Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Photocatalytic, nitrite sensing and antibacterial studies of facile bio-synthesized nickel oxide nanoparticles C.R Rajith Kumar a, Virupaxappa S Betageri a, G Nagaraju b, G.H Pujar c, B.P Suma d, M.S Latha a, * a Research Centre, Department of Chemistry, G M Institute of Technology, Davangere, Karnataka, 577006, India Energy Materials Research Laboratory, Department of Chemistry, SIT, Tumakuru, Karnataka, 572103, India Research Centre, Department of Physics, G M Institute of Technology, Davangere, Karnataka, 577006, India d Department of Chemistry, Bangalore University, Central College Campus, Bengaluru, 560001, India b c a r t i c l e i n f o a b s t r a c t Article history: Received 15 October 2019 Received in revised form February 2020 Accepted 11 February 2020 Available online xxx In the present work, Nickel oxide nanoparticles (NiO NPs) were synthesized using leaves extract of C 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 8e1700 mM and good stability with a lower detection limit of 1.2 mM © 2020 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: NiO nanoparticles Calotropis gigantea Dye degradation Antibacterial activity Nitrite sensing 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 (3.4e4.0 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, * Corresponding author GM Institute of Technology Davangere, Karnataka, 577006, India E-mail address: lathamschem97@gmail.com (M.S Latha) Peer review under responsibility of Vietnam National University, Hanoi 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 C gigantea plant The C 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, https://doi.org/10.1016/j.jsamd.2020.02.002 2468-2179/© 2020 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Please cite this article as: C.R Rajith Kumar et al., 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 C.R Rajith Kumar et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx 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 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, electroluminescent and capillary electrophoresis techniques However, some of the above quantitative techniques lack sensitivity and, high detection limits and 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 Experimental section 2.1 Materials All the chemicals (analytical grade) were purchased from SDeFine Chemicals Pvt Ltd and Hiemedia 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 alphaP) was used to examine the functional groups Absorption spectra were recorded with the UVeVisible 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 C gigantea were washed, dried and grinded well The Soxhlet extractor with water as solvent was used for the extraction for h 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 C gigantea leaves extract as a fuel In this process, gm of the extract dissolved in 100 mL of double distilled water was, constantly stirred for 10 to get a homogenous solution Ni (NO3)26H2O of 0.5 M was dissolved in 10 mL of C 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 The obtained NiO NPs were subjected for calcinations at 500 C for h 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 ppm The solution was continuously air bubbled for complete mixing of the MB dye and the photocatalyst Then, mL was taken out from the above solution, the first time after 15 and then at regular intervals of 30 The percentage of degradation of the cationic MB dye has been calculated using the BeereLambert law as follows [21]: % of degradation ¼ Ci À Cf  100 Ci (1) 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 bacteriaNCIM5051 through the Agar well diffusion method [22] 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 15e20 NA plates with 100 ml of 24 h mature broth culture of each individual bacterial strains were prepared and swabbed using a sterile L-shaped glass rod In each petri - plate 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 h 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 Modification of the surface of the bare glassy carbon electrode was carried out by drop coating 10 mL 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 Please cite this article as: C.R Rajith Kumar et al., 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 C.R Rajith Kumar et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Fig (a) XRD pattern (b) EDAX spectrum (c, d) SEM images of synthesized NiO NPs electrode, a platinum disc electrode as a counter electrode and saturated Ag/AgCl electrode as a reference electrode [25] 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 Debye-Scherer's formula [32]: Results and discussion 3.1 Structural and morphological analysis The diffractogram of green synthesized NiO NPs is depicted in Fig (a) The XRD peaks coincide with the rhombohedral structure and match well with the standard value of JCPDS (No 22e1189), with lattice parameters (a ¼ 2.954, c ¼ 7.236) and Space group R-3m D¼ 0:9l bcosq (2) where, ‘D’ is the crystallite size of synthesized NPs, ‘l’ is the wavelength of X-ray radiation (1.54 Å), ‘b’ is the full width at half maximum (FWHM) of the diffraction peak and ‘q’ is Bragg's Fig (a) TEM, (b) HR-TEM images, (c) Interplanar spacing (d) SAED pattern of synthesized NiO NPs Please cite this article as: C.R Rajith Kumar et al., 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 C.R Rajith Kumar et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Fig FT-IR spectrum of synthesized NiO NPs Fig Time dependent absorbance spectrum of synthesized NiO NPs against Methylene blue dye diffraction angle The average crystallite size of NiO NPs was found to be 31 nm In fig Fig (b) EDAX report confirms the elemental composition of Ni and O The SEM micrographs (Fig 1(c and d) show the agglomeration with irregularly shaped nanoparticles The TEM micrograph (Fig (a)) confirms that sizes of crystallites are in the range of about 10e30 nm which is in good agreement with the estimated value of XRD the analysis Fig 2(b and 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 3410 cmÀ1corresponds to (OeH) stretching of water and at 1632 cmÀ1 to (HeOeH) bending vibrations The band at 1114 cmÀ1is due to (CeO) bonds of carbon dioxide adsorbed on the NPs surface The bands corresponding to stretching and bending vibrations of (CeH) were observed at 2912 and 1381 cmÀ1, respectively In addition, the significant absorption band at 430 cmÀ1 is attributed to metaleoxygen (NieO) stretching vibrations [37].Thus, the expected structure and functional groups are confirmed by the above results 3.2 Fourier transform infrared spectroscopy analysis Fig (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 (b) The optical energy bandgap was determined using the KubelkaeMunk equation [22]: The FT-IR spectrum of NiO NPs is shown in Fig The spectrum is scanned in the range 400e4000 cmÀ1 to analyse the various functional groups The absorption band that appeared at 3.3 Diffuse reflectance spectroscopic (DRS) analysis Fig (a) Diffuse reflectance spectrum (DRS) (b) Optical energy band gap (Eg) of synthesized NiO NPs Please cite this article as: C.R Rajith Kumar et al., 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 C.R Rajith Kumar et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Table Comparison of results with published data: photocatalytic activity (MB dye) with different metal oxide NPs Sl Photocatalyst Synthesis method No % of dye references average degradation crystal size (nm) 30 23 20 11 14 20 20 2e3 31 ZnO NPs Sol gel Co-precipitation Solution combustion Solution combustion Microwave assisted Hydrothermal Green Synthesis precipitation Solution combustion Ag2O NPs MgO NPs NiO NPs 81 90 81 84 88 92 97 97 98 [32] [33] [25] [34] [35] [36] [37] Present work Bold signifies the current work details/data compared to published data FRị ẳ Rị 2R (3) 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 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 The degradation efficiency has been calculated using eq (1) The calculated efficiency is found to be 97.76% at 180 against MB dye [21] The degradation mechanism in dye solution is stated in the following equations (4e11) Comparable results of the degradation efficiency of MB dye with other metal oxide nanoparticles are tabulated in Table NiO ỵ hv / NiO (e-cb ỵ hỵvb) (4) NiO (e-cb) ỵ O2 / NiO ỵ O2 (5) H2O / Hỵ ỵ OH (6) O2 ỵ H / HO2 (7) NiO (e-cb) ỵ HO2 þ Hþ / H2O2 (8) NiO (hþvb) þ Dye / Degraded product (9) HO2 ỵ Hỵ / H2O2 (10) HO2 þ eÀ / HO2- (11) 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- Table Antibacterial activity of synthesized NiO NPs Treatment Bacterial strains Sample Concentration Escherichia coli (mean ± SE) Staphylococcus aureus (mean ± SE) Ciprofloxacin NiO NPs 10 mg/mL 500 mg/mL 1000 mg/mL 9.26 ± 0.28 2.95 ± 0.48 6.14 ± 0.37 14.13 ± 0.67 4.63 ± 0.41 7.86 ± 0.52 Values are the mean ± SE of inhibition zone in mm Fig Antibacterial activity of NiO NPs against E.coli and S.aureus bacterial strains (S) Standard antibiotic (C) control (a) 500 mg/mL (b) 1000 mg/mL Please cite this article as: C.R Rajith Kumar et al., 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 C.R Rajith Kumar et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Fig 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 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 mg/mL) with reference to the positive control (Ciprofloxacin) is shown in Fig and corresponding data are tabulated in Table The antibacterial activity of NiO NPs shows a significant inhibition to both bacterial strains compared to standard antibiotic Ciprofloxacin [25,26] 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 Fig 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 standard redox standard potassium ferricyanide solution From the voltammogram in Fig 7, it is observed that the DE 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 DE 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 [27e30] 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 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 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 ¼ 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 Fig Overlaid differential pulse voltammograms at NiO NPs modified electrode with increasing nitrite concentration in an acetate buffer under optimized conditions Insetecalibration plot of the peak current versus concentration Please cite this article as: C.R Rajith Kumar et al., 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 C.R Rajith Kumar et al / Journal of Science: Advanced Materials and Devices xxx (xxxx) xxx Table Comparison of reported values with other modified electrodes Modifier Technique Linearity range (mM) Limit of detection (mM) Reference Cu/MWCNTs/GC SPCE/anodized/CuAgNP poly (4-aminobenzoic acid/o-toluidine) (4-AB/OT)/CPE (AuNPs/MoS2/GN) NiO/GCE Differential pulse voltammetry Hydrodynamic chronoamperometry Amperometry Amperometry Differential pulse voltammetry 5e1260 20e370 6e600 5.0e5000 8e1700 1.8 11.1 3.5 1.0 1.2 [38] [39] [40] [41] Present work Bold signifies the current work details/data compared to published data potential:0.4 V and deposition time:15 s All the graphs are depicted in Fig.S1 (b-d) (in ESI) detection limit of 1.2 mM, which allows the exploration of NiO NPs as a novel non-enzymatic nitrite sensor for biological applications 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 The anodic peak currents linearly increase with the successive addition of nitrite in the concentration range 8e1700 mM with linear regression co-efficient of 0.998 The detection limit (3s) was found to be 1.2 mM These results convey that the NiO nanoparticles modified electrode can act as a novel non-enzymatic sensor in trace level quantification of nitrite 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 Conclusion In this study, NiO NPs have been synthesised through a solution combustion method using C gigantea leaves extract as a fuel NiO NPs and were characterised using X-RD, 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 10e30 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 8e1700 mM with a 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 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/GRD531/2016e17) 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 Appendix A Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jsamd.2020.02.002 References [1] V.K Prashant, Photophysical, photochemical and photocatalytic aspects of metal nanoparticles, J Phys Chem B 106 (2002) 7729e7744 [2] K.C Patil, S.T Aruna, S Ekambaram, Combustion synthesis, Curr Opin Solid State Mater Sci (1997) 158e165 [3] L Brus, Capped nanometer silicon electronic materials, Adv Mater (1993) 286e288 [4] F Zheng, S Xu, Y Zhang, Facile fabrication of hierarchically porous NiO microspheres as anode materials for lithium ion batteries, J Mater Sci Mater Electron 27 (2016) 3576e3582 [5] M Hassanpour, H Safardoust-Hojaghan, M Salavati-Niasari, J Mater Sci Mater Electron (2017), https://doi.org/10.1007/s10854-017-6860-3 [6] X Xu, Z Liu, Z Zuo, M 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Rajith Kumar et al., 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... Rajith Kumar et al., 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... pattern of synthesized NiO NPs Please cite this article as: C.R Rajith Kumar et al., Photocatalytic, nitrite sensing and antibacterial studies of facile bio- synthesized nickel oxide nanoparticles,