The use of engineered, single-stranded DNA molecules to direct the self-assembly of nano- and microscale particles is a powerful approach for making interesting ordered structures. The underlying idea is that single-stranded DNA oligomer brushes grafted onto spherical (or other) particles induce an interparticle attraction by the process of DNA hybridization (see figure below). A key advantage of this method is the possibility of producing highly specific and tunable interactions among mixed populations of particles, thereby forming multi-component structures that are difficult to achieve with other assembly methods (binary examples are shown in the Figure below).
Figure: DNA-mediated assembly of binary systems. (a).i – “A” and “B” spheres of the same size are distinguished by different grafted single-stranded DNA; (a).ii – linker DNA strands create dynamic bridges between particles; (b).i. – equal interactions between all particles, , leads to random close-packed (CP) crystals with the system stoichiometry; (b).ii. – the case leads to demixed CP crystals with substitutional minority concentrations determined by interaction strengths and growth kinetics; In both (b).i. and (b).ii., the body-centered cubic (BCC) phase is expected to be unfavorable relative to CP; (b).iii. – for , ordered superlattice structures are expected. Both CP and BCC superlattices are possible.
We are using a suite of computational tools to study the nucleation and growth of ordered multi-component superlattice structures based on DNA-mediated interactions between particles. These studies are being used to provide a predictive framework for engineering specific interaction combinations among different particles in order to assemble any number of desired ordered phases. Recently, for example, we demonstrated that a complex interplay can exist between kinetic and thermodynamic effects in which the lowest free energy phase is not necessarily the one that crystallizes. For example, the figure below shows a “phase diagram” for a two-component system as a function of the binding energies between two types of same-sized spherical particles “A” and “B”. When interaction between unlike particles (i.e. between “A”s and “B”s) are dominant, the resulting binary, ordered crystal phase (a “superlattice”) is the so-called CsCl superlattice in the BCC crystal structure. As the “like” (i.e. A-A or B-B) interaction strength is increased, however, the equilibrium phase becomes a CP superlattice structure (CuAu). This transition is denoted by the orange line. However, due to the fact that the CuAu phase is prone to defect formation during growth, in which some “A” particles take positions that should contain “B” particles (and vice versa), it is possible to grow CsCl to the right of the orange boundary and up to the cyan boundary. In the intermediate region between these two boundaries, kinetic limitations prevent the most thermodynamically favorable phase from forming.
Understanding this behavior in detail may provide routes for making non-equilibrium structures. We are currently studying how DNA-mediated interactions can be used to assemble more complex ordered lattices based on combinations of differently-sized and even non-spherical particles.
(Project is in collaboration with Prof. J. C. Crocker at the University of Pennsylvania.)