We report the development of a reagentless electrochemical impedance biosensor for glucose that employs the D-glucose/galactose receptor from E. coli for direct glucose detection. The biological platform for this sensor is an Au surface to which the protein is immobilized through formation of an Au-S bond to a genetically engineered cysteine residue at the N-terminus. The impedance signal detects the extensive ligand-induced domain motion within the protein upon glucose binding. We show the applicability of impedance spectroscopy in conjunction with periplasmic binding proteins as a general method for detecting small molecules. (c) 2005 The Electrochemical Society.