Naznin, Shakila
(2023)
Characterisation and phononic image reconstruction of gold
nanorods.
PhD thesis, University of Nottingham.
Abstract
The optical and mechanical properties of metal nanoparticles depend mostly on their sizes and shapes. However, a non-destructive size characterisation technique for metal nanoparticles which can work in any external environment with high precision is currently unavailable. It is well known that optical microscopy is diffraction limited which means that two close objects are not resolvable because of the bending of light waves while passing through small apertures. The resolution of an optical system is limited to a few hundred nanometres for visible wavelengths, even using the highest available numerical aperture (1.4). This PhD research focused on both these problems: the unavailability of a precise and non-destructive size characterisation technique for metal nanoparticles applicable for all environments and the resolution limit of optical microscopy.
In this PhD work, size, orientation, and location of gold nanorods were determined to reconstruct their acoustic or phononic images in both air and water media by using time-resolved pump-probe picosecond ultrasonics. Metal nanoparticles are of sub-optical dimensions and a large number of them can be placed inside the same optical point spread function. Using acoustic frequencies and orientation differences of gold nanorods, it was possible to separate and image them, even inside the same optical point spread function.
The approach involved the exploitation of gold nanorods as optoacoustic transducers when they were excited by focused circularly polarised femtosecond optical pump pulses. They became hot and vibrated due to the created stress, producing acoustic waves in the GHz range. Circularly polarised focused probe pulses were transmitted through them, and the modulation of the scattered probe light intensity caused by the vibration was used to determine their vibrational frequencies. These vibrational frequencies were then converted into sizes by using an established analytical model developed by Hu et al. (2003). A polarisation-controlled detection system was developed to change the polarisation of the probe light to determine the orientation of the rod. The nanorods were scanned spatially during the experiments and later an amplitude map of the scanned area at the frequency of the rod was extracted. Then, the centroid algorithm was applied to find the location of the rod. Experimental parameters such as pump and probe wavelengths were estimated by simulating the optical and mechanical responses of gold nanorods. A non-commercial Matlab code package SMARTIES and a commercial finite element model tool COMSOL Multiphysics were used to simulate the optical response of gold nanorods in specific external environments. The mechanical responses of gold nanorods were simulated by using an established analytical model developed by Hu et al. (2003) and finite element models designed by using COMSOL Multiphysics.
In this thesis, it was found that the characterised sizes of gold nanorods in both air and water media were close to the sizes measured from the scanning electron microscopy (SEM) images. Obtained worst-case length and width precisions were approximately 1 nm and 0.3 nm compared to SEM measurements, respectively. The size characterisation results presented in this thesis showed that the worst case SEM precision was ±36 nm. However, the SEM measurement was a function of human error because measurements were done manually from the pixels of the SEM images. In addition, the precision of SEM measurements also depends on the setting of the machine, noise, magnification, contrast, and aberration, among others. The reconstructed acoustic images of gold nanorods also matched reasonably well with the SEM images. Obtained minimum location and angle precisions were 2 nm and 0.4°, respectively. The result presented in this thesis showed that the minimum angle precision was ±7° from SEM. The results showed that the proposed technique was in good agreement with the SEM.
The proposed technique can work with high precision in any external medium without requiring a vacuum environment and conductive surface. Its applicability to the water environment also suggests that the technique can be used in bio-environments. Hence, the technique is simple, non-destructive and ideal for imaging living specimens.
The motivation behind this PhD research was to help make progress towards a phonon-based super-resolution imaging technique in biology. Although imaging biological nanostructures using gold nanorods was out of the scope of this PhD work, the achievements of the present work are significant steps towards offering a phonon based super-resolution imaging technique that can image biological nanostructures.
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