Optical micro-spectroscopy characterisation of individual colloidal nanoparticles for biomedical applications

Alabdullah, Furqan 2024. Optical micro-spectroscopy characterisation of individual colloidal nanoparticles for biomedical applications. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

bioimaging, biosensing, drug delivery, sensing, and diagnostics. All these applications necessitate an understanding of the nanoparticle size and shape. However, current methods for particle characterization are either time-consuming, expensive, or not very accurate. Borri’s group at Cardiff University has developed a method to overcome these limitations. The method measures the optical extinction cross-section σext (the sum of absorption and scattering) of individual nanoparticles using wide-field transmission microscopy. It allows the simultaneous acquisition of hundreds of nanoparticles for statistical analysis. The method is straightforward, rapid, and quantitative. The precise quantification of individual particle sizes and morphologies is derived from the measurement of σext and its relationship to the wavelength and polarisation of light. In this thesis, we applied and further developed the method on two types of sliver nanoparticles. These are commercially available nanoparticles of nominal nanoplate shape, coated with either PVP or with a silica shell. Plates with nominal 40 nm or 70nm edge length and 10 nm thickness were investigated. Spectrally-resolved measurements were carried out in the wavelength range from 500 nm to 750 nm. A rotatable linear polariser was used to manipulate the polarisation direction of the illumination in the sample plane. This allowed us to investigate the shape of individual nanoparticles. Additionally, we utilised a high numerical aperture annular ring illumination along with a radial polariser to create a powerful polarisation component in the vertical direction. This enabled us to differentiate between thin nanoplates and thicker particles. Measurements were carried out on hundreds of individual nanoparticles, allowing for a statistically relevant analysis. In particular, we developed and applied a cluster analysis, to categorize silver nanoparticles into different groups, based on the optical cross-section and its wavelength and polarisation dependence, as predictors. For silica-coated nanoplates, we found that the substantial silica shell surrounding these particles prevents them from lying flat on a glass surface. Hence, the cluster analysis had to be modified, by introducing new predictors, effectively averaging over the particle orientation. As well as using a polarisation-averaging approach, post-acquisition, with particles deposited on a surface, Brownian rotational diffusion of sliver nanoparticles in a viscous medium was investigated, as a physical way to measure an orientation-averaged cross-section. To validate the thickness accuracy of the technique, we collected a set of data using both optical extinction and AFM techniques. For these measurements, we used silver nanoplates coated with PVP, with a nominal edge size of 40 nm or 70 nm and a thickness of 10 nm. Finally, beyond linear optical microscopy, the application of nonlinear transient pump-probe micro-spectroscopy was explored, to investigate the thermal and mechanical interaction between individual silver nanoparticles and their surrounding environment. For these experiments, PVPcoated silver nanoplates with a nominal size of 70 nm were studied. The results indicated that there was a significant particle reshaping upon pulsed laser exposure during the pump-probe experiments, albeit a detailed interpretation of this behaviour was beyond the scope of this thesis. Overall, this work has helped advancing optical extinction micro-spectroscopy as a – i – powerful way to determine individual nanoparticle sizes and shapes, further boosting the utility of the method as an alternative to expensive and time-consuming electron microscopy analysis.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Engineering
Uncontrolled Keywords:	1). Optical extinction cross section 2). Plasmonic nanoparticles (NPs) 3). Silver nanoplates 4). Radial polarization 5). Polarisation 6).Optical characterization
Date of First Compliant Deposit:	12 February 2025
Last Modified:	12 Feb 2025 14:00
URI:	https://orca.cardiff.ac.uk/id/eprint/176033

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