Magnetic field directed self-assembly of gold Pickering emulsion for preparing patterned film.

Okpozo, Oghenefego Paul

Abstract

Patterning plays a vital in role in sensor-based devices like surface-enhanced Raman spectroscopy (SERS), surface-enhanced infrared absorption (SEIRA), radio frequency (RF) antennas and many others. The linear array spacing and width of gold strips has been shown to increase the local intensity through near-field coupling with diffracted electromagnetic waves. This rise in local charge boosts vibrational energies of molecules in close-surface contact or proximity, resulting in increased IR absorption. The strip-like or any other types of patterns are efficiently achieved through top-down nanofabrication processes like atomic-force-deposition, nanoimprinting, UV-Lithography etc., which involve high capital cost, complex processing and occasionally low throughput. This research was therefore undertaken with the aim of reducing the process complexities and improving scalability, by applying a magnetic and spin coating directed self-assembly (MSCDS) to prepare optically sensitive dipole-dipole chain-like ordered arrays of the gold nanoparticle Pickering ferrofluid in polyvinyl alcohol (PVA) emulsion, in the form of a thin film on glass and silicon substrate. Previously-conducted MSCDS processes lacked the control over the dimensions of the prepared patterns. Here, the static magnetic field approach was taken to modify the MSCDS process to overcome the limitation of pattern dimension control, providing tuneability for optical applications. Quantitative image analysis of the patterned thin film allowed for the measurement of pattern geometrical dimension (chain length-CL, chain gap-CG and chain thickness-CT), which was then correlated with processing parameters such as magnetic field configurations (single, compound and concentric), spinning speeds and viscosities of Pickering emulsion. Upon optimization, spectroscopical characterisation was performed on prepared patterned thin film to demonstrate the capability of the modified MSDS process in enhancing the molecular detection at low concentrations. The UV-vis spectra of the patterns demonstrated the impact of CT and CG on the degree of gold-iron oxide nanoscale interactions leading to tuneability of absorption bands between 390-700nm. The coupling of the increased optical sensitivity through enhanced charge transfer dynamics with the mid-infra-red range grating order (CT+CG) resulted in an amplification in vibrational band excitation of molecular bonds. For example, SEIRA measurements of thin film patterns showed a vibrational signal enhancement in asymmetric vibration of -CH2 (2920cm-1) bonds of PVA by 40%, as CT increased by 178% from 1.2μm at probing 45 degree grazing angle. Furthermore, the magneto-optical SERS phenomenon - involving local polarization of gold nanoparticles through the neighbouring magnetised iron oxide nanoparticle in the presence of external magnetic field - was exploited to reveal the varying degree of enhancement in peaks related to Rhodamine 6G (R6G) coated on thin film nanostructure, which was dependent on magnetized CT/CG morphology; especially the C-C-C ring (671 cm-1), for which the Raman peak increased by 12,000% when magnetized by a 43mT field. In summary, the modified MSCDC process is cheap with an expandable throughput rate ( > 0.1 m2/h) and flexible designs, offering both nanoscale and microscale tuneability of pattern dimensions. Even with higher defectivity (~14%) in comparison to the nanoimprinting method, this method can potentially be used to create repetitive array-like structure. Furthermore, the use of iron oxide reduces the cost without sacrificing the optical performance and thus contributes to the optical tuneability of the thin film nanostructure, thereby making the entire product a potential absorbing antenna and microfluidics thin film for biomolecule detection.

Citation

OKPOZO, O.P. 2022. Magnetic field directed self-assembly of gold Pickering emulsion for preparing patterned film. Robert Gordon University, PhD thesis. Hosted on OpenAIR [online]. Available from: https://doi.org/10.48526/rgu-wt-1987909

• Thesis Type: Thesis
• Deposit Date: Jun 15, 2023
• Publicly Available Date: Jun 15, 2023
• DOI: https://doi.org/10.48526/rgu-wt-1987909
• Keywords: Nanofabrication; Nanoengineering; Pickering emulsion; Ferrofluids; Iron oxide; Gold; Sensors
• Public URL: https://rgu-repository.worktribe.com/output/1987909
• Award Date: Dec 31, 2022