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Gutenberg-style printing of self-assembled metal nanoparticle Aarays: electrostatic nanoparticle immobilisation and DNA-mediated transfer
thesisposted on 06.02.2017, 02:49 by Zheng, Yuanhui
The main objective of the work presented in this thesis was to combine top-down and bottom-up techniques to develop a nanofabrication method that would allow for the highly parallel and repetitive production of any desired nanostructure. This was realised by (1) immobilising gold nanoparticle (AuNP) building blocks onto lithographically produced nanopatterns (stamps) by electrostatic self-assembly, followed by (2) transfer of the surface-confined AuNP assemblies from the stamps to non-patterned capture substrates by a DNA-directed nanoprinting technique. A surfactant Tween 20 mediated AuNP surface modification was developed as a rapid synthesis of highly stable negatively charged AuNP conjugates. Three different types of AuNP conjugates bearing different surface ligands - deoxyribonucleic acid (DNA), SH-PEG-COOH, and a mixture of DNA and SH-PEG-COOH (denoted AuNP/DNA, AuNP/SH-PEG-COOH and AuNP/DNA/SH-PEG-COOH) - were successfully synthesised at salt concentration of 0.5 M using this approach. The AuNP/DNA conjugates showed high colloidal stability in environments of high ionic strength (up to 1.5 M NaCl in the presence of Tween 20). The DNA loading densities on different sized AuNPs were investigated and found to be approximately 37, 102 and 231 DNA strands per NP for 20, 30 and 40 nm AuNP/DNA conjugates, respectively. The AuNP/SH-PEG-COOH conjugates showed sharp size-dependent colloidal stability thresholds as a function of ionic strength. A size-selective AuNP separation method was developed based on the colloidal stability of AuNP/SH-PEG-COOH conjugates of different sizes. A colloidal AuNP solution with bimodal size distribution (20 and 40 nm) was successfully separated using this approach, with more than 99.4% accuracy. Templating nanopatterns onto which the AuNP building blocks were immobilised comprised of elevated and recessed gold features on silica-coated silicon substrates were fabricated by means of electron beam lithography (EBL). Various geometries of the templating nanostructures, including nanolines, nanocrosses, nanotriangles, and nanodisks, were produced. Plain gold substrates were fabricated by evaporating chromium (Cr) and gold (Au) onto planar silicon wafers. The gold nanopatterns and the plain gold substrates were used as stamps and capture substrates for the DNA-directed AuNP printing, respectively. Protocols for surface modification of the stamps and capture substrates were developed; the former were surface-selectively modified with PEG-silane and SH-PEG-NH2 and the latter modified with DNA strands. The negatively charged AuNP conjugates were electrostatically assembled onto the EBL produced nanopatterns. When using AuNP/DNA conjugates as building blocks, the NPs were specifically immobilised onto positively charged gold nanopatterns, with high NP loadings. The EBL defined nanopatterns were well replicated by the self-assembled AuNP/DNA conjugates and non-specific adsorption was found to be extraordinarily low. When using AuNP/SH-PEG-COOH conjugates as building blocks, hexagonal close-packed (hcp) ordered AuNP superstructures and significant non-specific adsorption of AuNPs were observed. This is believed to be caused by AuNP agglomeration and precipitation during the self-assembly process. When AuNP/DNA/SH-PEG-COOH conjugates were used as NP building blocks, the NPs were specifically adsorbed onto the nanopatterns, forming closely packed AuNP assemblies on the nanopatterns. A two-step self-assembly strategy, that made use of electrostatic and DNA-directed AuNP immobilisation, was furthermore developed, enabling the fabrication of core-satellite AuNP assemblies (one 30 nm NP surrounded by six to nine 20 nm NPs). An affinity-based nanoprinting technique that exploits electrostatic AuNP assembly and DNA-directed replication of lithographically defined nanostructures was developed. The negatively charged AuNP/DNA conjugates were electrostatically immobilised onto SH-PEG-NH2 modified templating nanostructures and then printed onto non-patterned capture substrates, functionalised with DNA complementary to the AuNP-confined DNA, through DNA-DNA interaction. Single lines of AuNPs and regular NP patterns with close to single-particle resolution could be printed. Dense AuNP loading and high transfer yields were observed over three consecutive printing cycles. A UV-ozone surface cleaning method was developed to remove the AuNP-bound DNA strands, which could make the replicated AuNP nanopatterns applicable to a wide range of applications that require pristine metal NP arrays, e.g. plasmonic biosensors. The core-satellite assemblies were studied as substrates for Surface Enhanced Raman Scattering (SERS). The SERS signals of benzenethiol on the core-satellite AuNP assemblies were shown to be dependent on the excitation wavelength: the SERS effect was negligible when excited at 514 nm, while the SERS signals were significantly enhanced compared with the widely spaced core AuNPs and planar gold substrates when excited at 633 nm and 782 nm, respectively. The larger SERS enhancements for the core-satellite AuNP assemblies could be due to the formation of “hot spots” between core and satellite AuNPs. The detection limit of benzenethiol was found to be between 1-10 nM when using the core-satellite AuNP assemblies as SERS substrates.