Self-Assembled Monolayers as Models for Studying Protein Adsorption to Polymer Surfaces
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SELF-ASSEMBLED MONOLAYERS AS MODELS FOR STUDYING PROTEIN ADSORPTION TO POLYMER SURFACES KEVIN L. PRIME AND GEORGE M. WHITESIDES* Department of Chemistry, Harvard University, Cambridge, MA 02138
ABSTRACT Self-assembled monolayers (SAMs) of functionalized alkanethiolates on gold are a wellcharacterized system for studying the interfacial properties of organic materials. We have used SAMs as models for the surfaces of organic polymers and used them to study the adsorption of proteins onto organic materials. We have formed SAMs from mixtures of alkanethiols in which one alkanethiol is hydrophobic and the other is terminated by a short (2•5 n < 17) oligomer of poly(ethylene oxide). These "mixed" SAMs effectively resist the adsorption of fibrinogen from moderately concentrated (1 mg/mL) solutions. Protein adsorption begins when • 5% of the accessible area of the surface consists of hydrophobic groups. These findings suggest that real protein-resistant monolayers must present an almost defect-free surface of oligo(ethylene oxide) groups in order to eliminate adsorption.
[NTRODUCTION When long-chain alkanethiols [HS(CH 2)aR, n Ž 10, R no larger than the cross-section of the polymethylene chain] adsorb from solution onto the surface of a metal (Au, Ag, Cu), they create an oriented, ordered monomolecular film [1, 2]. The properties of the interface between such a selfassembled monolayer (SAM) and the water or air in contact with it are dominated by the properties of the tail group, R [3]. The surface-air interface of the SAM can be varied widely by changing the tail groups, either by synthesis prior to assembly, or by the co-adsorption of two or more alkanethiols from solutions containing mixtures of alkanethiols [4-6]. The Au-S bond, in particular, is very :specific: the Au-S bond is formed preferentially in the presence of a wide variety of tail groups, .including many that are of biological importance (carboxylic acids [7], alcohols [3], amides [8], carbohydrates [9], etc.). SAMs are, therefore, potentially useful systems for studying the interactions of biological materials with the surfaces of organic solids. We have reported [9] that SAMs can be used to study the non-specific adsorption of proteins to organic surfaces, a phenomenon of practical significance: non-specific adsorption has been implicated as one cause of failure in biomedical materials [10, 11] and in the fouling of membranes and other separation devices [12]. Furthermore, the design of sensors for use in biological milieux "requiresthat these sensors not be fouled by the adsorption of undesired proteins. In the last decade, several groups have shown that the amount of protein adsorbed to bulk hydrophobic polymers is dramatically reduced by grafting poly(ethylene oxide) (PEO) chains to the surface of the polymer [13-19]. The PEO is usually attached to the the surface at one end of the chain. We recently demonstrated that SAMs comprising mixtures of two alkanethiols ("mixed" SAMs) in which one component bore a hydrophobic tail group [HS(CH2)t0CH3] and the
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