Template Synthesis of Nanostructured Polymeric Membranes by Inkjet

Jan 19, 2016 - S.B. gratefully acknowledges support for this project from the Center for Environmental Science and Technology (CEST)/Bayer Predoctoral...
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Template Synthesis of Nanostructured Polymeric Membranes by Inkjet Printing Peng Gao, Aaron Hunter, Sherwood Benavides, Mark J. Summe, Feng Gao, and William A. Phillip* Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: The fabrication of functional nanomaterials with complex structures has been serving great scientific and practical interests, but current fabrication and patterning methods are generally costly and laborious. Here, we introduce a versatile, reliable, and rapid method for fabricating nanostructured polymeric materials. The novel method is based on a combination of inkjet printing and template synthesis, and its utility and advantages in the fabrication of polymeric nanomaterials is demonstrated through three examples: the generation of polymeric nanotubes, nanowires, and thin films. Layer-by-layer-assembled nanotubes can be synthesized in a polycarbonate track-etched (PCTE) membrane by printing poly(allylamine hydrochloride) and poly(styrenesulfonate) sequentially. This sequential deposition of polyelectrolyte ink enables control over the surface charge within the nanotubes. By a simple change of the printing conditions, polymeric nanotubes or nanowires were prepared by printing poly(vinyl alcohol) in a PCTE template. In this case, the high-throughput nature of the method enables functional nanomaterials to be generated in under 3 min. Furthermore, we demonstrate that inkjet printing paired with template synthesis can be used to generate patterns comprised of chemically distinct nanomaterials. Thin polymeric films of layer-by-layer-assembled poly(allylamine hydrochloride) and poly(styrenesulfonate) are printed on a PCTE membrane. Track-etched membranes covered with the deposited thin films reject ions and can potentially be utilized as nanofiltration membranes. When the fabrication of these different classes of nanostructured materials is demonstrated, the advantages of pairing template synthesis with inkjet printing, which include fast and reliable deposition, judicious use of the deposited materials, and the ability to design chemically patterned surfaces, are highlighted. KEYWORDS: template synthesis, inkjet printing, layer-by-layer assembly, nanofiltration membranes, nanotubes, nanowires

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INTRODUCTION Nanomaterials, such as nanotubes and nanowires, have been explored by the scientific and engineering communities for use in many industrial arenas, including water treatment,1−3 energy storage devices,4 and pharmaceutical applications.5 The two main strategies for the fabrication of nanomaterials can be classified broadly as top-down and bottom-up methods. Topdown approaches reduce bulk materials to the nanometer scale by using chemical or mechanical techniques, e.g., lithography and milling. Bottom-up methods construct nanomaterials through the deposition of atoms or molecules that are directed into place by self-assembly,6 directed assembly,7 or template synthesis.8,9 Template synthesis, which will be the focus of this study, uses a sacrificial template, such as polycarbonate tracketched (PCTE) membranes, to guide the deposition of material onto the surface of the template. In the case of PCTE membranes, polymeric, carbon, metallic, and semiconducting materials have been deposited within the pores of the membrane to form nanotubes or nanowires.10−19 Despite the versatility of the template synthesis method, the fabrication of nanomaterials with complex structures or functionality can be time-consuming and costly. For example, we recently implemented template synthesis to generate polyelectrolyte © 2016 American Chemical Society

nanotubes that were subsequently used for the fabrication of charge mosaic membranes.20 The layer-by-layer (LbL) process used to deposit the polyelectrolytes within the template took roughly 5 days to complete. Furthermore, the fabrication of patterned nanostructures generally requires lithography, which is laborious.21−23 As such, the continued development of reliable, high-throughput, and economic fabrication methods for the preparation of nanomaterials with complex structures is needed. Inkjet printing is a technology that offers a rapid and reliable method for depositing precise amounts of materials to specific locations on a substrate. Since its commercialization in the 1970s,24 inkjet-printing devices for both small-scale home usage and large-scale industrial applications have been developed.25−29 As the technology has become more widespread, the use of these devices has been extended beyond the printing of graphical images, and the trend toward printing functional materials is increasing.30−35 Examples of useful devices that have been printed from functional materials include polymeric Received: November 23, 2015 Accepted: January 19, 2016 Published: January 19, 2016 3386

DOI: 10.1021/acsami.5b11360 ACS Appl. Mater. Interfaces 2016, 8, 3386−3395

Research Article

ACS Applied Materials & Interfaces light-emitting-diode displays36 and electronic circuits,37 microbatteries,38 thin-film transistors,39 tissues,40 and drug-release systems.41 These devices can be printed as two-dimensional and three-dimensional structures.42 The critical dimension of materials printed using currently available printing techniques has a lower limit near 20 μm because the accurate deposition of ink by an inkjet printer relies on droplet ejection from a signal-responsive printer head. The resolution of the printer depends on many aspects, including the nozzle size, physical and chemical properties of the substrate, and properties of the ink. Ultimately, the resolution of current inkjet technology is in the micrometer range because of capillary forces.43 A fast and reliable method to move beyond this limitation and print materials with nanometer scale via inkjet printing would enable numerous future applications at the nanoscale. In this paper, we develop a novel method for fabricating nanostructured polymeric materials that combines inkjet printing with template synthesis. We demonstrate that the method is versatile and easy to operate by preparing polymeric nanotubes, nanowires, and multilayer thin films. Importantly, the printed nanomaterials retain the same functionality as their dip-coated counterparts, which require significantly longer fabrication times, make less efficient use of precursor materials, and cannot produce patterned surfaces. Incorporating template synthesis with inkjet printing is a promising route toward shortening and simplifying the fabrication, patterning, and modification of nanomaterials with complex structures and functionality.

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template by preventing the template from contacting the impermeable plastic sleeve of the vacuum device. The four sides of the PCTE membrane were sealed with tape, and a constant vacuum of 12 psig was applied throughout the printing process. The number of PAH/ PSS bilayers was controlled by the number of programmed printing cycles. Another input of the program is the number of overprints, which is the number of times that the printer ejects a droplet of ink at the same location. In this study, 20 overprints of the PAH and PSS solutions were applied and 40 overprints of water were used for rinsing. A total of 5 PAH/PSS bilayers were printed in the PCTE membrane. After printing, the membrane was dried in an oven at 100 °C for 1 h. Inkjet Printing of Poly(vinyl alcohol) (PVA) Nanowires and Nanotubes. A 0.3 wt % aqueous solution of a PVA solution was used to print nanowires and nanotubes. A PCTE membrane with pores of 200 nm diameter was used as the substrate. The PCTE template was fixed in the vacuum device, and a constant vacuum of 12 psig was pulled throughout the printing process. A total of 20 overprints of the PVA solution were applied over a 1 cm × 1 cm square to prepare the PVA nanowires; 5 overprints of the PVA solution were used to make the PVA nanotubes. After printing, the membrane template was placed in the oven at 100 °C for 1 h. Inkjet Printing of PAH/PSS Thin Films. Aqueous solutions of PAH and PSS at 20 mM (based on repeat unit molecular weight) with 0.5 M NaCl as the supporting electrolyte or with no supporting electrolyte were prepared. The pH of the solutions was unadjusted. A PCTE membrane with pores of 50 nm diameter was used as the permeable substrate for the printed PAH/PSS thin films so that their performance as nanofiltration membranes could be evaluated. Porous PCTE membranes were used as substrates because of their welldefined pore structures and narrow pore-size distributions. Depending upon the ultimate application of the thin films, they could also be printed on a nonporous flat surface, as demonstrated by Andres and Kotov.44 A total of 3 overprints of the PAH solution were applied to the membrane, which was then allowed to dry and rinsed with water. Because no vacuum was applied during the fabrication of thin films, the samples were dried between deposition steps to prevent the excessive accumulation of solution on the PCTE membrane surface. The rinsing step has been demonstrated to rinse away loosely bound polyelectrolyte and stabilize the LbL film.44 Additionally, a similar filmpreparation route that omitted the rinsing step resulted in thin films covered with crystallized salt. The process was repeated with PSS, which completed one printing cycle and resulted in 1 bilayer of PAH/ PSS on top of the PCTE membrane. The number of bilayers was controlled by the number of printing cycles. After printing, the membrane was placed in the oven at 100 °C for 1 h. The PCTE membrane was not dissolved when PAH/PSS thin films were fabricated. Printing Patterns. A 20 mM solution of FITC-PAH with 0.5 M sodium chloride as the supporting electrolyte was used to print patterned LbL structures. A PCTE membrane with 200-nm-diameter pores was used as the template. A total of 4 bilayers of PAH and PSS were deposited within the PCTE template using the process detailed above for printing PAH/PSS nanotubes. Chemical patterns were then printed using FITC-PAH as the terminal layer. The membrane was rinsed between deposition steps but was not dried. Three different patterns were printed on the PCTE membrane: dots, stripes, and the University of Notre Dame (ND) logo. The printed patterns were visualized in an EVOS fluorescent microscope with the green fluorescent protein light cube. A 0.3 wt % solution of PVA mixed with 5 mM FITC-PAH and a 0.05 wt % aqueous solution of PEO were used as inks for the printing of stripes of PVA nanowires with interstitial gaps. PCTE membranes with pores of 200 nm diameter were used as substrates. For the printing of alternating stripes of different chemical composition, a 2 wt % solution of PVA mixed with 6 mM PAH and 100 mM potassium permanganate and a 2 wt % solution of PVA mixed with 6 mM PSS and 100 mM copper chloride were used. A total of 20 overprints were used, and the membranes were placed in the oven at 100 °C for 1 h after printing. The membranes were then laid flat onto an APTES-treated glass slide and

EXPERIMENTAL PROCEDURES

Materials. Polycarbonate track-etched (PCTE) membranes (pore diameter, 50 and 200 nm; membrane thickness, 10 μm; porosity, ∼3 × 108 pores cm−2) were purchased from Whatman. Nonwoven membranes (Cranemat, CU 414) were purchased from Crane & Co., Inc. Poly(allylamine hydrochloride) (PAH; 15 and 120 kDa), fluorescein isothiocyanate labeled poly(allylamine hydrochloride) (FITC-PAH), poly(styrenesulfonate) (PSS; 70 kDa), poly(ethylene oxide) (PEO; 1000 kDa), (3-aminopropyl)triethoxysilane (APTES), sodium chloride, sodium sulfate, magnesium sulfate, magnesium chloride, copper chloride, and potassium permanganate were purchased from Sigma-Aldrich and used as received. The water used in all experiments was obtained from a Millipore water purification system. Modification of the Inkjet Printer. An Epson WorkForce 30 inkjet printer was modified for this study. Both the printer lid and the cartridge cover were removed from the setup so a continuous ink supply system made by CISinks could be installed. A vacuum device was fabricated by fixing two plastic sheets together using double-sided tape. A 1 cm × 1 cm hole and a 0.2 cm × 0.2 cm hole were cut on the top sheet. A plastic tube was inserted into the smaller hole and sealed with epoxy (3M, DP8010). The vacuum device was connected to an in-house vacuum system through tubing, and a digital pressure transducer (Omega Engineering, PX409) was used to monitor the pressure. LbL Inkjet Printing of PAH/PSS Nanotubes. Repeated deposition of PAH and PSS was used to fabricate nanotubes. Aqueous solutions of the polyelectrolytes at 20 mM (based on repeat unit molecular weight) with 0.5 M NaCl as the supporting electrolyte were prepared. The pH of the PAH solution was adjusted to 5.5 using 1 M HCl; the pH of the PSS solution was unadjusted. A PCTE membrane with a pore diameter of 200 nm was used as the template. The PCTE membrane with a nonwoven membrane underneath was placed over the 1 cm × 1 cm hole of the vacuum device. The nonwoven membrane supports the PCTE templates during printing. This support helps to promote an even flow distribution through the pores of the 3387

DOI: 10.1021/acsami.5b11360 ACS Appl. Mater. Interfaces 2016, 8, 3386−3395

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Figure 1. Schematic of the nanomaterials generated by coupling inkjet printing with template synthesis. (a) Polymeric nanotubes are prepared by printing PAH and PSS alternately on a PCTE membrane template while pulling a vacuum on the downstream side of the template. (b) Polymeric nanowires are generated by simply printing PVA on a membrane template while pulling a vacuum. (c) LbL thin films are fabricated on top of a PCTE membrane by printing alternating layers of PAH and PSS in the absence of an applied vacuum. placed in the oven at 100 °C for another 1 h. Heating cross-links APTES and helps to affix the nanotubes to the glass slide, which makes subsequent imaging analysis easier to execute. Finally, the PCTE templates were dissolved in dichloromethane, and the samples were taken for imaging by fluorescent and scanning electron (SEM) microscopies. SEM. The printed nanostructures were imaged using a FEIMagellan 400 field-emission scanning electron microscope. For nanotubes and nanowires, the template membrane was plasma-etched to remove any residual polymer on the upper and lower surfaces of the membrane. The membrane was attached to an APTES-treated glass slide and placed in the oven at 100 °C for 1 h. Subsequently, the membrane template was dissolved in dichloromethane, and the glass slide was rinsed with ethanol. A total of 2 nm of iridium was sputtered onto the nanotubes by a Cressington 208 HR sputter coater to prevent sample charging during imaging. Surface Charge Measurements of the Printed Nanotubes. Streaming-current measurements were used to determine the sign of the surface charge of the printed nanotubes. A PCTE membrane containing nanotubes was mounted between the halves of a U-tube cell. Potassium chloride (10 mM) was filled in both halves of the cell. Pressure was applied to the side of the cell connected to the positive terminal of the sourcemeter. As solution flows through the membrane, the surface charge restricts the passage of coions (i.e., ions with the same sign as the membrane charge), which results in a streaming current. The applied pressure was measured by a pressure transducer (Omega Engineering, PX409). The resulting current was measured with two Ag/AgCl wires by a Keithley 2400 sourcemeter. LabVIEW

software was used to record both the value of the pressure and the current as a function of time. Water Permeability and Ion-Rejection Measurements for the PAH/PSS Thin Films. The PAH/PSS thin film was placed in a stirred cell (Amicon model 8003). Water was filled in the stirred cell, and a pressure of 4 bar was applied to drive water through the membrane. The solution that permeated through the membrane was collected in a small beaker. The mass of the collected water was weighed over time using a balance and recorded by LabVIEW software. The slope of the mass of collected water over time, the membrane area, and the applied pressure were used to calculate the hydraulic permeability of the membrane. In ion-rejection measurements, 10 mM solutions of single salts (i.e., NaCl, MgCl2, Na2SO4, and MgSO4) were used as the feed solutions. A pressure of 4 bar was applied to drive flow. The solution that permeated through the membrane was collected in a glass beaker. The concentrations of ions in the feed and permeate solutions were analyzed using ion chromatography (Dionex ICS-5000). The measured concentrations were used to calculate the percent rejection, R, according to eq 1,where cp and cf are the concentrations of ions measured in the permeate and feed, respectively. ⎛ cp ⎞ R (%) = ⎜1 − ⎟ × 100 cf ⎠ ⎝ 3388

(1) DOI: 10.1021/acsami.5b11360 ACS Appl. Mater. Interfaces 2016, 8, 3386−3395

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RESULTS AND DISCUSSION Combining Inkjet Printing and Template Synthesis To Fabricate Polymeric Nanomaterials. In order to highlight the versatility of incorporating template synthesis with inkjet printing, the fabrication of polymeric nanotubes, nanowires, and thin films is examined (Figure 1). Only simple modifications to the printing solution and/or process were needed to generate these different nanostructures when combining inkjet printing and template synthesis. Solutions with a viscosity of less than 20 mPa·s can be used as functional “inks” when printing from a standard inkjet printer. This study utilized polymers dissolved in deionized water, namely, the polyelectrolytes PAH and PSS and the neutral polymer PVA. PAH and PSS were used for printing nanotubes and thin films because the LbL assembly of polyelectrolytes is a straightforward method for preparing multilayer polymeric films. PVA was selected as a model polymer because it can form nanowires in anodized alumina oxide membranes through dip-coating processes.45 All polymers were dissolved at concentrations that produce aqueous solutions with viscosities of around 1 mPa·s. This corresponds to 20 mM (based on repeat units) solutions of PAH and PSS and a 0.3 wt % solution of PVA. Although only water-soluble materials were used in this work, it is reasonable to expect that other materials and solvents can be implemented as long as the resulting solutions have a viscosity and a vapor pressure within the suitable range for printing and the printing device being implemented is compatible with the solvent of choice. PCTE membranes with pore sizes of 50 and 200 nm were used as templates. The pores in these membranes have a wellcontrolled and well-defined size, which make them ideal for producing nanotubes and nanowires.46 Dip-coating methodologies rely on the diffusive transport of polymeric building blocks into the pores of the template.46,47 This results in manually intensive protocols that require long periods of time to implement. Printing processes may have an advantage in the fabrication of these nanomaterials because of their highthroughput nature and reduced labor. In particular, when vacuum-assisted template synthesis is coupled with printing, the ballistic transport of the constituent polymers into the pores of the PCTE template reduces the times necessary to produce nanostructures greatly. Alternatively, when an applied vacuum is not used to assist the process, a thin film is deposited on top of the PCTE. Figures 2 and 3 display SEM micrographs of the different nanostructures generated when template synthesis was combined with an inkjet-printing process. Figure 2a shows a SEM micrograph of the printed LbL PAH/ PSS nanotubes. With a constant vacuum of 12 psig applied, PAH and PSS were printed sequentially on a PCTE membrane with a pore diameter of 200 nm. The number of droplets ejected at one location during each pass of the print head over the PCTE surface (defined as the number of overprints in this report) was set to 20. With vacuum applied during this process, no accumulation of printed solution on the PCTE surface was observed by visual inspection. The process was repeated five times in order to deposit 5 bilayers of PAH/PSS inside the pores. After the PCTE template is dissolved, the outer diameter of the nanotubes in Figure 2a is 220 ± 20 nm, which is in good agreement with the pore size of the template. The thickness of the nanotube wall is 70 ± 10 nm, which is comparable to that of nanotubes prepared by the dip-coating method,20,48 indicating that the nanotubes formed by dip coating and inkjet template synthesis are structurally similar.

Figure 2. SEM micrographs of printed PAH/PSS nanostructures. (a) Nanotubes were prepared by printing PAH and PSS sequentially in a PCTE membrane with pores that are 200 nm in diameter while pulling a constant vacuum of 12 psig on the downstream side of the membrane. The PCTE membrane template was dissolved in dichloromethane to liberate the nanotubes. (b) Top and (c) crosssectional views of thin films that were fabricated by printing 5 PAH/ PSS bilayers on top of a PCTE membrane with pores that are 50 nm in diameter.

The vacuum-assisted deposition of polyelectrolyte is faster compared to the diffusion-based dip-coating method. It takes less than 17 min to print 1 PAH/PSS bilayer in a 1 cm × 1 cm template using inkjet printing. In comparison, it takes at least 50 min to deposit a bilayer of the same material using dipcoating methods.49 Additionally, the volume of the polyelectrolyte solution used to print a 1 cm × 1 cm membrane with 5 bilayers of PAH/PSS (∼1 μL/layer) is significantly less than that used in standard dip-coating methods (∼5−10 mL/layer). The more efficient use of materials in the inkjet-printing 3389

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Figure 3. SEM micrographs of PVA (a) nanowires and (b) nanotubes. (a) Nanowires were prepared by printing 20 overprints of PVA in a template with 200 nm pore diameter while pulling a constant vacuum of 12 psig on the downstream side of the membrane. (b) Nanotubes were prepared by printing 5 overprints of PVA in a template with 200 nm pore diameter while pulling a constant vacuum of 12 psig on the downstream side of the membrane. The PCTE membrane was dissolved in dichloromethane prior to SEM characterization.

process has the additional benefit of reducing the effort needed to rinse away loosely absorbed polyelectrolytes. Last, because the printer executes deposition of the bilayers, the manual labor required is greatly reduced. In the absence of an applied vacuum, the LbL polyelectrolyte thin film is printed on top of the PCTE membrane. Figure 2b displays SEM micrographs of a PAH/PSS thin film printed on a PCTE membrane with pores of 50 nm diameter. The top-view image demonstrates that all pores of the PCTE template are completely blocked and covered by a thin film. The crosssectional view (Figure 2c) does not show a clear boundary between the thin film and PCTE membrane, but the thickness of the thin film is less than 200 nm. The time it takes to print 1 layer of PAH or PSS with 3 overprints is about 40 s. The concept of inkjet printing in template membranes can be extended to other polymeric materials and other nanostructures. Figure 3a shows an SEM micrograph of PVA nanowires that were printed in a PCTE membrane with pores of 200 nm diameter. The fabrication of these nanowires highlights that simple changes in the printing process can change the ultimate nanostructure of the deposited material. PVA nanotubes can be prepared by applying 5 overprints of the PVA solution to a PCTE template (Figure 3b). With an increase in the number of overprints to 20, nanowires were fabricated instead of nanotubes. Even though the nanowires fill the pore volume of the template, no accumulation of the printed solution was observed on the PCTE surface when printing the nanowires. In a process where only a single material is being deposited, printing nanowires over a 1 cm × 1 cm area takes under 3 min (170 s) and printing nanotubes over the same area takes under 1 min (45 s). Combining Inkjet Printing and Template Synthesis Enabling Control over the Spatial Distribution of Nanomaterials. A significant advantage of using inkjet printing to fabricate polymeric nanomaterials is the ability to control the spatial distribution of domains of unique chemical design over the surface of the substrate.50,51 This allows nanomaterials of varying chemical composition to be fabricated and oriented next to each other with relative ease. We demonstrate this ability by printing patterns of dots and an ND logo (Figure 4) that consist of nanotubes or nanowires. In these experiments, FITC-PAH was used so that the domains are visible in a fluorescent microscope. Printing the dots and stripes

Figure 4. Fluorescent images of different patterns: (a) dots; (b) ND logo printed with FITC-PAH on PCTE membranes with 200 nm pore diameter. (a) The printer was programmed to print dots with a diameter of 45 μm and a center-to-center spacing of 560 μm. (b) A digital image of the ND logo was drawn in a standard graphics software package.

(see below) was accomplished by writing a program in Epson Standard Code for printers (ESC/P). Figure 4a shows a cubic array of printed dots that are 45 μm in diameter and have a center-to-center distance of 560 μm. When a user-written program is implemented, the dot size and center-to-center distance can be controlled. The smallest dot size (40 μm) that can be generated is limited by the surfacetension-driven phenomena that set the resolution of the printer. These same phenomena prevent the direct printing of nanostructured materials in the absence of a structural 3390

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ACS Applied Materials & Interfaces template.52 The dots can be packed more densely by increasing the dots per inch (dpi) to 180, in which instance the center-tocenter distance is reduced to 140 μm, while a dpi higher than 180 results in the overlapping of dots. Printing these patterns from a user-defined protocol offers greater control over many printing parameters (e.g., droplet size, spacing, and number of overprints) and more accurate control over nozzle movement, but it becomes time-consuming to program and print complicated patterns. In these instances, printing directly from a digital image is simpler than programming complex patterns even though it offers less control over the printing process. For example, Figure 4b shows a printed ND logo that consists of FITC-PAH/PSS nanotubes. The logo was printed directly from an image file drawn in a standard graphics program. When vacuum was applied, no liquid accumulation was observed on the surface of the membrane during the printing process. This is likely because the printed solution permeates through the PCTE membranes quickly relative to the solution deposition rate. Operating in this regime, limits the spread of the printed solution on the membrane surface and results in the formation of the programed patterns with high fidelity. Regardless of how the spatial distribution of unique chemical components is programed, the resulting membranes can have unique domains of chemical design that comprise useful nanostructures. For example, Figure 5a shows an example of 200-μm-wide stripes that consist of printed PVA nanowires. The 200-μm-wide dark regions between the stripes were made by printing a solution of PEO, which dissolves when the PCTE template is removed. PEO was used as a filler to prevent the APTES solution from entering the pores of the PCTE membrane template. This generated blank gaps between the PVA nanowires. Figure 5b shows a higher resolution SEM micrograph of the boundary between a stripe and a gap. PVA nanowires are clearly visible. Additionally, patterns consisting of different chemical components can be printed. Figure 5c shows a SEM−energy-dispersive X-ray (EDX) image at the boundary of two stripes consisting of PVA nanotubes with different chemical components. PVA nanotubes printed with potassium permanganate are shown in red, while PVA nanotubes with copper chloride added are shaded green. The images show that ink solutions can be deposited selectively to form patterned nanomaterials from chemically distinct precursors with relative ease. Combining Inkjet Printing with Template Synthesis Providing Control over the Surface Functionality. The deposition of functional materials, such as polymers, proteins,53 dendrimers, inorganic, and biological materials, has been explored for numerous potential applications including nanobiosensing,54 controlled release,55 and ionic separation.56 The inkjet template synthesis method discussed above is potentially a viable method for processing these functional materials into useful nanostructures as long as the materials retain their functionality upon deposition. We use the example of the LbL assembly of polyelectrolytes in PCTE membranes to modify the surface charge of the nanotubes20 and demonstrate that the printed materials retain their functionality. Because of the residual charge on the dangling ends and loops associated with the innermost layer of deposited polycations or polyanions, the surface of the pore will possess either a positive or a negative charge, respectively. In order to demonstrate that inkjet template synthesis produces nanomaterials that retain their functionality, the surface charge modification of the LbL-

Figure 5. Spatial control and selective deposition of functional nanomaterials. A PCTE membrane with 200 nm pore diameter was implemented. (a) The printer is programmed to print fluorescent PAH stripes with widths of 200 and 200 μm spacing. (b) A highermagnification SEM micrograph is given at the stripe−gap boundary of printed PVA nanowires. A 200 μm stripe width and a 400 μm gap distance were used. The PCTE membrane was dissolved in dichloromethane prior to imaging. (c) A SEM−EDX image at the boundary of two 200 μm PVA stripes is given. One stripe was printed from PVA blended with potassium permanganate, and the other stripe was printed from PVA blended with copper chloride. Regions rich in manganese are shaded red, and regions rich in copper are shaded green. The PCTE membrane was dissolved in dichloromethane prior to imaging.

assembled PAH/PSS nanotubes was studied using streamingcurrent measurements. The sign of the surface charge of the PAH/PSS nanotubes fixed within a PCTE template can be determined from streaming-current measurements.57 The streaming current is generated by forcing a salt solution through a charged membrane, which sits between two solutions connected through an electrical circuit. The streaming current is a result 3391

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charge mosaic membranes.20 This result provides strong evidence that the combination of inkjet printing and template synthesis provides control over the surface charge of the nanotubes, which can subsequently be used for the fabrication of charge mosaic membranes.20 It is interesting to note that the absolute value of the current/ pressure ratio decreases slowly with the addition of more layers. The same decrease is observed if a 15 or 120 kDa PAH sample is used, which suggests that it is not the result of steric hindrance preventing polyelectrolyte deposition. To investigate the cause of this decrease further, Figure 6b plots the normalized hydraulic permeability of the membranes as well as the normalized values of the current/pressure ratio as a function of the increasing number of bilayers. The observed decrease in the current could be caused by the addition of bilayers reducing the effective pore size and permeability of the nanotubes, or it could be caused by the ionic cross-linking between PAH and PSS becoming more effective with the addition of each layer, which would result in fewer dangling ends and loops extending into the center of the nanotubes. The initial rapid drop in the normalized hydraulic permeability within 1 bilayer suggests the rapid buildup of PAH/PSS inside the pores. Subsequently, smaller changes in the permeability are observed, which suggests that smaller changes in the inner diameter of the nanotubes occur after the addition of 1 bilayer. On the other hand, the normalized values of the current/ pressure ratio do not vary significantly for the 0.0−1.0 bilayer systems, but for systems with more than 1 bilayer deposited, the values of the current/pressure ratio decrease. Taken together, these data suggest rearrangement of the polyelectrolytes within the confined nanopores of the PCTE template, and the loss of dangling ends and loops caused by this rearrangement leads to the reduced current/pressure ratio that is observed as more bilayers are added to the walls of the PAH/ PSS nanotubes. This polymer rearrangement of the PAH/PSS nanotubes in the pores of the PCTE membrane may result in a reduction of the membrane surface charge.47 Combining Inkjet Printing and Template Synthesis Generating Functional Nanomaterials. Multilayer thin films comprised of PAH/PSS can be fabricated by executing inkjet template synthesis in the absence of an applied vacuum. Such thin films generated using dip-coating LbL have shown promise as nanofiltration membranes59 and selective coatings that enhance the efficacy of ion-exchange membranes60 in electrodialysis. These promising characteristics of LbL thin films are retained when the constituent polyelectrolytes are deposited by inkjet printing (Figure 7). The hydraulic permeability of the printed thin film decreases as the number of PAH/PSS bilayers deposited increases (as shown in Figure S3). The concentration of the supporting electrolyte used during the preparation of the multilayer thin film can influence the amount of salt rejected by the thin film. This is true in the case of thin films made by dip coating,59 and we observe the same to be true for thin films made by inkjet printing. Figure 7 demonstrates the effect of the supporting electrolyte on the ion-rejection performance of the resulting thin films. One polymer ink was prepared with the addition of 0.5 M NaCl, and the other ink solution was prepared without the addition of any salt. The hydraulic permeability of the thin film prepared without a supporting electrolyte is lower than that of the membrane prepared with 0.5 M NaCl. One possible explanation for this observation is that salt crystallizes within the thin film as it dries between printing steps (Figure S4).

of the need to maintain electroneutrality. The ratio of the measured streaming current to the applied pressure used to drive flow is directly related to the surface charge inside the nanotubes. In the experimental design implemented here, because the positive terminal of the sourcemeter is connected to the side of the cell where pressure is applied, the sign of the current/pressure ratio is opposite that of the surface charge; i.e., a negative surface charge in the nanotubes results in a positive value for the ratio and vice versa. Figure 6 displays how the

Figure 6. Streaming current and water permeability versus number of deposited bilayers for the LbL-printed nanotubes. (a) Nanotubes were fabricated by printing PAH (red squares, 120 kDa; blue squares, 15 kDa) and PSS on a PCTE template with 200-nm-diameter pores. The streaming current was measured using a 10 mM KCl solution adjusted to pH 3. Pressure was applied on the side of the apparatus connected to the positive terminal of the sourcemeter. Values of the applied pressure and streaming current were recorded using a computer (Figure S1). Error bars represent the standard deviations between three measurements. (b) Nanotubes were fabricated by printing PAH (15 kDa) and PSS on a PCTE template with 200-nm-diameter pores. The streaming-current test is the same as that described in part a, and the hydraulic permeability was measured in a stirred cell (Figure S2). The values of the hydraulic permeability were normalized by the hydraulic permeability at the PCTE template. The streaming current/ applied pressure ratio were normalized by the ratio measured at 0.0 and 0.5 bilayers for the negative and positive values, respectively.

surface charge changes with the printing of alternating layers of PAH and PSS in PCTE membrane templates. The parent PCTE membrane has residual negative charges because of the poly(vinylpyrrolidone) coating applied during manufacturing.58 Every layer of PAH or PSS that is printed adds 0.5 bilayers and should cause the surface charge within the nanotubes to switch sign. This is precisely what is observed in Figure 6a, where each addition of 0.5 bilayers causes the streaming current/applied pressure ratio to alternate between a positive and a negative value. Additionally, the magnitude of this ratio is the same as that measured for polyelectrolyte nanotubes used to generate 3392

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Detailed experimental methods, solute rejection and hydraulic permeability of thin films as a function of the bilayers deposited, and fluorescent and scanning electron micrographs of patterned surfaces (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Figure 7. Water permeability and salt rejection of LbL thin films prepared with 0 and 0.5 M NaCl supporting electrolyte solutions. The first two columns display the water permeability, corresponding to the left y axis. The remaining columns show salt rejection data and correspond to the right y axis. PCTE membranes with 50 nm pore diameter were used as the printing substrates. A total of 5 bilayers of PAH/PSS were printed on the PCTE membrane. All salt feed solutions for the rejection tests were 1000 ppm in concentration. An applied pressure of 4 bar was used to drive flow. Error bars were obtained by three measurements with the same membrane.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.G. thankfully acknowledges support from the ND Energy Postdoctoral Fellowship Program at the University of Notre Dame. S.B. gratefully acknowledges support for this project from the Center for Environmental Science and Technology (CEST)/Bayer Predoctoral Research Fellowship at the University of Notre Dame. W.A.P. gratefully acknowledges support from the DuPont Young Faculty Award. We thank Dr. Thomas Gohndrone for assistance with viscosity measurements, Prof. Joan Brennecke for use of her viscometer, and Prof. Jeremiah Zartman for use of his fluorescent microscope. We thank the Notre Dame Integrated Imaging Facility and the CEST at the University of Notre Dame; portions of this research were performed with instruments at these facilities.

After the completed membranes are immersed in water, these salt crystals dissolve but leave cavities within the film that increase the hydraulic permeability. This hypothesis is supported by the rejection of sodium chloride and magnesium chloride displayed by the membranes made using the two different supporting electrolyte solutions. The rejection of these salts is greater when no supporting electrolyte is used during the printing of the thin films than when a 0.5 M NaCl solution is implemented. On the contrary, the film prepared with 0.5 M NaCl shows a larger rejection of sodium sulfate than that of the film prepared without any supporting electrolyte.59 These experimental results, which are in good qualitative agreement with the results obtained from similar thin films made by dip coating,59 demonstrate that inkjet printing combined with LbL is a promising route toward the fabrication of selective multilayer thin films.

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CONCLUSIONS The present investigation reports the fabrication of polymeric nanomaterials through a combination of inkjet printing and template synthesis. We demonstrated the successful fabrication of nanostructured materials using polymeric nanowires, polyelectrolyte nanotubes, and LbL thin films as examples. Through these examples, it was demonstrated that, when tested in membrane applications, the nanostructure and functionality of the materials made using a combination of inkjet printing and template synthesis are comparable to those of their dipcoated counterparts. This key result highlights the advantages of using inkjet printing for the fabrication of nanostructured polymeric materials, which include greatly reduced labor, materials requirements, and processing times and the ability to form chemically patterned functional surfaces. As such, the method reported in this study offers a promising way to fabricate, pattern, and modify nanomaterials with complex structures and functionalities.

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REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11360. 3393

DOI: 10.1021/acsami.5b11360 ACS Appl. Mater. Interfaces 2016, 8, 3386−3395

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