Controlling Protein Ad-and Desorption by an Electrical Switch on Polymer-Coated Carbon Electrodes

Capacitive Deionization (CDI) has been widely researched and applied to remove salt ions from brackish water in an energy-efficient way. During operation, two charged electrodes electrostatically attract ions, which are released again to the bulk solution once the potential is removed, resulting in a brine and freshwater solution. Carbon is generally used as electrode material, because of its high specific surface area and electrical conductivity. Building upon this principle, our research focuses on the separation of whey protein isolate (WPI) using a similar setup. We have successfully achieved reversible adsorption of up to 10 mg of protein per gram of electrode material on polyelectrolyte-covered carbon electrodes.<br/>To achieve this, positively and negatively charged polyelectrolytes are attached to the working and counter electrode, respectively, to create a surface charge in the absence of an electric field. The created surface charge enables spontaneous adsorption on the working electrode by electrostatic attraction with the negatively charged protein. Protein desorption is initiated by an electric field of -1.2 V, which neutralizes the positive chemical charges of the polyelectrolyte-covered working electrode through negative electronic charges. The negatively charged polyelectrolyte on the counter electrode prevents re-adsorption during this desorption phase. This set-up, with a spontaneous adsorption step, can be referred to as inverted capacitive deionization (iCDI). Adsorption is controlled by diffusion while desorption is mainly migration-driven due to the applied electric field, causing desorption to occur faster than adsorption.<br/>Interestingly, salt adsorption and desorption occur in phases opposite to those of protein adsorption and desorption. This unique behaviour allows for simultaneous concentration and desalination of the protein solution. Furthermore, we have successfully enriched β-lactoglobulin during the active desorption phase, which represents a significant step towards a separation system with high capacity and selectivity.<br/>Looking ahead, we envision the potential of implementing stimuli-responsive polymers in the porous carbon structure to improve both the selectivity and reversibility of these electrically-driven separation systems. Stimuli-responsive polymers exhibit unique properties, such as the ability to reversibly alter their charge, conformation, and/or wettability in response to an external stimulus (e.g., an electric field). This capability provides greater control over the charge distribution and surface properties of the electrodes, and consequently allows tuning of the interaction forces with the proteins. Our research highlights that a comprehensive understanding of the surface properties and the underlying interaction forces is essential in selecting an appropriate responsive polymer coating for proteins or other target molecules.<br/><br/>The ultimate objective is to combine stimuli-responsive polymers with porous carbon structure, exploiting their respective strengths, to develop a separation system characterized by a high adsorption capacity, reversibility, and selectivity. While the focus of the current system is on the food industry, its potential applications extend to other industries, including biotechnology, wastewater treatment and the biomedical industry.