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Vol. 8. Num. 1.
Pages 1-1592 (January - March 2019)
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Vol. 8. Num. 1.
Pages 1-1592 (January - March 2019)
Original Article
DOI: 10.1016/j.jmrt.2017.12.005
Open Access
Synthesis, characterization and antibacterial activity of cobalt doped cerium oxide (CeO2:Co) nanoparticles by using hydrothermal method
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Y.A. Syed Khadara,
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syedkhadar_ya@yahoo.co.in

Corresponding author.
, A. Balamuruganb, V.P. Devarajana, R. Subramanianc, S. Dinesh Kumarb
a Department of Physics, K.S.R. College of Arts and Science for Women, K.S.R. Educational Institutions, Tiruchengode, Namakkal-637 215, Tamil Nadu, India
b Department of Physics, Government Arts College, Udhagamandalam-643 002, Tamil Nadu, India
c Department of Chemistry, Sun Arts and Science College, Tiruvannamalai-606 755, Tamil Nadu, India
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Tables (3)
Table 1. XRD content for undoped and Co doped CeO2 nanoparticles.
Table 2. UV bandgap energy values of undoped and Co doped CeO2 nanoparticles.
Table 3. Antibacterial activity of undoped and Co doped CeO2 nanoparticles.
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Abstract

Different concentrations (2, 4, 6, and 8 mole %) of cobalt doped cerium oxide nanoparticles (CeO2:Co NPs) were synthesized by hydrothermal method. The synthesized samples were characterized by using various techniques to understand their structural, optical and surface morphological properties. The face-centred cubic (FCC) structure of the CeO2:Co NPs was identified from the X-ray diffraction (XRD) analysis. The calculated crystallites size of the CeO2:Co NPs were decreased from 20nm to 17nm on increased the concentration of cobalt from 2 mole % to 8 mole %. The bonding formation between cerium and oxygen (CeO) was confirmed using Fourier transform infra-red spectroscopy (FTIR). The surface morphology and shape of the CeO2:Co NPs were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM images have revealed a cube shaped, uniformly distributed and well dispersed CeO2:Co NPs. Further, a slight distortion of surface morphology was obtained when increased the concentration of cobalt. The optical properties were investigated by using ultra-violet visible (UV–vis) spectroscopy. A optical absorption band nature was observed from CeO2:Co NPs when compared with bulk spectrum. The bandgap energy of the CeO2:Co NPs were increased from 3.64eV to 3.69eV on increased the cobalt concentration. The photoluminescence (PL) emission spectrum of CeO2:Co NPs showed an enhanced defect of reduced emission by using spectrofluorometer. The CeO2:Co NPs resulted good antibacterial activity against pathogenic bacteria such Escherichia coli, Staphylococcus aureus, Bacillus Cereus and Salmonella Typhi. Hence, the CeO2:Co NPs could be used as biomaterial in nano-biotechnology applications.

Keywords:
Cerium oxide
Cobalt
Hydrothermal method
Cubic shape
Antibacterial activity
Full Text
1Introduction

In recent years, the design, synthesis and characterization of cerium oxide (CeO2) nanoparticles have drawn the attention of researchers due to their peculiar properties in the field of material science and technology. It is an attractive material which is used in various industrial products such as polishing agents, sun screens, fuel cells, photocatalysts and sensors applications [1–3]. CeO2 nanostructures have attracted extensive attention on the electronics and biological applications such as device fabrications, anti-oxidant activity, anti-bacterial activity and anti-cancer activities, which depends on the particle size, morphology and biocompatible nature. For the synthesis of CeO2 nanostructures, various chemical synthesis methods such as co-precipitation, sol–gel, chemical vapour deposition and hydrothermal techniques are used to fabricate CeO2 nanostructures with controlled morphology [4–8]. Among these techniques, the hydrothermal method was getting more important, because of efficient synthesis process and morphology controlled growth. Moreover, the CeO2 nanoparticles played an important role in the remediation of toxicity against microorganisms of bacteria, yeast and fungi [9]. Antibacterial activity of transition metal doped various metal oxide nanostructures has been reported to an electrostatic induction nature between nanoparticles and bacteria cell membrane [10–12]. Therefore, in the present research work, we have synthesized cobalt (Co) doped cerium oxide (CeO2) nanoparticles by using hydrothermal method and their structural, optical, morphology and anti-bacterial activity results were discussed in detail.

2Materials and methods2.1Reagents

Cerium nitrate Ce(NO3), cobalt chloride (CoCl2) and trisodium phosphate Na3(PO4) were purchased from Merck products, Mumbai, India. All the chemicals were used without purification. The double distilled water was used as solvent to prepare the solutions.

2.2Synthesis of cobalt doped CeO2 nanoparticles

Cobalt doped cerium oxide nanoparticles were synthesized using cobalt chloride and cerium nitrate as precursors. A 0.02 mole of trisodium phosphate solution (20ml) was slowly added dropwise to 0.1 mole of cerium nitrate solution (60ml) under constant stirring condition. During the synthesis, 2, 4, 6 and 8 mole % cobalt chloride solution was added to the above reaction mixture solution. The reaction was allowed for 30min at same condition resulted white colloidal solution. The residue was transferred to the suitable autoclave container and the hydrothermal treatment carried out at 180°C for 15h. After the reaction, the autoclave container was brought to room temperature. Then, the colloid was separated by using centrifugation and the obtained product was thoroughly washed with double distilled water followed by ethanol and dried at 80°C for 4h. For comparison purpose, the pure cerium oxide nanoparticles were synthesized by using the same procedure without cobalt.

2.3Characterization

Transformation of functional group present in the cerium oxide and cobalt doped cerium oxide nanoparticles were recorded using FTIR spectrophotometer (FTIR, Brucker-Tensor 27). The crystalline nature of the synthesized nanoparticles was analyzed by using X-ray diffractometer (XRD, Shimadzu-6000). The absorption of the nanoparticles was measured using spectrophotometer (UV–vis, Jasco V530). The emission spectra of the nanoparticles were measured using spectrofluorometer (Horiba Jobin, Flouromax-4). The formation of nanoparticles size and surface morphology of the nanoparticles were studied by scanning electron microscope (SEM, JOEL JSM-6390) and transmission electron microscope (TEM, Tecnai F-12).

2.4Antibacterial activity

The antibacterial activity of pure cerium oxide and cobalt doped cerium oxide nanoparticles were tested against Escherichia coli, Staphylococcus aureus, Bacillus Cereus and Salmonella Typhi using disc diffusion method. The pure CeO2 and Co doped CeO2 nanoparticles were prepared in appropriate concentration of 1mg/ml with dimethylsulfoxide solution for this process. Then, the dispersed nanoparticles were impregnated to each sterile disc by using micro-pipette. After that the discs were kept on culture swapped Mueller Hinton Agar medium using sterile force and allowed to incubate for 24h. The average zone of inhibition diameter was measured in millimetre (mm).

3Result and discussion3.1FTIR study

The FTIR spectra of the pure CeO2 NPs and Co doped CeO2 NPs were shown in Fig. 1. The transmittance peaks obtained below the range of 700cm−1[13], including the presence of peaks at 475, 545, 615cm−1 is attributed due to the CeO stretching mode and it confirms the formation of CeO2 structure. Further, the transmittance peaks obtained at 715, 735, 950 and 1052cm−1 is attributed to CO2 asymmetric stretching vibration and CO32− bending vibration, and CO stretching vibration. In addition, the peak at 818cm−1 with the OCO was absorbed due to the stretching frequencies. In addition, the band at 1381cm−1 was due to NO stretch formation of oxygen present in cerium with nitrogen and the band at 1629cm−1 corresponds to the bending of HOH, which was partly overlapping the OCO stretching band [14]. The band at 2427cm−1 was due to the presence of dissolved or atmospheric CO2 in samples and the absorption band at 3410 and 3500cm−1 was due to the OH stretching vibration/physical absorbed H2O/surface OH groups [15]. Based on previous report, a weak band around at 2037cm−1 was attributed to CO bonded to metallic fraction of Co ions [16] and therefore, we believed that it was occurred in Co doped CeO2 nanoparticles also. Moreover, the NO stretches and HOH bending nature were getting strengthened when increasing the concentration of Co doping in steps of 2, 4, 6 and 8 mole %. Then, these observations show the bonding nature of Co ions with CeO2 nanoparticles. Hence, the chemical structure and other functional groups were confirmed.

Fig. 1.

FTIR spectra of (a) undoped CeO2, (b) 2M% Co, (c) 4M% Co, (d) 6M% Co and (e) 8M% Co doped CeO2 nanoparticles.

(0.12MB).
3.2XRD study

The XRD patterns of the pure CeO2 NPs and Co doped CeO2 NPs were shown in Fig. 2. The XRD patterns are in tremendous concurrence with standard values of CeO2 (JCPDS card no. 43-1002). According to distinct diffraction peaks in the XRD patterns were clearly indicates face-centred cubic structure. The obtained 2θ values of 28.5, 33.0, 47.4, 56.3, 59.0, 69.4, 76.7 are corresponding to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1) planes and it is also similar to the previous reports [17,18]. In the CeO2:Co NPs, the lack of Co2+ or Co ions may be diluted in the CeO2 and, therefore, the Co peaks were not appeared. However, the crystallites size was slightly decreased when increasing the concentration of Co doping in steps of 2, 4, 6 and 8 mole %, while the (1 1 1) plane takes a gradual peak shift towards lower angle side [19,20]. Actually, the ionic radii of Ce4+ ions has lager size of 1.03Å than smaller size ionic radii of Co2+ ions has 0.74Å and, therefore, the Co ions were possible to substituted in the CeO2 crystal lattice [21]. Hence, the peak broaden was increased due to the decreased particles size [8,22] as shown in Fig. 3. The crystalline sizes or average particles sizes were calculated using Debye-Scherrer's formula [23] and their sizes were 17–20nm with d-spacing values of 3.14–3.08Å. The calculated crystalline sizes and their corresponding d-spacing values are given in Table 1.

Fig. 2.

XRD pattern of (a) undoped CeO2, (b) 2M% Co, (c) 4M% Co, (d) 6M% Co and (e) 8M% Co doped CeO2 nanoparticles.

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Fig. 3.

XRD crystallite sizes of undoped CeO2, 2M% Co, 4M% Co, 6M% Co and 8M% Co doped CeO2 nanoparticles.

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Table 1.

XRD content for undoped and Co doped CeO2 nanoparticles.

Samples  2θ (degree)  FWHM (radians)  Crystallite size (nm)  d-Spacing (Å) 
CeO2  28.90  0.43  19.9  3.0869 
CeO2:Co (2M%)  28.59  0.44  19.44  3.1197 
CeO2:Co (4M%)  28.77  0.46  18.6  3.1005 
CeO2:Co (6M%)  28.38  0.49  17.4  3.1423 
CeO2:Co (8M%)  28.47  0.50  17.1  3.1325 
3.3SEM analysis

The SEM images of the pure CeO2 NPs and Co doped CeO2 NPs were shown in Fig. 4. A cube shaped surface morphology was observed from pure CeO2 NPs and from the CeO2:Co NPs showed a slight distortion in the surface morphology on increasing the cobalt doping concentration, which has been reported earlier [24]. The incorporation of cobalt doping was indicated by small leaves or feathers like structures on the surface of the cube shaped CeO2 NPs. The slight alteration on the shape of CeO2 strongly indicates the effect of Co ions in the CeO2 crystal lattice, which made an agreement with the XRD results of average particles size. Further, the EDAX spectrum was recorded to know the chemical composition of Ce, O and Co ions distributions in the CeO2 NPs, shown in Fig. 5. The observed result indicates the chemical composition nature and also confirms the doping effect of cobalt ions. Moreover, the TEM images were recorded to observe the particles size and particles distribution. From that, a well grown nanoparticle was observed with well distributed nature shown in Fig. 6. However, the surface morphology was not clear, because of the embedded nature of particles in the copper grid.

Fig. 4.

SEM images of (a) undoped CeO2, (b) 2M% Co, (c) 4M% Co, (d) 6M% Co and (e) 8M% Co doped CeO2 nanoparticles.

(1.06MB).
Fig. 5.

EDAX pattern of (a) undoped CeO2, (b) 2M% Co, (c) 4M% Co, (d) 6M% Co and (e) 8M% Co doped CeO2 nanoparticles.

(0.27MB).
Fig. 6.

TEM images of (a) undoped CeO2, (b) 2M% Co, and (c) 8M% Co doped CeO2 nanoparticles.

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3.4Optical studies

UV absorption spectra of the pure CeO2 NPs and Co doped CeO2 NPs were shown in Fig. 7. The pure CeO2 NPs show a strong absorption at 345nm and their corresponding bandgap energy (Eg) value was 3.62eV, which was calculated using the Eg=1242/λabsorption relation. In the CeO2:Co NPs, the absorption intensity of CeO2 nature was quenched [25], which was maximum at visible region when increasing the Co doping concentration from 2 mole % to 8 mole % shown in Fig. 8. As the mean time, it may be the distributed nature of Co ions otherwise un-reacted nature of some Co doping ions. Therefore, the obtained bandgap energy value was slightly increased to 3.67eV and it is believed to be the decreased particles size [26,27]. Hence, the enhanced bandgap energy was due to the charge-transfer from O2− to Ce4+ energy levels in CeO2 crystal lattice [28]. The calculated absorption wavelengths and their corresponding bandgap energy values are given in Table 2.

Fig. 7.

UV absorption spectra of (a) undoped CeO2, (b) 2M% Co, (c) 4M% Co, (d) 6M% Co and (e) 8M% Co doped CeO2 nanoparticles.

(0.09MB).
Fig. 8.

UV Bandgap energy values of undoped CeO2, 2M% Co, 4M% Co, 6M% Co and 8M% Co doped CeO2 nanoparticles.

(0.06MB).
Table 2.

UV bandgap energy values of undoped and Co doped CeO2 nanoparticles.

Samples  Absorption wavelength (nm)  Band gap energy (eV) 
CeO2  343  3.62 
CeO2:Co (2M%)  341  3.64 
CeO2:Co (4M%)  340  3.65 
CeO2:Co (6M%)  339  3.66 
CeO2:Co (8M%)  338  3.67 
3.5Photoluminescence

The PL emission spectra of the pure CeO2 NPs and Co doped CeO2 NPs were shown in Fig. 9, using the excitation wavelength of 340nm corresponding to the absorption spectrum values. The pure CeO2 NPs exhibit a strong emission peak at 375nm, it was related to the transition of electron from localized Ce4f state to O2p of valence band. In addition, an emission peak obtained at 392nm was originated from defect states widely existing between Ce4f states to O2p of valence band [21]. Moreover, an emission of CeO2:Co NPs was gradually decreased around at 400nm and also it extended up to 550nm. The localized Ce4f state to O2p of valence band in CeO2:Co NPs related to the emission, which was gradually decreased in the CeO2 crystal lattice due to doping effect of Co ions. On the other hand, the emission band was gradually increased in the range of 400nm to 450nm and also it was extended upto 550nm. Meanwhile, the emission nature was red-shifted, because of reduced defect levels caused by the Co ions. Hence, the emission intensity of CeO2 nature got gradually quenched [29] and it was due to the formation of more oxygen defects or increases of surface defects and suppresses of electron transfer from Ce4f to O2p levels [21,30] when increasing the Co doping concentration from 2 mole % to 8 mole %.

Fig. 9.

PL emission spectra of (a) undoped CeO2, (b) 2M% Co, (c) 4M% Co, (d) 6M% Co and (e) 8M% Co doped CeO2 nanoparticles.

(0.11MB).
3.6Antibacterial activity

The antibacterial activity pure CeO2 NPs and Co doped CeO2 NPs were tested against four pathogenic bacteria's of E. coli, S. aureus, B. cereus and S. typhi. The antibacterial result of the pure CeO2 NPs and Co doped CeO2 NPs were shown in Fig. 10. The Co doped CeO2 NPs exhibits a higher zone of inhibition (ZOI) as compared with pure CeO2 NPs. These observations indicate that the interaction between CeO2:Co NPs and bacteria cell membrane induces the toxicity to bacteria and caused cell death. Both the pure CeO2 NPs and CeO2:Co NPs shows a good death effect against all the bacteria. The zone of inhibition was increased as the doping concentration of Co was increased. Hence, the comparison of antibacterial activity with different bacteria was shown in Fig. 11 and their zones of inhibition results are given in Table 3. Interestingly, the CeO2:Co NPs shows an enhanced antibacterial activity as compared with the previously reported on Co doped metal oxide nanoparticles for antibacterial and anticancer activities [31,32]. Among the four concentrations (2, 4, 6 and 8 mole %), the 8 mole % of CeO2:Co NPs showed higher zone of inhibition against E. coli (24mm), B. cereus (27mm), S. aureus (23mm) and S. typhi (25mm). It was believed to be the electrostatic interaction between positive charged nanoparticles and negative charged bacteria, which leads the small-sized nanoparticles penetration inside the cell wall and cause the cell damage. While the generation of reactive oxygen species (ROS), large surface area or small-sized particle nature and the release of constituent ions through efflux mechanism [33]. Therefore, the antibacterial activity of the CeO2:Co NPs were found an enhanced rate.

Fig. 10.

Anti-bacterial activity of (a) undoped CeO2, (b) 2M% Co, (c) 4M% Co, (d) 6M% Co and (e) 8M% Co doped CeO2 nanoparticles.

(0.12MB).
Fig. 11.

Anti-bacterial activity comparison of undoped CeO2, 2M% Co, 4M% Co, 6M% Co and 8M% Co doped CeO2 nanoparticles.

(0.1MB).
Table 3.

Antibacterial activity of undoped and Co doped CeO2 nanoparticles.

Bacteria  Zone of inhibitions (mm)
  CeO2  Coa (1)  Cob (2)  Coc (3)  Cod (4)  CFe 
E. coli  –  13  17  21  24  17 
B. cereus  –  12  16  21  27  19 
S. aureus  –  13  18  20  23  19 
S. typhi  –  15  15  19  25  18 
a

Co (1) – 2M% Co doped CeO2.

b

Co (2) – 4M% Co doped CeO2.

c

Co (3) – 6M% Co doped CeO2.

d

Co(4) – 8M% Co doped CeO2.

e

CF – Ciprofloxacin.

4Conclusions

The pure CeO2 and Co doped CeO2 were successfully synthesized by using hydrothermal method. From XRD patterns, a face-centre cubic crystal structure was confirmed, and their crystalline sizes were calculated approximately 17–20nm. FTIR spectrum shows the CeO chemical bonding nature and the presence of functional groups were confirmed. From SEM images, a cubic shaped surface morphology was observed. From UV optical spectrum, a blue-shifted absorption nature was observed, and their calculated bandgap energy values were approximately 3.62–3.67eV. The PL emission spectrum deals a strong blue emission nature and a reduced defect level was observed due the doping of Co ions. Further, the anti-bacterial activity of Co doped CeO2 nanoparticles were studied against different types of bacteria. An enhanced anti-bacterial activity killing effect was observed due to the increased concentration of Co doping from 2% to 8%.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

One of the author (SK) would like to thank Dr. P. Ponpandian, Professor and Head, Department of Nanoscience and Nanotechnology, Bharathiar University, Coimbatore, Tamil Nadu, India, for his support throughout this work.

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