In this work electrolyte insulator semiconductor (EIS) structures based on lithographically fabricated Si SiO2 micropillars and pores are studied. The samples are characterized by means of impedance spectroscopy (IS) and they are compared to samples with a planar SiO2 layer on a Si substrate in order to determine whether the increase in active surface is directly related to an increase in sensitivity of the device. Our initial results confirm this question in the case of the pillar samples but not for the pore ones, most likely due to some fabrication issues that must be improved. Afterwards the SiO2 surface is functionalized with posphonates and the influence of the modification is studied using IS. Also, the stability and limits of applicability of the functionalized devices is studied. Analysis with X-ray photoelectron spectroscopy (XPS) is used to show that the deposition of phosphonates was successful. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim