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Article

Research on a Humidity Sensor Based on Polymerizable Deep Eutectic System-Modified Filter Paper

by
Mengyao Shen
1,
Bo Zhang
1,*,
Qi Lu
1,
Yanan Xiao
1,
Hao Shen
2,3,
Yi Ni
1,
Yuechen Liu
3 and
Haitao Song
2,3,*
1
Engineering Research Center of IoT Technology Applications (Ministry of Education), School of Integrated Circuits, Jiangnan University, Wuxi 214122, China
2
Shanghai Artificial Intelligence Research Institute Co., Ltd., Shanghai 200240, China
3
Artificial Intelligence Technology Center, Institute of Translational Medicine, Shanghai Jiao Tong University, Shanghai 200240, China
*
Authors to whom correspondence should be addressed.
Chemosensors 2025, 13(9), 354; https://doi.org/10.3390/chemosensors13090354
Submission received: 9 August 2025 / Revised: 12 September 2025 / Accepted: 17 September 2025 / Published: 22 September 2025
(This article belongs to the Section Nanostructures for Chemical Sensing)
Browse Figures
Versions Notes

Abstract

In recent years, paper-based humidity sensors have emerged as a highly promising technology for humidity detection. In this work, a polymerizable deep eutectic solvent (PDES) was prepared via a one-step blending method, which was applied to modify filter paper. The modification process did not alter the overall structure of the paper cellulose but rather targeted only its internal cellulose channels, thereby minimizing any impact on the paper’s original moisture-independent properties. The filter paper functioned both as the substrate and the humidity-sensing material in the fabricated sensor. The finger-like electrodes were designed using AutoCAD 2018 software and then printed onto the modified paper using screen-printing technology to fabricate the humidity sensor. Different saturated salt solutions were used to simulate corresponding humidity environments and evaluate the humidity performance of sensors. Compared with that of the blank paper-based humidity sensor, the sensitivity of the sensor modified by the PDES was significantly greater, and the recovery time was greatly shorter. Specifically, the sensitivity increased from 1.34 to 10.36 at 54% RH and from 166.24 to 519.2 at 98% RH. Additionally, the sensor response time was reduced from 728 s to 137 s. PDES modification significantly improved the moisture-sensitive characteristics and detection performance of the sensor.

1. Introduction

Humidity sensors play an important role in environmental monitoring, agriculture, construction, and industry. Flexible devices offer numerous advantages, particularly in applications such as wearable electronics [1], smart sensors [2], and biomedical technologies [3]. However, the increasing number of flexible electronic devices produces much nondegradable plastic waste, causing damage to the environment on which human beings depend [4].
To better fit the concept of sustainable development, scientific researchers have begun to look for “green” substrates to replace existing plastics to fabricate environmentally friendly flexible electronic devices [5]. Shen et al. [6] developed a multifunctional biocompatible hydrogel–based e-skin I–skin (P(AA−APA)−Fe3+), which can be attached to fingers, wrist joints, etc., and used to monitor joint movement during daily activities. In 2017, Tao et al. [7] mixed paper towels with a graphene oxide solution and finally obtained conductive reduced graphene oxide paper, which could monitor the respiratory rate by measuring humidity changes. In 2020, Pataniya et al. [8] proposed an efficient and low-cost manufacturing strategy using filter paper and tissue paper for multifunctional sensing applications. Paper-based humidity sensors provide a lightweight, inexpensive, and easy-to-make humidity monitoring solution by taking advantage of the hygroscopic properties and printable circuit technology of paper [9].
Paper is made up of cellulose [10], and the functional groups of cellulose molecules are easily chemically modified, thus forming multifunctional semicrystalline fibers [11,12]. The modification of cellulose can be divided into physical modifications and chemical modifications. Chemical modification typically targets the abundant hydroxyl groups in the cellulose chain, functionalizing them with various organic compounds as needed. Nonpolar or hydrophobic groups may be introduced to enhance interfacial compatibility in composites, while positively or negatively charged groups can also be added to improve cellulose dispersion in solvents [13,14].
Deep eutectic solvents (DESs) are new types of liquid solvents associated with hydrogen bond donors (HBDs) and hydrogen bond acceptors (HBAs) through internal hydrogen bonding and have the characteristics of environmental protection, designability, and biocompatibility [15]. PDESs stands for Polymerizable Deep Eutectic Solvents, which are a type of functional material that introduces polymerizable functional groups (such as double bonds, epoxy groups, etc.) to DESs or is formed through polymerization reactions. Its core features combine the green solvent properties of DESs with the structural stability and processability of polymers, enabling the dissolution or loading of various substances through hydrogen bonding and triggering polymerization under conditions such as heat, light, or initiators to form polymers or polymer-based composites with specific structures. It has widespread applications in fields such as sensors, electrolytes, adsorbent materials, and catalytic carriers—for example, in modified filter paper and the preparation of electrolytes for flexible electronic devices. At present, DESs have been extensively applied in many fields, such as biomass pretreatment [16], extraction [17], catalysis [18,19], and biomedicine [20]. Compared with traditional ionic liquids, DESs not only have comparable conductivity and thermal stability but are also more cost-effective and environmentally friendly. Not only do PDESs possess the characteristics of DESs, but also the solid or quasi-solid structure formed after their polymerization makes them more stable than DESs in high-temperature and high-humidity environments [21]. PDESs contain polymerizable functional groups, which can introduce more functional groups through polymerization reactions, thereby enabling precise control of material properties. For instance, the degree of polymerization and crosslinking density of PDESs can be adjusted to optimize the material’s ion conductivity, gas separation selectivity, etc. The functional regulation of DESs is relatively limited. In some specific applications, PDESs can exhibit unique advantages. Therefore, PDESs can be used as alternatives to ionic liquids to enhance the hydrophilicity of materials and improve their moisture sensitivity. Similarly, PDESs also maintain their stability in high-humidity environments. PDES monomers are typically synthesized by substituting HBDs or HBAs with one or more polymerizable monomers. Hou et al. [22] combined PDESs and cellulose to make ionic conductive elastomers via complex crosslinking methods. The crosslinker N,N′-methylene bisacrylamide and the photoinitiator 2959 were added to the prepolymer solution in the dark, followed by ultraviolet irradiation to synthesize the PDES–cellulose paper composite material (PP3−E). In PP3−E, PDESs formed a conductive pathway inside the cellulose paper to improve the sensitivity of the cellulose paper. However, the polymerization process in PDES synthesis is complex and difficult to control, so the application of PDESs is greatly limited.
For the fabrication of paper-based electrodes, the hygroscopicity, conductivity, and stability of the paper must be considered. The choice of electrode materials is critical, with carbon-based materials [23], metallic materials [24], and conductive polymers [25] being commonly used. Analisa et al. [26] developed a simple method for fabricating flexible printed electronics by writing on paper with a ballpoint pen filled with conductive silver ink. Yuan et al. [27] prepared a low-cost carbon black (CB) conductive paper by drip coating, but the use of copper tape as an electrode still had certain influences and limitations to its flexibility. Xue et al. [28] developed a flexible temperature sensor by utilizing highly heat-sensitive graphene nanoribbon (GNR) ink, which could be directly written onto common paper or applied via spraying with a mask. The sensor was also humidity sensitive and could effectively monitor the respiratory rate owing to its fast response, high-precision resolution, and excellent flexibility. Kim et al. [29] demonstrated that black conductive ink (BCI) could be used for writing and drawing traditional handmade Korean paper (Hanji) as high-performance electrodes. This method provides a new way to draw or print electrodes directly on modified filter paper, and other commonly used methods include inkjet printing [30,31], screen-printing [32,33], and coating methods [34,35].
In order to enhance the hydrophilicity and moisture sensitivity of paper while also considering environmental friendliness and simplifying the manufacturing process of sensors, in this study, a PDES was employed to modify filter paper, serving both as the sensitive material and the substrate on which silver electrodes were screen-printed. In PDES synthesis, ChCl increases the hydrophilicity of PVA, which is advantageous for humidity sensitivity. The screen-printing technique is simple and can preserve the original form of the paper to the greatest extent possible. Compared with the blank filter paper sensor, the modified humidity sensor exhibited a nearly tenfold improvement in humidity sensitivity in the midhumidity range. In the highhumidity range, the sensitivity increased fourfold. Additionally, the response time was reduced to one-sixth of that of the original sensor. The PDESmodified paper-based humidity sensor demonstrated high sensitivity and enhanced overall performance, underscoring the effectiveness of PDES modification for humidity-sensing applications.

2. Materials and Methods

2.1. Materials

The materials used in this work included the following chemical reagents: sodium chloride (NaCl, AR, ≥99.5%), lithium chloride (LiCl, AR, 98.0%), potassium chloride (KCl, AR, ≥99.5%), potassium sulfate (K2SO4, AR, ≥99.0%), nickel (II) chloride hexahydrate (NiCl2·6H2O, AR, 99.0%), sodium bromide (NaBr, AR, ≥99.0%), magnesium chloride anhydrous (MgCl2, AR, ≥99.0%), zinc sulfate heptahydrate (ZnSO4·7H2O, AR, ≥99.0%), polyvinyl alcohol (PVA, type 1799, AR, 99.0%), choline chloride (C5H14ClNO, AR, 99.0%), silver oil, and deionized water. The diameter of the filter paper was 7 cm. The template of the interdigital electrode used in screen printing was designed via Auto–CAD 2018 software.

2.2. Materials Preparation

The PDES was easily prepared via a one-step blending method using polyvinyl alcohol (PVA) and choline chloride (ChCl) as the hydrogen bond donor and acceptor, respectively, thereby avoiding the need for a complex polymerization process.
First, 5 g of PVA and different amounts of ChCl were stirred and heated in a 95 °C water bath for 120 min to synthesize different types of PDES solvents. Then, blank filter papers of the same specifications were immersed in the prepared PDES solutions and treated in a water bath at 65 °C for 60 min. Afterward, the modified filter papers were removed and baked in an oven at 80 °C for 120 min to obtain the final products.

2.3. Sensor Fabrication

The humidity sensor was fabricated by printing silver interdigital electrodes on the paper surface using a screen-printing technique. The sensor was placed in an oven at 90 °C for 30 min after printing, and the silver electrode was heatcured. The process of sensor fabrication described above is summarized in Figure 1. The humidity sensors whose precursors contained 1 g, 1.5 g, 2 g, 2.5 g, and 3 g of ChCl were named PDES−1, PDES−2, PDES−3, PDES−4, and PDES−5, respectively.
Figure S1 presents a comparison of the macroscopic morphology before and after the modification of the filter paper. On the left side of the image, the filter paper underwent water treatment in the experiment, while on the right side it withstood PDES modification. From the magnified area, it can be observed that the color of the PDESmodified filter paper surface slightly lightened. The number of fibers decreased, and it presented a slight semi-transparent state. These phenomena collectively indicate that PDES successfully achieved an effective combination with the filter paper matrix. However, it is worth noting that the aforementioned changes mainly occurred at the micro level, while the overall macroscopic structure and appearance of the filter paper were maximally preserved before and after the modification.
Figure S2 shows physical images of sensors based on the blank and five modified filter papers. To objectively investigate the effect of PDES modification on sensor performance, the template for printing the interdigital electrode for all the sensors was fixed such that the size of each sensor was the same.

2.4. Characterizations

Morphological characterization and elemental analysis were performed via field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDS) on a scanning electron microscope (Hitachi SU1510, Tokyo, Japan). The Fourier Transform–Infrared Spectroscopy (FT-IR) analysis was performed on the infrared spectrometer equipment (Thermo Fisher Scientific Nicolet iS20, Waltham, MA, America). The hydrophilic contact angle test was conducted using the video contact angle measuring instrument (Dingsheng JY−82C, Chengde, China). The C−V curve was measured using the electrochemical workstation (Chenhua CHI600E, Shanghai, China).

2.5. Measurement of Humidity Sensors

In the humidity measurement experiment, the controlled humidity environment was provided by saturated salt solutions in a sealed container, with each solution corresponding to a specific relative humidity. The prepared sensors were placed in these environments and connected to the testing instrument to evaluate their performance under different humidity conditions.
All the sensors were tested at a sinusoidal voltage of 1 V with no DC bias. The ReZ–ImZ curves of each humidity sensor were recorded over a frequency range of 100 Hz to 20 MHz, while other performance tests were conducted at 1000 Hz. The humidity sensitivity of the paper-based humidity sensors were all measured using the Keysight E4990A inductance impedance analyzer. All tests were conducted at room temperature (25 °C).

3. Results and Discussion

3.1. Morphological Features

To investigate the morphological changes on the cellulose surface following chemical modification with the PDES, SEM was employed. As shown in Figure 2a, the filter paper’s cellulose structure exhibits a complex network, with the surface displaying microscopic burr-like particles. Figure 2b–f shows the cellulose structure of the modified paper, and compared with that of the blank filter paper, the original cellulose distribution of the modified filter paper does not change much. Wrinkles and membrane-like structures can be observed within and on the surface of the cellulose channels, respectively. The synthesized PDES displays a transparent membrane structure under SEM, which acts not only on the cellulose channels to make them smoother but also on the cellulose walls to make them more transparent. This indicates that the filter paper was successfully modified, with internal structural channels altered due to PDES attachment [36,37]. As shown in Figure 2d, the fibers appear smoothest and most transparent, suggesting the most pronounced adsorption effect after modification. These findings indicate that the PDES ratio under PDES−3 synthesis has the best effect on the chemical modification of the cellulose structure inside the filter paper, and the humidity-sensitive performance of PDES−3 can be especially focused on.
High-magnification SEM images provide a clearer view of the changes in cellulose channels. Figure 3 presents the images of the blank filter paper and PDES−3. As shown in Figure 3a, the average channel width in the blank filter paper is significantly narrower than that in PDES−3 (Figure 3b), with two representative channel widths annotated for comparison. This finding indicates that PDES modification increased the width of the cellulose channels inside the filter paper. Furthermore, the comparison between blank paper and PDES−3 reveals the presence of smooth folds in the modified channels and a reduction in burr-like fibers at the outer edges of the channels. The cellulose channels exhibit transparent membrane-wrapping characteristics, and the mesh spacing of the modified network structure decreases. These changes enhance the moisture absorption performance of the filter paper.
The EDS experiment in Figure S3 provides a better illustration of the elemental distribution within PDES−3. The composition of cellulose is relatively simple and is composed of three main elements (C, H, and O), whose chemical formula is (C6H10O5)n, which are bound together by specific chemical bonds to form a unique polysaccharide structure. Figure S3a shows the overall element distribution, and Figure S3b,c show the distributions of C and O, respectively. The cellulose channel is apparently mainly composed of C and O. The distribution of Cl in Figure S3d is derived from ChCl, one of the PDES precursors, so it can directly show the modification effect of filter paper by the PDES. The results show that PDES modification acts not only on the cellulose channels but also on the network structures formed by their intersections, making them more compact.

3.2. Structural Characterization

By detecting the absorption peaks of different wavelengths of light spectra, the different functional groups or molecular bond types present in the sample can be determined. The FT-IR spectroscopy of Blank, PDES, PDES−3, and PDES−5 is shown in Figure 4.
First, the chemical modification effect of PDES on the filter paper was evaluated. As shown in Figure 4a, it is observed that the blank filter paper has a −OH stretching vibration peak at 3328.05 cm−1. After the optimized PDES modification, this peak shifts towards higher wavenumbers to 3330.46 cm−1. This shift in the characteristic peak indicates the formation of hydrogen bond interactions between the hydroxyl groups of the PDES and those of the filter paper cellulose, thereby confirming the successful chemical modification of the filter paper surface [38]. The blank filter paper exhibits a characteristic peak of C−O−C stretching vibration at 1103.57 cm−1 and a characteristic peak of C−O stretching vibration at 1025.46 cm−1. After PDES modification, the peak position at 1103.57 cm−1 remains unchanged, while the peak position at 1025.46 cm−1 shifts by 0.48 cm−1 towards a higher wavenumber to 1025.94 cm−1. This provides strong evidence for the successful loading of PDES carbon–oxygen bonds and their interaction with the carbon–oxygen bonds of the filter paper. At the same time, it also confirms that the original carbon–oxygen bond structure of the filter paper has not been damaged [39]. In conclusion, the analysis results of the characteristic peaks related to hydroxyl groups and carbon–oxygen bonds in the infrared spectrum indicate that the PDES successfully achieves chemical modification of the filter paper, and the original structural integrity of the filter paper is effectively maintained.
Furthermore, this study conducted a further analysis of the FT-IR spectra to support the successful synthesis of the PDES. When investigating the intermolecular interactions between polymers, as shown in Figure 4b, ChCl exhibits a characteristic peak at 875 cm−1, which belongs to the asymmetric stretching vibration of NC4 [40]. Due to the formation of hydrogen bonds between ChCl and PVA, this interaction alters the electronic environment and bond strength constants of the NC4 group, resulting in a redshift of the characteristic peak from 876.93 cm−1 to 874.88 cm−1. As shown in Figure 4c, the stretching vibration absorption peak of the methine group (−CH−) of PVA is located at 2900 cm−1. When PVA is mixed with ChCl, the chloride ions in ChCl form hydrogen bonds with the hydroxyl groups of PVA, restricting the vibration freedom of the methylene group, thereby causing a redshift of this peak. For example, as the content of ChCl increases, the peak position shifts from 2900.90 cm−1 of PDES−3 to 2900.41 cm−1 of PDES−5. Additionally, the absorption intensity at 3020 cm−1 can be attributed to the methyl group (−CH3) of ChCl [38], and its intensity increases with the increase in the amount of ChCl added. Based on the above FT-IR spectral results, it can be confirmed that PVA and ChCl form intermolecular hydrogen bonds and successfully transform to PDES.

3.3. Humidity-Sensing Performance

The hydrophilic contact angle test quantifies the interaction between the surface of the device and water, which can reveal the core mechanism of humidity sensitivity, quantify the humidity sensitivity performance, verify the humidity sensitivity stability, and provide a crucial basis for evaluating the humidity sensitivity characteristics of the device. As shown in Figure 5a, the hydrophilic contact angle of the sensor made from blank filter paper is 71.74°. Figure 5b,c are the hydrophilic contact angle test diagrams of PDES−3 and PDES−5, respectively. During the process of manufacturing the sensor, the conductive ink was screen-printed and then subjected to thermal curing. This process had an impact on the hydrophilic property of the filter paper. The hydrophilic contact angle of the filter paper is relatively large, at 71.74°. Its surface has a relatively weak ability to adsorb water molecules. After the optimal modification, the hydrophilic contact angle of the sensor PDES−3 decreases to 52.83°, indicating enhanced hydrophilicity of the material. As shown in Figure 5c, the hydrophilic contact angle of the PDES−5-modified filter paper is 57.85°. It is worth noting that compared with the PDES−3, the hydrophilicity of PDES−5 has weakened, indicating that PDES-3 exhibits better performance in enhancing the hydrophilicity of the filter paper. However, compared with the blank filter paper shown in Figure 5a, the contact angle of PDES−5 still significantly decreases, confirming that its hydrophilicity is also effectively improved. By comparing the results of Figure 5a with those of 5b and 5c, it can be observed that regardless of the type of PDES used for modification, the sensors obtained have significantly smaller hydrophilic contact angles compared to the unmodified filter paper, and their humidity sensitivity is significantly enhanced. This provides strong evidence for the significant effect of PDES modification on improving the humidity sensitivity of filter paper.
The humidity sensitivity characteristics of the manufactured sensors were evaluated by measuring the impedance modulus under different RHs at room temperature (Figure 6). The formula for calculating sensitivity is Z0/Z, where Z0 and Z represent the impedance modulus values of the device at 11% RH and the target RH, respectively. As shown in Figure 6, PDES−3 exhibits the most favorable humidity-sensing performance. According to the measurement data, the sensitivities of the blank filter paper and the PDES−3 sensor at 54% RH are 1.34 and 10.36, respectively, whereas at 98% RH (high humidity), they reach 166.24 and 519.2, respectively. For other sensors obtained in this study, such as PDES−4, the sensitivities under medium- and high-humidity environments are 2.09 and 411.14, respectively, while for PDES−5, they are 1.23 and 253.53, respectively. In summary, the greatest improvement in sensitivity is achieved by PDES−3, with a nearly eightfold increase in sensitivity under moderate-humidity conditions and a significant improvement under high-humidity conditions.
In addition to PDES−3 exhibiting the most significant improvement in humidity sensitivity, Figure 6 also indicates that the impedance modulus changes in the other modified sensors are more pronounced than those of the blank filter paper under the same humidity variation. Within the range of 33% RH to 85% RH, the humidity sensitivity is significantly improved, indicating that the PDES modification notably enhances the humidity-responsive characteristics of filter paper, particularly in medium-humidity environments. This transformation effectively elevates the originally low-sensitivity filter paper into a high-sensitivity device, thereby broadening its practical application in the field of humidity sensing.
The response/recovery time is also a crucial indicator for evaluating the performance of humidity sensors and is generally defined as the time when the impedance changes by 90% within a certain humidity variation range [41]. In this work, the humidity environment was set from 11% RH to 98% RH and back to 11% RH. Figure 7a shows that the response time of the blank filter paper sensor is approximately 728 s, whereas Figure 7b indicates that the response time of PDES−3 is approximately 137 s, which is only 18.8% of that of the blank filter paper sensor, thus meeting some production and daily life needs. In terms of recovery time, the blank filter paper sensor requires approximately 79 s, while PDES−3 takes around 201 s. Although the recovery time increases slightly, the overall response and recovery performance of PDES−3 remains significantly superior to that of the blank filter paper sensor.

3.4. Flexibility Characteristics

Although the surface properties of the modified paper showed no significant changes, further experiments are necessary to verify whether the modification affects the inherent flexibility of the paper. Figure 8 illustrates the sensor under four different bending conditions, where the lengths indicated represent the distances between the sensor ends after bending. Figure 8a represents the state without bending, whereas Figure 8b–d correspond to increasing degrees of bending. The impedance modulus of the sensor was tested at different bending angles, and the results are shown in Figure 9. The test results indicate that while changes in the degree of bending have some impact on the impedance modulus, the effect is minimal and can be considered negligible. In conclusion, the modified paper-based material retains good flexibility, and its humidity sensitivity remains virtually unaffected under varying bending conditions, demonstrating high stability and reliability for practical applications.

3.5. Stability and Repeatability

Stability is a crucial indicator for evaluating sensor performance. The temperature-dependent properties and long-term stability of PDES−3 were tested. For the temperature dependence test (Figure 10a), the sensor was placed at different temperatures of 20 °C, 30 °C, and 40 °C, and the change in its impedance modulus was measured at a humidity range of 11% RH to 98% RH. The measurement data at different temperatures indicate that the impedance modulus of the sensor changes slightly as the temperature changes, even in high-humidity environments, where the impedance value at 40 °C decreases compared with that at 20 °C. In fact, in low- and medium-humidity environments, the impedance modulus of the sensor changes little, and the humidity sensitivity curves almost coincide. In conclusion, the influence of temperature on the sensor is relatively limited, and PDES−3 has a small temperature dependence, which enables it to maintain relatively stable performance in environments with temperature fluctuations.
Figure 10b shows the long-term stability test results of the PDES−3 humidity sensor under different humidity conditions. Although some fluctuations in impedance are observed over time, the overall variation remain minimal, with no significant signs of performance degradation or instability. The test results of the temperature dependence and long-term stability show that the PDES−3 humidity sensor has excellent stability under various environmental conditions, which is suitable for humidity sensing applications with high reliability and long-term stable operation.
The repeatability of the output signal is one of the key factors in evaluating the performance of a sensor, which directly affects its reliability and sustainability in practical applications. As shown in Figure 11, the humidity range was set between 11% RH and 98% RH, encompassing both low- and high-humidity conditions. Cyclic tests were performed for transitions from 11% RH to 98% RH and back. After five consecutive response–recovery cycles, the impedance modulus remains consistent across the humidity range, showing no significant degradation or fluctuation. The results in Figure 10b and Figure 11 demonstrate that the PDES−3 sensor not only maintains stable humidity-sensing characteristics over multiple humidity cycles but also has excellent long-term reliability and cycle stability. It can be reused under different humidity conditions and has good recovery performance.

3.6. Sensing Mechanism

Complex impedance spectroscopy is an important method for analyzing the internal electrochemical process of a humidity sensor. In an environment ranging from 11% RH to 98% RH, the ReZ–ImZ curves and equivalent circuits of the blank filter paper and PDES−3 sensors were analyzed in the frequency range of 100 Hz to 20 MHz to explore the changes in hydrophilicity and conductivity mechanism during humidity sensing.
The mechanism of the blank filter paper sensor is similar to that of the traditional impedance humidity sensor. Figure 12a shows that the impedance curve of the blank filter paper sensor is in the shape of a large radius arc under all the test humidity environments, and the arc trajectory can only be clearly observed at 98% RH. This indicates that the blank filter paper sensor developed in this study functions as a capacitive device across all humidity levels, with its equivalent circuit comprising a constant phase element (CPE) and contact resistance (RS) connected in series, as illustrated in Figure 13a. The sensor has a small adsorption capacity for water molecules and low ionic conductivity. Specifically, the device is in a high impedance state [42,43]. Figure 12b shows the ReZ–ImZ curve of PDES−3, which is similar to that of the blank filter paper sensor at 11% RH−90% RH, so they are all traditional impedance humidity sensors, and their equivalent circuit diagrams are the same. However, at 98% RH, the complex impedance spectrum shows a complete semicircle, so the equivalent circuit of the sensor material is a parallel circuit composed of capacitors and resistors (Figure 13b) [44]. At this point, the sensor exhibits high sensitivity due to the interaction between ionized cations in the electrolyte and H+ from water molecules with electrons in the metal electrode. These charged species accumulate densely near the interface, forming an electric double layer [45]. As the humidity increases, the ReZ–ImZ curve shows a quarter arc in the left half and a falling arc in the right half (Figure 12b). The ReZ–ImZ curve radius of the PDES−3 humidity sensor manufactured in this work at 75% RH is almost the same as that of the blank filter paper sensor at 90% RH, indicating that proton jump conduction can occur at a lower RH after PDES modification; that is, the prepared PDES can ionize Cl− and promote the conductive process. At 98% RH, the chemisorbed water molecules gradually form a continuous water layer with more Cl−, and the conductive particles are H+ and Cl−.
In a Nyquist plot, if the system trajectory lies entirely in the left half of the complex plane (i.e., it does not encircle the origin), the system is considered stable [46]. Furthermore, within a stable system, a smaller arc radius that does not intersect or approach the origin indicates better overall system stability [47]. For the sensor prepared in this work, the circular radius of PDES−3 at 98% RH is much smaller than that of the blank filter paper, indicating that the modified sensor is more stable than the unmodified filter paper sensor in terms of system stability.
The components were tested via cyclic voltammetry, and the internal structure was analyzed according to the test results. As shown in Figure S4a, the charge–discharge behavior of the blank filter paper capacitor was scanned via cyclic voltammetry at different scanning rates of 50 mV/s, 100 mV/s, and 150 mV/s. The results show that the sensor has stable charge–discharge characteristics as a capacitor. To compare the electrochemical performance of paper-based materials before and after modification, the C–V curves of different devices were obtained at a scanning rate of 100 mV/s. All tests were conducted at 98% RH and 25 °C. The blank filter paper sensor, PDES−1 (with a typical modification effect), and PDES−3 (exhibiting the best performance) were selected as test samples. The test results of the tested sensors are shown in Figure S4b. For different types of PDE-modified humidity sensors, the C–V curves of the sensors change significantly. The improved sensor image is rectangular, and the volt–ampere curve is generally flat, indicating that the capacitance characteristics of the modified sensor are significantly improved.
The rectangular shape of the C–V curves indicates a balanced charging and discharging process in the capacitor, leading to improved charge storage and discharge efficiency [20]. Furthermore, a larger enclosed area within the curve corresponds to greater energy storage capacity, signifying enhanced energy storage performance [48]. These results demonstrate that PDES modification not only improves the capacitive characteristics of the humidity sensor but also strengthens its electrochemical stability and overall energy storage capability. Notably, when modified with PDES−3, the sensor exhibits the flattest C–V curve, suggesting that its capacitive characteristics are optimized to the greatest extent possible (Figure S4b). The PDES−3 modification enables the humidity sensor to achieve optimal energy storage effectiveness, with more stable and efficient capacitance performance. Compared with other modification types, PDES−3 achieves superior charge storage efficiency by refining the material structure and enhancing conductivity, thereby yielding the best energy storage results.

3.7. Performance Comparison

In Table 1, several reports on humidity sensors that use cellulose as a substrate or sensitive material are listed. Compared with those of Ref. [48], the internal chemical properties of the paper base in this work were modified. Unlike in Ref. [49], the original appearance of the paper in this work was retained, and no extra materials were added to the paper. Compared with Refs. [42,50], the synthesis method in this work was more convenient, and no additional crosslinkers were introduced in the preparation of modified solvents. In Refs. [51,52], the paper served solely as the sensing substrate, without any modification. In contrast, this study employs a simple approach to modifying filter paper with a PDES, resulting in a humidity sensor that uses the paper both as the sensitive material and as the substrate. This dual functionality contributes to the sensor’s excellent humidity-sensitive performance.

4. Conclusions

In conclusion, a one-step blending method was used to prepare a PDES for filter paper modification. Different types of PDES were synthesized by varying the proportions of hydrogen bond donors and acceptors. The optimal ratio of PVA to ChCl was determined within a certain range, and electrodes were printed on the modified filter paper to prepare the sensor. The modified PDES−3 sensor had an increased sensitivity of 10.36 and a reduced 90response time in a 54% RH environment. After PDES modification, proton jump conduction can occur at lower RH levels. At higher humidity, the chemical adsorption of water molecules gradually forms a continuous water layer, ionizing more Cl− ions and thereby enhancing the conduction process. The PDES-modified humidity sensor also demonstrated good stability and repeatability, highlighting its strong application potential. Combined with its low cost and excellent flexibility, it represents an ideal candidate for the fabrication of high-performance humidity sensors. The modified paper-based humidity sensor has shown great potential for application in areas such as intelligent packaging, wearable devices, and environmental monitoring. It provides an innovative solution for achieving cost-effective real-time humidity monitoring in various scenarios.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/chemosensors13090354/s1, Figure S1. Images of filter paper: (a) before treatment (with pure water) and (b) after treatment (with PDES−3). Figure S2. Photographs of all prepared paper-based humidity sensors. Figure S3. EDS images of PDES−3: (a) total view, (b–d) elemental mapping. Figure S4. C–V curves of (a) the blank paper sensor at different scanning rates under ambient humidity, (b) three humidity sensors at the same scan rate (100 mV/s) under 98% RH.

Author Contributions

Conceptualization, B.Z., H.S. (Haitao Song), and M.S.; methodology, M.S., Y.X., and Y.L.; software, Y.X., Y.L., and M.S.; validation, B.Z., H.S. (Hao Shen), and Q.L.; formal analysis, M.S., Q.L., and Y.X.; investigation, B.Z., H.S. (Haitao Song), and Y.N.; resources, Y.N. and H.S. (Haitao Song); data curation, Y.N., H.S. (Hao Shen), and M.S.; writing—original draft preparation, M.S. and B.Z.; writing—review and editing, B.Z., Q.L., and Y.L.; visualization, B.Z., Y.X., and M.S.; supervision, Y.N. and H.S. (Hao Shen); project administration, B.Z., Y.X., H.S. (Hao Shen), and H.S. (Haitao Song); funding acquisition, B.Z., H.S. (Haitao Song), and Q.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Yangtze River Delta Innovation and Entrepreneurship Joint Carrier Demonstration Project (23002430100), the Wuxi Science and Technology Development Fund Project (K20241036), the National Natural Science Foundation of China (61903159), and the Natural Science Foundation of Jiangsu Province (BK20190617).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

Author Hao Shen and Haitao Song was employed by the company Shanghai Artificial Intelligence Research Institute Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

References

  1. Sadeq, H.Z.; Michael, W.S.; Reza, G.; Chen, Z. Thermoelectric coolers as thermal management systems for medical applications: Design, optimization, and advancement. Nano Energy 2021, 90, 2211–2855. [Google Scholar]
  2. Zhou, J.; Wu, Q.; Chen, X.; Fei, Q.; Zhang, G.; Rong, W.M.; Xiao, F.H.; Yao, L.; Dong, Y.H.; Lin, L. Two-component ratiometric sensor for Cu2+ detection on paper-based device. Anal. Bioanal. Chem. 2019, 411, 6165–6172. [Google Scholar] [CrossRef]
  3. Huang, L.; Jiang, P.; Wang, D.; Luo, Y.; Li, M.; Lee, H.; Gerhardt, R.A. Novel paper-based flexible ammonia gas sensor via silver and SWNT−PABS inkjet printing. Sens. Actuators B 2014, 197, 308–313. [Google Scholar] [CrossRef]
  4. Morais, R.M.; Klem, M.S.; Nogueira, G.L.; Gomes, T.C.; Alves, N. Low-cost humidity sensor based on PANI/PEDOT:PSS printed on paper. IEEE Sens. J. 2018, 18, 2647–2651. [Google Scholar] [CrossRef]
  5. Alatraktchi, F.A.; Noori, J.S.; Tanev, G.P.; Mortensen, J.; Dimaki, M.; Johansen, H.K.; Madsen, J.; Molin, S.; Svendsen, W.E. Paper-based sensors for rapid detection of virulence factor produced by Pseudomonas aeruginosa. PLoS ONE 2018, 13, e0194157. [Google Scholar] [CrossRef] [PubMed]
  6. Shen, K.; Xu, K.; Zhang, M. Multiple hydrogen bonds reinforced conductive hydrogels with robust elasticity and ultra-durability as multifunctional ionic skins. Chem. Eng. J. 2023, 451, 138525. [Google Scholar] [CrossRef]
  7. Tao, L.Q.; Zhang, K.N.; Tian, H.; Liu, Y.; Wang, D.Y.; Yang, Y.Q.; Ren, T.L. Graphene-paper pressure sensor for detecting human motions. ACS Nano 2017, 11, 8790–8795. [Google Scholar] [CrossRef] [PubMed]
  8. Pataniya, M.; Sumesh, C.K.; Tannarana, M. Flexible paper-based piezo-resistive sensor functionalised by 2D-WSe2 nanosheets. Nanotechnology 2020, 31, 435503. [Google Scholar] [CrossRef]
  9. Qu, Y.; He, W.; Li, S.; Liu, T.; Wang, S.; Yin, Q.; Zhu, Z. Paper-based colorimetric sensor for hydrogen peroxide based upon a graphene oxide/platinum-cobalt nanocomposite. Anal. Lett. 2024, 58, 152–166. [Google Scholar] [CrossRef]
  10. Eryanto, G.; Tseng, S.F. Self-powered paper-based humidity sensors with MgCl2/CNTs composites. Sens. Actuators A 2024, 376, 115606. [Google Scholar] [CrossRef]
  11. Park, H.; Kim, W.; Lee, S.W.; Park, J.; Lee, G.; Yoon, D.S.; Lee, W.; Park, J. Flexible and disposable paper-based gas sensor using reduced graphene oxide/chitosan composite. J. Mater. Sci. Technol. 2022, 101, 165–172. [Google Scholar] [CrossRef]
  12. Chavda, H.V.; Patel, C.N. Preparation and characterization of swellable polymer-based superporous hydrogel composite of poly(acrylamide-co-acrylic acid). Trends Biomater. Artif. Organs 2010, 24, 83–89. [Google Scholar]
  13. Russler, A.; Wieland, M.; Bacher, M.; Henniges, U.; Miethe, P.; Liebner, F.; Potthast, A.; Rosenau, T. AKD-modification of bacterial cellulose aerogels in supercritical CO2. Cellulose 2012, 19, 1337–1349. [Google Scholar] [CrossRef]
  14. Arnata, I.W.; Suprihatin, S.; Fahma, F.; Nur, R.; Candra, S.T. Cationic modification of nanocrystalline cellulose from sago fronds. Cellulose 2020, 27, 3121–3141. [Google Scholar] [CrossRef]
  15. Sawhney, D.; Vaid, S.; Bangotra, R.; Sharma, S.; Dutt, H.C.; Kapoor, N.; Mahajan, R.; Bajaj, B.K. Proficient bioconversion of rice straw biomass to bioethanol using a novel combinatorial pretreatment approach. Bioresour. Technol. 2023, 375, 128791. [Google Scholar] [CrossRef]
  16. Alshammari, O.A.O.; Almulgabsagher, G.A.A.; Ryder, K.S.; Surbhi, S.; Chander, D.H.; Nisha, K.; Ritu, M.; Kumar, B.B. Effect of solute polarity on extraction efficiency using deep eutectic solvents. Green Chem. 2021, 23, 5097–5105. [Google Scholar] [CrossRef]
  17. Song, X.; Hu, W.; Huang, W.; Hui, W.; Hai, Y.S.; Tao, Y.S.; Sheng, L.F. Methanolysis of polycarbonate into bisphenol A using choline chloride-based deep eutectic solvents. Chem. Eng. J. 2020, 388, 124324. [Google Scholar] [CrossRef]
  18. Zhou, L.; Lu, X.; Ju, Z.; Bo, L.; Yu, Y.H.; Li, X.J.; Zhang, Q.; Feng, H.Y.; Jiang, Z.S. Alcoholysis of polyethylene terephthalate to dioctyl terephthalate using choline chloride-based deep eutectic solvents. Green Chem. 2019, 21, 897–906. [Google Scholar] [CrossRef]
  19. Angsantikul, P.; Peng, K.; Curreri, A.M.; Ye, C.; Kevin, Z.C.; Jessica, E.; Samir, M. Ionic liquids and deep eutectic solvents for enhanced delivery of antibodies in the gastrointestinal tract. Adv. Funct. Mater. 2020, 31, 2002912. [Google Scholar] [CrossRef]
  20. Li, X.; Liu, J.; Guo, Q.; Zhang, X.; Tian, M. Polymerizable deep eutectic solvent-based skin-like elastomers with dynamic schemochrome and self-healing ability. Small 2022, 18, 2201012. [Google Scholar] [CrossRef]
  21. Chen, W.; Sun, W.; Li, J.C.; Qiu, Y.X.; Qiu, L.Y.; Ma, J.; Li, N.; Ji, X.X. Multifunctional conductive elastomers based on tannic acid and polymerizable 1-butyl-3-methylimidazolium chloride/acrylic acid deep eutectic solvent. ACS Appl. Polym. Mater. 2024, 6, 180–191. [Google Scholar] [CrossRef]
  22. Zhao, H.; Lin, X.; Qi, R.; Dai, J.; Liu, S.; Fei, T.; Zhang, T. A composite structure of in situ cross-linked poly(ionic liquid)s and paper for humidity-monitoring application. IEEE Sens. J. 2018, 19, 833–837. [Google Scholar] [CrossRef]
  23. Tao, L.J.; Zu, Z.M.; Hong, N.P.; Lin, H.J. Application of deep eutectic solvents in polymer synthesis. Prog. Chem. 2022, 34, 2159–2172. [Google Scholar]
  24. Holman, J.B.; Shi, Z.; Fadahunsi, A.A.; Li, C.; Ding, W. Advances on microfluidic paper-based electroanalytical devices. Biotechnol. Adv. 2023, 63, 108093. [Google Scholar] [CrossRef] [PubMed]
  25. Ana, R.; Ana, M.A.; Alexandre, P.; Anarita, D.C. Polymer science and engineering using deep eutectic solvents. Polymers 2019, 11, 912. [Google Scholar] [CrossRef] [PubMed]
  26. Analisa, R.; Yeop, A.B.; Jacob, J.A.; Eric, D.B.; Jennifer, B.T.; Jennifer, L.A. Pen-on-paper flexible electronics. Adv. Mater. 2011, 23, 3426–3430. [Google Scholar]
  27. Yuan, H.Z.; Fei, L.H.; Liang, X.J.; Lie, S.H.; Bo, L.; Sheng, C.S.; Ying, C.; Ji, M.Y.; Xue, F. Ultralow-cost, highly sensitive, and flexible pressure sensors based on carbon black and airlaid paper. ACS Appl. Mater. Interfaces 2019, 11, 33370–33379. [Google Scholar] [CrossRef] [PubMed]
  28. Xue, G.; Long, Z.; Nan, H.Y.; Guang, W.S.; Bo, P.G.; Qiang, L.L. Directly writing flexible temperature sensor with graphene nanoribbons for disposable healthcare devices. RSC Adv. 2020, 10, 22222–22229. [Google Scholar] [CrossRef]
  29. Kim, Y.J.; Yoon, S.; Cho, Y.H.; Gyewon, K.; Han, K.K. Paintable and writable electrodes using black conductive ink on traditional Korean paper (Hanji). RSC Adv. 2020, 10, 24631–24641. [Google Scholar] [CrossRef]
  30. Huang, Z.; Tang, Y.; Guo, H.; Feng, X.; Zhang, T.; Li, P.; Qian, B.; Xie, Y. 3D printing of ceramics and graphene circuits-on-ceramics by thermal bubble inkjet technology and high-temperature sintering. Ceram. Int. 2020, 46, 10096–10104. [Google Scholar] [CrossRef]
  31. Jia, Y.; Zhang, L.; Tan, K.; Dong, S.; He, Y.; Ye, L.; Zhao, W.; Gu, L.; Prati, S.; Bai, J. Deep eutectic solvent-based green gel for removal of polymeric coating from mural paintings. J. Cult. Herit. 2025, 71, 51–60. [Google Scholar] [CrossRef]
  32. Mi, X.; Zhang, C.; Zhang, N.; Wang, Z.; Wang, J. Recyclable flexible pressure, temperature and humidity multimodal sensors based on micro-pyramidal structures and PVA/choline chloride/ethylene glycol. Sens. Actuators B 2025, 440, 137866. [Google Scholar] [CrossRef]
  33. Oliveira, A.E.; Pereira, A.C.; Resende, M.A. Fabrication of Low-Cost Screen-Printed Electrode in Paper Using Conductive Inks of Graphite and Silver/Silver Chloride. Electroanalysis 2022, 35, 202200093. [Google Scholar] [CrossRef]
  34. Zhao, W.; Li, Q.; Wang, Q.; Li, Z.; Liang, J.; Wu, W. All-printed flexible self-powered humidity sensor. Sens. Actuators B 2025, 440, 137868. [Google Scholar] [CrossRef]
  35. Tan, C.; Cao, Y.; Xie, N.; Zhang, M.; Liu, L.; Yu, H.; Wang, C.; Jiang, Y.; Wu, Y.; Yuan, Z.; et al. Intelligent respiratory rate detection using disposable paper-based humidity sensor and precise peak-seeking algorithm. Sens. Actuators B 2025, 436, 137738. [Google Scholar] [CrossRef]
  36. Korotcenkov, G. Paper-based humidity sensors as promising flexible devices: State of the art—Part 1. General considerations. Nanomaterials 2023, 13, 1110. [Google Scholar] [CrossRef]
  37. Kim, S.; Jeong, E.; Kim, J.M.; Ayoung, P.; Thiruvengadam, P.; Sik, E.M.; Su, H.M.; Woo, L.S. Simple, fast and easy assay for transition-metal-catalyzed coupling reactions using a paper-based colorimetric iodide sensor. Chem. Commun. 2012, 48, 8751–8753. [Google Scholar] [CrossRef]
  38. Zheng, L.; Hua, H.; Zhang, Z.; Zhu, Y.; Wang, L.; Li, Y. PVA/ChCl deep eutectic polymer blends for transparent strain sensors with antifreeze, flexible, and recyclable properties. ACS Appl. Mater. Interfaces 2022, 14, 49212–49223. [Google Scholar] [CrossRef]
  39. Wang, Y.; Wang, Y.; Yan, L. Deep eutectic solvent-induced microphase separation and entanglement of PVA chains for tough and reprocessable eutectogels for sensors. Langmuir 2022, 38, 12189–12197. [Google Scholar] [CrossRef]
  40. Che, W.; He, X.; Zheng, X.; Wang, K.; Shi, J.; Song, Z.; Liu, B.; Zeng, X.; Zuo, M. Rational design of anti-freezing dual-network hydrogels with superior mechanical and self-healing properties for advanced flexible electronics. Chem. Eng. J. 2025, 515, 163607. [Google Scholar] [CrossRef]
  41. Guan, X.; Hou, Z.N.; Wang, K.; Zhao, H.R.; Liu, S.; Fei, T.; Zhang, T. Flexible humidity sensor based on modified cellulose paper. Sens. Actuators B 2021, 339, 129879. [Google Scholar] [CrossRef]
  42. Zhu, P.; Kuang, Y.; Wei, Y.; Wu, Z.; Guo, C.F.; Wang, S.; Yin, Q.; Zhu, Z. Electrostatic self-assembly enabled flexible paper-based humidity sensor with high sensitivity and superior durability. Chem. Eng. J. 2021, 404, 127105. [Google Scholar] [CrossRef]
  43. Zhang, D.; Zong, X.; Wu, Z.; Su, Y.; Wang, D.; Zhang, X.; Zhang, Y. Hierarchical self-assembled SnS2 nanoflower/Zn2SnO4 hollow-sphere nanohybrid for humidity-sensing applications. ACS Appl. Mater. Interfaces 2018, 10, 32631–32639. [Google Scholar] [CrossRef]
  44. Gaspar, C.; Olkkonen, J.; Passoja, S.; Lehto, M.; Hakonen, H.; Töyryläinen, J.; Lampinen, J. Paper as active layer in inkjet-printed capacitive humidity sensors. Sensors 2017, 17, 1464. [Google Scholar] [CrossRef]
  45. Zhang, S.; Zhang, Z.; Chen, S.; Zhu, R. Measurement of electric double layer capacitance using dielectrophoresis-based particle manipulation. Anal. Chem. 2021, 93, 5882–5889. [Google Scholar] [CrossRef]
  46. Weng, Z.; Su, Y.; Wang, D.; Li, F.; Du, J.; Cheng, H. Graphene-cellulose paper flexible supercapacitors. Adv. Energy Mater. 2011, 1, 917–922. [Google Scholar] [CrossRef]
  47. Hu, J.; Wu, C.; Liu, B.; Zhang, Y.; Wu, Z.; Wang, S.; Yin, Q.; Zhu, Z. Dual-mode paper-based humidity sensor: Optical and electrical dual-signal sensing platform for multifunctional applications. Sens. Actuators B 2025, 440, 137890. [Google Scholar] [CrossRef]
  48. Duan, Z.; Jiang, Y.; Yan, M.; Wang, S.; Yuan, Z.; Zhao, Q.; Sun, P.; Xie, G.; Du, X.; Tai, H. Facile, flexible, cost-saving and environment-friendly paper-based humidity sensor for multifunctional applications. ACS Appl. Mater. Interfaces 2019, 11, 21840–21849. [Google Scholar] [CrossRef] [PubMed]
  49. Wang, Y.; Zhang, L.; Zhou, J.; Lu, A. Flexible and transparent cellulose-based ionic film as a humidity sensor. ACS Appl. Mater. Interfaces 2020, 12, 7631–7638. [Google Scholar] [CrossRef]
  50. Guan, X.; Yu, Y.; Hou, Z.; Wu, K.; Zhao, H.; Liu, S.; Fei, T.; Zhang, T. A flexible humidity sensor based on self-supported polymer film. Sens. Actuators B 2022, 358, 131438. [Google Scholar] [CrossRef]
  51. Suman, J.; Adib, T.; Aarsh, P.; Masoud, M.S. Laser-assisted dry printing eco-friendly paper-based humidity and temperature sensors. Laser Appl. 2025, 37, 012025. [Google Scholar]
  52. Lin, X.Z.; Xue, H.; Li, F.; Zhao, H.R.; Zhang, T. An electrolyte-mediated paper-based humidity sensor fabricated by an office inkjet printer. IEEE Electron. Device Lett. 2024, 45, 244–247. [Google Scholar] [CrossRef]
Chemosensors 13 00354 g001
Figure 1. Schematic illustration of fabrication procedure for humidity sensors in this work.
Figure 1. Schematic illustration of fabrication procedure for humidity sensors in this work.
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Figure 2. SEM images of (a) blank filter paper, (b) PDES−1, (c) PDES−2, (d) PDES−3, (e) PDES−4, and (f) PDES−5.
Figure 2. SEM images of (a) blank filter paper, (b) PDES−1, (c) PDES−2, (d) PDES−3, (e) PDES−4, and (f) PDES−5.
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Figure 3. High-magnification SEM images of (a) blank filter paper and (b) PDES−3 sensors.
Figure 3. High-magnification SEM images of (a) blank filter paper and (b) PDES−3 sensors.
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Figure 4. (a) Full, (b) low wavenumber, and (c) mid–high wavenumber FT-IR spectra of Blank (filter paper + pure water), PDES (5 g PVA + 2 g ChCl), PDES−3 (filter paper + 5 g PVA + 2 g ChCl), and PDES−5 (filter paper + 5 g PVA+3 g ChCl).
Figure 4. (a) Full, (b) low wavenumber, and (c) mid–high wavenumber FT-IR spectra of Blank (filter paper + pure water), PDES (5 g PVA + 2 g ChCl), PDES−3 (filter paper + 5 g PVA + 2 g ChCl), and PDES−5 (filter paper + 5 g PVA+3 g ChCl).
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Figure 5. Water contact angles of (a) Blank, (b) PDES−3, and (c) PDES−5.
Figure 5. Water contact angles of (a) Blank, (b) PDES−3, and (c) PDES−5.
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Figure 6. Impedance modulus–RH correlation curves of the blank filter paper and PDES−1~PDES−5 sensors.
Figure 6. Impedance modulus–RH correlation curves of the blank filter paper and PDES−1~PDES−5 sensors.
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Figure 7. Response–recovery curves of (a) blank paper and (b) PDES−3 sensors between 11% and 98% RH.
Figure 7. Response–recovery curves of (a) blank paper and (b) PDES−3 sensors between 11% and 98% RH.
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Figure 8. Photographs of PDES−3 sensor under varying bending degrees: (a) no bending, and the distance between the sensor ends after bending (b) 24.0 mm, (c) 20.5 mm, and (d) 18.0 mm.
Figure 8. Photographs of PDES−3 sensor under varying bending degrees: (a) no bending, and the distance between the sensor ends after bending (b) 24.0 mm, (c) 20.5 mm, and (d) 18.0 mm.
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Figure 9. Impedance modulus–RH correlation curves of the PDES−3 sensor under various bending degrees: D0–D3 correspond to the sensors in Figure 8a–d (±0.5 mm), respectively.
Figure 9. Impedance modulus–RH correlation curves of the PDES−3 sensor under various bending degrees: D0–D3 correspond to the sensors in Figure 8a–d (±0.5 mm), respectively.
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Figure 10. (a) Impedance modulus–RH curves of the PDES−3 sensor recorded under different test temperatures. (b) Long-term impedance stability of the PDES−3 sensor under different RHs.
Figure 10. (a) Impedance modulus–RH curves of the PDES−3 sensor recorded under different test temperatures. (b) Long-term impedance stability of the PDES−3 sensor under different RHs.
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Figure 11. Continuous response–recovery tests of the PDES−3 sensor between 11% and 98% RH.
Figure 11. Continuous response–recovery tests of the PDES−3 sensor between 11% and 98% RH.
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Figure 12. Impedance spectra of (a) blank paper sensor and (b) PDES−3 sensor under different RHs.
Figure 12. Impedance spectra of (a) blank paper sensor and (b) PDES−3 sensor under different RHs.
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Figure 13. (a,b): Two forms of an equivalent circuit diagram.
Figure 13. (a,b): Two forms of an equivalent circuit diagram.
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Table 1. Comparison of the humidity-sensing performance of the PDES−3 sensor in this work with other cellulose-based humidity sensors.
Table 1. Comparison of the humidity-sensing performance of the PDES−3 sensor in this work with other cellulose-based humidity sensors.
Mater.Prep.T. R. (% RH)Sens.τres./τrec. (s)Ref.
Printing paperPaste41–911647 a, –19/472[48]
Cellulose/KOHImmersion11–97>200 b, –6/10.8[49]
EPTAC/CNFsSolution blending11–988.41 c, 54% RH25/188[41]
Crosslinked polyelectrolyteClick syndicate11–95103.75 c, 95% RH12.5/700[50]
Printing paperDry additive nanomanufacturing20–902.46 d, 70% RH−[51]
Printing paper/NaClInkjet printing11–9572.36 c, 95% RH1623/70[52]
PDES−3 One-step solution blending 11–98 10.36 c, 54% RH;
519.2 c, 95% RH 		137/201 This work
Mater.: sensitive materials; Prep.: preparation method; T. R.: test range; Sens.: sensitivity; τres./τrec.: response/recovery time; Ref.: references. a I/I0; b σ/σ0; c Z0/Z; d C0/C.
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MDPI and ACS Style

Shen, M.; Zhang, B.; Lu, Q.; Xiao, Y.; Shen, H.; Ni, Y.; Liu, Y.; Song, H. Research on a Humidity Sensor Based on Polymerizable Deep Eutectic System-Modified Filter Paper. Chemosensors 2025, 13, 354. https://doi.org/10.3390/chemosensors13090354

AMA Style

Shen M, Zhang B, Lu Q, Xiao Y, Shen H, Ni Y, Liu Y, Song H. Research on a Humidity Sensor Based on Polymerizable Deep Eutectic System-Modified Filter Paper. Chemosensors. 2025; 13(9):354. https://doi.org/10.3390/chemosensors13090354

Chicago/Turabian Style

Shen, Mengyao, Bo Zhang, Qi Lu, Yanan Xiao, Hao Shen, Yi Ni, Yuechen Liu, and Haitao Song. 2025. "Research on a Humidity Sensor Based on Polymerizable Deep Eutectic System-Modified Filter Paper" Chemosensors 13, no. 9: 354. https://doi.org/10.3390/chemosensors13090354

APA Style

Shen, M., Zhang, B., Lu, Q., Xiao, Y., Shen, H., Ni, Y., Liu, Y., & Song, H. (2025). Research on a Humidity Sensor Based on Polymerizable Deep Eutectic System-Modified Filter Paper. Chemosensors, 13(9), 354. https://doi.org/10.3390/chemosensors13090354

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