Fabrication of Polymer-Metal Nanohybrids - Routes Towards Functional Materials
by Darius Rohleder
Date of Examination:2021-10-07
Date of issue:2021-11-15
Advisor:Prof. Dr. Philipp Vana
Referee:Prof. Dr. Philipp Vana
Referee:Prof. Dr. Alec M. Wodtke
Persistent Address:
http://dx.doi.org/10.53846/goediss-8877
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Description:Dissertation Darius Rohleder: Fabrication of Polymer-Metal Nanohybrids - Routes Towards Functional Materials
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Abstract
English
Within this thesis various nanohybrid systems constructed from metal nanocrystal cores and polymer shells were specifically tailored towards distinct practical applications. The first part of this work addresses the straightforward design of a recyclable palladium-core–silica-shell nanocatalyst applied in Heck-reactions. Therefore, a mesoporous silica (mSi) shell was tailored onto palladium nanocubes (Pd-NCubes) in a single-step coating approach. Further functionalization of the porous silica platform via polyethylene glycol (PEG)-silanes is presented. This hierarchal strategy offers an excellent balance between nanohybrid stability and sufficient permeability for small organic molecules. To ensure reusability not only of the nanocatalyst but also of the solvent, unfunctionalized PEG was used as reaction medium. The overall process – design, catalysis and purification – is characterized by its simplicity and sustainability accompanied by a great recycling potential and ultra high yields in C–C-coupling reactions. Palladium nanocubes were further applied in seed-mediated growth processes to obtain larger nanocubes by tuning the seed-to-feed ratio. The variously sized PdNCubes were coated with poly(N-isopropyl acrylamide) (poly(NIPAM)) fabricated via reversible addition–fragmentation chain transfer (RAFT) polymerization. The inherently formed trithiocarbonate group of the RAFT-polymer served as anchor to immobilize the polymer onto the surface. Thus, prolonged stability and improved processability of palladium nanocubes compared to their cetyltrimethylammoniumbromide-capped (CTAB) counterpart were achieved. These nanohybrids were provided to the Pundt-group at the Karlsruher Institute of Technology (KIT) for investigations regarding its size dependent properties in hydrogen storage applications. Gold nanorods (AuNRs) were coated with linear and 3-arm star RAFT-polymers with tunable chain lengths constructed from NIPAM building blocks. AuNR nanohybrids self-assembled into 2D nanoarrays by a straightforward dropcasting approach onto an amorphous carbon film observed via transmission electron microscopy (TEM). Here, interparticle distances could be tuned over a wide range by altering the average molecular weight of the grafted polymer. Dynamic light scattering (DLS) experiments indicated that the rotational diffusion of gold nanorods may be a good relative measure regarding the size of polymer-grafted nanorods. AuNRs functionalized with 3-arm star RAFTpolymers were further decorated with spherical gold nanoparticles by taking advantage of free anchor groups on the outside of the polymer shell which were not immobilized on the AuNR planet within the first step. Tailorable planet–satellite nanostructures were fabricated in this hierarchal self-assembly approach while spacings between planet and satellites were tuned by different polymer chain lengths. Thus, optical properties in solution were altered, attributed to a plasmonic coupling between the AuNR core and the AuNPs offering potential applications for biological sensing or surface-enhanced Raman spectroscopy (SERS). At last, AuNRs were decorated with thermoresponsive copolymers constructed from N-isopropyl acrylamide and acrylamide (AAM) building blocks. Copolymers of tailored architecture and chain length were prepared via RAFTpolymerization using either a monofunctional or 3-arm star RAFT-agent and immobilized on gold nanorods over the trithiocarbonate groups. Twodimensional arrays of the self-assembled core–shell nanostructures were fabricated by drop-casting showing tunable interparticle spacings in TEM. Nanohybrids dispersed in pure water showed a continuous shrinkage of the polymer shell along an increasing temperature due to a lost of bound water molecules. In a simulated blood fluid, the discrete lower critical solution temperature (LCST) of the nanohybrids could be modified over a significant temperature range around body temperature by adjusting the copolymer composition, the architecture, and/or the size of the polymer. The intrinsic photothermal properties of the gold nanorods were utilized to trigger particle aggregation by irradiation at 808 nm in the optical window of human tissues. Thus, a new nanohybrid system with remotely controllable aggregation via an external NIR-light stimulus for nanomedical applications was developed.
Keywords: Polymer; Nanocrystal; Catalysis; Cancer Therapy; Self-assembly