Fabrication of pyrolytic carbon micro- and nanogap electrodes for electrochemical sensing

Our society is on a broad scale moving towards increased use of wearables, implants, point-of-care, internet-of-things and lab-on-a-chip systems. All of which require device miniaturization. Electrochemical sensing integrated within these systems offer huge benefits ranging from research settings for investigation and monitoring of e.g. bacteria and cells to improved health in the future and energy conversion, to chemical manufacturing such as in power-to-X plants and in consumer-oriented settings including monitoring of glucose levels in diabetes patients. One of the key factors for miniaturization of devices is to improve or maintain performance as the size is reduced.
Electrochemical redox cycling is a phenomenon in which the current generation from a single molecule can be tremendously amplified. The signal enhancement arises as the molecule is repeatedly oxidized and reduced at two electrically separate electrodes as it cycles back and forth. The amplification greatly increases as the gap between the electrodes grows smaller. At a gap of ten micrometers, the molecule can travel back and forth only a couple of times within a second, but reducing the gap to below one micrometer can increase that to several thousands of times. Pyrolytic carbon is an electrode material offering many advantages such as chemical inertness and mechanical stability. Compared to planar metal electrodes, pyrolytic carbon facilities the fabrication of electrodes with complex topographies, geometries and 3D structuring that can further enhance the performance.
This thesis explores the fabrication of different pyrolytic carbon micro- and nanogap electrode platforms to improve electrochemical sensing performance and approaches that can facilitate device integration and miniaturization. Specifically, the main objective of the thesis is to fabricate electrically separate pyrolytic carbon electrodes with gaps ultimately below one micrometer. Cleanroom fabrication methods were employed to fabricate pyrolytic carbon electrodes with micro- and nanogaps, which were characterized electrochemically to verify electrical separation and assess performance and redox cycling amplification.
The main pyrolytic carbon electrode systems developed were interdigitated microelectrodes with microgaps (IDME) and stacked layer electrodes (SLNE) with nanogaps. IDME were realized by a combination of implementing adhesion structures, optimization of pyrolysis as well as optimization of maskless lithography to achieve a high chip high yield with approximately 5 micrometer gaps between the interspersed fingers. SLNE were achieved with a film of atomic layer deposited alumina separating two microporous electrodes followed by reactive ion etching of alumina rendering access of analyte two both electrodes and resulting in a vertical, below 100 nm gap. Fabrication of additional systems were also investigated including the use of reactive ion etch of pyrolytic carbon for improved IDME fabrication, reactive ion etching of silicon followed by spray-coating and pyrolysis of photoresist to achieve membrane nanogap electrodes, suspended layer interdigitated electrodes with vertical microgaps and improvements within metallization of electrodes for better electrical contacts.
Electrical separation and redox cycling were demonstrated with IDME and SLNE with the latter having superior amplification performance. Redox cycling electrochemical techniques were examined and optimized for improved background signal correction and signal-to-noise ratio resulting in detection of Dopamine at low nanomolar concentrations of 25 nM.
Overall, the research conducted during the project provided significant improvements within pyrolytic carbon electrode micro- and nanofabrication enabling amplification of lectrochemical signals which in turn can aid in the advancement of miniaturization of electrochemical sensor devices.