Abstract
A reference electrode is necessary for the working of ion-sensitive field-effect transistor (ISFET)-type sensors in electrolyte solutions. The Ag/AgCl electrode is normally used as a reference electrode. However, the Ag/AgCl reference electrode limits the advantages of the ISFET sensor. In this work, we fabricated a two-channel graphene solution gate field-effect transistor (G-SGFET) to detect pH without an Ag/AgCl reference electrode in the electrolyte solution. One channel is the sensing channel for detecting the pH and the other channel is the reference channel that serves as the reference electrode. The sensing channel was oxygenated, and the reference channel was fluorinated partially. Both the channels were directly exposed to the electrolyte solution without sensing membranes or passivation layers. The transfer characteristics of the two-channel G-SGFET showed ambipolar field-effect transistor (FET) behavior (p-channel and n-channel), which is a typical characteristic curve for the graphene ISFET, and the value of VDirac was shifted by 18.2 mV/pH in the positive direction over the range of pH values from 4 to 10. The leakage current of the reference channel was 16.48 nA. We detected the real-time pH value for the two-channel G-SGFET, which operated stably for 60 min in the buffer solution.
Highlights
Since the invention of the ion-sensitive field-effect transistor (ISFET) by Bergveld (1970), field-effect transistor (FET)-based ion sensors have been widely studied for detecting many types of specific ions [1]
The Ag/AgCl reference electrode is difficult to miniaturize to micrometer size, and its fabrication is incompatible with the semiconductor technology used to manufacture ISFETs [10]
Oxygenated and fluorinated graphene were directly exposed to the electrolyte solution for sensing pH and serving as the reference electrode in the two-channel graphene solution gate field-effect transistor (G-SGFET), respectively
Summary
Since the invention of the ion-sensitive field-effect transistor (ISFET) by Bergveld (1970), field-effect transistor (FET)-based ion sensors have been widely studied for detecting many types of specific ions [1]. The ion-sensitive field-effect transistor (ISFET) has advantages such as high sensitivity, rapid response, high input impedance, low output impedance, miniaturization, and low cost. The Ag/AgCl electrode has been used as the reference electrode in the ISFET sensing system [6,7,8,9]. The Ag/AgCl reference electrode limits the advantages of the ISFET, especially miniaturization and low cost. The Ag/AgCl reference electrode is difficult to miniaturize to micrometer size, and its fabrication is incompatible with the semiconductor technology used to manufacture ISFETs [10]. Some studies have reported methods using solid-state reference electrodes or back gates to replace the Ag/AgCl reference electrode [10,11]
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