A method for nanofluidic device prototyping using elastomeric collapse

Abstract

Nanofluidics represents a promising solution to problems in fields ranging from biomolecular analysis to optical property tuning. Recently a number of simple nanofluidic fabrication techniques have been introduced that exploit the deformability of elastomeric materials like polydimethylsiloxane (PDMS). These techniques are limited by the complexity of the devices that can be fabricated, which can only create straight or irregular channels normal to the direction of an applied strain. Here, we report a technique for nanofluidic fabrication based on the controlled collapse of microchannel structures. As is demonstrated, this method converts the easy to control vertical dimension of a PDMS mold to the lateral dimension of a nanochannel. We demonstrate here the creation of complex nanochannel structures as small as 60 nm and provide simple design rules for determining the conditions under which nanochannel formation will occur. The applicability of the technique to biomolecular analysis is demonstrated by showing DNA elongation in a nanochannel and a technique for optofluidic surface enhanced Raman detection of nucleic acids.

Fig. 1.

Schematic describing “roof-collapse” technique for nanochannel fabrication. (A) Two-dimensional representation of the fabrication process. For clarity, the thicknesses of the various layers on the substrate are not drawn to scale. A thin layer of photoresist or evaporated metal is deposited on the substrate and lithographically pattered to the desired channel layout. Since the channel size is dependent on the height of the master, the resolution with which the channels can be patterned is not important. Patterning different thicknesses allows interfacing of different channel size. After patterning, the PDMS solution is cast onto the master. The PDMS mold is then removed and bonded to a plain substrate. (B) Three-dimensional representation of the fabrication process. Various pattern shapes can be used as a nanochannel template. “Roof collapse” of the PDMS leaves behind a nanochannel at the edge. Inset indicates triangular shape nanochannels were formed after the roof collapse.

Fig. 2.

SEM images of nanochannels. (A) Bonding of a PDMS (mold) to a PDMS (substrate) results in nanochannels with a triangular cross-section. (B) PDMS (mold)-cover glass (substrate) bonding results in larger channel width due to its elastic moduli mismatch. (C) With a controlled 50 kPa normal stress is applied to a PDMS (mold)-PDMS (substrate) system, much smaller channels result. The same PDMS mold as (A) was used to form the smaller channel shown here. (D) Compressed nanochannel in PDMS (mold)-cover glass (substrate). Again the same PDMS mold as (B) was used.

Fig. 3.

Nanochannel characterizations. (A) Channel width is plotted against master's height for both 10:1 PDMS and 5:1 PDMS. Due to their different elastic moduli, generally 10:1 PDMS nanochannel yields smaller widths than that of 5:1. (B) The nanochannel formation rate is characterized as a function of a/h², for different elastic moduli (5:1, 10:1, and 15:1) and oxidized vs. non-oxidized PDMS. Each dash line represents the “No collapse,” “Metastable collapse,” “Stable collapse,” and “Annihilation” regions. (C) Schematic representations of four regions in (B). Each region is accompanied with its SEM counterpart. The red arrows indicate the location of the nanochannels (Scale bars, 10 μm).

Fig. 4.

Optical images of complex nanochannels (A) and manipulation of a single DNA molecule along a straight nanochannel (B). (A) Nanofluidic representations of “nano” and “NBTC” (the logo of the Nanobiotechnology Center from Cornell University). Complex nanofluidic networks are possible with this technique. All these nanochannels are formed with at least one axis in nanometer regime (Scale bar, 100 and 50 μm, respectively). For a demonstration purpose, each letter is taken individually and then combined together. (B) Electrophoretic migration of the λ-DNA in a straight nanochannel by 40 V cm^-1 (0.5-s interval between the images.) Electric field is represented by E (Scale bar, 5 μm).

Fig. 5.

Interfacing nanofluidics with microfluidics and demonstration of a nanofluidic concentration device. (A) The nanofluidic concentration system used here consists of two functionally interfaced channels: A micrometer scale channel fabricated from a double thickness master and two nanoscale channels fabricated from the collapse of a single layer thickness master. Schematic representation of the operation of the device shows the concentration of nanoparticles at the interface between the microchannel and the nanochannel. (B) SEM images of (–2) the nanofluidic channel (400 nm width × 300 nm height) and (3) microfluidic channel (10 μm width × 1.4 μm height). (C) Time-lapse images of concentration of 700-nm polystyrene nanoparticles at the interface between microchannel and nanochannel.

Fig. 6.

SERS spectra of 3 nM Cy3-labeled DENV-4a. After introducing the 60-nm Au colloid solution by applying electric field of 40 V cm^-1, the SERS emission at the interface between nanochannels and microchannel was monitored as a function of time: (1) 5 s, (2) 10 s, (3) 20 s, (4) 40 s, and (5) 60 s. As expected, the intensity of the signal increases with the accumulation and aggregation of SERS emitters.