Self-assembled rnonolayers (SAMs) of single-stranded peptide nucleic acid (ssPNA) molecules have been grown on gold surfaces, with efficient capability for recognizing complementary ssDNA. In spite of their remarkably long length of 6-7 nm, ssPNA molecules can assemble standing-up on the surface similarly to the SAMs of short alkanethiols. The equilibrium between lying and standing-up molecules on the surface is a function of the molecular coverage. These structural and functional studies have been performed by means of powerful label-free techniques for surface characterization such as synchrotron radiation-based X-ray photoemission spectroscopy and atomic force microscopy. (C) 2004 Elsevier B.V. All rights reserved.