Templated DNA-directed nanofabrication

Abstract

The main objective of this thesis was to develop a method to place individual building- blocks onto pre-ordinate positions on a templating nanopattern so that the building- blocks replicated the templating structures. The templating substrate was fabricated by a series of different processes conducted in the following sequence: electron-beam lithography, etching, chromium/gold evaporation and resist lift-off. The gold nanopatterns were functionalised with oligonucleotides (DNA) using thiol-gold chemistry and the building-blocks (DNA modified gold nanoparticles) were assembled onto the templating nanostructure by DNA-DNA interaction. The templating nanopattern consisted of gold features on a silicon substrate. A method to passivate the surrounding silicon surface was developed and the gold nanoparticles (AuNPs) hybridisation conditions were optimised. An assessment of the DNA-directed self-assembly of AuNPs was conducted. The hybridisation efficiency onto each adsorption site was 80% while the non-specific adsorption was 0.7%. Approximately 50% of all the six-dot lines had five AuNPs immobilised, while roughly 20% had six particles. The occurrence of defects could be repressed by modifying the geometry of the templating nanostructures. An additional objective was to develop a method to assemble AuNP structures into more complex structures that potentially could be used as building-blocks for additional self-assembly. For the nanostructures to be used as building-blocks they would have to be covalently interconnected (e.g. by cross-linking) prior to being released from the templating substrate, to ensure that their configuration remained intact upon release. Following the release, the templating substrate could be reused for additional AuNP self-assembly cycles. The release of the AuNPs and the reusability of DNA functionalised substrates was investigated on non-patterned substrates. It was found that AuNPs could be immobilised and released ten times without a statistically significant decrease in the number of particles immobilised per μm2. A covalent cross-linking concept was developed and investigated for AuNPs im- mobilised onto non-patterned substrates. A clear difference in the release behaviour between AuNPs immobilised on substrates subjected to and not subjected to the cross- linking conditions was observed, which suggested that the cross-linking strategy was successful.When released AuNPs were recollected on capture substrates, no difference in the configuration of the released AuNPs was established between nanopatterns subject to and not subjected to the cross-linking conditions. It was found that the AuNPs were not stable at the temperatures required to drive the AuNPs release from the templating substrate and thus the temperature induced release of assembled nanostructures was not a viable option. Alternative systems, in which temperature is not used to drive the release of cross- linked nanostructures, could potentially circumvent the thermal instability of the particles. For example by using fuel-DNA to drive the desorption of cross-linked nanostructures, according to the work of Hazarika et al., or by using nanoprinting techniques to directly transfer the nanoparticle assemblies onto a capture substrate.