Micelle templated photochemistry

Kundrat, Franziska (2016): Micelle templated photochemistry: a new approach to fabricate functional photothermal gold nanoarrays. Dissertation, LMU München: Fakultät für Chemie und Pharmazie

Abstract

Individual nanoparticles as well as assembled nanostructures play a crucial role in the development of new materials. The reason for this is that nanomaterials, with a size ranging from 1 to 100 nm, display significantly different chemical and physical properties in comparison to their bulk (or microscopic) counterpart, including altered optical, magnetic, or electric properties. Especially, gold nanoparticles proved to be very attractive platforms in a broad range of applications from optics to biology because of their inertness, well-established surface chemistry and unique optical properties. Gold nanoparticles feature enhanced light absorption when excited at their plasmonic resonance wavelength, which can be tuned from visible to near infrared light by varying size, shape or interparticle distance. Moreover, nonradiative absorption of gold nanoparticles can rapidly be converted into heat - the photothermal effect - turning them into ideal nano-sources of heat. Although plasmonic applications focused for a long time on the use of the optical properties of gold nanoparticles, light-induced heat has gained more and more attention over the past years. It allows the investigation of thermal events down to the nanoscale remotely controlled through laser irradiation. Hitherto, photothermal applications commonly utilize gold nanoparticles dispersed in various chemical environments, while the use of nanoparticle assemblies on planar substrates would allow the design of functional interfaces. Such substrates would offer a better-defined photothermal profile with potential applications in the fields of nanochemistry and biomolecular manipulation. However, the fabrication of nanoparticle-based surfaces with tuneable optical properties still rely on complex and not scalable procedures, which constitute a major limitation to prepare uniform and robust plasmonic substrates. Therefore, new strategies need to be established to simplify multi-step procedures and simultaneously control the growth, the shape and the arrangement of gold nanoparticles into functional plasmonic interfaces. In the first part of this thesis, homogeneous and micropatterned arrays of gold nanoparticles with different morphologies were generated with a new synthetic technique, called micelle templated photochemistry (manuscript I). By irradiating 6surfaces made of block copolymer micellar monolayers with ultraviolet light, it is possible to synthesize gold nanoparticles without requiring any commonly used reagents, such as photosensitizers or photoresists. In this method, micelles play the dual role of nanocarriers and reactive templates in order to simultaneously synthesize and organize gold nanoparticles with a high spatial resolution. This novel technique enables the growth, the arrangement and the shaping of gold nanoparticles with tuneable plasmonic resonance wavelengths on glass substrates. Explicitly, it leads to the formation of particle arrays over arbitrarily large areas decorated with either gold deformed nanoparticles (“potatoids”) or nanorings featuring enhanced photothermal properties and high heat-sustainability. Moreover, control over particle distribution is possible at the microscale level in combination with photolithographic techniques. Micropatterning structures enables surface labelling necessary for serial experiments and correlative microscopy. This makes them ideal platforms for fine-tuned heat-manipulation of chemical and biological reactions. In the second part of this work, a new application based on the use of plasmonic substrates was introduced with substantial implications for solvothermal synthesis (manuscript II). Solvothermal synthesis constitutes a well-established chemical method that involves the use of a solvent heated above its boiling point under moderate or high pressure to synthesize inorganic nanomaterials. Experimentally, this approach requires the use of a pressure sealed chamber (autoclave) to avoid solvent evaporation and sustain high pressure. Using illuminated gold nanoparticles as nanosources of heat allows performing solvothermal chemistry at ambient pressure and in an open reaction medium. This method benefits from the possibility for chemical exchange and monitoring during the reaction. The concept was successfully demonstrated by the synthesis of In(OH) 3 nanocrystals at the micrometric scale using optical and thermal microscopy means. The underlying mechanism involves plasmon-assisted superheating of water above its boiling point and displays faster reaction times by decreasing the heated/illuminated region. Due to much faster kinetics and higher spatiotemporal resolution than standard synthetic approaches, this plasmon-assisted method is highly attractive for fundamental 7research in solvothermal synthesis and could lead to novel applications in the fields of nanomaterials deposition and patterning.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: Gold Nanoparticles, Micropatterning,block copolymer, solvothermal synthesis, photothermal
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 540 Chemie
• Fakultäten: Fakultät für Chemie und Pharmazie
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 20. Oktober 2016
• 1. Bericht­erstatter:in: Fässler, Reinhard