Synthesis, Characterization and Antifungal Activity of Fe(III) Metal–Organic Framework and its Nano-composite
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Metal–organic frameworks (MOFs) have gained developing interest due to their high specific surface area and pore volume, which has been exploited for gas storage, sensors and, drug delivery. This study presents the synthesis of a non-toxic, biocompatible and thermally stable MIL-53(Fe) and the preparation of its silver(I) nitrate nano-composite. This MIL-53(Fe) is a three-dimensional porous solid composed of infinite FeO4(OH)2 cluster connected by 1,4-benzenedicarboxylate (H2BDC) ligand using solvothermal method of synthesis and the encapsulation process was also carried out to produce a composite composed of silver nanoparticle (AgNP). The synthesized materials were characterized using Powder X-ray Diffractometer (PXRD), Scanning Electron Microscope coupled with Electron Diffraction X-ray Spectrometer (SEM–EDS) and Fourier Transform Infrared (FT-IR) Spectroscopy. The Ag@MIL-53(Fe) composite exhibits a remarkable antifungal activity against Aspergillus flavus using a poison plate method. This can be attributed to the therapeutic nature of nanoparticle with a range of 55–64% growth inhibition rate as the concentration of the Ag@MIL-53(Fe) was increased. Minimum lethal concentrations (MLC) were observed to be 40 μg/mL and 15 μg/mL for the prepared MIL-53(Fe) and the Ag@MIL-53(Fe) composite, respectively.
KeywordsMetal–organic frameworks Nano-composite Aspergillus flavus Antifungal test
Metal–organic frameworks (MOFs) are coordination polymers that extend into two, three-dimensional networks . These materials need to be strong bonding metal centers linked by organic ligands to form a geometrically well-defined structure . Metal organic frameworks (MOFs) have received great attention in recent years, due to their fascinating architectures and topologies (low density, high specific surface and pore volume) as well as their increasing properties and potential applications such as functional materials, catalysis, separation (adsorption), gas storage and drug delivery [2, 3, 4, 5]. Metal–organic frameworks also known as porous coordination networks and porous coordinated polymers refer to similar but not the same general type of materials [6, 7]. MOFs’ well-defined and large pore structure makes it possible for them to be used to stabilize and control the formation of metal nano-particles within their structure . The MIL (Material Institute Lavoiser) series of MOFs have been especially reported as versatile materials which have been tested in various applications ranging from gas adsorption/separation, gas storage, catalysis, and drug loading. Moreover, there is continued search for materials which can achieve controlled ion release thereby exhibiting antimicrobial properties. MOFs have been proposed as materials which show promising possibility for antimicrobial application. Recently, silver-based MOFs have been reported to show excellent antimicrobial activity, just as two cobalt-imidazolate MOFs were reported to show interesting anti-bacterial activity against the growth of Pseudomonas putida and Escherichia coli. The HKUST-1 (i.e. Hong Kong University of Science and Technology metal organic framework (MOF) consisting of copper ion linked by 1,3,5-benzenetricarboxylic acid) has been reported to have a pore system which provides access to the binuclear metal centres. The Cu ion metal centres are capable of been disconnected resulting in the MOF acting as a means of ions which are biologically active [9, 10, 11].
Silver nanoparticles (AgNPs) have been presented as broad-spectrum antimicrobial agents that have been widely utilized in products such as personal care and pharmaceutical products. The silver ions present in the core of the nanoparticles are reported to be responsible for the biological activity of the NPs. However, experimental findings reveal that the silver ions alone are not always responsible for the biological action of the NPs .
A typical MOF which has found several applications in a variety of processes is the iron−benzenedicarboxylate (MIL-53(Fe)) MOF. This material is constructed from a combination of 1,4-benzenedicarboxylate (BDC) linker and FeO4(OH). Some potential applications of the MIL-53(Fe) MOF include gas storage/separation, drug loading and delivery, and adsorption. The MIL-53(Fe) has been recently presented as having photocatalytic property in the degradation of organic dyes, and introduction of functionalities into the MIL-53(Fe) material is able to achieve great improvements in their photocatalytic performance [13, 14, 15].
The incorporation of active metal nano-particles into metal–organic frameworks is relevant for a number of potential applications involving heterogeneous catalysis and gas storage [16, 17]. Intercalation is usually achieved via decomposition of volatile organic precursors although it can be achieved using wetness impregnation, mechanical and co-precipitation methods [16, 18, 19, 20, 21, 22, 23, 24]. Palladium and ruthenium nanoparticles have been reportedly incorporated into MOF-5 using the chemical vapour deposition technique  while Cu-Pd nanoparticles were successfully intercalated into MIL-101 , and both materials were reported to exhibit enhanced catalytic activity in the oxidation of CO than the unincorporated MOFs or the separate nano-particles, and the incorporation of these nano-particles into the MOFs structure allowed for effective reactions at low temperature. The presence of Pd-Ru and Cu-Pd nanoparticles were confirmed by TEM and XRD analyses . Obtaining catalysts having optimally tuned adsorbate binding property is of great research interest. The study of this category of catalysts involves using density functional theory to determine the ensembles, the ligand, and effects of strain in the close-packed material which has been alloyed using transition metals particularly RhAu, PdAu, and PtAu bimetallic materials. The preparation of Ag–Ir (silver–iridium) alloys as solid-solution nanoparticles and their use as catalysts has been reported. The Ag–Ir NPs were found to have higher selectivity toward the C═O hydrogenation in α,β-unsaturated aldehydes and croton-aldehyde, resulting in crotyl alcohol which has high industrial value. Also, the incorporation of Ag–Ir NPs in the pores of Co3O4 leads to about 56% enhancement in selectivity. The performance of bimetallic and single metal surfaces for the reduction of nitrite has been shown to be rapidly enhanced using binding energies of ammonia (NH3), nitrogen (N), and nitrogen gas (N2) to describe the reactivity through catalyst modeling using the density functional theory (DFT) calculations [26, 27, 28].
There have been reports of preparation of Ag nanoparticles of MOFs such as MIL-101, MIL-53(Al), and silver phosphate composite of MIL-53(Fe), however, to the best of our knowledge, there is no literature report of the antifungal activity of MIL-53(Fe) silver-nanocomposite against the Aspergillus flavus, in particular.
The disease Aspergillosis is a common fungal infection which is ubiquitous in nature and occurs in birds and occasionally in man by exposure to Aspergillus fungi. The genus Aspergillus comprises of about 185 species of which 20 species of these have been reported to cause opportunistic infections in man. The species include Aspergillus fumigatus which is the most common specie isolated, Aspergillus flavus and Aspergillus niger, Aspergillus clavatus, Aspergillus glaucus group, Aspergillus terreus, Aspergillus oryzae, Aspergillus nidulans. Less commonly isolated species include the Aspergillus ustus and Aspergillus versicolor which have been reported to be less opportunistic pathogens. Disease states associated with aspergillosis include cutaneous aspergillosis, cerebral aspergillosis, meningitis, endocarditis, myocarditis, pulmonary aspergillosis, osteomyelitis, otomycosis, onychomycosis, sinusitis, endophthalmitis, hepatosplenic aspergillosis, and Aspergillus fungemia, disseminated aspergillosis may arise thereof [29, 30].
In this study, we report the synthesis and characterization of MIL-53(Fe), and the incorporation of silver-nanoparticles into its framework. The antifungal activity of the synthesized MOF and its nano-composite were investigated against Aspergillus flavus using disc diffusion method (poison plate method). The antifungal activity of the MOFs materials, [MIL-53(Fe)] and Ag@MIL-53(Fe), were compared based on the zone of inhibition.
Terephthalic acid (99%), Silver(I)nitrate (AgNO3, 99%), and iron(III)nitrate nonahydrate (Fe(NO3)3∙9H2O, 99%) were obtained from Sigma Aldrich Ltd., Germany. N,N-dimethylformamide (DMF, 99%) and triethylamine (TEA, 99%) were obtained from was obtained from British Drug House Ltd., England. All chemicals obtained were used as received.
2.2 Synthesis of MIL-53(Fe)
2.3 Characterization of the Synthesized MIL-53(Fe)
The powder X-ray diffraction (PXRD) analysis was carried out on an Empyrean XRD X-ray diffractometer using a CuKα-radiation operating at 30 kV and 40 mA. Fourier transform infrared (FTIR) spectrum was measured using a Shimadzu 8400 s spectrophotometer with KBr. The samples were mixed with KBr in the ratio 1:10 and pelletized, and the spectra were recorded over a range of 400–4000 cm−1. Scanning electron microscopy (SEM) images and energy-dispersive X-ray (EDX) analysis of the synthesized MOFs were obtained using a TESCAN Vega 3 XMU scanning electron microscope.
2.4 Preparation of MIL-53(Fe) Silver-nanoparticles (Ag@MIL-53(Fe))
2.5 Antifungal Activity
The inhibitory or stimulatory activity of the synthesized MIL-53(Fe) and its composites, Ag@MIL-53(Fe), on micro-organisms was determined by following the procedure described by Obaleye et al. . The antifungal activity was studied using a potato dextrose agar on which 1.0 cm diameter walls was punched and three different concentrations (5%, 10%, 15% m/v in distilled water) of the MOFs and its composite were utilized.
2.6 Determination of Minimum Lethal Concentration (MLC)
The procedure described by Obaleye et al.  was adopted in order to determine the MLC. Sterile stoppered test tubes were utilized and the growth medium for the Aspergillus flavus added. This was followed by subsequent addition of 0.05 mL aliquots of MIL-53(Fe) and its composites, Ag@MIL-53(Fe), from a volume of 0.1 mL to 5.0 mL. This represented 10 to 500 μg/mL, in a final mixture of 10 mL. Standard volume to represent the inoculum (0.2 mL each of the test), in which the MIL-53(Fe) and its composites, Ag@MIL-53(Fe), are omitted, and another in which the Aspergillus flavus test organism is omitted was also prepared. The tubes were all incubated at a temperature of 35 °C for a period of 24 h and evidence of growth observed. The MLC was thus noted.
3 Results and Discussion
3.1 Fourier Transform Infrared (FT-IR) Spectra
The FTIR spectra of the synthesized MIL-53(Fe) and that of the Ag@MIL-53(Fe) showed similar peaks indicating that the incorporated silver nano-particles did not alter the structure of the MIL-53(Fe) .
3.2 Powder X-ray Diffraction (PXRD) Analysis
3.3 SEM–EDX Analysis
3.4 Antifungal Activity
Antifungal activity of MOFs and its nano-composite against Aspergillus flavus
MIL-53(Fe) was synthesized and its Ag@MIL-53(Fe) composite successfully prepared using polyvinylpyrrolidone (PVP) surfactant. The antifungal activity of the MOFs and its composite was tested against Aspergillus flavus and the activities of the MOFs were compared based on the zone of inhibition. The Ag@MIL-53(Fe) was observed to have better antifungal activity against the Aspergillus flavus fungi at the various concentrations used which may be due to the therapeutic nature of the silver nanoparticles present in the framework. This work thus presents the Ag@MIL-53(Fe) as an effective antifungal agent against Aspergillus flavus. This indicates the capability of the Ag@MIL-53(Fe) to be used as an antifungal agent in the treatment of fungal infections arising from the Aspergillus flavus.
Prof. A. C. Tella is grateful to the Royal Society of Chemistry for the award of 2015 research fund. Authors Dr. H. K. Okoro and Prof J. C. Ngila are grateful to the U.J. Global Excellence and Stature Scholarship for running cost paid by Water Research Commission WRC Project No; K5/2365. Dr. Caliphs Zvinowanda thanks NRF-SA/Egypt collaboration grants No; 108685.
Compliance with Ethical Standards
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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