Microwave-Assisted Green Synthesis of Silver Nanoparticles Using Orange Peel Extract Genevieve A. Kahrilas, †,§ Laura M. Wally, ‡ Sarah J. Fredrick, ‡ Michael Hiskey, † Amy L. Prieto, ‡ and Janel E. Owens* ,† † Department of Chemistry and Biochemistry, University of Colorado, Colorado Springs, 1420 Austin Bluffs Parkway, Colorado Springs, Colorado 80918, United States ‡ Department of Chemistry, Colorado State University, Ft. Collins, Colorado 80523, United States * S Supporting Information ABSTRACT: Silver nanoparticles (AgNPs) were prepared in a one-step microwave-assisted synthesis guided by the principles of green chemistry. Microwave parameters were optimized using the Box-Benhken design for three factors (time, temperature, and pressure). Aqueous extracts from the peels of citrus fruits (orange, grapefruit, tangelo, lemon, and lime) were used for the synthesis of AgNPs using microwave technology, though the synthesis of AgNPs was only successful using the orange peel extract. Nanospheres of TEM mean diameter (with standard deviation) of 7.36 ± 8.06 nm were successfully synthesized in 15 min by reducing Ag + ions (from AgNO 3 ) with orange peel extract, which also served as a capping agent. Creation of AgNPs was confirmed using UV- visible spectroscopy, fluorescence emission spectroscopy, powder X-ray diffraction, and transmission electron microscopy, while size analysis was gathered from both transmission electron microscopy as well as dynamic light scattering. Analysis of all citrus peel extracts by gas chromatography-mass spectrometry indicated that the putative compounds responsible for successful AgNP synthesis with orange extract were aldehydes. The creation of AgNPs using environmentally benign reagents in minimal time paves the way for future studies on AgNP toxicity without risking interference from potentially toxic reagents and capping agents. KEYWORDS: Silver nanoparticles, Green chemistry, Microwave synthesis, Citrus peel ■ INTRODUCTION There has been much recent interest in using silver nanoparticles (AgNPs) in new technologies owing to their drastically enhanced properties over bulk silver, especially particles of diameters 30 nm and smaller. 1 These NPs are increasingly being incorporated into consumer products 2 despite rising evidence suggesting AgNPs have toxic effects on humans and experimental animal models meant to mimic human bio- and neurochemistry such as mice, rats, and Drosophila. 3-30 Many studies also suggest that AgNPs are quite harmful to the aquatic environment should they be inadvertently released into wastewater. 31-44 However, toxicity studies of this nature are often hindered by the AgNPs themselves used in the studies. In lieu of fast and convenient synthesis methods, many studies have utilized AgNPs, but the specification by which these materials have been synthesized and/or purified may not be included. Many current protocols for the synthesis of AgNPs utilize harsh and/or toxic chemicals. 45 The presence of these harsh synthetic conditions or contaminants may confound sensitive toxicity studies. Furthermore, AgNPs available for purchase are most often shipped as dry powders, and many studies fail to report the methods in which these AgNPs were redispersed in aqueous solutions. 46 The process of redispersion is quite crucial to the nature of the suspended AgNPs, 46,47 and inconsistencies in this methodology may also confound toxicity studies. As an alternative to purchased powders, the method for synthesizing AgNPs reported here was done so in a cost- and time-efficient manner and keeping with the principles of green chemistry. The preparation of AgNPs using plant-based extracts 48 is widely growing in popularity; recently proposed syntheses use reagents such as many types of leaf extract, 49-54 including menthol, 55 aloe vera, 56 clove extract, 57 edible mushroom extract, 58 and extracts from coffees and teas. 59 AgNP synthesis using the extract of the navel orange (Citrus sinensis) was first proposed by Kaviya et al. in 2011. 60 AgNPs were both reduced from silver nitrate (AgNO 3 ) and capped by the compounds present in the orange peel extract. Received: April 26, 2013 Revised: November 15, 2013 Research Article pubs.acs.org/journal/ascecg © XXXX American Chemical Society A dx.doi.org/10.1021/sc4003664 | ACS Sustainable Chem. Eng. XXXX, XXX, XXX-XXX