pH Gradient Formation In An Insulator-Based Dielectrophoresis Device Used In Protein Trapping Applications

pH Gradient Formation in an Insulator-Based Dielectrophoresis Device Used in Protein Trapping Applications

Aytuğ Gen¨oğlu¹, Fernanda Camacho-Alanis², Vi Thanh Nguyen², Asuka Nakano², Alexandra Ros², Adrienne Minerick¹

1: Department of Chemical Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI, 49931 USA

2: Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ, 85287-1604 USA

Insulator-based direct current (DC) dielectrophoretic (iDEP) microdevices have the potential to replace traditional alternating current (AC) dielectrophoretic devices for many cellular and biomolecular separation applications. These microdevices employ large DC fields, under which the electrode reactions and electrokinetic ion transport mechanisms become significant enough to affect ion distributions in the nanoliters of fluid in the microdevice. The most commonly encountered of these electrode reactions is the electrolysis of water. With most electrode materials, when an electric field is applied, H⁺ and OH^- ions are generated at anode and cathode surfaces, respectively. Both of these ions have high diffusional and electrophoretic mobilities. Therefore, diffusional and electrokinetic transport of these species can be significant enough in micro- and nanoscale systems that H⁺ and OH^- concentration gradients can develop within the microfluidic channel. This leads to what is termed a "natural pH gradient," and can be exploited for applications such as isoelectric focusing (IEF). However, natural pH gradients can also cause unexpected fluid behavior in micro- or nanofluidic systems by causing spatial changes, such as nonuniform wall surface charges. Moreover, the properties of analytes may be dependent on pH and the formation of natural pH gradients may not be desirable.

This work shows the formation of natural pH gradients in an iDEP microdevice with pt wire electrodes, under conditions applied during iDEP protein manipulation experiments. pH changes were observed by measuring the fluorescence intensities of pH sensitive dye FITC Isomer I and the pH insensitive dye TRITC and correlating the FITC/TRITC fluorescence intensity ratio to pH. A dependence of natural pH gradient formation on the phosphate buffer solution concentration was observed under 100 V/cm electric fields. When the channel was filled with relatively low concentration solutions (Conductivity: s=0.01S/m or 0.05 S/m), pH was found to drop below 4 in 5 to 10 minutes. However, this behavior was more consistent in the case of 0.01 S/m solution. pH gradient formation was not observed in the case of a relatively high concentration solution (s=0.10 S/m), due to the higher buffering capacity of the solution. pH was observed to drop dramatically in seconds under 3000 V/cm electric fields with all buffer solutions. It was also observed that when the Pt wire electrodes had been in use for more than 1 hour, pH gradient formation did not occur under 100 V/cm electric fields, even with the 0.01 S/m buffer solution. However, 3000 V/cm electric fields still caused rapid pH changes in the microchannels when the used Pt wire electrodes were employed, regardless of the solution concentration.

This work shows that pH gradients can form in iDEP devices, possibly affecting the operation of such devices. It is shown that pH gradient formation is influenced by electric field strength, buffering capacity of the medium, and the properties of the electrode surface. Based on this work, pH gradient formation can be prevented by using solutions with high buffering capacity, by passivating the electrodes prior to use, or by using electrode materials on which water electrolysis reactions do not occur.