ABSTRACT
The monitoring of Na+ ions distributed in the body has been indirectly calculated by the detection of Na+ ions in urine. We fabricated a two-dimensional (2D) Na+ ion sensor using a graphene ion-sensitive field-effect transistor (G-ISFET) and used fluorinated graphene as a reference electrode (FG-RE). We integrated G-ISFET and FG on a printed circuit board (PCB) designed in the form of a secure digital (SD) card to fabricate a disposable Na+ ion sensor. The sensitivity of the PCB tip to Na+ ions was determined to be -55.4 mV/dec. The sensor exhibited good linearity despite the presence of interfering ions in the buffer solution. We expanded the evaluation of the PCB tip to real human patient urine samples. The PCB tip exhibited a sensitivity of -0.36 mV/mM and linearly detected Na+ ions in human patient urine without any dilution process. We expect that G-ISFET with FG-RE can be used to realize a disposable Na+ ion sensor by serving as an alternative to Ag/AgCl reference electrodes.
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.
ABSTRACT
Electrical stimulation therapy is a promising method for treating neurological diseases. This method induces the activity and differentiation of nerve cells by the direct or indirect transmission of an electrical signal through biomedical electrodes. We demonstrated the efficacy of a graphene sheet as a bioelectrode to differentiate neurites from hippocampal neuron, through electrical stimulation. In order to the artificially induce the differentiation of hippocampal neurons, we directly transmitted electrical signals of square pulse through the graphene electrode to directly stimulate neurons cultured onto graphene surface. Compared to cell culture plates, the average length of differentiated neurites increased 111.1% on pristine graphene with electrical stimulation. And the average number of differentiated neurites on a single cell increased to 281.9% on oxygenated graphene with electrical stimulation. Electrical stimulation with graphene electrodes promoted the differentiation of neurites and activated the production of intercellular networks of hippocampal neurons.
