Evolution of Self-Organized Microcapsules with Variable

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Evolution of Self-Organized Microcapsules with Variable Conductivities from Self-Assembled Nanoparticles at Interfaces Voichita Mihali, and Andrei Honciuc ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b09625 • Publication Date (Web): 13 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019

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Evolution of Self-Organized Microcapsules with Variable Conductivities from Self-Assembled Nanoparticles at Interfaces Voichita Mihali, † Andrei Honciuc†* †Institute

of Chemistry and Biotechnology, Zurich University of Applied Sciences, Einsiedlerstrasse

31, 8820 Waedenswil, Switzerland

ABSTRACT Self-organization dramatically affects the surface properties of materials on a macroscopic scale, such as wettability and adhesion. Fundamentally, it is equally interesting when self-organization at the nanoscale affects the bulk properties and thus provides means to engineer the optoelectronic properties of the materials on larger scales. In this work we report the evolution of conductive selforganized polymer microcapsules from a monomer emulsion droplet stabilized by a monolayer of conductive Janus nanoparticles (JNPs) via a mechanism resembling morphogenesis. The wall of the resulting conductive microcapsule has a honeycomb-like structure with highly oriented JNPs occupying each hollow cell. The JNPs consist of an electrically conductive and an insulating lobe; due to their orientation and presence in the honeycomb the conductivity of the microcapsule is greatly enhanced as compared to each of the constituting materials. The current method is universally applicable to induce self-organization in conductive polymers forming by oxidative addition.

KEYWORDS: PEDOT microcapsules, semiconductive Janus nanoparticles, Pickering emulsions, interfacial polymerization, cooperative doping

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Multifunctionality in nanomaterials emerges from a manifold of combinations between surface properties (amphiphilicity, polarity), material’s intrinsic bulk properties (optic, electronic, magnetic) and geometry (shape, size).1 The ability of snowman-type Janus nanoparticles (JNPs) to self-assemble into a large variety of suprastructures constitute such an example of multifunctionality that emerged through coupling of a amphiphilicity with shape.2,3 Coupling of amphiphilicity, electronic properties and variable particle geometry was also achieved in snowman-type JNPs constituted of one electrically conductive and another electrically insulating lobes; by changing the relative size ratio of the lobes, powders with tunable wettability and conductivity were obtained.4 Further, JNPs are notorious for their ability to stabilize and generate Pickering emulsions,5–9 preventing the coalescence of droplets through the formation of a protective armor by self-assembly into monolayers10 at the oil-water interface.5,11 Pickering emulsions can also be stabilized by homogeneous nanoparticles (HNPs) with appropriate surface polarities,12 but JNPs proved superior due to their enhanced interfacial activity and the possibility to tune the amphiphilicity by simply changing the relative lobe size ratio without surface chemical modifications.13 In fact, the resulting Pickering emulsions are so stable5,12 that can even be polymerized at higher temperatures if the oil used is a vinylic-type monomer, resulting in the so-called colloidosomes,8,15 micron sized beads with a nanostructured surface.5,12,14 The self-organization that normally arises in the evolution of natural systems is quickly developing into a strategy for designing multifunctional materials. Thus far, several mechanisms are believed to induce self-organization in chemical systems, among which morphogenesis is believed to be the expression of the reaction-diffusion, where controlling of hierarchical structuring can be obtained through directional flows of reactants as described by Cera & Schalley.1 In this work we report the evolution of a conductive microcapsule with a self-organized structure and tunable optoelectronic properties from self-assembled JNPs at oil-water interfaces via a mechanism resembling morphogenesis. We start from snowman JNPs consisting of one semiconductive polyaniline (PANI) lobe and one electrically insulating lobe. The PANI-JNPs have a 2


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good ability to stabilize emulsions of 3,4-ethylenedioxythiophene (EDOT) monomer in water. The EDOT-in-water (EDOT/water) Pickering emulsions are subsequently polymerized. Polymerization of oil-monomer droplets in Pickering emulsions works particularly well if vinyl type monomers that can polymerize via radical polymerization are used.5,8,14,15 In contrast, polymerization of Pickering emulsions with other types of monomers, which polymerize via other mechanisms such as oxidativeaddition, is very challenging and to the best of our knowledge has not been reported. Polymerization drives the unidirectional flow of EDOT monomer reactants from the inside of the EDOT oil droplet to the interface through the armor of self-assembled JNPs, which evolves into a microcapsule with self-organized wall architecture. The structure of the wall consists of highly ordered and oriented PANI-JNPs individually located into PEDOT wells. The optoelectronic properties of the PANI-JNPs and PEDOT in the microcapsule are coupled. The relative size between Janus lobes as well as the density of PANI-JNPs in the microcapsule wall provides means for precisely tuning the properties of the material.

RESULTS AND DISCUSSIONS The starting polystyrene (PS) seed with an average diameter 300 ± 10 nm and JNPs were synthesized in surfactant-free conditions. The synthesis of JNPs proceeded via seeded emulsion polymerization and phase separation of the 3-(triethoxysilyl)propyl-methacrylate (TSPM) monomer as previously reported.15 The relative size of the JNP lobes could be changed by adjusting the volume of the TSPM monomer, 1 mL or 2 mL, which is added to 1 g of seed HNPs, resulting in JNPs-1 and JNPs-2. The SEM images of the obtained particles are presented in Figure 1a-c. Selective Modification of the JNPs with PANI. The asymmetric modification of JNPs with PANI occurs in situ during the oxidative-addition polymerization of the anilinium salt via electrostatic interaction between the PANI nuclei and the negatively charged PS lobe, Figure 1e-g.4 From the SEM images in Figure 1e,f it can be clearly observed that the selective modification of the PS Janus lobe with PANI was achieved, while the poly(3-(triethoxysilyl)propyl-methacrylate) (PTSPM) Janus lobe 3


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remained pristine. In the same reaction conditions, the PS seed HNPs, Figure 1a, were also modified with PANI resulting in a core-shell like structure, Figure 1d. The thickness of the PANI shell is ≈ 25 nm, measured from SEM, Figure 1a-f.

Figure 1. Structure of nanoparticles and formation of microcapsules. SEM images of: (a) seed PSHNPs, (b) JNPs-1, (c) JNPs-2, (d) PANI-HNPs, (e) PANI-JNPs-1, (f) PANI-JNPs-2. (g) Reaction 4


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scheme for the selective modification of JNPs with PANI. The PANI-JNPs are next used for the emulsification of EDOT. (h) Interfacial polymerization and formation of microcapsules with a honeycomb structure of the inner wall. (b-c) The PTSPM Janus lobe appears brighter than the PS Janus lobe due to a higher electron density. (e-f) The conductive PANI Janus lobe appears rough while the electrically insulating PTSPM lobe appears smooth.

Microcapsules Obtained by Pickering Emulsion Polymerization. The three types of particles constituting a homologous series were used to construct microcapsules by Pickering emulsion polymerization, one PANI modified homogeneous nanoparticle (PANI-HNPs) and two PANI-JNPs with different relative size ratio of the Janus lobes. The nanoparticles were first used for stabilization of Pickering emulsions of EDOT monomer. During emulsification, the nanoparticles self-assemble into a monolayer at the oil-water interface forming protective armor around the emulsion droplet, as demonstrated in Figure S1 for molten wax. The stability of the self-assembled monolayers of nanoparticles at the oil-water interface plays a key role in re-enforcing and preserving the structural integrity of the oil-droplet and its evolution into a self-organized microcapsule. The EDOT oil droplets were observed with the fluorescence microscope and the obtained microcapsules were observed by SEM; their average diameter decreased by increasing the amount of nanoparticles in emulsification from 10 to 70 mg. (Figure 2, Table S1). The average diameter of the droplets and microcapsules obtained with different nanoparticles also decreased in the homologous series from PANI-HNPs to PANI JNPs-1 or PANI-JNPs-2. The emulsification efficiency, i.e. smaller oil droplets, increased with the increase in particle concentration. The emulsification efficiency of the PANI-JNPs was also better than that of PANI-HNPs, due to their amphiphilicity.5,13,15

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Figure 2. Oil droplets and PEDOT microcapsules. Fluorescence microscopy and SEM images of EDOT emulsion droplets and microcapsules stabilized by different nanoparticles: (A) PANI-HNPs, (B) PANI-JNPs-1, (C) PANI-JNPs-2 and concentrations 10, 25, 50 and 70 mg in 7.5 mL total emulsion volume. The emulsion composition used was EDOT/water (v/v = 0.5/7). The emulsion droplet diameter decreases from left to right. The scale bar in the fluorescence microscopy images is 40 µm and in SEM images 25 µm.

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The oxidative-addition polymerization of the EDOT starts at the interface of the nanoparticle stabilized emulsion droplet, initiated by the ammonium peroxydisulfate (APS) from the water phase. The PEDOT grows at the interface16 of the oil-water while continuously supplied by the monomer diffusing from the interior of the droplet as depicted in Figure 1h. The PEDOT grows in between and around the exterior armor of nanoparticles until the oxidant or monomer are consumed. The growth of the PEDOT film around the tightly assembled nanoparticles at the surface of the oil droplet, results in microcapsules with a self-organized PEDOT PANI-JNPs structure with different morphologies on the inner and the outer sides, as shown in Figure 3 for PANI-JNPs-2. The wall on the interior side has a honeycomb-like morphology with one nanoparticle occupying each hollow cell and has the appearance of chain-mail armor on the outer side of the wall due to the PEDOT film that grew around the nanoparticles (Figure 3). The water-soluble oxidant initiator APS triggers the polymerization reaction of EDOT. In a Pickering emulsion, for the EDOT polymerization to take place, the monomer must diffuse at the emulsion droplet interface (EDOT-water interface) to meet the oxidant, as depicted in the cartoon of Figure S3. At the beginning of the reaction the JNPs constitute an armor surrounding and stabilizing the oil droplet. During initial stage, EDOT monomer at the EDOT-water interface begins to polymerize under the action of the APS oxidant from the water phase. As the PEDOT layer grows thicker, EDOT monomer must diffuse through the formed PEDOT layer to meet the APS initiator, Figure S3. From the SEM images in Figure 3 it can be observed that at the end of polymerization reaction the JNPs, which were initially constituting an armor around the emulsion droplet Figure S1, become covered with PEDOT. This is evidence that the migration of EDOT monomer took place from inside of the droplet to the interface through the PEDOT barrier, Figure S3. It is not clear at this point how the diffusion of the EDOT to the newly formed PEDOT-water interface to meet the oxidant initiator occurs and whether PEDOT membrane rupturing and healing to enable EDOT diffusion is necessary, as it is often the case in morphogenesis. The large volcanolike protuberances observed on the outer side of the PEDOT material obtained from polymerization of an EDOT suspension in water, in the absence of nanoparticles, suggests that indeed membrane 7


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rupturing is necessary for the expulsion of the monomer, Figure S4. In the case of nanoparticle stabilized Pickering emulsion polymerization, no such large volcano-like formations are observed on the surface of PEDOT microcapsules, rather the PEDOT layer grows uniformly around the nanoparticles. This suggests that EDOT monomer can diffuse out of the microcapsule, most probably through the juncture formed between the growing PEDOT layer and the nanoparticles. The mechanism described above leading to self-organization and formation of the chain-mail armor on the outer side of the microcapsule wall, around the JNPs, resembles the mechanism of hierarchical structures formation via morphogenesis, also known as chemical garden. The mechanism of morphogenesis requires:1,17,18 (1) compartmentalization of reactants, (2) asymmetric flow of reactants under the action of some forces or gradients and (3) formation of hierarchical structures from reactant diffusion through a membrane into a second compartment followed by immediate precipitation or solidification reactions in the newly met environmental conditions. An illustrative example, a semipermeable metal silica hydride membrane compartmentalizes a metal salt crystal grain in a sodium silicate solution. The combination between membrane rupturing and unidirectional ejection of metal salts to the exterior followed by their precipitation upon contact with the silicate solution, leads to the formation of complex self-organized biomorphic silica structures.1,17,18 In the current case the same conditions are fulfilled, compartmentalization of the monomer in the emulsion droplet protected by an armor of nanoparticles, the unidirectional diffusion of EDOT monomer to the interface and interfacial polymerization with the growth of PEDOT layer around the nanoparticles. The emulsion EDOT droplets are spherical before the reaction, but the PEDOT microcapsules appear squeezed and strained, likely due to the monomer migration from the interior of the droplet to the interface. SEM images of the microcapsules generated with PANI-HNPs and PANI-JNPs-1 are reported in Figure S5, S6. Interestingly, the asymmetrically modified PANI-JNPs are oriented with the PANI lobe toward the PEDOT layer and the PTSPM lobe is oriented toward the inside of the microcapsule. This orientation is probably preserved after polymerization from the initially orientation of the PANI8


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JNPs at the EDOT-water interface in Pickering emulsions, where the more hydrophilic PANI Janus lobe is oriented toward water and the more hydrophobic PTSPM lobe toward EDOT. To show that, we synthesized colloidosomes from molten wax and we observed that a large fraction of PANI-JNPs2 have a preferred orientation (see Figure S1). In order to better understand the fine structure of the self-organized PEDOT PANI-JNPs microcapsule wall, the following parameters will be defined: T1 - thickness of the PEDOT layer covering the nanoparticles, T2 - thickness of the PEDOT layer between nanoparticles (Figure 3e), D1 and D2 - distances between apex of protuberances on the outer wall and between the nanoparticles occupying the cells of the honeycomb on the inner wall respectively. The T1 and D1 are always larger than T2 and D2 due to the curvature of the wall. The magnitude of T1, T2, D1 and D2 decreases by increasing the amount of nanoparticles in emulsification and by decreasing the APS/EDOT molar ratio in polymerization (see Table S2). The reason why the magnitude of the above parameters decreases with increasing the amount of the nanoparticles is because the average diameter of EDOT droplets in emulsions also becomes smaller (Figure 2, Table S2), a smaller reservoir of EDOT monomers leads to microcapsules with thinner PEDOT layer. Therefore, changing the PEDOT layer thickness changes the distance between nanoparticles and the effective surface density defined as the number of particles per unit of surface area, ρA (see Table S2). The type of nanoparticle also influences the structural parameter of the microcapsules, namely we observe that the magnitude of T1, T2, D1 and D2 increases and ρA decreases with the increasing of the PTSPM lobe size from PANI-HNPs to PANI-JNPs-2.

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Figure 3. Self-organization in microcapsule walls. SEM images of the self-organized PEDOT PANIJNPs microcapsule walls obtained with variable amount of PANI-JNPs-2: (a) 10 mg, (b) 25 mg, (c) 50 mg, (d) 70 mg in 7.5 mL. The first row of images show the structure of the outer wall, and second row of images shows the inner structure of the wall with oriented PANI-JNPs occupying each cell. (e) Brocken microcapsule showing inner and outer wall obtained with 70 mg PANI-JNPs-2 and APS/EDOT 0.25. (f) Cross section of the microcapsule wall obtained with 70 mg PANI-JNPs-2 and APS/EDOT 0.25. The orientation of PANI-JNPs is such that the PANI Janus lobe (appearing darker) is pointing toward the PEDOT layer. (g) Cartoon depicting the cross-section of the microcapsule wall indicating PEDOT layer thickness (T1, T2) and inter-particle distances (D1, D2).

Tunable electrical properties of microcapsules. The conductivity of all the self-organized PEDOT PANI-JNPs microcapsules, as well as of the nanoparticles was done by first measuring the current vs. voltage (I-V) (Figure S7-S9). The properties of conductive polymers obtained by oxidativeaddition polymerization in aqueous solutions can be tuned by changing a wide variety of synthesis parameters, such as pH, monomer/oxidant ratios, polymerization temperature, time of reaction, surfactants, type/concentration of oxidizing agents and protic acids.19–22 In the current case, all of nanoparticles and microcapsules were made in surfactant-free conditions, therefore only ionic species 10


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such as HSO4-, SO42- and Cl-, resulting from the degradation of APS oxidant and addition of HCl for pH adjustment of the reaction23–25, can act as dopants. In the current system, we have measured the electrical properties of the microcapsules produced in the same conditions, while only the concentration of APS and nanoparticle were varied. The conductivity of the microcapsules as a function of APS/EDOT molar ratio, nanoparticle type and concentration are summarized in Figure 4a-c. Influence of the amount of nanoparticles on conductivity: the main parameter influencing the conductivity of the microcapsules is the amount of nanoparticles. Increasing the amount of nanoparticles leads to enhancement in the conductivity of the microcapsules, as seen in the Figure 4a-c. For example, for PANI-JNPs-1, the highest conductivity 3.51 ± 0.02 mS cm-1 was obtained for the maximum amount of 70 mg nanoparticles and the lowest conductivity of 1.30 ± 0.01 mS cm-1 for the smallest amount of nanoparticles 10 mg in 7.5 mL emulsion (Table S3, entries 7, 10). The same is true for all the other nanoparticles. Clearly, the concentration of nanoparticles has a profound role in the enhancement of the conductivity of microcapsules that cannot be explained by a mere averaging of the conductivities between PEDOT and PANI; the conductivity of the PANI-HNPs and PANIJNPs is significantly lower than that of the microcapsules (Table S3). As previously discussed, the amount of nanoparticles used in the emulsification affects the surface density of nanoparticles, ρA. The results indicate that the electronic properties of the PANI-JNPs and PEDOT wall in the microcapsule are coupled leading to enhancement in conductivity, a mechanism we refer to as “cooperative doping”. Previously Dimitriev26 reported a “cooperative doping” mechanism between alternating layers of PEDOT and PANI and observed an enhancement in the electrical conductivity of the respective bilayers. The cooperative doping effect was also reported by others.27,28 Influence of nanoparticle type on conductivity: for a constant APS/EDOT molar ratio and amount of nanoparticles, the conductivity of the microcapsules changes with the type of nanoparticle. For example, the PANI-JNPs-2 has the largest relative size ratio of the electrically insulating PTSPM Janus lobe to the conductive PANI lobe, followed by PANI-JNPs-1 and PANI-HNPs with no lobe. 11


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With the increase in the relative size ratio of the insulating Janus lobe, at the highest surface density of particles, the conductivity decreases from 4.62 ± 0.01 mS cm-1 for PANI-HNPs to 3.51 ± 0.02 mS cm-1 for PANI-JNPs-1 and 3.20 ± 0.02 mS cm-1 for PANI-JNPs-2 (Table S3 entries 4, 10 & 16). The clear difference in conductivity of the microcapsules obtained with different types of nanoparticles (Figure 4a-c, Table S3) can be explained in two ways. First, the structural parameters of the microcapsule should be considered, T1 increases with the increase in the PTSPM lobe size in the homologous series of particles, consequently the A decreases (Table S2) and the level of cooperative doping decreases. Secondly, in PANI-HNPs the conductive PANI layer covers the whole surface of the particle, while in the case of PANI-JNPs-2 the conductive PANI polymer covers only a part of the nanoparticle surface, as a consequence the level of “cooperative doping” is reduced with the increase in the size of the insulating PTSPM lobe. Influence of APS on conductivity: for a constant amount of nanoparticles used the increase in APS/EDOT molar ratio produces a change in the conductivity of microcapsules. When the APS/EDOT molar ratio 0.5 was used the conductivity of microcapsules was always larger, for all nanoparticle types, than that of the microcapsules produced with APS/EDOT molar ratio of 0.25 (Table S3, Figure 4a-c). For even larger APS/EDOT molar ratio of 1 the conductivity begins to dramatically decrease, so a clear trend could not be identified. The lack of trend in this case can be explained by the contribution of several mechanisms operating simultaneously. A quick inspection of the corresponding data in Table S2 (entries 13,15 and 16) reveals that T1 increases and A decreases with increasing the APS/EDOT molar ratio; as a consequence the conductivity should decrease due to loss in “cooperative doping” by the PANI-JNPs-2. Concurrently, the increase in the oxidant amount is known to produce higher doping levels in PEDOT29 with increasing the conductivity but also creates shorter chains30 which has an opposite effect.

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Figure 4. Optoelectronic properties of microcapsules. 3D graphic representation of conductivity of microcapsule vs. concentration of APS and amount of nanoparticles: (a) PANI-HNPs, (b) PANIJNPs-1, (c) PANI-JNPs-2. UV-Vis-NIR absorption spectra: (d) microcapsule obtained with different amounts of PANI-JNPs-2 (I) 10 mg, (II) 25 mg (III) 50 mg and (IV) 70 mg; (e) Optical properties of microcapsules obtained with different types of nanoparticles at the same concentration 70 mg and constant APS/EDOT molar ratio 0.25: (I) PANI-JNPs-2, (II) PANI-JNPs-1 and (III) PANI-HNPS; (f) Optical properties of microcapsules obtained with 50 mg PANI-JNPs-2 and different APS/EDOT molar ratio: (I) 1, (II) 0.5 and (III) 0.25. The colormap represents the 7 rainbow colors from the violet (the highest conductivity 6.08 ± 0.02 mS cm-1) to red (the lowest conductivity 1.05 ± 0.01 mS cm-1).

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Optical Properties of the Self-Organized Microcapsules. PEDOT can exist in three different states, which depend on the oxidation state of the polymer backbone. PEDOT in neutral state shows an absorption band around 600 nm, due to π-π* transitions, PEDOT+ in polaron state show absorption around 900 nm and the bipolaron PEDOT2+ state show broad absorption in the infrared region from 1050 nm to higher wavelengths.31–34 Influence of the amount of nanoparticles on optical properties: the UV-vis-NIR spectra of the microcapsules obtained with varying PANI-JNPs-2 concentrations, 10 mg to 70 mg, are reported in Figure 4 and those obtained with PANI-HNPs and PANI-JNPs-1 are given in Figure S10. In our case, the absorption bands related to π-π* transition exhibited a red shift with the increase in the amount of nanoparticles; for example in absorption spectrum of the microcapsule in Figure 4d the maximum of the adsorption band at 572 nm (microcapsule with 10 mg PANI-JNPs-2) shifts to 629 nm (microcapsule with 70 mg PANI-JNPs-2) owing to a decrease in the band-gap energy as a result of the “cooperative doping” by nanoparticles.26,28 The max of the polaron and bi-polaron absorption bands red shifts from 1000 to 1060 nm and the intensity of these broad bands relative to the π-π* transition band increases with the increase in the amount of nanoparticles and of A. This increase in the intensity of the polaron bands and the corresponding red-shift of max indicates a transition toward the quinoid structure of PEDOT with increase in the A of PANI-JNPs-2, which translates in a higher electrical conductivity, Figure 4a-d. Note that in the same time the structural parameters of microcapsule such as T1 decreases with the increase in the amount of nanoparticles, therefore a thinner PEDOT layer should have decreased the conductivity of the microcapsule in the absence of the “cooperative doping” effect. Influence of the type of nanoparticles on optical properties: the type of nanoparticle has a great influence also on the optical properties of the microcapsules, namely with the decrease in the relative size between the electrically insulating PTSPM and PANI Janus lobes the polaron and bi-polaron band increase in intensity. The highest intensity of polaron and bipolaron absorption bands 14


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correspond to microcapsules obtained with PANI-HNPs, followed by PANI-JNPs-1 and PANI-JNPs2, Figure 4e; this trend is very well reflected in the electrical properties: microcapsules obtained with PANI-HNPs are most conductive followed by PANI-JNPs-1 and by PANI-JNPs-2. In addition, a strong red-shift in the neutral absorption band from 627 nm for PANI-JNPs-2 to 682 nm for PANIHNPs is also observed in Figure 4e. Also, in this case the optical and electrical data support the evidence that the optoelectronic properties of the PEDOT layer and PANI-HNPs and PANI-JNPs are strongly coupled. Influence of APS concentration on optical properties: by increasing the molar ratio between APS/EDOT from 0.25 to 1 while keeping the concentration of PANI-JNPs-2 constant a blue-shift of the π-π* transition band from 625 to 584 nm as well as a blue-shift for a bipolaron band from 1037 to 996 nm can be observed in the absorption spectra, Figure 4f. This could imply that the conjugation length of PEDOT decreases with increasing the concentration of oxidant.35 The relative decrease in the intensity of the polaron, bi-polaron absorption bands when the APS/EDOT molar ratio increases from 0.25 to 1 (Figure 4f) is reflected well in the decrease in conductivity, Figure 4c.

CONCLUSIONS We have reported a phenomenon where EDOT oil droplets with self-assembled Janus nanoparticles at oil-water interfaces evolved into a conductive microcapsule with a self-organized wall structure via a process resembling hierarchical morphogenesis. The microcapsules have PEDOT walls with a honeycomb-like structure where each cell is occupied by a highly oriented PANI-JNPs or PANIHNPs. This emulsification-polymerization method could prove useful as strategy to induce selforganization in other polymers that form via an oxidative addition mechanism from nanoparticle stabilized Pickering emulsions. In fact, we suggest that numerous other optic or electronic phenomena could be observed in different types of polymer-nanoparticle systems due to self-organization and coupling of properties between nanoparticles and the nanostructured polymer. In the current case the optoelectronic properties between PEDOT layer and nanoparticles are coupled and the conductivity 15


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of the microcapsules could be tuned and precisely controlled by the level of “cooperative doping”, which can be adjusted by the surface density nanoparticle and the relative size ratio between the conductive and insulating Janus lobe. We have therefore presented an alternative technology to produce materials with a self-organized structure and with tunable optoelectronic properties, by hierarchical coupling of various functionalities1 in surfactant-free conditions that can be used in applications such as sensors and light harvesting devices.36–42

EXPERIMENTAL SECTION Synthesis of Nanoparticles. The synthesis of the surfactant-free seed PS nanoparticles (PS-HNPs) and JNPs in surfactant-free conditions was performed according to a procedure we have previously reported15 and also briefly described in Supporting Information. Synthesis of Semiconductive PANI-HNPs, PANI-JNPs-1 and PANI-JNPs-2. Solution A was prepared: 200 mg HNPs or JNPs-1 or JNPs-2 were added to 40 mL deionized water. Next, an anilinium solution, Solution B, was prepared by adding 50 µL aniline (0.548 mmol) into 5 mL deionized water and the pH was adjusted to 1 with HCl (37%). The Solution B was injected into Solution A under stirring at 800 rpm at 0 °C. Subsequently, an APS solution (0.548 mmol in 5 mL of UPW) was added dropwise to the solution mixture and the reaction proceeded for 12 h at 0 °C. The color of the reaction changed from white to dark green. After polymerization, the PANI-HNPs, PANIJNPs-1 and PANI-JNPs-2 were purified by centrifugation and washed with UPW and ethanol; the cleaning procedure was repeated twice. Synthesis of PEDOT Microcapsules with PANI-HNPs and PANI-JNPs as Emulsion Stabilizers. First, in a scintillation vial, varying amount of nanoparticle stabilizers, PANI-HNPs, PANI-JNPs-1 and PANI-JNPs-2, namely 10, 25, 50, 70 mg were dispersed in 5 mL UPW. Then, 0.5 mL EDOT (4.67 mmol) was added to the above solution and the EDOT/water Pickering emulsions were prepared by emulsifying this mixtures by ultrasonication (with a Branson 450, with a ½ inch processing horn at 50% intensity amplitude) for 30 seconds. Next, for the polymerization reaction, stoichiometric and 16


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non-stoichiometric amounts of APS were used, namely the molar ratio between the oxidant and monomer (APS/EDOT) were 0.25, 0.5 and 1 mmol. The corresponding amount of APS was first dissolved in 2 mL UPW and added to the Pickering emulsion, prepared above, at room temperature. After the addition of APS initiator, the Pickering emulsion color changed from dark green to dark blue. The polymerization was carried at 45°C for 12 h without any stirring. The obtained PEDOT microcapsules were filtered and washed with ethanol and UPW to remove the residues and the nonreacted monomer and oxidant. The product was dried under vacuum at 60°C for 24 h. The yield of the reaction was quantitative in all cases, calculated in reference to the molar amount of APS.

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS publication website at DOI:… Details on the synthesis of the JNPs. SEM images of the wax Pickering emulsion droplets surrounded by an armor of JNPs assembled in a monolayer. Table containing the dimension of the EDOT emulsion droplets stabilized by JNPs. Fluorescence images of the EDOT droplets. SEM images of the microcapsules obtained by Pickering emulsion polymerization from PANI-HNPs and PANI-JNPs-1 for different concentrations. Table of the wall thickness and surface density of nanoparticles. Electrical properties of microcapsules: current-voltage graphics and table of conductivities. Optical properties of the microcapsules, UV-Vis-NIR absorption spectra for microcapsules obtained with from PANI-HNPs and PANI-JNPs-1. The authors declare no competing interests. AUTHOR INFORMATION Corresponding Author

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* [email protected] Present Addresses † Institute of Chemistry and Biotechnology, Zurich University of Applied Sciences, Einsiedlerstrasse 31, 8820 Waedenswil, Switzerland ACKNOWLEDGMENT We gratefully acknowledge the financial funding from Metrohm Foundation (Herisau).

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Evolution of Self-Organized Microcapsules with Variable

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