Single-Layered Two-Dimensional MetalâOrganic Framework

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Functional Nanostructured Materials (including low-D carbon)

Single-Layered 2-D MOF Nanosheets as In-Situ Visual Test Paper for Solvents Yang-Hui Luo, Chen Chen, Chang He, Ying-Yu Zhu, Dan-Li Hong, Xiao-Tong He, Pei-Jing An, Hong-Shuai Wu, and Bai-Wang Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08739 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 28, 2018

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ACS Applied Materials & Interfaces

Single-Layered 2-D MOF Nanosheets as In-Situ Visual Test Paper for Solvents

This paper was dedicated to the 80th birthday of professor Dai-Zheng Liao

Yang-Hui Luo, * Chen Chen, Chang He, Ying-Yu Zhu, Dan-Li Hong, Xiao-Tong He, Pei-Jing An, Hong-Shuai Wu and Bai-Wang Sun*

School of Chemistry and Chemical Engineering, Southeast University, Nanjing, 211189, PR. China. E-mail: [email protected]; [email protected].

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Abstract: Through an facile-operating ultrasonic force-assisted liquid exfoliation technology, the single-layered 2-D [Co(CNS)2(pyz)2]n (pyz = pyrazine) nanosheets, with thickness of sub-1.0 nm, have been prepared from the bulk precursors. The atomically thickness and the presence of abundant sulfur atoms with high electronegativity arrayed on the double surfaces of the sheets, making this kind of 2-D MOF (metal-organic framework) nanosheets are highly sensitive to intermolecular interactions. As a result, it can be well dispersed in all kinds of solvent to give a stable colloidal suspension that can maintained for at least one month, accompanied by significant solvatochromic behavior and various optical properties, which thus have shown the potential to be practically applicated as in-situ visual test paper for solvent identification and solvent polarity measurements. More importantly, combined with a smartphone, this kind of 2D-MOF nanosheets can be developed into in-situ visual test paper to identify isomers and determine the polarity of mixed solvents quantitatively and qualitatively, suggesting the promising application of a portable, economical and in-situ visual test strategy in real-world. Keywords: 2-D nanosheets, MOF, exfoliation, application, solvent polarity, in-situ visual test paper.

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INTRODUCTION The investigations of ultrathin two-dimensional (2-D) nanosheets beyond graphene have attracted great attentions in recent years, attributing to their unique properties and promising functionalities derived from their atomically thickness and 2-D morphology, when compared with their bulk counterparts.1-6 Unlike the well-studied 2-D nanomaterials, such as graphitic carbon nitride (g-C3N4),7 transition metal dichalcogenides (TMDs),8 hexagonal boron nitride (h-BN),9,10, layered metal oxides and hydroxides (LDHs),11,12 and black phosphorus (BPs),13 the 2-D metal-organic framework (MOF) nanosheets have emerged as a new competitive member of the 2D family owing to their adjustable structure and functionality, highly ordered pore arrays in plane, and highly accessible active sites on their large surface,14-20 which thus have shown great potential for real-world applications in surface-active related fields, such as sensing platforms,21,22 catalysis,23,

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gas separations,4,13,25 and so

on.24,27 However, despite the significant efforts have been performed, only a handful example of 2D-MOF nanosheets have been investigated, including [Cu2Br(IN)2]n (IN = isonicotinato),14 CuBDC (BDC = 1,4-benzenedicarboxy late), M-TCPP (M = Zn, Cu, Cd, Co, TCPP = tetrakis(4-carboxyphenyl) porphyrin),28 poly[Zn2(benzimidazole)4]4 and poly[Zn2 (benz-imidazole)3(OH) (H2O)].29 The current investigation on ultrathin 2D-MOF nanosheets is far from mature, the main limitations go to:1,17 a). The development of convenient strategy for high-yield and massive production; b). The improvement of

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stability in liquid solution, in ambient conditions and during applications; c). The seek for the most suitable application field for each ultrathin 2D-MOF nanosheets. Hence, searching for the most effective preparation strategies with high-yield and well-stability, then seeking for the most suitable application field, is thus of fundamental importance for the development of ultrathin 2D-MOF nanosheets. Solvent polarity, which refers to the separation of positive and negative charge centers of solvent molecules and measured as dipole moments, is an important constant for solvents.30 It is arguably that solvent polarity belongs to one of the most important factors that has profound impact on various chemical and material fabricating processes.31,32 For instances, on the one hand, solvent polarity triggering the conformational switching of π-conjugated junction;33 affecting the twisted intramolecular charge transfer (TICT) in donor-acceptor

molecules;34

determining

the

rotation

types

of

photo-switches,35 quantum yield photochromic reactions,36 as well as lifetimes of luminescence.37 The above-mentioned phenomenon can be classified as solvato-chromic behavior which originated from the presence of various dipole moment-dependent molecular interactions. Control of these processes through accurate choose of solvent polarity can be viewed as solvent engineering strategy.38, 39 On the other hand, solvent polarity controlling the nucleation and growth rate, as well as the collision interactions between primary nucleation centers,40 which thus can modify the final functionality and

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morphology

of

nanostructures

through

the

so-called

solvent-polarity-engineering.41-43 However, despite the sound functions, no methods have been reported for rapid measure the polarity of solvent, especially for the mixed solvents, let alone the correlation between the solvent polarity and functionalities. Hence, the development of a convenient method to accurate measure the solvent polarity, and further correlate the dipole moments with specific chemical and material fabricating processes is thus of significant importance for guiding the chemical reactions and designing the desired nanomaterials. Here in this work, we demonstrate that the 2-D [Co(CNS)2(pyz)2]n (pyz = pyrazine) nanosheets with thickness of sub-1.0 nm, which were obtained via an facile-operating top-down method from the layered bulk Co(CNS)2(pyz)2 precursors, can be developed into visual test papers that have shown promising application for in-situ solvent identification and solvent polarity measurements. Note that, this ambition was implemented through in-situ monitoring by a smartphone of the specific solvato-chromic behavior and optical properties of this 2D-MOF nanosheets responded to solvent polarity. More importantly, this 2D-MOF nanosheets based test paper can be applicated for rapidly identification of isomers and determine the polarity of mixed solvents. It is thought that, the atomically thickness and the abundant sulfur atoms with high electronegativity arrayed on the double surfaces of the sheets, are primary responsible for the highly stability and highly sensitive of

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this single-layered 2D-MOF nanosheets to solvents with various polarity. To the best of our knowledge, this is the first report of single-layered 2D-MOF nanosheets as portable, economical and in-situ visual test paper for solvents.

Figure 1. Crystal structure of layered bulk Co(CNS)2(pyz)2 precursor: (a) the molecular structure of the Co(CNS)2(pyz)4 unit; (b) and (c) the connecting style of the 2-D single-layered structure viewed from different direction, the thickness of it was highlighted; (d) the stacking style of the 2-D layers into 3-D framework; and (e) charge distribution of the Co(CNS)2(pyz)4 unit. 6


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Results Preparation and characterization of the single-layered 2-D MOF nanosheets. The pink-colored material of bulk Co(CNS)2(pyz)2 precursor, which was initially synthesized by Jacobson et al.,44 exhibits layered crystalline structure and shows potential to be exfoliated into ultrathin 2D nanosheets. In the 2-D layered structure, each Co ion is coordinated by four pyz ligands located at the equatorial direction and two CNS- ions along the trans axial positions (Figure 1a), and each pyz ligand is connected by two Co metallic nodes with a bis-monodendate connectivity, forming a square-like structure within the 2-D layer (Figure 1b). For each single-layered nanosheet, the CNS groups were arrayed centrosymmetric as “baffles” along the two sides of nanosheet, with SJS distance of 9.476 Å (Figure 1c and Figure S1a). Note that, this “baffles” configuration lead to an JABABJ stacking geometry of the 2-D nanosheets along the (110) crystallographic direction, with “baffles” from adjacent layers arrayed interlaced (with distances of 4.218 Å) within the clearance of nanosheets, giving rise to a 3-D (three-dimensional) porous framework (Figure 1d and Figure S1b). To be exfoliated into ultrathin 2D nanosheets through top-down strategy, one should break the intermolecular interactions (hydrogen bonding, pi-pi, van der Waals forces, etc.) between adjacent layers. Thus, the weaker the interlayer interactions within 3-D structure, the easier to be exfoliated into 2-D nanosheets.17 Note that, the interlayer interactions within bulk Co(CNS)2(pyz)2

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precursors were dominated by sulfur atoms, including S-S, S-N, S-C and S-H contacts. 3-D Hirshfeld surface and 2-D fingerprint plot analysis 45-47 (Figure S2) revealed longer interlayer interactions exactly the van der Waals separation, and only 2.3, 5.6, 17.5 and 14.8% contributions of these sulfur atoms dominated contacts to the total Hirshfeld surfaces of the [Co(CNS)2(pyz)4] unit. In addition, charge distribution calculations and zeta potential measurements (-32.92 mV) have demonstrated strong electronegativity of the 2-D sheets (Figure 1e). These characteristics endow a potential facile top-down exfoliation strategy to disintegrate the bulk Co(CNS)2 (pyz)2 precursors into single-layered nanosheets. By using an ultrasonic force-assisted liquid exfoliation technology in the ethanolic solution (please see the Methods section), for the first time, the single-layered 2-D

[Co(CNS)2(pyz)2]n nanosheets

were obtained, the

green-colored powdered samples of 2-D nanosheets were collected by rotary evaporation (Figure 2a). A fine-dispersed colorless colloidal suspension with significant Tyndall effect (Figure 2b) was observed after ultrasonic the pink-colored crystalline bulk Co(CNS)2(pyz)2 precursors for about 2h and followed by sedimentation of the unexfoliated particles for at about one week. Note that, this obviously pink to green color change of the solid-state materials (Figure 2a), as well as the colorless ethanolic colloidal suspension, have demonstrated that: on the one hand, the successful exfoliation of 2-D MOF nanosheets; on the other hand, the highly sensitive response of optical

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properties (Figure S3a) for the 2-D MOF nanosheets to intermolecular interactions (including interlayer contacts and interactions with solvent molecules). More importantly, this ethanolic colloidal suspension can be maintained for at least one month without any sedimentation, indicating the excellent stability of this kind ultrathin 2D-MOF nanosheets.

Figure 2. (a) An image of the powder samples of bulk Co(CNS)2(pyz)2 precursor and 2-D [Co(CNS)2(pyz)2]n nanosheets; (b) The Tyndall effect of a ethanolic colloidal suspension; (c) SEM image of bulk Co(CNS)2(pyz)2 precursors; (d) and (e) TEM images of the 2-D MOF nanosheets. To further demonstrate the successful preparation of ultrathin nanosheets and reveal the exfoliation mechanism, the bulk precursors and the obtained 2-D [Co(CNS)2(pyz)2]n nanosheets were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atom force microscopy (AFM), powder X-ray diffraction (PXRD) and thermos-gravimetric 9



Figure 3. (a) AFM topological image and (b) height profile of a single-layered nanosheet along the white line; Comparison between the (C) PXRD patterns and (D) TGA profiles of the bulk precursor and the 2-D MOF nanosheets. analysis (TGA). The SEM images of the bulk precursors demonstrated the lamellar structures, with nanosheets are stacked together via weak van der Waals forces into block 3-D morphology (Figure 2c and Figure S4a), indicating the potential to obtained ultrathin 2-D nanosheets with large lateral area via top-down exfoliation. TEM images of the 2-D MOF nanosheets have proved the aforementioned speculation, Figure 2d-e and Figure S4b revealed the random stacked giant ultrathin nanosheet with unambiguous outlines and

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somewhat curling edges, this randomly stacking has resulted apparent contrast, which can be attribute to the overlapping of nanosheets during the gradual evaporation of the solvent on porous TEM grid at room temperature. This phenomenon suggested the highly flexible and easily stacking of these ultrathin 2D-MOF nanosheets upon removal of media solvents. The morphology of ultrathin 2D-MOF nanosheets was further confirmed by AFM topological image (Figure 3a), where, a giant ultrathin nanosheet with lateral area about 0.5 µm was observed. What’s more, the height profile of this nanosheets displayed a thickness of sub-1.0 nm (Figure 3b), corresponding to a single-layered nanosheet as verified by the crystal structure analysis (the SJS distances along the trans axial positions is found to be 9.476 Å). The successfully exfoliation of the layered bulk Co(CNS)2(pyz)2 precursors were also demonstrated by PXRD measurements (Figure 3c), where, the (110) crystallographic planes was disappeared completely for the 2-D nanosheets, indicating the layered-by-layered exfoliation mechanism from the bulk precursors to single-layered nanosheets, and the re-stacking of nanosheets during rotary evaporation also completely avoided the (110) planes. This phenomenon can be interpreted as follows: during the exfoliation process, the ethanolic solvent was intercalated into the interlayer space with the help of ultrasonic force, and then absorbed solidly onto the surfaces of single-layered nanosheets to prevent the restacking back to bulk precursors. It is thought that, the absorption of solvent on the surfaces of single-layered nanosheets was too

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solidly to be removed by rotary evaporation, as have been demonstrated by the 9% mass-loss in temperature range 60-80 ºC (Figure 3d) upon heating the powder samples of 2-D nanosheets, which can be assigned to removal of ethanol molecules. One thing should be stressed that, during the rotary evaporation process, the single-layered 2-D nanosheets were re-stacked randomly, which give birth to the new crystal faces in the powdered 2-D samples. All the aforementioned results have proved the successfully preparation of single-layered 2-D MOF nanosheet through the facile ultrasonic force-assisted liquid exfoliation technology, a strategy that can be applied to prepare various kinds of 2-D MOF nanosheets of this series.

Figure 4. (a) Color display of the colloidal suspension in the selected nonpolar, protonic and polar solvents taken by a smartphone; (b): Tyndall effect and (c)-(e): UV-vis absorption spectra of the colloidal suspension in the selected three different kinds of solvents.

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Solvent identification. From the above analysis, the atomically thickness and the abundant sulfur atoms with high electronegativity arrayed on the double surfaces, may making all kinds of solvents to be absorbed on the surfaces and endow the nanosheets with tunable optical properties. As we have expected, this single-layered 2-D nanosheets can be suspended in all kinds of solvent with significant Tyndall effect (Figure 4a, b), demonstrating its excellent dispersion ability. Note that, each solvent has shown a unique effect on the color and optical properties of these colloidal suspensions, as have been revealed by the UV-vis absorption (Figure 4c-e) and fluorescence spectroscopy (Figure S5). There different kinds of solvents have been investigated. For the nonpolar solvents (hexane, benzene, toluene, etc.), the colloidal suspensions displayed green-colors which were similar with that of the solid-state 2-D MOF nanosheets (Figure 4a), indicating the slight influence of nonpolar solvents on the electronic state within the 2-D MOF nanosheets. However, despite the similar color, the different suspensions shown various absorption properties. Compared with absorption properties of raw materials Co(CNS)2, pyz, and bulk Co(CNS)2(pyz)2 precursors (Figure S3b), the π → π* transition bands for the colloidal suspensions were red-shifted with the solvent polarity (Figure 4c and Table 1). While for the polar solvents (DMSO, DMF, Acetone, CH2Cl2, etc.), all these colloidal suspensions displayed the similar bright blue-colors, which may attribute to the d-d transition of the Co2+ center within the 2-D sheets induced by dipole moments of these solvents. Again,

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each colloidal suspension shown unique absorption properties despite the similar colors (Figure 4d and Table 1), suggesting that this kind of single-layered 2-D MOF nanosheets can provide unique absorption spectra for each solvent, that have the potential to be practically applicated for solvent identify. Table 1. Summary of the solvent polarity, absorption peak and emission peak of the colloidal suspensions in the selected solvents. Solvent

Solvent polarity

Absorption peak (nm)

Emission peak(nm)

H2O

10.2

260

296

346

MeOH

6.6

260

292

340

EtOH

6.2

260

291

359

n-propanol

4

260

290

354

465

i-propanol

4.3

318

351

467

DMSO

7.2

262

DMF

6.4

265

Acetone

5.4

CH2Cl2

3.4

Benzene

3.0

Toluene

2.4

Hexane

0.06

360

270

280

310

356

329

416

468

324

380

465

330

362

324

420

275

410

We further tried the protonic solvents (H2O, MeOH, EtOH). Interestingly, white or light-pinked colors were observed (Figure 4a). Note that, this phenomenon can be attribute to the strong O-HJS hydrogen bonding contacts between the protonic solvents and CNS- groups on the surfaces of nanosheets. It is thought that, upon the hydrogen bonding contacts, the electron density within the 2-D MOF nanosheets is transferred to the 14


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hydrogens of the protonic solvents, resulting the reduction π → π* and d-d transition. In addition, this conclusion can be verified by the UV-vis absorption and fluorescence spectroscopy (Figure 4e and Figure S5), where, the π → π* and d-d transition band, as well as the fluorescence emission intensity were decreased significantly with the increasing of solvent polarity, demonstrating the sensitive response of this single-layered 2-D MOF nanosheets to solvent polarity. Thus, an effective sensing platform based on this 2-D MOF nanosheets for solvent polarity measurements can be expected. Isomers identification. The aforementioned results have demonstrated the effectiveness of this single-layered 2-D MOF nanosheets as sensing platform for solvents identify and solvent polarity measurements. These encouraging results then inspired us to explore the possibility of this 2-D MOF nanosheets to identify the components of mixed solvents, or even the isomers. The isomers n-propanol and i-propanol were then selected. To our surprise, in

n-propanol solution, this colloidal suspension displayed white color (Figure 5a) with π → π* and d-d transition band at 261 and 294 nm, and fluorescence emission at 355 nm, respectively. While in i-propanol solution, the colloidal suspension displayed bright-blue color (Figure 5a) with d-d transition band at 318 nm and fluorescence emission at 352 and 457 nm, respectively (Figure 4e, Figure S5 and Table 1), demonstrating the effectiveness of this 2-D MOF nanosheets for isomers identification. To further reveal the identification mechanism for isomers, variation of UV-vis absorption spectroscopy for mixed

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solvents by adding i-propanol into n-propanol with stoichiometry (v/v) from 5:0, 4.5:0.5J to 0:5 (Figure 5b), and in-turn by adding n-propanol into i-propanol with stoichiometry (v/v) from 5:0, 4.5:0.5J to 0:5 (Figure 5c), have been

Figure 5. (a) Color display of the colloidal suspension in n-propanol and i-propanol taken by a smartphone; (b) Variation of the UV-vis absorption spectroscopy for the colloidal suspension in mixed solvents by adding i-propanol into n-propanol and (c) by adding n-propanol into i-propanol; (d) The components identify curves for n-propanol/i-propanol mixed solvents. investigated. For both the cases, in the early stage, adding of the counterisomer induced the reduction of the character absorption band of the original

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isomer, until disappear. Then, the character absorption band of the added counter-isomer has been generated and increased with continue addition. Thus, the components identify curves for n-propanol/i-propanol mixed solution can be figure out (Figure 5d). These results indicating that: on the one hand, the different solvents have shown distinct interactions with this 2-D MOF nanosheets, even for isomers; on the other hand, the polar and/or protonic solvents can be absorbed firmly on the surfaces of nanosheets, which then changed the electronic state of the nanosheets to various extent and displayed different optical properties. Thus, the practical application of this 2-D MOF nanosheets as a rapid, convenient, economical and in-situ visual test paper for mixed solvents can be expected. In-situ Visual Test paper for solvents identification and solvent polarity measurements. To demonstrate the aforementioned prospects of this 2-D MOF nanosheets for practical application, paper strips coated with this single-layered 2-D MOF nanosheets as the test paper for solvents identification and solvent polarity measurements were developed, through the analysis of solvato-chromic behavior by using a smartphone equipped with color-scanning APP. This kind of test paper strips were prepared by infiltrating the filter paper strips into the ethanolic suspension of 2-D [Co(CNS)2(pyz)2]n nanosheets for one day, during this time, the 2-D MOF nanosheets were absorbed onto the filter paper. Then the 2-D MOF nanosheets coated paper strips were dried in an oven under 80°C for 1 h. After that, the

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light-green-colored test papers were obtained. Identification of solvents was operated by dropping two or three drops of solvent onto the test paper squares, the latter were placed on a spot plate separately under ambient condition (Figure 6). Immediately color change of the test paper squares was observed by naked eyes, and the specific color intensity was then scanned by a smartphone, the unique red-green-blue (RGB) intensity of each photos was thus calculated by a color-scanning APP. And finally, the G/B ratio of each color was corelated with the solvent polarity.

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Figure 6. Scheme illustration of the in-situ visual solvent identification and solvent polarity measurements by using 2-D MOF nanosheets coated test papers combined with a smartphone. The color change on the test paper was scanned by the smartphone, the G/B value of each photos was thus correlated with the solvent polarity. As shown in Figure 6, the 2-D [Co(CNS)2(pyz)2]n nanosheets coated test paper squares showed sensitive response to solvents with different polarity. As is evident by the immediately color switching of the test paper squares from light-green to white, pink, blue and green, upon exposure with H2O, MeOH, EtOH, n-propanol, DMSO, DMF, Acetone, CH2Cl2, benzene and toluene, respectively. The G/B ratio of each color was then calculated and correlated with the polarity of solvents. As a result, a calibration linearity curve of G/B = 1.260 – 0.023 [solvent polarity] has been obtained within solvent polarity range from 2.4 to 10.2. Hence, combined with a smartphone, a portable, economical and visual test papers have been developed for in-situ solvent identification and solvent polarity measurements application.

Conclusions In this work, we have proposed an easy operating and robust preparation strategy to produce the single-layered 2-D MOF nanosheets from bulk Co(CNS)2(pyz)2 precursors with high yield. The obtained 2-D nanosheets have act as excellent platforms for intermolecular interactions, which have shown significant solvatochromic behavior and unique optical properties for different 19



solvents. Combined with a smartphone, the present 2-D MOF nanosheets can be practically applicated as in-situ visual test paper for solvent identification and solvent polarity measurements. On considering the simple preparation method; the low cost and non-toxic of the raw materials; and the rapid scanning process, the present work has developed a portable, economical and in-situ visual test strategy, which has broadened the application of 2D MOFs nanosheets. Inspired by these attractive results, visual detection of toxic cations, anions, or other noxious chemicals by using this single-layered 2-D MOF nanosheets will be performed in our further work.

Experimental Section General considerations. All syntheses were performed under ambient conditions and all the chemicals were of analytical grade and used without further purification. The raw materials for bulk precursors Co(CNS)2 and pyrazine were purchased from Sigma Aldrich and all the used solvents were commercially available from Sinopharm Chemical Reagent Co. Ltd (China). Characterization. The morphologies of the 2-D MOF nanosheets were characterized by using a Field emission scanning electron microscope (FE-SEM, HITACHI S-4800 20 kV), transmission electron microscope (TEM; Tecnai-G2 20 E-TWIN 200 KV) and atom force microscopy (AFM, Cypher, Asylum Research). Before these microscope characterization, the ethanolic suspension of 2-D MOF nanosheets was dropping onto the holey

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carbon-coated carbon support copper grids, Si/SiO2, and piranha-cleaned Si/SiO2, respectively, and then naturally dried. The chemical composition of bulk precursors and 2-D MOF nanosheets were characterized using X-Ray powder diffractions (PXRD) on an Ultima IV diffractometer equipped with Cu Kα radiation (λ = 1.5418 Å) in the range 5-50°at room temperature. UV-vis absorption and the fluorescence spectra of the colloidal suspensions were recorded with a Shimadzu UV-3150 double-beam spectrophotometer and a Horiba Fluoro-Max4 Spectro-fluorometer, respectively. Thermogravietric analysis (TGA) of bulk precursors and 2-D MOF nanosheets were performed by using a Mettler-Toledo TGA/DSC STARe System at a heating rate of 10K min-1, under an atmosphere of dry N2 flowing at a rate of 20 cm3min-1 over a temperature range from 50 °C to 800 °C. Preparation of bulk Co(CNS)2(pyz)2 precursors. Bulk precursors were synthesized according to literature.40 In a typical procedure, a methanol solution (12 mL) of pyrazine (0.405 g, 5.0 mmol) was added slowly into an aqueous solution (8 mL) of Co(NCS)2 (0.434 g, 2.5 mmol) with continuous stirring. After stirred for about 1h, pink-colored solids were formed, which was separated by suction filter and washed with methanol before dried vacuum. The typical yield was found to be 86% based on Co(NCS)2, and the purity of them was checked by X-ray powder diffraction. Preparation of the single-layered 2-D MOF nanosheets by using an ultrasonic force-assisted liquid exfoliation technology. In a typical 21



experiment, 15 mg of bulk Co(CNS)2(pyz)2 precursors were dispersed in 30 mL ethanol. The mixture was sonicated in an ultrasonic bath (Brandson, CPX2800H-E, 110 W, 40 KHz) for 30 mins, then the obtained suspension was kept vigorous stirring overnight. After that, the colloidal suspension was let standing for one week. The upper colloidal suspension of the exfoliated 2-D [Co(CNS)2(pyz)2]n nanosheets was collected by centrifugate to remove the sedimentation of bulked samples. The powdered samples of 2-D nanosheets was obtained by rotary evaporation of the colloidal suspension at room-temperature. Preparation of the in-situ visual test paper for solvents identification solvent polarity measurements. Filter paper strips was used to prepare the desired in-situ visual test paper. In a typical procedure, this kind of in-situ visual test paper strips were prepared by infiltrating the filter paper strips into the ethanolic colloidal suspension of 2-D [Co(CNS)2(pyz)2]n nanosheets (0.2 mg/mL) for one day, then, the 2-D nanosheets coated paper strips were taken out and dried in an oven under 80°C for 1 h. After that, the light-green-colored test papers were obtained. Before solvents identification solvent polarity measurements, the obtained 2-D nanosheets coated paper strips were tailored into small squares, the latter were placed in the spot plate separately. Then, the solution with different polarity was dropped onto the small squares. The rapidly color change of the small squares can be observed by naked eyes in ambient condition, and the color photos were scanned by a smartphone 22


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equipped with a color-scanning APP. Thus, the G/B value (RGB color model) of each specific color can be quickly calculated, and its correlation with solvent polarity can be quickly obtained by using the calculator in the smartphone. As a result, a portable, economical and in-situ visual test papers for solvents identification solvent polarity measurements have been developed. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional crystal structure of bulk Co(CNS)2(pyz)2 precursors, 3-D Hirshfeld surface and 2-D fingerprint plot analysis of the [Co(CNS)2 (pyz)4] unit, additional SEM images of bulk Co(CNS)2(pyz)2 precursors and TEM images of single-layered 2-D MOF nanosheets, UV-vis absorption spectra of Co(CNS)2, pyz, the bulk Co(CNS)2(pyz)2 precursors and 2-D MOF nanosheets both in solution and in solid state, the fluorescence spectroscopy of colloidal suspensions in all kinds of solvents.

Notes The authors declare no competing financial interest.

Acknowledgements

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This research was supported by the Natural Science Foundation of China (Grant No. 21701023), Natural Science Foundation of Jiangsu Province (Grant No. BK20170660), Fundamental Research Funds for the Central Universities (No.3207048427) and PAPD of Jiangsu Higher Education Institutions.

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