Acacia nilotica (Babool) leaf extract mediated size-controlled rapid synthesis of gold nanoparticles and study of its catalytic activity
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The leaf extract of Acacia nilotica (Babool) is rich in different types of plant secondary metabolites such as flavanoids, tannins, triterpenoids, saponines, etc. We have demonstrated the use of the leaf extract for the synthesis of gold nanoparticles in water at room temperature under very mild conditions. The synthesis of the gold nanoparticles was complete in several minutes, and no extra stabilizing or capping agents were necessary. The size of the nanoparticles could be controlled by varying the concentration of the leaf extract. The gold nanoparticles were characterized by HRTEM, surface plasmon resonance spectroscopy, and X-ray diffraction studies. The synthesized gold nanoparticles have been used as an efficient catalyst for the reduction of 4-nitrophenol to 4-aminophenol in water at room temperature.
KeywordsGold nanoparticle Green synthesis Acacia nilotica Polyphenols Catalytic reduction
The synthesis of gold nanoparticles (AuNPs) and their utilization in diversified areas such as catalysis, drug delivery, biodiagnostics, medicine, and electronics have become an area of tremendous investigations during the last two decades because of their unique optical, physical, chemical, and magnetic properties compared to the bulk solids [1, 2, 3, 4]. Among various synthetic methods, the solution phase synthesis involving the reduction of Au(III) to Au(0) by plant extracts has gained profound significance in recent years because of the renewable and nontoxic nature of the plant extracts, eco-friendly aqueous medium, and mild reaction condition . Moreover, this method becomes more advantageous over other synthetic methods since the plant extract itself acts as a stabilizer, and no additional stabilizers or capping agents are needed . The extracts of Punica granatum, Breynia rhamnoides, Saraca indica, Piper betle, etc. have been utilized for the synthesis of AuNPs. During our investigations on the utilization of triterpenoids (C30s) as renewable functional nanoentities [11, 12, 13, 14], it occurred to us that the medicinally important leaf extract of Acacia nilotica, rich in polyphenolic compounds, can be utilized for the synthesis of AuNPs from HAuCl4. Herein, we report a very mild and environment-friendly method for the synthesis of AuNPs from the leaf extract of A. nilotica at room temperature without any additional capping or stabilizing agents. The AuNPs were characterized by surface plasmon resonance (SPR) spectroscopy, high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) studies. The synthesized AuNPs have been utilized as an efficient catalyst for the sodium borohydride reduction of 4-nitrophenol to 4-aminophenol at room temperature, and the reduction kinetics have been investigated spectrophotometrically.
Au(III) solution, HAuCl4, was purchased from Sisco Research Laboratory, Mumbai, India, and used without further purification. HAuCl4 (35.4 mg) was dissolved in distilled water (10 mL) to obtain a 10.42-mM Au(III) stock solution. The leaves of A. nilotica were collected from the Vidyasagar University, Midnapore, India campus. Doubled distilled water was used for experimental purposes.
Preparation of the A. nilotica leaf extract
Dried and finely powdered A. nilotica leaf (2.9 g) was suspended in ethanol (30 mL) and stirred magnetically at room temperature for 1 h and then filtered. Volatiles of the greenish filtrate was removed under reduced pressure to afford a sticky solid (0.390 g) that was purified by column chromatography (Si gel, 100 to 200 mesh) using 30% methanol-ethyl acetate as the eluant affording a greenish solid (0.3 g). The column-purified leaf extract (0.005 g) was suspended in distilled water (10 mL) and sonicated in an ultrasonicator bath for 10 min to get a semitransparent solution (500 mgL?1).
Synthesis of nanoparticles
Aliquots of Au(III) solution (0.16 mL, 10.42 mM each) were added dropwise to the leaf extract solution (containing polyhydroxy aromatic compounds) to prepare a series of stabilized AuNPs where the concentration of the leaf extract varied from 20 to 200 mg L?1, and the concentration of Au(III) was fixed at 0.42 mM. UV-visible measurements of the solutions were carried out after 3 h of HAuCl4, and A. nilotica leaf extract were mixed.
Transmission electron microscopy of AuNPs was carried out in Tecnai G2 20 instrument (FEI, Hillsboro, OR, USA). XRD patterns of the stabilized AuNPs were studied in Panalytical X'pert Pro diffractometer (Almelo, The Netherlands) with Cu-K? radiation (? = 1.54 ?). Mass spectral analysis of the left extract was carried out in Shimadzu GCMS QP 2100 plus (Kyoto, Japan). Shimadzu 1601 spectrophotometer was used for UV-visible spectroscopic measurements. The FTIR spectra of samples were recorded in Perkin Elmer FTIR Spectrum-II model (Waltham, MA, USA) using KBr pellet.
Results and discussion
UV-visible spectroscopy studies
Due to charge transfer interactions between metal and the chloro ligands of HAuCl4, the UV-visible spectrum showed two peaks at 220 and 288 nm (Figure?1a). On reaction with increasing concentration of the A. nilotica leaf extract, disappearance of these two peaks were observed with concomitant appearance of a SPR band between 533 to 529 nm region (Figure?1b,c,d,e,f,g). The variation of color of the AuNP colloids has been reported arising due to the change in size, shape, composition, crystallinity, etc. . In our present studies, a shift of the SPR band was observed from 533 to 529 nm and the intensities increased with the increasing concentration of the leaf extract from 20 to 200 mgL?1. The strong peak at 272 nm at 200 mgL?1 concentration is due to the formation of quinone moieties formed due to the oxidation of the phenolic compounds by Au(III) (Figure?1g).
HRTEM, EDX, FTIR, and XRD studies
With the increasing concentration of the leaf extract, smaller sized AuNPs can be effectively stabilized by the polyphenolic compounds and the oxidized quinones. As the steric bulk of the surrounding ligands of the AuNPs prevents aggregation, the size of the AuNPs were smaller at higher concentration of the leaf extract. Lattice fringes of the AuNPs with a d-spacing of 0.24 nm were observed from the HRTEM image (Figure?2c). The fringe spacing matched with the expected d-spacing of the  plane of face-centered cubic crystalline Au (JCPDS, no. 04?0784). Selected area electron diffraction (SAED) pattern obtained from a AuNP (Figure?2h) showed the diffraction rings from inner to outer associated with the , , , and  atomic planes of Au indicating the formation of crystalline gold nanoparticles. The elemental composition of the synthesized AuNPs was determined by energy dispersive X-ray analysis (EDX). Area profile analysis of the synthesized nanoparticles showed strong peaks of Au (Additional file 1: Figure S3) confirming the formation of AuNPs. The presence of organic matrix was evident from characteristic carbon peak. The AuNP samples were coated over a glass plate, and X-ray diffraction analysis of the AuNPs was carried out after removal of the volatiles. The characteristic reflections of the planes (111), (200), (220), and (311) at 2? = 38.3?, 44.3?, 64.8?, and 77.8?, respectively, (Additional file 1: Figure S4) indicated the crystallinity of the metallic face-centered cubic AuNPs. These values are in agreement with the reported standards JCPDS file no. 04?0784 for crystalline gold. The comparatively greater peak intensity of the (111) plane is indicative of the predominant orientation of the (111) plane.
Mechanism of the formation of stabilized AuNPs
Study of the catalytic activity of stabilized AuNPs
Catalytic activity of colloidal AuNPs (synthesized freshly with 60 mgL?1of leaf extract)
Concentration of 4-nitrophenol (mM)
Concentration of sodium borohydride (mM)
Volume of colloidal AuNPs
Reaction completion time (min)
Catalytic rate constant (k) (min?1)
A very mild and efficient method for the green synthesis of colloidal gold nanoparticles has been demonstrated using the medicinally important A. nilotica leaf extract. According to our knowledge, this is the first report of the synthesis of AuNPs using A. nilotica leaf extract. The polyphenolic compounds present in the leaf extract act as an effective reducing agent as well as stabilizing agent, and AuNPs of 6 to 12 nm size were formed. Increasing the concentration of the leaf extract resulted in the formation of smaller sized nanoparticles as evident from HRTEM analysis. The synthesized colloidal AuNPs were highly efficient as catalyst for the reduction of p-nitrophenol to p-aminophenol at room temperature as evident from the spectrophotometric kinetic studies. As A. nilotica leaf extract has tremendous medicinal significance and AuNPs are biocompatible, the results described here will be useful in biomedical applications as well as nanoscience and nanotechnology.
RM carried out the experiments, analyzed the data and participated in the manuscript preparation. BGB as the corresponding author participated throughout the investigations and manuscript writing. NM carried out preliminary studies.
BGB thanks CSIR for funding. RM thanks UGC, New Delhi for senior research fellowship.
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