Detection of non-specific protein adsorption at artificial surfaces by the use of acoustic plate mode sensors

The interaction of proteins with artificial surfaces is important to many medical and biochemical applications. Such examples involve the incorporation of catheters and prostheses as well as the non-specific adsorption of pharmacological proteins at the walls of a container, which may drastically reduce their activity. A fast analytical tool capable of determining the specific adsorption characteristics at these surfaces would, therefore, support technological progress. Contrary to traditional immunoassays, acoustic wave-based sensors allow an on-line and direct detection of label-free proteins, thus saving time and providing the opportunity to monitor the kinetics of the binding process. In this study, Cr/Au-coated acoustic plate mode (APM) sensors have been used to investigate the interaction of immunoglobulin G (IgG) and fibrinogen with differently terminated self-assembled monolayers (SAMs) of thiols. By this method, both the low affinity of hexa(ethylene glycol)-terminated (HS-(CH₂)₁₁-(O-CH₂-CH₂) ₆-OH) alkanethiol SAMs and the high affinity of methyl-terminated (HS-(CH₂)₁₁-CH₃) surfaces towards protein adsorption were confirmed. It was found that the amount of bound proteins depends on the pH of the solution. At low pH values, protein binding to methyl-terminated surfaces is drastically reduced. The adsorption characteristics of fibrinogen at methyl-terminated surfaces are explained by a kinetic model which involves the initial binding of native proteins and a subsequent unfolding process. Complete regeneration of the sensor element is achieved by the use of sodium dodecylsulfate.