Effect of DNA Aptamer Concentration on the Conductivity of a Water-Gated Al:ZnO Thin-Film Transistor-Based Biosensor

Abstract

Field-effect transistor-based biosensors (bio-FETs) are promising candidates for the rapid high-sensitivity and high-selectivity sensing of various analytes in healthcare, clinical diagnostics, and the food industry. However, bio-FETs still have several unresolved problems that hinder their technological transfer, such as electrical stability. Therefore, it is important to develop reliable, efficient devices and establish facile electrochemical characterization methods. In this work, we have fabricated a flexible biosensor based on an Al:ZnO thin-film transistor (TFT) gated through an aqueous electrolyte on a polyimide substrate. In addition, we demonstrated techniques for establishing the operating range of such devices. The Al:ZnO-based devices with a channel length/width ratio of 12.35 and a channel thickness of 50 nm were produced at room temperature via magnetron sputtering. These Al:ZnO-based devices exhibited high field-effect mobility (μ = 6.85 cm²/Vs) and threshold voltage (V_th = 654 mV), thus showing promise for application on temperature-sensitive substrates. X-ray photoelectron spectroscopy was used to verify the chemical composition of the deposited films, while the morphological aspects of the films were assessed using scanning electron and atomic force microscopies. The gate-channel electric capacitance of 40 nF/cm² was determined using electrochemical impedance spectroscopy, while the electrochemical window of the gate-channel system was determined as 1.8 V (from -0.6 V to +1.2 V) using cyclic voltammetry. A deionized water solution of 10 mer (CCC AAG GTC C) DNA aptamer (molar weight -2972.9 g/mol) in a concentration ranging from 1-1000 pM/μL was used as an analyte. An increase in aptamer concentration caused a proportional decrease in the TFT channel conductivity. The techniques demonstrated in this work can be applied to optimize the operating parameters of various semiconductor materials in order to create a universal detection platform for biosensing applications, such as multi-element FET sensor arrays based on various composition nanostructured films, which use advanced neural network signal processing.

Keywords: DNA; biosensor; electrochemistry; thin-film transistor; zinc oxide.

Figure 1

(a) Schematic representation of the principle of operation of bio-FET. (b) A photograph of the actual polyimide substrate hosting an array of 9 WGFETs. (c) A schematic of a single device unit: 1—gate, source and drain contact pads; 2—gate, source and drain electrodes passivated with Ta₂O₅; 3—Al:ZnO layer on top of the drain-source channel; 4—Al:ZnO layer; 5—the gate electrode, 6—DNA aptamers attached to the Al:ZnO surface. (d) SEM image of the transistor channel dimension measurements.

Figure 2

High-resolution XPS spectra and peak fits of the Al:ZnO thin film: (a) Zn 2p scan, (b) O 1s scan, and (c) Al 2p scan.

Figure 3

(a) Anisotropic AFM image of the room-temperature magnetron-sputtered Al:ZnO layer surface profile; (b) thickness measurements.

Figure 4

(a) Cyclic I−V curve for DI water in contact with a metal/semiconductor (red vertical lines indicate the electrochemical window for the particular device (chromium/DI water/Al:ZnO) that is equal to 1.8 V); (b) Nyquist plot of the impedance spectra for Al:ZnO EGFET and the equivalent electrical circuit. An AC signal is applied between the gate electrode and the shorted together drain-source electrodes.

Figure 5

(a) Output characteristics (I_d–V_d) of the WGFET at gate voltages ranging from 100 mV to 500 mV and drain voltages from 0 V to 1 V; (b) logarithmic representation of transfer characteristics (I_d–V_g) of the thin-film transistor; (c) results from the extrapolation of the linear region (ELR) method for determination of V_th; (d) square root plot of the drain current vs. gate voltage.

Figure 6

(a) Transfer (I_d−V_g) curves at 0.4 V drain voltage for aptamer containing analyte at concentrations of 1–1000 pM/μL, and (b) the relative sensor response level.