Precise Nanoengineering of Polymer Functionalized Nanomaterials with Silica as Coating and Templating Tools
by Yingying Cai
Date of Examination:2021-12-16
Date of issue:2022-05-10
Advisor:Prof. Dr. Philipp Vana
Referee:Prof. Dr. Philipp Vana
Referee:Prof. Dr. Frauke Alves
Referee:Prof. Dr. Andreas Janshoff
Files in this item
This file will be freely accessible after 15.12.2022.
EnglishThe construction of a well-defined hybrid nanostructure becomes possible if functional polymer meets inorganic nanomaterials in a well-designed way. To fully control the structure of the nanohybrid and create the synergy between the surface-bound polymer and the nanomaterial, a vast variety of engineering options must be understood in-depth and practically optimized for both functional polymer and nanomaterials. These options include: I) size (distribution), colloidal stability, and surface ligand exchange for the nanomaterials; II) type of monomer(s), chain-length, topological design, and end-group functionalization for the surface-grafted polymer; III) the anchoring strategies between the polymer and surface of the nanomaterials. In this thesis, four projects, focusing on the design and fabrication of four different nanohybrids with polymers functionalization for different applications, are presented. Within these projects, reversible addition-fragmentation chain transfer (RAFT) polymerization technique was employed to fabricate the functional polymer with full control of the macromolecular architecture. These RAFT polymers are introduced onto different types of nanoparticles (NPs) with high capping density, yielding hybrid nanocomposite with well-defined structures. Another important strategy used in this thesis is the silica coating/fabrication method. It includes the classic one-phase coating approach and reversed microemulsion technique. Choosing suitable methods and conditions, the silica coating can be performed in a controlled fashion. For those nanomaterials without effective surface chemistry, the thin silica shell brings its well-established surface chemistry to them, e.g., for introducing polymer brushes. The reversed microemulsion method for silica coating found its application in the first project: superparamagnetic ~ 8 nm magnetite nanoparticles (MNPs) bearing hydrophobic ligands were uniformly coated with a thin silica shell of a thickness of ~ 12 nm in a one-to-one fashion. From the silica surface, the surface-initiated RAFT polymerization was performed to introduce a dense hydrophilic and thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) shell. The highlight of this project is the dramatic change of the superparamagnetic properties of the MNPs after silica coating and polymer functionalization. The superconducting quantum interference device (SQUID) measurements impressively demonstrated the complete suppression of the magnetic interaction between the MNPs even in the dry-powder state. This result is very intriguing for magnetic hyperthermia applications where the magnetic interaction is highly unfavored for its strong jeopardizing effect on the heating performance. The thesis further focused on the nanoengineering of Egyptian blue nanosheets (EBNS). Egyptian blue (EB) is a near-infrared (NIR) fluorescent material with excellent photoluminescence (PL) performance. However, the lack of effective production of this nanomaterial and the absence of a surface modification method are the bottlenecks of EBNS to be applied as a cost-effective NIR fluorophore for a wide audience, especially in the NIR bioimaging field. To unfold this feature, high-energy ball milling technique was first applied to enable the mass production of the nanomaterial, with a significantly reduced size of EBNS. Again, silica coating was chosen here to introduce effective surface modification on EBNS. During the project, two new challenges were found for EBNS: the significant size-dependent PL performance, and the strong light scattering effect of EBNS in colloid caused by the mismatch of the refractive index of EBNS and water. These insights are very important for optimizing PL performance of EBNS for detailed application scenarios, e.g., choosing the suitable size of EBNS and its dispersion media. Under this consideration, two types of polymers, hydrophobic PNIPAM, and hydrophilic poly(methyl methacrylate) (PMMA) brushes were introduced to the surface of EBNS respectively, to enhance the dispersibility of EBNS in the diverse media. Notably, the good match of the refractive index of PMMA, silica, and EBNS in one nanohybrid can significantly reduce the scattering event thus increasing the PL output of the material. The role of silica is very flexible in the fabrication of hybrid nanomaterials. Rather than as coating material for further functionalization, in the project of creating circular AuNPs nanopattern, ~ 46 nm silica NPs served as a temporary template for the colloidal self-assembly of polyethylene glycol (PEG) capped ~ 13 nm AuNPs, by utilizing the adsorption between the surface-grafted PEG and the silica surface. Surprisingly, after depositing the silica-Au nanostructure on the substrate, the AuNPs were perfectly arranged into a 2D circular nanopattern. To completely remove the silica content without alerting the circular AuNPs pattern, an etching condition was established using an extremely concentrated NaOH solution at increased temperature. Both high ionic strength from NaOH and increased temperature can significantly raise the efficiency of silica etching while immobilizing PEG capped AuNPs on the substrate without any diffusion, since PEG is unsolvable under this condition. The perfect circular AuNP pattern has great potential to be used as an innovative nanopixel for advanced nanopatterning. Furthermore, this thesis focused on the fabrication of novel nanostructure for in vivo phase-contrast computed tomography (CT) experiments under synchrotron radiation. In this project, the impact of the geometry of AuNPs on the contrast performance was studied by comparing “hollow AuNPs” with solid AuNPs with the same penetration thickness. To mimic a “hollow” AuNP, the self-assembly approach was used to fabricate a dense layer of ~ 5 nm AuNPs onto thiol functionalized ~ 37 nm silica NPs. Both nanostructures were functionalized with PEG to increase the colloidal stability under extremely high concentration (100 mgAu/mL). The in vivo experiment confirmed the strong CT enhancement of “hollow” AuNPs for lung imaging.
Keywords: Nanoparticles; Polymer; Self-assembly; Silica; Nanoengineering