Abstract
Recently introduced nanoscale electrokinetic phenomenon called ion concentration polarization (ICP) has been suffered from serious pH changes to the sample fluid. A number of studies have focused on the origin of pH changes and strategies for regulating it. Instead of avoiding pH changes, in this work, we tried to demonstrate new ways to utilize this inevitable pH change. First, one can obtain a well-defined pH gradient in proton-received microchannel by applying a fixed electric current through a proton exchange membrane. Furthermore, one can tune the pH gradient on demand by adjusting the proton mass transportation (i.e., adjusting electric current). Secondly, we demonstrated that the occurrence of ICP can be examined by sensing a surrounding pH of electrolyte solution. When pH > threshold pH, patterned pH-responsive hydrogel inside a straight microchannel acted as a nanojunction to block the microchannel, while it did as a microjunction when pH < threshold pH. In case of forming a nanojunction, electrical current significantly dropped compared to the case of a microjunction. The strategies that presented in this work would be a basis for useful engineering applications such as a localized pH stimulation to biomolecules using tunable pH gradient generation and portable pH sensor with pH-sensitive hydrogel.
Highlights
The theory of electrokinetic transportation in microchannels opens a new era for precise diagnostics of bio- and chemical-substances and environmental monitoring, etc
In order to check the concentration of universal pH indicator, I-V characteristics were measured. −V was applied to the main microchannel from 0 V to 25 V at 0.2 V/20 s, while the buffer microchannel was electrically grounded
ion concentration polarization (ICP) phenomenon has been utilized for various interesting engineering applications using zero-Reynolds number characteristics such as portable scale desalination / purification devices, selective preconcentration devices of biomolecules
Summary
The theory of electrokinetic transportation in microchannels opens a new era for precise diagnostics of bio- and chemical-substances and environmental monitoring, etc. Together with the nanofluidic channels and electricity, new nanoscale electrokinetics phenomena have been actively studied for high efficient (bio-)molecular preconcentrator, portable desalination devices, energy harvesting and fluidic circuit elements. The representing phenomenon is ion concentration polarization (ICP). ICP conventionally referred the polarization of electrolyte concentration at the sides of nanoporous membrane under direct current (dc) bias [1,2]. Since the nanoporous membrane has a perm-selectivity, only counter-ions can pass through the membrane, while co-ions are rejected from the membrane [3]. Note that ICP is a branch of concentration polarization (CP) phenomenon because they share similar physical origins of perm-selective ion transportation through nanoporous membrane.
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