Electrochemistry of Redox-Active Guest Molecules at β-Cyclodextrin-Functionalized Silicon Electrodes

Functionalization of silicon-based sensing devices with self-assembled receptor monolayers offers flexibility and specificity towards the requested analyte as well as the possibility of sensor reuse. As electrical sensor performance is determined by electron transfer, we functionalized H-terminated silicon substrates with β-cyclodextrin (β-CD) molecules to investigate the electronic coupling between these host monolayers and the substrate. A trivalent (one ferrocene and two adamantyl moieties), redox-active guest was bound to the β-CD surface with a coverage of about 10⁻¹¹ mol/cm² and an overall binding constant of 1.5⋅10⁹ M⁻¹. This packing density of the host monolayers on silicon is lower than that for similar β-CD monolayers on gold. The monolayers were comparable on low-doped p-type and highly doped p++ substrates regarding their packing density and the extent of oxide formation. Nonetheless, the electron transfer was more favorable on p++ substrates, as shown by the lower values of the peak splitting and peak widths in the cyclic voltammograms. These results show that the electron-transfer rate on the host monolayers is not only determined by the composition of the monolayer, but also by the doping level of the substrate.