A Novel Composite of Zinc-based Metal Organic Framework Embedded with SnO2 Nanoparticle as a Photocatalyst for Methylene Blue Dye Degradation as well as Fluorometric Probe for Nitroaromatic Compounds Detection

A facile bottom up synthesis technique is opted for the preparation of novel composite SnO2@Zn-BTC. This synthesized composite is fully characterized by Fourier Transform Infrared (FTIR) Spectroscopy, Powder X-Ray Diffraction (PXRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), and Elemental mapping techniques. Optical analysis was performed using UV–Visible absorption spectroscopy and fluorescence studies. Further this composite was utilized for the first time as a photocatalyst for methylene blue (MB) dye degradation under sunlight irradiation. This photocatalyst shows degradation efficiency of 89% within 100 min of exposure of sunlight. In addition to that, the synthesized composite can be utilized as a fluorescence probe for detection of NACs via ‘turn-off” quenching response. This composite is extremely sensitive towards 3-NA in aqueous medium with quenching efficiency of 75.42%, which is highest quenching rate till reported. There occurs no interference for detecting 3-NA in the presence of other NACs. The linear fitting of the Stern–Volmer plot for 3-NA shows large quenching constant (KSV) of 0.0115 ppb−1 with correlation coefficient R2 = 0.9943 proves higher sensitivity of composite in sensing process. The outstanding sensitivity of composite for 3-NA is certified by the low detection limit (LOD) of 25 ppb (0.18 µM). Photoinduced Electron Transfer (PET) and Fluorescence Resonance Energy Transfer (FRET) are the mechanisms used for clarification of quenching response of PL intensity by NACs via density functional theory (DFT) calculations and extent spectral overlap, respectively. Hence, synthesized composite is verified as multi-component system to act as excellent photocatalyst as well as fluorescent sensor.


Introduction
The regular increase in pollution in terms of agriculture, non-agriculture and industrial pollutants is problematic issue for human beings and an environment. Environment pollutants in the form of dyes is major issue to be under concern these days [1]. Contaminated water results in death of 1800 children under the age of 5 years due to diarrhea reported by united state report [2]. Many organic pollutants are released by industries in water causing harmful effects on human health [3]. These contaminated water from industries is directly released into fresh water bodies without treatment [4][5][6]. The main organic pollutant released by industries is to provide a color wide product selection, which is achieved 1 3 through dyes used in clothing, leather, plastic and cosmetic industries. WHO reported more than 7,00,000 tons production, use and discharge of dyes in form of industrial effluents [7]. So, one of major challenge is removal of these toxic dyes from ground as well as underground water. During past decades advanced oxidation technologies (AOTs) has been utilized for the complete degradation of organic dyes into non-toxic products in waste water [8,9]. Nitroaromatic compounds (NACs) are not only well-known explosives, but also notorious environmental pollutants [10]. Hence, there is a need to develop sensitive, selective and effective fluorescent sensor for the security of environment and human [11]. For the detection of NACs modern instrumental methods like high-performance liquid chromatography (HPLC) [12], Liquid chromatography-mass spectrometry (LC-MS) [13], gas chromatography-mass spectrometry (GC-MS) [14] and capillary electrochromatography coupled to mass spectrometry (CEC-MS) [15] can be opted, but these are highly expensive and laborious to operate. Hence there is need of cheap and easy operative method for the detection of NACs. So, fluorescent detection of NACs via luminescence quenching or enhancement is an easy technique to operate and economically adapted [16].
From above discussion it is clear that there is need of such material, which can act both as photocatalyst as well as fluorescent sensor. Wide range of applications can be shown by metal oxide nanoparticles having desirable optical, catalytic, electronic and sensing properties. In the recent decades, these can be utilized for photocatalytic degradation of dyes and sensing of pollutants like heavy metal ions, explosives etc. However, their use in such applications is restricted due to thermodynamic instability and tendency to aggregate owing to their high surface free energy, which leads to less stability and sensitivity [17]. To resolve this problem, there is need of immobilization of metal oxide nanoparticles on support structures. For the support Metal-organic frameworks (MOFs) can act as an appropriate material, which are highly porous material as an assembly of metal ions or clusters and organic linkers. MOFs have wide range of features like large specific surface area, uniform and well-ordered structures, high porosity and chemical stability. These all make them effective host for significant amounts of metal oxide nanoparticles with prevention in their aggregation in order to increase the stability.
Studies of metal oxide nanoparticle/MOF composites shows that there exist interactions between metal oxide nanoparticles and MOF, which result in complex and diverse synergistic effect. There are basically three synergistic mechanisms, which can be opted to explain photocatalytic and sensing applications: (1) Metal oxide nanoparticles acts as an active site, stabilised by MOFs. This results in enhancing their stability making them potential candidate for the purpose of photocatalytic and sensing applications. (2) Confinement of aggregated metal oxide nanoparticles in MOFs results in enhancing their detectable limit. As metal oxide nanoparticles alone shows low detectable limits, but their impregnation in MOFs restrict their free movement leading to aggregation-induced emission (AIE) resulting in enhancement of quantum yield. (3) Metal oxide nanoparticles serve as active site while porous MOFs determine size selectivity. The core-shell structure of encapsulated MOFs allows the passage of only those molecules, which are smaller than their pore while preventing the entry of larger molecules. This sieving function of metal oxide nanoparticles/MOFs composite results in improvement in their selectivity and anti-interference ability [17]. The above-mentioned mechanisms for synergistic effects of metal oxide nanoparticles/MOFs composites produce sensing and photocatalytic systems with enhancement in sensitivity, selectivity and effectivity compared to previous reported traditional systems. Hence metal oxide/MOFs composites can act as excellent photocatalyst as well as sensor due to their synergistic effects. Studies of metal oxide nanoparticles/MOF composites shows that there exist interactions between metal oxide nanoparticles and MOFs, which result in complex and diverse synergistic effect. From the previous reported composites such as CDS/MIL-101 [18], Au@ CdS/MIL = 101 [19], UiO-66/g-C 3 N 4 [20], MoS 2 /UiO-66/CdS [21], MIL-101-Ni/NiO X [22] and CdS/UiO-66 [23] were utilized for photocatalytic hydrogen production under visible light irradiation. Further ZnO@ZIF-8 [24] and Aminopyrene@ZIF-8 [25] were utilized for sensing purpose. Still, to the best of our knowledge, SnO 2 @Zn-BTC have not been reported and studied for photocatalytic degradation of organic dyes in water and fluorescent sensing of nitroaromatic compounds (NACs).
Zn-BTC one of the zinc-based MOFs, have features like stability, non-toxicity, inexpensiveness and visible light response. Here in, we report for the first time SnO 2 @ Zn-BTC composites for the photocatalytic degradation of methylene blue (MB) dye in aqueous media with the degradation efficiency of 89% and sensing of NACs via fluorescence studies. This composite is selective towards 3-NA through turn-off response with quenching efficiency and limit of detection (LOD) to be 75.42% and 25 ppb (0.18 µM), respectively. As the continuous of our research work, SnO 2 @Zn-BTC composite has been successfully synthesized by "Bottle in the ship" approach in which zinc nitrate hexahydrate: metal ion salt and trimesic acid: organic linker are used as MOF precursor and SnO 2 is utilised as such pre synthesized, This composite is characterized by various analytical techniques and utilized for photocatalytic degradation and sensing of NACs. In addition, the fluorescent sensing mechanisms have been also proposed.

Instrumentation and Characterization
For functional group detection in dried sample of SnO 2 , Zn-BTC and SnO 2 @Zn-BTC, Fourier Transform Infrared Spectra were scanned in 400-4000 cm −1 on a Perkin Elmer FT-IR Spectrophotometer. The powder X-ray Diffraction (PXRD) patterns of the dried samples were measured using XPERT PRO Powder X-ray Diffractometer [CuK α , X-ray (λ = 1.5406°Å), 1800 W (45 kV, 40 mA)] with step size-0.013°, scan speed-0.001° sec −1 and 2θ range 10° to 60°. Scanning electron microscopy (SEM) images were performed using JEOL, JSM-6510LV for detailed morphological and topographical analysis. Elemental analysis is done with additional accessary of Energy dispersive X-ray analysis (EDXA) having gold coated sample rigidly mounted on the specimen stub was recorded using Oxford instrument (INCA X-act). Fluorescence measurements were done using Shimadzu RF-5301PC spectrofluorometer equipped with quartz cell in the emission mode. The ultraviolet-visible (UV-vis) spectrophotometer (Shimadzu UV-Vis 1600) was utilized to measure adsorption/degradation. Gaussian 09 software was opted for theoretical studies.

Synthesis of Capped SnO 2
SnO 2 nanoparticles were prepared by addition of tin chloride pentahydrate (SnCl 4 .5H 2 O, 8 mmol, 0.90256 g) and trisodium citrate dihydrate (C 6 H 5 Na 3 O 7 .2H 2 O, 8 mmol, 2.064 g) into 30 ml of ethanol:water in (1:1) followed by magnetic stirring for 1 h. After vigorous stirring next is pouring of reaction mixture into teflon-lined stainless-steel autoclave. The autoclave was properly sealed and heated in vacuum oven by maintaining temperature to be 180 °C for 24 h and cooled to room temperature naturally. At last precipitates were collected and washed with mixture of ethanol and distilled water followed by annealing at 600 °C for 2 h.

Synthesis of SnO 2 @Zn-BTC Composite
"Bottle around the ship" approach can be utilized for the synthesis of composite (SnO 2 @Zn-BTC). In brief, equimolar (1 mol each) mixture containing zinc acetate monohydrate and trimesic acid were dissolved in 20 ml DMF, magnetically stirred for 10 min and ultrasonicated for 5 min followed by addition of 2 mol capped SnO 2 nanoparticles again ultrasonicate for 20 min. Then mixed solution was placed into Teflon-lined autoclave and optimized at 100 °C for 24 h under static conditions. After all this solution was slowly cooled to room temperature and collected through centrifugation. The collected powder was washed with DMF-water mixture several times. Finally, the white powder was dried at ambient temperature and stored in air tight bottle for further use [26]. Schematic representation for the preparation of SnO 2 @Zn-BTC composite is shown in Fig. 2.

Procedure for Photocatalytic Experiment
15 mg of nano-photocatalyst (SnO 2 @Zn-BTC) was added into 100 ml of MB dye solution for the photocatalytic degradation studies under sunlight irradiation at geographical coordinates: 30.33° N and 76.38° E (average of the solar power incident on a surface was 400 W/m 2 for 12 h). Prior to sunlight exposure, adsorption-desorption equilibrium is maintained between MB dye, nano-photocatalyst and water through magnetic stirring in dark for 1 h. After specific time intervals of sunlight exposure, 3-5 ml of sample was collected from reaction chamber followed by analysis of remnant dye concentration through UV-Visible absorption through a prominent absorption peak at λ max = 667 nm.

Procedure for Luminescent Sensing
Firstly, stock solution was formed by dissolving SnO 2 @Zn-BTC composite (1 mg) in 3 ml of DMF, water, methanol and ethanol followed by sonication of 30 min for detailed photoluminescence (PL) studies. All PL studies were done at room temperature. Due to its excellent fluorescent behaviour, it can be utilized as a fluorescent sensor for the detection of different NACs. Next, solution of different NACs of concentration 140 ppb each (2,4-DNT, 2-NA, 4-NT, 4-NA and 3-NA) were prepared in water. Suspension of 1 mg of SnO 2 @Zn-BTC composite in 3 ml of each NACs was made for selective studies. The resulting suspensions were sonicated prior to PL studies. Various concentrations of analyte 3-NA were prepared through dilution of 140 ppb stock solution with the aim of its selective studies. All luminescence studies were done by keeping excitation wavelength (λ ex = 380 nm) and 10 nm of slit widths for both excitation and emission monochromators.

FT-IR
This Fourier Transform Infrared (FT-IR) Spectroscopy has been opted to detect chemical bonding in elements, functional groups and impurities present in the synthesized materials. Figure 3a-c represents FT-IR spectra of synthesized SnO 2 nanoparticles, Zn-BTC and SnO 2 @Zn-BTC composite. In Fig. 3a, the spectral band at 578 cm −1 can be ascribed to Sn-O vibrations in SnO 2 [27]. Also, a sharp peak present at 1561 cm −1 corresponds to C-C stretching. Next, in Fig. 3b, the characteristic peak at 742 cm −1 is due to Cu-O stretching vibration in which O is bonded to Cu [28]. The bands at 1620 cm −1 and 3328 cm −1 are due to C = O and O-H stretching vibrations, respectively. Moreover, one more peak at 1367 cm −1 in Zn-BTC is due to binding of carboxylate group with zinc metal. A peak at 1620 cm −1 in Zn-BTC becomes more intense and splitted into two peaks with wavenumber 1569 cm −1 and 1628 cm −1 [29]. These changes may be due to either bonding of free carboxylate oxygen from the capping agent or the co-ordination of oxygen from Sn-O of SnO 2 to the zinc metal of Zn-BTC proves formation of composite shown in Fig. 3c.
Also, there are other additional peaks, which corresponds to Zn-BTC present at 11.36° (220), 20 [33][34][35][36]. From sharp peaks in the diffractogram, it is proved that sample SnO 2 @Zn-BTC composite have crystalline nature with uniform porosity [37]. From diffraction peaks presence of both SnO 2 and Zn-BTC in the composite can be verified.

SEM and EDS
The SEM micrographs taken at magnifications of 500 × and 5000 × for the morphological study of synthesised SnO 2 @ Zn-BTC composite are shown in Fig. 5a, b. The captured micrographs show morphology resembling sphere with a diverse distribution. Figure 6 displays the energy dispersive X-ray spectrum that was taken to ascertain the purity and chemical make-up of SnO 2 @Zn-BTC composite. Carbon  According to a detailed analysis of the EDS spectrum of SnO 2 @Zn-BTC composite, the formation of good superiority of sample with the right stoichiometric ratio of constituting elements was proved.

UV-Vis. Absorption Spectra
The UV-Vis. absorption spectra and the Tauc plot of synthesized material are shown in Fig. 7. This absorbance spectra of shows a strong absorbance at wavelength 384 nm. The band gap energy (E g ) of synthesized sample can be calculated from UV-Vis. Spectra by Tauc plot of (αhv) 2 (eV cm −1 ) 2 versus hv via extrapolation of linear portions of the curve to the energy axis according to [38]: where α is the absorption coefficient, hv is the photon energy, E g is the direct band gap energy and B is a constant.

Photocatalytic Activity
To check the photocatalytic degradation of organic pollutants, photocatalytic degradation of MB dye was conducted under sunlight exposure. In this paper, the effect of photocatalyst dosage and pollutant concentration was studied. Without using photocatalyst dye degradation is less than 5%   Fig. 8. Hence, photocatalytic dye degradation using photocatalyst was opted for further investigations.
First of all, the effect of amount of photocatalyst (5, 10 and 15 mg) on degradation of dye was studied. As shown in Fig. 9a, there occurs an increase in degradation of dye with increase in concentration of the photocatalyst. This destruction of organic pollutant MB dye is due to production of powerful oxidants which includes radicals like hydroxyl radicals, superoxide, etc. In addition to that there occur formation of more and more active surface sites for photocatalytic degradation with increase in concentration of photocatalyst [39,40]. Next, another important parameter is effect of initial dye concentration was studied. Here, we have opted the different pollutant concentration (2, 4, 6 and 8 mg/L) to examine the effect of this on dye degradation by keeping 15 mg of photocatalyst to be fixed. Figure 9b, shows the decolourization of different concentrations of dye with respect to time with continuous exposure of sunlight. The absorption spectra shown in Fig. 10a represents the absorption measured during degradation of dye by photocatalyst (SnO 2 @Zn-BTC composite) under sunlight exposure at 200 nm ≤ λ ≤ 800 nm at different time intervals. The reason of decrease in absorbance of dye with time is destruction in dye structure. From this it has been observed that the dye degrades upto 97.05%. These experiments were performed to check efficiency of photocatalyst to degrade organic pollutant. The prominent peak used for degradation efficiency analysis is 667 nm. Further degradation efficiency is calculated using the following equation: where, D represents degradation efficiency, C o & C t represents initial concentration & concentration after sunlight exposure for time t of the dye. Figure 10b represents the degradation efficiency of photocatalyst in form of histogram after particular interval of time. From this it is concluded that degradation efficiency increase with the passage of time. Along with this schematic representation of degradation of dye can be shown in Fig. 11. Recycling reactions were done to check the stability of photocatalysts for the use of SnO 2 @Zn-BTC composite for the photodegradation of MB dye under sunlight irradiation. Keeping other factors constant, the photocatalyst was reused after washing and drying at 80℃. The time interval plot of MB dye degradation for consecutive four cycles is shown in Fig. 12a. There occurs no loss in activity of SnO 2 @Zn-BTC composite towards degradation of MB dye during four consecutive cycles, indicating its excellent long-term stability. For further justification, PXRD were recorded before and after four consecutive = − ∕ Fig. 12 a Photodegradation of MB dye under sunlight irradiation using SnO 2 @Zn-BTC composite as a photocatalyst during four repeated process; b PXRD patterns for SnO 2 @Zn-BTC composite before and after proceed of reaction Fig. 13 Proposed photocatalytic degradation mechanism of MB dye cycles as shown in Fig. 12b, which remains same shows no structural transformation was happened.

Dye Degradation Mechanism
Photocatalyst surface becomes active through formation of electron (e − ) and hole (h + ) pair through sunlight irradiation resulting in excitation of SnO 2 @Zn-BTC composite. It is predicted that the initial process used in the destruction of dye is formation of e − /h + pair. This pair is formed as a result of absorption of energy by valence band (VB) and transition of electron from VB to CB (conduction band) resulting in h + left behind in VB. This e − participate in reduction of O 2 into O 2 −• and finally hydroxyl radical (OH • ) is formed. In addition to that OH • can also be generated via oxidation of H 2 O through h + . At last this OH • produced through oxidation as well as reduction degrades the dye effectively [41]. The whole mechanism of degradation of dye is shown in Fig. 13.

Water Stability
For successful future applications, the stability of SnO 2 @Zn-BTC composite towards water is checked through immersion of particles in water for 24 h and then analysed by PXRD analysis.

Photoluminescent Properties and Detection of 3-NA
The emission spectra of SnO 2 @Zn-BTC composite dispersed in different solvents like DMF, water, methanol and ethanol to achieve maximum luminescence quantum yield.1 mg of synthesized SnO 2 @Zn-BTC composite was dispersed into the 3 ml of different solvents followed by ultrasonication before PL measurements. Maximum emission intensity has been observed for DMF as shown in Fig. 15a. Due to better PL intensity of composite in DMF it has been selected as appropriate solvent for further sensing studies. In addition to that, concentration gradient experiment has been performed to select appropriate concentration of composite for sensing studies. From Fig. 15b it has been clearly seen that the recorded spectra has maximum PL intensity at 3000 ppm (1 mg of composite in 3 ml of DMF) and it has been utilized as a appropriate concentration for further PL studies.
Thus, the sensing properties of SnO 2 @Zn-BTC composite towards different NACs was investigating via selecting DMF as a solvent due to its good dispersion, high stability and excellent luminescence in DMF at 3000 ppm concentration. The emission spectra of SnO 2 @Zn-BTC composite dispersed in DMF solvent via variation of excitation wavelength from 300-420 nm with maximum emission intensity observed at 830 nm at λ ex = 380 nm as shown in Fig. 16a. Moreover, the excitation and emission spectra of SnO 2 @ Zn-BTC composite are shown in Fig. 16b. From this it can be depicted that upon excitation at 380 nm, a broad emission peak is presented at 447 nm, which may arise from π − π* or n − π* electron transition within the BTC. From above it can be evaluated that for further PL studies, 380 nm has been selected as a suitable excitation wavelength for the further PL studies to detect different NACs as shown in Fig. 17a. 1 mg of synthesized SnO 2 @Zn-BTC composite was dispersed into the 3 ml aqueous solution of 140 ppb of different NACs (2,4-DNT, 2-NA, 4-NT, 4-NA and 3-NA) to form stable suspension after ultrasonication for 30 min. For comparing the luminescence of SnO 2 @Zn-BTC composite effected by various NACs, histogram has been drawn as shown in Fig. 17b. From this, it can be depicted that there occurs a prominent suppression in luminescence intensity of composite after adding different NACs. This "suppression effect" can be seen maximum in 3-NA due to good contact between 3-NA and composite as compared to other NACs (2,4-DNT, 2-NA, 4-NT and 4-NA). Next, quenching efficiencies of different NACs was calculated using [(I o -I)/I o ] × 100%, where I o and I are luminescence intensities before and after addition of analyte (NACs). Through this, it has been observed that 3-NA dispersion in SnO 2 @Zn-BTC composite shows maximum quenching of 75.42% followed by 4-NA (~ 65.30%), 4-NT (~ 35.78), 2-NA (~ 24.09%) and 2,4-DNT (~ 12%) as shown in Fig. 17c. From this, it can be evaluated that SnO 2 @Zn-BTC composite can act as a selective fluorescent sensor for 3-NA. So, SnO 2 @Zn-BTC composite can be explored for the selective detection of 3-NA via fluorescence quenching. Different concentrations of 3-NA are prepared and 1 mg of SnO 2 @Zn-BTC composite was immersed. Next, its fluorescent quenching response of different concentrations of 3-NA towards SnO 2 @Zn-BTC composite has been measured as shown in Fig. 18a. From this it is clear that there is strong dependence of luminescence intensity of SnO 2 @Zn-BTC composite on 3-NA concentrations and PL intensity seems to decrease gradually with increase in 3-NA concentration in the range of 20-140 ppb. For more detail Stern-Volmer (SV) equation can be opted to study the quenching effect of 3-NA. This SV equation is shown below: To check the time dependence for the detection of 3-NA, emission spectra were recorded with time interval of 1 min upto 10 min by dispersing 1 mg of SnO 2 @Zn-BTC composite in 3 ml, 3-NA (140 ppb) in aqueous solution with excitation wavelength of 380 nm. From emission spectra it is evaluated that luminescence intensity remains constant after 1 min as shown in Fig. 19a. Moreover, fluorescence quenching reaction proved to be very fast through unchanged emission intensity of suspension after 1 min. So, synthesized material is proved

Mechanisms for Detection of 3-NA
Here, SnO 2 @Zn-BTC composite is an electron rich, while 3-NA is an electron deficient. As there are more free -COOH groups in SnO 2 @Zn-BTC composite due to which it can form more number of hydrogen bonds with the -NO 2 group present in NACs. Hence this composite can act as excellent fluorescent sensor for different NACs. So, to explain the quenching of SnO 2 @Zn-BTC composite particularly towards 3-NA, following three mechanisms are possible: (1) Destruction of crystalline structure of SnO 2 @Zn-BTC composite; (2) Photo-induced electron transfer (PET); (3) Fluorescence resonance electron transfer (FRET). FT-IR experiments have been performed for SnO 2 @Zn-BTC composite after sensing towards 3-NA. From this, it is evaluated that there occurs no change in the crystallinity and the structural integrity of SnO 2 @ Zn-BTC composite was preserved because there occur no change in FT-IR spectra (Fig. 20A) and PXRD (Fig. 20B).
As proposed from previous reports, PET might govern the highly selective detection of NACs by composite. To provide the reason of decrease in PL intensity of SnO 2 @Zn-BTC composite, density functional theory (DFT) theoretical calculations have been adopted. These calculations were performed at B3LYP level followed by calculating the values of HOMO-LUMO energies of ligand as well as various NACs used in this work (Table 1 and Fig. 21). The reason behind the decrease in PL intensity of SnO 2 @Zn-BTC composite, when different NACs dispersed in suspension is charge transfer.  [37]. Moreover, FRET mechanism might be recognized for the quenching process. For FRET to take place, there must be efficient spectral overlap between emission spectrum of fluorophore and absorption spectrum of the analyte. As Fig. 22 represents the overlap of absorption spectra of different NACs and emission spectrum of composite and large overlap can be seen between the absorption spectrum of 3-NA (λ max = 417 nm) and emission spectrum of composite (λ em = 447 nm). Hence, from this it can be evaluated that highest quenching efficiency is for 3-NA towards composite. Due to this spectral overlap reason, this FRET supports the fact that the fluorescence quenching by 3-NA take place by an energy transfer mechanism.

Conclusion
In summary, bottom up technique has been used for the synthesis of novel SnO 2 @Zn-BTC composite having remarkable stability even when soaked in water for 24 h. Due to high chemical stability this composite has been utilized for degradation of MB dye in aqueous medium under sunlight exposure with degradation efficiency of 89%. Further, it acts as fluorescent sensor for various NACs with selective detection of 3-NA with quenching efficiency, LOD and quenching constant (K SV ) as 75.42%, 25 ppb (0.18 µM) and 0.0115 ppb −1 respectively. The assynthesized composite shows high sensitivity and selectivity towards 3-NA without suffering any interference from other NACs in aqueous media. The highest selectivity of 3-NA in presence of other NACs can be related through electron and energy transfer mechanisms. Easy preparation, photocatalytic activity and efficient sensing make composite as a potential candidate for homeland security and environment governance. ment, Punjabi University, Patiala, for providing lab and instrument facilities. The authors Deepika and Manpreet kaur are highly obliged to the UGC, New Delhi, India, for UGC-SRF fellowship.

Fig. 22
Spectral overlap of emission spectra of SnO 2 @Zn-BTC composite and absorption spectra of 3-NA Author Contributions Deepika, Manpreet Kaur has performed the experiment and written the paper. Heena, Karamjit Singh, AKM and MY has written and edited the paper. All authors reviewed the manuscript.
Funding No funding was available supporting this research.

Data Availability
All relevant data will be provided on request.

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