(237f) pH Gradient Formation In An Insulator-Based Dielectrophoresis Device Used In Protein Trapping Applications

Authors: 
Gencoglu, A., Michigan Technological University
Camacho-Alanis, F., Arizona State University
Nakano, A., Arizona State University
Nguyen, V. T., Arizona State University
Ros, A., Arizona State University
Minerick, A., Michigan Technological University


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.