Highly Sensitive DNA Sensing in Subthreshold Regime with ITO-Based One-Piece Thin Film Transistor
Authors: Katayama Ritsu , Sakata Toshiya 
Published: August 2024

INTRODUCTION
Solution-gated field-effect transistors (FETs) with silicon or other semiconductive materials as channels can specifically and selectively detect ions and biomolecules in biological samples by chemically modifying the gate electrode surface, and these are known as biologically coupled FETs (Bio-FETs) [1]. Since Bio-FETs can directly detect the charges of ions and biomolecules, they do not require to label fluorescent dyes and to induce redox reactions based on enzymes. Therefore, Bio-FETs are expected to be used as in vitro diagnostic devices for a label-free monitoring of health conditions in daily life.
Our research group is investigating the application of thin-film transistors (TFTs) composed of indium tin oxide (ITO), which has been widely used as an electrode material for displays owing to its transparency and conductivity, to Bio-FETs. Considering the fact that the ITO thin film is not only conductive but also semiconductive, depending on the thickness, we have found that solution-gated FETs can be fabricated by one-step sputtering of ITO [2]. Moreover, by etching a part of the conductive ITO film with the thickness of 100 nm) to the optimal thickness (< 30 nm), which induces semiconductive characteristics, a solution-gated ITO-TFT can be fabricated, which is composed of one piece of ITO thin film and whose shape and properties can be precisely controlled [3]. The ITO-based one-piece TFT has no interface among the source/channel/drain electrodes and exhibits a steep subthreshold slope (SS) of about 80 mV/decade, which is based on the large capacitance of the electric double layer formed by the direct contact of electrolyte solutions with the ITO channel surface,. Therefore, the ITO-based one-piece TFT is expected to be applied to biosensors with high sensitivity.
In this study, we demonstrate the possibility of a highly sensitive DNA detection in the steep SS with the ITO-based one-piece TFT for the biosensing application. In particular, DNA probes are chemically modified on the ITO channel surface, and then DNA hybridization is detected in the near subthreshold regime with the ITO-based one-piece TFT.
EXPERIMENTAL
First, photoresist was patterned on a glass substrate by photolithography. ITO thin film (100 nm thickness) was deposited on the patterned area by sputtering, and photoresist was removed using a solvent. Next, as shown in Figure 1, photolithography was used again to deposit the resist film so that the channel portion was exposed. Subsequently, a drop of 0.1 M hydrochloric acid was dropped onto the exposed channel area, and then the exposed area was etched. Etching was controlled by monitoring the conductivity between the source and drain over time. Moreover, thiolated DNA probes were immobilized on the channel surfaces of the fabricated devices using m-maleimidobenzonyl-N-hydroxysuccinimide ester (MBS) as a crosslinker to the amino groups at the poly-serotonin surface as the anchor layer (Figure 1). Thus, DNA probes were fixed on the surface to detect the target DNA, and various bioaffinity parameters such as detection limit and sensitivity were investigated in the near-subthreshold region of ITO-based one-piece TFTs.
A semiconductor parameter analyzer was used to monitor conductivity during etching and to evaluate electrical properties such as gate voltage (VG)-drain current (ID) transfer characteristics of the fabricated ITO-based one-piece TFTs. AFM (Bruker) was used to evaluate the shape of the surface chemical modification.
RESULTS and DISCUSSION
The ITO-based one-piece TFT functionalized with DNA probes was used to evaluate the potential response to the addition of target DNA. The measurement was conducted in the near-subthreshold regime. As a result, the DNA probe-modified device showed the larger electrical signals than the device without the modification of DNA probes. This suggests that the functionalized ITO-based one-piece TFT enables the highly sensitive detection of biomolecules in the subthreshold regime.
CONCLUSIONS
In this study, we demonstrated the relatively highly sensitive DNA detection even in the near subthreshold regime with the ITO-based one-piece TFT with DNA probes. This indicates that the functionalized ITO-based one-piece TFT would allow the specific detection of various biomarkers in the subthreshold regime, which has the potential to be applied to biosensors with high sensitivity in the future.
REFERENCES
[1] Sakata, T. ACS Omega
2019, 4, 11852.
[2] Sakata, T.; Nishitani, S.; Saito, A.; Fukasawa, Y. ACS Appl. Mater. Interfaces
2021, 13, 38569.
[3] Katayama, R.; Sakata, T. ECS Trans.
2023, 111, 37.

Figure 1