Self-Assembling Hydrophilic Magnetic Covalent Organic Framework

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Functional Nanostructured Materials (including low-D carbon)

Self-Assembling Hydrophilic Magnetic Covalent Organic Framework Nanospheres as a Novel Matrix for Phthalate Esters Recognition Yinghua Yan, Yujie Lu, Baichun Wang, Yiqian Gao, Lingling Zhao, Hongze Liang, and Dapeng Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08934 • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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ACS Applied Materials & Interfaces

Self-Assembling

Hydrophilic

Magnetic

Covalent

Organic Framework Nanospheres as a Novel Matrix for Phthalate Esters Recognition Yinghua Yan*, Yujie Lu, Baichun Wang, Yiqian Gao, Lingling Zhao, Hongze Liang*, Dapeng Wu School of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, China *Corresponding author: Yinghua Yan, Prof. Hongze Liang E-mail: [email protected], [email protected]

KEYWORDS: covalent organic framework (COF); Fe3O4@PDA@TbBd; Phthalic acid esters (PAEs); Magnetic solid-phase extraction (MSPE); Detection; GC-MS

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ABSTRACT: The development of covalent organic framework (COF)-derived materials with additional functions and applications are still in highly desired. In this work, a unique COF-functionalized hydrophilic magnetic nanosphere (Fe3O4@PDA@TbBd) with Fe3O4 as a magnetic core, polydopamine (PDA) as a hydrophilic middle layer, TbBd as an outer COF shell was facilely prepared as a novel hydrophilic platform for efficient detection of phthalic acid esters. The resultant Fe3O4@PDA@TbBd displayed strong magnetic response, high surface area and good hydrophilicity. Accordingly, the new synthesized COF exhibited great potential in phthalates analysis with wide linearity (50-8000 ng/mL), good recovery (92.3-98.9%), low limit of detection (LOD, 0.0025-0.01 ng/mL) and small relative standard deviation (RSD, for intra-day less than 4.6%, for inter-day less than 6.8%). More excitingly, the new COF was applied to analyze 9 phthalic acid esters (PAEs) in human plasma sample. This work opens up new avenues for development and application of functionalized COF-derived materials. 1. INTRODUCTION Covalent organic frameworks (COFs), as a burgeoning kind of crystalline porous material, have become a brilliant star in modern materials science since their first discovery in 2005.1-3 COFs distinguished themselves by low density, large surface area, excellent structural regularity, and adjustable pore structure.4-7 These attractive properties enable COFs with superior potential applications in many fields such as catalysis,8 adsorption,9 optoelectricity10 and gas storage.11 Recently, the great


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development of COF functionalized materials have improved the potential performance of COF materials and extended their applicability.12-14 Among these COF functionalized materials, magnetic COF material have been developed as great potential materials for the detection of peptides and bisphenols.15,16 Although these magnetic COF extended the application field of COFs, further development of potential functionalized COF-derived materials and exploration their applications in new research fields still remain highly urgent. Phthalic acid esters (PAEs) are commonly applied in a variety of plastic products as plasticizers to improve their flexibility and properties.17 PAEs are easy to escape into environment since they are not chemically bonded to plastic products. So far, PAEs have become the most common aromatic chemicals that people are exposed to everyday due to their widely used in plastic packages, plastic container and cosmetics.18-20 Unfortunately, these aromatic chemicals can cause carcinogenicity or tumorigenicity as well as contribute to disruption and malfunction in endocrine system and reproductive organs development,21,22 a high level of PAEs may even do potential harm to viscera and testicles development.23,24 Owing to the potential threat to environment and human health, it is urgent and crucial to explore an efficient and sensitive strategy for PAEs detection from complicated biosamples. To date, several approaches

for

PAEs

detection

mainly

include

LLME

(liquid–liquid

microextraction),25 SPME (solid phase microextraction),26 SPE (solid phase extraction) 27

and MSPE (magnetic solid phase extraction).28 Thereinto, MSPE has become more

and more attractive for PAEs detection due to its unique features of facile, rapid,



effective and environmentally friendly. Recently, metal organic framework (MOF)-based MPSE techniques have been greatly developed for detection of PAEs because of their tunable cavities and controllable chemical affinities.29,30 As a rapidly rising star in material fields, COFs could be perfect candidate for PAEs enrichment due to their unique properties as MOFs. However, there are a lot of inconveniences for the directly application of unfunctionalized COFs in PAEs detection. Besides the time-consuming and tedious centrifugation separation, the water dispersibility is very poor, largely discounting the performance of the COF materials. Therefore, the development of additional functionalized COF materials with a strong magnetic response and good hydrophilicity is essential for time-saving and high efficiency of PAEs recognization. Magnetic particles have drawn widely attention owing to the unique chemical properties, good biocompatibility, and magnetic separability. Based on these remarkably merits, magnetic particles are widely used in cell sorting, bio-separation, enrichment science and drug delivery.31,32 Recently, many studies have reported magnetic composites exhibited remarkable performance for PAEs detection.33,34 Dopamine, a novel coating material, moved into the spotlight as it can deposite on a variety of organic and inorganic substrates to form a unique hydrophilic polydopamine (PDA).35 This unique natural adhesive have drawn increasing attention in virtue of its versatile surface modification and has stimulated extensive research. The PDA layer exhibits prominent environmental stability, excellent water dispersibility and good biocompatibility.36 Meanwhile, many reports indicated that


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COFs have been successfully assembled on different substrates such as graphene, alumina and so on by a charge transfer interaction.37,38 Therefore, it is fascinating to design COF-derived magnetic composites for specific detection of PAEs from complicated biological samples. Herein, we prepared a hydrophilic magnetic covalent organic framework (Fe3O4@PDA@TbBd) nanosphere and for the first time applied COFs for selectively recognizing PAEs. The as-prepared Fe3O4@PDA@TbBd nanospheres showed many advantages beneficial to PAEs detection. First, the external layer of COFs offered large surface area and active sites for PAEs adsorption by π–π and hydrophobic interactions. Second, the PDA middle layer endowed the new prepared magnetic COF with good biocompatibility and dispersibility, which is beneficial for dispersing in aqueous solution. Third, the magnetic cores made the whole experimental process simply and fast just with the help of a magnet. Taking these advantages together, Fe3O4@PDA@TbBd nanospheres were expected to have great potential performance for PAEs analysis. 2. EXPERIMENTAL SECTION 2.1. Reagents and Chemicals. DMP (Dimethyl phthalate), DPRP (dipropyl phthalate), BBP (benzyl butyl phthalate), DIBP (diisobutyl phthalate), DPP (dipentyl phthalate), DEHP (di-2-ethylhexyl phthalate), DBP (dibutyl phthalate), DNOP (di-n-octyl

phthalate) and

DEP (diethyl phthalate)

were

purchased

from

Sigma-Aldrich. Tb (1,3,5-Triformylbenzene) and FeCl3·6H2O were obtained by J&K Chemical Ltd. Dopamine hydrochloride and Bd (benzidine) were obtained from



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Aladdin Chemistry. Trisodium citrate, sodium hydroxide, sodium acetate andydrous, Trifluoroacetic

acid

(TFA),

concentrated

nitric

acid,

ethanol,

methanol,

dichloromethane and acetone were purchased from Shanghai Chemical Corporation. Human plasma was supplied by school Hospital of Ningbo University. 2.2. Preparation of Samples. PAEs were dispersed in methanol to form stock standard solution with concentration of 1000 mg/L and stored at 4◦C. HCl (200 µL, 37% w/w) and TFA (200 µL) were used to precipitate proteins of human plasma (3.0 mL). The suspension was collected by centrifugation and stored at 4 °C. 2.3. Synthesis of Fe3O4@PDA@TbBd. The Fe3O4@PDA nanospheres were synthesized following a previous report.39 Fe3O4@PDA@TbBd nanospheres were prepared following a previous work.40 In brief, 0.3 mmol of Tb and 0.45 mmol of Bd were dispersed in 50 mL of DMSO, following adding 0.15g of Fe3O4@PDA, sonicated for 5min. Then 2 mL of anhydrous acetic acid was added slowly, incubating at room temperature for 15 min. At last, the new prepared Fe3O4@PDA@TbBd nanospheres were washed throughly with tetrahydrofuran and anhydrous ethanol successively and dried in vacuum at 50 ◦C for further use. 2.4. Instrument and Chromatographic Conditions. SEM image was characterized on an electron microscope (Philips XL30, Netherlands). TEM image was characterized on a microscope (JEOL 2011, Japan). The zeta potential was tested using a Zetasizer (Nano Series, UK). FT-IR spectra were tested by using a Fourier spectrophotometer (Nicolet 6700, USA). Agilent 5973N/6890N GC-MS was used to analyze the samples, which were


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separated by HP-5MS capillary column (30 m × 0.25 mm, 0.25 m). The initial temperature of column oven was 80 ◦C, keeping for 1 min, then raised to 280 ◦C with the rate of 20 ◦C/min and kept for 4 min. The source temperature and the quadrupole temperature were set at 230 ◦C and 150 ◦C, respectively. The carrier gas was high pure helium with a flow rate of 1 mL/min. 2.5. Procedure of MSPE. A certain amount of Fe3O4@PDA@TbBd was dispersed into PAEs standards (100 ng/mL) or real samples aquous solution, vortexing for 20 min. The nanospheres with PAEs were collected under magnetic field. Afterwards, the captured PAEs were removed by vibration for 20 min using acetone (500 µL). At last, 1 µL of the eluent was analyzed by GC-MS. 2.6. Optimization of the parameters. For the optimization process, Fe3O4@PDA@TbBd nanospheres (10-30 mg) were dispersed into 100 ng/mL of PAEs standards solution with different pH values (3-11, adjusted by 0.01 M HCl or NaOH solution), or different ionic strength (with NaCl concentration of 0-20%, W/V), vortexing for different extraction time (5-30 min). Then nanospheres with PAEs were collected under magnetic field. Afterwards, 500 µL of eluent (methanol, dichloromethane, acetone) was used to remove PAEs by vibration for different time (5-30 min). At last, 1 µL of the eluent was analyzed by GC-MS. 3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Fe3O4@PDA@TbBd. The strategy for construction of Fe3O4@PDA@TbBd nanospheres was displayed in Scheme1. The Fe3O4 nanospheres were prepared via a hydrothermal reaction, dopamine coated on



Fe3O4 nanospheres by self-polymerization to form a PDA layer (Fe3O4@PDA), then Fe3O4@PDA@TbBd was obtained via a one pot reaction by using DMSO as solvent, Fe3O4@PDA as the hydrophilic magnetic core, TbBd as the COF shell. The strategy of construction of Fe3O4@PDA@TbBd nanospheres was efficient, facile and time-saving.

Scheme 1. The synthetic strategy of Fe3O4@PDA@TbBd.

SEM and TEM were first used to characterize the morphology of Fe3O4@PDA and Fe3O4@PDA@TbBd. Figure 1a showed Fe3O4@PDA had uniform diameter, after modified with COF (Figure1c), the surface of Fe3O4@PDA became rough, indicating the formation of COF shell. TEM image of Fe3O4@PDA (Figure1b) revealed a core-shell structure with about a 10 nm PDA layers, after coated with COF, the outer layer became approximate 20 nm (Figure 1d), suggesting TbBd COF was modified on the surface of Fe3O4@PDA successfully.


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Figure 1. SEM (a and c) and TEM (b and d) images of Fe3O4@PDA and Fe3O4@PDA@TbBd.

FT-IR spectroscopy was used to testify the successful preparation of Fe3O4@PDA@TbBd (Figure S1). Compared with infrared adsorption of Fe3O4 and Fe3O4@PDA microspheres, similar adsorption vibration was obtained in the spectrum of Fe3O4@PDA@TbBd nanospheres. Furthermore, the adsorption at 1621 cm-1 was ascribed to C=N vibration, adsorption at 1495 cm-1 belonged to C–C ring stretching, which demonstrated that Fe3O4@PDA nanospheres was successfully modified by the TbBd COF shell. The increase of zeta potential (Figure S2) was further demonstrated the modification of PDA and COF layer. To investigate the hydrophilicity, the new prepared Fe3O4@PDA@TbBd nanospheres were dispersed in milli-Q water, which could maintain a uniform dispersion for a longer time (Figure S3). Wide-angle XRD of the Fe3O4@PDA@TbBd nanospheres was displayed in Figure S4, the peaks with 2θ at 30.1° (220), 35.5° (311), 43.4° (400), 53.4° (422), 57.1° (511) and 62.8° (440) were attributed to magnetite, which indicated that the Fe3O4@PDA@TbBd nanospheres were well crystallized.



Brunauer–Emmett–Teller (BET) was utilized to investigate the porous structure. The Fe3O4@PDA@TbBd nanospheres indicated a typical IV isotherm, it is indicative of a mesoporous character (Figure 2). The pore size distribution was displayed in the inlet of Figure 2, which indicated that Fe3O4@PDA@TbBd microspheres had narrow pore size with average pore size of 2.6 nm, which was in accordance with the bulk COFs (2.6 nm).41 The surface area of the new prepared COF was 146.47 m2 g-1, and the pore volume was 0.45m2 g-1. In contrary, the surface area of Fe3O4 and Fe3O4@PDA was 64 m2 g-1 and 79 m2 g-1, which were far less than that of Fe3O4@PDA@TbBd nanospheres.

Figure 2. N2 adsorption–desorption isotherms of Fe3O4, Fe3O4@PDA and Fe3O4@PDA@TbBd nanospheres; pore size distribution of Fe3O4@PDA@TbBd (the inlet).

Thermogravimetric analysis (TGA) curve is a good indicator of mass ratios and thermal stability of different components (Figure S5). A weight-loss of 16% was obtained for Fe3O4@PDA nanospheres, which belonged to the loss of small molecules


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such as H2O, CO2 and NH2, for Fe3O4@PDA@TbBd nanospheres, the weight losses of about 27 wt% was observed, implying the loss of TbBd COF layers. It was worth noting that a long plateau was observed for Fe3O4@PDA@TbBd under 400 ℃, exhibiting that the new prepared COFs had excellent thermal stability. 3.2. Optimization of the MSPE Conditions. Several parameters influence the adsorption efficiency, such as the pH value, amounts of adsorbents, extraction time, ionic strength, eluting solvent and eluting time, to get the maximum efficiency, these parameters were optimized.

Figure 3. (a) Effect of pH value of loading buffer (b) amount of sorbent, (c) extraction time, (d) ionic strength.

3.2.1. Optimization of the pH Value. The pH value influence the existing form of targeted analyte, so the pH value of sample solution between 3 and 11 was



optimized. As shown in Figure 3a, the extraction efficiency was best in pH = 7. As a result, the pH value of 7 was selected for further extractions. 3.2.2. Optimization of the Amount of sorbents. The amount of sorbents has a great influence on extraction efficiency, so different amount of Fe3O4@PDA@TbBd (10, 20 and 30 mg) was investigated. The results in Figure 3b indicated that 20 mg was enough for most PAEs extraction because Fe3O4@PDA@TbBd had large surface area to achieve high extraction efficiency. So 20 mg of Fe3O4@PDA@TbBd was used in this work. 3.2.3. Optimization of the Extraction Time. Extraction time affected the distribution equilibrium of PAEs in adsorbents and sample solution, so it played an important role in extraction efficiency. The extraction time from 5 to 30 min was studied, as displayed in Figure 3c, from 5 to 10 min, the extraction efficiency enhanced remarkably, which mainly owing to the large surface area and mutual Π–Π interaction between the PAEs and COFs. However, as extraction time was further extended to 30 min, there was no significant increasing of the adsorption efficiency. So the extraction time of 10 min was used for further experiments. 3.2.4. Optimization of the Ionic Strength (NaCl Addition). The solubility of analytes in aqueous solution was influenced by ionic strength of the solution, which played an important role in the distribution of analytes in organic phase and aqueous phase. To get the best efficiency, different concentration of NaCl solution (0-20%, W/V) was optimized. The results in Figure 3d displayed that the enrichment efficiency was affected little by ionic strength. Therefore, the loading buffer with no


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NaCl addition was used in following experiment.

Figure 4. (a) Effect of desorption solvents, (b) desorption time.

3.2.5. Optimization of the Eluting Solvent. Elution efficiency may be largely influenced by eluting solvent. Different eluting solvents including methanol, acetone and dichloromethane were optimized. The results (Figure 4a) exhibited that the elution efficiency was the best when acetone was used to remove the captured analytes. Hence, acetone was used in following work. 3.2.6. Optimization of the Desorption Time. Desorption time had a great impact on desorption efficiency. In this study, desorption time varied from 5-30 min was investigated. Figure 4b showed 10 min was enough to achieve high elution efficiency, the elution efficiency increased little as eluting time further prolonged to 30 min. So 10 min was used in this study. 3.3. Method Validation. Under the above optimum conditions, the validation parameters were investigated to evaluate the proposed MSPE strategy and the results were displayed in Table 1. The linear range was 50-8000 ng/mL with correlation coefficient varied from 0.9990 to 0.9998. The limits of detection (LOD) was 0.0025-0.01 ng/mL (S/N=3), the limits of quantification (LOQ, S/N=10) varied from



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0.1 ng/mL to 0.5 ng/mL. Recoveries and RSD were investigated in the blank tap water sample. The recoveries of tap water sample were 92.3-98.9%. The RSDs for intra-day was lower than 4.6%, for inter-day was lower than 6.8%. To test whether the proposed strategy can be recycled and reused for detection of PAEs, Fe3O4@PDA@TbBd was recycled by washing with ultrapure water. The regenerated nanospheres were reused to detect PAEs five times. The peak area of the fifth time was very consistent with the first time, indicating the adopted method had good repeatability. Table 1. Validation parameters of the proposed method. Analytes

Linear

Linearity equation

Correlation

RSDs

RSDs

LOD

LOQ

Recovery

range

coefficient

(intra-day, %)

(inter-day, %)

(ng/mL)

(ng/mL)

(%)

(ng/mL)

(R2)

DMP

50-8000

Y=15.3764X+78.378

0.9990

3.1

4.2

0.01

0.5

92.3

DEP

50-8000

Y=18.2864X+225.832

0.9996

2.7

5.8

0.005

0.2

95.2

DPRP

50-8000

Y=1.3468X+411.482

0.9991

4.2

4.2

0.005

0.2

98.9

DIBP

50-8000

Y=0.9326X+34.167

0.9993

3.7

5.6

0.0025

0.1

92.6

DBP

50-8000

Y=1.4682X+187.363

0.9995

4.6

5.1

0.0025

0.1

95.5

DPP

50-8000

Y=0.3671X+235.124

09990

3.2

4.2

0.0025

0.1

98.3

BBP

50-8000

Y=10.9632X+169.821

0.9998

2.3

3.4

0.0025

0.1

93.9

DEHP

50-8000

Y=5.6812X+108. 853

0.9992

2.8

5.1

0.005

0.2

95.6

DNOP

50-8000

Y=20.3481X+68.424

0.9997

2.9

6.8

0.005

0.2

94.7


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Figure 5. The representative chromatograms of (a) human plasma; (b) its spiked solution with 10 ng/mL phthalates. (1) DMP, (2) DEP, (3) DPRP, (4) DIBP, (5) DBP, (6) DPP, (7) BBP, (8) DEHP, (9) DNOP.

3.4. Real Sample Analysis. To test the applicability of the proposed approach, Fe3O4@PDA@TbBd was applied to detect PAEs in human plasma sample. The real sample was analyzed three times. As the results showed (Figure 5), in this plasma sample, five PAEs such as DMP, DIBP, DBP, DEHP and DNOP were observed. To test the recovery and the accuracy of the proposed method, the human plasma sample was spiked with PAEs standards concentration of 10 ng/mL. The results in Table 2 showed the recovery ranges were 90.5-98.7%, the RSDs was in the range of 2.3-4.9%. The typical chromatograms for plasma sample before and after being spiked with PAEs are displayed in Figure 5.



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Table 2. Recoveries and precision in human plasma sample. Compound

Spiked (ng/mL)

Average recovery

RSD (%)

DMP

10

90.5

3.1

DEP

10

95.2

2.9

DPRP

10

97.8

4.2

DIBP

10

91.9

3.9

DBP

10

95.1

4.5

(%)

DPP

10

95.2

2.3

BBP

10

98.7

3.1

DEHP

10

96.2

3.5

DNOP

10

97.3

4.9

3.5. Comparison with previously Reported strategies. To evaluate the proposed method, a comparison with other reported strategies for phthalates detection was displayed in Table 3. The results in terms of extraction material, samples, linear range, LOD, recovery and RSD were summarized. The current approach exhibited comparable sensitivity, improved linear range, lower LOD and better RSDs. In this wok, 9 PAEs standards were tested, which was much more than other works, besides, human serum sample were used to evaluate the current strategy, which was more complicated than other samples. Furthermore, there were rare report on the enrichment of PAEs using COF material, so our strategy was much novel, meanwhile, due to the presence of COF and PDA layers, Fe3O4@PDA@TbBd possessed good hydrophilicity, large surface area and Π–Π interaction between the COF and PAEs, the novel adsorbent was anticipated to have great potential in enrichment process with high efficiency and specificity. The proposed strategy provides a new avenue for developing new methods for PAEs analysis.


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Table 3. Comparison of current strategy with previously reported methods for PAEs analysis. strategies

Matrix

PAEs

Linear range

RSD (%)

(ng/mL) Fe3O4@PDA@TbBd

Human

DMP, DPRP, DPP,

plasma

DEHP, DEP, DBP, DIBP,

Water

DEP, DIBP, DBP, BBP,

50-8000

2.3-4.9

LOD

Recovery

(ng/mL)

(%)

0.0025-0.01

90.5-98.7

Ref.

This work

DNOP, BBP PA6-MnO

0.5-500

1.09-107

0.04-0.193

90.3-107

[20]

0.01-100

3.63-13.41

0.019-0.051

61.5-106.7

[21]

0.5-100

3.7-4.8

0.26-0.45

99-104

[23]

0.2-10.0

2.6-15.9

0.02-0.09

43.5-116.7

[33]

0.2-50

3.7-8.4

125-350 pg

77.8-102.1

[34]

DEHP Fiber probe

soil

DMP, DEHP, DNOP, DEP, DNP, DBP, BBP, DCHP

Fe@SiO2@PEI

Water

DPP, DCHP, BBP, DPIP, DBP

Fe3O4@SiO2@MDN

Water

DEP, DIBP, DNOP, DBP,

PILs-PMNPs

Water

DMP, DEP, BBP, DNOP

BBP, DEHP

4. CONCLUSIONS In conclusion, we proposed a facile and efficient strategy for preparation of hydrophilic Fe3O4@PDA@TbBd nanospheres. The new synthesized nanospheres perfectly not only possessed unique π–π electron system, good hydrophilicity and strong magnetism, but also owned regular porous structure and high surface area. By virtue of these advantages, the Fe3O4@PDA@TbBd nanospheres exhibited incredible high sensitivity and good reliability for PAEs detection. Particularly, in practical application, 9 PAEs were detected human plasma sample. This work may provide a new direction to develop new MSPE strategy for detection of aromatic compound in real samples. ACKNOWLEDGEMENTS This work is supported by the K. C. Wong Magna Fund in Ningbo University and research funds of NBU (No. 401708090).



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Graphics abstract


Self-Assembling Hydrophilic Magnetic Covalent Organic Framework

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