In this work we present an extensive investigation of the behaviour of a pH sensor fabricated according to an extended gate configuration by exploiting the superior sensing properties of a zinc oxide nanostructured membrane (ZnO nanowalls) and the high performing capabilities of a polysilicon thin film transistor integrated directly onto an ultra-flexible polyimide substrate. The sensor response is then compared with other two materials generally used for this purpose: silicon nitride and zinc oxide in thin film arrangement. The pH sensor based on ZnO nanowalls shows an near-ideal Nernstian response (~59 mV/pH), indicating an ideality factor α~1 according to the conventional site binding model.