Modification of microscale electrodes with carbon nanomaterials for biomedical sensing applications

The modification of electrode surfaces with high-surface-area electrocatalyst materials presents an exciting opportunity to enhance the sensitivity, limit of detection and selectivity of electrochemical sensors. Micro and nanoscale electrodes modified with these electrocatalyst materials have the added benefit of enabling high spatial resolution measurements which give great insight into electrochemical sensing of cell-scale biological systems. However, the use of modified microscale electrodes in this context remains relatively underexplored. In this thesis, platinum nanoparticles (supported on high-surface-area carbon nanotubes) are investigated as modification agents for micro and nanoscale electrodes. Electrophoretic deposition (EPD) is investigated as a key technique for electrode modification with platinum/carbon-nanotube electrocatalysts, as traditional coating techniques (e.g. drop-casting) are insufficiently precise on micro and nanoscale surfaces. Hydrogen peroxide is selected as a model analyte for electrochemical detection due to its relevance in many biological environments as an indicator of cell stress and metabolism.
This work demonstrates that EPD is a highly tuneable and precise tool for modifying ultra-small electrode surfaces with electrocatalyst nanoparticles. Using a glassy carbon macro-electrode model system, a range of nanoparticle/nanocarbon coatings were pre-screened and optimised towards hydrogen peroxide sensing. The macroscale study was then used as a platform to transpose EPD-based modification to platinum and carbon-fibre microelectrodes, resulting in substantial improvements in sensitivity. EPD-based modification was also explored for carbon nanoelectrodes, and challenges associated with the functionalisation of ultra-small electrode surfaces were identified. A Design-of-Experiments (DoE) methodology was developed to further optimise EPD-based microelectrode modification. DoE provides a systematic, data-driven approach for the improvement of microelectrode coatings and is shown to enable more robust and repeatable microelectrode modification (important for future technological translation of modified microelectrode sensors). The newly developed DoE methodology was used to identify optimal EPD parameters for modification of both platinum and carbon-fibre microelectrodes, resulting in substantial improvements in hydrogen peroxide sensing sensitivity and limit of detection. These optimised microelectrodes were then modified further to improve their selectivity before being used in a preliminary in vitro study to detect hydrogen peroxide released upon the stimulation of live cancer cells. This study provides the first steps towards the development of a robust, highly sensitive and minimally invasive electrochemical sensor for biomedical applications.