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MATERIALS SCIENCE:
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Self-Assembly of Unusual Nanoparticle Crystals

The crystallization [HN1] of matter on any length scale, from atoms and ions to biomolecules to nano- and microparticles, has long been a major thrust in science and technology. On page 420 of this issue, Kalsin et al. [HN2] (1) report the cocrystallization of equally sized metallic nanoparticles into large crystals with diamond-like symmetry [HN3]. The oppositely charged gold and silver nanoparticles attract each other at very short distances and assemble into unusual lattices [HN4]. This work provides new insights into crystallization on the nanoscale, and fills in a gap in the overall picture of particle and biomolecule crystallization.

It has been known for decades that micrometer- and submicrometer-sized spheres suspended in liquids readily form "colloidal crystals" [HN5] during sedimentation or drying. The spheres crystallize when their free volume is restricted below a certain threshold, but this occurs only when the interactions between the spheres are repulsive, which allows their rearrangement. Such closely packed crystals allow facile fabrication of materials with controlled porosity and long-range organization (2). Volume-restricted repulsive spheres, however, always crystallize in a trivial lattice of hexagonally close-packed layers [HN6]. This limits the range of their application as other types of crystal symmetries are required for photonic, optoelectronic, and memory storage applications.

The formation of colloid crystals with other symmetries can, in principle, be achieved if the particles are assembled by attractive interactions. Two seemingly simple ideas for crystallization by particle attraction have been considered, yet they have proven notoriously difficult to realize experimentally. The first idea is to use binary mixtures of oppositely charged particles that could cocrystallize in a manner broadly similar to crystallization of ionic salts from liquid solutions. The problem with this system is that strongly attractive particles rapidly and irreversibly stick to each other, forming gel-like aggregates. Only recently have Leunissen et al. [HN7] designed a procedure whereby micrometer-sized colloidal spheres having small positive or negative charges are synthesized and cocrystallized in density-matched organic liquids (3). The particles, whose attractive interaction energies are estimated to be on the order of a few k_BT units (where k_B is Boltzmann's constant and T is temperature), come together in mixed CsCl-type lattices [HN8] of alternating positive and negative charges. A variety of crystals of other symmetries and particle compositions have been assembled, and the method could be versatile enough to be used in the routine synthesis of ionic colloidal crystals.

A second idea for particle crystallization by attractive interactions that has also proven difficult to realize is the crystallization of particles by functionalizing them with complementary DNA strands. DNA hybridization locks the particles together when they come into contact; however, the strong irreversible "snapping" into place does not allow crystallization. The key to making this idea work has been to reduce the strength of the interactions by adjusting the temperature of the suspension very near the melting point of DNA, where hybridization is weak and reversible (4) [HN9]. Thus, colloidal crystallization may be achieved by various attractive forces, but only when the interaction energy is precisely adjusted within a certain small range (see the figure).

Systems of nanoparticles 1 to 10 nm in size provide a natural link between the areas of molecular and colloidal crystallization. Crystals from such particles can find applications in nanoelectronics, plasmonics, high-density data storage, catalysis, and biomedical materials. The formation of binary crystals from nanoparticle mixtures as a likely result of cocrystallization under restricted volume conditions was reported some time ago (5). Only recently has the role of electrostatics in the formation of nanoparticle crystals emerged as a parameter that can be controlled in order to assemble various crystals of new symmetry and composition (6) [HN10]. The report by Kalsin et al. conclusively proves that large crystals can be produced by controlled electrostatic self-assembly [HN11]. The crystallization has been achieved by precise adjustment of the attraction between the oppositely charged nanoparticles, but the data also point to the existence of unusual effects of electrostatic screening of the larger particles by the smaller ones that do not scale up to interactions between microspheres.

Packing together. The crystallization of colloidal particles and biomacromolecules is intrinsically related to the interactions between the particles, which often can be controlled by their charge (graph at left). Repulsive spheres can easily be crystallized by restricting their volume. Proteins and binary mixtures of oppositely charged particles can be crystallized by precise adjustment of the interactions into a weakly attractive regime. However, if the interactions between the particles are strongly attractive, rapid precipitation of amorphous aggregates occurs. The micrograph images are of colloidal crystals assembled by restricting the free volume of repulsive latex spheres (left), crystals of the protein lysozyme obtained under slightly attractive interactions (center), and a nanoparticle crystal assembled under controlled electrostatic attraction (1) (right). CREDITS: O. VELEV

Future research in nanoparticle assembly may bring closer the areas of biomacromolecule and nanoparticle crystallization. Could a similar charge-balancing approach be applied to binary mixtures of proteins, or mixtures of proteins and nanoparticles? A large variety of nanoparticles of special shape and properties have been synthesized in the past few years, but little is yet known about their self-assembly [HN13]. New "zwitterionic" [HN14] particles, having patches of negative and positive charges on their surfaces, could soon be synthesized and crystallized by adjustment of the interactions in a manner similar to the crystallization of proteins. Thus, nanoparticle crystallization and assembly may not only yield new nanomaterials, but could also provide insights into how to control colloidal forces on the nanoscale.

References

1. A. M. Kalsin et al., Science 312, 420 (2006); published online 23 February 2006 (10.1126/science.1125124).
2. O. D. Velev, A. M. Lenhoff, Curr. Opin. Colloid Interface Sci. 5, 56 (2000) [CrossRef].
3. M. E. Leunissen et al., Nature 437, 235 (2005) [CrossRef].
4. P. L. Biancaniello, A. J. Kim, J. C. Crocker, Phys. Rev. Lett. 94, 058302 (2005) [CrossRef].
5. C. J. Kiely, J. Fink, M. Brust, D. Bethell, D. J. Schiffrin, Nature, 396, 444 (1998) [CrossRef].
6. E. V. Shevchenko, D. V. Talapin, N. A. Kotov, S. O'Brien, C. B. Murray, Nature 439, 55 (2006) [CrossRef].
7. A. George, W. Wilson, Acta Crystallogr. D. 50, 361 (1994) [CrossRef].

10.1126/science.1125800

HyperNotes
Related Resources on the World Wide Web

General Hypernotes

Dictionaries and Glossaries

A glossary of materials science is provided by the University of Liverpool's MATTER Project.

A glossary is provided by the Polymers and Liquid Crystals (PLC) Web site.

The Compendium of Chemical Terminology is provided by the International Union of Pure and Applied Chemistry (IUPAC).

Web Collections, References, and Resource Lists

The Open Directory Project provides links to Internet resources on nanotechnology.

The Yahoo Search Directory offers links to nanotechnology and material science Internet resources.

PSIgate, a physical sciences information gateway, offers links to materials science Internet resources.

The Incomplete Guide to Web Resources on Nanotechnology and Nanomaterials is provided by M. Lu, Australian Research Council Centre for Functional Nanomaterials, University of Queensland, Brisbane.

Frank Potter's Science Gems provides links to educational resources on materials science.

Online Texts and Lecture Notes

Making Matter: The Atomic Structure of Materials is presented by the Institut Laue-Langevin, Grenoble, France.

Discovering Materials Science and Engineering is a presentation of the Department of Materials Science and Engineering, Cornell University.

Nanoparticles.org is a resource for particle technology information.

The National Nanotechnology Initiative provides an education center and links to links to nanotechnology research centers.

P. Howell, Department of Materials Science and Engineering, Pennsylvania State University, makes available in PDF format lecture notes for a course on materials science.

The Department of Materials Science and Engineering, University of Illinois, offers lecture notes by J. Bullard for a materials science course. A section on crystal structures is included.

S. J. Heyes, Department of Chemistry, University of Oxford, UK, provides lecture notes on the structure of simple inorganic solids.

The Department of Materials Science and Engineering, Massachusetts Institute of Technology, makes available lecture slides in PDF format for an introduction to solid state chemistry course.

General Reports and Articles

S. C. Glotzer, Department of Chemical Engineering, University of Michigan, makes available in PDF format a December 2004 article by S. C. Glotzer, M. J. Solomon, and N. A. Kotov titled "Self-assembly: From nanoscale to microscale colloids."

The Vol. 74, No. 9, 2002 issue of Pure and Applied Chemistry was a Special Topic Issue on the theme of nanostructured advanced materials. Included was an article by M. Sastry titled "Assembling nanoparticles and biomacromolecules using electrostatic interactions."

B. A. Grzybowski, Department of Chemical and Biological Engineering, Northwestern University, makes available in PDF format a 2004 article by B. A. Grzybowski and C. J. Campbell titled "Complexity and dynamic self-assembly."

The 29 March 2002 Science was a special issue on supramolecular chemistry and self-assembly that included a Viewpoint article by G. Whitesides and B. Grzybowski titled "Self-assembly at all scales."

The 15 October 2004 issue of Science had a Perspective by S. C. Glotzer titled "Some assembly required."

Numbered Hypernotes

1. Crystallization. Wikipedia has an article about crystals. Cheresources.com provides an introduction to crystallization. A course on solid state chemistry at the Massachusetts Institute of Technology makes available lecture notes on the nature of crystalline solids. S. A. Nelson, Department of Earth and Environmental Sciences, Tulane University, provides a series of lecture notes on crystallization and crystal structure for a course on Earth materials.

2. Alexander M. Kalsin, Marcin Fialkowski, Maciej Paszewski, Stoyan K. Smoukov, Kyle J. M. Bishop, and Bartosz A. Grzybowski are in the Department of Chemical and Biological Engineering and at the Northwestern Institute on Complex Systems, Northwestern University.

3. Diamond-like symmetry. Crystal Structures of Elements from Lebanon Valley College's Molecules Web site has an entry for diamond structures. H. Föll's Defects in Crystals includes information on diamond or sphalerite structure in the section on ionic crystals.

4. Crystal lattices. C. Nave's HyperPhysics offers an introduction to the crystal lattice. H. Föll's Defects in Crystals provides an introduction to lattice and crystal. An index of common crystal lattice structures is provided by the Center for Computational Materials Science, Naval Research Laboratory.

5. Colloidal crystals. Wikipedia has an article on colloids. W. Poon, School of Physics, University of Edinburgh, offers a presentation on colloidal crystals. The Soft Condensed Matter Group, Debye Institute, University of Utrecht, offers a research presentation on colloidal matter and a 2 February 2006 Nature News & Views article by A. van Blaaderen titled "Colloids get complex." O. D. Velev makes available in PDF format the March 2000 review article by O. D. Velev and A. M. Lenhoff titled "Colloidal crystals as templates for porous materials" (2). The 24 March 2000 issue of Science had a Report by O. D. Velev, E. W. Kaler, and A. M. Lenhoff titled "A class of microstructured particles through colloidal crystallization."

6. An entry for hexagonal (crystal system) is included in Wikipedia. A collection of crystalline structures related to the hexagonal close-packed crystal structure is included in the presentation on crystal lattice structures provided by the Center for Computational Materials Science, Naval Research Laboratory.

7. Mirjam E. Leunissen and colleagues are in the Soft Condensed Matter Group, Debye Institute, Utrecht University. The Soft Condensed Matter Group makes available in PDF format the 8 September 2005 Nature article by M. E. Leunissen et al. titled "Ionic colloidal crystals of oppositely charged particles" (3) and the accompanying News & Views article.

8. CsCl-type lattices. WebElements has an entry for CsCl. Crystal Structures of Binary Inorganic Compounds from Lebanon Valley College's Molecules Web site includes an entry for CsCl. H. Föll's Defects in Crystals includes an entry on CsCl structure. Wikipedia has an entry for CsCl that displays the structure. The chemical structure presentation of the Avogadro Web Site includes a section on CsCl structure.

9. Crystallization with DNA. Scholarly Commons @ Penn makes available in PDF format the February 2005 Physical Review Letters article by P. L. Biancaniello, A. J. Kim, and J. C. Crocker titled "Colloidal interactions and self-assembly using DNA hybridization" (4). A nanostructured materials course at Rensselaer Polytechnic Institute makes available a 1998 student presentation by T. Pickles titled "Using DNA as a template for nanomaterial synthesis." N. C. Seeman's Lab at the Department of Chemistry, New York University, offers a presentation on DNA nanotechnology. J. H. Reif, Department of Computer Science, Duke University, makes available in PDF format a 2000 conference paper by J. H. Reif, T. H. LaBean, and N. C. Seeman titled "Challenges and applications for self-assembled DNA nanostructures" and presentation slides. Y. Brun at the Laboratory for Molecular Science, University of Southern California, makes available in PDF format a 2004 meeting presentation by Y. Brun et al. titled "Building blocks for DNA self-assembly."

10. Electrostatics in the formation of nanoparticle crystals. D. Frenkel, FOM-Institute for Atomic and Molecular Physics, Amsterdam, makes available in PDF format a News & Views article from Nature Materials titled "Colloidal crystals: Plenty of room at the top" about the 5 January 2006 Nature article by E. V. Shevchenko, D. V. Talapin, N. A. Kotov, S. O'Brien, and C. B. Murray titled "Structural diversity in binary nanoparticle superlattices" (6).

11. Electrostatic self-assembly. NanoSonic Inc. offers a presentation about electrostatic self-assembly. The condensed matter section of the arXiv.org preprint archive makes available a 2005 paper by J.-F. Berret et al. titled "Electrostatic self-assembly: A new route towards nanostructures." The Grzybowski Group makes available in PDF format an April 2003 Nature Materials article by B. A. Grzybowski et al. titled "Electrostatic self-assembly of macroscopic crystals using contact electrification."

12. Protein crystallization. An introduction to protein crystallization is provided by the Science Outreach Program of the University of Alabama in Huntsville. The Macromolecular Structure Group at the University of Florida offers lecture notes on crystallization for a course on macromolecular structure determination by x-ray crystallography. The Structural Medicine Division, Department of Haematology, University of Cambridge, UK, makes available lecture notes on the theory and practice of crystallization for a course on protein crystallography.

13. Self-assembly. Self-assembly is defined in the PLC Virtual Textbook glossary. A presentation on self-assembly is provided by the Modeling, Experiment, and Computation Laboratory at the University of Delaware. A 2004 student project on self-assembly was prepared by J. Beech, P. Sommansson, and M. Jansson for a course on experimental biophysics at Lunds University, Sweden. E. Rabani, School of Chemistry, Tel Aviv University, makes available in PDF format a 2003 article titled "Self-assembly of nanoparticles: Theory and experiments meet."

14. Zwitterionic compounds (zwitterions) are defined in the IUPAC Compendium of Chemical Terminology. Wikipedia has an entry for zwitterion.

15. Orlin D. Velev is in the Department of Chemical and Biomolecular Engineering, North Carolina State University.