ABSTRACT
In this short review, I summarize the literature on a class of self-assembled monolayers (SAMs) that exhibit nonfouling surface properties when exposed to biological media. My focus will be on semicrystalline oligo(ethylene glycol) (OEG)-functionalized SAMs on gold substrates, which are the most thoroughly studied nonfouling molecular monolayers. In this review I will first summarize briefly the various SAM architectures containing OEG moieties which inhibit irreversible protein adsorption, and the spectroscopic characterization of these films, giving their lateral density, thickness, and conformation of the molecular entities. In the second part the focus will be on experimental and theoretical work that was conducted with the specific aim to gain a mechanistic understanding
of protein and biofouling resistance. Finally, in reply to the frequent statement made in the literature that the “mechanism by which OEG SAMs resist protein adsorption is still poorly understood,” I will summarize the present state of knowledge on the mechanism of fouling resistance and invite the reader to apply Ocam’s razor1 to derive his or her own conclusion. 3.1 Introduction The understanding of the interaction of biomolecules-and eukaryotic and prokaryotic cells-with various surfaces is essential for developing new materials for biomedical and environmental applications. Of particular interest are surfaces that reduce-if not prevent-biofouling, that is, resist the irreversible adsorption of biomolecules and adhesion of cells and bacteria. Alkanethiolate SAMs provide an universal toolbox for modifying surface properties as reviewed by Ulman2 and more recently by Love.3 These monolayer films allow reproducible access to functionalized interfaces and the application of a large range of surface analytical techniques to verify their successful assembly and to characterize their properties. Moreover, one major advantage is that the complexity in trying to disentangle surface chemistry and elastic properties is reduced, since in contrast to polymeric bulk materials the elastic modulus (which is a very important parameter in cell adhesion and foul-release coatings) of the SAM-coated surface is dominated by the underlying substrate. The first systematic search and classification of nonfouling SAM surfaces was carried out by Whitesides et al.4-6, who concluded that all hydrogen bridge bond acceptors promote nonfouling properties, whereas hydrogen bridge bond donors are not effective. This has, however, been shown to be a rule with exceptions, since mannose-functionalized surfaces also show protein-resistant properties.7 The work by Whitesides et al. guided most of the later studies on nonfouling chemistries, such as phosphocholine, polysaccharides, and other zwitterionic surface functions8-10 in SAMs and in surface-grafted polymeric systems. Several older experiments to control biofouling were repeated with well-defined SAM surfaces and chemical gradients made by self-assembly.
