The use of FTO samples as an extended gate field effect transistor biosensor is presented. The FTO samples were produced by spray pyrolysis technique. The cleaning process is shown to have a fundamental importance for the final sensitivity of the samples when multiple re-usage is adopted. The role of electrical resistivity and morphology of the films are investigated. The influence of pH sequence of measurements from 2 to 12 is presented. Both increasing and decreasing the pH values sequence of measurements are compared. Electrical, morphological, time evolution and electrochemical experiments are correlated in the main discussion. A physical-chemical model is presented to explain the main mechanisms of charge adsorption and desorption. Parameters not commonly reported in the literature are proven to have fundamental importance in sensors behavior and characterization.

Fluorine-doped tin oxide (SnO2:F) was investigate as the sensitive part of a pH sensor in the extended-gate field effect transistors (EGFET) device, which provided a linear response for pH range from 2 to 12; the sensitivity was 37 mV.pH-1 for experiments performed in absence of light. Neutral pH, leads to a transistor’s electric current remained practically constant, suggesting that pH 7 correspond to the isoelectric point of the SnO2:F samples. For acid and alkaline pH, the power law varied along time and stabilized at about 10 min. UV-vis light did not alter the results. The transistor’s electric current increased with the operating temperature increases. Once that small change in the operating conditions can alter the results, the physical mechanisms underlying the sensing process must be clearly understood. It is essential to monitor transient response and measurements conditions.