Plasmonic nanostructures: synthesis, functionalization & sensing applications

Nobel metal nanoparticles possess unique materials properties different from their corresponding bulk materials, which have sparked extensive research developments in the field of nanofabrication over the past few decades. In particular, the rational design of plasmonic nanoparticles (“artificial atoms”) is emerging as an exciting route for engineering material properties with high accuracy. Synthetic advances enable the sophisticated control over their size, shape, composition, and morphology, which has led to a wide spectrum of applications including miniaturized optical and electronic device, sensors and photonic circuits, and medical diagnostics and therapeutics. It is expected that the ability to assemble these elementary plasmonic nanoparticle building blocks into well-defined assemblies such as “artificial molecules”, “artificial polymers” or “superacrystals” will further impact the way materials are synthesized and devices are fabricated. Despite the encouraging progresses in synthesizing metallic nanoparticles, it is still far from the capability of constructing any arbitrary nanostructures in a well-controlled manner. This thesis is dedicated to synthesis and characterisation of a few novel plasmonic nanostructures including hairy plasmonic nanorods, hairy bacteria, nanoparticle pyramids, etc. I have also thoroughly investigated the soft DNA corona structures via small angle neutron scattering (SANS) for the first time. Soft organic ligand plays a critical role in synthesis and assembly of plasmonic nanoparticles. In chapter 3, SANS was used to investigate soft ligand corona structures using DNA-capped nanoparticles as the model system. Two 15mer DNA strands with palindromic sequence and poly(dT) sequence under high number density packing on gold nanoparticle surfaces, the influence of ionic strength and temperature on DNA corona structures and resultant hybridization has been investigated. Poly(dT) sequences were found to maintain globular corona structures across a range of ionic strengths and temperatures but the corona thickness decreased with increasing salt concentration and increased with increasing temperature. In contrast, palindromic sequenced DNA had globular corona structures in the absence of salt but quickly evolved into dimeric and multimeric structures under high ionic strength or under low annealing temperatures. The structural insights revealed by SANS can help us better understand how DNA controls nanoparticle interaction, which in further guide the design of tailor-made DNA corona structures for customizable designer materials and devices. In the chapter 4, synthesis and characterization of a new metal nanoarchitecture, hairy gold nanorods (HGNRs), are described. HGNRs were obtained by a seed-mediated growth of nanowires on gold nanorod templates. The hairy nanowires could be obtained in a wider range of ratios of gold precursor to ligand than that reported on solid surfaces or silica beads in the literature. The HGNRs have the unique soft ‘hairs’ and rigid ‘core’, allowing for the fabrication of patches with controllable percolation conductivity networks. The soft conducting patches could be used as elastic strain sensors with high stretchability and durability. Remarkably, this nanotemplated approach appears to be general. We found that E. coli bacteria could be employed as the template as well, leading to generation of ‘hairy plasmonic bacteria’ for the first time. In chapter 5, a combined top-down and bottom-up approach is developed to fabricate structurally well-defined nanoparticle pyramids. The top-down fabricated silicon pyramid well arrays are used as the template to confine self-assembly of pre-synthesized gold nanoparticles. Three types of monodisperse nanoparticles, nanospheres, octahedron, romboic dodecahedron, are used as elementary building blocks. For all the three type of nanoparticles, well-defined pyramids could be successfully constructed. The plasmonic properties of these pyramids were thoroughly investigated by micro-UV-visible spectroscopy and dark field spectroscopy. These novel plasmonic pyramids show highly tunable Surface Enhanced Raman Scattering (SERS) enhancements