Nanofluidic Devices and Their Applications

Nanofluidic Devices and Their Applications Patrick Abgrall* and Nam Trung Nguyen Singapore-MIT Alliance/School of Mechanical and Aerospace Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798 Recent developments in micro- and nanotechnologies Excellent reviews have been already dedicated to nanofluidics3,4 made possible the fabrication of devices integrating a and the fabrication of nanochannels.5,6 Practically, a technology deterministic network of nanochannels, i.e., with at least is chosen according to the geometry of a device and its application. one dimension in a range from 1 to 100 nm. The In this perspective, we choose to present the different technologies proximity of this dimension and the Debye length, the size available according to geometrical parameters to assist researchers of biomolecules such as DNA or proteins, or even the slip toward the most adequate choice. A second objective is to length, added to the excellent control on the geometry highlight the differences between micro- and nanofluidics and the gives unique features to nanofluidic devices. This new new opportunities offered by this emerging field. class of devices not only finds applications wherever less According to the main objectives stated above, this perspective well-defined porous media, such as electrophoresis gels, article consists of two parts: (i) fabrication technologies for have been traditionally used but also give a new insight nanochannels and (ii) nanofluidic applications. The first part aims into the sieving mechanisms of biomolecules and the fluid to help readers to navigate through the large number of techno- flow at the nanoscale. Beyond this, the control on the logical choices. The applications highlighted in the second part geometry allows smarter design resulting, among others, illustrate how nanofluidic systems and devices are powerful tools, in new separation principles by taking advantage of the not only to study fundamental nanoscale science but also for anisotropy. This perspective gives an overview on the practical biological and chemical analysis. Finally, in the conclu- fabrication technologies of nanofluidic devices and their sion we discuss the challenges ahead and the future applications applications. In the first part, the current state of the art of nanofluidics. of nanofluidic fabrication is presented. The second part first discusses the key transport phenomena in nanochan- FABRICATION TECHNOLOGIES nels. Current applications of nanofluidic devices are next discussed. Finally, future challenges and possible ap- Definitions. Nanochannels are channels with at least one plications are highlighted. dimension in the nanoscale which is usually defined in this field as the range from 1 to 100 nm, while the term submicrometer has been reserved for objects in the range from 100 nm to 1 µm. Nanofluidics has emerged as a discipline of science and Figure 1 defines the two parameters used for our classification, engineering, where a fluid flows in structures with at least one namely, the aspect ratio (AR) between the height and the width transversal dimension approaching the nanometer range. Al- of the cross section and the dimensionality of the channel network. though, transport phenomena of fluids at the nanocale have Numerous nanofluidic devices are based on low AR or planar already been studied in the past, the terminology of nanofluidics nanochannels. The term 1D (one-dimensional) has also been used has been appearing and becoming popular only in the past few but may lead to confusion. Keeping the width in the micrometer years. This growing interest stems from the new opportunities scale allows the use of standard photolithography while nanometer offered by micro- and nanotechnologies. While known materials scale in the depth can easily be obtained using well-defined thin such as zeolites naturally include a random network of pores, new film deposition and etching methods. Planar nanochannels have technologies allow the fabrication of well-defined, deterministic also the advantage of a higher flow rate for an equivalent pressure networks of nanochannels. This was well illustrated by the works compared to square nanochannels with two dimensions in the 1 conducted in the group of Austin at Princeton aiming to create nanometer range and an AR close to 1. On the other hand, the an ideal porous medium, as a substitute for the gel in electro- fabrication of square nanochannels involves the use of specific phoresis. Beyond this aspect, integration of more complex nanopatterning techniques, which are more or less complicated functions becomes feasible, as it was demonstrated with the and expensive depending on the dimensionality of the network. realization of a nanofluidic transistor by Gajar and Geis from MIT From the technological point of view, high aspect ratio (HAR) reported as early as 1992.2 (3) Eijkel, J. C. T.; van den Berg, A. Microfluid. Nanofluid. 2005, 1, 249-267. * Corresponding author. Phone: +65-6790-6174. Fax: +65-6792-4062. E- (4) Yuan, Z.; Garcia, A. L.; Lopez, G. P.; Petsev, D. N. Electrophoresis 2007, mail: [email protected] 28, 595-610. (1) Volkmuth, W. D.; Austin, R. H. Nature 1992, 358, 600-602. (5) Mijatovic, D.; Eijkel, J. C. T.; van den Berg, A. Lab Chip 2005, 5, 492-500. (2) Gajar, S. A.; Geis, M. W. J. Electrochem. Soc. 1992, 139, 2833-2840. (6) Perry, J. L.; Kandlikar, S. G. Microfluid. Nanofluid. 2006, 2, 185-193. 10.1021/ac702296u CCC: $40.75 © xxxx American Chemical Society Analytical Chemistry A Published on Web 03/06/2008 PAGE EST: 15.3 Figure 1. Geometrical definitions used in this article: 1D and 2D networks of planar, square, and high aspect ratio (HAR) nanochannels. nanochannels are even more challenging but would allow a higher level of integration and a higher throughput than low AR nanochannels. Beyond the AR and the dimensionality, the structural material and the length will also affect the choice of the right fabrication technology. Long channels usually necessitate a very high homogeneity of the depth and are often fabricated using bonding techniques rather than time-consuming sacrificial techniques, which additionally may lead to a slight tapering.7 As for the materials, silicon is well established and benefits from the long experience gained in microelectronics over the last 50 years. Silicon dioxide channels are naturally hydrophilic and can be simply filled by capillarity. Furthermore surface chemistry of silica is well-known and can be conveniently controlled using alkylsilane- based self-assembled monolayers (SAM).8 Polymers represent an attractive alternative due to the wide range of properties available (e.g., optical, mechanical). The lower cost associated with polymer molding techniques along with the lower price of some common polymers are also advantages for an eventual marketing of Figure 2. Fabrication of planar nanochannels in silicon/glass technology. (1) Bulk micromachining: nanochannels (b), microchan- nanofluidic devices. nels, and access holes (c) are etched in the substrate, then closed The Simplest Case: Planar Nanochannels. Because the by a second substrate (d). (2) Spacer technique: a thin layer is lateral dimension can be defined by conventional ultraviolet (UV) deposited and patterned (b), microchannels and access holes (c) are photolithography, patterning of 1D and 2D networks of planar etched in the substrate, then closed by a second substrate (d).20 (3) nanochannels is not a big issue. The challenge is then to avoid Sacrificial technique: a thin layer is deposited and patterned (b), a structural layer is deposited and patterned, then the sacrificial layer the collapse of very wide and shallow channels. This collapse can is etched away (d). be due to the process itself or other effects such as capillary and van der Waals forces. Similar situations have been encountered previously in the case of microcontact printing (µCP)9,10 and in Bonding-Based Techniques. Common ways to fabricate planar the well documented problem of stiction which has been known nanochannels are shown schematically in Figure 2. As early as as one of the major sources of failure of microelectromechanical 1992, Volkmuth et al. described a device consisting of an array of - systems (MEMS) and hard drives.11 13 pillars 2 µm center-to-center, 150 nm high, and 1 µm in diameter.1 The pillars were etched in a silicon dioxide layer and capped with (7) Mela, P.; Tas, N. R.; Berenschot, E. J. W.; van Nieuwkasteele, J.; van den Berg, A. Electrophoresis 2004, 25, 3687-3693. a pyrex wafer by anodic bonding [Figure 2(1)]. Han et al. reported (8) Pallandre, A.; de Lambert, B.; Attia, R.; Jonas, A. M.; Viovy, J. L. Electro- a similar fabrication technique in 1999.14 Channels with 1 µm depth phoresis 2006, 27, 584-610. and 90 nm depth were etched by reactive ion etching (RIE) in (9) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002-2004. (10) Hui, C. Y.; Jagota, A.; Lin, Y. Y.; Kramer, E. J. Langmuir 2002, 18, 1394- silicon substrate. Access holes were opened using anisotropic wet 1407. etching; an insulation silica layer was thermally grown and the (11) Bhushan, B. J. Vac. Sci. Technol., B 2003, 21, 2262-2296. channels were finally closed with a glass coverslip by anodic (12) Delrio, F. W.; De Boer, M. P.; Knapp, J. A.; Reedy, E. D.; Clews, P. J.; Dunn, M. L. Nat. Mater. 2005, 4, 629-634. bonding. This approach led to structures with depths and widths (13) Parker, E. E.; Ashurst, W. R.; Carraro, C.; Maboudian, R. J. Microelectromech. Syst. 2005, 14, 947-953. (14) Han, J.; Craighead, H. G. J. Vac. Sci. Technol., A 1999, 17, 2142-2147. B Analytical Chemistry of 20 nm and 5 µm using silicon-glass anodic bonding (AR of integrated. Channels with a very thin capping layer will result in 0.004) and depths and widths of 25 nm and 50 µm using glass- a low light absorbance but are relatively fragile.

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