Defect dependent antioxidant activity of nanostructured nickel oxide synthesized through a novel chemical method

Colloids and Surfaces A: Physicochem. Eng. Aspects 429 (2013) 44–50

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Defect dependent antioxidant activity of nanostructured nickel oxide synthesized through a novel chemical method G. Madhu a,b , Vipin C. Bose a , A.S. Aiswaryaraj a , K. Maniammal a , V. Biju a,∗ a b

Department of Physics, University of Kerala, Kariavattom Campus, Thiruvananthapuram, Kerala 695581, India Department of Physics, University College, Thiruvananthapuram, Kerala 695034, India

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Enhanced antioxidant activity of NiO. • Antioxidant activity of NiO is defect dependent.

• The presence of O2− vacancies in NiO enhances the antioxidant activity.

• Use of dc conductivity under controlled environment to explain DPPH scavenging. • Possibility of tuning antioxidant activity by changing processing conditions.

a r t i c l e

i n f o

Article history: Received 28 December 2012 Received in revised form 22 March 2013 Accepted 25 March 2013 Available online 1 April 2013 Keywords: Antioxidant activity Nickel oxide DPPH Nanomaterials Defects

a b s t r a c t Nanostructured nickel oxide samples with different average crystallite sizes in the range 32–45 nm are synthesized through a novel chemical route using nickel chloride and ethanol amine as starting materials. The samples are characterized using XRD, TEM and XPS analysis. The antioxidant activity estimated by measuring the DPPH scavenging activity is found to increase with decrease in particle size. A study of the variation of antioxidant activity with particle size and comparison of the results with DC conductivity measurements lead to the conclusion that the antioxidant activity in NiO is defect dependent. The origin of antioxidant activity is traced to the presence of anion O2− vacancies in the nanostructured NiO samples as inferred from the study of variation of DC electrical conductivity of the samples in vacuum and air. The study leads to the possibility of tuning the antioxidant activity of transition metal oxides by changing the processing conditions. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Magnetic nanoparticles with suitable surface coatings are used clinically for many important biomedical applications. These include contrasting agents in magnetic resonance imaging, hyperthermia, targeted drug delivery, tissue repair, cell and tissue targeting, etc. [1]. They are also used for early detection of diseases including cancer, diabetes, atherosclerosis etc. [2]. Many transition metal oxides in the nanostructured form show remarkable

∗ Corresponding author at: Department of Physics, University of Kerala, Kariavattom Campus, Thiruvananthapuram, Kerala 695581, India. Tel.: +91 471 2308920. E-mail addresses: [email protected], [email protected] (V. Biju). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.03.055

magnetic, optical and electrical properties which can be controlled effectively by altering the synthesis routes and processing conditions and hence have potential use in the biomedical and allied fields [3]. The physical properties of these nanostructures are determined to a great extent by the electrons in the localized ‘d’ band and defect levels associated with cation and/or oxygen vacancies [4]. Nickel oxide (NiO) is an antiferromagnetic transition metal oxide with Neel temperature TN = 523 K, which crystallizes in FCC symmetry [5]. Pure single crystalline NiO is an insulator at ordinary temperatures and has excellent chemical stability. Nanostructured NiO is reported to be a p-type semiconductor due to the presence of cation (Ni2+ ) vacancies primarily associated with the large surface area to volume ratio and exhibits super paramagnetism at ordinary temperatures [6,7]. The common applications

(220)

(111)

N600

Intensity (arb.units)

of NiO include temperature sensors, smart windows, switching devices, thermistors, electrode materials in fuel cells, supercapacitors, Li-ion batteries, ␥-ray detectors, etc. [8–13]. In the biomedical field, NiO is used as contrasting agent for magnetic resonance imaging, carriers, adsorbents, etc. Further, Ni/NiO nanoparticles stabilized with the amphipathic peptide have potential properties for Alzheimer’s disease therapy [14]. With the increased use of NiO for biomedical activities, antioxidant activity of NiO became an important question and Saikia et al. [15] had first reported the antioxidant activity of chemically synthesized NiO nanoparticles with a size range of 10–60 nm. Herein, we report studies on the antioxidant activity of nanostructured NiO with different average crystallite sizes in the range 32–45 nm, synthesized through a novel chemical method. The possible origin of antioxidant activity in nanostructured NiO is explained by correlating the results with the DC electrical conductivity measurements under controlled conditions.

45

(200)

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N500

N400

* (112)

*

2.1. Synthesis, TGA, XRD and TEM analysis

# All the chemicals used were of analytical grade and were used without further purification. Nickel chloride, [NiCl2 ·4H2 O] was completely dissolved in ethanol amine [CH2 (OH) CH2 NH2 ] with the help of an ultrasonic bath to obtain 0.1 M solution. Then, 60 ml of distilled water was added to 60 ml of the NiCl2 solution and the mixture was stirred using a magnetic stirrer at room temperature for 5 h. The resulting mixture was dried at 175 ◦ C, to get a dark brown NiCl2 –(EA) complex. The drying temperature was kept greater than the boiling point of ethanol amine (170 ◦ C) to ensure the evaporation of unreacted solvent. The dried complex was finely ground using an agate mortar and pestle. In order to study the decomposition scheme of the NiCl2 –(EA) complex, Thermo Gravimetric Analysis (TGA) was carried out over the temperature range 30–1000 ◦ C with a step size of 10 ◦ C/min in air ambience using a SDT -2960, TA Instrument and the results are shown in Fig. 1. The decomposition of the NiCl2 –(EA) complex was done at 350 ◦ C in air atmosphere to obtain nanostructured nickel oxide. The as prepared oxide samples were annealed at different temperatures (Table 1). X-ray diffraction (XRD) patterns of the samples were recorded in the 2 range 10–70 ◦ using a Philips X’Pert Pro X-ray ˚ as the Diffractometer employing Cu K␣ radiation ( = 1.540560 A) source and the results are shown in Fig. 2. Transmission Electron Micrography (TEM) of the samples was done using a Philips CM200 Electron Microscope. The Ni 2p and O 1s X-ray photoelectron

100

Measured Data

Weight(%)

80

60

40

20

0 0

200

400

600

800

Temperature (ºC) Fig. 1. TGA curve of the NiCl2 –EA complex.

1000

30

NiO # Ni2 O3

*

(102)

2. Experimental

40

*

# 50

N350 60

70

2θ (degree) Fig. 2. X-ray diffraction pattern of nickel oxide samples.

spectra (XPS) for sample N350 were recorded in a M/s SPECS make X-ray Photoelectron spectrometer under an operating pressure of 6 × 10−8 mbar using Al K␣ (1486.7 eV) radiation as the source. 2.2. Evaluation of antioxidant activity The antioxidant activities of soluble samples can be evaluated by estimating the free radical scavenging activity using 1,1-diphenyl2-picryl hydrazyl (DPPH). DPPH is normally a paramagnetic solid soluble in alcohol. The odd electron in the DPPH free radical gives a strong absorption maximum at 517 nm usually in a deep violet color. The color turns from deep violet to yellow as the molar absorptivity of the DPPH radical at 517 nm reduces from 9660 to 1640 when the odd electron of DPPH radical becomes paired with a cation from a free radical scavenging antioxidant to form the reduced DPPH-H. The resulting decolorization is stoichiometric with respect to the number of electrons captured. Thus the DPPH radical has a deep violet color in solution, and it becomes colorless or pale yellow when neutralized. This property allows visual monitoring of the reaction using a UV–vis spectrophotometer and the number of initial radicals can be counted from the change in the optical absorption at 517 nm [16]. Modified DPPH method [17] for insoluble materials was used to measure the antioxidant activity of naostructured NiO in the present study. The antioxidant activity of the sample N400 was studied as a function of mass. In a typical experiment 5 mg of the well ground sample (N400) was applied to 3 ml, 100 ␮M DPPH reagent. The surface reaction between the nanoparticles and DPPH was effected by vortexing for 5 min. The mixture was centrifuged and the absorbance of the supernatant was measured at 517 nm using a SHIMADZU UV-2550 double beam UV–vis spectrophotometer. The radical scavenging activity (RSA) was calculated as percentage of DPPH discoloration using the equation, RSA (%) = {(Ac − As )/Ac } × 100 where Ac is the absorbance of the control (blank DPPH) and As is the absorbance with sample after vortexing. The experiment was repeated for different mass of the sample, viz., 10 mg, 15 mg, 20 mg and 25 mg with

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Table 1 Details of annealing and results of XRD and TEM analysis of nanostructured NiO. Sample code

Annealing temp (◦ C)

Duration (h)

Phases present

Size from Scherrer equ (nm)

N350 N400 N500 N600

350 400 500 600

8 2 2 2

NiO, Ni2 O3 NiO, Ni2 O3 NiO, Ni2 O3 NiO

32 37 40 45

a

± ± ± ±

0.8 0.5 0.5 0.3

Size from TEM analysis (nm) 35 36 37 40

± ± ± ±

3 2 3 4

Surface area to volume ratioa (%) 18.75 16.22 15.00 13.33

Surface area to volume ratio is calculated by assuming spherical shape for the nanoparticles.

the time of votex kept same as 5 min. Ultrasonication is reported to enhance the scavenging activity of nanoparticles [18]. In order to study the effect of ultrasonication, the experiment was repeated with the vortexing procedure replaced by ultrasonication for 5 min. For studying the effect of annealing temperature, the antioxidant activity of samples N350, N500 and N600 were estimated using 5 mg each of the samples. Experiments were done using both vortexing and ultrasonication procedures as in the case of sample N400 with the vortexing/ultrasonication time kept as 5 min. 2.3. DC electrical conductivity measurements In order to measure the dc electrical conductivity, the nanostructured samples were consolidated in the form of cylindrical pellets of 13 mm diameter and about 1 mm thickness using a hydraulic press by applying a force of 4 tons. Conducting silver epoxy was coated on the opposite faces to ensure good electrical contact. The dc electrical conductivity of the samples were measured in a dielectric cell evacuated to a vacuum of the order of 0.05 mbar at different temperatures ranging from 303 K to 423 K using a KEITHLEY 2400 source meter. The electrical conductivity of the samples in air ambience was also measured at the same temperatures. 3. Results and discussion In the TGA curve (Fig. 1), it is observed that ∼29% weight loss occur in the temperature range 250–450 ◦ C. This can be attributed to the decomposition of the NiCl2 –(EA) complex into the oxide phase. The complex was decomposed by annealing at 350 ◦ C for 8 h. In order to synthesize naostructured oxide samples with different average crystallite sizes, the as prepared oxide sample was annealed at higher temperatures up to 600 ◦ C. Sample codes were assigned for convenience and are listed in Table 1 together with the details of annealing. The three intense peaks in the XRD pattern (Fig. 2) of all the four samples can be indexed with the JCPDSICDD Pattern numbers 75-0197 corresponding to NiO with cubic symmetry (space group-Fm3m). Further, two less intense peaks corresponding to the (1 0 2) and (1 1 2) planes of Ni2 O3 phase with hexagonal symmetry (JCPDS-ICDD Pattern number 14-0481) are present in the case of samples N350, N400 and N500. Thus the XRD analysis leads to the conclusion that traces of Ni2 O3 is present in the case of samples N350, N400 and N500 while the amount of Ni2 O3 in N600 is below the detectable limit of XRD analysis. The full widths at half maxima (FWHM) of the peaks were estimated by a nonlinear curve fitting routine assuming a Pseudo-Voigt function for the line profile [19]. The average crystallite sizes were estimated using Scherrer equation [19] for all the samples after applying corrections for instrumental contribution and are included in Table 1. The average crystallite sizes mentioned are the averages of the size measured from the three most intense peaks corresponding to NiO and the deviation is the standard deviation between the sizes estimated along the three different crystallographic directions. It can be noted that there is a gradual increase of crystallite size with annealing temperature.

A crystallite is the periodic arrangement of atoms up to a specific boundary from where the next crystallite starts with different orientation and the number of these crystallites oriented in various directions can be recognized using XRD. If the sample is not monodispersed, the combination of crystallites oriented in various directions forms the particle and it can be substantiated through TEM analysis. Hence a comparison of crystallite size measured from XRD analysis and particle size measured from TEM analysis could give information on the state of agglomeration in the sample. The TEM image for sample N400 is shown in Fig. 3(A) and the particle size distribution is shown in Fig. 3(B). The particle size distribution follows a log normal profile. The average particle size and the size distribution for all the samples estimated by curve fitting a log normal profile to the observed histograms are listed in Table 1. The average particle size estimated from the TEM analysis is in good agreement with the average crystallite size estimated from XRD analysis showing that the samples are monodispersed with minimum agglomeration. Also the TEM analysis shows that the particles are nearly spherical. The radical scavenging activity of sample N400 measured after vortexing and ultrasonication are plotted in Fig. 4. The scavenging activity for 5 mg sample on vortexing is 25.76% which increases exponentially with increase in mass. For 25 mg, the corresponding value is 85.63%. The mass of N400 sample for scavenging 50% of the free radical DPPH (SC50 ) was calculated as 9.15 mg from the graph. The scavenging activity of N400 is enhanced when the reactions were carried out under ultrasonication (Fig. 4(A)). The scavenging activity for 5 mg and 25 mg are respectively 35.31% and 92.81%. The exponential dependence on mass is still evident and the mass of sample to scavenge 50% of free radical DPPH (SC50 ) decreases to 7.31 mg. Saikia et al. had studied the antioxidant activity of NiO nanoparticles with average particle size in the range 10–60 nm synthesized through self propagating hightemperature synthesis method [18]. The vorxing/ultrasonication time was same as in the present study. The SC50 values on vortexing and ultrasonication was respectively 32.93 mg and 7.49 mg. Thus the SC50 value on vortexing in the present case is much smaller than that compared to the case of Saikia et al., while the corresponding values on ultrasonication are nearly equal even though the average crystallite sizes are different. The free radical scavenging activity of nanoparticles is a surface reaction since only the surface comes in contact with the free radicals. As the particle size decreases, the surface area to volume ratio increases and the scavenging activity is expected to increase. On comparing the results for sample N400 with reports of Saika et al. an enhancement in scavenging activity with decrease in particle size is not evident [18]. This point to the possibility that the free radical scavenging activity of nanostructured NiO is defect dependent and hence could depend on the synthesis route and thermal history of the sample. For a comparative study of the dependence of antioxidant activity on particle size and to get an insight into the possible origin of antioxidant activity of naostructured NiO, the scavenging activity of 5 mg each of all the samples in the present study with vortexing and ultrasonication are shown in Fig. 4(B). In each case, the scavenging

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47

Fig. 3. TEM image (A) and the particle size distribution histogram (B) of sample N400.

activity increases on ultrasonication in comparison to vortexing as expected. It can be noted that the scavenging activity decreases with increase in the crystallite size/annealing temperature. One obvious reason for the enhancement of free radical scavenging activity with decrease in annealing temperature is the larger surface area to volume ratio due to smaller crystallite size. However, from Fig. 4(B), it can be noted that the activity almost doubles from 20.61% for sample N600 (45 nm) to 38.36% for sample N350 (32 nm) though the increase in the surface area to volume ratio is only about 5.42% (Table 1). This combined with the comparison of SC50 values for sample N400 and those reported by Saika et al., [18] lead to the

conclusion that the free radical scavenging activity of nanostructured NiO is defect dependent. In the following section, the possible origin of the observed anti oxidant activity and its variation with annealing temperature is discussed. The cation (Ni2+ ) vacancies are the most probable kind of defects in NiO [24–28]. The presence, concentration and distribution of Ni2+ vacancies is known to precipitously affect the electrical, optical, magnetic, catalytic, etc., properties of NiO. Pure NiO is an insulator and the electrical conduction in NiO at ordinary temperatures is due to the presence of Ni2+ vacancies [25]. Presence of each Ni2+ vacancy causes two adjacent Ni2+ ions to transform into Ni3+ ions

Fig. 4. DPPH scavenging activity of N400 on vortexing and sonicating (A) and DPPH scavenging activity of different nickel oxide samples (B).

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Table 2 Results of DC electrical conductivity study of nanostructured NiO samples. Sample code

N350 N400 N500 N600

 DC (−1 m−1 ) (in vacuum)

 DC (−1 m−1 ) (in air)

At 303 K

At 423 K

At 303 K

7.06 × 10−10 1.72 × 10−09 1.32 × 10−06 1.27 × 10−05

9.22 × 10−08 2.08 × 10−07 3.62 × 10−05 1.26 × 10−04

4.33 × 10−09 8.19 × 10−09 3.84 × 10−06 2.15 × 10−05

Percentage change in  DC At 423 K

Activation energy (eV)

At 423 K

Percentage change in  DC At 303 K

In vacuum

In air

4.95 × 10−07 9.24 × 10−07 8.85 × 10−05 2.02 × 10−04

513.31 376.16 190.91 69.29

436.88 344.23 144.48 60.32

0.68 0.54 0.36 0.28

0.44 0.40 0.32 0.26

to maintain charge neutrality and introduces a lattice distortion [26]. The presence of traces of Ni3+ can be confirmed by the XPS measurements. Fig. 5(A) and (B) shows Ni 2p and O 1s XPS spectra of sample N350 [20–24]. The two sharp peaks obtained at binding energies of 872.7 eV and 854.9 eV corresponds to Ni 2p1/2 and Ni 2p3/2 while their satellite peaks due to shake-up process appear at about 879.4 eV and 860.5 eV respectively. The O 1s spectrum of sample N350 deconvoluted by Gaussian curves is shown Fig. 5(B). The intense peak corresponding to the binding energy of 530.32 eV indicates the presence of Ni2+ and the less intense peak corresponding to binding energy of 531.56 eV indicates the presence of Ni3+ in the nickel oxide sample [22,23]. The electrical conduction in NiO in the temperature range 200–1000 K is reported to be due to the large polarons associated with the holes in the oxygen 2p band which originate due to the transfer of electrons into the neighboring Ni3+ ions. The activation energy for the conduction due to the large polarons associated with Ni2+ vacancies is reported to be ∼0.6 eV [28]. Each Ni2+ vacancy contributes two charge carriers for conduction and hence the electrical conductivity is a direct measure of the concentration of Ni2+ vacancies in the sample. The dc electrical conductivity,  DC of the nanostructured NiO samples at 303 K and 423 K in a vacuum of 0.05 mbar are tabulated in Table 2. The  DC value for the sample N350 is 7.06 × 10−10 −1 m−1 at 303 K which increases to 9.22 × 10−8 −1 m−1 at 423 K. Further,  DC is found to increase with increase in annealing temperature. The  DC values for sample N600 at 303 K and 423 K are respectively 1.27 × 10−5 −1 m−1 and 1.26 × 10−4 −1 m−1 . Thus the  DC values for the nanostructured NiO samples are enhanced by about four to five orders of magnitude with increase in annealing temperature. The  DC values at room temperature for single crystalline NiO without defects are reported to be of the order of 10−11 −1 m−1 which is about ten times the ␴DC value of sample N350 which is the least conducting one in the present study. Further, this is about six orders of magnitude higher than the  DC value for sample N600 which is the most conducting

(A)

854.9

one in the present case. The enhanced  DC values for the annealed samples clearly show the presence of Ni2+ vacancies. Also, the more or less similar values of activation energies for the samples (Table 2) confirms that the conductivity mechanism in all the samples are the same, viz. due to the large polarons associated with the Ni2+ vacancies [25]. However, it may be noted that annealing in air in the temperature range 400–600 ◦ C could not lead to creation of Ni2+ vacancies. Hence it can be concluded that the observed increase in  DC with annealing temperature is influenced by the presence of O2− vacancies. The presence of one O2− vacancy will annihilate the effect of one Ni2+ vacancy [28]. Hence, if in a sample both Ni2+ vacancies and O2− vacancies are present, the effect of Ni2+ vacancies will be evident only if the number of Ni2+ vacancies is more than the number of O2− vacancies and quantitatively the conductivity will be proportional to the number of uncompensated Ni2+ vacancies [28]. In the present case, the synthesis of nanostructued NiO was done by the thermal decomposition of NiCl2 –(EA) complex. The presence of the amine group in the decomposing atmosphere could lead to possible O2− vacancies in the sample N350. On annealing the as prepared sample N350 at higher temperatures in air ambience, the oxygen vacancies may be filled up by O2− ions due to the presence of oxygen, thereby increasing the number of uncompensated Ni2+ vacancies. The slight decrease in the activation energy with increase in annealing temperature is also an indication of the increase in the uncompensated Ni2+ vacancies in support of our argument [28]. Further, it is reported that the dependence of  DC on oxygen partial pressure is an indication of the presence of O2− vacancy with the dependence increasing with increase in the concentration of O2− vacancy [28]. The  DC values for the samples at 303 K and 423 K in air ambience are also included in Table 2. It can be noted that the  DC values for all the samples increases in comparison to the values measured in vacuum. In order to compare the influence of air ambience in the  DC values for different samples, the percentage change in conductivity in air and vacuum

(B)

Ni 2p

O 1s 530.32

872.7

900

890

880

860.5

870

860

Binding energy (eV)

Intensity (a.u)

Intensity (a.u)

879.4

850

840

531.56

520 522 524 526 528 530 532 534 536 538 540

Binding energy (eV)

Fig. 5. X-ray photoelectron spectrum of Ni 2p (A) and O 1s (B) of sample N350.

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the DPPH scavenging method increases with decrease in annealing temperature/particle size. The comparison of the variations in the antioxidant activity and DC electrical conductivity leads to the conclusion that the antioxidant activity in NiO is defect dependent. The origin of antioxidant activity of nanostructured NiO is traced to the presence of O2− vacancies. As the electrical, optical and magnetic properties of nanostructured NiO which are made use of in various applications are determined primarily by the concentration of uncompensated Ni2+ vacancies, the estimation of O2− vacancy is important from both academic and application points of view. The estimation of antioxidant activity of NiO could be developed as a potential analytical technique for estimating the O2− vacancy concentration. Possible extension of the technique to similar system could be attempted in the future. Acknowledgements

Fig. 6. Variation of DPPH scavenging activity (A), percentage change in  DC (B) and surface area to volume ratio (C) with average particle size.

{ DC (air) −  DC (vac)/ DC (vac)} at 303 K and 423 K were estimated and are included in Table 2. For sample N350, the increase is ∼500% which gradually decrease with increase in annealing temperature and for sample N600, the increase is only 70%. This observation also supports our argument that on annealing the O2− vacancies are filled up thereby increasing the number of uncompensated Ni2+ vacancies. Fig. 6(A)–(C) shows the variation of antioxidant activity (Free radical scavenging activity), percentage change in conductivity in air compared to vacuum and the surface area to volume ratio as a function of particle size. All the three quantities decrease with increase in particle. It is clear that the trends of variation of antioxidant activity and the percentage change in conductivity resemble one another. The correlation coefficient between the antioxidant activity and change in conductivity is estimated to be 0.97 while the correlation coefficient between the antioxidant activity and surface area to volume ratio is 0.91. This also justifies our argument that the antioxidant activity in nanostructured NiO is originating due to the presence of O2− vacancies. 4. Conclusions NiCl2 –(EA) complex was synthesized by a chemical reaction at 175 ◦ C. TGA analysis revealed the decomposition of the complex occur in the temperature range 250–450 ◦ C. Nanostructured NiO (32 nm) was synthesized by the decomposition of the complex at 350 ◦ C. The as prepared sample contained traces of Ni2 O3 . The as prepared sample was annealed at different temperatures up to 600 ◦ C. Annealing resulted in an increase of crystallite size and a decrease in the percentage of Ni2 O3 . The average particle sizes measured from the TEM analysis were found to be in good agreement with the results from XRD analysis revealing that the samples are monodispersed with minimum agglomeration. The XPS measurement confirmed the presence of trace of Ni3+ indicating presence of Ni2+ vacancies. The antioxidant activity estimated by measuring

G. Madhu acknowledges University Grants Commission (UGC), Government of India for Financial Assistance in the Form of FDP teacher fellowship. Vipin C. Bose acknowledges University of Kerala for financial assistance in the form of Junior Research Fellowship. A.S. Aiswaryaraj acknowledges University Grants Commission (UGC), Government of India for Financial Assistance in the Form of Rajiv Gandhi National Fellowship. K. Maniammal acknowledges Government of Kerala for Junior Research Fellowship under the SC/ST development scheme. The authors gratefully acknowledge SAIF Indian Institute of Technology (IIT), Bombay, India for TEM analysis and Dr. U. Kamachi Mudali, Associate Director and C. Thinaharan of Corrosion Science and Technology Group of Indira Gandhi Center for Atomic Research (IGCAR), Kalpakkam, India for XPS measurements. References [1] A.K. Gupta, R.R. Naregalkar, V.D. Vaidya, M. Gupta, Recent advances on surface engineering of magnetic iron oxide nanoparticles and their biomedical applications, Nanomedicine (Lond.) 2 (2007) 23–29. [2] S. Rodríguez-Llamazares, J. Merchán, I. Olmedo, Ni/Ni oxides nanoparticles with potential biomedical application obtained by displacement of a nickel-organometallic complex, J. Nanosci. Nanotechnol. 8 (2008) 3820–3827. [3] S. Mohapatra, P. Pramanik, Synthesis and stability of functionalized iron oxide nanoparticles using organophosphorus coupling agents, Colloids Surf. A: Physicochem. Eng. Aspects 339 (2009) 35–42. [4] H. Gleiter, Nanostructured materials: basic concepts microstructure, Acta Mater. 48 (2000) 1–29. [5] M.T. Hutchings, E.J. Samuelson, Inelastic neutron scattering measurement of spin waves and magnetic interactions in NiO, Solid State Commun. 9 (1971) 1011–1014. [6] J.T. Richardson, D.I. Yiagas, B. Turk, K. Forster, Origin of superparamagnetism in nickel oxide, J. Appl. Phys. 70 (1991) 6977–6982. [7] M. Abdul Khadar, V. Biju, A. Inoue, Effect of finite size on the magnetization behavior of nanostructured nickel oxide, Mater. Res. Bull. 38 (2003) 1341–1349. [8] A.I. Inamdar, Y.S. Kim, S.M. Pawar, J.H. Kim, Chemically grown, porous, nickel oxide thin-film for electrochemical supercapacitors, J. Power Sources 196 (2011) 2393–2397. [9] Yan-Na Nuli, Sheng-Li Zhao, Qi-Zong Qin, Nanocrystalline tin oxides and nickel oxide film anodes for Li-ion batteries, J. Power Sources 114 (2003) 113–120. [10] K. Arshak, O. Korostynska, F. Fahim, Various structures based on nickel oxide thick films as gamma radiation sensors, Sensors 3 (2003) 176–186. [11] W.J. Moore, Seven Solid States, W.A. Benjamin, Inc., New York, 1967, p. 134. [12] V.V. Bakovets, L.N. Trushnikova, I.V. Korol Kov, V.V. Sokolov, Synthesis of nanostructured nickel oxide, Russ. J. Gen. Chem. 79 (2009) 356–361. [13] S. Lei, K. Tang, Z. Fang, H. Zheng, Ultrasonic-assisted synthesis of colloidal Mn3 O4 nanoparticles at normal temperature and pressure, Cryst. Growth Des. 6 (2006) 1757–1760. [14] C. Wilhelm, F. Gazeau, J.C. Bacri, Magnetophoresis and ferromagnetic resonance of magnetically labeled cells, Eur. Biophys. J. 31 (2002) 118–125. [15] J.P. Saikia, S. Paul, B.K. Konwar, S.K. Samdarshi, Nickel oxide nanoparticles: a novel antioxidant, Colloids Surf. B: Biointerfaces 78 (2010) 146–148. [16] P. Molyneux, The use of the stable free radical diphenylpicrylhydrazyl (DPPH) for estimating antioxidant activity, Songklanakarin J. Sci. Technol. 26 (2004) 211–219.

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