Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1467–1474
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EGCG assisted green synthesis of ZnO nanopowders: Photodegradative, antimicrobial and antioxidant activities D. Suresh a,⇑, Udayabhanu a, P.C. Nethravathi a, K. Lingaraju b, H. Rajanaika b, S.C. Sharma c, H. Nagabhushana d a
Department of Studies and Research in Chemistry, Tumkur University, Tumkur, Karnataka 572 103, India Department of Studies and Research in Environmental Science, Tumkur University, Tumkur, Karnataka 572 103, India Chattisgarh Swami Vivekanand Technical University, Bhilai, Chattisgarh, India d Prof. C.N.R. Rao Centre for Advanced Materials, Tumkur University, Tumkur, Karnataka 572 103, India b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Green synthesis of ZnO nanopowders
200 mg
130 mg
was achieved using EGCG. NPs have hexagonal structure with agglomeration and sponge like structure. Photoluminescence spectrum exhibits peak at 590 nm. Excellent photocatalytic activity toward malachite green and methylene blue. NPs exhibit significant antioxidant and bactericidal activity.
a r t i c l e
i n f o
Article history: Received 30 July 2014 Received in revised form 13 September 2014 Accepted 13 October 2014 Available online 4 November 2014 Keywords: Combustion method Methylene blue Photodegradation Luminescence Zinc oxide nanopowders
⇑ Corresponding author. Tel.: +91 9886465964. E-mail address: [email protected] (D. Suresh). http://dx.doi.org/10.1016/j.saa.2014.10.038 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
a b s t r a c t Zinc oxide nanopowders were synthesized by solution combustion method using Epigallocatechin gallate (EGCG) a tea catechin as fuel. The structure and morphology of the product was characterized by Powder X-ray Diffraction, Scanning Electron Microscopy, photoluminescence and UV–Visible spectroscopy. The nanopowders (Nps) were subjected to photocatalytic and biological activities such as antimicrobial and antioxidant studies. PXRD patterns demonstrate that the formed product belongs to hexagonal wurtzite system. SEM images show that the particles are agglomerated to form sponge like structure and the average crystallite sizes were found to be 10–20 nm. PL spectra exhibit broad and strong peak at 590 nm due to the Zn-vacancies, and O-vacancies. The prepared ZnO Nps exhibit excellent photocatalytic activity for the photodegradation of malachite green (MG) and methylene blue (MB) indicating that the ZnO NPs are potential photocatalytic semiconductor materials. ZnO NPs exhibit significant bactericidal activity against Klebsiella aerogenes, Pseudomonas aeruginosa, Escherichia coli and Staphylococcus aureus using the agar well diffusion method. Furthermore, the ZnO nano powders show good antioxidant activity by potentially scavenging DPPH radicals. The study successfully demonstrates synthesis of ZnO NPs by simple ecofriendly route employing EGCG as fuel that exhibit superior photodegradative, antibacterial and antioxidant activities. Ó 2014 Elsevier B.V. All rights reserved.
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Introduction In recent years, nanomaterials have been widely studied compared to their bulk materials due to their interesting chemical and physical properties [1]. Zinc oxide is considered as an important material among nanomaterials of transition metal oxides. These metal oxides find application in various fields ranging from catalysis to drug delivery including optical, electrical electrochemical sensing, magnetic and biological properties. These make ZnO a multitasking material that finds applications in biosensors, light emitting material for spintronics solar cells, photocatalysts and antibacterials, etc. [2,3]. ZnO has a high excitation binding energy of 60 meV and wide semiconductor band gap of 3.37 eV [4,5]. It is one of the hardest materials in the II to VI group of elements. As a result of this ZnO devices do not suffer from dislocation degradation during working [6–8]. Several methods have been employed to synthesize ZnO nano particles such as sol–gel, direct precipitation, solvothermal and hydrothermal [9–13]. These methods require tedious procedures, expensive substrate and sophisticated equipment. Epigallocatechin gallate (EGCG) assisted Solution Combustion Synthesis (SCS) method was employed as it is a simple, easier and less energy and less time consuming method among the existing wet chemical routes. Important benefits of green synthesis are lesser or zero pollution to the environment and useful materials can be synthesized easily, environmental friendly and reasonable scale. The biomaterial based routes removes the essential to use toxic chemical agents. Generally waste products from the plants are usually low cost and are easily available in the environment. The reactants and combustible fuel are mixed for uniform distribution and subjected for combustion during SCS method that results in production of pure products in the form of powder. Combustible fuel plays an important role since it is responsible for the liberation of energy to produce particle of reduced size during combustion. It involves an aqueous solution containing suitable metal salts preferably metal nitrates and a suitable fuel which acts as a reducing agent. SCS method is an exothermic reaction between metal nitrates and combustible fuels which act as reducing agents. From the literature it is clear that till date most of the fuels used for combustion synthesis of ZnO nanoparticles are commercially available carbohydrates and amino acids [14–16]. EGCG is the ester of gallic acid and epigallocatechin, abundant catechin found in green tea and it is a potent antioxidant that has therapeutic uses in the treatment of many disorders like cancer and cardiovascular. It is found mainly in green tea, white tea and in smaller quantities in black tea. It is also found in several nuts, vegetables, as well as carob powder [17,18]. In the present work, we report the synthesis of ZnO nanocrystals via solution combustion method using EGCG as fuel and final product was characterized using XRD, SEM, PL and UV–Visible spectroscopy. Further antibacterial, antioxidant, and photodegradation properties of the product were studied.
Experimental EGCG powder of 100, 130, 160, 200 and 260 mg dissolved in 5 different beakers containing 10 ml of double distilled water, the mixture was stirred for 10 min to get a homogeneous solution. Zinc nitrate (1.45 g) and EGCG solutions mentioned above were taken in petri dish, mixed well then the solution was kept in a preheated muffle furnace pre heated to 400 °C, the reaction was completed in 4–5 min, the synthesized product was in the form of fluffy mass. The fluffy mass was introduced for calcination for 2 h to remove impurities and to form pure phase. The final nanocrystalline zinc oxide nano powders obtained was used for characterization and
evaluated for photocatalytic, activities.
antioxidant and
antimicrobial
Characterization The phase purity and the crystallinity of the nanopowders were examined by powder X-ray diffractometer employing the Shimadzu-7000 X-ray diffractometer with monochromatized Cu Ka radiation. The optical properties were characterized by employing the Thermo scientific Evolution-220 UV–Visible Spectrophotometer and photoluminescence studies were carried out using Horiba Spectroflourimeter. The surface morphology of the nanopowders were examined by Hitachi-7000 Table top Scanning Electron Microscope. Photocatalytic activity Photocatalytic experiments were carried out with the help of 150 75 mm batch reactor. A catalytic load of 50 mg nanopowders in 100 ml of 5 ppm dye was prepared. The dye solution and catalyst was placed in the reactor and magnetically stirred with simultaneous exposure to Sun light/UV-light. Then the known volume (3 ml) of slurry was drawn at specific intervals (30 min), centrifuged to remove the intervention of the catalyst and assessed using spectrophotometer (617 nm) for rate of degradation. By using the following formula, the percentage (%) of degradation of the dye can be determined.
% of degradation ¼
Ci Cf 100 Ci
where Ci and Cf are initial and final dye concentrations respectively. The experiment was repeated by varying various parameters such as dye concentration, catalytic load, irradiation time and pH. Evaluation of anti-bacterial activity Antibacterial activity was screened by agar well diffusion method [19–21] against Gram ve bacteria Klebsiella aerogenes (B1) (NCIM-2098), Escherichia coli (B2) (NCIM-5051), Pseudomonas aeruginosa (B5) (NCIM-2242) and Gram +ve bacteria Staphylococcus aureus (B4) (NCIM-5022). Nutrient agar plates were prepared in sterile conditions and spread plate method was finished using sterile L-shaped glass rod with 100 ll of 24 h old broth culture of respective bacterial strains. Wells (6 mm) were made into each petri-plate using the sterile cork borer. Different concentrations of ZnO nanopowders (500 and 1000 lg/well) were used to assess the dose dependent activity of the product. ZnO nano powders were dispersed in sterile water using sonicator (Digital Ultrasonic, Medica Instrument Co. Bangalore) and micropipettes were used for the addition of dispersed nano compounds into the wells. The standard antibiotic ciprofloxacin (Hi Media) as positive control was tested against the pathogens. Then the plates were incubated at 37 °C for 36 h in incubator. After the incubation period, the zone of inhibition of each well was measured and the values were noted. The experiments were carried out in triplicates with each compound and the average values were calculated for determining the antibacterial activity [22]. Evaluation of antioxidant activity Antioxidant activity was carried out by the DPPH assay using modified method of Brand-Williams [23]. 1,1-Diphenyl-2-picrylhydrazyl (oxidized form) is a stable free radical with purple color. In the presence of an antioxidant which can donate an electron to DPPH radical decays, and the change in absorbance at 520 nm was followed which can be measured spectrophotometrically. 39.4 mg
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of DPPH was dissolved in 100 mL of methanol to get 0.14 mM solution. 50% methanol was prepared by diluting methanol 1:1 with de-ionized water. ZnO nanopowder stock (conc. 10 mg/mL) was prepared by dispersing 100 mg of nanopowder in volume of 10 ml with de-ionized water. ZnO nanopowder concentrations of 5, 10, 15, 20, 25, 30 mg/2.5 ml of 50% methanol were considered. To this 280 lL of 1 mM DPPH was added and incubated at 37 °C for 30 min. The absorbance was measured at 540 nm against 50% methanol blank by spectrophotometer, a control reaction was carried out without addition of the test sample. The actual absorbance was taken as the absorbance difference of the control and the test sample and (Inhibitory Concentration 50) IC50 values was determined. Results and discussion Fig. 1 shows the powder XRD patterns of the ZnO nanopowder synthesized with various concentrations of EGCG by SCS method. All the diffraction patterns were indexed to be hexagonal wurtzite 0 structure of0 ZnO nanopowders with lattice constants a = 3.2417 Å A, c = 5.1876 Å A which are consistent with the values in standard card JCPDS 89-1397. The average crystallite size was estimated using Debye Scherrer’s formula
D¼
0:89k b cos h
ð1Þ
where D is the crystallite size, k is the wavelength and b is the full width at half maximum. The Debye Scherrer’s and Williamson–Hall plots (W–H plot) calculations are employed to determine the size of crystallites of nanoparticles. These calculations reveal that the average crystallite sizes of ZnO nanopowders prepared with different EGCG concentrations were found to be in the range 10–20 nm (Table 1).
260 mg
Intensity (a.u)
200 mg
160 mg
130 mg
Fig. S1 (supplementary information) shows the UV–Visible spectra of ZnO nanopowders prepared with various concentrations of 100, 130, 160, 200, 260 mg EGCG. The spectra exhibit a strong absorption band in the range of 369–377 nm. This is due to the transfer of electrons from the valence band to the conduction band. The estimated Eg values were found to be in the range 3.33–3.26 eV. Further, bulk ZnO has absorption band at 357 nm (3.45148) in the UV–Visible spectrum and is superior to the as prepared ZnO nanopowders indicating the clear blue shift [24]. Fig. 2 shows that the PL spectra of ZnO nanopowders prepared under various EGCG concentrations at room temperature. The excitation wavelength was 378 nm and the emission peaks were observed at 414 and 615 nm along with broad and strong peak at 590 nm. These emissions were due to the Zn-vacancies, and O-vacancies, the energy gap between the interstitial zinc energy level to the valence band and the conduction band to the antisite oxygen (O–Zn) defect energy levels [25]. The emission at 549 nm occurs due the recombination of a photogenerated hole with a singly ionized oxygen vacancy. Scanning Electron Micrographs of the ZnO Nps obtained under different EGCG concentrations as shown in Fig. 2. It clearly shows that the particles are agglomerated to form sponge like structures and well defined pores. This was due to escaping gases lost of during combustion. Further it was observed that the spherical nature was almost reduced and convert to porus nature with increase of EGCG concentration. The enhanced porous nature of the material could be due to release of large amounts of gases formed during combustion (Fig. 3). The photocatalytic activity of any organic dye are prejudiced by phase composition, crystallinity, particle size, band gap of the photocatalyst, surface area, etc., ZnO nanopowder prepared from 130 lg of EGCG was used due to their high yield and purity of the nanopowder. ZnO nanopowder was used as photocatalyst for studying the degradation of malachite green (MG) and methylene blue (MB) upon exposure to Sunlight as well as UV light. The rate of degradation was observed in terms of change in intensity at kmax of the organic dyes. The effect of increased dye concentration with fixed amount of nanopowders was studied. The results are as shown in Fig. 4(a and b). The concentration range of the dye studied was in the range of 5–25 ppm. The catalytic load was kept constant at 50 mg/100 mL. In the case of MG degradation with UV-light, it was observed that with increase in concentration of the dye the degradation of the MG was marginally decreased from 95% degradation to 86% degradation at the end of 150 min. In the case of sunlight exposure, the degradation was slightly lesser. However it followed the same trend as in the case of UV-light exposure. These observations clearly indicate that the decrease in dye degradation could be due to limited active sites on the catalyst as the concentration of catalyst was kept constant. In the case of MB degradation with UV-light, it was observed that with increase in concentration of the dye, the degradation of the MB was marginally decreased from 13% degradation to 0% degradation at the end of 120 min (Fig. 5a and b). In the case of Sunlight exposure, the degradation was much higher. It degrades 100% of the MB at the end of
(101) Table 1 Crystallite size calculations of ZnO Nps prepared with various concentrations of EGCG using Debye Scherrer’s and W-H Plots.
(100) (002) (102)
(110) (103) (112)
100 mg 20
30
40
50
60
70
80
2 θ (degrees) Fig. 1. Powder XRD patterns of the ZnO nanopowders prepared by various concentrations of EGCG extract.
EGCG (mg)
Average crystallite size (nm) Debye Scherrer’s
W–H plots
100 130 160 200 260
19 11 11 9 9
19 25 9 7 7
Strain (104)
11.3 90.0 16.7 38.4 12.4
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6x10 5
PL intensity (a.u)
5x10 5
590 nm
λexci = 375 nm
(a) 100 mg (b) 130 mg (c) 160 mg (d) 200 mg (e) 260 mg
(a)
4x10 5 3x10 5
(b) 2x10
5
414 nm
1x10 5
(d)
(c)
(e)
0
400
500
600
700
800
Wavelength (nm) Fig. 2. Photoluminescence spectra of ZnO nanopowders prepared under different EGCG concentration.
90 min. Also, it followed the same trend as in the case of UV-light exposure. These observations clearly indicate that the decrease in dye degradation. This could be due to limited active sites on the concentration of catalyst were kept constant. Fig. S2(a and b) (supplementary information) shows the UV– Visible spectra of both the dyes i.e., MG and MB. MG degradation was observed at zero min. up to 70% by the addition of the catalyst. But MB did not undergo degradation at the zero minute. However it degrades significantly with the increase in the irradiation time. This clearly indicates that the ZnO prepared from the EGCG is more efficient toward the degradation of MG and MB. From the spectra, it is clearly observed that as irradiation time increases the concentration of MB dye decreases, which is shown by the decrease in UV–Visible absorbance at 663 nm. The absorption of photon leads to charge separation due to jump of an electron (e) from the valence band of the semiconductor catalyst to the conduction band, thus a hole (h+) gets generated in the valence band. Light source offers photon energy required to excite the semiconductor electron from the valence band (VB) area to the conduction band (CB) area. Photodegradation efficiency increases with the increase in irradiation with sufficient intensity,
100 mg
130 mg
160 mg
200 mg
260 mg
Fig. 3. SEM images of ZnO nanopowders prepared under different EGCG concentrations.
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because electron–hole formation is predominantly increasing and electron–hole recombination is negligible at sufficiently higher intensity, however at lower light intensity, electron–hole pair separation contests with recombination which in turn decrease the formation of free radicals, thus causing lesser percentage degradation of the dyes [26]. However, bulk ZnO has shown only about 20% of the photocatalytic dye degradation activity compared to ZnO nanoparticles. ZnO hollow spheres were studied for their photocatalytic activities toward phenol oxidation and they show significantly high activity due to the increase in the surface defects in presence of UV and Sun light [27]. Among pure and transition metal ion doped ZnO nanoparticles, pure ZnO photocatalysts showed negligible visible-light photocatalytic activity due to the large band gap of ZnO and the lack of absorption in the visible region. However, Cu2+ modified ZnO nanoparticles showed UV and visible light photodegradation [28]. Mechanism for photodegradation of MB and MG dye in presence of Nps can be clarified as follows. When radiation of certain energy incidents on catalyst surface with sufficient energy equal to or higher than the band gap energy of the catalyst, creation of a hole (h+) in the valence band and an electron (e) in the conduction band takes place. The hole oxidizes water to produce OH radicals, while the electron in the conduction band lessens the oxygen adsorbed on the catalyst.
þ
ZnO þ hm ! ZnO hvb þ ecb þ
OHads þ hvb ! OHads ðin basic mediumÞ MB=MG þ OHads ! degradation of the dyes It is clear from the spectrum that, as irradiation time increases the efficiency of nanopowders to degrade the dye also increases. Here, ZnO nanopowders were found to be more efficient catalyst. The effect of increased pH of the solution with fixed concentration of nanopowders and concentration of the dye MB was evaluated. The results are as shown in Fig. S3 (a and b) (supplementary information). The pH of the solution was in the range of 2–12 pH. The catalytic load 50 mg and concentration of the dye 5 ppm was kept constant per 100 ml. It clearly indicates that the rate of degradation of the MB dye was effective in alkaline medium [29–32] and highest rate of degradation was observed at pH 10 [33–35] which can be clarified based on the zero potential charge (ZPC). The ZPC of ZnO found to be 9.0 ± 0.3 and beyond this value the surface is negatively charged due to adsorbed OH ions. Due to the occurrence of numerous OH ions on the surface of ZnO leads to the formation of OH radicals, which acts as primary oxidizing species and are responsible for the degradation of MB [31–36]. Due to
(a) 96
12
88
5 ppm 10 ppm 15 ppm 20 ppm 25 ppm
80
% degradation
% degradation
(a)
6
5 ppm 10 ppm 15 ppm 20 ppm
0
0
0
30
60
90
120
30
150
60
90
120
Time (min)
Time (min)
(b)
105
(b) 88
80
5 ppm 10 ppm 15 ppm 20 ppm 25 ppm
72
% degradation
% degradation
70
35
5 ppm 10 ppm 15 ppm 20 ppm 0
0
30
60
90
120
150
Time (min) Fig. 4. Degradation of MG under (a) UV-light (b) Sunlight.
0
30
60
90
Time (min) Fig. 5. Degradation of MB under (a) UV-light (b) Sunlight.
120
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Fig. 6. Photographs showing antibacterial activity in agar well diffusion method with concentration 500 and 1000 lg per well with Gram ve bacteria (a) Klebsiella aerogenes, (b) Escherichia coli, (c) Pseudomonas aeruginosa and with Gram +ve bacteria, and (d) Staphylococcus aureus.
Antioxidant activity assay of ZnO nanoparticles
% Inhibition
100
50
IC50of ZnO = 10000 μg/mL
0 0
5000
10000
15000
20000
25000
Sample concentration (μg) Fig. 7. Percentage inhibition of DPPH radical.
electrostatic repulsion between the surface charges on the adsorbent and the adsorbate prevents the absorption of MB and it is not protonated above pH 10. From Fig. S3(a and b) (supplementary information) it clearly specifies that pH value of the MB solution had significant impact on the photocatalytic activity of ZnO Fig. S4(a and b) (supplementary information). Hence above pH 10, the rate of degradation of MB was decreased.
The effect of increased catalytic load with fixed concentration of dyes MG and MB was evaluated in Figs. S5(a–c) and S6(a and b) (supplementary information) respectively. The concentration of catalytic load studies was in the range of 50–200 mg. The dye concentration was kept constant at 5 ppm/100 ml in the case of MG degradation in dark. It was observed that with increase in the catalytic load the rate of degradation increases marginally from 70% to 85% at the end of 150 min. In case of UV-light and Sunlight exposure, the degradation was slightly higher. However it followed the same trend as in the case of dark degradation. These observations clearly indicate that, increased catalytic load enhances the percentage degradation. This could be due to the availability of more active sites with the increase of the catalytic load. In the case of MB degradation and UV-light exposure, it was observed that with increase in the catalytic load the rate of degradation increases marginally from 0% to 70% at the end of 120 min. In case of Sunlight exposure, the rate of degradation was significantly higher than the UV-light exposure. It degrades completely at the end of 90 min. The effect on percent degradation with various parameters like UV-light, Sunlight, pH-2, 8 and 12 was evaluated. The results are as shown in Fig. S7 (supplementary information). When compared to UV-light, Sunlight was more efficient toward the percentage degradation. Lower pH and lowest catalytic load show similar results toward lesser percentage degradation. Higher catalytic load and higher pH show the similar results toward higher percentage degradation. During the photocatalytic degradation reaction hydroxyl radicals have been proved to be the major active species. By using coumarin (COU) as a probe molecule it was easy to detect the rate of
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formation of OH during the photocatalytic degradation reaction and it this technique was very easy, simple, sensitive, rapid, photoluminescence (PL) technique. OH reacts with Coumarin to produce highly fluorescent product 7-hydroxyl coumarin at a signal of 454 nm. Here 0.2 g of catalyst (ZnO nanopowders) was dissolved in 50 ml of 103 M aqueous solution in a Borosil trough. The mixture was allowed for adsorption–desorption equilibrium among the photocatalyst, water and coumarin before the irradiation. Using UV-light of intensity 125 W/m2 as a light source this reaction was irradiated. Aliquots were withdrawn for every 10 min and measure the photoluminescence spectrum by using Shimadzu RF-5301PC spectrofluorophotometer. It is evident from the spectra (Fig. S8 – supplementary information) that the PL intensity at 454 nm increased linearly with irradiation time and it clearly indicated that irradiation time proportional to the development of OH at the surface of ZnO. The antibacterial activity of the ZnO nanopowders was assessed against Gram ve E. coli, K. aerogenes, P. desmolyticum and Gram +ve bacteria S. aureus using agar well diffusion method. With the help of agar well diffusion method the ZnO nanopowders showed significant antibacterial activity on all the four bacterial strains. Zone of inhibition with the concentration 500 and 1000 lg per well is shown in Fig. 6. The anti-bacterial activities of the ZnO nanopowders are shown in the Table – TS1 (supplementary information). Mesoporous ZnO particle with high specific surface area was synthesized by a reverse microemulsion-based evaporationinduced self-assembly (EISA) technique which showed ZnO particle exhibits a strong antibacterial activity against both Gram-negative bacterium and E. coli and Gram-positive bacterium S. aureus [37]. ZnO crystals with twinned structures were prepared by using a hydrothermal reaction and their antibacterial activity of depend on morphology. Thick twinned hexagonal prisms of ZnO have the highest antibacterial activity and the (0 0 1) polar plane of ZnO plays an important role in its antibacterial activity against E. coli [38]. The Table-TS1 clearly shows that ZnO prepared from the EGCG was found to possess very significant antibacterial activity toward the different pathogenic bacterial strains. However it was most potent toward the K. aerogenes (B1) and indicates that its antibacterial activity was nearly equal to the standard. ZnO was also significantly potent toward other bacterial strains such as P. desmolyticum, S. aureus, E. coli and K. aerogenes as shown in Fig. 6. The molecule of 1,1-diphenyl-2-picryl hydrazyl (DPPH) is characterized as a stable free radical by virtue of the delocalization of the spare electron over the molecule as a whole, so that the molecules do not dimerise, as would be the case with most other free radicals. The delocalization also gives rise to the deep violet color, characterized by an absorption band in ethanol solution centered at about 520 nm. When a solution of DPPH is mixed with that of a substance that can donate an electron or hydrogen atom, then this gives rise to the reduced form with the loss of this violet color (although there would be expected to be a residual pale yellow color from the picryl group still present). This latter will then undergo further reactions which control the overall stoichiometry, that is, the number of molecules of DPPH reduced (decolorized) by one molecule of the reductant. The reaction is therefore intended to provide the link with the reactions taking place in an oxidizing system, such as the autoxidation of a lipid or other unsaturated substance; the DPPH molecule is thus intended to represent the free radicals formed in the system whose activity is to be suppressed by the substance. Antioxidant potential of ZnO nanoparticles synthesized using P. aeruginosa rhamnolipids was assessed through 2,2-diphenyl-1-picrylhydrazyl (DPPH), hydroxyl, and superoxide anion free radicals with varying concentration and time of the storage up to 15 months, while it was found to decline in bare ZnO nanoparticles. It was observed that the decline antiradical capacity by 6.9% of 200 lg/mL rhamnolipids nanoparticles up
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to15 months of storage as compared to 0 month; whereas it was declined by 87.8% in bare ZnO nanoparticles at the same concentration [39]. Our investigations revealed that IC50 value for quenching DPPH activity of EGCG was found to be 10,000 lg/mL (Fig. 7). This means that it shows considerable antioxidant activity at quenching the free radical scavenging of DPPH.
Conclusions Sponge like ZnO Nps were synthesized via solution combustion method with EGCG as fuel. The as-prepared ZnO Nps were highly porous in nature with large surface area. XRD patterns show that the ZnO nanopowders have hexagonal wurtzite structure. UV–Visible spectrum of Nps shows blue shift compared to bulk ZnO. SEM images reveal the formation of highly porous nanopowders. Debye Scherrer’s and W–H plots analysis indicate that the size of nanoparticles was in the range of 10–20 nm. Photoluminescence spectrum exhibits broad and strong peak at 590 nm. These emissions were due to the Zn-vacancies, and O-vacancies, the energy gap between the interstitial zinc energy level to the valence band and the conduction band to the antisite oxygen defect energy levels. The as-prepared ZnO Nps show prominent photocatalytic activity toward the photodegradation of malachite green and methylene blue. ZnO nanopowders exhibit significant bactericidal activity against K. aerogenes, P. aeruginosa, E. coli and S. aureus using the agar well diffusion method. Furthermore, the ZnO nano powders show good antioxidant activity by potentially scavenging DPPH radicals. The study fruitfully reveals simple, economical and ecofriendly method of synthesis of multifunctional ZnO Nps.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.10.038. References [1] C.R. Martin, Science 266 (1994) 1961–1966. [2] A. Umar, Y.B. Hahn, J. Microbiol. Biotechnol. Food Sci. 2 (1) (2012) 106–120. [3] G. Singh, E.M. Joyce, J. Beddow, T.J. Mason, J. Microbiol. Biotechnol. Food Sci. 2 (2012) 106–120. [4] Y.I. Alivov, E.V. Kalinina, A.E. Cherenkov, D.C. Look, B.M. Ataev, A.K. Omaev, M.V. Chuki Chev, D.M. Bagnall, Appl. Phys. Lett. 83 (2003) 4719–4721. [5] D. Calestani, M.Z. Zha, R. Mosca, A. Zappettini, M.C. Carotta, V. Di Natale, L. Zanotti, Sens. Actuat. B Chem. 144 (2010) 472–478. [6] L.L. Xia, T.Q. Xin, S.C. Lu, L.Y. Chun, Chin. Phys. Let. 22 (2005) 998–1001. [7] C. Suresh, J.M. Kelly, R. Ramesh, D.E. Mc, J. Mater. Chem. C 1 (2013) 3268–3281. [8] Z.L. Wang, J. Phys. Condens. Matter 16 (2004) R829–R858. [9] G. Nagaraju, S. Ashoka, P. Chithaiah, C.N. Tharamani, G.T. Chandrappa, Mater. Sci. Semicond. Proc. 13 (2010) 21–28. [10] S. Ashoka, G. Nagaraju, C.N. Tharamani, G.T. Chandrappa, Mater. Lett. 63 (2009) 873–876. [11] Y.H. Wang, S.J. Seo, B.S. Bae, J. Mater. Res. 25 (2010) 695–700. [12] S. Bhat, S.V. Shrisha, K.G. Naik, Arch. Phys. Res. 4 (2013) 61–66. [13] M. Moazzen, S.M. Borghei, F. Taleshi, J. Nano Sci. 2 (2012) 295–330. [14] Y. Tao, G. Zhao, W. Zhang, S. Xia, Mater. Res. Bull. 32 (1997) 501–506. [15] B. Nagappa, G.T. Chandrappa, Micropor. Mesopor. Mater. 106 (2007) 212–218. [16] K.C. Patil, Chemistry of Nanocrystalline Oxide Materials, World Sci Publi Com, 2008. [17] M. Lorenz, J. Urban, Basic Res. Cardiol. 104 (1) (2009) 100–110. [18] R. Wang, W. Zhou, X. Jiang, J. Agric. Food Chem. 56 (8) (2008) 2694–2701. [19] C. Perez, M. Paul, P. Bazerque, Actabiol. Med. Exp. 15 (1990) 113–115. [20] L.F. Qi, Z.R. Xu, X. Jiang, C.H. Zou, Carbohydr. Res. 339 (2004) 2693–2700. [21] K. Amarnath, J. Kumar, T. Reddy, V. Mahesh, S.R. Ayyappan, J. Colloids Surf. B 92 (2012) 254–261. [22] T. Liu, H.Q. Tang, X.M. Cai, J. Zhao, D.J. Li, X.L. Sun, Nucle. Instrum. Math. B 264 (2007) 282–286. [23] W. Brand-Williams, Food Sci. Technol. (London) 28 (1995) 25–30. [24] C. Wang, E. Shen, E. Wang, L. Gao, Z. Kang, C. Tian, Y. Lan, C. Zhang, Mater. Lett. 59 (2005) 2867–2871. [25] Q. Xiang, J. Yu, P.K. Wong, J. Colloid Interface Sci. 357 (2011) 163–167.
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[26] A. Jagannatha Reddy, M.K. Kokila, H. Nagabhushana, J.L. Rao, C. Shivakumara, B.M. Nagabhushana, R.P.S. Chakradhar, Spectrochim. Acta Part A 81 (2011) 59– 63. [27] Greta Patrinoiu, Madalina Tudose, Jose Maria Caldero´n-Moreno, Ruxandra Birjega, Petru Budrugeac, Ramona Ene, Oana Carp, J. Solid State Chem. 186 (2012) 17–22. [28] Raju Kumar, Srinivasan Anandan, Kaliyan Hembram, Tata Narasinga Rao, ACS Appl. Mater. Interfaces 6 (2014) 13138–13148. [29] A.K. Singh, Umesh T. Nakate, J. Nanopart. (2013) 1–7. [30] S. Anandan, A. Vinu, K. Lovely, N. Gokulakrishnan, P. Srinivasu, T. Mori, et al., J. Mol. Catal. A Chem. 226 (2007) 149. [31] A.A. Khodja, T. Sehili, J.F. Pilichowski, P. Boule, J. Photochem. Photobiol. Part A: Chem. 141 (2001) 231–239.
[32] M. Muruganandham, M. Swaminathan, Sol. Energy Mater. Sol. Cells 81 (2004) 439–457. [33] S. Rengaraj, X.Z. Li, Int. Environ. Pollut. 27 (2006) 20–37. [34] A. Mills, R.H. Davies, D. Worsley, Chem. Soc. Rev. 22 (1993) 417–425. [35] J. Liqiang, S. Xiaojun, X. Baifu, W. Baiqi, C. Weimin, F. Honggang, J. Solid State Chem. 177 (2004) 3375–3382. [36] J.C. Pivin, G. Socol, I. Mihailescu, P. Berthet, F. Singh, M.K. Patel, Thin Solid Films 517 (2008) 916–922. [37] Wen Zhang, T. Guoping, Huaji Zhang, Ying Zheng, Li Li Yang, Mater. Lett. 114 (2014) 119–121. [38] Da-Hae Jin, Dahye Kim, Youngsik Seo, Heonyong Park, Young-Duk Huh, Mater. Lett. 115 (2014) 205–207. [39] B.N. Singh, A.K.S. Rawat, W. Khan, A.H. Naqvi, B.R. Singh, PLoS One 9 (2014) 106937.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
EGCG assisted green synthesis of ZnO nanopowders: Photodegradative, antimicrobial and antioxidant activities D. Suresh a,⇑, Udayabhanu a, P.C. Nethravathi a, K. Lingaraju b, H. Rajanaika b, S.C. Sharma c, H. Nagabhushana d a
Department of Studies and Research in Chemistry, Tumkur University, Tumkur, Karnataka 572 103, India Department of Studies and Research in Environmental Science, Tumkur University, Tumkur, Karnataka 572 103, India Chattisgarh Swami Vivekanand Technical University, Bhilai, Chattisgarh, India d Prof. C.N.R. Rao Centre for Advanced Materials, Tumkur University, Tumkur, Karnataka 572 103, India b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Green synthesis of ZnO nanopowders
200 mg
130 mg
was achieved using EGCG. NPs have hexagonal structure with agglomeration and sponge like structure. Photoluminescence spectrum exhibits peak at 590 nm. Excellent photocatalytic activity toward malachite green and methylene blue. NPs exhibit significant antioxidant and bactericidal activity.
a r t i c l e
i n f o
Article history: Received 30 July 2014 Received in revised form 13 September 2014 Accepted 13 October 2014 Available online 4 November 2014 Keywords: Combustion method Methylene blue Photodegradation Luminescence Zinc oxide nanopowders
⇑ Corresponding author. Tel.: +91 9886465964. E-mail address: [email protected] (D. Suresh). http://dx.doi.org/10.1016/j.saa.2014.10.038 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
a b s t r a c t Zinc oxide nanopowders were synthesized by solution combustion method using Epigallocatechin gallate (EGCG) a tea catechin as fuel. The structure and morphology of the product was characterized by Powder X-ray Diffraction, Scanning Electron Microscopy, photoluminescence and UV–Visible spectroscopy. The nanopowders (Nps) were subjected to photocatalytic and biological activities such as antimicrobial and antioxidant studies. PXRD patterns demonstrate that the formed product belongs to hexagonal wurtzite system. SEM images show that the particles are agglomerated to form sponge like structure and the average crystallite sizes were found to be 10–20 nm. PL spectra exhibit broad and strong peak at 590 nm due to the Zn-vacancies, and O-vacancies. The prepared ZnO Nps exhibit excellent photocatalytic activity for the photodegradation of malachite green (MG) and methylene blue (MB) indicating that the ZnO NPs are potential photocatalytic semiconductor materials. ZnO NPs exhibit significant bactericidal activity against Klebsiella aerogenes, Pseudomonas aeruginosa, Escherichia coli and Staphylococcus aureus using the agar well diffusion method. Furthermore, the ZnO nano powders show good antioxidant activity by potentially scavenging DPPH radicals. The study successfully demonstrates synthesis of ZnO NPs by simple ecofriendly route employing EGCG as fuel that exhibit superior photodegradative, antibacterial and antioxidant activities. Ó 2014 Elsevier B.V. All rights reserved.
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Introduction In recent years, nanomaterials have been widely studied compared to their bulk materials due to their interesting chemical and physical properties [1]. Zinc oxide is considered as an important material among nanomaterials of transition metal oxides. These metal oxides find application in various fields ranging from catalysis to drug delivery including optical, electrical electrochemical sensing, magnetic and biological properties. These make ZnO a multitasking material that finds applications in biosensors, light emitting material for spintronics solar cells, photocatalysts and antibacterials, etc. [2,3]. ZnO has a high excitation binding energy of 60 meV and wide semiconductor band gap of 3.37 eV [4,5]. It is one of the hardest materials in the II to VI group of elements. As a result of this ZnO devices do not suffer from dislocation degradation during working [6–8]. Several methods have been employed to synthesize ZnO nano particles such as sol–gel, direct precipitation, solvothermal and hydrothermal [9–13]. These methods require tedious procedures, expensive substrate and sophisticated equipment. Epigallocatechin gallate (EGCG) assisted Solution Combustion Synthesis (SCS) method was employed as it is a simple, easier and less energy and less time consuming method among the existing wet chemical routes. Important benefits of green synthesis are lesser or zero pollution to the environment and useful materials can be synthesized easily, environmental friendly and reasonable scale. The biomaterial based routes removes the essential to use toxic chemical agents. Generally waste products from the plants are usually low cost and are easily available in the environment. The reactants and combustible fuel are mixed for uniform distribution and subjected for combustion during SCS method that results in production of pure products in the form of powder. Combustible fuel plays an important role since it is responsible for the liberation of energy to produce particle of reduced size during combustion. It involves an aqueous solution containing suitable metal salts preferably metal nitrates and a suitable fuel which acts as a reducing agent. SCS method is an exothermic reaction between metal nitrates and combustible fuels which act as reducing agents. From the literature it is clear that till date most of the fuels used for combustion synthesis of ZnO nanoparticles are commercially available carbohydrates and amino acids [14–16]. EGCG is the ester of gallic acid and epigallocatechin, abundant catechin found in green tea and it is a potent antioxidant that has therapeutic uses in the treatment of many disorders like cancer and cardiovascular. It is found mainly in green tea, white tea and in smaller quantities in black tea. It is also found in several nuts, vegetables, as well as carob powder [17,18]. In the present work, we report the synthesis of ZnO nanocrystals via solution combustion method using EGCG as fuel and final product was characterized using XRD, SEM, PL and UV–Visible spectroscopy. Further antibacterial, antioxidant, and photodegradation properties of the product were studied.
Experimental EGCG powder of 100, 130, 160, 200 and 260 mg dissolved in 5 different beakers containing 10 ml of double distilled water, the mixture was stirred for 10 min to get a homogeneous solution. Zinc nitrate (1.45 g) and EGCG solutions mentioned above were taken in petri dish, mixed well then the solution was kept in a preheated muffle furnace pre heated to 400 °C, the reaction was completed in 4–5 min, the synthesized product was in the form of fluffy mass. The fluffy mass was introduced for calcination for 2 h to remove impurities and to form pure phase. The final nanocrystalline zinc oxide nano powders obtained was used for characterization and
evaluated for photocatalytic, activities.
antioxidant and
antimicrobial
Characterization The phase purity and the crystallinity of the nanopowders were examined by powder X-ray diffractometer employing the Shimadzu-7000 X-ray diffractometer with monochromatized Cu Ka radiation. The optical properties were characterized by employing the Thermo scientific Evolution-220 UV–Visible Spectrophotometer and photoluminescence studies were carried out using Horiba Spectroflourimeter. The surface morphology of the nanopowders were examined by Hitachi-7000 Table top Scanning Electron Microscope. Photocatalytic activity Photocatalytic experiments were carried out with the help of 150 75 mm batch reactor. A catalytic load of 50 mg nanopowders in 100 ml of 5 ppm dye was prepared. The dye solution and catalyst was placed in the reactor and magnetically stirred with simultaneous exposure to Sun light/UV-light. Then the known volume (3 ml) of slurry was drawn at specific intervals (30 min), centrifuged to remove the intervention of the catalyst and assessed using spectrophotometer (617 nm) for rate of degradation. By using the following formula, the percentage (%) of degradation of the dye can be determined.
% of degradation ¼
Ci Cf 100 Ci
where Ci and Cf are initial and final dye concentrations respectively. The experiment was repeated by varying various parameters such as dye concentration, catalytic load, irradiation time and pH. Evaluation of anti-bacterial activity Antibacterial activity was screened by agar well diffusion method [19–21] against Gram ve bacteria Klebsiella aerogenes (B1) (NCIM-2098), Escherichia coli (B2) (NCIM-5051), Pseudomonas aeruginosa (B5) (NCIM-2242) and Gram +ve bacteria Staphylococcus aureus (B4) (NCIM-5022). Nutrient agar plates were prepared in sterile conditions and spread plate method was finished using sterile L-shaped glass rod with 100 ll of 24 h old broth culture of respective bacterial strains. Wells (6 mm) were made into each petri-plate using the sterile cork borer. Different concentrations of ZnO nanopowders (500 and 1000 lg/well) were used to assess the dose dependent activity of the product. ZnO nano powders were dispersed in sterile water using sonicator (Digital Ultrasonic, Medica Instrument Co. Bangalore) and micropipettes were used for the addition of dispersed nano compounds into the wells. The standard antibiotic ciprofloxacin (Hi Media) as positive control was tested against the pathogens. Then the plates were incubated at 37 °C for 36 h in incubator. After the incubation period, the zone of inhibition of each well was measured and the values were noted. The experiments were carried out in triplicates with each compound and the average values were calculated for determining the antibacterial activity [22]. Evaluation of antioxidant activity Antioxidant activity was carried out by the DPPH assay using modified method of Brand-Williams [23]. 1,1-Diphenyl-2-picrylhydrazyl (oxidized form) is a stable free radical with purple color. In the presence of an antioxidant which can donate an electron to DPPH radical decays, and the change in absorbance at 520 nm was followed which can be measured spectrophotometrically. 39.4 mg
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of DPPH was dissolved in 100 mL of methanol to get 0.14 mM solution. 50% methanol was prepared by diluting methanol 1:1 with de-ionized water. ZnO nanopowder stock (conc. 10 mg/mL) was prepared by dispersing 100 mg of nanopowder in volume of 10 ml with de-ionized water. ZnO nanopowder concentrations of 5, 10, 15, 20, 25, 30 mg/2.5 ml of 50% methanol were considered. To this 280 lL of 1 mM DPPH was added and incubated at 37 °C for 30 min. The absorbance was measured at 540 nm against 50% methanol blank by spectrophotometer, a control reaction was carried out without addition of the test sample. The actual absorbance was taken as the absorbance difference of the control and the test sample and (Inhibitory Concentration 50) IC50 values was determined. Results and discussion Fig. 1 shows the powder XRD patterns of the ZnO nanopowder synthesized with various concentrations of EGCG by SCS method. All the diffraction patterns were indexed to be hexagonal wurtzite 0 structure of0 ZnO nanopowders with lattice constants a = 3.2417 Å A, c = 5.1876 Å A which are consistent with the values in standard card JCPDS 89-1397. The average crystallite size was estimated using Debye Scherrer’s formula
D¼
0:89k b cos h
ð1Þ
where D is the crystallite size, k is the wavelength and b is the full width at half maximum. The Debye Scherrer’s and Williamson–Hall plots (W–H plot) calculations are employed to determine the size of crystallites of nanoparticles. These calculations reveal that the average crystallite sizes of ZnO nanopowders prepared with different EGCG concentrations were found to be in the range 10–20 nm (Table 1).
260 mg
Intensity (a.u)
200 mg
160 mg
130 mg
Fig. S1 (supplementary information) shows the UV–Visible spectra of ZnO nanopowders prepared with various concentrations of 100, 130, 160, 200, 260 mg EGCG. The spectra exhibit a strong absorption band in the range of 369–377 nm. This is due to the transfer of electrons from the valence band to the conduction band. The estimated Eg values were found to be in the range 3.33–3.26 eV. Further, bulk ZnO has absorption band at 357 nm (3.45148) in the UV–Visible spectrum and is superior to the as prepared ZnO nanopowders indicating the clear blue shift [24]. Fig. 2 shows that the PL spectra of ZnO nanopowders prepared under various EGCG concentrations at room temperature. The excitation wavelength was 378 nm and the emission peaks were observed at 414 and 615 nm along with broad and strong peak at 590 nm. These emissions were due to the Zn-vacancies, and O-vacancies, the energy gap between the interstitial zinc energy level to the valence band and the conduction band to the antisite oxygen (O–Zn) defect energy levels [25]. The emission at 549 nm occurs due the recombination of a photogenerated hole with a singly ionized oxygen vacancy. Scanning Electron Micrographs of the ZnO Nps obtained under different EGCG concentrations as shown in Fig. 2. It clearly shows that the particles are agglomerated to form sponge like structures and well defined pores. This was due to escaping gases lost of during combustion. Further it was observed that the spherical nature was almost reduced and convert to porus nature with increase of EGCG concentration. The enhanced porous nature of the material could be due to release of large amounts of gases formed during combustion (Fig. 3). The photocatalytic activity of any organic dye are prejudiced by phase composition, crystallinity, particle size, band gap of the photocatalyst, surface area, etc., ZnO nanopowder prepared from 130 lg of EGCG was used due to their high yield and purity of the nanopowder. ZnO nanopowder was used as photocatalyst for studying the degradation of malachite green (MG) and methylene blue (MB) upon exposure to Sunlight as well as UV light. The rate of degradation was observed in terms of change in intensity at kmax of the organic dyes. The effect of increased dye concentration with fixed amount of nanopowders was studied. The results are as shown in Fig. 4(a and b). The concentration range of the dye studied was in the range of 5–25 ppm. The catalytic load was kept constant at 50 mg/100 mL. In the case of MG degradation with UV-light, it was observed that with increase in concentration of the dye the degradation of the MG was marginally decreased from 95% degradation to 86% degradation at the end of 150 min. In the case of sunlight exposure, the degradation was slightly lesser. However it followed the same trend as in the case of UV-light exposure. These observations clearly indicate that the decrease in dye degradation could be due to limited active sites on the catalyst as the concentration of catalyst was kept constant. In the case of MB degradation with UV-light, it was observed that with increase in concentration of the dye, the degradation of the MB was marginally decreased from 13% degradation to 0% degradation at the end of 120 min (Fig. 5a and b). In the case of Sunlight exposure, the degradation was much higher. It degrades 100% of the MB at the end of
(101) Table 1 Crystallite size calculations of ZnO Nps prepared with various concentrations of EGCG using Debye Scherrer’s and W-H Plots.
(100) (002) (102)
(110) (103) (112)
100 mg 20
30
40
50
60
70
80
2 θ (degrees) Fig. 1. Powder XRD patterns of the ZnO nanopowders prepared by various concentrations of EGCG extract.
EGCG (mg)
Average crystallite size (nm) Debye Scherrer’s
W–H plots
100 130 160 200 260
19 11 11 9 9
19 25 9 7 7
Strain (104)
11.3 90.0 16.7 38.4 12.4
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6x10 5
PL intensity (a.u)
5x10 5
590 nm
λexci = 375 nm
(a) 100 mg (b) 130 mg (c) 160 mg (d) 200 mg (e) 260 mg
(a)
4x10 5 3x10 5
(b) 2x10
5
414 nm
1x10 5
(d)
(c)
(e)
0
400
500
600
700
800
Wavelength (nm) Fig. 2. Photoluminescence spectra of ZnO nanopowders prepared under different EGCG concentration.
90 min. Also, it followed the same trend as in the case of UV-light exposure. These observations clearly indicate that the decrease in dye degradation. This could be due to limited active sites on the concentration of catalyst were kept constant. Fig. S2(a and b) (supplementary information) shows the UV– Visible spectra of both the dyes i.e., MG and MB. MG degradation was observed at zero min. up to 70% by the addition of the catalyst. But MB did not undergo degradation at the zero minute. However it degrades significantly with the increase in the irradiation time. This clearly indicates that the ZnO prepared from the EGCG is more efficient toward the degradation of MG and MB. From the spectra, it is clearly observed that as irradiation time increases the concentration of MB dye decreases, which is shown by the decrease in UV–Visible absorbance at 663 nm. The absorption of photon leads to charge separation due to jump of an electron (e) from the valence band of the semiconductor catalyst to the conduction band, thus a hole (h+) gets generated in the valence band. Light source offers photon energy required to excite the semiconductor electron from the valence band (VB) area to the conduction band (CB) area. Photodegradation efficiency increases with the increase in irradiation with sufficient intensity,
100 mg
130 mg
160 mg
200 mg
260 mg
Fig. 3. SEM images of ZnO nanopowders prepared under different EGCG concentrations.
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because electron–hole formation is predominantly increasing and electron–hole recombination is negligible at sufficiently higher intensity, however at lower light intensity, electron–hole pair separation contests with recombination which in turn decrease the formation of free radicals, thus causing lesser percentage degradation of the dyes [26]. However, bulk ZnO has shown only about 20% of the photocatalytic dye degradation activity compared to ZnO nanoparticles. ZnO hollow spheres were studied for their photocatalytic activities toward phenol oxidation and they show significantly high activity due to the increase in the surface defects in presence of UV and Sun light [27]. Among pure and transition metal ion doped ZnO nanoparticles, pure ZnO photocatalysts showed negligible visible-light photocatalytic activity due to the large band gap of ZnO and the lack of absorption in the visible region. However, Cu2+ modified ZnO nanoparticles showed UV and visible light photodegradation [28]. Mechanism for photodegradation of MB and MG dye in presence of Nps can be clarified as follows. When radiation of certain energy incidents on catalyst surface with sufficient energy equal to or higher than the band gap energy of the catalyst, creation of a hole (h+) in the valence band and an electron (e) in the conduction band takes place. The hole oxidizes water to produce OH radicals, while the electron in the conduction band lessens the oxygen adsorbed on the catalyst.
þ
ZnO þ hm ! ZnO hvb þ ecb þ
OHads þ hvb ! OHads ðin basic mediumÞ MB=MG þ OHads ! degradation of the dyes It is clear from the spectrum that, as irradiation time increases the efficiency of nanopowders to degrade the dye also increases. Here, ZnO nanopowders were found to be more efficient catalyst. The effect of increased pH of the solution with fixed concentration of nanopowders and concentration of the dye MB was evaluated. The results are as shown in Fig. S3 (a and b) (supplementary information). The pH of the solution was in the range of 2–12 pH. The catalytic load 50 mg and concentration of the dye 5 ppm was kept constant per 100 ml. It clearly indicates that the rate of degradation of the MB dye was effective in alkaline medium [29–32] and highest rate of degradation was observed at pH 10 [33–35] which can be clarified based on the zero potential charge (ZPC). The ZPC of ZnO found to be 9.0 ± 0.3 and beyond this value the surface is negatively charged due to adsorbed OH ions. Due to the occurrence of numerous OH ions on the surface of ZnO leads to the formation of OH radicals, which acts as primary oxidizing species and are responsible for the degradation of MB [31–36]. Due to
(a) 96
12
88
5 ppm 10 ppm 15 ppm 20 ppm 25 ppm
80
% degradation
% degradation
(a)
6
5 ppm 10 ppm 15 ppm 20 ppm
0
0
0
30
60
90
120
30
150
60
90
120
Time (min)
Time (min)
(b)
105
(b) 88
80
5 ppm 10 ppm 15 ppm 20 ppm 25 ppm
72
% degradation
% degradation
70
35
5 ppm 10 ppm 15 ppm 20 ppm 0
0
30
60
90
120
150
Time (min) Fig. 4. Degradation of MG under (a) UV-light (b) Sunlight.
0
30
60
90
Time (min) Fig. 5. Degradation of MB under (a) UV-light (b) Sunlight.
120
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Fig. 6. Photographs showing antibacterial activity in agar well diffusion method with concentration 500 and 1000 lg per well with Gram ve bacteria (a) Klebsiella aerogenes, (b) Escherichia coli, (c) Pseudomonas aeruginosa and with Gram +ve bacteria, and (d) Staphylococcus aureus.
Antioxidant activity assay of ZnO nanoparticles
% Inhibition
100
50
IC50of ZnO = 10000 μg/mL
0 0
5000
10000
15000
20000
25000
Sample concentration (μg) Fig. 7. Percentage inhibition of DPPH radical.
electrostatic repulsion between the surface charges on the adsorbent and the adsorbate prevents the absorption of MB and it is not protonated above pH 10. From Fig. S3(a and b) (supplementary information) it clearly specifies that pH value of the MB solution had significant impact on the photocatalytic activity of ZnO Fig. S4(a and b) (supplementary information). Hence above pH 10, the rate of degradation of MB was decreased.
The effect of increased catalytic load with fixed concentration of dyes MG and MB was evaluated in Figs. S5(a–c) and S6(a and b) (supplementary information) respectively. The concentration of catalytic load studies was in the range of 50–200 mg. The dye concentration was kept constant at 5 ppm/100 ml in the case of MG degradation in dark. It was observed that with increase in the catalytic load the rate of degradation increases marginally from 70% to 85% at the end of 150 min. In case of UV-light and Sunlight exposure, the degradation was slightly higher. However it followed the same trend as in the case of dark degradation. These observations clearly indicate that, increased catalytic load enhances the percentage degradation. This could be due to the availability of more active sites with the increase of the catalytic load. In the case of MB degradation and UV-light exposure, it was observed that with increase in the catalytic load the rate of degradation increases marginally from 0% to 70% at the end of 120 min. In case of Sunlight exposure, the rate of degradation was significantly higher than the UV-light exposure. It degrades completely at the end of 90 min. The effect on percent degradation with various parameters like UV-light, Sunlight, pH-2, 8 and 12 was evaluated. The results are as shown in Fig. S7 (supplementary information). When compared to UV-light, Sunlight was more efficient toward the percentage degradation. Lower pH and lowest catalytic load show similar results toward lesser percentage degradation. Higher catalytic load and higher pH show the similar results toward higher percentage degradation. During the photocatalytic degradation reaction hydroxyl radicals have been proved to be the major active species. By using coumarin (COU) as a probe molecule it was easy to detect the rate of
D. Suresh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1467–1474
formation of OH during the photocatalytic degradation reaction and it this technique was very easy, simple, sensitive, rapid, photoluminescence (PL) technique. OH reacts with Coumarin to produce highly fluorescent product 7-hydroxyl coumarin at a signal of 454 nm. Here 0.2 g of catalyst (ZnO nanopowders) was dissolved in 50 ml of 103 M aqueous solution in a Borosil trough. The mixture was allowed for adsorption–desorption equilibrium among the photocatalyst, water and coumarin before the irradiation. Using UV-light of intensity 125 W/m2 as a light source this reaction was irradiated. Aliquots were withdrawn for every 10 min and measure the photoluminescence spectrum by using Shimadzu RF-5301PC spectrofluorophotometer. It is evident from the spectra (Fig. S8 – supplementary information) that the PL intensity at 454 nm increased linearly with irradiation time and it clearly indicated that irradiation time proportional to the development of OH at the surface of ZnO. The antibacterial activity of the ZnO nanopowders was assessed against Gram ve E. coli, K. aerogenes, P. desmolyticum and Gram +ve bacteria S. aureus using agar well diffusion method. With the help of agar well diffusion method the ZnO nanopowders showed significant antibacterial activity on all the four bacterial strains. Zone of inhibition with the concentration 500 and 1000 lg per well is shown in Fig. 6. The anti-bacterial activities of the ZnO nanopowders are shown in the Table – TS1 (supplementary information). Mesoporous ZnO particle with high specific surface area was synthesized by a reverse microemulsion-based evaporationinduced self-assembly (EISA) technique which showed ZnO particle exhibits a strong antibacterial activity against both Gram-negative bacterium and E. coli and Gram-positive bacterium S. aureus [37]. ZnO crystals with twinned structures were prepared by using a hydrothermal reaction and their antibacterial activity of depend on morphology. Thick twinned hexagonal prisms of ZnO have the highest antibacterial activity and the (0 0 1) polar plane of ZnO plays an important role in its antibacterial activity against E. coli [38]. The Table-TS1 clearly shows that ZnO prepared from the EGCG was found to possess very significant antibacterial activity toward the different pathogenic bacterial strains. However it was most potent toward the K. aerogenes (B1) and indicates that its antibacterial activity was nearly equal to the standard. ZnO was also significantly potent toward other bacterial strains such as P. desmolyticum, S. aureus, E. coli and K. aerogenes as shown in Fig. 6. The molecule of 1,1-diphenyl-2-picryl hydrazyl (DPPH) is characterized as a stable free radical by virtue of the delocalization of the spare electron over the molecule as a whole, so that the molecules do not dimerise, as would be the case with most other free radicals. The delocalization also gives rise to the deep violet color, characterized by an absorption band in ethanol solution centered at about 520 nm. When a solution of DPPH is mixed with that of a substance that can donate an electron or hydrogen atom, then this gives rise to the reduced form with the loss of this violet color (although there would be expected to be a residual pale yellow color from the picryl group still present). This latter will then undergo further reactions which control the overall stoichiometry, that is, the number of molecules of DPPH reduced (decolorized) by one molecule of the reductant. The reaction is therefore intended to provide the link with the reactions taking place in an oxidizing system, such as the autoxidation of a lipid or other unsaturated substance; the DPPH molecule is thus intended to represent the free radicals formed in the system whose activity is to be suppressed by the substance. Antioxidant potential of ZnO nanoparticles synthesized using P. aeruginosa rhamnolipids was assessed through 2,2-diphenyl-1-picrylhydrazyl (DPPH), hydroxyl, and superoxide anion free radicals with varying concentration and time of the storage up to 15 months, while it was found to decline in bare ZnO nanoparticles. It was observed that the decline antiradical capacity by 6.9% of 200 lg/mL rhamnolipids nanoparticles up
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to15 months of storage as compared to 0 month; whereas it was declined by 87.8% in bare ZnO nanoparticles at the same concentration [39]. Our investigations revealed that IC50 value for quenching DPPH activity of EGCG was found to be 10,000 lg/mL (Fig. 7). This means that it shows considerable antioxidant activity at quenching the free radical scavenging of DPPH.
Conclusions Sponge like ZnO Nps were synthesized via solution combustion method with EGCG as fuel. The as-prepared ZnO Nps were highly porous in nature with large surface area. XRD patterns show that the ZnO nanopowders have hexagonal wurtzite structure. UV–Visible spectrum of Nps shows blue shift compared to bulk ZnO. SEM images reveal the formation of highly porous nanopowders. Debye Scherrer’s and W–H plots analysis indicate that the size of nanoparticles was in the range of 10–20 nm. Photoluminescence spectrum exhibits broad and strong peak at 590 nm. These emissions were due to the Zn-vacancies, and O-vacancies, the energy gap between the interstitial zinc energy level to the valence band and the conduction band to the antisite oxygen defect energy levels. The as-prepared ZnO Nps show prominent photocatalytic activity toward the photodegradation of malachite green and methylene blue. ZnO nanopowders exhibit significant bactericidal activity against K. aerogenes, P. aeruginosa, E. coli and S. aureus using the agar well diffusion method. Furthermore, the ZnO nano powders show good antioxidant activity by potentially scavenging DPPH radicals. The study fruitfully reveals simple, economical and ecofriendly method of synthesis of multifunctional ZnO Nps.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.10.038. References [1] C.R. Martin, Science 266 (1994) 1961–1966. [2] A. Umar, Y.B. Hahn, J. Microbiol. Biotechnol. Food Sci. 2 (1) (2012) 106–120. [3] G. Singh, E.M. Joyce, J. Beddow, T.J. Mason, J. Microbiol. Biotechnol. Food Sci. 2 (2012) 106–120. [4] Y.I. Alivov, E.V. Kalinina, A.E. Cherenkov, D.C. Look, B.M. Ataev, A.K. Omaev, M.V. Chuki Chev, D.M. Bagnall, Appl. Phys. Lett. 83 (2003) 4719–4721. [5] D. Calestani, M.Z. Zha, R. Mosca, A. Zappettini, M.C. Carotta, V. Di Natale, L. Zanotti, Sens. Actuat. B Chem. 144 (2010) 472–478. [6] L.L. Xia, T.Q. Xin, S.C. Lu, L.Y. Chun, Chin. Phys. Let. 22 (2005) 998–1001. [7] C. Suresh, J.M. Kelly, R. Ramesh, D.E. Mc, J. Mater. Chem. C 1 (2013) 3268–3281. [8] Z.L. Wang, J. Phys. Condens. Matter 16 (2004) R829–R858. [9] G. Nagaraju, S. Ashoka, P. Chithaiah, C.N. Tharamani, G.T. Chandrappa, Mater. Sci. Semicond. Proc. 13 (2010) 21–28. [10] S. Ashoka, G. Nagaraju, C.N. Tharamani, G.T. Chandrappa, Mater. Lett. 63 (2009) 873–876. [11] Y.H. Wang, S.J. Seo, B.S. Bae, J. Mater. Res. 25 (2010) 695–700. [12] S. Bhat, S.V. Shrisha, K.G. Naik, Arch. Phys. Res. 4 (2013) 61–66. [13] M. Moazzen, S.M. Borghei, F. Taleshi, J. Nano Sci. 2 (2012) 295–330. [14] Y. Tao, G. Zhao, W. Zhang, S. Xia, Mater. Res. Bull. 32 (1997) 501–506. [15] B. Nagappa, G.T. Chandrappa, Micropor. Mesopor. Mater. 106 (2007) 212–218. [16] K.C. Patil, Chemistry of Nanocrystalline Oxide Materials, World Sci Publi Com, 2008. [17] M. Lorenz, J. Urban, Basic Res. Cardiol. 104 (1) (2009) 100–110. [18] R. Wang, W. Zhou, X. Jiang, J. Agric. Food Chem. 56 (8) (2008) 2694–2701. [19] C. Perez, M. Paul, P. Bazerque, Actabiol. Med. Exp. 15 (1990) 113–115. [20] L.F. Qi, Z.R. Xu, X. Jiang, C.H. Zou, Carbohydr. Res. 339 (2004) 2693–2700. [21] K. Amarnath, J. Kumar, T. Reddy, V. Mahesh, S.R. Ayyappan, J. Colloids Surf. B 92 (2012) 254–261. [22] T. Liu, H.Q. Tang, X.M. Cai, J. Zhao, D.J. Li, X.L. Sun, Nucle. Instrum. Math. B 264 (2007) 282–286. [23] W. Brand-Williams, Food Sci. Technol. (London) 28 (1995) 25–30. [24] C. Wang, E. Shen, E. Wang, L. Gao, Z. Kang, C. Tian, Y. Lan, C. Zhang, Mater. Lett. 59 (2005) 2867–2871. [25] Q. Xiang, J. Yu, P.K. Wong, J. Colloid Interface Sci. 357 (2011) 163–167.
1474
D. Suresh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1467–1474
[26] A. Jagannatha Reddy, M.K. Kokila, H. Nagabhushana, J.L. Rao, C. Shivakumara, B.M. Nagabhushana, R.P.S. Chakradhar, Spectrochim. Acta Part A 81 (2011) 59– 63. [27] Greta Patrinoiu, Madalina Tudose, Jose Maria Caldero´n-Moreno, Ruxandra Birjega, Petru Budrugeac, Ramona Ene, Oana Carp, J. Solid State Chem. 186 (2012) 17–22. [28] Raju Kumar, Srinivasan Anandan, Kaliyan Hembram, Tata Narasinga Rao, ACS Appl. Mater. Interfaces 6 (2014) 13138–13148. [29] A.K. Singh, Umesh T. Nakate, J. Nanopart. (2013) 1–7. [30] S. Anandan, A. Vinu, K. Lovely, N. Gokulakrishnan, P. Srinivasu, T. Mori, et al., J. Mol. Catal. A Chem. 226 (2007) 149. [31] A.A. Khodja, T. Sehili, J.F. Pilichowski, P. Boule, J. Photochem. Photobiol. Part A: Chem. 141 (2001) 231–239.
[32] M. Muruganandham, M. Swaminathan, Sol. Energy Mater. Sol. Cells 81 (2004) 439–457. [33] S. Rengaraj, X.Z. Li, Int. Environ. Pollut. 27 (2006) 20–37. [34] A. Mills, R.H. Davies, D. Worsley, Chem. Soc. Rev. 22 (1993) 417–425. [35] J. Liqiang, S. Xiaojun, X. Baifu, W. Baiqi, C. Weimin, F. Honggang, J. Solid State Chem. 177 (2004) 3375–3382. [36] J.C. Pivin, G. Socol, I. Mihailescu, P. Berthet, F. Singh, M.K. Patel, Thin Solid Films 517 (2008) 916–922. [37] Wen Zhang, T. Guoping, Huaji Zhang, Ying Zheng, Li Li Yang, Mater. Lett. 114 (2014) 119–121. [38] Da-Hae Jin, Dahye Kim, Youngsik Seo, Heonyong Park, Young-Duk Huh, Mater. Lett. 115 (2014) 205–207. [39] B.N. Singh, A.K.S. Rawat, W. Khan, A.H. Naqvi, B.R. Singh, PLoS One 9 (2014) 106937.
