Low‐cost fabrication of indium tin oxide (ITO) FETs for sodium detection in electrolytes and human urine

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Abstract
Here, robust and low‐cost sodium sensors based on the ion‐sensitive field‐effect transistor (ISFET) are fabricated using indium tin oxide (ITO) thin film. The effect of the presence of oxygen during the sputtering of the ITO thin film is characterized on the sensitivity of sodium ion (Na + ) detection. A sodium ionophore III membrane is applied on the ITO substrate to fabricate ISFET (ITO‐ISFET). The results reveal that the increase in the oxygen gas flow rate (0–1 sccm) with a fixed argon flow rate at 20 sccm during sputtering increased the sensitivity of ITO‐ISFET to Na + from 15.8 to 100.9 mV/decade. In addition, the proper condition with a nearly Nernstian slope is at 0.4 sccm oxygen flow rate. Furthermore, the experimental results reveal that the sensors exhibit a high sensitivity of 58.5 ± 2.1 mV/decade in a wide range (10 −10 to 10 −1 M). The limit of detection (LOD) of ITO‐ISFET is 1.8 nM, which is lower than the minimum allowable Na + level in the human body and urine. Furthermore, the proposed ITO‐ISFET has the capability to detect Na + in real human patient urine without any dilution process.

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