A biomimetic synthesis of stable gold nanoparticles derived from aqueous extract of Foeniculum vulgare seeds and evaluation of their catalytic activity
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A facile biomimetic approach for the synthesis of gold nanoparticles (AuNPs) using aqueous extract of fennel (Foeniculum vulgare) seeds have been reported in this article. The seeds of F. vulgare are rich in various plant secondary metabolites (phytochemicals) such as polyphenolic acids, flavonoids, and saponins. The phytochemicals of F. vulgare seeds play dual role of reducing and stabilizing agents. The formation of gold nanoparticles was evidenced from the appearance of intense purple color at room temperature with λ max around 550 nm in the UV–Vis absorption spectra. The stable AuNPs were further characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy and transmission electron microscopy (TEM) analysis. The synthesized nanoparticles were observed to be polydispersed, spherical and ranged from 10 to 30 nm with an average size of 20 ± 2 nm, as obtained from TEM images. The catalytic activity of gold nanoparticles was investigated by studying the reduction of anthropogenic dyes such as methylene blue (MB) and rhodamine B (Rh-B) with sodium borohydride. Results showed the possible applications of biogenic AuNPs in environment related problems.
KeywordsGold nanoparticles Biogenic synthesis Catalytic reduction Anthropogenic dyes Fennel seed
Several water soluble anthropogenic dyes are extensively utilized in various industrial processes such as cosmetics, textiles, printing, ceramics, pharmaceuticals and food processing (Kant 2012). For example, methylene blue, a thiazine dye, is readily used in textile, data storage and holographic industries (Rajesh et al. 2014). It is also utilized for staining tissues and body fluids during diagnosis and surgeries (American Pharmacists Association 2012). Moreover, it also has applications as anti-malarial agent and chemotherapeutic agent (Schirmer et al. 2003; Mwangi et al. 2016). Similarly, Rhodamine B, a fluorescent staining dye, has applications in the field of flow cytometry (Temmerman and Nickel 2009), fluorescence microscopy and ELISA tracer techniques (Shelley 1969; Forster et al. 2012). Further, it is also utilized as tracer for determination of transport and flow direction of water (Mull 1993; Hardy et al. 2016). All these processes, at various stages, require large quantity of water, and thus produce effluents which are fortified with various hazardous colorants. These waste water discharges with various dye stuffs lead to elevated demand of oxygen chemically as well as biologically (i.e., COD and BOD).
Most of the anthropogenic dyes are carcinogenic and mutagenic and cause high level toxicity to humans, aquatic organisms and whole eco-system. Therefore, the degradation of these dyes is important industrially and environmentally. Generally, coagulation, flocculation, adsorption, chemical precipitation and oxidation methods are employed for the removal of dyes. More recently, noble metal nanoparticles because of their enhanced catalytic activity and high stability have gained importance in the area of catalytic degradation of toxic coloring agents, such as dyes and pigments. For instance, the catalytic reduction and degradation of various dye pollutants using metal nanoparticles (MNPs) such as Cu, Ag, Au, Pd and Pt has been reported (Gangula et al. 2011; Sahoo et al. 2014; Kumari et al. 2015; Yang et al. 2015). Various biogenic synthesis routes have been employed in the fabrication of noble MNPs using Cylindrocladium floridanum and Penicillium fungus (Sadowski et al. 2008; Narayanan et al. 2013), Vigna radiata seed extract (Choudhary et al. 2016), Catharanthus roseus leaf extract (Kalaiselvi et al. 2015), Solanum lycopersicums fruit extract(Umadevi et al. 2013), Cocos nucifera coir extract (Roopan et al. 2013), Artemisia nilagirica leaf extract (Vijayakumar et al. 2013), etc.
In this report, we have reported the phytochemical preparation of (AuNPs) utilizing aqueous seeds extract of Foeniculum vulgare (fennel seeds). Fennel is a flowering herbaceous plant which belongs to the Apiaceae family. The F. vulgare seeds are native to Southern Europe and popular as most sought-after ingredient in Chinese, Turkey, and Indian cuisines. The seeds of fennel are extensively grown in all over Europe, Middle-East and all over the Mediterranean regions for their distinctive sweet, anise-flavor. Moreover, the seeds of F. vulgare have been reported to exhibit antimicrobial (Purkayastha et al. 2012), antioxidant (Roby et al. 2013), antitumor, antiallergic and memory enhancing properties (Badgujar et al. 2014). The major phytochemicals present in the seeds are saponins, tannins, polyphenolic acids and flavonoids (Purkayastha et al. 2012).
The as-synthesized AuNPs were analyzed using various spectroscopic and microscopic techniques such as UV–Vis; TEM, XRD and FT-IR. Further, the AuNPs were employed as nanocatalysts for the borohydride reduction of MB and Rh-B. The catalytic reduction of these carcinogenic dyes was investigated periodically by studying their decline in intensity of absorption using UV–Vis spectroscopy.
Materials and methods
Hydrogen tetra chloroaurate trihydrate (HAuCl4.3H2O) of reagent grade was obtained from Sigma-Aldrich while methylene blue (MB), rhodamine B (Rh-B) dyes and NaBH4 for catalytic reduction reactions were purchased from Fisher Scientific, Mumbai. All solutions used in the synthesis procedure were prepared with double distilled water.
Preparation of fennel seeds extract
Foeniculum vulgare seeds were obtained from the super market of Ludhiana, Punjab, India. The seeds were washed with deionized water three times to remove any dust particles and left overnight to dry. 5 g of dried seeds were refluxed at 80 °C for 30 min in a 250-ml Erlenmeyer flask containing 150 ml of deionized water. After cooling, the aqueous extract was filtered through Whatman no. 1 filter paper and stored in refrigerator for future use.
Biogenic synthesis of gold nanoparticles
For synthesis of AuNPs using fennel seeds, first, 1 mM aqueous HAuCl4.3H2O solution was prepared by dissolving 98.5 mg of HAuCl4.3H2O in 250 ml of deionized water and then gold solution and aqueous seeds extract in the ratios (7:3, 8:2, and 9:1 v/v) were carefully mixed in three different 250 ml flasks and labeled as FA-3, FA-2 and FA-1 (where the number 3, 2 and 1 corresponds to volume of seeds extract). The flasks containing reaction mixture were stirred magnetically under dark with 700–800 rpm. During the course of reaction, the color of colloidal solution was changed from yellow to purple which indicated the formation of AuNPs.
Characterization of biogenic AuNPs
The periodical monitoring of AuNPs formation at a regular interval of 15 min was carried out using Shimadzu 2550 UV–Vis spectrophotometer. An intense peak around 540–550 nm in the visible region was observed which corresponds to surface plasmon resonance (SPR) absorption of nanogold. To investigate the stability of the biogenic AuNPs, the λ max value was also recorded after 24 h. For analysis, 0.3 ml of colloidal AuNPs solution was withdrawn and diluted to 3.0 ml, by adding deionized water in a quartz cuvette. The SPR absorption of AuNPs was recorded in the wavelength range between 250 and 800 nm.
The involvement of active molecules of fennel seeds in the reduction and capping of NPs were determined using Perkin Elmer RX-I, FT-IR spectrophotometer. The dried extract and synthesized AuNPs were mixed with KBr and pellets were formed which were subjected to FT-IR analysis. For KBr pellet formation, 5 mg each of dried extract and AuNPs were separately mixed with 100 mg of KBr and condensed into pellet using hydraulic press.
To investigate the crystalline nature of prepared biogenic AuNPs, analysis through XRD was performed. The AuNPs suspension was washed thrice with double distilled water and collected by repeated centrifugation on a clean glass slide and measurement was made through a PANalytical X’PERT-PRO X-ray diffractometer by employing monochromatic Cu Kα radiation (1.5406 Å) running at 45 kV and 40 mA.
The stable washed AuNPs suspensions were sonicated and diluted with deionized water to get the intensity of absorbance around 0.50 a.u. and a drop of suspensions was taken on 200 mesh carbon coated copper grids (Ted pella, USA) and placed the grids for 2 min, and removed the remaining solution with Whatman paper, and allowed the grids to dry before measurement. After drying, the particles were visualized through Hitachi H-7500 TEM instrument.
Catalytic efficacy of biogenic AuNPs
The catalytic efficacy of synthesized gold nanoparticles was examined for borohydride reduction of two anthropogenic dyes MB and Rh-B as model reaction. Initially, a 20 ml of colloidal solution of AuNPs was centrifuged and washed thrice with double distilled water and twice with ethanol. Afterwards, the gold nanopellet obtained, was dissolved ultrasonically in 20 ml of distilled water to make gold nanoparticles solution free from biomolecules. The degradation of each dye was studied by recording the UV–Vis spectrum for 2 min. The degradation was also studied without AuNPs and NaBH4 as control reactions.
Results and discussion
X-Ray diffraction analysis
Mechanism for the formation of AuNPs by phytoconstituents of fennel seeds
Catalytic activity of AuNPs for reduction of anthropogenic dye pollutants
A facile, biomimetic and environment friendly procedure for the preparation of stable gold nanoparticles (AuNPs) was described using aqueous extract of fennel (Foeniculum vulgare) seeds. The bioreduction of gold ions to crystalline nanoparticles with an average size of 20 ± 2 nm and subsequent capping was mediated by polyphenolic compounds of fennel seeds. Further, these surfactant free biogenically prepared gold nanoparticles showed an excellent catalytic activity towards the degradation of anthropogenic dye pollutants. Hence, the efficient dye degradation efficacy of biogenic AuNPs can have wide applications in environment remediation of waste water polluted with toxic synthetic dyes and pigments.
The work is supported by Northern Regional College Bureau, University Grants Commission (NRCB-UGC) New Delhi, under minor research project scheme [F. 8-4 (107)/2015(MRP/NRCB)]. MKC would like to thank Principal and Management of Guru Nanak National College, Doraha (Ludhiana), Punjab, for their constant support, encouragement and providing required infrastructure and laboratory facilities for smoothly carry out the research work. We are also grateful to the SAIF, Panjab University, Chandigarh and Thapar University, Patiala for extending the TEM and XRD facilities, respectively.
Compliance with ethical standards
Conflict of interest
The authors declare no competing financial interest.
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