Multi-stimulus responsive polyoxometalate–viologen hybrid ionic liquids for rewritable inkless printing and environmental monitoring

Abstract

Multi-stimulus responsive materials have attracted significant attention due to their ability to respond dynamically to multiple external stimuli, including light, temperature, and chemical signals. Achieving both rapid response times and precise selectivity is essential for advancing the functionality of such materials. In this study, five polyoxometalate-based viologen ionic liquids (ILs) were synthesized through ion exchange between H₃PMo₁₂O₄₀ and viologen ILs with varying substituents. These ILs demonstrated multi-stimulus responses to light and ammonia gas. Under Xe lamp irradiation, all ILs exhibited prominent photochromism, with C₁₂–PMo₁₂ undergoing a rapid change from light yellow to light green within just 5 seconds. Exposure to an H₂O₂ atmosphere for 3 hours resulted in the complete decolorization of C₁₂–PMo₁₂. Remarkably, after five cycles of coloring and decoloring, its absorbance remained virtually unchanged, highlighting its excellent reversibility and stability. Additionally, as the alkyl chain length of the viologen-based cation increased, the coloring rate accelerated, and the color contrast enhanced, likely due to the suppression of viologen radical quenching by longer carbon chains. These ILs also exhibited gas-chromic behavior in the presence of ammonia, showing high detection selectivity. Furthermore, the materials were successfully applied in inkless erasable printing and ultraviolet detection. The C₁₂–PMo₁₂-based carbon paper displayed clear and readable prints in only 10 seconds and could be bleached in an H₂O₂ atmosphere within 1.5 hours. These findings demonstrate that the synthesized ILs possess versatile multi-stimulus responsive properties, making them highly promising for applications in inkless printing, ultraviolet detection, and amine vapor sensing.

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1. Introduction

To address the demands of an increasingly intelligent society, stimulus-responsive polymers^1,2 have garnered significant scientific interest for a wide range of applications,^3,4 particularly in the domain of stimulus-responsive color-changing materials.^5–9 These materials undergo color changes in response to external stimuli, such as light, temperature, gas, electricity, and pH, and hold considerable potential for applications in diverse areas, including information storage and encryption,^10,11 sensing,^12–14 and anti-counterfeiting.^15,16 Viologens exhibit excellent electron-accepting capabilities and possess Lewis acidic sites.^17–19 The electron-deficient viologen cation (V²⁺) can undergo one-electron transfer from an appropriate electron donor to form the viologen cationic radical (V˙⁺). A distinct color change typically accompanies this process. Consequently, viologens and their derivatives exhibit notable stimulus-responsive color-changing properties,^20–23 showing great promise in fields such as optical switching,²⁴ sensors,^25–27 anti-counterfeiting,^28,29 and inkless printing.^30,31 A significant challenge for viologen-based chromic materials is achieving quick response times and high stability. A practical approach to developing stable and rapid multi-stimulus responsive chromic materials is the incorporation of viologen derivatives into high-performance electron donors.

Polyoxometalates (POMs), known for their favorable redox properties and abundance of electrons,^32,33 are regarded as excellent electron donors. This makes them compatible with the electron-acceptor properties of viologens, facilitating efficient electron transfer.^34,35 Several noteworthy studies have been published on polyoxometalate–viologen hybrid chromic materials.^36–39 For instance, Zhang's group developed a new crystalline polyoxometalate–viologen hybrid,⁴⁰ which demonstrated remarkable ultraviolet light detection capabilities. The material exhibited an apparent color change from pale yellow to blue and a rapid response to ultraviolet light, with detection at intensities as low as 0.006 mW cm⁻² in narrow-band UV regions. However, it can only maintain a longer duration in the dark, and as the light intensity increases, the exposed cations are easily oxidized, leading to a decrease in fluorescence intensity. Jun's group achieved fast-response photochromism by utilizing two types of viologen ligands,⁴¹ resulting in the synthesis of three POM-based compounds under hydrothermal and solvothermal conditions. These compounds proved to be effective as visible UV detectors and inkless erasable printing materials, while the spectra of each element overlap before and after the XRS spectral test changes. Chen's group synthesized a thermochromic polyoxometalate-based metal–organic framework (POMOF) using Keggin-type polyoxometalates and metalloviologens,⁴² which exhibited reversible color changes from yellow to black at 150 °C. Despite these advances, the development of novel multifunctional stimuli-responsive hybrid chromic materials remains a significant challenge. The above studies focus on maximizing and stabilizing color changes, while overlooking the difficulty of material reproducibility. The hard and brittle nature of metal framework structures, along with their poor solubility, limits their applications in flexible devices. Therefore, constructing multifunctional stimulus-responsive hybrid color-changing materials with fast response, long-term retention, and good flexibility remains a significant challenge.

Ionic liquids (ILs) are organic molten salts at room temperature, composed of organic cations and either organic or inorganic anions. These materials have garnered significant attention due to their wide electrochemical window, high ionic conductivity, and tunable structural properties. Polyoxometalate-based ionic liquids (POM-ILs) are organic–inorganic hybrid materials that combine various cations with polyoxometalate (POM) anions. These POM-ILs combine the unique characteristics of polyoxometalates and ionic liquids, and hold great promise for realizing multifunctional stimulus-responsive hybrid color-changing materials with fast response, long-term stability, and good flexibility.⁴³ In this study, five ionic liquids (C_n–PMo₁₂, where n = 3, 4, 5, 6, 12) were prepared using POMs with multi-electron donor characteristics as anions and 4,4′-bipyridine, which is prone to reversible redox reactions, as cations. The color-changing behavior of these five ILs in response to external stimuli, such as light and ammonia were also investigated. All five ILs exhibited rapid and noticeable photochromic behavior upon exposure to light. Remarkably, C₁₂–PMo₁₂ demonstrated the most pronounced photochromism, transitioning from yellow to light green within 5 seconds. These ILs show significant potential for applications in inkless erasable printing and UV detection technologies. Furthermore, they also displayed discoloration when exposed to ammonia and organic amines, with high selectivity, suggesting their promising use as organic amine detectors.

2. Experimental

2.1. Materials and methods

Phosphoric acid (85 wt% in water solution), hydrochloric acid (36–38wt%), Na₂WO₄·2H₂O (purity, >99%), 4,4′-bipyridine, 1-bromopropane, 1-bromobutane, 1-bromopentane, 1-bromohexane, 1-bromodecane, acetone, N,N-dimethylformamide (DMF), ethanol and diethyl ether were all supplied by Aladdin Reagent Co., Ltd China. Polydimethylsiloxane (PDMS; SYLGARD® 184 elastomer) was purchased from Gelest Inc., USA. Proton nuclear magnetic resonance (¹H NMR) spectra were recorded on a Bruker 500 spectrometer at room temperature. FT-IR spectroscopy was performed using a VERTEX-70 spectrometer with a test range of 4000–400 cm⁻¹. PXRD patterns were generated using Bruker D8 Advance. The ultraviolet-visible (UV-vis) absorption spectra were obtained by making use of a UH4150 ultraviolet spectrophotometer at room temperature. Electron paramagnetic resonance (EPR) spectra were recorded at RT with a Bruker EMX PLUS apparatus. X-ray photoelectron spectroscopy (XPS) was performed using monochromatic radiation on the ESCALB250Xi spectrometer.

2.2. Syntheses and characterization

2.2.1. Preparation of viologen-based ionic liquid. As shown in Fig. 1, C_nbpy·Br (where n = 3, 4, 5, 6, 12) was synthesized following the method previously reported.⁴⁴ The successful synthesis was confirmed by the ¹H NMR spectrum, as presented in Fig. S1–S5.

Fig. 1 Synthetic routes of viologen-based ionic liquids.

2.2.2. Preparation of polyoxometalates-based ionic liquid. H₃PMo₁₂O₄₀ (0.1 mmol) was synthesized using a previously reported method⁴⁵ and dissolved in distilled water, while the viologen-based ionic liquid (C_nbpy·Br) (0.3 mmol) was dissolved in anhydrous ethanol. The ionic liquid solution was then added dropwise to the H₃PMo₁₂O₄₀ solution and stirred for 2 hours at room temperature. The resulting mixture was transferred to a hydrothermal synthesis reactor and heated at 80 °C for 12 hours. The yellow solid product was subsequently dried in a vacuum oven at 60 °C for 12 hours, yielding C_n–PMo₁₂. The successful synthesis of the products was confirmed by FT-IR, XRD, and SEM analyses (Fig. S6–S8).

2.2.3. Preparation for PDMS film. To improve operational convenience and expand the practical applications of these ionic liquids (ILs), C₆–PMo₁₂ and C₁₂–PMo₁₂ were utilized to fabricate composite membranes. Initially, C₆–PMo₁₂ and C₁₂–PMo₁₂ were ground into fine powders, which were then uniformly dispersed in dimethyl sulfoxide (DMSO) using ultrasonication. This dispersion was subsequently blended with polydimethylsiloxane (PDMS). The resulting mixture was applied to 200 μm films onto a glass substrate using a film applicator and cured in an oven at 80 °C. This process resulted in uniform composite films, which were labeled as C₆–PMo₁₂-a and C₁₂–PMo₁₂-b, respectively.

3. Results and discussion

3.1. Photochromic properties

The photochromic properties of the five ionic liquids (ILs) were studied by irradiating them with a Xe lamp. As shown in Fig. 2a, the color of C₃–PMo₁₂ and C₄–PMo₁₂ exhibited slight changes after 180 s of exposure. In contrast, C₅–PMo₁₂ and C₆–PMo₁₂ showed significant color changes within 10 s of irradiation. Notably, C₁₂–PMo₁₂ displayed a quick response, transitioning from light yellow to light green in just 5 s. Both C₆–PMo₁₂ and C₁₂–PMo₁₂ reached a color saturation (deep olive green) after 10 minutes of irradiation. The CIE1931 chromatic diagrams, derived from the L*, a*, b* values of C₃–PMo₁₂, C₆–PMo₁₂ (Fig. S9), and C₁₂–PMo₁₂ (Fig. 2b) before and after 10 min irradiation, clearly demonstrate that C₁₂–PMo₁₂ displays the most prominent color changes. The calculated color difference values ∇E of C₃–PMo₁₂, C₆–PMo_12, and C₁₂–PMo₁₂ are 29.260, 72.360, and 85.040, respectively (Table S1). These values further confirm the high contrast and vivid photochromic behavior of C₁₂–PMo₁₂. As the alkyl chain length of the viologen-based cation increased, the coloring rate accelerated, and the color contrast enhanced, likely due to the suppression of viologen radical quenching by longer carbon chains. This remarkable photochromic response highlights the potential of C₁₂–PMo₁₂ as a candidate for dynamic optical applications that require high-visibility color switching under Xe lamp excitation.

Fig. 2 (a) Photographs of the color change of the samples before and after irradiation with a Xe lamp. (b) CIE chromaticity diagrams of C₁₂–PMo₁₂ before and after irradiation with a Xe lamp.

The photochromic behavior of the five ILs was investigated using UV-vis absorption spectroscopy. As shown in Fig. 3a and b, and the SI (Fig. S10), none of the five ILs exhibited significant absorption peaks in the 500–800 nm range before UV irradiation. However, upon exposure to UV light, a new broad absorption peak appeared within the 600–750 nm range, characteristic of viologen radicals.⁴⁶ Notably, the intensity of this absorption peak increased progressively with prolonged irradiation, indicating a gradual rise in the concentration of viologen radicals. These results suggest that the observed color change in these compounds is due to the light-induced formation of viologen radicals. Additionally, when UV-irradiated C₆–PMo₁₂ and C₁₂–PMo₁₂ were exposed to an H₂O₂ atmosphere for 3 hours, both compounds decolorized and returned to their original color. Upon reirradiation, the samples displayed a dark green coloration again, confirming that the photochromic properties of the ILs are reversible (Fig. S11). As illustrated in Fig. 3c and d, the coloration–decoloration cycling tests of C₆–PMo₁₂ and C₁₂–PMo₁₂ were performed over five cycles. It was found that the absorbance at 728 nm and 717 nm remained nearly unchanged for both C₆–PMo₁₂ and C₁₂–PMo₁₂, demonstrating excellent photochromic reversibility and fatigue resistance.

Fig. 3 (a and b) UV-vis absorption spectra of compound C₆–PMo₁₂ (a), C₁₂–PMo₁₂ (b) before and after irradiation with a Xe lamp. (c and d) UV-vis absorption value of the C₆–PMo₁₂ at 728 nm (c) and C₁₂–PMo₁₂ at 717 nm (d) according to photochromic repeatability. (e and f) Photochemical reaction kinetic plots monitored at 728 nm (e) and 717 nm (f).

To further assess the photochromic performance, detailed kinetic studies of C₆–PMo₁₂ and C₁₂–PMo₁₂ were conducted. The kinetic calculation equation used in this study is provided below:

ln[(A₀ − A_∞/A_t − A_∞] = kt

Here, k is the rate constant, A₀ is the initial absorbance, A_t is the absorbance at time t, and A_∞ is the absorbance at photochemical saturation.⁴⁷

In Fig. 3e and f, the rate constants derived from the absorption peaks at 728 nm and 717 nm indicate that both C₆–PMo₁₂ and C₁₂–PMo₁₂ follow a two-stage first-order kinetic behavior. The K₁ values for the initial stage are higher than the K₂ values for the second stage, suggesting that the primary photochromic process occurs on the surface at the beginning. As irradiation time increases, photochromic behavior progresses deeper into the material. Specifically, the K₁ and K₂ values for C₁₂–PMo₁₂ are 0.0186 s⁻¹ and 0.00784 s⁻¹, respectively, which are higher than those for C₆–PMo₁₂, with K₁ and K₂ values of 0.0158 s⁻¹ and 0.00621 s⁻¹, respectively. These findings demonstrate that C₁₂–PMo₁₂ exhibits a faster photochromic response compared to C₆–PMo₁₂.

3.2. Analysis of the photochromic mechanism

To investigate the photochromic mechanism, the electron paramagnetic resonance (EPR) of C₁₂–PMo₁₂ was measured. As shown in Fig. 4a, C₁₂–PMo₁₂ displayed a weak but detectable EPR signal at g = 2.0028 before irradiation, which is attributed to trace viologen radical species.⁴⁸ This pre-existing signal indicates the intrinsic photosensitivity of the compound, as the viologen moiety exhibits partial electron activity even in the ground state. After irradiation, this signal was significantly enhanced. Furthermore, EPR analysis provided additional spectroscopic evidence of photoreduction, as a new signal appeared at g = 1.9426, characteristic of Mo(V) species formed by the partial reduction of Mo(VI) centers.⁴⁸ FT-IR and PXRD analyses were conducted on C₁₂–PMo₁₂ before and after irradiation. The results showed that both the infrared spectrum (Fig. 4b) and the XRD (Fig. 4c) remained essentially unchanged, indicating that the structure and composition of C₁₂–PMo₁₂ were unaffected by irradiation. After irradiation, the peak signal around 3500 cm⁻¹ increased in intensity, which can be attributed to O–H stretching vibrations, possibly arising from trace amounts of water absorbed in the sample. In addition, a small peak appeared around 1200 cm⁻¹, which may correspond to the C–N stretching vibration in the reduced radical species. These findings suggest that the photochromic process involves reversible electron transfer without any structural reorganization.

Fig. 4 (a and c) EPR, FT-IR, XRD spectra of C₁₂–PMo₁₂ before and after irradiation.

The photochromic mechanism of C₁₂–PMo₁₂ was further investigated using X-ray photoelectron spectroscopy (XPS). As shown in Fig. S12, the binding energies of C 1s and P 2p remained nearly unchanged before and after irradiation, indicating that these elements do not participate in the electron transfer process. However, significant changes were observed in the O 1s, N 1s, and Mo 3d spectra upon irradiation (Fig. 5). After exposure to UV light, the binding energy of O 1s (Fig. 5d) increased compared to that before irradiation (Fig. 5a), suggesting that oxygen acts as an electron donor.⁴⁹ In contrast, the binding energy of N 1s slightly decreased (Fig. 5e), indicating that nitrogen functions as an electron acceptor. For the Mo 3d spectra (Fig. 5c), two prominent peaks at 232.45 eV and 235.54 eV were observed before irradiation, corresponding to Mo(VI). After light exposure (Fig. 5f), new peaks appeared at 233.16 eV and 236.24 eV. This suggests that electron transfer from O_Mo–O → Mo_(IV) and O_POMS to the viologen-based ionic liquids is responsible for the photochromism observed in C₁₂–PMo₁₂.

Fig. 5 (a–c) High resolution XPS spectra for C₁₂–PMo₁₂ O 1s, N 1s, Mo 3d before irradiation. (d–f) High resolution XPS spectra for C₁₂–PMo₁₂ O 1s, N 1s, Mo 3d after irradiation.

3.3. Applications

3.3.1. Inkless and erasable printing. The above results indicate that C₆–PMo₁₂ and C₁₂–PMo₁₂ exhibit excellent photochromic properties and reversibility, making them highly suitable for ink-free erasable printing applications. Consequently, these materials were further applied in this study for inkless erasable printing. The C₆–PMo₁₂ and C₁₂–PMo₁₂ powders were dispersed in ethanol through ultrasonic treatment for 30 minutes to create stable suspensions, which were then uniformly drop-cast onto filter paper⁵⁰ and dried, repeating the process three times. Upon exposure to a Xe lamp (Fig. 6a), distinct “HENU” patterns appeared within 15 seconds for C₆–PMo₁₂ (Fig. S13) and 10 seconds for C₁₂–PMo₁₂. These patterns demonstrated substantial stability under ambient conditions, retaining their color for extended periods, thus offering advantages for long-term information storage. Moreover, the irradiated patterns could be erased in a controlled manner by placing the paper in a H₂O₂ atmosphere for 2 hours (C₆–PMo₁₂) and 1.5 hours (C₁₂–PMo₁₂). Repeating the printing and erasing process over five cycles (Fig. 6b) preserved the maximum contrast of the photochromic paper, demonstrating exceptional reversibility and durability. This is because the oxidation by H₂O₂ not only eliminates the radical but also completely removes their characteristic absorption band. Thus, even though the IL molecules remain on the filter paper, they are already in the colorless initial oxidized state and do not interfere with the coloration of the new patterns. These findings emphasize the potential of C₆–PMo₁₂ and C₁₂–PMo₁₂ as ink-free erasable printing materials, suitable for high-performance information storage and exchange applications.

Fig. 6 (a) Schematic diagram of light printing. (b) Photos of samples in a cyclic light printing process.

3.3.2. UV light detection. Ultraviolet detectors are crucial for applications in environmental monitoring, medical diagnostics, and advanced security systems, enabling precise detection of UV radiation. In this work, flexible composite films based on C₆–PMo₁₂ and C₁₂–PMo₁₂ were designed and applied to UV photodetectors. The compounds C₆–PMo₁₂ and C₁₂–PMo₁₂ were homogeneously mixed with PDMS in DMSO and cast into uniform films using a film applicator, yielding membranes named C₆–PMo₁₂-a (Fig. 7a) and C₁₂–PMo₁₂-b (Fig. 7b), respectively. Upon irradiation with a Xe lamp for 2 min and 1.5 min, respectively, the C₆–PMo₁₂-a and C₁₂–PMo₁₂-b films exhibited a distinct color change from pale yellow-green to deep blue. Both membranes demonstrated excellent flexibility, being portable (Fig. 7c and d) and easily bendable without structural damage. Fig. 7e shows that these membranes also possess favorable mechanical properties, and the addition of C₆–PMo₁₂ and C₁₂–PMo₁₂ did not destroy the flexibility of the PDMS. UV-vis absorption spectra further confirmed that both C₆–PMo₁₂-a (Fig. 7f) and C₁₂–PMo₁₂-b (Fig. 7g) exhibited the appearance of new absorption bands in the 600–750 nm range after Xe lamp irradiation. The intensity of these absorption peaks increased systematically with prolonged exposure, consistent with the solid-state UV-vis absorption spectra. These results demonstrate that flexible membranes are easy to fabricate and operate, making them ideal for applications involving ultraviolet radiation monitoring.

Fig. 7 (a and b) Color change of C₆–PMo₁₂-a and C₁₂–PMo₁₂-b before and after photochromism. (c and d) Flexibility of C₆–PMo₁₂-a and C₁₂–PMo₁₂-b. (e) Stress–strain curves of PDMS, C₆–PMo₁₂-a, and C₁₂–PMo₁₂-b. (f and g) The UV-vis spectra of compound C₆–PMo₁₂-a and C₁₂–PMo₁₂-b before and after irradiation with a Xe lamp.

3.3.3. Amine vapor detection. Organic amines are highly corrosive and toxic volatile compounds that pose significant risks to human health and contribute to air pollution. Consequently, the development of effective detection methods for organic amines and ammonia is essential.⁵¹ To evaluate the detection performance of five ionic liquids (ILs) toward organic amines and ammonia, several representative organic amine compounds were selected, including ammonia (NH₃), diethylamine (DEA), N,N-dimethylformamide (DMF), and acetonitrile (CH₃CN). When these five ILs were exposed to ammonia vapor, distinct color changes were observed over time (Fig. S14). The color of C₆–PMo₁₂ and C₁₂–PMo₁₂ transitioned from yellow to off-white after 1.5 minutes in the NH₃ atmosphere (Fig. 8a) and reverted to yellow after 4 minutes in the HCl atmosphere (Fig. S15). C₃–PMo₁₂, C₄–PMo₁₂, and C₅–PMo₁₂ also changed from yellow to white-gray within 3 minutes. Additionally, the colour of C₆–PMo₁₂-a and C₁₂–PMo₁₂-b shifted from light green to gray-white after 1.5 minutes in the NH₃ atmosphere, with recovery upon exposure to concentrated HCl for 4 minutes (Fig. 8b and c). After three cycles of coloration and decoloration, the color contrast remained stable (Fig. S16). When C₁₂–PMo₁₂ was placed in ammonia (NH₃·H₂O), N,N-dimethylformamide (DMF), diethanolamine (DEA), and acetonitrile (CH₃CN) atmospheres for 10 minutes, significant color changes were observed in NH₃·H₂O and DEA, while minimal changes occurred in DMF and CH₃CN after 1 hour (Fig. S17). This behavior can be attributed to the relationship between the detection efficiency of the ionic liquid and the molecular size of the target amines. Smaller molecules, such as ammonia and primary amines, readily form hydrogen bonds with the ionic liquid, facilitating electron transfer and generating viologen free radicals. In contrast, bulkier secondary and tertiary amines introduce steric hindrance, which limits hydrogen bond formation and hinders electron transfer.⁵² These results indicate that C₁₂–PMo₁₂ demonstrates high selectivity for amine detection.

Fig. 8 (a) Color change of C_n–PMo₁₂ before and after 1.5 min detection of NH₃. (b and c) Color change of C₆–PMo₁₂-a, C₁₂–PMo₁₂-b in HCl and organic amine.

4. Conclusion

In this study, five polyoxometalate-based viologen ionic liquids (ILs) with various substituents were synthesized, each exhibiting outstanding multi-stimuli responsive capabilities. Among these, the IL with the longest alkyl chain, C₁₂–PMo₁₂, demonstrated the fastest coloration rate and highest color contrast, transitioning from light yellow to light green in just 5 seconds under xenon lamp irradiation, likely due to the suppression of viologen radical quenching by longer carbon chains. All the colored ILs were able to revert to their original state in an H₂O₂ atmosphere, with no change in color contrast after five cycles of coloration and decoloration, indicating exceptional fatigue resistance. These ionic liquids were further applied as photochromic layers for the fabrication of rewritable paper, among which C₁₂–PMo₁₂ displayed clear and legible text within only 10 seconds. The printed text exhibited excellent stability, retaining its colored state for extended periods under ambient conditions, thereby demonstrating suitability for long-term information storage. Remarkably, the written content could be fully erased within 1.5 hours in an H₂O₂ atmosphere, highlighting the potential of these materials for inkless and erasable printing. In addition, flexible composite membranes incorporating these ILs were fabricated, showing promising performance as portable UV photodetectors. These compounds also exhibited pronounced and highly selective responses toward organic amines. Overall, this work broaden the application range of polyoxometalate–viologen ILs, providing theoretical insights for their future applications in photochromism, UV detection, and organic amine monitoring.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: including 1H NMR, FT-IR, XRD and SEM data of these five ILs; CIE chromaticity diagrams of C3-PMo12, C6-PMo12; color change of C_n-PMo₁₂ with UV light, amine; and so on. See DOI: https://doi.org/10.1039/d5ra05884h.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22408084), Young Elite Scientists Sponsorship Program by CAST (2022QNRC001) and Henan Provincial Science and Technology Research Project (242102231082).

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