Substrate temperature induced enhanced selectivity and sensitivity for nanomolar gallic acid detection on RF magnetron sputtered ZnO/GS thin film electrode

Abstract

Zinc oxide (ZnO) thin films possessing average thickness of 176 nm were deposited onto Graphite Sheet (GS) substrates by a radio frequency magnetron sputtering (RFMS) technique at substrate temperatures of 100 °C (T1), 200 °C (T2) and 400 °C (T3). X-ray diffraction (XRD) patterns of the ZnO thin films revealed polycrystalline hexagonal wurtzite structure. Field emission scanning electron microscopy (FE-SEM) showed the existence of aggregated crystallites with nanoflakes structures in the films. Increasing the substrate temperature lead to larger grain size and enhanced crystallinity. The RFMS ZnO/GS thin film electrodes (TFEs) sputtered at 400 °C (TFET3) resulted in Gallic acid (GA) electrochemical detection at 0.35 V in pH 7 phosphate buffer solution with high sensitivity, stability, reproducibility and repeatability compared to thin films sputtered at other temperatures. Significant selectivity for GA detection from the other closely related polyphenols and biological analytes was achieved for the TFET3. The GA sensing was obtained resulted in appreciable sensitivity of 0.0671 μA.μM−1 for the linear range of 30–360 μM and a lowest detection limit value (LOD) of 3.51 nM. The developed electrode demonstrated practical application in GA detection in grape juice and water samples with appreciable recovery values.