Disintegration-controllable stimuli-responsive polyelectrolyte multilayer microcapsules via covalent layer-by-layer assembly

Colloids and Surfaces B: Biointerfaces 82 (2011) 385–390

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Disintegration-controllable stimuli-responsive polyelectrolyte multilayer microcapsules via covalent layer-by-layer assembly Bin Mu, Chunyin Lu, Peng Liu ∗ State Key Laboratory of Applied Organic Chemistry and Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China

a r t i c l e

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Article history: Received 20 August 2010 Received in revised form 1 September 2010 Accepted 9 September 2010 Available online 11 November 2010 Keywords: Multilayer microcapsule Stimuli-responsive Covalent layer-by-layer assembly Disintegration-controllable

a b s t r a c t The disintegration-controllable stimuli-responsive polyelectrolyte multilayer microcapsules have been fabricated via the covalent layer-by-layer assembly between the amino groups of chitosan (CS) and the aldehyde groups of the oxidized sodium alginate (OSA) onto the sacrificial templates (polystyrene sulfonate, PSS) which was removed by dialysis subsequently. The covalent crosslinking bonds of the multilayer microcapsules were confirmed by FTIR analysis. The TEM analysis showed that the diameter of the multilayer microcapsules was <200 nm. The diameter of the multilayer microcapsules decreased with the increasing of the pH values or the ionic strength. The pH and ionic strength dual-responsive multilayer microcapsules were stable in acidic and neutral media while they could disintegrate only at strong basic media. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The polymeric micro- and nanocapsules have attracted intense attention due to their various potential applications, such as delivery vesicles for drugs [1], protection shield for proteins, enzymes, or DNA [2], and catalysts [3], and so on. Many excellent strategies have been developed for the fabrication of the polymeric micro- and nanocapsules, including the layer-by-layer assembly [4,5], emulsion/interfacial polymerization [6,7], core–shell micelles [8,9] or vesicles [10,11] of block copolymers, and polymerization from template techniques [12–14]. Among them, the layer-by-layer assembly of the oppositely charged polyelectrolytes onto the sacrificial templates via the electrostatic interaction has become a well-established approach, which was extensively employed for the preparation of the polymeric microcapsules after the removal of the sacrificial templates [15,16]. It was found that the crosslinked multilayers could potentially display enhanced stability compared with the LbL ionicassembled films [17]. Recently, several excellent strategies had been developed for the fabrication of the crosslinked multilayer microcapsules via single-component [18–20] or two-component [21,22] covalent crosslinking. However, the disintegration or stability of these crosslinked multilayer microcapsules had not been studied. Furthermore, the crosslinkers were used in the twocomponent covalent crosslinking systems.

∗ Corresponding author. Tel.: +86 0931 8912516; fax: +86 0931 8912582. E-mail address: [email protected] (P. Liu). 0927-7765/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2010.09.024

In this paper, we have successfully prepared the disintegrationcontrollable stimuli-responsive polyelectrolyte multilayer microcapsules via the alternate adsorption and covalent layer-by-layer assembly by the electrostatic interaction between the amino groups of chitosan (CS) and carboxyl groups of oxidized sodium alginate (OSA) from the polystyrene sulfonate (PSS) templates, and the covalent crosslinking of the polyelectrolyte shells via the amino groups of CS and aldehyde groups of OSA from the polystyrene sulfonate (PSS) templates (Scheme 1). Compared with the crosslinked multilayer microcapsules reported [18–22], no extra crosslinkers were needed and the oppositely charged polyelectrolytes used could covalently crosslink with each other in the present work. The pH and ionic strength dual-responsive and the controlleddisintegration of the multilayer microcapsules were investigated.

2. Experimental 2.1. Materials Chitosan with viscosity-average molecular weight of 6.0 × 105 and N-deacetylation degree of 90% was purchased from Yuhuan Ocean Biochemical, Co., Ltd., Zhejiang, China. Sodium alginate (viscosity 350 cps for a 1 mg/mL solution) was purchased from Xudong, Co., Ltd., Beijing. Sodium metaperiodate (analytical reagent grade) was obtained from Haichuan, Co., Ltd., Zhejiang. Other reagents used were all of analytical reagent grade from Tianjin Chemical, Co., China, and were used as received. Deionized water was used throughout.

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Scheme 1. Synthetic route to the multilayer microcapsules (CS-OSA)7 .

2.2. Oxidized sodium alginate (OSA)

2.4. Measurements

Oxidized sodium alginate was prepared according to the method reported previously [23]. Sodium alginate (3.0 g) was dissolved in 300 mL deionized water by being stirred magnetically in a beaker. Sodium metaperiodate (2.0 g), dissolved in 50 mL deionized water, was added into the sodium alginate solution, and then the solution was stirred magnetically in the dark at the room temperature for 12 h. The product was precipitated by adding 400 mL ethanol into the solution. The precipitate was filtered, washed with ethanol/water (1:1, v/v ratio). The product was then dried under vacuum at room temperature for 48 h. The degree of oxidation (DO%) of OSA, defined as the number of oxidized guluronate residues per 100 guluronate units, was found to be 28%, determined by the method described by Bouhadir et al. [24].

Bruker IFS 66 v/s infrared spectrometer (Bruker, Karlsruhe, Germany) was used for the Fourier transform infrared (FT-IR) spectroscopy analysis in the range of 400–4000 cm−1 with the resolution of 4 cm−1 . The KBr pellet technique was adopted to prepare the samples. The morphologies of the PS particles, the polystyrene sulfonate (PSS) templates, the core–shell PSS@(CS-OSA)7 microspheres, and the crosslinked multilayer microcapsules (CS-OSA)7 were characterized with a JEM-1200 EX/S transmission electron microscope (TEM) (JEOL, Tokyo, Japan). They were dispersed in water and stirred for 30 min, and then deposited on a copper grid covered with a perforated carbon film. The zeta potentials of the crosslinked multilayer microcapsules (CS-OSA)7 at different pH values were determined with Zetasizer Nano ZS (Malvern Instruments Ltd, UK). The details of the process of measurement can be depicted as follow. The (CS-OSA)7 microcapsules were dispersed in aqueous solution (about 0.2 wt.%) with different pH values in an ultrasonic bath for 5 min, and then the supernatants were adopted to measure at 25 ◦ C. The mean particle size of the crosslinked multilayer microcapsules (CS-OSA)7 was conducted with BI-200SM laser light scattering system (LLS, Brookhaven Instruments, Co., Holtsville, NY) [25].

2.3. Multilayer microcapsules The sacrificial templates (polystyrene sulfonate, PSS) were prepared according to the procedure as reported previously [5]. The covalent layer-by-layer assembly technique was applied for the preparation of the crosslinked multilayer microcapsules by the electrostatic interaction between the amino groups of CS and carboxyl groups of OSA, and the covalent crosslinking of the polyelectrolyte shells via the amino groups of CS and aldehyde groups of OSA from the polystyrene sulfonate (PSS) templates, starting with CS. The adsorption of CS was conducted in a solution of 400 mL deionized water containing 0.5 g polystyrene sulfonate (PSS) templates and 0.25 g chitosan at pH around 4 for 8 h, followed by being centrifuged and washed with water for three times to obtain PSS-CS1 . Next the PSS-CS1 was dispersed into 200 mL deionized water. And 200 mL aqueous solution containing 0.25 g oxidized sodium alginate (pH = 4) was added into the dispersion at 50 ◦ C under magnetic stirring for 8 h. Then the core/shell microspheres (PSS@(CS-OSA)1 ) were centrifuged and washed three times with water. The adsorption procedure was repeated for further six cycles to prepare the PSS@(CS-OSA)7 (Scheme 1). After the desired number of layers was reached, the sacrificial template (PSS) was removed by dialysis to obtain the multilayer crosslinked microcapsules: The PSS@(CS-OSA)7 aqueous dispersion was dialyzed against N,N-dimethyl formamide (DMF) and deionized water using a dialysis membrane (MWCO = 13,000) for 5 days with several changes of the ratio of DMF and deionized water. The complete removal of the templates was confirmed by mixing the final dialysate with three times volume water ensuring the absence of any precipitate. Then the microcapsules were centrifuged, washed with ethanol, and dried under vacuum at the room temperature for 48 h.

3. Results and discussion 3.1. Preparation of multilayered microcapsules The uniform polystyrene (PS) particles were prepared by the emulsifier-free emulsion polymerization of styrene (St) with 2% methacrylic acid (MAA) as the surfmer, and then the PS particles were immersed into the concentrated sulfuric acid under stirring at 45 ◦ C for 8 h to introduce the sulfonic acid groups onto their surface for the adsorbing of chitosan (CS). The typical TEM images of the PS particles and the sodium polystyrene sulfonate templates (PSS) are given in Fig. 1(a) and (b), respectively. The microspheres are spherical and mono-disperse in size, with diameter of about 300 nm. The crosslinked multilayer microcapsules (CS-OSA)7 were fabricated by the covalent layer-by-layer assembly technique from the polystyrene sulfonate (PSS) templates. Originally, chitosan (CS) was adsorbed onto the templates by the electrostatic interaction between the amino groups of CS and the carboxyl groups and sulfonic acid groups on the surfaces of the PSS templates. Then the oxidized sodium alginate (OSA) was bonded to the PSSCS1 by the electrostatic interaction between the amino groups of CS and carboxyl groups of oxidized sodium alginate (OSA) from the polystyrene sulfonate (PSS) templates, and the covalent crosslinking of the polyelectrolyte shells via the amino groups of

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Fig. 1. TEM images of (a) PS particle, (b) polystyrene sulfonate (PSS) templates, (c) PSS@(CS-OSA)7 microspheres, and (d) multilayer microcapsules (CS-OSA)7 .

CS and aldehyde groups of OSA. Repeating this processing cycle for the desired number will yield the core/shell chitosan-oxidized sodium alginate multilayers encapsulated templates (PSS@(CSOSA)7 ) microspheres. After the templates were removed by dialysis in DMF, the crosslinked multilayer microcapsules were obtained, as illustrated in Scheme 1. The IR spectrum of core–shell PSS@(CS-OSA)7 microspheres, shown in Fig. 2, reveals that the well-defined characteristic bands of PS at 3061, 3026, 1602, 1493, 1425, and 699 cm−1 , and two characteristic peaks at 1699 and 1117 cm−1 attributed to the carboxyl and sulfonic acid groups, respectively. It also can be found that a new characteristic band appears at 1654 cm−1 due to the –C N– stretching, which confirms that the Schiff-base structure was formed by the covalent cross-linking reaction between the amino groups of CS and the aldehyde groups of OSA. That is to say that the covalent layer-by-layer assembly had taken place between CS and OSA on the templates (PSS). Furthermore, Fig. 1 distinctly demonstrates that the typical core–shell morphology of the PSS@(CS-OSA)7 microspheres after the templates were covered alternately by chitosan and oxidized sodium alginate by the covalent layer-by-layer assembly. It could be found that the size is

Fig. 2. FTIR spectra of the core–shell PSS@(CS-OSA)7 microspheres, and the multilayer microcapsules (CS-OSA)7 .

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a

30

100

C NaCl = 0 C NaCl = 0.005 C NaCl = 0.012

80

10 0

f(D h)

Zeta potential (mV)

20

-10 -20

40

20

-30 -40

60

2

4

6

8

10

0

12

0

pH

around 300–320 nm, which is consistent with the result of the TEM analysis (Fig. 1(c)). The characteristic absorbance bands of the templates disappeared after the templates were etched by dialysis, as shown in Fig. 2. It indicates that the PSS templates are removed completely. The hollow structure of the crosslinked polyelectrolyte multilayer microcapsules (CS-OSA)7 could be observed in the TEM image (Fig. 1(d)). The diameter of the crosslinked multilayer microcapsules was around 150–180 nm, which was small than the sizes of the templates particles (300 nm). It might be mainly attributed to the shrinking of the polymeric shells in the dried state after the templates were removed.

Zeta potential measurement gives the information of the surface charge associated with the microspheres. It was observed that the zeta potentials of the crosslinked polyelectrolyte multilayer microcapsules (CS-OSA)7 changed as a function of pH value of the dispersion media (Fig. 3). It demonstrates that the negative value of the zeta potential was achieved in the dispersion medium with pH value of about 5 due to the ionization of the carboxylic acid groups of OSA on the surface of the multilayered microcapsules. The further increasing of pH values decreased the negative value of the zeta potential due to the masking of the carboxylate ions by the Na+ counterions from NaOH added to adjust the pH values. However, the negative value of the zeta potential increased at the pH value of 10, which might attribute to the hydrolysis of the Schiff-base structures. 3.3. Ionic strength-responsive The diameter of the multilayer microcapsules (CS-OSA)7 shows the significant decrease with the increase of the ionic strength, as shown in Fig. 4. However, in the media with the higher salt concentrations, the diameter of the crosslinked multilayer microcapsules remains constant. The typical hydrodynamic diameter distributions (f(Dh )) of the multilayer microcapsules (CS-OSA)7 are different with different ionic strengths. The hydrodynamic diameter (Dh ) is found to be in the range 0.07–2.0 ␮m, with the average hydrodynamic diameter of 2.0 ␮m without NaCl added. While the salt concentration increasing to 0.012, the hydrodynamic diameter distributions become broad, and the hydrodynamic diameter (Dh ) is in the range 25–408 nm, the average hydrodynamic diameter is about 73 nm. This might be due to the different crosslinking degrees of the multilayer crosslinked microcapsules.

1500

2000

b 2000

1500

1000

500

0

3.2. Zeta potentials analysis

1000

D h (nm)

D h(nm)

Fig. 3. Zeta potential of the multilayer microcapsules (CS-OSA)7 at different pH values.

500

0.000

0.004

0.008

0.012

0.016

C NaCl (mol/L) Fig. 4. Ionic strength dependence of (a) the typical hydrodynamic diameter distributions f(Dh ) and (b) the average hydrodynamic diameter (Dh ) obtained for 4 × 10−5 g/mL aqueous solution of the multilayer microcapsules (CS-OSA)7 .

It is well-known that the increasing ionic strength shields intramolecular electrostatic repulsions and causes the chains to swell for the polyelectrolyte multilayer microcapsules via only the electrostatic interaction. So the hydrodynamic diameter of the polyelectrolyte multilayer microcapsules with only the electrostatic interaction increases with the increasing of the ionic strength [26]. However, different with the polyelectrolyte multilayer microcapsules with only the electrostatic interaction, for the polyelectrolyte multilayer microcapsules via covalent crosslinking in the present work, the addition of salt induced the enhancement in the hydrophobic character of the polyelectrolyte chains due to the screening of the macroion charges so their swelling degree decreased, as in the hydrogels. Among the two factors, the decreased swelling degree of the covalent crosslinked shells was the decisive factor, thereby the diameter of the multilayer microcapsules decreased with the increasing of the ionic strength [27,28]. 3.4. pH-responsive In order to investigate the effect of the pH values on the crosslinked multilayer microcapsules (CS-OSA)7 , which were dispersed in aqueous solutions with different pH values (pH = 2, 4, 6, 10, and 12) by being stirred for 30 min to measure their diameters. It can be seen from Fig. 5 that the diameter of the multilayer microcapsules (CS-OSA)7 decreases from 535 to 86 nm with the increase

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a

100 pH=4 pH=12

80

f(D h)

389

60

40

20

0

0

50

100

150

200

250

300

350

D h(nm)

b

600 500

D h(nm)

400 Fig. 6. TEM image of the multilayer microcapsules (CS-OSA)7 dispersed in aqueous solution at pH = 12 for 1 h.

300 200

C

N R + H 2O

HO C

H N R

(1)

100 0

-

0

2

4

6

8

10

OC

12

pH Fig. 5. pH dependence of (a) the typical hydrodynamic diameter distributions f(Dh ) and (b) the average hydrodynamic diameter (Dh ) obtained for 4 × 10−5 g/mL aqueous solution of the multilayer microcapsules (CS-OSA)7 .

of the pH value from 2 to 12. This fact can be explained by the following reason. These multilayer microcapsules (CS-OSA)7 were fabricated by chitosan (CS) and the oxidized sodium alginate (OSA) via the alternating adsorption electrostatic interaction between the amino groups of CS and carboxyl groups of OSA and the covalent crosslinking of the polyelectrolyte shells via the amino groups of CS and aldehyde groups of OSA. CS is a kind of weak alkali and OSA is a kind of weak acid. The pKa values of the guluronic acid and the mannuronic acid of sodium alginate (SAG) are 3.65 and 3.38, respectively. And the pKa value of CS is about 6.5 [29–31]. In the stronger acidic conditions, such as pH < 4.0, most of the carboxyl groups of OSA are in the form of –COOH, the remanent amino groups of CS are in the form of –NH3 + . The interaction between –NH3 + and –COO– in the multilayer microcapsules (CS-OSA)7 could be disrupted by the small molecule acid [32,33], which leads to the chain shrink of OSA because that OSA would be flocculated at low pH values (at about 2.0). Furthermore, the Schiff-base structure was stable at low pH values. However, it would be destroyed at high pH value media, the relevant mechanism of hydrolysis of the Schiff-base structures derived from aliphatic amines can be illustrated as reported previously [34]. The TEM image of the multilayer microcapsules (CS-OSA)7 after being dispersed in aqueous solution at pH = 12 for 1 h was shown in Fig. 6. The electrostatic interaction and the covalent crosslinking between CS and OSA were destroyed in the stronger basic media, as shown schematically in Scheme 2. The solid

H+ N R H

CO

+ RNH2

(2)

Scheme 2. Mechanism of hydrolysis of the Schiff-base structure derived from aliphatic amines.

particles with size less than 40 nm might be the complex formed by the CS and OSA molecules. 4. Conclusions In summary, an efficient strategy was developed for the preparation of the pH and ionic strength dual-responsive multilayer microcapsules with the diameter of around 150–180 nm via the alternating adsorption and covalent layer-by-layer assembly by the electrostatic interaction between the amino groups of CS and carboxyl groups of OSA and the covalent crosslinking of the polyelectrolyte shells via the amino groups of CS and aldehyde groups of OSA, after the polystyrene sulfonate (PSS) templates were etched. The diameter of the multilayer microcapsules (CS-OSA)7 decreased significantly from 2.0 ␮m to 73 nm with the increase of the ionic strength from the range of 0–0.010 mol/L NaCl, which might be attributed to the increase in the hydrophobic character of the polyelectrolyte chains due to the screening of the macroion charges. Furthermore, the multilayered microcapsules are pH-sensitive in the acidic and neutral media and can only disintegrate in the strong basic media, so they are expected to be promising intelligent drugcarriers for alimentary canal. Acknowledgments This project was granted financial support from the National Nature Science Foundation of China (Grant No. 20904017) and Program for New Century Excellent Talents in University (Grant No. NCET-09-0441).

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References [1] Y. Wang, V. Bansal, A.N. Zelikin, F. Caruso, Nano Lett. 8 (2008) 1741. [2] A.N. Zelikin, Q. Li, F. Caruso, Angew. Chem. Int. Ed. 45 (2006) 7743. [3] S.D. Miao, C.L. Zhang, Z.M. Liu, B.X. Han, Y. Xie, S.F. Ding, Z.Z. Yang, J. Phys. Chem. C 112 (2008) 774. [4] J. Schwiertz, W. Meyer-Zaika, L. Ruiz-Gonzalez, J.M. González-Calbet, M. ValletRegíand, M. Epple, J. Mater. Chem. 18 (2008) 3831. [5] B. Mu, P. Liu, Y. Dong, C.Y. Lu, X.L. Wu, J. Polym. Sci. A: Polym. Chem. 48 (2010) 3135. [6] C.I. Zoldesi, A. Imhof, Adv. Mater. 17 (2005) 924. [7] S. Yang, H.R. Liu, J. Mater. Chem. 16 (2006) 4480. [8] Q. Zhang, E.E. Remsen, K.L. Wooley, J. Am. Chem. Soc. 122 (2000) 3642. [9] F. Checot, S. Lecommandoux, Y. Gnanou, H.A. Klok, Angew. Chem. Int. Ed. 41 (2002) 1339. [10] L.S. Zha, Y. Zhang, W.L. Yang, S.K. Fu, Adv. Mater. 14 (2002) 1090. [11] J.Z. Du, Y.M. Chen, Macromolecules 37 (2004) 5710. [12] Y.W. Zhang, M. Jiang, J.X. Zhao, X.W. Ren, D.Y. Chen, G.Z. Zhang, Adv. Funct. Mater. 15 (2005) 695. [13] B. Mu, R.P. Shen, P. Liu, Colloids Surf. B: Biointerfaces 74 (2009) 511. [14] P. Liu, G.F. Liu, W. Zhang, F. Jiang, Nanotechnology 21 (2010) 015603. [15] A.P.R. Johnston, C. Cortez, A.S. Angelatos, F. Caruso, Curr. Opin. Colloid Interface Sci. 11 (2006) 203. [16] Y.J. Wang, A.S. Angelatos, F. Caruso, Chem. Mater. 20 (2008) 848. [17] I. Pastoriza-Santos, B. Scholer, F. Caruso, Adv. Funct. Mater. 11 (2001) 122.

[18] M.K. Park, C. Xia, R.C. Advincula, Langmuir 17 (2001) 7670. [19] Y. Zhang, Y. Guan, S. Zhou, Biomacromolecules 6 (2005) 2365. [20] L. Duan, Q. He, X. Yan, Y. Cui, K. Wang, J. Li, Biochem. Biophys. Res. Commun. 354 (2007) 357. [21] S. Ye, C. Wang, X. Liu, Z. Tong, J. Biomater. Sci.: Polym. Ed. 16 (2005) 909. [22] H. Lee, Y. Jeong, T.G. Park, Biomacromolecules 8 (2007) 3705. [23] N.Q. Wu, C.Y. Pan, B.J. Zhang, Y.P. Rao, D. Yu, Acta Polym. Sin. 6 (2007) 497. [24] K.H. Bouhadir, D.S. Hausman, D.J. Mooney, Polymer 40 (1999) 3575. [25] Y. Jin, L.N. Zhang, M. Zhang, L. Chen, P.C.K. Cheung, V.E.C. Oi, Y. Lin, Carbohydr. Res. 338 (2003) 1517. [26] C.Y. Gao, S. Leporatti, S. Moya, E. Donath, H. Mohwald, Chem. Eur. J. 9 (2003) 915. [27] A. Bartkowiak, Colloids Surf. A: Physicochem. Eng. Aspects 204 (2002) 117. [28] S. Forster, N. Hermsdorf, C. Bottcher, P. Lindner, Macromolecules 35 (2002) 4096. [29] G. Orive, S. Ponce, R.M. Hernandez, A.R. Gascon, M. Igartua, J.L. Pedraz, Biomaterials 23 (2002) 3825. [30] J.L. Drury, D.J. Mooney, Biomaterials 24 (2003) 4337. [31] H.L. Lai, A. Abu’Khalil, D.Q.M. Craig, Int. J. Pharm. 251 (2003) 175. [32] Y. Hu, X.Q. Jiang, Y. Ding, H.X. Ge, Y.Y. Yuan, C.Z. Yang, Biomaterials 23 (2002) 3193. [33] T. Mauser, C. Dejugnat, G.B. Sukhorukov, Macromol. Rapid Commun. 25 (2004) 1781. [34] E.H. Cordes, W.P. Jencks, J. Am. Chem. Soc. 85 (1963) 2843.