (4-AP) and N tS is the total number of Au surface atoms of the catalyst produce the product (4-AP) and NtS is the total number of Au surface atoms of the catalyst that that are are available available for for the the reaction. reaction. The The details details of of the the method method and and calculations calculations are are included included in in the the Electronic Electronic Supplementary Information, ESI. Supplementary Information, ESI. As expected,thethe of reduction of nitrophenol linearly with concentration the catalyst As expected, raterate of reduction of nitrophenol increases increases linearly with the catalyst concentration (Figure 8) [59–61]. (Figure 8) [59–61].

Figure Plot of of apparent apparent rate versus concentration of AuNPs (data from Table 1). Figure 8. 8. Plot rate constant constant kkapp app versus concentration of AuNPs (data from Table 1).

Comparable rates were observed by Panigrahi et al. [39] for the reduction of a series of aromatic nitrocompounds by by core-shell core-shellnanocomposites nanocomposites(R-Au) (R-Au)bearing, bearing,atatthe the surface, well-defined AuNPs surface, well-defined AuNPs of of variable sizes (8–55 [39] by Gangula et al. the reduction 4-NP with to 4-AP with variable sizes (8–55 nm)nm) [39] or byorGangula et al. [57] for[57] the for reduction of 4-NP of to 4-AP naturally naturally AuNPs (fromextract the stem extractrhamnoides), of Breynia rhamnoides), although a larger produced produced AuNPs (from the stem of Breynia although with a largerwith average size average thusthan lessthe active, thanprepared the AuNPs prepared (30 nm), size and (30 thusnm), lessand active, AuNPs in our work. in our work. The influence 4 concentration waswas alsoalso investigated for the of 4-NP, for a influence ofofNaBH NaBH investigated for reduction the reduction of 4-NP, 4 concentration constant concentration of AuNPs (Table(Table 2, Figures 9 and9S5). the very 4 concentration for a constant concentration of AuNPs 2, Figure andFor Figure S5). low For NaBH the very low NaBH4 of 4 × 10−5 M, noofconversion observed, even afteris40 min of reaction time. 40 Increasing [NaBHtime. 4] to concentration 4 × 10−5isM, no conversion observed, even after min of the reaction − 4 −4 8Increasing × 10 M allows a 30% achieved after the sametotime (entry 3, Table 2), same whereas a the [NaBH to 8 × 10 to Mbe allows a 30% conversion be achieved after the time 4 ] conversion conversion of 94% is obtained after only of 6 min 5, Table 2) only for [NaBH of 1.65,× Table 10−3 M. (entry 3, Table 2), whereas a conversion 94% (entry is obtained after 6 min4](entry 2) No for [NaBH4 ] of 1.6 × 10−3 M. No catalyst poisoning seems to occur, since the characteristic strong UV–vis adsorption band of the AuNPs at ca. 538 nm remains with the same intensity. Calculation of the

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catalyst poisoning seems to occur, since the characteristic strong UV–vis adsorption band of the apparentatrate assuming that the reaction follows pseudo-first law app ) is performed AuNPs ca. constant 538 nm (k’ remains with the same intensity. Calculation of the aapparent rateorder constant (Equations (4) and (5)), where c and c are the concentrations of 4-nitrophenolate at time t and at t = 0, 0 reaction follows a pseudo-first order law (Equations (4) and (k’app) is performed assuming that the respectively, and cln(c/c ln(A − Ab /A0of−4-nitrophenolate Ab ), where A0 and A aret and the absorbances at 400 nm at 0 ) =concentrations (5)), where c and 0 are the at time at t = 0, respectively, and instant t = 0 and t, respectively, and A is the background. All the values presented show the correction ln(c/c0) = ln(A − Ab/A0 − Ab), where Ab 0 and A are the absorbances at 400 nm at instant t = 0 and t, due to the background at 400 nm. respectively, and Ab is absorbance the background. All the values presented show the correction due to the background absorbance at 400 nm.

dc − = k0 app · c (4) dt (4) − = ′ .   c = −k0′app ·∙ t (5) ln = (5) c0 Higher concentrationsofof NaBH 4 for the 4-nitrophenol-catalyzed reduction induce faster Higher concentrations NaBH 4 for the 4-nitrophenol-catalyzed reduction induce faster kinetics kinetics from zeroto first-order kinetics (ESI,S5 Figures S5 and S6). from zero- to first-order kinetics (ESI, Figures and S6). The rate of reaction also increases with the concentration sodium borohydride, observed The rate of reaction also increases with the concentration ofof sodium borohydride, as as observed by by other authors [34,39,59]. other authors [34,39,59].

Figure 9. Plot of apparent rate constant k’app obtained from Table 2 versus concentration of NaBH4 Figure 9. Plot of apparent rate constant k’app obtained from Table 2 versus concentration of NaBH4 (data from Table 2). (data from Table 2). Table 2. Reduction of 4-NP to 4-AP with variable concentrations of NaBH4 a . Table 2. Reduction of 4-NP to 4-AP with variable concentrations of NaBH4 a. Entry Entry 1

[NaBH [NaBH 4 ]4] (M) (M) 3.0 × 10−4

k’app ××10 k'app 1033 − 1 −1) (min (min )

Conversionb b(6 Conversion (6 min)(%) (%) min)

1.0

0.0

d d Time Time (min) (min) 40 40 40 40 40 40 14 14 6

c c Conversion Conversion TOFTOF (%) (%) (h−1)(h−1 ) 2.4 8.5 2.4 8.5 18.5 43.9 18.5 43.9 30.2 130 30.2 130 88.2 76.3 88.2 76.3 94.0 127

1 1.0 0.0 3.0 × 10−4−4 2 5.0 × 10 38.1 2.4 2 38.1 2.4 5.0 × 10−4−4 3 8.0 × 10 168 22.8 − 4 3 168 22.8 8.0 × 10 −3 4 1.2 × 10 315 55.9 4 315 55.9 1.2 × 10−3−3 5 1.6 × 10 464 94.0 5 464 94.0 6 94.0 127 1.6 × 10−3 a Reaction conditions: [4-NP] = 3.8 × 10−5 M and [AuNPs 1%] = 3.2 × 10−5 M. b 6 min reaction time. c TOF a Reaction conditions: [4-NP] = 3.8 × 10−5 M and [AuNPs 1%] = 3.2 × 10−5 M. b 6 min reaction time. c TOF = =TON TON hour (estimated from Equation (3)); TON =ofnumber molesper of product percatalyst. mol of dmetal perper hour (estimated from Equation (3)); TON = number moles ofof product mol of metal Time d Time beyond which no beyond further conversion significant. catalyst. which nowas further conversion was significant.

3.4.2. Reduction of Other Nitro Compounds The catalytic catalyticactivity activityofof AuNPs evaluated forreduction the reduction of 2-nitrophenol thethe AuNPs 1%1% waswas alsoalso evaluated for the of 2-nitrophenol (2-NP), (2-NP), nitroanilines and nitrobenzene (NB),4 with 4 asagent the reducing agent (Table nitroanilines (NA), and(NA), nitrobenzene (NB), with NaBH as theNaBH reducing (Table 3, Figures S7–S9). 3, Figures S7–S9). In all cases, the conversion is more efficient for the compounds with the nitro group in the In all in cases, the conversion is more for the compounds with the nitro group in the 4-position comparison with those in the efficient 2- and 3-positions (Table 3). For instance, for the reduction 4-position in comparison with(4-PD), those 77% in the 2- and is3-positions (Tableafter 3). 8For for the of 4-NA to 4-phenylenediamine conversion achieved shortly mininstance, (entry 4, Table 3); reduction 4-NA to (entries 4-phenylenediamine 77% conversion achieved shortly after 8 min while for 2-ofand 3-NA 2 and 3, Table(4-PD), 3), the conversion is 40%isand 15%, respectively. (entry 4, Table 3); while for 2and 3-NA4-phenylenediamine (entries 2 and 3, Table 3), theisconversion is 40% and 15%, The product of 4-NA reduction, (4-PD), an important member of respectively. aminobenzenes, with applications in dyes [62] and rubber antioxidants [63], and can also be ofto4-NA reduction, 4-phenylenediamine (4-PD), an important member of usedThe as a product precursor aramid textile fibers, thermoplastics, and cast iselastomers [64]. In industry, aminobenzenes, with applications in dyes [62] and rubber antioxidants [63], and can also be used as

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the preparation of 4-PD involves the catalytic hydrogenation of 4-NA [64]. Thus, developing an

precursor to aramid textile fibers, thermoplastics, and cast elastomers [64]. In industry, the precursor to aramid textile fibers, thermoplastics, and cast elastomers [64]. In industry, the a precursor precursor to aramid textile fibers, thermoplastics, and cast elastomers [64]. In industry, the aramid textile fibers, thermoplastics, and cast elastomers [64]. industry, the a aaaaprecursor toto aramid textile fibers, thermoplastics, and cast elastomers [64]. InIn industry, the precursor to aramid textile fibers, thermoplastics, and cast elastomers [64]. In industry, the economic and simple route to its preparation is ofand great interest. precursor to aramid textile fibers, thermoplastics, and cast elastomers [64]. In industry, the aaapreparation precursor to aramid textile fibers, thermoplastics, cast elastomers [64]. In industry, the precursor to aramid textile fibers, thermoplastics, and cast elastomers [64]. In industry, the of 4-PD involves the catalytic hydrogenation of 4-NA [64]. Thus, developing an a precursor to aramid textile fibers, thermoplastics, and cast elastomers [64]. In industry, the preparation of 4-PD involves the catalytic hydrogenation of 4-NA [64]. Thus, developing an preparation of 4-PD involves the catalytic hydrogenation of 4-NA [64]. Thus, developing an preparation of 4-PD involves the catalytic hydrogenation of 4-NA [64]. Thus, developing an preparation of 4-PD involves the catalytic hydrogenation of 4-NA [64]. Thus, developing an preparation of4-PD 4-PDinvolves involvesthe thethe catalytic hydrogenation of4-NA 4-NA [64].Thus, Thus, developing an preparation of 4-PD involves the catalytic hydrogenation of 4-NA [64]. Thus, developing an preparation of catalytic hydrogenation of [64]. developing an The intensity decrease of absorbance peak at 380 nm, due to 4-NA, during the preparation of 4-PD involves the catalytic hydrogenation of 4-NA [64]. Thus, developing an economic and simple route to its preparation is of great interest. preparation of 4-PD involves the catalytic hydrogenation of 4-NA [64]. Thus, developing anreduction economic and simple route to its preparation is of great interest. economic and simple route to its preparation is of great interest. economic and simple route to its preparation is of great interest. economic and simple route to its preparation is of great interest. economicand and simple route to its preparationisisis of great interest. economic and simple route to its preparation of great interest. economic simple route to its preparation of great interest. (Figure S8) is accompanied by the appearance of a new one at 308 nm, corresponding to the economic and simple route to its preparation is of great interest. The intensity decrease of the absorbance peak at 380 nm, due to 4-NA, during the reduction economic and simple route to its preparation is of great interest. The intensity decrease of the absorbance peak at 380 nm, due to 4-NA, during the reduction The intensity decrease of the absorbance peak at 380 nm, due to 4-NA, during the reductionformation The intensity decrease the absorbance peak 380 nm, due 4-NA, during the reduction The intensity decrease ofof the absorbance peak atat 380 nm, due toto 4-NA, during the reduction The intensity decrease of the absorbance peak at 380 nm, due to 4-NA, during the reduction The intensity decrease of the absorbance peak at 380 nm, due to 4-NA, during the reduction The intensity decrease of the absorbance peak at 380 nm, due to 4-NA, during the reduction The intensity decrease of the absorbance peak 380 nm, due to 4-NA, during the reduction (Figure S8) is accompanied by the appearance of a new one at 308 nm, corresponding to the of 4-PD. Another characteristic peak of 4-PD at 238 nm is not visible due to the overlap The intensity decreasebyof the absorbance peak at 380one nm, tonm, 4-NA, during the reduction (Figure S8) is accompanied by the appearance of new one at 308 nm, corresponding to the (Figure S8) is accompanied by the appearance of a new new one at 308 nm, corresponding to thewith a tea (Figure S8) accompanied by the appearance one at 308 nm, corresponding the (Figure S8) isis accompanied the appearance ofof a aaanew atdue 308 corresponding toto the (Figure S8) is accompanied by the appearance of new one at 308 nm, corresponding to the (Figure S8) is accompanied by the appearance of a new one at 308 nm, corresponding to the (Figure S8) is accompanied by the appearance of a new one at 308 nm, corresponding to the (Figure S8) is accompanied by the appearance of a new one at 308 nm, corresponding to the formation of 4-PD. Another characteristic peak of 4-PD at 238 nm is not visible due to the overlap (Figure absorption S8) is accompanied by the appearance of aatnew one 308 nm, corresponding to theperiod of extract band. Initially, no other products were detected, but after aoverlap longer formation of 4-PD. Another characteristic peak of 4-PD at 238 nm isnot not visible due to the overlap formation of 4-PD. Another characteristic peak of 4-PD at 238 nm is not visible due to the overlap formation of 4-PD. Another characteristic peak 4-PD at 238 nm is not visible due to the overlap formation of 4-PD. Another characteristic peak ofof 4-PD 238 nm isat visible due to the formation of 4-PD. Another characteristic peak of 4-PD at 238 nm not visible due to the overlap formation of 4-PD. Another characteristic peak of 4-PD at 238 nm isisnot not visible due the overlap formation of 4-PD. Another characteristic peak of 4-PD at 238 nm is visible due to the overlap formation of 4-PD. Another characteristic peak of 4-PD at 238 nm is not visible due toto the overlap with atea tea extract absorption band. Initially, no other products were detected, but after alonger longer formation of 4-PD. Another characteristic peak of 4-PD at 238 nm is not visible due to the overlap with a tea extract absorption band. Initially, no other products were detected, but after a longer with a tea extract absorption band. Initially, no other products were detected, but after a longer with a tea extract absorption band. Initially, no other products were detected, but after a longer with a extract absorption band. Initially, no other products were detected, but after a reaction, reoxidation of the amino group of 4-PD to give benzoquinone and/or azobenzene withaaatea teaextract extractabsorption absorptionband. band.Initially, Initially,no noother otherproducts productswere weredetected, detected,but butafter afteraaalonger longermay occur with tea extract absorption band. Initially, no other products were detected, but after longer with with aoftea extract absorption band. Initially, no other products were detected, but after a longer period of reaction, reoxidation of the amino group of 4-PD to give benzoquinone and/or azobenzene with a tea extract absorption band. Initially, no other products were detected, but after a longer period of reaction, reoxidation of the amino group of 4-PD to give benzoquinone and/or azobenzene period of reaction, reoxidation of the amino group of 4-PD to give benzoquinone and/or azobenzene period of reaction, reoxidation of the amino group of 4-PD to give benzoquinone and/or azobenzene period reaction, reoxidation of the amino group of 4-PD to give benzoquinone and/or azobenzene (with appearance of a new of absorption band at ca. 486 nm) [65]. period of reaction, reoxidation of the amino group of 4-PD to give benzoquinone and/or azobenzene period reaction, reoxidation of the amino group of 4-PD to give benzoquinone and/or azobenzene period of reaction, reoxidation the amino group of 4-PD to give benzoquinone and/or azobenzene period ofofof reaction, reoxidation of the amino group of 4-PD to give benzoquinone and/or azobenzene may occur (with appearance of aof new absorption band at ca. 486 nm) [65]. period reaction, reoxidation the amino group of 4-PD to give benzoquinone and/or azobenzene may occur (with appearance of a new absorption band at ca. 486 nm) [65]. may occur (with appearance of a new absorption band at ca. 486 nm) [65]. may occur (with appearance of a new absorption band at ca. 486 nm) [65]. may occur (with appearance of a new absorption band at ca. 486 nm) [65]. may occur (withappearance appearance of anew new absorption band at ca. 486 nm) [65]. by Haber [66], is a well-studied The(with reduction of nitrobenzene (NB), since the classical work may occur (with appearance absorption band ca. 486 nm) [65]. may occur of aaanew absorption band at ca. 486 nm) [65]. may occur (with appearance ofofof new absorption band atatat ca. 486 nm) [65]. The reduction of nitrobenzene (NB), since the classical work by Haber [66], is aawell-studied well-studied may occur (with appearance a new absorption band ca. 486 nm) [65]. The reduction of nitrobenzene (NB), since the classical work by Haber [66], is well-studied The reduction of nitrobenzene (NB), since the classical work by Haber [66], well-studied The reduction of nitrobenzene (NB), since the classical work Haber [66], is The reduction of nitrobenzene (NB), since the classical work byby Haber [66], isthe aisaawell-studied The reduction of nitrobenzene (NB), since the classical work by Haber [66], well-studied reaction. In the presence of AuNPs, the reduction proceeds only along so-called direct route, The reduction nitrobenzene (NB), since the classical work by Haber [66], isisis aawell-studied The reduction of nitrobenzene (NB), since the classical work by Haber [66], is aadirect The reduction ofofof nitrobenzene (NB), since the classical work by Haber [66], isdirect well-studied reaction. In the presence of AuNPs, the reduction proceeds only along the so-called route, via The reduction nitrobenzene (NB), since the classical work by Haber [66], awell-studied well-studied reaction. In the presence of AuNPs, the reduction proceeds only along the so-called direct route, via reaction. In the presence of AuNPs, the reduction proceeds only along the so-called direct route, via reaction. In the presence of AuNPs, the reduction proceeds only along the so-called direct route, via reaction. In the presence of AuNPs, the reduction proceeds only along the so-called route, via reaction. In the presence of AuNPs, the reduction proceeds only along the so-called direct route, via via nitrosobenzene and phenylhydroxylamine [67–69]. reaction. the presence AuNPs, the reduction proceeds only along the so-called direct route, via reaction. In the presence of AuNPs, the reduction proceeds only along the so-called direct route, via reaction. InInIn the presence ofofof AuNPs, the reduction proceeds only along the so-called direct route, via nitrosobenzene and phenylhydroxylamine [67–69]. reaction. the presence AuNPs, the reduction proceeds only along the so-called direct route, via nitrosobenzene and phenylhydroxylamine [67–69]. nitrosobenzene and phenylhydroxylamine [67–69]. nitrosobenzene and phenylhydroxylamine [67–69]. nitrosobenzene and phenylhydroxylamine [67–69]. nitrosobenzene and phenylhydroxylamine [67–69]. nitrosobenzene and phenylhydroxylamine [67–69]. nitrosobenzene and phenylhydroxylamine [67–69]. In our catalytic system, nitrobenzene is less reactive than 4-NP against NaBH in the presence of nitrosobenzene and phenylhydroxylamine [67–69]. 4 In our catalytic system, nitrobenzene is less reactive than 4-NP against NaBH in the presence of nitrosobenzene and phenylhydroxylamine [67–69]. In our catalytic system, nitrobenzene is less reactive than 4-NP against NaBH 444in in the presence of In our catalytic system, nitrobenzene is less reactive than 4-NP against NaBH in the presence of our catalytic system, nitrobenzene is less reactive than 4-NP against NaBH 4 the presence of InIn our catalytic system, nitrobenzene is less reactive than 4-NP against NaBH 4 in the presence of In our catalytic system, nitrobenzene less reactive than 4-NP against NaBH 4 in the presence of −isis 5less In our catalytic system, nitrobenzene less reactive than 4-NP against NaBH 4in in the presence of In our catalytic system, nitrobenzene is reactive than 4-NP against NaBH 44min the presence of −5 In our catalytic system, nitrobenzene is less reactive than 4-NP against NaBH in the presence of the same amount of AuNPs (3.2 × 10 M), and provides, after 8 and 14 (ESI, Figure S9), −5 the same amount of AuNPs (3.2 10 M), and provides, after and 14 min (ESI, Figure S9), ca. 61% −5M), Inamount our catalytic system, nitrobenzene is less reactive than against NaBH 4 in the presence of ca. 61% −5 −5 the same amount of AuNPs (3.2 ××××10 10 M), and provides, after 8884-NP and 14 min (ESI, Figure S9), ca. 61% the same amount of AuNPs (3.2 10 M), and provides, after 8and and 14 min (ESI, Figure S9), ca. 61% the same amount AuNPs (3.2 and provides, after min (ESI, Figure S9), ca. 61% the same ofof AuNPs (3.2 × ×10 M), and provides, after 8 8and 1414 min (ESI, Figure S9), ca. 61% −5 the same amount of AuNPs (3.2 10 M), and provides, after and 14 min (ESI, Figure S9), ca. 61% −5 the same amount of AuNPs (3.2 × 10 M), and provides, after 8 and 14 min (ESI, Figure S9), ca. 61% the same amount of AuNPs (3.2 × 10 and provides, after 8 and 14 min (ESI, Figure S9), ca. 61% −5−5M), −5 the same amount of AuNPs (3.2 × 10 M), and provides, after 8 and 14 min (ESI, Figure S9), ca. 61% (entry 5, Table 3) and 70% conversion, respectively. After 40 min, the conversion is not yet complete. the same amount of AuNPs (3.2 × 10respectively. M),respectively. and provides, after 840 and 14 minconversion (ESI, S9), ca.yet 61%complete. (entry 5,Table Table and 70% conversion, min, the is not (entry 5, Table 3) and 70% conversion, respectively. After 40 min, the conversion is not yet complete. (entry 5, Table 3)3) and 70% conversion, respectively. After 40 min, the conversion isFigure not yet complete. (entry and 70% conversion, respectively. After 40 min, the conversion not yet complete. (entry 5,5, Table 3)3) and 70% conversion, After 40After min, the conversion isis not yet complete. (entry 5, Table 3) and 70% conversion, respectively. After 40 min, the conversion is not yet complete. (entry Table and 70% conversion, respectively. After 40 min, the conversion yet complete. (entry 5, 3) and 70% conversion, respectively. After 40 min, the conversion is yet complete. (entry 5,5,Table Table 3)3)3) and 70% conversion, respectively. After 40 min, the conversion isisnot not yet complete. (entry 5, Table and 70% conversion, respectively. After 40 min, the conversion isnot not yet complete. a a. Table 3. Reduction of various substrates using AuNPs 1% tea as catalyst Table 3. Reduction of various substrates using AuNPs 1% tea as catalyst Table 3. Reduction of various substrates using AuNPs 1% tea aas Table 3.Reduction Reduction ofvarious various substrates using AuNPs 1% tea as catalyst . Table substrates using AuNPs 1% tea catalyst Table 3.3. Reduction ofof various substrates using AuNPs 1% tea asas catalyst . aa.aa..catalyst Table 3. Reduction of various substrates using AuNPs 1% tea as catalyst a a . Entry Entry Entry Entry Entry Entry Entry Entry Entry Entry Entry

Table various substrates usingAuNPs AuNPs 1%tea tea catalyst Table 3. of substrates using 1% as catalyst a.. . a Table 3.3.Reduction Reduction ofofvarious various substrates using AuNPs 1% tea asasas catalyst Table 3.Reduction Reduction of various substrates AuNPs tea catalyst . kapp × using 1022 Time 1%Conversion Substrate Substrate Substrate Substrate Substrate Substrate Substrate Substrate Substrate Substrate Substrate

Product Product Product Product Product Product Product Product Product Product Product

1 11111 111 11 (2-NP) (2-NP) (2-NP) (2-NP) (2-NP) (2-NP) (2-NP) (2-NP) (2-NP) (2-NP) (2-NP)

5 555555 555 5

8 88888 888 8

80 80 80 8080 80 80 80 80 8 80

17.3 17.3 17.3 17.3 17.3 17.3 17.3 17.3 17.3 17.3 17.3

8 88888 888 8

40 8 40 404040

30.5 30.5 4030.5 30.5 30.5

30.5

2.5 2.5 2.5 2.52.5 2.5 2.5 2.5 2.5 2.5 2.5

8 88888 888 8

15 15 15 8 15 1515

15 15 15 15

11.4 11.4 11.4 1511.4 11.4 11.4

11.4 11.4 11.4 11.4

11.4

37.8 37.8 37.8 37.8 37.8 37.8 37.8 37.8 37.8 37.8 37.8

8 88888 888 8

77 77 77 8 7777 77

58.7 58.7 58.7 7758.7 58.7 58.7

58.7

4.6 4.6 4.64.6 4.64.6 4.6 4.6 4.6 4.6 4.6

8 88888 888 8

61 8 6161 61 61

46.5 6146.5 46.5 46.5 46.5

46.5

61.1

40 40 40 4040

30.5 30.5 30.5 30.5 30.5

(3-PD) (3-PD) (3-PD) (3-PD) (3-PD) (3-PD) (3-PD) (3-PD) (3-PD) (3-PD) (3-PD)

4 444444 444 4

(4-NA) (4-NA) (4-NA) (4-NA) (4-NA) (4-NA) (4-NA) (4-NA) (4-NA) (4-NA) (4-NA)

23.9 23.9 23.9 23.9 23.9 23.9 23.9 23.9 23.9 23.9 23.9

(2-PD) (2-PD) (2-PD) (2-PD) (2-PD) (2-PD) (2-PD) (2-PD) (2-PD) (2-PD) (2-PD)

3 33333 333 3 3 (3-NA) (3-NA) (3-NA) (3-NA) (3-NA) (3-NA) (3-NA) (3-NA) (3-NA) (3-NA) (3-NA)

61.1 61.1 61.1 61.1 61.1 61.1 61.1 61.1 61.1 80 61.1

TOF b (h−1 )

(2-AP) (2-AP) (2-AP) (2-AP) (2-AP) (2-AP) (2-AP) (2-AP) (2-AP) (2-AP) (2-AP)

2 22222 222 2 2 (2-NA) (2-NA) (2-NA) (2-NA) (2-NA) (2-NA) (2-NA) (2-NA) (2-NA) (2-NA) (2-NA)

b TOF 210 b bbb app Time Conversion TOF app 10222 Time Time Conversion Conversion TOF kkkkapp ×××10 Time Conversion TOF kapp × 10 TOF b app × 10 Time Conversion TOF 2 2 bb 2Time −1 −1 k app ×10 10 Conversion TOF kkapp ××app Time Conversion TOF 2× b−1) b (min ) (min) (%) (h −1 −1 −1 k 10 −1 −1 −1 −1 2 app 10 Time Conversion TOF (min ) (min) (%) (h ) (min ) (min) (min) (%) (h (min )10 (min) (%) ))) (min (%) (h(h ) (%) k(min app )×−1 TimeTime Conversion TOF −1 ) (min) (%) (h (min) Conversion −1−1 −1−1 (min (min) (%) (min (%) (h −1)) )−1 −1 (min) −1)) )−1 (min (%) (h(h (min (min ) )(min) (min) (%) (h )

77 777777

58.7 58.7 58.7 58.7

(4-PD) (4-PD) (4-PD) (4-PD) (4-PD) (4-PD) (4-PD) (4-PD) (4-PD) (4-PD) (4-PD)

61 61 616161

46.5 46.5 46.5 46.5 46.5

(NB) (NA) (NB) (NA) (NB) (NA) (NB) (NA) (NB) (NA) (NB) (NA) (NB) (NA) (NB) (NA) (NB) (NA) −5− −5 M; −3 M − (NB) (NA) 5 M; −5 [NaBH −5 −5 −3 Reaction conditions: [AuNPs 1%] ===3.2 3.2 10 M; [substrate] 3.8 10 4]] = ==1.6 1.6 10 at (NB) (NA) −5M; −5 −3M −5 −5 −5 −3 Reaction conditions: [AuNPs 1%] 3.2 10 [substrate] = ×−5M; 10 M; [NaBH ]×=× × 10 Reaction conditions: [AuNPs 1%] = 3.2 ××××× 10 [substrate] ====3.8 3.8 ××××10 10 M; [NaBH ××××1.6 10 at Reaction conditions: [AuNPs 1%] 3.2 10 M; [substrate] 3.8 10 M; [NaBH 1.6 10−3 M at3 M at 25 ◦ C. Reaction conditions: [AuNPs 1%] = 10 [substrate] ×10 M; [NaBH ]44]]=1.6 10 Reaction conditions: [AuNPs 1%] = 3.2 × ×10 M; [substrate] = =3.8 ×3.8 [NaBH 4] 44= 10 M atat −5M; −5 −3M 41.6 Reaction conditions: [AuNPs 1%] = 3.2 10 M; [substrate] 3.8 10 M; [NaBH = 1.6 10 M at

a a aaaaa aa a b a

−5 −5−5M; −3−3 Matat Reaction conditions: [AuNPs 1%] ×10 M; [substrate] =3.8 10 M; [NaBH =1.6 Reaction conditions: [AuNPs 1%] ==3.2 ××of [substrate] ==per ×××per 10 [NaBH 44]]4]= ×××10 −5−5M; b TOF −5 −5 −3 M Reaction conditions: [AuNPs 1%] 10 [substrate] 3.8 10 M; [NaBH =catalyst. 10 M at at TOF TON per hour; TON = number moles of product mol of−5 metal catalyst. 25 °C. TON per hour; TON ===3.2 number of moles of product mol of metal catalyst. bbbb= b TOF Reaction [AuNPs 1%] 3.2 ×10 10 [substrate] =3.8 3.8 ×mol 10 [NaBH 4]1.6 =1.6 1.6 ×10−310M 25 °C. TOF TON per hour; TON number of moles of product per mol of metal catalyst. 25 °C. TOF TON per hour; TON number ofM; moles ofproduct product per mol ofmetal metal °C. TOF =====TON per hour; TON ==3.2 number of moles per mol of catalyst. 2525 °C. =conditions: TON per hour; TON == number ofM; moles ofof product per ofM; metal catalyst. 25 °C. TOF TON per hour; TON = number of moles of product per mol of metal catalyst. b b 2525 °C. TOF = TON per hour; TON = number of moles of product per mol of metal catalyst. 25 °C. = TON per hour; TON = number of moles of product per mol of metal catalyst. b TOF b 25 °C. TOF = TON per hour; TON = number of moles of product per mol of metal catalyst. °C. TOF = TON per hour; TON = number of moles of product per mol of metal catalyst.

3.4.3. Recycling Studies for AuNPs in 4-NP Reduction To check the possibility of reutilization and recycling of our AuNPs as the catalyst in the reduction of nitro compounds, we repeated the reduction experiments of 4-NP via two different procedures

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To check the possibility of reutilization and recycling of our AuNPs as the catalyst in the reduction of nitro compounds, we repeated the reduction experiments of 4-NP via two different andfirst II). In the onereduction (I), 4-NP reduction was performed through sequential (Iprocedures and II). In(Ithe one (I),first 4-NP was performed through sequential additionaddition of new of new portions of substrate the one previous one consumed) had been consumed) to an aqueous solution portions of substrate (after the (after previous had been to an aqueous solution containing containing NaBH4 in excess and AuNPs 1% tea (Figure 10). The AuNPs become less efficient upon NaBH 4 in excess and AuNPs 1% tea (Figure 10). The AuNPs become less efficient upon each new each new addition of 4-NP.even However, even after the third addition (Figure of substrate (Figure 10c) and in addition of 4-NP. However, after the third addition of substrate 10c) and in the presence thethe presence of the catalyst, initial AuNPs catalyst, conversion to 4-AP is still observed, although of initial AuNPs conversion of 4-NP to 4-APofis 4-NP still observed, although taking longer to taking longer to almost complete the transformation (26 min in comparison with the 10 min of the almost complete the transformation (26 min in comparison with the 10 min of the second addition second 10b) addition (Figure or first the 2one min(Figure for the10a)). first one (Figure 10a)). (Figure or the 2 min 10b) for the After the fourth addition of substrate (Figure 10d), itit was was found found that thatthe theconversion conversion stagnated stagnated After the fourth addition of substrate (Figure 10d), after 12 min. However, upon addition of a new amount of the reducing agent NaBH 4 , a full descent after 12 min. However, upon addition of a new amount of the reducing agent NaBH4 , a full descent of of characteristic the characteristic of the 4-nitrophenolate was demonstrating detected, demonstrating theconversion complete the band band of the 4-nitrophenolate was detected, the complete conversion of the substrate that the are and still active andavailable thereforefor available for recycling. of the substrate and that theand AuNPs 1% AuNPs are still 1% active therefore recycling. The recycling of the AuNPs 1% catalyst was also studied (procedure II), by separating the The recycling of the AuNPs 1% catalyst was also studied (procedure II), by separating the AuNPs AuNPs the reaction mixture (by centrifugation) after a first reduction reaction10a). (Figure 10a). from thefrom reaction mixture (by centrifugation) after a first reduction reaction (Figure The deposited AuNPs were washed with distilled water, and new portions of 4-NP and The deposited AuNPs were washed with distilled water, and new portions of 4-NP and reducing reducing were added (Figure 11). The volume was completed withwater. distilled water. agent wereagent added (Figure 11). The volume was completed with distilled The reduction of 4-NP takes 32 min to be completed, whereas in the method, it happens The reduction of 4-NP takes 32 min to be completed, whereas in the first first method, it happens after after 10 min (Figure 10b). It was also found that since the tea solution was removed after 10 min (Figure 10b). It was also found that since the tea solution was removed after centrifugation, centrifugation, theband characteristic of 4-AP at 290 nm and is now and nottea masked tea the characteristic of 4-AP atband 290 nm is now visible notvisible masked with extractwith bands. extract bands. Moreover, the tendency to form agglomerates using the last method could lead to the Moreover, the tendency to form agglomerates using the last method could lead to the reduced reactivity reduced reactivity and stability of [70]. theseIn nanoparticles [70]. In fact, in the first538 cycles, a shift from 538 and stability of these nanoparticles fact, in the first cycles, a shift from nm to 630 nm in the nm to 630 nm in the characteristic band of AuNPs was observed, indicating that the size ofdue the characteristic band of AuNPs was observed, indicating that the size of the nanoparticles increased nanoparticles increased due to aggregation. With the progression of the reaction, a new shift in the to aggregation. With the progression of the reaction, a new shift in the band, now to lower wavelengths, band, now to wavelengths, is toobserved. Theofchange could agent be due to AuNPs, the reaction of the is observed. Thelower change could be due the reaction the reducing with promoting reducing agent with AuNPs, promoting their decrease in size. their decrease in size.

Figure 10. 10. Reutilization of AuNPs addition of Figure AuNPs 1% 1%tea teaas asthe thecatalyst catalystininthe thereduction reductionofof4-NP. 4-NP.(a)(a)First First addition 4-NP; (b)(b) second 4-NP and and of 4-NP; secondaddition additionofof4-NP; 4-NP;(c) (c)third thirdaddition addition of of 4-NP; 4-NP; (d) (d) fourth addition of 4-NP subsequent addition additionof ofNaBH NaBH44.. All additions were were in in identical identical conditions conditions to to those those of of entry entry 5, 5, Table Table1.1. subsequent

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catalytic process waswas alsoalso proven as follows. AfterAfter the first The heterogeneous heterogeneousnature natureofofthe the catalytic process proven as follows. thecycle, first the AuNPs were separated from the finalthe reaction mixture mixture by centrifugation (Figure 11). The resulting cycle, the AuNPs were separated from final reaction by centrifugation (Figure 11). The solution was also tested for tested the reduction of 4-NP, and no activity was observed. Moreover,Moreover, the AuNP resulting solution was also for the reduction of 4-NP, and no activity was observed. characteristic absorptionabsorption band was not detected. the AuNP characteristic band was not detected. The use of a heterogeneous nanocatalyst produced by an eco-friendly procedure is still an underdeveloped field, field, and a comparison comparison with other related cases shows that our catalyst is more underdeveloped active than a similar one supported in multiwalled carbon nanotubes [71], where the reduction of 4-NP was not complete even after 2 h. In the case of the AuNPs synthesized from the stem extract of Breynia rhamnoides [57], 90% conversion was observed after 7 min, whereas over 94% conversion was observed in 6 min with the AuNPs 1% tea produced in in this work. work.

Figure asas the catalyst in in thethe reduction of Figure 11. 11. Recycling Recycling of ofAuNPs AuNPs1% 1%tea tea(separated (separatedby bycentrifugation) centrifugation) the catalyst reduction 4-NP. of 4-NP.

4. 4. Conclusions Conclusions The results show show that that AuNPs AuNPs were were successfully successfully synthesized single step step process upon The results synthesized in in aa single process upon reduction by polyphenols and flavonoids present in black tea, without using a reduction of of HAuCl HAuCl44∙3H ·3H2O 2 O by polyphenols and flavonoids present in black tea, without using surfactant. These gold nanoparticles a surfactant. These gold nanoparticleswere wereutilized utilizedasasefficient efficient heterogeneous heterogeneous catalysts catalysts for for the the reduction of nitro compounds. High conversions into the corresponding amino-products reduction of nitro compounds. High conversions into the corresponding amino-products were were obtained, AuNP catalyst catalyst can obtained, no no particle particle aggregation aggregation occurred occurred during during the the catalytic catalytic process, process, and and the the AuNP can be easily reused and recycled. The black tea method used for the synthesis of nanoparticles be easily reused and recycled. The black tea method used for the synthesis of nanoparticles is is quite quite simple good simple and and environmentally environmentally benign, benign, and and the the work work shows shows that that the the AuNPs AuNPs obtained obtained thusly thusly act act as as good catalysts for nitro reductions, thus promoting greener and safer methodologies for such reactions. catalysts for nitro reductions, thus promoting greener and safer methodologies for such reactions. Supplementary Materials:The Thefollowing following available online at www.mdpi.com/xxx/s1, Figure S1: Gold Supplementary Materials: areare available online at http://www.mdpi.com/2079-4991/8/5/320/s1, Figure S1: Gold nanoparticles (AuNPs and AuNPsby 10% tea) prepared by4.3H addition of nanoparticles (AuNPs 1% tea, AuNPs 5%1% teatea, andAuNPs AuNPs5% 10%tea tea) prepared addition of [HAuCl 2O] = 1.0 .3Hblack 1.0extracts × 10−1with M todifferent black tea extracts with(1, different concentrations (1, 5image or 10%), S2: ×[HAuCl 10−1 M4to concentrations 5 or 10%), Figure S2: EDS and Figure spectrum 2 O] =tea EDS image andto spectrum corresponding a selected area for(b) (a) AuNPs AuNPs 1% and AuNPs 10% tea; (c) EDS corresponding a selected area for (a) to AuNPs 1% tea and 10%tea tea; (c)(b) EDS image and spectrum image and spectrum corresponding to a selected area for tea extract, Figure S3: FTIRS of (a) 1% tea stock solution; corresponding to a selected area for tea extract, Figure S3: FTIRS of (a) 1% tea stock solution; (b) AuNPs 1% tea, (b) AuNPs 1% tea, Figure S4: UV-Vis spectra for the reduction of 4-NP to 4-AP with variable concentrations of Figure S4: UV-Vis spectra for the reduction of 4-NP to 4-AP with variable concentrations of AuNPs. Reaction AuNPs. Reaction conditions: [4-NP] = 3.8 × 10−5 M and [NaBH4 ] = 1.6 × 10−3 M, Figure S5: Sucessive UV-Vis −3 M, Figure S5: Sucessive UV-Vis spectra for the conditions: [4-NP] = 3.8of× 4-nitrophenol 10−5 M and [NaBH ] =AuNPs 1.6 × 10 spectra for the reduction (4-NP)4by 1% tea with different concentrations of reducing agent reduction of 4-nitrophenol (4-NP) by AuNPs teavariable with different concentrations reducing [NaBH 4], [NaBH4 ], Figure S6: Reduction of 4-NP to 4-AP1% with concentrations of NaBHof S7:agent UV-Vis spectra 4 , Figure run along reduction 2-nitrophenol (2-NP) by AuNPs 1% tea, Figure S8: 4Sucessive UV-Vis spectra for run the Figure S6: the Reduction of of 4-NP to 4-AP with variable concentrations of NaBH , Figure S7: UV-Vis spectra reduction 4-nitroaniline (4-NA) by AuNPs 1%AuNPs tea, Figure spectra for the reduction of along the of reduction of 2-nitrophenol (2-NP) by 1% S9: tea, Sucessive Figure S8:UV-Vis Sucessive UV-Vis spectra for the nitrobenzene by AuNPs1% tea, Table S1: Calculation of the number of AuNPs, of the number of Au surface atoms reduction of 4-nitroaniline (4-NA) by AuNPs 1% tea, Figure S9: Sucessive UV-Vis spectra for the reduction of and of turnover frequency. nitrobenzene by AuNPs1% tea, Table S1: Calculation of the number of AuNPs, of the number of Au surface Author Contributions: A.P.C.R. and E.C.B.A.A. conceived and designed the experiments; M.M. prepared the Au atoms and of turnover frequency. NPs. A.M.F and A.M.B.d.R. performed the XPS analyses. A.P.C.R., M.M. and E.C.B.A.A. performed the catalytic experiments; A.P.C.R. and E.C.B.A.A. analyzed the data andand wrote the paper; and A.J.L.P. provided Author Contributions: A.P.C.R. and E.C.B.A.A. conceived designed the E.C.B.A.A. experiments; M.M. prepared the the means needed forA.M.B.d.R. the realization of this work. All authors and approved the E.C.B.A.A. manuscript.performed the Au NPs. A.M.F and performed the XPS analyses.read A.P.C.R., M.M. and catalytic experiments; A.P.C.R. and E.C.B.A.A. analyzed the data and wrote the paper; E.C.B.A.A. and A.J.L.P. provided the means needed for the realization of this work. All authors read and approved the manuscript.

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Acknowledgments: This work has been partially supported by the Fundação para a Ciência e a Tecnologia (FCT), Portugal, their projects UID/QUI/00100/2013 and UID/NAN/50024/2013, fellowships SFRH/BPD/90883/2012 to A.P.C.R and SFRH/BPD/108338/2015 to A.M.F., and MechSynCat Project-Concurso Anual de IDI&CA-710044/2016 from Instituto Politécnico de Lisboa. Conflicts of Interest: The authors declare no conflict of interest.

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