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. 2021 Sep 20;7(10):1698–1706. doi: 10.1021/acscentsci.1c00941

Mechanical Bond Approach to Introducing Self-Adaptive Active Sites in Covalent Organic Frameworks for Zinc-Catalyzed Organophosphorus Degradation

Xianghui Ruan 1, Yajie Yang 1, Weixu Liu 1, Xujiao Ma 1, Cheng Zhang 1, Qinghao Meng 1, Zeyu Wang 1, Fengchao Cui 1, Jiahui Feng 1, Fuli Cai 1, Ye Yuan 1,*, Guangshan Zhu 1,*
PMCID: PMC8554822  PMID: 34729413

Abstract

graphic file with name oc1c00941_0006.jpg

Mechanically interlocked molecules (MIMs) with discrete molecular components linked through a mechanical bond in space can be harnessed for the operation of molecular switches and machines, which shows huge potential to imitate the dynamic response of natural enzymes. In this work, rotaxane compounds were adopted as building monomers for the synthesis of a crown-ether ring mechanically intercalated covalence organic framework (COF). This incorporation of MIMs into open architecture implemented large amplitude motions, whose wheel slid along the axle in response to external stimulation. After impregnation with Zn2+ ions, the relative locations of two zinc active sites (crown-ether coordinated Zn(II) and bipyridine coordinated Zn(II)) are endowed with great flexibility to fit the conformational transformation of an organophosphorus agent during the hydrolytic process. Notably, the resulting self-adaptive binuclear zinc center in a crown-ether-threaded COF network is endowed with a record catalytic ability, with a rate over 85.5 μM min–1 for organophosphorus degradation. The strategy of synthesis for porous artificial enzymes through the introduction of mechanically bound crown ether will enable significant breakthroughs and new synthetic concepts for the development of advanced biomimetic catalysts.

Short abstract

Based on a flexible pocket, natural enzymes with conformational dynamics corresponding to the important transition-state barrier crossing steps enable the unparalleled catalytic rate and selectivity.

1. Introduction

Organophosphate (OP) compounds, one of the most toxic agents, are illegally but widely used in chemical warfare, destroy the neurotransmitter acetylcholine, and damage the nervous system.1,2 Frequent military events have led to an urgent requirement for the rapid degradation of these banned chemical weapons.3 Naturally occurring organophosphorus hydrolase (OPH) exhibits an outstanding ability to hydrolyze various organophosphate esters.4−6 In its active domain, a responsive structural transformation for two zinc fragments is observed within 3.3–4.6 Å of the distance, which corresponds to the important transition-state barrier crossing and the product-releasing steps.7−9 Due to these conformational dynamics, OPHs hydrolyze OP compounds at rates several orders of magnitude higher than artificial catalysts. However, the poor stability under harsh environments (including high temperatures, heavy metal ions, and acids/bases) makes their industrial utilization unfeasible.10−12 Recently, a remarkable example was explored to construct a biomimetic covalent organic framework (COF) catalyst with porphyrin active sites, which involves the design and development of catalysts that resemble the structure and functionality of natural enzymes.13−15 The feasible synthesis of COFs allows for the modulation of pore size, network topology, and chemical functionality, thus endowing COFs with huge potentials for biocomposite performance. However, COF architectures with highly rigid skeletons and strong intermolecular π–π interactions restrict the structural flexibility specifically at active sites, leading to a limited catalytic performance.16,17

Mechanically interlocked molecules (MIMs) refer to linking two or more molecular components through a mechanical bond in space, such that their constituent parts can be separated and combined without breaking the structural integrity.18−21 Based on these characteristics, the translation (ring along axle) or rotation (ring about axle) can be harnessed for the operation of molecular switches and machines in the fields of molecular electronic devices and nanoelectromechanical systems.19−26 Introducing mechanical bonds to lattice frameworks facilitates the incorporation of the responsive structural transformation of enzymes and host–guest interactions of porous networks, improving the flexibility of biomimetic catalysts to make them more proteinlike. However, the application of MIMs as a functional unit to mimic the conformational transformation of enzyme catalysis has not yet been reported.

Herein, we introduced mechanically interlocked molecules as building units, whose wheel could slide along the axle in response to external stimulation to provide a host–guest complex. The traditional COF building linker—2,2′-bipyridine—was protonated to intercalate into the crown-ether ring by self-assembly through a hydrogen bond in the form of pseudorotaxane. The crown-COF was synthesized through a Schiff-base reaction, with a rotaxane as the linker and a triangle building monomer, 1,3,5-tris(4-aminophenyl)benzene, as the stopper. After being impregnated with Zn2+ ions, crown-ether rings, coupled with COF bipyridine linkers, cooperate in Zn2+ ion binding, while zinc-coordinated bipyridine groups are left in the adjacent layer. Since the components of the binding site are split between the robust skeleton and the wheel, the coordination structure for the Zn2+ ion can easily change as the wheel pirouettes and slides along the axle. This is advantageous for a strong guest association, because the relative location of two Zn2+ ions (one is coordinated by the crown-ether ring and bipyridine unit, and the other is fixed by the bipyridine unit) can adjust the geometry to fit the structural transformation of the organophosphorus agent. The resulting crown-ether-threaded COF network, with a self-adaptive binuclear zinc center, gains an ultrahigh capability for organophosphorus degradation.

2. Results and Discussion

To intercalate the bipyridine linker into the crown-ether ring, 2,2′-bipyridyl-5,5′-dialdehyde (BPDA) was acidized with trifluoromethanesulfonic acid into the protonated BPDA-salt, denoted as 2H+-BPDA (Figure 1). According to previous research,27,28 dibenzo-24-crown-8 (DB-24-C-8) and 2H+-BPDA, at a stoichiometric ratio of 1:1, were mixed in CH3CN solution to synthesize the pseudorotaxane complex, denoted as Crown-BPDA, composed of a DB-24-C-8 ring and intercalated 2H+-BPDA. As shown by Fourier-transform infrared spectroscopy (FT-IR, Figure S6), the C=N stretching vibration in bipyridine was reduced from 1682.1 cm–1 for BPDA to 1635.4 cm–1 for Crown-BPDA, proving the hydrogen bonding interaction between the O atom of DB-24-C-8 and the protonated H atom of 2H+-BPDA. Correspondingly, the observed peaks at 2876 and 1028 cm–1 were ascribed to the C—H and C—O stretching vibrations of crown ether, respectively, in pseudorotaxane. The preparation of a pseudorotaxane structure was further proved by the large chemical shifts in the methylene hydrogen atoms in crown ether, with a value up to ∼0.07 ppm in 1H nuclear magnetic resonance (1H NMR) spectrum analysis (Figure S7).

Figure 1.

Figure 1

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Synthesis of a crown-ether ring mechanically intercalated COF network, Crown-COF, and zinc(II) ion coordinated COF solid, Crown-COF-Zn.

To prepare a COF network, a common cross-linking approach—the Schiff-base reaction in an acidic environment—was selected, and we synthesized a molecule model to probe the feasibility of coupling reactions. 4-Tritylaniline (stopper) and Crown-BPDA (pseudorotaxane) were poured into mesitylene and acetonitrile (v/v = 1:1) at 60 °C to prepare the protonated rotaxane by a Schiff-base reaction, which served as the model molecule. After being continually washed with a mixture of Et3N/CH3CN/H2O for 20 min, the protonated rotaxane was neutralized into the deprotonation state, denoted as [2]rotaxane. The H1–4 resonances in the dumbbell molecule underwent upfield shifts as compared with those in [2]rotaxane, as depicted in the 1H NMR spectra (Figure 2a). These upfield shifts were ascribed to the fact that these protons in the dumbbell molecule were placed in a shielded magnetic environment, demonstrating the encircled linear group in the dumbbell molecule by the crown-ether macrocycle. The structure of [2]rotaxane was also proved by identical calculations, and values of 439.8619 and 439.8633 m/z were found for [2]rotaxane [M + 2H + Na]3+ using high-resolution mass spectrometry (HRMS, Figure S13). These results proved the success of introducing pseudorotaxane as a building monomer into the construction of COF networks.

Figure 2.

Figure 2

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(a) 1H NMR spectra of a dumbbell, [2]rotaxane, and DB-24-C-8 in CDCl3 at 500 MHz. (b) XPS spectra of [2]rotaxane-Zn and some reference molecules: [(bpy–OH)Zn(Trop)2] (CCDC 922725), [(bpy–OH)Zn(Trop)Cl] (CCDC 922725). (c) 13C solid NMR spectra of [2]rotaxane-Zn and paraoxon-methyl adsorbed [2]rotaxane-Zn. (d) Enlarged 13C solid NMR spectra for analyzing the movability of the crown-ether ring.

Subsequently, [2]rotaxane was added in methyl alcohol solution; after ultrasonic dispersion, ZnCl2 powder was poured into the solution to load the Zn2+ ion on the [2]rotaxane molecule, named [2]rotaxane-Zn. The known structures of [(bpy–OH)Zn(Trop)Cl], [(bpy–OH)Zn(Trop)2] were selected as the references to probe the chemical state of the Zn species in [2]rotaxane-Zn (Figure 2b).29,30 X-ray photoelectron spectroscopy (XPS) indicated that the Zn 2p3/2 and Zn 2p1/2 binding energies (BE) for [2]rotaxane-Zn were located at 1044.8 and 1021.6 eV, respectively. Using the Zn 2p3/2 signal as an object, the curve-fitting analysis showed that the BE values at 1021.3 and 1022.1 eV were assigned to the Zn–N and Zn–O bonds,31 which confirmed that the Zn2+ ion in the [2]rotaxane-Zn was coordinated by both crown-ether and dipyridyl groups. The Zn 2p3/2 and Zn 2p1/2 BE values for [2]rotaxane-Zn were identical to those for [(bpy–OH)Zn(Trop)Cl], confirming the 4-fold coordination structure of the zinc ion in [2]rotaxane-Zn.

The crown-ether-threaded COF (Crown-COF) solid was synthesized using the same method used for [2]rotaxane. The pseudorotaxane complex, together with BPDA at a molar ratio of 1:5, was reacted with the trigonal building monomer 1,3,5-tris(4-amino-phenyl)benzene (TAPB) in a mixture of mesitylene and acetonitrile (v/v = 1:1) at 60 °C for 3 days (Figure 1). For a comparison, a conventional COF network derived from BPDA and TAPB building monomers was also synthesized via the same procedures, termed BP-COF. The occurrence of the Schiff-base coupling reaction was confirmed by the appearance of C=N stretching vibrations centered at 1699 cm–1 for BP-COF and 1698 cm–1 for Crown-COF in the FT-IR spectra (Figure 3a). Identical signals for Crown-COF, as compared with [2]rotaxane, were observed at 2876 and 1028 cm–1, corresponding to the C—H(—CH2) and C—O stretching vibrations of the crown-ether ring, proving the structural integrity of the crown-ether ring mechanically intercalated frameworks. The 13C magic angle spinning nuclear magnetic resonance (MAS NMR) carbon resonances, with chemical shifts in the range 110–165 ppm, were associated with aromatic carbon atoms in the BP-COF framework (Figure 3b). Meanwhile, a wide resonance of —CH2— for the crown-ether ring in the Crown-COF framework was observed at around 69 ppm. The results showed that the pseudorotaxane building units remained intact in the porous architecture during the coupling reaction.

Figure 3.

Figure 3

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(a) FT-IR spectra of BP-COF, Crown-COF, and Crown-COF-Zn. (b) 13C solid NMR spectra of BP-COF, Crown-COF, and Crown-COF-Zn. (c) XPS spectrum of the Zn element in the Crown-COF-Zn. (d) Pore size distribution profiles of BP-COF and Crown-COF from N2 adsorption calculated via the NL–DFT method.

Respective Crown-COF or BP-COF powder is impregnated into the solution of Zn2+ ions to yield the metalated COF material, namely, Crown-COF-Zn and BP-COF-Zn (Figure 1). Using the C=C FT-IR stretching bond (at 1589.1 cm–1) as the internal standard, the ratio of the C=N stretching band (in the range 1618.9–1623.8 cm–1) to the C=C bond (1589.1 cm–1) is calculated to determine the relative intensity. As shown in Figure 3a, the increase in the relative intensity from 0.3 for Crown-COF to 0.9 for Crown-COF-Zn demonstrates the successful coordination of the Zn2+ ion on the bipyridyl group.32 The vanishing peak at 1028.8 cm–1 when comparing Crown-COF-Zn with Crown-COF is attributed to the coordination structure of the —O— group crown ether to the Zn2+ ion. In addition, the Zn 2p3/2 XPS peak is fitted with symmetrical components to verify the coordination environment of Zn atoms in Crown-COF-Zn, as shown in Figure 3c. The curve-fitting analysis of the Zn 2p3/2 spectrum shows BE values centered at 1021.3 eV (mass ratio, 43.0%), 1022.1 eV (mass ratio, 8.8%), and 1023.6 eV (mass ratio, 48.2%), assigned to the Zn—N, Zn—O, and Zn—Cl bonds (Figure S14).31,33 In addition, the mass ratios for the Zn atoms in the dumbbell-Zn (Figure S15) and [2]rotaxane-Zn states are calculated to be 83.0% (1022.4 eV) and 17.0% (1021.3 eV), respectively.

Thermogravimetric analysis (TGA) showed 13.0% weight loss as the temperature increased from 400 to 490 °C because of the decomposition of the crown-ether ring (boiling point in the range 280–380 °C) from the Crown-COF skeleton (Figure S17). The elevated temperature for Crown-COF is ascribed to the fact that the interlocked structure inhibits the escape of the crown ether from the porous architecture. After that, the COF structure gradually loses weight within the range 490–670 °C, and there is no residue left after burning to 670 °C, indicating that no inorganic impurities exist in the COF solid. For Crown-COF-Zn, ca. 12.1 wt % residue remains after heating the samples to 800 °C, ascribed to the leftover zinc compound. The loading amount of the Zn element is estimated to be ca. 9.5 wt % by ICP element analysis. Both the TGA and ICP element analysis are in good agreement with the theoretical value of 10.7 wt % for the zinc element.

The powder X-ray diffraction (PXRD) patterns indicate an amorphous structure for both BP-COF and Crown-COF solids (Figure S18). This phenomenon is attributed to the fact that pseudorotaxane monomer (Crown-BPDA) maintains its crown-ether interlocked structure in the acetonitrile solvent. Despite the occurrence of the Schiff-base reaction, the reversibility of the coupling process is destroyed in the presence of acetonitrile solvent, resulting in a disordered structure for the COF skeleton. To explore the porosity of BP-COF and Crown-COF, N2 adsorption–desorption isotherms were measured at 77 K (Figure S19). According to the Brunauer–Emmett–Teller (BET) calculation, BP-COF and Crown-COF possessed specific surface areas of 235 and 280 m2 g–1, respectively. According to nonlocal density functional theory (NL–DFT), both BP-COF and Crown-COF exhibited parallel trend curves coupled with similar pore size distributions (3.3–3.7 nm) compared with the previously reported crystalline BP-COF solid (Figure 3d).34 This result suggests that the AA stacking mode of the 2D layers occurs in both BP-COF and Crown-COF samples. At the same time, the pore volume of mesopores for Crown-COF increased significantly compared with BP-COF, which proved that the crown ether interlocked building unit effectively prevents the interpenetration and distortion of the COF network.

According to previous reports, various stimulations including acid–base, light, and metal ions will result in the movement of the crown-ether ring along the COF bridge. The high movability of mechanically interlocked crown ether will drive the crown-ether ring coordinated Zn(II) ion ([2]rotaxane-Zn) to cooperate with the bipyridine unit coordinated Zn(II) ion (dumbbell-Zn) from the adjacent COF layer. Therefore, all of the COF samples were tested using paraoxon-methyl (POM) as an organophosphorus agent to evaluate their performance in the hydrolysis of an organophosphorus agent under the common conditions of a Tris-HCl buffer (pH = 9.0) at 30 °C.32,35−42 As illustrated in Figure S26, the characteristic peak in POM at 273 nm in the UV spectrum diminished rapidly within 10 min, indicating the strong enrichment effect of porous channels on the substrate molecules. Over time, the peak at 400 nm, belonging to the hydrolysate (p-nitrophenol), remained unchanged (Figure S26a), which indicates that Crown-COF without zinc cores and two model molecules ([2]rotaxane-Zn and dumbbell-Zn) has no catalytic activity for the degradation of POM. Regarding BP-COF-Zn with dumbbell-Zn only, the hydrolysate (p-nitrophenol) is produced in a small quantity (Figure S26b), implying a weak capability for POM degradation.

For Crown-COF-Zn containing both [2]rotaxane-Zn and dumbbell-Zn with a molar ratio of 1/6, 63.8% of the POM substrates were hydrolyzed in 3 h, demonstrating an ultrahigh catalytic activity (Figure S26d). Using Beer–Lambert’s law, the initial hydrolysis rates for POM were calculated to be 6.7 and 62.8 μM min–1 in the presence of BP-COF-Zn and Crown-COF-Zn, respectively. The enhanced activity for Crown-COF-Zn suggests that the two active sites of [2]rotaxane-Zn and dumbbell-Zn in Crown-COF-Zn exhibit a cooperative effect in the hydrolysis of an organophosphorus agent. To verify this cooperation, a physical mixture of BP-COF-Zn and [2]rotaxane-Zn, with a similar number of functional sites to Crown-COF-Zn, was tested for the catalytic reaction. Subsequently, the hydrolysis rate (6.1 μM min–1) of the mixture was far less than that of Crown-COF-Zn (62.8 μM min–1), confirming that the two sites work cooperatively in the Crown-COF-Zn structure.

The steady-state kinetic assay of methyl paraoxon was conducted using the Michaelis–Menten model, by fixing the Crown-COF-Zn concentration while varying the concentration of the POM substrate. The Michaelis–Menten constants (Km = 2.26 mM; Kcat = 47.4 min–1; Kcat/Km = 2.1 × 104 mM–1 min–1) (Figure S28a) approached the values for some natural organophosphorus hydrolases, such as Sulfolobus solfataricus (Km = 0.21 mM; Kcat = 78 min–1; Kcat/Km = 3.80 × 105 mM–1 s–1).43

In order to elucidate the exceptional activity of Crown-COF-Zn, long-term molecular dynamics simulations were conducted. A key feature of the active site in naturally occurring organophosphorus hydrolase is a self-adaptive binuclear zinc center, where two zinc ions become close through the coordination of the nascent OH–, with a distance of ∼3.4 Å.8−10 Once the reaction occurs, the bridging OH– ion is substituted by a phosphate group of POM substrates into the form of the Zn(II)··O–P = O·Zn(II) intermediate state, and the Znα–Znβ distance increases to ∼4.0 Å. As the hydrolysis proceeds further, the dissociated Znα–Znβ centers release the degraded phosphate from the active site and restore their original state for the next cycle. This mechanism is in line with our design philosophy, where the interlocked crown-ether ring slides along the COF skeleton to simulate the behavior of natural hydrolase. In a weakly alkaline environment (pH = 9), the OH– ion dislodges the crown ester ring from the bipyridyl group and narrows the distance between the two Zn(II) centers. To confirm this assumption, the situation of the binuclear zinc center in the cage of COF was examined by all-atom simulations using the Forcite module in Materials Studio 7.0 (Accelrys, Inc., San Diego, CA). The optimization of the all-atom configuration was carried out with a universal force field and after 1000 max iterations (Figure 4a and Figure S34). As the OH– ion was replaced by the phosphate group of POM substrates, the two Zn(Znα–Znβ) units were stuffed by the relapsed crown ester ring back to the appropriate pattern (∼4.8 Å). This self-adaptive behavior of the binuclear zinc center contributes to the simulation of the enzymatic catalysis for the realization of efficient POM degradation.

Figure 4.

Figure 4

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(a) Possible patterns of a coordination complex fixed onto a Crown-COF-Zn skeleton according to the energy minimization optimization by Materials Studio simulation and analysis of the distance in the binuclear zinc center along the hydrolysis reaction. (b–d) XPS spectra of Zn, N, and O elements for both Crown-COF-Zn and paraoxon-methyl adsorbed Crown-COF-Zn, respectively. (e) 3D in situ infrared fragment of absorbance versus time for species involved in the C=N and C—O of Crown-COF-Zn.

To verify the mechanism used in the hydrolysis of organophosphate agents, we examined the interaction between POM substrates and the molecule model [2]rotaxane-Zn. 13C solid NMR spectra showed that there was a “g” peak variation from 112.5 ([2]rotaxane-Zn) to 114.0 ppm ([2]rotaxane-Zn + POM) for aromatic carbon (Figure 2c). This is ascribed to the fact that the “g” carbon on the benzene plug undergoes a π–π conjugation, which reduces the electron cloud density of the benzene ring, leading to an increased chemical shift and a decreased field value. Regarding the methylene groups of the crown-ether ring, the “r” chemical shift of 68 ppm ([2]rotaxane-Zn) increases to 71 ppm ([2]rotaxane-Zn + POM), which proves the divorced structure of the crown-ether ring and Zn(II) ion after the coordination of POM to [2]rotaxane-Zn.

The XPS Zn 2p1/2 spectrum of the POM-adsorbed [2]rotaxane-Zn was studied at room temperature, and a lower energy shift was observed from 1021.3 to 1018.8 eV (Figure S30a). The change was attributed to the oxygen atom of paraoxon-methyl, an element capable of easily clinging to a zinc surface, pulling the electron cloud of the Zn(II) ion to itself, which reinforced the shielding effect and decreased the binding energy. As shown in Figure S30b, the N 1s binding energies of 398.7 and 400.3 eV corresponded to Schiff-base N and Zn(II) coordinated pyridinic N, respectively. After interacting with POM molecules, the decrease in the binding energy of the Zn(II) coordinated pyridinic N atom, from 400.3 to 399.7 eV, indicated the strong complexing ability of the POM substrate to the active Zn atom, demonstrating the activation of the Zn(II) center. In addition, the curve-fitting analysis for the O 1s signal of [2]rotaxane-Zn showed a two-peak feature, assigned to O–C (532.5 eV) and O–Zn (537.0 eV) bonds (Figure S30c). An obvious variation in the BE value, from 537.0 to 535.9 eV, for the O 1s (O–Zn) signal suggested a weakened affinity between the crown-ether ring and Zn(II) ion, because the crown-ether wheel slid along the pyridinic bridge upon POM exposure.44,45

Similar to the [2]rotaxane-Zn model, the XPS Zn 2p spectra of the POM adsorbed Crown-COF-Zn showed lower energy shifts (for dumbbell-Zn, Zn 2p1/2, 1024.4 → 1021.3 eV; for [2]rotaxane-Zn, Zn 2p1/2, 1021.3 → 1018.8 eV) (Figure 4b), suggesting that the oxygen atom of paraoxon-methyl pulled the electron cloud of Zn(II) ion toward itself. As shown in Figure 4c, the decrease in the binding energy of the Zn(II) coordinated pyridinic N atom, from 400.3 to 399.7 eV, also proved the strong ability of the POM substrate to form a complex with the active Zn atom. The curve-fitting analysis for the O 1s signal of pristine Crown-COF-Zn showed a three-peak feature, assigned to O—C (532.5 eV), O=C (531.5 eV), and O—Zn (537.0 eV) bonds (Figure 4d). The variation in the O 1s (O—Zn) BE value from 537.0 to 535.9 eV demonstrated the weakened affinity between the crown-ether ring and Zn(II) ion during the hydrolysis reaction, because the crown-ether wheel slid along the COF bridge upon POM exposure.44

This varied structure of the crown-ether-wheel-threaded COF network in the presence of POM was further demonstrated by monitoring the changes in the relative intensities of the C=N of bipyridyl (1629.6 cm–1) and C—O of crown-ether ring (1010.6 cm–1) bands, which coordinated with the Zn(II) ion in Crown-COF-Zn; the reference peak C=C stretching of the benzene ring was at 1588.6 cm–1, which did not participate in the reaction. As illustrated in Figure 4e, the frequently changing peak intensity for the C—O band confirmed that the crown-ether wheel slid along the COF bridge as the reaction progressed. Meanwhile, the coordinated Zn(II) ion of rotaxane-Zn constantly changed its location, accompanied by the movement in the crown-ether wheel, leading to the labile intensity of the C=N band.46,47 FT-IR coupled with an XPS analysis demonstrated that the binuclear zinc center, composed of rotaxane-Zn and bipyridyl-Zn, facilitated a self-adaptive structural transformation for interacting with POM molecules.48,49

The thermal stability of the artificial enzyme is a key parameter for its practical usage; most natural-microorganism enzymes lose their activity as the temperature increases.50,51 It can be seen in Figure 5a and Figure S35a–d that the activity of Crown-COF-Zn increased when the temperature was raised from 0 to 60 °C. Calculated from the hydrolysis yield in the first 10 min, the observed hydrolysis rates were 9.05, 14.30, 31.40, and 47.20 μM mg–1 min–1 at 0, 20, 30, and 60 °C, respectively. As the temperature increased, the flexibility of the crown-ether ring increased the movability along the COF bridge, which provided enough energy for the conformational switching to fit the hydrolyzation process, resulting in the acceleration of the degradation.

Figure 5.

Figure 5

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(a) Degradation performance of Crown-COF-Zn for POM at different temperatures. (b) Hydrolysis experiments performed with various substrates in the presence of Crown-COF-Zn. (c) Comparison of the catalytic capacities of both paraoxon-methyl and paraoxon-ethyl in similar conditions (25–30 °C, pH = 8.5–9), where the values are plotted from remarkable references, and the rates for each sample are listed in Table S1.

The hydrolysis propensity of Crown-COF-Zn was tested in aqueous solutions of various pHs, in order to correlate the acid–base stability with regard to catalytic activity. Crown-COF-Zn had the highest hydrolytic performance at pH = 9, defined as 100% (Figure S35e). As the acid–base environment affects the relative position of the crown-ether ring on the dipyridyl group (COF bridge) in the [2]rotaxane unit, the changes in pH seriously affected the catalytic activity of the Crown-COF-Zn catalyst.

Notably, some other organophosphorus agents such as paraoxon-ethyl and parathion-methy can be hydrolyzed by Crown-COF-Zn. Figure 5b clearly illustrates that 76.3% of paraoxon-ethyl and 28.7% of parathion-methyl are hydrolyzed within 3 h of testing. The hydrolysis rate (rate per unit mass) by Crown-COF-Zn is much higher than the rates for all other reported artificial enzymes under similar conditions (25–30 °C, pH = 8.5–9), including POFs, MOFs, and nanoparticles (Figure 5c). Using parathion-methyl as a reference, the hydrolytic activity of Crown-COF-Zn (31.4 μM mg–1 min–1, 30 °C) far outweighed those of MIPAF-9 (13.75 μM mg–1 min–1), MTV-UiO-66-BE (2.00 μM mg–1 min–1), and LT-nCeO2 (0.97 μM mg–1 min–1) (Table S1, Figure 5c), greatly surpassing that of some natural organophosphorus hydrolases such as OPH-PCD (0.74 μM mg–1 min–1), by ca. 42 times, and EC 3.1.8.1-aryldialkylphosphatase (0.97 μM mg–1 min–1), by 32 times.32

Reusability is critical for industrial applications, so the regeneration of the catalyst after each cycle should be evaluated. After the first degradation, the Crown-COF-Zn solid was washed with acetonitrile and then used for nine more cycles under the same conditions. As depicted in Figure S35f, the degradation efficiency was still above 90% after 10 cycles. This collection of high performances demonstrates that crown-ester-threaded porous artificial enzymes can withstand harsh conditions in practical applications.

3. Conclusions

For the first time, a mechanically interlocked building unit (rotaxane) was successfully introduced in the construction of a covalent organic framework. The development of rotaxane-based architectures offers better control with respect to the design of the COF substituents, which may help in exploring their applications in the fields of nanoscience and nanotechnology. By conferring excellent movability, a self-adaptive binuclear zinc center endows great flexibility for the degradation of organophosphorus agents. The excellent properties of the crown-ether-ring-threaded COF solid, such as its high activity, substantial stability, and excellent recyclability, confirm its considerable potential as porous artificial enzymes in real industrial applications.

Acknowledgments

The authors are grateful for financial support of the experimental work from the National Natural Science Foundation of China (Grants 21975039, 21604008, 21531003, and 91622106), the “111” project (B18012), and the Fundamental Research Funds for the Central Universities (2412020ZD008).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscentsci.1c00941.

  • Linker syntheses and characterization, PXRD patterns, TGA curves, NMR spectroscopy, and adsorption isotherms (PDF)

The authors declare no competing financial interest.

Supplementary Material

oc1c00941_si_001.pdf (2MB, pdf)

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