Selectivity and Real-Time Potassium Ion Sensor Based on dsDNA Sequences Aptamer in ZnO Thin-Film Transistors with Floating Structure

The level of potassium ions in clinical diagnosis is most associated with chronic kidney disease (CKD) and the pathophysiology of insulin secretion. The normal concentration in human blood of potassium ions is 2.5 to 5.5 mEq/L. In the past, the assay was mostly done by Blood Potassium Content Assay Kit, which provided that the potassium ions in the serum interact with sodium tetraphenylborate to form water-insoluble, which is time-consuming. Therefore, a rapid and immediate of detecting potassium ions is helpful for this issue. Recently, thin film transistors (TFTs) such as zinc oxide (ZnO) for biosensors have become the focus of increased research interest due to their high transparency and non-toxicity, and several reports have discussed the impact of floating gate structures on the sensing and reproducibility aspects of biosensors in solution.<br/>In this study, unique double-stranded deoxyribonucleic acid (dsDNA) was used in combination with different concentrations of K ions detected through ZnO-based biosensors with a floating-gate structure. The TFTs were successfully operated as a switching device at a low operating voltage of 5 V. Here, ZnO TFTs with floating gate structures were used to sense dsDNA bound to metal ions in solution by non-contact sensing, and more than 50 detection times were achieved by removing solutions and low-temperature annealing, which is useful for biosensing. The dsDNA at 100 μM was dissolved in DI water and TE buffer to form a solution. The ratio of DI water to Tris-EDTA (TE) buffer was 1:10, and the molecular weight of the dsDNA (GGTTGGTGTGGTTGG) was 359.8. The KCl concentrations were 0.01 M (0.8 mEq/L), 0.03 M (2.5 mEq/L), and 0.1 M (5.5 mEq/L), respectively. To understand the relationship between the number of combinations of different concentrations of potassium ions and the potential of the DNA sequence, the number of zeta potentials for each concentration was measured. The zeta potential of dsDNA was -23.63 mV. The stability of 0.01 M KCl solution was tested with -15.01 and -14.41 mV, respectively. The zeta potential was measured within an inaccuracy of 0.6 mV because a few ions flowing into the solution were unaffected. A positive charge is generated when the DNA sequence binds to potassium ions showing a glove-like shape and can completely wrap around K⁺. When the DNA sequence was bound to K⁺in the 0.01 M KCl solution, the zeta potential was -15.01 mV. The zeta potential was -2 mV when the concentration of the KCl solution was 0.03 M; the zeta potential was 52.53 mV when the concentration was 0.1 M. These results showed that the KCL elevated by 6.75 mV per 0.01 M concentration.<br/>To investigate ZnO-TFT-based potassium ion detectors, the performance of the ZnO TFT was assessed by analyzing the field-effect mobility (µFE), threshold voltage (V_TH), and threshold voltage shift (△V_TH) for carrier transport. In the initial state, the TFTs exhibited typically electrical performance. Specifically, I_on/I_off was 2.3 × 10⁴ A, the threshold voltage (V_TH) was -2 V, and the field-effect mobility (µFE) was 0.006 cm² V^-1s^-1. A unique ssDNA sequence combined with 0.01, 0.03, and 0.1 M KCl. The TFTs exhibited selective and sensitive electrical performance: I_on/I_off = 3.8 × 10⁴ A, V_TH = 2.5 V, and = 0.0052 cm² V^-1s^-1; I_on/I_off = 2.9 × 10⁴ A, V_TH = 2 V, and = 0.0059 cm² V^-1s^-1; and I _on/I_off = 3.6 × 10⁴ A, = -0.5 V, and = 0.0068 cm²V^-1s^-1. The ZnO-based TFT showed that KCL was elevated by 0.5 V per 0.01 M of concentration of K⁺. As the concentration rises, dsDNA binds to more K⁺, suggesting that the detected positively and negatively charged objects affect the turn-on voltage of ZnO TFTs. The increase in ΔV_THalso indicated increased binding of dsDNA to K+ with potassium concentrations ranging from 0.01 M to 0.1 M and showed a linear correlation (0.99706). This ZnO-based TFT with DNA sequences aptamer could be used as a biosensor.