Self-Assembly
Supramolecular Assembly of Macroscopic Building Blocks Through Self-Propelled Locomotion by Dissipating Chemical Energy Mengjiao Cheng, Guannan Ju, Yingwei Zhang, Mengmeng Song, Yajun Zhang, and Feng Shi* The formation of ordered, three-dimensional structures from separate components has been shown to improve the mechanical and/or optical properties of reinforcing materials,[1,2] optical crystals,[3,4] and tissue scaffolds.[5] However, few methods, including microfabrication through micro-electromechanical systems,[3] three-dimensional printing,[6] twophoton polymerization,[7,8] and self-assembly techniques,[9–11] have been developed to fabricate such structures. Among these, macroscopic supramolecular self-assembly is an advantageous, bottom-up strategy in which building blocks can be tailored and designed, making the assembly process flexible.[12,13] Although macroscopic supramolecular assembly and assembly at the molecular and nanoscale levels occur via similar interaction processes, the diffusion process that leads to effective collisions between building blocks presents several challenges. At the molecular and nanoscale levels, building blocks move freely via Brownian motion, resulting in a high collision frequency (≈1021 impacts per second)[14] and high efficiency for supramolecular assembly. In contrast, the assembly of components larger than ten micrometers is not affected by Brownian motion, and the collision frequency is remarkably reduced. Therefore, to realize macroscopic supramolecular assembly, the challenge to effectively construct ordered and advanced structures by self-propulsion of the building blocks should be addressed first. Currently, the major strategies that are available toward this end are mechanical rotating or shaking and the application of magnetic fields. By simply introducing external mechanical forces, Whitesides et al. used a rotating process to assemble macroscopic polymeric objects by capillary forces.[15] Harada and co-workers used a mechanical shaker
M. J. Cheng, G. N. Ju, Y. W. Zhang, M. M. Song, Prof. Y. J. Zhang, Prof. F. Shi State Key Laboratory of Chemical Resource Engineering & Key Laboratory of Carbon Fiber and Functional Polymer Ministry of Education Beijing University of Chemical Technology 15 Beisanhuan East Road Chaoyang Distrist, Beijing 100029, China E-mail: [email protected] DOI: 10.1002/smll.201400922 small 2014, 10, No. 19, 3907–3911
to induce the assembly of host/guest hydrogel building blocks.[13,16] Xu[17–19] and Khademhosseini[20,21] showed directed self-assembly of hydrogels by involving mechanical forces through shaking, acoustic, agitation as driving forces to propel building blocks, respectively. Similarly, by applying a magnetic field as an energy supply, Whitesides’ and Demirci's groups reported the direct assembly of metallic rods with magnetic segments[22] and the assembly of hydrogels loaded with magnetic nanoparticles,[23] respectively. In the above methods, the building blocks were forced to locomote by an external energy source, which acted as a passive feature of the assembly system. Although the building blocks in these studies were well designed, their assembly was triggered by the introduction of an external energy source. The building blocks alone showed little spontaneous tendency to selfassemble. Self-assembly processes should involve as little external intervention as possible,[24] thus exhibiting a spontaneity similar to that of the natural world. Life in nature presents dynamic, energy-dissipating, and self-adaptive features such as deformable cell structures, the bacterial trait of multicellularity, and fish (sardine) schooling phenomena, among many other examples. Learning from nature, researchers have begun to understand the collective motion or selfassembly behaviors of life.[25–27] This knowledge, in turn, has been used to enhance the complexity and intelligence of selfassembled systems such that building blocks can automatically form designed and ordered structures independent of human intervention. Such systems may include dynamic selfassembly features through the dissipation of energy.[24] However, to the best of our knowledge, the spontaneous, macroscopic supramolecular assembly of building blocks into advanced ordered structures has not been reported. The key challenges to achieving this goal are the introduction of energy into the assembly system to enable self-propulsion, high collision frequencies, and the formation of ordered structures rather than the formation of simple aggregates. The current study describes a self-assembly technique that harnesses the chemical energy supplied by the catalytic decomposition of hydrogen peroxide into macroscopic building blocks. These then underwent self-propulsion, interacted with each other, and finally assembled into ordered and advanced structures. The locomotion tracks and assembly behavior of the building blocks indicated dynamic and energy-dissipative
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Scheme 1. A schematic shows the fabrication of PDMS cuboids with coatings of platinum catalyst.
characteristics similar to those found in complex biological assembly systems. In a hydrogen peroxide solution, oxygen bubbles were released through decomposition of hydrogen peroxide which was catalyzed by platinum loaed on the building blocks. The building blocks were propelled by the continuous generation of bubbles. By adjusting the location of the catalyst on the building blocks, different ordered architectures were obtained, including pairs, lines, and triangles. The macroscopic building blocks showed locomotive characteristics similar to those of molecular systems during the assembly process, demonstrating the spontaneous nature of this macroscopic supramolecular assembly. The assembly process also consumed the energy of motion and exhibited dynamic assembly features of energy-dissipative systems. In order to drive and realize macroscopic supramolecular assembly, the building block was designed with catalyst of platinum for its self-propulsion through oxygen bubbles released from the decomposition of hydrogen peroxide solution, which has been well developed in various microscale systems but not yet at macroscopic scale.[28–34] The fabrication process is illustrated in Scheme 1. Two major factors were considered in the building block design: 1) the chemical composition of the building block itself and 2) the method by which the platinum catalyst is introduced. Thermally cured, biocompatible polydimethylsiloxane (PDMS) cuboids measuring 8 mm × 8 mm × 3.5 mm were chosen as the main bodies of the building blocks. The density of PDMS is comparable to that of water and its surface is hydrophobic; thus, the PDMS cuboids float on the surface of dilute aqueous solutions and aggregate according to the principle of minimizing interfacial free energy.[35] An electrochemical deposition method was used to form the catalyst.[36] To obtain rough gold structures, a gold-coated glass slide was immersed in a mixture of the H2SO4 and HAuCl4, and electrochemical deposition was conducted at –200 mV for 1600 s. From Supporting Information Figure S1a,b, we can observe that microscale dendritic structures form on the surface, which is assigned to gold clusters by corresponding energy dispersive spectrometer (EDX) pattern. The same slide deposited with gold aggregates was then immersed in a mixture of H2SO4 and H2PtCl6 for 800 s to form a layer of platinum aggregates, which was characterized by SEM image and EDX pattern in Supporting Information Figure S1c,d; the surface presents a rough surface morphology with scattered aggregates and the platinum
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Figure 1. a–d) Snapshots show the process of locomotion and assembly of two PDMS cuboids with platinum catalyst deposited on opposite side surfaces in a hydrogen peroxide solution.
element appeared accompanying with aurum. The pre-deposition of rough gold clusters served to increase the final surface roughness. The resulting hierarchical platinum structure further increased surface roughness, thereby amplifying the hydrophilicity of the film, and also increased the specific surface area, which improved the catalytic efficiency and bubble release rate.[37,38] The platinum catalyst was transferred to the PDMS cuboids by sticking the glass slides onto the 8 mm× 3.5 mm-side faces of the cuboids with a thin layer of PDMS fluid, followed by a second thermal crosslinking. The type of structure formed can be adjusted by varying the number of slides bearing platinum structures and/or by changing the side of the PDMS cuboid onto which the platinum catalyst is attached. PDMS building blocks were fabricated with catalyst deposited on opposite faces. When two such cuboids were placed in a hydrogen peroxide solution (aq, 5 wt%), numerous bubbles were continuously formed and released at the catalyst-bearing side faces, as shown in Figure 1a. This created an imbalance between the forces exerted on the two opposing sides of the cuboids, inducing random motion of the building blocks, similar to Brownian motion at the molecular level. Propelled by the bubbles, the PDMS cuboids moved across the surface of the solution, largely increasing the chance of collision and providing more opportunities for assembly. The movements of the building blocks were tracked in 5-s intervals, and the tracks are shown in Supporting Information Figure S2. A video recording of the motion of the blocks is also provided in Supporting Information Movie S1. At the onset, one of the cuboids moved in a helical fashion while the others progressed along the wall of the container. A string of bubbles released from one side of the PDMS cuboid overlapped with an analogous string from another cuboid. Next, the overlap of numerous bubble streams pushed the PDMS cuboids toward the boundary of the container, resulting in the detachment of two strings of bubbles accompanied by opposite movements of the cuboid. Although both the glass wall of the Petri dish and the platinum-coated side
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of the PDMS cuboids are hydrophilic within the interactive distance,[39] the bubbles prevented the cuboids from attaching to the Petri dish wall. The asymmetric generation and attachment of bubbles on opposing sides of the cuboids resulted in a force imbalance that propelled the cuboids, causing them to assemble. The driving force for this supramolecular assembly is attributed to capillary forces between the building blocks within their interactive distance.[39,40] As shown in Figure 1b–d, after their initial locomotion and collision, two PDMS cuboids formed a pair with two hydrophobic surfaces facing each other. This phenomenon was interpreted as follows. The uncoated and platinum-coated surfaces of the PDMS cuboids have different wetting properties. The bare, uncoated PDMS surfaces are hydrophobic while the surfaces with platinum are hydrophilic. When in contact with water, the two surfaces form positive or negative menisci. As two surfaces with the same wettability approach their interactive distance under self-propulsion, these menisci gradually overlap and the resulting lateral force pulls the two building blocks together (Figure S3a, Supporting Information). In this way, the system minimizes the total interfacial free energy.[35,39,40] We have measured and estimated the critical interactive distance by hunging two building blocks above water surface and gradually moving them towards each other until their assembly as shown in Supporting Information Figure S3b,c, and the resulted distance is about 18.0 mm. Although the assembly of two PDMS cuboids demonstrates the supramolecular assembly of macroscopic building blocks through self-propelled locomotion and the dissipation of chemical energy, the possibility of coincidental assembly must be addressed. It is reasonable to assume that since there were only two building blocks in the container, they would sooner or later associate with one another. A single pair of cuboids does not necessarily represent an ordered and advanced structure with a periodic arrangement. The number of building blocks was therefore increased in order to further demonstrate the energy-dissipating self-assembly of these PDMS cuboids. As shown in Figure 2, four identical PDMS cuboids with platinum catalyst deposited on opposing sides were placed in a solution of hydrogen peroxide. As observed in previous experiments, rapid decomposition of the hydrogen peroxide upon contacting the platinum surface released streams of bubbles from the coated cuboid surfaces. As discussed above, the imbalanced forces applied to opposite sides of the cuboids induced random locomotion. The locomotive paths of these four PDMS cuboids are shown in Supporting Information Figure S4. The cuboids meet and assemble to form two separate pairs with their hydrophobic faces oriented toward each other. Bubbles were continually forming on the opposite side surfaces, which provided the driving force for assembly. Finally, the two pairs assembled into an ordered line structure composed of four PDMS cuboids connected by hydrophobic surfaces. The line structure was held stable for a relatively long time due to the decrease in the area of the hydrophobic surface exposed to the peroxide solution and the resulting minimization of the overall interfacial free energy of the system. Thus, four identical PDMS cuboids formed an ordered structure resulting from selective self-assembly as opposed to the coincidental small 2014, 10, No. 19, 3907–3911
Figure 2. Snapshots show four PDMS cuboids bearing platinum catalyst on opposite side surfaces when a) placed on a hydrogen peroxide solution, b,c) randomly moving, colliding, and assembling, and d) assembled in a final line structure.
formation of random aggregates (for the entire process, please see Supporting Information Movie S2). The spontaneous supramolecular assembly of macroscopic building blocks is the first step in the creation of more massive features composed of a large collection of subunits.[41] The design and tailoring of specific building blocks can not only benefit the large-scale self-assembly of ordered structures, but can also be used to create specific advanced geometries. For example, when hydrophilic platinum coatings were deposited on adjacent surfaces of PDMS cuboids, the oxygen bubbles released from the hydrogen peroxide solution attached to the two adjacent sides. This kind of PDMS building block should act as a linker, connecting other building blocks along the remaining hydrophobic sides. To confirm this hypothesis, we placed three PDMS cuboids into a solution of hydrogen peroxide. Two of the cuboids were identical, with platinum catalyst deposited on opposing faces. The other cuboid was coated with platinum catalyst on two adjacent faces. Once placed in the peroxide solution, the three cuboids moved across the solution surface as described above. The linker cuboid approached and attached to one of the other cuboids along their hydrophobic surfaces. The assembled pair was then driven by the force imbalance at the bubbling surfaces and collided with the third PDMS cuboid, which attached to the remaining hydrophobic surface of the linker cuboid. As seen in Figure 3, this resulted in the formation of a triangular structure with the linker cuboid at the vertex (Movie S3, Supporting Information). After formation of the triangle, both of the PDMS cuboids bearing platinum catalyst on opposite sides presented a hydrophobic surface for further assembly. It is important to note that the assembled pair, the line, and the triangle structures each left exposed hydrophobic surfaces that would be available for further assembly. This indicates the potential for assembly
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Experimental Section
Figure 3. Snapshots show three PDMS cuboids: two had platinum catalyst on opposite sides and one had platinum deposited on adjacent sides (linker). a) The cuboids were placed successively into a peroxide solution. b) The linker formed a pair with one cuboid and c) went on to assemble with the other cuboid to form d) the final triangular geometry.
following pre-formed geometries. In addition, the geometry of the assembled structures could be “frozen” by creating “end blocks” with an exposed hydrophilic platinum surface. Such flexibility in building block design allows the control required to create ordered structures. The platinum catalyst not only supplies the energy of motion to the macroscopic building blocks but also provides a means of designing the assembling components and determining the final assembled geometry. To summarize, supramolecular assemblies of macroscopic building blocks were formed through the self-propelled motion of PDMS cuboids driven by the catalytic decomposition of hydrogen peroxide on a platinum surface. Rough platinum aggregates were deposited on various sides of the cuboids and used to generate bubbles upon contact with a hydrogen peroxide solution. The random locomotion of the building blocks, driven by bubble formation, at the macroscopic scale is similar to the Brownian motion of molecules observed at the molecular or nanoscale. This type of motion significantly enhanced the collision frequency between macroscopic PDMS cuboids. Propelled by oxygen bubbles released from the decomposition of hydrogen peroxide, the PDMS building blocks formed various structures, including pairs, lines, and triangles depending on which sides of the individual cuboids had been coated with platinum catalyst. The assembly process joined the hydrophobic surfaces of the cuboids, thereby minimizing the overall interfacial free energy of the system. In particular, the line and triangle structures exhibited the dynamic and energy-dissipating characteristics of natural biological assembly systems. This demonstrates that spontaneous macroscopic supramolecular assembly may provide novel strategies for the parallel assembly of massive building blocks.
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Materials: PDMS (Sylgand 184) and its curing agent were purchased from Dow Corning. Polymethyl methacrylate (PMMA) templates with arrays of cubic cavities (8 mm × 8 mm × 3 mm) were commercially manufactured. Glass slides coated with a gold layer of 100 nm through evaporating gold under vacuum were obtained from Changchun institute of applied chemistry Chinese academy of sciences. Fabrication of PDMS Cuboids Loaded with Pt Catalyst: Firstly the PDMS pre-polymer and its corresponding curing agent were mixed by stirring for 15 min and underwent a bubble removing process under vacuum for 0.5 h. Secondly the mixture was poured into the PMMA template with flat PMMA plates on both sides, followed by heating at 65 °C overnight. Thirdly glass slides coated with gold was deposited with gold aggregates through an electrochemical deposition process by immersing in a mixture of the H2SO4 (aq, 0.5 m) and HAuCl4 (4 mg/mL). The electrochemical deposition was conducted under a impedance-time mode at –200 mV for 1600 s, using a platinum electrode as the counter-electrode and Ag/AgCl as the reference electrode. Subsequently, the glass slides were deposited with plainum aggregates by immersing in a mixture of the H2SO4 (aq, 0.5 m) and H2PtCl6 (4 mg/mL) for 800 s under identical mode. The slides were cut into small pieces (7 mm × 3 mm) for use. Fourthly the pieces bearing platinum structures above gold aggregates were attached onto designated side surfaces of the PDMS cuboids with a thin layer of PDMS fluid, followed by a second thermal crosslinking.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements M.J.C. and G.N.J. contributed equally to this work. This work was supported by NSFC (21374006), the Program of the Co-Construction with Beijing Municipal Commission of Education of China, Program for New Century Excellent Talents in University (NCET10–0211), the Fok Ying Tung Education Foundation (131013), Open Project of State Key Laboratory of Supramolecular Structure and Materials (SKLSSM201401) and Beijing Young Talents Plan (YETP0488).
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[5] P. X. Ma, Mater. Today 2004, 7, 30. [6] D. W. Hutmacher, J. Biomater. Sci., Polym. Ed. 2001, 12, 107. [7] R. G. Wylie, S. Ahsan, Y. Aizawa, K. L. Maxwell, C. M. Morshead, M. S. Shoichet, Nat. Mater. 2011, 10, 799. [8] C. A. DeForest, B. D. Polizzotti, K. S. Anseth, Nat. Mater. 2009, 8, 659. [9] D. H. Gracias, Science 2000, 289, 1170. [10] Z. Zhang, P. Pfleiderer, A. B. Schofield, C. Clasen, J. Vermant, J. Am. Chem. Soc. 2010, 133, 392. [11] J. Y. Wang, Y. Wang, S. S. Sheiko, D. E. Betts, J. M. DeSimone, J. Am. Chem. Soc. 2012, 134, 5801. [12] B. Schulte, M. Tsotsalas, M. Becker, A. Studer, L. De Cola, Angew. Chem. 2010, 122, 7033; Angew. Chem. Int. Ed. 2010, 49, 6881. [13] A. Harada, R. Kobayashi, Y. Takashima, A. Hashidzume, H. Yamaguchi, Nat. Chem. 2011, 3, 34. [14] S. Chandrasekhar, Rev. Mod. Phys. 1943, 15, 1. [15] J. Tien, T. L. Breen, G. M. Whitesides, J. Am. Chem. Soc. 1998, 120, 12670. [16] H. Yamaguchi, Y. Kobayashi, R. Kobayashi, Y. Takashima, A. Hashidzume, A. Harada, Nat. Commun. 2012, 3, 603. [17] F. Xu, C. M. Wu, V. Rengarajan, T. D. Finley, H. O. Keles, Y. Sung, B. Q. Li, U. A. Gurkan, U. Demirci, Adv. Mater. 2011, 23, 4254. [18] Y. L. Han, Y. S. Yang, S. B. Liu, J. H. Wu, Y. M. Chen, T. J. Lu, F. Xu, Biofabrication 2013, 5, 035004. [19] F. Xu, T. D. Finley, M. Turkaydin, Y. Sung, U. A. Gurkan, A. S. Yavuz, R. O. Guldiken, U. Demirci, Biomaterials 2011, 32, 7847. [20] H. Qi, M. Ghodousi, Y. Du, C. Grun, H. Bae, P. Yin, A. Khademhosseini, Nat. Commun. 2013, 4, 2275. [21] Y. N. Du, E. Lo, S. Ali, A. Khademhosseini, Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 9522. [22] J. C. Love, A. R. Urbach, M. G. Prentiss, G. M. Whitesides, J. Am. Chem. Soc. 2003, 125, 12696. [23] S. Tasoglu, D. Kavaz, U. A. Gurkan, S. Guven, P. Chen, R. Zheng, U. Demirci, Adv. Mater. 2013, 25, 1137. [24] G. M. Whitesides, B. Grzybowski, Science 2002, 295, 2418. [25] J. A. Shapiro, Annu. Rev. Microbiol. 1998, 52, 81.
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[26] N. Shimoyama, K. Sugawara, T. Mizuguchi, Y. Hayakawa, M. Sano, Phys. Rev. Lett. 1996, 76, 3870. [27] Y. Itino, T. Ohta, J. Phys. Soc. Jpn. 2012, 81, 104007. [28] R. F. Ismagilov, A. Schwartz, N. Bowden, G. M. Whitesides, Angew. Chem. 2002, 114, 674; Angew. Chem. Int. Ed. 2002, 41, 652. [29] A. A. Solovev, E. J. Smith, C. C. Bof’ Bufon, S. Sanchez, O. G. Schmidt, Angew. Chem. 2011, 123, 11067; Angew. Chem. Int. Ed. 2011, 50, 10875. [30] W. Wang, W. Duan, A. Sen, T. E. Mallouk, Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 17744. [31] M. Guix, J. Orozco, M. García, W. Gao, S. Sattayasamitsathit, A. Merkoçi, A. Escarpa, J. Wang, ACS Nano 2012, 6, 4445. [32] S. Ebbens, R. A. L. Jones, A. J. Ryan, R. Golestanian, J. R. Howse, Physical Review E 2010, 82, 015304. [33] G. A. Ozin, I. Manners, S. Fournier-Bidoz, A. Arsenault, Adv. Mater. 2005, 17, 3011. [34] A. A. Solovev, W. Xi, D. H. Gracias, S. M. Harazim, C. Deneke, S. Sanchez, O. G. Schmidt, ACS Nano 2012, 6, 1751. [35] N. B. Bowden, M. Weck, I. S. Choi, G. M. Whitesides, Acc. Chem. Res. 2001, 34, 231. [36] a) M. Xiao, M. J. Cheng, Y. J. Zhang, F. Shi, Small 2013, 9, 2509; b) M. Xiao, X. P. Guo, M. J. Cheng, G. N. Ju, Y. J. Zhang, F. Shi, Small 2014, 10, 859. [37] Y. Gao, M. Cheng, B. Wang, Z. Feng, F. Shi, Adv. Mater. 2010, 22, 5125. [38] G. Ju, M. Cheng, M. Xiao, J. Xu, K. Pan, X. Wang, Y. Zhang, F. Shi, Adv. Mater. 2013, 25, 2915. [39] N. Bowden, Science 1997, 276, 233. [40] E. P. Lewandowski, J. A. Bernate, A. Tseng, P. C. Searson, K. J. Stebe, Soft Matter 2009, 5, 886. [41] D. S. Goodsell, Bionanotechnology: Lessons From Nature, Wiley, Hoboken, NJ, USA 2004.
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Received: April 3, 2014 Revised: May 4, 2014 Published online: May 19, 2014
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Supramolecular Assembly of Macroscopic Building Blocks Through Self-Propelled Locomotion by Dissipating Chemical Energy Mengjiao Cheng, Guannan Ju, Yingwei Zhang, Mengmeng Song, Yajun Zhang, and Feng Shi* The formation of ordered, three-dimensional structures from separate components has been shown to improve the mechanical and/or optical properties of reinforcing materials,[1,2] optical crystals,[3,4] and tissue scaffolds.[5] However, few methods, including microfabrication through micro-electromechanical systems,[3] three-dimensional printing,[6] twophoton polymerization,[7,8] and self-assembly techniques,[9–11] have been developed to fabricate such structures. Among these, macroscopic supramolecular self-assembly is an advantageous, bottom-up strategy in which building blocks can be tailored and designed, making the assembly process flexible.[12,13] Although macroscopic supramolecular assembly and assembly at the molecular and nanoscale levels occur via similar interaction processes, the diffusion process that leads to effective collisions between building blocks presents several challenges. At the molecular and nanoscale levels, building blocks move freely via Brownian motion, resulting in a high collision frequency (≈1021 impacts per second)[14] and high efficiency for supramolecular assembly. In contrast, the assembly of components larger than ten micrometers is not affected by Brownian motion, and the collision frequency is remarkably reduced. Therefore, to realize macroscopic supramolecular assembly, the challenge to effectively construct ordered and advanced structures by self-propulsion of the building blocks should be addressed first. Currently, the major strategies that are available toward this end are mechanical rotating or shaking and the application of magnetic fields. By simply introducing external mechanical forces, Whitesides et al. used a rotating process to assemble macroscopic polymeric objects by capillary forces.[15] Harada and co-workers used a mechanical shaker
M. J. Cheng, G. N. Ju, Y. W. Zhang, M. M. Song, Prof. Y. J. Zhang, Prof. F. Shi State Key Laboratory of Chemical Resource Engineering & Key Laboratory of Carbon Fiber and Functional Polymer Ministry of Education Beijing University of Chemical Technology 15 Beisanhuan East Road Chaoyang Distrist, Beijing 100029, China E-mail: [email protected] DOI: 10.1002/smll.201400922 small 2014, 10, No. 19, 3907–3911
to induce the assembly of host/guest hydrogel building blocks.[13,16] Xu[17–19] and Khademhosseini[20,21] showed directed self-assembly of hydrogels by involving mechanical forces through shaking, acoustic, agitation as driving forces to propel building blocks, respectively. Similarly, by applying a magnetic field as an energy supply, Whitesides’ and Demirci's groups reported the direct assembly of metallic rods with magnetic segments[22] and the assembly of hydrogels loaded with magnetic nanoparticles,[23] respectively. In the above methods, the building blocks were forced to locomote by an external energy source, which acted as a passive feature of the assembly system. Although the building blocks in these studies were well designed, their assembly was triggered by the introduction of an external energy source. The building blocks alone showed little spontaneous tendency to selfassemble. Self-assembly processes should involve as little external intervention as possible,[24] thus exhibiting a spontaneity similar to that of the natural world. Life in nature presents dynamic, energy-dissipating, and self-adaptive features such as deformable cell structures, the bacterial trait of multicellularity, and fish (sardine) schooling phenomena, among many other examples. Learning from nature, researchers have begun to understand the collective motion or selfassembly behaviors of life.[25–27] This knowledge, in turn, has been used to enhance the complexity and intelligence of selfassembled systems such that building blocks can automatically form designed and ordered structures independent of human intervention. Such systems may include dynamic selfassembly features through the dissipation of energy.[24] However, to the best of our knowledge, the spontaneous, macroscopic supramolecular assembly of building blocks into advanced ordered structures has not been reported. The key challenges to achieving this goal are the introduction of energy into the assembly system to enable self-propulsion, high collision frequencies, and the formation of ordered structures rather than the formation of simple aggregates. The current study describes a self-assembly technique that harnesses the chemical energy supplied by the catalytic decomposition of hydrogen peroxide into macroscopic building blocks. These then underwent self-propulsion, interacted with each other, and finally assembled into ordered and advanced structures. The locomotion tracks and assembly behavior of the building blocks indicated dynamic and energy-dissipative
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Scheme 1. A schematic shows the fabrication of PDMS cuboids with coatings of platinum catalyst.
characteristics similar to those found in complex biological assembly systems. In a hydrogen peroxide solution, oxygen bubbles were released through decomposition of hydrogen peroxide which was catalyzed by platinum loaed on the building blocks. The building blocks were propelled by the continuous generation of bubbles. By adjusting the location of the catalyst on the building blocks, different ordered architectures were obtained, including pairs, lines, and triangles. The macroscopic building blocks showed locomotive characteristics similar to those of molecular systems during the assembly process, demonstrating the spontaneous nature of this macroscopic supramolecular assembly. The assembly process also consumed the energy of motion and exhibited dynamic assembly features of energy-dissipative systems. In order to drive and realize macroscopic supramolecular assembly, the building block was designed with catalyst of platinum for its self-propulsion through oxygen bubbles released from the decomposition of hydrogen peroxide solution, which has been well developed in various microscale systems but not yet at macroscopic scale.[28–34] The fabrication process is illustrated in Scheme 1. Two major factors were considered in the building block design: 1) the chemical composition of the building block itself and 2) the method by which the platinum catalyst is introduced. Thermally cured, biocompatible polydimethylsiloxane (PDMS) cuboids measuring 8 mm × 8 mm × 3.5 mm were chosen as the main bodies of the building blocks. The density of PDMS is comparable to that of water and its surface is hydrophobic; thus, the PDMS cuboids float on the surface of dilute aqueous solutions and aggregate according to the principle of minimizing interfacial free energy.[35] An electrochemical deposition method was used to form the catalyst.[36] To obtain rough gold structures, a gold-coated glass slide was immersed in a mixture of the H2SO4 and HAuCl4, and electrochemical deposition was conducted at –200 mV for 1600 s. From Supporting Information Figure S1a,b, we can observe that microscale dendritic structures form on the surface, which is assigned to gold clusters by corresponding energy dispersive spectrometer (EDX) pattern. The same slide deposited with gold aggregates was then immersed in a mixture of H2SO4 and H2PtCl6 for 800 s to form a layer of platinum aggregates, which was characterized by SEM image and EDX pattern in Supporting Information Figure S1c,d; the surface presents a rough surface morphology with scattered aggregates and the platinum
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Figure 1. a–d) Snapshots show the process of locomotion and assembly of two PDMS cuboids with platinum catalyst deposited on opposite side surfaces in a hydrogen peroxide solution.
element appeared accompanying with aurum. The pre-deposition of rough gold clusters served to increase the final surface roughness. The resulting hierarchical platinum structure further increased surface roughness, thereby amplifying the hydrophilicity of the film, and also increased the specific surface area, which improved the catalytic efficiency and bubble release rate.[37,38] The platinum catalyst was transferred to the PDMS cuboids by sticking the glass slides onto the 8 mm× 3.5 mm-side faces of the cuboids with a thin layer of PDMS fluid, followed by a second thermal crosslinking. The type of structure formed can be adjusted by varying the number of slides bearing platinum structures and/or by changing the side of the PDMS cuboid onto which the platinum catalyst is attached. PDMS building blocks were fabricated with catalyst deposited on opposite faces. When two such cuboids were placed in a hydrogen peroxide solution (aq, 5 wt%), numerous bubbles were continuously formed and released at the catalyst-bearing side faces, as shown in Figure 1a. This created an imbalance between the forces exerted on the two opposing sides of the cuboids, inducing random motion of the building blocks, similar to Brownian motion at the molecular level. Propelled by the bubbles, the PDMS cuboids moved across the surface of the solution, largely increasing the chance of collision and providing more opportunities for assembly. The movements of the building blocks were tracked in 5-s intervals, and the tracks are shown in Supporting Information Figure S2. A video recording of the motion of the blocks is also provided in Supporting Information Movie S1. At the onset, one of the cuboids moved in a helical fashion while the others progressed along the wall of the container. A string of bubbles released from one side of the PDMS cuboid overlapped with an analogous string from another cuboid. Next, the overlap of numerous bubble streams pushed the PDMS cuboids toward the boundary of the container, resulting in the detachment of two strings of bubbles accompanied by opposite movements of the cuboid. Although both the glass wall of the Petri dish and the platinum-coated side
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of the PDMS cuboids are hydrophilic within the interactive distance,[39] the bubbles prevented the cuboids from attaching to the Petri dish wall. The asymmetric generation and attachment of bubbles on opposing sides of the cuboids resulted in a force imbalance that propelled the cuboids, causing them to assemble. The driving force for this supramolecular assembly is attributed to capillary forces between the building blocks within their interactive distance.[39,40] As shown in Figure 1b–d, after their initial locomotion and collision, two PDMS cuboids formed a pair with two hydrophobic surfaces facing each other. This phenomenon was interpreted as follows. The uncoated and platinum-coated surfaces of the PDMS cuboids have different wetting properties. The bare, uncoated PDMS surfaces are hydrophobic while the surfaces with platinum are hydrophilic. When in contact with water, the two surfaces form positive or negative menisci. As two surfaces with the same wettability approach their interactive distance under self-propulsion, these menisci gradually overlap and the resulting lateral force pulls the two building blocks together (Figure S3a, Supporting Information). In this way, the system minimizes the total interfacial free energy.[35,39,40] We have measured and estimated the critical interactive distance by hunging two building blocks above water surface and gradually moving them towards each other until their assembly as shown in Supporting Information Figure S3b,c, and the resulted distance is about 18.0 mm. Although the assembly of two PDMS cuboids demonstrates the supramolecular assembly of macroscopic building blocks through self-propelled locomotion and the dissipation of chemical energy, the possibility of coincidental assembly must be addressed. It is reasonable to assume that since there were only two building blocks in the container, they would sooner or later associate with one another. A single pair of cuboids does not necessarily represent an ordered and advanced structure with a periodic arrangement. The number of building blocks was therefore increased in order to further demonstrate the energy-dissipating self-assembly of these PDMS cuboids. As shown in Figure 2, four identical PDMS cuboids with platinum catalyst deposited on opposing sides were placed in a solution of hydrogen peroxide. As observed in previous experiments, rapid decomposition of the hydrogen peroxide upon contacting the platinum surface released streams of bubbles from the coated cuboid surfaces. As discussed above, the imbalanced forces applied to opposite sides of the cuboids induced random locomotion. The locomotive paths of these four PDMS cuboids are shown in Supporting Information Figure S4. The cuboids meet and assemble to form two separate pairs with their hydrophobic faces oriented toward each other. Bubbles were continually forming on the opposite side surfaces, which provided the driving force for assembly. Finally, the two pairs assembled into an ordered line structure composed of four PDMS cuboids connected by hydrophobic surfaces. The line structure was held stable for a relatively long time due to the decrease in the area of the hydrophobic surface exposed to the peroxide solution and the resulting minimization of the overall interfacial free energy of the system. Thus, four identical PDMS cuboids formed an ordered structure resulting from selective self-assembly as opposed to the coincidental small 2014, 10, No. 19, 3907–3911
Figure 2. Snapshots show four PDMS cuboids bearing platinum catalyst on opposite side surfaces when a) placed on a hydrogen peroxide solution, b,c) randomly moving, colliding, and assembling, and d) assembled in a final line structure.
formation of random aggregates (for the entire process, please see Supporting Information Movie S2). The spontaneous supramolecular assembly of macroscopic building blocks is the first step in the creation of more massive features composed of a large collection of subunits.[41] The design and tailoring of specific building blocks can not only benefit the large-scale self-assembly of ordered structures, but can also be used to create specific advanced geometries. For example, when hydrophilic platinum coatings were deposited on adjacent surfaces of PDMS cuboids, the oxygen bubbles released from the hydrogen peroxide solution attached to the two adjacent sides. This kind of PDMS building block should act as a linker, connecting other building blocks along the remaining hydrophobic sides. To confirm this hypothesis, we placed three PDMS cuboids into a solution of hydrogen peroxide. Two of the cuboids were identical, with platinum catalyst deposited on opposing faces. The other cuboid was coated with platinum catalyst on two adjacent faces. Once placed in the peroxide solution, the three cuboids moved across the solution surface as described above. The linker cuboid approached and attached to one of the other cuboids along their hydrophobic surfaces. The assembled pair was then driven by the force imbalance at the bubbling surfaces and collided with the third PDMS cuboid, which attached to the remaining hydrophobic surface of the linker cuboid. As seen in Figure 3, this resulted in the formation of a triangular structure with the linker cuboid at the vertex (Movie S3, Supporting Information). After formation of the triangle, both of the PDMS cuboids bearing platinum catalyst on opposite sides presented a hydrophobic surface for further assembly. It is important to note that the assembled pair, the line, and the triangle structures each left exposed hydrophobic surfaces that would be available for further assembly. This indicates the potential for assembly
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Experimental Section
Figure 3. Snapshots show three PDMS cuboids: two had platinum catalyst on opposite sides and one had platinum deposited on adjacent sides (linker). a) The cuboids were placed successively into a peroxide solution. b) The linker formed a pair with one cuboid and c) went on to assemble with the other cuboid to form d) the final triangular geometry.
following pre-formed geometries. In addition, the geometry of the assembled structures could be “frozen” by creating “end blocks” with an exposed hydrophilic platinum surface. Such flexibility in building block design allows the control required to create ordered structures. The platinum catalyst not only supplies the energy of motion to the macroscopic building blocks but also provides a means of designing the assembling components and determining the final assembled geometry. To summarize, supramolecular assemblies of macroscopic building blocks were formed through the self-propelled motion of PDMS cuboids driven by the catalytic decomposition of hydrogen peroxide on a platinum surface. Rough platinum aggregates were deposited on various sides of the cuboids and used to generate bubbles upon contact with a hydrogen peroxide solution. The random locomotion of the building blocks, driven by bubble formation, at the macroscopic scale is similar to the Brownian motion of molecules observed at the molecular or nanoscale. This type of motion significantly enhanced the collision frequency between macroscopic PDMS cuboids. Propelled by oxygen bubbles released from the decomposition of hydrogen peroxide, the PDMS building blocks formed various structures, including pairs, lines, and triangles depending on which sides of the individual cuboids had been coated with platinum catalyst. The assembly process joined the hydrophobic surfaces of the cuboids, thereby minimizing the overall interfacial free energy of the system. In particular, the line and triangle structures exhibited the dynamic and energy-dissipating characteristics of natural biological assembly systems. This demonstrates that spontaneous macroscopic supramolecular assembly may provide novel strategies for the parallel assembly of massive building blocks.
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Materials: PDMS (Sylgand 184) and its curing agent were purchased from Dow Corning. Polymethyl methacrylate (PMMA) templates with arrays of cubic cavities (8 mm × 8 mm × 3 mm) were commercially manufactured. Glass slides coated with a gold layer of 100 nm through evaporating gold under vacuum were obtained from Changchun institute of applied chemistry Chinese academy of sciences. Fabrication of PDMS Cuboids Loaded with Pt Catalyst: Firstly the PDMS pre-polymer and its corresponding curing agent were mixed by stirring for 15 min and underwent a bubble removing process under vacuum for 0.5 h. Secondly the mixture was poured into the PMMA template with flat PMMA plates on both sides, followed by heating at 65 °C overnight. Thirdly glass slides coated with gold was deposited with gold aggregates through an electrochemical deposition process by immersing in a mixture of the H2SO4 (aq, 0.5 m) and HAuCl4 (4 mg/mL). The electrochemical deposition was conducted under a impedance-time mode at –200 mV for 1600 s, using a platinum electrode as the counter-electrode and Ag/AgCl as the reference electrode. Subsequently, the glass slides were deposited with plainum aggregates by immersing in a mixture of the H2SO4 (aq, 0.5 m) and H2PtCl6 (4 mg/mL) for 800 s under identical mode. The slides were cut into small pieces (7 mm × 3 mm) for use. Fourthly the pieces bearing platinum structures above gold aggregates were attached onto designated side surfaces of the PDMS cuboids with a thin layer of PDMS fluid, followed by a second thermal crosslinking.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements M.J.C. and G.N.J. contributed equally to this work. This work was supported by NSFC (21374006), the Program of the Co-Construction with Beijing Municipal Commission of Education of China, Program for New Century Excellent Talents in University (NCET10–0211), the Fok Ying Tung Education Foundation (131013), Open Project of State Key Laboratory of Supramolecular Structure and Materials (SKLSSM201401) and Beijing Young Talents Plan (YETP0488).
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Received: April 3, 2014 Revised: May 4, 2014 Published online: May 19, 2014
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