Photonic crystal hydrogels based on highly reproducible molding method

Abstract

A lot of sensors using structural color are based on periodic nanostructures, or photonic crystals. So the nanostructures need to be fabricated with high reproducibility so that those sensors can be suitable for practical and commercial applications. Furthermore, achieving the reproducible fabrication is more challenging for hydrogel-based devices with structural color. In this study, we propose a novel molding approach to fabricate photonic crystal hydrogels with high reproducibility. A silicon wafer with a monolayer of self-assembled nanoparticles is used as a mold to transfer nanostructures onto the hydrogel surface. Since the molding technique is sensitive to the mechanical properties of the hydrogel, we optimized these properties by adjusting the monomer-to-crosslinker ratio. The ratio of 50:1 was identified as the optimal composition for the molding method to ensure both mechanical stability and chemical responsiveness. In order to demonstrate reproducibility, the molding processes were performed for over 50 cycles, resulting in hydrogel exhibiting structural colors with optical and mechanical integrity. Additionally, hydrogels showed reversible color changes in response to various solvents. Volume change of the hydrogel caused variation of periodicity of photonic crystal, which led to red-shifted colors upon swelling and blue-shifted colors upon contraction. This study shows that photonic crystal hydrogels can be fabricated with enhanced reproducibility by molding method. And it also shows that they can be applied to structural color-based sensors. The principle of this study can be extended to biosensing and environmental monitoring applications by incorporating selective molecules such as antibodies.

Introduction

With recent advancement of nature-inspired technologies, research on structural color has been conducted in various fields¹. Structural colors, commonly observed in nature on butterfly wings and chameleon skins, result from periodic nanostructures that selectively reflect or transmit certain wavelengths of light^2,3. These periodic nanostructures, known as photonic crystals, produce vivid colors.

Structural color-based sensors mainly depend on the periodic nanostructure of colloidal particles such as silica (SiO₂) or polystyrene. If the sensor is deformed, the periodicity of the nanostructure varies, inducing optical phenomena like Bragg reflection or bandgap shifts, which manifest as observable color changes^4,5. These distinctive optical properties of structural colors are widely used in various fields such as chemical^{6, 7–8}, mechanical and biological signal detection^{9, 10–11}, display^12,13 and anti-counterfeiting technology^{14, 15, 16–17}.

Even slight changes in periodicity can lead to noticeable color variations. Due to this optical responsiveness, structural colors offer high sensitivity when used as a sensing platform for detecting subtle chemical and biological reactions. Based on the advantages of structural colors, hydrogel is widely adopted as a platform for chemical and biosensors. Hydrogel is a cross-linked polymer network that can absorb up to 1,000 times its dry weight in water, with reversible volumetric changes in response to environmental stimuli such as pH, temperature, or ionic strength.

Recently, combination of polystyrene (PS) colloidal nanoparticles and hydrogel is used to fabricate photonic crystal hydrogel sensors^{18, 19–20}. For example, Kim et al. reported a hydrogel sensor with two-dimensional (2D) photonic crystal that utilized structural color for monitoring glucose concentrations in tear fluid in real-time²¹. Additionally, Zhang et al. fabricated a photonic crystal hydrogel sensor capable of selectively detecting trace mercury ions in seawater. By utilizing the interaction between mercury ions and hydrogel, the sensor showed volumetric changes. These changes caused shifts in Bragg diffraction peaks and enabled highly sensitive and quantitative detection of mercury ion²². Both studies commonly focus on the development of sensor technology that detects specific chemical substances based on variation of structural color resulting from the volumetric change of the hydrogel.

The methods for fabricating photonic crystal hydrogels depend on their dimensionality (1D, 2D, or 3D) and their specific application. Self-assembly is the most widely used method for fabricating colloidal photonic crystals. Monodisperse nanoparticles such as silica (SiO₂) or polystyrene form periodic structures through self-assembly²³. This method is simple and scalable but faces challenges with large-scale uniformity and reproducibility. For example, even under optimized conditions, spin-coated polystyrene (PS) nanoparticle monolayers frequently suffer from multilayer regions, cracking, and domain boundaries that limit pattern fidelity and reproducibility across large areas^1,23. These defects result in inconsistent optical properties and hinder practical deployment in reproducible sensor systems.

Top-down lithography using UV light provides high precision and control but is limited by its high cost and time-intensive process. Techniques like Direct Laser Writing (DLW)²⁴ and etching demand precise particle arrangements, which reduces reproducibility. These challenges limit the potential for mass production and require improved fabrication methods to support the commercial application of PC hydrogels.

Soft lithography, a microfabrication technique that uses elastomeric stamps to transfer patterns onto substrates, has been widely used for structuring various materials including hydrogels. The technique offers advantages such as low cost, simple equipment requirements, and compatibility with a wide range of materials^25,26. However, soft lithography reveals several limitations when applied to the fabrication of periodic nanostructures such as photonic crystals on hydrogels. Since soft lithography requires the fabrication of an elastomeric stamp from a rigid master mold, the overall process inherently involves at least two replication steps^27,28. Each step introduces the risk of cumulative errors or imperfections, making it more challenging to preserve nanoscale fidelity in periodic structures. In addition, repeated use of the soft stamp can result in gradual mechanical degradation, reducing pattern fidelity over time.

Nanoimprint lithography (NIL) has also been explored for patterning nanoscale structures and offers high resolution and fidelity through direct physical embossing^29,30. However, NIL involves several practical limitations when applied to hydrogel-based or flexible substrates. The use of rigid molds requires high-pressure contact and precise alignment, which often leads to mechanical stress and damage, particularly in soft and deformable materials like hydrogels^31,32. In addition, the demolding process can cause pattern distortion or sticking, while the molds themselves are prone to wear or contamination over repeated cycles. These issues, combined with the high cost of mold fabrication and the complexity of the imprinting process, make NIL less suitable for scalable and reproducible fabrication of structurally colored hydrogels.

In this study, we propose a highly reproducible molding method to fabricate hydrogels with photonic crystal. Based on the previously reported study³³, our study demonstrates higher reproducibility and reusability compared to conventional photonic crystal hydrogels. In order to investigate the reproducibility, molding processes were performed over 50 cycles, which shows that the mold and molded hydrogels maintained their integrity. Finally, the fabricated photonic crystal hydrogels were tested for different concentrations of solvents, where the color of the structure changed as it swells or contracts in the solvents. These findings suggest the potential application of the hydrogel as a structural color-based chemical indicator.

Methods and materials

Fabrication of mold

The mold used in this study has nano concave structure fabricated by polystyrene nanoparticles and metal layer on a silicon wafer substrate³³ and the overall process is shown in Fig. 1a. The silicon wafer was prepared by washing and plasma treatment so that its surface became hydrophilic. Then, a monolayer of polystyrene nanoparticles (diameter: 780 nm, Bangs Laboratories, Inc.) was self-assembled by spin coating technique at 1000 rpm, as seen in Fig. 1b. And oxygen (O₂) reactive ion etching (RIE) was performed to reduce the average diameter of the nanoparticles. A chromium (Cr) thin film of 200 nm thickness was deposited onto the gaps between the nanoparticles using an electron beam evaporator. After deposition, the polystyrene particles were removed using adhesive tape which left concave nanostructure on the wafer surface. The concave nanostructures formed on the wafer surface were observed using a scanning electron microscope (SEM, JSM-7800 F, JEOL). The wafer was then cut into pieces with an average size of 1.5 × 2 mm² to be used as molds. These molds were placed at the bottom of casting frames made of silicon rubber with unit dimensions of 2.8 × 2.8 × 0.35 cm³ as shown in Fig. 1c and used to fabricate hydrogels with surface nanostructures.

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Fig. 1

Hydrogel molding process. (a) Schematic of mold fabrication using polystyrene nanoparticles and molding process for hydrogel. (b) Si wafer with particle layer before deposition of Cr layer. (c) Casting of hydrogel with mold in casting frame. (d) Hydrogel after demolding (Scale bar = 1 cm).

Synthesis of hydrogel

The synthesis mechanism of polyacrylamide (PAAm) hydrogel is shown in Fig. 2. PAAm was selected among candidate hydrogels because it offers an optimal combination of high swelling capacity, mechanical flexibility for defect-free molding, and consistent structural color visibility after repeated fabrication. Acrylamide (AAm, Sigma Aldrich) was used as the monomer, and N, N′-Methylenebis(acrylamide) (MBAA, Sigma Aldrich) was used as the crosslinker. Curing was performed under UV light, and 2-Hydroxy-4’-(2-hydroxyethoxy)−2-methylpropiophenone (Irgacure 2959, Sigma Aldrich) was used as the photoinitiator. The monomers, crosslinkers, and photoinitiator were mixed in deionized water and sonicated for 20 s. The well-dissolved solution was dispensed onto the silicone rubber casting frame, with the pre-cut mold positioned at the bottom. It was then covered with a slide glass to maintain humidity, and the solution was cured for 7 min under UV light at 365 nm (Vilber Lourmat, VL-6LC). After curing, photonic crystal hydrogels with nanostructures on the surface were synthesized (Fig. 1d). The synthesized hydrogels were dried at room temperature for 5 min before being used in experiments.

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Fig. 2

Schematic illustration of the polyacrylamide hydrogel synthesis mechanism.

Monomer conversion and water content for swelling behavior

To evaluate the swelling behavior of the synthesized hydrogels, the monomer conversion and water content were quantified. The monomer conversion was determined using the following Eq. 

1

where ​ is the weight of the monomer used in the synthesis, and gel​ is the dried weight. The dry weight was measured by soaking the synthesized hydrogels in water for 2 days at room temperature to maximize swelling and then drying them in an oven for 2 days to maximize dryness.

The dry weight was measured after placing the synthesized hydrogels in water at room temperature for up to 2 days to reach maximum expansion, followed by oven drying for up to 2 days until fully dried. The swelling mass ratio of the hydrogel serves as a key parameter for evaluating its swelling behavior, providing a quantitative measure of the extent to which the hydrogel expands upon absorbing water or solvent. The swelling mass ratio is determined by the following Eq. 

2

where is the maximum swollen weight and is the same as the dry weight above.

Water content represents the proportion of water within the hydrogel’s structural composition and is defined as follows:

3

and ​ were calculated using the same dry and expanded conditions.

Hydrogel surface characterization and optical analysis

Reflectance spectra were measured using a spectrometer (Flame-s-vis–nir, Ocean Optics, Inc.) to examine the optical properties of the hydrogel. A tungsten halogen light source was used for illumination. Spectral data were normalized and analyzed to evaluate peak shifts associated with changes in structural color. All digital photos and videos were taken with a cell phone (Samsung Galaxy S24).

The incident angle (θ₁), defined as the angle between the incoming light and the normal to the hydrogel surface, was set to 30°. The camera angle (θ₂), defined as the angle between the camera’s optical axis and the hydrogel normal, was set to 0°, meaning the camera was positioned perpendicular to the hydrogel plane.

Atomic force microscopy (AFM) was used in tapping mode with a scan area of 3.1 × 3.1 μm² to analyze nanoscale topography (NX10, Park Systems). Surface roughness (RMS) values were extracted from AFM images using Gwyddion software.

Result

Mechanical properties of hydrogel for various mixing ratios

Polymers like hydrogels rely on interactions between monomers and crosslinkers to form polymer networks. Crosslinkers play a crucial role in linking monomers and form a structured network that influences the mechanical properties of the synthesized hydrogel, such as stiffness and elasticity. A higher monomer-to-crosslinker ratio means fewer crosslinks per unit monomer. This leads to increased flexibility or stretchability while higher ratio of crosslinker leads to a stiffer and more rigid hydrogel. Based on these characteristics, this study aims to determine the optimal monomer-to-crosslinker molar ratio for fabricating hydrogels which are suitable for molding and mechanically stable. The goal is to achieve a balance between flexibility and strength, ensuring that the hydrogel can be easily released, or demolded, from the mold after the curing while it has sufficient strength to prevent fracture.

Therefore, tensile tests were conducted using hydrogel specimens fabricated according to ASTM D638 standards (Fig. 3a-b). To avoid possible defects on surfaces from cutting, the specimens were made with the molding method. The detailed fabrication process is provided in supplementary information. Supplementary Movie S1 shows that the dog-bone-shaped hydrogel specimen was fractured at the gauge section during tensile loading. This confirms that the specimens were fabricated properly for tensile tests.

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Fig. 3

Experiments on mechanical properties of hydrogel samples. (a) Photograph of tensile specimen (hydrogel) and its mold (PDMS)(Scale bar = 20 mm). (b) Photo of tensile test. (c) Stress–Strain curves for various monomer-to-crosslinker ratios. (d) Comparison of Young’s moduli and yield strengths.

Stress-strain curves were obtained from the tests as seen in Fig. 3c. All the specimens show linear variation until they were fractured, which means that they undergo elastic deformation only. The slope of each curve represents elastic modulus and the end point represents fracture strength, which is identical to yield strength for hydrogels in this study (Fig. 3d). Higher modulus (e.g., 10:1, 20:1) indicates higher stiffness while lower one (e.g., 50:1, 100:1) indicates better flexibility or stretchability.

In the demolding process after the curing, the hydrogels inevitably undergo both local and overall deformation. So better flexibility, or low stiffness, is required for easy demolding. On the other hand, a higher yield strength allows the hydrogel to withstand larger forces without breaking. Therefore, a composition with a higher yield point is preferable to prevent possible fracture during demolding. Consequently, balancing between lower modulus and higher yield strength is critical for molding process of hydrogels. Our results show that the modulus decreases greatly from 10:1 to 20:1, and that the yield strength decreases gradually as the monomer-to-crosslinker ratio increases. This indicates that a ratio of at least 20:1 is recommended to achieve this balance and is suitable for molding-based fabrication.

Hydrogel composition and swelling behavior

The synthesized hydrogels are intended for structural color-based sensors that rely on the periodic nanostructures. Since periodicity is influenced by volumetric changes, understanding the factors that affect swelling behavior is crucial. However, simply increasing the monomer content to induce swelling is inefficient, as it can lead to a lower conversion rate and an excess of unreacted residual monomers. To determine the optimal hydrogel formulation for structural color-based sensing, the swelling behavior was evaluated by investigating swelling ratio, dry weight, conversion rate and shape recovery.

Swelling behavior was first examined as it directly affects structural color performance through volume change. Figure 4a shows the swelling weight and water content which both increase as the monomer proportion increases. These parameters are commonly used to characterize hydrogel network density and water uptake capacity since swelling behavior reflects how loosely or tightly the polymer chains are connected. At the 10:1 ratio the mass expansion was 2.85 times and the water content was 64%. At the 100:1 ratio the values increased to 7.28 times and 86% respectively. A more relaxed polymer network was caused by lower crosslinking densities, which allowed greater expansion and water absorption.

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Fig. 4

Analysis of swelling behavior and network formation in hydrogels. (a) Swelling mass ratio and water content with various monomer-to-crosslinker ratios. (b) Comparison of monomer weight before and after drying. (c) Conversion rate with various monomer-to-crosslinker ratios. (d) Top-view images of hydrogels during swelling and drying steps(grid: 1 cm × 1 cm per square).

Reduced crosslinker concentration also led to lower dry weight after polymerization despite the same monomer amount (Fig. 4b). Dry weight represents the amount of monomer successfully incorporated into the network and is used to estimate conversion efficiency and network formation quality in hydrogel systems. A lower dry weight suggests that fewer monomers participated in network formation which can lead to weak mechanical stability and excessive residual monomer.

The conversion quantifies the proportion of monomers incorporated into the polymer network compared to the total amount of initial monomers. Figure 4c demonstrates that the conversion decreases as the proportion of monomers increases. At the 10:1 ratio, more than 80% of the monomers were converted into the polymer network. However, at the 100:1 ratio, the conversion dropped to around 60%. This reduction at higher monomer proportions results from lower crosslinking density, which reduces polymerization efficiency. In addition, excess monomers do not effectively participate in the reaction due to limited initiator availability or steric hindrance. While introduction of washing step could remove unreacted monomers, it would not resolve the fundamental issue of inefficient network formation. Given these limitations, the 100:1 ratio was considered less favorable for molding applications due to its compromised structural integrity.

To further analyze volumetric expansion, Fig. 4d presents a visual comparison of hydrogel expansion at different monomer ratios. The grid in the figure corresponds to 1 cm per division in both horizontal and vertical directions. Since hydrogels undergo three-dimensional volume changes, weight-based measurements and top-view observations provide complementary insights into expansion behavior. At lower monomer ratios like 10:1, the dense polymer network restricted water uptake, resulting in minimal swelling. In contrast, higher monomer ratios created more open networks due to reduced crosslinking density, allowing greater water absorption and leading to significant volumetric expansion.

The optimal monomer-to-crosslinker ratio was determined by evaluating both mechanical and chemical properties. In the analysis of mechanical properties, a ratio of at least 20:1 was recommended to ensure structural recovery in demolding. As shown in Fig. 4d, the volumetric expansion at 20:1 was not sufficient to induce variation of structural color so it was not suitable for sensor applications (Fig. S2). In contrast, the 100:1 composition showed excessive swelling but its low conversion rate resulted in a high amount of unreacted monomers which reduced the stability of the polymer network. Considering the trade-off between mechanical stability and volumetric expansion, the 50:1 composition was identified as the most suitable candidate for structural color sensors. It provided sufficient mechanical strength while allowing enough volumetric change for colorimetric variation. So, this composition was used in the subsequent experiments to ensure consistent performance and reproducibility in molding process.

Moldability and reproducibility of photonic crystal hydrogels

The molding method in this study makes it possible to replicate structure of photonic crystals of the mold onto the surface of polymeric materials. By this way, the molding results exhibit the same structural color as the mold. So the moldability of the hydrogel has been investigated by comparing its molding results with those of polydimethylsiloxane (PDMS) which is widely used for nanoscale replication.

Figure 5 shows pictures of the molded surfaces of both PDMS and hydrogel. Structural colors appear because light interacts with nanostructures on the surface. Although the colors are iridescent or solid according to distance to light source or observer, both of the hydrogel and PDMS emitted identical structural colors. This indicates that the hydrogel in this study has moldability comparable to that of PDMS. This result confirms that hydrogel can successfully replicate nanoscale structures and preserve structural coloration.

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Fig. 5

Comparison of structural colors in hydrogels and PDMS and the reproducibility of hydrogels.

Reproducibility was evaluated by repeating the molding process using the same mold (Fig. 6). SEM images of the mold, optical images of the hydrogel, and reflectance spectra were used to assess reproducibility. The SEM images of the mold surface in Fig. 6a and b indicate that the mold used for replication remained stable, and the arrangement of the concave structures was not deformed after 50 cycles. Since nanoscale structural deformation directly affects optical appearance, the consistent structural color observed in Fig. 6c also indicates that the mold maintained its integrity throughout the 50 molding cycles. This optical consistency further supports the conclusion that both the mold and the replicated nanostructures remained stable without degradation. As shown in Fig. 6d and e, the structural color of the hydrogel was compared at every 10 cycle, and reflectance spectra were measured to analyze optical stability. While some positions showed slight blue-shifted features due to local variation, most samples consistently exhibited green peaks centered near 520–540 nm. AFM measurements also show reproducibility of the molding process (Fig S3). The RMS surface roughness was measured at 7.194 nm at first molding and 6.522 nm after 50th molding, with standard deviations of 1.392 nm and 1.43 nm, respectively. These results confirm that the nanostructures were replicated uniformly without degradation even after 50 cycles, demonstrating that the hydrogel system can serve as a reproducible structural color platform suitable for sensing applications.

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Fig. 6

Morphological and optical characteristics of molded hydrogels. (a, b) SEM images of mold surface before and after 50 molding cycles.(c) Optical images of hydrogel surface after 1st and 50th cycles. (d) Structural colors of hydrogel across molding cycles. (e) Reflectance spectra over cycles 1 to 50.

Effect of solvent-induced volume changes on structural color

Hydrogels undergo significant volume changes in response to chemical or biological stimuli, which makes them promising materials for sensor applications. So we performed tests by immersing the hydrogels in various solvents to observe color changes resulting from volume swelling or contraction.

Structural color from nanostructure is generated by diffraction, interference, and scattering of light. This phenomenon is governed by Bragg’s law.

4

where is the refractive index, is wavelength of the incident wave, is the distance between lattice planes (periodicity), and is the angle of incidence. In this study, the incident angle and observation angle are identical (), simplifying the Bragg equation to the symmetric form shown above. According to this equation, when the incident angle remains constant, structural color is primarily influenced by variations in .

Meanwhile, solvents are categorized based on their dielectric constants and hydrogen bonding properties. Polar solvents are further classified as protonated or non-protonated, depending on their ability to solvate anions through hydrogen bonding. In this study, responses of hydrogel are expressed or visualized as variations in structural color. And they were compared in protonated polar, non-protonated polar, and non-polar solvents.

Ethanol, a protonated polar solvent, and acetone, a non-protonated polar solvent, were used for comparison. As shown in Fig. 7a, hydrogels immersed in water swelled, whereas those in ethanol and acetone turned white because of dehydration. Ethanol and acetone both cause dehydration via different mechanisms, respectively. Ethanol competes with water through hydrogen bonding, while acetone displaces water due to its small molecular size and volatility.

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Fig. 7

Effect of solvent-induced volume changes on structural color. In the figures, the percentages indicate the proportion of the solvent in distilled water, where control represents pure distilled water. (a) Optical images of hydrogels immersed in water ethanol and acetone. (b) Structural color changes of hydrogels in dimethyl sulfoxide (DMSO) at different concentrations over 24 h. (c) Structural color changes of hydrogels in hexane. (d) Schematic illustration of solvent-induced swelling and contraction in hydrogels.

A different response was observed with dimethyl sulfoxide (DMSO), which is an amphiphilic solvent (Fig. 7b)³⁴. Because of its dual polar and non-polar characteristic, DMSO is highly miscible with water and can interact with both hydrophilic and hydrophobic regions of the hydrogel network^35,36. This allows DMSO to penetrate deeply into the hydrogel and affect its swelling behavior. At lower concentrations (< 50%), DMSO primarily interacts with the hydrophilic network of hydrogel, that enhances water retention and causes swelling. Swelling increases the periodicity of nanostructure, shifting the reflected structural color from green to red. However, at higher concentrations (> 70%), DMSO preferentially interacts with the hydrophobic domains of hydrogel. This interaction reduces water retention and causes dehydration. As water is removed, the hydrogel loses the hydration force to maintain its polymer network. Consequently, the removal of water reduces the intermolecular spacing within the hydrogel, leading to contraction of nanostructure. This contraction reduces the periodicity of the structure, resulting in color shift to blue. And the contraction also reduces the height of individual nanostructure, which decreased the intensity of the structural color.

Hexane was selected as a representative non-polar solvent. Since hexane is immiscible with water, we performed experiments using pure hexane without surfactants. As shown in Fig. 7c, osmotic pressure led to some water leaving the hydrogel, resulting in a slight decrease in volume. As a result, the structural color shifted to a shorter wavelength, showing a blue color. Additional experiments were done without surfactants and they are shown in Fig. S4, where the samples in the presence of water exhibited swelling.

Above experiments show that solvent-driven volume changes directly affect the structural color of hydrogel. Swelling increases periodicity of nanostructure, shifting the color from green to red, while contraction reduces periodicity causing blue shift. This suggests that the chemical response of hydrogels can be visualized with structural colors as illustrated in Fig. 7d.

Reflectance spectra of the hydrogels were measured so that the color shift was quantified as seen in Fig. 8a. Swelling caused a peak shift from 517 nm (green) to 627 nm (red), while contraction caused the shift to 462 nm (blue). According to the previous study³³ the relationship between strain and wavelength shift in a 2D photonic crystal can be explained based on Bragg diffraction. Since the structural color originates from the air–surface interface, the refractive index n is often approximated as 1 for calculation. When uniaxial strain is applied the periodic spacing of the nanostructure increases as and the diffracted wavelength shifts proportionally. Under fixed optical conditions, where the sum of the incident and viewing angles remains constant, the Bragg equation simplifies to a linear form:

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Fig. 8

(a) Reflectance spectra of hydrogels in original (black), swelling (red), and contraction (blue) states. Inset images show the corresponding optical appearance at each peak. (b) Reversible color changes of the hydrogel in response to volumetric transitions. The hydrogel returns to its original green state after repeated cycles between swollen (red) and contracted (blue) states in water. Scale bar = 1 cm.

5

where λ₀ is the initial wavelength, ε is the applied strain, and λ′ is the wavelength after deformation. This model assumes that the periodicity of the surface structure expands uniformly with the applied strain. In our experiment, the hydrogel expanded from 2.51 cm to 3.02 cm in the vertical direction upon swelling, which corresponds to a 20.32% strain (= 0.2032). The structural color of original state was observed at 517 nm in the green region. Based on the Eq. (5), the expected shift in the diffracted wavelength is calculated as 622.75 nm. The color shift from 517 nm to 627 nm, as shown in Fig. 8a, agrees well with the calculated value, supporting the predictability of Bragg’s law in response to solvent-induced swelling.

Additionally, using the same sample, a reversibility test was performed by dehydrating the hydrogel at 48% relative humidity for 24 h and subsequently rehydrating it in water. As shown in Fig. 8b, both contraction and re-swelling induced color changes consistent with the previously observed spectral shifts, confirming the reversible optical response of the structured hydrogel.

In conclusion, the structural color of the hydrogel changed in response to external stimuli due to variations in periodicity of the molded nanostructure. Swelling caused red shifts while contraction led to blue shifts or fading which indicates a relationship between periodicity and optical response. The nanostructure fabricated through the proposed method demonstrated these optical changes clearly and showed consistent behavior with Bragg’s law.

Discussion

In this study, we demonstrated that the molding method enables high reproducibility in hydrogel fabrication and explored its potential for colorimetric sensing. Conventional photonic crystal hydrogels often struggle with reproducibility issues due to the arrangement of colloidal particles and variations in their dispersion. By employing a molding-based fabrication method, we overcame these limitations and achieved highly uniform and stable structural coloration in hydrogel.

Our experimental results highlight three key findings. Firstly the mechanical and chemical properties of the hydrogel were optimized by adjusting the monomer-to-crosslinker ratio. A 50:1 composition provided a balance between flexibility and structural integrity. Secondly the molding approach demonstrated high reproducibility and moldability, maintaining the nanostructure and optical characteristics after repeated molding cycles. Finally, the hydrogel exhibited solvent-responsive color variation. Swelling and contraction occurred depending on the type of solvent and caused predictable shifts in structural color.

The proposed method demonstrates that the developed photonic crystal hydrogel maintains both mechanical stability and colorimetric visualization. Consequently, the molding-based fabrication makes it well-suited for future applications of colorimetric sensors.

Acknowledgements

This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (Grant No. RS-2023-00254093), the National Research Foundation of Korea(NRF) and the Commercialization Promotion Agency for R&D Outcomes(COMPA) grant funded by the Korea government(Ministry of Science and ICT))RS-2025-02309252) and the Institute Project (Grant No. NK255D).

Author contributions

J. K., J. S. Y. conceived and designed the experiments. J.K., N. H. M., D. I. K., K. K., D. H. K. carried out the experiments and analyzed the data. J.K. wrote the paper. Y. Y. commented on the manuscript. J. S. Y. revised and modified the manuscript.

Data availability

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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