NGUYEN ET AL. VOL. 5 ’ NO. 11 ’ 8892–8903 ’ 2011 www.acsnano.org 8892 September 27, 2011 C 2011 American Chemical Society Self-Assembly and Reconfigurability of Shape-Shifting Particles Trung Dac Nguyen, † Eric Jankowski, † and Sharon C. Glotzer †,‡, * † Department of Chemical Engineering and ‡ Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States N ext generation materials will be dis- tinguished by their ability to adapt in novel ways both on demand and to environmental cues to perform impor- tant functions. The ability of materials to carry out molecular recognition, autono- mous sensing and reporting, and to change properties when needed, all require that intrinsic building blocks be able to reconfi- gure from one structure to another. While adaptability is ubiquitous in biological sys- tems, few if any examples of this ability can be found in traditional synthetic materials. For instance, it is well-known that proteins, the basic building blocks of biology, are the canonical switchable particle with a config- uration that is entirely dependent upon solvent pH, the presence of ligands, and the presence of other proteins. Numerous studies have recently been conducted in attempts to mimic such protein-like switch- ability in nano- and microscale systems with implications to adaptive, functional materi- als. An interesting model of switchable self- assembling objects was developed recently by Bishop et al. 1 They studied the formation of hexamers of triangular robotic pucks, which are able to move on an air hockey table and detach from/attach to other pucks. Klavins et al. utilized this model to demonstrate how switchable components can be used to direct the assembly of a target structure. 2 Today's materials are primarily static: once formed they retain the same structure throughout their lifetime, aside from aging, fatigue, corrosion and other deleterious ef- fects. Shape memory alloys remember their original shape and after deformation can return to it through rearrangement of the constituent atoms via heating. Only recent- ly have techniques become available to consider the synthesis and fabrication of dynamically switchable nanoparticle-based materials, that is, materials whose structure can dynamically reconfigure between two or more states through controlled changes of the nanoparticle itself. Materials self- assembled from shape-changing building blocks could be among the first experimen- tally realized materials in this class. For example, experiments have shown that col- loidal gold nanorods can be shortened or shifted to other shapes such as spheres, bent, twisted, or φ-shaped by using laser pulses with different wavelengths and widths. 3À5 As the colloidal particles change shape from rodlike to spherical, their pack- ing pattern transforms from nematic to triangular lattices accordingly. 3À5 The trans- formations, however, were irreversible. Kim et al. reported thermally responsive capsule structures with 25 nm diameter pores on the shell formed by hierarchical self-assem- bly of double tethered rod amphiphiles. Upon heating or cooling, the hydrophilic oligo-(ethylene oxide) coils at one end of the rods shrink or expand, respectively, resulting in a reversible closed/open gating motion of the nanopores. 6 They also de- monstrated a reversible transformation be- tween two-dimensional sheets and tubular structures assembled by laterally grafted rod amphiphiles upon heating via a similar mechanism. 7 Alternatively, polypeptide- based block copolymers can also be used as stimuli responsive building blocks due to the ability of the polypeptide segments to * Address correspondence to sglotzer@umich.edu. Received for review August 10, 2011 and accepted September 27, 2011. Published online 10.1021/nn203067y ABSTRACT Reconfigurability of two-dimensional colloidal crystal structures assembled by anisometric particles capable of changing their shape were studied by molecular dynamics computer simulation. We show that when particles change shape on cue, the assembled structures reconfigure into different ordered structures, structures with improved order, or more densely packed disordered structures, on faster time scales than can be achieved via self-assembly from an initially disordered arrangement. These results suggest that reconfigurable building blocks can be used to assemble reconfigurable materials, as well as to assemble structures not possible otherwise, and that shape shifting could be a promising mechanism to engineer assembly pathways to ordered and disordered structures. KEYWORDS: reconfigurable materials . self-assembly . shape-shifting particles . computer simulation ARTICLE