Design of Polymeric Stabilizers for Size-Controlled Synthesis of Monodisperse Gold Nanoparticles in Water Zhenxin Wang, ²,‡ Bien Tan, ² Irshad Hussain, ²,§ Nicolas Schaeffer, ² Mark F. Wyatt, | Mathias Brust, ² and Andrew I. Cooper* ,² Centre for Nanoscale Science and Centre for Materials DiscoVery, Department of Chemistry, The UniVersity of LiVerpool, Crown Street, LiVerpool, L69 3BX, United Kingdom, State Key Laboratory of Electro-Analytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, China, National Institute for Biotechnology and Genetic Engineering (NIBGE), Jhang Road Faisalabad, Pakistan, and EPSRC National Mass Spectrometry SerVice Centre (NMSSC), School of Medicine, Swansea UniVersity, Singleton Park, Swansea, SA2 8PP, United Kingdom ReceiVed September 7, 2006 A new methodology is described for the one-step aqueous preparation of highly monodisperse gold nanoparticles with diameters below 5 nm using thioether- and thiol-functionalized polymer ligands. The particle size and size distribution was controlled by subtle variation of the polymer structure. It was shown that poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) were the most effective stabilizing polymers in the group studied and that relatively low molar mass ligands (∼2500 g/mol) gave rise to the narrowest particle size distributions. Particle uniformity and colloidal stability to changes in ionic strength and pH were strongly affected by the hydrophobicity of the ligand end group. “Multidentate” thiol-terminated ligands were produced by employing dithiols and tetrathiols as chain- transfer agents, and these ligands gave rise to particles with unprecedented control over particle size and enhanced colloidal stability. It was found throughout that dynamic light scattering (DLS) is a very useful corroboratory technique for characterization of these gold nanoparticles in addition to optical spectroscopy and TEM. Introduction Gold nanoparticles have a wide range of uses in modern nanoscale science, and it is therefore important to understand and control their physical and chemical properties, which are generally size dependent. 1,2 Gold nanoparticles are commercially available in many forms, and numerous preparative methods are documented in the literature for particles from about 1 nm to several micrometers diameter. 3-7 Nonetheless, only a handful of standard procedures are employed routinely to prepare gold particles for a multitude of applications. These methods are reliable and simple to carry out and lead to uniform particles with a narrow size distribution in the desired range. The most widely applied procedures to obtain gold hydrosols are variations of the classic Turkevich-Frens citrate reduction route. 8,9 Most hy- drophobic (and some hydrophilic) particles are prepared by borohydride reduction in an organic solvent in the presence of thiol capping ligands using either a two-phase liquid/liquid system or a suitable single-phase solvent. 10-19 The latter approach is usually employed for particles in the 1 to ca. 8 nm range. 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