**2.2 Synthesis of tragacanth gum/chitosan/ZnO nanoprism**

Tragacanth gum/chitosan/ZnO nanoprisms were prepared using a green sonochemical method at 25°C (35 kHz frequency, 320 W, Sonoplus, Bandelin, Germany). In the first step, 0.1 g of tragacanth gum was dissolved in 50 ml of deionized pure water on a magnetic stirrer in 2 h at 25°C. In the second step, 0.1 g of chitosan was dissolved in 50 ml of 2% glacial acetic acid. In the third step, 25 ml of the prepared solutions was taken and mixed. In the fourth step, O.1 M of Zn(NO3)2·6H2O and 0.2 M of NaOH was prepared in deionized pure water. In the fifth stage, the solution of Zn(NO3)2·6H2O was added in the solutions, and then the solution of NaOH was added dropwise under the sonicator at 25°C for 40 min (%30 amp). The solution was kept at 24 h in the dark at room temperature. Then, it was first filtered through membrane filters of 0.45 μm and 0.22 μm pore size, respectively. The final solution (viscosity: 1.28 cP and pH 4) was stored at 25°C for use in the sterile container for analysis.

## **2.3 Preparation and measurements of electrochemical sensors**

Electrochemical gold transducers were rinsed with ultrapure water, dried with nitrogen gas, and coated with tragacanth gum/chitosan/ZnO nanoprism solution by dropping and drying the solution. The 3D model structure and the property of dyes used are shown in **Figure 1**.

The potential of the working electrode was varied linearly with time, while the reference electrode was maintained at a constant potential. The potential was

#### **Figure 1.**

*3D model structure and property of the reactive dyes.*

applied between the reference electrode and the working electrode, and the current was measured between the working electrode and the counter electrode. Ebtro voltammetric electrochemical workstation with a three-electrode configuration was used for all electrochemical tests. Cyclic voltammetry (CV) was performed in [−1, +1] V range with a scan rate of 50 mV/s.
