Name: Microfluidic-based Synthesis of Covalent Organic Frameworks (COFs): A Tool for Continuous Production of COF Fibers and Direct Printing on a Surface (Video) | JoVE Journal
Uploaded: 2017-07-10T11:30:00.000+00:00
Duration: 8 min 42 s
Description: Learn microfluidic-based synthesis of covalent organic frameworks for continuous fiber production and direct surface printing. This method enables creation of 2D and 3D COF structures with enhanced processability.

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5 minutes

The authors acknowledge the Swiss National Science Foundation (SNF) for financial support through project no. 200021_160174.

1. Master Mold Fabrication
<ol>
	<li>Perform the photolithographic fabrication of a 4 inch silicon master mold as described in detail previously19; the master mold used in this study has been fabricated using the same protocol. 
	NOTE: Microfluidic devices are typically fabricated through a multi-step process. The first step is the design of the microfluidic channel using a conventional drawing software. Then, high-resolution film photomasks containing the microfluidic network are produced with a feature precision of approximately 5 &#181;m. Next, master molds are fabricated on a 4-in silicon wafer through standard photolithography techniques. SU-8, a negative photoresist, is employed for the fabrication of the master molds in the current investigations. The height of SU-8 structures is defined to be 50 &#181;m in our devices. Finally, microfluidic devices are fabricated by directly casting a transparent polymer, normally polydimethylsiloxane (PDMS), against the master mold.</li>
</ol>
2. Fabrication of Single-layer Microfluidic Devices
NOTE: The protocol requires an oven operating at 70 &#176;C. The temperature of the oven should be stabilized at 70 &#176;C before initiating the fabrication protocol. Lower temperatures can lead to poorly bonded and non-functional devices.
<ol>
	<li>Place the fabricated master mold in a desiccator, equipped with a vacuum pump. Then, pour 100 &#181;L of chlorotrimethylsilane in a glass vial and place this inside the desiccator. 
	NOTE: CAUTION! Chlorotrimethylsilane is a corrosive, hazardous and toxic substance. Accordingly, all handling steps should be performed under a well-ventilated fume hood, and appropriate protective goggles, gloves and lab coat must be worn.</li>
	<li>Close the desiccator and put under vacuum (in this experiment, 51 mbar). Wait for at least 1 h to ensure the deposition of vaporized chlorotrimethylsilane on the surface of master mold. After 1 h, gently open the air valve of the desiccator to equilibrate it to atmospheric pressure and open it. 
	NOTE: CAUTION! As soon as the desiccator is opened, chlorotrimethylsilane vapor leaks out; do not breath directly above the desiccator and always perform the above in a ventilated fume hood.</li>
	<li>Carefully take out the silanized master mold by hand and close the desiccator. Store the master mold in a closed box (or inside a laminar flow hood) to avoid deposition of particulates on its surface. 
	NOTE: All proceeding steps must be performed under a laminar flow hood operating with a uniform air velocity.</li>
	<li>Prepare a mixture of PDMS pre-polymer and curing agent (10:0.9 in weight) in a disposable cup and mix vigorously with a plastic spatula. As a guide, use 20 g of elastomer and 1.8 g curing agent to fabricate four PDMS microfluidic devices approximately 5 mm thick.</li>
	<li>Place the cup containing the well-mixed PDMS in a desiccator under vacuum to degas and remove air bubbles. Once the PDMS is degassed, open the desiccator and remove the cup. 
	NOTE: In this experiment, it takes approximately 30 min at 51 mbar.</li>
	<li>Gently place four square frames (e.g., polytetrafluoroethylene (PTFE) frames with inner dimensions of 24 mm x 24 mm) on the master mold such that each forms a wall around a single patterned structure on the master mold.</li>
	<li>Pour the degassed PDMS into the frames and on top of the master mold until full. Place the master mold with the filled square frames in an oven at 70 &#176;C for 2 h.</li>
	<li>After 2 h, remove the master mold from the oven and leave the assembly to cool to room temperature.</li>
	<li>Manually peel off the structured PDMS slabs (or PDMS chips) and the square frames by carefully separating them from the master mold and slide the PDMS chips out of the square frames.</li>
	<li>Punch inlet and outlet holes using a 1.5 mm biopsy puncher at desired positions in the design, e.g., at the end of the microfluidic channels. Cut the extra pieces of PDMS and remove any debris from the surface of the structured PDMS chips using adhesive tape.</li>
	<li>Place the PDMS chips (with the open channels facing up) as well as the glass coverslips, into the chamber of a plasma generator and close the chamber.</li>
	<li>Put the plasma generator under vacuum (1.4 mbar here); switch the plasma generator on for 1 min.</li>
	<li>After 1 min, switch the plasma generator off, ventilate the chamber and take out the treated PDMS chips and glass coverslips. Bond the PDMS chips (from the side with structured channels) and glass coverslips together to close the channels; at this point the single-layer microfluidic devices are fabricated.</li>
	<li>Finally, place the bonded microfluidic devices in an oven at 70 &#176;C for at least 4 h to enhance the bonding between the PDMS and glass, substantially.</li>
</ol>
3. Preparation of the Microfluidic Set-up and Precursor Solutions
<ol>
	<li>Prepare a 0.040 M solution of BTCA in acetic acid. 
	NOTE: CAUTION! Acetic acid is a hazardous, corrosive and flammable compound and its vapor is extremely irritating to the eyes and respiratory system. Accordingly, handling steps must be performed in a fume hood. Also, the user must wear a protective lab coat, goggles and gloves.</li>
	<li>Prepare a 0.040 M solution of TAPB in acetic acid. 
	NOTE: The microfluidic device used in the current experiments has four inlet channels (Figure 1B and Figure 2).</li>
	<li>Load BTCA and TAPB solutions into two different syringes (5 mL syringes loaded with 3 mL solution here), place and secure the syringes onto a syringe pump and connect them to the two middle inlets of the fabricated microfluidic chip (one reagent per inlet) using PTFE tubing (0.8 mm inner diameter).</li>
	<li>Load two other syringes with pure acetic acid (here 5 mL syringes fully loaded), place and secure the syringes onto the syringe pump and connect them to the two side inlets of the microfluidic chip using the same type of PTFE tubing.</li>
	<li>Connect a sufficiently long PTFE tubing (in the current experiment, ~15 cm) to the outlet of the microfluidic chip. Use a computer-controlled syringe pump to motivate fluid flows as described in the following steps.</li>
</ol>
4. Continuous Synthesis of MF-COF Fibers
<ol>
	<li>Using the syringe pump introduce two sheath flows of acetic acid each at a flow rate of 100 &#181;L/min; the sheath flows are located on the outer side of the reagent flows (Figure 2).</li>
	<li>Wait for 1 min and inject the two reagents (TAPB and BTCA) via the two middle inlets (one reagent per inlet) each at a flow rate of 50 &#181;L/min. Wait for 1 min until stable flows are established.</li>
	<li>Observe the formation of yellow fibrous microstructures previously characterized as MF-COF by Fourier transform infrared (FT-IR) spectroscopy, elemental analysis and solid state 13C CP-MAS-NMR6; under these conditions the formation of MF-COF is not continuous.</li>
	<li>Increase the flow rate of TAPB and BTCA to 200 &#181;L/min and maintain the two sheath flows of acetic acid at 100 &#181;L/min. Now wait for 1 min until the flow stabilizes. Observe the formation of a highly-concentrated suspension of yellow MF-COF fibers, which ultimately leads to blockage of the outlet.</li>
	<li>As the chip and outlet tubing are now non-functional, use a new chip and prepare it for experiment according to steps 3.3-3.6.</li>
	<li>Introduce two sheath flows of acetic acid each at a flow rate of 100 &#181;L/min and wait for 1 min. Set the flows of TAPB and BTCA each to 100 &#181;L/min and observe the formation of a continuous yellow MF-COF fiber.</li>
	<li>Place the outlet of the tubing in a Petri dish containing acetic acid. For example, place 10 mL of acetic acid in a round glass Petri dish (60 mm in diameter). Once the synthesized fiber exits the tube located at the outlet of the microfluidic device, move the tube over a surface to facilitate the exit of the continuous MF-COF fiber.</li>
</ol>
5. Direct Printing of 2D and 3D MF-COF Structures
NOTE: As the synthetized fiber may not be completely homogeneous, the deposition speed must be adjusted to ensure continuous printing.
<ol>
	<li>Prepare the microfluidic set-up as described in section 3 and inject all four solutions each at a flow rate of 100 &#181;L/min.</li>
	<li>Wait for 1 min until the flows are stabilized and the synthesized MF-COF fiber exits the tube located at the outlet of the microfluidic device. Prepare a clean substrate next to the exit of the tube located at the outlet of the microfluidic device for direct printing of MF-COF fibers. 
	NOTE: In our investigations, 24 mm x 76 mm glass coverslips were employed for all printing experiments.</li>
	<li>Hold the tube connected to the outlet of the microfluidic device so that its end is a few millimeters above the glass coverslip. Slowly move the tube over the glass coverslip to facilitate the exit of the MF-COF fiber and avoid aggregation.</li>
	<li>Once the flows are stabilized, slowly lift the tube located at the outlet of the microfluidic device approximately 2-3 cm away from the glass coverslip to observe a freestanding and stable MF-COF fiber.</li>
	<li>To continue printing, bring the outlet of the tube back towards the glass coverslip and manually move the tube on the surface to draw the desired 2D or 3D MF-COF structure.</li>
</ol>

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The authors have nothing to disclose.

The microfluidic-based synthetic method reported here provides a novel and simple approach for direct printing of COF materials on surfaces. Synthesis is performed using a single-layer microfluidic device, comprised of a microfluidic PDMS chip bonded to a glass coverslip. The fabrication of the microfluidic device can be achieved through conventional casting of PDMS against a silicon master mold and subsequently bonding the PDMS with the imprinted microchannels against a glass coverslip.
For t...

Covalent organic frameworks (COFs) are a well-established class of porous and crystalline material in which the organic building blocks are firmly held together by covalent bonds1,2,3,4,5. COFs are typically assembled following supramolecular chemistry principles, where the constituent molecular building blocks are selectively reacted to define a final and predetermined porous assembly. Such an approach allows the synthesis of materials with controlled and ordered structure (e.g.,with defined pore dimensions) and composition3,6,7,8. Compared to other porous materials, COFs are unique since they are comprised of light elements (C, H, B, N and O) and have tunable porosities1,5. Inspired by these unique and intrinsic characteristics, COFs have been assessed for potential application in chemical separations9, gas storage10and catalysis11, sensors12, optoelectronics13, clean energy technologies14 and electrochemical energy devices15.
To date, the vast majority of methods used for the preparation of COF materials are based on solvothermal self-condensation and co-condensation reactions, where high temperatures and pressures are the standard. Although COFs are thermally robust, they commonly suffer from limited processability, i.e., COFs are usually insoluble and unprocessable crystalline powders, and this significantly limits their use in a range of potential and practical applications2,6,8,16,17. Despite the remarkable progress made in COF synthesis, a major challenge in the field is to develop a method enabling the preparation of COFs in appropriate reaction conditions (e.g., temperature and pressure), which can then facilitate their processability on surfaces.
Recently, studies have shown that Shiff-base chemistry can be used to synthesize an imine-based COF at room temperature. The COF produced, named RT-COF-1, forms due to the fast and efficient reaction between 1,3,5-tris(4-aminophenyl)benzene (TAPB) and 1,3,5-benzenetricarbaldehyde (BTCA)17 (Figure 1A). The efficacy of this synthetic method was demonstrated by the direct printing of micron and submicron patterns of RT-COF-1 on both rigid and flexible surfaces using lithography or inkjet printing techniques. More recently, and making use of microfluidics, we have demonstrated an effective approach for the continuous synthesis of fibers of the same imine-based COF hereafter called MF-COF6. Unlike other reported synthetic approaches for the generation of COFs18, this microfluidic-based synthetic method enabled the rapid synthesis of MF-COF fibers at ambient temperatures and pressures within a few seconds. Furthermore, and owing to the mechanical stability of the synthesized MF-COF fibers, we have demonstrated how such microfluidic-based method can enable the direct printing of 2D and 3D structures on surfaces. Herein, we demonstrate that this method can be used to draw COF structures on various surfaces having different chemical and physical properties. We believe that this novel method opens new avenues for the well-controlled patterning and direct printing of COFs in different orientations and on various surfaces.

<table><tbody><tr><td>High resolution film masks</td><td>Microlitho, UK</td><td>-</td><td>Features down to 5um</td></tr><tr><td>Silicon wafers</td><td>Silicon Materials Inc., Germany</td><td>4&quot; Silicon Wafers</td><td>Front surface: polished, back surface: etched</td></tr><tr><td>Silicone Elastomer KIT (PDMS)</td><td>Dow Corning, USA</td><td>Sylgard 184</td><td>-</td></tr><tr><td>Chlorotrimethylsilane</td><td>Sigma-Aldrich, Switzerland</td><td>386529</td><td>&amp;ge;97%, CAUTION: Handle it only under fume hood.</td></tr><tr><td>Biopsy puncher</td><td>Miltex GmBH, Germany</td><td>33-31A-P/25</td><td>1.5 mm</td></tr><tr><td>Glass coverslip</td><td>Menzel-Glaser, Germany</td><td>BB024040SC</td><td>24 mm &amp;times; 40 mm, #5</td></tr><tr><td>Plasma generator instrument</td><td>Diener</td><td>Zepto B</td><td>Frequency: 40 kHz and plasma generator power: 0-30 W</td></tr><tr><td>PTFE tubing</td><td>PKM SA, Switzerland</td><td>AWG-TFS-XXX</td><td>AWG 20TFS, roll of 100 m</td></tr><tr><td>neMESYS Syringe Pumps</td><td>Cetoni GmbH, Germany</td><td>Low Pressure (290N)</td><td>-</td></tr><tr><td>Disposable Cup</td><td>Semadeni, Switzerland</td><td>8323</td><td>PS, 200 ml</td></tr><tr><td>Plastic Spatula</td><td>Semadeni, Switzerland</td><td>3340</td><td>L &amp;times; W : 135 mm x 14 mm</td></tr><tr><td>Disposable Scalpels</td><td>B. Braun, Switzerland</td><td>233-5320</td><td>Nr. 20</td></tr><tr><td>Disposable Syringes</td><td>VWR, Switzerland</td><td>613-3951</td><td>5 ml, Discardit II</td></tr><tr><td>Acetic Acid</td><td>Sigma-Aldrich, Switzerland</td><td>695092-500</td><td>&gt;=99.7%, CAUTION: Handle it only under fume hood.</td></tr><tr><td>1,3,5-benzenetricarbaldehyde</td><td>Aldrich-Fine Chemicals</td><td>753491</td><td>97%</td></tr><tr><td>1,3,5-Tris(4-aminophenyl)benzene</td><td>Tokyo Chemical Industry</td><td>T2728-5G</td><td>&gt;93.0%</td></tr></tbody></table>

microfluidic-based synthesis of covalent organic frameworks (cofs): a tool for continuous production of cof fibers and direct printing on a surface

Microfluidic-based Synthesis of Covalent Organic Frameworks (COFs): A Tool for Continuous Production of COF Fibers and Direct Printing on a Surface

Covalent Organic Frameworks (COFs) are a class of porous covalent materials which are frequently synthesized as unprocessable crystalline powders. The first COF was reported in 2005 with much effort centered on the establishment of new synthetic routes for its preparation. To date, most available synthetic methods for COF synthesis are based on bulk mixing under solvothermal conditions. Therefore, there is increasing interest in developing systematic protocols for COF synthesis that provide for fine control over reaction conditions and improve COF processability on surfaces, which is essential for their use in practical applications. Herein, we present a novel microfluidic-based method for COF synthesis where the reaction between two constituent building blocks, 1,3,5-benzenetricarbaldehyde (BTCA) and 1,3,5-tris(4-aminophenyl)benzene (TAPB), takes place under controlled diffusion conditions and at room temperature. Using such an approach yields sponge-like, crystalline fibers of a COF material, hereafter called MF-COF. The mechanical properties of MF-COF and the dynamic nature of the approach allow the continuous production of MF-COF fibers and their direct printing onto surfaces. The general method opens new potential applications requiring advanced printing of 2D or 3D COF structures on flexible or rigid surfaces.

The microfluidic device used in our investigations is fabricated using conventional PDMS replica molding20 and incorporates four microfluidic inlet channels that merge into a main microchannel. The final microfluidic device consists of a structured PDMS layer and a glass coverslip used to close the imprinted microchannels, as shown in Figure 1B.
<img alt="Chemical structures ...

We present a novel microfluidic-based method for synthesis of covalent organic frameworks (COFs). We demonstrate how this approach can be used to produce continuous COF fibers, and also 2D or 3D COF structures on surfaces.

Covalent Organic Frameworks (COFs) are a class of porous covalent materials which are frequently synthesized as unprocessable crystalline powders. The ...

The overall goal of this approach is to demonstrate how a microfluidic-based platform can be used to produce continuous fibers of covalent organic frameworks that can be printed on surfaces to create 2D and 3D structures. Despite the remarkable progress in the synthesis of covalent and organic frameworks, developing a method to facilitate the process ability of these materials remains a challenge, which makes this approach very promising. This method effectively contributes to the field of crystalline materials through introduction of a noble, synthetic approach to produce continuous fibers of covalent and organic frameworks, thereby enhancing their process ability.To begin the device fabrication, in a fume hood, place a silicon master mold fabricated from a four inch silicon wafer in a vacuum desiccator. Dispatch 100 microliters of trimethylsilyl chloride into a glass vial and place the vial in the vacuum desiccator. Close the desiccator and put it under vacuum.Allow the trimethylsilyl chloride vapor to deposit on the master mold surface for one hour. Then, slowly vent the desiccator. Remove the silanised master mold and close the desiccator.Store the silanised mold in a closed container or in a laminal flow hood. Next, in a disposable cup, use a plastic spatula to mix together PDMS elastomer and curing agent in a 10:0.9 weight ratio. De-gass the mixture in a vacuum desiccator for 30 minutes.Then, carefully place four PTFE frames on the master mold, so that each frame encloses a single pattern on the mold. Pour the PTMS mixture into the frames and over the mold until full. Cure the PDMS in an oven at 70 degrees celsius for two hours, then remove the mold from the oven and allow the PDMS chips to cool to room temperature in the laminar flow hood.Once cool, carefully separate the frames from the mold by hand. Gently slide the PDMS chips from the frame. Use a 1.5 millimeter biopsy puncture to punch inlet and outlet holes at the end to the micro-fluidic channels in the chips.Trim excess PDMS from the chips and clean the chip and cover slip surfaces with adhesive tape. Place the clean chips in a plasma generator with the open channels facing up, along with four glass cover slips. Evacuate the sample chamber and run the generator for one minute.Then, vent the chamber and remove the plasma treated chips and cover slips. Bond each chip to a treated cover slip with the open channels faced down by gently pressing the chip onto the cover slip. Place the resulting micro-fluidic chips in an oven at 70 degree celsius for at least four hours to strengthen the bonding between the PDMS and the glass.To begin preparing for the synthesis, draw three milliliters of 40 milli molar BTCA in acetic acid into a five milliliter syringe. Draw three milliliters of 40 milli molar TAPB in acetic acid into another five milliliter syringe. Use PTFE tubing with an inner diameter of 0.8 millimeters and connect the tubings from one side to the syringes.Mount both syringes on a syringe pump. Next, fill two five milliliters syringes with acetic acid, use PTFE tubing with an inner diameter of 0.8 millimeters and connect the tubings from one side to the syringes. Secure these syringes into syringe pumps.Then, obtain a single layer four channel micro-fluidic chip. Connect the other side of the PTFE tubing to the inlets of the micro-fluidic device and connect the syringes to the outer channel inlets. Connect the 15 centimeter length of PTFE tubing to the chip outlet.Fix the end of the tubing in a 60 millimeter petri dish containing 10 milliliters of acetic acid. Place a glass cover slip next to the petri dish. Set each syringe pump to a flow rate of 100 microliters per minute and start the pumps.Wait one minute for the flows to stabilize while monitoring the MF-COF fiber formation in the chip. Change the flow rates of the acetic acid and check the outlet tubing. If the sheath flows of acetic acid are too high relative to the BTCA and TAPB flows, the MF-COF fiber's microstructure suspension will be too dilute to form a continuous fiber.If the sheath flows of acetic acid are too low relative to the reagent flows, the high concentration of MF-COF fibers will clog the outlet tubing. Set each syringe to a flow rate of 100 microliters per minute and wait one minute for the flows to stabilize. Once the flows have stabilized and the continuous MF-COF fiber reaches the end of the outlet tube, hold the end of the tube, a few millimeters above the surface.Slowly move the tube to draw an MF-COF structure. The suitable mechanical properties drive from the microscopic organization of MF-COF, allow the use of MF-COF fibers for conformal creating of 2D and 3D structures on saraphasis. Slowly raise the tube two to three centimeters from the surface to produce a free standing stable MF-COF fiber.Slowly lower the tube to continue drawing a 2D or 3D structure directly on the cover slip. MF-COF fibers were synthesized from TAPB and BTCA, using a four channel micro-fluidic device. Scanning electron microscopy showed that the fibers consisted of inter-connected micro and nano fibers in 3D sponge-like, porous structures that grant the mechanical properties needed to print 2D and 3D structures.2D MF-COF structures were printed on glass surfaces by drawing shapes as the fiber was continuously generated. The relative precision of the printing process was further highlighted in writing experiments on glass. The fibers did not merge when patterned over each other, allowing printing of 3D structures.In addition to glass, MF-COF structures were printed on tissue paper, cardboard, aluminum foil and polystyrene. While attempting this procedure, remember to perform all the steps for the fabrication of the micro-fluidic device inside the laminar flow hood and all the synthesis steps that involve handling of acetic acid under a fume hood. After its development, this technique paves the way for the researchers in the field of crystalline materials to explore new technological applications requiring advance patterning of 2D and 3D structures made of crystalline fibers and various seraphasis.After watching this video, you should have a good understanding of how to synthesize the continuous fiber of covalent, organic frameworks using a micro-fluidic platform and how to print a produced fiber and seraphasis to make 2D and 3D structures.

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Learn microfluidic-based synthesis of covalent organic frameworks for continuous fiber production and direct surface printing. This method enables creation of 2D and 3D COF structures with enhanced processability.

Microfluidic-based Synthesis of Covalent Organic Frameworks (COFs): A Tool for Continuous Production of COF Fibers and Direct Printing on a Surface (Video) | JoVE Journal

Microfluidic-based Synthesis of Covalent Organic Frameworks (COFs): A Tool for Continuous Production of COF Fibers and Direct Printing on a Surface

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