CoreâShell Structured Magnetic Covalent Organic Framework

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22492−22500

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Core−Shell Structured Magnetic Covalent Organic Framework Nanocomposites for Triclosan and Triclocarban Adsorption Yanxia Li,†,‡ Hongna Zhang,‡ Yiting Chen,† Lu Huang,† Zian Lin,*,§ and Zongwei Cai*,‡ †

Department of Chemical Engineering, Ocean College, Minjiang University, Fuzhou 350108, China State Key Laboratory of Environmental and Biological Analysis, Department of Chemistry, Hong Kong Baptist University, Hong Kong, China § Ministry of Education Key Laboratory of Analytical Science for Food Safety and Biology, Fujian Provincial Key Laboratory of Analysis and Detection Technology for Food Safety, College of Chemistry, Fuzhou University, Fuzhou 350116, China Downloaded via BUFFALO STATE on July 19, 2019 at 03:12:49 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Triclosan (TCS) and triclocarban (TCC) are widely used as bactericides in personal-care products. They are frequently found in environmental water and have the potential to cause a number of environmental and human health problems. In this study, we investigated adsorption and magnetic extraction for efficient removal of TCS and TCC from water and serum samples by core−shell structured magnetic covalent organic framework nanocomposites (Fe3O4@COFs). The asprepared Fe3O4@COFs was fabricated on the Fe3O4 nanoparticles in situ growth strategy at room temperature via condensation reaction of 1,3,5-tris(4-aminophenyl) benzene (TAPB) and terephthaldicarbox-aldehyde (TPA) in the presence of dimethyl sulfoxide (DMSO). The whole process of adsorption was monitored by ultrahigh performance liquid chromatography−tandem mass spectrometry (UHPLC-MS/MS) analysis with high sensitivity. The adsorption behaviors showed high adsorption capacity and fast adsorption. Furthermore, the adsorption performance through Langmuir and Freundlich isotherms showed multilayer adsorption through the interactions of space embedding effect, van der Waals forces, and benzene ring π−π stacking at a low concentration range and monolayer adsorption through strong π−π stacking at a high concentration range between the interface of TCS or TCC and Fe3O4@COFs at a high concentration range. Results indicated that the adsorption of TCS and TCC onto Fe3O4@COFs can be better represented by the pseudo-second-order model. Good removal efficiencies (82.3∼95.4%) and recoveries (92.9∼109.5%) of TCS and TCC in fetal bovine serum (FBS) and reusability at least 10 times were achieved. The Fe3O4@COFs exhibited high stability and excellent performance for the removal of TCS and TCC from water and biological samples. The results presented here thus reveal the exceptional potential of COFs for high-efficient environmental remediation. KEYWORDS: triclosan, triclocarban, Fe3O4, covalent organic framework nanocomposites, adsorption

1. INTRODUCTION Pharmaceuticals and personal care products (PPCPs) are a series of compounds which have been paid much attention as environmental pollutants in recent years.1,2 PPCPs have the characteristics of biological activity and bioaccumulation.3,4 They have potential harms to ecological safety and human health. It is imperative to reduce the pollution levels of PPCPs in the environment.5 Triclosan (TCS) and triclocarban (TCC) are two representative chlorinated antibacterial agents in PPCPs.6,7 As two persistent antimicrobials with similar chemical features (i.e., two benzene rings carrying multiple © 2019 American Chemical Society

chlorines, Figure S1), they continue to be produced and consumed to this day in high volumes.8 There is normally 0.1− 2% of TCS or TCC by weight in PPCPs. It was reported that TCS and TCC were in large consumption of 0.6−10 million kg year −1, which could be easily released into the environment and act as potential sources of contamination.9 Incomplete removal of TCS and TCC has the potential to cause a number Received: April 21, 2019 Accepted: June 10, 2019 Published: June 10, 2019 22492

DOI: 10.1021/acsami.9b06953 ACS Appl. Mater. Interfaces 2019, 11, 22492−22500

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ACS Applied Materials & Interfaces

reusable, simple in operation, free of secondary pollution, easy to separate, short in adsorption and desorption time, and high in adsorption efficiency. Therefore, it is desirable to be used for the separation and enrichment of chlorinated antimicrobials in environmental sewage, which is conducive to the resource utilization of wastewater.

of environmental and health problems, including, bioaccumulation in algae and snails; algal growth inhibiting effects; endocrine-disrupting effects; the formation of toxic degradation products, and the development of microbial resistance.10−12 TCS and TCC have been reported in wastewaters and surface waters with contamination levels ranging from 9 ng/L to 6.7 g/L.13 Therefore, to protect human health, there is an urgent need to establish green, effective, and reliable treatment methods to remove these pollutants in environmental and biological samples. Nanoscience is an important area of research and promising direction in modern science.14,15 Because of the unique size and physical properties, nanomaterials have many advantages in the fields of biomedicine, biotechnology, materials science, and environment.16 Adsorption by various materials is often studied as a potential tool for the purification of water and industrial effluents.17,18 Magnetic nanomaterials have attracted much attention due to their easy synthesis and modification, simple separation, and environmental friendliness. Magnetic nanomaterials have small size, large specific surface area, good dispersion, and strong adsorption capacity. It is easy to realize solid−liquid separation under the action of external magnetic field. Therefore, magnetic nanomaterials can be used as magnetic solid-phase extraction adsorbents for the separation and enrichment of trace components in samples, and the selective adsorption of specific components can be achieved by modifying magnetic nanoparticles.19,20 Covalent organic frameworks (COFs) are a novel crystalline porous materials formed by the reaction of organic molecules as structural units to create predesigned skeletons and nanopores.21 Different from the metal−organic frameworks (MOFs) materials that have rapidly developed in recent years, COFs are completely composed of H, B, C, O, and other light elements through covalent bonds. In addition, the COFs possess remarkable high specific surface area and thermal stability (up to 500 °C). They have been utilized as novel materials22 for gas and storage,23 catalysis,24 and optoelectronic applications.25 COF porous materials have the right combination of properties with enhanced binding affinity and uptake capacity giving them potential as excellent sorbents for environmental remediation.26,27 Sun et al. demonstrated a twodimensional COFs by modifying sulfur derivatives on a newly designed vinyl-functionalized mesoporous COFs to produce synergistic effects arising from densely populated chelating groups with a strong binding ability and thus allow rapid diffusion of mercury species.28 In our previous studies, a series of magnetic COFs materials were designed and synthesized for highly efficient enrichment of peptides and bisphenols from complex biological samples.29−31 These studies demonstrated that the magnetic COFs can serve as ideal absorbents. However, the interfacial adsorption mechanism of COFs materials needs further investigation. At the same time, there are few reports on the adsorption of TCS or TCC, especially when they coexist. The objective of this study was to develop a novel magnetic COFs material (Fe3O4@COFs) for rapid removal and separation of TCS and TCC from water and serum samples through UHPLC-MS/MS analysis. At first, we developed a facile approach for rapid room-temperature synthesis of core− shell structured Fe3O4@COFs was developed. Subsequently, this research focused on the evaluation of the Fe3O4@COFs behaviors as adsorbents for removing TCS and TCC. The asprepared Fe3O4@COFs are simple to prepare, cheap in cost,

2. EXPERIMENTAL SECTION 2.1. Materials. Triclosan (TCS), triclocarban (TCC), 1,3,5-tris(4-aminophenyl)benzene (TAPB), and terephthaldicarboxaldehyde (TPA) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Ferric chloride hexahydrate (FeCl3·6H2O), sodium citrate dehydrate (Na3Cit·2H2O), ethylene glycol (EG), anhydrous ethanol, acetic acid, dimethyl sulfoxide (DMSO), and sodium chloride (NaCl) of analytical grade were purchased from Xinyuhua (Fuzhou, China). Fetal bovine serum was purchased from Dingguo changsheng Biotechnology Co. Ltd. (Beijing, China). Milli-Q purified water was used for all experiments described here. All experiments were performed in triplicate. The error bars showed the standard deviation (SD) in all figures. 2.2. Synthesis of Fe3O4@COFs. Monodisperse Fe3O4 magnetic nanoparticles (MNPs) were synthesized according to our previous report with minor modification.31 Briefly, FeCl3·6H2O (3.4 g), Na3Cit·2H2O (1.0 g), and sodium acetate (6.0 g) were dissolved in EG (100 mL). The obtained homogeneous yellow solution was transferred to a Teflon-lined stainless-steel autoclave and then heated to 200 °C for 12 h. After reaction, the product was collected by magnet, washed with ethanol and water for several times, and then dried at 25 °C. The obtained Fe3O4 MNPs (0.15 g) were suspended in 50 mL of DMSO containing TAPB (0.106 g) and TPA (0.06 g). After sonication for 5 min, acetic acid (2 mL) was slowly added to the mixture. The brown precipitates (Fe3O4@COFs) were formed during this process (∼10 min). After vigorous stirring for 30 min, the obtained brown precipitates were collected by magnetic separation and washed with anhydrous tetrahydrofuran and methanol for several times. The resultant Fe3O4@COFs were dried at 60 °C for further usage. 2.3. Adsorption Experiments. Briefly, 0.8 mg of Fe3O4@COFs were incubated with 1.0 mL of TCS and TCC mixed aqueous solution at different concentrations under optimized adsorption conditions. After separation of magnetic field, the supernatant was diluted with methanol, and the final TCS and TCC concentrations in it were detected by UHPLC-MS/MS and calculated by peak areas. The amount of TCS and TCC adsorbed by the Fe3O4@COFs was calculated from the following formula:

Q=

(Ci − Cf )V m

(1)

The extraction efficiency of Fe3O4@COFs to TCS and TCC (η) was calculated from the following formula:

η=

Ci − Cf Ci

(2)

−1

where Q (μg g ) is the mass of TCC and TCS adsorbed by unit mass of dry particles, η (%) is the extraction efficiency of Fe3O4@COFs to TCS and TCC, Ci (mg mL−1) and Cf (mg mL−1) are the concentrations of the initial and final solutions, respectively, V (mL) is the total volume of the adsorption mixture, and m is the mass of the particles used. All experiments were performed in glass bottles. 2.4. Desorption Experiments. After adsorption TCC and TCS, the above sediment was desorbed by 1 mL of organic solvent under optimized desorption conditions. After separation of magnetic field, the final TCS and TCC concentrations in it were analyzed by UHPLC-MS/MS after dilution with methanol. 2.5. UHPLC-MS/MS Analysis. Analysis of TCS and TCC was performed on an Ultimate 3000 ultrahigh performance liquid chromatography (UHPLC) system coupled with a Thermo Scientific TSQ Quantiva Triple Quadrupole tandem mass spectrometer (MS/ 22493


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ACS Applied Materials & Interfaces Scheme 1. Synthesis of Fe3O4@COFs and efficient adsorption and removal of TCS and TCC

Figure 1. TEM images of Fe3O4 MNPs (A), Fe3O4@COFs (B), EDS spectra (C), and elemental percentage (D) of Fe3O4 MNPs and Fe3O4@ COFs. MS). Chromatographic separation was achieved on a BEH C18 column (1.7 μm particles, 100 mm × 2.1 mm, Waters) with acetonitrile/water gradient mobile phase at a flow rate of 0.30 mL/ min. Acetonitrile was initialized at 20% for 1.0 min, then ramped to 80% at 2.0 min and held for 3.5 min, and ramped back to 20% in 0.5 min and held for 2.0 min. Column temperature was maintained at 35 °C. The sample injection volume was 10 μL. MS/MS analysis was achieved using an electrospray ionization source (ESI) in the negative ion mode. The following optimized parameters were applied: spray voltage, 2600 V; sheath gas, 40 arbitrary units; auxiliary gas, 10 arbitrary units; ion transfer tube temperature, 350 °C; vaporizer temperature, 300 °C. Precursor ion and product ion mass for selective reaction monitoring (SRM) detection and collision energy (CE) are listed in Table S1. Xcalibur 4.1 Software (Thermo Fisher Scientific, U.S.A.) was used for data acquisition and processing. 2.6. Characterization. The morphologies and structures of the magnetic nanomaterials were examined by TEM (Tecnai F30 G2 STWIN 300 kV). TEM coupled to energy dispersive spectroscopy (TEM-EDS) was used to analyze relative contents of C, N, O, and Fe elements. Fourier transform infrared (FT-IR) spectra of magnetic nanomaterials were recorded using the Nicolet 360 FT-IR spectrophotometer (Nicolet, USA). Thermogravimetric analysis

(TGA) was performed for power samples (3 mg) with a heating rate of 40 °C min−1 up to 1100 °C using a STA 449C thermogravimetric analyzer (Netzsch, Germany) under nitrogen atmosphere. Magnetization was detected with a LDJ9600 vibrating sample magnetometer (VSM, Troy, MI) at ambient temperature. The nitrogen adsorption and desorption isotherms were measured by using an ASAP 2020 (Micromeritics, U.S.A.). Powder X-ray diffraction (XRD) patterns were carried out on an X’Pert Pro MPD (Philips, Holland) using Cu Kα radiation at 40 mA to 40 kV.

3. RESULTS AND DISCUSSION 3.1. Synthetic Strategy of the Fe3O4@COFs. Scientists have made wide and deep research studies on the interactions between molecules and nano materials over the last decades. The adsorption properties of nanomaterials are highly affected not only by the weak intermolecular interactions (e.g., hydrophobicity, hydrophilicity, electrostatic interactions, hydrogen bonding, and van der Waals), but also by the intrinsic characteristics (e.g., charge, size, shape, electronic states, crystallinity, surface-to-mass ratio, surface modifications with 22494


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Figure 2. Optimization of adsorbent dosage (A), pH (B), and ion strength (C). V = 1.0 mL, m = 0.8 mg (in B and C), pH 7.0, adsorption time 30 min. The curves of adsorption capacities changed with TCS and TCC concentrations (D). The linear fitting of Langmuir (E) and Freundlich (F) models for the adsorption data of TCS and TCC on Fe3O4@COFs. V = 1.0 mL, m = 0.2 mg, pH 7.0.

percentages of C, N, O, and Fe elements. Figure 1C,D clearly indicates that both of the Fe3O4 MNPs and Fe3O4@COFs contains C, N, O, and Fe elements with mass percentage (Wt %) of 3.51, 0.06, 33.02, and 63.41 in the Fe3O4 MNPs and 10.51, 0.67, 29.19, and 59.62 in the Fe 3 O 4 @COFs, respectively. The contents of C and N elements increased significantly after the self-assembly of COF shells, implying the generation of COF shells via covalent reaction of TAPB and TPA. The synthesized magnetic materials were further characterized by FT-IR to determinate the functional groups. As shown in Figure S4A, the peak at 529 cm−1 is assigned to Fe− O−Fe vibration. The peak at 1134 cm−1 is the antisymmetric stretching vibration. The peaks at 1632 and 1402 cm−1 belong to the stretching vibration peaks of CO and C−O, respectively. The peak at 3430 cm−1 is the stretching vibration of −OH. These peaks show infrared characteristic absorption of Fe3O4 MNPs with carboxylic surface. Compared to Fe3O4 MNPs, two distinct peaks appeared in the IR spectra of Fe3O4@COFs. The peaks at 1700 and 1489 cm−1 are respectively assigned to C−N deformation vibration of amide bond and CC stretching vibration of benzene ring respectively, indicating the formation of COF shells in the condensation reaction. The mass ratios and thermal stability of COF shells were characterized by TGA. As shown in Figure S4B, the weight loss of Fe3O4 MNPs and Fe3O4@COFs from 40 to 1000 °C was about 23.1% and 47.5%, respectively. The weight loss of Fe3O4 MNPs could be attributed to the loss of adsorbed water and rich carboxyl groups. However, after being coated with COFs, Fe3O4@COFs showed about 24.4% mass percent higher weight loss than Fe3O4 MNPs. This result further indicated that COF shells successfully wrapped on the Fe3O4 MNPs. Moreover, obvious weight loss at temperature up to 800 °C showed good chemical and thermal stability of Fe3O4@COFs.

active groups, and surface wrapping in the biological medium).32 The general scheme for efficient adsorption and removal of TCS and TCC based on Fe3O4@COFs was illustrated in Scheme 1. Three major steps were used in the preparation of Fe3O4@COFs and removal of TCS and TCC, including the following. (1) Preparation of bare Fe3O4 MNPs with abundant carboxyl groups on the surface of the Fe3O4 MNPs by a typically solvothermal reaction. (2) One-step synthesis of the COF shells by Schiff-base condensation reaction of TPA and TPAB on the surface of the Fe3O4 MNPs at room temperature. (3) Evaluation of the Fe3O4@COFs adsorption behaviors as adsorbents for removing TCS and TCC by UHPLC-MS/MS analysis. To identify the role of the COF shells in the adsorption process, the adsorption capacities of the Fe3O4 and Fe3O4@ COFs to TCS and TCC were contrasted. As shown in Figure S2, both of the Fe3O4 and Fe3O4@COFs have a certain adsorption mass to TCS and TCC, but the Fe3O4@COFs exhibit distinct adsorption capacity compared to Fe3O4 NPs, indicating a specific interaction of TCS and TCC on the Fe3O4@COFs. 3.2. Characterization of the Fe 3 O 4 @COFs. The morphological structures of the Fe3O4 MNPs and the Fe3O4@COFs were characterized by TEM. Figure 1A shows that the Fe3O4 MNPs had spherical nanoclusters with an approximate size of 160 nm. In the Figure 1B a thin layer of COF shells could be clearly observed covering on the surface of the Fe3O4 MNPs with a thickness of ∼5 nm surrounding the magnetic core. Figure S3 shows the TEM images of Fe3O4 MNPs (A) and Fe3O4@COFs (B) with full view further certificate the COFs wrapped on the surface of Fe3O4 NPs successfully. EDS analysis was carried out for the analysis of C, N, O, and Fe elements. Figure 1C shows the EDS spectra of the Fe3O4 MNPs and Fe3O4@COFs. Figure 1D shows the relative mass 22495


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should be noted that there was no direct relationship between TCC extraction efficiencies and NaCl concentration (Figure 2C). However, it had a pronounced effect on TCS adsorption properties when NaCl concentration exceeded 0.4 mol/L. This may be because the high ionic strength affected the phenolic hydroxyl structure of TCS, thus weakening hydrophobic interaction between the analytes and the sorbents. 3.4. Adsorption Performance of the Fe3O4@COFs. 3.4.1. Adsorption Isotherms. The feasibility of the Fe3O4@ COFs as adsorbents for the enrichment of TCS and TCC was evaluated by investigating their adsorption isotherms. The adsorption capacity of Fe3O4@COFs for TCS and TCC was studied by static equilibrium experiments, which were carried out using TCS and TCC solutions at different initial concentrations. Because of the low solubility of TCS and TCC in aqueous solution, the upper limits of TCS and TCC concentration should within their solubility. Therefore, the sorbent dosages of 0.2 and 0.8 mg were examined simultaneously. As shown in Figure 2D, when the sorbent dosage was 0.8 mg, the adsorption capacity increased linearly with the concentration of adsorbent. When the sorbent dosage was reduced to 0.2 mg, it was observed that the adsorption amounts of TCS and TCC on the Fe3O4@COFs increased with the increase of TCS and TCC concentrations from 0 to 2000 ng/mL and came to equilibrium over 1500 ng/mL. Langmuir and Freundlich isotherms were used to analyze the adsorption equilibrium for understanding the adsorption behaviors of TCS and TCC. The Langmuir isotherm is based on the assumption that it predicts monolayer coverage of the adsorbate on the outer surface of the adsorbent.33 Linear form expression for the Langmuir model is34

The super para-magnetism nature of magnetic materials can be measured by the hysteresis loops. The VSM curves of Fe3O4 MNPs and Fe3O4@COFs are shown in Figure S4C. Both Fe3O4 MNPs and Fe3O4@COFs had high saturation magnetism with saturation magnetization values of 52.6 and 48.4 emu/g, respectively. Slightly reduced magnetism showed that COF shells formation affected magnetic nanomaterials. However, the high absolute magnetization value of Fe3O4@ COFs was conducive to magnetic field separation. The N2 sorption−desorption isotherm for Fe3O4@COFs displayed a characteristic of type IV isotherms with H1 hysteresis indicating the presence of mesopores. Whereas Fe3O4 MNPs could be assigned to type II isotherms, indicating the nonporous structure (Figure S4D). The BET surface areas and total pore volumes of the Fe3O4 MNPs and Fe3O4@COFs were 24.63 and 55.71 m2 g−1 and 0.068 and 0.12 cm3 g−1, respectively. Compared to Fe3O4 MNPs, the surface area and pore volume of Fe3O4@COFs were obviously increased. The adsorption performance of Fe3O4@COFs for TCS and TCC was greatly improved accordingly. The as-synthesized COFs were also characterized by XRD. As shown in Figure S5A, the bare Fe3O4 MNPs and the Fe3O4@COFs have identical peaks(30.51°, 35.78°, 43.56°, 53.97°, 57.52°, and 63.04°) in the wide angle which attribute to the (220), (311), (400), (422), (511), and (440) reflection planes of Fe3O4 (JCPDS No. 75-1609). Moreover, the XRD patterns exhibit the intense (100) peak at 2.74°, and additional peak (200) at 5.54 in the low angle indicating that the COFs on the surface of Fe3O4 MNPs are highly crystalline (Figure S5B). 3.3. Optimization of Adsorption Conditions. 3.3.1. Effect of the Adsorbent Dosage. The effect of adsorbent dose on the removal of TCS and TCC for different concentrations was investigated by agitating with different adsorbent dosage over the range of 0.05−1.2 mg, while the adsorption time was set as 30 min, and the supernatant was analyzed. The study revealed that extraction efficiency increased with the increase of adsorbent dosage (Figure 2A). This could be attributed to the increased surface area of Fe3O4@COFs and availability of more adsorption sites. When the adsorbent dose was 0.8 mg, the extraction efficiencies exceeded 90% at different concentrations of TCS and TCC. Therefore, 0.8 mg adsorbent was selected as the optimized dosage. 3.3.2. Effect of pH. The effect of pH was investigated by employing initial concentrations of TCS (1000 ng/mL) and TCC (500 ng/mL) in a pH range of 6.0−11.0. The initial pH values were adjusted with 0.1 M HCl or 0.1 M NaOH. As can be seen from Figure 2B, there was no significant change in the extraction efficiency of TCS and TCC within the pH range from 6.0 to 9.0. This indicated that weak acid or weak alkaline solution could not influence the adsorption effect. However, just a little decrease was observed when pH of solution was higher than 9.0. The pKa values of TCS and TCC are 7.9 and 12.8, respectively. Alkaline environment could increase the solubilities of TCS and TCC and weaken hydrophobic interaction between the analytes and the sorbents. Therefore, pH 7.0 of solution was chosen in the extraction experiments considering the operation convenience. 3.3.3. Effect of Ion Strength. Salt addition can increase the viscosity of the aqueous solution, which is unfavorable for the extraction. The effect of ionic strength on the TCS and TCC adsorption capacities of Fe3O4@COFs was investigated in NaCl solutions with concentrations from 0 to 1.0 mol/L. It

Ce C 1 = e + Qe Qm Q mkL

(3)

Another characteristic parameter of the Langmuir isotherm is the dimensionless factor RL (separation factor), related to the shape of the isotherm. Its value indicates either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0) adsorption and it was evaluated as35 RL =

1 1 + kLC0

(4)

where Ce (ng/mL) is the equilibrium concentration of TCS or TCC, Qe (μg/g) and Qm (μg/g) are the corresponding adsorption capacity in equilibrium adsorption and theoretical saturation adsorption, and kL (mL/ng) is the Langmuir constant. C0 (ng/mL) is the highest concentration of the adsorbate. The linear plots of Ce/Qe versus Ce are shown in Figure 2E, good linear correlations fitted the Langmuir isotherm in the high concentration region of TCS and TCC. The data points clearly deviated from linearity in the low concentration region of TCS and TCC. The adsorption parameters for TCS and TCC adsorption on Fe3O4@COFs were calculated and tabulated in Table S2. As can be seen from Table S2, the RL values were between 0 and 1 which showed the adsorption process was favorable. The Freundlich isotherm is based on multilayer adsorption on heterogeneous surface. Linear form of Freundlich equation is36 22496


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Figure 3. Adsorption kinetics of pseudo-first-order (A) and pseudo-second-order (B) kinetic models for TCS and TCC adsorption. V = 1.0 mL, m = 0.8 mg, Ci = 1.5 μg/mL, pH 7.0.

InQ e = InkF +

InCe n

and TCC showed a similar trend. Therefore, 20 min was the equilibrium adsorption time. The kinetic models of pseudo-first order and the pseudosecond order were adopted to simulate the adsorption kinetics of the Fe3O4@COFs to TCS and TCC. The pseudo-first order kinetics introduced by Lagergren for the adsorption of solid/ liquid systems can be written in the following alternative way:38

(5)

where kF is the binding energy constant and n is the Freundlich constant. The linear plots of InQe versus InCe are shown in Figure 2F. Good linear correlations fitted the Freundlich isotherm in the low concentration region of TCS and TCC (R2 > 0.99). The adsorption parameters of n and kF were calculated and given in Table S2. The values of n were less than 1 indicating easy absorption of both TCS and TCC on Fe3O4@COFs. Thus, both TCS and TCC had the similar adsorption process. The Freundlich model was more appropriate in the low concentration region and the Langmuir model was more appropriate in the high concentration region in describing the adsorption process. This phenomenon may be due to multilayer adsorption through the interactions of space embedding effect, van der Waals forces and benzene ring π−π stacking at a low concentration range, and monolayer adsorption through strong π−π stacking between TCS or TCC and Fe3O4@COFs. The reasons maybe due to the distances are far from each other between molecules and interface of COFs in low concentration region leading to weak interactions. Therefore, the interactions of space embedding effect, van der Waals forces and benzene ring π−π stacking work together synergistically to adsorb TCS and TCC. Moreover, low concentration is more suitable for space embedding. Multiple interactions tend to produce multilayer adsorption. When the concentration increases, the distances between COFs and molecules are shortened resulting increased interaction force of π−π stacking significantly. The adsorption behavior is dominated by π−π stacking. 3.4.2. Adsorption Kinetics. Adsorption kinetics describes the solute uptake rate which in turn controls the residence time of adsorbate uptake at the solid-solution interface. Therefore, the kinetics can provide valuable insights into the mechanism and reaction pathway of adsorption process.37 To understand the adsorption mechanism of Fe3O4@COFs, adsorption kinetics was investigated via the changing curves of adsorption amount with time. The adsorption experiments were performed on the mixed solution of TCS and TCC with 1.5 μg/mL at neutral aqueous solution. As can be seen from Figure 3, the adsorption occurred rapidly during the first 20 min, and then reached a platform. The adsorption behaviors of the TCS

Q t = Q m[1 − e(−k1t )]

(6)

Ho and McKay’s pseudo-second-order kinetic model can be expressed as39 t t 1 = + Qt Qm k 2Q m 2 −1

(7) −1

−1

where k1 (min ) and k2 (g μg min ) are the pseudo-first order and pseudo-second-order rate constant. Qm and Qt are the adsorption amounts of TCS and TCC (μg g−1) at equilibrium and at time t, respectively. The values of k1, k2, and the correlation coefficient (R) can be determined experimentally by plotting Qt versus t and t/Qt versus t, respectively. The fitting curves of Qt versus t according to the pseudofirst-order model are shown in Figure 3A. The adsorption kinetic parameters for TCS and TCC adsorption on Fe3O4@ COFs are shown in Table S3. Both TCS and TCC exhibited similar dynamic progress. It was observed that the Q was equilibrated at t ⩾ 20 min. The pseudo-first-order kinetic model substantiated a diffusion-based process. The determined values of Qm calculated from the equation were close to the experimental values. The correlation coefficients (R2) were poor especially for TCC. The data for the adsorption of TCS and TCC onto Fe3O4@COFs applied to pseudo-second-order models are shown in Figure 3B and the results are presented in Table S3. It showed that the values of R2 (R > 0.99) was higher than the pseudo-first-order kinetic model, suggesting that the adsorption of TCS and TCC onto Fe3O4@COFs can be better represented by the pseudo-second-order model. In general, the adsorption of TCS onto the Fe3O4@COFs was well fitted to both models. But the adsorption of TCC onto the Fe3O4@COFs was more suitable to the pseudosecond-order kinetic model. This phenomenon can be explained as the adsorption of TCS onto the Fe3O4@COFs affected the adsorption process of TCC at the initial 10 min. 22497


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Figure 4. Optimization of desorption solvent (A), time (B), desorption times (C). V = 1.0 mL, m = 0.8 mg, pH 7.0, adsorption time 20 min, TCS concentration 2500 ng/mL, TCC concentration 1250 ng/mL.

When the adsorption time exceeded 10 min, the adsorption rates gradually slowed down. Both TCS and TCC showed similar dynamic progress. The Fe3O4@COFs were more beneficial to TCS adsorption. The saturated adsorption capacity and rate constant of TCS were slightly higher than TCC. Therefore, adsorption process essentially may be divided into two steps. The first step was dominated by diffusion-based process via van der Waals forces at the initial 10 min. The second step was dominated by special intermolecular interactions of benzene ring π−π stacking between TCS or TCC and Fe3O4@COFs and the space embedding effect of TCS and TCC on Fe3O4@COFs mesoporous surface over 10 min. 3.5. Optimization of Desorption Conditions. 3.5.1. Effect of Desorption Solvent. To improve the regeneration of the Fe3O4@COFs, five different organic solvents including methanol, acetonitrile, acetone, chloroform, and iso-propanol were selected as desorption solvents. As shown in Figure 4A, all of the organic solvents demonstrated high desorption efficiency. Among these solvents, methanol provided the best desorption efficiency toward both TCS and TCC under the same extraction and desorption conditions. Hence, methanol was selected in the following study. It should also be noted that ultrapure water could not remove TCS and TCC from the Fe3O4@COFs, indicating a high stability of TCS and TCC adsorbed on the Fe3O4@COFs in water environment. 3.5.2. Effect of Desorption Time. The effect of desorption time was further studied using methanol as the desorption solvent. As shown in Figure 4B, both TCS and TCC had a rapid desorption effect. The desorption efficiencies were higher than 90% within 2 min and then reached the desorption equilibrium at 10 min with high desorption efficiency, indicating that TCS and TCC were almost completely desorbed from the Fe3O4@COFs. Thus, 10 min was chosen as the desorption time. 3.5.3. Effect of Multiple Desorption. Besides, multiple desorption was studied to improve the desorption effect of TCS and TCC from the Fe3O4@COFs using methanol as the desorption solvent. Figure 4C shows that triplicate desorption could completely remove TCS and TCC from the Fe3O4@ COFs. 3.6. UHPLC-MS/MS Validation. The quantitative analysis of TCS and TCC was evaluated by UHPLC-MS/MS with high sensitivity and selectivity. The selected ion chromatograms of standard solution (containing 50 ng/mL TCS and 25 ng/mL TCC in pure water) are shown in Figure 5. The peaks of TCS and TCC did not interfere with each other due to the different precursors and products in MS/MS analysis although the

Figure 5. Selected ion chromatograms of the standard solution (containing 50 ng/mL TCS and 25 ng/mL TCC in pure water).

baseline was not completely separated. UHPLC-MS/MS analysis was also studied here, including linearity, limits of detection (LODs, S/N = 3) and correlation coefficient (R2). The results are summarized in Table S4. Both TCS and TCC had good linearity with correlation coefficients (R2) higher than 0.9976 in the range of 0.1−50 ng/mL for TCS and 0.05− 25 ng/mL for TCC. The LODs for TCS and TCC were calculated to be 0.02 and 0.005 ng/mL, respectively, which showed high sensitivity for sample analysis. 3.7. Application of the Fe3O4@COFs. The Fe3O4@COFs was then applied to remove TCS and TCC in healthy fetal bovine serum (FBS) samples. To prepare these samples, different spiked concentrations of TCS and TCC were dissolved in 50-fold diluted FBS. After the solution adsorption by the Fe3O4@COFs and magnetic separation, the supernatants diluted with methanol were analyzed by UHPLC-MS/ MS and the peak areas were used to determine the extraction efficiency and recovery. Then the precipitates were desorbed by methanol followed by magnetic separation. The supernatants were analyzed by UHPLC-MS/MS to determine the recovery. We can see from Figure S6 that there was no TCS and TCC detected in blank FBS sample. Figure S7 shows the selected ion chromatograms of the standard added mixture solution in FBS (containing 500 ng/mL TCS and 250 ng/mL TCC) before (black line) and after (red line) treatment with the Fe3O4@COFs. It was noted that the signals of TCS and TCC were significantly weakened after adsorption by the Fe3O4@COFs. Results were calculated in Table 1. Good 22498


Research Article

ACS Applied Materials & Interfaces Table 1. Removal Efficiencies and Recoveries of TCS and TCC in FBSa analytes TCS

TCC

added, ng/mL

found after adsorption, ng/mL

0 50 500 2500 0 25 250 1250

ND 2.29 79.98 442.75 ND 2.08 44.07 210.78

removal efficiency, % 95.4 84.0 82.3 91.7 82.4 83.1

found after desorption, ng/mL ND 51.84 547.63 2352.07 ND 27.18 245.90 1161.63

Comparisons of different adsorbents for TCS and TCC adsorption; structures of TCS and TCC; TEM images of Fe3O4 MNPs and Fe3O4@COFs; adsorption capacities of TCS and TCC adsorbed by Fe3O4 and Fe3O4@COFs; FT-IR spectra, TGA curves, hysteresis loops, N2 adsorption−desorption isotherms and XRD of the Fe 3 O 4 MNPs and Fe 3 O 4 @COFs; total ion chromatography of the blank serum after treated with the Fe3O4@COFs; selected ion chromatograms of the standard added mixture solution in FBS; recycle test of the Fe3O4@COFs (PDF)

recovery, % 103.7 109.5 94.1 108.7 98.4 92.9

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a

ND: Not detected.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

extraction efficiencies and recoveries were obtained by adding 50, 500, and 2500 ng/mL TCS and 25, 250, and 1250 ng/mL TCC in the FBS respectively, indicating the capability of using the Fe3O4@COFs to remove the TCS and TCC in real serum samples without any pretreatment. It still showed good extraction efficiency when the sample volume was magnified to 10 and 50 mL (Table S5). 3.8. Regeneration of the Fe3O4 @COFs. Regeneration is another important property for the application of the Fe3O4 @ COFs. The adsorption−desorption cycle of the Fe3O4 @COFs was repeated 10 times. It can be seen from Figure S8 that the extraction efficiencies to TCS and TCC adsorbed by the Fe3O4@COFs were stable after 10 adsorption−desorption cycles, indicating that the Fe3O4@COFs had an excellent regeneration and physic stability.

ORCID

Zian Lin: 0000-0002-0866-0711 Zongwei Cai: 0000-0002-7013-5547 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was financially supported by NSFC (21405075 and 21806134), Fuzhou science and technology project (2018S-112), Industrial Technology Key Project of Fujian Province (2014H0040), Fujian Province Natural Science Foundation (2019J01757 and 2017J01418), Fujian provincial youth natural fund key project (JZ160468), Science and technology project of Minjiang University (MYK18008), and new century excellent talents support plan of Fujian province colleges and universities (2017).

4. CONCLUSIONS This work proceeds a simple one-pot synthesis COFs on Fe3O4 MNPs by condensation reaction of TAPB and TPA at room temperature and atmospheric pressure. The Fe3O4@ COFs as adsorbents are very effective for the simultaneous removal of TCS and TCC. It shows unique advantages compared to carbon nanotubes and soils (Table S6). The adsorption behaviors and mechanism were discussed in details. TCS and TCC have similar adsorption process. The Freundlich model was more appropriate in the low concentration region and the Langmuir model was more appropriate in the high concentration region in describing the adsorption process. Moreover, the adsorption of TCC onto the Fe3O4@COFs was more suitable to the pseudo-second-order kinetic model. Combined with highly sensitive UHPLC-MS/ MS analysis, the Fe3O4@COFs was successfully applied to remove TCS and TCC in real serum samples without any pretreatment. The Fe3O4@COFs possesses characteristics of magnetic field separation, rapid adsorption, easy preparation, and good regeneration and have great potential as adsorbents in the treatment of environmental and biological samples.

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REFERENCES

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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.9b06953. Dynamic SRM MS parameters of TCS and TCC; adsorption isotherms and kinetic parameters for TCS and TCC adsorption on Fe3O4@COFs; linear ranges, regression equations, LODs of TCS and TCC; extraction efficiencies of TCS and TCC in FBS; 22499


Research Article

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