Nanomaterials: DNA brings quantum dots to order.

An important goal in nanotechnology is to construct elaborate superassemblies of nanoscale components while maintaining strict control over the organization of these building blocks. Among the various types of potential building blocks are quantum dots, which are the focus of much research because they have unique optical and electronic properties that could prove useful in applications such as photovoltaics, lightemitting devices and medical imaging1. The primary challenge when constructing super-assemblies from quantum dots is to control the number of binding sites on the surface of the dots while simultaneously maintaining their physical stability and useful properties. Writing in Nature Nanotechnology, Shana Kelley, Ted Sargent and co-workers2 at the University of Toronto report that quantum dots coated with DNA ligands can be reliably organized into complex super-assemblies. DNA is a polymer in which various bases are attached to a backbone made of sugars and phosphate groups held together by phosphodiester bonds. There are four different bases (adenine, cytosine, guanine and thymine), and DNA can be programmed to form different twoand three-dimensional structures because adenine always pairs with thymine, and cytosine always pairs with guanine. Similarly, any material that can be attached to DNA can take advantage of this complementary base pairing to form precisely designed structures. Kelley and coworkers2 devised a simple but clever one-pot method to synthesize cadmium telluride quantum dots that had a discrete number of DNA ligands attached to their surface, with molecules of mercaptopropionic acid (MPA) acting as a co-ligand and occupying the sites left open by the DNA. The dots were stable and had good optical properties, and their size (and hence the wavelengths at which they absorb and emit light) could be controlled by adjusting the experimental conditions. The DNA ligands consisted of three domains: a domain containing between 5 and 20 guanine bases that was attached to the quantum dot (shown in purple in Fig. 1); a spacer region (blue); and a domain in which the sequence of bases was chosen by the researchers (yellow or pale pink). The backbone of the quantum-dotbinding domain contained 5–20 sulphur atoms because it was held together by phosphorothioate bonds, rather than the normal phosphodiester bonds (which do not contain sulphur). These sulphur atoms reacted with the cadmium atoms at the surface of the quantum dot to form a CdS shell around the CdTe core; this monolayer of CdS ensured that the dot was stable and that the fluorescence-emission efficiency was high. The presence of the guanine bases also helped to secure the quantumdot-binding domain to the surface of the dot. The backbones of the other two domains were held together by normal phosphodiester bonds, leaving the DNA binding domain (yellow) free to hybridize with complementary DNA sequences. The key to achieving strict control over the organization of the quantum dots in any super-assembly is to control the ‘valency’ of the dots (that is, the number of DNA ligands attached to their surface). Kelley and co-workers found that the valency was 1 when there were 20 phosphorothioate linkages in the quantumdot-binding domain, and that it increased to 4 or 5 as the number of linkages was reduced to 5 (depending on the size of the dot). The right panel of Fig. 1 illustrates the complexity of quantum dot assemblies that can be constructed with high yield; quantum dots with asymmetric bivalency can also be seen. Kelley and co-workers then studied the transfer of energy between the quantum dots within these assemblies. In particular, they found that the colour of the fluorescent emission from a linear ternary complex could be reversibly switched by changing the pH of the solution. This can NANOMATERIALS