Coordination-controlled self-assembled multilayers on gold

A new kind of multilayers based on metal-ion coordination was constructed on gold surfaces, where molecular layers are successively added using a highly controlled step-by-step procedure. A bifunctional ligand is used as the base layer, bearing a cyclic disulfide group to attach to the gold surface and a bishydroxamate group capable of ion binding. An 8-coordinating metal ion such as Zr4+ or Ce4+ is then coordinated to the bishydroxamate site, followed by exposure to a second ligand possessing four hydroxamate groups. The tetrahydroxamate molecule ligates to the metal ion (bound to the base layer) using two of its four hydroxamate groups and is free to bind a second metal ion at its other end. A sequence of adsorption steps using metal ions and tetrahydroxamate ligands was carried out, resulting in an ordered metal-organic multilayer. Multilayer structures comprising up to 10 tetrahydroxamate/metal ion layers were constructed, with full characterization at each step of multilayer formation using ellipsometry, contact angle measurements, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. The multilayer morphology and mechanical properties were studied by scanning force microscopy. It is shown that different base ligands induce dramatic differences in the morphology and stiffness of the final multilayer. The possibility to construct segmented multilayers containing Zr4+ and Ce4+ ions at defined locations is presented.