Materials Letters 148 (2015) 37–40
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Green synthesis of gold nanoparticles using Pogestemon benghalensis (B) O. Ktz. leaf extract and studies of their photocatalytic activity in degradation of methylene blue Bappi Paul, Bishal Bhuyan, Debraj Dhar Purkayastha, Madhudeepa Dey, Siddhartha Sankar Dhar n Department of Chemistry, National Institute of Technology Silchar, Silchar 788010, Assam, India
art ic l e i nf o
a b s t r a c t
Article history: Received 16 December 2014 Accepted 11 February 2015 Available online 20 February 2015
Biosynthesis of gold nanoparticles has been accomplished via reduction of aqueous chloroauric acid solution with the leaf extract of Pogestemon benghalensis (B) O. Ktz., as both reductant and stabilizer. The synthesized nanoparticles were characterized by UV–visible, powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier transform infrared (FT-IR) studies. The UV–visible spectrum of the synthesized gold nanoparticles showed surface plasmon resonance (SPR) around 555 nm after 12 h. The powder XRD pattern furnished evidence for the formation of face-centered cubic structure of gold having average crystallite size 13.07 nm. The shapes of synthesized gold nanoparticles are mostly spherical and triangular with sizes 10–50 nm. The photocatalytic degradation of methylene blue dye was monitored spectrophotometrically using gold nanoparticles as catalyst under visible light illumination. & 2015 Elsevier B.V. All rights reserved.
Keywords: Biosynthesis Nanoparticles Pogestemon benghalensis (B) O. Ktz. Electron microscopy Photocatalysis
1. Introduction Nowadays synthesis of nanoparticles (NPs) involving one or more principles of Green Chemistry has garnered significant interest among the scientific community [1–3]. It has been well established that there exists a strong correlation between sizes and shapes of NPs with their electric, magnetic, optical and chemical properties [4–7]. Among many metal NPs, gold nanoparticles (Au NPs) are especially important as they find wide application in life sciences, detection of microorganisms, targeted drug delivery, medical diagnoses, imaging and therapy [8,9]. Current strategies for ‘Green’ synthesis include use of nonhazardous chemicals, bio-degradable polymers, environmentally friendly solvents like plant extracts [1,10–13]. The significant advantages of using plant extracts as biogenic sources for synthesis of metal NPs are easy availability of such plants, protocols applicable at room temperature and pressure etc. In general, conventional methodologies for production of Au NPs involve chemical techniques that use expensive and environmentally hazardous organic solvents [14]. With the increasing demand for Au NPs, development of clean, non-toxic and eco-friendly processes of their synthesis has become a challenging issue for
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http://dx.doi.org/10.1016/j.matlet.2015.02.054 0167-577X/& 2015 Elsevier B.V. All rights reserved.
researchers [6]. The biogenic syntheses of Au NPs are well-known and there are reports in the literature where a variety of organisms like fungi, algae, bacteria, and plants are used [14–17]. Metal NPs including Au NPs are known to promote degradation of dyes. Au NPs are known to catalyze many reductions [8]. Environmental hazards from discharge of dyes from textile and other industries are considered to be major causes of concern due to their impact on human health. Therefore, development of newer technologies for degradation of dyes before their release into the environment will always be important in the present day context. In the present work, we wish to report biosynthesis of Au NPs using hitherto unused plant extract from the leaves of Pogestemon benghalensis (B) O. Ktz. (Family-Lamiaceae) and application of these newly synthesized Au NPs for degradation of methylene blue (MB). P. benghalensis is a traditional medicinal plant, with elliptic leaves, abundant in Indo-Burma border specially north-east corner of India [18]. Its young shoots and leaves are used to cure stomach ailments, increase digestive power, in cuts and injuries to stop bleeding and mainly for menstrual disorders.
2. Experimental Materials and physical measurements: The leaves of P. benghalensis were cleaned thoroughly in fresh water followed by distilled water. The leaf extract was prepared by boiling 20 g of finely cut
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leaves in 100 ml of distilled water for 10 min and filtered. Chloroauric acid and sodium borohydride (SB) were purchased from Sigma-Aldrich. MB was obtained from Merck India Ltd. Double distilled water was used throughout the experiment. Absorption spectrum was recorded on a Carry Varian-450 UV– visible spectrophotometer. XRD measurements were carried out on a Bruker AXS D8-Advance powder X-ray diffractometer with CuKα radiation (λ ¼ 1.5418 Å) with a scan speed of 21/min. Transmission electron microscopy images were obtained on a JEOL, JEM2100 equipment. TEM grids were prepared using a few drops of the nanoparticles followed by drying. FT-IR spectrum was recorded on KBr matrix with a Bruker 3000 Hyperion Microscope with a Vertex 80 FT-IR system. Biosynthesis of Au NPs: 10 ml of leaf extract was added to 90 ml (10 3 M) aqueous solution of chloroauric acid and was stirred for 12 h. The progress of the reaction was monitored by observing color change as well as recording UV–visible spectrum. The initial light-yellow solution turned to purple, indicating formation of colloidal gold. The supernatant containing Au NPs was collected by centrifugation at 10,000 rpm. The solution was dried in a vacuum desiccator and the solid Au NPs was collected. Catalytic performance test: In the typical run, 2 ml of freshly prepared SB solution (0.2 M) was mixed with 50 ml (10 mg L 1) aqueous solution of MB at room temperature (30 71 1C). To this, 1 mg of catalyst was added and mixed quickly. At a regular interval of time, 4 ml of the suspension was withdrawn and centrifuged immediately. The absorbance of the supernatant was then measured using a UV–visible spectrophotometer. The reaction is also monitored without catalyst.
after 12 h. This red-shift along with steady increase in intensity of SPR clearly suggested gradual increase of size and yield of the nanoparticles with time. The inset (Fig. 1) shows the change in solution color (light yellow to purple). The synthesized nanoparticles covered with biomolecules are well dispersed in solution and fairly stable (up to 3 months) as indicated by retention of red color of the solution and steady SPR band position. The XRD pattern (Fig. 2(a)) of the synthesized nanomaterials showed five diffraction peaks at 2θ ¼38.151, 44.261, 64.571, 77.721, and 81.511 which corresponds to (111), (200), (220), (311), and (222) planes of the face-centered cubic gold (space group Fm3m, JCPDS File no.89-3697). No peaks attributable to impurities were observed. The broad diffraction peaks clearly indicated reduced crystallite size. The average crystallite size estimated by the Debye–Scherrer formula using a Gaussian fit was found to be 13.07 nm. The FT-IR spectrum of the Au NPs (Fig. 2(b)) showed bands at 3433 and 1055 cm 1 due to N–H and C–O stretching, respectively. The bands at 2920 and 2854 cm 1 are due to C–H stretching, those at 1633 and 1437 cm 1 correspond to the amide I of the polypeptides
3. Results and discussion Au NPs were synthesized via bioreduction of aqueous chloroauric acid solution with the leaf extract of P. benghalensis. The protein molecules present in the leaf extract served both as reductant and stabilizer. The as-synthesized NPs were characterized by UV–visible, XRD, TEM, and FT-IR studies. Au NPs show SPR around 510–560 nm [19]. The time-dependent UV–visible spectra (Fig. 1) of the synthesized Au NPs showed SPR band around 550 nm after 1 h which red shifted and appeared around 555 nm
Fig. 1. The time-dependent UV–visible spectra of gold nanoparticles (inset showing (A) aqueous chloroauric acid solution (B) gold nanoparticles solution).
Fig. 2. (a) XRD pattern of gold nanoparticles and (b) FT-IR spectrum of gold nanoparticles.
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Fig. 3. (a, b) TEM images (c) HRTEM image and (d) ED pattern of gold nanoparticles.
and symmetric stretching of the carboxylate groups in the amino acid residues of the protein molecules. This gives clear indication of the presence of proteins and other organic molecules in the material, which might have been produced by P. benghalensis extracellularly. Thus surface capped protein molecules provided stability and prevented agglomeration of the nanoparticles [20]. The TEM image (Fig. 3) showed that the synthesized Au NPs were well dispersed, predominantly spherical and triangular in shape with sizes 10–50 nm. The HRTEM image showed the lattice fringes between the two adjacent planes to be 0.231 nm apart which corresponds to the interplanar separation of the (111) plane of face-centered cubic gold. The electron diffraction (ED) pattern indicated polycrystalline nature of the synthesized material. The catalytic efficiency of Au NPs in reduction of MB to leuco methylene blue (LMB) was investigated in presence of SB. UV– visible spectra (Fig. 4(a)) of an aqueous solution of MB showed the absorption peak (λmax) at 662 nm which corresponds to n–πn transition and a shoulder peak at 612 nm. The catalytic reaction was monitored spectrophotometrically by following the decrease of absorbance at λmax662 nm with time. The reduction of MB dye is a thermodynamically favorable process. The discoloration of dye occurred within 8 min, indicating the structural changes and the removal of chromophoric group for the entire amount of dye taken. The absorbance of MB decreases only slightly without the catalyst indicating very slow reaction rate, whereas those with catalyst decreases abruptly, suggesting the faster reaction rate. Au NPs act as an electron relay and initiate shifting of electron from
BH4 ion (donor B2H4/BH4 ) to MB (acceptor LMB/MB) and thus causing reduction of dye. BH4 ion simultaneously adsorbed on the surface of NPs and thus electron transfer occurs from BH4 ion to MB through NPs [21]. The degradation of MB fitted well with the pseudo-first order equation, ln(Co/C)¼ kt, where ‘Co’ and ‘C’ correspond to the concentration of solution at different illumination time, ‘k’ is the rate constant of the reaction and ‘t’ is the reaction time (min). The value of ‘k’ was obtained from the slope of the graph between ln(Co/C) and time (Fig. 4(b)) and was found to be 0.1758 min 1. Pertinent here is to mention that Au NPs synthesized by reduction of chloroauric acid with external reducing agents tend to form agglomerates due to the high surface energy of the NPs, which in turn reduce the access of the reactants to catalytically active sites. This reduces catalytic activity of NPs. However, Au NPs synthesized without any external reducing are free from agglomeration and act effectively as photocatalyst in the degradation of MB.
4. Conclusion Gold nanoparticles have been successfully synthesized using the leaf extract of P. benghalensis following a green procedure. No external reducing agent or surfactants were needed for the synthesis of nanoparticles. The protein molecules present in the leaf extract served a dual role as reducing and stabilizing agent. The synthesized nanoparticles showed pronounced photocatalytic activity.
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Fig. 4. (a) UV–visible spectra of reduction of MB by sodium borohydride in presence of gold nanoparticles as catalyst and (b) plot of ln(Co/C) vs. time (min).
Acknowledgment The authors acknowledge SAIF IIT Bombay for providing analytical results of the sample. The financial support from the Director NIT Silchar is gratefully acknowledged.
