A Luminescent Cu(II)-MOF with Lewis Basic Schiff Base Sites for the Highly Selective and Sensitive Detection of Fe3+ Ions and Nitrobenzene

A Schiff base functionalized Cu(II)-based metal–organic framework (MOF) denoted as Cu-L, was developed via a solvothermal method using low-cost starting material, i.e., Schiff base linker, 4,4'-(hydrazine-1,2-diylidenedimethylylidene)dibenzoic acid (L). Good crystallinity and thermal stability of synthesized Cu-L was confirmed by the crystallographic and thermogravimetric studies. An excellent photoluminescent properties of Cu-L ensure their suitability for the ultrafast detection of Fe3+ ions and nitrobenzene via a turn-off quenching response. The remarkable sensitivity of Cu-L towards Fe3+ ions and nitrobenzene was certified by the low limit of detection (LOD) of 47 ppb and 0.004 ppm, respectively. With incorporated free azine groups, this MOF could selectively capture Fe3+ ions and nitrobenzene in aqueous solution. The plausible mechanistic pathway for the quenching in the fluorescence intensity of the Cu-L in the presence of Fe3+ ions and nitrobenzene have been explained in detail through the density functional theory calculations, photo-induced electron transfer (PET), fluorescence resonance energy transfer (FRET), and competitive energy adsorption. This present study open a new avenue to synthesize novel crystalline MOF-based sensing materials from cheap Schiff base linkers for fast sensing of toxic pollutants.


Introduction
The booming development in industries and human society discharged diverse range of toxic pollutants including small organic molecules, metal ions, anions, and explosives etc. into the environment, which have a lethal influence on living organisms. Among diverse range of pollutants, there is urgent demand for the sensing of toxic small organic molecules, which are used as raw materials in valuable organic compounds synthesis. The excessive ingestion of these toxic organic pollutants like nitrobenzene may cause severe disorders like vomiting, coma, and even death. Similarly, metal ions, particularly Fe 3+ is common inorganic pollutant, whose recommended standard level in water is 0.3 ppm [1]. It is the most indispensable transition metal ion in human body which play a crucial role for normal functioning of central nervous system and transports oxygen [2]. However, its concentration must be controlled because excess iron leading to an DNA damage, choroiditis, conjunctivitis, and retinitis [3]. On the other hand, iron deficiency results in the sleep loss, low oxygen supply to cells (which results in anemia), skin diseases, and lower blood pressure. Therefore, developing highly sensitive and selective sensors for rapid and effective detection of small organic molecules and metal ions is an extremely urgent issue concerning homeland security, environmental protection and humanitarian concern. 1 3 Presently, different techniques, e.g., voltammetry, atomic absorption spectroscopy, and spectrophotometry etc. are used to detect Fe 3+ and nitrobenzene [4]. However, majority of these techniques required complicated sample preparation and exhibit low sensitivity. Therefore, the development of effective methodologies for the quick and sensitive determination of Fe 3+ ions and nitrobenzene is much more meaningful. Fluorescence based chemosensor owing to their simplicity, short response time, and high sensibility have attracted much attraction from researchers [5]. In this context, metal-organic frameworks (MOFs) have been recently emerged as viable platform owing to their porosities, adjustable functionalities, and good emission properties [6,7]. Despite a large number of fluorescent MOF materials, only a few fluorescent MOF materials have capability to detect Fe 3+ ions in aqueous media. This may be due to their poor water stability, which limits their utility to organic solvents only [1]. This restriction motivates us to build the desired water stable MOF sensor. To develop such type of MOF, a Schiff base type ligand was selected in this study due to the subsequent motives: (1) low-cost and ease of synthesis; (2) this type of ligands offer chemically stable materials; and (2) it also contains electron pair rich nitrogen for interactions with electron poor metal ions, which in turn change the emission signal intensity.
Herein, we report a Schiff base decorated Cu(II)-based MOF material, coded as Cu-L MOF prepared using inexpensive 4,4'-(hydrazine-1,2-diylidenedimethylylidene)dibenzoic acid (L) Schiff base ligand. The as-synthesized Cu-L has been utilized as a hurriedly responsive sensor for the identification of metal ions and small organic molecules. Interestingly, Cu-L selectively detect Fe 3+ ions and nitrobenzene via significant turn-off quenching response. Moreover, Fe 3+ ions detection by Cu-L was not interfered by the competing metal ions, which is highly encouraging for potential applications in wastewater treatment and water-quality monitoring. Detailed mechanistic studies and density functional theory (DFT) calculations were done to endorse the mode of action of Cu-L.

Characterizations
The Fourier transform infrared (FTIR) spectrum of L and Cu-L MOF were achieved on FTIR Spectrophotometer (RXIFT, Perkin Elmer). 1 H and 13 C nuclear magnetic resonance (NMR) spectra for the L was gathered on a Bruker Avance NEO 500 MHz spectrometer calibrated with respect to the reference material tetramethylsilane (TMS) in DMSOd 6 solvent. Powder X-ray diffraction (PXRD) patterns were collected in the 2θ range 5° to 50° using XPERT PRO Powder X-ray Diffractometer [CuK α X-ray (λ = 1.5406 Å)] for crystal structure determination. Thermogravimetric analysis (TGA) analysis was performed using Hitachi STA7300, Japan in the temperature range from 35 °C to 700 °C with a heating rate of 10 °C/min under a pure nitrogen atmosphere. Brunauer-Emmett-Teller (BET) surface area analyzer (Microtrac Belsorp Mini-II, Bel, Japan, Inc) was used for the measurement of specific surface area and pore size distribution of Cu-L sensor. Morphological analyses of Cu-L was done on JEOL, JSM-6510LV scanning electron microscope (SEM). EDS spectrum of gold coated sample rigidly mounted on the specimen stub was recorded using Oxford instrument (INCA X-act) EDS spectrometer equipped as an additional accessary to SEM. Absorption and photoluminescence studies were conducted using spectrophotometer (Shimadzu UV-Vis 1800 model) and spectrofluorophotometer (Shimadzu RF-5301PC), respectively.

Computational Methods
The theoretical calculations were performed using Gaussian software of GaussView version 5.08 to support the experimental results. All theoretical calculations were done using Gaussian 09 Version. Calculations to determine geometrical optimization, the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) of Schiff base ligand L and small organic molecules were performed at DFT levels by using the Becke, 3-parameter, and Lee-Yang-Parr (B3LYP)/6-311G(d, p) method.

Synthesis of Cu-L MOF
The Cu-L MOF was synthesized according to the route as shown in Fig. 1. A mixture of L (0.1 mmol, 0.0296 g) and Cu(NO 3 ) 2 ·3H 2 O (0.1 mmol, 0.0241 g) was dissolved in 10 mL DMF in a 15 mL glass vial via ultrasonication for

Results and Discussion
Characterization of L and Cu-L

H NMR
The formation of L was established by the 1 H NMR results recorded in DMSO-d 6 . The L ligand being a symmetrical molecule, its proton count comes out to be half because all the protons at symmetrical position resonates at same position in 1 H NMR. As shown in Fig. 3, a sharp singlet appearing at 8.80 ppm was attributed to the imine -CH = N-proton and confirmed the L formation. A broad singlet appearing at 13.22 ppm was attributed to the protons of carboxylic acid groups. A two proton doublet located at 8.07 ppm was assigned to the H-2 and -6 of the benzene ring. Another two proton doublet appeared at 8.02 ppm was ascribed to the H-3 and -5 of the benzene ring. Sharp peaks located at 2.50 and 3.40 ppm and they were ascribed to the solvent DMSO and H 2 O, respectively. The absence of extra peaks confirmed the purity of the synthesized L.

C NMR
The structure of the L was also verified by 13 C NMR conducted in DMSO-d 6 . As shown in Fig. 4, the carbonyl carbon of carboxyl acid group of the L appeared as a sharp singlet at 166.70 ppm. The imine carbon appeared at 160.85 ppm confirmed the successful formation of L. The carbon atom of the benzene ring directly connected to the nitrogen of imine group appeared as a singlet at 137.38 ppm. The carbon atoms directly connected to the two carboxylic acid group appeared as a singlet at 132.97 ppm. The carbon atom of the benzene ring adjacent to the carbon connected to the carboxylic acid group appeared as a singlet at 129.69 ppm. The carbon atom of the benzene ring adjacent to the imine

FTIR
FTIR spectra of L showed a new peak at 1620 cm −1 corresponding to imine group (-CH = N-) compared with FTIR spectra of reactant 4-FBA, can also confirmed the successful formation of L (Fig. 2). FTIR analysis was also provide the conclusive evidence of formation of Cu-L. The FTIR spectra of Cu-L showed no peak at 1678 cm −1 corresponding to -COOH group, indicates that L coordinates with copper in Cu-L MOF through -COOH group (Fig. 2). This was also confirmed by the appearance of new symmetric and antisymmetric metal carboxylate stretching bands at 1,382 and 1,588 cm -1 , respectively (Fig. 2). The difference in the antisymmetric and symmetric carbonyl stretching frequencies (v) for Cu-L was 251 (Δν = 1588 − 1382 = 206) significantly lower than L (Δν = 1678 -1428 = 250), indicating the chelating bidentate coordination mode of carboxylate moieties in this MOF [8]. The appearance of peak at Cu-O at 753 cm −1 , which is another conclusive evidence of coordination between L and Cu 2+ . These results indicated that high purity Cu-L MOF was synthesized successfully. Band centering near 1,704 and 3,395 cm -1 was ascribed to v C = O and O-H vibrations indicating the presence of lattice DMF and water molecules, respectively.  Figure 5a shows the PXRD pattern of the synthesized Cu-L. The observed diffraction peaks of Cu-L at the 2ϴ scale was in good agreement with the simulated pattern reported for the UoB-5 indicating that both of them (Cu-L & UoB-5) are isostructural with the cubic lattice structure [8]. This result indicated that Cu-L having cubic lattice structure was successfully synthesized. Moreover, PXRD analysis was also used to evaluate the phase purity and crystallinity of the synthesized MOF material. As shown in Fig. 5a, the recorded PXRD pattern indicated the sharp peaks, confirmed the highly crystalline Cu-L MOF material was produced. Moreover, the sharp peaks appeared at the low angel area (2θ < 10) showed a high degree of mesoscopic ordering. Absence of extra peaks confirmed the high phase purity of Cu-L. Unfortunately, all efforts used for the preparation of single crystal of Cu-L MOF from Schiff base ligand L such as use of series of modulators (like acetic acid, benzoic acid, and formic acid) in varying concentrations and mixture of solvents has been failed. Hence, based on PXRD, the structure of Cu-L was proposed which is presented in Fig. 6.

Thermogravimetric Analysis (TGA)
TGA curve of Cu-L showed its thermal stability up to 280 °C (Fig. 5b). As is clear from the TGA curve, Cu-L was released two main weight loss. The first weight loss of about 7.1% happened around 300 °C, due to the loss of solvent molecules residing in the pores. Another sharp weight loss of about 34.99% occurred in 500-600 °C that signified the decomposition of Cu-L framework structure. Thus, these results showed that Cu-L MOF was thermally stable up to 300 °C, after which its thermal degradation occurred.

Brunauer-Emmett-Teller (BET)
The N 2 adsorption/desorption at 77 K was used to investigate the surface area of the synthesized Cu-L. Figure 7 shows the N 2 adsorption-desorption isotherm of Cu-L sensor. The sorption curve showed a typical IV isotherms characteristics, which revealed a hierarchically mesoporous structure of Cu-L sensor. The analysis of the sorption curve was done using the Brunauer-Emmett-Teller (BET) method to measure the surface area, pore-volume, and pore-size distribution of Cu-L (Fig. 7). The results demonstrated that the Fig. 7 The adsorption-desorption curves of Cu-L and inset showing the pore diameter distribution surface area and pore value of the synthesized Cu-L were 94 m 2 /g and 0.48 cm 3 /g, respectively. The relevant distribution curve of the pore diameter, calculated by BJH method, displayed that the sample exhibits the average pore diameter of 19.23 nm (Fig. 7 inset).

SEM
The structural and topographical morphology of Cu-L was explored by SEM analysis. As shown in Fig. 8a, b, the SEM micrographs of Cu-L MOF was recorded at different magnifications of × 1,000, and × 2,000, respectively. According to these micrographs, the Cu-L has cube-like morphology, which is quite similar to the cube-shaped morphology reported for the isostructural MOF, ie., UoB-5 in literture [8]. Moreover, the cubic morphology of Cu-L was in complete agreement with the obtained results of PXRD pattern. This comparison confirmed the successful formation of Cu-L MOF. Thus, it can be concluded that both PXRD and SEM analysis validated the sucessful formation of high quality crystalline Cu-L MOF.

EDX
Energy dispersive X-ray (EDX) analysis was carried out to explore the elemental compositions of Cu-L. The EDX spectra of Cu-L is shown in Figs

Photoluminescent (PL) Properties
MOFs gathered from conjugated organic ligands are known to exhibit excellent PL properties. Hence, the PL emission spectra's of aqueous suspensions of L and Cu-L were recorded. L showed a broad emission peak centered at 397 nm under 302 nm excitation, which can be attributed to the ligand centered π* → π or π* → n electronic transitions (Fig. 10a). However, as clear from Fig. 10a, Cu-L showed a significantly red shifted emission peak at 413 nm under 308 nm excitation compared to L. This attributed to the coordination of L to Cu within framework, which enhanced the stiffness of the ligand L, thereby energy loss decreases and electron transitions were effected.

Quantum Yield
As shown in Fig. 11, the as-prepared Cu-L displayed good photoluminescent behavior with the ~ 67% of fluorescence quantum yield using quinine sulfate in 0.5 M H 2 SO 4 (QY = 54%) as a reference standard. The calculated high fluorescence quantum yield was a satisfactory for fluorescent probe to be used as fluorescence sensing

Selectivity and Sensitivity of Cu-L Towards Fe 3+
To evaluate its metal ions sensing ability, the powdered sample of Cu-L MOF (3 mg) was dispersed in 2 mM aqueous solutions (3 mL) of 12 different metal ions of Ca 2+ , Ni 2+ , Zn 2+ , Cd 2+ , Na + , Al 3+ , K + , Ba 2+ , Cu 2+ , Pb 2+ , Co 2+ , and Fe 3+ , respectively, to form the Cu-L MOF/ M n+ suspensions. The emission intensities of these metal ion fused MOF suspensions were noted and compared (Fig. 10b-d). Excitingly, it was establish that the emission intensity of Cu-L is prominently reliant on the metal ions identities. Ca 2+ , Ni 2+ , K + , Pb 2+ , and Na + have insignificant quenching effect on the emission of Cu-L MOF. On the other hand, Ba 2+ , Cd 2+ , Zn 2+ , Co 2+ , Cu 2+ , and Al 3+ have reasonable quenching effect, but Fe 3+ ion implies the most noteworthy quenching effect based on the emission intensity compared with itself. Fe 3+ induced the 86.3% quenching of emission of Cu-L MOF. These outcomes designate that Cu-L MOF selectively sense Fe 3+ ions in water.
The sensitivity of Cu-L towards ferric was explored by the recording the emission quenching effects of Cu-L as a function of 10-350 μM concentrations of Fe 3+ (Fig. 12a). It was observed that the emission intensity for Cu-L at 413 nm progressively decline as the concentration of Fe 3+ increases. As is renowned, the emission quenching efficiency can be quantitatively considered by the Stern-Volmer (SV) equation: (I 0 /I) = 1 + Ksv[M], where [M] represents the molar concentration of Fe 3+ ions (μM); I 0 and I signify the emission intensities of Cu-L in the presence and absence of Fe 3+ , respectively; and Ksv is the quenching constant (M −1 ) [9]. The SV plot of Cu-L towards Fe 3+ was linear over all concentration range (Fig. 12b). The K sv value was calculated to be 2.0 × 10 3 M -1 , which is analogous to another Fe 3+ luminescent MOF sensors (Table 1). Considering the K sv value It should be imperative to be remark here that many metal ions often coexist in environmental systems. To fix the sensing selectivity for Fe 3+ , the influence of other metal ions was explored. Excitingly, mixed aqueous solution of previously used 11 metal ions (2 mM) expect Fe 3+ moderately quenched the emission intensity of Cu-L by about 20% compared to blank suspension (3 mg Cu-L in 3 mL pure water). Such unpredicted results demonstrate that the quenching effects of these metal ions are mutually interfered. However, the addition of Fe 3+ along with mixed aqueous solution of 11 metal ions dramatically quenched the emission intensity by 86% (Fig. 13a). These outcomes discloses that Fe 3+ can be solely detected by Cu-L via turn-off quenching, without interference by other metal ions. This solely Fe 3+ detection is prominent, because utilization of such sensors can evade challenging pretreatment processes to exclude the interference from other cations.

Mechanism for Fe 3+ Detection
In general, the luminescent MOFs based detection of metal ions was centered on four reasons: (1) ruining of crystalline structure; (2) ion-exchange of metal node of MOF with target metal ion; (3) competition between target metal ion and MOF for excitation energy; 4) electron and energy transfer between target metal ion and MOF [10][11][12][13]. Considering these reasons, the mechanisms for detection of Fe 3+ by Cu-L were probed in detail. Consistent PXRD pattern of Cu-L/Fe 3+ and original Cu-L excluded the ruining of crystal structure of Cu-L in the presence of Fe 3+ (Fig. 14). The quenching of emission of Cu-L happened within short time in just 1 min implies that detection of Fe 3+ should not be attributed to the metal-exchange between Cu-L framework and Fe 3+ (Fig. 13b) [13][14][15]. Partial spectral overlap between the excitation spectrum of the Cu-L and the absorption spectrum of Fe 3+ signifies that Fe 3+ partially absorbed the excitation light of Cu-L, thereby quenched the emission intensity (Fig. 13c). Hence, it should be noted that competitive energy absorption is active in our case.   Additionally, definitive overlapping between emission of Cu-L and the absorption of Fe 3+ , implies that light emitted by Cu-L is absorbed by the Fe 3+ and ultimately lead to quenching of emission of Cu-L (Fig. 13d). However, other metal ions have no evident overlap. Thus, we can say that spectral overlapping is fully in accordance with the maximum quenching efficiency observed for Fe 3+ . These outcomes suggest that fluorescence resonance energy transfer (FRET) also participate in quenching of emission of Cu-L.
Then, photo-induced electron transfer (PET) might be another possible reason for quenching of emission of Cu-L. The presence of free electron-rich Lewis basic Schiff base sites in the pores of Cu-L can donates their lone-pair electrons to the Fe 3+ via coordination interactions (Fig. 15). So, PET may also quenched the emission of Cu-L in the presence of Fe 3+ ions. Thus, it can be establish that the detection of Fe 3+ by Cu-L MOF was be sure of combination of PET, FRET, and competitive energy absorption mechanisms.

Comparison of Cu-L with Reported Sensors for Fe 3+ Detection
As clear from Table 1, most of literature of Fe 3+ identification via MOFs are principally rely on the fluorescent lanthanide MOFs. Majority of these MOFs work in organic solvents such as CH 3 OH, DMSO, and DMF, owing to their low water stability, which definitely hamper their practical utilization [1,[16][17][18]. On the other hand, some water stable lanthanide MOFs and MIL-53(Al) recognize Fe 3+ in water, however, their action mainly based on the ionexchange between the framework metal ions of MOFs and Fe 3+ ions in water [11,19]. This ion-exchange route often time-consuming in that way dropped the effectiveness and applicability of the detection. Notably, such category of MOF-based sensors cannot be reused unless additional treatment was done though ion exchange.
To  [20][21][22][23][24]. Generally, detection limit are used to access the sensing performance of these luminescent sensors. Small detection limit are the features of good fluorescent sensor. As shown in Table 1

Selectivity and Sensitivity of Cu-L Towards Nitrobenzene
As encouraging sensing materials, luminescent MOFs have been extensively used to identify small organic pollutants through the change of emission signals. This change was mainly relies on the replacement of solvent molecules in pores with other different organic solvent molecules due to the complex structure of MOFs. To probe the organic molecules sensing ability of Cu-L, different small organic molecules, such as ethanol (EtOH), methanol (MeOH), benzene, acetone, toluene, dimethyl amide (DMA), N,N'dimethylformamide (DMF), nitrobenzene (NB), and isopropyl alcohol (IPA) were chosen. Interestingly, the emission intensity of Cu-L is mainly centered on the solvent molecules, specifically nitrobenzene, which showed a remarkable turn-off effect (Fig. 16a, b). It was observed that the emission of Cu-L decreases progressively with the increasing concentration of nitrobenzene from 10-50 ppm (Fig. 17a). The maximum quenching calculated to be 94.96% of emission of Cu-L in the presence of 50 ppm concentration of nitrobenzene. The SV plot of Cu-L towards nitrobenzene was linear over whole concentration range (Fig. 17b). The K sv value and LOD for nitrobenzene detection by Cu-L was calculated to be 417 × 10 3 M -1 and 0.004 ppm, respectively.

Mechanism for Nitrobenzene Detection
Small organic small molecules typically influences the ligand-tometal energy transfer (LMET) efficiency which in turn govern the emission intensity. Since nitrobenzene is electron-poor acid owing to the presence of electron-withdrawing nitro group, it can steadily move to the bare electron-rich Lewis basic Schiff base sites in the pores, which apparently lower the LMET efficiency and diminish the fluorescence intensity. As proposed for the earlier reported MOFs sensors electron transfer mechanism has been offered to elucidate the quenching of emission intensity of MOFs by small organic molecules. Thus, the PET might govern the highly selective detection of nitrobenzene by the Cu-MOF. To endorse the PET mechanism appropriate for this turn-off quenching, the E HOMO and E LUMO energies of the L and small organic molecules were calculated using DFT with the basis set B3LYP/6-311G(d, p) and the Gaussian 09 package program. Figure 18a represent the optimized structures of organic molecules and L used in this study. It is well known that the species with high energy HOMO can easily donate electrons and those with low-lying LUMO can easily accept electrons. From the HOMO − LUMO energy gaps, it is inferred that PET can takes place from the HOMO of the L of Cu-MOF to the lowest energy LUMO of electron-deficient nitrobenzene (-2.6307 eV) compared to other organic molecules   Fig. 18b). This agrees well with the observed maximum quenching efficiency for nitrobenzene. These outcomes suggest that PET mechanism can be accounted for the highest quenching of emission of Cu-L by nitrobenzene.

Recyclability
The recyclability of the Cu-L sensor towards Fe 3+ ions and nitrobenzene was studied by conducting a simple fluorescence   Subsequently, fluorescence spectra of these aqueous suspensions were recorded in the 350 − 600 nm spectral range under the excitation wavelength at 308 nm, while the emission intensity was examined at 413 nm for Cu-L. After every fluorescence sensing experiment, each MOF was isolated by centrifugation and then washed many times with water, methanol, and acetone before using in next sensing recovery cycle. It was found that the recovered Cu-L showed no substantial changes of the fluorescence intensity over four sensing-recovery cycles, in either case, indicating the good recyclability of Cu-L (Fig. 19).

Conclusion
In summary, we have successfully synthesized a luminescent MOF (Cu-L) by employing low-cost Schiff base ligand. Our results validate that the luminescence intensities of Cu-L are highly sensitive to the Fe 3+ and nitrobenzene. The LODs was found to be as low as 47 ppb and 0.004 ppm, respectively. The detection of Fe 3+ ions and nitrobenzene can be related to the existence of electron and energy transfer processes between the Cu-L and target analytes (Fe 3+ ions and nitrobenzene). Eventually, the present study provides the new insight into the design of cost effective MOF-based sensors, which may be useful under more realistic conditions. Clearly, this work also demonstrate that porous MOFs decorated with bare Lewis basic Schiff base sites are very promising materials for detection of small organic molecules and metal cations. It is expected that introducing the bare Lewis basic sites into luminescent MOFs may generate highly sensitive MOF sensors. These studies are still underway in our group. Fig. 19 Bar diagram showing the recyclability of Cu-L for fluorescence quenching experiment with Fe 3+ ions and nitrobenzene up to 4 cycles Author Contributions All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Manpreet Kaur, Mohamad Yusuf and, Ashok Kumar Malik.The first draft of the manuscript was written by Manpreet Kaur. All authors read and approved the final manuscript.
Funding No funding was available supporting this research.
Data Availability All relevant data will be provided on request.

Declarations
Ethics Approval There is no ethical approval required.

Consent to Participate
All authors have given consent to participate in publication process of this paper.

Consent to Publish
All authors give consent to publish the paper.