In situ decoration of Au NPs over polydopamine encapsulated GO/Fe3O4 nanoparticles as a recyclable nanocatalyst for the reduction of nitroarenes

A new and efficient catalyst has been designed and prepared via in situ immobilization of Au NPs fabricated polydopamine (PDA)-shelled Fe3O4 nanoparticle anchored over graphene oxide (GO) (GO/Fe3O4@PDA/Au). This novel, architecturally interesting magnetic nanocomposite was fully characterized using different analytical techniques such as Field Emission Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy, elemental mapping, Transmission Electron Microscopy, Fourier Transformed Infrared Spectroscopy, X-ray Diffraction and Inductively Coupled Plasma-Atomic Electron Spectroscopy. Catalytic activity of this material was successfully explored in the reduction of nitroarenes to their corresponding substituted anilines, using NaBH4 as reducing agent at ambient conditions. The most significant merits for this protocol were smooth and clean catalysis at room temperature with excellent productivity, sustainable conditions, ease of separation of catalyst from the reaction mixture by using a magnetic bar and most importantly reusability of the catalyst at least 8 times without any pre-activation, minimum loss of activity and considerable leaching.

In material science graphene is considered as a remarkable member with a single atom distance across and a densely packed two-dimensional honeycomb like matrix 32 . It brings in several unique properties like high thermal and mechanical stability, exceptional electrical conductivity within credibly large surface area and adsorption ability. Consequently, graphene has recently acquired significant attention for being used as support in the preparation of efficient catalysts [33][34][35][36][37][38] . In its oxidized form, so called graphene oxide (GO), contains large number of diverse oxygenated functional groups like -OH, -COOH, carbonyl and epoxy on its surface which facilitates the immobilization of different organo-funationalities and metal nanoparticles (MNP) towards the modified solid acid nanocomposites 39,40 . The synergistic effects of the MNP and GO sheet adjoin several extraordinary features to these novel architectured materials thus, undoubtedly could be considered as one of the marvelous effective catalysts of future 41 .
Gold catalysis has been an enthralling research field due to its well-known fascinating characteristics. They exhibit excellent photocatalytic activities under both UV and visible lights 42 . A wide variety of Au materials find applications in the degradation of organo-pollutants, biological transmission electron microscopy, colorimetric DNA sensing, biomedical applications and catalysis 43,44 . Their unique catalytic activity which relied on its negative redox potential, found being much smaller than bulk gold [43][44][45][46][47][48][49][50][51] .
In continuation to our current research on the precise designing and sustainable development of novel heterogeneous nano materials as effective catalysts 15,18,20,21,[52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69] , we thought, it is worthwhile to construct a noble metal adorned magnetically isolable high surface area nanocomposite and examine its catalytic activity. We are specifically interested in nanocatalyzed organic transformations being conducted under green conditions in order to keep the environment safe and clean. We usually try to use water as the most abundant and cheap greenest solvent to carry out the reactions at room temperature conditions 10,[70][71][72][73][74][75][76][77] . Reduction of nitroarenes to corresponding amines is one of the elementary but very important reaction having outstanding industrial implications. The aromatic amines are relatively safer chemical and have broad range of synthetic and biological applications like photographic development, synthesis of dye intermediates, optical brightening, corrosion inhibition, anticorrosion lubrication and in agrochemicals, in pharmaceuticals for the preparation of analgesic, antipyretic and other drugs [78][79][80] . In addition, nitrophenols are recognized as significant organopollutant of water and highly toxic for human and marine lives. They cause severe damage of liver, kidney and central nervous system. The reduced product, the aminophenols are non-toxic and have many other applications 81,82 .
Herein, we wish to disclose our experiences in the design and synthesis of a new hybrid nanomaterial, the in situ synthesized Au NP decorated on polydopamine (PDA) functionalized magnetic Fe 3 O 4 nanoparticles grafted over GO nanocomposite (GO/Fe 3 O 4 @PDA/Au). Its structure was analyzed based on the data obtained using different standard techniques. After unambiguous structural elucidation, we examined its catalytic activity of this novel nanocomposite in the reduction of nitoarenes using conventional reductive agent such as NaBH 4 under ambient reaction conditions in water (Scheme 1). It is worthy to mention, the reduction of 4-nitrophenol Catalytic reduction of nitrobenzene. In the typical synthesis, an emulsion of nitrobenzene (0.003 M) in H 2 O (0.03 mL) was stirred in presence of water suspended GO/Fe 3 O 4 @PDA/Au nanocomposite (0.5 mL, 0.001 g/mL) for 5 min. Then, 0.05 mL aqueous solution of NaBH 4 (0.001 g/mL) was added to it and stirring was continued. As the reaction progressed, the yellow color of the solution gradually faded out. The entire course of reaction was monitored over UV-Vis spectroscopy. After completion, the catalyst was isolated using a magnet, regenerated and reused in further cycles.

Results and discussion
Study of catalyst characterizations. The GO/Fe 3 O 4 @PDA/Au nanocomposite was synthesized following a stepwise post-functionalization approach. At the outset, magnetic graphene oxide (GO/Fe 3 O 4 ) was synthesized according the experimental. The NPs were then covered using PDA, being synthesized by in situ polymerization. PDA organizes a suitable polar environment to anchor Au(III) ions over them and reduces to metallic Au NPs promoted by its active catechol and amine functionalities. Scheme 1 depicts the graphic preparative scheme. The as designed nanocomposite was characterized using FT-IR, FESEM, EDX, elemental mapping, TEM, XRD and ICP-OES techniques.
In order to justify the sequential synthesis of GO/Fe 3 O 4 @PDA/Au nanocomposite, FT-IR spectra of all the corresponding intermediates have been presented in Fig. 1. In Fig. 1a the strong broad band observed in the region ∼3100 to 3450 cm −1 were attributed to combined C-OH stretching, O-H coupled and the water intercalated stretching vibrations. The C=O stretching vibrations for carbonyl functions and carboxylic acids appeared at 1741 cm −1 . The absorptions at 1378 cm −1 and 1060 cm −1 were due to carboxyl O-H and epoxy C-O groups respectively. In Fig. 1b (Fig. 2). GO exhibited a typical folded and wrinkled thin sheet-like appearance (2a). The incorporation of Fe 3 O 4 NPs into GO surface results an increase in the wrinkles over the surface. It also restrains the stacking of GO planes towards polymeric strictures. The globular magnetite NPs are clearly visible over the GO sheet in Fig. 2b. Due to higher concentration during sampling, it seems somewhat agglomerated. In Fig. 2c the polymeric DA is found to immobilize homogeneously over the GO/Fe 3 O 4 surface. Au NPs were generated in situ by reduction and capped over PDA and decorated on the GO/Fe 3 O 4 @PDA composite (Fig. 2d).
The elemental constitution of GO/Fe 3 O 4 @PDA/Au was further confirmed by EDX analysis. Figure 3 displays the EDX profile where Fe and Au are present as metallic component. The C, N and O element validates the PDA and GO attachment in the nanocomposite.
In addition to EDX, elemental mapping of GO/Fe 3 O 4 @PDA/Au nanocomposite was further carried out to study the atomic composition and their distribution over the whole surface. From the SEM image a small surface section is chosen and is analyzed via X-ray dispersion. The results are presented in Fig. 4. The mapping displayed a homogeneous dispersion of C, N, O, Fe and Au atoms in the composite. The occurrence of C, N and O also justifies the organic functionalization over GO.
More detail of the structural framework was ascertained by TEM analysis of the GO/Fe 3 O 4 @PDA/Au nanocomposite (Fig. 5). It demonstrates the Fe 3 O 4 @PDA/Au conjugates, being seen as dark spots, are uniformly Study of catalytic application. The catalytic exploration of GO/Fe 3 O 4 @PDA/Au nanocomposite was started in the reduction of 4-nitrophenol as model reaction at room temperature using NaBH 4 as reducing agent. The entire process was quantitatively monitored over a UV-Vis spectrophotometer. Initially, in the absence of catalyst, when NaBH 4 was added, the pale yellow color was intensified due to the formation of 4-nitrophenate ion and a red shift was identified. The characteristic absorption maxima of 317 nm were shifted to 400 nm. Just after the addition of catalyst, the color as well as peak intensity started diminishing, which indicated the initiation of reduction of 4-NP. Without the addition of catalyst, reaction did not proceed at all. As the reaction progressed, the bell shaped curve, corresponding to λ max 400 nm, gradually flattened and concurrently a new peak was generated at 295 nm due to the formation of 4-AP (Fig. 7a). The reduction was completed in 16 min as visually indicated by the decoloration of solution 85 . A kinetic study for the reaction was also carried out using the spectroscopic data. It represented a linear relationship when − ln (A t /A 0 ) was plotted against reaction time (t) for the process (Fig. 7b) where A t and A 0 being the absorbance of 4-NP at time t and 0, respectively. The curve fitted absolutely with pseudo first-order reaction kinetics 86 . The rate constants (k) was obtained from the slope as being 0.15 min −1 (R 2 = 0.985).
The control experiments for the reduction of 4-NP to 4-AP were tested. With the purpose of having optimized catalytic conditions, the experiment was also investigated with bare Fe 3 O 4 NPs, GO/Fe 3 O 4 and GO/Fe 3 O 4 @ PDA nanocomposites as catalyst keeping other conditions intact. Interestingly, no noticeable transformation was detected even after 2 h, which evidently demonstrates the role of Au NPs being stabilized over GO/Fe 3 O 4 @ PDA. The results showed that in both conditions without NaBH 4 and in the presence of the GO/Fe 3 O 4 @PDA/ Au there is no progress in the reduction reaction after 5 h. Also, the reduction reaction was done in the presence of Fe 3 O 4 NPs, GO/Fe 3 O 4 and GO/Fe 3 O 4 @PDA nanocomposites, which showed the yield was low and end time of the reduction reaction was longer than GO/Fe 3 O 4 @PDA/Au nanocomposite (Table 1). Now, in order to generalize, we further extended our catalytic explorations with our developed catalyst in the reduction of several other nitroarenes, being monitored over UV-Vis spectrometry and the results have been documented in Table 2 Notwithstanding the type and location of substituent (1-Cl, 2/3/4-CH 3 , 2/4-OH, 2/3/4-NH 2 , 2-OCH 3 ) in the aryl ring, all the substrates were highly compatible in the reaction conditions and afforded excellent conversions. The productivity was in the range of 90-98%. Notably, the electron rich nitroanilines ( Table 2, entries 11-13) were found to undergo reduction at a relatively faster rate (10-15 min) as compared to electron deficient chloro (Table 2, entry 2) or dinitro substrates (Table 2, entry 6) (90 min). Among the nitrophenols, the reduction of 2-susbtituted nitroarene was sluggish, might be due to field effect or spatial electron inhibition. A wide number of nitroarenes have been found to be compatible at the developed conditions resulting  Discussion of reaction mechanism. For mechanism discussion, more attention should be paid to the catalytic kinetics. The catalytic process is a complex process, which involves the diffusion process, adsorption process, catalyst wetting angle and etc. [87][88][89] . The plausible catalytic pathway for the reduction of nitrobenzene over GO/Fe 3 O 4 @PDA/Au nanocomposite in presence of NaBH 4 can be explained based on the Langmuir-Hinshelwood model 90,91 . Initially, the BH 4 ions get adsorbed on the catalyst surface and generate hydride ions (H -) in situ towards the formation an Au-hydride complex. Subsequently, the substrate nitrobenzene (NB) also approaches the nano Au surface. The adsorption of both H − and NB occurs reversibly following Langmuir isotherm. Then, interfacial electron transfer occurs from hydrides to NB. The rate of electron transfer is proportional to the conversion rate. The reduction pathway to aniline involves two fast intermediate steps via nitrosobenzene and hydroxylamine 92,93 . A slow hydro-deoxygenation step is followed thereafter in the reduction from hydroxylamine to aniline, being considered as the rate determining step. Finally, desorption of aniline takes place from the catalyst surface to make it free for a new cycle. These whole process of diffusion of reactants, adsorption/desorption equilibria are very facile over the Au catalyst (Scheme 2).   methodology, facile isolation, regeneration and reusability of catalyst is an indispensable task. After the successful demonstration of catalytic efficiency of GO/Fe 3 O 4 @PDA/Au nanocomposite in the reduction of nitroarenes, it was the turn to prove those said criteria. Due to the strong inner ferromagnetic core, it was isolated totally with ease from the reaction mixture 4-NP reduction by using an external magnet. The catalyst was then washed with aqueous ethanol, dried and recycled for 8 times with no appreciable loss in activity (Fig. 8). To emphasize the fact, a TEM analysis was conducted with the reused catalyst after 8th run. Amusingly, it retained its structural morphology as initial, which can be seen from Fig. 9a. Also, a FT-IR spectrum for reused catalyst after 8th run (Fig. 9b) shown the same signals without changes in functionality with fresh catalyst. Hence, significant stability of the material validates its outstanding reusability. A leaching test was carried out as well in order to prove the robustness of our catalyst. After the isolation of the catalyst from reaction mixture, an ICP-AES analysis was performed with the reaction filtrate. It was gratifying to ensure that only a marginal amount of Au has been leached out. After the 7th cycle, the Au content in the nanocomposite was greater than 90%. The slight decrease in yield in the 8th cycle is probably due to this loss of Au and the adsorption of product (4-AP) over the catalyst surface 94 .

Distinctiveness of our result.
To prove the uniqueness of our devised protocol, we justified our results in the reduction of 4-NP with some previously reported methods which have been shown in Table 3. As can be seen, the GO/Fe 3 O 4 @PDA/Au catalyst definitely has superiority over others in terms of rate constant.

Conclusions
In summary, we have prepared an Au NP implanted PDA coated magnetic GO nanocomposite (GO/Fe 3 O 4 @PDA/ Au). The material was prepared following stepwise post-synthetic approach involving the in situ green reduction of Au (III) ions, without using any harsh conditions. PDA acts as the green reductant as well as the stabilizer of Au NPs. The enormous surface area of GO was exploited for grafting the Au(0)/PDA@Fe 3 O 4 complex. After full characterization of this novel catalyst using different standard techniques, its catalytic activity was examined towards the reduction of nitroarenes to their corresponding amines. Initially, 4-nitrophenol was selected as a  www.nature.com/scientificreports/ model compound and treated with sodium borohydride in the presence of aforementioned composite at room temperature. The reduction proceeds smoothly leading to the formation of p-hydroxyaniline, a useful starting material for production of acetoaminophen (paracetamols), an over -counter analgesic medication. Then, the as synthesized nanocatalyst was employed in the reduction of a wide range of nitroaromaticssusingNaBH 4 as the hydrogen source with outstanding conversions and great selectivity. After the completion of the reaction, the catalyst was separated easily, just by using a magnet bar and without any pre-activation were reused for 8successive cycles with almost consistent reactivity. The material was stable enough towards leaching as confirmed  www.nature.com/scientificreports/ by ICP-AES analysis. Its excellent catalytic performance is assumed to be due to synergistic bridged interaction between Au(0),Fe 3 O 4 and GO sheets, facilitating the faster electron transport to the substrate towards the reduction.