HOST-GUEST INTERACTION AT A SELF-ASSEMBLED MONOLAYER SOLUTION INTERFACE - AN ELECTROCHEMICAL ANALYSIS OF THE INCLUSION OF 11-(FERROCENYLCARBONYLOXY)UNDECANETHIOL BY CYCLODEXTRINS/

A host-guest equilibrium involving the redox guest 11-(ferrocenylcarbo nyloxy)undecanethiol (FcSH) in a self-assembled monolayer on gold elec trode surfaces and a beta-cyclodextrin (beta-CD) host in solution is i nvestigated. The effect of varying the ratio of FcSH to a diluent (n-d ecanethiol) in the self-assembly solution on the electrochemical chara cteristics of the ferrocene/ferricenium redox process and the inclusio n properties of FcSH self-assembled monolayers (FcSH-SAMs) is examined using cyclic voltammetry and chronoamperometry. alpha- and gamma-cycl odextrin have a negligible effect on both the thermodynamics and kinet ics of the redox reaction. The electrochemical and ion-pairing behavio r of FcSH-SAMs in solutions containing beta-CD suggests that an inclus ion complex is formed and that the included form of the ferrocenyl gro up is electroinactive in the potential window investigated. Inclusion results in a decrease in cyclic voltammetric peak currents (surface co verage of electroactive FcSH) and a slight increase in the standard el ectron transfer rate constant for the ferrocene/ferricenium redox proc ess. The decrease in surface coverage of electroactive FcSH upon addit ion of beta-CD exhibits a response similar to that expected for a Lang muir adsorption isotherm. Stability constants for the inclusion comple xes of beta-CD and the ferrocenyl or ferricenium forms of an FcSH-SAMs , containing a FcSH surface coverage of 5.2 x 10(-11) mol cm(-2), have been evaluated as 3.6 (0.4) x 10(4) and 3.2 (0.4) x 10(4), respective ly. The stability constants thus evaluated were shown to be dependent on the surface coverage of FcSH in the FcSH-SAMs.