ScienceWeek

MATERIALS SCIENCE: ON SELF-ASSEMBLY OF NANOPARTICLE CRYSTALS

The following points are made by Orlin D. Velev (Science 2006 312:376):

1) The crystallization 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. New work [1] reports the cocrystallization of equally sized metallic nanoparticles into large crystals with diamond-like symmetry. The oppositely charged gold and silver nanoparticles attract each other at very short distances and assemble into unusual lattices. This work provides new insights into crystallization on the nanoscale, and fills in a gap in the overall picture of particle and biomolecule crystallization.

2) It has been known for decades that micrometer- and submicrometer-sized spheres suspended in liquids readily form "colloidal crystals" 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. This limits the range of their application as other types of crystal symmetries are required for photonic, optoelectronic, and memory storage applications.

3) 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 researchers 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 kBT units (where kB is Boltzmann's constant and T is temperature), come together in mixed CsCl-type lattices 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.

4) 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]. Thus, colloidal crystallization may be achieved by various attractive forces, but only when the interaction energy is precisely adjusted within a certain small range.

References (abridged):

1. A. M. Kalsin et al., Science 312, 420 (2006)

2. O. D. Velev, A. M. Lenhoff, Curr. Opin. Colloid Interface Sci. 5, 56 (2000)

3. M. E. Leunissen et al., Nature 437, 235 (2005)

4. P. L. Biancaniello, A. J. Kim, J. C. Crocker, Phys. Rev. Lett. 94, 058302 (2005)

5. C. J. Kiely, J. Fink, M. Brust, D. Bethell, D. J. Schiffrin, Nature, 396, 444 (1998)

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