Delivered by Ingenta to: MICHIGAN TECHNOLOGICAL UNIVERSITY IP : 141.219.207.167 Tue, 17 Nov 2009 21:49:13 Copyright © 2009 American Scientific Publishers All rights reserved Printed in the United States of America RESEARCH ARTICLE Advanced Science Letters Vol. 2, 360–363, 2009 Synthesis and Characterization of Highly Fluorescent Water Dispersible ZnO Quantum Dots Prachi Joshi 1 2 , Z. A. Ansari 2 , Surinder P. Singh 3 , and Virendra Shanker 1 ∗ 1 National Physical Laboratory (CSIR), Dr. K. S. Krishnan Marg, New Delhi 110012, India 2 Centre for Interdisciplinary Research in Basic Sciences, Jamia Millia Islamia, New Delhi 110025, India 3 Department of Engineering Science and Materials, University of Puerto Rico, Mayaguez, PR 00680, USA We report the synthesis of stable and water dispersible fluorescent zinc oxide (ZnO) quantum dots (QDs). Hydrophilic polyethyleneimine (PEI) was used to stabilize QDs in water and trisodiumcitrate was used as linker between ZnO and PEI. X-ray diffraction reveals the nanocrystalline nature and hexagonal wurtzite structure of as synthesized ZnO QDs. High-resolution transmission electron micrograph suggests nearly spherical particles of size 3–6 nm and lattice spacing of 0.28 nm corresponding to the (100) plane of zinc oxide. Water dispersed ZnO QDs exhibit efficient yellow-green fluorescence centered at 555 nm (2.23 eV) with an excitation wavelength of 360 nm, which is found to be stable for two month revealing the high stability of QDs in water. 1. INTRODUCTION Recently, nanostructures such as nanoclusters, nanotubes, nanowires and quantum dots (QDs) have attracted attention due to their low dimensionality and size dependent properties use- ful for various applications. 1–3 Semiconductor QDs such as CdS, CdSe, CdTe are promising fluorescent materials for biologi- cal labeling including diagnostic and therapeutic applications with high quantum efficiency, long term photostability, narrow emission, and continuous absorption spectra depicting several advantages over conventional chemical fluorophores and visible fluorescent proteins in stability and sensitivity. 4–6 However, the Cd based QDs have been reported to become toxic due to release of Cd ions even in the protected core–shell structures and are also expensive. 7 High water dispersity of QDs is another fac- tor that limits their bioapplications as the organic solvents have their adverse effect on cells and tissues. 8 Therefore, there is a need to search Cd free, nontoxic, water dispersible substitute for traditional QDs that actively participate in bioconjugation reactions. ZnO has emerged as a potential QD for bioapplications because of its biocompatibility, high excitonic band gap of 3.3 eV and binding energy of 60 MeV. 9–11 It has been reported that the solubility of ZnO wires in biofluids has important implications in biomedical applications. 12 More recently, ZnO nanoparticles have been used for killing of cancer cells and activated human T ∗ Author to whom correspondence should be addressed. cells suggesting ZnO as an alternative to the chemotherapy and radiation therapy. 13 Despite of the reports on excellent monodis- persity, stability and fluorescence of ZnO QDs in various organic mediums, the major drawback still associated with ZnO QDs for their efficient bioapplications is low quantum yield, poor solubil- ity and stability in water and water-based solvents because of the limitations of the synthesis processes. 14 15 The fluorescence and stability of ZnO QDs in water remains a challenge because the ZnO QDs with highly active surface undergo irreversible agglom- eration in water resulting into poor dispersion and loss of all their important fluorescence properties. Therefore, various stud- ies are done to solve these difficulties by modifying surface of ZnO nanoparticles using silanes, water soluble/organic ligands, or by embedding ZnO nanoparticles into the polymer. 14 15 However, only a few reports are available on protection of fluorescence in water based ZnO QDs. In recent studies, water based ZnO nanoparticles have been reported with blue and green luminescence with complicated synthesis steps and use of hazardous chemicals to stabilize ZnO in aqueous medium. 16 17 To utilize ZnO in bioimaging, a simple, fast and reproducible technique to synthesize water dispersible fluorescent ZnO QDs, free of hazardous organic moi- eties, is necessary. In the present paper, we introduce a new facile wet chemical route to synthesize water dispersible, stable and highly fluorescent polyethyleneimine (PEI) capped ZnO QDs and nanopowder that can be readily redispersed in water to have dis- persions of known concentrations. Due to high affinity of ZnO 360 Adv. Sci. Lett. Vol. 2, No. 3, 2009 1936-6612/2009/2/360/004 doi:10.1166/asl.2009.1043