Creating Sub-50 Nm Nanofluidic Junctions in PDMS Microfluidic Chip via Self-Assembly Process of Colloidal Particles

Summary

We propose a simple self-assembly technique of silica colloidal nanoparticles to create a nanofluidic junction between two microchannels in polydimethylsiloxane (PDMS). Using this technique, a nanoporous bead membrane with a pore size down to ~45 nm was built inside a microchannel and applied to electrokinetic preconcentration of DNA samples.

Abstract

Polydimethylsiloxane (PDMS) is the prevailing building material to make microfluidic devices due to its ease of molding and bonding as well as its transparency. Due to the softness of the PDMS material, however, it is challenging to use PDMS for building nanochannels. The channels tend to collapse easily during plasma bonding. In this paper, we present an evaporation-driven self-assembly method of silica colloidal nanoparticles to create nanofluidic junctions with sub-50 nm pores between two microchannels. The pore size as well as the surface charge of the nanofluidic junction is tunable simply by changing the colloidal silica bead size and surface functionalization outside of the assembled microfluidic device in a vial before the self-assembly process. Using the self-assembly of nanoparticles with a bead size of 300 nm, 500 nm, and 900 nm, it was possible to fabricate a porous membrane with a pore size of ~45 nm, ~75 nm and ~135 nm, respectively. Under electrical potential, this nanoporous membrane initiated ion concentration polarization (ICP) acting as a cation-selective membrane to concentrate DNA by ~1,700 times within 15 min. This non-lithographic nanofabrication process opens up a new opportunity to build a tunable nanofluidic junction for the study of nanoscale transport processes of ions and molecules inside a PDMS microfluidic chip.

Introduction

Nanofluidics is an emerging research area of µTAS (Micro Total Analysis Systems) to study biological processes or transport phenomena of ions and molecules at the length scale of 10¹- 10² nm. With the advent of the nanofluidic tools such as nanochannels, transport processes of molecules and ions can be monitored with unprecedented precision and manipulated, if needed, by exploiting features that are available only at this length scale for separation and detection.^1,2 One of these characteristic nanoscale features is a high ratio of surface to bulk charge (or Dukhin number) in nanochannels that can cause a charge imbalance....

Protocol

1. Preparation of the Silica Colloidal Bead Suspensions

1. Preparation of 300 nm and 500 nm silica bead suspensions
   1. Vortex the silica bead stock suspension (10% w/v in water) for 30 sec. to obtain a homogeneous suspension. Pipette a total of 600 µl stock suspension into a 1.5 ml tube and centrifuge it at 2,600 x g for 1 min.
   2. Substitute the supernatant with 400 µl of 1 mM sodium phosphate buffer (PB, pH 7.0).
   3. Suspend the silica beads into a final concentration of 15% in 1 mM sodium phosphate solution at pH 7.0 through vortexing.
2. Surface functionalize 500 nm silica carb....

Results

An electrokinetic concentrator chip in PDMS that contains a self-assembled nanofluidic junction between two microchannels is shown in Figure 1A). The channel in the middle of the device is filled with a DNA sample solution and flanked by two buffer solution channels on each side via a 50 µm wide bead delivery channel (Figure 1B). The silica colloidal suspension is flown into the bead delivery channel immediately after plasma bonding to create a nanofluidi.......

Discussion

Following the common device design scheme to study nanofluidics, we fabricated a nanofluidic junction between two microfluidic channels by using the evaporation-driven self-assembly of colloidal nanoparticles instead of lithographically patterning an array of nanochannels. When flowing the colloidal particles into the bead delivery channel, an array of nanotraps with a depth of 700 nm and a width of 2 µm on both sides of the bead delivery channel at a total width of 100 μm prevented the bead suspensio.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Poly(Styrenesulfonic Acid) Sodium Salt	Polysciences	08772
Poly(allylamine) Solution	Sigma Aldrich	479144-5G
Silica Microsphere - 300 nm	Polysciences	24321
Silica Microsphere - 500 nm	Polysciences	24323
Silica Microsphere Carboxyl Functional - 500 nm	Polysciences	24753
Silica Microsphere Amine Functional - 500 nm	Polysciences	24756
Sylgard 184 Silicone Elastomer kit	Dow Corning
Trichlorosilane	Sigma Aldrich	175552
Ultrasonic Cleaner	Branson	3510
Tube Rotator	VWR	10136-084
Vortex Mixer	WiseMix	VM-10
Microcentrifuge	VWR	Micro 1207
Plasma Cleaner	Harrick Plasma	PDC-001-HP
PDMS Mixer	Thinky	ARE-250
Oven	Thermo Scientific	PR305220M
Epi-fluorescence Microscope	Nikon	Eclipse Ti
CCD Camera	Andor	Clara
Platinum Electrodes	Alfa Aesar	43014
Source Meter	Keithley	2400
Digital Multimeter	Extech	410
Microscopy Glass Slides	Thermo Scientific	2951-001
Tween 20	Merck Millipore	822184
Sodium chloride	Fisher Scientific	7646-14-5
Sodium phosphate monobasic	Sigma Aldrich	71505
Sodium phosphate dibasic	Sigma Aldrich	S3264
DNA	IDT		CAA CCG ATG CCA CAT CAT TAG CTA C
B-Phycoerythrin	Life Technologies	P-800
Dynamic light scattering system for Zeta Potential Measurement	Malvern	Zetasizer Nano S
Photoresist	Shipley	SPR700-1.0
Projection lithography	Nikon	NSR2005i9
Reactive Ion Etcher	Applied Materials	AME P5000
ICP deep reactive ion etcher	STS	STS-6"
Contact lithography	Electronic Visions	EV620
Photoresist Coater Developer	SSI	SSI 150
Non-contact surface profiler	Wyko	NT 9800