Colloids and Surfaces B: Biointerfaces 122 (2014) 611–616
Contents lists available at ScienceDirect
Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
Effect of exopolysaccharides on photocatalytic activity of ZnO nanoparticles Preethy Chandran, Suhas Netha, Aswathy Ravindran, S. Sudheer Khan ∗ CeNTAB, School of Chemical and Biotechnology, SASTRA University, Thanjavur 638 401, India
a r t i c l e
i n f o
Article history: Received 23 April 2014 Received in revised form 17 July 2014 Accepted 24 July 2014 Available online 2 August 2014 Keywords: ZnO NPs Exopolysaccharides Methylene blue Visible light Photocatalytic activity
a b s t r a c t Zinc oxide nanoparticles (ZnO NPs) are largely used in consumer products and industrial applications. The increased use of such materials may lead to its release into the environment. The study used chemically synthesized ZnO NPs and characterized by using UV–visible spectrophotometer, scanning electron microscopy, particle size analyzer and X-ray diffraction (XRD) analysis. The mean diameter of the particles was found to be 55 ± 1.2 nm. The XRD patterns exhibited hexagonal structure for ZnO NPs. The photocatalytic property of ZnO NPs was evaluated based on the UV–vis spectra changes of the methylene blue solution as a function of reaction time in the presence of ZnO NPs under visible light. The study suggests that ZnO NPs can be used as an efficient photocatalyst and the environmental factor such as exopolysaccharides could mask the photocatalytic activity of NPs. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Nanotechnology is a rapidly growing field of research that has already introduced into variety of commercial application such as cosmetics, suntan lotions, paints and self-cleaning materials. NPs are size range from 1 to 100 nm and they have been attracted by their unique surface properties, and their potential use in photocatalysis and biomedicine applications [1–3]. Among them metal oxide NPs received considerable attention and they are being incorporated into variety of products based on catalytic capacity, optoelectronic and antimicrobial properties [4]. ZnO NPs are wide band-gap semiconductor and possess a large excitation binding energy. The particles received the specific attention for electronic sensor, solar voltaic, and transducer applications due to its piezoelectricity and wurtzite crystal structure. ZnO is an efficient photocatalyst and this property is utilized for the remediation of environmental pollutants to medical disinfection [5]. Currently ZnO NPs are largely used in personal care products such as toothpaste, beauty products, sunscreens and textile products [4,6,7]. The use of NPs in sunscreen products alone is estimated to be 1000 tons during 2003/2004, consisting TiO2 and ZnO NPs [8]. According to Box all et al. [9] the estimated amount of ZnO NP concentrations in the UK sewage was less than 100 g/L. The indiscriminate use of ZnO NPs has resulted for the release of such
∗ Corresponding author. Tel.: +91 9047286362; fax: +91 4362 264120. E-mail address: [email protected] (S. Sudheer Khan). http://dx.doi.org/10.1016/j.colsurfb.2014.07.039 0927-7765/© 2014 Elsevier B.V. All rights reserved.
materials to the environment which increases the environmental levels and ecotoxicity of these materials [10]. The eco-toxicological evaluation of ZnO NPs toward a broad range of organisms has been studied extensively. ZnO NPs possess toxicity to environmentally relevant bacterial, algal, vertebrates and invertebrate species [1,10–12]. Being a photocatalyst, ZnO NPs promotes ROS generation under irradiation [4]. The toxic potential of ZnO NPs can also be induced due to its photocatalytic effect [10]. The toxicological effect of the NPs is limited by environmental factors such as the presence of exopolysaccharides (EPS). Most of the bacteria present in the environment produce EPS as their metabolic by-products. EPS are produced by many bacterial cells from environmental habitats to protect bacterial cells [13]. EPS has the ability to inhibit the aggregation of NPs due to its adsorption onto NPs surface [14]. There is an urgent need to evaluate the factors influencing the toxicity of nanoparticles [15]. The researchers are reported that the photocatalytic activity of NPs increases the toxic potential of NPs. From this point of view, the present study evaluates the effect of EPS on the photocatalytic activity of ZnO NPs.
2. Materials and methods 2.1. Materials All the chemicals were obtained from Merck Chemicals Ltd., India. All chemicals possess 99% purity. All the chemicals used for the study were of analytical grade. UV–vis absorption
612
P. Chandran et al. / Colloids and Surfaces B: Biointerfaces 122 (2014) 611–616
spectra was recorded by using a double beam Lambda 25 UV–visible spectrophotometer (Perkin Elmer, USA).
2.2. Preparation of ZnO NPs The ZnO NPs were prepared by sol–gel method where zinc acetate dihydrate (Zn (CH3 COO)2 ·2H2 O) and tri-ethanolamine (TEA) were chosen as precursor and stabilizing agent respectively. Ethanol and ammonium hydroxide takes care for the homogeneity and pH value of the solution and helps to make a stoichiometric solution to get ZnO NPs. Briefly, 20 ml of water was added with 30 ml of TEA, followed by the addition of ethanol (2 ml) drop wise under continuous stirring to get a homogeneous solution. Thereafter, 0.5 M zinc acetate solution (50 ml) was added and stirred at 80 ◦ C for 30 min. Ammonia was added drop wise until the white milky precipitate is formed. The precipitate was collected and dried in hot-air oven at 60 ◦ C. The obtained sample was calcined at 500 ◦ C for 60 min in a muffle furnace.
2.3. Characterization of ZnO NPs The preliminary characterization of NPs was done by using a double beam Lambda 25 UV–visible spectrophotometer. For XRD analysis, lyophilized nanoparticles were coated on XRD grid and the spectra was recorded using Bruker AXS Diffractometer (D8 Focus, Germany) operated at the voltage of 40 kV using Cu K␣ radiation. The surface state, morphology and structure of NPs were recorded using a field emission scanning electron microscopy (JEOL JSM6701F, Japan) at a magnification level of 6000× with an acceleration voltage of 3–35 kV and transmission electron microscopy (Joel JEM2100F field emission electron microscope, Japan). The surface area was measured using a Smart Sorb 93 Single point BET surface area analyzer (Smart Instruments Co. Pvt. Ltd., Mumbai, India). The zeta potential of the synthesized nanoparticles was determined by zeta sizer (Nanoseries, Nano-ZS, UK). Size distribution of the particles was determined using particle size analyzer (Microtrac Blue Wave, Nikkiso, Japan).
2.4. Extraction of EPS The bacterial strain used in this study is Pseudomonas Putida MTCC 4910. The freeze dried form of bacteria is revived its life by growing in Luria–Bertani broth medium at 37 ◦ C, under shaking at 150 rpm for 24 h. For the extraction of EPS, overnight culture was grown in a medium containing 3% of sucrose to induce the EPS production. The flaks were left in a shaker at 150 rpm until culture color changes into green-yellowish. The culture was centrifuged at 10,000 × g for 10 min and the supernatant was collected. To the supernatant equal volume of 95% ethanol was added, and kept at −20 ◦ C overnight. The formed precipitate was collected by centrifugation at 10,000 rpm for 30 min. The pellet was collected and air-dried.
proportional to the absorbance. Hence the degradation efficiency can be calculated by following the equation [16]: R=
C − C 0 C0
× 100
(1)
where C0 and C are the concentration of MB at time 0 and t, respectively. The effect of EPS on the photocatalytic activity of ZnO NPs was evaluated in presence of 1, 10 and 100 mg/L EPS. 3. Results and discussion 3.1. Characterization of ZnO NPs UV–visible spectroscopy technique is used for the preliminary characterization of NPs. UV–vis absorption spectra of ZnO NPs showed absorption maxima at 385 nm (Fig. 1a). Size and morphology of NPs were characterized by SEM (Fig. 1b). The microscopic images showed that NPs were poly dispersed. The particle size distribution analysis of NPs showed a mean diameter of 55 ± 1.2 nm. The histogram of size distribution of ZnO NPs is shown in Fig. 1c. Fig. 1d shows the TEM image of ZnO NPs and hexagonal shaped NPs can be clearly seen in the image. The XRD pattern of ZnO NPs revealed that the synthesized particles were pure and crystalline in nature. The facets (2 value) observed at 31.8, 34.2, 36.2, 47.5 and 56.6 were assigned to (1 0 0), (0 0 2), (1 0 1), (1 0 2) and (1 1 0) reflection lines of hexagonal ZnO NPs respectively (Fig. 1e). From this it is clear that the characteristic peaks represent ZnO NPs with hexagonal phase. By using Scherrer’s equation the average crystalline size was obtained from the XRD peaks [17] and the calculated particles diameter was 52 nm. The broadening of the diffraction peaks indicates the formation of particles in nano range. 3.2. Adsorption process The study of adsorption characteristics of MB on the surface of photocatalyst is very important in the photo-oxidation or photodegradation process. The adsorption process is usually influenced by pH of the interaction medium due to the modification of the electrical double layer of the solid electrolyte interface [18]. At acidic condition, the photocatalyst possess positive surface charge whereas, it possess negative surface charge at alkaline condition [19]. Cationic dye favors the adsorption at alkaline pH, where as anionic dye favors the adsorption at acidic pH. Since MB is a cationic dye, the alkaline pH favors the adsorption on the surface of photocatalyst. The electrostatic repulsive force exists between the photocatalyst and dye did not favor the adsorption of MB on photocatalyst surface. The adsorption of MB on ZnO NPs surface was tested using suspensions of MB and NPs in dark condition. The pH of samples was adjusted by using 0.1 N HCl or NaOH solutions. The adsorption process completed in 40 min of interaction and further no more adsorption occurred. Fig. 2 shows the adsorption of MB onto the photocatalysts surface. The highest adsorption was noted at pH 10, while the lease adsorption was noted at pH 5. Intermediate adsorption situation was observed at neutral pH.
2.5. Evaluation of photocatalytic activity
3.3. Photodegradation process
To determine the photocatalytic activity of ZnO NPs, 10 mg/L of MB solution was interacted with 5 mg/L of NPs. The solution was exposed to a 400 W Xenon lamp placed 30 cm above the dishes. The photocatalytic efficiency was evaluated based on the degradation of MB at 10 min time intervals by measuring the absorbance at 665 nm using UV–visible spectrophotometer. A control set was run without photocatalyst to evaluate the photolysis of MB. According to the Beer–Lambert law, the concentration of MB is directly
The photocatalytic degradation process was performed at pH 5, since pH 5 showed the least adsorption and its slight adsorption onto the photocatalyst surface was neglected. However, the photodegradation efficiency is expected to increase with pH, since higher pH provides higher concentration of hydroxyl ions to react with holes to form hydroxyl radicals [20]. The irradiated NPs are capable of destroying many organic contaminants in the presence of air or oxygen. The activation of ZnO NPs by light (h) produces
P. Chandran et al. / Colloids and Surfaces B: Biointerfaces 122 (2014) 611–616
613
Fig. 1. Characterization of ZnO NPs. (a) UV–vis absorption spectra, (b) scanning electron microscopic image, (c) particle size distribution and (d) transmission electron microscopic image and (e) X-ray diffraction pattern for ZnO NPs.
electron–hole pairs which are powerful agents to oxidize and reduce the compound. When the particle size is small, the photo-generated electrons and holes can transfer to the surface of the crystal and then react with H2 O and oxygen [21]. The hydroxyl radical comes from the oxidation of adsorbed water or adsorbed OH− , is the primary oxidant that degrading the dye molecules. The re-combination of holeelectron pairs can be prevented by the presence of oxygen.
The UV–vis spectral changes of MB due to photocatalytic degradation as a function of irradiation time in the presence of ZnO NPs are presented in Fig. 3. Here the photodegradation of MB by ZnO NPs was evaluated at 10 min intervals under visible light exposure. The results show that the intensity of the MB absorption peaks diminished gradually as the exposure time increased. After 60 min of NPs exposure under visible light, the absorption spectra of MB almost disappeared. Photolysis of MB in the absence of NPs
614
P. Chandran et al. / Colloids and Surfaces B: Biointerfaces 122 (2014) 611–616
0.8
Original Blank 5 mg/L NPs 1 mg/L EPS 10 mg/L EPS 100 mg/L EPS
Adsorption (1-C/C0)
0.7 Absorbance
0.6 0.5 0.4 0.3 0.2 0.1 0
450
6
7
8
9
10
Fig. 2. Effect of pH on the adsorption of MB on the surface of ZnO NPs in dark condition.
was also evaluated at different time intervals as mentioned earlier, and the reduction in the peak intensity was negligible. The photodegradation of MB was evaluated based on the changes in the concentration of MB and its degradation efficiency. The intersection of C/C0 and 1 − C/C0 indicates the half-life of MB, which is the time taken for the concentration of MB to reduce or degrade by its half. The half life period of MB by ZnO NPs was calculated to be 36 min. Rahman et al. [22] studied the photocatalytic activity of ZnO NPs by the degradation of rhodamine B dye, and they found that NPs could degrade 95% dye in 70 min. According to Wahab et al. [23], ZnO NPs exhibited excellent photocatalytic activity than that of commercial Degussa P-25 powder. In another study by Dutta and Basak [24] showed 64% photocatalytic degradation of methyl orange. The present study shows 95% photocatalytic degradation of MB at 60 min of visible light exposure. Tian et al. [25] developed an easy method to prepare large quantity of ZnO NPs with excellent photocatalytic performance by direct calcination of zinc acetate. Hong et al. [26] synthesized sheet-like ZnO assembly composed of small ZnO particles, which could effectively combine with Ag particles, resulting in a large increase in the photocatalytic activity of NPs. Fig. 4 shows the UV–vis absorption spectra of MB in the presence of 5 mg/L NPs with different EPS concentration. The peak intensity of MB was gradually increased upon the increase of EPS concentration. In the absence of EPS, the degradation of MB was almost done (95%) and it showed very low peak intensity. Where as in the presence of 1, 10 and 100 mg/L EPS, the degradation of MB were 93, 61, and 34%, respectively. The UV–vis absorption spectra of MB in the presence of 100 mg/L EPS was almost similar to blank (MB exposed to visible light in absence of ZnO NPs). The data indicated that 100 mg/L EPS can completely mask the photocatalytic property of ZnO NPs. It was reported that the bacterial exopolysaccharides 0.8
a
0.7
0 min 10 min 20 min 30 min 40 min 50 min 60 min
0.6 Absorbance
750
Fig. 4. UV–vis absorption spectra of MB solutions and the degraded dye solutions under different conditions at the end of 60 min under visible light. Blank indicates the MB solution simulated under visible light without NPs.
Initial pH value
0.5 0.4 0.3 0.2
have the ability to cap the NPs [14,27,28]. The capping may be so strong that reduces the photocatalytic effect of NPs. It was expected that the photocatalytic activity of ZnO NPs to be increased with increase in ZnO NPs content. For this reason and to find the optimal condition, a series of experiments were carried out with different NPs concentrations. Fig. 5a illustrates the photodegradation of MB treated with different concentrations of ZnO NPs, under exposure to visible light. Fig. 5a shows the plot of C/C0 vs time indicates the degradation of methylene blue at different time intervals. From the figure it is clear that the degradation of MB increased with increase in concentration of NPs. Blank (MB without NPs) shows very little degradation after 60 min of exposure to light. 50 mg/L NPs was able to degrade the dye completely with in 20 min of exposure to light. The rate of degradation of MB in the presence of ZnO NPs under visible light irradiation could be compared in terms of first-order rate constants. The photocatalytic reaction rate depends on concentration of the MB and can be described by the following kinetic model [29]: rate = −
dC kKC = 1 + KC dt
(2)
where C is the concentration of MB (mol/L) at any time, t is the irradiation time, k is first-order rate constant of the reaction and K is adsorption constant. This equation can be simplified to a pseudofirst-order equation: ln
C = −kKt = kobs t C0
(3)
where kobs is the observed first-order rate constant of the photodegradation reaction which can be calculated using the plot of ln C/C0 vs illumination time [30]. Fig. 5b shows the linear plots of ln(C/C0 ) for the photodegradation of MB with different NPs concentrations. The slops indicate the photodegradation rate constants and it can be seen that the 1.2 C/C0 and 1-C/C0
5
550 650 Wavelength (nm)
b
1
C/C0 1-C/C0
0.8 0.6
0.4 0.2
0.1
0
0
450
550 650 Wave length (nm)
750
0
20
40 Time (min)
60
Fig. 3. (a) UV–vis absorption spectral changes and (b) photodegradation of MB by ZnO NPs under visible light. Values are the mean of n = 3 (mean ± standard error).
P. Chandran et al. / Colloids and Surfaces B: Biointerfaces 122 (2014) 611–616
1.2
b 0.2
1 0.8 C/C0
0
Blank 5 mg/L NPs 10 mg/L NPs 25 mg/L NPs 50 mg/L NPs
0.6 0.4
-0.2 ln(C/C0)
a
615
20
0
40
60
-0.4 -0.6 -0.8
Blank 5 mg/L NPs 10 mg/L NPs 25 mg/L NPs 50 mg/L NPs
-1
0.2
-1.2
0 0
20
40 Time (min)
60
80
80
-1.4 Time (min)
Fig. 5. (a) The normalized concentration change of MB in the absence and presence of different concentration of ZnO NPs under visible light and (b) linear plots of ln(C/C0 ) for the photodegradation of MB under visible light in the absence and presence of different concentration of ZnO NPs. Values are the mean of n = 3 (mean ± standard error).
0.2
1
0
Blank 5 mg/L NPs 1 mg/L EPS 10 mg/L EPS 100 mg/L EPS
0.8 0.6 0.4
b 0
20
40
-0.6
Blank 5 mg/L NPs 1 mg/L EPS 10 mg/L EPS 100 mg/L EPS
-1
0 0
20
40 Time (min)
60
80
80
-0.4
-0.8
0.2
60
-0.2 ln(C/C0)
C/C0
a 1.2
-1.2 Time (min)
Fig. 6. (a) The normalized concentration change of MB by ZnO NPs in the absence and presence of different concentration of EPS under visible light and (b) linear plots of ln(C/C0 ) for the photodegradation of MB by ZnO NPs under visible light in the absence and presence of different concentration of EPS. Values are the mean of n = 3 (mean ± standard error).
rate of reaction in the degradation of MB increases with increase in NPs content. The photocatalytic degradation reaction constant for 5, 10, 25 and 50 mg/L ZnO NPs was determined to be 16 × 10−3 m−1 , 19.9 × 10−3 m−1 , 28.5 × 10−3 m−1 and 57.9 × 10−3 m−1 respectively. Song et al. [31] synthesized wurtzite ZnO with special morphologies such as nanoparticles, nano wires, dandelion-like, peanut-like, and micro spheres and he found dandelion-like ZnO had a highest efficiency of 96.2%. In another study nano disks had the best photocatalytic performance among the other morphologies [32]. Here we synthesized wurtzite NPs with spherical shape and it exhibited 100% degradation in 20 min. In order to evaluate the effect of EPS on photocatalytic activity of ZnO NPs, the photodegradation phenomenon observed under different EPS conditions and photolysis (in the absence of photocatalyst) is presented in Fig. 6a. It can be clearly seen that the photodegradation of MB decreased with increase in EPS content. The photodegradation of MB in the presence of 100 mg/L EPS was almost similar to blank solution. The photocatalytic efficiency of NPs is based on the ability to generate • OH radicals. Curri et al. [33] reported that Besides the essential role played by oxygen in generating the oxidizing species (H2 O2 and • OH), the photogenerated charge carriers at the semiconductor/liquid interface, is influenced to some extent by the surface properties of the semiconductor, depending on pH, on surface hydroxyl groups and on adsorbed charged molecules. EPS is reported to be negatively charge molecule [14]. Yu et al. [34] reported that fluoride influenced the photoactivity of mesoporous anatase hollow microspheres. Here we can say that the capped EPS might prevent the photogenerated holes and electrons to react with OH− /H2 O and O2 to form • OH and O2 − , respectively. The reduction in • OH amount obviously resulted in the reduction in photoactivity of ZnO NPs. Soltani et al. [19] studied the effect of inorganic capping on
photocatalytic effect of NPs, and they found that the inorganic capping increased the photocatalytic effect of NPs. At the same time Luo and Gao [35] reported the decrease in the photoactivity of TiO2 pigment on doping with transition metals. They suggested that decrease in photoactivity of TiO2 might be due to the d electrons of molybdenum (4d) and vanadium (3d), can effectively quench the high energy photogenerated holes at the impurity levels introduced by doping within the band gap of TiO2 . Fig. 6b shows the linear plots of ln(C/C0 ) for the photodegradation of MB with different EPS concentrations. It can be seen that the rate of reaction decreased when the concentration of EPS increased. The rate constant for 5 mg/L NPs was determined to be 16 × 10−3 m−1 . In the presence of EPS the rate constant of the reaction was decreased to 14.8 × 10−3 m−1 , 5 × 10−3 m−1 and 1.9 × 10−3 m−1 for 1, 10 and 100 mg/L EPS respectively. The very low rate constant value indicates the low level of photodegradation of MB. 4. Conclusion The present study evaluated the effect of EPS on the photocatalytic activity of ZnO NPs. ZnO NPs possess excellent photocatalytic activity, but the presence of EPS reduced or masked the photocatalytic activity of NPs. The study suggests that the presence of EPS in the environment may limit the photocatalytic activity of NPs once the NPs are released into the environment; thereby reduce its toxic effect on environmental organisms. Acknowledgment Authors sincerely thank Dr. Soudamini Menon, Professor at School of Education, SASTRA University for proof reading our manuscript.
616
P. Chandran et al. / Colloids and Surfaces B: Biointerfaces 122 (2014) 611–616
References [1] M. Premanathan, K. Karthikeyan, K. Jeyasubramanian, G. Manivannan, Nanomed. Nanotechnol. 7 (2011) 184–192. [2] R. Wahab, N.K. Kaushik, A.K. Verma, A. Mishra, I.H. Hwang, Y.B. Yang, H.S. Shin, Y.S. Kim, J. Biol. Inorg. Chem. 16 (2011) 431–442. [3] G.S. Mital, T. Manoj, Chin. Sci. Bull. 56 (2011) 1639–1657. [4] H. Ma, P.L. Williams, S.A. Diamond, Environ. Pollut. 172 (2013) 76–85. [5] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69–96. [6] N. Serpone, D. Dondi, A. Albini, Inorg. Chim. Acta 360 (2007) 794–802. [7] T. Yuranova, D. Laub, J. Kiwi, Catal. Today 12 (2007) 109–117. [8] P. Borm, F.C. Klaessig, T.D. Landry, B. Moudgil, J. Pauluhn, K. Thomas, R. Trottier, S. Wood, Toxicol. Sci. 90 (2006) 23–32. [9] A. Boxall, Q. Chaudhry, C. Sinclair, A. Jones, R. Aitken, B. Jefferson, C. Watts, Current and Future Predicted Environmental Exposure to Engineered Nanoparticles, Central Science Laboratory, York, UK, 2007. [10] W.-M. Lee, Y.-J. An, Chemosphere 91 (2013) 536–544. [11] L.Z. Li, D.M. Zhou, W.J.G.M. Peijnenburg, C.A.M. van Gestel, S.Y. Jin, Y.J. Wang, P. Wang, Environ. Int. 37 (2011) 1098–1104. [12] I. Blinova, A. Ivask, M. Heinlaan, M. Mortimer, A. Kahru, Environ. Pollut. 158 (2010) 41–47. [13] A. Miao, K.A. Schwehr, C. Xu, S. Zhang, Z. Luo, A. Quigg, P.H. Santschi, Environ. Pollut. 157 (2009) 3034–3041. [14] S. Sudheer Khan, A. Mukherjee, N. Chandrasekaran, Colloid Surf. A: Physicochem. Eng. Asp. 381 (2011) 99–105. [15] R. Behra, H. Krug, Nat. Nanotechnol. 3 (2008) 253–254. [16] D. Sun, M. Gu, R. Li, S. Yin, X. Song, B. Zhao, C. Li, J. Li, Z. Feng, T. Sato, Appl. Surf. Sci. 280 (2013) 693–697.
[17] A. Becheri, M. Dürr, P.L. Nostro, P. Baglioni, J. Nanopart. Res. 10 (2008) 679–689. [18] A. Franco, M.C. Neves, M.M.L.R. Carrott, M.H. Mendonca, M.I. Pereira, O.C. Monteiro, J. Hazard. Mater. 161 (2009) 545–550. [19] N. Soltani, E. Saion, W.M.M. Yunus, M. Erfani, M. Navasery, G. Bahmanrokh, K. Rezaee, Appl. Surf. Sci. 290 (2014) 440–447. [20] H.R. Rajabi, O. Khani, M. Shamsipur, V. Vatanpour, J. Hazard. Mater. 250–251 (2013) 370–380. [21] X. Li, C. Hu, X. Wang, Y. Xi, Appl. Surf. Sci. 258 (2012) 4370–4376. [22] Q.I. Rahman, M. Ahmad, S.K. Misra, M. Lohani, Mater. Lett. 91 (2013) 170–174. [23] R. Wahab, I.H. Hwang, Y.-S. Kim, J. Musarrat, M.A. Siddiqui, H.-K. Seo, S.K. Tripathy, H.-S. Shin, Chem. Eng. J. 175 (2011) 450–457. [24] M. Dutta, D.A. Basak, Nanotechnology 20 (2009) 475602. [25] C. Tian, Q. Zhang, A. Wu, M. Jiang, Z. Liang, B. Jiang, H. Fu, Chem. Commun. 48 (2012) 2858–2860. [26] Y. Hong, C. Tian, B. Jiang, A. Wu, Q. Zhang, G. Tian, H. Fu, J. Mater. Chem. A 1 (2013) 5700. [27] S. Sudheer Khan, A. Mukherjee, N. Chandrasekaran, Water Res. 45 (2011) 5184–5190. [28] S. Sudheer Khan, P. Srivatsan, N. Vaishnavi, A. Mukherjee, N. Chandrasekaran, J. Hazard. Mater. 192 (2011) 299–306. [29] V. Taghvaei, A. Habibi-Yangjeh, M. Behboudnia, Physica E 42 (2010) 1973–1978. [30] X. Xu, R. Lu, X. Zhao, S. Xu, X. Lei, F. Zhang, D.G. Evans, Appl. Catal. B 102 (2011) 147–156. [31] L. Song, Q. Jiang, P. Du, Y. Yang, J. Xiong, Mater. Lett. 123 (2014) 214–216. [32] M. Farbod, E. Jafarpoor, Ceram. Int. 40 (2014) 6605–6610. [33] M.L. Curri, R. Comparelli, P.D. Cozzoli, G. Mascolo, A. Agostiano, Mater. Sci. Eng. C 23 (2003) 285–289. [34] J. Yu, S. Liu, H. Yu, J. Catal. 249 (2007) 59–66. [35] Z. Luo, Q.-H. Gao, J. Photochem. Photobiol. A 63 (1992) 367–375.
Contents lists available at ScienceDirect
Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
Effect of exopolysaccharides on photocatalytic activity of ZnO nanoparticles Preethy Chandran, Suhas Netha, Aswathy Ravindran, S. Sudheer Khan ∗ CeNTAB, School of Chemical and Biotechnology, SASTRA University, Thanjavur 638 401, India
a r t i c l e
i n f o
Article history: Received 23 April 2014 Received in revised form 17 July 2014 Accepted 24 July 2014 Available online 2 August 2014 Keywords: ZnO NPs Exopolysaccharides Methylene blue Visible light Photocatalytic activity
a b s t r a c t Zinc oxide nanoparticles (ZnO NPs) are largely used in consumer products and industrial applications. The increased use of such materials may lead to its release into the environment. The study used chemically synthesized ZnO NPs and characterized by using UV–visible spectrophotometer, scanning electron microscopy, particle size analyzer and X-ray diffraction (XRD) analysis. The mean diameter of the particles was found to be 55 ± 1.2 nm. The XRD patterns exhibited hexagonal structure for ZnO NPs. The photocatalytic property of ZnO NPs was evaluated based on the UV–vis spectra changes of the methylene blue solution as a function of reaction time in the presence of ZnO NPs under visible light. The study suggests that ZnO NPs can be used as an efficient photocatalyst and the environmental factor such as exopolysaccharides could mask the photocatalytic activity of NPs. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Nanotechnology is a rapidly growing field of research that has already introduced into variety of commercial application such as cosmetics, suntan lotions, paints and self-cleaning materials. NPs are size range from 1 to 100 nm and they have been attracted by their unique surface properties, and their potential use in photocatalysis and biomedicine applications [1–3]. Among them metal oxide NPs received considerable attention and they are being incorporated into variety of products based on catalytic capacity, optoelectronic and antimicrobial properties [4]. ZnO NPs are wide band-gap semiconductor and possess a large excitation binding energy. The particles received the specific attention for electronic sensor, solar voltaic, and transducer applications due to its piezoelectricity and wurtzite crystal structure. ZnO is an efficient photocatalyst and this property is utilized for the remediation of environmental pollutants to medical disinfection [5]. Currently ZnO NPs are largely used in personal care products such as toothpaste, beauty products, sunscreens and textile products [4,6,7]. The use of NPs in sunscreen products alone is estimated to be 1000 tons during 2003/2004, consisting TiO2 and ZnO NPs [8]. According to Box all et al. [9] the estimated amount of ZnO NP concentrations in the UK sewage was less than 100 g/L. The indiscriminate use of ZnO NPs has resulted for the release of such
∗ Corresponding author. Tel.: +91 9047286362; fax: +91 4362 264120. E-mail address: [email protected] (S. Sudheer Khan). http://dx.doi.org/10.1016/j.colsurfb.2014.07.039 0927-7765/© 2014 Elsevier B.V. All rights reserved.
materials to the environment which increases the environmental levels and ecotoxicity of these materials [10]. The eco-toxicological evaluation of ZnO NPs toward a broad range of organisms has been studied extensively. ZnO NPs possess toxicity to environmentally relevant bacterial, algal, vertebrates and invertebrate species [1,10–12]. Being a photocatalyst, ZnO NPs promotes ROS generation under irradiation [4]. The toxic potential of ZnO NPs can also be induced due to its photocatalytic effect [10]. The toxicological effect of the NPs is limited by environmental factors such as the presence of exopolysaccharides (EPS). Most of the bacteria present in the environment produce EPS as their metabolic by-products. EPS are produced by many bacterial cells from environmental habitats to protect bacterial cells [13]. EPS has the ability to inhibit the aggregation of NPs due to its adsorption onto NPs surface [14]. There is an urgent need to evaluate the factors influencing the toxicity of nanoparticles [15]. The researchers are reported that the photocatalytic activity of NPs increases the toxic potential of NPs. From this point of view, the present study evaluates the effect of EPS on the photocatalytic activity of ZnO NPs.
2. Materials and methods 2.1. Materials All the chemicals were obtained from Merck Chemicals Ltd., India. All chemicals possess 99% purity. All the chemicals used for the study were of analytical grade. UV–vis absorption
612
P. Chandran et al. / Colloids and Surfaces B: Biointerfaces 122 (2014) 611–616
spectra was recorded by using a double beam Lambda 25 UV–visible spectrophotometer (Perkin Elmer, USA).
2.2. Preparation of ZnO NPs The ZnO NPs were prepared by sol–gel method where zinc acetate dihydrate (Zn (CH3 COO)2 ·2H2 O) and tri-ethanolamine (TEA) were chosen as precursor and stabilizing agent respectively. Ethanol and ammonium hydroxide takes care for the homogeneity and pH value of the solution and helps to make a stoichiometric solution to get ZnO NPs. Briefly, 20 ml of water was added with 30 ml of TEA, followed by the addition of ethanol (2 ml) drop wise under continuous stirring to get a homogeneous solution. Thereafter, 0.5 M zinc acetate solution (50 ml) was added and stirred at 80 ◦ C for 30 min. Ammonia was added drop wise until the white milky precipitate is formed. The precipitate was collected and dried in hot-air oven at 60 ◦ C. The obtained sample was calcined at 500 ◦ C for 60 min in a muffle furnace.
2.3. Characterization of ZnO NPs The preliminary characterization of NPs was done by using a double beam Lambda 25 UV–visible spectrophotometer. For XRD analysis, lyophilized nanoparticles were coated on XRD grid and the spectra was recorded using Bruker AXS Diffractometer (D8 Focus, Germany) operated at the voltage of 40 kV using Cu K␣ radiation. The surface state, morphology and structure of NPs were recorded using a field emission scanning electron microscopy (JEOL JSM6701F, Japan) at a magnification level of 6000× with an acceleration voltage of 3–35 kV and transmission electron microscopy (Joel JEM2100F field emission electron microscope, Japan). The surface area was measured using a Smart Sorb 93 Single point BET surface area analyzer (Smart Instruments Co. Pvt. Ltd., Mumbai, India). The zeta potential of the synthesized nanoparticles was determined by zeta sizer (Nanoseries, Nano-ZS, UK). Size distribution of the particles was determined using particle size analyzer (Microtrac Blue Wave, Nikkiso, Japan).
2.4. Extraction of EPS The bacterial strain used in this study is Pseudomonas Putida MTCC 4910. The freeze dried form of bacteria is revived its life by growing in Luria–Bertani broth medium at 37 ◦ C, under shaking at 150 rpm for 24 h. For the extraction of EPS, overnight culture was grown in a medium containing 3% of sucrose to induce the EPS production. The flaks were left in a shaker at 150 rpm until culture color changes into green-yellowish. The culture was centrifuged at 10,000 × g for 10 min and the supernatant was collected. To the supernatant equal volume of 95% ethanol was added, and kept at −20 ◦ C overnight. The formed precipitate was collected by centrifugation at 10,000 rpm for 30 min. The pellet was collected and air-dried.
proportional to the absorbance. Hence the degradation efficiency can be calculated by following the equation [16]: R=
C − C 0 C0
× 100
(1)
where C0 and C are the concentration of MB at time 0 and t, respectively. The effect of EPS on the photocatalytic activity of ZnO NPs was evaluated in presence of 1, 10 and 100 mg/L EPS. 3. Results and discussion 3.1. Characterization of ZnO NPs UV–visible spectroscopy technique is used for the preliminary characterization of NPs. UV–vis absorption spectra of ZnO NPs showed absorption maxima at 385 nm (Fig. 1a). Size and morphology of NPs were characterized by SEM (Fig. 1b). The microscopic images showed that NPs were poly dispersed. The particle size distribution analysis of NPs showed a mean diameter of 55 ± 1.2 nm. The histogram of size distribution of ZnO NPs is shown in Fig. 1c. Fig. 1d shows the TEM image of ZnO NPs and hexagonal shaped NPs can be clearly seen in the image. The XRD pattern of ZnO NPs revealed that the synthesized particles were pure and crystalline in nature. The facets (2 value) observed at 31.8, 34.2, 36.2, 47.5 and 56.6 were assigned to (1 0 0), (0 0 2), (1 0 1), (1 0 2) and (1 1 0) reflection lines of hexagonal ZnO NPs respectively (Fig. 1e). From this it is clear that the characteristic peaks represent ZnO NPs with hexagonal phase. By using Scherrer’s equation the average crystalline size was obtained from the XRD peaks [17] and the calculated particles diameter was 52 nm. The broadening of the diffraction peaks indicates the formation of particles in nano range. 3.2. Adsorption process The study of adsorption characteristics of MB on the surface of photocatalyst is very important in the photo-oxidation or photodegradation process. The adsorption process is usually influenced by pH of the interaction medium due to the modification of the electrical double layer of the solid electrolyte interface [18]. At acidic condition, the photocatalyst possess positive surface charge whereas, it possess negative surface charge at alkaline condition [19]. Cationic dye favors the adsorption at alkaline pH, where as anionic dye favors the adsorption at acidic pH. Since MB is a cationic dye, the alkaline pH favors the adsorption on the surface of photocatalyst. The electrostatic repulsive force exists between the photocatalyst and dye did not favor the adsorption of MB on photocatalyst surface. The adsorption of MB on ZnO NPs surface was tested using suspensions of MB and NPs in dark condition. The pH of samples was adjusted by using 0.1 N HCl or NaOH solutions. The adsorption process completed in 40 min of interaction and further no more adsorption occurred. Fig. 2 shows the adsorption of MB onto the photocatalysts surface. The highest adsorption was noted at pH 10, while the lease adsorption was noted at pH 5. Intermediate adsorption situation was observed at neutral pH.
2.5. Evaluation of photocatalytic activity
3.3. Photodegradation process
To determine the photocatalytic activity of ZnO NPs, 10 mg/L of MB solution was interacted with 5 mg/L of NPs. The solution was exposed to a 400 W Xenon lamp placed 30 cm above the dishes. The photocatalytic efficiency was evaluated based on the degradation of MB at 10 min time intervals by measuring the absorbance at 665 nm using UV–visible spectrophotometer. A control set was run without photocatalyst to evaluate the photolysis of MB. According to the Beer–Lambert law, the concentration of MB is directly
The photocatalytic degradation process was performed at pH 5, since pH 5 showed the least adsorption and its slight adsorption onto the photocatalyst surface was neglected. However, the photodegradation efficiency is expected to increase with pH, since higher pH provides higher concentration of hydroxyl ions to react with holes to form hydroxyl radicals [20]. The irradiated NPs are capable of destroying many organic contaminants in the presence of air or oxygen. The activation of ZnO NPs by light (h) produces
P. Chandran et al. / Colloids and Surfaces B: Biointerfaces 122 (2014) 611–616
613
Fig. 1. Characterization of ZnO NPs. (a) UV–vis absorption spectra, (b) scanning electron microscopic image, (c) particle size distribution and (d) transmission electron microscopic image and (e) X-ray diffraction pattern for ZnO NPs.
electron–hole pairs which are powerful agents to oxidize and reduce the compound. When the particle size is small, the photo-generated electrons and holes can transfer to the surface of the crystal and then react with H2 O and oxygen [21]. The hydroxyl radical comes from the oxidation of adsorbed water or adsorbed OH− , is the primary oxidant that degrading the dye molecules. The re-combination of holeelectron pairs can be prevented by the presence of oxygen.
The UV–vis spectral changes of MB due to photocatalytic degradation as a function of irradiation time in the presence of ZnO NPs are presented in Fig. 3. Here the photodegradation of MB by ZnO NPs was evaluated at 10 min intervals under visible light exposure. The results show that the intensity of the MB absorption peaks diminished gradually as the exposure time increased. After 60 min of NPs exposure under visible light, the absorption spectra of MB almost disappeared. Photolysis of MB in the absence of NPs
614
P. Chandran et al. / Colloids and Surfaces B: Biointerfaces 122 (2014) 611–616
0.8
Original Blank 5 mg/L NPs 1 mg/L EPS 10 mg/L EPS 100 mg/L EPS
Adsorption (1-C/C0)
0.7 Absorbance
0.6 0.5 0.4 0.3 0.2 0.1 0
450
6
7
8
9
10
Fig. 2. Effect of pH on the adsorption of MB on the surface of ZnO NPs in dark condition.
was also evaluated at different time intervals as mentioned earlier, and the reduction in the peak intensity was negligible. The photodegradation of MB was evaluated based on the changes in the concentration of MB and its degradation efficiency. The intersection of C/C0 and 1 − C/C0 indicates the half-life of MB, which is the time taken for the concentration of MB to reduce or degrade by its half. The half life period of MB by ZnO NPs was calculated to be 36 min. Rahman et al. [22] studied the photocatalytic activity of ZnO NPs by the degradation of rhodamine B dye, and they found that NPs could degrade 95% dye in 70 min. According to Wahab et al. [23], ZnO NPs exhibited excellent photocatalytic activity than that of commercial Degussa P-25 powder. In another study by Dutta and Basak [24] showed 64% photocatalytic degradation of methyl orange. The present study shows 95% photocatalytic degradation of MB at 60 min of visible light exposure. Tian et al. [25] developed an easy method to prepare large quantity of ZnO NPs with excellent photocatalytic performance by direct calcination of zinc acetate. Hong et al. [26] synthesized sheet-like ZnO assembly composed of small ZnO particles, which could effectively combine with Ag particles, resulting in a large increase in the photocatalytic activity of NPs. Fig. 4 shows the UV–vis absorption spectra of MB in the presence of 5 mg/L NPs with different EPS concentration. The peak intensity of MB was gradually increased upon the increase of EPS concentration. In the absence of EPS, the degradation of MB was almost done (95%) and it showed very low peak intensity. Where as in the presence of 1, 10 and 100 mg/L EPS, the degradation of MB were 93, 61, and 34%, respectively. The UV–vis absorption spectra of MB in the presence of 100 mg/L EPS was almost similar to blank (MB exposed to visible light in absence of ZnO NPs). The data indicated that 100 mg/L EPS can completely mask the photocatalytic property of ZnO NPs. It was reported that the bacterial exopolysaccharides 0.8
a
0.7
0 min 10 min 20 min 30 min 40 min 50 min 60 min
0.6 Absorbance
750
Fig. 4. UV–vis absorption spectra of MB solutions and the degraded dye solutions under different conditions at the end of 60 min under visible light. Blank indicates the MB solution simulated under visible light without NPs.
Initial pH value
0.5 0.4 0.3 0.2
have the ability to cap the NPs [14,27,28]. The capping may be so strong that reduces the photocatalytic effect of NPs. It was expected that the photocatalytic activity of ZnO NPs to be increased with increase in ZnO NPs content. For this reason and to find the optimal condition, a series of experiments were carried out with different NPs concentrations. Fig. 5a illustrates the photodegradation of MB treated with different concentrations of ZnO NPs, under exposure to visible light. Fig. 5a shows the plot of C/C0 vs time indicates the degradation of methylene blue at different time intervals. From the figure it is clear that the degradation of MB increased with increase in concentration of NPs. Blank (MB without NPs) shows very little degradation after 60 min of exposure to light. 50 mg/L NPs was able to degrade the dye completely with in 20 min of exposure to light. The rate of degradation of MB in the presence of ZnO NPs under visible light irradiation could be compared in terms of first-order rate constants. The photocatalytic reaction rate depends on concentration of the MB and can be described by the following kinetic model [29]: rate = −
dC kKC = 1 + KC dt
(2)
where C is the concentration of MB (mol/L) at any time, t is the irradiation time, k is first-order rate constant of the reaction and K is adsorption constant. This equation can be simplified to a pseudofirst-order equation: ln
C = −kKt = kobs t C0
(3)
where kobs is the observed first-order rate constant of the photodegradation reaction which can be calculated using the plot of ln C/C0 vs illumination time [30]. Fig. 5b shows the linear plots of ln(C/C0 ) for the photodegradation of MB with different NPs concentrations. The slops indicate the photodegradation rate constants and it can be seen that the 1.2 C/C0 and 1-C/C0
5
550 650 Wavelength (nm)
b
1
C/C0 1-C/C0
0.8 0.6
0.4 0.2
0.1
0
0
450
550 650 Wave length (nm)
750
0
20
40 Time (min)
60
Fig. 3. (a) UV–vis absorption spectral changes and (b) photodegradation of MB by ZnO NPs under visible light. Values are the mean of n = 3 (mean ± standard error).
P. Chandran et al. / Colloids and Surfaces B: Biointerfaces 122 (2014) 611–616
1.2
b 0.2
1 0.8 C/C0
0
Blank 5 mg/L NPs 10 mg/L NPs 25 mg/L NPs 50 mg/L NPs
0.6 0.4
-0.2 ln(C/C0)
a
615
20
0
40
60
-0.4 -0.6 -0.8
Blank 5 mg/L NPs 10 mg/L NPs 25 mg/L NPs 50 mg/L NPs
-1
0.2
-1.2
0 0
20
40 Time (min)
60
80
80
-1.4 Time (min)
Fig. 5. (a) The normalized concentration change of MB in the absence and presence of different concentration of ZnO NPs under visible light and (b) linear plots of ln(C/C0 ) for the photodegradation of MB under visible light in the absence and presence of different concentration of ZnO NPs. Values are the mean of n = 3 (mean ± standard error).
0.2
1
0
Blank 5 mg/L NPs 1 mg/L EPS 10 mg/L EPS 100 mg/L EPS
0.8 0.6 0.4
b 0
20
40
-0.6
Blank 5 mg/L NPs 1 mg/L EPS 10 mg/L EPS 100 mg/L EPS
-1
0 0
20
40 Time (min)
60
80
80
-0.4
-0.8
0.2
60
-0.2 ln(C/C0)
C/C0
a 1.2
-1.2 Time (min)
Fig. 6. (a) The normalized concentration change of MB by ZnO NPs in the absence and presence of different concentration of EPS under visible light and (b) linear plots of ln(C/C0 ) for the photodegradation of MB by ZnO NPs under visible light in the absence and presence of different concentration of EPS. Values are the mean of n = 3 (mean ± standard error).
rate of reaction in the degradation of MB increases with increase in NPs content. The photocatalytic degradation reaction constant for 5, 10, 25 and 50 mg/L ZnO NPs was determined to be 16 × 10−3 m−1 , 19.9 × 10−3 m−1 , 28.5 × 10−3 m−1 and 57.9 × 10−3 m−1 respectively. Song et al. [31] synthesized wurtzite ZnO with special morphologies such as nanoparticles, nano wires, dandelion-like, peanut-like, and micro spheres and he found dandelion-like ZnO had a highest efficiency of 96.2%. In another study nano disks had the best photocatalytic performance among the other morphologies [32]. Here we synthesized wurtzite NPs with spherical shape and it exhibited 100% degradation in 20 min. In order to evaluate the effect of EPS on photocatalytic activity of ZnO NPs, the photodegradation phenomenon observed under different EPS conditions and photolysis (in the absence of photocatalyst) is presented in Fig. 6a. It can be clearly seen that the photodegradation of MB decreased with increase in EPS content. The photodegradation of MB in the presence of 100 mg/L EPS was almost similar to blank solution. The photocatalytic efficiency of NPs is based on the ability to generate • OH radicals. Curri et al. [33] reported that Besides the essential role played by oxygen in generating the oxidizing species (H2 O2 and • OH), the photogenerated charge carriers at the semiconductor/liquid interface, is influenced to some extent by the surface properties of the semiconductor, depending on pH, on surface hydroxyl groups and on adsorbed charged molecules. EPS is reported to be negatively charge molecule [14]. Yu et al. [34] reported that fluoride influenced the photoactivity of mesoporous anatase hollow microspheres. Here we can say that the capped EPS might prevent the photogenerated holes and electrons to react with OH− /H2 O and O2 to form • OH and O2 − , respectively. The reduction in • OH amount obviously resulted in the reduction in photoactivity of ZnO NPs. Soltani et al. [19] studied the effect of inorganic capping on
photocatalytic effect of NPs, and they found that the inorganic capping increased the photocatalytic effect of NPs. At the same time Luo and Gao [35] reported the decrease in the photoactivity of TiO2 pigment on doping with transition metals. They suggested that decrease in photoactivity of TiO2 might be due to the d electrons of molybdenum (4d) and vanadium (3d), can effectively quench the high energy photogenerated holes at the impurity levels introduced by doping within the band gap of TiO2 . Fig. 6b shows the linear plots of ln(C/C0 ) for the photodegradation of MB with different EPS concentrations. It can be seen that the rate of reaction decreased when the concentration of EPS increased. The rate constant for 5 mg/L NPs was determined to be 16 × 10−3 m−1 . In the presence of EPS the rate constant of the reaction was decreased to 14.8 × 10−3 m−1 , 5 × 10−3 m−1 and 1.9 × 10−3 m−1 for 1, 10 and 100 mg/L EPS respectively. The very low rate constant value indicates the low level of photodegradation of MB. 4. Conclusion The present study evaluated the effect of EPS on the photocatalytic activity of ZnO NPs. ZnO NPs possess excellent photocatalytic activity, but the presence of EPS reduced or masked the photocatalytic activity of NPs. The study suggests that the presence of EPS in the environment may limit the photocatalytic activity of NPs once the NPs are released into the environment; thereby reduce its toxic effect on environmental organisms. Acknowledgment Authors sincerely thank Dr. Soudamini Menon, Professor at School of Education, SASTRA University for proof reading our manuscript.
616
P. Chandran et al. / Colloids and Surfaces B: Biointerfaces 122 (2014) 611–616
References [1] M. Premanathan, K. Karthikeyan, K. Jeyasubramanian, G. Manivannan, Nanomed. Nanotechnol. 7 (2011) 184–192. [2] R. Wahab, N.K. Kaushik, A.K. Verma, A. Mishra, I.H. Hwang, Y.B. Yang, H.S. Shin, Y.S. Kim, J. Biol. Inorg. Chem. 16 (2011) 431–442. [3] G.S. Mital, T. Manoj, Chin. Sci. Bull. 56 (2011) 1639–1657. [4] H. Ma, P.L. Williams, S.A. Diamond, Environ. Pollut. 172 (2013) 76–85. [5] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69–96. [6] N. Serpone, D. Dondi, A. Albini, Inorg. Chim. Acta 360 (2007) 794–802. [7] T. Yuranova, D. Laub, J. Kiwi, Catal. Today 12 (2007) 109–117. [8] P. Borm, F.C. Klaessig, T.D. Landry, B. Moudgil, J. Pauluhn, K. Thomas, R. Trottier, S. Wood, Toxicol. Sci. 90 (2006) 23–32. [9] A. Boxall, Q. Chaudhry, C. Sinclair, A. Jones, R. Aitken, B. Jefferson, C. Watts, Current and Future Predicted Environmental Exposure to Engineered Nanoparticles, Central Science Laboratory, York, UK, 2007. [10] W.-M. Lee, Y.-J. An, Chemosphere 91 (2013) 536–544. [11] L.Z. Li, D.M. Zhou, W.J.G.M. Peijnenburg, C.A.M. van Gestel, S.Y. Jin, Y.J. Wang, P. Wang, Environ. Int. 37 (2011) 1098–1104. [12] I. Blinova, A. Ivask, M. Heinlaan, M. Mortimer, A. Kahru, Environ. Pollut. 158 (2010) 41–47. [13] A. Miao, K.A. Schwehr, C. Xu, S. Zhang, Z. Luo, A. Quigg, P.H. Santschi, Environ. Pollut. 157 (2009) 3034–3041. [14] S. Sudheer Khan, A. Mukherjee, N. Chandrasekaran, Colloid Surf. A: Physicochem. Eng. Asp. 381 (2011) 99–105. [15] R. Behra, H. Krug, Nat. Nanotechnol. 3 (2008) 253–254. [16] D. Sun, M. Gu, R. Li, S. Yin, X. Song, B. Zhao, C. Li, J. Li, Z. Feng, T. Sato, Appl. Surf. Sci. 280 (2013) 693–697.
[17] A. Becheri, M. Dürr, P.L. Nostro, P. Baglioni, J. Nanopart. Res. 10 (2008) 679–689. [18] A. Franco, M.C. Neves, M.M.L.R. Carrott, M.H. Mendonca, M.I. Pereira, O.C. Monteiro, J. Hazard. Mater. 161 (2009) 545–550. [19] N. Soltani, E. Saion, W.M.M. Yunus, M. Erfani, M. Navasery, G. Bahmanrokh, K. Rezaee, Appl. Surf. Sci. 290 (2014) 440–447. [20] H.R. Rajabi, O. Khani, M. Shamsipur, V. Vatanpour, J. Hazard. Mater. 250–251 (2013) 370–380. [21] X. Li, C. Hu, X. Wang, Y. Xi, Appl. Surf. Sci. 258 (2012) 4370–4376. [22] Q.I. Rahman, M. Ahmad, S.K. Misra, M. Lohani, Mater. Lett. 91 (2013) 170–174. [23] R. Wahab, I.H. Hwang, Y.-S. Kim, J. Musarrat, M.A. Siddiqui, H.-K. Seo, S.K. Tripathy, H.-S. Shin, Chem. Eng. J. 175 (2011) 450–457. [24] M. Dutta, D.A. Basak, Nanotechnology 20 (2009) 475602. [25] C. Tian, Q. Zhang, A. Wu, M. Jiang, Z. Liang, B. Jiang, H. Fu, Chem. Commun. 48 (2012) 2858–2860. [26] Y. Hong, C. Tian, B. Jiang, A. Wu, Q. Zhang, G. Tian, H. Fu, J. Mater. Chem. A 1 (2013) 5700. [27] S. Sudheer Khan, A. Mukherjee, N. Chandrasekaran, Water Res. 45 (2011) 5184–5190. [28] S. Sudheer Khan, P. Srivatsan, N. Vaishnavi, A. Mukherjee, N. Chandrasekaran, J. Hazard. Mater. 192 (2011) 299–306. [29] V. Taghvaei, A. Habibi-Yangjeh, M. Behboudnia, Physica E 42 (2010) 1973–1978. [30] X. Xu, R. Lu, X. Zhao, S. Xu, X. Lei, F. Zhang, D.G. Evans, Appl. Catal. B 102 (2011) 147–156. [31] L. Song, Q. Jiang, P. Du, Y. Yang, J. Xiong, Mater. Lett. 123 (2014) 214–216. [32] M. Farbod, E. Jafarpoor, Ceram. Int. 40 (2014) 6605–6610. [33] M.L. Curri, R. Comparelli, P.D. Cozzoli, G. Mascolo, A. Agostiano, Mater. Sci. Eng. C 23 (2003) 285–289. [34] J. Yu, S. Liu, H. Yu, J. Catal. 249 (2007) 59–66. [35] Z. Luo, Q.-H. Gao, J. Photochem. Photobiol. A 63 (1992) 367–375.
