Patterned Arrays of Ordered Peptide Nanostructures

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The unique chemical and physical properties of the peptide nanotubes make them excel- lent component in various devices and the useful application were ...
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Journal of Nanoscience and Nanotechnology Vol. 8, 1–8, 2008

Patterned Arrays of Ordered Peptide Nanostructures Lihi Adler-Abramovich1 † , D. Aronov2 † , E. Gazit1 ∗ , and G. Rosenman2 ∗ 1

Department of Molecular Microbiology and Biotechnology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 69978, Israel 2 Department of School of Electrical Engineering-Physical Electronics, Tel Aviv University, 69978 Tel Aviv, Israel Methods for the deposition of ordered nanostructures on various substrates are a key factor in nanotechnological devices. There is a special interest in the development of methods for the organization of organic nanostructures that are not compatible with some of the conventional fabrication methods. The unique chemical and physical properties of the peptide nanotubes make them excellent component in various devices and the useful application were already demonstrated in the case of biosensors. Here we demonstrate the ability to deposited aromatic dipeptide nanotubes using electron beam treatment of surfaces to control their wettability. The use of a low energy electron irradiation results in the formation of pre-defined surfaces with controlled level of wettability. This treatment allows the precise patterning of the organic tubular assemblies at high resolution. The differential wettability of the surface resulted in organization of the peptide assemblies according to the properties of the different areas of the surface. In the current work, we describe the use of wettability patterned surfaces for the control patterning of horizontal peptide nanotubes and nanospheres. Furthermore, lift-off lithography is used to make patterned arrays of peptide nano-forests, vertically aligned peptide nanotubes. In summary, the novel patterning techniques together with the unique properties of the peptide nanostructures represent an important step in the integration of these assemblies into functional nanosystems and devices.

1. INTRODUCTION Self-assembled nanostructures are envisioned to serve as major building-blocks in future nanotechnological applications. One challenge in this field is the ability to control the orientation and distribution of these nanostructures. Most self-assembling materials are macroscopically disordered, a property which limits their bulk properties and potential uses.1–3 In order to improve the performances and enable new functions, patterning at the microscale is needed and will extend order and organization in a predictable manner over large areas.4–9 Much effort has been invested in the development of fabrication techniques for the growth of organized arrays of carbon and inorganic nanostructures.1–3 10–12 Nanostructures that are based on protein and peptide building blocks offer the advantages of chemical diversity deriving from the extensive structure variability of the building blocks. Moreover, these structures could also be decorated through their fusion to functional and protective groups. Furthermore, amino acid motifs can enable the binding of metals and other ∗ †

Authors to whom correspondence should be addressed. These authors contributed equally to this work.

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inorganic materials to the biological structures, for example the high affinity binding of nickel or cobalt to Histidine stretches. The chemical diversity together with biocompatibility, biological recognition abilities and facile synthesis make the protein and peptide building blocks very attractive for bionanotechnology applications.13–19 The recently identified aromatic dipeptide nanotubes [ADNT] represent a unique class of organic nanostructures. These bio-inspired structures are formed by the self-assembly of diphenylalanine, the core recognition motif of the -amyloid polypeptide into individual hollow entities with a remarkable micrometer-scale persistence length. Biocompatible and water-soluble ADNT are readily formed under mild conditions from inexpensive starting materials.20 Although the ADNT are bioinspired materials they are remarkably stable at the presence of organic solvent and have extraordinary thermal stability properties.21 22 Furthermore the ADNT were suggested to be among the stiffest bio-inspired materials presently known in view of their unique mechanical strength, which was directly measured through indentation type experiments using atomic force microscopy. The averaged point stiffness of the nanotubes calculated in this study is 160 N/m, and they have a correspondingly high Young’s

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modulus of ∼19 GPa as calculated by finite element analysis.23 Furthermore, by applying the bending beam model to atomic force microscopy images of diphenylalanine nanotubes suspended across cavities, Young’s modulus of 27 ± 4 GPa and a shear modulus of 021 ± 003 GPa were obtained.24 This is most likely due the aromatic interactions that stabilize the structures as observed with aromatic polyamides such as Kevlar® . Moreover, tubular structures can assemble from non-charged aromatic dipeptide analogue Ac-Phe-Phe-NH2 , in which the N-terminal amine is acetylated and the C-terminal carboxyl is amidated, as well as the assembly of other amine-modified analogues.25 Two types of nanostructures, i.e., nanotubes and nanospheres were produced by the self-assembly of the same aromatic dipeptide, tertbutoxycarbonyl-Phe-PheOH (Boc-Phe-Phe-OH), under different conditions.26 The ADNT can serve as a degradable mold for the fabrication of silver nanowires, as a scaffold for the organization of platinum nanoparticles and as a template for the formation of coaxial nanocables.20 27 28 Furthermore, the tubes were used for the improvement of the sensitivity of electrochemical biosensors,29 30 and the formation of ADNT biocompatible hydrogel has been demonstrated.31 A limiting factor in the utilization of the ADNT system was the ability to temporally control the assembly process. This was resolved by the use of a self-immolative dendritic system as a platform for the controlled assembly of peptide nanotubes that was enzymatically activated. The extremely short length of the peptide building blocks and their ability to self-assemble enable the controlled assembly applications.32 Various methodologies were also developed for the horizontal and vertical alignment of the ADNT. Vertically aligned nanoforest was formed by axial unidirectional growth of a dense array of these peptide tubes.33 Furthermore, horizontal alignment of the tubes was achieved through noncovalent coating of the tubes with a ferrofluid and the application of an external magnetic field.33 In addition, the alignment of the ADNT without any additional coating in an external strong magnetic field was demonstrated. The alignment was attributed to the effect of the magnetic torque associated with the diamagnetic anisotropy of the aromatic rings of phenylalanine.34 Recently the inkjet technology has been applied for the patterning of peptide nanostructures on non-biological surfaces. The ADNT were used as an ‘ink’ and patterned on transparent foil and ITO-coated plastic surfaces by modified commercial inkjet printer.26 The ability to organize matter on a surface is one of the major enabling principles in the field of micro- and nanotechnology. The microelectronics industry developed photolithography and associated techniques to fabricate integrated circuits. Silicon micromachining had been developed for the fabrication of micro- nanoelectromechanical systems (NEMS), and these techniques were the first to be adapted to the fabrication of microstructures 2

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for biological research. In addition, diverse surface modification techniques have been found to permanently alter the basic material properties. These properties variation has been recently demonstrated by the use of a number of methods including electrical,35 36 light-induced37 38 and electrochemical surface modifications.39–41 However, the most highly developed technologies for micro- and nanopatterning are limited in their application to biotechnology and biology, due to their high expenses, limited control over surface properties.42 Recently, we have proposed another approach to design and engineer functionality of the material surfaces of different origin.43 This new concept is based on chargeinduced wettability modification performed by a low energy electron irradiation.43 We have demonstrated that using this method, the surface wettability properties may be varied in controllable wide range by variation of incident electron flux and electron energy. The developed electron-beam-based technique allows tailoring surface wettability properties both on unlimited surface area and imprinting in micro/nanoscale. This phenomenon has been observed by us in many solid state materials, such as amorphous S3 N4 , glass, mica, n- and p-Si, metal oxides (Al2 O3 , TiO2 ) and biomimetic materials (sea shells, hydroxyapatite and related calcium phosphates).43 44 Recently, the electron-induced method of the surface modification has allowed high-resolution wettability patterning, selective adhesion of biomolecules such as bovine serum albumin proteins and DNA,45 different bacteria,46 patterning of deposited polymers.47 In the current work we present simple techniques suitable for the patterning and alignment of bio-inspired nanostructures. We describe the use of wettability patterned surfaces for the control patterning of horizontal peptide nanotubes and nanospheres and the use of lift-off lithography technique for the control patterning of vertically aligned peptide nanotubes.

2. EXPERIMENTAL DETAILS 2.1. Preparation of Studied Substrates Two different groups of representative materials were used: Silicon (Si) and glass, as conductive and dielectric materials, respectively. P -type Si substrates with resistivity in the range of 11 to 17 /cm were preliminary thoroughly cleaned. Standard cleaning method using NH4 OH/H2 O2 /H2 O and HCl/H2 O2 /H2 O solutions (RCA, USA) at temperature of around 70–80  C was exploited for Si samples.48 After the RCA cleaning the Si samples were rinsed well under de-ionized water and blown dry using nitrogen stream to remove any traces of the cleaning solution attached to the sample surface. Glass substrates, specimens of 10 mm2 were cut off from a standard microscope glass slides for surface treatment. Prior to surface modification exposure, the glass specimens were cleaned J. Nanosci. Nanotechnol. 8, 1–8, 2008

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by sonication in 95% isopropanol (Sigma) for 2 minutes and rinsed with de-ionized water. 2.2. Formation of Hydrophobic Pattern Surface In order to fabricate hydrophobic patterning, electron irradiation was performed in vacuum 10−6 Torr at room temperature, using a commercially available electron gun (EFG-7, Kimball Physics Inc. USA) with invariable electron energy of 150 eV and incident charge density of 200 C/cm2 . Hydrophobicity patterning (Fig. 1(a)-iii) was tailored by exposing both, Si and glass, samples to local electron irradiation through specifically designed Si-shadow masks. The mask contained arrays of squareshaped holes of 100 m length and spaces of 200 m (Fig. 1(a)-ii).

2.3. Formation of Hydrophilic Pattern Surface In order to fabricate hydrophilic patterning, the electron irradiation treatment was combined with UV-illumination in air. Hydrophilicity patterning (Fig. 1(b)-iv) was tailored by exposing the preliminary electron irradiated both, Si and glass, samples to local UV illumination through the aforementioned Si shadow mask (Fig. 1(b)-iii). The light source (non-filtered UV light 185–2000 nm) utilized a Hamamatsu UV spot light device equipped with 200 W Hg-Xe lamp. The conventional illumination duration was 2 minutes. 2.4. Preparation of Initial Peptides Solutions The studied peptide diphenylalanine (H-Phe-Phe-OH) and the N-terminal modified analogues: Boc-Phe-Phe-OH,

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were purchased from Bachem (Bubendorf, Switzerland). Fresh stock solutions were prepared by dissolving lyophilized form of the peptides in 1,1,1,3,3,3-hexafluoro2-propanol, HFP (Sigma-Aldrich, USA) at a concentration of 100 mg/ml. To avoid any pre-aggregation, fresh stock solutions were prepared for each experiment. 2.5. Preparation of Boc-Phe-Phe-OH Nanospheres Solution for Patterned Deposition For the preparation of nanospheres, the Boc-Phe-Phe-OH peptide stock solution in HFP was diluted in 50% ethanol to a final concentration of 5 mg/ml. 2.6. Preparation of Diphenylalanine Peptide Nanotubes Solution for Patterned Deposition For the nanotubes preparation the 100 mg/ml peptide stock solutions in HFP was diluted into a final concentration of 2 mg/ml in ddH2 O. Then, peptide solution was allowed to dry at room temperature over the wettability treated Si and glass samples and viewed using scanning electron microscopy analysis.

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2.7. Preparation of Vertically Aligned Diphenylalanine Peptide Nanotubes Solution for Patterned Deposition Using Lift-Off Lithography Technique A flexible aluminum foil (3M™ 431) and Kapton™ polyimide (3M™ 5413) tapes with the desired void structure were used as a contact mask on both, Si and glass, surfaces. The peptides stock solution in HFP was deposited in the voids, and the contact mask was finally peeled away. 2.8. Scanning Electron Microscopy (SEM) Morphology features of the patterned samples, preliminary coated with gold, were studied by conventional scanning electron microscopy using a JSM JEOL 6300 SEM operating at 5 kV.

3. RESULTS Three different techniques were used for the patterning of the aromatic dipeptide nanostructure. Two of them are related to substrate wettability properties modification, the third technique is based on lift-off lithography process. 3.1. Formation of Wettability Patterned Surface We utilized a recently developed low-energy electron irradiation-based approach for modifying the wettability properties of surfaces of different origin.43 In this method, electron/hole charges, generated by the electron beam, are trapped in the vicinity of the irradiated substrate 4

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surface. Such treatment results in the formation of organic hydrocarbon layer, which exhibits a significant increase in hydrophobic and oleophilic properties in the irradiated areas.49 50 This effect is consistent with the well known fact that the alkyl groups such as CH2 are concurrently both strongly hydrophobic and oleophilic.51 This method allows fabricating either homogenous distribution or patterning of the hydrophobic states with high resolution (Fig. 1(a)). 3.2. Peptide Nanotubes Patterning by the Repulsion of Hydrophobic Areas The diphenylalanine peptide was dissolved in fluorinated alcohol then diluted in water and self assembled to form nanotubes, this solution was deposited on the wettability modified Si and glass surfaces and allowed to dry at room temperature. The water based solution was drawn to the hydrophilic areas, while drying it allowed the deposition of the nanotubes only at the hydrophilic areas (Fig. 2). Peptide nanotubes droplets were deposited on the stripe between the hydrophilic and the hydrophobic areas resulted in the formation of peptide nanotube pattern at the shape of half a circuit due to the repulsion of the solution from the hydrophobic area (Fig. 2(a)). Next we covered all the electron treated Si and glass specimens with the peptide nanotubes solution after the sample was dried a cover of horizontally deposited nanotube was observed only at the hydrophilic areas (Figs. 2(b–c)). It should be mentioned that no significant difference were found between peptide nanotubes patterning on conductive Si and dielectric glass samples. 3.3. High-Resolution Wettability Patterning The developed methods of surface wettability modification allowed tailoring a high-resolution wettability patterning. Figures 1(a)-iii and (b)-iv present the Si and glass surfaces with hydrophobic and hydrophilic patterning, which were fabricated by a low-energy electron irradiation and electron irradiation combined with UV-illumination, respectively. Both, patterned surfaces were exposed to water vapor at a 50% relative humidity. After cooling to a temperature of 5  C below the dew point, the water condensed only on the hydrophilic regions.52 3.4. Peptide Nanospheres Patterning by the Repulsion of Hydrophobic Areas We used the wettability pattern surface to control patterning of peptide nanospheres (Fig. 3(a)). The peptide nanospheres were self assembled by the amine modified analogue of diphenylalanine peptide the Boc-PhePhe-OH.53 The nanospheres solution was deposited on a wettability patterned surface containing array of 100 m hydrophobic square. The surface high resolution wettability J. Nanosci. Nanotechnol. 8, 1–8, 2008

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Fig. 2. Peptide nanotubes patterning on wettability treated surface (a) peptide nanotubes aliquot on the edge of the hydrophobic area forming half a circle. Red dots represent the line between the hydrophobic and the hydrophilic area. (b) Thick cover of peptide nanotubes on the hydrophilic area. (c) SEM image of the peptide nanotubes deposited on the hydrophilic area.

pattern was formed by exposing the Si and glass samples to local electron irradiation through a specifically designed Sishadow mask. The nanospheres, similarly to the nanotubes were deposited only at the hydrophilic areas, which are in this case, outside of the 100 m squares (Figs. 3(b–c)). Here, as described above, no preference between conductive Si and dielectric glass surfaces was observed. J. Nanosci. Nanotechnol. 8, 1–8, 2008

Fig. 3. Peptide nanospheres patterning on wettability treated surface. (a) Peptide nanospheres array deposited on glass surface. (b–c) Peptide nanospheres deposited on treated glass outside of the 100 m hydrophobic square. (d) Peptide nanospheres deposited on treated glass on the 100 m hydrophilic square.

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Next we wanted to form a mirror image of the peptide nanospheres pattern by forming a 100 m array of hydrophilic square. It was previously shown that UV illumination of the preliminary electron irradiated sample led to complete restoration of the wettability properties in the studied samples.49 50 This fact is in accordance with well known data on UV light-induced discharge of the charged states54 and decomposition of organic contaminations on solid surfaces.55 Therefore, removal of the observed UV-stimulated hydrocarbon layer reverses the electron-irradiated sample to the initial state, resulting in hydrophilic patterning (Fig. 1(b)). Following this procedure, the nanospheres solution were deposited on the surface, this time the structure were withdrawn to the 100 m squares and deposited on it (Fig. 3(d)). It should be mentioned that the same results were observed on both, Si and glass, substrates.

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3.5. Lift-Off Lithography of Aligned Peptide Nanotubes Lift-off lithography method was used (Fig. 4) to control patterning of vertically aligned peptide nanotubes. The aligned tube array was achieved by spreading monomeric diphenylalanine peptide building blocks dissolved in HFP over the Si and glass surfaces. We used the Si and glass

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Lift-off patterning Fig. 5. Vertically align peptide nanotubes patterning. (a) Peptide nanotubes deposited on the hard mask. (b) Peptide nanotubes patterning after the hard mask removal. (c–d) High resolution images of vertically aligned peptide nanotubes patterned.

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surfaces covered with a contact mask made of flexible aluminum foil or polyimide tapes containing desired void structures (Fig. 5(a)). The fluorinated alcohol solution of diphenylalanine peptide was deposited on the voids, allowing the formation of nanoforest, a vertically aligned array of nanotubes.33 Then, the contact mask was finally peeled away resulting in the desirable control patterning of vertically aligned nanotubes (Figs. 5(b–d)).

Controlling the distribution, patterning and orientation of nanostructures is essential for many of the technological applications envisioned for supramolecular self-associated assemblies. In the current study we present novel approaches for the patterning of vertically and horizontally aligned peptide nanotubes and peptide nanospheres on various surfaces. One of the advantages of peptide nanostructure in general and ADNT in particular is their ability to disperse in aqueous solution, here we used this characteristic of the ADNT and deposited them on pre-designed wettability J. Nanosci. Nanotechnol. 8, 1–8, 2008

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removed chemically, leaving the biomolecule film patterned with the original void structure of the photoresist. To avoid the use of aggressive reagents in the final step, we use a flexible aluminum foil and polyimide tapes instead of photoresist with the desired void structure (Figs. 4(b–c)). The peptide monomer solution then deposited in the voids, allowing the formation of the nanoforests and the contact mask is finally peeled away, resulting in vertically aligned controlled patterned ADNT (Fig. 4). It should be noted that since the lift-off technique is not depends on surface properties of the substrate material, every material may be used as a substrate for the patterning of vertically aligned array of ADNT. The ability to fill and/or coat the peptide nanotubes with metallic materials was previously demonstrated, suggesting their use in nanoelectronic devices. The ADNT can serve as a degradable mold for the fabrication of silver nanowire20 and allow the formation of platinumnanoparticle composites.28 Moreover, a coaxial metal nanocable formation using the ADNT scaffold was demonstrated.27 The patterning of ADNT on conductive surfaces (Si), as demonstrated in this work, is needed in order to use the ADNT in nanoelectronic applications.

5. CONCLUSION The patterning of vertically and horizontally aligned peptide nanotubes as well as the peptide nanospheres opens a variety of possibilities for using these nanostructures in technological devices. The developed electron-induced wettability surface modification and lift-off techniques to either conductive or dielectric materials open an avenue for the applications of different origin in electronics and medicine.

ABBREVIATIONS ADNT Boc ddH2 O HFIP NEMS Phe SEM Si

Aromatic dipeptide nanotubes Tert-butoxycarbonyl Double distilled water Hexafluoro-2-isopropanol Nanoelectromechanical systems Phenylalanine Scanning electron microscopy Silicon

Acknowledgments: We thank Dr. Zahava Barkay for help with the SEM experiments and members of the Gazit’s laboratory for helpful discussions. E. Gazit and Lihi Adler-Abramovich acknowledge the financial support of the European Commission 6FP BeNatural consortium. E. Gazit acknowledge the support of the Israel Science Foundation (ISF). Lihi Adler-Abramovich gratefully acknowledges the support of the Colton Foundation. 7

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patterned surface to form the desirable patterning of nanotubes and nanospheres. We studied the patterning of aromatic dipeptides, diphenylalanine (H-Phe-Phe-OH) and the N-terminal modified analogues, Boc-Phe-Phe-OH. While, the diphenylalanine peptide self-assembles in solution to form nanotubes, the amine-modified analogue, BocPhe-Phe-OH, can self-assemble into nanospheres. The first two patterning techniques that were used are related to surface wettability properties modification. In the hydrophobic patterning technique (Fig. 1(a)) the hydrophobic area was modified by exposing the surface to local electron irradiation through a specifically designed Si shadow masks. It should be mentioned that the used electron energy is lower than the threshold of defects generation in the irradiated material, to exclude the contribution of the radiation-induced defects.49 Thus the applied electron irradiation does not alter defect structure of a material substrate, its phase state, and basic bulk properties such as optical transparency, or resistivity. Based on a Monte-Carlo simulation method, we estimated a penetration depth of the incident electrons of approximately 4 nm, consistent with the analytical solution.56 In the second technique, hydrophilic patterning (Fig. 1(b)), the electron irradiation treatment was combined with UV-illumination. The hydrophilic areas were modified by exposing the preliminary electron irradiated surface to local UV illumination through the aforementioned Si shadow mask. It was found that UV illumination of the preliminary electron irradiated samples leads to complete restoration of the wettability properties in the studied samples.49 50 We demonstrate the deposition of horizontally array of ADNT using control surface modification by the hydrophobic patterning technique. As the nanotubes dispersed in aqueous solution they were deposited on the non hydrophobic regions. Furthermore, peptide nanospheres were patterned at high resolution either on the 100 m squares or on the exterior surface using the hydrophilic patterning or the hydrophobic patterning technique, correspondingly. Next we were interested in the patterning of vertically aligned array of ADNT. The nanoforests, vertically aligned ADNT, were formed by evaporation-initiated unidirectional axial growth of ADNT under mild conditions. This controlled patterning of aligned tubes is achieved by spreading monomeric diphenylalanine peptide building blocks dissolved in HFP on a surface. The highly volatile fluorinated alcohol allows the existence of the peptide entities as monodispersed building blocks. Upon the rapid evaporation of the HFP, a thin layer is formed on the substrate.33 The third technique used for patterning the vertically aligned ADNT in this study is based on lift-off lithography process. In conventional lift-off lithography the photoresist is deposited directly on the substrate and used as a contact mask, i.e., the photoresist is patterned, the biomolecules layer is then deposited in the voids of the photoresist, and the photoresist is then

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