Facile synthesis of PDMAEMA-coated hollow mesoporous silica nanoparticles and their pH-responsive controlled release

Microporous and Mesoporous Materials 173 (2013) 64–69

Contents lists available at SciVerse ScienceDirect

Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Facile synthesis of PDMAEMA-coated hollow mesoporous silica nanoparticles and their pH-responsive controlled release Faqi Yu a, Xinde Tang b,⇑, Meishan Pei a a b

School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, PR China School of Material Science and Engineering, Shandong Jiaotong University, Jinan 250023, PR China

a r t i c l e

i n f o

Article history: Received 31 October 2012 Received in revised form 26 January 2013 Accepted 10 February 2013 Available online 17 February 2013 Keywords: Hollow mesoporous silica nanoparticles Stimuli-responsive polymer Surface-initiated atom transfer radical polymerization Storage capacity pH-responsive controlled release

a b s t r a c t A versatile method of grafting poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) on the surface of hollow mesoporous silica nanoparticles (HMSN) via surface-initiated atom transfer radical polymerization (SI-ATRP) has been presented. TEM, FT-IR, TGA, XRD, and N2 adsorption–desorption analysis were used for the characterization. The results state that the stimuli-responsive polymer has been successfully grafted onto the surface of HMSN. These hybrid nanoparticles have been proven to have a large storage capacity which can be applied in drug delivery. The external coated PDMAEMA layers act as a storage gate as well as a release switch in response to the stimuli of environment. In this paper, pH-responsive controlled release behavior was focused. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction Mesoporous silica nanoparticles (MSN) with well-defined structures have attracted considerable attention due to their potential applications such as delivery carriers [1,2], biosensors [3,4], biomarkers [5–9], enzyme supporters [10,11]. Hollow mesoporous silica nanoparticles (HMSN) possess huge hollow interiors and penetrating pores in the shell that endow them with a sustained release property and a much higher drug loading capacity than conventional MSN such as MCM-41 and SBA-15 [12,13]. Therefore, many research efforts have focused on designing the HMSN with desired structures and particle sizes. Template-assisted methods are typical routes for synthesizing various HMSN [14,15]. Hard templates or soft templates were used to double-coat different materials by sol–gel processes, the layerby-layer approach, or chemical deposition. The use of soft templates can facilitate the formation of the hollow core during the sol–gel process. For example, surfactant [16,17], bacteria [18], lysozymes [19], emulsions [20–23], and gas bubble [24–27] have been employed as soft templates. Due to their large surface areas, tunable pore sizes and volumes, bio-inert and biocompatible properties, these HMSN have been widely used as efficacious drug and gene carriers. However, unmodified silica nanoparticles showed a low drug loading content ⇑ Corresponding author. Tel.: +86 531 82765394. E-mail address: [email protected] (X. Tang). 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.02.012

and rapid drug release from them [28]. Therefore, the fabrication of hybrid mesoporous silica nanoparticles to fully control over the release procedure is much desired. Lin et al., employed magnetic nanoparticles [29], cadmium sulfide [30], and gold nanoparticles [31] as gatekeepers to regulate the release of guest molecules encapsulated in the pores of MSN by the introduction of the disulfide-reducing trigger or by cleavage of a disulfide linkage under physiological conditions. Stimuli-responsive polymers are grafted onto the internal or external surface of mesoporous silica materials to regulate the transport of encapsulated molecules. When a stimulus, such as a change of pH, temperature, or light is applied, their physical/chemical properties change as a response, and further these ‘‘smart’’ polymers could control access to the pores. Apart from the simple procedure of fabrication, these nanoparticles could also control the drug release repeatedly [32]. It seems that smart polymers should be better as a nanovalve for controlled drug release. Poly(N-isopropylacrylamide) (PNIPAM), as an extensively studied thermal-responsive polymer, has been reported to graft onto the mesoporous silica nanoparticles by atom transfer radical polymerization (ATRP) [33–35], reversible addition–fragmentation chain transfer (RAFT) polymerization [36,37] and chemical coupling reaction [38]. Poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) as a thermal- and pH-responsive polymer with biocompatibility and antibacterial activity, has been used to fabricate functional materials for medical and biomedical applications [39], and graft onto the

65

F. Yu et al. / Microporous and Mesoporous Materials 173 (2013) 64–69

mesoporous silica nanoparticles by atom transfer radical polymerization with activators regenerated by electron transfer [40]. PDMAEMA is a weak electrolyte (pKa  7.3) [41], at low pH, such as pH 1, PDMAEMA is entirely protonated to form polyelectrolyte with positive charge, and the polymer chains are extending fully due to the electrostatic repulsions between PDMAEMA chains. Nevertheless, PDMAEMA chains are deprotonated and then became hydrophobic at high pH, so the polymer chains are collapsed [42]. As for the surface modification of HMSN, to the best of our knowledge, there have been few reports concerning thermal- and pH-responsive PDMAEMA-coated HMSN nanoparticles so far. Herein, a facile strategy of grafting PDMAEMA on the surface of HMSN via surface-initiated atom transfer radical polymerization (SI-ATRP) has been described. The modified HMSN could have potential applications in drug delivery systems.

2.4. Synthesis of PDMAEMA-coated HMSN (HMSN-PDMAEMA)

2. Experimental methods

Doxorubicin hydrochloride was dissolved in phosphate buffer saline (PBS) at pH 4.0 with a concentration of 1 mg/mL. Five milligrams of HMSN was ultrasonically dispersed in 1.25 mL DOX solution. The mixture was stirred 37 °C for 24 h. Then the dispersion was centrifuged to collect the DOX loaded HMSN and the products were washed with deionized water twice to remove the DOX absorbed on the surface. The DOX loaded HMSN-PDMAEMA was prepared according to the identical procedure. Three aliquots (2 mg) of DOX loaded HMSN or HMSN-PDMAEMA were respectively dispersed in 4 mL of PBS solutions of different pH (4.0, 6.9, and 9.0) and the samples were transferred into dialysis bags (molecular weight cut off 8000–14,000). Then, the dialysis bags were kept in a 250 mL lucifugal beaker with 200 mL of PBS (pH 4.0, 6.9, or 9.0) and stirred at 37 °C. At timed intervals, 2 mL of the solution was extracted periodically and then 2 mL of fresh PBS was added to keep the volume constant. The concentration of the remaining DOX solution was determined by using a fluorescence spectrophotometer at kex = 542 nm and kem = 600 nm, respectively. A standard plot was prepared under identical conditions to confirm the amount of drug loaded by the nanoparticles. The amount of released drug was analyzed by fluorescence spectrophotometer. The drug loading content and entrapment efficiency were calculated using Eqs. (1) and (2), respectively:

2.1. Materials and reagents Poly(vinylpyrrolidone) (PVP-10, Aldrich), dodecylamine (DDA, Aldrich), anhydrate ethanol (Sinopharm Chemical Reagent Co. Ltd. 98%), and tetraethyl orthosilicate (TEOS, Aldrich) were used as received. Tetrahydrofuran (THF) was distilled before use. 2Bromoisobutyryl bromide (BIBB) (98%, Aldrich) was freshly distilled at room temperature under vacuum. Triethylamine (TEA) was refluxed with tosyl chloride to remove the primary amines and secondary amines. N,N-Dimethylaminoethyl methacrylate (DMAEMA, Aldrich, 99%) was purified by passing it through an alumina column for removal of inhibitor, followed by vacuum distillation. Tris((N,N-dimethylamino)ethyl)amine (Me6TREN, Sigma– Aldrich) was used as received. Cu(I)Cl (Sinopharm Chemical Reagent Co. Ltd. 98%) was purified by stirring in glacial acetic acid, washed with methanol, and then dried in a vacuum oven. Doxorubicin hydrochloride (DOX) was purchased from Zhejiang Hisun Pharmaceutical Co. Ltd. of China. All other reagents were of analytical grade and used as received. 2.2. Synthesis of HMSN HMSN was synthesized by the sol–gel method according to the literature [43], which using DDA as the main template and PVP-10 as the co-template. PVP-10 (0.5 g) was dissolved in a mixture of ethanol (20.0 mL) and water (80.0 mL) under stirring for 30 min. DDA (1.17 g) and TEOS (5.0 mL) were then added into the solution and the mixture was stirred for 24 h at room temperature. The solid product was recovered by centrifugation, and then dried. Removal of both templates (PVP-10 and DDA) was achieved by solvent extraction: silica particles (1.5 g) were suspended in anhydrous ethanol (150.0 mL), and the mixture was heated under reflux for 24 h. The solvent-extracted particles were washed extensively with anhydrous ethanol and collected via centrifugation. 2.3. Synthesis of HMSN-supported ATRP initiator (HMSN-Br) HMSN (0.6 g), anhydrous THF (10.0 mL), and triethylamine (0.85 mL, 6 mmol) were added into a 50 mL flask, and then 2bromoisobutyryl bromide (2.5 mL, 20.2 mmol) in 10 mL of anhydrous THF was added drop wise into the mixture at 0 °C. The resulting mixture was stirred for 2 h at 0 °C and then at room temperature for 24 h. The solid was then separated by centrifugation and washed three times with THF, deionized water, and CH2Cl2, respectively. The HMSN-Br was collected and dried overnight under vacuum at 60 °C.

Typically, HMSN-Br (100.0 mg), Cu(I)Cl (10 mg, 0.1 mmol), Me6TREN (28.0 lL, 0.1 mmol), DMAEMA (2.5 mL, 15 mmol), and anisole (0.5 mL) were placed in a 20 mL dry flask, which was then sealed with a rubber plug. The solution was degassed by three freeze-pump-thaw cycles, and then placed in a thermostatted oil bath at 90 °C for 5 h. The polymerization was stopped by opening the polymerization tube to air. To ensure that no ungrafted polymer or free reagents were mixed in the product, the mixture was washed with THF by centrifugation (5000 rpm) several times; HMSN-PDMAEMA was obtained after dried overnight under vacuum at 60 °C. 2.5. Drug loading and in vitro release

Loading contentðwt%Þ ¼ Weight of drug in HMSN=Weight of drug loaded HMSN

ð1Þ

Entrapment efficiencyðwt%Þ ¼ Weight of drug in HMSN= Initial weight of drug

ð2Þ

2.6. Characterization Transmission electron microscopy (TEM) was obtained with a JEM-2100F microscope using an accelerating voltage of 200 kV. The samples were lightly ground and then dispersed ultrasonically in ethanol. A drop of the suspension was evaporated on ‘‘holey’’ carbon films, pre-deposited on 200-mesh copper grids. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker VECTOR-22 IR spectrometer. Thermal gravimetric analyses (TGA) were conducted on a Perkin–Elmer Diamond TG/DTA Instruments with a heating rate of 10 °C/min under a nitrogen flow. The X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max 2500 kV PC X-ray diffractometer with a Cu tube anode. Nitrogen adsorption isotherms were measured on an ASAP 2020 micromeritics porosimeter at 77 K using a 10 s equilibrium interval. Luminescence spectra were recorded on a Hitachi F-4600 fluorescence spectrophotometer.

66

F. Yu et al. / Microporous and Mesoporous Materials 173 (2013) 64–69

3. Results and discussion

3.4. Thermal gravimetric analysis

3.1. Preparation of HMSN-PDMAEMA

The thermogravimetric analysis of HMSN, HMSN-Br, and HMSN-PDMAEMA were performed after these samples dried under vacuum at 60 °C for 24 h. The results in Fig. 3 shows that the weight losses of HMSN, HMSN-Br, and HMSN-PDMAEMA are 90.5 wt.%, 83.0 wt.%, 65.6 wt.%, respectively, while the samples heated to 750 °C. From Fig. 3(b), the weight loss of ATRP initiator (HMSN-Br) is of about 7.50 wt.%. The different weight loss between HMSN-Br and HMSN-PDMAEMA is 17.4 wt.%, which is obviously indicated that PDMAEMA was successfully grafted on the surface of HMSN.

The PDMAEMA-modified HMSN (HMSN-PDMAEMA) was prepared according to Scheme 1. The HMSN-Br was firstly prepared by reaction of hydroxyl groups on the surface of HMSN with 2bromoisobutyryl bromide, and then it was used as the initiator in the ATRP of DMAEMA to obtain HMSN-PDMAEMA. 3.2. TEM observation The HMSN was obtained by a sol–gel process with an average diameter of 500 nm and almost uniform pore size. The clear hollow mesoporous morphology can be observed in Fig. 1, in which the obvious contrast between the dark edges and the pale center confirms the hollow structure. From Fig. 1(d), the spots in the shell indicate the silica nanoparticles have many mesopores through the shell. 3.3. FT-IR detection The FT-IR spectroscopy was employed to provide direct identification of chemical groups in HMSN, HMSN-Br, and HMSN-PDMAEMA. As shown in Fig. 2(a), the absorption peak at 1650 cm1 is attributed to the bending O–H bands of the adsorbed water. A new weak absorption peak at 1750 cm1 (Fig. 2b) is ascribed to C=O stretching vibration, indicating the attachment of 2-bromoisobutyryl bromide group to the silica surface. After polymerization of DMAEMA on the surface of HMSN, the ratio of ethyl bending (1450 cm1) to methyl bending (1400 cm1) peak areas increases, and an intense increase of the absorption peak appears at 1730 cm1 (Fig. 2c) corresponding to the stretching vibration of ester carbonyl group in the PDMAEMA.

3.5. X-ray diffraction measurement The physical state of silica in HMSN, HMSN-Br, and HMSNPDMAEMA were investigated by X-ray diffractometer. The XRD pattern (Fig. 4) confirms that HMSN are amorphous silicon dioxide on account of the broad peak with 2h ranging from 15° to 30°, which is similar to that of standard silica. Owing to the less contrast between the polymers and the pores, the diffraction peaks in HMSN-PDMAEMA are weaker than the corresponding peaks in HMSN. 3.6. N2 adsorption–desorption analysis Nitrogen adsorption–desorption method was employed to measure the pore structures of HMSN and HMSN-PDMAEMA. The results were summarized in Table 1. The specific surface area is 831.9 m2/g for HMSN. The BET isotherm of HMSN exhibits the characteristic type IV adsorption– desorption pattern in Fig. 5(a). The BJH pore size distribution of HMSN is shown in Fig. 6(a) and the pore size is about 2.59 nm. The specific surface area is 403.3 m2/g for HMSN-PDMAEMA. The

Scheme 1. The preparation of PDMAEMA-modified HMSN.

67

F. Yu et al. / Microporous and Mesoporous Materials 173 (2013) 64–69

Fig. 1. HRTEM images of HMSN (a and b), and HMSN-PDMAEMA (c and d).

100

a 1650

1750

b

1470 1400

1650

Weight (%)

Transmittance

a

90

b

c 1400

80

70

c

1650 1470 1730

60

50 100 4000

3500

3000

2500

2000

1500

1000

500

Fig. 2. FT-IR spectra of HMSN (a), HMSN-Br (b), and HMSN-PDMAEMA (c).

BET isotherm of HMSN exhibits microporous adsorption–desorption pattern in Fig. 5(b) and the BJH pore size distribution of HMSN-PDMAEMA is shown in Fig. 6(b). The pore size decreases, which demonstrate that ATRP of DMAEMA took place on internal surface of the mesopores and the compact polymer layers on the surface of HMSN prevent penetration of the nitrogen. 3.7. pH-dependent release studies With the potential application of HMSN in drug delivery in mind, the effect of mesoporous on drug release behavior was

200

300

400

500

600

700

o

Temperature ( c) Fig. 3. TGA curves for HMSN (a), HMSN-Br (b), and HMSN-PDMAEMA (c).

investigated. On the basis of the calibration curves of fluorescent intensity changed with the concentration of DOX at different pH, the fluorescent intensities were converted to the concentrations. The HMSN and HMSN-PDMAEMA were loaded DOX for 24 h at 37 °C, and ultimately afforded DOX loading contents of 2.4 wt.%, and 15.2 wt.%, respectively. Also, their entrapment efficiencies are estimated to be 9.9 wt.%, and 66.3 wt.%, respectively. It is obviously that the loading content increases after modification with PDMAEMA. The released DOX curves at different pH are shown in Fig. 7. At pH 4.0, 6.9, or 9.0, the amount of released DOX increases in the first

68

F. Yu et al. / Microporous and Mesoporous Materials 173 (2013) 64–69

1.0

120

a

b 100

Pore Volume (cm /g)

0.8

3

80

Counts

a 60

c

40

0.6

0.4

0.2

b

20 0.0

0 10

20

30

40

50

1

2

3

2θ (deg)

4

5

6

7

8

Pore Diameter (nm)

Fig. 4. XRD patterns of HMSN (a), HMSN-Br (b), and HMSN-PDMAEMA (c).

Fig. 6. BJH pore size distribution plot of the HMSN (a) and HMSN-PDMAEMA (b).

Table 1 Mesopore parameters of HMSN and HMSN-PDMAEMA.

80

Sample

BET surface area (m2/g)

Average pore diameter by BJH (nm)

Pore volume (cm3/g)

70

HMSN HMSN-PDMAEMA

831.9 403.3

2.59 <2

0.81 0.29

60

6 h for HMSN. As the process is prolonged by 24 h, HMSN gives rise to the release of 23.2% of the DOX at pH 4.0, 18.3% of the DOX at pH 6.9, and attains a release of 9.2% of the DOX at pH 9.0, which indicates that the release amount is quite small and that the difference in release amount between different pH is not significant. The result may be ascribed to the solubility of DOX dropping rapidly as the pH of the aqueous solution increases [44]. At high pH, the poor solubility becomes the main factor in the impedance of the drugrelease process, resulting in the released amounts of DOX being lower level [45]. At pH 4.0, the amount of released DOX increased rapidly in 12 h for HMSN-PDMAEMA and the release content reached to 51.3%. At pH 6.9 or 9.0, the amount of released DOX increased in the first 8 h.

40 30 20 10 0 0

5

10

15

20

25

Time (h) Fig. 7. DOX release from HMSN-PDMAEMA at pH 4.0 (j), pH 6.9 (d), and pH 9.0 (N); DOX release from HMSN at pH 4.0 (h), pH 6.9 (s), and pH 9.0 (4).

As the process was prolonged by 24 h, the HMSN-PDMAEMA gave rise to the release of 11.1% of the DOX at pH 6.9, 6.1% of the DOX at pH 9.0. It is apparent that DOX loaded HMSN-PDMAEMA exhibited a more pronounced pH-dependent drug release behavior than DOX loaded HMSN. Although a little DOX leached out based on a diffusion-controlled release mechanism similar to DOX loaded HMSN at higher pH, most DOX had been effectively confined inside the pore of HMSN-PDMAEMA. It can be seen that the DOX release behavior depends on pH of the medium, which indicates that the pH-responsive polymer shell can control the drug release by adjusting pH of the solution. In acidic solution, the pores are open and the drug is released owing to extending of the polymer chains, while the compact polymer layers block the pores and confine the drug in the pores in alkaline or neutral medium.

600

a 500

3

Quantity Adsorbed (cm /g)

Release (%)

50

400

300

b 200

100

0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po) Fig. 5. Nitrogen adsorption–desorption isotherm of HMSN (a) and HMSN-PDMAEMA (b).

4. Conclusions Hollow mesoporous silica spheres with uniform size and morphology have been successfully synthesized by a facile route using PVP and DDA as the dual templates at room temperature. The

F. Yu et al. / Microporous and Mesoporous Materials 173 (2013) 64–69

formed ordered pore channels endow the mass exchange between the internal and the external of HMSN. PDMAEMA grafted on HMSN can act as a good gatekeeper to control access to the pores via a pH-dependent open–close mechanism, which is confirmed by the well-controlled release of DOX from the mesopores through adjusting pH of the solution. This nanodevice should have potential applications in site-selected drug release and gene delivery. Acknowledgement This research was supported by Shandong Provincial Natural Science foundation of China (ZR2010BM006).

[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

References [1] M. Manzano, M. Vallet-Regi, J. Mater. Chem. 20 (2010) 5593–5604. [2] Q.J. He, J.L. Shi, F. Chen, M. Zhu, L.X. Zhang, Biomaterials 31 (2010) 3335–3346. [3] I.I. Slowing, B.G. Trewyn, S. Giri, V.S.Y. Lin, Adv. Funct. Mater. 17 (2007) 1225– 1236. [4] B.G. Trewyn, S. Giri, I.I. Slowing, V.S.Y. Lin, Chem. Commun. 31 (2007) 3236– 3245. [5] Y.S. Lin, C.P. Tsai, H.Y. Huang, C.T. Kuo, Y. Hung, D.M. Huang, Y.C. Chen, C.Y. Mou, Chem. Mater. 17 (2005) 4570–4573. [6] C.P. Tsai, Y. Hung, Y.H. Chou, D.M. Huang, J.K. Hsiao, C. Chang, Y.C. Chen, C.Y. Mou, Small 4 (2008) 186–191. [7] S.H. Wu, Y.S. Lin, Y. Hung, Y.H. Chou, Y.H. Hsu, C. Chang, C.Y. Mou, ChemBioChem 9 (2008) 53–57. [8] C.W. Lu, Y. Hung, J.K. Hsiao, M. Yao, T.H. Chung, Y.S. Lin, S.H. Wu, S.C. Hsu, H.M. Liu, C.Y. Mou, C.S. Yang, D.M. Huang, Y.C. Chen, Nano Lett. 7 (2007) 149–154. [9] H.M. Liu, S.H. Wu, C.W. Lu, M. Yao, J.K. Hsiao, Y. Hung, Y.S. Lin, C.Y. Mou, C.S. Yang, D.M. Huang, Y.C. Chen, Small 4 (2008) 619–626. [10] C.H. Lee, T.S. Lin, C.Y. Mou, Nano Today 4 (2009) 165–179. [11] K.C. Kao, C.H. Lee, T.S. Lin, C.Y. Mou, J. Mater. Chem. 20 (2010) 4653–4662. [12] J. Zhou, W. Wu, D. Caruntu, M.H. Yu, A. Martin, J.F. Chen, C.J. O’Connor, W.L. Zhou, J. Phys. Chem. C 111 (2007) 17473–17477. [13] A.H. Lu, W.C. Li, A. Kiefer, W. Schmidt, E. Bill, G. Fink, F. Schuth, J. Am. Chem. Soc. 126 (2004) 8616–8617. [14] H.M. Buchold, C. Feldmann, Nano Lett. 7 (2007) 3489–3492. [15] Y.J. Li, X.F. Li, Y.L. Li, H.B. Liu, S. Wang, H.Y. Gan, J.B. Li, N. Wang, X.R. He, D.B. Zhu, Angew. Chem., Int. Ed. 45 (2006) 3639–3643.

[28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45]

69

P.T. Tanev, T.J. Pinnavaia, Science 271 (1996) 1267–1269. S.S. Kim, W. Zhang, T.J. Pinnavaia, Science 282 (1998) 1302–1305. S.A. Davis, S.L. Burkett, N.H. Mendelson, S. Mann, Nature 385 (1997) 420–423. T. Shiomi, T. Tsunoda, A. Kawai, F. Mizukami, K. Sakaguchi, Chem. Commun. 42 (2007) 4404–4406. W.J. Li, X.X. Sha, W.J. Dong, Z.C. Wang, Chem. Commun. 20 (2002) 2434–2435. Y.S. Li, J.L. Shi, Z.L. Hua, H.R. Chen, M.L. Ruan, D.S. Yan, Nano Lett. 3 (2003) 609– 612. S. Schacht, Q. Huo, I.G. Voigt-Martin, G.D. Stucky, F. Schuth, Science 273 (1996) 768–771. J. Wang, Q. Xiao, H. Zhou, P. Sun, Z. Yuan, B. Li, D. Ding, A.-C. Shi, T. Chen, Adv. Mater. 18 (2006) 3284–3288. R.K. Rana, Y. Mastai, A. Gedanken, Adv. Mater. 14 (2002) 1414–1418. M. Ogawa, N. Yamamoto, Langmuir 15 (1999) 2227–2229. J.G. Wang, F. Li, H.J. Zhou, P.C. Sun, D.T. Ding, T.H. Chen, Chem. Mater. 21 (2009) 612–620. J. Wang, Y. Xia, W. Wang, M. Poliakoff, R.J. Mokaya, J. Mater. Chem. 16 (2006) 1751–1756. Z.Z. Li, L.X. Wen, L. Shao, J.F. Chen, J. Controlled Release 98 (2004) 245–254. S. Giri, B.G. Trewyn, M.P. Stellmaker, V.S.Y. Lin, Angew. Chem., Int. Ed. 44 (2005) 5038–5044. C.Y. Lai, B.G. Trewyn, D.M. Jeftinija, K. Jeftinija, S. Xu, S. Jeftinija, J. Am. Chem. Soc. 125 (2003) 4451–4459. F. Torney, B.G. Trewyn, K. Wang, Nat. Nanotechnol. 2 (2007) 295–300. J.T. Sun, C.Y. Hong, C.Y. Pan, J. Phys. Chem. C 114 (2010) 12481–12486. Q. Fu, G.V.R. Rao, L.K. Ista, Y. Wu, B.P. Andrzejewski, L.A. Sklar, T.L. Ward, G.P. Lopez, Adv. Mater. 15 (2003) 1262–1266. Z. Zhou, S. Zhu, D. Zhang, J. Mater. Chem. 17 (2007) 2428–2433. Y. Yang, X. Yan, Y. Cui, Q. He, D. Li, A. Wang, J. Fei, J. Li, J. Mater. Chem. 18 (2008) 5731–5737. P.W. Chung, R. Kumar, M. Pruski, V.S.Y. Lin, Adv. Funct. Mater. 18 (2008) 1390– 1398. C.Y. Hong, X. Li, C.Y. Pan, J. Phys. Chem. C 112 (2008) 15320–15324. Y.Z. You, K.K. Kalebaila, S.L. Brock, D. Oupicky, Chem. Mater. 20 (2008) 3354– 3359. J.O. You, D.T. Auguste, Nano Lett. 9 (2009) 4467–4473. L. Cao, T. Man, J.Q. Zhuang, M. Kruk, J. Mater. Chem. 22 (2012) 6939–6946. S.W. An, P.N. Thirtle, R.K. Thomas, F.L. Baines, N.C. Billingham, S.P. Armes, J. Penfold, Macromolecules 32 (1999) 2731–2738. X. Li, C.Y. Hong, C.Y. Pan, Polymer 51 (2010) 92–99. L. Du, H.Y. Song, S.J. Liao, Appl. Surf. Sci. 255 (2009) 9365–9370. L.B. Chen, F. Zhang, C.C. Wang, Small 5 (2009) 621–628. Y.F. Jiao, J. Guo, S. Shen, B.S. Chang, Y.H. Zhang, X.G. Jiang, W.L. Yang, J. Mater. Chem. 22 (2012) 17636–17643.