Letter pubs.acs.org/macroletters
Shape Memory Actuation of Janus Nanoparticles with Amphipathic Cross-Linked Network Jinlong Zhang, Xiaotong Zheng,* Fengluan Wu, Bingyun Yan, Shaobing Zhou, Shuxin Qu, and Jie Weng School of Materials Science and Engineering, Key Laboratory of Advanced Technologies of Materials, Ministry of Education, Southwest Jiaotong University, Chengdu 610031, China S Supporting Information *
ABSTRACT: Preparation of nanoscale Janus particles that can respond to external stimulation and, at the same time, be prepared using an easily achievable method presents a significant challenge. Here, we have demonstrated the shape memory of Janus nanoparticles (SMJNPs) with a multifunctional combination of Janus nanostructure and a shape memory effect, composed of a welldefined amphipathic sucrose-poly(ε-caprolactone) cross-linked network. A sudden negative pressure method was first used to prepare the Janus-shaped nanoparticles (temporary shape), which can switch their shape and wettability. The Janus-shaped nanoparticle is an amphipathic structure composed of hydrophilic and hydrophobic parts. Moreover, in response to temperature, the nanoparticle can recover their nanosphere state via a shape memory process. The novel Janus nanoparticles with the shape memory property also show a great potential for application such as drug delivery.
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SMP particles down to 200 nm. They buried poly(ωpentadecalactone)−poly(ϵ-caprolactone) (PPDL−PCL) particles in a water-soluble PVA film and stretched the film at a temperature of 70 °C to create a temporary ellipsoid shape. The ellipsoid particle was heated again to test the shape memory effect. The result illustrated that the particle may show shape memory effect on the nanoscale. However, the aim here is how to prepare the functional Janus particles with an easily achieved methodology and have the particles able to respond quickly to situations at the same time. An emulsion method is easily used to prepare the Janus particle by swelling the polymer core from a core−shell structure, but the Janus particle cannot respond to the stimulation.18 Accordingly, Solomon et al.37 reported a kind of Janus particle assembled from colloidal fibers that can switch its shape because of the shape memory behavior of the colloidal fiber. Hence, in this study, we developed the Janus-shaped nanoparticles that possess a biomimetic structural shape composed of a hydrophilic body and hydrophobic tentacles. The hydrophobic part of the Janus-shaped nanoparticles can recoil into the center of the particle via an entropy-driven process. Importantly, a novel sudden negative pressure was encountered in the fabrication of the Janus nanoparticle with shape memory property. Compared with the above-mentioned approaches, our strategy to obtain Janus nanoparticles is a
anus particle, defined as an anisotropic particle consisting of opposite shape, composition, and surface chemistry.1,2 Functionally, the distinct surfaces of Janus particles have attracted great attention because of their anisotropic wettability and anisotropic optical,3,4 electrical,5 and magnetic properties.6−8 Taking advantage of these anomalous properties, Janus particles have actually been used in diverse applications such as drug delivery,9 micro/nanomotors,10,11 particulate surfactants,12 imaging nanoprobes,13 and self-motile colloidal materials,14 thereby demonstrating the particular usefulness of Janus particles. Because of the numerous application, extensive studies such as selective surface modification using the particle monolayer,15,16 evaporative crystallization,17 Pickering emulsion,18,19 partially masked particles as templates,20 various seeded polymerization methods,21−23 selective cross-linking and subsequent dissolution of a microphase separated copolymer,24,25 microfluidic technique,26 and a hydrodynamic jetting process27 have focused on the precise synthesis of Janus particles. Shape memory polymers (SMPs) are defined by their ability to store a temporary shape and recover to its permanent shape in response to simulation.28,29 SMPs have attracted broad interest because of their extensive applications. Recently, most researchers have focused on macroscopic shape memory polymers.30 Because the morphological control of polymer particles is crucial for the creation of functional nanoparticles,31 it is more significant to synthesize nanoparticles with a tunable shape in response to certain stimuli.32−35 Thus, Lendlein et al.36 raised the question of whether SMPs would be suitable for miniaturized applications by the preparation of submicrometer © XXXX American Chemical Society
Received: September 25, 2016 Accepted: November 7, 2016
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DOI: 10.1021/acsmacrolett.6b00730 ACS Macro Lett. 2016, 5, 1317−1321
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Figure 1. Synthetic route for amphipathic SU-PCL copolymer with controlled hydrophilic and hydrophobic units.
Figure 2. TEM image of (a) cross-linked SU-PCL nanoparticles and (b) Janus-shaped nanoparticles. (c) Natural structure of jellyfish (left) and SEM of Janus-shaped nanoparticles (right). (d) TEM image of cross-linked SU-PCL nanoparticles without THF in the core under sudden negative pressure. (e) Growth diagram of both hydrophilic and hydrophobic parts of cross-linked SU-PCL nanosphere during the negative pressure process. (f) Schematic of the programming memory of cross-linked PCL segment with hydrophobic lobe shape in SU-PCL nanosphere under sudden negative pressure. (g) TEM image and (h) schematic of cross-linked SU-PCL nanoparticles with recovery state.
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DOI: 10.1021/acsmacrolett.6b00730 ACS Macro Lett. 2016, 5, 1317−1321
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fixed by freezing the sudden deformation so that the Janus shape (temporary shape) of the cross-linked SU-PCL nanoparticle could be obtained. Because a certain amount of THF was encapsulated in the center of the nanoparticles, the core of the cross-linked SU-PCL nanoparticles can be regarded as the PCL-THF solution encapsulated by a hydrophilic surface.40 The pressure of the surrounding environment (Ps) was far below the pressure at the core of nanoparticles, so THF volatilized violently across the hydrophilic surface of the nanospheres. According to the Flory−Huggins (FH) theory,41 polymer−solvent interaction exists between the hydrophobic polymer segment and the solvent in the condensed mixture. In addition, the presence of solvent decreases the Tm of the crosslinked SU-PCL nanoparticles (Figure S3). Therefore, the frozen PCL segment was able to transfer the initial position with the movement of the THF molecules. Thus, the crosslinked PCL segment would be induced to move along the path of the escaping THF during the process of the sudden negative pressure at Thigh, suggesting that the cross-linked PCL segment gushes through the hole left by the THF and forms a hydrophobic lobe on the open surface of the particle.42 In view of this, we can hypothesize that (1) the cloud-like lobe is the core of the initial cross-linked SU-PCL nanoparticles; (2) the Janus nanoparticles with the cross-linked structure have a surprising shape memory effect. We further demonstrated the shape memory effect of the Janus nanoparticles during the recovery process (Figure 2g,h). As shown by TEM images, the Janus-shaped nanoparticles recovered their initial spherical shape in response to temperature, this demonstrated the memory-recovery characteristics of the cross-linked amphipathic nanoparticles. When heated to Thigh again, the entropy driven recoil of the switching segments took place,43 and the Janus nanoparticles recovered to the hydrophilic spherical state. This transformation strongly supported our hypothesis that the cross-linked SU-PCL nanoparticles were capable of remembering a Janus temporary shape and recovering to their permanent shape again at Thigh. For a quantitative description of the SME,37 the diameter of the hydrophobic lobe was used to characterize the program and recovery of the spherical particles from the Janus particles (temporary shape; see Figures S5−S6, SI). Figure 3 shows the influence of cross-link degree of PCL segment on the shape ratio and recovery ratio of Janus
simple processing technique for a smart-responsive nanomaterial. A well-defined network with biodegradable sucrose-poly(εcaprolactone) (SU-PCL) copolymer was used to prepare the initial spherical hydrophilic nanoparticles. SU-PCL is an amphipathic copolymer with good biocompatibility that has great potential in drug delivery.38 The amphipathic multiarm SU-PCL was synthesized using the protection/deprotection technique for sucrose via trimethylsilyl groups and ring-opening polymerization (ROP) of ε-caprolactone according to the following steps in Figure 1. Compared to previously used polysaccharose, sucrose was first used as the initiator of a PCL ring-open polymerization to obtain an amphipathic copolymer. The synthesis results for SU-PCL are discussed in Figures S1− S3 of Supporting Information (SI). The disaccharide has the advantage of a quite smaller steric effect, and a high PCL molecular weight, which is an important parameter to shape memory performance, is easy to achieve. We prepared SU-PCL nanoparticles by a microemulsion method and PCL segment was cross-linked with benzophenone (BP) under UV. Thus, the cross-linking networks of PCL molecules and the amphipathic micelle structure copolymer from the SU-PCL nanoparticles were considered as the stationary phase so as to obtain shape memory property. Subsequently, a sudden negative pressure method was used to prepare the shape memory Janus nanoparticles. In addition, the bioinspired micro/nanomaterials with special functions have an extensive influence on material applications. Janus nanoparticles with the shape memory effect (SME) at a diameter of ≈80 (±10) nm (characterized by DSL) were prepared via an oil-in-water emulsion technique producing the vortex for the favorable dispersion of the polymer solution (Figure S4). Amphipathic SU-PCL molecules were assembled to form a micelle in water with the hydrophobic PCL segment gathering in the center and hydroxyl groups of sucrose toward the outside. Subsequently, UV (365 nm) was used to cross-link the PCL segment for 60 min to chemically fabricate the crosslinking SU-PCL nanoparticle. Morphology of the cross-linked SU-PCL nanoparticles was observed by TEM (Figure 2a). The good dispersibility and stability of the cross-linked SU-PCL nanoparticles in water indicates that the surface of the nanospheres is hydrophilic. In addition, the stability of the nanoparticles is also quite good so that the nanoparticles can keep their spherical morphology and dispersibility more than 1 week. We further found that the surface of the nanoparticles is rough (a common feature of the nanoparticles) due to the solvent uptake in the swollen domains.37 It should be noted that, because the PCL segments aggregate in the center of the particle, as reported in many studies,39 a certain amount of hydrophobic solvent THF is encapsulated in the nanoparticles, which is key to fabricating Janus-shaped nanoparticles. We further observed that spherical cross-linked SU-PCL nanoparticles changed their shape to a “jellyfish”-like particle when exposed to a sudden negative pressure environment (Figure 2b). The SEM images (Figure 2c, right) also show that the morphology of the Janus-shaped nanoparticle is just like the profile of a jellyfish. In addition, Figure 2d demonstrates that the nanospheres cannot transform into the Janus-shaped particles in the absence of THF, indicating that the THF encapsulated in the center of the particles plays a crucial role during the shape programming process. We further investigated the movement process of the cross-linked PCL segment (Figure 2e,f). It indicated that the created “lobe” shape could be
Figure 3. Diagram of the influence of BP contents on shape changing ratio (green) and recovery ratio (red). 1319
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Figure 4. Drug-loading behavior of SMJNPs simulating jellyfish. (a) Schematic of the cis-DDP-loading process of the cross-linked SU-PCL SMJNPs. TEM image (b) and EDS mapped images of O (c) and Pt (d) of Janus-shaped nanoparticle loading cis-DDP in the initial phase of shape recovery (temporary state). TEM image (e) and EDS mapped images of O (f) and Pt (g) of Janus-shaped nanoparticle loading cis-DDP at the mid phase of shape recovery (recovering state). TEM image (h) and EDS mapped images of O (i) and Pt (j) of Janus-shaped nanoparticle loading cis-DDP at the final phase of shape recovery (recovered state).
entropy-driven shape memory process, and we could demonstrate the potential application for smart drug loading. In summary, we have developed a simple approach to synthesize SMJNPs using sudden negative pressure. The novel Janus-shaped nanoparticles possess a “jellyfish”-like morphology, while are an amphipathic structure composed of hydrophilic polyhydroxy saccharide and hydrophobic crosslinked PCL parts, which possesses the ability to recover to the initial spherical shape at a temperature above Tm via an entropy-driven recovery process. We also demonstrated that SMJNPs as nanocarriers can be used in drug loading. More importantly, the shape memory actuation of the Janus nanoparticle as an interesting biomimetic nanomaterial shows the potential application in biomedical and other fields such as a bioinspired route to the decontamination of nanowaste.
nanoparticles. The content of BP changed from 0 to 8 (wt %) and the volume of hydrophobic lobe of the achieved nanoparticles was calculated. With the increase of BP content, Rδ decreased from 110 to 89%, which means that the volume of the hydrophobic lobe of the Janus-shaped nanoparticles decreased. This result indicates that the high cross-link degree hinders the movement of PCL segment (further discussion in SI). We further investigated the potential application of SMJNPs. Because many drugs are hydrophobic, the hydrophobic part of the Janus-shaped nanoparticle may have a good interaction with hydrophobic drugs and take the drugs into the center of the nanoparticles using the shape memory effect of the novel Janus structure. cis-Dichlorodiamineplatinum(II) (cis-DDP), a kind of hydrophobic anticancer medicine was used to investigate the drug-loading behavior of SMJNPs. As shown in Figure 4, the red points corresponding to the mapped distribution of O (Figure 4c,f,i) and the blue points attributing to Pt (Figure 4d,g,j) illustrated the distribution of nanocarriers and cis-DDP. Figure 4b,e,h perfectly described the shape recovery process of the intelligent drug loading: the Janus shape (temporary shape) with the hydrophobic “lobe” capturing the drug (Figure 4b), the ellipsoidal shape (recovering shape) with the recovering “lobe” delivering the drug (Figure 4e) and the spherical shape (recovered shape) with the central “core” storing the drug (Figure 4h). We could also strongly demonstrate this process from the mapping images of Pt from cis-DDP loaded in nanoparticles shown in Figure 4d,g,j. The results illustrated that cis-DDP was successfully loaded into the nanoparticles by an
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00730. Experimental details and additional data and discussion. Figures S1−S7 (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]. Notes
The authors declare no competing financial interest. 1320
DOI: 10.1021/acsmacrolett.6b00730 ACS Macro Lett. 2016, 5, 1317−1321
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China under Grant (51203130) and the Central Universities under Grant (2682013CX001).
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