Comparative study of ZnO and ZnFe2O4 micro and nanoparticle-based screen-printed electrodes in pH sensing

This work presents the application of zinc oxide (ZnO) and zinc ferrite (ZnFe2O4) for electrochemical pH sensing. ZnO and ZnFe2O4 are synthesized by auto-combustion synthesis method. Field emission scanning electron microroscopic (FESEM) images revealed that ZnO particles have pyramid- and spherical-shaped morphology with micrometer dimensions, while ZnFe2O4 particles have spherical shape at the nanoscale. The surface-modified screen-printed electrodes with ZnO and ZnFe2O4 particles are initially characterized by the ferri/ferrocyanide redox couple. Significant improvement in sensitivity (bare carbon: 6.3 ± 0.4 μ A/mM, ZnO: 8.5 ± 0.3 μ A/mM, ZnFe2O4: 8.9 ± 0.5 μ A/mM) and rate constant (bare carbon: 10 ± 1 ms−1 , ZnO: 46 ± 4 ms−1 , ZnFe2O4: 42 ± 3 ms−1 ) is observed with the surface-modified sensors. Chronopotentiometric pH response of the sensors showed hysteresis behavior with pH loop. No interference effects are observed, and the pH sensitivity of the bare carbon sensor ( 23.9 ± 1.4 mV/pH) is increased by the introduction of ZnO ( 38.1 ± 1.3 mV/pH) and ZnFe2O4 ( 37.2 ± 1.1 mV/pH) particles. Stability of the pH response is discussed, and ways for its improvement are proposed.