Azide-functionalized hollow silica nanospheres for removal of antibiotics

Journal of Colloid and Interface Science 444 (2015) 38–41

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Azide-functionalized hollow silica nanospheres for removal of antibiotics Jinsuo Gao a,⇑, Jingjing Chen a, Xiaona Li a, Meiwen Wang c, Xueying Zhang a, Feng Tan a, Shutao Xu b,⇑, Jian Liu c,⇑ a Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, China b National Engineering Laboratory for Methanol to Olefins, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China c Department of Chemical Engineering, Curtin University, Perth, WA 6845, Australia

g r a p h i c a l a b s t r a c t

a r t i c l e

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Article history: Received 11 October 2014 Accepted 17 December 2014 Available online 25 December 2014 Keywords: Hollow sphere Mesoporous materials Click reaction Antibiotics adsorption

a b s t r a c t Antibiotics, which are hardly removed from polluted water by conventional water-treatment technologies, adsorption has been deemed as one of the efficient and promising method to resolve the problems of antibiotics pollution. Herein, we reported a synthesis of filtration separable hollow nanostructured silicas (HNSs) with efficient click functionalization property for antibiotics adsorption. The clickable HNSs were synthesized by the co-condensation and assembling of tetramethoxysilane (TMOS) and 3-azidopropyltrimethoxysilane (AzPTMS) around F127 single micelle template. Alkynyl compounds such as phenylacetylene (Ph), propargyl alcohol (PA), 1-heptyne (Hep), and 2-butyne-1,4-diol (BD) have been linked to the materials through click reaction with high efficiency. Antibiotic adsorption results reveal that functional groups play an important role in adsorption properties of adsorbents and phenyl was found to be the optimal functional group due to the p–p stacking effect. Excellent adsorption capacity and recyclability indicate that the clickable hollow nanostructured silicas exhibit potential application for antibiotics removal. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Antibiotics pollutants have been considered as one of the emerging contaminants in regards to their potential risks to human health, which have been detected in aquatic environment [1] including surface water, ground water, and even drinking water. To date, various treatment technologies including photocatalysis ⇑ Corresponding authors. Fax: +86 411 84707965. E-mail addresses: [email protected] (J. Gao), [email protected] (S. Xu), jian. [email protected] (J. Liu). http://dx.doi.org/10.1016/j.jcis.2014.12.054 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.

[2,3], advanced oxidation [4], ozonation [5] and adsorption [6,7] have been developed for antibiotics removal. From these diverse treatment methods, adsorption is recognized as one of the reliable methods owing to its recyclable nature and simplicity; and more importantly, is its relatively low energy consumption and operation costs. However, the traditional adsorbents used in industries such as active carbons have many shortcomings such as flammability, regeneration difficulty and pore size inconformity for antibiotics. Therefore, exploring new adsorbents with low cost and high efficiency, is of scientific and technological importance.

J. Gao et al. / Journal of Colloid and Interface Science 444 (2015) 38–41

Hollow nanostructured silicas (HNSs) have shown promising applications as drug delivery carriers [8–12], catalyst supports [13–16], electrodes for energy storage [17] and adsorbents [18] due to their low density, large surface area, high thermal and mechanical stability, and short diffuse lengths. To date, various templates [19–23] including vesicles, polymer micelles, emulsion, and polystyrene spheres (PS) were involved to fabricate HNSs either through a soft templating or hard templating method. However, centrifugation is usually needed to separate HNSs from the synthetic solution. Considering application of HNSs as adsorbents, ideal HNSs should be easily separated from the adsorption solution by filtration rather than centrifugation. For instance, a single micelle template method has been developed to fabricate HNSs with various functionalities [13–15,24–26]. The Huigen [3 + 2] azide-alkyne cycloaddition, termed as one typical click reaction [27–29], has been widely used to incorporate organic functional groups onto mesoporous silicas for diverse applications [30–33]. Recently, we have synthesized clickable mesoporous SBA-15 silicas and PMOs as functional groups screening materials and excellent adsorbents for antibiotics adsorption [34,35]. Inspired by the above work, herein, we further extend the strategy to develop filtration separable clicked hollow silica nanospheres for antibiotics adsorption. Azide-functionalized HNS (N3HNS), has been firstly prepared by a co-condensation method, using tetramethyl orthosilicate (TMOS) and 3-azidopropyltrimethoxysilane (AzPTMS) as silica precursors, as shown in Scheme 1. The clickability of the obtained azido HNS is evaluated by click reaction between azido HNS and different alkynyl compounds, such as phenylacetylene (Ph), propargyl alcohol (PA), 1-heptyne (Hep), and 2butyne-1,4-diol (BD) (Scheme 1). Finally, the clicked HNSs have been evaluated by the adsorption of ciprofloxacin hydrochloride.

2. Results and discussion Co-condensation method has been adapted to synthesize azidefunctionalized HNS, an advantage of which is that the organo-functional groups would distribute uniformly on the surface of the hollow nanospheres as shown in Scheme 1. A TEM image shows azidefunctionalized HNS (N3-HNS) have hollow spherical morphology with an average size of 25 nm (Fig. 1), the average cage size is 16 nm. After click reaction, as a typical example, Ph-N3-HNS maintains the hollow nanostructure as evidenced from TEM image (Fig. 1), which indicated the click reaction with phenylacetylene (Ph) did not alter the morphology and structure. Nitrogen sorption isotherms and pore size distribution curves of the hollow spheres before and after click reaction are shown in Fig. 2. The shape of the isotherms is similar to previous reported

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HNSs: a typical IV isotherms with a H2 hysteresis loop. [13,24,25] The similarity of the isotherms of N3-HNS and click HNSs indicates the click reaction has little influence on the nanostructure of the materials. The pore size distribution shown in Fig. 2 calculated from BJH method is with a peak centered at 14– 15 nm, which is attributed to the inner void of hollow nanospheres [24]. It should be noted that BJH model is usually more suitable for calculating size of cylindrical channel like pore. For cage-like pore, BdB or DFT method is more suitable. However, in this case, the pore size calculating from the BJH model is more close to the hollow cage size determined by TEM images similar as the previous report [13,14,24,25]. The N3-HNS show a high specific surface area of 437 m2 g 1 and total volume of 1.94 cm3 g 1. After click reaction, the surface area of the resultant clicked HNSs is decreasing, indicating the successful introduction of organic functional groups using click chemistry (Table S1) [35]. The integration of organic functional groups were further evidenced by composition characterization methods including IR and 13C CP-MAS NMR. An obvious adsorption peak at 2100 cm 1, which is characteristic stretching vibration of azido groups, is observed in the FTIR spectrum (Fig. 3). The peaks at 2976, 2940 cm 1 are attributed to the stretching vibration adsorbance of the CH of propyl, whose bending vibration can be observed at 1407 cm 1. The IR spectrum shows the clickable organosilane, AzPTMS, has been incorporated into the materials. After the click reaction, the adsorbance corresponding to the azido group decreases significantly and even disappears in the IR spectra of clicked materials. A new peak at 763 cm 1 corresponding to phenyl is observed in the spectrum of Ph-N3-HNS. The decrease of azido groups adsorbance together with the appearance of organic groups adsorbance indicate the facile process of click reaction for introducing organic functional groups. The solid-state 13C cross-polarized magic angle spinning (CPMAS) NMR spectra of N3-HNS and Ph-N3-HNS are presented in Fig. 4. For N3-HNS, the peaks at d = 9.4, 22.3 and 53.9 ppm are ascribed to the C signals of azidopropyl. After the click reaction, new peaks ascribed to the reaction products are observed at around d = 127 and 147 ppm. The 13C CP-MAS NMR characterizations further confirm that phenylacetylene has been anchored onto the hollow nanospheres during the click reaction. Thermogravimetric (TG) analysis was also used to confirm the incorporation of organic functional groups. As shown in Fig. S1, the first weight loss stage below 170 °C is caused by the loss of physical adsorbed water. The second weight loss stage between 170 and 250 °C is ascribed the decomposition of the residual surfactant. The weight loss stage from 250 to 600 °C can be ascribed to the decomposition of the functional groups. TG analyses show Ph-N3-HNS has 35 wt% weight losses in compare with 20 wt%

Scheme 1. Schematic description of the synthesis of clickable HNS and clickable modifications.

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J. Gao et al. / Journal of Colloid and Interface Science 444 (2015) 38–41

Volume adsorbed (cm3 g-1, STP)

Fig. 1. TEM images of N3-HNS and Ph-N3-HNS.

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Fig. 2. Nitrogen sorption isotherm (left) and pore size distribution (right) of HNSs materials.

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Chemical shift (ppm) Fig. 4. Solid-state 13C CP/MAS NMR spectra of N3-HNS and Ph-N3-HNS. #Surfactant signals. ⁄Spinning side band.

Fig. 3. IR spectra of HNSs materials.

for N3-HNS from the temperature between 250 and 600 °C, which suggests phenylacetylene has been incorporated onto the materials. Adsorption is an efficient treatment method for resolving antibiotics pollution, because the concentration of antibiotics in the aquatic environment is in the unit of lg/L, which is extremely low and cannot be easily removed by traditional methods. Ciprofloxacin hydrochloride (CFHCl), one of the usually used fluoroquinolone antibiotics which has been widely used for treating infectious symptoms, is selected as a representative example. The

clicked materials were used to investigate the effect of functional groups on the adsorption properties at the adsorbate concentration of 0.1, 1 and 5 mg/L, which are close to the realistic level of antibiotics in the environment. As we have previously disclosed that the adsorption of CFHCl was fast and could reach equilibrium within 8 h [34,35], thus, the adsorption experiments were conducted for 24 h to ensure the adsorption equilibrium. As can be determined from the histograms in Fig. 5, a significant enhancement of adsorption amount has been induced by increasing the adsorbate concentration. At high CFHCl concentration (5 mg/L), the adsorption

J. Gao et al. / Journal of Colloid and Interface Science 444 (2015) 38–41

12 10

Adsorption (mg/g)

0.1 mg/L 1 mg/L 5 mg/L

11.75

9.53

8 6

5.80

5.43

Acknowledgments We gratefully acknowledge financial support for National Natural Science Foundation of China (21107009 and 21103180). The study was also supported by Open Foundation of Key Laboratory of Industrial Ecology and Environmental Engineering (MOE) (KLIEEE-13-01) and the Fund of State Key Laboratory of Catalysis in DICP (N-13-04). Appendix A. Supplementary material

4 2.45

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2 0

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1.48 0.12

0.13

Ph-N3-HNS

PA-N3-HNS

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BD-N3-HNS

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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.12.054. References

Hep-N3-HNS

Fig. 5. Adsorption capacity of the adsorbents.

capacity of the clicked materials follows the sequence: BD-N3HNS < HEP-N3-HNS < PA-N3-HNS < Ph-N3-HNS, illustrating that the functional groups in the adsorbents indeed influence the adsorption properties due to different interaction. The more hydrophobic of the functional group is, the stronger interaction between the functional group and CFHCl will be, the higher adsorption capacity can be achieved. All the functionalized adsorbents exhibit excellent adsorption properties for CFHCl at different concentration. In particularly, Ph-N3-HNS can reach the adsorption capacities of 11.75 mg/g under CFHCl concentration of 5 mg/L. It is also noted that when the adsorption is conducted at higher adsorbate concentration, the difference of adsorption between Ph-N3-HNS and other functionalized adsorbents becomes more obvious, which might be due to the p–p stacking effect is pronounced under higher adsorbate concentration [34,35]. The recycle tests suggest the adsorbent can be reused for at lease 4 times with gradually decreasing adsorption capacity (Fig. S2). In comparison of traditional adsorbents, silica hollow nanospheres are easily separation, also can be used as filler materials for column treatment of waste water. In addition, the hollow cage will provide large space for accommodate and storage of antibiotics molecules. These special features make clickable silica hollow nanospheres very promising for industrial treatment of antibiotics pollution. 3. Conclusions In conclusion, clickable hollow nanostuctured silicas have been successfully synthesized by a co-condensation between tetramethoxysilane and 3-azidopropyltrimethoxysilane templated by F127 single micelle, the resultant hollow nanostructured silicas can be harvested by filtration without the need of centrifugation which benefits for the adsorption application. Various organic functional groups, have been integrated into the materials through click reaction, among them, silica hollow nanospheres with phenyl functional group exhibit the highest adsorption capacity due to the strong p–p stacking interaction. The facile synthesis of functionalizing HNSs would show promising application in the area of scaling nanomaterials for environmental protection.

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