Gold-Nanoparticle-Stabilized Pluronic Micelles Exhibiting Glutathione

Oct 13, 2010 - Jian-Ping Xu, Xi Yang, Li-Ping Lv, Yu Wei, Fang-Min Xu, and Jian Ji* ... Xiaoxiao Yao , Yuan Yao , Minhua Zong , Xuehui Li , Wenyong Lo...
4 downloads 0 Views 2MB Size
Download PDF
pubs.acs.org/Langmuir © 2010 American Chemical Society

Gold-Nanoparticle-Stabilized Pluronic Micelles Exhibiting Glutathione Triggered Morphology Evolution Properties Jian-Ping Xu, Xi Yang, Li-Ping Lv, Yu Wei, Fang-Min Xu, and Jian Ji* MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China Received July 22, 2010. Revised Manuscript Received September 25, 2010 Nanocomposites constructed from metallic nanoparticles and amphiphilic copolymers have attracted substantial interest for various potential applications. Here we report on the nanocomposites prepared through cross-linking pluronic micelles with gold nanoparticles. The covalent binding of gold nanoparticles onto the micelles and the thermoresponsibility of the system was followed via ultraviolet-visible spectroscopy, dynamic light scattering, transmission electron microscopy, and fluorescence spectroscopy. The gold-nanoparticle-stabilized pluronic micelles can take thiol-exchange reaction with glutathione and their morphology spontaneously evolved and reassembled into large “vesicular”-like nanocapsules. Obvious temperature responsibility was followed in the gold-nanoparticlestabilized pluronic micelles system and also the glutathione triggered nanocapsules systems. It is believed that the high stability and glutathione responsibility of the Au-NPs shell-cross-linked micelles allowed for high potential in drug delivery and biosensors.

Introduction The controlled self-assembling of block copolymers at nanometer and micrometer scale has attracted substantial interest for various potential applications, including drug and gene delivery, catalysis, and biosensors.1-11 Rationally designed molecular building blocks allow for the precise control of morphology of the supramolecular aggregate, and various defined structures, including spherical micelles, rodlike micelles, or vesicles can be obtained.12-19 Furthermore, copolymers with stimulus responsibility have attracted growing attention due to their diverse self-assembly *Corresponding author. Tel: (þ86)-571-87953729. Fax: (þ86)-57187953729. E-mail: [email protected]. (1) Cunliffe, D.; Alarcon, C. D.; Peters, V.; Smith, J. R.; Alexander, C. Langmuir 2003, 19, 2888. (2) Hoffmann, J.; Plotner, M.; Kuckling, D.; Fischer, W. J. Sens. Actuators, A 1999, 77, 139. (3) Ista, L. K.; Lopez, G. P. J. Ind. Microbiol. Biotechnol. 1998, 20, 121. (4) Kim, S. J.; Park, S. J.; Lee, S. M.; Lee, Y. M.; Kim, H. C.; Kim, S. I. J. Appl. Polym. Sci. 2003, 89, 890. (5) Liu, S. Y.; Armes, S. P. Langmuir 2003, 19, 4432. (6) Nandkumar, M. A.; Yamato, M.; Kushida, A.; Konno, C.; Hirose, M.; Kikuchi, A.; Okano, T. Biomaterials 2002, 23, 1121. (7) Cabral, H.; Kataoka, K. Sci. Technol. Adv. Mater. 2010, 11, 014109. (8) Ding, Y.; Hu, Y.; Zhang, L.; Chen, Y.; Jiang, X. Biomacromolecules 2006, 7, 1766. (9) Wen, F.; Zhang, W.; Wei, G.; Wang, Y.; Zhang, J.; Zhang, M.; Shi, L. Chem. Mater. 2008, 20, 2144–2150. (10) Zhang, M.; Kataoka, K. Nano Today 2009, 4, 508. (11) Kim, S. H.; Jeong, J. H.; Chun, K. W.; Park, T. G. Langmuir 2005, 21, 8852. (12) Zhang, L. F.; Eisenberg, A. Science 1995, 268, 1728. (13) Zhang, L. F.; Eisenberg, A. J. Am. Chem. Soc. 1996, 118, 3168. (14) Hu, Y.; Jiang, Z.; Chen, R.; Wu, W.; Jiang, X. Biomacromolecules 2010, 11, 481. (15) Zhao, L.; Ma, R.; Li, J.; Li, Y.; An, Y.; Shi, L. Biomacromolecules 2008, 9, 2601. (16) Wang, J.; Jiang, M. J. Am. Chem. Soc. 2006, 128, 3703. (17) Yu, Y.; Wu, G.; Liu, K.; Zhang, X. Langmuir 2010, 26, 9183. (18) Duan, H.; Chen, D.; Jiang, M.; Gan, W.; Li, S.; Wang, M.; Gong, J. J. Am. Chem. Soc. 2001, 123, 12097. (19) Chen, D.; Jiang, M. Acc. Chem. Res. 2005, 38, 494. (20) Giacomelli, C.; Men, L. L.; Borsali, R.; Lai-Kee-Him, J.; Brisson, A.; Armes, S. P.; Lewis, A. L. Biomacromolecules 2006, 7, 817. (21) Rijchen, C. J.; Snel, C. J.; Schiffelers, R. M.; Nostrum, C. F.; Hennink, W. E. Biomaterials 2007, 28, 5581. (22) Hassan, P. A.; Raghavan, S. R.; Kaler, E. W. Langmuir 2002, 18, 2543.

Langmuir 2010, 26(22), 16841–16847

behavior in response to stimuli, such as pH, temperature, salt, etc.20-24 The stimulus responsibility allows for tailoring the supramolecular aggregates with desirable morphologies and properties from the same precursory copolymers. Recently, specific chemical reactions, such as the disulfide reduction, have emerged as an alternative stimulus for tuning the self-assembling of copolymers. For example, monodispersed polymeric nanocapsules have spontaneously evolved from reducible hetero-PEG PIC micelle disulfide bond reduction.25 In this paper, a novel glutathione-mediated morphology evolution strategy will be introduced to prepare uniform “vesicular”-like nanocapsules. As a promising nanobiomaterial, gold nanoparticles have found a burst of research interest in biological applications, for example, transfection vectors, DNA-binding agents, protein inhibitors, and spectroscopic markers.26-37 Among the unique features of gold particles, facile place-exchanging reaction makes them well suited for biomedical applications.38 As the most abundant thiol species in the cytoplasm and the major reducing agent in biochemical processes, glutathione (GSH) showed special (23) Wang, Y.; Han, P.; Xu, H.; Wang, Z.; Zhang, X.; V.; Kabanov, A. Langmuir 2010, 26(2), 709. (24) Wang, B.; Ma, R.; Liu, G.; Li, Y.; Liu, X.; An, Y.; Shi, L. Langmuir 2009, 25 (21), 12522–12528. (25) Dong, W. F.; Kishimura, A.; Anraku, Y.; Chuanoi, S.; Kataoka, K. J. Am. Chem. Soc. 2009, 131, 3804. (26) Sandhu, K. K.; McIntosh, C. M.; Simard, J. M.; Smith, S. W.; Rotello, V. M. Bioconjugate Chem. 2002, 13, 3. (27) Rosi, N. L.; Mirkin, C. A. Chem. Rev. 2005, 105, 1547. (28) Cao, Y. W.; Jin, R. C.; Mirkin, C. A. Science 2002, 297, 1536. (29) Storhoff, J. J.; Elghanian, R.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1998, 120, 1959. (30) Dubertret, B.; Calame, M.; Libchaber, A. J. Nat. Biotechnol. 2001, 19, 365. (31) Galow, T. H.; Boal, A. K.; Rotello, V. M. Adv. Mater. 2000, 12, 576. (32) Levy, R.; Thanh, N. T. K.; Doty, R. C.; Hussain, I.; Nichols, R. J.; Schiffrin, D. J.; Brust, M.; Fernig, D. G. J. Am. Chem. Soc. 2004, 126, 10076. (33) Faulk, W. P.; Taylor, G. M. Immunochemistry 1971, 8, 1081. (34) Alivisatos, A. P. Nat. Biotechnol. 2004, 22, 47. (35) Katz, E.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 6042. (36) Rashid, M. H.; Bhattacharjee, R. R.; Kotal, A.; Mandal, T. K. Langmuir 2006, 22, 7141. (37) Guo, R.; Zhang, L.; Zhu, Z.; Jiang, X. Langmuir 2008, 24, 3459. (38) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293.

Published on Web 10/13/2010

DOI: 10.1021/la102929k

16841

Article

Xu et al.

interest for the manipulation of place-exchanging reaction.39,40 The glutathione-mediated selective intracellular release system has been developed on the basis of the fact that the intracellular GSH concentration is substantially higher than extracellular levels.40 Vast studies have concerned aqueous solutions of the amphiphilic “pluronics” type of triblock copolymers, PEO-PPO-PEO, for pharmaceutical use and drug delivery systems.41 It is well established that the pluronic copolymers with higher PEO/PPO ratio self-assembly into micelles composed of a PPO-rich core and a PEO-rich corona above the critical micellar concentration (CMC) and a critical micellar temperature (CMT).42-44 To improve the stability of polymeric micelles against dilution in the bloodstream, various methods, by chemically cross-linking either in the inner core domain or within the outer shell layer, have been developed.45-49 For example, Park et al. recently reported on the shell cross-linked pluronic micelles using gold nanoparticles that exhibit a reversibly thermosensitive swelling/shrinking behavior.42 The study on the evolution and morphology transition of the shell cross-linked micelles is, however, scarce. We demonstrate herein that the gold-nanoparticle-stabilized pluronic micelles spontaneous evolve and reassemble into large “vesicular”-like nanocapsules using glutathione as an effective trigger. We demonstrate also that the evolved nanocapsules showed obvious temperature responsibility.

Experimental Section Materials. Chemicals and solvents were reagent grade and all were commercially available. Pluronic F127 ((PEO)100(PPO)65(PEO)100) was purchased from Aldrich. HAuCl4 3 3H2O powders were purchased from J&K Chemicals. Trisodium citrate, sodium borohydride (NaBH4), and glutathione (GSH) were supplied from Sinopharm Chemical Reagent Co., Ltd.. Synthesis of Thiolated Pluronic F127. Thionyl chloride (3.0 mL) was dissolved in 30 mL of anhydrous THF. This solution was slowly added into 20 mL of anhydrous THF solution containing 1.06 g of 3,30 -dithiodipropionic acid under nitrogen atmosphere. The mixture was stirred at 65 °C for 24 h, and the excess thionyl chloride was removed by reduced pressure distillation. The product and trimethylamine (1.3 mL) were sealed in a flask. To this solution was slowly added 3.6 g of F127 powder dissolved in THF, and the mixture was stirred for 1 h. deionized water (0.18 mL) was added, and 4 h of stirring was followed. The reaction mixture was then filtered, and the filtrate was concentrated in vacuo by a rotary evaporator at room temperature. Excess ether was added to precipitate the product. The product (1 g) was dissolved in ethanol (8 mL), and 1,4-dithio-D,L-threitol (DTT, excess) was added. The pH of the solution was adjusted to 9 with concentrated NH3 3 H2O. The solution was stirred for 1 h and purified in vacuo by a rotary evaporator at room temperature. The product was precipitated by adding excess ether. The final product was dried in vacuo. (39) Meister, A.; Anderson, M. E. Annu. Rev. Biochem. 1983, 52, 711. (40) Hong, R.; Han, G.; Fernandez, J. M.; Kim, B. J.; Forbes, N. S.; Rotello, V. M. J. Am. Chem. Soc. 2006, 128, 1078. (41) Bae, K. H.; Lee, Y.; Park, T. G. Biomacromolecules 2007, 8, 650. (42) Bae, K. H.; Choi, S. H.; Park, S. Y.; Lee, Y.; Park, T. G. Langmuir 2006, 22, 6380. (43) Linse, P.; Malmsten, M. Macromolecules 1992, 25, 5434. (44) Mortensen, K.; Pedersen, J. S. Macromolecules 1993, 26, 805. (45) Read, E. S.; Armes, S. P. Chem. Commun. 2007, 3021. (46) Bronich, T. K.; Keifer, P. A.; Shlyakhtenko, L. S.; Kabanov, A. V. J. Am. Chem. Soc. 2005, 127, 8236. (47) Zhang, J. Y.; Jiang, X. Z.; Zhang, Y. F.; Li, Y. T.; Liu, S. Y. Macromolecules 2007, 40, 9125. (48) Sanji, T.; nakatsuka, Y.; Ohnishi, S.; Sakurai, H. Macromolecules 2000, 33, 8524. (49) Xu, H. X.; Xu, J.; Jiang, X. Z.; Zhu, Z. Y.; Rao, J. Y.; Yin, J.; Wu, T.; Liu, H. W.; Liu, S. Y. Chem. Mater. 2007, 19, 2489.

16842 DOI: 10.1021/la102929k

Synthesis of 3.5 nm Gold Nanoparticles. The gold nanoparticles with a diameter of 3.5 nm were synthesized according to Murphy’s report.50 Typically, an aqueous of HAuCl4 (2.06 mL 10 mg/mL) was mixed with the aqueous of trisodium citrate (200 mL, 0.25 mM); to this solution was rapidly added 6 mL ice cold NaBH4 (100 mM) with vigorous stirring. The color of the solution turned from pale yellow into burgundy, indicating the formation of gold nanoparticles. Stirring was continued for 1 h and then stored in the dark for use. Synthesis of Gold Nanoparticle Shell Cross-Linking Pluronic Micelles. The pluronic micelles were shell cross-linked by gold nanoparticles wrapped on their surfaces. The procedure of cross-linking was similar to the Park report except for some modifications.42 Typically, in a 50 mL round-bottom flask purged with nitrogen, 10 mg of thiolated pluronic F127 was dissolved in 24 mL of triply distilled water. The solution was kept at 37 °C for 30 min to form the pluronic micelles. Then, in the protection of nitrogen, the aqueous gold nanoparticles (16 mL) were added dropwise into the micelle solution with vigorous stirring. The stirring was maintained overnight at 37 °C.

Glutathione-Mediated Micelle to Nanocapsule Transition. In a 10 mL flask, 1 mL of freshly made GSH solution (1 mg/mL) was added into 3 mL of shell cross-linked goldpluronic micelles and purged with nitrogen for 5 min. The flask was sealed and incubated in a 37 °C water bath for 4 h.

Instrumentation and Measurements. UV-Vis Spectra Studies. UV-visible spectra were carried out with a UV-vis Shimadzu UV-2505 spectrometer using 1 cm path length quartz cuvettes. Spectra were collected within a range of 400-800 nm. Photoluminescence Spectra Studies. Photoluminescence spectra were recorded on a LS55 luminescence spectrometer with a voltage of 775 mV and a slit of 2.5 nm. TEM Measurements. Transmission electron microscopy (TEM) analysis were conducted on a JEM-1200EX TEM operating at 200 kV in bright field mode. For AuNPs, the samples were prepared by placing a drop of the colloidal solutions on a 400 mesh carbon-coated copper grid and air-drying the grid at 25 °C. Micelle (before and after shell cross-linking) and vesicle samples were prepared in the same way except that all samples were prepared at 15 and 37 °C, respectively. Micelles before shell crosslinking negatively were stained with aqueous phosphotungstic acid solution 2% (by w/v) before observe. Dynamic Light Scattering Measurements. The hydrodynamic diameter of micelles (before and after shell cross-linking) and nanocapsules were evaluated on a Brookhaven 90 Plus size analyzer at the 90° scattering angle and the wavelength was 658 nm. The hydrodynamic diameters of all samples were recorded at 15 and 37 °C, respectively.

Results and Discussion The pluronic F127-SH polymer used in this work possessed thiol groups at both ends, i.e., HS-PEO-PPO-PEO-SH, where PEO stands for poly(ethylene oxide) (Mn = 4400), PPO for poly(propylene oxide) (Mn = 3770), and SH for the thiol end group. F127-SH was prepared via reaction between F127 and an excess amount of 3,30 -dithiodipropionyl chloride, followed by disulfide reduction by DTT (Supporting Information, Figure S1). Ellman’s assay and H NMR experiments indicated that all the terminal OH end groups of F127 have been conjugated with free thiol groups. Figure S2 (Supporting Information) showed the 1H NMR spectroscopy of F127 and F127-SH in CDCl3, respectively. New peaks at δ 1.5 characteristic of thiol protons and δ 2.7 characteristic of methylene protons next to the thiol end group (-CH2-SH) were detected, which demonstrated that thiolated pluronic F127 was successful synthesized. The micelles with a (50) Gole, A.; Murphy, C. J. Chem. Mater. 2004, 16, 3633.

Langmuir 2010, 26(22), 16841–16847

Xu et al.

Article

Figure 1. TEM micrographs of citrate stabilized gold nanoparticles with sizes of ∼3.5 nm (A), F127-SH micelles with sizes of ∼45 nm (B), and gold-nanoparticle-decorated monodispersed F127 micelles with sizes of ∼80 nm (C).

Figure 2. UV-vis absorption spectra in the absence (a) and presence (b) of cysteamine (1 mM): (A) gold nanoparticles; (C) gold nanoparticles stabilized pluronic micelles. (B) Samples following the same cross-linking process in (C) except that the pluronic micelles possess no SH in their outer shells. All the data were collected at room temperature. Scheme 1. Procedure of Pluronic Micelle Shells Cross-Linked by Gold Nanoparticles

thiol-functionalized outer shell layer were formed by dissolving F127-SH copolymer in water at 37 °C under argon. Different from the shell cross-linking procedure in Park’s report,42 the shell cross-linking process presented here was performed by incubating the F127-SH micelles with citrate stabilized gold nanoparticles, which feature the size of ∼3.5 nm (Figure 1A) and were synthesized by reducing HAuCl4 with NaBH4 in water using citrate as stabilizer, Scheme 1. In 37 °C, which is higher than the CMT of F127, micelles with a thiol-functionalized outer shell layer were formed. As surface-exposed thiol groups around the F127-SH micelles can form covalent bonding with gold nanoparticles, the thiol-functionalized pluronic copolymers were cross-linked in the outer shell layer by bridging between neighboring gold nanoparticles.42,49 The presence of gold nanoparticles on the micelles was observed via TEM. While the F127-SH micelles showed sizes of ∼45 nm (Figure 1B), the binding of gold nanoparticles onto the F127-SH micelles produced gold-nanoparticle-decorated monodispersed micelles with sizes of ∼80 nm, Figure 1C. Gold nanoparticles with sizes of ∼3.5 nm could be clearly found on the micelles, Figure 1C, inset. Langmuir 2010, 26(22), 16841–16847

The binding of gold nanoparticles onto the thiol-functionalized F127-SH micelles can also be followed taking advantage of the characteristic plasmon resonance at 508 nm of gold nanoparticles. The as-prepared citrate stabilized gold nanoparticles precipitated quickly in the presence of cysteamine, which can be attributed to the strong binding of gold nanoparticles with both amine and thiol groups on the cysteamine, Figure 2A. As is known, a covalent bond can be formed between Au-S or Au-N. F127 micelles without thiol functional groups cannot form covalent binding with Au NPs, and the micelles could not be shell crosslinked. Instead, only weak physical interaction between Au NPs and F127 micelles existed, which is instable. Consequently, F127 micelles without thiol functional groups did not show an obvious stabilizing effect for the gold nanoparticles since the gold nanoparticles in the mixture aggregated upon the addition of the cysteamine, Figure 2B. The gold nanoparticles on the F127-SH micelles showed improved stability, and no precipitation was observed after incubation with 1 mM cysteamine for several days, Figure 2C. Also, the maximum amount of gold nanoparticles that can bind onto the surface of the F127-SH micelles can easily be DOI: 10.1021/la102929k

16843

Article

Xu et al.

Figure 3. UV-vis absorption spectra of gold-nanoparticle-stabilized pluronic micelles in the absence (a) and presence (b) of cysteamine

(1 mM). The molar ratio between the gold elements to the thiol groups was 2:1 (A), 5:1 (B), 10:1 (C), and 15:1 (D), respectively. (E) ΔA/A at different molar ratios of gold elements to the thiol groups. A is the absorbance of (a); ΔA is the decrease of absorbance from (a) to (b). All the data were collected at room temperature.

Figure 4. (A) Absorption spectrum of the gold-nanoparticle-cross-linked F127-SH micelles solution (a) and F127-SH micelles solution (b). (B) Maximum wavelength of the surface plasmon peak of gold-nanoparticle-cross-linked F127-SH micelles as a function of solution temperature. (C) Reversible surface plasmon peak position of gold-nanoparticle-cross-linked F127-SH micelles when the temperature was cycled between 12 and 60 °C. 16844 DOI: 10.1021/la102929k

Langmuir 2010, 26(22), 16841–16847

Xu et al.

followed spectroscopically using cysteamine as precipitator. For this purpose, we simply change the molar ratio between the gold elements to the thiol groups. The quantity of gold element was calculated from HAuCl4 used to prepare Au NPs, while the quantity of thiol groups can be easily evaluated from the copolymers used. When the molar ratio between the gold elements to the thiol groups was less than 5:1, all the gold nanoparticles could bind onto the surface of the F127-SH micelles and no obvious change in the absorbance spectrum of the system was observed, Figure 3. With the increase of the gold nanoparticles, the free gold nanoparticles in solution increased. These unbounded gold nanoparticles aggregated in the presence of cysteamine, and the absorbance of the system decreased consequently, Figure 3. Furthermore, the temperature-induced collapse process of the cross-linked assembly structure was proved by checking the distance dependence of plasmon resonance of the gold nanoparticles, Figure 4. The absorption spectrum of the gold-nanoparticle-cross-linked F127-SH micelles solution exhibited a maximum at 508 nm, Figure 4A, which is characteristic of gold nanoparticles. When the solution temperature rose from 15 to 60 °C, the size of the micelles shrinked gradually. The shrinkage of micelles will Scheme 2. Synthetic Scheme of Nanocapsules from GoldNanoparticle-Stabilized Pluronic Micellesa

a Both the gold-nanoparticle-stabilized pluronic micelles and nanocapsules exhibit a thermosensitive swelling/shrinking behavior.

Article

concomitantly decrease the average distance between gold nanoparticles and the induced the red shift of surface plasmon peak, Figure 4B. It should be noted that the surface plasmon band can be shifted back to the original position upon the temperature decreases from 60 to 15 °C and this process can be reliably repeated, Figure 4C. The success in cross-linking of F127-SH micelles by gold nanoparticles was followed by using pyrene as fluorescence probe for micropolarity, which enables the characterization of assembly/deassembly process of the polymeric micelles.51 For the noncross-linked F127-SH micelles solution, the intensity ratio I339/ I334 from pyrene excitation spectra dramatically decreased between the F127-SH concentrations 1.0 and 0.001 mg/mL at 37 °C, Figure S3(A) (Supporting Information). This indicated that the non-cross-linked F127-SH micelles structure deassembled and the critical micellar concentration was calculated to be 2.32  10-3 mg/mL. After the F127-SH micelles with gold nanoparticles were cross-linked, no obvious decrease in the intensity ratio of I339/I334 was observed until the concentration reached 1.0  10-7 mg/mL, Figure S3(B) (Supporting Information). This can be ascribed to the improved stability against dilution due to gold nanoparticle cross-linking of the F127-SH micelles. As the most abundant thiol species in the cytoplasm and the major reducing agent in biochemical processes, GSH has been proved to be an efficient in situ releasing triggering signal in nanoparticle-based drug delivery systems.52,53 It has been demonstrated that GSH could perform place exchange reactions of thiols on gold nanoparticle surfaces.40 We are interested to explore whether the GSH could be used as a triggering signal to deassemble the gold-nanoparticle-cross-linked F127-SH micelles. Since both F127-SH micelles and GSH own thiol groups, we keep GSH excess for the successful place exchange.40 However, it was intriguing to find that morphology of the micelles evolved during the GSH triggered deassembly process, Scheme 2. When triggered by GSH, the gold-nanoparticle-stabilized pluronic micelles took the thiol-exchange reaction with glutathione and their morphology spontaneously evolved and reassembled into large “vesicular”-like nanocapsules. In addition, both pluronic micelles and “vesicular”-like nanocapsules exhibited a thermosensitive assembly/ deassembly behavior. Dynamic light scattering (DLS) at 15 °C of an aqueous solution (1 mg/mL) of cross-linked F127-SH micelles showed an intensity-average hydrodynamic radius (Rh) of 320 nm. When the GSH (1 mM) was added as a triggering signal, the Rh increased gradually and reached 385 nm in 3.5 h, Figure 5C, curve a. It should be noted that the F127 micelles did

Figure 5. TEM micrographs of gold-nanoparticle-stabilized pluronic micelles (A) and vesicular assemblies evolved from gold-nanoparticlestabilized pluronic micelles (B). (C) Time-resolved intensity-average hydrodynamic radius (Rh) of gold-nanoparticle-cross-linked micelles (a) and F127 micelles (b), triggered by the GSH. Langmuir 2010, 26(22), 16841–16847

DOI: 10.1021/la102929k

16845

Article

Figure 6. Reversible swelling/shrinking behavior of F127 micelles (A), gold-nanoparticle-stabilized pluronic micelles (B), and nanocapsules formed with GSH as triggering signal (C) when the temperature was cycled between 15 and 37 °C.

not show obvious change in Rh under the same conditions, Figure 5C, curve b. The evolution of the gold-nanoparticlecross-linked micelles triggered by the GSH was further explored in detail by TEM visualization. TEM micrographs of goldnanoparticle-cross-linked micelles prepared from aqueous solutions at room temperature and stained with aqueous phosphotungstic acid solution show spherical micelles with a mean radius of ∼80 nm, Figure 5A. The evolved aggregates were visualized by adding a drop of this solution to a TEM grid at room temperature and allowing it to dry for a few minutes before staining. TEM micrographs of the resulting samples confirm the formation of (51) Thurmond, K. B.; Kowalewski, T.; Wooley, K. L. J. Am. Chem. Soc. 1996, 118, 7239. (52) Verma, A.; Simard, J. M.; Worrall, J. W. E.; Rotello, V. M. J. Am. Chem. Soc. 2004, 126, 13987. (53) Han, G.; Chari, N. S.; Verma, A.; Hong, R.; Martin, C. T.; Rotello, V. M. Bioconjugate Chem. 2005, 16, 1356.

16846 DOI: 10.1021/la102929k

Xu et al.

large “vesicular”-like assemblies with a mean radius of ∼180 nm, Figure 5B. The formation of the “vesicular”-like nanocapsule structure is not surprising if the free energy of the assembly is taken into consideration. During the place-exchange reactions of thiols on gold nanoparticle surfaces, the cross-linking points of the micelles decreased and the free energy of the micelles corona decreased. The decreased free energy of micelles corona might induce the transformation from micelle to “vesicular”-like nanocapsule structures.25 During the evolution, most of Au NPs were removed from the micelles after GSH treatment, leaving only a small number of Au NPs on the nanocapsule surface. The free Au NPs aggregated in the solution due to the lack of effective protecting ligands, which was proved by the red-shifted characteristic surface plasmon resonance (SPR) peak of Au NPs (from 508 to 540 nm), Figure S4 in the Supporting Information. It has been well documented that pluronic micelles exhibited a thermosensitive assembly/deassembly behavior.42,54 DLS experiments indicated that F127 micelles had a number-averaged hydrodynamic diameter of 38 nm at 37 °C. This micelle structure deassembled, and no signal could be measured when the temperature was decreased to 15 °C, Figure 6A. This temperature sensitive assembly/deassembly behavior can be ascribed to the thermally triggered hydrophobic interactions between the PPO segments in the core structure. After the F127-SH micelles were cross-linked with gold nanoparticles, significant volume transition between 15 and 37 °C was found in DLS examine. Goldnanoparticle-cross-linked micelles had a number-averaged hydrodynamic diameter of 80 ( 15.6 nm at 15 °C but collapsed and shrunk to 48 ( 5.5 nm when the temperature was raised to 37 °C, Figure 6B. The intriguing temperature responsibility of goldnanoparticle-cross-linked micelles has been attributed to the selfassociation of pluronic copolymer chains cross-linked or grafted within the shell layer of the micelles.42,49 It was worthwhile to note that this temperature responsibility was preserved after the morphology evolution using GSH as the triggering signal, Scheme 2. The morphology of the gold-nanoparticle-cross-linked micelles revolved into nanocapsule structure and concomitantly the number-averaged hydrodynamic diameter increased to 180 ( 20.6 nm at 15 °C. Increasing the temperature to 37 °C induced the collapse of the “vesicular”-like nanocapsule structure and the number-averaged hydrodynamic diameter shrunk to 40 ( 3.5 nm, Figure 6C. Importantly, this volume transition in aqueous solution in response to temperature change was fully reversible.

Conclusions In conclusion, the present study has demonstrated the success of preparation of shell cross-linked micelles using Au-NPs as cross-linker. The Au-NP shell-cross-linked micelles show improved stability against dilution. The gold-nanoparticle-stabilized pluronic micelles spontaneous evolve and reassemble into large nanocapsules using glutathione as an effective trigger. The high stability and glutathione responsibility of the Au-NP shellcross-linked micelles allow for a wide potential of biomedical applications. Acknowledgment. This research was financially supported by Natural Science Foundation of China (NSFC-20774082, 50703036, 50830106), National Science Fund for Distinguished Young Scholars (51025312), Zhejiang Provincial Natural Science Foundation of China (Y4080024, Y4080250), Fundamental (54) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414.

Langmuir 2010, 26(22), 16841–16847

Xu et al.

Research Funds for the Central Universities (2009QNA4039, KYJD09005), and Qianjiang Excellence Project of Zhejiang Province (2009R10051). Supporting Information Available: Synthetic scheme of thiolated pluronic F127, 1H NMR spectra of pluronic F127

Langmuir 2010, 26(22), 16841–16847

Article

and thiolated pluronic F127, I339/I334 intensity ratio from pyrene excitation spectra for noncross-linked F127-SH micelles and cross-linked F127-SH micelles solution, and UV-vis spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la102929k

16847