Single-Layered Two-Dimensional MetalâOrganic Framework

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 28860−28867

Single-Layered Two-Dimensional Metal−Organic Framework Nanosheets as an 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* School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, P. R. China

ACS Appl. Mater. Interfaces 2018.10:28860-28867. Downloaded from pubs.acs.org by 79.133.106.153 on 09/01/18. For personal use only.

S Supporting Information *

ABSTRACT: Through a facile-operating ultrasonic force-assisted liquid exfoliation technology, the single-layered two-dimensional (2D) [Co(CNS)2(pyz)2]n (pyz = pyrazine) nanosheets, with a 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 are making this kind of 2D MOF (metal−organic framework) nanosheets highly sensitive to intermolecular interactions. As a result, it can be well dispersed in all kinds of solvents to give a stable colloidal suspension that can be 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: 2D nanosheets, MOF, exfoliation, application, solvent polarity, in situ visual test paper

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INTRODUCTION The investigations of ultrathin two-dimensional (2D) nanosheets beyond graphene have attracted great attentions in recent years, attributing to their unique properties and promising functionalities derived from their atomically thickness and 2D morphology, when compared with their bulk counterparts.1−6 Unlike the well-studied 2D nanomaterials, such as graphitic carbon nitride (g-C3N4),7 transition-metal dichalcogenides,8 hexagonal boron nitride (h-BN),9,10 layered metal oxides and hydroxides,11,12 and black phosphorus,13 the 2D 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 realworld applications in surface-active related fields, such as sensing platforms,21,22 catalysis,23,24 gas separations,4,13,25,26 and so on.24,27 However, despite the significant efforts have been performed, only a handful example of 2D-MOF nanosheets has been investigated, including [Cu2Br(IN)2]n (IN = isonicotinato),14 CuBDC (BDC = 1,4-benzenedicarboxy late), M-TCPP (M = Zn, Cu, Cd, and Co, TCPP = tetrakis(4carboxyphenyl) 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 © 2018 American Chemical Society

convenient strategy for high yield and massive production; (b) the improvement of 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 and then seeking for the most suitable application field are 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 arguable 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 in donor−acceptor molecules;34 determining the rotation types of photoswitches,35 quantum yield photochromic reactions,36 and lifetimes of luminescence.37 The above-mentioned phenomenon can be classified as solvatochromic behavior which originated from the presence of various dipole moment-dependent molecular interactions. Received: May 26, 2018 Accepted: July 26, 2018 Published: July 26, 2018 28860

DOI: 10.1021/acsami.8b08739 ACS Appl. Mater. Interfaces 2018, 10, 28860−28867

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ACS Applied Materials & Interfaces Control of these processes through accurate choose of solvent polarity can be viewed as a solvent engineering strategy.38,39 On the other hand, solvent polarity controls the nucleation and growth rate, as well as the collision interactions between primary nucleation centers,40 which thus can modify the final functionality and morphology of nanostructures through the so-called solvent polarity engineering.41−43 However, despite the sound functions, no methods have been reported to rapid measure the polarity of the solvent, especially for the mixed solvents, let alone the correlation between the solvent polarity and functionalities. Hence, the development of a convenient method to accurately 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 2D [Co(CNS)2(pyz)2]n (pyz = pyrazine) nanosheets with a thickness of sub-1.0 nm, which were obtained via a facile-operating topdown 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 solvatochromic behavior and optical properties of these 2D-MOF nanosheets responded to solvent polarity. More importantly, this 2D-MOF nanosheet-based test paper can be applicated for rapid 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 high stability and high sensitivity of this single-layered 2D-MOF nanosheets to solvents with various polarities. 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) molecular structure of the Co(CNS)2(pyz)4 unit; (b) and (c) connecting style of the 2D single-layered structure viewed from different directions, the thickness of it was highlighted; (d) stacking style of the 2D layers into three-dimensional (3D) framework; and (e) charge distribution of the Co(CNS)2(pyz)4 unit.

into 2D nanosheets.17 Note that the interlayer interactions within bulk Co(CNS)2(pyz)2 precursors were dominated by sulfur atoms, including S−S, S−N, S−C, and S−H contacts. 3D Hirshfeld surface and 2D fingerprint plot analysis45−47 (Figure S2) revealed that 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 2D 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 Experimental Section), for the first time, the single-layered 2D [Co(CNS)2(pyz)2]n nanosheets were obtained; the greencolored powdered samples of 2D nanosheets were collected by a rotary evaporator (Figure 2a). A fine-dispersed colorless colloidal suspension with a significant Tyndall effect (Figure 2b) was observed after ultrasonic the pink-colored crystalline bulk Co(CNS)2(pyz)2 precursors for about 2 h and followed by sedimentation of the unexfoliated particles for about one week. Note that this obvious pink to green color change of the solid-state materials (Figure 2a) and the colorless ethanolic colloidal suspension have demonstrated that: on the one hand, the successful exfoliation of 2D MOF nanosheets; on the other hand, the highly sensitive response of optical properties (Figure S3a) for the 2D MOF nanosheets to intermolecular interactions (including interlayer contacts and interactions

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RESULTS Preparation and Characterization of the SingleLayered 2D MOF Nanosheets. The pink-colored material of bulk Co(CNS)2(pyz)2 precursor, which was initially synthesized by Jacobson et al.,44 exhibits a layered crystalline structure and shows potential to be exfoliated into ultrathin 2D nanosheets. In the 2D 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 bismonodendate connectivity, forming a square-like structure within the 2D layer (Figure 1b). For each single-layered nanosheet, the CNS groups were arrayed centrosymmetric as “baffles” along the two sides of nanosheet, with a S···S distance of 9.476 Å (Figures 1c and S1a). Note that this “baffle” configuration leads to an ...ABAB... stacking geometry of the 2D nanosheets along the (110) crystallographic direction, with “baffles” from adjacent layers arrayed interlaced (with a distance of 4.218 Å) within the clearance of nanosheets, giving rise to a 3D porous framework (Figures 1d and S1b). To be exfoliated into ultrathin 2D nanosheets through the 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 the 3D structure, the easier to be exfoliated 28861


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Figure 2. (a) Image of the powder samples of bulk Co(CNS)2(pyz)2 precursor and 2D [Co(CNS)2(pyz)2]n nanosheets; (b) Tyndall effect of an ethanolic colloidal suspension; (c) SEM image of bulk Co(CNS)2(pyz)2 precursors; and (d,e) TEM images of the 2D MOF nanosheets.

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 2D MOF nanosheets.

nanosheets that are stacked together via weak van der Waals forces into a block 3D morphology (Figures 2c and S4a), indicating the potential to obtain ultrathin 2D nanosheets with large lateral area via top-down exfoliation. TEM images of the 2D MOF nanosheets have proved the aforementioned speculation; Figures 2d,e and S4b reveal the random stacked giant ultrathin nanosheet with unambiguous outlines and somewhat curling edges; this random stacking has resulted apparent contrast, which can be attributed to the overlapping of nanosheets during the gradual evaporation of the solvent on the porous TEM grid at room temperature. This phenomenon suggested the high flexibility and easy stacking of these

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 of ultrathin 2D-MOF nanosheets. To further demonstrate the successful preparation of ultrathin nanosheets and reveal the exfoliation mechanism, the bulk precursors and the obtained 2D [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 thermogravimetric analysis (TGA). The SEM images of the bulk precursors demonstrated the lamellar structures, with 28862


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

ultrathin 2D-MOF nanosheets upon removal of media solvents. The morphology of ultrathin 2D-MOF nanosheets was further confirmed by the AFM topological image (Figure 3a), where a giant ultrathin nanosheet with a lateral area about 0.5 μm was observed. What is more, the height profile of these 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 S···S distances along the trans axial positions are found to be 9.476 Å). The successful exfoliation of the layered bulk Co(CNS)2(pyz)2 precursors was also demonstrated by PXRD measurements (Figure 3c), where the (110) crystallographic planes were disappeared completely for the 2D nanosheets, indicating the layered-by-layered exfoliation mechanism from the bulk precursors to singlelayered nanosheets, and the restacking 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 solid to be removed by rotary evaporation, as have been demonstrated by the 9% mass loss in temperature range 60−80 °C (Figure 3d) upon heating the powder samples of 2D nanosheets, which can be assigned to the removal of ethanol molecules. One thing should be stressed that during the rotary evaporation process, the single-layered 2D nanosheets were restacked randomly, which give birth to the new crystal faces in the powdered 2D samples. All of the aforementioned results have proved the successfully preparation of single-layered 2D MOF nanosheet through the facile ultrasonic force-assisted liquid exfoliation technology, a strategy that can be applied to prepare various kinds of 2D MOF nanosheets of this series. Solvent Identification. From the above analysis, the atomically thickness and the abundant sulfur atoms with high

electronegativity arrayed on the double surfaces may make 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 2D nanosheets can be suspended in all kinds of solvents with a 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). These different kinds of solvents have been investigated. For the nonpolar solvents (hexane, benzene, toluene, etc.), the colloidal suspensions displayed green colors which were similar to that of the solid-state 2D MOF nanosheets (Figure 4a), indicating the slight influence of nonpolar solvents on the electronic state within the 2D MOF nanosheets. However, despite the similar color, 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), whereas for the polar solvents (DMSO, DMF, acetone, CH2Cl2, etc.), all of these colloidal suspensions displayed the similar bright blue colors, which may attribute to the d−d transition of the Co2+ center within the 2D sheets induced by dipole moments of these solvents. Again, each colloidal suspension shown unique absorption properties despite the similar colors (Figure 4d and Table 1), suggesting that this kind of single-layered 2D MOF nanosheets can provide unique absorption spectra for each solvent that have the potential to be practically applicated for solvent identification. We further tried the protonic solvents (H2O, MeOH, and EtOH). Interestingly, white or light-pinked colors were observed (Figure 4a). Note that this phenomenon can be attributed to the strong O−H···S hydrogen-bonding contacts between the protonic solvents and CNS− groups on the surfaces of nanosheets. It is thought that upon the hydrogen28863


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Isomer Identification. The aforementioned results have demonstrated the effectiveness of these single-layered 2D MOF nanosheets as a sensing platform for solvent identification and solvent polarity measurements. These encouraging results then inspired us to explore the possibility of these 2D MOF nanosheets to identify the components of mixed solvents, or even the isomers. The isomers n-propanol and ipropanol 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 bands at 261 and 294 nm and fluorescence emission at 355 nm, respectively, whereas in i-propanol solution, the colloidal suspension displayed bright-blue color (Figure 5a) with d−d transition band at 318 nm and fluorescence emissions at 352 and 457 nm (Figures 4e, S5 and Table 1), demonstrating the effectiveness of these 2D MOF nanosheets for isomer identification. To further reveal the identification mechanism for isomers, variation of UV−vis absorption spectroscopy for mixed solvents by adding i-propanol into n-propanol with stoichiometry (v/v) from 5:0, 4.5:0.5... to 0:5 (Figure 5b), and in-turn by adding n-propanol into i-propanol with stoichiometry (v/v) from 5:0, 4.5:0.5... to 0:5 (Figure 5c), has been investigated. For both the cases, in the early stage, addition of the counter isomer induced the reduction of the character absorption band of the original isomer, until disappears. Then, the character absorption band of the added counter isomer has been generated and increased with continuous addition. Thus, the components that identify curves for n-propanol/i-propanol mixed solution can be figured out (Figure 5d). These results indicating that on the one hand, different solvents have shown distinct interactions with these 2D MOF nanosheets, even for isomers and on the other hand, the polar and/or protonic

Table 1. Summary of the Solvent Polarity, Absorption Peak, and Emission Peak of the Colloidal Suspensions in the Selected Solvents solvent

solvent polarity

H2O MeOH EtOH n-propanol i-propanol DMSO DMF acetone CH2Cl2 benzene toluene hexane

10.2 6.6 6.2 4 4.3 7.2 6.4 5.4 3.4 3.0 2.4 0.06

absorption peak (nm) 260 260 260 260 262 265 270 280

296 292 291 290 318 310 329 324 330 324 275

emission peak (nm) 346 340 359 354 351 360 356 416 380 362 420 410

465 467

468 465

bonding contacts, the electron density within the 2D MOF nanosheets is transferred to the 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 (Figures 4e and S5), where the π → π* and d−d transition bands, as well as the fluorescence emission intensity, were decreased significantly with the increase of solvent polarity, demonstrating the sensitive response of these single-layered 2D MOF nanosheets to solvent polarity. Thus, an effective sensing platform based on these 2D MOF nanosheets for solvent polarity measurements can be expected.

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; and (d) components identify curves for n-propanol/i-propanol mixed solvents. 28864


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ACS Applied Materials & Interfaces solvents can be absorbed firmly on the surfaces of nanosheets, which then changed the electronic state of the nanosheets to various extents and displayed different optical properties. Thus, the practical application of these 2D 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 Solvent Identification and Solvent Polarity Measurements. To demonstrate the aforementioned prospects of these 2D MOF nanosheets for practical applications, paper strips coated with these singlelayered 2D MOF nanosheets as the test paper for solvent identification and solvent polarity measurements were developed, through the analysis of solvatochromic behavior by using a smartphone equipped with a color-scanning APP. This kind of test paper strips was prepared by infiltrating the filter paper strips into the ethanolic suspension of 2D [Co(CNS)2(pyz)2]n nanosheets for one day; during this time, the 2D MOF nanosheets were absorbed onto the filter paper. Then, the 2D MOF nanosheet-coated paper strips were dried in an oven under 80 °C for 1 h. After that, the lightgreen-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 was placed on a spot plate separately under ambient condition (Figure 6). Immediate 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 photo was thus calculated by a color-scanning APP. Finally, the G/B ratio of each color was correlated with the solvent polarity. As shown in Figure 6, the 2D [Co(CNS) 2 (pyz) 2 ] n nanosheet-coated test paper squares showed sensitive response to solvents with different polarities. As is evident by the immediate color switching of the test paper squares from lightgreen to white, pink, blue, and green, upon exposure with H2O, MeOH, EtOH, n-propanol, DMSO, DMF, acetone, CH2Cl2, benzene, and toluene. 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 the solvent polarity range from 2.4 to 10.2. Hence, combined with a smartphone, portable, economical, and visual test papers have been developed for in situ solvent identification and solvent polarity measurement application.

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CONCLUSIONS In this work, we have proposed an easy operating and robust preparation strategy to produce the single-layered 2D MOF nanosheets from bulk Co(CNS)2(pyz)2 precursors with high yield. The obtained 2D nanosheets have act as excellent platforms for intermolecular interactions, which have shown significant solvatochromic behavior and unique optical properties for different solvents. Combined with a smartphone, the present 2D 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 nontoxicity 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 MOF nanosheets. Inspired by these attractive results, visual detection of toxic cations, anions, or other noxious chemicals by using these single-layered 2D MOF nanosheets will be performed in our further work.

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EXPERIMENTAL SECTION

General Considerations. All syntheses were performed under ambient conditions, and all of 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 of the used solvents were commercially available from Sinopharm Chemical Reagent Co. Ltd (China). Characterization. The morphologies of the 2D MOF nanosheets were characterized by using a field emission scanning electron microscope (Hitachi S-4800 20 kV), transmission electron microscope (TEM; Tecnai-G2 20 E-TWIN 200 kV), and AFM (Cypher, Asylum Research). Before these microscopy characterizations, the ethanolic suspension of 2D MOF nanosheets was added dropwise onto the holey carbon-coated carbon support copper grids, Si/SiO2, and piranha-cleaned Si/SiO2 and then naturally dried. The chemical composition of bulk precursors and 2D MOF nanosheets were characterized using X-ray powder diffractions (PXRDs) on a 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 FluoroMax4 Spectro-fluorometer, respectively. TGA of bulk precursors and 2D MOF nanosheets were performed by using a MettlerToledo TGA/DSC STARe System at a heating rate of 10 K min−1, under an atmosphere of dry N2 flowing at a rate of 20 cm3 min−1 over a temperature range from 50 to 800 °C.

Figure 6. Scheme illustration of the in situ visual solvent identification and solvent polarity measurements by using 2D MOF nanosheetcoated test papers combined with a smartphone. The color change on the test paper was scanned by the smartphone; the G/B value of each photo was thus correlated with the solvent polarity. 28865


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ACS Applied Materials & Interfaces Preparation of Bulk Co(CNS)2 (pyz)2 Precursors. Bulk precursors were synthesized according to the 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) under continuous stirring. After being stirred for about 1 h, pink-colored solids were formed, which was separated by a suction filter and washed with methanol before dried in 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 2D MOF Nanosheets by Using a Ultrasonic Force-Assisted Liquid Exfoliation Technology. In a typical experiment, 15 mg of bulk Co(CNS)2(pyz)2 precursors was dispersed in 30 mL of ethanol. The mixture was sonicated in an ultrasonic bath (Brandson, CPX2800H-E, 110 W, 40 KHz) for 30 min; 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 2D [Co(CNS)2(pyz)2]n nanosheets was collected by centrifugation to remove the sedimentation of bulked samples. The powdered samples of 2D nanosheets were obtained by rotary evaporation of the colloidal suspension at room temperature. Preparation of the in Situ Visual Test Paper for Solvent Identification and Solvent Polarity Measurements. Filter paper strips were used to prepare the desired in situ visual test paper. In a typical procedure, these kind of in situ visual test paper strips were prepared by infiltrating the filter paper strips into the ethanolic colloidal suspension of 2D [Co(CNS)2(pyz)2]n nanosheets (0.2 mg/ mL) for one day; then, the 2D nanosheet-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 solvent identification and solvent polarity measurements, the obtained 2D nanosheet-coated paper strips were tailored into small squares; the latter was placed in the spot plate separately. Then, the solution with different polarities was dropped onto the small squares. The rapid color change of the small squares can be observed by naked eyes in ambient condition, and the color photos were scanned by a smartphone 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, portable, economical, and in situ visual test papers for solvent identification and solvent polarity measurements have been developed.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b08739. Additional crystal structure of bulk Co(CNS)2(pyz)2 precursors, 3D Hirshfeld surface and 2D 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 2D MOF nanosheets, UV−vis absorption spectra of Co(CNS)2, pyz, the bulk Co(CNS)2(pyz)2 precursors and 2D MOF nanosheets both in solution and in solid state, and the fluorescence spectroscopy of colloidal suspensions in all kinds of solvents (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.-H.L.). *E-mail: [email protected] (B.-W.S.). ORCID

Yang-Hui Luo: 0000-0002-5555-2510 Bai-Wang Sun: 0000-0001-8724-0055 28866


Research Article

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