Unraveling the origin of the “Turn-On” effect of Al-MIL-53-NO₂ during H₂S detection

Abstract

Nitro-functionalized metal–organic frameworks (MOFs), such as Al-MIL-53-NO₂, have been widely used in quantitative hydrogen sulfide (H₂S) detection based on the “turn-on” effect, where fluorescence enhancements were observed upon contact with H₂S. This was believed to be caused by the fact that the electron-withdrawing –NO₂ groups in the initial non-luminescent MOFs were reduced to electron-donating –NH₂ groups in the sensing process. However, since most H₂S detection is conducted in a suspension system consisting of MOFs and solvents, it is still unclear whether these –NH₂ groups are on MOFs or in the liquid. Using Al-MIL-53-NO₂ as a model MOF, this work aims to answer this question. Specifically, the supernatant and undissolved particles separated from the Al-MIL-53-NO₂ suspensions after being exposed to H₂S were analyzed systematically. The results showed that it is the free BDC-NH₂ (2-aminobenzene-1,4-dicarboxylic acid) in the solution rather than the formation of Al-MIL-53-NH₂ that really caused the fluorescence enhancement. In particular, the formed BDC-NH₂ was reduced from the shedded BDC-NO₂ (2-nitrobenzene-1,4-dicarboxylic acid) during the decomposition of Al-MIL-53-NO₂, which was attacked by OH⁻ in the NaHS solution. We anticipate that this work will offer new ways of tracing fluorophores for MOF-based sensing applications in aqueous systems.

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1. Introduction

Hydrogen sulfide (H₂S), known by its characteristic odor of rotten eggs, has been receiving considerable attention because it is highly associated with the physiological and pathological processes in biological systems.^1,2 Abnormal levels of H₂S have been identified to be related to many complex diseases such as Alzheimer's disease,³ diabetes,⁴ Down's syndrome,⁵ arterial and pulmonary hypertension,⁶ Parkinson's disease,⁷ and periodontal disease,⁸ which makes H₂S detection significant in disease prevention and diagnosis. Various methods towards H₂S sensing have been proposed, including semiconductor-based electrochemical tests,⁹ direct gas chromatography measurements,⁸ and piezoelectric transducer detection.¹⁰ However, the development of these methods is hampered by expensive instruments, complexity in sample preparation, and complicated operating procedures. The fluorescence-based detection method has grown rapidly in recent years because of its high selectivity and sensitivity, short response time, simplicity of operation, and easy observation in real-time imaging.^11,12 Up till now, plenty of fluorescent materials have been designed and used for H₂S sensing.^13–15

Among these fluorescent materials, metal–organic frameworks (MOFs), a class of porous crystalline materials composed of bridged organic linkers and centered metal ions/clusters,¹⁶ are interesting candidates because the great abundance of functional ligands and metals endow MOFs with very promising physical and chemical properties.^17–20 Several luminescent MOFs have been used for H₂S detection in aqueous or real physiological systems.^21–37 Among these explored mechanisms of H₂S sensing, the “turn-on” effect is the most popular one, where the original non-luminescent MOFs in the “turn-off” state are activated and enhanced upon being exposed to H₂S. Nitro-functionalized MOFs have been proven to be effective probes for H₂S detection based on the “turn-on” effect.^22,23,35 Specifically, the nitro groups (–NO₂) in nitro-functionalized MOFs decrease the electron density of the aromatic ring because of the unpaired electrons on nitrogen, which makes MOFs emit very weak fluorescence and stay in the “turn-off” state. After being treated by H₂S, electron-donating –NH₂ groups are obtained and the “turn-on” fluorescence signal is captured.

Based on this mechanism, Zr-UiO-66-NO₂ was first synthesized by Ghosh et al. to realize the “turn-on” response towards 0.0–4.0 mM H₂S through the transformation of Zr-UiO-66-NO₂ to Zr-UiO-66-NH₂.²¹ Later on, Biswas et al. reported Ce-UiO-66-NO₂,²³ DUT-52-(NO₂)₂,²⁶ and Zr-UiO-66-(NO₂)₂ (ref. 32) for quantitative H₂S detection in physiological systems and living cells. The lowest detection limit at 92.31 × 10⁻⁹ M was recently reported by Qian et al. using polymer mixed-matrix membranes (MMM) developed by the combination of Al-MIL-53-NO₂ nanoparticles and PVDF (polyvinylidene fluoride).³⁵ Among all these nitro-functionalized MOF compounds, linear relationships were obtained in the different ranges of H₂S concentrations with the fluorescence intensity, which can be applied for real quantitative H₂S sensing.

As most of these “turn-on” assays have been conducted in a suspension system, where the solid-state nitro-functionalized MOFs were dispersed in NaHS/Na₂S solutions, the fluorescence obtained was actually attributed to the combined effects of real fluorophores and non-luminescent particles in the suspension. Even though the researchers ascribed the fluorescence enhancement in the suspension to the formation of NH₂-functionalized MOFs, the direct evidence of –NH₂ existence within the MOFs after H₂S treatment is still lacking. In addition, misinterpretation of fluorescent signals may also arise for the following reasons: 1. the precipitation of undissolved particles in the suspension will change the pathway of excitation radiation, which further leads to fluctuations in the fluorescence intensity; 2. the irradiation could be absorbed by the non-luminous undissolved particles in the suspensions, which decreases the effective light for absorption by real fluorophores. This phenomenon is called inner-filter effect and the existence of high concentrations of non-luminous undissolved particles can reduce the fluorescence intensity; 3. except for absorption, the irradiation light is also scattered in all directions from the small molecules in the suspensions. In a case of a suspension with a small Stokes shift, the broad scattering peak near the excitation wavelength sometimes may overlap with the emission peak, which makes the fluorescence intensity from fluorophores difficult to separate. Therefore, it is sometimes difficult to identify the real fluorophores in such a complex suspension system.

To further explore the mechanism of the “turn-on” effect, Al-MIL-53-NO₂ was utilized in this study to react with H₂S in an aqueous system. Furthermore, the supernatant and the undissolved particles were isolated for the first time to locate the possibly formed –NH₂ groups. Meanwhile, the effect of the undissolved MOF particles on the fluorescence intensity in the sensing application was also analyzed. Interestingly, after being treated with H₂S (0.1–1.0 mM), the undissolved nanoparticles that remained in the suspension were proved to be Al-MIL-53-NO₂ rather than Al-MIL-53-NH₂ and no –NH₂ groups were formed at the surface of the residual solids, which made no contribution to fluorescence enhancement. The fluorescence “turn-on” effect was actually caused by the free BDC-NH₂ in the solution, which was reduced from BDC-NO₂ in the collapse of Al-MIL-53-NO₂ when exposed to H₂S. With the existence of the fluorophore BDC-NH₂ in the aqueous system, both the supernatant (limit of detection, LOD = 69.7 μM) and suspension (i.e., Al-MIL-53-NO₂ particles were dispersed evenly in aqueous solutions of NaHS by ultrasonication) (LOD = 69.3 μM) can be used to realize quantitative H₂S sensing within 0–0.7 mM H₂S. The result of this work unravels the origin of the “turn-on” effect of Al-MIL-53-NO₂ in H₂S sensing and explores new ways (separation of suspension into supernatant and undissolved residues) of fluorophore tracing in MOF-based detection.

2. Materials and methods

2.1 Chemicals

Sodium hydrosulfide hydrate (NaHS·xH₂O, ∼70%), 2-nitrobenzene-1,4-dicarboxylic acid (C₈H₅NO₆, BDC-NO₂, ≥99%), 2-aminobenzene-1,4-dicarboxylic acid (C₈H₇NO₄, BDC-NH₂, ≥99%), and aluminum chloride hexahydrate (AlCl₃·6H₂O, ≥99%) were obtained from Sigma Aldrich. N-Dimethylformamide (HCON(CH₃)₂, DMF) was purchased from VWR Corporation. Ethanol (C₂H₅OH, 190 proof) was obtained from Gold Shield. Deionized (DI) water was used to prepare the solutions. All chemicals were used without further purification.

2.2 Synthesis of Al-MIL-53-NO₂ and Al-MIL-53-NH₂

2.2.1 Al-MIL-53-NO₂. Al-MIL-53-NO₂ nanoparticles were synthesized according to a previous report.³⁵ Typically, BDC-NO₂ (42.2 mg, 0.2 mmol) was dissolved in 10 mL DMF by sonication for 1 minute. Then, AlCl₃·6H₂O (48.3 mg, 0.2 mmol) with the above solution was transferred into a 25 mL Teflon lined steel autoclave and heated at 180 °C for 12 hours. The obtained yellow particles were separated by centrifugation at 10 000 rpm for 10 minutes and washed with 15 mL DMF and 15 mL ethanol three times to remove unreacted chloride species and BDC-NO₂. Finally, the collected particles were dried in air at 200 °C for 6 hours to completely remove the attached DMF molecules.

2.2.2 Al-MIL-53-NH₂. Al-MIL-53-NH₂ was prepared based on a reported synthetic procedure.³⁸ AlCl₃·6H₂O (1.45 g, 6.0 mM) and 2-aminobenzene-1,4-dicarboxylic acid (BDC-NH₂) (1.1 g, 6.0 mM) were mixed in 30 mL DMF and then kept in a 100 mL Teflon lined steel autoclave at 150 °C for 5 hours. After being cooled to room temperature, the obtained yellow particles were washed with 15 mL DMF and 15 mL ethanol three times. The as-synthesized Al-MIL-53-NH₂ was activated in 60 mL DMF solution at 155 °C for 24 hours and then heated in air at 155 °C for 24 hours.

2.3 Materials' characterization

The morphologies of the samples were analyzed using a scanning electron microscope (SEM) with energy dispersive X-ray (EDX) spectroscopy (Su-70, Hitachi). The structure and crystallinity of Al-MIL-53-NO₂ were determined by powder X-ray diffraction (PXRD) (PANalytical X'Pert Pro MPD). The analysis of vibrations of the functional groups was carried out with a Fourier transform infrared (FT-IR) spectrometer (Nicolet iS50, Thermo Scientific). The photoluminescence properties of the samples were characterized by a fluorospectrometer (PTI QuantaMaster-400). X-ray photoelectron spectroscopic (XPS) measurement was conducted with a Thermo Scientific ESCALAB 250 to investigate the chemical state of the element on the surface of Al-MIL-53-NO₂. The pore size and surface area of Al-MIL-53-NO₂ were measured by using Autosorb iQ (Quantachrome Instruments).

2.4 H₂S fluorescence sensing measurements

NaHS was used as the H₂S source and dissolved in DI water to form a transparent solution.^23,25,35 In this paper, “H₂S treated” or “H₂S treatment” means that Al-MIL-53-NO₂ is suspended in NaHS solution with a specific concentration. In a typical experiment, 1.5 mg Al-MIL-53-NO₂ was suspended into the cuvette containing 3.0 mL NaHS solution with various concentrations (0.0–5.0 mM) for 2 hours before centrifugation. The corresponding supernatant and the suspended undissolved particles were separated by centrifugation at 12 000 rpm for 10 minutes. The re-collected particles were washed with water three times and re-dispersed into 3.0 mL DI water for further photoluminescence measurement. For all the fluorescence measurements, the excitation wavelength (λ_ex) was 339 nm and the emission spectra were recorded in the range of 380–580 nm. Both the emission and excitation slits were set to be 2 nm.

3. Results and discussion

3.1 Characterization of Al-MIL-53-NO₂

The crystallinity and structure of Al-MIL-53-NO₂ were determined by PXRD. As shown in Fig. 1a, all the peaks are consistent with those as reported in the previous references, where Al-MIL-53-NO₂ has a monoclinic crystal structure.^35,39,40 The main peak was located at ∼8.5°, which is ascribed to the (100) plane.³⁹ The crystals and morphologies were also characterized by scanning electron microscopy (SEM). It was found that the Al-MIL-53-NO₂ powders are composed of irregular nanoparticles with sizes ranging from 20 nm to 300 nm (Fig. 1b), which is also similar to that reported previously.³⁵ In addition to the crystallinity and morphology, the vibrational analysis of the surface functional groups was directly carried out by FT-IR. The absorption peak at 1542 cm⁻¹ (Fig. 1c) originated from the N=O stretching vibration of the nitro groups attached to the coordinated ligands, which demonstrated the successful incorporation of –NO₂ group into the MOF crystals.²⁶ The strong absorption bands at 1421 cm⁻¹ and 1619 cm⁻¹ are attributed to the symmetric and asymmetric –CO₂ stretching vibrations from the coordinated BDC-NO₂ linkers, respectively. It should be noted that the small peak at 3662 cm⁻¹ is ascribed to the μ₂-OH group.³⁹ The porosity of Al-MIL-53-NO₂ was further confirmed by the N₂ sorption measurements at 77 K. As shown in Fig. 1d, the rapid increase in the low relative pressure regions demonstrated the existence of micropores in Al-MIL-53-NO₂. The surface area of Al-MIL-53-NO₂ based on the Brunauer–Emmett–Teller (BET) theory was calculated to be 1157 m² g⁻¹, which provides a large number of active sites to interact with H₂S in the sensing process (detailed BET surface area calculation and plots are shown in ESI† S1 and Fig. S1).^41–44 Overall, the above results indicated that Al-MIL-53-NO₂ was synthesized successfully.

Fig. 1 (a) PXRD patterns and digital image (inset), (b) SEM image, (c) FT-IR spectrum, and (d) gas sorption isotherms of the Al-MIL-53-NO₂ nanoparticles.

3.2 H₂S sensing performance

The detection capability of Al-MIL-53-NO₂ for H₂S was investigated by performing fluorescence “turn-on” experiment in the aqueous suspension system. NaHS was used as the source of H₂S in this study.^25,35 Owing to the existence of electron-withdrawing nitro groups in the coordinated BDC-NO₂ ligand, Al-MIL-53-NO₂ is non-luminous and remains in the fluorescence “turn-off” state.³⁵ Once exposed to H₂S, –NO₂ is reduced to the electron-donating –NH₂ groups and fluorescence enhancement was observed based on the following reaction:

For H₂S sensing quantification, 1.5 mg Al-MIL-53-NO₂ was suspended in 3 mL DI water with various NaHS concentrations from 0.0 mM to 5.0 mM. To ensure the complete conversion of –NO₂ to –NH₂ in the sensing process, the suspension was kept in a sealed glass vial for 2 hours and then shaken to form a uniform suspension for photoluminescence measurement. The excitation wavelength was determined at 339 nm by scanning 1.0 mM H₂S treated Al-MIL-53-NO₂ suspension from 380 nm to 580 nm with the maximum emission peak located at 450 nm, as shown in Fig. S2.† All the fluorescence emission spectra were recorded in the range from 380 nm to 550 nm at the excitation wavelength of 339 nm in this study. It is found that H₂S showed a significant “turn-on” response to the Al-MIL-53-NO₂ suspensions (Fig. 2a–c). An excellent linear correlation (R² = 0.98498) was obtained between the fluorescence intensities and H₂S concentrations within 0.7 mM (Fig. 2d). Moreover, the limit of detection (LOD) was calculated to be 69.3 μM (detailed LOD calculations are shown in ESI† S2), which indicated that Al-MIL-53-NO₂ can be used for the quantitative detection of H₂S in the aqueous suspension system. It is noteworthy that the fluorescence intensity was vastly enhanced when the H₂S concentration was larger than 1.0 mM and tended to be constant within the range from 2.0 mM to 5.0 mM. The possible mechanisms for such significant “turn-on” fluorescence increment will be discussed in the next section.

Fig. 2 (a) Digital images of Al-MIL-53-NO₂ suspensions with various NaHS concentrations under visible light (up) and UV light (339 nm, down); (b) emission spectra of Al-MIL-53-NO₂ suspensions with various NaHS concentrations (excitation wavelength: 339 nm); (c and d) emission intensity versus NaHS concentration.

3.3 Exploration of the “Turn-On” effect

Most studies ascribe the “turn-on” effect to the conversion of –NO₂ to –NH₂ groups by “MOF transformation” and fluorescence signals of MOFs in the suspension were used for quantitative H₂S detection. Few researchers separated the undissolved particles to give direct evidence of –NH₂ groups on the surface of MOF after exposure to H₂S. In order to determine whether luminescence enhancement comes from the –NH₂ groups in MOF or the free ones in the solution, the suspensions were centrifuged at 12 000 rpm for 10 minutes to separate the undissolved particles and supernatant for further analysis (see Scheme 1). The exploration of the “Turn-On” effect was carried out through the analysis of both undissolved nanoparticles (section 3.3.1) and supernatant (section 3.3.2).

Scheme 1 Undissolved particles and supernatant were separated by centrifugation for further analysis.

3.3.1 Analysis of undissolved nanoparticles. The undissolved nanoparticles separated from the suspensions were washed with DI water three times and dried in air at 120 °C for two hours to remove the water molecules. The 0.5 mM and 1.0 mM H₂S treated Al-MIL-53-NO₂ were characterized by PXRD. The PXRD patterns matched well with the pristine Al-MIL-53-NO₂ nanoparticles and no apparent phase of the as-synthesized Al-MIL-53-NH₂ was observed (Fig. S3†). According to the Scherrer equation,
where τ is the mean size of the crystallite; K is the dimensionless shape factor (K = 0.89); λ is the X-ray wavelength (λ = 1.5406 Å); β is the line broadening at the maximum intensity or the so-called full width at half maximum (FWHM); θ is the Bragg angle, it was found that with H₂S concentration increased from 0.0 mM to 0.5 mM, and 1.0 mM, the FWHM of the peak at 8.5° becomes broader, and the corresponding calculated mean crystallite size within the (100) domain is reduced from 19.69 nm to 18.52 nm and 17.16 nm, respectively (Fig. 3(a–c)). The small shrinkage of the Al-MIL-NO₂ crystals could be caused by the original Al-MIL-53-NO₂ being degraded from outside to the core, where the peripheral bridged Al–O bonds were broken in the alkaline NaHS solution.⁴⁵ Similar morphologies of the recollected Al-MIL-53-NO₂ nanoparticles treated with 0.5 mM and 1.0 mM H₂S as compared to the pristine Al-MIL-53-NO₂ (Fig. 3(e–g)) confirmed that the residual Al-MIL-53-NO₂ nanoparticles kept the same shapes with the 1.0 mM H₂S treatment. When the H₂S concentration increased to 2.0 mM, the initial Al-MIL-53-NO₂ crystalline materials collapsed and an amorphous PXRD pattern was obtained (Fig. 3d). The large crystals disappeared, leaving much smaller amorphous particles, as shown in Fig. 3h. Interestingly, similar peak broadening phenomena were also observed in other nitro-functionalized MOFs (Zr-UiO-66-NO₂, Ce-UiO-66-NO₂, and Zr-UiO-66-(NO₂)₂) during H₂S detection,^22,23,32 which indicated that all these MOFs could be partially consumed and the particle sizes became smaller after being treated with H₂S. The particles with reduced crystal size and the released species into the solution could cause a dramatic change in the fluorescence intensity, which will be further discussed using Al-MIL-53-NO₂ in the next section (section 3.3.2).

Fig. 3 PXRD patterns and SEM images of undissolved nanoparticles collected from Al-MIL-53-NO₂ suspensions with various NaHS concentrations: 0.0 mM (a and e), 0.5 mM (b and f), 1.0 mM (c and g), and 2.0 mM (d and h). The scale bar for (e–h) is 500 nm.

The functional groups of the undissolved Al-MIL-53-NO₂ nanoparticles treated with 0.5 mM and 1.0 mM H₂S were analyzed by an FT-IR spectrometer. As shown in Fig. S4,† two major peaks were observed for Al-MIL-53-NH₂ at 3387 and 3499 cm⁻¹, corresponding to symmetric and asymmetric N–H stretching vibration, respectively. However, no apparent peaks were observed for the undissolved particles, which indicated that –NO₂ groups on undissolved Al-MIL-53-NO₂ nanoparticles were not reduced to –NH₂ groups after H₂S sensing.

Al-MIL-53-NO₂ before and after 1.0 mM H₂S treatment was further analyzed by X-ray photoelectron spectroscopy (XPS) (Fig. 4). For pristine Al-MIL-53-NO₂, the high-resolution spectrum of N 1s can be divided into two peaks located at 399.1 eV and 405.5 eV (Fig. 4a), which are attributed to N–C and –NO₂ groups within Al-MIL-53-NO₂, respectively.⁴⁶ After being exposed to 1.0 mM H₂S, no other peaks related to N 1s appeared (Fig. 4b) and the ratio of the peak area of N–O to C–N remained almost the same (before: 1.71 : 1; after 1.62 : 1), indicating that the undissolved Al-MIL-53-NO₂ nanoparticles in 1.0 mM H₂S solution shared similar surface chemical properties with the pristine unreacted Al-MIL-53-NO₂. Similar survey XPS pattern was also obtained for Al-MIL-53-NO₂ treated with 1.0 mM H₂S (Fig. S5†), where the peaks of binding energy at ∼532 eV, ∼405 eV, ∼285 eV, ∼119 eV, and ∼74 eV are ascribed to C 1s, N 1s, O 1s, Al 2s, and Al 2p, respectively. Moreover, the as-synthesized Al-MIL-53-NH₂ was used for comparison (Fig. S6†). The high-resolution N 1s spectrum of Al-MIL-53-NH₂ exhibits two peaks at 399.3 eV and 398.5 eV, which can be assigned to C–N bonds and NH₂ groups, respectively (Fig. S6†).⁴⁷ It is not surprising that this N 1s information related to Al-MIL-53-NH₂ cannot be observed in Fig. 4b since the 1.0 mM H₂S treated Al-MIL-53-NO₂ shared almost the same chemical properties with the unreacted one.

Fig. 4 High-resolution spectra of N 1s of Al-MIL-53-NO₂ before (a) and after (b) being treated with 1.0 mM H₂S.

Additionally, the recollected Al-MIL-53-NO₂ nanoparticles after interaction with various concentrations of H₂S were resuspended in 3.0 mL DI water for further photoluminescence measurement. It turns out that all the undissolved Al-MIL-53-NO₂ nanoparticles dispersed in water showed no fluorescence intensity and stayed in the “turn-off” mode (Fig. 5), which further confirmed that there were no –NH₂ groups at the surface of the undissolved H₂S-treated Al-MIL-53-NO₂ nanoparticles.

Fig. 5 Emission intensities at 450 nm (excitation wavelength: 339 nm) of aqueous solutions of undissolved nanoparticles (treated with 0.0 mM to 5.0 mM H₂S) (black columns) and Al-MIL-53-NO₂ suspension with 1.0 mM NaHS (red column).

Overall, the recollected particles from H₂S treatment suspension were analyzed by XRD, SEM, FT-IR, PL, and XPS, and these nanoparticles were proved to be almost identical to the unreacted Al-MIL-53-NO₂ (0.1–1.0 mM). In addition, there were no –NH₂ groups attached to these remaining Al-MIL-53-NO₂ based on the FT-IR and XPS data, indicating that –NO₂ on the surface of Al-MIL-53-NO₂ were not converted to –NH₂. Therefore, it can be concluded that these remaining unreacted suspended Al-MIL-53-NO₂ particles made no contribution to the fluorescence enhancement in the H₂S sensing process.

3.3.2 Supernatant analysis. Since the fluorescence enhancement in the initial suspension system does not come from the undissolved Al-MIL-53-NO₂ nanoparticles treated with H₂S, the isolated supernatants were used for further analysis. The photoluminescence properties of the isolated supernatants were measured and the fluorescence enhancement was also observed under the supernatant system (Fig. 6(a–c)). It is interesting to find that a good linear relationship was also obtained upto 0.7 mM and the LOD of H₂S detection with the supernatant was calculated to be 69.7 μM (Fig. 6d, S2†), which indicated that the supernatant can be used to realize quantitative H₂S sensing as well.

Fig. 6 (a) Digital images of Al-MIL-53-NO₂ supernatants separated from Al-MIL-53-NO₂ suspensions with various NaHS concentrations under visible light (up) and UV light (339 nm, down); (b) emission spectra of Al-MIL-53-NO₂ supernatants from Al-MIL-53-NO₂ supernatants with various NaHS concentrations (excitation wavelength: 339 nm); (c and d) emission intensity versus NaHS concentration.

To analyze the fluorophores in the supernatant, the transparent supernatant from 1.0 mM H₂S treated Al-MIL-53-NO₂ suspension was heated at 120 °C in air to evaporate the solvent and the extracts were characterized by FT-IR. Sodium 2-aminoterephthalate, which serves as the real fluorophore in the solution, was detected in the FT-IR spectrum of the residues (Fig. S7†).⁴⁸ The broad peak with a maximum at 3370 cm⁻¹ can be assigned to the amino groups interacting with the metal sodium after evaporation.^48,49 The completely deprotonated asymmetric and symmetric COO⁻ vibrations were observed at 1590 cm⁻¹ and 1371 cm⁻¹, respectively.⁴⁸ Moreover, a very small amount of –NO₂ groups was also detected, indicating that BDC-NO₂ from Al-MIL-53-NO₂ existed in the supernatant after 1.0 mM H₂S treatment (Fig. S7†). Considering that no –NH₂ groups were observed on the undissolved Al-MIL-53-NO₂ nanoparticles (section 3.3.1), these results suggest that the linker BDC-NO₂ was first released because of Al-MIL-53-NO₂ degradation in the solution and then reduced to BDC-NH₂ by H₂S treatment. Such partial structural decomposition of the initial Al-MIL-53-NO₂ (Al–(OOC–NO₂⁻) → Al–OH⁻ + NO₂–COO⁻ (BDC-NO₂)) could stem from the replacement of the linkers by hydroxide ions (OH⁻), which competitively bind to metal cations from the MOF clusters in the basic NaHS solution.^45,50 The conversion of –NO₂ to –NH₂ that occurs in the solution rather than on the MOF is probably because free BDC-NO₂ in the aqueous solution is more easily reduced by NaHS while the center of the Al–O clusters in solid-state Al-MIL-53-NO₂ increase the resistance towards NaHS attack. Therefore, it can be concluded that after H₂S treatment, a part of Al-MIL-53-NO₂ collapsed and the released BDC-NO₂ was reduced to BDC-NH₂, which gave rise to the fluorescence “turn-on” effect (Scheme 2).

Scheme 2 Schematic illustration of Al-MIL-53-NO₂ in H₂S sensing.

The results of the above supernatant analysis matched well with the peak broadening in the XRD pattern, as discussed in the previous section (section 3.3.1). The remaining particles with reduced crystal size and the released fluorophores in the solution could also have an effect on the fluorescence signal. It is noteworthy that, for Al-MIL-53-NO₂ treated with 1.0 mM H₂S, a much greater fluorescence intensity at 450 nm of the supernatant was achieved, as compared to that of the suspension (Fig. 7a). Such an enhancement in the emission of the supernatants can be also observed for Al-MIL-53-NO₂ upon treatment with H₂S at other concentrations (Fig. S8†). The reduced fluorescence of the suspension, as compared to the supernatant, is mainly caused by the unreacted non-luminescent Al-MIL-53-NO₂ dispersed in the aqueous system. The unreacted Al-MIL-53-NO₂ would absorb and scatter the incident excitation light and hence, decrease the photoluminescence intensity.⁵¹ To further understand the emission difference between the supernatant and the suspension, we define relative reduction (η) as the ratio of fluorescence intensity difference between the supernatant and the suspension to that of the suspension,

where I_sup is the fluorescence intensity of the suspension and I_sup is the fluorescence intensity of the supernatant (Fig. 7a). As shown in Fig. 7b, η increased from 2.25 at 0.1 mM (H₂S) and reached the maximum value of 3.64 at 0.7 mM (H₂S), and then decreased with further increment of H₂S concentration until the minimum value of 0.15 was obtained at 5.0 mM. Such variations in the relative reduction (η) could be explained by the consumption of Al-MIL-53-NO₂ in the H₂S sensing process. When the H₂S concentration was less than 0.7 mM, only a small amount of Al-MIL-53-NO₂ was consumed, so the scattering effect of the remaining Al-MIL-53-NO₂ in the suspension system (0.1–0.7 mM) was almost the same. Therefore, with more interaction with H₂S (0.1–0.7 mM), more corresponding reaction product BDC-NH₂ was released into the solution and the linear relationship was maintained from 0.1 mM to 0.7 mM (Fig. 2c), considering that the scattering could be ignored. When Al-MIL-53-NO₂ was exposed to 1.0 mM H₂S, more initial Al-MIL-53-NO₂ collapsed and the scattering effect weakened sharply, so a much larger fluorescence enhancement in the suspension system was obtained and the linear relationship between the fluorescence intensity and H₂S concentration could not be maintained (Fig. 2c). This “turning point” was also reflected in the sharp reduction in η from 3.64 at 0.7 mM to 2.36 at 1.0 mM (Fig. 7b). For 2.0–5.0 mM H₂S treatment suspensions, the initial Al-MIL-53-NO₂ was completely destroyed and the corresponding suspensions became more transparent (Fig. 2a), indicating that the scattering effect in this range could be neglected. Therefore, η tends to be stable and extremely small after being exposed to 2.0–5.0 mM H₂S (Fig. 7b).

Fig. 7 (a) Emission spectra (monitored at 339 nm) of Al-MIL-53-NO₂ suspended in 1.0 mM NaHS solution (black line), Al-MIL-53-NO₂ supernatant separated from 1.0 mM NaHS treated Al-MIL-53-NO₂ suspension (red line), undissolved particles separated from 1.0 mM NaHS treated Al-MIL-53-NO₂ suspension re-dispersed in aqueous solution (blue line); (b) the relative reduction in the fluorescence intensity of Al-MIL-53-NO₂ treated with various NaHS concentrations.

4. Conclusion

In summary, the photoluminescence properties of Al-MIL-53-NO₂ for H₂S sensing have been systematically analyzed in both the suspension and the supernatant system. The mechanism of Al-MIL-53-NO₂ probe for H₂S detection unraveled that the fluorescence enhancement was ascribed to BDC-NH₂ reduction from BDC-NO₂ with the collapse of Al-MIL-53-NO₂ on exposure to H₂S. The undissolved nanoparticles treated with 0.1–1.0 mM H₂S were proved to be Al-MIL-53-NO₂ rather than Al-MIL-53-NH₂ with XRD, FT-IR, and XPS measurements, and these non-luminescence nanoparticles make no contribution to fluorescence enhancement in the “turn-on” process. In particular, both the suspension (LOD = 69.3 μM) and the supernatant (LOD = 69.7 μM) can be used for quantitative H₂S detection within 0–0.7 mM. This work provides a new insight into the ways of fluorophore location exploration in probe-consuming sensing applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are very grateful for the financial support from National Science Foundation (CMMI-1727553).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ce01595g

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