Novel SnO2@Cu3(BTC)2 Composites as a Highly Efficient Photocatalyst and Fluorescent Sensor

A novel SnO2@Cu3(BTC)2 composite was synthesized using a quick and affordable bottom-up approach via impregnation of SnO2 nanoparticles into the porous Cu3(BTC)2 metal-organic framework (MOF). This composite material is characterized by Fourier transform infrared (FTIR) spectroscopy, powder X-ray diffraction (PXRD) spectra, scanning electron microscope (SEM) analysis, and energy-dispersive X-ray spectroscopy (EDS) analysis. SnO2@Cu3(BTC)2 degraded the methylene blue (MB) dye within 80 min under sunlight with a maximum degradation efficiency of 85.12%. This composite easily recyclable up to five cycles with the retention of its MB degradation efficiency. Moreover, SnO2@Cu3(BTC)2 can be also used efficiently for fast sensing of 2,4,6-trinitrophenol (TNP) in water with noticeable turn-off quenching response. Its limits of detection (LOD) for TNP was 2.82 µM with enhanced selectivity toward TNP (over other NACs) as verified by competitive nitro explosive tests. Density functional theory (DFT) calculations and spectral overlap were used to assess the sensing mechanism. This composite fluorescent sensing system for TNP are demonstrated to have high selectivity and sensitivity. Our findings imply that the prepared low cost SnO2@Cu3(BTC)2 composite can be used as a superior fluorescence sensor and photo catalyst for large scale industrial applications.


Introduction
Metal ions along with organic linkers are used to create one, two, or three dimensional metal-organic frameworks (MOFs) [1,2].MOFs have porous structure which offers wide range of industrial applications, including photocatalytic degradation [3,4], catalysis [5][6][7], molecular separations [8][9][10], drug delivery [11,12], sensing [13,14], and many others [15][16][17][18].However, the usage of these porous crystalline materials have certain obstacles in the aforementioned industrial applications such as electrical non-conductivity [19], weak dispersive forces [20], and significant chemical reactivity towards water and moisture, which lead to distortion of MOFs structure.Recently, these problems have been solved by the incorporation of nanoparticles (NPs) into pore cages of MOFs.However, NPs synthesis also suffer from numerous shortcomings like agglomeration and size distribution management, which lead to the loss of their remarkable surface features.After introduction into MOF support, the NPs stability can be enhanced by the surface confinement effect [21].

Highlights
• Using a bottom-up approach, a unique SnO 2 @Cu 3 (BTC) 2 composite has been prepared.This produces a composite material which showed enhanced performance in industrial applications including gas storage and separation, heterogeneous catalysis, chemical sensing, photocatalytic degradation, and others [22]."Ship in the bottle," is one of the approach which encapsulates NPs in MOF cavities.In this method, pre-synthesized MOF and NP precursors are treated together by the mechanical grinding [27,28], vapor deposition [25,26], or solution or wet impregnation [23,24].Alternatively, "bottle around ship" method in which pre-synthesized NPs are treated with the MOF precursors [29].Since NACs are not only explosives but also notoriously harmful to the environment, so there is an utmost need of selective and sensitive chemical sensors for their detection in terms of homeland security, environmental protection, and humanitarian considerations [30,31].Although there are different MOFs that can only be used for the fluorescence quenching-based detection of NACs.But here, we have focused our research on the firstever use of NPs@MOFs for NACs detection due to their sensitivity, simplicity, and short reaction time.
Moreover, besides detection, the removal of harmful toxins from water is also very important.To date, several techniques like biological, chemical, physical, adsorption, and photodegradation etc. have been used for water decontamination.Among these, photodegradation has been found to be most effective as it degrade the harmful organic pollutants to less harmful substances compared to previously described techniques.Recently, NPs@MOFs composites appeared as most effective photocatalysts due to their high surface area with extraordinary absorption capacity [32][33][34].
In this work, novel SnO 2 @Cu 3 (BTC) 2 was prepared by "bottle around ship" method in which precursors of Cu 3 (BTC) 2 i.e., trimesic acid linker and Cu 2+ metal ion was treated with the pre-synthesized SnO 2 .Here, we have first time explored the photocatalytic degradation of the methylene blue (MB) dye and the sensing application towards distinct NACs.The goal of the present study is to synthesize novel materials that can be used for water purification and homeland security applications as fluorescence sensors and photocatalysts in real-world settings.

Materials
The following chemicals tin(IV) chloride pentahydrate (SnCl 4 .5H  .After magnetically stirring for one hour the resulting mixture was poured into an autoclave made of stainless steel with a Teflon liner.The autoclave was properly sealed, heated in a hot-air oven by keeping the temperature at 180℃ for 24 h, and then naturally cooled to ambient temperature.The produced precipitates were then taken out of the autoclave and repeatedly rinsed with ethanol and distilled water to get rid of any contaminants.These precipitates were then annealed for two hours at 600℃.

SnO 2 @Cu 3 (BTC) 2 Composite
Add an equimolar (1 mol each) mixture of copper nitrate trihydrate and trimesic acid to a glass vial along with 20 ml of DMF followed by ultrasonication for 5 min.Then, add 2 mol of capped SnO 2 nanoparticles.This vial is heated at 100 °C for 24 h in an airtight oven.The solution was filtered and washed several times with a DMF-water solution, dried, and then placed in an airtight bottle for later use after cooling at room temperature [35].Scheme 1 can serve as an illustration of the complete synthesis process.In essence, this strategy is "bottle around the ship," as depicted in Fig. 1.

Characterizations
For the identification of functional groups the dried samples of SnO 2 , Cu 3 (BTC) 2 , and SnO 2 @ Cu 3 (BTC) 2 were explored under Fourier Transform Infrared Spectroscopy (FT-IR).Using the XPERT PRO Powder X-ray Diffractometer [CuK α X-ray (λ = 1.5406Å)], 1800W (45 kV, 40 mA)] in the 2ϴ range 10-60 o keeping step size of 0.013 o at scan speed of 0.001 o sec −1 , the powder X-ray Diffraction (PXRD) patterns of the dried samples were recorded.Use of the JEOL JSM-6510LV scanning electron microscope (SEM) was done for microstructure and morphological investigations.Analytical approach for elemental analysis was performed using energy dispersive X-ray analysis (EDX).The Oxford instrument (INCA X-act) EDX spectrometer was used in the current study to record the EDX spectra of a gold coated sample that was rigidly attached on the specimen stub.UV-Vis spectrophotometer (Shimadzu UV-Vis 1600) and spectrofluorometer (Shimadzu RF-5301PC) were used for optical investigations.

Photocatalytic Dye Degradation
At coordinates 330.33 o N and 76.38 o E (average of the solar power incident on a surface was 400 W/m 2 for 12 h), sunlight was utilized to irradiate the SnO 2 @Cu-BTC composite to test its photocatalytic activity.In this experiment, 100 ml of MB dye aqueous solution containing 15 mg of the MB dye was prepared.The suspension was stirred for an hour in the dark before exposure of sunlight to maintain equilibrium between MB dye, nano-photocatalyst, and water.After being exposed to sunlight for 10 min, 3-5 ml of the sample were taken from the reaction chamber and subjected to an analysis of the residue dye concentration using UV-Visible Spectrophotometer through a significant absorption peak at λ max = 667 nm.

Luminescent Sensing
The solution of finely ground SnO 2 @Cu 3 (BTC) 2 was prepared for photoluminescent (PL) measurements by dissolving 1 mg of SnO 2 @Cu 3 (BTC) 2 in 3 mL of water followed by sonication for 30 min.Further, it is utilized for a prospective fluorescence sensor for the detection of various NACs.In order to learn more about its intriguing sensitivity and selective behaviour towards luminescence, 1 mg of SnO 2 @ Cu 3 (BTC) 2 composite was added in 3 mL (20 mM) of each NACs aqueous solution.This was followed by 30 min of ultrasonication to create a stable suspension for PL analysis.The subsequent step was to dilute the 20 µM aqueous solution of TNP to prepare varied concentrations of the TNP for its selective sensing.For all luminescence studies, the PL spectra of the aforementioned solutions were recorded using a spectrofluorometer at λ ex = 360 nm with slit widths of 10 nm for both the excitation and emission monochromators.

Computational Methodology
Gauss View 5.08 was used to prepare the input files for H 3 BTC and various NACs for the theoretical experiments.Utilizing the tools from the Gaussian 09 programme package, geometrical optimization, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) computations have been performed.At the B3LYP/631G level, the density functional theory (DFT) calculations have been carried out, followed by a geometry optimization.

Ftir Spectroscopy
The use of FT-IR spectroscopy has been chosen for the establishment of functional groups, detail of chemical bonding in existing elements, and presence of contaminants even at low levels.A pure sample of capped SnO 2 (Fig. 2a) displays a distinctive peak at 637 cm −1 corresponding to the vibration mode of O-Sn-O stretching and 1628 cm −1 corresponding to C-C stretching.The FT-IR spectra of Cu 3 (BTC) 2 are shown in Fig. 2b, where the band at 1702 cm −1 relates to the stretching vibration of the acidic C = O contained in trimesic acid and the peak at 1633 cm −1 relates to the deprotonation of the acidic C = O after complexation with Cu 2+ .O-H stretching in the carboxylic group is associated with the band between 3100-3600 cm −1 .Additionally, the Cu-O stretching vibration, in which the oxygen atom is coordinated with Cu, is responsible for the peak at 724 cm −1 .As seen in Fig. 2c, after encapsulation, the peak at 637 cm −1 gets shifted and divided into two peaks, one at 698 cm −1 and the other at 619 cm −1 .In this composite, the peak at 724 cm −1 vanished and the peak at 1633 cm −1 diminished and moved to 1610 cm −1 .Additionally, a new peak at 935 cm −1 has been observed as a result of the carboxylate group's deformation vibration.Both the bonding of free carboxylate oxygen from the capping agent or coordination of oxygen from Sn-O of SnO 2 to the copper metal of Cu 3 (BTC) 2 may be the reason of these modifications.

Photocatalytic Activity
MB dye was subjected to photocatalytic degradation for testing the photo degradation efficiency of SnO 2 @ Cu 3 (BTC) 2 composite towards organic contaminants.Firstly, a control experiment was conducted to ensure that MB dye was not degraded much in the absence of a photocatalyst (Fig. 8).Firstly, composite degradation efficiency is compared with Cu 3 (BTC) 2 alone.Figure 9 depicts the time-dependent absorption spectra of MB dye degraded by Cu 3 (BTC) 2 photocatalyst over equal duration of sunlight irradiation.These spectra indicate the decline in dye absorbance with time.It demonstrates that as irradiation time is increased, the degradation efficiency rises  [36] and reaches its maximum value of 88.96% in 4.5 h.In this experiment dye solution is exposed to sunlight for 80 min, the concentration of methylene blue dye hardly changes, demonstrating that the dye cannot degraded on its own when exposed to sunlight.The time-dependent UV-Visible spectra for the MB dye degradation curves of the SnO 2 @ Cu 3 (BTC) 2 composite during sunlight exposure are further explained in Fig. 10a.The relative intensity at maximum absorbance at 667 nm in UV-visible spectra is used to estimate how much methylene blue dye has been degraded on the surface of the composite.The degradation of methylene blue dye across the surface of the SnO 2 @Cu 3 (BTC)  (1) SnO 2 @Cu − BTC + hυ → SnO 2 @Cu − BTC(e − ) + SnO 2 @Cu − BTC h + To check the efficiency of this composite the photocatalytic degradation of synthesized composite can be (2)  1].

Recyclability Test
After being removed from the MB aqueous solution through centrifugation, the synthesized photocatalyst's (SnO 2 @ Cu 3 (BTC) 2 composite) ability to be recycled was assessed [40].The photocatalyst was then repeatedly cleaned with water and pure C 2 H 5 OH before being heated in an oven at 60 °C for three hours.Then, it was used in a fresh MB aqueous solution for the subsequent photocatalytic process.Whole method was the same as it was in the initial phase.The results of the experiment after five repetitions are shown in the Fig. 12a.The photodegradation of MB dye aqueous solution using SnO 2 @Cu 3 (BTC) 2 composite as a photocatalyst was almost constant from the first to the fifth run.The photocatalyst XRD spectra from the first and fifth recycling experiments of the photocatalytic degradation of MB dye are shown in Fig. 12b.

Fluorescence Analysis
The synthesized sample [SnO 2 @Cu 3 (BTC) 2 ] had great stability and dispersion and water was chosen as the solvent to investigate luminous characteristics.SnO 2 @Cu 3 (BTC) 2 exhibits good luminous characteristics in water at 360 nm, as illustrated in Fig. 13 with emission peak centered at 465 nm.This may be caused by an electronic transition between π-π * or n-π * with in the ligand and coordination interactions inside the framework as well as with nanoparticles (SnO 2 ), which may make the ligands more rigid.

TNP Detection
Through the use of water as a solvent, the sensing characteristics of the synthesized sample [SnO 2 @Cu 3 (BTC) 2 ] towards various NACs were examined.The variation in emission intensity is shown in Fig. 14a by adjusting the excitation wavelength from 260 to 360 nm, where the maximum emission intensity of 750 a.u. is found at λ ex = 360 nm.Therefore, 360 nm has been chosen as the excitation wavelength to detect NACs in further PL experiments. 1 mg of the SnO 2 @Cu 3 (BTC) 2 composite powder was dissolved in three ml of a 20 μM solution of several NACs (TNP, 4-NA, 2-NA, 4-NT, 3-NA, 2,6-DNT, 1,3-DNB, and 2,4-DNT) before being ultrasonically processed for 30 min.Figure 14b displays the recorded and related PL spectra of various NAC suspensions.Additionally, it is found that all NACs under examination exhibit a quenching in luminescence intensity with the most effect on suppressing luminescence on TNP compared to other NACs.The histogram in Fig. 14c, which shows a nearly sevenfold reduction in luminescence intensity compared to original luminescence intensity was created to examine the suppression impact on luminescence intensity of the sample SnO 2 @Cu 3 (BTC) 2 composite by various NACs in detail.For a more in-depth analysis, the fluorescence quenching efficiency was estimated using the formula [(I o -I)/I o × 100], where I o and I represent the luminescence intensity before and after the addition of analyte, respectively.In comparison to 2,4-DNT (60.3%), 4-NA (55.8%), 2-NA (52.1%), 4-NT (44.6%), 2,6-DNT (38.3%), 3-NA (29.1%), and 1,3-DNB (19.9%), the maximum PL quenching has been observed in TNP 88.2%, as illustrated in Fig. 14d.The prepared sample [SnO 2 @Cu 3 (BTC) 2 ] can therefore function well as a "turn-off" fluorescence sensor for TNP.
For the additional sensitivity tests, several concentrations of an aqueous solution of TNP with 1 mg of SnO 2 @ Cu 3 (BTC) 2 were analysed to look at the fluorescence   R 2 = 0.9901.This demonstrates that the Stern-Volmer mode is being followed effectively.Stern-Volmer quenching constant (K SV ) value of 1.04 × 10 4 M −1 can also be used to demonstrate the strong quenching effect of the synthesized sample SnO 2 @Cu 3 (BTC) 2 (Fig. 15b).The limit of detection LOD = 3σ/m, where m is the slope of the linear curve drawn at low concentration, and 'σ' is slope of the linear curve.LOD calculated for this analyte (TNP) is 2.82 μM.As a result, the LOD and KSV values show that synthetic composite makes a great sensor.
A time dependent detection of TNP experiment can be carried out to demonstrate the quick detection of TNP by the synthetic material SnO 2 @Cu 3 (BTC) 2 .To do this, emission spectra were initially recorded using 1 mg of SnO 2 @ Cu 3 (BTC) 2 dissolved in 3 ml of 20 µM at intervals of 1 min, proceeded to 10 min (λ ex = 360 nm).According to Fig. 16a, the PL intensity becomes nearly constant after 1 min, indicating that the "suppression effect" for the analyte TNP's quick quenching of the luminescence of SnO 2 @Cu 3 (BTC) 2 is present.It is possible to analyze a competing effect of NACs for further sensing investigations.SnO 2 @Cu 3 (BTC) 2 fluorescence quenching is greatest with TNP was proved by an experiment of specificity of SnO 2 @Cu 3 (BTC) 2 towards TNP in the presence of other NACs.In this experiment, the addition of 3 mL (20 μM) of each NACs aqueous solution was followed by the addition of 3 mL (20 µM) of TNP aqueous solution before recording the PL intensity of SnO 2 @ Cu 3 (BTC) 2 .Figure 16b makes it abundantly evident that the fluorescence intensity significantly decreases with the addition of TNP.Thus, it has been demonstrated that the presence of additional nitroaromatic compounds would not interfere with the extraordinary selectivity of TNP.

Mechanism for Fluorescence Quenching
The following sensing mechanisms were looked at in order to understand how NACs, which are electron-deficient, quench the fluorescence of SnO 2 @Cu 3 (BTC) 2 which is electron-rich.
(1) SnO 2 @Cu 3 (BTC) 2 crystalline structure has been damaged (2) Photoinduced Electron Transfer (PET), and (3) Fluorescence Resonance Energy Transfer (FRET).FTIR patterns of SnO 2 @Cu 3 (BTC) 2 were examined, which appear to be maintained before and after fluorescence quenching by TNP as shown in Fig. 17.Thus, the interaction between the synthetic material SnO 2 @Cu 3 (BTC) 2 and TNP prevented the structure from being destroyed and only the fluorescence quenching arose as a result.In order to understand how NACs cause SnO 2 @Cu 3 (BTC) 2 to lose its fluorescence intensity, another process PET can be examined.PET may control how specifically NACs are detected by the composite NPs@MOFs [41].LUMO of NACs essentially lies between the valence and conduction bands of BTC.Thus, an electron can simply travel from the CB of a BTC to the LUMO of a NACs resulting in the fluorescence quenching.DFT with basis set B3LYP/6-311G and Gaussian 09 package programme were used to calculate the energies (E HOMO and E LUMO ) of BTC and NACs as shown in Fig. 16.Here, the E HOMO and E LUMO constituents of the Frontier Molecular Orbital (FMO) theory can be used to explain how well NACs quench.Higher HOMO energies and lower LUMO energies are capable of donating and receiving electrons, respectively.According to the HOMO-LUMO gaps in Fig. 18, electron transport from the CB of the BTC to the LUMO of the NACs causes the luminescence to "switch off" with the TNP having the lowest LUMO energy experiencing the most quenching (-5.84 eV).The order of LUMO energies follows order: 2,4-DNT > 2,6-DNT > 1,3-DNB > 4-NA > 4-NT > 3-NA > 2-NA > TNP and this results in reverse order of quenching efficiencies as: TNP > 2-NA > 3-NA > 4-NT > 4-NA > 1,3-DNB > 2,6-DNT > 2,4-DNT, which is not in fully agreement with the experimental results.Therefore, it may be inferred from this that the theoretical and experimental results of quenching efficiencies do not coincide well.Therefore, the observed PL quenching cannot be fully explained by PET.Additionally, the PL quenching process can be further investigated using FRET.Here, FRET is crucial for the thorough description of the quenching mechanism.FRET predicts that the emission spectrum of the donor and the absorber should have significant overlap (analyte).As shown in Fig. 19, TNP exhibits a strong absorbance maximum λ max = 429 nm, and the SnO 2 @ Fig. 17 FTIR spectra of (a) fresh SnO 2 @Cu 3 (BTC) 2 composite (b) after TNP sensing in aqueous medium Cu 3 (BTC) 2 composite has a significant emission at 472 nm.This shows that the emission spectrum of SnO 2 @Cu 3 (BTC) 2 composite [donar] and the absorption spectrum of TNP [acceptor/analyte] have a larger spectral overlap, which results in a higher probability of electron transfer from SnO 2 @ Cu 3 (BTC) 2 to TNP, leading to the highest fluorescence quenching and selective detection phenomenon towards TNP.Thus, it can be inferred from all of this that energy transfer and electron transfer-via PET and FRET, respectively both reasonably account for fluorescence quenching of SnO 2 @ Cu 3 (BTC) 2 in presence of different NACs.

Conclusions
In summary, the SnO 2 @Cu 3 (BTC) 2 composite was successfully synthesized and investigated for its photocatalytic and luminous properties.It has been evaluated for the degradation of MB dye in sunlight and showed great stability, with a degradation efficiency of 85.12% and good recyclability even after being used five times.This combination has been used to create a fantastic method for TNP detection using water-based fluorescence quenching.Stern-Volmer quenching constants (K SV = 1.04 × 10 4 M −1 ) and limit of detection (LOD = 2.82 μM) were both detected by aqueous phase detection.With no influence from other NACs, the synthesised composite exhibits good selectivity towards TNP.The processes involve the transport of electrons and energy to explain the quenching efficiencies of composites toward various NACs.

Fig. 9
Fig. 9 Time dependent absorption spectra of methylene blue dye degraded in presence of Cu 3 (BTC) 2

Fig. 10 a
Fig. 10 a UV-Vis.absorbance spectrum of degradation of methylene blue (MB) dye under sunlight irradiation for time interval of 0 to 80 min.10 b Histogram for degradation efficiency of photocatalyst as a function of time

Fig. 12 aFig. 13
Fig. 12 a Recyclability test of the photocatalytic degradation of MB in five catalytic cycles under sunlight irradiation; b XRD patterns of the Sno 2 @Cu 3 (BTC) 2 before and after the photocatalytic reaction, indicating stability of photocatalyst

Fig. 16 a
Fig. 16 a Variation in fluorescent intensities as a function of response time upon addition of aqueous solution of TNP (20 µM, 3 mL) into SnO 2 @Cu 3 (BTC) 2 composite.b Comparison of luminescence inten-

Table 1
Performance comparison of photodegradation of MB dye of present composite with traditional composite materials