A Novel Tb@Sr-MOF as Self-Calibrating Luminescent Sensor for Nutritional Antioxidant

Sesamol, is well-known antioxidant and can reduce the rate of oxidation and prolong expiration date. It is also potentially antimutagenic and antihepatotoxic, the detection of sesamol is important and remains a huge challenge. Herein, a new 3D alkaline earth Sr metal organic framework [Sr(BDC)·DMAC·H2O]n (BDC = benzene-1,4-dicarboxylate; DMAC = N,N-dimethylacetamide) is synthesized and a probe based on Tb3+ functionalized Sr-MOF. The Tb(3+)@Sr-MOF showed good luminescence and thermal property. Due to the energy competition between sesamol and ligand, the luminescence intensity of sesamol increases meantime luminescence intensity of Tb3+ decreases, the ratio of the emission intensities (I344/I545) linearly increases with sesamol in concentrations ranging from 1 × 10−7 to 8 × 10−4 M. Furthermore, the fluorescence-detected circular test shows that the composite Tb(3+)@Sr-MOF can serve as ratiometric sensor for sensing of sesamol. This is the first example for self-calibrated detecting sesamol based on metal-organic framework (MOF).


Introduction
Sesame oil is a high-priced, high-quality health food that is popular in China and India because it contains a number of bioactive phytochemical; it is very high in natural antioxidants in the form of lignans. Antioxidant compounds in sesame seed oil that are beneficial impacts on health have attracted increasing attention. Sesamol (3,4-methylenedioxyphenol) is a natural phenolic lignan found in sesame seed or sesame oil as well as has shown promising antioxidant and neuroprotective effects [1,2]. Recently, tremendous research has shown that sesamol can weaken injury in endotoxemic rats, lower serum lipids and blood pressure; it also has potentially anti-hypertensive and anti-inflammatory activities in humans. Sesamol content plays an important role human health and the flavor of sesame oil, therefore there is a need for sesamol determination for evaluate lignan content in sesame oil or other food, meanwhile, determination of sesamol in different environment with high selectivity and sensitivity has also become a major research topic. Some detection methods for sesamol have been developed [3,4] such as high-performance liquid chromatography (HPLC) or ultraviolet (UV) detection ultraviolet (UV) detection. However, many disadvantages have various limitations such as time-consuming, cost, complicated preparation process, and the need for professionals. Therefore, it is exigent to explore a kind of simple, rapid, highly selective, and sensitive access for detecting sesamol [5].
Metal-organic frameworks (MOFs), are crystalline porous architectures that are composed of metal ions or clusters and organic ligand, have been emerging as very promising materials which can

Detection of Sesamol
The obtained Tb(3+)@Sr-MOF (2.00 mg) dispersed in 4 mL ethanol and ultrasonicated for 5 min. Different concentrations of sesamol ethanol solutions were prepared and mixed with suspension of Tb(3+)@Sr-MOF for the detection of sesamol. For the selectivity of sesamol detection, 1 × 10 −3 M for sesamol, 4-Methylcatechol, catechol, guaiacol, carvacrol, paeonol, thymol, vanillin, resorcinol, and 1,3-dichlorophenol were also prepared and added to each of the suspension of Tb(3+)@Sr-MOF, respectively. In order to examine the cycle performance of Tb(3+)@Sr-MOF, the suspension is formed by dispersing the sample (1 mg/mL) into ethanol. After detection of sesamol, the suspensions of Tb(3+)@Sr-MOF/sesamol are obtained by filtration and rinsed several times with ethanol, then the Tb(3+)@Sr-MOF was dried naturally and ready for the next cyclic test.

X-ray Crystallography
Crystal of Sr-MOF was collected from the mother liquor. Single-crystal data of Sr-MOF were collected on a Rigaku Oxford CCD diffractometer equipped with graphite-monochromatic Mo-K α radiation (λ = 0.71073 Å) at 293 K. The structure was solved by direct methods, and refined by full-matrix least-square method with the SHELX-2016 program package. The crystallographic data and refinements and the selected bond lengths and angles for Sr-MOF are listed in Tables 1 and 2.

Results and Discussion
The crystals of Sr-BDC belong to the orthorhombic space group Pnma, the asymmetric unit is made up of half a Sr 2+ ion, half a BDC 2− ligand, half a DMA, and half a water molecules (Figure 1a). The Sr 2+ ion is bound to eight O atoms from one H 2 O, one DMAC and four BDC 2− ligands, which form an octahedron which adopted distorted bicapped coordination. The DMAC and H 2 O are monodentate, and the COO − of a BDC 2− adopted two coordination modes with Sr 2+ ions: η1:η1 and η2:η2-bridging mode, which link one and three Sr 2+ ion. In Figure 1b, in the bicapped octahedron, the Sr-O bond distance vary from 2.490(6) to 2.687(5) Å. A zigzag chain is formed by adjacent octahedra along the b axis, and the chains are connected by the BDC 2− (µ4,η1:η1:η2:η2-bridging mode) forming a three-dimensional framework, (Figure 1), which form quadrangular channel (two kinds of triangular channels), DMAC molecules are filled and connected directly to the Sr 2+ ions in the channels.
The XRD patterns of simulated and as-synthesized Sr-MOF, Tb(3+)@Sr-MOF are shown in Figure 2. All the diffraction peaks (the Sr-MOF and Tb(3+)@Sr-MOF) were well corresponded to those in the simulated PXRD pattern of Sr-MOF(CCDC:1551141). The introduction of Tb 3+ will not influence the crystal form of Sr-MOF. The XRD patterns of simulated and as-synthesized Sr-MOF, Tb(3+)@Sr-MOF are shown in Figure 2. All the diffraction peaks (the Sr-MOF and Tb(3+)@Sr-MOF) were well corresponded to those in the simulated PXRD pattern of Sr-MOF(CCDC:1551141). The introduction of Tb 3+ will not influence the crystal form of Sr-MOF. As shown in Figure 3, the TG measurement show that Tb@Sr-MOF and Sr-MOF similar thermal stability and exhibit three events of mass (Tb@Sr-MOF and Sr-MOF) reduction. The TG curve shows that Tb@Sr-MOF and Sr-MOF starts to reduce mass at ~130 °C due to the removal of water molecules and complete dehydration is at about 200 °C. The second plateau of reducing mass start from 200 °C to 310 °C corresponds to the loss of DMAC. The decomposition of the organic ligand begins at 580 °C As shown in Figure 3, the TG measurement show that Tb@Sr-MOF and Sr-MOF similar thermal stability and exhibit three events of mass (Tb@Sr-MOF and Sr-MOF) reduction. The TG curve shows that Tb@Sr-MOF and Sr-MOF starts to reduce mass at~130 • C due to the removal of water molecules and complete dehydration is at about 200 • C. The second plateau of reducing mass start from 200 • C to 310 • C corresponds to the loss of DMAC. The decomposition of the organic ligand begins at 580 • C and ends at 630 • C. The final stage of reducing mass start from 630 • C corresponds to oxide. As shown in Figure 3, the TG measurement show that Tb@Sr-MOF and Sr-MOF similar thermal stability and exhibit three events of mass (Tb@Sr-MOF and Sr-MOF) reduction. The TG curve shows that Tb@Sr-MOF and Sr-MOF starts to reduce mass at ~130 °C due to the removal of water molecules and complete dehydration is at about 200 °C. The second plateau of reducing mass start from 200 °C to 310 °C corresponds to the loss of DMAC. The decomposition of the organic ligand begins at 580 °C and ends at 630 °C. The final stage of reducing mass start from 630 °C corresponds to oxide.   As seen in Figure 5, Tb(3+)@Sr-MOF exhibits characteristic emission of the Tb 3+ ion when excited 294 nm. Tb(3+)@Sr-MOF exhibits three peaks at 489, 545, and 592 nm originated from 5 D4→ 7 FJ (J = 6, 5, 4) transitions, respectively. The emission bands of the Tb(3+)@Sr-MOF at 545 nm show a bright green light. The results suggested Tb(3+)@Sr-MOF can act as a luminescence sensor. As seen in Figure 5, Tb(3+)@Sr-MOF exhibits characteristic emission of the Tb 3+ ion when excited 294 nm. Tb(3+)@Sr-MOF exhibits three peaks at 489, 545, and 592 nm originated from 5 D 4 → 7 F J (J = 6, 5, 4) transitions, respectively. The emission bands of the Tb(3+)@Sr-MOF at 545 nm show a bright green light. The results suggested Tb(3+)@Sr-MOF can act as a luminescence sensor.
The sensing ability of the Tb(3+)@Sr-MOF was investigated in the presence of different molecules. As shown in Figure 6, on the addition of 1 × 10 −3 M of biomolecules (sesamol, 4-Methylcatechol, catechol, guaiacol, carvacrol, paeonol, thymol, vanillin, resorcinol, and 1,3-dichlorophenol), however, the luminescence intensity of Tb(3+)@Sr-MOF at 545 nm exhibit the strongest luminescence quenching in the presence of sesamol, meantime, and luminescent intensity (the emission at 330 nm) increases significantly with the increasing the concentration of sesamol, We speculate that the emission spectrum at 330 nm is ascribed to sesamol. As seen in Figure 5, Tb(3+)@Sr-MOF exhibits characteristic emission of the Tb 3+ ion when excited 294 nm. Tb(3+)@Sr-MOF exhibits three peaks at 489, 545, and 592 nm originated from 5 D4→ 7 FJ (J = 6, 5, 4) transitions, respectively. The emission bands of the Tb(3+)@Sr-MOF at 545 nm show a bright green light. The results suggested Tb(3+)@Sr-MOF can act as a luminescence sensor. The sensing ability of the Tb(3+)@Sr-MOF was investigated in the presence of different molecules. As shown in Figure 6, on the addition of 1 × 10 −3 M of biomolecules (sesamol, 4-Methylcatechol, catechol, guaiacol, carvacrol, paeonol, thymol, vanillin, resorcinol, and 1,3-dichlorophenol), however, the luminescence intensity of Tb(3+)@Sr-MOF at 545 nm exhibit the strongest luminescence quenching in the presence of sesamol, meantime, and luminescent intensity (the emission at 330 nm) increases significantly with the increasing the concentration of sesamol, We speculate that the emission spectrum at 330 nm is ascribed to sesamol. In order to overcome such disadvantages of the traditional single emission sensing, Tb(3+)@Sr-MOF were synthesized and could be served as ratiometric luminescent sensor for sesamol. In Figure  7, the change of luminescent intensity (Tb(3+)@Sr-MOF) displayed with a concentration of sesamol increases. The luminescent intensity at 330 nm increased meantime the fluorescence intensity of Tb 3+ at 545 nm decreased. The plot of the luminescent intensity ratio I330/I545 against the concentration of added sesamol was shown in Figure 7a,b, the luminescent intensity ratio I330/I545 has a good linear relationship to the concentration of sesamol varying from 1 × 10 −7 to 2 × 10 −4 M and 3 × 10 −4 to 8 × 10 −4 M, which was described by calibrating function of I330/I545 = 0.00538 + 0.0184 × C and I330/I545 = 0.005 × Csesamol-0.18 with a correlation coefficient of 0.99966 and 0.9887. Interestingly, when the concentration of sesamol reaches 3 × 10 −4 M, luminescent intensities of I330 and I545 decreases, respectively. The luminescent intensity ratio I330/I545 also has a good linear correlation to the concentration of sesamol in the range from 1 × 10 −7 to 8 × 10 −4 M calibrating function of I330/I545 = 0.02 + 0.005 × C sesamol with a correlation coefficient of 0.9977. The limit of detection (LOD = 3δ/S, δ represents the blank solution was measured ten times, and S stands for the slope of the calibration curve was about 4.2 μM. [50] The above results illustrated that Tb(3+)@Sr-MOF is an excellent candidate for self-calibrating luminescent sensor (sesamol) and is not influenced by environmental factors. In order to overcome such disadvantages of the traditional single emission sensing, Tb(3+)@Sr-MOF were synthesized and could be served as ratiometric luminescent sensor for sesamol. In Figure 7, the change of luminescent intensity (Tb(3+)@Sr-MOF) displayed with a concentration of sesamol increases. The luminescent intensity at 330 nm increased meantime the fluorescence intensity of Tb 3+ at 545 nm decreased. The plot of the luminescent intensity ratio I 330 /I 545 against the concentration of added sesamol was shown in Figure 7a,b, the luminescent intensity ratio I 330 /I 545 has a good linear relationship to the concentration of sesamol varying from 1 × 10 −7 to 2 × 10 −4 M and 3 × 10 −4 to 8 × 10 −4 M, which was described by calibrating function of I 330 /I 545 = 0.00538 + 0.0184 × C and I 330 /I 545 = 0.005 × Csesamol-0.18 with a correlation coefficient of 0.99966 and 0.9887. Interestingly, when the concentration of sesamol reaches 3 × 10 −4 M, luminescent intensities of I 330 and I 545 decreases, respectively. The luminescent intensity ratio I 330 /I 545 also has a good linear correlation to the concentration of sesamol in the range from 1 × 10 −7 to 8 × 10 −4 M calibrating function of I 330 /I 545 = 0.02 + 0.005 × C sesamol with a correlation coefficient of 0.9977. The limit of detection (LOD = 3δ/S, δ represents the blank solution was measured ten times, and S stands for the slope of the calibration curve was about 4.2 µM [50]. The above results illustrated that Tb(3+)@Sr-MOF is an excellent candidate for self-calibrating luminescent sensor (sesamol) and is not influenced by environmental factors. concentration of sesamol in the range from 1 × 10 −7 to 8 × 10 −4 M calibrating function of I330/I545 = 0.02 + 0.005 × C sesamol with a correlation coefficient of 0.9977. The limit of detection (LOD = 3δ/S, δ represents the blank solution was measured ten times, and S stands for the slope of the calibration curve was about 4.2 μM. [50] The above results illustrated that Tb(3+)@Sr-MOF is an excellent candidate for self-calibrating luminescent sensor (sesamol) and is not influenced by environmental factors. The CIE (Commission International deLEclairage) diagram of the Tb(3+)@Sr-MOF treated with different concentrations of sesamol was performed. As shown in Figure 8, luminescent color of Tb(3+)@Sr-MOF tuned from blue to green when excited at 294 nm. The results show that the luminescent ratio (I344/I545) is highly sensitive to the concentration of sesamol. The feature could be used served for sensing of different concentrations of sesamol with high selectivity and sensitivity and without any addition. The CIE (Commission International deLEclairage) diagram of the Tb(3+)@Sr-MOF treated with different concentrations of sesamol was performed. As shown in Figure 8, luminescent color of Tb(3+)@Sr-MOF tuned from blue to green when excited at 294 nm. The results show that the luminescent ratio (I 344 /I 545 ) is highly sensitive to the concentration of sesamol. The feature could be used served for sensing of different concentrations of sesamol with high selectivity and sensitivity and without any addition. From a practical standpoint, the probe should have good response and high selectivity to the detecting. As seen in Figure 9, to access the selectivity of Tb(3+)@Sr-MOF, the competitive experiment was performed by adding 1 × 10 −3 M sesamol to the Tb(3+)@Sr-MOF in the presence of 1 × 10 −3 M other biomolecules (including 4-Methylcatechol, catechol, guaiacol, thymol, carvacrol, resorcinol, vanillin, and paeonol). The addition of biomolecules will not influence the changed trend of the ratio of I330/I545. (colorful columns in Figure 4), However, when added 1 × 10 −3 M sesamol to the Tb(3+)@Sr-MOF containing other biomolecules, the luminescent intensity ratio I330/I545 increased remarkably (blue columns in Figure 4). Therefore, the results show that the Tb(3+)@Sr-MOF is a reliable and highefficient self-calibrating sensor for sesamol. Furthermore, the cycling ability is an important indicator to access the sensor's practicability. The Tb(3+)@Sr-MOF can be reused five times (Figure 10). After five cycles, the results show that the luminescence intensity of the recycled Tb(3+)@Sr-MOF almost agrees with those of the initial From a practical standpoint, the probe should have good response and high selectivity to the detecting. As seen in Figure 9, to access the selectivity of Tb(3+)@Sr-MOF, the competitive experiment was performed by adding 1 × 10 −3 M sesamol to the Tb(3+)@Sr-MOF in the presence of 1 × 10 −3 M other biomolecules (including 4-Methylcatechol, catechol, guaiacol, thymol, carvacrol, resorcinol, vanillin, and paeonol). The addition of biomolecules will not influence the changed trend of the ratio of I 330 /I 545 . (colorful columns in Figure 4), However, when added 1 × 10 −3 M sesamol to the Tb(3+)@Sr-MOF containing other biomolecules, the luminescent intensity ratio I 330 /I 545 increased remarkably (blue columns in Figure 4). Therefore, the results show that the Tb(3+)@Sr-MOF is a reliable and high-efficient self-calibrating sensor for sesamol. From a practical standpoint, the probe should have good response and high selectivity to the detecting. As seen in Figure 9, to access the selectivity of Tb(3+)@Sr-MOF, the competitive experiment was performed by adding 1 × 10 −3 M sesamol to the Tb(3+)@Sr-MOF in the presence of 1 × 10 −3 M other biomolecules (including 4-Methylcatechol, catechol, guaiacol, thymol, carvacrol, resorcinol, vanillin, and paeonol). The addition of biomolecules will not influence the changed trend of the ratio of I330/I545. (colorful columns in Figure 4), However, when added 1 × 10 −3 M sesamol to the Tb(3+)@Sr-MOF containing other biomolecules, the luminescent intensity ratio I330/I545 increased remarkably (blue columns in Figure 4). Therefore, the results show that the Tb(3+)@Sr-MOF is a reliable and highefficient self-calibrating sensor for sesamol. Furthermore, the cycling ability is an important indicator to access the sensor's practicability. The Tb(3+)@Sr-MOF can be reused five times (Figure 10). After five cycles, the results show that the luminescence intensity of the recycled Tb(3+)@Sr-MOF almost agrees with those of the initial Furthermore, the cycling ability is an important indicator to access the sensor's practicability. The Tb(3+)@Sr-MOF can be reused five times (Figure 10). After five cycles, the results show that the luminescence intensity of the recycled Tb(3+)@Sr-MOF almost agrees with those of the initial Tb(3+)@Sr-MOF, Meanwhile, These results reveal that Tb(3+)@Sr-MOF displays well reusability of sensing sesamol, suggesting its practical use in sesamol detection. Tb(3+)@Sr-MOF, Meanwhile, These results reveal that Tb(3+)@Sr-MOF displays well reusability of sensing sesamol, suggesting its practical use in sesamol detection. While the quenching mechanism for biomolecules is still not very clear, it is necessary to study the possible quenching mechanism. (1) The emission spectra of sesamol was monitored when excited 294 nm, as shown in Figure 11, the I344 is consistent with I344 in Figure 7c,d. The result shows that luminescent signal(I344) in Figure 7c,d is assigned to sesamol. (2) As shown in Figure S1, the excitation spectra of the ligand within Tb(3+)@Sr-MOF is overlapped by the excitation spectra of sesamol, which suggests an excitation energy competition between the ligand and sesamol exists. Sesamol absorbs most of the energy and only a small fraction of energy will be transferred from the linker to the Tb 3+ ions. The PXRD patterns of the Tb(3+)@Sr-MOF treated with sesamol reveal that its crystal structure is not changed and is consistent with the original Tb(3+)@Sr-MOF (as shown in Figure S2). (3) To better understand why luminescent intensities of I344 and I545 decreases when concentration of sesamol reached 3 × 10 −4 M, we monitored the excitation spectra of Tb(3+)@Sr-MOF treated with various concentrations of sesamol under the monitoring wavelength(545 nm). As shown in Figure 12, with the increased concentration of sesamol, the intensities of excitation spectra of Tb(3+)@Sr-MOF decreases and a blue shift in the excitation maxima(294 to280 nm) could be observed for Tb(3+)@Sr-MOF treated with different concentrations of sesamol, leading to decline in luminescent intensity(I344 and I545), respectively, the fluorescence intensity ratio I330/I545 has also a good linear relationship to the concentration of sesamol vary from 1 × 10 −7 to 8 × 10 −4 M, The results suggested that Tb(3+)@Sr-MOF can serve as a self-calibrating luminescent sensor for sesamol and is not influenced by environmental factors.  While the quenching mechanism for biomolecules is still not very clear, it is necessary to study the possible quenching mechanism. (1) The emission spectra of sesamol was monitored when excited 294 nm, as shown in Figure 11, the I 344 is consistent with I 344 in Figure 7c,d. The result shows that luminescent signal(I 344 ) in Figure 7c,d is assigned to sesamol. (2) As shown in Figure S1, the excitation spectra of the ligand within Tb(3+)@Sr-MOF is overlapped by the excitation spectra of sesamol, which suggests an excitation energy competition between the ligand and sesamol exists. Sesamol absorbs most of the energy and only a small fraction of energy will be transferred from the linker to the Tb 3+ ions. The PXRD patterns of the Tb(3+)@Sr-MOF treated with sesamol reveal that its crystal structure is not changed and is consistent with the original Tb(3+)@Sr-MOF (as shown in Figure S2). (3) To better understand why luminescent intensities of I 344 and I 545 decreases when concentration of sesamol reached 3 × 10 −4 M, we monitored the excitation spectra of Tb(3+)@Sr-MOF treated with various concentrations of sesamol under the monitoring wavelength(545 nm). As shown in Figure 12, with the increased concentration of sesamol, the intensities of excitation spectra of Tb(3+)@Sr-MOF decreases and a blue shift in the excitation maxima(294 to280 nm) could be observed for Tb(3+)@Sr-MOF treated with different concentrations of sesamol, leading to decline in luminescent intensity(I 344 and I 545 ), respectively, the fluorescence intensity ratio I 330 /I 545 has also a good linear relationship to the concentration of sesamol vary from 1 × 10 −7 to 8 × 10 −4 M, The results suggested that Tb(3+)@Sr-MOF can serve as a self-calibrating luminescent sensor for sesamol and is not influenced by environmental factors. Tb(3+)@Sr-MOF, Meanwhile, These results reveal that Tb(3+)@Sr-MOF displays well reusability of sensing sesamol, suggesting its practical use in sesamol detection. While the quenching mechanism for biomolecules is still not very clear, it is necessary to study the possible quenching mechanism. (1) The emission spectra of sesamol was monitored when excited 294 nm, as shown in Figure 11, the I344 is consistent with I344 in Figure 7c,d. The result shows that luminescent signal(I344) in Figure 7c,d is assigned to sesamol. (2) As shown in Figure S1, the excitation spectra of the ligand within Tb(3+)@Sr-MOF is overlapped by the excitation spectra of sesamol, which suggests an excitation energy competition between the ligand and sesamol exists. Sesamol absorbs most of the energy and only a small fraction of energy will be transferred from the linker to the Tb 3+ ions. The PXRD patterns of the Tb(3+)@Sr-MOF treated with sesamol reveal that its crystal structure is not changed and is consistent with the original Tb(3+)@Sr-MOF (as shown in Figure S2). (3) To better understand why luminescent intensities of I344 and I545 decreases when concentration of sesamol reached 3 × 10 −4 M, we monitored the excitation spectra of Tb(3+)@Sr-MOF treated with various concentrations of sesamol under the monitoring wavelength(545 nm). As shown in Figure 12, with the increased concentration of sesamol, the intensities of excitation spectra of Tb(3+)@Sr-MOF decreases and a blue shift in the excitation maxima(294 to280 nm) could be observed for Tb(3+)@Sr-MOF treated with different concentrations of sesamol, leading to decline in luminescent intensity(I344 and I545), respectively, the fluorescence intensity ratio I330/I545 has also a good linear relationship to the concentration of sesamol vary from 1 × 10 −7 to 8 × 10 −4 M, The results suggested that Tb(3+)@Sr-MOF can serve as a self-calibrating luminescent sensor for sesamol and is not influenced by environmental factors.

Conclusions
In summary, a new 3D alkaline earth Sr metal organic framework is synthesized and chosen as a host to sensitize via encapsulating Tb 3+ in Sr-MOF. Tb(3+)@Sr-MOF display excellent luminescent property and thermal stability. Due to energy competition between sesamol and ligand, the luminescent intensity of sesamol (I344) increases meantime luminescence intensity of Tb 3+ (I545) decreases. The Tb(3+)@Sr-MOF can be used as ratiometric sensor for sesamol. It is the first time reported that the rational design and preparation of luminescent MOFs for ratiometric sensing of sesamol relying on the ratio of emission-peak-height of analyte (sesamol) to lanthanide ions (Tb 3+ ) as the detectable signals. In addition, this strategy may promote the development of lanthanide functionalized MOF for self-calibrating sensing and broaden the application of alkaline earth metal organic framework.