Nanofluidics: Systems and Applications 443

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IEEE SENSORS JOURNAL, VOL. 8, NO. 5, MAY 2008 441 Nanoﬂuidics: Systems and Applications Shaurya Prakash, Aigars Piruska, Enid N. Gatimu, Paul W. Bohn, Jonathan V. Sweedler, and Mark A. Shannon

Abstract—Nanofluidics presents growing and exciting oppor- Symbol Description tunities for conducting fundamental studies for processes and systems that govern molecular-scale operations in science and Absolute temperature. engineering. In addition, nanofluidics provides a rapidly growing platform for developing new systems and technologies for an Applied electric potential. ever-growing list of applications. This review presents a summary Local electric potential. of the transport phenomena in nanofluidics with a focus on several systems and applications important to problems of public health Current density. and welfare. Special emphasis is afforded to the role of the electric double layer and the molecular-scale interactions that occur Total number of ionic species. within confined nanoscale systems. Inverse Debye length. Index Terms—Microfluidics, nanofluidics, nanomedicine, sur- Debye length. faces, water purification. Diameter of nanochannel/nanopore. Radius of th ionic species. NOMENCLATURE Viscosity. Symbol Description Length of nanochannel/nanopore. Molar flux of th species. Hydraulic diameter. Diffusion coefficient of th species. Pressure drop. Molar concentration of th species. Body force. Velocity of bulk flow. Density. Mobility of th species. Electric field. Valence of th species. Bessel function of the first kind. Faraday’s constant. Zeta potential. Electrical potential. Volumetric flow rate. Universal gas constant.

I. INTRODUCTION Manuscript received June 25, 2007; accepted August 12, 2007. This work was supported in part by the National Science Foundation through the HE DIGITAL AGE has seen tremendous advances in sci- Center for Nano-Chemical-Electrical-Mechanical Manufacturing Systems Tence and technology that have propelled human imagina- (Nano-CEMMS) under DMI-0328162, in part by the Science and Technology tion to new heights. The vision of developing massively parallel Center of Advanced Materials for the Purification of Water with Systems (WaterCAMPWS) under Agreement CTS-0120978, in part by the Department microscale systems for automating operations in the chemical of Energy under Grant DE FG02 07ER15851, and in part by the Strategic and biological sciences now seems within reach, and the re- Environmental Research and Development Program (SERDP). The associate search community wants to move to the next level—building editor coordinating the review of this paper and approving it for publication was Dr. Richard Fair. systems to probe, analyze, and use individual molecules to ad- S. Prakash was with the Department of Mechanical Science and Engineering, dress issues in public health and welfare. Toward these goals, University of Illinois at Urbana–Champaign, Urbana, IL 61801 USA. He is now with the Department of Mechanical and Aerospace Engineering, Rutgers, nanofluidics will play an important role as chemistry and bi- The State University of New Jersey, Piscataway, NJ 08854-8058 USA (e-mail: ology occur at molecular length scales. The defining feature [email protected]). of nanofluidics is that transport processes occur within systems A. Piruska, E. N. Gatimu, and P. W. Bohn are with the Department of Chem- ical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN with critical dimensions on the order of characteristic physical 46556 USA (e-mail: [email protected]; egatimu@nd. du; [email protected]). scaling lengths, typically 1–100 nm. In some nanofluidic appli- J. V. Sweedler is with the Department of Chemistry, University of Illinois at cations, the key length scales are determined by the shielding Urbana–Champaign, Urbana, IL 61801 USA (e-mail: [email protected]). M. A. Shannon is with the Department of Mechanical Science and Engi- properties of aqueous solutions. For example, the Debye length, neering, University of Illinois at Urbana–Champaign, Urbana, IL 61801 USA is nm for deionized (DI) water (limited by (e-mail: [email protected]). the autoprotolysis of water), and is less than 1 nm for concen- Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. trated salt solutions. Therefore, in many systems and applica- Digital Object Identifier 10.1109/JSEN.2008.918758 tions the effects of shielding and the electric double layer (EDL)

Fig. 1. A schematic description of the length scales and lithographic techniques used in micro and nanosystems. Figure from [1] and [2]. govern transport and reactions in nanofluidic structures. Using II. THEORETICAL BACKGROUND nanoscale systems and functional nanoscale components in mi- In contrast to the recent explosion of interest in nanoflu- croscale devices provide exciting opportunities for building sys- idics, the underlying concepts that govern nanofluidics have tems that can probe or manipulate individual molecules. Fig. 1 been known for some time and are discussed next. The theory shows the length scales important for transport at the micro and assumes that the continuum descriptions of fluid transport are nanoscales. The length scales are presented from three perspec- valid. Once the continuum assumption breaks down, advanced tives: those based on representative devices and applications, the numerical techniques are generally needed for accurate descrip- objects that are important for these devices and applications, and tions of transport phenomena. A discussion of noncontinuum lithographic and fabrication techniques that can be used to fab- models and some key advances can be found in several excel- ricate the devices. lent references [13], [23]–[30]. The advantages of microfluidics, such as limited reagent Most transport phenomena at the nanoscale are driven either consumption, high throughput, and development of disposable by electrokinetic flow or surface mediated transport. Pressure low-cost devices, are well documented [1], [3]–[6]. In addition, driven flows, more common for macroscale systems, are rarely transport physics at the microscale are also well-understood employed for nanoscale systems due to the large driving pres- [1], [7]–[10]. However, the story for nanofluidics is quite sures required, as can be seen from the dependence of pres- different. The walls, due to the large surface-area-to-volume sure drop, , on channel diameter in the Hagen–Poiseuille ratios (as high as m ), dominate behavior giving rise to equation [31] interesting phenomena such as ion permselectivity, ion enrich- ment (or depletion), fast injection of small volumes of reagents, (1) rapid mixing, and pH, charge and concentration gradients stable for long periods of time. New experimental tools such where is the viscosity of the fluid, the volumetric flow rate, as scanning probe microscopy (SPM), confocal microscopy, and the length of the channel. For instance, with water the and single-molecule fluorescence along with the advances in pressure drop across a 100 m long channel 1 nm in diameter for numerical calculations and atomic scale simulation methods only an attoliter ( m ) per second incompressible laminar provide an increased clarity of understanding of the principles flow would need a pressure drop from (1) greater than 3 GPa, on which new applications can be based [8], [10]–[14]. which is impractical for any device. Electrokinetic flows can The purpose of this review is to summarize the transport phe- sustain higher flow rates through nanometer channels without nomena in nanofluidics, with special emphasis on the distinctive excessive pressures. Thus, electrokinetic flows are a preferred role of the EDL and the molecular-scale interactions that occur method for driving fluid and ions through nanochannels. within confined nanoscale systems. The range of devices, how they are fabricated, and the applications differ in many ways A. Electrokinetic Flow from their microscale constructs. Theoretical investigations of A charged surface in contact with a liquid forms an EDL. nanofluidics, including numerical simulations, have focused on The concept of the EDL has been developed over the last cen- nanoscale transport and several excellent references exist [8], tury [16], [17], [32], [33], and several excellent reviews have [10], [12], [13], [15]–[21]. Experimental studies, which con- been presented [18], [19], [34], [35]. A schematic diagram il- stitute the majority of this review, may require special fabrica- lustrating the classic EDL structure is shown in Fig. 2, showing tion approaches, with a recent review highlighting methods of the positions to which different characteristic surface charge re- nanofabrication [22]. lated potentials are referenced within the EDL. Ionic transport PRAKASH et al.: NANOFLUIDICS: SYSTEMS AND APPLICATIONS 443

Fig. 2. A schematic depiction of the classic double-layer structure with the locations where key potentials are measured. Figure based on the work of [16], [23], [25], and [35].

Fig. 3. A simple form of the molecular gate construct. The NCAM acts as is classically described by the Nernst–Planck equation and the a fluidic bridge between microfluidic channels. The NCAM presents exciting velocity profiles are often obtained by using the Navier–Stokes new possibilities in micro-nanoscale systems due to the interesting physics as equations, which are the equations of motion. The potential dis- discussed in this review. (a) shows some SEM images of the NCAM and images of Au nanotubes that can be formed within these NCAMs. (b) depicts the tribution in the nanostructure is critical because the dominant nearly instantaneous mixing that can occur after injection through an NCAM. transport mechanisms are electrokinetically driven. The ionic The mixing is complete within a few hundred microns after injection into the and potential distributions are obtained from the Poisson equa- separation channel. The fluorescence images are collected as a function of time. tion. Frequently, a Boltzmann distribution of charged species is assumed yielding the Poisson–Boltzmann (P-B) equation. In most applications, these equations are coupled, and a numerical which combined with (2) yields solution is required [7], [8], [10], [13], [15], [20], [21], [36] [37]. The terms used in the equations to follow are defined in the (6) nomenclature section. The molar flux of the th species is given by a form of 1-D Nernst–Planck equation such that Now, let us look at some important limiting cases as these often arise in the microfluidic and nanofluidic systems being (2) reviewed in this paper. In the case of a concentration gradient being the only driving force, an electrical current arises such that where is the molar concentration of the th species, is the electrical potential, is a bulk fluid velocity, and is the valence of the th species. Einstein’s relation connects the ionic (7) mobility to the diffusion coefficient such that This equation can also be obtained from Fick’s laws of diffu- (3) sion. For purely electrokinetic flow, the diffusion and electroosmotic terms can be often neglected in comparison to the elec- where is Faraday’s constant. The electrical potential is gen- trophoretic term, yielding erally considered the sum of two potentials

(4) (8) where is the applied and is the local potential. Summing ﬂuxes over all the species and accounting for the valence of each Consider a single, cylindrical nanocapillary with the applied species provides the total current density within a ﬂuidic potential across it being the only driver for transport of ionic channel species. In such a case, the velocity of the species in the axial direction is given by

(5) (9) 444 IEEE SENSORS JOURNAL, VOL. 8, NO. 5, MAY 2008 for which the solution is orientation of fluids and ions with changes in fluid density near surfaces due to surface energy interactions [51]–[54], and effect (10) of surface roughness [55] in nanochannels have been investigated. An overall conclusion from these studies is that water where is the radius of the channel, is the electrical permit- and ions organize within a nanochannel that is reminiscent of tivity, and is the hyperbolic Bessel function of the first kind. classical P-B and N-P solutions for potential, but with signifi- Equation (10) is valid for small zeta potentials, , i.e., typically cant departures in the details that can give rise to unexpected mV. The potential is a phenomenological measure of results in transport of species through nanochannels, as will be the electric potential at a “slip” plane near a surface, beyond the highlighted in the following discussions. fixed Stern layer, where the first layer of molecules from the surface are thought to have significant motion. As (10) implies, III. APPLICATIONS the transport velocity is proportional to the potential and the permittivity of the solution, varies inversely with the viscosity, A. Transport Phenomena and is exponentially affected by the shielding length. One caveat Several interesting phenomena based on the fundamental dif- for transport in nanochannels is that the concept of the slip or ferences in physics at the nanoscale have been observed. For ex- shear plane (Fig. 2) becomes ill-defined when the EDLs from ample, the channel diameter, , and the inverse Debye length, , opposed walls within nanopores begin to overlap, and/or struc- may be combined into the dimensionless parameter . This pa- tural layering of molecules and charges are a significant portion rameter may be tuned to control the dominant form of nanoflu- of the molecules within the channel. In these circumstances of idic transport between electroosmosis and electro- nanoscale transport with interacting or overlapping EDLs, it is migration [56]. This tuning can be achieved by more useful to consider the surface charge or the surface poten- changing the EDL thickness relative to the pore diameter ei- tial [38]–[41]. ther by altering the electrolyte concentration of the solution or by changing the surface charge density by changing the solu- B. Advanced Numerical Techniques tion pH [56], [57], and functionalizing or applying an electrical The theoretical background presented in the previous section bias to the channel walls (as will be discussed later). An implicit assumes the validity of the continuum assumptions, i.e., that assumption in altering the EDL thickness by changing the bulk the ions are treated as hard spheres and point charges and concentration is that the concentration within the nanopores also water is considered as a continuum in the P-B equation. In changes in concert with the bulk concentration. However, deter- order to evaluate transport characteristics, the Navier–Stokes mining the exact concentration within the nanopores can require and Nernst–Planck equations are used that consider water as detailed numerical calculations [13], [35], [58] to account for a continuous medium with a constant viscosity. Any inter- factors such as ion-ion interactions, changes in hydration level actions between the surface and the analytes are considered of aqueous species in confined nanoscale spaces, and the molec- through the boundary conditions. These assumptions neglect ular nature of water. The role of the EDL in reducing electroos- ion-ion, ion-wall, ion-water interactions, and effects of hy- motic flow (EOF) has also been investigated. It has been shown drogen bonding between water molecules. Advanced numerical that once the EDL is greater than approximately 10% of the techniques such as molecular dynamic simulations present an critical channel dimension, the effect of EDL on EOF can no attractive method to answer some questions about the atomistic longer be neglected, and at EDL thickness greater than % nature of the interactions in confined nanoscale transport. One of the channel dimension EOF is reduced [59]. In a related ef- of the major downsides to advanced numerical techniques is the fect, ion permselectivity occurs due to the excess charge density high computational cost [42]. Therefore, most advanced simu- present on the inner walls of the nanochannels. For example, lations are limited in scope. Typically, for confined nanoscale Au-plated nanocapillary array membranes (NCAMs) reject ions transport if the size of the species of interest is about ten of the same charge (co-ions) and transport ions of the opposite times smaller than the characteristic system dimension the sign (counter-ions). This charge-selective transport is only pos- continuum approach has been found to be approximately valid. sible when the EDLs within the nanopores overlap or interact. For example, for simple hydrated ions such as Na :H Oina This permselectivity effect of the surface potential has been nanochannel with critical dimensions on the order of 10 nm or demonstrated by biasing the membrane, so that by switching greater, the continuum assumptions hold for dilute solutions. polarity of the applied bias leads to either preferential cation or However, contrasting criteria for the breakdown of the con- anion transport [60]. The effect of changing length scales on tinuum methods have also been presented [13], [37], [43]–[46]. transport and separation of polyelectrolytes such as DNA has Employing advanced numerical techniques to nanofluidics also been investigated [61]. One notable point about separations has demonstrated several interesting phenomena spurring is that often the need for selectivity and high permeability are in greater interest in developing better systems. In addition, the conflict. Few studies have discussed competing roles of selec- understanding of atomistic behavior governing the transport tivity versus permeability across nanofluidic structures. characteristics has also been enhanced. Effects of finite ion size One of the key factors that affect transport at the nanoscale and hydration on ion distribution and velocity [24], [42], [47], is the surface charge [39]–[41], [62]. Surface charge can be charge inversion and flow reversal [23], density fluctuations controlled by either functionalizing the inner surfaces of the [48], role of fluid density on particle transport [49], role of nanofluidic systems through physical or chemical modifica- surface charge on transport of ions and water [50], layering and tions or by applying potentials to metal-coated interior pore PRAKASH et al.: NANOFLUIDICS: SYSTEMS AND APPLICATIONS 445 walls. Several different surface modification schemes have Three-dimensional systems using nanofluidic interconnects, been developed to affect several parameters for microscale which provide fluidic communication among microfluidic transport [63]–[68]; however, the use of surface coatings to channels in vertically separated layers, have been of interest for affect transport at the nanoscale is relatively new. Using surface the purpose of fluid handling in mass-transported limited sam- coatings, size-based separations have been demonstrated by ples [83], [84]. In these micro-nano hybrid systems size-based controlling the plating time of Au within an NCAM template to selectivity for various dextrans has been demonstrated [85], tune the internal nanopore diameter. In addition, by using thiol [86]. Size-based selectivity can be understood based on the monolayers, chemical transport selectivity can be achieved Stokes–Einstein relation. For a chemical species in bulk solu- [69]. Thiol-based surface modifications of Au-NCAMs have tion the diffusion coefficient is governed by been used to generate chemical transport selectivity for different experimental conditions, including selective transport (11) of hydrophilic or hydrophobic species [70], pH-dependent As the size of a nanochannel, nanopore, or nanotube decreases, transport [71], separation of proteins by exploiting changes in with increasing molecule size, one expects to observe the phys- the isoelectric point of a functionalized surface [72], or direct ical effects of hindered diffusion. For a molecule of radius molecular recognition mediated by thiolated-DNA used to diffusing within a nanoscale channel of comparable radius , detect base-pair mismatches [73]. Most surface modification the center of the molecule cannot approach the wall within a schemes to control the surface properties including surface distance . The extent to which the diffusion coefficient of charge utilize either Au-thiol self-assembled monolayers or a molecule in the confined nanoscale space is reduced in con- Si-based monolayers for glass-like systems. Modification of trast to its value in the bulk solution is given by the Renkin confined surfaces at the nanoscale with synthetic polymers equation remains a challenge to researchers, although recent advances in derivatization strategies for biological macromolecules have (12) seen both enzymes [74] and antibodies [75] derivatized within nanopores. where . A similar analysis has been presented by The transport phenomena through nanostructures can also be Bayley and Martin in their review article on resistive-pulse influenced by applying an electrical potential across or to the sensing [87]. nanostructures. An early demonstration of controlling ionic per- The transport characteristics of multilevel 3-D hybrid micro- meability through a nanoporous membrane was shown over two nano structures are related to the electrical characteristics of the decades ago by using an embedded electrode [76]. Using this fluidic network. This fluidic network was initially modeled as a approach, ion movement through membranes containing fixed simple resistive network in order to explain the polarity reversal ionic sites was shown to depend on the nature and number of in the direction of electrokinetic transport of 200 nm pore di- charged sites, and ionic resistance of the membrane was electro- ameter NCAMs compared with 15 nm pore diameter NCAMs chemically controlled via the oxidation state on the redox sites [80], [85]. In a later study, the capacitance of the NCAM was within the polymer [76]. Embedded electrode systems are also included [88] in the models. However, further development of being developed to probe potential distributions within single equivalent circuit models may be necessary for nanofluidic sys- nanopores [66]. tems, since it has been shown recently that simple RC circuits, Rapid progress in microfluidics has led to an increasing under certain conditions, are not sufficient to explain the trans- demand for developing systems that can handle complex flu- port characteristics at the nanoscale [66]. These 3-D microfluidic manipulations in confined nanoscale systems. Integrating idic structures with nanofluidic interconnects exhibit a variety of nanofluidic components allows for not only further reduction of interesting and useful electrical characteristics stemming from volumes but also permits manipulation of fluidic samples based their nanoscale architecture. For example, they have been shown on size and charge of analyte species using valves, molecular to maintain large concentration gradients for extended periods gates, and fluidic architectures similar to VLSI circuits [9], [66], of time [89], [90], possess near-diode like behavior with control [77]–[81]. Developing multilayer 3-D integrated microfluidic over rapid injection, mixing, and to preconcentrate attomolar systems [77] constitutes one viable method to enhance com- concentration solutions [81], [91], [92] and other mass-limited plexity in fluid handling operations, because disparate fluidic samples [93], [94]. manipulations such as preparation, concentration, tagging, separation, or affinity recognition may be accomplished in B. Chemical Analysis different planes. Optimal use of these capabilities requires a Nanofluidics can add functionality for sample manipulations thorough understanding of nanofluidic transport at its most in analytical chemistry, such as sample injections, separa- basic level. To this end, single nanopores, microfabricated in tion, purifications, and preconcentration for quantitative and PMMA films can be used as individual interconnects or as qualitative identification. For example, NCAMs functioning platforms to study fundamental nanofluidics [82]. However, as controllable molecular gates can mediate digital transfer several open questions such as the role of surface charge, [95] of fluid voxels from one microfluidic channel to another. surface potentials, specific interactions between the walls In addition, NCAMs presenting different pore sizes [96] can and the translocating species, mechanisms of transport across transfer analytes with disparate mass characteristics at different nanofluidic structures, and visualization of confined nanoscale rates to achieve intelligent fluidic control. This capability has flows remain a challenge to the research community. consequently found use in a miniaturized lead sensor that uses 446 IEEE SENSORS JOURNAL, VOL. 8, NO. 5, MAY 2008

DNAzyme [97] as a probe and for preparative post-separation from 30–500 nm DNA and proteins have been separated using processing for mass limited samples [93]. Control of fluidic steric effects that exploit the principle of entropic minimization transport in nanochannels is crucial to the development of inte- in the transport of biomacromolecules within confined geome- grated components nanofluidic-microfluidic chemo-electronic tries [123]. Nanostructures and single nanopores have been devices. In this regard, Karnik et al. reported a metal oxide used to reduce the entropic contribution to the total free energy solution-based system analogous to a metal oxide semicon- of DNA, thereby modulating entry into a separation channel ductor field effect transistor [39] that achieves flow switching according to the oligonucleotide length [127], [128]. via active control of the surface potential in the interior of the Silicon has been extensively used as a material for building nanochannel, thereby demonstrating the ability to control trans- systems for applications in biochemical systems at the port of charged species in a nanostructure that could potentially nanoscale due to its versatility in microfabrication [129]. lead to fluiditronics. However, chemical modification can impart novel properties Another area in which nanofluidics can have a profound im- of the solid-liquid interface that can be usefully exploited. pact on chemical analysis is sample preparation. For example, Many materials and methods are currently being researched for an analyte may need to be isolated from a complex matrix and drug-delivery applications [130] including dendrimers, such made available for analysis in a suitable chemical environment. as poly(amido amine) or PAMAM [131]. Dendrimers have Iannacone et al. [98] demonstrated that NCAMs can be used to recently been attached to confined surfaces within microflu- enhance the intensity of an insulin peak introduced into a mass idic devices and show a possible method for attachment of spectrometer from a microfluidic channel using electrospray ion- PAMAM dendrimers to nanofluidic devices [68]. Development ization; this occurs by reducing the salt adducts inherent in the of systems relying on bacteria as molecular motors for pumping sample preparation process. Additionally, various receptors can small volumes of liquids has also been investigated [132]. be incorporated into NCAMs. This strategy could be beneficial for sample preconcentration and purification of biomolecules D. Water Purification in microfluidic devices. Both antibodies [75] and DNA probes Recent reports have highlighted the growing challenge of [99] have been successfully incorporated in NCAMs for appli- providing clean, safe water for a variety of applications in- cations in protein purification and preconcentration. Preconcen- cluding potable water, energy generation, and agricultural use tration factors of up to 300 have been achieved for single polarity [133]–[135]. These problems are central to the general public species [100] in NCAMs and up to for proteins and peptides health and welfare and, therefore, have led to a resurgence of [101] in simple nanofluidic structures. interest in researching new materials, systems, and methods Beyond these demonstrated applications of micro-nanoflu- for water purification and detection of various chemical and idic architectures have enormous potential for novel chemical biological species in water supplies [90], [136]–[139]. Mem- separations by reducing required sample amount and improving branes, consisting of nanopores or nanocapillaries ranging from resolution. This issue has been addressed both theoretically 1nmto1 m, have historically played an important role in ap- [102], [103] and experimentally, and is mainly driven by plications related to water purification [10], [140]–[142]. They interest in DNA separation. Periodically constricted nanochan- have been studied extensively for their role in aqueous systems nels [104] have shown successful DNA separations and close including studies on the importance of chemical composition, packed beads with nanometer spacing [105] have been utilized surface charge and morphology [62], [143]–[145], fouling to accomplish both DNA and protein separations. [146], [147], and desalination and filtration [90], [135], [139], [148], [149]. Structures based on the purposeful manipulation C. Nanomedicine of nanofluidics present excellent opportunities for growth in the One of the reasons the scientific community is excited about area of detection and sensing for water purification as illustrated advances in nanofluidics and nanotechnology centers on the by the use of an integrated microsystem for injection analysis possibility of manipulating processes, especially in biological of ammonia in surface waters for environmental applications systems, at the molecular level. This possibility has led to rapid [150]. Detection of heavy metal ions has found a new pathway development of new materials, methods, and systems for appli- in the development of DNA-based sensors for lead and uranium cations in biomedicine [106]–[110]. A recent review explains ions [151]–[154]. Nanoparticles have been used for removal the concept of nanomedicine and some of the early advances in of trace organic contaminants such as trichloroethylene and applying the principles of nanotechnology to medicine [111]. chloroform [155] and mesoporous materials with pores in the Nanomaterials, such as silica or gold nanoparticles, quantum sub-10 nm range have been used for removal of polyelectrolyte dots, functionalized micro and nanocantilevers, and artificial species such as humic acids from water [156]. nanopores are being increasingly used for detection of DNA [2], [112]–[115], RNA [116]–[118], and proteins [119]–[122]. In IV. SUMMARY some cases, the asymmetry within artificial nanostructures is Many applications and scientific advances in nanotech- being exploited to mimic the behavior of natural systems, such nology have created an intense debate both in the scientific as ion-channels [123]–[125]. Multiple fabrication techniques, and socio-political forums about the implications and future for example, e-beam lithography, focused ion-beam milling, and directions of the multitude of new technologies. Nanofluidics nanoimprint lithography have been used to construct nanostruc- is a rapidly evolving discipline with wide and varied appli- tures for trapping and transporting DNA and focusing of proteins cations. A few of these applications with direct relevance to using dielectrophoresis [126]. Using well-defined nanochannels problems of public health and welfare such as nanomedicine PRAKASH et al.: NANOFLUIDICS: SYSTEMS AND APPLICATIONS 447 and water purification have been discussed in this paper. The [26] R. Qiao, J. G. Georgiadis, and N. R. 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[149] G. T. Gray, R. L. McCutcheon, and M. Elimelech, “Internal concentra- Paul W. Bohn received the B.S. degree in chemistry tion polarization in forward osmosis: Role of membrane orientation,” from the University of Notre Dame, Notre Dame, IN, Desalination, vol. 197, pp. 1–8, 2006. in 1977 and the Ph.D. degree in chemistry from the [150] H. Gardeniers and A. V. D. Berg, “Micro- and nanofluidic devices University of Wisconsin-Madison, in 1981. for environmental and biomedical applications,” Int. J. Environ. Anal. After a two-year stint at Bell Laboratories, he Chem., vol. 84, pp. 809–819, 2004. joined the faculty at the University of Illinois at [151] J. W. Liu, A. K. Brown, X. Meng, D. M. Cropek, J. D. Istok, D. B. Urbana–Champaign (UIUC). While at UIUC, he Watson, and Y. Lu, “A catalytic beacon sensor for uranium with parts- served as Centennial Professor in Chemical Sci- per-trillion and million-fold selectivity,” Proc. Nat. Acad. Sci., USA, ences, Professor of Chemistry, and Professor in 2007. the Beckman Institute. He also served as Interim [152] J. W. Liu and Y. Lu, “A colorimetric lead biosensor using DNAzyme- Director of the School of Chemical Sciences from directed assembly of gold nanoparticles,” J. Am. Chem. Soc., vol. 125, 1993 to 1994, and Head of the Chemistry Department from 1994 to 1999. pp. 6642–6643, 2003. From 2001 2002, he was Interim Vice Chancellor for Research, the Senior [153] C. B. Swearingen, D. P. Wernette, D. M. Cropek, Y. Lu, J. V. Sweedler, Research Officer of the UIUC campus. In August 2006, he left UIUC to join and P. W. Bohn, “Immobilization of a catalytic DNA molecular beacon the faculty at the University of Notre Dame as the Arthur J. Schmitt Professor on Au for Pb(II) detection,” Anal. Chem., vol. 77, pp. 442–448, 2005. of Chemical and Biomolecular Engineering and Professor of Chemistry. He [154] D. P. Wernette, C. B. Swearingen, D. M. Cropek, Y. Lu, J. V. Sweedler, and P. W. Bohn, “Incorporation of a DNAzyme into has authored/coauthored over 165 publications, 4 patents issued, and delivered au-coated nanocapillary array membranes with an internal standard over 200 invited lectures throughout the world. His research interests include for Pb(II) sensing,” The Analyst, vol. 131, pp. 41–47, 2006. understanding and control of molecular transport on the nanometer length [155] Y. Zhongren and J. Economy, “Nanoparticle and nanoporous carbon scale, developing new optical spectroscopic measurement strategies for surface adsorbents for removal of trace organic contaminants from water,” J. and interfacial structure-function studies, optoelectronic materials and devices Nanoparticle Res., vol. 7, pp. 477–487, 2005. and chemical sensors, and molecular approaches to nanotechnology. [156] C. Liu, N. Naismith, and J. Economy, “Advanced mesoporous Dr. Bohn has received several awards. organosilica material containingmicroporous cyclodextrins for the removal of humic acid from water,” J. Chromat. A, vol. 1036, pp. 113–118, 2004. Jonathan V. Sweedler is a Lycan Professor of Chemistry, the Director of the Carver Biotechnology Center, and has appointments in the Physiology, Shaurya Prakash received the B.S. degree in Neuroscience and Bioengineering Programs, all at mechanical engineering from the University of the University of Illinois at Urbana–Champaign. Arkansas, Fayetteville, in 2001, the M.S. degree in His research emphasizes analytical neurochemistry, mechanical engineering working on laser-induced and involves developing analytical methods for fluorescence measurements of hydroxyl radicals assaying complex microenvironments that include over various inert surfaces from the University of micro/nanofluidic sampling, electrophoresis sep- Illinois at Urbana–Champaign, Urbana, in 2003. He aration methods, mass spectrometric detection is currently working towards the Ph.D. degree from techniques, and nanoliter volume NMR. The second the University of Illinois under the supervision of group theme is the application of these technologies to the study neuropeptides Prof. M. A. Shannon, where has worked extensively and classical transmitters, as well as their metabolism in a cell-specific manner. with surface modification, impedance spectroscopy, micro and nanofluidics, and flame structure studies. He brings together a multidisciplinary skill set with his areas of interest and expertise, which include micro and nanofabrication, microscale combustion, Mark A. Shannon received the B.S., M.S., and and transport in confined micro and nanosystems. Ph.D. degrees in mechanical engineering from the University of California at Berkeley, in 1989, 1991, and 1993, respectively. He is the James W. Bayne Professor of Me- Aigars Piruska was born in Riga, Latvia, in 1979. He chanical Engineering at the University of Illinois received the B.S. degree in chemistry from the Uni- at Urbana–Champaign. His goal is to advance the versity of Latvia, Riga, in 2001, and the Ph.D. degree state-of-knowledge in coupled phenomenon at the from the University of Cincinnati, Cincinnati, OH, in micro to nanoscale, with applications to high-tem- 2006. perature combustion reactors, water purification, Currently, he is a Postdoctoral Researcher with the microfabrication, nano and microelectromechanical Department of Chemical Engineering, University of systems (NEMS and MEMS), micro and nanofluidic devices, and mesoscale Notre Dame du Lac. His research interests include machines that bridge the nano and microscales to the normal scales. Students environmental sensing and micro/nanofluidics. in his group conduct research related to microfabrication process development, experimental measurements, and computational research. Currently, he is the Director of a NSF STC the WaterCAMPWS, which is a multiple university and government laboratory center for advancing the science and technology of materials and systems for revolutionary improvements in water purification for human use. He is also the Director of the Micro-Nano-Mechanical Systems Enid N. Gatimu was born in Nairobi, Kenya, in 1970. She received the B.S. (MNMS) Laboratory at the University of Illinois at Urbana–Champaign, a 2000 and Ph.D. degrees in chemistry from Clark Atlanta University, Atlanta, GA, in sq. ft class 10 and 100 clean room laboratory devoted to research and education 1994 and 2003, respectively. in the design and fabrication of a variety of MEMS and NEMS. He also chairs In 2004, she joined Prof. P.l Bohn’s group at the University of Illinois at the Instrument Systems Development Study Session for the National Institutes Urbana–Champaign campus as a Postdoc and in 2006 moved to the University of Health. He has published over 150 papers. of Notre Dame in the same capacity. Her research is focused on studying the Dr. Shannon has received numerous awards for excellence in teaching and hindered transport characteristics of electrolytes flowing through nanocapillary research. array membranes and PMMA single pores incorporated in 3-D nanomicrofluidic devices.