Diphenylalanine-based microribbons for piezoelectric applications via

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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 Pidko, Alexander Vinogradov, and Andrei Leonidovitch Kholkin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19668 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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ACS Applied Materials & Interfaces

Diphenylalanine-based microribbons for piezoelectric applications via inkjet printing Sofia Safaryan1, Vladislav Slabov1, Svitlana Kopyl2, Konstantin Romanyuk2,3, Igor Bdikin4, Semen Vasilev3, Pavel Zelenovskiy3, Vladimir Ya. Shur3, Evgeny A. Uslamin5, Evgeny A. Pidko1,5, Alexander V. Vinogradov1, and Andrei L. Kholkin2,3* 1

Laboratory of Solution Chemistry of Advanced Materials and Technologies, ITMO University, St. Petersburg 197101, Russian Federation

2

Department of Physics & CICECO-Aveiro Institute of Materials, University of Aveiro, 3810193 Aveiro, Portugal

3

School of Natural Sciences and Mathematics, Ural Federal University, Ekaterinburg 620000, Russian Federation 4

Department of Mechanical Engineering & TEMA- Centre for Mechanical Technology and Automation, University of Aveiro, 3810-193 Aveiro, Portugal 5

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 due to 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 selfassemble into different structures such as nano- and microtubes and microrods, was modified to be used as an efficient ink for the printing of aligned FF-based structures. Furthermore, during the development of the suitable ink we were able to produce a novel type of diphenylalanine conformation with high piezoelectric response and excellent stability. By using this method, ribbon-like microcrystals based on diphenylalanine 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, micro-ribbon

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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 selfassembled structures based on peptides include: formation of vertically aligned nanoforest by physical and chemical vapor deposition techniques,6,7 using seed layer for further epitaxial growth of vertical microrods8, confined organization of nanoscale tubular structures during slow crystallization or aggregation9, controlled growth deposition on patterned hydrophilic surfaces10 or under external electric field,11 nanoscale patterning via evaporative dewetting12 and, finally, inkjet printing (IJP) of self-assembled crystals.13 Inkjet printing 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 in order 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 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 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 due to the problems with solvents required for 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 2 ACS Paragon Plus Environment

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from the used ink, which was developed for this deposition method.

RESULTS AND DISCUSSION

Printing process Preparation of the suitable ink for printing process was done 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. Since 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

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.

An optical image taken with a camera mounted on DMP 2831 shows a representative array of 3 ACS Paragon Plus Environment


printed drops. The picture demonstrates that the all 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 (Fig. 2a,b), and with a more elongated shape, in an area with a smaller radius of curvature crystals grow almost parallel to each other (Fig. 2c,d).

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

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 4 ACS Paragon Plus Environment

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important, since, upon aligning the crystals, the efficiency of the piezoelectric transduction can be simultaneously increased.21,22 Based on the above discussion, it is logical to assume that crystal growth in an evenly printed line will occur along the normal to its edges as shown in Fig. 3.

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.

The optical images presented in Fig. 3 show that for almost evenly printed lines (Fig. 3a) the crystals grow practically parallel to each other and perpendicular to the printing direction (Fig. 3b).

Ink preparation and inkjet printing Stable printing process implementation always requires tailoring the optimal rheological properties of the ink. For the inkjet printing 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. The 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:

√ ,


(1)


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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):

,

(2) ,

(3)

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:

(4)

one can derive an estimate of the printability of the given composition. Since 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 cPs) (Fig. 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 (Fig. 4b) on the concentration, which would ensure ease of use and stable printing.


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Figure 4. (a) Surface tension of the DX4000 surfactant vs. its concentration; (b) viscosity of the ink vs. ethylene glycol / FF solution ratio. 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 Since the velocity of the drop during printing should lie in the range from 6 to 10 m/s, the hydrodynamic 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.



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, i.e., 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. Concentration of 40% FF solution ensures the growth of thin elongated needles, whereas its increase promotes the formation of a greater number of crystallization centers, due to which they start to grow by "overlapping" each other. Thus, for the preparation of ink a solution with the following rheological parameters was chosen to be the working one: viscosity 6.1 Cps, surface tension 25.8 mN/m, Z number = 3.82 at a drop velocity of 6 m/s. Due to 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. E.g., Fig. 6 clearly demonstrates how the morphology of the grown crystals changes after the introduction of appropriate additives into the solution.


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Figure 6. Optical micrographs of FF crystals obtained from the solutions of a) ddH2O, b) ddH2O in the presence of fluorine-based surfactant and c) ethylene glycol substrate: microscope slide. The pure FF solution forms disordered crystals with a predominance of micro-sized crystallites (Fig. 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 (Fig. 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 9 ACS Paragon Plus Environment


structure, Fig. 7 After drying, both samples were studied by optical and scanning electron microscopy (SEM), which revealed different crystal forms.

Figure 7 Optical (a,b) and SEM (c, d) images of the samples: (a, c) – initiating peptide solution withoutcethylene glycol; (b, d) – peptide ink containing 60 vol.% ethylene glycol.

Figure 6a shows that the sample without ethylene glycol consists of long thin thread-like structures. In the sample with ethylene glycol we observed wide ribbon-like microcrystals (Fig. 7b). Scanning electron microscopy revealed the elongated hollow tubes and rods in the initial solution (Fig. 7c), whereas in the case of printing ink we observed completely different, ribbonlike crystal shapes (Fig. 7d). Raman measurements In general, the middle frequency range of Raman spectra of peptide microtubes and ribbon-like crystals are similar, indicating the presence of FF molecules in the structure of microribbons (Fig.8) . However, there are certain differences apparently related to different packing of the molecules. Comparison of the vibration lines of the phenyl rings in-plane vibrations observed in spectra of ribbon-like 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.


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Figure 8. Comparison of Raman spectra of FF nanotubes and ribbon-like crystals formed from the ink.

X-ray diffraction measurements As has been demonstrated earlier6,26 small diphenylalanine peptide monomers are able to selfassemble into supramolecular peptide nanotube-like structures. The process of self-assembly of FF nanotubes begins with the self-association of six FF monomers into the macrocycle structures where amino and carboxyl groups form the inner core of the cycle. This material crystallizes in 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 ribbon-like crystals formed from FF solution (Figs. 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 self-assembled 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 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 (Figs. 7c and 9a) in the cross-section corresponds to the C6 point group symmetry (space group P61), observation of a rectangular cross-section (Fig. 7d and 9b, d) suggests lower symmetry of the internal structure for the ribbon-like forms of FF, for 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 11 ACS Paragon Plus Environment


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are shown in Fig. 9c, d; due to crystal symmetry the opposite side facets are equal and we compared only two neighboring facets: d and h (Fig. 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 (Fig. 9b- left image). It should be noticed here that the presented variant allows piezoreponse component along z-direction. For mm2 class only morphology with facets perpendicular to the basis lattice vectors (Fig. 9d) satisfies experimental conditions: d ≠ h (d

Diphenylalanine-based microribbons for piezoelectric applications via

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