Introduction

Janus particles have recently attracted considerable attention [1, 2]. Because of the lack of centrosymmetry, Janus systems can form superstructures on different length scales [3]. This behavior opens up many applications in a wide variety of fields ranging from medicine [4], biochemistry [5], and physics [6] to colloidal chemistry [7]. Janus particles can be mainly divided into spherical and nonspherical particles. By now, spherical Janus particles have been investigated intensively [3, 8, 9]. More recently, nonspherical Janus particles have been synthesized including cylinders [10], disks [11, 12], and dumbbell-shaped particles [1, 2, 13]. In general, nonspherical Janus particles have interesting colloidal properties. Thus, Park et al. [14] have reported that nonspherical Janus particles can be used for the stabilization of multiphasic fluid mixtures. The two opposite wetting surfaces of Janus dumbbells may hence be used to obtain emulsions with long-term stability [15]. Janus dumbbells can not only form self-assembled [16] structures but also serve as model systems to investigate the phase behavior and the crystallization [17, 18] of shape anisotropic systems.

Recently, the synthesis of hollow Janus dumbbells has been reported [13, 19, 20]. As nonspherical capsules, hollow Janus dumbbells have potential applications in catalysis [2], delivery of encapsulated materials [1], and as model system for the study of soft objects like red blood cells in motion [21]. Up to now, hollow asymmetrical Janus composite particles were synthesized by template-assisted methods [13, 20, 22]. Incorporation of stimuli-responsive properties [23–25], triggered by pH, temperature, or ions, would be highly useful for hollow Janus particles when discussing their use in, e.g., controlled delivery [26]. Poly(N-isopropylacrylamide) (PNIPAM) is the most widely used polymer for this purpose [27, 28]. However, to the best of our knowledge, thermosensitive hollow Janus dumbbells have not been reported yet.

Here, we present the first report on the synthesis and analysis of hollow thermosensitive Janus colloids. The hollow Janus dumbbells consist of two partially fused PNIPAM spheres: one PNIPAM sphere consists of a copolymer hybrid hollow sphere as core while the other one is a purely hollow PNIPAM sphere. The synthesis is carried out following four main steps shown in Fig. 1: (I) polystyrene (PS) spheres are prepared as seeds and then coated by a copolymer layer of styrene and 3-(trimethoxysilyl) propyl methacrylate (MPS) [29]. In step II, the resulting core-shell particles are swollen with free styrene and then polymerized. The PNIPAM shell cross-linked by N,N′-methylene-bis (acrylamide) (BIS) is attached in step III onto the surface of dumbbell-shaped core particles via seeded emulsion polymerization. Finally, the PS is selectively dissolved in tetrahydrofuran (THF) (step IV). The copolymer layer coated in step I is stable in THF whereas the PS of the core can be removed by THF, resulting in well-defined and monodisperse hollow Janus dumbbells with thermosensitivity. Special emphasis is put on a precise structural analysis of all intermediates used for the synthesis of these particles. Hollow Janus microgel particles will open the way to a wide variety of stimuli-sensitive Janus structures and applications, e.g., as carriers for drugs or nanoreactor for catalysts with temperature-controlled release.

Fig. 1
figure 1

Preparation procedures for thermosensitive hollow Janus dumbbells. The template is prepared in steps I and II and subsequently covered by a shell of PNIPAM. Dissolution of the template in step IV leads to hollow Janus dumbbells

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Experimental section

Materials

Sodium dodecyl sulfate (SDS) (Fluka), potassium persulfate (KPS) (Fluka), MPS (Aldrich), 4-styrenesulfonic acid sodium salt hydrate (NaSS) (Aldrich), 2,2′-azobis (2-methylpropionitrile) (AIBN) (Aldrich), N-isopropylacrylamide (NIPAM) (Aldrich), BIS (Aldrich), and THF (Aldrich) were used as received; styrene (Aldrich) was purified by inhibitor remover (Aldrich). Water used was purified through reverse osmosis and ion exchange (MilliΩ, Millipore).

Synthesis of partial hollow dumbbell-shaped microgels

Dumbbell-shaped microgel

The dumbbell-shaped PS cores were synthesized based on the phase separation and emulsion polymerization [29]. An analysis of TEM images of the corresponding particles gave a length of the long axis of 261 ± 8 nm whereas the radius of one sphere is determined as 91 ± 6 nm. Using the dumbbell-shaped PS core particles as seeds, a shell of PNIPAM cross-linked by BIS is coated onto the surface through seeded emulsion as described in the Supporting information and in our previous work [30].

Thermosensitive Janus hollow dumbbells

Thermosensitive hollow Janus dumbbells were prepared via the removal of the PS part from dumbbell-shaped microgels by dissolution in THF. The dumbbell-shaped microgels were purified via centrifugation with a speed of 8,000 rpm for 20 min and re-dispersed in THF again. The microgels were kept in THF overnight under stirring. Three runs of centrifugation, decantation, and re-dispersion process were scheduled every day until all free PS was completely removed. Finally, the purified dumbbell-shaped microgels were re-dispersed in Millipore water for further investigations.

Methods

Transmission electron microscopy

Transmission electron microscopy (TEM) specimens were prepared by putting ca. 5 μl of a 0.05 wt% solution on a TEM copper grid with carbon support film (200 mesh, Science Services, Munich, Germany). The carbon-coated copper grids have been pretreated by 10 s of glow discharge. The excess of liquid was blotted with a filter paper after 2 min, and the TEM grid was then dried at room temperature for at least 1 h. The specimen was inserted into a sample holder (EM21010, JEOL GmbH, Eching, Germany) and transferred into a JEOL JEM-2100 (JEOL GmbH, Eching, Germany). The TEM was operated at an acceleration voltage of 200 kV. All images were recorded digitally by a bottom-mounted 4k CMOS camera system (TemCam-F416, TVIPS, Gauting, Germany) and processed with a digital imaging processing system (EM-Menu 4.0, TVIPS, Gauting, Germany).

Cryogenic transmission electron microscopy

Cryogenic transmission electron microscopy (cryo-TEM) specimens were prepared by vitrification of thin liquid films supported on a TEM copper grid with lacey carbon film (200 mesh, Science Services, Munich, Germany) in liquid ethane at its freezing point using a Vitrobot Mark IV (FEI, Eindhoven, Netherlands). The specimens were inserted into a cryo transfer holder (Gatan 914, Gatan, Munich, Germany) and transferred into a JEOL JEM-2100 (JEOL GmbH, Eching, Germany). Examinations were carried out at temperatures around 90 K. The TEM was operated at an acceleration voltage of 200 kV, and a defocus of the objective lens of about 4–8 μm was used to increase the contrast. All images were recorded digitally with a bottom-mounted 4*4k CMOS camera (TemCam-F416, TVIPS, Gauting, Germany).

Scanning force microscopy

Scanning force microscopy (SFM) specimens were prepared on silicon wafer substrates by spin coating with 50 rpm if not stated otherwise. Silicon wafers were cleaned in piranha solution. For hollow Janus dumbbells, one monolayer was first formed on the surface of water [31] and then transferred onto the silicon wafers. Images were recorded by operating in PeakForce QNM mode (MultiMode SFM, Bruker Corporation; Nanoscope 8 SFM controller). Silicon cantilevers were used with typical resonance frequencies of 70 kHz and spring constants of 2 N/m (Olympus Corporation). The tips exhibited asymmetric cone shape with the maximum opening angle of 35° and typical apex radius of 7 nm, with an upper limit of 10 nm, as provided by the manufacturer.

Depolarized/dynamic light scattering

Depolarized/dynamic light scattering DDLS/DLS experiments were carried out on an ALV/DLS/SLS-5000 compact goniometer system equipped with a He-Ne laser (632.8 nm). The temperature of the samples was controlled by a thermostat (Rotilabo, ±0.1 °C). For every temperature, at least 1 h was set for the sample to reach equilibrium state. All DLS measurements were done at scattering angles between 20° and 150° with steps of 10°. For DDLS measurements, three runs per angle were set and averaged with an angular step of 1° for scattering angles between 20° and 30°. For scattering angles between 30° and 50°, three runs were set and averaged per angle with a step of 2.5°. The intensity autocorrelation functions were evaluated using the CONTIN software provided by the manufacturer (ALV Correlator Software 3.0).

Results and discussion

Analysis of the core particles

As shown in Fig. 1, dumbbell-shaped microgels were firstly synthesized with a monodisperse dumbbell-shaped PS core and a PNIPAM shell cross-linked by BIS attached onto its surface [30]. TEM and cryo-TEM were used to investigate the dumbbell-shaped PS core particles. TEM images in Fig. S1 indicate that the PS dumbbell-shaped cores are homogeneous by means of the overall size and their aspect ratio [30]. Moreover, the fused spheres in one dumbbell-shaped core particle are nearly identical in size. Based on more than 500 particles in TEM images, the length of the long axis is measured to be 261 ± 8 nm and the radius of one sphere is determined as 91 ± 6 nm.

The cryo-TEM micrograph in Fig. 2a demonstrates that the dumbbell-shaped PS core particles consist of two partially fused spheres with different structures: One side consists of a homogeneous sphere while the other side reveals a double shell structure. Figure 2b shows the gray scale profile plot which takes the background arising from the vitrified water into account [32]. Firstly, the minimum of the gray scale profile of the core-shell sphere (right-hand side in Fig. 2b) is higher than that for the other sphere, which reveals that the right sphere shows a lower contrast in cryo-TEM. The difference in the gray scale profile points clearly to a difference in composition. Secondly, the gray scale profile shows a shoulder at the edge of the right-hand sphere of the dumbbell (red arrow in Fig. 2b) indicating a core-shell structure with a double shell. From step I (see Fig. 1) of the synthesis, it is evident that the shell is a copolymer layer of MPS and styrene. This copolymer shell is designed to facilitate the growth of the second PS sphere. The cryo-TEM image in Fig. 2 proves that this copolymer shell is well kept after the growth of the second PS sphere thus corroborating the mechanism of the synthesis proposed by Park et al [29]. Hence, Fig. 2 provides the first direct proof for the Janus-type nature of the dumbbell-shaped core particles using cryo-TEM.

Fig. 2
figure 2

Cryo-TEM micrograph of a PS dumbbell core particle (a) and its corresponding normalized gray scale profile (b) along the long axis (indicated by the solid line in a). Along the gray profile, a red arrow was used to mark the shoulder on the right side. A homogeneous shell of the core-shell-structured part (right-hand side) is visible. This shell corresponds to the random copolymer of styrene and MPS that was incorporated in step I of the synthesis

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Removal of the cores

In the following, the dissolution process of the Janus-type dumbbell cores using THF will be discussed in detail since this process is a crucial step for the generation of hollow PNIPAM Janus microgels (see Fig. 1). We first treat the dumbbell-shaped core particles with THF. This leads to the formation of hollow spheres as shown by TEM analysis (see Fig. S1a). These spheres indicate that the pure PS sphere can be easily dissolved in THF while the copolymer hybrid shell retains its spherical morphology. The dissolution of PS is not complete after the first cycle cleaning with THF which can be deduced by PS leftover around the spherical particles showing as white domains as observed in Fig. S2a. By monitoring the cleaning process after each cycle, it was found that in total 30 runs of centrifugation, decantation, and re-dispersion are necessary to achieve complete removal of the core material. No traceable amounts of polymer are found around the spherical particles anymore after this procedure (see Fig. S2b). The cleaning process finally leads to the formation of spherical particles showing a rather low contrast in TEM (Fig. S2). These spherical particles correspond to thin hybrid shells of PS-co-PMPS copolymer, which is originally coated onto the PS sphere surface in step I.

To address this point in further detail, the particles were vitrified after removal of the PS core directly in THF. A cryo-TEM micrograph of a particle vitrified in THF (Fig. 3a) indicates a spherical particle with a hollow structure after the removal of PS via THF from the dumbbell-shaped core. The hybrid shell of the resulting hollow sphere exhibits only ca. 10 nm thickness and is difficult to discern from the background. Nevertheless, the gray scale profile along the center of the particle (see Fig. 3b) indicates clearly the decrease in contrast towards the center of the gray scale profile, which is typically found for hollow spheres. Thus, a hollow structure of the copolymer sphere is formed after the removal of the PS core. We therefore conclude that the PS-co-PMPS copolymer layer cannot be dissolved by THF whereas pure PS from the inner part is able to diffuse through this layer, resulting in hollow spheres. The insolubility of the hybrid shell is due to the hydrolytically cross-linked methoxy silyl groups of MPS copolymer. The organic-inorganic network of the hybrid shell has already been reported in detail by E. Bourgeat-Lami et al. [33].

Fig. 3
figure 3

a Cryo-TEM micrograph of a dumbbell-shaped core vitrified in THF after the removal of PS via THF. Spherical hollow particles are remaining. b The corresponding gray profile plot (yellow line) shows a characteristic profile for hollow spherical particles. c SFM height image of dumbbell-shaped cores after the removal of PS via THF and d the corresponding cross section along the blue line in c showing different collapsed states of isolated (marked by 1 and 3) and agglomerated particles (marked by 2) in the dried state

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Further details can be deduced from SFM measurements. Figure 3c displays the SFM height image of a selected area demonstrating two different morphologies of particles on the surface. The isolated particles have mostly pancake-like morphology. The one exemplified in Fig. 3c (marked by 1) has the height of ca. 70 nm and diameter of ca. 280 nm. The upper limit of the tip broadening by the conical tip can be estimated as tan(α)h, where h is the height of the object and α is the cone opening, which is 50 nm for the tips we used. Substantial flattening of isolated particles on the surface indicates their hollow nature. Particles in clusters exhibit typically donut or red blood cell topography (marked by 2 in Fig. 3c). Some isolated particles are slightly deformed, as the one exemplified in Fig. 3c (marked by 3). SFM measurement demonstrates that single, hollow particles lying dry on the substrate collapse, resulting in the pancake surface morphology because the thin shell of hollow spheres cannot support their spherical morphology. In a more dense packing, a supporting of the wall by neighboring hollow particles results in donut or red blood cell topography.

Hollow thermosensitive dumbbells

The effective cleaning procedure via THF now facilitates the preparation of thermosensitive hollow Janus dumbbells. The synthesis and characterization of the microgels are described in detail in previous work [30]. PNIPAM networks cross-linked by BIS are reported to be stable in THF [34]. Thus, thermosensitive hollow Janus dumbbells are prepared using dumbbell-shaped microgels as a template using THF to remove PS, as shown in step IV in Fig. 1.

Figure 4 displays the surface morphology of thermosensitive Janus dumbbells in the dried state as investigated by SFM (a) and TEM (b). The SFM height image reveals dumbbell-shaped particles with one sphere obviously having a larger height (brighter color) than the other. This is due to the hollow copolymer sphere from the dumbbell-shaped core after the removal of PS via THF as described above. This finding is verified by TEM micrographs shown in Fig. 4b. Dumbbell-shaped particles are clearly visible, having one side with a high contrast due to the copolymer layer and one rather light-colored part that consists only of the collapsed PNIPAM shell.

Fig. 4
figure 4

a SFM height image and b TEM image of thermosensitive hollow Janus dumbbell-shaped microgels after the removal of polystyrene. The inset shows one single particle at higher magnification

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Cryo-TEM measurements of the dumbbell-shaped, hollow Janus microgels are shown in Fig. 5, demonstrating that the dumbbell-shape morphology of the PNIPAM network was well maintained after the removal of PS. This shows that PS can be effectively removed from the dumbbell-shaped core particles through the PNIPAM network without destroying the network. This is in full accord with the TEM and SFM measurements displayed in Fig. 4.

Fig. 5
figure 5

Cryo-TEM micrograph of thermosensitive hollow Janus dumbbell-shaped microgels at 20 °C in water. The image proves the Janus character of the particles showing a hollow side of the microgel (light gray) and partially hollow part (dark gray) composed of PNIPAM shell and PS-co-PMPS copolymer layer

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Characterization of the hollow dumbbells in solution

DLS and DDLS can now be used to investigate the change of the size of the thermosensitive hollow Janus dumbbells with temperature [30]. Rotational and translational diffusion coefficients of the hollow PNIPAM microgels have been firstly measured by DLS and DDLS in a temperature regime from 10 to 25 °C. In order to screen the electrostatic interactions from possible residual charges from the synthesis [30], the Janus dumbbells were dispersed in 50 mM KCl solution. Figure 6a presents the result from the CONTIN analysis of the autocorrelation function (see in Fig. S3), which includes two discrete exponential decays. The slow relaxation mode characterizes the translational motion (Γ slow = D T q 2), and the fast mode is related to the rotational motion (Γ fast = D T q 2 + 6D R) [35]. A very weak third mode can be observed at some low angles which may be due to the fluctuation of the PNIPAM network [36].

Fig. 6
figure 6

a DDLS-relaxation time distributions (CONTIN plots) calculated from the intensity autocorrelation functions for Janus hollow dumbbells in 50 mM KCl solution at 20 °C. From scattering angle θ = 20° (solid line in panel a) to θ = 30° (dashed line in panel a). b The corresponding relaxation rate is plotted as a function of the square of the scattering vector (q 2) showing DLS slow mode (open triangle) for Janus hollow dumbbells, DDLS slow mode (filled circles), and DDLS fast mode (open circles). The solid line is the linear fitted data according to the corresponding mode. c Scheme of one Janus hollow dumbbell: L is the center to center distance, R C is the radius of the dumbbell-shaped core before the removal of PS, R H and D are the hydrodynamic radius and diameter of the Janus hollow dumbbell, respectively

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Figure 6b displays the corresponding relaxation rate plotted as a function of the square of the scattering vector. The slow mode measured from DLS is comparable to that from DDLS within experimental error. Based on the slow mode, the translational diffusion coefficient D T can be calculated. Our previous work [30] on dumbbell-shaped microgels showed that the resulting rotational diffusion of dumbbell-shaped microgels has a rather large error due to the weak signal of the fast mode. However, DDLS can accurately measure the D T and D R of dumbbell-shaped PS cores due to the reliable signal. Thus, the radius of one sphere R C and center to center distance L (see Fig. 6c) can be calculated with excellent accuracy. Applying the shell model [30], R C and L are determined to be 98.80 and 105.12 nm, respectively. Combined with the fixed length L, D T at various temperatures can provide reliable calculation for dimensional information based on the shell model [30]. The resulting D T of the Janus hollow dumbbells at temperatures of 10, 20, and 25 °C and their comparison with dumbbell-shaped microgels are listed in Table 1.

Table 1 Characterization of Janus hollow dumbbells (JHD) and the comparison with that of dumbbell-shaped microgels (DPM), experimental diffusion coefficients D T measured from both DLS and DDLS experiments
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This comparison demonstrates that D T of hollow Janus dumbbells differs only by 5 % from that of dumbbell-shaped microgels. This difference is within the experimental error. It indicates that dimensional information of Janus dumbbells is more or less identical with that of the dumbbell-shaped microgels before the removal of the PS core. Hence, the shape imposed by the core is well kept for Janus hollow dumbbells. This is in agreement with the cryo-TEM image shown in Fig. 5.

To further investigate the thermosensitivity of the Janus hollow dumbbells, DLS and DDLS measurements have been performed at temperature range from 10 to 50 °C for the hollow dumbbells dispersed in aqueous solution. The analysis of D T as the function of temperature shows that the R H of Janus hollow dumbbells has a similar temperature dependency as the dumbbell-shaped microgels. Thus, the shell thickness decreases linearly with increasing temperature (T from 10 to 22.5 °C) as shown in Fig. 7. When T is above LCST, the shell shrinks further and the thickness decreases from ca. 140 nm at 10 °C to ca. 50 nm at 45 °C, leading to the aspect ratio (L*, the ratio of the center to center distance (L) to the diameter of the hollow Janus dumbbells (D)) increased from 0.21 to 0.38. Hence, the DDLS measurements demonstrate that hollow dumbbells maintain the thermosensitivity after the removal of PS.

Fig. 7
figure 7

The dependence of R H on temperature (ranging from 10 to 50 °C): dumbbell-shaped microgels (DPM; filled triangles) and hollow Janus dumbbells (JHD; open triangles). With increasing T, R H decreases correspondingly while the aspect ratio (L*; open squares) of the thermosensitive hollow Janus dumbbells increases

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Conclusion

Homogeneous thermosensitive Janus hollow dumbbells have been synthesized for the first time. They exhibit a well-defined Janus structure with two partially fused hollow PNIPAM spheres. One PNIPAM sphere is hollow while the other has a hollow core with a hybrid wall consisting of the copolymer of PS and PMPS with thickness of ca. 10 nm. DLS and DDLS measurements prove that hollow Janus dumbbells exhibit a thermosensitivity comparable to the dumbbell-shaped microgels. Hence, the radius and aspect ratio of a hollow Janus dumbbell is tunable within a certain range by temperature.

The present work opens the way to a wide variety of Janus structures and applications. The hollow hybrid sphere core in one PNIPAM sphere has silanol groups on its surface [33], and surface modifications can easily be provided by reaction with various silane reagents or grafting of other metal oxides (i.e., TiOx [37]). Moreover, the two different hollow cores can be used as nanoreactors. Work along these lines is in progress.