Optimization of High-Yield Biological Synthesis of Single-Crystalline Gold Nanoplates B. Liu, ² J. Xie, ² J. Y. Lee,* ,²,‡ Y. P Ting, ‡ and J. Paul Chen ‡ Singapore-MIT Alliance, and Department of Chemical & Biomolecular Engineering, National UniVersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260 ReceiVed: March 21, 2005; In Final Form: June 21, 2005 In this work, single-crystalline gold nanoplates were obtained by reducing aqueous chloroauric acid solution with the extract of Sargassum sp. (brown seaweed) at room temperature. The gold nanoplates so obtained were characterized by UV-vis spectroscopy, X-ray diffraction, atomic force microscopy, and transmission electron microscopy. The formation of gold nanoplates was found to depend on a number of environmental factors, such as the time taken to age the seaweed extract, pH of the reaction medium, reaction temperature, reaction time, and initial reactant concentrations. The size of the gold nanoplates could be controlled to between 200 and 800 nm by manipulating the initial reactant concentrations. The yield of the flat gold nanocrystals relative to the total number of nanoparticles formed was as high as ∼80-90%. Introduction The synthesis of metal nanoparticles with controlled chemical composition, size, and shape distributions is an important first step toward the realization of nanotechnology. Size and shape are pivotal in determining the physical and chemical properties of materials on the nanoscale. 1 The synthesis of metal nano- particles with well-defined size and shape can be classified broadly into wet and dry methods. Wet methods often require the use of an aggressive chemical reducing agent such as sodium borohydride, hydroxylamine, or tetrakishydroxymethylphos- phonium chloride (THPC), 2 a capping agent such as trioctyl phosphine oxide (TOPO), 3 and may additionally involve an organic solvent such as toluene or chloroform. 4 Dry methods, on the other hand, generate nanoparticles by ultraviolet irradia- tion, aerosol technology, or physical deposition into a solid template. 5 Although these methods may successfully produce pure, well-defined metal nanoparticles, the cost of production is relatively high both materially and environmentally. There is hence an unequivocal need to develop more cost-effective and environmental benign (“green chemistry”) alternatives to these existing methods. 6 The choice of an environmentally compatible solvent system, an eco-friendly reducing agent, and a nonhazardous capping agent for the stabilization of the nanoparticles are three main criteria for a totally “green” nanoparticle synthesis. For these reasons researchers in the field of nanoparticle synthesis turn to the biological systems where many organisms, both unicellular and multicellular, are known to produce inorganic nanostructures either intracellularly 7 or extracellularly, 8 even though the actual mechanisms are not yet known because of the complexity of most biological reactions. Bacteria, yeasts, and fungi have been used to fabricate silver and gold nanoparticles. 9-11 While the intracellular synthesis in principle may accomplish a better control over the size and shape distributions of the nanoparticles, product harvesting, and recovery are more cumbersome and expensive. In addition, only a few types of nanoparticles of biological significance to the cellular organisms can be produced intracellularly. The extra- cellular synthesis by comparison is more adaptable to the synthesis of a wider range of nanoparticle systems. Among various metal nanoparticles, gold nanoparticles are of particular interest to applications that leverage on their strongly size- and shape-dependent properties. 12 Hence, it is highly desirable to be able to produce gold nanoparticles with different morphology and size at high yields. Although there has been a large volume of work on the synthesis of gold nanoparticles and nanorods, 1g,2,4 there are relatively fewer attempts to produce gold nanoplates. 13-18 Among the few published successful efforts, either a complex multistep chemical process was involved 13 or the yield of the nanoplates relative to the total number of nanoparticles formed was low. 14-16 More recently, a wet chemistry route to scaling-up the production of micrometer-scale single-crystalline gold plates has been re- ported. 17 However, the lateral size and the thickness of the plates were relatively large and could not be easily manipulated by controlling the reaction conditions. Very recently, Sastry and co-workers developed a biological method to produce gold nanoprisms at 50% yield by using the extract of lemongrass leaves as a reducing agent cum shape-directing agent. 18 Since boiling was used to release the biochemical molecules in a noncontrollable manner, the analysis of the system response and process optimization are a daunting task. This study reports a simple biological synthesis of large quantities of gold nanoplates using a single step, room-temperature reduction of aqueous chloroaurate ions (AuCl 4 - ) by seaweed extract. The yield of the gold nanoplates was remarkably high, at ∼80-90%, and the lateral size of the nanoplates could be controlled in the range of 200-800 nm. Experimental conditions such as the aging time of the seaweed extract, pH of the reaction medium, reaction temperature, reaction time, and initial reactant concentrations, were systematically varied to arrive at the optimal growth conditions for the gold nanoplates. Experimental Section Materials Preparation. Sargassum sp. belonging to the Phaeophyta division was collected from the west coast of Singapore and carefully washed with deionized water and dried * To whom correspondence should be addressed. Email: cheleejy@ nus.edu.sg. ² Singapore-MIT Alliance. ‡ National University of Singapore. 15256 J. Phys. Chem. B 2005, 109, 15256-15263 10.1021/jp051449n CCC: $30.25 © 2005 American Chemical Society Published on Web 07/23/2005