Grafting of Single, Stimuli-Responsive Poly(ferrocenylsilane) Polymer Chains to Gold Surfaces

Redox-responsive poly(ferrocenylsilane) (PFS) polymer molecules were attached individually to gold surfaces for force spectroscopy experiments on the single molecule level. By grafting ethylenesulfide-functionalized PFS into the defects of preformed self-assembled monolayers (SAMs) of different omega-mercaptoalkanols on Au(111), the surface coverage of PFS macromolecules could be conveniently controlled. Atomic force microscopy (AFM), contact angle, as well as cyclic and differential pulse voltammetry measurements were carried out to characterize the morphology, wettability, and surface coverage of the grafted layers. The values of the PFS surface coverage were found to depend on the chain length of the omega-mercaptoalkanol molecules and on the concentration of the PFS solution but not on the insertion time or on the molar mass of PFS. The equilibrium surface coverages were successfully described by Langmuir adsorption isotherms. For low-surface coverage values (< 6.2 x 10(-4) chain/nm2), achieved by PFS insertion from very dilute solutions (8 x 10(-6) M) into long-chain SAMs, AFM and differential pulse voltammetry showed that surfaces exposing isolated individual polymer chains were obtained. The isolated PFS macromolecules were subjected to in situ AFM-based single molecule force spectroscopy (SMFS) measurements. The single chain elasticity of PFS in isopropanol (and ethanol) was fitted with the modified freely jointed chain (m-FJC) model. This procedure yielded a Kuhn segment length of 0.33 +/- 0.05 nm and a segment elasticity of 32 +/- 5 nN/nm.