Diphenylalanine Peptide Nanotube Energy Harvesters - ACS Nano

Aug 2, 2018 - Piezoelectric materials are excellent generators of clean energy, as they can harvest the ubiquitous vibrational and mechanical forces...
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Diphenylalanine Peptide Nanotube Energy Harvesters Ju Hyuck Lee, Kwang Heo, Konstantin Schulz-Schönhagen, Ju Hun Lee, Malav S Desai, Hyo-Eon Jin, and Seung-Wuk Lee ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b03118 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 4, 2018

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Diphenylalanine Peptide Nanotube Energy Harvesters Ju-Hyuck Lee†,‡,§, Kwang Heo†,‡,§,∥, Konstantin Schulz-Schönhagen†,‡, Ju Hun Lee†,‡, Malav S. Desai†,‡, Hyo-Eon Jin†,‡,#, and Seung-Wuk Lee*,†,‡ †

Department of Bioengineering, University of California, Berkeley, CA, 94720 USA, and

Tsinghua Berkeley Shenzhen Institute, University of California, Berkeley, CA, 94720 USA ‡

Biological Systems and Engineering Division, Lawrence Berkeley National Laboratory,

Berkeley, CA, 94720 USA Corresponding Author *E-mail: [email protected]

Present Addresses ∥

Department of Nanotechnology and Advanced Materials Engineering, Sejong University, 98

Gunja-Dong, Gwangjin-Gu, Seoul 05006, Republic of Korea #

College of Pharmacy, Ajou University, Suwon 16499, Republic of Korea

KEYWORDS : diphenylalanine, self-assembly, piezoelectric, uni-polarization, energy harvester.

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ABSTRACT : Piezoelectric materials are excellent generators of clean energy as they can harvest the ubiquitous vibrational and mechanical forces. We developed large-scale unidirectionally polarized, aligned diphenylalanine (FF) nanotubes and fabricated peptide-based piezoelectric energy harvesters. We first used the meniscus driven self-assembly process to fabricate horizontally aligned FF nanotubes. The FF nanotubes exhibit piezoelectric properties as well as unidirectional polarization. In addition, the asymmetric shapes of the self-assembled FF nanotubes enable them to effectively translate external axial forces into shear deformation to generate electrical energy. The fabricated peptide based piezoelectric energy harvesters can generate voltage, current and power of up to 2.8 V, 37.4 nA and 8.2 nW, respectively, with 42 N of force, and can power multiple liquid-crystal display panels. These peptide-based energy harvesting materials will provide a compatible energy source for biomedical applications in the future.

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The piezoelectric effect can be defined as an interconversion between mechanical and electrical energies induced by charge redistribution and separation when mechanical or electrical stimulus is applied to materials that lack inversion symmetry.1-3 Various inorganic piezoelectric materials, such as lead zirconate titanate (PZT), lithium niobate, gallium nitride, and barium titanate exhibit strong piezoelectric properties and have been used to generate electrical energy through the ubiquitous vibrational and mechanical forces.4-7 Although previous piezoelectric materials are practical for energy generation, their fabrication often requires environmentally harmful components (e.g., heavy metals and organic solvents) and/or energy intensive conditions such as high temperatures, high electric fields and extreme-pressure.8 Contrastingly, many natural biomaterials (e.g., virus, fibrillar collagen, DNA, amino acids and cellulose) that can be synthesized in an environmentally friendly manner have also been shown to have piezoelectric properties.9-15 In addition to safe synthesis schemes, such bio-piezoelectric materials are often highly uniform and are potentially more compatible alternatives for future biomedical applications. However, compared to their inorganic and synthetic counterparts, bio-piezoelectric materials have not yet been widely explored for typical piezoelectric applications (such as actuators, sensors and energy harvesting devices) due to poor piezoelectric responses of the major piezoelectric components (d33, d31 or d11); which are parameters utilized in common compressive or tensile stress-based devices. In addition, there is a limited control over the orientation and polarization directions in many biological piezoelectric materials suitable for device fabrication. Recently, diphenylalanine (FF) has been reported to form nanotubes with strong piezoelectric property comparable to conventional piezoelectric materials.15-17 However, the difficulty of fabricating scalable unidirectionally polarized structures that can convert external mechanical force to electric energy has been a major impediment to the realization of a

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practical piezoelectric device.18-22 Here, we develop a strategy to synthesize large-scale aligned and uni-polarized piezoelectric FF nanotubes, and create an FF peptide-based piezoelectric energy harvester (PEH). We use a meniscus-driven self-assembly process to synthesize tunable FF nanostructures (Figure 1).23 By controlling the composition of the peptide solution and pulling speeds of the substrates, we synthesized FF nanostructures ranging from vesicles to aligned peptide nanotubes. The resulting in-plane aligned FF nanotubes have unidirectional polarization and their asymmetric shapes can translate external axial forces into shear deformation to generate electrical energy. Furthermore, our facile synthesis approach allows us to easily scale-up to form large-area coatings that we use to develop PEHs. The resulting FFbased PEHs can generate maximum electrical voltage, current, and power output of 2.8 V, 37.4 nA and 8.2 nW, respectively, upon application of 42 N of force. Thus, the self-assembly process provides a convenient way to produce polarized piezoelectric peptide nanotubes in large-scale without energy intensive synthesis conditions. RESULTS AND DISCUSSION Meniscus-driven FF nanotube self-assembly. We synthesize large-scale aligned FF nanotubes with asymmetric shapes using meniscus-driven nanotube self-assembly process (Figure 1a).23 Briefly, we made FF solutions and produced self-assembled FF nanostructures by pulling a substrate vertically from those solutions. During the self-assembly process, evaporation proceeds rapidly at the air-liquid-solid interface resulting in a locally increased concentration of FF molecules that leads to nanostructure formation. The crystallization mechanism of FF nanotubes has previously been studied, both, experimentally and theoretically.18,

22, 24-27

It has been

proposed that six FF molecules form ordered rings through hydrogen bonds between the FF molecules.28-29 These rings self-assemble to form tubes through a combination of stacking of the

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aromatic side chains and hydrogen bonding of the peptide backbones (Figure 1a). In order to fully realize the synthesis strategy to achieve dense and organized FF nanotubes, we investigated the effects of thermodynamic and kinetic parameters on the nanostructure formation. First, we investigated the type of solvent and the solubility of FF, and found that DDW:HFIP co-solvent mixture yielded highly aligned FF nanotubes (Figure S1). The co-solvent ratio of DDW:HFIP also greatly affects the shape, density, and alignment of the FF nanotubes during assembly as it influences the nanotube growth rate. Specifically, the 1:3 DDW:HFIP co-solvent ratio was identified to form the densest and the most aligned FF nanotubes (at a constant pulling speed of 50 µm min-1 and a FF concentration of 2 mg ml-1) (Figure 2a-2d and Figure S2). Next, we investigated the effect of pulling speed on the morphology of the assembled FF nanostructures (at fixed DDW:HFIP solvent ratio of 1:3 and the FF concentration of 2 mg ml-1) (Figure 2e and Figure S3). A high pulling speed of 900 µm min-1 resulted in vesicular nano- and microstructures, while slower pulling speeds led to increasingly elongated and aligned nanotubes. When we evaluated the alignment of the FF nanotubes on the substrates by calculating orientation order parameter, the highly aligned FF nanotubes at slow pulling speeds showed near-perfect alignment toward the preferential direction (Figure S1-S4). The increase in density and alignment of FF nanotubes saturated at a pulling speed of 50 µm min-1, which was used for subsequent experiments. Furthermore, when the concentration of FF was increased from 1 mg ml-1 to 8 mg ml-1 at the DDW:HFIP solvent ratio of 1:3 and pulling speed of 50 µm min-1, the coverage area of the FF nanotubes on the substrate increased from 39.3 % to 89.7 % (Figure 2f). Higher FF concentration of 16 mg ml-1 resulted in thicker FF nanotube bundle formation but poor coverage of nanostructures on the substrates (Figure S4). An additional feature of the meniscus driven self-assembly process is the stick-slip motion of the meniscus as the substrate is

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pulled out.23 The oscillating meniscus movement and shear forces consist of a slow ‘stick’ phase that results in thick and wide FF nanotube bundles followed by fast ‘slip’ phase that drastically decreases the bundle size. This phenomenon creates the observed nanotube bundle structure with asymmetrical thickness and width (Figure 2c, Figure S5 and Figure S6). Moreover, our strategy can be used to produce FF nanotube arrays on a number of substrates, such as metal (Au), metalloid (Si), oxides (ITO and SiO2) and polymer films (PEN, PET) and mica (Figure S7). When we characterized the crystalline structure of the resulting FF nanotubes using X-ray diffraction (XRD) analysis, we confirmed the formation of hexagonal crystal structure with the space group P61, which is non-centrosymmetric without an inversion center. This indicated that the synthesized FF nanostructures have piezoelectric characteristics (Figure S8).30 Piezoelectric properties of self-assembled FF nanotubes. We verified the piezoelectric nature of FF nanotubes and their polarity using piezoresponse force microscopy (PFM; Figure 3a). For samples fabricated using drop-cast method as a control, we observed randomly oriented nanotubes with uncontrolled polarization (Figure 3b and Figure 3c). On the other hand, the FF nanotubes fabricated by meniscus-driven self-assembly process exhibited unidirectionally aligned piezoelectric polarization. Figure 3d shows a three-dimensional (3D)-atomic force microscopy (AFM) image of the aligned FF nanotubes on Au/Si substrate. Following the topography acquisition, PFM characterization was performed by measuring FF nanotube response to an applied electric field. When the scan was performed along the long axis of the FF nanotube, a strong shear component of the piezo tensor d15 corresponding to the in-plane signal was observed to be 46.6 pm V-1, which is similar to previously reported values.16, 31 The aligned FF nanotubes exhibited unidirectional polarization (Figure 3e, Figure S9 and Figure S10). We further verified the polarity of the aligned nanotubes after physically rotating the sample by 180°

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and observing a corresponding reversal of polarity (Figure 3f). We also compared the FF nanostructure responses with those of two piezoelectric materials, periodically poled lithium niobate (PPLN) and type I collagen, and found the d33 values of PPLN and collagen to be 17.0 and 1.1 pm V-1, respectively (Figure S11). The formation and spontaneous uni-polarization of the FF nanotubes can be attributed to the synergistic interactions between dipolar FF nanotubes and charged substrates coupled with the meniscus induced self-assembly process. The noncentrosymmetric hexagonal structure (P61) of the FF nanotubes imparts a net dipole;16,

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whereas, the gold substrates typically possess a negative surface charge.32 Thus, once an FF nanotube is nucleated at the meniscus, the positive side of the nanotube adheres to the negatively charged Au surface. The nanotubes continue to grow at the negative pole towards the out-ofplane direction (c-axis) and simultaneously coalesce into bundles. During the meniscus-induced self-assembly process, the meniscus force consequently aligns the nanotubes along the pulling direction. When we introduced amine-functionalized Au surfaces through cysteamine treatment,33 we observe mixed polarization of aligned FF nanotubes (Figure 3g and Figure S12). The result indicated that both negative and positive sides of the FF molecules are simultaneously attracted to the cysteamine coated surface that still contains exposed Au (negatively charged). We also investigated the effects of solution ionic strength (Na+ and Cl- ions) and found that the polarization of the FF nanotube and their piezoelectric properties are significantly decreased with increasing ionic strength (Figure 3h and Figure S13). The observed changes likely result due to charge screening from the Na+/Cl- ions in solution. As the solution ionic strength increases, the Debye length decreases and leads to diminished charge-charge interactions between Au surfaces and dipolar FF nanotubes, and consequent random polarization.34 These results support that the uni-directional polarization and alignment of the FF nanotubes are achieved through synergistic

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interactions between dipolar nanotubes and charged substrates as well as shear force at the meniscus during the self-assembly process. Piezoelectric energy harvesters based on FF nanotubes. We fabricated piezoelectric energy harvesters (PEHs) using the resulting large-scale aligned FF nanotubes (Figure 4a). We first assembled horizontally aligned FF nanotubes on Au/Cr coated flexible polyethylene naphthalate (PEN) substrate. A polyvinylpyrrolidone (PVP) layer was spin-coated on the aligned FF nanotubes as a protection layer, followed by the addition of an Au/Cr coated PEN substrate as the top electrode (for detailed methods see the Supporting Information).35 The resulting FF nanotube devices produce electrical energy upon application of mechanical force. We measured the output voltages with a load resistance of 1 GΩ and short-circuit currents from the FFnanotube PEHs by periodically applying compressive loads using a mechanical tester. The devices generated a negative output when force was applied, followed by a positive output when released (Figure 4a-4d). The peak voltage reached 2.8 V and the current reached 37.4 nA with a force of 42 N. The performance of this device is comparable to or higher than that of a piezoelectric device with vertically oriented FF nanotubes (The previously-reported vertically aligned FF nanotube energy harvesters generated an open-circuit voltage of 1.4 V and a shortcircuit current of 39.2 nA upon the application of 60 N).15 Indeed, the uni-polarization of the FF nanotubes is critical for generating higher piezoelectric potentials in the PEH. Moreover, we also confirmed that the observed signals were generated by the PEH by measuring signals after switching device polarity (Figure S14). As a control, we tested the output performance of a device with randomly oriented FF nanotubes (Figure S15), and PVP only device (without FF), and confirmed that they do not produce comparable electrical performance. We also observed that the output voltage and current of the PEH enhanced with increases in the coverage of FF

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nanostructures (Figure S16) as well as the applied force on the device (Figure 4e). In addition, our measurements showed that as the load resistance was increased from 1 kΩ to 1 GΩ, the output current decreased due to ohmic loss, while the output voltage increased. Peak power of 8.2 nW was achieved at a load resistance of 44 MΩ (Figure 4f and Figure 4g). Additional piezoelectric output performance of the resulting PEHs upon bending confirmed that the generation of the piezoelectric signal was from the PEHs and affirmed the piezoelectric behavior of the FF-nanotube PEHs (Figure S17). We further investigated the piezoelectric phenomenon observed experimentally in the FF nanotubes using finite element modeling simulations. Our simulations indicated the importance of the asymmetric geometric shapes of FF nanotubes to generate the strong piezoelectric potentials observed. The asymmetric shape consisting of a sloped geometry enables the nanotubes to translate axial forces into strong shear stresses that are converted to electrical energy (Figure S18). Additionally, the simulations revealed that increasing the slope of the FF nanotubes correlates with generation of greater piezoelectric potentials (Figure S19). Indeed, the synthesized FF nanotubes possess asymmetric shapes and the observed strong piezoelectric phenomena matches well with the theoretical model (See the supporting information). Finally, in order to demonstrate the efficacy of FF-nanotube PEHs as energy generators, we powered two liquid crystal display (LCD) panels to display “FF PEH” by simply pressing the PEH device using a finger (Figure 4h). Supporting Information movie 1 shows “FF PEH” displayed on the LCD panels in response to the applied load and frequency of mechanical stimulation. When the device is pressed lightly, the LCD only shows a few letters compared to a full display of “FF PEH” with firm pressing. Similarly, higher frequency of stimulation produces a consistent and effective response from the PEHs we developed.

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Conclusion In this study, we developed a facile method to fabricate large-scale aligned peptide nanotube arrays through the meniscus-driven self-assembly process. By controlling the FF concentration, type of solvent and pulling speeds, we can control the morphology of FF peptide nanostructures from vesicles to elongated and aligned nanotubes. Furthermore, the in-plane aligned FF nanotubes show uni-polarization with asymmetric shapes that enable them to translate axial forces into shear deformation, which is critical for the fabrication of practical piezoelectric energy harvesters. Upon application of force, the FF-nanotube PEH can generate voltage, current and power of up to 2.8 V, 37.4 nA and 8.2 nW, respectively, and can power multiple liquidcrystal display panels. Our work will be helpful to study the self-assembly of these biomaterials and their application in bioelectronics and biomedical energy generation.

Methods FF nanotube formation process. A home-built apparatus was constructed by modifying a syringe pump that is controlled by a custom computer program (C++). Various substrates were vertically dipped into an FF solution and pulled out at specific speeds using the computercontrolled syringe pump. Optical microscopy. Optical microscopic images were collected using an IX71 Inverted Microscope (Olympus, Tokyo, Japan) equipped with a digital CCD camera (Q-imaging, Surrey, Canada). Imaging was also performed using Celestron 44302 handheld digital microscope. Atomic force microscopy. Atomic force microscopy (AFM) images were collected using MFP3D AFM (Asylum Research, Santa Barbara, CA) and analysed using Igor Pro 6.0 (Wave Metrics, Inc. Lake Oswego, OR) and the Asylum software package (Asylum Research, CA).

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X-ray Diffraction. Crystal structures of the synthesized FF-nanotubes were characterized using XRD (Bruker D8 Advanced diffractometer) with Cu Kα radiation (λ = 1.54 Å). Piezoelectric response characterization of FF nanotubes. Piezoelectric response of FF nanotubes was characterized using MFP-3D AFM (Asylum Research) with a Pt-coated AC240TS (Olympus) tip with a nominal spring constant of ~2 N m-1 and a free-air resonance frequency of ~70 kHz, and using XE-100 (Park Systems) with a Pt-coated Multi75E-G (Budget sensors) tip with a spring constant of 3 N m-1 and free-air resonance frequency of ~75 kHz. While in contact with FF nanotubes, a driving AC voltage of 5 V was applied to the tip and the mechanical amplitude was measured. Piezoelectric device fabrication. Gold-coated flexible PEN substrates were prepared by depositing 10 nm chromium and 50 nm gold by sputtering. FF nanotubes were self-assembled on the substrates using dip-coating process from an FF solution (2 - 8 mg ml-1) at a pulling speed of 50 µm min-1. After the completion of dip-coating process, PVP solution (100 mg ml-1) in water was spin coated as a protection layer on the FF nanotube coating. Once dried, Cr/Au coated PEN substrate was placed on the PVP layer as the top electrode. Piezoelectric energy harvester characterization. The FF nanotube based PEH was mounted on a dynamic mechanical test system (Electroforce 3200, Bose, MN) and a predefined displacement was applied. Force was monitored with a 50 lb-f load cell and displacement was adjusted until the desired amount of force was reached. A semiconductor parameter analyser (4200-SCS, Keithley, OH) was used for electrical measurements.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions §

J.H.L. & K.H. contributed equally to this work.

ACKNOWLEDGMENT This work was supported by the CJ Global Grant Program (CGP) by CJ CheilJedang Research Institute of Biotechnology (Grant number CGP-30-16-01-0001). This work was also supported by Tsinghua Berkeley Shenzhen Institute Fund. J.H.L acknowledges the support of the Postdoctoral Research Program of Sungkyunkwan University (2016).

ASSOCIATED CONTENT Supporting Information Available: Additional details and figures about experiments and methods; fabrication and characterization of horizontally aligned FF nanotube; piezoelectric force microscopy characterization, finite element modeling simulation (PDF). These materials are available free of charge via the Internet at http://pubs.acs.org The authors declare no competing financial interest

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20. Wang, M.; Du, L.; Wu, X.; Xiong, S.; Chu, P. K. Charged Diphenylalanine Nanotubes and Controlled Hierarchical Self-Assembly. ACS Nano 2011, 5, 4448-4454. 21. Hill, R. J. A.; Sedman, V. L.; Allen, S.; Williams, P. M.; Paoli, M.; Adler-Abramovich, L.; Gazit, E.; Eaves, L.; Tendler, S. J. B. Alignment of Aromatic Peptide Tubes in Strong Magnetic Fields. Adv. Mater. 2007, 19, 4474-4479. 22. Reches, M.; Gazit, E. Controlled Patterning of Aligned Self-Assembled Peptide Nanotubes. Nat. Nanotechnol. 2006, 1, 195-200. 23. Chung, W. J.; Oh, J. W.; Kwak, K.; Lee, B. Y.; Meyer, J.; Wang, E.; Hexemer, A.; Lee, S. W. Biomimetic Self-Templating Supramolecular Structures. Nature 2011, 478, 364-368. 24. Levin, A.; Mason, T. O.; Adler-Abramovich, L.; Buell, A. K.; Meisl, G.; Galvagnion, C.; Bram, Y.; Stratford, S. A.; Dobson, C. M.; Knowles, T. P. J.; Gazit, E. Ostwald's Rule of Stages Governs Structural Transitions and Morphology of Dipeptide Supramolecular Polymers. Nat. Commun. 2014, 5219. 25. Amdursky, N.; Beker, P.; Koren, I.; Bank-Srour, B.; Mishina, E.; Semin, S.; Rasing, T.; Rosenberg, Y.; Barkay, Z.; Gazit, E.; Rosenman, G. Structural Transition in Peptide Nanotubes. Biomacromolecules 2011, 12, 1349-1354. 26. Bdikin, I.; Bystrov, V.; Kopyl, S.; Lopes, R. P. G.; Delgadillo, I.; Gracio, J.; Mishina, E.; Sigov, A.; Kholkin, A. L. Evidence of Ferroelectricity and Phase Transition in Pressed Diphenylalanine Peptide Nanotubes. Appl. Phys. Lett. 2012, 100, 043702. 27. Diaz, J. A. C.; Cagin, T. Thermo-Mechanical Stability and Strength of Peptide Nanostructures from Molecular Dynamics: Self-Assembled Cyclic Peptide Nanotubes. Nanotechnology 2010, 21, 115703. 28. Heredia, A.; Bdikin, I.; Kopyl, S.; Mishina, E.; Semin, S.; Sigov, A.; German, K.; Bystrov, V.; Gracio, J.; Kholkin, A. L. Temperature-Driven Phase Transformation in Self-Assembled Diphenylalanine Peptide Nanotubes. J. Phys. D: Appl. Phys. 2010, 43, 462001. 29. Bystrov, V. S.; Paramonova, E.; Bdikin, I.; Kopyl, S.; Heredia, A.; Pullar, R. C.; Kholkin, A. L. Bioferroelectricity: Diphenylalanine Peptide Nanotubes Computational Modeling and Ferroelectric Properties at the Nanoscale. Ferroelectrics 2012, 440, 3-24. 30. Huang, R. L.; Wang, Y. F.; Qi, W.; Su, R. X.; He, Z. M. Temperature-Induced Reversible Self-Assembly of Diphenylalanine Peptide and the Structural Transition from Organogel to Crystalline Nanowires. Nanoscale Res. Lett. 2014, 9, 653. 31. Vasilev, S.; Zelenovskiy, P.; Vasileva, D.; Nuraeva, A.; Shur, V. Y.; Kholkin, A. L. Piezoelectric Properties of Diphenylalanine Microtubes Prepared from the Solution. J. Phys. Chem. Solids 2016, 93, 68-72. 32. Wang, J.; Bard, A. J. Direct Atomic Force Microscopic Determination of Surface Charge at the Gold/Electrolyte Interface - The Inadequacy of Classical GCS Theory in Describing the Double-Layer Charge Distribution. J. Phys. Chem. B 2001, 105, 5217-5222. 33. Zhang, M.; Liu, Y. Q.; Ye, B. C. Colorimetric Assay for Sulfate Using PositivelyCharged Gold Nanoparticles and Its Application for Real-Time Monitoring of Redox Process. Analyst 2011, 136, 4558-4562. 34. Xue, X.; Nie, Y.; He, B.; Xing, L.; Zhang, Y.; Wang, Z. L. Surface Free-Carrier Screening Effect on the Output of a ZnO Nanowire Nanogenerator and Its Potential as a Self-Powered Active Gas Sensor. Nanotechnology 2013, 24, 225501. 35. Lee, S.; Hinchet, R.; Lee, Y.; Yang, Y.; Lin, Z. H.; Ardila, G.; Montes, L.; Mouis, M.; Wang, Z. L. Ultrathin Nanogenerators as Self-Powered/Active Skin Sensors for Tracking Eye Ball Motion. Adv. Funct. Mater. 2014, 24, 1163-1168.

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Figures:

Figure 1. Schematic diagram depicting the fabrication process to create large-scale peptide nanotube arrays through the meniscus-driven self-assembly. a, Meniscus driven dip-coating process to synthesize aligned FF nanotubes with unidirectional piezoelectric polarization. b, Photograph and optical microscopy image of the fabricated aligned FF nanotubes on a flexible substrate.

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Figure 2. Synthesis and characterization of aligned FF nanotubes. a-c, SEM images of the FF nanostructures. FF nanostructures were modulated by control of DDW:HFIP solvent ratio. The pulling speed and the concentration of FF suspension were 50 µm min-1 and 2 mg ml-1, respectively. The yellow dashed-line in image in c indicates boundary of stick-slip motion where the FF nanotubes show asymmetric shape. The inset image in c shows cross-sectional SEM image of an FF nanotubes. d-f, Coverage of FF nanotubes depending on variation of thermodynamic and kinetic parameters. Coverage of FF nanotubes depending on, d, co-solvent ratio of DDW:HFIP, e, pulling speed, and f, concentration of FF solution. Error bars in d-f indicate the standard deviation.

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Figure 3. Unidirectional Polarization of FF piezoelectric nanotubes. a, Schematic representation of AFM/PFM characterization. b, AFM topography image and c, PFM image of randomly distributed FF nanotubes on Au-coated substrate prepared by drop-casting. Randomly oriented FF nanotubes exhibit random piezoelectric polarization. d, 3D-AFM topography image of aligned FF nanotubes on Au-coated substrate. e, PFM images of FF nanotubes exhibit unidirectional polarization. f, PFM image of the sample e after 180º rotation exhibits opposite piezoelectric polarization. Asterisks indicate the same location after 180º rotation. Graphs under

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the PFM images show the cross-sectional profiles of piezoelectric polarization responses along the red dashed line in PFM images in e and f. g, Piezoelectric polarity of aligned FF nanotubes on bare Au and cysteamine treated Au surfaces (n = 10). h, Effect of NaCl concentration on piezoelectric polarity of aligned FF nanotubes (n = 10 for 0 mM NaCl, otherwise n = 4).

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Figure 4. Piezoelectric characterization of FF nanotube piezoelectric energy harvester. a, Schematic illustration of FF nanotube PEH. b, Axial force applied to the device. c, Output

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voltage responses with 1 GΩ external load and d, Output short-circuit current responses of the PEH under periodic compressive force. Typical electric response of a single signal is shown on the right side. e, Piezoelectric peak voltages with 1 GΩ of load resistance and short-circuit currents of the PEH as a function of applied force. f, Piezoelectric peak voltages and currents outputs from the PEH with 42 N of force as a function of load resistance. g, Piezoelectric peak power output depending on the load resistance. h, Photograph of the LCD driven by pressing the PEH with a finger.

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