DNA‐ and Field‐Mediated Assembly of Magnetic Nanoparticles into High‐Aspect Ratio Crystals

Abstract

Under an applied magnetic field, superparamagnetic Fe 3 O 4 nanoparticles with complementary DNA strands assemble into crystalline, pseudo-1D elongated superlattice structures. The assembly process is driven through a combination of DNA hybridization and particle dipolar coupling, a property dependent on particle composition, size, and inter-particle distance. The DNA controls inter-particle distance and crystal symmetry, while the magnetic field leads to anisotropic crystal growth. Increasing the dipole interaction between particles by increasing particle size or external field strength leads to a preference for a particular crystal morphology (e.g., rhombic dodecahedra, stacked clusters, and smooth rods). Molecular dynamics (MD) simulations show that an understanding of both DNA hybridization energetic and magnetic interactions are required to predict the resulting crystal morphology. Taken together, the data show that applied magnetic fields with magnetic nanoparticles can be deliberately used to access nanostructures beyond what is possible with DNA hybridization alone.

Probing the Consequences of Cubic Particle Shape and Applied Field on Colloidal Crystal Engineering with DNA

“…Since the cubes with the 66 bp DNA design assembled with the same symmetry as spheres, we hypothesized that a similar series of morphologies would form after assembly in a field, consistent with what has been observed with spherical particles [29] . When slowed cooled as before, the morphology of the resulting crystals varied as a function of magnetic field strength from more isotropic clusters (0 G; Figure 2 c), to aligned crystals (140 G; Figure S5), and finally to defined, smooth rods (3800 G; Figure 2 e).…”

“…A clear orientation alignment along the ⟨111⟩ SL with the field direction is observed for both samples grown inside (3800 G, Figures 4 b,d, S9) and outside a field (0 G, Figure S10 b,c). This alignment is in contrast to what is observed for spherical Fe 3 O 4 PAE crystals with bcc symmetry which have their ⟨100⟩ SL oriented along the field direction [29] . The alignment in a field also contrasts with what observed for oleate coated cubes with SC symmetry, where the ⟨100⟩ SL is oriented along the field direction and ||⟨100⟩ SL (Figure S11, see SI for details).…”

“…Recent work has investigated the use of external stimuli to influence colloidal crystal growth of spherical, DNA‐coated Fe 3 O 4 NPs. This work found that applying magnetic fields to the magnetic PAEs resulted in the formation of elongated nanostructures, [29] highlighting the unique ability of DNA to separate the interactions of dipole‐dipole coupling and DNA hybridization interactions to form high‐aspect ratio structures with tailored crystal symmetry. However, the effect of nanoscale anisotropy on these interactions could not be investigated as spherical PAEs were used.…”

Probing the Consequences of Cubic Particle Shape and Applied Field on Colloidal Crystal Engineering with DNA

“…Since the cubes with the 66 bp DNA design assembled with the same symmetry as spheres, we hypothesized that a similar series of morphologies would form after assembly in a field, consistent with what has been observed with spherical particles. [29] When slowed cooled as before, the morphology of the resulting crystals varied as a function of magnetic field strength from more isotropic clusters (0 G; Figure 2 c), to aligned crystals (140 G; Figure S5), and finally to defined, smooth rods (3800 G; Figure 2 e). The rod shape indicates that the superlattices are macroscopically aligned with respect to the applied magnetic field.…”

“…[25][26][27][28] Recent work has investigated the use of external stimuli to influence colloidal crystal growth of spherical, DNA-coated Fe 3 O 4 NPs. This work found that applying magnetic fields to the magnetic PAEs resulted in the formation of elongated nanostructures, [29] highlighting the unique ability of DNA to separate the interactions of dipoledipole coupling and DNA hybridization interactions to form high-aspect ratio structures with tailored crystal symmetry. However, the effect of nanoscale anisotropy on these interactions could not be investigated as spherical PAEs were used.…”