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Thermoreversible Surface Polymer-Patches: A Cryo-TEM Investigation Christian Rossner, Ilse Letofsky-Papst, Andreas Fery, Albena Lederer, and Gerald Kothleitner Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01742 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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Thermoreversible Surface Polymer-Patches: A CryoTEM Investigation Christian Rossner,*,† Ilse Letofsky-Papst, †,‡ Andreas Fery,§,ǁ Albena Lederer,# and Gerald Kothleitner†,‡ †Institute for Electron Microscopy & Nanoanalysis and ‡Center for Electron Microscopy, Graz University of Technology, NAWI Graz, Steyrergasse 17, A-8010 Graz, Austria. §Leibniz-Institut für Polymerforschung Dresden e.V., Institute of Physical Chemistry and Polymer Physics, Hohe Str. 6, D-01069 Dresden, Germany. ǁCluster of Excellence Centre for Advancing Electronics Dresden (cfaed), Technische Universität Dresden, D-01062 Dresden, Germany. #Leibniz-Institut für Polymerforschung Dresden e.V., Institute of Macromolecular Chemistry, Hohe Str. 6, D-01069 Dresden, Germany. Corresponding Author *(C.R.) E-mail: [email protected].

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ABSTRACT

Hybrid

core–shell

type

nanoparticles

from

gold

nanoparticle

cores

and

poly(N-

isopropylacrylamide) shells had been investigated with regard to their structural plasticity. Reversible Addition–Fragmentation Chain Transfer (RAFT) polymerization was used to synthesize well-defined polymers that can be readily anchored onto the gold nanoparticle surface. The polymer shell morphologies were directly visualized in their native solution state at high resolution by cryogenic transmission electron microscopy and the microscopic results were further corroborated by dynamic light scattering. Different environmental conditions and brush architectures are covered by our experiments, which lead to distinct thermally induced responses. These responses include constrained de-wetting of the nanoparticle surface at temperatures above the lower critical solution temperature of poly(N-isopropylacrylamide), leading to surface polymer-patches. This effect provides a novel approach toward breaking the symmetry of nanoparticle interactions and we show first evidence for its impact on the formation of colloidal superstructures.


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1 Introduction Hybrid Nanoparticles and, in particular, polymer-coated inorganic nanoparticles1–3 represent a class of materials with a rich structural diversity and phenomenology. This diversity mostly stems from the possibility to tailor these materials by means of targeted macromolecular design of the polymer component, enabling control over polymeric architectures on nanoparticle surfaces4–7 and allowing the introduction of desired functionalities,8–10 which in turn determine the properties of the hybrid nanomaterials. For example, the soft polymer coating can be used to generate adaptive systems, in which external stimuli regulate interactions with the environment, resulting in specific responses.11,12 This can lead to e.g. triggered self-assembly,13,14 switchable nanoparticle arrangement structures,15–17 induced activation of surface functions,18 and many more. Besides, adaptivity to the environment can also be used to impart structural heterogeneity to polymer layers on surfaces by controlling the interface regime between the surface and the polymer coating. In situations where the surface wettability of a polymer is bad, this polymer will retract from the surface and de-wetting will occur.19–21 When the macromolecules are not end-grafted to the surface, the associated length scales for the dewetting will typically be orders of magnitude larger compared with the size of the macromolecules. For surface-grafted macromolecules, on the other hand, their de-wetting will be constrained, and as a result of that a variety of distinct nanoscale surface patterns can be theoretically predicted to result from such constrained de-wetting,22,23 including (in the order of increasing polymer grafting density) pinned micelles,24 pancake micelles,25 worm-like micelles,24,26 and holey layers.25,26 This predicted behavior can be reproduced in experiments.27–29 The group of Eugenia Kumacheva has recently reported a new


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paradigm for nanoparticle patterning, by demonstrating how this behavior can be translated to nanoparticle surfaces.30–32 According to this recent work, distinct nanoparticle surface patterns can be obtained by controlling the polymer’s degree of polymerization, grafting density, and the nanoparticle’s dimension and shape.30,31 This versatile approach offers exciting opportunities in the realm of polymer–inorganic hybrid nanoparticles, which are yet to be explored. For comparably small nanoparticles, the constrained de-wetting approach can lead to single pinned micelles on nanoparticle surfaces,30,32 i.e. Janus-type nanoparticles.33 Janus-type nanoparticles recently received much focus and had been suggested as promising candidates for surface modification34 and catalytic applications,35 among others. Hence, from an application-oriented perspective but also from a fundamental point of view, the availability of Janus-type structures with controllable dynamic properties is highly desirable. Based on these precedents, we intended to broaden the range of polymers and available external stimuli to trigger polymer surface pattern formation via constrained de-wetting. Because of their lower critical solution temperature (LCST) in water which results in a hydrophilic-to-hydrophobic transition at approx. 33 °C and atmospheric pressure,36,37 polymers of N-isopropylacrylamide (NiPAAM) were envisaged for the formation of thermally activated Janus-type nanoparticles. Such thermally induced surface structure formation is advantageous, as this stimulus can be easily applied and avoids the drawbacks of waste accumulation associated with chemical triggers. Although hybrid systems of gold nanoparticles (AuNPs) with grafted poly(NiPAAM) chains have been excessively studied,7,37–43 structures with laterally inhomogeneous PNiPAAM shells have not been reported to date. To address this point, the structural plasticity of gold–


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poly(NiPAAM) hybrid nanoparticles was evaluated in this work, with particular emphasis on the possibility of thermally induced surface patterning.

Scheme 1. Possible scenarios for poly(NiPAAM) brushes grafted on a gold nanoparticle surface with their trithiocarbonate groups44 upon raising the temperature above the polymer’s LCST: Uniform brush collapse as illustrated on top and already described by others39,40 and constrained de-wetting of the particle surface as shown at the bottom (this work). [Note: This schematic is not drawn to scale. The gold nanoparticles studied in this work accommodate approx. 200 or 300 polymeric chains on their surface. Also, the average contour length7,45 of the poly(NiPAAM) macromolecules employed here is about 4.5 times larger compared with the average diameter of the gold cores.]


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2 Experimental Section Materials 2-Bromopropionic acid, carbon disulfide, cetyltrimethylammonium bromide, n-hexanethiol, and sodium citrate tribasic dihydrate (Aldrich) were purchased at the highest purity available and used as received. Hydrogen tetrachloroaurate trihydrate (ABCR, 99.9 %) and magnesium sulfate (Grüssing) were used as received. The RAFT agent 1 (see Scheme 2) used in this work was synthesized in analogy to a previously reported procedure46 with slight modifications (for details see supporting information). Gold nanoparticles used in this work were obtained from citratereduction, which is described in detail in the supporting information. AIBN (Akzo Nobel) and Nisopropylacrylamide (Aldrich) were recrystallized from methanol and toluene/petrolether (3:1), respectively, and stored at 4 °C prior to use. Acetone, diethyl ether, hexane, methanol, and toluene (p.a. grade, Fisher) were used as received. N,N-Dimethylformamide (p.a. grade, Fisher) was freshly distilled prior to use. Nanopure (type I) water was obtained using a Millipore filtration system. Instrumentation Cryogenic Transmission Electron Microscopy (Cryo-TEM). Cryo-TEM characterization was performed on a FEI T12 microscope applying an acceleration voltage of 120 kV. To enable visualization of the polymer shell, samples were stained with phosphotungstic acid47 (1 %wt) before being applied onto Pacific Grid-Tech® copper grids holding a film of lacey carbon at temperatures of 21 °C or 45 °C and 99 % relative humidity using a Leica® EM GP plunge freezer (Wetzlar, Germany), and then blotted on filter paper for


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1 s. The blotted samples were allowed to rest for 1 s before immersion into liquid ethane. The vitrified samples were stored in liquid nitrogen prior to the TEM characterization. Dynamic Light Scattering (DLS). DLS measurements were carried out partly with a WYATT DynaPro NanoStar (Wyatt Technologies, USA), and partly with an Anton Paar LitesizerTM 500 Particle Analyzer. Both DLS setups used a laser of wavelength λ = 658 nm and scattered intensity was detected at an angle at an angle of 90°. All measurements were performed in Millipore® water at a gold concentration of 0.1 mg/mL. Size-Exclusion Chromatography (SEC). Molar mass characterization was performed using a SEC system consisting of HPLC pump 1200 (Agilent Technologies, US) and PolarGel-M column (300x7,5 mm, Polymer Labs, UK) coupled to differential refractive index detector K-2301 (Knauer, DE) and multiangle light scattering detector TREOS II (Wyatt Technology, US). As an eluent N,N-dimethylacetamide with 3 g/L LiCl in a flow rate of 1.00 mL/min was used. UV/visible Extinction Spectroscopy. UV/visible

extinction

spectroscopy

was

performed

with

a

Shimadzu

UV-1800

photospectrometer, using Hellma quartz cuvettes. The measurements were performed at room temperature, in steps of 0.5 nm. Measurements of the pure solvent were used for background subtraction.


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Polymerization See scheme 2. The RAFT agent, AIBN, NiPAAM, and DMF were weighed into polymerization flasks (the amounts are given in Table 1) and the mixture was purged with nitrogen for 10 minutes. Polymerization was initiated by heating to 60 °C and stopped after 7 hours by cooling on ice and exposing the mixture to air. The polymeric material was isolated by threefold precipitation into diethylether. Table 1: Polymerization conditions and characterization results for the RAFT polymer of NiPAAM used in this work.

a

[DMF] : [NiPAAM]0 : [RAFT]0 : [AIBN]0

t / min

θ / °C

Mn / (103 g mol–1)a

Đb

1800 : 400 : 1 : 0.1

420

60

31

1.04

By end group analysis based on the UV absorbance of the trithiocarbonate chromophore.

b

Determined via SEC.

Scheme 2: Synthetic scheme for the preparation of the RAFT polymer of NiPAAM used in this work.


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Functionalization of gold nanoparticles Surface pre-functionalizatiion with CTAB. An aqueous CTAB solution (250 mM, 0.7 mL) was added to a solution of citrate-reduced gold nanoparticles (2.8 mL) under sonication at room temperature. The mixture was incubated for 1 hour and then subjected to two centrifugation–redispersion steps (14800 g, 20 min, room temperature, redispersion in aqueous CTAB solution (50 mM)). Preparation of sample I. An aqueous solution of poly(NiPAAM) (1 mL, 3 mg mL–1) was added to an as-prepared citratereduced gold nanoparticle dispersion (4 mL) under sonication at room temperature. The mixture was incubated overnight in the dark at room temperature before being subjected to three centrifugation–redispersion steps (14800 g, 20 min, room temperature, first and second step: redispersion in methanol, third step: redispersion in water). Preparation of sample II. Gold nanoparticles pre-functionalized with CTAB were washed by centrifugation and 90 % of the supernatant were replaced by water. An aqueous solution of poly(NiPAAM) (225 µL, 0.15 mg mL–1) was added to this washed solution (900 µL) of CTAB-functionalized gold nanoparticles under sonication at room temperature. The mixture was incubated overnight in the dark at room temperature before being purified by centrifugation–redispersion (14800 g, 20 min, room temperature, redispersion water).


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3 Results and Discussion Synthesis of Hybrid Nanoparticles Herein, we used RAFT polymerization48 for the synthesis of polymers of NiPAAM with narrowly dispersed molar masses and high end-group fidelity.49,50 Number average molar mass, Mn, of the produced polymer was analyzed by UV/visible extinction spectroscopy,51 based on the extinction coefficient of the trithiocarbonate chromophore (ε307 nm (MeOH) = 15100 L mol–1 cm–1,

Mn(poly(NiPAAM)) = 31 kg mol–1,

see

supporting information). Besides controlling the polymerization, the trithiocarbonate RAFT groups on the ω-end of the polymeric chain also allow efficient immobilization of such polymers onto the surfaces of AuNPs.44,52,53 In this work, we employed AuNPs from citrate-reduction (Turkevich method,54 15 nm), either as-prepared (i.e. capped with citrate ligands55 and oxidized form of citrate)56 or after surface modification with cetyltrimethylammonium

bromide

(CTAB),

as

substrates

for

the

grafting

of

poly(NiPAAM). The functionalization of citrate-reduced AuNPs with organic halide salts is a known surface modification reaction, which proceeds via binding of the halogen ion to gold.57,58 Successful AuNP surface modification with CTAB can be judged from a redshift of the plasmon band maximum from approx. 518 nm for citrate-capped AuNPs to approx. 524 nm for CTAB-functionalized AuNPs, which can be attributed to the changing dielectric environment upon binding of CTAB (see Figure S6). Recent experimental and theoretical work30 disclosed the importance of grafting density for polymer brush collapse on nanoparticle surfaces under bad solvent conditions. In particular, surface de-wetting and formation of polymer patches is expected only below a


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threshold value of grafting density, whereas above this threshold value bad solvent conditions lead to uniform brush collapse. Considering this, our experiments cover two different grafting densities that were controlled by adjusting the polymer feed in the nanoparticle functionalization reactions. Table 2 displays the associated experimental conditions for the fabrication of two types of hybrid polymer–gold nanoparticles that were investigated in detail in this study.

Table 2. Synthesis conditions for the preparation of gold– poly(NiPAAM) hybrid nanoparticles and results from the determination of polymer grafting densities, σ. sample

Surface pre-functionalization

poly(NiPAAM) feeda / nm–2

σb / nm–2

I

None (as-prepared citrate-reduced AuNPs)

7.7 (large excess)

0.42

II

Pre-functionalization with CTAB

0.38

0.27

a

Number of poly(NiPAAM) macromolecules per nm2 AuNP surface in the grafting

procedure (based on the amount of gold salt used in the AuNP synthesis). bPolymer grafting density of the purified hybrid particles (determined by cryo-TEM, vide infra).

Polymer Shell Morphologies of Hybrid Nanoparticles To investigate the structure of the hybrid nanoparticles in the colloidally dispersed state at different temperatures, we used plunge freezing and cryo-TEM,59–61 allowing for in-situ characterization of polymer shell morphologies at solid surfaces.62 Figure 1 displays such cryoTEM images obtained from aqueous particle dispersions that were vitrified below and above the LCST of poly(NiPAAM) (after heating to 45 °C for 90 minutes). In both situations, the core– shell structure is clearly visible. Results from the analysis of several micrographs are shown in


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the form of diameter histograms at the bottom of Figure 1. These data reveal a shrunken and thus compacted polymer shell at 45 °C in comparison to the system being quenched at 21°C (i.e., a contraction of the average diameter of the core–shell particles from 36.2 nm to 31.2 nm, corresponding to a volume contraction of the polymer shell to approx. 60 % of its initial value). This shrinkage can be attributed to the fact that the grafted polymer chains are no longer swollen by the solvent (water) above the LCST,63,64 but expel the water and precipitate toward the particle surface. It is reasonable to assume a spherical 3-dimensional shape for the polymer canopy at temperatures above the LCST, as this would minimize unfavorable interactions with the solvent. With the assumption of a spherical shape and of a compact, not swollen polymer shell, polymer grafting density, σ, can be obtained from cryo-TEM images as such:

=

CS AuNP ∙∙A

(1)

∙AuNP

Here, VCS is the average volume of the core–shell particles, VAuNP and AAuNP are average volume and average surface of gold nanoparticle cores, respectively, ρ and Mn are the polymers density and number average molar mass, respectively, and NA is the Avogadro constant. Values obtained in such way are given in Table 2. The σ-value for sample I, i.e. after adding large excess of poly(NiPAAM) to citrate-capped AuNPs, is in reasonable agreement with a value recently determined by thermogravimetric analysis using equal preparation conditions, but slightly lower polymer molar mass (σ = 0.50 nm–2 for Mn(poly(NiPAAM)) = 26 kg mol–1),65 thus confirming the suitability of determining grafting densities from the analysis of cryo-TEM images via equation 1.


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Figure 1. Exemplary cryo-TEM images of hybrid gold– poly(NiPAAM) nanoparticles (sample I) after plunge freezing at 21 °C and 45 °C (top) and associated diameter histograms, together with sample mean diameter, standard deviation, and Gauß-fit to the data (bottom). 45 particles were analyzed for the sample that was quenched at 21 °C and 28 particles were analyzed for the sample that was quenched at 45 °C. To corroborate the TEM results for sample I, the volume transition of the grafted polymer was also investigated by dynamic light scattering. Average hydrodynamic diameters determined via DLS are consistently larger compared with the cryo-TEM values (Table 3), which can be attributed to the overrepresentation of large structures in the (intensity-weighted) DLS data. Furthermore, DLS measurements may be more sensitive to few macromolecular chains that


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extend farther from the nanoparticle surface than the average of chains, thus enlarging the hydrodynamic diameter. Such single dangling chains may not be detectable in cryo-TEM images.60,66 Nevertheless, intensity-weighted particle size distributions (as exemplarily shown in Figure 2a) reveal the same effect as already observed by means of cryo-TEM, i.e. the sizereduction of the hybrid core–shell particles upon heating above the LCST of poly(NiPAAM), as had already been observed by others.39,40 This transition is fully reversible: Upon cooling to 21 °C again, the initial solvated and swollen polymer shell is recovered. This can be repeated for several heating/cooling cycles (Figure 2b). 55

(b)

(a) 50

0.8 0.6

45

0.4

40 35

0.2 0.0 0 10

21 °C 45 °C

Dh / nm

1.0

intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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21 °C 45 °C 1

10

2

10

3

10

hydrodynamic diameter / nm

4

10

30 0

1

2

3

4

5

no. of cycle

Figure 2. (a) Exemplary DLS results for hybrid gold–poly(NiPAAM) nanoparticles (sample I) above and below the LCST of poly(NiPAAM). (b) Repeated contraction and swelling of the hybrid particles during several heating/cooling cycles (30 min equilibration time at each temperature). Interestingly, for sample II which is freshly prepared and directly measured, a heating-induced increase in the average apparent hydrodynamic particle diameter as obtained from DLS measurements is observed (Table 3), which is opposite to the behavior found for sample I. This


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result can be understood after evaluating scattering intensities (Figure 3). For sample I, the light scattering intensity remains almost constant upon increasing the temperature, indicating that no particle agglomeration occurs, whereas the increased scattering intensity at elevated temperature observed for sample II suggests particle agglomeration.40 When sample II is stored at room temperature and measured two weeks after its preparation, DLS also reveals a relatively large fraction agglomerated structures (see Figure S7). These observations indicate a poor colloidal stability of the core–shell particles in sample II. Table 3. Overall size of gold–poly(NiPAAM) hybrid nanoparticles as obtained from DLS and cryo-TEM.

a

sample

DDLS (21 °C)a

DDLS (45 °C)a Dcryo-TEM (21 °C)

Dcryo-TEM (45 °C)

I

49 nm

34 nm

36 nm

31 nm

II

36 nm

58 nm

27 nm

27 nm

Obtained as average value of intensity-weighted size distributions.


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count rate × 10−2 / kcounts s−1

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12 10 II I

8 6 4 2 0 20

25

30

35

40

45

Temperature / °C Figure 3. Light Scattering intensities for samples I and II at temperatures of 21 °C and 45 °C detected at a scattering angle of 90°. The formation of particle agglomerates in sample II at 45 °C is not the sole difference to sample I. For aliquots of sample II that were vitrified at 21 °C, polymer shells uniformly surrounding the gold cores were always found, as already observed for sample I. However, for core–shell particles of sample II quenched at 45 °C (after heating at 45 °C for 90 minutes), nonuniform polymer shells in the form of a Janus-type structure were also found. Because cryoTEM provides a random sampling of all conceivable particle orientations, this non-uniform polymer shell was not observed for all spotted particles of the sample. This is however not surprising but expected: Different two-dimensional projections from a three-dimensional object will be produced, when a non-symmetric object is projected from different orientations in space. The experimentally found projections include polymer shells appearing symmetric (as exemplarily shown on the right hand side of Figure 4, micrograph displayed on top, found 18 times experimentally, corresponding to the Janus-type structure illustrated in Scheme 1 being


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projected horizontally), polymer shells appearing located on one side of the particle and fully depleted on one other side (as exemplarily shown on the right hand side of Figure 4, micrograph displayed at the bottom, found 22 times experimentally, corresponding to the Janus-type structure illustrated in Scheme 1 being projected vertically), and intermediate cases (right hand side of Figure 4, micrograph displayed in the middle, found 24 times experimentally). (Micrographs of all counted particles can be found in the supporting information, Figures S8– S10.) The observation of non-uniform polymer shells points to a surface de-wetting which is triggered by reducing the solvent quality for the poly(NiPAAM) polymer brushes via heating the aqueous solutions above the polymer’s LCST. Because the polymers are strongly affixed to the gold surface due to the trithiocarbonate−gold interaction,44,52,53,65 the de-wetting is constrained and surface polymer-patches evolve. This surface patch formation can be reverted. When samples of II that were previously heated (45 °C, 90 minutes) were cooled to room temperature and left undisturbed overnight, plunge freezing at 21 °C of these samples reveals fully symmetric polymer shells for all spotted particles without exception (see Figure 4).


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Figure 4. Exemplary cryo-TEM images of hybrid gold– poly(NiPAAM) nanoparticles (sample II) after plunge freezing at 21 °C, after heating to 45 °C for 90 minutes, and after cooling the heated samples overnight. The scale bar is valid for all micrographs. No reduction in the overall size of the polymer shell was observed in cryo-TEM images for sample II (see Table 3), which is in contrast to sample I. We can only speculate about the reasons for this unexpected behavior. One reason may be the lower polymer grafting density of sample II. This low polymer surface coverage may lead to a polymer shell which becomes too dilute at regions remote from the spherical particle surface to give considerably more contrast compared with the vitreous water background, particularly below the LCST of poly(NiPAAM).


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Besides the observed single isolated core–shell particles (with surface polymer-patches), secondary agglomerated structures—as already anticipated from the DLS results—in which several particles were fused together were also found (see Figure 5).30 Heat-induced agglomeration of Janus-type nanoparticles with poly(NiPAAM) patches was already described by Isojima et al.67 Herein, however, these agglomerates (which dominate the light scattering data) were very rarely observed during the TEM characterization of freshly prepared samples of II.

Figure 5. Exemplary cryo-TEM image of an agglomerated structure composed of individual hybrid core–shell particles at 45 °C.


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4 Conclusion We have studied thermally induced volume transitions of poly(NiPAAM) brushes confined at a nanoparticle surface with regard to emerging polymer shell morphologies. Our results reveal the possibility to thermally induce polymer patch formation of poly(NiPAAM) brushes on nanoparticle surfaces. This provides a convenient alternative to existing chemical triggers for achieving surface polymer-patches. Furthermore, using cryogenic transmission electron microscopy, this work provides—to the best of our knowledge—the first direct evidence of surface polymer-patch formation on nanoparticles via constrained de-wetting in the native solvent environment. This is important as in such cases the patch formation is dominated by polymer–solvent interactions. Our contribution hence underlines the possibility and versatility of using environmentally adaptive polymers to achieve the formation of surface patches on nanoparticles. The surface patch formation could as well be triggered by illumination of the particles at the localized surface plasmon resonance frequency, since excitation of the LSPR is a dissipative process accompanied by local heating of the particle and its environment.68,69 The investigation of these effects is subject to further work and may provide a novel approach toward dynamic colloidal superstructures. ASSOCIATED CONTENT Supporting Information. Additional synthetic procedures, material characterization, UV-vis data, and cryo-TEM images. AUTHOR INFORMATION Corresponding Author


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*(C.R.) E-mail: [email protected]. Funding Sources Leopoldina Fellowship Programme, German National Academy of Sciences Leopoldina (Project No. LPDS 2017-02). Notes The Authors declare no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge Professor Gregor Trimmel and the TU Graz for providing access to their laboratories and equipment and the Zentrum für Elektronenmikroskopie Graz (ZFE) for access to their microscopes. C.R. acknowledges support from the Leopoldina Fellowship Programme, German National Academy of Sciences Leopoldina (Project No. LPDS 2017-02).

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