A Stable Luminescent Covalent Organic Framework Nanosheet for Sensitive Molecular Recognition

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES15 Nov 2022A Stable Luminescent Covalent Organic Framework Nanosheet for Sensitive Molecular Recognition Yaozu Liu†, Junxia Ren†, Yujie Wang, Xin Zhu, Xinyu Guan, Zisheng Wang, Yida Zhou, Liangkui Zhu, Shilun Qiu, Shengxiong Xiao and Qianrong Fang Yaozu Liu† State Key Laboratory of Inorganic Synthesis and Preparative Chemistry College of Chemistry, Jilin University, Changchun 130012 , Junxia Ren† State Key Laboratory of Inorganic Synthesis and Preparative Chemistry College of Chemistry, Jilin University, Changchun 130012 The Education Ministry Key Laboratory of Resource Chemistry, Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Frontiers Science Center of Biomimetic Catalysis, College of Chemistry and Materials Science, Shanghai Normal University, Shanghai 200234 , Yujie Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Inorganic Synthesis and Preparative Chemistry College of Chemistry, Jilin University, Changchun 130012 , Xin Zhu The Education Ministry Key Laboratory of Resource Chemistry, Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Frontiers Science Center of Biomimetic Catalysis, College of Chemistry and Materials Science, Shanghai Normal University, Shanghai 200234 , Xinyu Guan State Key Laboratory of Inorganic Synthesis and Preparative Chemistry College of Chemistry, Jilin University, Changchun 130012 , Zisheng Wang State Key Laboratory of Inorganic Synthesis and Preparative Chemistry College of Chemistry, Jilin University, Changchun 130012 , Yida Zhou State Key Laboratory of Inorganic Synthesis and Preparative Chemistry College of Chemistry, Jilin University, Changchun 130012 , Liangkui Zhu State Key Laboratory of Inorganic Synthesis and Preparative Chemistry College of Chemistry, Jilin University, Changchun 130012 , Shilun Qiu State Key Laboratory of Inorganic Synthesis and Preparative Chemistry College of Chemistry, Jilin University, Changchun 130012 , Shengxiong Xiao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] The Education Ministry Key Laboratory of Resource Chemistry, Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Frontiers Science Center of Biomimetic Catalysis, College of Chemistry and Materials Science, Shanghai Normal University, Shanghai 200234 and Qianrong Fang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Inorganic Synthesis and Preparative Chemistry College of Chemistry, Jilin University, Changchun 130012 https://doi.org/10.31635/ccschem.022.202202352 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Despite rapid advances in fluorescence detectors over the past decade, the development of a highly stable, sensitive, and selective fluorescence platform for molecular recognition remains a considerable challenge. Here we report a stable carbazole-based sp2 carbon fluorescence covalent organic framework (COF) nanosheet, termed a JUC-557 nanosheet. Owing to the synergistic effect of aggregation-induced emission- and aggregation-caused quenching-based chromophores, the architecture of the JUC-577 shows high absolute quantum yields (up to 23.0%) in the solid state and when dispersed in various solvents as well as excellent sensing performance toward specific analytes, such as iodine (Ka: 2.10 × 105 M−1 and LOD: 302 ppb), 2,4,6-trinitrotoluene (Ka: 4.38 × 105 M−1 and LOD: 129 ppb), and especially nitrobenzene (Ka: 6.18 × 106 M−1 and LOD: 5 ppb) that is superior to that of fluorescence detection materials reported so far. Furthermore, the JUC-557 nanosheet preserves strong luminescence and sensitive recognition, even under harsh conditions, and allows trace detection of various analytes via a handheld UV lamp. These findings pave the way for developing stable ultrathin COF nanomaterials for highly sensitive and selective molecular detection. Download figure Download PowerPoint Introduction Sensitive and selective detection of various analytes has been widely applied in many scientific and technological fields, such as life science, environmental monitoring, and public security.1–4 Therefore, there is a growing demand for convenient, low-cost, real-time, and highly efficient methods based on portable and wearable sensors. Among these sensors, the optical sensor is a cost-effective and fast detection pathway for transmitting signals in intricate situations.5,6 Progress in materials science and chemistry has led to several distinct classes of optical materials, for example, molecular probes, supramolecules, amorphous polymers, and metal–organic frameworks (MOFs), which have remarkably advanced of detection of chemical and biological analytes.7–11 For example, the Zhao group12 synthesized an ultrathin 2D nanosheet (NUS-25) with tetraphenylethylene molecular rotors that has an effective chemical sensor for acenaphthylene in polycyclic aromatic hydrocarbons. Li et al.13 reported two stable fluorescent Zr(IV)-based MOFs, BUT-12 and BUT-13, for selective detection and removal of antibiotics and organic explosives in water solution. These outstanding fluorescence sensors include some important advantages, such as selectivity, sensitivity, and durability in complex and even harsh chemical environments.14,15 Notably, however, most of the current optical materials have problems meeting these requirements, and thus the development of stable fluorescence materials for highly selective and sensitive sensors is still of tremendous significance. Covalent organic frameworks (COFs), as an emerging class of crystalline porous polymers comprising light elements linked by covalent bonds, have recently attracted considerable attention.16–21 Due to the virtue of well-ordered pore structures, large specific surface, and high chemical stability, COF materials exhibit a wide range of potential applications, such as in gas sorption and separation, energy storage, catalysis, optoelectronics, and many others.22–33 In addition to the above advantages, these architectures also possess abundant organic building blocks, specific pore environments, and tunable chemistry, making them easy to modify to create ideal functionalized materials.34–36 Theoretically, highly designable construction of COFs is more accessibly used as a monitor of molecular recognition via increasable adsorption sites and adjustable fluorescence for targeted analytes.37–42 For instance, the Wang group43 reported a thioether-functionalized COF with multifunctionality, COF-LZU8, for the selective detection and facile removal of mercury(II). Qiu et al.44 developed a sp2 carbon-conjugated fluorescent COF, termed TFPT-BTAN-AO, which shows a fast response time and a low detection limit suitable for detection of UO22+ in H2O. Nevertheless, because of the scarcity of a suitable linkage that combines fluorescence with stability, most fluorescent COFs are chemically fragile, and stable COFs are less illuminating.45–47 Therefore, the COF materials with both stable and fluorescent for chemical detection remains a great challenge. Herein, we design a carbazole-based COF nanosheet connected by sp2 carbons, termed a JUC-557 nanosheet, which is exfoliated from its bulk COF (JUC-557, JUC = Jilin University China). Due to the complementary characteristic of phenyl ethylene (PE) as an aggregation-induced emission (AIE) group and carbazole as an aggregation-caused quenching (ACQ) chromophore in the same framework, the JUC-557 nanosheet has been proven to have high absolute quantum yields of ∼23.0% in both the solid-state and in solution. Therefore, the JUC-661 557 nanosheet as a sensitive fluorescence sensor exhibits excellent molecular recognition of the essential element in the human body, radioactive contaminants, explosives, and toxic nitro compounds, bringing forth I2 with an association constant (Ka) of 2.10 × 105 M−1 and limit of detection (LOD) of 302 ppb, 2,4,6-trinitrotoluene (TNT) with Ka of 4.38 × 105 M−1 and LOD of 129 ppb, and especially nitrobenzene (NB) with Ka of 6.18 × 106 M−1 and LOD of 5 ppb, which is much better than those optical detection materials reported to date, including fluorescence porous materials, small molecule probes, and inorganics. Furthermore, a combined theoretical and experimental study, including time-resolved photoluminescence (TRPL) measurements, ultraviolet–visible (UV–vis) absorption spectroscopy, and density functional theory (DFT) calculations, explains the fluorescence quenching mechanism of the JUC-557 nanosheet. Due to its high chemical stability, the JUC-557 nanosheet also shows excellent luminescence and sensitive molecular recognition, even under extreme conditions (e.g., in HCl with pH 1 or NaOH with pH 14), and allows optical trace detection of various analytes using visual detection equipment. Experimental Section Synthesis of JUC-557 A solution of 4,4′,4″,4″-([9,9′-bicarbazole]-3,3′,6,6′-tetrayl)tetrabenzaldehyde (BCTB-4CHO, 37.4 mg, 0.05 mmol 1.0 equiv), and 1,4-phenylenediacetonitrile (PDAN, 31.2 mg, 0.1 mmol 1.0 equiv) in dioxane (1.0 mL) containing NaOH (4 M, 0.1 mL) was degassed in a glass tube using three freeze/pump/thaw cycles. The tube was sealed off by flame and heated at 120 °C for 5 days. After cooling to room temperature, the tube was opened, and the precipitate was filtered off and washed with acetone (extracted with Soxhlet). The solid was dried under vacuum at 100 °C overnight to afford the JUC-557 as a yellow powder (48.7 mg, yield 71%). Solid-state 13C NMR (500 MHz) δ: 141.8, 131.3, 129.2, 125.8, 121.9, 117.2, 107.9. IR (KBr): 3032, 2214, 1597, 1480, 1451, 1416, 1364, 1276, 1231, 1228, 1194, 1132, 1002, 897, 840, 808, 724, 649 cm−1. Anal. Calcd: C, 86.37; H, 5.23; N, 8.39. Found: C, 86.68; H, 5.11; N, 8.21. Synthesis of JUC-557 nanosheet JUC-557 (100.0 mg) was dispersed into the methanol (200.0 mL) and sonicated for 10 h, and the supernatant was removed after centrifugation. The obtained supernatant was sonicated for 2 h and again removed after centrifugation. Then, the ultrathin JUC-557 nanosheet was dried by a vacuum. Solid-state 13C NMR (500 MHz) δ: 141.7, 131.2, 129.3, 125.5, 121.6, 117.3, 107.8. IR (KBr): 3032, 2215, 1599, 1480, 1451, 1417, 1378, 1279, 1231, 1228, 1194, 1130, 1004, 897, 840, 808, 724, 649 cm−1. Anal. Calcd: C, 86.37; H, 5.23; N, 8.39. Found: C, 86.54; H, 5.20; N, 8.26. Fluorescence recognition The titration experiments were conducted in a 3 mL standard quartz reactor equipped with the excitation wavelength of 352 nm. Typically, the JUC-557 or JUC-557 nanosheet (180.0 μg) was dispersed in 3.0 mL of water/EtOH (v:v = 9:1) solution with different analytes. The concentrations of cation and anion ions, I2, and aromatic nitro compounds were 1.0 × 10−2 M, 5 × 10−3 M, and 5 × 10−4 M, respectively. Recycling test The recycling test was carried out by washing the JUC-557 nanosheet with acetonitrile and methanol to remove guest molecules in the cavities. Before the detecting test, the JUC-557 nanosheet was centrifuged and dried in a vacuum at 100 °C. Highest occupied molecular orbital–lowest unoccupied molecular orbital energy calculations Initially, the structure of JUC-557 was optimized by Forcite using Materials Studio to remove geometric distortions. Then, a JUC-557 fragment was used in DFT calculations. The nitro explosives molecules and the JUC-557 fragment were optimized using the M06 functional with the 6-31G(d) basis set. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the JUC-557 fragment and nitro explosives molecules were calculated using the M06 functional with the 6-311G** basis set.48,49 All the DFT calculations were performed using Gaussian 16 software. Adsorption energy calculations The adsorption energy is defined by Ead = Etotal − ECOF − Em, where Etotal, ECOF, and Em are the energies of the whole system, COF, and one small molecule, respectively. All computations were performed within the framework of DFT as implemented in the Vienna Ab-initio Simulation Package code by using the projector augmented wave method with the Perdew–Burke–Ernzerhof exchange-correlation functional.50–52 The influence of vdW interactions was considered by using a modified version of vdW-DF, referred to as "optB86b-vdW."53,54 The projector augmented wave potentials55 were used with a kinetic energy cutoff of 500 eV. Brillouin zone integration was sampled by a 1 × 1 × 1 gamma-centered k-mesh. The system was simulated with a periodic boundary condition. Initially, the COF was fully relaxed using the conjugate gradient method until the force on each atom was less than 0.05 eV Å−1. Then, one specific small molecule (Fe3+, I2, and TNT) was adsorbed on the carbazole top site or –CN lateral site of optimized COF. The whole system was relaxed again. Finally, the static calculations were executed to obtain the single-point energy. Results and Discussion Designed synthesis and characterizations of JUC-557 To achieve an excellent fluorescence material, we introduced the carbazole-based derivative, BCTB-4CHO (Figure 1a), as a planar four-connected building block. Due to the attractive rich electron properties and high chemical stability, carbazolyl and its derivatives have been widely used as famous chromophores.56,57 Furthermore, PDAN (Figure 1a) was chosen as a linear linker, a typical building unit for the formation of C=C bonds. We thus developed a novel sp2 carbon-linked COF (JUC-557) with sql net and microporous channels of 1.8 nm (Figure 1a). Notably, the inherent sp2 C=C linkages simultaneously improved both in-plane π-electron delocalization and the chemical stability of COF materials.58–60 More importantly, after the construction of JUC-557, a typical AIE functional moiety, PE group, was formed in the framework (Figure 1a). Therefore, a novel two-dimensional fluorescence material, JUC-557, was successfully established, based on the combination of characteristics of ACQ- and AIE-based fluorophores, making it a promising fluorescent material (Figure 1b). Figure 1 | (a) Synthesis of JUC-557 based on the condensation reaction of BCTB-4CHO and PDAN. (b) Vertical projection view of the crystal structure of JUC-557 with square pores of 1.8 nm. (c) Experimental and refined PXRD profiles of JUC-557. (d) N2 adsorption–desorption isotherm with a BET surface area of 1115 m2/g and the pore-size distribution of JUC-557 indicating a microporous width of ∼1.8 nm. (e) PXRD patterns of JUC-557 before and after the treatment in 12 M HCl, 14 M NaOH, and boiling H2O for 3 days. Download figure Download PowerPoint The crystal structure of JUC-557 was analyzed by using its powder X-ray diffraction (PXRD) pattern together with its structural simulation. After a geometrical energy minimization performed on the Materials Studio software package, the unit cell parameters based on an AA stacking sql net were obtained (a = 35.0907 Å, b = 41.6802 Å, c = 4.0831 Å, and α = β = γ = 90°). The experimental PXRD peaks were quite consistent with the simulated ones (Figure 1c). Full profile pattern matching (Pawley) refinement was also implemented, which featured three main peaks at 2θ = 3.3°, 6.5°, and 9.8°, assigned to the (110), (220), and (330) Bragg peaks of the space group P222 (No. 16). The refinement results revealed that the unit cell parameters were close to ideal agreement factor predictions (wRP = 1.36% and RP = 1.08%). In contrast, the simulated PXRD pattern of an alternative staggered 2D arrangement (AB stacking) showed distinct deviations compared with the experimental ( Supporting Information Figures S1–S3 and Tables S1 and S2). Its porosity and textural characteristics were analyzed based on the nitrogen (N2) adsorption–desorption isotherm at 77 K (Figure 1d). A sharp gas uptake was observed at low pressure (below 0.05 P/P0), which revealed its microporous nature. The Brunauer–Emmett–Teller (BET) surface area was 1115 m2/g, and the total pore volume was calculated to be 0.637 cm3 g−1 at P/P0 = 0.9 ( Supporting Information Figure S4). The inclination of isotherms in the 0.8–1.0 P/P0 range and small hysteresis can be attributed to the presence of textural mesopores, which was the consequence of the agglomeration of COF crystals.61,62 The pore-size distributions were estimated by the nonlocal DFT and demonstrated the microporous width of ∼1.8 nm, which matched well with that from its crystal structure (Figure 1d, inset). The thermal gravimetric analysis of JUC-557 showed only 4% weight loss, up to 400 °C, indicating good thermal stability ( Supporting Information Figure S5). Furthermore, JUC-557 was inspected by immersing it in different organic solvents, boiling water, and strong acid/base (12 M HCl and 14 M NaOH aqueous solutions) for 3 days or by UV irradiation for 3 h. Unchanged PXRD patterns were observed, manifesting the high chemical stability of sp2 carbon-linked COFs (Figure 1e and Supporting Information Figures S6 and S7). Preparation and analysis of JUC-557 nanosheet Compared with bulk materials, fluorescent nanosheets usually have higher quantum yields and easily accessible combining sites for molecular recognition.12,42,48 Thus, we exfoliated JUC-557 into an ultrathin nanosheet (termed the JUC-557 nanosheet) by sonication in the presence of methyl alcohol (Figure 2a). Notably, the highly twisted molecular conformation of the carbazole unit can weakened interlayer π–π stacking, leading to easy formation of nanosheets. As we expected, the JUC-557 nanosheet displayed a graphene-like morphology (Figure 2c and Supporting Information Figures S8–S10). Lattice fringes between layers (∼0.4 nm) were observed by the high-resolution transmission electron microscopy (HR-TEM, Figure 2d). The atomic force microscopy (AFM) measurements further confirmed a molecular-level understanding of anticipated ultrathin nanostructure. The thickness of the JUC-557 nanosheet was in the range of 2.2∼2.6 nm, and thus the number of layers was speculated to be 5–6 layers (Figure 2e,f and Supporting Information Figure S11). The homogeneous lateral size of the JUC-557 nanosheet ranged from 0.5 to 1.0 μm ( Supporting Information Figure S12). Furthermore, the suspension of JUC-557 nanosheet in acetonitrile solution exhibited a typical Tyndall effect, indicating the colloidal feature of the solution with uniformly dispersed nanosheets ( Supporting Information Figure S13). Figure 2 | (a) Schematic diagram of JUC-557 nanosheet from the exfoliation of its bulk layered material JUC-557. (b) A comparison of the UV–vis absorption band and the fluorescent emission of JUC-557 and JUC-557 nanosheet. (c) TEM image of JUC-557 nanosheet. (d) HR-TEM image of JUC-557 nanosheet featuring the lateral lattice structure. (e) AFM image and thickness (∼2.5 nm) of JUC-557 nanosheet. Top: seven layers of JUC-557 nanosheet based on the optimized AA stacking modeling structure. Thickness measurements of five locations (f) from Figure (e). (g) The colors of JUC-557 and JUC-557 nanosheet on the CIE chromaticity diagram. Download figure Download PowerPoint Unlike bulk JUC-557, its nanosheet counterpart did not present an apparent PXRD pattern ( Supporting Information Figure S14), indicating that the pristine bulk was converted to a dissociative nanosheet.38 Besides, the JUC-557 nanosheet was by and solid-state 13C and these that the exfoliated nanosheet the same chemical ( Supporting Information Figures and The JUC-557 nanosheet still good thermal stability ( Supporting Information Figure Nevertheless, the specific surface area of the JUC-557 nanosheet remarkably from 1115 to m2/g in comparison with its material ( Supporting Information Figures and Due to the structural from the showed a toward a higher ( Supporting Information Figure and the UV–vis absorption band was from to nm (Figure Furthermore, we conducted the UV–vis to the optical between the bulk powder and nanosheet. As in Supporting Information Figures and the absorption band of the JUC-557 nanosheet be attributed to its π–π interactions by the and the calculated was to eV compared with the bulk COF Fluorescence characteristics of the JUC-557 nanosheet As expected, the fluorescent emission of the JUC-557 nanosheet = nm) exhibited a of nm compared to that of bulk JUC-557 = nm, Figure The observed emission colors were on the chromaticity yellow for the JUC-557 and for the JUC-557 nanosheet (Figure As in Supporting Information the fluorescence absolute quantum yield of bulk JUC-557 in the solid state was only which can be attributed to the of PE as AIE compared with its bulk the JUC-557 nanosheet in the solid state showed a of fluorescence absolute quantum yield Supporting Information which is better than those of COF materials of only is because of an emission by the luminescence quenching of carbazole and of the fluorescence of PE Furthermore, the JUC-557 nanosheet dispersed in showed a more improved absolute quantum yield (up to 23.0%) because the nanosheets can be well in leading to further fluorescence ( Supporting Information the of the JUC-557 nanosheet showed an compared with its bulk material Supporting Information Figure Notably, the JUC-557 nanosheet also a stable fluorescence in harsh chemical environments, such as in strong pH and aqueous solution pH Supporting Information Figures Fluorescence recognition of JUC-557 nanosheet for by the outstanding fluorescence we the and of the JUC-557 nanosheet for recognition of including and and and and only led to an apparent fluorescence quenching of the JUC-557 nanosheet. The quenching of the JUC-557 nanosheet for the was up to the quenching for were (Figure and Supporting Information Figure Furthermore, the showed a linear = between the fluorescence quenching of the JUC-557 nanosheet and and the Ka was × M−1 (Figure For ions, Ka were much than that of such as × M−1 for × M−1 for and × M−1 for (Figure and Supporting Information Figures The fluorescence quenching of the JUC-557 nanosheet for was still at a titration as low as 1.0 × M, and the LOD is the standard of detection was calculated to be × M ppb) in Supporting Information Figures In the quenching of the JUC-557 nanosheet for showed even after the stability and of the sensor based on JUC-557 nanosheet (Figure As a fluorescence recognition of bulk JUC-557 for the was also carried The quenching was and its Ka was only × which is one of than that of the JUC-557 nanosheet (Figure and Supporting Information Figures Figure 3 | Fluorescence emission (a) and (b) of JUC-557 nanosheet = titration with solution M in = at room = 352 A comparison of Ka of (c) JUC-557 nanosheet titration with different M in = Recycling test of (d) JUC-557 nanosheet for the detection of A comparison of Ka of (e) JUC-557 and JUC-557 nanosheet titration with solution M in = The fluorescence of (f) JUC-557 nanosheet combined with = titration with different M in = at room = 352 Download figure Download PowerPoint To further the quenching was to monitor quenching The of the JUC-557 nanosheet was after with different concentrations Supporting Information Figure indicating the static of the fluorescence The apparent quenching constant = was to be × M−1 which is two of higher than those of quenching M−1 revealed that the fluorescence quenching of the JUC-557 nanosheet for can be attributed to the formation of a which be an adsorption quenching mechanism and has also been by the of UV–vis absorption peaks of the JUC-557 nanosheet after the addition of in Supporting Information Figures and Furthermore, the fluorescence of in water with that of the JUC-557 nanosheet, which that the energy also fluorescence quenching ( Supporting Information Figure Therefore, the synergistic mechanism of static quenching and energy quenching to an excellent recognition of the by the JUC-557 nanosheet. JUC-557 nanosheet combined with for the detection of we fluorescence sensing and selective recognition of JUC-557 nanosheet for including and JUC-557 nanosheet showed low to various ( Supporting Information Figure the system of the JUC-557 nanosheet combined with was much more sensitive to specific such as and (Figure The fluorescence of JUC-557 nanosheet after = was when the was into system, the fluorescence of the JUC-557 nanosheet with was remarkably (up to which be attributed to the formation of a stable The fluorescence of the JUC-557 nanosheet with not be fully because there be a between the group and with different from the detection of the addition of in even more of the fluorescence of the JUC-557 nanosheet with low as can be to I2 by were the formation of I2, the of fluorescence of the JUC-557 nanosheet with was which be because the nanosheet with was more sensitive to the to the of material for the detection of Fluorescence recognition of the JUC-557 nanosheet for I2 iodine and are typical that which is a for environmental and human Therefore, it is important to a sensitive to on the above we further the of the

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