Role of Absorbed Solvent in Polymer Pen Lithography - American

Dec 10, 2013 - absorbed in the pen. To explore the transport of materials in the absence of environmental solvent, a hydrophobic polymer was patterned...
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Role of Absorbed Solvent in Polymer Pen Lithography Daniel J. Eichelsdoerfer,†,‡ Keith A. Brown,†,‡,§ Mary X. Wang,∥ and Chad A. Mirkin†,§,∥ †

Department of Chemistry, §International Institute for Nanotechnology, ∥Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ABSTRACT: We report on the dynamic role of solvents in molecular printing and show that material transport can be mediated by both environmental solvent (i.e., humidity) and solvent absorbed in the pen. To explore the transport of materials in the absence of environmental solvent, a hydrophobic polymer was patterned using a polydimethylsiloxane (PDMS) pen array that had been soaked in undecane, a nonpolar solvent that readily absorbs into PDMS. We also explored the patterning of the hydrophilic polymer polyethylene glycol (PEG) and found that, even though PDMS only absorbs trace amounts of water, soaking a PDMS pen array in water enables PEG deposition in completely dry environments for over 2 h. We find that the length of time one can pattern in a dry environment is determined by the availability of absorbed solvent, a relationship that we elucidate by comparing the performance of pens with varying ability to absorb water. Furthermore, a calculation accounting for the dynamics of retained water captures these effects completely, allowing for generalization of this result to other solvents and providing a way to tune the desired solvent retention profile. Taken together, this work explores the subtle and dynamic role of solvent on molecular printing and provides an alternative to strict environmental humidity control for reliable molecular printing.

1. INTRODUCTION Patterning soft or bioactive materials is an increasingly important task that is best achieved through direct deposition techniques such as dip-pen nanolithography (DPN),1−3 polymer pen lithography (PPL),4,5 or microcontact printing.6,7 In DPN, materials are commonly transferred from a scanning probe to a surface through the water meniscus that forms between the pen and the surface under ambient conditions.8 Because of the importance of environmental moisture, DPN is normally performed in a humidity-controlled environment. Although the majority of molecular printing research has focused on cantilever-based probes composed of silicon or silicon nitride,8,9 the cantilever-free architecture10 underlying PPL allows one to print with massively parallel arrays of polydimethylsiloxane (PDMS) probes (Figure 1A).4 The differences between PPL and DPN largely originate from their material constituents; for example, while feature size in DPN is force independent, the deformable nature of the pens in PPL allows one to rapidly toggle between nanoscale and microscale features by changing the force4,11 and even apply pressure to locally accelerate chemical reactions.12 PPL also has the advantage that the exceptionally high coefficient of thermal expansion of PDMS allows the use of local heating to physically actuate individual probes in an array.13,14 Another advantage of PPL is that the constituent material of the probes can be readily altered, for example, by adjusting the cross-link density to change the spring constant of individual probes,15 by including a second, more rigid elastomer at the tip of the pen,16 or by substituting different polymer materials altogether.17 Despite extensive research into molecular printing and the recognized importance of moisture in DPN, the role of solvents in PPL has not been studied. This fact is particularly striking since, unlike © XXXX American Chemical Society

Figure 1. (A) Schematic showing polymer pen lithography (PPL) where the ink (purple) is being printed from a solvent-filled (blue) pen array onto a surface. (B) Cross-section of an inked pen in contact with a surface where the solvent contained within the ink is exchanged with both the pen and the environment.

DPN probes which are impermeable to solvents, PDMS is permeable to many common solvents,18,19 including water.20−22 Here, we investigate the influence of solvent absorption into a PPL pen array on its ability to print molecules. When the pen is made of a permeable material, the solvent required to facilitate molecular transport can come from the environment and/or the pen itself (Figure 1B). To evaluate the importance of tip-absorbed solvent, we investigate various types of solvents, including water and nonpolar solvents, and quantify their ability to enter the tips and subsequently be used for molecular printing. As PDMS is known to readily absorb organic solvents, Received: October 23, 2013 Revised: November 26, 2013

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in an ink solution for 10 s, followed by gently blowing dry with N2. After inking, pen arrays were placed into a scanning probe instrument for printing (XE-150, Park Systems). Hexamethyldisilazane (HMDS)-coated Si ⟨100⟩ was used as a substrate for all patterning. Prior to patterning, the Si substrate was vapor coated with HMDS by placing the wafer in a desiccator with a vial of 1:1 v/v HMDS:toluene, evacuating the chamber, and then leaving the desiccator under vacuum overnight. Arrays were leveled using an optical method, and patterning was directed by commercial lithography software. Patterning was conducted at ambient temperature, which was ∼21 °C. For experiments conducted at RH ≈ 0%, dry N2 was flowed into the environmental chamber. After patterning, sample characterization was performed by optical microscopy (Zeiss Axiovert) and atomic force microscopy (AFM, Bruker Dimension ICON). Polymer samples were prepared for thermogravimetric analysis (TGA) by thoroughly mixing the prepolymer components, degassing the mixture, pouring it into a Petri dish, and then curing overnight at 80 °C. After curing, samples were sectioned with a 4 mm biopsy punch to obtain cylindrical samples of consistent size. In a typical TGA experiment, a disk was soaked in a solvent for 10 min, blotted dry, and then placed into the TGA (Discovery TGA, TA Instruments), which immediately heated the sample to 40 °C at 10 °C/min and held that temperature for 8 h under a N2 atmosphere. In order to determine the quantity of retained water, the weight change of a dry sample that had been held in a vacuum chamber for ∼24 h prior to TGA was used as a baseline.

we screen nonpolar solvents for those that are strongly absorbed by PDMS and retained for hours. Drawing analogy to a felt pen, we find that pen arrays soaked in the topperforming solvent (undecane) are able to pattern a hydrophobic polymer that otherwise could not be transferred at any relative humidity (RH). While this effect might be expected for nonpolar solvents, which are readily absorbed into the hydrophobic tip arrays, what is truly remarkable is that water, despite only being absorbed in trace quantities, elicits the same writing ability. Soaking a PPL pen array in water allows one to pattern materials in a totally dry environment for several hours that cannot be patterned at RH < 70% in the absence of absorbed solvent. By comparing the patterning performance of three different tip materials that absorb different quantities of water, we find that the quantity of water stored in the tip array determines how long a tip array can pattern. A calculation accounting for the dynamics of retained water captures these effects completely, allowing for generalization of this result to other solvents and providing a way to tune the desired solvent retention profile. These results provide a pen array treatment strategy that enables patterning both hydrophilic and hydrophobic materials while providing new insight into the dynamic role of solvents in molecular printing.

2. EXPERIMENTAL METHODS Pen arrays for PPL were fabricated according to literature methods.4,23 Briefly, a master consisting of an array of pyramidal holes in a Si ⟨100⟩ wafer was fabricated using photolithography, subsequent anisotropic etching and coating with a hydrophobic silane. To fabricate pen arrays, 1−4 drops of the prepolymer mixture were drop cast onto the master and a plasma-cleaned glass slide was placed on top of the prepolymer. Arrays were then cured at 80 °C for 24−36 h. Prepolymer for PDMS pen arrays was made by thoroughly mixing 3.4 g of a stock precursor mixture with 1.0 g of (25− 35% methylhydrosiloxane)-dimethylsiloxane copolymer (Gelest, HMS-301). The stock precursor mixture consisted of 500 g of (7−8% vinylmethylsiloxane)-dimethylsiloxane copolymer (Gelest, VDT-731), 20 μL of Pt divinyltetramethyldisiloxane (Gelest, SIP6831.2), and 344 μL of 1,3,5,7-tetramethyl-1,3,5,7tetravinylcyclotetrasiloxane (Gelest, SIT7900.0), which was stirred for 3 days before use. Prepolymer for composite PDMS−PEG arrays was made by mixing finely ground PEG (MW = 2000 g/mol, Fluka) with the aforementioned PDMS prepolymer, degassing the mixture for ∼15 min, and then curing at 80 °C for 24−36 h. Two inks were used in this study: (1) an ethanolic solution of 10 mM fluorescein isothiocyanate (AnaSpec, Inc., FITC) mixed with 0.5 wt/vol % PEG (MW = 2000 g/mol, Fluka), and (2) a chloroform solution of 0.4 wt/vol % polystyrene-bpoly(4-vinylpyridine) (PS-b-P4VP, MW = 47 000/10 000 g/ mol, Polymer Source). Both inks were sonicated for ∼10 min prior to use. Prior to inking the pen arrayswhich were polymer pen arrays for PPL or one-dimensional probe arrays (type M, NanoInk, Inc.) for DPNwere rendered hydrophilic through treatment with an air plasma at 10 W for 2 min. To spin coat a polymer pen array, 100 μL of the ink was cast onto the array followed by spinning for 60 s at 1000 rpm with a 1000 rpm/s ramp rate. To drop coat a polymer pen array, 100 μL of the ink was dispensed onto the array, which was then blown dry with N2 after standing for a period of 10 min and several seconds for the ethanol- and chloroform-based solutions, respectively. To dip coat a DPN pen array, arrays were dipped

3. RESULTS/DISCUSSION In order to evaluate the role of absorbed solvent in PPL, it is first necessary to quantify how much solvent is absorbed and how long it is retained. PDMS is known to absorb a wide variety of liquids,18,19 so we performed TGA on samples of PDMS soaked in one of a variety of nonpolar solvents chosen for their ability to permeate PDMS. Additionally, solvents were selected from two ranges of vapor pressures: low (1000 Pa). In all cases, the measured weight decayed and saturated to a stable value as the solvent evolved from the PDMS (Figure 2A), with the low vapor pressure solvents (undecane, chlorooctane, decane) exhibiting a slower rate of mass loss and thus longer retention time. The solvent retention time, quantified here as a retention half-life (Figure 2B), is important to consider for molecular printing as it typically takes tens of minutes to ink an array, approach the substrate, level the array, and perform PPL. Solvent retention half-lives show a strong dependence on vapor pressure, with the high vapor pressure solvents exhibiting retention half-lives of less than 0.5 h, which makes them poor choices for molecular printing. On the other hand, the low vapor pressure solvents have half-lives greater than 1 h, with undecane showing a retention half-life of over 2 h, thus making it an excellent candidate for patterning. To test whether material could be patterned using undecane absorbed in the pen array, we selected PS-b-P4VP as an ink material both because it is water insoluble and because it is widely used for block copolymer micelle lithography; therefore, successfully patterning it could be useful for that community.24,25 To explore whether PS-b-P4VP could be patterned using solvent absorbed in the pen array, we soaked a plasmatreated PPL pen array in undecane for 5 min, blew off excess undecane with N2, and then drop cast the array with the PS-bB

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Figure 3. (A) Dark field micrograph of the PS-b-P4VP pattern written by one PDMS pen, where all dwell times are 1 s. Here, the pen was used to write dot features in a serpentine pattern starting at the top left. (B) Atomic force microscopy (AFM) image of a similar pattern written by a different pen.

during patterning in a humidity-dependent manner. We attribute this to a depletion of solvated ink at the tip of the pen, a hypothesis that is supported by the spike in feature diameter observed near point 300 of Figure 4C, which is due to the pen array patterning over a leveling feature, presumably allowing the pens to reclaim some solvated ink. (3) Patterning eventually reaches a steady state transport rate that is dependent on RH. In order to further explore the role of absorbed water in the pen arrays, we designed a series of patterning experiments that utilized three pen arrays with differing water uptake capacities. Specifically, TGA characterization (Figure 5A) revealed that DPN pen arrays retain 0.02% water by mass, while PPL pen arrays retain 0.10% water. In order to fabricate pen arrays that hold more water, we constructed chemically modified PPL pen arrays that were composed of 5% PEG in PDMS by weight; these were found to retain 0.29% water. As the DPN pen arrays do not retain water after soaking, it was necessary to explore a different pretreatment procedure to allow for patterning. Given that PEG is hygroscopic,28 we developed a method to take advantage of that fact: prior to patterning, pen arrays were inked and then stored in a RH = 100% environment for 12 h, allowing the PEG to absorb ambient water. Following this treatment, these pen arrays were used to pattern 3 × 3 blocks of dot features at 0% RH with a 1 s dwell time and a 14 min wait between blocks. This pattern was designed to elucidate the dynamics of transport over 2 h. For patterns generated with DPN (Figure 5B), the feature size for a given pen decreased steadily throughout the pattern and did not vary within each block. Interestingly, not all pens wrote the whole time, with the longevity of all 12 pens being 76 ± 25 min. Here, longevity is defined as the time at which transport is no longer observable by fluorescence microscopy. In contrast, PPL pens wrote for slightly longer with the longevity of 96 pens being 90 ± 40 min; however, a more telling comparison is that just 8% of DPN pens wrote the whole time while 40% of PPL pens wrote over the full 2 h experiment. Furthermore, when patterning with composite PEG−PDMS PPL pen arrays (Figure 5D), all pens were observed to pattern throughout the entire experiment. The results in Figure 5 show that there is a correlation between the quantity of water that can be stored in a pen array and how long it could be used to pattern in a dry environment. Interestingly, a decrease in transport rate over the first few points of a pattern was only apparent in the case of PDMS pens (Figure 5C). Furthermore, between blocks of points, the ink appears to “regenerate”, as can be seen by the consistently large features at the beginning of each block. This behavior hints at a subtle relationship between the rate of solvent entering the ink from the pen array and evaporating into the environment.

Figure 2. (A) Weight change of solvent-soaked PDMS samples as the absorbed solvent evaporates as measured by thermal gravimetric analysis (TGA). (B) Half-life vs swelling ratio from the TGA data in A.

P4VP ink. Patterning an array of dot features resulted in an array of features that was visible by dark field optical microscopy (Figure 3A). AFM characterization of these features (Figure 3B) revealed an array of domed features where the feature diameter decreases after the first point. As a control, no transport was observed for pen arrays that had not been soaked in undecane, even with RH as high as 90%. This experiment suggests that it is possible to pattern inks that are not water soluble by storing solvent in the pen array. While patterning a hydrophobic polymer is an important validation of the concept of patterning with solvent absorbed in a pen array, the majority of inks that have been studied are hydrophilic.8 One particularly important hydrophilic ink material is PEG, which can act as a carrier matrix for a wide variety of nanomaterials and biomolecules.26,27 Unfortunately, despite being permeable to water,20 PDMS absorbs only trace water18 and PEG can reportedly only be patterned at RH > 70%.8,26,27 In order to explore whether PPL pen arrays can retain enough water to facilitate transport of hydrophilic materials, we plasma treated a PPL pen array to render it hydrophilic, soaked it in water for 10 min, drop cast PEG/ FITC ink onto it, dried the ink, and attempted to pattern at RH = 0%. Surprisingly, polymer patterns were deposited on the surface, and all 400 points in a 20 × 20 array were visible by fluorescence microscopy (Figure 4A). AFM characterization (Figure 4B) revealed that the feature diameter decreased rapidly (Figure 4C) throughout the first row of points before stabilizing at diameters of 244 ± 12 nm, which is qualitatively identical to patterning PS-b-P4VP with an undecane-soaked pen array. In contrast, when environmental solvent was introduced by repeating the experiment at RH = 60%, a much more uniform array of features was observed (Figure 4D and 4E) that more gradually decreased in size (Figure 4F) to diameters of 435 ± 19 nm. These experiments led us to three conclusions. (1) PEG can be patterned at 0% RH if the pen array is soaked in water prior to inking. It is important to note that in controls where the soaking step was omitted, no material transfer was observed. (2) The transport rate decreases C

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Figure 4. (A) Fluorescence micrograph and matching AFM image (B) of a poly(ethylene glycol) (PEG) pattern written by a single PPL pen at 0% RH, where all dwell times are 1 s. Deposition was obtained by soaking the PPL array in water prior to inking. (C) Diameters of the features written by three different pens in the same experiment as A and B. (D) Fluorescence micrograph and matching AFM image (E) of a PEG pattern written by a single PPL pen at 60% RH. Again, inking was preceded by soaking the array in water, and all dwell times are 1 s. (F) Diameters of the features written by three different pens in the same experiment as D and E.

Figure 5. (A) Weight change of solvent-soaked silicon, PDMS, and composite PEG−PDMS samples as the absolvent solvent evaporates as found by TGA. Fluorescence micrographs of PEG deposited by (B) dip-pen nanolithography (DPN), (C) PPL pen arrays made of PDMS, and (D) PPL pen arrays made of PEG−PDMS at 0% RH. (B−D) Inked pen arrays were held RH = 100% overnight prior to patterning. Each column represents the pattern written by a different pen in a DPN or PPL array, and all dwell times are 1 s. Blue curve in C shows the serpentine order of feature deposition within each block. Bottom panels schematically show the water flow between the pen, the ink, and the environment for each condition.

suggesting that the PDMS tip array will retain water for at least 20 min, a remarkably close number to the observed 14 min difference in mean longevity between PPL and DPN. Similarly, the PEG−PDMS composites are expected to retain >6 μg/cm2 water and lose water at a rate of ∼0.1 μg/(cm2·min), indicating that PEG−PDMS composite tip arrays should provide at least an extra hour of patterning time under dry conditions, consistent with our experiments. In contrast, while a 20 μm thick PDMS tip array is expected to hold up to 2 mg/cm2 of undecane, the tip array will lose undecane at a rate of 0.2 mg/ (cm2·min), indicating that undecane will only be held for ∼10 min. However, as tip arrays are predicted to retain solvent for a time proportional to the thickness of their PDMS films, thicker

By performing calculations to account for solvent dynamics, one can gain important insights into the printing process governing the behavior observed in Figures 3−5. Since it was possible to print PEG with a silicon tip array that retained no water, it is clear that the PEG itself retained enough water to remain viscous for tens of minutes, a length of time that is commensurate with the stabilization time required when measuring environmental solvent exchange with PEG.28 The PPL tip arrays considered here consisted of a 20 μm thick PDMS film supporting an array of tips, indicating that there is ∼2 mg/cm2 of PDMS in the array. TGA indicates that these PDMS tip arrays can hold at least 2 μg/cm2 of water and that PDMS will lose water at a rate of ∼0.09 μg/(cm2·min), D

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Figure 6. (A) Large-area fluorescence micrographs of PEG deposited for 12 h with a PDMS pen array at RH = 45%. Prior to inking, the PPL pen array was soaked in water. Within each pattern, dwell times are 0.3, 1, and 3 s. (B) Flourescence micrographs of patterns from five representative pens, where all dwell times are 1 s.

4. CONCLUSIONS

PDMS could be very useful for long duration printing in solvent-poor environments. In order to determine whether solvent loss through evaporation or feature deposition determines the total patterning time, one can estimate the amount of solvent printed to the surface in a given feature. To estimate this for a typical experiment, such as that shown in Figure 3 where a 300 nm diameter feature with a 7:1 diameter:height aspect ratio (∼2 aL volume) was patterned once a second per pen, it is important to note that PEG retains approximately its weight in water at RH = 90%.28 These data, combined with the fact that there are typically 10 000 pens/cm2, amounts to ∼1 ng/(cm2· min) of lost water, a rate nearly 2 orders of magnitude smaller than the rate of water lost through evaporation. This implies two important things: (1) when patterning in a dry environment, solvent lost to the environment dominates solvent transferred to the surface; (2) it takes very little solvent to pattern, an important result considering the small quantity of water that PDMS retains. While patterning under dry conditions is useful for studying ink dynamics, stable and long-term transport is desired for molecular patterning experiments. On the basis of the results in Figures 4 and 5, this can be achieved in solvent-poor systems by (1) waiting long enough between each feature for the ink to regenerate, (2) blotting the pen at the start of a pattern to enter the steady state regime, or (3) providing additional solvent in the pen material, e.g., using a thicker PDMS backing layer. It is reasonable to predict that just as additional pen-absorbed solvent allows for stable patterning, so will additional environmental solvent. Therefore, we performed an experiment in which a PDMS pen array was soaked with water prior to inking and then used to pattern PEG at ambient RH (∼45%). As expected, the observed features (Figure 6) did not exhibit the decrease in feature size present in solvent-poor systems. Surprisingly, in this regime, PDMS pen arrays were able to robustly pattern PEG/FITC for up to 12 h (Figure 6B). This result, together with the result from Figure 5D, suggests that through proper chemical modification and treatment a pen array can be designed to retain all solvent necessary for patterning, obviating the need to control RH during deposition.

In this work, we have shown that it is necessary to consider the influence of solvent absorbed in the pen array as well as solvent in the environment in order to understand the molecular printing process. This observation highlights a major advantage of using elastomeric pens for molecular printing, namely, that solvent can be stored in the pen array, allowing for molecular deposition without the requirement of strict environmental control. This work encompasses three results of particular interest: (1) Soaking a PPL pen array in undecane prior to inking allows it to pattern a hydrophobic block copolymer that is water insoluble, a capability that may prove extremely useful for patterning the myriad water-insoluble polymers used in organic electronics and photovoltaics. (2) Soaking a PPL pen array in water prior to inking allows it to pattern a hydrophilic polymer for many hours, even at 0% RH. Given the importance of PEG as an ink, this property alone makes this an appealing technique for rapidly fabricating high-density bioassays without the need for humidity control, which is a compelling advantage as the main difference between an AFM and a DPN tool is the presence of a humidity-controlled chamber. (3) As the patterning dynamics in a solvent-poor environment is well described by a versatile model, these results provide guidelines for designing tip arrays that retain solvents for desired amounts of time, reflecting a degree of customizability that is not present in hard systems.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (847) 491-5784. Fax: (847) 491-3721. E-mail: [email protected]. Author Contributions ‡

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

The authors declare no competing financial interest. E

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ACKNOWLEDGMENTS We thank Donald Aubrecht and C. Michael McGuirk for helpful discussions. This material is based upon work supported by DARPA/MTO Award N66001-08-1-2044, AFOSR Awards FA9550-12-1-0280 and FA9550-12-1-0141, and NSF award DMB-1124131. D.J.E. gratefully acknowledges the DoD, Air Force Office of Scientific Research, for a National Defense Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a. K.A.B. gratefully acknowledges support from Northwestern University’s International Institute for Nanotechnology. M.X.W. gratefully acknowledges a National Science Foundation Graduate Research Fellowship (NSFGRFP) and a Northwestern University Ryan Fellowship.

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