Optimization of green synthesis of gold nanoparticles using Delonix regia leaf extract and evaluation of their catalytic activity

Preeti Dauthal, Mausumi Mukhopadhyay*
Department of Chemical Engineering, S.V. National Institute of Technology, Surat-395007, Gujarat, India
Tel.: +91 261 2201645; Fax: +91 261 2227334
*Email: mmu@ched.svnit.ac.in; mausumi_mukhopadhyay@yahoo.com

Graphical abstract

Bio-fabrication methods for nanoparticles synthesis was reported to be more ecologically sound and sustainable alternative to chemical and physical methods. Based on the aforesaid premise, present study deals with the optimization of gold nanoparticles (Au-NPs) fabrication using Delonix regia (D. regia) leaf extract. The effects of the amount of D. regia leaf extract, reaction temperature, reaction time and pH on the bio-fabrication of Au-NPs were also reported. UV-visible (UV-vis) spectroscopy, Dynamic Light Scattering (DLS), ? potential, Transmission Electron Microscopy (TEM), X-ray diffraction (XRD), Energy Disperse X-ray Spectroscopy (EDX) and Fourier Transform Infrared Spectroscopy (FTIR) techniques were adopted for evaluation of size, stability, morphology, crystal nature and purity of the bio- fabricated Au-NPs. UV-vis spectra represented the surface Plasmon resonance (SPR) peak at
542 nm which suggested the reduction of gold ions (Au3+) into zerovalent gold (Au0). The
results were rapid for the bio-fabrication of spherical Au-NPs of about 4-24 nm (by TEM) size within 10 min of reaction time. In the TEM micrograph the edges of the nanoparticles were lighter compared to that of the centre, suggested capping and adherence of bio-organics of D. regia leaf extract on the surface of Au-NPs. This observation was further supported by the FTIR, EDX and DLS analysis. Crystalline nature of nanoparticles was evident from bright circular spot in the Selected Area Electron Diffraction (SAED) pattern and broad

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diffraction peaks in the XRD pattern. XRD spectra represented the five distinctive peaks present at 38.46? (111), 44.43? (200), 65.01? (220), 77.81? (311) and 82.25? (222) position, suggested the presence of crystalline gold at zero oxidation state. The lattice constant calculated from these patterns was 4.069 Ã?, confirmed the face centered cubic (fcc) geometry oriented along the (111) plane. The average particles size was calculated as 25.77 nm with the line width of the maximum intensity (111) diffraction peak by Debye Scherer equation. Thus, the XRD pattern proved strong evidence in favour of the TEM analysis for the presence of crystalline Au-NPs. The appearance of optical absorption peak at 2.2 keV in EDX spectra further confirmed the presence of Au-NPs. High negative ? potential (-15 mV) on the surface
of bio-fabricated Au-NPs suggested long term stability of colloidal Au-NPs. Particle size distribution of colloidal Au-NPs in DLS analysis was found in the range of 12-55 nm with an average particle size of about 39 nm with polydispersity index (PdI) 0.279, which appears to be higher as compare to TEM and XRD measurements. This was due to the fact that the measured size also included the bioorganic compounds enveloping in the core of the Au-NPs. FTIR analysis confirmed further the role of polar polyphenolic compounds of D. regia leaf extract for bio-fabrication of Au-NPs and their chemical framework were also provided effective wrapping around the nanoparticles and robustness against agglomeration. Long term stability (upto 10 days) of the bio-fabricated Au-NPs was also confirmed by UV-vis, DLS and ? potential analysis. Catalytic activity of bio-fabricated Au-NPs was evaluated for the reduction of toxic pollutant 2-Nitroaniline (2-NA). These nanoparticles were demonstrated a good catalytic activity for the reduction of 2-NA to 1, 2 Benzenediamine. The reduction of reaction followed pseudo first-order kinetics with respect to the substrates. It was also found that high concentration of Au-NPs and high temperature reduced the induction time for catalytic reduction and increased the reduction rate. Catalytic reduction of 2-NA was found to
increase by a factor of 1.6-1.9 for every 10 °C rise in temperature. The pseudo first-order rate

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constant was estimated to be 12.28 � 10?2 min?1 at room temperature (28 ± 2 °C) for this reduction reaction. The activation energy (Ea) for this catalytic reduction was calculated as
44.26 kJ mol?1. Au-NPs were recycled up to 10 cycles without significant changes in
reduction rate constant. Thus, bio-fabricated Au-NPs proved as a potent recyclable catalyst for the industrial applications. These results suggested the fabrication of stable Au-NPs with the available biological resource D. regia, which offers an inexpensive, sustainable and eco- friendly alternative for fabrication of catalytically active Au-NPs.
Keywords: Delonix regia; Biofabrication; Gold; Nanoparticle; Reduction

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