Effects of cetyltrimethylammonium bromide on the morphology of green synthesized Fe3O4 nanoparticles used to remove phosphate

Materials Science & Engineering C 82 (2018) 41–45

Contents lists available at ScienceDirect

Materials Science & Engineering C journal homepage: www.elsevier.com/locate/msec

Effects of cetyltrimethylammonium bromide on the morphology of green synthesized Fe3O4 nanoparticles used to remove phosphate

MARK

Li Gana, Zeyang Lua, Dan Caoa, Zuliang Chena,b,⁎ a Fujian Key Laboratory of Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Fujian Normal University, Fuzhou 350007, Fujian Province, China. b Global Centre for Environmental Remediation, University of Newcastle, Callaghan, NSW 2308, Australia.

A R T I C L E I N F O

A B S T R A C T

Keywords: Green synthesis Iron oxide nanoparticles Cetyltrimethylammonium bromide Phosphate removal

In this paper, iron oxide nanoparticles (IONPs) are successfully synthesized using Eucalyptus leaf extract in the presence of cetyltrimethylammonium bromide (CTAB) to enhance the dispersion and reduce aggregation of IONPs. CTAB was used as a stabilizing and capping agent in biosynthesis of IONP was observed. The particle size decreased from 183.9 ± 30.1 nm to 89.8 ± 17.1 nm as the concentration of CTAB increased from 0 to 0.4 mM CTAB, indicating that CTAB reduce the aggregation of IONPs and enhance the reactivity. In addition, the removal efficiency of phosphate declined from 95.13% to 89.58% when the CTAB concentration increased from 0.4 to 10 mM, indicating that a CTAB impacted on micelles and lipophilic biomolecules in Eucalyptus leaf extract, and hence affected the formation of IONPs. Furthermore, SEM image showed that the smaller spherical with some irregularly shaped CTAB-IONPs having a diameter of 80–90 nm in the presence of 0.4 mM CTAB were observed. The date from EDS, FTIR and TGA suggested that the CTAB capped on the surface of CTAB-IONPs, while XRD showed that zero-valent iron and iron oxide were formed. Finally, the formation mechanism of IONPs was proposed.

1. Introduction Phosphate is an essential macronutrient [1] in aquatic environments but excess phosphate can cause eutrophication and algal blooms, thereby distorting aquatic biology [2]. For some decades now, phosphate removal from wastewater has been regarded as a vital environmental sustainability problem. Various techniques, such as chemical precipitation, adsorption, biological removal, reverse osmosis, membrane, ion exchange, and constructed wetlands, have been tested for their ability to remove phosphate from wastewater [3]. Among them, phosphate adsorption is one satisfactory solution from an economic perspective of water treatment. This is due to the fact that adsorption process has stable phosphate removal outcomes and produces little sludge under simple operating conditions [3]. To date, various adsorbents have been used to remove phosphate including fly ash [4], active red mud [5], natural wollastonite [6], iron humate [7], biogenic iron oxides [8] and other waste materials [9]. In the past few decades, the synthesis of inorganic nanomaterials has made great progress and applied to various fields, particularly in medicine and biology [10]. For example, inorganic nanoparticles, due to their excellent properties, can be used as medicine in treating antibiotic resistant bacteria [11], as

⁎

versatile drug carriers [12,13], as electrochemical sensing of biomolecules [14–16]. Additionally, they have shown great potential in bioimaging and photodynamic therapy [10]. Recent times have witnessed the increasing use of engineered magnetic nanoparticles for remediation and water treatment [17,18] because of their simplicity to operation and separation [19]. Iron oxide nanoparticles, as biocompatible and recyclable magnetic adsorbents, have received much attention [20]. Currently, various chemical and physical routes are available for the synthesis of IONPs such as co-precipitation, micro-emulsion, thermal decomposition, hydrothermal and sonochemical techniques [21]. However, these methods are limited due to low production rates, high energy consumption and high cost, as well as toxicity of the used chemicals and by-products [22,23]. A suggested alternative with much potential is the green synthesis using plant leaf extract since it is generally cost-effective, biocompatible, non-toxic, and eco-friendly [22,23]. This method can convert plant waste into high-value products and realize sustainable development. In our previous study, Eucalyptus leaf extract containing polyphenols and flavonoids has been shown to act as a reducing and capping agent in the plant-mediated synthesis of iron-based nanoparticles (Fe NPs), including zero-valent iron and iron oxide

Corresponding author at: Global Centre for Environmental Remediation, University of Newcastle, Callaghan, NSW 2308, Australia. E-mail address: [email protected] (Z. Chen).

http://dx.doi.org/10.1016/j.msec.2017.08.073 Received 6 July 2017; Received in revised form 14 August 2017; Accepted 16 August 2017 Available online 17 August 2017 0928-4931/ © 2017 Elsevier B.V. All rights reserved.

Materials Science & Engineering C 82 (2018) 41–45

L. Gan et al.

out at an initial phosphate concentration of 20 mg/L with a sample dose of 2 g/L in 25 mL aqueous solution. Then the solutions were agitated on a rotary shaker at 25 °C and 250 rpm without any pH adjustment. Following different reaction times, they were filtered through a 0.45 μm filter paper. The concentration of phosphate was determined using a UV-Spectrophotometer (752 N, Shanghai, China) at 700 nm, which was done according to the ascorbic acid method. In addition, the amount of CTAB in the biosynthesis of IONPs was considered and a series of phosphate removal experiments in the presence of CTAB with different concentrations were executed. All the experiments were repeated in duplicate and the mean values were reported.

nanoparticles [24]. Yet the easy agglomeration and oxidation of IONPs (especially magnetite) generally resulted in loss of reactivity. Therefore, the surfaces of IONPs are often modified to keep their stability by coating or grafting, such as silica, metal oxides, polymers and surfactants [21]. A typical cationic surfactant, namely cetyltrimethylammonium bromide (CTAB), is chosen to improve the properties of a nanomaterial. CTAB with a hydrophilic head and hydrophobic tail can serve as a capping or protecting agent for nanoparticles against external forces [24,25]. Previous studies have shown that CTAB as a capping agent can greatly inhibit particle aggregation in the synthesis of metallic nanoparticles [25,26]. Furthermore, the capping of nanoparticles with CTAB enhances the adsorption capacity owing to the interactions between surface CTAB molecules and contaminants [27]. To date, there is no report on using CTAB as a stabilizing and capping agent in the biosynthesis of IONPs to: firstly, improve their dispersion; and secondly, form the multifunctional IONPs. Therefore, we hypothesized that the size and dispersion of IONPs could be tuned by CTAB in plant-mediated synthesis, and consequently, their capacity to remove phosphate could be enhanced. To understand the potentially useful applications of CTAB in green synthesis of IONPs, this study investigates CTAB in terms of a stabilizing and capping agent. We evaluated the reactivity between CTAB-modified IONPs and uncapped IONPs in removing phosphate from aqueous solutions. The effects of various CTAB concentrations on the ability to remove phosphate were investigated. Characterizations including SEM, EDS, XRD, FTIR and TGA were employed to demonstrate the role of CTAB in the formation of IONPs. Finally, a possible mechanism is proposed.

2.4. Characterization The morphology, size distribution and chemical composition of CTAB-IONPs and IONPs were examined using a scanning electron microscope (SEM) (JSM 7500F, JEOL, Japan) equipped with an energydispersive X-ray spectrometer (EDS) (Inca, Oxford Instruments, UK). Fourier transform infrared (FTIR) spectra were collected using a FTIR spectrophotometer (Nicolet 5700, Thermo Corp., USA) with KBr as background with a range of 4000–400 cm− 1. Thermogravimetric analysis (TGA) was done using a TGA analyzer (TGA-SDTA851, Mettler Toledo, Switzerland) from 30 °C to 700 °C in a N2 flow of 80 mL/min at a heating rate of 10 °C/min. The phase and purity of the product were determined using a X-ray powder diffractometer (XRD) (X'Pert Pro MPD, Philips, Netherlands) with Cu Kα radiation (λ = 0.154 nm) at 40 kV and 40 mA. 3. Results and discussion

2. Experiment

3.1. Phosphate removal experiment

2.1. Preparation of Eucalyptus leaf extract

As shown in Fig. 1, phosphate was removed from an aqueous solution containing an initial concentration of 20 mg/L using CTAB, IONPs and CTAB-IONPs after 60 min. In fact, 95.0% of phosphate was removed from the solution within the hour using CTAB-IONPs, while only 81.0% and 23.0% were removed using IONPs and CTAB, respectively. When pure CTAB was added, electrostatic interactions between CTA+ and negatively charged phosphate groups induced phosphatesurfactant complex formation and phosphate removal. For uncapped IONPs, the phosphate groups were adsorbed onto the surface oxide layers just by ligand exchange, where phosphates displaced water or hydroxyls from the surface of hydrous iron oxides [20]. Meanwhile, CTAB-modified IONPs shows a higher removal

Eucalyptus leaves (EL) were collected from a local farm in Fuzhou, China. After being thoroughly washed with de-ionized water and sundried, 15 g of EL was added to 250 mL aqueous solution and the mixture was heated at 80 °C. Then it was vacuum-filtered to obtain the extract. In making it ready for further experiments, the extract was stored at 4 °C. 2.2. Synthesis of CTAB-IONPs and uncapped IONPs 4.32 g of FeCl3·6H2O and 13.12 g of sodium acetate were dissolved in 80 mL of Eucalyptus leaf extract. Following this a specified amount of CTAB was added to the solution, and subsequently the mixture was stirred vigorously at 70 °C for 2 h. The color of the solution turned homogenous black, indicating the formation of IONPs colloid. After being filtered the obtained IONPs were washed with de-ionized water and ethanol, and vacuum dried at 45 °C overnight. The obtained IONPs, mainly containing magnetite and capped with CTAB were described as CTAB-IONPs. For comparison purposes, uncapped IONPs were synthesized employing the same procedure without CTAB. The formation of IONPs can be described by two steps. Firstly, FeCl3·6H2O hydrolyzes to form Fe(OH)3 precipitate and releases H+ ions in the proper pH at 70 °C. Secondly, Fe(OH)3 was partially reduced by polyphenols and flavonoids of Eucalyptus leaf extract to form IONPs and oxydates [28]. All chemicals used in this investigation were of analytical grade and utilized without further purification.

Removal efficiency/%

100

2.3. Phosphate removal experiment

80 60

IONP CTAB-IONP CTAB

40 20 0 0

20

40

60

80

100

T/min

To evaluate the removal capacity of CTAB-IONPs and uncapped IONPs, phosphate removal experiments were conducted under identical conditions. The desired phosphate solutions were prepared by diluting the KH2PO4 stock solution (50 mgP/L). The experiments were carried

Fig. 1. Comparison of efficiency in removing phosphate using various materials Conditions: C(PO43 −) = 20 mg·L− 1; initial pH; temperature: 298 K; CTAB = 0.4 mM; rotative speed: 250 rpm; sample dose: 2 g·L− 1.

42

Materials Science & Engineering C 82 (2018) 41–45

L. Gan et al.

presence of 0.4 mM CTAB were observed as shown in Fig. 3(a). This indicates that the size, shape and size distribution of CTAB-IONPs are significantly different in the presence of CTAB, which was confirmed in the previous section on the efficiency in removing phosphate when various concentrations of CTAB were present [29]. This is possibly due to the fact that the micelles of CTAB influenced the reduction, solubilization, adsorption, and incorporation of reactants with the positive head group of the surfactant among the different pseudo phases present in the reaction medium. The explanation lies in the micelles not acting as a true template for the growth of nanoparticles [26]. To compare the chemical composition between CTAB-IONPs and IONPs, the element compositions of CTAB-IONPs and IONPs were determined by EDS as shown in Fig. 4. The C, O and Fe loadings of CTABIONPs were 15.39, 37.85 and 46.76 wt% respectively, while those of IONPs were 15.04, 44.47, and 40.48 wt%. Compared to that of IONPs, the increase of C element composition in CTAB-IONPs suggested that a CTAB monolayer existed, resulting from the CTAB capping on the surface of CTAB-IONPs [29]. The O element was attributed mainly to biomolecules in Eucalyptus leaf extract and CTAB, and these capping subsequently resulted in less oxidation of IONPs [22]. The decrease of O element and increase of Fe supported the explanation. Hence, it can be concluded that CTAB may act as a stabilizer that reduces the oxidation of IONPs. As shown in Fig. 5, the XRD pattern of the CTAB-IONPs agreed well with the standard XRD pattern of Fe3O4 (Magnetite, JCPDS 99-0073). Additionally, an obvious peak at 44.67° corresponding to α-Fe (Iron, JCPDS 06-0696) indicated that IONPs synthesized by Eucalyptus leaf extract was not pure and zero-valent iron was present [22]. The FTIR spectra of CTAB-IONPs, IONPs and CTAB are presented in Fig. 6. As shown in Fig. 6(a), the characteristic bands at 474 and 614 cm− 1 referring to FeeO bending vibration [27] confirmed the formation of iron oxide. The band at 660 cm− 1 is attributed to out-ofplane bending of OeH in polyphenols and flavonoids [22]. The bands at 1068 and 1193 cm− 1 were ascribed to CeO stretching vibration of polyphenol, and the bands at 1338 and 1415 cm− 1 were identified as CeH bending vibration of hydrocarbon. The band at 1563 cm− 1 belonged to the C]C stretching vibration in the phenol compounds' aromatic ring. Furthermore, a broad band at 3383 cm− 1 assigned to the OH stretching vibration had originated from the OH-group on IONPs. When comparing the CTAB-IONPs (Fig. 6(a)) and IONPs (Fig. 6(b)), no obvious differences in the spectra were observed except for two sharp peaks at around 2850 and 2919 cm− 1. The bands corresponded to the characteristic symmetric and asymmetric stretching vibrations of CeCH2 in the CTAB chain [19,30], which was confirmed by bands corresponding only to CTAB (Fig. 6(c)). This indicates that CTAB was capped on the IONPs' surface, an outcome consistent with the results obtained from SEM and EDS. Moreover, the FTIR spectra of CTAB showed characteristic bands at 728 cm− 1 for the rocking mode of methylene chain ((CH2)n, n > 4), 937 and 961 cm− 1 for the CeN+ stretching bands, 1432 and 1480 cm− 1 for asymmetric and symmetric CeH scissoring vibration of CH3eN+ moiety of CTAB, respectively. However, these peaks were not shown in the spectrum of CTAB-IONPs, due to overlapping with the CeO stretching vibration and CeH bending vibration in the biomolecules. The unchanged frequency positions suggested that the capping effect did not enhance IONPs' intermolecular interaction [30,31]; neither did the hydrophobic tail of CTAB impact on the reduction of Fe(III). It could therefore be concluded that CTAB adsorbs on the IONPs' surface via their head group while their cetyl chains are titled to the surface [32]. The TGA curves of CTAB-IONPs and IONPs are both presented in Fig. 7. A weight loss in both samples from room temperature to 200 °C corresponded to the evaporation of physically absorbed water. When the temperature increased to 550 °C, mass loss was mainly attributed to the decomposition of organic matter. Beyond 550 °C, the bulk of IONPs without CTAB was prone to remaining constant, while CTAB-IONPs experienced continuous weight loss due to the CTAB monolayer on the

Removal efficiency/%

100 80 60 without CTAB 0.0004M CTAB 0.002M CTAB 0.01M CTAB

40 20 0 0

20

40

60

80

100

T/min Fig. 2. Effect of CTAB concentration in the synthesis process for removing phosphate. Conditions: C(PO43 −) = 20 mg·L− 1; initial pH; temperature: 298 K; rotative speed: 250 rpm; sample dose: 2 g·L− 1.

efficiency. We hypothesized that: First, the charged CTAB molecules on the surface of IONPs also adsorbed phosphate via electrostatic interactions; Second, CTAB as a stabilizing and capping agent can improve the dispersion and keep the stability of IONPs. Consequently, the phosphate removal of CTAB-IONPs was higher than that of IONPs as shown in Fig. 1. This is also observed that CTAB has the shape-directing role in biosynthesis, where the most interesting feature of the present study is the micellar catalytic effect of CTAB on the formation and morphology of Ag NPs [29]. CTAB-IONPs were modified by covalent binding between CTAB head groups and IONPs surface. This particular action formed a monolayer structure (excess-free CTAB was removed by washing). It can be concluded that CTAB's many functions include stabilizing nanoparticles and introducing electrostatic interaction. To investigate the effects of CTAB on the phosphate removal capacity of CTAB-IONPs, the removal of phosphate was done using CTABIONPs synthesized in the presence of various CTAB concentrations as shown in Fig. 2. Initially, the introduction of CTAB increased the efficiency in removing phosphate from 81.34% to 95.13%. However, the removal efficiency declined from 95.13% to 89.58% when the CTAB concentration increased from 0.4 to 10 mM. This indicates that the reactivity of CTAB-IONPs strongly depended on the CTAB concentrations. In aqueous, a high CTAB concentration produces micelles and lipophilic biomolecules like polyphenols, flavonoids and tannins [22] tend to be incorporated into the micellar phase [29], leading to the decrease of biomolecules involved the formation of CTAB-IONPs. Based on these results, we proposed that CTAB may act as a stabilizing and capping agent in the plant-mediated synthesis, in which well-dispersed IONPs with smaller size and higher adsorption capacity were obtained. We applied the following characterization techniques to confirm our hypothesis.

3.2. Characterizations The morphology and size distribution of the IONPs observed by SEM is shown in Fig. 3. Fig. 3(b) illustrates the IONPs synthesized in the absence of CTAB are mainly spherical in shape with particles ranging in size from 130 to 180 nm and their size distribution is relatively wide. The biomolecules such as polyphenols, flavanoids and tannins that are present in Eucalyptus leaf extract are responsible for the formation of iron oxide nanoparticles (IONPs). The inspection of SEM clearly indicates that a thin layer of other material was visualized on the IONPs which may be due to the capping organic material of Eucalyptus leaf extract. It is concluded that the leaf extract acts as a reducing, stabilizing and capping agent. However, the smaller spherical with some irregularly shaped CTAB-IONPs having a diameter of 80–90 nm in the 43

Materials Science & Engineering C 82 (2018) 41–45

L. Gan et al.

Fig. 3. SEM image of plant-mediated synthesized IONPs (a) in the presence of CTAB, (b) in the absence of CTAB.

a

Element CK OK Fe K

Wt% 15.39 37.85 46.76

At% 28.58 52.75 18.67

c

b

a

b

Element CK OK Fe K

Wt% 15.04 44.47 40.48

At% 26.33 58.43 15.24

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber/cm

Fig. 6. FTIR spectra of (a) CTAB-IONP, (b) IONP, (c) pure CTAB.

% Weight

100

Fig. 4. EDS of plant-mediated synthesized IONPs (a) in the presence of CTAB, (b) in the absence of CTAB.

80 b 60

Intensity(arb.units)

(110)

a

(440)

100

(511)

(422)

(222)

(220)

(311)

(400)

40 200

300

400

500

600

700

Temperature/ C O

Fig. 7. TGA of plant-mediated synthesized IONPs (a) in the presence of CTAB, (b) in the absence of CTAB.

4. Conclusion 20

30

40

50

60

70

This study achieved the synthesis of iron oxide nanoparticles using Eucalyptus leaf extract, and the cationic surfactant CTAB successfully served as a stabilizing and capping agent in biosynthesis. Characterizations confirmed that the formation of a CTAB monolayer on the surface greatly inhibited the aggregation and enhanced the dispersion. Consequently, the capacity of CTAB-IONPs to remove phosphate was significantly improved. The many roles of CTAB in removing phosphate include stabilizing nanoparticles and introducing electrostatic interaction. Finally, it was demonstrated that the formation of CTAB-IONPs improved the efficiency in removing phosphate.

2-Theta-Degrees Fig. 5. XRD of plant-mediated synthesized CTAB-IONP.

surface [30]. From the TGA data, the total CTAB content was determined to be as high as 23.45 wt%. Thus it was confirmed that a capping CTAB monolayer [32] had formed on the IONPs (as shown in Fig. 8). In summary, the formation of iron oxide nanoparticles (IONPs) using Eucalyptus leaf extract in the presence of CTAB is proposed as the following.

44

Materials Science & Engineering C 82 (2018) 41–45

L. Gan et al.

Fig. 8. The formation of as-synthesized biogenic Fe3O4@CTAB by Eucalyptus leaf extracts.

Mater. Sci. Eng. C 70 (2017) 1018–1030. [16] S.Y. Zhang, E. Ye, S. Liu, S.H. Lim, S.Y. Tee, Z. Dong, M.Y. Han, Temperature and chemical bonding-directed self-assembly of cobalt phosphide nanowires in reaction solutions into vertical and horizontal alignments, Adv. Mater. 24 (2012) 4369–4375. [17] L. Zeng, X. Li, J. Liu, Adsorptive removal of phosphate from aqueous solutions using iron oxide tailings, Water Res. 38 (2004) 1318–1326. [18] Y. Seida, Y. Nakano, Removal of phosphate by layered double hydroxides containing iron, Water Res. 36 (2002) 1306–1312. [19] S.C. Tang, I.M. Lo, Magnetic nanoparticles: essential factors for sustainable environmental applications, Water Res. 47 (2013) 2613–2632. [20] A. Zach-Maor, R. Semiat, H. Shemer, Adsorption–desorption mechanism of phosphate by immobilized nano-sized magnetite layer: interface and bulk interactions, J. Colloid Interface Sci. 363 (2011) 608–614. [21] A.H. Lu, E.E.L. Salabas, F. Schüth, Magnetic nanoparticles: synthesis, protection, functionalization, and application, Angew. Chem. Int. Ed. 46 (2007) 1222–1244. [22] T. Wang, X. Jin, Z. Chen, M. Megharaj, R. Naidu, Green synthesis of Fe nanoparticles using eucalyptus leaf extracts for treatment of eutrophic wastewater, Sci. Total Environ. 466 (2014) 210–213. [23] X. Weng, L. Huang, Z. Chen, M. Megharaj, R. Naidu, Synthesis of iron-based nanoparticles by green tea extract and their degradation of malachite, Ind. Crop. Prod. 51 (2013) 342–347. [24] T. Wang, J. Lin, Z. Chen, M. Megharaj, R. Naidu, Green synthesized iron nanoparticles by green tea and eucalyptus leaves extracts used for removal of nitrate in aqueous solution, J. Clean. Prod. 83 (2014) 413–419. [25] Y. Wang, R. Cheng, Z. Wen, L. Zhao, Facile preparation of Fe3O4 nanoparticles with cetyltrimethylammonium bromide (CTAB) assistant and a study of its adsorption capacity, Chem. Eng. J. 181 (2012) 823–827. [26] Z. Khan, J.I. Hussain, A.A. Hashmi, Shape-directing role of cetyltrimethylammonium bromide in the green synthesis of Ag-nanoparticles using Neem (Azadirachta indica) leaf extract, Colloids Surf. B Biointerfaces 95 (2012) 229–234. [27] Y. Jin, F. Liu, M. Tong, Y. Hou, Removal of arsenate by cetyltrimethylammonium bromide modified magnetic nanoparticles, J. Hazard. Mater. 227 (2012) 461–468. [28] S. Venkateswarlu, Y.S. Rao, T. Balaji, B. Prathima, N. Jyothi, Biogenic synthesis of Fe3O4 magnetic nanoparticles using plantain peel extract, Mater. Lett. 100 (2013) 241–244. [29] Z. Khan, O. Bashir, J.I. Hussain, S. Kumar, R. Ahmad, Effects of ionic surfactants on the morphology of silver nanoparticles using Paan (Piper betel) leaf petiole extract, Colloids Surf. B: Biointerfaces 98 (2012) 85–90. [30] H. Kavas, Z. Durmus, M. Şenel, S. Kazan, A. Baykal, M.S. Toprak, CTAB-Mn3O4 nanocomposites: synthesis, NMR and low temperature EPR studies, Polyhedron 29 (2010) 1375–1380. [31] J. Tang, J. Huang, S.-Q. Man, Preparation of gold nanoparticles by surfactantpromoted reductive reaction without extra reducing agent, Spectrochim. Acta A Mol. Biomol. Spectrosc. 103 (2013) 349–355. [32] Z. Sui, X. Chen, L. Wang, L. Xu, W. Zhuang, Y. Chai, C. Yang, Capping effect of CTAB on positively charged Ag nanoparticles, Physica E 33 (2006) 308–314.

Acknowledgment The authors are grateful for the financial support from the Fujian Sciences and Technology projects (No.2015J05082; No.2016Y0026), and “Shuang Chuang” program, Fujian, China. References [1] I. De Vicente, A. Merino-Martos, F. Guerrero, V. Amores, J. de Vicente, Chemical interferences when using high gradient magnetic separation for phosphate removal: consequences for lake restoration, J. Hazard. Mater. 192 (2011) 995–1001. [2] S. Sengupta, A. Pandit, Selective removal of phosphorus from wastewater combined with its recovery as a solid-phase fertilizer, Water Res. 45 (2011) 3318–3330. [3] Y. Su, H. Cui, Q. Li, S. Gao, J.K. Shang, Strong adsorption of phosphate by amorphous zirconium oxide nanoparticles, Water Res. 47 (2013) 5018–5026. [4] S. Lu, S. Bai, L. Zhu, H. Shan, Removal mechanism of phosphate from aqueous solution by fly ash, J. Hazard. Mater. 161 (2009) 95–101. [5] C.-J. Liu, Y.-Z. Li, Z.-K. Luan, Z.-Y. Chen, Z.-G. Zhang, Z.-P. Jia, Adsorption removal of phosphate from aqueous solution by active red mud, J. Environ. Sci. 19 (2007) 1166–1170. [6] J.P. Gustafsson, A. Renman, G. Renman, K. Poll, Phosphate removal by mineralbased sorbents used in filters for small-scale wastewater treatment, Water Res. 42 (2008) 189–197. [7] P. Janoš, A. Kopecká, S. Hejda, Utilization of waste humate product (iron humate) for the phosphorus removal from waters, Desalination 265 (2011) 88–92. [8] J.A. Rentz, I.P. Turner, J.L. Ullman, Removal of phosphorus from solution using biogenic iron oxides, Water Res. 43 (2009) 2029–2035. [9] N. Moelants, I. Smets, J. Van Impe, The potential of an iron rich substrate for phosphorus removal in decentralized wastewater treatment systems, Sep. Purif. Technol. 77 (2011) 40–45. [10] Q.Q. Dou, H.C. Guo, E. Ye, Near-infrared upconversion nanoparticles for bio-applications, Mater. Sci. Eng. C 45 (2014) 635–643. [11] C.P. Teng, T. Zhou, E. Ye, S. Liu, L.D. Koh, M. Low, X.J. Loh, K.Y. Win, L. Zhang, M.Y. Han, Effective targeted photothermal ablation of multidrug resistant bacteria and their biofilms with NIR-absorbing gold nanocrosses, Adv. Healthc. Mater. 5 (2016) 2122–2130. [12] K.Y. Win, E. Ye, C.P. Teng, S. Jiang, M.Y. Han, Engineering polymeric microparticles as theranostic carriers for selective delivery and cancer therapy, Adv. Healthc. Mater. 2 (2013) 1571–1575. [13] K.Y. Win, C.P. Teng, E. Ye, M. Low, M.Y. Han, Evaluation of polymeric nanoparticle formulations by effective imaging and quantitation of cellular uptake for controlled delivery of doxorubicin, Small 11 (2015) 1197–1204. [14] S.Y. Tee, E. Ye, P.H. Pan, C.J.J. Lee, H.K. Hui, S.-Y. Zhang, L.D. Koh, Z. Dong, M.Y. Han, Fabrication of bimetallic Cu/Au nanotubes and their sensitive, selective, reproducible and reusable electrochemical sensing of glucose, Nanoscale 7 (2015) 11190–11,198. [15] S.Y. Tee, C.P. Teng, E. Ye, Metal nanostructures for non-enzymatic glucose sensing,

45