Quantification of redox-induced thickness changes of 11-ferrocenylundecanethiol self-assembled monolayers by electrochemical surface plasmon resonance

Redox-induced orientation changes (or monolayer thickness changes) of self-assembled monolayers (SAMs) of 11-ferrocenylundecanethiol (FcC(11)SH) were quantified by the electrochemical surface plasmon resonance (EC-SPR). EC-SPR enables one to determine the collective effect of the monolayer thickness and refractive index changes resulted from the oxidation of ferrocene (Fc) to ferrocenium. To measure the monolayer volume variation associated with the molecular orientation change, an electrochemical quartz crystal microbalance (EQCM) was used to determine the total number of water molecules accompanying with the ion-pairing between the ferrocenium cation and the counteranion in the solution. With the maximum void space within the SAM for water incorporation known, the Lorentz-Lorenz equation was used to correlate the SPR dip shift to the maximum monolayer thickness variation. In the presence of 0.1 M HClO4 and 0.1 M HNO3, the monolayer thickness changes were deduced to be 0.09 and 0.08 nm, respectively. Thus, upon electrochemical oxidation of the FcC(11)SH SAM, the swinging of the alkyl chain farther away from the electrode (Ye et al., Langmuir, 1997, 13, 3157) or the rotation or flipping of the Fc cyclopentyldiene ring around the bond between the Fc group and the alkyl chain (Viana et al., J. Electroanal. Chem. 2001, 500, 290) can both lead to the observed film thickness changes, with the former probably being the more important process.