POTENTIAL-DEPENDENT WETTING OF ELECTROACTIVE FERROCENE-TERMINATED ALKANETHIOLATE MONOLAYERS ON GOLD

The electrochemical and electrowetting behavior of ferrocene (Fc)-terminated alkanethiol (FcCO(2)C(11)H(22)SH) monolayers adsorbed on gold is studied in 1 M HClO4. The concentration of the ferrocene groups in the monolayer is varied by diluting the adsorbed ferrocenethiol with alkanethiol (CnH2n+1SH with n = 8, 12, 16, or 22). The Fc group of FcCO(2)C(11)H(22)SH can be oxidized to the Fc(+) cation while leaving the monolayer intact. Upon oxidation, the wettability of the surface increases: the advancing contact angle of the FcCO(2)C(11)H(22)SH monolayer with the electrolyte (theta(a)(red)) decreases from 74 degrees to 49 degrees, the latter being the advancing contact angle of the oxidized Fc(+) cation, theta(a)(ox). Upon renewed reduction the meniscus recedes and the receding contact angle, theta(r)(red), becomes 56 degrees and can be continuously stepped between theta(r)(red) and theta(a)(ox). Due to contact angle hysteresis this potential-dependent change in contact angle is only partly reversible. Within the number of applied steps, a decrease in theta(r)(red) is observed due to disordering of the monolayer on continuous oxidation/reduction. This disruptive effect is reduced for a mixed FcCO(2)C(11)H(22)SH/C12H25SH monolayer. The C12H25SH thiol increases the distance between the bulky Fc groups and consequently the monolayer becomes more stable. For a mixed monolayer with 60% FcCO(2)C(11)H(22)SH and 40% C12H25SH, the contact angle can reversibly be switched between 49 degrees and 59 degrees. We suggest that the change in wettability aff er oxidation/reduction is due to specific interactions of the charged Fc(+) ion with anions from the electrolyte. Due to the hydrophilic nature of the ClO4- anion, the wettability of the surface increases.