Study of Surface Charge Instabilities by EOF Measurements on a Chip: A Real-Time Hysteresis and Peptide Adsorption Based Methodology.

This paper describes the measurement of the electroosmotic mobility (EOF) in a Wheatstone fluidic bridge (μFWB) as a direct probe of the surface instability. The variation of EOF known as one major contribution of the electrokinetic migration has been determined with a real-time measurement platform after different conditionings on chips. We also scan the pH of the background electrolytes with three different ionic strengths to evaluate the dependencies of the EOF as a function of the pH. A hysteresis methodology has been developed for probing the surface charge instabilities. EOF mobility has been recorded during on-a-chip electrophoresis to estimate the effect of such instability on the analytical performance. As expected, our experimental curves show that a decrease in the ionic strength increases the surface charge stability of the hybrid microchip. This result demonstrates that ionic exchanges between the surface and the fluid are clearly involved in the stability of the surface charge. With this original method based on real-time EOF measurement, the surface state can be characterized after hydrodynamic and electrophoresis sequences to mimic any liquid conditioning and separation steps. Finally, as a demonstrative application, isotherms of the adsorption of insulin have been recorded showing the change in surface charge by unspecific adsorption of this biomolecule onto the microfluidic channel's wall. These methodologies and findings could be particularly relevant to investigating various analytical pathways and to understanding the molecular mechanisms at solid/liquid interfaces.