Synthesis of Encapsulated Zn(8-hydroxyquinoline)2(H2O)2 in the Pore of BioMOF1 for Sensing Dissolved Oxygen in Water
The Zn(8-hydroxyquinoline)2(H2O)2, ZnQ2·2H2O, encapsulated in the porous BioMOF1 (ZnQ2@BioMOF1) host was synthesized by solid-solid and solid-solution reaction between Zn2+@BioMOF1 and 8-hydroxyquinoline. To prepare Zn2+@BioMOF1, dimethylammonium (DMA+), guests in the pores of BioMOF1 were replaced by Zn2+ ions via ion exchange process. The synthesized compound was characterized by XRD and TGA to confirm stability of BioMOF1 host. The ZnQ2·2H2O forming by metal-cation-directed 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 ZnQ2·2H2O 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.
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 , catalysis , drug delivery , and optical and chemical sensing . 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 [7–13]. 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 , chemical vapor deposition , cation exchange , 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, “metal-cation-directed de novo assembly” or “ship-in-a-bottle” [21–23]. 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 m2/g) and pore aperture of 5.2 Å . 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 . They are useful materials for electroluminescent application especially for light emitting diodes (LED). Recent reports have shown that Zn(8-hydroxyquinoline)2(H2O)2, or (ZnQ2·2H2O), can be synthesized simply by a room temperature solid-state reaction . As it is well known that oxygen molecules are able to quench fluorescence, therefore the fluorescence properties of ZnQ2·2H2O 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 [27–31] during the early stage. Later, polypyridine complexes of Osmium(II) , Rhenium(I) , Platinum(II) , and Irridium(II)  have also been investigated. These transition metal complexes possess high emission metal-to-ligand 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) , silicone matrix , 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 . Recently, Zn(8-hydroxyquinoline)2 intercalated in calcium bentonite coated on a polystyrene sheet was used to measure dissolved oxygen .
Herein, we report the facile synthesis and characterization of a novel oxygen sensing material comprising Zn(8-hydroxyquinoline)2(H2O)2, or (ZnQ2·2H2O), encapsulated in the pores of BioMOF1 (Scheme 1). The incorporation of ZnQ2·2H2O into the MOF structure was done via metal-cation-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.
2. Materials and Methods
2.1. 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 N2. 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 X-ray diffraction patterns (PXRD) were collected on a D8 Advance Bruker X-ray diffractometer using Cu-Kα radiation (λ = 1.54060 Å). 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.
2.2. Synthesis of Zn2+@BioMOF1
BioMOF1 was synthesized according to the previous work . Typically, Adenine (0.125 mmol), 4,4′-biphenyl dicarboxylic acid (BPDC) (0.25 mmol), zinc acetate dihydrate (0.375 mmol), nitric acid (1 mmol), DMF (13.5 mL), and water (1 mL) were added to a Teflon-lined autoclave at 130°C for 24 h. Rod-shaped colorless crystals were produced. The crystals were collected, washed with DMF (3 mL × 3), and dried. The prepared BioMOF1 was soaked in DMF solution of Zn(NO3)2 and heated at 65°C for 24 h. Then the resulting product was washed with DMF three times and dried in the oven. The obtained solid was called Zn2+@BioMOF1.
2.3. Synthesis of ZnQ2@BioMOF1
2.3.1. Solid-Solution Reaction
Zn2+@BioMOF1 powder was immersed in the DMF solution of 8-hydroxyquinoline for 24 h. The supernatant solution was decanted and fresh 8-hydroxyquinoline in DMF solution was refilled. This procedure was repeated a time a day for 3 days.
2.3.2. Solid-Solid Reaction
A 1 : 2 ratio of Zn2+@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. ZnQ2·2H2O was also prepared as a reference via solid-solid reaction according to the previously reported procedure .
2.4. Preparation of ZnQ2@BioMOF1 Film
The sensor film was prepared using the method adopted from the literature . Briefly, 1 g of polystyrene pellets was dissolved in 5 ml toluene and the obtained solution was mixed with ZnQ2@BioMOF1 (2 : 1 by weight, respectively); then the mixture was coated manually as a film on cleaned polystyrene sheets.
2.5. 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 O2. 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.
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.
3. Results and Discussion
XRD patterns of the prepared BioMOF1 host and its guest-encapsulated 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 Zn2+@BioMOF1. The XRD pattern of Zn2+@BioMOF1 was not changed after Zn2+ 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 ZnQ2-encapsulated BioMOF1, 8-hydroxyquinoline (Q) molecules were incorporated into Zn2+@BioMOF1 using two different approaches including solid-solution and solid-solid reactions, later yielding ZnQ2@BioMOF1(S) and ZnQ2@BioMOF1(G), respectively. For both reactions, the white color of Zn2+@BioMOF1 turned to yellow after Zn2+@BioMOF1 reacted with 8-hydroxyquinoline, confirming the formation of Zn-8-hydroquinoline complex, ZnQ2·2H2O, and corresponding ZnQ2@BioMOF1. The XRD patterns of ZnQ2@BioMOF1(S) and ZnQ2@BioMOF1(G) are in good agreement with Zn2+@BioMOF1, which suggests that the encapsulation of ZnQ2 did not disrupt the crystal structure of BioMOF1 and the formation of ZnQ2@BioMOF1 can be achieved either by using solid-solution or solid-solid reaction. No diffraction peaks from ZnQ2·2H2O were detected; this is likely because of the low content and the small size of ZnQ2·2H2O 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, (Zn8(ad)4(BPDC)6O·2Me2NH2·8DMF·10H2O), are C, 46.53; H, 4.83; N, 12.80 (Anal. Calc.: C, 46.76; H, 5.07; N, 12.40). For the parent BioMOF1, the first weight loss up to 170°C, approximately 23.85%, was due to the loss of 10 water and 8 DMF molecules as calculated from elemental analysis results (calc. 22.5%). The second weight loss of about 2.13% observed 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 ZnQ2 complexes even at high temperature.
According to elemental analysis, the percentages of elements in Zn2+@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 Zn2+@BioMOF1·4DMF·8H2O (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 Zn2+@BioMOF1 compared to the starting BioMOF1 implied that DMA+ cations were replaced by Zn2+ ions during the cation exchange process.
The TGA curves of ZnQ2@BioMOF1(S) and ZnQ2@BioMOF1(G) show the similar weight loss stages compared to parent BioMOF1 and Zn2+@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 ZnQ2 guests were encapsulated in its structure.
FTIR spectra in Figure 3 showed the vibration of functional groups in ZnQ2·2H2O, Zn2+@BioMOF1, ZnQ2@BioMOF1(S), and ZnQ2@BioMOF1(G). The characteristic bands at around 1390, 1600 cm−1, and 3000–3500 cm−1 were observed for all the encapsulated BioMOF1 and attributed to the O-C-O stretching of carboxylate and N-H stretching of adenine ligands and water molecules, respectively. The presence of ZnQ2·2H2O can be confirmed by focusing on the bands existing in the region of 1000–1750 cm−1. From Figure 3 (expand), the characteristic band at 1500 cm−1 which corresponds to the vibrations of pyridyl and phenyl groups of 8-hydroxyquinoline  was observed in the spectrum ZnQ2@BioMOF1(S) and ZnQ2@BioMOF1(G) but was absent from the spectrum of Zn2+@BioMOF1. This clearly shows that ZnQ2·2H2O was formed in ZnQ2@BioMOF1(S) and ZnQ2@BioMOF1(G). In addition, the bands at 1327 cm−1 attributed to the quinoline group of ZnQ2·2H2O were found in ZnQ2@BioMOF1(S) and ZnQ2@BioMOF1(G) spectrum. However, this band for ZnQ2@BioMOF1(S) and ZnQ2@BioMOF1(G) was red-shifted, which indicates an interaction between quinoline complex and BioMOF1 frameworks.
According to Figure 4, BioMOF1 shows the absorption band in UV region, corresponding with the white color of the compound. The prepared ZnQ2·2H2O shows two absorption bands at 270 and 400 nm which is in agreement with literature . These bands can be attributed to a - transition and metal-to-ligand charge transfer (MLCT), respectively. For ZnQ2@BioMOF1(G) and (S), the formation of ZnQ2·2H2O 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 of ZnQ2·2H2O (Figure 4), indicating that encapsulation of ZnQ2·2H2O in the host BioMOF1 was successful. This confirmed that ZnQ2·2H2O 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 432 nm ( 380 nm) (Figure 5(a)) while maximum emission peak of ZnQ2·2H2O is found at 495 nm  ( 380 nm) with quantum yield 29.9% and life time 60.2μs. After ZnQ2·2H2O were encapsulated in the pores forming ZnQ2@BioMOF1, under the same condition ( 380 nm), the emission band at 432 nm disappears. Only emission band at 499 nm for ZnQ2@BioMOF1(S) (quantum yield 7.23% and life time 69.3μs) and 511 nm for ZnQ2@BioMOF1(G) (quantum yield 6.97% and life time 57.4μs) are present which is corresponding to ZnQ2·2H2O (Figure 5(b)). The fluorescence photographs of the prepared materials are shown in Figures 5(c)–5(f) with respect to fluorescence spectra of those materials. Red shift was observed in the fluorescence spectrum of ZnQ2@BioMOF1 prepared by grinding compared to the one prepared by soaking. We think that it was possible from the different particle size of ZnQ2·2H2O . The disappearance of the band at 432 nm could be assigned to photoinduced electron transfer from BioMOF1 to ZnQ2 [41, 42] and total electron transfer was proposed. It may be close together between BioMOF1 and ZnQ2·2H2O.
To confirm intercalation of ZnQ2·2H2O in the host BioMOF1, confocal fluorescence microscopy was employed. Figures 6(a) and 6(b) show photograph of ZnQ2@BioMOF1(S) and ZnQ2@BioMOF1(G), respectively, comparable with fluorescence images (Figures 6(b) and 6(d)) to understand the distribution of ZnQ2·2H2O. The inset images with respect to orthogonal cross sections reveal that fluorescence species located at some depth from the surface of materials, but not on their surface, which suggests that ZnQ2·2H2O guests were formed within the framework of BioMOF1. Figures 7(a) and 7(b) show scanning electron microscopy (SEM) images of the microsized BioMOF1 crystal, which exhibit regular shape and smooth surface. After ZnQ2·2H2O was encapsulated in BioMOF1’s pore by soaking (Figures 7(c) and 7(d)), the morphology and surface of ZnQ2@BioMOF1(S) change slightly compared to the parent BioMOF1.
The Study of Dissolved Oxygen in Water Sensing. In order to study potential application of the ZnQ2@BioMOF1(G) as a luminophore in a sensor, luminescence was measured after treating ZnQ2@BioMOF1(S) with dissolved oxygen. The fluorescence intensity of ZnQ2@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:
It can be concluded that Zn(8-hydroxyquinoline)2(H2O)2 or ZnQ2·2H2O can be synthesized in the pores of BioMOF1 via solid-solid and solid-solution reactions by metal-cation-directed de novo assembly method without structural collapse. The photoinduced electron transfer from BioMOF1 to ZnQ2·2H2O occurred providing new emission band near 500 nm belonging to ZnQ2·2H2O. 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 () is relatively low.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this article.
The authors would like to thank the Department of Chemistry, Faculty of Science, Kasetsart University, the Science Achievement Scholarship of Thailand, and Kasetsart University Research and Development Institute for financially supporting this work.
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