Nanoplate biosynthesis DOI: 10.1002/smll.200600612 Identification of Active Biomolecules in the High-Yield Synthesis of Single-Crystalline Gold Nanoplates in Algal Solutions Jianping Xie, Jim Yang Lee,* Daniel I. C. Wang, and Yen Peng Ting I n this work, single-crystalline gold nanoplates were produced by treating an aqueous solution of chloroauric acid with the extract of the unicellular green alga Chlorellavulgaris at room temperature. The results suggest proteins as the primary biomolecules involved in providing the dual function of Au III reduction and the size- and shape-controlled synthesis of the nanogold crystals. A protein with a molecular weight of approxi- mately 28 kDa was isolated and purified by reversed-phase HPLC; this protein tested positive for the reduction of chloroauric acid in aqueous solution. The isolated protein (named gold shape-directing protein, or GSP for convenience) was then used to produce gold nanoplates with distinctive triangular and hexagonal shapes in high yields (  90%). The kinetics of the reduction reaction could be manipulated through changes in the GSP concentration to produce plates with lateral sizes ranging from nanometers to micrometers. The growth of gold nanoplates in the GSP solution with time was monitored by microscopic and spectroscopic techniques, thereby allowing the detection of several key intermediates in the growth process. Keywords: · biosynthesis · gold · green chemistry · nanomaterials · proteins 1. Introduction The current interest in nanomaterials is firmly founded on their size- and shape-tunable properties, which allow very diverse applications to be designed based on the same material. [1,2] Metal nanoparticles have in the past been pro- ducedmainlybychemicalprocesses,whichmayinvolvethe use of an aggressive chemical reducing agent (for example, sodium borohydride, hydroxylamine, or tetrakishydroxyme- thylphosphonium chloride (THPC)), [3] a capping agent (such as trioctyl phosphine oxide (TOPO)), [4] and some- times an organic solvent (for example, toluene or chloro- form) [5] forbetterresults.Theenvironmentalcostofproduc- tion can be relatively high. There is therefore the need to developenvironmentallysustainable(“greenchemistry”)al- ternativestotheexistingmethods. [6] Theselectionofanen- vironmentally acceptable solvent system, an ecofriendly re- ducingagent,andabenignparticle-stabilizingcappingagent arethreequalifyingcriteriaforatotally(green)nanoparticle [*] J. Xie, Prof. J.Y. Lee, Prof. D. I. C. Wang Singapore–MIT Alliance National University of Singapore 4 Engineering Drive 3, Singapore 117576 (Singapore) Fax: (+ 65)6779-1936 E-mail: cheleejy@nus.edu.sg Prof. J.Y. Lee, Prof. Y. P. Ting Department of Chemical and Biomolecular Engineering National University of Singapore 10 Kent Ridge Crescent, Singapore 119260 (Singapore) Prof. D. I. C. Wang Department of Chemical Engineering Massachusetts Institute of Technology 77 Massachusetts Ave., Cambridge, MA 02139 (USA) Supporting information for this article is available on the WWW under http://www.small-journal.com or from the author. 672 # 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim small 2007 , 3, No. 4, 672 – 682 full papers J. Y. Lee et al.