Detection of SO₂ using a hybrid LDH-MOF material

Abstract

A hybrid MgAlFe-LDH/UiO-66-NH₂ material has been developed for the efficient detection of SO₂, showing an exceptionally low detection limit of only 0.72 ppm, which is considerably lower in comparison to UiO-66-NH₂ (739 ppm). Spectroscopic studies suggest that the preferential oxidation of SO₂ to SO₄²⁻, catalysed by the Fe³⁺ centres within MgAlFe-LDH, is the key factor in the enhanced fluorescence response. This process in situ generates Fe²⁺ sites and favours the interlaminar sequestration of SO₂, preventing direct interaction with the active sites of UiO-66-NH₂.

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Sulphur dioxide (SO₂) is a colourless, pungent-smelling, highly irritating gas that is found in a gaseous state under standard ambient conditions (25 °C and 1 atmosphere). It is classified as a primary pollutant and its presence in the atmosphere is due to direct emissions from natural sources and human activities.¹

Natural sources of emissions include volcanoes, which release significant volumes of this gas during their eruptions, together with the oxidation of organic materials in vegetation fires and agricultural practices involving biomass burning.² However, in the current context, industrial activity is the main source of emissions, particularly the combustion of sulphur rich energy resources such as certain types of coal and heavy oil fractions.³ Even at low concentrations, uncontrolled SO₂ emissions pose risks to the environment and human health.⁴ Their role in forming acid rain, which damages ecosystems, is exacerbated by their correlation with respiratory and cardiovascular diseases in exposed populations.⁵ This duality of effects has prompted the need for highly accurate and sensitive monitoring systems. Therefore, measurement strategies that can accurately identify SO₂ at minimum thresholds (≥1.5 ppm) in both dry and humid environments are required.⁶

Currently, resistive chemical sensors based on metal oxide semiconductors (such as SnO₂, WO₃ and TiO₂) are gaining prominence, due to their high sensitivity and selectivity towards SO₂, as well as their fast response and recovery kinetics.^7–11 However, their main limitation is their high operating temperature of 200–600 °C, which results in high energy consumption and limits their practical applications.^12–15 Therefore, developing materials that can detect SO₂ at room temperature and at low concentrations has become a priority. This breakthrough would mitigate problems associated with energy consumption and mark a fundamental milestone in the design of next-generation materials.

In this context, various porous materials have been investigated, among which lamellar double hydroxides (LDHs) and metal–organic frameworks (MOFs) stand out due to their potential for SO₂ adsorption and detection. LDHs correspond to lamellar ionic compounds with a brucite-like structure, where positively charged layers alternate with interlaminar regions containing compensating anions and water molecules.^16–18 Due to their unique structural properties, LDHs have attracted great attention for sensor and SO₂ capture applications.^19–21 However, they have certain disadvantages, such as limited selectivity and lower efficiency compared to other more developed materials.²²

On the other hand, MOFs represent a class of highly crystalline porous materials, constituted by the coordinated bonding between metal centres and organic ligands, predominantly carboxylates.²³ These structures are distinguished by their high surface area, the possibility to modulate the pore size and the ability to adjust their physicochemical properties.^24–27 Thanks to these characteristics, MOFs are emerging as promising candidates for use in detecting and capturing toxic and corrosive gases such as SO₂. Recently, MOFs have emerged as promising materials in the field of luminescence due to their structure. These characteristics allow them to modulate their fluorescence, providing a flexible possibility to design materials with specific optical responses.²⁸ Luminescence in MOFs can be originated through different mechanisms, which postulates them as ideal candidates for the selective detection of analytes.²⁹ However, most MOFs exhibit marked instability upon exposure to moisture and acid gases (e.g., SO₂). Thus, the development of hybrid MOF-LDH-type materials with unique properties that can improve both the specific interaction with SO₂ and their detection through specific fluorescence-mediated changes has been studied, opening new possibilities for the development of efficient sensors.

With this target in mind, an effective MOF-LDH hybrid material was successfully developed. The MgAlFe-LDH/UiO-66-NH₂ composite was synthesised through the surface functionalisation of MgAlFe-LDH using the coupling agent CPTMS (3-chloropropyltrimethoxysilane), which acted as a molecular bridge for the covalent anchoring of UiO-66-NH₂via in situ growth. The methoxy (–OCH₃) groups of the CPTMS reacted with the hydroxyl (–OH) groups on the surface of MgAlFe-LDH, while the chlorinated end (–CH₂Cl) of the silane generated an electrophilic centre after chlorine removal, capable of reacting with the carboxyl (–COOH) groups from the UiO-66-NH₂ ligand (Fig. S1, SI). This covalent coupling mechanism enabled a stable interface to be formed between the two materials.

Powder X-ray diffraction (PXRD), FT-IR spectroscopy, scanning electron microscopy (SEM) and thermogravimetric analysis (TGA) were used to confirm the phase purity of MgAlFe-LDH, MgAlFe-LDH-CPTMS, UiO-66-NH₂ and MgAlFe-LDH/UiO-66-NH₂ (Fig. S2–S6, SI). Additionally, XPS analysis was performed to confirm the effective functionalisation of the materials. High-resolution XPS analysis of the O 1s region (Fig. S7a, SI) confirms the surface modification of MgAlFe-LDH with CPTMS, as indicated by the appearance of a new peak at 533.1 eV (Si–O) and a shift of the O–M peak (M=Mg, Al, Fe) towards higher energies, indicative of the formation of covalent bonds between MgAlFe-LDH and the organosilane. In the case of MgAlFe-LDH/UiO-66-NH₂ (Fig. S7b, SI), the shift of the O–M peak (530.5 eV) confirms the presence of the O–M bonds (O–Al, O–Mg and O–Zr) of both MgAlFe-LDH and UiO-66-NH₂. The changes in the signals at 532.0 eV (decrease in area) and 532.9 eV (an increase in area) reflect covalent interactions between the carboxyl groups of the UiO-66-NH₂ ligand and CPTMS, as well as the formation of new Si–O bonds during the in situ growth of UiO-66-NH₂ on the modified MgAlFe-LDH. These results validate the successful functionalisation of the hybrid system through covalent bonds.

The surface area and pore diameter of the different sintered materials were subsequently evaluated using N₂ physisorption at −196 °C (77 K) (Fig. S8 and Table S1, SI). SO₂ adsorption–desorption isotherms were carried out using a DVS vacuum (Surface Measurement Systems Ltd) dynamic gravimetric gas/vapour sorption analyser from 0 to 0.1 bar at 25 °C for the activated samples MgAlFe-LDH, UiO-66-NH₂ and MgAlFe-LDH/UiO-66-NH₂. Fig. 1 shows the resulting isotherms. Typical type I behaviour was observed, with pronounced SO₂ sorption in the 0.03 bar range. Values reached 1.4 mmol g⁻¹ for UiO-66-NH₂, 1.2 mmol g⁻¹ for MgAlFe-LDH/UiO-66-NH₂, and 0.49 mmol g⁻¹ for MgAlFe-LDH. As the pressure increased from 0.03 to 0.1 bar, the adsorption capacity increased gradually to 2.1 mmol g⁻¹ (UiO-66-NH₂), 1.78 mmol g⁻¹ (MgAlFe-LDH/UiO-66-NH₂) and 0.59 mmol g⁻¹ (MgAlFe-LDH), respectively.

Fig. 1 The SO₂ isotherms of MgAlFe-LDH/UiO-66-NH₂, MgAlFe-LDH and UiO-66-NH₂ at 25 °C and 0.1 bar. Filled circles = adsorption; hollow circles = desorption.

The results indicate that UiO-66-NH₂ exhibits a higher SO₂ adsorption capacity than MgAlFe-LDH/UiO-66-NH₂ and MgAlFe-LDH. This difference between UiO-66-NH₂ and MgAlFe-LDH/UiO-66-NH₂ was mainly attributed to structural modifications induced during the functionalisation process. In MgAlFe-LDH/UiO-66-NH₂, the incorporation of MgAlFe-LDH could alter the crystalline orientation of the MOF, generating a partial blockage of its porosity and reducing accessibility to the active sites. In contrast, UiO-66-NH₂ retains its porous integrity and homogeneous crystalline arrangement, favouring more efficient interactions with SO₂ molecules. These observations are consistent with BET surface area analyses, which indicates a substantial decrease in the specific surface area of MgAlFe-LDH/UiO-66-NH₂ relative to UiO-66-NH₂. By contrast, the results for MgAlFe-LDH were consistent with those reported in the literature.³⁰ The most significant property of a material for SO₂ detection is probably its adsorption capacity under low pressures (P < 0.1 bar), since the typical concentration ranges for SO₂ detection (generally measured in ppm) have a direct correlation with the low-pressure regime of the gas.³¹ In this context, the SO₂ adsorption behaviour of MgAlFe-LDH, UiO-66-NH₂ and MgAlFe-LDH/UiO-66-NH₂ was evaluated and the possibility of using these materials as SO₂ detectors was investigated. Specifically, it was expected that the MgAlFe-LDH/UiO-66-NH₂ hybrid material would demonstrate superior fluorescent properties for SO₂ detection.

First, the UV-Vis spectra of UiO-66-NH₂ and MgAlFe-LDH/UiO-66-NH₂ (Fig. S9, SI) reveal bands that are characteristic of electronic transitions within different spectral ranges. Between 250 and 300 nm, bands are observed that are attributable to π → π* transitions, while in the 300–400 nm region, the bands correspond to n → π* transitions.^32–34 Conversely, MgAlFe-LDH shows an absorption band below 300 nm attributable to π transitions from OH⁻ orbitals to Fe³⁺ 3d orbitals, and between 300 and 500 nm, absorption bands are observed that could be related to iron clusters.³⁵

In order to evaluate the emission properties and determine the optimum excitation wavelength, solid-state emission spectra were obtained for both UiO-66-NH₂ and MgAlFe-LDH/UiO-66-NH₂. For both materials, the best emission was found to be obtained at 380 nm, so this excitation wavelength was chosen for all fluorescence experiments. The emission spectra of the 2-aminoterephthalic acid ligand and the UiO-66-NH₂ and MgAlFe-LDH/UiO-66-NH₂ materials reveal marked differences in fluorescent behaviour (Fig. 2a). While the free ligand exhibits a characteristic emission at 467 nm associated with n–π* transitions,^36,37 the formation of UiO-66-NH₂ through coordination with Zr⁴⁺ results in significant fluorescent quenching due to ligand-metal charge transfer (LMCT). This phenomenon occurs when the excited electrons of the ligand are transferred to the empty orbitals of Zr⁴⁺, preventing radiative recombination and consequently suppressing the emission.^38–41 Interestingly, MgAlFe-LDH/UiO-66-NH₂ exhibits increased emission intensity compared to UiO-66-NH₂, possibly due to the presence of Fe³⁺ in the MgAlFe-LDH structure. This inhibits the LMCT process characteristic of UiO-66-NH₂ by acting as a less favourable electron-accepting centre than Zr⁴⁺. This reduces the non-radiative deactivation pathways. This behaviour is consistent with that observed in the UV-Vis spectra (Fig. S9, SI), where the band associated with the n → π transitions (the LMCT-associated band) in MgAlFe-LDH/UiO-66-NH₂ is found to decrease in intensity relative to UiO-66-NH₂.

Fig. 2 (a) Comparison of the emission spectra of the 2-aminoterephthalic acid linker, UiO-66-NH₂ and MgAlFe-LDH/UiO-66-NH₂. (b) Comparison of the emission spectra of activated UiO-66-NH₂ and MgAlFe-LDH/UiO-66-NH₂ samples, exposed to 0.1 bar SO₂. (c) Comparison of the RAMAN spectra of activated UiO-66-NH₂ and MgAlFe-LDH/UiO-66-NH₂ samples, exposed to 0.1 bar SO₂. (d) Emission spectra of MgAlFe-LDH/UiO-66-NH₂ suspensions in THF exposed to SO₂.

Once the fluorescence behaviour of UiO-66-NH₂ and MgAlFe-LDH/UiO-66-NH₂ was observed, solid-state SO₂ detection was evaluated, where activated samples (120 °C for 3 hours) of UiO-66-NH₂ and MgAlFe-LDH/UiO-66-NH₂ were exposed to 0.1 bar SO₂ to observe what changes occur in their photoluminescence properties. Interestingly, opposite behaviours were observed between the materials: UiO-66-NH₂ exhibited a fluorescent quenching of 42.5% (Fig. 2b), attributed to the polarisation of the lone pairs (n) of the –NH₂ group by the SO₂ dipole moment. RAMAN spectra confirm this interaction (Fig. 2c), showing a decrease in the characteristic bands of the –NH₂ group (1300–1500 cm⁻¹); conversely the MgAlFe-LDH/UiO-66-NH₂ hybrid compound exhibited a 191.45% increase in the fluorescence emission intensity, which was attributed to the catalytic oxidation of SO₂ to SO₄²⁻, mediated by the Fe³⁺ centres of the MgAlFe-LDH.³⁰ This was confirmed by the disappearance of the CO₃²⁻ band (∼1060 cm⁻¹) in the Raman spectra (Fig. 2c and Fig. S10, SI) and the appearance of the asymmetric S–O stretch (∼1100 cm⁻¹) in the FTIR spectra (Fig. S11, SI). This process restricts SO₂ diffusion to the active sites of UiO-66-NH₂, while in situ Fe²⁺ generation (Fig. S12, SI) acts as an electron donor to the ligand's π-conjugated system. This demonstrates that the synergy between the hybrid material's components allows its fluorescent response to be modulated by a redox mechanism and the physical blocking effect of SO₂.

Finally, the limit of detection (LOD) of MgAlFe-LDH/UiO-66-NH₂ was experimentally determined in a suspension of the material in THF, obtaining a value of 0.72 ppm (Fig. 2d), demonstrating its high potential as an efficient SO₂ sensor. This result represents a huge improvement compared to the LOD of UiO-66-NH₂ (739 ppm) (Fig. S20, SI), showing that the synergistic interaction between MgAlFe-LDH and UiO-66-NH₂ significantly enhances the sensitivity of the material.

To summarise, the MgAlFe-LDH/UiO-66-NH₂ hybrid material was demonstrated to be capable of adsorbing SO₂ under low pressures (1.2 mmol g⁻¹ at 0.03 bar) with an excellent fluorescence sensitivity response. In contrast to UiO-66-NH₂, which exhibited turn-off behaviour in fluorescence, the hybrid compound demonstrated a turn-on effect in fluorescence emission, achieving an exceptional SO₂ detection limit of 0.72 ppm. Spectroscopic studies revealed that this behaviour is due to the catalytic oxidation of SO₂ being mediated by the Fe³⁺ centres of MgAlFe-LDH, as well as the capture of SO₂ in the interlaminar part of MgAlFe-LDH, which modulates the interaction with UiO-66-NH₂. Overall, this study demonstrates that the strategic combination of the properties of LDHs and MOFs can overcome the limitations of individual materials, providing a fundamental design principle for the development of hybrid materials, in which optimising the ratio and arrangement of the components will enable superior sensitivity and selectivity to be achieved in gas detection.

J. A. F. A. and M. L. M. thank SECIHT for the PhD fellow ships (1176665 and 1080048). J. A. F. A. thanks V. M. S. P. for her valuable scientific inspiration and assistance with artistic consultation. E. L. thanks Secihti (CBF 2023-2024-227) and I. A. I. thanks PAPIIT UNAM (IN201123), México, for financial support. We thank U. Winnberg (Euro Health) for scientific discussions and G. Ibarra-Winnberg for scientific encouragement.

Conflicts of interest

There are no conflicts to declare.

Data availability

The datasets supporting this article are provided in the supplementary information (SI). Supplementary information: synthesis, characterization, and additional figures and tables. See DOI: https://doi.org/10.1039/d5cc03612g.

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