DNA-programmable particle superlattices: Assembly, phases, and dynamic control
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Introduction
Crystallization of superlattices
The field of colloidal assembly was driven for a long time by fundamental interest in understanding crystallization in simple model systems. More recently, colloids and their nanoscale analogs, nanoparticles (NPs), have emerged as new functional blocks for optical, energy, and biomedical applications. These developments have opened a new frontier of rational self-assembly of materials made from such particles. The structures of the assemblies are determined by the interplay among interaction forces, entropic effects, and kinetics. Unlike atoms, colloids and NPs can vary widely in their sizes, shapes, and interactions. Nevertheless, it has been challenging to establish a broadly applicable platform for “a la carte” assembly of ordered particle arrays and their dynamic control.1–3 One of the most controllable assembly strategies relies on programming interparticle interactions using the complementarity of DNA strands.3 This provides selective and reversible interactions between particles of different sizes and shapes (Figure 1). DNA strands can also act as tailored structural elements in particle crystals, smart assembly guides, and reconfigurable structural elements. In recent years, much progress has been made in DNA-guided assembly of particle superlattices. By tuning the interactions, sizes, and shapes of NPs, a wide variety of structures have been assembled. Here, we discuss the most significant achievements and challenges in this field and future directions.
DNA was proposed as a programmable “glue” for the assembly of NPs about two decades ago.8,9 The basic idea is that DNA-functionalized NPs with complementary strands can hybridize due to the Watson–Crick base pairing between the strands grafted to two different particles. One can consider two approaches to link particles with DNA: direct hybridization (DH) and linker-mediated hybridization (LH) (see Figure 1). In the DH approach, the particles are functionalized by DNA terminated with mutually complementary single-stranded segments (also known as a “sticky end”). In the LH approach, the grafted DNA pairs are not complementary, and appropriately encoded DNA linkers are required to connect them. In addition, the grafted DNA can vary in their rigidity, length, and grafting density, all of which lead to modification of the shell, formed by DNA around the particle core, and, consequently, it will affect particle interaction characteristics. DNA-mediated particle binding without additional optimization or annealing would normally lead to amorphous aggregates due to unfavorable interactions and complex kinetic pathways to crystallization.10,11 Crocker’s group reported the first example of crystallization for DNA-coated particles for micrometer-scale colloids12 (see formation of small crystallites in Figure 2a–c). The Gang13,14 and Mirkin15 groups independently reported a major breakthrough, an assembly of crystalline 3D lattices from NPs using DNA.
Oleg Gang, Center for Functional Nanomaterials, Brookhaven National Labora
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