Dedicated to Professor Gerhard Wegner on the occasion of his 68th birthdayThe tuneable NIR absorbance of gold in conjunction with its low cytotoxicity has fueled research in the synthesis of rodlike gold nanocrystals for a wide range of biomedical applications such as sensing, imaging, and photothermal therapy.[1] However, a fundamental problem in the realization of these technologies is the need for (cytotoxic) surfactants-such as cetyltrimethylammonium bromide (CTAB)-in order to induce the anisotropic particle growth in aqueous solution.[2]Herein we present an alternate synthetic strategy based on ionic liquid solvents that alleviates the need for shaperegulating surfactants.Ionic liquids (ILs) have attracted interest as benign solvent systems for the synthesis of nanomaterials as they combine several attractive characteristics, for example inherent conductivity, stability over a broad range of electrochemical potentials, and environmental benefits deriving from the low vapor pressure and straightforward separation procedures. [3,4] Two major strategies have been pursued for the synthesis of metal nanoparticles in IL solution: 1) the addition of auxiliary capping agents (in analogy to the reactions in aqueous solution) to stabilize the formation of nanosized particles, and 2) the use of modified ILs capable of acting both as solvent and capping agent. For example, thioland alcohol-substituted ILs were applied for the synthesis of Au and Pt nanoparticles by reduction of the respective metal salts with a strong reducing agent (NaBH 4 ). [5,6] Common to these prior studies is the use of strong reducing agents and the (predominantly) covalent linkage of the capping agent to stabilize the growing metal nuclei.We report herein that under conditions of decelerated particle growth (by use of weak reducing agents) the stabilization of gold nanocrystals is facilitated by solvent coordination in unmodified imidazolium ionic liquids. [7,8] Imidazolium cations are particularly intriguing stabilizing agents for gold nanocrystals since related aromatic heterocycles have been shown to preferentially bind to high-energy crystallographic orientations of gold surfaces such as the {100}, {110}, and {311} orientations. [8][9][10] This suggests that imidazolium-based ILs may stabilize non-equilibrium particle shapes (such as nanorods) that exhibit fewer low-energy {111} facets than the equilibrium (Wulff) shape.We demonstrate herein that anisotropic gold nanocrystals can be synthesized in 1-ethyl-3-methylimidazolium ethylsulfate ([EMIM][ES]) in very high yield in the absence of auxiliary shape-regulating surfactants. Dependent on the amount of Ag I present in the reaction mixture, the particle aspect ratio can be controlled within the range a = L/d = 1-15 (where L and d denote the particle length and thickness); this is comparable to the range of shape anisotropy that has been demonstrated in aqueous solutions.The synthetic approach is based on the seeded-growth method originally developed by the Murphy group for the synthesis of go...
