Diphenylalanine-Based Microribbons for Piezoelectric Applications via

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 10543−10551

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Diphenylalanine-Based Microribbons for Piezoelectric Applications via Inkjet Printing Sofia Safaryan,† Vladislav Slabov,† Svitlana Kopyl,‡ Konstantin Romanyuk,‡,§ Igor Bdikin,∥ Semen Vasilev,§ Pavel Zelenovskiy,§ Vladimir Ya. Shur,§ Evgeny A. Uslamin,⊥ Evgeny A. Pidko,†,⊥ Alexander V. Vinogradov,† and Andrei L. Kholkin*,‡,§ †

Laboratory of Solution Chemistry of Advanced Materials and Technologies, ITMO University, St. Petersburg 197101, Russian Federation ‡ Department of Physics & CICECO-Aveiro Institute of Materials, University of Aveiro, 3810-193 Aveiro, Portugal § School of Natural Sciences and Mathematics, Ural Federal University, Ekaterinburg 620000, Russian Federation ∥ Department of Mechanical Engineering & TEMA- Centre for Mechanical Technology and Automation, University of Aveiro, 3810-193 Aveiro, Portugal ⊥ Inorganic Materials Chemistry Group, Eindhoven University of Technology, PO Box 513, Eindhoven 5600 MB, The Netherlands ABSTRACT: Peptide-based nanostructures are very promising for nanotechnological applications because of their excellent self-assembly properties, biological and chemical flexibility, and unique multifunctional performance. However, one of the limiting factors for the integration of peptide assemblies into functional devices is poor control of their alignment and other geometrical parameters required for device fabrication. In this work, we report a novel method for the controlled deposition of one of the representative self-assembled peptides diphenylalanine (FF)using a commercial inkjet printer. The initial FF solution, which has been shown to readily self-assemble into different structures such as nano- and microtubes and microrods, was modified to be used as an efficient ink for the printing of aligned FFbased structures. Furthermore, during the development of the suitable ink, we were able to produce a novel type of FF conformation with high piezoelectric response and excellent stability. By using this method, ribbonlike microcrystals based on FF could be formed and precisely patterned on different surfaces. Possible mechanisms of structure formation and piezoelectric effect in printed microribbons are discussed along with the possible applications. KEYWORDS: self-assembly, diphenylalanine, piezoelectricity, inkjet printing, microribbon

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crystals.13 IJP technique has been successfully used14,15 for lateral structuring of biomaterials on flat surfaces in the range from several tens to several hundred microns. Apparently, IJP can be used for directed self-assembly of peptides to convert initially random geometry of the grown structure into the controlled shape, useful for microelectronic applications. Unlike the case of vertical growth, which is quite limited in height and is intrinsically porous, horizontal IJP structuring of peptides would allow covering large areas to prepare dense films and the formation of functional components in the predetermined locations.16 In addition, IJP would also enable easy patterning and electroding of peptide nanostructures, which is required for microelectronic applications.17 However, this method of peptide deposition strongly depends on functional capabilities of the used printer.17 Inkjet office printers are more affordable

INTRODUCTION The formation of the ordered arrays of bionanostructures is a very promising solution for many nanotechnological applications.1,2 Biocompatibility, chemical versatility, ordinary modifiability, biological recognition abilities, and availability for bottom-up fabrication of self-assembled peptides make them attractive organic structures for biotechnological applications.3,4 One of the major challenges in this field is the ability to precisely control the organization and orientation of peptide structures on the substrate surface.5 Popular deposition methods to form oriented self-assembled structures based on peptides include the formation of vertically aligned nanoforest by physical and chemical vapor deposition techniques,6,7 using seed layer for further epitaxial growth of vertical microrods,8 confined organization of nanoscale tubular structures during slow crystallization or aggregation,9 controlled growth deposition on patterned hydrophilic surfaces10 or under external electric field,11 nanoscale patterning via evaporative dewetting,12 and, finally, inkjet printing (IJP) of self-assembled © 2018 American Chemical Society

Received: December 26, 2017 Accepted: March 2, 2018 Published: March 2, 2018 10543

DOI: 10.1021/acsami.7b19668 ACS Appl. Mater. Interfaces 2018, 10, 10543−10551

Research Article

ACS Applied Materials & Interfaces and simple devices, but they are rather limited in the precise monitoring of the growth process. Although high-density peptide arrays manufactured with an inkjet printer have been patented as early as 1994,18 no significant progress has been reported since then. Slow advancement in this area might be because of the problems with solvents required for the peptide synthesis. These solvents are usually viscous, which makes it difficult to print them with common inkjet or piezoelectric printers. In particular, using common solutions often lead to detrimental side effects, such as nozzle blockage19 or poor droplet break-up performance, resulting in decreased operational efficiency and enhanced cost. Here, we demonstrate a novel method for the controlled deposition of one of the most popular peptidediphenylalanine (FF)using a Fujifilm Dimatix Material Printer 2831, which features drop-on-demand piezoelectric printing technology combined with high resolution and reproducibility. We also demonstrate a new conformation of the printed FF-based crystals grown from the used ink, which was developed for this deposition method.

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RESULTS AND DISCUSSION Printing Process. Preparation of the suitable ink for printing process was carried out following a few important steps. Before refilling the cartridge, the ink goes through the filtration and degassing stage. The filtration was carried out through a syringe filter with a pore size of 0.45 μm. Because a large number of air microbubbles are present in the ink volume, the liquid was degassed under vacuum. After removing all air from the solution volume, the cartridge was refilled and installed in the printer. The key to start printing is adjusting the printing settings. The cartridge setting section implies creating a driving waveform individually for each type of ink. This setting is necessary to ensure the most stable printing process. The waveform adjustment regulates the behavior of the piezoceramic membrane in the capillary of the cartridge. Figure 1 shows the used driving waveform for our process.20 An optical image taken with a camera mounted on DMP 2831 shows a representative array of printed drops. The picture demonstrates that all of the droplets are equidistant, and they are stable with correctly selected printing parameters. Figure 2 represents the drops of the solution of arbitrary shape with grown crystals. Since the crystal growth occurs in the direction of drying drops (from the edges to the center) on the substrate, the initial form determines the orientation of the crystals. Thus, the circular shape of the drop directs crystals to the center (Figure 2a,b), and with a more elongated shape, in an area with a smaller radius of curvature crystals growing almost parallel to each other (Figure 2c,d). After completing the preparation of ink and adjustment of printing settings, direct printing of FF-based ink was carried out. The inkjet method allows one to create patterns for printing using a graphic editor, opening the possibility of a simple preparatory stage for the experiment. The physics of the crystal growth process in the drop (from its edges to the center) enabled us to control the growth direction. Thus, with the proper selection of the printing pattern, one can arrange the crystals so that they are almost parallel to each other. This feature is important, because, upon aligning the crystals, the efficiency of the piezoelectric transduction can be simultaneously increased.21,22 On the basis of the above discussion, it is

Figure 1. (a) Typical driving waveform for the used ink based on FF solution; (b) optical image of the printed drops using optimized printing parameters.

Figure 2. (a,b) Crystals grow in the center of a circular drop; (c) crystals aligned normal to the straight sections of the droplet; and (d) increase in the volume of the drop leads to the formation of arbitrary crystals in the central part of the drop.

logical to assume that crystal growth in an evenly printed line will occur along the normal to its edges, as shown in Figure 3. The optical images presented in Figure 3 show that for almost evenly printed lines (Figure 3a), the crystals grow practically parallel to each other and perpendicular to the printing direction (Figure 3b). 10544


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Figure 3. (a) Optical image of the lines printed by the ink based on FF solution (before drying); (b) optical image of the grown crystals ordered by the inkjet method.

Ink Preparation and IJP. Stable printing process implementation always requires tailoring the optimal rheological properties of the ink. For the IJP method, there is a general methodology for selecting rheological properties. This technique allows using a wide range of surface tension values, as well as viscosity. This method is based on the calculation of hydrodynamic constants included in the formula of the Ohnesorge number (1), which is responsible for the physical properties of jetted drop Oh =

We Re

Figure 4. (a) Surface tension of the DX4000 surfactant vs its concentration and (b) viscosity of the ink vs ethylene glycol/FF solution ratio.

(1)

where We is the Weber number and Re is the Reynolds number. The Reynolds number is a dimensionless hydrodynamic parameter that shows the ratio of inertia and viscous forces of the fluid (eq 2). The Weber number is also a hydrodynamic constant showing the relationship between inertia and surface tension of the fluid (eq 3) Re =

ρVd η

(2)

We =

ρV 2d γ

(3)

constants were calculated at the minimum value. Thus, the main parameters by which the printing process can be adjusted are the parameters of viscosity and surface tension. Adjustment of rheological properties of the ink was carried out based on calculating the Z number for each ethylene glycol/FF solution and surfactant ratio. Figure 5 shows the dependence of the Oh number on Re. Each zone in this figure characterizes the ink printability.

where ρ is the density of the ink, V is the velocity of the drop, d is the diameter of the nozzle, η is the viscosity, and γ is the surface tension of the ink. Having calculated the reciprocal of the Ohnesorge number, the so-called Z number 1 Z= (4) Oh one can derive an estimate of the printability of the given composition. Because the pristine FF dipeptide solution has low viscosity, ethylene glycol and cationic surfactant Dynax DX4000 were used to change its rheological properties. Ethylene glycol was selected as a solvent because it has high viscosity (18 cP) (Figure 4a) and low evaporation rate, which allows the crystal to form the necessary structure until the solvent evaporates completely. The choice of surfactant was due to the linear dependence of its surface tension (Figure 4b) on the concentration, which would ensure ease of use and stable printing. The literature data suggest that the value of the Z number should lie in the range from 1 to 10 for the best ink printability.23 Because the velocity of the drop during printing should lie in the range from 6 to 10 m/s, the hydrodynamic

Figure 5. Plot for determining the properties of ink for printing. Altering rheological parameters, one can adjust the Z parameter and, hence, the position of the point on the plot.

The data obtained indicate that the experimental compositions are suitable for printing, however, another factor in the preparation of ink is the possibility of obtaining the desired result, that is, crystal growth. To this end, the composition corresponding to Z = 3.82 was chosen to be the working one. In addition to being as close as possible to the center of the printable fluid zone, it also has the most relevant composition in terms of FF solution concentration. The concentration of 10545


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scanning electron microscopy (SEM), which revealed different crystal forms.

40% FF solution ensures the growth of thin elongated needles, whereas its increase promotes the formation of a greater number of crystallization centers because of which they start to grow by “overlapping” each other. Thus, for the preparation of an ink solution with the following rheological parameters was chosen to be the working one: viscosity 6.1 cP, surface tension 25.8 mN/m, and Z number = 3.82 at a drop velocity of 6 m/s. Because of the need for selecting rheological properties by introducing new components in the solution, such as viscosity and surface tension regulators, there is a possibility of a change in the morphology of the crystals and their phase. For example, Figure 6 clearly demonstrates how the morphology of the grown crystals changes after the introduction of appropriate additives into the solution.

Figure 7. Optical (a,b) and SEM (c,d) images of the samples: (a,c) initiating peptide solution without ethylene glycol and (b,d)peptide ink containing 60 vol % ethylene glycol.

Figure 6a shows that the sample without ethylene glycol consists of long thin threadlike structures. In the sample with ethylene glycol, we observed wide ribbonlike microcrystals (Figure 7b). SEM revealed the elongated hollow tubes and rods in the initial solution (Figure 7c), whereas in the case of printing ink, we observed completely different, ribbonlike crystal shapes (Figure 7d). Raman Measurements. In general, the middle frequency range of Raman spectra of peptide microtubes and ribbonlike crystals is similar, indicating the presence of FF molecules in the structure of microribbons (Figure 8). However, there are

Figure 6. Optical micrographs of FF crystals obtained from the solutions of (a) ddH2O and (b) ddH2O in the presence of fluorinebased surfactant and (c) ethylene glycol; substrate: microscope slide.

The pure FF solution forms disordered crystals with a predominance of microsized crystallites (Figure 6a). The compacted edges with a high concentration of microcrystals indicate that the drying of the solution was proceeding at a high speed, not allowing the particles to be distributed in the volume. The introduction of the surfactant provides the formation of a more ordered structure, distributing the crystallization centers throughout the sample volume24 (Figure 6b) and distributing the drying rate uniformly throughout the entire volume of the drop. The mechanism of crystal growth in this case is spherulitic, with distinct boundaries. At the last stage of the preparation of ink, ethylene glycol was introduced, the addition of which substantially affected the crystal growth. When ethylene glycol was added to the ink, the evaporation time of the solvent increased, thereby enabling the growth of large crystals having a complete needle structure, as shown in Figure 7 After drying, both samples were studied by optical and

Figure 8. Comparison of Raman spectra of FF nanotubes and ribbonlike crystals formed from the ink.

certain differences apparently related to different packing of the molecules. Comparison of the vibration lines of the phenyl rings in-plane vibrations observed in the spectra of ribbonlike crystals and typical microtubes showed that all phenyl rings were in the same environment, and there was no water in the structure of the new crystals.25 More detailed information on molecule packing was obtained by X-ray diffraction (XRD) measurements. XRD Measurements. As has been demonstrated earlier,6,26 small FF peptide monomers are able to self-assemble into supramolecular peptide nanotube-like structures. The process of self-assembly of FF nanotubes begins with the selfassociation of six FF monomers into the macrocycle structures 10546


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Figure 9. Schematic of crystal morphology of FF forms: (a) hexagonal crystal morphology; (b) orthorhombic crystal morphology with noncentrosymmetric classes: mm2 and 222; (c,d) possible morphologies with four (low-index) side facets for orthorhombic crystal symmetry; the morphology in the presence of ethylene glycol is shown in (d). The fastest growth direction is defined as the z-axis (last index in the hkl notation), sizes of neighboring facets are d and h.

9b-left image). It should be noticed here that the presented variant allows lateral piezoresponse component along the z direction. For the mm2 class, only morphology with facets perpendicular to the basis lattice vectors (Figure 9d) satisfies the experimental conditions: d ≠ h (d ≪ h) and α = 90°. In cases with facets nonorthogonal to the basis lattice vectors, the normal cross section takes a rhombic shape (because of mirror plane symmetry) with d = h (Figure 9c) and it does not satisfy anisotropic conditions, d ≪ h. The conclusions are basically the same for the variants with mirror plane perpendicular to the fastest growth direction (z) and for centrosymmetric class mmm. The cross section for the 222 symmetry class, in general case, takes rhombic shape with d = h and α ≠ 90° (because of rotation symmetries, Cx2 or/and Cy2·Cz2) if side facets are nonorthogonal to the basis lattice vectors (Figure 9c). Thus, the condition of rectangular cross section with d ≠ h and α = 90° is fulfilled if all facets are orthogonal to the basis lattice vectors. The XRD data presented in Figure 10 could be used to identify possible symmetries of the crystal lattice and allowed us to determine the elementary cell parameters. The XRD pattern shown in Figure 9 is compatible with the orthorhombic symmetry (point groups: mmm, mm2, or 222) with unit cell parameters a = 6.1717 Å, b = 10.355 Å, and c = 23.736 Å. It should be noted that the presented results and extracted unit cell parameters are very similar to those obtained in thermally annealed FF tubes21,26,34 and cyclo-FF peptides (cyclo-FF)35,36 for which the orthorhombic space group P22121 has been proposed. The role of ethylene glycol in the formation of ribbonlike FF crystals with an orthorhombic symmetry still remains an open question. The proposed mechanism is based on the transition of linear-FF molecule to cyclo-FF, the effect being commonly observed in the FF nanotubes annealed at 150 °C.36 However, we believe that there can be other mechanisms different from those based solely on cyclo-FF structures. Our assumption is established based on our XRD data that show some deviation in the peak intensities as compared to the case of FF nanotubes after annealing.26,34 The observed peak

where amino and carboxyl groups form the inner core of the cycle. This material crystallizes in a hexagonal crystal system and belongs to the P61 space group with elementary cell parameters: a = b = 24.071 Å, and c = 5.456 Å.21,26,27 The obtained ribbonlike crystals formed from FF solution (Figure 7b,d) have a rectangular cross section. It is obvious that the addition of ethylene glycol to the initial FF solution has a significant effect on the packing of FF in structures with different growth symmetry. As is known28 the ethylene glycol may induce a conformational change in the materials. The main driving force for the self-assembly process of our dipeptide is intermolecular H-bonding interaction.29 The hydrogen bonds between dipeptide and solvent can be formed mainly by the donation of hydrogen atoms of amino group in each dipeptide to the solvent molecule.30,31 Compared to water, the ethylene glycol has different solvation properties, which affect the selfassembled process of FF;32 in particular, each molecule of ethylene glycol can form twice as many hydrogen bonds with other molecules as compared with water. The equilibrium and/or the growth crystal shape usually (under uniform conditions) have a close relation between the crystal lattice symmetry and its morphology.33 If the internal structure of a unit cell of typical hexagonal tubes (Figures 7c and 9a) in the cross section corresponds to the C6 point-group symmetry (space group P61), the observation of a rectangular cross section (Figures 7d and 9b,d) suggests lower symmetry of the internal structure for the ribbonlike forms of FF; for the rectangular cross section, they may have C2v or C2 point-group symmetries. The observed reduction of the shape symmetry is obviously an indication of the reduction of the crystal lattice symmetry. Possible morphologies for orthorhombic 1D crystal structures33 with four side facets are shown in Figure 9c,d; because of the crystal symmetry, the opposite-side facets are equal and we compared only two neighboring facets: d and h (Figure 9c,d). For simplicity, we show variants with low-index surfaces, but main consequences can be extended on arbitrary facets orientation. For mm2 class, the variant with mirror planes parallel to the fastest growth direction (z) is present (Figure 10547


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polarization component in X1 direction) was absent. We determined the corresponding piezoelectric coefficient deff = 40 ± 5 pm/V; this value is smaller than that for typical hexagonal FF microtubes (60 ± 10 pm/V), but it is still higher than that in many organic piezoelectrics including polyvinylidene fluoride.38 Axial symmetry of the PFM problem is schematically expressed with E-field vector components; the total contribution is provided normal to the surface vector componentE1, contributions from lateral components (E2 and E3) are compensated, and these components are present by pairs of equal and opposite vectors along X2 and X3 directions, respectively. In the configuration shown in Figure 11, the component of the shear deformation is parallel to the microribbon axis (X3) and proportional to

Figure 10. XRD patternstop: XRD patterns of ribbonlike crystal. Simulated curve (red line) was calculated for cyclo-FF structure with the space group P22121 applying Rietveld refinement method. Atomic coordinates were taken from ref 35. Green line (bottom)error curve (divided by 2).

deff Vac sin α

(5)

where α is the angle between the microribbon axis X3 and the main axis of the cantilever and deff is the shear piezocoefficient (or combination of thereof). We have measured the dependence of lateral piezoresponse on the angle between X3 and the main axis of the cantilever (Figure 12). The observed orientational dependence was fitted with the sine function,39 thus confirming that the polarization vector in the crystal lies in the plane of the microribbon, and it is parallel to its long side (direction X3). From XRD measurements, we know that possible structure of the printed microribbons belongs to the orthorhombic group with 222 or mm2 symmetry classes. The point-group mmm is excluded because it is centrosymmetric. For the crystallographic class mm2, the piezoelectric matrix can be written as40

intensities in our data (Figure 9, black circles) have different ratios in comparison with the simulated curve for cyclo-FF structure,29,30 in particular, it can be clearly seen for peaks (211), (204), and (023) (Figure 10, red curve). The observed change in the ratios of the peak intensities (Figure 10) can be associated with different structural factors responsible for different atom arrangement in the unit cell. Thus, we do not exclude that the obtained structures can differ in their atomic structure from thermally synthesized cyclo-FF samples. Piezoelectricity in Ribbonlike FF Microcrystals. Recently, strong shear piezoelectric activity has been observed in FF peptide nanotubes, polarization being directed along the tube axis.21,26,37 Piezoresponse force microscopy (PFM) images were obtained by applying an electrical ac bias voltage while detecting the local lateral vibrational piezoresponse of the samples, which is proportional to the d15 component of the piezotensor.37 The obtained FF ribbonlike crystals had sufficiently high piezoelectric response measured by PFM. In particular, lateral piezoresponse (along the major face oo microribbons) was observed, which is related to the component of polarization along the ribbon’s major axis (direction X3 in Figure 11), whereas vertical piezoresponse (related to the

⎛ 0 0 0 0 d15 0 ⎞ ⎜ ⎟ d = ⎜ 0 0 0 d 24 0 0 ⎟ ⎜⎜ ⎟⎟ ⎝ d31 d32 d33 0 0 0 ⎠

(6)

and for the class 222 (space group P22121), it is ⎛ 0 0 0 d14 0 0 ⎞ ⎜ ⎟ d = ⎜ 0 0 0 0 d 25 0 ⎟ ⎜⎜ ⎟⎟ ⎝ 0 0 0 0 0 d36 ⎠

(7)

In the geometry of the experiment (Figure 11), no signal should be observed for the 222 class, but in the case of mm2, the apparent piezoresponse because of d15 component should be registered.40 On the basis of the above-said and the results of XRD measurements, it can be inferred that the grown microribbons belong to mm2 class of orthorhombic crystal symmetry. This configuration is advantageous for the piezoelectric applications37 because the in-plane electrodes can be deposited on the major face, and electromechanical deformation will be caused by the longitudinal piezoeffect. Further measurement (similar to those described in ref 41) should be performed to extract the exact values of all piezoelectric coefficients in the matrix.

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CONCLUSIONS In conclusion, we found that adding the ethylene glycol to the original FF peptide stock solution can be used to prepare an efficient ink for the consequent IJP. Well-aligned lines of the oriented peptide microribbons were obtained with the

Figure 11. Schematic of the nanoscale in-plane measurements of a single microribbon by PFM. 10548


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Figure 12. (a) Phase-contrast of the as-grown microribbons. (b) Topography and lateral piezoresponse (color scale) under ac voltage 5 V. (c) Schematic of the microribbon polarizations deduced from PFM images. (d) Dependence of the lateral piezoresponse on the angle between the crystal and the cantilever. deposited on a glass slide and allowed to dry for further SEM and optical microscopy analyses. IJP of Peptide Arrays. For peptide deposition, we used a Dimatix materials printer DMP 2831 with 10 pl cartridges and a conductive slide of indium tin oxide plastic as a substrate. The printing was performed with a jetting frequency of 1 kHz and dots per inch of 423. The resulting drop velocity was 6 m/s. The printed samples were kept to dry and used in the subsequent analysis by optical, scanning probe, and scanning electron microscopes. Characterization Techniques. High-resolution investigation of the grown microtubes and microribbons was performed by SEM in secondary electron mode using an AURIGA CrossBeam workstation (Ural Federal University) and Hitachi SU-70 (University of Aveiro). For SEM investigation, the samples were covered with a 6 nm Cr layer by a magnetron-sputtering system (Q150T, Quorum Technologies, UK). The samples were characterized by the XRD and Raman spectroscopy methods. To characterize the piezoelectric properties via PFM, we used commercial atomic force microscope Asylum MFP-3D (Asylum Research, Oxford Instruments, UK) and Ntegra Aura (NTMDT Spectrum Instruments, Russia). The PFM images of peptide microcrystals were acquired by applying a range of ac voltage (0.1−10 V, peak-to-peak) with a frequency of 20 kHz. Conductive probes (stiffnesses 3−10 N/m, resonance frequency 45−200 kHz) were used for the PFM measurements under ambient conditions. The cantilever response was detected using the internal lock-ins of the atomic force microscope because of the electromechanical coupling. The values of piezoelectric coefficients were determined by using the alreadyreported procedure.41

morphology, considerably different from the common nano- or microtube structure. In this way, we obtained a new conformation of FF peptides orthorhombic structure, possibly belonging to the mm2 symmetry class. Oriented ribbonlike crystals based on FF were characterized by different techniques. The resultant FF structure showed sufficiently strong piezoelectric response comparable to that previously observed in hexagonal FF microtubes. These functional properties of FFbased nanostructures can be further used in various microdevices.

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METHODS

Preparation of Initial Peptide Solution. The studied peptide, FF (H-D-Phe-D-Phe-OH, ff), was purchased from Bachem (Bubendorf, Switzerland). Fresh peptide stock solution was prepared by modifying the previously reported procedure.6 Given the fact that peptides in acetic acid are crystallized much slower42 in contrast to commonly used hexafluoroisopropanol to avoid premature formation of peptide nanostructures, the peptide powder was dissolved in acetic acid (99.8%, Carl Roth) at a concentration of 100 mg/mL. After that, the peptide stock solution was diluted in ethanol to a concentration of 10 mg/mL and immediately diluted in ddH2O to a concentration of 5 mg/mL. After dilution in ddH2O, the peptide solution was allowed to dry at room temperature on a glass slide for SEM and optical microscopy analyses. Preparation of Peptide Solution for IJP. The peptide solution prepared as described above was finally diluted in ethylene glycol (99%, Sigma-Aldrich) in different ratios so that the percentage of ethylene glycol in the total ink volume was 34, 50, 60, 67, 72, and 75%. After that, viscosity and surface tension were measured in all mixtures. We used the Z number to calculate the optimal rheological parameters of the ink (see below). Thus, we determined the optimal ink viscosity and surface tension to be 6.1 cP and 25.8 mN m−1, respectively, in the sample with 60% of ethylene glycol. The resultant ink was also

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +351 234 247025. Fax: +351 234 401 470. 10549


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Svitlana Kopyl: 0000-0003-3347-8628 Semen Vasilev: 0000-0002-3103-1438 Pavel Zelenovskiy: 0000-0003-3895-4785 Andrei L. Kholkin: 0000-0003-3432-7610 Author Contributions

The manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the joint project Portugal-Turkey (TUBITAK/0006/2014). I.B. and K.R. wish to thank the Portuguese Foundation for Science and Technology (FCT) for the financial support (grants IF/00582/2015 and FRH/BPD/ 88362/2012, respectively). E.A.P. thanks the Ministry of Education and Science of the Russian Federation (Project 11.1706.2017/4.6) for the financial support. A.V.V. and S.M.S. thank the Russian Science Foundation (project 360964), and V.S.S. thanks the Ministry of Education and Science of the Russian Federation (project 417004). We would also like to thank Erasmus plus program for the mobility grants of the coauthors of this paper. The equipment of the Ural Center for Shared Use “Modern nanotechnology” of Ural Federal University was used. The authors are grateful to Dr. Dmitry Chezganov for the help with SEM imaging.

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