Abstract
The in situ control of reversible protein adsorption to a surface is a critical step towards biofouling prevention and finds utilisation in bioanalytical applications. In this work, adsorption of peptides is controlled by employing the electrode potential induced, reversible change of germanium (100) surface termination between a hydrophobic, hydrogen terminated and a hydrophilic, hydroxyl terminated surface. This simple but effective 'smart' interface is used to direct adsorption of two peptides models, representing the naturally highly abundant structural motifs of amphipathic helices and coiled-coils. Their structural similarity coincides with their opposite overall charge and hence allows the examination of the influence of charge and hydrophobicity on adsorption. Polarized attenuated total reflection infrared (ATR-IR) spectroscopy at controlled electrode potential has been used to follow the adsorption process at physiological pH in deuterated buffer. The delicate balance of hydrophobic and electrostatic peptide/surface interactions leads to two different processes upon switching that are both observed in situ: reversible adsorption and reversible reorientation. Negatively charged peptide adsorption can be fully controlled by switching to the hydrophobic interface, while the same switch causes the positively charged, helical peptide to tilt down. This principle can be used for 'smart' adsorption control of a wider variety of proteins and peptides and hence find application, as e.g. a bioanalytical tool or functional biosensor.
Highlights
Electro-responsive interfaces alter their properties in response to an electric potential trigger. Such interfaces are an effective approach to reversibly control protein and peptide adsorption on interfaces. 1–8 such ’smart’ interfaces offer exciting possibilities for applications in, for instance, microfluidics, separation systems, biosensors and -analytics. 9–14 An especially interesting application is the employment of such hydrophobic/hydrophilic switchable interfaces to trigger and study protein adsorption, which can lead to important new insights. 7,8 little is known about the protein dynamics that govern the adsorption and desorption upon the sudden switch of the surface properties
The cyclic voltammogram (CV) shows rising anodic current at ca. −0.3 V assigned to oxide formation. 20–22 Cathodic peaks have been assigned to the change in the surface termination from a hydrophilic OH(OD) to a hydrophobic H(D) termination. 18–20,22 Here an increased cathodic current plateau was observed at ca. −1.0 V along with minor shoulders around −0.75 V and −0.55 V (Figure 2)
The hydrophobicity change induced by the reversible electrochemical termination switch of germanium surfaces –shown here for germanium (100) –in aqueous solutions can be employed as a ’smart’ interface to reversibly trigger hydrophobic adsorption or adsorbate reorientation processes
Summary
Electro-responsive interfaces alter their properties in response to an electric potential trigger. 15,16 on a hydrophobic/hydrophilic switchable interface in specific cases structural changes of adsorbed proteins or even reversible adsorption is to be expected Such interfaces can be prepared for instance from self-assembled monolayers (SAMs) of long chain alkanethiols with charged terminal groups 1,2,17 or brushed polymers. 8 experimental studies that reveal in situ structural details or the dynamics of adsorbed proteins reacting to switchable interfaces are lacking This may be in part owing to the chemical complexity of most of these interfaces, which complicates the interpretation of data from powerful in situ structural analysis methods. To overcome these issues, the (100) surface of germanium is used here, because of its ability to reversibly switch between hydroxylated and hydrogenated surface termination dependent on the applied electrode potential. This method allows in situ quantification, structural analysis and orientational analysis of adsorbates from polarised spectra. 26–28 its applicability in fibre coupled probes, flow cells, micro- or nanochannel cells makes it interesting for smart sensor applications. 29–31
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