Microfluidic Devices Containing ZnO Nanorods with Tunable Surface

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Microfluidic Devices Containing ZnO Nanorods with Tunable Surface Chemistry and Wetting Independent Water Mobility Mirit Hen, Eitan Edri, Ortal Guy, Dorit Avrahami, Hagay Shpaisman, Doron Gerber, and Chaim N Sukenik Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02826 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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Microfluidic Devices Containing ZnO Nanorods with Tunable Surface Chemistry and Wetting Independent Water Mobility Mirit Hen1, Eitan Edri1, Ortal Guy2, Dorit Avrahami2, Hagay Shpaisman1, Doron Gerber2*, Chaim N. Sukenik1* 1Chemistry

Department and Institute of Nanotechnology and Advanced Materials, Bar

Ilan University, Ramat Gan 5290002, Israel 2Mina

and Everard Goodman Life Science Faculty and Institute of Nanotechnology

and Advanced Materials, Bar Ilan University, Ramat Gan 5290002, Israel *Corresponding authors: Chaim N. Sukenik

[email protected]

Doron Gerber [email protected]

Abstract Interest in PDMS microfluidic devices has grown dramatically in recent years, particularly in the context of improved performance lab-on-a-chip devices with decreasing channel size enabling more devices on ever smaller chips. As channels become smaller, the resistance to flow increases and the device structure must be able to withstand higher internal pressures. We report herein the fabrication of microstructured surfaces that promote water mobility independent of surface static wetting properties. The key tool in this approach is the growth of ZnO nanorods on the bottom face of the microfluidic device. We show that water flow in these devices is similar

whether

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superhydrophobic; i.e. the device tolerates a wide range of surface wetting properties without changing the water flow within the device. This is not the case for smooth surfaces with different wetting properties, wherein hydrophilic surfaces result in slower flow rates. The ability to create monolayer-coated ZnO nanorods in a PDMS microfluidic device also allows for a variety of surface modifications within standard, mass produced, devices. The inorganic ZnO nanorods can be coated with alkyl phosphonate monolayers. These monolayers can be used to convert hydrophilic surfaces into hydrophobic and even super-hydrophobic surfaces that provide a platform for further surface modification. We also report photopatterned biomolecule immobilization within the channels on the monolayer-coated ZnO rods.

Keywords: Super-hydrophobic Surface wetting Biomolecule immobilization Self-assembled monolayers Mass flow in microfluidic channels Textured surfaces

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Introduction Microfluidics is a technological platform that enables scale reduction in sample volume and measurement times1-3, versatility in design, improved sensitivity and selective reactions for surface patterning4-10. The use of microfluidic devices to conduct biological and chemical research and to create clinically useful technologies, has a number of significant advantages. One such advantage is that because the volume of fluids within these channels is very small, often in the nanoliter range, the amount of sample and/or reagents used is quite small. This is especially significant for expensive and/or hard to obtain materials wherein smaller channels allow for the use of less material. PDMS elastomer is the most widely used material in the construction of microfluidic devices11-13. A big advantage of PDMS is its straightforward manufacturing and fabrication14-16. PDMS also has important materials properties: it is elastomeric, optically transparent, and is a soft material. These advantages allow for the integration of complex processes and diverse analytical tools into microfluidic lab-on-a-chip devices that serve as multifunctional platforms for a variety of analytical assays17. Interest in the potential application of microstructured surfaces for drag reduction has grown18. Microstructured materials showing superhydrophobic behavior (contact angles of >150°) can be based on micro- or nano-scale surface roughness, combined with hydrophobic surface chemistry. These surfaces exhibit extremely high water contact angles and very low flow resistance19. The surface wetting characteristics of these surfaces involve two states: the Cassie Baxter state20 and the Wenzel state21. In the Cassie - Baxter state , the drop sits on top of the microstructured area with trapped air underneath it. When the water drop starts to wet the surface, it moves into the Wenzel state. 3 ACS Paragon Plus Environment

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The possible role for superhydrophobic surfaces within microfluidic devices has received increased attention as engineering applications reach down to the micrometer and nanometer scale, and as we improve our ability to probe the physics of fluid surface interactions at the molecular scale. Drag reducing properties of superhydrophobic surfaces have been observed experimentally in microfluidic devices22-24 and for coated objects, such as hydrofoils25, settling spheres26 and cylinders27, covering flow regimes from laminar to turbulent. Superhydrophobic PDMS microfluidic devices have been previously reported28-31. Generally, these devices are open, one-channel systems. Usually, PDMS microfluidic devices for biology or biochemistry are closed-channel systems which enable the control of surface chemistry and bioactivity1-3. However, the utility of these devices is affected by changes in water mobility resulting from surface modifications. Additionally, downscaling devices to sub-micron channels32, typically leads to higher internal pressure. Thus, improved water mobility within these devices is important. Previous research has shown that superhydrophobic rough surfaces can improve water mobility33-36. It has also been suggested that hydrophilic rough surfaces can provide similar improvement28-31,37 and that the effect of rough surfaces on water mobility depends on their geometry and dimensions. In the work reported herein we have studied the effect of surface roughness on water mobility within closed channel PDMS microfluidic devices. We examined water mobility at high pressures on both smooth and textured surfaces with different wetting properties. Microstructured ZnO films were fabricated38 on the silicon base of a PDMS microfluidic device. The hydrophilic ZnO nanorods can be made hydrophobic using a variety of monolayer coatings38-41. We have used long-chain organophosphonates41 and adjusted the. deposition conditions to be compatible with the PDMS device 4 ACS Paragon Plus Environment

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environment. This enabled the modification of the ZnO nanorods within a preassembled microfluidic device. The use of an olefin terminated alkylphosphonate enabled additional in situ chemistry, including the covalent anchoring of biomolecules on the device surface. Materials and Methods All the reagents and solvents were obtained from Sigma-Aldrich, Acros Organics, or Bio-Lab Ltd. and used as is (unless otherwise specified). Polydimethylsiloxane (PDMS) SYLGARD 184 was obtained from Dow Corning, USA. Silicon wafers were obtained from Virginia Semiconductor (N-Type; undoped, 100, >1000 -cm). Silicon samples were washed with chloroform, acetone and ethanol and dried with nitrogen. The PDMS device was washed with acetone and ethanol and dried with nitrogen. Then, the substrates were put in a plasma cleaner for 5 min (unless otherwise stated). Hydrophobic treatment of smooth silicon surface within PDMS microfluidic device Ocatadecyltrichlorosilane (OTS) deposition onto smooth silicon surface within a microfluidic device A monolayer of OTS molecules was created within the device using a previously reported vapor deposition method42. Commercial OTS was put into a vial with a septum. A tube was connected with a small needle inserted through the septum and it was connected to the device input. A syringe was connected via a piece of tubing to the device output. When the plunger was pulled, it created a vacuum in the device, which pulled OTS vapors into the device. The vial was heated to 60 C and the vapors were


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allowed to remain in the device overnight. Then the device was washed with acetone for 5 min. ZnO nanorod growth and modification MnO2 seeding to promote ZnO nanorod growth Creation of the required MnO2 seed layer (as per reference 38) was done in two different ways. Method 1: To 15 mL of 5 mM KMnO4 in water was added 60 μL of butanol. This solution was used to create an MnO2 layer by dipping the sample into the solution for 25 min at room temperature and rinsing with water. Method 2: A MnO2 thin film was created by sputtering (CUSTOM. Hjush, BesTec) using a manganese target (99.9%, 2'' diameter and 6 mm thickness) that was obtained from Holland Moran Ltd. The vacuum chamber was pumped down to 3*10-3 mbar. The flows of argon and oxygen into the chamber were 1.4 and 15 SCCM respectively. The plasma power was 100 W and the substrate temperature was 25 ºC. Method 2 is suitable for the direct production of ZnO modified micro-fluidic devices, while Method 1 could be used for the deposition of ZnO layers within already sealed devices. ZnO nanorods grown by chemical bath deposition (CBD) ZnO nanorods were grown from deposition solutions prepared as previously reported18. A 10 mL portion of the deposition solution was prepared by adding 1 mL of 1 M Zn(OAc)2, 2 mL ethanol amine 50% (vol) and 1.5 mL of 4 M NH4OH to 5.5 mL H2O. Flat silicon wafers bearing the MnO2 primer layer were immersed into the deposition solution and heated to 80 °C for 10 min. They were then washed with water and sonicated in water for 10 min and dried under a flow of N2.


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Alkylphosphonic acid deposition on ZnO nanorods A silicon substrate coated with ZnO nanorods was immersed into a solution of 1 mg of commercial ODPA (octadecylphosphonic acid; an 18-carbon saturated alkyl chain) or C16DB-PA (15-hexadecenylphosphonic acid; a 16-carbon chain with a terminal double bond) in 10 mL of dry acetone at room temperature and left to stand overnight. It was then withdrawn from the solution and sonicated for 6 min in acetone and dried in a nitrogen stream. Microfluidic device with ZnO nanorods within the microfluidic channel Integrated microfluidic device fabrication A mixture of silicone based elastomer and curing agent in a 10:1 ratio, was prepared. The PDMS mixture was poured into a silicon mold of one microchannel (100 µm width, 2 cm length, 15 µm height) with input and output ports for fluid flow. The PDMS is degassed and baked for 30 min at 80 °C. This yielded a 5 mm thick device. Holes were made in the PDMS so as to accommodate the input and output ports. This methodology allows for the convenient production of channels down to 5 µm width. ZnO within the device The textured surface based on ZnO nano-rods within a microfluidic channel was achieved in two ways. ZnO nanorods grown within a pre-assembled PDMS microfluidic device The silicon wafer substrate was coated with a seed layer of MnO2 (as described above). A plasma activated PDMS device was then bonded to it. ZnO deposition solution was prepared as described above. The solution was injected into the device and the channel was filled at ~4 psi backpressure. The device was placed on a hot plate at 80 C and the solution flowed for 10 min. After 10 min, water 7 ACS Paragon Plus Environment

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was flowed through the device for 5 min to remove residual Zn-containing solution and the device was dried under an air flow. Silicon based ZnO patterning and PDMS device alignment SPR-220-7 resist was spin-coated on one side of a polished 4" silicon wafer and put on a hot plate at 105 C for 6 min. The wafer was then exposed to UV light (365 nm) through a chrome photomask for 1 min. After exposure, the wafer was put on a hot plate at 110 C for 5 min. The wafer was allowed to stand for 45 min at room temperature prior to resist development. Then the wafer was put in AZ-726 development solution (aqueous solution of 2.38% tetramethyl ammonium hydroxide). The wafer was put in a sputtering machine for MnO2 seeding on the patterned wafer after being activated in O2 plasma for 5 min. After sputtering, the wafer was cut into 8 patterned pieces with one channel each, sonicated in water for 10 min and put in a ZnO CBD solution as described above38. After the ZnO was grown, the resist was removed using acetone. Device sealing A PDMS microfluidic device and a patterned ZnO silicon wafer were put in an O2 plasma for 40 sec. After plasma treatment, the two pieces were aligned and bonded. They were then put on a hot plate for 2 h at 80 oC. Organophosphonate deposition on ZnO nanorods within the microfluidic device A solution of alkyl phosphonate was prepared as described above. The solution was flowed overnight through the microfluidic device at room temperature. The device was washed with acetone and dried in an air flow.


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Adhesion of hydrophophilic particles to superhydrophobic ZnO nanorods within the device Fluorescent particles (multi-fluorophore 0.1-0.3 μm, Spherotech, Inc.) with hydrophilic groups like acids and esters on their surface were suspended in water in a ratio of 1:10 (V:V). This solution was then flowed into the microchannel device for 5 min at 4 psi backpressure and incubated for 20 min. The channel was then washed with water and the fluorescence was measured using a fluorescence microscope. The image was analyzed using Image J software. Mass flow rate measurements within the microfluidic device The one channel PDMS device described above was connected to a backpressure system with water flow. The backpressure was raised in 250 mbar increments every 5 min from 250 mbar to 2000 mbar and the water that came out of the device was collected and weighed in tared vials. The flow rate experiments reported herein were only done on the system in Figure 1B due to occasional clogging issues when the nanorod deposition was done within the device (as per Figure 1A). The water mass flow rate through four kinds of devices (differing only in the characteristics of the base of the channel) were measured: bare silicon wafer (hydrophilic smooth), OTS coated (smooth hydrophobic), ZnO nanorods (hydrophilic rough), ODPA-coated ZnO nanrods (superhydrophobic rough). Each flow experiment was repeated on 2 separate devices of each type. Each pressure measurement was done 3 times. The data that is plotted is the average of all 6 measurements.


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Patterned-Biomolecule immobilization within device Thiol-ene click chemistry of terminal olefin with 3-mercaptopropionic acid In one channel of a PDMS microfluidic device onto which had been deposited ZnO, 15-hexadecenyl phosphonic acid (C16DB PA) was deposited on the nanorod surfaces using a solution of 1 mg of C16DB PA in 10 mL acetone. Thiol-ene click chemistry43,44 was carried out by flowing through the channel a solution of 3mercaptopropionic acid (100 µL) in methanol (10 mL). Then the device was placed in the mask-aligner under a chrome photo-mask as shown in Figure 2 and was exposed to light at 365 nm. Biotin attachment The areas in the device to which 3-mercaptopropionic acid was attached were functionalized with molecules having an amine on one end and biotin on the other end (Figure 2). The coupling procedure was done by flowing within the device overnight at ~1 psi backpressure a 10 mL water solution of 1 mL Dulbecco's phosphate buffered saline-02-023-1 (PBS), 100 µL of amine-PEG-biotin in PBS (1 mg/mL) and 0.01 g of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC)-hydrochloride. It was then cleaned by flowing DD water for 5 min. Fluorescent Streptavidin attachment A solution of 1 µL of labeled Cy3'-streptavidin in 99 µL of PBS solution was flowed for 30 min at ~5 psi. It was then cleaned by flowing a PBS solution for 5 min. Fluorescence intensity was measured using a Tecan LS400 Microarray scanner with an excitation wavelength of 532 nm and an emission wavelength 560 nm.


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Characterization methods Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) The spectra were measured using a Bruker Tensor 27 Infra-Red spectrometer. Twoside polished (500 μm thick) silicon wafers were polished on the short edges to a 45° angle and used as ATR-FTIR internal reflection elements (IRE). The incident angle (45°) of the infrared beam was controlled to be normal to the bevel surface of the internal reflection element. The spectra (80 scans) were then acquired using 4 cm–1 resolution. Each spectrum of coated samples was obtained by subtracting a background obtained using cleaned substrates (silicon or silicon + ZnO). Contact Angle Measurements (CA) The CA was measured using a Rame-Hart Model 100 contact angle goniometer under ambient conditions. A drop of double distilled water (~3 μL) was placed on the sample using a micro-syringe. Reported values are the average of three measurements per sample taken at different points on >7 samples. Scanning Electron Microscopy (SEM) The morphology of the sample surface was assessed by Environmental-SEM (FEI, Quanta-E-SEM) using an accelerating voltage of 10kV-15kV. Focused ion beam (FIB) The ZnO nanorod layer thickness was determined using a dual beam FIB (FEI, Helios 600), with electron and ion beams of up to 30 kV each. The combination of two beams with different angles (52 degree between the beams) enables simultaneous work with both beams.Surface Coverage Nanorod film density was analyzed using Image-J software (1.47 V Wayne Rasband, National Institutes of Health, USA). A threshold


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operator was applied to each image and the surface coverages were calculated by comparing the black spots to the white spots on the image. Results and Discussion We have examined water mobility at high pressures on both smooth and textured surfaces with different wetting properties. For testing the effect on water flow, we used a device with a silicon wafer as its base substrate. The ZnO nanorods require an MnO2 primer layer and depending on how the primer layer is created, the ZnO can be created either by deposition within a pre-assembled device (Figure 1A) or by lithography before closing the device (Figure 1B).

Figure 1: (A) Device preparation and sealing after MnO2 seeding on a silicon surface. (B) PDMS microfluidic device aligned on silicon surface with patterned ZnO nanorods

To address the possibility of immobilizing biomolecules onto monolayer-coated nanorod films, we used thiol-ene photochemistry. This enabled us to attach thiol terminated molecules onto an olefin decorated monolayer covering the surface of the nanorods in the textured channel (Figure 2). Photo-attachment also allows for patterned molecular anchoring.


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Figure 2: Illustration of device exposure to UV for patterning biomolecules and thiol-ene click chemistry under UV irradiation and biotin group immobilization within the channel

Alkyl Phosphonate-Coated ZnO nanorods as a superhydrophobic surface ZnO nanorods were deposited from solution onto a silicon wafer with a MnO2 layer. They could be made superhydrophobic by organophosphonate deposition. Figure 3A shows the SEM image of the nanorods (spike-like projections) and Figure 3B shows the FIB cross-section of such a sample (thickness of layer 1.1-1.6 µm). Analysis of the SEM image showed 96.5% surface coverage (the density percent refers to the top surface nanorod density) after seeding with a sputtered primer layer of MnO2. The asdeposited ZnO (before ODPA treatment) was completely wetted by water. The superhydrophobicity of this sample (after ODPA deposition) was characterized by an an average advancing contact angle of 158 (Figure 3C). We acknowledge that our measurements of wetting are done in air and not with a fluid filled channel and that this might influence the results as well45,46.


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Figure 3: (A) SEM image of ZnO nanorods on silicon (B) FIB cross section image of ZnO nanorods (C) Optical microscope image of water contact angle on ZnO nanorods surface coated with an ODPA monolayer

The ODPA coating was characterized by FTIR-ATR. The methylene peaks of the aliphatic chains for ODPA appear at 2917 and 2850 cm-1. This indicates a dense, wellpacked organic monolayer film47. This packing undoubtedly contributes to the observed hydrophobic/superhydrophobic character of the coated textured surface. The olefin terminated thin film coating was also characterized by FTIR and showed the expected band for a vinyl CH stretch at 3080 cm-1. Microfluidic Channel with a ZnO bottom Photolithography on a silicon wafer surface Patterned ZnO nanorods could be created using photolithography. Figure 4 shows the patterned ZnO nanorods using SEM (A and B) and FIB (C), after photolithography on silicon. From the characterization of the patterned film it can be seen that the film grows uniformly and over the entire width of the channel of the PDMS device. Thus, we could align a PDMS device over a pre-defined photo-patterned ZnO-coated floor of the device channel (Figure 1B).


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Figure 4: Patterned ZnO nanorods created using photolithography: (A) and (B) are SEM images of the rods (surface coverage 93.2%) and (C) is the FIB cross-section of a 1.3-1.4 micron thick film.

ZnO nanorods grown within a pre-assembled PDMS microfluidic device Figure 1A shows the experimental set-up that enabled us to grow ZnO within a preassembled microfluidic device. Seeding was done on the open silicon surface either by sputtering or by solution treatment. The device was sealed after the PDMS was activated by plasma and the MnO2 layer did not adversely affect the adhesion of the PDMS to the wafer substrate. Flowing the ZnO deposition solution into a channel with a suitably seeded substrate gave similar nanorod formation. We note that in this work the ZnO films are not coated onto the PDMS, as has been reported elsewhere48,49. This was confirmed by SEM analysis (Figure 5), which showed a ~2 micron layer of ZnO nanorods grown within the device. The structure of the nanorods in this method is slightly different, probably due to the fact that ZnO nanorod growth is very sensitive to reaction conditions50. Due to the very high adhesion forces between the plasma-activated PDMS and the MnO2 layer on the device, it could not be removed cleanly from the silicon surface and residue of the torn PDMS can be seen in the SEM image in Figure 5A.


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Figure 5: (A) and (B) SEM images of ZnO nanorods grown on silicon surface within the chip by flowing ZnO solution within the chip; (C) cross section of the ZnO nanorods

In summary, the ZnO texturing could be applied either before device assembly or within a pre-assembled device. While nanorod fabrication within a pre-assembled device is convenient and allows us to avoid the photopatterning step, it has the disadvantage that, in some cases, the ZnO precipitates and clogs the device's inputs and outputs. Hence, the flow experiments described below were all performed on devices wherein the ZnO layer was created before device assembly. Reduced adhesion of hydrophilic fluorescent particles to superhydrophobic nanorods within microfluidic channels The impact of the hydrophobicity of the monolayer-coated nanorods within the device was assessed based on the adhesion of hydrophilic particles to their surface. We measured the binding of these particles to the superhydropobic surface of the treated nanorods and compared it to their binding to the hydrophilic surface of the asdeposited ZnO. The adhesion of fluorescent particles to these surfaces was measured by fluorescence microscopy within the microfluidic channel (Figure 6). There was clearly more fluorescence in the unmodified ZnO channel than in the superhydrophobic ZnO channel; the total fluorescent area measured in the superhydrophobic channel is half of that in the unmodified (hydrophilic) channel, i.e.


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13,058 versus 25,160 micron squared. Clearly, the superhydrophic surface is less prone to attracting the hydrophilic particles by approximately a factor of 2.

Figure 6: Fluorescent images of (a) particles on ZnO (b) hydrophobic modified ZnO

Flow velocity measurements within a microfluidic device Microfluidic channels with a rough film of ZnO nanorods on their silicon substrate surface were sealed into a PDMS device as described above. The flow velocity of water within the device was measured. Devices with 4 different kinds of bottom surfaces were compared: rough hydrophilic (as-deposited ZnO nanorods), rough hydrophobic (ZnO nanorods coated with ODPA), smooth hydrophilic (clean silicon with native oxide) and smooth hydrophobic (OTS monolayer coated silicon). Figure 7 compares the mass flow rate of water as the pressure increases for each of the four kinds of surfaces (for pressures between 250 and 2000 mbar). The experiment was limited by the fact that at higher pressures the PDMS can deform. We note that according to SEM cross section measurements (Figure 4c), the ZnO layer reduces the original channel height by ~10%. We therefore did not directly compare the textured to the non-textured surfaces.


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Figure 7: Comparison of mass flow rate between hydrophilic and hydrophobic device surface (a) rough hydrophilic vs hydrophobic and (b) smooth hydrophilic vs hydrophobic

Examining the differences at the highest backpressure (2000 mbar) allows for the most detailed comparison. Leaving the silicon smooth but making it hydrophobic (OTS treatment) improved the flow by 52%. However, for the rough surfaces the impact of differences in surface wetting is small (less than 10%) and may even favor the hydrophilic surface. Thus, control of either texture or hydrophobicity can lead to comparable flow improvement and could significantly enhance the performance of a microfluidic device by minimizing the pressure increase within small channels. In the uncoated (hydrophilic) rods, the improved flow can be due to water completely filling the gaps between the nanorods and becoming a uniform lubricant for the flowing water. The flowing layer interacts only with the tips of the nanorods (if at all). Therefore the friction, compared to a smooth surface, is reduced, as per Lee’s prediction for a system with small porosity and small diameter rods37. The lack of additional advantage for the superhydrophobic rough surface may be a reflection of turbulence induced by the surface roughness51. Rather than creating a uniform water-filled lubricant layer, the topography of the hydrophobic surface allows for the water to partiality percolate into the air between the tops of the nanorods but the lubricant effect of a uniform water layer is not present. Deformation of the water 18 ACS Paragon Plus Environment

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surface causes the interface to bulge as shown by Lee et al37 for short, densely packed, pillars on the micro scale and this increases the turbulence and the drag. Biomolecule anchoring and patterning on ZnO nanorod surfaces Finally, we demonstrate biomolecule attachment to the ZnO nanrods. Since flow rate in channels with textured surfaces is independent of changes in hydrophobicity, biomolecule attachment, which is often accompanied by changes in surface wetting, can now be done without impairing the flow rate within a PDMS microfluidic device. Biomolecule attachment on the textured surfaces was done by coating the ZnO nanorods with an olefin terminated alkylphosphonate which was then reacted with a thiol terminated carboxylic acid by shining a UV light through a chrome mask onto an area within the device (Figure 2). Biotin attachment was then achieved by reacting the acid-functionalized surface with a biotin derivative bearing a terminal amine. Figure 8 shows the fluorescence image (I) and signal distribution (II) through the channel of Cy3-labeled streptavidin after coupling to the biotinylated surface. This image shows the fluorescence signal intensity distribution expected for a single slit pattern52, reflecting the diffraction of the UV light irradiated through the single slit mask. Eliminating the slit, would result in the surface modification chemistry being achieved in a uniform fashion across the entire surface. Whether patterned or not, the surface biomolecules should not change the flow enhancement provided by the surface texturing.


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II

Figure 8: Fluorescence of streptavidin on biotinylated ZnO surface within device. (I) Fluorescent image from fluorescence scanner and (II) Fluorescence signal through the channel.

Conclusions The work reported above focused on creating microstructured surfaces that promote water mobility independent of static surface wetting properties. A zinc oxide nanorod layer provides a textured surface that promotes water mobility and can facilitate the down-scaling of microfluidic devices. The ZnO nanorods can be coated with organophosphonate molecules to provide superhydrophobic surfaces that can also be used as a general template for further surface modification. Both the ZnO deposition and the alkylphosphonate coating use conditions that allow for their application within a PDMS microfluidic device. Microfluidic devices with smooth surfaces at the base of the channels show a big difference between the hydrophobic and the hydrophilic surfaces in terms of device back pressure. We have shown that rough surfaces can enhance water flow without being sensitive to the wetting properties of the surfaces thus enhancing the applicability of biomolecule immobilization within PDMS microfluidic devices.


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Acknowledgements C.N.S. gratefully acknowledges the support of the Edward and Judith Steinberg Chair in Nanotechnology. D.G. gratefully acknowledges the support of the European Research Council (ERC) 309600 and Israel Science Foundation (ISF) 715/11. We would like to thank Elitsour Rozen and Erel Lasnoy for their ongoing help and encouragement. References (1) Thorsen, T.; Maerkl, S. J.; Quake, S. R. Microfluidic large-scale integration. Science 2002, 298, 580-584. (2) Fordyce, P. M.; Gerber, D.; Tran, D.; Zheng, J.; Li, H.; DeRisi, J. L.; Quake, S. R. De novo identification and biophysical characterization of transcription-factor binding sites with microfluidic affinity analysis. Nat Biotech 2010, 28, 970-975. (3) Gerber, D.; Maerkl, S. J.; Quake, S. R. An in vitro microfluidic approach to generating protein-interaction networks. Nat Meth 2009, 6, 71-74. (4) Bates, S. R.; Quake, S. R. Highly parallel measurements of interaction kinetic constants with a microfabricated optomechanical device. Applied Physics Letters 2009, 95, 073705. (5) Braslavsky, I.; Hebert, B.; Kartalov, E.; Quake, S. R. Sequence information can be obtained from single DNA molecules. Proceedings of the National Academy of Sciences of the United States of America 2003, 100, 3960-3964. (6) Zhong, J. F.; Chen, Y.; Marcus, J. S.; Scherer, A.; Quake, S. R.; Taylor, C. R.; Weiner, L. P. A microfluidic processor for gene expression profiling of single human embryonic stem cells. Lab on a Chip 2008, 8, 68-74. (7) Lee, C.-C.; Sui, G.; Elizarov, A.; Shu, C. J.; Shin, Y.-S.; Dooley, A. N.; Huang, J.; Daridon, A.; Wyatt, P.; Stout, D.; Kolb, H. C.; Witte, O. N.; Satyamurthy, N.; Heath, J. R.; Phelps, M. E.; Quake, S. R.; Tseng, H.-R. . Multistep synthesis of a radiolabeled imaging probe using integrated microfluidics. Science 2005, 310, 1793-1796. (8) Ben-Ari, Y. a.; Glick, Y.; Kipper, S.; Schwartz, N.; Avrahami, D.; BarbiroMichaely, E.; Gerber, D. Microfluidic large scale integration of viral-host interaction analysis. Lab on a Chip 2013, 13, 2202-2209. (9) Drayman, N.; Glick, Y.; Ben-nun-Shaul, O.; Zer, H.; Zlotnick, A.; Gerber, D.; Schueler-Furman, O.; Oppenheim, A. Pathogens use structural mimicry of native host ligands as a mechanism for host receptor engagement. Cell Host & Microbe 2013, 14, 63-73. (10) Chiu, D. T.; deMello, A. J.; Di Carlo, D.; Doyle, P. S.; Hansen, C.; Maceiczyk, R. M.; Wootton, R. C. R. Small but perfectly formed? Successes, challenges, and opportunities for microfluidics in the chemical and biological sciences. Chem 2017, 2, 201223. (11) Duffy, D. C.; McDonald, J. C.; Schueller, O. J. A.; Whitesides, G. M. Rapid prototyping of microfluidic systems in poly(dimethylsiloxane). Analytical Chemistry 1998, 70, 4974-4984. (12) Anderson, J. R.; Chiu, D. T.; Jackman, R. J.; Cherniavskaya, O.; McDonald, J. C.; Wu, H.; Whitesides, S. H.; Whitesides, G. M. Fabrication of topologically complex threedimensional microfluidic systems in pdms by rapid prototyping. Analytical Chemistry 2000, 72, 3158-3164. 21 ACS Paragon Plus Environment

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4 microchannel systems 108x60mm (300 x 300 DPI)

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Microfluidic Devices Containing ZnO Nanorods with Tunable Surface

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