Nanoparticles are now available that are attractive for a wide range of materials and devices, but novel fabrication methods are also required to take full advantage of the interesting properties of Nanoparticulates.
Approaches based on the self-assembly of systems from individual components offer tremendous cost advantages and an almost a magical "ease of manufacture" compared to lithographic methods. Self-assembly techniques can also address tasks that are intrinsically challenging for conventional lithography processes for example the fabrication of thee dimensional architectures or structures containing pre-fabricated nano-objects.
Conversely, self-assembly methods typically have significant limitations: they allow for the formation of relatively simple structures from similar components, and they rarely permit a rational design of systems. The ability to arrange different nanocomponents into a system with a specific architecture is a key requirement for enabling many emerging functional properties of nanosystems. The question therefore arises, can this be achieved via self-assembly?
Figure 1. A concept of programmable self-assembly. Nano-components of various types (lego-blocks) are directed by "smart" glue, biomolecules (for example, DNA), that encode how components interact, that leads to formation of final structure.
One of the promising strategies for the fabrication of complex systems from components of multiple types via self-assembly is based on the use of biomolecules.
The incorporation of biomolecules into nano-object designs provides an opportunity to assign selective attractions between various nano-components. Biomolecular encoding establishes the rules which regulate how different nano-objects interact with each other and how the final structure appears.
One of the most attractive implementation of this approach is based on the use of DNA, which is chemically stable and can be synthesized by various means. DNA also provides a high degree of encoding and can be conveniently used to adjust inter-component distances.
The use of DNA for the nanotechnology applications was pioneered by the group of Ned Seeman, who explored the idea of DNA branched structures1, or so called scaffolds, and by the groups of Chad Mirkin and Paul Alivisatos, who demonstrated that two types of particles functionalized with complementary DNA can recognize each other2.
Despite the ostensible clarity of the bio-programmable concept, the fabrication of well determined structures from nano-objects remains a challenge as a result of additional inter-component charge related interactions, various molecular interactions, geometric and entropic. Consequently, a complex phase interaction is anticipated even for relatively simple systems3.
Several groups have demonstrated in both 1-D and 2-D, that DNA scaffolds, i.e. DNA patterns with recognition sites, can be used to direct an attachment of encoded particles onto pre-determined sites4. In three dimensions however, a different approach is more practical, the final structure with all the relevant information should be "programmed" using DNA motifs attached to nano-objects.
Figure 2. (Left) Small angle x-ray scattering pattern obtained from a superlattice of nanoparticles assembled with DNA. (Right) The corresponding structure, body centered cubic lattice, formed by particles of two types, containing two complementary DNA; interparticle distances are determined by DNA.
A recent discovery has shown that certain DNA motifs can indeed lead to the formation of highly organised structures - crystalline superlattices of nanoparticles with order propagating over tens or hundreds of particles sizes5. Phase diagram of DNA mediated assemblies investigated experimentally shows that the degree of order depends on the details of the DNA shells, the number of DNA linking particles and DNA length6.
The crystalline assemblies are thermodynamically reversible and temperature-tunable, with body centered cubic (bcc) lattices where particles occupy only ~3-5% of the unit cell. Such open structures potentially permit the incorporation of various functional elements at specific locations on the 3D superlattice, as well as the opportunity to perform post-assembly modifications. For instance, if nanoparticles are linked in a superlattice using a reconfigurable DNA device, the interparticle distances can be switched "on-demand"7 using molecular stimuli and simple DNA strands.
Apart from assembling millions of nanoparticles into 3D assemblies, it is a quite advantageous to fabricate precisely structured clusters of several particles. When a few nanoparticles are arranged in a particular structure, new material properties can emerge. Nanoparticles in this case are analogous to atoms, which, when connected in a molecule, often exhibit properties not found in the individual atoms.
Conventional solution-based methods typically result in a broad population of multimers of clusters; furthermore, the efficiency of fabrication is limited. A high-throughput method has recently been demonstrated for the fabrication of clusters using DNA-encoded nanoparticles assembled on a solid support via recognition8. This approach allows the construction of nanoparticle clusters with a high yield. Using this method, two-component clusters have been fabricated from gold and silver particles to examine their optical properties. More complex structures containing several types of nano-objects with regulated inter-objects positions can be assembled using this approach.
Research on programmable self-assembly of nanosystems promises to bring a new paradigm in relation to the fabrication of materials and devices. Although significant progress is evident, several major challenges still need to be understood and resolved: (i) how to balance selective biological interactions and non-selective physical factors; (ii) how to program a global structure of a system using encoding techniques for individual components; (iii) how to implement error-correction in the self-assembly process.
1. Seeman, N. C. DNA in a material world. Nature 421, 427-431 (2003)
2. C. A. Mirkin, R. L. Letsinger, R. C. Mucic, and J. J. Storhoff, "A DNA-based method for rationally assembling nanoparticles into macroscopic materials" Nature 382, 607 (1996); A. P. Alivisatos, K. P. Johnsson, X. G. Peng, T. E. Wilson, C. J. Loweth, M. P. Bruchez, and P. G. Schultz, "Organization of 'nanocrystal molecules' using DNA" Nature 382, 609 (1996).
3. A. V. Tkachenko, "Morphological diversity of DNA-colloidal self-assembly" Physical Review Letters 89, 148303 (2002)
4. Y. Y. Pinto, J. D. Le, N. C. Seeman, K. Musier-Forsyth, T. A. Taton, and R. A. Kiehl, "Sequence-encoded self-assembly of multiple-nanocomponent arrays by 2D DNA scaffolding" Nano Letters 5, 2399 (2005); Zhang, J. P., Liu, Y., Ke, Y. G. & Yan, H. Periodic square-like gold nanoparticle arrays templated by self-assembled 2D DNA nanogrids on a surface. Nano Lett. 6, 248_251 (2006); Deng, Z. X., Tian, Y., Lee, S. H., Ribbe, A. E. & Mao, C. D. DNA-encoded self-assembly of gold nanoparticles into one-dimensional arrays. Angew. Chem. Int. Ed. 44, 3582 (2005)
5. D. Nykypanchuk, M. M. Maye, D. van der Lelie, and O. Gang, "DNA-guided crystallization of colloidal nanoparticles", Nature 451, 549 (2008); S. Y. Park, A. K. R. Lytton-Jean, B. Lee, S. Weigand, G.C. Schatz, C.A. Mirkin, "DNA-Programmable Nanoparticle Crystallization," Nature 451, 553 (2008)
6. H. M. Xiong, D. van der Lelie, and O. Gang, "Phase Behavior of Nanoparticles Assembled by DNA Linkers", Physical Review Letters 102, 015504 (2009).
7. M. M. Maye, D. Nykypanchuk, M. Cusiner, D. van der Lelie, and O. Gang, "Stepwise surface encoding for high-throughput assembly of nanoclusters", Nature Materials, 8, 388 (2009)
8. M. M. Maye, K. Mudalidge, D. Nykypanchuk, W. Sherman, and O. Gang "Molecularly Switchable Nanoparticle Superlattices and Clusters with Binary States", Nature Nanotechnology, DOI:10.1038/nnano.2009.378
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