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

Published: March 13th, 2016



1Division of Engineering, New York University Abu Dhabi (NYUAD), 2Department of Chemical and Biomolecular Engineering, New York University Tandon School of Engineering, 3Newomics, Inc., 4Department of Electrical Engineering and Computer Science, Department of Biological Engineering, MIT

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.

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.

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....

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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 concen.......

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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 nanof.......

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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.......

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This work was supported by NIH R21 EB008177-01A2 and New York University Abu Dhabi (NYUAD) Research Enhancement Fund 2013. We express our thanks to the technical staff of MIT MTL for their support during microfabrication and James Weston and Nikolas Giakoumidis of NYUAD for their support in taking SEM pictures and building a voltage divider, respectively. The device fabrication in PDMS was conducted in the microfabrication core facility of NYUAD. Lastly, we would like to thank Rebecca Pittam from the NYUAD Center for Digital Scholarship for video shooting and editing.


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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
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

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