Covalent Organic Framework-Functionalized Magnetic CuFe2O4/Ag Nanoparticles for the Reduction of 4-Nitrophenol

In this work, magnetic CuFe2O4/Ag nanoparticles activated by porous covalent organic frameworks (COF) was fabricated to evaluate the heterogenous reduction of 4-nitrophenol (4-NP). The core-shell CuFe2O4/Ag@COF was successfully prepared by polydopamine reduction of silver ions on CuFe2O4 nanoparticles, followed by COF layer condensation. By integrating the intrinsic characteristics of the magnetic CuFe2O4/Ag core and COF layer, the obtained nanocomposite exhibited features of high specific surface area (464.21 m2 g−1), ordered mesoporous structure, strong environment stability, as well as fast magnetic response. Accordingly, the CuFe2O4/Ag@COF catalyst showed good affinity towards 4-NP via π-π stacking interactions and possessed enhanced catalytic activity compared with CuFe2O4/Ag and CuFe2O4@COF. The pseudo-first-order rate constant of CuFe2O4/Ag@COF (0.77 min−1) is 3 and 5 times higher than CuFe2O4/Ag and CuFe2O4@COF, respectively. The characteristics of bi-catalytic CuFe2O4/Ag and the porous COF shell of CuFe2O4/Ag@COF made a contribution to improve the activity of 4-NP reduction. The present work demonstrated a facile strategy to fabricate COF-activated nano-catalysts with enhanced performance in the fields of nitrophenolic wastewater treatment.


Introduction
Nitrophenols are mutagenic and refractory aromatic pollutants commonly found in various industrial and agricultural wastewaters. Among the three isomeric nitrophenols, 4-nitrophenol (4-NP) is more detrimental to pose a threat to both the environment and the public [1]. Generally, two conventional clean-up techniques are used for the removal of 4-NP: (1) permanent removal of target contaminants, and (2) conversion into less-or non-toxic forms. A technique related to the latter pattern would be desirable because of the production of p-animophenol (4-AP), which is an important intermediate in the productions of pharmaceuticals, dyes, pigments and pesticides [2][3][4]. Therefore, high-efficiency and recycled catalysts have been desired for 4-NP reduction.
Catalytic reduction using noble metals as catalysts have been widely employed for 4-NP reduction due to the high surface areas and the exposed active atoms. However, due to the high surface free energy, noble metals tend to agglomerate, thus leading to obvious decrease of the catalytic activities [5].

Synthesis of CuFe 2 O 4 /Ag Nanoparticles
CuFe 2 O 4 nanoparticles were synthesized according to the reported method [31]. Then the Ag nanoparticle was assembled on CuFe 2 O 4 according to a polydopamine (PDA)-assisted reduction process [32]. Typically, 30 mg of CuFe 2 O 4 nanoparticles were dispersed in 25 mL Tris buffer solution (10 mM, pH = 8.5), followed by adding 50 mg of DA·HCl into the mixture and mechanically stirred for 2 h at room temperature. Then the black powder was collected by a magnet, washed with ethanol and deionized water. For the preparation of CuFe 2 O 4 /Ag nanoparticles, silver ammonia solution was firstly prepared by adding ammonia aqueous solution (2 wt%) into 10 mg·mL −1 AgNO 3 solution until the brown precipitation was just dissolved. Then 50 mg of PDA-modified CuFe 2 O 4 were added to 25 mL silver ammonia solution, and the mixture was mechanically stirred for 6 h at room temperature. The product was collected with a magnet and washed with ethanol and deionized water, then dried under vacuum at 60 • C over night.

Preparation of Core-Shell Structured CuFe 2 O 4 /Ag@COF Nanocomposites
The synthesis of core-shell structured CuFe 2 O 4 /Ag@COF nanocomposites was carried out as a modified method [33]. Firstly, CuFe 2 O 4 /Ag nanoparticles were stabilized with PVP. In brief, the CuFe 2 O 4 /Ag nanoparticles (50 mg) were suspended in water (50 mL), and a solution of PVP (25 mg) in water (50 mL) was added dropwise to the above solution under stirring. The mixture was further stirred at room temperature for 12 h, washed three times with methanol, and final dispersed in methanol (10 mL). Secondly, the PVP-stabilized CuFe 2 O 4 /Ag nanoparticles were sonicated for 30 min under stirring, and 4 mL 1,4-dioxane-butanol (v/v = 1/1) containing TAPB (0.03 mmol) and DMTP (0.045 mmol) was added to the suspension. After that, aqueous acetic acid (12 M, 0.15 mL) was slowly appended to the mixture. Then, the reaction was allowed to conduct at room temperature for 2 h. After another aqueous acetic acid (12 M, 0.45 mL) was added to the suspension, the reaction was further stirred at 70 • C for 24 h. The product (CuFe 2 O 4 /Ag@COF) was naturally cooled to room temperature and collected with the help of a magnet. After being washed with tetrahydrofuran and acetone several times, the product was dried in vacuum at 80 • C overnight.
The CuFe 2 O 4 @COF nanocomposite was prepared according to the above method by adding CuFe 2 O 4 nanoparticles.

Catalytic Reduction of 4-Nitrophenol (4-NP)
Catalytic activities of the CuFe 2 O 4 /Ag@COF nanocomposite were evaluated by UV-Vis spectroscopy through the reduction of 4-NP to 4-AP at room temperature, using NaBH 4 as a reductant. 200 µL of 4-NP solution (5 mM), 0.45 mL of freshly prepared NaBH 4 aqueous solution (200 mM), and 2.35 mL deionized water were added into a standard quartz cuvette. The mixture solution turns from light yellow to bright yellow. Subsequently, 2 mg of CuFe 2 O 4 /Ag@COF was dispersed to the above mixture. Then the intensity of the absorption peak at 400 nm was monitored by UV-Vis spectroscopy as a function of time. The as-prepared CuFe 2 O 4 nanoparticles, CuFe 2 O 4 /Ag nanoparticles, and CuFe 2 O 4 @COF nanocomposite were also applied for 4-NP reduction under the same conditions. For the catalyst recycling test, the catalyst was collected with an extra magnet after each cycle, and another 4-NP and NaBH 4 solution was added for the next catalytic cycle. The UV-Vis absorption of reaction solution was measured at 400 nm.

Fabrication and Characterization of CuFe 2 O 4 /Ag@COF Nanocomposite
The magnetic CuFe 2 O 4 /Ag@COF was synthesized by using CuFe 2 O 4 /Ag as a core, and TAPB-DMTP COF as a shell (Scheme 1). CuFe 2 O 4 /Ag was synthesized by depositing Ag on CuFe 2 O 4 nanoparticles, which was assisted by PDA reduction. This bi-catalyst composed of CuFe 2 O 4 and in situ synthesized Ag nanoparticles would accelerate the 4-NP reduction [34]. The COF shell encapsulation process was assisted by polyvinylpyrrolidone (PVP)-stabilized CuFe 2 O 4 /Ag strategy. This is because it has been proved that the amphiphilic PVP can stabilize the nanoparticles in the reaction solution and enhance the affinity between apolar groups of PVP and organic linkers [30]. At last, the TAPB-DMTP was assembled on the surface of CuFe 2 O 4 /Ag based on Schiff-base reaction to obtain the core-shell CuFe 2 O 4 /Ag@COF nanocompsite [33]. (200 mM), and 2.35 mL deionized water were added into a standard quartz cuvette. The mixture solution turns from light yellow to bright yellow. Subsequently, 2 mg of CuFe2O4/Ag@COF was dispersed to the above mixture. Then the intensity of the absorption peak at 400 nm was monitored by UV-Vis spectroscopy as a function of time. The as-prepared CuFe2O4 nanoparticles, CuFe2O4/Ag nanoparticles, and CuFe2O4@COF nanocomposite were also applied for 4-NP reduction under the same conditions. For the catalyst recycling test, the catalyst was collected with an extra magnet after each cycle, and another 4-NP and NaBH4 solution was added for the next catalytic cycle. The UV-Vis absorption of reaction solution was measured at 400 nm.

Fabrication and Characterization of CuFe2O4/Ag@COF Nanocomposite
The magnetic CuFe2O4/Ag@COF was synthesized by using CuFe2O4/Ag as a core, and TAPB-DMTP COF as a shell (Scheme 1). CuFe2O4/Ag was synthesized by depositing Ag on CuFe2O4 nanoparticles, which was assisted by PDA reduction. This bi-catalyst composed of CuFe2O4 and in situ synthesized Ag nanoparticles would accelerate the 4-NP reduction [34]. The COF shell encapsulation process was assisted by polyvinylpyrrolidone (PVP)-stabilized CuFe2O4/Ag strategy. This is because it has been proved that the amphiphilic PVP can stabilize the nanoparticles in the reaction solution and enhance the affinity between apolar groups of PVP and organic linkers [30]. At last, the TAPB-DMTP was assembled on the surface of CuFe2O4/Ag based on Schiff-base reaction to obtain the core-shell CuFe2O4/Ag@COF nanocompsite [33]. The core-shell structure and morphologies of the as-prepared CuFe2O4/Ag@COF nanocomposite was verified by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Figure 1A and B revealed that the as-prepared CuFe2O4/Ag nanoparticles exhibited good monodispersity, and the CuFe2O4 were spherical in shape with an average diameter of 200 nm. After being deposited with Ag nanoparticles, small Ag nanoparticles (∼20 nm) were densely and uniformly assembled on the surface of CuFe2O4. After being coated with a COF layer, the COF shell (brighter) of TAPB-DMTP networks with a rough surface was formed and the thickness of ca. 30 nm surrounding the CuFe2O4/Ag core (darker) was clearly observed ( Figure 1C). Meanwhile, at the edges of CuFe2O4/Ag@COF spheres consisted of domains that contain regular channels along the radial direction ( Figure S1). It is due to the self-assembly of the rod-like crystallites of the COF shell [35] and revealed a highly ordered porous structure of the CuFe2O4/Ag@COF. Figure 1D displayed the high-resolution TEM images (HRTEM) and the corresponding fast Fourier transform (FFT) pattern of the CuFe2O4/Ag@COF nanocomposite. The spacing of the crystallographic plane of the Ag nanoparticles is 0.24 nm, which could be assigned to the (111) crystal planes of face-centered cubic (fcc) bulk Ag (0.235 nm). A clear inter-planar spacing of the lattice fringes is 0.25 nm, corresponding to the (311) crystal planes of CuFe2O4 with cubic spinel structure. The core-shell structure and morphologies of the as-prepared CuFe 2 O 4 /Ag@COF nanocomposite was verified by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Figure 1A,B revealed that the as-prepared CuFe 2 O 4 /Ag nanoparticles exhibited good monodispersity, and the CuFe 2 O 4 were spherical in shape with an average diameter of 200 nm. After being deposited with Ag nanoparticles, small Ag nanoparticles (∼20 nm) were densely and uniformly assembled on the surface of CuFe 2 O 4 . After being coated with a COF layer, the COF shell (brighter) of TAPB-DMTP networks with a rough surface was formed and the thickness of ca. 30 nm surrounding the CuFe 2 O 4 /Ag core (darker) was clearly observed ( Figure 1C). Meanwhile, at the edges of CuFe 2 O 4 /Ag@COF spheres consisted of domains that contain regular channels along the radial direction ( Figure S1). It is due to the self-assembly of the rod-like crystallites of the COF shell [35] and revealed a highly ordered porous structure of the CuFe 2 O 4 /Ag@COF. Figure 1D  Furthermore, the SEM image of CuFe2O4/Ag@COF also shows that the nanocomposite consisted of spheres with a rough and coarse surface ( Figure 2A). The obtained nanocomposite is somewhat visibly sticky and exhibited convex domains due to the presence of porous TAPB-DMTP. To confirm the elemental composition of the CuFe2O4/Ag@COF, the EDX elemental mapping confirmed the presence of Cu, Fe, Ag, C, N, and O element ( Figure 2B). Fourier transform infrared (FTIR) spectrosopy was performed to confirm the successful Furthermore, the SEM image of CuFe 2 O 4 /Ag@COF also shows that the nanocomposite consisted of spheres with a rough and coarse surface ( Figure 2A). The obtained nanocomposite is somewhat visibly sticky and exhibited convex domains due to the presence of porous TAPB-DMTP. To confirm the elemental composition of the CuFe 2 O 4 /Ag@COF, the EDX elemental mapping confirmed the presence of Cu, Fe, Ag, C, N, and O element ( Figure 2B). Furthermore, the SEM image of CuFe2O4/Ag@COF also shows that the nanocomposite consisted of spheres with a rough and coarse surface ( Figure 2A). The obtained nanocomposite is somewhat visibly sticky and exhibited convex domains due to the presence of porous TAPB-DMTP. To confirm the elemental composition of the CuFe2O4/Ag@COF, the EDX elemental mapping confirmed the presence of Cu, Fe, Ag, C, N, and O element ( Figure 2B). Fourier transform infrared (FTIR) spectrosopy was performed to confirm the successful preparation of the CuFe2O4/Ag@COF nanocomposite ( Figure 3A). In the spectrum of CuFe2O4/Ag, the absorption broad band at 3430 cm −1 represented the stretching mode of -OH groups and another peak at 1620 cm −1 corresponded to the bending vibration of H2O. In the lower zone, peak at 590 cm −1 Fourier transform infrared (FTIR) spectrosopy was performed to confirm the successful preparation of the CuFe 2 O 4 /Ag@COF nanocomposite ( Figure 3A). In the spectrum of CuFe 2 O 4 /Ag, the absorption broad band at 3430 cm −1 represented the stretching mode of -OH groups and another peak at 1620 cm −1 corresponded to the bending vibration of H 2 O. In the lower zone, peak at 590 cm −1 can be related to the vibration of the metal (tetrahedron)-oxygen (M-O), in which the atom M can be copper or iron. Another absorption peak at 428 cm −1 was referred to the vibration of octahedral sites of spinel-type oxide [36]. Compared with CuFe 2 O 4 /Ag, similar adsorption bands were observed in the spectrum of the as-prepared CuFe 2 O 4 /Ag@COF nanocomposite. New peaks at 1460 cm −1 , 1501 cm −1 and 1405 cm −1 were assigned to the aromatic C-C ring stretch and methoxy group stretching the of TAPB-DMTP, respectively. Furthermore, the characteristic peaks at 3430 cm −1 and 1620 cm −1 became strong and sharp due to the existence of N-H bonds and C=N vibration of COF coating, validating the successful formation of COF shell via Shiff-base condensation reaction. can be related to the vibration of the metal (tetrahedron)-oxygen (M-O), in which the atom M can be copper or iron. Another absorption peak at 428 cm −1 was referred to the vibration of octahedral sites of spinel-type oxide [36]. Compared with CuFe2O4/Ag, similar adsorption bands were observed in the spectrum of the as-prepared CuFe2O4/Ag@COF nanocomposite. New peaks at 1460 cm −1 , 1501 cm −1 and 1405 cm −1 were assigned to the aromatic C-C ring stretch and methoxy group stretching the of TAPB-DMTP, respectively. Furthermore, the characteristic peaks at 3430 cm −1 and 1620 cm −1 became strong and sharp due to the existence of N-H bonds and C=N vibration of COF coating, validating the successful formation of COF shell via Shiff-base condensation reaction. The crystallinity of CuFe2O4/Ag and CuFe2O4/Ag@COF was investigated by XRD measurements and the result was shown in Figure 3B. As shown in Figure 3B, the characteristic diffraction peaks at 2θ = 30.2°, 35.5°, 43.4°, 57.3° and 62.6° corresponding to the (220), (311), (400), (511) and (440) crystal planes of CuFe2O4 [36]. The pattern at 38.0°, 44.2° and 64.4° can be attributed to the (111), (200) and (220) crystalline planes of the synthesized Ag [37]. After the coating of TAPB-DMTP COF, the moderate new peaks at 2θ = 5.60° and 7.4°, and weak peak at 9.7° belonging to the TAPB-DMTP was observed, which is consistent with that of TAPB-DMTP COF previously reported [29]. These results suggested that the CuFe2O4/Ag@COF nanocomposite exhibited the crystalline structure feature of magnetic CuFe2O4/Ag and TAPB-DMTP COF. TGA was further recorded to confirm the content of TAPB-DMTP coatings on the magnetic CuFe2O4/Ag ( Figure 4A). As for CuFe2O4/Ag nanoparticles, a distinct weight-loss step of 1.7 wt% up to ca. 300 °C was observed, which can be attributed to the volatilization of the absorbed water and solvent molecule. Another sharply weight loss profile (5.0 wt%) in the range of 300-400 °C was observed due to the decomposition of PDA. Then it remained constant until the temperature rose to 800 °C, and demonstrated a remarkable thermo-stability of CuFe2O4/Ag. Compared with CuFe2O4/Ag, the CuFe2O4/Ag@COF nanocomposite had almost 14 wt% weight loss in the range of 400-700 °C, implying the decomposition of TAPB-DMTP and further demonstrated the presence of TAPB-DMTP COF shell. Meanwhile, the CuFe2O4/Ag@COF also exhibited excellent thermal stability in nitrogen up to 300 °C.
The pore structure of the CuFe2O4/Ag@COF was further evaluated by N2 adsorption-desorption The crystallinity of CuFe 2 O 4 /Ag and CuFe 2 O 4 /Ag@COF was investigated by XRD measurements and the result was shown in Figure 3B. As shown in Figure 3B 200) and (220) crystalline planes of the synthesized Ag [37]. After the coating of TAPB-DMTP COF, the moderate new peaks at 2θ = 5.60 • and 7.4 • , and weak peak at 9.7 • belonging to the TAPB-DMTP was observed, which is consistent with that of TAPB-DMTP COF previously reported [29]. These results suggested that the CuFe 2 O 4 /Ag@COF nanocomposite exhibited the crystalline structure feature of magnetic CuFe 2 O 4 /Ag and TAPB-DMTP COF.
TGA was further recorded to confirm the content of TAPB-DMTP coatings on the magnetic CuFe 2 O 4 /Ag ( Figure 4A). As for CuFe 2 O 4 /Ag nanoparticles, a distinct weight-loss step of 1.7 wt% up to ca. 300 • C was observed, which can be attributed to the volatilization of the absorbed water and solvent molecule. Another sharply weight loss profile (5.0 wt%) in the range of 300-400 • C was observed due to the decomposition of PDA. Then it remained constant until the temperature rose to CuFe2O4/Ag), respectively (Table S1). The high external surface area with mesoporous structure indicates the superior concentration performance for the substrates, and thus made it possible for CuFe2O4/Ag@COF to facilitate efficient catalysis reduction of 4-NP. The vibrating sample magnetization (VSM) curves of CuFe2O4/Ag nanoparticle and CuFe2O4/Ag@COF nanocomposite are shown in Figure S2. All the materials obtained exhibited a superparamagnetic nature due to no obvious coercivity and remanence. After being assembled with TAPB-DMTP, the saturation magnetization of CuFe2O4/Ag@COF decreased from 62.7 emu g −1 to 35.1 emu g −1 . However, very fast aggregation (about 1 min) of CuFe2O4/Ag@COF from the homogeneous dispersion was observed with the help of an extra magnet (as shown in the bottom-right inset of Figure S2).
To further analyze the chemical composition of the magnetic CuFe2O4/Ag@COF, XPS was carried out. In Figure 5A, the wide scan spectra of the magnetic CuFe2O4/Ag@COF displayed photoelectron lines at binding energies of 285, 402, and 528 eV, which can be assigned to C 1s, N 1s, and O 1s, respectively. The disappearing Fe, Cu, Ag peaks in the wide scan spectra indicating the inner CuFe2O4/Ag core was coated with a shell which was thicker than the analysis depth of XPS (~10 nm). The C 1s core-level photoelectron spectrum was split into three peaks located at 284.6, 286.3, and 290.8 eV, respectively ( Figure 5B), which can be attributed to C-C/C=C, C-N, and C=O, respectively. The binding energies observed at Figure 5C showed the appearance of -NH-and -N= with their characteristic peaks at 398.4 and 400.0 eV. These results confirmed the successful formation of the COF shell. The pore structure of the CuFe 2 O 4 /Ag@COF was further evaluated by N 2 adsorption-desorption at 77 K ( Figure 4B). The isotherm of CuFe 2 O 4 /Ag presented a typical II characteristics, indicating the obviously non-porous structure of the solid sphere. Compared to CuFe 2 O 4 /Ag, a typical type IV isotherm was observed by CuFe 2 O 4 /Ag@COF, which is indicative of a mesoporous character. From pore size distribution curve (the inset of Figure 4B), the CuFe 2 O 4 /Ag@COF contained an average pore size of 3.15 nm with narrow sized distribution, which was well in agreement with theoretical value of the bulk COF (3.2 nm) [33]. In addition, the prepared CuFe 2 O 4 /Ag@COF gave an enhanced BET surface area and pore volume of 464.21 m 2 g −1 and 0.396 m 3 g −1 (38.60 m 2 g −1 and 0.0862 m 3 g −1 for CuFe 2 O 4 /Ag), respectively (Table S1). The high external surface area with mesoporous structure indicates the superior concentration performance for the substrates, and thus made it possible for CuFe 2 O 4 /Ag@COF to facilitate efficient catalysis reduction of 4-NP.
The vibrating sample magnetization (VSM) curves of CuFe 2 O 4 /Ag nanoparticle and CuFe 2 O 4 /Ag@COF nanocomposite are shown in Figure S2. All the materials obtained exhibited a superparamagnetic nature due to no obvious coercivity and remanence. After being assembled with TAPB-DMTP, the saturation magnetization of CuFe 2 O 4 /Ag@COF decreased from 62.7 emu g −1 to 35.1 emu g −1 . However, very fast aggregation (about 1 min) of CuFe 2 O 4 /Ag@COF from the homogeneous dispersion was observed with the help of an extra magnet (as shown in the bottom-right inset of Figure S2).
To further analyze the chemical composition of the magnetic CuFe 2 O 4 /Ag@COF, XPS was carried out. In Figure 5A, the wide scan spectra of the magnetic CuFe 2 O 4 /Ag@COF displayed photoelectron lines at binding energies of 285, 402, and 528 eV, which can be assigned to C 1s, N 1s, and O 1s, respectively. The disappearing Fe, Cu, Ag peaks in the wide scan spectra indicating the inner CuFe 2 O 4 /Ag core was coated with a shell which was thicker than the analysis depth of XPS (~10 nm). The C 1s core-level photoelectron spectrum was split into three peaks located at 284.6, 286.3, and 290.8 eV, respectively ( Figure 5B), which can be attributed to C-C/C=C, C-N, and C=O, respectively. The binding energies observed at Figure 5C showed the appearance of -NH-and -N= with their characteristic peaks at 398.4 and 400.0 eV. These results confirmed the successful formation of the COF shell.

Catalytic Reduction of 4-NP
The as-prepared CuFe2O4/Ag@COF nanocomposite has advantages of large surface area, rapid magnetic response, abundant catalytic sites, which can make sure the high catalytic performance and the catalyst stability towards 4-NP reduction. The catalytic performance of CuFe2O4/Ag@COF was tested for reducing 4-NP in the presence of excess NaBH4 in water. The reaction kinetics were monitored by UV-vis absorption spectroscopy of the reaction mixture after adding the catalyst. Notably, the absorption peak of 4-nitrophenol shifts from 319 to 400 nm (the aqueous solution turns from light yellow to bright yellow rapidly) after adding NaBH4 due to the formation of 4-nitrophenolate ions in the alkaline medium. Figure 6A and B showed the successive UV-vis spectra of 4-nitrophenolate catalyzed by CuFe2O4/Ag and CuFe2O4/Ag@COF in the presence of NaBH4, respectively. The absorption peak at 400 nm was observed to decrease in intensity and a new peak at ~300 nm increased along with the reaction time, suggesting the reduction of 4-NP to give 4-AP as the sole product [38]. Figure 6A shown that the catalytic reduction with CuFe2O4/Ag nanoparticles could be completed within 11 min. When CuFe2O4/Ag@COF was used as the catalyst ( Figure 6B), a significant catalytic activity was observed and conversion of 4-NP to 4-AP was finished within 4 min. We can speculate that the momentous activity shown by CuFe2O4/Ag@COF over CuFe2O4/Ag own to the porous COF matrix,

Catalytic Reduction of 4-NP
The as-prepared CuFe 2 O 4 /Ag@COF nanocomposite has advantages of large surface area, rapid magnetic response, abundant catalytic sites, which can make sure the high catalytic performance and the catalyst stability towards 4-NP reduction. The catalytic performance of CuFe 2 O 4 /Ag@COF was tested for reducing 4-NP in the presence of excess NaBH 4 in water. The reaction kinetics were monitored by UV-vis absorption spectroscopy of the reaction mixture after adding the catalyst. Notably, the absorption peak of 4-nitrophenol shifts from 319 to 400 nm (the aqueous solution turns from light yellow to bright yellow rapidly) after adding NaBH 4 due to the formation of 4-nitrophenolate ions in the alkaline medium. Figure 6A,B showed the successive UV-vis spectra of 4-nitrophenolate catalyzed by CuFe 2 O 4 /Ag and CuFe 2 O 4 /Ag@COF in the presence of NaBH 4 , respectively. The absorption peak at 400 nm was observed to decrease in intensity and a new peak at~300 nm increased along with the reaction time, suggesting the reduction of 4-NP to give 4-AP as the sole product [38]. Figure 6A shown that the catalytic reduction with CuFe 2 O 4 /Ag nanoparticles could be completed within 11 min. When CuFe 2 O 4 /Ag@COF was used as the catalyst ( Figure 6B), a significant catalytic activity was observed and conversion of 4-NP to 4-AP was finished within 4 min. We can speculate that the momentous activity shown by CuFe 2 O 4 /Ag@COF over CuFe 2 O 4 /Ag own to the porous COF matrix, which provided a large surface area of the encapsulated CuFe 2 O 4 /Ag nanoparticles and high particle number per unit mass for the catalyst. The increased fraction of the substrates at the surface of CuFe 2 O 4 /Ag@COF led to a significantly higher catalytic activity.

Plausible Mechanisms
Scheme 2 depicts the performance of CuFe2O4/Ag@COF as catalyst in reducing 4-NP to 4-AP by NaBH4. In aqueous medium, NaBH4 was first diffused through the mesoporous COF shell and adhered onto the surface of CuFe2O4/Ag, then electrons transferred from CuFe2O4/Ag to the adsorbed BH4 -with hydride ions' production. Simultaneously, the 4-NP were adsorbed onto the mesoporous COF shell via π-π stacking interactions because 4-NP is π-rich in nature [38]. Subsequently, the hydrogen atom formed from the hydride ions attacked 4-NP molecules to reduce them based on electron transfer to the CuFe2O4/Ag nanoparticles, forming the product 4-AP [38]. The surface of dual-catalyst CuFe2O4/Ag is presumed to play a role in electrically connecting two adsorbates through the surface so that electrons can be transferred from the oxidation site (NaBH4) to the reduction site (4-NP). Such preconcentration of BH4 -and 4-NP in the COF shell would result in more frequent contact of the inner CuFe2O4/Ag catalyst with reactants, thereby facilitating a highly efficient catalysis. Moreover, it has been revealed that CuFe2O4 with 'd n ' (n = 5-9) electronic configuration exhibited better catalytic activity for reduction of nitrophenol [30]. Specifically, both Cu 2+ and Fe 3+ ions present in the octahedral sites exposed on the surface of particles, thus the reduction of hydride ions would be accelerated by the enhanced electron transfer boosted by Ag and neighboring CuFe2O4 nanoparticles (Scheme 2). Since the concentration of BH 4 − added in the system is in excess compared to the concentration of 4-NP, pseudo-first-order kinetics is expected for the catalytic reaction under this circumstance ( Figure 6C). The pseudo-first-order kinetics can be described as: where C t is the concentration of 4-NP at time t, C 0 is the initial concentration of 4-NP, and k is the rate constant. From Figure 6C we can see all composite samples can catalyze 4-NP reduction but display different activities. Basically, single-catalyst showed lower catalytic activity than the dual-catalyst.  (Table S2). Additionally, the reusability of CuFe 2 O 4 /Ag@COF was further displayed. With high saturation magnetization, the CuFe 2 O 4 /Ag@COF can be easily separated in a few seconds using an external magnetic field. Figure 6D displayed the conversion for the reduction of 4-NP in an estimated time span of 4 min. The result displayed excellent reusability of the catalyst for more than 6 catalytic cycles with yields over 95%, indicating excellent reusability and stability towards the as-prepared CuFe 2 O 4 /Ag@COF nanocomposite. The slightly decreased conversions in the later catalysis cycles are presumably caused by the loss of catalyst during the washing process between cycles. The SEM image manifested that the structure of the recycled CuFe 2 O 4 /Ag@COF was preserved and no obvious aggregation was observed, further confirming the excellent stability of the prepared catalyst ( Figure S3). Hence, the prominent catalytic activity and high stability of CuFe 2 O 4 /Ag@COF can be attributed to the highly stable mesoporous architecture (TAPB-DMTP), which held the encapsulated nanoparticles to a high extent and provided docking sites for 4-NP.

Plausible Mechanisms
Scheme 2 depicts the performance of CuFe 2 O 4 /Ag@COF as catalyst in reducing 4-NP to 4-AP by NaBH 4 . In aqueous medium, NaBH 4 was first diffused through the mesoporous COF shell and adhered onto the surface of CuFe 2 O 4 /Ag, then electrons transferred from CuFe 2 O 4 /Ag to the adsorbed BH 4 − with hydride ions' production. Simultaneously, the 4-NP were adsorbed onto the mesoporous COF shell via π-π stacking interactions because 4-NP is π-rich in nature [38]. Subsequently, the hydrogen atom formed from the hydride ions attacked 4-NP molecules to reduce them based on electron transfer to the CuFe 2 O 4 /Ag nanoparticles, forming the product 4-AP [38]. The surface of dual-catalyst CuFe 2 O 4 /Ag is presumed to play a role in electrically connecting two adsorbates through the surface so that electrons can be transferred from the oxidation site (NaBH 4 ) to the reduction site (4-NP). Such preconcentration of BH 4 − and 4-NP in the COF shell would result in more frequent contact of the inner CuFe 2 O 4 /Ag catalyst with reactants, thereby facilitating a highly efficient catalysis. Moreover, it has been revealed that CuFe 2 O 4 with 'd n ' (n = 5-9) electronic configuration exhibited better catalytic activity for reduction of nitrophenol [30]. Specifically, both Cu 2+ and Fe 3+ ions present in the octahedral sites exposed on the surface of particles, thus the reduction of hydride ions would be accelerated by the enhanced electron transfer boosted by Ag and neighboring CuFe 2 O 4 nanoparticles (Scheme 2).

Scheme 2.
The mechanism of the reduction of 4-NP to 4-AP by NaBH4 in the presence of CuFe2O4/Ag@COF.

Conclusions
In summary, we have presented a facile and efficient strategy to construct core-shell magnetic CuFe2O4/Ag@COF nanocomposite. The obtained nanocomposite not only retained its high crystallinity, large surface area and thermal stability of the COF shell, but held high catalytic properties and strong magnetic effect of magnetic CuFe2O4/Ag nanoparticles. By virtue of these merits, the CuFe2O4/Ag@COF exhibited enhanced catalytic activity, robust recyclability and stability for the catalytic reduction of 4-NP by NaBH4. Specifically, the apparent reaction rate of CuFe2O4/Ag@COF was 3 times higher than that by CuFe2O4/Ag, which can be ascribed to the enhanced CuFe2O4/Ag dispersion and the strengthened interfacial interaction. Moreover, the magnetic CuFe2O4/Ag@COF can be recycled easily by a magnet and exhibited robust reusability for six cycles with minimal loss of catalytic activity but held its structural integrity. Encouraged by the effective catalysis of 4-NP with this nanocomposite, it can be expected that the rational design of the promising catalytic material with magnetic platforms and a tunable porous COF shell will be developed and applied for many redox-active environmental contaminants.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1: Figure S1: TEM image of the CuFe2O4/Ag@COF, Figure S2: Magnetic hysteresis loops of CuFe2O4/Ag and CuFe2O4/Ag@COF (the inset shows the magnetic separation behavior of CuFe2O4/Ag@COF in aqueous solution), Figure S3: SEM image of the recycled CuFe2O4/Ag@COF after six times reuse, Table S1: Nitrogen adsorption-desorption data of CuFe2O4/Ag and CuFe2O4/Ag@COF, Table S2

Conflicts of Interest:
The authors declare no conflict of interest.

Conclusions
In summary, we have presented a facile and efficient strategy to construct core-shell magnetic CuFe 2 O 4 /Ag@COF nanocomposite. The obtained nanocomposite not only retained its high crystallinity, large surface area and thermal stability of the COF shell, but held high catalytic properties and strong magnetic effect of magnetic CuFe 2 O 4 /Ag nanoparticles. By virtue of these merits, the CuFe 2 O 4 /Ag@COF exhibited enhanced catalytic activity, robust recyclability and stability for the catalytic reduction of 4-NP by NaBH 4 . Specifically, the apparent reaction rate of CuFe 2 O 4 /Ag@COF was 3 times higher than that by CuFe 2 O 4 /Ag, which can be ascribed to the enhanced CuFe 2 O 4 /Ag dispersion and the strengthened interfacial interaction. Moreover, the magnetic CuFe 2 O 4 /Ag@COF can be recycled easily by a magnet and exhibited robust reusability for six cycles with minimal loss of catalytic activity but held its structural integrity. Encouraged by the effective catalysis of 4-NP with this nanocomposite, it can be expected that the rational design of the promising catalytic material with magnetic platforms and a tunable porous COF shell will be developed and applied for many redox-active environmental contaminants.