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Cite this: Chem. Commun., 2014, 50, 1000 Received 6th October 2013, Accepted 11th November 2013
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Polyacrylic acid@zeolitic imidazolate framework-8 nanoparticles with ultrahigh drug loading capability for pH-sensitive drug release† Hong Ren,a Lingyu Zhang,a Jiping An,b Tingting Wang,*c Lu Li,a Xiaoyan Si,a Liu He,a Xiaotong Wu,a Chungang Wang*a and Zhongmin Su*a
DOI: 10.1039/c3cc47666a www.rsc.org/chemcomm
The polyacrylic acid@zeolitic imidazolate framework-8 (PAA@ZIF-8) nanoparticles (NPs) were first fabricated using a facile and simple route. It is worthwhile noting that the as-fabricated PAA@ZIF-8 NPs possessed ultrahigh doxorubicin (DOX) loading capability (1.9 g DOX g
1
NPs),
which were employed as pH-dependent drug delivery vehicles.
Nanoscale coordination polymers (NCPs) have been studied for a wide range of applications.1 For example, Maspoch et al. have fabricated NCPs for encapsulating and delivering drugs (DOX, SN-38, camptothecin and daunomycin) by connecting Zn2+ metal ions through 1,4-bis(imidazol-1-ylmethyl)benzene organic ligands.2 The final encapsulation efficiency is 21% of the initial drug concentration. Junior and co-workers have incorporated DOX into ZIF-8 with a loading of 0.049 g DOX g 1 ZIF-8.3a In addition, a pH-controlled drug delivery system is considered beneficial for targeting cancerous tissues, because of lower extracellular pH values of tumors, especially, inside endosomal (pH = 5.5– 6.0) and lysosomal (pH = 4.5–5.0) compartments than normal tissues and bloodstream.4 Huang et al. have recently reported the pHresponsive NCPs which displayed a DOX loading content of nearly 40%.4c However, the primary drawback of these NCPs is their limited drug loading of DOX, an effective anticancer drug. Design of functional NCPs to realize the high DOX loading and pH-responsive release is therefore of great significance. Recently, we have developed a facile and simple method to fabricate poly(acrylic acid sodium salt) (PAAS) NPs, which exhibit the potential high DOX-loading capacity through electrostatic interaction and the pH-responsive drug release property. However, the bare PAAS NPs lack stability in aqueous solution due to their good water solubility. Moreover, nanosized ZIF-8 is suggested to be feasible for use as a drug carrier owing to
its exceptional thermal and chemical stability and pH-sensitive release. This motivates us to explore PAA@ZIF-8 NPs with ultrahigh DOX loading capacity to achieve pH-responsive drug delivery. Here we for the first time report the synthesis of PAA@ZIF-8 NPs with ultrahigh drug loading capability using PAA-Zn NPs as templates, which are used as pH-controlled drug delivery vehicles. As summarized in Scheme 1, PAA-Zn NPs were firstly fabricated by ion-exchange between Zn2+ and Na+ on the surface of PAAS NPs. Then PAA-Zn NPs were dropped into a methanol solution of 2-methylimidazole (HMeIM) to form PAA@ZIF-8 NPs. Furthermore, DOX as a model drug was used to confirm the drug loading and pH-sensitive release of PAA@ZIF-8 NPs which were employed as drug carriers with excellent biocompatibility. In a typical synthesis, PAAS NPs with a diameter of B100 nm were prepared via a mild and facile route reported in our previous study (Fig. 1A).5 Subsequently, the as-synthesized PAAS NPs with –COO groups were simply mixed with Zn(NO3)2 to prepare PAA-Zn NPs in the presence of methanol at room temperature for 5 min. Transmission electron microscopy (TEM) reveals that dispersed PAA-Zn NPs have an average diameter of 100 nm (Fig. 1B). The fabrication of PAA-Zn NPs was owing to the coordination
a
Faculty of Chemistry, Northeast Normal University, Changchun 130024, P. R. China. E-mail: [email protected], [email protected] b Hospital of Stomatology, JiLin University, Changchun 130021, P. R. China c School of Chemistry & Environmental Engineering, Changchun University of Science and Technology, Changchun, 130022, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental procedures, TEM, XRD, EDS and TG curves. See DOI: 10.1039/c3cc47666a
1000 | Chem. Commun., 2014, 50, 1000--1002
Scheme 1 Schematic illustration of the synthetic procedure for the PAA@ZIF-8 NPs as drug vehicles for loading and pH-controlled release of DOX. HMeIM: 2-methylimidazole.
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Fig. 1 TEM images of (A) PAAS NPs, (B) PAA-Zn NPs and (C) PAA@ZIF-8 NPs, respectively. SEM images of (D) PAA-Zn NPs and (E) PAA@ZIF-8 NPs. (F) FT-IR spectra of (a) PAAS NPs, (b) PAA-Zn NPs and (c) PAA@ZIF-8 NPs, respectively.
between –COO groups provided by PAAS NPs and Zn2+. However, not all –COO groups combine with Zn2+ ions, although the content of Zn2+ ions exceeds –COO groups. The percentage of Zn2+ in PAA-Zn NPs is 20.5 wt% as confirmed by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Assuming that all Zn2+ ions in PAA-Zn combined with –COO groups, 60 mol% –COO groups in samples combined with Zn2+ ions.6 As is well known, Zn2+ ions will form [Zn(H2O)6]2+ complexes and then change to [Zn(OH)(H2O)5]+ ions as in the following equation, when Zn(NO3)2 is added into PAAS NPs solution. [Zn(H2O)6]2+ " [Zn(OH)(H2O)5]+ + H+ The generated H+ ions react with the –COO groups and form –COOH, leading to the presence of PAA.6 Energy dispersive X-ray spectroscopy (EDS) and Fourier transform infrared measurements (FT-IR) were also performed to prove the fabrication of PAA-Zn. Fig. 1F(a) and (b) show the FT-IR spectra of PAAS and PAA-Zn, respectively, where the carbonyl asymmetric stretching band (CQO) of –COOH can be found at 1710 cm 1. The peaks around 1550 cm 1 are assigned to the shift of the CQO bands caused by coordination of –COO groups and metal cations. Furthermore, the Zn element can be obviously found from the EDS (Fig. S1, ESI†). Finally, the as-obtained PAA-Zn NPs were used as templates and Zn sources to form PAA@ZIF-8 NPs (Fig. 1C). The TEM and SEM images show an average diameter of 128 nm for PAA@ZIF-8 NPs, compared to that of around 100 nm for PAA-Zn NPs (Fig. 1D and E). The hydrodynamic diameters of the PAAS, PAA-Zn and PAA@ZIF-8 NPs were determined by dynamic light scattering (DLS) to be 111.7, 109.4 and 138.6 nm, respectively (Fig. S2, ESI†). In addition, no obvious aggregation and changes in size and morphology were observed when the PAA@ZIF-8 NPs were dispersed into serum and water, respectively, (Fig. S3, ESI†), reflecting
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their good stability. To prove the successful growth of ZIF-8, FT-IR spectroscopy and X-ray diffraction (XRD) (Fig. S4, ESI†) were performed. Several bands are observed for ZIF-8 in the FT-IR spectrum. As shown in Fig. 1F(c), the absorption band at 422 cm 1 is observed for Zn-N stretching mode and the absorption bands in the 1100–1400 cm 1 region are associated with C–N stretch.7 Note that in the XRD pattern (Fig. S4(a), ESI†) the diffraction peaks at ca. 7.3 and 12.71 confirm the presence of ZIF-8.4b However, it doesn’t thoroughly crystallize. Control experiments at 70 1C for 0.5, 12, 24 and 36 h were carried out to investigate the effect of the reaction time on the crystallization of ZIF-8. Along with the reaction time, better crystalline ZIF-8 structures could be obtained (Fig. S4, ESI†). However, it should be pointed out that aggregation of PAA@ZIF-8 NPs occurred to different degrees with the reaction time prolonged (Fig. S5, ESI†). Based on these results, the dispersed PAA@ZIF-8 NPs synthesized at 70 1C for 0.5 h and the dispersed PAA@ZIF-8 NPs synthesized at 70 1C for 0.5 h were considered as the ideal structures as drug delivery carriers. To further demonstrate the synthesis of PAA@ZIF-8 NPs, thermogravimetric (TG) analysis of PAA@ZIF-8 NPs was done and showed three weight loss events (%) from room temperature to 616 1C (Fig. S6, ESI†). The first event is typical of the loss of hydration water molecules from room temperature to 100 1C. The PAA partly displays weight loss at a temperature range of 200–390 1C. The third event corresponding to the decomposition of PAA and ZIF-8 yields 21% residual zinc oxide.3 To evaluate the loading and controlled release behaviour of the PAA@ZIF-8 NPs, DOX, an anti-cancer drug, has been selected as a model drug. The high loading efficiency of encapsulation of DOX into PAA@ZIF-8 carriers could reach 95%, and the extremely high drug loading content was 190 mg of DOX per 100 mg of the PAA@ZIF-8 support, which were determined by the characteristic DOX absorption peak at B480 nm (Fig. 2A).8 The high drug loading capacity for DOX of NPs may be attributed to two types of forces: (1) the electrostatic interaction between the negatively charged carboxylic acid groups and positively charged DOX; (2) the coordination bonding of Zn(II)-DOX.9 The in vitro release rate of DOX from the DOX-loaded PAA@ZIF-8 could be controlled by changing pH values at 37 1C. In neutral PBS (pH = 7.4), DOX was released in a very slow fashion and the cumulative release of DOX was only about 35.6% even after 60 h at pH = 7.4. In contrast, it
Fig. 2 (A) UV-Vis absorption spectra and photographs (inset) of DOX solutions before (a) and after (b) interaction with PAA@ZIF-8 NPs. (B) DOX-release profiles for DOX-loaded PAA@ZIF-8 NPs measured at pH 5.5 and 7.4 at 37 1C. (C) In vitro cytoxicity of empty PAA@ZIF-8 NPs, DOX-loaded PAA@ZIF-8 NPs and free DOX against MCF-7 cells at different concentrations after 24 h.
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Fig. 3 CLSM images of MCF-7 cells incubated with DOX-loaded PAA@ZIF-8 ([DOX] = 20 mg mL 1) for 3 h (A–C), 12 h (D–F) and 24 h (G–I) at 37 1C, respectively. Each series can be classified to cell nucleus (being dyed in blue by Hoechst 33342), DOX-loaded PAA@ZIF-8 NPs and the merged images of both above, respectively. All scale bars are 10 mm.
can be clearly seen that the system shows a faster drug release rate, 75.9% of DOX was released after 60 h at pH = 5.5 (Fig. 2B). Thus, the pH-dependent drug release PAA@ZIF-8 system is promising for use as a drug carrier and is beneficial for targeting cancerous tissues,10 since the extracellular pH of tumors is lower than that of normal tissues and the microenvironments of tumors are acidic.4 Furthermore, it is very important to evaluate the biocompatibility of the PAA@ZIF-8 NPs for their potential bio-applications. Hence, the standard 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) cell assay was performed on MCF-7 cells to characterize the cytotoxicity of the PAA@ZIF-8 NPs without DOX, DOX-loaded PAA@ZIF-8 NPs and free DOX. These data shown in Fig. 2C reveal that the cytotoxic efficacy of the DOX-loaded PAA@ZIF-8 NPs was similar to that of the free DOX and enhanced with an increase of DOX released from the DOX-loaded PAA@ZIF-8 NPs within the MCF-7 cells. In addition, more than 90.8% cell viability is observed in the varying concentration ranges, even at a high concentration of up to 50 mg mL 1 of PAA@ZIF-8 NPs after incubation for 24 h. The above MTT assay results confirm that the PAA@ZIF-8 NPs are nearly nontoxic and can be applied in the biomedical field with good biocompatibility. Confocal laser scanning microscopy (CLSM) analyses were used to evaluate the intracellular release behaviour of DOX-loaded PAA@ZIF-8 NPs and further confirm whether the pH-responsive function is maintained intracellularly11 (Fig. 3). In the first 3 h, only a few of the DOX-loaded PAA@ZIF-8 NPs could be uptaken by MCF-7 cells via the endocytosis process and localized in the cytoplasm12 as shown in Fig. 3A–C. Increasing amounts of DOX released and delivered by DOX-loaded PAA@ZIF-8 NPs passed through the nucleus membrane and eventually assembled in the nucleus to kill cells12 at 12 and 24 h. The cells incubated with DOX-loaded PAA@ZIF-8 NPs for 24 h (Fig. 3G–I) show stronger red luminescence signals, suggesting that more drug carrier systems were uptaken specifically by MCF-7 cells than at 12 h (Fig. 3D–F). The above results demonstrate that PAA@ZIF-8 NPs are promising
1002 | Chem. Commun., 2014, 50, 1000--1002
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for use as drug delivery candidates and enhance the therapeutic efficacy. In summary, a synthetic strategy for the designed preparation of PAA@ZIF-8 NPs for the first time by using PAA-Zn NPs as templates has been reported. The resulting PAA@ZIF-8 NPs possessed an ultrahigh loading capacity of DOX (1.9 g DOX g 1 NPs) and the pH-sensitive drug release property. The DOX-loaded PAA@ZIF-8 NPs could be taken up by MCF-7 cells through endocytosis and release DOX much faster in mild acidic buffer solution (pH = 5.5) than at a neutral pH of 7.4 in the cytoplasm by CLSM. The MTT assay results indicated that the PAA@ZIF-8 NPs were nearly non-toxic to live cells. This pH-responsive release property may contribute to the selective release of an encapsulated drug in acidic tumor tissues or cell interiors. Moreover, the general and convenient synthetic method can be extended to synthesize other functional nanomaterials for the various potential biological applications such as biological imaging probes, drug delivery systems and therapeutic applications. This work was supported by the National Natural Science Foundation of China (Grant No. 21173038 and 21301027), Natural Science Foundation and Science and Technology Development Planning of Jilin Province (201215003, 201201115 and 20130522136JH), Science Foundation for Young Teachers of Changchun University of Science and Technology (XQNJJ-2011-11), Program for New Century Excellent Talents in University (NCET-13-0720) and Fundamental Research Funds for the Central Universities (12SSXM005).
Notes and references 1 (a) S. Keskin and S. Kizilel, Ind. Eng. Chem. Res., 2011, 50, 1799; (b) J. D. Rocca, D. M. Liu and W. B. Lin, Acc. Chem. Res., 2011, 44, 957; (c) Z. B. Ma and B. Moulton, Coord. Chem. Rev., 2011, 255, 1623; (d) W. L. Rieter, K. M. Pott, K. M. L. Taylor and W. Lin, J. Am. Chem. Soc., 2008, 130, 11584; (e) M. V. Regı´, F. Balas and D. Arcos, Angew. Chem., Int. Ed., 2007, 46, 7548. ´ndez, F. Garcı´a, D. R. Molina, 2 I. Imaz, M. R. Martı´nez, L. G. Ferna J. Hernando, V. Puntes and D. Maspoch, Chem. Commun., 2010, 46, 4737. 3 (a) I. B. Vasconcelos, T. G. D. Silva, G. C. G. Militao, T. A. Soares, N. M. Rodrigues, M. O. Rodeigues, N. B. D. Costa, R. O. Freire and S. A. Junior, RSC Adv., 2012, 2, 9437; (b) L. L. Chen, L. Li, L. Y. Zhang, S. X. Xing, T. T. Wang, Y. A. Wang, C. G. Wang and Z. M. Su, ACS Appl. Mater. Interfaces, 2013, 5, 7282. 4 (a) L. Xing, H. Q. Zheng, Y. Y. Cao and S. A. Che, Adv. Mater., 2012, 24, 6433; (b) C. Y. Sun, C. Qin, X. L. Wang, G. S. Yang, K. Z. Shao, Z. M. Su, P. Huang, C. G. Wang and E. B. Wang, Dalton Trans., 2012, 41, 6906; (c) P. F. Gao, L. L. Zheng, L. J. Liang, X. X. Yang, Y. F. Li and C. Z. Huang, J. Mater. Chem. B, 2013, 1, 3202; (d) D. M. Yang, X. J. Kang, P. A. Ma, Y. L. Dai, Z. Y. Hou, Z. Y. Cheng, C. X. Li and J. Lin, Biomaterials, 2013, 34, 1601. 5 H. Ren, L. Y. Zhang, T. T. Wang, L. Li, Z. M. Su and C. G. Wang, Chem. Commun., 2013, 49, 6036. 6 Y. Gotoh, Y. Ohkoshi and M. Nagura, Sen’i Gakkaishi, 1999, 55, 522. 7 H. J. Lee, W. Cho and M. Oh, Chem. Commun., 2012, 48, 221. 8 T. T. Wang, F. Chai, Q. Fu, L. Y. Zhang, H. Y. Liu, L. Li, Y. Liao, Z. M. Su, C. G. Wang, B. Y. Duan and D. X. Ren, J. Mater. Chem., 2011, 21, 5299. 9 H. Q. Zheng, L. Xing, Y. Y. Cao and S. A. Che, Coord. Chem. Rev., 2013, 257, 1933. 10 (a) L. Xing, Y. Y. Cao and S. A. Che, Chem. Commun., 2012, 48, 5995; (b) E. Y. Yan, Y. Ding, C. J. Chen, R. T. Li, Y. Hu and X. Q. Jiang, Chem. Commun., 2009, 2718. 11 Z. W. Chen, Z. H. Li, Y. H. Lin, M. L. Yin, J. S. Ren and X. G. Qu, Biomaterials, 2013, 34, 1364. 12 (a) X. Y. Yuan, B. S. Zhu, X. F. Ma, G. S. Tong, Y. Su and X. Y. Zhu, Langmuir, 2013, 29, 12275; (b) Y. L. Dai, P. A. Ma, Z. Y. Cheng, X. J. Kang, X. Zhang, Z. Y. Hou, C. X. Li, D. M. Yang, X. F. Zhai and J. Lin, ACS Nano, 2012, 6, 3327.
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Polyacrylic acid@zeolitic imidazolate framework-8 nanoparticles with ultrahigh drug loading capability for pH-sensitive drug release† Hong Ren,a Lingyu Zhang,a Jiping An,b Tingting Wang,*c Lu Li,a Xiaoyan Si,a Liu He,a Xiaotong Wu,a Chungang Wang*a and Zhongmin Su*a
DOI: 10.1039/c3cc47666a www.rsc.org/chemcomm
The polyacrylic acid@zeolitic imidazolate framework-8 (PAA@ZIF-8) nanoparticles (NPs) were first fabricated using a facile and simple route. It is worthwhile noting that the as-fabricated PAA@ZIF-8 NPs possessed ultrahigh doxorubicin (DOX) loading capability (1.9 g DOX g
1
NPs),
which were employed as pH-dependent drug delivery vehicles.
Nanoscale coordination polymers (NCPs) have been studied for a wide range of applications.1 For example, Maspoch et al. have fabricated NCPs for encapsulating and delivering drugs (DOX, SN-38, camptothecin and daunomycin) by connecting Zn2+ metal ions through 1,4-bis(imidazol-1-ylmethyl)benzene organic ligands.2 The final encapsulation efficiency is 21% of the initial drug concentration. Junior and co-workers have incorporated DOX into ZIF-8 with a loading of 0.049 g DOX g 1 ZIF-8.3a In addition, a pH-controlled drug delivery system is considered beneficial for targeting cancerous tissues, because of lower extracellular pH values of tumors, especially, inside endosomal (pH = 5.5– 6.0) and lysosomal (pH = 4.5–5.0) compartments than normal tissues and bloodstream.4 Huang et al. have recently reported the pHresponsive NCPs which displayed a DOX loading content of nearly 40%.4c However, the primary drawback of these NCPs is their limited drug loading of DOX, an effective anticancer drug. Design of functional NCPs to realize the high DOX loading and pH-responsive release is therefore of great significance. Recently, we have developed a facile and simple method to fabricate poly(acrylic acid sodium salt) (PAAS) NPs, which exhibit the potential high DOX-loading capacity through electrostatic interaction and the pH-responsive drug release property. However, the bare PAAS NPs lack stability in aqueous solution due to their good water solubility. Moreover, nanosized ZIF-8 is suggested to be feasible for use as a drug carrier owing to
its exceptional thermal and chemical stability and pH-sensitive release. This motivates us to explore PAA@ZIF-8 NPs with ultrahigh DOX loading capacity to achieve pH-responsive drug delivery. Here we for the first time report the synthesis of PAA@ZIF-8 NPs with ultrahigh drug loading capability using PAA-Zn NPs as templates, which are used as pH-controlled drug delivery vehicles. As summarized in Scheme 1, PAA-Zn NPs were firstly fabricated by ion-exchange between Zn2+ and Na+ on the surface of PAAS NPs. Then PAA-Zn NPs were dropped into a methanol solution of 2-methylimidazole (HMeIM) to form PAA@ZIF-8 NPs. Furthermore, DOX as a model drug was used to confirm the drug loading and pH-sensitive release of PAA@ZIF-8 NPs which were employed as drug carriers with excellent biocompatibility. In a typical synthesis, PAAS NPs with a diameter of B100 nm were prepared via a mild and facile route reported in our previous study (Fig. 1A).5 Subsequently, the as-synthesized PAAS NPs with –COO groups were simply mixed with Zn(NO3)2 to prepare PAA-Zn NPs in the presence of methanol at room temperature for 5 min. Transmission electron microscopy (TEM) reveals that dispersed PAA-Zn NPs have an average diameter of 100 nm (Fig. 1B). The fabrication of PAA-Zn NPs was owing to the coordination
a
Faculty of Chemistry, Northeast Normal University, Changchun 130024, P. R. China. E-mail: [email protected], [email protected] b Hospital of Stomatology, JiLin University, Changchun 130021, P. R. China c School of Chemistry & Environmental Engineering, Changchun University of Science and Technology, Changchun, 130022, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental procedures, TEM, XRD, EDS and TG curves. See DOI: 10.1039/c3cc47666a
1000 | Chem. Commun., 2014, 50, 1000--1002
Scheme 1 Schematic illustration of the synthetic procedure for the PAA@ZIF-8 NPs as drug vehicles for loading and pH-controlled release of DOX. HMeIM: 2-methylimidazole.
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Fig. 1 TEM images of (A) PAAS NPs, (B) PAA-Zn NPs and (C) PAA@ZIF-8 NPs, respectively. SEM images of (D) PAA-Zn NPs and (E) PAA@ZIF-8 NPs. (F) FT-IR spectra of (a) PAAS NPs, (b) PAA-Zn NPs and (c) PAA@ZIF-8 NPs, respectively.
between –COO groups provided by PAAS NPs and Zn2+. However, not all –COO groups combine with Zn2+ ions, although the content of Zn2+ ions exceeds –COO groups. The percentage of Zn2+ in PAA-Zn NPs is 20.5 wt% as confirmed by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Assuming that all Zn2+ ions in PAA-Zn combined with –COO groups, 60 mol% –COO groups in samples combined with Zn2+ ions.6 As is well known, Zn2+ ions will form [Zn(H2O)6]2+ complexes and then change to [Zn(OH)(H2O)5]+ ions as in the following equation, when Zn(NO3)2 is added into PAAS NPs solution. [Zn(H2O)6]2+ " [Zn(OH)(H2O)5]+ + H+ The generated H+ ions react with the –COO groups and form –COOH, leading to the presence of PAA.6 Energy dispersive X-ray spectroscopy (EDS) and Fourier transform infrared measurements (FT-IR) were also performed to prove the fabrication of PAA-Zn. Fig. 1F(a) and (b) show the FT-IR spectra of PAAS and PAA-Zn, respectively, where the carbonyl asymmetric stretching band (CQO) of –COOH can be found at 1710 cm 1. The peaks around 1550 cm 1 are assigned to the shift of the CQO bands caused by coordination of –COO groups and metal cations. Furthermore, the Zn element can be obviously found from the EDS (Fig. S1, ESI†). Finally, the as-obtained PAA-Zn NPs were used as templates and Zn sources to form PAA@ZIF-8 NPs (Fig. 1C). The TEM and SEM images show an average diameter of 128 nm for PAA@ZIF-8 NPs, compared to that of around 100 nm for PAA-Zn NPs (Fig. 1D and E). The hydrodynamic diameters of the PAAS, PAA-Zn and PAA@ZIF-8 NPs were determined by dynamic light scattering (DLS) to be 111.7, 109.4 and 138.6 nm, respectively (Fig. S2, ESI†). In addition, no obvious aggregation and changes in size and morphology were observed when the PAA@ZIF-8 NPs were dispersed into serum and water, respectively, (Fig. S3, ESI†), reflecting
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their good stability. To prove the successful growth of ZIF-8, FT-IR spectroscopy and X-ray diffraction (XRD) (Fig. S4, ESI†) were performed. Several bands are observed for ZIF-8 in the FT-IR spectrum. As shown in Fig. 1F(c), the absorption band at 422 cm 1 is observed for Zn-N stretching mode and the absorption bands in the 1100–1400 cm 1 region are associated with C–N stretch.7 Note that in the XRD pattern (Fig. S4(a), ESI†) the diffraction peaks at ca. 7.3 and 12.71 confirm the presence of ZIF-8.4b However, it doesn’t thoroughly crystallize. Control experiments at 70 1C for 0.5, 12, 24 and 36 h were carried out to investigate the effect of the reaction time on the crystallization of ZIF-8. Along with the reaction time, better crystalline ZIF-8 structures could be obtained (Fig. S4, ESI†). However, it should be pointed out that aggregation of PAA@ZIF-8 NPs occurred to different degrees with the reaction time prolonged (Fig. S5, ESI†). Based on these results, the dispersed PAA@ZIF-8 NPs synthesized at 70 1C for 0.5 h and the dispersed PAA@ZIF-8 NPs synthesized at 70 1C for 0.5 h were considered as the ideal structures as drug delivery carriers. To further demonstrate the synthesis of PAA@ZIF-8 NPs, thermogravimetric (TG) analysis of PAA@ZIF-8 NPs was done and showed three weight loss events (%) from room temperature to 616 1C (Fig. S6, ESI†). The first event is typical of the loss of hydration water molecules from room temperature to 100 1C. The PAA partly displays weight loss at a temperature range of 200–390 1C. The third event corresponding to the decomposition of PAA and ZIF-8 yields 21% residual zinc oxide.3 To evaluate the loading and controlled release behaviour of the PAA@ZIF-8 NPs, DOX, an anti-cancer drug, has been selected as a model drug. The high loading efficiency of encapsulation of DOX into PAA@ZIF-8 carriers could reach 95%, and the extremely high drug loading content was 190 mg of DOX per 100 mg of the PAA@ZIF-8 support, which were determined by the characteristic DOX absorption peak at B480 nm (Fig. 2A).8 The high drug loading capacity for DOX of NPs may be attributed to two types of forces: (1) the electrostatic interaction between the negatively charged carboxylic acid groups and positively charged DOX; (2) the coordination bonding of Zn(II)-DOX.9 The in vitro release rate of DOX from the DOX-loaded PAA@ZIF-8 could be controlled by changing pH values at 37 1C. In neutral PBS (pH = 7.4), DOX was released in a very slow fashion and the cumulative release of DOX was only about 35.6% even after 60 h at pH = 7.4. In contrast, it
Fig. 2 (A) UV-Vis absorption spectra and photographs (inset) of DOX solutions before (a) and after (b) interaction with PAA@ZIF-8 NPs. (B) DOX-release profiles for DOX-loaded PAA@ZIF-8 NPs measured at pH 5.5 and 7.4 at 37 1C. (C) In vitro cytoxicity of empty PAA@ZIF-8 NPs, DOX-loaded PAA@ZIF-8 NPs and free DOX against MCF-7 cells at different concentrations after 24 h.
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Fig. 3 CLSM images of MCF-7 cells incubated with DOX-loaded PAA@ZIF-8 ([DOX] = 20 mg mL 1) for 3 h (A–C), 12 h (D–F) and 24 h (G–I) at 37 1C, respectively. Each series can be classified to cell nucleus (being dyed in blue by Hoechst 33342), DOX-loaded PAA@ZIF-8 NPs and the merged images of both above, respectively. All scale bars are 10 mm.
can be clearly seen that the system shows a faster drug release rate, 75.9% of DOX was released after 60 h at pH = 5.5 (Fig. 2B). Thus, the pH-dependent drug release PAA@ZIF-8 system is promising for use as a drug carrier and is beneficial for targeting cancerous tissues,10 since the extracellular pH of tumors is lower than that of normal tissues and the microenvironments of tumors are acidic.4 Furthermore, it is very important to evaluate the biocompatibility of the PAA@ZIF-8 NPs for their potential bio-applications. Hence, the standard 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) cell assay was performed on MCF-7 cells to characterize the cytotoxicity of the PAA@ZIF-8 NPs without DOX, DOX-loaded PAA@ZIF-8 NPs and free DOX. These data shown in Fig. 2C reveal that the cytotoxic efficacy of the DOX-loaded PAA@ZIF-8 NPs was similar to that of the free DOX and enhanced with an increase of DOX released from the DOX-loaded PAA@ZIF-8 NPs within the MCF-7 cells. In addition, more than 90.8% cell viability is observed in the varying concentration ranges, even at a high concentration of up to 50 mg mL 1 of PAA@ZIF-8 NPs after incubation for 24 h. The above MTT assay results confirm that the PAA@ZIF-8 NPs are nearly nontoxic and can be applied in the biomedical field with good biocompatibility. Confocal laser scanning microscopy (CLSM) analyses were used to evaluate the intracellular release behaviour of DOX-loaded PAA@ZIF-8 NPs and further confirm whether the pH-responsive function is maintained intracellularly11 (Fig. 3). In the first 3 h, only a few of the DOX-loaded PAA@ZIF-8 NPs could be uptaken by MCF-7 cells via the endocytosis process and localized in the cytoplasm12 as shown in Fig. 3A–C. Increasing amounts of DOX released and delivered by DOX-loaded PAA@ZIF-8 NPs passed through the nucleus membrane and eventually assembled in the nucleus to kill cells12 at 12 and 24 h. The cells incubated with DOX-loaded PAA@ZIF-8 NPs for 24 h (Fig. 3G–I) show stronger red luminescence signals, suggesting that more drug carrier systems were uptaken specifically by MCF-7 cells than at 12 h (Fig. 3D–F). The above results demonstrate that PAA@ZIF-8 NPs are promising
1002 | Chem. Commun., 2014, 50, 1000--1002
Communication
for use as drug delivery candidates and enhance the therapeutic efficacy. In summary, a synthetic strategy for the designed preparation of PAA@ZIF-8 NPs for the first time by using PAA-Zn NPs as templates has been reported. The resulting PAA@ZIF-8 NPs possessed an ultrahigh loading capacity of DOX (1.9 g DOX g 1 NPs) and the pH-sensitive drug release property. The DOX-loaded PAA@ZIF-8 NPs could be taken up by MCF-7 cells through endocytosis and release DOX much faster in mild acidic buffer solution (pH = 5.5) than at a neutral pH of 7.4 in the cytoplasm by CLSM. The MTT assay results indicated that the PAA@ZIF-8 NPs were nearly non-toxic to live cells. This pH-responsive release property may contribute to the selective release of an encapsulated drug in acidic tumor tissues or cell interiors. Moreover, the general and convenient synthetic method can be extended to synthesize other functional nanomaterials for the various potential biological applications such as biological imaging probes, drug delivery systems and therapeutic applications. This work was supported by the National Natural Science Foundation of China (Grant No. 21173038 and 21301027), Natural Science Foundation and Science and Technology Development Planning of Jilin Province (201215003, 201201115 and 20130522136JH), Science Foundation for Young Teachers of Changchun University of Science and Technology (XQNJJ-2011-11), Program for New Century Excellent Talents in University (NCET-13-0720) and Fundamental Research Funds for the Central Universities (12SSXM005).
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