Attachment of amine- and maleimide-containing ferrocene derivatives onto self-assembled alkanethiol and alkanedithiol monolayers: Voltammetric evaluation of cross-linking efficiencies and surface coverage of electroactive groups

Ferrocene derivatives containing primary amines and maleimide groups were attached covalently onto N-hydrosuccinimidyl (NHS)-terminated alkanethiol self-assembled monolayers (SAMs) and SAMs of alkanedithiol. The surface coverage and efficiencies of the two cross-linking reactions were evaluated with cyclic voltammetry. All the ferrocene derivatives attached onto the alkanethiol or alkanedithiol SAMs exhibit reversible redox waves. The surface coverage of the aminated ferrocene groups was compared to that of N-hydrosuccinimidyl (NHS) -terminated alkanethiol SAM. The covalent attachment of beta-ferrocenylethylamine onto a 11,11'-dithio-bis(succinimidylundecanoate) SAM yielded an efficiency as high as 63.1%. The cross-linking efficiency of this reaction was found to increase with the nucleophilicity of the amino groups. SAMs of longer alkyl chains favor the attachment of a greater number of ferrocene derivatives. As for the Michael-type electrophilic addition between the sulfhydryl groups of the alkanedithiol SAMs and the ferrocenyl maleimide, the cross-linking efficiencies were found to range from 6.5% to 25.7%, depending on the alkanedithiol chain length. The difference in the efficiencies between the two types of cross-linking reactions might be partially attributable to the steric hindrance imposed by the SAMs and the relative sizes of the functional groups.