Impedance and QCM analysis of the protein resistance of self-assembled PEGylated alkanethiol layers on gold

In this study, we describe the formation and characterization of self-assembled layers of undecanthiol with poly(ethylene glycol) (PEG) moieties of 350 Da (PEG350-undecanthiol) and 2000 Da (PEG2000-undecanthiol) on gold surfaces. The functionalized surfaces were investigated by means of electrical impedance spectroscopy, which allows calculating the surface coverage from the obtained capacitance values of the formed layers. For both, PEG350-undecanthiol and PEG2000-undecanthiol layers, a surface coverage well above 90% was obtained. Protein resistance of those layers was investigated using the quartz crystal microbalance (QCM) technique, which enables one to monitor protein adsorption label free and in a time resolved manner. The change in resonance frequency of the quartz plate was monitored upon addition of fetal bovine serum indicating that PEG-functionalized surfaces are partly protein resistant compared to hydroxyundecanthiol- and non-functionalized gold surfaces. From QCM experiments, where only a single protein component was added to the PEG-functionalized gold surface, we conclude that the surfaces are fully resistant against serum albumins, while the main protein components that adsorb are globulins. A kinetic analysis reveals that PEG modified gold surfaces do not only significantly diminish the overall amount of bound protein but also significantly slows down the adsorption process.