DOI: 10.1002/adma.200600118 The Microemulsion Synthesis of Hydrophobic and Hydrophilic Silicon Nanocrystals** By Richard D. Tilley* and Kenji Yamamoto 1. Introduction Research in the field of semiconductor nanocrystals synthe- sized in the liquid phase has been intense since the formation of CdSe quantum dots by Murray et al. in 1993. [1] Like CdSe quantum dots, silicon particles have intense visible lumines- cence because of quantum confinement, as first reported in porous silicon by Canham. [2] The most promising applications include biological imaging agents, [3–8] optoelectronic devices, and photovoltaic devices. [9–13] The main difference between quantum dots made in solution and those made in the vapor phase is that those in solution are capped or coated by surface molecules. Thus, applications best suiting solution-phase quantum dots are those where the presence of a coating of surface molecules is an advantage rather than a hindrance. For example, particles capped with hydrophobic groups misci- ble with conducting polymers could be used in the manufac- ture of optoelectronic devices. Quantum dots are becoming popular as replacements for fluorescent dyes in biological fluorescence imaging because of their superior resistance to photobleaching. To date, consider- able emphasis has been placed on using CdSe quantum dots with a ZnS shell as biological chromophores, since they emit light that can be tuned throughout the visible spectrum. [1] However, concerns have been raised about the toxicological issue of quantum dots in living systems. Quantum dot toxicity can stem from two sources: i) the quantum dot core, and ii) the capping molecule. A recent study by Derfus et al. showed that quantum dots with a CdSe core and without a ZnS shell, after exposure to UV light, were toxic to liver cells. [14] The potential biocompatibility of silicon makes photoluminescent silicon-core quantum dots an ideal candi- date for biological fluorescence imaging and should eliminate any potential toxicology problems of quantum dots that might arise from having a CdSe core. [15] For quantum dots to be used as fluorescence imaging agents in biological systems, surface capping is of critical importance to produce particles that are hydrophilic and biologically compatible. The solution-phase synthesis of silicon nanocrystals has been previously reported by Kauzlarich and co-workers using a variety of reducing agents, [16,17] by Korgel and co-workers at high temperatures [18,19] and pressures, and by Wilcoxon et al. RESEARCH NEWS Silicon nanocrystals, also called quantum dots, have unique optical properties when in the quantum-confinement regime. These optical properties make silicon nanocrystals promising materials for a wide variety of applications ranging from optoelectronic devices to fluoro- phores in biological imaging. A liquid-phase synthetic approach is re- ported using surfactant molecules to control particle growth, producing highly monodisperse silicon particles. The surface of the nanocrystals are capped by functional organic molecules that passivate and protect the silicon particles from oxidation, enabling the particles to be used in hydrophobic and hydro- philic applications. The use of hydrophilic silicon quantum dots as optical probes is illustrated by the imaging of Vero cells. – [*] Dr. R. D. Tilley School of Chemical and Physical Sciences MacDiarmid Institute of Advanced Materials and Nanotechnology Victoria University of Wellington P.O. Box 600, Wellington (New Zealand) E-mail: Richard.tilley@vuw.ac.nz Prof. K. Yamamoto Department of Medical Ecology and Informatics, Research Institute International Medical Center of Japan Toyama 1-21-1, Shinjuku, Tokyo 162-8655 (Japan) [**] The authors thank Jamie Warner, Akiyoshi Hoshino, and Amane Shiohara for contributing to this research. R. D. T. thanks the Mac- Diarmid Institute of Advanced Materials and Nanotechnology for funding. Adv. Mater. 2006, 18, 2053–2056 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2053