Greener approach for synthesis of antibacterial silver nanoparticles using aqueous solution of neem gum (Azadirachta indica L.)
Graphical abstract
Introduction
Nanotechnology is considered as one of the most important areas for scientific development in the 21st century. In nanomaterials study, the development of noble nanoparticles is increasing interest due to their attractive applications in the field of multidisciplinary research activities (Bhattacharya and Mukherjee, 2008). Metal nanoparticles have been studied in different applications owing to their specific characteristics such as catalytic activity, optical properties, electronic properties, antimicrobial properties and magnetic properties (Narayanan and Sakthivel, 2010, Yilmaz et al., 2011, Das and Velusamy, 2014). Traditionally UV irradiation, aerosol technologies, lithography, laser ablation, ultrasonic fields, and photochemical reduction techniques have been used successfully to produce various metal nanoparticles such as gold, silver, platinum and palladium (Jain and Jain, 2005, Kharissova et al., 2013). However, considering the fast growth in the usage of nanomaterials in diverse fields, there is an urgent need to develop clean, nontoxic, simple and eco-friendly procedures for their synthesis.
Synthesis of noble metal nanoparticles, in particular silver nanoparticles (AgNPs), using plant gum has become a major focus for researchers due to their simplicity of procedures, stability, and their potential applications in chemical sensing, biological imaging, antimicrobial, gene silencing, and drug delivery (Wei and Qian, 2008). Recently, several studies have reported natural polymers such as chitosan, starch and tannic acid as reducing agents for the synthesis of silver and gold nanoparticles (Vigneshwaran et al., 2006, Dadosh, 2009). Plant-based exudates and gums such as gum olibanum (Kora et al., 2012), gum acacia (Mohan et al., 2007), gum kondagogu (Kora et al., 2010) have been shown to act as reducing and stabilizing agents for the biosynthesis of AgNPs. The plant based gum is a heteropolysaccharide of natural gum and its morphological, compositional, physico-chemical, thermal, rheological, and emulsifying properties have been well studied (Srivastava and Rai, 1963, Aspinall et al., 1965, Jefferies et al., 1977). The biopolymer of gum is a rich-arabinose, acidic protein mixed calcium, magnesium, potassium and sodium salts (Tischer et al., 2002).
Neem gum is exudates of naturally occurring water soluble, complex polysaccharides derived from the bark of Azadirachta indica (family Meliaceae). The neem gum is clear, bright and brown colored which was used in many industries such as cosmetics, paper, textile, and pharmaceuticals (Srivastava and Rai, 1963, Ogunjimi and Alebiowu, 2013). Such interesting features of the neem gum encouraged us to use this biopolymer as a template for the synthesis and stabilization of AgNPs due to its (i) water solubility and easy availability; (ii) non-toxic and natural materials; (iii) abundance of hydroxyl, acetyl, carbonyl and carboxylic functional groups; and (iv) metal-biosorption properties. Therefore, in the present study we have reported for the first time the detailed investigation of the biosynthetic procedure for production of AgNPs using the aqueous extract of neem gum. The synthesis was carried out in aqueous suspension of the gum extract by autoclaving, without the addition of any external chemical reducing agent. Moreover, the synthesized nanoparticles have also demonstrated antibacterial activity against Gram positive and Gram negative bacteria, revealing the potential of the nanoparticles for biomedical applications.
Section snippets
Chemicals and bacterial pathogens
All the chemicals were purchased from Sigma–Aldrich (St. Louis, USA). BacLight® Bacterial Viability Kit (L-7012) was purchased from Invitrogen. The clinical isolates of Salmonella enteritidis and Bacillus cereus were obtained from SRM Medical College Hospital and Research Center, Kattankulathur, Chennai. Double sterilized Milli-Q water was used throughout the experiments.
Synthesis of silver nanoparticles
Dried gum was collected from the neem plants in the forest area of Chengalpattu, India. The gum was dried under direct
Results and discussion
Gum is a renewable plant product that can be exploited as a reducing agent for the green synthesis of noble metal nanoparticles. In order to study the effects of gum concentration on the synthesis of AgNPs, different concentrations of gum extract (ranging from 0.1–0.5% w/v) were used. A mixture of these gum solutions and 1 mM of AgNO3 was autoclaved for 15 min. After autoclaving, the color of the reaction mixture changed from colorless to yellowish brown, indicated the formation of AgNPs. As
Conclusions
The present study demonstrates an autoclave-assisted green approach to synthesize silver nanoparticles using neem gum extract. The hydroxyl and carboxyl groups present in the gum facilitates the formation of silver nanoparticles during autoclaving as evident from the FTIR analysis. The powder diffraction study confirmed the formation of face centered cubic AgNPs and microscopic images revealed that the synthesized AgNPs were monodispersed spherical particles with an average size <30 nm.
Acknowledgement
The author wishes to express his hearty thanks to SRM University for providing research supports and laboratory facilities.
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