We report on the direct electrochemical detection of aptamer-protein interactions, namely between a DNA aptamer and lysozyme (LYS) based on electrochemical impedance spectroscopy (EIS) technique. First, the affinity of the aptamer to LYS and control proteins was presented by using filter retention assay. An amino-modified version of the DNA aptamer-recognizing lysozyme was covalently immobilized on the surface of multiwalled carbon nanotube-modified screen-printed electrodes (MWCNT-SPEs), which were employed for measurements and have improved properties compared with bare SPEs. This carbon nanotube setup enabled the reliable monitoring of the interaction of lysozyme with its cognate aptamer by EIS transduction of the resistance to charge transfer (R-ct) in the presence of 2.5 mM [Fe(CN)(6)](3-/4-). This assay system provides a means for the label-free, concentration-dependent, and selective detection of lysozyme with an observed detection limit of 12.09 mu g/ml (equal to 862 nM). (C) 2011 Elsevier Inc. All rights reserved.