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November 14, 2013

C 2013 American Chemical Society

Virus-Directed Design of a FlexibleBaTiO3 NanogeneratorChang Kyu Jeong,†,§ Insu Kim,†,§ Kwi-Il Park,† Mi Hwa Oh,‡ Haemin Paik,†,^ Geon-Tae Hwang,†

Kwangsoo No,† Yoon Sung Nam,†,‡,* and Keon Jae Lee†,*

†Department of Materials Science and Engineering, and ‡Department of Biological Sciences, Korea Advanced Institute of Science and Technology (KAIST),291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea. §These authors contributed equally to this work. ^Present address: Department of MaterialsScience, California Institute of Technology, Pasadena, California 91125, United States.

Energy harvesting from renewable re-sources became a critical issue in ourcivilization because of environmental

problems associated with fossil fuel-basedenergy technologies.1 In particular, me-chanical resources are very common andeasily accessible in our everyday lives, mak-ing them distinctive from other renewableenergy sources (e.g., solar, geothermal, andbiomass energies).2,3While severalmethodshave been developed to convert mechan-ical energy into electricity,4 piezoelectricsystems have attracted increasing attentiondue to the direct conversion from mechan-ical energy to electricity.3,5,6 A nanogenera-tor, which is a piezoelectric material-basedenergy-harvesting device,7�12 has been stu-died using perovskite-structured ceramics,such as PbZrxTi1�xO3 (PZT)

13�15 and BaTiO3

(BTO).3,16 BTO is a particularly attractivematerial due to its lead-free biocompatibil-ity with high piezoelectricity.17 However,typical methods to synthesize perovskitenanomaterials use toxic organic solventsand alkoxide precursors.18,19 Furthermore,the surface of resultant nanomaterials is

often contaminated by polymeric residuesand surfactants, which can cause detrimentaleffects to the original properties ofmaterials.20

Recently, as an alternative to conven-tional techniques, a new synthetic approachbased on biological templates has gainedpopularity to generate new inorganic archi-tectures with unique properties under am-bient conditions.21,22 For instance, DNA,23

virus,24,25 and microorganisms26 have beenexplored as a template to control thematerials properties (e.g., composition, crystal-linity, phase, morphology, etc.) as well asdevelop environmentally benign and energy-efficient fabrication processes (e.g., bio-mineralization).27,28 The biogenic routes tothe fabrication of functional nanomaterialshave been investigated for a wide range ofapplications, including energy storage,29,30

photovoltaics,31 water oxidation,32,33 mag-netic resonance imaging,34 etc.35,36 In parti-cular, viruses have been successfully utilizedas a genetically programmable toolkit for theself-assembly of nanoarchitectured inorganicstructures through specific peptide-mediatedinteractions.37 For instance, a filamentous M13

* Address correspondence toyoonsung@kaist.ac.kr,keonlee@kaist.ac.kr.

Received for review September 5, 2013and accepted November 14, 2013.

Published online10.1021/nn404659d

ABSTRACT Biotemplated synthesis of functional nanomaterials has received increasing attention

for applications in energy, catalysis, bioimaging, and other technologies. This approach is justified by

the unique abilities of biological systems to guide sophisticated assembly and organization of

molecules and materials into distinctive nanoscale morphologies that exhibit physicochemical

properties highly desirable for specific purposes. Here, we present a high-performance, flexible

nanogenerator using anisotropic BaTiO3 (BTO) nanocrystals synthesized on an M13 viral template

through the genetically programmed self-assembly of metal ion precursors. The filamentous viral

template realizes the formation of a highly entangled, well-dispersed network of anisotropic BTO

nanostructures with high crystallinity and piezoelectricity. Even without the use of additional

structural stabilizers, our virus-enabled flexible nanogenerator exhibits a high electrical output up to ∼300 nA and ∼6 V, indicating the importance of

nanoscale structures for device performances. This study shows the biotemplating approach as a facile method to design and fabricate nanoscale materials

particularly suitable for flexible energy harvesting applications.

KEYWORDS: M13 bacteriophage . biosynthesis . virus template . barium titanate (BaTiO3) . piezoelectric . nanogenerator

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virus encapsulates a single-stranded DNA with 2700copies of pVIII major coat proteins along the long-itudinal viral axis. The genetic modification of pVIII torender an affinity toward various inorganic materialsallows the formation of anisotropic nanomaterialsthrough biotemplated synthesis in an aqueous solu-tion under mild conditions. This noteworthy biologicalscaffold has been investigated to realize variousenergy devices (e.g., battery and solar cells) with out-standing performances because the filamentous shapeof an M13 virus can generate a percolated network forwell-dispersed nanostructures.29�31,38 Likewise, innanostructure-embedded piezoelectric devices, thehomogeneous dispersion of piezoelectric nanomater-ials has been regarded as a key issue for high electricaloutputs as demonstrated in our recent works. Forexample, Park et al. used graphitic carbon nanomaterialsas a dispersant for nanogenerators.3,15 On the contrary,we emphasize that a percolated network of piezoelectricnanomaterials dispersed in a flexible elastomer matrixcan be generated without toxic dispersion enhancers byemploying a simple M13-based biosynthesis for effectiveenergy harvesting, which enables one to power themicro/nanoelectronic devices for indoor and in vivo en-vironments. Recently, it was reported that the piezo-electric properties of the M13 virus can be utilized togenerate electrical energy,39 although the energy levelfrom the virus was significantly lower than that frominorganic-based energy harvesters.3,13,15,40

In this work, we propose a high-performance, flexiblenanogenerator device based on the stable network ofanisotropic BTO nanostructures prepared using an M13virus as a template. An M13 virus was genetically en-gineered to display a glutamate trimer (EEE or E3) at theN-terminus of each pVIII protein. The virus is denoted as'E3-M13'. The same virus was previously utilized for thebiomineralization of amorphous iron phosphate (a-FePO4).

29 In this work, it was found that barium andtitanium precursor ions were also strongly associatedwith the E3 peptide viametal ion�peptide coordination.The prepared virus-metal ionic complexes were used tosynthesize virus-templated BTO nanostructures. The fila-mentous shape of M13 can guide the formation of auniform dispersion of anisotropic piezoelectric materials,which is vitally important to generate effective and well-distributed piezopotential. The interaction of the metalions with the viral capsid proteins, forming virus-metalionic complexes, and the subsequent formation of aniso-tropic BTO nanocrystal structures were carefully char-acterized. The virus-templated BTO nanostructures withhigh crystallinity and effective piezoelectricity wereincorporated into an elastomeric matrix to fabricate avirus-templated flexible piezoelectric nanogenerator.

RESULTS AND DISCUSSION

The overall synthesis processes of virus-templatedBTO nanostructures are presented in Figure 1a. The

geneticallymodifiedM13 viruswas incubated in ligandsolutions to form a virus-metal ionic complex, achiev-ing highly percolated and well-dispersed nanostruc-tures obtained via calcination. The simulation for piezo-electric potential with a simplified virus-templated BTOmodel in an elastomer matrix indicates that the en-tangled network structure of the anisotropic virus-templated BTO nanostructures is crucial to the energyharvesting performances of a nanogenerator (Figure 1b).The piezopotential distribution of anisotropic BTO nano-structures is presented by a color code with the x-axialtensile stress. With this finite element analysis (FEA), thevirus-templated filamentous BTO nanostructure caneffectively generate electrical energy from mechanicalagitationmuchmore effectively than BTO nanoparticles(NPs), which show poor dispersion stability (see Sup-porting Information).Figure 1c�e describes the detailed synthetic proce-

dures of virus-templated BTO nanostructures. The pVIIImajor coat proteins can strongly interact with posi-tively charged metal ion precursors because of thegenetically incorporated extra carboxylates, as illu-strated in Figure 1c. To prepare a virus-metal ioniccomplex (Figure 1d), a 200 mM barium glycolatesolution was initially incubated with 1013 pfu 3mL�1

E3-M13 for 2 h at ambient temperature, and then200 mM titanium glycolate was added to the virussolution. The titanium precursor solution was added at80 �C and pH g10 for condensation and diffusion ofthe two glycolate precursors onto the viral surface. Thesequential addition is very important because titaniumglycolate chelate, [TiIV(OCH2CH2O)3]

2�, is negativelycharged and thus can be electrostatically repulsed byhighly negative charged E3-M13. In contrast, bariumglycolate chelate, [BaII(HOCH2CH2OH)4(OH2)]

2þ, is po-sitively charged,41 so this barium precursor can beeasily attracted by the virus. Sequentially, the bariumglycolate complexed with the virus can draw thetitanium glycolate through electrostatic attractionand hydrogen bonding.41,42 It is well-known that octa-hedral titanium glycolate and the nine-coordinatedbarium glycolate can be linked by hydrogen-bondedpairs.41 If these two precursors were added together atthe same time, the titanium glycolate would be rapidlyself-polymerized in an aqueous solution, generatinglarge precipitated aggregates, while the barium glyco-late would exist as a dissolved ligand. In our biotem-plate approach for a BTO-based nanogenerator, theheat treatment is needed to form a perovskite crystal-line structure due to the high crystallization energy ofBTO.43 Although the virus-template was eliminatedduring calcination, the anisotropic filamentous shapefor BTO nanostructure was well maintained with highcrystallinity (Figure 1e). As we expected, the calcinedspecies using the single-step incubation processwere only titanium-rich materials, such as BaTi4O9

and BaTi2O5 (Figure S2).

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The overall morphologies and properties of theproduced materials during the virus-templated syn-thesis were analyzed, as shown in Figure 1f�h.Figure 1f is a transmission electron microscope (TEM)image of E3-M13 virus negatively stained capsids byuranyl acetate, which shows a well-entangled filamen-tous structure having a diameter of about 7 nm. Thezeta-potential analysis of both E3-M13 and wild-typeM13 (M13KE) in tris buffered saline (TBS) at pH 10confirms that the E3-M13 is more negatively chargedthan M13KE (inset of Figure 1f). After the incubation ofglycolates for the assembly of metal ligands onto theviral surface, the width of the resulting structure in-creased to about 20�30 nm, while the percolatedstructure of viruses was well maintained (Figure 1g).Some crystallites in an amorphous matrix of the virus-metal ionic complex were observed presumably dueto the hydrolysis of titanium glycolates, as presented byhigh resolution TEM (HRTEM) analysis (inset of Figure 1g).

These crystallites seem to be tiny TiO2 domains asdetermined by selected-area electron diffraction (SAED)rings; however, the entire X-ray diffraction (XRD) pat-terns of this virus-templated intermediate showno peakbecause most of the complex consists of glycolates/metal oxides-based amorphous regions (Figure S3).After calcination, the final virus-templated BTO nano-structures with the well-dispersed morphology andanisotropic shape were 50�100 nm in width as a resultof the heat treatment (Figure 1h and Figure S5). Thisgrain size of nanocrystals was large enough to form thetetragonal phase, which is essential for piezoelectricproperties.44 HRTEM and rectangular patterns in thepower spectrum of virus-templated BTO nanostructures(insets of Figure 1h) show the formation of the tetra-gonal perovskite structure of BTO from the virus-templated complex. A comparison of syntheses withoutthe viral template and with wild-type M13KE is alsodescribed in Supporting Information.

Figure 1. Schematic diagram and TEM images of virus-templated BTO synthesis for nanogenerator. (a) Overall fabricationprocess of a virus-templated BTO based piezoelectric nanogenerator. (b) Simulation for the piezoelectric potential ofdispersed virus-shaped BTO in a matrix (mechanical tensile stress of 0.33%). (c�e) Schemes of each step to explain syntheticprocesses. (c) M13 bacteriophage is shown with the major capsid proteins (pVIII) genetically engineered. Three glutamates(E3) are expressed on the N-terminus of pVIII. (d) Virus-metal ion complexation occurs when barium/titanium glycolatesinteract with the E3 modified coat proteins. (e) Virus-templated BTO is formed by calcination for not only crystallization butalso elimination of phage templates. BTO crystals have a perovskite structure that can show excellent piezoelectricity. (f�h)TEMmicrographs of each step in the virus-templated BTO synthesis. (f) The well-entangled E3-M13 viruses negative stained.(Inset) Zeta-potential analysis showing that the E3-M13 virus hasmore net negative charges than thewild-type virus (M13KE).(g)Morphology of the virus-metal ion complexwith glycolate precursors. (Inset) TheHRTEM image presents that the complexmaterials are composed of an amorphous matrix of oxides/glycolates and some titanium oxide crystallites. (h) Virus-templated BTO nanocrystals after calcination. (Inset) The crystal fringe image by HRTEM corresponding to the latticeparameter of BTO perovskite structure (a = b = 3.994 Å, c = 4.038 Å). The power spectrum from the fast-Fourier transformverifies the rectangular patterns, exhibiting the tetragonal phase of the synthesized BTO.

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The M13 virus expressing trimeric or tetramericglutamates have been proved to be highly useful forthe templated synthesis of metal oxides; however, thechemical nature of the virus-metal ionic complexes hasnot been well examined. The regions of negativelycharged amino acid lumps on a viral surface have beenjust regarded as an origin of Coulombic attractive forcewith metal cations. Moreover, our experiment andsome previously reportedmethods have used chelatesor nonpristine ions as precursors, rather than just usingpurely aqueous metal cations. To investigate the che-mical properties of virus-templated materials, we car-ried out attenuated total reflection infrared (ATR-IR)spectroscopy and X-ray photoelectron spectroscopy(XPS). In the ATR-IR spectra of the virus-metal ioniccomplex (Figure 2a�i), the two humps near 2987 and2901 cm�1 are attributed to the alkanes in the boundglycolates. The bands at 1395 and 1066 cm�1 areassigned to the hydroxyl groups and C�O bonds inEG-based precursors, respectively. In addition, there isonly a carboxylic C�O bond at 1252 cm�1 without aCdO bond. These results indicate that both of the twooxygen atoms in the carboxylic group of glutamate areinvolved in the chelation of the metal-center, as in thesimilar manner with the EG ligand. The strong chelat-ing capability of glutamate is reasonable since theelectron density of carboxylate is higher than that ofthe hydroxyl group in EG. The metal�oxygen coordi-nating bond of chelates is visible as a multitude ofpeaks45 below 670 cm�1. The small band at 860 cm�1

corresponding to the Ti�O stretching vibrations isobserved46 in not only as-calcined final virus-templatedBTO but also the virus-metal ionic complex. Since thechelation of our approach is achieved by oxygen atomssurrounding a metal element, the O1s XPS analysis isvital to inspect the chemical state and binding config-uration of the complex (Figure 2b�i). In the broadspectra, the oxygen�metal chelating bond can bedetected at a lower binding energy (530.8 eV) thanC�Obinding energy (531.9 eV).47 Moreover, there is anobvious envelope of metal oxide at the lowest energy(529.5 eV), which accords with the crystalline andamorphous TiO2 observed in the results of HRTEMand SAED from the virus-metal ionic complex. Theshallow shoulder at 533.6 eV corresponds to physicallyadsorbed water molecules on organic elements.48 TheXPS curve of pure virus shows no metal-related envel-ope (Figure S8); that is, the complexation and chelationevents between the virus scaffold and precursorsdefinitely may happen during the incubation process.On the other hand, the deep and wide trenches below600 cm�1 in the IR spectra (out of window) are char-acteristic to BTO as reported previously (Figure 2a-ii).46

Some deviated signals of the IR analysis of BTOare induced by oxidized hydrocarbons. Figure 2b-ii onlyshows that the BTO specific XPS peak exists at 528.9 eVafter calcination. The down-shift of the oxide peak

compared to the virus-metal ion complex results fromthe elimination of organic residues. A good agreementbetween the theoretical value and measurement of theO1s peak from BTO is observed despite the inevitablecontamination with carbonaceous species from equip-ment (530.7 eV).49

The intermediate compositions or phases duringheat treatments in various BTO synthesis methodsare highly important because of the large activationenergy of BTO crystallization.43 Thermogravimetry (TG)and differential scanning calorimetry (DSC) analysesshow that the vaporization of water and the burning ofM13 phage occur at 100�200 and 200�400 �C, re-spectively (Figure 3a). Although the carbon sources ofglycolates are burned even after 400 �C (see TG), thereis a broad endothermic region in the DSC due to theannealing and diffusion of the twometal elements. Forcalcination at 600�900 �C, the thermal treatment isinsufficient, producing various titanium-rich composi-tions or phases, as observed by Raman spectroscopy(Figure S9).50 The bands of Raman shift for final virus-templated BTO nanostructures correspond to the gen-eral result of tetragonal perovskite BTO (Figure 3b). Inparticular, only the tetragonal phase can show the B1and E modes at 307 cm�1 and the high-frequencylongitudinal optic mode at 715 cm�1, which are

Figure 2. Chemical characterization of virus-metal ionscomplex and virus-templated BTO. ATR-IR spectroscopyandO1sXPS analysismeasuredbefore and after calcination.(a) There are bands suggestive of complexation at the as-incubated state, whereas only the BTO-specific spectrum isshown at the as-calcined state. (b) The deconvolution of theO1s envelope before the calcination process displays chela-tion between the virus-template and the metal-ions fromglycolates. After calcination, the sharp BTO peak at lowbinding energy is emitted with a small hydrocarbon shoulder.

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experimental evidence of the cubic-to-tetragonalphase transformation.51 The XRD patterns also definethe P3 mm crystalline structure of BTO (inset ofFigure 3b). The perfect crystallinity of virus-templatedBTO is presented by the HRTEM and SAED alongthe [111] zone-axis, showing near 3-fold symm-etry (Figure 3c-i,c-ii). The existing grain boundaries(GBs) and large growth dislocation,52 as indicated inFigure 3c-iii, confirm that the synthesized virus-templated BTO nanostructures are derived from thevirus-metal ionic complex, not from just the agglom-eration of BTO particles. Piezoresponse hysteresismea-surement is performed to identify the switchingbehavior of polarization in the resultant virus-tem-plated BTO domains (Figure 3d). This remanent hyster-esis loop is the result of detecting piezoresponse afterturning off the electric field. The phase hysteresis dis-plays perfectly switchable dipoles (inset of Figure 3d), sothe retention properties of the piezoelectricity are re-vealeddespite a slight deviationof piezoresponse causedby the mechanical instability of a silver paste electrode.To harvest energy using the piezoelectric virus-

templated BTO, a well-established nanogeneratorfabrication process is utilized,3 as illustrated by theschematics of Figure 4a. The stirred and infiltratedvirus-templated BTO nanostructures are mixed withpolydimethylsiloxane (PDMS) to achieve the BTOpiezoelectric layer (Figure 4a-i). The indium tin oxide(ITO)-coated thick (∼175 μm) polyethylene terephtha-late (PET) substrate is covered with PDMS dielectric

layer to prevent the electric breakdown of device(Figure 4a-ii). Next, the virus-templated BTO-basedpiezoelectric layer (∼200 μm in thickness, Figure S10)was spin-casted on a PDMS-coated flexible substrate.For the top electrode, an ITO-deposited thin PET sub-strate (∼50 μm) is subsequently placed on the piezo-electric layer (Figure 4a-iii). Figure 4a-iv shows thephotograph of fabricated virus-templated BTO nano-generator, which ismalleableandflexible. It is theoreticallyand experimentally well-established that percolated nano-structures of one-dimensional fibrousmaterials can bewelldistributed in three-dimensional active regions.29,30,53�55

Figure S11 also shows a well-dispersed, virus-templatedBTOpiezo-layer thatoriginated fromthepercolationofone-dimensional nanostructures, whereas BTO NPs are signifi-cantly aggregated in the PDMS matrix.3,15,40 Due to theanisotropic structure of M13, the well-percolated nano-clusters of the virus-templated BTO were well-distributedin a soft elastomeric matrix without any dispersion agents,as shown in the high resolution scanning electron micro-scope (SEM) image of Figure 4a-v.Figure 4b shows the output performance of the

virus-templated BTO nanogenerator by periodicalbending/releasing motion. The short-circuit currentand open-circuit voltage measured from the nanogen-erator device with an effective area of 2.5 � 2.5 cm2

reach up to ∼300 nA and ∼6 V, respectively, whichcorrespond to the previously reported theoreticalstudy.40 In addition, this virus-templated BTO nano-generator exhibits high output compared to that

Figure 3. Thermal analysis during calcination and material characterization of virus template BTO. (a) TG and DSC analysisduring calcination from 50 to 1150 �C with 0.1 �C 3 s

�1. (b) Raman scattering spectrum of the virus-templated BTOnanocrystals. The indexed bands are in agreement with the typical tetragonal phase of perovskite BTO. The inset presentsXRDpatterns of virus-templatedBTO, corresponding to P4mm crystal symmetry (JCPDS#005-0626). (c) (i and ii) HRTEM imageand SAED patterns along the [111] zone-axis of the nanostructured BTO. (iii) GBs and one-dimensional defect are indicated inthe TEM micrograph. (d) Remanent piezoresponse hysteresis property as a function of DC voltage bias. The inset hysteresiscurve shows the inversion of the piezoelectric phase (dipole switching). Virus-templated BTO is denoted as 'vt-BTO'.

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reported in other works, as summarized in Table S1. Toconfirm that the electrical signals are produced by thenanogenerator, the output current and voltage weremeasured under both forward and reverse connec-tions with measurement equipment. The polarity ofelectrical signals in forward connection was invertedwhen the connection is reversely switched, as shown inFigure 4b-i,b-ii. These electrical outputs are higher thanthe results of our previously reported nanocompositegenerator using BTO NPs with carbon nanotubes(CNTs) as dispersion improvers. This enhanced perfor-mance may be caused by the inherently dispersed

structure of the virus-templated nanocrystals, whilethere is only physical mixing in the previous nanocom-posite fabrication.Figure 4c depicts the mechanism of energy conver-

sion by the nanogenerator device. During the polingprocess (see Methods), the direction of electrical di-poles in the virus-templated BTO piezoelectric domaincan be aligned along the external electrical field inparallel (Figure 4c-i). When the nanogenerator suffersfrom the mechanical deformation, the piezoelectricpotential is induced between the top and bottomelectrodes owing to the stress of dipoles (Figure 4c-ii).

Figure 4. Nanogenerator fabrication and energy harvesting from mechanical deformation. (a) (i�iii) Schematics of thefabricationprocess forananogeneratordevice. (iv) Photographof thefinalnanogeneratorand (v) SEMmicrographofwell-entangledBTO nanocrystal clusters in the PDMS matrix. (b) The measured (i) short-circuit current and (ii) open-circuit voltage signals of thevirus-templated nanogenerator device in both forward and reverse connectionswith a curvature radius of 5 cm, frequency of 0.3Hz,and rate of 0.2m 3 s

�1. The inversion of signals (switching polarity) indicates that the generated outputs are the genuine results fromthe nanogenerator. (c) The mechanism of energy harvesting from the nanogenerator device. (i) Dipole moments of the virus-templated BTO are definitely aligned in the direction of external electric field during the poling process. (ii) Built-in potential andcurrent (electrons flow) are generated by the mechanical bending due to the produced piezoelectric potential. (iii) Since thepiezopotential disappears when the stress is released, the accumulated charges flow back to the original state.

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The formed built-in potential results in the flow of freeelectronswhich neutralizes the electric field produced bythe dipoles. Since the virus-templated BTO piezoelectriclayer and the dielectric PDMS are insulators, the currentonlyflows throughexternal circuitry, so there is nochargeleakage throughout the periodical measurement. At astatic strain, thebuilt-inpotential is fadedoutbecause thepiezopotential is balanced by the accumulated freecharges at both electrodes. When the strain disappears,the piezoelectric potential vanishes and an oppositebuilt-in potential is formed by the free charges accumu-lated at both ends of the external circuit (Figure 4c-iii).The free electrons flow back in the opposite direction,and subsequently the current diminishes to zero.The output performance of virus-templated BTO

nanogenerator is affected by the strain rate, as thepiezoelectric bound charges are constant at the samedegree of strain. For the constant strain, the fasterstrain rate results in a higher output voltage (Figure 5a).Likewise, the outputs also increase with the bendingcurvature at the given strain rate (Figure 5b) since amore deformed piezoelectric layer can produce alarger built-in potential along the circuit. The stabilityof the virus-templated BTO based nanogeneratoris estimated through measurement over extended

cycling times (∼21000 cycles) at fast frequency.Figure 5c,d shows that the output signals are relativelystable with no significant degradation of performanceduring the durability test. The generated current andvoltage of the nanogenerator device under fast fre-quency (3.5 Hz) were almost the same as the measure-ment under the original frequency. These resultsdemonstrate that the nanogenerator fabricated byvirus-templated BTO nanostructure is highly stableand suitable for energy harvesting from irregular agita-tions (insets of Figure 5c,d). The slight deviation ofsignals may be attributed to the shortage in theresponse time of the measurement unit caused bythe harsh mechanical conditions. Also, the generatedelectrical energy can be used to operate commercialdevices without an external energy source. Three 1 mFcapacitors connected in parallel were charged bybending/releasing motions of the virus-templatedBTO nanogenerator device with bridge rectifier circui-try (Figure S15). The stored voltage of each capacitorunder the externalmechanical deformation reaches upto about 1.5 V generated from the nanogeneratordevice (the total stored voltage is about 4.6 V). Finally,the white light emitting diode (LED)-based opticalfibers are successfully driven (Figure 5e and Video S1).

Figure 5. Characteristics and application of the virus-templated BTO nanogenerator device. Output voltage signalsgenerated by the virus-templated nanogenerator device according to (a) strain rate (0.2 vs 0.005 m 3 s

�1) and (b) radius ofcurvature (r = 5.0�10.5 cm). A larger radius of curvature means a smaller degree of strain. (c and d) Stable energy generationtest performed to confirm the mechanical durability of the virus-templated BTO based nanogenerator device. (e) Capturedphotograph of the moment of lighting up the white LED-based optical fibers using the electrical energy stored by the virus-templated BTO-nanogenerator. The inset shows the three capacitors in series charged by mechanical deformation of thenanogenerator for 4 h (charged voltage is about 4.6 V). (f) Commercial LCD driven by electric power from bending andreleasing motions of the virus-templated BTO nanogenerator. Logo use permitted by Copyright 2013 KAIST.

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Moreover, the LCD was fully operated by the repeatedpositive/negative electrical signals from the virus-templated BTO based nanogenerator (Figure 5f). Thealternating signals can turn on the LCD device with norectifier, as an LCD is a nonpolar device. SupportingInformation Video S2 presents the LCD screen driven bybending/releasingmotions of thenanogeneratordevicewith frequencies of 0.3 and 5 Hz.

CONCLUSION

In this study, we have developed a biological self-assembly method for dispersed and high crystallineperovskite material using a genetically modified virusto fabricate the self-powered flexible energy harvest-ing device. The virus-templated BTO nanostructureoriginated from the virus-metal ionic complex based

on the crucial interactions between metal-based li-gands and genetic engineered proteins. The com-plexation and calcination events are characterized toinvestigate their chemical states and materials proper-ties. Excellent output performance (∼300 nA and∼6 V)is successfully obtained from the virus-templated BTOnanogenerator by periodical mechanical motions. Thecommercial electrical devices are driven by the harvestedenergy from the virus-templated BTO nanogenerator. Weexpect that the chargeeffectof ligands andchelatingeffectof glutamates studied in our research can help to under-stand the versatility of biotemplate synthesis. Further-more, we believe that our biosynthetic route for inorganicnanostructures can open a new era of bioinspired self-assembly technology, such as biomechanical energy har-vesting, thermoelectric applications, and biofuel cells.

METHODSVirus-Templated BTO Synthesis. BaCl2 3 2H2O (Sigma-Aldrich)

and TiCl4 (Sigma-Aldrich) were dissolved into ethylene glycol(Sigma-Aldrich) with magnetic stirring overnight to makebarium and titanium glycolate solutions, respectively. The E3-M13virus solution was prepared as a concentration of 1013 pfu 3mL�1

suspended in TBS (pH 7.5).First, 0.1mL of the bariumglycolate solutionwasmixedwith

20 mL of the virus solution, and then 5 M NaOH (Sigma-Aldrich)solution was slowly added to adjust pH to 10. The virus solutionwas incubated with the barium precursor withmagnetic stirringat 200 rpm for 2 h. Next, 0.1 mL of the titanium glycolatesolution was added to the barium glycolate preincubated virussolution. The total solutionwasmaintained at 80 �Covernight toform a virus-metal ionic complex. The obtained virus-metalionic complex was purified by centrifugation (RFC 11000g)and washing with deionized water (DI water, Milli-Q, Millipore)three times. The freeze-drying process was performed to eva-porate water from the purified complex. After calcination of thevirus-metal ionic complex at 1000 �C for 1�2 h in ambient aircondition, virus-templated high-crystalline BTO was formed.

Nanogenerator Fabrication. The synthesized virus-templatedBTO nanocrystals were mixed with a PDMS elastomer (Sylgard184,DowCorning)matrix for thepiezoelectric layer (1:5w/w ratio ofBTO to PDMS). Then, this piezoelectric layer was spin-casted onto aPDMS/ITO (100 nm)/PET (175 μm, Sigma-Aldrich) flexible substrateat a rate of 1500 rpm for 40 s and cured at 75 �C for 5 min. Next,another ITO-deposited PET film (50 μm, SKC) was placed on abottom piezoelectric layer/PDMS/ITO/PET substrate and fully har-dened at 75 �C for 24 h. Copper cables were connected to ITOelectrodes by using conductive epoxy (Chemtronics) to measurethe output current and voltage signals. Finally, to enhance thepiezoelectric effect of device, the virus-templated BTO nanogen-erator devicewas poled at 130 �Cwith a direct current (DC) voltageof 2 kV for 12 h.

Conflict of Interest: The authors declare no competingfinancial interest.

Supporting Information Available: Texts explaining detailedvirus-engineering steps, analyses equipment, and calculationfor yield of synthesis. Figures and texts of an additional COM-SOL-FEA simulation, TEM images and Raman spectra of varioussynthesis conditions, SAED and XRD patterns before calcination,issue of bundling and aggregation, a low-magnified TEMmicro-graph for showing well-dispersed virus-templated BTO, XPSdata of bare E3-M13 bacteriophage, generated electrical out-puts from the virus-templated BTO nanogenerator poled withvarious external fields, generated outputs from the only BTONPs composite-based nanogenerator, and the rectified voltageof virus-templated BTO nanogenerator. A table providing a

comparison of this study with previously reported nanogenera-tors. Videos showing operated LED-based optical fibers and LCDscreen by generated electrical energy from the virus-templatedBTO nanogenerator. This material is available free of charge viathe Internet at http://pubs.acs.org.

Acknowledgment. C. K. Jeong and I. Kim contributed equallyto this work. This work was financially supported by the BasicScience Research Program (NRF-2012R1A2A1A03010415) fundedby the Korea government (MSIP) through the National ResearchFoundation of Korea (NRF), and the Center for Integrated SmartSensors fundedby theMinistry of Science, ICT& Future PlanningasGlobal Frontier Project (CISS-2012M3A6A6054187). Also, thiswork was supported under the framework of international co-operation programmanaged by National Research Foundation ofKorea (NRF-2012K2A1A2033167) and by Basic Science ResearchProgram through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT & Future Planning(NRF-2013R1A1A1009626).

REFERENCES AND NOTES1. Aubrecht, G. J. Energy: Physical, Environmental, and Social

Impact; Pearson Prentice Hall: Upper Saddle River, NJ, 2006.2. Priya, S.; Inman, D. J. Energy Harvesting Technologies;

Springer: New York, 2009.3. Park, K. I.; Lee, M.; Liu, Y.; Moon, S.; Hwang, G. T.; Zhu, G.;

Kim, J. E.; Kim, S. O.; Kim, D. K.; Wang, Z. L.; et al. FlexibleNanocomposite Generator Made of BaTiO3 Nanoparticlesand Graphitic Carbons. Adv. Mater. 2012, 24, 2999–3004.

4. Beeby, S. P.; Tudor, M. J.; White, N. M. Energy HarvestingVibration Sources for Microsystems Applications. Meas.Sci. Technol. 2006, 17, R175–R195.

5. Wang, Z. L. Nanogenerators for Self-Powered Devices andSystems; Georgia Institue of Technology: Atlanta, GA, 2011.

6. Qi, Y.; McAlpine, M. C. Nanotechnology-Enabled Flexibleand Biocompatible Energy Harvesting. Energy Environ. Sci.2010, 3, 1275–1285.

7. Wang, Z. L.; Song, J. H. Piezoelectric Nanogenerators BasedonZincOxideNanowire Arrays. Science2006, 312, 242–246.

8. Qin, Y.; Wang, X. D.; Wang, Z. L. Microfibre-Nanowire HybridStructure for Energy Scavenging.Nature 2008, 451, 809–813.

9. Yang, R. S.; Qin, Y.; Dai, L. M.; Wang, Z. L. Power Generationwith Laterally Packaged Piezoelectric Fine Wires. Nat.Nanotechnol. 2009, 4, 34–39.

10. Xu, S.; Qin, Y.; Xu, C.; Wei, Y. G.; Yang, R. S.; Wang, Z. L. Self-Powered Nanowire Devices. Nat. Nanotechnol. 2010, 5,366–373.

11. Persano, L.; Dagdeviren, C.; Su, Y.; Zhang, Y.; Girardo, S.;Pisignano, D.; Huang, Y.; Rogers, J. A. High Performance

ARTIC

LE

JEONG ET AL. VOL. 7 ’ NO. 12 ’ 11016–11025 ’ 2013

www.acsnano.org

11024

Piezoelectric Devices Based on Aligned Arrays of NanofibersofPoly(vinylidenefluoride-co-trifluoroethylene).Nat. Commun.2013, 4, 1633.

12. Chung, S. Y.; Kim, S.; Lee, J. H.; Kim, K.; Kim, S. W.; Kang, C. Y.;Yoon, S. J.; Kim, Y. S. All-Solution-Processed Flexible ThinFilm Piezoelectric Nanogenerator. Adv. Mater. 2012, 24,6022–6027.

13. Gu, L.; Cui, N. Y.; Cheng, L.; Xu, Q.; Bai, S.; Yuan, M. M.; Wu,W. W.; Liu, J. M.; Zhao, Y.; Ma, F.; et al. Flexible FiberNanogenerator with 209 VOutput VoltageDirectly Powersa Light-Emitting Diode. Nano Lett. 2013, 13, 91–94.

14. Qi, Y.; Kim, J.; Nguyen, T. D.; Lisko, B.; Purohit, P. K.;McAlpine, M. C. Enhanced Piezoelectricity and Stretch-ability in Energy Harvesting Devices Fabricated fromBuckled PZT Ribbons. Nano Lett. 2011, 11, 1331–1336.

15. Park, K. I.; Jeong, C. K.; Ryu, J.; Hwang, G. T.; Lee, K. J. Flexibleand Large-Area Nanocomposite Generator Based on LeadZirconate Titanate Particles and Carbon Nanotubes. Adv.Energy Mater. 2013, 10.1002/aenm.201300458.

16. Park, K. I.; Xu, S.; Liu, Y.; Hwang, G. T.; Kang, S. J. L.; Wang,Z. L.; Lee, K. J. Piezoelectric BaTiO3 Thin Film Nanogenera-tor on Plastic Substrates. Nano Lett. 2010, 10, 4939–4943.

17. Park, J. B.; Kelly, B. J.; Kenner, G. H.; Vonrecum, A. F.; Grether,M. F.; Coffeen, W. W. Piezoelectric Ceramic Implants -Invivo Results. J. Biomed. Mater. Res. 1981, 15, 103–110.

18. Um, M. H.; Kumazawa, H. Hydrothermal Synthesis of Ferro-electric Barium and Strontium Titanate Extremely FineParticles. J. Mater. Sci. 2000, 35, 1295–1300.

19. O'Brien, S.; Brus, L.; Murray, C. B. Synthesis ofMonodisperseNanoparticles of Barium Titanate: Toward a GeneralizedStrategy of Oxide Nanoparticle Synthesis. J. Am. Chem.Soc. 2001, 123, 12085–12086.

20. Nuraje, N.; Dang, X. N.; Qi, J. F.; Allen, M. A.; Lei, Y.; Belcher,A.M. BiotemplatedSynthesis of PerovskiteNanomaterials forSolar Energy Conversion. Adv. Mater. 2012, 24, 2885–2889.

21. Sarikaya, M.; Tamerler, C.; Jen, A. K. Y.; Schulten, K.; Baneyx,F. Molecular Biomimetics: Nanotechnology through Biol-ogy. Nat. Mater. 2003, 2, 577–585.

22. Kim, S.; Park, C. B. Bio-Inspired Synthesis of Minerals forEnergy, Environment, and Medicinal Applications. Adv.Funct. Mater. 2013, 23, 10–25.

23. Yan, H.; Park, S. H.; Finkelstein, G.; Reif, J. H.; LaBean, T. H.DNA-Templated Self-Assembly of Protein Arrays and HighlyConductive Nanowires. Science 2003, 301, 1882–1884.

24. Mao, C. B.; Solis, D. J.; Reiss, B. D.; Kottmann, S. T.; Sweeney, R. Y.;Hayhurst, A.; Georgiou, G.; Iverson, B.; Belcher, A. M. Virus-Based Toolkit for the Directed Synthesis of Magnetic andSemiconducting Nanowires. Science 2004, 303, 213–217.

25. Lee, S. W.; Mao, C. B.; Flynn, C. E.; Belcher, A. M. Ordering ofQuantum Dots Using Genetically Engineered Viruses.Science 2002, 296, 892–895.

26. Malvankar,N. S.; Vargas,M.;Nevin, K. P.; Franks, A. E.; Leang, C.;Kim, B. C.; Inoue, K.;Mester, T.; Covalla, S. F.; Johnson, J. P.; et al.Tunable Metallic-Like Conductivity in Microbial NanowireNetworks. Nat. Nanotechnol. 2011, 6, 573–579.

27. Belcher, A. M.; Wu, X. H.; Christensen, R. J.; Hansma, P. K.;Stucky, G. D.; Morse, D. E. Control of Crystal Phase Switch-ing and Orientation by Soluble Mollusc-Shell Proteins.Nature 1996, 381, 56–58.

28. Naik, R. R.; Stringer, S. J.; Agarwal, G.; Jones, S. E.; Stone,M. O. Biomimetic Synthesis and Patterning of Silver Na-noparticles. Nat. Mater. 2002, 1, 169–172.

29. Lee, Y. J.; Yi, H.; Kim, W. J.; Kang, K.; Yun, D. S.; Strano, M. S.;Ceder, G.; Belcher, A. M. Fabricating Genetically Engi-neered High-Power Lithium-Ion Batteries Using MultipleVirus Genes. Science 2009, 324, 1051–1055.

30. Lee, Y.; Kim, J.; Yun, D. S.; Nam, Y. S.; Shao-Horn, Y.; Belcher,A. M. Virus-Templated Au and Au-Pt Core-Shell Nanowiresand Their Electrocatalytic Activities for Fuel Cell Applica-tions. Energy Environ. Sci. 2012, 5, 8328–8334.

31. Dang, X. N.; Yi, H. J.; Ham,M. H.; Qi, J. F.; Yun, D. S.; Ladewski,R.; Strano, M. S.; Hammond, P. T.; Belcher, A. M. Virus-Templated Self-Assembled Single-Walled Carbon Nano-tubes for Highly Efficient Electron Collection in Photo-voltaic Devices. Nat. Nanotechnol. 2011, 6, 377–384.

32. Nam, Y. S.; Magyar, A. P.; Lee, D.; Kim, J. W.; Yun, D. S.; Park, H.;Pollom, T. S.; Weitz, D. A.; Belcher, A. M. Biologically Tem-plated Photocatalytic Nanostructures for Sustained Light-DrivenWater Oxidation.Nat. Nanotechnol. 2010, 5, 340–344.

33. Nam, Y. S.; Shin, T.; Park, H.; Magyar, A. P.; Choi, K.; Fantner,G.; Nelson, K. A.; Belcher, A. M. Virus-Templated Assemblyof Porphyrins into Light-Harvesting Nanoantennae. J. Am.Chem. Soc. 2010, 132, 1462–1463.

34. Ghosh, D.; Lee, Y.; Thomas, S.; Kohli, A. G.; Yun, D. S.;Belcher, A. M.; Kelly, K. A. M13-Templated Magnetic Nano-particles for Targeted in Vivo Imaging of Prostate Cancer.Nat. Nanotechnol. 2012, 7, 677–682.

35. Tseng, R. J.; Tsai, C. L.; Ma, L. P.; Ouyang, J. Y. DigitalMemoryDevice Based on Tobacco Mosaic Virus Conjugated withNanoparticles. Nat. Nanotechnol. 2006, 1, 72–77.

36. Nam, Y. S.; Park, H.; Magyar, A. P.; Yun, D. S.; Pollom, T. S.;Belcher, A. M. Virus-Templated Iridium Oxide-Gold HybridNanowires for Electrochromic Application. Nanoscale2012, 4, 3405–3409.

37. Pokorski, J. K.; Steinmetz, N. F. The Art of Engineering ViralNanoparticles. Mol. Pharmaceutics 2011, 8, 29–43.

38. Nam, K. T.; Kim, D. W.; Yoo, P. J.; Chiang, C. Y.; Meethong, N.;Hammond, P. T.; Chiang, Y. M.; Belcher, A. M. Virus-EnabledSynthesis and Assembly of Nanowires for Lithium IonBattery Electrodes. Science 2006, 312, 885–888.

39. Lee, B. Y.; Zhang, J. X.; Zueger, C.; Chung, W. J.; Yoo, S. Y.;Wang, E.; Meyer, J.; Ramesh, R.; Lee, S. W. Virus-BasedPiezoelectric Energy Generation. Nat. Nanotechnol. 2012,7, 351–356.

40. Lin, Z. H.; Yang, Y.; Wu, J. M.; Liu, Y.; Zhang, F.; Wang, Z. L.BaTiO3 Nanotubes-Based Flexible and Transparent Nano-generators. J. Phys. Chem. Lett. 2012, 3, 3599–3604.

41. Day, V. W.; Eberspacher, T. A.; Frey, M. H.; Klemperer, W. G.;Liang, S.; Payne, D. A. Barium Titanium Glycolate: A NewBarium Titanate Powder Precursor. Chem. Mater. 1996, 8,330–332.

42. Day, V. W.; Eberspacher, T. A.; Klemperer, W. G.; Liang, S.Barium TitaniumAlkoxides for Barium Titanates: Synthesis,Characterization, and Applications. Mater. Res. Soc. Symp.Proc. 1994, 335, 177–182.

43. Walton, R. I.; Millange, F.; Smith, R. I.; Hansen, T. C.; O'Hare,D. Real Time Observation of the Hydrothermal Crystal-lization of Barium Titanate Using in Situ Neutron PowderDiffraction. J. Am. Chem. Soc. 2001, 123, 12547–12555.

44. Zhao, Z.; Buscaglia, V.; Viviani,M.; Buscaglia,M. T.;Mitoseriu, L.;Testino, A.; Nygren, M.; Johnsson, M.; Nanni, P. Grain-SizeEffects on the Ferroelectric Behavior of Dense Nanocrystal-line BaTiO3 Ceramics. Phys. Rev. B 2004, 70, 024107.

45. Han, J. J.; Zhang, H.; Li, Y. Z.; Zhao, X. J.; Chen, H.; Wu, Z. K.;Kim, S. J.; Park, K. S. Fabrication of TiO2 Microrod withDesired Shapes from Rod-like Titanium Glycolate. Chem.Lett. 2007, 36, 1352–1353.

46. Yu, P. F.; Cui, B.; Shi, Q. Z. PreparationandCharacterizationofBaTiO3 Powders and Ceramics by Sol-Gel Process UsingOleic Acid as Surfactant.Mater. Sci. Eng., A 2008, 473, 34–41.

47. Huang, C. J.; Lin, J. J.; Shieu, F. S. FormationMechanism andCharacterization of Ag-Metal Chelate Polymer Preparedby a Wet Chemical Process. Jpn. J. Appl. Phys. 2005, 44,6332–6340.

48. Polzonetti, G.; Battocchio, C.; Dettin, M.; Gambaretto, R.; DiBello, C.; Carravetta, V.; Monti, S.; Iucci, G. Self-AssemblingPeptides: A Combined XPS and NEXAFS Investigation onthe Structure of Two Dipeptides Ala-Glu, Ala-Lys. Mater.Sci. Eng. C 2008, 28, 309–315.

49. Wegmann, M.; Watson, L.; Hendry, A. XPS Analysis ofSubmicrometer Barium Titanate Powder. J. Am. Ceram.Soc. 2004, 87, 371–377.

50. Rossel, M.; Hohe, H. R.; Leipner, H. S.; Voltzke, D.; Abicht,H. P.; Hollricher, O.; Muller, J.; Gablenz, S. Raman Micro-scopic Investigations of BaTiO3 Precursors with Core-ShellStructure. Anal. Bioanal. Chem. 2004, 380, 157–162.

51. Li, A. D.; Ge, C. Z.; Lu, P.; Wu, D.; Xiong, S. B.; Ming, N. B.Fabrication and Electrical Properties of Sol-Gel DerivedBaTiO3 Films with Metallic LaNiO3 Electrode. Appl. Phys.Lett. 1997, 70, 1616–1618.

ARTIC

LE

JEONG ET AL. VOL. 7 ’ NO. 12 ’ 11016–11025 ’ 2013

www.acsnano.org

11025

52. Yang, Y. D.; Priya, S.; Li, J. F.; Viehland, D. Two-PhaseCoexistence in Single-Grain BaTiO3-(Mn0.5Zn0.5)Fe2O4

Composites, via Solid-State Reaction. J. Am. Ceram. Soc.2009, 92, 1552–1555.

53. Cheng, X.; Sastry, A. M.; Layton, B. E. Transport in StochasticFibrous Networks. J. Eng. Mater. Technol. 2001, 123, 12–19.

54. Berhan, L.; Sastry, A. M. On Modeling Bonds in Fused,Porous Networks: 3D Simulations of Fibrous-ParticulateJoints. J. Compos. Mater. 2003, 37, 715–740.

55. Berhan, L.; Sastry, A. M. Modeling Percolation in High-Aspect-Ratio Fiber Systems. I. Soft-Core versus Hard-CoreModels. Phys. Rev. E 2007, 75, 041120.

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Supporting Information

Virus-Directed Design of a Flexible BaTiO3 Nanogenerator

Chang Kyu Jeong,†,§ Insu Kim,†,§ Kwi-Il Park,† Mi Hwa Oh,‡ Haemin Paik,†,¶ Geon-Tae Hwang,† Kwangsoo No,† Yoon Sung Nam,†,‡,* and Keon Jae Lee†,*

†Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea‡Department of Biological Sciences, Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea§These two authors contributed equally to this work.¶Present address: Department of Materials Science, California Institute of Technology, Pasadena, California 91125, United States.

*E-mail correspondence to yoonsung@kaist.ac.kr (Y.S.N.) and keonlee@kaist.ac.kr (K.J.L.).

1

Engineering pVIII of M13 phage. To insert three glutamate (denoted as ‘E3’) in pVIII,

M13KE phage vector was modified by site-directed mutagenesis. Pre-existing PstI site at

position 6250 was deleted by replacing A with T by mutagenesis. The specific sites of

restriction enzymes for DNA insertion into pVIII were also created by mutagenesis; PstI site

and BamHI site were added by replacing T at position 1372 with A and replacing C at position

1381 with G, respectively. The sequences for E3 were synthesized as follows: forward, 5´-

[phos] GAA GAG GAA-3´ and reverse, 5´-[phos] GAT CTT CCT CTT CTG CA-3´. Annealed

DNA fragment was ligated into pVIII-modified M13KE phage vector which were prepared by

PstI/BamHI digestion, followed by transformation into XL1-blue competent cells. Engineered

M13 phages which display E3 on their pVIII surfaces were confirmed by DNA sequencing and

amplified using ER2738 growing media. M13KE phage, PstI, and BamHI were purchased from

New England Biolabs (Ipswich, MA). QuikChange II XL Site-Directed Mutagenesis Kit and

T4 DNA ligase were purchased from Agilent Technologies (Santa Clara, CA) and Invitrogen

(Carlsbad, CA), respectively. XL1-blue chemically competent cells were manufactured in

house. All components concerning growth media were purchased from BD Bioscience

(Franklin Lakes, NJ). All primers were synthesized from Bioneer Co. (Daejeon, South Korea).

2

Materials Characterization. Field emission TEM (Tecnai G2 F30, FEI Co.) was used to

observe the morphologies of each synthesis step. Zeta-potential analysis (ELSZ-1000, Photal

Otsuka, Japan) was performed to compare the negative charges of M13KE and E3-M13. The

chemical states of virus-metal ionic complex and virus-templated BTO were characterized by

ATR-IR spectroscopy (IFS66V/S & HYPERION 3000, Bruker Optiks, Germany) and XPS

(Sigma Probe, Thermo VG Scientific). The thermal analyses during calcination were

performed by TG and DSC (Setsys 16/18, Setaram, France). High resolution dispersive-Raman

microscope (ARAMIS, Horiba Jobin Yvon, France) was used to investigate highly

sophisticated phase characterizations of materials. The crystallinity of materials was

characterized by XRD (D/MAX-2500, Rigaku, Japan) and TEM-SAED. Piezoelectric

hysteresis property was measured by a commercial atomic force microscope (XE-100, Park

systems, South Korea) with lock-in amplifier (SR830, Stanford Research System) under

ambient conditions. SEM (S-4800, Hitachi, Japan) was utilized to show the virus-templated

BTO in PDMS matrix of the nanogenerator device.

3

Measurement for output signals of the nanogenerator. The virus-templated BTO

nanogenerator was periodically deformed by a linear motor for bending and releasing motions

to measure the generated electrical signals. The maximum deformation was 5 cm in a radius of

curvature at a frequency of 0.3 Hz (a strain rate of 0.2 m�s-1). The output short-circuit current

and open-circuit voltage signals were measured by a measurement instrument (Keithley 2612A)

and recorded directly by a computer. All measurement processes were conducted in a Faraday

cage on a vibration isolation platform to exclude any external mechanical effects.

4

Calculation for the yield of virus-templated BTO synthesis. When we used the 200 mM

Ba(EG) of 0.l mL, 200 mM Ti(EG) of 0.1 mL, and 1013 pfu/mL E3-M13 virus solution of 15

mL, about 4.0 mg virus-templated BTO was synthesized in the given conditions described in

Method section.

To obtain the theoretical mass of virus-templated BTO, following calculations were performed.

The total number of Ba and Ti atoms in the glycolate precursors:

200 �� × 0.1 �� = 20 ����

The amount of Ba and Ti atoms in the precursors are 20 ��������� �����

If whole Ba and Ti atoms in the precursors can be reacted, the total number of BTO unit cells:

20 ���� × (6.02 × 1023 ���-1) = 1.20 × 1019

where 6.02 × 1023 mol-1 is Avogadro number.

The theoretical mass of virus-templated BTO from whole used precursors:

1.2 ×1019 × �3.87 ×10-22 = 4.64 �

where 3.87 × 10-22 g is the mass of one BTO peroveskite crystalline unit cell. The calculated

theoretical amount of virus-templated BTO is about 4.64 mg. This calculation is based on the

assumption of infinite oxygen source from air during calcination process.

The yield of virus-templated BTO:

Yield

= The amount of obtained virus-templated BTO through the real experiment

The theoretical amount of virus-templated BTO through the calculation× 100

= 4.00 �4.64 �

× 100

= 86.21 %

5

Figure S1. Simulation for the piezopotential of piezoelectric layer with only BTO NPs.

6

Figure S2. Raman spectroscopy of resultant materials after calcination when the two

precursors, barium glycolate and titanium glycolate, are added at same time.

The bands of Raman shift show that barium dititanate (BaTi2O5) and barium

tetratitanate (BaTi4O9) are obtained. In this one-step addition method, negatively charged

titanium glycolate molecules are rapidly aggregated and polymerized before the negatively

charged E3-M13 viruses are screened by positively charged barium glycolate molecules. So,

the titanium glycolate molecules have less chances to be bound to the filamentous E3-M13

body compared to the case of sequential addition method. Eventually, titanium-rich

composition materials could be obtained by the one-step addition method because viruses and

barium glycolates exist as only dissolved states in aqueous solution without condensation by

titanium glycolates.

7

Figure S3. i) The magnified version of inset in Figure 1g, which shows HRTEM of virus-metal

ionic complex. ii) SAED and XRD (inset) of the virus-metal ionic complex.

In the HRTEM of virus-metal ionic complex, some crystallites are observed despite of

the majority consists of amorphous phases. The crystallites are considered as TiO2 caused by

the hydrolysis of titanium glycolate. The distance of crystal planes (~ 3.7 Å) approximately

corresponds with the lattice parameter of anatase. Moreover, dispersed ring patterns are shown

in the SAED of virus-metal ionic complex, which accord with the crystal planes of TiO2 phases

(anatase and rutile are denoted by A and R, respectively). However, the overall XRD patterns

of bulky virus-metal ionic complex show no crystalline peak since most of the complex is

composed of amorphous oxides and glycolates.

8

Figure S4. i) TEM image of the E3-M13 virus with bundling shapes (blue arrows). ii-iv) TEM

images of entangled and percolated virus-metal ionic complex nanostructures. Some bundling

or thickening (aggregations) events are indicated by red circles (There is no aggregation site in

the iv image).

Due to the inherent bundling of the filamentous virus (blue arrows, Fig. S4i), the virus-

metal ionic complex can also be slightly aggregated. Despite some aggregations (red circles,

Fig. S4-ii and iii), the percolated and entangled anisotropic shapes were maintained over all

sites (Fig. S4-ii, iii, and iv).

9

Figure S5. A low-magnified TEM image of well-dispersed virus-templated BTO

nanostructures after calcination. Some regions of aggregation are indicated using red circles.

Inset: A high magnification TEM image of the isolated virus-templated BTO unit.

Some virus-templated BTO nanostructures were aggregated by the intrinsic bundling

of the E3-M13 and virus-metal ionic complex (Fig. S4). Additional aggregates could be formed

during the calcination process. However, most of the virus-templated BTO structures were well

dispersed, and the entire network of virus-templated BTO nanostructures was also maintained,

as shown in the TEM images.

10

Figure S6. TEM image of morphology after incubation processes without M13 viruses. Inset:

Raman spectrum of TiO2 which is synthesized without M13 viruses.

There is only aggregated morphology when the synthesis is conducted without M13

virus templates. The Raman spectroscopy analysis after calcination presents that the resultant

material is TiO2. This means that the aggregated intermediate materials are almost constructed

by titanium glycolates. If there is no virus-anchored barium glycolates which can bind titanium

glycolates, the condensation and polymerization of only titanium glycolates happen rapidly

themselves, while barium glycolates exist as dissolved species in the aqueous solution.

11

Figure S7. TEM micrograph of intermediate complex after incubation processes with M13KE

viruses. Inset: Raman shift of BaTi4O9 which is synthesized with M13KE.

As shown in the zeta-potential analysis (Figure 1f, inset), the M13KE virus (wild-type

M13) is less negatively charged than the E3-M13 (genetically engineered M13 in our research).

The M13KE is not effective to self-assemble positively charged barium glycolates, that is,

titanium glycolates are also hard to be bound to the virus scaffold. For this reason, there are

still a lot of aggregates composed of titanium glycoaltes although a little of filamentous

structures by the M13KE are observed, as presented by Figure S6. Because the M13KE can

bind barium glycolates a little more than the case of no virus, the Raman spectrum of calcined

materials shows BaTi4O9.

12

Figure S8. O1s XPS data of E3-M13 virus before the synthesis. Inset: P2p XPS peak of E3-

M13 induced by phosphates from viral genes.

In the XPS analysis of pristine E3-M13 virus before the formation of complex with

metal ligands, the overall peak of O1s is placed at the higher binding energy than that of virus-

metal ionic complex and virus-templated BTO. This means that there are only organic elements

without metallic composition. The envelope of amide (peptide) bond is very high because the

virus mostly consists of capsid proteins which totally are exposed to the X-ray beam of XPS.

Moreover, the peak of relatively low binding energy in O1s XPS can be regarded as the trace

of oxygen atoms in phosphates from viral DNA due to the existence of P2p envelope.

13

Figure S9. Raman spectroscopy analysis according to calcination temperature.

Due to the sequential addition processes of our biological approaches for BTO

synthesis, a sufficient temperature must be needed to not only crystallize the material but also

uniformly diffuse barium and titanium atoms. During the calcination for BTO, the Raman

spectroscopy for the phase analysis shows the appearance and disappearance of the various

phases as a function of annealing temperature. At 600 °C, there are many titanium-rich phases

(e.g., TiO2, BaTi5O11, BaTi4O9, and BaTi2O5). However, BTO can be formed as increasing

calcination temperature close to 1000 °C.

14

Figure S10. The cross-sectional SEM image of virus-templated BTO-embedded PDMS matrix.

From this micrograph, the developed nanogenerator device can be very flexible.

15

Figure S11. i) Low magnification cross-sectional SEM image of the piezo-layer prepared using

virus-templated BTO nanostructures. Most regions are covered by the PDMS polymer matrix

without significant aggregations of the virus-templated BTO nanostructures. The

nanostructures are well dispersed in the piezo-layer. ii) High-resolution SEM image of some

virus-templated BTO nanostructures exposed to the surface of the cross-sectional piezo-layer

with infiltrated PDMS matrix. iii) Low magnification SEM image of the piezo-layer prepared

using BTO nanoparticles (NPs, diameter ~100 nm). A number of BTO NPs are significantly

aggregated in the PDMS matrix. iv) High-resolution SEM image of BTO NPs aggregates

exposed to the surface of the piezo-layer. The BTO NPs were synthesized by the hydrothermal

process as described in our previous report.1

16

Figure S12. Output i) current and ii) voltage signals of the virus-templated BTO nanogenerator

without the poling process.

Figure S13. Output i) current and ii) voltage signals of the virus-templated BTO nanogenerator

with the poling process at 1 kV.

Output electrical signals are increased with the poling voltage conditions (the final

poling voltage condition is 2 kV) since piezoelectric dipoles in BTO can be more aligned at

higher poling voltage. The poling process cannot be conducted above 2 kV because the

electrical breakdown occurs in the piezo-layer of nanogenerator device (current flows through

the piezo-layer). 17

Figure S14. Output i) current and ii) voltage signals of the nanogenerator fabricated by only

BTO NPs, which were synthesized by same methods as our previous report.

It is a very important issue to uniformly disperse BTO piezoelectric materials inside

the piezo-layer of energy harvesting device for high-performance output signals. The zero-

dimensional BTO NPs in PDMS elastomer cannot avoid the agglomeration and poor dispersion;

thus they cause the low output signals. In contrast, the virus-templated BTO nanostructures can

be well-dispersed due to the anisotropic shape; so, the high output signals can be achieved, as

indicated in Figure 1b and 4.

18

Figure S15. i) The measured output voltage signals after rectification. ii) Rectifying and

charging circuitry for the potential utilization of our virus-templated BTO nanogenerator (NG).

19

Materials Morphology Applied stress Output voltage(Max.)

Reference

M13 Virus Self-assembled film

Pressing 0.4 V 2

PVDF# Nanofibers Bending 1.5 V 3

BTO NPs with CNTs dispersant

Bending 3.2 V 1

NaNbO3 Nanowires Pressing 3.2 V 4

KNbO3 Nanorods Pressing 3.2 V 5

ZnO NPs with CNTs dispersant

Pressing 7.5 V 6

PMN-PT## Nanowires Pressing 7.8 V 7

PZT Particles with CNTs dispersant

Bending 10 V 8

BTO Nanotubes Pressing 5.5 V 9

BTO Virus-templated nanostructures

Bending 6 V This work

#Polyvinylidene fluoride##������������1/3Nb2/3)O3�xPbTiO3

Table S1. Comparison of the performance with other reported organic and composite-based

nanogenerators.

The output voltage of our nanogenerator was higher than that of other flexible

nanogenerators based on lead-free materials without any dispersion agents. Note that

mechanical stress under a pressing condition is much larger than that induced by a bending

stage.10

20

Videos of Supporting Information for Publication

Video S1. Commercial LED-based optical fibers lit up by the power generated from a virus-

templated BTO nanogenerator device.

Video S2. An LCD screen of electronic clock directly driven by a virus template BTO

nanogenerator device.

Supporting References

1. Park, K. I.; Lee, M.; Liu, Y.; Moon, S.; Hwang, G. T.; Zhu, G.; Kim, J. E.; Kim, S. O.; Kim, D. K.; Wang, Z. L., et al., Flexible Nanocomposite Generator Made of BaTiO3 Nanoparticles and Graphitic Carbons. Adv. Mater. 2012, 24, 2999-3004.

2. Lee, B. Y.; Zhang, J. X.; Zueger, C.; Chung, W. J.; Yoo, S. Y.; Wang, E.; Meyer, J.; Ramesh, R.; Lee, S. W., Virus-Based Piezoelectric Energy Generation. Nat. Nanotechnol. 2012, 7,351-356.

3. Persano, L.; Dagdeviren, C.; Su, Y.; Zhang, Y.; Girardo, S.; Pisignano, D.; Huang, Y.; Rogers, J. A., High Performance Piezoelectric Devices Based on Aligned Arrays of Nanofibers of Poly(vinylidenefluoride-co-trifluoroethylene). Nat. Commun. 2013, 4, 1633.

4. Jung, J. H.; Lee, M.; Hong, J. I.; Ding, Y.; Chen, C. Y.; Chou, L. J.; Wang, Z. L., Lead-Free NaNbO3 Nanowires for a High Output Piezoelectric Nanogenerator. ACS Nano 2011, 5,10041-10046.

5. Jung, J. H.; Chen, C. Y.; Yun, B. K.; Lee, N.; Zhou, Y. S.; Jo, W.; Chou, L. J.; Wang, Z. L., Lead-Free KNbO3 Ferroelectric Nanorod Based Flexible Nanogenerators and Capacitors. Nanotechnology 2012, 23. 375401.

6. Sun, H.; Tian, H.; Yang, Y.; Xie, D.; Zhang, Y. C.; Liu, X.; Ma, S.; Zhao, H. M.; Ren, T. L., A Novel Flexible Nanogenerator Made of ZnO Nanoparticles and Multiwall Carbon Nanotube. Nanoscale 2013, 5, 6117-6123.

7. Xu, S.; Yeh, Y.; Poirier, G.; McAlpine, M. C.; Register, R. A.; Yao, N., Flexible Piezoelectric ��������������-Based Nanocomposite and Device. Nano Lett. 2013, 13, 2393-2398.

8. Park, K. I.; Jeong, C. K.; Ryu, J.; Hwang, G. T.; Lee, K. J., Flexible and Large-Area Nanocomposite Generator Based on Lead Zirconate Titanate Particles and Carbon Nanotubes. Adv. Energy Mater. 2013, 10.1002/aenm.201300458.

9. Lin, Z. H.; Yang, Y.; Wu, J. M.; Liu, Y.; Zhang, F.; Wang, Z. L., BaTiO3 Nanotubes-Based Flexible and Transparent Nanogenerators. J. Phys. Chem. Lett. 2012, 3, 3599-3604.

10. Lee, K. Y.; Kim, K.; Lee, J. H.; Kim, T. Y.; Gupta, M. K.; Kim, S. W., Unidirectional High-Power Generation via Stress-Induced Dipole Alignment from ZnSnO3 Nanocubes/Polymer Hybrid Piezoelectric Nanogenerator. Adv. Funct. Mater. 2013, 10.1002/adfm.201301379.

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