Facile Synthesisof Silver Nanoparticles and Their Synergistic Antibacterial Activity in Combination with Commercial Antibiotics Jahar Dey, 1 Koel Biswas, 2 Uttam Thapa, 1 Santaram Joshi, 2 Ibapalei Shisha Kharbangar, 1 Nazima Sultana, 1 and Kochi Ismail* 1 1 Department of Chemistry, North-Eastern Hill University, Shillong 793022, India 2 Microbiology Laboratory, Department of Biotechnology and Bioinformatics, North-Eastern Hill University, Shillong 793022, India E-mail:kismail@nehu.ac.in; kinehu@hotmail.com Received: August 28, 2014; Accepted: November 20, 2014; Web Released: December 1, 2014 We demonstrate a facile method for the synthesisof silver nanoparticles (Ag NPs) by using silver nitrate (AgNO 3 ) as precursor, p-toluidine as reducing agent and cetyltrimethylammonium bromide (CTAB) as capping agent. The main highlight of the present synthesis is the formation of NPs at near neutral pH. The effect of the concentrations of precursor, reducing agent and capping agent, and of pH on the stability and size of the NPs has been studied using UVvis spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM) techniques. Critical concentrations of AgNO 3 , reducing agent and CTAB required for the synthesisof Ag NPs are found to be 0.002 mol dm ¹3 . The NPs prepared have been tested for antibacterial properties for a series of bacteria and they are found to have good antibacterial efficacy. In combination with some commercially available antibiotics, synergism in the antibacterial activity of the NPs has also been observed. Metal nanoparticles have been extensively investigated because of their unique properties and potential applications inwide range of areas such as catalysis, optoelectronics, chem- ical sensing, and drug delivery. 16 Because of the immense importance of nanotechnology, economical and eco-friendly methods are continuously explored for the synthesisof nano- particles. In addition to the research on novel methods of synthesis, a lot of emphasis isalso being paid to the anti- bacterial and antimicrobial studies of the synthesized nano- particles. Antibacterial studies have become important primar- ily due to the bacterial resistance to conventional medical antibiotics. Moreover, the need for higher doses of the anti- biotics has also been a matter of concern. In this respect, synergistic action of different NPs in combination with different antibiotics isalso being investigated. Silver nanoparticles (Ag NP) are one of the metal nano- particles that show antimicrobial properties 711 and newer synthetic methods are being reported for their preparation. 1125 In a recently reported method, 15 Ag NPs have been synthesized using aniline as reducing agent and cetyltrimethylammonium bromide (CTAB) as stabilizer in aqueous medium of basic pH and at 33 °C. The primary motive of the present work is to synthesize Ag NPs in aqueous medium having nearly neutral pH at a faster rate than the rate achieved using aniline and to test the NPs for antibacterial activity in combination with different commercial antibiotics. Generating NPs in neutral aqueous medium would have far reaching impact since biological processes demand the maintenance of the solution pH near a neutral value. With this motive inmind, in this paper we have synthesized Ag NPs using p-toluidine as reducing agent and CTAB as capping agent, and the antibacterial activity of the synthesized Ag NPs has been investigated in the absence and presence of different antibiotics. Experimental Materials. We purchased CTAB (99%) from Fluka, nitric acid (HNO 3 ) and silver nitrate (AgNO 3 ) from Merck, p- toluidine (assay 98%) from s. d. fine chemicals, and sodium hydroxide (NaOH) from Sisco Research Laboratories. Milli- pore water was used for preparing all solutions. Method. Ag NPs were prepared using an aqueous solution of AgNO 3 as precursor and p-toluidine as reducing agent in the presence of CTAB at room temperature. The pH of the reaction mixture was monitored with a Systronics pH-meter using a combined glass electrode. In this method, a stock solution of aqueous 0.01 M (M: mol dm ¹3 ) p-toluidine was prepared. Then aqueous solutions of AgNO 3 , p-toluidine, and CTAB were added in such a way that the total volume of the mixture was always 10 mL. The experiments were performed in such a way as to keep the concentrations of all the components fixed except one whose effect is to be studied. In all the cases, the reducing agent was added at the end. The antibacterial activity of the synthesized Ag NPs was evaluated using the agar well diffusion method. 26 The indi- cator strains used for this test were Staphylococcus aureus MTCC96, Enterococcus faecalis MTCC2729, Escherichia coli MTCC730, Streptococcus pyogenes MTCC1925, and Klebsiella pneomoniae subsp.pneumoniae MTCC109. 100 ¯L of overnight culture of the test strain was spread evenly on Muller Hilton agar (MHA). 0.5 mm diameter wells were cut on 352 | Bull. Chem. Soc. Jpn. 2015, 88, 352–357 | doi:10.1246/bcsj.20140255 © 2014 The Chemical Society of Japan