- Email: [email protected]
Photocatalytic, nitrite sensing and antibacterial studies of facile bio-synthesized nickel oxide nanoparticles
Journal Pre-proof Photocatalytic, nitrite sensing and antibacterial studies of facile bio-synthesized nickel oxide nanoparticles C.R. Rajith Kumar, Virupaxappa S. Betageri, G. Nagaraju, G.H. Pujar, B.P. Suma, M.S. Latha PII:
S2468-2179(20)30011-3
DOI:
https://doi.org/10.1016/j.jsamd.2020.02.002
Reference:
JSAMD 272
To appear in:
Journal of Science: Advanced Materials and Devices
Received Date: 15 October 2019 Revised Date:
5 February 2020
Accepted Date: 11 February 2020
Please cite this article as: C.R Rajith Kumar, V.S Betageri, G Nagaraju, G. Pujar, B.P Suma, M.S Latha, Photocatalytic, nitrite sensing and antibacterial studies of facile bio-synthesized nickel oxide nanoparticles, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/ j.jsamd.2020.02.002. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.
Photocatalytic, nitrite sensing and antibacterial studies of facile bio-synthesized nickel oxide nanoparticles Rajith Kumar C R1, Virupaxappa S Betageri1, G Nagaraju2, G H Pujar3, Suma B P4, Latha M S1* 1
Research Centre, Department of Chemistry, G M Institute of Technology, Davangere, Karnataka, India-577006 2 Energy Materials Research Laboratory, Department of Chemistry, SIT, Tumakuru, Karnataka, India-572103 3 Research Centre, Department of Physics, G M Institute of Technology, Davangere, Karnataka, India-577006 4 Department of Chemistry, Bangalore University, Central College Campus, Bengaluru, India-560001 *Corresponding Author: Dr Latha M S Associate professor GM Institute of Technology Davangere, Karnataka India -577006 Mob: 9964428253 Email: [email protected]
Acknowledgments Dr. G. Nagaraju thanks the DST-Nano mission (SR/NM/NS-1262/2013) Govt of India, New Delhi for providing characterization techniques and also the VGST, Govt of Karnataka (CISEE-VGST/GRD-531/2016-17) for UV-DRS studies. Rajith Kumar C R thanks the Department of Biotechnology, GM Institute of Technology, Davangere and Siddaganga Institute of Technology, Tumakuru for providing lab facility.
Photocatalytic, nitrite sensing and antibacterial studies of facile bio-synthesized nickel oxide nanoparticles Abstract: In the present work, Nickel oxide nanoparticles (NiO NPs) were synthesized using leaves extract of Calotropis gigantea through a solution combustion method. The NiO NPs were characterized through analytical techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FT-IR). The XRD results revealed rhombohedral structured crystallites with average size of 31 nm. SEM and TEM images indicate that the nanoparticles are agglomerated with an asymmetrical shape. The optical energy bandgap of 3.45 eV was estimated using UV-diffused reflectance spectroscopy (UV-DRS). The synthesized NiO NPs have shown superior photodegradation for methylene blue (MB) dye. Further, the antibacterial activity of the prepared nanoparticles was tested against E.coli and S.aureus bacterial strains. In addition, nanoparticles were utilized for electroanalytical applicability as a novel non-enzymatic sensor in the trace level quantification of nitrite. The proposed nitrite sensor showed wide linearity in the range 8-1700 µM and good stability with a lower detection limit of 1.2 µM.
Key words: NiO nanoparticles, Calotropis gigantea, Dye degradation, Antibacterial activity, Nitrite sensing
Graphical Abstract
1. Introduction Nanoscience and nanotechnology have acquired an excellent impetus in the rapidly growing technological era by covering the basic understanding of physicochemical and biological properties in atomic/sub-atomic levels with promising applications in various fields [1]. In the last few years, various researchers investigated on transition metal oxide nanoparticles due to their increasing importance and potential applications [2]. Among all, NiO an interesting p-type, wide direct bandgap semiconductor (3.4–4.0 eV), has caught more attention owing to its key applications. Indeed, nano-sized NiO materials have gained great interest with respect to bulk NiO because of their size quantization and large surface-area ratio [3]. Due to their unique and remarkable properties NiO NPs gained significant importance in various fields, as battery cathodes/ anodes [4], catalysis [5], solar cells [6], materials for sensors [7], electrochemical super capacitors [8]. Various plants have been increasingly employed in the synthesis of nanoparticles due to their ample advantages in elimination of elaborate processes of maintaining cell cultures, cost-effectiveness and easy scale up for large-scale synthesis. During the bioproduction of NPs, plant extracts act as both reducing and stabilizing agents [9]. Kumar et. al. [10], and Vidya et. al. [11], have reported about the synthesis of Ag NPs, and ZnO NPs using leaf extract of Calotropis gigantea. In the present study, NiO NPs have been synthesized using leaves extracts of Calotropis gigantea plant. The Calotropis gigantea, also called as Arka, Madara, etc., belongs to the family of Apocynaceae and is available throughout India, especially in the dry and vast land. Various phytochemical constituents are present in different parts of the Calotropis plant, mainly in the leaves, which acts as a reducing and stabilizing agents during the synthesis of NPs. Highly toxic dyes play a major role in polluting water. These, are frequently being used in the industries like textile, food, cosmetics, paper, plastics, etc., [12]. The natural degradation of such dyes is very difficult due to their complex structure. However, recently, various semiconductor photocatalysts NiO, Cu2O, FeO, etc., have been developed to degrade the organic pollutants. [13, 14]. In the present study, the synthesized NiO NPs have been used to study the photocatalytic degradation of methylene blue dye. Various nanostructure materials have shown good antibacterial activity against human pathogens [15]. Earlier reports have demonstrated the possibility of utilization of metal oxide NPs, in particular, NiO NPs in biomedicines due to their unique therapeutic and biological properties such as adsorbing and metal ion releasing ability, cytotoxic effects and surface-area ratio [16]. Hence, the antibacterial activity of NiO NPs has been demonstrated in the present study.
In the past decades, the highly sensitive detection of ‘nitrite’ has caught increasing interest because of its harmful effect on both human health and global environment. Further, ground water pollution is rapidly increasing by ‘nitrates’ due to the anthropogenic activities [17]. The World Health Organization (WHO) recommends, the maximum limit of ‘nitrite’ should be 3 mg/L in drinking water [18], hence, it is an important task of chemists to monitor the existing levels/limits of nitrite in water and environment. Generally, the analysis of nitrites can be quantified by using various techniques such as chromatography, spectrophotometry/spectrofluorimetry,
electroluminescy
and
capillary
electrophoresis
techniques. However, some of the above quantitative techniques lack sensitivity and, high detection limits and do require extensive instrumentation. In contrast, electrochemical methods give better precision quantification over all these methods in terms of sensitivity/selectivity [19]. In a quantitative analysis, the thorough exploitation of CMEs within the field of electrochemistry and surface manipulation with selective indicator moieties is desirable to achieve the tailored properties. Such CMEs have found to be very sensitive, easy to fabricate and target specific in electrochemical applications [20]. Here, the synthesized NiO NPs have been used as a modifier molecule in the fabrication of electrode. The modifier electrode has been explored for its electroanalytical applicability as a novel non-enzymatic sensor in trace level quantification of nitrite.
2. Experimental Section 2.1. Materials All the chemicals (analytical grade) were purchased from SD-Fine Chemicals Pvt. Ltd. and Hi-media and used without any further purification.
2.2. Instrumentation and experimental methods The Crystalline nature and phase purity was identified with the aid of the X-ray diffractometer (Rigaku Smart Lab). The morphology and elemental composition of the material was examined using SEM and EDAX (Hitachi S3400n), respectively. The HR-TEM with SAED (Jeol/JEM 2100) was used to measure shape and size of the nanoparticles, respectively. The FT-IR spectrometer (Bruker alpha-P) was used to examine the functional groups. Absorption spectra were recorded with the UV–Visible spectrophotometer (Agilent technology cary-60 spectrophotometer). The diffuse reflectance spectrum was measured using the Lab India UV 3092, UV-VIS spectrophotometer. Electrochemical measurements were achieved using the CH instrument.
2.3. Synthesis of NiO NPs Freshly collected leaves of Calotropis gigantea were washed, dried and grinded well. The Soxhlet extractor with water as solvent was used for the extraction for 5 hours and the obtained extract was dried using a rotary evaporator. The combustion synthesis method was used to synthesize NiO NPs using Nickel nitrate hexahydrate (Ni (NO3)26H2O) as an oxidizer and Calotropis gigantea leaves extract as a fuel. In this process, 2 gm of the extract dissolved in 100 mL of double distilled water was, constantly stirred for 10 min to get a homogenous solution. Ni (NO3)26H2O of 0.5 M was dissolved in 10 mL of Calotropis gigantea extract and was placed in a preheated muffle furnace (400 ± 10 °C). A smouldering reaction takes place and the entire process was completed within 10 min. The obtained NiO NPs were subjected for calcinations at 500°C for 3 hours to eliminate the impurities. Until further use, the obtained product was stored in an airtight container.
2.4. Photo catalytic studies The photocatalytic studies of NiO NPs were assessed by the degradation of cationic methylene blue (MB) dye in aqueous media using a 250 W UV-light irradiation source. For the photocatalytic experiments, a visible annular photoreactor was used, which consists of cylindrical tubes with transparent interior to employ complete radiation. In this process, 50 mg of NiO NPs as a photocatalyst was added to quartz tubes of 100 mL capacity, which contains 100 mL MB solution of concentration 5 ppm. The solution was continuously air bubbled for complete mixing of the MB dye and the photocatalyst. Then, 2 mL was taken out from the above solution, the first time after 15 min and then at regular intervals of 30 minutes. The percentage of degradation of the cationic MB dye has been calculated using the Beer-Lambert law as follows [21]: % of degradation =
C − C × 100 (1) C
Where, Ci and Cf are the initial and final concentration of the dye solution, respectively.
2.5. Antibacterial studies The antibacterial activity of NiO NPs was screened against Gram positive bacteria NCIM-5022 and Gram negative bacteriaNCIM-5051 through the Agar well diffusion method [43]. The bactericidal activity of NiO NPs was tested in Nutrient Agar (NA) media, the NA plates were prepared using 28 gm of NA media. Then, it was dissolved in 1000 mL of double distilled water and subjected to pasteurization at 121˚C with pressure of 15 lbs during 15-20
minutes. NA plates with 100 µl of 24 hours mature broth culture of each individual bacterial strains were prepared and swabbed using a sterile L-shaped glass rod. In each petri - plate 6 mm wells were made using a sterile cork bore. The NiO NPs were dispersed in sterile double distilled water and loaded onto the well. The zone of inhibition (ZOI) was measured after the incubation of NA plates for 24 hours at 37˚C [23, 24].
2.6. Fabrication of the electrode for electrochemical sensing Prior to fabrication, the glassy carbon electrode was uniformly polished using an alumina slurry on polishing pads to get a mirror like shiny surface. To remove physically adhered impurities on the surface of the electrode, it was washed and ultrasonicated with double distilled water and ethanol respectively for 15 minutes. Modification of the surface of the bare glassy carbon electrode was carried out by drop coating 10 µL of a NiO NPs dispersed solution (1 mg/mL). The modified electrode was dried at room temperature and used as it is in further experiments. The electrocatalytic behaviour of the NiO modified glassy carbon electrode was evaluated by using the CH Instrument with a three electrode configuration comprising of the NiO particles modified glassy carbon electrode as the working electrode, a platinum disc electrode as a counter electrode and saturated Ag/AgCl electrode as a reference electrode [25].
3. Results and discussion 3.1. Structural and morphological analysis The diffractogram of green synthesized NiO NPs is depicted in Fig.1 (a) The XRD peaks coincide with the rhombohedral structure and match well with the standard value of JCPDS (No. 22-1189), with lattice parameters (a=2.954, c=7.236) and Space group R-3m 166. From the XRD pattern, it was confirmed that NiO NPs exhibited a crystalline nature with no impurity peaks. The crystallite size of NiO NPs was estimated using the DebyeScherer’s formula [32]: =
0.9λ
(2)
where, ‘D’ is the crystallite size of synthesized NPs, ‘λ’ is the wavelength of X-ray radiation (1.54 Å), ‘β’ is the full width at half maximum (FWHM) of the diffraction peak and ‘θ’ is Bragg’s diffraction angle. The average crystallite size of NiO NPs was found to be 31nm.
In fig Fig.1 (b) EDAX report confirms the elemental composition of Ni and O. The SEM micrographs (Fig.1 (c,d) show the agglomeration with irregularly shaped nanoparticles. The TEM micrograph (Fig.2 (a)) confirms that sizes of crystallites are in the range of about 10–30 nm which is in good agreement with the estimated value of XRD the analysis. Fig.2 (b-c) represent the HR-TEM micrographs that show particles in hexagonal and rhombohedral shape with interplanar spacing of 0.21 nm. The SAED pattern depicted in Fig. 2(d) indicates the presence of (111) (200) and (220) planes of the synthesized rhombohedral NiO NPs.
Fig.1. (a) XRD pattern (b) EDAX spectrum (c, d) SEM images of synthesized NiO NPs
Fig.2. (a) TEM, (b) HR-TEM images, (c) Interplanar spacing (d) SAED pattern of synthesized NiO NPs
3.2. Fourier transform infrared spectroscopy analysis The FT-IR spectrum of NiO NPs is shown in Fig.3.The spectrum is scanned in the range 400-4000 cm-1 to analyse the various functional groups. The absorption band that appeared at 3410 cm−1corresponds to (O–H) stretching of water and at 1632 cm−1 to (H–O– H) bending vibrations. The band at 1114 cm−1is due to (C–O) bonds of carbon dioxide adsorbed on the NPs surface. The bands corresponding to stretching and bending vibrations of (C–H) were observed at 2912 and 1381cm−1, respectively. In addition, the significant absorption band at 430 cm−1 is attributed to metal-oxygen (Ni–O) stretching vibrations [37].Thus, the expected structure and functional groups are confirmed by the above results.
Transmittance (%)
-1
3410 cm
-1
2912 cm
-1
1632 cm
-1
1114 cm -1
1381 cm
-1
430 cm
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
Fig.3. FT-IR spectrum of synthesized NiO NPs
3.3. Diffuse reflectance spectroscopic (DRS) analysis Fig.4 (a) shows the DRS spectrum of green synthesized NiO NPs. A blue shifted strong absorption peak is observed at 305 nm. DRS Spectral data can be used to estimate the optical energy bandgap of biosynthesised NiO NPs as shown in Fig.4 (b). The optical energy bandgap was determined using the Kubelka-Munk equation [22]: (1 − R)$ !(") = (3) 2" where, R is the reflection coefficient of the sample. From eq. (3), plot of F(R)2 vs the photon energy (eV) gives an optical energy bandgap (Eg) of 3.45 eV. Thus, nanoscale NiO exhibits directly a wide bandgap semiconductor nature.
(b)
F (R ) 2
D iffu se R e fe lc ta n c e (% )
(a)
Eg= 3.42 eV
200
300
400
500
600
700
800
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0
Wavelength (nm)
Energy (eV)
Fig.4. (a) Diffuse reflectance spectrum (DRS) (b) Optical energy band gap (Eg) of synthesized NiO NPs
3.4. Photocatalytic studies The photocatalytic behavior of green synthesized NiO NPs is assessed through the photo-degradation of the MB dye with the aid of visible annular type photoreactor under UV light irradiation. The actual trail starts when the light is irradiated and, the photon of energy is consumed by the semiconducting NiO in which the band gap is higher. Electrons and hole pairs are generated in the conduction and valence bands. If the charge carriers are not put together again, then the migration of free electrons on the surface leads to the oxygen reduction and formation of peroxides and superoxides. The newly generated holes can oxidizes water and forms OH free radicals. Such radicals are unstable and highly reactive in nature, which eventually leads to the organic dye degradation. The photocatalytic action on dyes is enhanced by factors like particle size, morphology, composition, size distribution, surface area, band gap, etc. The steady decrease in the absorption peak intensity at 663 nm by the time exposed to UV light indicates the dye degradation as shown in Fig.5. The degradation efficiency has been calculated using eq. (1). The calculatedefficiency is found to be 97.76% at 180 minutes against MB dye [21]. The degradation mechanism in dye solution is stated in the following equation (4-11). Comparable results of the degradation efficiency of MB dye with other metal oxide nanoparticles are tabulated in Table 1.
-
NiO + hv→ NiO (e cb+ h+vb) . NiO (e cb) + O2 → NiO + O2
(4) (5)
.
H2O→H++ OH
-
O2 + H
(6)
.
→ HO2
(7)
.
-
NiO (e cb) + HO2 + H+→ H2O2
(8)
+
NiO (h vb) + Dye → Degraded product HO2 + H+→H2O2 . HO2 + e →HO2
(9) (10) (11)
Absorbance (a.u.)
0 min 15 min 30 min 60 min 90 min 120 min 150 min 180 min
400
450
500
550
600
650
700
750
800
Wavelength (nm)
Fig.5. Time dependent absorbance spectrum of synthesized NiO NPs against Methylene blue dye
Table 1 Comparison of results with published data: photocatalytic activity (MB dye) with different metal oxide NPs Sl. no Photocatalyst 1 2
ZnO NPs
3 4
Ag2O NPs
5
MgO NPs
6 7 8
NiO NPs
average crystal size (nm)
% of dye degradation
references
Sol gel
30
81
[32]
Co-precipitation
23
90
[33]
Solution combustion
20
81
[25]
Solution combustion
11
84
[34]
Microwave assisted
14
88
Hydrothermal
20
92
Green Synthesis
20
97
[36]
precipitation
2-3
97
[37]
Solution combustion
31
98
Present work
Synthesis method
[35]
3.5. Antibacterial studies The antibacterial study of the synthesized NiO NPs was tested against the human pathogenic bacteria’s Staphylococcus aureus and Escherichia coli, employing the Agar well diffusion method. Generally, the antibacterial activity depends upon the reactive oxygen species (ROS), surface area, particle size, etc. NiO NPs produce ROS (hydroxyl, superoxide radical, singlet oxygen, and alpha-oxygen) through the Fenton reaction, which leads to lipid peroxidation, DNA damage and protein oxidation which can eliminate the bacteria. The zone of inhibition formed by the NiO NPs of known concentrations (500 and 1000 µg/µL) with reference to the positive control (Ciprofloxacin) is shown in Fig.6 and corresponding data are tabulated in Table 2. The antibacterial activity of NiO NPs shows a significant inhibition to both bacterial strains compared to standard antibiotic Ciprofloxacin [25].
Fig.6. Antibacterial activity of NiO NPs against E.coli and S.aureus bacterial strains (S) Standard antibiotic (C) control (a) 500µg/mL (b) 1000µg/mL
Table 2 Antibacterial activity of synthesized NiO NPs Bacterial strains Escherichia coli Staphylococcus aureus (mean ± SE) (mean ± SE) Ciprofloxacin 10 µg/mL 9.26±0.28 14.13 ± 0.67 NiO NPs 500 µg/mL 2.95± 0.48 4.63±0.41 1000 µg/mL 6.14±0.37 7.86±0.52 Values are the mean ±SE of inhibition zone in mm. Sample
Treatment Concentration
3.6. Electrochemical investigation of NiO nanoparticles The initial electrochemical characterization of the NiO nanoparticles modified glassy carbon electrode surface was carried out by using the most powerful electrochemical techniques such as cyclic voltammetry (CV). The redox activity of the NiO nanoparticles modified electrode was studied in the presence of a standard redox standard potassium ferricyanide solution. From the voltammogram in Fig.7, it is observed that the ∆E value of 136 mV for NiO NPs modified electrode (peak b) shows a better redox activity with increased current density than the bare glassy carbon electrode with ∆E value of 263 mV (peak a). The decrease in peak potentials has increased effect on conductivity. This increased activity might be attributed to the high surface area provided by the nanoparticles in comparison to the bare glassy carbon electrode [27].
45 30
b
Bare GCE NiO/GCE
a
Current (µ A)
15 0 -15 -30 -45 -0.2
0.0
0.2
0.4
0.6
Potential (V) Fig.7. Overlaid Cyclic voltammograms at (a) bare (b) NiO NPs modified electrode in presence of a potassium ferricyanide solution and 0.1 M KCl as supporting electrolyte. Scan rate: 50 mV/s. The NiO NPs modified electrode was utilized to investigate its electrocatalytic property in the electro oxidation of nitrite. The voltammograms at modified interface were recorded in the presence of a nitrite in acetate buffer of pH 4 at the scan rate of 50 mV/s. From Fig.8, it is clear that the NiO nanoparticles modified electrode in the absence of nitrite did not show any redox signature (peak c) suggesting that the modified electrode is inactive in absence of nitrite under the potential window studied. However, in the presence of nitrite the modified electrode showed an enhanced current response responsible for the electro oxidation of nitrite with potential at 0.93 V (peak a) in comparison to the unmodified electrode at 1.03 V (peak b). The observed results illustrate the electrocatalytic behaviour of the modified electrode towards the electro oxidation process. Hence, the NiO NPs modified electrode can be used in the electrochemical quantification of nitrite at trace level.
16
a Bare GCE in presence of nitrite NiO/ GCE in presence of nitrite NiO/ GCE in absence of nitrite
Current (mA)
12
b
8 c
4
0 0.2
0.4
0.6
0.8
1.0
1.2
Potential (V)
Fig.8. Overlaid Cyclic voltammograms at a) bare, b) NiO NPs modified electrode in presence and c) absence of nitrite in acetate buffer and 0.1 M KCl. As presented Fig.S1 (a) (in ESI), with increasing scan rate from 10 to 300 mV/s the anodic peaks were shifting towards more positive potentials with increase in peak current response with R2= 0.98 showing that the process of nitrite oxidation at NiO NPs modified electrode is a diffusion controlled process.
3.7. Optimization of experimental parameters Owing to the excellent analytical sensitivity and resolved responses of the differential pulse voltammetry (DPV) technique over cyclic voltammetry, the experimental parameters were optimized. The factors which affect the analytical responses such as pH, deposition potential, deposition time and the concentration were varied and their effect on the current responses were studied. The optimized parameters are as follows- pH:4, deposition potential:0.4 V and deposition time:15 seconds. All the graphs are depicted in Fig.S1 (b-d) (in ESI).
3.8. Calibration plot and linearity The determination of nitrite has been done using differential pulse voltammetry (DPV) due to its high current sensitivity and better resolution compared to cyclic
voltammetry. Hence, under the optimized experimental conditions, the performance of the NiO NPs modified electrode on increasing nitrite concentration has been studied as shown in Fig.9. The anodic peak currents linearly increase with the successive addition of nitrite in the concentration range 8 - 1700 µM with linear regression co- efficient of 0.998. The detection limit (3σ) was found to be 1.2 µM. These results convey that the NiO nanoparticles modified electrode can act as a novel non-enzymatic sensor in trace level quantification of nitrite.
16 10 9
Current (µµ A)
8
12
7 6 5
Current (µ A)
4 3 100
200
300
400
500
600
Concentration (µµM)
8
4
0 0.4
0.6
0.8
1.0
1.2
Potential (V)
Fig.9. Overlaid differential pulse voltammograms at NiO NPs modified electrode with increasing nitrite concentration in an acetate buffer under optimized conditions. Inset – calibration plot of the peak current versus concentration
3.9. Stability of the modified electrode The stability of the modified electrode was studied by continuously recording the responses at the modified electrode up to 10 cycles as depicted in Fig.S5 and S6 (ESI). The modified electrode showed significant analytical responses responsible for the electro oxidation of nitrite even after 10 cycles. However, the peak current density decreased which might be due to an oxide layer formation on the electrode surface [31]. This reveals that the modified electrode is very stable and can be used in the continuous monitoring of nitrite. The modified electrode showed excellent analytical performance in comparison to other reported nitrite sensors and is given in Table 3.
Table 3 Comparison of reported values with other modified electrodes Modifier
Cu/MWCNTs/GC SPCE/anodized/ CuAgNP poly(4-aminobenzoic acid/o-toluidine) (4AB/OT)/CPE (AuNPs/MoS2/GN) NiO/GCE
Linearity range (µM)
Limit of detection (µM)
Reference
5 - 1260
1.8
[38]
20 - 370
11.1
[39]
Amperometry
6–600
3.5
[40]
Amperometry Differential pulse voltammetry
5.0 - 5000
1.0
8-1700
1.2
[41] Present work
Technique Differential pulse voltammetry Hydrodynamic chronoamperometry
Conclusion In this study, NiO NPs have been synthesised through a solution combustion method using Calotropis gigantea leaves extract as a fuel. NiO NPs and were characterised using XRD, SEM with EDAX, HR-TEM with SAED and FT-IR spectroscopy. The synthesised NiO NPs were utilized to study their diversified applications in dye degradation, anti-bacterial activity and in electrochemical sensing. The X-RD pattern confirms the rhombohedral structure of NiO NPs with a particle size in the range 10-30 nm. The EDAX spectrum confirms the presence of Ni and O as major elements in its elemental composition. The NiO NPs exhibited very good photocatalytic activity in the degradation of methylene blue dye. The anti bacterial activity studies revealed that the nanoparticles have good ability to inhibit the growth of E.coli and S.aureus pathogens. The electrochemical investigation of the NiO NPs modified electrode depicts an excellent electro catalytic behaviour in the quantification of nitrite at trace level in comparison to the bare electrode. The modified electrode showed wide linearity in the concentration range 8 – 1700 µM with a detection limit of 1.2 µM, which allows the exploration of NiO NPs as a novel non-enzymatic nitrite sensor for biological applications.
Acknowledgments Dr. G. Nagaraju thanks the DST-Nano mission (SR/NM/NS-1262/2013) Govt of India, New Delhi for providing characterization techniques and also the VGST, Govt of Karnataka (CISEE-VGST/GRD-531/2016-17) for UV-DRS studies. Rajith Kumar C R thanks the Department of Biotechnology, GM Institute of Technology, Davangere and Siddaganga Institute of Technology, Tumakuru for providing lab facility.
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Highlights •
Nickel oxide nanoparticles (NiO NPs) were synthesized using Calotropis gigantea leaves through solution combustion method.
•
The prepared NPs were characterized using FT-IR, XRD, SEM, EDAX, HR-TEM and voltammetric techniques.
•
The NiO NPs showed good performance in degradation of methylene blue dye and better anti-bacterial activity in inhibiting the growth of E. coli and S. aureus bacterial strains.
•
The NiO NPs modified electrode showed the excellent electro catalytic behavior in the electrochemical sensing of nitrite with detection limit of 1.2 µM.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
S2468-2179(20)30011-3
DOI:
https://doi.org/10.1016/j.jsamd.2020.02.002
Reference:
JSAMD 272
To appear in:
Journal of Science: Advanced Materials and Devices
Received Date: 15 October 2019 Revised Date:
5 February 2020
Accepted Date: 11 February 2020
Please cite this article as: C.R Rajith Kumar, V.S Betageri, G Nagaraju, G. Pujar, B.P Suma, M.S Latha, Photocatalytic, nitrite sensing and antibacterial studies of facile bio-synthesized nickel oxide nanoparticles, Journal of Science: Advanced Materials and Devices, https://doi.org/10.1016/ j.jsamd.2020.02.002. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.
Photocatalytic, nitrite sensing and antibacterial studies of facile bio-synthesized nickel oxide nanoparticles Rajith Kumar C R1, Virupaxappa S Betageri1, G Nagaraju2, G H Pujar3, Suma B P4, Latha M S1* 1
Research Centre, Department of Chemistry, G M Institute of Technology, Davangere, Karnataka, India-577006 2 Energy Materials Research Laboratory, Department of Chemistry, SIT, Tumakuru, Karnataka, India-572103 3 Research Centre, Department of Physics, G M Institute of Technology, Davangere, Karnataka, India-577006 4 Department of Chemistry, Bangalore University, Central College Campus, Bengaluru, India-560001 *Corresponding Author: Dr Latha M S Associate professor GM Institute of Technology Davangere, Karnataka India -577006 Mob: 9964428253 Email: [email protected]
Acknowledgments Dr. G. Nagaraju thanks the DST-Nano mission (SR/NM/NS-1262/2013) Govt of India, New Delhi for providing characterization techniques and also the VGST, Govt of Karnataka (CISEE-VGST/GRD-531/2016-17) for UV-DRS studies. Rajith Kumar C R thanks the Department of Biotechnology, GM Institute of Technology, Davangere and Siddaganga Institute of Technology, Tumakuru for providing lab facility.
Photocatalytic, nitrite sensing and antibacterial studies of facile bio-synthesized nickel oxide nanoparticles Abstract: In the present work, Nickel oxide nanoparticles (NiO NPs) were synthesized using leaves extract of Calotropis gigantea through a solution combustion method. The NiO NPs were characterized through analytical techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FT-IR). The XRD results revealed rhombohedral structured crystallites with average size of 31 nm. SEM and TEM images indicate that the nanoparticles are agglomerated with an asymmetrical shape. The optical energy bandgap of 3.45 eV was estimated using UV-diffused reflectance spectroscopy (UV-DRS). The synthesized NiO NPs have shown superior photodegradation for methylene blue (MB) dye. Further, the antibacterial activity of the prepared nanoparticles was tested against E.coli and S.aureus bacterial strains. In addition, nanoparticles were utilized for electroanalytical applicability as a novel non-enzymatic sensor in the trace level quantification of nitrite. The proposed nitrite sensor showed wide linearity in the range 8-1700 µM and good stability with a lower detection limit of 1.2 µM.
Key words: NiO nanoparticles, Calotropis gigantea, Dye degradation, Antibacterial activity, Nitrite sensing
Graphical Abstract
1. Introduction Nanoscience and nanotechnology have acquired an excellent impetus in the rapidly growing technological era by covering the basic understanding of physicochemical and biological properties in atomic/sub-atomic levels with promising applications in various fields [1]. In the last few years, various researchers investigated on transition metal oxide nanoparticles due to their increasing importance and potential applications [2]. Among all, NiO an interesting p-type, wide direct bandgap semiconductor (3.4–4.0 eV), has caught more attention owing to its key applications. Indeed, nano-sized NiO materials have gained great interest with respect to bulk NiO because of their size quantization and large surface-area ratio [3]. Due to their unique and remarkable properties NiO NPs gained significant importance in various fields, as battery cathodes/ anodes [4], catalysis [5], solar cells [6], materials for sensors [7], electrochemical super capacitors [8]. Various plants have been increasingly employed in the synthesis of nanoparticles due to their ample advantages in elimination of elaborate processes of maintaining cell cultures, cost-effectiveness and easy scale up for large-scale synthesis. During the bioproduction of NPs, plant extracts act as both reducing and stabilizing agents [9]. Kumar et. al. [10], and Vidya et. al. [11], have reported about the synthesis of Ag NPs, and ZnO NPs using leaf extract of Calotropis gigantea. In the present study, NiO NPs have been synthesized using leaves extracts of Calotropis gigantea plant. The Calotropis gigantea, also called as Arka, Madara, etc., belongs to the family of Apocynaceae and is available throughout India, especially in the dry and vast land. Various phytochemical constituents are present in different parts of the Calotropis plant, mainly in the leaves, which acts as a reducing and stabilizing agents during the synthesis of NPs. Highly toxic dyes play a major role in polluting water. These, are frequently being used in the industries like textile, food, cosmetics, paper, plastics, etc., [12]. The natural degradation of such dyes is very difficult due to their complex structure. However, recently, various semiconductor photocatalysts NiO, Cu2O, FeO, etc., have been developed to degrade the organic pollutants. [13, 14]. In the present study, the synthesized NiO NPs have been used to study the photocatalytic degradation of methylene blue dye. Various nanostructure materials have shown good antibacterial activity against human pathogens [15]. Earlier reports have demonstrated the possibility of utilization of metal oxide NPs, in particular, NiO NPs in biomedicines due to their unique therapeutic and biological properties such as adsorbing and metal ion releasing ability, cytotoxic effects and surface-area ratio [16]. Hence, the antibacterial activity of NiO NPs has been demonstrated in the present study.
In the past decades, the highly sensitive detection of ‘nitrite’ has caught increasing interest because of its harmful effect on both human health and global environment. Further, ground water pollution is rapidly increasing by ‘nitrates’ due to the anthropogenic activities [17]. The World Health Organization (WHO) recommends, the maximum limit of ‘nitrite’ should be 3 mg/L in drinking water [18], hence, it is an important task of chemists to monitor the existing levels/limits of nitrite in water and environment. Generally, the analysis of nitrites can be quantified by using various techniques such as chromatography, spectrophotometry/spectrofluorimetry,
electroluminescy
and
capillary
electrophoresis
techniques. However, some of the above quantitative techniques lack sensitivity and, high detection limits and do require extensive instrumentation. In contrast, electrochemical methods give better precision quantification over all these methods in terms of sensitivity/selectivity [19]. In a quantitative analysis, the thorough exploitation of CMEs within the field of electrochemistry and surface manipulation with selective indicator moieties is desirable to achieve the tailored properties. Such CMEs have found to be very sensitive, easy to fabricate and target specific in electrochemical applications [20]. Here, the synthesized NiO NPs have been used as a modifier molecule in the fabrication of electrode. The modifier electrode has been explored for its electroanalytical applicability as a novel non-enzymatic sensor in trace level quantification of nitrite.
2. Experimental Section 2.1. Materials All the chemicals (analytical grade) were purchased from SD-Fine Chemicals Pvt. Ltd. and Hi-media and used without any further purification.
2.2. Instrumentation and experimental methods The Crystalline nature and phase purity was identified with the aid of the X-ray diffractometer (Rigaku Smart Lab). The morphology and elemental composition of the material was examined using SEM and EDAX (Hitachi S3400n), respectively. The HR-TEM with SAED (Jeol/JEM 2100) was used to measure shape and size of the nanoparticles, respectively. The FT-IR spectrometer (Bruker alpha-P) was used to examine the functional groups. Absorption spectra were recorded with the UV–Visible spectrophotometer (Agilent technology cary-60 spectrophotometer). The diffuse reflectance spectrum was measured using the Lab India UV 3092, UV-VIS spectrophotometer. Electrochemical measurements were achieved using the CH instrument.
2.3. Synthesis of NiO NPs Freshly collected leaves of Calotropis gigantea were washed, dried and grinded well. The Soxhlet extractor with water as solvent was used for the extraction for 5 hours and the obtained extract was dried using a rotary evaporator. The combustion synthesis method was used to synthesize NiO NPs using Nickel nitrate hexahydrate (Ni (NO3)26H2O) as an oxidizer and Calotropis gigantea leaves extract as a fuel. In this process, 2 gm of the extract dissolved in 100 mL of double distilled water was, constantly stirred for 10 min to get a homogenous solution. Ni (NO3)26H2O of 0.5 M was dissolved in 10 mL of Calotropis gigantea extract and was placed in a preheated muffle furnace (400 ± 10 °C). A smouldering reaction takes place and the entire process was completed within 10 min. The obtained NiO NPs were subjected for calcinations at 500°C for 3 hours to eliminate the impurities. Until further use, the obtained product was stored in an airtight container.
2.4. Photo catalytic studies The photocatalytic studies of NiO NPs were assessed by the degradation of cationic methylene blue (MB) dye in aqueous media using a 250 W UV-light irradiation source. For the photocatalytic experiments, a visible annular photoreactor was used, which consists of cylindrical tubes with transparent interior to employ complete radiation. In this process, 50 mg of NiO NPs as a photocatalyst was added to quartz tubes of 100 mL capacity, which contains 100 mL MB solution of concentration 5 ppm. The solution was continuously air bubbled for complete mixing of the MB dye and the photocatalyst. Then, 2 mL was taken out from the above solution, the first time after 15 min and then at regular intervals of 30 minutes. The percentage of degradation of the cationic MB dye has been calculated using the Beer-Lambert law as follows [21]: % of degradation =
C − C × 100 (1) C
Where, Ci and Cf are the initial and final concentration of the dye solution, respectively.
2.5. Antibacterial studies The antibacterial activity of NiO NPs was screened against Gram positive bacteria NCIM-5022 and Gram negative bacteriaNCIM-5051 through the Agar well diffusion method [43]. The bactericidal activity of NiO NPs was tested in Nutrient Agar (NA) media, the NA plates were prepared using 28 gm of NA media. Then, it was dissolved in 1000 mL of double distilled water and subjected to pasteurization at 121˚C with pressure of 15 lbs during 15-20
minutes. NA plates with 100 µl of 24 hours mature broth culture of each individual bacterial strains were prepared and swabbed using a sterile L-shaped glass rod. In each petri - plate 6 mm wells were made using a sterile cork bore. The NiO NPs were dispersed in sterile double distilled water and loaded onto the well. The zone of inhibition (ZOI) was measured after the incubation of NA plates for 24 hours at 37˚C [23, 24].
2.6. Fabrication of the electrode for electrochemical sensing Prior to fabrication, the glassy carbon electrode was uniformly polished using an alumina slurry on polishing pads to get a mirror like shiny surface. To remove physically adhered impurities on the surface of the electrode, it was washed and ultrasonicated with double distilled water and ethanol respectively for 15 minutes. Modification of the surface of the bare glassy carbon electrode was carried out by drop coating 10 µL of a NiO NPs dispersed solution (1 mg/mL). The modified electrode was dried at room temperature and used as it is in further experiments. The electrocatalytic behaviour of the NiO modified glassy carbon electrode was evaluated by using the CH Instrument with a three electrode configuration comprising of the NiO particles modified glassy carbon electrode as the working electrode, a platinum disc electrode as a counter electrode and saturated Ag/AgCl electrode as a reference electrode [25].
3. Results and discussion 3.1. Structural and morphological analysis The diffractogram of green synthesized NiO NPs is depicted in Fig.1 (a) The XRD peaks coincide with the rhombohedral structure and match well with the standard value of JCPDS (No. 22-1189), with lattice parameters (a=2.954, c=7.236) and Space group R-3m 166. From the XRD pattern, it was confirmed that NiO NPs exhibited a crystalline nature with no impurity peaks. The crystallite size of NiO NPs was estimated using the DebyeScherer’s formula [32]: =
0.9λ
(2)
where, ‘D’ is the crystallite size of synthesized NPs, ‘λ’ is the wavelength of X-ray radiation (1.54 Å), ‘β’ is the full width at half maximum (FWHM) of the diffraction peak and ‘θ’ is Bragg’s diffraction angle. The average crystallite size of NiO NPs was found to be 31nm.
In fig Fig.1 (b) EDAX report confirms the elemental composition of Ni and O. The SEM micrographs (Fig.1 (c,d) show the agglomeration with irregularly shaped nanoparticles. The TEM micrograph (Fig.2 (a)) confirms that sizes of crystallites are in the range of about 10–30 nm which is in good agreement with the estimated value of XRD the analysis. Fig.2 (b-c) represent the HR-TEM micrographs that show particles in hexagonal and rhombohedral shape with interplanar spacing of 0.21 nm. The SAED pattern depicted in Fig. 2(d) indicates the presence of (111) (200) and (220) planes of the synthesized rhombohedral NiO NPs.
Fig.1. (a) XRD pattern (b) EDAX spectrum (c, d) SEM images of synthesized NiO NPs
Fig.2. (a) TEM, (b) HR-TEM images, (c) Interplanar spacing (d) SAED pattern of synthesized NiO NPs
3.2. Fourier transform infrared spectroscopy analysis The FT-IR spectrum of NiO NPs is shown in Fig.3.The spectrum is scanned in the range 400-4000 cm-1 to analyse the various functional groups. The absorption band that appeared at 3410 cm−1corresponds to (O–H) stretching of water and at 1632 cm−1 to (H–O– H) bending vibrations. The band at 1114 cm−1is due to (C–O) bonds of carbon dioxide adsorbed on the NPs surface. The bands corresponding to stretching and bending vibrations of (C–H) were observed at 2912 and 1381cm−1, respectively. In addition, the significant absorption band at 430 cm−1 is attributed to metal-oxygen (Ni–O) stretching vibrations [37].Thus, the expected structure and functional groups are confirmed by the above results.
Transmittance (%)
-1
3410 cm
-1
2912 cm
-1
1632 cm
-1
1114 cm -1
1381 cm
-1
430 cm
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
Fig.3. FT-IR spectrum of synthesized NiO NPs
3.3. Diffuse reflectance spectroscopic (DRS) analysis Fig.4 (a) shows the DRS spectrum of green synthesized NiO NPs. A blue shifted strong absorption peak is observed at 305 nm. DRS Spectral data can be used to estimate the optical energy bandgap of biosynthesised NiO NPs as shown in Fig.4 (b). The optical energy bandgap was determined using the Kubelka-Munk equation [22]: (1 − R)$ !(") = (3) 2" where, R is the reflection coefficient of the sample. From eq. (3), plot of F(R)2 vs the photon energy (eV) gives an optical energy bandgap (Eg) of 3.45 eV. Thus, nanoscale NiO exhibits directly a wide bandgap semiconductor nature.
(b)
F (R ) 2
D iffu se R e fe lc ta n c e (% )
(a)
Eg= 3.42 eV
200
300
400
500
600
700
800
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0
Wavelength (nm)
Energy (eV)
Fig.4. (a) Diffuse reflectance spectrum (DRS) (b) Optical energy band gap (Eg) of synthesized NiO NPs
3.4. Photocatalytic studies The photocatalytic behavior of green synthesized NiO NPs is assessed through the photo-degradation of the MB dye with the aid of visible annular type photoreactor under UV light irradiation. The actual trail starts when the light is irradiated and, the photon of energy is consumed by the semiconducting NiO in which the band gap is higher. Electrons and hole pairs are generated in the conduction and valence bands. If the charge carriers are not put together again, then the migration of free electrons on the surface leads to the oxygen reduction and formation of peroxides and superoxides. The newly generated holes can oxidizes water and forms OH free radicals. Such radicals are unstable and highly reactive in nature, which eventually leads to the organic dye degradation. The photocatalytic action on dyes is enhanced by factors like particle size, morphology, composition, size distribution, surface area, band gap, etc. The steady decrease in the absorption peak intensity at 663 nm by the time exposed to UV light indicates the dye degradation as shown in Fig.5. The degradation efficiency has been calculated using eq. (1). The calculatedefficiency is found to be 97.76% at 180 minutes against MB dye [21]. The degradation mechanism in dye solution is stated in the following equation (4-11). Comparable results of the degradation efficiency of MB dye with other metal oxide nanoparticles are tabulated in Table 1.
-
NiO + hv→ NiO (e cb+ h+vb) . NiO (e cb) + O2 → NiO + O2
(4) (5)
.
H2O→H++ OH
-
O2 + H
(6)
.
→ HO2
(7)
.
-
NiO (e cb) + HO2 + H+→ H2O2
(8)
+
NiO (h vb) + Dye → Degraded product HO2 + H+→H2O2 . HO2 + e →HO2
(9) (10) (11)
Absorbance (a.u.)
0 min 15 min 30 min 60 min 90 min 120 min 150 min 180 min
400
450
500
550
600
650
700
750
800
Wavelength (nm)
Fig.5. Time dependent absorbance spectrum of synthesized NiO NPs against Methylene blue dye
Table 1 Comparison of results with published data: photocatalytic activity (MB dye) with different metal oxide NPs Sl. no Photocatalyst 1 2
ZnO NPs
3 4
Ag2O NPs
5
MgO NPs
6 7 8
NiO NPs
average crystal size (nm)
% of dye degradation
references
Sol gel
30
81
[32]
Co-precipitation
23
90
[33]
Solution combustion
20
81
[25]
Solution combustion
11
84
[34]
Microwave assisted
14
88
Hydrothermal
20
92
Green Synthesis
20
97
[36]
precipitation
2-3
97
[37]
Solution combustion
31
98
Present work
Synthesis method
[35]
3.5. Antibacterial studies The antibacterial study of the synthesized NiO NPs was tested against the human pathogenic bacteria’s Staphylococcus aureus and Escherichia coli, employing the Agar well diffusion method. Generally, the antibacterial activity depends upon the reactive oxygen species (ROS), surface area, particle size, etc. NiO NPs produce ROS (hydroxyl, superoxide radical, singlet oxygen, and alpha-oxygen) through the Fenton reaction, which leads to lipid peroxidation, DNA damage and protein oxidation which can eliminate the bacteria. The zone of inhibition formed by the NiO NPs of known concentrations (500 and 1000 µg/µL) with reference to the positive control (Ciprofloxacin) is shown in Fig.6 and corresponding data are tabulated in Table 2. The antibacterial activity of NiO NPs shows a significant inhibition to both bacterial strains compared to standard antibiotic Ciprofloxacin [25].
Fig.6. Antibacterial activity of NiO NPs against E.coli and S.aureus bacterial strains (S) Standard antibiotic (C) control (a) 500µg/mL (b) 1000µg/mL
Table 2 Antibacterial activity of synthesized NiO NPs Bacterial strains Escherichia coli Staphylococcus aureus (mean ± SE) (mean ± SE) Ciprofloxacin 10 µg/mL 9.26±0.28 14.13 ± 0.67 NiO NPs 500 µg/mL 2.95± 0.48 4.63±0.41 1000 µg/mL 6.14±0.37 7.86±0.52 Values are the mean ±SE of inhibition zone in mm. Sample
Treatment Concentration
3.6. Electrochemical investigation of NiO nanoparticles The initial electrochemical characterization of the NiO nanoparticles modified glassy carbon electrode surface was carried out by using the most powerful electrochemical techniques such as cyclic voltammetry (CV). The redox activity of the NiO nanoparticles modified electrode was studied in the presence of a standard redox standard potassium ferricyanide solution. From the voltammogram in Fig.7, it is observed that the ∆E value of 136 mV for NiO NPs modified electrode (peak b) shows a better redox activity with increased current density than the bare glassy carbon electrode with ∆E value of 263 mV (peak a). The decrease in peak potentials has increased effect on conductivity. This increased activity might be attributed to the high surface area provided by the nanoparticles in comparison to the bare glassy carbon electrode [27].
45 30
b
Bare GCE NiO/GCE
a
Current (µ A)
15 0 -15 -30 -45 -0.2
0.0
0.2
0.4
0.6
Potential (V) Fig.7. Overlaid Cyclic voltammograms at (a) bare (b) NiO NPs modified electrode in presence of a potassium ferricyanide solution and 0.1 M KCl as supporting electrolyte. Scan rate: 50 mV/s. The NiO NPs modified electrode was utilized to investigate its electrocatalytic property in the electro oxidation of nitrite. The voltammograms at modified interface were recorded in the presence of a nitrite in acetate buffer of pH 4 at the scan rate of 50 mV/s. From Fig.8, it is clear that the NiO nanoparticles modified electrode in the absence of nitrite did not show any redox signature (peak c) suggesting that the modified electrode is inactive in absence of nitrite under the potential window studied. However, in the presence of nitrite the modified electrode showed an enhanced current response responsible for the electro oxidation of nitrite with potential at 0.93 V (peak a) in comparison to the unmodified electrode at 1.03 V (peak b). The observed results illustrate the electrocatalytic behaviour of the modified electrode towards the electro oxidation process. Hence, the NiO NPs modified electrode can be used in the electrochemical quantification of nitrite at trace level.
16
a Bare GCE in presence of nitrite NiO/ GCE in presence of nitrite NiO/ GCE in absence of nitrite
Current (mA)
12
b
8 c
4
0 0.2
0.4
0.6
0.8
1.0
1.2
Potential (V)
Fig.8. Overlaid Cyclic voltammograms at a) bare, b) NiO NPs modified electrode in presence and c) absence of nitrite in acetate buffer and 0.1 M KCl. As presented Fig.S1 (a) (in ESI), with increasing scan rate from 10 to 300 mV/s the anodic peaks were shifting towards more positive potentials with increase in peak current response with R2= 0.98 showing that the process of nitrite oxidation at NiO NPs modified electrode is a diffusion controlled process.
3.7. Optimization of experimental parameters Owing to the excellent analytical sensitivity and resolved responses of the differential pulse voltammetry (DPV) technique over cyclic voltammetry, the experimental parameters were optimized. The factors which affect the analytical responses such as pH, deposition potential, deposition time and the concentration were varied and their effect on the current responses were studied. The optimized parameters are as follows- pH:4, deposition potential:0.4 V and deposition time:15 seconds. All the graphs are depicted in Fig.S1 (b-d) (in ESI).
3.8. Calibration plot and linearity The determination of nitrite has been done using differential pulse voltammetry (DPV) due to its high current sensitivity and better resolution compared to cyclic
voltammetry. Hence, under the optimized experimental conditions, the performance of the NiO NPs modified electrode on increasing nitrite concentration has been studied as shown in Fig.9. The anodic peak currents linearly increase with the successive addition of nitrite in the concentration range 8 - 1700 µM with linear regression co- efficient of 0.998. The detection limit (3σ) was found to be 1.2 µM. These results convey that the NiO nanoparticles modified electrode can act as a novel non-enzymatic sensor in trace level quantification of nitrite.
16 10 9
Current (µµ A)
8
12
7 6 5
Current (µ A)
4 3 100
200
300
400
500
600
Concentration (µµM)
8
4
0 0.4
0.6
0.8
1.0
1.2
Potential (V)
Fig.9. Overlaid differential pulse voltammograms at NiO NPs modified electrode with increasing nitrite concentration in an acetate buffer under optimized conditions. Inset – calibration plot of the peak current versus concentration
3.9. Stability of the modified electrode The stability of the modified electrode was studied by continuously recording the responses at the modified electrode up to 10 cycles as depicted in Fig.S5 and S6 (ESI). The modified electrode showed significant analytical responses responsible for the electro oxidation of nitrite even after 10 cycles. However, the peak current density decreased which might be due to an oxide layer formation on the electrode surface [31]. This reveals that the modified electrode is very stable and can be used in the continuous monitoring of nitrite. The modified electrode showed excellent analytical performance in comparison to other reported nitrite sensors and is given in Table 3.
Table 3 Comparison of reported values with other modified electrodes Modifier
Cu/MWCNTs/GC SPCE/anodized/ CuAgNP poly(4-aminobenzoic acid/o-toluidine) (4AB/OT)/CPE (AuNPs/MoS2/GN) NiO/GCE
Linearity range (µM)
Limit of detection (µM)
Reference
5 - 1260
1.8
[38]
20 - 370
11.1
[39]
Amperometry
6–600
3.5
[40]
Amperometry Differential pulse voltammetry
5.0 - 5000
1.0
8-1700
1.2
[41] Present work
Technique Differential pulse voltammetry Hydrodynamic chronoamperometry
Conclusion In this study, NiO NPs have been synthesised through a solution combustion method using Calotropis gigantea leaves extract as a fuel. NiO NPs and were characterised using XRD, SEM with EDAX, HR-TEM with SAED and FT-IR spectroscopy. The synthesised NiO NPs were utilized to study their diversified applications in dye degradation, anti-bacterial activity and in electrochemical sensing. The X-RD pattern confirms the rhombohedral structure of NiO NPs with a particle size in the range 10-30 nm. The EDAX spectrum confirms the presence of Ni and O as major elements in its elemental composition. The NiO NPs exhibited very good photocatalytic activity in the degradation of methylene blue dye. The anti bacterial activity studies revealed that the nanoparticles have good ability to inhibit the growth of E.coli and S.aureus pathogens. The electrochemical investigation of the NiO NPs modified electrode depicts an excellent electro catalytic behaviour in the quantification of nitrite at trace level in comparison to the bare electrode. The modified electrode showed wide linearity in the concentration range 8 – 1700 µM with a detection limit of 1.2 µM, which allows the exploration of NiO NPs as a novel non-enzymatic nitrite sensor for biological applications.
Acknowledgments Dr. G. Nagaraju thanks the DST-Nano mission (SR/NM/NS-1262/2013) Govt of India, New Delhi for providing characterization techniques and also the VGST, Govt of Karnataka (CISEE-VGST/GRD-531/2016-17) for UV-DRS studies. Rajith Kumar C R thanks the Department of Biotechnology, GM Institute of Technology, Davangere and Siddaganga Institute of Technology, Tumakuru for providing lab facility.
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Highlights •
Nickel oxide nanoparticles (NiO NPs) were synthesized using Calotropis gigantea leaves through solution combustion method.
•
The prepared NPs were characterized using FT-IR, XRD, SEM, EDAX, HR-TEM and voltammetric techniques.
•
The NiO NPs showed good performance in degradation of methylene blue dye and better anti-bacterial activity in inhibiting the growth of E. coli and S. aureus bacterial strains.
•
The NiO NPs modified electrode showed the excellent electro catalytic behavior in the electrochemical sensing of nitrite with detection limit of 1.2 µM.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.