Electrochemical pH sensing characteristics of nitrogen-doped zinc oxide nanostructures using a screen-printed platform**

Monitoring pH variations is of vital importance in the field of medical diagnostics and healthcare devices. Employing zinc oxide (ZnO) nanomaterials as active sensing elements allows sensitivity enhancement by increasing the surface area of the nanomaterials used and improving the charge transfer mechanism in the sensor. In this study, an electrochemical pH sensor based on nitrogen-doped zinc oxide nanosheets was developed using a single step hydrothermal technique. The results obtained show the successful incorporation of nitrogen into the crystal structure of ZnO nanosheets. The sensing platform was fabricated using a simple mask-printing technique using carbon electrodes on a polyimide substrate. The sensing characteristics of nitrogen-doped ZnO (N−ZnO) nanosheets as pH sensors are evaluated for the first time. The results show that the response time and performance improved with nitrogen doping, for a lower analyte volume of just 5 mL. Furthermore, the detailed mechanism of pH sensing was formulated using electrochemical impedance spectroscopy (EIS). The resistance obtained was directly proportional to the charge-transfer resistance at the electrode-electrolyte interface for N−ZnO. The sensitivity as determined by charge-transfer resistance is 0.512 MΩ/pH. Further, the chronoamperometric studies show a sensitivity of 0.156 μA/pH. The response characteristics also reveal a linearity of 0.965 over a pH range of 3 to 9. Hence, the study shows the exceptional response of N−ZnO nanostructures in pH sensing applications. The advantages of the N−ZnO nanosheets include higher sensitivity, flexibility, and a smaller volume of testing fluid that promotes their easy integration into various analytical applications.