Development of 3D printed flexible electrodes for gas sensing applications
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Date
2026-07-07
Authors
Μαρούδας, Ευστράτιος
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Abstract
Gas sensors are increasingly important for environmental monitoring, industrial safety, and indoor air-quality control, as they allow the detection of harmful or volatile compounds in real time.Conventional gas-sensing technologies can provide reliable measurements, but they often rely on rigid platforms, high operating temperatures, or complex fabrication processes. For this reason, there is growing interest in flexible, low-cost, and easily customizable sensing devices that can be produced by simpler manufacturing routes and adapted to different applications.The aim of this diploma thesis was the development of flexible 3D-printed electrodes for gas sensing applications, using conductive GNPs/HDPE nanocomposites as the electrode material and MIL-53(Fe)-based metal organic frameworks (MOFs) as the active sensing layers. The work focused on combining the electrical conductivity of graphene nanoplatelets (GNPs) with the processability and flexibility of high-density polyethylene (HDPE), to produce a printable conductive filament suitable for 3D printing.Initially, the raw BeDimensional GNPs were characterized by Raman spectroscopy, indicating their graphitic multilayer structure. Their lateral size was then reduced by using two different mechanical processing routes, shear mixing and tip sonication and the effect of processing time was evaluated through optical microscopy and image analysis. Tip sonication led to stronger lateral size reduction, while shear mixing provided a more balanced reduction of agglomerates while preserving larger platelet dimensions.GNPs/HDPE nanocomposite films with different filler contents and processing routes were prepared by hot pressing and evaluated through mechanical, electrical, and thermal characterization.The electrical measurements showed that increasing GNPs loading strongly decreased the sheet resistance of the nanocomposites, due to the formation of conductive pathways within the HDPE matrix. However, the mechanical tests showed that high GNPs contents increased stiffness and reduced ductility, making highly loaded materials less suitable for filament extrusion. Based on the combined electrical, mechanical, and thermal results, the 40 wt.% shear-mixed GNPs/HDPE nanocomposite was selected as the most suitable formulation for filament fabrication and 3D printing.Three MIL-53(Fe)-based sensing materials were synthesized using different solvothermal routes: pristine MIL-53(Fe), acetic acid modulated MIL-53(Fe), and NaOH modulated MIL-53(Fe). XRD, FTIR, and TGA analysis confirmed the formation of MIL-53(Fe)-based structures, while also showing that the synthesis additives affected the crystallization behavior and composition of the final powders.MIL-53(Fe)-AA showed better defined structural features, whereas MIL-53(Fe)-NaOH showed evidence of additional phases and higher inorganic residue.The sensing layers were deposited onto the 3D-printed electrodes by drop casting, and the gas sensing behavior was evaluated through capacitance measurements. Preliminary vapor screening showed that the MIL-53(Fe)-AA coated sensor responded more clearly to ammonia andformaldehyde, while toluene produced almost no response. The coating volume study showed that increasing the amount of deposited MOF increased the capacitance response under saturated ammonia vapor conditions. Humidity measurements indicated that the sensor was affected mainly by very high humidity, close to 100% RH.Under controlled gas concentrations, MIL-53(Fe)-AA showed the most effective sensing behavior toward both ammonia and formaldehyde, while the pristine MIL-53(Fe) sensor also responded to both analytes, but with a generally weaker and less defined signal. In contrast, the MIL-53(Fe)-NaOHcoated sensor did not provide a clear response at lower controlled concentrations, and a detectable signal was observed only at higher concentrations. Overall, the results show that 3D-printed GNPs/HDPE electrodes combined with MIL-53(Fe)-based coatings can operate as flexible capacitive gas sensing devices, with the MIL-53(Fe)-AA showing the best practical sensing performance among the materials examined.
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Keywords
3D-printed electrodes, Gas sensors, Metal-organic frameworks (MOFs), Graphene nanocomposites