DOI: 10.1002/adma.200700116 Optical Analysis of Beads Encoded with Quantum Dots Coated with a Cationic Polymer** By Claudine Ni Allen, Nicolas Lequeux, Christophe Chassenieux, Gilles Tessier, and Benoit Dubertret* Multiplexed screening methods are becoming an essential tool for research, diagnosis, and drug screening. The best ex- ample of such methods is probably epitomized by the DNA microarrays. An alternative to conventional DNA microar- rays is emerging with the use of polymeric beads optically en- coded with fluorophores. This approach has shown to be more flexible, more sensitive and probably cheaper than conven- tional microarrays. [1,2] So far, both organic [3] and inorganic fluorophores [4] have been used to optically encode polymeric beads. However, inorganic fluorophores such as semiconduc- tor nanocrystals (quantum dots or QDs) have several advan- tages in terms of coding capacities compared to organic fluo- rophores. First, QDs have high quantum yield and good photostability. Second, their photoluminescence (PL) emis- sion spectrum is narrow and tunable with QD size. Finally, they can all be excited with a unique blue or ultraviolet light source independently of their emission wavelength. Because of these advantages, many efforts have recently been devoted to the synthesis of optical coded QD beads. The main difficul- ties are twofold: produce beads with homogenously dispersed QDs in or on it, so that a given optical code is bead- indepen- dent, and preserve the optical properties of the QDs during the production process. Although QDs are known for their photostability, their resistance to both photobleaching and dif- ferent surface chemistries is highly dependent on the quality of the QD synthesis, more specifically on the type of core/ shell used. So far, three approaches have been tested: (i) diffusion of hydrophobic QDs into swelled polystyrene beads; (ii) incor- poration of QDs during bead polymerization; (iii) coating of bead surfaces with QDs. The main advantage of the first method is that QDs may be used as synthesized (generally in non-polar solvents) with the native ligands. [4–7] Conversely, the main problem of this approach is the nonuniform distribu- tion of QDs inside the beads resulting in significant color in- tensity fluctuations from bead to bead. This method has re- cently been improved by using mesoporous beads: hydrophobic silica [8] or polystyrene; [9] or by immobilizing QDs within polystyrene beads by demixing two nonmiscible solvents. [10] For the second method, efforts have been focused on the encapsulation of QDs into polystyrene microspheres by emulsion [11–13] or suspension polymerization. [14,15] The phase separation between polystyrene microspheres and QDs occurring during polymerization [16] is however an important drawback. Uniform distribution of QDs has been obtained with a ligand exchange prior to polymerization, [13,14] but infor- mation such as quantum yield stability or emission intensity fluctuations from bead to bead has not been reported. The last encoding method is the immobilization of QDs onto inorganic or polymeric beads via covalent bounding [17,18] or electrostatic interaction. This strategy first requires water- soluble QDs. They can be obtained using QD synthesis in water. For example, thioglycerol-stabilized CdTe, HgTe and ZnSe QDs have been deposited on silica beads, [19] polystyrene beads, [20–22] Fe 3 O 4 magnetic nanoparticles, [23] microcap- sules [24,25] and glass substrates. [26] However, QDs synthesized in water are not as photostable as the ones synthesized in or- ganic solvents and this is a serious limitation for the bead cod- ing application. Alternatively, QDs can first be synthesized in organic solvent to yield highly photostable nanocrystals and then be made water soluble and charged using either ligand exchange techniques [27] or encapsulation in amphiphilic co- polymers. We show here that the latter technique can be easily implemented using a cationic statistical copolymer. QDs of any size, obtained from an organometallic synthesis method, can be made positively charged, water soluble and subse- quently adsorbed homogeneously on the surface of various polymeric beads, including magnetic ones. We investigated the coding potential of these beads using single bead fluorescence spectra measurements on a specially designed optical set up. Using an EM-CCD device, we mea- sured the fluctuations in color and relative intensity from bead to bead. With only two colors, the coding capability of our beads is excellent. It is mainly limited by the quality – spe- COMMUNICATION 4420 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2007, 19, 4420–4425 – [*] Dr. B. Dubertret, Dr. C. Ni Allen,Dr. G. Tessier Laboratoire de Photons et Matiére, UPR A0005 (CNRS) École Supérieure de Physique et de Chimie Industrielles 10 rue Vauquelin, 75005 Paris (France) E-mail: benoit.dubertret@espci.fr Dr. N. Lequeux Laboratoire Physicochimie des Polymères et Milieux Dispersés, UMR 7615 École Supérieure de Physique et de Chimie Industrielles 10 rue Vauquelin, 75005 Paris (France) Dr. C. Chassenieux Laboratoire Polymères, Colloïdes, Interfaces, Université du Maine 1 Avenue Olivier Messiaen, 72085 Le Mans (France) [**] The authors would like to thank X. Xu for assistance with TEM imag- ing. C. N. A. acknowledges the financial support from Le Fonds Québécois de la Recherche sur la Nature et les Technologies (FQRNT) and from the Natural Sciences and Engineering Research Council of Canada (NSERC). B. D. would like to thank the Région Ile de France and the Human Frontier Science Program for funding. Supporting Information is available online from Wiley InterScience or from the author.