INVESTIGATION OF FLUOR TIN OXIDE (FTO) AS EGFET FOR PH SENSOR

FQ9

Pablo Diniz Batista and Marcelo Mulato 

Departamento de Física e Matemática, FFCLRP-USP,

Many chemical and biological processes depend on pH value, what makes it one of the commonest laboratory measurements. A recent development in pH measurement was the introduction of ion-sensitive field effect transistor (ISFET) technology as an alternative to the glass electrode.  Recently,  the extended gate field effect transistor (EGFET) was introduced as an alternative for the fabrication of ISFET. The EGFET is a device composed of a conventional ion-sensitive electrode and a MOSFET device, which can be applied to the measurement of ion content in a solution. Therefore, the EGFET is fabricated connecting the sensitive membrane to a comercial MOSFET. This structure has a lot of advantages when compared to ISFET because the sensitive membrane can be optimized without the fabrication of the transsitor. In this work, we investigated the comercial tin oxide doped with fluor (FTO) as sensitive membrane to EGFET. The device is separated in two parts. The sensitive part is made of a commercial FTO film, with surface area of 1 cm², deposited on a 1mm-thick glass. The system is completed with a commercial CD4007UB MOSFET. The comercial FTO shows a low resistivity and a crystalline phase, both determined using conductivity set-up and by X-ray diffraction experiments, respectively. So far, it has been known that an amorphous structure is desirable to obtain high sensitivity. Despite of the a crystalline phase, we have fabricated the FTO as EGFET for pH sensor and carried out experiments in order to obtain the  response of the device inserted into solutions with pH values from 2 up to 12. In this range, we have quantified a sensitivity of 50 mV/pH, which may have large potential applications as pH and biosensors.  In addition, both the film and the structure of the sensor are cheaper and easier than common techniques.