Restriction of Molecular Rotors in Ultrathin Two-Dimensional Covalent

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Restriction of Molecular Rotors in Ultrathin Two-Dimensional Covalent Organic Framework Nanosheets for Sensing Signal Amplification Jinqiao Dong, Xu Li, Shing Bo Peh, Yi Di Yuan, Yuxiang Wang, Dongxiao Ji, Shengjie Peng, Guoliang Liu, Shaoming Ying, Daqiang Yuan, Jianwen Jiang, Seeram Ramakrishna, and Dan Zhao Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03685 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 10, 2018

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Chemistry of Materials

Restriction of Molecular Rotors in Ultrathin Two-Dimensional Covalent Organic Framework Nanosheets for Sensing Signal Amplification Jinqiao Dong,†§ Xu Li,†§ Shing Bo Peh,† Yi Di Yuan,† Yuxiang Wang,† Dongxiao Ji,‡ Shengjie Peng,‡,¶ Guoliang Liu,† Shaoming Ying,† Daqiang Yuan, Jianwen Jiang,† Seeram Ramakrishna,‡ and Dan Zhao*,† †

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4

Engineering Drive 4, 117585, Singapore ‡

Department of Mechanical Engineering, National University of Singapore, 117574, Singapore



State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of

Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ¶ Jiangsu

Key Laboratory of Electrochemical Energy Storage Technologies, College of Materials

Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China *E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Covalent organic frameworks (COFs) have emerged as promising crystalline porous materials with well-defined structures, high porosity, tunable topology and functionalities suitable for various applications. However, studies of few-layered ultrathin two-dimensional (2D) COF nanosheets, which may lead to unprecedented properties and applications, are still limited. Herein we report the targeted synthesis of three azine-linked and imine-linked 2D COFs named NUS 30-32 using monomers containing aggregation-induced emission (AIE) rotor-active tetraphenylethylene (TPE) moieties, affording micro- and meso- dual pores in NUS-30 and NUS-32, and triple pores in NUS-31. For the first time, we demonstrate that these isostructural bulk COF powders can be exfoliated into ultrathin 2D nanosheets (2 – 4 nm thickness) by temperature-swing gas exfoliation approach. Compared with TPE monomers and COF model compounds, the AIE characteristic of NUS 30-32 nanosheets is distinctly suppressed due to the covalent restriction of the AIE molecular rotors in the confined 2D frameworks. As a result, the enhancement of conjugated conformations of NUS 30-32 nanosheets with unusual structure relaxation show signal amplification effect in biomolecular recognition of amino acids and small pharmaceutical molecules (L-dopa), exhibiting much higher sensitivity than their stacked bulk powders, TPE monomer, and COF model compound. Moreover, the binding affinity of the COF nanosheets toward amino acids can be controlled by increasing the number of azine moieties in the structure. Density functional theory (DFT) calculations reveal that binding affinity control results from the crucial geometric roles and stronger host-guest binding between azine moieties and amino acids. In addition, we demonstrate that minimal loading of the NUS-30 nanosheets in composite membranes can afford excellent performance for biomolecule detection. Our findings pave a way for the development of functional ultrathin 2D COF nanosheets with precise control


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over the nature, density, and arrangement of the binding active sites involved in enhanced molecule recognition.

INTRODUCTION Since the initial report of covalent organic frameworks (COFs) in 2005,1 these materials have become an emerging class of crystalline porous polymers with predictable structures, good chemical stability, high specific surface area, and tunable topology and functionalities.2-3 They have gained increasing attention recently for potential applications in gas adsorption and separation,4-9

chiral

separation,10-11

chemical

sensing,12-19

heterogeneous

catalysis,20-29

photocatalytic hydrogen generation,30-32 proton conduction,33-35 drug delivery,36-39 and energy storage.40-42 In particular, two-dimensional (2D) COFs exhibit unique architectures wherein the monomers that make up their 2D layers stack almost perfectly into 1D channels, creating an ideal platform for the transport of guest molecules. However, although a large number of 2D COFs have been reported, such materials have rarely been explored as few-layered ultrathin 2D nanosheets for practical applications. In particular, thin layered COF nanosheets with high aspect ratios are expected to exhibit dimensionally related chemical sensing properties, such as large external surface, amplified fluorescent signal, and more accessible active site, above that of their stacked bulk counterparts.16,39,43 Recently, Banerjee’s group has reported the fluorescent sensing of volatile organic compounds (VOCs) using covalent organic nanosheets (CONs) obtained by exfoliation from COF bulk powder.16 Zhang and co-workers have also realized highly sensitive DNA sensing using ultrathin fluorescent 2D COF nanosheets.43 Compared with bulk COF powder, those exfoliated 2D COF nanosheets have more exposed external surface and accessible


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active binding sites, which may contribute to enhanced selectivity and sensitivity in fluorescence-based chemical sensing.16,43 However, this area of research is still in its infancy, and there are mainly two issues to be resolved: (1) The strong π–π stacking interactions between adjacent COF layers make it extremely difficult to obtain highly ordered crystalline 2D COF nanosheets with few-layer thickness; (2) The energy migration process in fluorescent COFs needs to be further improved for enhanced detection sensitivity.44-47 Therefore, it is crucial to develop new linkages and effective approaches to prepare high-quality ultrathin fluorescent 2D COF nanosheets that can overcome the above issues for practical sensing applications. The aggregation-induced emission (AIE) phenomenon was discovered by Tang’s group in 2001,48 which is a useful strategy to prepare highly emissive materials in aggregated state. AIE molecular rotors play an important role in fluorescence emission behavior.49 Very recently, Tang and co-workers have reported that tetraphenylethylene (TPE)-based molecules can act as AIE-active fluorescent molecular rotors for humidity sensors with high sensitivity.50 Unlike other monomers, TPE and its derivatives, possessing twisted molecular conformation in the aggregated state, have become one of the most important AIE luminogens (AIEgens),51 and they have been demonstrated to facilitate exciton migration and improve luminescence activity in organic porous materials.52 So far, they have been widely used as molecular building blocks to synthesize fluorescent materials as chemical sensors or biosensors,53-54 such as polymers,55-58 metal-organic frameworks (MOFs),59-63 and supramolecular coordination complexes (SCCs).64-67 In addition, Bein and co-workers have demonstrated that propeller-shaped TPE building blocks can be employed to construct highly crystalline 2D COFs by generating well-defined periodic docking sites.68-69 Zhao and co-workers have realized triangular microspores and hexagonal mesopores in COFs based on TPE monomers.70-72 Jiang’s group has obtained highly emissive boronate-linked


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COFs by introducing TPE monomers, and demonstrated their highly sensitive fluorescence in the presence of ammonia vapor.73 Recently, Loh and co-workers have reported TPE-based COF with frustrated bonding network for enhanced carbon dioxide storage.74 Despite the above progress, fundamental investigation of AIE characteristic of those TPE-based COFs has yet to be reported. This type of study is important to understand the dynamic behavior of phenyl rings that may have profound influence on emission behavior,75 as well as molecular sensing and recognition.76 Furthermore, due to their large external surface area, accessible binding sites, and inhibition of aggregation-caused quenching (ACQ) phenomenon, we envision that TPE-based 2D COF nanosheets with few-layer thickness may have advantages over 3D bulk powder in chemical sensing as well as practical membrane sensing applications. In our previous work, we have demonstrated that flexible TPE units confined in porous materials can act as AIE-active molecular rotors.63,77-78 Further investigations show that partially restricted AIE molecular rotors in 2D porous organic nanosheets can enhance molecular sensing and recognition.76 Inspired by Rodríguez-Molina’s recent study of gas sorption enhancement arising from restricted molecular rotors at 196 K,79 and Swager’s study of amplified fluorescent quenching in conjugated polymers,44-45 we envision that the full restriction of AIE molecular rotors in few-layered highly ordered 2D COF nanosheets may facilitate pronounced signal amplification of chemical sensing or biosensing due to the enhancement of conjugation in 2D sheets for electron or energy migration. In this work, we report the targeted synthesis of three highly crystalline 2D COFs, namely NUS 30-32, containing long conjugated TPE units. For the first time, we have successfully prepared the ultrathin 2D nanosheets of these COFs with thicknesses of around 2 – 4 nm by the temperature-swing gas exfoliation approach. The AIE characteristics of exfoliated NUS 30-32 nanosheets decrease distinctly owing to the


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conformational covalent locking of TPE molecular rotors in the confined 2D frameworks. Because of their highly ordered and partially conjugated structures, these 2D COF nanosheets exhibit signal amplification effect in fluorescence-based biomolecular recognition of amino acids and small pharmaceutical molecules. In addition, the binding affinity can be tuned by increasing the number of azine moieties in the COF structure. Density functional theory (DFT) calculations were conducted to provide structural and microscopic insights into the controlled binding of amino acids. Finally, we demonstrate that composite membranes containing only minimal loading of NUS-30 nanosheets can exhibit excellent performance in biomolecule detection for practical applications. RESULTS AND DISCUSSION Synthesis and characterization of NUS 30-32. Starting from commercially available tetraphenylethylene

monomer,

we

synthesized

the

1,1,2,2-tetrakis(4-formyl-(1,1’-

biphenyl))ethane (TFBE) monomer via bromination reaction and Pd-catalyzed Suzuki crosscoupling reaction (Figure S1-S2).80 The first step to demonstrate our concept is to obtain highly crystalline 2D COFs containing TPE units. We chose TFBE as the COF monomer due to its twisted molecular conformation that can easily generate highly ordered 2D frameworks by the flexible molecular docking sites.68 To microscopically elucidate the dynamic feature of the phenyl rings, we conducted DFT calculations to evaluate the rotational energy barrier of TFBE linker (see the Supporting Information for calculation details). Figure 1a illustrates the relative energy as a function of torsional angle of the phenyl ring. The energy profile shows four minima and three maxima corresponding to stable and less favorable conformations. Three rotational energy barriers exist, with the highest barrier of 8.7 kcal mol-1 at 11°, and two other barriers of 6.3 and 6.7 kcal mol-1 (Figure 1a). The energy barriers of TFBE, the TPE rotor in this study, is


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slightly higher than the recently reported phenyl ring molecular rotor (5.21 kcal mol-1).75 We inferred that the rotational barriers should increase with the extension of the arms of the TPEtype molecules. To prove our hypothesis, we further examined the dynamic behavior of 1,1,2,2tetrakis(4-(N-methylene-aniline)-(1,1’-biphenyl))ethane (TMBE), another TPE rotor with longer rotational arms (Figure S3). DFT calculations show that TMBE has similar stable and less favorable conformations as a function of torsional angle (Figure 1b), but with higher rotational energy barriers compared to that of TFBE which has shorter arms. For example, the highest energy barrier of TMBE is 9.2 kcal mol-1 at 15°, and the other two barriers are also higher than that of TFBE (7.6 vs. 6.3 and 7.2 vs. 6.7 kcal mol-1). Therefore, the molecular rotation is disfavored with the increase in arm length of TPE linker. Accordingly, the application of TPE linkers of suitable length will allow the molecular rotors in the confined 2D COFs to be fully restricted, leading to enhancement of the intrinsic emissive and signal amplification of molecular recognition and sensing.


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Figure 1. Theoretical calculations of the energy barrier of TPE rotors in the monomer TFBE (a) and the model compound TMBE (b) under ground-state. Based on the above results, TFBE was employed to construct four-fold connected and star-shaped COFs named NUS 30-32. NUS-30 and NUS-32 exhibit dual porosity with both microporous (triangular) and mesoporous (hexagonal) pores distributed in the frameworks, while a triple-pore arrangement was realized in NUS-31 through a heterostructural mixed linker synthesis strategy (Figure 2). Typically, syntheses were carried out by suspending TFBE and hydrazine or 1,4-diaminobenzene in 1,4-dioxane/acetic acid (aq., 6 M) (10/1, v/v) binary solvent followed by heating at 120 °C for 5 days (see more synthetic details in Supporting Information), affording yellow crystalline solids in yields of 85%, 81%, and 78% for NUS-30, NUS-31, and


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NUS-32, respectively. The COF bulk powders are stable in ambient air, aqueous solutions, and common organic solvents such as dichloromethane, tetrahydrofuran, acetone, ethanol, n-hexane, and N,N-dimethylformamide (Table S1). The atomic-level formation of NUS 30-32 was assessed by solid state

13C

CP-MAS

nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR), and X-ray photoelectron spectroscopy (XPS). The characteristic signal of C=N groups was observed at 163 ppm in NUS30, 161 ppm in NUS-31, and 157 ppm in NUS-32 from NMR spectroscopy (Figure S4). The FTIR spectra of NUS 30-32 indicate the stretching bands of C=N at 1674 – 1698 cm−1 (Figure S5), further confirming the formation of azine-bonds and imine-bonds in these COFs. Elemental analysis of the NUS-30 bulk powder reveals that contents of carbon, nitrogen, and hydrogen are 85.63%, 6.62%, and 4.60%, respectively, which are close to its corresponding theoretical values (88.31% for C, 6.44% for N, and 5.25% for H). Similar results were also observed in NUS-31 and NUS-32 bulk powders (Table S2). In addition, compared to TFBE monomers, the XPS spectra of NUS 30-32 contain new peaks of N1s (~399.6 eV, Figure S6), demonstrating the success of the Schiff-base reactions. Thermogravimetric analyses (TGA) reveal that the asprepared COFs are thermally stable up to 350 °C (Figure S7).


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Figure 2. Synthetic approach for the formation of the 2D COFs (NUS 30-32) and COF model compound (TMBE). Crystallinity and porosity of NUS 30-32. Successful formation of crystalline frameworks of NUS 30-32 was confirmed by powder X-ray diffraction (PXRD). As shown in Figure 3, the diffraction patterns exhibit a number of well-defined reflections with only weak background, highlighting the high degree of long-range order in these materials. NUS-30 shows four


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prominent diffraction peaks, with the most intense one at 2.3° and the other two peaks at 4.6° and 20.4° (Figure 3g). NUS-31 exhibits three intense peaks at 2.2°, 4.5° and 20.7° (Figure 3h), and NUS-32 shows four intense peaks at 1.9°, 3.8°, 5.8°, and 20.3° (Figure 3i). In addition, their high crystallinity was further verified by high-resolution transmission electron microscopy (HRTEM) (Figure S8-S10). In order to study the possible stacking modes of the TFBE moieties being incorporated into COFs, we synthesized TMBE (Figure 2), a COF molecular model compound containing the TFBE moiety by imine condensation between TFBE linker and four equivalents of aniline (see the Supporting Information for details). Single X-ray crystal structure shows that rod-like yellow crystals of TMBE were crystallized in the typical “propeller” configuration (Figure 3a-c, Figure S11a and Table S3-S5). There is a dihedral angle of 43.4° between twisted phenyl rings (Figure S11c), which is caused by the sterically demanding edgeon-face configuration of the four phenyl rings (Figure S11a). The CH···CH distance of two phenyl rings close to each other in this screw-like arrangement is approximately 3.0 Å (Figure S11b). Therefore, strong CH−π interactions (3.0 – 3.6 Å) are expected (Figure S11e-f), which can direct the packing of TMBE molecules in parallel along the crystallographic a-axis into robust supramolecular frameworks without interpenetration. The crystal structure of the COF model compound TMBE indicates that the twisted spatial TPE units may easily trigger AA stacking of resultant COFs (Figure 3b). Obviously, PXRD data show that NUS 30-32 are different from that of the monomer TFBE and COF model compound TMBE (Figure S12). With the above results, we then proceeded to simulate possible COF structures. After a geometrical energy minimization of the eclipsed stacking (AA-H and AA-O) and staggered stacking (AB-H and AB-O) on the basis of hexagonal and orthorhombic systems for NUS 30-32 (Figure S13-S15 and Table S6-S14), we


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found that the hexagonal system of eclipsed structure (AA-H) agrees well with the experimentally observed PXRD patterns (Figure S16), resulting in unit cell parameters of a = b = 47.3537 Å, c = 4.7263 Å, α = β = 90°, γ = 120°, Rwp = 4.30 % and Rp = 3.39 % for NUS-30 (a = b = 51.0869 Å, c = 5.2739 Å, α = β = 90°, γ = 120°, Rwp = 4.94 % and Rp = 3.87 % for NUS-31; a = b = 56.2068 Å, c = 4.6625 Å, α = β = 90°, γ = 120°, Rwp = 4.46 % and Rp = 3.46 % for NUS32). The simulated PXRD patterns of NUS-30, NUS-31 and NUS-32 show the first (also the strongest) peaks at 2θ = 2.2°, 2.0° and 1.8°, respectively, which correspond to the reflection from the (100) planes matching well with the experimental data (Figure 3g-i). Furthermore, the broad peaks of experimental PXRD at 2θ = ~ 20.4°, 20.7°and 20.3° could be indexed to the π−π stacked (001) planes of NUS-30, NUS-31, and NUS-32, respectively (Figure S17). The dihedral angles of twisted phenyl rings in these simulated COF structures are around 46.5 − 48.5°, and the CH···CH distances of two adjacent phenyl rings are approximately 3.0 – 3.3 Å (Figure S18), which are close to that observed in the crystal structure of the TMBE compound. In comparison, the dihedral angles in typical TPE-based MOFs,81-82 coordination cages,83 and organic cages84 are considerably larger than that in our ultrathin 2D COF nanosheets. This spatial restriction behavior contributes to the enhanced conjugation degree, which is beneficial to electron or energy migration for chemical sensing. Our COF structures are also consistent with other TPEbased COFs.70 In addition, hierarchical porosity was observed in those COFs. For example, in the AA-H stacking model of NUS-30, the TPE monomers are located at the vertices, and the hydrazine linkers occupy the edges of the polygons to generate periodic columnar TPE π−arrays and a dual-pore Kagome lattice with triangular micropores (8.1 Å) and hexagonal mesopores (36.1 Å) in azine-linked NUS-30 (Figure 3d and Figure S19a). When 1,4-diaminobenzene was used to replace the hydrazine linker, larger micro- and meso-pores (12.3 Å and 44.6 Å) in imine-


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linked NUS-32 were obtained (Figure 3f and Figure S19c). The d-space of (100) planes in NUS32 enlarges, which can be confirmed by the change of the 2θ from 2.3° to 1.9° in the experimental PXRD data. A heterostructural mixed linker strategy71 was used to construct azinelinked and imine-linked NUS-31. Theoretical simulation suggests the generation of three pores (8.2, 12.0, and 40.1 Å) in the 2D framework of NUS-31 (Figure 3e and Figure S19b). Experimental PXRD pattern of NUS-31 shows the diffraction of (100) planes at 2θ = 2.2°, which is in good agreement with the simulated PXRD pattern generated from triple-pore structure with AA stacking (Figure 3h). The fact that this peak neither belongs to (100) diffraction of NUS-30 (2.3°) nor corresponds to the (100) diffraction of NUS-32 (1.9°) strongly suggests the formation of a COF with a new structure. Although the heterostructural mixed-linker strategy has been used to construct imine-linked COFs with three different kinds of pores,71,85-86 our study demonstrates for the first time the feasibility of preparing azine-linked and imine-linked heterostructural COFs using this approach. Furthermore, the stability tests suggest excellent chemical stability of NUS 30-32, as indicated by the unchanged intensities and positions of their PXRD peaks after being soaked in various solutions for one week, including tetrahydrofuran, acetonitrile, dimethyl sulfoxide, chloroform, water, aqueous HCl (1 M), and aqueous NaOH (1 M) solutions (Figure S20).


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Figure 3. (a) The X-ray crystal structure of the model compound TMBE (CCDC: 1835603). Color coding: C, blue; N, pink; H, white. (b) Packing of TMBE to generate a supramolecular structure viewed along the c-axis. (c) Fluorescence photograph of TMBE crystals (λex = 365 nm). View of the slipped AA stacking crystal structures of NUS-30 (d), NUS-31 (e), and NUS-32 (f). Experimental (black), Pawley-refined (red), and simulated (blue) PXRD patterns of NUS-30 (g),


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NUS-31 (h), and NUS-32 (i). The difference plots are presented in green. 77 K N2 sorption isotherms of NUS-30 (j), NUS-31 (k), and NUS-32 (l) (inset: the corresponding pore width distribution).

To further investigate the assigned crystal structures of NUS 30-32, nitrogen sorption measurements were carried out at 77 K to evaluate their porosities and pore size distributions (Figure 3j-l). The sorption curves of NUS 30-32 can be classified as a combination of Type I and Type IV isotherms, which are characteristics of micropores and mesopores, respectively. Brunauer-Emmett-Teller (BET) surface areas of NUS-30, NUS-31, and NUS-32 were calculated to be 1386, 685, and 249 m2 g-1, and their total pore volumes were determined to be 1.25, 0.60, and 0.33 cm3 g-1, respectively. NUS-30 exhibits a much faster N2 uptake at a low pressure range of P/P0 < 0.1 than that of NUS-31 and NUS-32, confirming its smaller pores. Another possible reason for the sharp decrease of BET surface areas from NUS-30 to NUS-32 would be the reduced reactivity of 1,4-diaminobenzene compared to hydrazine, which may result in higher defect concentration in NUS-32. The pore size distribution calculated using nonlocal density functional theory (NLDFT) indicates dominant porosity at 6.0 Å and 29.6 Å in NUS-30 (inset of Figure 3j), and 12.6 Å and 30.5 Å in NUS-31 (inset of Figure 3k). These values are close to the simulated pore size distributions based on the proposed structures. Although the observed mesopores at 37.1 Å are close to the theoretical simulation (44.7 Å) in NUS-32 (inset of Figure 3l), the existence of defects probably reduce the extent of microporosity in the structure. Temperature-swing gas exfoliation and characterization of NUS 30-32 nanosheets. Up to now, due to the strong π–π stacking interactions between adjacent layers of 2D COFs, there are


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only few examples of 2D COF nanosheets, which are prepared by various approaches such as liquid exfoliation,16,41,43 mechanical exfoliation,87 and self-exfoliation.88 However, drawbacks associated with these exfoliation techniques include extended sonication time in toxic organic solvents or the addition of large amounts of surfactant. An ideal approach for the synthesis of ultrathin 2D COF nanosheets should have a short preparation period with minimal application of chemical additives or surfactants. Recently, Dai and co-workers presented a highly efficient gas exfoliation strategy relying on high local temperature difference for the scalable synthesis of few-layered hexagonal boron nitride (h-BN) nanosheets.89 In our previous work, we also applied a related temperature-swing strategy to obtain few-layer nanosheets of 2D MOFs.90 In this study, we for the first time applied the temperature-swing gas exfoliation approach toward the fabrication of ultrathin 2D COF nanosheets. The bulk powders of NUS 30-32 were first heated to 300 °C for 10 min in air and then immediately immersed into liquid N2 until the liquid N2 gasified completely (see more details in Supporting Information). In this process, the high temperature expands the interlayer distance and volume of bulk COFs, allowing the subsequent liquid N2 gasification to efficiently exfoliate the COFs (Figure 4a). Consequently, free-standing 2D nanosheets of NUS 30-32 can be successfully prepared by this method. In contrast, our efforts to fabricate ultrathin NUS 30-32 nanosheets by surfactant-induced bottom-up methods were unsuccessful, emphasizing the additional fabrication challenges presented by COFs over other 2D materials.


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Figure 4. (a) The schematic diagram for the temperature-swing gas exfoliation of NUS-30 from bulk layered powder to ultrathin 2D nanosheets. FE-SEM images of COF bulk powder of NUS30 (b), NUS-31 (c), and NUS-32 (d) (inset: corresponding optical photographs). TEM images of exfoliated ultrathin 2D nanosheets of NUS-30 (e), NUS-31 (f), and NUS-32 (g) (inset: the


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corresponding SAED patterns). HR-TEM images of 2D nanosheets of NUS-30 (h), NUS-31 (i), and NUS-32 (j) featuring the planar lattice structure (inset: the corresponding lattice distances and the Fast Fourier transformation).

The morphology of bulk powder and nanosheets of NUS 30-32 was studied by fieldemission scanning electron microscopy (FE-SEM) and HR-TEM. FE-SEM images show that NUS 30-32 bulk powders exhibit a layered stacking morphology (Figure 4b-d and Figure S21S22). The HR-TEM images of NUS 30-32 nanosheets clearly indicate their ultrathin sheet-like 2D morphology (Figure 4e-g). During the exfoliation process, NUS 30-32 nanosheets maintained the rectangular shape of the bulk materials and micrometer-scale size (Figure S23). The substantial retention of structural integrity in NUS-30, NUS-31 and NUS-32 nanosheets can be attributed to the strong covalent bonds that can link the monomers and protect the frameworks from extensive degradation, which is rarely observed in the exfoliation of MOF nanosheets.90-92 Furthermore, selected area electron diffraction (SAED) patterns of exfoliated NUS-30 nanosheets (inset of Figure 4e) reveal the highly ordered crystallographic plane, which is consistent with the simulated SAED pattern based on AA stacking model (inset of Figure 4e), confirming the crystalline nature of the nanosheets. Although the SAED patterns of NUS-31 and NUS-32 nanosheets are weaker than that of NUS-30 nanosheets (inset of Figure 4f-g), crystal lattices can still be identified in the HR-TEM images of those COF nanosheets, indicating the presence of crystalline order in small domains. The experimental lattice distances are around 2.6 nm, 2.9 nm, and 3.6 nm in NUS-30, NUS-31, and NUS-32 nanosheets (Figure 4h-j), respectively, matching well with the (110) planes of their modeled AA stacking structures. These results indicate that the in-plane crystal structures do not change from the bulk powders to


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exfoliated nanosheets. Overall, the morphological characterization evidences the high efficiency of the temperature-swing gas exfoliation approach in producing ultrathin ordered COF nanosheets.

Figure 5. AFM images of 2D nanosheets obtained by temperature-swing exfoliation of NUS-30 (a), NUS-31 (b), and NUS-32 (c). Inset: the theoretical thickness of NUS-30 (5 layers, 2.3 nm), NUS-31 (6 layers, 3.0 nm), and NUS-32 (7 layers, 3.2 nm) based on the AA stacking structures. Selected AFM height profiles of the exfoliated nanosheets of NUS-30 (d), NUS-31 (e), and NUS-32 (f). (g) DLS results of NUS 30-32 nanosheets suspended in acetonitrile solution. (h) The increase of optical band gap of NUS 30-32 after exfoliation. (i) The optical photograph of NUS


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30-32 nanosheets in acetonitrile solution indicating a strong Tyndall effect (inset: the fluorescence photograph of NUS 30-32 nanosheets in acetonitrile solution, λex = 365 nm).

Molecular level understanding of the ultrathin 2D nanostructure is further confirmed by atomic force microscopy (AFM). The flat and rectangular morphology of NUS 30-32 nanosheets transferred on silicon wafers was observed (Figure 5a-c and Figure S24-S26). The average thicknesses of these nanosheets are around 2.4 nm, 2.8 nm, and 3.1 nm for NUS-30, NUS-31, and NUS-32 nanosheets (Figure 5d-f), respectively, corresponding to structures with 5 – 7 unit cell thicknesses on the basis of their optimized AA stacking models (inset of Figure 5a-c). Notably, the lateral sizes of NUS 30-32 nanosheets range from 1 μm to 15 μm, resulting in high aspect ratios confirming their 2D layered features. Dynamic light scattering (DLS) data reveal that the average sizes of NUS-30, NUS-31, and NUS-32 nanosheets suspended in acetonitrile solutions are around 4.2 μm, 1.3 μm, and 2.1 μm, respectively (Figure 5g), matching well with their HR-TEM and AFM results. Furthermore, acetonitrile solutions containing NUS 30-32 nanosheets exhibit typical Tyndall effect (Figure 5i), indicating the colloidal feature of the solutions containing freestanding and homogeneous ultrathin 2D nanosheets. Both bulk powder and nanosheets of NUS 30-32 were further characterized by various techniques. Basically, these exfoliated ultrathin nanosheets keep the same chemical compositions as that of their bulk powder, which is evidenced by FT-IR spectra (Figure S5), XPS spectra (Figure S6), TGA measurements (Figure S7), and Raman spectra (Figure S27). Notably, we can observe a structure relaxation of those ultrathin 2D nanosheets after exfoliation. For example, the ultraviolet-visible (UV-Vis) spectrum of NUS-30 nanosheets shows a blue shift of about 15 nm


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compared to that of the bulk powder (426 nm vs. 411 nm, Figure S28a). Similar blue shifts of 8 nm and 9 nm (Figure S28b-c) were also found in NUS-31 and NUS-32 nanosheets, respectively. The shifts can be attributed to the reduced π−π interactions caused by exfoliation.78 We also observed distinct decreases in the BET surface area of NUS 30-32 nanosheets after exfoliation (208, 186, and 127 m2 g-1 for the exfoliated nanosheets of NUS-30, NUS-31, and NUS-32, respectively, Figure S29), indicating that 1D channels in bulk COF powders have been largely eliminated in exfoliated ultrathin 2D nanosheets. This result further demonstrates the disruption of the π−π stacking among the layers after exfoliation. Accordingly, the crystallinity of NUS 3032 nanosheets is also weakened compared to that of the bulk powders (Figure S30). Moreover, the UV-Vis diffuse reflectance measurements were conducted to investigate the changes of optical band gap (Eg) between the bulk powder and the 2D nanosheets. As shown in Figure 5h, the corresponding Eg of NUS-30, NUS-31, and NUS-32 bulk powders were calculated to be 2.48, 2.52, and 2.42 eV, respectively. After exfoliation, the Eg increases to 2.57, 2.60, and 2.50 eV for NUS-30, NUS-31, and NUS-32 nanosheets (Figure S31), respectively, indicating weakened conjugation due to the disruption of interlayer π‒π interactions caused by exfoliation.76,78 In addition, the unusual structure relaxation of those ultrathin 2D COF nanosheets can affect the emission behavior as well. For example, blue shifts of 5 − 15 nm were occurred in the fluorescent emissions of the 2D nanosheets compared to the bulk powders due to the reduction of electronic decoupling between adjacent layers (Figure S32). Furthermore, the quantum yields of the 2D nanosheets are also higher than that of their stacked bulk powders (Table S15). Similar structural relaxation phenomena have also been observed in other ultrathin 2D nanomaterials such as atomically thin 2D hybrid perovskites93 and 2D ultrathin nanosheets prepared in our previous work.76,78


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AIE characteristics of NUS 30-32 nanosheets. We next sought to understand the different dynamic properties of TPE phenyl rings as molecular rotors among TFBE monomer, COF model compound TMBE, and the prepared ultrathin 2D COF nanosheets. The AIE characteristics of these compounds and materials were investigated using mixtures of THF or acetonitrile with water at different water fractions (fw). As expected, the TFBE monomer exhibits a typical AIE characteristic (Figure S33a-b). The highest fluorescence intensity was obtained at fw = 90 %, which is 163.2-fold higher than that in THF solution (Figure 6a). Such AIE characteristic can be attributed to the rotational restriction of the phenyl rings (AIE molecular rotor) in the aggregated state in poor solvents. As expected, the AIE characteristic dramatically decreases in the TMBE model compound. In addition, increasing the water fraction results in a stepwise enhancement in emission intensity as well as a large blue-shift (~ 50 nm, Figure 6a and Figure S33c), which may due to the formation of reduced reorganization energy after the rigidification of molecular structure in the aggregated state.94 The maximum fluorescence intensity at fw = 90 % in this case is only 4.4-fold higher than that in THF solution (Figure S33d), which is much smaller than the TFBE monomer (Figure 6b). Thus, the increased rotational energy barrier in TMBE caused by the extension of TPE arm length can lead to partial restriction of the molecular rotors, masking the AIE characteristic (Figure 6c). Having achieved substantial suppression of AIE characteristic in the TMBE model compound, we postulated that near-complete masking of AIE characteristic would be possible from the thorough covalent restriction of rotor dynamics in the confined COF environment. The rotational freedom of the molecular rotors was subsequently evaluated by comparing the AIE behavior of the NUS 30-32 nanosheets at different fw, similar to our early analysis of 2D porous organic nanosheets.76,78 Photoluminescence experiments were performed on the NUS 30-32


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nanosheets suspended in acetonitrile/water solutions with fw of 0 and 90 %, respectively. The AIE behavior was compared by taking the ratio of relative intensities at the two states. When fw was increased to 90 %, we observed red-shifts with 10, 8, and 5 nm for NUS-30, NUS-31, and NUS-32 nanosheets, respectively (Figure 6a and Figure S34), which is different from the large blue-shifts of TMBE model compound. This is probably because the rigid 2D COF nanostructure cannot generate reduced reorganization energy, suggesting that the red-shifts of exfoliated nanosheets can be attributed to the π–π restacking with increasing water content. The AIE enhancement ratios for NUS-30, NUS-31 and NUS-32 were calculated to be 1.2, 1.1 and 1.5 (Figure 6b), respectively, which are much smaller than that of TMBE model compound and TFBE monomer, as well as our previously reported NUS-24 2D nanosheets (1.87)78 and NUS-25 2D nanosheets (1.38).76 The results indicate that the four arms of TPE molecular rotors in TFBE monomer can be almost fully locked in the COF structures through covalent bonds, leading to the greatly diminished AIE characteristic. Although the dynamic properties of TPE molecular rotors cannot work in the confined space, conformational locking of these highly conjugated TPE units in highly ordered ultrathin 2D nanosheets would be beneficial for signal amplification of molecular recognition and sensing via electron or energy transfer processes.44


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Figure 6. (a) Fluorescent spectra of TFBE (c = 6.7 × 10-6 M in THF), TMBE (c = 4.0 × 10-6 M in THF), and NUS-32 nanosheets (c = 80 µg mL-1 in acetonitrile) in THF (or acetonitrile) and 90 % water fractions (λex = 365 nm). (b) Relative fluorescence intensity [I(water

= 90%)/I(water = 0%)]

of

TFBE, TMBE, and NUS 30-32 nanosheets in THF (or acetonitrile)/water mixtures. (c) The schematic diagram of stepwise restriction of AIE molecular rotor from TPE monomer (TFBE) and COF model compound (TMBE) to NUS-32 COF nanosheets on the basis of AIE characteristic study results.


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Biomolecular recognition by NUS 30-32 nanosheets. Inspired by the work of Swager’s group demonstrating that enhanced energy migration in polymer could amplify the signal of chemical sensing,46-47 we explored the fluorescent sensing signal amplification in the ultrathin 2D nanosheets of NUS 30-32, which are supposed to have more pathways for energy migration compared to 1D linear polymers. In this study, we focused on the detection of amino acids (Figure 7a), which are indispensable components of modern medicinal chemistry.95 When NUS30 nanosheets were treated with L-phenylalanine (1.0 mM), the emission was rapidly quenched to 61.3 % of the original value. In accordance with the Stern−Völmer (S-V) equation,96 the measured absorbance I0/I at 465 nm varied as a function of quencher concentration in a linear relationship (R2 > 0.9900), suggesting 1:1 stoichiometry of the interaction between Lphenylalanine (guest) and NUS-30 nanosheets (host).97 The quenching constant (KSV) representing the binding affinity was calculated to be 13529 M-1. Similar quenching by Lphenylalanine was also observed in the other COF nanosheets, albeit with lower KSV constants (11225 M-1 for NUS-31 and 4078 M-1 for NUS-32, Figure 7b). Furthermore, emission quenching of NUS-30 was also observed using L-alanine, L-threonine, and L-tryptophan, with the calculated KSV constants of 8305, 9325, and 12638 M-1 (Figure S35 and Table S16), respectively. Remarkably, the binding affinity toward these amino acids decreases in the order of NUS-30 > NUS-31 > NUS-32 (Figure 7d, Figure S35-37 and Table S16), which is consistent with the total number of azine moieties within the COF structures, indicating the key role of the azine moieties in the binding and recognition of biomolecules. Similar phenomenon has also been observed for special binding of chloro(pyridine)cobaloxime co-catalyst with the azine moieties (Co−N) of another 2D COF.32 Based on the above results, we tested the recognition of L-dopa (Figure S38), a drug for Parkinson's disease. As shown in Figure 7c, the fluorescence


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intensity of NUS-30 nanosheets decreased stepwise during titration with L-dopa. The S-V plot shows a linear trend with a KSV value of 15022 M-1. Furthermore, the apparent quenching constant Kq (Kq = KSV/τ0, where τ0 is lifetime) for NUS-30 nanosheets with L-dopa system was evaluated to be 2.13 × 1013 M-1 s-1, which is 3 orders of magnitude higher than that of conventional bimolecular diffusion (∼ 1010 M-1 s-1).13 Given that the detection of drug analytes occurs almost immediately upon mixing with ultrathin 2D COF nanosheets without any prior sample treatment, the COF materials may be potentially useful in medicinal drug monitoring. Again, NUS-30 nanosheets also exhibit higher binding affinity toward L-dopa than NUS-31 nanosheets (Ksv: 11958 M-1) and NUS-32 nanosheets (Ksv: 3713 M-1, Figure S38). To contextualize the sensing performance of the ultrathin 2D COF nanosheets, we evaluated the quenching performance of TFBE, TMBE, and the 3D bulk COF powder for comparison. The NUS-30 bulk powder exhibited slower fluorescence quenching than NUS-30 nanosheets upon titration with L-phenylalanine and L-threonine (Figure S39), along with lower KSV values of 6771 M-1 for L-phenylalanine and 5864 M-1 for L-threonine, which are 50.0 % and 37.1 % lower than that of the nanosheets, respectively. Meanwhile, fluorescence quenching was not observed in TFBE and TMBE upon titration with L-phenylalanine and L-threonine (Figure S40). The distinct sensitivity improvement in NUS-30 nanosheets over small molecules (TFBE and TMBE) can be attributed to the covalent conformational locking of AIE molecular rotors amplifying the quenching signal for biomolecules due to facile electron or energy transfer. Compared with stacked COF bulk powder, the enhanced sensing sensitivity may be caused by the largely exposed surface in the ultrathin 2D nanosheets allowing sufficient contact and interaction with amino acids for host-guest energy transfer.


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Figure 7. (a) The chemical structures of amino acids used in this study. (b) The Stern−Völmer plots of TFBE, TMBE, and NUS 30-32 nanosheets being titrated with L-phenylalanine. (c) Fluorescence emission spectra of NUS-30 nanosheets (c = 40 μg mL-1) upon titration with Ldopa solution (1 × 10-3 M) at room temperature (λex = 365 nm). (d) The Ksv constants of NUS 3032 nanosheets upon titration with amino acids and L-dopa.

We inferred that the molecular recognition may be induced by hydrogen bonds between guest molecules and host COF nanosheets. To prove our hypothesis, we tested the binding affinity of NUS-30 nanosheets toward L-mandelic acid and CO2H- or OH-protected mandelic


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acid (L-methyl mandelate and S-1-methoxyphenylacetic acid, Figure S41). As expected, Lmethyl mandelate and S-1-methoxyphenylacetic acid exhibit reduced Ksv values of 7687 M-1 and 9233 M-1, respectively, compared to that of L-mandelic acid (12825 M-1). This result suggests the involvement of both carboxylic acid and hydroxyl groups in the generation of a ground-state complex. Therefore, we believe that the signal amplified biomolecular recognition of NUS 30-32 nanosheets can be attributed to highly efficient hydrogen-bonded energy transfer with amino acids.96,98 Furthermore, the static nature of the hydrogen-bond-induced fluorescent quenching is supported by time-resolved photoluminescence (TRPL) measurements, which show the unchanged lifetime after titration with L-phenylalanine and L-threonine (Figure S42). Additionally, the specific recognition is enhanced by azine moieties in the COF structures (NUS30 > NUS-31 > NUS-32), suggesting that the azine moieties may play an important role in generating hydrogen bonds. This point was further proved by contact angle measurements of the three COF bulk powders (Figure S43), in which NUS-30 exhibits a smaller contact angle (61.5°) than that of NUS-31 (66.8°) and NUS-32 (77.9°).


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Figure 8. Electrostatic potential surfaces of NUS-30 (a), NUS-31 (b), and NUS-32 (c) fragments obtained by DFT calculations. DFT calculations indicating the binding of L-phenylalanine molecule to NUS-32 (d, via the imine group) and NUS-30 (e, via the azine group) fragment in forming N−H···N hydrogen bonds, and NUS-32 (f, via the imine group) and NUS-30 (g, via the azine group) fragment in forming O−H···O hydrogen bonds. Reduced density gradient figures show the hydrogen bonds with blue isosurfaces, van der Waals interactions with green isosurfaces, and steric effect with red isosurfaces.


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DFT calculations of binding energy. In order to understand why NUS-30 nanosheets have a stronger binding affinity than the other two nanosheets, DFT calculations were conducted to determine the electrostatic surface potential of NUS 30-32 fragments. Figure 8a-c show regions of low potential (red) at the imine and azine moieties (nitrogen atom), and high potential (blue) at TPE phenyl rings (carbon atom), indicating that the N-atoms are the most likely binding sites for the accommodation of amino acid molecules through the formation of hydrogen bonds. In the case of L-phenylalanine, the host-guest interactions for NUS-30 and NUS-32 may occur through two types of hydrogen bonds, namely N−H···N (2.2 – 2.4 Å) and O−H···N (1.7 – 1.9 Å). Following DFT optimization, the binding energies for N−H···N hydrogen bond were calculated to

be

-4.5

and

-5.1

kcal

mol-1

for

L-phenylalanine@NUS-32

fragment

and

L-

phenylalanine@NUS-30 fragment, respectively (Figure 8d-e), whereas the binding energies for O−H···N bond were -8.6 and -11.4 kcal mol-1 for L-phenylalanine@NUS-32 fragment and Lphenylalanine@NUS-30 fragment, respectively (Figure 8f-g). The higher binding energies for Lphenylalanine@NUS-30 fragment in both hydrogen-bonding scenarios confirm that the azine moiety does have a higher binding affinity toward L-phenylalanine than the imine functional group. In addition, reduced density gradient (RDG) calculations further show the stronger O−H···N hydrogen bond (blue isosurfaces) in L-phenylalanine@NUS-30 fragment. Notably, that strong intermolecular π−π interaction (green isosurfaces) is also observed between the phenyl ring of L-phenylalanine and NUS-30 fragment (Figure 8g). Thus, the configuration in Figure 8g possesses the strongest binding energy among the four configurations due to the cooperative hydrogen−bonding and intermolecular π−π interactions. All these theoretical calculation results agree well with the previous experimental observations. Hence, the heightened binding affinities for L-phenylalanine at the azine moieties underpin the superior sensing performance of NUS-30


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nanosheets among the COF nanosheets in this work. Notably, this study represents the first example of controlled binding of biomolecules by azine moieties in ultrathin 2D COF nanosheets. Composite membrane containing NUS-30 nanosheets fabricated by electrospinning. The amplified quenching signal of NUS-30 nanosheets by amino acids in solutions, alongside their freestanding and dispersible nature, represent promising features for the integration of NUS-30 nanosheets into composite membranes for practical applications. In this study, such a composite membrane was prepared by electrospinning (Figure 9a), which is a facile method to fabricate nanofiber membranes with high-flexibility and excellent gas or liquid permeability.99 Polyvinylidene fluoride (PVDF) was chosen as the polymeric matrix to prepare the composite membrane due to the absence of electron-deficient groups in this polymer preventing any interference with the fluorescent emission of the nanosheets. In addition, the high strength and excellent chemical stability of PVDF enable membrane operation under corrosive conditions. The prepared composite membrane has a large portion of porosity (Figure 9b) with a thin thickness (~120 μm, Figure 9c), facilitating the fast diffusion and transport of analytes. In addition, it also exhibits excellent mechanical strength suitable for practical applications (Figure S44). The uniform yellow fluorescence emission of the membrane suggests a homogeneous dispersion of NUS-30 nanosheets within the PVDF matrix (Figure 9d-e).


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Figure 9. (a) Schematic illustration of the preparation of composite membrane containing NUS30 nanosheets by electrospinning. (b) FE-SEM image of the front view of the composite membrane. (c) FE-SEM image of the cross-section part of the composite membrane. Optical (d) and corresponding fluorescent photographs (e) of the composite membrane. (f) Time-dependent fluorescence intensity changes (I/I0) of the composite membrane upon 10 min exposure to amino acid solutions (λex = 365 nm, λem = 465 nm). (g) The quenching percentages of the composite membrane by amino acid after 2 min and 10 min exposure.


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Fluorescence quenching was observed when the membrane was exposed to various amino acids including L-phenylalanine, L-tryptophan, L-alanine, L-threonine, and L-dopa. As shown in Figure 9f, the membrane exhibits an extended fluorescence quenching response time compared with solution-based sensing of these amino acids. This can be attributed to the slow diffusion of analytes from the solution phase into the membrane matrix. The quenching kinetics were studied by measuring the fluorescence intensity of the membrane (λex = 365 nm, λem = 465 nm) in various amino acid solutions up to 10 min. Notably, L-dopa causes the fastest fluorescence quenching among all the tested amino acids. The extent of quenching for L-dopa, as estimated by the formula (I0–I)/I0 × 100%, is 20.5 % after 2 min exposure, which is substantially higher than that of the other amino acids such as L-phenylalanine (10.9 %), L-tryptophan (8.3 %), L-alanine (3.9 %), and L-threonine (7.9 %). When the exposure time was prolonged to 10 min, the quenching extent of L-dopa increased to 43.6 %, which still remains higher than that of L-phenylalanine (31.1 %), L-tryptophan (17.1 %), L-alanine (9.6 %), and L-threonine (15.2 %, Figure 9g). The remarkably quenched fluorescence of composite membrane upon exposure to Ldopa is consistent with the behavior of free-standing NUS-30 nanosheets in solution. Although many synthetic fluorescence biosensors for the recognition of amino acids have been studied, reports on ultrathin 2D COF nanosheets that can specifically recognize L-dopa remain limited. Thus, our results suggest that 2D COF nanosheets are promising material candidates for the fabrication of composite materials or devices for practical biosensing applications. CONCLUSION In conclusion, we report the controlled synthesis of azine-linked and imine-linked 2D COFs named NUS 30-32 using monomers containing AIE rotor-active TPE moieties, and demonstrate the successful preparation of ultrathin 2D nanosheets with micrometer-size and


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thicknesses of around 2 – 4 nm by temperature-swing gas exfoliation approach. The molecular structure and morphology of NUS 30-32 nanosheets were analyzed by various techniques including spectroscopic methods, FE-SEM, HR-TEM, and AFM. In particular, we found that the dynamic behaviour of the AIE molecular rotors is partially restricted in a model compound with TPE core but longer arms, and can be fully locked in the confined 2D COF structures, leading to the suppressed AIE characteristics of NUS 30-32 nanosheets. Benefiting from the enhancement of conjugated conformations and unusual structure relaxation of the ultrathin 2D nanosheets, the fluorescence emission of NUS-30 nanosheets can be effectively quenched by a series of amino acids and pharmaceutical molecules (L-dopa). Control experiments reveal the amplified quenching signal of NUS-30 nanosheets compared to the monomer compound, COF model compound, and bulk COF powder. Moreover, the binding affinity of the COF nanosheets toward amino acids can be controlled by increasing the number of azine moieties in the COF structures, which can be attributed to the crucial geometric roles and stronger host-guest binding between azine moieties and amino acids revealed by DFT calculations. Finally, we demonstrate the electrospinning-based fabrication of composite membrane containing minimal loading of NUS30 nanosheets for the practical detection of biomolecules. The synthetic strategy demonstrated in this study can be used for other ultrathin 2D COF nanomaterials, which may have promising applications in gas sensors, biosensors, photocatalysis, electronics, and light emitting materials.


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ASSOCIATED CONTENT Supporting Information Experimental details on the synthesis and characterization of NUS 30-32 bulk powder and nanosheets, crystals of COF model compound TMBE, fluorescence sensing of amino acids, and DFT calculations as well as X-ray crystallography (CCDC: 1835603) are provided in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION J. Dong and X. Li contributed equally to this work. Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by National University of Singapore (CENGas R-261-508-001-646), Ministry of Education - Singapore (MOE AcRF Tier 1 R-279-000-472-112, R-279-000-540114), and Agency for Science, Technology and Research (PSF R-279-000-475-305, IRG R-279000-510-305).


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Restriction of Molecular Rotors in Ultrathin Two-Dimensional Covalent

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