Self-assembly of colloidal molecules that respond to light and a magnetic field

Sven Sagebiel , Lucas Stricker , Sabrina Engel and Bart Jan Ravoo *
Organic Chemistry Institute and Center for Soft Nanoscience, Westfälische Wilhelms-Universität Münster, Corrensstraße 40, 48149, Münster, Germany. E-mail:

Received 13th June 2017 , Accepted 20th July 2017

First published on 20th July 2017

This communication reports a novel method to prepare Janus particles with light-responsive arylazopyrazole (AAP) polymer caps, which can be reversibly cross-linked to chain-like colloidal oligomers in the presence of cyclodextrin (CD) functionalized superparamagnetic nanoparticles. The resulting colloidal molecules are light-responsive and can be controlled by an external magnetic field.

Anisotropic particles have properties that are completely different from isotropic particles.1,2 With some creativity in the control of shape and chemical composition, an almost infinite collection of anisotropic particles of various sizes and “patchy” nature can be imagined.1–4 One interesting class of “patchy” particles are Janus particles, named after the ancient Roman god Janus, who was believed to have two opposing faces. Currently, much attention is drawn to Janus particles since they are amphiphilic character as a result of their two distinct faces giving rise to improved and even unprecedented material properties.5 Due to the anisotropic character of Janus particles, they tend to assemble in anisotropic structures very different from the common packing modes observed for isotropic particles. Well-defined clusters of Janus particles have been described as colloidal molecules:6 in analogy to molecules that have functional groups, Janus particles have distinct surface regions that can interact selectively with other particles. In analogy with the valency and hybridization of atoms, patchy particles can be described as sp, sp2 and sp3 type colloids. Impressive examples were reported by Weck and Pine, who described three dimensional colloids and superstructures.7–9 Alternatively, Janus particles may be classified as AB, ABA and ABC type colloids, where A, B and C denote caps or patches with orthogonal properties. In pioneering experiments, Granick et al. reported the self-assembly of Janus particles into very unusual surface patterns10 and colloidal clusters.11 They also studied the magnetic properties of Janus particles.12,13 Particularly interesting examples of the self-assembly of Janus particles reported in the last few years include patchy particles with distinct valency and specific directional bonding due to DNA hybridization,14 single patch polymer beads with orthogonal functionalization and biorecognition,15 dimerization of Janus particles via host–guest interactions16 and reversible aggregation of Janus particles via light-responsive host–guest interactions.17 Granick et al. recently summarized the key advances in the directional assembly of Janus particles and pointed out that the design of arbitrarily selected structures by bottom up self-assembly of Janus particles remains a great challenge.18 It is clear that this challenge can only be met if efficient methods for the preparation of Janus particles with a high density of addressable non-covalent binding sites are available.

We recently reported a versatile strategy for the preparation of Janus particles by post-modification using sandwich microcontact printing. In this procedure, microparticles are sandwiched between two inked poly(dimethylsiloxane) (PDMS) stamps in a way that both caps of the particles are functionalized simultaneously yet orthogonally.19 We have applied this method to various larger and smaller polymer microparticles using in particular click chemistry to immobilize various inks exclusively on the caps of the polymer particles.20 The size of the resulting caps mainly depends on pressure, time, temperature and stiffness of the stamps. Increasing the pressure on the stamps leads to larger caps, while longer time and higher temperature favour diffusion of the inks, promoting large and ill-defined caps. Fast reactions at room temperature are preferred for sandwich printing.20 The stiffness of the PDMS stamps can be adjusted by the cross-linker content and curing time and should match the stiffness of the particles.21 We now demonstrate that using a similar sandwich printing approach, initiators for polymerization can be immobilized in well-defined caps on silica microparticles, so that functional polymer brushes can be grown by atom transfer radical polymerization (ATRP) at the caps. This approach facilitates the controlled growth of hydrophilic polymer brushes containing fluorescent reporter units as well as light-responsive arylazopyrazole (AAP) units. Compared to conventional azobenzenes, AAPs are superior photoswitches that show nearly quantitative E/Z-isomerization in both directions upon irradiation.22 However, the light-responsive host–guest interaction of AAPs to cyclodextrins (CD) is identical to the well-established azobenzene CD interaction (i.e. E-AAP binds to CD while Z-AAP does not).22,23 We note that the host–guest interaction of CDs has been proven to be a versatile binding motif in mesoscopic self-assembly.24–26 The combination of Janus microparticles functionalized with hydrophilic light-responsive caps containing a high density of AAPs and magnetite nanoparticles functionalized with CDs gives rise to unprecedented dual responsive self-assembly of colloidal molecules in aqueous solution (Fig. 1).

image file: c7cc04594h-f1.tif
Fig. 1 Dual responsive self-assembly of Janus microparticles functionalized with PAAPA-PNBDA-PHEA co-polymer brushes grown from ATRP-TAD and CD stabilized magnetite nanoparticles. AAP can be photoswitched from the adhesive trans-isomer to the non-binding cis-isomer.22

As a first proof of concept, we printed an ATRP initiator exclusively on one or both caps of 6.65 μm silica microparticles. To this end, the silica beads were coated with a monolayer of n-octenyl trichlorosilane and subsequently subjected to sandwich microcontact printing using an ATRP initiator with a triazolindione (TAD) tag. TAD has been recently introduced by Du Prez and co-workers as a versatile click motif that can react with dienes or alkenes in a short reaction time with high yields.27 Together with Du Prez and co-workers, we also immobilized an ATRP-TAD initiator on a flat substrate, which enabled us to grow dense polymer brushes from these surfaces.28 Finally, ATRP with hydroxyethyl acrylate (HEA) and the fluorescent co-monomer nitrobenzoxadiazole acrylate (NBDA, 0.5 mol%) provided AB Janus particles, in case the ATRP-TAD initiator was printed on one cap of each microparticle and ABA Janus particles, if the initiator was printed on both caps of each particle. Fluorescence microscopy showed the formation of uniform caps of fluorescent polymer brushes on either a single cap or two opposite caps of the particle (Fig. 2a–d). Furthermore, arylazopyrazole acrylate (AAPA) was included as a third co-monomer (7.5 mol%) to integrate a light-responsive guest for CD in the caps. Indeed, ABA Janus particles equipped with two PAAPA-PNBDA-PHEA co-polymer caps (Fig. 2e and f) are indistinguishable from those functionalized with PNBDA-PHEA caps. Negative controls, in which the initiator was not printed on the particles, are provided as ESI (see Fig. S1). In the absence of an initiator, no fluorescence can be detected and hence physisorption of monomer can be excluded.

image file: c7cc04594h-f2.tif
Fig. 2 Optical and fluorescence microscopy of AB and ABA Janus particles made by sandwich printing and grafting-from polymerization on silica microparticles. (a and b) AB Janus particles with ATRP-TAD initiator printed on a single cap followed by PNBDA-PHEA co-polymerization; (c and d) ABA Janus particles with ATRP-TAD initiator printed on both caps followed by PNBDA-PHEA co-polymerization; (e and f) ABA Janus particles with ATRP-TAD initiator printed on both caps followed by PAAPA-PNBDA-PHEA co-polymerization. Scale bar: 10 μm. Fluorescent mirror specifications: dichroic mirror: DM500; excitation filter: BP460-490C; barrier filter: BA520 IF.

Magnetite nanoparticles with a diameter of 10 nm, functionalized with per-6-deoxy-per(carboxylpropyl)thio-β-cyclodextrin (CD-MNPs),29 were used as supramolecular magnetic glue to connect the ABA Janus particles in aqueous solution. Indeed, the multivalent host–guest interaction of the AAPs on the caps of the microparticles and the CDs on the nanoparticles resulted in the spontaneous formation of chains consisting mostly of two to four microparticles. We observed chains with a maximum of seven particles. The particle concentration is rather low, the particles sediment rather quickly and hence no extensive supramolecular association can occur. We note that not all oligomers are perfectly linear. This can be attributed to the fact that the caps are rather large, compared to the particle diameter, so that particles can adhere at various angles. It is likely that rotational flexibility of the Janus particles in the oligomers is restricted due to the formation of multiple host–guest complexes in the contact area. We assume that smaller patches lead to mostly linear structures and one-to-one interactions, whereas larger patches increase the number of CD-MNPs binding to the caps and the probability of two caps interacting in solution. Even sonication does not affect the stability and shape of the microparticle oligomers.

The particle chains were also investigated by scanning electron microscopy (SEM, see Fig. S2, ESI). SEM clearly reveals the connection of the caps of the silica microparticles with the magnetite nanoparticles. Moreover, it can be observed that only the caps of the silica particles are covered with the nanoparticles. Thus, SEM indicates that CD-MNPs adhere preferentially on the AAP-functionalized polymer brushes and that oligomerization of the Janus particles is a consequence of multivalent host–guest interactions. This interpretation was further supported by negative control experiments, in which the functionalization of one or both of the particle types is omitted. Microparticles printed without ATRP-TAD initiator (see Fig. S3, ESI), microparticles functionalized with PNBDA-PHEA (but no AAPA, see Fig. S4, ESI), and also MNPs without CD (see Fig. S5 and S6, ESI) do not form any type of colloidal clusters in aqueous solution. These experiments show that the functionalization on both particles with complementary guest and host units is a necessary condition for the self-assembly of linear oligomers. If any part of the functionalization is omitted, no interaction of the particles is observed. Thus, any non-specific hydrophobic or polar interactions that could potentially lead to aggregation of particles can be excluded.

The linear oligomers of ABA Janus particles and CD-MNPs can be easily manipulated by application of an external magnetic field. The superparamagnetic nanoparticles will tend to arrange according to the direction of the applied magnetic field. Consecutive snap shots of the rotation of a linear ABA particle oligomer are shown in Fig. 3. It can be seen that the entire oligomer moves with the direction of the field and that no significant changes in the relative orientation of the Janus particles can be detected. This observation underscores the strength of the multivalent host–guest interactions between the particles. It should be noted that, although the non-magnetic silica microparticles are more than 300 times larger than the CD-MNPs, magnetization of the CD-MNPs results in a significant magnetic force on the entire assembly. Occasionally, residual individual ABA Janus particles with CD-MNPs in the polymer caps can be observed. In addition, these Janus particles can be rotated, if a magnetic field is applied (see Video supplied as ESI).

image file: c7cc04594h-f3.tif
Fig. 3 Magnetic manipulation of Janus particle oligomers. Light microscopy shows consecutive snap shots (a–d) of the rotation of a linear ABA particle oligomer in an external magnetic field. For this purpose, microparticles with two caps functionalized with PAAPA-PNBDA-PHEA co-polymer brushes and CD-MNPs (0.6 mg mL−1) were used. Scale bar: 10 μm.

Remarkably, even if the oligomers of ABA Janus particles are held together by multiple host–guest interactions between the AAPs in the polymer brushes and the CDs on the MNPs, the excellent light-responsive characteristics of AAPs facilitated the reversible disassembly and reassembly of the oligomers. Irradiation with UV light (365 nm, 3 h) was used to isomerize the AAPA in the polymer brushes from the E-isomer to the Z-isomer. Brief sonification after photoisomerization resulted in complete disassembly of the oligomers. Upon irradiation with green light (520 nm, 2 h), the E-isomer is re-obtained and linear particle oligomers are observed again. The light-responsive assembly and disassembly of linear Janus particle oligomers could be repeated over at least three cycles (Fig. 4).

image file: c7cc04594h-f4.tif
Fig. 4 Light-responsive self-assembly of Janus particle oligomers. (a and b) Optical and fluorescence microscopy of ABA Janus particles with ATRP-TAD initiator printed on both caps, followed by PAAPA-PNBDA-PHEA co-polymerization, mixed with CD-MNPs (0.6 mg mL−1); (c) light microscopy of particle mixtures after irradiation with UV light (365 nm); (d) light microscopy of particle mixtures after irradiation with green light (520 nm); (e) light microscopy of particle mixtures after second irradiation with UV light (365 nm); (f) light microscopy of particle mixtures after second irradiation with green light (520 nm); scale bar = 10 μm. Overview pictures are provided as ESI (see Fig. S7).

In conclusion, we have developed a method to synthesize Janus particles with polymer caps that display a high density of functional groups. Since in principle any acrylate can be used as co-monomer for the polymerization, this approach to post-modification of microparticles is highly versatile. If light-responsive guest units are embedded in the polymer caps, the addition of host-modified nanoparticles as supramolecular glue leads to the self-assembly of highly stable linear particle oligomers due to the formation of multiple host–guest complexes. The oligomers can be manipulated in a magnetic field and they can be assembled and disassembled in response to irradiation. Thus, we have explored new strategies for the dual responsive assembly of well-defined colloidal molecules. We foresee that this assembly strategy can be extrapolated to more complex structures involving multiple components and orthogonal interaction motifs.

This work was funded by the Volkswagen Foundation. Sabrina Engel is grateful for a fellowship from the Fonds der Chemischen Industrie. We thank Lukas Ibing at MEET in Münster for SEM and Dr Oliver Roling, and Dr Philipp Seidel for helpful discussions.

Notes and references

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Electronic supplementary information (ESI) available: Experimental procedures, supporting data and control experiments. See DOI: 10.1039/c7cc04594h

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