Letter pubs.acs.org/macroletters
Light-Mediated Reversible Assembly of Polymeric Colloids Elizabeth Elacqua, Xiaolong Zheng, and Marcus Weck* Molecular Design Institute and Department of Chemistry, New York University, New York, New York 10003, United States S Supporting Information *
ABSTRACT: This contribution highlights the functionalization of colloidal particles featuring high-symmetry patches with telechelic block copolymers and subsequent reversible self-assembly of the resulting particles into longer chain and branched structures using host−guest complexation. The 3-(trimethoxysilyl)propyl methacrylate (TPM)-based anisotropic particles, obtained through a clusterencapsulation process, consist of poly(styrene) patches and are sitespecifically functionalized with block copolymers bearing pendant viologen or azobenzene motifs. Key to the design is the engineering of heterotelechelic α-hydroxy-ω-formyl-poly(norbornene)s via ringopening metathesis polymerization (ROMP). The block copolymers feature both main chain anchor points to the particle surface, as well as orthogonal reactive sites for cyanine dye conjugation. The polymeric particles undergo directed and reversible supramolecular assembly in the presence of the host cucurbit[8]uril.
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to manipulate supramolecular assembly. In all cases, the assembled architectures were dictated by the patch geometry. We recently investigated the use of host−guest pairing cucurbit[7]uril (CB[7]) and diphenylviologen (DPV) to facilitate the assembly of sulfonated poly(styrene).48 We demonstrated that sulfonated anisotropic dimers form small daisy-chain-like architectures through CB[7]−DPV−CB[7] bridging interactions. Herein, we present a directional and versatile method to assemble colloids through the combination of host−guest complexation and polymer chemistry. Host−guest chemistry has been integrated into synthetic systems to engineer polymeric materials for applications ranging from drug delivery to chemical sensing.43,49−52 Common hosts based upon supramolecular containers, such as cyclodextrins43,53 and cucurbit[n]urils,52,54−57 have been assembled with electronpoor (i.e., viologen) and electron-rich (e.g., azobenzene, naphthol) guests.57 Using soft matter, Scherman and coworkers have generated hybrid light responsive raspberry-like colloids from azobenzene-based and viologen-studded nanoparticles,54 while Huskens and co-workers fabricated colloidosomes from viologen-functionalized poly(styrene) nanoparticles in the presence of cucurbit[8]uril (CB[8]).58 While CB[8] has engineered self-assembled polymers and dendrimers,52,56,57,59 the use of CB[8] to direct colloidal selfassembly into open lattices remains unexplored and can afford reversible assemblies. Recent studies have also utilized host− guest complexation of β-cyclodextrin-functionalized Janus particles to fabricate light-responsive assemblies.60 Additional
he preparation of polymeric colloidal particles has received considerable attention in the past decade, owing to applications ranging from catalysis1 and drug delivery2 to materials for plasmonics3,4 and photonics.5−7 Most applications require monodisperse particles that are uniform in not only shape and size,8 but also composition and surface properties.9 Spherical colloidal particles have emerged as prototypical models since they feature highly tunable, yet inexpensive platforms with the ability to control size, shape, and bonding interactions,10−13 while giving rise to a variety of materials that exhibit long and short-range order.3,14−18 Recent efforts feature desirable surface functionalities17,19−21 while obtaining different topologies via self-assembly.11,12,16,22−26 Employing molecular recognition is an advantageous platform for the synthesis of colloidal architectures. For one, the directional strategy is versatile, as multiple complementary motifs, differing in connectivity and association strengths, are available.27 Supramolecular interactions are also modular and can be fine-tuned and regulated/reversed with stimuli,28−30 leading to adaptive and responsive materials.31,32 Whereas supramolecular engineering is prominent both in solid-state and polymer chemistry,33−35 affording materials such as artificial machines,36−38 porous-organic frameworks,39,40 biomaterials,41,42 and stimuli-responsive polymers,43−46 the application of molecular recognition to engineer colloidal architectures is still in its infancy. We have demonstrated the ability to endow bonding specificity and directionality into polymeric colloidal particles, achieving open colloidal assemblies that feature lock-and-key-based depletion,15 DNA hybridization,13,19−21,47 metal-coordination,17 and bridging interactions with a host−guest complex.48 Central to our approach is the ability to fabricate molecular geometry-mimicking patchy particles13 while installing site-specific directional interactions © XXXX American Chemical Society
Received: July 22, 2017 Accepted: September 6, 2017
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Figure 1. Host−guest strategy featuring both synthetic polymer chemistry and colloidal assembly: (A) engineering of heterotelechelic side-chainfunctionalized block copolymers through ROMP; (B) fabrication of patchy particles from carboxylated poly(styrene) colloidal clusters through encapsulation and polymerization with TPM, followed by polymer conjugation; and (C) target host−guest driven assembly mediated by cucurbit[8]uril.
tropic particles in a site-specific manner (Figure 1).17 Host− guest complexation of pendant Vio and Azo moieties in the presence of CB[8] should induce self-assembly and afford colloidal clusters and chains. The Vio and Azo recognition units are utilized as receptors owing to strong associations with CB[8].57 Our choice of Vio and Azo functionalities is founded upon the ability to facilely integrate both groups within a norbornene backbone, while fabricating a switchable 52 anisotropic particle assembly that can be controlled with light. To achieve the targeted heterotelechelic block copolymers, we engineered a hydroxyl-functionalized Grubbs initiator via cross-metathesis. The polymer is designed to include a block containing either Vio or Azo pendant units, followed by a spacer to separate the self-assembly event from the particle surface. Termination of the ROMP with 1,3-dioxol-2-one installs an aldehyde moiety that facilitates conjugation of Cy3 and Cy5 hydrazides. ROMP initiator 3 is synthesized through EDC-mediated coupling between aminopropanol and 1, followed by cross metathesis with Grubbs’ first generation initiator (GI, Scheme 1A). Target monomers VioNB and AzoNB are synthesized in two steps from exo-5-norbornenecarboxylic acid (4). Their common precursor 5 and octyl estercontaining comonomer (OctNB) are synthesized according to the literature.63 Subsequently, Vio and Azo block copolymers are synthesized via ROMP in the presence of GI derivative 1.
work with cyclodextrin and Janus particles bearing arylazopyrazole caps has demonstrated light-mediated assembly and disassembly.61 While these studies combine specific supramolecular interactions with responsive assembly on the colloidal scale, they cannot sustain three-dimensional (3D) lattices. We have reported the functionalization of patchy particles with synthetic polymers that interact through metal coordination and their subsequent assembly into chain-like structures.17 While the directional self-assembly of colloids is not trivial, synthetic polymers provide a versatile platform to introduce a variety of molecular recognition moieties that can be fine-tuned for directional assembly toward different architectures. Herein, we further our synthetic polymer platform to install fluorescently-labeled polymers that are capable of host−guest complexation and a directed disassembly in the presence of a noninvasive stimulus (i.e., light). The use of polymer chemistry is key to the design, as all of the requirements for directional (and observable) molecular recognition are addressed using synthesized monomers, initiators, and terminators. In our design, ROMP62 is utilized owing to the ability to introduce a variety of functionalized norbornenes and initiators that facilitate directional self-assembly33 while maintaining a controlled polymerization. In this approach, ROMP can be exploited to alter the density of molecular recognition elements through block length, while functional initiators and terminators provide versatility to the process. The use of synthetic polymers is advantageous, as they are versatile and can accommodate supramolecular recognition motifs, while the anisotropic colloids are not limited to Janus-like geometry and, thus, are amenable to 3D architectures. Our research design is based on the synthesis of telechelic polymers via a functionalized Grubbs initiator, leading to block copolymers comprising viologen (Vio) and azobenzene (Azo) pendant side chains. The copolymers also contain spacer blocks, a hydroxylated terminus, and aldehyde motifs. The aldehydes are functional handles to conjugate to a cyanine dye (Cy3 or Cy5), affording a terminal moiety that confirms selfassembly specificity in the presence of CB[8]. The alcohol groups installed through the initiator can covalently attach the polymers to carboxylic acid-functionalized patches of aniso-
Scheme 1. Synthesis of (A) ROMP Initiator and (B) Functional Monomers
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feature distinct −COOH functionalities on the patches and are stable in both aqueous and organic media, making them suitable to investigate host−guest-driven directional selfassembly. Prior to functionalization, the patchy particles are dispersed in THF. The fluorescent block copolymers containing pendant viologen or azobenzene moieties and hydroxyl end groups (pVioNB-Cy3 and pAzoNB-Cy5, respectively) are conjugated to the acid-functionalized patches using EDC-mediated esterification. Site-specific functionalization of the patches of the colloidal particles is essential to enable a directional and predictable self-assembly. To verify that the patches are, indeed, selectively fluorescently labeled, we turned to confocal microscopy (Figure 2) after repeated washing/centrifugation cycles of the reaction mixture in THF. Since the matrix has no −COOH groups and therefore is not able to conjugate covalently to any polymers, the matrix does not exhibit fluorescence. After installation of the synthetic polymers featuring Vio or Azo motifs selectively on the colloidal patches (Figure 2C), we investigate the self-assembly behavior of the colloidal particles. Cucurbit[8]uril forms ternary complexes with a variety of electron-rich and electron-poor systems based on viologens, naphthalenes, and stilbene analogues;55,65 thus, we examined the assembly of pVioNB- and pAzoNB-functionalized patchy particles in the presence of CB[8]. Whereas the self-assembly is most prominent in aqueous media, even-numbered CB[n] systems exhibit poor solubility in water, often having to be assisted with acid or alkali salts to encounter moderate solubility; CB[n] complexes doped with a suitable guest also can display enhanced solubility.55 While we did investigate different mixtures of particles (Figure 3), we focused mainly on the two-patch particles and expect the assembly to lead to chain-like structures,13,17 capable
To furnish formyl end groups, we quenched the polymerizations with 1,3-dioxol-2-one. The resulting α-hydroxy-ω-formyl-poly(norbornene) copolymers (pVioNB and pAzoNB) were analyzed via 1H NMR spectroscopy to determine block ratios and assess end-group fidelity. Using signals corresponding to the initiator and pendant groups (Figures S-2 and S-3), we confirm that the block lengths correspond well with the feed ratios (OctNB/ funct. NB: 15:20 for pAzoNB and 20:15 for pVioNB). In addition, both block copolymers demonstrate high end-group fidelity, wherein the initiator end-group and the aldehyde are present on all polymer chains. Whereas pAzoNB displayed a monomodal distribution and low dispersity (Đ = 1.3; Mn = 19700, Mw = 25500; Figure S-1), as determined by SEC analysis in CHCl3, the exclusive solubility of pVioNB in MeOH/H2O or DMSO/DMF precluded SEC analysis. Using 1H NMR spectroscopic endgroup analysis, the polymer molecular weights were found to be 13740 and 16750 for pAzo and pVio, respectively. The polymers are conjugated with Cy3- and Cy5-hydrazide, forming the corresponding hydrazone linkage on the ω-terminus. Successful reaction is evidenced in the 1H NMR spectra, wherein the newly formed N−H protons are visible between 11.9 and 12.1 ppm. In both cases, near quantitative conversion is observed, as the aldehyde resonance is indistinguishable in the corresponding 1H NMR spectra (Figures S-7 and S-8). Colloidal particles are then fabricated following a previously reported method.13 Briefly, poly(styrene) microspheres with carboxylated termini (650 nm diameter, 3% cross-linked with divinylbenzene) are synthesized via emulsion polymerization in the presence of 4,4′-azobis(cyanovaleric acid).17 Patchy particles are fabricated using a modified cluster64 encapsulation method. Carboxylated poly(styrene) clusters (containing 1−5 close-packed spheres) are partially encapsulated with TPM (Figure 2). The resultant particles (separable into dimers, trimers, and so on, through density gradient centrifugation)
Figure 2. Scanning electron micrographs of (A) carboxylated poly(styrene) clusters, (B) TPM-encapsulated patchy particles, and (C) purified batches of dimers, trimers, and tetramers, and (D) confocal micrographs of individual (red) pAzoNB-Cy5 and (green) pVioNB-Cy3 functionalized patchy particles (scale bars = 1 μm).
Figure 3. Bright-field and confocal overlay (left) and confocal images of self-assembled particles (right): (A, B) monopatch, (C, D) dipatch, and (E, F) mixed patch assemblies depicting clear patch−patch interactions (scale bars = 5 μm). 1062
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ACS Macro Letters of both being linear and containing branch points (Figure S11), pending patch size. As is typical for CB[8] complexes, the electron-poor guest (pVioNB-Cy3) was first mixed with the CB[8] under acidic pH, wherein it was expected that Vio would interact with the CO portal. The ensuing colloidal suspension is evaluated using both bright field and confocal microscopies, wherein a stable colloidal assembly is observed; two methyl viologen-based moieties are not capable of interacting within the CB[8] framework (Figures S-8 and S9).66 As we sought to induce an element of reconfigurability within the colloidal architectures, we integrated the pAzoNB-Cy5 particles within the assembled architectures as a photoresponsive element that can alter an assembled lattice. Azofunctionalized colloids were added to the suspension and subsequently agitated. The assembled materials are evaluated using both bright field and confocal microscopy. The majority of particles form branched chains (Figures 4 and S-11−S-14)
Figure 5. Bright-field images illustrating reversible nature of host− guest interactions: (A) dimer particles prior to assembly; (B) clustering effects seen after 15 min mixing with CB[8]; (C) assembled chains forming after 2 h; (D) particle mixture after 8 h of UV irradiation; and (E) reassembled particles after 8 h of visible light irradiation. Panel (F) features assembled particles after 12 h, while (G) displays reassembled particles after 16 h of visible light irradiation (scale bars = 5 μm).
bright field micrographs that roughly 85% disassemble within the 8 h period, wherein remaining particles are engaged in twoparticle interactions that may be reversed with further irradiation. The particles reassemble upon exposure to visible light, thus reactivating the supramolecular complexation within 8 h (Figure 5E). Larger assemblies are seen after 16 h of visible light irradiation (Figure 5G). As expected, the reassembly process is slower relative to the assembly process, thus, providing an additional potential to reconfigure colloidal assembly processes between kinetic to thermodynamic selfassembly. In conclusion, this report describes the use of host−guest chemistry to direct the self-assembly of functionalized colloidal particles. Our modular design uses ROMP to obtain telechelic block copolymers that are used to further functionalize colloidal patchy particles. The strategy allows for exclusive control over the identity and quantity of recognition motifs featured, while providing points for both dye and particle conjugation. The colloidal networks feature heteroternary complexation of Vio and Azo motifs within the CB[8] cavity, thus, providing a simple photoswitch to reconfigure materials. The resultant complexes undergo reversible disassembly/self-assembly in the presence/absence of UV light. Owing to a difference in time scale of the CB[8] related assembly/disassembly/reassembly process, we hypothesize that the integration of these molecular recognition elements into higher valency particles (i.e., trimers, tetramers, etc.) can provide a facile and noninvasive method to reconfigure 3D particle assemblies.
Figure 4. Bright-field and confocal images of (A, B) functionalized dimers, and (C−F) assembled pVioNB and pAzoNB-functionalized dimers in the presence of CB[8] (scale bars = 5 μm).
based on interpatch interactions. Since the patches are large and can facilitate multiple binding partners, we observe branching and some clustering effects, wherein particles approach each other to assemble from an orthogonal orientation. Branched clusters of 4−6 particles, along with chains of at least six two-patch particles are observed. In assemblies containing some three-patch particles, more branch points and larger assemblies are observed (Figures 5 and S-12). Azo moieties are known to undergo a UV-mediated trans−cis photoisomerization43,54,58 that has been exploited to fabricate stimuli-responsive materials.46 As a result of the isomerization, cis-Azo is not compact enough to reside within the CB[8] cavity, leading to disassembly of any supramolecular structure. We evaluated whether the colloidal assemblies would maintain the photoresponsive nature of Azo in the presence of UV-light (Figures 5 and S-15). Upon UV exposure for 8 h, the majority of the particles disassemble (Figure 5D). We estimate from 1063
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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.7b00539. Experimental details including functional initiator, monomers and polymer synthesis and characterization, polymer−particle conjugation, assembly trials, and additional micrographs (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Elizabeth Elacqua: 0000-0002-1239-9560 Xiaolong Zheng: 0000-0002-7749-6265 Marcus Weck: 0000-0002-6486-4268 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work is primarily supported by the Donors of the American Chemical Society Petroleum Research Fund under Grant No. 56280-ND7. We acknowledge support from the MRI program of the National Science Foundation under Award Number DMR-0923251 for the purchase of a Zeiss field emission SEM. We also acknowledge the NIH for the purchase of the Avance III-600 CPTCI-cryoprobe head (S10 Grant, OD016343).
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DOI: 10.1021/acsmacrolett.7b00539 ACS Macro Lett. 2017, 6, 1060−1065
