Tailoring the catalytic activity of Au nanoparticles synthesized by a naturally occurring green multifunctional agent

2.1 Materials

Chloroauric acid (HAuCl₄) (Sigma Aldrich) was used as the source of Au³⁺ ions required for the synthesis of Au NPs. NaBH_4, 4-Nitrophenol (4-NP), 2-Nitroaniline (2-NA), 3-Nitroaniline (3-NA) and 1,3-dinitrobenzene (1,3-DNB) were purchased from Sigma Aldrich. All other chemicals used for the present study were of analytical grade and Milli-Q (Milli-Q Academic with 0.22 mm Millipak R-40) water was used for all the analyses.

2.2 Instrumentation

FT-IR studies were conducted on a Shimadzu FTIR-8400S PC instrument in the diffuse reflectance mode in KBr (IR spectroscopic grade, FLUKA) pellets. Prior to the FT-IR measurements, Au NP solutions were centrifuged at 14,000 rpm for 30 min and the NPs were separated from the supernatant part. All the IR-samples (Au NPs, GMA and the supernatant) were dried under vacuum before measuring their IR spectra. Absorption spectra were recorded on UV–vis (Perkin Elmer Lambda35) spectrophotometer. Fluorescence spectra of six sets of Au NP solutions (synthesized using the GMA at six different pH values) were measured in a Perkin Elmer LS55 fluorescence spectrophotometer at an excitation wavelength (λ_ex) of 308 nm to get the maximum fluorescence intensity (Abdelhalim et al., 2011, 2012). In addition to these Au NP solutions, we have also measured the fluorescence activity of corresponding supernatant solutions and GMA itself at different pH (λ_ex 308 nm). All spectroscopic measurements were performed at 25 °C. The spectra of each of the solutions remained unchanged for sufficiently long periods of time during which the spectroscopic experiments were completed. Therefore any possibility of decomposition of samples that might affect the spectroscopic results can be safely ruled out.

The shape and size of the resultant particles were elucidated with the help of Transmission Electron Microscopy (TEM). Samples for TEM were prepared by drop-coating the Au NPs solution onto carbon-coated copper grids (400 mesh size). The films on the grids were allowed to dry prior to the TEM measurement in a JEOL JEM-2100 instrument.

Thermal responses of the Au NPs synthesized by Burmese grape fruit juice as GMA at pH = 4.3 were monitored by Thermo Gravimetric Analysis (TGA) using a Pyris Diamond TG/DTA (PerkinElmer, STA-6000) thermal analyzer. The experiment was done in the temperature range of 40–900 °C and at a heating rate of 15 °C min⁻¹ under nitrogen atmosphere.

For powder X-ray Diffraction (XRD) study, Au NPs solutions were centrifuged at 14,500 rpm for 30 min. After discarding the supernatant, water was added to the Au NPs and vortexed. These steps were repeated for three times. Finally, the residue part was separated and dried well. The dry powder obtained was spread evenly on a quartz slide to perform XRD studies using the Rigaku-Smart Lab diffractometer attached with D/tex ultra detector and Cu Kα source operating at 50 mA and 40 kV. The scan range was fixed at 2θ = 30–80° with a stepwise size of 0.02°.

Zeta potential of all six sets of Au NPs was measured by Zetasizer Nano ZS-ZEN 3600 (Malvern Instruments Ltd, UK). Measurements were done using disposable cuvettes (1 mL volume) specific for this instrument. Prior to the experiment, all samples were diluted with the appropriate amount of nanopure water so that we could get optimum signal intensity. Five replications were done in each.

2.3 Method of preparation of GMA

GMA was prepared from unripe Burmese grape fruits according to the method reported earlier (Alam et al., 2015).

2.4 Method of synthesis of Au NPs by Burmese grape fruit juice as GMA

In a typical procedure, 1 ml of GMA was diluted to 5 ml (0.216 g ml⁻¹) by adding water. 50 μl of 0.05 M of HAuCl₄ was added to it whereby the final concentration of Au³⁺ ions in reaction mixture became 0.5 mM. The pH of the reaction mixture was found to be 4.3. Then the reaction mixture was sonicated using a sonicator bath (40 kHz) at room temperature. Bluish-red coloration was observed within 2 min indicating the onset of formation of Au NPs. The progress of the reaction was monitored by measuring the absorbance of the reaction mixture at regular intervals of time. The absorption peak was assigned to the surface plasmon resonance (SPR) band of Au NPs formed by the reduction of Au³⁺ ions (Cao, 2004; Rao et al., 2002; Abdelhalim et al., 2012; He et al., 2002; Link and El-Sayed, 2000).

Similar method was followed to synthesize Au NPs by the GMA at different pH conditions (i.e. at pH 6, 7, 8, 10 and 12). In each case, pH of the reaction medium was adjusted by adding appropriate volume of aqueous solution of NaOH [6 (M)] at the beginning of the reaction.

2.5 Method of borohydride reduction of aromatic nitro compounds using GMA synthesized Au NPs as catalyst

We have tested the catalytic activity of six sets of Au NPs synthesized by the GMA at six different pH of the reaction medium in the reduction of aromatic nitro compounds to their respective amine derivatives by NaBH₄ (Scheme 1), which is not feasible in the absence of the catalyst (Gangula et al., 2011; Zhu et al., 2011).

NaBH₄ reduction of 4-Nitrophenol (4-NP) to 4-aminophenol (4-AP) was selected as the model reaction for optimizing the reaction condition and absorption spectroscopy was used to monitor the progress of this catalytic reduction reaction. For a typical reduction reaction, 24 ml of 0.125 mM of 4-NP was taken in a 50 ml round bottom flask and the pH of the reaction medium was adjusted at 10 by adding aqueous solution of NaOH. 3 ml of freshly prepared ice-cold aqueous solution of NaBH₄ (100 mM) and 3 ml of the Au NPs solution (193 μg ml⁻¹) were added to it and mixed properly. The reaction mixture was stirred at room temperature and progress of the reaction was monitored by UV–vis spectroscopy. After completion of the reaction within 9 min, the reaction mixture was centrifuged at 15,000 rpm at room temperature and the supernatant was collected for the isolation and identification of the product. Then it was acidified by dilute HCl to decompose the excess NaBH₄ followed by the addition of dilute aqueous NaOH solution. The colorless solution thus obtained was made saturated by adding brine solution. The product was extracted times with by diethyl ether (20 ml in each time) and then it was dried with Na₂SO₄. The resultant reaction product was evaporated to dryness under vacuum whereby the solid product (4-AP) was obtained. The catalytic activity of the Au NPs synthesized by the GMA at pH 12 was further investigated for the NaBH₄ reduction of other aromatic nitro compounds e.g. 2-NA, 3-NA and 1,3-DNB under the same reaction condition as shown in Scheme 1.

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Scheme 1 Reduction of aromatic nitro compounds to their respective amine derivatives by NaBH₄ catalyzed by Au NP.

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In each case, the structure of the product was confirmed on the basis of m.p. and ¹H NMR data (Fig. S1). The residual mass (recovered Au NP catalyst) in the centrifuge tube was washed with water, dried under vacuum and weighed.

3.1 Absorption spectroscopy and morphology of the Au NPs synthesized by Burmese grape fruit juice as GMA at different pH conditions

Stability and morphology are the two most important parameters for NPs which control their structure-dependent properties and subsequent technological applications. The aggregation behavior of the NPs under the specific reaction condition strongly influences these two structural parameters of NPs. We have tested the efficacy of the present protocol over a wide pH range of the reaction medium and we have observed that, pH of the reaction medium played a strong role in controlling the morphology of the GMA synthesized Au NPs and hence their applicability as catalyst (discussed later).

The UV–vis spectroscopy was our primary tool to monitor the progress of the reaction between Au³⁺ ions and GMA leading to the formation and growth of Au NPs. Fig. 1 shows the absorption spectra as a function of time for the reaction between Au³⁺ ion containing solution and GMA at pH = 4.3. The broken curve represents the absorption spectrum of GMA at t = 0 i.e. at the instant of addition of Au³⁺ ions. Upon sonication at room temperature, onset of formation of Au NPs was indicated by the appearance of violet-red color (inset (i) of Fig. 1). Within 4 min, a narrow peak at 540 nm with a broad shoulder at ∼696 nm appeared. The main peak at 540 nm steadily increased in intensity as a function of time and the saturation was observed within 30 min (inset (ii) of Fig. 1).

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Figure 1 UV–vis spectra of Au NPs prepared by Burmese grape fruit juice as GMA at pH 4.3. Broken line represents absorbance curve of reaction medium in the absence of Au³⁺ ions. Inset (i) shows the color of the Au NPs solutions and inset (ii) shows the change in peak absorbance maxima with time.

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The appearance of more than one peak can be an indication of the formation of NPs with different shapes and sizes (He et al., 2002; Link and El-Sayed, 2000; Jin et al., 2003; Mandal et al., 2005). Moreover, appearance of more than one SPR bands can be due to the formation of anisotropic NPs (Taleb et al., 1998; Tan et al., 2002). It is quite possible that the smaller particles formed coalesced together to generate some anisotropic structures of the NPs (Taleb et al., 1998; Tan et al., 2002; Zhang et al., 2014). This was further confirmed on the basis of TEM images of the corresponding NPs.

Fig. 2 shows the absorption spectra of the Au NPs synthesized by GMA at different pH of the reaction medium.

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Figure 2 UV–vis spectra of Au NPs prepared using Burmese grape fruit juice as GMA at different pH: 4.3 (a), 6 (b), 7 (c), 8 (d), 10 (e) and 12 (f) respectively. Inset shows the color of the corresponding Au NP solutions.

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When the same method was repeated at pH 6.0, the reaction mixture turned violet-red indicating the onset of formation of Au NPs within 4 min of sonication and in this case also, the saturation was observed after 30 min. But, the difference was observed in the nature of the SPR band (more broad and appeared at 544 nm) (Fig. 2). We have also tested the efficacy of the present protocol at pH 7. At this pH, the color of the Au NPs formed was found to be blue and saturation time for the formation of NPs was 25 min. However, a drastic blueshift and increased broadness of the SPR band at 521 nm were noticed in this case (Fig. 2). Higher yield and faster time of formation of the Au NPs with progressive blueshift of the SPR band were noticed when the same method was done at pH 8, 10 and 12 (Fig. 2 and Table 1). The effect of pH on the rate of formation and morphology of the GMA synthesized Au NPs is summarized in Table 1. The alkaline pH (especially pH 12) was found to be better choice for the controlled synthesis of Au NPs by GMA in terms of both yield and time taken for formation. We have observed an increase in the zeta potential value (negative) of the Au NPs synthesized by GMA at alkaline pH (Table 1). Alkaline pH responsive functionalities of active chemical constituents of GMA may impart high negative surface charge which prevent self aggregation of the NPs (AbelHamid et al., 2013) and thus small sized Au NPs with more uniform shape (mostly spherical) were obtained at higher pH. Moreover, use of sonication may also influence the structural properties of these synthesized NPs as we have observed in our earlier study (Alam et al., 2015).

This continuous blueshift of the SPR band may be associated with the formation of smaller particles (Mie, 1908). Absorption spectra provided solid evidence of the pH dependent formation and growth of these NPs. This was further confirmed with the help of TEM study. TEM images and corresponding particle size distribution histograms are shown in Fig. 3(a–f) and insets (i) of these figures respectively.

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Figure 3 TEM images of Au NPs synthesized by Burmese grape fruit juice as GMA at different pH: 4.3 (a), 6 (b), 7 (c), 8 (d), 10 (e) and 12 (f). Insets (i) and (ii) in each case show particle size distribution histogram and SAED pattern of Au NPs respectively.

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At pH 4.3 and 6, particles formed are comparatively larger in size and mostly spheroidal in shape although few anisotropic structures formed by occasional aggregation of the smaller particles are visible in the both the cases [Fig. 3(a and b)], as it is evident from the TEM images [Fig. 3(c and d)] that comparatively smaller sized spheroidal particles predominate at pH 7 and 8. However there are some particles with pronounced anisotropic morphologies, such as nanorod, nanotriangle and nanodiscs. Aggregations are almost rare in these TEM images. This wide distribution of shape and size is in accordance with the nature of the absorption curves as shown in Fig. 2(a–d). At alkaline pH 10 and 12, majority of the particles appeared spherical and smaller in size [Fig. 3(e and f)]. Aggregation of the small particles and anisotropic structures almost disappeared in these two cases.

Energy Dispersive X-ray study (EDX) spectra of the Au NPs synthesized by GMA at pH 4.3, 6, 7, 8, 10 and 12 are shown in Fig. S2(a–f). These spectra were found to be identical and there were clear signals for elemental Au with a strong signal for Cu which were due to the Cu present in Cu-grid used for the experiment [Fig. S2(a–f)]. These results confirmed the presence of elemental Au in these NPs.

The selected area electron diffraction (SAED) and X-ray diffraction (XRD) studies were carried out to study the crystal structure of GMA synthesized Au NPs and results are shown in insets (ii) of Figs. 3(a–f) and 4 respectively. The diffraction rings shown in SAED patterns [inset (ii) of Fig. 3(a–f)] correspond to the crystalline plane of these NPs synthesized at various pH values. XRD analyses also confirmed the crystalline nature of these NPs. There are four lattice planes viz., (1 1 1), (2 0 0), (2 2 0) and (3 1 1) corresponding to a fcc crystal structure of Au NPs which gave the diffraction peaks at almost 38.3°, 44.3°, 64.7° and 77.6° respectively. These results indicate the formation of Au NPs (metallic gold, JCPDS 04-0784) (Alam et al., 2014; Palashuddin et al., 2013).

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Figure 4 XRD patterns of synthesized Au NPs at different pH values of reaction medium.

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With further decrease or increase in the pH of the reaction medium, the formation of Au NPs totally stopped. For the present method, we have not used any other stabilizing agent. So it can be assumed that, here GMA served two purposes: 1. reduction of Au³⁺ to Au^o and 2. stabilization of synthesized Au NPs. From our previous study, we have observed that the chemical components of GMA (mainly polyphenol, flavonoids, ascorbic acid and phytosterols) show strong pH dependent redox behavior and control the nucleation and growth of NPs synthesized by it (Alam et al., 2015). So it is quite evident that pH of the reaction medium played a strong role in controlling the reducing and stabilizing activity of GMA which is crucial in maintaining surface chemistry and structural properties of the GMA synthesized Au NPs and this is explained further on the basis of TGA, FT-IR and fluorescence studies as discussed later.

3.2 Role of Burmese grape fruit juice as GMA in controlling the structural properties of the Au NPs synthesized by it at different pH conditions

We have done the TGA of the synthesized NPs to understand the involvement of the active components of GMA toward the surface modification and thermal stability of these NPs.

TGA plot is shown in Fig. 5(a). Initially, there was a weight loss of 2.75% up to 100 °C which may be due to the loss of water molecules associated with these Au NPs. A steady weight loss of the Au NPs powder was continued at 100–900 °C. Total weight loss was found to be 59.1%. This weight loss may be due to the surface desorption of the active chemical components of GMA which adhered to the surface of the Au NPs to give these NPs stability and surface morphology.

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Figure 5 (a) Thermo gravimetric analysis plot of Au NPs synthesized by Baccaurea ramiflora fruit juice as GMA at pH 4.3. (b) FT-IR spectra of (i) GMA before the reduction of Au³⁺ to Au⁰, (ii) Au NPs synthesized by GMA and (iii) supernatant part separated from the Au NP solution.

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To confirm further the role of these biomolecules in the nucleation, growth and stabilization of Au NPs synthesized by the present protocol, we have taken the help of FT-IR spectroscopic measurements of GMA, GMA synthesized Au NPs and supernatant part separated from the NP solution and the corresponding IR spectra are indicated as (i), (ii) and (iii) respectively in Fig. 5(b). There is a similarity between the IR spectra of the GMA itself (i.e. before the reduction of Au³⁺ to Au⁰) and the supernatant solution which was removed from the Au NPs solution synthesized by GMA [(i) and (iii) in Fig. 5(b) respectively]. Prominent IR peaks observed with GMA itself and supernatant of Au NPs were as follows: 1700, 1535, 1302, 1182, 945 and 893 cm⁻¹. The IR peaks at 1700 cm⁻¹ may be associated with stretching vibrations for —C⚌O (keto and ester) groups. On the other hand, IR peaks at 1535 cm⁻¹, 1182 cm⁻¹ and 945 cm⁻¹ can be associated with stretching vibrations for —C⚌C— [(in-ring) aromatic], C—O (ester, ether), and C—O (polyol) respectively (Gupta et al., 2010; Gangula et al., 2011; Aromal and Philip, 2012; Alam et al., 2015; He et al., 2002; Link and El-Sayed, 2000).

However, IR spectrum of GMA synthesized Au NPs [(ii) in Fig. 5(b)] was found to be distinctly different from the IR spectra of GMA itself and supernatant. Some new peaks appeared with slight shifting of the peaks in case of Au NPs. These were found to be at 1710, 1641, 1545, 1480, 1405, 1333, 1175 and 931 cm⁻¹. So it is quite possible that the biomolecules present in the GMA reduced Au³⁺ to Au⁰ and thereby, they themselves got oxidized. The oxidized form of these active biomolecules took prominent role in the stabilization of synthesized NPs by adhering to their surface and this phenomenon gave a characteristic IR spectrum. On the other hand, biomolecules that did not take part in either the synthesis or the stabilization of the NPs remained intact and went to the supernatant part. So it is evident that the oxidized form of the active components of GMA played a strong role in the controlling the structural properties of GMA synthesized Au NPs.

This fact was further confirmed from a detailed comparative fluorescent spectroscopic study of GMA (before the reduction of Au³⁺ to Au⁰), Au NPs synthesized by GMA at pH 4.3, 6, 7, 8, 10 and 12 and the supernatant solutions separated from the corresponding Au NP solutions. The results are shown in Fig. 6(a–f). GMA itself at all the pH values has a very strong fluorescence intensity [denoted by (i) in Fig. 6(a–f)] but a change in overall characteristic of the spectrum was observed with the change of pH. This further supports the pH responsive behavior of the chemical constituents of GMA. In all the cases, broad emission bands were observed. This broadness may be due to the presence of wide varieties of chemical constituents present in GMA. There were overlaps of the emission spectrum originated due to each of these active species of the GMA. This ultimately gave broad emission bands. It is interesting to note that the fluorescence behavior of the GMA and that of the supernatant [marked as (i) and (iii) in Fig. 6(a–f)] were very much similar in nature except for the pH 4.3 [Fig. 6(a)]. However, GMA synthesized NPs also showed fluorescence activity but their fluorescence behavior in all the pH conditions [marked as (ii) in Fig. 6(a–f)] was found to be distinctly different from those of the GMA and that of the supernatant [marked as (i) and (iii) in Fig. 6(a–f)]. This may be due to the difference in the chemical composition of GMA and GMA synthesized Au NP solution. GMA itself (before the reduction of Au³⁺ to Au⁰) and the supernatant solution contain same type of chemical components which are either in unreacted molecules/their unoxidized forms. The intensity of the emission band of the GMA synthesized Au NPs diminished drastically with repeated centrifugation and washing of the corresponding samples. The fluorescence spectra of the repeatedly washed NP samples are shown by the (iv) in Fig. 6(a–f). So it is evident that the fluorescence activity of these synthesized NPs was originated from the strong fluorescence intensity of surface adsorbed functionalities which may be the part of oxidized form of the chemical components of GMA and these results further support the outcomes of TGA and FT-IR studies of the GMA synthesized Au NPs.

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Figure 6 (a–f) Fluorescence spectra of (i) GMA at pH 4.3, 6.0, 7.0, 8.0, 10.0, and 12.0 respectively, (ii) Au NPs synthesized by GMA at respective pH, (iii) supernatant part separated from the Au NP solution and (iv) Au NPs synthesized by GMA at respective pH after repeated centrifugation and washing. Excitation wavelength 308 nm.

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3.3 Evaluation of catalytic activity of Au NPs synthesized by Burmese grape fruit juice as GMA at different pH conditions

The catalytic activity of Au NPs depends critically on their size, shape and stabilization which in turn, is found to be very much dependent on the surface adsorbed functionalities of the stabilizing agents (Fenger et al., 2012). In the present case, we have explored the catalytic activity of the GMA synthesized Au NPs with well defined structural properties and surface adsorbed functionalities. Nitroaromatics in most of the cases come out as industrial wastes but at the same time various nitroaromatics are the precursors of several industrially and pharmaceutically important organic compounds.

In the present case, these Au NPs showed appreciable catalytic activity toward NaBH₄ reduction of the nitroaromatics to corresponding amine derivatives (Scheme 1). We have selected 4-NP as the model substrate which showed an absorption maximum at 401 nm at pH 10. With the progress of the reduction, this peak gradually vanished and a new peak appeared at 296 nm due to the formation of the product i.e. 4-aminophenol (4-AP) or corresponding phenolate ion as the reaction was under alkaline condition. The completion of the reaction can be tracked by monitoring a visual color change (yellow to colorless) [inset of Fig. 7(a)]. The progress of the NaBH₄ reduction of other nitroaromatics (2-NA, 3-NA and 1,3-DNB) (Scheme 1) catalyzed by GMA synthesized Au NPs to corresponding amine derivatives is shown in Fig. 7(b). We have done a detailed kinetic study for the GMA synthesized Au NPs catalytic hydride transfer reduction reaction of 4-NP to 4-AP. For this reaction, we have taken NaBH₄ ∼100 times excess than that of 4-NP so that we can assume that the reaction rate is independent of the concentration of NaBH₄ and 4-NP should be treated as limiting reagent (Zhu et al., 2011). We showed the change of absorbance of 4-NP (λ_max = 401 nm) with time in Fig. 7(c) and a good linear fitting was obtained in the time dependent absorbance (ln A_t vs. time) plot. Combining these two facts, it is quite logical to apply pseudo-first order rate law (Eq. (1)) to determine the rate constant for the present reduction reaction (Gangula et al., 2011; Zhu et al., 2011; Fenger et al., 2012).

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Figure 7 (a) Time dependent UV–vis spectra of the reduction of 4-Nitrophenol (4-NP) to 4-Aminophenol (4-AP) by NaBH4 in the presence of Au NPs synthesized by GMA at pH 4.3 as catalyst. Inset shows the change of color after completion of reaction. (b) UV–vis spectra of the reduction of 2-Nitroaniline (2-NA); 3-Nitroaniline (3-NA) to 1,3-Diaminobenzene (1,3-DAB); 1,3-Dinitrobenzene (1,3-DNB) to 1,3-DAB. (c) ln A_t vs time plot for the catalytic activity of Au NPs (conc. 193 μg ml⁻¹) synthesized by GMA at different pH for the NaBH4 reduction of 4-NP to 4-AP. (d) Effect of concentration on the catalytic activity of the Au NP synthesized by GMA at pH 12. Inset shows the % yield and rate constant with different conc. of Au NP (pH 12).

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The pseudo first-order rate equation:

• [A] = Absorption intensity of 4-NP.

• k = Rate constant (min⁻¹).

Rate constants for all the six sets of catalyst i.e. Au NPs synthesized by GMA at six different pH are shown in Table 2 whereas the UV–vis absorption profile for the progress of the reduction of 4-NP to 4-AP and corresponding ln A_t vs. time plots are shown in Figs. S3 and S4. In all the cases, first order linear fitting profiles were observed. However with the decrease in the size of the Au NPs, both rate constants and % yield of the product increased. The reaction became most facile when we have used Au NPs synthesized by GMA at alkaline pH. But there was drastic decrease in size of the Au NPs when the pH was increased to 12 from 4.3. However, rate of the reaction and product yield did not drastically increase rather a slow increment in rate constants was observed with the decrease in size of Au NPs (Table 2). However, the yield of the product in some cases, did not match the trend of the change of reaction rates. This may be due to the difficulty in the complete separation of the product from the GMA-Au NP catalyst matrix.

We have made a comprehensive survey of the literature to make a comparative performance study of the catalytic study of these GMA synthesized Au NPs toward the reduction of 4-NP to 4-AP by NaBH₄ reduction with other similar systems. However, very few examples are found which are listed in Table S1 of the supplemental material.

Recovery rate of the catalyst is an important parameter for testing the efficacy of any catalytic system. In all the six cases, 90–93% catalyst was recovered (Table 2). To track the change in the morphology of the recovered Au nanocatalyst, we have performed their TEM studies (Fig. S5 in the supplemental material) which indicated no appreciable change. This indicates the possible reusability of the present Au nanocatalyst. We have also studied the effect of the concentration of the catalyst on the rate of the reduction of 4-NP to 4-AP. As shown in the inset of Fig. 7(d), the rate of the reaction was enhanced with the catalyst concentration. But an optimum concentration (64–322 μg ml⁻¹) level of the catalyst was found to be more favorable and the effect was found to be maximum at catalyst concentration of 193 μg ml⁻¹ [inset of Fig. 7(d)]. We have also tested the catalytic activity of Au NPs synthesized by GMA at pH 12 (which gave the best result in the previously discussed reaction) in the reduction of other nitroaromatics (2-NA, 3-NA and 2-DNB) (Scheme 1) and appreciable efficacy of these NPs as catalyst was observed. The progress of the reaction is shown in Fig. 7(b).