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Directed Self-Assembly of Dipeptide Single Crystal in A Capillary Bingbing Sun, Hans Riegler, Luru Dai, Stephan Eickelmann, Yue Li, Guangle Li, Yang Yang, Qi Li, Meifang Fu, Jinbo Fei, and Junbai Li ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b08925 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018
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Directed Self-Assembly of Dipeptide Single Crystal in A Capillary † ⊥
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Bingbing Sun, , Hans Riegler, Luru Dai, Stephan Eickelmann, Yue Li, Guangle Li, , Yang Yang,§ Qi Li,†, ⊥ Meifang Fu,†, ⊥ Jinbo Fei,† and Junbai Li,†, ⊥* †
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid, Interface
and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡
Max Planck Institute of Colloids and Interfaces, Potsdam 14424, Germany
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National Center for Nanoscience and Technology, Chinese Academy of Sciences, Beijing
100190, China ⊥
University of Chinese Academy of Sciences, Beijing, 100049, China
*Email:
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ABSTRACT: Controlled growth of one-dimensional nanostructure is playing a key role in creating type of materials for functional devices. Here, we report procedures for controlled assembly, as well as disassembly of the dipeptide diphenylalanine (FF) into aligned and ultralong single crystals in a capillary. With the evaporation of solvent, nucleation of the crystal occurred in the confined region and the crystals grew continuously with supply of molecules from the concentration-gradient system inside the capillary. Based on the "Knudsen regime", an ultralong aligned individual FF single crystal possessing active optical waveguide property at a macroscopic length scale could be obtained. Moreover, the capillary is also an effective microdevice to investigate the disassembly process of the FF single crystals. This strategy has the potential to broaden the range of applications of aligned organic nanomaterials.
KEYWORDS: self-assembly, diphenylalanine, capillary, crystal growth, disassembly
One-dimensional (1D) micro- and nanostructures of organic materials have attracted extensive attention due to their promising applications in high-performance functional devices.1-4 This is especially true for 1D organic single-crystal nanomaterials because they have no defects, which is essential for the manufacture of this type of advanced materials.5-8 Controlling the growth and assembly of organic single crystals is therefore important for improving and optimizing the performance of functional devices. Self-assembled organic crystalline nanomaterials are flexible building blocks employable in functional optical and electronic nanoscale circuits.9-12 Much attention has lately been directed toward diphenylalanine (FF), a dipeptide which is the core recognition motif of the Alzheimer's β-amyloid polypeptide.13-15 FF is attractive as a molecular building block because of its biocompatibility, biodegradability and functional flexibility.16-18 Different routes have been
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published for fabrication of FF single-crystals. Our group has obtained FF single-crystalline dipeptide structure by introduction of an aldehyde into the fibrous networks.19 As a small molecule with two aromatic rings, FF easily self-assembles to single crystal nanostructures either from solution or by vapor deposition methods. These crystals can selfassemble into supramolecular structures; however, they generally do a disordered manner giving macroscopic assemblies with random position and orientation of the single crystals.20,21 Such an uncontrolled self-assembly process limits the usefulness of the FF single crystals because many applications require unidirectional single crystals. Recently, a time-programmed control of the self-assembly and the subsequent growth of these structures has been developed.22,23 The crystal growth mechanism consists of three steps: nucleation, adsorption, and diffusion and growth.24 Both thermodynamic and kinetic factors have been found to be essential in the self-assembly process.25-27 Based on this crystal growth mechanism, we have proposed that a confined nucleation and a continuous supply of molecules are prerequisites for a controlled fabrication of FF single crystals. In this work, we report a method for a controlled directed growth of ultralong FF single crystals in capillaries (Figure 1). Capillary has been used to control evaporation of nanotubes aqueous dispersion to prepare a patterned surface.28 We used a capillary to provide a confined space, controlling the growth direction of the single crystals and the evaporation of solvent gives the concentration gradient of FF that is required in order to limit the number of nucleation sites. The loss of solvent triggers the growth of the FF crystals inside the capillary, resulting in a theoretically unlimited length of the single crystal. RESULTS AND DISCUSSION
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Figure 1. Growth of an individual FF fiber inside a capillary starts within a few minutes after the beginning of the evaporation at room temperature. a) Bright field images of a single FF fiber at different times. The inner diameter of the capillary is 50 µm. b) Schematic of the FF fiber growth process in a capillary. FF was dissolved in a mixture of HFP (1,1,1,3,3,3-hexafluor-2-propanol) and water and this solution was soaked into a capillary by capillary pressure. One end of the capillary was sealed and the other end kept open. Bright field microscopy images of this process at two different times are shown in Figure 1 together with a schematic of the process. As the solvent evaporates,
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the meniscus recedes further into the capillary. At one point FF crystals start to form inside the capillary. With the continuous evaporation of the solvent, a long FF fiber is generated (Movie S1). The crystallization of FF was significantly influenced by the diameter of the capillary. Figure S1c,f,i shows that only one individual FF microwire was formed inside a capillary with an inner diameter of 50 µm. For a capillary with an inner diameter of 100 µm, several microwires were obtain (Figure S1b,e,h). For the 200 µm capillary, the number of microwires was even larger (Figure S1a,d,g). The SEM images reveal that the microwires have a kind of trapezoid cross sectional shape with lateral dimensions of about 5 µm, corresponding to cross sectional areas of about 20 µm2 (Figure S1d-f). The uniform brightness in the polarized optical microscopy (Figure S1g-i, t) indicates that the single fiber in the case of the smallest capillary (inner diameter of 50 µm) is a single crystal. The microwire-bundles that form in the case of the capillaries with larger diameters seem to consist of sections of single crystals. The following scenario is proposed for formation of FF fibers in capillaries with different diameters. It is assumed that the density of the dipeptide is approximately the same as that of the solution in weight (c0, in relative volume content of FF per solvent volume). A single fiber with a fixed cross section of af , and material conservation for each capillary cross section, ac, all the FF in solution per capillary inner diameter, ∗ , gets crystalline with a total FF crystal cross section of ∗ , with n= number of fibers (Equation 1): ∗ , = ∗ , or =
∗
(1)
Based on this scenario, when c0 is constant, n mainly depends on the cross section of the capillary. If only one fiber forms inside a capillary of 50 µm inner diameter, the number of fibers
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formed inside capillaries of 100 and 200 µm inner diameter should be four and sixteen, respectively. The number of fibers seen in Figure S1 correspond closely to these theoretical values, demonstrating that the number of fibers obtained with our procedure is controllable. The obtained FF fiber was analyzed with X-ray diffraction (XRD) to investigate the crystal pattern. Figure 2a shows the results at three different positions on the FF fiber formed inside a long capillary (Figure S2). The XRD patterns are similar and show hexagonal structures, as has previously been determined for FF single crystals. The results indicate that the degree of crystallinity is high and uniform along the long FF fiber.29 The FF fiber was also investigated by single crystal X-ray diffractometry. As shown in Figure 2c, the X-ray was focused on the fiber and not on the whole capillary in order to avoid interference from the capillary walls. The experimental conditions and the structural XRD parameters are shown in Table S1. The experiment confirmed a hexagonal lattice of the FF single crystal.30 The selected-area electronic diffraction pattern (Figure 2b inset) also suggests a single-crystalline structure of the FF fiber.
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Figure 2. Characterization of the FF single crystals and the growth process. a) XRD patterns at three positions (P1, P2, P3) of a FF single crystal residing inside a long capillary. The three positions are shown in Figure S1. b) Electron diffraction pattern of a FF single crystal. The scale bar is 1 µm. c) Analysis of the FF fiber by single crystal X-ray diffractometry. The red circle shows the region at which the X-ray was focused. The scale bar is 100 µm. d) Evaporation rate of solvent vs. length of the capillary. e) "Knudsen regime" in capillary. f) The ultralong FF single crystal in a capillary. Rectangles 1, 2 and 3 represent three positions on the capillary and optical images are shown at these positions. The vapor pressure is -85 kPa.
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Movie S1 shows the growth of the FF fibers in the smallest capillary (50 µm) at different times. At the beginning, the meniscus recedes rather fast (>1 µm/s). The generated FF fiber remains as a deposit at the dry capillary side wall, while the meniscus recedes. At this early stage, no growth of the FF fiber within the solution can be detected within the resolution of the microscope (µm). At later stages, the meniscus recedes much slower than in the beginning due to the reduced evaporation rate. Then the FF fiber grows in the solution at some distance (>100 µm) "ahead" of the meniscus. We assume a typical nucleation and growth scenario for this system. The fiber starts to grow when the FF concentration exceeds the (equilibrium) saturation concentration. The growth rate and the growth direction are determined by the supply of FF from the surrounding solution. We neglect that the evaporation and solubility properties of the solution depend on the solvent composition (i.e. on the relative amounts of water and HFP). It is simply assumed that the evaporation drives the concentration of FF closer to the saturation limit. We also neglect gravity effects (the Bond number
