One-Step Fabrication of Microchannels Lined with a Metal Oxide

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One-Step Fabrication of Microchannels Lined with a Metal Oxide Coating Sandip Patil, Amit Ranjan, Tanmoy Maitra, and Ashutosh Sharma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00413 • Publication Date (Web): 01 Apr 2016 Downloaded from http://pubs.acs.org on April 4, 2016

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ACS Applied Materials & Interfaces

One-Step Fabrication of Microchannels Lined with a Metal Oxide Coating Sandip Patil,† Amit Ranjan,‡ Tanmoy Maitra,† and Ashutosh Sharma* † [†] Department of Chemical Engineering and Center for Nanosciences Indian Institute of Technology Kanpur Kanpur-208016, U.P (India) [‡] Department of Chemical Engineering Rajiv Gandhi Institute of Petroleum Technology Raebareli Ratapur Chowk, Raebareli-229010, U.P (India)

ABSTRACT

We demonstrate a simple, single step method for metal/metal oxide coating on interior walls of microchannels in an elastomeric material like PDMS, which is the mainstay of microfluidic devices. The fabrication process involves electrodeposition of cuprous oxide on a metallic wire or a sheet, embedding it inside a PDMS matrix along with the cross-linker, curing and then swelling the PDMS elastomer, and finally pulling out the template metal wire or the metal sheet from the PDMS matrix. Stronger attachment of the metal oxide layer to PDMS allows the transfer of the metal oxide coating originally present on the template surface (wire or sheet) to

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the channel wall resulting in a microchannel/microslit lined with the metal/metal oxide layer. In view of the catalytic activity associated with transition metal oxides, this simple method offers a cost-effective and versatile technique to fabricate microfluidic and lab-on-a-chip devices which can be utilized as microcatalytic reactors or chemical filters. As a proof of concept, we have successfully tested the metal oxide coated microchannels and microslits as active sites for adsorption of iodide ions.

KEYWORDS Microchannel, Electrodeposition, PDMS, Copper Oxide Microchannel, Templating


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1. INTRODUCTION Microchannels are the main building blocks of microfluidic and lab-on-a-chip devices and have also attracted considerable scientific interest in areas ranging from electronics,1 medicine,2 automobiles,3 heat exchangers,4 micro-reactors,5 valves,6 microfluidic adhesive.7 The research efforts have been directed to devising ways of fabricating microchannels in different materials with suitable internal structures and desired functionality. The materials of the medium hosting the microchannels can range from metals, carbon and ceramics to polymers. The polymeric materials, especially elastomers like polydimethylsiloxane (PDMS), are the mainstay of microfluidic devices because of their mechanical flexibility, inertness and ease of processing and fabrication. The structural elements fabricated in microchannels include features such as line arrays, helices and knots embedded in a matrix.8–12 Three dimensional complex elements are particularly challenging to fabricate,6,8,9,13–15 but are useful in diverse applications such as mixing by chaotic flows, biosensors, separation, and waveguides.16–19 The techniques currently in use to fabricate mostly open microchannels employ tools such as photolithography,20 nano-imprint lithography,21,22 soft lithography,23–25 etching,26,27 and techniques based on inducing instability such as those by contact instability,28,29 electric field30 and by release of stresses.31,32 All these techniques are expensive and time-consuming as they involve multiple processing steps. Moreover, rendering 3-dimensional structural elements using the aforementioned techniques is often either not possible or very difficult. Further, the functionality of the microchannel walls is limited by the host material as deposition of another material in embedded channel walls is inefficient and difficult. Cross-linked PDMS is the most attractive and used material for the host material because it is chemically stable, mostly inert, offers ease of fabrication, mechanical flexibility and optical


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transparency. One of the attractive and simple methods proposed by Verma et al.9,12 for the fabrication of microchannels, including complex 3-D shapes, in PDMS is based on embedding a template (micro-wire or other complex shapes) in a PDMS block followed by its curing and the retraction or destruction of the sacrificial template.9,11 The method is useful in the creation of circular and other complex cross-section embedded channels, quite unlike the rectangular crosssections of the open channels produced by the conventional photolithography and micro-molding based methods. The method has also been used for creation of microchannels in materials other than PDMS, for example, in PAN polymer and by its pyrolysis, in carbon.33 We propose and show that the template method can be extended to fabricate microchannels with their walls decorated by a functional material such as for adsorption, catalysis, sensing, etc. Deposited metal electrodes on the walls of microchannels are used in regulation of electrokinetics.34,35 We illustrate the method by producing a metal/metal oxide coating on PDMS microchannel inner walls that provide the active sites for the adsorption of iodide ions. We demonstrate that the drawback of poor adhesion between PDMS and metal/metal oxides due to lower surface energy of PDMS material (~20 mN/m)36 and a large mismatch in the elastic strengths of metals and the polymer can be circumvented by our method. Various steps in the proposed method involve electrochemical deposition of copper and copper oxide film on a copper wire substrate, immersion of these wires in non-crosslinked PDMS mixed with the cross-linking agent, and curing of PDMS with the embedded coated template. After curing, the cross linked PDMS blocks with the embedded wires is slightly swollen using an appropriate solvent and the wire substrate is gently pulled off leaving back the electrodeposited film adhered to the cross-linked PDMS matrix. A schematic of the procedure is depicted in Figure 1a. It is to be noted here that this method offers versatility in terms of the micro-channel


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shapes, for example, using an electrodeposited metal sheet in place of wire, micro-slits coated with metal/metal oxide films can also be fabricated. Organization of this paper is as follows. In the next section, we present the methodology and describe the materials required. Section 3 presents characterization of the deposited films. In the subsequent section we show an application of the coated microchannels and microslits by demonstrating their ability to adsorb iodide ions.

2. MATERIALS AND METHODS We used copper wires (diameter is ~100±3.9 µm) and copper sheets (thickness is ~80±2 µm) as the electrodes for electro-deposition of a copper oxide coating. Electro-deposition of copper/copper oxide on these substrates was performed by immersing the substrates and a copper rod in a copper sulfate solution with 0.1 M solution concentration and choosing the substrates to be cathodes (Figure 1(a)). Passing the voltage led to deposition of copper/cuprite films on the substrates. The analytical grade compounds KI and CuSO4 (Loba Chemie) were used. The copper solution was prepared in DI-water (Milli-Q) with resistivity of 18.2 mΩ. cm-1. Electrodeposition was performed at a DC voltage of 0.4 V for around 5 minutes. The deposited cuprous oxide layer on copper substrate (in forms of a thin copper sheet and a thin copper wire) was dried and embedded into commercially available 10% cross-link PDMS (Sylgard 184, Dow corning). The elastomer and cross-linking agents were mixed in a ratio of 100:10 w/w (or 10% crosslinked PDMS) separately in a beaker and degassed under vacuum in order to remove dissolved air from the PDMS solution.28,37–40 First, the 10% cross-linked PDMS solution was cast between two clean glass plates with cuprous oxide coated metal wire and sheet embedded (Figure 1(a)). Complete block of cast PDMS with electrodeposited metal wire and metallic sheet embedded in


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it was kept for curing for 12 hrs at 80˚C. This step results into an embedded template inside the PDMS which attains a strong bonding with the metal coating of the template during curing. Subsequently the PDMS block containing the immersed substrate/film was swollen by chloroform to allow an easy removal of the template without its coating. We will refer to this procedure in short by TAFoCµC (Template Assisted Fabrication of Coated Microchannels). The cuprous oxide film lost its adhesion with the template and remained strongly adhered to the PDMS micro-channel wall. Adhesion quality of the deposited film on PDMS was also tested by applying scotch adhesive tape on cuprous oxide deposited surfaces and peeling it off multiple times from the surface. The adhesive tape peeled off in this manner was examined under microscope to check the transfer of the cuprous oxide layer. Comparing the image in Figure 1(b), which shows a fresh scotch tape, and Figure 1(c), which shows the same tape after peeling it repeatedly (ten times) off the PDMS coated surface, shows that the cuprous oxide has not been transferred to the scotch tape, thereby confirming good adhesion between the PDMS and the coated oxide layer. The difference in the images in Figures 1(b) and (c) is solely due to the altered adhesive viscous film partially destroyed by the peeling steps.


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Figure 1. (a) Schematic of electrodeposition procedure, PDMS preparation, casting, swelling and micro-channel fabrication. The thickness of the cuprous oxide micro-channel is 23.4± 6 µm. (b) Optical micrograph showing the image of a fresh scotch tape. (c) Optical micrograph of the same tape as in part (b) after peeling off the cuprous oxide coated PDMS surface ten times. Difference in the image is due to the altered film of the adhesive material on the tape resulting from multiple peel-off.

3. RESULTS AND DISCUSSION We now present the characterization of coated microchannels and iodine absorption application as a proof-of-principle for a functional micro-channel coating. In particular, we show that the coated channels produced by TAFoCµC technique can be used to adsorb iodide ions. The results obtained were satisfactorily reproducible when experiments were repeated under the same conditions. 3.1 Characterization We first present the characterization of the electrodeposited layers on the metal wire (or sheet) followed by the results on the fabricated micro-channel. We characterized the surface of the electrodeposited layer by SEM analysis on metal wire in order to check uniformity of the coated layer. Figure 2a shows the SEM micrograph of the crystals formed on the metallic substrate immediately after the electrodeposition. Several electrodeposition experiments were performed and each time a similar morphology on copper wire and foil were obtained. Voltage (0.4 V)


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across the electrodes was kept constant for all experiments. After SEM characterization, the copper templates (wire or sheet) carrying the electrodeposited layers were embedded in the PDMS in order to form microchannels (or microslits) in the PDMS. The cross sectional view of fabricated micro-channel is shown in the optical micrograph presented in Figure 2b, where the coating transferred on the inner wall of the channel is clearly seen. Cross sectional view of micro-channel shows two different color areas where the inner reddish area indicates the presence of coated layer from the metallic wire. The PDMS block adheres strongly to this layer as evident by the adhesive failure of the coating with the template. The cuprous oxide layer is non-uniform and its thickness is found to be 23.4±6 µm under the conditions stated above. The roughness of the inner wall was studied by cutting these channels open so as to expose the inner wall and use optical profilometry to characterize the exposed surface. The result shown in Figure 2(c) suggests RMS roughness to be 11.02 ± 1.2 µm. The roughness originates from non uniform deposition of cupric oxide on metal wire during electrodeposition step.


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Figure 2. (a) SEM image of cuprous oxide layer on copper wire. Inset shows electrodeposited copper wire. (b) Cross sectional view of micro-channel with two distinct color identification patterns in order to indentify polymer and metal layer. Dark red color indicates the cuprous oxide layer and outside of that is the PDMS support. (c) Optical profilometry image of a representative micro-channel. The scan was taken on horizontal cross section of the channel. The thickness profile shows a rough inner wall with RMS roughness of 11.02± 1.2 µm.

The wide angle X-ray diffractogram of the coated surface presented in Figure 3(a) reveals the inner coating to be the copper/cuprous oxide layer. Peaks were satisfactorily indexed by assuming that both metallic copper as well as cuprous oxide are present in the coating. To further support the XRD results, the elemental mapping was performed by using the EDX technique along with the SEM. The results shown in Figure 3(b) reveal nearly uniform presence of copper and oxygen in the coated layer.


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Figure 3. (a) The XRD diffractogram of the inner wall of the PDMS microchannels and microslits. (b) Elemental analysis and mapping by EDX. Top left panel is the SEM image of the area of interest. Other panels show elemental maps of other concerned elements. Red dots represent copper, green dots represent Si, and grey dots represent oxygen. Oxygen signals may originate from both copper oxide and PDMS.

Figure 4 shows the SEM micrograph of the film bonded to PDMS surface after curing. As this figure suggests, the crystallites are well assimilated by the PDMS matrix. Although the cured elastic PDMS shows very week adhesion strength with a metal,41,42 our method results into a strong adhesion between the PDMS and metals/metal oxides. Origin of the strong bonding lies in the processing as the micro-crystallites are likely to be incorporated in the matrix during the curing step. The stick-and-peel-off test using scotch tape already discussed in Section 2 confirms strong adhesion.


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Figure 4. SEM image of cuprous oxide layer deposited on PDMS surface. Inset shows high magnification SEM image of cuprous oxide layer deposited on PDMS surface.

3.2 Adsorption of iodide ions: an illustrative application Iodide ions are harmful environmental contaminant from nuclear fission reactions products43 and a flexible and cost effective device controlling their migration could find its use in environmental applications. Here we demonstrate that films prepared by the TAFoCµC technique can be used to immobilise the iodide ions. In an elaborate study Lefevre et al.43 suggested that Cu2O can immobilize the iodide ions by adsorbing them. In our study, we immerse our films in aqueous KI solution and study the concentration of the mobile iodide ions by UV-visible spectrometry at different times by taking samples from the solution. Jortner et al.44 have reported that UV-visible spectrum of iodide ions in aqueous solution is formed of two bands, with one having its peak at around 226 nm and the other slightly below 200 nm. Figure 5 shows the UV-visible spectrum of the solution in the concerned range of wavelengths and clearly shows the peaks corresponding to the iodide ions. The same figure also shows the absorption spectra of the solution after 17 hours and 41 hours. It is quite clear that the iodide ion concentration decreases which might be attributed to their adsorption on to the cuprous oxide films dipped inside the solution.


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Figure 5. UV-Visible spectra of the aqueous KI solution at different times with TAFoCµC deposited films immersed inside.

4. CONCLUSIONS AND SCOPE In summary, we have designed a novel and cost-effective method to fabricate microchannels and microslits in PDMS elastomers with copper and cuprous oxide films strongly adhered to their inner walls. Usefulness of the technique lies in its ability to fabricate microcrystallite coated microslits and microchannels in flexible elastomers in relatively easy and costeffective manner. With a proper control, a coating of metal and metal oxide in a nano-particulate form can also be produced which will improve the functionality of the microchannels and microslits by increasing the catalytic activity of the coating. This technique can be easily extended to fabricate multiple nano-particle coated slits and channels inside the PDMS matrix which could be utilized for applications such as micro-filtering, micro-reactors, solar cells, and reactive micro-fluidics. PDMS is a widely established material in biological applications and a coating of biocompatible metals such as gold, silver, iron, titanium would be highly desirable.45


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This method is sufficiently general and can be extended to other metals and metal oxides to impart a desired functionality to these surfaces for intended biological applications.

AUTHOR INFORMATION Corresponding Author [*] Ashutosh Sharma Professor Department of Chemical Engineering Indian institute of Technology Kanpur Kanpur-208016 U.P (India) *E-mail: [email protected] Telephone: +91-512-2597026 Fax:+91-512-2590104 ACKNOWLEDGMENT AS acknowledges the support of the DST through its grants to the Thematic Unit of Excellence on Soft Nanofabrication with Application in Energy and Bioplatform at IIT Kanpur and an IRHPA grant.


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(38) Patil, S.; Ranjan, A.; Sharma, A. Prefracture Instabilities Govern Generation of SelfAffine Surfaces in Tearing of Soft Viscoelastic Elastomeric Sheets. Macromolecules 2012, 45 (4), 2066–2073. (39) Patil, S.; Mangal, R.; Malasi, A.; Sharma, A. Biomimetic Wet Adhesion of Viscoelastic Liquid Films Anchored on Micropatterned Elastic Substrates. Langmuir 2012, 28 (41), 14784– 14791. (40) Pangule, R. C.; Banerjee, I.; Sharma, A. Adhesion Induced Mesoscale Instability Patterns in Thin PDMS-Metal Bilayers. J. Chem. Phys. 2008, 128 (23), 234708–6. (41) Schmid, H.; Wolf, H.; Allenspach, R.; Riel, H.; Karg, S.; Michel, B.; Delamarche, E. Preparation of Metallic Films on Elastomeric Stamps and Their Application for Contact Processing and Contact Printing. Adv. Funct. Mater. 2003, 13 (2), 145–153. (42) Loo, Y. L.; Willett, R. L.; Baldwin, K. W.; Rogers, J. A. Additive, Nanoscale Patterning of Metal Films with a Stamp and a Surface Chemistry Mediated Transfer Process: Applications in Plastic Electronics. Appl. Phys. Lett. 2002, 81 (3), 562–564. (43) Lefèvre, G.; Walcarius, A.; Ehrhardt, J. J.; Bessière, J. Sorption of Iodide on Cuprite (Cu2O). Langmuir 2000, 16 (10), 4519–4527. (44) Jortner, J.; Raz, B.; Stein, G. The Far U.-V. Absorption Spectrum of the Iodide Ion in Aqueous Solution. Trans. Faraday Soc. 1960, 56, 1273–1275. (45) Sperling, R. A.; Gil, P. R.; Zhang, F.; Zanella, M.; Parak, W. J. Biological Applications of Gold Nanoparticles. Chem. Soc. Rev. 2008, 37, 1896-1908.


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TABLE OF CONTENTS

One-Step Fabrication of Microchannels Lined with a Metal Oxide Coating Sandip Patil,†Amit Ranjan,‡ Tanmoy Maitra,† and Ashutosh Sharma* †


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One-Step Fabrication of Microchannels Lined with a Metal Oxide

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