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Solvatochromic Coatings with Self-Cleaning Property from Palygorskite@Polysiloxane/Crystal Violet Lactone Yujie Zhang, Jie Dong, Hanxue Sun, Bo Yu, Zhaoqi Zhu, Junping Zhang, and Aiqin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09252 • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 23, 2016

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ACS Applied Materials & Interfaces

Solvatochromic Coatings with Self-Cleaning Property from Palygorskite@Polysiloxane/Crystal Violet Lactone Yujie Zhang,†,§,┴ Jie Dong,†,┴ Hanxue Sun,‡ Bo Yu,‡ Zhaoqi Zhu,‡ Junping Zhang,†* and Aiqin Wang†* †

State Key Laboratory for Oxo Synthesis & Selective Oxidation, and Center of Eco-material and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, 730000, Lanzhou, P.R. China, ‡Department of Chemical

Engineering, College of Petrochemical Engineering, Lanzhou University of Technology, 730050, Lanzhou, P.R. China, and § School of Chemistry and Chemical Engineering, Huaiyin Normal University, 223300, Huaian, Jiangsu, P.R. China, ┴Graduate University of the Chinese Academy of Sciences, 100049, Beijing, P.R. China. KEYWORDS: superhydrophobic, wettability, attapulgite, surface chemistry, silanes ABSTRACT: Organic allochroic materials have wide potential applications in various fields, but have so far been limited because of their low stability, and low and slow switching reversibility. Inspired by the extraordinarily high durability of Maya Blue and the superhydrophobic lotus leaves, the new solvatochromic and self-cleaning coatings with intense blue color are fabricated by the combination of polysiloxane-modified palygorskite (PAL@POS) and crystal violet lactone (CVL) via solid-state grinding. The coatings are characterized using scanning electron microscopy, diffusive reflection UV-Vis spectra and other analytical techniques. The hydrogen bonding of the hydroxyl groups of PAL@POS with the carboxylate groups of CVL+ is the origin of the intense blue color. The interruption of the hydrogen bonding by the vapor of solvents results in rapid discoloration of the coating. On the other hand, the evaporation of the solvents from the coating results in complete recovery of the original color by restoring the hydrogen bonding between PAL@POS and CVL+. The polarity, hydrogen bonding ability and volatility of the solvents determine the solvatochromic properties of the coating. The PAL@POS/CVL coatings feature high reversibility and rapid switching between the colored and colorless states induced by the vapor of various solvents. Also, the coatings are superhydrophobic with fine self-cleaning properties and high durability in different environments. Moreover, the CVL content in the coating is controllable and can be as high as 4.8 wt%.

INTRODUCTION Organic allochroic materials are a well-known class of materials that change color in response to light, temperature, force and/or chemicals.1-3 This unique property makes them very interesting candidates for many potential applications, e.g., smart coatings for windows, information storage and security printing ink, etc.4, 5 However, the practical applications of organic allochroic materials have so far been limited because of their low stability, and low and slow switching reversibility.6 For example, the application of photochromic spiropyrans is often hindered owing to the oxygen-mediated degradation of the molecules.7 Although important achievements have been obtained in the synthesis of new allochroic molecules in the past decades,8 it is still challenging to design and synthesize allochroic materials exhibiting rapid switching in bulk materials. Compared to the mobile environment of a solution, it is generally difficult to obtain complete and rapid switching between the colored and colorless states in a rigid matrix, which hinders the changes in the structure and/or conformation of the molecules. Moreover, in most cases the switching reaction can only occur when the concentration of the molecules is very low in the matrix. So, it is usually difficult to achieve loading of a high content of molecules in conventional allochroic materials.4 The stability of organic allochroic materials can be enhanced by incorporating them into rigid matrices such as polymers, inorganic materials (silicates and aluminosilicate) and mesostructured materials, which could restrict mobility of

the colored species and the oxygen diffusion.7, 9 Organic-inorganic hybrid materials are of great interests among these systems on account of combining advantages of organic components, such as great variety and high availability, and those of inorganic components, e.g. high stability, eco-friendliness and surface modificability.10 The inorganic hosts can enhance stability of organic allochroic materials. Also, the color changing properties of organic allochroic materials may be improved because of the charge transfer between organic and inorganic species. For example, Maya Blue, prepared by ancient Maya, is a hybrid pigment of palygorskite (PAL) and indigo.11-14 The host-guest interactions between PAL and indigo generate Maya Blue with vivid color and extraordinarily high stability against acids, alkalis and UV irradiation, etc. Compared to the other clays, PAL showed a special rod-like structure.14 The high aspect ratio, large surface area and excellent adsorption properties of PAL are very conducive to its applications in the acceptance of guest molecules,15, 16 and preparation of hybrid materials.17, 18 Wetting of a solid surface is the basis for its interaction with a liquid. So, the low durability of organic allochroic materials should be partly because of the fact that aqueous solutions can wet these materials. On the contrary, the stability of conventional organic allochroic materials might be enhanced by introducing a superhydrophobic coating. Superhydrophobic coatings with high water contact angles (CAs > 150°) and low sliding angles (SAs < 10°) have received great interests.19-23 It is expected that the durability of organic allochroic materials could be evidently enhanced via introduction of a stable

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superhydrophobic coating. Moreover, a superhydrophobic coating may also endow traditional allochroic materials with a self-cleaning property. Herein, we report the first allochroic coating with self-cleaning property. The coatings were prepared by the combination of polysiloxane-modified PAL (PAL@POS) and crystal violet lactone (CVL) via solid-state grinding. PAL@POS was formed by condensation of n-hexadecyltrimethoxysilane (HDTMS) and tetraethoxysilane (TEOS) onto PAL. CVL is a white precursor and can convert to a triphenylmethane dye (CVL+) with intense blue color.24 Solid-state grinding is frequently used for preparing functional hybrid materials in order to enhance guest-host interactions.11, 25-27 Different from the previously reported chromic materials, the as-prepared intense blue coatings feature high reversibility and rapid switching between the colored and colorless states induced by the vapor of various solvents. Also, the coatings are superhydrophobic with fine self-cleaning properties and high durability in different environments. Moreover, the CVL content in the coating is controllable and can be as high as 4.8 wt%, which is rare for organic allochroic materials.

EXPERIMENTAL SECTION Materials. PAL was obtained from Xuyi (Jiangsu, China), and was treated using 2% H2SO4 according to our previous work.[14] The strong diffraction peak at 2θ = 8.38° indicates high purity of PAL. However, the peak at 2θ = 26.63° implies that there is still some quartz in the purified PAL. HDTMS (95%) and TEOS (99.9%) were purchased from Gelest. CVL (3,3-bis-(4-dimethylaminophenyl)-6-dimethylaminophthalide), H2SO4, NaOH, ammonia, HCl, anhydrous ethanol, acetone and other reagents were purchased from Sinopharm Co. Ltd. (Shanghai, China). Glass slides 24 × 50 mm in size was purchased from Menzel, Germany. Preparation of PAL@POS/CVL. PAL@POS was prepared according to a previously reported approach.28 PAL (5.0 g) was homogeneously dispersed in the mixture of ethanol (90 mL) and ammonia saturated ethanol (10 mL) under magnetically stirring in a flask. Subsequently, TEOS (3.12 mL) and HDTMS (3.12 mL) were added, which was ultrasonicated at 50 °C for 30 min. Then, water (14.4 mL) was added, and the flask was ultrasonicated at 50 °C for 1 h. After reacting under room conditions with magnetically stirring for 24 h, the product was obtained by centrifugation at 10,000 rpm for 10 min. The product was washed with 50 mL of ethanol and oven-dried at 60 °C for 4 h. PAL@POS/CVL was prepared by solid-state grinding the mixtures of a given amount of CVL and 0.5 g of PAL@POS for a period of time (2-30 min). The weight ratio of CVL to PAL@POS is designated as mCVL/mPAL@POS. Preparation of PAL@POS/CVL Coatings. The coatings were fabricated by spray-coating the uniform suspension of PAL@POS/CVL in toluene onto glass slides according to our previously reported method.15 PAL@POS/CVL (60 mg) was dispersed in toluene (4 mL), and then ultrasonicated at 25 °C for 30 min to form a uniform suspension. Finally, the PAL@POS/CVL coatings were obtained by spray-coating the suspension to the glass slides using an airbrush with a spraying pressure of 0.2 MPa N2 and a spraying distance of 15 cm. Measurements of Solvatochromic Properties. The PAL@POS/CVL coating was positioned upside down on a vial containing the given solvent at room temperature so that the coating was exposed to the vapor of the solvent. The reversible switching process between the colored and colorless

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states was video-recorded. The time span in each switching process and the degree of color change were observed using the videos. Characterization. The Fourier transform infrared (FTIR) spectra, scanning electron microscopy (SEM), transmitting electron microscopy (TEM), X-ray photoelectron spectra (XPS) and powder X-ray diffraction (XRD) analysis of the samples, and the CAs and SAs of water droplet (10 µL) were obtained according to our previously reported methods.15 Diffusive reflection UV-Vis (DRV) spectra of samples were collected on a UV-Vis-NIR spectrometer (PerkinElmer Lambda 950, USA).

RESULTS AND DISCUSSION Preparation of PAL@POS/CVL Coatings. PAL@POS/CVL was prepared by modification of PAL with HDTMS and TEOS, followed by solid-state grinding with CVL, a white powder (Figure 1). In the ethanol-water-ammonia solution, PAL was coated with a layer of POS.28, 29 Then, the mixture of PAL@POS and CVL was ground in an agate mortar. During grinding, the grey white mixture gradually became intense blue and PAL@POS/CVL was formed. Finally, the solvatochromic PAL@POS/CVL coatings with self-cleaning property were obtained via spray-coating.

Figure 1. Schematic illustration for preparation of the reversibly solvatochromic and self-cleaning PAL@POS/CVL coatings, and digital images of the corresponding samples.

Figure 2. FTIR spectra of (a) PAL, PAL@POS, CVL and PAL@POS/CVL, and (b) PAL@POS, CVL and PAL@POS/CVL in the range of 1340 to 1800 cm-1. mCVL/mPAL@POS = 5%, grinding time = 10 min. The reactions occurred in the formation of PAL@POS/CVL were studied by FTIR spectroscopy (Figure 2). In the spectrum of PAL@POS (Figure 2a), the bands at 2923, 2852 and 1468 cm-1 are attributed to the hexadecyl group, which indicates successful modification of PAL with POS. The characteristic absorption bands of POS and CVL (1350-1750 cm-1) were detected in the spectrum of PAL@POS/CVL. In addition, compared to the FTIR spectra of PAL@POS and CVL, a new


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band at 1587 cm-1 was observed in the spectrum of colored PAL@POS/CVL (Figure 2b), indicating that a new compound was formed in the process of solid-state grinding. It was reported that the lactone ring of CVL could be opened and the intense blue CVL+ species, sensitive to heat, could be formed by a weak acid or proton donor such as dilute HNO3 solutions and silica (Figure 3a).30, 31 Thus, this new band should be assigned to the asymmetric stretching vibration of the carboxylate groups of CVL+ in PAL@POS/CVL.31-33 PAL has a formula of [(OH2)4(Mg, Al,Fe,)5(OH)·2Si8O20]·4H2O.34 To further explore the origin of the intense blue color, CVL was ground with SiO2, Al2O3, CaO and MgO powders, respectively, under the same conditions. The order of color depth of the resulting hybrids is SiO2/CVL > Al2O3/CVL >> MgO/CVL ≈ CaO/CVL (Figure 3b), which is in accordance with the order of hydrogen bonding donating capability of the oxides.35, 36 In contrast to MgO and CaO, SiO2 and Al2O3 are rich in hydroxyl groups. The new absorption band at 1587 cm-1 can be clearly identified in the FTIR spectra of SiO2/CVL and Al2O3/CVL. Also, the intensity of the band of SiO2/CVL is higher. However, this peak cannot be detected in the FTIR spectra of MgO/CVL and CaO/CVL (Figure S1). The intensity of this band is consistent with the color depth of the resulting hybrids. This indicates that the break of the CVL’s lactone ring is largely affected by the hydrogen bonding donating capability of the oxides. Thus, the hydrogen bonding of the abundant hydroxyl groups of PAL@POS with the carboxylate groups of CVL+ is the origin of the intense blue color.

The surface chemical composition of PAL, PAL@POS and PAL@POS/CVL was analyzed using XPS (Figure S2). There are Si, C and O elements on the surface of all the samples. The integrated peak intensities showed an evident increase of the C/O atomic ratio from 0.46 for PAL to 3.55 for PAL@POS owing to the abundant hexadecyl groups of POS, which could efficiently reduce the surface energy. In addition, PAL@POS showed a very strong O 1s peak, which assured its interaction with CVL in the process of solid-grinding. The further increase of the C/O atomic ratio to 7.92 for PAL@POS/CVL is owing to the high C content of CVL. The surface modification with HDTMS and TEOS, and the subsequent solid-state grinding with CVL have no influence on the crystalline structure of PAL (Figure S3).

Figure 4. SEM images of (a) PAL, (b) PAL@POS and (c) PAL@POS/CVL, and (d) TEM image of PAL@POS/CVL. mCVL/mPAL@POS = 5%, grinding time = 10 min.

Figure 3. (a) Temperature-induced reversible switching between CVL and CVL+, and (b) images of SiO2/CVL, Al2O3/CVL, MgO/CVL and CaO/CVL. mCVL/mpowder = 10%, grinding time = 10 min. The changes in microstructures in the preparation of PAL@POS/CVL were observed using SEM and TEM as shown in Figure 4. The rod-like PAL crystals with high aspect ratio tightly packed together. The POS layer can be observed on POS/PAL and the aspect ratio of the PAL crystals decreased, indicating successful condensation of HDTMS and TEOS onto the PAL crystals. The surface roughness of the PAL@POS/CVL coating decreased compared to that of the PAL@POS coating because of introduction of CVL. In addition, some PAL crystals became shorter due to the solid-state grinding, which is also responsible for the decrease of the surface roughness. The TEM observation showed that PAL@POS/CVL is uniform with fairly well-distributed PAL crystals.

Solvatochromic Properties. The PAL@POS/CVL coating is reversibly and rapidly solvatochromic (Figures 1 and 5a-b, Movie S1). Once encountered the vapor of acetone, the color of the coating faded instantly within 7-11 s from intense blue to grey white, the original color of PAL@POS. Considering the origin of the intense blue color as mentioned above, it is acceptable to assume that discoloration of the coating takes place when the hydrogen bonding between CVL+ and PAL@POS is interrupted. For example, another molecule (acetone) with appreciable hydrogen bond accepting ability competitively participates in hydrogen bonding with the hydroxyl groups of PAL@POS. The discoloration of PAL@POS/CVL requires a much shorter time than those previously reported allochroic materials which often need over 10 min or even several hours in some cases.4, 37-40 Interestingly, the faded PAL@POS/CVL coating could completely recover its original color in air in 140 s, i.e. from grey white to intense blue (Figure 5b). The evaporation of acetone from the coating, i.e. the elimination of the competitive hydrogen bonding, restores the hydrogen bonding between PAL@POS and CVL+ (Figure 5c). Compared to the discoloration of the coating in the vapor of acetone, the recovery of the coating to the original intense blue color takes



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a longer time, but the recovery is very fast in the first 10 s. This means the reconstruction of the hydrogen bonding between PAL@POS and CVL+ is slower than interruption by the vapor of acetone. However, the recovery of the PAL@POS/CVL coating is still much faster than the other allochroic materials.41 Moreover, the solvatochromic behavior of the PAL@POS/CVL coating is highly reversible. After 100 cycles of the discoloration-recovery test, the DVR spectrum of the coating was virtually consistent with that of the original coating in the range of 500-700 nm (Figure 6a), indicating complete recovery of the colored species. Meanwhile, the discoloration time and recovery time of the coating are almost invariant during the test (Figure 6b).

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accepting ability. The third category is incapable of hydrogen bonding, hence shows little effect on the discoloration, in view of their non-polar property.43 The polar aprotic solvents like chloroform, hardly possessing any hydrogen bonding property, perform quite similarly to hydrocarbons.

Figure 7. Discoloration behavior of the PAL@POS/CVL coating in the vapor of different solvents. mCVL/mPAL@POS = 5%, grinding time = 10 min.

Figure 5. (a) Discoloration of the PAL@POS/CVL coating in the vapor of acetone and (b) recovery in air. (c) Schematic illustration for the reversible hydrogen bonding between PAL@POS and CVL+ induced by the vapor of acetone. mCVL/mPAL@POS = 5%, grinding time = 10 min.

The kind of solvents in the first and second categories also has great influences on the discoloration time and recovery time of the PAL@POS/CVL coating as shown in Table S1. A solvent with lower boiling point results in faster discoloration and recovery. The discoloration time and recovery time is highly dependent on volatility of the solvents in each category, which controls the reversible hydrogen bonding between PAL@POS and CVL+.

Figure 6. (a) DRV spectra and (b) solvatochromic properties of the PAL@POS/CVL coating in 100 cycles of the discoloration-recovery test induced by the vapor of acetone. mCVL/mPAL@POS = 5%, grinding time = 10 min. The degree of discoloration induced by the vapor of various solvents is distinctively different (Figure 7). The sequence of the degree of discoloration is ammonia > acetone > acetonitrile > tetrahydrofuran > methanol ≈ ethanol > acetic acid ≈ ethyl acetate >> chloroform ≈ diethyl ether ≈ tetrachloromethane ≈ toluene ≈ n-hexane. This difference is attributed to the polarity and the hydrogen bonding ability of the solvents with the hydroxyl groups of PAL@POS. The abovementioned solvents, except for ammonia, can be divided into the following categories: 1) polar aprotic solvents (acetone, acetonitrile and tetrahydrofuran) 42; 2) polar protic solvents (methanol, ethanol and acetic acid); and 3) non-polar solvents (diethyl ether, tetrachloromethane, toluene and n-hexane). The first category affords the most remarkable discoloration, on account of their inability to donate hydrogen bonding, but exceptionally strong hydrogen bond accepting ability. Ammonia also results in effective discoloration due to its alkalinity. The second category produces moderate discoloration, due to their strong hydrogen bonding donating capability, which hampers the corresponding hydrogen bond

Figure 8. (a) A 10 µL water droplet on the PAL@POS/CVL coating, (b) a 10 µL water droplet rolling off the 9.5° tilted coating, (c) a jet of water bounces off the coating, (d) the coating immersed in water and (e) self-cleaning behavior of the coating. mCVL/mPAL@POS = 5%, grinding time = 10 min. Superhydrophobicity. The PAL@POS/CVL coating is superhydrophobic. The spherical water droplets on the coating


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have a CA of 164.5° and an SA of 9.5° (Figure 8a-b). In addition, a water jet could bounce off without any trace (Figure 8c and Movie S2, parts 1-2). Also, a water drop from a height of 10 mm can bounce five times on the coating (Movie S2, part 3), implying that the interaction between them is very weak. Further, the coating is reflective in water and keeps dry (Figure 8d). The above phenomena indicate the presence of the air cushion at the solid-liquid interface. The PAL@POS/CVL coating also displays excellent self-cleaning property. Various kinds of powdered dirts including sand microparticles and dust at the surface of the coating could be effectively removed with water (Figure 8e and Movie S3).

Figure 9. Variation of (a) DRV spectra, (b) solvatochromic properties and (c) superhydrophobicity of the PAL@POS/CVL coatings with the mCVL/mPAL@POS. Grinding time = 10 min. Effects of Preparation Parameters on Color Depth, Solvatochromic Properties and Wettability. Color depth, solvatochromic properties and wettability of the PAL@POS/CVL coatings depend strongly on the preparation parameters, e.g., mCVL/mPAL@POS and the grinding time. An mCVL/mPAL@POS of 5% leads to the maximum absorbance of PAL@POS/CVL according to the DRV spectra, indicating the intensest blue color (Figure 9a). Once the employable binding sites of PAL@POS were completely consumed by CVL, the excessive CVL lightened the color of PAL@POS/CVL. The mCVL/mPAL@POS has no obvious influence on the discoloration time (in 7-11 s) as shown in Figure 9b. All the faded PAL@POS/CVL coatings could recover their original color in air regardless of the mCVL/mPAL@POS. However, the recovery time is highly dependent on the mCVL/mPAL@POS. A higher CVL content significantly slows down recovery of the color. This might be attributed to the retention of the vapor of solvents by the functional groups of CVL, as a result of various interactions such as hydrogen bonding and π-δ interaction.44 All the PAL@POS/CVL coatings are superhydrophobic (CA > 160°, SA < 17°) when the mCVL/mPAL@POS is in the range of 1% to 20% (Figure 9c). The SA is even less than 7° when the mCVL/mPAL@POS is not more than 7%. The surface wettability of a coating is highly dependent on its surface microstructure and chemical composition.45 The surface microstructure of the PAL@POS/CVL coatings with different mCVL/mPAL@POS has no obvious difference (Figure S4). Therefore, the slight increase in the SA is assumed to be owing to the changes in chemical composition as proved by XPS analysis (Figure S5). The intensity of the N 1s peak increases with increasing the

mCVL/mPAL@POS, implying a higher content of CVL on the surface of the coating. During grinding, the mixture changed from grey white to light blue, and became intense blue as the grinding continued. This is consistent with the variation of the DRV spectra of the samples (Figure S6a). A longer grinding time can greatly enhance the hydrogen bonding between PAL@POS and CVL, and then leads to PAL@POS/CVL with a higher color depth. A longer grinding time results in a slight increase of the discoloration time from 7 to 12 s (Figure 6b). Also, the recovery time increases to about 140 s with the increase of the grinding time to 10 min, and then exhibits small fluctuation with further increasing the grinding time to 30 min. However, excessive grinding undermines superhydrophobicity of the coatings as the SA gradually increases from 4° to 16° (Figure S6c). This is because the PAL crystals were damaged by grinding, and the surface roughness of the coatings decreases with the increase of the grinding time as shown in Figure S7.46, 47 In addition, new hydrophilic silanols of PAL are generated accompany damage of the PAL crystals, which should also be responsible for the decline of the superhydrophobicity. Durability of PAL@POS/CVL Coatings. Regarding durability of the PAL@POS/CVL coatings, some experiments were performed. After immersed in the 0.1 M HCl aqueous solution or 0.1 M NaOH aqueous solution for 1 h, no obvious change could be observed from the DRV spectra of the coating (Figure S8a). Also, the coatings remained superhydrophobic in spite of a slight decrease in the CAs and increase in the SAs (Figure S8b). The coatings are even resistant to wetting of concentrated acid and base. The droplets of 85% H3PO4 and 60% NaOH are spherical on the coatings (Figure S9). The coatings remained completely dry without any damage after the droplets rolling off the tilted coatings owing to the air cushion (Movie S4). Furthermore, the coating kept its superhydrophobicity in 100 cycles of the discoloration-recovery test. The CA was above 162° and the SA was in the range of 8° to 12° (Figure S10). The high durability of the PAL@POS/CVL coating is attributed to its superhydrophobicity and self-cleaning property, which is unusual for the allochroic materials. On the other hand, the high durability of the PAL@POS/CVL coating ensures the reversibly and rapidly solvatochromic properties.

CONCLUSIONS The solvatochromic PAL@POS/CVL coatings with self-cleaning property were successfully prepared for the first time by the combination of PAL@POS and CVL. CVL, a previously reported thermochromic dye, was newly found to be solvatochromic upon combining with PAL@POS. The hydrogen bonding between PAL@POS and CVL+ is the origin of the intense blue color. The interruption of the hydrogen bonding by the vapor of solvents results in rapid discoloration of the coating. On the other hand, the evaporation of the solvents from the coating results in complete recovery of the original color by restoring the hydrogen bonding. The polarity, hydrogen bonding ability and volatility of the solvents determine the solvatochromic properties of the coating. In addition, the color depth, solvatochromic properties and wettability of the PAL@POS/CVL coatings depend strongly on the preparation parameters, e.g., mCVL/mPAL@POS and the grinding time. The PAL@POS/CVL coatings feature high reversibility and rapid switching between the colored and colorless states induced by the vapor of various solvents. Also, the coatings are superhydrophobic with excellent self-cleaning property and high durability. We believe that the



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PAL@POS/CVL coatings can be used as a new generation of allochroic materials, and hold great potential for applications in many fields, e.g., information storage and security printing ink.

ASSOCIATED CONTENT Supporting Information FTIR spectra, XPS spectra, XRD patterns, SEM images, DRV spectra, solvatochromic properties, superhydrophobicity, images and videos. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], Phone: +86 931 4968251

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We are grateful for financial support of the “Hundred Talents Program” of the Chinese Academy of Sciences and the Key Technology R&D Program of Jiangsu (BE2014102).

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