JACS: Micrometer‑scale precision—Aerosol jet printing enables controlled patterning and integration of COF thin films.
Introduction
In the field of materials science, covalent organic frameworks (COFs) are regarded as promising candidates for next-generation high-performance materials, owing to their high specific surface area, tunable pore structures, and exceptional functional properties. However, conventional synthesis methods for COFs often encounter significant challenges—namely, poor processability, difficulty in shaping, and limited patterning capabilities—since these materials are typically insoluble in common solvents, making it difficult to fabricate them into thin films or complex architectures using standard techniques.
Recently, an important study published in the Journal of the American Chemical Society has introduced an innovative strategy: first synthesizing a soluble polyimine precursor, which is then transformed into a highly crystalline COF material via dynamic covalent bond exchange. In this process, aerosol jet printing (AJP)—a high-precision, patternable microfabrication technique—has played a pivotal role, opening new avenues for the “customized fabrication” of COFs.
Main text
Why is COF difficult to process?
COFs are crystalline porous materials composed of organic building blocks linked by strong covalent bonds, featuring well-defined pore channels and exceptional stability. However, their highly ordered crystalline structure renders them poorly soluble in most solvents and incapable of melt processing, thereby making it difficult to fabricate thin films or complex shapes using conventional techniques such as spin coating, spray coating, or printing.
Previous studies have attempted to fabricate COFs using colloidal inks, microfluidic devices, and even custom inkjet printers; however, these approaches are typically characterized by complex processes, high costs, and limited capability for high‑resolution patterning.
Aerosol jet printing technology
This technology offers the following advantages:
• High resolution: capable of producing features on the order of tens of micrometers;
• Wide applicability: Supports printing with a variety of functional inks;
• Operation at ambient temperature and pressure: suitable for heat-sensitive materials;
• Multi-layer printing: suitable for manufacturing complex structures.
When aerosol jet printing meets COF precursors
The key breakthrough of this study lies in the fact that, rather than directly printing the inherently difficult-to-process COF itself, we instead print its soluble polyimine precursor.
Researchers synthesized two soluble linear polyimides: 4MMCA-PDA and 4MMCA-F4PDA. These polymers exhibit excellent solubility in common organic solvents such as THF and chloroform, enabling them to be formulated into print‑compatible inks.
The printing process is as follows:
• Preparation of the ink: Dissolve the polyimide precursor and a crosslinking amine monomer (such as TAPB) in a chloroform/terpineol mixed solvent;
• Aerosol-jet printing: Using a 300 μm nozzle, the desired pattern (e.g., a 1 cm × 1 cm square) is printed onto a heated substrate at 60°C.
• Gas-phase annealing conversion: The printed precursor film is placed in a vapor mixture of 1,4-dioxane, mesitylene, and acetic acid and heated at 70 °C. Through a dynamic imine‑bond exchange reaction, it is in situ converted into a highly crystalline COF film.
Powder X-ray diffraction analysis confirmed that the printed and post‑conversion material exhibits a crystalline structure identical to that of the bulk COF, with high crystallinity and a well‑preserved porous framework.
Technological Advantages
• Avoiding Rapid Curing: In conventional COF synthesis, amines and aldehydes react rapidly to form insoluble polymers, leading to nozzle clogging. In contrast, the polyimine precursor remains stable during printing and undergoes chemical conversion only after printing is complete, thereby effectively circumventing curing issues.
• Realization of Complex Shapes and Patterned Structures: This technology not only enables the printing of simple geometries but also, through multi‑layer printing, composite printing, and other approaches, facilitates the fabrication of COF components with intricate three‑dimensional architectures or functional gradients, thereby opening up possibilities for the fabrication of devices such as sensors and microelectrodes.
• Applicable to composite material fabrication: The study also demonstrates that carbon nanotubes can be incorporated into the precursor ink to print COF/CNT composites. While preserving the porous structure of the COF, these materials exhibit a substantial enhancement in electrical conductivity, opening new avenues for the development of electronic devices and energy‑storage materials.
Why is COF difficult to process?
The integration of aerosol jet printing with convertible precursors offers unprecedented flexibility for the micro‑ and nanofabrication, patterned integration, and functional multiplexing of COF materials. In the future, this technology is expected to play an even more significant role in the following areas:
• Flexible electronics: Printing high-surface-area COF electrodes for supercapacitors or sensors;
• Separation membranes: Preparation of COF composite membranes with oriented channels for gas separation or water treatment;
• Catalytic devices: COFs patterned with catalytically active sites enable the fabrication of microreactors;
• Optoelectronic materials: Composed with luminescent or conductive materials, they are used in display or photovoltaic devices.
Image and Text Guide
Figure 1. Synthesis of COFs via dynamic bond exchange of polyimides.
Figure 2. (a) Synthesis and characterization of the newly reported polyimides, 4MMCA-PDA and 4MMCA-F4PDA, in this study. (b) GPC analysis and summary of the thermal properties of 4MMCA-PDA and 4MMCA-F4PDA.
Figure 3. Mechanical property characterization of 4MMCA-PDA and 4 mmca-f4pda.
Figure 4. (a) Vertex‑amine structures used in this study: TAPB, TAPT, and ETTA. (b) Synthesis and structure of the TAPB‑PDA COF, TAPTPDA COF, and ETTA‑PDA COF prepared from 4MMCA‑PDA. (c) The TAPB‑F4PDA COF synthesized from 4MMCA‑F4PDA, along with its structure. (d) Powder X‑ray diffraction.
Figure 5. (a) TAPB–PDA thin film fabricated by casting from TAPB and 4MMCA-PDA. (b) SEM images of the 4MMCA-PDA/TAPB film (top) and the TAPB–PDA COF film (bottom) on a leaf‑shaped mold. (c) GIWAXS patterns of the TAPB–PDA COF film at different reaction times. (d) 1D projections of the GIWAXS scattering patterns.
Figure 6. Process for fabricating COFs in desired shapes. (a) B2‑ and aircraft‑shaped COFs, prepared by blade casting and laser cutting. (b) SEM images of a two‑layer precursor film, with a 4MMCA‑PDA bottom layer and a 4MMCA‑PDA/TAPB top layer, and a TAPB‑PDA COF film at the bottom. (c) Square‑shaped COFs fabricated via aerosol jet printing of the precursor mixture. (d) PXRD patterns of TAPB‑PDA COFs with various shapes, obtained through thin‑film processing and aerosol jet printing.
Figure 7. (a) PXRD patterns of the ttap‑pda COF composite membranes, COF/CNT‑0.9 and COF/CNT‑4.5. (b) SEM cross-sectional images of the ttap‑pda COF/CNT composites, COF/CNT‑0.9 (top) and COF/CNT‑4.5 (bottom). (c) Resistivity of the 4MMCA‑PDA/TAPB/CNT film and the TAPB‑pda COF/CNT composite film at initial carbon nanotube loadings of 0.9 wt % and 4.5 wt %.
Conclusion
From “difficult to process” to “printable manufacturing,” research on COF materials is advancing from synthetic chemistry toward the integration of functional devices. Aerosol jet printing, with its high precision, broad material compatibility, and patterning capabilities, is emerging as a crucial bridge linking COF materials science with micro- and nanofabrication technologies.
In the future, as printing processes continue to be refined and material systems are further expanded, we can expect to see more high‑performance, customized, and multifunctional COF devices transition from the laboratory to real‑world applications, thereby truly realizing the vision of “materials as devices.”
Original article link: https://doi.org/10.1021/jacs.5c03079
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