Ordered microstructured metal array electrodes for bioelectrochemical systems

• Main Author: Astorga, Solange Elizabeth
• Other Authors: Huang Yizhong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Environmental engineering; Engineering::Materials
• Online Access: https://hdl.handle.net/10356/137452
• Institution: Nanyang Technological University

Innovative applications like energy production from wastewater via microbial fuel cells, production of high-added value products, biosensors for bacteria detection in food, environment, and clinical samples, as well as understanding microbially influenced corrosion can be achieved through bioelectrochemical systems (BES). BES are powered by the extracellular electron transfer (EET) from bacteria into electrodes. Bacteria accumulate at interfaces to form biofilms. When they grow on electrodes, biofilms are termed electrochemically active biofilms (EAB). While biofilm formation and electrochemical activity are thought to be affected by the material and the microstructured of the interface, this interaction is not well-understood for early bacterial biofilms. This dissertation aims to investigate early bacterial biofilms at electrode surfaces, through the fabrication and testing of ordered array microstructured electrodes with different topography and materials. The goals of this research were to understand (i) the EET from bacteria to the electrode, (ii) the role of the metal surface in the transfer mechanism, and (iii) the effect of electrode microstructure on early biofilm formation. The first study involved three different gold microstructured electrodes fabricated with photolithography. The charge production showed that micropillar structures enhanced the charge transfer up to 22% higher than without any microstructure. The characterization of the electrode surface using electrochemical impedance spectroscopy (EIS) showed a higher conductivity and lower impedance for the microstructured electrodes after early biofilm growth at 24 h. Microscope analysis of the electrodes showed that Escherichia coli biofilm formation ensued at the base of the pillar microstructures. Confocal laser scanning microscopy images revealed that 41% of the cells on the electrode were alive, composing an early biofilm of 400 nm thickness. Further, the combination of the microstructures, electrochemical analysis, and imaging shed light on the EET at the biofilm/gold electrode interface. The second study employed nickel microstructured electrodes analog to the gold electrodes used in the first study, to assess the effect of a different material on early biofilm formation. Microstructured electrodes showed a minimal enhancement on the charge transferred (3%) for the chronocoulometry test, although the surface area was increased by 3 and 6% for small and large pillars, respectively. The cell/electrode interface was monitored through EIS at 0, 8, and 24 after cell inoculum at three different temperatures (23, 30, and 37 °C). The results showed a drop of the impedance during the first hours of the experimental runs, followed by a constant increase of the impedance with time. EIS analysis was carried on a range of electric potentials, temperatures and stirring conditions, to gain further information on the electrochemical signature at the cell/electrode interface. The results showed that non-stirring conditions favored reproducible results, rather than stirring conditions. Experiments at 37 °C displayed a redox active species at 0 V vs. Ag/AgCl, which is formed after 24 h. The third study tested topographic arrangement of the features on the electrode. Microstructures of 10 µm height spaced by approximately 10 µm produced more current than bigger 100 µm gaps. However, the higher surface area available when using 10 µm vs 100 µm gaps (91% vs 3%) did not correspond with the increase in current measured (38% vs. 15%). This implies that the available surface is under-utilized during early bacterial biofilm detection, which suggests that the use of shorter pillars may be more efficient. Thus, efforts should be put into optimizing gap distance rather than the surface area of the pillars. Additionally, chronocoulometry revealed an increase in current with glucose concentration, while EIS analysis showed that capacitance increased when glucose concentration decreased. These results highlighted the sensibility of E. coli to the substrate concentrations in bioelectrochemical studies. Overall, this dissertation advances the understanding of the bacteria-surface interaction in early biofilm formation, for a weak electrogenic model organism, using microstructured array electrodes fabricated on Au and Ni. The findings in this dissertation should have implications for bacterial biofilm sensing developments, as well as for preventing microbially influenced corrosion in drinking water systems and biomedical devices.