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Title: Development of quantitative scanning electrochemical microscopy (SECM) imaging of single and array micro and nanoband edge platinum electrodes
Author: Mitrikeviciute, Ugne
ISNI:       0000 0004 8508 920X
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2019
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Imaging by scanning electrochemical microscopy (SECM) is an established technique that mixes electrochemistry with microscopy. To name a few examples of its broad applicability, it is used for imaging nanoparticles, enzymes, monitoring corrosion processes and redox processes in living cells. However, the information gathered from such images is mostly qualitative, rather than quantitative. Gold band arrays have been used as a standard substrate of nominally defined dimensions for conventional SECM imaging, but they are known to be non-uniform across the electrode surface and suffer ageing, producing significantly timevariant currents among other uncertainties. This thesis addresses this by imaging robust single and array micro and nanoband edge electrodes of controlled design, shape, size dimension and spacing and assists the quantitative analysis of the response. Both qualitative and quantitative analysis was performed on these SECM images. First, data from experimental probe approach curve (PAC) experiments obtained before imaging were compared to the established analytical model reported by Lefrou et al., confirming its applicability. This was then used to fit COMSOL simulated data to extract real tip working distances in μm above the insulator surface specific for these experiments. Then, qualitative analysis of SECM images of micro array, single micro and nanoband edge platinum (Pt) electrodes allowed the evaluation of the impact of effects such as convection, sample orientation and changes in the response of the SECM tip with time in feedback (FB) and substrate generator-tip collector (SG-TC) mode. From single electrode analysis, differences between the imaging modes regarding image resolution and imaging artefacts, also disc and square geometry impact, and differences between imaging micro and nanoband edge electrodes are discussed. Two types of arrays, hexagonal and square, were used as SECM substrates, also allowing to evaluate hemispherical diffusion field overlap evolution from differently arranged arrays. This allowed to visually evaluate the quality of the in-house fabricated electrodes which has not been reported before, together with visual evaluation and direct evidence that hemispherical diffusion field evolves from both disc and square geometry micro and nanoband edge electrodes. Extraction of line profiles from various parts of the images was then used to further compare SG-TC and FB modes. This lead to the quantitative analysis of the tip response of selected scan lines (both vertical and horizontal) in these 2D images collected at fixed tip working distances. The responses of square nanoband edge electrodes were shown to fit to Gaussian distributions and to be consistent with a combination of diffusional broadening and convolution of the sample and tip response. Further, the tip currents were shown to follow the expected concentration profiles of diffusion from the ring nanoband generated analytically using modified Bessel function. Finally, imaging settings and substrate and tip size were varied to evaluate their effect on image spatial resolution, on artefact occurrence and the effectiveness of the above quantitation. Images of a smaller disc nanoband edge electrode of 50 μm diameter instead of the previously used 100 μm were collected using SECM FB and SG-TC modes and were comparable to the 100 μm diameter electrodes. A Pt tip of 1 μm diameter, which is 10 times smaller than the original Pt tip, was used to probe the effect of the tip size. Finally, the effectiveness of quantitative approaches using Gaussian and modified Bessel functions on disc nanoband substrate of 50 μm diameter was evaluated and compared to 100 μm fitting results. Together this data analysis has enabled the evaluation of such electrodes as a benchmark system for SECM probe response validation, method development, optimisation and quantitation.
Supervisor: Mount, Andrew ; Underwood, Ian ; Walton, Anthony Sponsor: Engineering and Physical Sciences Research Council (EPSRC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: scanning electrochemical microscopy ; SECM ; substrates ; probe working distance ; scanning speed ; diffusion ; image resolution