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

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.