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Stimulus-Responsive Planet−Satellite Nanostructures as Colloidal Actuators: Reversible Contraction and Expansion of the Planet− Satellite Distance Christian Rossner,*,† Otto Glatter,‡ and Philipp Vana§ †

Institut für Elektronenmikroskopie und Nanoanalytik, Technische Universität Graz, Steyrergasse 17, A-8010 Graz, Austria Institut für Anorganische Chemie, Technische Universität Graz, Stremayrgasse 9/V, A-8010 Graz, Austria § Institut für Physikalische Chemie, Georg-August-Universität Göttingen, Tammannstraße 6, D-37077 Göttingen, Germany ‡

S Supporting Information *

ABSTRACT: Structural plasticity and its control at the nanoscale are a vivid area of material science. In this contribution, we report a conceptually simple and versatile strategy for the formation of reconfigurable nanoparticle arrangements. The key role in our approach is played by star block copolymers from controlled radical RAFT polymerization, which fulfill the dual task of guiding the particle arrangement and also of equipping the nanomaterials with stimulus-responsiveness. By virtue of their block structure, the star polymers provide at the same time colloidal stability and responsive properties. Structural switching in response to the applied stimulus was investigated by means of small-angle X-ray scattering and dynamic light scattering. The developed approach is general, easy to implement, and may provide new prospects for the development of colloidal actuators, nanoscale materials with switchable properties, and nanoscale machines.

1. INTRODUCTION Molecular actuators, whose structural and dynamic properties can be controlled by applying external stimuli, have been widely studied in the fields of supramolecular1−3 and macromolecular chemistry.4−6 Bringing these concepts from the (macro)molecular to the colloidal scale provides a strong impetus for the development of new nanomaterials7 because switching of the arrangement structure in multiparticle systems can be associated with remarkable changes in the material properties.8,9 For developing synthetic strategies toward reconfigurable particle arrangements, gold nanoparticles (AuNPs) are most frequently used as model particles because their surface is chemically stable and can be easily modified. A few studies have demonstrated reversibly reconfigurable AuNP arrangements including nanoparticle dimers10,11 and core−satellite structures12 with switchable particle nanogaps, using chemical triggers in the form of DNA.10−13 Although these DNAbased approaches provide high structural precision, they are on the other hand hardly expected to be suited for providing materials in multi-milligram scale in the near future, and their switching by means of chemical triggers poses the further downside of waste accumulation. Therefore, it is attempted in this work to demonstrate the potential of carefully designed synthetic polymers for the scalable preparation of higher-order nanostructures with thermoswitchable arrangement structure. Polymers of N-isopropylacrylamide (NiPAAM) are by far the most studied stimulus-responsive synthetic polymers. Their aqueous solutions feature a lower critical solution temperature © XXXX American Chemical Society

(LCST), at which a hydrophilic-to-hydrophobic transition occurs.14 This phase transition has been used for preparing a range of unique responsive microgel structures,15−17 responsive polymer−protein conjugates,18 templates for the preparation of mesoporous SiO2 structures with controllable pore sizes,19 and many more. The LCST behavior is retained when PNiPAAM chains are assembled onto the surface of AuNPs,20 and it can be exploited in the fabrication of core−shell particles with switchable surface properties.21,22 In these cited and many other conceivable cases, one requirement for making use of the temperature-responsive properties of PNiPAAM in colloidal hybrid structures is that the resulting aqueous dispersions are stable both below and above the LCST. In fact, for many colloidal hybrid structures featuring a stimulus-responsive organic part, which may be PNiPAAM23 or something else,24−26 particle agglomeration is observed when applying the stimulus and thereby altering the solvophilicity of the responsive organic part. Already the question of whether aqueous dispersions of simple AuNP-core−PNiPAAM-shell hybrid particles are colloidally stable in both states is difficult because the answer to it depends on how the hybrid particles had been prepared and purified.27,28 We had recently developed a strategy to prepare more complex hybrid nanostructures, in which two different types of AuNPs are assembled into planet− Received: June 14, 2017 Revised: August 28, 2017

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DOI: 10.1021/acs.macromol.7b01267 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. (Top) Synthetic Scheme for Preparing 3-Arm Star Block Copolymers by Controlled Radical RAFT Polymerization through Successive Monomer Insertions; (Bottom) Chain Assembly of the Star Block Copolymers on the Surface of Large (16 nm) AuNPs and Planet−Satellite Structure Formation after Addition of Smaller (4 nm) AuNPs and Stabilizing Linear Polymera

a

The RAFT polymers attach to the gold nanoparticle surfaces with their trithiocarbonate termini (red dots).38

satellite arrangements by RAFT star polymers of NiPAAM.29,30 Similar systems were achieved by Dey et al.,31 employing hyperbranched RAFT polymer to join AuNPs of different sizes into this type of arrangement structure; by Fan et al.,32 who used telechelic RAFT polymer to assemble smaller satellite AuNPs around gold nanorods; by Wu et al.,33 following surface-initiated RAFT polymerization from silica nanoparticles (R group approach) followed by addition of satellite AuNPs, thus forming silica-planet−gold-satellite arrangement structures; and recently by us34 demonstrating the potential of RAFT star polymers for the formation of gold-planet−silversatellite nanostructures. Biopolymers can also be applied for the formation of planet−satellite nanostructures, as is for example demonstrated for proteins35 and DNA.12,36 The higher-order nanostructures studied by us29,30 were found experimentally to be unstable above the LCST of PNiPAAM, resulting in the aqueous dispersions first turning their color from red to blue followed by precipitation upon heating. Hence, in order to use the thermoresponsive properties of PNiPAAM for reversible structural switching of these unique particle arrangements, a remaining challenge is to develop systems which at the same time remain responsive and colloidally stable. By tailoring of macromolecules at the molecular level, it is possible to guide the formation of (hybrid) structures in a rational fashion and to impart some level of hierarchy to these structures: Because powerful tools for targeted polymer

fabrication are at hand, synthetic polymers can serve as platform for precisely locating specific building units into desired positions both within the individual macromolecules, by virtue of controlling polymeric architecture,37 and also within hybrid nanostructures, by exploiting the macromolecular chain assembly on (inorganic) particle surfaces.38 The versatility of the RAFT process to form stimulus-responsive polymers with controlled polymeric architecture has been reviewed recently,39 and RAFT polymers are known to attach to AuNPs with their sulfur-containing end groups.38,40,41 On the basis of these considerations, we introduce here planet−satellite arrangements of AuNPs which are connected by star block copolymers comprising inner blocks of thermoresponsive PNiPAAM and outer blocks of poly(N,N-dimethylacrylamide) (PDMAAM), a star polymer design which is already known in the literature.42 The PDMAAM blocks remain hydrophilic in the temperature range used in this study and may hence provide colloidal stability to the hybrid structures (see Scheme 1). We demonstrate that such approach allows for the preparation of stimulus-responsive multiparticle nanostructures.

2. RESULTS AND DISCUSSION Synthesis of Nanohybrid Materials from Star Block Copolymers. The star block copolymers used in this work as both colloidal stabilizer and stimulus-responsive entities had been prepared by the RAFT technique,43 polymerizing the two B


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Macromolecules different monomers in two consecutive steps, following a corefirst R group approach:44,45 Star polymers of NiPAAM were prepared as a precursor material for subsequent chain extension with DMAAM (Scheme 1). The star block copolymers obtained in such a way will be composed of the PNiPAAM blocks attached to the core at the star polymer center and of outer PDMAAM blocks forming the star polymer corona. The individual star polymer arms are terminated with trithiocarbonate moieties. Successful star block copolymer formation can be judged from the molar mass dispersity remaining low (see Table 2) together with almost-complete reinitiation after chain extension with the second monomer DMAAM (see Figure S1 for SEC traces). Hybrid AuNP-core−star-block-copolymer-shell particles were obtained following a “grafting-to” approach, during which the RAFT polymers attach to the gold surface with their TTC end groups.38,46,47 The AuNPs used here for core−shell particle formation had an average radius of approximately 8 nm: RTEM = 8.2 ± 0.4 nm; RSAXS = 7.5 ± 0.8 nm (see Supporting Information Figures S2 and S3 for details). We have demonstrated recently that a significant proportion of star polymer termini in 3-arm star polymers remain free (not surface-bound) after grafting to the AuNP surface and can therefore be exploited as binding sites for the addition of smaller (satellite) particles.30 Hence, planet−satellite nanostructures (see Figure 2a for an exemplary TE micrograph) were formed by adding smaller AuNPs to the precursor core− shell particles and stabilizing their reactive surfaces with linear PDMAAM (see Table 2). The planet−satellite nanostructures formed as smoothly as in our previous work, in which starshaped PNiPAAM homopolymer was used as particle linker.29,30 (For detailed information about the material synthesis see the Supporting Information.) Thermoresponsive Properties of the Nanohybrid Materials. Hybrid particles with core−shell architecture from star block copolymers and AuNPs were analyzed regarding their temperature-induced switching behavior with dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS). The SAXS measurements were undertaken in order to test if the core−shell particles form larger agglomerates in colloidal dispersion after heating, which would result in the presence of a particle structure factor.48,49 Data from such measurements are shown in the bottom part of Figure 1. The scattering curves for the core−shell particles are almost identical at both temperatures and so are consequently also the corresponding pairdistance distribution functions (PDDFs), which we obtained via indirect Fourier transformation50 of the measured scattering data. From this we can conclude that the individual core−shell particles remain colloidally stable above the LCST of PNiPAAM and do not agglomerate into larger structures. These SAXS data do however not contain information about structural changes in the polymer shell, which is due to the negligible contribution from the PNiPAAM to the scattering in comparison with the gold. In this sense, DLS measurements can complement the SAXS experiments for studying these systems and were therefore used for evaluating a possible swelling or contraction of the polymeric layer. These DLS measurements confirm the colloidal stability of the core−shell particles, which is evident from a constant average detector count rate (see Table S1). This count rate is proportional to the average aggregation number to a good approximation, and from its constant value in the temperature range from 20 to 40 °C we can conclude that no aggregation takes place throughout these

Figure 1. Top: schematic scheme illustrating the expected morphological changes of the 3-arm star block copolymer shell on the surface of AuNPs (16 nm). Middle: dynamic light scattering results for the illustrated core−shell particles in a temperature range from 20 to 40 °C (the dashed gray lines serve the purpose of only guiding the eye). The apparent hydrodynamic radii, RHapp, correspond to average values of volume-weighted size distributions. Bottom: small-angle Xray scattering results for the core−shell particles at 20 and 40 °C. The main graph displays PDDFs obtained from the scattering data, which is shown as (smeared) scattering curves in the inset in the upper right corner.

measurements, which is in perfect agreement with the SAXS data. Volume-weighted average apparent hydrodynamic radii were obtained from the DLS data and are displayed in the middle part of Figure 1. These data clearly demonstrate a shrinking of the core−shell particles from around 33 nm to approximately 29 nm after increasing the temperature beyond 32 °C. The obtained value of 33 nm for the core−shell structure in the fully swollen state is in reasonable agreement with simulated radial polymer shell densities which we had obtained earlier for such systems.30 The value of the LCST of PNiPAAM is known to decrease with increasing polymer concentration51 and arm number in the case of star polymers,52 and it is known to be molar mass dependent for relatively low molar masses.53 All of these factors are hence expected to contribute to the LCST behavior observed here. The expansion and contraction of the polymer layer can be performed C


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Figure 2. (a) Exemplary TE bright-field image showing the planet−satellite arrangement in the dried state after drop-casting from colloidal dispersion. (b) Schematic scheme illustrating the expected structural changes in planet−satellite nanostructures upon heating to 40 °C in aqueous dispersion. (c) Exemplary small-angle X-ray scattering results for the planet−satellite structures at 20 and 40 °C. The PDDFs shown in the main plot reveal a contraction of the planet−satellite distance upon heating. The inset in the upper right shows the corresponding (smeared) scattering curves.

reveal a repeated contraction and expansion of the planet− satellite distance during the heating/cooling cycles. In this sense these nanostructures show thermoreversible properties, at least during two full heating/cooling cycles. The planet− satellite edge-to-edge distance can be expanded and contracted by approximately 20% in these colloidal actuators studied here, which is comparable to the strain typically produced by mammalian muscle cells.55

repeatedly, as demonstrated from a heating/cooling cycle experiment performed with the core−shell particles and followed by DLS (see Figure 3A). SAXS measurements were also performed with planet− satellite nanostructures derived from the core−shell particles by addition of smaller AuNPs. Here, subtle differences can be clearly observed in the scattering, which can be attributed to temperature-induced changes in the AuNP arrangement structure (see exemplarily in Figure 2). These differences can be best understood by comparison of the small-angle data in real space: The PDDFs in Figure 2 reveal a second peak at larger r values together with a peak centered at around 8 nm. The peak at approximately 8 nm contains contributions from the individual planet and satellite particles, where the planet particles dominate however due to their size (see for comparison also the PDDF for individual planet particles in Figure 1); the peak at larger r contains contributions from planet−satellite distances.30,54 Hence, information about the planet−satellite (and satellite−satellite) distances in these nanostructures are contained in these PDDFs, and we had recently described how this information can be extracted together with the average number of satellite particles per planet.30 We performed such analysis for SAXS measurements in which the temperature was repeatedly switched between 20 and 40 °C (Figure 3B). These data reveal a constant average number of 7−8 satellite particles per planet particle, thus proving the structural integrity of the planet−satellite arrangements during switching. They also reveal a modulation of the planet−satellite distance during the heating/cooling cycles (see Table 1 for these SAXS results). This modulation is manifested by, first, an increased planet−satellite distance after a full heating/cooling cycle, which indicates a rearrangement of the satellite particle configuration. Second, the SAXS results also

3. CONCLUSION We have developed a strategy for the postformation regulation of higher-order nanoparticle arrangements, exploiting the thermoresponsive properties of PNiPAAM. For this purpose, we introduced here RAFT star block copolymers as nanoparticle linker and responsive entities, in which the responsive PNiPAAM is located around the star core, whereas nonresponsive PDMAAM blocks in the star corona provide colloidal stability to the prepared nanoarchitectures. Our concept has been demonstrated using PNiPAAM as thermoresponsive polymer and AuNPs as inorganic particles, but we expect it to be useful for the preparation of different reconfigurable nanohybrid materials as well. This work therefore highlights the potential and versatility of RAFT polymers for preparing well-defined multiparticle systems: RAFT polymers allow a straightforward modular nanomaterial synthesis, and at the same time they can provide chemical flexibility, structural fidelity, and stimulus-responsiveness. 4. METHODS Small-Angle X-ray Scattering (SAXS). SAXS measurements were conducted employing a SAXSess camera (Anton Paar, Graz, Austria) and sealed tube X-ray generator (DebyeFlex3000) operating at 40 kV and 50 mA. The divergent polychromatic X-rays were focused D


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into a line-shaped X-ray beam (λ = 1.54 Å, Cu Kα radiation) using a Goebel mirror. Scattering profiles were recorded with a CCD camera (2084 × 2084 pixels, pixel size 24 μm × 24 μm, PI-SCX, Roper Scientific, Germany). The measurements were performed in aqueous dispersion four times for 10 min and averaged. The planet−satellite nanostructures were found to interact with the inside wall of glass capillaries at elevetaed temperature, which is the reason why measurements on planet−satellite nanostructures were performed in plastic capillaries. Scattering patterns were edited by subtracting solvent background and integrated into one-dimensional scattering functions. Dynamic Light Scattering (DLS). DLS measurements were performed in aquaeous dispersion using a He−Ne laser (λ = 633 nm) at a detection angle of 90°. Intensity distributions were obtained by inverse Laplace transformation of the correlation data using the Ortlight program package (TU Graz)56 and subsequently transformed into volume-weighted distributions. This procedure avoids artifacts resulting from afterpulsing of the photodiode used in this work. Synthesis of RAFT Star Block Copolymers. For the preparation of the precursor star polymer of NiPAAM, star RAFT agent (1 equiv), NiPAAM monomer (250 equiv), and AIBN (0.1 equiv) were dissolved in 72 wt % DMF. The mixture was purged with argon before it was heated to 60 °C for 180 min. Polymerization was stopped by cooling and exposing the mixture to air. The polymeric material was isolated by 3-fold precipitation into cold diethyl ether and used as precursor for the chain extension with DMAAM monomer in the next step. The block copolymers were obtained by mixing PNiPAAM precursor (100 mg), DMAAM (198.3 mg), and ACCN (0.244 mg) in 67 wt % DMF. The mixture was purged with argon before it was heated to 100 °C for 60 min. The polymeric material was isolated by 3-fold precipitation into cold diethyl ether. For SEC characterization results of the polymer samples see Table 2.

Table 2. Characterization Results (i.e., Number-Average Molar Masses, Mn, and Dispersity Values, Đ) for the Different Polymer Samples Used in This Worka polymer

Mn/(103 g/mol)

Đ

3-PNiPAAM 3-(PNiPAAM-b-PDMAAM) 1-PDMAAMb

15 30 8

1.18 1.41 1.33

a

Apparent average molar masses and dispersity values as measured by SEC (N,N-dimethylacetamide as mobile phase, PMMA calibration, refractive index detection). For the star polymer samples, average molar mass values were corrected by multiplication with a form factor in order to compensate for the star topology.57 bThis polymer had been obtained in previous work.30

Figure 3. (A) Reversible switching of the star block copolymer shell in responsive core−shell structures as monitored by DLS. The apparent hydrodynamic radii, RHapp, correspond to average values of volumeweighted size distributions. (B) Reversible switching of the planet− satellite distance monitored by SAXS in planet−satellite structures derived from the core−shell structures shown in (A). The dashed gray lines serve the purpose of only guiding the eye.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01267. Details about the chemicals employed, synthetic procedures, SEC traces, and additional TEM, SAXS, and DLS data (PDF)

Table 1. Results from SAXS Measurements of Planet− Satellite Structures in Which the Temperature Was Repeatedly Switched below and above the LCST of PNiPAAM no. of cycle

T/ °C

planet−satellite edge-to-edge distance

no. of satellite particles

0 0.5 1 1.5 2

20 40 20 40 20

14 11.5 15.5 13.5 16.5

7.5 7.5 7.5 7.5 7.5

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AUTHOR INFORMATION

Corresponding Author

*(C.R.) E-mail [email protected]; phone +43 (0) 316 873 8346. ORCID

Christian Rossner: 0000-0002-3428-3542 E


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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the TU Graz for access to the SAXS setup and Dr. Manfred Kriechbaum for support with the measurements. C.R. acknowledges support from the Leopoldina Fellowship Programme, German National Academy of Sciences Leopoldina (Project No. LPDS 2017-02).

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