Nanowire Oriented On-Surface Growth of Chiral Cystine Crystalline

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Nanowire Oriented On-surface Growth of Chiral Cystine Crystalline Nanosheets

Shenxiang Zhang,†,‡,§ Feng Zhang,†,§ Haili Qin,† Liang Hu,† and Jian Jin†,* †

Nano-Bionics Division and i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics,

Chinese Academy of Sciences, Suzhou 215123, China ‡

University of Chinese Academy of Sciences, Beijing 100049, China

KEYWORDS: free-standing films; organic nanosheets; self-assembly; cystine crystalline nanosheets; on-surface growth. ABSTRACT: Exploration of an effective route to achieve the controlled growth of 2D molecular crystal is of scientific significance yet greatly underdeveloped due to the complexity of weak intermolecular interactions thus difficulty of inducing anisotropic 2D growth. We report here a facile nanowire oriented on-surface growth strategy for the fabrication of cystine crystalline nanosheets with finely controlled thickness (1.1 nm, 1.9 nm, 2.9 nm, and 4.8 nm which correspond to one layer, two layers, three layers, and five layers of crystal cystine) and large areas (>100 µm2). The cystine crystalline nanosheets display chirality delivered by chiral cysteine monomers, either L-cysteine or D-cysteine. The chiral nanosheets with structural precision and chemical diversity could serve as a novel 2D platform for constructing advanced hybrid materials.

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1. INTRODUCTION The study of two-dimensional (2D) nanomaterials has been flourishing during the past decade enlightened by the discovery of graphene and its amazing properties of 2D effect.1-6 To date, most of reported 2D nanomaterials, including graphene, graphitic boron nitride, metal oxides and chalcogenides as well as clay and silicates, are inorganic crystals since they have intrinsically layered crystal structures with strong in-plane chemical bonds.7-9 A simple physical or chemical exfoliation process could thus be applied for the fabrication of various inorganic 2D crystals.10-15 Besides, the recent studies have also witnessed the success of the creation of inorganic 2D crystals without intrinsically layered crystal structures by means of the 2D oriented attachment and space/surface induced anisotropic growth strategy.16-18 Compared with inorganic 2D crystals, organic 2D crystals, or called 2D molecular crystals are still less-reported due to complexity of intermolecular interactions.19-26 2D molecular crystals are important class of 2D nanomaterials due to their abundant species and adjustable architectures, which are the keys to tailor their structures and chemical functionalities.27-33 The exploration of a new route to achieve the controlled growth of 2D molecular crystals is thus of scientific significance and highly desired. As an important component of organisms, amino acid plays a key role in processes such as neurotransmitter transport and biosynthesis.34-36 Amino acids composed of amine (-NH2) and carboxylic acid (-COOH) functional groups, along with a side-chain specific to each amino acid, present diverse assembly forms which are directly related to the functions, properties and performance.37 Gaining insight


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into the crystallization and self-assembly behaviors of amino acid is an urgent demand in modern medical science. Cystine is an essential amino acid formed by the oxidation of two cysteine molecules that covalently link via a disulfide bond.38 It serves an important structural role in many proteins. For instance, the crystallization and growth of cystine is a critical step in the pathogenesis of cystine kidney stone.39-42 A series of crystal growth inhibitors have been designed for the prevention of cystine kidney stones through the use of the specific binding of inhibitors at the crystal surface to frustrate the attachment of cystine molecules.43, 44 However, this does not always provide an effective and precise solution to control over the crystal growth at nano/atomic scale. The self-assembly of cystine crystals in the form of 3D ordered superstructures such as dendritic, flowerlike and other hierachical structures has been thoroughly studied previously through the oxidation of cysteine.45 However, in contrast to the 3D ordered superstuctures, the rational design and controlled synthesis of cystine crystalline nanosheets is still less of report due to the lack of ability to well control the self-assembly of cystine into 2D structure. Here, we report the fabrication of chiral cystine crystalline nanosheets with controlled thickness via a facile nanowire oriented on-surface growth strategy. The formation of cystine nanosheets is induced by a trace amount of Cu2+ ions releasing from Cu(OH)2 nanowires perpendicularly grown on a copper mesh, which act as catalyst from cysteine to cystine. The intermolecular hydrogen bond directed self-assembly into cystine nanosheets then occurs along the nanowire surface. Through such a template-oriented process, cystine crystalline


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nanosheets with well-controlled thicknesses including 1.1 nm, 1.9 nm, 2.9 nm, and 4.8 nm which correspond to one layer, two layers, three layers, and five layers of crystal cystine, respectively, and with micrometer scale dimensions are achieved.

2. EXPERIMENTAL SECTION 2.1 Materials. L-cysteine (98%, Alfa), D-cysteine (98%, Alfa), anhydrous ethanol (AR, Sinopharm Chemical Reagent Co., Ltd), NaOH (AR, Sinopharm Chemical Reagent Co., Ltd), (NH4)2S2O8 (98%, Alfa), and PdSO4 (AR, Alfa) were used as received. Millipore purified water was used in all the experiment.

2.2 Synthesis. Preparation of Cu(OH)2 nanowire-wrapped copper mesh: Pre-cleaned copper mesh with a mesh number of 200 was immersed in an aqueous solution of 2.5 M NaOH and 0.13 M (NH4)2S2O8 at room temperature for 30 minute. The membrane was then taken out and washed with deionized water repeatedly. Synthesis of cystine nanosheets: 3.0 mg L-cysteine was dissolved in 5 mL aqueous solution containing 10 µM NaOH, then the solution was mixed with 5 mL anhydrous ethanol to form a uniform solution. A piece of Cu(OH)2 nanowire-haired copper mesh (1 cm × 4 cm) was immersed into the solution at 28 oC for 1 hour, 2 hours, 3 hours, 4 hours and 5 hours, respectively. The products on the surface can be peeled from Cu(OH)2 nanowire-haired mesh by mild ultrasonic and dispersed in anhydrous ethanol. D-cystine could also be obtained through the same procedure.


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Growing Pd nanocrystals on cystine nanosheets: 2 mg cystine nanosheets formed from L-cysteine were dispersed into 5 mL anhydrous ethanol and stirred for 30 min. 0.5 mL PdSO4 solution (50 mM) was added into the above solution under stirring. The reaction was continued for 30 min and the products were washed by anhydrous ethanol three times. 2.3 Characterization. Scanning electron microscopy (SEM) images were obtained on a field-emission scanning electron microscope (Quanta FEG 250) at acceleration voltages of 20 kV. Transmission electron microscopy (TEM) was measured on a Tecnai G2 F20 S-Twin field-emission transmission electron microscope at acceleration voltages of 120 kV. Power X-ray Diffraction pattern (XRD) was acquired with a Bruke D8 diffractometer with Cu Kα (λ = 0.15418 nm). Atomic Force Microscopy (AFM) was obtained on a ICON Bruker operating in ScanAsyst mode at room temperature. Raman measurements were acquired with a Horiba Raman (model LABRAM HR, λ = 533 nm). Circular dichroism (CD) was obtained with Applied Photophysics Chirascan Plus at room temperature.

3. RESULTS AND DISCUSSION 3.1. Synthesis of cystine nanosheets on Cu(OH)2 nanowire-haired copper mesh The growth of cystine nanosheets was carried out via a special nanowire oriented on-surface process where positively-charged Cu(OH)2 nanowires perpendicularly grown on a copper mesh were used as a template. Cu(OH)2 nanowires were firstly grown on a commercial copper mesh with 200 mesh. To grow Cu(OH)2 nanowires,


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copper mesh was oxidized by (NH4)2S2O8 in an alkaline aqueous solution and hairy Cu(OH)2 nanowires were produced along the frames of the copper mesh according to our previous report.46 Scanning electron microscopy (SEM) image shows that all frames of the copper mesh are wrapped thickly and uniformly by Cu(OH)2 nanowires in a length of 10-15 µm and a width of 200-500 nm (Figure 1a-1d). The nanowires grow vertically along the frames and intertwine each other. The composition of the nanowires is verified by X-ray diffraction (XRD) spectrum as shown in Figure S1. It indicates an orthorhombic-phase Cu(OH)2 crystals (JCPDS No. 80-0656).

Figure 1. (a) Photograph and (b) SEM image of bare copper mesh. (c) low-magnification and (d) enlarged cross-sectional SEM images of Cu(OH)2 nanowire-wrapped copper mesh. (e) and (f) SEM images of cystine nanosheets formed from L-cysteine grown on the surface of Cu(OH)2 nanowire-wrapped copper mesh. (g) Corresponding schematic drawing of the growth of cystine nanosheets.


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Our previous report has demonstrated that the nanowire-wrapped copper mesh behaves superhydrophilic property so that water can completely wet and infiltrate into the inner of nanowire network.46 It assures that cysteine monomer in aqueous solution could fully contact with individual Cu(OH)2 nanowire to accomplish the oxidation reaction from cysteine to cystine. To produce cystine nanosheets, a piece of Cu(OH)2 nanowire-wrapped copper mesh (1 cm × 3 cm) was immersed into an alkaline solution containing 5 mM cysteine (see detail description in experimental section). A white precipitate on the mesh surface forms immediately. SEM images reveal that the white precipitate is composed of an enormous amount of ultrathin nanosheets clustering on the surface of copper mesh (Figure 1e and 1f). By taking use of such a facile and one-pot process, both the cystine nanosheets formed from L-cysteine and D-cysteine could be obtained, respectively, and the cystine nanosheets could be easily peeled off from the mesh via ultrasonic treatment. 3.2. Characterization of cystine nanosheets The two cystine nanosheets formed from L-cysteine and D-cysteine were subjected to further structure characterizations. Figure 2a-2c are SEM and atomic force microscopy (AFM) images of the cystine nanosheets formed from L-cysteine. Flexible and ultrathin nanosheets with high transparency and smoothness could be clearly observed in large-view SEM image (Figure 2a). Figure 2b displays an individual nanosheet with well-defined edge and large area in several micrometers covering a naked porous grid. The thickness of the nanosheet as measured by AFM is


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about 1.9 nm (Figure 2c). Correspondingly, the cystine nanosheets formed from D-cysteine exhibit the similar morphology and thickness (Figure 2d-2f) as those formed from L-cysteine. The as-prepared cystine nanosheets are robust enough to be characterized by transmission electron microscopy (TEM) directly. Figure 2g is a typical TEM image of a cystine nanosheet formed from L-cysteine spanning over a micrometer hole. The corresponding selected-area electron diffraction (SAED) pattern of this nanosheet displays hexagonal diffraction spots, indicating the single crystalline properties of the nanosheet (inset in Figure 2g). The component of the as-prepared nanosheets was evaluated by Raman spectroscopy (Figure 2h). A typical peak at 2550 cm-1 attributed to S-H stretching is observed for L-cysteine monomer. This peak disappears and a new peak at 497 cm-1 associated with S-S stretching is observed for the nanosheet, indicating the nanosheet is composed by cystine. Powder X-ray diffraction (XRD) pattern of the nanosheets shows two intense peaks at 2θ = 18.9° and 28.5° corresponding to the (0012) and (0018) crystal planes, respectively, which is in well agreement with the previous report of L-cystine crystals (Figure S2).42 In view of the above analysis, the molecular structure of cystine nanosheets formed from L-cysteine is drawn schematically in Figure 2i. The crystal structure of the nanosheet follows hexagonal P6122 space group (a = b = 5.42 Å, c = 56.28 Å). Considering the molecular length of a cystine is around 0.94 nm, a 1.9 nm thick cystine nanosheet fits well a bilayer of cystine molecules. Along the nanosheet plane, adjacent cystine molecules form intermolecular -NH2 ··· -COOH hydrogen bond to anchor-hold each molecules firmly to give a 2D structure.47


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Figure 2. Morphology and structure characterization of cystine nanosheets. (a) SEM image, (b) enlarged SEM image, and (c) AFM image and corresponding height profile of cystine nanosheets formed from L-cysteine grown on the surface of Cu(OH)2 nanowire-wrapped copper mesh. (c) SEM image, (d) enlarged SEM image, and (e) AFM image and corresponding height profile of cystine nanosheets formed from D-cysteine grown on the surface of Cu(OH)2 nanowire-wrapped copper mesh. (g) TEM image and (inset) SAED pattern of cystine nanosheets formed from L-cysteine. (h) Raman spectra of L-cysteine powder and cystine nanosheets formed from L-cysteine. (i) Structure model of L-cystine molecules packing in the nanosheet.

3.3. Growth mechanism of cystine nanosheets As

for

the

growth

mechanism

of cystine

nanosheets

on

Cu(OH)2

nanowire-wrapped copper mesh, we propose a two-step process by combination of oxidation reaction and hydrogen bond directed self-assembly in a localized region of


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Cu(OH)2 nanowire surfaces (Figure 3a). In a weak alkaline environment (pH ≈ 9), a trace amount of Cu2+ions in several µM are slowly and evenly released from Cu(OH)2 nanowires into the reaction solution. In the presence of oxygen, these Cu2+ ions act as catalyst to induce the formation of cysteine to cystine.48 Owning to the dense and interlaced array of Cu(OH)2 nanowires, the released Cu2+ ions tend to concentrate at the region close to nanowire surfaces where the oxidation reaction prefer to occur. Therefore, the cystine molecules have a relatively higher concentration near to nanowire surfaces. This creates the condition for the following self-assembly from cystine molecules to cystine nanosheet along nanowire surfaces driven by intermolecular hydrogen bond. A series of control experiments were further used to verify the growth mechanism of nanosheet. The amount of Cu2+ ions released by Cu(OH)2 nanowires into the solution as a function of time was firstly measured via inductive coupled plasma emission spectrometer (ICP). The result shows that the concentration of Cu2+ ions increases gradually with time from 1.3 µM at 1 h to 4.0 µM at 5 h (Figure S3). As a catalyst, the slow release of Cu2+ ions could effectively slow down the course of oxidation reaction which is advantageous to maintain the concentration of cystine molecules stable and finely control the self-assembly process superior to common solution reaction systems. Secondly, to further confirm the function of Cu(OH)2 nanowires instead of copper mesh, Cu(OH)2 nanowires were peeled off from copper mesh and used solely for the growth of cystine nanosheet under the same reaction condition as that of Cu(OH)2 nanowires-wrapped copper mesh. SEM image shows that cystine nanosheets with similar structure and quality


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could be produced at this condition (Figure S4). What’s more, nanosheets are only observed on the surface of Cu(OH)2 nanowire-haired copper mesh and almost no free nanosheets in the solution. It indicates that the dense Cu(OH)2 nanowires on the mesh membrane serve as a site of nucleation for cystine crystal growth through slow release of Cu2+ during the reaction process. Thirdly, the effect of locally concentrated Cu2+ ions close to nanowire surfaces on the growth of cystine nanosheets was investigated. A contrast test where a magnetic stirring was exerted on the reaction solution during the whole reaction process was done. As shown in Figure S5, different from the reaction solution without stirring, the reaction solution with stirring turns out to be cloudy after 3 h. SEM images reveals the formation of large cystine crystals in the solution rather than ultrathin nanosheets. Our specific reaction system with features of slow-released oxidant and nanowire-forest as template makes the fine control of the thickness of cystine nanosheets feasible. The control of nanosheet thickness was carried out by adjusting the reaction time. In our experiment, cystine nanosheets with well-tuned thicknesses of 1.1 nm, 1.9 nm, 2.9 nm, and 4.8 nm are achieved corresponding to the reaction time of 2 h, 3 h, 4 h and 5 h, respectively. Figure 3b shows four representative AFM images of cystine nanosheets obtained under different reaction time and their corresponding height profiles. The time-dependent thickness was plotted as shown in Figure 3c. It can be seen that the thickness of cystine nanosheets increases almost linearly with increasing reaction time. The thickness of cystine nanosheets is a multiple of the length of cystine molecule. The obtained four thicknesses correspond


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to 1 layer, 2 layers, 3 layers, and 5 layers of crystal cystine, respectively. Meanwhile ，the size of the L-cystine nanosheets increased with increasing reaction time (figure S7). As can be seen in Figure S6, L-cysteine and D-cysteine show opposite peaks at ~ 210 nm in the CD spectra while the L-cystine and D-cystine nanosheets exhibit similar but distinct new peak at ~ 250 nm, suggesting the highly ordered assembly of cystine molecular in nanosheets.

Figure 3. (a) A schematic showing the formation process of cystine nanosheets on Cu(OH)2 nanowire-wrapped copper mesh. (b) AFM images and corresponding height profiles of cystine nanosheets at different reaction time (The nanosheets are formed from L-cysteine). (c) Plot of nanosheet thickness as a function of reaction time.


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3.4. Constructing advanced hybrid materials Cystine is formed by the oxidation of two cysteine molecules that covalently link via a disulfide bond. It thus possesses a symmetric molecular structure with a carboxyl and an amine group at each end. The carboxyl and amine group exposed to the outside of cysteine nanosheet allows the post-functionalization of the nanosheet toward building hybrid 2D nanomaterials. We show here a representative work of in-situ growing Pd nanocrystals on cystine nanosheet. As shown in Figure 4, 5 nm Pd nanocrystals are uniformly dispersed on cystine nanosheet with high density through reduction of PdSO4. It is considered that the Pd nanocrystals tend to attach selectively on the surface of (006) plane of cystine crystals due to the higher density of polar residues (006) plane than (100) plane. Previous study also confirmed that gold nanoparticles are prior to attach on the surface of (006) plane.49

Figure 4. (a) A schematic of growing Pd nanocrystals on the surface of cystine nanosheets. (b) TEM and (c) HR-TEM images and SAED pattern (inset) of of Pd nanocrystals on cystine nanosheet.

4. CONCLUSIONS In summary, free-standing, ultrathin cystine crystalline nanosheets with finely controlled thickness and micrometer scale dimension have been successfully


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synthesized through a nanowire oriented on-surface growth strategy where dense and interlaced Cu(OH)2 nanowire-wrapped copper grid was utilized as template. The chiral nanosheets with well-defined structure and high-density arranged functional groups exposed to the outer surfaces could serve as a novel 2D platform for building advanced hybrid materials. The on-surface directed growth of 2D molecular crystals provides a flexible and effective way for the construction of 2D molecular-based nanomaterials with a device-processable scale and might contribute widely to supermolecular chemistry.

ASSOCIATED CONTENT Supporting Information: XRD caracterization of nanowire-wrapped copper mesh and cysteine nanosheet, concentration of Cu ion released into solution, control experiment of cystine nanosheetsgrown on Cu(OH)2 nanowires and in solution with stirring, and CD spectra of cystine nanosheets formed from L-cysteine and D-cysteine. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes §

These authors contributed equally to this work. The authors declare no

competing financial interest.


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ACKNOWLEDGEMENT This work was supported by the National Natural Science Foundation of China (21433012, 21273270, 51403231), the National Basic Research Program of China (2013CB933000), and the Key Development Project of Chinese Academy of Sciences (KJZD-EW-M01-3).


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Table of Contents Graphic and Synopsis

We report a facile nanowire oriented on-surface growth strategy for the fabrication of chiral cystine crystalline nanosheets with finely controlled thickness and large areas. The cystine crystalline nanosheets display enhanced chirality delivered by chiral cysteine monomers. The chiral nanosheets with structural precision and chemical diversity could serve as a novel 2D platform for constructing advanced hybrid materials.


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Nanowire Oriented On-Surface Growth of Chiral Cystine Crystalline

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