I. Abstract of the Paper’s Core Research

This study establishes a comprehensive aerosol‑jet‑based fabrication process for MOF thin films, using oxalic acid and a calcium hydroxide–water‑based precursor solution as feedstocks. By employing a saturated carrier gas to suppress premature crystallization of droplets, the study systematically optimizes three key parameters—substrate temperature, precursor concentration, and the number of printing passes—and conducts a full‑dimensional characterization of the films’ crystallinity, microporous adsorption properties, and sensing performance. The key findings are as follows:

Water-based stable MOF precursor aerosol ink system

Calcium hydroxide and squaric acid are dissolved in a two-component aqueous solution; after 5 hours of mixing, no precipitate forms, and water is used as the solvent without any flammable organic solvents. Saturated steam‑nitrogen gas is employed as the carrier gas to prevent premature crystallization of droplets during atomization, which could otherwise clog the nozzle.

Low-Temperature In-Situ Crystallization and Forming Mechanism

At 30°C, the optimal printing substrate enables slow solvent evaporation following droplet deposition, facilitating in-situ growth of UTSA-280 ultramicroporous MOF crystals. The resulting thin film retains the native one-dimensional pore channels aligned parallel to the substrate, eliminating the need for high‑temperature thermal treatment that would otherwise compromise the porous structure.

Precise, controllable thickness for grayscale graphic fabrication

The number of printed layers—1, 2, 5, or 10—corresponds to a graded film thickness profile, eliminating the need for grayscale lithography. A single CAD file can generate multi‑thickness patterned features, with a minimum resolvable line width of 150 μm; after adding an octanol additive, the resolution is improved to 100 μm.

Excellent molecular sieving and adsorption performance

The aerosol‑printed MOF thin film exhibits excellent agreement with the adsorption isotherms of the corresponding powder, enabling selective separation of ethane and ethylene. QCM measurements on the thin film reveal stable, reversible adsorption of CO₂ and water vapor, with only minimal performance degradation after six weeks of storage.

Integration MOF Capacitive Sensor Verification

Aerosol printing is used directly on platinum interdigitated electrodes to fabricate the UTSA-280 thin film, enabling one-step formation of the sensing layer. This approach yields a response sensitivity to water vapor that far exceeds that for CO₂, with highly reversible signals and excellent reproducibility. The process can be extended to a variety of MOF materials, including ZIF‑8.

II. Complete Preparation Process for MOF Aerosol Jet Printing

1) Preparation of the precursor solution: Prepare aqueous solutions of calcium hydroxide and squaric acid separately, mix them at desired concentrations of 2.5, 5, or 10 mM, and add a small amount of hexanol to optimize spreading.

2) Equipment modification: A pneumatic atomization system is employed, with the carrier gas introduced into the water tank in advance to saturate it with water vapor, thereby preventing premature crystallization of the spray droplets.

3) Plasma cleaning of silicon wafers and interdigitated electrodes, with the printing platform maintained at a constant temperature of 30°C;

4) G-code enables customization of micro‑lines and grayscale patterns, with film thickness controlled by adjusting the number of print passes.

5) After deposition, it naturally crystallizes at room temperature, eliminating the need for high-temperature annealing.

6) XRD, AFM, SEM, QCM, and a capacitance-sensing system were used to characterize the thin films and test the devices.

III. Five Core Process Advantages of Aerosol-Printed MOFs

Maskless digital patterning eliminates the photolithography and lift-off processes.

No photoresist or sacrificial layers are required; MOFs can be deposited precisely at desired locations, with no organic impurity contamination of the micropores, thereby preserving the material’s intrinsic adsorption properties.

The ink compatibility window is extremely broad and is not subject to stringent viscosity constraints.

Unlike inkjet printing, which imposes stringent viscosity requirements, both high- and low-concentration aqueous precursor solutions can be stably atomized, significantly broadening the range of available MOF precursors.

Single-process fabrication of grayscale gradient-thickness thin films

By controlling the number of printed lines to regulate film thickness, an equivalent gray-scale lithography effect is achieved, enabling low-cost fabrication of partitioned functional porous coatings.

Mild, ambient-temperature shaping without compromising the MOF’s microporous framework.

The entire process is carried out at a low temperature of 30°C, with no high-temperature sintering, thereby preventing pore collapse and ligand decomposition and preserving the ultramicroporous sieving capability.

Multi-substrate, multi-MOF–compatible, conformal printing on curved surfaces

Silicon, metal electrodes, and flexible thin films can all be fabricated, while remaining compatible with various MOF systems such as UTSA‑280 and ZIF‑8, thereby broadening the scope of material applications.

IV. Five Core Technological Innovations of This Study

Innovation 1: Aerosol-Direct-Crystallization MOF Forming Process

It is demonstrated that aerosol‑atomized precursor solutions can be used to grow continuous, crystalline MOF thin films directly on the substrate, overcoming the limitation of conventional spray coating, which is restricted to producing only powder coatings.

Innovation 2: A saturated steam carrier gas anti-crystallization scheme

It addresses the industry’s key challenges of premature crystal precipitation during the atomization process and nozzle clogging, enabling long‑term, continuous, and stable printing.

Innovative 3D printing: a new method for precise layer‑by‑layer thickness control and grayscale fabrication.

By controlling the number of deposition cycles to tune the film thickness, we can replace costly grayscale lithography and fabricate partitioned porous functional coatings at low cost.

Innovation: A low‑surface‑tension additive enhances micrometer‑level print resolution.

Trace amounts of hexanol reduce the contact angle of the precursor solution, optimizing the minimum resolvable line pitch from 150 μm to 100 μm.

Innovative One-Step Integrated MOF Microsensor Device

Without the need for a second transfer membrane, the sensing layer can be directly deposited on interdigitated electrodes, streamlining the entire fabrication process of gas sensors.

V. Current Industry Challenges and Corresponding Solutions

Challenge 1: In the lithographic fabrication of MOFs, residual photoresist can clog the micropores, thereby reducing adsorption selectivity.

✅ Aerosol-based direct writing at specific locations, with no organic masking materials used throughout the process, yielding a pristine thin film that fully preserves the ultra‑microscopic pore‑sieving structure.

Challenge 2: Inkjet printing places extremely stringent demands on the viscosity of the precursor solution; high-concentration formulations are highly prone to nozzle clogging.

✅ Pneumatic atomization is independent of liquid viscosity, enabling stable, continuous printing even with high-concentration precursor solutions of 5–10 mM.

Challenge 3: Conventional processes can only apply a uniform laminated coating, making it impossible to fabricate regionally graded functional films.

✅ Digital program control enables precise adjustment of print counts, allowing multi-layered grayscale MOF patterns with varying thicknesses on the same substrate.

Challenge 4: Post‑synthesis high‑temperature treatment can easily lead to the collapse of MOF pore channels and the decomposition of ligands, resulting in loss of functionality.

✅ In-situ crystallization at ambient temperature of 30°C, without calcination, with the native one-dimensional microporous structure fully preserved.

Challenge 5: Current MOF thin-film fabrication methods are limited, making it difficult to accommodate a wide range of porous framework materials.

✅ A universal aqueous atomization system; various MOFs, including UTSA‑280 and ZIF‑8, can all be prepared using the same equipment.

VI. Future Diversified Industrial Application Scenarios

1 Miniature High-Precision Gas-Sensing Chip

MOF aerosol-sensitive-layer capacitive sensor, designed for indoor CO₂ and humidity micro‑monitoring modules and wearable environmental monitoring devices.

2 Microfluidic Small-Molecule Separation Chip

Micrometer-scale patterned MOF separation membranes enable precise micro‑separation of low‑carbon hydrocarbons such as ethylene and ethane, and serve as laboratory‑scale trace‑detection devices.

3 Microcatalytic Array Thin Film

Custom-patterned MOF catalytic layers, microreactors, on-chip catalytic screening platforms, and high-throughput catalyst development.

4 Flexible Electronic Gas-Sensing Patch

PI‑based conformal printing of MOF coatings enables noninvasive wearable sensing of human volatiles.

5 High-Throughput MOF Thin-Film Synthesis Platform for Research

Rapidly print MOF samples of varying thicknesses and compositions, significantly shortening the screening cycle for adsorption and catalytic materials.

Image and Text Guide

Figure 1. Schematic diagram of the AJP device (UTSA-280 MOF deposition)

Figure 2. Optimization and characterization of the UTSA-280 coating process (a–g)

Figure 3. Grayscale direct-write pattern and IDE capacitive sensor demonstration