Paper BI-TuP10
Specific versus Non-Specific Protein Adsorption: Effects of Chain Length and Tailgroup in Functionalized Poly(ethylene glycol)-Terminated Self-Assembled Monolayers

Tuesday, October 21, 2008, 6:30 pm, Room Hall D

In recent years, substantial efforts have been made to develop surface coatings which limit or even suppress non-specific adsorption of proteins. While for many technological applications the surfaces are designed to exclusively repel proteins, an even more complex situation exists in biomedical analysis, where immobilized probe molecules, e.g. antibodies, are used to specifically bind target proteins from solution. Here, the coating must fulfil a two-fold function: (i) effectively suppress non-specific adsorption processes, which may both result in false interpretation of binding events and significantly lower the detection limit of the analytical techniques, and (ii) additionally facilitate the integration of the probe molecules without loss of non-specific protein repulsion. Hereby the question arises, to what extent efficient immobilization of probe molecules and suppression of non-specific interactions can actually be achieved with a single material as the repulsion mechanism inhibiting non-specific adsorption of biomolecules might also compromise the integration of the probes. In a recent study we have shown that COOH-functionalized poly(ethylene glycol) (PEG) alkanethiolate self-assembled monolayers (SAMs) [HS-(CH₂)₁₁-(OCH₂CH₂)_n-COOH] with a mean number, n, of 33 EG units suppress non-specific protein adsorption while facilitating covalent coupling of antibodies via the terminal COOH-group.¹ We now synthesized the same type of molecule with various EG chain lengths (n = 13-40) and different tailgroups (-OH, -NH₂, and -COOH), and compared both the antigen binding capacity and the protein resistance of the corresponding SAMs by ellipsometry, X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (FT-IRRAS), and enzyme-linked immunosorbent assays (ELISA). It is observed that protein repulsion depends on the tailgroup selected and increases with increasing EG chain length. In parallel, however, the maximum amount of probe molecules that can be coupled to the films significantly decreases with enhanced EG content, thus, reducing the antigen binding capacity of the films. The results, therefore, show that for bioanalytical applications the number of EG units has to be properly adjusted in order to obtain an optimum signal-to-noise-ratio. The best performance has been observed for a chain length of about 30 EG moieties.

¹ S. Herrwerth et al., Langmuir 2003, 19, 1880.