Switchable assembly of stable, ordered molecular layers

Bipolar amphiphiles belonging to alpha,omega-bis(4-amidinophenoxy)alkanes s elf-assemble in neutral or alkaline aqueous solution on acid-functionalised surfaces to form pH-switchable layers, with the amphiphiles oriented nearl y perpendicular to the substrate. Layer thicknesses estimated by in situ el lipsometry, surface plasmon spectroscopy (SPS) or neutron reflection measur ements correlated, within a certain amphiphile size range, with the molecul ar length of the amphiphile. Above a certain chain length, the characterisa tion by infrared reflection absorption spectroscopy (IRAS) indicated a crys tallinelike order of the layered amphiphiles and a tendancy for bilayer for mation was observed. These layers were stable and charge selective towards plasma proteins and charged surfactants. The layer stability was strongly p H dependant and no self-assembly was observed in acidic solutions. By cycli ng the pH between 9 and 2 rapid disassembly and reassembly of the layers wa s thus observed. The pH-switchable assembly can be used to restore a chemic ally selective interface after binding of a target molecule, for enhancemen t of sensor responses and potentially for optoelectronic or molecular elect ronic applications.