A squaramide-based anionic hydrogen-bonded organic framework: enhancing sensing performance for pesticides by post-metallization with in situ imaging

Abstract

An anionic hydrogen-bonded organic framework (HOF) (named DBDA) is formed by the solvothermal reaction of squaramide-based tetracarboxylic acid organic building units (OBUs). Thanks to the diketone uncoordinated units in the squaramide functionalized groups and anionic frameworks of DBDA, Eu³⁺-functionalized HOF hybrid materials (named Eu@DBDA) were successfully fabricated and characterized. After post-synthetic modification (PSM), the synthesized hybrid materials retain the advantages of both lanthanide ions and DBDA to exhibit dual-emission luminescence properties. Therefore, we designed Eu@DBDA as a fluorescence sensor and attempted to utilize it to detect 2,6-dichloro-4-nitroaniline (DCN). It was found that the synthesized Eu@DBDA could rapidly detect DCN through luminescence quenching upon Eu³⁺-based emission, and its limit of detection (LOD) for DCN can reach 0.13 μM. Compared to the mother HOF materials, the sensing performance of DCN has been greatly improved. Furthermore, Eu@DBDA was employed to imitate fast and in situ imaging in the detection of pesticide residues in fresh produce nondestructively. This novel design approach for improving the detection performance provides a good way to prepare sensing materials related to HOFs.

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Introduction

In recent years, pesticides have played a pivotal role in agricultural productivity with their widespread employment in modern agriculture, boosting the output of the agro-industry. However, the irrational utilization of pesticides can contaminate the soil, the atmosphere, water bodies and even the ecological environment, raising the risk of their residues being present in food and drinking water.¹ Due to the inherent high toxicity and non-degradability of pesticides, their long-term residual accumulation poses an extremely serious threat to the ecological environment and human health.² Among various pesticides, 2,6-dichloro-4-nitroaniline (DCN) is an essential raw material used to manufacture various agrochemicals, such as herbicides, insecticides, fungicides, etc. Even at extremely low concentrations, DCN remains potentially very detrimental to human health.³ DCN can induce diseases that impair the central nervous system and the liver and kidneys. Chronic poisoning is often manifested as neurological disorders syndromes, and some patients appear to have polyneuropathy and toxic liver disease.⁴ Until now, the detection of DCN has been accomplished by gas chromatography, capillary electrophoresis and flow injection spectrophotometric analysis, which has shown remarkable sensitivity and exclusive selectivity.⁵ However, considering instrument costs and complicated operation steps, as well as the restrictions on detection methods mentioned before, it is urgent to find a suitable sensing method that can rapidly detect traces of DCN.

In order to solve these problems, fluorescence-based detection methods emerged to meet the demands of the time. After making great strides in the past decade, fluorescence sensors serve as a pillar in the fields of environmental pollution monitoring, life sciences, and electronic information.^6,7 Due to the unique luminescence properties of rare earth (RE) ions, such as their large Stokes shift, high luminescence quantum yield, long lifetime and narrow emission peaks, they are regarded as great assets for optical materials.^8,9 In the ultraviolet and visible regions, organic ligands have a strong absorption effect; when coordinating with lanthanide ions, organic ligands can transfer the absorbed energy to lanthanide ions to make them luminous, thereby increasing the luminous efficiency of lanthanide ions, which is the so-called “antenna effect”.^10,11

α- or β-Diketone derivatives are excellent rare earth ion coordination and sensitizing molecules; if rare earth luminescence can be introduced into diketone-functionalized materials, they would present their luminescence advantage, so as to improve selectivity and sensitivity.^12,13 Nevertheless, in some cases, locating free diketone sites in materials is constrained by sophisticated methods of organic synthesis and also limited by the fact that diketone sites are easy to coordinate with the ions of the materials needed for preparations. Therefore, new crystalline hydrogen-bonded organic frameworks (HOFs) are required to solve these thorny problems.

HOFs are crystalline framework materials consisting of organic building blocks often connected by O–H⋯O or N–H⋯O hydrogen-bonds.^14–20 Apart from hydrogen bonding, other intermolecular forces such as π–π interactions, electrostatic interactions and van der Waals forces are integral to the construction and stability of HOFs. Like most metal–organic frameworks (MOFs)^21–23 and covalent organic backbones (COFs),^24–26 HOFs also have an ordered void structure and void modifiability, and HOFs have shown their talents in a wide variety of fields, such as gas storage and separation,^27,28 chemical sensing^29–34 and catalysis.^35–38 It is difficult to control the coordination of metal ions in the pre-synthesis process. If the diketone groups are introduced into the HOF structure, the diketone sites can be effectively protected. After the post-synthetic modification (PSM) of HOFs, the target RE@HOFs will be afforded.

Post-synthetic modification (PSM), extensively reviewed by Cohen³⁹ and Burrows⁴⁰et al., is widely used in the functionalization of MOFs and COFs and can contribute to the interference reduction of functional groups in MOF/COF assemblies; in addition, PSM of MOFs/COFs can produce new active sites beneficial for promoting selectivity. Moreover, PSM can strengthen the sensitivity of detection.^41–44 However, few reports have delved into the application of PSM based on HOF materials. For instance, Yan and co-workers designed and synthesized a Eu³⁺-functionalized HOF using PSM to achieve sensing of methylamine.⁴⁵ They also proposed a Eu³⁺-functionalized HOF as a fluorescence detection platform for SO₂ gas.⁴⁶ Therefore, further development of research on HOFs in the field of fluorescence detection is of great significance and imperative.

Inspired by these efforts, we fabricated a new squaramide-based anionic HOF, namely DBDA, as the matrix, where the free squaramide diketone groups of DBDA are good for sensitizing the luminescence emission of Eu³⁺, to construct a lanthanide complex functionalized HOF Eu@DBDA as a fluorescence sensor. The potential of Eu@DBDA for DCN sensing was also explored. The results indicate that Eu@DBDA boasts superior photoluminescence performance and fluorescence quenching response to DCN with high sensitivity. Further attention should be paid to detecting pesticide residues in fruits and vegetables. According to what we searched, no research about HOF-based fluorescence sensors for DCN detection has been published. This study presents a representative case for using HOF-based materials in environmental detection and offers innovative directions and strategies for the design of lanthanide functionalized HOF hybrid materials.

Results and discussion

Crystal description

The ligand 5,5′-((3,4-dioxocyclobut-1-ene-1,2-diyl)bis(azanediyl))diisophthalic acid (H₄DBDA, Fig. 1b) was readily synthesized on a gram scale through the aminolysis reaction of ester. {(H₂DBDA)[(CH₃)₂NH₂]₂}_n (DBDA) with rodlike light yellow crystals is produced by the reaction of H₄DBDA and dimethylformamide/ethanol (as the solvent) under solvothermal conditions. X-ray single crystal structure analysis revealed that DBDA crystallized in the triclinic P1̄ space group with the asymmetric unit of one H₂DBDA²⁻ and two dimethylamine cations (CH₃)₂NH₂⁺ leading to the formation of an anionic hydrogen-bonded organic framework material. Each H₂DBDA²⁻ building block connects with four other blocks by eight hydrogen bonds to generate a two-dimensional (2-D) layer parallel to the [011] plane (Fig. 1a). These hydrogen bonds include one pair of intermolecular dimer –COOH⋯HOOC– hydrogen bonds (O⋯O distance: 2.675 Å, O–H⋯O angle: 164.93°), two intermolecular –COOH⋯OOC– hydrogen bonds (O⋯O distance: 2.494 Å, O–H⋯O angle: 169.16°), four intermolecular N–H⋯O hydrogen bonds (O⋯O distance: 2.853 Å and 2.719 Å, respectively; O–H⋯O angles: 158.39° and 156.35°, respectively), which fall into a strong hydrogen bond range according to the literature (distances: 2.49–3.15 Å).⁴⁷

Fig. 1 (a) The two-dimensional (2-D) layer formed by the H₂DBDA²⁻ building block via hydrogen bonds. (b) The structure of the H₄DBDA building block. (c) The three-dimensional (3-D) supramolecular structure of DBDA through C–H⋯π interaction and π⋯π interaction. (d) The view of the packing structure of DBDA along the c direction. (e) Schematic representation with the 3,7-connected topology of DBDA.

Adjacent layers pack together by C–H⋯π interaction (distance 3.26 Å) between hydrogen atoms in the benzene ring and benzene rings from two different H₂DBDA²⁻ ligands and π⋯π interaction (distance 3.28 Å) between two squaric acid rings from two different H₂DBDA²⁻ ligands (Fig. 1c), giving rise to a three-dimensional (3-D) supramolecular structure (Fig. 2c and S1†). Dimethylamine cations are linked with H₂DBDA²⁻ moieties for charge balancing to further stabilize the whole framework structure via hydrogen-bonding interactions with N–H⋯O distances of 1.812 Å, 2.454 Å, 1.937 Å, and 2.719 Å and O–H⋯O distances of 2.519 Å, 2.498 Å, 2.682 Å, and 2.682 Å, respectively (Fig. 1d). The whole network can thus be viewed as a novel topology of a 3,7-c binodal net with an extended Schlafli symbol of {3^2.4}{3^4.6^8.5^5.6^4} (Fig. 1e). The powder X-ray diffraction (PXRD) pattern of DBDA is consistent with the simulated profile, indicating the phase purity of the synthesized sample (Fig. 2a). In addition, the thermal stability of DBDA was examined by thermogravimetric analysis (TGA) under an air atmosphere from 25 to 800 °C. Almost no weight loss was observed below 200 °C, and then DBDA slowly lost weight due to the decomposition of the ligand (Fig. S3 in the ESI†).

Fig. 2 (a) Powder X-ray diffraction of the simulated DBDA, as-synthesized DBDA and Eu@DBDA. (b) FT-IR spectra of DBDA, Eu@DBDA and Eu(NO₃)₃ itself.

Preparation and characterization of Eu@DBDA

Thanks to the anionic framework and abundant free diketone groups of DBDA, it is possible to modify the rare earth cations. Since diketone derivatives are excellent sensitizing molecules for rare earth ions, the abundant diketone units in the squaramide groups in the DBDA framework can provide clear action sites and the possibility of sensitizing the luminescence of rare earth ions. Therefore, another fluorescence emission will occur for the composite material, providing a clear advantage for studying its sensing performance. It can be seen from the crystal structure of DBDA that, as we expected, the diketone sites were well preserved, so we prepared Eu@DBDA by the post-synthesis modification of DBDA. Briefly, Eu@DBDA could be fabricated by immersing the powder of DBDA in a certain amount of Eu(NO₃)₃·6H₂O solution, and the PXRD curves of Eu@DBDA and DBDA were in good agreement, thus confirming that the framework was well preserved (Fig. 2a). From the FT-IR spectra in Fig. 2b, the peak at 1784 cm⁻¹ is ascribed to the stretching vibration of the free carbonyl group in the squaramide units of the DBDA framework. Compared to DBDA, the 1784 cm⁻¹ peak of Eu@DBDA is obviously weaker due to the emerging Eu–O coordination bonds, and the Eu–O stretching vibration can be confirmed at 415.1 cm⁻¹ as reported in the literature.⁴⁸ In addition, we could recognize a distinct peak at 1385 cm⁻¹, which belongs to the N=O stretching vibration of NO₃⁻ ions. As a consequence, this result demonstrates that Eu³⁺ is coordinated with DBDA, and NO₃⁻ ions appear on the DBDA surface to keep the charge balance of the whole structure.

In addition, to further confirm the coordination effect between Eu³⁺ and the carbonyl group in squaramide, we performed X-ray photoelectron spectroscopy (XPS). The characteristic peaks of Eu³⁺ ions appearing on the curve of Eu@DBDA as shown in the complete XPS plot proved that Eu³⁺ was successfully introduced into DBDA (Fig. S6 in the ESI†). More remarkably, two peaks were obtained by the peak fitting of O 1s; the one at 531.5 eV belonged to the carboxylate oxygen of the benzene ring, while the other at 533.5 eV belonged to the squaramide carbonyl oxygen of DBDA. As shown in Fig. 3a, after immersion in Eu³⁺, there appeared a shift from 533.5 eV to 532.9 eV at one O 1s peak, while another at 531.5 eV of the carboxylate group remained unchanged. This difference in the shift suggests that coordination interactions existed between the Eu³⁺ ions and the oxygen atom of the squaramide carbonyl oxygen rather than between Eu³⁺ and the carboxylate group.⁴⁹ Additionally, the 1s peak of N (400.1 eV) also stayed the same, which confirmed that Eu³⁺ did not interact with N in the squaramide part, but a new peak appeared at 407.1 eV, attributed to the N 1s peak of NO₃⁻ (Fig. S6 in the ESI†). The external morphology of the material was analyzed by SEM, and it was found that the morphology of Eu@DBDA remained almost identical to that of DBDA, as shown in Fig. 3b. From the energy dispersive spectroscopy (EDS) mapping images, it is known that the elements N, O and Eu are uniformly distributed, indicating that Eu³⁺ was successfully introduced into DBDA. All these results can effectively demonstrate the successful synthesis of Eu@DBDA through the coordinated interaction of Eu and the squaramide carbonyl oxygen of DBDA.

Fig. 3 (a) O 1s XPS spectra of DBDA and Eu@DBDA. (b and c) SEM images of DBDA and Eu@DBDA, respectively. (d) SEM/EDX images of Eu@DBDA.

Luminescence properties and sensing performance of DNC

Because the vibration of hydroxyl groups in water can readily trigger the fluorescence quenching of Eu³⁺, the fluorescence detection of the composite material was studied in ethanol. Typically, 1 mg of DBDA or Eu@DBDA was dispersed into 3 mL of ethanol, and then the fluorescence spectra of the suspension were recorded; the emission spectra of DBDA and Eu@DBDA are shown in Fig. S4.† Upon excitation at 350 nm, DBDA displayed an emission peak at 440 nm, while Eu@DBDA exhibited characteristic Eu³⁺ emissions at 590 and 617 nm besides the emission peak at 435 nm. Given the excellent luminescence of Eu@DBDA, we further investigated its potential for detecting DCN as a fluorescent probe. To gain insight into the DCN-induced luminescence changes of Eu@DBDA, we explored the luminescence titration performance of Eu@DBDA as a fluorescence sensor for DCN and recorded its fluorescence spectra after 15 s at room temperature in each case.

As shown in Fig. 4a, as DCN concentration increased, the fluorescence quenching behaviour of the Eu@DBDA material was observed. The emission at 440 nm was quenched by only about 24%, while the typical Eu-based fluorescence at 617 nm was quenched by >97% when DCN at a concentration of approximately 0.38 mM was added to the Eu@DBDA ethanol emulsion. As shown in Fig. 4b, there was an excellent linear relationship between the relative fluorescence intensity of Eu@DBDA and the DCN concentration in the range of DCN concentrations from 0 to 60 μM. This linear correlation fitting results in a function of eqn (1) and the correlation coefficient (R²) is 0.993.

I_em = −7.92 × 10⁹C_DCN + 99 988 (1)

where I_em presents the relative emission intensity of Eu@DBDA and C_DCN is the concentration of DCN. According to the 3δ IUPAC standard,⁵⁰ the limit of detection (LOD) of Eu@DBDA for DCN can reach 0.13 μM, and the relative standard deviation (RSD) was calculated as 2.17%. Compared with the detection limits in some reports of DCN detection based on MOFs (Table S2†), we could find that our system exhibits the advantage of a lower detection limit. These data indicate that Eu@DBDA can be applied to the quantitative measurement of DCN. Importantly, upon adding the same amounts of DCN to the original DBDA ethanol suspension, the emission at 440 nm was quenched by only about 35%; it is shown that the PSM of Eu³⁺ in DBDA can effectively promote the detection sensitivity. As we know, this is the first report of HOF chemosensors for DCN pesticide in solution.

Fig. 4 (a) Fluorescence spectra of Eu@DBDA in the presence of different concentrations of DCN. (b) Fluorescence intensity of Eu@DBDA at 617 nm at different concentrations of DCN. (Inset) The corresponding linear relationship in the low concentration range.

To investigate whether Eu@DBDA exhibited selective sensing for DCN, we implemented fluorescence detection experiments with common aromatic and nitro-containing analysts, including aniline (A), toluene (T), phenol, chlorobenzene (CB), nitrobenzene (NB), 2,4-dinitrotoluene (DNT), m-nitrotoluene (m-NT), 3-nitropropionic acid (3-NPA) and 1-chloro-3-nitrobenzene (CN) (Scheme S1 in the ESI†). As shown in Fig. 5, the presence of T and CN only led to a weak increase in the fluorescence intensity of Eu@DBDA (max. = 4%). Almost no significant spectral changes were observed for Eu@DBDA in the presence of phenol, A, 3-NPA and CB. The addition of DNT, NB and m-NT resulted in 5.0%, 5.0% and 15.0% fluorescence quenching, respectively, of the typical Eu(III)-based emission of Eu@DBDA. Importantly, other harmful pesticides, including hexaconazole (HCZ), triclosan (TCS), 2,4-dichlorophenol (DCP), isoproturon (IPU) and diuron (DCMU), only caused slight luminescence changes of Eu@DBDA. These results demonstrate that Eu@DBDA can selectively sense DCN. Meanwhile, the anti-interference capability of Eu@DBDA for DCN sensing was further explored by competing experiments (Fig. 5, red columns). The experimental result reveals that the coexistence of interferences had no obvious effect on DCN detection, indicating the excellent anti-interference capability of Eu@DBDA.

Fig. 5 Luminescence response data of Eu@DBDA among various analysts at a concentration of 0.38 mM. The green bars stand for the luminescence intensities of Eu@DBDA when the selected analysts (0.38 mM) are present. The red ones stand for the change of emission when continuing to add 0.38 mM DCN.

Mechanism discussion

We investigated the mechanism of the fluorescence response of Eu@DBDA toward DCN in detail. First, we recorded the related UV–vis spectra, and it can be seen from Fig. 6a that the excitation peaks of Eu@DBDA can overlap almost completely with those of DCN. It indicates that there is competition between DCN and Eu@DBDA for absorption excitation energy. Thus, the result caused a decrease in the absorbed energy of Eu@DBDA, which further weakened the antenna effect and gave rise to the energy reduction transferred from DBDA to Eu³⁺, and finally the fluorescence quenching of Eu³⁺ was observed. However, it is clearly shown in Fig. 4a that the emission peaks at 440 nm did not quench completely, indicating that there may be other interactions that offset the competitive absorption effect. Based on the structure of DCN, it can be divided into two sub-units: CN and p-nitroaniline (p-NA). Since CN hardly caused the fluorescence change of Eu@DBDA, p-NA may be a key component contributing to the selective detection of DCN, especially the amino group of p-NA.

Fig. 6 (a) UV-vis spectra of the related analysts and the excitation spectrum of Eu@DBDA. (b) SEM/EDX images of DCN-treated Eu@DBDA.

In order to verify our speculation about the mechanism of highly selective DCN sensing in Eu@DBDA, fluorescence titrations were further performed with p-NA. Interestingly, the quenching efficiency of the fluorescence intensity by p-NA shows nearly the same quenching responses as DCN (Fig. S7 in the ESI†). Meanwhile, when the same amounts of isomer analytes (o-NA and m-NA) as p-NA were added, the quenching efficiency for Eu@DBDA fluorescence was 22% and 18%, respectively. These results demonstrated that only the amino group in nitroaniline with a suitable configuration can potentially form hydrogen bonding with the host framework. In addition, the SEM/EDX of Eu@DBDA impregnated with an ethanol solution of DCN exhibited a uniform distribution of Eu and Cl (Fig. 6b), suggesting the absorbance of DCN in the channels of Eu@DBDA. These data further supported that the detection of DCN originated not only from the surface contact in solution but also from the existence of the hydrogen-bonding interaction between the frameworks of Eu@DBDA and the amino group of DCN. Furthermore, such an interaction can slow down the ligand-based emission of Eu@DBDA at 440 nm. All these results provide favourable evidence for the selective detection of DCN by synergistic effects such as hydrogen bonding interactions and competition absorption.

Detection and imaging of DCN in test paper and garden stuff

Since Eu@DBDA exhibits a high efficiency in detecting DCN, it is possible to conduct the on-site detection of DCN. The test paper was prepared by immersing Whatman filter paper with a size of 1.0 × 1.0 cm² in the dispersion of ground Eu@DBDA in ethanol and drying it at room temperature. As shown in Fig. 7a, under UV light irradiation of 365 nm, we can observe that the luminescence of the test paper changed from red to almost no luminescence with the increase of DCN concentration. Besides, we applied Eu@DBDA to imitate the direct imaging detection of pesticide residues on the surface of garden stuff. As DCN has not been detected in the green apples and lettuce purchased, we sprayed a solution of different concentrations of DCN on the surface of the green apples and lettuce, so as to simulate different degrees of pesticide contamination. Then, we added a Eu@DBDA suspension and luminescence appeared under UV light. As we can see, Fig. 7b and c manifest obvious differences in green apples’ surfaces when sprayed with 0 (blank) and 0.38 mM DCN solution, respectively. Similarly, on the surface of lettuce, as DCN concentration increased from 0.03 mM to 0.17 mM, the luminescence of Eu@DBDA plunged; when DCN concentration was up to 0.38 mM, it totally quenched. All these results suggest that Eu@DBDA has the potential for rapid and in situ imaging detection of pesticide residues in actual samples.

Fig. 7 (a) Photographs (under 365 nm UV light) of the fluorescence quenching of Eu@DBDA on test strips for the visual detection of a small amount of DCN on a (i) test strip; DCN of different concentrations: (ii) 0.03 mM, (iii) 0.1 mM, (iv) 0.2 mM, and (v) 0.38 mM. (b) Eu@DBDA suspension (0.33 g L⁻¹) was added to the surface and visual signals appeared under ultraviolet light within 30 s. (c) An Eu@DBDA-treated green apple was sprayed with 0.38 mM DCN solution to simulate pesticide residues. (d) Lettuce was sprayed with 0, 0.03, 0.17 and 0.38 mM DCN solutions to simulate pesticide residues.

Conclusions

In summary, a squaramide-based anionic hydrogen-bonded organic framework, namely DBDA, has been prepared and structurally characterized. Single crystal structure analysis shows that the diketone units in the squaramide groups of DBDA were fully exposed, providing clear coordination sites and sensitization function for europium ions. By way of post-synthetic modification, we fabricated Eu@DBDA, the dual-emission HOF hybrid material, and applied it to fluorescence detection. Remarkably, Eu@DBDA exhibited a specific fluorescence quenching response to DCN among common nitro compounds. It is worth noting that the detection efficiency of Eu@DBDA is much higher than that of its parent material. As a sensing material, the advantages of Eu@DBDA are mainly reflected in these aspects: fast response (15 s), high selectivity, and low detection limits (0.13 μM). The mechanism for the selective detection of DCN was attributed to synergistic effects such as hydrogen bonding interactions and competition absorption. The sensor was also applied to imitate rapid and in situ imaging detection of pesticide residues in fresh produce nondestructively. In accordance with what has been analysed, this work represents the first example of DCN detection by post-modification synthesis of HOF hybrid materials, which is essential to protecting human health and pursuing a better life.

Experimental

Synthesis of DBDA

In the process of fabricating DBDA crystals, a mixture of the H₄DBDA ligand (20 mg, 0.04 mmol), 5 mL N,N-dimethylformamide (DMF), 10 mL ethanol and hydrochloric acid (11.0 mM) was added to a Teflon-lined steel autoclave and kept in an oven at 120 °C for 72 h. After that, the reaction system was cooled to room temperature. The rodlike light yellow crystals of DBDA collected by filtration were washed several times with the original solution and dried in air. Yields: 92%. Anal. calc. (%) for C₂₄H₂₆N₄O₁₀: C 54.29, H 4.90, N 10.56; found (%): C 54.51, H 4.95, N 10.62. CCDC: 2206854.†

Synthesis of Eu@DBDA

The crystalline DBDA (50 mg) was dispersed into 1 mL DMF solution, which contained 0.11 M Eu(NO₃)₃·6H₂O at room temperature for 4 days. The collected solid was washed several times with DMF in order to remove the Eu³⁺ that was adsorbed on the surface of Eu@DBDA and dried at room temperature.

Luminescence sensing experiment

Typically, 1 mg of a well ground Eu@DBDA sample was dispersed into 3 mL of ethanol. After sonication for 5 min, the initial fluorescence spectra of Eu@DBDA were recorded. The fluorescence spectra of Eu@DBDA were again recorded after the addition of DCN at an excitation wavelength of 350 nm. The selectivity experiments of Eu@DBDA for DCN were performed with aniline (A), toluene (T), phenol, chlorobenzene (CB), nitrobenzene (NB), 2,4-dinitrotoluene (DNT), m-nitrotoluene (m-NT), 3-nitropropionic acid (3-NPA), 1-chloro-3-nitrobenzene (CN), hexaconazole (HCZ), triclosan (TCS), 2,4-dichlorophenol (DCP), isoproturon (IPU) and diuron (DCMU) that completed a similar sensing process in solution.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge financial support from the Major Basic Research Project of the Natural Science Foundation of Jiangsu Higher Education Institutions (No. 21KJA150001), the Natural Science Foundation of Jiangsu Province for Outstanding Youth (No. BK20180105), TAPP of Jiangsu Higher Education Institutions, and the Postgraduate Research & Practice Innovation Program of Jiangsu Normal University.

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Footnotes

† Electronic supplementary information (ESI) available: Crystal data, experimental details and related spectra. CCDC 2206854. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi01054f

‡ These authors contributed equally to this paper.

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