Nanowires
Generating Aligned Micellar Nanowire Arrays by Dewetting of Micropatterned Surfaces Piotr J. Glazer, Leo Bergen, Laurence Jennings, Arjan J. Houtepen, Eduardo Mendes, and Pouyan E. Boukany* Macroscopic fabrication of one-dimensional (1D) nanostructures, including nanowires, nanofibers, nanorods, and nanotubes, with well-ordered arrangement is of great interest for physics, chemistry, and biology not only due to the fundamental questions they raise but also their technological implication in mesoscopic functional devices.[1–3] Exhibiting high aspect ratios (length to diameter ratio of 100 or more), quantized conductance, and size-dependent emission in sub100 nm regime, 1D nanowires are promising for construction of nanoscale transistors, sensors and optoelectronics.[4–10] Many different types of nanowires, based on organic and inorganic material, exist in diverse fields with different structure-property relationships. In comparison to inorganic materials, organic 1D nanowires are advantageous in general for producing high-performance nanodevices, due to their lowcost, facile and large-scale synthesis, flexibility, simple processability (by solutions or vapor methods) and tunability by molecular design.[2,11,12] However, they display a disordered alignment with random positions during and after large-scale processing.[13,14] This disorder strongly hinders possible future applications in functional devices, where often cross-linking and overlapping of nanowires takes place. As a result, the ability to position and align the assembly of organic nanowires into desired architectures with low-cost, no defects, and high-throughput is a prerequisite for their integration into functional devices.[15–20] A number of mature techniques have been developed for the assembly of nanowires through either top-down lithography or bottom-up self-assembly or a combination of both. The classical ‘top-down’ fabrication approach, based on lithography and etching techniques, is typically expensive and limited by the system resolution which makes patterning of nanowires below 10 nm experimentally challenging.[21] In case of electron-beam lithography[21] based methods the low throughput is also an issue. In contrast with this, a ’bottomup’ approach based on spontaneous self-assembly of organic
Dr. P. J. Glazer, L. Bergen, L. Jennings, Dr. A. J. Houtepen, Dr. E. Mendes, Dr. P. E. Boukany Department of Chemical Engineering Delft University of Technology Julianalaan 136, 2628 BL, Delft, The Netherlands E-mail: [email protected] DOI: 10.1002/smll.201303414 small 2014, 10, No. 9, 1729–1734
material, which is the main organizational mechanism found in nature, provides a simpler route to form desired nanostructures. Moreover, these organic nanowires are also highly deformable since they are assembled from molecular units with weak intermolecular interactions, such as hydrogen bonds, π–π stacking and van der Waals forces.[22–24] Although these organic nanowires can be prepared easily, most of them end up in a disordered orientation after the fabrication process. So far, in order to control, align and integrate 1D nanowires into devices, the most popular strategy has been the use of electric/magnetic fields in a desired region between metallic electrodes[25–28] or magnetic poles.[29] Recently, nanomanipulating instruments such as custom-made AFM tips were employed to draw and suspend single polymeric nanofibers on solid substrates in a desired position.[30,31] Although partial alignment and orientation could be generated through the above approaches, they are still suffering from several limitations: i) requirement for expensive and complex equipment (electric/magnetic field introduction, AFM, and nanoimprint lithography), ii) unpredictable positions, iii) lack of reproducibility, and iii) nonuniform density and spacing of nanostructures. In recent years, molecular combing on patterned surfaces has emerged as a simpler approach for stretching, controlling, and producing highly ordered nanowires.[32–35] In the previous studies based on this micropatterned guiding approach, ordered nanowires were made from DNA and polyvinyl formal (PVF), but not from amphipilic materials (such as diblock-copolymer micelles). In contrast with polymeric nanowires derived from similar monomeric species, diblockcopolymer (two chemically dissimilar polymers joined together) approach is more suitable for bio/chemical sensing applications, due to their availability in different shapes, affordability, and capability of functionalization with hydrophilic, hydrophobic species. Despite progress at the molecular level, the ability to align and suspend nanowires made from self-assembled block-copolymer into highly ordered arrays remains the main issue for their successful implementation into sensing devices.[36–38] We report here that molecular combing can be adopted to generate large arrays of amphiphilic nanowires, which contain both hydrophobic core and hydrophilic corona. Below, we introduce a low-cost, simple and rapid method to form aligned nanowires from ultralong self-assembled polystyrenepolyethylene oxide block-copolymer (PS-b-PEO) micelles
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based on a guided deweting technique (see Figure 1a–f). The mechanism of the wormlike micelles alignment is similar to the mechanism involved to generate DNA nanowires by surface-mediated dewetting.[35,39] During the dewetting process the receding waterline slides on the air cushion trapped between pillar-like features of the patterned surfaces and in so doing orients the worm-like micelles. The alignment directionality is controlled by dragging the micelles solution along the array with a paper tissue that serves as an external capillary force source. Finally, we demonstrate that the micellar nanowires can be successfully decorated with hydrophobic molecules that incorporate into the micelle core or with hydrophilic nanoparticles such as quantum dots entrapped in the corona. We believe that the creation of an aligned blockcopolymer nanowire arrays, in this simple manner, offers a unique and powerful platform, which should enable new opportunities in functional nanoscale devices, especially biochemical sensors. The process for creating aforementioned micelle nanowire arrays is illustrated in Figure 1(a–f). This nanowire fabrication consists of two major steps: generating micropatterned
surfaces and formation of aligned nanowires by gentle dragging of micelle solution on micropatterned surfaces. Squarepillar structured polydimethylsiloxane (PDMS, Sylgard 184) stamps (about 1.5 × 1 cm2 laterally) and 4 mm in thick with two types of micro features were prepared by standard lithography techniques. The first type consisted of an array of square pillars of 1.5 µm in width, 1.0 µm in gap, and 3 µm in height (See Figure 1a–d). The second consisted of an array of square pillars of 3.5 µm in width, 2.0 µm in gap, and 3 µm in height (as shown in Figure S2). In this study, we attempted to create aligned amphiphilic nanowire arrays based on the employment of ultralong PS-b-PEO micelles, with contour lengths up to 300 µm and diameter of around 60 nm. Using the wormlike micelle formation protocol (see Experimental Section), we can obtain stable and long micelles as illustrated in Figure 1e (more SEM images are presented in SI, Figure S1). For the formation of highly ordered micelle nanowire arrays, the micelle solutions are dewetted gently on micropillar-structured surfaces. First, a small drop (∼30 μL) of micelle solution was dropped onto micropatterned PDMS substrate. Next, a paper tissue was employed to touch the
Figure 1. The PDMS micropillars preparation steps and aligning approach for generating a micellar nanowire. (a) First a silicon wafer (master mold) with rectangular prisms is created by means of deep reactive ion etching (DRIE) (a). The PDMS is then cast on the master mold (b). When the curing is complete the PDMS, with reproduced pillar structures, can gently be released from the master (c). Scanning electron microscope (SEM) image of the PDMS micropillar array is illustrated in (d). (e) Ultralong PS-b-PEO worm-like micelles: SEM images of PS-b-PEO worm-like micelle. Length of micelle is approximately 150 µm. (f) Schematic (not to scale) representation of the experimental procedure for the generation of nanowire arrays: a droplet of micelles solution is placed on top of PDMS micropillars and dragged along the array by an external source of capillary force (paper tissue). The drag of the droplet aligns the worm-like micelles creating ordered nanowires.
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droplet and drag it gently along the micropatterned pillars in one direction. As a result, an ordered nanowire array was formed between adjacent pillars, as shown in Figure 2(a-b). The nanowires aligned according to the dewetting direction creating a bridge between adjacent pillars. During dewetting, micro-features here acted as wetting defects to hold the receding contact line locally. A liquid bridge between two adjacent micropillars was created temporarily in the dewetting direction. Then, breakage of liquid bridges, due to evaporation of water, led to order and suspends nanowires of micelles between micropillars. The suspended nanowires have nearly uniform diameters (∼60 nm) as confirmed by SEM imaging. Figure 2c shows a titled scanning electron microscopy image of suspended PS-b-PEO nanowire between two adjacent pillars. The bridges linking the pillars were indeed “hanging micelles”—micelles suspended above the surface— as experimentally confirmed with an AFM measurement (Figure 2d). In addition, a landed nanowire on the top of micropillar is displayed in Figure 2e. The nanowire density can be controlled with the micelle solution concentration. By reducing the solution concentration, single and well-separated nanowires were fabricated on patterned surface (see Figure S8). At high concentration the micelles form bundled wires with a high tendency for branching. In this case, by changing the flow pattern, determined by the contact area between the tissue and droplet, branched-like nanowire arrays, towards the dragging force, can be created (see the Supporting Information for more details). We found that, it was extremely difficult to generate
aligned micellar nanowires through well established combing techniques such as peeling off and dip coating method used for generating DNA nanowires (see Figures S5-S7). An important advantage of block-copolymer based systems over traditional solid-state nanowires is that due to micelle amphiphilic character selective targeted entrapment inside the nanowire structure can easily be achieved. By just mixing the micelles with the fluorescent hydrophobic molecule (Dil) the dye incorporates into to micelle core allowing visualization by means of fluorescence based methods as illustrated in Figure 3a. The same approach allowed us to incorporate hydrophilic quantum dots into the micelle corona (Figure 3b). Magnified fluorescence micrographs of modified nanowires are presented in Figure 3c-d. Those two examples illustrate the ease of nanowire structural modification by simple molecular entrapment due to hydrophilic/ hydrophobic interactions (Figure 3e). In conclusion, we have developed a novel, simple and low-cost method capable of generating a large array of laterally ordered soft nanowires from worm-like micelles (see Figure S4). The nanostructures were approximately 60 nm in lateral size with a length of hundreds of micrometers. The presented block-copolymer patterning process based on surface dewetting has several advantages over conventional wire formation methods. The PDMS fabrication is a simple and affordable technique for which neither clean-room facility nor specialized training are required. In addition, the structure of the wires can easily be modified either chemically or by simple physical interactions (hydrophobic/hydrophilic)
Figure 2. (a) SEM image of large arrays of dewetted PS-b-PEO nanowires on PDMS micro pillars (square pillars of 3.5 µm in width, 2 µm in gap and 3.0 µm in height), (b) magnified SEM image of dewetted nanowires on PDMS micropillars (square pillars of 3.5 µm in width, 2 µm in gap and 3.0 µm in height), (c) 45 ° inclined angle view of SEM image of suspended micelle nanowire between two micropillars (square pillars of 3.5 um in width, 2 µm in gap and 3.0 µm in height), (d) 3D AFM image of the micelle nanowire hanging between two pillars (square pillar of 1.2 µm in width, 1.0 µm in gap and 3.0 µm in height), and (e) SEM image of landed nanowire on the top of 1.2 µm square pillar (landed nanowire is specified by white arrow). small 2014, 10, No. 9, 1729–1734
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retain the stretched conformation, due to their glassy PS core that reduces micelle flexibility.[45] The glassy polystyrene core is also responsible for the stability of the system. The shape of micellar structure was further confirmed by the Cryogenic Transmission Electron Microscopy (Cryo-TEM) imaging (Figure S2). Multiple fluorescence imaging control experiments showed that the PS-b-PEO micelles are very stable and robust. No size reduction was observed within a month. The unique PS-PEO micelles length and stability made PS-PEO system perfect candidate for nanowire formation. Worm-like micelles of PS-PEO were prepared using the following protocol: first a stock solution of 10 mg of PS-PEO polymer (PS16k-PEO7,5k, Polymer Source, P13139-SEO, PDI = 1.09) and 1 ml of CHCl3 (Sigma-Aldrich, 288306, anhydrous ≥ 99% purity, ethanol stabilized) was prepared. The stock solution was left in the refrigerator for 1 hour during which the 10−2 M salt solution (NaCl, Sigma-Aldrich, S5886, ≥ 99.5% purity) was prepared. To a thoroughly cleaned vial 0.5 mL of salt solution was added, followed by 100 µl of PS-bPEO solution. The solution was then stirred vigorously for 1 hour. Afterwards 1.8 mL of salt solution were added followed by vigorous stirring for another hour. The stirring speed Figure 3. Fluorescence microscope images of Ultralong worm-like micelles decorated with was then reduced to the lowest possible (a) hydrophobic Dil molecules incorporated intoworm-like micelle core and (b) hydrophilic quantum dots (QDs) associated to the corona. Scale bars are 5 µm. Magnified fluorescence rate and the cap of the vial was removed for micrographs of micellar nanowires (specified by white arrow) decorated with (c) hydrophobic 2 hours to allow chloroform evaporation. After Dil molecule and (d) hydrophilic QDs (scale bar is 3.5 µm). (e) Schematic illustration of 2 hours the cap was replaced. Ultralong flexsuspended micellar nanowire on the micropillars which is decorated by hydrophobic dye and ible micelles presence was experimentally hydrophilic QDs. confirmed at that moment, however to ensure that the system is in quasi-equilibrium state creating new opportunities for specific chemical and bio- the solution was stirred very gently for another 2 days before conducting experiments. The worm-like micelles were visualized by sensing applications. means of fluorescence microscopy, scanning electron microscopy (SEM), cryogenic transmission electron microscopy (Cryo-TEM), and atomic force microscopy (AFM). Directly before visualization Experimental Section the micelle solution was diluted 10 or 100 times. Fluorescent labeling was achieved by incorporating fluoSynthesis of the Ultralong PS-b-PEO Wormlike Micelles: hydrophobic molecule 1,1′-dioctadecyl-3,3,3′,3′Amphiphilic block-copolymers can self-assemble into cylindrical rescent structures under certain conditions (above critical micelle concen- tetramethylindocarbocyanine perchlorate (Dil, Sigma-Aldrich, tration and critical micelle temperature) through the single or multi 42364) into the micelles core. Therefore a 0.2 g/L stock solution (immiscible) solvent process.[40–42] Typically, the self-assembly of Dil in ethanol was prepared and stored at room temperature. is driven by the hydrogen bridges, polarity, or ionic interactions Directly before measurement 1 weight% was added to the micellar between the two joined dissimilar polymers.[43,44] The copolymers solution. The fluorescence signal was observed in the Dil (dichroic are easy to process and to modify chemically in great detail, down mirror 530–560) channel. Synthesis of QDs: Water soluble CdTe quantum dots with to the monomer level, allowing for tuning of their mechanical, electrical or magnetic properties for different applications. Besides thioglycolic acid ligands and zinc blende crystal structure were polymer block chemical modification, combined hydrophilic/ synthesized via the optimized method reported in ref. [46]. For hydrophobic nature of block-copolymers creates possibilities for the experiments reported here quantum dots with a first exciton molecular entrapment in the micelle ‘core’ and/or ‘corona’. The absorption maximum at 614 nm were selected, corresponding to a nonpolar polystyrene (PS) groups cluster to form a polymer shell particle diameter of 4.0 nm.[47] The particles were washed twice by with the polar polyethylene oxide (PEO) groups. The polystyrene- addition of isopropanol and centrifugation and were redissolved b-polyethylene oxide (PS-b-PEO) micelles are relatively stiff and in water at a concentration of 1.2 µM and at pH 7.6. The solution
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was stored under ambient conditions in a freezer. For QDs labeling, a solution of micelles was mixed gently with QDs solution at a 100:1 (v/v) ratio. Characterization: Fluorescence microscopy was performed on a Zeiss LSM 710 microscope. For scanning electron microscopy FEI/Philips XL30 SFEG microscope was used. Dried samples were sputtered with gold (gold layer thickness: 4 nm, deposition rate: 1.3 nm/min) and imaged. The AFM measurements were preformed on an NT-MDT (NTEGRA) microscope. For Cryo-TEM imaging, 5 µL of the solution is deposited onto 300 mesh Cu lacey carbon grids (Ted Pella) rendered hydrophilic with a freshly made glow discharge (Elmo, Cordouan Technology). The grid is rapidly plunged into liquid ethane cooled by liquid nitrogen with a homemade freezing machine. So vitreous ice embedding the sample is obtained. The grid is mounted onto a Gatan cryo holder and transferred into a Tecnai G2 microscope and observed under low dose conditions. Images are recorded with an Eagle ssCCD camera. Ordered Pillar-Structured Microfabrication: The micropillar array used for worm-like micelles alignment was produced by Poly(dimethyl siloxane) (PDMS) based soft lithography. First a master silicon wafer, was created by etching square prisms (width: 1.5 µm and 3.5 µm; depth: 1 µm and 3 µm) in silica by means of Deep Reactive Ion Etching technique. In order to reduce PDMS adhesion to the master its surface was treated with the vapors of a silanazing agent (Tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (ABCR, AB111444). The master stamp was then placed in a Petri dish and a polymer was cast on top. The PDMS filled the microwells molding a negative image of the master stamp. After the polymer has been degassed and cured the stamp was carefully released from the master. The molded PDMS contained the negative image of the master, thus the micropillar structure. It is important to stress that due to the silanization process, the silicon master can be re-used multiple times without any structural damage. Figure 1a shows a graphical representation of the fabrications steps and the SEM image of a PDMS pillar array. Preparation of Suspended Nanowire Arrays: The worm-like micelles alignment procedure and mechanism involved is similar to the method used for DNA alignment as described elsewhere.[32,33,35] Shortly, the pillars act as a wetting defects when the solution droplet slides on the “air cushion” between the pillars. To prevent nanowire misalignment originating from the droplet curvature and edge effects it is important to completely cover the micropillar array with the micelle solution. When the wetting front passes from one pillar to another a liquid bridge, which is responsible for nanowires alignment, is formed. However, due to the nature of the self-assembled system investigated here some specific modifications of the protocol were required as described below. In particular, in order to reduce the number of nanowires broken, by the receding waterline, the formed liquid bridges between the pillars had to be stabilized. Therefore, we have reduced the speed of the receding waterline by dragging the micelles solution with the capillary force of an external source (with precision wipes, KimberlyClark), as schematically shown in Figure 1f. The capillary force exerted by the precision wipe induces a gentle flow of the droplet that pulls the receding water line (with speed of ∼ 1 mm/s). The resulting wires were continuous and showed a clear directionality. Some nanowires on the top surface of the micropillars are reoriented, due to formation of small droplets on the top of micropillars during dewetting process and weak interaction between PDMS small 2014, 10, No. 9, 1729–1734
and nanowires. Similar phenomena has been observed in the DNA nanowires formation on the micropatterned surfaces.[39]
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work is supported by a Marie Curie Career Integration Grantno [334419] to P.E.B. We thank Marc Schmutz at Institute Charles Sadron (ICS, CNRS) for assisting Cryo-TEM Imaging. We would like to thank the reviewers for their constructive comments that greatly improved our presentation.
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Received: November 2, 2013 Revised: December 15, 2013 Published online: February 13, 2014
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Generating Aligned Micellar Nanowire Arrays by Dewetting of Micropatterned Surfaces Piotr J. Glazer, Leo Bergen, Laurence Jennings, Arjan J. Houtepen, Eduardo Mendes, and Pouyan E. Boukany* Macroscopic fabrication of one-dimensional (1D) nanostructures, including nanowires, nanofibers, nanorods, and nanotubes, with well-ordered arrangement is of great interest for physics, chemistry, and biology not only due to the fundamental questions they raise but also their technological implication in mesoscopic functional devices.[1–3] Exhibiting high aspect ratios (length to diameter ratio of 100 or more), quantized conductance, and size-dependent emission in sub100 nm regime, 1D nanowires are promising for construction of nanoscale transistors, sensors and optoelectronics.[4–10] Many different types of nanowires, based on organic and inorganic material, exist in diverse fields with different structure-property relationships. In comparison to inorganic materials, organic 1D nanowires are advantageous in general for producing high-performance nanodevices, due to their lowcost, facile and large-scale synthesis, flexibility, simple processability (by solutions or vapor methods) and tunability by molecular design.[2,11,12] However, they display a disordered alignment with random positions during and after large-scale processing.[13,14] This disorder strongly hinders possible future applications in functional devices, where often cross-linking and overlapping of nanowires takes place. As a result, the ability to position and align the assembly of organic nanowires into desired architectures with low-cost, no defects, and high-throughput is a prerequisite for their integration into functional devices.[15–20] A number of mature techniques have been developed for the assembly of nanowires through either top-down lithography or bottom-up self-assembly or a combination of both. The classical ‘top-down’ fabrication approach, based on lithography and etching techniques, is typically expensive and limited by the system resolution which makes patterning of nanowires below 10 nm experimentally challenging.[21] In case of electron-beam lithography[21] based methods the low throughput is also an issue. In contrast with this, a ’bottomup’ approach based on spontaneous self-assembly of organic
Dr. P. J. Glazer, L. Bergen, L. Jennings, Dr. A. J. Houtepen, Dr. E. Mendes, Dr. P. E. Boukany Department of Chemical Engineering Delft University of Technology Julianalaan 136, 2628 BL, Delft, The Netherlands E-mail: [email protected] DOI: 10.1002/smll.201303414 small 2014, 10, No. 9, 1729–1734
material, which is the main organizational mechanism found in nature, provides a simpler route to form desired nanostructures. Moreover, these organic nanowires are also highly deformable since they are assembled from molecular units with weak intermolecular interactions, such as hydrogen bonds, π–π stacking and van der Waals forces.[22–24] Although these organic nanowires can be prepared easily, most of them end up in a disordered orientation after the fabrication process. So far, in order to control, align and integrate 1D nanowires into devices, the most popular strategy has been the use of electric/magnetic fields in a desired region between metallic electrodes[25–28] or magnetic poles.[29] Recently, nanomanipulating instruments such as custom-made AFM tips were employed to draw and suspend single polymeric nanofibers on solid substrates in a desired position.[30,31] Although partial alignment and orientation could be generated through the above approaches, they are still suffering from several limitations: i) requirement for expensive and complex equipment (electric/magnetic field introduction, AFM, and nanoimprint lithography), ii) unpredictable positions, iii) lack of reproducibility, and iii) nonuniform density and spacing of nanostructures. In recent years, molecular combing on patterned surfaces has emerged as a simpler approach for stretching, controlling, and producing highly ordered nanowires.[32–35] In the previous studies based on this micropatterned guiding approach, ordered nanowires were made from DNA and polyvinyl formal (PVF), but not from amphipilic materials (such as diblock-copolymer micelles). In contrast with polymeric nanowires derived from similar monomeric species, diblockcopolymer (two chemically dissimilar polymers joined together) approach is more suitable for bio/chemical sensing applications, due to their availability in different shapes, affordability, and capability of functionalization with hydrophilic, hydrophobic species. Despite progress at the molecular level, the ability to align and suspend nanowires made from self-assembled block-copolymer into highly ordered arrays remains the main issue for their successful implementation into sensing devices.[36–38] We report here that molecular combing can be adopted to generate large arrays of amphiphilic nanowires, which contain both hydrophobic core and hydrophilic corona. Below, we introduce a low-cost, simple and rapid method to form aligned nanowires from ultralong self-assembled polystyrenepolyethylene oxide block-copolymer (PS-b-PEO) micelles
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based on a guided deweting technique (see Figure 1a–f). The mechanism of the wormlike micelles alignment is similar to the mechanism involved to generate DNA nanowires by surface-mediated dewetting.[35,39] During the dewetting process the receding waterline slides on the air cushion trapped between pillar-like features of the patterned surfaces and in so doing orients the worm-like micelles. The alignment directionality is controlled by dragging the micelles solution along the array with a paper tissue that serves as an external capillary force source. Finally, we demonstrate that the micellar nanowires can be successfully decorated with hydrophobic molecules that incorporate into the micelle core or with hydrophilic nanoparticles such as quantum dots entrapped in the corona. We believe that the creation of an aligned blockcopolymer nanowire arrays, in this simple manner, offers a unique and powerful platform, which should enable new opportunities in functional nanoscale devices, especially biochemical sensors. The process for creating aforementioned micelle nanowire arrays is illustrated in Figure 1(a–f). This nanowire fabrication consists of two major steps: generating micropatterned
surfaces and formation of aligned nanowires by gentle dragging of micelle solution on micropatterned surfaces. Squarepillar structured polydimethylsiloxane (PDMS, Sylgard 184) stamps (about 1.5 × 1 cm2 laterally) and 4 mm in thick with two types of micro features were prepared by standard lithography techniques. The first type consisted of an array of square pillars of 1.5 µm in width, 1.0 µm in gap, and 3 µm in height (See Figure 1a–d). The second consisted of an array of square pillars of 3.5 µm in width, 2.0 µm in gap, and 3 µm in height (as shown in Figure S2). In this study, we attempted to create aligned amphiphilic nanowire arrays based on the employment of ultralong PS-b-PEO micelles, with contour lengths up to 300 µm and diameter of around 60 nm. Using the wormlike micelle formation protocol (see Experimental Section), we can obtain stable and long micelles as illustrated in Figure 1e (more SEM images are presented in SI, Figure S1). For the formation of highly ordered micelle nanowire arrays, the micelle solutions are dewetted gently on micropillar-structured surfaces. First, a small drop (∼30 μL) of micelle solution was dropped onto micropatterned PDMS substrate. Next, a paper tissue was employed to touch the
Figure 1. The PDMS micropillars preparation steps and aligning approach for generating a micellar nanowire. (a) First a silicon wafer (master mold) with rectangular prisms is created by means of deep reactive ion etching (DRIE) (a). The PDMS is then cast on the master mold (b). When the curing is complete the PDMS, with reproduced pillar structures, can gently be released from the master (c). Scanning electron microscope (SEM) image of the PDMS micropillar array is illustrated in (d). (e) Ultralong PS-b-PEO worm-like micelles: SEM images of PS-b-PEO worm-like micelle. Length of micelle is approximately 150 µm. (f) Schematic (not to scale) representation of the experimental procedure for the generation of nanowire arrays: a droplet of micelles solution is placed on top of PDMS micropillars and dragged along the array by an external source of capillary force (paper tissue). The drag of the droplet aligns the worm-like micelles creating ordered nanowires.
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droplet and drag it gently along the micropatterned pillars in one direction. As a result, an ordered nanowire array was formed between adjacent pillars, as shown in Figure 2(a-b). The nanowires aligned according to the dewetting direction creating a bridge between adjacent pillars. During dewetting, micro-features here acted as wetting defects to hold the receding contact line locally. A liquid bridge between two adjacent micropillars was created temporarily in the dewetting direction. Then, breakage of liquid bridges, due to evaporation of water, led to order and suspends nanowires of micelles between micropillars. The suspended nanowires have nearly uniform diameters (∼60 nm) as confirmed by SEM imaging. Figure 2c shows a titled scanning electron microscopy image of suspended PS-b-PEO nanowire between two adjacent pillars. The bridges linking the pillars were indeed “hanging micelles”—micelles suspended above the surface— as experimentally confirmed with an AFM measurement (Figure 2d). In addition, a landed nanowire on the top of micropillar is displayed in Figure 2e. The nanowire density can be controlled with the micelle solution concentration. By reducing the solution concentration, single and well-separated nanowires were fabricated on patterned surface (see Figure S8). At high concentration the micelles form bundled wires with a high tendency for branching. In this case, by changing the flow pattern, determined by the contact area between the tissue and droplet, branched-like nanowire arrays, towards the dragging force, can be created (see the Supporting Information for more details). We found that, it was extremely difficult to generate
aligned micellar nanowires through well established combing techniques such as peeling off and dip coating method used for generating DNA nanowires (see Figures S5-S7). An important advantage of block-copolymer based systems over traditional solid-state nanowires is that due to micelle amphiphilic character selective targeted entrapment inside the nanowire structure can easily be achieved. By just mixing the micelles with the fluorescent hydrophobic molecule (Dil) the dye incorporates into to micelle core allowing visualization by means of fluorescence based methods as illustrated in Figure 3a. The same approach allowed us to incorporate hydrophilic quantum dots into the micelle corona (Figure 3b). Magnified fluorescence micrographs of modified nanowires are presented in Figure 3c-d. Those two examples illustrate the ease of nanowire structural modification by simple molecular entrapment due to hydrophilic/ hydrophobic interactions (Figure 3e). In conclusion, we have developed a novel, simple and low-cost method capable of generating a large array of laterally ordered soft nanowires from worm-like micelles (see Figure S4). The nanostructures were approximately 60 nm in lateral size with a length of hundreds of micrometers. The presented block-copolymer patterning process based on surface dewetting has several advantages over conventional wire formation methods. The PDMS fabrication is a simple and affordable technique for which neither clean-room facility nor specialized training are required. In addition, the structure of the wires can easily be modified either chemically or by simple physical interactions (hydrophobic/hydrophilic)
Figure 2. (a) SEM image of large arrays of dewetted PS-b-PEO nanowires on PDMS micro pillars (square pillars of 3.5 µm in width, 2 µm in gap and 3.0 µm in height), (b) magnified SEM image of dewetted nanowires on PDMS micropillars (square pillars of 3.5 µm in width, 2 µm in gap and 3.0 µm in height), (c) 45 ° inclined angle view of SEM image of suspended micelle nanowire between two micropillars (square pillars of 3.5 um in width, 2 µm in gap and 3.0 µm in height), (d) 3D AFM image of the micelle nanowire hanging between two pillars (square pillar of 1.2 µm in width, 1.0 µm in gap and 3.0 µm in height), and (e) SEM image of landed nanowire on the top of 1.2 µm square pillar (landed nanowire is specified by white arrow). small 2014, 10, No. 9, 1729–1734
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retain the stretched conformation, due to their glassy PS core that reduces micelle flexibility.[45] The glassy polystyrene core is also responsible for the stability of the system. The shape of micellar structure was further confirmed by the Cryogenic Transmission Electron Microscopy (Cryo-TEM) imaging (Figure S2). Multiple fluorescence imaging control experiments showed that the PS-b-PEO micelles are very stable and robust. No size reduction was observed within a month. The unique PS-PEO micelles length and stability made PS-PEO system perfect candidate for nanowire formation. Worm-like micelles of PS-PEO were prepared using the following protocol: first a stock solution of 10 mg of PS-PEO polymer (PS16k-PEO7,5k, Polymer Source, P13139-SEO, PDI = 1.09) and 1 ml of CHCl3 (Sigma-Aldrich, 288306, anhydrous ≥ 99% purity, ethanol stabilized) was prepared. The stock solution was left in the refrigerator for 1 hour during which the 10−2 M salt solution (NaCl, Sigma-Aldrich, S5886, ≥ 99.5% purity) was prepared. To a thoroughly cleaned vial 0.5 mL of salt solution was added, followed by 100 µl of PS-bPEO solution. The solution was then stirred vigorously for 1 hour. Afterwards 1.8 mL of salt solution were added followed by vigorous stirring for another hour. The stirring speed Figure 3. Fluorescence microscope images of Ultralong worm-like micelles decorated with was then reduced to the lowest possible (a) hydrophobic Dil molecules incorporated intoworm-like micelle core and (b) hydrophilic quantum dots (QDs) associated to the corona. Scale bars are 5 µm. Magnified fluorescence rate and the cap of the vial was removed for micrographs of micellar nanowires (specified by white arrow) decorated with (c) hydrophobic 2 hours to allow chloroform evaporation. After Dil molecule and (d) hydrophilic QDs (scale bar is 3.5 µm). (e) Schematic illustration of 2 hours the cap was replaced. Ultralong flexsuspended micellar nanowire on the micropillars which is decorated by hydrophobic dye and ible micelles presence was experimentally hydrophilic QDs. confirmed at that moment, however to ensure that the system is in quasi-equilibrium state creating new opportunities for specific chemical and bio- the solution was stirred very gently for another 2 days before conducting experiments. The worm-like micelles were visualized by sensing applications. means of fluorescence microscopy, scanning electron microscopy (SEM), cryogenic transmission electron microscopy (Cryo-TEM), and atomic force microscopy (AFM). Directly before visualization Experimental Section the micelle solution was diluted 10 or 100 times. Fluorescent labeling was achieved by incorporating fluoSynthesis of the Ultralong PS-b-PEO Wormlike Micelles: hydrophobic molecule 1,1′-dioctadecyl-3,3,3′,3′Amphiphilic block-copolymers can self-assemble into cylindrical rescent structures under certain conditions (above critical micelle concen- tetramethylindocarbocyanine perchlorate (Dil, Sigma-Aldrich, tration and critical micelle temperature) through the single or multi 42364) into the micelles core. Therefore a 0.2 g/L stock solution (immiscible) solvent process.[40–42] Typically, the self-assembly of Dil in ethanol was prepared and stored at room temperature. is driven by the hydrogen bridges, polarity, or ionic interactions Directly before measurement 1 weight% was added to the micellar between the two joined dissimilar polymers.[43,44] The copolymers solution. The fluorescence signal was observed in the Dil (dichroic are easy to process and to modify chemically in great detail, down mirror 530–560) channel. Synthesis of QDs: Water soluble CdTe quantum dots with to the monomer level, allowing for tuning of their mechanical, electrical or magnetic properties for different applications. Besides thioglycolic acid ligands and zinc blende crystal structure were polymer block chemical modification, combined hydrophilic/ synthesized via the optimized method reported in ref. [46]. For hydrophobic nature of block-copolymers creates possibilities for the experiments reported here quantum dots with a first exciton molecular entrapment in the micelle ‘core’ and/or ‘corona’. The absorption maximum at 614 nm were selected, corresponding to a nonpolar polystyrene (PS) groups cluster to form a polymer shell particle diameter of 4.0 nm.[47] The particles were washed twice by with the polar polyethylene oxide (PEO) groups. The polystyrene- addition of isopropanol and centrifugation and were redissolved b-polyethylene oxide (PS-b-PEO) micelles are relatively stiff and in water at a concentration of 1.2 µM and at pH 7.6. The solution
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was stored under ambient conditions in a freezer. For QDs labeling, a solution of micelles was mixed gently with QDs solution at a 100:1 (v/v) ratio. Characterization: Fluorescence microscopy was performed on a Zeiss LSM 710 microscope. For scanning electron microscopy FEI/Philips XL30 SFEG microscope was used. Dried samples were sputtered with gold (gold layer thickness: 4 nm, deposition rate: 1.3 nm/min) and imaged. The AFM measurements were preformed on an NT-MDT (NTEGRA) microscope. For Cryo-TEM imaging, 5 µL of the solution is deposited onto 300 mesh Cu lacey carbon grids (Ted Pella) rendered hydrophilic with a freshly made glow discharge (Elmo, Cordouan Technology). The grid is rapidly plunged into liquid ethane cooled by liquid nitrogen with a homemade freezing machine. So vitreous ice embedding the sample is obtained. The grid is mounted onto a Gatan cryo holder and transferred into a Tecnai G2 microscope and observed under low dose conditions. Images are recorded with an Eagle ssCCD camera. Ordered Pillar-Structured Microfabrication: The micropillar array used for worm-like micelles alignment was produced by Poly(dimethyl siloxane) (PDMS) based soft lithography. First a master silicon wafer, was created by etching square prisms (width: 1.5 µm and 3.5 µm; depth: 1 µm and 3 µm) in silica by means of Deep Reactive Ion Etching technique. In order to reduce PDMS adhesion to the master its surface was treated with the vapors of a silanazing agent (Tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (ABCR, AB111444). The master stamp was then placed in a Petri dish and a polymer was cast on top. The PDMS filled the microwells molding a negative image of the master stamp. After the polymer has been degassed and cured the stamp was carefully released from the master. The molded PDMS contained the negative image of the master, thus the micropillar structure. It is important to stress that due to the silanization process, the silicon master can be re-used multiple times without any structural damage. Figure 1a shows a graphical representation of the fabrications steps and the SEM image of a PDMS pillar array. Preparation of Suspended Nanowire Arrays: The worm-like micelles alignment procedure and mechanism involved is similar to the method used for DNA alignment as described elsewhere.[32,33,35] Shortly, the pillars act as a wetting defects when the solution droplet slides on the “air cushion” between the pillars. To prevent nanowire misalignment originating from the droplet curvature and edge effects it is important to completely cover the micropillar array with the micelle solution. When the wetting front passes from one pillar to another a liquid bridge, which is responsible for nanowires alignment, is formed. However, due to the nature of the self-assembled system investigated here some specific modifications of the protocol were required as described below. In particular, in order to reduce the number of nanowires broken, by the receding waterline, the formed liquid bridges between the pillars had to be stabilized. Therefore, we have reduced the speed of the receding waterline by dragging the micelles solution with the capillary force of an external source (with precision wipes, KimberlyClark), as schematically shown in Figure 1f. The capillary force exerted by the precision wipe induces a gentle flow of the droplet that pulls the receding water line (with speed of ∼ 1 mm/s). The resulting wires were continuous and showed a clear directionality. Some nanowires on the top surface of the micropillars are reoriented, due to formation of small droplets on the top of micropillars during dewetting process and weak interaction between PDMS small 2014, 10, No. 9, 1729–1734
and nanowires. Similar phenomena has been observed in the DNA nanowires formation on the micropatterned surfaces.[39]
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work is supported by a Marie Curie Career Integration Grantno [334419] to P.E.B. We thank Marc Schmutz at Institute Charles Sadron (ICS, CNRS) for assisting Cryo-TEM Imaging. We would like to thank the reviewers for their constructive comments that greatly improved our presentation.
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Received: November 2, 2013 Revised: December 15, 2013 Published online: February 13, 2014
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