DNA-nanoparticle superlattices formed from anisotropic building blocks
Entry by Sandeep Koshy, AP 225, Fall 2010
Title: DNA-nanoparticle superlattices formed from anisotropic building blocks
Authors: Matthew R. Jones, Robert J. Macfarlane, Byeongdu Lee, Jian Zhang, Kaylie L. Young, Andrew J. Senesi & Chad A. Mirkin
Journal: Nature Materials
Jones et. al explore the effect of shape and DNA-linker length on the assembly of DNA-functionalized nanoparticle systems. They show that particle shape has a significant influence on the dimensionality and lattice parameters of assembled crystals. The effect of DNA linker length was found to influence the crystal structure formed. This work shows that such systems can produce precisely patterned systems with varying crystal properties with applications ranging from optics to energy harvesting.
Soft Matter Keywords: colloids, crystallization, phase transition, assembly
Interest in the assembly of particles is ever-growing since the assembly of nanoparticles into superlattices may result in the creation of materials with unique properties. The shape of the particles composing the assembled structure is thought to be an important parameter in generating novel materials. Functionalization of particles with complementary DNA ligands which hybridize and facilitate assembly is gaining favor in the field due to its ability to generate intense specificity and impart nanoscale precision on the assembly process. Since most previous work in this area was performed using spherical particles, the authors of this work aimed to investigate the role of using anisotropic particles of different shapes on crystallization.
Gold nanorods were synthesized using a silver assisted growth procedure. Triangular nanoparticles were synthesized using a previously described method and were found to contain spherical nanoparticle impurities which were separated out. Octahedra and dodecohedra were synthesized using a previously described method relying on the use of the surfactant cetylpyridinium chloride (CPC). All particles were functionalized by ligand replacement between cetyltrimethylammonium bromide (CTAB) and thiolated oligonucleotides. The investigators then performed small angle X-ray scattering (SAXS) and transmission electron microscopy to characterize systems after crystallization.
The authors hypothesized that particles with flat surfaces could facilitate higher amounts of bonding between DNA linkers on the particle surface than spherical shapes (Figure 1 – A). They then synthesized particles of various geometries (Figure 1 – B) and explored the resulting crystal structures using TEM. The geometries used were rods, triangular prisms, rhombic docahedra and rhombic octahedral. They showed that the extension coefficient of systems containing these particles showed a rapid melting transition corresponding to dehybridization of the DNA linkers (Figure 1 – C). A schematic of the hybridization process of the complementary DNA ligands on the nanoparticle surface is also shown (Figure 1 – D).
A 1D particle was defined as one having a length much greater than its width or diameter. The authors used a rod shape as a prototype for such a particle. They hypothesized that the rods would assemble with their long axes in contact to maximize the interactions between their DNA sticky ends. This was indeed observed by TEM. They saw that the rods assembled into 2D sheets and occasionally observed the formation of 3D lattices with hexagonal close packed geometry by using SAXS (Figure 2 – A,B).
A 2D particle was defined as one where the length and width are of at least an order of magnitude larger than their depth. Flat, triangular nanoparticles were used as the prototype for this geometry. They predicted that face-to-face contacts would be established in order to maximize DNA hybridization. SAXS showed a 1D arrangement of particles stacked face-to-face (Figure 2 – C,D). These 1D lamellae did not stack into higher order structures. The authors hypothesized that this was due to the thinness of the prisms relative to the rigidity of the DNA linkers. Since the exposed side of the stack did not contain a high density of DNA ligands, it is unlikely that the sparse DNA links formed between adjacent lamellae could cause stable assembly. The authors showed that the stacking of the triangular prisms showed highly precise spacing and could control the distance between adjacent particles at a resolution of 0.281+/-0.002 nm, which is a resolution that cannot be achieved by other methods.
A 3D particle was defined as one where all 3 spacial dimensions of the particle were of the same order in size. The authors first used rhombic dodecahedron shaped particles that are capable of 100% packing efficiency, unlike spheres, which can only pack to 74% at most. These particles should pack face to face very precisely to maximize DNA hybridization. It was seen that these particles pack in a face centered cubic configuration with the crystallization parameters tunable using various DNA linker lengths (Figure 2 – E,F). The packing of these structures was compared to previous spherical particles and showed increased positional order. Interestingly, although the hydrodynamic size of the DNA shell was larger than the particle, the packing order was retained, indicating that the shape of the particle had a strong influence on system behavior.
The authors then used octahedral, which do not favor face-to-face interactions. With short DNA lengths, long range order was not formed (Figure 3). With longer DNA lengths, body centered and face centered cubic patterns were observed. This indicates that particle shape and DNA length both play a role in the crystallization of DNA linked nanoparticle assemblies.
A summary of the crystallization parameters for the various particles tested is given below in Figure 4.
This paper shows that shape and DNA length have influences on the crystallization parameters of DNA-functionalized nanoparticles. These factors effect the dimensionality, phase behavior and symmetry of these systems. They also show that ultraprecise resolutions may be achieved in such systems. This knowledge may be important in designing such systems for plasmonic-based circuitry, waveguides, photonic bandgap materials and energy harvesting or storage devices.