Researchers have developed a transparent, copper-based MOF thin film that can detect ethanol vapor using a smartphone camera, offering a low-cost, portable alternative to traditional sensing tools.
Study: Solvato- and Vapochromism-Based Alcohol Sensing through Metal–Organic Framework Thin Films with Coordinatively Unsaturated Metal Sites. Image Credit: chemical industry/Shutterstock.com
A recent article in Small Science presents a promising approach to ethanol (EtOH) detection using metal-organic frameworks (MOFs), a class of porous, crystalline materials. The researchers emphasize ethanol's importance across industries, from pharmaceuticals to food, fuels, and lab research, and demonstrate how MOFs with coordinatively unsaturated metal sites can enable responsive and reversible sensing.
Why MOFs?
MOFs are porous, crystalline materials formed by metal ions or clusters connected via organic ligands. Their high surface area, tunable chemistry, and ordered pore networks have attracted interest for various sensing applications. A key advantage lies in their ability to respond visibly, through changes in color or light absorption, when they interact with specific guest molecules.
Some MOFs exhibit vapochromism, meaning they change color in response to vapor-phase molecules. This effect stems from alterations in the framework’s electronic structure, particularly at coordinatively unsaturated metal sites. Prior work has shown that MOFs containing metals like copper or cobalt can recognize specific molecules via hydrogen bonding or coordination shifts, which in turn influence their optical properties.
Building on these insights, the research team focused on a copper-based MOF—Cu-MOF-74—known for its strong affinity to alcohols and potential for colorimetric sensing.
Building the Sensor
The sensor fabrication process began with the synthesis of copper hydroxide (Cu(OH)2) nanobelts. Using copper sulfate, ammonia, and sodium hydroxide in a controlled chemical reaction, the team produced nanobelt suspensions, which were drop-cast onto silicon substrates and dried to form smooth, uniform films.
These precursor films were then transformed into Cu-MOF-74 through a thermal treatment in methanol at 100 °C. This topochemical conversion maintained the film’s transparency and structural integrity while embedding the metal-organic framework directly into the substrate.
To confirm successful transformation, the researchers employed several characterization techniques:
- X-ray diffraction (XRD) verified crystallinity
- Fourier-transform infrared (FT-IR) and Raman spectroscopy assessed chemical bonding and vibrational modes
- Scanning electron microscopy (SEM) revealed consistent surface morphology
Together, these methods demonstrated that the films were not only structurally sound but also optically suitable for sensing applications.
How it Works
The key sensing mechanism hinges on the film’s solvatochromic and vapochromic behavior. When the Cu-MOF-74 thin films are exposed to ethanol-water vapor mixtures, ethanol molecules are adsorbed into the framework, triggering measurable shifts in the material’s absorption spectrum—visible as a distinct color change.
The underlying chemistry involves hydrogen bonding between ethanol molecules and phenolate oxygen sites in the MOF. This interaction subtly alters bond lengths within the framework, leading to a shift in its electronic bandgap and hence its optical properties.
Smartphone Integration
To make this technology practical and accessible, the team developed a smartphone-based detection system. The phone’s camera captures real-time color changes in the film, while a dedicated app analyzes the image to extract RGB (red-green-blue) values. The change in the green channel (ΔG) proved to be a reliable indicator of ethanol concentration.
This calibration-based method successfully quantified ethanol levels across the full range—from pure water to pure ethanol—and was even tested on commercial alcoholic beverages. The results were consistent, reproducible, and easy to interpret without any specialized lab equipment.
Results
The Cu-MOF-74 films offered excellent optical clarity and surface uniformity, both of which enhance sensitivity to color shifts. More importantly, the system was shown to be reversible and stable, all of which are key traits for sensors intended for repeated use or continuous monitoring.
By combining material responsiveness with a familiar interface—smartphone cameras—this sensor provides a compelling alternative to more complex, instrument-heavy detection systems. It opens up possibilities for in-field measurements in quality control, safety checks, and even consumer use.
Conclusion
This work demonstrates how advanced MOF materials can be integrated with consumer technology to create practical, high-performance chemical sensors. The ethanol detection system developed here is portable, cost-effective, and easy to operate, offering meaningful value across industries that rely on accurate alcohol monitoring.
Looking ahead, the researchers note that the same strategy could be extended to detect other molecules by modifying the MOF composition, laying the groundwork for a new generation of accessible, MOF-based sensing platforms.
Journal Reference
Toki Y., Okada K., et al. (2025). Solvato- and Vapochromism-Based Alcohol Sensing through Metal–Organic Framework Thin Films with Coordinatively Unsaturated Metal Sites. Small Science, DOI: 10.1002/smsc.202400634, https://onlinelibrary.wiley.com/doi/10.1002/smsc.202400634