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Publication Title | Assembly of reconfigurable one-dimensional colloidal superlattices due to a synergy of fundamental nanoscale forces

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Assembly of reconfigurable one-dimensional

colloidal superlattices due to a synergy

of fundamental nanoscale forces

Kaylie L. Younga,b, Matthew R. Jonesb,c, Jian Zhanga,b, Robert J. Macfarlanea,b, Raul Esquivel-Sirventa,d, Rikkert J. Nape, Jinsong Wuc, George C. Schatza,b, Byeongdu Leef,1, and Chad A. Mirkina,b,c,e,1

aDepartment of Chemistry, and bInternational Institute for Nanotechnology, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208; cDepartment of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, IL 60208; dInstituto de Fisica, Universidad Nacional Autonoma de Mexico, Apartado Postal 20-364, DF 01000, Mexico; eDepartment of Biomedical Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208; and fX-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439

Contributed by Chad A. Mirkin, December 5, 2011 (sent for review September 24, 2011)

We report that triangular gold nanoprisms in the presence of attractive depletion forces and repulsive electrostatic forces assem- ble into equilibrium one-dimensional lamellar crystals in solution with interparticle spacings greater than four times the thickness of the nanoprisms. Experimental and theoretical studies reveal that the anomalously large d spacings of the lamellar superlattices are due to a balance between depletion and electrostatic interactions, both of which arise from the surfactant cetyltrimethylammonium bromide. The effects of surfactant concentration, temperature, ionic strength of the solution, and prism edge length on the lattice parameters have been investigated and provide a variety of tools for in situ modulation of these colloidal superstructures. Additionally, we demonstrate a purification procedure based on our observations that can be used to efficiently separate triangular nanoprisms from sphe- rical nanoparticles formed concomitantly during their synthesis.

anisotropic ∣ tunable ∣ small angle X-ray scattering ∣ depletion interaction

The ability to form ensembles of inorganic nanoparticles with a high degree of control has become one of the main areas of focus in nanoscience research (1). This interest stems from the fact that nanocrystal superlattices often exhibit electronic (2), optical (3), and magnetic (4) properties that are distinct from both the corresponding individual particles and the bulk solid as a result of the interactions between the excitons, surface plas- mons, or magnetic moments of the assembled particles (5). Superlattices composed of spherical building blocks have been extensively studied, and researchers now have the ability to synthesize a wide variety of structures (6). However, as new tech- niques are developed to synthesize high-quality anisotropic nano- particles with new physical properties that cannot be obtained with spheres alone, researchers are increasingly interested in the rich assembly behavior of particles with reduced symmetry (7–9). Indeed, periodic arrays of these anisotropic building blocks have been shown to possess unique collective properties with applica- tions in various fields including plasmonics (10) and photonics (11). However, to take full advantage of these collective proper- ties, it is necessary to understand the relationship between the architectural parameters of the ensemble and the emergent phy- sical properties. For this purpose, it is crucial to be able to “engi- neer” the various interactions that exist between nanoparticle building blocks to produce a desired structure (12, 13). The assem- bly of nanocrystals into ordered arrays can be induced via the manipulation of interparticle interactions including van der Waals (14), electrostatic (15), entropic (16–21), and through highly spe- cific biological interactions (22–24). Herein, we report the assem- bly of colloidal triangular gold nanoprisms protected by a cetyltrimethylammonium bromide (CTAB) bilayer in a solution of CTAB micelles into highly ordered 1D crystals with unexpected structural features that emerge from a synergy of attractive deple-

tion and repulsive electrostatic interactions. This system is unique in that it allows access to a regime wherein several interparticle forces are of comparable strength, leading to an unusual example of a one-dimensional superlattice of inorganic disc-like nanostruc- tures that is stable in solution. Furthermore, these columnar assemblies are highly reconfigurable through modification of sev- eral intensive and extensive variables, providing a means to create tunable stimuli-responsive materials (18, 25).

Depletion forces are purely entropic in nature and arise when small, nonadsorbing molecules, such as surfactants, are added to a colloidal solution of particles (26). Two large particles of radius R immersed in a dispersion of small particles (depletants) with radius r possess an exclusion layer whose thickness is equal to r. As the surfaces of the large particles reach a separation smaller than the diameter of the depletant (2r), their exclusion layers be- gin to overlap and the total volume available to the depletants is increased, thereby decreasing the free energy of the system by

Edep 1⁄4 −nkBTΔV; [1]

where n is the number density of the depletant, kB is the Boltz- mann’s constant, T is the temperature, and ΔV is the volume gained by the overlap of exclusion layers (13). The depletion force can also be thought of in terms of osmotic pressure: The exclusion of the depletant from the space between the particles results in a local concentration gradient that produces a net osmotic pressure acting to push the particles together. The strength of the interaction energy is proportional to the magni- tude of the volume gained by bringing two particles together, and therefore depletion forces are particularly attractive for assem- bling anisotropic building blocks because they can potentially have a more efficient overlap of their exclusion layers compared to spherical building blocks due to directionally dependent inter- actions (27). Additionally, depletion forces are maximized for smooth surfaces because very little free volume is gained when two rough surfaces approach each other (28). This fact makes plate-like triangular gold nanoprisms an ideal structure to probe depletion force-based assembly because they are extremely ani- sotropic (aspect ratio of ca. 19) and have atomically flat triangular faces on the top and bottom (29), allowing the strength of the

Author contributions: K.L.Y., M.R.J., B.L., and C.A.M. designed research; K.L.Y., M.R.J., J.Z., R.J.M., J.W., and B.L. performed research; K.L.Y., M.R.J., and B.L. analyzed data; R.E.-S., R.J.N., G.C.S., and B.L. performed theoretical calculations; and K.L.Y., M.R.J., B.L., and C.A.M. wrote the paper.

The authors declare no conflict of interest.

1To whom correspondence may be addressed. E-mail: chadnano@northwestern.edu or blee@aps.anl.gov.

This article contains supporting information online at www.pnas.org/lookup/suppl/ doi:10.1073/pnas.1119301109/-/DCSupplemental.

2240–2245 ∣ PNAS ∣ February 14, 2012 ∣ vol. 109 ∣ no. 7

www.pnas.org/cgi/doi/10.1073/pnas.1119301109

Image | Assembly of reconfigurable one-dimensional colloidal superlattices due to a synergy of fundamental nanoscale forces



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