Specific Recognition and Adsorption of Volatile Organic Compounds by Using MIL-125-Based Porous Fluorescence Probe Material

The severity of the volatile organic compounds (VOCs) issue calls for effective detection and management of VOC materials. Metal-organic frameworks (MOFs) are organic-inorganic hybrid crystals with promising prospects in luminescent sensing for VOC detection and identification. However, MOFs have limitations, including weak response signals and poor sensitivity towards VOCs, limiting their application to specific types of VOC gases. To address the issue of limited recognition and single luminosity for specific VOCs, we have introduced fluorescent guest molecules into MOFs as reference emission centers to enhance sensitivity. This composite material combines the gas adsorption ability of MOFs to effectively adsorb VOCs. We utilized (MIL-125/NH2-MIL-125) as the parent material for adsorbing fluorescent molecules and selected suitable solid fluorescent probes (FGFL-B1) through fluorescence enhancement using thioflavin T and MIL-125. FGFL-B1 exhibited a heightened fluorescence response to various VOCs through charge transfer between fluorescent guest molecules and ligands. The fluorescence enhancement effect of FGFL-B1 on tetrahydrofuran (THF) was particularly pronounced, accompanied by a color change from yellow to yellowish green in the presence of CCl4. FGFL-B1 demonstrated excellent adsorption properties for THF and CCl4, with saturated adsorption capacities of 655.4 mg g−1 and 811.2 mg g−1, respectively. Furthermore, FGFL-B1 displayed strong luminescence stability and reusability, making it an excellent sensing candidate. This study addresses the limitations of MOFs in VOC detection, opening avenues for industrial and environmental applications.


Introduction
Environmental pollution has become a pressing global issue with the release of hazardous chemical pollutants into the atmosphere through industrial emissions, steam leakage, and fossil fuel combustion [1,2].Among these pollutants, volatile organic compounds (VOCs) pose a significant threat to human health [3].There is an urgent need to develop VOC detection sensors with high sensitivity capable of detecting a variety of gases.The selection of sensor materials plays a crucial role in achieving efficient VOC detection [4].Metalorganic frameworks (MOFs) are highly effective porous materials for the removal of harmful gases [5][6][7].They offer numerous advantages over traditional materials such as zeolite-type materials [8], mesoporous silica [9], resins [10], and activated carbons with additives [11,12].Unlike zeolite-related inorganic hybrid materials that require a template for formation, MOFs rely primarily on a solvent as the main templating molecule [13,14].To ensure efficient capture of harmful gases through interaction with MOFs, it is crucial to employ MOFs with appropriate pore size and shape [15].MOFs are functional materials known for their tailorable porosity, making them attractive candidates for VOC capture and sensing [16].
Introducing luminescent groups in MOFs as reference emission centers is a simple and effective strategy to enhance the luminescence sensitivity of MOF sensors, allowing for widespread applications.Yao et al. used a porous supramolecular framework to adsorb guest dye molecules, creating solid materials with high fluorescence.This provides a simple, low-cost, and efficient way to create luminescent solid materials [38].Similarly, Tian created solid luminous materials by adsorbing guest dye molecules into a symmetrical urea-based polypore supramolecular assembly of tetramethyl calabinus [6] for the detection of VOCs [39].By adopting this method, luminescent groups can be introduced into MOFs as reference emission centers, effectively addressing the issue of limited luminescence and specific VOC recognition in traditional MOFs.Furthermore, taking advantage of the excellent gas adsorption performance of MOFs, there is potential to utilize them as parent materials for simultaneous VOC detection and adsorption.MIL-125, a popular Ti-based MOF, stands out due to its high thermal stability, solid framework, large specific surface area, and two types of pore cages (effective reachable diameters of 12.55 Å and 6.13 Å, respectively, are octahedral and tetrahedral cages, connected by triangular windows of 5-7 Å).These properties make MIL-125 an exceptional material for gas adsorption [40].In 2014, Kim et al. reported the adsorption and catalytic properties of MIL-125 and NH 2 -MIL-125 [41].Recently, Kim et al. found that the formaldehyde adsorption ability of NH 2 -MIL-125 was significantly better than that of other MOF materials [42].However, MIL-125 and NH 2 -MIL-125 have not been widely utilized for VOC detection.Our objective is to employ MIL-125 and NH 2 -MIL-125 as parent materials for developing fluorescent materials capable of detecting and adsorbing VOCs.
The fluorescence probe FGFL-B 1 (composed of MIL-125 and thioflavin T dye molecules) showed the strongest fluorescence enhancement effect on tetrahydrofuran (THF), and it exhibited distinct fluorescence discoloration when specifically recognizing CCl 4 .This method not only has a simple material fabrication step and strong operability but also expands the application range of MIL-125 and other MOF materials as fluorescent probes.The study aimed to address practical challenges in VOC detection and treatment, offering potential applications in industrial and living environments.

Precursor Synthesis
The MIL-125 and NH 2 -MIL-125 were prepared by a solvothermal method, which was modified from a previous report [43].
Terephthalate acid (H 2 BDC, 3 mmol) was completely dissolved in N, N-dimethylformamide (DMF, 9 mL), and anhydrous methanol (3 mL).Then, 0.75 mmol of tetrabutyl titanate (TBT) was added to the solution under an N 2 atmosphere to form a suspension.After vigorous stirring for 30 min, the suspension was reacted in a 50 mL Teflon-sealed autoclave at 423 K for 24 h.Finally, the material was cooled down to room temperature.The white MIL-125 powders were obtained after being washed with DMF and anhydrous methanol, dried at 333 K, and then vacuum dried at 423 K for 3 h (In order to remove water and DMF).Yellow NH 2 -MIL-125 powders were prepared in the same way, except that H 2 BDC was replaced with NH 2 -terephthalate acid (H 2 ATA).

Adsorption and Fluorescence Response of FGFL-A 1-5 /FGFL-B 1-5 for VOCs
A glass bottle (10 mL) containing 50 mg of FGFL-A 1-5 /FGFL-B 1-5 was placed into the sealable glass container (100 mL) and vacuumed with a vacuum pump until the weight of the FGFL-A 1-5 /FGFL-B 1-5 remained the same.Then, another glass bottle (10 mL) containing a small amount of volatile organic compounds (C 6 H 6 , C 6 H 5 CH 3 , THF, CCl 4 , C 2 Cl 2 , HCHO) was placed in the vacuum-sealed glass container and sealed for 3.5 h.After the adsorption of different VOCs, the fluorescence of FGFL-A 1-5 /FGFL-B 1-5 was detected successively.The weight variation was measured, and corresponding solid-state fluorescence spectra were determined at intervals of approximately 20-240 min over several hours in order to obtain the vapor adsorption profile.

Measurement of Fluorescence Spectra of Solid
Fluorescence spectra of solid FGFL-A 1-5 /FGFL-B 1-5 before and after adsorbing VOCs were recorded at room temperature by using a fluorescence spectrometer (F-320 PL, Guangdong Technology, Guangzhou, China), respectively.

Characterization
A powder X-ray diffractometer (PXRD, Bruker D8 Advance) was used to characterize the crystal structure of the synthesized catalysts by Cu Kα radiation operated at 40 kV and 40 mA.Patterns were collected using a scan speed of 2 s/step and a step size of 0.02 • at a 2θ range from 5 • to 50 • .The morphologies and microstructures of the prepared materials were observed using a scanning electron microscope (SEM, Gemini SEM300, ZEISS, Germany).
The Brunauer-Emmett-Teller (BET) surface areas of the samples were measured using a nitrogen adsorption instrument (Micromeritics ASAP 2460 analyzer, Beishide, Beijing, China) at liquid-nitrogen temperature.The IR experiments were carried out on a Nicolet 380 FT-IR spectrometer.The IR spectra of pure samples were collected without diluting with KBr.Photoluminescence (PL) spectra were recorded using a Guangdong Technology F-320 PL spectrophotometer with an excitation wavelength of 385 nm.All the liquid-state 1 H NMR spectra in this work were obtained on a Bruker 400 MHz spectrometer in dimethyl sulfoxide solution.Ultraviolet-visible (UV-vis) diffuse reflectance spectra (DRS) of the prepared samples were recorded using a UV-vis spectrometer (UV-2600, Shimadzu, Kyoto, Japan) with a background of BaSO 4 .We successfully synthesized MIL-125 and NH 2 -MIL-125 by the solvothermal method [40].The powder X-ray diffraction (PXRD) patterns are shown in Figure 1.The main diffraction peaks of NH 2 -MIL-125 (Figure 1a) and MIL-125 (Figure 1b) were consistent with the simulated spectra, indicating that both MIL-125 and NH 2 -MIL-125 have been successfully prepared [40].Different fluorescent probe materials were prepared by introducing dye molecules into MIL-125/NH 2 -MIL-125 by the impregnation method.The PXRD pattern of NH 2 -MIL-125/MIL-125 (FGFL-A 1-5 /FGFL-B 1-5 ) remained unchanged after the adsorption of fluorescent dye molecules, indicating that the crystal structure of NH 2 -MIL-125/MIL-125 was not affected by the adsorption process.

Characterization
A powder X-ray diffractometer (PXRD, Bruker D8 Advance) was used to characterize the crystal structure of the synthesized catalysts by Cu Kα radiation operated at 40 kV and 40 mA.Patterns were collected using a scan speed of 2 s/step and a step size of 0.02° at a 2θ range from 5° to 50°.The morphologies and microstructures of the prepared materials were observed using a scanning electron microscope (SEM, Gemini SEM300, ZEISS, Germany).The Brunauer-Emmett-Teller (BET) surface areas of the samples were measured using a nitrogen adsorption instrument (Micromeritics ASAP 2460 analyzer, Beishide, Beijing, China) at liquid-nitrogen temperature.The IR experiments were carried out on a Nicolet 380 FT-IR spectrometer.The IR spectra of pure samples were collected without diluting with KBr.Photoluminescence (PL) spectra were recorded using a Guangdong Technology F-320 PL spectrophotometer with an excitation wavelength of 385 nm.All the liquid-state 1 H NMR spectra in this work were obtained on a Bruker 400 MHz spectrometer in dimethyl sulfoxide solution.Ultraviolet-visible (UV-vis) diffuse reflectance spectra (DRS) of the prepared samples were recorded using a UV-vis spectrometer (UV-2600, Shimadzu, Kyoto, Japan) with a background of BaSO4.

Structure and Property of FGFL-A1-5/FGFL-B1-5
We successfully synthesized MIL-125 and NH2-MIL-125 by the solvothermal method [40].The powder X-ray diffraction (PXRD) patterns are shown in Figure 1.The main diffraction peaks of NH2-MIL-125 (Figure 1a) and MIL-125 (Figure 1b) were consistent with the simulated spectra, indicating that both MIL-125 and NH2-MIL-125 have been successfully prepared [40]  To investigate the impact of dye molecules on the morphology of NH2-MIL-125/MIL-125, we examined both the pristine materials and composites using a Scanning Electron Microscope (SEM).Figure 2 shows that MIL-125 and FGFL-B1-5 exhibit similar disc-like shapes with a diameter of approximately 300 nm and a thickness of around 100 nm.Similarly, the morphology and size of NH2-MIL-125 and FGFL-A1-5, as shown in Figure S1, remained unchanged.The surface morphology and structure, as observed from the PXRD and SEM patterns, also did not show significant alterations, and no dye was detected.To investigate the impact of dye molecules on the morphology of NH 2 -MIL-125/MIL-125, we examined both the pristine materials and composites using a Scanning Electron Microscope (SEM).Figure 2 shows that MIL-125 and FGFL-B 1-5 exhibit similar disc-like shapes with a diameter of approximately 300 nm and a thickness of around 100 nm.Similarly, the morphology and size of NH 2 -MIL-125 and FGFL-A 1-5 , as shown in Figure S1, remained unchanged.The surface morphology and structure, as observed from the PXRD and SEM patterns, also did not show significant alterations, and no dye was detected.

Fluorescence of FGFL-A 1-5 /FGFL-B 1-5
The fluorescence spectra of FGFL-A 1-5 /FGFL-B 1-5 were tested to identify the optimal fluorescent probe materials.The results revealed that ThT exhibited a strong fluorescence enhancement effect when adsorbed onto   [48][49][50].In order to confirm this phenomenon, we tested the solid UV-vis absorption spectra of ThT, MIL-125, and FGFL-B1, respectively.As can be seen from Figure S7a, the absorption peak of FGFL-B1 formed by the combination of ThT and MIL-125 had a significant red shift compared with MIL-125.As a chromophore, ThT is prone to electron transition.The FGFL-B1 material formed by  [48][49][50].In order to confirm this phenomenon, we tested the solid UV-vis absorption spectra of ThT, MIL-125, and FGFL-B 1 , respectively.As can be seen from Figure S7a, the absorption peak of FGFL-B 1 formed by the combination of ThT and MIL-125 had a significant red shift compared with MIL-125.As a chromophore, ThT is prone to electron transition.The FGFL-B 1 material formed by the combination of MIL-125 and ThT had a significant red shift relative to the absorption wavelength of MIL-125 and a significant blue shift relative to the absorption wavelength of ThT, which corresponded to the above phenomenon [51,52].The band gaps of ThT, MIL-125, and FGFL-B 1 can be calculated from the solid UV-vis absorption spectra using the Kubelka-Munk function [53].It can be seen from Figure S7b that by combining with ThT, the band gap of MIL-125 was reduced from 3.4 eV to 2.4 eV.

Fluorescence Sensors and Storage Performance of VOCs over FGFL-B 1
Environmental pollution is a pressing global issue caused by the discharge of harmful chemical pollutants into the atmosphere.Volatile organic compounds (benzene, toluene, tetrahydrofuran, carbon tetrachloride, dichloromethane, methylene chloride, formaldehyde) are the main cause of environmental air pollution and pose risks to human health [54][55][56].For instance, THF exhibits stimulatory and anesthetic effects.Inhalation causes upper respiratory irritation, nausea, dizziness, headache, and central nervous system depression [57,58].CCl 4 and its decomposition products can be absorbed through the respiratory tract, and skin contact can result in rapid absorption.CCl 4 is particularly damaging to peripheral nerves, especially the liver [59].Therefore, the development of fluorescent probes and porous materials for the detection and adsorption of VOC gases is crucial to addressing this issue.
Nanomaterials 2023, 13, x FOR PEER REVIEW 7 of the combination of MIL-125 and ThT had a significant red shift relative to the absorpti wavelength of MIL-125 and a significant blue shift relative to the absorption waveleng of ThT, which corresponded to the above phenomenon [51,52].The band gaps of Th MIL-125, and FGFL-B1 can be calculated from the solid UV-vis absorption spectra usi the Kubelka-Munk function [53].It can be seen from Figure S7b that by combining wi ThT, the band gap of MIL-125 was reduced from 3.4 eV to 2.4 eV.

Fluorescence Sensors and Storage Performance of VOCs over FGFL-B1
Environmental pollution is a pressing global issue caused by the discharge harmful chemical pollutants into the atmosphere.Volatile organic compounds (benzen toluene, tetrahydrofuran, carbon tetrachloride, dichloromethane, methylene chlorid formaldehyde) are the main cause of environmental air pollution and pose risks to h man health [54][55][56].For instance, THF exhibits stimulatory and anesthetic effects.Inh lation causes upper respiratory irritation, nausea, dizziness, headache, and centr nervous system depression [57,58].CCl4 and its decomposition products can be a sorbed through the respiratory tract, and skin contact can result in rapid absorptio CCl4 is particularly damaging to peripheral nerves, especially the liver [59].Therefo the development of fluorescent probes and porous materials for the detection and a sorption of VOC gases is crucial to addressing this issue.
In this paper, fluorescent molecules were selected through fluorescence spect testing to determine the most suitable candidates.FGFL-A1 and FGFL-B1 were chosen fluorescent probes for investigating the detection and adsorption properties of six com mon volatile organic compounds (VOCs): dichloromethane (CH2Cl2), carbon tetrach ride (CCl4), tetrahydrofuran (THF), formaldehyde (HCHO), benzene (C6H6), and tolue (C6H5CH3).As shown in Figure 6a,b, FGFL-B1, and FGFL-A1 demonstrated varying d grees of fluorescence response towards the VOCs.However, the fluorescence enhanc ment effect of FGFL-A1 was very low, which was negligible compared with that FGFL-B1 (Figure 6c,d).Therefore, FGFL-B1 is considered the most suitable fluoresce probe material.After FGFL-B1 and FGFL-B1 adsorbed with different VOCs were form into a suspension in acetonitrile solvent, different fluorescence responses could be clea ly observed under UV light (Figure 6e).Notably, FGFL-B1 showed the most significa fluorescence enhancement effect towards THF gas, with fluorescence intensity 36 tim higher than FGFL-B1 (Figure 6b).Additionally, the adsorption of CCl4 gas induced a n ticeable red shift in FGFL-B1 (Figure 6b).This red shift resulted in a distinct transfo mation in fluorescence color from yellow to yellowish green (After the adsorption CCl4, the maximum emission peak of FGFL-B1 undergoes a red shift from 530 nm to 5 nm, while it remained unchanged at 530 nm after the adsorption of other VOCs).demonstrate the color transformation more effectively, test strips of FGFL-B1 were fab cated, which visibly turned yellowish green upon adsorption of CCl4 (Figure 6f).Th In this paper, fluorescent molecules were selected through fluorescence spectra testing to determine the most suitable candidates.FGFL-A 1 and FGFL-B 1 were chosen as fluorescent probes for investigating the detection and adsorption properties of six common volatile organic compounds (VOCs): dichloromethane (CH 2 Cl 2 ), carbon tetrachloride (CCl 4 ), tetrahydrofuran (THF), formaldehyde (HCHO), benzene (C 6 H 6 ), and toluene (C 6 H 5 CH 3 ).As shown in Figure 6a,b, FGFL-B 1, and FGFL-A 1 demonstrated varying degrees of fluorescence response towards the VOCs.However, the fluorescence enhancement effect of FGFL-A 1 was very low, which was negligible compared with that of FGFL-B 1 (Figure 6c,d).Therefore, FGFL-B 1 is considered the most suitable fluorescent probe material.After FGFL-B 1 and FGFL-B 1 adsorbed with different VOCs were formed into a suspension in acetonitrile solvent, different fluorescence responses could be clearly observed under UV light (Figure 6e).Notably, FGFL-B 1 showed the most significant fluorescence enhancement effect towards THF gas, with fluorescence intensity 36 times higher than FGFL-B 1 (Figure 6b).Additionally, the adsorption of CCl 4 gas induced a noticeable red shift in FGFL-B 1 (Figure 6b).This red shift resulted in a distinct transformation in fluorescence color from yellow to yellowish green (After the adsorption of CCl 4 , the maximum emission peak of FGFL-B 1 undergoes a red shift from 530 nm to 560 nm, while it remained unchanged at 530 nm after the adsorption of other VOCs).To demonstrate the color transformation more effectively, test strips of FGFL-B 1 were fabricated, which visibly turned yellowish green upon adsorption of CCl 4 (Figure 6f).This test strip method offers the potential for faster and easier detection of simulated factory or indoor gas leaks.
FGFL-B 1 was formed by the recombination of ThT (4 Å) within the MIL-125 channel [60,61].Upon the entry of VOC gas into the FGFL-B 1 channel, collisions with ThT molecules generate an effect [62].The interaction between ThT and MIL-125 restricts the rotation of the benzene ring, thereby limiting non-radiative energy consumption pathways and reinforcing radiative transitions.As a result, fluorescence is substantially enhanced [63][64][65].To illustrate this effect, we present the example of THF adsorption (Figure S8).Following the adsorption of THF, the fluorescence intensity of FGFL-B 1 was significantly enhanced, accompanied by a change in the full width at half maximum (FWHM) from 73.63 nm to 85.04 nm [66].Electronegative molecules induce a pronounced red shift in the spectra of the material.CCl 4 , known for its strong electronegativity, triggers intramolecular charge transfer of ThT, resulting in a red shift effect and fluorescence color transformation [67,68].Therefore, FGFL-B 1 can selectively recognize CCl 4 .These findings demonstrate that the incorporation of fluorescent guest molecules (ThT) as luminescent clusters significantly improves the fluorescence detection performance of materials and overcomes the limitations of conventional MOFs, which only exhibit a fluorescence response to specific gases (Figure 7).anomaterials 2023, 13, x FOR PEER REVIEW 8 of 14 test strip method offers the potential for faster and easier detection of simulated factory or indoor gas leaks.FGFL-B1 was formed by the recombination of ThT (4 Å) within the MIL-125 channe [60,61].Upon the entry of VOC gas into the FGFL-B1 channel, collisions with ThT mole cules generate an effect [62].The interaction between ThT and MIL-125 restricts the rota tion of the benzene ring, thereby limiting non-radiative energy consumption pathways and reinforcing radiative transitions.As a result, fluorescence is substantially enhanced transformation [67,68].Therefore, FGFL-B1 can selectively recognize CCl4.These findings demonstrate that the incorporation of fluorescent guest molecules (ThT) as luminescent clusters significantly improves the fluorescence detection performance of materials and overcomes the limitations of conventional MOFs, which only exhibit a fluorescence response to specific gases (Figure 7).The titrated fluorescence spectra of FGFL-B1 were analyzed in detail to monitor the real-time adsorption of THF and CCl4 (Figure 8a,b).The fluorescence intensity of solid FGFL-B1 at 530 nm and 560 nm increased with the increase in the adsorption time of THF and CCl4.As shown in Figure 8e,f, it took approximately 2 h for FGFL-B1 to reach its maximum fluorescence intensity with THF and CCl4, while only about 20 min were needed to shift the peak position to 560 nm.Although the fluorescence intensity and peak position of FGFL-B1 remained unchanged, the absorption rates of THF and CCl4 continued to increase within 3 h (Figure 8c,d).Comparing Figure 8c-f, it can be observed that the adsorption amount and time of THF and CCl4 required to achieve the maximum equilibrium value of FGFL-B1 fluorescence intensity are much lower than those required by saturated adsorption.The size of ThT (4 Å) is large, and the barrier entering porous MIL-125 is much larger than that of VOCs molecules.Therefore, the amount of ThT that can interact with MIL-125 is significantly lower than that of VOC molecules.Therefore, the time required for fluorescence to reach maximum equilibrium and the amount of gas is far less than the time of saturation adsorption and gas volume.By comparing the mass of FGFL-B1 before and after the saturated adsorption of THF and CCl4, we calculated that the adsorption properties of FGFL-B1 for THF and CCl4 were 655 mg g -1 and 812 mg g -1 , respectively (Table S2).The adsorption performance of other VOC gases is shown in Table S1.Additionally, the ΔI curves representing the adsorption capacity of THF and CCl4 on FGFL-B1 (Figure 8g,h) were plotted using the adsorption-time curve data (Figure 8c,d) and intensity (ΔI) versus time curve data (Figure 8e,f) obtained from the interpolation method.The detection limits (DL) [39] of FGFL-B1 for THF and CCl4 were determined to be 1.41 × 10 -4 mol g -1 and 4.66 × 10 -5 mol g -1 , respectively.The titrated fluorescence spectra of FGFL-B 1 were analyzed in detail to monitor the real-time adsorption of THF and CCl 4 (Figure 8a,b).The fluorescence intensity of solid FGFL-B 1 at 530 nm and 560 nm increased with the increase in the adsorption time of THF and CCl 4 .As shown in Figure 8e,f, it took approximately 2 h for FGFL-B 1 to reach its maximum fluorescence intensity with THF and CCl 4 , while only about 20 min were needed to shift the peak position to 560 nm.Although the fluorescence intensity and peak position of FGFL-B 1 remained unchanged, the absorption rates of THF and CCl 4 continued to increase within 3 h (Figure 8c,d).Comparing Figure 8c-f, it can be observed that the adsorption amount and time of THF and CCl 4 required to achieve the maximum equilibrium value of FGFL-B 1 fluorescence intensity are much lower than those required by saturated adsorption.The size of ThT (4 Å) is large, and the barrier entering porous MIL-125 is much larger than that of VOCs molecules.Therefore, the amount of ThT that can interact with MIL-125 is significantly lower than that of VOC molecules.Therefore, the time required for fluorescence to reach maximum equilibrium and the amount of gas is far less than the time of saturation adsorption and gas volume.By comparing the mass of FGFL-B 1 before and after the saturated adsorption of THF and CCl 4 , we calculated that the adsorption properties of FGFL-B 1 for THF and CCl 4 were 655 mg g −1 and 812 mg g −1 , respectively (Table S2).The adsorption performance of other VOC gases is shown in Table S1.Additionally, the ∆I curves representing the adsorption capacity of THF and CCl 4 on FGFL-B 1 (Figure 8g,h) were plotted using the adsorption-time curve data (Figure 8c,d) and intensity (∆I) versus time curve data (Figure 8e,f) obtained from the interpolation method.The detection limits (DL) [39] of FGFL-B 1 for THF and CCl 4 were determined to be 1.41 × 10 −4 mol g −1 and 4.66 × 10 −5 mol g −1 , respectively.
The detection of VOCs by FGFL-B 1 is based on a fluorescence reaction triggered by the interaction of gases after entering the material.VOCs can be detached from FGFL-B 1 , allowing the material to be recycled [39].To validate this, we conducted adsorption and desorption experiments using THF and CCl 4 as examples.Lifetime tests demonstrated that the adsorption and desorption of THF and CCl 4 on FGFL-B 1 were not only reversible but also highly stable.Numerous adsorption and desorption experiments consistently indicated that the detection properties of FGFL-B 1 for THF and CCl 4 , such as fluorescence intensity, remained unchanged (Figure S9c,d).Furthermore, a comparison of the PXRD patterns of FGFL-B 1 before and after the adsorption and desorption of THF and CCl 4 revealed no significant changes, suggesting that the adsorption or desorption of THF and CCl 4 had minimal impact on FGFL-B 1 (Figure S9a,b).These results provided strong evidence for the excellent stability of FGFL-B 1 .

Conclusions
We investigated the fluorescence enhancement effect of FGFL-B 1 , a composite of the ThT dye molecules and MIL-125 H•••O conjugation, on VOCs by introducing various fluorescent dye molecules into MIL-125 and NH 2 -MIL-125 synthesized via the solvothermal method.The presence of steric hindrance in ThT molecules both hampers and enhances their intramolecular rotation, resulting in a remarkable fluorescence enhancement effect on THF.The fluorescence intensity increases by a factor of 36 upon THF adsorption.Moreover, due to the strong electronegativity of CCl 4 , FGFL-B 1 exhibits selective recognition of CCl 4 , leading to a distinct yellow-to-yellowish-green fluorescence color change.Remarkably, FGFL-B 1 demonstrates excellent adsorption capacities for THF and CCl 4 , with values of 655.4 mg g −1 and 811.2 mg g −1 , respectively.As a result, the prepared porous fluorescence probe material, FGFL-B 1 , holds great potential for detecting and adsorbing mixed gases in both industrial and domestic environments.This method opens up new possibilities for fluorescence detection of adsorption properties in Ti-based MOF materials and other MOF materials.

Figure 4 .
Figure 4. (a) Fluorescence spectra of MIL-125 and FGFL-B1-5, (b) Fluorescence spectra of FGFL-A1 and FGFL-B1.To analyze whether ThT undergoes simple physical adsorption or chemical coupling with MIL-125/NH2-MIL-125, we conducted IR spectroscopy on ThT, NH2-MIL-125/MIL-125, and FGFL-A1-5/FGFL-B1-5.As shown in Figure5, the carboxyl group characteristic peak of NH2-MIL-125/MIL-125 showed at 3.140-3.204(log1380-log1600 cm -1 )[40].An evident larger wavenumber shift at 3.23 (log1700 cm -1 ) was observed in the infrared spectra of FGFL-A1-5/FGFL-B1-5, indicating the adsorption and coupling of ThT molecules with NH2-MIL-125/MIL-125.The larger wavenumber shift was more pronounced in the FGFL-B1 infrared spectra.Figure S5 displays the Raman spectra of NH2-MIL-125/MIL-125, ThT, and FGFL-A1/FGFL-B1, with FGFL-A1/FGFL-B1 revealing peaks from both NH2-MIL-125/MIL-125 and the fluorescent dye ThT.Additionally, Figure S6a,b illustrates the liquid nuclear magnetic hydrogen spectra of ThT, MIL-125, and FGFL-B1.The peak at 8.14 ppm of ThT belongs to the H peak on the benzene ring on the benzylamine ring; the H peak (8.06 ppm) on the benzene ring of the organic ligand terephthalic acid of MIL-125 was significantly offset in FGFL-B1, indicating that ThT forms H•••O conjugations with MIL-125 [44].The ThT molecule consists of a benzylamine ring and a phenyl sulfide ring.When the benzylamine ring and phenyl sulfide ring rotate freely around the C-C bond in the natural state, the ThT fluorescence signal is weak.Once this rotation is limited by some structures, the ThT fluorescence is enhanced [45-47].When ThT fluorescent guest molecules were introduced into NH2-MIL-125/MIL-125 as light sources and coupled with them via H•••O interaction, the restriction of intramolecular rotation (RIR) of ThT fluorescence was caused to some extent.Charge transfer from fluorescent guest molecules to organic ligands was formed, resulting in a strong fluorescence response[48][49][50].In order to confirm this phenomenon, we tested the solid UV-vis absorption spectra of ThT, MIL-125, and FGFL-B1, respectively.As can be seen from FigureS7a, the absorption peak of FGFL-B1 formed by the combination of ThT and MIL-125 had a significant red shift compared with MIL-125.As a chromophore, ThT is prone to electron transition.The FGFL-B1 material formed by

Figure 4 .
Figure 4. (a) Fluorescence spectra of MIL-125 and FGFL-B 1-5 , (b) Fluorescence spectra of FGFL-A 1 and FGFL-B 1 .To analyze whether ThT undergoes simple physical adsorption or chemical coupling with MIL-125/NH 2 -MIL-125, we conducted IR spectroscopy on ThT, NH 2 -MIL-125/MIL-125, and FGFL-A 1-5 /FGFL-B 1-5 .As shown in Figure 5, the carboxyl group characteristic peak of NH 2 -MIL-125/MIL-125 showed at 3.140-3.204(log1380-log1600 cm −1 ) [40].An evident larger wavenumber shift at 3.23 (log1700 cm −1 ) was observed in the infrared spectra of FGFL-A 1-5 /FGFL-B 1-5 , indicating the adsorption and coupling of ThT molecules with NH 2 -MIL-125/MIL-125.The larger wavenumber shift was more pronounced in the FGFL-B 1 infrared spectra.Figure S5 displays the Raman spectra of NH 2 -MIL-125/MIL-125, ThT, and FGFL-A 1 /FGFL-B 1 , with FGFL-A 1 /FGFL-B 1 revealing peaks from both NH 2 -MIL-125/MIL-125 and the fluorescent dye ThT.Additionally, Figure S6a,b illustrates the liquid nuclear magnetic hydrogen spectra of ThT, MIL-125, and FGFL-B 1 .The peak at 8.14 ppm of ThT belongs to the H peak on the benzene ring on the benzylamine ring; the H peak (8.06 ppm) on the benzene ring of the organic ligand terephthalic acid of MIL-125 was significantly offset in FGFL-B 1 , indicating that ThT forms H•••O conjugations with MIL-125 [44].The ThT molecule consists of a benzylamine ring and a phenyl sulfide ring.When the benzylamine ring and phenyl sulfide ring rotate freely around the C-C bond in the natural state, the ThT fluorescence signal is weak.Once this rotation is limited by some structures, the ThT fluorescence is enhanced [45-47].When ThT fluorescent guest molecules were introduced into NH 2 -MIL-125/MIL-125 as light sources and coupled with them via H•••O interaction, the restriction of intramolecular rotation (RIR) of ThT fluorescence was caused to some extent.Charge transfer from fluorescent guest molecules to organic ligands was formed, resulting in a strong fluorescence response[48][49][50].In order to confirm this phenomenon, we tested the solid UV-vis absorption spectra of ThT, MIL-125, and FGFL-B 1 , respectively.As can be seen from FigureS7a, the absorption peak of FGFL-B 1 formed by the combination of ThT and MIL-125 had a significant red shift compared with MIL-125.As a chromophore, ThT is prone to electron transition.The FGFL-B 1 material formed by the combination of MIL-125 and ThT had a significant red shift relative to the absorption wavelength of MIL-125 and a significant blue shift relative to the absorption wavelength of ThT, which corresponded to the above phenomenon[51,52].The band gaps of ThT, MIL-125, and FGFL-B 1 can be calculated from the solid UV-vis absorption spectra using the Kubelka-Munk function[53].It can be seen from FigureS7bthat by combining with ThT, the band gap of MIL-125 was reduced from 3.4 eV to 2.4 eV.

Figure 6 .
Figure 6.A general survey of the fluorescence spectra of (a) FGFL-A 1 and (b) FGFL-B 1 loaded with the six selected VOCs.Relative fluorescence intensities of (c) FGFL-A 1 and (d) FGFL-B 1 in response to the six selected VOCs.(e) Photos under UV (365 nm) of FGFL-B 1 and FGFL-B 1 loaded with six selected VOCs.(f) Photos under UV (365 nm) of FGFL-B 1 (left) and FGFL-B 1 loaded with CCl 4 (right).

Figure 8 .
Figure 8.The titrated fluorescence spectra of FGFL-B1 loaded (a) THF and (b) CCl4.Adsorption profile of the loading of (c) THF and (d) CCl4 in FGFL-B1 upon increasing the adsorption time.Change in the fluorescence intensity of FGFL-B1 by loading (e) THF and (f) CCl4 upon increasing the adsorption time.Plot of ΔI versus the amount of (g) THF and (h) CCl4 adsorbed by solid FGFL-B1.

Figure 8 .
Figure 8.The titrated fluorescence spectra of FGFL-B 1 loaded (a) THF and (b) CCl 4 .Adsorption profile of the loading of (c) THF and (d) CCl 4 in FGFL-B 1 upon increasing the adsorption time.Change in the fluorescence intensity of FGFL-B 1 by loading (e) THF and (f) CCl 4 upon increasing the adsorption time.Plot of ∆I versus the amount of (g) THF and (h) CCl 4 adsorbed by solid FGFL-B 1 .