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Nanoscience is one of the most important research anddevelopment frontiers in modern science1. The word‘nano’ means one billionth, and the nanometre (nm,10–9 m) defines the length scale that is used to measuresystems being studied in nanoscience. In the most sim-plistic sense, nanoscience is the science of small particlesof materials. Such small particles are of interest from afundamental viewpoint because all properties of amaterial, such as its melting point, and its electronic andoptical properties, change when the size of the particlesthat make up the material become nanoscopic. Withnew properties come new opportunities for technologi-cal and commercial development, and applications ofnanoparticles have been shown or proposed in areas asdiverse as microelectronics, coatings and paints, andbiotechnology.

For example, one application that is now in use inthe commercial sector involves using gold nanoparti-cles as visual indicators in over-the-counter medicaldiagnostic kits2. This application is a good example ofhow the unique properties of a nanoparticle can leadto new technological opportunities. Macroscopicsamples of pure gold have only one colour (gold), butnanoparticles of gold can show essentially all thecolours of the rainbow, depending on the size and the shape of the nanoparticle3. Furthermore, theintensity of the optical absorption of gold nanoparti-cles is extraordinarily strong, which means that when

suspended in a solution or deposited onto a surface,the naked eye can detect a very small quantity of theseparticles. These properties make gold nanoparticlesideally suited as visual indicators.

Other applications of micro- and nanoparticles inthe biomedical sciences and in biotechnology includetheir use as vehicles for enzyme encapsulation4, DNAtransfection5–7, BIOSENSORS8–10 and drug delivery11–13.For example, drugs can be incorporated into nano-spheres that are composed of a biodegradable poly-mer, and this allows for the timed release of the drugas the nanospheres degrade11,12. The circumstancesthat cause the particle to degrade can be adjusted byvarying the nature of the chemical bonding withinthe particle. For example, when acid-labile bonds areused, the particles degrade in acidic microenviron-ments, such as would exist in tumour cells or arounda site of inflammation13, and so this approach allowssite-specific drug delivery. In another recent study,polymeric nanoparticles were labelled on their outersurfaces with a viral peptide sequence that promotesthe permeation of substances through cell mem-branes14. These peptide-derivatized nanoparticlespassed through cell membranes, and were incorpo-rated into living cells at much higher levels thannanoparticles without the surface-bound peptide.

Surface-functionalized nanoparticles can also beused to deliver genetic material into living cells, a process

THE EMERGING FIELD OFNANOTUBE BIOTECHNOLOGYCharles R. Martin and Punit Kohli

Nanoparticles are being developed for a host of biomedical and biotechnological applications,including drug delivery, enzyme immobilization and DNA transfection. Spherical nanoparticlesare typically used for such applications, which reflects the fact that spheres are easier to makethan other shapes. Micro- and nanotubes — structures that resemble tiny drinking straws —are alternatives that might offer advantages over spherical nanoparticles for some applications.This article discusses four approaches for making micro- and nanotubes, and reviews thecurrent status of efforts to develop biomedical and biotechnological applications of thesetubular structures.

Department of Chemistryand Center for Research at the Bio/Nano Interface,University of Florida,Gainesville, Florida 32611-7200, USA.Correspondence to C.R.M.e-mail:crmartin@chem.ufl.edudoi:10.1038/nrd988

BIOSENSOR

A chemical or electronic devicethat, when immersed in asolution (for example, a watersample or an air sample), bindsa particular chemical com-ponent of the solution andproduces an electronic signalthat is proportional to theconcentration of thecomponent in the solution.

© 2002 Nature Publishing Group

CHIRAL

The property of a molecule thathas no plane of symmetry suchthat it cannot be superimposedon its mirror image. Thistypically occurs when themolecule has at least one carbonatom that is bonded to fourdifferent groups. A chiralmolecule is like a person’s hands:the right and left hands appearthe same, but they are, in fact,non-superimposable mirrorimages of each other.

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We review here some emergent technologies forpreparing micro- and nanotubes, and give examples ofrecent biotechnological applications of these tubularsystems. This review is not intended to be all encom-passing; instead, it discusses recent trends and empha-sizes some particularly promising technologies. Systemsreviewed include self-assembling lipid microtubes,fullerene carbon nanotubes, cyclic peptide nanotubesand template-synthesized nanotubes.

NomenclatureIn the nanotechnology literature, the tubular structuresof interest to this review are referred to variously asmicrotubules, microtubes, nanotubules and nanotubes.This nomenclature can be simplified by noting that, inthis context, there is no difference between a tube andtubule; in this review we use only the term tube. The dis-tinction between nano and micro is a more difficultissue to resolve, as there is no universally accepteddimension scale above which a particle is designated amicroparticle, and below which it is deemed to be ananoparticle. In our research group we have agreed thatif a tube has at least one dimension that is 100 nm orless, it is called a nanotube.

Self-assembling lipid microtubesIn 1984, Yagar and Schoen of the US Naval ResearchLaboratory showed that CHIRAL lipid molecules of thetype shown in FIG. 2 self-assemble in solution to frommicrotubes16. These microtubes have diameters ofabout 500 nm, lengths varying from about 50 to severalhundred micrometers, and wall thicknesses of 10–60nm or more17. The theory of this self-assembly processhas been discussed by Selinger and colleagues18. In themost simplistic interpretation, the lipids self-assemblein solution to form planar bilayer sheets, which thencurl up to make microtubes.

One proposed application of lipid microtubes is asvehicles for controlled drug release. For example, Priceand Patchan19 coated such tubes with metallic copper toimprove their mechanical strength, and then loaded thesetubes with antibiotics that are used to prevent marinefouling. The tubes were then incorporated into a paint,which was applied to fibreglass rods. This microtube-based paint successfully inhibited marine fouling duringthe six-month testing of these rods in ocean water.Microtubes of this type have also been used for controlledrelease of testosterone in living rats20. Although theseapplications provide a proof-of-concept for the use oflipid microtubes as drug-delivery vehicles, lipid micro-tubes are mechanically weak, and, as indicated above,must be coated before use. In addition, because the abilityto form tubes is connected with the unique chemistry andchirality of the lipids used, it would be difficult to use thisapproach to make tubes with any desired set of chemical,biochemical or physical properties.

Fullerene carbon nanotubesIn 1985, Smalley and co-workers at Rice University dis-covered a new form of carbon — buckminsterfullereneor C

60 (REF. 33) — a discovery that ultimately led to the

known as transfection. For example, silica nanosphereslabelled on their outer surfaces with cationic ammoniumgroups bind DNA — a polyanion — through electro-static interactions5. These nanoparticles were then usedto deliver the surface-bound DNA into cells. That trans-fection was successful was proven by showing that thecells produced the protein encoded by the attached DNAsequences. There is also tremendous interest in usingcationic lipid-based nanospheres — liposomes — asDNA transfection agents6,7. Finally, the gold nanoparti-cles introduced above have been used extensively in thedevelopment of new types of gene sensor8–10.

Spherical nanoparticles are typically used in suchapplications, but this only reflects the fact that spheres areeasier to make than other shapes. Micro- and nano-tubes — structures that resemble tiny drinking straws(FIG. 1) — are alternatives to spherical nanoparticles.Examples include organosilicon polymer nanotubes15,self-assembling lipid microtubes16–20, fullerene carbonnanotubes21–24, template-synthesized nanotubes25–28 andpeptide nanotubes29–32. Nanotubes offer some interestingadvantages relative to spherical nanoparticles forbiotechnological applications. For example, nanotubeshave large inner volumes (relative to the dimensions ofthe tube), which can be filled with any desired chemicalor biochemical species, ranging in size from small mole-cules through to proteins25,26. In addition, nanotubeshave distinct inner and outer surfaces, which can bedifferentially modified for chemical or biochemicalfunctionalization25. This creates the possibility, for exam-ple, of loading the inside of a nanotube with a particularbiochemical payload, and at the same time impartingchemical features to the outer surface that render it bio-compatible. Finally, nanotubes have open mouths, whichmakes the inner surface accessible and subsequent incor-poration of species within the tubes particularly easy.

100 nm

Figure 1 | Scanning electron micrograph of an array of template-synthesized carbonnanotubes. These nanotubes are composed of disordered graphitic carbon.

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depending on the way they are rolled up, they haveelectronic properties similar to both metals and semi-conductors23,35,36. Several groups have shown thatfullerene nanotubes can be used as components ofelectronic circuit elements24,36,37.

In addition, there have been several recent reportsdescribing the attachment of biomolecules onto andinto fullerene carbon nanotubes38–42. For example, Daiand colleagues38 used a simple non-covalent route toattach a reactive molecule to the sidewalls of single-walled fullerene nanotubes. This reactive moleculecould then be used to attach proteins to the walls ofthese nanotubes. This and related work is of interest tothe development of biosensors based on fullerene nan-otubes39.

Lieber’s group has pioneered the use of single-walledfullerene carbon nanotubes (SWNTs) as probe tips foratomic-force microscopy (AFM) imaging of biomacro-molecules, such as antibodies, DNA and β-amyloidprotofibrils (a constituent of the amyloid plaques that arecharacteristic of Alzheimer’s disease)42,43.As discussed inthat recent review43, AFM is a scanning-probemicroscopy technique that has molecular-level resolutioncapabilities. Lieber and colleagues have shown thatSWNTs are ideal probe tips for AFM because of theirsmall diameter (which maximizes resolution), highaspect ratio, excellent chemical and mechanical robust-ness, and extraordinary stiffness43. Lieber’s group hasattached biomolecules to SWNTs and used them as‘molecular-recognition’AFM tips42, an approach that isused to study chemical forces between molecules. Forexample, the molecule biotin was attached to a SWNT,and this tip was then used to explore the binding interac-tion between biotin and the protein streptavidin42.

Nobel Prize in Chemistry being awarded to Smalley,Kroto and Curl in 199633. Fullerene molecules of thistype can be prepared by a carbon-arc discharge method,and in 1991, Iijima21 discovered nanotubes in the prod-ucts obtained from such a reactor (FIG. 3). These tubesare in essence rolled-up, highly ordered graphene sheets,and they can be single-walled or multi-walled. They aretypically referred to in the literature as simply carbonnanotubes; however, as there are examples of nanotubescomposed of more disordered forms of carbon (FIG. 1)34,the more precise name, fullerene carbon nanotubes, isused here. These nanotubes have diameters rangingfrom one to tens of nanometres and their length canrange from a few micrometres to hundreds of microme-tres, or more22. Since their discovery, there has been amassive international research effort aimed at under-standing the properties of fullerene carbon nanotubesand at developing applications for these nanotubes, andseveral authoritative reviews on fullerene nanotubeshave recently been written22–24.

One proposed non-biomedical application is in thearea of microelectronics, in which the ultimate objec-tive is to make electronic circuit elements that are com-posed of these nanoscopic carbon tubes24,35–37. It iswidely believed that the current silicon-based litho-graphic technology will reach its limit during the nextdecade, and this research effort is driven by the need tofurther miniaturize electronic circuits so that increasingnumbers of circuits can be accommodated on a singlechip. Fullerene nanotubes are interesting alternatives,not only because they are small, but also because

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Figure 2 | Typical lipids used to make self-assembling lipid microtubes. These lipids self-assemble in solution to yield planar sheets that then roll-up into microtubes.

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Figure 3 | Transmission electron micrograph of fullerenecarbon nanotubes. The three converging objects are each asmall bundle (‘rope’) of hexagonally close-packed single-wallcarbon nanotubes. These ropes are a little greater than 10 nm in diameter and so contain ~ 50–100 nanotubes, eachapproximately 1.35 nm in diameter. Image kindly provided byAndrew Rinzler, University of Florida, USA.

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point — living systems have evolved and routinely usenanotubes, more commonly referred to as proteinchannels. The most prevalent examples are the ligand-gated and voltage-gated ion channels that are used forelectrical signalling in our brains, nerves and muscles46.Another type of protein channel, the α-haemolysin(αHL) channel (FIG. 5a), is used as the sensing element instochastic sensors. In these devices, a lipid bilayer mem-brane containing a single αHL channel is mountedbetween two electrolyte solutions, a constant transmem-brane potential is applied and the resulting transmem-brane current is measured. This current is associated withtransport of ions down the central pore that runsthrough the αHL channel44,45.

In the simplest terms, when the chemical species tobe sensed (the ‘analyte’) enters the channel, it acts as acork and blocks the pathway for ionic conductionthrough the channel. This results in a transient decrease,or downward pulse, in the measured transmembraneion current. The term ‘stochastic’ (associated withrandomness) is used because the analyte enters andleaves the channel by diffusion — that is, as a result of arandom walk47. The number of such current pulses isrelated to the concentration of the analyte, and the dura-tion of the pulse provides clues as to the identity of theanalyte. Analytes detected in this way include metal ions,DNA chains and small organic molecules44,45.

However, small organic molecules by themselvescannot appreciably block the ion current, and so alarger ‘molecular adaptor’ that binds such analytes mustbe used. Bayley, Ghadiri and colleagues demonstratedthat cyclic peptides can be used as molecular adaptors44.In this case, the cyclic peptide becomes transientlylodged within the lumen of the channel (FIG. 5b), andthis can be detected as a pulse in the transmembrane

Peptide nanotubesIn 1993, Ghadiri and co-workers at the ScrippsResearch Institute described a fascinating class of nano-tubes that are based on cyclic peptide molecules thatconsist of an even number of alternating D- and L-AMINO

ACIDS (FIG. 4)29. They showed that these molecules self-assemble through HYDROGEN-BONDING interactions intonanotubes, which in turn self-assemble into orderedparallel arrays of nanotubes. The number of amino-acidresidues in the ring determines the inside diameter ofthe nanotubes obtained; as an example, a 12-amino-acid ring produces nanotubes with an inside diameterof 1.3 nm30. The chemical functional groups on theoutside of the walls of these nanotubes can be varied byvarying the amino-acid composition of the ring.

Ghadiri and colleagues have shown a number ofbiomedical/biotechnological applications for thesenanotubes. Perhaps the most interesting is as a new classof antibiotics against bacterial pathogens31. Fernadanez-Lopez et al. showed that cyclic peptides comprising sixand eight amino-acid residues act preferentially on bothGram-positive and Gram-negative bacteria relative tomammalian cells31. These data indicate that nanotubesformed from these peptides insert into the cell wall ofthe bacterium, which results in rapid cell death. Ghadiriand colleagues have also shown that these nanotubescan function as artificial ion channels in lipid bilayermembranes by self-assembling across the membrane32,and ion-transport rates comparable to those of naturallyoccurring ion channels were observed.

These cyclic peptides have also been used as compo-nents in stochastic sensors — an elegant new chemicaland biochemical sensing technology that was devel-oped by Bayley’s group at Texas A&M University44,45.This example raises an interesting fundamental

D- VERSUS L- AMINO ACIDS

The amino acids that make upproteins are CHIRAL (see above).The two forms are calledenantiomers. One enantiomer is called the D-enantiomer of theamino acid and the other iscalled the L-enantiomer. Theseterms come from the directionthat a solution of theenantiomer rotates the plane of plane-polarized right.

HYDROGEN BONDING

A strong type of dipole–dipoleinteraction that can occurbetween two molecules. Onemolecule must have a hydrogenatom bound to a veryelectronegative atom, such asoxygen, nitrogen or fluorine;the other must have a lone pairof electrons on such an atom.

NANOWIRE

A wire of a material (forexample, a metal), the diameterof which is less than 100 nm.

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Figure 4 | Cyclic peptide nanotubes. A typical chemical structure for a cyclic peptide and schematic illustrations of the self-assemblyof such peptides into nanotubes and nanotube arrays.

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nanostructures with monodisperse diameters andlengths are obtained, and, depending on the membraneand synthetic method used, these can be solid NANOWIRES

or hollow nanotubes. This method has been used toprepare nanowires and nanotubes composed of manytypes of material, including metals, polymers, semicon-ductors and carbons3,48. In addition, the templatemethod can be used to prepare composite nanostruc-tures, including both concentric tubular composites49

and segmented composite nanowires50.How nanotubes are made within the pores of a

template membrane is shown by the carbon nanotubesin FIG. 1 (REF. 34). An alumina template (FIG. 6a) washeated to 670 ºC and ethylene gas was passed through

ion current. The analyte in turn becomes transientlybound within the annulus of the cyclic peptide (FIG. 5b),and these binding and unbinding events produce asecond set of current pulses. This is an interesting casein which a small molecule (the analyte) is inserted intoa small tube (the cyclic peptide), which is in turninserted into a larger tube (the αHL channel).

Template-synthesized nanotubesThe template method is a general approach for prepar-ing nanomaterials that involves the synthesis or deposi-tion of the desired material within the cylindrical andmonodisperse pores of a nanopore membrane or othersolid surface3,48 (see, for example, FIG. 6a). Cylindrical

IN-PORE POLYMERIZATION

A polymerization process thattakes place within the pores ofone of the template membranes,which is used to make apolymeric nanowire ornanotube.

ELECTROLESS DEPOSITION

A method for coating metalfilms onto non-metallic surfaces,such as plastics. It involvesreducing an ionic form of themetal at the surface using achemical reducing agent.

Figure 5 | The α-haemolysin protein channel, an example of a naturally occuring nanotube. a | Structure of α-haemolysin,(based on an image kindly provided by H. Bayley, Texas A&M University, USA). b,c | Schematic representation of the use of acyclic peptide as a ‘molecular adaptor’ for stochastic sensing with the α-haemolysin channel.

a Surface Cross-section b

100 nm

Figure 6 | Scanning electron micrographs. a | The surface and cross-section of a typical nanopore alumina template membraneprepared in the lab of C. R. Martin and P. Kohli. Pores with monodisperse diameters that run like tunnels through the thickness of themembrane are obtained. b | Silica nanotubes prepared by sol–gel template synthesis within the pores of a template like that shownin part a. After sol–gel synthesis of the nanotubes, the template was dissolved and the nanotubes were collected by filtration.

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outside diameters determined by the diameter of thepores in the template; the inside diameter is determinedby the carbon deposition time. Other synthetic methodsused include IN-PORE POLYMERIZATION to make polymericnanotubes, ELECTROLESS DEPOSITION to generate metal nano-tubes, and SOL–GEL CHEMISTRY to manufacture nanotubescomposed of silica and other inorganic materials3,48.Templated silica nanotubes25,26 are shown in FIG. 6b.These nanotubes are all the same length, which is deter-mined by the thickness of the template membrane.

Silica nanotubes have been used as test vehicles toshow the power of the template method for preparingnanotubes for biotechnological applications25,26. Silicananotubes are ideal for such proof-of-concept experi-ments because they are easy to make and readily suspend-able in aqueous solution. Furthermore, silica surfacescan be derivatized with an enormous variety of differentchemical functional groups using simple silane chem-istry with commercially available reagents.

Attaching functional groups to nanotube surfaces.As noted above, one of the most important attributes ofa nanotube is that it has distinct inner and outer surfacesthat can be differentially modified for chemical and bio-chemical functionalization. The template method pro-vides a particularly easy route to accomplish this differ-ential functionalization25. FIGURE 7a represents across-section of the alumina template membrane (FIG. 6a)

and shows two pores. FIGURE 7b illustrates this mem-brane after sol–gel template synthesis of the silica nano-tubes within the pores (blue colour). Note that thesurfaces of the membrane are coated with thin silicafilms, which are removed by a brief mechanical polish25.While still embedded within the pores of the templatemembrane, the inner nanotube surfaces are reacted witha silane reagent that contains the functional group to beattached to the inner surfaces (green in FIG. 7c). Thissilane cannot attach to the outer nanotube surfacesbecause the outer surfaces are in contact with the porewall and are therefore masked. The template is then dis-solved to liberate the nanotubes, which unmasks theouter nanotube surfaces (FIG. 7d). The liberated nan-otubes are then exposed to a second silane that containsthe functional group to be attached to the outer nan-otube surfaces (red in FIG. 7e).

Nanotubes for biological extraction and biocatalysis. Oneapplication for such differentially functionalized nano-tubes is as smart nanophase extractors to remove specificmolecules from solution. For example, nanotubes withhydrophilic chemistry on their outer surfaces andLIPOPHILIC chemistry on their inner surfaces can be usedto extract lipophilic chemicals and drugs from aqueoussolution25. The ability to sequester lipophilic moleculescan be viewed as a generic type of extraction selectivity,which might be useful in some applications. However,nanotubes that have molecular-recognition capabilitiesand extract only one particular molecule from solutionmight also be useful. We have shown that antibody-functionalized nanotubes can provide the ultimate inextraction selectivity — the extraction of one enantiomer

the membrane. This causes the ethylene to decomposeon the pore walls to yield graphitic carbon nanotubeswithin the pores. The alumina template membrane canthen be dissolved away and the carbon nanotubes arecollected by filtration. These tubes have monodisperse

SOL–GEL CHEMISTRY

A versatile route for synthesizingtypically inorganic materials,such as semiconductors andsilica. This method involveshydrolysis of a molecularprecursor followed by thermaltreatment, typically in air.

LIPOPHILIC

The propensity of a molecule to dissolve in a non-polar,oil-like solvent. This termoriginates from the fact thatnaturally occurring lipidmolecules have this propensity.

RACEMIC MIXTURE

An equimolar mixture of the twoenantiomers of a chiral molecule.

ENANTIOSEPARATIONS

The separation of oneenantiomer of a chiral moleculefrom the other. This is animportant issue in drugdevelopment because drugmolecules are typically chiral,and only one of theenantiomers tends to beefficacious.

STEREOISOMERS

Isomers are chemicalcompounds that have the samemolecular formula but differentmolecular structures (structuralisomers) or differentarrangements of atoms in space (stereoisomers).

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Figure 7 | Schematic of the procedure used to attachdifferent chemical functional groups to the inner versusouter surfaces of template-synthesized nanotubes.a | Cross-section of the template membrane. b | Membraneafter template synthesis of nanotubes within the pores (blue).c | Membrane after attachment of the first functional group(green) to the inner nanotube surfaces. d | Liberatednanotubes after dissolution of the template membrane. e | Membranes after attachment of the second functionalgroup to the outer nanotube surfaces (red).

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of the SR and RS enantiomers of FTB. The antibody-containing nanotubes extracted only the RS enantiomerfrom the racemic mixture25.

The silica nanotubes can also be used as bioreactors.For example, the enzyme glucose oxidase was attachedto 60-nm diameter nanotubes, which were then used tocatalyse the oxidation of glucose.

Finally, biocompatibility is an important issue forpossible in vivo applications of nanotube technology. Itis well known that materials can be made biocompati-ble, as evidenced by decreased protein adsorption, byattaching poly(ethylene glycol) (PEG) chains to theirsurfaces53. We have shown that when a PEG–silane isattached to the silica nanotubes, adsorption of IgGimmunoglobulins is strongly suppressed relative tonanotubes that do not contain the attached PEG (S. B.Lee and C. R. Martin, unpublished observations).

Nanotube membranes for bioseparations. In all of theexamples cited above, the template membrane was dis-solved away and the liberated nanotubes were collectedby filtration. The nanotubes can also be left embeddedwithin the pores of the template to yield a free-standingnanotube-containing membrane. We have shown thatthe nanotubes can act as conduits for the highly selectivetransport of molecules and ions between solutions thatare present on either side of the membrane26–28. Forexample, membranes containing gold nanotubes withinside diameters of molecular dimensions — that is,<1 nm — cleanly separate small molecules on the basisof molecular size28. Gold nanotubes with larger insidediameters (20–60 nm) can be used to separate proteins,and, in this instance, as with silica nanotubes, renderingthe nanotubes biocompatible is essential to prevent pro-tein adsorption53. Generic chemical transport selectivity(for example, lipophilic versus hydrophilic) can also beimparted to these gold nanotube membranes54.

More recently, it has been shown that membranescontaining the silica nanotubes and the enantioselec-tive FTB antibody Fab fragment discussed above canbe used for enantioseparations26. In this case, the nano-tube membrane separated a feed half-cell containing aracemic mixture of the RS and SR enantiomers of FTBfrom a permeate half-cell that initially contained onlybuffer solution. The time course of permeation of thetwo enantiomers across the membrane was deter-mined by periodically assaying the permeate solution.Results for a membrane containing silica membranesthat were ~15-nm in diameter are shown in FIGURE 8b

as plots of moles of enantiomer transported againstpermeation time. The flux of the RS enantiomer (theone bound by the antibody) is five times higher thanthe flux of the SR enantiomer. Membranes containingsilica nanotubes with larger inside diameters showlower RS versus SR transport selectivity.26 Hence, thisis an example in which the nanoscopic inside diameterof the silica nanotubes is crucial to achieving selectiv-ity in the separation.

Enantioseparations are an important issue in drugdevelopment because, similar to FTB, most drug mole-cules are chiral, and only one of the enantiomers is

from a RACEMIC MIXTURE of a chiral drug molecule. This isthe ultimate in selectivity because the two enantiomersof a chiral molecule are chemically identical. As mostmethods of chemical separation exploit differences inthe chemical properties between molecules, ENANTIOSEPA-

RATIONS are particularly difficult.In collaboration with Hans Soderlund of VTT

Biotechnology, we have been investigating an antibodythat selectively binds one enantiomer of the drug diary-lalkyltriazole (FTB) (FIG. 8a), an inhibitor of aromataseenzyme activity51. This molecule has two chiral centresand therefore four STEREOISOMERS — RR, SS, SR and RS.Soderlund supplied us with the Fab fragment52 of anantibody that selectively binds the RS relative to the SRenantiomer. This antibody fragment was immobilizedto both the inner and outer surfaces of the silica nano-tubes, which were then dispersed into a racemic mixture

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Figure 8 | Enantioseparations. a | Three-dimensional structures of the RS enantiomer (left) andthe SR enantiomer (right) of the drug diarylalkyltriazole. The black, white, blue, red and yellow ballsare carbon, hydrogen, nitrogen, oxygen and fluorine, respectively, and * denote the chiral centres.The geometry optimization was performed by ab initio calculation with minimal basis set inHyperChem 6.03. The drug is also called Finrozole and is being developed by Hormos MedicalCorporation to treat lower-urinary-tract symptoms. b | Plots of moles of each enantiomertransported versus time for a silica nanotube membrane containing the enantioselective antibodyFab fragment. The inside diameter of the nanotubes was ~15 nm.

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interactions between the payload and the inside walls ofthe nanotubes. However, in some applications it mightbe useful to simply fill the nanotube with a payload andthen apply caps to the nanotube ends to keep the pay-load encapsulated. Furthermore, it might be useful forthese caps to fall off, so spilling the payload, when a par-ticular chemical or biochemical signal is detected. Weare now exploring routes for preparing such cappednanotubes. The issues of cost of production and massproduction of nanotubes must also be addressed.

Finally, the synthetic nanotubes discussed here canbe thought of as mimics of naturally occurring nano-tubes — protein channels46. Indeed, the cyclic peptidenanotubes have been used as artificial ion channels32

that open and close in response to chemical and electri-cal stimuli. We are developing synthetic nanotubes withsimilar voltage27,55 and chemical56 gating characteristics.These nanotubes can be used in smart membranes, thetransport properties of which change in response to anelectrical stimulus27,55, and in sensing devices in which achemical stimulus turns on a current that can be mea-sured in an external circuit56.

efficacious; indeed, there are examples in which theother enantiomer has deleterious effects. As antibodiescan, in principle, be obtained that selectively bind toany desired molecule or enantiomer, this concept mightprovide a general approach for obtaining selectivelypermeable membranes for a host of enantio- and otherbioseparations.

ConclusionsWe believe that nanotubes offer some important advan-tages for biotechnological applications of nanoparticles.As a result of its tremendous versatility in terms ofmaterials that can be used, sizes that can be obtainedand chemistry and biochemistry that can be applied thetemplate method might prove to be a particularlyadvantageous approach for preparing nanotubes forsuch applications. However, this field of nanotubebiotechnology is in its infancy, and there is much workto be done before products based on this technology arebrought to the marketplace. For example, in our appli-cations so far, we have incorporated the payload into thenanotubes by either covalent bonding or other chemical

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2. Martin, C. R. & Mitchell, D. T. Nanomaterials in analyticalchemistry. Anal. Chem. 70, 322A–327A (1998).

3. Martin, C. R. Nanomaterials — a membrane-basedsynthetic approach. Science 266, 1961–1966 (1994).A review of the concepts that underlie templatesynthesis.

4. Chang, T. M. S. & Prakash, S. Procedures formicroencapsulation of enzymes, cells and geneticallyengineered microorganisms. Mol. Biotechnol. 17, 249–260(2001).

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AcknowledgementsThis work was supported by the National Science Foundation,(Nanoscale Interdisciplinary Research Teams for Biomedical

Nanotube Technology) the Office of Naval Research and the UFEngineering Research Center for Particle Science and Technology.We gratefully acknowledge our collaborator H. Soderlund for con-tributions to the antibody research projects.

Online links

DATABASESThe following terms in this article are linked online to:LocusLink: http://www.ncbi.nlm.nih.gov/LocusLink/Glucose oxidaseSWISS-PROT: http://ca.expasy.org/sprot/α-haemolysinOnline Mendelian Inheritance in Man:http://www.ncbi.nlm.nih.gov/Omim/Alzheimer’s disease

FURTHER INFORMATIONC. R. Martin’s lab:http://www.chem.ufl.edu/~crmartin/ Access to this interactive links box is free online.