Preparation and characterization of dendrimer monolayers and dendrimer-alkanethiol mixed monolayers adsorbed to gold

Single-component monolayers of dendrimers and two-component monolayers consisting of dendrimers and n-alkanethiols immobilized on Au substrates are described. Single-component monolayers are prepared by exposing an Au substrate to ethanolic solutions of amine-or hydroxy-terminated polyamidoamine (PAMAM) dendrimers. The resulting monolayers are highly stable and nearly close-packed for dendrimer generations ranging from 4 to 8 (G4-G8). Electrochemical ac-impedance measurements indicate that the dendrimer surface is very porous toward the electroactive redox couple Fe(CN)(6)(3-/4-). Ferrocene-terminated dendrimer monolayers have also been investigated. Exposure of higher-generation dendrimer monolayers to ethanolic solutions of hexadecanethiol (C16SH) results in a dramatic compression of the dendrimers, and causes them to reorient on the surface from an oblate to prolate configuration. The dendrimers originally present on the surface do not desorb as a consequence of this configurational change. Comparison of the extent of adsorption of C16SH in different media (vapor-phase N-2, hexane, and ethanol) shows that solvation of the dendrimers is the primary driving force for the structural change. Finally, the reactivity and stability of the dendrimer monolayers is investigated by on-surface functionalization of the dendrimer monolayer with 4-(trifluoromethyl)benzoyl chloride. The physical and chemical properties of the single- and two-component monolayers are evaluated by using reflection infrared spectroscopy, ellipsometry, contact-angle measurements, ac-impedance spectroscopy, cyclic voltammetry, and surface acoustic wave (SAW)-based analyte-dosing experiments.