made to orderThe DNA-mediated assembly of anisotropic gold nanoparticles shows the importance of particle shape in the controlled formation of DNA–nanoparticle superlattices.
sharon c. Glotzer and Joshua a. anderson
since the early days of the nanoscience revolution, scientists have recognized the great potential in exploiting the
highly specific and recognitive properties of DNA to assemble nanomaterials. The four nucleotide bases of DNA bind only with their complementary counterparts, providing lock-and-key specificity that traditional materials lack. Attaching a DNA oligonucleotide at one end to a nanoparticle confers a ‘sticky patch’ or binding site to
the particle that will stick only to its ‘mate’ (see Fig. 1). In parallel work published in 1996, Mirkin1 and Alivisatos2 showed that mixing together batches of nanoparticles functionalized with complementary DNA strands resulted in hybridization of the DNA linkers, consequently binding the patchy particles3 to one another. Without the DNA, the gold nanoparticles would not ordinarily assemble, at least not into anything interesting. In this early work, binding and
unbinding was demonstrated1, and even nanoparticle dimer and trimer ‘molecules’ were achieved2. However, extended aggregates formed from many particles lacked any long-range order.
The first crystal lattices assembled from DNA-linked nanospheres were reported, using slightly different binding strategies, in 20084–6. An ordered gold-nanoparticle binary crystal with a CsCl structure was obtained by Gang and co-workers5, and in a separate
Zimmerman notes that this is not the case in tetracene, because fission occurs in that material only through thermally activated processes. Can singlet fission therefore occur in rubrene crystals, which are chemical derivatives of tetracene, and if so, by what mechanism? In model tetracene dimers, the yield of this process depends sensitively on the molecular nature of a bridge linking the two molecules6. One might therefore expect that in rubrene the side-group substituents could increase the rate of singlet fission with respect to that in unsubstituted tetracene crystals. Further ultrafast spectroscopic measurements in high-purity rubrene crystals are thus warranted.
Generally, it is not yet clear to what extent triplets can contribute to the photocurrent in organic semiconductor heterostructures. Recent ultrafast spectroscopic measurements by Rao and co-workers7 have shown for pentacene:fullerene bilayer heterostructures that charge is generated following slow diffusion of triplet excitons to the heterojunction. Triplets were shown to be produced by singlet fission on subpicosecond timescales, but charge-induced absorption was only observed on nanosecond timescales, consistent with a diffusion-limited process involving triplets. Considering the short singlet-exciton lifetime, and therefore the long singlet diffusion length that would be required in bulk pentacene, pentacene:fullerene bilayers display surprisingly high power conversion efficiencies in solar cells8. Podzorov and co-workers suggest that, on generation
of triplets in the bulk of the crystal, the triplets diffuse to the surface by a Dexter mechanism. Taken together, the studies
by Rao and colleagues and by Podzorov and co-workers point towards the view that triplet states may indeed be useful to produce photocurrent in photovoltaic diodes. If so, the mechanism of charge separation of triplet excitons at fullerene heterojunctions clearly requires substantial further investigation.
We continue to find new opportunities to understand in greater depth the rich many-body physics of organic semiconductors. A detailed understanding of triplet-exciton generation mechanisms, their transport properties and their dynamics at heterojunctions, will be crucial to the fabrication of organic photovoltaic devices. Certainly, if we could learn to make use of singlet fission as an effective energy up-conversion scheme in solar cells, exciton energies would then really go a long way. ❐
Carlos Silva is in the Department of Physics and Regroupement québécois sur les matériaux de pointe, Université de Montréal, C.P. 6128, Succursale centre-ville, Montréal (Québec) H3C 3J7, Canada. e-mail: [email protected]
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78, 973–989 (2006).2. Sundar, V. C. et al. Science 303, 1644–1646 (2004).3. Najafov, H., Lee, B., Zhou, Q., Feldman, L. C. & Podzorov, V.
Nature Mater. 9, 938–943 (2010).4. Pope, M. & Swenberg, C. E. Electronic Processes in Organic
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Figure 2 | Illumination of a single rubrene crystal produces current at the surface of the crystal if the surface is functionalized with a layer that splits excitons. a, Podzorov and co-authors3 propose that triplet excitons, T1, are produced by fission of singlet excitons, S1, on excitation of the crystal with light of energy ћω. Triplet excitons then diffuse to the surface, where they produce photocurrent. b, On the basis of their observations, the authors conclude that the diffusion length, Lex, of T1 must be on the micrometre length scale.
study by Mirkin, both a CsCl structure and a face-centred-cubic crystal structure were formed6. These findings revealed, at long last, that DNA-programmed nanoparticle
assembly could controllably organize spheres into structures with ordered crystal lattices.
Writing in this issue of Nature Materials, Mirkin and co-workers now report
the DNA-mediated assembly of crystal structures from non-spherical gold nanoparticles7. They have synthetically realized and numerically rationalized the assembly of various anisotropic nanoparticles into ordered arrangements that had not been achieved by the assembly of spherical particles. In going beyond spheres, the researchers demonstrate a new level of complexity and control for DNA-mediated nanoparticle self-assembly — an important step in the struggle to manipulate matter at the nanoscale.
By exploiting the same self-complementary DNA linking strategies as before, the Northwestern team is able to self-assemble hexagonal packing of gold rods, columns of triangular prisms and face-centred-cubic crystals of rhombic dodecahedrons. These structures are easily predicted given the shapes of the particle building blocks8. Maximal DNA hybridization occurs when the particles pack face-to-face because of a preponderance of DNA on the particle faces, as opposed to the edges or vertices, making the faces ‘sticky’ (Fig. 2).
They also obtained both body-centred-cubic and face-centred-cubic crystals of gold octahedrons depending on the length of the DNA linkers, demonstrating the exciting possibility of tailoring both the crystal structure and the lattice constants by simply changing the length of the DNA. Their analysis shows that the face-centered-cubic structure formed at long DNA linker lengths results from the increased flexibility of the longer DNA. With enough flexibility, the outside surface of ‘sticky patches’ around an octahedron becomes spherical, removing the tendency for face-to-face ordering. This result shows that both the shape of the particle and the flexibility of the DNA contribute to the final structure. Also writing in this issue, Cigler et al. report on a similar system of linked nanoparticles with two types of particles — gold nanospheres and protein virus capsids9. Cigler et al. show that the interaction of the DNA linkers alone results in the previously seen CsCl structure, but the different interactions between the two types of nanoparticles with each other enables the formation of the NaTl lattice — a composition of two interpenetrating diamond lattices10. Computer simulations revealed aggregation of the particles, followed by crystallization of the structures.
Interestingly, Mirkin and co-workers show that their anisotropic nanostructures initially form disordered aggregates and then, on thermal annealing, form ordered lattice structures, similar to the process
Gold nanoparticle
Double-stranded DNA
Single-stranded DNA
Figure 1 | A schematic of a functionalized gold nanoparticle. Double-stranded DNA is attached to the gold nanoparticle, covering the surface. At the end of this DNA linker is a small strand of self-complementary single-stranded DNA that acts as a ‘sticky patch’ to which the DNA on another nanoparticle can attach.
Figure 2 | An assembled crystal of DNA-functionalized nanoparticles. The preferred face-to-face interactions lead to the formation of a hexagonal columnar structure in this hypothetical system of hexagonal prisms.
reported by Cigler et al. In the case of the gold nanorods reported by Mirkin and co-workers, single-angle X-ray scattering data shows that the nanorods’ shape directs crystallization into a two-dimensional lattice, and then on further annealing, three-dimensional order is gradually introduced to form a hexagonal-close-packed lattice.
Although the crystal structures reported by Mirkin and co-workers are relatively simple ones as crystals go, the work opens the door to far more interesting structures that are possible now that the linking of non-spherical building blocks by DNA is in hand. Future work will no doubt explore the self-assembly of more complex structures, where different DNA sequences are used to obtain anisotropic patches on the particles, conferring unique orientations to assemble unconventional crystal structures, or even quasicrystalline structures11. Indeed, binding of complementary DNA pairs alone has been exploited to achieve highly complex DNA structures12–14, and dreams of self-replicating materials are being pursued with DNA-linked colloids15. Non-spherical building blocks introduce possibilities
of directional bonding and packing into the problem16.
With a virtually limitless variety of particle shapes, and the ability to create specific interparticle bonds using complementary DNA pairs, the design space is practically infinite3,16,17. In this situation, the need for guidance from theory, modelling and simulation could not be greater. Some of the questions posed at this stage are: What structures are possible? How complex can they be? What building block shapes — or mixtures of shapes — are most likely to form the most interesting structures? Where should the DNA ‘bonds’ be placed, how long should they be, and how many are needed? What other assembly ‘knobs’ could be tuned? The parameters are limitless, and computer simulations can rapidly search the vast design space of DNA-programmed building-block assembly to identify candidate building blocks and structures. Modern computer chips built for fast graphics now allow molecular dynamics simulations of DNA-linked nanoparticle assembly with unprecedented speed and fidelity18, allowing for rapid prototyping and design. In any case, with continued synthetic
advances, such as those reported by Mirkin’s team and by Cigler et al., ‘made to order’ nanomaterials are closer than ever. ❐
Sharon C. Glotzer and Joshua A. Anderson are in the Department of Chemical Engineering, University of Michigan, Ann Arbor, 2300 Hayward Street, Ann Arbor, Mchigan 48109-2136, USA. e-mail: [email protected]
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Nature 451, 549–552 (2008).6. Park, S. Y. et al. Nature 451, 553–556 (2008).7. Jones, M. R. et al. Nature Mater. 9, 913–917 (2010).8. Zhang Z. L., Horsch, M. A., Lamm, M. H. & Glotzer, S. C.
Nano. Lett. 3, 1341–1346 (2003).9. Cigler, P. et al. Nature Mater. 9, 918–922 (2010).10. Iacovella, C. R. & Glotzer, S. C. Nano Lett. 9, 1206–1211 (2009).11. Haji-Akbari, A. et al. Nature 462, 773–777 (2009).12. Dietz, H., Douglas, S. M. & Shih, W. M. Science
325, 725–730 (2009).13. Seeman, N. C. Nature 421, 427–431 (2003).14. Seeman, N. C. Ann. Rev. Biochem. 79, 65–87 (2010).15. Leunissen, M. E. et al. Soft Matter 5, 2422–2430 (2009).16. Glotzer, S. C. & Solomon, M. J. Nature Mater. 6, 557–562 (2007).17. Tkachenko, A. V. Phys. Rev. Lett. 89, 148303 (2002).18. http://codeblue.umich.edu/hoomd-blue/
