Comparative Study of ZnO and ZnFe₂O₄ Microparticle 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: <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6.3~\pm ~0.4~\mu $ </tex-math></inline-formula>A/mM, ZnO: <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8.5~\pm ~0.3~\mu $ </tex-math></inline-formula>A/mM, ZnFe2O4: <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8.9~\pm ~0.5~\mu $ </tex-math></inline-formula>A/mM) and rate constant (bare carbon: <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10~\pm ~1~{\text {ms}}^{-{1}}$ </tex-math></inline-formula>, ZnO: <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$46~\pm ~4~{\text {ms}}^{-{1}}$ </tex-math></inline-formula>, ZnFe2O4: <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$42~\pm ~3~{\text {ms}}^{-{1}}$ </tex-math></inline-formula>) 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 (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$23.9~\pm ~1.4$ </tex-math></inline-formula> mV/pH) is increased by the introduction of ZnO (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$38.1~\pm ~1.3$ </tex-math></inline-formula> mV/pH) and ZnFe2O4 (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$37.2~\pm ~1.1$ </tex-math></inline-formula> mV/pH) particles. Stability of the pH response is discussed, and ways for its improvement are proposed.