A porous hydrogel composite prevents MXene collapse and rapid oxidation, creating a stable semiconducting network that achieves reliable low level gas sensing at room temperature.
(Nanowerk Spotlight) Materials that deliver the strongest electrical response to trace gases, including graphene, transition metal dichalcogenides, and MXenes, also tend to be the most fragile. Their sensitivity comes from surfaces fully exposed at the atomic level, but that same exposure makes them easy to damage. Oxygen can alter their chemistry. Humidity can make the sheets clump or collapse into stacks that hide the active sites needed for sensing.
Attempts to protect these materials often weaken the very interactions that produce their signals. This link between high responsiveness and poor stability has slowed efforts to create room temperature gas sensors that can operate in real environments rather than controlled laboratory settings.
Hydrogels offer a different strategy. These soft polymer networks change their volume in response to specific triggers such as temperature, pH, or dissolved compounds. They can be tuned precisely and can show strong selectivity. Yet most hydrogels depend on absorbing water to move. That limits their usefulness in air, where they absorb too little liquid to produce clear or reliable responses. Early attempts to adapt them to dry conditions often created networks that were unstable or too slow to respond.
Progress came only after researchers developed methods to introduce permanent pores into hydrogels. These pores stay open even when the material is dry, which allows gases to travel through the network. Around the same time, improved processing techniques helped stabilize MXenes such as Ti₃C₂Tₓ and made it possible to control their structure more reliably.
MXenes interact strongly with volatile organic compounds because their surfaces contain active chemical groups. But they still oxidize rapidly and tend to form dense stacks that limit sensitivity. Advances in porosity engineering and composite fabrication raised the possibility that combining MXenes with porous hydrogels could balance sensitivity with durability.
The study investigates how integrating the two materials during synthesis affects MXene’s structure, electrical behavior, and stability. By comparing pure MXene with composite versions, the researchers show how the hydrogel environment changes the pathway through which MXene degrades and how it shapes the material’s response to acetone, which serves as a model volatile organic compound.
Porous MXene/PNIPAAm composite: a) Fabrication process, b) bulk sample images (inset: optical microscope; main: SEM). Due to the synthesis process, the MXene is fully integrated with the polymer matrix, and consequently, no individual MXene sheets can be identified. The homogeneous MXene distribution is instead evidenced by the EDX-mapping of the titanium element depicted in Figure 4. Optical microscope (inset), SEM images, and chemiresistive analysis of MXene/PNIPAAm composite samples on IDE (fabricated by molding): c) air-dried compact and d) freeze-dried porous composite in response to gaseous acetone with a high humidity background. IDE measurements were performed on as-fabricated samples without prolonged storage. Both samples were fabricated with the same amount of precursor solution and a MXene concentration of 30 mg/mL. (Image: Reproduced from DOI:10.1002/advs.202516529, CC BY) (click on image to enlarge)
The researchers first examined pure MXene. Air dried MXene collapsed into thin, dense layers with almost no internal space. Freeze drying produced a large three-dimensional network formed by ice crystals that created and then left behind interconnected pores. This porous network exposed much more MXene surface area. It also made the sheets more accessible to oxygen.
Over six weeks of storage, the freeze dried MXene shifted from a conductive state to one with extremely high resistance. At that point, the material could no longer be measured. Chemical analysis confirmed a clear increase in oxygen content, showing that oxidation progressed across the sample.
The composite followed a different process. The researchers mixed MXene flakes into a PNIPAAm precursor along with a porogen, polyethylene glycol. A porogen is a substance that creates pores when removed. As the polymer chains formed, they restricted the movement of the MXene flakes, which prevented the usual stacking seen in pure MXene. Removing the porogen and freeze drying created a stable three-dimensional structure with pores about 3 to 4 micrometers across. Chemical mapping showed that MXene flakes were distributed evenly throughout the hydrogel, both at fabrication and after extended testing.
Drying conditions determined how well the composite functioned as a sensor. Air dried composite samples formed a compact outer layer that limited gas entry. These samples showed almost no response to acetone. Freeze dried samples preserved their open pores, allowing gas molecules to reach the embedded MXene. When exposed to acetone in a humid environment, these samples showed a sharp drop in resistance followed by a slower decline.
The sharp drop reflected the MXene’s electronic interaction with acetone. The slower decline came from the hydrogel swelling as it absorbed molecules, which increased spacing between MXene flakes. These two processes created a clear two stage signal at acetone concentrations as low as 20 ppm.
Mechanical tests revealed how MXene affected the hydrogel’s stiffness. Pure PNIPAAm had the highest storage and loss moduli. Adding porogen reduced these values. Low MXene content reduced them further, since the flakes disrupted the polymer network. At higher concentrations, interactions between MXene flakes and the polymer chains added physical crosslinks and increased stiffness again. When MXene concentration rose above 30 mg mL⁻¹, the mixture became too viscous to form a uniform polymer network.
Electrical tests showed how the hydrogel environment changed MXene’s behavior. Pure air dried MXene acted like a metal. Its resistance increased when exposed to acetone because the gas scattered charge carriers. Pure freeze dried MXene acted like a semiconductor because partial oxidation created barriers that controlled charge movement. In that case, exposure to acetone lowered the barriers and decreased resistance. The composite showed the same semiconducting pattern even though it had not been heated.
This indicates that the hydrogel environment and the porous structure were associated with a partially oxidized state that supported strong gas sensitivity while slowing the full oxidation that destroys conductivity.
The long-term tests showed the benefit of this environment. Pure freeze dried MXene became unmeasurable after six weeks. The composite remained measurable and responsive for at least twelve weeks under the same conditions. It eventually oxidized fully after about fifteen months, but its lifetime was much longer than that of the pure MXene.
Taken together, the results show that embedding MXene in a porous hydrogel can stabilize its structure, slow degradation, and maintain the partially oxidized state needed for strong semiconducting responses. The hydrogel prevents the flakes from collapsing into dense stacks and preserves open pathways for gases to reach them. Because porosity engineering can be applied to many hydrogels, this strategy may support the design of room temperature gas sensors tuned for a variety of volatile organic compounds.
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