A Zn(II)-Organic Framework as a Fluorescent Probe for the Aqueous Phase Detection of Nitroaromatic Explosive and Cr(III) ion

doi:10.21203/rs.3.rs-934804/v1

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A Zn(II)-Organic Framework as a Fluorescent Probe for the Aqueous Phase Detection of Nitroaromatic Explosive and Cr(III) ion

https://doi.org/10.21203/rs.3.rs-934804/v1

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From the perspective of environment, civilian safety and human health it is utmost important to selectively and sensitively detect hazardous substances in the running water. In the last few decades, metal organic frameworks (MOFs) have been utilized for the detection of lethal substances in the aqueous phase using fluorescence method. Herein, a highly fluorescent MOF, Zn-MOF-1, namely, {[Zn₂(pydc)₂(DMF)]·2DMF}_n, based on the ligand pyridine-2,5-dicarboxylic acid (H₂pydc), was synthesized by employing solvothermal conditions. The Zn-MOF-1 was thoroughly characterized by crystallographic, PXRD, elemental, FTIR, and TG analysis. Single crystal data elucidated the exact structure of Zn-MOF-1. Due to its better emission spectra, the fluorescence sensing behavior of Zn-MOF-1 was checked in the aqueous medium. It was found that Zn-MOF-1 could detect Cr³⁺ with high sensitivity and selectivity and serve as an excellent fluorescent probe for TNP among other interfering nitroaromatic compounds in aqueous media. Furthermore, a plausible sensing mechanism has been demonstrated employing UV-Visible, fluorescence, and theoretical data. The observed K_sv values for TNP and Cr³⁺ are 2.1×10⁸ M^-1 and 1.46×10⁷ M^-1, respectively. Selective fluorescence quenching with high K_sv values and low detection limits validates the superior sensing performance shown by Zn-MOF-1 toward TNP and Cr³⁺. Thus, the fine-tuning of MOF for the detection of hazardous substances in wastewater is a challenging task and needs to address in future endeavors.

Inorganic Chemistry

Organic Chemistry

Metal-organic framework

TNP detection

Fluorescence quenching

Cr3+ sensing

From the point of constantly increasing pollution problems worldwide, novel functional materials have attracted considerable attention towards chemist due to its fast response time, high sensitivity, selectivity, and low limit of detection to sense the pollutants. Metal-organic frameworks (MOFs), a kind of evolving crystalline materials, comprises organic linkers and inorganic building blocks, and these rigid, stable, and emerging materials continue to be a subject of intense research [1]. Owing to their structural diversity, tunable pore size, and potential applications in a broad range of areas; for instance, drug delivery and release [2], heterogeneous catalysis [3], adsorption [4], gas storage, separation [5] and fluorescence sensing [6]. MOFs, as fluorescent probes, possess great superiorities including, low cost, simplicity, rapid response, high sensibility, liable recyclability, ease of handling and sample preparation [7]. On the other hand, the other detection methods such as atomic absorption spectroscopy, cyclic voltammetry, inductively coupled plasma mass spectrometry (ICPMS), gas chromatography − mass spectrometry, high-pressure liquid chromatography, ion mobility spectroscopy and raman spectroscopy [8] are costly, time consuming and tough to operate [9]. The abundance of fluorescent MOF materials has been scrutinized in the literature to date for sensing purpose which involves sensing of various hazardous substances such as cations-anions, volatile organic compounds, small molecules, Nitroaromatic compounds (NACs) [10]. NACs are among the most common malignant organic pollutants due to their toxic nature, highly explosive, and difficult to decompose naturally [11]. Among others, 2,4,6-trinitrophenol (TNP) is severely hazardous due to their high explosive nature, low safety coefficient, and widespread applications in fireworks, glass, matches, dyes, rocket fuels, and leather industries [12]. The effluents released by the industries into the environment containing TNP can results in severe environmental problems. Metabolism of TNP develops picramic acid as a byproduct which shows the tenfold mutagenic activity compared to TNP [13]. Moreover, the carcinogenic activities of TNP can results in various health related issues such as headache, anemia, improper liver functions, skin irritation and so forth [14]. Hence, to decrease the adverse belongings of TNP on the atmosphere, it is reasonably necessary to evolve such kind of sensing material that is reliable, convenient and efficient, in order to detect NACs for the above concerns. Fluorescence-based chemical sensors; for instance, conjugated polymers [15], nanoparticles [16], MOF based fluorescent probes [17] are highly in demand for selective and sensitive detection of NACs.

On the flip side, the inorganic pollutant, hefty metal cations, are becoming a topical issue due to their high toxicity and bioaccumulation [18]. Among the essential heavy metal ions, detection of Cr³⁺ ion is also crucial since it significantly impacts the metabolism of carbohydrates, fats, nucleic acids, and proteins. Cr³⁺ is an essential trace element for human health. The insufficiency of Cr³⁺ might cause diabetes, cardiovascular diseases, and atherosclerosis [19]. However, Cr³⁺ in excess is still detrimental to human health, which can combine with DNA in the body and results in mutation or malignant cells [20]. Chromium is employed in industries on a large scale, for instance, steel fabrication, electroplating, tanning, paint manufacturing. Chromium waste is developed from burning coal, oxidative dyeing, cooling towers and sanitary landfills, and the use of corrosion inhibitors in water pipes [21]. The improper disposal of industrial waste containing chromium can pollute soil and water. Consequently, damaging the environment and human health. Therefore, it is essential to trace Cr³⁺ ions fastly and sensitively, as it is beneficial for the protection of environment. Hence, it remains a challenge to design and synthesize a bifunctional fluorescent MOFs to detect Cr³⁺ ion and TNP with rapid response time, high sensitivity, and selectivity in water. Following the points mentioned above, we designed and synthesized a 3-D Zn-MOF using Zn(NO₃)₂·6H₂O and planer pydc ligand, namely, {[Zn₂(pydc)₂(DMF)]·2DMF}_n Zn-MOF-1, which was fortunately manufactured through a solvothermal method. The MOF was characterized using various spectroscopic technique and the definite structure was drawn by single crystal method. The Zn-MOF-1 can exploit as a dual fluorescent sensor to detect TNP and Cr³⁺ ion with a extremely low limit of detection in the aqueous phase. The better stability of Zn-MOF-1 can be the reason for its better sensing ability towards the hazardous substances in the water. By its good selectivity and sensitivity in sensing, it is competitive with current sensory materials.

2.1 Materials and methods

The reagents used in this paper were analytical grade and procured from trade sources and used as received. pyridine-2,5-dicarboxylic acid (pydc), zinc nitrate hexahydrate, Nitroaromatic compounds (NACs), and the metal salts of nitrate were procured from Sigma-Aldrich Chemical Co. India.

Caution

NACs are very reactive and explosives in nature and should be tested carefully and only in minute quantity. To prevent explosion, nitroaromatics were treated in safe manner.

2.2 Physical Methods

The solid state FTIR spectrum of Zn-MOF-1 within the range of 4000 − 400 cm^− 1 on the Perkin Elmer Model spectrum GX spectrophotometer. The Melting point of the compound was checked employing open capillary method. Further, the elemental analysis was obtained from Micro-Analytical Laboratory in CDRI, Lucknow, India. Perkin Elmer λ-45 UV visible spectrophotometer was used to record the electronic spectrum of the compound. The stock solution was made in 10^− 3 M solution in aqueous phase by maintaining the 1cm length of the cuvettes. Hitachi F-2700 spectrophotometer were used to record the emission spectra. “Miniflexll X-ray diffractometer” with Cu-Kα radiation was used to determine the PXRD patterns. The thermal behavior of MOF was checked using Shimadzu TGA-50H instrument with the heating of 20˚C min^− 1 from 25˚C to 800˚C.

2.3 Single crystal X-ray data

The Single Crystal X-ray diffraction of Zn-MOF-1 at 100 K was performed on Bruker SMART APEX CCD diffractometer. The graphite monochrome and Mo-Kα radiation (λ = 0.71073 Å) was used for X-ray data collection [22]. Employing SAINT SOFTWARE, the reduction and integration of data were created [23]. The SADABS was used for empirical absorption correction [24] and XPREP was used to investigate the space group in Zn-MOF-1 [25]. The structure refinement was done by exploiting least-squares techniques on F² by employing the OLEX-2 program package [26]. Furthermore, the non-hydrogen atoms were refined with anisotropic displacement parameters. The different hydrogen positions were fixed at calculated positions and refined isotopically. The CCDC reference numbers for Zn-MOF-1 is 2077312.

2.4 Sensing Experiments

In usual setup, the stock solution (1mM) of Zn-MOF-1 was prepared. The 3ml aqueous solution of Zn-MOF-1 was placed in a 1cm quartz cuvette on emission spectrophotometer. After excitation on 272 nm the fluorescence response was recorded. The incremental addition of nitro analytes and metal salts were added in the solution of Zn-MOF-1 and the emission spectra was observed. In the entire experiment the slit width of 2 nm was kept constant for both source and detector.

2.5 Synthesis of {[Zn₂(pydc)₂(DMF)]·2DMF}_n

The Zn-MOF-1 was synthesized by mixing 0.2 mmol (0.033 gm) of H₂pydc, 0.2 mmol (0.059 gm) of Zn(NO₃)₂·6H₂O in 3mL of DMF. Then the solution was sealed in a Teflon- line stainless-steel autoclave and heated for 72h at 100˚C. After that it was gradually cooled to room temperature over 12h. The small colorless round-shaped crystals were obtained in a yield of 48%.

Yield: 48%, m.p. 315˚C, Anal.Calcd (%) for C₁₇H₁₃N₃O₉Zn₂: C = 52.92; H = 11.02; N = 5.51; Observed: C = 52.89; H = 11.06; N = 5.54. IR (KBr, cm^− 1): 3438br, 2925m, 1614s, 1383s, 1281w, 763m, 533w, 423w.

3.1 Synthetic approach

To uncover the chemistry behind the luminescent metal organic system in terms of its sensitive and selective sensing of hazardous substance in the water. We have employed our understanding and constructed a Zn(II)-based metal organic system which is better than other MOF based materials in terms of their sensing ability using fluorescence method. Fortunately, we have successfully synthesized a fluorescent Zn-MOF-1, by employing a planer pydc ligand which possess various coordination modes (Scheme 1). In this work we applied solvothermal method to synthesize Zn-MOF-1. We choose this method because of high solubility of precursors and thereby leads to the production of high-quality MOF crystals. The choice of solvent plays an important role as well. The solvent utilized in the fabrication process which may assist in binding with metal ions and thus act as a coordinating ligand or can behave as a guest solvent molecule in the environment of the complex [28]. Therefore, to look over the impact of coordination affinity towards the solvent, Junhua Luo’s group carried out several reactions with distinct solvents such as DMF, DEF and DMA of H₃BPT and Cd(NO₃)₂·4H₂O [29], among which some coordination architecture have been reported; for instance, {[Cd₃(BPT)₂(DMF)₂]·2H₂O}, which possess solvent as a coordinating agent, comparable to our designed MOF Zn-MOF-1 in which DMF act as both coordinating ligand and guest molecule in the lattice environment (Scheme 2).

3.2 Structural explanation of Zn-MOF-1

The Single crystal x-ray data (SCXRD) was used to analyze the exact structure of Zn-MOF-1. The SCXRD analysis revelas that Zn-MOF-1 crystallizes in a monoclinic system with P1 21/n1 space group. Table 1 enlisted the crystal data and different refinement parameters, and selected bond lengths, angles are given in Table S1 of Zn-MOF-1. The structure of Zn-MOF-1, i.e., {[Zn₂(pydc)₂(DMF)]·2DMF}_n is assembled by the repeating units of binuclear Zn(II) species. The labeled molecular structure of Zn-MOF-1 is dispensed in Fig. 1. The interesting feature of Zn-MOF-1 is that the two different Zn atoms, Zn1 and Zn2, exist in the structure in two different coordination environments. Each Zn1 atom is coordinated by one N- atom of pydc ligand, four O- atom from four different pydc ligands, and one O- atom of DMF ligand. Zn1 acquire distorted octahedral geometry (Fig. 2a) with O4, O9, N1, O1 form the equatorial plane, while O8 and O5 occupying the axial position. Few important bond angles that describe the order of distortion around Zn1 atom are found as O9-Zn1-O4 = 166.81(6)˚, O1-Zn1-N1 = 170.13(7)˚, O5-Zn1-O8 = 177.78(6)˚, it supports the distorted octahedral geometry of Zn-MOF-1 [30]. On the other hand, similar coordination environment is found in Zn2 in terms of coordination to the pydc ligands. The coordination environment around Zn2 is filled by one N- atom of pydc ligand and four O- atoms of carboxylate moiety from four different pydc ligands with five coordination sites. The five-coordinated system can attain either triangular bipyramidal (tbp) geometry or square pyramidal (sq-py) geometry of Zn(II) ion. The Addison and co-workers introduced t parameter to distinguish between these two geometries [31]. The t parameter is in the range of 0–1 and at the utmost values giving perfect sq-py or tbp geometry, respectively. The closer the value of t to 0, the more relatable it is to sq-py geometry. We can calculate this parameter by applying the difference between the two largest coordination center angles and divide it by 60˚. Our calculation shows that Zn(2) has a distorted sq-py geometry with t = (159.25-147.84)/60 = 0.19 (Fig. 2b). The quasi-square plane i.e., base is formed by O4, O6, O7, and N2. The top of the pyramid i.e., apex is formed by O2. The important bond angles are obtained as O4-Zn2-N2 = 147.84(7)˚ and O7-Zn2-O6 = 159.25(7)˚. The oxygen (O4) of the same pydc ligand bridges the Zn1 and Zn2 atom. The bond angle between Zn2 with its neighboring atom is N2-Zn2-O4 = 147.84(7)˚, O7-Zn2-O4 = 100.38(6)˚, O6-Zn2-O7 = 159.25(7)˚, N2-Zn2-O2 = 110.59(7)˚; it also supports the distorted sq-py. For the sake of clarity, two uncoordinated DMF molecules in the lattice environment are squeezed. It should be noteworthy that, in Zn-MOF-1 pydc ligand exhibits two distinct coordination modes as shown in Fig. 2c. In mode-I, µ₂-η¹:η¹ bridging mode is adopted by one carboxylate group. In contrast, the O-atom of the second carboxylate group and N-atom of pyridine adopts the µ₁-η¹:η¹ chelating mode, and the other O-atom of the same carboxylate group is present in µ₁:η¹ bridging mode, while in mode-II pydc shows similar coordination modes as that of mode-I except for the second carboxylate group, which adopts µ₁-η¹:η² bridging mode. In both cases, the whole pydc ligands act as a µ₄- bridge to bind four different Zn(II) atoms in adjoining chains, resulting in a three-dimensional network structure (Fig. 3) [32, 33].

Table 1

Crystal structure and refinement data for **Zn-MOF-1**.
Compound	Zn-MOF-1
Empirical formula	C₁₇ H₁₃ N₃ O₉ Zn₂
Formula weight	534.10
Crystal color	Colorless
Temperature/K	100(2)
Crystal system	Monoclinic
Space group	P 1 21/n 1
a/Å	14.0774(10)
b/Å	12.1211(9)
c/Å	14.6325(10)
α/˚	90
β/˚	106.079(2)
γ/˚	90
Volume/Å³	2399.1(3)
Z	4
ρ_calcg/cm³	1.4786
µ/mm^− 1	2.047
F(000)	1075.0356
Crystal size/mm³	0.37
Radiation	Mo-Kα
Independent reflections	5975
WR₂(all)	0.0896
WR₂	0.0841
Final R Indexes [I > = 2σ(I)]	0.0294
Final R indexe(all data)	0.0363
Θ_min	2.18
Θ_max	21.91

3.3 FTIR, UV-Visible, PXRD and TG analysis of Zn-MOF-1

FT-IR spectrum of Zn-MOF-1 is given in Fig. 4a. The broadband observed at 3438 cm^− 1 can be consigned to the ν(C-H) pyridyl ring stretching vibration. Again, the weak band at 2925 cm^− 1 is characteristic of the ν(C-H) vibrations of the coordinated DMF molecule. The absence of characteristics band at around 1700 cm^− 1 which is assignable to –COOH groups designate that all carboxyl groups is completely deprotonated in Zn-MOF-1 upon reaction with metal ion [34]. For Zn-MOF-1, the two strong bands at 1614 cm^− 1 and 1383 cm^− 1 (Scheme S1), corresponding to the stretching vibrations of asymmetric and symmetric carboxyl groups, respectively [35]. The weak band at 1281 cm^− 1 are assigned to ν(C-O) vibrations and the absorption band observed at 763 cm^− 1 are due to δ(O-C-O) vibrations of the pydc [36]. The absorption at 533 cm^− 1 corresponds to ν(Zn-O) vibration, and the band in weak intensity at 423 cm^− 1 consigned to the ν(Zn-N) vibration [37]. The infrared spectra of Zn-MOF-1 are compatible with its structural features as resolved by SCXRD. The absorption spectrum of Zn-MOF-1 exhibited well resolved absorption band at 272 nm which is assignable to intraligand (n→π*, π→π*) transition. Zn-MOF-1 doesn’t show any d-d transition due to the closed shell d¹⁰ configuration as illustrated in Fig. 4b.

Thermal gravimetric analysis (TGA) was performed in presence of N₂ atmosphere, to inspect the thermal disintegration of Zn-MOF-1. The model pattern of TG curve exhibits two decomposition steps as shown in Fig. 4c. In very first step, TG curve displays the weight loss of 27.36% at 380˚C, corroborates the loss of two uncoordinated DMF in the lattice (Calc.: 27.50%), while in case of second step it exhibits weight loss of 76.26% at 550˚C which is corresponds to the loss of one coordinated DMF and two units of pydc ligand (Calc.: 76.52%). Then, no changes were found from 550˚C-800˚C and the final product left may be the metal residue. Additionally, the PXRD investigation were done on Zn-MOF-1 to affirm the phase purity of the sample. The simulated and as-synthesized patterns are quite similar to each other as display in Fig. 5. The PXRD patterns of Zn-MOF-1 was also recorded before and after immersing the sample in water for 24h. Both the pattern is almost identical confirming the robust nature and structural integrity of Zn-MOF-1 [38].

3.4 Fluorescence ability of Zn-MOF-1

After excitation at 272 nm the emission spectrum of Zn-MOF-1 in solution state shows a sharp intense peak at 385 nm Fig. 4b. The Zn(II) ions possess d¹⁰ configuration, thus it is not easily get oxidize or reduce, it indicates the absence of MLCT and LMCT behavior of Zn-MOF-1 [39]. Consequently, the band of Zn-MOF-1 can be consigned to intraligand fluorescence emissions. Additionally, to check out either the emission spectra are based only on ligand or depend on the entire moiety of Zn-MOF-1, the emission spectra of free ligand pydc upon excitation at 270 nm shows a moderate peat at 345 nm (Fig. 6.). Accordingly, it could be deduced that intense emission spectrum of Zn-MOF-1 doesn’t depend merely on ligand moiety instead its entire moiety is the reason behind the intense emission, thus Zn-MOF-1 is a fluorescent material. This inspire us to explore its potential application in the field of nitroaromatic compounds sensitive sensing and some other vital and lethal metal ions.

3.5 Selective and sensitive recognition of TNP

In order to understand the potential application of Zn-MOF-1 to sense a minute amount of nitro explosives, fluorescence-quenching titration was implemented by the incremental addition of nitro analytes to Zn-MOF-1 in H₂O. For fluorescence sensing experiment, specified nitro aromatics compounds were selected: TNP (2,4,6-trinitrophenol), ONP (2-nitrophenol), ONA (2-nitroaniline), MNA (3-nitroaniline), MDNB (1,3-dinitrobenzene). The fluorescence intensity of Zn-MOF-1 had a little effect on incremental addition of ONA, MNA, and MDNB (Fig. S1-S2, ESI), except for ONP, which shows considerable quenching behavior (Fig. S2b, ESI). Impressively, Zn-MOF-1 showed good sensing behavior and high fluorescence quenching (95%) for TNP on incremental addition of 10^− 3M (50µL) analyte (Fig. 7b). The marvelous fluorescence quenching of 54% was observed at a very little concentration of TNP i.e., 0.12 ppb in H₂O (details in ESI). As far as we know, Zn-MOF-1 shows the highest quenching efficiency as compare to that of reported MOFs [40] for sensitive sensing of TNP employing Zn-MOF-1 (Table 2). The quenching efficiencies of the selected analytes in descending order are as follows: TNP (95%) > ONP (87%) > MDNB (50%) > MNA (47%) > ONA (41%) (Fig. 7d). This indicates a strong interaction between TNP and Zn-MOF-1 with a detection limit of 0.12 ppb. This signify that Zn-MOF-1 exihibits super sensitivity for TNP detection in the aquatic phase, which is vital for environmental application purpose [41]. Using Stern-Volmer (S-V) equation [42] (details in ESI) we analyzed the quenching efficiencies of all the analytes to discover the cause behind the quenching mechanism for the selective and sensitive sensing of TNP. The plots with TNP and ONP show a linear increase at low concentration, which deviates from linearity and turn in an upwards direction with increasing concentrations. Whereas the other nitro aromatic analytes showed only linear trends in the S-V plots (Fig. 7c). A 3-D model of the S-V plots of Zn-MOF-1 for different nitro analytes in water is shown (Fig. 8). On the other hand the linear fitting plot (R² = 0.97) displays the significant quenching ability of Zn-MOF-1 towards TNP in water with Stern-Volmer quenching constant K_sv of 2.1×10⁸ for TNP and among the reported MOFs this is the highest value obtained for selective detection of TNP till date [43]. Two possible mechanisms for quenching the fluorescence intensity of electron-rich Zn-MOF-1 by electron-deficient TNP can be suggested on the basis of steady-state fluorescence measurements. It includes (1) photo-induced electron transfer (PET) mechanism and (2) fluorescence resonance energy transfer (FRET) mechanism. The highly selective detection of TNP by Zn-MOF-1 might be assigned to PET mechanism. In general, the lowest unoccupied MOs (LUMO) of nitro analyte (electron-deficient) lies between the valence band (VB) and conduction band (CB) of the MOFs [44]. Upon excitation, the excited electron resided in the CB of Zn-MOF-1 can easily transfer to the LUMO of electron-deficient analytes. Therefore, there is quenching in fluorescence intensity of Zn-MOF-1. However, in case of analytes having electron-rich systems, the LUMO lies above the CB of the MOFs, excited-state electrons from the LUMO of the analytes will be transferred to the CB of the MOFs which leads to the enrichment of the fluorescence intensity. This was well elaborated by various research articles published in the literature. Mukul et al. reported the insight of PET mechanism by employing the theoretical calculations such as DFT. In this article the 2,6-pyridine dicarboxylic acid (pda²⁻) ligand has been employed for the formation of three Ln-based MOF materials. The DFT calculations can be employed to calculate the HOMO-LUMO energy gap. The energy gap calculated using DFT does not in good agreement of PET mechanism which means that the PET mechanism is not the solely mechanism responsible for the sensitive and selective detection of TNP. This was also well established in the few other articles in which author used DFT calculations for confirming the mechanism associated with the sensitive and selective detection of nitro analytes [45]. Though, the fluorescence quenching for all other nitro analytes is not in line with their LUMO energy trend except TNP, this indicate that there were some other reasons for drastic quenching which synchronized with electron transfer. The FRET mechanism transfer resonance energy from fluorophore to non-emissive analyte. The absorption bands of the nitro analytes must overlap effectively with the emission band of the fluorophore [46]. To examine this, the normalized emission spectra and UV spectra of the Zn-MOF-1 and for all the nitro analytes were recorded. The overlap graph was plotted between the relative intensity and the wavelength. As shown in Fig. 9 it is found that TNP showed maximum overlap with the fluorescence emission spectrum of Zn-MOF-1. On the other hand, it was relatively less or almost no overlap was found for other nitro analytes. This is in plausible accordance with the high quenching efficiency observed for TNP as compare to other nitro analytes. Consequently, it is eminent in TNP that it involves both electron and energy-transfer mechanisms where as only electron-transfer mechanisms could employ by other nitro analytes. Thereby collaborative involvement of such processes, viz. PET and FRET, that may lead to the enhancement of quenching efficiency of TNP with a very good detection-sensitivity limit. Furthermore, with the help of X-ray structure it was well established that there is no evidence of porosity in the structure of Zn-MOF-1. Hence the trapping of nitroaromatic analytes on the pores of Zn-MOF-1 was nullified. Moreover, the identical PXRD patterns of Zn-MOF-1, before and after soaking in the solution of TNP for 24h specify that there is no evidence of degradation of Zn-MOF-1 after immersing it in TNP as specified in Fig. 5 which authenticate the robust nature and stability of the MOF. The particular Zn-MOF-1 can be used for different cycle of fluorescence sensing and to trace the TNP in water.

3.6 Selective and sensitive detection of Cr³⁺

To check the sensing behavior of Zn-MOF-1, we prepared stock solution of different metal ions M(NO₃)_x ( M = Na⁺, K⁺, Ba²⁺, Sr²⁺, Mn²⁺, Co²⁺, Ca²⁺, Zn²⁺, Mg²⁺, Sn²⁺, Cd²⁺, Pb²⁺, Cu²⁺, Hg²⁺, Cr³⁺, Cr⁶⁺) in aqueous phase and gradually it was added to the aqueous solution of Zn-MOF-1 and then the fluorescence spectra were recorded. It was detected that the fluorescence intensity was moderately enhanced in Zn²⁺, Sr²⁺, Na⁺, Mn²⁺, Co²⁺, Ca²⁺, Mg²⁺, Cr⁶⁺, Ba²⁺, Sn²⁺, Cd²⁺, Pb²⁺, Cu²⁺, and K⁺ ions except in the case of Hg²⁺ ion which shows a mild quenching behavior. As shown in Fig. 10a, a notable quenching was detected by Cr³⁺ ion. The quenching efficiency was calculated to be 43% after the addition of only 10^− 3M (50µl) of Cr³⁺ ions. The quenching efficiency affiliated with Hg²⁺ ions was 11% which is the highest value in comparative to other metal ions, Fig. 11a. This suggested that Zn-MOF-1 can be used selectively for the sensing of Cr³⁺ ions in an aqueous solution. From the practical point of view selectivity in sensing is a very crucial factor in biological and environmental systems where numerous metal ions may coexist. To further explore the sensitivity of Zn-MOF-1 as a Cr³⁺ sensor the fluorescence quenching titrations experiment were done by gradual addition of different concentrations of Cr³⁺ ions. Fluorescence intensity of Zn-MOF-1 moderately decreased as the concentration of Cr³⁺ ion increases (Fig. 10b). Moreover, the Stern-Volmer constant, K_sv = 1.46×10⁷ obtained from linear fitting of the plot (R² = 0.93) (details in ESI). Zn-MOF-1 exhibits the highest K_sv value reported for selective and sensitive sensing of Cr³⁺ ion (Table 3). Competitive experiments were carried out to explore the selectivity for sensing Cr³⁺ ions over various interfering metal cations. As shown in Fig. 11b, upon the addition of Cr³⁺ ions to the mixture of Zn-MOF-1 and other interfering cations, it was observed that the fluorescence is remarkably quenched, indicating that Zn-MOF-1 can selectively and effectively detect Cr³⁺ ion. It was illustrating that Cr³⁺ ions can still be sensed when it is accompanied by other metal ions.

As reported in previous literature, the quenching mechanisms of the fluorescent MOFs of various metal ions mainly attributed to (i) the structure of MOF being collapsed by the metal ions; (ii) exchange of cations between the metal centers of MOFs and the objective cations; (iii) competetive absorption and fluorescence resonance energy transfer (FRET) between fluorophores in MOFs and incoming metal ions [47]. The following experiments were performed to elucidate the mechanism of Zn-MOF-1 as a sensor towards Cr³⁺ ion through quenching effects. PXRD patterns of Zn-MOF-1 were recorded before and after soaking in the solution of Cr³⁺ for 24h to authenticate the extra stability of Zn-MOF-1 (Fig. 5). Secondly, Considering the neutral framework of Zn-MOF-1, it is not possible that the fluorescence sensing results from cations exchange. As shown in Fig. 12 the UV-visible absorption spectrum of various metal cations was measured, which showed that solely Cr³⁺ significantly overlapped with the excitation band of Zn-MOF-1, which suggested that the competitive energy absorption between Zn-MOF-1 excitation and Cr³⁺ ion absorption probably happened and resulted in fluorescence quenching of Zn-MOF-1. Therefore, it can be deduced that the fluorescence quenching mechanism of Zn-MOF-1 for sensing of Cr³⁺ can be attributed to both competitive energy absorption and FRET.

To calculate the detection limit, TNP (5–50µl, 1mM stock solution) and Cr³⁺ (50–300µl, 1mM stock solution) were added to Zn-MOF-1, and noted the equivalent fluorescent intensities by plotting the graph between fluorescence intensity and increasing concentration of the analyte, then calculated the slope (m) of the analogue graph (Fig S3, S4, ESI). Moreover, from five blank measurements standard deviation (σ) was also calculated of Zn-MOF-1 (Table S2, S3, S4, ESI), and the detection limit of Zn-MOF-1 was calculated by employing the formula: Detection limit = (3σ/m). TNP has detection limit of 0.12 ppb while 0.62 ppb was found in the case of Cr³⁺.

Table 2

Selected MOF with TNP sensing and their K_sv Value.
Fluorescent sensing Materials	Analyte	Ksv(M^− 1)	Detection limit	Ref.
[Zr₆O₄(OH)₄(BTDB)₆]·8H₂O·6DMF	TNP	2.49 × 10⁴	1.63ppm	48
[Eu₂(L)₂(DMA)₂]n·nH₂O	TNP	3.58 × 10⁴	4.66 × 10^− 4mM	49
Bio-MOF-1	TNP	4.6× 10⁴	2.9ppb	50
[Tb(1,3,5-BTC)]	TNP	3.41× 10⁴	8.1 × 10^− 8 M	51
UiO-68@NH₂	TNP	5.8 × 10⁴	0.4 ppm	52
Y-MOF:Eu	TNP	3.21× 10⁴	4µM	53
Y-MOF:Tb	TNP	3.19 10⁴	6.5µM	53
Zn-MOF-1	TNP	2.1×10⁸	0.12ppb	This Work

Table 3

Selected MOF with Cr³⁺ sensing and their K_sv Value.
Fluorescent sensing Materials	Analyte	Ksv(M^− 1)	Detection limit	Ref.
[Zn₂(TCBPDC)_0.5(H₂O)₂]_n	Cr³⁺	7.32×10³	0.56ppm	54
[Eu₂(tpbpc)₄·CO₃·4H₂O]·DMF·solvent	Cr³⁺	5.14×10²	3.64ppm	55
[Tb(hfac)₃(NITPh-Pa)₂][0.5CH₃(CH₂)₅CH₃]	Cr³⁺	1483.82	2.34ppm	56
[Zn (tbda)]_n	Cr³⁺	2.68×10³	9.36ppm	57
[CH₃-dpb]₂[Mg₃(1,4NDC)₄(µH₂O)₂(CH₃OH)(H₂O)]·1.5H₂O	Cr³⁺	0.15 × 10⁵	960ppm	58
Zn₃(bpdc)₂(pdc) (DMF)·6DMF	Cr³⁺	3.870×10³	25.1ppm	59
Zn-MOF-1	Cr³⁺	1.46×10⁷	0.62ppb	This work

In summary, we successfully constructed and structurally characterized a fluorescent 3D Zn-MOF-1. The MOF was synthesized to explore sensing ability towards different hazardous substances. Using slow evaporation method, the MOF was synthesized and with the help SCXRD the exact structure of MOF was confirmed. The PXRD pattern and TG analysis reveals the bulk purity as well as thermal stability of the Zn-MOF-1. Significantly, Zn-MOF-1 displays strong fluorescence emission and can be used as a potential fluorescent probe for efficiently detecting TNP and Cr³⁺ ion in aqueous solution. The detection limit of Zn-MOF-1 towards TNP and Cr³⁺ were calculated with the values of 0.12 ppb and 0.62 ppb, respectively. Furthermore, the possible fluorescence quenching mechanism of Zn-MOF-1 for TNP and Cr³⁺ were also elucidated. Consequently, the present study revealed that fluorescent Zn-MOF-1 can be rationally constructed and scrutinized as a potential fluorescent sensing material for multi-functional applications in future endeavors.

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scheme1.jpg
Different coordination modes of pydc2- ligand.
scheme2.jpg
Synthetic route of Zn-MOF-1.
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GRAPHICALABSTRACTpictogram.docx
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A Zn(II)-Organic Framework as a Fluorescent Probe for the Aqueous Phase Detection of Nitroaromatic Explosive and Cr(III) ion

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental Protocols

2.1 Materials and methods

2.2 Physical Methods

2.3 Single crystal X-ray data

2.4 Sensing Experiments

2.5 Synthesis of {[Zn₂(pydc)₂(DMF)]·2DMF}_n

3. Results And Discussions

3.1 Synthetic approach

3.2 Structural explanation of Zn-MOF-1

3.3 FTIR, UV-Visible, PXRD and TG analysis of Zn-MOF-1

3.4 Fluorescence ability of Zn-MOF-1

3.5 Selective and sensitive recognition of TNP

3.6 Selective and sensitive detection of Cr³⁺

4. Conclusion

References

Supplementary Files

Status:

Version 1

A Zn(II)-Organic Framework as a Fluorescent Probe for the Aqueous Phase Detection of Nitroaromatic Explosive and Cr(III) ion

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental Protocols

2.1 Materials and methods

2.2 Physical Methods

2.3 Single crystal X-ray data

2.4 Sensing Experiments

2.5 Synthesis of {[Zn2(pydc)2(DMF)]·2DMF}n

3. Results And Discussions

3.1 Synthetic approach

3.2 Structural explanation of Zn-MOF-1

3.3 FTIR, UV-Visible, PXRD and TG analysis of Zn-MOF-1

3.4 Fluorescence ability of Zn-MOF-1

3.5 Selective and sensitive recognition of TNP

3.6 Selective and sensitive detection of Cr3+

4. Conclusion

References

Supplementary Files

Status:

Version 1

2.5 Synthesis of {[Zn₂(pydc)₂(DMF)]·2DMF}_n

3.6 Selective and sensitive detection of Cr³⁺