Merging Ion Concentration Polarization between Juxtaposed Ion Exchange Membranes to Block the Propagation of the Polarization Zone

Summary

The protocol for a novel ion concentration polarization (ICP) platform that can stop the propagation of the ICP zone, regardless of the operating conditions is described. This unique ability of the platform lies in the use of merging ion depletion and enrichment, which are two polarities of the ICP phenomenon.

Abstract

The ion concentration polarization (ICP) phenomenon is one of the most prevailing methods to preconcentrate low-abundance biological samples. The ICP induces a noninvasive region for charged biomolecules (i.e., the ion depletion zone), and targets can be preconcentrated on this region boundary. Despite the high preconcentration performances with ICP, it is difficult to find the operating conditions of non-propagating ion depletion zones. To overcome this narrow operating window, we recently developed a new platform for spatiotemporally fixed preconcentration. Unlike preceding methods that only use ion depletion, this platform also uses the opposite polarity of the ICP (i.e., ion enrichment) to stop the propagation of the ion depletion zone. By confronting the enrichment zone with the depletion zone, the two zones merge together and stop. In this paper, we describe a detailed experimental protocol to build this spatiotemporally defined ICP platform and characterize the preconcentration dynamics of the new platform by comparing them with those of the conventional device. Qualitative ion concentration profiles and current-time responses successfully capture the different dynamics between the merged ICP and the stand-alone ICP. In contrast to the conventional one that can fix the preconcentration location at only ~5 V, the new platform can produce a target-condensed plug at a specific location in the broad ranges of operating conditions: voltage (0.5-100 V), ionic strength (1-100 mM), and pH (3.7-10.3).

Introduction

Ion concentration polarization (ICP) refers to a phenomenon that occurs during ion enrichment and ion depletion on a permselective membrane, resulting in an additional potential drop with ion concentration gradients^1,2. This concentration gradient is linear, and it becomes steeper as a higher voltage is applied (Ohmic regime) until the ion concentration on the membrane approaches zero (limiting regime). At this diffusion-limited condition, the gradient (and corresponding ion flux) has been known to be maximized/saturated¹. Beyond this conventional understanding, when the voltage (or cur....

Protocol

1. Fabrication of Cation Exchange Membrane-integrated Microfluidic Chips

1. Preparation of silicon masters
   1. Design two kinds of silicon masters: one for patterning a cation exchange resin and the other for building a microchannel with polydimethylsiloxane (PDMS).
      NOTE: The detail geometry will be described in the steps 1.3.1 and 1.4.1.
   2. Fabricate the silicon masters by using either conventional photolithography or deep reactive ion etching²⁷.
   3. Silanize the micropatterned silicon masters with trichlorosilane (~30 μL) in a vacuum jar for 30 min.
      CAUTION:

Results

The schematic fabrication steps of a membrane-integrated microfluidic preconcentrator are shown in Figure 1. A detailed description of the fabrication is given in the Protocol. The designs and device images of the spatiotemporally defined preconcentrator²⁶ are contrasted with those of a conventional preconcentrator¹¹ (Figure 2). The ICP phenomenon in the spatiotemporally defined preconcentrator was investiga.......

Discussion

We have described the fabrication protocol and the performance of a spatiotemporally defined preconcentrator in a range of the applied voltage (0.5-100 V), ionic strength (1-100 mM), and pH (3.7-10.3), achieving a 10,000-fold preconcentration of dyes and protein within 10 min. As like previous ICP devices, the preconcentration performance becomes better at higher voltage and at lower ionic strength. One additional parameter we can consider here is the distance between two cation exchange membranes. If we increase the int.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Sylgard 184 Silicone Elastomer kit	Dow Corning
Trichlorosilane	Sigma Aldrich	175552	Highly toxic
Nafion perfluorinated resin, 20 wt%	Sigma Aldrich	527122
Sodium chloride	Sigma Aldrich	71394
Potassium chloride	Sigma Aldrich	60121
Alexa Fluor 488 carboxylic acid, succinimidyl ester	Invitrogen	A20000
Isothiocyanate-conjugated albumin	Sigma Aldrich	A9771
Phosphate buffer saline, 1x	Wengene	LB004-02
Tween 20	Sigma Aldrich	P1379
Epifluorescence microscope	Olympus	IX-71
Charged-coupled device camera	Hamamtsu Co.	ImageEM X2
Source measurement unit	Keithley Instruments	2635A
Covance-MP	Femto Science