Redox Cycling-Based Field-Effect Transistor: Fabrication Strategies and Characterization

In spite of the fact that the nanofluidic FET has achieved promising applications in iontronics, biosensing, etc., the transconductance of nanofluidic FET is still low, which limits its abilities in fast response and ultrasensitive sensing. Nanopore electrode arrays (NEAs), allow for fast response, small capacitive currents, enhanced steady-state voltammetric response as well as sensitive manipulation of ion transport. Considering the similarity of nanofluidic FETs to NEAs in their functional mechanism (manipulation of ion transport through nano-sized channels), it is desirable to extend the abovementioned benefits of NEAs to other applications such as the fluidic field-effect transistor, with the aim to establish a redox cycling-based FET that possesses higher transconductance and can achieve ultrasensitive chemical sensing. The ultimate goal of this research is to fabricate the NEAs with three embedded electrodes, of which the middle electrode is specifically designed to be coated with electrochemical blocking layers. The middle electrode, thereafter, serves as the gate electrode which manipulates the ion transport inside the nanopores. This requires the development of effective blocking layers, which can be applied within the interior space of an NEA nanopore. Initial experiments explored the behavior of candidate blocking layers on planar Au electrodes. Notably, this work has shown that the planar gold electrodes can be modified with passivating layers of Al2O3 and HfO2 deposited via atomic layer deposition (ALD) and that these ALD layers can be selectively removed either by electrochemical desorption or wet etching. The electrochemical performance of a variety of 3-electrode NEAs was explored using different concentrations of redox species/supporting electrolyte. At an inter-electrode distance of 100 nm, significant current enhancements resulting from redox cycling, 4-30×, were achieved depending on the ionic strength. In addition, operation of these devices in the two-configuration produced hysteresis-free, rectified currents at low ionic strength. The perspective on the potential approaches to improve the selective ALD on NEAs is also provided.