DNA Nanotechnology: Growing Gold Crystals

DNAWe all know that DNA (deoxyribonucleic acid) encodes genetic information and is essential for all known forms of life. It also is an amazing biological polymer with unique structural characteristics, offering numerous possibilities for rational design on molecular level.

A combination of crystals and polymers often results in exciting technological applications. Now, using DNA nanotechnology, scientists from Northwestern University in Evanston, Ill., have demonstrated the growth of micrometer-size crystals out of gold nano particles  (see  simulation video). Professor Chad Mirkin, a recognized leader DNA nanotechnology, explains:

Crystallization is a fundamental and ubiquitous process much studied over the centuries. But although the crystallization of atoms is fairly well understood, it remains challenging to predict reliably the outcome of molecular crystallization processes that are complicated by various molecular interactions and solvent involvement. This difficulty also applies to nanoparticles: high-quality three-dimensional crystals are mostly produced using drying and sedimentation techniques that are often impossible to rationalize and control to give a desired crystal symmetry, lattice spacing and habit (crystal shape). In principle, DNA-mediated assembly of nanoparticles offers an ideal opportunity for studying nanoparticle crystallization […] Here we show that very slow cooling, over several days, of solutions of complementary-DNA-modified nanoparticles through the melting temperature of the system gives the thermodynamic product with a specific and uniform crystal habit.

This work directly follows formation of DNA-programmable nano particle superlattices, obtained by the same group earlier this year, and is based on more than a decade of research. To form crystal lattices, the scientists used controlled self-assembly of gold nanoparticles of 10-20nm, connected by short stretches of synthetic DNA (oligonucleotides).

Nanoparticle self-assembly was driven by an inherent property of DNA to form double stranded helices, which comes from its chemical structure. Each DNA strand consists of a backbone of alternating sugar (deoxyribose) and phosphate groups. Sugar units carry side functional groups called nucleobases (adenine, guanine, thymine, cytosine (A, G, T, C), which  form hydrogen bonds with each other in complementary pairs (A&T, G&C). Cooperative hydrogen bond formation assembles the strands of DNA into double helix. The hydrogen bonds are reversible and can be either disrupted by elevating temperature (DNA melting), or re-formed by decreasing temperature (DNA hybridization, or annealing).  The property of DNA fragments to reversibly form double-stranded structures is the core of an exiting discipline of DNA Nanotechnology, simply explained in a following video:

Back to nanoparticle-DNA assembly: The scientists from Mirkin’s group used end-thiolated oligonucleotides (DNA fragments) to modify gold nanoparticles. Oligonucleotides, attached through thiol to the gold surface, formed a dense brush with protruding single-stranded, so-called sticky ends (think of Velcro). During the crystal formation, these sticky ends slowly were attached to each other, following the rule of complementarity, bringing nanoparticles close together. The distance between the nanoparticles was determined by the design of connecting synthetic oligonucleotides.

The researchers found that nanoparticle assembly followed predicted equilibrium crystal structure, “thus establishing that DNA hybridization can direct nanoparticle assembly along a pathway that mimics atomic crystallization.” The assembly was triggered by temperature decrease and the speed of assembly was the key to the quality of the crystal: the more slowly it grew, the better its structure. The crystal structure can been verified by SEM and TEM electron microscopy (download file). This exciting research was published in Nature.

Looking deeper into such a crystal, one might wonder about the building blocks, or metal nanoparticles. Highly ordered nanoparticles result in interesting applications, like quantum dots (semiconductor nanocrystals with a very stable, size-dependent fluorescence). Apparently, polymers can help control the shape of growing nanocrystals. As was published in Nature Nanotechnology earlier this year, scientists from Georgia Institute of Technology in Atlanta came up with a “general strategy for crafting a large variety of functional nanocrystals with precisely controlled dimensions, compositions and architectures by using star-like block co-polymers as nanoreactors.”

The star-shaped block co-polymers with a central beta-cyclodextrin core formed unimolecular micelles, which served as reaction vessels and templates for the formation of the nanocrystals. Micelles made with di-block and tri-block copolymers of polyacrylic acid, polystyrene, polyethylene oxide, polyvinyl pyridine, and polybutyl acrylate were used for preparing plain and hollow nanoparticles. Using this approach, the scientists were able to obtain metallic, ferroelectric, magnetic, semiconductor, and luminescent  “nanocrystals with desired composition and architecture, including core-shell and hollow nano structures.” It is amazing what polymers can do at the nanolevel!

Source: DNA-mediated nanoparticle crystallization into Wulff polyhedra, Auyeung E, Li TI, Senesi AJ, Schmucker AL, Pals BC, de la Cruz MO, Mirkin CA. Nature, doi: 10.1038/nature12739. [Epub ahead of print], Source: Senesi, A. J., Eichelsdoerfer, D. J., Macfarlane, R. J., Jones, M. R., Auyeung, E., Lee, B. and Mirkin, C. A. (2013), Stepwise Evolution of DNA-Programmable Nanoparticle Superlattices. Angew. Chem. Int. Ed., 52: 6624–6628. doi: 10.1002/anie.201301936
Source: A general and robust strategy for the synthesis of nearly monodisperse colloidal nanocrystals, X.Pang,L. Zhao,W. Han,X. Xin, Z. Lin, Nature Nanotechnology, 8: 426–43doi:10.1038/nnano.2013.85
Video: Building nano structures with DNA, Wyss Institute, youtube
Image by Zephyris, Wikipedia Commons