Simultaneous Printing of Two Inks by Contact Lithography - ACS

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Simultaneous Printing of Two Inks by Contact Lithography David Moore, and Ravi F. Saraf ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03038 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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ACS Applied Materials & Interfaces

Simultaneous Printing of Two Inks by Contact Lithography David Moore, Ravi F. Saraf* Department of Chemical and Biomolecular Engineering, University of Nebraska-Lincoln Lincoln, NE, 68588 *Corresponding author ([email protected]) Keywords: Microcontact printing; soft lithography; cell culture; wetting modulationnon; contact printing; surface patterning

Abstract

Microcontact printing (µCP) is a valuable technique used to fabricate complex patterns on surfaces for applications such as sensors, cell seeding, self-assembled monolayers of proteins and nanoparticles, and micromachining. The process is very precise but is typically confined to depositing a single type of ink per print, which limits the complexity of using multifunctionality patterns. Here we describe a process by which two inks are printed concomitantly in a single operation to create an alternating pattern of hydrophobic and hydrophilic characteristics. The hydrophobic ink, PDMS is deposited by evaporation on the non-contact region while the hydrophilic polyelectrolyte is transferred on contact. We demonstrate that there is no gap

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between the two patterns using an optical-electrochemical method. We describe some potential applications of this method, including layer-by-layer deposition of polyelectrolytes for sensors and creation of a scaffold for cell culture.

Introduction Fabrication is at the heart of developing novel devices using established principles. As the requirements for complex functionality increase, it becomes increasingly necessary to have structures made from multiple materials with significantly different properties at micron scales. Soft lithography microcontact printing is a well-developed technique1 capable of creating fine structures on surfaces used for fabricating photonic,2 electronic,3–5 electrochemical,6 and optical7 devices. One application of microcontact printing is patterning scaffolds for the immobilization of cells, which is vital for tissue engineering and co-culturing in the study of the biology of diseases.8 In addition, self-assembled monolayers,9 polymers,10 proteins,11 and nanoparticle patterns12 are fabricated via this method for a variety of applications. Repeated printing13,14 and subsequent dip coating10,15,16 and backfilling with self-assembled monolayers14,17 have been developed to incorporate more than one material. However, repeated printing requires precise alignment, which can be limiting because of the flexible nature of the stamp. Seamless deposition of "two color" printing has been shown by patterning one thiol containing molecule on Au surface followed by backfilling a self-assembly of the second thiolated moieties,17 but patterning a hydrophilic polymer on top of a hydrophobic surface, or vice versa, by dip coating results in a dewetting-induced cleft between the two materials as they will repel each other.5,18–20 Dip coating polymers with an affinity for each other can cause one material to deposit over the other.16


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For applications such as (electronic) sensors, these effects impede electric probing of patterned substances, as the exposed electrode creates large current leaks and overlapping materials can create complicated capacitive signals and undeterminable coverage areas. Thus, there is a need for a clean and continuous material interface. Here, we describe a method of creating a pattern of alternating hydrophilic and hydrophobic materials in a single-step process. The technique builds on a previously described principle of microcontact “negative printing” where the material from a polydimethylsiloxane (PDMS) stamp was observed to exclusively deposit where it did not touch the substrate. Specifically, low molecular weight PDMS evaporates from the stamp (under the appropriate conditions) to deposit on the substrate.21 Two inks with different properties can be printed simultaneously by stamping one ink using a standard contact transfer and the other ink by using the above mentioned anomalous negative printing. Specifically, the “positive” printing, i.e., by contact, is hydrophilic poly(allylamine hydrochloride) (PAH); and the "negative" printing, i.e., by noncontact, is hydrophobic PDMS. As we will describe in the following section, the major technical challenge to performing dual printing was “inking” the two disparate materials on the stamp, i.e., PAH and (native) PDMS. These challenges were overcome with a modification to the traditional stamping methods using Tween® 20 modification to print fine patterns of hydrophilic and hydrophobic lines. We demonstrate two potential applications of selectively depositing and seeding nanoparticles and cells, respectively, on hydrophilic regions.



Experimental Fabrication SU-8 Mold Fabrication: Molds for the microcontact stamp were fabricated on a silica wafer using photolithography and negative photoresist (SU-8, MicroChem). The SU8 mold was approximately 25 µm thick and was baked overnight at 150 °C to fully cure the photoresist. Details are described in the Supporting Information (SI), Section S.1.1. PDMS Stamp Fabrication: The SU-8 mold was treated with an ethanol solution of 1% vol/vol (v/v) Tween 20 (Sigma-Aldrich) placed in the grooves by pipette. The Tween 20 solution was wicked in the channels between the raised SU-8 ridges by capillary action (see the SI, Section S.1.2 and Video R1). A 10:1 ratio by mass of PDMS (viscosity 1500 cSt) and a crosslinking agent (Dow Corning) were thoroughly mixed and poured on the SU-8 mold (Fig. 1c, discussed later). The assembly was degassed at 160 Torr and cured for 25 minutes at 60 °C to ensure that the PDMS was not sticky (see the SI, Section S.1.2.and Video R2). The stamp was manually peeled from the mold (see the SI, Section S.1.2 and Video R3). Characterization SEED Characterization: Scanning Electrometer for Electrical Double-layer (SEED) is an (inhouse developed) optical method of quantitatively measuring the local electrochemical process on a 6 µm spot defined by the probe laser during a cyclic voltammetry (CV) scan (see the SI, Section S.1.3).22,23 Briefly, the method is a differential interferometer where the probe and sample beams are incident on the PDMS/PAH pattern and a polymethyl methacrylate (PMMA) passivation layer, respectively. During the CV scan, the sample is subjected to an AC potential (100 mV RMS at 2 KHz) in a redox active or inactive salt solution, causing oscillations of the


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ions at the electrolyte/electrode interface. The amplitude of oscillation in the optical path length, ∆ due to the ion motion at the interface is measured by SEED. The ∆ at certain DC potential during the CV ramp between the solution and the electrode is maximum, i.e., ∆max. Keeping the DC potential at ∆max condition, the laser beam was scanned over the pattern to obtain the profile of ∆max. See the SI, Section S.1.3, for details. Escherichia Coli (E. coli) Cells: A solution of E. coli cells and lysogeny broth (LB) was placed on the stamped gold electrodes overnight in a water bath held at 25 °C. The electrodes were carefully removed. The excess E. coli and broth were cleaned off by gingerly dipping the electrodes in deionized (DI) water. Details are provided in the SI, Section S.1.4. The resulting pattern was then characterized by field emission scanning electron microscopy (FESEM). The viability of the patterned cells was determined using a BacLight™ Bacterial Viability Kit (Thermo Fisher) (see the SI, Section S.1.5). Deposition of Nanoparticle Necklace Pattern: Nanoparticle necklaces were made in solution by directed self-assembly.24 Briefly, an aqueous colloid of 10 nm gold (Au) nanoparticles (BB International, GC 20) was mixed with Cd(ClO4)2 to a concentration of 3.2 mM. The solution was shaken for 12 hours, until the color changed from red to blue indicating the formation of nanoparticle necklaces. The patterned PDMA/PAH electrode sample was immersed in the necklace colloid solution for 18 hr. The sample was washed with DI water to remove unbound particles. The samples were characterized by FESEM.



Results and Discussion The stamp mold was produced by standard photolithography using 25 µm thick SU-8 photo resist (Fig. 1a). Three patterns were primarily used: 20 µm lines with 80 µm gaps, the converse, and 150 µm lines with 100 µm gaps. Previous experiments with similar methods successfully achieved features of 5 µm.21 Selectively modifying the PDMS stamp with Tween 20 was the most critical and challenging aspect of the process. Specifically, inking the contact region of the stamp with an aqueous ink required a selective modification of the PDMS surface of the stamp to create a hydrophilic surface without disrupting the rest of the surrounding hydrophobic PDMS surface. This selective hydrophilicity was created by wicking Tween 20 (Fig. 1b) SU-8 patterned channels by capillary action. The selective deposition is remarkably straightforward (see the SI, Section S.2.1 and Video R1). Tween 20 is not volatile (vapor pressure of ~100 Pa at room temperature) leading to insignificant condensation on (nonwetting) SU-8 ridges (in Step 1b). The wicking in the grooves was demonstrated by placing a drop of 100 µM methylene blue (MB) in ethanol on the pattern. The MB dye was exclusively observed in the grooves (see the inset of Fig. 1b and the SI, Section S.2.2 and Fig. S7). Subsequently, the PDMS stamp is molded as described above. For this study the stamp was cured for 25 min so that it could be released from the SU8 mold and (also) have a reasonable amount of PDMS oligomers for negative printing.21 Tween 20 is an amphiphilic molecule that adhered to the PDMS to form the hydrophilic surface (see the SI, Fig. S8). As a result, the PDMS in contact with the trough of the SU8 mold incorporated Tween 20 (Fig. 1c). The resultant contact angle due to Tween 20 modification was reduced from 85°, for the PDMS stamp (with no Tween 20), to 44.9° (see the SI, Fig. S9). The


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discrepancy in the literature regarding contact angle values for untreated PDMS25 is likely due to the incomplete curing process necessary to facilitate the negative printing.21 An aqueous ink composed of 0.1% weight/weight (w/w) solution of 1.2 x 105 daltons PAH (Sigma-Aldrich) of pH adjusted to pH 7 by NaOH was prepared. The ink was gently spread across the surface of the stamp. Optimization of this process is discussed in the SI, Section S.2.3. The stamp was placed under a low vacuum (~160 Torr) at room temperature (~22 °C) with a desiccant and allowed to dry (Fig. 1d). After the modification, water wet the ridges over the grooves (see Fig. 1d inset and the SI, Fig. S10a), indicating that the Tween 20 coating was exclusively on the former region. The methylene blue solution in ethanol, which has no preference, wicked in the grooves (see the SI, Fig. S10b). For the untreated stamp, the water favored the grooves due to topography. That the PAH only transferred to the raised region of the stamp was further confirmed by the application of Reactive Red 120 dye that selectively deposits only on PAH (see the SI, Fig. S11). The inked stamp was then placed on the Au electrode on SiO2/Si substrate (Fig. 1e). The stamp/substrate assembly was heated to 60 °C for 5 min. The stamp was carefully peeled parallel to the lines. Shear parallel to the surface was kept to a minimum. The patterned Au electrode (Fig. 1f) was then thoroughly washed in DI water to remove excess material. The stamp contacted the substrate via its own weight with a nominal contact pressure of ~33.5 Pa. Sagging due to its weight is estimated in the SI, Section S.1.2. From the stiffness measurement by nanoindentation at different curing conditions (see the SI, Fig. S.1), for a minimum stiffness of 0.44 MPa at 25 min curing and the largest lateral gap of 150 µm (between the contacts), the sagging of the "roof" was estimated to be less than 1 nm. However, applying an external load of ~100 gm led to an effective pressure of ~10 KPa to cause the roof to sag as observed by scanning probe microscopy (SPM) (see the SI, Fig. S12).



The development of the Tween 20 modification process was critical to the success of the dualprinting process. Many techniques to create hydrophilic PDMS surfaces have been developed while investigating microfluidics fabrication methods;25–27 however, negative printing requires partially uncured PDMS. A common method is to treat the stamp with oxygen plasma,26,27 which coats the surface with silanol groups.25 However, the oxygen plasma treatment is at very low pressure (550 millitorr) causing the low molecular weight PDMS that is necessary for noncontact deposition to deplete from the stamp's surface by leeching.21,28,29 The oxygen plasma process also creates microfissures on the surface of the stamp,30,31 which makes “noncontact printed” nonuniform (see the SI, Fig. S13). Tween 20 modification via dip coating has also been used before,25,32 but modification of the noncontact region was found to limit PDMS transfer. The dual-printed features were characterized by three methods. In the first method, the printed surface was exposed to an anionic amine-reactive dye, Reactive Red 120 (Sigma-Aldrich). Reactive Red 120 has sulfonate groups that selectively react to the amine groups of PAH. On the printed surface, only the contact region was stained indicating selective deposition of PAH (see the Fig. 1f inset and the SI, Fig. S.14). No staining was observed on PDMS, which is not expected to interact with sulfonate groups. This suggests that PAH was patterned successfully. In the second method, the quality of printing was assessed at the molecular level by SEED as describe above (Fig. 2).22,23 The sample was immersed in 100 mM of NaCl solution. The measurement was made using the standard three-electrode CV arrangement, where the reference electrode (RE) was Ag/AgCl and the counter electrode (CE) was the Pt-coated chamber wall (see the SI, Section S.1.3). The sample was the working electrode (WE). The DC potential between the WE and RE was fixed at ~250 mV, close to the potential of zero charge (PZC) for the NaCl solution where ∆ ~ ∆max.33–35 As the probe beam scanned over the pattern, the ∆max modulated


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due to changes in the local electroactivity (Fig. 2a). For the beam over the passivated region (i.e., covered with PDMS), ∆max was nominally zero, while the electrode coated with polyelectrolyte had a high signal because of ionic conductivity. SEED is extremely sensitive to pinholes. Small pinholes in PDMS film would lead to a large ∆max in a passivated region (in Fig 2a). The SEED sensitivity to pinholes is explicitly demonstrated for an electrochemical DNA chip study where incomplete passivation by 6-Mercapto-1-hexanol (MCH) of the Au electrode surrounding the DNA leads to local redox (SI, Section S.1.6 and Fig. S5).36 The sharp modulation of ∆max in Fig. 2a indicates that the interface is robust and pinhole-free. The rounding of the profile is due to the ~6 µm diameter laser beam. SEED did not show pinholes in the PDMS regions, indicating the passivation is pinhole free. The ∆max of film without the PAH deposition (Fig. 2a, red line) is significantly lower than dual-ink-printed sample with PAH (Fig. 2a, blue line) due to more charge accumulation in the latter. The ratio ∆max between the contact and noncontact region is ~160 (for blue line). The enhancement is consistent with previously observed SEED results where the signal for PAH/Au is higher than bare gold because the polyelectrolyte’s charge causes a higher Dukhin number,37 which causes a decrease in the apparent zeta potential of the double layer.38 As a result, the double layer has higher ionic strength leading to a larger signal as if the bulk concentration were increased.39 Replacing the NaCl solution with NaBr reduced the effective contrast (see the SI, Fig. S4). The window for curing time (in Step c in Fig. 1) to ensure deposition of PDMS in the noncontact region (Fig. 1e) and have adequate stiffness to release the stamp from the mold is broad. For a curing time above 24 min, the PDMS is adequately cross-linked to peel off from the mold (see the SI, Section S.2.5 and Video R3). The quality of PDMS deposition based on curing time was determined by SEED.

The ratio of average ∆max between passivation and no passivation



(averaged over 10 CV cycles) for a curing time between 25 and 60 min is large (Fig. 2b). The ∆max ratio for a curing time of 25 min was 160 (Fig. 2a) and for 0.5 and 1 hr was 74.7 and 17.3, respectively (Fig. 2c), indicating a reasonable pinhole-free deposition of PDMS. Beyond 60 min, the contrast in ∆max reduces significantly; and the deposition of PDMS is not uniform due to depletion of PDMS oligos in the stamp (Fig. 2b and 2c). Thus, a curing time of 25 min is reasonable to obtain a high contrast (i.e., large ∆max ratio) and mechanical stiffness of PDMS to release the stamp from the mold, and, as discussed above, to prevent the roof from collapsing during stamping. SEED ensures a local pinhole-free PDMS film. The pinhole-free passivation on the whole electrode can be ensured electrochemically as follows. Negatively charged [Fe(CN)6]4- ions can exclusively be embedded in positively charged PAH film by application of cyclic potential (see the SI, Section S.1.7).40 During the embedding process, the redox peak in CV monotonically reduces down to zero due to reduction in the turnover reaction caused by repulsion between the [Fe(CN)6]4- ions in the solution and those embedded in the film (see the SI, Fig. 6).40 The PDMS/PAH patterned electrode was placed in 100 mM K4Fe[CN]6 (Sigma Aldrich), and CV was performed from -200 mV to 500 mV at 800 mV/s at 22 °C. The embedding process showed characteristic monotonic reduction in the peak currents of a cyclic voltammogram (Fig. 3). After 150 cycles, the redox current was entirely quenched to zero, indicating no pinholes. A small, accessible, underlying, exposed Au electrode would have enough current to prevent the monotonic reduction in redox peaks to zero. Thus, the printed pattern had no significant unfilled regions over the entire 1.1 mm by 8 mm electrode. In the third method, the deposition thicknesses were measured using SPM. Consistent with the previous observation,20 the PDMS deposition was 18 nm thick (Fig. 4a.) without the PAH ink.21


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To demonstrate the PAH transfer without PDMS, a Tween 20 treated stamp was fully cured for 24 hours to avoid PDMS deposition. The SPM images showed a thickness of 150 nm that corresponded to only PAH deposition (Fig. 4b). The lack of PDMS transfer was confirmed with SEED, which showed no passivation of the noncontact area. After thorough washing, the sample was too thin to be observed by SPM due to its limited scan distance. This is consistent with the literature where a subnanometer PAH monolayer on gold is expected after washing.41,42 The full structure, with PAH and PDMS deposition, showed a more gradual slope for the PAH than previously and without the ridges at the edges (Fig. 4c). Since SPM only shows topography by relative height, dual transfer could not be determined. However, it is noteworthy that there was no dip in between the PAH and PDMS due to dewetting. The combined observations of SEED and SPM confirm the codeposition of PDMS and PAH in single-step printing. E. coli is known to have negative surface charge43 and is, therefore, expected to immobilize on an amine-rich surface.44 Upon deposition, there was obvious clustering on the contact region with a clean noncontact region (Fig. 5). This was reliably repeated over several successive features, demonstrating a possible application for cell immobilization or growth using patterned surfaces.45,46 The coverage remained excellent even after gingerly washing in DI water, indicating that the PAH had good coverage. The viability of the cells was confirmed using BacLight L7007 stain immediately after deposition (Fig. 5d and the SI, Section S.1.5. As Au has high autofluorescence, the measurement was performed on glass using stamping conditions identical to those described above. Because there was no gap at the PAH/PDMS interface, the patterned seeding could be used to electrically probe the cells while eliminating any interfering signal from bare gold.



As a second application, negatively charged Au nanoparticle necklaces were deposited selectively on the PAH surface of the patterned structure. The PAH/PDMS pattern was printed on a Si chip metalized with a Au electrode pattern about 200 nm thick (Fig. 6). On immersing the chip in the necklace solution. Deposition of Au nanoparticle necklaces showed high selectivity to PAH-coated regions (Fig. 6a). The FESEM image shows that the necklaces cover the PAH region at high density (Fig. 6b) with good coverage across the electrode/SiO2 interface. The interface between percolating necklace deposition on the PAH and PDMS region (Fig. 6c) is sharp with some stray clusters close to the interface indicating that the method can be used to developing conducting nanoparticle necklace array. The percolating array can potentially be used as an electrochemical nanoelectrode for sensor applications47 and an interface to live cells.48,49 To note is that with the high contrast due to Au nanoparticle deposition, it was possible to demonstrate the quality of the printing in terms of uniformity over a large area. Conclusion In summary, we have demonstrated a method by which a hydrophilic and hydrophobic ink can be printed simultaneously on an electrode using microcontact printing. A stamp fabrication process was developed where the hydrophilic ink transferred by traditional contact printing and a hydrophobic phase was deposited by noncontact transfer to a thickness of 18 nm. Repeatable 20 µm features have been demonstrated, and previous experiments suggest that features as small as 5 µm would be possible. A pinhole-free coverage of the pattern was achieved on an electrode measuring 1.1 mm by 8 mm. The process was characterized by microscopy using a dye, by surface probe microscopy, and electrochemically to show that the patterning was pinhole free. It was demonstrated electrochemically that the PDMS region was entirely passivated with no exposed Au gap between the hydrophobic and hydrophilic inked patterns (which occurs due to


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the dewetting effect in stage-wise printing). Selective deposition of live microorganisms and Au nanoparticles onto the PAH regions was demonstrated for potential applications in co-culture and microelectrode sensors. The process may be expanded to (hydrophobic) ink other than PDMS oligomers by soaking/adsorbing the ink in Step 1(c) (in Fig. 1) to other oligomers followed by temperature-assisted deposition during the stamping Step 1(e). Acknowledgements The authors would like to acknowledge the Biological Process Development Facility for assisting with the production of the E. coli culture. RFS acknowledges funding from NSF/CBET support (Award #1353125). Supporting Information (SI) SI contains details on sample preparation and precautions. The SI has 15 illustrations. Derivation of a formula to estimate sagging of the roof during printing. There are four videos on various aspect of the the process, such as, deposition of Teween 20 by capillarity, curing conditions, and mold release of the PDMS stamp.



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Figures

Fig. 1. A diagram of the dual-printing process. a) Silica substrate being patterned. b) Mold being filled with Tween 20. c) PDMS poured into the mold. d) PAH solution dried on the stamp. e) Stamp applied to a gold electrode. f) Finished sample. Inset optical microscope images of the associated steps. Larger versions of the inset images are available in the SI.



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(a)

(b)

(c)

Fig. 2. (a) SEED scan of electrodes in 100 mM NaCl at the potential of zero charge. The PAH (black) shows a significantly higher ∆max compared to bare gold (red). Scans are overlaid on a microscopy image of the patterned electrode for visual reference. (b) SEED scans at various curing times of the stamp. No PAH was deposited. (c) Histogram of the average ∆max values taken at nominally the center of the contact and noncontact regions. Ratios of contact to noncontact activity are displayed.


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Fig. 3. CV of patterned electrode in 100 mM K4[Fe(CN)6] solution.

Fig. 4. Scanning probe microscopy scans of the PAH and PDMS depositions, cross-section averages are inset. a) PDMS alone. b) PAH alone. c) PAH/PDMS together. PAH images were taken before the vigorous rinsing step that appears to remove excess PAH buildup. Larger versions of the insets are in provided in the SI, Fig. S13.



Fig. 5. (a) Optical microscopy image of E. coli deposited on PAH lines of PAH/PDMS pattern. (b) FESEM image of E. coli pattern. (c) FESEM close-up of the edge of the pattern. (d) Flourescence microscopy image of BacLight-dyed E. coli pattern on PAH-stamped glass. Green indicates living cells and red indicates dead cells.


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Fig. 6. a) Image of 20 nm nanoparticles deposited directly onto the PAH/PDMS surface. b) FESEM image of 20 nm nanoparticle chains on the PAH/PDMS surface. c) FESEM image at the edge of the deposition indicating sharp definition.



Figure for Table of Contents


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Simultaneous Printing of Two Inks by Contact Lithography - ACS

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