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Chemoresponsive Colloidosomes via Ag+ Soldering of Surface-Assembled Nanoparticle Monolayers Miao Liu, Qian Tian, Yulin Li, Bo You, An Xu, and Zhaoxiang Deng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b00298 • Publication Date (Web): 13 Apr 2015 Downloaded from http://pubs.acs.org on April 15, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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Chemoresponsive Colloidosomes via Ag+ Soldering of Surface-Assembled Nanoparticle Monolayers Miao Liu,† Qian Tian,† Yulin Li,† Bo You,† An Xu,‡ and Zhaoxiang Deng*,† †
CAS Key Laboratory of Soft Matter Chemistry, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China
‡
Key Laboratory of Ion Beam Bioengineering, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China
ABSTRACT: Colloidosomes with a hollow interior and a porous plasmonic shell are highly desired for many applications including nanoreactors, surface-enhanced Raman scattering (SERS), photothermal therapy, and controlled drug release. We herein report a silica nanosphere templated electrostatic self-assembly in conjunction with a newly developed Ag+ soldering to fabricate gold colloidosomes towards multi-functionality and stimuli-responsibility. The gold colloidosomes are capable of capturing a nanosized object, and releasing it via structural dissociation upon responding a biochemical input (GSH, glutathione) at a concentration close to its cellular level. In addition, the colloidosomes have a tunable nanoporous shell composed of strongly coupled gold nanoparticles which exhibit broadened near infra-red plasmon resonance. These features along with the simplicity and high tunability of the fabrication process make the gold colloidosomes quite promising for applications in a chemical or cellular environment.
INTRODUCTION
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Colloidosome, with the name coined in 2002, represents an emerging class of micro- or nanostructures with a hollow interior and an inherent surface porosity coming from the close packing of colloidal particles.1 These unique vesicular structures exhibit controllable permeability and mechanical stability, and provide an efficient encapsulation of various guest objects (including even live cells) of nanometer to micrometer sizes. The past years have witnessed various innovative applications of this novel structure focused on drug delivery and controlled release.2-4 Classical ways for colloidosome fabrication are based on Pickering-stabilized emulsion where emulsion-stabilizing latex particles spontaneously adsorb at an immiscible liquid-liquid interface.1,5-8 Another route for colloidosome fabrication is based on a temperature-sensitive microgel template.9 Negatively charged PS particles adsorb on the positively charged microgel particles via electrostatic attractions and form uniform vesicular shells after a thermal treatment. Gold nanoparticles (AuNPs) are another important class of materials for the construction of hollow permeable nanovesicles with very interesting plasmonic and catalytic properties.10-11 For example, a quick one-pot strategy has been developed to make sub-100 nm hollow gold colloidal spheres through a chemical deposition templated by a gold-binding peptide-conjugate.12 Another method to prepare pH-responding AuNP vesicles is based on amphiphilicity-driven selfassembly of polymer-decorated AuNPs, and the as-obtained structures have showed an interesting light-triggered drug release ability.13 In addition, a linear block-copolymer has been used to modify AuNPs toward the assembly of them into nanosized colloidosomes.14 By altering the co-polymer and the size of AuNPs, gold nanovesicles with strong plasmonic coupling were realized for photoacoustic imaging and phototherapy.15 Bifunctional submicron colloidosomes co-assembled from fluorescent and paramagnetic nanoparticles have also been fabricated through an emulsion template method.16
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Apart from the above processes and benefitting from a newly developed Ag+ soldering process in our group,17 we herein report a much simpler, widely controllable, and highly efficient silicananosphere-templated electrostatic self-assembly strategy to build a special class of plasmonic colloidosomes (Figure 1). The Ag+ soldering process takes advantage of the strong Ag+ affinity of the bis(p-sulfonatophenyl)phenyl phosphine (BSPP) ligand decorated on AuNPs to create BSPP-Ag+-BSPP bridging bond between adjacent AuNPs.17 The colloidosomes feature a porous wall and exhibit a strong near infrared (NIR) absorbance due to closely coupled AuNPs in their metallic shells, which undergo a self-disruption in the presence of millimolar range of glutathione (a widespread thiol molecule in cells). These properties make them quite promising for various applications in vitro or in vivo.
Figure 1. A silica-nanosphere-templated electrostatic assembly and Ag+ soldering of a gold nanoparticle colloidosome, which disassembles upon responding glutathione (GSH) molecules. BSPP-Ag+-BSPP bonding is generated between adjacent gold nanoparticles during the soldering process, which guarantees the structural stability of the colloidosomes after template removal. APTES and BSPP represent 3-aminopropyltriethoxysilane and bis(p-sulfonatophenyl)phenyl phosphine, respectively. EXPERIMETNAL SECTION Our experiments started with the synthesis and amino-functionalization of a silica nanosphere template. In a NaAc/HAc buffer (pH 6.0), the amino groups were protonated, resulting in a positively charged silica surface. Negatively charged bis(p-sulfonatophenyl)phenyl phosphine
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(BSPP)-capped AuNPs (BSPP-AuNPs, 13.8 nm diameter, Figure S1a, b) were then electrostatically adsorbed on the silica surface to form a dense monolayer. Further treating the nanoassembly by Ag+ helped build robust interconnects among AuNPs due to BSPP-Ag+-BSPP bonding based on a recent finding in our group.17 The Ag+ treatment ensured the mechanical stability of the AuNP nanovesicles after removing the silica template by HF acid etching. The resulting nanovesicles showed an interesting responsibility towards glutathione (GSH) (a bioactive thiol molecule, with a concentration close to its intracellular level), leading to a loss of the Ag+ bonding and complete structural disassembly, with encapsulated objects (could be doped in the SiO2 spheres during their synthesis) being released. RESULTS AND DISCUSSION We first used commercial 500 nm SiO2 nanospheres to template the assembly of AuNPs. One challenge in this process was to achieve a full coverage of closely packed AuNP monolayer on the SiO2 surface, which was critical for the following Ag+ locking step. We used NaCl to elevate the ionic strength of the NaAc-HAc buffer, which reduced electrostatic repulsions between neighbouring AuNPs to achieve a densely packed AuNP monolayer.18 Silver ions were then introduced to form BSPP-Ag+-BSPP bridges between adjacent AuNPs.17
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Figure 2. TEM (a, b) and SEM (c, d) images of gold colloidosomes prepared by silica nanosphere-templated electrostatic assembly and Ag+ soldering of gold nanoparticles. Transmission electron microscopy (TEM) imaging (Figure 2a) showed that the AuNP-based colloidosomes after an HF acid etching of the silica core appeared much darker at their edge parts, indicating enclosed spherical shells. The existence of some partially ruptured structures in Figure 2a evidenced the flexibility of the vesicular shells. The spherical shape of the AuNP colloidosomes was more clearly revealed by scanning electron microscope (SEM) imaging (Figures 2c and 2d). Occasionally, some structures were disrupted during the template removal process so that their hollow interiors were exposed (Figure 2c). These results evidenced the effectiveness of our strategy, which resulted in robust and free-standing vesicular structures. High magnification TEM (Figure 2b) and SEM (Figure 2d) images revealed the mesoporosity of the vesicular shells, related to their monolayer structures containing closely packed AuNPs. By varying the diameter of the AuNPs, it would be possible to fine-tune the pore size. Benefiting from the silica nanosphere-guided self-assembly, the diameter of the colloidosomes could be facilely tuned by altering the size of the silica templates. To demonstrate this, SiO2 nanospheres with a much smaller diameter of about 160 nm were prepared by a standard Stöber’s method,19,20 which were then utilized to direct the colloidal assembly. Figure 3a showed the monodisperse SiO2 nanospheres with a diameter of 160 nm. Figures 3b and 3c were TEM images of as-fabricated colloidosomes featuring hollow interiors and porous shells. SEM image in Figure 3d evidenced the enclosed cage-like 3D structure of the colloidosomes (the surface porosity was also visible from the SEM image). The colloidosomes had a diameter of 18020 nm based on the TEM data, which was consistent with dynamic light scattering (DLS) data (Figure
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S2). The pore size was estimated to be 2-6 nm from TEM data, which was slightly larger than a geometrical consideration due to some packing disorders of the AuNP shells.
Figure 3. TEM (a-c) and SEM (d) images of as-synthesized 160 nm SiO2 nano-template (a), and the assembled AuNP colloidosomes (b-d). In addition to the size tunability, the involvement of a SiO2 nanotemplate provided us a freedom to encapsulate a guest object in the colloidosome. Considering the tunable shell porosity (depending on the size of the AuNPs), this unique rattle-type structure might be used as environment-isolated bio- or chemo-nanoreactors, and to build plasmonic and drug delivery devices. We chose AuNPs with a diameter of 28.4 nm (Figures S1c, d) as the guest objects for the encapsulation experiment. The overall process was identical to the SiO2-templated process, with Au@SiO2 yolk-shell structures (130 nm, Figure S3) as the assembly templates. The Au@SiO2 yolk-shell nanoparticles based on the hydrolysis of tetraethylorthosilicate (TEOS) in the presence of the 28.4 nm guest AuNPs were shown as an inset in Figure 4a. The correct formation of a rattle-type AuNP colloidosome encapsulating a 28.4 nm AuNP was verified by TEM imaging (Figure 4a). Because the SiO2 layers had been completely removed, the guest AuNPs (28.4 nm) were found randomly distributed inside the hollow structures composed of
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13.8 nm AuNPs, in sharp contrast to the Au@SiO2 nanoparticles where the guest AuNPs stayed at the center (Figure 4a, inset). Also, we found that most of the colloidosomes maintained their spherical shape (Figure 4b), suggesting a good mechanical robustness.
Figure 4. TEM (a) and SEM (b) images of the as-synthesized 13.8 nm AuNP-based colloidosome encapsulating a 28.4 nm AuNP object. Inset in (a) shows the Au@SiO2 yolk-shell structure as the assembly template (scale bar: 200 nm). We have to emphasize that, to obtain a stable AuNP colloidosome, the Ag+ soldering was a key step. To reveal the critical role of the Ag+ locking, HF etching was applied to the electrostatically assembled 13.8 nm AuNPs in the absence of an Ag+ treatment. A sharp color transition of the sample solution from blue to red (Figure S4) was observed upon removal of SiO2, implying a dissociation of the assembled AuNPs. TEM images revealed an efficient assembly of the AuNPs on the SiO2 and Au@SiO2 templates (Figures S5a, b). After removing the SiO2 template, discrete and scattered AuNPs were seen on the TEM grid (Figures S5c, d, in sharp contrast to Figures S5a, b). This control experiment clearly verified the important role of the Ag+ soldering process during the formation of the AuNP colloidosomes. Benefiting from the reversibility of such an Ag+ locking process,17 a glutathione (GSH)responsive structural disruption of the colloidosomes could be realized. GSH, a tri-peptide, is an antioxidant widely existing in cells, serving to prevent damages to various cellular components caused by reactive oxygen species.21 The thiol moiety in the GSH molecule could compete for
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Ag+ in the assembled structures, and thus disrupt the BSPP-Ag+-BSPP bonding which held the colloidosomes. Another possibility that could not be excluded in this experiment was that GSH could strongly interact with the gold surface via its thiol group so that the BSPP ligands responsible for the Ag+ soldering would be displaced. Both of these two processes could contribute to the GSH-triggered disassembly of the colloidosomes. Note that such a chemoresponsive structural disruption will be highly desirable if applications such as controlled drug release are considered in a cellular environment. As shown in Figure 5a, the reduced interparticle gaps in the colloidosomes led to a significant red-shift and broadening of the plasmon resonance peak (blue v.s. black curve). Upon introducing GSH, such a near infra-red (NIR) plasmon band quickly diminished and a sharp peak around 524 nm (red curve) appeared, close to that of unassembled AuNPs (black curve). This phenomenon was a visual sign of the GSH-induced colloidosome dissociation. The slightly broadened peak of the dissociated structures compared with unassembled AuNPs indicated the existence of small AuNP aggregates, consistent with TEM observation (Figure 5b). Besides the TEM and spectral evidences, DLS data showed a gradual dissociation of the colloidosomes with increased GSH concentrations (Figure S6). In addition to GSH, other thiols could also lead to a similar disassembly of the as-formed colloidosomes (Figure S7). However, chloride, ethylenediaminetetraacetic acid (EDTA) disodium salt, and phosphate did not seem to be effective to trigger the disassembly (Figures S7 and S8).
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Figure 5. (a) Optical absorbance of AuNP colloidosomes before (blue) and after (red) interacting with GSH, in comparison with unassembled AuNPs (black). Inset in (a) is a photo of intact (blue) and disassembled (red) colloidosomes. (b) TEM image of disrupted AuNP colloidosomes upon reacting with GSH (see Figure S9 for a GSH-induced release of a guest nanoparticle). OUTLOOK We have developed a very simple, highly controllable, and widely adaptable hard template (SiO2) strategy, instead of emulsion or gel templates, to fabricate monodisperse AuNP colloidosomes with a hollow or rattle-like structure. Plasmonic coupling of the AuNPs in the colloidosome shell provides many possibilities of novel applications including optical sensors, surface-enhanced Raman scattering, light-induced hyperthermia therapy, and light scattering based cell imaging. The chemoresponsivity of the structures, along with the inherent tunable shell porosity, is especially desired for controlled drug release in a cellular environment where GSH is commonly existent. The use of the colloidosome as a nanosized reactor is also interesting since catalyst poisoning might be avoided by preventing harmful species from entering the colloidosome which encapsulates a catalyst particle. One obvious merit of the gold-based structure is that further surface functionalizations towards a special selectivity or a targeting process will be very easy. ASSOCIATED CONTENT
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Supporting Information. Experimental details and extra data. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author. *Email: [email protected]
ACKNOWLEDGMENTS This work was supported by NSFC (Grants 21425521, 21273214 and 91023005), the Hefei Center for Physical Science and Technology (2014FXCX010), and the Collaborative Innovation Center of Suzhou Nano Science and Technology. GSH was a kind gift from G. L. group.
REFERENCES (1) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Colloidosomes: Selectively permeable capsules composed of colloidal particles. Science 2002, 298, 10061009. (2) Peer, D.; Karp, J. M.; Hong, S.; FaroKHzad, O. C.; Margalit, R.; Langer, R. Nanocarriers as an emerging platform for cancer therapy. Nat. Nanotechnol. 2007, 2, 751-760. (3) Yang, X. C.; Samanta, B.; Agasti, S. S.; Jeong, Y.; Zhu, Z. J.; Rana, S.; Miranda, O. R.; Rotello, V. M. Drug delivery using nanoparticle-stabilized nanocapsules. Angew. Chem. Int. Ed. 2011, 50, 477-481. (4) Niikura, K.; Iyo, N.; Matsuo, Y.; Mitomo, H.; Ijiro, K. Sub-100 nm gold nanoparticle vesicles as a drug delivery carrier enabling rapid drug release upon light irradiation. ACS Appl. Mater. Interfaces 2013, 5, 3900-3907. (5) Noble, P. F.; Cayre, O. J.; Alargova, R. G.; Velev, O. D.; Paunov, V. N. Fabrication of “hairy” colloidosomes with shells of polymeric microrods. J. Am. Chem. Soc. 2004, 126, 8092-8093.
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(6) Takahara, Y. K.; Ikeda, S.; Ishino, S.; Tachi, K.; Ikeue, K.; Sakata, T.; Hasegawa, T.; Mori, H.; Matsumura, M.; Ohtani, B. Asymmetrically modified silica particles: A simple particulate surfactant for stabilization of oil droplets in water. J. Am. Chem. Soc. 2005, 127, 6271-6275. (7) Velev, O. D.; Furusawa, K.; Nagayama, K. Assembly of latex particles by using emulsion droplets as templates. 1. microstructured hollow sphers. Langmuir 1996, 12, 2374-2384. (8) Fialkowski, M.; Bitner, A.; Grzybowski, B. A. Self-assembly of polymeric microspheres of complex internal structures. Nat. Mater. 2005, 4, 93-97. (9) Kim, J. W.; Fernandez-Nieves, A.; Dan, N.; Utada, A. S.; Marquez, M.; Weitz, D. A. Colloidal assembly route for responsive colloidosomes with tunable permeability. Nano Lett. 2007, 7, 2876-2880. (10) Niikura, K.; Iyo, N.; Higuchi, T.; Nishio, T.; Jinnai, H.; Fujitani, N.; Ijiro, K. Gold nanoparticles coated with semi-fluorinated oligo(ethylene glycol) produce sub-100 nm nanoparticle vesicles without templates. J. Am. Chem. Soc. 2012, 134, 7632-7635. (11) Sanles-Sobrido, M.; Exner, W.; Rodriguez-Lorenzo, L.; Rodriguez-Gonzalez, B.; Correa-Duarte, M. A.; Alvarez-Puebla, R. A.; Liz-Marzan, L. M. Design of SERS-encoded, submicron, hollow particles through confined growth of encapsulated metal nanoparticles. J. Am. Chem. Soc. 2009, 131, 2699-2705. (12) Song, C. Y.; Zhao, G. P.; Zhang, P. J.; Rosi, N. L. Expeditious synthesis and assembly of sub-100 nm hollow spherical gold nanoparticle superstructures. J. Am. Chem. Soc. 2010, 132, 14033-14035. (13) Song, J. B.; Fang, Z.; Wang, C. X.; Zhou, J. J.; Duan, B.; Pu, L.; Duan, H. W. Photolabile plasmonic vesicles assembled from amphiphilic gold nanoparticles for remote-controlled traceable drug delivery. Nanoscale 2013, 5, 5816-5824. (14) He, J.; Liu, Y. J.; Babu, T.; Wei, Z. J.; Nie, Z. H. Self-assembly of inorganic nanoparticle vesicles and tubules driven by tethered linear block copolymers. J. Am. Chem. Soc. 2012, 134, 11342-11345.
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(15) Huang, P.; Lin, J.; Li, W. W.; Rong, P. F.; Wang, Z.; Wang, S. J.; Wang, X. P.; Sun, X. L.; Aronova, M.; Niu, G.; Leapman, R. D.; Nie, Z. H.; Chen, X. Y. Biodegradable gold nanovesicles with an ultrastrong plasmonic coupling effect for photoacoustic imaging and photothermal therapy. Angew. Chem. Int. Ed. 2013, 125, 14208-14214. (16) Bollhorst, T.; Shahabi, S.; Wörz, K.; Petters, C.; Dringen, R.; Maas, M.; Rezwan, K. Bifunctional submicron colloidosomes coassembled from fluorescent and superparamagnetic nanoparticles. Angew. Chem. Int. Ed. 2015, 127, 120-125. (17) Wang, H. Q.; Li, Y. L.; Liu, M.; Gong, M.; Deng, Z. X. Overcoming the coupling dilemma in DNA-programmable nanoparticle assembly by “Ag+ soldering”. Small 10.1002/smll.201403108. (18) Liu, J. B.; Fu, S. H.; Yuan, B.; Li, Y. L.; Deng, Z. X. Toward a universal “adhesive nanosheet” for the assembly of multiple nanoparticles based on a protein-induced reduction/decoration of graphene oxide. J. Am. Chem. Soc. 2010, 132, 7279-7281. (19) You, L.; Wang, T. Y.; Ge, J. P. When mesoporous silica meets the alkaline polyelectrolyte: A controllable synthesis of functional and hollow nanostructures with a porous shell. Chem. Eur. J. 2013, 19, 2142-2149. (20) Stober, W.; Fink, A.; Bohn, E. Controlled growth of monodisperse silica spheres in micron size range. J. Colloid Interface Sci. 1968, 26, 62. (21) Pompella, A.; Visvikis, A.; Paolicchi, A.; De Tata, V.; Casini, A. F. The changing faces of glutathione, a cellular protagonist. Biochem. Pharmacol. 2003, 66, 1499-1503.
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Table of Contents Chemoresponsive Colloidosomes via Ag+ Soldering of Surface-Assembled Nanoparticle Monolayers Miao Liu, Qian Tian, Yulin Li, Bo You, An Xu, Zhaoxiang Deng*
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Letter
Chemoresponsive Colloidosomes via Ag+ Soldering of Surface-Assembled Nanoparticle Monolayers Miao Liu, Qian Tian, Yulin Li, Bo You, An Xu, and Zhaoxiang Deng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b00298 • Publication Date (Web): 13 Apr 2015 Downloaded from http://pubs.acs.org on April 15, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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Chemoresponsive Colloidosomes via Ag+ Soldering of Surface-Assembled Nanoparticle Monolayers Miao Liu,† Qian Tian,† Yulin Li,† Bo You,† An Xu,‡ and Zhaoxiang Deng*,† †
CAS Key Laboratory of Soft Matter Chemistry, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China
‡
Key Laboratory of Ion Beam Bioengineering, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China
ABSTRACT: Colloidosomes with a hollow interior and a porous plasmonic shell are highly desired for many applications including nanoreactors, surface-enhanced Raman scattering (SERS), photothermal therapy, and controlled drug release. We herein report a silica nanosphere templated electrostatic self-assembly in conjunction with a newly developed Ag+ soldering to fabricate gold colloidosomes towards multi-functionality and stimuli-responsibility. The gold colloidosomes are capable of capturing a nanosized object, and releasing it via structural dissociation upon responding a biochemical input (GSH, glutathione) at a concentration close to its cellular level. In addition, the colloidosomes have a tunable nanoporous shell composed of strongly coupled gold nanoparticles which exhibit broadened near infra-red plasmon resonance. These features along with the simplicity and high tunability of the fabrication process make the gold colloidosomes quite promising for applications in a chemical or cellular environment.
INTRODUCTION
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Colloidosome, with the name coined in 2002, represents an emerging class of micro- or nanostructures with a hollow interior and an inherent surface porosity coming from the close packing of colloidal particles.1 These unique vesicular structures exhibit controllable permeability and mechanical stability, and provide an efficient encapsulation of various guest objects (including even live cells) of nanometer to micrometer sizes. The past years have witnessed various innovative applications of this novel structure focused on drug delivery and controlled release.2-4 Classical ways for colloidosome fabrication are based on Pickering-stabilized emulsion where emulsion-stabilizing latex particles spontaneously adsorb at an immiscible liquid-liquid interface.1,5-8 Another route for colloidosome fabrication is based on a temperature-sensitive microgel template.9 Negatively charged PS particles adsorb on the positively charged microgel particles via electrostatic attractions and form uniform vesicular shells after a thermal treatment. Gold nanoparticles (AuNPs) are another important class of materials for the construction of hollow permeable nanovesicles with very interesting plasmonic and catalytic properties.10-11 For example, a quick one-pot strategy has been developed to make sub-100 nm hollow gold colloidal spheres through a chemical deposition templated by a gold-binding peptide-conjugate.12 Another method to prepare pH-responding AuNP vesicles is based on amphiphilicity-driven selfassembly of polymer-decorated AuNPs, and the as-obtained structures have showed an interesting light-triggered drug release ability.13 In addition, a linear block-copolymer has been used to modify AuNPs toward the assembly of them into nanosized colloidosomes.14 By altering the co-polymer and the size of AuNPs, gold nanovesicles with strong plasmonic coupling were realized for photoacoustic imaging and phototherapy.15 Bifunctional submicron colloidosomes co-assembled from fluorescent and paramagnetic nanoparticles have also been fabricated through an emulsion template method.16
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Apart from the above processes and benefitting from a newly developed Ag+ soldering process in our group,17 we herein report a much simpler, widely controllable, and highly efficient silicananosphere-templated electrostatic self-assembly strategy to build a special class of plasmonic colloidosomes (Figure 1). The Ag+ soldering process takes advantage of the strong Ag+ affinity of the bis(p-sulfonatophenyl)phenyl phosphine (BSPP) ligand decorated on AuNPs to create BSPP-Ag+-BSPP bridging bond between adjacent AuNPs.17 The colloidosomes feature a porous wall and exhibit a strong near infrared (NIR) absorbance due to closely coupled AuNPs in their metallic shells, which undergo a self-disruption in the presence of millimolar range of glutathione (a widespread thiol molecule in cells). These properties make them quite promising for various applications in vitro or in vivo.
Figure 1. A silica-nanosphere-templated electrostatic assembly and Ag+ soldering of a gold nanoparticle colloidosome, which disassembles upon responding glutathione (GSH) molecules. BSPP-Ag+-BSPP bonding is generated between adjacent gold nanoparticles during the soldering process, which guarantees the structural stability of the colloidosomes after template removal. APTES and BSPP represent 3-aminopropyltriethoxysilane and bis(p-sulfonatophenyl)phenyl phosphine, respectively. EXPERIMETNAL SECTION Our experiments started with the synthesis and amino-functionalization of a silica nanosphere template. In a NaAc/HAc buffer (pH 6.0), the amino groups were protonated, resulting in a positively charged silica surface. Negatively charged bis(p-sulfonatophenyl)phenyl phosphine
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(BSPP)-capped AuNPs (BSPP-AuNPs, 13.8 nm diameter, Figure S1a, b) were then electrostatically adsorbed on the silica surface to form a dense monolayer. Further treating the nanoassembly by Ag+ helped build robust interconnects among AuNPs due to BSPP-Ag+-BSPP bonding based on a recent finding in our group.17 The Ag+ treatment ensured the mechanical stability of the AuNP nanovesicles after removing the silica template by HF acid etching. The resulting nanovesicles showed an interesting responsibility towards glutathione (GSH) (a bioactive thiol molecule, with a concentration close to its intracellular level), leading to a loss of the Ag+ bonding and complete structural disassembly, with encapsulated objects (could be doped in the SiO2 spheres during their synthesis) being released. RESULTS AND DISCUSSION We first used commercial 500 nm SiO2 nanospheres to template the assembly of AuNPs. One challenge in this process was to achieve a full coverage of closely packed AuNP monolayer on the SiO2 surface, which was critical for the following Ag+ locking step. We used NaCl to elevate the ionic strength of the NaAc-HAc buffer, which reduced electrostatic repulsions between neighbouring AuNPs to achieve a densely packed AuNP monolayer.18 Silver ions were then introduced to form BSPP-Ag+-BSPP bridges between adjacent AuNPs.17
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Figure 2. TEM (a, b) and SEM (c, d) images of gold colloidosomes prepared by silica nanosphere-templated electrostatic assembly and Ag+ soldering of gold nanoparticles. Transmission electron microscopy (TEM) imaging (Figure 2a) showed that the AuNP-based colloidosomes after an HF acid etching of the silica core appeared much darker at their edge parts, indicating enclosed spherical shells. The existence of some partially ruptured structures in Figure 2a evidenced the flexibility of the vesicular shells. The spherical shape of the AuNP colloidosomes was more clearly revealed by scanning electron microscope (SEM) imaging (Figures 2c and 2d). Occasionally, some structures were disrupted during the template removal process so that their hollow interiors were exposed (Figure 2c). These results evidenced the effectiveness of our strategy, which resulted in robust and free-standing vesicular structures. High magnification TEM (Figure 2b) and SEM (Figure 2d) images revealed the mesoporosity of the vesicular shells, related to their monolayer structures containing closely packed AuNPs. By varying the diameter of the AuNPs, it would be possible to fine-tune the pore size. Benefiting from the silica nanosphere-guided self-assembly, the diameter of the colloidosomes could be facilely tuned by altering the size of the silica templates. To demonstrate this, SiO2 nanospheres with a much smaller diameter of about 160 nm were prepared by a standard Stöber’s method,19,20 which were then utilized to direct the colloidal assembly. Figure 3a showed the monodisperse SiO2 nanospheres with a diameter of 160 nm. Figures 3b and 3c were TEM images of as-fabricated colloidosomes featuring hollow interiors and porous shells. SEM image in Figure 3d evidenced the enclosed cage-like 3D structure of the colloidosomes (the surface porosity was also visible from the SEM image). The colloidosomes had a diameter of 18020 nm based on the TEM data, which was consistent with dynamic light scattering (DLS) data (Figure
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S2). The pore size was estimated to be 2-6 nm from TEM data, which was slightly larger than a geometrical consideration due to some packing disorders of the AuNP shells.
Figure 3. TEM (a-c) and SEM (d) images of as-synthesized 160 nm SiO2 nano-template (a), and the assembled AuNP colloidosomes (b-d). In addition to the size tunability, the involvement of a SiO2 nanotemplate provided us a freedom to encapsulate a guest object in the colloidosome. Considering the tunable shell porosity (depending on the size of the AuNPs), this unique rattle-type structure might be used as environment-isolated bio- or chemo-nanoreactors, and to build plasmonic and drug delivery devices. We chose AuNPs with a diameter of 28.4 nm (Figures S1c, d) as the guest objects for the encapsulation experiment. The overall process was identical to the SiO2-templated process, with Au@SiO2 yolk-shell structures (130 nm, Figure S3) as the assembly templates. The Au@SiO2 yolk-shell nanoparticles based on the hydrolysis of tetraethylorthosilicate (TEOS) in the presence of the 28.4 nm guest AuNPs were shown as an inset in Figure 4a. The correct formation of a rattle-type AuNP colloidosome encapsulating a 28.4 nm AuNP was verified by TEM imaging (Figure 4a). Because the SiO2 layers had been completely removed, the guest AuNPs (28.4 nm) were found randomly distributed inside the hollow structures composed of
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13.8 nm AuNPs, in sharp contrast to the Au@SiO2 nanoparticles where the guest AuNPs stayed at the center (Figure 4a, inset). Also, we found that most of the colloidosomes maintained their spherical shape (Figure 4b), suggesting a good mechanical robustness.
Figure 4. TEM (a) and SEM (b) images of the as-synthesized 13.8 nm AuNP-based colloidosome encapsulating a 28.4 nm AuNP object. Inset in (a) shows the Au@SiO2 yolk-shell structure as the assembly template (scale bar: 200 nm). We have to emphasize that, to obtain a stable AuNP colloidosome, the Ag+ soldering was a key step. To reveal the critical role of the Ag+ locking, HF etching was applied to the electrostatically assembled 13.8 nm AuNPs in the absence of an Ag+ treatment. A sharp color transition of the sample solution from blue to red (Figure S4) was observed upon removal of SiO2, implying a dissociation of the assembled AuNPs. TEM images revealed an efficient assembly of the AuNPs on the SiO2 and Au@SiO2 templates (Figures S5a, b). After removing the SiO2 template, discrete and scattered AuNPs were seen on the TEM grid (Figures S5c, d, in sharp contrast to Figures S5a, b). This control experiment clearly verified the important role of the Ag+ soldering process during the formation of the AuNP colloidosomes. Benefiting from the reversibility of such an Ag+ locking process,17 a glutathione (GSH)responsive structural disruption of the colloidosomes could be realized. GSH, a tri-peptide, is an antioxidant widely existing in cells, serving to prevent damages to various cellular components caused by reactive oxygen species.21 The thiol moiety in the GSH molecule could compete for
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Ag+ in the assembled structures, and thus disrupt the BSPP-Ag+-BSPP bonding which held the colloidosomes. Another possibility that could not be excluded in this experiment was that GSH could strongly interact with the gold surface via its thiol group so that the BSPP ligands responsible for the Ag+ soldering would be displaced. Both of these two processes could contribute to the GSH-triggered disassembly of the colloidosomes. Note that such a chemoresponsive structural disruption will be highly desirable if applications such as controlled drug release are considered in a cellular environment. As shown in Figure 5a, the reduced interparticle gaps in the colloidosomes led to a significant red-shift and broadening of the plasmon resonance peak (blue v.s. black curve). Upon introducing GSH, such a near infra-red (NIR) plasmon band quickly diminished and a sharp peak around 524 nm (red curve) appeared, close to that of unassembled AuNPs (black curve). This phenomenon was a visual sign of the GSH-induced colloidosome dissociation. The slightly broadened peak of the dissociated structures compared with unassembled AuNPs indicated the existence of small AuNP aggregates, consistent with TEM observation (Figure 5b). Besides the TEM and spectral evidences, DLS data showed a gradual dissociation of the colloidosomes with increased GSH concentrations (Figure S6). In addition to GSH, other thiols could also lead to a similar disassembly of the as-formed colloidosomes (Figure S7). However, chloride, ethylenediaminetetraacetic acid (EDTA) disodium salt, and phosphate did not seem to be effective to trigger the disassembly (Figures S7 and S8).
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Figure 5. (a) Optical absorbance of AuNP colloidosomes before (blue) and after (red) interacting with GSH, in comparison with unassembled AuNPs (black). Inset in (a) is a photo of intact (blue) and disassembled (red) colloidosomes. (b) TEM image of disrupted AuNP colloidosomes upon reacting with GSH (see Figure S9 for a GSH-induced release of a guest nanoparticle). OUTLOOK We have developed a very simple, highly controllable, and widely adaptable hard template (SiO2) strategy, instead of emulsion or gel templates, to fabricate monodisperse AuNP colloidosomes with a hollow or rattle-like structure. Plasmonic coupling of the AuNPs in the colloidosome shell provides many possibilities of novel applications including optical sensors, surface-enhanced Raman scattering, light-induced hyperthermia therapy, and light scattering based cell imaging. The chemoresponsivity of the structures, along with the inherent tunable shell porosity, is especially desired for controlled drug release in a cellular environment where GSH is commonly existent. The use of the colloidosome as a nanosized reactor is also interesting since catalyst poisoning might be avoided by preventing harmful species from entering the colloidosome which encapsulates a catalyst particle. One obvious merit of the gold-based structure is that further surface functionalizations towards a special selectivity or a targeting process will be very easy. ASSOCIATED CONTENT
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Supporting Information. Experimental details and extra data. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author. *Email: [email protected]
ACKNOWLEDGMENTS This work was supported by NSFC (Grants 21425521, 21273214 and 91023005), the Hefei Center for Physical Science and Technology (2014FXCX010), and the Collaborative Innovation Center of Suzhou Nano Science and Technology. GSH was a kind gift from G. L. group.
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(6) Takahara, Y. K.; Ikeda, S.; Ishino, S.; Tachi, K.; Ikeue, K.; Sakata, T.; Hasegawa, T.; Mori, H.; Matsumura, M.; Ohtani, B. Asymmetrically modified silica particles: A simple particulate surfactant for stabilization of oil droplets in water. J. Am. Chem. Soc. 2005, 127, 6271-6275. (7) Velev, O. D.; Furusawa, K.; Nagayama, K. Assembly of latex particles by using emulsion droplets as templates. 1. microstructured hollow sphers. Langmuir 1996, 12, 2374-2384. (8) Fialkowski, M.; Bitner, A.; Grzybowski, B. A. Self-assembly of polymeric microspheres of complex internal structures. Nat. Mater. 2005, 4, 93-97. (9) Kim, J. W.; Fernandez-Nieves, A.; Dan, N.; Utada, A. S.; Marquez, M.; Weitz, D. A. Colloidal assembly route for responsive colloidosomes with tunable permeability. Nano Lett. 2007, 7, 2876-2880. (10) Niikura, K.; Iyo, N.; Higuchi, T.; Nishio, T.; Jinnai, H.; Fujitani, N.; Ijiro, K. Gold nanoparticles coated with semi-fluorinated oligo(ethylene glycol) produce sub-100 nm nanoparticle vesicles without templates. J. Am. Chem. Soc. 2012, 134, 7632-7635. (11) Sanles-Sobrido, M.; Exner, W.; Rodriguez-Lorenzo, L.; Rodriguez-Gonzalez, B.; Correa-Duarte, M. A.; Alvarez-Puebla, R. A.; Liz-Marzan, L. M. Design of SERS-encoded, submicron, hollow particles through confined growth of encapsulated metal nanoparticles. J. Am. Chem. Soc. 2009, 131, 2699-2705. (12) Song, C. Y.; Zhao, G. P.; Zhang, P. J.; Rosi, N. L. Expeditious synthesis and assembly of sub-100 nm hollow spherical gold nanoparticle superstructures. J. Am. Chem. Soc. 2010, 132, 14033-14035. (13) Song, J. B.; Fang, Z.; Wang, C. X.; Zhou, J. J.; Duan, B.; Pu, L.; Duan, H. W. Photolabile plasmonic vesicles assembled from amphiphilic gold nanoparticles for remote-controlled traceable drug delivery. Nanoscale 2013, 5, 5816-5824. (14) He, J.; Liu, Y. J.; Babu, T.; Wei, Z. J.; Nie, Z. H. Self-assembly of inorganic nanoparticle vesicles and tubules driven by tethered linear block copolymers. J. Am. Chem. Soc. 2012, 134, 11342-11345.
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(15) Huang, P.; Lin, J.; Li, W. W.; Rong, P. F.; Wang, Z.; Wang, S. J.; Wang, X. P.; Sun, X. L.; Aronova, M.; Niu, G.; Leapman, R. D.; Nie, Z. H.; Chen, X. Y. Biodegradable gold nanovesicles with an ultrastrong plasmonic coupling effect for photoacoustic imaging and photothermal therapy. Angew. Chem. Int. Ed. 2013, 125, 14208-14214. (16) Bollhorst, T.; Shahabi, S.; Wörz, K.; Petters, C.; Dringen, R.; Maas, M.; Rezwan, K. Bifunctional submicron colloidosomes coassembled from fluorescent and superparamagnetic nanoparticles. Angew. Chem. Int. Ed. 2015, 127, 120-125. (17) Wang, H. Q.; Li, Y. L.; Liu, M.; Gong, M.; Deng, Z. X. Overcoming the coupling dilemma in DNA-programmable nanoparticle assembly by “Ag+ soldering”. Small 10.1002/smll.201403108. (18) Liu, J. B.; Fu, S. H.; Yuan, B.; Li, Y. L.; Deng, Z. X. Toward a universal “adhesive nanosheet” for the assembly of multiple nanoparticles based on a protein-induced reduction/decoration of graphene oxide. J. Am. Chem. Soc. 2010, 132, 7279-7281. (19) You, L.; Wang, T. Y.; Ge, J. P. When mesoporous silica meets the alkaline polyelectrolyte: A controllable synthesis of functional and hollow nanostructures with a porous shell. Chem. Eur. J. 2013, 19, 2142-2149. (20) Stober, W.; Fink, A.; Bohn, E. Controlled growth of monodisperse silica spheres in micron size range. J. Colloid Interface Sci. 1968, 26, 62. (21) Pompella, A.; Visvikis, A.; Paolicchi, A.; De Tata, V.; Casini, A. F. The changing faces of glutathione, a cellular protagonist. Biochem. Pharmacol. 2003, 66, 1499-1503.
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Table of Contents Chemoresponsive Colloidosomes via Ag+ Soldering of Surface-Assembled Nanoparticle Monolayers Miao Liu, Qian Tian, Yulin Li, Bo You, An Xu, Zhaoxiang Deng*
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