Detection of enantiomeric impurities in a simple membrane-based optical immunosensor Oliver Hofstetter * , Jay K. Hertweck, Heike Hofstetter Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115-2862, USA Received 29 November 2004; accepted 13 March 2005 Abstract Currently available methods for the detection of enantiomeric impurities generally require expensive and sophisticated instrumentation. Here, we describe a simple and inexpensive membrane- based chiral immunosensor that allows quantitative determination of chiral analytes up to an enantiomer excess of 99.9%. The experimental setup is based on a competitive reaction between the analyte and a biotin-derivatized analog for the binding sites of a stereoselective antibody, which is immobilized onto a membrane. The antibody-bound analog is detected with peroxidase-conjugated avidin that converts a colorless substrate into an insoluble dye on the membrane surface. The color intensity, which is inversely related to the concentration of analyte in a sample, can be evaluated with standard image analysis programs. D 2005 Elsevier B.V. All rights reserved. Keywords: Chiral discrimination; Antibody; Immunosensor; Membrane; Enantiomer excess; Avidin-biotin system 1. Introduction The physiological effects of drugs, nutrients, pesticides, and other bioactive compounds invariably depend on their three-dimensional structure. Due to their stereoselective interaction with, e.g., receptors, enzymes, and transport proteins, the individual stereo- 0165-022X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jbbm.2005.03.003 * Corresponding author. Tel.: +1 815 753 6898; fax: +1 815 753 4802. E-mail address: ohofst@niu.edu (O. Hofstetter). J. Biochem. Biophys. Methods 63 (2005) 91 – 99 www.elsevier.com/locate/jbbm