Constrained Iron Catalysts for Single-Walled CarbonNanotube Growth

Ryan M. Kramer, Laura A. Sowards, Mark J. Pender, Morley O. Stone, andRajesh R. Naik*

Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-PattersonAir Force Base, Dayton, Ohio 45433-7702

Received March 11, 2005. In Final Form: June 7, 2005

The diameter of single walled carbon nanotubes (SWNTs) determines the electronic properties of thenanotube. The diameter of carbon nanotubes is dictated by the diameter of the catalyst particle. Here wedescribe the use of iron nanoparticles synthesized within the Dps protein cage as catalysts for the growthof single-walled carbon nanotubes. The discrete iron particles synthesized within the Dps protein cageswhen used as catalyst particles gives rise to single-walled carbon nanotubes with a limited diameterdistribution.

Introduction

Carbon nanotube synthesis has stimulated the materialscience community by offering many potential applicationsincluding high-strength polymer composites, batteryelectrode materials, nanoscale sensors, and for use innanoelectronics.1-6 Structurally distinct single-walledcarbon nanotubes (SWNTs) occur when the graphene sheetis cylindrically rolled along the (n,m) lattice vector in thegraphene plane to form a single rolled sheet one atomthick. Nanotube chirality and diameters corresponddirectly to their unique wrapping vectors associated withthe n and m lattice integers giving rise to SWNTs withdistinct properties.7-9 Most SWNT production involvesthe decomposition of a volatile carbon species by a metalcatalyst. Typical catalyst materials used for SWNTproduction are metal oxide nanoparticles. It is known thatSWNT diameters and its related chiralities are dependenton the size and quality of the catalyst particle.10 Therefore,well-defined catalyst particles are crucial for the produc-tion of monodispersed SWNT populations.

The controlled formation of metal nanoparticles withprecise nanoscale dimensions has been an actively growingfield. Polydispersed metal nanoparticles used in CNTsynthesis results in theproduction ofnanotubes withbroadsize distributions. Catalysts that give rise to narrow size

and chirality distributions in the nanotube productionprocess are desired. Examples of catalyst synthesistechniques include preformed molybdenum-iron clusters,microporous aluminophosphate templates, and combina-tions of thin metal film evaporation and annealingtechniques to produce localized metal clusters of similarsizes.11-15 Biologically derived metal nanoparticles canalso serve as catalysts for the production of carbonnanotubes. For example, ferritin, a spherical proteincomplex containing an iron nanoparticle in the form of ahydrous ferric oxide, has also been employed as a catalystfor both multi- and single-walled carbon nanotubegrowth.13,16 The hollow cage of the ferritin complexcomprises an ideal space for the constrained depositionof iron and can also be selectively loaded to control ironnanoparticle size. Additionally, the protein shell helps toprevent aggregation of iron particles in both samplepreparation and SWNT growth stages.13 Dictating catalystsize in combination with the ability to deposit the proteinshell in precise locations through micro-contact printingand other soft lithography techniques makes biologicalconstructs a promising avenue in nanotube device con-struction.

The mammalian ferritin complex used in previousstudies is comprised of 24 identical or structurally similarsubunits to form a spherical shell with a hollow core. Theinner core of the protein complex is approximately 8 nmand is able to incorporate roughly 4500 iron atoms in theform of a paracrystalline iron hydroxide.17,18 When fullyloaded and annealed, the complex gives rise to Fe particlethat is capable of catalyzing growth of monodispersemultiwalled nanotube structures.16 Partial iron loading

* To whom correspondence should be addressed. E-mail:Rajesh.Naik@wpafb.af.mil. Fax: (937) 255-4913. Tel: (937) 255-3808.

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10.1021/la0506729 CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 08/05/2005

of the ferritin core has been demonstrated by Dai andco-workers, resulting in the creation of iron catalyst withdiscrete particle size in the production of SWNTs withnarrow size distributions.13 However, variations in thepartial loading of the ferritin cage could possibly accountfor the variations found in the nanotube dimensions.Although partial loading of the ferritin complex has beenfound to be an important contribution to production ofstructurally similar SWNTs, the goal remains to produceidentical SWNTs with the same diameter and chirality ina single production process.

In this study, we use a protein cage termed Dps, whichbelongs to a superfamily of ferritin-like protein complexes,as a nanocontainer for growing discrete iron catalysts foruse in the production of carbon nanotubes with a muchnarrower size distribution. Typically ferritin-like Dpscomplexes are comprised of only 12 identical subunits(compared to 24 for ferritin) that also self-assemble intoa spherical structure, albeit with a reduced inner corediameter of ∼4 nm.19 When fully loaded, the dodecamericstructure of Dps protein cages incorporates far less ironthan ferritin. All ferritin-like particles including the Dpsfamily of proteins show strong sequence, structural, andfunctional homology throughout prokaryotic species andincorporate anywhere between 250 and 400 iron atoms.20,21

The Dps protein from Bacillus subtilis has previously beenshown to display DNA-binding properties, but the exactquaternarystructureand inorganic irontemplatingnatureof the Dps protein has not been elucidated.22 Here weshow that the recombinantly expressed Dps protein cagefrom Bacillus subtilis is able to self-assemble into aspherical shell and can sequester iron into its centralhollow cavity. Due to the nature of the constrained reactionenvironment, formation of the iron nanoparticle is limitedto the confines of the hollow cavity. In contrast to thepartial loading of the larger ferritin particle that givesrise to particle sizes with limited size distributions, loadingthe Dps protein under iron saturating conditions ensuresdiscrete nanoparticles with a narrower size distribution.Investigation into other ferritin-like protein cages andtheir slight variations in iron incorporation, in conjunctionwith optimization of growth conditions, is the first stepin the identification of a subset of bio-derived catalystparticles that give rise to SWNTs with limited sizedistributions.

The Dps family of proteins has long been shown to beboth structurally and functionally related to the ferritin-like family of proteins and has been theorized to be theevolutionary precursor to ferritins found in higher eu-karyotes. The Dps complex, when fully assembled, is ableto protect its host cell from free radical species in vivothrough a bimodal action. Inside the cell, Dps is able tobind and condense DNA in oxidative stress conditionsproviding a physical barrier limiting access to the geneticmaterial. The secondary protective effect of the Dpscomplex arises from its ability to actively sequesterintracellular iron and deposit it within its core.22 Here weutilized the Dps complex as a size constrained proteincage in order to produce a homogeneous iron nanoparticle.To ensure that each catalyst particle contained preciselythe same amount of iron atoms, we loaded the protein

cage under iron saturating conditions. Following miner-alization, the protein shell is removed by oxidization atelevated temperatures, thereby exposing the iron nano-particle. When used as a catalyst for nanotube growth,SWNT populations with similarly sized diameters wereobtained.

Results and DiscussionThe Dps protein from Bacillus subtilis was previously

shown to have amino acid sequence identity to bacterio-ferritins and other Dps proteins, though the exactstructural nature and iron incorporation properties hadnot been explored.22 Initial purification of the proteincomplex used a combination of heat denaturation andammonium-sulfate precipitation techniques. The proteinstability under these highly denaturing conditions can beattributed to the fairly stable structure of the Dps complex.Further purification of the protein complex used acombination of ion exchange and gel-filtration chro-matographies. Analysis of the relative sizes of the re-combinantlyexpressedDpsproteinwereundertakenusingdynamic light scattering (DLS) and transmission electronmicroscopy (TEM). DLS measurements estimate the outerdiameter of the Dps protein cage to be 9.0 ( 1.1 nm (datanot shown). The TEM micrographs revealed a self-assembled spherical protein cage with an outer diameterof ∼9 nm and an inner cavity diameter of 4 nm (Figure1A). Based on the molecular weight and homology to otherDps proteins for which the crystal structure has beensolved, the Dps complex studied in this report shares theself-assembling properties of the Dps family and iscomprised of 12 subunits with outer and inner dimensionsof approximately 9 and 4 nm respectively.

Iron loading of the recombinant Dps protein cage wasconsequently performed as described in the ExperimentalSection and size exclusion chromotography separationusing gel-filtration allowed for the removal of excess ironfree in solution. UV-vis spectroscopy of the gel filtrationeluant was then monitored to detect absorption of pro-teinaceous material (at 280 nm) and Fe oxide mineral (at350 nm) (data not shown). The overlap of these twospectroscopic peaks during elution indicated that thecoelution represented a composite protein-mineral frac-tion. Additionally, because the elution profile remained

(19) Ilari, A.; Stefanini, S.; Chiancone, E.; Tsernoglou, D. Nat. Struct.Biol. 2000, 7, 38-43.

(20) Ilari, A.; Pierpaolo, C.; Ferrari, D.; Rossi, G. L.; Chiancone, E.J. Bio. Chem. 2002, 277, 37619-37623.

(21) Grant, R.; Filman, D. J.; Finkel, S. E.; Kolter, R.; Hogle, J. M.Nat. Struct. Biol. 1998, 5, 294-303.

(22) Antelmann, H.; Engelmann, S.; Schmid, R.; Sorokin, A.; Lapidus,A.; Hecker, M. J. Bacterilo. 1997, 179, 7251-7256.

Figure 1. Characterization of Dps protein cage. (A) Transmis-sion electron micrograph of negatively stained empty Dpsprotein cages. (B) Prussian blue staining following iron loadingof the DPS cage. A native-polyacrylamide gel with Ferritin asa positive control(lane 1), Apoferritin as a negative control (lane2) iron loaded Dps (lane 3) and unloaded Dps (lane 4). (C)Elemental analysis of lyophilized Dps protein following ironloading and purification. SEM micrograph of the lyophilizedDps and the chlorine and iron EDS maps. 50 µm × 50 µm maps.

Constrained Iron Catalysts Langmuir, Vol. 21, No. 18, 2005 8467

unchanged when compared to the unmineralized Dps cage,we expected any iron mineralization to occur within theprotein cage. The iron loaded protein samples wereanalyzed using nondenaturing polyacrylamide gel elec-trophoresis (PAGE) and stained with Coomassie forprotein and prussian blue for the presence of iron.26 Bothiron loaded ferritin and iron loaded Dps stained darklywhen exposed to the prussian blue stain (Figure 1B: lanes1 and 3), whereas apoferritin (lacks iron) and unloadedDps did not (Figure 1B: lanes 2 and 4). Iron incorporationwas additionally confirmed using EDAX elemental analy-sis of the gel-purified and lyophilized iron loaded Dpsprotein. Figure 1C shows a scanning electron microscopy(SEM) image of the lyophilized bulk protein and elementalmaps for chlorine (from NaCl contained in the buffersolution) and iron. The prussian blue staining in conjunc-tion with the EDAX analysis confirms that the Dps proteinwas loaded with iron.

Iron loaded Dps samples were repetitively dialyzedagainst water before use in SWNT catalysis on both SiO2/Si and quartz substrates. Dps was spotted on thesesubstrates and oxidized at 900 °C in air for 30 min toremove the proteinaceous shell that surrounded the ironnanoparticle. AFM measurements of the discrete particlesleft after the heat treatment revealed the average particleheights were 1.6 ( 0.1 nm as determined by measuring50 individual nanoparticles (Figure 2A). Apparent topo-graphic heights were used in measuring catalyst andnanotube dimensions due to AFM tip convolution inmeasuring apparent widths. Calculation of particle vol-umes based on height measurements, assuming a spheri-cal nature, suggests that each catalyst is comprised of∼182 iron atoms. Because the Dps particles were repeti-tively loaded under saturating conditions we can assumethat each catalytic particle is representative of a fullyloaded protein cage. The diameter distribution of the ironparticles after heat treatment at 900 °C for 10 min inargon as determined by AFM analysis was found to bearound 1.05 ( 0.11 nm (n ) 20).

Nanotube growth using the Dps-derived iron catalystwas performed using chemical vapor deposition (CVD)with a 1:1 mixture of methane and hydrogen (250 sccm

each) at 900 °C for 10 min to allow for nanotube growth.Both SEM and AFM analysis were used to confirm thegrowth of carbon nanotubes on the substrate. Carbonnanotubes could often be seen emanating from a Dps-derived iron catalyst as shown in Figure 2B. Our resultsshow that the majority of catalysts failed to give rise tonanotubes and the catalyst efficiency was estimated to be∼20% as determined by AFM analysis. Improvement ofcatalyst treatments using a combination of ozonation,ultraviolet irradiation, surfactants, and proteases couldbe used in conjunction with calcination to further increasethe efficiency. The majority of SWNTs grown from theCVD process appeared as individual nanotubes with anapproximate nanotube densitiy of 50 nanotubes/µm2 andwith varied lengths from 20 nm to 1 mm as measuredfrom composite SEM images. Almost all of the unusualcurvatures and morphologies associated with SWNTgrowth were observed to include “crop circles”, “shepherd’shooks”, and long curving nanotubes that arise due to theirflexibility and high aspect ratios.

Perhaps the greatest potential in using the iron loadedDps protein cage as a source as a catalyst in SWNT growtharises from the monodisperse populations it can produce.We measured diameters of nanotubes grown from the Dps-derived iron catalyst using tapping mode AFM. To obtainquantitative data, we took averages of nanotube heightsof long stretches (10-600 nm) of straight nanotubes sothat slight variations in background roughness could beminimized (Figure 3A). The average diameter of carbonnanotubes found using this technique was 1.0 ( 0.1 nm.The size distribution of 50 individual nanotubes is shownin Figure 4A. Based on the AFM results, the majority ofthe nanotubes had a diameter of ∼1 nm. To elucidate theexact nature of the dispersity in our nanotube populations,we used Raman spectroscopy to characterize the radialbreathing vibrational modes (RBM) associated withSWNTs grown on SiO2. Excitation wavelengths of 514,633, and 725 nm were used in the micro-Raman studiesusing a 50× objective and a 1 µm spot size. Of all theRBM’s detected, the majority showed a single peakcentered around 248 cm-1 which is indicative of a fairlymonodisperse population with occasional peaks appearingat slightly lower and higher frequencies (Figure 4B). TheRaman shift (ωRBM, in cm-1) of this vibrational mode canbe used as a rough estimate of the nanotube diameter (d,in nm) using the relationship d ) 248 cm-1 nm/ωRBM.23

Based on the spectroscopic measurements, the nanotubesthat gave rise to RBM peaks had a diameter of ∼1.0 nm.The tangential modes associated with the RBM peaks inFigure 4B typically showed a strong peak at 1590 cm-1

with smaller peaks centered at 1571 and 1551 cm-1. It isimportant to note that RBM peaks were only seen in asmall percentage of the scans we performed despite thefact that they were areas rich in nanotubes with diametersof less than 1.4 nm as determined by AFM. Notably, thediameters represented through Raman experiments mightonly consist of a sampling of the possible diameters becauseonly nanotubes whose electronic energy spacing betweenvan Hove singularities matching the laser excitationenergies used for our micro-Raman experiments can beresonant.

Lattice oriented growth of carbon nanotubes has beendemonstrated on Si (111), Si (100) surfaces (6-3) and Au(111) surfaces.24,25 Due to the flexibility and small diameterof the tubes, they align along the crystal grooves of thesubstrate. We also observed oriented nanotubes growthon quartz substrate. Of all nanotubes grown on the quartzsurface using Dps using typical growth procedures, morethan 60% show oriented growth in a single direction.

(23) Jorio, A.; Pimenta, M. A.; Souza Filho, A. G.; Saito, R.;Dresselhaus, G.; Dresselhaus, M. S. N. J. Phys. 2003, 5, 139.1-139.17.

(24) Su, M.; Li, Y.; Maynor, B.; Buldum, A.; Lu, J. P.; Liu, J. J. Phys.Chem. B 2000, 104, 6505-6508.

(25) Tominaga, M.; Ohira, A.; Kubo, A. ChemComm 2004, 1518-1519.

(26) Levi, S.; Santambrogio, P.; Cozzi, A.; Rovida, E.; Albertini, A.;Yewdall, S. J.; Harrison, P. M.; Arosio, P. Biochem. J. 1992, 288, 591-596.

Figure 2. Dps-derived iron particles. (A) AFM topographyimage of discrete iron catalyst nanoparticles after heat treat-ment. (B) AFM image showing two carbon nanotubes grownDps derived iron catalyst nanoparticles.

8468 Langmuir, Vol. 21, No. 18, 2005 Kramer et al.

Figure 5 shows AFM images of oriented SWNTs on thequartz substrate. Because there was only a weak cor-relation to tube growth in other directions, no directdetermination could be drawn between the crystal surfaceof the quartz and overall orientation. Further investigationinto this phenomenon is currently being explored.

Conclusions

In conclusion, we have demonstrated that the dodeca-meric Dps protein cage can be used to produce discreteiron nanoparticles, unlike selective loading of ferritin thatgives rise to iron particles with a larger size distributionin catalyst populations. The nature of the decreasedpolydispersity results from the iron loading under satu-rating conditions ensuring the iron particles have littlevariation in iron content. Dps gives rise to monodisperse

populations of SWNTs with limited size variation assupported by the data presented here. Table 1 summarizesprotein derived catalysts and the populations of nanotubesthey can produce. Currently the effects of varying growthparameters and its effects on size dimensions of Dps-derived SWNTs are being investigated. In addition, theDps protein cage can also be loaded with cobalt, as analternative catalyst source for SWNT synthesis. Thedevelopment of a uniform catalyst particle that is inex-pensive, durable, and functional represents a crucial stepin eliminating variability in SWNT production.

Figure 3. AFM measurements of nanotubes. (A) AFM imagesof six different carbon nanotubes grown using the Dps derivediron catalysts. Tube heights were averaged (boxed area in lowerpicture) along their lengths and background subtracted. (B)AFM image of two overlapping nanotubes.

Figure 4. Dps-derived iron catalysts give rise to carbonnanotubes with limited size distribution. (A) Histogram showingthe diameter distribution as determined from AFM images of50 SWNTs. (B) Representative micro-Raman spectra of threeindependent nanotubes grown on a Si/SiO2 substrate using a514 nm excitation wavelength. Peaks centered around 248 cm-1

comes from the radial breathing mode (RBM) of the nanotubes,whereas peaks centered at 300 cm-1 are from the Si/SiO2substrate.

Figure 5. Oriented growth of carbon nanotubes on a quartzsurface. AFM image of SWNTs grown on quartz substrate usingDps-derived iron catalyst.

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Experimental SectionCloning. The Dps gene was PCR amplified from the Bacillus

subtilis genome using primers (Integrated DNA Technologies)flanking the Dps open reading frame. The gene was subsequentlycloned into the bacterial expression vector pET21b (Novagen,San Diego, CA). The sequence was confirmed using an automatedDNA sequencer (Applied Biosystems 3100).

Expression and Purification. BL21 (DE3) cells weretransformed with the pET21b plasmid carrying the Dps gene.For expression, an overnight culture was diluted (1:50) into freshLB medium containing ampicillin (100 µg/mL). Expression wasinduced by the addition of 1 mM IPTG when cells reached anoptical density at 600 nm of ≈0.5. The cells were grown forapproximately 4-5 h and harvested in 25 mL of buffer A (20 mMTris-Cl pH 8.0, 100 mM NaCl) and frozen for storage. Proteinwas extracted by incubating the thawed cell suspension in 5 mgof lysozyme and ultrasonicated 5 times for 10 s. The lysed crudecell suspension was centrifuged at 5000 rpm for 30 min and thesupernatant was transferred to a fresh tube and heat treated at60 °C for 10 min. Again the sample was centrifuged at 10K rpmfor 15 min to remove any denatured protein. The supernatantwas subjected to ammonia sulfate precipitation and mixed to afinal concentration of 2.0 M. Following a 10 min incubation thesample was centrifuged as above. The Dps protein was isolatedby anion exchange chromatography (MonoQ Amersham Phar-macia) and eluted using a liner 0.1-0.6 NaCl gradient. The elutedfractions were concentrated for gel filtration chromatography(Superdex 200) using a column equilibrated with buffer B (20mM phosphate buffer pH 7.0, 50 mM NaCl). Eluted fractionswere analyzed using SDS-PAGE and purity was estimated tobe greater than 90%. Prussian blue staining to confirm thepresence of iron was done as previously described.

Fe Loading into the Dps Protein Complex. A 10 mLsolution of Dps (0.20 mg/mL) in buffer B was heated to 42 °C andstirred constantly. A 0.05 M ammonium iron (II) sulfate solutionwas added dropwise to the Dps solution followed by addition ofH2O2 to ensure full oxidation of Fe(II) to Fe(III). This processwas repeated several times to ensure complete mineralizationof the Dps cavity (2000 Fe/Dps protein). Following the final

addition the solution was centrifuged to remove bulk precipitate(10 min, 15K rpm). The solution was subsequently dialyzedovernight against a total of 6 L of water to remove ammoniumiron (II) sulfate still in solution. Dps dodecameric self-assemblywas confirmed using dynamic light scattering (Dyna-Pro -MS/X)and transmission electron microscopy (TEM) using a PhillipsEM208 operating at 200 kV of negatively stained samples usinguranyl acetate. Ferrocyanide staining of native PAGE gels wasundertaken to confirm iron incorporation into the protein complexin addition to elemental analysis of lyophilized protein usingXL30 Environmental Scanning Microscope field emmision gunby FEI Company equipped with EDAX Genesis elementalanalysis software. Following dialysis, the sample was centrifuge(10 min, 15K rpm) and stored at 4 °C for further use.

Synthesis of Single-Walled Carbon Nanotubes. Mineral-ized Dps was spotted onto a silicon (3 µm thick silicon oxidelayer, P-type, Boron doped) or quartz (3′′ 500 µm thick, singleside polished) wafers and placed into the center of Type F21100Barnstead International tube furnace. The substrate was heatedto 900 °C to burn off the protein shell and oxidize the particle.For nanotube production, the oxidized substrate was again heatedin the tube furnace in Ar (150 sccm, 99.999%). Once the furnacereached 900 °C, methane (225 sccm, 99.999%) and hydrogen (225sccm) were allowed to pass through the tube reactor for 10 min,followed by cooling to room temperature under Ar (150 sccm).SWNT production was confirmed using a combination of scanningelectron microscopy (XL30 Environmental Scanning Microscope),atomic force microscopy (Digital Instruments MultiMode witha Nanoscope IIIa controller with NSC15 and HI′RES (DP14)µMaschprobes), andRamansprectroscopy (Renishaw1000micro-Raman system).

Acknowledgment. Funding for this work was pro-vided by the Air Force Office of Scientific Research. Theauthors thank Dr. Song Tan from the Gene RegulationCenter at the Pennsylvania State University for helpfuldiscussion and use of equipment.

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Table 1. Biologically Derived Iron Catalysts

loaded Dpspartially loaded

ferritinloadedferritin

outer diameter 9 nm 12 nm 12 nminner cavity diameter 4 nm 8 nm 8 nmnanotube type catalyzed single-walled single-walled multiwalleddiameter distribution of nanotubes 1.0 ( 0.1 nm 1.5 ( 0.4 nm 5.2 ( 0.6 nm

3.0 ( 0.9 nm

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