Printing Non‐Polar Polymers Using the BioForce Nano eNabler™ Open Channel Micro‐patterning Tool William Montes and Thomas C. Marsh University of St. Thomas, Dept. of Chemistry Abstract The Nano eNabler™ (NeN) is a versatile micro/nano‐scale printing tool for creating arrays of materials with high precision and accuracy. An open channel microfluidic device is used to deliver very small volumes of solution to a surface. The majority of current applications for the NeN are focused on creating patterns of water‐soluble polymers, biomolecules, viral particles and living cells on various surfaces. In order to use the NeN for creating arrays of non‐polar molecules, a suitable solvent with low vapor pressure is required. This work describes the development of a sample preparation method and instrument parameters that enable printing arrays of polystyrene (PS) and polymethylmethacrylate (PMMA) onto substrates such as SiO2, Au, Mica and Indium Tin Oxide. Introduction The ability to reproducibly pattern a surface with a variety of material would be of great utility to biotechnology and nanotechnology industries. The NeN is a precise and extremely accurate molecular printer. Its motion control platform is attached to the surface patterning tool (SPT) which can create 1‐
30 μm spots in a matter of milliseconds and a 10 x 10 array of spots in a matter of seconds. A high‐
resolution optical zoom microscope and CCD camera with video recording capabilities allows constant real time observation of the spotting. Computer software permits a high level of control over a number of variables that determine spot size, such as humidity and contact time. The majority of applications for the NeN have involved creating arrays of water‐soluble materials. This technology has the potential to spot solutions of non‐polar molecules on the micro/nano‐scale. The primary goal of this project was to successfully pattern hydrophobic polymers on a variety of surfaces. Dilute solutions of polymer using a plasticizer as the solvent should have a low enough vapor pressure to permit patterning using the open channel device. Adjusting the surface wetting properties of a given substrate should also allow pattering of a polymer array. Methods Polymer solutions tested include PS, PMMA, Poly[3‐(3‐carboxypropyl)thiophene‐2,5‐diyl] (P3CPT) as well as the more polar polyethylene glycol (PEG 400) as a neat liquid. The polystyrene solution preparation consisted of first dissolving PS (17 kD, Mn) in toluene, then diluting that solution into diethylpthalate (DEP). The toluene was allowed to evaporate leaving a 0.1% w/w solution of PS in DEP. PMMA (120 kD, Mn) solutions were prepped using chlorobenzene and DEP in a similar manner. The P3CPT was dissolved in dimethylformamide (DMF) to prepare a 10% concentrated solution that was then diluted to 1% P3CPT in tetraethyleneglycol diethyl ether (TEGDE). A 10 μm SPT was loaded with the polymer solutions and then used to produce arrays. Humidity, contact time, tip speed and tip height were manipulated to obtain reproducible spot sizes. The effect of the SPT holder angle on spot size was also investigated. The substrates tested were silicon wafers with a native oxide layer (SiO2), atomically flat gold (AFG) evaporated onto SiO2, freshly cleaved mica and Indium Tin Oxide (ITO) were the primary substrates tested. Substrate surface energies were modified either by ozone treatment prior to pattering or, in the case of SiO2, adding a layer of 3‐Aminopropyltriethoxysilane (APTES) through vapor diffusion under low vacuum. Results The ability to routinely spot a solvent (PEG 400, TEGDE, DEP, DMF) on a variety of substrates was consistent with expected surface wetting properties of the solvent for a given substrate (AFG, Mica, ITO SiO2, APTES‐SiO2). A neat solution of PEG 400 was patterned in a 10 x 20 spot array on AFG, SiO2 and ITO with the smallest average spot size of 1.5 μm achieved using a 24° SPT holder @ 34% relative humidity on an AFG substrate (Figure 1). It was not possible to spot the PEG 400 on freshly cleaved mica due to surface wetting. The average spot diameter of PEG 400 on AFG increased to 2.4 μm as the relative humidity decreased to 14%. A 1.0% solution of P3CPT in TEGDE, a solvent chemically similar to PEG 400, had markedly different substrate wetting properties on AFG. The smallest spot size achieved on this substrate was 25 μm. The addition of polymer and the presence of DMF in the mixture most likely account for this result. The best surface for pattering the polythiophene was ITO on which arrays with an average spot size of 7 μm were obtained using the standard 12° SPT holder. In certain cases the surface wetting of a given solvent was overcome by either increasing the humidity in the chamber or by treating the surface of the substrate to alter its wetting properties. This enabled reproducible patterning of solutions of non‐polar polymers such PMMA in DEP onto SiO2. The only native substrate tested that was amenable to spotting with PMMA/DEP solution was ITO (5 mm average spot size at 15% relative humidity. Significant substrate wetting occurred with DEP solutions on native AFG and SiO2 at low relative humidity. Surface wetting was no longer observed at 37% relative humidity yielding 25 μm spots. Average spot size decreased with an increase in humidity up to 58% that yielded 10 μm average spot size of PMMA in DEP. Similar results were obtained with PS in DEP. Treating the SiO2 surface with APTES vastly improved patterning of the polymer solutions producing highly uniform arrays of PMMA with an average diameter of 7 μm (Figure 2).
Figure 1. Array of PEG 400 on Au surface. Figure 2. Array of PMMA on APTES‐SiO2. Evaporation of the DEP from the spots can be seen in the lower left corner of the array Conclusion and Discussion Discussion and Conclusion The results of this work demonstrate that solutions of non‐polar polymers may be prepared in a common plasticizer to facilitate their use in an open channel microfluidic device. Solution/surface compatibility greatly impacted the ability to form stable spots. Altering the surface to increase or decrease its wettability is a likely necessary step for patterning new other new materials. In general a hydrophilic substrate surface, such as APTES coated silicon, would be expected to have a greater ability to hold smaller non‐polar polymer spot size compared to relatively hydrophobic surfaces like untreated silicon. It was also observed that the cleanliness of the substrate had a significant effect on solution beading, particularly on silicon and AFG. Substrate surface preparation is crucial to the spot size and quality of the array. The pattern defects seen on AFG (Figure 1.) are likely do to surface undulations and or poor spot adhesion that lead to scooting of the spot due to competing adsorption to the SPT tip during withdrawal from the surface. Solving these problems is matter of selecting flattest possible substrates. The spot scooting problem was not observed with PMMA on APTES‐SiO2 and a highly ordered array was obtained on a substrate with similar roughness to the AFG. This suggests that substrate The ability to pattern non‐polar macromolecules with high precision and accuracy opens up many new applications for the NeN. Arrays of polymers that have different functional group properties could be used to construct micro‐scale sensor arrays, micro/nano‐scale electronic devices and solar cells as evidenced by the ability to construct arrays of a polythiophene. It will be interesting to investigate and further characterize combinations of solvent‐polymer‐and substrate to expand the utility of the NeN. Acknowledgments We extend our special thanks to Eric Henderson, Michael Lynch, Jun Wang and the support staff at BioForce Nanosciences for provision of the NeN as well as helpful advice. We would also like to thank Patrick Hawk for his assistance and the University of St. Thomas for research funding.