I. Abstract of the Paper’s Core Research

This study uses borate-based COF‑5 and COF‑10 nanoparticles as precursors to fabricate a dual‑solvent colloidal ink comprising acetonitrile and benzonitrile. Employing aerosol jet printing (AJP), we achieve maskless, micrometer‑scale direct writing of COF thin films. By integrating carbon nanotubes, we develop fully printed flexible temperature sensors and conduct comprehensive characterization of ink stability, film crystallinity, thermally responsive electrical properties, and mechanical durability. The key findings are as follows:

Low-Volatility Dual-Solvent COF Colloid-Specific Ink

Benzonitrile regulates the evaporation rate of ink droplets, suppressing coffee-ring formation and particle agglomeration, while enabling the ink to remain stable for several weeks. Throughout aerosolization and deposition, the COF’s intrinsic high crystallinity is preserved, eliminating the need for high-temperature polymerization or any additional post‑processing steps after printing.

Micrometer‑level high‑precision maskless direct‑write patterning

Optimizing the carrier‑gas/auxiliary‑gas ratio ensures stable printing of uniform 36‑μm lines, enabling the fabrication of custom patterns on a variety of substrates, including PI flexible films and silicon wafers. The resulting large‑area thin films exhibit no pinholes and demonstrate crystallinity that closely matches that of the powdered raw material.

COF-CNT composite ultra-high-sensitivity temperature-sensitive conductive system

Carbon nanotubes serve as conductive bridging pathways, with optimal performance achieved at a CNT doping level of 0.36 wt%; the electrical conductivity spans four orders of magnitude between 25°C and 300°C, and the COF‑5 device exhibits a temperature coefficient of resistance (TCR) of −2.6%/°C, significantly exceeding that of conventional carbon–polymer composite thermosensitive materials.

Reversible, temperature‑wide sensing performance with mechanical stability

The heating–cooling cycling electrical signal is fully reversible; under a 12 mm bending radius, the device exhibits no degradation in electrical performance after 1,000 bends, combining both high-temperature stability and flexible adaptability.

Universal Composite Ink Platform

This ink system is compatible with various two-dimensional nanofillers (carbon nanotubes, graphene, and metal nanoparticles), enabling the development of COF‑based printed sensors for gases, pressure, and other stimuli.

II. Aerosol Spraying COF Complete Process Flow for Thin-Film Fabrication

1) COF nanoparticles were synthesized via a solvothermal method, followed by centrifugation, washing, and purification.

2) A mixed solvent of acetonitrile and benzonitrile in a 7:3 ratio is used to disperse the COF powder, and ultrasonic treatment yields a stable colloidal ink; carbon nanotubes can be added as needed to formulate composite sensing inks.

3) The substrate is activated by plasma cleaning, and the equipment generates an aerosol beam via ultrasonic atomization.

4) Carrier gas: 40 sccm; sheath gas: 50 sccm; printing speed: 0.5–1 mm/s; directly write the target pattern layer by layer.

5) The material dries naturally at room temperature, eliminating the need for sintering or high-temperature polymerization; device assembly and electrical testing can be performed directly.

III. Five Key Advantages of Aerosol-Based Fabrication Processes (Comparison with Spin Coating, Inkjet Printing, and Template-Assisted Methods)

One-step molding of highly crystalline COF films without post‑polymerization.

Traditional COF printing requires first printing monomers followed by high-temperature curing; AJP, on the other hand, directly prints pre‑crystallized colloids, thereby avoiding high temperatures that could damage flexible substrates.

Micrometer-level high-precision digitalization and visualization

Maskless CAD direct writing achieves a line resolution of 36 μm and enables the customization of interdigitated electrodes and dot‑array sensor arrays—features that are difficult to realize with conventional template‑based methods for fabricating fine, complex patterns.

It exhibits excellent substrate compatibility and is compatible with flexible PI films.

The solvent is mild, enabling room-temperature processing throughout the entire process; it can be used to fabricate both heat‑unstable polyimides and paper substrates, whereas the solvothermal method is limited to high‑temperature‑resistant rigid substrates.

Compatible multi-material blending, enabling one-stop preparation of composite functional membranes.

The same device can seamlessly switch between pure COF and COF/carbon nanotube ink, enabling rapid tuning of its electrical conductivity and sensing performance.

Fixed-point deposition with low consumable usage, enabling rapid iteration in small batches.

Only the functional area deposits COF, a costly powder; spin-coating the entire wafer results in significant waste, while R&D prototyping costs are substantially reduced.

IV. Five Core Technological Innovations of This Study

Innovation 1: A low-volatility COF colloidal ink formulation tailored for aerosols

Introducing benzonitrile-based controlled‑rate drying to address the issues of rapid evaporation and particle agglomeration in aerosol droplets, enabling long‑duration printing with uniform lines free of coffee‑ring effects.

Innovative 2‑step post‑processing‑free COF direct printing process

Breaking the bottleneck of in-situ polymerization in COF printing, maintaining the intact crystalline structure after deposition of pre‑formed nanocolloidal aerosols, and streamlining the device fabrication process.

Innovation 3 COF – A Novel Mechanism of Carbon Nanotube–Based Synergistic High Sensitivity and Wide-Temperature-Range Thermosensitivity

The COF high-temperature insulating substrate, combined with carbon‑nanotube conductive pathways, significantly enhances current‑carrying transition efficiency as temperature rises, enabling an ultra‑wide detection range of 25–300°C.

Innovative 4‑in‑1 Flexible High-Temperature‑Resistant Integrated Printed Temperature Sensor

It combines 300°C thermal stability with robust bendability over a thousand cycles, filling a critical gap in high-temperature flexible printed sensing materials.

Innovation 5: A Universal COF Composite Printing and Manufacturing Platform

The ink system can be combined with various nano-fillers, enabling rapid advancement in the development of gas‑sensing, humidity‑sensing, and optoelectronic COF‑printed devices.

V. Current Industry Challenges in the Process and Corresponding Breakthrough Solutions

Challenge 1: COFs are typically poorly soluble powders that cannot be directly printed; conventional methods require high-temperature in-situ polymerization, which can damage flexible substrates.

✅Pre‑fabricated, highly crystalline COF colloidal particles; direct‑write deposition in the aerosol phase at room temperature, with no high‑temperature processing throughout the entire process; compatible with flexible substrates such as PI.

Challenge 2: In aerosol printing, uneven solvent evaporation can lead to coffee-ring effects and rough, porous films.

✅ A dual-solvent formulation with high and low boiling points ensures uniform control of the droplet drying rate, yielding a dense, smooth, and defect-free deposited film.

Challenge 3: Pure COFs exhibit extremely poor electrical conductivity, making them unsuitable for direct use in electrical sensing.

✅ In-situ blending of carbon nanotubes constructs a continuous conductive network, and precise control of the doping level enables ultra‑high‑temperature responsiveness.

Challenge 4: Conventional COF processing is limited to rigid planar substrates and cannot fabricate flexible, bendable sensors.

✅ Aerosol-based, non-contact deposition at room temperature; direct fabrication on a polyimide flexible substrate; stable performance after thousands of bending cycles.

Challenge 5: COF-based devices exhibit a narrow temperature detection range and low sensitivity.

✅COF high-temperature‑resistant skeleton combined with carbon‑nanotube conductivity synergizes to extend the operating temperature range up to 300°C, boosting TCR performance by several times.

VI. Future Diversified Industrial Application Scenarios

1 Flexible High-Temperature Wearable Temperature-Measuring Patch

Surface temperature measurement for industrial equipment and high-temperature personal protective‑equipment sensing—ultrathin, flexible, and capable of long‑term, stable monitoring of elevated‑temperature operating conditions.

2 Miniature Temperature Sensor Array for Electronic Devices

In-situ printed temperature‑sensing array on PCBs and power devices enables real-time monitoring of chip thermal distribution and provides early warning of overheating faults.

3 Aerospace High-Temperature-Resistant Flexible Sensing

An onboard lightweight, flexible temperature‑sensing film that withstands a wide range of temperatures and is compatible with the surfaces of compact, irregularly shaped devices.

4 Distributed Temperature-Sensing Tags for Chemical Pipelines

A COF sensing film that can be printed in a single pass, adhered to pipelines for monitoring reaction temperatures, and resistant to chemical corrosion and high temperatures.

5 Laboratory COF Device Rapid Development Platform

Aerosol-based equipment enables the rapid fabrication of various COF‑based printed sensor prototypes, eliminating the need for large solvothermal reactors.

Image and Text Guide

Figure 1. (A) Schematic illustration of the previous approach, which involved inkjet printing COF precursors followed by polymerization to form crystalline COF patterns. (B) Schematic of the method employed in this work, wherein colloidal COFs are directly patterned via aerosol jet printing. (C) Molecular structures of the 2D boronate ester COFs (COF‑5 and COF‑10) used in this study, as depicted in their unit cells.

Figure 2. (A) Plot of the average thickness of COF‑5 as a function of the CH3CN:PhCN ratio for different numbers of printing cycles. Prints were deposited at a flow rate of 40 sccm and a printing speed of 2 mm/s. (B) (Top) Schematic illustrating the formation of a rougher film resulting from the uncontrolled, rapid solvent evaporation when CH3CN is used alone. (Bottom) Schematic depicting the formation of a more uniform film due to the controlled, slower solvent evaporation upon addition of PhCN to the ink formulation. (C, D) Cross-sectional SEM images of COF‑5 films printed from inks composed of (C) a 9:1 CH3CN:PhCN co‑solvent mixture and (D) a 7:3 CH3CN:PhCN co‑solvent mixture.

Figure 3. (A) COF‑5 lines printed on a silicon substrate (0.5 mm/s, one pass) and (B) the corresponding profile measurement of the vertical height (i.e., thickness). (C) COF‑10 lines printed on a silicon substrate (0.5 mm/s, one pass) and (D) the corresponding profile measurement of the vertical height (i.e., thickness). (E) Intensity profiles from grazing-incidence X-ray diffraction along qz = 0 for the printed COF film. Also included are colloidal inks in sol–capillary X-ray diffraction and simulated Pawley refinement patterns.

Figure 4. (A) The relationship between the measured current and temperature for COF‑5 films deposited on graphene interdigitated electrodes on a polyimide substrate, under different CNT loadings. COF‑5 films containing 0 wt% CNT, 0.12 wt% CNT, and 0.36 wt% CNT were measured at an applied bias of 40 V due to their low electrical conductivity. In contrast, films containing 2.66 wt% and 9 wt% CNT were measured at 1 V, while the 100 wt% CNT film was measured at 0.1 V. (B) Schematic illustrating the effect of CNT percolation on the electrical conductivity of COF‑CNT nanocomposite films.

Figure 5. (A) Current–voltage curves of the printed COF‑5‑CNT nanocomposite device as a function of temperature, at a CNT loading of 0.36 wt%. The device was fabricated using a flow rate of 40 sccm, a printing speed of 1 mm/s, and five printing passes (photograph shown in the inset). (B) Electrical conductivity as a function of temperature, in units of A, extracted from the current–voltage data. (C) A reversible temperature dependence of electrical conductivity is observed after thermal cycling between room temperature and 300 °C. (D) Electrical conductivity remains essentially unchanged as a function of bending cycles (with a bending radius of 12 mm), demonstrating the mechanical flexibility and resilience of the all‑printed COF‑CNT nanocomposite device.