Facile green synthesis of silver oxide nanoparticles and their electrochemical, photocatalytic and biological studies

Journal Pre-proof Facile green synthesis of silver oxide nanoparticles and their electrochemical, photocatalytic and biological studies

B.N. Rashmi, Sujatha F. Harlapur, B. Avinash, C.R. Ravikumar, H.P. Nagaswarupa, M.R. Anil Kumar, K. Gurushantha, M.S. Santosh PII:

S1387-7003(19)30785-3

DOI:

https://doi.org/10.1016/j.inoche.2019.107580

Reference:

INOCHE 107580

To appear in:

Inorganic Chemistry Communications

Received date:

2 August 2019

Revised date:

11 September 2019

Accepted date:

12 September 2019

Please cite this article as: B.N. Rashmi, S.F. Harlapur, B. Avinash, et al., Facile green synthesis of silver oxide nanoparticles and their electrochemical, photocatalytic and biological studies, Inorganic Chemistry Communications (2018), https://doi.org/10.1016/ j.inoche.2019.107580

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© 2018 Published by Elsevier.

Journal Pre-proof Facile Green synthesis of Silver Oxide Nanoparticles and their Electrochemical, Photocatalytic and Biological Studies B. N. Rashmi1,2, Sujatha F. Harlapur1, B. Avinash3, C.R. Ravikumar3,*, H.P. Nagaswarupa4, M.R. Anil Kumar3, K. Gurushantha5, M.S. Santosh6* 1Department 2

of Chemistry, Vemana Institute of Technology, VTU, Bangalore 560 064, India. Department of Science, East West College of Engineering, VTU, Bangalore 560 064, India. 3 Research Centre, Department of Science, East West Institute of Technology, VTU, Bangalore-560091, India. 4 Department of Chemistry, Davanagere University, Davanagere-577001, India. 5 Department of Chemistry, Sapthagiri College of Engineering, Bangalore-560057, India. 6 Centre for Incubation, Innovation, Research and Consultancy (CIIRC), Jyothy Institute of Technology, Thataguni, Off Kanakapura Road, Bangalore - 560082, Karnataka, India

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Abstract In the recent times, cost-effective and eco-friendly processes of synthesizing

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nanoparticles (NPs) have emerged as a substitute to conventional synthetic methods. Several research groups in the past have synthesized silver oxide (Ag2O) nanoparticles from various

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plant extracts and have been extensively reported. A first of its kind facile green combustion

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method has been adopted here to synthesize Ag2O NPs using Centella Asiatica and Tridax plant powder. The unique pentacyclic and triterpene constituents of Centella Asiatica and

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tridax powder has kindled our curiosity to explore their electrochemical, photocatalytic and biological activities that may contribute to various environmentally benign industrial

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applications. The cyclic voltammetric studies show that the cathodic and anodic peak potentials for the synthesized Ag2O NPs exhibit hysteresis in the range of 0.3V to – 0.1V. A

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high percentage of degradation of acid orange 8 (AO8) dye by the synthesized Ag2O NPs offers new hope on its potential photocatalytic activity. Finally, their anti-bacterial and antifungal activities carried out against S. epidermidis and S. aureus and A. fumigates and A. aureus respectively, have shed new light on the ability of the synthesized Ag2O NPs to inhibit the growth of various disease-causing pathogens. Hence, it is believed that this research work will act as a fundamental basis to various researchers and industrialists working in the energy, water and biological fields to carry out advanced studies in the aforementioned domains.

Key words: Ag2O nanoparticles; Centella asiatica; Tridax; cyclic voltammetry; photocatalysis; antimicrobial activity *

Corresponding authors: [email protected]| [email protected]

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1. Introduction Due to the unique optical, electrochemical, photocatalytic, biological and magnetic properties of nano-sized metal particles, nanomaterials have attracted much attention of late. Specifically, in this domain, the development of nanoparticles using eco-friendly techniques has gained a greater significance. The applications of silver oxide nanoparticles are wide ranging from catalysis to sensors and therapeutics to photovoltaics [1]. Greener methods of

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synthesis of Ag2O NPs have attracted many researchers because of their cost-effectiveness

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and non-toxic nature [2, 3]. The shape and size of the nanoparticles produced by plant

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sources are highly stable and are quite popular [4]. Using eco-friendly and green processes, nanoparticles have been synthesized in the recent times using various plants such as neem, Cinnamomumcamphora,

Emblica

Officinalis,

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Alfalfa,

lemongrass,

tamarind,

and

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Euphorbiatirucalli [5]. Considering the enormous potential of green plants, the present work emphasizes on the synthesis of silver oxide nanoparticles using two important medicinal

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plants Centella Asiatica and Tridax and has investigated their electrochemical, photocatalytic

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and microbial activities. Among the chosen plants, Centella asiatica (Supplementary Fig. 1(a)), belongs to the Apiaceae family of flowering plants and is commonly known as Centella or gotu kola. These species are largely found in the Indian subcontinent, Southeast Asia and wetland regions of the US. Pentacyclic triterpenoids, asiaticoside, brahmoside, Asiatic acid and Brahmic acid being some of the prominent constituents of Centella, it has been used to treat various disorders, minor wound and to enhance lactation [6]. Similarly, Tridax (Supplementary Fig. 1(b)) hails from Asteraceae and is popular as coat buttons or Jayanthi. Tridax is commonly found in the tropical regions of North and South America. Using the aerial parts of Tridax procumbens, flavonoids and many other chemical compounds have been isolated [7].

Journal Pre-proof Taking advantage of the pentacyclic structure of these two medicinal species, we wanted to extend their applications to electrochemical, photocatalytic and biological activities [8-11]. Hence, the synthesized Ag2O nanoparticles were used to cover the electrode surface to increase the power and area of application of electrochemically modified electrodes [12, 13]. These nanoparticles immobilized on the surface can lower the overpotential, amplify the reaction rate and sensitivity, and enhance selectivity [14, 15]. Further, we tested these nanoparticles for their ability to remove the dyes from the polluted water by photocatalytic

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degradation as they are safe, non-toxic and can reduce the silver ions in the process [16].

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Also, since times immemorial, silver compounds and their salts have been used as

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antimicrobial agents and wound care products for their effective antimicrobial properties [17,

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18]. Here, we have investigated the antibacterial and anti-fungal properties of Ag2O nanoparticles and reported their efficiency.

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These studies are believed to offer new avenues for using both C. asiatica and Tridax in

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from these results.

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applications that were unknown until recently and can attract various industries to benefit

2. Materials and Methods

2.1. Synthesis of Ag2O NPs:

The dried leaves of Centella Asiatica plant were used to green synthesize Ag2O NPs by combustion. The plant leaves were subjected to drying in an autoclave and their fine powder was obtained when grounded in a mortar. Stoichiometric ratios of silver nitrate (analytical grade, 99.0% purity - Sigma Aldrich) and finely ground powder of C. Asiatica plant leaves were transferred into a silica crucible and was subjected to 5-10 minutes of stirring using a magnetic stirrer. A muffle furnace was pre-heated to 600 ± 10 oC before placing the mixture in it. The reaction yielded a highly porous white powder. The same

Journal Pre-proof procedure was used to synthesize Ag2O NPs using dried Tridax plant leaves. Then, the synthesized Ag2O NPs were subjected to various characterizations.

2.2. Preparation of modified carbon-paste electrode 70% graphite powder, 15% Ag2O and 15% silicone oil were hand mixed in a mortar for nearly 30 minutes to produce a homogeneous paste of the modified carbon-paste electrode

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(MCPE). The cavity of the working electrode was filled with the above prepared paste and

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was smoothened with a piece of butter paper [19]. 2.3. Photocatalytic activity

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Using Shimadzu’s UV-Vis spectrophotometer model 2600, photocatalytic activities

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were performed. Acid Orange-8 dye was used to evaluate the photocatalytic activity of the

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synthesized Ag2O NPs. We used a circular glass reactor in which a mercury vapour lamp acted as the UV light source, where a 20 ppm of 250 ml aqueous solution of AO-8 and 60 mg

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of the photocatalyst was subjected to continuous stirring in a magnetic stirrer. The reaction mixture was irradiated by UV light in an open air condition and was monitored in the UV-

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Visible range of 200-800 nm [20].

2.4. Antimicrobial Studies:

The standard agar well diffusion method was used to study the antimicrobial activity [21]. Gram-positive bacterial strains such as Staphylococcus aureus, Staphylococcus epidermidis and fungal strains Aspergillus aureus and Aspergillus were used in the present study. The Department of Biotechnology, Sahyadri Science College, Shivamogga, Karnataka, India was kind enough to lend the Fumigatus for our study. The obtained pure cultures were inoculated onto nutrient and potato dextrose broth respectively, and incubated at optimum conditions.

The

different

extracted

silver

oxide

nanoparticles

(0.1mg/ml)

were

Journal Pre-proof resuspended/reconstituted in DMSO. A lawn of each test bacteria on nutrient agar plates with a sterile cotton swab was created for the antibacterial studies. With the help of a sterile cork borer, the media containing plates were punched to create a well of 0.5 cm. Each well was filled with 2 ml of two different extracts of Ag2O NPs. Tetracycline (0.1mg/ml) was used as a reference standard and DMSO as a solvent control. Before their use, the plates were incubated at room temperature for 24 hrs.

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The antifungal activity was carried out by potato dextrose agar well diffusion method [21]. 2

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ml of two different extracts of Ag2O NPs were added to each well. Fluconazole (0.1mg/ml) was used as a reference standard and DMSO as a solvent control. Plates were observed after

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48 hrs of incubation at 30 °C. In both the activities, experiments were performed in triplicates

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and the inhibition zone was expressed in millimeter.

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2.5. Characterization

Shimadzu’s X-ray diffractometer (PXRD-7000) equipped with CuKα radiation (λ=1.541

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Ǻ) and a nickel filter was used to obtain the phase of the products. Similarly, Perkin Elmer Spectrophotometer (Spectrum-1000) with KBr pellets was used to record FT-IR spectra of

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the samples from 400-4000 cm-1. In order to estimate the energy band gap, Shimadzu’s UVVis spectrophotometer model 2600 was used to obtain the UV–visible absorption spectra of NPs. An electrochemical analyzer CHI608E potentiostat with a tri-electrode system was used to perform the CV and EIS measurements which consisted of modified carbon paste electrode, platinum wire and Ag/AgCl as working, counter and reference electrodes, respectively and 1.0 M KOH solution as the electrolyte.

3. RESULTS AND DISCUSSION: 3.1. Powder X-ray Diffraction (PXRD) Analysis

Journal Pre-proof The intense x-ray diffraction peaks observed at (Supplementary Fig. 2) 2θ values 32.46, 38.08, 44.26, 46.52, 64.39 and 77.36 correspond to (110), (111), (200), (211), (220) and (311) planes of Ag2O NPs. These peaks are in agreement with the standard Ag2O (JCPDS 76 - 1393) that correspond to the characteristic face-centered cubic structure of silver oxide [22]. By using the Debye-Scherrer's equation, the size of the synthesized Ag2O NPs was found to be in the range of 11-12 nm and the corresponding SEM images also supported

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3.2. Scanning Electron Microscopy (SEM) Analysis

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this fact.

The structure and morphology of the synthesized Ag2O NPs were analysed using

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SEM (VEGA3 TESCAN with a magnification of x67.6K (Accelerating voltage 25 kV)) and

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have been shown in Supplementary Fig. 3. The spherical shapes of the particles are clearly

during combustion [23].

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evident from the SEM images although agglomerated, due to the rapid evolution of gases

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3.3. Energy Dispersive X-ray analysis (EDAX)

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The EDAX analysis was carried out to confirm the presence of Ag and to estimate the percentage of Ag2O NPs present. The presence of silver in both the samples was confirmed from the peaks shown in Supplementary Fig. 4. Elemental composition showed a highintensity signal for silver (Ag) and weak signals for O & Ca atoms [24]. Moreover, from the EDAX analysis, it was observed that the sample contained 52.4% silver by weight. 3.4. FTIR analysis: The FTIR spectrum of green synthesized Ag2O NPs has been shown in Supplementary Fig. 5. FTIR Spectrum shows major peaks at 3450 cm−1, 2971 cm−1, 2392 cm−1, 1652 cm−1, 1463 cm−1, 1102 cm−1, 1049 cm−1, 806 cm−1, 694 cm−1 and 445 cm−1. The peaks at 3450 cm−1 and 2392 cm−1 correspond to O-H and C-H stretching. 1652 cm−1 and 1463 cm−1, 1423 peaks indicate the presence of C=O bond and symmetric bending of CH3

Journal Pre-proof respectively. The intense peaks at 1049 cm−1 and 1035 cm−1 are attributed to C-C and N-H vibrations. The peak at 806 cm−1 is associated with C-H bending, and the one at 694 cm−1 corresponds to C-O stretching. The formation of Ag-O was confirmed by the peaks at 492 cm−1 and 445 cm−1 [25, 26].

3.5. UV-Visible Spectral Analysis:

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The energy structures of Ag2O NPs were described using UV–Vis absorption

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spectrum. Supplementary Fig. 6 shows a sharp increase in the diffuse reflectance spectrum of

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Ag2O NPs at 360 cm-1. The Kubelka–Munk function was used to determine the energy gap

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[27]:

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(F(R) hυ)n= A(hυ-Eg) -------- (1) where h is the Planck’s constant, ν is the frequency of vibration, Eg is the energy gap, A is the

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value of reflectance.

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proportionality constant, F(R) is equivalent to the absorption coefficient, R is the absolute

The direct band gap of Ag2O was determined by plotting F(R)(hν)2 vs hν (eV) as shown in the inset of Supplementary Fig. 6. According to the graph, the optical band gap of Ag2O NPs prepared using C. Asiatica and Tridax plant extract was found to be 1.408 eV and 1.310 eV respectively, which is apparently lesser than that of bulk Ag2O (1.6 eV) [28]. 3.6. Cyclic Voltammetry Studies Cyclic voltammograms of Ag2O NPs synthesised by (a) Centella Asiatica and (b) Tridax plant extract were carried out for different scan rates from 10 to 50 mV/s. Fig. 1 shows that the scan rate has an immense effect on the peak current of Centella and Tridax. In the CV scans, a pair of strong redox peaks is due to the presence of a reduction and an

Journal Pre-proof oxidation peak [29]. Fig. 1 shows the voltammogram of Ag2O NPs with a peak at during the cathodic scan, and a minor reversible anodic peak at

0.3V

-0.1V. The hysteresis

between the main cathodic and anodic peak potentials of Ag2O NPs was

0.2V [30].

One of the measures of the redox reaction reversibility is the peak potential difference (EO – ER) between the anodic (EO) and cathodic (ER) peaks [31]. The CV data in Fig. 1 is tabulated in Supplementary Table 1, which describes the characteristics of the two electrodes. In the

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Ag2O electrode, which was prepared using Tridax plant extract, the peak potential of the

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anode (EO) and cathode (ER) were found to reduce with a consequent lowering of the potential (EO-ER) between the peaks. This behaviour suggests that the Tridax plant extract

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used during the synthesis leads to a marked development in the reversibility of the oxidation

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and reduction states as compared to C. Asiatica plant extract.

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Silver oxide undergoes charging during the oxygen evolution reaction (OER) which significantly affects the structure and charge efficiency of the electrode. The resultant oxygen

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evolution reaction (2) is specified below:

Alternatively, an important factor to moderate the reversibility is the difference (EO.E – EO) between the potential of oxygen evolution (EO.E) and the oxidation potential [32]. It was observed that there was a significant difference between the EOE and EO values for Ag2O NPs synthesized using Tridax plant extract as compared to C. Asiatica, which means that the oxygen evolution potential increases with a decrease in the oxidation potential. Here, Ag0 undergoes complete oxidation to Ag1+ allowing full charge of the electrodes, making them suitable for higher utilization with larger capacity [33]. 3.7. Electrochemical Impedance Spectroscopy (EIS)

Journal Pre-proof Fig. 2 shows the electrochemical impedance spectrum of CPE of Ag2O NPs synthesized by C. Asiatica and Tridax plant extract. EIS is the most authoritative tool to scrutinize the surface property of the electrode. The impedance behaviour of Ag2O NPs was studied in 1M KOH solution as an electrolyte. A semicircle with a large diameter was observed at Ag2O NPs synthesized using Centella extract, representing the high charge resistance of Ag2O NPs (140±5Ω) and the diameter of the semicircle diminishes appreciably for Ag2O NPs synthesized by Tridax plant extract (49 ± 5Ω). It was notable that the charge

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transfer resistance (49±5Ω) of the electrode surface decreased, while the charge transfer rate

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increased. On the whole, the impedance study suggests that electron transfer is easy on the

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surface of Ag2O NPs synthesized from Tridax plant as compared to Ag2O NPs synthesized by

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Centella [34-36].

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In Fig. 2, resistance was offered at the electrode-electrolyte interface in the region of high frequency where the solution resistance (Rs) intercepts the semicircle on the real axis of the

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Nyquist plot. Among the semicircles seen in Fig. 2, the double-layer capacitance (C) was

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connected in parallel to the interfacial charge transfer resistance (Rct) or the polarization resistance (Rp). Further, the Nyquist plot shows the Warburg element, W, by a straight line in the low-frequency region [37]. Silver ions and the electrons diffuse into the pores on the electrode surface from the electrolyte and the working electrode respectively, are represented by W. It is important to note that during the transition from high-frequency semicircle to midfrequency point, ions undergo an electrolytic diffusion [31]. As the constant phase element (Q1) and the charge-transfer resistance (Rct) are parallel to one another, the low-frequency capacitance (Q2) and the leakage resistance (Rl) also are in parallel. Here, the Nyquist plots clearly show that the high frequency semicircle is quantified by the charge transfer resistance (Rct) and double layer capacitance (C) [38, 39].

Journal Pre-proof R, Cdl, RS, R1, Q1 and Q2 are the EIS fitted circuit parameters of the prepared Ag2O electrodes obtained by fitting the experimental data to the equivalent circuit as shown in Supplementary Table 2. Moreover, the Ag2O electrode prepared using Tridax plant extract indicated a reduction in the solution and leakage resistance which is also evident from a decrease in the RCt and an increase in the Cdl values as compared to the ones prepared using Centella Asiatica plant extract. Similarly, there is an improvement in the surface electrochemical activity of Ag2O electrode as seen by a decrease in the charge transfer resistance and an increase in the

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capacitance of Ag2O NPs synthesized using Tridax plant extract [40].

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3.8. Photocatalytic study

The photocatalytic activity of Ag2O NPs synthesized by Centella and Tridax plant

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extract was evaluated by studying the photocatalytic degradation of AO-8 dye under UV light

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irradiation at room temperature for 150 min. The intensity of the absorption peaks gradually decreases under UV light irradiation with an increase in time without any change in the

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position of the peaks. Moreover, the absorbance of dye degradation is proportional to the concentration. Fig. 3 (a) and (b) shows the absorbance spectra of Ag2O NPs using different plant extracts and Fig. 3 (c) shows the percentage of degradation of dye with successive time intervals. Although the percentage of dye degradation was initially found to be low, it subsequently increased with an increase in the exposure time as evident from the result. The percentage degradation versus time intervals graph indicate that Ag2O NPs synthesised by Tridax plant shows the highest photocatalytic activity (70 %) than C. Asiatica plant at an absorbance range of 480-490 nm [41, 42]. A noteworthy observation in the photocatalysis of AO-8 under UV irradiation was that, in the photochemical process using UV light irradiation, decolorization of Ag2O NPs synthesised by

Journal Pre-proof Tridax and C. Asiatica respectively was permitted after 150 min of treatment. Based on this, a mechanism of photochemical activity of AO-8 when Ag2O was subjected to irradiation suggests that the formation of h+vb as the electron gets promoted from the VB to the CB (e-cb) of Ag2O. The material acts as a strong Lewis acid because of the presence of half-filled dorbitals with well trapped electrons in CB and slows down the recombination with holes in the VB. The population of Ag+ ions (with 4d electrons) decreases, eventually creating instability and thereby, the electrons in CB can be re-trapped and taken up by the O2

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molecules promoting the formation of O•2- which gets converted to active OH- ions. The

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following are the steps involved in the decolourization mechanism of Ag2O [32, 43, 44]:

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Ag2O + hν → Ag2O ( (e-cb + h+vb )

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Ag+ + (e-cb) → Ag0 (electron trapping)

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Ag0 + O2 → Ag+ + O2*- (electron transfer) O2*- + H+ → *OOH

OOH + H+ + e-cb → H2O2

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*

H2O2 + e-cb → *OH + OH-

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On the other hand, the photogenerated h+vb may get captured on the surface of the catalyst, transferring its charge with the OH- ions present on the surface or with surface adsorbed H2O to produce active OH* species as shown below: h+vb + H2O → H+ + *OH h+vb

+ OH- → *OH

Hence, the Ag2O NPs led to the formation of charge carriers. Subsequently, the high yield of hydroxyl ions amounts to decolorization of AO-8 dye and the proposed mechanism of the photocatalytic activity of AO-8 dye decolorization has been show in Supplementary Fig. 7. It is well known that Ag2O is responsive to light and decolorizes under UV-light. Nevertheless, it has been proposed that Ag(0) being there in Ag/Ag2O operates as an electron

Journal Pre-proof submerged and receives the conduction band electron of Ag2O, thus restraining the reduction of Ag+ and Ag2O stabilization. Conversely, the lessening of Ag+ during the processes cannot be prohibited, yielding Ag0, since no other suitable electron acceptor is available. Regardless of whether the electrons get reduced to Ag+ or stored in Ag0, Ag/Ag2O will not perform as a photocatalyst, since the material modifies irreversibly throughout the reaction [45]. This fact has been supported by our experiment, where Ag2O was subjected to visible light to test its ability to decolorize AO-8 dye. However, the results were not so encouraging as Ag2O

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showed a lower photocatalytic activity in the visible region as compared to the UV-region.

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3.9. Antimicrobial Activities

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The results of antimicrobial activity were shown by the zone of inhibition, which revealed that the antimicrobial ability of Ag2O NPs synthesized by C. Asiatica and Tridax

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plant extract depends on the tested organisms. The antibacterial activity and antifungal

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activity are shown in the below Tables 1(a) and 1(b).

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The experimental results of the antibacterial activity showed that S. epidermidis [24] was more susceptible to tetracycline, used as a standard. S. aureus (20±0.01) and S. epidermidis

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(25±0.02) were more sensitive for Ag2O NPs synthesized by Tridax. Among the Ag2O NPs synthesized by Centella Asiatica and Tridax plant extracts, Tridax extract proved to be more effective than C. Asiatica extract for the synthesis of Ag2O NPs (Table 1 (a), Supplementary Fig. 8(a)). Similarly, antifungal activity carried out for A. fumigatus (22 ± 0.10) was more sensitive to Ag2O NPs synthesised by Tridax plant, whereas A. Aureus (18 ± 0.02) showed more susceptibility to Ag2O NPs synthesized by C. Asiatica (Table 3b) and A. Fumigates [46] seemed sensitive to the standard drug fluconazole. From the above data, it is evident that the Ag2O NPs synthesized by Tridax was more effective against both bacterial and fungal

Journal Pre-proof pathogens (Table 1a & 1b; Supplementary Fig. 8(b)) and hence, can be used as an effective antimicrobial agent. 4. Conclusions Eco-friendly synthesis of metal oxide NPs has a wide range of applications in different domains of science and technology among which electrochemical, photocatalytic and biological applications have been reported here. From the analytical data, the size of Ag2O NPs was found to be 11-12 nm, although in an agglomerated form. Under UV light

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irradiation, Ag2O NPs were able to degrade AO-8 dye very well exhibiting excellent photocatalytic activity. Antimicrobial activity evaluated via agar-well diffusion assay showed

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that Ag2O NPs synthesized using Tridax plant extract emerged as the most efficient plant material among the two to be used as an antimicrobial agent. Finally, the obtained results

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suggest that the synthesized Ag2O nanoparticles have excellent electrochemical properties,

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exhibit good photocatalytic activity for AO-8 under UV light and have commendable

Acknowledgement

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antimicrobial properties.

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The authors wish to thank VGST, Govt. of Karnataka, India, (No: VGST/CISEE/201415/282) and (VGST/K-FIST-L1/2014-15/GRD-360) for extending their support to carry out

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S. Rajendrachar, B.E. Kumara Swamy, S.Reddy, D. Chaira, Synthesis of Ag2O NPs and their Applications, Anal. Bioanal. Electrochem., 5 (2013) 455 – 466 P. Singh, Y.J. Kim, D. Zhang, D.C. Yang, Review Biological Synthesis of Nanoparticles from Plants and Microorganisms, Trends in Biotechnology 34 (2016) 7. S.W. Kim, J.H. Jung, K. Lamsal, Y.S. Kim, J.S. Min, Y.S. Lee, Antifungal effects of silver nanoparticles (AgNPs) against various plant pathogenic fungi. Microbiology, 40 (2012) 53-58. S. Joshi, U. Kaushik, A. Upadhyaya, P. Sharma, Green Technology Mediated Synthesis Of Ag2O NPs From Momordica Charantia Fruit Extract And Its Bactericidal Activity, Asian J. Pharm Clin Res. 3 (2017) 196-200 R. Palani velana, P.M. Ayyasamya, R. Kathiravanb, B. Subashnic, Rapid decolorization of synthetic melanoidin by bacterial extract and their mediated Ag2O NPs as support, J. Applied Biology & Biotechnology 3 (2015) 006-011 V. Manikandan, P. Yi, P. Velmurugan, P. Jayanthi, S.C. Hong, S.H. Jang, J.M. Suh, S. Sivakumar, Production, optimization and characterization of silver oxide nanoparticles using Artocarpus heterophyllus rind extract and their antifungal activity”, African J. Biotechnology 16 (2017)1819-1825 Is Fatimah, Green synthesis of Ag2O NPs using extractof Parkia speciosa Hassk pods assisted by microwave irradiation, J. Advanced Research 7 (2016) 961–969 C.R. Ravikumar, P. Kotteeswaran, A. Murugan, V. Bheema Raju, M.S. Santosh, H.P. Nagaswarupa, H. Nagabhushana, S.C. Prashantha, M.R. Anil Kumar, K. Gurushantha, Electrochemical Studies of Nano Metal Oxide Reinforced Nickel Hydroxide Materials for Energy Storage Applications, Journal of materials today proceedings 4 (2017) 12205–12214 F. Pei, S. Wu, G. Wang, M. Xu, S.Y. Wang, L.Y. Chen, Electronic and Optical Properties of Noble Metal Oxides M2O (M = Cu, Ag and Au): First-principles Study, J. Korean Physical Society, Vol. 55, No. 3 (2009) 1243-1249 S. Rajendrachar, B.E. Kumara Swamy, S. Reddy, D. Chaira, Synthesis of Ag2O NPs and their Applications, Anal. Bioanal. Electrochem., 4 (2013) 455 – 466 M.V. Reddy, R. Jose, T.H. Teng, B.V.R. Chowdari, S. Ramakrishna, Preparation and electrochemical studies of electrospun TiO2 nanofibers and molten salt method nanoparticles, Electrochimica Acta 55 (2010) 3109–3117. C.R. Ravikumar, P. Kotteeswaran, V. Bheema raju, A. Murugan, M.S. Santosh, H.P. Nagaswarupa, S.C. Prashantha, M.R. Anil kumar, M.S. Shivakumar, Influence of zinc additive and pH on the electrochemical activities of β-nickel hydroxide materials and its applications in secondary batteries, Journal of Energy Storage, Elsevier: 9 (2017) 12–24. M.R. Anil Kumar, H.P. Nagaswarupa, C.R. Ravikumar, S.C. Prashantha, H. Nagabhushana, Aarti S. Bhatt, Green engineered nano MgO and ZnO doped with Sm3+: Synthesis and a comparison study on their characterization, PC activity and electrochemical properties, J. Physics and Chemistry of Solids 127 (2019) 127–139. C.R. Ravikumar, M.R. Anil Kumar, H.P. Nagaswarupa, S.C. Prashantha, Aarti S. Bhatt, M.S. Santosh, Denis Kuznetsov, CuO embedded β-Ni(OH)2 nanocomposite as

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Table Captions Table 1(a). Antibacterial activity of Ag2O nanoparticles synthesized by Centella Asiatica and Tridax. (b). Antifungal activity of Ag2O NPs synthesized by Centella asiatica and Tridax

Table 1(a).

of

Zone of Inhibition in mm

ro

Bacterial Strains Compounds

S. aureus

Ag2O (C. Asiatica)

18 ± 0.12

20 ± 0.06

-p

re

DMSO

25 ± 0.02

-

-

25

30

20 ± 0.01

lP

Ag2O (Tridax)

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Std (Tetracycline)

S. epidemidis

Table 1(b).

Zone of Inhibition in mm Fungal Strains

Compounds

A. aureus

A. fumigatus

Ag2O (C. Asiatica)

18 ± 0.02

16 ± 0.08

Ag2O (Tridax)

14 ± 0.14

22 ± 0.10

DMSO

-

-

Std (Tetracycline)

24(b)

25(b)

Journal Pre-proof Figure Captions Fig. 1. Cyclic Voltammogram of Ag2O sample prepared using (a) Centella asiatica (b) Tridax plant extract. Fig. 2. Nyquist plots of Ag2O electrodes prepared using Centella asiatica and Tridax plant extract and proposed equivalent circuit for Ag2O electrodes (Inset).

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Fig. 3. Absorbance spectra of Ag2O NPs a) Centella asiatica (b) Tridax plant extract using AO-8 dye photocatalyst under UV light irradiation (c) percentage of degradation of dye with successive time intervals of Ag2O NPs.

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0.0004

0.0002

(a)

10 mV/s 20 mV/s 30 mV/s 40 mV/s 50 mV/s

0.0008 0.0006

(b)

0.0004

0.0000

0.0002 0.0000 -0.0002

-0.0002

ro

-0.0004

of

Current (A)

Current (A)

0.0010

10 mV/s 20 mV/s 30 mV/s 40 mV/s 50 mV/s

0.4

0.2

0.0

-0.2

re

Potential (V)

na

lP

Fig. 1.

Jo ur

0.6

-p

-0.0006

0.6

0.4

0.2

0.0

Potential (V)

-0.2

-0.4

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-800

Centella Tridax

Z" (ohm)

-600

-400

1

0 20

40

60

80

100

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na

lP

re

-p

Fig. 2.

120

ro

Z' (ohm)

of

-200

140

160

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0.5

Absorbance (Orb. units)

0.2

0.1

0.2

0.1

0.0 450

500

360

550

380

400

70

(c)

Centella Tridax

420

ro

60 50

-p

40

re

30 20

0 0

20

lP

10

40

60

440

460

480

Wavelength (nm)

Wavelength (nm)

80

100

Time in minutes

na

400

% of Degradation

0.0 350

0.3

of

0.3

(b)

0 min 15 min 30 min 45 min 60 min 75 min 90 min 105 min 120 min 135 min 150 min

0.4

Jo ur

Absorbance (Orb. units)

0.4

(a)

0 min 15 min 30 min 45 min 60 min 75 min 90 min 105 min 120 min 135 min 150 min

Fig. 3.

120

140

160

500

520

540

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of

The Authors declares that they have no Conflict of Interest.

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Graphical abstract

0.0004

0.0002

10 mV/s 20 mV/s 30 mV/s 40 mV/s 50 mV/s

0.0008 0.0006 0.0004

Current (A)

Current (A)

0.0010

10 mV/s 20 mV/s 30 mV/s 40 mV/s 50 mV/s

0.0000

0.0002 0.0000 -0.0002

0.4

0.2

0.0

-0.2

-p

ro

Potential (V)

Antimicrobial activity

0.5

lP

0.2

0.1

0.0 350

400

450

Wavelength (nm)

500

550

na

0.3

0 min 15 min 30 min 45 min 60 min 75 min 90 min 105 min 120 min 135 min 150 min

Jo ur

Absorbance (Orb. units)

0.4

-0.0006 0.6

0.4

0.2

0.0

-0.2

-0.4

Potential (V)

SEM

re

SEM

-0.0004

0.5 0 min 15 min 30 min 45 min 60 min 75 min 90 min 105 min 120 min 135 min 150 min

0.4

Absorbance (Orb. units)

0.6

of

-0.0002

0.3

0.2

0.1

0.0 360

380

400

420

440

460

480

Wavelength (nm)

500

520

540

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Highlights Two different Silver oxide NPs synthesized by combustion method.



Electrochemical executions of Ag2O NPs were examined by CV and EIS studies.



Ag2O exhibit excellent photo catalyst prepared using Tridax as a fuel.



It is highly useful electrochemical activity and dye degradation.

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