Multi‐Hydration Induced Zwitterionic Hydrogel with Open Environment Stability for Chemical Sensing

Hydrogels with open environment stability are of great significance in the fields of microelectronics, organ regeneration, and hydrogel‐based sensors. Although a series of strategies for exploring highly robust hydrogels have been proposed, it is still challenging to improve environmental stability while maintaining the original dispersed medium feature. Here, 1‐vinyl‐3‐butylimidazolium (VBIM) and acrylic acid (AA) are used as the anionic and cationic monomers to prepare zwitterionic hydrogels through the thermal initiation polymerization. The prepared PAA‐IL (polyacrylic acid‐co‐ionic liquid) hydrogel forms a large number of multi‐hydration, including hydrogen bonds and ion‐dipole interactions with water to inhibit the freezing and volatilization of water, and it possesses good structural stability due to the formation of ion bond self‐association through interchain charge interaction. The robust stability of the chemical sensor with PAA‐IL hydrogels as the substrate at different pH and temperature verifies the validity of the proposed design strategy. It is expected that the present zwitterionic hydrogel design strategy would shine a light on the exploration of environment‐stable hydrogels and hydrogel‐based chemical sensors.


Introduction
Internal liquid-like properties and external solid-like properties of hydrogels have endowed it with wide spread applications in various fields, including biomedicine, [1,2] soft electronics, [3,4] sensors, [5,6] and actuators. [7] The application of hydrogel depends on its flexibility and its internal conductivity and mass transfer capability. However, the volatilization or freezing of water in DOI: 10.1002/adsr.202200061 hydrogel would lead to the loss of the intrinsic properties, which greatly limits the application of hydrogels. [8,9] At present, the environment stability of hydrogels is retained mainly by changing the dispersing medium, such as injecting cryoprotectants or adding inorganic base/salt into the hydrogel. [10][11][12][13] For example, inspired by the cryopreservation of biological samples at extremely cold conditions, a "one pot solvent replacement" strategy taking glycerol, ethylene glycol, and sorbitol as the cryoprotectant was reported to replace water in hydrogels, and a non-drying and anti-freezing organic hydrogel was obtained, which remained unfrozen and mechanically flexible as well as being long term stable even at −70°C when exposed to ambient conditions or even stored under vacuum. [14] Ionic compounds (LiCl) were also added into the ductile polyacrylamide sodium alginate double network hydrogels and the freezing point of the liquid phase was thus reduced, and it was demonstrated to be used as an ionic conductor and combined with other soft materials (such as dielectric elastomer) to form a functional soft machine at low temperature. [15] In some occasions, high concentration of acid, alkali, or salt solution would cause hydrogel to be invalid, usually leading to the performance degradation and fault detection of hydrogel-based sensor. [16,17] Besides, most non-aqueous solvents face with the toxicity and flammability problems, and thus require complex and strict preparation process. [18] For hydrogel based chemical sensors, the change of disperse medium would affect the reaction rate and reaction process, and thus the influence of disperse medium on reaction should be considered while improving the environment stability. [19][20][21][22] Therefore, it is urgent to optimize the intrinsic polymer chains in hydrogels from the point of the construction procedure rather than from the perspective of adding other components into the disperse medium to ensure the hydrogel is stable in open environment.
Similar to the swelling process of hydrogels, the water loss process generally could be classified into two stages. [23,24] One stage refers to the rapid free water diffusion while the other represents slow adsorption and desorption of the binding water. [25] In the first stage, the hydrogel is nearly swelling, the free-flowing water occupies a relatively high proportion, and the moisturizing performance mainly comes from the liquid disperse medium. In the second stage, the drying rate is significantly reduced, and increasing the hydration of polymer chains in the unit volume of the hydrogel will retard the water loss process. Therefore, by introducing the polymer groups with more hydration in the solution phase, the continuous loss of moisture in hydrogels could be effectively prevented. Generally, the adsorption effect of polymer on water in hydrogels mainly comes from the formation of hydrogen bond. Increasing the crosslinking density is one of the most common method to enhance the hydrogen bond concentration, while it might seriously affect the mechanical properties and morphology of the hydrogels. The addition of hydration in polymers other than hydrogen bonds is conducive to improving the hydrophilicity of polymers without affecting the structural strength. As a promising strategy, ion-dipole interaction, has been pointed out to form a "flowing" hydration between water and cations through electrostatic interaction. [26] Inspired by the ion-dipole interaction, the interaction between polymer and water can be similarly equivalent to that between inorganic ions and water, and thus can effectively enhance the binding ability of hydrogel to water. [27] In light of the limitations of traditional methods for devising open environment stable hydrogels, whether ionic liquid could be employed as the polymer monomer and thus to ensure the environment stability and avoid the influence from the loss or the replacement of the disperse medium on the hydrogel device performance, remains unknown.
Herein, to explore hydrogels with open environment stability, PAA-IL (polyacrylic acid-co-ionic liquid) zwitterionic hydrogel was designed and synthesized with VBIM (1-vinyl-3butylimidazolium) and AA (acrylic acid) as the cation-anion pair. The PAA-IL hydrogel forms a large number of hydrogen bonds and ion-dipole interaction with water and forms ionic bond through charge interaction between chains without changing the dispersing medium, greatly inhibiting the freezing and volatilization of water. The radial distribution function and the average interaction energy of water molecules in PAA-IL system investigated by molecular dynamics simulation verified the strong binding ability of PAA-IL to water molecules. By characterizing the dehydration process, mechanical properties, and the electrical properties of the hydrogel at different temperatures, the PAA-IL hydrogel presents exceptionally consistent elastic properties and impedance in the non-breaking tensile range. Furthermore, chemical sensors constructed for permanganate, nitrate, and nitrite detection by functionalizing the PAA-IL hydrogel with the reagents indicate good stability and effective detection properties. Due to the decent stability in open environment over a wide temperature range, this PAA-IL hydrogel is expected to have broad applications in boosting the performances of chemical sensors, flexible electrical devices, etc.

Fabrication of the PAA-IL Hydrogel
The hydrogel is formed by copolymerization of acrylic acid with VBIM, and the interaction between the carboxyl group and the N cation was used to form the chain (Figure 1a). Due to the unique properties of imidazole ions, water molecules directly bonded by hydrogen bonding and water clusters spontaneously gathered by ion-dipole interaction are combined on the polymer surface (Figure 1b). When PAA-IL hydrogel is swelling, the block with a point load and the negatively charged block will form ionic bonds in the form of end-to-end connection, and the same charge block will be fully connected with each other due to the mutual exclusion effect, and it will form a network in the form of polymer rings with abundant branches in the hydrogel. When the hydrogel gradually loses water, the degree of freedom of the polymer chain drops and is arranged in a more ordered way. Thus, the ionic bond is mainly composed of anion and cation pairs intersecting the parallel chains, leading to the formation of multiple micro regions in the polymer chains and further increase of the bound water content.
The effect of zwitterionic monomer ratio on the structure of ionic hydrogels was studied by changing the molar ratios of acrylic acid (anionic monomer) and [VBIM]Br (cationic monomer) to 1:4, 1:2, 1:1, 2:1, and 4:1, respectively. With the increase of the ratio of cationic and anionic monomers, the saturated swelling of the hydrogel gradually increased until the ratio reached 1:2 where the swelling ratio reached the maximum (Figure S1, Supporting Information and Figure 1c). This is because the hydrophilic property of acrylic monomer improves the water absorption of hydrogel. When the acrylic acid content continued to increase, the saturation swelling of the hydrogel gradually decreased and the hydrogel appeared phase separation and turned to opaque with the ratio of 4:1. The reason for this phenomenon could be ascribed to the poor ionization capacity of the acrylic acid. When the content of the acrylic acid is low, it mainly exists in the polymer chains in the form of acrylic acid, and the hydrogen bond plays a dominant role in its non-covalent bond interaction. When its content increases to a certain proportion, the content of the ionized anionic monomer increases to form a strong ionic bond with cationic monomer, which reduces the free movement ability of the polymer chain, forms the polymer crystal region, and initiates the phase separation which makes the hydrogel white. Compared to the much larger pore sizes of the other hydrogels, the pore size of the white one with the molar ratio of cation and anion monomers of 4:1 is significantly decreased, and the obvious polymer aggregation area can be found through the field emission scanning electron microscope (FE-SEM) observation, implying the formation of a phase separation area between multiple crosslinked grids. [28] Furthermore, according to the water content of the hydrogel obtained by the mass relationship between the lyophilized hydrogel and the fully swollen hydrogel (Figure 1d), the white hydrogel with a molar ratio of cation and anion monomer of 4:1 has about 70% water content, while the others have more than 90% water content, of which the hydrogel with a molar ratio of monomer of 1:2 has the highest water content, reaching to 97.3%. The hydrogel with relatively low water content is conducive to the construction of a more stable sensor and the white hydrogel as the sensing substrate of a visual sensor is more favorable to the observation of the surface signal. Therefore, the white hydrogel with a molar ratio of cation and anion monomer of 4:1 is intentionally selected as the sensor substrate.

Formation Mechanism of the PAA-IL Hydrogel
To investigate the influence mechanism of the ionic monomer on the properties of the PAA-IL hydrogel, the interaction between polymer and water in aqueous environment was simulated by molecular dynamics. The water distribution in the polymer network can be studied through the radial distribution function g(r), which gives the local density of water molecules at the distance from the polymer model r divided by the average density of the whole system. In a homogeneous system, g(r) will gradually be unified at a distance. In the PAA polymer system (Figure 2a), it can be found that the water molecules begin to distribute around the polymer at 1.45 Å, indicating that the distance here exceeds the characteristic size of AA monomer, and the interaction with water begins to appear. The extreme value of g(r) at 1.75 Å indicates that this is the first hydrated shell of PAA, and its interaction mainly exists in the form of hydrogen bond. The density of water molecules began to increase at 2.05 Å, mainly because the water molecules were in a relatively free state due to the van der Waals force interaction exceeding the hydrated shell of the polymer. It can be seen that poly-ionic liquid (PIL) has no obvious first shell formed by the interaction between polymer and water, and its water starts to appear at 1.825 Å, which is significantly smaller than that of the van der Waals force in PAA (Figure 2b). Considering that the molecular size of [VBIM]Br monomer is significantly larger than that of the AA monomer, the relative distance of this value is even lower. This demonstrates that there is a strong interaction between the bond between PIL and water, and proves that the ion-dipole interaction of imidazolyl monomers enhances their interaction. The ion-dipole interaction is essentially an orientation force, and its existence will significantly enhance the intermolecular van der Waals force, which is the reason why the g(r) of PIL does not appear at the characteristic extreme point of the first water shell but reduces the interaction distance between water molecules and polymers. The PAA-IL system has the extreme point at 1.775 Å brought by the characteristic hydrogen bonded water shell of the PAA system (Figure 2c). At the same time, the water shell dominated by the van der Waals (vdW) force between molecules appears at 2.025 Å, which is shorter than the distance under the pure AA monomer, indicating that the imidazolyl cations do contribute to the interaction in the system where the anionic and cationic ionomers coexist. The extreme values of the radial distribution function at 253 and 323 K are basically the same as that at 298 K ( Figure S2, Supporting Information), indicating that these hydration processes are temperature-stable in a short distance. The energy analysis of the three polymer models was simulated at 253, 298, and 323 K with a relaxation of 20 ns (Figures S3-S5, Supporting Information). It can be seen that the interaction between the three polymer models and water is dominated by the Coulomb force, while the L-J potential occupies a secondary position. This phenomenon comes from the fact that hydrogels are formed by the anionic and anionic monomers AA and [VBIM]Br, and the interaction between the charged monomers and water mainly comes from the electrostatic force. Through the average noncovalent interactions (aNCI) analysis of the PAA-IL molecule (Figure 2d), it can be seen that the main part of the hydrogen bond interaction with water in the hydrogel is the carboxyl segment (blue region) of AA monomer, while the vdW potential is distributed in the other regions of the molecule. The dynamic energy analysis of the molecular model shows that the Coulomb and L-J potential of the PIL system are much larger than those of the other two systems, because its molecular weight and molecular volume are significantly larger than those of the other two molecules. To measure the distribution of the intermolecular forces more accurately, the Coulomb force is averaged over the system charge and the L-J potential is averaged over the molecular surface (the ratio of the system interaction energy to the area of vdW). After molecular surface averaging, the average L-J potential of PIL is higher than that of the other two systems, verifying that the van der Waals force of PIL is stronger than that of the acrylic group (Figure 2e). The main reason is that the ionic dipole interaction between the imidazole cation and water enhances the orientation force. Through the above molecular dynamics analysis, it is shown that after adding the [VBIM]Br monomer, the hydrogel has an obvious improvement in the interaction energy with water, and it is verified that the improvement mainly comes from the vdW force dominated by the orientation force, that is, the contribution of the ion-dipole interaction formed by the imidazole cations to the hydration of the polymer chains.

Open Environment Stability of the PAA-IL Hydrogel
In PAA-IL hydrogel, the main way of network combination is chemical crosslinking through MBAA. In order to analyze the unique properties of PAA-IL hydrogel from the structural point of view and prove this design, the chemical structure of this PAA-IL hydrogel was characterized by Fourier transform infrared (FT-IR) spectroscopy (Figure 3a). Compared with the [VBIM]Br and PAA hydrogel, the C─O stretching bands of AA (1632 cm −1 ) shifted to ≈1702 cm −1 and the corresponding peaks of imidazolium groups shifted from 1467 cm −1 to 1447 cm −1 , suggesting the existence of the ionic associations between the imidazolium and ─COO groups.
Hydrogels are usually made up of large amounts of water, which are evaporated (high temperature) or frozen (low temperature) under environment impact. [29,30] The loss of moisture in these hydrogels will adversely affect the performance of hydrogels, such as mechanical deformation, slow mass transfer, and low mechanical properties. Therefore, the binding effect of hydrogel on water in the disperse medium would affect its application. Due to the hydration feature, the water retention property of the PAA-IL hydrogel was improved. The mechanical properties of the PAA-IL hydrogel stored at different temperatures for 72 h were studied. When the storage temperature increases from −20 to 20°C, the PAA-IL hydrogel shows a higher strain failure point and a maximum tensile stress (Figure 3b). At the loading rate of 15 mm min −1 , the PAA-IL can be stretched to 2.87 times of its original length at 20°C. With the decrease of the storage temperature, the strain index increases to 3.03 times at 20°C and 3.29 times at 20°C. This phenomenon can be explained as the antifreezing property of hydrogel at low temperature, which makes the proportion of crystal water in the hydrogel system increase with the decrease of temperature. The combined hydration of the imidazole cations and the carboxyl anions also brings about a stronger resistance to the tensile damage for the ionic hydrogels. Due to the strong interaction between anion and cation, the ions in the hydrogel network form stable chain entanglement, which is reflected in the linear correlation relationship in the full www.advancedsciencenews.com www.advsensorres.com strain range of the stress strain-curve of the PAA-IL hydrogel, indicating that the hydrogel was endowed with almost completely elastic mechanical behavior with a maximum Young's modulus as 158 kPa.
After 72 h storage at −20°C, the PAA-IL hydrogel still showed good mechanical properties and high elasticity and can withstand various deformations, including bending, torsion, and compression (Figure 3c). Once the external force disappears, the deformed shape recovers rapidly, indicating the high elasticity and good stability at −20°C due to the disperse medium keeping unfrozen. The improvement of the mechanical properties of the PAA-IL is mainly due to the energy dissipation mechanism of phase separation structure formed by ion bond, which is verified by the tensile cycle experiment (Figure 3d). When thehydrogel first stretched to 200% strain, the recovery curve and the tensile curve could not coincide during the recovery process, which means that the hysteresis occurred. As a sacrificial bond, the chemical cross-linked structure in the PAA-IL breaks in the process of stretching, causing external energy dissipation, while the second structure formed by ionic bond can connect the original network through extension. In addition, when the temperature of the PAA hydrogel drops below 0°C in the DMA test, its storage modulus (G') and loss modulus (G"") both rise rapidly to GPa level, which indicates that when the temperature is lower than the freezing point of water, the rigidity of the PAA hydrogel increases significantly (Figure 3e). PAA-IL hydrogel has no obvious modulus change from −15-40°C, and maintains the viscoelastic state. Thus, the PAA-IL zwitterionic hydrogel always has the ability to protect water, demonstrating the good environmental stability in a wide temperature range.
To test the open environment stability, the PAA-IL hydrogel samples were placed together with different saturated salt solutions, and there was no direct contact between each hydrogel sample and the salt solution. It can be seen that when the humidity is more than 33%, the water loss rate of the PAA-IL hydrogel after 3 h of storage at room temperature is less than 0.2, while when the humidity is only 11%, the water loss rate after 3 h is almost 50% (Figure 4a). Higher relative humidity increases the time required for water weight loss, but under all air relative humidity conditions, water weight loss increases linearly with the initial time. It indicates that the PAA-IL is satisfied with the condition of free water evaporation at the initial stage of water loss, that is, when the water content is high, the water molecules in the hydrogel can move freely. [31] The water loss characteristics of the PAA-IL and PAA hydrogels stored at 33% humidity for 48 h were further studied (Figure 4b). The difference in water loss rate between the PAA and PAA-IL hydrogels indicates that the crystalline domain in the PAA-IL hydrogels appears earlier. [32] This phenomenon proves that in the PAA-IL zwitterionic hydrogel, there are bonds and functions different from the formation of hydrogen bonds between PAA chains, and the ion bonds in the hydrogel help to form micro regions that can inhibit water evaporation.
To verify that the ionic bond in PAA-IL hydrogel plays an essential role in protecting the chemical cross-linking structure as a sacrificial bond of the dissipation mechanism during the stretching process, the stability of the resistance change of the PAA-IL hydrogel tested by the stretching cycle is used to explain the recovery ability of the interaction in the polymer after stretching ( Figure 4c). Under 200% and 300% strain, the PAA-IL hydrogel showed rather good stability and recovery ability of the resistance change in 8 cyclic tensile tests (RSD 5.5% and 3% for 200% and 300% strain), which proved that it mainly consumed recoverable ionic bonds in the hydrogel during the non-breaking tensile process, and ionic bonds can be recovered and formed in a short time after the unloading tension (Figure 4d). In addition, the stability of the PAA-IL hydrogel after low-temperature storage was further verified by resistance test. The original resistance value of the PAA-IL hydrogel after storage at −20°C for 12 h and 72 h, as well as the resistance value after 200% stretching are highly consistent with the hydrogel stored at 20°C, which proves that it has excellent environment stability and shows that there is no increase in resistance due to water freezing at low temperature (Figure 4e). For most elastic materials, when the tensile length increases, the cross-sectional area decreases, and the resistance value becomes larger. For the initial resistance R 0 , the initial length L 0 , the resistivity for the PAA-IL hydrogel, and the resistance value R after stretching obey the following formula: From the above formula, it can be seen that within the elastic limit, the resistance change of the PAA-IL hydrogel has a linear relationship with the strain change rate. The linear relationship between the resistance change of the PAA-IL hydrogel stored at −20°C for 72 h and the strain with a fitting coefficient R 2 as 0.9707, which indicates that the PAA-IL hydrogel stored at low temperature still has a good elasticity, verifying the excellent environment stability (Figure 4f).

Chemical Sensing Performances Employing the PAA-IL Hydrogel Substrate
For most chemical sensors, the structural stability of the substrate is the primary consideration, and it is necessary to study the properties of hydrogel in the presence of ions. The selfassociation effect in the zwitterionic hydrogel will be weakened due to the presence of ions (Figure 5a). The electrostatic interaction between the positive and negative charges in the polymer is to some extent shielded by ions, which leads to the relaxation, volume expansion, and even dissociation of the polymer chain segments. [33,34] To prove the structural stability of the PAA-IL, the morphology after full swelling in pH 1-14 solution was tested (Figure 5b). The morphology of the PAA-IL hydrogel does not change significantly in the range of pH 2-12, and the swelling ratio is between 3.1 to 6.7. In strong acid and alkali solutions, the swelling ratio and volume of the PAA-IL hydrogel increased significantly, but it still maintains a complete white appearance and the elastic structure, indicating its environment adaptability as a chemical sensor substrate. The detection reagents of MnO 4 − (consisting of NaOH and Na 2 S 2 O 3 ) and NO 3 − (consisting of H 2 SO 4 and diphenylamine) were selected to functionalize the PAA-IL hydrogel and verify its stability as the sensor substrate. It can be seen that the hydrogel sensor shows a similar response rate (Figure 5c) at 25 and −20°C regardless of whether it carries acidic or alkaline reagents. From the color value analysis, it can be seen that the R-values of the two hydrogel sensors have a decreasing trend at different temperatures, corresponding to the ongoing the green and blue chromogenic reaction with MnO 4 − and NO 3 − , respectively (Figure 5d). To better illustrate the applicability of the PAA-IL hydrogel as a sensing substrate, a colorimetric fluorescence dual-mode probe (7-amino-4-methylcoumarin) for nitrite detection was used to functionalize the hydrogel. With the increase of the nitrite concentration, the hydrogel sensor shows obvious yellow deepening (colorimetric channel, Figure 5e-i) and brightness reduction (fluorescence quenching channel, Figure 5e-iii). The difference processing between the sensing images of different concentrations and the initial images shows that there are obvious color generation (Figure 5e-ii) and brightness changes (Figure 5e-iv). The B-value of the difference image contrast in the colorimetric chan-nel and the L (lightness) value of the difference image contrast in the fluorescence quenching channel both show good linearity in the nitrite concentration range of 0.5-6 mm (Figure 5f). This result demonstrates that the PAA-IL hydrogel is not only applicable in a wide range of chemical environments, but also can realize semi-quantitative analysis by non-destructive mapping of the optical images to the concentration of the analytes. Thus, this feature makes the PAA-IL hydrogel of great significance for the construction of highly sensitive and environmentally stable chemical sensors.

Conclusion
In summary, the interaction between water molecules and polymer skeleton is fully utilized to endow the zwitterion hy- drogel with environment stability through the construction of a zwitterion hydrogel with multi-hydration. Anionic and cationic monomers anchor water molecules through hydrogen bonds and ion-dipole interactions respectively, and intertwine each other through ionic bonds to form a unique hydrogel structure. The radial distribution function, average interaction energy, and binding energy of water molecules show that the PAA-IL structure greatly enhances the van der Waals force and the combination of the hydrogen bonds. The binding energy between the PAA-IL hydrogel and water molecule is −23.41 kcal mol −1 , which is much higher than those of PAA (−6.85 kcal mol −1 ) and PIL (−18.77 kcal mol −1 ), demonstrating that the PAA-IL has rather strong binding ability to water molecules. It has also been proven that the PAA-IL hydrogel has consistent elastic properties in the non-breaking tensile range, and excellent environment stability.
When employing the PAA-IL as the substrate of chemical sensors, the excellent structural stability ensures the good sensing performance to MnO 4 − and NO 3 − at 25 and −20°C, as well as the great potential for quantitative analysis both in the colorimetric and fluorescent channel. We expect that the present design of zwitterionic hydrogels could shine light on the exploration of anti-freezing hydrogels and hydrogel-based chemical sensors.
Computational Method: To study the interaction strength of anionic and cationic monomers with water molecules, three short chain models consisting of three monomers, PAA, PIL, and PAAIL, were respectively constructed. Under the premise of fully considering the influence of hydrogen bond as long-range dispersion, the B3LYP functional with BJ damping D3 dispersion correction. [35,36] The structure of PAA, PIL, PAA-IL, and the model of their combination with water molecules were optimized under Def2-SVP, and the optimized configuration was frequency-analyzed to exclude the virtual frequency configuration. [37,38] Then, 4-zeta basis setDef2-QZVP was used to calculate the single point energy of the optimized configuration to minimize the basis set superposition error (BSSE), and the binding energies of three polymer short chain models and water molecules were obtained.
All molecular dynamics simulations were completed using the GRO-MACS 2021.5 software package. [39,40] The generalized Amber force field parameter was used to describe the bonding and non-bonding interactions of all molecules in the system. [41] The Gaussian 09 software package was used to optimize the molecules of the system at the B3LYP/6-311G (d, p) calculation level, [42][43][44][45] and Multiwfn software was used to fit the electrostatic potential atomic charge in the form of restricted fitting electrostatic potential (RESP). [46] SPC/E model was adopted for water molecules and the initial configuration was constructed with the PAA/PAA-IL/PIL model at the center of 7 nm cubic box filled with water molecules. [47] The charge of the system was ensured to neutral state by filling Br − according to the charged condition.
Before simulation, the conjugate gradient method was used to minimize the energy and eliminate the inferior contact in the system and then run for 1 ns NPT simulation. The time coupling constants of Velocity-rescale method and Berendsen method were 0.2 and 0.5 ps, respectively. [48,49] Finally, the Velocity-rescale and the Parrinello-Rahman method were used for 20 ns NPT simulation, [50] and the coupling time constants were 0.2 and 2.0 ps, respectively. The generating phase of the final equilibrium state was obtained and the data were analyzed and processed.
Particle-mesh Ewald (PME) method was used to deal with long-range electrostatic interaction, [51] and the truncation method was used to calculate the van der Waals interaction, with the truncation distance of 1.0 nm. When calculating the interaction energies of PAA/PAA-IL/PIL with water respectively, the truncation method was used with a truncation distance of 3.3 nm. When calculating the average non covalent interaction between PAA and water, a 10 ns restricted NVT simulation was carried out for PAA. The coordinates of PAA were frozen in the center of the 3 nm cubic box. Multiwfn software was used to calculate the grid data of aNCI isosurface, and VMD software was used to render the drawing. [52] Synthesis of 1-Vinyl-3-Butylimidazolium: 1-vinyl-3-butylimidazolium was synthesized by adding 1-vinylimidazole (4.70 g, 0.0494 mol) and n-butyl bromide (6.85 g, 0.05 mol) into a round-bottom flask and heated at 60°C for 24 h with refluxing. The product was washed three times with ethyl acetate and steamed in a rotary evaporator after adding dichloromethane. Then the transparent solution was dried to get the crude product. Finally, the crude product was washed with ethanol for three times and dried at room temperature to obtain the white product powder ( 1 H NMR spectrum in Figure S6 Synthesis of the PAA-IL Zwitterionic Hydrogel: 1-vinyl 3-butyl imidazole and acrylic acid in different proportions were added into the beaker with water, stirring until the mixture was completely dissolved. Then, 0.3% cross-linking agent and 0.2% thermal initiator (azodiisobutyronitrile thermal initiator was a pre-prepared ethanol solution) and part of PVP were added to the mixture. The mixture was purged with nitrogen for 30 min and poured into a glass mold and heated at 70°C for 3 h to fabricate the PAA-IL hydrogel. Fourier transform infrared (FT-IR) spectra of the hydrogels were recorded by the FT-IR spectrometer (PerkinElmer Frontier) with a smart orbit diamond crystal ATR attachment to characterize the hydrogel composition.
Synthesis of PAA and PIL Hydrogels: 1.4 g acrylic acid was added into the beaker with 10 mL water. Then, 0.3% cross-linking agent and 0.2% thermal initiator (azodiisobutyronitrile thermal initiator was a pre-prepared ethanol solution) and part of PVP were added to the solution. The mixture was purged with nitrogen for 30 min and poured into a glass mold and heated at 70°C for 3 h to fabricate the PAA hydrogel; 1 g 1-vinyl 3butyl imidazole was added into the beaker with 10 mL water, stirring until the mixture was completely dissolved. Then, 0.3% cross-linking agent and 0.2% thermal initiator (azodiisobutyronitrile thermal initiator was a preprepared ethanol solution) and part of PVP were added to the solution. The mixture was purged with nitrogen for 30 min and poured into a glass mold and heated at 70°C for 3 h to fabricate the PIL hydrogel.
Scanning Electron Microscope Test of the Hydrogels: All samples were pretreated by freeze-drying machine (SCIENTZ-10ND) for 48 h to completely remove the water in the hydrogel and maintain the original shape of the polymer skeleton. Field emission scanning electron microscope (FE-SEM, JEOL JSM-7610F Plus) operating at 4.0-6.0 kV was performed on the freeze-dried hydrogel to characterize the network morphology of the hydrogel.
Mechanical Property Test of Hydrogels: Dynamic Thermomechanical Analysis (DMA, NETZSCH DMA 242 E) was used to measure the mechanical properties of the hydrogels with temperature change. The tensile mode was selected for the test, the preload was 0.01 N, the frequency was 1, 2, 5, 10 HZ, the strain was 1%, the scanning temperature range was −15-45°C, and the scanning rate was 20°C min −1 . A universal testing machine (MTS C43-104) was used to measure the mechanical properties of the PAA-IL hydrogel. The tensile test sample was rectangular, the length of the working area was 30 mm, the width was 7 mm, the thickness was 0.8 mm, and the tensile rate was 30 mm min −1 . During the cyclic test, the test was resumed for 2 min in a humid environment after each stretch.
Electrical Performance Test of Hydrogels: The resistance test of hydrogels was measured by a digital source meter (KEITHLEY 2636b). The hydrogel sample was cut into rectangles of 15 mm long, 5 mm wide, and 0.8 mm thick. The stretching was controlled by the marked position of the mold with different lengths. The resistance change of the sample was detected in real time using a digital source meter. The resistance change of the sample was represented by R/R 0 , where R 0 was the self resistance of the hydrogel when it relaxes.
Water Content of the PAA-IL Hydrogels: The ratio of water vapor pressure p to the saturated water vapor pressure p 0 was designated as the relative humidity (RH). Known water vapor pressures can be generated by saturated aqueous salt solutions. [53] The hydrogel samples were equilibrated with air of known water vapor pressure in a large sealed Petri dish, with no direct contact between each hydrogel sample and the salt solutions. The saturated aqueous salt solutions and hydrogel samples were separately placed in small open Petri dishes. One large Petri dish contained six small dishes, from which two dishes were hydrogel samples and the other four dishes were salt solutions to ensure that a constant vapor pressure was maintained. The time scale shown in the final results and data analysis was the real recording time, including the opening disturbance time and weight measurement time. A digital hygrometer was used to check the RH of the air in contact with the saturated salt solutions.
Preparation of the Functionalized Hydrogels with Reagents: The reagent for MnO 4 − detection was prepared by mixing 10 mL 1 m NaOH solution and 0.01 mol Na 2 S 2 O 3 with a reaction principle that Mn (VII) was reduced to Mn (VI) under alkaline conditions, showing green color. The reagent for NO 3 − was prepared with H 2 SO 4 and diphenylamine, showing blue color. [54] The reagent for nitrite was prepared with AMC, ethanol, and acetic acid, showing yellow color and fluorescence quenching. The PAA-IL hydrogels were soaked in different reagents for 12 h and then were wiped and stored hermetically.
Functionalized Hydrogel Detecting MnO 4 − and NO 3 − Solution: 1 L MnO 4 − and NO 3 − solution were measured and dropped to the function-alized hydrogels with MnO 4 − and NO 3 − reagent, respectively. The optical images and fluorescent images before and after the reaction of the functionalized hydrogels were videoed with an iPhone 12 camera and screenshotted by PotPlayer. The room temperature tests were conducted at 20°C and 15% humidity, and the low temperature tests were conducted at −20°C freezer.
Functionalized Hydrogel Detecting Nitrite Solution: 1 mL of sodium nitrite solutions (0.5, 1, 2, 4, 6, and 10 mm) were measured and dropped to the centrifuge tube by pipette gun respectively, and then the functionalized hydrogels with nitrite reagent were put into the centrifuge tube with 30 s reaction. The optical images and fluorescent images before and after the reaction of the functionalized hydrogels were recorded with an iPhone 12 camera under the irradiation of LED and 365 nm ultraviolet light, and the temperature and humidity during the test was 20°C and 15% respectively.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.