research papers J. Synchrotron Rad. (2007). 14, 477–482 doi:10.1107/S0909049507044743 477 Journal of Synchrotron Radiation ISSN 0909-0495 Received 22 February 2007 Accepted 13 September 2007 # 2007 International Union of Crystallography Printed in Singapore – all rights reserved Structural properties of ‘naked’ gold nanoparticles formed by synchrotron X-ray irradiation Chang-Hai Wang, a Chia-Chi Chien, a,b Yen-Lu Yu, a,c Chi-Jen Liu, a Cheng-Feng Lee, a Chih-Hsiung Chen, a Y. Hwu, a,b,d,e * Chung-Shi Yang, f Jung-Ho Je g and G. Margaritondo h a Institute of Physics, Academia Sinica, Nankang, Taipei, Taiwan, b Department of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan, c Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, d Institute of Opto-Electronics Sciences, National Taiwan Ocean University, Keelung, Taiwan, e National Synchrotron Radiation Research Center, Hsinchu, Taiwan, f Nanomedicine Research Center, National Health Research Institute, Chunan, Taiwan, g X-ray Imaging Center, Pohang Univeristy of Science and Technology, Pohang, Korea, and h Ecole Polytechnique Fe ´de ´rale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland. E-mail: phhwu@sinica.edu.tw The formation of colloidal unmodified (‘naked’) gold nanoparticles is investigated by irradiation of a precursor solution with X-rays from a synchrotron source. An interesting morphological evolution as a function of exposure time, from cross-linked network-like structure to individual particles, has been discovered. The particle size decreased with the exposure time and was influenced by the ionic strength of the precursor solution. Contrary to  -ray exposure, an OH radical scavenger was not required for cluster formation. Keywords: structure; morphology; gold nanoparticles. 1. Introduction Potential applications including biosensor (Chah et al., 2005; Siwy et al. , 2005), bio-imaging (Lee, Oldenburg et al., 2003; Osawa et al., 2005; Loo, Hirsch et al., 2005), tumor treatment (Sun et al., 2004; Loo, Lowery et al. , 2005) and many others stimulate the interest in colloidal gold particles. Their fabri- cation and properties have been intensively investigated (Daniel & Astruc, 2004; Schmid & Corain, 2003; Glomm, 2005) with specific emphasis on possible use in cancer therapy (O’Neal et al., 2004; Zharov et al. , 2005). Practical applications often require a good control of the nanoparticle size, morphology, colloidal stability and specifi- city, which constitutes a serious challenge in materials science. To meet this challenge, chemical reduction methods are most often used. Radiolytic approaches have also been extensively explored since the 1960s (Fujita et al., 1962) and the corre- sponding reactions were systematically studied (Belloni et al., 1998; Gachard et al., 1998). Photon-stimulated processes indeed offer several advantages: no need for the preliminary use of reducing agents, well characterized reaction parameters and the production of highly reduced pure nanoparticles. The radiolytic preparation of metal particles was recently extended to X-rays (Rosenberg et al., 1998; Ma et al., 2000; Lee, Je et al. , 2003; Borse et al. , 2004). Karadas et al. (2005) analyzed the precipitation of Au nanoparticles from tetra- chloriauric acid induced by a conventional X-ray source in terms of the reduction dynamics. Owing to the low reaction rate, exposure times in excess of 30 h were required. We recently developed a new and more rapid method using X-rays from a synchrotron source to prepare well dispersed Au particles in aqueous solutions (Yang et al., 2006; Wang et al., 2007). This X-ray-based approach compares favorably with  -ray irradiation. Reducing agents are created by the solvent radiolysis, but for -ray exposure stabilizing agents are still needed to prevent coalescence and obtain dispersed metal clusters. On the contrary, no pre-added stabilizer is required for our method. The investigations conducted so far explored the role of the pH value and of the exposure conditions as well as the bio- compatibility of the final products. The present study deals with the structure and morphology of the X-ray-produced Au nanoparticles. We analyzed the effects of the exposure time, of the ionic strength and of radical scavengers, and discovered interesting phenomena that are discussed in terms of X-ray- induced synthesis. 2. Experimental 2.1. Materials and methods All the chemicals were of reagent grade and were used without additional purification. The gold precursor was hydrogen tetrachloroaurate trihydrate (HAuCl 4 3H 2 O) from