Generation of Natural pH Gradients in Microfluidic Channels for Use in Isoelectric Focusing

Abstract

As a part of an ongoing investigation of the use of isoelectric focusing (IEF) in microfluidic devices, pH gradients were electrochemically formed and optically quantified in microfluidic channels using acid-base indicators. The microchannels consisted of two parallel 40-mm-long electrodes with an interelectrode gap of 2.54 mm; top and bottom transparent windows were separated by 0.2 mm. Gradients in pH were formed as a result of the electrochemical decomposition of water at an applied potential not higher than 2.5 V to avoid generation of gas bubbles. Solutions contained low concentrations of a single buffer. The stability of the pH gradients and their sensitivity to changes in initial conditions were investigated under static (nonflow) conditions. Isoelectric focusing of sample biological analytes, bovine hemoglobin and bovine serum albumin, was performed to illustrate the potential of "microfluidic transverse IEF" for use in continuous concentration and separation systems.

An Analysis of Eliminating Electroosmotic Flow in a Microfluidic PDMS Chip

“…A gold-sputtered electrode design was used successfully by previous a mylar IEF chip design (Macounova, Cabrera et al 2000). A micromachined lift-off mask was fabricated to be placed on the mylar during the sputtering to create a design of the individual electrodes.…”

“…Gold electrodes only allow for a voltage of 2.5V or less to be applied to avoid the formation of bubbles. Using platinum or palladium electrodes could enable the application of a higher voltage (Macounova, Cabrera et al 2000).…”

“…Macounova, Cabrera et al created a chip in which the electrodes were placed paralelly directly in the separation chamber (see Section 1.2.7.1) (Macounova, Cabrera et al 2000). Despite the low potential that was possible to be used, the analytes adhered directly to the electrodes.…”

“…Since there was no physical barrier between the electrode and separation chamber, only 2.5V needed to be applied to the device without losing any potential across a separation barrier. Due to the low potential, no bubbles due to Joule heating were formed in the separation chamber (Macounova, Cabrera et al 2000).…”

“…Also, the surface-to-volume ratio would allow for heat generated by the electrodes to dissipate quickly and not inhibit the experiment in the separation chamber (Fonslow, Barocas et al 2006). The lack of a high amount of heat generated by the electrodes, and the thin geometry of the chamber decreases turbulent flow, and would improve the resolution of the experiment (Macounova, Cabrera et al 2000).…”