Reactivity of self-assembled monolayers: formation of organized amino functionalities

The reactivity of self-assembled monolayers (SAMs) of mercaptodocosanol (MDO) on gold has been screened by a variety of wet chemical reactions. Changes in thickness and composition were monitored using X-ray photoelectron spectroscopy (XPS). The order and tilt angles were studied using near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Sulfonyl chlorides could not be reacted to form sulfonates, and p-toluenesulfonyl chloride (TosCl) at longer reaction times even resulted in a partial desorption of the alkanethiol film. A coupling reaction of silanes to the hydrophilic surface was very much dependent on the reagent. Whereas chlorodimethylphenylsilane (CPS) at elongated reaction times leads to an almost complete removal of the monolayer, p-aminophenyltrimethoxysilane (APTS) attaches to the OH terminated surface. From the orientation analysis a broad distribution of tilt angles of the aromatic moiety is concluded. Attachment of the APTS moieties induces disorder in the thiol layer. The average tilt angle of the hydrocarbon chains of the MDO layer changes from 30° for the unperturbed, well-ordered layer to 44° for the reacted film and indicates an increasing number of gauche conformations.