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Synthesis of conducting asymmetric hydrogel particles showing autonomous motion A. Srinivasana, J. Rochea, V. Ravainea, A. Kuhna*
Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/
In the present work, we introduce a new approach for the synthesis of asymmetric particles made from electrically conducting polyaniline-alginate hydrogels by using bipolar electrochemistry. Such an intrinsic break of symmetry allows the soft beads to exhibit tuneable motion at the air/water interface when loaded with ethanol due to a controllable directed release of the solvent.
Asymmetry is the crucial ingredient for generating directed motion of objects, however the efficient synthesis of asymmetric particles is still a challenge in many respects [1]. Self-propelled particles, which are relying on an intrinsic asymmetry, are currently in the spotlight of various fields of science due to some promising potential applications for example in biosensing, drug release and lab-on-chip devices [2-4]. Over the last decade, a large number of artificial systems showing autonomous motion have been developed. Controlling their movement in certain environments is not straightforward, especially with respect to the directionality along predetermined trajectories. As the key criterion for propulsion of these swimmers lies in breaking the symmetry of the system, the majority of previously reported approaches use a built-in asymmetry in order to control the motion [5-8]. The synthesis of asymmetric structures requires, with few exceptions [9], surfaces or interfaces in order to break the symmetry. With this respect, bipolar electrochemistry (BE) offers in a general sense an attractive alternative for the generation of asymmetric objects. As a complement to classical approaches (i.e. electrodeposition, template based synthesis, masking etc…), BE is a simple and cost effective method for the production of asymmetric objects [10,11]. It is a technique which induces two spatially separated electrochemical reactions (oxidation and reduction) on the surface of conductive objects [12]. This allows controlling the shape, position and size of the asymmetric feature easily by changing the electric field in the solution [13]. So far the concept has been exclusively used for the modification of objects made out of solid materials such as different metals, carbon [11], polymers [14] and semiconductors [15]. In the present work, bipolar electrochemistry is applied for the first time to particles made out of soft materials, namely hydrogels. Asymmetric hybrid polyaniline-alginate beads are synthesized by this approach, where the hydrogel part offers the porous and soft structure needed to entrap and release a molecule, whereas the interpenetrated polyaniline brings the required electrical conductivity for symmetry breaking and modulating the permeability. As a result of their asymmetry, these beads present non-symmetrical release profiles. Therefore, when loaded with a
This journal is © The Royal Society of Chemistry 2012
solvent cargo, in the present case ethanol, they can undergo autonomous motion at a water/air interface. There are several reports describing the fabrication of electroconductive hydrogels (ECHs) in the literature [16-18]. In the present case a straight forward two-step process has been employed for the synthesis of hybrid polyaniline-alginate electroconductive hydrogels with tunable electrical conductivity and morphology (Fig. 1a). Alginate hydrogel spheres were prepared in the first step by the drop-wise addition of sodium alginate sol (1.0 wt %) into a gelation bath containing 0.05 M calcium chloride [19,20]. The formed droplets of sodium alginate immediately solidify into gel particles in the calcium chloride solution. These beads were kept in the gelation bath for 1 minute during which the beads reached a size in the mm range, involving the rearrangement of the gel structure, due to the crosslinking induced by the calcium ions [21, 22]. Subsequently the preformed hydrogel spheres were loaded with aniline monomer (1 M in 1 M HCl) for 14 h and then chemically polymerized with ammonium persulfate (APS) (0.125 M in 1 M HCl) for 1 h. During the synthesis, we observed that the morphology of the polyaniline-alginate based electro-conductive hydrogel beads can be tuned by changing the concentration of cross-linking agent (CaCl2), while simultaneously altering the conductivity of these beads. At high concentrations of calcium chloride (0.2 M) an extensively cross-linked hydrogel structure was formed, which hinders the penetration of aniline monomer and oxidant molecules into the hydrogel matrix, and resulted in core-shell morphologies with a poor conductivity due to the deficiency of conductive polyaniline inside the hydrogel (Fig. 1b). In contrast, when using lower calcium concentrations (0.05 M) a highly permeable gel structure was obtained, allowing an easy diffusion of aniline and oxidizing agent, so that an interpenetrating network of polyaniline and alginate is formed (Fig. 1c), leading to high conductivity.
J. Name., 2012, 00, 1-3 | 1
Soft Matter Accepted Manuscript
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Fig. 1 (a) Summary of the sequential steps involved in the preparation of PANi-alg electroconductive hydrogel beads. The morphology of the resulting soft beads can be readily tuned by altering the calcium chloride concentration during the production of Ca-alginate beads. Optical images of beads with different morphologies: (b) 0.2 M CaCl2 results in core-shell structures (c) 0.05 M CaCl2 leads to interpenetrating PANi-alg conductive hydrogel beads. potential difference ∆V that is proportional to the electric field E and the characteristic length l of the object [10,11], in that case the bead diameter, according to the following equation: ∆V = E l
Fig. 2 (a) Scheme of the experimental system illustrating the bipolar electrode behavior of a spherical conducting object immersed in an electrolyte solution and exposed to an electric field. (b) corresponding asymmetrically modified PANi-alg electroconductive hydrogel bead, obtained by exposure for 5 min to an electric field of 66V/cm (c) fluorescence microscope image of a bead releasing fluorescein dye in an asymmetric way, synthesized by exposure to an electric field of 66V/cm for 15min. The obtained conductive beads were asymmetrically modified by bipolar electrochemistry, which relies on the fact that when the soft beads are immersed in an electrolyte solution and are exposed to an external electric field, the polarization of the substrate with respect to the surrounding medium generates a This journal is © The Royal Society of Chemistry 2012
(1)
For a sufficiently high ∆V, two electrochemical reactions (i.e. oxidation at the anodic pole and reduction at cathodic pole) can occur simultaneously at the two extremities of the object (Fig. 2a). The intrinsic break of symmetry provided by this technique was used to synthesize asymmetric soft particles in a controlled manner (Fig. 2b). The oxidation of the conductive polyaniline-alginate hydrogel beads by BE was carried out according to Fig. 2a after loading them with ethanol for various periods of time (5, 10, 15 and 30 minutes respectively). The modification with the aforementioned technique usually involves a high potential, depending on the distance between the feeder electrodes and the size of the particles. Actually, the experimental key parameter is the electric field. In order to estimate the necessary amplitude of the electric field one has to consider the actual potentials needed to drive the two redox reactions on the right and the left side of the particle respectively (Fig.2a). In order to oxidize polyaniline into its pernigraniline form, a typical potential of around 0.8 V vs SCE has to be used. On the other hand reduction of polyaniline into its leucoemeraldine form occurs around 0V vs SCE and proton reduction on such surfaces is rather difficult and needs significant overpotentials (E < - 0.2 V vs SCE). This means that between the two sides of the particle a potential difference of at least 1 V needs to be established in order to trigger oxidation and reduction reactions simultaneously. Consequently a minimum electric field of 10 V/cm has to be applied to the solution when assuming a typical dimension of the beads in the mm range. However this is only the lower threshold value, because such an electric field would only lead to reactions at the very extremities of the particle. Figure 2b illustrates that the modification of the particle reaches almost the middle of the bead, meaning that for those parts which are close to the center, the polarisation has still to be high enough in order to trigger the redox reaction. Therefore, in the present example a field of 66 V/cm has been used. When exposed to J. Name., 2012, 00, 1-3 | 2
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this field strength, polyaniline undergoes electrochemical overoxidation, leading to degradation of the polymer backbone due to the formation of quinone derivatives namely hydroquinone (HQ) and benzoquinone (BQ) [23] and forming the typical green polymer visible in Fig. 2b. This leads to a controlled degradation at the oxidized side of the modified bead, increasing its permeability. The degree of permeability on the oxidized end can also be tuned by adjusting the exposure time to the high potential. For short exposure times only a color change is observed due to oxidation of the conducting polymer [24] as illustrated in Fig. 2b for a 5min exposure to the electric field, whereas longer exposure (15min) results in more degraded beads as shown in Fig. 2c. In the latter case one can see that part of the initially quasi-spherical bead has been removed by oxidation. Due to the size of the particles and the distance between the electrodes only a few particles can be modified simultaneously with this specific setup. As the density of the gel beads is higher than ethanol they are located at the bottom of the cell during modification, which has the advantage to prevent rotation while the electric field is applied. However in order to generate many particles in parallel and in the whole volume of the reactor, a similar approach as the one already reported for solid objects might be applied by artificially increasing the viscosity of the medium [25]. Interestingly, the hydrophilic/hydrophobic character of the two sides of the beads is not significantly changed during this process as we could observe by contact angle measurements (see Figure S4, SI). Most likely the hydrophilicity is solely governed by the alginate gel network and this doesn’t change upon electrochemical oxidation. In order to demonstrate the resulting asymmetric permeability, the bead has been loaded with ethanol/fluorescein before the bipolar modification. When the bead is placed in water, after having been exposed to the electric field, the asymmetric release of fluorescein from the degraded side of the bead is clearly visible (upper left part in Fig. 2c). We assume that if fluorescein is able to leak out preferentially on one side of the bead, the same is true also for smaller molecules such as ethanol. In the following this is used to induce directed motion. The resulting modified beads were floated on the water surface in a plastic dish (12 cm diameter) and motion of the beads was recorded with a digital camera. The beads exhibited two different types of motion (periodic and continuous motion) depending on the duration of oxidation and the amount of ethanol inside the beads. Typical trajectories of beads are illustrated in the Figs. 3 a & b as well in Video 1&2 in the SI. In order to better illustrate that ethanol is released in an asymmetric way due to a higher permeability created on the oxidized side, fluorescein dye was
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again added to the ethanol during the modification step to track later the ethanol/dye release. In a prototype experiment a dye loaded bead was carefully immobilized in a UV-Vis cuvette filled with water. We observed continuous release of ethanol/fluorescein at the oxidized end (Video 3 in SI). The released ethanol is transported to the air/water interface due to density differences of the two liquids (ρୌ =0.789 g/cm3). An UV-Vis absorption study allowed us to further illustrate the differential release of ethanol from both ends of a hydrogel that has been treated by bipolar electrochemistry (Figs. S1 and S2 in SI). The propulsion of the hydrogel beads on the water surface originates from this asymmetric release of ethanol due to the increased permeability created at the oxidized side of the modified bead. The released ethanol rapidly spreads over the water surface due to low surface energy and low density, thus giving rise to surface tension gradients. This phenomenon is well known as Marangoni effect, which is responsible for the self-propulsion of various kinds of swimmers [5, 26-34]. In the case of the pulsating mode, also observed by other authors for other hydrogel systems [5], the motion of the bead leads to a gradual change of surface tension, while it escapes the initial ethanol rich area. This restores the original surface tension of pure water and temporarily terminates the bead’s motion. As the bead comes to rest, ethanol is accumulated at one end of the bead, restarting the motion. This process keeps repeating itself. Once all the stored ethanol is released from the bead, the motion of the bead comes to a final rest. The pattern of the bead motion can be readily tuned by changing the experimental parameters such as the duration of oxidation time and the amount of ethanol loaded inside the bead. Beads with longer oxidation time and with a high amount of ethanol loading propelled continuously, because the release of ethanol is so fast that the bead is not able to reach the conditions of reestablishing the initial surface tension of pure water. In general the motion is relatively fast initially, and eventually slows down with time. Velev et al. developed a model in order to calculate the release rate of ethanol from self-propelled hydrogel objects at the water-air interface [5]. Based on this model and making a couple of simplifying assumptions we have roughly estimated the characteristic time scale of propulsion for the asymmetric beads studied here (see SI). The ethanol entirely diffuses out of the bead into a zero concentration bath in approx. 3000 s, which is in good agreement with the experimentally observed time scale of self-propelling motion, because typically the beads are moving for up to one hour.
Fig. 3 Tracking of trajectories of a (a) bead soaked in ethanol for 30 min and oxidized for 10 min, which propels in a continuous fashion, (b) bead loaded with ethanol for 10 min and oxidized for 10 min exhibited pulsating motion, (c) unmodified bead loaded with ethanol showing whirling type motion. This journal is © The Royal Society of Chemistry 2012
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In contrast, no long range motion is observed for an unmodified bead loaded with ethanol (Fig. 3c and Video 4 in SI), supporting the fact that the asymmetric structure is the crucial ingredient to achieve directed motion. For the pulsating motion the effect of oxidation on the self-propelling behavior has been further characterized by analyzing the relationship between the waiting time (time between two consecutive jumps) and the distance travelled by the beads in each jump (see Fig. S3). This study showed that beads having the same amount of ethanol loading, but longer oxidation times, initially travel longer distances (~2cm) during every jump, which decrease gradually with time. The waiting period, which was initially quite short, increased considerably later on, due to the depletion of ethanol inside the beads. This illustrates that in addition to changing the ethanol loading, the dynamic behavior can also be modulated by changing the oxidation time, which influences the degree of permeability in the oxidized area of the bead. Another possibility is to change the applied electric field during the oxidation step, which will change the size of the oxidized area and therefore also the ethanol release kinetics. Increasing the electric field also allows addressing smaller beads. It has been shown in the past that even quite small conducting objects, such as short pieces of carbon nanotubes (l = 200nm), can still be modified by BE [35], so in principle a down-scaling is possible. In the present case of moving hydrogel particles, however, it makes less sense to scale them down to such small dimensions, because the surface of the sphere, which is used for the ethanol release, scales with the square of the radius, whereas the volume of stored ethanol decreases with the cube of the radius. The practical consequence is that a very small particle will release the stored amount of ethanol in a very short time, so it should move only for a couple of seconds before it is empty. In conclusion, we have successfully designed hybrid polyaniline– alginate beads by integrating the unique and inherent properties of conductive polyaniline with those of straightforward generated alginate hydrogels. Due to their conductivity, such hydrogel beads could be for the first time asymmetrically modified by BE. The intrinsic anisotropy of the bipolar electrochemical process results in soft beads with a partially oxidized structure on one side. They exhibit pulsating and continuous motion depending on the degree of the generated asymmetry and the ethanol loading. Analysis of the motion allowed us to conclude that this is due to the emergence of dynamic surface tension gradients. They are established by the asymmetric release of ethanol from one end of the conductive hydrogel, which lowers the surface tension locally and drives the beads in the direction of higher surface tension. The type of motion of these beads can be readily tuned by varying the oxidation time and concentration of ethanol inside the beads. Self-propulsion of such Janus beads offers a promising application potential for selfassembly, sensors, environmental monitoring and detoxification of water [36, 37]. As such motors can be custom designed, the approach enriches the spectrum of self-propelled objects [38] and they may encapsulate for example various chemical and pharmaceutically active compounds [39], which also opens up interesting perspectives in the field of controlled dynamic drug delivery [40]. This journal is © The Royal Society of Chemistry 2012
COMMUNICATION DOI: 10.1039/C5SM00273G
Acknowledgments This work was partially funded by the ANR Program Emergence (Project PROJANUS) under contract ANR-2011EMMA-007-01. The authors also thank the French laboratory of excellence AMADEus (Advanced Materials by Design) for a grant supporting A.S.
Notes and references a
Univ. Bordeaux, ISM, UMR 5255, Site ENSCBP, 16 avenue Pey Berland, 33607 Pessac, France * Phone: 0033540006573. E-mail: [email protected]
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S. Sánchez, L. Soler, J. Katuri, Angew.Chem.Int.Ed., 2015, 54, 1414. J. Wang, Nanomachines: Fundamentals & Applications, 2013. F. Lamberti, S. Giulitti, M. Giomo and N. Elvassore, J. Mater. Chem. B 2013, 1, 5083. M. Guix , C. C. Mayorga-Martinez, A. Merkoçi, Chem. Rev., 2014, 114, 6285. R. Sharma, S.T. Chang, O. D. Velev, Langmuir, 2012, 28, 10128. R. F. Ismagilov, A. Schwartz, N. Bowden, and G. M. Whitesides, Angew. Chem. Int. Ed., 2002, 41(4), 652. W. F. Paxton, P. T. Baker, T. R. Kline, Y. Wang, T. E. Mallouk, A. Sen, J. Am. Chem. Soc., 2006, 128, 14881. J. G. Gibbs and Y. P. Zhao, Appl. Phys. Lett., 2009, 94, 163104. Z. Rozynek, A. Mikkelsen, P. Dommersnes, J. O. Fossum, Nat. Commun., 2014, 5, 3945. S. E. Fosdick, K. N. Knust, K. Scida, R. M. Crooks, Angew.Chem.Int.Ed., 2013, 52, 10438 G. Loget, D. Zigah, L. Bouffier, N. Sojic, and A. Kuhn, Acc. Chem. Res., 2013, 46, 2513. M. Fleischmann, J. Ghoroghchain, S. Pons, J. Phys. Chem., 1985, 89, 5530. J. Roche, G. Loget, D. Zigah, Z. Fattah, B. Goudeau, S. Arbault, L. Bouffier, A. Kuhn, Chem.Sci., 2014, 5, 1961 S. Inagi, Y. Ishiguro, M. Atobe, T. Fuchigami, Angew. Chem.Int.Ed., 2010, 49, 10136 M. Ongaro, J. Roche, A. Kuhn, P. Ugo, ChemElectroChem, 2014, 1, 2048 W. Zhao, L. Glavas, K. Odelius, U. Edlund, A. C. Albertsson, Chem. Mater., 2014, 26, 4265. W. Zhao, L. Glavas, K. Odelius, U. Edlund, A. C. Albertsson, Polymer, 2014, 55, 2967. B. Guo, A. Finne-Wistrand, A. C. Albertsson, Biomacromolecules, 2011, 12, 2601. B. B. Lee, P. Ravindra, E. S. Chan, Chem. Eng. Technol., 2013, 36, 1627. J. Name., 2012, 00, 1-3 | 4
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20 21 22 23
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24 25 26 27 28 29 30 31 32 33 34 35
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J. Sun, H. Tan, Materials, 2013, 6, 1285. L. W. Chan, Y. Jin, P. W. S. Heng, Int. J. Pharm., 2005, 242, 255. P. Aslani, R. A. Kennedy, J. Control. Release, 1996, 75. M. Gao, G. Zhang, G. Zhang, X. Wang, S. Wang, Y. Yang, Polym. Degrad. Stab., 2011, 96, 1799. Y. Ishiguro, S. Inagi, T. Fuchigami, Langmuir, 2011, 27, 7158. G. Loget, J. Roche, A. Kuhn, Adv.Mater., 2012, 24, 5111 H. Jin, A. Marmur, O. Ikkalaa and R. H. A. Ras, Chem. Sci., 2012, 3, 2526. D. Pesach and A. Marmur, Langmuir, 1987, 3, 519. R. Tadmor, J. Coll. Interf. Sci., 2009, 332, 451. N. Bassik, B. T. Abebe, D. H. Gracias, Langmuir, 2008, 24, 12158. N. J. Suematsu, T. Sasaki, S. Nakata, H. Kitahata, Langmuir, 2014, 27, 8101. V. I. Kovalchuk, H. Kamusewitz, D. Vollhardt, N. M. Kovalchuk, Phys. Rev. E, 1999, 60, 2029. G. Zhao, M. Pumera, J. Phys. Chem. B, 2012, 116, 10960. G. Zhao, T. H. Seah, M. Pumera, Chem. Eur. J., 2011, 17, 12020 . J. P. Gong, S. Matsumoto, M. Uchida, N. Isogai, Y. Osada, J. Phys. Chem., 1996, 100, 11092. C. Warakulwit, T. Nguyen, J. Majimel, M.-H. Delville, V. Lapeyre, P. Garrigue, V. Ravaine, J. Limtrakul, A. Kuhn, NanoLett. 2008, 8, 500. J. Chen, H. Zhang, X. Zheng, H. Cui, Appl. Phys. Lett., 2014, 4, 031325. L. Soler, S. Sanchez, Nanoscale, 2014, 6, 7175. G. Zhao, M. Pumera, Chem. Asian. J., 2012, 7, 1994 L. K. E. A. Abdelmohsen, F. Peng, Y. Tu, D. A. Wilson, J. Mater. Chem. B, 2014, 2, 2395. D. Patra, S. Sengupta, W. Duan, H. Zhang, R. Pavlick, A. Sen, Nanoscale, 2013, 5, 1273.
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COMMUNICATION DOI: 10.1039/C5SM00273G
TOC graphic Electrically conducting hydrogel particles are synthesized and transformed into Janus particles by bipolar electrochemistry. The generated asymmetric permeability of the particles allows a site selective release of the solvent stored inside, leading to different types of motion as a function of the degree of asymmetry.
Soft Matter Accepted Manuscript
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Soft Matter Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: A. Srinivasan, J. Roche, V. Ravaine and A. Kuhn, Soft Matter, 2015, DOI: 10.1039/C5SM00273G.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Journal Name
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Cite this: DOI: 10.1039/x0xx00000x
Synthesis of conducting asymmetric hydrogel particles showing autonomous motion A. Srinivasana, J. Rochea, V. Ravainea, A. Kuhna*
Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/
In the present work, we introduce a new approach for the synthesis of asymmetric particles made from electrically conducting polyaniline-alginate hydrogels by using bipolar electrochemistry. Such an intrinsic break of symmetry allows the soft beads to exhibit tuneable motion at the air/water interface when loaded with ethanol due to a controllable directed release of the solvent.
Asymmetry is the crucial ingredient for generating directed motion of objects, however the efficient synthesis of asymmetric particles is still a challenge in many respects [1]. Self-propelled particles, which are relying on an intrinsic asymmetry, are currently in the spotlight of various fields of science due to some promising potential applications for example in biosensing, drug release and lab-on-chip devices [2-4]. Over the last decade, a large number of artificial systems showing autonomous motion have been developed. Controlling their movement in certain environments is not straightforward, especially with respect to the directionality along predetermined trajectories. As the key criterion for propulsion of these swimmers lies in breaking the symmetry of the system, the majority of previously reported approaches use a built-in asymmetry in order to control the motion [5-8]. The synthesis of asymmetric structures requires, with few exceptions [9], surfaces or interfaces in order to break the symmetry. With this respect, bipolar electrochemistry (BE) offers in a general sense an attractive alternative for the generation of asymmetric objects. As a complement to classical approaches (i.e. electrodeposition, template based synthesis, masking etc…), BE is a simple and cost effective method for the production of asymmetric objects [10,11]. It is a technique which induces two spatially separated electrochemical reactions (oxidation and reduction) on the surface of conductive objects [12]. This allows controlling the shape, position and size of the asymmetric feature easily by changing the electric field in the solution [13]. So far the concept has been exclusively used for the modification of objects made out of solid materials such as different metals, carbon [11], polymers [14] and semiconductors [15]. In the present work, bipolar electrochemistry is applied for the first time to particles made out of soft materials, namely hydrogels. Asymmetric hybrid polyaniline-alginate beads are synthesized by this approach, where the hydrogel part offers the porous and soft structure needed to entrap and release a molecule, whereas the interpenetrated polyaniline brings the required electrical conductivity for symmetry breaking and modulating the permeability. As a result of their asymmetry, these beads present non-symmetrical release profiles. Therefore, when loaded with a
This journal is © The Royal Society of Chemistry 2012
solvent cargo, in the present case ethanol, they can undergo autonomous motion at a water/air interface. There are several reports describing the fabrication of electroconductive hydrogels (ECHs) in the literature [16-18]. In the present case a straight forward two-step process has been employed for the synthesis of hybrid polyaniline-alginate electroconductive hydrogels with tunable electrical conductivity and morphology (Fig. 1a). Alginate hydrogel spheres were prepared in the first step by the drop-wise addition of sodium alginate sol (1.0 wt %) into a gelation bath containing 0.05 M calcium chloride [19,20]. The formed droplets of sodium alginate immediately solidify into gel particles in the calcium chloride solution. These beads were kept in the gelation bath for 1 minute during which the beads reached a size in the mm range, involving the rearrangement of the gel structure, due to the crosslinking induced by the calcium ions [21, 22]. Subsequently the preformed hydrogel spheres were loaded with aniline monomer (1 M in 1 M HCl) for 14 h and then chemically polymerized with ammonium persulfate (APS) (0.125 M in 1 M HCl) for 1 h. During the synthesis, we observed that the morphology of the polyaniline-alginate based electro-conductive hydrogel beads can be tuned by changing the concentration of cross-linking agent (CaCl2), while simultaneously altering the conductivity of these beads. At high concentrations of calcium chloride (0.2 M) an extensively cross-linked hydrogel structure was formed, which hinders the penetration of aniline monomer and oxidant molecules into the hydrogel matrix, and resulted in core-shell morphologies with a poor conductivity due to the deficiency of conductive polyaniline inside the hydrogel (Fig. 1b). In contrast, when using lower calcium concentrations (0.05 M) a highly permeable gel structure was obtained, allowing an easy diffusion of aniline and oxidizing agent, so that an interpenetrating network of polyaniline and alginate is formed (Fig. 1c), leading to high conductivity.
J. Name., 2012, 00, 1-3 | 1
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Fig. 1 (a) Summary of the sequential steps involved in the preparation of PANi-alg electroconductive hydrogel beads. The morphology of the resulting soft beads can be readily tuned by altering the calcium chloride concentration during the production of Ca-alginate beads. Optical images of beads with different morphologies: (b) 0.2 M CaCl2 results in core-shell structures (c) 0.05 M CaCl2 leads to interpenetrating PANi-alg conductive hydrogel beads. potential difference ∆V that is proportional to the electric field E and the characteristic length l of the object [10,11], in that case the bead diameter, according to the following equation: ∆V = E l
Fig. 2 (a) Scheme of the experimental system illustrating the bipolar electrode behavior of a spherical conducting object immersed in an electrolyte solution and exposed to an electric field. (b) corresponding asymmetrically modified PANi-alg electroconductive hydrogel bead, obtained by exposure for 5 min to an electric field of 66V/cm (c) fluorescence microscope image of a bead releasing fluorescein dye in an asymmetric way, synthesized by exposure to an electric field of 66V/cm for 15min. The obtained conductive beads were asymmetrically modified by bipolar electrochemistry, which relies on the fact that when the soft beads are immersed in an electrolyte solution and are exposed to an external electric field, the polarization of the substrate with respect to the surrounding medium generates a This journal is © The Royal Society of Chemistry 2012
(1)
For a sufficiently high ∆V, two electrochemical reactions (i.e. oxidation at the anodic pole and reduction at cathodic pole) can occur simultaneously at the two extremities of the object (Fig. 2a). The intrinsic break of symmetry provided by this technique was used to synthesize asymmetric soft particles in a controlled manner (Fig. 2b). The oxidation of the conductive polyaniline-alginate hydrogel beads by BE was carried out according to Fig. 2a after loading them with ethanol for various periods of time (5, 10, 15 and 30 minutes respectively). The modification with the aforementioned technique usually involves a high potential, depending on the distance between the feeder electrodes and the size of the particles. Actually, the experimental key parameter is the electric field. In order to estimate the necessary amplitude of the electric field one has to consider the actual potentials needed to drive the two redox reactions on the right and the left side of the particle respectively (Fig.2a). In order to oxidize polyaniline into its pernigraniline form, a typical potential of around 0.8 V vs SCE has to be used. On the other hand reduction of polyaniline into its leucoemeraldine form occurs around 0V vs SCE and proton reduction on such surfaces is rather difficult and needs significant overpotentials (E < - 0.2 V vs SCE). This means that between the two sides of the particle a potential difference of at least 1 V needs to be established in order to trigger oxidation and reduction reactions simultaneously. Consequently a minimum electric field of 10 V/cm has to be applied to the solution when assuming a typical dimension of the beads in the mm range. However this is only the lower threshold value, because such an electric field would only lead to reactions at the very extremities of the particle. Figure 2b illustrates that the modification of the particle reaches almost the middle of the bead, meaning that for those parts which are close to the center, the polarisation has still to be high enough in order to trigger the redox reaction. Therefore, in the present example a field of 66 V/cm has been used. When exposed to J. Name., 2012, 00, 1-3 | 2
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this field strength, polyaniline undergoes electrochemical overoxidation, leading to degradation of the polymer backbone due to the formation of quinone derivatives namely hydroquinone (HQ) and benzoquinone (BQ) [23] and forming the typical green polymer visible in Fig. 2b. This leads to a controlled degradation at the oxidized side of the modified bead, increasing its permeability. The degree of permeability on the oxidized end can also be tuned by adjusting the exposure time to the high potential. For short exposure times only a color change is observed due to oxidation of the conducting polymer [24] as illustrated in Fig. 2b for a 5min exposure to the electric field, whereas longer exposure (15min) results in more degraded beads as shown in Fig. 2c. In the latter case one can see that part of the initially quasi-spherical bead has been removed by oxidation. Due to the size of the particles and the distance between the electrodes only a few particles can be modified simultaneously with this specific setup. As the density of the gel beads is higher than ethanol they are located at the bottom of the cell during modification, which has the advantage to prevent rotation while the electric field is applied. However in order to generate many particles in parallel and in the whole volume of the reactor, a similar approach as the one already reported for solid objects might be applied by artificially increasing the viscosity of the medium [25]. Interestingly, the hydrophilic/hydrophobic character of the two sides of the beads is not significantly changed during this process as we could observe by contact angle measurements (see Figure S4, SI). Most likely the hydrophilicity is solely governed by the alginate gel network and this doesn’t change upon electrochemical oxidation. In order to demonstrate the resulting asymmetric permeability, the bead has been loaded with ethanol/fluorescein before the bipolar modification. When the bead is placed in water, after having been exposed to the electric field, the asymmetric release of fluorescein from the degraded side of the bead is clearly visible (upper left part in Fig. 2c). We assume that if fluorescein is able to leak out preferentially on one side of the bead, the same is true also for smaller molecules such as ethanol. In the following this is used to induce directed motion. The resulting modified beads were floated on the water surface in a plastic dish (12 cm diameter) and motion of the beads was recorded with a digital camera. The beads exhibited two different types of motion (periodic and continuous motion) depending on the duration of oxidation and the amount of ethanol inside the beads. Typical trajectories of beads are illustrated in the Figs. 3 a & b as well in Video 1&2 in the SI. In order to better illustrate that ethanol is released in an asymmetric way due to a higher permeability created on the oxidized side, fluorescein dye was
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COMMUNICATION DOI: 10.1039/C5SM00273G
again added to the ethanol during the modification step to track later the ethanol/dye release. In a prototype experiment a dye loaded bead was carefully immobilized in a UV-Vis cuvette filled with water. We observed continuous release of ethanol/fluorescein at the oxidized end (Video 3 in SI). The released ethanol is transported to the air/water interface due to density differences of the two liquids (ρୌ =0.789 g/cm3). An UV-Vis absorption study allowed us to further illustrate the differential release of ethanol from both ends of a hydrogel that has been treated by bipolar electrochemistry (Figs. S1 and S2 in SI). The propulsion of the hydrogel beads on the water surface originates from this asymmetric release of ethanol due to the increased permeability created at the oxidized side of the modified bead. The released ethanol rapidly spreads over the water surface due to low surface energy and low density, thus giving rise to surface tension gradients. This phenomenon is well known as Marangoni effect, which is responsible for the self-propulsion of various kinds of swimmers [5, 26-34]. In the case of the pulsating mode, also observed by other authors for other hydrogel systems [5], the motion of the bead leads to a gradual change of surface tension, while it escapes the initial ethanol rich area. This restores the original surface tension of pure water and temporarily terminates the bead’s motion. As the bead comes to rest, ethanol is accumulated at one end of the bead, restarting the motion. This process keeps repeating itself. Once all the stored ethanol is released from the bead, the motion of the bead comes to a final rest. The pattern of the bead motion can be readily tuned by changing the experimental parameters such as the duration of oxidation time and the amount of ethanol loaded inside the bead. Beads with longer oxidation time and with a high amount of ethanol loading propelled continuously, because the release of ethanol is so fast that the bead is not able to reach the conditions of reestablishing the initial surface tension of pure water. In general the motion is relatively fast initially, and eventually slows down with time. Velev et al. developed a model in order to calculate the release rate of ethanol from self-propelled hydrogel objects at the water-air interface [5]. Based on this model and making a couple of simplifying assumptions we have roughly estimated the characteristic time scale of propulsion for the asymmetric beads studied here (see SI). The ethanol entirely diffuses out of the bead into a zero concentration bath in approx. 3000 s, which is in good agreement with the experimentally observed time scale of self-propelling motion, because typically the beads are moving for up to one hour.
Fig. 3 Tracking of trajectories of a (a) bead soaked in ethanol for 30 min and oxidized for 10 min, which propels in a continuous fashion, (b) bead loaded with ethanol for 10 min and oxidized for 10 min exhibited pulsating motion, (c) unmodified bead loaded with ethanol showing whirling type motion. This journal is © The Royal Society of Chemistry 2012
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In contrast, no long range motion is observed for an unmodified bead loaded with ethanol (Fig. 3c and Video 4 in SI), supporting the fact that the asymmetric structure is the crucial ingredient to achieve directed motion. For the pulsating motion the effect of oxidation on the self-propelling behavior has been further characterized by analyzing the relationship between the waiting time (time between two consecutive jumps) and the distance travelled by the beads in each jump (see Fig. S3). This study showed that beads having the same amount of ethanol loading, but longer oxidation times, initially travel longer distances (~2cm) during every jump, which decrease gradually with time. The waiting period, which was initially quite short, increased considerably later on, due to the depletion of ethanol inside the beads. This illustrates that in addition to changing the ethanol loading, the dynamic behavior can also be modulated by changing the oxidation time, which influences the degree of permeability in the oxidized area of the bead. Another possibility is to change the applied electric field during the oxidation step, which will change the size of the oxidized area and therefore also the ethanol release kinetics. Increasing the electric field also allows addressing smaller beads. It has been shown in the past that even quite small conducting objects, such as short pieces of carbon nanotubes (l = 200nm), can still be modified by BE [35], so in principle a down-scaling is possible. In the present case of moving hydrogel particles, however, it makes less sense to scale them down to such small dimensions, because the surface of the sphere, which is used for the ethanol release, scales with the square of the radius, whereas the volume of stored ethanol decreases with the cube of the radius. The practical consequence is that a very small particle will release the stored amount of ethanol in a very short time, so it should move only for a couple of seconds before it is empty. In conclusion, we have successfully designed hybrid polyaniline– alginate beads by integrating the unique and inherent properties of conductive polyaniline with those of straightforward generated alginate hydrogels. Due to their conductivity, such hydrogel beads could be for the first time asymmetrically modified by BE. The intrinsic anisotropy of the bipolar electrochemical process results in soft beads with a partially oxidized structure on one side. They exhibit pulsating and continuous motion depending on the degree of the generated asymmetry and the ethanol loading. Analysis of the motion allowed us to conclude that this is due to the emergence of dynamic surface tension gradients. They are established by the asymmetric release of ethanol from one end of the conductive hydrogel, which lowers the surface tension locally and drives the beads in the direction of higher surface tension. The type of motion of these beads can be readily tuned by varying the oxidation time and concentration of ethanol inside the beads. Self-propulsion of such Janus beads offers a promising application potential for selfassembly, sensors, environmental monitoring and detoxification of water [36, 37]. As such motors can be custom designed, the approach enriches the spectrum of self-propelled objects [38] and they may encapsulate for example various chemical and pharmaceutically active compounds [39], which also opens up interesting perspectives in the field of controlled dynamic drug delivery [40]. This journal is © The Royal Society of Chemistry 2012
COMMUNICATION DOI: 10.1039/C5SM00273G
Acknowledgments This work was partially funded by the ANR Program Emergence (Project PROJANUS) under contract ANR-2011EMMA-007-01. The authors also thank the French laboratory of excellence AMADEus (Advanced Materials by Design) for a grant supporting A.S.
Notes and references a
Univ. Bordeaux, ISM, UMR 5255, Site ENSCBP, 16 avenue Pey Berland, 33607 Pessac, France * Phone: 0033540006573. E-mail: [email protected]
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S. Sánchez, L. Soler, J. Katuri, Angew.Chem.Int.Ed., 2015, 54, 1414. J. Wang, Nanomachines: Fundamentals & Applications, 2013. F. Lamberti, S. Giulitti, M. Giomo and N. Elvassore, J. Mater. Chem. B 2013, 1, 5083. M. Guix , C. C. Mayorga-Martinez, A. Merkoçi, Chem. Rev., 2014, 114, 6285. R. Sharma, S.T. Chang, O. D. Velev, Langmuir, 2012, 28, 10128. R. F. Ismagilov, A. Schwartz, N. Bowden, and G. M. Whitesides, Angew. Chem. Int. Ed., 2002, 41(4), 652. W. F. Paxton, P. T. Baker, T. R. Kline, Y. Wang, T. E. Mallouk, A. Sen, J. Am. Chem. Soc., 2006, 128, 14881. J. G. Gibbs and Y. P. Zhao, Appl. Phys. Lett., 2009, 94, 163104. Z. Rozynek, A. Mikkelsen, P. Dommersnes, J. O. Fossum, Nat. Commun., 2014, 5, 3945. S. E. Fosdick, K. N. Knust, K. Scida, R. M. Crooks, Angew.Chem.Int.Ed., 2013, 52, 10438 G. Loget, D. Zigah, L. Bouffier, N. Sojic, and A. Kuhn, Acc. Chem. Res., 2013, 46, 2513. M. Fleischmann, J. Ghoroghchain, S. Pons, J. Phys. Chem., 1985, 89, 5530. J. Roche, G. Loget, D. Zigah, Z. Fattah, B. Goudeau, S. Arbault, L. Bouffier, A. Kuhn, Chem.Sci., 2014, 5, 1961 S. Inagi, Y. Ishiguro, M. Atobe, T. Fuchigami, Angew. Chem.Int.Ed., 2010, 49, 10136 M. Ongaro, J. Roche, A. Kuhn, P. Ugo, ChemElectroChem, 2014, 1, 2048 W. Zhao, L. Glavas, K. Odelius, U. Edlund, A. C. Albertsson, Chem. Mater., 2014, 26, 4265. W. Zhao, L. Glavas, K. Odelius, U. Edlund, A. C. Albertsson, Polymer, 2014, 55, 2967. B. Guo, A. Finne-Wistrand, A. C. Albertsson, Biomacromolecules, 2011, 12, 2601. B. B. Lee, P. Ravindra, E. S. Chan, Chem. Eng. Technol., 2013, 36, 1627. J. Name., 2012, 00, 1-3 | 4
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20 21 22 23
Published on 15 April 2015. Downloaded by University of New England on 21/04/2015 13:32:44.
24 25 26 27 28 29 30 31 32 33 34 35
36 37 38 39 40
J. Sun, H. Tan, Materials, 2013, 6, 1285. L. W. Chan, Y. Jin, P. W. S. Heng, Int. J. Pharm., 2005, 242, 255. P. Aslani, R. A. Kennedy, J. Control. Release, 1996, 75. M. Gao, G. Zhang, G. Zhang, X. Wang, S. Wang, Y. Yang, Polym. Degrad. Stab., 2011, 96, 1799. Y. Ishiguro, S. Inagi, T. Fuchigami, Langmuir, 2011, 27, 7158. G. Loget, J. Roche, A. Kuhn, Adv.Mater., 2012, 24, 5111 H. Jin, A. Marmur, O. Ikkalaa and R. H. A. Ras, Chem. Sci., 2012, 3, 2526. D. Pesach and A. Marmur, Langmuir, 1987, 3, 519. R. Tadmor, J. Coll. Interf. Sci., 2009, 332, 451. N. Bassik, B. T. Abebe, D. H. Gracias, Langmuir, 2008, 24, 12158. N. J. Suematsu, T. Sasaki, S. Nakata, H. Kitahata, Langmuir, 2014, 27, 8101. V. I. Kovalchuk, H. Kamusewitz, D. Vollhardt, N. M. Kovalchuk, Phys. Rev. E, 1999, 60, 2029. G. Zhao, M. Pumera, J. Phys. Chem. B, 2012, 116, 10960. G. Zhao, T. H. Seah, M. Pumera, Chem. Eur. J., 2011, 17, 12020 . J. P. Gong, S. Matsumoto, M. Uchida, N. Isogai, Y. Osada, J. Phys. Chem., 1996, 100, 11092. C. Warakulwit, T. Nguyen, J. Majimel, M.-H. Delville, V. Lapeyre, P. Garrigue, V. Ravaine, J. Limtrakul, A. Kuhn, NanoLett. 2008, 8, 500. J. Chen, H. Zhang, X. Zheng, H. Cui, Appl. Phys. Lett., 2014, 4, 031325. L. Soler, S. Sanchez, Nanoscale, 2014, 6, 7175. G. Zhao, M. Pumera, Chem. Asian. J., 2012, 7, 1994 L. K. E. A. Abdelmohsen, F. Peng, Y. Tu, D. A. Wilson, J. Mater. Chem. B, 2014, 2, 2395. D. Patra, S. Sengupta, W. Duan, H. Zhang, R. Pavlick, A. Sen, Nanoscale, 2013, 5, 1273.
This journal is © The Royal Society of Chemistry 2012
COMMUNICATION DOI: 10.1039/C5SM00273G
TOC graphic Electrically conducting hydrogel particles are synthesized and transformed into Janus particles by bipolar electrochemistry. The generated asymmetric permeability of the particles allows a site selective release of the solvent stored inside, leading to different types of motion as a function of the degree of asymmetry.
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