Spectrophotometric evaluation of surface morphology dependent catalytic activity of biosynthesized silver and gold nanoparticles using UV–vis spectra: A comparative kinetic study
Graphical abstract
Introduction
Metal nanoparticles are of great importance due to their remarkable physical and chemical properties as well as their potential applications in optics, electronics, sensing, catalysis, biodiagnostics and surface-enhanced Raman scattering (SERS) [1], [2], [3], [4], [5]. The various properties of metal nanoparticles can be tuned by controlling their shapes and sizes. Silver nanoparticles are of tremendous interest to researchers due to their excellent optical, electrical, catalytic and antibacterial properties [6], [7], [8], [9]. The antibacterial activities of silver ions and salts have studied since early days and they can be utilized to control the bacterial growth in various applications such as prosthese, catheters, burn wounds, etc. [10], [11]. Silver nanoparticles can also exhibit excellent catalytic activity due to its high surface area to volume ratio and high surface energy making them extremely reactive. Therefore, silver nanoparticles of various sizes and shapes can be used as an effective catalyst in various catalytic reactions including chemical, electrochemical and photochemical [12], [13], [14].
Therefore, developing new and novel synthetic protocol for the preparation of silver nanoparticles is very important in nanotechnological research with special emphasis on the controlling of their sizes and shapes. The most common method for the synthesis of silver nanoparticles is the chemical reduction of silver salts by reducing agents in the presence of different stabilizing agents such as polymers and surfactants. However, the chemical reduction method is highly tedious, expensive and non-eco-friendly due to the use of toxic and harmful chemicals.
The natural constituents of the environment, especially the secondary metabolites present in the plant and fruit extracts as well as in micro-organisms such as bacteria and fungi can be a possible sources of reducing and stabilizing agents to synthesize metal nanoparticles. The biosynthesis of metal nanoparticles can provide a cost effective eco-friendly alternative to the most commonly used chemical routes due to its biocompatibility, simplicity and low cost. Several groups have used plant and fruit extracts to synthesize both silver and gold nanoparticles of various sizes and shapes [15], [16], [17], [18], [19], [20]. In this article, we have reported the synthesis of silver nanoparticles using the plant extract of Piper betle. The synthesized silver nanoparticles were characterized by UV–vis spectroscopy, X-ray diffraction (XRD), Fourier-transform infra-red (FTIR) spectroscopy and transmission electron microscopy (TEM). We have used the biosynthesized silver nanoparticles as effective surface-enhanced Raman scattering (SERS) active substrate. We have also carried out a systematic comparative studies on the catalytic activity of as synthesized silver nanoparticles along with the gold nanoparticles synthesized by the same plant extract to study the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of NaBH4 at 0–4 °C. It is important to mention here that 4-AP is an important intermediate in the synthesis of various analgesic and antipyretic drugs like paracetamol, acetaniline, phenacetin, etc. As 4-NP is used as a common precursor, the conversion of 4-NP to 4-AP is of great importance to study further. Metal nanoparticles such as gold and silver can exhibit catalytic activity towards this reaction.
The catalytic activities of gold and silver nanoparticles were studied using UV–vis spectra and the reaction was monitored spectrophotometrically varying the nanoparticles concentrations. We find that reaction has a pseudo-first order rate constant of 2.18 × 10−3 s−1 and corresponding activity parameter of 436 s−1 g−1 in the presence of 0.5 × 10−5 g of silver nanoparticles as the whole reaction completed within 85 min.
Section snippets
Materials and methods
All chemicals used in our studies were of analytical grade and used without further purification. All aqueous solutions were prepared with de-ionized water (DI). Sodium borohydride (NaBH4) (Sigma–Aldrich) solution was freshly prepared in ice cold water. 4-NP (Sigma–Aldrich) was purified by repeated crystallization in petroleum ether and ethyl acetate.
Results and discussion
The bio-reduction of Ag+ ions to Ag0 was monitored by UV–vis spectroscopy. Fig. 1(A) shows the UV–vis spectra of the biosynthesized Ag nanoparticles along with Piper betle broth (curve 1). A strong surface-plasmon resonance band (SPR) at 434 nm is clearly seen in curve 2 and arises due to the excitation of localized surface-plasmon oscillations of conduction electrons in silver nanoparticles whereas, the SPR band typically observed for silver nanoparticles is totally absent in the broth of the
Conclusions
We have synthesized silver nanoparticles using Bombay Piper betle plant extract as the source for reducing and stabilizing agents at room temperature. The biosynthesized silver nanoparticles were characterized by UV–vis spectroscopy, XRD, FTIR spectroscopy and TEM studies. We have also reported SERS measurements of R6G molecules adsorbed on these biosynthesized silver nanoparticles. The SERS results suggest that silver nanoparticles can be used as an efficient SERS active substrate. We have
Acknowledgements
BA thanks the Board of College and University Development (BCUD), (BCUD, Finance/2013-14/1776/dated: 20/01/2014) University of Pune for provision of financial support and UKS would like to thank Indian National Science Academy (INSA), New Delhi, India for INSA Visiting Scientist Fellowship (SP/VF-9/2014-15/273/01 April, 2014) under the supervision of BA at Bio-inspired Materials Science Laboratory, Department of Chemistry, Savitribai Phule Pune University, Ganeshkind, Pune-411007, India. UKS
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