Interfacial self-assembly of surface-accessible plasmonic assemblies: from 3D Pickering emulsion to 2D film

Abstract

The focus of this thesis lies in the fabrication of multi-dimensional plasmonic nanomaterials through interfacial self-assembly. These prepared nanomaterials offer great potential for various applications, such as surface-enhanced Raman spectroscopy (SERS) measurements or interfacial catalysis.

Depending on the surface chemistry of the nanoparticles (NPs), multi-dimensional nano assemblies such as 2D film or 3D Pickering emulsion or colloidosomes could be generated at the oil-water interface. Traditionally, these interfacial assemblies are composed of NPs which have been modified with strongly adsorbing modifiers to remove electrostatic repulsion or alter hydrophobicity. However, such surface modification process shows a passive potential in various applications since these modifiers can impede or hinder subsequent interactions between the functional nanosurface and its surrounding environment. In contrast, our group’s previous research showed that charged NPs can be induced to pack tightly at the liquid-liquid interface by utilizing promoters that carry an opposite charge to the NPs, which allows them to screen electrostatic repulsion between adjacent NPs. This method enables the creation of closely packed surface-exposed NPs at the interface without any surface modification.

3D Pickering emulsions with biphasic property endows them with substantial potential across diverse applications. Building upon the promoter-assisted self-assembly approach, this thesis introduces a novel concept for creating surface-accessible Pickering emulsions. This innovative approach utilizes a co-assembly method, combining stabilizers such as carbon nanotubes (CNT) and a promoter. The stabilizers are NPs which stabilize the curved interface, while the promoter effectively eliminates interparticle electrostatic repulsion. Importantly, this unique fabrication technique means that functional plasmonic NPs can be placed at the interface alongside the stabilizer particles without the need for surface modification unoccupied. This preserves the inherent surface properties of the functional materials. Since this general approach does not require chemically specific interactions, it can act as a versatile platform technology for creating diverse modifier-free Pickering emulsions by simply changing the particulate stabilizers, functional NPs or oil phase. Importantly, this newly proposed method for synthesizing Pickering emulsion overcomes the long-standing dilemma between emulsion stability and functionality imposed by surface-blocking modifiers. The significant enhancement in the functionalities of the plasmonic Pickering emulsions achieved through the elimination of modifiers was demonstrated through applications in SERS sensing and interfacial catalysis. More specifically, the surface-accessible Pickering emulsions could be used as the SERS substrate for a variety of weakly adsorbing analytes that do not adsorb to the modified emulsions. For analytes such as adenine which adsorbed to both the modified and surface-accessible emulsions, the limit of detection was found to be 1000 times better when the surface-accessible emulsions were used as the enhancing substrate. Similarly, the surface-accessible emulsions outperformed other state-of-the-art interfacial catalysts in the reduction of 4-nitrophenol with NaBH4, while the modifier-capped emulsions could not remain stable and act as a catalyst for this reaction. We further expanded our horizons by introducing PB as an internal standard into the realm of 3D Pickering emulsion and 2D nanosheets, addressing signal reproducibility issues in complex environments.

Thesis is embargoed until 31 December 2026.

Date of Award	Dec 2023
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Chinese Scholarship Council (CSC)
Supervisor	Chunfei Wu (Supervisor) & Yikai Xu (Supervisor)