Journal ofMaterials Chemistry A

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A trifluoromethy

aState Key Laboratory of Marine Resource

University, Haikou, 570228, China. E-ma

foxmail.combSchool of Chemistry & Chemical Enginee

ChinacDepartment of Bioengineering, Zunyi Medi

519041, China. E-mail: jcyczh@126.com

† Electronic supplementary informa10.1039/d0ta07708a

‡ These authors contributed equally to th

Cite this: DOI: 10.1039/d0ta07708a

Received 6th August 2020Accepted 8th November 2020

DOI: 10.1039/d0ta07708a

rsc.li/materials-a

This journal is © The Royal Society

l-grafted ultra-stable fluorescentcovalent organic framework for adsorption anddetection of pesticides†

Yuxiang Zhao,‡a Zhuyin Sui,‡b Zhaosen Chang,‡a Shunli Wang,a Ying Liang,a Xin Liu,a

Lijuan Feng,*c Qi Chen *a and Ning Wang *a

A fluorine-rich covalent organic framework, COF-(CF3)2, was designed and prepared by post-

functionalization via the Povarov reaction in a flask in order to take up fluorinated pesticides. Compared

with those obtained by a synthetic method performed in a Pyrex tube, the as-prepared COFs in our

process not only possess higher specific surface areas, but also can be obtained on a large scale. It's

worth noting that COF-(CF3)2 exhibits strong chemical stability even in KMnO4 or NaBH4 solutions,

which is highly beneficial for its practical applications. Considering the strong and selective fluorine–

fluorine interactions, COF-(CF3)2 was used for the uptake of fluorinated pesticides and showed higher

adsorption capacities for trifluralin (151 mg g�1) and fipronil (171 mg g�1). We also found unexpectedly

that COF-(CF3)2 possesses strong fluorescence due to the enhanced conjugated system via

cycloaddition functionalization. This unique property can be applied in detecting trifluralin through

fluorescence quenching induced by their interaction.

Introduction

A variety of insecticides and herbicides developed during agri-cultural production and daily life have led to great ecologicaland environmental damage because of their excessive usage.1–3

For example, triuralin4 is easy to accumulate and difficult todegrade, and thus can cause cancer to humans and livestock atacute concentrations. Fipronil5 is unfriendly to butteries anddragonies, threatening biodiversity. These two pesticidesexisting in soil and water2,6 are absorbed as pollutants by plantsand aquatic animals.7 Subsequently, they circulate through thefood chain and eventually accumulate in human beings andanimals, which can induce negative effects on their life andhealth.8 Therefore, the removal of pesticide residues in theenvironment has drawn great attention from scientists andresearchers.

Currently, adsorption with porous adsorbents is the cheap-est and most effective treatment approach to remove pesticideresidues.9 The adsorption efficiency and selectivity are closely

Utilization in South China Sea, Hainan

il: chenqi@hainanu.edu.cn; wangn02@

ring, Yantai University, Yantai 264005,

cal University (Zhuhai Compus), Zhuhai

tion (ESI) available. See DOI:

is work.

of Chemistry 2020

related to the specic surface area, pore size and specicrecognition of porous materials.10 So it is very signicant todesign new functional materials with a high specic surfacearea, a suitable pore size and specic active sites. Up to now,adsorption materials have included activated carbon,11,12

molecular sieves,13 MOFs,14 and porous organic polymers.15

Covalent organic frameworks (COFs) are a kind of crystalizedporous organic polymer and possess the advantages mentionedabove, especially in the aspects of post-functionalization andmodication.16,17 They are widely used in energy storage,18–22

catalysis,23–26 adsorption,27–33 luminescence34–36 and sensing.37–40

As far as we know, there have been no reports on adsorbingpesticides using COFs. Meanwhile, the stability of COFs is alsoa hotspot that researchers keep an eye on when consideringtheir applications.41 Since, for most of the imine COFs, thereversible imine bond is unstable and leads to breaking down inpractical applications, it's very challenging to develop ultra-stable COFs. Recently, Liu42 and Dong43 explored similarmethods to stabilize the reversible imine bond through thePovarov reaction, individually, which can also introduce activefunctional groups on COF matrices at the same time. However,their materials are all prepared in a Pyrex tube which can onlyproduce small amounts of nal COFs and limits the practicalapplication due to complicated operation to seal the glass tube.In this work, we simplied the experimental procedure andprepared a triuoromethyl-graed COF in a ask on a largescale. The new uorine-modied COF (COF-(CF3)2) exhibitsultrastability under strongly acidic and basic conditions and itsweight and crystallinity barely changed. COF-(CF3)2 with a high

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specic surface area (SBET ¼ 1533 m2 g�1) and abundant tri-uoromethyl groups can take up uorinated pesticides rapidlyand selectively, because triuoromethyl groups can interact wellwith pollutants through the strong and selective uorine–uo-rine interaction44,45 as well as hydrophobic interaction.46,47

Surprisingly, since the highly polarized imine bond is convertedto a quinoline unit aer cycloaddition, highly conjugated COF-(CF3)2 exhibited excellent uorescence performance36 whenexcited at 365 nm wavelength. This unique property can beapplied in detecting triuralin through uorescence quenchinginduced by their interaction.

Experimental sectionMaterials

2,5-Dimethoxy-1,4-benzenedicarboxaldehyde and 1,3,5-tris(4-aminophenyl)benzene were purchased from Jilin ChineseAcademy of Sciences-Yanshen Technology Co., Ltd. p-Chloraniland boron triuoride diethyl etherate were obtained fromShanghai McLean Biochemical Technology Co., Ltd. Fipronil and3,5-bis (triuoromethyl)phenylacetylene were purchased fromShanghai bide Pharmaceutical Technology Co., Ltd. Triuralinwas purchased from Shanghai Aladdin Biochemical TechnologyCo., Ltd. None of the reagents and chemicals were further puri-ed. Unless otherwise specied, they were all used directly.

Instrumental characterization

Details can be acquired in the ESI.†

Synthesis of the COFs

Functional reagents containing acetylene (0.5 mmol) and TPB-DMTP-COFs (40 mg) synthesized according to a report (see theESI†) were added to toluene (10 mL). BF3$Et2O (40 mL, 0.3 mmol)and chloranil (80 mg, 0.3 mmol) were also added to it. Aerstirring in a ask at 110 �C for 72 hours under nitrogen protec-tion, the collected solid was ltered with THF and saturatedNaHCO3 solution and then washed with THF. In the end, thefunctional COF was obtained by drying under vacuum for 12 h.When phenylacetylene was used as a functional reagent, COF-Ph(50 mg) was obtained as a dark yellow powder. When the func-tional reagent was 3,5-bis (triuoromethyl)phenylacetylene, COF-(CF3)2 (52 mg) was obtained as a tan solid. In our actual experi-ment, the maximum preparation amount reached 200 mg.

Stability test

COF-(CF3)2 was placed in a strong alkali (14 M NaOH at 80 �C, 2days), a strong acid (12 M HCl at 80 �C, 2 days), a strong oxidant(KMnO4 in H2O, 2 days) and strong reducing agents (NaBH4 inMeOH at 60 �C, 2 days). Materials weighing and PXRD studieswere used to determine whether the structure of COF-(CF3)2 wasdamaged.

Fluorinated pesticide sorption studies

The pesticide solution (30 ppm) was prepared according toa volume ratio of 1 to 9 of anhydrous methanol and deionized

J. Mater. Chem. A

water. First, triuralin (30 mg) and pronil (30 mg) were dis-solved in anhydrous methanol (100 mL), respectively. Thendeionized water (900 mL) was added and it was shaken fully ina volumetric ask to ensure that the mixture was evenly mixed.Adsorption experiments were carried out at room temperature.The adsorbing material (5 mg) and solution (100 mL) wereadded into a 250 mL conical ask, and the mixture wasconstantly stirred. Appropriate solutions were selected atdifferent time intervals and ltered through a 0.2 mmmembrane lter. The ltrate was tested with an ultravioletvisible spectrophotometer for the content of the remainingpollutants.

Fluorescence detection study

Finely ground COF-(CF3)2 (10 mg) was dispersed in ethanol (60mL), which was then ultrasonically treated for 10 minutes toensure a uniform dispersion. The mixture was divided evenlyinto 6 parts of 10 mL each and different weights of triuralinwere added to form analytes with concentrations of 0.1 mM to1 mM. Aer being shaken for one minute, the uorescenceintensity of the solution under UV excitation at 365 nm could beanalyzed with a uorescence spectrometer.

Results and discussion

The TPB-DMTP-COF was synthesized according to the previousliterature (see the ESI†).41,48 In this study, COF-(CF3)2 and COF-Ph are prepared via the Povarov reaction.42 The difference is thatwe simplify the experimental operation and achieve the large-scale preparation in a ask instead of a Pyrex tube (Scheme1). This can increase the amounts of products synthesized byone or two orders of magnitude. The reaction rate can also beaccelerated by fully stirring the reaction system to increase theeffective collision between the TPB-DMTP-COF and the func-tional reagent.

The successful modication of the TPB-DMTP-COF can beproved by a variety of methods. Six distinct diffraction peaks canbe observed in the powder X-ray diffraction (PXRD) pattern ofCOF-(CF3)2 and COF-Ph (Fig. 1a). All of them are close to thepeaks of the TPB-DMTP-COF. The peak at 2.79� that representsthe (100) facet is more intense than others. The ve other peaksat 4.82�, 5.61�, 7.37�, 9.75�, and 24.8� correspond to the (110),(200), (210), (220), and (001) facets, respectively. The PXRDpatterns of COF-Ph prepared in the ask and a Pyrex tube arealmost indistinguishable (Fig. S1, ESI†). This shows that thesame products can be obtained in different systems. TheFourier-transform infrared (FT-IR) spectra of COF-(CF3)2 arequite different from those of the TPB-DMTP-COF (Fig. 1b). Onenew strong absorption peak at �1300 cm�1 corresponds to thetriuoromethyl stretching.42 The TPB-DMTP-COF is free ofuorine, so triuoromethyl was successfully added to the TPB-DMTP-COF.

X-ray photoelectron spectroscopy (XPS) further conrms thesuccessful synthesis of COF-(CF3)2 in two aspects. On the onehand, the distinct peak at �688 eV corresponds to F 1sdemonstrating the existence of uorine (Fig. 1c). On the other

This journal is © The Royal Society of Chemistry 2020

Scheme 1 Synthetic route to COF-(CF3)2 via the Povarov reaction.

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hand, the spectrum of N 1s (Fig. 1d) shows that the imine bondis converted to a quinoline unit. The two peaks at 398.8 and399.5 eV belong to the imine49 and quinoline.42,50 The conver-sion rates were also calculated using the area ratios of imineand quinoline in the XPS data, which was according to methodsmentioned in the literature.42 The calculated conversion rate is

Fig. 1 (a) Powder X-ray diffraction (PXRD) pattern of the original TPB-DMFourier-transform infrared (FT-IR) spectra; (c) XPS survey spectrum of C

This journal is © The Royal Society of Chemistry 2020

just for a reference and may not reect the real extent of post-functionalization, since it can only be analyzed semi-quantitatively. Up to now, it has been hard to accurately char-acterize the conversion rate of functionalization. All of theabove information proves that triuoromethyl groups havebeen graed onto the TPB-DMTP-COF via the Povarov reaction.

TP-COF and post-synthetically modified COF-Ph and COF-(CF3)2; (b)OF-(CF3)2; (d) N(1s) XPS spectra of COF-(CF3)2.

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The morphology and channel information on COF-(CF3)2can be observed by high-resolution transmission electronmicroscopy (HRTEM). As shown in Fig. 2a–d and S2 in the ESI,†large areas of the ordered porous structure can be seen clearly.It is arranged in a honeycomb pattern along the (111) direc-tion.42 By enlarging the selected area, we observed hexagonswith the same aperture which is about 2.6 � 0.2 nm (Fig. 2c).This result is consistent with a previous study conducted withnonlocal density functional theory. Such an ordered porestructure is not easy to observe, which also reects the strongstability of COF-(CF3)2.38,51 It can be seen from the TEM-EDXelemental mapping images (Fig. 2f–i and S3 in the ESI†) intui-tively that the uorine element is evenly distributed in thematerial.

The N2 adsorption isotherms at 77 K reect the porosityinformation on the TPB-DMTP-COF, COF-(CF3)2, and COF-Ph,including the Brunauer–Emmett–Teller (BET) surface area,

Fig. 2 (a–c) HR-TEM characterization of COF-(CF3)2; (d) HR-TEM chara(CF3)2.

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pore volume and pore size distribution. As can be seen fromFig. 3a and b, the specic surface areas and pore size of the TPB-DMTP-COF, COF-Ph and COF-(CF3)2 decrease signicantly insequence. Their specic surface areas are 2174, 1817, and 1533m2 g�1 (Fig. S4 and 5, ESI†), respectively. The pore sizes of COF-(CF3)2 and COF-Ph are 2.67 and 2.89 nm, respectively. Thesedata conrm that two kinds of functional groups are graedonto the TPB-DMTP-COF. The porosity data of all the materialsare listed in Tables 1 and S1 in the ESI.† In addition, the specicsurface area of COF-Ph prepared in a ask is higher than thoseprepared in a Pyrex tube according to the literature (SBET ¼ 955m2 g�1).42

The contact angles of the TPB-DMTP-COF, COF-Ph and COF-(CF3)2 are 67�, 100� and 110� (Fig. 4a). Contact angles increaseobviously aer functionalization because triuoromethyl isa strong hydrophobic group, and the benzene ring is weakslightly. First of all, this result conrmed the success of the

cterization of COF-Ph; (e–i) EDX elemental mapping images of COF-

This journal is © The Royal Society of Chemistry 2020

Fig. 3 Porosity analysis of the TPB-DMTP-COF, COF-Ph and COF-(CF3)2. (a) Nitrogen adsorption and desorption isotherms curves; (b) the poresize distributions.

Table 1 Porosity and adsorption data of the prepared COFs

Materials SBETa (m2 g�1) SLangmuir

b (m2 g�1) Vtotalc (m3 g�1) Dpore

d (nm) Qe,triuraline (mg g�1) Qe,pronil

f (mg g�1)

COF-(CF3)2 1533 1948 0.78 2.67 151 171COF-Ph 1817 2975 1.17 2.89 104 129

a Specic surface area calculated according to BET theory. b Specic surface area calculated using a Langmuir theoretical model. c Total porevolume at P/P0 ¼ 0.99. d Data calculated with the NLDFT method. e Maximum adsorption for triuralin. f Maximum adsorption for pronil.

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functionalization. More importantly, the enhancement ofhydrophobicity was also conducive to the adsorption of pollut-ants. In terms of stability, COF-(CF3)2 shows excellent perfor-mance under different chemical conditions. Its weight loss is allless than 10% (Fig. 4b) and the intense peak at 2.79� in thePXRD images (Fig. 4c) proves that the structure shows still two-dimensional ordered stacking. The crystallinity is also retained,especially in 12 M HCl and NaBH4 solution. Aer acid treat-ment, the N2 adsorption–desorption isotherms of COF-(CF3)2have been shown in Fig. S6 (in the ESI).† We can see that the

Fig. 4 (a) Water contact angles of a water droplet on the pressed pelletCOF-(CF3)2; (b and c) residual weight and PXRD of COF-(CF3)2 after trea

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change of the N2 adsorption–desorption isotherms and PSDplot is not obvious, which is another evidence of the stability ofCOF-(CF3)2. We believe that there are some reasons for its ultra-stability. First of all, the starting COF material TPB-DMTP-COFwe chose is very stable, and can be stable in strong acids, strongbases and organic solvents for a long time. Second, parts ofreversible imine bonds were transformed into more stablequinoline units that are not easy to decompose. We think thatthis is the most important reason for the enhanced stability.Meanwhile, the enhancement of hydrophobicity is crucial in

of the TPB-DMTP-COF and post-synthetically modified COF-Ph andtment in different solutions.

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improving the stability of materials in water, which has beenwidely reported in the work of improving the stability ofMOFs.52,53 In addition, the newly formed quinoline units extendthe p-conjugated systems of the material, thus increasing thep–p stacking interlaminar force of the COF-(CF3)2. Therefore,the material exhibits great tolerance to the harsher conditionssuch as KMnO4 and NaBH4 solutions, in which most of COFsshould be dissociated or decomposed. Compared with normalorganic solvents such as THF, MeOH and DMF, the conditionsof a strong acid and strong base are much harsher, and it isdifficult for most COFs to maintain stability under suchconditions. Therefore, in the stability test, we mainly focus onthemedia such as a strong acid, strong base, strong oxidant andstrong reducing agent and so on. COF-(CF3)2 is almost the moststable compared to other COFs linked by imine bonds. Thissuper stability is crucial for the commercial application of COFsunder complex conditions in the future.

There is a kind of hydrophobic attraction between twohydrophobic molecules because of the entropic tendency ofaggregation in water. Triuralin and pronil contain one andtwo triuoromethyl groups, respectively. So, both of them willconverge on the hydrophobic surfaces of COF-(CF3)2 and COF-Ph. The saturation adsorption capacity can be calculatedaccording to an equation (see the ESI†). Compared with COF-Ph, COF-(CF3)2 showed a better adsorption effect on triuralinand pronil. Aer shaking in a shaker for an hour, the saturatedadsorption capacities of COF-(CF3)2 and COF-Ph to pronil are171 and 129 mg g�1 in a batch experiment using [pronil]0 of30 mg L�1, and [COF] of 100 mg L�1 (Fig. 5a and S7†). Thesaturated adsorption capacities to triuralin are 151 and

Fig. 5 (a and b) The adsorption curves of COF-(CF3)2 and COF-Ph for fipof COF-(CF3)2 for the two pollutants; (d) pseudo-first-order adsorptioemission spectra of COF-(CF3)2; (f) Stern–Volmer plots for COF-(CF3)2 a

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104 mg g�1 at the same concentration (Fig. 5b and S7†). On theone hand, the difference of adsorption capacity is due to thedifference of hydrophobic properties between the twomaterials.COF-Ph is not as hydrophobic as COF-(CF3)2 because its contactangle is 10 degrees less than that of COF-(CF3)2.54,55 Moreimportantly, there is a strong and selective uorine–uorineinteraction between triuoromethyl and uorinated contami-nants.56 COF-(CF3)2 with both of these forces has betterperformance. In addition, COF-(CF3)2 has a better adsorptioneffect on pronil than triuralin because the benzene ring ofpronil has an amine group that will form intermolecularhydrogen bonds with nitrogen atoms on the COFs. Theadsorption kinetics of the two kinds of materials for the twokinds of pollutants was analyzed using pseudo-second-orderand pseudo-rst-order models (see the ESI†). The two linearplots of COF-(CF3)2 are more consistent with pseudo-second-order models with a high correlation coefficient (R2) of 0.998and 0.996 (Fig. 5c). Therefore, chemisorption is the adsorptionmode of COF-(CF3)2 for the two pollutants. While the physicaladsorption is more consistent with COF-Ph because the linearplots (ln(qe� qt) vs. t) have a high R2 of 0.994 and 0.997 (Fig. 5d).

The imine bond is highly polarized, so the conjugate effect ofthe TPB-DMTP-COF is not strong. However, the quinoline unitof COF-(CF3)2 solves the problem well. When an imine bond isconverted to quinoline units via the Povarov reaction, the p-conjugated system is enhanced. It is well understood that p-systems undergo electronic excitations more easily than theirsaturated counterparts. This is due to the lower energy differ-ence (DE) for a p–p* transition than for a s–s* transition.When an excited electron returns to its ground state, the energy

ronil and trifluralin; (c) pseudo-second-order adsorption kinetics fittingn kinetics fitting of COF-Ph for the two pollutants; (e) fluorescencet different concentrations of trifluralin solution.

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Scheme 2 The process of fluorescence sensing for trifluralin.

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is converted to uorescence.57 In addition, the intramolecularrotation is limited aer a cycloaddition reaction,58 whichdiminishes the consumed energy of the excited state due tointramolecular vibrations or rotations and the non-radiativetransition rate.59,60 This is another reason for uorescenceenhancement of the material. COF-(CF3)2 dispersed in theethanol evenly emits a distinct blue uorescence under UVexcitation at 365 nm. It can be seen from Fig. 5e that theemission wavelength of COF-(CF3)2 is 455 nm, corresponding tothe wavelength of blue light. During the adsorption process oftriuralin, we accidentally found that it had a uorescencequenching effect to COF-(CF3)2 (Scheme 2). Due to linking ofelectron withdrawing triuoromethyl and nitro, the benzenerings are electron-decient for triuralin. So, the energy willtransfer from the COF-(CF3)2 to the electron-decient triuralin.The Stern–Volmer equation ((I0/I) � 1 ¼ Ksv [C]) can be used forevaluation of the sensing ability of COF-(CF3)2 to triuralin, andit exhibits a better correlation with R2 over 0.99 (Fig. 5f). There isanother phenomenon that the uorescence is redshied duringquenching. This Stokes shi61–63 is the result of the combinationof COF-(CF3)2 and triuralin with strong electron-withdrawingproperties. For the two different pesticides, we found that theuorescence of COF-(CF3)2 can only be effectively quenchedusing triuralin. When pronil was added, the uorescenceintensity of COF-(CF3)2 was almost unchanged. This result isalso expressed in the form of a bar chart, which is included inFig. S8 (see the ESI).† We have prepared counterpart COFs withother functional groups by the same method. Interestingly, theyall exhibit intrinsic uorescence, suggesting that it is a commonphenomenon for the cycloaddition functionalization of frame-works to induce uorescence. A deeper investigation on theoptical properties and performance is under way and will bereported in our following work.

Conclusions

In summary, we rst veried the feasibility of post-functionalization of COFs via the Povarov reaction in a ask,

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and the large-scale preparation of this functional COF has alsobeen achieved. The triuoromethyl-graed COF has the prop-erties of hydrophobicity, ultrastability and uorescence, whichare well applied in the adsorption of uorinated pesticides andthe detection of triuralin. The preparation of uorescent COFsby enhancing conjugation provides a good strategy for futureresearch. More importantly, large-scale preparation and ultra-stability are decisive factors in promoting the industrialproduction and application of COFs.

Conflicts of interest

There are no conicts to declare.

Acknowledgements

This work was supported by the Hainan Science and TechnologyMajor Project (ZDKJ2019013), the National Natural ScienceFoundation of China (51873053, 52063014, 21975058,51775152, 61761016 and U1967213), the National Key R&Dprogram of China (2018YFE0103500), and the Science andTechnology Project of Guizhou Province (Grant QKH Platformtalents [2017]5733–067). Zhuyin Sui also acknowledges theTaishan Scholars Program (Grant No. tsqn201909087).

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