In this paper, we report the fabrication of ion sensitive field effect transistor which can detect nitrate in the water, and we clarify the sensing mechanism using a membrane-solution potential change model. For the device, CVD graphene is used for a sensing material and a polymer based nitrate selective membrane is used for the selectivity. While graphene can very sensitively response to the surrounding environment, graphene needs the selectivity to detect specific ions in water. To solve the problem in graphene based sensor, the selective membrane including nitrate ionophore, PVC and plasticizer is directly applied to the graphene surface. The quantity of nitrate in water is estimated by measuring Dirac point shift of graphene with a semiconductor analyzer because Dirac point sensitively responses to a graphene's carrier density which can be changed by the membrane-solution potential difference. Nitrate is the only ion that can be recognized by the selective membrane. Therefore, the sensor can detect the quantity of nitrate in water without the interference of other ions. In the experiment, four different solutions (phosphate, sulfate, chloride and nitrate) are applied to the sensors to test the selectivity and four different concentrations (0.1, 1, 10 and 100 mg/L) are used for the sensitivity test. The detection limit is 0.1 mg/L, and the response time is about 10 s.
- Polymer selective membrane