Ligand-triggered antenna effect and dual emissions in Eu(III) MOF and its application in multi-mode sensing of 1,4-dioxane

Abstract

A new set of metal–organic frameworks was designed by functionalizing g-C₃N₄ with benzoic acid and using them as structure-directing ligands during the metal–organic framework (MOF) formation. One such MOF exhibited dual emissions, both metal- and ligand-centered, enabling ratiometric sensing of the carcinogenic industrial solvent dioxane. The fabricated MOFs possessed a unique fluffy spherical morphology that enabled atomic level resolution in transmission electron microscopy—a rarity in MOFs due to the ‘Knock-on’ effect. Sensor experiments showed a rapid response within 5 s of analyte introduction and achieved a low limit of detection (LOD) of 0.026 ppm, well below the FDA-approved level of 10 ppm. In addition, the sensor exhibited exceptional selectivity, discriminating 1,4-dioxane from a pool of 16 solvents. This increased sensing capability was attributed to the formation of complexes and precise alignment of energy levels between the host and analyte, facilitating photoinduced electron transfer (PET). This material is equally efficient for colorimetric detection of the same solvent under excitation of UV light as well as gas phase detection of this volatile organic compound through I–V characteristics. Density functional theory (DFT) analysis supported the crucial role of Eu and the ligand system in efficiently detecting 1,4-dioxane by fluorescence spectroscopy, as shown in the energy level diagram. Future research could focus on optimizing these metal–organic frameworks for enhanced industrial applications in the detection of dioxane and exploring their potential applications in real-world environmental monitoring and public health safety.

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Introduction

Metal–organic frameworks (MOFs) are porous materials formed by the coordination between metal ions and organic linkers through self-assembly.^1–5 MOFs have found wide applications in molecular sensing,⁶ gas storage/separation⁷ and photocatalysis.⁸ This is because of their fascinating characteristics, namely, high porosity, large surface area, and facile modification of the surface. Among them, lanthanide-MOF (Ln-MOFs) have acquired widespread applications due to strong emissions, large Stokes shifts and comparatively longer lifetimes.⁹ However, a few drawbacks exist for pure MOFs such as low photoresponsivity, high recombination rate of electron–hole pair and poor stabilities. Therefore, efforts have been made to develop composites of MOF with semiconductor compounds to form hybrid materials for better performance efficiency.^10,11

In this regard, Zhou et al. reported a novel composite material of NH₂-MIL-125(Ti) with g-C₃N₄ functionalized by benzoic acid.¹² Hence, the composite was formed through covalent bonds for the first time, where benzoic acid functionalized g-C₃N₄ acts as the structure-directing agent in MOF formation as well. Their results show that with an increasing amount of g-C₃N₄, the NH₂-MIL-125(Ti) surface is gradually covered with nanosheets, indicating good connectivity between the two and also six times higher H₂ productivity by the composite than the pure MOF. Another reference is made to Gao et al., wherein a composite was prepared by combining the aminated flower-like ZnIn₂S₄ with g-C₃N₄ functionalized by benzoic acid through amide bonds. The composite possesses improved photocatalytic activity due to intimate interfacial contact and well-matched band-gap structure between the two motifs.¹³ Also, Liao et al. fabricated shape-controlled Fe-based MOF with the assistance of benzoic acid functionalized g-C₃N₄, where the structure directing property of the latter was envisaged.¹⁴ However, none of the above work has tried the fabrication of lanthanide-based MOF with a composition of functionalized g-C₃N₄. Another drawback of the above-mentioned products is that the functionalized g-C₃N₄ plays the role of a co-structure directing agent along with the primary ligand. Whether benzoic acid functionalized g-C₃N₄ alone can participate in the MOF formation was not examined.

Volatile organic compounds (VOCs) have emerged as a major concern to environmental contamination because of their outspread utilization as industrial solvents. 1,4-Dioxane is an industrial solvent for inks, adhesives, fats, and waxes and is an environmental water toxin due to its high solubility at low vapour pressures and hydrophilicity.^15,16 It has been identified as a probable human carcinogen by the IARC (International Agency for Research on Cancer).¹⁷ Therefore, its detection by a suitable scientific method is useful. There are a number of reports on sensing of 1,4-dioxane by different methods. Among them, reference is made to Karim et al., wherein PAni–SiO₂ nanocomposite was deposited onto a glassy carbon electrode, and the sensor analytical performance was carried out in the presence of different concentrations of 1,4-dioxane.¹⁶ A research group from King Abdulaziz University, Saudi Arabia, developed various systems for electrochemical sensing of 1,4-dioxane selectively; namely, NiO@Nd₂O₃ nanocomposite,¹⁸ mixed metal oxide of ZnO/NiO/MnO₂ nanoparticle¹⁷ and doped ZnO/GO nanocomposite.¹⁹ Also, fluorescence-based detection of 1,4-dioxane was reported using porous organic polymer,²⁰ Tröger's base functionalized triazine covalent polymer,²¹ carbon nano-onion²² and d¹⁰-MOFs.²³

However, electrochemical sensor technology also has some disadvantages, e.g., its applicability in a limited temperature range despite its reliability and accuracy. Drying of the electrodes due to high temperatures and low humidity shortens the life of such technology. The disadvantages of this method include the laborious development of the electrodes and the selectivity towards the specific analyte. In addition, the products formed during the electrode reactions must be removed. In contrast, fluorescence-based measurement is simple, fast and inexpensive. However, the simple fluorescence-based measurement by monitoring the intensity change of a single fluorescence peak in the presence of an analyte is highly influenced by external conditions, i.e., instrument parameters, phototransformation, receptor concentration, competing analytes and also autofluorescence.

Lanthanide-based MOFs are generously used for fluorescence-based sensing applications.^24–29 To design a desired MOF for selective sensing applications, appropriate ligand construction is crucial. In the case of Ln-MOF, the ‘antenna effect’ of the ligand plays a vital role in achieving the requisite fluorescence properties. Keeping these considerations in mind, we developed benzoic acid functionalized 2-D sheets of g-C₃N₄ to act as structure-directing agents in Ln-MOF formation. Another Ln-MOF was developed using a mixture of ligands (both benzoic acid functionalized g-C₃N₄ and terephthalic acid) to obtain a dual-emitting MOF. Both of these Eu(III) ion-based MOFs exhibit good chemical stability, water dispersibility and strong fluorescence emission. Remarkably, one of the developed MOFs demonstrated ratiometric quenching behavior in the presence of trace amounts of volatile organic solvent 1,4-dioxane with a limit of detection as low as 0.3 μM. Moreover, this detection capability was extended to the gaseous phase by conducting I–V characterization of the sensor MOF in the presence of dioxane vapor.

Experimental

Materials and methods

All the commercial reagents and solvents were used directly without further purification. Synthesis of (i) g-C₃N₄, (ii) composite of BCN with MoS₂ (MSBCN), (iii) MOF from Eu³⁺ ions and BCN (MZN-1), (iv) MOF from Eu³⁺ ions, terephthalic acid (BDC) and MSBCN (MZN-2), functionalization of g-C₃N₄ with benzoic acid (BCN), characterization techniques, method of fluorescence sensing study and gas phase sensing study are presented in the ESI.†

Results & discussion

Functionalization is a versatile method to improve the properties of g-C₃N₄ and thus make it more adaptable for various applications. One of the techniques used for this purpose is surface grafting, in which functional groups are attached to the g-C₃N₄ surface to improve its compatibility, optical properties and stability. To achieve the property of structural control of MOF formation by a ligand, a molecule must have at least one functional group capable of coordinating with the metal ion. Therefore, it is our top priority to validate the surface functionality of g-C₃N₄ before we can elucidate its role in MOF formation. Since benzoic acid functionalization was applied to functionalize 2-D sheets like graphene oxide,^11,30,31 we employed the same approach to functionalize g-C₃N₄. We have subsequently confirmed the presence of benzoic acid functionalization on g-C₃N₄, as well as its coordination with the metal ion during MOF formation, through various experimental techniques.

The compositional study and phase identification of CN and BCN were performed through XRD analysis and the results are shown in Fig. S1, ESI.† From the comparative XRD graphs of Fig. S1(a),† mainly, peaks at 2θ = 13° and 28° were observed in both cases, corresponding to (100) and (002) planes, characteristic of the graphitic material as interplanar stacking peaks of the conjugated aromatic systems.³² However, two weak peaks at 18° and 22° were also observed, which indicated the presence of carbon dots as well.³³ This observation confirms that functionalization does not cause any major change in the composition of g-C₃N₄. From the Brunauer–Emmett–Teller (BET) analysis of CN, the surface area and pore distribution were explored. The adsorption–desorption isotherm of the system exhibited the type IV curve, as shown in Fig. S1(b), ESI,† demonstrating the presence of uniform mesopores with high specific surface area and large total pore volume. From the data presented in Table S1, ESI,† the surface area of g-C₃N₄ is observed to increase twice that of the reported bulky g-C₃N₄ (16.15 m² g⁻¹). Pore volume also increased compared to the reported bulky g-C₃N₄ (0.097 cm³ g⁻¹). However, in the case of benzoic acid functionalized BCN, the BET plot was found to be uninterpretable. As per the literature, if the material has a heterogeneous surface with different types of adsorption sites, it does not conform to the uniformity assumed by the BET theory.³⁴ From the BJH data in Table S1,† the pore diameter and volume were seen to be decreased, which can be assumed to be due to the confinement effect caused by the introduction of organic molecules.^35,36

Next, the morphology and in-depth crystallinity of the developed 2-D materials were analysed through TEM results. The TEM image of CN (Fig. 1) shows nanosheets consisting of highly crystalline macro-fragments of size below 100 nm. This kind of morphology is plausible because of the high-temperature pyrolysis of the precursor in two steps. The bulky g-C₃N₄ produced from the heating of thiourea was further calcined at the same temperature for another 2 h. Therefore, the formation of graphitic carbon species leading to nanodomains is possible. This interpretation is corroborated by XRD and XPS results. Increase in the carbon content leads to an increase in the interlayer distance. The value of d-spacing, as calculated from HRTEM images in Fig. 1(c), is 0.47 nm, much higher than the conventional 0.336 nm in g-C₃N₄. This observation suggests the presence of both crystalline g-C₃N₄ and graphitic carbon, which can be termed as carbon-rich carbon nitride.

Fig. 1 TEM images of CN (a, b, c) and BCN (d, e, f) in different scales and insets of (a, d) show SAED patterns, and (c, f) show FFT images of lattice planes.

Furthermore, the selected area diffraction pattern (SAED) of g-C₃N₄ shows a polycrystalline nature with rings consisting of many bright spots (Fig. 1a). These bright spots are not observed in the case of BCN, showing only diffuse concentric rings of maximum intensity (Fig. 1d). This indicates that long-range order in the atomic lattice is decreased in functionalized g-C₃N₄.³⁷ This structural transformation due to grafting an aromatic species on g-C₃N₄ sheets leads to the possibility of interaction of the organic moiety in perpendicular direction to the triazine ring hindering the stacking of g-C₃N₄ layers and hence decreasing the long-range order crystallinity.³⁸

The XPS results of g-C₃N₄ and BCN are presented in Fig. 2. In the case of g-C₃N₄, the C 1s spectra exhibited three peaks (Fig. 2a). The first peak appeared at 284.6 eV, which arises due to graphitic carbon, the most common defect introduced during the synthesis of g-C₃N₄. This observation is in agreement with XRD and HRTEM results, revealing the presence of crystalline carbon nanodomains. The C–(N)₃ peak at 287.8 eV corresponds to the main peak. On the other hand, the peak at 293.5 eV is assigned to two-coordinated carbon atoms of g-C₃N₄.³⁹ However, on BCN formation, a minor peak was observed at 286.1 eV, which usually does not appear in the case of pure g-C₃N₄. It may have come from C–O or other defects in benzoic acid functionalized C₃N₄.⁴⁰ However, the main peak at 287.8 eV corresponding to the C–N=C moiety is shifted to a higher binding energy of 288.1 eV in the case of BCN. The N 1s spectra for both are presented in Fig. 2(b). The C–N=C peak is observed at 398.4 eV and 398.6 eV in the case of g-C₃N₄ and BCN, respectively. The N–(C)₃ peak is observed at 399.7 eV for g-C₃N₄. Another peak at 400.7 eV corresponds to an NH(x) kind of terminal nitrogen present in triazine morphology.⁴⁰ The subpeak observed at 403 eV indicates the N–O kind of bonding if present. To specify the presence of oxygen in BCN, the survey spectra of both are presented together in Fig. S2(a), ESI,† from where the O 1s peak is clearly seen in the case of BCN. On high resolution (Fig. 2c), this peak seems to consist of two peaks at 531.7 eV corresponding to C=O and 533.3 eV corresponding to the COOH moiety in BCN.⁴¹ All the above results point towards the successful functionalization of benzoic acid over g-C₃N₄ in BCN.

Fig. 2 High-resolution XPS spectra of (a) C 1s, (b) N 1s of CN and BCN, (c) O 1s and (d) Cl 2p XPS spectra of BCN.

To check whether chlorine from diazo salt has played any role in the functionalization of benzoic acid with C₃N₄, we have studied the high-resolution XPS spectra of the Cl 2p peak. Although from the survey spectra of BCN in Fig. S2(a),† no significant presence of Cl 2p at 200 eV is observed, still, the high-resolution XPS spectra of Cl 2p exhibit multiple peaks (Fig. 2d). Among these, higher intensity peaks are at binding energy <200 eV, corresponding to inorganic chlorides used during diazo salt synthesis.⁴² However, a small peak at 201.6 eV is present, which is ascribed to physisorbed chlorine.⁴³ This observation indicates that chlorine does not play any vital role in functionalization rather than remaining as a physisorbed entity over the graphitic surface in less content.

The FTIR spectra and their interpretations are presented in Fig. S2, ESI.† Based on the observations made from the characterization results, it is assumed that BCN formation is occurring through hydrogen bonding between benzoic acid and g-C₃N₄ moieties. However, the benzoic acid molecule is deprotonated and the triazine N atom of C₃N₄ is protonated, leading to the NH⋯O kind of hydrogen bonding in a perpendicular direction as explained in HRTEM analysis. This kind of interaction was studied earlier in the case of benzoic acid and melamine.^44,45 The schematic diagram of plausible BCN is shown in Fig. S3, ESI.†

Once the BCN formation is confirmed, it is required to understand the MOF formation from BCN with metal ions. The comparative FTIR spectra of MZN-1 and BCN, as shown in Fig. 3a, exhibit mainly the COO symmetric and asymmetric stretching bands at 1398 and 1578 cm⁻¹, respectively, corroborating the presence of metal carboxylate linkages in the MOF. Also, the C–N stretch at 1203 and 1234 cm⁻¹, C–O stretch at 1125 cm⁻¹ and bending vibration of the heptazine unit at ∼800 cm⁻¹ reveal the formation of MZN-1 with BCN as ligand. In the FTIR spectra of MSBCN, as shown in Fig. 3b, all the stretches of BCN are observed with slight shifting in some cases. On the other hand, MZN-2 spectra (Fig. 3b) consist of the peaks of MSBCN along with that of COO symmetric and asymmetric stretching, C–H_bend at 742 cm⁻¹, C–N_stretch at 670 cm⁻¹ and metal–oxygen stretch at 496 cm⁻¹. After the functionalities of the ligand and developed MOFs are confirmed, it is now necessary to identify the composition of the MOF; therefore, X-ray diffraction studies were carried out. The XRD curves of MZN-1 and MZN-2 presented in Fig. 3(c) and (d) are in good agreement with that of the MOF-LIC-1(Eu) type.^10,46,47 Due to the presence of MoS₂ alongwith functionalized g-C₃N₄ in MZN-2, slight variations were observed than that of MZN-1. The pore volume and diameter of the MOFs are presented in Table S1, ESI,† confirming the microporous nature of the MOFs.

Fig. 3 FTIR spectra of (a) BCN and MZN-1 and (b) MSBCN and MZN-2; XRD peaks of (c) MZN-1 and (d) MZN-2.

Then, XPS was used to determine the type of interaction between MOF and BCN. Fig. 4 shows the XPS spectra of MZN-1 and MZN-2, while the comparative peak positions of the elements in three samples are tabulated in Tables S2 and S3.† The graphitic carbon-related XPS peak at 284.6 eV seems to be enhanced in both the C 1s spectra (Fig. 4a) of MZN-1 and MZN-2. However, the main peak for the C–N bond is decreased in intensity and is shifted to a higher binding energy of 288.8 eV than BCN. This indicates coordination between BCN and Eu³⁺.

Fig. 4 Comparative XPS spectra of (a) C 1s, (b) N 1s, (c) O 1s, (d) Eu 3d and (e) S 2p in developed MOFs.

On the other hand, the peak for the O=C–O linkage is observed only in the case of MZN-1 at 289.9 eV. In MZN-1, the ligand role was solely played by BCN, whereas in MZN-2, it was a composite of BCN and MoS₂, termed here as MSBCN. This ligand variation was designed to form a specific heterostructure, aiming to develop a fluorescent MOF with both ligand and metal-centered emissions. During the MOF formation, the combination of MSBCN, Eu³⁺ and terephthalic acid in MZN-2 may suppress the O=C–O peak compared to the S 2p-related peaks. The N 1s spectra are presented in Fig. 4(b). Both the C–N=C peak at 397. 6 eV and the N–(C)₃ peak at 399.4 eV (ref. 48) of CN are shifted to higher binding energies in the case of MZN-1, pointing towards effective coordination between BCN and Eu³⁺ ions. The subpeak was observed at 403 eV in the case of only BCN, indicating the N–O kind of bonding is absent in MOFs.⁴⁹ From the O 1s spectra in Fig. 4(c), the C=O peak is shifted to a higher binding energy than 531.6 eV of BCN spectra. In the case of MZN-1, the inorganic O peak at 530.9 eV is the major peak, while the C=O peak at 532.6 eV appears as hump. However, in MZN-2, both inorganic and organic O 1s peaks appear with equivalent intensity.

Moreover, Eu²⁺ 3d_5/2 and Eu³⁺ 3d_5/2 peaks at 1125 eV and 1135 eV are present in the Eu 3d spectra of both MZN-1 and MZN-2 (Fig. 4d), corroborating the presence of both Eu²⁺ and Eu³⁺ ions.⁵⁰ In the S 2p spectra of MZN-2 in Fig. 4e, the peak at 166.4 eV accompanied by a peak at 161.4 eV corresponds to sulphite ([SO₃]²⁻) and sulphide (S^2–).^51,52 This indicates an S–O interaction between MoS₂ and BCN in MSBCN. This observation supports the assumption that metal coordination with Eu³⁺ occurs through S–O kind of linkages in MZN-2. Although Mo⁴⁺ peaks are not detected in the survey spectra, elemental compositional analysis reveals the presence of low composition in MZN-2 (Table S3, ESI†).

The morphology of the developed MOFs was studied through SEM characterization, and the images are presented in Fig. 5. It is observed that MZN-1 is a mixture of big and small spheres of sizes ∼1 μm and 250 nm whereas the range of particle size of MZN-2 is 2–6 μm. Also, the images reveal that the morphologies of the MOFs are not similar. In the case of MZN-2, the surface of MOF particles was seen to be fluffy in nature, which may be due to the composite use of BCN and MOS₂ along with terephthalic acid as co-ligand. For the distribution study of Eu(III) in the MOFs, their EDS spectra were recorded and shown in Fig. S4(a) and (b), ESI.† Also, the survey spectra along with the atomic composition table are presented in Fig. S4(c), (d) and Table S4, ESI.†

Fig. 5 SEM images of (a) MZN-1 and (b) MZN-2, respectively. TEM and HRTEM images of MZN-1 (c and e) and MZN-2 (d and f). Inset shows the SAED patterns, which are analyzed in detail in ESI.†

In general, transmission electron microscopy imaging of MOF is not achievable in nano-scale resolution due to the ‘knock-on’ damage effect. There are only a few reports of HR-TEM images of known MOFs taken with low-voltage electron beams.^53–55 Here, we have obtained atomic-scale resolution of both MZN-1 and MZN-2 with distinct lattice planes under conventional HR-TEM conditions, with an accelerating voltage of 200 kV and resolution up to 5 nm. The TEM images are shown in Fig. 5(c) and (d), from where the particle sizes are calculated to be 85 nm and 1 μm for MZN-1 and MZN-2, respectively. This result is in accordance with the particle size obtained from SEM results, as MZN-1 seems to consist of small and large-sized particles. Meanwhile, the d-spacing of MZN-1 was calculated to be 0.34 nm, which is in agreement with that obtained from PXRD of the graphitic C₃N₄ for the (100) plane.⁵⁶ Then, with the aid of a DDEC camera, SAED patterns were captured for the developed MOFs. The SAED pattern of MZN-1 was analyzed, and the spots were assigned for Eu MOF and g-C₃N₄ phase based on the literature. The details of the SAED analysis are given in ESI (Fig. S5 and Tables S5, S6†). Furthermore, the dot pattern of SAED was obtained for MZN-2, which appeared to be a single crystal pattern and was analyzed to be due to Eu-MOF (Fig. S5b, ESI†).

Optical characterization of BCN and MSBCN was done by UV-visible absorption and fluorescence spectroscopy. From the absorption spectra of BCN and MSBCN in Fig. 6a, peaks are observed at 210 nm, 248 nm and 317 nm, which are due to π to π* and n to π* transitions within carboxylic as well graphitic C₃N₄ entities.⁵⁷ While in the absorption spectra of MZN-1, the 248 nm peak is slightly red-shifted to 254 nm, indicating metal–ligand complex formation, whereas the 317 nm peak is intact but decreases in intensity significantly (Fig. 6b). On the other hand, although the ligand MSBCN exhibits the same absorption peaks as BCN, in the absorption spectra of MZN-2, the peak is blue-shifted to 240 nm, and the peak at 317 nm disappears, with a new peak emerging at 288 nm (inset of Fig. 6c). This confirms the interaction between Eu³⁺ and the ligand system in MZN-2.

Fig. 6 UV-Visible absorption spectra of (a) BCN, MSBCN, (b) MZN-1 and (c) MZN-2.

From the fluorescence spectra of BCN and MSBCN, as shown in Fig. 7a and b, the emission peaks are observed at 460 nm, 490 nm and 504 nm, corresponding to three excitation peaks at 248 nm, 310 nm and 342 nm, respectively (inset of Fig. 7c). This is in corroboration with the UV-visible absorption spectra of Fig. 6(a), indicating the electronic transition of ligands being the source of origin for these emissions. Whereas the emission spectra of MZN-1 possess only ligand-centered emissions, as shown in Fig. 7a, MZN-2 possesses both ligand (490 nm, 504 nm) and metal-centered emissions at 580 nm (⁵D₀ to ⁷F₂), 593 nm (⁵D₀ to ⁷F₁), 617 nm (⁵D₀ to ⁷F₂), 652 nm (⁵D₀ to ⁷F₃) and 700 nm (⁵D₀ to ⁷F₄) of Eu³⁺ ion (Fig. 7b). This indicates the effective ‘antenna effect’ in the formation of MZN-2 MOF. The lanthanide transitions are basically forbidden, while the effective antenna effect through energy transfer from the ligand system leads to strong emissions from metal centers in MOF. Here, the strong pink coloration of the MZN-2 solution was observed under UV lamp excitation of 270 nm, as shown in Fig. 8(d). Hence, MSBCN, along with terephthalic acid, was optimized to be the best combination of ligands for developing this kind of MOF with dual emissions. Further, the surface charge of the developed MOF was studied through zeta potential measurement, and the values were obtained as 16.0 mV and 24.4 mV for MZN-1 and MZN-2, respectively. The results are shown in Fig. S6, ESI.† The surface characteristics will be helpful for understanding the mechanism of complex formation in sensing applications.

Fig. 7 Fluorescence spectra of (a) BCN, MZN-1, (b) MSBCN, MZN-2 and (c) excitation spectra of MSBCN, MZN-1 and MZN-2.

Fig. 8 (a) and (b) Sensing activity study of MZN-2 with the different microliter addition of dioxane, (c) time-dependent study of sensing activity and (d) photographs of MZN-2 (i) before and (ii) after dioxane addition.

Sensing activity of the developed MOF

Among the developed MOFs, MZN-2 was found to be an efficient sensor for the environmental toxicant 1,4-dioxane in both the liquid and vapor phases. To evaluate the sensing activity of MOF for the detection of 1,4-dioxane in the liquid phase, fluorescence spectroscopy was performed on the MOF solution spiked with the analyte. Selectivity studies and limit of detection (LOD) calculation experiments were performed, and the results are described below.

For the fluorescence-based detection of 1,4-dioxane, a solution of MZN-2 was prepared by dispersing 2.5 mg of it in 2.5 mL of water and then sonicated. Fluorescence spectra were recorded by excitation at 270 nm. The sensing experiment was performed by adding an increasing volume of the dioxane solution by stirring and recording the fluorescence spectra at the same excitation wavelength. It was observed that the MZN-2 solution exhibited ratiometric sensing behavior in response to 1,4-dioxane. Upon adding the analyte to the MOF solution, the metal-centered emission peaks were attenuated while the ligand-centered emission peak increased in intensity. The results are shown in Fig. 8.

As shown in Fig. 8a and b, the metal-centered emission at 593, 617, and 700 nm decreased upon the addition of dioxane to the MOF solution in water. As the concentration of the analyte solution increased, the emission intensity decreased to zero. On the other hand, the intensity of the emission peak at 504 nm, which originated either from the ligand emission interfered by the metal or from the ligand to metal charge transfer (LMCT), gradually increased and eventually saturated after a certain period. The response time, as shown in the plot in Fig. 8(c), was observed to be as fast as 5 s. This type of ratiometric sensing was well illustrated by photographs of the MOF solution in the dark, excited with UV light. The photos are shown in Fig. 8d. It can be seen that the solution had a pink color, while it became blue after the addition of dioxane. The solution initially appeared pink but turned blue after the addition of dioxane. This colorimetric change, visible to the naked eye, enhances the method's practicality for easily detecting 1,4-dioxane in the liquid phase.

Having observed the spectacular sensing phenomenon of dioxane by the MOF we developed, it is now imperative to explore the mechanism behind the ratiometric sensing behavior towards 1,4-dioxane. In this context, the possibilities of quenching by the inner filter effect (IFE) and Förster resonance energy transfer (FRET) were first examined. The comparative plot between the absorption spectra of dioxane (absorber) and the excitation (Fig. S7a†) and emission (Fig. S7b†) of MZN-2 (fluorophore) shows that there is no overlap, which rules out an inner filter effect (IFE). The possibility of FRET is also excluded by comparing the absorption spectra of MZN-2 (acceptor) and the emission spectra of dioxane (donor) without any overlap (Fig. S7c†).⁵⁸ Then, the Stern–Volmer diagram with a linear characteristic is obtained (Fig. 9a), indicating that the quenching can be static or dynamic or a mixture of both. The quenching constant K_SV was found to be 2.26 × 10⁸ M⁻¹ (eqn (S1), ESI†). Next, time-resolved photoluminescence (TRPL) was performed with MZN-2 before (Fig. 9b) and after the addition of dioxane (Fig. 9c) to determine the nature of the quenching of the metal-centered emissions, whereas the comparative plot between the two is presented in Fig. S8, ESI.† In particular, the emission peak at 617 nm was considered, and the TRPL plots were fitted with monoexponential decay curves. It was observed that the excited state lifetime remained almost constant (0.87 ns to 1.0 ns) upon the addition of the dioxane analyte, indicating the nature of quenching as static. Static quenching is caused by the formation of a complex between the analyte and the fluorophore, which forms a non-fluorescent complex. This leads to a reduction in the number of emitting fluorophores and thus quenching, but not all fluorophores form a complex so that the lifetime of the remaining fluorophores in the excited state remains almost the same.

Fig. 9 (a) S–V plot, (b) TRPL of MZN-2 before (emission 617 nm) and (c) after dioxane addition (emission at 617 nm).

The limit of detection was calculated by considering the quenching effect through the Stern–Volmer plot. As shown in Fig. 9a, the S–V plot shows a linear response towards dioxane with an LOD value of 0.3 μM or 0.026 ppm. The comparative table for LOD, as reported in the literature, is described in Table S7, ESI.† The S–V equation and LOD calculation are given in eqn (S1) and (S2), ESI.† In this regard, it is worthwhile to mention that the Food and Drug Administration (FDA) has set a limit of 10 ppm for 1,4-dioxane in many food additives and cosmetic products.⁵⁹

Once the type of quenching appears to be static, the next step is to find the mechanism of complex formation between MZN-2 and 1,4-dioxane. In this context, zeta potential values of MZN-2 and dioxane solution were found to be 24.4 mV (Fig. S6, ESI†) and −1.61 mV (graph not obtained), respectively. This result indicates the possibility of electrostatic interaction between the two entities. The FTIR spectra of MZN-2 treated with dioxane were recorded and are shown in Fig. S9, ESI.† From the spectra, it is seen that the COO symmetric and antisymmetric stretches at 1372 and 1560 cm⁻¹ disappear, which may be due to the replacement of the coordination bond between the carboxylate groups and the Eu³⁺ ions in MZN-2 by the dioxane coordination. The dioxane complexes of magnesium recovered from the Grignard solution are well established, where both the oxygen atoms of the dioxane are involved in the coordination.⁶⁰ A similar interaction is presumed to occur in this case, as indicated by the appearance of four new absorption peaks in the 400–600 cm⁻¹ fingerprint region, corresponding to metal–oxygen bonds in the FTIR spectra of dioxane-treated MZN-2. Moreover, the presence of the C–O stretch for cyclic ether at 933 cm⁻¹ and the C–O stretches in the 1100–1200 cm⁻¹ region confirms the presence of dioxane in the treated MZN-2. In addition, extended S-related peaks in the 2500 cm⁻¹ region are observed in the FTIR spectra of this sample compared to MZN-2, as shown in Fig. S9(b), ESI.† This observation again suggests the possibility of an interaction between the S-containing groups of MZN-2 and dioxane. However, the XPS spectra of MZN-2 and dioxane-treated MZN-2 reveal that the C 1s spectrum of the latter (Fig. 10a) shows a peak at 285 eV corresponding to the O–C–O bond, indicating the presence of dioxane. Moreover, the N 1s (Fig. 10b), Eu 3d (Fig. 10c) and S 2p (Fig. 10d) XPS spectra show unchanged properties after treatment. Moreover, the O 1s spectra (Fig. 10e) of this sample exhibit an additional peak at 532 eV, characteristic of loosely bound oxygen on the surface, such as metal–OH groups.⁶¹ The observed decrease in peak intensities in the case of Eu 3d spectra in Fig. 10(c) can be explained again in terms of complex formation between the sensor and analyte. Complex formation can result in the analyte covering the surface of the sensor, thereby reducing the exposure of the sensor's atoms to the X-ray source.

Fig. 10 XPS spectra of MZN-2 added with dioxane. (a) C 1s, (b) N 1s, (c) Eu 3d, (d) S 2p and (e) O 1s.

However, the UV absorption study of MZN-2 with dioxane addition showed that absorption increased to manifold just after adding the analyte, reached a maximum and then started decreasing, and the results are shown in Fig. 11a. This observation also suggests the possibility of MZN-2 and dioxane complex formation. In this context, it is important to note that the UV-visible absorption spectra of MSBCN alone were recorded with increasing volumes of 1,4-dioxane, showing no changes (Fig. S10, ESI†). This confirms that the interaction of dioxane occurs specifically with MZN-2 and not with MSBCN alone. From the Benesi–Hilderbrand plot in Fig. 11b, the association constant, K_a, is calculated to be 1.605 × 10⁻⁴ M⁻¹.

Fig. 11 (a) Fold change in the UV-visible absorption spectra of MZN-2 with dioxane addition, where A₂₄₀ and A₀ correspond to the absorbance of the MOF solution at 240 nm and in the absence of dioxane addition, respectively, and (b) Benesi–Hilderbrand plot.

Mechanism study of energy transfer

A plausible pathway for quenching by photoinduced electron transfer (PET) due to dioxane coordination with the Eu³⁺ ion has been realized by developing an energy-level diagram of the species involved. The diagram is shown in Fig. 12a. The valence band positions of MSBCN and MZN-2 were determined using valence band XPS (Fig. 12b), the energy band gap were calculated from the corresponding Tauc plot (Fig. 12c) and conduction band positions were calculated using eqn (1). For the analyte dioxane, on the other hand, cyclic voltammetry (Fig. 12d) was performed to determine the oxidation and reduction potentials, and the following equation (eqn (2)) was used to calculate the HOMO and LUMO energy levels.⁵⁷ Standard data was available for the other ligand BDC. In the MOF composition, g-C₃N₄ is present with a highly conjugated 3-D network system. Also, the presence of a carboxyl functional group with enriched electron density in BCN provides a pool of delocalized electron density to the MOF composition. This constitutes the HOMO of the MOF sensor, while vacant d-orbitals of the Eu³⁺ ion contribute to the LUMO of the same. From the energy level diagram in Fig. 12a, it is observed to be a Type I band arrangement after dioxane addition, where the electrons from the conduction band of MZN-2 can easily transfer to the conduction band of dioxane, causing quenching in fluorescence due to relaxation of singly excited electrons from the conduction band to the valence band.

E_CB = E_VB − E_g (1)

Fig. 12 (a) Energy level diagram, (b) valence band XPS of MSBCN and MZN-2, (c) Tauc plot and (b) CV of dioxane.

Hence, the formation of a ground state complex (GSC) results in more effective PET, causing efficient quenching of metal-centered emissions. Meanwhile, on GSC formation between dioxane and MZN-2, the electronic transition viz. n to π* increases, ultimately enhancing the absorbance in the UV-visible spectra and ligand-centered emission at 504 nm. As a consequence of all these phenomena, a ratiometric sensing behavior is exhibited by MOF MZN-2 towards 1,4-dioxane. This activity is correlated by the colorimetric change of the MOF solution, which is easily detectable even by the naked eye.

Selectivity and recyclability study

The selectivity study was conducted among a pool of 16 solvents. The results shown in Fig. 13(a) clearly demonstrate the discriminative sensing ability of the MOF towards dioxane with the maximum fold change in intensity of both 504 and 617 nm emission peaks in the pool of solvents. The XRD analysis of MZN-2 immersed in dioxane was carried out, the result is shown in Fig. 13(c), and the crystallinity of the sample seems to remain intact after the application. Additionally, the effect of interfering solvents on the dioxane sensing experiment by MZN-2 was investigated. The emission intensities of four peaks of the MOF were compared in the presence of dioxane alone and dioxane with an interfering solvent. The results, shown in Fig. S11 (ESI†), indicate that no significant changes were observed in most cases, except for the 617 nm peak in the presence of THF and toluene. This study highlights the robustness of the sensing behavior, demonstrating selectivity towards dioxane. For the recyclability study, the used sample was separated by centrifuging the solution at 8000 RPM for 10 min, followed by drying at 60 °C for 12 h. From the results shown in Fig. 13(b) the activity of the used sample was seen to be slightly decreased at both the metal and ligand-centered emission wavelengths because of the loss of sample mainly caused by the high dispersibility of the MOF in water. The XRD analysis of the recycled sample for three cycles was carried out, and the comparative plot with native MZN-2 is given in Fig. 13(d). It is observed that in the recycled sample, a few XRD peaks shifted, and two new peaks arose compared to MZN-2. These shifts can be attributed to several factors such as phase transformation,⁶² leaching out of elements during repeated washing inducing impurities,⁶² surface reconstruction⁶³etc. A similar observation was made in the case of BJH parameters of recycled MZN-2 sample. From the data presented in Table S1, ESI,† the surface area, pore volume and diameter of MZN-2R were seen to be increased than the pure one, which corroborated with the surface reconstruction viewpoint during the recycling process.

Fig. 13 (a) Selectivity result of dioxane sensing by MZN-2 and (b) recyclability study of the used samples for sensing. XRD graphs of MZN-2 (c) immersed in dioxane and (d) recycled after 3 cycles.

Then, the sensing activity of MZN-2 MOF was carried out in real samples spiked with dioxane. From the results shown in Table 1, the recovery% was calculated to be 95–99% mostly.

Table 1 Calculation of found concentration (mM) and recovery percentage (%)

Source of water	Dioxane concentration (mM)	Found concentration (mM)	Recovery percentage (%)
1. Distilled water	19.52	18.72	95.88
	38.99	36.53	93.71
	77.71	74.44	95.78
	135.33	133.80	98.87
	192.38	190.67	99.11
2. River water	19.52	19.46	99.68
	38.99	38.57	98.92
	77.71	77.00	99.07
	135.33	134.40	99.31
	192.38	189.24	98.37
3. Industrial water	19.52	18.59	95.21
	38.99	36.27	93.03
	77.72	74.05	95.28
	135.33	133.14	98.38
	192.38	188.82	98.15

---

Gaseous phase detection of VOC

Next, the results for detecting dioxane in the gas phase using the synthesized MOF are presented through I–V characteristic curves. Detailed procedure for gaseous sensing is provided in ESI.†Fig. 14(a) shows the schematic diagram of the vacuum chamber that we developed in the laboratory for gas phase sensing. The I–V characteristic curve in Fig. 14(b) clearly shows that the sensor works efficiently for dioxane detection as the measured current (in microAmp) increases with increasing voltage (in V) applied across the sample when exposed to dioxane vapour. However in air, the signal remains almost constant in the same setup (Fig. 14(c)). It is assumed that the developed MOF possesses high oxygen functionalities with hydrophilicity on its surface. Therefore, the adsorption of volatile organic compounds is triggered by hydrogen bonding interactions, which ultimately affect the electrical properties.

Fig. 14 (a). Schematic of the vacuum chamber, (b) I–V characteristic curve of the MOF sample in the presence of dioxane vapour and (c) in air.

Density functional theory (DFT) study

The essential collaboration between experiments and computational methods seems to be crucial for material design. Our strategy is, therefore, to carry out calculations to provide important information for the material design process. To complement the experimental results, DFT calculations were performed to reveal the inherent mechanism behind the adsorption process that alters the electronic properties of both pure and Eu-conjugated BCN upon the adsorption of dioxane. First, we adsorbed the dioxane molecule onto different adsorption sites of BCN and identified the most stable site by evaluating the adsorption energy.

The adsorption energy of dioxane adsorbed on BCN and Eu-conjugated BCN can be estimated using the relations as given below:

E_ads = E_dioxane@BCN − E_BCN − E_dioxane (3)

E_ads = E_{dioxane@Eu-BCN} − E_Eu-BCN − E_dioxane (4)

where E_dioxane@BCN, E_{dioxane@Eu-BCN}, E_BCN, E_Eu-BCN and E_dioxane represents the energies of dioxane adsorbed on BCN, dioxane adsorbed on Eu-conjugated BCN, BCN, Eu-conjugated BCN and dioxane molecule, respectively.

The adsorption energy of dioxane on BCN is 0.415 eV, suggesting a physisorption process. Subsequently, we introduced the Eu atom as a dopant into the BCN system to explore its impact on the dioxane molecule's adsorption process on Eu-conjugated BCN. Remarkably, the adsorption energy for the most stable site notably rises to −1.426 eV, following the inclusion of the Eu atom within the host substrate. The negative adsorption energy observed in both cases implies a spontaneous and exothermic process.

Subsequently, we investigated the frontier molecular orbitals, i.e., the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of pure and Eu-doped BCN before and after dioxane adsorption. Fig. 15 represents the optimized structure, along with their frontier molecular orbitals and energy level diagram of Eu-BCN and dioxane adsorbed on Eu-BCN. The analysis of these energy levels provides insights into the electronic properties influencing the sensing performance. Moreover, these energy levels offer a glimpse into the nature of the species by elucidating the types of interactions occurring during the sensing process. Furthermore, we computed the HOMO–LUMO gap (E_g) to assess the chemical activity of the dioxane molecule upon its adsorption onto pure and Eu-doped BCN surfaces. The calculated E_g for pure BCN is 0.836 eV, showing a slight reduction (−0.832 eV) upon the adsorption of the dioxane molecule. Conversely, in the case of Eu-conjugated BCN, the E_g is measured at 1.355 eV, significantly increasing to 1.549 eV after the dioxane molecule is adsorbed onto the Eu-doped BCN surface. Both adsorption energy and HOMO–LUMO gap suggest that the introduction of the Eu atom onto the BCN surface enhances the sensing capability of the dioxane molecule significantly, surpassing that of the pure BCN surface.

Fig. 15 Optimized structure and frontier molecular orbital plots of (a) Eu-BCN, (b) dioxane@Eu-BCN and (c) energy level diagram of Eu-BCN and dioxane@Eu-BCN.

Conclusion

In summary, this study demonstrates the successful development of a novel ratiometric sensing approach for detecting 1,4-dioxane, an industrial solvent with significant health concerns. By using a tailored metal–organic framework (MOF) consisting of functionalized g-C₃N₄ and Eu salt, the sensing capabilities were improved, enabling efficient detection in both the liquid and vapor phases. The unique heterostructure of functionalized g-C₃N₄, MoS₂, terephthalic acid and Eu salt exhibited remarkable sensitivity, achieving a fast response time and an impressively low limit of detection (LOD) well below regulatory limits. Importantly, the sensor has excellent selectivity, effectively distinguishing 1,4-dioxane from a range of other solvents. The success of this sensor platform can be attributed to the precise alignment of energy levels between the MOF and the analyte, which enables photo-induced electron transfer (PET). Computer-aided analyses underpinned the pivotal role of Eu and the ligand system in the efficient detection of 1,4-dioxane. Overall, this study underlines the potential of ratiometric sensing using MOFs for detecting hazardous compounds and offers promising perspectives for advanced environmental monitoring and industrial safety applications. Moreover, the future potential of this technology is promising, as it paves the way for developing a point-of-care device based on the colorimetric difference of the MOF solution in the presence and absence of the analyte 1,4-dioxane.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Department of Science & Technology, Govt. of India, via the Women Scientist Fellowship (Ref. no. SR/WOSA/CS-15/2019). Authors are thankful to the ex-Director, CSIR NEIST, Dr G. N. Sastry, for his guidance and support in executing the theoretical validation of the proposed mechanism in the manuscript. MG acknowledges BIRAC, Department of Biotechnology, Govt. of India, for the BIG-18 startup grant (Ref. no. BIRAC/KIIT01346/BIG-18/21), SJK thanks the Department of Science & Technology, Govt. of India for INSPIRE Fellowship (IF 210516) and JD thanks the Department of Chemicals and Petrochemicals, Ministry of Chemicals and Fertilizers, Govt. of India, for the postdoctoral fellowship (Project No.: GPP0373). Authors are thankful to CSIR-NEIST in-house project OLP-2078 for the financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4dt01709a

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