Using DNA to assemble nanowires and nanoswitches

We have become used to buying electronic goods that do more and cost less than the last time we went to the store. This reflects progress in miniaturizing the integrated electronic circuits, or ‘chips’, at the heart of personal computers, cell phones and other electronic devices. Miniaturization yields chips that are faster, because the charge carriers have a shorter distance to move— and cheaper to make— because it is possible to fit more integrated circuits on a silicon wafer.

Gordon Moore, one of the founders of Intel,¹ predicted that the number of circuits on a silicon wafer would double every eighteen months. This prediction, referred to as Moore's Law, has held good for more than four decades and is expected to hold true for at least another.

Thereafter, however, it may be impossible to go on making chips that are faster and cheaper.² There are two reasons why this might be so. First, major advances in materials and manufacturing technology will be required to continue miniaturization at the rate predicted by Moore's Law. Second, as miniaturization proceeds, chips will not work as expected due to the mobile charge carriers within them being confined to smaller spaces and spending more time close to surfaces.

Faced with this prospect, research groups in academia and industry are working to develop new approaches to chip manufacturing, allowing further miniaturization, and chip design, to accommodate or even exploit the novel properties of highly miniaturized chips.

The nanochemistry group based in both the School of Chemistry and Chemical Biology at University College Dublin, and the Centre for Adaptive Nanostructures and Nanodevices at Trinity College Dublin, is one such team. We are working to develop a new approach to manufacturing chips.^3,4 Our approach involves assembling electronic circuits, from nanoscale molecular and condensed-phase components, and organizing these components into electronic circuits on physically and chemically patterned substrates.

We have focused a lot of effort on using DNA to template the assembly of nanoscale wires and switches from nanoparticles.^5,6

There are two main reasons we use DNA.^7,8 First, automated and scalable technologies exist that can be used to prepare DNA templates. Second, DNA templates can be prepared with increasingly complex three-dimensional structures and with finely-tuned properties that depend on the base-pair sequence chosen.

There are two main reasons we use nanoparticles.⁹ First, it is possible to prepare them in a wide range of sizes and shapes and from a wide range of materials. Second, the surface of these nanoparticles can be modified with molecules that control their interaction with the DNA template.

Figure 1 shows a gold nanowire⁵ that was prepared by depositing a DNA template on a silicon-wafer substrate and then dipping this substrate in an aqueous dispersion of surface-modified gold nanoparticles. These positively-charged gold nanoparticles are adsorbed at the negatively-charged DNA template. The individual nanoparticles are subsequently enlarged and enjoined by electroless deposition of gold. The resulting wires are uniform, with a diameter of about 30nm, and show good electrical properties.

Figure 2 shows a gold nanoswitch⁶ that was prepared in the same way as the nanowire described above. At the centre of this DNA template is a biotin molecule that recognizes and strongly binds the protein streptavidin. On exposing the DNA template-nanoparticle assembly to a solution of streptavidin, the biotin binds the protein and the nanoparticles adsorbed at the center of the template are displaced. Again, the the remaining nanoparticles are enlarged and enjoined by electroless deposition of gold, yielding a protein-functionalized nanogap. Finally, exposing this structure to a dispersion of biotin-modified gold nanoparticles causes a single nanoparticle to be fixed in the gap. The resulting nanoscale switch is expected to possess novel and potentially-interesting electrical properties that are currently being studied in detail.

Future work will focus on using DNA and other biological molecules to template the assembly of nanoscale wires and switches from nanoparticles. Our work will also focus on preparing patterned substrates on which these templates may be pre-organized, with a view to templating the assembly of integrated arrays of nanoscale wires and switches. By these means we hope to contribute to developing new approaches to manufacturing chips that will allow Moore's Law be extended.