Novel Model of Field-Effect Sensing Mechanism for Hg (II) Ion Detection on ZnO-Based TFT Sensor with Conjugation of Au Nps and ssDNA Aptamer
Authors: Huang Yi-Cheng , Lai Yu-Wen , Lin Zhe-Yu , You Hsin-Chiang , Chao Keng-Hsien , Chen Yen-Shuo , Chen I-Hsin , Wen Kai-Chieh 
Published: December 2024

Mercury(Hg) has been regarded as a highly mobile, bio-accumulated, non-degradable, irreversible substance which pollute the ecological environment deeply and become the concern of global sustainability [1]. Although the aspect and approach for detect the mercury has developed for several decades, but it is uncomplicated for us to find all the pollutants since the mercury being the contaminant in the natural environment for such a long time. Traditional methods to detect the heavy metallic ions such as atomic absorption spectroscopy, inductively coupled plasma/ mass spectrometry, inductively coupled plasma/atomic emission spectrometry, ultraviolet−visible spectroscopy have been applied in this fields for decades [2,3] Among these brand-new approaches, five sorts of sensors are widely known9, fluorescent sensor, plasmonic sensor, SERS sensor, electrochemical sensor and FET sensors respectively. Changing in field, charge or electron change, transistor has instant signal for detection of heavy metals that no other physical environment parameters like temperature, humidity, brightness, heat can affect the sensing process. The ultra-stable of detection method provides us another facility in the field not only heavy metals but also other kinds of analytes. To date, not only bare semiconductor layer applies for detect the anatypes like heavy metals but also utilize nanomaterials and biological technologies conjugate with semiconductor layer to expand the diversity of detection possibility. Nanomaterials participate into the transistor sensor of heavy metals provide novel opportunities for being real time and precise measurement. Owing to the chip-based state, TFT sensor becomes more convenient for human being to take facile test of possibly harmful environment. The sol-gel solution of the high-k 0.1 M HfO2 were prepared by dissolving hafnium chloride (HfCl4) in ethanol (EtOH) under nitrogen-filled florence flask and stirred at 60 °C with HfCl4 weight ratio of 4%. The homogeneous solution stirred at 80 °C to remove byproducts which finally forming the transparent high-k HfO2. We synthesized ssDNA aptamer solution conjugated with Au NPs. At first, mercury specific-ssDNA aptamer containing thymine sequence (TTCTT TCTTC CCCTT GTTTG TT 5′ -C6SH, MDBio Inc. from Taiwan) centrifuged in 3000 rpm for 5 minutes. Second, dissolved 5 OD260 aptamer into 243.1μL DI-H2O to form 100μM ssDNA aptamer solution which 1 OD260 has 33μg oligonucleotide. After completely dissolved and diluted to various concentration we need, poured ssDNA aptamer solution into 10nM Au NPs solution and incubated for 24 hours in room temperature. ssDNA aptamer linked to Au NPs in the reason of surface modification of gold nanoparticles with Van der waals force.
The structure of the components is shown in Figure 1a. The exact thickness of the HfO2-based TFT is discussed by the focus ion beam. Figure 1b shows the thicknesses of the spin-coated three-layer HfO2 and five-layer ZnO are 42.58 nm and 27.10 nm, respectively. For the real capacitance and dielectric constant used in thin-film transistors, the k-value of the 3-layer HfO2 capacitor is 18.39, which proves that we succeeded in fabricating a high-k-value capacitor (Figure 1c). The UV-Vis absorption spectrum of 1×10-8 M Au NPs conjugated with 1×10-6 M ssDNA aptamer (TTCTT TCTTC CCCTT GTTTG TT 5′-C6SH). The thymine sequence with OD260 concept had imported for the ssDNA aptamer in UV-Vis spectrum, which means this specific sequence has 33 μg oligonucleotide within light radiation length of 1 cm, absorbance under 260 nm. The result of the UV-Vis spectrum corresponded to the OD concept (Figure 1d). The TFT sensor's representative electrical characteristics transfer curve (ID-VG curve) after detecting 10 nM Au NPs is shown in Figures 1e and 1f. The off-current (IOFF) is 2.06 ×10-9 A at VG = -2 V, the saturation on-current (ION) is 3.31 ×10-5 A at VG = 5 V, and the on/off ratio (ION / IOFF) is approximately 6.24×104. Figure 1g is the representative electrical characteristics transfer curve (ID-VG curve) for the TFT sensor after detecting different concentrations of Hg2+ with the conjugation of Au NPs and ssDNA aptamer. Initially, the performances on the square root of the ID saturation current versus the VG gate voltage curve were compared first because we had to confirm the change in threshold voltage (VTH). After detecting 10 μM, 5 μM, 1 μM, 0.5 μM, 0.1 μM, 50 nM, 10 nM, 2 nM and 1 nM Hg2+ ions, the VTH of them is 1.45 V, 1.45 V, 1.47 V, 1.50 V, 1.53 V, 1.55 V, 1.65 V, 2.3 V, 2.35 V, respectively. The stability of these six states in the electrical characteristics transfer curve without evident changes after 10 repeated cycles confirms that the TFT sensor is highly stable after different detection states (Figure 1h).

Figure 1