ARTICLES 482 nature materials | VOL 3 | JULY 2004 | www.nature.com/naturematerials T he size 1 and shape-dependent 2,3 physicochemical and optoelectronic properties of metal and semiconductor nanoparticles have important applications in catalysis 4 , biosensing 5,6 , recording media 7 and optics 8 . Nanocubes 9 , nanorods/nanowires 10,11 , nanodisks 12 , nanotapes/nanobelts 13 and nanoteardrops/nanoarrows/nanotetrapods 14 can be routinely synthesized by chemical and physical methods. The synthesis of triangular metal nanoprisms (nanotriangles) in large quantities has proved more difficult,often requiring elaborate and time- consuming sphere–triangle shape transformation 2,13,15 or seeded growth 16 . Biological methods using bacteria and fungi for the synthesis of metal 17–21 and semiconductor 22–24 nanoparticles represent a relatively unexplored and underexploited alternative, but have hitherto yielded little by way of size and shape control. Biological systems synthesize and assemble a range of inorganic nanomaterials such as amorphous silica (diatoms) 25 , magnetite (magnetotactic bacteria) 26 and minerals such as calcite 27 into functional superstructures. Understanding biochemical processes that lead to the formation of nanoscale inorganic materials is therefore potentially appealing as environmentally friendly alternatives to chemical methods for nanoparticle synthesis. Although biotechnological applications such as remediation of toxic metals have used microorganisms such as bacteria and yeast (the detoxification often occurring through reduction of the metal ions/formation of metal sulphides), the use of microorganisms as possible ecofriendly nanofactories has now been realized 17–24 . Shape control of inorganic materials in biological systems is achieved either by growth in constrained environments such as membrane vesicles 25 , or through functional molecules such as polypeptides that bind specifically to inorganic surfaces 21 . Specific polypeptide repeat sequences in proteins secreted by the bacterium Escherichia coli have been shown to induce growth of flat, triangular gold nanocrystals at a 4% yield relative to the total nanoparticle formation 21 . Here we demonstrate the biological synthesis of large amounts of triangular gold nanoprisms by a single-step, room-temperature reduction of aqueous chloroaurate ions (AuCl 4 – ) by the extract of the plant lemongrass (Cymbopogon flexuosus). The nanotriangles are formed by assemblies of spherical nanoparticles that seem to be ‘liquid-like’; this fluidity The optoelectronic and physicochemical properties of nanoscale matter are a strong function of particle size. Nanoparticle shape also contributes significantly to modulating their electronic properties. Several shapes ranging from rods to wires to plates to teardrop structures may be obtained by chemical methods; triangular nanoparticles have been synthesized by using a seeded growth process. Here, we report the discovery that the extract from the lemongrass plant, when reacted with aqueous chloroaurate ions, yields a high percentage of thin, flat, single-crystalline gold nanotriangles. The nanotriangles seem to grow by a process involving rapid reduction, assembly and room-temperature sintering of ‘liquid-like’ spherical gold nanoparticles. The anisotropy in nanoparticle shape results in large near-infrared absorption by the particles, and highly anisotropic electron transport in films of the nanotriangles. Biological synthesis of triangular gold nanoprisms S. SHIV SHANKAR 1 , AKHILESH RAI 1 , BALAPRASAD ANKAMWAR 2 , AMIT SINGH 1 , ABSAR AHMAD 3 AND MURALI SASTRY 1 1 Materials Chemistry, National Chemical Laboratory, Pune – 411 008, India 2 Chemistry Department, Abasaheb Garware College, Pune – 411 004, India 3 Biochemical Sciences Division, National Chemical Laboratory, Pune – 411 008, India *e-mail: sastry@ems.ncl.res.in Published online: 20 June 2004; doi:10.1038/nmat1152 ©2004 Nature Publishing Group