Real Time Interrogation of Surface Charge to Investigate Surface Chemistry in Solution

April 7, 2017

Julio Alvarez, Department of Chemistry, Virginia Commonwealth University

The surface charge of a microchannel can be readily determined by measuring the voltage developed at two electrodes located at the outlets of the microchannel, when a liquid solution is pumped through by pressure driven flow. The spontaneous generation of this electrical potential difference, ∆E, also known as streaming potential, is proportional to the liquid pressure and the surface charge. This is a result of the ionic and polar structure of the electrical double layer (EDL) at the surface-solution interface, which can extend up to ~30 nm into the solution. As the forward flow perturbs the local concentration of EDL counterions, a longitudinal gradient of charge is formed thus creating a ∆E along the flow axis. Given that this phenomenon is surface driven and confined within the EDL, the effect is only detectable in micro-cells or conduits with small volume and large surface area. However, the polarity of ∆E is a direct measure of the surface charge in contact with the solution. Values of ∆E can range from a few mV up to several V depending on the pressure.
 
This seminar is about the use of this principle to track real-time adsorption of biomolecules on surfaces and its application to biosensors. For the examples described, the pressure was maintained constant while changes on the surface charge induced by surface sensing and binding were monitored in real time. The most important aspect of this approach is that the analytical signal arises without the need of labeling the probes of interest and kinetic information about surface binding can be extracted. The extension of this technique to study other surface phenomena that rely on changes on the surface charge is briefly discussed.