The reversible self-assembly of a series of bipolar amphiphiles, alpha,omega-bis(3- or 4-amidinophenoxy)alkanes (chain length n = 5-12), on mercaptoalkanoic acid-functionalized gold surfaces (chain length n = 10, 11, 14, 15) has been studied by in-situ ellipsometry, IR reflection absorption spectroscopy (IRAS), and atomic force microscopy (AFM). The layer order, amphiphile orientation, and tendency to form bilayers depends on the position of the amidine substituent, the alkyl chain length of both the amidine amphiphile and the underlying acid self-assembled monolayer (SAM), and whether the amidine alkyl chain contained an even or odd number of methylene groups. Thus, para-substituted bisbenzamidines containing more than six methylene groups (n>6) and with an odd number (n = 7, 9, 11) tended to form bilayered structures, whereas those containing an even number formed monolayers when adsorbed on SAMs of the long-chain acids (n = 14, 15). This behavior also correlated with the average tilt angle of the benzene moieties of the amphiphiles, as estimated by IRAS. The odd-numbered chains gave lower tilt angles than the even-numbered ones, and a possible model that accounts for these results is proposed. IRAS also revealed a higher order of the odd-numbered chains and an increasing hydrogen-bonding contribution with increasing chain length. Additional evidence for the proposed bilayered assemblies and their reversibility was obtained by AFM. Images obtained from the assembly of decamidine on a SAM of mercaptohexadecanoic acid in a pH 9 borate buffer revealed domains of similar size to that of the underlying acid SAM (20-30 nm), but less densely packed. By acidifying the solution, the second layer was destabilized and a very smooth layer with few defects appeared. Further acidification to pH 3 also destabilized the first layer.

Bipolar amphiphiles belonging to a,w-bis(4-amidinophenoxy)alkanes self-assemble in neutral or alkaline aqueous solution on acid-functionalised surfaces to form pH-switchable layers, with the amphiphiles oriented nearly perpendicular to the substrate. Layer thicknesses estimated by in situ ellipsometry, surface plasmon spectroscopy (SPS) or neutron reflection measurements correlated, within a certain amphiphile size range, with the molecular length of the amphiphile. Above a certain chain length, the characterisation by infrared reflection absorption spectroscopy (IRAS) indicated a crystallinelike order of the layered amphiphiles and a tendancy for bilayer formation was observed. These layers were stable and charge selective towards plasma proteins and charged surfactants.The layer stability was strongly pH dependant and no self-assembly was observed in acidic solutions. By cycling the pH between 9 and 2 rapid disassembly and reassembly of the layers was thus observed. The pH-switchable assembly can be used to restore a chemically selective interface after binding of a target molecule, for enhancement of sensor responses and potentially for optoelectronic or molecular electronic applications.

Bis-benzamidines were studied as linkers for the formation of gold nanoparticle assemblies on planar gold surfaces modified with ω-mercaptohexadecanoic acid. In situ ellipsometry, external reflection Fourier transform spectroscopy, and atomic force microscopy showed that the packing density and order of the bis-benzamidine layer exerted a pronounced influence on the thickness and density of the nanoparticle layers. These layers exhibited optical properties similar to those of solid gold.

Short oligonucleotides adsorb rapidly and selectively on pHswitchable self-assembled layers of bisbenzamidines on modified gold substrates.