Evaporation-Driven Crystallization of Diphenylalanine Microtubes for

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Evaporation-Driven Crystallization of Diphenylalanine Microtubes for Microelectronic Applications Alla Nuraeva,† Semen Vasilev,† Daria Vasileva,† Pavel Zelenovskiy,† Dmitry Chezganov,† Alexander Esin,† Svetlana Kopyl,‡ Konstantin Romanyuk,†,‡ Vladimir Ya. Shur,† and Andrei L. Kholkin*,†,‡ †

Institute of Natural Sciences, Ural Federal University, 620000 Ekaterinburg, Russia Physics Department & CICECO, Materials Institute of Aveiro, 3810-193 Aveiro, Portugal

‡

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S Supporting Information *

ABSTRACT: Self-assembly of supramolecular biomaterials such as proteins or peptides has revealed great potential for their use in various applications ranging from scaffolds for cell culture to light-emitting diodes and piezoelectric transducers. Many of these applications require controlled growth of individual objects in the configuration allowing simple transfer to the desired device. In this work, we grew millimeter-long diphenylalanine (FF) self-assembled microtubes with high aspect ratio via evaporation-driven crystallization of nonsaturated FF solutions, making use of the Marangoni flow in the drying droplets. The growth mechanism was investigated by measuring the microtube length as a function of time. Jerky (steplike) growth behavior was observed and explained by a self-activated process in which additional activation energy is provided through condensation. The calculated growth rate due to the diffusion-controlled process is in agreement with the experimentally measured values. The grown microtubes were successfully transferred to metallized patterned substrates, and their specific conductivity and piezoelectric properties were evaluated as a function of the applied voltage and frequency. A number of piezoelectric resonances were observed and attributed to different vibrational modes excited by the piezoelectric effect inherent to the FF structure.

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efforts have been focused on understanding the mechanical,16 electric,17 piezoelectric,18 optical,19 electrochemical,20 and other properties of the produced structures with the final aim of using them in nano- and microdevices for biomedical applications (see, e.g., refs 3 and 4). The polymorphic nature of FF-based structures was shown to be a serious factor impeding the applications of microtubes and microwires since in a typical experiment a variety of cross-linked nanofibers, nano- and microtubes, and/or microrods are obtained.12 In order to be used in electronic devices, isolated microtubes or microrods with controlled dimensions have to be grown and consequently transferred to the patterned substrate. On the other hand, the anisotropic growth of microtubes has to be better understood because it should depend not only on the supersaturation conditions but also on the Marangoni21 or capillary flow22 effects during the evaporation-driven crystallization process. In addition, the details of FF growth are important for understanding the aggregation mechanism in β-amyloid peptides.23

INTRODUCTION Self-assembly of bioorganic materials is currently an emerging field of research because of the excellent possibilities offered by their unique anisotropic structure combined with the variability due to non-covalent interactions between the molecules.1−4 Self-assembled amino acid- and peptide-based nanostructures are of special importance because they can be used in various applications ranging from scaffolds for cell culture5 to lightemitting diodes,6,7 field-effect transistors,8 and biosensors.9 Specifically, the smallest self-assembled dipeptide, diphenylalanine (FF), has been widely investigated because of its fascinating physical properties and biocompatibility combined with its ability to bind to several metals and other functional groups.10 In addition, this aromatic dipeptide is derived from the core recognition motif of the Alzheimer’s disease β-amyloid polypeptide and thus presents significant interest for biomedical applications. Recent investigations have shown that FF-based peptides can easily self-assemble into tubular,10 spherical,11 rodlike,12 or fibrous nanostructures13 depending on the deposition conditions and solvent type. On the basis of a detailed X-ray diffraction study, Gorbitz14 was the first to find that the phenyl rings in the FF structure can stack into fascinating tubular arrangements with a hydrophilic internal hole and hydrophobic exterior surface.15 Since then, numerous © 2016 American Chemical Society

Received: November 12, 2015 Revised: January 27, 2016 Published: February 2, 2016 1472

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Figure 1. Sequence of optical images demonstrating the evolution of the microtube growth process at different times after droplet deposition on the Pt/Ti/SiO2/Si substrate. surface was sputtered with a 6 nm Cr layer to prevent charging. SEM images of FF microtubes representative of the presented growth method have been published earlier.25 Electric Conductivity. Current−voltage (I−V) characteristics were acquired with a model 6430 Sub-Femtoamp Remote SourceMeter (Keithley, Cleveland, OH, USA). A special test fixture was used to prevent leakage background current. We used a guarding electrode for the same purpose. The measurements consisted of the application of voltage pulses and measurements of the current after it reached a steady-state value. Piezoelectric Measurements. Piezoelectric resonance measurements were carried out by atomic force microscopy (AFM) using a commercial MFP-3D instrument (Asylum Research, Oxford Instruments, Oxford, UK). Triangular Pt-covered conductive probes with force constants of 3−5 N/m and resonance frequencies of 45−75 kHz were used. In this method, a sharp tip in contact with the microtube surface is periodically biased with an ac voltage in the range 10−1000 kHz. The piezoelectrically actuated displacements were measured by the same cantilever using the piezoresponse force microscopy (PFM) option of the microscope.

In this work, we studied evaporation-driven self-assembly of FF microtubes based on drying of nonsaturated solutions in droplets. The growth starts at the solution−substrate contact line and continues inside the droplet until it completely dries up. Individual microtubes with a very high aspect ratio can be grown in this way and successfully transferred to two-terminal devices for further use. The growth mechanisms and physical properties of the fabricated microtubes are discussed in this paper.

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EXPERIMENTAL SECTION

Sample Fabrication. FF lyophilized powder was purchased from Bachem (Bubendorf, Switzerland). The powder was dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol (HFP) to prepare a solution with an FF concentration of 100 mg/mL. After that, 2 μL of the obtained solution was put on a Pt/Ti/SiO2/Si substrate (Inostek, Ansan, South Korea) and diluted with deionized water to an FF monomer concentration of 2 mg/mL. The substrate was cleaned with acetone and deionized water. The droplet was dried under ambient conditions at about 50% relative humidity. After drying, the tubes of sufficiently big dimensions (length 800−1000 μm) were manually transferred to the two-terminal device and fixed using silver paste. X-ray Diffraction. The powder X-ray diffraction (XRD) measurements on the FF nanostructures were made using an X’Pert MPD 2θ diffractometer (Philips Analytical) with a Cu anode operating at 1.2 kW. The XRD pattern (Figure S1 in the Supporting Information) shows very good crystallinity of the FF hexagonal structure in full agreement with literature results.14 Optical Characterization. Optical images and videos were obtained using a BX-51 optical microscope (Olympus, Tokyo, Japan) in reflection mode. We used 5× and 20× objectives to capture the images. A video of the tube growth was acquired with a 5× objective and a CCD camera (shown with 60× acceleration). Raman spectra were acquired with an Alpha 300AR instrument (WiTec, Ulm, Germany). A 488 nm solid-state laser (27 mW) was used for the excitation of Raman scattering, which was collected by a 100× objective in the 180° backscattering geometry. A representative unpolarized Raman spectrum of the grown FF microtubes is shown in Figure S2 in the Supporting Information. The spectrum is characteristic of the FF structure, and the shifts of the vibrational frequencies of the amide I and amide III bands indicate that the FF molecules interact by hydrogen bonding at the N−H sites and not at the CO sites.24 Scanning Electron Microscopy. High-resolution scanning electron microscopy (SEM) imaging of microtubes was performed using a commercial microscope (AURIGA CrossBeam Workstation, Carl Zeiss, Oberkochen, Germany) in secondary electron mode. The

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RESULTS AND DISCUSSION Growth of Diphenylalanine Microtubes. In a typical evaporative crystallization experiment, a drop of the diluted FF solution was placed on a glass substrate. The crystallization and growth of FF microtubes were then monitored in situ using time-resolved optical microscopy. In the optical setup, the spatial distribution of nucleation centers and geometrical dimensions of the grown tubes could be examined from the top. Figure 1 shows a sequence of optical images demonstrating the evolution of the nucleation and growth process of a number of tubes appearing from the same nucleation site at the perimeter of a drying drop. A video of the tube growth (images taken every 30 s, time acceleration 60×) is presented in the Supporting Information. The contact line of the substrate− solution interface does not move during the experiment because of a well-known pinning effect.26 After a certain induction time required for nucleation (10−30 s in our case), needlelike tubes of microscopic dimensions appear and grow out of the contact line, where the solution−air interface meets the substrate and supersaturation exists as a result of intensive evaporation of the solution. The process continues until the drop completely dries up (Figure 1f), leaving tubes with lengths of up to about 1−2 mm. 1473

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Figure 2. (a) Optical image of dried microtubes. (b) Schematic illustration of the liquid circulation due to the Marangoni effect. (c) Time dependence of the tube length during the growth process in a drying droplet.

Figure 3. (a−d) Schematic illustrations and optical images of the tube growth process during a plateau of the steplike growth. (e−g) SEM images of the tube ends showing their sharpening.

to 300. The growth rate ranged from 1 to about 4.5 μm/s (Figure S1 in the Supporting Information). However, the spread of the growth rates was significantly larger for radially grown tubes. In rare cases we observed heterogeneous nucleation in the droplet bulk (Figure 1e) and splitting of existing tubes rather than their continuous growth (Figure 1f). Fast registration of optical images allowed monitoring of the tube lengths as a function of time during growth (Figure 2). It is worth noting that circumferential tubes show the highest growth rates (e.g., tube 1 in Figure 2a) but reach sufficiently fast saturation because the solution is depleted by many

It is clearly seen that the nucleation and growth of the tubes first starts in the circumferential direction to the contact line because of the availability of monomers along the perimeter of the drop. The high supersaturation state enables rapid growth of the tube bundles (Figure 1a−c). However, these are not useful for applications because of their short lengths and extremely complex morphology. After some period of time (about 10 min; Figure 1c), the growth of more-or-less isolated tubes occurs at a big enough angle to the circumferential direction. The longest tubes appear close to the radial direction and can reach a millimeter in length with an aspect ratio of up 1474

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involved self-assembly mechanisms. In general, the growth dynamics is defined by the supersaturation conditions near growing facets and depends on a number of factors such as surface defect structure, presence of growth steps, surface orientation, diffusion of adsorbed particles along the growth surface, etc. Herewith we use the term supersaturation to refer to the excess FF concentration (i.e., Ci − C̃ i, where Ci is the FF monomer concentration and C̃ i is the equilibrium FF monomer concentration at the ith growth facet). Diphenylalanine microand nanotubes have a high aspect ratio due to the pronounced anisotropy of their structure, resulting in high aspect ratio of the shape of the Wulff plot32 between the axial and six equal radial directions. The anisotropy is defined by the specificity of the FF monomers as building blocks. It also assumes the existence of preferred directions for molecular attachment during growth in the basis of one molecular building block or/and ordered polymer structure. By analogy with crystal growth, ordered organic polymer structures can have growth steps and kinks or their analogues on the surface (Figure 3). As has been shown by several authors,14,17 diphenylalanine microtubes consist of hollow FF nanofibers with a unit cell size of about 1 nm. In the following, we propose a model that can explain all of the phenomena observed during the growth of FF microtubes. Evaporation of a liquid droplet has been considered in a number of papers,22,33,34 which assume an evaporative flux J(r) as a function of the radial position r. According to ref 22, J(r) can be described by the relationship

microtubes that rapidly consume monomers. Most of the “radial” tubes demonstrate pronounced jerky growth (Figure 2c, tubes 2−4). There are several plateaus in the length versus time dependences. It can be seen that during the plateau only the diameter of the tube increases (Figure 3a−c; also see the video in the Supporting Information), while the length does not change. Figure 3e−g illustrates the ends of some tubes at higher magnification. It can be concluded that the growth is inhomogeneous (Figure 3e) and that individual fibers (or tubes) seem to grow first close to the inner hole. Sharpened ends of some tubes (Figure 3f) reveal the steps of several tens of nanometers in size (Figure 3g). These steps could be the individual nanotubes constituting the microtubes, as will be discussed below. The top panel of Figure 3 presents proposed schematics of the microtube configurations corresponding to the optical images at different stages of the growth process. The analysis of growth rates in Figure 2c allowed us to conclude that the growth of “radial” tubes is on average linear, and thus, it cannot be explained by the common diffusionlimited mechanism, which predicts a decrease in the growth rate with time.27 It is hypothesized that there is significant transport of the FF monomers from the center of the droplet to its periphery that supplies enough monomers to maintain a sufficiently high average concentration at the tube end. This could be due to capillary flow resulting from the extensive evaporation at the droplet edge.21 This is the famous “coffee ring” effect that should be observed for the droplet, whose perimeter is pinned by defects during evaporation. In this case, the mass of the drying droplet should obey a power law with an exponent depending on the contact angle.21 To verify this assumption, we measured the time dependence of the mass of the drying droplet and found that there are no straight line segments in the log−log plot of the mass versus time dependence (Figure S2 in the Supporting Information). Therefore, we attribute the monomer flow to the well-known Marangoni effect, which takes into account not only the intensive evaporation near the contact line but also the circulation of the liquid due to stress and temperature gradients (see the schematic illustration in Figure 2b).28 The Marangoni effect may supply enough monomers for the growth and thus maintain high growth rates of individual microtubes. The existence of the flow is proved by the trajectories of small particles from the center of the drop to its periphery seen in the video in the Supporting Information. No sign of the intermediate phases during growth was detected in our experiments. Summarizing this section, we conclude the following: (i) The growth of FF microtubes is a two-stage process and always starts from the appearance of a large number of small tubes growing in the circumferential direction. These demonstrate the highest growth rate of about 4.5 μm/s. (ii) In the second stage, isolated tubes grow closer to the radial direction and exhibit a pronounced jerky growth with plateaus in the length versus time dependence (Figure 2c). During the plateaus, the tubes stop growing in length and just increase their diameters. (iii) The tube ends are sharpened and sometimes exhibit steps with nanometer-sized dimensions. (iv) The observed circulation of the liquid due to the Marangoni effect helps the growth process by delivering monomers to the depleted solution. Model of Evaporation-Induced Growth. The application of crystal growth theory29−31 to the crystallization of supramolecular complexes can advance our understanding of the

J(r ) ∝ (R − r )−λ

(1)

where R is the radius of the contact line and λ = (π − 2θc)/(2π − 2θc), in which θc is the contact angle of the drop. For λ > 0, the highest evaporative flux J(r) is expected at the drop’s periphery, i.e., at r → R. Therefore, supersaturation of the solution increases in going from the center to the periphery of the droplet, as it depends on the evaporative flux J(r). In this case, the nucleation at the contact line of the droplet and the initial growth of nanotubes in the circumferential direction are naturally expected. In spite of the circulation processes induced by temperature/stress gradients (the Marangoni effect), furthering a uniform distribution of solute in the droplet, the oversaturation of the solution near the periphery is higher, so virtually all of the microtubes observed in the experiment nucleate and grow at the droplet periphery. As a result, the growth rates of the tubes grown in the circumferential direction are also greater than those of radially grown tubes (Figure 2c). For a long period of time (more than 5 min), the growth can be characterized by average rates ranging from 1 to 4.5 μm/s (Figure S1 in the Supporting Information). The faster growth of small tubes near the periphery can be explained by the smaller size of the depletion zone required to supply the material to such tubes and consequently by the shorter diffusion time required for the material to be transported to the tube’s end. Furthermore, we consider a simplified model of the microtube growth where the growth parameters are assumed to be averaged values. With a stationary flux of FF monomers to the tube’s end, the length of the tube increases at a constant speed. In the simplified model of diffusion-controlled growth (see the schematic illustration in Figure 4), where the tube’s end is a sphere of radius r0 and the boundary conditions are C(r0) = C̃ and C(rd) = C0, in which rd is the size of depletion zone, the solution of the diffusion problem for the FF monomer concentration, C(r), in the spherically symmetrical case is 1475

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⎤ ⎡⎛ rd ⎞⎛ rd 3 − r0 3 ⎞ 1 r1rd ⎟+ C 0 ⎢⎜1 − (rd 2 − r0 2)⎥ ⎟⎜ ⎥⎦ ⎢⎣⎝ rd − r1 ⎠⎝ 3 2 rd − r1 ⎠ =

(ρ − C0)r0 3 3

For r0 ≈ r1, the above equation can be solved to give rd ≈ r0 2

ρ −1 C0

(6)

where ρ is the density of the nanotube = 1233 kg/m . Then expression for the expected growth rate takes the form 3

DC ∂l =4 0 r0ρ ∂t

ΔC(r ) = 0

⇒

(2)

(3)

The integrated flux to the tip can be estimated as follows: F = D·grad(C(r ))· 4πr 2 rr ⎛1 ∂⎡ 1 ⎞⎤ = D ⎢C0 0 d ⎜ − ⎟⎥ ·4πr 2 ∂r ⎢⎣ rd − r0 ⎝ r0 r ⎠⎥⎦ rr = 4πDC0 0 d rd − r0

(4)

Then the longitudinal growth rate ∂l/∂t of a solid tube can be expressed in terms of the flux as follows: DC rd ∂l F = =4 0 2 ∂t r0ρ rd − r0 ρπr0

(5)

where ρ is the average density of the material in the tube. A similar relation will be valid for a hollow tube with fixed ratio of its internal and external diameters. The size of the depletion zone, rd, can be estimated from the balance between the shortage of material in the depletion zone and the amount of material in the end of the tube as follows:

∫r

0

L

[C0 − C(r )]4πr 2 dr =

C0 ρ

(7)

If we take C0 = 0.2 kg/m , ρ = 1233 kg/m , r0 ≈ 2 μm, and ∂l/ ∂t = 1−4 μm/s, eq 8 provides the diffusion coefficient as D ≈ (0.75−3) × 10−9 m2/s. This value is close to the typical values of diffusion coefficients of organic molecules in water.35 It directly follows from eq 8 that the length of smaller tubes (with smaller r0) increases faster than that of bigger ones, with this effect also being enhanced by faster penetration of small tubes into nondepleted areas of the solvent. The jerky growth observed in our experiments (Figure 2c) can be explained if we assume that the attachment of FF monomers to the fast growth facet has a self-activated character. We suggest that at the plateaus the growth process is limited by the rate of attachment because of high barriers for incorporation or/and low steric factors (responsible for orientation effects). The chain reaction of monomer attachment can be induced via positive feedback, by which additional energy received from condensation process of FF monomers activates the attachment process and accelerates the condensation. The increased rate of attachment thus drives the system into the diffusion-controlled regime. The initiation of the self-activated process is possible only at supersaturations higher than some sufficiently high threshold value. As a result of heat dissipation into the environment, the self-activated character of attachment implies two values for the supersaturation: a “triggered” value, which is the minimal supersaturation where the chain reaction of monomer attachment can be initiated, and a “locked” value, which is the minimal value below which the chain reaction stops. At supersaturations lower than the locked value, the attachment of FF monomers still happens but at a very low rate. Evidently the triggered value is higher than the locked one. The hypothesis of self-activated monomer attachment is supported by the experimental observation of the same value of the growth rate between plateaus (Figure 2c), which corresponds to the maximal possible growth rate of tubes (about ∼4.5 μm/s). As can be seen in the growth video in the Supporting Information and Figure 3e, the thin central part of the microtube grows first, and when this growth stops, the growth of outside layers is started (with the same longitudinal growth rate of about 4.5 μm/s), resulting in the increase in the microtube diameter. When the outside layers reach the external diameter, the central part starts to grow again, as shown in

in which C̃ is the equilibrium concentration near the sphere. We consider the case where C̃ ≪ C0, for which r1 ≈ r0 and r0rd ⎛ 1 1⎞ ⎜ − ⎟ rd − r0 ⎝ r0 r⎠

1/2

)

(8) 3

C 0 − C̃ C0 − C̃(r0/rd)

C(r ) = C0

C0 ρ

DC ∂l =4 0 ∂t r0ρ

where r is the distance from the center of the sphere (Figure 4) and r1 is given by r1 = r0

(

1− 2−

For C0 ≪ ρ, eq 7 simplifies to

Figure 4. Schematic illustration of the simplified model of diffusioncontrolled growth. ∂l/∂t is the longitudinal growth rate, rd is the size of the depletion zone, r0 is the radius of the tube, and C0 and C̃ are the concentrations of FF monomers at rd and r0, respectively.

rr ⎛1 1⎞ C(r ) = C0 1 d ⎜ − ⎟ rd − r1 ⎝ r1 r⎠

1

4 (ρ − C0)πr0 3 3 1476

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Figure 5. (a) Schematic illustration and (b) optical image of the suspended tube. (c) I−V characteristics of the tube.

Figure 6. (a) Schematic illustrations of the tube arrangements (tube 1 supported by the substrate and tube 2 suspended between the electrodes) in comparison with the dummy sample (substrate only). (b) Corresponding piezoelectric resonances measured by AFM.

limited regime, where the rates of attachment of FF monomers are controlled by the energy barriers for attachment, the shape of the tubes can be related to the growth velocities of the facets via the kinematic Wulff construction. According to the tube’s shape, the side facets are the facets with the lowest incorporation rate, and the facet whose normal is parallel to the axial direction is a facet with a higher incorporation rate. This facet with a higher incorporation rate grows faster in the direction perpendicular to the surface and can in some cases finally disappear, being overgrown by the nearest (sidewall) facets with lower growth rates. The overgrowth by the nearest (side) facets happens via the formation of downward surface steps on the side facets (Figure 3g) and results in the sharpening of the tube end (Figure 3f). At low supersaturation in the diffusion-limited regime, the existing steps at the base of the tip adsorb more material. This changes the tube ends to a more equilibrium compact shape mainly consisting of two facets, the sidewall and butted facets. Testing of Electrical and Piezoelectric Properties. Since the grown tubes had sufficiently big dimensions and appeared isolated from each other, they could be manually separated from the dried droplets and transferred to metallized patterned substrates for further testing. The tests could be conducted either with tubes lying on the substrate with their ends fixed using silver paste or with tubes suspended between

Figure 3a−c. This is in a good agreement with our hypothesis. In spite of the fact that the supersaturation in the central part of the tube is lower than triggered value (but higher than locked one), the self-activated monomer attachment is possible when the activation process in the outside layers reaches the microtube end and triggers self-activated monomer attachment on the central part. Since the central part (next to the hollow) is a more preferable place for attachment during self-activated growth, the growth of the outside layers is suppressed. The fast growth of small central tubes (see the schematic illustration in Figure 3) is possible until the supersaturation near the growth facets is high and kept at a sufficiently high level. When the supersaturation decreases to a value lower than the locked value, the activation process stops. As a result of competition, the self-activated growth of the central part can suppress the self-activated growth of the outside layers, and vice versa, the growth of the outside layers can suppress the growth of the central part beyond the diffusion length. The diffusion length, ∼ Dt , can be associated with the depletion zone rd. Estimation of the length of the depletion zone using eq 6 yields rd ≈ 110 μm. This value is close to the tube length increments between cessations of the longitudinal growth (Figure 2c). The sharpening of the tube ends observed in the experiment (Figure 3) could be a result of step generation on tube side facets during longitudinal growth. In the case of the kinetically 1477

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devices43 but still lower than those of traditional Si-based MEMS devices. The major advantage of using self-assembled dipeptides as piezoelectric resonance biosensors44 is their much easier biofunctionalization. In inorganic sensors based on piezoelectric oxides, the common practice is to cover the piezoelectrically active material with a covalently bonded metal such as gold (and because of poor adhesion a buffer layer such as Cr might be needed). This is attributed to the fact that the thiol groups frequently used for self-assembly and functionalization have a high affinity for gold, forming strong bonds that help to ensure that the gold−ligand interaction is stronger than the ligand−receptor interaction, not to mention that the high quality of a gold layer and necessity of its annealing are typically required. Therefore, a non-piezoelectric buffer layer is often required to decouple the driving and sensing parts.27 On the contrary, the surfaces of peptides can be readily functionalized. For example, this can be done by absorbing free amide and carboxylic groups on the surfaces of peptides to serve as binding sites to anchor biological molecules. The high binding capabilities of peptide tubes have been already used for labelfree detection of antibodies and heavy metals.42,43 The observed piezoelectric resonance behavior in FF microtubes is also beneficial for easy removal of the biomolecules nonspecifically adsorbed on the peptide nanotube surface. This can be done by driving the peptide with high amplitude, thus leading to possible self-cleaning of the biosensor.45 Apparently, further studies are needed for the implementation of the described piezoelectric material in real biosensor devices.

two electrodes with a gap between the tube and substrate. Figure 5a presents a schematic illustration of the suspended tube, and Figure 5b shows the corresponding optical image. It can be seen that the tube retained its integrity and initial geometry, so that various electrical tests could be conducted. First, we measured the current−voltage (I−V) characteristics of a tube with a length of 1.5 mm (Figure 5c). We see that the dependence is perfectly linear, meaning that the current is due to the bulk conductivity rather than being limited by the electrodes. The specific conductivity calculated from the slope of the I−V dependence is G ≈ 4.2 × 10−9 Ω−1·m−1 and does not depend on the direction of the applied electric field and polarization. This is expected because of the extremely high coercive field necessary for polarization switching under dark conditions at room temperature.36,37 The relatively high conductivity value (for a material with the forbidden band over 4 eV38) could be explained either by an intrinsic mechanism, e.g., charge transfer between aromatic side chains,39 or by the presence of water in hydrophilic channels.40 It is well-known that quasi-one-dimensional hydrogen-bonded wires of water molecules formed inside carbon nanotubes are excellent conductors of protons. Simulations of quantummechanical energy surfaces using density functional theory and empirical valence bond approaches show that the proton mobility along those wires exceeds that in bulk water by more than an order of magnitude.41 The predicted conductivity matches the observed experimental value taking into account the fact that the nanochannels occupy only a small part of the microtube’s cross section. Additional investigations are obviously required in order to uncover the mechanism of the dc conductivity in peptide microtubes. These are extremely important in view of the applications of FF peptides as biosensors.42 The described growth method largely facilitates the fabrication of peptide-based microdevices. Figure 6 shows a schematic illustration and measurement results for the electromechanical behavior of FF microtubes as a function of frequency. Two types of devices were fabricated: with the tube supported by the substrate (tube 1) and with the tube suspended between the electrodes (tube 2). In the first case, the substrate was grounded, and all of the resonances except for the bending ones could be excited. In the second case, the tube ends were clamped to the electrodes, and all of resonances, including the bending ones, could be revealed via the voltage applied through the AFM tip. Only out-of-plane (i.e., radial) displacements were detected in this case. In comparison, the signal excited by the bare substrate was also registered. These displacements were based on the electrostatic interaction between the tip and the substrate and reflect only clamped cantilever resonances. Accordingly, they should be excluded from the discussion because they are not related to the properties of the tubes. The lowest resonance frequency of the suspended tube (at 82.5 kHz) was assigned to the fundamental bending mode, while the peak at about 251 kHz could be due to the second-order bending resonance. The lowest resonance peak of the free tube was lower than that of the clamped tube because of the different boundary conditions. A detailed analysis of the piezoelectric resonances is beyond the scope of this paper and will be reported elsewhere. It should be only noted that the high quality of our piezodevices is confirmed by the sufficiently high Q factors for higher vibrational modes, such as for tube 2 at about 1.6 MHz. The Q factor is about 260, which is comparable to those of commercial organic microelectromechanical system (MEMS)

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CONCLUSION

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ASSOCIATED CONTENT

We have used a simple method of evaporation-induced crystallization to fabricate high-quality individual diphenylalanine (FF) microtubes with high aspect ratios. The mechanism of the growth was thoroughly investigated by measuring the microtube lengths as a function of time. A model of selfactivated growth was proposed to explain the observed effect of steplike behavior of the microtube length versus time. The growth rate of the tubes with arbitrary diameter during the diffusion-controlled process was calculated, and the diffusion coefficient for FF monomers in the solution was estimated. Grown tubes were successfully transferred to metallized patterned substrates, and their conductivity and piezoelectric properties were evaluated as a function of the applied voltage and frequency. A number of piezoelectric resonances were observed and attributed to different vibrational modes excited by the piezoelectric effect inherent to FF. The extraordinary physical properties of FF-based nanostructures can be now used in various microdevices.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01604. XRD pattern and Raman spectrum of FF microtubes, plot of growth rate versus angle, plot of mass of the droplet versus time, and optical images of an FF solution droplet (PDF) Video of the microtube growth (AVI) 1478

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The equipment of the Ural Center for Shared Use “Modern Nanotechnology” at UrFU was used. A.L.K., S.V., D.V., P.Z., and A.E. acknowledge support from the Russian Scientific Foundation (Grant 14-12-00812). K.R. is grateful to the Portuguese Foundation for Science and Technology (FCT) for support within his grant SFRH/BPD/88362/2012. Part of this work was developed in the scope of Project CICECO-Aveiro Institute of Materials (ref. FCT UID/CTM/50011/2013), financed by national funds through the FCT/MEC and, when applicable, cofinanced by FEDER under the PT2020 Partnership Agreement.

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DOI: 10.1021/acs.cgd.5b01604 Cryst. Growth Des. 2016, 16, 1472−1479