NATURE NANOTECHNOLOGY | VOL 5 | JANUARY 2010 | www.nature.com/naturenanotechnology 15 REVIEW ARTICLE PUBLISHED ONLINE: 24 DECEMBER 2009 | DOI: 10.1038/NNANO.2009.453 C urrent interest in ensembles of inorganic nanoparticles is motivated by our ability to exploit their collective properties and the possibility of using these properties in functional devices. Ensembles of nanoparticles can be used to improve the mechanical properties of composite materials, and they can also allow multiple tasks to be performed simultaneously or in sequence. Ensembles of nanoparticles can also display new electronic, magnetic and optical properties as a result of interactions between the excitons, magnetic moments or surface plasmons of individual nanoparticles. It should be possible to exploit these properties in devices, and also the direc- tionality and long-range order found in ensembles, if the spacing and alignment of individual nanoparticles can be controlled. Self-assembly provides a simple and low-cost method for pro- ducing ensembles of nanoparticles in a controllable manner, and in this review we summarize recent advances in the ield. We start with a summary of various methods for the self-assembly or assisted assembly of nanoparticles (see refs 1–6 for more detailed reviews), and then go on to review the properties of self-assembled nano- particle structures and possible applications for these structures. We restrict ourselves to systems in which the distance between nano- particles is comparable to the nanoparticle size and do not, therefore, discuss nanoparticle patterning or alignment, or the self-assembly of secondary nanoparticle structures (such as nanowires), unless these methods are used to create close-packed structures. Strategies for self-assembly of nanoparticles Self-assembly in solution. Figure 1 illustrates the self-assembly of nanoparticles in solutions in the absence of templates, interfaces or external ields. Assembly is governed by the balance of attractive forces (such as covalent or hydrogen bonding, electrostatic attraction between oppositely charged ligands, depletion forces or dipole–dipole interactions) and repulsive forces (such as steric forces and electro- static repulsion between ligands of like charge) 7 . Self-organization of nanoparticles generates a variety of structures, including chains 8–11 , sheets 12,13 , vesicles 9,14,15 , three-dimensional (3D) crystals 16–19 or more complex 3D architectures 20 . One approach to solution-based self-assembly exploits site-spe- ciic interactions of chemically heterogeneous nanoparticles 9,21 . For Properties and emerging applications of self-assembled structures made from inorganic nanoparticles Zhihong Nie 1† , Alla Petukhova 1† and Eugenia Kumacheva 1,2,3 * Just as nanoparticles display properties that difer from those of bulk samples of the same material, ensembles of nanoparticles can have collective properties that are diferent to those displayed by individual nanoparticles and bulk samples. Self-assembly has emerged as a powerful technique for controlling the structure and properties of ensembles of inorganic nanoparticles. Here we review diferent strategies for nanoparticle self-assembly, the properties of self-assembled structures of nanoparticles, and potential applications of such structures. Many of these properties and possible applications rely on our ability to control the interactions between the electronic, magnetic and optical properties of the individual nanoparticles. example, end-by-end or side-by-side assembly of gold nanorods can be driven by triggering attraction between the distinct ligands attached to the long and short facets of the nanorod 9 . Figure 1b shows assemblies of gold nanorods carrying cetyl trimethyl ammonium bro- mide (CTAB) on the long nanorod side and polystyrene molecules at the nanorod ends. his method used the analogy between the nano- rods and ABA triblock copolymers 9,22,23 : the addition of water (a bad solvent for polystyrene) to a solution of nanorods in dimethyl forma- mide produced nanorod chains, whereas the addition of water to a solution of nanorods in tetrahydrofurane (a bad solvent for CTAB) triggered nanorod assembly in bundles. Chemical heterogeneity of nanoparticles was also induced by phase separation between ‘immiscible’ organic ligands 8,24 or consecu- tive attachment of diferent ligands 21,25 . Phase separation in a mixture of nonanoic acid and 4-phenylbutyric acid produced two distinct sin- gularities on the surface of γ-Fe 2 O 3 nanoparticles, which allowed for the subsequent reaction with the molecular linker and the formation of nanoparticle chains (Fig. 1c) 24 . Alternatively, a balance of the anisotropic hydrophobic attraction and electrostatic interactions (originating from a dipole moment and a small positive charge of nanoparticles) governed the spontaneous formation of close-packed monolayer sheets of CdTe nanoparticles coated with 2-(dimethylamino)ethanethiol (Fig. 1d) 12 . he experi- mental results were supported by computer simulations of interpar- ticle interactions. Recently, the formation of 3D nanoparticle crystals with face-centred or body-centred cubic lattice structures was mediated by hybridizing complementary DNA molecules attached to the nano- particle surface 17,18 . he variation in DNA sequences or length of DNA linkers, and the absence or presence of a non-bonding single-base lexor, was used to tune interactions between the nanoparticle–DNA conjugates. Figure 1e shows a fragment of the crystal with the body- centred cubic structure formed by gold nanoparticles 17 . In a difer- ent strategy, crystals with a diamond-like structure were grown from oppositely charged gold and silver nanoparticles 16 . Crystallization of nanoparticles was achieved by screening electrostatic interactions, so that each nanoparticle was surrounded by a layer of counter-ions and the nanoparticles interacted by short-range potentials. 1 Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, Ontario M5S 3H6, Canada, 2 Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario M5S 3E5, Canada, 3 Institute of Biomaterials and Biomedical Engineering University of Toronto, 4 Taddle Creek Road, Toronto, Ontario M5S 3G9, Canada. † These authors contributed equally to this article. *e-mail: ekumache@chem.utoronto.ca © 20 Macmillan Publishers Limited. All rights reserved 10