Abstract
The ion depletion zone of ion concentration polarization has a strong potential to act as an immaterial barrier, separating delicate submicron substances, including biomolecules, without causing physical damage. However, the detailed mechanisms of the barrier effect remain incompletely understood because it is difficult to visualize the linked behavior of protons, cations, anions, and charged molecules in the thin ion depletion zone. In this study, pH distribution in an ion depletion zone was measured to estimate the role of proton behavior. This was done in order to use it as a tool with good controllability for biomolecule handling in the future. As a result, a unique pH peak was observed at several micrometers distance from the microchannel wall. The position of the peak appeared to be in agreement with the boundary of the ion depletion zone. From this agreement, it is expected that the pH peak has a causal connection to the barrier effect of the ion depletion zone.
Highlights
Understanding the ionic characteristics of molecules in solution is a useful tool for their handling, such as in concentration and separation
The pH distribution around an ion depletion zone in a microchannel was measured by a dual excitation ratio method with fluorescein isothiocyanate (FITC) to estimate proton behavior
In a microchannel of PDMS without Ion concentration polarization (ICP), pH is slightly decreased near the microchannel wall due to the electric double layer
Summary
Understanding the ionic characteristics of molecules in solution is a useful tool for their handling, such as in concentration and separation. Ion concentration polarization (ICP), which can be used by applying voltage to a solution across an ion-exchange membrane, is a well-known and convenient concentration technique for dialysis [5,6,7]. This concentration technique, but another technique based on an almost negligible phenomenon called ion depletion, which occurs near a membrane under steady-state ICP, have attracted attention [8,9]. The microscale barrier can be effectively employed by building an ICP system into a microfluidic device, in which a microchannel provides sufficient space for using the valid range of the barrier [13]
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