We used an easy method to assemble a biosensor using simple and low cost materials. Commercial SnO 2 :F thin films were used as the basic sensing part of an extended-gate ion-sensitive field effect transistor. When used as a pH sensor, the oxide film showed a sensitivity of 59 ± 4 mV.pH −1 , and linear response in the pH range 2.0 to 10.0. Since urea detection and quantification is important for the control of many pathologies, as proof of concept the samples were further used as platform for urea sensing by immobilizing urease protein onto the surface of the film. For urea sensing, the modified electrodes showed a linear response in the pUrea range of 2.1 to 3.0 (1.00 mM to 7.94 mM, which covers a suitable screening interval for clinical exams) and a sensitivity of 109 ± 3 mV.pUrea −1 . A steady-state was reached after about 60 seconds. Varied pHs and buffer concentrations were also investigated to assess the biosensor behavior toward changes in the environmental conditions. The proposed device could be applied in clinical trials in developing countries as well as in areas hit by natural disasters. Its fabrication consists in the removal of the gate electrode of a MOS-FET, thus exposing the oxide insulator layer to an electrolyte solution. The device is then liquid-gated by a reference electrode immersed in the same solution. Therefore, the gate-source potential (V GS ), which modulates the drain-source current (I DS ), has a contribution due to the reference electrode (V Ref ) and a parcel provided by ions adsorbed at the oxide surface in the inner Helmholtz plane of the Stern-GouyChapman model of the electrical double layer, as given by the site binding model.
2,3The possibility of mass fabrication and miniaturization using thin film technology turned ISFET into a new research field. Van der Spiegel et al., and further Chi et al. developed and upgraded the concept of Extended Gate Field Effect Transistor (EGFET). 4,5 In this design, the sensing element was built apart of the FET structure, solving limitations regarding system encapsulation that arises due to the FET close contact with the electrolyte solution. Furthermore, EGFET permits to obtain a new device by only replacing the sensitive film instead of the complete electronic component, which facilitates its fabrication and disposability.
5With the combination of the ISFET concept and the enzyme electrode proposed by Clark and Lyons, Janata and Caras introduced the Enzymatic Field Effect Transistor (EnFET).6,7 Selectivity toward penicillin was conferred to a pH sensitive field effect transistor through penicillinase enzyme immobilization at the gate of an ISFET. Penicillinase catalyzes the hydrolysis of penicillin to penicilloic acid, which releases protons thus depressing the pH at the surface of the oxide electrode. This charge variation promoted by the end products of the catalyzed reaction could be used as the main parameter for EnFET operation.