基于合成后修飾策略構(gòu)建高靈敏度和選擇性金屬有機(jī)骨架基熒光靜默系統(tǒng)用于抗壞血酸的檢測(cè)

張 婕 劉 欣 李梓欣 裴玉婷 楊宇琦 李慧敏 劉志強(qiáng)

(1太原工業(yè)學(xué)院化學(xué)與化工系，太原 030008)

(2南開(kāi)大學(xué)化學(xué)學(xué)院，先進(jìn)能源材料化學(xué)教育部重點(diǎn)實(shí)驗(yàn)室，天津 300071)

(3安慶師范大學(xué)化學(xué)化工學(xué)院，光電磁功能材料安徽省重點(diǎn)實(shí)驗(yàn)室，安慶 246133)

0 Introduction

Ascorbic acid (AA), an essential vitamin for animals and humans, is renowned for its valuable antioxidant properties[1-3]. Not only does it play a crucial role in the prevention of scurvy, wound healing, and treatment of anemia, but it is also an indispensable helper for various enzymes involved in the metabolism of tryptophan, folic acid, and tyrosine[4-5]. However, the deficiency of AA can lead to weak immunity, infertility,and increased cancer risk,while an overdose can cause stomach cramps,diarrhea,and kidney stones[6-8].Therefore, the quantitative analysis of AA is gaining significant attention in the food industries, pharmaceuticals,and clinicals. The traditional methods for analyzing or detecting AA are often time-consuming procedures,and specialized equipment, and require skilled personnel[9-10].Hence,it is imperative to investigate straightforward and highly sensitive approaches for swift monitoring of AA, which holds significance in the domains of chemistry,medicine,and nutrition,pharmacy[11-13].

Metal-organic frameworks(MOFs)are porous crystalline materials formed by the coordination of organic ligands with metal clusters or metal ions. Due to their highly adjustable nature and diversity, MOFs play a crucial role in luminescence sensing[14-15]and gas separation[16]. However, there are several challenges in the practical application of directly synthesized MOF materials, including non-obvious experimental phenomena,insufficient stability, and difficulties in loading[17-20].Addressing these constraints is crucial for constructing molecular models with a wider range of functional groups and complex structures. Post-synthetic modification (PSM) is a process that enables different functional groups, such as anions, cations, or a variety of organic functional groups into the structures of the original MOFs, thereby increasing functionality and stability[21]. Currently, several methods, including cosynthesis, ion-doping, covalent post-synthesis, and ion exchange, have been adopted to enhance polymer conductivity. In the context of luminescence sensing technologies,PSM enables the introduction of luminescencesensitive groups for rapid detection of target molecules.For example, groups with selective luminescence responses to target molecules can be introduced, allowing for targeted detection. In recent years, there is a surge of research has been made to design and synthesize MOFs based on sensors for the detection of AA.One such noteworthy MOF is the 3D MOF [{(H2O)[Eu(SBDB)(H2O)2]}n], also known as ZJU-137, which was proposed by Qian and his co-worker[23]. This MOF exhibited an obvious “turn-off” luminescence quenching effect, making it ideal for the sensitive and selective detection of AA. In addition, Shyam Biswas and his co-worker synthesized a guest-free 3D IITG-6 MOF(named IITG-6a)to detect AA via the turn-off luminescence response in the water system[24]. The majority of MOF materials have been used for AA detection through a turn-off mechanism, although the experimental phenomenon was not always readily observable.Therefore, the current focus of this work is developing a new MOF material that enables the “turn-on” detection of AA in the water system, utilizing the PSM method.

We designed a porous luminescence cadmium （Ⅱ）MOF[Cd(L)(H2O)0.5]·DMF·2.5H2O(1)with high-density nitrogenous groups semi-rigid 3-(tetrazol-5-yl)triazole (H2L) ligand under hydrothermal conditions. Importantly, 1 exhibits high thermal stability and water stability and can detect Cr（Ⅵ）ions (CrO42-and Cr2O72-)with fast response time and high sensitivity. The incorporation of Cr（Ⅵ）ions into the cavities of 1 led to a significant decrease in the luminescence intensity, which can be attributed to the luminescence Forster resonance energy transfer (FRET) mechanism occurring between the absorption bands of Cr （Ⅵ）(Cr2O72-and CrO42-) and the excitation band of 1. Thus, a luminescence silencing system, referred to as Cr （Ⅵ）＠1, was formed. However, the addition of AA eliminates the FRET process, disrupting the luminescence silencing system and enhancing the original luminescence intensity.Thus,the Cr（Ⅵ）＠1 can be utilized as an open luminescence sensing platform, providing a versatile tool for the detection of AA in water with enhanced luminescence (Scheme 1). This system offers advantages such as simplicity, reliability, and a wide dynamic range, making it a potentially valuable analytical tool in various fields. In addition, the high sensitivity of the Cr （Ⅵ）＠1 luminescence silencing sensor led us to explore the construction of a series combination logic gate to improve its practicality and convenience in AA sensing processes.

Scheme 1 Illustration of the synthesis of MOF 1 and the detection of Cr（Ⅵ）and AA

1 Experimental

1.1 Materials and methods

All commercially available reagents are used as receipts. Elemental analyses for C, H, and N were performed on an Elementar Vario MICRO Elemental analyzer. FTIR spectral measurements were carried out on a Bruker Vector 22 Fourier transform infrared spectrometer. Thermogravimetric analyses (TGA) were taken on a Mettler-Toledo (TGA/DSC1)thermal analyzer under nitrogen with a heating rate of 10 ℃·min-1.Powder X-ray diffraction (PXRD)analyses were carried out on a Bruker D8 Advance using CuKαradiation(λ=0.154 18 nm) with the X-ray tube operated at 40 mA and 40 kV (2θ=5°-50°). The scanning electron microscopy (SEM)and energy-dispersive X-ray (EDX)images were collected using a Zeiss Merlin Compact (FE -SEM). UV-Vis spectra were collected on a UV 2300 analyzer. The luminescence spectra were recorded on a Hitachi FL-4600 fluorescence spectrometer.

1.2 X-ray crystallography

Diffraction data collections for MOF 1 were finished on a Bruker Smart Apex ⅡCCD area-detector diffractometer with graphite-monochromated MoKαradiation (λ=0.071 073 nm). The integration of the diffraction data,as well as the intensity corrections for the Lorentz and polarization effects, were carried out using the SAINT program.Semi-empirical absorption correction was performed using the SADABS program. The structure of 1 was solved by direct methods and all the non-hydrogen atoms were refined anisotropically onF2by the full-matrix least-squares technique with OLEX.The hydrogen atoms except for those of water molecules were generated geometrically and refined isotropically using the riding model. Because the guest solvent molecules in 1 are highly disordered and impossible to refine using conventional discrete-atom models, the SQUEEZE subroutine of the PLATON software suite was used to remove the scattering from the highly disordered solvent molecules.The formula of 1 was obtained based on volume/count electron analysis,TGA, and elemental analysis. The reported refinements were of the guest - free structures obtained by the SQUEEZE routine, and the results were attached to the CIF file. The details of the crystal data for 1 are summarized in Table S1 (Supporting information), and selected bond lengths and angles are listed in Table S2.

CCDC：2286017.

1.3 Synthesis of MOF 1

A mixture of Cd(NO3)2·6H2O (11.64 mg, 0.04 mmol)and H2L(0.1 mmol,0.014 g)was dissolved in 10 mL of solvent(VDMF∶VH2O=2∶1),sealed in a high-pressure reactor,and heated at 397 K for 48 h.After cooling naturally to room temperature,the resulting filter cake was washed with DMF several times to obtain the title Cd-MOF 1 (Yield： 60% based on H2L ligand). Anal.Calcd.for C6H14CdN8O4(%)：C,19.24;H,3.77;N,29.91.Found(% )： C, 19.25; H, 3.74; N, 29.95. IR (KBr)：3 438 (w), 3 125 (w), 2 891 (m), 2 352 (s), 1 649 (w),1 508 (m), 1 391 (m), 1 309 (m), 1 181 (w), 1 156 (m),1 061 (w), 919 (w), 817 (w), 689 (m), 661 (w), 614 (m)(Fig.S1).

1.4 Luminescent sensing

First, 2 mg of fully ground powder of MOF 1 was evenly dispersed into a 2 mL water solution containing 2 mmol·L-1of Cr （Ⅵ）(1 mL of Cr2O72-or 150 μL of CrO42-, 1 mmol·L-1) and ultrasonicated for 30 min.Subsequently, the luminescence silencing systems Cr2O72-＠1 and CrO42-＠1 were fabricated.

1.5 Detection of AA

The gradient titration experiment was carried out by adding different amounts of AA to the CrO42-＠1 sensor system suspension. After 30 min of ultrasonic treatment, the luminescence spectra of the suspension were collected. After filtering, cleaning, and drying, the recovered samples were prepared for the next test cycle.

2 Results and discussion

2.1 Structural description of MOF 1

Structural analysis shows that MOF 1 crystallizes in the Pbcm space group of the orthorhombic crystal system. The asymmetry unit contains one Cd（Ⅱ）cation(Cd1 and Cd2 with the occupation of 0.5), one semirigid L2-ligand, a half of coordinated water molecules,one lattice DMF molecule, and two and a half lattice water molecules (Fig.1a). Cd1（Ⅱ）ions form a binuclear anion [Cd2N6] SBU by bridging N atoms in the L2-ligands, while Cd2（Ⅱ）ion is coordinated by N atoms in L2-ligand and coordinated water molecules to constitute the neutral binuclear octahedron [CoN4(H2O)2]nSBU (Fig.1b). These two different SBUs are further extended by L2-ligands to build the final 3D skeleton structure with two different pore sizes (0.875 nm×1.290 nm and 1.261 nm×0.846 nm) (Fig.1c and 1d). The potential porosity of 1 can reach 24.4% after removing the free solvents, according to the free software PLATON.

Fig.1 (a)Coordination environment of Cd-MOF 1;(b)Binuclear octahedron[CoN6]and neutral[CoN4(H2O)2]n SBU;(c)3D network of 1;(d)3D framework of 1

2.2 Characterization of MOF 1

As illustrated in Fig.S2, the PXRD patterns of MOF 1 showed distinct characteristic peaks at 12.5°,14.7°, 17.17°, 19.68°, 22.74°, 25.00°, 27.71°, 31.03°,34.19°, 35.70°, 36.76°, 39.77°. The positions of the characteristic peaks of PXRD were in agreement with those of the simulated ones, which shows that the highcrystalline Cd-MOF materials have been successfully synthesized. Fig.S3 presents the TGA curves of 1. It displayed a significant weight loss of 20.01% below 270 ℃, which is attributed to the absence of two and a half lattice water molecules, one lattice DMF molecule,and a half of coordinated water molecules in the pore and on the surface(Calcd.19.80%)

To assess the changes in morphology before and after the formation of the luminescence silencing system, element maps of 1 and Cr2O72-＠1 were also analyzed by SEM. As can be seen from Fig.2, both 1 and Cr2O72-＠1 exhibited uniform and regular tetrahedron morphology, which indicates that the process of PSM process does not cause any significant morphological changes in 1. Additionally, EDX element mapping was performed to further investigate the distribution of elements within the Cr2O72-＠1 sample. As exhibited in Fig.3,the C,N,O,Cd,and Cr elements were uniformly distributed throughout the fabricated Cr2O72-＠1. Thus,the above characterization confirmed that Cr2O72-ions have been successfully introduced into the confined space of 1 according to the PSM strategy without changing the structure of 1.

Fig.2 SEM images of MOF 1(a-b)and Cr2O72-＠1(c-d)

Fig.3 SEM image(a),and C(b),N(c),O(d),Cd(e),and Cr(f)EDX elemental mappings of Cr2O72-＠1 luminescence silencing system

2.3 Luminescent and sensing properties

The initial luminescence spectra of MOF 1 and ligand L in the solid state were studied at room temperature (λex=327 nm). As exhibited in Fig.4a, the initial weak band near 446 nm corresponds to the emission of the L ligand, while the stronger band near 450 nm corresponds to the emission of charge transition between the ligand and the metal ion in 1. The emission peaks of 1 were typically stronger compared to those of the L2-ligand in Fig.4b, which can be attributed to the coordination interaction between cadmium and the ligands[25]. In addition, the stable and rigid framework helps to reduce the loss of vibrational energy, allowing 1 to be efficiently luminescent. As a result, the luminescence intensity of 1 has been enhanced, leading to stronger emission peaks compared to those of the individual ligands.

Fig.4 Solid-state luminescence spectra of ligand L(a)and MOF 1(b)

2.4 Water and pH stability properties

To demonstrate the luminescence stability of MOF 1, the ground samples of 1 were soaked in an aqueous solution with varying pH values at room temperature for 10 d, followed by luminescence measurements. The results demonstrated that the relative luminescence intensity of 1 remained stable within the pH range of 3-13 under various conditions (Fig.S4a). Specifically, the luminescence intensity of 1 was strongest when the pH value was equal to 6. Furthermore, after being soaked in deionized water for 10 d, there was no change in the luminescence intensity of the suspension(Fig.S4b).Therefore,1 is a potential candidate for practical sensing applications, with a wide pH range and excellent luminescence stability.

2.5 Luminescence properties

To facilitate the efficient evaluation of MOF 1 for AA turn-on detection, a luminescence silencing system based on 1 has been constructed through a straightforward PSM. Additionally, the similarity in the diffraction peaks between the Cr（Ⅵ）-treated samples and the synthesized samples implied that the presence of Cr（Ⅵ）ions did not lead to any significant phase transformations (Fig.S5). The luminescence spectra of CrO42-＠1 and Cr2O72-＠1 are shown in Fig.5. This observation was intriguing as it suggested that the presence of the Cr2O72-ions has a greater impact on the luminescence intensity of 1 compared to that of CrO42-ions.The addition of AA effectively disrupts the luminescence silencing system,allowing the luminescence intensity of Cr（Ⅵ）＠1 to be restored to its original state. Furthermore, 1 exhibited a high quantum yield of 16.30%. Upon the addition of Cr（Ⅵ）ions, the quantum yield decreased to 1.61% for CrO42-＠1 and 1.59% forCr2O72-＠1, signifying a significant reduction in luminescence efficiency,while recovered or exceeded the original quantum yields (16.10% for CrO42-＠1-AA and 16.33% for Cr2O72-＠1-AA)with the presence of AA,which coincided with a change in luminescence intensity. It has also been demonstrated that the system of Cr（Ⅵ）＠1 showed an exceptionally rapid response time of 10 s for the species CrO42-ions and Cr2O72-ions (Fig.S6). This indicates that the formation of a luminescence silencing system of Cr （Ⅵ）＠1 can be easily achieved through a straightforward method. Additionally, based on the quenching effect and response time, the Cr2O72-＠1 was employed for the following sensing applications.

Fig.5 Luminescence emission spectra of CrO42-＠1 and CrO42-＠1-AA(a),and Cr2O72-＠1 and Cr2O72-＠1-AA(b)

2.6 Determination of AA

As all know the highly reducible nature of AA has generated substantial interest in its role in the reduction reaction of Cr2O72-ions[26]. Interestingly, due to the elimination of FRET, the addition of AA can excite the luminescence silencing system. Subsequently, we conducted a series of luminescent titration tests to investigate the detection capability of Cr2O72-＠1 in the water system. In addition, with the increasing AA concentration, the luminescence intensity at 450 nm (I450)increased overtly (Fig.6). Based on the AA concentrations andI450values, an obvious linear correlation between them was compiled well matched with the equation ofI/I0=12.8cAA+0.89 (R2=0.99) in the concentration range from 0 to 0.2 mmol·L-1. The limit of detection (LOD) was determined to be 0.78 μmol·L-1using the calculation method 3σ/KSV, whereσis the standard deviation ofI0in the blank solution andKSVrepresents the slope of the plot (1.28×104L·mol-1).According to our understanding, the LOD of Cr2O72-＠1 was comparable to, or even surpassed, that of MOFbased AA sensors described in the literature[27-30].Additionally, the recyclability and sensitivity of Cr2O72-＠1 in AA sensing have been conducted to comprehensively evaluate the performance of the fabricated sensor.As shown in Fig.7,theI450values increased rapidly and stabilized within 10 s. Furthermore, samples were added to the water solution containing 2 mmol·L-1of urine substances (Val, Gly, Thr, Arg, Ser, Hse, Glu, Met,Lys, Asp) (Fig.S7). It is noteworthy that the presence of other small biological molecules had minimal impact on the AA sensing process. The fast response time,excellent stability, selectivity and high sensitivity, reliable reusability, exceptional anti-interference capability, and low LOD collectively indicate that Cr2O72-＠1 is an outstanding sensor for the quantitative monitoring of AA.

Fig.6 (a)Emission spectra of Cr2O72-＠1 in the presence of AA solution(in water)with various concentrations;(b)Stern-Volmer plot for Cr2O72-＠1 detecting AA

Fig.7 Response time of Cr2O72-＠1 to AA

The above results indicate that the fabricated sensor exhibits a remarkable selectivity for AA. Hence,the Cr2O72-＠1 system could be deemed as a turn-on luminescence probe for sensing AA quickly. Moreover,the detailed “on-off-on” detection process has been exhibited in Scheme 1.

2.7 Fabrication of the “IMPLICATION” logic gate

Molecular logic gates, as a demonstration based on luminescence signals, were first conducted by Silva and his collaborators in 1993[31]. Since then, a large number of research about molecular logic gates have appeared in the literature[32-33]. Due to changes in the luminescence signal caused by AA and Cr （Ⅵ）, an “IMPLICATION” molecular logic gate based on Cr（Ⅵ）and AA was constructed (Fig.8). The output of this logic gate is represented by a colorimetric signal change induced by Cr（Ⅵ）and AA.The absence and presence of AA and Cr（Ⅵ）are defined as logic “0” and “1”,respectively. As for the output, when the luminescence intensity at 450 nm exceeds a specified threshold (0.2, relative intensity),it is defined as logic “1”;otherwise,it is defined as logic “0”. Therefore, the luminescence intensity of MOF 1 can display four possible input combinations： (0, 0), (1, 0), (0, 1), and (1, 1). The truth table and the corresponding combination logic circuit of the IMPLICATION logic gate are shown in Fig.8.This logic gate can achieve specific logic operations by evaluating the presence of Cr（Ⅵ）and AA.

Fig.8 “IMPLICATION” logic gate with AA and Cr（Ⅵ）as inputs：relative intensity of Cr2O72-＠1(a)and CrO42-＠1(b)in the form of a 3D bar for different input combinations,and the truth table(c)

2.8 Possible sensing mechanism

First, the XRD analysis revealed that the diffraction peaks obtained from the Cr（Ⅵ）ion-treatment samples were very similar to those of the as-synthesized sample. This indicated that the Cr（Ⅵ）ion treatment did not significantly alter the crystal structure of the Cd-MOF 1 (Fig.S5). Thus, the absorption bands of Cr（Ⅵ）(Cr2O72-and CrO42-) and the excitation band of 1 were measured. The absorption bands of Cr2O72-and CrO42-significantly overlapped with the excitation band of 1 (Fig.S8). This indicated that the absorbed energy by Cr（Ⅵ）hinders the efficiency of energy transfer between the metal central ion and ligands[34-35]. Consequently,1 fails to emit the luminescence,resulting in luminescence quenching. Furthermore, the luminescence decay lifetimes of 1 before and after the Cr2O72-ions treated were calculated. As shown in Fig.S9, the luminescence decay curve could be well adapted to the two-exponential function, yielding average lifetimes of 10.20 ns for 1 and 8.68 ns for Cr2O72-＠1. Overtly, the decay lifetime of Cr2O72-＠1 was shorter than that of the initial sample. Thus, the luminescent quenching mechanism was due to the combination of dynamic and static quenching[36-37]. Additionally, theζpotentials of 1 and Cr2O72-＠1 were demonstrated to be -20.1 and-18.3 mV,respectively,providing the existence of electrostatic interactions (Fig.S10). As illustrated in Fig.S11, the UV-Vis spectra indicated that there was no energy overlap between the excitation band of 1 and the absorption bands of CrO42-＠1-AA or Cr2O72-＠1-AA. Moreover, the luminescence lifetimes have significantly improved and almost recovered to the initial level. Thus, the potential mechanism for detecting AA in the luminescence silencing system of CrO42-＠1 or Cr2O72-＠1 has been elucidated as the disappearance of the FRET interaction between Cr（Ⅵ）and 1, increasing the luminescence intensity[38-39].

2.9 Application to real sample analysis

To verify the effectiveness of the Cr2O72-＠1 sensor in real samples, a spiked recovery rate of AA in vitamin C tablets was carried out. Thus, different concentrations of AA (4.0, 8.0, and 12.0 μmol·L-1) were added to the system of vitamin C tablets to determine the recovery rate. As shown in Table 1, the recovery rates of AA in vitamin C tablets were from 98.20% to 103.33%, and the relative standard deviations (RSDs)were from 1.78% to 3.42%.The results showed that the fabricated Cr2O72-＠1-based sensor could be used for the detection of AA in pharmaceutical formulation.

Table 1 Detection of AA in vitamin C tablets by Cr2O7 2-@1 sensor(n=3)

3 Conclusions

In summary, a unique 3D MOF (1) has been synthesized successfully by solvothermal assembly of a semi-rigid 3-(tetrazol-5-yl)triazole (H2L) ligand with Cd2+ions. Subsequently, the luminescence silencing system Cr2O72-＠1 was then obtained to detect AA by PSM,because 1 had remarkable luminescence quenching properties for Cr（Ⅵ）(Cr2O72-ions) in aqueous solution.Therefore,after the addition of AA to the Cr2O72-＠1 system, the luminescent silencing system was excited and enabled the quantitative detection of AA with low LOD. Thus, the potential mechanism for detecting AA in the luminescence silencing system of Cr2O72-＠1 can be attributed to the disappearance of the FRET interaction between Cr （Ⅵ）and 1. Additionally, based on the multi-response characteristics of Cr （Ⅵ）and AA, an “IMPLICATION” molecular logic gate has been fabricated. Thus, a convenient, effective, and low-cost AA detection strategy has been developed and could be used to detect AA in vitamin C tablets with recoveries and RSDs from 98.20% to 103.33% and 1.78% to 3.42%, respectively. The novel design route will direct the development of a luminescence silencing system to detect specific chemicals based on MOFs, which is important to protect the environment, health, and social security.

Acknowledgments:This work was supported by the Natural Science Foundation of Shanxi Province (Grant No.202203021212331), the Science and Technology Innovation Project of Colleges and Universities of Shanxi Province (Grant No.2022L532), the Scientific Research Project of Colleges and Universities in Anhui Province (Grant No.2022AH040142), and the project of Youth Elite Support Plan in Universities of Anhui Province(Grant No.2022-194).

Competing interests:The authors declare no competing financial interest.

Supporting information is available at http：//www.wjhxxb.cn