The over Nernstian response of Porous Silicon (PS) devices towards sodium ions have been reported [1]. This paper aims to give interpretations and to proposal model associated to a non-classical electrochemical behaviour. The capacity measurements were made to test the functionalised pS samples with regard to their porosities, porous layer thicknesses and pores morphologies. We have studied samples with different resistivities of bulk silicon (Si). Both PS samples prepared from, either, heavily or lightly doped substrates have shown an increase of the sensitivity with the porosity.