whitby, m. and n. quirke (2007). fluid flow in carbon nanotubes and nanopipes. nature nanotechnology...

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Fluid fl ow in carbon nanotubes and nanopipes

Nanoscale carbon tubes and pipes can be readily fabricated using self-assembly techniques and

they have useful electrical, optical and mechanical properties. The transport of liquids along their

central pores is now of considerable interest both for testing classical theories of fl uid fl ow at the

nanoscale and for potential nanofl uidic device applications. In this review we consider evidence for

novel fl uid fl ow in carbon nanotubes and pipes that approaches frictionless transport. Methods for

controlling such fl ow and for creating functional device architectures are described and possible

applications are discussed.

M. WHITBY AND N. QUIRKE Chemistry Department, Imperial College, South Kensington, London SW7 2AZ, UK.

e-mail: [email protected]

Understanding and controlling the fl ow of liquids at the nanoscale is currently a subject of great interest. More than 100 papers describing work in nanofl uidics have been published in the past fi ve years. Recent experimental measurement of fl uid transport through channels with widths of a few nanometres has confi rmed predictions derived from computer simulations that such fl ow occurs at a much greater rate than expected. Th is discovery has signifi cant implications, both for our understanding of how fl uids behave at very small length scales and for the design of nanofl uidic devices. Here we review selected papers with an emphasis on fl uid fl ow through the central pores of carbon tubes.

Th e practical investigation of fl uid fl ow in nanoscale channels has been facilitated by the availability of tubular carbon structures with open central pore diameters in the range of one to several hundred nanometres. Carbon off ers a number of attractive features for the fabrication of nanofl uidic devices including: the existence of several well studied processes for self-assembly of tubular structures; useful electronic properties for sensing and signalling; biocompatibility, ready chemical modifi cation to allow functionalization; and low-friction transport thanks to graphitic surfaces.

Two types of nanoscale carbon tubes have been used experimentally, each with distinct properties and methods of synthesis. The first are carbon nanotubes, both single and multiwalled, with the fullerene molecular structure reported by Ijima in 1991 (ref. 1). The second are carbon nanopipes produced by chemical vapour deposition (CVD) of amorphous carbon in alumina templates with a honeycomb pore morphology (Fig. 1). Throughout this review we reserve the term ‘nanotube’ to refer to true molecular carbon structures. The term ‘nanopipe’ will be used to describe carbon pipes produced in templates using CVD. The latter are generally larger and composed mainly of amorphous rather than well ordered graphitic carbon. The properties and fabrication of both types of carbon tube, and their

arrays when assembled as nanoporous membranes, are discussed in later sections.

Our understanding of the static properties of liquids in macroscopic capillaries is based on the nineteenth century work of Laplace, Poisson and Young2. Steady-state flow of simple incompressible fluids in a channel width 2h, driven for example by gravity ρg or a pressure gradient dP/dy, can be described by the Navier Stokes equation3. The solution for the velocity in the direction of flow, y, as a function of the distance from the wall, z, has a parabolic profile (Fig. 2) given by:

Uy(z) = (ρg/2η)[(δ + h)2 – z2]

where η is the viscosity and δ, the slip length, is the distance into the wall at which the velocity extrapolates to zero. Conventionally, the slip length is assumed to be zero. However, if it is not then we are said to have slip boundary conditions at the wall, which for very small channels can signifi cantly enhance the fl uid fl ow. Integrating the velocity profi le over the cross-sectional area of the pipe gives the Hagen-Poiseuille law for the fl ux through a pipe. Th is law states that the fl ow rate of a fl uid passing through a tube is directly proportional to the pressure diff erence between the tube ends and to the fourth power of the tube’s internal radius. Th e fl ow rate is inversely proportional to the tube length and to the viscosity of the fl uid.

Th e dynamics of capillary fi lling were elucidated by Washburn4,5 using the Hagen-Poiseuille law with the driving force for fl ow given by the Laplace equation for the pressure diff erence across the invading liquid meniscus. Th us the penetration length L at time t of a fl uid in a capillary of radius R is given by:

L2 = (Rγ/2η)t

with γ being the liquid/vapour surface tension and η the shear viscosity of the invading liquid. In deriving the Washburn equation it is assumed that there is no fl uid motion at the wall (that is, stick boundary conditions). Th e Washburn equation is very successful on time scales suffi cient to establish viscous fl ow but breaks down for short times. A more general solution can be found for

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non-steady viscous incompressible fl ow (the Bosanquet equation6), which approaches the Washburn equation at long times.

For nanoscale capillaries the fl ow behaviour is dominated not by bulk properties (γ, η) but by the interaction of the fl uid with the capillary walls. In this regard it is important to include the role of surface friction due to molecular corrugation, especially for graphitic surfaces where the high surface density of atoms leads to a very low corrugation. A convenient parameter here is the Maxwell coeffi cient7 α, which represents the fraction of the molecular collisions with the capillary walls that undergo diff use scattering and/or trapping–desorption, the remainder being specularly refl ected (no energy loss). Th e Maxwell coeffi cient has been shown to provide a useful description of molecular friction8 for a variety of materials9. However, although it is clear

that the smaller the value of α, the greater the slip length, a direct relationship between the two only exists at low density10.

SIMULATION AND THEORY OF NANOSCALE FLOW

A central question in nanofl uidics concerns the extent to which the classical equations describing the way fl uids interact with nanomaterials hold at the nanoscale. Molecular simulation is ideally suited to shed light on this problem and has shown that static properties such as surface tensions and contact angles of simple fl uids obey classical relations down to almost single nanometre dimensions11. Steady-state Poiseuille fl ows maintain a parabolic velocity profi le down to widths of several nanometres12. Deviations from this behaviour can typically be linked to density inhomogeneities in the confi ning walls.

In performing simulations relevant to experimental work on fl ow in nanopores, a major challenge has been to choose realistic potentials between the molecules in the fl uid and in the pore walls. Although it has proved possible to be reasonably confi dent about, say, determining alkane/carbon potentials by fi tting to heats of adsorption, for fl uids such as water considerable uncertainty has arisen. For example, reports of the measured contact angle of water on a graphitic surface diff er by as much as a factor of two13. Th e choice of intermolecular potential determines whether a model nanotube can be fi lled with water14,15 or not. However recent work has shown how it may be possible to calibrate solvent–nanotube potentials using Raman shift s in solution16. Th e uncertainty in water potentials notwithstanding, some general features of nanoscale fl ow have been determined from molecular dynamics using simple fl uids relevant to the experimental work. For example Sokhan et al.8,17 were the fi rst to show that fl uids fl owing through (slit) carbon nanopores experience very low surface friction, that is very small Maxwell coeffi cients (~0.01) or equivalently large slip boundary conditions, whereas pores made from other materials show higher values and stick boundary conditions.

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Figure 1 Scanning electron microscope images of carbon nanopipes produced by the authors using standard chemical vapour deposition. a, Nanopipes partially released from an anodic aluminium oxide template following sonication in NaOH. b, Cross section of intact carbon coated membrane. c, Higher magnifi cation view of individual aligned carbon pipes. d, Surface of carbon membrane showing open pores (diameter ~160 nm). The growth procedures are described in the section on “Flow through nanoscale channels including carbon nanopipes”.

+h

–h

Uy(z )z

y

Figure 2 Parabolic velocity profi le Uy(z) for Poiseuille fl ow in a capillary of width 2h.


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Subsequent theoretical work shows that in a cylindrical geometry, decreasing the radius can reduce the Maxwell coeffi cients by orders of magnitude compared with a slit (~0.000002 for decane in a (7,7) nanotube, diameter 0.75 nm) and that carbon nanotubes are predicted to be essentially frictionless pipes18,19. Skoulidas et al.20 compared transport diff usivities for carbon nanotubes with zeolites using molecular dynamics and found exceptionally high transport rates for the nanotubes due to the smooth carbon surface. We should expect therefore that crystalline carbon channels (as opposed perhaps to amorphous surfaces) show signifi cantly increased fl ow. Another consequence is the extremely rapid imbibition of wetting fl uids (decane is predicted to fi ll (7,7) nanotubes, at 800 m s–1; ref. 18) observed in non-equilibrium molecular dynamics. Note that the same model predicts external wetting to be much slower by factors of roughly 30 for a (13,13) tube with diameter 1.765 nm. Th is has implications for nanotube arrays, where fl ow may occur along either the external or internal surfaces of the tubes.

From a Bosanquet-like equation (where we approximate the surface friction with an expression involving the Maxwell coeffi cient and the surface tension and the driving force is represented by the wall-wetting liquid tension), it is possible to obtain a solution for the imbibition velocity and penetration length L at all times18. Th ere is an L ∝ t dependence for short times tending to t1/2 at long times (as in the Washburn equation), the timescale for which depends on the value of the Maxwell coeffi cient. Figure 3 shows how the imbibition speed for decane varies as a function of time with ultrafast uptake for t<μs falling to cm s–1 at longer times.

Th e situation is more complex for hydrogen bonding fl uids such as water. In very narrow tubes the water molecules form a one-dimensional chain with linking hydrogen bonds. At equilibrium, conduction through such tubes occurs in bursts14 and this behaviour is attributed to density fl uctuations at the openings of the tube.

FLOW THROUGH NANOSCALE CHANNELS INCLUDING CARBON NANOPIPES

Initial indications of rapid fl uid fl ow through very small channels in an experimental system were published by Pfahler et al. in 1990 (ref. 21). Working with rectangular channels etched in a silicon substrate, they found an n-propanol fl ux approximately three times greater than expected in the smallest channels (height 0.8 μm and width 100 μm). Following up on this observation in 2002, Cheng and Gordanio22 reported measurements for Newtonian fl uids fl owing under pressure through nanoscale channels produced

lithographically. Th e height of the channels varied from 2.7 μm down to 40 nm. Th e results show a marked and progressive departure from Poiseuille fl ow as the channel height falls below 200 nm. Th is eff ect was observed most strongly for hexadecane and is also evident for decane and for silicone oil. Slip lengths were estimated at 25 to 30 nm, ~9 nm and ~14 nm respectively. Th e eff ect was not seen for water at the length scales investigated.

Several groups have recently reported results from experiments involving fl ow of liquids and gases through membranes with nanoscale pores composed of carbon. Two principal methods have been used to construct suitable membranes. Th e fi rst involves CVD of carbon in an anodic aluminium oxide (AAO) template. Th e second is based on production of polymer composites containing an aligned array of carbon nanotubes spanning the thickness of the membrane. Both techniques allow liquid fl ow through the central pores of carbon nanotubes and nanopipes to be studied without the complication of external wetting.

Th e fi rst method for creating carbon nanomembranes takes advantage of the self-assembly of highly-ordered nanopores arranged in a dense hexagonal pattern in alumina when aluminium foil is etched in acid23,24. Using this approach, monodisperse-sized pores with densities as high as 1011 cm–2 are possible. To line the nanopores, the AAO template is placed in a furnace and a hydrocarbon gas is fl owed for several hours, whereupon carbon is deposited on the hot surface of the alumina25. Carbon pipes with an outside diameter as small as 10 nm have been formed in this way although larger 100 to 200 nm nanopipes are more usual. Th e length of these pipes is typically 50 to 100 μm.

It is important to appreciate that, as synthesized, the typically 20-nm-thick walls of carbon pipes produced by the AAO templating method will be at least partly amorphous. Che et al.26 reported electron diff raction studies in a transmission electron microscope (TEM), which produced a pattern characteristic of non-aligned graphite fragments. Th ere are reports that high-temperature annealing can improve the order of the tube walls through graphitization27,28, which may enhance transport properties. Miller et al.25 confi rmed electro-osmotic fl ow through these carbon nanomembranes and reported a linear relationship between electro-osmotic fl ow velocity and applied current density.

FLUID FLOW THROUGH CARBON NANOTUBES

Hinds et al.29 published the fi rst account of experiments involving fl uid fl ow through a membrane composed of true molecular nanotubes. Th is

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Figure 3 Imbibition of decane in a (7,7) carbon nanotube (diameter = 0.951 nm). Left: Simulated imbibition, showing a snapshot of the fl uid after 60 ps50. Right: plot of the imbibition velocity obtained from the analytic model using parameters derived from nanoscale simulation, the upper (blue) line shows results for a carbon nanotube, the bottom (black) line, a non-carbon tube50.


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group used dense, well aligned arrays of multiwalled carbon nanotubes (MWNT) grown using CVD on a fl at quartz substrate (Fig. 4). One advantage of this approach is that the pore diameters are much smaller (4.3 ± 2.3 nm) than with the AAO template method. Another is that the crystallinity of the inner carbon nanotube walls is greater, with consequent benefi ts for low friction fl uid fl ow.

As synthesized MWNT arrays are permeable not only through the central pores of the nanotubes, but also extensively through the gaps between the individual tubes. Furthermore, many nanotubes will be

closed by end caps or plugged by residual iron catalyst. To overcome these problems, a polystyrene solution was spin coated onto the array to fi ll the gaps. Th en plasma etching was used to remove the top layer of the composite and to open the nanotube central pores. Th e pore density was estimated to be 6 (±3) × 1010 cm–2.

Using gas fl ow apparatus, the MWNT composite array showed a linear relationship between fl ow rate and pressure drop across the membrane. Th e authors also studied diff usion of aqueous ionic species (Ru(NH3)6

3+ diameter = 0.55 nm) and noted that the enhanced diff usion coeffi cient was near the bulk aqueous-solution diff usion for the Ru cation. Th ey infer only limited interaction between the ion and the nanotube tip and walls.

Majumder et al.30 published the results of further experiments by the same group on fl uid transport through the MWNT composite membranes. A simple pressure-driven fl ow apparatus was used, with the accumulated mass of transported fl uids including water, ethanol and alkanes being measured aft er a fi xed time period. Th eir results demonstrate dramatically enhanced fl ow through ~7-nm-diameter nanotube cores compared with conventional fl uid fl ow theory. Observed fl ow rates were four to fi ve orders of magnitude greater than predicted by hydrodynamics based on macroscale behaviour (Table 1). Th e authors conclude that the implied slip lengths (3 to 70 μm), which are much greater than the tube diameter, are consistent with a nearly frictionless interface.

In the studies reviewed so far, increasingly interesting fl uid fl ow results were found at progressively smaller length scales. In 2006, Holt et al.31 achieved a milestone in measuring water fl ow through the central pores of double walled carbon nanotubes (DWNT). Th ese tubes had inner diameters less than 2 nm with defect-free graphitic walls that are expected from simulation (see previous section) to present a low friction surface to transported fl uids.

Th e practical challenges involved in studying transport through very small carbon nanotubes are formidable. Th e approach used by Holt et al. was to use fabrication techniques adapted from the semiconductor industry. A dense array of carbon nanotubes were fi rst grown on a silicon substrate onto which metal catalyst particles had been deposited. Th e spaces between the tubes were then completely fi lled with silicon nitride from low-pressure chemical vapour. Finally the closed ends of the tubes and catalyst particles were removed by a series of etching and ion milling steps. Th e resulting membranes had a thickness in the range of 2.0 to 3.0 μm and pore densities ≤0.25 × 1012 cm–2. Pore diameters, calibrated by passage of gold nanoparticles, were in the range of 1.3 to 2.0 nm.

Gas fl ow and water fl ow measurements were taken in fl ow cells sealed with o-rings. Five hydrocarbon and eight non-hydrocarbon gases were tested to determine fl ow rates and to demonstrate molecular weight selectivity compared with helium. Water fl ow was pressure-driven at 0.82 atm and measured by following the level of the meniscus in a feed tube. Th e results for both gas and liquid show dramatic enhancements over fl ux rates predicted with continuum fl ow models. Gas fl ow rates were between 16 and 120 times that expected according to the Knudsen diff usion model in which fl uid molecule–wall collisions dominate the fl ow. Water fl ow rates were 560 to 8,400 times greater than those calculated according to the Hagen–Poiseuille equation. Minimum slip lengths are estimated in the range of 140 to 1,400 nm. To express these fi ndings in a practical context, the nanotube membranes showed fl ow rates several orders of magnitude greater than those of conventional membranes, despite having pore sizes an order of magnitude smaller. Th e authors of these studies point out the signifi cance of this fi nding for separation applications.

MICROSCOPE STUDIES OF FLUID FLOW IN NANOTUBES

Permeation experiments allow quantitative measurement of fl uid transport through nanoscale carbon pores. A more qualitative

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Figure 4 Carbon nanotube arrays and membranes. a, An as-grown, dense, multiwalled carbon nanotube array produced with an Fe-catalysed chemical vapour deposition process. b, The cleaved edge of the nanotube-polystyrene membrane after exposure to H2O plasma oxidation. The polystyrene matrix is slightly removed to contrast the alignment of the nanotubes across the membrane. c, Schematic of the target membrane structure. With a polymer embedded between the nanotubes, a viable membrane structure can be readily produced, with the pore being the rigid inner-tube diameter of the nanotube. Reproduced with permission from ref. 29. Copyright (2004) AAAS.


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approach is to observe fl uid interaction using optical and electron microscopes. It should be noted that the structures studied are generally large (tube diameter > 200 nm). Th e results are therefore unlikely to show the more unusual surface-dominated fl ow behaviour expected in true nanoscale channels.

In 2004, Rossi et al.32 reported a series of observations of liquid/carbon-pipe interactions made in an environmental scanning electron microscope (ESEM). Th e advantage of using an ESEM is that in situ experiments involving fl uids can be undertaken. Furthermore, by carefully varying the pressure in the sample chamber and by controlling the temperature using a Peltier cooling stage, it is possible to cause condensation and evaporation to occur at will in the sample. Using this method, the team captured the behaviour of liquid menisci inside 200 to 300 nm diameter CVD grown carbon pipes (Fig. 5).

A key result was that water condensed on the internal walls of the carbon tubes in preference to the external steel surface of the stage. Th e authors concluded that the disordered walls of the AAO template-grown tubes are hydrophilic and they measured contact angles with water between 5 and 20°. Th ey also reported a slight deformation of tubes containing plugs of water, indicating negative pressure inside the channel due to tensile capillary forces. Carbon tubes of this type may be useful for guiding aqueous fl uids to and from specifi c locations at the nanoscale and for collecting attolitre and picolitre samples.

Also in 2004, Kim et al.33 reported work on liquid interactions with similar carbon pipes and plotted fi lling length against time. Working with ≥200 nm nanopipes, they found a good fi t with x(t) = At1/2 (the Washburn equation) as expected for the long (ms) observation times.

A year later, the same group published a follow-on paper34 reporting on imbibition experiments that involved introducing fl uorescent nanoparticles (diameter ~50 nm) into a variety of liquids. Th e suspended particles were taken up by carbon pipes with 500-nm diameters through a combination of capillary action and evaporation forces. Th e introduced particles could be observed moving along the tubes as their fl uorescence was visible through the tube walls. Th e authors speculate that this technique may be useful for observing the interactions of biological macromolecules in the vacuum environment of an electron microscope.

CONTROLLING LIQUID FLOW THROUGH NANOTUBES

In the previously discussed study by Miller et al.25, electrochemical derivitization was used to change the surface functionalization of carbon nanopipes. Using this technique, both the magnitude and the direction of electro-osmotic fl ow could be modifi ed. Simple application of a potential across the membrane can also act as an immediate transport switch as reported by several researchers29,35,36.

Miller and Martin35 describe a more sophisticated redox modulation of ionic transport, which they used to switch both the rate and direction of electro-osmotic fl ow. Working again with carbon nanopipes, they coated the carbon surfaces with the redox polymer poly(vinylferrocene) (PVFc). A variable potential was used to totally oxidize, totally reduce or to set the redox state of the PVFc to an intermediate value. Once set, the membrane would remember its state until altered. Th e authors compare their approach to the fi eld-eff ect concept in semiconductors.

Similar fi ndings are reported by Majumder et al.37 using their fi ner MWNT composite membranes (described earlier). Instead of functionalizing the entire length of the nanopores, the authors selectively modifi ed just the tip region of the nanotubes. Th e tips were exposed by plasma etching and functionalized using a carbodiimide mediated coupling. Th ey showed that diff erent chemical end groups attached near the entrance to the channels could selectively modulate ionic fl ux.

Babu et al.38 report a technique for guiding the entry of water into carbon nanopipes which were selectively modifi ed by the electrodeposition of polypyrrole on their tips. Th e authors conclude that this may be useful for constructing devices capable of controlling liquid fl ows at the nanoscale.

Molecular dynamics simulations of water in the pores of single-walled carbon nanotubes39 show that an electric fi eld can also modulate fi lling through dipole interactions. Th is eff ect might also be exploited to achieve selective control of liquid fl ow at the nanoscale and may play a part in natural biological pore transport mechanisms.

CHALLENGES IN BUILDING NANOFLUIDIC DEVICES

Many of the papers reviewed above describe novel phenomena and methods that will be directly applicable to the design and fabrication of nanofl uidic devices with practical application in medical diagnosis, sensing and materials processing. Before these ambitions can be achieved, a considerable number of practical hurdles will need to be overcome. In this section some approaches are considered that may have useful architectural or procedural application in the implementation of a viable nanofl uidics technology.

Microfluidics is a rapidly advancing field with emerging analytical and synthetic applications40. Cao et al.41 report one of the few experimental investigations of the problem of microfluidic to nanofluidic interfacing. Liquid samples for characterization or processing using nanoscale components are likely to start out as macroscale droplets. These must be guided into progressively smaller channels for ultimate delivery to the nanoscale device components. A number of potential problems arise. One is channel blocking due to large macromolecules or insoluble debris. This can already be a limiting hazard with microscale lab-on-a-chip systems. Work is currently taking place to understand the dynamics of particulate transport at the

Table 1 Comparison of observed fl ow velocity with predictions for pressure-driven fl ow through aligned MWNT membranes30. These results indicate liquid fl ow rates four to fi ve orders of magnitude faster than would be predicted from conventional fl uid-fl ow theory. The slip lengths are calculated from observed data. * Units: cm3 per cm2 at minimum pressure. †Flow velocities in cm s–1 at 1 bar.

Liquid Initial permeability* Observed flow velocity† Expected flow velocity† Slip length (µm)

Water 0.58 25 0.00057 541.01 43.9 0.00057 680.72 9.5 0.00015 39

Ethanol 0.35 4.5 0.00014 28Iso-Propanol 0.088 1.12 0.00077 13Hexane 0.44 5.6 0.00052 9.5Decane 0.053 0.67 0.00017 3.4


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nanoscale and to find low friction coatings or channel materials that help to reduce blocking42.

Another problem is more fundamental: the large size of polymers, including biologically relevant molecules such as DNA, which are oft en tangled and tightly folded in vivo. As Cao et al.41 point out, a typical DNA molecule from a virus has a length of 100 to 200 kilobases

and will form a random coil with a radius of some 700 nm in aqueous solution at 20 °C. Th is is several times greater than the pore diameter of even large carbon pipes and two orders of magnitude greater than the diameter of a single walled carbon nanotube such as might be functionalized to detect the presence of specifi c base sequences43. DNA molecules will fi t into the central channel of even small nanotubes,

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Figure 5 ESEM images of the dynamic behaviour of a water plug in a carbon nanopipe. a–e, The meniscus shape changes when the stage temperature is constant but the vapour pressure of the water in the chamber is changed: a, 5.5 Torr, b, 5.8 Torr, c, 6.0 Torr, d, 5.8 Torr and e, 5.7 Torr (where the meniscus returns to the shape seen in a). The asymmetrical shape of the meniscus, especially the complex shape of the meniscus on the right side in a and e is a result of the difference in the vapour pressure caused by the open left end and closed right end of the tube. Estimated contact angles between the meniscus and the tube wall are indicated at two locations. f, TEM image showing a similar plug shape in a closed carbon nanotube under pressure. Reproduced with permission from ref. 32. Copyright (2004) ACS.


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but only when unravelled and fed into the pore opening lengthwise. Th e entropic barrier to achieve this from the disordered state is very high and therefore such long molecules are normally excluded.

To overcome this problem of interfacing the microscale to the nanoscale, Cao et al. used optical lithography to fabricate an array of microchannels that form a gradient from wide to narrow fluid passages (Fig. 6a)41. They used a novel modified form of diffraction gradient lithography involving a photosensitive blocking mask resist on a silicon wafer substrate. The technique is inherently parallel and much faster and more efficient than using electron-beam lithography. The end result is a massive array of microposts, with a continuous reduction in the gaps, which form fluidic channels as the chip is traversed from one side to the other.

To test the device, long DNA strands stained with a fl uorescent dye were introduced on the microscale side (right-hand side of Fig. 6b). Diff usion of the molecules was then observed under ultraviolet light and captured on video. Still frames from the recording show individual DNA molecules straightening out and moving through the interface in an extended confi guration towards the nanoscale region (left -hand side of Fig. 6b). Th e authors report transport of the stretched DNA molecules with signifi cantly greater effi ciency compared with random diff usion through comparable nanoscale pores in the absence of any gradient interface.

Melechko et al.36 working at Oak Ridge National laboratory in Tennessee report a general method for creating patterned arrays of silica nanopipes precisely positioned over pores in a silicon nitride membrane on a silicon substrate. Th e method requires costly equipment and top-down fabrication techniques, but allows a high degree of control over device architecture. Critically, precise control of nanopipe location is achieved by deterministic positioning of catalyst particles for CVD growth.

Bau et al.44 describe another method for constructing a nanofl uidic device comprising a single carbon nanopipe (diameter 250 nm) connecting two fl uid reservoirs. Dielectrophoresis was used to orient and manipulate the nanopipe into position prior to depositing a dividing wall on top. Th ese general approaches enable prototyping of specifi c functional devices for research and testing. Once eff ective architectures have been developed, less expensive methods can be investigated for large-scale manufacture.

Within the past two years, a number of papers have started to appear reporting experimentally realized nanofl uidic devices. For example Flashbart and co-workers45 describe fabrication using contact printing of a microfl uidic array with nanochannel interconnects. Th is allows their device to take advantage of nanocapillary eff ects such as in situ size-based separation, enhanced mixing and molecular concentration. Han et al.46 have used surface micromachining on silicon nitride, silica and polysilicon in combination with micromoulded polydimethylsiloxane to fabricate fl exible nanofl uidic device architectures. Th ey have demonstrated attolitre electrokinetic injection using this technique. Finally, Wang et al.47 have created a nanoscale preconcentration device using standard photolithography and etching methods, which has achieved factors in the range of 106 to 108. Th ey exploit the electrokinetic trapping eff ect found in nanofl uidic fi lters.

CONCLUSIONS AND FUTURE DIRECTIONS

Molecular dynamics simulations16,17,48 have suggested that carbon nanotubes have very low surface friction with respect to fl uid fl ow. Th is is now being confi rmed by direct experiment on timescales which show fast fl ow corresponding to large slip lengths18,28,25. Water appears to wet carbon nanopipes with fl ow governed by the Washburn equation, but not for subnanosecond timescales. Control of liquid uptake has been achieved by applying a potential diff erence27,32,33

across the tubes and also by functionalizing tube entrances34,35. Th e problem of the transition from micro to nanofl ows is being tackled, for example, by the creation of patterned surfaces, which guide biomolecules through progressively smaller apertures36. Th e more control that can be engineered into nanopore arrays, the greater the prospect of using them in applications requiring high selectivity and fast throughput.

It is signifi cant that the size range of nanopores discussed in this review is essentially the size range of many important biological entities (antibodies, viruses). Larger molecules such as DNA can be uncoiled36 to fi t. Th us carbon nanopipes are potential conduits, collimators, sensors, encapsulators and probes for medical applications. Clearly there are still many challenges ahead before such devices become viable including: controlling the mechanical strength of nanoelements in contact with cells and tissue; methods for the assembly of huge numbers of very small components; fouling of the nanopipes and surfaces; management of defects in components; fl uid/biomolecule/disease marker input and output; and managing the information fl ow from large arrays of nanoscale sensors to the outside world.

In a biological context, it is worth noting the vital importance of nanoscale pores in natural systems, particularly the channels in cell membranes involved in ion and water transport. Aquaporin proteins, for the elucidation of which Peter Agre earned the Nobel Chemistry Prize in 2003 (ref. 49), are one example. As is oft en the case, science is only now starting to investigate and appreciate long-established natural mechanisms.

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Figure 6 Interfacing microfl uidics with nanofl uidics. a, Optical image of fl uidic channels defi ned by microposts on a chip (after photoresist development). A continuous reduction of the gaps (light areas) between microposts occurs in the gradient zone. b, Integrated CCD video images of DNA fl ow through the channels. The right-hand side of the image shows partially stretched long DNA molecules passing through the micropost array and the gradient zone and continuously entering the nanochannels on the left-hand side, becoming fully stretched in the process. Reproduced with permission from ref. 41. Copyright (2002) AIP.


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doi: 10.1038/nnano.2006.175

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AcknowledgementsM. Whitby acknowledges support from the EPSRC through a Doctoral Training Award and thanks

F. Barclay and T. Cotter at RGB Research for helpful discussions. The authors are members of the EU

network Inside POReS.


whitby, m. and n. quirke (2007). fluid flow in carbon nanotubes and nanopipes. nature nanotechnology...

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