Potential-Switchable Viscoelasticity of Protein Nanolayers at a Liquid/Liquid Interface

Protein nanolayers (PNLs) formed at an electrochemical liquid|liquid interface between water (W) and a fluorous solvent (F) were examined by using interfacial rheological measurement (IRM) and neutron reflectometry (NR) under the externally controlled condition of the phase boundary potential differences E_F^W(= φ^W - φ^F + const.), where F contained a hydrophobic ionic liquid (IL) as a supporting electrolyte and W, whose pH was 7.4, contained a protein, bovine serum albumin (BSA). The IRM and NR results illuminated that both static and dynamic properties of the PNL at the electrochemical F|W interface were varied by applying E_F^W. NR found minimal E_F^W dependence on the adsorption amount of BSA in the PNL. In contrast, IRM revealed that although the interfacial shear loss moduli G″ of the PNL was constant regardless of E_F^W, the interfacial shear storage G' of the PNL increased dramatically at more negative E_F^W, showing a more elastic response. This difference between static and dynamic properties results from the increase in intermolecular and intramolecular interactions between BSA molecules in the PNL at more negative E_F^W due to the accelerated denaturation of negatively charged BSA that formed complexes with IL cations accumulated on the F side of the F|W interface. The G' and G″ reversibly responded to switching between different potentials (a positive and a negative E_F^W). These IRM results unveiled that the viscoelasticity of the PNL at the electrochemical F|W interface is reversibly potential-switchable. The present interface-specific method using the potential control is a new promising method to diversify and switch the PNL structure reversibly. The reversible structural control of the PNL would enable us to perform real-time observation of cells reacting to environmental changes at liquid|liquid interfaces.