Research article
Biosynthesis of cobalt oxide nanoparticles using endophytic fungus Aspergillus nidulans

https://doi.org/10.1016/j.jenvman.2018.04.032Get rights and content

Highlights

  • First attempt to synthesize cobalt oxide nanoparticles through endophytic fungus.

  • The average size of synthesized nanoparticles is 20.29 nm.

  • Protein capping act as stabilizing agent for nanoparticles.

  • Extracellular synthesis makes the process simple.

Abstract

Metallic oxide nanoparticles have profound applications in electrochemical devices, supercapacitors, biosensors and batteries. Though four fungi were isolated from Nothapodytes foetida, Aspergillus nidulans was found to be suitable for synthesis of cobalt oxide nanoparticles, as it has proficient tolerance towards metal under study. The broth containing precursor solution and organism Aspergillus nidulans had changed from pink to orange indicating the formation of nanoparticles. Characterization by x-ray diffraction analysis (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and energy dispersive x-ray analysis (EDX) confirmed the formation of spinel cobalt oxide nanoparticles at an average size of 20.29 nm in spherical shape with sulfur-bearing proteins acting as a capping agent for the synthesized nanoparticles. The nanoparticles could be applied in energy storage, as a specific capacitance of 389 F/g showed competence. The study was a greener attempt to synthesize cobalt oxide nanoparticles using endophytic fungus.

Introduction

Nanomaterials are materials having dimensions on a nanoscale. They are the materials in the size of 10−9 m. When compared to their bulk material counterparts, nanoparticles illustrate peculiar physical and chemical properties (Ullah et al., 2014). In addition, they reveal superior electric, mechanical, thermal, catalytic and magnetic properties than macrostructures due to the greater surface area to volume ratio. The predominant types of nanomaterials are carbon-based nanomaterials, metal-based nanomaterials, semiconductor nanoparticles, fullerenes, dendrimers and composites. They are synthesized by both top-down and bottom-up methods. In top-down methods (lithography, photolithography, micromachining, laser machining), macroscopic initial structures are reduced to nano-scale structures. While in bottom-up methods (atomic layer deposition, inert-gas expansion, inert-gas condensation, ultrasonic dispersion), self-assembly of miniature compounds are performed. Cobalt oxide is one of the metallic oxides, being cobalt as a transition metal. Its nanoparticle counterparts are considered reliable in the field of nanotechnology as they have erected favorable applications in optoelectronics (Salavati-Niasari et al., 2009).

In chemical methods of synthesis of nanoparticles, separate capping and stabilization agents are required to stabilize and control the growth of nanoparticles and prevent agglomeration. Further, extraction and purification problems exist (Seabra and Durán, 2015). Nevertheless, chemical methods deliver generous yield in relatively short times, they employ toxic chemicals and exercise stringent protocols (Yadav et al., 2015), that engage severe reaction conditions like decomposition and combustion. They also embroil more than one step and involve processes that are highly energy intensive (Loo et al., 2012). The instance of contamination is inevitable affecting the quality of the nanomaterials obtained. Owing to these disadvantages, biological methods have gained importance recently, as they are clean, non-toxic, pollution free, economically viable and eco-friendly, even though the yield is less. The proteins secreted by microorganisms can act as both capping and stabilizing agent to control the size of nanoparticles, thus yielding biocompatible particles of lesser toxicity. Some of the microorganisms used for the synthesis of nanoparticles are given in Table 1.

Generally, biological synthesis involves synthesis using bacteria, yeast, plant extract and fungi. In plants, the extracts from leaves, flowers and fruits have been used for the synthesis. Lactic acid bacteria synthesize nanoparticles through non-enzymatical methods. In case of microorganisms like fungus, it is easy to isolate and culture and the yield of biomass is more which has a direct relevance in the production of nanoparticles (Saxena et al., 2014). Endophytic fungi secrete a significant number of bioactive metabolites and antimicrobial compounds that are responsible for the synthesis of nanoparticles in presence of precursor compounds via bioactive principles tailoring the elemental compositions (Baker and Satish, 2012). The endophytic fungus is fancied over bacterium as it is noticeable, providing high yield of proteins, large amount of biomass to be handled (Ingale and Chaudhari, 2013) and optimum growth of mycelium with large surface area (Tomar et al., 2015). Fungal biomass along with the supernatant serves as a reduction medium for the formation of the nanoparticles from the precursor solution (Gupta and Bector, 2013). Biosynthesis techniques involving fungi are usually free from toxic chemicals (Kashyap et al., 2013). Nothapodytes foetida has been chosen to isolate the endophytic fungi, which is a medicinal plant distributed in the Western Ghats, used in the treatment of cancer and bacterial infections and classified as vulnerable species (Musavi and Balakrishnan, 2014). It is a producer of camptothecin, which is an alkaloid topoisomerase I-DNA inhibitor (Namdeo and Sharma, 2012). It is reported to house more than 170 species of endophytes and the dominant ones are highly tolerant to toxic metals even at high concentrations and harsh conditions (Musavi and Balakrishnan, 2013). Moreover, their adaptability is one of the major reasons to be used to produce metal-oxide nanoparticles.

The biological methods used are given in Table 2. The reactors for the synthesis process are usually beakers or smaller conical flasks kept in an incubator shaker (Kora and Rastogi, 2016; Rahmatpour et al., 2017). Disposal of cobalt oxide nanoparticles has yet to be reported. In life cycle assessment of magnetite nanoparticles to outweigh the environmental benefits to costs, nanoparticles from used solution were recovered (Sashukan et al., 2017). Silver nanoparticles were removed by electrocoagulation (Matias et al., 2015). Cobalt oxide nanoparticles are primarily obtained by acidic treatment of chemical precursor sources in the solvent environment. Still, endophytic fungi are yet to be explored for the synthesis of cobalt oxide nanoparticles. Hence, synthesis can be bidden using potential microorganisms, possibly endophytic fungi that are relatively tolerant to these metals and respond quickly. The current study illustrates an attempt made to biosynthesize and characterize cobalt oxide nanoparticles via an endophytic fungus Aspergillus nidulans isolated from Nothapodytes foetida.

Section snippets

Materials

Samples of plant Nothapodytes foetida were collected from Agumbe forest located in the Western Ghats (13°30′N, 75°02′E), Shimoga District, Thirthahalli taluk in the Malnad region of Karnataka, India. Fresh and healthy parts of the plant like stem, seed and leaves were cut with a sterile knife and refrigerated for further use. Potato dextrose agar (PDA) was purchased from Sisco Research Laboratories, Mumbai, India and potato dextrose broth (PDB) and chloramphenicol were acquired from Himedia

Isolation of endophytic fungi

After 4 days of inoculation of explants in petri plates supplemented with PDA, a few colonies of fungi began to erupt. While monitoring the growth of fungal isolates, it was observed that color and size of the colonies varied according to the fungi in the consortium. Subculturing of these isolates on petri plates was done once the hyphal tips emerged from explants. Only the viable fungi were repeatedly subcultured until pure isolates were obtained. Thus, pure isolates obtained were subcultured

Practical application, future research prospects and limitations

The stable nanoparticle dispersions are called nanofluids which exhibits the property of quantum confinement. They are discrete pockets of energy sites, which trap the energy and store it in them. Superior solar absorption capability of cobalt oxide nanofluids makes it attractive for solar and thermal applications because of its capability to transform radiative heat influx into useful thermal energy, which will be distributed throughout the fluid. The nanofluids possess high surface area to

Conclusion

Four fungi were isolated from Nothapodytes foetida and tolerance studies were performed to identify the suitable fungus for the synthesis of cobalt oxide nanoparticles. Fluorescent, metallic oxide nanoparticles had been synthesized successfully through Aspergillus nidulans. The nanoparticles were synthesized extracellularly by the fungus at room temperature. Morphological and optical characterizations confirmed the formation of nanoparticles. It had been inferred from FTIR studies that protein

Competing interests

The authors declare that there is no competing interest.

Acknowledgement

The authors are thankful to Dr. Udaya Bhat. K., Head of the Department of Metallurgical and Materials Engineering and Dr. M. N. Satya Narayan, Head of the Department of Physics, National Institute of Technology Karnataka, Surathkal for providing their facilities.

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