Metal-organic frameworks, or MOFs, are highly porous materials whose structure can be engineered at the molecular level. Their combination of large internal surface area, adjustable pore size and tunable chemistry has generated interest in applications ranging from catalysis and molecular separation to sensing, electronics and photonics.
But for many of these applications, producing the material itself is only part of the challenge.
How the material grows on a surface, and particularly how its crystalline structure is oriented, can influence its properties.
Researchers from Dresden University of Technology and TU Bergakademie Freiberg recently investigated this question using copper gallate, a biobased metal-organic framework, deposited as a thin surface-mounted film. Their study explored whether the chemistry of the underlying surface could be used to control the growth and orientation of these structures.
To build the films, they used a continuous flow layer-by-layer synthesis process, supported by automated fluid switching with an AMF rotary distribution valve.
A surface-mounted metal-organic framework, or SURMOF, is essentially a MOF grown directly as a thin film on a substrate. This is particularly interesting when the material needs to interact with its environment through a surface, for example in sensing, electronic or photonic systems.
An easy way to picture the importance of orientation is to think about the grain in wood.
The material is chemically the same, but its behaviour can differ depending on the direction of its internal structure.
Something similar can occur in crystalline thin films. If their pores, channels or structural elements preferentially align in one direction, the resulting material can become anisotropic, meaning that some of its properties depend on direction.
Controlling this orientation is therefore an important part of engineering functional MOF coatings.
The researchers wanted to understand whether different cellulosic surface chemistries could influence how copper gallate SURMOFs organize during growth.
Copper gallate is a metal-organic framework formed using copper ions and gallic acid.
The use of gallic acid is particularly interesting because it provides a biobased alternative to more commonly studied MOFs based on aromatic carboxylate or imidazole linkers, such as HKUST-1 or ZIF-8.
Previous research has investigated gallate-based MOFs for several potential applications, including biological and antimicrobial uses. In this study, however, the objective was different: to explore copper gallate as an oriented thin-film material.
According to the authors, the formation and orientation of copper gallate SURMOFs had not previously been described.
Their question was therefore relatively fundamental:
Can copper gallate be grown as a controlled thin film, and can the underlying surface influence its orientation?
Building a material one layer at a time
To investigate this, the researchers used a layer-by-layer synthesis approach.
Rather than forming the complete material in a single reaction, the surface is exposed successively to different chemical solutions.
In this study, the sequence was:
Copper acetate → ethanol rinse → gallic acid → ethanol rinse → repeat
Real-time QCM-D monitoring of copper gallate SURMOF formation during repeated copper acetate, ethanol rinsing and gallic acid steps. From Elschner T. et al., RSC Advances 16, 31877–31884 (2026).
Copper ions are first introduced onto the surface. After rinsing, gallic acid is added to contribute to the formation of the framework. Another rinsing step removes excess material before the next cycle begins.
Repeated cycles progressively build the SURMOF film.
This is where continuous flow synthesis becomes particularly useful.
Instead of repeatedly immersing the sample in different solutions, the reagents can be delivered through the same flow chamber in a controlled sequence. Reaction time, flow rate and reagent exposure can then become defined process parameters rather than a series of manual operations.
The principle is simple.
The practical execution is less so.
Every layer requires several successive fluid changes, and the sequence must be repeated many times under comparable conditions.
The researchers performed experiments with up to 45 deposition cycles.
That means repeatedly selecting the correct reagent, introducing it for a defined period, switching to ethanol for rinsing, introducing the second reagent, rinsing again, and starting the next cycle.
For this type of continuous flow process, the fluidic system must therefore be able to:
The researchers initially used 10-minute exposure periods for copper acetate and gallic acid. During process optimization, they found that these deposition times could later be shortened to 5 minutes without affecting the deposited mass.
The fluidic sequence therefore becomes an integral part of the experimental method.
To automate reagent selection, the researchers integrated an AMF rotary distribution valve into the continuous flow system.
For the coating experiments, a peristaltic pump drew the selected solution through the valve and into a PDMS flow chamber containing the substrate.
The flow rate was maintained at 100 µL/min, while the rotary valve enabled automated switching between copper acetate, gallic acid and ethanol.
A similar fluidic architecture was used during quartz crystal microbalance with dissipation monitoring, or QCM-D. Four inlet tubes were connected to the AMF rotary distribution valve, enabling the different solutions to be introduced automatically during real-time measurements.
Instead of manually changing fluid connections throughout a long experiment, reagent selection could therefore become part of the programmed workflow.
For layer-by-layer synthesis, this is particularly relevant because fluid switching is not peripheral to the experiment, it defines the sequence by which the material is built.
The continuous flow setup also allowed the researchers to monitor what happened during individual deposition cycles. Using QCM-D, they measured changes in frequency and dissipation as copper acetate, ethanol and gallic acid successively passed over the sensor.
The resulting signal showed repetitive and clearly identifiable steps corresponding to the different stages of the process. From these measurements, the researchers calculated that approximately 500 ng/cm² of material was deposited per individual layer, with similar overall SURMOF growth across the different support materials.
This real-time monitoring also helped them investigate the contribution of each reagent and optimize the duration of individual synthesis steps.
The continuous flow process was therefore not only used to produce the material, but also to observe and understand its formation.
Once the copper gallate films had been produced, the researchers investigated their crystalline orientation using grazing incidence wide angle X-ray scattering, or GIWAXS.
The results revealed a clear influence of surface chemistry.
On SiO₂ and pure cellulose, the copper gallate SURMOFs showed partial orientation. A dominant diffraction plane associated with the pillars of copper octahedra was predominantly oriented parallel to the surface.
The degree of orientation was approximately 0.6 to 0.65.
GIWAXS pattern of a copper gallate SURMOF grown on cellulose, showing anisotropic orientation, alongside a representative MIL-53-type framework illustrating the rhombic channel structure. From Elschner T. et al., RSC Advances 16, 31877–31884 (2026).
However, when the cellulose surface contained catecholic groups, specifically cellulose caffeate or cellulose protocatechuate, the resulting SURMOFs showed little preferential orientation.
SEM images of copper gallate SURMOFs grown on SiO₂ and different cellulose-based thin films, revealing changes in morphology depending on surface chemistry. From Elschner T. et al., RSC Advances 16, 31877–31884 (2026).
The researchers attribute this difference to stronger complexation between copper ions and catechol groups distributed along randomly oriented polymer chains.
In other words, changing the chemistry of the surface changed how the material organized itself during growth.
This ability to influence crystalline orientation could eventually be relevant when designing functional thin films where directional properties matter, particularly for sensing, photonic or electronic applications.
Could the flow itself influence material orientation?
The study raises another intriguing question.
Could the direction of fluid flow also influence the way the material organizes?
Some of the GIWAXS observations suggested an orientation related to the direction of flow. The authors considered whether fluidic forces inside the chamber might favour the alignment of the rhombic channels along the flow direction.
However, the available data did not allow them to identify the corresponding diffraction planes precisely enough to confirm this mechanism.
The hypothesis therefore remains open.
Still, it illustrates an interesting aspect of continuous flow material synthesis: the fluidic environment may potentially contribute not only to how reagents reach the surface, but also to the conditions under which the material forms.
This study demonstrates how continuous flow layer-by-layer synthesis can be used to investigate material formation under controlled conditions.
For the researchers, automation allowed a repetitive multi-reagent protocol to become a programmable workflow, while QCM-D provided real-time information about what happened during each cycle.
The result was a process capable of:
And the scientific result goes beyond the fluidics itself.
The researchers demonstrated that functional cellulosic surfaces can influence the orientation of copper gallate SURMOFs, opening an interesting route for controlling the structure of biobased MOF thin films.
For continuous flow synthesis, the example also illustrates a broader principle.
As chemical workflows become more repetitive and involve more reagents, fluid selection becomes part of process control.
Programmable valves can help transform these complex sequences into automated workflows, whether the goal is advanced material synthesis, surface functionalization or another multi-step chemical process.
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