Self-Assembled Monolayers Generated from Cholesterol-Based Thiols

Self-assembled monolayers (SAMs) generated from adsorption of the single-chained cholesterol-based and unsymmetrical double-chained cholesterol-based thiols on gold substrates were examined. The single-chained cholesterol-based thiols have a thiol headgroup, hydrocarbon spacer consisting of 3 – 12 methylene units, and a cholesterol tailgroup. The double-chained cholesterol-based thiols consist of a thiol headgroup and two tailgroups: an alkyl chain and a cholesterol-based moiety. Two series of single-component SAMs were formed from the cholesterol-based thiols. Analysis of the pure SAM shows that the added methylene spacer for the single-chained thiols allows for the formation of a better monolayer than that formed from thiocholesterol. The pure SAMs formed from the double-chained cholesterol-based thiols also produce a monolayer that is denser than the SAM generated from thiocholesterol, with interfacial properties resembling a SAM formed from normal alkanethiols. A series of binary SAMs generated from the single-chained cholesterol-based thiols and n-octadecanethiol exhibit properties of the cholesterol condensing effect. The SAMs were characterized using ellipsometry, contact angle goniometry, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). Overall, these cholesterol-based adsorbates were used to generate dense monolayers despite a mixed composition of cholesterol moieties and trans-extended alkyl chains. The cholesterol condensing effect was observed in self-assembled monolayers for the first time.