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Highly Fluorescent Polyimide Covalent Organic Nanosheets as Sensing Probes for the Detection of 2,4,6-Trinitrophenol Cuiling Zhang,† Shiming Zhang,‡ Yinghan Yan,† Fei Xia,†,§ Anni Huang,† and Yuezhong Xian*,† †

Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200241, China ‡ Institute of Electrochemical and Energy Technology, Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China § NYU-ECNU Center for Computational Chemistry at New York University Shanghai, East China Normal University, Shanghai 200062, China S Supporting Information *

ABSTRACT: A new fluorescent polyimide covalent organic framework (PI-COF) has been successfully synthesized through solvothermal route using tetra(4-aminophenyl) porphyrin and perylenetracarboxylic dianhydride, which possesses porous crystalline and excellent thermal stability (>500 °C). Furthermore, few-layered PI covalent organic nanosheets (PI-CONs) can be easily obtained from the fluorescent PI-COF through a facile liquid phase exfoliation approach, which were confirmed by atomic force microscopy and transmission electron microscopy analysis. It is interesting that the fluorescent intensity of PI-CONs is obviously enhanced relative to that of PI-COF. The PI-CONs have been successfully utilized as an efficient fluorescent probe for the highly sensitive and selective detection of 2,4,6-trinitrophenol (TNP). The mechanism might be attributed to the combination of electron transfer and inner filter effect based on DFT calculations and spectral overlap data. The system exhibits a good linear response toward TNP over the range from 0.5 to 10 μM with a detection limit of 0.25 μM. KEYWORDS: polyimide covalent organic nanosheets, solvothermal, fluorescence, chemosensor, TNP

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INTRODUCTION As an emerging class of porous crystalline materials, covalent organic frameworks (COFs) have attracted considerable attention and shown promising applications in gas storage,1,2 chemosensors,3,4 catalysis,5,6 and electricity.7,8 COFs were made entirely from light elements (typically H, B, N, C, and O) and could crystallize into networks with highly ordered structures through the formation of strong covalent bonds. Since the pioneering work by Yaghi’s group9 in 2005, COFs have been prepared through organic reactions, including boronic acid self-condensation10 and esterification,11 Schiff base reaction,12 and squaraine-linked reaction.13 New reactions are desired to broaden the structural diversity and applications of COFs. Recently, novel polyimide (PI) COFs were synthesized by Yan’s group through the imidization reaction, and were used to load dye molecules and deliver drugs.14,15 As compared to other COFs, PI-COFs show excellent thermal stability, well crystalline, large pore sizes, and high surface area. Nevertheless, the investigation on the synthesis of various units of PI-COFs with unique properties is still challenging; for example, PI-COFs with fluorescent features and their applications in chemical and biological sensing have not been reported until now. As one of the most dangerous nitroaromatic explosives, 2,4,6trinitrophenol (TNP) possesses superior explosive efficacy as © XXXX American Chemical Society

compared to its well-known counterpart 2,4,6-trinitrotoluene (TNT).16,17 TNP has mutagenic activities to cause acute health problems, such as headache, liver injury, and diarrhea.18,19 In addition, it is also a major source of environmental pollution.20 Hence, highly selective and sensitive detection of TNP has attracted great research concern. Currently, a variety of approaches have been developed to detect TNP, such as surface-enhanced Raman and infrared absorption spectroscopy,21 chromatography−mass spectrometry,22 electrochemistry,23 and so on. Although these methods have some advantages, they are time-consuming, lack portability, and are expensive.24 Reliable methods for the detection of TNP are more desirable. Fluorescent-based sensors play a vital role for detecting TNP due to its high sensitivity, simplicity, and short response time. However, TNP is not inherently fluorescent. Ultrasensitive methods usually involve exogenous fluorescent reagents as probes. Organic dyes and inorganic nanomaterials (e.g., quantum dots, carbon dots, upconversion nanocrystals, and metal organic frameworks) have been employed for the construction of fluorescence-quenching-based sensors.25 Besides, fluorescent COFs have opened new horizons for trace Received: December 21, 2016 Accepted: April 4, 2017 Published: April 4, 2017 A

DOI: 10.1021/acsami.6b16423 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces detection of TNP.3,4,26,27 Das et al.3 synthesized imide-based COFs for fast and highly selective detection (about 1 ppm) of TNP. Wang’s group26 prepared a 3D pyrene-based COF for TNP detection (20 ppm). Cao’s group27 reported the sensing of TNP using covalent-organic polymers as fluorescent probes with low concentration (10−5 M). However, a relative low sensitivity of these sensors may limit their applications. Therefore, it is still desired to create new fluorescent probes for the determination of TNP with high sensitivity and selectivity. Herein, we designed and synthesized a porous crystalline PICOF with high thermal stability and surface area via the imidization reaction between tetra(4-aminophenyl) porphyrin (TAPP) and perylenetracarboxylic dianhydride (PTCA) (Scheme 1). The as-synthesized PI-COF displayed strong

Synthesis of PI-CONs. The PI-COF in this Article was synthesized by solvothermal route similar to the previously reported method.14 Typically, TAPP (5 mmol, 33.7 mg) and PTCA (10 mmol, 39.2 mg) were dissolved in 1 mL of mesitylene/1 mL of NMP/0.1 mL of isoquinoline, which was bottled with a 25 mL glass tube. The tube was flash frozen at 77 K through liquid N2 bath, and then was evacuated to an internal pressure of 0.15 mmHg. Next, the top of the glass tube was flame-sealed. The reaction mixture was heated at 180 °C for 4 days, and a porous precipate was obtained. The product was isolated by filtration and washed with THF. After further purification with THF for 24 h, the final product was dried in a vacuum oven at 65 °C for 24 h. The PI-CONs were prepared from PI-COF through sonication for 8 h in ethanol. To remove large-size masses, the yellow dispersion was then centrifugated at 3000 rpm for 5 min. The as-obtained supernatant was dispersed in ethanol for further characterizations and applications. Sensing Procedure for TNP. Different concentrations (0, 0.5, 1.5, 2.5, 5, 8, 10, 13, 16, 18, and 21 μM) of TNP were added to PI-CONs (0.01 mg/mL) in ethanol solution. The mixture was incubated for 2 min at room temperature. The fluorescence spectra of PI-CONs then were recorded immediately under excitation at λ = 375 nm. As for the calibration curve, the experiments were repeated three times to get average values. Theoretical Calculations. All of the calculations for PI-CONs, TNP−, TNP, and TNT were carried out with the Gaussian 09 software.28 To perfrom the quantum mechanics calculation for PICONs, the model was constructed by extracting a unit from the PICONs, and the boundaries of the truncated unit were capped with hydrogen atoms. The unit model for PI-CONs as well as the TNP−, TNP, and TNT molecules were optimized using the empirical PM6 method.29 On the basis of the optimized structures by PM6, the single point calculations were conducted by using the B3LYP/6-31G* method30,31 to obtain the related electronic information such as the energy levels of HOMO and LUMO orbitals. Apparatus and Characterization. Power X-ray diffraction (PXRD) data were collected on a D8 Discover X-ray diffractometer (Bruker, Germany) using Cu Kα radiation over the range of 2−30° at a scanning rate of 2° per minute. The size and morphology of PI-COF were characterized by a JEM-2100F transmission electron microscope (TEM) (JEOL, Japan) and by field-emission scanning electron microscopy (FE-SEM) (Zeiss Supra 55, Germany). Thermal behavior of PI-COF was investigated at Jupiter simultaneous thermal analyzer (Netzsch Sta 449 F3, Germany) with a heating rate of 5 °C min−1 from room temperature to 1000 °C. The adsorption isotherm for N2 was measured with a Micromeritics ASAP 2020 analyzer (Micromeritics, U.S.). A Tarazona nonlocal density functional theory (NLDFT) was employed to analyze slit pore geometry. Atomic force microscope (AFM) (Bruker, Germany) analysis of layered PICONs was acquired with tapping mode. The sample was prepared by depositing a droplet of dispersion on a freshly cleaved mica surface and dried at room temperature in a vacuum drying oven. FT-IR spectrum was recorded by a Nexus 670 optical bench (Nicolet, U.S.). The solidstate 13C NMR spectrum was recorded on a VARIAN VNMRS 400WB NMR spectrometer (Varian, U.S.). Thermogravimetric analysis (TGA) was characterized by STA 449 F3 Jupiter simultaneous thermo-analyzer (Netzsch, Germany). Fluorescence spectra were measured on a F-7000 fluorescence spectrophotometer (Hitachi, Japan). Fluorescence decay experiments were performed on an FLS 980 system (Edinburgh, UK).

Scheme 1. Schematic Representation for the Preparation of the Fluorescent PI-CONs and Their Application in the Detection of TNP

fluorescence due to the existence of a p−n heterojunction between TAPP and PTCA units. It could be exfoliated to fewlayered PI covalent organic nanosheets (CONs) with the help of ultrasonic. Interestingly, the fluorescence intensity of PICONs was obviously enhanced relative to the PI-COF structure. Furthermore, the PI-CONs were further employed as a fluorescent probe to detect TNP with high sensitivity and selectivity.

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EXPERIMENTAL SECTION

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Materials. 3,4,9,10-Perylenetracarboxylic dianhydride (PTCA) was purchased from J&K Scientific (Beijing, China). Tetra(4aminophenyl)porphyrin (TAPP), nitrobenzene (NB), nitrotoluene (NT), dinitrotoluene (DNT), 4-chlorophenol (MCP), hydroquinone (HQ), p-phenylenediamine (PPD), nitrophenol (NP), aniline (AN), dinitrophenol (DNP), and TNT (1 mg/mL in methanol) were obtained from Aladdin Industrial Co. Ltd. (Shanghai, China). 2,4,6Trinitrophenol (TNP) was obtained from Taishan Chemical Reagent Factory (Guangdong, China). The other chemical reagents were commercially available from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Ultrapure water was obtained from a Millipore purification system (Millipore, U.S.) and used throughout.

RESULTS AND DISCUSSION Synthesis and Characterization of PI-COF and PICONs. PI-COF was synthesized employing the solvothermal route by the imidization effect of TAPP and PTCA. First, the PXRD was performed to investigate the crystallinity of PI-COF. As shown in Figure 1A, the PXRD patterns possess intense diffraction peaks, indicating highly crystalline phase of PI-COF. We then carried out the structural simulation with an eclipsed AA- (blue curve) and AB-stacking (red curve) structure using B


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Figure 1. (A) PXRD pattern of PI-COF (black curve, experimental pattern; blue curve, AA-eclipsed pattern; red curve, AB-eclipsed pattern). Inset: Rietveld refinements of PI-COF (red cross, experimental pattern; black curve, refined pattern; green curve, difference pattern), the Bragg positions are labeled as pink bars; (B) FE-SEM image of PI-COF; and (C) TEM and (D) AFM images of PI-CONs.

image in Figure 1B confirms the layered structure. TGA is performed to check the weight loss (%) of PI-COF at high temperature. As observed from Figure S5, PI-COF is very stable up to 500 °C, displaying the good thermal stability. These results demonstrate that the crystalline PI-COF is successfully synthesized with high thermal stability and surface area. More importantly, the layered structure provides the possibility for the preparation of PI-CONs. The CONs from exfoliation of COF could reduce the aggregation and improve the availability of electron density among the layers as compared to COFs.3,32−34 In our work, the PI-CONs were prepared from PI-COF through one-step ultrasonic assisted exfoliation. The nanosheets were further characterized using TEM and AFM. As shown in Figure 1C, the TEM image exhibits the existence of flat thin PI-CONs. The AFM (Figure 1D) shows that the thickness of the as-prepared PI-CONs is about 1 nm, indicating about few-layered CONs.3 Fluorescence Properties of the PI-CONs. Figure 2 shows the fluorescence features of PI-CONs with the maximum excitation at 375 nm and the maximum emission at 500 nm. Under the same excitation wavelength, the PTCA and TAPP display fluorescence emission peaks at 605 and 673 nm, respectively (Figure S6). To the best of our knowledge, this is the first fluorescent PI-CON. Interestingly, PI-CONs exhibit a stronger fluorescence than that of PI-COF under the same excitation (Figure S7). The quantum yield of PI-CONs is calculated as 8%, which is higher than that of PI-COF (4%) using rhodamine 6G in ethanol as the reference standard (QY = 95%). The as-produced fluorescence was mainly attributed to the existence of a p−n heterojunction between TAPP and PTCA units in the PI-COF. The COF was exfoliated to be few

the Materials Studio software package, and the results display AA-stacking mode due to the existence of domain peak at around 3 °C (2θ) in agreement with the experimental profile. In addition, the predicted PI-COF structure was also validated with the Rietveld refinements. The inset in the Figure 1A shows the refined pattern (black curve) and difference pattern (green curve) between the experimental patterns and refined ones, indicating the experimental pattern in accord with the refined profile. The FT-IR spectrum of PI-COF (Figure S1) shows the absorptions at 1773 and 1722 cm−1, which correspond to the asymmetric and symmetric vibrations of CO groups, respectively.15 The peak at 1298 cm−1 is attributed to inplane bending vibration of C−H in phenyl rings. Besides, the band at 1398 cm−1 is due to the presence of the C−N−C moiety,14 whereas the peak of anhydride is not observed, demonstrating the formation of imide from anhydride units and amino groups. Furthermore, the structure of PI-COF was confirmed at the molecular level by solid-state 13C MAS NMR (Figure S2). The signal at 160.9 ppm could be ascribed to the carbonyl carbon of imide ring, and the other peaks at 116.8, 126.3, and 133.9 ppm were assigned to phenyl carbons.14 The permanent porosity of PI-COF is measured using the nitrogen absorption isotherms at liquid nitrogen temperature (77 K). As shown in Figure S3, PI-COF exhibits hysteresis reflecting abundant mesoporous structure. In addition, the dominant pore diameter is concentrated at ∼2.7 nm, which is almost in agreement with the crystal model (2.8 nm). The porous properties are summarized in Table S1, and the specific surface area of PI-COF is up to 894 m2 g−1. The external morphology is investigated by FE-SEM, and irregular block-like shapes are observed (Figure S4). Furthermore, the side view of FE-SEM C


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and picrate anion (TNP−). Thus, the fluorescence of PI-CONs was quenched by TNP. The speculation is supported by theoretical studies. DFT calculation was performed with B3LYP/6-31G* method using the Gaussion 09 program. As shown in Figure 4, it is clear that the lowest unoccupied

Figure 2. Excitation (red curve) and emission spectra (blue curve) of PI-CONs.

layered nanosheets, minimizing the aggregation and maximizing the availability of electron density among the layers. In addition, donor−acceptor charge transfer among the CONs was extended. All of these could enhance the fluorescence intensity of the CONs.3 Fluorescence Determination of TNP. Given the electronrich benzene ring, electron-deficient imide structure, and good fluorescent properties, PI-CONs were utilized to investigate the chemosensing ability for TNP, an explosive nitro-aromatic compound. As shown in Figure 3A, the fluorescence intensity of PI-CONs is gradually reduced along with the increasing concentration of TNP (0−21 μM). The plot of fluorescence intensity at 500 nm versus different concentration of TNP is shown in Figure S8, and a linear relationship is achieved in the range of 0.5−10 μM (Figure 3C). The detection limit is calculated to be 0.25 μM based on S/N = 3. In addition, a linear Sterm−Volmer curve is observed as shown in Figure 3B, and the quenching constant (Ksv) is found to be 1 × 107 M−1. Accordingly, the fluorescence quenching efficiency with TNP is about 64%. As compared to other fluorescence covalent organic materials-based sensing toward TNP (Table S2), the assay shows higher Ksv and sensitivity due to the enhancement of the fluorescence of PI-CONs. Mechanisms for the Detection of TNP. To demonstrate the phenomenon of fluorescence quenching, one possible mechanism is that the TNP, as an electron-deficient compound with nitro groups, can form a PI-CONs-picrate complex through the electron transfer between protonated PI-CONs

Figure 4. Pictorial representation of the electron transfer in the ground state occurring from the HOMO of picrate anion to the LUMO of protonated PI-CONs.

molecular orbital (LUMO) energy level of PI-CONs (−3.731 eV) is below the highest occupied molecular orbital (HOMO) of TNP− (−3.352 eV), indicating the possibility of ground-state electron transfer from TNP− to PI-CONs and following fluorescent quenching. In addition, we have also carried out theoretical studies to explain the difference between TNT and TNP both with three nitro groups. As shown in Figure 5, it can be observed obviously that the HOMO energy of TNT (−8.840 eV) is much lower than the LUMO energy of PICONs (−3.731 eV). Thereby, the electron transfer is forbidden, and TNT hardly impacts the fluorescence of PICONs. It was further proved by the fluorescence lifetime investigation. The fluorescence decay curves are shown in Figure 6, indicating that the fluorescence lifetime of PI-CONs decreases with the increasing TNP concentration (0 μM, 5.28 ns; 5 μM, 4.75 ns; 10 μM, 4.21 ns). The phenomenon reveals the occurrence of dynamic quenching. Besides, inner filter effect (IFE) plays a vital role in fluorescence quenching if the excitation and/or emission light of fluorophore is absorbed by the absorbers.35−37 It is observed from Figure S9 that the UV−

Figure 3. (A) Fluorescence spectra of PI-CONs in ethanol phase in the presence of different concentrations of TNP (0, 0.5, 1.5, 2.5, 5, 8, 10, 13, 16, 18, and 21 μM); (B) Sterm−Volmer plots for the quenching of PI-CONs using TNP; and (C) the linear relationship between the fluorescence intensity and the TNP concentration (0, 0.5, 1.5, 2.5, 5, 8, and 10 μM). D


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Figure 7. Changes in fluorescence intensity induced by different analytes; the concentration of them is 10 μM. The fluorescence intensity of PI-CONs is defined as F0, and F is the fluorescence intensity in the presence of tested analyses. Excitation wavelength: 375 nm.

Figure 5. HOMO−LUMO energy profile of PI-CONs, picrate (TNP−), TNP, and TNT, respectively.

the PI-CONs-ased sensing platform shows excellent selectivity for TNP detection over other nitroaromatics.

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CONCLUSIONS We demonstrate that a new fluorescent PI-COF is successfully synthesized through imidization reaction between TAPP and PTCA. The PI-COF exhibits well crystalline, porous, and good thermal stability (>500 °C). We find that the PI-CONs derived from the bulk PI-COF by ultrasonic assistance exfoliation display enhanced fluorescence quantum yield (∼8%) as compared to that of PI-COF. It is further employed as a novel fluorescent probe to detect TNP due to the electron transfer and IFE mechanism. In particular, the Ksv of TNP toward PI-CONs is 1 × 107 M−1, which is higher than that of COFs. The approach has a good linear relationship toward TNP from 0.5 to 10 μM with the detection limit of 0.25 μM. We expect that the fluorescent PI-CONs will benefit future research for environmental and biological applications.

Figure 6. Fluorescence decay curves of PI-CONs in the presence of different amounts of TNP.

vis absorption spectrum of TNP has an effective overlap with the excitation spectrum of PI-CONs. The spectral overlap reveals that the IFE mechanism is also an important factor for the fluorescence quenching. On the basis of the abovementioned experimental results, the combination of electron transfer and IFE may be attributed to the high sensitivity of PICONs for the detection of TNP. Selectivity of TNP Detection. To validate the selectivity of the chemosensor toward TNP, other interferences including NB, NT, DNT, TNT, MCP, HQ, AN, PPD, NP, and DNP were tested with the same procedure as in the case of TNP. As shown in Figure 7, NB, NT, DNT, TNT, MCP, and HQ have negligible impact on the detection of TNP. Although other analytes, such as PPD, NP, and DNP, can trigger some fluorescence changes, the variation is very small while relative to TNP. The LUMO and HOMO orbital energies of other interferences were listed in Table S3. The HOMO energies (Table S3) were much less than the LUMO energy of PICONs (Figure 5), which would not occur during electron transfer. In addition, UV−vis absorption bands of TNP and other interferences were determined at the same concentration. As shown in Figures S9 and S10, the absorption spectra of TNP and DNP have an overlap with the excitation spectrum of PICONs. However, the absorption intensity of TNP was much higher than that of DNP at the same concentration. The phenomenon revealed that other interferences could not induce evidently the IFE effect. The experimental results suggest that

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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.6b16423. Characterization of PI-COF, including FT-IR, BET, SEM, and TGA; NMR; fluorescence spectra of PI-COF and PI-CONs; comparison of various determination methods toward TNP; UV−vis absorption spectrum of TNP and the excitation spectrum of PI-CONs; LUMO and HOMO orbital energies of other interferences; and UV−vis absorption bands of TNP and other interferences (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +86-021-54340046. E-mail: [email protected]. cn. ORCID

Fei Xia: 0000-0001-9458-9175 Yuezhong Xian: 0000-0001-9535-3658 Notes

The authors declare no competing financial interest. E


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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (nos. 21605050, 21606149), the Shanghai Natural Science Foundation (no. 15ZR1411600), and the China Postdoctoral Science Foundation (nos. 2015M570349, 2016M591678).

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Research Article

ACS Applied Materials & Interfaces (37) Li, J.; Li, X.; Shi, X.; He, X.; Wei, W.; Ma, N.; Chen, H. Highly Sensitive Detection of Caspase-3 Activities via a Nonconjugated Gold Nanoparticle−Quantum Dot Pair Mediated by an Inner-Filter Effect. ACS Appl. Mater. Interfaces 2013, 5, 9798−9802.

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