COMMUNICATION www.rsc.org/greenchem | Green Chemistry Glutathione promoted expeditious green synthesis of silver nanoparticles in water using microwaves Babita Baruwati, Vivek Polshettiwar and Rajender S. Varma* Received 2nd February 2009, Accepted 27th April 2009 First published as an Advance Article on the web 5th May 2009 DOI: 10.1039/b902184a Silver nanoparticles ranging from 5–10 nm in size have been synthesized under microwave irradiation conditions using glutathione, an absolutely benign antioxidant that serves as the reducing as well as capping agent in aqueous medium. This rapid protocol yields the nanoparticles within 30–60 s at a power level as low as 50 W. The effect of microwave power on the morphology of ensuing silver nanoparticles is investigated for this green and sustainable procedure which is adaptable for the synthesis of palladium, platinum and gold nanoparticles. Introduction Over the past decade, the increased emphasis on developing green and sustainable chemical processes has led to numerous efforts toward the elimination or at least minimization of waste generation. Implementing sustainable methodologies in almost all areas of chemistry, including nanomaterial synthesis 1 entails eliminating toxic reagents and solvents. The choice of an environmentally benign solvent, the use of a multipurpose agent that serves the purpose of a reducing, capping and dispersing agent are some of the key issues that may be addressed in green synthesis of nanomaterials. 1 Synthesis of silver nanoparticles is of much interest to the scientific community because of their wide range of applications in catalysis, 2 electronics, 3 photonics, 4 optoelectronics, 5 sensing, 6 and pharmaceuticals. 7 Specifically, these nanoparticles are strong candidates for Surface Enhanced Raman Spectroscopic (SERS) studies 8 that yet again prompts the interest of the scientific community to develop newer green synthetic methods for obtaining these nanoparticles. Numerous pathways have been employed for the synthesis of silver nanoparticles with different morphologies as well as size distributions including NaBH 4 reduction, 9 polyol process, 10–12 use of plant extracts, 13,14 photoreduction 15 etc. Most of these proceed via wet chemistry methods with the use of highly reactive reducing agents such as sodium borohydride, hydrazine etc. that are not environmentally friendly. Some of them use noxious and highly volatile organic solvents. To eliminate the use of toxic reducing agents, the use of amino acids, 16 vitamins, 17 and other eco-friendly biological agents in the synthesis of metal nanopar- ticles have been reported. 18–20 In conjunction with the use of these Sustainable Technology Division, National Risk Management Research Laboratory, U. S. Environmental Protection Agency, MS 443, Cincinnati, Ohio 45268, USA. E-mail: varma.rajender@epa.gov; Fax: +1 513-569-7677; Tel: +1 513-487-2701 eco-friendly reducing agents and solvents, microwave irradiation (MW) is emerging as a rapid and environment friendly mode of heating for the generation of nanomaterials. It offers a rapid and volumetric heating of solvents, reagents, and intermediates, that provides uniform nucleation and growth conditions for nanomaterial synthesis. 21–25 Engaged in the development of greener and sustainable pathways for organic synthesis and nanomaterials, 26–33 herein we report an easy and rapid synthesis of silver nanoparticles using glutathione as a reducing as well as capping agent under MW irradiation conditions in pure aqueous medium. To the best of our knowledge, glutathione has not been reported for the synthesis of silver nanoparticles. The choice of glutathione (Fig. 1, GHS), as a reducing agent was made because of its benign nature and the presence of a highly reactive thiol group that can be used to reduce the metal salts. GHS is a tripeptide consisting of glutamic acid, cysteine and glycine units and is an ubiquitous antioxidant present in human and plant cells. Besides the thiol group, each GSH molecule also contains amine and carboxylate functionalities that provide coupling possibilities for further cross-linking to other molecules of biological or sensing interest. Fig. 1 Molecular structure of glutathione, GSH (reduced). Results and discussions The first step in developing this protocol, was the optimization of MW power, exposure time and glutathione concentration. Reactions were conducted at three different power levels i.e. 50, 75 and 100 W for 30, 45 and 60 s (Table 1). Under all of the above reaction conditions (Table 1), silver nanoparticles with spherical morphology in the size range 5–50 nm were obtained. At lower power levels (50 W) and shorter reaction times (30 s), mostly unreacted silver nitrate remained in the reaction mixture. With increase in reaction time to 60 s, nanoparticles of 5–10 nm size with spherical morphology was observed (Fig. 2). At higher power levels (75 and 100 W) with 30 s reaction times, nanoparticles of 4–10 nm were obtained (Fig. 3a, b). However, with longer reaction times (60 s), increased 926 | Green Chem., 2009, 11, 926–930 This journal is © The Royal Society of Chemistry 2009