Light-Triggered Reversible Self-Engulfing of Janus Nanoparticles

Letter Cite This: ACS Macro Lett. 2018, 7, 1475−1479

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Light-Triggered Reversible Self-Engulfing of Janus Nanoparticles Xiaojuan Hou,† Song Guan,† Ting Qu,† Xuefei Wu,|| Dong Wang,|| Aihua Chen,*,†,‡ and Zhenzhong Yang*,§ †

School of Materials Science and Engineering, Beihang University, Beijing 100191, People’s Republic of China Beijing Advanced Innovation Centre for Biomedical Engineering, Beihang University, Beijing 100191, People’s Republic of China § Department of Chemical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China || State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China Downloaded via CALIFORNIA STATE UNIV FRESNO on November 30, 2018 at 00:28:23 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Block copolymers containing azobenzene liquid crystalline (LC) mesogen are used to prepare snowman-like Janus nanoparticles (NPs) by emulsion solvent evaporation. The azobenzene-containing poly(methacrylate) (PMAAz) head of the Janus NPs is in the smectic LC phase with ordered stripes, which becomes amorphous and enlarged due to trans/cis transformation under UV irradiation. The expanded PMAAz can consequently engulf the other head. The self-engulfed NPs can recover to their original state in both shape and LC state via visible-light irradiation. This strategy is promising for programmable load and release of different payloads by remote trigger using light.

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irradiation or heating. In the case of micellar aggregates from azobenzene-containing BCPs, a reversible dissociation/association can be triggered by alternative UV/visible-light irradiation.30−32 Along with the LC−isotropic phase transition, polarity of the azobenzene moiety changes.33 The LC ordered structure from the azobenzene rigid moiety has been used to prepare nonspherical polymeric NPs.34 It is noticed that the deformation is realized of the target spheres supported onto the substrate upon polarized laser irradiation.35−40 Yield of deformed spheres is rather low. Herein, we propose the azobenzene-containing Janus NPs from PMAAz-based amphiphilic BCPs via the confined assembly of a BCP solution emulsion droplet after evaporation. The Janus NP is snowman-like in shape by controlling the shear strength during the formation of emulsion droplets. Onto one head of the Janus NPs, the hydrophobic azobenzene LC segments form ordered stripes. The trans-to-cis photoisomerization of azobenzene induces the transformation from an LC ordered more compact state to a disordered loose state upon UV irradiation. Eventually, the hydrophilic part is covered to be completely enveloped by the hydrophobic PMAAz chain. This self-engulfing transformation can be reversibly driven by visible-light irradiation (Scheme 1). Janus balance of the NP is thus broadly tuned by light with

anus nanoparticles (NPs) consisting of two different parts in composition and (or) performance within the same entity have gained considerable attention due to their diversified potential applications,1,2 such as functional solid surfactants,3,4 optical biosensors,5,6 and building blocks toward superstructures.7−9 Janus NPs derived from amphiphilic block copolymers (BCPs) can exhibit very abundant architectures with tunable affinity, specific interaction, and responsive performance arising from functional polymer chains.10,11 It is important to develop methods to prepare BCP Janus NPs with tunable microstructure and composition. Disintegration of selfassembled supramolecular structures of BCPs has been extensively employed to prepare nanoscale polymeric Janus NPs.12−14 The method is more suitable for those copolymers containing a cross-linkable block. Recently, we have reported a robust approach toward snowman-like Janus NPs from polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) via emulsion solvent evaporation induced assembly.15−17 The approach is effective to a broader range of BCPs because a cross-linkable block is not a prerequisite. It is highly desired to achieve responsive Janus NPs. For polymeric NPs, reversibly responsive performance is of great importance due to their applications in biomedicine,18,19 drug delivery systems,20−23 and nanoreactors.24,25 Azobenzene is an interesting rigid liquid crystalline (LC) unit, which displays remarkable trans/cis photoisomerization.26−29 Azobenzenecontaining BCPs demonstrate LC behavior in the trans state, which can convert into the cis state by UV irradiation. The cisto-trans isomerization can be driven by either visible-light © XXXX American Chemical Society

Received: September 26, 2018 Accepted: November 28, 2018

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DOI: 10.1021/acsmacrolett.8b00750 ACS Macro Lett. 2018, 7, 1475−1479

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ACS Macro Letters

reported the formation of smectic LC ordering stripes from azobenzene-containing BCPs via conventional self-assembly in solution or polymerization-induced self-assembly.34,41 In the current report, the smectic LC ordered stripes are present within one head of the Janus NP. Under polarized optical microscope (POM), the colorful dots further verify the LC ordered phase (Figure S2). AFM phase image of the Janus NP indicates that the dark part corresponds to the soft PEO head while the bright part to the rigid PMAAz head (Figure 1d), further confirming the asymmetric Janus structure. Different from our previous formation of snowman-like Janus NPs of LC unit-free copolymers such as PS-b-P4VP, after more than 40 extrusion cycles,15 the current Janus NP can be more easily achieved after a 10-cycle extrusion owing to the LC mesogen. After more cycles of extrusion, for example, 20, the Janus NPs are prone to coalescence at the PMAAz heads (Figure S3a). After 40-cycle extrusion, nanowires form with a striped PMAAz core which is covered with a PEO sheath (Figure S3b). In the following experiments, the extrusion cycle is fixed at 10 to achieve individual Janus NPs. The PMAAz head size and thus volume fraction (Φ) of the Janus NPs is tunable by alternation of PMAAz chain length at a fixed PEO length of, for example, 114 (Table S1). In the case of PEO114-b-PMAAz20 ( f PMAAz = 68.1%), the PMAAz head is too small to distinguish the LC ordered stripes (Figure 2a). The Janus NP is small, ∼50 nm, and the volume fraction of the PMAAz head (Φ) is estimated as ∼15%. In comparison, the Janus NPs of PEO114-b-PMAAz120 (f PMAAz = 92.8%) display a higher fraction of ∼42% (Figure 2b). The Janus NP is longer, ∼80 nm. Especially, the PMAAz head becomes highly ordered with regular stripes. The Janus NPs become larger yet uniform with PMAAz chain length (Figure 2c). Accordingly, the volume fraction of the PMAAz head increases with PMAAz chain fraction of the BCPs (Figure 2d). It is worth noting that the volume fraction keeps below 50% even when f PMAAz is as high as 92.8%. This is quite different from our previous report on Janus NPs from PS-b-P4VP, whose Janus balance can be easily adjusted across 1:1 by changing one block fraction.15 In the current case, the PMAAz contains rigid LC mesogen side chains and is prone to forming more compact ordered stripes upon subjection to extrusion, while PEO chains are soft to easily relax the stress resulting in the free chain conformation.42 Azobenzene mesogens can display light -triggered trans/cis photoisomerization, which has been extensively used to stimulate many transformations such as polarity, shape, and size.26−34 It is expected that the Janus NPs should demonstrate some morphological transformation by light irradiation. After exposure to UV irradiation for 1 h, the PMAAz head of the Janus NP of PEO114-b-PMAAz48 becomes less ordered and much larger. The NP is long, ∼150 nm, but the gray PEO head remains the same size (Figures 3a, S4a, and S5). The snowman-like shape is preserved. No bright head is found from the Janus NP under AFM phase image (Figure S4b), indicating the smaller phase difference between two blocks upon UV irradiation. Volume fraction of the PMAAz head (Φ) significantly increases to ∼68% from the original ∼26% (Figure S6). It is reasonable that the expanded PMAAz can further spread along the PEO head and thus is partially covered. This process looks like a self-engulfing PEO head, yet partially with the expanded PMAAz block. After visible-light irradiation, the deformed Janus NPs can almost completely recover to the original shape and size (Figures 3b, S5, and S7). It is important

Scheme 1. (a) Chemical Structures of the PMAAz-Based Amphiphilic Block Copolymers and (b) Light-Triggered Reversible Self-Engulfing of One Example Janus NP

adjustable affinity and interfacial activity. The reversible transformation is performed in dispersion, which can guarantee a high yield of deformed Janus NPs. One example Janus NP of PEO114-b-PMAAz48 was prepared by the BCP self-assembly after solvent evaporation from the emulsion droplets.15−17 The emulsion droplets containing the copolymer solution in chloroform were stabilized with CTAB. The dispersion of the Janus nanoparticles was obtained after a 10-cycle membrane extrusion. The Janus NPs were collected by centrifugation, and no further washing was performed to ensure dispersibility of the NPs in water by the residual CTAB. DLS analysis shows one narrow peak of the Janus NP dispersion, indicating that the NP is uniform (Figure S1). As shown in Figure 1a, the representative Janus NP appears

Figure 1. Morphological characterization of the snowman-like Janus NP of PEO114-b-PMAAz48. (a) TEM image and inset SEM image with the bar scale of 50 nm; (b) a magnified TEM image; (c) AFM height image and the height profiles; and (d) AFM phase image.

snowman-like in shape with two heads, and the NP is long, ∼70 nm, and wide, ∼50 nm, which is confirmed by AFM height image analysis (Figure 1c) and DLS results (Figure S1). The dark and gray heads are assigned to PMAAz and PEO, respectively. The snowman-like shape is further confirmed by an SEM image (inset Figure 1a). A magnified TEM image (Figure 1b) shows that the PMAAz domain is highly ordered with regular stripes spacing ∼3.5 nm. We have previously 1476

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Figure 2. (a, b) TEM images of Janus NPs from PEO114-b-PMAAz20 and PEO114-b-PMAAz120, respectively; (c) DLS traces of the three Janus NPs. The curves 1, 2, 3 are corresponding to PEO114-b-PMAAz20, PEO114-b-PMAAz48, and PEO114-b-PMAAz120, respectively;. (d) The plot of volume fraction (Φ) of the PMAAz head as a function of f PMAAz.

It is required to systematically characterize the lighttriggered self-engulfing. Transformation of the Janus NP of PEO114-b-PMAAz120 in aqueous dispersion with UV irradiation was monitored by UV−vis spectroscopy and POM observation. Upon UV irradiation, the trans−cis isomerization (corresponded to π−π* transition) occurs, and the transazobenzene mesogen absorption intensity at 335 nm becomes weakened (Figure 4a). At the initial stage (within seconds) of UV irradiation, the intensity decreases fast and levels off to a plateau at 2 min (inset Figure 4a). Under POM, the Janus NP looks less colorful and eventually colorless after 2 min UV irradiation (Figure S10). On the other hand, the cisazobenzene can recover toward the trans-azobenzene after visible-light irradiation (Figure S11a). The transition is rather slow, and as long as 1 h is required for the complete recovery (Figure S11b), which is confirmed by POM image results (Figure S12). Morphological evolution of the Janus NP with UV irradiation was recorded by TEM observation. After 10 s irradiation, although the snowman-like shape is preserved, the dark PMAAz head becomes highly expanded from the periphery (Figure 4b). The LC stripes disappear. After UV irradiation for 30 s, the PMAAz head is further expanded (Figure 4c) yet keeping the Janus structure. After 60 s irradiation, the expanded PMAAz starts to envelop the PEO head forming an eccentric shell (Figure 4d). The eccentric shell becomes slightly matured with further prolonging irradiation for 120 s (Figure S13a). After 10 min UV irradiation, the eccentric shell is less influenced (Figure S13b). The PMAAz head volume fraction (Φ) increases significantly from 42% to 64% within the early 30 s and reaches a constant until 2 min (Figure S13c). The light-triggered morphological evolution of the Janus NP is general and applicable to other systems. The similar snowman-like Janus NP was derived from P4VP 50 -bPMAAz68 (Figure S14a). The LC ordered stripes are present in the Janus NP. The morphological transformation is similar to PEOm-b-PMAAzn. The PMAAz head is expanded after UV

Figure 3. (a) TEM image of the Janus NP from PEO114-b-PMAAz48 after UV irradiation and (b) after visible-light irradiation. (c, d) TEM images of the Janus NPs from PEO114-b-PMAAz20 and PEO114-bPMAAz120 after UV irradiation.

that the LC ordered stripes appear again as the darker domain (inset Figure 3b). In the case of snowman-like Janus NP of PEO114-b-PMAAz20 with small fraction of PMAAz, the PMAAz dark head is slightly expanded upon UV irradiation (Figure 3c), and the volume fraction is ∼40% (Figure S6). Upon visible-light irradiation, the deformed Janus NP can completely recover to the original size and shape (Figure S8). When the PMAAz fraction is higher, for example, in the case of PEO114-bPMAAz120, the UV light triggered expansion extent becomes dramatically large which can completely envelop the whole PEO head (Figure 3d). An eccentric core/shell structure forms, where no ordered stripes are found. In this case, the PEO head is completely engulfed. The core/shell structure can be transformed into the original snowman-like Janus NP with the same size and shape and microstructure of distinctly discerned LC ordered stripes after visible-light irradiation (Figure S9). This implies that the self-engulfment is reversible by light irradiation. 1477

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Figure 4. (a) UV−vis spectra of the Janus NP of PEO114-b-PMAAz120 with UV irradiation. Inset: the peak intensity at 335 nm as a function of UV irradiation time. TEM images of morphological evolution of the Janus NP after UV irradiation for varied time: (b) 10 s, (c) 30 s, (d) 60 s.

*E-mail: [email protected] (Zhenzhong Yang).

irradiation (Figure S14b). The expanded Janus NP can be completely recovered to the original snowman-like one after visible-light irradiation (Figure S14c). Another snowman-like Janus NP was achieved from PMAA75-b-PMAAz65 (Figure S15a). The reversible morphological evolution of the Janus NP by UV and visible-light irradiation is similar to that aforementioned (Figure S15b,c). In summary, we have designed and synthesized snowmanlike Janus NPs with azobenzene-containing LC mesogen-based PMAAz heads. The Janus NPs are achieved by emulsion solvent evaporation from the BCP solution droplets. Especially, the PMAAz head is highly ordered with regular smectic stripes containing optical active azobenzene groups. Upon UV irradiation, the PMAAz head becomes amorphous and expanded. In the case of a sufficiently long PMAAz chain, an eccentric shell forms and envelops the whole area of the other head after UV irradiation, like a self-engulfing behavior. Upon visible-light irradiation, the deformed Janus NPs can completely recover to the original size and LC state. This approach is general and applicable for other Janus NPs of azobenzene-containing BCPs. The reversible self-engulfing of Janus NPs can be used for light-triggering contact and separation of different species loaded onto the two heads. It is promising for remote manipulation of controlled encapsulation and release.

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ORCID

Dong Wang: 0000-0003-2326-0852 Aihua Chen: 0000-0002-9609-988X Zhenzhong Yang: 0000-0002-4810-7371 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51272010, 51472018), Beijing Nova Program (XX2013009), and Fundamental Research Fund for the Central Universities.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00750. Materials and methods, characterization, and supporting figures and table (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Aihua Chen). 1478

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