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

Summary

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.

Abstract

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.

Introduction

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 bonds^1,2,3,4,5. 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 ....

Protocol

1. Master Mold Fabrication

1. 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 µm. Next, master molds are fabricated on a 4-in silicon wafer through standard photolithography techn....

Results

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.

Discussion

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.

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Materials

List of materials used in this article

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