Tackling Humidity with Designer Ionic Liquid‐Based Gas Sensing Soft Materials

Abstract

Relative humidity is simultaneously a sensing target and a contaminant in gas and volatile organic compound (VOC) sensing systems, where strategies to control humidity interference are required. An unmet challenge is the creation of gas‐sensitive materials where the response to humidity is controlled by the material itself. Here, humidity effects are controlled through the design of gelatin formulations in ionic liquids without and with liquid crystals as electrical and optical sensors, respectively. In this design, the anions [DCA]− and [Cl]− of room temperature ionic liquids from the 1‐butyl‐3‐methylimidazolium family tailor the response to humidity and, subsequently, sensing of VOCs in dry and humid conditions. Due to the combined effect of the materials formulations and sensing mechanisms, changing the anion from [DCA]− to the much more hygroscopic [Cl]−, leads to stronger electrical responses and much weaker optical responses to humidity. Thus, either humidity sensors or humidity‐tolerant VOC sensors that do not require sample preconditioning or signal processing to correct humidity impact are obtained. With the wide spread of 3D‐ and 4D‐printing and intelligent devices, the monitoring and tuning of humidity in sustainable biobased materials offers excellent opportunities in e‐nose sensing arrays and wearable devices compatible with operation at room conditions.

Synergy between silk fibroin and ionic liquids for active gas-sensing materials

“…The chloride anion was selected due to its properties. It is a strong H-bond acceptor, known to break inter-strand hydrogen bonds between neighbour backbone polypeptide chains, thus critical for fibroin dissolution, but also able to act as a cross-linker in H-bonds, thus important for gel formation [ 22 ]. Regarding the cation moiety, we selected methylimidazolium cations with varying lengths of the alkyl chain, namely C n mimCl with n ​= ​2, 6, 10.…”

“…The chloride anion is constant among all tested ILs. This anion is hydrophilic and a strong hydrogen bond acceptor [ 22 ], that can undertake multiple H-bonding interactions as doubly ionic [ 32 ] which act as a cross-linker between neighbour polypeptide chains. Regarding the cation moiety, the imidazolium ring is constant.…”

“…3 A). Such behaviour is likely attributable to the high hygroscopy of the ILs containing the chloride anion [ 22 ]. While C 6 F and C 10 F remain gels that can be handled with tweezers after 30 days, C 2 F turned into a liquid state after 1 day of exposure to ambient air, likely due to the low self-assembling propensity of the IL alone.…”

Organic ionic fluid-based wearable sensors for healthcare

“…Most organic ionic fluids based on ionic bonds are humiditysensitive, especially ionic liquids, which have been widely applied in human respiratory-related humidity sensors. [38][39][40] When water molecules are absorbed into the sensing medium, the ionic interaction is weakened, promoting ion migration to a certain extent, which is reflected in the corresponding electrical output signal as a reduction in the resistance value.…”

“…Human exhaled gas contains not only conventional substances such as water vapor, carbon dioxide, and oxygen but also toxic gases such as ammonia, hydrogen sulfide and acetone in some special cases, which are generally used as biomarkers to diagnose human diseases. 23,40,43,82 People with oral diseases often exhale hydrogen sulfide gas, which always corresponds to halitosis and can seriously affect an individual's social image. 23 Ammonia has also been linked to liver and kidney disease, and toluene is often present in the breath of lung cancer patients.…”