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Characterization of Iron Oxide Nanoparticles

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The mysteries gathered from nature have led to the expansion of biomimetic approaches for the growth of advanced nanomaterials. Biological methods for nanoparticle production using microorganisms, enzymes, and plants or plant extracts have been recommended as possible ecofriendly alternatives to chemical and physical methods. Here, we report extracellular mycosynthesis of γ-Fe2O3 nanoparticles by Alternaria alternata (Fr.) Keissl (1912). On treating iron (III) chloride (FeCl3, 6H2O) solution with fungal culture filtrate, rapid reduction of FeCl3 was observed leading to the formation of highly stable iron oxide (γ-Fe2O3) nanoparticles in the solution and up-to-date literature survey showed this was the first report of biosynthesis of γ-Fe2O3 nanoparticles using this fungus. The influence of these biosynthesized γ-Fe2O3 nanoparticles on the properties of hydroxypropylmethylcellulose (HPMC) was also investigated. The biosynthesized γ-Fe2O3 nanoparticles and HPMC- γ-Fe2O3 nanocomposite films were characterized by the transmission electron microscopy (TEM), X-ray diffraction (XRD) and fourier transform infrared (FTIR) spectroscopy. The TEM image clearly showed the surface morphology of the γ-Fe2O3 nanoparticles with size range of 75−650 nm and also the agglomeration of the γ-Fe2O3 nanoparticles into the HPMC polymer matrix. The dynamic light scattering (DLS) experiment supported the size of the nanoparticles. The crystallinity of dispersed γ-Fe2O3 nanoparticles was assured by XRD analysis. Energy dispersive X-ray (EDX) spectrum revealed the presence of iron and oxygen in the nanoparticles. The FTIR peaks confirmed the presence of a protein shell outside the nanoparticles and also designated the strong interaction between γ-Fe2O3 nanoparticles and HPMC matrix. Nano-sized γ-Fe2O3 modified HPMC showed enhanced mechanical properties i.e. strengthening and toughening of HPMC matrix.

Introduction

Iron oxide nanoparticles are of particular interest due to their fundamental properties associated to their multivalent oxide state and polymorphism1. Several technological applications have been reported for these nanoparticles, for example, magnetic data storage2, catalysis3, magnetic fluids4, medical diagnosis5, controlled drug delivery6, and magnetic-induced cancer therapy7. Iron oxide magnetic nanoparticles have been synthesized by different methods such as sol-gel8, chemical co-precipitation9 and microemulsion10. So, the advancement of the conventional synthetic processes for oxide nanomaterials is an issue of considerable newsworthy interest. While a number of chemical methods are available and are extensively used, the collaborations are often energy intensive and produce hazardous by-products. But on the other hand, the synthesis of inorganic materials by biological means is categorized by the methods that occur at close to ambient temperatures and pressures, and at neutral pH (magnetotactic bacteria, diatoms, and S-layer bacteria)11. Thus, there is a necessity for ‘green chemistry’ that includes a clean, nontoxic and environment-friendly method of nanoparticle synthesis. Among different bio-organisms, fungi in general possess some distinctive advantage over others, because of their high metal tolerance, easy to scale-up, low cost downstream processing, handling of biomass and economic viability. Furthermore, fungi are extremely efficient secretor of extracellular enzymes and possible to easy large-scale production. The cell free culture filtrates of different fungi are used for biosynthesis of different nanoparticles like silver12–14, selenium15,16, gold17,18, ZnO19 etc.

Recently polymer hybrid nanocomposites have attracted much attention because of the unique properties that can be achieved with these materials. Polymers are considered as a good host material for metal and semiconductor20–29 nanoparticles, which, on the other hand, exhibit exceptional optical and electrical properties. At the same time, metal nanoparticles with high surface to bulk ratio drastically affect the polymer matrix leading to some unique properties which are not present in either of the pure materials. Therefore, the investigation of the influence of nanoparticles on the properties of a polymer matrix is necessary in order to be able to better predict the final properties of the composite22,23.

The present study has two fold objectives: firstly the biological synthesis and characterization of iron oxide nanoparticles by using the culture filtrate of a phytopathogenic fungus Alternaria alternata (Fr.) Keissl. (1912) (strain number: MAMP/C/51) and secondly, to find out the effect of these bio-synthesized nanoparticles in the polymerization of the HPMC particles as well as the characteristic features of the HPMC polymer.

Materials and Methods

Materials

Iron (III) chloride hexahydrate (FeCl3, 6H2O) was reagent of E. Merck., India. A cellulose derivative, HPMC (viscosity 50 cPs) was obtained from Central Drug House (P) Ltd., India. The water used throughout this work was triple distilled water.

Fungal Culture

The pathogen, A. alternata (strain number: MAMP/C/51) was isolated previously by Maiti et al.30. The strain of the fungus was maintained by sub-culturing in PDA medium [potato extract (40%), glucose (2%) and agar (2%)] for further use.

Preparation of fungal culture filtrate (FCF)

The fungal strain was grown in PDB medium which was composed of potato extract (40%), glucose (2%) at pH 7.4 and kept for 15 days at room temperature (30°C). After the incubation period, 100 ml of that filtered FCF was collected to synthesize iron oxide nanoparticles.

Synthesis of γ-Fe2O3 nanoparticles

In the present study, we used the FCF as reducing agent to reduce iron (III) chloride hexahydrate (FeCl3, 6H2O). Hundred ml of FCF was taken in each different Erlenmeyer flask and mixed with iron (III) chloride hexahydrate solution (0.1 M final concentration). The FCF containing flask was agitated for 24 h at room temperature. Simultaneously, only the culture filtrate of A. alternata and only iron (III) chloride solution were maintained under same conditions. The reaction mixture was routinely monitored by visual colour change.

The iron oxide nanoparticles (γ-Fe2O3) were separated out by centrifugation at 12000g for 10 min, and the settled nanoparticles were washed with deionized water (three times). The purified γ-Fe2O3 nanoparticles were resuspended in deionized water and ultrasonicated by Piezo-u-sonic ultrasonic cleaner (Pus-60w). Synthesis of γ-Fe2O3 nanoparticles was repeated for three times (n = 3) and subsequently utilized for characterization of the particles.

Preparation procedure of HPMC-γ-Fe2O3 nanocomposite film

A homogeneous solution of 1% aqueous HPMC (w/v) was prepared by mixing a definite amount of HPMC in triple distilled water with continuous stirring. Various amounts (wt %) of γ-Fe2O3 nanoparticles were added to the polymeric solution with continuous stirring for approximately 10 h. Then, the admixture solution was ultra-sonicated for 4h. In order to fabricate the nanocomposite films, the solutions were then transferred into a glass petri plate at ambient temperature for the complete evaporation of water. The films were dried at room temperature in air for 4 day. Finally, the nanocomposite films of appropriate thickness were obtained. Pure HPMC film samples were prepared in similar manner.

Characterization

The size of the particles was measured by using Zen 1600 Malvern nano size particle analyser (range 0.6 nm–6.0 mm). X-ray diffraction measurements of γ-Fe2O3 nanoparticles, virgin HPMC polymer and HPMC-γ-Fe2O3 nanocomposite films were performed at room temperature (30°C) by PW 3040/60 Panalytical X-ray diffractometer operating at an accelerating voltage of 45 KV and current of 30 mA using Cu Kα radiation ( λ 1.54443) as X-ray source. The diffracted intensities were recorded from 20° to 90° 2θ angles. EDX analysis of vacuum-dried γ-Fe2O3 nanoparticles was made by the Hitachi S 3400N instrument to know the elemental compositions of the nanoparticles. The results presented in this study were reproducible with the accuracy of ± 5% error. FTIR spectrophotometer (Shimadzu FTIR-8400S) was used to identify the ingredients as well as the capping agent of the bioreduced γ-Fe2O3 nanoparticles, pure HPMC and HPMC- γ-Fe2O3 nanocomposite films. The thickness of the films was prepared in the range of 0.03 ± 0.005 mm. For this analysis KBr (potassium bromide) was used to make the pellet at a ratio of 1:100. The spectra were recorded by using a diffuse reflectance accessory in the range between 4000 and 400 cm-1 and the scanning data were obtained from the average of 50 scans. TEM measurements were performed on a Tecnai G2 spirit Biotwin (FP 5018/40) instrument operated at an accelerating voltage at 80 kV. A specimen for TEM study was made by casting a drop of sample suspension on a carbon coated copper grid allowed to air dry. The tensile strength, elongation at break and tensile modulus values of virgin HPMC and HPMC-γ-Fe2O3 nanocomposite films were investigated using a universal testing machine (Zwick Roell) with a cross head speed of 5.0 mm/min for a gauge length of 50 mm and 14 mm in width at ambient temperature. The reported values were taken from an average of five measurements.

Results and discussion

Production and characterization of γ-Fe2O3 nanoparticles

The culture filtrate mediated synthesis of γ-Fe2O3 nanoparticles was validated by visually monitoring three flasks containing only the culture filtrate of A. alternata, reaction mixture of the culture filtrate with iron (III) chloride and only iron (III) chloride solution. Only the reaction mixture displayed an instantaneous colour change in the solution from yellow to intense brown, indicating the formation of iron-containing nanoparticles31, whereas the FCF and the iron (III) chloride solution were observed to retain their original colour (Fig. 1). The colour did not change further with increasing incubation time. The appearance of the intense brown colour indicated the occurrence of the reaction and the formation of iron-containing nanoparticles.

Particle size measurement

Particle size was determined by dynamic light scattering measurement. Laser diffraction revealed that particle size obtained in the range of 75−650 nm (Fig. 2).

EDX observation of γ-Fe2O3 nanoparticles

Fig. 3 showed the EDX spectrum recorded in the spot-profile mode from one of the densely populated γ-Fe2O3 nanoparticles area. Strong signal of iron from the examined field was observed. The sharp optical absorption peak in the range of 1−3 and 6−7 keV signified the presence of iron in the nanoparticles32.

XRD observation of γ-Fe2O3 nanoparticles

Fig. 4 showed the XRD pattern of the γ-Fe2O3 nanoparticles, with the (220), (310), (400), (500) and (622) peaks from the fcc structure, clearly identified (JCPDS file, No. 04-0755)33. The peak positions agreed with those of the spinel structure. Thus, the synthesized nanoparticles were Fe3O4 and/or γ-Fe2O3 phases. Although the patterns of Fe3O4 and γ-Fe2O3 phases were similar33, but some peaks corresponding to γ-Fe2O3 phase, such as (220), (310) and (622) peaks, were also seen in the XRD patterns. Therefore, the synthesized nanoparticles seemed to be γ-Fe2O3 phase. The highest peak was shifted towards 35° also indicating γ-phase of Fe2O3 compared with the standard JCPDS data (card no. 39-1346)27.

FTIR analysis of γ-Fe2O3 nanoparticles, virgin HPMC polymer & HPMC-γ-Fe2O3 nanocomposites

FTIR absorption spectra of biosynthesized vacuum-dried γ-Fe2O3 nanoparticles were shown in the Fig. 5. The spectra showed the presence of bonds due to O−H stretching (around ~3,430 cm-1) and asymmetric aldehydic stretching vibration of C−H in pyranoid ring (around ~2,920 cm-1)34. These peaks indicated the presence of proteins and other organic residues, which might have produced extracellularly by A. alternata. FTIR spectrum of γ-Fe2O3 nanoparticles showed absorption band at 1,628 and 1,585 cm-1 corresponding to the amide I and amide II of polypeptides34 respectively. The FTIR peak in between 1,240−1,260 cm-1 present in both the cases signified amide III band of the random coil of protein35. Peaks at 1,405.1 and 815.45 cm-1 might be assigned to the symmetric stretching of the carboxyl side groups in the amino acid residues of the protein molecules36. Band at around 1,058.21 cm-1 indicated C−O−C stretching37. The bands visible in between 500 and 749 cm-1 signified the presence of R−CH group38. Thus, it could be concluded that the γ-Fe2O3 nanoparticles were stabilized by surface bound protein molecules that also prevented aggregation39. In addition to that microscopic fungi could generate different extracellular nanoparticles by a process involving the enzyme NADH-reductase18.

The virgin HPMC polymer and HPMC- γ-Fe2O3 nanocomposites were further examined by FTIR analysis, as shown in Fig. 6. All the samples had almost similar peaks and exhibited the characteristics peaks of HPMC. For the HPMC molecule the band at 3,439 cm−1 could be attributing to the stretching vibration of hydroxyl group whereas the band at 2,920 cm−1 was assigned to the asymmetric stretching vibration of C−H in pyranoid ring. 1641 cm−1 corresponded to the C−H bending mode; the absorption band at 1050 cm−1 was ascribed to C−O−C stretching mode from glucosidic units40. The peaks at round 1,310 cm−1, 1,376 cm−1 and 1,453 cm−1 were signified the corresponding C−C stretching, symmetric –COO stretching and C−O stretching respectively41. The peak at 940 cm−1 was related to −OCH3 group. In the HPMC-γ-Fe2O3 nanocomposite, the absorption bands related to O−H stretching was at 3,429 cm−1, –COO stretching at 1,373 cm−1, C−H and C−O stretching at 2,920 cm−1 and 1,457 cm−1 respectively. The peaks at 1,047 cm−1 and 942 cm−1 were due to C−O−C stretching mode from the glucosidic unit and the −OCH3 group of HPMC respectively. For all the HPMC-γ-Fe2O3 films had been desiccated in vacuum to remove water before carrying out the FTIR measurement. A similar phenomenon in cellulose-based nanocomposites was reported for cellulose–Fe2O3 nanocomposites42. The band, found in the 1,739 cm−1 in the HPMC molecule, signified C−O stretching37, did not seen in the HPMC-γ-Fe2O3 films. All absorption bands in nanocomposite films were little more shifted and the band at 3,429 cm−1 in nanocomposite films became broader, further indicating a strong interaction between γ-Fe2O3 nanoparticles and HPMC matrix.

TEM image of γ-Fe2O3 nanoparticles & HPMC-γ-Fe2O3 nanocomposites

The morphology and size of the synthesized γ-Fe2O3 nanoparticles and the agglomeration of the γ-Fe2O3 nanoparticles during their incorporation into the polymer matrix were determined by the transmission electron microscopic images. TEM image shown in the Fig. 7(a) recorded different sizes of γ-Fe2O3 nanoparticles which arose from the bio-reduction of iron (III) chloride by FCF at room temperature (37°C) for 24 h. The particles, formed in the reaction solution, were irregular in shape presenting an overall quasi-spherical morphology11. The diameters of these γ-Fe2O3 nanoparticles were measured and the size was in the range of 35−150 nm. The average diameter of these γ-Fe2O3 nanoparticles was of 95 ± 5 nm. A small difference was observed in the diameter value obtained from TEM and DLS measurement mainly due to the process involved in the sample preparation. The particle size determined by TEM represented the actual diameter of the nanoparticles as it was measured at the dry state of the sample, whereas the size measured by the laser light scattering method (DLS) was a hydrodynamic diameter (hydrated state); therefore, the nanoparticles would have a larger hydrodynamic volume due to solvent effects in the hydrated state43.

On the other hand, a typical TEM image of the HPMC-γ-Fe2O3 nanocomposite film was shown in Fig. 7(b). This observation indicated that agglomeration of the γ-Fe2O3 nanoparticles (0.03 wt% γ-Fe2O3 nanoparticles at HPMC) took place during their incorporation into the polymer matrix. Despite the observed agglomeration our films maintained optical clarity23.

Mechanical properties of the HPMC-γ-Fe2O3 nanocomposites

The mechanical properties such as elongation at break (%), tensile strength and Young’s modulus of the HPMC-γ-Fe2O3 nanocomposite films at different concentration of γ-Fe2O3 nanoparticles were measured and compared with the virgin HPMC film. Fig. 8(a) showed the elongation at break (%) of pure HPMC and HPMC-γ-Fe2O3 nanocomposite films. Fig. 8(a) showed a clear indication that the elongation at break (%) of pure HPMC was increased from 25.18 to 31.6. Fig. 8(b) showed tensile properties of pure HPMC and its nanocomposite films. Tensile tests were performed on the sample in a relative humidity of 50%. It was clear that the tensile strength of γ-Fe2O3 nanocomposite films increased with increasing concentration of γ-Fe2O3 nanoparticles (Fig. 8(b)). The tensile strength of pure HPMC film was 43.57 Mpa and increased to 73.15 Mpa in case of HPMC-γ-Fe2O3 nanocomposites. So, it could be concluded that the tensile strength of γ-Fe2O3 nanocomposite films prepared increases by 67.89% compared to pure HPMC film. This indicated the strong interactions between HPMC molecules and γ-Fe2O3 nanoparticles. The Young’s modulus of the γ-Fe2O3 film increased linearly with increasing concentration of the Fe2O3 solution. It was also observed that the tensile modulus at 1% strain was found to increase from 2,760.07 MPa for pure HPMC to 3,440.73 MPa for HPMC-γ-Fe2O3 nanocomposite films (Fig. 8(c)). Therefore the tensile modulus was increased by 24.66% compared to that of pure HPMC.

Thus, from the above observation at increasing concentration range of γ-Fe2O3 solution, it could be concluded that the introduction of γ-Fe2O3 would lead to both strengthening and toughening of the HPMC matrix. The nanoparticles affected the structural rearrangements during the post-elastic deformation to induce semi crystalline like mechanical behaviour. This type of feature sometimes observed in case of polymer–metal nanocomposites23.

Conclusions

A simple, eco-friendly, energy-conserving nature of fungus-based biological processes for magnetic iron oxide synthesis was developed to synthesize γ-Fe2O3 nanoparticles directed by particle size using the culture filtrate of A. alternata. The advantage of biosynthesis of nanoparticles using this protocol over other methods currently in use was that the nanoparticles were quite stable in solution. TEM analysis confirmed the presence of γ-Fe2O3 nanoparticles of average size 265 ± 5 nm and these nanoparticles were agglomerated in the HPMC-γ-Fe2O3 nanocomposites films. The incorporation of these γ-Fe2O3 nanoparticles into the HPMC matrix induced significant changes in the mechanical properties of the HPMC, even when the content of the inorganic phase was extremely low (<1 wt %). Nano-sized γ-Fe2O3 modified HPMC which showed enhanced mechanical properties like tensile strength, young modulus etc. and the introduction of γ-Fe2O3 nanoparticles led to both strengthening and toughening of HPMC matrix. FTIR characterization also indicated a strong interaction between γ-Fe2O3 nanoparticles and HPMC matrix. Finally, it was concluded that the biosynthesized γ-Fe2O3 nanoparticles showed excellent capability to enhance the mechanical properties of the HPMC polymer.


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