Synthesis of Encapsulated Zn ( 8-hydroxyquinoline ) 2 ( H 2 O ) 2 in the Pore of BioMOF 1 for Sensing Dissolved Oxygen in Water

The Zn(8-hydroxyquinoline) 2 (H 2 O) 2 , ZnQ 2 ⋅2H 2 O, encapsulated in the porous BioMOF1 (ZnQ 2 @BioMOF1) host was synthesized by solid-solid and solid-solution reaction between Zn@BioMOF1 and 8-hydroxyquinoline. To prepare Zn@BioMOF1, dimethylammonium (DMA), guests in the pores of BioMOF1were replaced byZn ions via ion exchange process.The synthesized compound was characterized by XRD and TGA to confirm stability of BioMOF1 host. The ZnQ 2 ⋅2H 2 O forming by metal-cationdirected de novo coassembly approach was confirmed by UV, IR, Fluorescence, BET, and confocal microscopy. Scanning electron microscopy images show slight change in morphology of BioMOF1 after introducing ZnQ 2 ⋅2H 2 O by solid-solution reaction into its pores. Thin films of the produced materials were used to sense dissolved oxygen in water by using fluorescence technique.


Introduction
Metal-organic frameworks (MOFs) are a new class of synthetic porous materials built up with metal ions/clusters and organic linkers.Depending on the selected metal ions, molecular linkers, and the synthetic conditions, MOFs develop a variety of molecular topologies, usually with large internal space, which contributes to their prominent features including high specific surface area and pore volume.These materials have attracted attentions from academic and industry around the globe over this period.They have shown a wide range of potential applications such as gas storage [1], catalysis [2], drug delivery [3], and optical and chemical sensing [4].Recently, new MOF materials with tunable luminescent properties have been designed [5,6].Encapsulation of guest molecules in space of MOFs has been considered an effective concept for fabricating novel functional materials.By using MOFs as a host, functional guest species, for example, metal ions, organometallic complexes, quantum dots, nanoparticles, and fluorescent dyes, are stabilized and confined within the pores, leading to specific behaviors that improve and broaden properties of MOFs towards advanced gas storage materials, optical devices, heterogeneous catalysts, and sensors [7][8][9][10][11][12][13]. Interestingly, the fluorescence behavior of MOFs can even be further tuned by encapsulating guest species such as cations, anions, and solvent molecules [14,15].
There are several methods to include functionalized guest molecules into the pores of MOFs such as impregnation [16], chemical vapor deposition [17], cation exchange [18], and coassembly [19,20].However, the ability to incorporate the guests is limited by the size of the pores as the pores are likely to be inaccessible to guests with larger size compared to pore size of MOF hosts.To solve that problem, a new alternative approach has been developed, namely, "metalcation-directed de novo assembly" or "ship-in-a-bottle" [21][22][23].This approach is based on the formation of functional guest molecules from its fragments by introduction of metal ions to MOF prior to the assembly of component fragments under the direction of metal ions.
In this research BioMOF1 was selected to be a host because it has high specific surface area (∼1700 m 2 /g) and pore aperture of 5.2 Å [24].In addition, the BioMOF1 is an anionic complex encapsulated with dimethylammonium cation (the product of DMF decomposition) in the cavity.
8-Hydroxyquinoline metal chelate, Zn-bis(8-hydroxyquinoline), is under intensive research due to its excellent fluorescence properties.Quinoline derivatives have gained a lot of attention due to their photoluminescence and electroluminescence properties.These properties are very sensitive to the change of electron vibration shift in their structures [25].They are useful materials for electroluminescent application especially for light emitting diodes (LED).Recent reports have shown that Zn(8-hydroxyquinoline) 2 (H 2 O) 2 , or (ZnQ 2 ⋅2H 2 O), can be synthesized simply by a room temperature solid-state reaction [26].As it is well known that oxygen molecules are able to quench fluorescence, therefore the fluorescence properties of ZnQ 2 ⋅2H 2 O will be quenched when oxygen molecules are present.This complex can be an alternative dye for oxygen sensor.
Dissolved oxygen in water plays an important role in biological, clinical, environmental, and industrial applications.Several oxygen detection methods have been reported, mainly based on electrochemical, optical, and chemical processes.Electrochemical method is the most popular due to the conventional procedure in measuring oxygen concentration.However, this method tends to suffer from the instability of electrode surface.Optical oxygen sensor offers inexpensive highly sensitive measurement and fast response.Most optical oxygen sensors are based on luminescence quenching of sensitive dyes, which are commonly transition metal complexes.Ru(II) polypyridine complexes immobilized in polymer films have been employed more frequently [27][28][29][30][31] during the early stage.Later, polypyridine complexes of Osmium(II) [32], Rhenium(I) [33], Platinum(II) [34], and Irridium(II) [35] have also been investigated.These transition metal complexes possess high emission metal-toligand charge transfer (MLCT), therefore representing good oxygen sensitive dyes.
Moreover to improve the quantum yield and lifetime of the organic dyes, oxygen permeability of polymer matrixes is more concentrated on such as using fluoropolymer [35,36], organically modified silicates (ORMOSIL) [37], silicone matrix [38], and so on.Optical oxygen sensors have been further developed in many procedures.Material based on a Ru(II) polypyridine complex encapsulated in zeolite Y was reported compared with one adsorbed on the surface.It was found that oxygen quenching was higher in encapsulating material [39].Recently, Zn(8-hydroxyquinoline) 2 intercalated in calcium bentonite coated on a polystyrene sheet was used to measure dissolved oxygen [40].
Herein, we report the facile synthesis and characterization of a novel oxygen sensing material comprising Zn(8hydroxyquinoline) 2 (H 2 O) 2 , or (ZnQ 2 ⋅2H 2 O), encapsulated in the pores of BioMOF1 (Scheme 1).The incorporation of ZnQ 2 ⋅2H 2 O into the MOF structure was done via metalcation-directed de novo assembly through solid-state reaction and solid-solution reaction.Dissolved oxygen sensing property of this material was studied by fluorescence-based measurement.

Materials and Measurements.
All reagents were purchased from commercial sources and used as received.Infrared (IR) spectra were measured in KBr pellets on a Bruker Model Equinox 55 spectrophotometer in the region 400-4000 cm −1 .Thermogravimetric analyses (TGA) were studied by a Perkin Elmer TGA7 with a heating rate of 10 ∘ C per min under N 2 .UV-VIS diffuse reflectance spectra were recorded between 200 and 1000 nm by using UV-VIS Spectrophotometer Shimadzu UV-2600.Fluorescence emission spectra were measured on a Perkin Klmer model LS55 Luminescence spectrometer.Elemental analyses (C, H, and N) were studied by a LECO CHNS-932.Powder Xray diffraction patterns (PXRD) were collected on a D8 Advance Bruker X-ray diffractometer using Cu-K radiation ( = 1.54060Å).Scanning electron microscopy (SEM) was carried out on a Quanta 450 FEI with Tungsten filament electron source operated at 25 kV.Fluorescence quantum yield and fluorescence lifetime were measured in solid-state with Spectrofluorometer HORIBA Fluoromax model.The fluorescence images were obtained with a Nikon C2-si laser scanning confocal microscope.
2.3.Synthesis of ZnQ 2 @BioMOF1 2.3.1.Solid-Solution Reaction.Zn 2+ @BioMOF1 powder was immersed in the DMF solution of 8-hydroxyquinoline for 24 h.The supernatant solution was decanted and fresh 8hydroxyquinoline in DMF solution was refilled.This procedure was repeated a time a day for 3 days.

Solid-Solid Reaction.
A 1 : 2 ratio of Zn 2+ @BioMOF1 and 8-hydroxyquinoline was mixed and ground in a mortar for 20 min.The color of the mixture changed from white to yellow within a few seconds.Then the mixture was washed with ethanol to remove the unreacted starting materials.The product was dried at 363 K and used for further characterization.ZnQ 2 ⋅2H 2 O was also prepared as a reference via solidsolid reaction according to the previously reported procedure [26].

Preparation of ZnQ
2 @BioMOF1 Film.The sensor film was prepared using the method adopted from the literature [40].Briefly, 1 g of polystyrene pellets was dissolved in 5 ml toluene and the obtained solution was mixed with ZnQ 2 @BioMOF1 (2 : 1 by weight, respectively); then the mixture was coated manually as a film on cleaned polystyrene sheets.

Preparation of Dissolved Oxygen Water Samples.
To obtain 0%, 10%, 20%, 30%, 50%, 60%, 70%, 80%, 90%, and 100% dissolved oxygen in water (DO water), different ratios of nitrogen and oxygen gas mixtures were flown into deionized (DI) water.For example, 50 ml/min of oxygen gas was mixed with 450 ml/min of nitrogen, the flow rate of which was controlled by mass flow controllers, in order to produce the gas mixture containing 10% of O 2 .This gas mixture was then allowed to flow into the chamber containing deionized for 30 min.The resultant water sample was labelled as 10% dissolved oxygen in water.

Results and Discussion
XRD patterns of the prepared BioMOF1 host and its guestencapsulated versions are shown in Figure 1.The pattern of the prepared BioMOF1 is in agreement with the simulated pattern, indicating that BioMOF1 framework was successfully synthesized.After the prepared BioMOF1 was immersed in DMF solution of Zn(II) ions, dimethylammonium cation (DMA + ) guests in the pores of BioMOF1 were substituted by Zn(II) ions from the solution through cation ion exchange process, allowing the formation of Zn 2+ @BioMOF1.The XRD pattern of Zn 2+ @BioMOF1 was not changed after Zn 2+ incorporation and showed the same reflections as the parent BioMOF1, which illustrates that the framework of BioMOF1 host was stable upon cation ion exchange during soaking step.
In order to form ZnQ 2 -encapsulated BioMOF1, 8hydroxyquinoline (Q) molecules were incorporated into Zn 2+ @BioMOF1 using two different approaches including solid-solution and solid-solid reactions, later yielding ZnQ 2 @BioMOF1(S) and ZnQ 2 @BioMOF1(G), respectively.For both reactions, the white color of Zn 2+ @BioMOF1 turned to yellow after Zn 2+ @BioMOF1 reacted with 8-hydroxyquinoline, confirming the formation of Zn-8-hydroquinoline complex, ZnQ 2 ⋅2H 2 O, and corresponding ZnQ 2 @BioMOF1.The XRD patterns of ZnQ 2 @BioMOF1(S) and ZnQ 2 @BioMOF1(G) are in good agreement with Zn 2+ @BioMOF1, which suggests that the encapsulation of ZnQ 2 did not disrupt the crystal structure of BioMOF1 and the formation of ZnQ 2 @BioMOF1 can be achieved either by using solid-solution or solid-solid reaction.No diffraction peaks from ZnQ 2 ⋅2H 2 O were detected; this is likely because of the low content and the small size of ZnQ 2 ⋅2H 2 O formed inside the pores of BiOMOF1 framework.
Figure 2 shows thermal gravimetric analysis of the parent BioMOF1 and its guest-encapsulated products.The percentages of elements found in the prepared BioMOF1, (Zn 8 (ad) Temperature ( ∘ C) Zn１ 2 @BioMOF1(G) Figure 2: TGA plots of prepared BioMOF1, Zn 2+ @BioMOF1, ZnQ 2 @BioMOF1(S), and ZnQ 2 @BioMOF1(G).at 250 ∘ C was assigned to the loss of DMA cations (calc.2.72%) and the third weight loss at 380-550 ∘ C indicated the decomposition of the structure to become ZnO.It can be explained by the fact that thermal gravimetric analysis is corresponding to the elemental analysis.Therefore, from the thermal decomposition analysis, it is clear that the structure of BioMOF1 is thermally stable up to quite high temperature.Therefore, we can expect BioMOF1 to be a good host for encapsulating ZnQ 2 complexes even at high temperature.
According to elemental analysis, the percentages of elements in Zn 2+ @BioMOF1 are found to be C, 45.28; H, 4.63; N, 12.21, which corresponds to the presence of 8 water and 4 DMF molecules.As a result, the formula can be written as Zn 2+ @BioMOF1⋅4DMF⋅8H 2 O (Anal.Calc.: C, 45.90; H, 4.08; N, 11.08).This result is in agreement with the TGA analysis where the first weight loss observed from 30 to 180 ∘ C was approximately equal to 17.90%, due to the loss of 8 water and 4 DMF molecules (calc.14.37%).The disappearance of the second-step weight loss in Zn 2+ @BioMOF1 compared to the starting BioMOF1 implied that DMA + cations were replaced by Zn 2+ ions during the cation exchange process.
The TGA curves of ZnQ 2 @BioMOF1(S) and ZnQ 2 @BioMOF1(G) show the similar weight loss stages compared to parent BioMOF1 and Zn 2+ @BioMOF1, especially the last stage which corresponds to the decomposition of the BioMOF1 structures.This indicates that the structure of BioMOF1 remained intact even though ZnQ 2 guests were encapsulated in its structure.
According to Figure 4, BioMOF1 shows the absorption band in UV region, corresponding with the white color of the compound.The prepared ZnQ 2 ⋅2H 2 O shows two absorption bands at 270 and 400 nm which is in agreement with literature [26].These bands can be attributed to a - * transition and metal-to-ligand charge transfer (MLCT), respectively.For ZnQ 2 @BioMOF1(G) and (S), the formation of ZnQ 2 ⋅2H 2 O in the pores of BioMOF1 resulted in the absorption spectrum in two absorption regions: one in the UV region similar to BioMOF1 and the other one in visible region at the same  max of ZnQ 2 ⋅2H 2 O (Figure 4), indicating that encapsulation of ZnQ 2 ⋅2H 2 O in the host BioMOF1 was successful.This confirmed that ZnQ 2 ⋅2H 2 O complex was formed in the host BioMOF1 from both solid-solid reaction and solid-solution reaction.
The solid-state fluorescence of BioMOF1 shows blue emission band at  em 432 nm ( ex 380 nm) (Figure 5(a)) while maximum emission peak of ZnQ 2 ⋅2H 2 O is found at 495 nm [26] ( ex 380 nm) with quantum yield 29.9% and life time 60.2 s.After ZnQ 2 ⋅2H 2 O were encapsulated in the pores forming ZnQ 2 @BioMOF1, under the same condition ( ex 380 nm), the emission band at 432 nm disappears.Only emission band at  em 499 nm for ZnQ 2 @BioMOF1(S) (quantum yield 7.23% and life time 69.3 s) and 511 nm for ZnQ 2 @BioMOF1(G) (quantum yield 6.97% and life time 57.4 s) are present which is corresponding to ZnQ The Study of Dissolved Oxygen in Water Sensing.In order to study potential application of the ZnQ 2 @BioMOF1(G) as a luminophore in a sensor, luminescence was measured after treating ZnQ 2 @BioMOF1(S) with dissolved oxygen.The fluorescence intensity of ZnQ 2 @BioMOF1 was varied as a function of amount of dissolved oxygen in water as shown in Figure 8(a).The data were also fitted to a Stern-Volmer equation: and  represent the luminescent intensities in the absence and presence of O 2 gas. sv is the Stern-Volmer quenching constant which can be obtained from Stern-Volmer plot [43] (Figure 8(b)).

Conclusions
It can be concluded that Zn

Scheme 1 :
Scheme 1: Procedure for encapsulating guest molecule into metal-organic framework.

2. 6 .
Fluorescence Measurement.The sensor films were placed into diagonally fluorescent cuvette.The efficacy of sensor films was measured by adding dissolved oxygen water (DO water) at various concentrations.The changes in fluorescent response were recorded by excitation at wavelength of 380 nm.

Zn１ 2 @Figure 1 :
Figure 1: Comparison of the experimental powder XRD patterns of the synthesized materials.
(8-hydroxyquinoline) 2 (H 2 O) 2 or ZnQ 2 ⋅2H 2 O can be synthesized in the pores of BioMOF1 via solid-solid and solid-solution reactions by metal-cationdirected de novo assembly method without structural collapse.The photoinduced electron transfer from BioMOF1 to ZnQ 2 ⋅2H 2 O occurred providing new emission band near 500 nm belonging to ZnQ 2 ⋅2H 2 O.A film of synthesized material on polystyrene sheet shows sensing to dissolved oxygen in water by fluorescence quenching.The fluorescence intensities can be fitted to Stern-Volmer equation but, however, Stern-Volmer quenching constant ( sv ) is relatively low.