ZnO nanoparticles for better thermal, photocatalytic and antibacterial activity

Accepted Manuscript Synthesis of bio-surfactant based Ag/ZnO nanoparticles for better thermal, photocatalytic and antibacterial activity

S. Rajaboopathi, S. Thambidurai PII:

S0254-0584(18)30992-1

DOI:

10.1016/j.matchemphys.2018.11.034

Reference:

MAC 21118

To appear in:

Materials Chemistry and Physics

Received Date:

24 October 2017

Accepted Date:

14 November 2018

Please cite this article as: S. Rajaboopathi, S. Thambidurai, Synthesis of bio-surfactant based Ag /ZnO nanoparticles for better thermal, photocatalytic and antibacterial activity, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys.2018.11.034

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Synthesis of bio-surfactant based Ag/ZnO nanoparticles for better thermal, photocatalytic and antibacterial activity S. Rajaboopathi, S. Thambidurai* Bio-nanomaterials Research Lab, Department of Industrial Chemistry, School of Chemical Sciences, Alagappa University, Karaikudi-630003, Tamil Nadu, India. Abstract In recent years, synthesis of nanoparticles using natural materials is the essential constituent in the medicinal and clinical laboratory due to inexpensive, recyclable and without use of toxic chemicals. In this present work, SW-Ag/ZnO nanoparticles were synthesized by simple in situ chemical precipitation approach. Padina gymnospora was taken as a bio-surfactant material for preparing Ag nanoparticles and these are further intercalated with ZnO particles. The synthesized nanoparticles were charecterized and confirmed by Fourier transform infra-red spectroscopy, X-ray diffraction analysis, UV-Visible Diffuse reflectance spectroscopy, High resolution scanning electron microscopy and Transmission electron microscopy techniques. The SW-Ag/ZnO was appeared in large clusters with different shaped particles, the size of them was obtained in 14-40 nm. Seaweed capped Ag/ZnO particles showed the greater UV protection values achieved the 45.2% than RSW and SW-Ag. Thermal degradation characteristics of SW-Ag/ZnO have greater thermal stability than RSW and SW-Ag nanoparticles. Moreover, the photocatalytic degradation performances of nanoparticles were tested under the sunlight irradiation using phenol pollutants. Their photocatalytic degradation reaction was well fitted with the first order reaction kinetics. In addition, the antimicrobial activity of nanoparticles was also tested against (Gram (+ve) and Gram (-ve) microbial pathogens and it could be exhibited the higher bacterial inhibition activity obtained for SW-Ag/ZnO than that of RSW and SW-Ag nanoparticles. 1

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Keywords: Biomaterials, Spherical structure, Ag/ZnO nanoparticles, Thermal stability, Antibacterial activity. *Corresponding author Tel/Fax: +914565228836 Email: [email protected] (S.Thambidurai) 1. Introduction Marine algae are the important and inexpensive constituent existing in all ecological units on earth [1]. Marine algae extract has a wide variety of bioactive components and different health benefits. These were due to the presence of many functional binding sites, such as sulphonate, carboxyl, amine and hydroxyl groups in their structural edges [2, 3]. Furthermore, these can be existed with the good anti-diabetic and nutritional properties, which are extensively used in various applications, such as anti-tumor, anticoagulant, anti-thrombosis and antiviral agents and are applied in food ingredients or natural health products [4-7]. In the earlier report, polysaccharide constituents were isolated separately from the seaweed species, which is more useful in antioxidant and anti-inflammatory agents [8]. Photocatalytic degradation experiment carried by using Congo Red and Direct Brown 95 dyes with use of silver nanoparticles photocatalyst [9], it can be synthesized by using Padina tetrastromatica species. Seaweed capping agent mediated synthesis of Ag/TiO2 nanocomposites was formerly studied by Jegadeeswaran et al. [10]. Silver nanoparticles is one of the attractive inorganic materials, it can be synthesized by reducing the Ag-ions by many methods, such as chemical reduction [11], reduction in solutions [12], thermal decomposition and biological or greener route [13]. Several methods have been proposed to synthesize silver nanoparticles using different reducing agents such as ascorbic acid, hydrazine, dimethyl formamide and sodium borohydride [14]. These all are useful methods for obtaining the particles with different shape and property to the diverse applications. 2

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Zinc oxide (ZnO) nano-crystalline material has the potential exciting properties of bio compatibility, ultraviolet protection, biosafety, high refractive index and antimicrobial activity with new potential applications in sensors, solar cells and electric materials [15, 16]. Recent years ZnO based composite modified cotton fabric possess excellent UV protection, when compared with untreated one due to the incorporation of ZnO component [17]. In currently, mixed oxides such as N doped TiO2, Ag-ZnO, Ag2S-ZnO, CuS and Fe2O3 have developed significantly as a photocatalyst and antibacterial agents [18-25]. To increase the ZnO contents is more effective to increase the antibacterial activity of the composite materials, which is studied by Alswat et al., [26]. Many efforts have been taken to enhance the antibacterial efficiency of ZnO using various metal dopants, which was reported by Adhikari et al, Rokesh et al, Gupta et al, [27-30] and other reported articles based on the materials of Ag, ZnO and some twodimensional materials is more important for rapid photo-induced disinfection fields [31-35]. Based on the above knowledge, there are no reports available for the bio-surfactant based synthesize of Ag/ZnO nanoparticles and studied their thermal, UV protection property, photocatalytic and antibacterial activity. In this present work, we are the synthesize Ag nanoparticles in greener approach and these were intercalated with ZnO particles, role of seaweed as the bio-surfactant component, silver nitrate, zinc nitrate and sodium hydroxide as the precursors. The synthesized nanoparticles was characterized and confirmed by various instrumentation techniques. Thus synthesized nanoparticles can be utilized to analyze the UV protecting and photo catalytic activity. Its bactericidal activity was investigated by using S.aureus (Gram positive) and E.coli (Gram negative) pathogens.

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2. Experimental details 2.1. Materials Silver nitrate (99%) was purchased from Nice Chemicals Pvt. Ltd., Kochi, Kerala. Zinc nitrate hexahydrate (95%) and Sodium hydroxide (98%) were purchased from Fisher Chemic Ltd, Chennai; the seaweeds of Padina gymnospora (brown) were collected from mandapam coastal area in Gulf of manner, south India. The freshly collected seaweeds were washed with tap water and then with Millipore water to eliminate all the extraneous materials. Then the sample was dried under air shade. Finally the dried samples were ground to fine powder and stored in an air tight container for future analysis. 2.2. Synthesis of SW-Ag/ZnO nanoparticles About 0.5g of seaweed powder was poured into 50 mL of millipore water was taken in a 250 mL beaker. The resultant solution was allowed to stir for 4 h under the magnetic stirrer at 80˚ C, after that it was allowed to stand for 30 minutes at room temperature, the extract was sonicated for 30 min using ultrasonic cleaner bath and filtered through the whatmann 41 filter paper. Finally obtained the pale green colored solution was taken for further process. 25 mL of 0.05M AgNO3 solution was poured into the above prepared extract; then it was continuously stirred at 60 °C. The solution mixture was gradually changed from light brown to dark brown colour. Which indicates the reduction of silver ion; after the complete formation of silver nanoparticles, the sample was designated as SW-Ag nanoparticles. In addition, 25 mL of Zn(NO3)2 (0.2M) solutions was slowly added to the above synthesized SW-Ag colloidal solution under constant stirring at 60 ˚C. The colour was begun to decrease from dark brown to light brown colour, owing to the interaction of SW biomolecule on the Zn2+ ion. Then it was slowly added with freshly prepared (0.4M) NaOH solution, brownish white coloured precipitate was regenerated. The 4

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obtained product was left undisturbed for 24 hours; the resultant precipitate was washed more times with Millipore water, then it was dried in a hot air oven at 110 ˚C for 5h. The sample was designated as SW-Ag/ZnO nanoparticles. The schematic representation of the formation of SWAg/ZnO nanoparticles was explained in scheme.1. Raw seaweed (RSW) powder was taken for the comparison purpose. 2.3 Characterization Thus synthesized SW-Ag/ZnO nanoparticles was characterized and confirmed by Fourier Transform Infrared spectroscopy (FT/IR- 4600 type A, Detector-TGS), X-ray diffraction (XRD) patterns were carried out by using X-ray diffractometer (model XPERT-PRO) operating at 40 KV and 30 mA (Cu Kα radiation (k=1.5406 Å)) were measured in the 2θ values ranging from 10 to 80˚ with a scanning rate of 2˚/min. Surface morphological structures were analyzed by using High Resolution Scanning Electron Microscope (FEI quanta FEG 250 instrument operated in an accelerating voltage at 20kV and EDX analysis) and Transmission Electron Microscope (TEM 300 kV). UV protection activity was tested by UV-Vis/NIR spectrophotometer (JASCO model: V-670) using absorbance and transmittance mode and thermal characteristics by thermo gravimetric analyzer (Model: STA 409 PC/PG, NETZSCH), The BET surface area and pore volume were studied from nitrogen adsorption/desorption isotherm using Gemini model: 2380. 2.4. Photo catalytic degradation of phenol The photo catalytic degradation experiment was tested by using the pollutants of phenol molecule. The aqueous solution of phenol containing (0.1g/ L) concentration and 0.5 g L-1 of photocatalyst nanoparticles was taken for the experiment. Then the suspension was exposed under the direct sunlight irradiation, it was conducted on June 2017 between 12.00 PM to 2.30 PM, the intensity of sunlight radiation during the experiments was 0.4-0.6 kW/m2. For each experiment 5

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proceeding to irradiation, the suspension was magnetically stirred for 1 h in dark to ensure the adsorption–desorption equilibrium. The photo catalytic experiment was carried out with an appropriate temperature of 36-40 ˚C. At appropriate time intervals, 5 ml of the aliquots was withdrawn for each 30 min intervals, then the suspension was filtered and analyzed under UV-Vis spectrophotometer in the wavelength range of 200 to 800 nm. Volume of the mixture solution was kept constant during the entire experiment. The residual concentration of phenol solution was analyzed for each interval during the 150 min irradiation period. 2.5. UV protection study Transmittance behavior of RSW, SW-Ag and SW-Ag/ZnO samples were tested by DRSUV visible spectrophotometer. UV radiation can photo chemically react with polymer structures or nanoparticles matrix, these can leads to the absorption of high energy UV rays [13]. UV light with the region of 280-400 (UVB) and 315-400 (UVA) radiated to the tanning of epidermis, long period emissions of the exposed rays into the sensitive skin causes the skin wrinkles, photo allergic reaction and loss of skin elasticity. The UPF (Ultraviolet Protection Factor) is one kind of measurement to calculate the average protection properties of the samples, testing was carried by using powdered form of the sample was analyzed under the spectra. The calculation of the UPF measurement was elaborately discussed in our previous chapter [36], the each test was taken in a triplicate measurement and their values are given by the standard deviation of three tests. 2.6. Testing of antibacterial assessment The agar diffusion method is a relatively quick and easy detection method to determine the antibacterial activity (Suresh et al. 2015) [37]. The bacterial inhibition test was carried with two strains of bacteria such as Gram positive (Staphylococcus aureus) and Gram negative (Escherichia coli) microbial pathogen. The bacterial culture was prepared by the Mueller-Hinton agar medium, 6

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the fresh bacterial micro organism was swapped on the nutrient agar plates and then 200 μg/ mL stock solution of RSW, SW-Ag and SW-Ag/ZnO samples was impregnated on the Muller Hinton agar plates were located on the center of microbiological substrate to get in touch with growing microbial pathogens. Then the plates were incubated for 24 h at 37 0C in the bacteriological incubator. The disc contains Amikacin antibiotic was taken as a positive control and to compared this effect to our synthesized nanomaterials. The bacterial inhibition zone was measured and recorded directly in and around the sample, the each experiment was carried in a triplicate measurement and their values were provided in standard deviation. 3. Results and discussion 3.1. HR-SEM with EDX analysis The surface morphological image of the nanoparticles was analyzed by using HR-SEM analysis. The images of RSW, SW-Ag and SW-Ag/ZnO particles were shown in Fig.1. RSW biomolecules were shown the interconnected fiber like structure; some places are appeared like a layered form, the whole structural units having multiple pores in between their fiber and layered morphology. This obtained topography is more useful for binding with other metal nanoparticles. The morphology of SW-Ag (Fig.1b) revealed the spherical shape and some places showing the agglomerated clusters are appeared on their surface; the obtained image is seemed like a ball shaped appearance. The image of SW-Ag/ZnO (Fig.1c) shown the surface having agglomerated spherical and undefined sized particles was presented on their SW matrix. EDX spectra of SW-Ag/ZnO particles shown in (Fig.1d) confirm the presence of elemental peaks of C, N, O, Ag and Zn with the presence of other elemental peak of Ca is detected; these may be come from the source materials

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of SW component. Those obtained elemental signals were confirmed the existence of biocomponents, silver and zinc on the hybrid nanoparticles. 3.2. TEM analysis The surface structural features of nanoparticles were further examined by TEM analysis. Fig.2 showed the TEM images of RSW, SW-Ag and SW-Ag/ZnO nanoparticles, the surface of RSW component were shown the layer like structure and some places appeared the hollow space in between the layered morphology. However, the image of SW-Ag exhibited the uneven sized spherical shaped clusters was obtained on the surface of layered SW matrix. Whereas the appearance of SW-Ag/ZnO is different from SW-Ag, the particles are appeared in various shaped large clusters. In addition, the structures of the particles are shown in spherical, hexagonal and grain size. TEM image of SW-Ag and SW-Ag/ZnO with higher magnification was additionally included in (Fig. 2c and e), respectively, this image was marked the particle size in nm scale, the particle size of SW-Ag and SW-Ag/ZnO materials was obtained to be in 6-20 nm and 9-25 nm range, respectively. In figure.2f shows the SAED pattern of SW-Ag/ZnO hybrid material, the appeared concentric rings are corresponds to (101) and (111) crystalline orientation of Ag and ZnO particles [38]. Thus the results are more accordance with HR-SEM analysis results. 3.3. FTIR spectroscopy The functional group of synthesized nanoparticles was studied by FTIR spectroscopy. Fig.3 showed the FTIR spectra of RSW, SW-Ag and SW-Ag/ZnO nanoparticles, the RSW molecule depicted the sharp absorption peak at 3405 cm-1 is assigned to the stretching vibration of O-H group and the slight appearance of C-H stretching vibrational peak at 2928 cm-1 [39]. The deformation band of hydroxyl group with asymmetric and symmetric stretching vibration of (C=O) 8

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group was observed at 1636 and 1488 cm-1, respectively. Furthermore, the C-O and C-C stretching vibrational band was appeared in broad and sharp peak at 1157 and 1043 cm-1 of pyranose ring [40, 41]. While the peaks of in-plane and out plane C-H bending vibration mode showed at 857 and 693 cm-1, which is the characteristic vibrational bands for mannuronic (M) and gluronic (G) units [42], respectively. The spectrum of SW-Ag NPs was appeared the all characteristic vibrational peaks of SW biomolecule and those peaks are shifted to lower wave number; which is possibly due to the reduction of Ag ion and these were stabilized with the SW molecule [36, 43]. The spectra of SW-Ag/ZnO revealed the all vibrational bands of biomolecules are appeared and their relevant peaks are shifted to higher wave number. It indicates that the strong complex formation between (SW-Ag) functional groups connecting with Zn2+ ions [29]. Such kind of vibrational shifting was previously recorded by Vidhya et al, Padiselvi et al and Rajendran et al., [44-46]. They have been synthesized the ZnO nanoparticles using natural carbohydrates were taken as the template materials. The main functional group stretching vibration of Zn-O was observed in broad intense with peak shoulder obtained at 441cm-1[47]. These results are supported with the HR-SEM analysis. In the present synthesize method, we found that the important source materials of polysaccharides, carbohydrates, proteins and polyphenolic compounds were derived from the marine macro algae, which are the significant reducing components for the nanoparticle synthesize. 3.4. X-ray diffraction analysis X-ray diffraction study was provided the crystalline phase-structural information of the nanoparticles, the diffraction images of RSW, SW-Ag and SW-Ag/ZnO nanoparticles were emerged in Fig.4. The XRD pattern of RSW wasn’t shown any apparent crystalline peaks, which indicates the biomolecules are in amorphous nature. The crystalline representation of SW-Ag 9

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nanoparticles shown the characteristic crystalline peaks at 38.10°, 44.43°, 64.43° and 77.51° are corresponds to the reflection planes of (111), (200), (220) and (311) lattice structure, these are in good agreement with the face centered cubic (FCC) geometry of standard Ag crystal (JCPDS file no: 04-0783) [36, 48, 49]. There is no observing any other extra peaks belong to SW molecule, but their obtained crystalline peak intensities are slightly suppressed due to the incorporation of SW biopolymer. But for the crystal structure of SW-Ag/ZnO nanoparticles shown the well intense crystalline peaks at 31.65°, 34.30°, 36.12°, 47.41°, 56.46° and 62.72° are matched to the plane values of (100), (002), (101), (102), (110) and (103), which are analogous to hexagonal wurzite structured ZnO nanoparticles and these are well matched to the (JCPDS data card no.36-1451) values [47]. Additionally showed the small crystalline peaks at 38.10°, 44.43°, 64.43° and 77.51° are corresponding to the plane values of (111), (200) , (220) and (311). Those obtained peaks are fine matched to the (FCC) geometry of standard Ag crystal. The crystalline peak intensity of nanoparticles was fairly suppressed due to the incorporation of biopolymer component. Such kind of characteristic crystalline peak changes was previously recorded using biomaterials as a capping molecule [44, 48]. The average crystalline size of SW-Ag and SW-Ag/ZnO nanoparticles was calculated by using Debye sherrer’s equation from the FWHM value of (111) and (101) plane values; these were estimated to be 20.5 and 31.2 nm, respectively. 3.5. DRS-UV visible spectra Diffuse reflectance UV-Vis spectrometer was employed to study the optical absorption properties of synthesized nanoparticles. Fig. 5 shows the UV-Vis absorption spectra of RSW, SWAg and SW-Ag/ZnO nanoparticles, the spectral response of RSW molecule exhibited the strong 10

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absorption in UV region with absorption maxima at 385 nm. Their absorption characteristics were further extended to visible region with the peak shoulder at 670 nm. The absorption spectrum of SW-Ag nanoparticle was obtained the optical absorption peak in both UV and visible region. The presented most important broad visible region peak centered at 435 nm, are due to the SPR band of Ag particles. This was confirmed the formation of Ag nanoparticles. The surface plasmon resonance band of SW-Ag nanoparticles has appeared in higher intensity as compared to SW component. Their strong visible absorption is greatly influenced to the photo physical and chemical properties. While, the SW-Ag/ZnO nanoparticles showed the absorption maxima are broadly covered the UV region, and their absorption peak appeared at 370 nm, besides that the initial absorption peak is red shifted as compared to SW-Ag nanoparticles. These may due to the Zn2+ ion are electro statically connected on the strong metal binding SW component [45, 50]. 3.6. UPF evaluation Fig.6. shows the UV-Visible transmittance curve of RSW, SW-Ag and SW-Ag/ZnO nanoparticles, RSW alone are able to absorb most of the UV regions with transmittance of almost covered 22% of UV-A and UV-B region, and the transmittance behavior of SW-Ag nanoparticle was obtained the peak for both UV and visible region. The emerged important broad visible region peak is due to the SPR nature of Ag nanoparticles. The surface plasmon resonance band of SW-Ag nanoparticles was acquired in lower transmittance value when compared with SW component, which indicates the improvement of UV protection capacity [51]. Whereas the SW-Ag/ZnO nanoparticles shown the transmittance of 17 % with the maximum absorption wavelength reached to 380 nm covered (both of UV-A and UV-B region) when compared with SW-Ag nanoparticles, the transmittance values are mostly blocked the 11

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harmful region when compared with SW and SW-Ag nanoparticles, which is confirmed that the presence of UV blocking materials (ZnO nanoparticles). This may be due to the higher energy absorption occurred with the help of incorporated materials. The UPF values of RSW, SW-Ag and SW-Ag/ZnO nanoparticles are 32.6, 43.4 and 45.2 %, respectively and their values are tabulated in (Table.1). The outcome of the results indicated that the UV blocking efficiency of SW-Ag nanoparticles increases by introducing the ZnO nanoparticles [17]. 3.7. TG analysis The thermo-gram study (TG) was performed to investigate the thermal stability of RSW, SW-Ag and SW-Ag/ZnO nanoparticles were operated at 35-770 ºC, and their representative weight changes were shown in (Fig.7A). Thermal responses of RSW were shown the three stages of weight loss. The first step weight loss observed from 35-230 ºC, reported that was attributed to the expulsion of moisture [52]. The weight loss of RSW was 15 % and the weight loss of SW-Ag and SW-Ag/ZnO nanoparticles was 8 and 5%, respectively. Obviously, SW biomolecules possess higher weight loss than SW-Ag and SW-Ag/ZnO nanoparticles, which is attributed to the more water bonding in their molecules. The second enormous weight loss up to 500 ºC attributed to the dehydration of saccharide rings, degradation and decomposition of side chain groups presented in the seaweed structure [53, 54]. The total weight loss was obtained in 73.9 %. The lost decomposition corresponds to the complete decomposition of organic residues of SW component. The total residual weight content of RSW, SW-Ag and SW-Ag/ZnO nanoparticles was obtained to be 26.1, 82.6 and 93.6 %, respectively. Moreover, it was found that to raise the temperature, the obtained residual weight of SW-Ag and SW-Ag/ZnO nanoaprticles was quite greater than SW component. The thermal stability of seaweed component may be increased by 12

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introducing Ag and ZnO particles, those differences were possibly due to the incorporation of high thermally stable Ag and ZnO nanoparticles [47, 50]. Concerning the UV protection effectiveness of SW component has greatly influenced for incorporating of UV protecting component as well as more thermally stable Ag and ZnO nanoparticles, the fabricated sample was more stable for longer last uses to irradiating the samples under the sunlight exposure. 3.8. DTA analysis DTA measurement has provided the information about the quantity and their constitution of chemical substance upon heated in presence of nitrogen atmosphere, operated at the temperature range of 35-770 ºC. Thermal characteristic changes of RSW, SW-Ag and SW-Ag/ZnO nanoparticles were shown in Fig.7B. RSW component was obtained the three stages of exothermic weight change, the first step exothermic peak at 60-210 ºC, was due to the evaporation of water molecules [50]. The second exothermic peak showed at 210-430 ºC, which is acquired to the complete dehydration of saccharide ring, these were consistent with the TGA results. The third large and sharp exothermic peak obtained at 450-550 ºC, this enormous weight loss was reasonably due to degradation and decomposition of SW molecule [53, 54]. Where as the SW-Ag and SW-Ag/ZnO nanoparticles showed the three same exothermic peaks, but they were observed in lower temperature and less peak width, which is due to the incorporation of Ag/ZnO nanoparticles in SW matrix. Two of the exothermic peaks observed around 450 ºC, is due to simultaneous dehydration and decomposition of water and SW component. Above 500 ºC, was related to decomposition of residual biomolecule and transition of amorphous phase to crystalline state of Ag/ZnO nanoparticles [47]. The obtained results are similar to the results of TG analysis.

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3.9. BET analysis To study the surface textural properties and porous features of the SW-Ag/ZnO nanoparticles was further examined by BET analysis, with the operating temperature at 77K. In Fig.8 represents the nitrogen adsorption/ desorption isotherm plot of BET analysis, the obtained hysteresis loops would be ascribed to the H4 type based on the IUPAC classification, the loop observed at the relative pressure of about 0.2-1.0. This indicated the presence of mesoporous materials with uniform interparticle distribution. The BET surface area and the pore radius of SWAg/ZnO nanoparticles were estimated to be 68.36 m2/g and 17.4 nm. These obtained surface areas are larger than previous reported ZnO and polysaccharide-ZnO based nanoparticles [55, 41]. This may be due to the reduction of particles size, which is related to increase in surface area. These are supported with HR-SEM and TEM analysis. The pore volume distribution was found to be 0.1322 cm3/g, by the BJH method. Hence, the greater surface area of SW-Ag/ZnO nanoparticles is expected for providing the excellent surface properties with lower particles size are greatly influenced to the better bacterial inhibition activity [56]. 3.10. Photocatalytic degradation of phenol The photo catalytic degradation experiment was executed under the sunlight irradiation using the catalyst materials of (RSW, SW-Ag and SW-Ag/ZnO nanoparticles). The absorption spectral pattern of phenol molecule in presence of catalyst nanoparticles was displayed in Fig.9 (ac). The concentration of phenol was noticeably decreased for adding the photocalalyst material irradiated under sunlight illumination. The photo catalytic reaction mechanism of SW-Ag/ZnO nanoparticles can be illustrated as follows, initially the catalyst loaded phenol solution was irradiated under the direct sunlight, the electrons from the valance bond of Zn-O moves easily to the conduction band, due to the incorporation of silver. At the moment, create a hole in the valance 14

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bond. The above generated holes act as an oxidizing agent and it can be react with water molecules to provide hydroxyl radicals. The conduction band electrons behave as a reducing agent that reduces the oxygen molecule to produce the superoxide radical (O2-) [57, 58]. Those produced oxygenous radicals (O2and OH*-) are a powerful oxidizers, that is capable to degrade the pollutant organic molecule [57]. When phenol molecules are adsorbed on the surface of SW-Ag/ZnO matrix, there is activation of phenol molecules by reacting with O2- and OH*- radicals, that can leads to forming the phenolic intermediates. Furthermore, the active radical species OH• and O2•, are further oxidizes the phenolic intermediates into simple byproducts (CO2 and H2O) [58, 59]. The photocatatic degradation reaction of phenol was indicated below the equation.

The photo catalytic degradation efficiency of phenol was obtained to be 70, 80.9 and 87.1 % for the catalyst materials of RSW, SW-Ag and SW-Ag/ZnO nanoparticles, respectively. The photocatalytic degradation efficiency of SW-Ag/ZnO has higher than the others nanoparticles. In recent years, many of the researchers have focused to utilize the Ag/ZnO based composite materials as the heterogeneous photocatalyst for the organic pollutants removal applications [6064]. The photo degradation performance and their first order reaction kinetics of nanoparticles were displayed in Fig.10a-c. The linear plot of ln(C0/C) vs t plot demonstrate the photodegradation of phenol follows the pseudo first order kinetics. The rate constant value of SW-Ag/ZnO nanoparticles was obtained in 0.0143 min-1, it was two times higher than SW and slightly higher than SW-Ag nanoparticles, respectively. The results of photo catalytic degradation percentage, R 15

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square and kinetic rate constant values of the nanoparticles are summarized in Table.2. The photo degradation efficiency was improved from 80 % to 87 %, due to the incorporation of ZnO nanoparticles on SW-Ag matrix. The overall results confirmed that the intercalation of ZnO on SW-Ag matrix has enhances the photo catalytic activity. 3.11. Antibacterial activity and mechanism The microbial resistant activities of seaweed functionalized Ag/ZnO nanoparticles was evaluated by using Staphylococcus aureus (Gram positive) and Escherichia coli (Gram negative) pathogens. The antibacterial inhibition zone images of (RSW, SW-Ag and SW-Ag/ZnO nanoparticles) were presented in Fig.11. The bactericidal effect was evaluated based on the appearance of clear zone around the sample in discs. Inhibition zone of E.coli bacterial strain was 8.6mm and for S.aureus bacterial strain 7.2mm for RSW component, the obtained bacterial resistivity may due to the electrostatic interaction of bacterial membrane with the SW constituent [65, 66]. The antimicrobial activity of seaweed depends on some factors such as habitat and the season of algae collection, experimental methods, etc. while their seaweed components having the reactive functional groups of O-H, C-O-S, N-H and N=O in (carbohydrates, alkaloids and protein) [67, 68] presents on their structure, that are strongly interacted to the microbial cell membrane and to inactivate most of the respiratory chain enzymes, that leads to self destruction of the bacterial cell [69, 70]. While the SW-Ag nanoparticles showed the greater bactericidal effect against E.coli (15.4mm) over the S.aureus (13.4mm) bacterial strain. This is due to the penetration of Ag ion released from AgNPs into bacterial cell membrane as well as the interaction of SW component, which causes to retarding of DNA reproduction and consequently, apoptosis of bacterial unit [71, 16

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72]. Furthermore, it was previously discussed in BET analysis results, provided the results about the surface area of nanoparticles is larger and then the particles sizes are become lower, resulting to increase the diffusion of metal ions into the bacterial cell that can causes to destroy the bacterial growth [56]. This phenomenon can be ascribed to the differences in structure of the cell and their existing functional groups. The antimicrobial activity of Ag is dependent on the Ag+ ion that binds strongly to electron donor groups on biological molecules containing sulphur, oxygen or nitrogen [72, 73]. In recent study, the specificity of Ag nanoparticles towards the bacterial inhibition study was elaborately discussed in the previous article [74]. The bacterial inhibition zone diameter values were provided in Table.3. In consequences, the antibacterial efficiency of antibiotics like amikacin was taken as the positive control and DMF (dimethyl formamide) solvent was taken as the negative control. Moreover, SW-Ag/ZnO nanoparticles getting a greater inhibition effect against both the Gram (+ve) and Gram (-ve) strain, obtained in 18.1 and 19.5mm than the RSW and SW-Ag nanoparticles. In previous reports, examined the antibacterial activity of pure ZnO and Cd doped ZnO thin films, the results achieved the Cd doped ZnO has greater activity [75]. Other related research about the ZnO based materials having the superior antibacterial results showed in the previous articles [76-78]. Consequently, another main important reason for the bacterial resistivity is due to the generation of reactive radical species like OH-, O2- and H2O2- produced on their surface of nanoparticles. It can react fastest with the bacterial cell, ensuing to leakage of intracellular fluid that can cause to destruction of bacterial cells [79]. The antimicrobial activity mechanism of SW-Ag/ZnO nanoaprticles was described in scheme.2.

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4. Conclusion SW-Ag/ZnO nanoparticles were synthesized by using Padina gymnospora seaweed extract; which is taken as the surfactant agent and NaOH was taken as the precipitating agent. FTIR study were confirmed the detection of functional groups of SW molecule and Zn-O stretching vibration. The crystalline peaks intensity was slightly suppressed for the incorporation of SW matrix, their average crystalline size was found to be 20.5 and 31.2 nm for SW-Ag and SW-Ag/ZnO nanoparticles. The UV protection efficiency of SW-Ag/ZnO nanoparticles was achieved to 45.2 % and their antibacterial activity of SW-Ag/ZnO exhibited the better bacterial inhibition activity against Gram (+ve) and Gram (-ve) microbial pathogens compared to SW and SW-Ag nanoparticles. The photocatalytic activity of nanoparticles was tested by using phenol pollutants, these were followed the first order reaction kinetics, the rate constant value was in 0.0143 min-1 for SW-Ag/ZnO nanoparticles. These results indicated that the SW surfactant based Ag/ZnO nanoparticles could be employed as a proficient material for UV protecting, antibacterial agents and better photocatalyst material for the pollutants removal applications. Acknowledgement The authors greatly acknowledge the Department of Science and Technology, New Delhi, for awarding the DST-INSPIRE Fellowship (INSPIRE Reg. No: IF140650, Ref: DST/INSPIRE fellowship/2014/ 263, dt. 18.12.2014) for the financial support to one of the authors (S.R.).

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Captions for Figures Fig.1. HR-SEM images of (a) RSW, (b) SW-Ag, (c) SW-Ag/ZnO and (d) EDX image of SWAg/ZnO nanoparticles. Fig.2. TEM images of (a) RSW, (b & c) SW-Ag, (d & e) SW-Ag/ZnO nanoparticles for lower and higher magnification and (f) SAED pattern of SW-Ag/ZnO nanoparticles. Fig.3. FTIR spectra of (a) RSW, (b) SW-Ag and (c) SW-Ag/ZnO nanoparticles. Fig.4. XRD patterns of (a) RSW, (b) SW-Ag and (c) SW-Ag/ZnO nanoparticles. Fig.5. DRS UV-Visible absorbance spectra of (a) RSW, (b) SW-Ag and (c) SW-Ag/ZnO nanoparticles. Fig.6. DRS UV-Visible transmittance spectra of (a) RSW, (b) SW-Ag and (c) SW-Ag/ZnO nanoparticles. Fig.7. (A) TGA and (B) DTA curves of (a) RSW, (b) SW-Ag and (c) SW-Ag/ZnO nanoparticles. Fig.8. Nitrogen adsorption-desorption isotherms and the inset pore size distribution curve of SWAg/ZnO nanoparticles. Fig.9. Absorption spectral pattern of phenol in presence of (a) RSW, (b) SW-Ag and (c) SWAg/ZnO photocatalyst irradiated under the direct sunlight irradiation. Fig.10. (a) photo catalytic degradation performance of phenol, (b) % degradation and (c) First order photo degradation reaction kinetics of phenol solution using RSW, SW-Ag and SW-Ag/ZnO nanoparticles. Fig.11. Bacterial inhibition zone images of (1.RSW, 2.SW-Ag and 3.SW-Ag/ZnO nanoparticles) tested against the (A) Gram positive (S.aureus) and (B) Gram negative (E.coli) bacterial pathogens by agar well diffusion method.

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Captions for Schemes Sheme.1. The schematic representation of the formation of SW-Ag/ZnO nanoparticles. Scheme.2. Proposed mechanism of bacterial inhibition activity of SW-Ag/ZnO nanoparticles.

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Highlights 

Bio-surfactant based Ag/ZnO nanoparticles were synthesized using seaweed extract.



Layered surface of SW component was covered on the agglomerated spherical and undefined sized Ag-ZnO particles.



SW-Ag/ZnO nanoparticles have higher UPF values than the SW-Ag and RSW materials.



SW-Ag/ ZnO nanoparticles revealed the better bacterial inhibition activity.



Phenol degradation was also tested using the catalyst nanoparticles.

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Table.1. UPF % values of RSW, SW-Ag and SW-Ag/ZnO nanoparticles. S.No.

Samples

UPF %

UV protection category

1.

RSW

32.6

Good

2.

SW-Ag

43.4

Good

3.

SW-Ag/ZnO

45.2

Excellent

UPF-Ultraviolet protection factor

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Table.2. Summary of reaction rate constants (min-1), R square and % degradation of phenol for RSW, SW-Ag and SW-Ag/ZnO nanoparticles. S. No

Samples

R2 value

Kinetic rate constant

% Degradation of

k (min-1)

phenol

1.

RSW

0.975

0.0075

70.0 %

2.

SW-Ag

0.952

0.0106

80.9 %

3.

SW-Ag/ZnO

0.942

0.0143

87.1 %

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Table.3. The inhibition zone values (mm scale) of RSW, SW-Ag and SW-Ag/ZnO nanoparticles tested against the Gram positive (S.aureus) and Gram negative (E.coli) bacterial pathogens. S.No

Samples

Gram positive

Gram negative

S.aureus

E.coli

1.

RSW

7.2±0.2

8.6±0.3

2.

SW-Ag

13.4±0.1

15.4±0.1

3.

SW-Ag/ZnO

18.1±0.1

19.5±0.2

4.

Standard

17

17