scaffolding and ald on porous templates

Phys. Status Solidi B 247, No. 10, 2412–2423 (2010) / DOI 10.1002/pssb.201046208 p s sb

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Preparation and magnetoviscosity ofnanotube ferrofluids by viralscaffolding and ALD on porous templates

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Zhenyu Wu1, Robert Zierold2, Anna Mueller3, S. Emil Ruff3, Chenchen Ma1, Abid A. Khan4, Fania Geiger3,Bernd A. Sommer3, Mato Knez5, Kornelius Nielsch*,2, Alexander M. Bittner**,4,6, Christina Wege***,3,and Carl E. Krill III****,1

1 Institute of Micro and Nanomaterials, Ulm University, Albert-Einstein-Allee 47, 89081 Ulm, Germany2 Institute of Applied Physics, University of Hamburg, Jungiusstraße 11, 20355 Hamburg, Germany3 Institute of Biology, University of Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany4 CIC nanoGUNE Consolider, Tolosa Hiribidea 76, 20018 Donostia – San Sebastian, Spain5 Max Planck Institute of Microstructure Physics, Am Weinberg 2, 06120 Halle, Germany6 IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain

Received 23 April 2010, revised 30 July 2010, accepted 3 August 2010

Published online 8 September 2010

Keywords atomic layer deposition, ferrofluids, nanotubes, synthesis, templates, Tobacco mosaic virus

* Corresponding authors: e-mail [email protected], Phone: þ49 40 42838 6521, Fax: þ49 40 42838 3589** e-mail [email protected], Phone: þ34 943 574 000, Fax: þ34 943 574 001*** e-mail [email protected], Phone: þ49 711 685 65073, Fax: þ49 711 685 65096**** e-mail [email protected], Phone: þ49 731 50 25476, Fax: þ49 731 50 25488

Current models for magnetoviscosity suggest that replacing

the spherical nanoparticles of a conventional ferrofluid with

magnetic nanotubes would lead to a stronger field-induced

viscosity enhancement and a much-improved stability against

shear thinning – two important parameters for technological

exploitation of the magnetoviscous effect. We report the

development of positive and negative templating strategies

for the synthesis of magnetic nanotubes out of a variety of

materials. Our positive template is Tobacco mosaic virus

(TMV) – in natural form or genetically engineered to express

specific surface chemistries and lengths – which we exploit as a

template for the electroless deposition (ELD) of nanosized

clusters of nickel and as a scaffold for magnetic particles in

a conventional ferrofluid. Our negative templating strategy

employs porous anodic aluminum oxide (AAO) as a substrate

for the atomic layer deposition (ALD) of a conformal coating of

iron oxide, offering precise control over the length and wall

thickness of the resulting nanotubes. Both strategies were

scaled up to produce the mass quantities of uniform-aspect-ratio

nanotubes that are needed for macroscopic ferrofluid volumes.

The magnetoviscosity of these ‘‘nanotube ferrofluid’’ samples

was studied as a function of applied magnetic field and shear

frequency, and a particularly strong effect was found to be

induced by viral scaffolding.

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction First successfully prepared in the1960s, ferrofluids are composite substances that combinethe magnetic properties of magnetic nanoparticles with thefluid properties of the carrier liquid in which the nanopar-ticles are suspended [1]. The latter, which are typically on theorder of 10 nm in diameter, consist of single magneticdomains, either in the superparamagnetic or ferromagneticstate at room temperature. Owing to the fact that the particlesare free to rotate, the magnetic hysteresis loops of a ferrofluidmanifest the Langevin shape of an ideal paramagnet, but with

a much larger susceptibility [2] – a property that makes itpossible for ferrofluids to be held in place or manipulated byexternally applied magnetic fields. This characteristic has, inturn, led to niche applications of ferrofluids in areas asdiverse as vacuum technology (creating low-friction, air-tight seals around rotating shafts) and high-end audiosystems (transferring heat from the voice coils of high-power loudspeakers) [3]. Recent interest in ferrofluids can betraced to their potential uses in biomedical contexts, such asattacking cancer cells via magnetic hyperthermia or in the


Phys. Status Solidi B 247, No. 10 (2010) 2413

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detection and removal of pathogens from liquid suspensionsby magnetic filtration [4, 5]. Furthermore, ferrofluids havefound application in ‘‘lab-on-a-chip’’ devices as a keyelement in microfluidic valves and pumps [6].

The technological exploitation of ferrofluids relies notonly on the interaction between magnetic fields andnanoparticles, but also on the coupling between thesuspended nanoparticles and the surrounding liquid. Onesurprising consequence of this coupling is the fact that evenmodest magnetic fields have the potential to increase theviscosity of a ferrofluid by several orders of magnitude [7, 8].This so-called magnetoviscous effect has recently foundapplication, for example, in the suspension systems ofautomobiles outfitted with MagneRide1 shock absorbers[9], which feature a damping behavior (i.e., stiffness) that istunable via a built-in electromagnetic coil.1 The firstattempts to understand the origin of the magnetic-field-induced viscosity enhancement in ferrofluids were based ona model of noninteracting monodisperse nanoparticles [10];however, the predicted increase in viscosity turned out to beorders of magnitude smaller than the effect actuallymeasured. Current theoretical models [11] – supported bycomputer simulations as well as cryogenic transmissionelectron microscopy (TEM) [12] and small-angle neutronscattering (SANS) [13, 14] studies of ferrofluid microstruc-ture as a function of the applied magnetic field strength –posit the field-induced formation of chain-like aggregates ofnanoparticles, which align themselves with the applied fieldand thereby exert a much stronger drag on the surroundingfluid than would the same nanoparticles if suspended inisolation. An appealing aspect of this model is its ability toaccount for the strong decrease in magnetoviscosity thatis observed in ferrofluids subjected to shear forces [8], asthe latter (partially) overcome the interactions holdingthe nanoparticle chains together, thus counteracting themechanism for viscosity enhancement.

The decrease in viscosity of a complex fluid followingexposure to shear forces – known as shear thinning – is amajor obstacle to the use of ferrofluids in situations callingfor the transmission of forces or torques, such as in activedamping systems. If it were possible to replace the looselybound nanoparticle chains of conventional ferrofluids withstiff magnetic nanorods or nanotubes, then the resultingferrofluid might be expected to retain its field-inducedviscosity enhancement even at high shear rates [15]. Not onlyshould aspherical magnetic nanoparticles induce significantviscosity enhancement when held in place by an appliedmagnetic field, but a suspension of such particles would beexpected to be less susceptible to shear thinning, since theparticles will not lose their shape anisotropy in the event of

1 The working substance in these shock absorbers is a magnetorheological

fluid (MR) rather than a ferrofluid. Because the ferromagnetic particles in

MR fluids are typically micrometer-sized, they do not remain in suspension

indefinitely; consequently, MR fluids tend to suffer from sedimentation

and agglomeration to a much greater extent than do ferrofluids.

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shearing forces, as is the case with fragile chain-likeagglomerates of spherical nanoparticles.

Conventional ferrofluids generally contain 1015–1017

nanoparticles per milliliter of carrier liquid. Obviously, ifmacroscopic quantities of a ‘‘nanorod/nanotube ferrofluid’’are to be prepared, then an approach must be found to massproduce aspherical magnetic particles at reasonable cost. Ofthe numerous techniques that have been developed todate for the synthesis of rod or tube-shaped nanoparticles[16–19], the only ones that appear to lend themselves tobeing scaled up to the extent required for a ferrofluid arethose based on positive or negative templating [19]. Whenthe goal is to synthesize linear structures like rods or tubes,then positive templating would employ one-dimensionalobjects, such as nanowires, nanorods, or carbon nanotubes,as substrates onto which functional materials are deposited(Fig. 1). Negative templating in the one-dimensional casewould rely on objects having cylindrical pores, the insidesurfaces of which can be coated by functional materials.Typical negative templates are porous anodic aluminumoxide (AAO), porous silicon, nanoporous gold thin films, ortrack-etched polymers. After removal of the negativetemplate by selective etching or pyrolysis (also a possibilityin the case of positive templates), the functional material isleft behind with the desired morphology. Either approachmay be appropriate for the synthesis of ferrofluids, provideda way can be found to prepare templates in sufficient numberwith adequate uniformity, and to coat them with a magneticsubstance.

1.1 Positive templating: metallization of virusparticles Biotemplates are an example of a class of positivetemplates that ought to be well-suited to the large-scalesynthesis of tube-shaped magnetic particles. In general,the genetic code that ensures that each copy of a givenbiotemplate has the same shape also provides a mechanismfor its rapid replication, thus meeting the main requirementsfor this study. A prime example of such a biotemplate is

Figure 1 (online colorat: www.pss-b.com) Schematiccomparisonof positive and negative templating for the synthesis of nanotubes ofdefined length and diameter. Removal of the template (second step)is optional in the case of positive templating but mandatory fornegative templating.


2414 Z. Wu et al.: Preparation and magnetoviscosity of nanotube ferrofluidsp

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Tobacco mosaic virus (TMV) – a macromolecular complexconsisting of a shell of 2130 coat protein (CP) monomersbound to a helical strand of RNA, resulting in the formationof a 300 nm-long, tube-shaped structure with outer diameterof 18 nm and inner diameter of 4 nm [20]. TMV is stable withrespect to temperature and pH, it manifests proven long-termstability, and it can be harvested at high yield [21].Furthermore, the biology and structure of TMV have beenstudied for more than 120 years [22] – indeed, with suchsuccess that today’s molecular models for TMV are accurateto the atomic level [23, 24]. The deposition of metals,nanoparticles, or semiconducting oxides on various sites ofthe viral particles was demonstrated both in our laboratories[25–31] and elsewhere [32–36].

Carrying out the functionalization of virions while theyare suspended in a liquid offers the potential for completecoating, since all parts of the virion surface are exposed to thesolution. Whereas oxides can be precipitated selectively onTMV, metals are conveniently produced by the reduction ofmetal ions. Here, autocatalytic electroless deposition (ELD)is the method of choice [30, 37], because it assures acontinuous growth of layers (Fig. 2). ELD requires a‘‘sensitization’’ by noble metal clusters or particles, whichare highly active catalysts for the redox reaction betweenreductant and metal ion. The noble metal is quicklyencapsulated in the deposited metal, which is a similarlygood catalyst for the reaction. Thus, wherever a noble metalcluster or a deposited metal cluster is present, it can growto nanometer – or even micrometer – sizes, eventuallycoalescing with neighboring clusters to form a closed layer.In this manner, we attempted to prepare nanotube ferrofluidsout of TMV particles sheathed with Ni, Co, or Fe oxide(Section 3.2).

Figure 2 (online color at: www.pss-b.com) Electroless metalliza-tion of TMV. Upper sequence: binding phosphate (Pho) followed bybinding Pd(II) complexes, reducing Pd(II) to Pd and growing Ni onPd,andbycontinuationof thegrowthofNionNi.Lowersequence: inthe presence of surfactants, growing Ni clusters are encapsulated inmicelles consisting of amphiphilic surfactant molecules. Furthergrowth is hindered by the dense molecular layer on each Ni cluster.


Owing to the coalescence step, it can be a challenge toachieve a smooth coating of the virion, especially at thenanoscale. Several deposition processes have been discoveredthat reproducibly result in clusters some tens of nanometers indiameter [31, 33, 38]. In this case, parts of the viral surfaceremain exposed to the bath. The colloidal properties of thevirion are at least partially retained, which can be advan-tageous when re-suspending the coated virus particles in otherliquids. Moreover, some solid-state properties (optical andmagnetic) are better defined for clusters than for layers. Forthis reason, we did not restrict our metallization efforts only tothose promising a smooth coating of virus particles.

An alternative and much simpler route for the productionof quasi-linear magnets suspended in a carrier fluid waspursued in parallel to the above concept: TMV particles wereintroduced as bio-additives into a commercially availableferrofluid. This novel approach to tuning the rheologicalproperties of a magnetic fluid – experimentally realized andsystematically studied for the first time in this study – yieldedthe surprising results detailed in Section 3.3.

1.2 Negative templating: ALD coating ofnanoporous templates In the engineering and develop-ment of novel functional nanostructures, hexagonally self-ordered porous alumina membranes prepared by a two-stepelectrochemical oxidation of aluminum have been widely usedas building blocks and templates [39]. Self-organization of theporosity in AAO has been studied experimentally [40–43] aswell as theoretically [44, 45]. Alternatively, ordering of poresin AAO membranes can be achieved by pre-structuring thealuminum surface by nanoindentation with SiC molds [46] orwith metal stamps prepared by laser interference lithography[47] or electron-beam lithography [48]. Pre-patterning thestarting aluminum surface has also been carried out usingfocused ion beams (FIB) [49], microbeads [50, 51], and opticaldiffraction gratings [52]. More recently, diameter-modulatedpores have been obtained by switching between hard and mildanodization conditions [53–56]. Review articles by Huczko[57], Lei et al. [58], and Piao and Kim [59] describe the widerange of applications of porous AAO membranes, includingtheir use as masks or templates for chemical or physicaldeposition and their function as templates for the electro-chemical synthesis of nanowires.

Combination of AAO membranes with atomic layerdeposition (ALD) enables high-quality nanotubes [60, 61] tobe prepared from a wide range of materials [62]. In contrastto all other common thin-film deposition techniques, such assputtering, thermal evaporation, chemical vapor deposition(CVD), or molecular beam epitaxy (MBE), the self-limitingnature of ALD enables coating of pores with aspect ratiosabove 1000 in a conformal, layer-by-layer fashion.Sequential, consecutive pulses of vapor-phase precursorslead to surface-limited reactions that avoid shadowingeffects even on highly structured substrates. Since eachcycle of precursor pulses results in the deposition of a well-defined amount of material, the film thickness is a linearfunction of the number of cycles.

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In the case of nanotubes of iron oxide, it is known that themagnetic properties of an individual tube are influenced bythe wall thickness [63]; therefore, the precise control overdeposition thickness afforded by ALD represents a signifi-cant advantage over competing techniques. Together withthe ability to tailor the pore depth and diameter of AAOmembranes, ALD of Fe3O4 [64] enables the experimentalistto control all geometrical tube parameters of relevance tomagnetic properties.

In order to adapt this approach to the preparation ofnanotube ferrofluids, two challenges had to be overcome.First, the constraints of quantity and homogeneity ofmagnetic nanotubes had to be met by the AAO template.Secondly, the tubes synthesized by ALD had to be separatedfrom each other and released from the supporting template –without losing a significant fraction of the sample andwithout etching away the magnetic layer. Figure 3 offers aschematic view of the approach that we developed to solveboth of these problems: scale-up allows for the synthesis ofmore than 1011 nanotubes at once, and a complex procedureinvolving reactive ion etching (RIE), argon sputtering andwet-chemical etching yields well-separated nanotubes readyfor suspension in a carrier fluid.

2 Measuring themagnetoviscosity of small fluidvolumes The rheological properties of complex fluidsdepend in a complex manner on the dynamics of theirconstituent components. As such, measurements of theviscosity of suspensions of colloidal particles or polymersolutions – to name just a few classes of complex fluids – canoffer valuable insight into their underlying microstructure[65, 66], particularly when such studies are performed as afunction of time, frequency, or temperature. In order to carryout such measurements on the small ferrofluid sample

Figure 3 (online color at: www.pss-b.com) Negative-templateapproach to the large-scale preparation of highly uniform magneticnanotubes for novel ferrofluids. Anodization of aluminum sheetleads to large-areaporousaluminamembranes,whicharecoatedbyamultilayerALDprocess, resulting in ironoxidesandwichedbetweensilica. A complex etching and releasing procedure yields individualnanotubes for suspension in a carrier liquid.

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volumes that were feasible to prepare in this study –�100mlor less – it was necessary to turn to specialized equipment.The piezo-membrane axial vibrator (PMAV) developed bythe Institute for Dynamic Materials Testing (IdM GmbH,Ulm, Germany) was designed and constructed to measure theviscosity of samples as small as 50ml [67]. In this apparatus,the fluid sample is confined between two horizontal plates(Fig. 4) and measured in the so-called ‘‘squeeze-flow’’ modethat results from the lower plate being placed in vibration bya piezoelectric actuator. From the response of the overallsystem (sampleþ plates), which is detected as a function ofthe driving frequency by a piezoelectric sensor, thefrequency-dependent complex viscosity h� vð Þ is calculated[67]. An electromagnetic coil mounted outside the sample isused to generate constant magnetic fields as strong as 200 mTperpendicular to the sample plane (Fig. 4). A thermocoupleembedded above the upper sample plate provides an in situmeasurement of the sample temperature. Despite the fact thatthe ‘‘squeeze-flow’’ geometry exerts inhomogeneous shearforces on the sample, this instrument has been found todeliver quantitatively reliable viscosity values for polymersolutions and colloidal suspensions [68]. For our apparatus,the reproducibility of viscosity measurements was deter-mined to be better than 0.3 mPa � s at vibrational frequenciesranging from 10 to 200 Hz. Unless otherwise noted, all datawere collected at a temperature of 20� 0.3 8C.

For calibration purposes, the frequency-dependentviscosity of a commercial ferrofluid (SusTech LCE-25,SusTech GmbH, Germany) was measured in the PMAV andcompared to values obtained from a much larger volume ofthe same sample by conventional cone/plate viscometry. Asillustrated in Fig. 5, the smooth crossover between the twodata sets between 6 and 10 Hz indicates that the PMAV iscapable of delivering quantitatively accurate viscosity datafor ferrofluids.

3 Virus-based magnetic fluids As outlined above,two independent strategies for employing TMV as a positivetemplate were followed in this study. In the first approach(Section 3.2), large quantities of native TMV particles –which are non-infectious to animals but widespread in the

Figure 4 (online color at: www.pss-b.com) Schematic diagramof the piezo-membrane axial vibrator (PMAV) with its axiallyvibrating piezoelements and gap between parallel flat surfaces,which establish a ‘‘squeeze flow’’ in the liquid sample.



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Figure 5 Viscosity of SusTech LCE-25 ferrofluid plotted as afunction of shear rate (cone/plate rheometer) or frequency (PMAV)in zero applied magnetic field. Solid curve is a guide to the eye.

Figure 6 (online color at: www.pss-b.com) SEM micrographsillustrating the growth of nickel on TMV during ELD for theindicated metallization times: (a) sensitized at pH 8.0; (b) through(f) sensitized at pH 6.5. Metallization bath: 180 mM Ni(CH3COO)2,230 mM lactic acid, 35 mM DMAB, pH 6.0.

plant world – were grown in tobacco with the help of suitableinfectious plasmid clones [69] and used primarily tooptimize the conditions for ELD of magnetic metals onthe viral particle’s outer surface. ELD is especially welldeveloped for Ni, but Co layers and even alloys can bedeposited in this manner, as well. Here, we focus exclusivelyon Ni in order to highlight the influence of surfactants andmetallization time on the morphology of ELD products.

In addition to attempting to achieve a uniform coating ofthe TMV outer surface – a challenging undertaking, as notedin Section 1.1 – we also investigated synthesis routes thatlead to partial coverage of the tubular virus particles withspherical clusters. When suspended in a carrier fluid, suchnanotubes could conceivably induce a magnetoviscouseffect comparable to that of smooth nanotubes – as long asthe clusters are close enough to each other to interactmagnetically. In order to investigate this idea, the conditionsfor ELD were deliberately modified in order to produce lownumbers of isolated metal clusters attached to the outer TMVsurface or specifically to the virion ends. Promising resultswere obtained by the addition of certain detergent variants(i.e., surfactants) into the metallization bath. To our knowl-edge, the role played by surfactants during ELD has yet to bestudied in detail at the nanoscale – a regime in whichsurfactants are indeed found to operate in a quite differentmanner than at the macroscale [37].

Our second strategy for producing virus-based magneticfluids (Section 3.3) focused primarily on the potentialbenefits of ‘‘artificial’’ – that is, biotechnologically produced– TMV-like nanotubes having various pre-defined lengthsand genetically tailored surface charge states. These virusparticles were available in reasonable quantities, owing tothe fact that the essential TMV components – the CP and thesingle encapsidated RNA strand – self-assemble intocomplete particles under appropriate conditions. Weexploited the self-assembly mechanism to synthesizenanotube biotemplates of altered lengths – governed by invitro-transcribed cloned RNA species – and of modifiedelectrostatic surface potential, which was expected to


influence the interaction strength between the bare biotem-plate and the magnetic particles of a conventional ferrofluid.

3.1 Experimental procedures ELD of Ni on TMVwas carried out by well-established procedures [30]; theupper sequence in Fig. 2 illustrates the basic principle.In our experiments, TMV was sensitized in 1.36 mMNa2PdCl4�xH2O (x� 3) and 1 M NaCl by mixing this solutionwith an equal volume of 0.6–0.7 mg/mL suspension of TMVfor 10–20 min at pH values of 3–6.5. After sensitization,surplus ions were removed by dialysis (30–90 min) againstwater. The metallization bath contained 180–230 mMNi(CH3COO)2�4H2O, 180–230 mM lactic acid, and 34 mMDMAB (dimethylamine borane). Metallization requires pHvalues between 6 and 9, which were achieved via the additionof 2 M NaOH solution. The bath was stable for months. ELDof Ni was initiated by mixing sensitized TMV suspension(0.3 mg/mL) with the metallization bath at a ratio of 1:10.

Investigations of the influence of anionic surfactants onELD were carried out with Igepal [poly(oxyethylene)-nonylphenyl ether] and RE610 [poly(oxyethylene)-nonyl-phenyl ether sulfonic acid]. The surfactants were added indefined amounts to the metallization bath, keeping all otherELD conditions unchanged.

Viral scaffolding experiments were performed with thesame commercial ferrofluid that was used in the calibrationmeasurements of Section 2 – namely, SusTech LCE-25,which consists of spherical nanoparticles (Ø 10–18 nm)of cobalt ferrite (CoFe2O4) suspended in a carrier fluid ofdiethylene glycol and deionized (DI) water. Samples ofLCE-25 were diluted to ten times the original volume andsupplemented with TMV particles at concentrations up to0.045mg TMV per microliter carrier fluid [70], a process thatyielded solutions that were stable for months.

3.2 Results: electroless deposition of Ni onTMV Figure 6 illustrates the typical morphological evol-ution observed during ELD of Ni on TMV [71]. Clearly, atmetallization times up to about 1 min in duration, isolated

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Figure 8 SEM micrograph of TMV particles (black) metallized byNi clusters (white), as obtained by ELD in the presence of thesurfactant Igepal. [Sensitization: 15 min at pH 5.0; 4� 25 mindialysis. Metallization: pH 6.5 in 230 mM Ni(CH3COO)2, 35 mMDMAB, 180 mM lactic acid, 10mM Igepal].

clusters of Ni form on the TMV surface rather thancontinuous Ni layers. On the one hand, this result supportsthe hypothesis that ELD progresses via the growth ofrelatively few, isolated nuclei located at Pd clusters; on theother hand, the finding demonstrates that deliberate inter-ruption of growth can yield products with a well-definedmorphology. Longer growth times (Figs. 6 and 7) quicklylead to coalescence of the growing clusters, which iscomplete after about 5 min [71].

Despite extensive efforts to optimize the parameters ofthe ELD process, we found in all cases that Ni layers havingthicknesses in the true nanoscale range (<20 nm) weredominated by grain boundaries formed by the coalescence ofneighboring Ni clusters; moreover, the metal layer thicknesswas always found to be inhomogeneous, apparently becausethe Ni clusters grow at different rates [71].

Incorporation of the surfactant Igepal into the metalliza-tion bath resulted in complete suppression of clustercoalescence and growth of continuous Ni layers; instead,isolated Ni clusters always formed (Fig. 8). Since most of theNi clusters are more than 20 nm in diameter, we expect themto be ferromagnetic at 300 K (smaller Ni clusters can besuperparamagnetic). The layer of NiO that grows on thecluster surface after removal from the reducing environmentof the ELD bath could help to improve the coercivity and theremanent magnetization in comparison to the values of bulkNi [72, 73]. Magnetometry measurements performed onthese samples (Fig. 9) find a saturation magnetization ofapproximately 0.004 emu per gram of deposited solid (whichcontains Ni but also salts from the bath). This very low valuecan be accounted for by a cluster microstructure consisting ofrather small Ni cores surrounded by NiO shells, which wouldbe indicative of extensive oxidation. However, the measuredcoercivity of 90 Oe is similar to that of bulk Ni (100 Oe [72]),suggesting that the cores are large enough to exhibit bulkferromagnetism.

The surfactant RE610 is an anionic surfactant that can beused as an additive in ELD baths to improve the smoothnessof deposited layers. We found that even after rather long

Figure 7 Average cluster diameter versus metallization time dur-ing ELD of Ni on TMV. Sensitization at pH 6.5 for 4 h was followedby dialysis for 1 h; metallization as in Fig. 6.

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ELD times the presence of RE610 promoted the growth ofclusters similar to those produced by Igepal (althoughcontinuous layers were obtained in some cases, as well). Anintriguing observation was the fact that the Ni clusters wereusually found to grow at specific ‘‘defects’’ – namely, at theends of virions and at locations where two virions meet end-to-end (virions are prone to linear or kinked aggregation), asseen in Fig. 10. This phenomenon – in some cases alsoobserved in the absence of RE610 – may be related to theformation of Au-TMV-Au ‘‘dumbbells’’ [74], prepared bymixing Au nanoparticles and TMV followed by electrolessAu deposition to produce Au particles of sufficient size tocover and attach firmly to the TMV tube openings. Incontrast, our Ni clusters were synthesized from ions ratherthan nanoparticles, and the main assembly step occurs duringELD. While the Au particles attach to viral RNA exposed at

Figure 9 SQUID magnetometry of a Ni/Igepal sample [15 minsensitization at pH 5.5, dialysis 2� 30 min, metallization at pH7.0 with 10mM Igepal]. It was verified that no part of the substratecontained large-scale Ni deposits. Magnetization is expressed interms of the overall mass of salts in the bath, which includes the massof Ni.



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Figure 10 SEM micrograph of TMV (black) metallized at virionendpoints by Ni clusters (white), as obtained by ELD in the presenceof the surfactant RE610. [Sensitization: 15 min at pH 5.5; dialysis2� 30 min, metallization at pH 7.0 in 230 mM Ni(CH3COO)2,35 mM DMAB, 180 mM lactic acid, 4.75 vol.% of a solution ofthe surfactant RE610.]

Figure 11 (online color at: www.pss-b.com) The properties of acommercial ferrofluid were tuned by supplementation with virus-likenanotubes.Notonlydid theTMV-derivedbio-additivesenhancethe ferrofluid magnetoviscosity dramatically (upper left), they alsoimproved its resistance to shear thinning (upper right). Geneticallyengineered nanotubes were produced in vitro by self-assembly ofTMV coat protein (CP) about RNA strands of various lengths (90 nmvariant shown bottom left). Interaction between the viral proteinsurface and the magnetic nanoparticles of the ferrofluid is thought toleadtoa‘‘scaffolding’’effect,asvisualizedatbottomright.Thelatterphenomenon can be tailored by gene-technological alteration of thesurfaceaminoacidsof theTMVCP.For furtherdetails, seeRef. [70].

the TMV endpoints, the mechanism of Ni cluster attachmentis unclear.

3.3 Results: viral scaffolding of conventionalferrofluids Owing to the difficulties encountered in coat-ing TMV particles with magnetic layers of relatively highmagnetization, we pursued an independent strategy ofinvestigating the ability of bare TMV additives to influencethe magnetoviscous properties of conventional ferrofluids.The magnetoviscous effect is commonly quantified bydividing the increase in viscosity induced by a givenmagnetic field B by the viscosity at zero field,

� 20

Dh0

h00¼ h0ðBÞ�h0ðB ¼ 0Þ

h0ðB ¼ 0Þ ;

where h0 denotes the real part of the complex viscosity h�.Upon supplementation of the commercial ferrofluid LCE-25with small amounts of TMV particles, the ratio Dh0

�h00 was

discovered to have increased by as much as one-order ofmagnitude, with concomitant improvement in the stabilityagainst shear thinning (Fig. 11) [70].

This rather surprising observation was investigated infurther detail by altering the length and surface chemistry ofthe viral nanotube additives. Such modifications are feasiblebecause, since the 1950s, TMV has been studied extensivelywith respect to the molecular processes underlying viral self-assembly, reconstitution from isolated monomeric com-ponents in vitro [75], and the effects of protein mutations onparticle formation as well as on physical characteristics ofthe resulting viral nanotubes [76]. Based on these data,different RNA species have been designed that are able toinduce and govern the growth of virus-like particles not onlywith natural TMV protein, but also with CP variantsgenetically engineered to expose various amino acids onthe outer surface [77]. Nucleation of nanotube assembly was

10 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

efficiently achieved via an RNA sequence exhibiting acomplex secondary structure, the viral origin of assembly(OAs, [78] and references therein), which was incorporatedinto different RNA species transcribed in vitro fromappropriately constructed plasmid clones (Ref. [69] andunpublished data). TMV CP was produced with modifiedsurface groups from plant-infectious TMV variants that werealso generated by genetic engineering or from fission yeastcell cultures transformed with suitable expression plasmidclones. After optimization of protein and RNA purificationprotocols, methods were established to promote the in vitroassembly of virus-derived nanotubes belonging to distinctlength classes and/or manifesting altered surface chargestates (to be published elsewhere).

Employing such biotechnological variants as ferrofluidadditives yielded results that were consistent with intuition:the magnitude of the viscosity enhancement in LCE-25increased with nanotube length as well as with the predictedinteraction strength between viral and cobalt ferrite surfaces.Moreover, such effects were stable over periods of severalmonths, suggesting that biologically engineered nanoparti-cles have advanced past the stage of being mere laboratorycuriosities: they now have the potential to play an integralrole in tailoring technical systems to meet specific

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Figure 12 (a) Cross-sectional measurements of pore lengthLwereperformed at various anodization times by SEM. (b) Pore lengthversusanodization time.The linear fit corresponds toagrowth rateof68 nm/min. Part (a) also includes a TEM image of released tubes.Length measurements of released tubes [gray circles in (b)] agreewell with SEM pore data (open circles).

application needs. Based on the measured influence ofvarious nanotube variants on the magnetoviscosity – andsupported by electron microscopic studies – we developed amolecular model to account for the experimental findings[70].

4 ALD-based mass production of magneticnanotubes

4.1 Experimental procedures The starting point formass production of magnetic nanotubes using AAOtemplates was enlargement of the anodization cell to enableit to handle template areas as large as 54 cm2. Prior tocarrying out the two-step anodization process, eachaluminum substrate (Goodfellow, 99.999% purity) wascleaned in ethanol, isopropanol, and DI water and electro-polished at room temperature in a mixture of 70% perchloricacid and ethanol (volume fraction 1:4) for 5 min. A powersupply consisting of two motorcycle batteries (12 V, 12 Ah)connected in series was used to generate the high currentsneeded at the beginning of the electropolishing process. Thealuminum sheet was fixed in position in a Teflon beaker andcontacted from the bottom by a copper plate, which acts as anelectrode during anodization. Additionally, heat generatedduring electrochemical oxidation is conducted away by apowerful chiller. The first anodization step (oxalic acid0.3 M, bath temperature 8 8C, applied potential 40 V) iscarried out for at least 20 h. Afterwards, an exposure to0.18 M chromic acid at 45 8C for 24 h removes the aluminalayer produced by the first anodization. The latter step doesnot attack the underlying aluminum; rather, it uncovers afresh, pre-structured aluminum surface that acts as atemplate for a subsequent anodization treatment. Thissecond anodization is performed under the same conditionsas the first but timed precisely in order to establish the desiredpore length.

From cross-sectional micrographs obtained by scanningelectron microscopy (SEM), the pore depth was measuredfor a series of samples as a function of anodization time[Fig. 12(a)]. The results reveal a strictly linear relationshipbetween pore length and time, with the slope correspondingto a growth rate of 68 nm/min [Fig. 12(b)].

Following preparation of the AAO sample, silicon oxidewas deposited on the negative template in a custom ALDreactor at a chamber temperature of 180 8C, using 3-aminopropyltri-ethoxysilane (heated to 100 8C), water (at40 8C), and ozone at room temperature as reactants [79].Next, ALD of a layer of iron oxide was performed usingferrocene (heated to 100 8C) and ozone (room temperature)as precursors at an ALD chamber temperature of 230 8C [64].Lastly, another layer of silica was deposited to protect theiron oxide layer during subsequent processing steps (namely,reduction and release).

Owing to the large template size, it was necessary tobuild a new oven for reducing the paramagnetic iron oxidetubes deposited by ALD [see inset Fig. 14(a)] to the magneticmagnetite phase. This apparatus consisted of a stainless steelvessel with a lid sealed by an O-ring. After insertion of a

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sample (negative template plus layers deposited by ALD),the vessel was evacuated at room temperature and thenbackfilled with a 95:5 mixture of argon and hydrogen gas.After 10 min of purging, the flow rate was set toapproximately 200 sccm by adjusting a needle valve on theoutlet side, and the vessel was placed on a hotplate pre-heated to 400 8C. Gas flow was maintained at a constant rateduring subsequent vessel cool-down on a thick metal plate.

At this point, adjacent nanotubes were still connected toone another by the multilayer regions located on the topsurface of the alumina template. A complex procedureconsisting of various dry and wet-etching steps wasnecessary to obtain fully separated nanotubes. An RIE step(25 sccm CHF3 flow, 10 mTorr, 75 W) was implementedbetween the first silica ALD process and the iron oxidedeposition to etch away the interconnects arising from thefirst silica layer. After reduction, the same procedure wasapplied to etch the silicon oxide layer deposited by the finalALD process. Subsequently, an argon sputter step (20 sccmAr flow, 10 mTorr, 200 W) was carried out to remove the ironoxide interconnect. At this point, all tubes embedded in thealumina template are fully separated. Chromic acid (0.18 M)was then used to dissolve the aluminum oxide matrix andthereby release the tubes in an acidic suspension. By meansof washing and filtration, the acidic liquid suspending thenanotubes was replaced by a pH-neutral fluid environment.

4.2 Results: microstructure and magneticproperties of ALD nanotubes Selective area electrondiffraction [SAED, 300 kV, CS-corrected, Fig. 13(a)] andgrazing-incidence X-ray diffraction [GIXRD, Cu radiation,Fig. 13(b)] were performed on a dried suspension of tubeshaving length 180 nm and diameter 90 nm. The tube wallconsisted of an outer silica shell (5 nm thickness), 8 nm of



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Figure 13 (online color at: www.pss-b.com) A dried suspension ofnanotubes (length 180 nm, diameter 90 nm, silica thickness 5 nm,iron oxide thickness 8 nm) was analyzed by (a) SAED and (b)GIXRD.Therings in theSAEDpatterncorrespondtopolycrystallinemagnetite (Fe3O4). Twopeaks fromthe same phase are evident in theGIXRD pattern, as well. The Si (311) peak arises from the sampleholder, and the aluminum peaks likely arise from contaminationintroduced when the nanotubes were released from the AAO tem-plate. The red line in the GIXRD plot is a smoothed curve meant toserve as a guide to the eye.

Figure 14 (online color at: www.pss-b.com) (a) Magnetizationisotherms measured at 5 K for nanotubes embedded in an AAOtemplate, with the applied magnetic field oriented parallel to thetube axis (filled red circles) or perpendicular to the tube axis(gray squares). The inset shows the magnetization isotherm ofembedded tubes prior to reduction. The magnified region illustratesthe differences in remanence and coercivity for the two fieldorientations. (b) Room-temperature measurement by vibrating sam-ple magnetometry (VSM) of a liquid suspension of nanotubes.Saturation occurs when the applied magnetic field suffices to alignthe nanotubes in the field direction (inset top left); at zero field, thetubes are randomly aligned and the vector sum of magnetic momentsvanishes (inset bottom right). Nanotube dimensions in (a) and (b) asin Fig. 13.

iron oxide, and a 5 nm thick protective SiO2 layer. Thepresence of polycrystalline Fe3O4 and the absence of Fe2O3

are confirmed by the SAED and GIXRD measurements. Thestrongest iron oxide peak at (311) is apparent in bothdiffraction patterns. Additionally, the (440) peak at 62.78 isevident in the GIXRD pattern. Furthermore, the (220), (222),(422), and (511) planes can be identified as diffraction ringsin the SAED pattern. The crystalline aluminum phasedetected by GIXRD is likely the result of contaminationintroduced during the nanotube releasing procedure.

Magnetization isotherms were recorded at 5 K in aQuantum Design MPMS2 SQUID magnetometer in appliedmagnetic fields up to 10 kOe. Magnetic hysteresis loopsrecorded for nanotubes still embedded in the AAO templateare displayed in Fig. 14(a). This sample was prepared in thesame way as the tubes analyzed in Fig. 13. The normalizedmagnetization is plotted as function of the applied field fortwo different orientations: parallel to the long axis of thenanotubes (filled red circles) and perpendicular to the tubeaxis (gray squares). In contrast to the paramagnetic behaviormanifested by unreduced tubes [inset Fig. 14(a)], thehysteresis loop for reduced nanotubes is characterized bynonzero coercivity, nonzero remanence and, at high applied

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fields, an approach toward saturation. In the magnifiedregion of Fig. 14(a), the two different magnetic fieldorientations can be distinguished. When the applied field isparallel to the tube axis, the coercive field amounts to280(�20) Oe, whereas for the perpendicular orientation thecoercive field is 200(�20) Oe. This observation is consistentwith the easy axis of the magnetization lying along thesymmetry axis of the nanotubes.

Dissolving the alumina template and suspending thenanotubes in a carrier solution (7:3 mixture of diethyleneglycol and DI water) changes the shape of the hysteresis loopsignificantly [Fig. 14(b)]: owing to the free rotation of tubesin the liquid (at room temperature), both the coercivity andthe remanence vanish. In an applied magnetic field, the tubescan rotate into the field direction, which aligns their magneticmoments – and, thereby, the overall sample magnetization –in the field direction, as well. When the magnetic field isswitched off, however, the tubes take on random directions(owing to Brownian motion of carrier fluid molecules), and

Figure 15 (online color at: www.pss-b.com) (a) Magnetoviscosityversus applied magnetic field of a liquid suspension of nanotubes, asmeasured by PMAV at vibrational frequencies of 10 Hz (open blackcircles) and 50 Hz (open blue squares). (b) Frequency dependence ofmagnetoviscosity at applied magnetic fields of 30 mT (open redsquares) and 100 mT (open blue circles). Solid curves are guides tothe eye in (a) and (b). Nanotube parameters: length 780 nm, diameter40 nm, silica layer thickness 5 nm, iron oxide thickness 8 nm, andreduction carried out for 15 h at 400 8C.

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the vector sum of magnetic moments fluctuates about zero –even though the magnetic moment of an individual nanotuberemains oriented along its long axis.

Preliminary PMAV measurements performed on asuspension of ALD nanotubes are presented in Fig. 15. Aplot of magnetoviscosity as a function of the appliedmagnetic field [Fig. 15(a)] reveals a nonlinear increase andincipient saturation in the same range as observed for thehysteresis loops [Fig. 14(b)]. With increase in vibrationalfrequency, shear thinning is evident in the concomitantdecrease in magnetoviscosity [Fig. 15(b)]. The observedsaturation behavior can be interpreted to result from all tubesbecoming aligned in the field direction, at which point themagnetoviscosity reaches its maximal value. The depen-dence of the latter value on the vibrational frequency – i.e.,the shear thinning – could result from shear forces rotatingthe nanotubes out of alignment with the external field.

5 Conclusions A two-pronged templating strategyhas established technologically feasible routes toward theproduction of magnetic nanotubes in sufficient quantity forferrofluid applications – with ample flexibility regarding thechoice of magnetic material and near complete control overmorphological dimensions and uniformity.

TMV particles were adopted as a ‘‘positive’’ template forfabricating magnetic nanotubes via ELD of Ni. By choosingappropriate activation agents and surfactants, themorphology of the Ni metallization could be changed froma continuous layer to isolated clusters. The latter weredemonstrated to be ferromagnetic, but the measuredmagnetization was lower than expected for pure Ni,suggesting that net magnetic moment was lost owing tooxidation of cluster surfaces. Further optimization of thetechnique will be necessary if nanotubes prepared in thismanner are ever to exert a measurable influence on therheological properties of a ferrofluid.

Surprisingly, the conceptually simpler approach ofadding bare TMV particles to a conventional ferrofluiddramatically enhanced the magnetoviscosity well beyondthe value manifested by the corresponding ferrofluid withoutTMV. Experimentation with TMV variants of variouslengths and surface charge states suggests that the strongermagnetoviscous effect can be traced to a ‘‘scaffolding’’ ofmagnetic nanoparticles by the virus – i.e., to the formation ofquasi-linear TMV/nanoparticle complexes.

Greater control over the shape of the suspendedmagnetic particles was achieved by combining porousAAO – a ‘‘negative’’ template – with the ALD of iron oxide.A scale-up procedure was developed to overcome theinherent size limitations of conventional AAO-basedsynthesis routes. The resulting nanotubes exhibited highlyuniform morphology (length, diameter, wall thickness), astrong magnetization when suspended in a carrier fluid and aclear magnetovisous response. Progressing beyond thisproof of principle will require suppression of sedimentationand agglomeration of the magnetic nanotubes, which interactvia their permanent magnetic moments.



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Acknowledgements This work was supported by theDeutsche Forschungsgemeinschaft (DFG) within the frameworkof Priority Program 1165 ‘‘Nanowires and Nanotubes – fromControlled Synthesis to Function,’’ by the Baden-Wurttemberg-Stiftung (Kompetenznetz Funktionelle Nanostrukturen) and by theMax Planck Society (MPG). Two of the authors (A. A. K. and A. M.B.) acknowledge the Basque Industria, Merkataritza eta TurismoSaila and the Spanish Ministerio de Ciencia e Innovacion (grant. no.CSD2006-53). We thank J. Biskupek, A. Chuvilin, and U. Kaiser forTEM characterization of various samples; D. Borin and S.Odenbach for performing the cone/plate rheometry calibrationmeasurement of Fig. 5; P. Martinoty and D. Collin for technicaladvice and comparative viscosity measurements; as well as H. Jeskeand J. Spatz for subsidiary support. We are indebted to H.-M. Sauer(SusTech GmbH) for kindly providing the samples of LCE-25ferrofluid. Finally, we express our gratitude for two samples ofRhodafac RE610 from C. H. Erbsloh (Gemany) and Rhodia Spain.

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scaffolding and ald on porous templates

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