Andrew Houlton, Benjamin Horrocks, Andrew Pike and Miguel Galindo of the Chemical Nanoscience Laboratories at Newcastle University, UK, explain how DNA can direct electronic materials to grow on the nanoscale.

DNA acts as a template for growing and organising nanomaterials

Industrial, research and consumer sectors are relying more and more on semiconductor materials. In both existing and newly emerging markets, these materials are required on an ever smaller scale - the nanoscale. The International Technology Roadmap for Semiconductors predicts that silicon-based electronics smaller than 30 nanometres will be in use by the end of the decade. A wide range of other semiconductors, providing us with everything from protection from the sun (titanium dioxide) to new insights into cellular processes (cadmium selenide in bio-markers), also rely on nanoscale materials.

Semiconductors become particularly interesting at such small sizes because their electronic structure changes dramatically. On the nanoscale, semiconductors becomes more molecule-like and their energy levels change from continuous to discrete. This phenomenon, known as quantum confinement, gives rise to unusual optical and electronic properties, such as luminescence at visible wavelengths in silicon. However, preparing nanoscale materials and assembling them into functional systems is challenging, both technically and economically.

Materials scientists are exploring a new approach to overcome this challenge that draws inspiration from biology, where complex hierarchical systems are assembled from simpler building blocks. One of the most promising strategies for synthesising and organising nanomaterials makes use of nature's own storage medium for the genetic code, DNA.

For a biomolecule, DNA is remarkably robust. It can be readily prepared at precise lengths using biological or chemical synthesis. The four letter genetic code, ATGC, allows scientists to build well-defined structures, including extended 2D frameworks and 3D objects. DNA's constituents, the phosphodiester backbone and nucleobases, ensure that a wide range of precursors, such as metal ions and monomer units, can interact with the biopolymer. This, along with DNA's long thin shape, means that single DNA molecules can act as excellent templates for growing technologically useful materials at the smallest of sizes.

Scientists have made inorganic compounds, such as cadmium sulfide, lead sulphide and copper sulfide, as nanoscale solids using DNA strands as templates. This is done in a two-step process, where DNA is first doped with metal ions and then incubated with sulphide anions. The size and shape of the resulting material can be controlled by adapting the reaction conditions. For example, it is possible to prepare continuous strands of cadmium sulfide - so-called nanowires - as well as chains of nanoparticles. The nanoparticles show clear evidence of quantum confinement and their assembly into chains represents the first step towards quantum logic arrays, an exciting new architecture for low-power electronics. The continuous nanowires, formed in solution-phase reactions, are electrically conducting and have been used as ultra-small components in simple electrical circuits.

Researchers have also demonstrated DNA-templating of conducting polymers. These materials are important because, being molecular-based, their properties can be easily tuned. They can also be readily functionalised using synthetic chemistry, which allows scientists to attach probe molecules and develop nanowire-based sensors. This is an exciting area for future application that takes advantage of the sensitivity of nanowires' electrical conductivity to external changes.

DNA-templating also provides traditional conducting polymer materials, such as polypyrrole, in a more processable form. Ordinarily, this material is prepared as highly insoluble films but when synthesised as nanowires, the resulting material can be manipulated by aligning or stretching. This type of DNA-polymer hybrid also self-organises into ordered rope-like strands and extended networks, features which may be useful in device fabrication and plastic electronics.

In other recent developments, as an alternative to using DNA as a template, researchers have used synthetic chemistry to modify the basic structure of DNA by incorporating new functional groups. This approach raises the fascinating scenario of biological processing of electronic materials and the possibility of 'growing' nanoscale structures and circuits using the intrinsic properties of biomolecules themselves.

In more ways than one, it looks like DNA will be central to nanoscience's evolution to nanotechnology and will bring about the low-cost, functional, nanoscale systems of the future.

The authors thank the ONe for funding.

Read more in 'DNA-based routes to semiconducting nanomaterials' in issue 12 of ChemComm

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