Molecular adsorption at solid/liquid interfaces using self-assembled monolayer films

Abstract

Many areas of technology rely on interfacial events that are controlled by nanometer-level interactions present at solid/liquid interfaces. Properties of wetting, corrosion inhibition, and molecular recognition provide convenient examples. To investigate such interactions at the molecular level, self-assembled monolayers (SAMs) have been employed as a model system as they offer the ability to produce well-defined organic surfaces of controlled composition. This thesis addresses the development and characterization of such films for controlling the adsorptive properties of surfaces toward various surfactant-like molecules and for proteins. Adsorption is controlled to facilitate the organized assembly of molecular precursors, retard the non-specific adsorption of proteins, provide a specificity for the adsorption of select proteins, and the use of molecular adsorption to generate local surface energy gradients useful for directing self propelled drop movement. A common theme in these studies is the importance of controlling the energetics and compositions of surfaces at the molecular level to influence microscopic events that translate into macroscopically observable changes in behavior. The first part of this thesis details the formation of monolayer films by the solution-phase adsorption of n-alkyl-chained adsorbates [CH 3(CH2)~ Y] onto the polar surfaces of terminally substituted SAMs [Au/S(CH)mX]. The polar tail groups (X and Y) of the adsorbate and SAM included amine, carboxylic acid, and amide groups, and the formation of the adsorbed monomolecular films on the SAMs relied on non-covalent interactions between X and Y. Highly organized monomolecular adlayers could be produced that were as densely packed as the alkanethiolate SAMs on gold comprising the first layer. This thesis also used this molecular adsorption process to cause liquid drops to move spontaneously on surfaces by creating local changes in surface energy. The drops could be directed to move along specified paths using patterned substrates that contained inner tracks of polar functionality and exterior domains of oleophobic methyl groups. The adsorption process allowed sequential transport of two drops on a common track and also regeneration of the initial high energy surface for reuse. The developed system provides an experimental platform for examining reactive flow and offers a novel "pumpless" method for sequentially delivering multiple drops along surfaces and within microfluidic devices. The second part of this thesis discusses various oligo(ethylene glycol)-terminated alkyltrichlorosilanes [C13Si(CH2)11(OCH2CHnX; X = -OCH 3 or -O 2CCH 3, n= 2- 4] that can form robust films on glass and metal oxide surfaces and control the adsorption of proteins. The adsorption of the methyl-capped trichlorosilanes produces densely packed, oriented monolayer films that are 2-3 nm in thickness. The trichlorosilyl group anchors the molecules to the surface, and the resulting film exposes the ethylene glycol units at its surface, as noted by its moderate hydrophilicity. The films are robust with stabilities similar to those of other alkylsiloxane coatings. These oligo(ethylene glycol)-terminated silane reagents produce films that exhibit resistances against the non-specific adsorption of proteins and that are better than for films prepared from octadecyltrichlorosilane. These oligo(ethylene glycol)-siloxane coatings offer performance advantages and could easily provide a direct and superior replacement for protocols that presently use silane reagents to generate hydrophobic, "inert" surfaces. This thesis also discusses the development of an acetate-capped oligo(ethylene glycol)-terminated silane to produce a HO-terminated oligo(ethylene glycol)-based coating on glass and metal oxide surfaces. The HO-termini of these films provide sites for covalently grafting biomolecules to the parent surface. As a demonstration, biotin and mannose moieties were covalently attached to the HO-surfaces to provide a means to induce the specific adsorption of proteins. For these surfaces, the presence of oligo(ethylene glycol) groups reduces the nonspecific adsorption of other competing proteins. The results indicate that the developed systems could offer a strategy to arrange biomolecules selectively on glass and metal oxide surfaces.