Abstract: Metal Organic Frameworks (MOFs) are frameworks of 1-3D formed by the interaction between organic molecules and metal ions/clusters. The high specific surface area (~1000-5000 m2 g-1) and large pore volume (~0.7-2.5 cc g-1) of these molecules render them as ideal candidates for catalysis, gas absorption, separation of gases and sensing applications. The pore size and surface area can be tailored by modifying the synthetic conditions. In this paper, we report the synthesis and characterization of MOF-199 for potential application in enzyme-based sensing. MOF-199 was synthesized under room temperature using benzenetricarboxylic acid and a 1:1:1 mixture of DMF/ethanol/copper (II) acetate. Further, triethylamine (0.5 mL) was added to the reaction mixture and the resultant mixture was stirred for 23 h followed by drying to obtain a bluish crystalline material of MOF-199. The crystalline material was characterized using various analytical and microscopic techniques.
INTRODUCTION
Researchers have been working on preparation of porous material for about more than half century leading to many large number of applications in home appliances, industries etc. Zeolites are one of the prominent porous material whose application has been widely explored in a variety of fields. Zeolites are microporous inorganic aluminosilicates of alkali earth metals such as sodium, potassium and calcium will find applications detergents, water softeners, ion exchange etc. (Yang and Doong, 1985).
In 1990s, researchers found a new class structure based on metal ions and organic binding named as Metal Organic Framework (MOF). The metal ion or a cluster of metal ions interacts with mono, di or trivalent organic compounds (linkers) to form the MOF. These are usually prepared by hydrothermal or solvothermal methods in DMF or DEF medium which decompose upon heating and generate bases capable of deprotonating organic linker molecules. The negative moieties of organic linkers react with metal ions to provide 2D network of MOFs. The choice of the metal ion and the organic linker determines the structure and properties of MOFs. Co-ordination number of the metal ions and its oxidation state determines the pore size and shape (Keskin and Kizilel, 2010). For a solid to be termed as MOF, it should display the following attributes (Yaghi et al., 2003):
| • | Strong bonding to provide robustness |
| • | Modifiable organic linkers by synthetic routes and |
| • | Geometrically well-defined structure (should be highly crystalline so that structure property correlation can be well established) (Li et al., 1999) |
This study has reported the synthesis of MOF-199 also called Cu-BTC Copper(II) acetate, BTC = 1,3,5-benzenetricarboxylate, which was first reported by Chui et al. (1999).
MATERIALS AND METHODS
Materials: Copper acetate monohydrate and benzene dicarboxylic acid were purchased from Sigma Aldrich, India. N, N-dimethylformamide was purchased from Merck Specialities Pvt. Ltd., India.
Triethylamine was purchased from Sisco Research Laboratory, India.
Preparation of MOF-199: Copper(II) acetate monohydrate (860 mg) was dissolved in 1:1:1 ratio of DMF, ethanol, and distilled water (4 mL each). Benzenetricarboxylic acid (500 mg) was also dissolved in the same proposition of solvent mentioned for copper. The two samples were mixed together in a magnetic stirrer to make a homogenous solution. About 0.5 mL of triethylamine was added to the same slowly while the solution was being stirred. The solution was left for 23 h to stir the mixture homogeneously. Then the sample was filtered and washed twice with DMF (25 mL) and kept for drying (Tranchemontagne et al., 2008). A product yield of 1.7 g was obtained.
Characterization of MOF-199: Scanning Electron Microscopy: The surface properties and topography were studied using scanning Electron Microscopy (JSM 6701F, JEOL, Japan). Samples were put up on a brass stub with double sided carbon tape and platinum was coated by using sputtering at 20 mA. Thin conducting layer was formed after which the sample was into the electron microscope’s specimen chamber through exchange chamber. At an accelerating voltage of 3 kV under ultrahigh vacuum, samples were imaged using secondary electron detector.
X-ray diffractometry: The crystal structure of synthesized material was studied by recording Powder X-ray Diffraction (PXRD) patterns using a diffractometer (D8 Focus, Bruker, USA) with Cu-Kα radiation.
Fourier transform infrared spectroscopy: The presence of functional group was identified by using FTIR spectrometry (Spectrum 100, Perkin-Elmer, USA). The sample was scanned over a range of 400-4000 cm-1 (10 scans).
Transmission electron microscopy: The porous nature of the material is characterized using TEM (JEM 2100F, JEOL, JAPAN) at 200 kV. The samples were dispersed in ethanol and placed in copper grid for imaging.
Surface area analysis: A commercial BET surface area analyser (ASAP 2020, Micrometrics, USA) was used for determination of surface of the synthesized sample.
RESULTS AND DISCUSSION
SEM analysis: Figure 1 shows the scanning electron micrograph of the synthesized material.
| Fig. 1: | Scanning electron micrograph of the synthesized material at 100,000 X showing the presence of rod-like structures |
The gross morphology of the material indicates the presence of rod-like structures with diameters ranging from 19 to 35 nm.
X-ray diffractometry: The X-ray diffractogram (Fig. 2) shows the presence of broad, intense peaks in the 2θ range of 10-20.
Fourier transform infrared spectroscopy: The FTIR spectra (Fig. 3) shows absorption in the wave numbers range of 400-500, 700-800, 1380-1390 and 1590-1600 cm-1.
Transmission electron microscopy: The transmission electron micrographs (Fig. 4) revealed the existence of high pore density in the MOF-199 sample. Most of the pores were found to be less than 2 nm.
BET analysis: Table 1 shows the results obtained from the BET analysis of the synthesized material. The adsorption-desorption isotherms are shown in Fig. 5.
| Table 1: | Results of BET surface area analysis |
| Fig. 2: | X-Ray diffractogram of MOF-199 |
| Fig. 3: | FTIR spectrum of MOF-199 |
| Fig. 4: | High resolution transmission electron micrographs of MOF-199 showing the presence of pores less than 2 nm in diameter |
| Fig. 5: | BET adsorption-desorption isotherms |
The presence of larger size crystals in scanning electron micrograph (Fig. 1) may be attributed to the uncontrolled crystal growth at few locations.
From the X-ray diffractogram shown in Fig. 2 it may be concluded that the synthesized material was crystalline in nature. A peak at‘2θ’ around 11.986 is characteristic of MOF-199 (Tranchemontagne et al., 2008).
The absorption band between 400-500 cm-1 (Fig. 3) may be attributed to the Copper-oxygen bond. The presence of band at 1590-1600 cm-1 confirms the presence of aromatic ring (Abbasi et al., 2011).
Transmission Electron Micrograph (Fig. 4) confirms the synthesized material is highly crystalline and porous with unordered pores. The presence of large number of pores contributes to the high surface area of MOF-199.
From Table 1, it is evident that the BET surface area and BJH pore volume are 950 and 0.95 cm3 g-1, respectively. The high surface area indicates that the material is suitable for a variety of applications like sensing, catalysis etc. Enzyme immobilization studies were also carried out which reveal high enzyme immobilization potential of this material. Hence it is a promising material for application in biosensors also.
CONCLUSION
Copper based metal organic framework (MOF-199) was synthesized successfully and confirmed through X-ray diffraction and FTIR analyses. The presence of micropores (pore diameter <2 nm) was evident from transmission electron micrograph. Owing to the high surface area MOF-199 can be used for sensing applications to efficiently sense various gases and biomolecules.
ACKNOWLEDGMENTS
This work was supported by PG teaching grant No. SR/NMPG-16/2007 of Nano Mission Council, Department of Science and Technology (DST), India.