BaFeO3-δ perovskite nanomaterials have been synthesized by two different methods: co-precipitation (Cop) and sol-gel (Sol) methods. Rietveld analysis of the X-Ray diffraction (XRD) shows that the samples are crystallized in rhombohedral perovskite structure with space group R3c. Scanning electron microscope (SEM) of these samples showed the agglomerations of various particles. Dynamic light scattering (DLS) showed that the average particle size of BaFeO3-Cop sample is larger than that of BaFeO3-Sol sample. The amount of oxygen deficient (δ) and the valence states of Fe ions in these samples were determined from Mössbauer spectroscopy. X-ray photoelectron spectroscopy (XPS) shows the elemental compositions and surface electronic states of these samples. The thermal, optical and magnetic properties of these samples depend on the amount of oxygen deficient (δ) and the valence states of Fe ions. Furthermore, the antibacterial and antibiofilm activity of these samples was systematically investigated. The present results suggest that BaFeO3-δ superparamagnetic perovskite can be used as antibacterial and antibiofilm agent.
Research Article
Effect of Synthesis Method on Antibacterial and Antibiofilm Activity Properties of BaFeO3-δ Perovskite Nanomaterials
https://doi.org/10.21203/rs.3.rs-427524/v1
This work is licensed under a CC BY 4.0 License
Version 1
posted 21 Apr, 2021
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BaFeO3-δ perovskite nanomaterials containing oxygen deficient (δ) have attracted attention because of their interesting applications which result from the presence of structural polymorphs and mixed valence states of Fe ions [1-3]. The crystallographic structures of BaFeO3-δ perovskite as a function of the anion deficient (δ) are cubic, triclinic, orthorhombic, rhombohedral, tetragonal and hexagonal [2-5]. Interestingly, BaFeO3-δ perovskite have unique electric, optical and magnetic properties which useful in elecronic applications such as oxygen transport membrane, electrocatalysts, photocatalytic activity, fuel cells cathodes, oxygen sensors, etc. [3, 6]. It is known that BaFeO3-δ perovskite nanomaterials exhibit charge-disproportionation (CD) phenomenon (2Fe4+→Fe3+ + Fe5+ or 2Fe3.5+→Fe3+ + Fe4+) accompanied by lattice distortion which affects the Fe-3d and O-2p electronic states hybridization [7, 8]. S. Morimoto et al. investigated the charge-disproportionation of BaFeO3-y by Mössbauer spectroscopy and they gave the evidence for the existence of charge-disproportionation and magnetic ordering in BaFeO3-y perovskite [5]. There are numerous methods for the determination of anion deficient (δ) in perovskite nanomaterials such as Mössbauer spectroscopy and titration method as well as there is well agreement between them [9, 10].
In spite of the extensive studies on antibacterial nanomaterials, new and less expensive nanomaterials are needed because of the increasing number of bacterial strains [11]. The perovskite nanomaterials as antibacterial and antibiofilm agent is a new and growing area in the field of medicine because of the size of bacterial cell and its membranes are micrometers and nanometers sizes, respectively [12]. Less expensive perovskite nanomaterials were used as antibacterial agent such as SrFeO3-δ, SrTi1-xFexO3 (x=0, 0.2, 0.4, 0.6, 0.8 and 1) and Ba(ZrxTi1-x)O3 (x=0.05) [10, 13, 14]. Abdel-Khalek et al. [10] investigated the microorganisms test, antimicrobial activity as well as minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of the SrFeO3-δ nanoparticles sample. They found that the antibacterial mechanism on Escherichia coli of SrFeO3-δ nanoparticles sample depends on nano-size of it, alkaline environment and Sr2+ dissociation. In spite of good results of antibiotics to kill the bacteria, there are some drawbacks such as it is impossible for the antibiotics to cross biofilm which making by some bacteria to protect themselves [12].
To overcome these problems in the biological field, perovskite nanomaterials were used as the inhibitions of biofilm. The synthesis methods for perovskite nanomaterials and the sintering temperature play an important role in the structural changes, particle size and the amount of oxygen deficient (δ) [10]. In view of the aforementioned aspects, BaFeO3-δ perovskite nanomaterials have been synthesized by co-precipitation (Cop) and sol-gel (Sol) methods. The aim of the present work is to investigate the effect of synthesis methods on structural, thermal, optical, and magnetic of BaFeO3-δ perovskite nanomaterials. In addition to, we investigated BaFeO3-δ perovskite nanomaterials as a less expensive antibacterial and antibiofilm agent for the first time. The antibacterial activity of investigated samples consists of two gram-positive (Bacillus subtilis and Staphylococcus aureus) and two gram-negative (Escherichia coli and Pseudomonas aeruginosa) as well as Enterococcus faecalis, Salmonella typhi, Candida albicans, and Aspergillus brasiliensis. Furthermore, MIC and MBC were determined for all bacterials as well as antibiofilm activity of BaFeO3-Cop and BaFeO3-Sol samples against Pseudomonas aeruginosa (ATCC 9027).
The synthesis of BaFeO3-δ perovskite samples were carried out by co-precipitation and sol-gel methods which were marked as BaFeO3-Cop and BaFeO3-Sol respectively.
2.1. Synthesis of the BaFeO3-δ sample by co-precipitation method
The synthesis process of BaFeO3-δ perovskite sample was begun by dissolving separately the stoichiometric amount of the metal salts (Ba(NO3)2) and (Fe(NO2)3·9H2O) in a distilled water. The salt solutions were mixed successively and then stirred. After that, the cations coprecipitated by slow addition of precipitating agent, a saturated ammonium carbonate (NH4)2CO3 solution, which was added drop by drop under stirring. The mixture salt solutions were stirred by magnetic stirrer until the ammonium carbonate was completely dissolve and heated at 423 K using hot plate until the precipitate is attained. The resulted precipitate was recovered by filtered and dried in an oven at 383 K for 1h in air for the removal of water from the precipitate. Finally, the obtained powders were calcined at a temperature of 1273 K for 12 h in air.
2.2. Synthesis of the BaFeO3-δ sample by sol-gel method
The synthesis process of BaFeO3-δ perovskite sample was begun by dissolving separately the stoichiometric amount of the metal salts (Ba(NO3)2), (Fe(NO2)3·9H2O) and citric acid (C6H8O7) (in the molar ratio 1 : 1 : 4 ratio respectively) in a distilled water. The salt solutions were mixed successively and then stirred. After that, the resultant mixture mixed with citric acid as a complexing agent and to remove the solvent, the gel is heated at 523 K using hot plate with magnetic stirrer. Moreover, the ethylene glycol (C2H6O2) was added to the mixture drop by drop under stirring. The resulting solution was continuously heated on hot plate with magnetic stirrer to evaporate H2O and polymerization organic compounds and until the gel is formed. The obtained powders were calcined at a temperature of 1273 K for 12 h in air to produce the required BaFeO3-δ perovskite.
2.3. Characterization techniques
Powder XRD patterns of the BaFeO3-Cop and BaFeO3-Sol samples at room temperature were obtained from SIEMENS D5000 diffractometer using Cu Kα radiation between 2θ ranges from 20º to 80º. The XRD patterns of the BaFeO3-Cop and BaFeO3-Sol samples were analyzed by employing Rietveld refinement using the FULLPROF WinPLOTR software suite. The morphology of the BaFeO3-Cop and BaFeO3-Sol samples was recorded using scanning electron microscope (SEM) model JEOL (JSM-7800F). The average particle size and polydispersity index (PdI) of BaFeO3-Cop and BaFeO3-Sol samples were determined by dynamic light scattering (DLS) using a Zetasizer Nano ZS (Malvern Instruments Ltd).
The Mössbauer spectra of BaFeO3-Cop and BaFeO3-Sol samples at room temperature were obtained using a conventional 57Fe constant acceleration spectrometer with 57Co (embedded in rhodium matrix) radioactive source. The elemental compositions and surface electronic states of the BaFeO3-Cop and BaFeO3-Sol samples were analyzed by X-ray photoelectron spectroscopy (XPS: a Thermo ScientificTM K-AlphaTM USA) using a monochromatic aluminum source (AL-Kα). Differential scanning calorimetric (DSC) of the BaFeO3-Cop and BaFeO3-Sol samples was carried out using Perkin-Elmer-DSC thermal analyser with a heating rate of 50 K/min in the 300–870 K temperature range. The band gaps for BaFeO3-Cop and BaFeO3-Sol samples have been obtained by diffuse reflectance spectroscopy in the wavelength range 250-800 nm using a double-beam Jasco-V- 570 UV–VIS-NIR spectrophotometer. The magnetic hysteresis M (H) loops measurement for BaFeO3-Cop and BaFeO3-Sol samples were recorded at room temperature by using vibrating sample magnetometer (VSM), Lakeshore 7410 with magnetic fields from -20 kG to 20 kG.
2.4. Test microorganisms
The test microorganisms, two gram-positive (Bacillus subtilis ATCC 6633 and Staphylococcus aureus ATCC 29213), two gram-negative (Escherichia coli ATCC 25922 and Pseudomonas aeruginosa ATCC27853), Enterococcus faecalis ATCC 29212, Salmonella typhi ATCC 6539 also, Candida albicans (ATCC 10231), and Aspergillus brasiliensis (ATCC16404), were used to define the antifungal activity tested samples.
2.4.1. Antibacterial activity
The antimicrobial potential of BaFeO3-Cop and BaFeO3-Sol perovskite nanomaterial samples were investigated towards the test microorganisms by the agar plate diffusion method and expressed as the inhibition zones diameter [15]. In brief, 100 μL from the tested bacteria/fungi were cultured in 10 μL of fresh media until they reached 108 and 105 cells/mL for bacteria and fungi, respectively. One mL from BaFeO3-Cop sample and one mL from BaFeO3-Sol sample were added to each well with 10 mm diameter holes cut in the agar gel. After incubation of the plates for 24 h at 37 °C for both bacteria and yeast and for 72 h at 27 °C for filamentous fungi, the growth of microorganism's was observed. The standard antibacterial drugs are Amoxicillin/Clavulanic acid while the standard antifungal drug is nystatin. Solvent controls (DMSO) were used to dissolve BaFeO3-Cop and BaFeO3-Sol perovskite nanomaterial samples and included in the experiment as negative controls which mean it has no effect on the growth of the tested microorganisms.
2.4.2. MIC and MBC determination
MIC of the active BaFeO3-Cop and BaFeO3-Sol samples was investigated by using paper disk diffusion method throughout employing filter paper with diameter 12.7 mm. In the present work, the bacteria were grown in a nutrient agar media while yeasts and fungi were grown in Sabouraud´s agar media. BaFeO3-Cop and BaFeO3-Sol perovskite nanomaterial samples were dissolved and put with different concentrations on the paper disks. The drying disks inoculated with the test microorganisms were loaded over the agar plates then the inhibition of growth was tested when the bacterial strains incubated for a day at 37 °C and fungi and yeasts incubated for three day at 27 °C. Every experiment was repeated three times for reproducibility [10, 16].
MBC test was carried out by using the broth microdilution test which explained above. To determine MBC, 10 μL of culture volume from MIC test was plated onto TSB agar plate and the formed colony was examined after 24 h at 37 °C. MBC is the least concentration of BaFeO3-δ perovskite nanomaterial that reduces the number of colony forming units [10, 17].
2.5. Antibiofilm activity (Microtitre plate assay)
The activity of synthesized BaFeO3-Cop and BaFeO3-Sol samples to inhibit the biofilm formation of Pseudomonas aeruginosa (ATCC 9027) was carried out by using a measure of efficacy which is referred as percentage reduction (PR). According to Pitts et al. [18], the PR values of the samples were estimated from the following equation
where B is the mean absorbance value for blank wells (no biofilm and no treatment), C is the mean absorbance value for control wells (biofilm and no treatment), and T is the mean absorbance value for treated wells (biofilm and treatment).
3.1. XRD, SEM and DLS studies
Fig. 1 shows the observed data and Rietveld refined of XRD patterns for BaFeO3-Cop and BaFeO3-Sol samples. From observed data, it can be seen that the intense diffraction peak is splitting into two peaks which indicating the rhombohedral perovskite structure [19]. The Rietveld analysis for BaFeO3-Cop sample showed that it crystallizes in rhombohedral perovskite structure (space group R3c) phase (Fig. 1a) with the presence of small amount of impurity (Barium oxide BaO) which marked by star in XRD pattern [2, 5]. This impurity is due to oxygen deficient (δ) which observed for BaFeO3-Cop sample [20]. The Rietveld analysis for the BaFeO3-Sol sample showed that it crystallizes in a single phase (Fig. 1b) with rhombohedral perovskite structure (space group R3c) [21]. These results are in agreement with the previous report [21]. From Fig. 1, it is clear that the crystallinity of BaFeO3-Cop sample is higher than that of BaFeO3-Sol sample which is due to the addition of precipitating agent in co-precipitation method [22]. The refined structural parameters for BaFeO3-Cop and BaFeO3-Sol samples including the lattice parameters, atoms, Wyckoff position, lattice coordinate, bond distances, bond angles and reliability factors are listed in Table 1. From this table, it can be seen that the lattice parameters, bond lengths, and bond angles of BaFeO3-Cop sample is higher than that of BaFeO3-Sol sample which is due to the presence of high oxygen deficient (oxygen vacancies) and the difference in size of both Fe3+ and Fe4+ ions [1]. The decrease of oxygen vacancies in BaFeO3-Sol leads to the space reduction around the cations. This observation is consistent with Mössbauer results, as mentioned below. The Fe—O bond length and Fe—O— Fe bond angle in BaFeO3-Cop sample is slightly large than that of BaFeO3-Sol sample which can be attributed to the increasing oxidation state of Fe ion. Moreover, the slightly decrease of Fe—O bond length in BaFeO3-Sol sample indicated the increase in the covalency [23].
The average crystallite size (D) from XRD data of BaFeO3-Cop and BaFeO3-Sol samples has been calculated by using Scherrer equation
where k =0.9 is the Scherrer constant (depends on the shape of crystallite size), λ=0.15406 nm is the X-ray wavelength of the Cu Kα radiation used for the diffraction, θ is the Bragg’s diffraction angle measured and β is the full width at half maximum intensity of the diffraction peaks which has been obtained from Rietveld refinement of XRD patterns [15]. The values of D are 28 and 29 nm for BaFeO3-Cop and BaFeO3-Sol samples, respectively. These results revealed that average crystallite size of BaFeO3-Cop and BaFeO3-Sol samples in nanometer size. The average crystallite size (D) of BaFeO3-Sol sample is slightly large than that of BaFeO3-Cop sample which can be attributed to chemical nature of the solvant [24].
Fig.2 shows SEM micrographs of BaFeO3-Cop and BaFeO3-Sol samples. From this figure, it can be seen that there is different in the surface morphology of BaFeO3-Cop sample than that BaFeO3-Sol sample which is due to the chemical nature of the used solvant in the precipitate [24]. The SEM micrograph of BaFeO3-Cop sample exhibits inhomogeneous dispersion and porous nature as well as there are some agglomerations of various particles with micrometric size [25]. Moreover, it is clear from Fig.2 that some particles covered with a porousity secondary phase which is observed in XRD for BFO-Cop sample. This result is similar to that reported by Penwell et al. of the BaFeO3-δ [26]. The SEM micrograph of BaFeO3-Sol samples exhibits agglomerations of various particles with micrometric size as well as it exhibits porous nature [10]. The agglomerations of various particles and particle size of BaFeO3-Sol sample is higher than that of BaFeO3-Cop sample because of the used solvant in the precipitate [24]. This result indicates that the method of synthesis has large effect on the morphology of BaFeO3 nanomaterials.
The average particle size of BaFeO3-Cop and BaFeO3-Sol samples was determined by dynamic light scattering (DLS) because of the formation of large aggregates in SEM micrographs. The DLS analysis of BaFeO3-Cop and BaFeO3-Sol samples are shown in Fig.3. From this figure, it can be seen that the particle size of BaFeO3-Sol sample (350 nm) is larger than that of BaFeO3-Cop (117. 4 nm) which may be due to the decrease in the lattice parameters and oxygen deficient in BaFeO3-Sol sample. It is known that, the value of particle size from DLS measurement is larger than the average crystallite size values obtained from XRD data because of the particle hydrodynamic radius that bounded by water molecules. The DLS analysis of BaFeO3-Cop sample showed the existence of secondary phase with sizes around 825 nm in the range from 0.3 % to 2.2% of the entire volume which is observed in XRD for BaFeO3-Cop sample [11]. The polydispersity index (PdI), indicates the size distrbution, of BaFeO3-Sol sample (0.775) is larger than that of BaFeO3-Cop (0.639). The PdI values of these samples are larger than 0.4 which means that there is no monodisperse nature in the samples and there is broad particle size distribution in BaFeO3-Sol sample [27].
3.2. Mössbauer and XPS studies
Fig.4. shows the Mössbauer spectra of BaFeO3-Cop and BaFeO3-Sol samples at room temperature. It is clear from this figure that the spectra are composed of two magnetic hyperfine sextets, one single peak, and one quadrupole doublet. The coexistence of magnetic order (two sextets) and paramagnetic (one single peak, and one quadrupole doublet) indicated the presence of superparamagnetic behavior in BaFeO3-Cop and BaFeO3-Sol samples [28, 29]. Moreover, the existence of magnetic sextets in Mössbauer spectra indicated that the Néel temperature (TN) of BaFeO3-Cop and BaFeO3-Sol samples is above the room temperature [29, 30]. This result is agreement with previous reports for barium ferrate and Ba1-xCaxFeO3 (where x=0.05) as well as it is consistent with the following magnetic measurement of the BaFeO3-Sol and BaFeO3-Cop samples [31, 32]. The single peak and quadrupole doublet in the Mössbauer spectra of BaFeO3-Cop and BaFeO3-Sol samples (Fig.4) are due to the presence of Fe ions in two different valence states in distorted octahedral sites with oxygen deficient [33, 34]. The Mössbauer hyperfine parameters of the BaFeO3-Cop and BaFeO3-Sol samples are summarized in Table 2. The magnetic sextets in the BaFeO3-Cop and BaFeO3-Sol samples with hyperfine magnetic field in the range of 47.1 ̶ 47.4 T and 33.1 ̶ 33.9 T are assigned to a disordered Fe3+ and magnetic relaxation Fe4+ ions, respectively [35, 36]. The single peak with an isomer shift (IS) in the range of 0.10-0.16 mms−1 and quadrupole splitting (QS) in the range of 0.35–0.46 mms-1 is assigned to Fe4+ ions in octahedral site [30, 37]. The doublet with IS (0.24 ̶ 0.25 mms−1) and QS (0.83–0.90 mms-1) values are intermediate between Fe3+and Fe4+ ions which are assigned to Fe3.5+ ions in oxygen deficient octahedral site [10, 37]. The half integer valence state Fe3.5+ results from a rapid transfer of an electron between Fe3+ and Fe4+ ions in the lattice [37, 38]. From above results, it can be concluded that there are different oxidation states of Fe ions corresponding to different positions inside one structure of BaFeO3-δ. The proportion of Fe valence states and the degrees of distortion around Fe ions are changed with oxygen content. This result is similar to that reported by Sedykh et al. of the SrFeO3-δ [33]. For electroneutrality in the BaFeO3-Cop and BaFeO3-Sol samples, oxygen deficient δ (oxygen vacancies) are created because of the conversion of unstable Fe4+ into Fe3.5+ and Fe3+ ions [39]. The oxygen deficient (δ) in the BaFeO3-Cop and BaFeO3-Sol samples can be estimated from Mössbauer data by using the following electroneutrality equation, starting from usual valance states of barium (Ba2+) and oxygen (O2-) [40],:
where and represent the fractions of Fe4+ and Fe3.5+ ions in the samples, respectively. The oxygen deficient (δ) values in the BaFeO3-Cop and BaFeO3-Sol samples are summarized in Table 2. From this table, it can be seen that the area of magnetic components of BaFeO3-Cop sample is higher than that of BaFeO3-Sol sample because of high oxygen deficient (δ) content [33, 32]. The IS and QS values of paramagnetic components of BaFeO3-Cop sample is lower than that of BaFeO3-Sol sample because of the increase of oxygen deficient (δ) content [32]. Based on the Mössbauer results, we concluded the presence of the new magnetic species and charge-disproportionation phenomenon (2Fe3.5+→Fe3+ + Fe4+) at room temperature in BaFeO3-Cop and BaFeO3-Sol samples [7, 8, 32].
Fig. 5 (a, b, and c) shows X-Ray photoelectron spectroscopy (XPS) spectra of Fe 2p, Ba 3d, and O 1s in BaFeO3-Cop and BaFeO3-Sol samples, respectively. It is clear that the XPS spectra are composed of the expected species Fe, Ba, and O witout any contamination in the experimental spectra. Fig. 5(a) shows the XPS spectra of the Fe 2p in BaFeO3-Cop and BaFeO3-Sol samples. It can be seen that the peaks of Fe 2p are very broad which may arise from spin orbit coupling between unpaired 3d electron and 2p core of Fe ions [10, 30]. The spin-orbit of Fe 2p peaks were deconvoluted into six peaks of 2p3/2 and 2p1/2. The asymmetric of 2p3/2 and 2p1/2 peaks means the presence of Fe ions in different valance states [3, 41]. This result is the same as that reported by Ahmed et al. [3]. The presence of Fe ions in different valance states are responsible for the oxygen deficient (δ) in BaFeO3-δ perovskite nanomaterials which are in agreement with Mössbauer results, as mentioned above [42]. The peak of 2p3/2 at higher binding energy 711.18 eV may be attributed to the existence of Fe3.5+ in the surface of BaFeO3-Cop sample [10]. This assignment is in agreement with Mössbauer results that reported above. Moreover, this peak may be attributed to the existence of Fe4+ [43, 44]. The satellite peak of Fe 2p3/2 at 719.58 eV may be atrributed to the presence of Fe3+ ions with oxygen vacancies in the surface of BaFeO3-Cop sample [3, 45]. The peak of 2p1/2 at 724.08 eV may be attributed to the existence of Fe3+ with oxygen vacancies in the surface of BaFeO3-Cop sample [45]. The peaks of 2p3/2 and 2p1/2 at higher binding energy at 715.28, and 727.68 eV can be attributed to the existence of Fe4+ ions in the surface of BaFeO3-Cop sample [10, 30]. The peak of Fe 2p1/2 at higher binding energy at 732.78 eV is characteristic of satellite structure which in agreement with the previous results [46]. The peaks of Fe 2p in XPS spectrum of BaFeO3-Cop sample is same as that in BaFeO3-Sol sample with small shift to higher binding energies. The higher binding energy in BaFeO3-Sol sample is due the increase of the coulombic interaction between the Fe ion core and the electron [41]. This shift leads to the increase in the relative amount of Fe4+ ions which is in agreement with Mössbauer results reported above [43].
Fig. 5(b) shows Gaussian fitting of the XPS spectra of Ba 3d peaks in BaFeO3-Cop and BaFeO3-Sol samples. It can be seen that the XPS spectra of Ba 3d for BaFeO3-Cop and BaFeO3-Sol samples are composed of two peaks of 3d5/2 and 3d3/2. The peak of Ba 3d5/2 at 779.68 eV can be atrributed to the Ba2+ ions in BaFeO3-Cop perovskite lattice [44, 46]. The peak at 795.08 eV can be atrributed to one electron state of Ba 3d3/2 in surface region of BaFeO3-Cop [47]. The peaks of Ba 3d in XPS spectrum of BaFeO3-Cop sample is same as that in BaFeO3-Sol sample with small shift to higher binding energies
Fig. 5(c) shows the deconvolution analysis of the oxygen O 1s peak in BaFeO3-Cop and BaFeO3-Sol samples. It can be seen that the XPS spectra of O 1s peak for BaFeO3-Cop sample were deconvoluted into four individual peaks located at 528.68, 530.88, 531.48 and 532.68 eV. The peak at 528.68 eV can be atrrbuted to the oxygen lattice of BaFeO3, and the intermediate peak of O 1s at 530.88 is due to the lattice O2- ions in perovskite related structure [3, 30]. The peak of O 1s at 531.48 eV is due to chemisorbed oxygen Oy- (where 0 ˂ y ˂ 2) which arise from oxygen vacancies (oxygen deficient (δ)) in the crystal structure [3, 10] The area of the peak at 531.48 eV for BaFeO3-Cop sample is higher than the area of the peak at 531.74 eV for BaFeO3-Sol sample which means that BaFeO3-Cop sample has large amount of oxygen deficient (δ) as compared to BaFeO3-Sol sample [48]. This observation is in agreement with Mössbauer results which mentioned above. The peak at 532.68 eV is due to the oxygen in chemically absorbed water [49]. Based on the results of XPS, we conclueded the existence of oxygen vacancies and mixed oxidation states of Fe ions on the surface of BaFeO3-Cop and BaFeO3-Sol samples.
3.3. Thermal, optical and magnetic studies
Fig.6. shows DSC curves of BaFeO3-Cop and BaFeO3-Sol samples in the temperature range of 300-875 K. It is evident that the BaFeO3-Cop sample exhibits two exothermic peaks (Tc1 and Tc2) and two endothermic peaks (Tm1 and Tm2) while the BaFeO3-Sol sample exhibits two exothermic peaks (Tc1 and Tc2) and one endothermic peak (Tm1). The two exothermic peaks are representing the two phase transitions [10, 30]. In order to obtain the first phase transition temperature of broad exothermic peak (Tc1), we make the first derivative of DSC curves of BFO-Cop and BFO-Sol samples as shown in the inset of Fig.6. The first exothermic peaks for BaFeO3-Cop and BaFeO3-Sol samples appeared at 379 and 377 K, respectively. These peaks are due to the oxygen loss and may be due to the antiferromagnetic Néel temperature (TN) temperature which is consistent with the following Mössbauer results [30, 50]. The second exothermic peaks for BaFeO3-Cop and BaFeO3-Sol samples appeared at 578 and 597 K, respectively. These peaks indicat the structure transition which may be correspond to ferroelectric-paraelectric transition [10]. Furthermore, the first endothermic peaks for BaFeO3-Cop and BaFeO3-Sol samples at 613 and 624 K, respectively. These peaks may be due to the formation of BaFeO3-δ perovskite while the second small endothermic peak at 650 K for BaFeO3-Cop sample may be due to impurity (BaO) formation which is observed in XRD and DLS results for BFO-Cop sample [14, 22]. The BaFeO3-δ perovskite phase begins to appear in BaFeO3-Cop sample at lower temperature than the BaFeO3-Sol sample because of the chemical nature of the used solvant in the precipitate [25].
Fig. 7 shows the absorption spectra of BaFeO3-Cop and BaFeO3-Sol samples in ultraviolet-visible (UV-Vis) range in the wavelength range from 250 to 800 nm which are derived from the diffuse reflectance according to Kubelka-Munk (K-M) method. It is notice that the absorption for BaFeO3-Cop sample is lower than that of BaFeO3-Sol sample becouse it is less opaque [51]. The two absorption bands nearly at 300 and 340 nm in UV range are atrributed to the charge transfer transition (O 2p level to 3eg orbital). [51]. Moreover, the broad absorption bands in visible range are atrributed to electronic transition overlapping (O 2p –Fe 3d charge transfer) thus can be used as excellent photocatalysts [10]. The optical energy band gaps Eg for the BaFeO3-Cop and BaFeO3-Sol samples were determined from Tauc plots which obtained from diffuse reflectance data by the following equation [30].
where F(R) is Kubelka-Munk (K-M) function, α is the absorption coefficient, A is the proportional constant, hν represents the incident photon energy, and n represents the nature of the transition which equal 1/2 for direct allowed transition [10]. The K-M function as given by the following equation
where R is the diffuse reflectance. Fig. 8 shows the polt of (F(R)hν)2 versus photon energy (hν) of BaFeO3-Sol and BaFeO3-Cop samples. By extrapolating (F(R)hν)2 to zero, the direct optical energy band gaps Eg values of the BaFeO3-Cop and BaFeO3-Sol samples have 0.9 and 1eV, respectively. The small values of Eg are atrributed to the presence of oxygen deficient (δ) in BaFeO3-Cop and BaFeO3-Sol samples where the oxygen deficient is observed from Mössbauer and XPS results as mentioned above [10, 30]. Hence, BaFeO3-Cop and BaFeO3-Sol samples could be used as potential applications promising in photocatalysts. The energy band gap Eg value of BaFeO3-Cop sample is lower than that of BaFeO3-Sol sample which may be due to the smaller particle size and the increase in the orbital overlapping between O 2p –Fe 3d levels [20]. In addition to, the increase in oxygen deficient in BaFeO3-Cop sample leads to the reduction in energy band gap because of the lower energy levels associated with oxygen deficient (oxygen vacancies) than adjacent Fe 3d levels [52]
Fig. 9 shows the magnetic hysteresis M (H) loops for BaFeO3-Cop and BaFeO3-Sol samples at room temperature up to an applied field 20 kOe. It can be seen that the shapes of M (H) loops for both samples are hysteretic and the saturation of magnetization not observed. These results indicate the presence of weak ferromagnetic coexisting with antiferromagnetic ordering of the spins [1, 53]. The presence of hysteresis loops for both samples suggest that the Néel temperature (TN) of them is above the room temperature [43]. This is consistent with Mössbauer results, as mentioned above. The presence of weak ferromagnetic in both samples at room temperature may be due to spin canting, the contribution of superexchange interaction Fe4+̶ O ̶ Fe4+ and the formation of oxygen vacancies which create magnetic moments at the surface [34, 54, 55]. On the other hand, the presence of antiferromagnetic in both samples at room temperature may be due to the sperexchange interaction between Fe cations and oxygen anion [56]. The magnetization parameters of the samples including the maximum magnetization (Mmax), remanent magnetization (Mr), and coercive force (Hc) are summarized in Table 3. From this table, the small value of remanent magnetization (Mr) for BaFeO3-Cop and BaFeO3-Sol samples indicates the presence of antiferromagnetic with weak ferromagnetic which result from the spin canting of antiferromagnetic order [20]. The remanent magnetization (Mr) of BaFeO3-Cop sample is lower than that of BaFeO3-Sol sample because of the inhancement of ferromagnetic [20]. The coercive force Hc of BaFeO3-Cop sample is higher than that of BaFeO3-Sol sample because of high magnetic anisotropy and oxygen deficient δ [34]. The increase in oxygen deficient (δ) in BaFeO3-Cop sample leads to the resistance of motion in the domain wall during switching [57]. The maximum magnetization Mmax of BaFeO3-Cop sample is higher than that of BaFeO3-Sol sample which due to the increase in oxygen deficient (δ), amount of Fe4+ ions, and the Fe—O— Fe bond angle [57]. This result is consistent with Mössbauer and XRD analysis, as mentioned above. Based on the above results, the BaFeO3-Cop and BaFeO3-Sol samples containing oxygen deficient and have high TN values above the room temperature can be used as a developed magnetic material.
3.4. Antibacterial properties studies.
Fig. 10 shows the antimicrobial activity such as Bacillus subtilis, Escherichia coli, Salmonella typhi, and Enterococcus faecalis of BaFeO3-Cop and BaFeO3-Sol samples as representative. The results of the antimicrobial activity, the Bacillus subtilis, Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Salmonella typhi, Enterococcus faecalis, Candida albicans, and Aspergillus brasiliensis values are listed in Table 4. It can be seen that the inhibition zone of two gram-positive (Bacillus subtilis and Staphylococcus aureus) higher than that of two Gram-negative (Escherichia coli and Pseudomonas aeruginosa) in both samples. This indicates that the gram-positive more sensitive than gram-negative [10]. This difference may be atrributed to the difference in cell structure, physiology, cell walls, and degree of contact of organisms with particles [58]. It is known that gram-positive bacteria has thick cell wall and contains groups with negative charge like amide, hydroxyl and carboxyl groups [59]. Thus, the present result is similar to that reported by Sonohara et al [60]. They found that the small negative charge in Staphylococcus aureus membrane allow the penetration of negative charge free radical like peroxide ions (Mn3+ and Fe3+) and superoxide radical anions which leads to damage and cell death to Staphylococcus aureus [60]. The inhibition zone of two gram-positive and two gram-negative in BaFeO3-Cop sample higher than that BaFeO3-Sol sample which means that BaFeO3-Cop sample has high antibacterial agent [10]. The antibacterial effect in BaFeO3-Cop sample may be atrributed to the interaction between nagative charged cell wall and positive charged nanoparticle [58]. Therefore, the toxic effect in BaFeO3-Cop sample higher than that BaFeO3-Sol sample because of the BaFeO3-Cop sample has high surface area which is agreement with SEM, dynamic light scattering (DLS) results as mentioned above [10]. In addition to, the formation of inhibition zone in BaFeO3-Cop sample may be attributed to alkaline environment which leads to the cell death [10].
In order to known the antibacterial agents for fighting wastewater pathogenic Bacteria, the Enterococcus faecalis (ATCC 29212) and Salmonella typhi(ATCC6539) of BaFeO3-Cop and BaFeO3-Sol samples were measurements as shown in Fig. 10. The Salmonella typhi, Enterococcus faecalis, Candida albicans (ATCC 10231), and Aspergillus brasiliensis (ATCC 16404) values of BaFeO3-Cop and BaFeO3-Sol samples are listed in Tabl 4. It can be seen that the Salmonella typhi, Enterococcus faecalis, Candida albicans, and Aspergillus brasiliensis values of BaFeO3-Cop sample higher than that of BaFeO3-Sol sample. These results indicated that the antibacterial agents for fighting wastewater pathogenic bacteria in BaFeO3-Cop sample are better than BaFeO3-Sol sample because of the decrease in the particle size and the increase of oxygen vacancies in BaFeO3-Cop sample [10].
3.5. MIC and MBC determination
In order to investigate and confirmed that the BaFeO3-Cop and BaFeO3-Sol samples as promising nanomaterials for antibactericidal, the MIC and MBC values for Bacillus subtilis, Staphyloccus aureus, Escherichia coli, Pseudomonas aeruginosa, Salmonella typhi, Enterococcus faecalis, Candida albicans, and Aspergillus brasiliensis were determined and listed in Table 4. From Table 4, it is found that the ratio MBC/MIC is in the range from 1 to 2 in both present samples which means that the samples exhibit fungicidal and bacterical properties compared with standards that give the same effect against tested microbial strains [61]. In addition to, it noticed that the MIC and MBC values of BaFeO3-Cop sample are lower than that of BaFeO3-Sol sample which means that the BaFeO3-Cop sample has high antibacterial agents (bactericidal and fungicidal) [62]. This result may be explained by the diffusion of the bacteria into oxygen vacancies (voids) in the BaFeO3-Sol sample where it containes large amount of oxygen vacancies than BaFeO3-Sol sample as in Mössbauer and XPS results [10]. These results are similar to that reported by Abdel-Khalek et al. which studied the antibacterial mechanism of SrFeO3 sample. They showed that the oxygen vacancies have critical effect on the antimicrobial activity and the zone of inhibition depends on the amount of oxygen vacancies in surface of the sample [10]. The diffusion through the cell wall in the present samples may be depend on the binding of the metal ions (Ba2+, Fe3+, and Fe4+) with the microorganisms [59]. Therefore, we can also explain the difference in antimicrobial activity between the present samples based on the different oxidation states of Fe ions, oxygen species on the surface and particle size [10, 63]. Based on above results, we conclueded that the BaFeO3-Cop and BaFeO3-Sol samples are promising nanomaterials for wastewater treatment applications
3.6. Antibiofilm activity
BaFeO3-δ perovskite nanomaterials have been used to inhibit the activity of biofilms. In the current study, the dose-dependent ability of BaFeO3-δ perovskite nanomaterials to inhibit the activity of biofilm formed by the human pathogens P. aeruginosa ATCC 9027 was determined under in-vitro conditions. These results showed that the BaFeO3-Cop and BaFeO3-Sol samples inhibited the activity of biofilm as shown in Table 5. The inhibition of biofilm activity of the BaFeO3-Cop sample was observed reduction for biofilm activity at percentage higher than that of BaFeO3-Sol sample. The treatment of P. aeruginosa ATCC 9027 for 24h with 100, 50, 25 and 12.5μg/ml decreased biofilm activity at 97.75%, 88.5% , 80.44% and 76.37 %, respectively. Based on these results, we conclueded the BaFeO3-δ perovskite nanomaterials may be used to inhibit the activity of biofilms [12].
BaFeO3-δ perovskite nanomaterials have been successfully synthesized by co-precipitation and sol-gel methods. The formation of BaFeO3-δ phase with rhombohedral distorted perovskite structure (space group R3c) depends on the method of synthesis. Characterization techniques show that co-precipitation method can produce agglomerations of particles with small particle size which makes it an important method to enhance optical and magnetic properties in comparison to sol-gel method. The change in the amount of oxygen deficient (δ) in BaFeO3-δ samples leads to the change in the valence states of Fe ions thus it can be controlled the properties of the samples. According to Mössbauer results, BaFeO3-δ samples exhibit new magnetic species, oxygen vacancies and CD phenomenon at room temperature. XPS results showed the existence of oxygen vacancies and mixed valence states of Fe ions on the surface of the samples. The inexpensive BaFeO3-δ perovskite nanomaterials display antibacterial activity against various species of species of bacteria and good antibacterial agent in the wastewater treatment applications. The obtained outcomes for BaFeO3-δ superparamagnetic perovskite nanomaterials indicate its commercial importance as antibacterial and antibiofilm.
Acknowledgements
The authors thank the Academy of Scientific Research and Technology (ASRT), Egypt, for financing and supporting this project. (ASRT) is the 2nd affiliation of this resarch.
Compliance with Ethical Standards
Conflict of interest: The authors declare that they have no conflict of interest.
Funding Information: This project was supported financially by ASRT, Egypt, Grant No 6668.
Author contribution: All listed authors have participated in the research presented in this paper.
Ethical statement: This article does not contain any studies with human participants or animals performed by any of the authors.
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Table 1. Rietveld refinement parameters for BaFeO3-Cop and BaFeO3-Sol samples with space group R3c.
Table 2. The Mössbauer hyperfine parameters of the BaFeO3-Cop and BaFeO3-Sol samples. IS is isomer shift, LW is line-widths, QS/ε is quadrupole splitting/shift, Bhf is the magnetic hyperfine field of magnetic sextet and A is the relative area.
Table 3. The magnetization parameters of the BaFeO3-Cop and BaFeO3-Sol samples. Mmax is the maxmum magnetization, Mr is remanent magnetization, and Hc is coercive force.
Table.4. Antimicrobial activity assessments of the BaFeO3-Cop and BaFeO3-Sol samples.
|
Bacterial test strains |
Mean diameter of inhibition zone (mm)/ minimum inhibitory concentration (MIC) (µg/ml). |
||||||||
|
BaFeO3 Sol |
BaFeO3 Cop |
(Amoxicillin / Clavulanic acid) / Nystatin (standard) |
|||||||
|
IZ |
MIC |
MBC |
IZ |
MIC |
MBC |
IZ |
MIC |
MBC |
|
|
Bacillus subtilis (ATCC 6633)
|
24 ±0.12 |
8.33 ±0.05 |
15.62 ±0.16 |
27 ±0.12 |
3.9±0.5 |
8.19 ±0.16 |
22±0.42 |
7.81±0.36 |
15.62±0.19 |
|
Staphylococcus aureus (ATCC 29213) |
21 ±0.02 |
14.28± 0.44 |
27.13 ± 0.92 |
23 ±0.02 |
9.46±0.12 |
19.86 ± 0.92 |
24±0.61 |
3.12±0.21 |
6.24±0.44 |
|
Escherichia coli (ATCC 25922) |
13 ±0.36 |
62.5 ± 0.32 |
118.75 ± 0.76 |
17 ±0.36 |
31.25 ± 0.22 |
53.12 ± 0.76 |
23±0.33 |
5.68±0.54 |
12.5±0.12 |
|
Pseudomonas aeruginosa (ATCC27853)
|
19 ±0.55 |
15.62 ±0.61 |
26.55 ± 0.43 |
21 ±0.55 |
12.5 ±0.61 |
20.5 ± 0.43 |
20±0.55 |
9.46±0.15 |
20.83±0.53 |
|
Enterococcus faecalis ATCC 29212 |
18 ±0.32 |
31.25 ± 0.22 |
46.87 ± 0.14 |
20 ±0.32 |
15.62 ±0.2 |
29.67 ± 0.14 |
22±0.2 |
6.12±0.43 |
8.56±0.22 |
|
Salmonella typhi ATCC 6539 |
23 ±0.5 |
12.5 ±0.61 |
22.5 ± 0.33 |
25 ±0.5 |
6.12 ±0.05
|
11.62 ± 0.33 |
30 ±0.41 |
3.9±0.5 |
5.07± 0.36 |
|
Candida albicans ATCC 10231
|
21 ±0.46 |
15.62 ±0.26 |
28.11 ± 0.77 |
24 ±0.46 |
7.81±0.36 |
14.83 ± 0.77 |
15±0.21 |
7.81±0.91 |
15.62±0.94 |
|
Aspergillus brasiliensis (ATCC16404) |
12 ±0.19 |
62.5 ± 0.33 |
131.25 ± 0.84 |
16 ±0.19 |
31.25 ± 0.32 |
65.62 ± 0.4 |
18±0.23 |
20.2±0.05 |
24.13±0.22 |
The results are represented as (Mean ± SD).
Table.5. Antibiofilm activity assessments of the BaFeO3-Cop and BaFeO3-Sol samples.
|
Concentration of NPs (μg/ml) |
Absorbance (λ517nm) |
Reduction percentage (RP) % |
||
|
BaFeO3 Sol |
BaFeO3Cop |
BaFeO3 Sol |
BaFeO3 Cop |
|
|
12.5 |
0.20 |
0.155 |
67.20 |
76.37 |
|
25 |
0.15 |
0.135 |
77.39 |
80.44 |
|
50 |
0.135 |
0.095 |
80.4 |
88.5 |
|
100 |
0.065 |
0.050 |
94.70 |
97.75 |