Multi-stimuli responsive photonic hydrogel based on a novel photonic crystal template containing gold nanorods

Abstract

Multi-stimuli responsive photonic hydrogels (MRPHs) fabricated by doping nanoparticles into hydrogels show promising potential value in the fields of visual detection and drug delivery. However, complicated surface chemical modification is selected to improve the compatibility between nanoparticles and a pre-gel solution of hydrogel. Herein, we developed a simple and convenient vertical deposition method to prepare a novel photonic crystal (PC) template containing gold nanorods (Au NRs) (Au NRs/PC template), which could respond to near-infrared (NIR) light due to the conversion capability of Au NRs from NIR light to heat. Additionally, carboxyl groups on the surface of polystyrene (PS) colloids endowed the Au NRs/PC template with pH-stimulus responsiveness. Based on the Au NRs/PC template, MRPH film was fabricated by infiltrating the pre-gel solution of poly(N-isopropylacrylamide) (PNIPAM) hydrogel into the gap of a ‘sandwich’ structure through capillary forces and then polymerizing at 25 °C for 24 h. The obtained MRPH film could respond to NIR light, pH and temperature. Under the irradiation of NIR light, only the irradiated position lost structural color while the film volume had no distinct change. With the increase of ambient temperature, the whole MRPH film completely lost structural color and shrank significantly, which was greatly different from the phenomenon irradiated by NIR light. Besides, the structural color of the MRPH film exhibited a red shift from green to orange-red as the pH increased. Overall, both the Au NRs/PC template and the MRPH film may have potential applications in visual detection, due to their multi-stimuli responsiveness.

---

1. Introduction

Photonic crystal (PC) with a periodic structure formed by the periodic arrangement of different refractive index media has attracted extensive attention on account of brilliant structural color through Bragg diffraction.^1–3 Therefore, a series of PC materials with vivid structural colors have been fabricated, which have significant application value in sensors,^4–6 drug delivery,^7–9 actuators,^10–12 printing^13–15 and other fields.^16–18 A hydrogel is a stimulus-responsive material due to its elastic network that can shrink and swell, so it is usually combined with PC to fabricate stimulus-responsive photonic hydrogels, which realize a visual conversion to optical signals by changing structural colors in response to external stimuli, such as light,^19,20 organic compounds and solvents,^21–24 metal ions,^25,26 temperature,^27–29 and pH.^30–32 For example, Zhao et al.³³ combined a polymeric hydrogel with an SiO₂ inverse opal structure to prepare an inverse opal hydrogel (IOH) sensor which can respond to pH, temperature and ultraviolet light. The polymeric hydrogel was composed of pH and temperature responsive poly(dimethylaminoethyl) methacrylate and a light responsive spiropyran-methacrylate segment. Matsubara et al.³⁴ chose thermosensitive monomer N-isopropylacrylamide and light sensitive monomer 4-acryloylaminoazobenzene to prepare a thermally adjustable multicolor photochromic hydrogel.

Stimulus-responsive photonic hydrogels can also be obtained by combining functional nanoparticles, such as graphene oxide quantum dots (GO QDs), gold nanorods (Au NRs), and carbon dots (CDs) with photonic hydrogels. Generally speaking, there are two methods to realize the combination of nanoparticles and photonic hydrogels. One is doping nanoparticles into hydrogel, followed by combining them with PC. For example, Xiao et al.³⁵ modified Au NRs by thiol terminal poly(ethylene glycol) so that they could disperse stably in poly(N-isopropyl acrylamide-co-acrylamide) P(NIPAM-co-AAm) pre-gel solution. Then the pre-gel solution was infiltrated into a two glass slide sandwiched polystyrene (PS) photonic colloid by capillary force. After UV light-initiated polymerization, the Au NRs-polymer hybrid photonic hydrogel was fabricated, which exhibited a near-infrared (NIR) light response. Wang et al.³⁶ dispersed graphene oxide quantum dots (GO QDs) into a biocompatible hydrogel of gelatin methacryloyl (GelMA) and hyaluronic acid methacryloyl (HAMA), and then combined the HAMA/GelMA hydrogel with SiO₂ photonic crystals to prepare self-bonded hydrogel inverse opal particles. Attributed to the photothermal conversion capability of GO QDs, amoxicillin and vascular endothelial growth factors (VEGFs) encapsulated in the inverse opal particles were released under NIR light, which were applied for wound tissue healing and drug delivery monitoring. It is worth noting that the compatibility between nanoparticles and pre-gel solutions may limit the choice of hydrogel systems. The other method is to use nanoparticle-functionalized monodisperse colloids to prepare PC templates, followed by the combination of the PC templates and polymeric hydrogels. For example, Xie et al.³⁷ grafted CDs on the surface of polystyrene@poly (methyl methacrylate-co-acrylic acid) (PS@P(MMA-AA)) colloidal particles through covalent bonding. The PC film from CDs/PS@P(MMA-AA) colloidal particles not only showed multiple structural color and fluorescence emission, but also illustrated the sensitive response toward stress and pH. However, nanoparticle-functionalized monodisperse colloids always require a complicated chemical preparation process.

Nanostructured metals such as Ag, Cu and Au, can be nice plasmonic nanostructures.^38–40 Moreover, their mesoporous structures show nice plasmonic characteristics like a localized surface plasmon.^38,39 Au NRs exhibit an excellent photothermal effect, derived from their plasmonic nanostructures and surface plasmon resonance properties.⁴¹ In this work, we present a multi-stimuli responsive photonic hydrogel (MRPH) film based on a novel PC containing an Au NR (Au NR/PC) template. Firstly, Au NR/PC templates are prepared using a simple and convenient vertical deposition co-assembly method. Attributed to the high efficiency of Au NRs in converting specific NIR light to heat, the Au NR/PC template can exhibit a NIR light response. Since the PS colloids used for preparing the Au NR/PC template are synthesized by copolymerization of styrene (St) and acrylic acid (AA), the surface of PS colloids has −COOH groups so that the Au NR/PC template can respond to pH. Next, a pre-gel solution of PNIPAM is infiltrated into a ‘sandwich’ structure formed by covering the Au NRs/PC template on the top of a glass slide. Finally, after polymerization for 24 h at 25 °C, the Au NR/PC template is combined with the PNIPAM hydrogel. Attributed to the temperature sensitivity of the PNIPAM hydrogel and the dual-stimuli responsiveness of the Au NR/PC template, the MRPH film can respond to NIR light, pH and temperature. Both the Au NR/PC template and the MRPH film are fabricated using low-cost and convenient methods. Thus, we can foresee that the MRPH film and the Au NR/PC template have potential application value in the field of visual detection.

2 Experimental section

2.1 Materials

Styrene (St), potassium persulfate (KPS), chloroauric acid (HAuCl₄), hexadecyltrimethyl ammonium bromide (CTAB), ascorbic acid, silver nitrate (AgNO₃), xanthan gum, N,N′-methylenebisacrylamide (MBA), N,N,N′,N′-tetramethylethylene diamine (TEMED), β-cyclodextrin and concentrated hydrochloric acid (HCl) were purchased from Sinopharm Chemical Reagent Co., Ltd. N-Isopropylacrylamide (NIPAM), acrylic acid (AA) and sodium borohydride (NaBH₄) and were purchased from Aladdin Reagent Co., Ltd. Concentrated sulfuric acid was provided by Xinyang chemical reagents. Carbon dots (C-dots) were prepared through low-temperature hydrothermal carbonization.⁴² Gold nanorods (Au NRs) were prepared using the improved seed mediated growth procedure,⁴³ in which the addition of 1.5 mL 1 mol L⁻¹ HCl was used to prepare Au NRs. The Au NRs were dispersed in water to obtain a burgundy Au NR suspension (Fig. S1(a), ESI†). The TEM image of the Au NR indicates that the Au NRs had a uniform size and a rod-like microstructure (Fig. S1(b), ESI†). The average size of Au NRs analyzed using the software Image-Pro Plus was 60.96 nm × 15.39 nm (length × diameter), and the ratio of the length to diameter was 3.96. The location of the longitudinal surface plasmon resonance (LSPR) absorption peak of the prepared Au NRs was at 808 nm (Fig. S1(c), ESI†). Styrene was washed three times with 10 wt% NaOH solution before use. Deionized water was produced by deionization and filtration using a Millipore device (resistivity = 18.2 MΩ cm⁻¹). The glass slides (76.2 × 25.4 mm) were treated using a mixed solution of concentrated sulfuric acid and H₂O₂ (7 : 3) for 48 h to improve their hydrophilicity.

2.2 Preparation of monodisperse PS colloids

The preparation of monodisperse PS colloids with different particle sizes was achieved by soap-free emulsion polymerization. First, 0.6 g KPS and 0.65 g AA were dissolved in 170 mL deionized water in a four-necked flask equipped with a mechanical stirrer and stirred for 10 min at a speed of 300 rpm under the protection of N₂ to mix the above solutions evenly and remove O₂. Then, the reaction kettle was slowly heated to 75 °C in an oil bath, followed by adding 15 mL of refined St into a constant pressure dropping funnel, in which 1/3 of monomer St was slowly dropped into the reaction kettle. When blue light appeared in the reaction system, the rest of St was dropped into the mixture. After the polymerization continued for 8 h, monodisperse PS colloid latex was obtained. Five kinds of PS colloids with different particle sizes (258, 295, 340, 396, and 458 nm) were fabricated by changing the amount of St, AA and KPS, which were named as PS₂₅₈, PS₂₉₅, PS₃₄₀, PS₃₉₆ and PS₄₅₈, respectively.

2.3 Preparation of the Au NRs/PC template

Typically, 1 mL of a 0.5 mg mL⁻¹ Au NR dispersion with an LSPR absorption peak at 808 nm and PS colloid latex were mixed in 80 mL of deionized water, in which the content of PS colloids was 0.1 wt%. Then the mixed dispersion was ultrasonically treated for 0.5 h, followed by vertically inserting the glass slide into the mixed dispersion. Then, the Au NRs/PC template was obtained through the vertical deposition self-assembled method at 60 °C. The Au NR/PC templates formed by depositing PS₂₅₈, PS₂₉₅, PS₃₄₀, PS₃₉₆ and PS₄₅₈ were labeled as Au NRs/PC₂₅₈, Au NRs/PC₂₉₅, Au NRs/PC₃₄₀, Au NRs/PC₃₉₆ and Au NRs/PC₄₅₈, respectively.

2.4 Preparation of the MRPH film

Synthesis of the pre-gel solution of PNIPAM hydrogel: briefly, 0.02 g C-dots were dispersed in 8 mL deionized water, and ultrasonic dispersion was carried out for 0.5 h. Then, 0.02 g xanthan gum, 1.5 g NIPAM and 0.02 g MBA were sequentially added into the above dispersion until it was completely dissolved with the help of magnetic stirring. Then, the mixed solution was moved to an ice bath environment and stirred continuously for 1 h under the protection of N₂. Additionally, 0.01 g KPS and 11 μL TEMED were added to the mixed solution and stirred vigorously for 1 min. Finally, the excess bubbles were removed by ultrasonic wave in an ice bath for 3 min and the pre-gel solution was obtained for later use.

Fabrication of the MRPH film. Firstly, two polyester film strips (0.3 mm in thickness) were placed on both sides of the glass slide and then the Au NRs/PC₂₅₈ template was covered on the top to form a ‘sandwich’ structure, where a gap of around 0.3 mm between the two slides was reserved. Then, the pre-gel solution of PNIPAM hydrogel was slowly injected into the gap through capillary forces until the template became transparent. Next, the pre-gel solution was polymerized at 25 °C for 24 h and finally the MRPH film was obtained.

2.5 Characterization

The morphology of Au NRs was observed with a field emission high-resolution transmission electron microscope (TEM, JEM-2100F, Electronics, Japan) at an accelerating voltage of 80 kV. Before the observation, 10 μL of the Au NR dispersion was diluted with deionized water, and then dropped onto a copper net to be dried naturally.

The characteristic plasmon resonance absorption peak of Au NRs was measured using an ultraviolet and visible spectrophotometer (UV-2550, Shimadzu, Japan) at room temperature. An Au NR dispersion was diluted three times before testing. The scanning wavelength range was 400–1000 nm and the incident light was perpendicular to the sample.

An optical fiber spectrometer (USB 2000+, Ocean Optical Company, USA) was used to record the reflection spectra of the Au NRs/PC template and the MRPH film at room temperature. The scanning wavelength range was 400–800 nm, and the incident light was perpendicular to the sample.

A cold field emission scanning electron microscope (SEM, JSM-7500F, Electronics, Japan) was used to observe the surface morphology of the Au NRs/PC template and the MRPH film at an accelerating voltage of 5.0 kV. Before observation, the MRPH film was frozen with liquid nitrogen, and then lyophilized for 24 h to obtain a completely freeze-dried film sample. All the samples were coated with gold using a sputter coater.

The 808 nm NIR laser (MDL-H-808-5W, Changchun New Industry Photoelectric Technology Co., Ltd, China) was used to explore the photothermal effect of Au NR/PC templates and MRPH films under different NIR intensities (1 W cm⁻², 2 W cm⁻² and 3 W cm⁻²). The distance between the sample and the light source probe was fixed at 5 cm. The temperature changes of the sample surface were recorded using an infrared thermometer (AT380, Xima industry, China). Before exploration, MRPH film was put into water (pH = 7) to reach swelling equilibrium and the water on the surface of the MRPH film was absorbed by filter paper.

After reaching the swelling equilibrium, the MRPH film was placed on a heating stage with a temperature controlling module to perform the thermosensitive test. The pH-sensitive test of MRPH films was carried out at 25 °C by touching aqueous solutions with different pH values.

The swelling ratio (SR) of MRPH film was calculated according to the mass change of hydrogel before and after swelling, as defined in formula (1)

where m refers to the mass of the MRPH film after swelling (g), and m₀ represents the mass of the MRPH film before swelling (g).

3 Results and discussion

3.1 Preparation and morphology of the Au NR/PC template

The vertical deposition method for the preparation of the Au NRs/PC template is schematically illustrated in Fig. 1(a). In this procedure, with the evaporation of water, PS colloids are uniformly self-assembled by capillary force on the glass slide. Since the average size of Au NRs is 60.96 nm × 15.39 nm and the widest voids of PS colloids are around 63 nm, Au NRs can be held in the voids of PS colloids.

Fig. 1 (a) Schematic diagram of the preparation for the Au NR/PC template. (b) Optical photos of the Au NR/PC template assembled by PS colloids with different particle sizes.

Fig. 1(b) exhibits the structural colors of Au NR/PC templates formed using different PS colloid sizes. Visibly, Au NR/PC₂₅₈ and Au NR/PC₂₉₅ templates show bright green and red structural colors, respectively. The structural colors of Au NRs/PC₃₄₀, Au NRs/PC₃₉₆ and Au NRs/PC₄₅₈ templates are pale blue, blue and white, respectively. With the increase of the particle size of PS colloids, the structural colors of Au NRs/PC templates become paler and even disappear. The theoretical calculation formula of the reflection wavelength is as follows:

λ_max = 2.397D (2)

where λ_max is the maximum reflection wavelength of the Au NR/PC template and D represents the diffraction plane lattice spacing.

Because the wavelength range of light visible to the naked eye is about 390–780 nm, the Au NR/PC template can show bright structural colors only when the theoretical particle size of the PS colloids is about 163–325 nm according to formula (1). The average sizes of PS₂₅₈ and PS₂₉₅ are between 163 and 325 nm so that Au NRs/PC₂₅₈ and Au NRs/PC₂₉₅ templates show bright green and red structural colors, respectively. However, the average sizes of PS₃₄₀, PS₃₉₆ and PS₄₅₈ are beyond the theoretical range, so the structural colors of the Au NRs/PC templates formed by them are pale or even lost.

In addition, both the concentration of PS colloids and the amount of Au NRs in the assembly solution affect the structural color of the Au NR/PC templates. In order to investigate the influence of the PS colloids’ concentration, PS₂₅₈ colloids were selected and the amount of Au NRs was fixed at 0.5 mg. As shown in Fig. 2, the Au NR/PC templates formed are flat and uniform, and also show bright and vivid structural colors, when the concentration of PS₂₅₈ colloids is between 0.1 wt% and 0.5 wt%. However, with the concentration of PS₂₅₈ colloids increasing from 0.7 wt% to 1.0 wt%, the Au NR/PC templates become rough and non-uniform, and the structural colors start disappearing, which is due to the irregular arrangement of PS colloids caused by the deposition acceleration with the increase of PS₂₅₈ colloid concentration. As a consequence, 0.1 wt% is an optimal concentration of PS₂₅₈ colloids and was selected for further studies.

Fig. 2 Optical photos of Au NR/PC₂₅₈ templates assembled with different PS₂₅₈ colloid concentrations.

Fig. 3(a) shows the optical photos of the Au NRs/PC₂₅₈ templates with different amounts of Au NRs when the concentration of PS₂₅₈ colloids was fixed at 0.1 wt%. When the amount of Au NRs varies from 0 to 2.5 mg, the structural colors of the Au NR/PC₂₅₈ templates are yellow-green and have little difference with each other. However, the structural colors of the Au NR/PC₂₅₈ templates are completely lost when the amount of Au NRs reaches 3.5 mg and 4.5 mg. The reflection spectra of the Au NRs/PC₂₅₈ templates with the corresponding amount of Au NRs were recorded in Fig. 3(b). It can be found that with the amount of Au NRs increasing from 0 mg to 4.5 mg, the reflection peaks of Au NRs/PC₂₅₈ templates are about 572 nm, with no obvious red shift or blue shift. The reflection of the reflection peak decreases slightly first, but drops greatly when the amount of Au NRs is over 2.5 mg. It is indicating that when the amount of Au NRs exceeds the void capacity of PS colloids, redundant Au NRs stop the formation of the three-dimensional face-centered cubic structure of the Au NR/PC₂₅₈ template, so the template impossibly presents structural colors.

Fig. 3 (a) Optical photos of the Au NR/PC₂₅₈ template with different amounts of Au NRs. (b) The reflection spectra of the Au NRs/PC₂₅₈ template under conditions with different amounts of Au NRs.

As a result, the optimal concentration of PS₂₅₈ colloids and the amount of Au NRs are 0.1 wt% and 0.5 mg, respectively, and Au NRs/PC₂₅₈ templates are selected for further study of dual-stimuli responsiveness.

SEM images of Au NR/PC₂₅₈ templates prepared under two different concentrations of PS₂₅₈ colloids are shown in Fig. 4. When the concentration of PS₂₅₈ colloids is 0.1 wt%, the PS₂₅₈ colloids are uniform in size and arranged orderly in the Au NRs/PC₂₅₈ template, as shown in Fig. 4(a). The average face center distance of PS colloids is 116 nm, calculated from Fig. 4(a). Fig. 4(b) presents a SEM cross-section image of the Au NRs/PC₂₅₈ template. Distinctly, the superposition of multiple layers of regular PS colloids constitutes a three-dimensional face-centered cubic Au NR/PC₂₅₈ template, which can illuminate the brilliant structural color of the template generated from 0.1 wt% PS₂₅₈ colloids (Fig. 2). When the PS₂₅₈ colloid concentration increases to 1.0 wt%, most PS₂₅₈ colloids are still arranged orderly, as shown in Fig. 4(c). However, the sectioned SEM image (Fig. 4(d)) shows that there are irregular arrangements in some areas. These obvious defects give a reason why the template from 1.0 wt% PS₂₅₈ colloids has a rough surface without brilliant structural color (Fig. 2).

Fig. 4 (a) Surface SEM image of the Au NR/PC₂₅₈ template generated from 0.1 wt% PS₂₅₈. (b) Sectioned SEM image of the Au NR/PC₂₅₈ template generated from 0.1 wt% PS₂₅₈. (c) Surface SEM image of the Au NR/PC₂₅₈ template generated from 1.0 wt% PS₂₅₈. (d) Sectioned SEM image of the Au NR/PC₂₅₈ template generated from 1.0 wt% PS₂₅₈.

3.2 Dual stimuli responsiveness of Au NR/PC templates

In order to explore the NIR-stimulus responsiveness of the Au NR/PC₂₅₈ template, we put the Au NR/PC₂₅₈ template under different NIR intensities and measured the temperature increase rate of the Au NR/PC₂₅₈ template, as shown in Fig. 5. When the NIR intensity is 1 W cm⁻², the temperature increase rate of the Au NR/PC₂₅₈ template is only 0.16 °C s⁻¹. When the NIR intensity is increased from 1 W cm⁻² to 2.0 W cm⁻², the temperature increase rate of the Au NR/PC₂₅₈ template changes slightly from 0.16 °C s⁻¹ to 0.88 °C s⁻¹. In particular, the temperature increase rate of the Au NR/PC₂₅₈ template changes significantly, from 0.88 °C s⁻¹ to 3.85 °C s⁻¹ when the NIR intensity reaches 3.0 W cm⁻².

Fig. 5 Histogram of the temperature increase rate of the Au NR/PC₂₅₈ template under different NIR intensities.

As PS colloids are prepared using a soap-free emulsion polymerization of St with a small amount of AA, the surface of PS colloids has hydrophilic −COOH groups, which provides the Au NR/PC₂₅₈ template with pH-stimulus responsiveness. To study the pH-stimulus responsiveness of the Au NR/PC₂₅₈ templates, aqueous solutions of pH 2.3, pH 7 and pH 12 were dropped on the Au NR/PC₂₅₈ templates respectively. As shown in Fig. 6(a), after touching aqueous solutions of pH 2.3 or pH 7, the structural colors of the Au NR/PC₂₅₈ templates change little, from light green to green. However, when the alkaline solution with a pH of 12 is dropped on the template, a great change happens, the structural color turns to orange. Moreover, the structural colors of the Au NR/PC₂₅₈ templates return back to green after the aqueous solutions of pH 2.3, pH 7 and pH 12 evaporate completely. Fig. 6(b) displays the changes in the reflection spectra of the Au NR/PC₂₅₈ templates under different pH conditions (2.3, 7 and 12). Compared to the original reflection peak of the Au NR/PC₂₅₈ template (at 554 nm), the reflection peaks have no obvious shift at pH 2.3 and 7. However, the reflection peak of the template shifts to longer wavelengths (from 554 nm to 589 nm) at pH 12. After the alkaline solution on the template evaporates completely, the maximum reflection wavelength returns to 552 nm, close to the initial value. Because the refractive index of water is 1.33, according to the modified Bragg diffraction formula, the theoretical value of the reflection peak after filling aqueous solutions is 578 nm, which is due to the fact that the air in the voids of PS₂₅₈ colloids is replaced by the water with a higher refractive index. However, the reflection peak moves to 589 nm at pH 12, because the −COOH groups on the surface of PS colloids are deprotonated to form carboxylate ions (−COO⁻) under alkaline conditions. The electrostatic repulsion between carboxylate ions causes the face-center distance between PS colloids in the template to increase, so the red shift of the reflection peak occurs. After the water evaporation of the alkaline solution, the electrostatic repulsion between PS colloids disappears. Therefore, the face-center distance between PS colloids returns to the initial value, which leads to a blue-shift of the reflection peak (from 589 nm back to 551 nm). Under the conditions of pH 7 or 2.3, the electrostatic repulsion between PS colloids is too small or disappears, so the structural color and the reflection peak of the Au NRs/PC₂₅₈ template are not obviously affected.

Fig. 6 (a) Optical photos of the Au NR/PC₂₅₈ template under conditions of three different pH values. (b) The reflection spectra of the Au NR/PC₂₅₈ template before and after adding an acidic solution with pH 2.3, water with pH 7, and alkaline solution with pH 12, respectively.

3.3 Preparation and morphology of the MRPH film

The schematic fabrication process of the MRPH film is shown in Fig. 7. We selected the green Au NR/PC₂₅₈ template to fabricate the MRPH film. The pre-gel solution of PNIPAM hydrogel infiltrates the gaps of PS colloids under the capillary action. After a free radical polymerization of NIPAM, an orange-red MRPH₂₅₈ film is obtained. The color difference between the Au NR/PC₂₅₈ template and the MRPH₂₅₈ film is due to the fact that the air in the voids of the PS colloids is replaced by the PNIPAM hydrogel with a greater refractive index than the air. Since PS₂₅₈ colloids are physically stacked on the glass slide, the face-centered cubic structure formed by them is easily destroyed by an external force. Therefore, how to keep the integrity of the face-centered cubic structure is the key problem in the infiltrating procedure of the pre-gel solution. In order to solve this problem, xanthan gum is introduced into the hydrogel system to adjust the viscosity of the pre-gel solution of PNIPAM.

Fig. 7 Schematic illustration of the formation process of the MRPH₂₅₈ film.

The optical photos of the MRPH₂₅₈ film prepared with different amounts of xanthan gum are shown in Fig. 8. The structural color of the MRPH₂₅₈-1 film without xanthan gum is orange-red but not bright, and there is some white crazing in the middle area, indicating that the face-centered cubic structure of the template is damaged during the infiltrating process of the pre-gel solution. After adding 0.01 g xanthan gum, the structural color of the MRPH₂₅₈-2 film becomes brighter and the crazing content decreases obviously. When the amount of xanthan gum increases to 0.02 g, a perfect MRPH₂₅₈-3 film gum is formed, with a bright orange-red structural color and smooth surface. The reason is that the addition of xanthan gum increases the viscosity of the pre-gel solution appropriately, which prevents the face-centered cubic structure from being destroyed during the infiltrating process. Therefore, the optimal amount of xanthan gum is 0.02 g.

Fig. 8 Optical photos of the MRPH₂₅₈ film with different amounts of xanthan gum.

In addition, the amount of MBA can influence the structural color of the MRPH₂₅₈ film through changing the crosslinking density of the MRPH₂₅₈ film. With the addition of 0.02 g xanthan gum, and varying the amount of MBA from 0.01 to 0.05 g, MRPH₂₅₈-4, MRPH₂₅₈-5, MRPH₂₅₈-6, MRPH₂₅₈-7 and MRPH₂₅₈-8 films can be obtained. As shown in Fig. 9(a), the structural colors of five MRPH₂₅₈ films are all red, with few changes at different MBA amounts before the films swell. However, after reaching the swelling equilibrium, the MRPH₂₅₈-4, MRPH₂₅₈-5, MRPH₂₅₈-6, MRPH₂₅₈-7 and MRPH₂₅₈-8 films exhibit different structural colors, white, red, orange, yellow and green, respectively. Fig. 9(b) shows the relationship between the swelling ratios of the MRPH₂₅₈ films and MBA amount. With the amount of MBA increasing, the swelling ratio decreases gradually. According to previous research reports,³³ the increase of swelling ratio can make the maximum reflection wavelength increase. When the amount of MBA is 0.01 g, the crosslinking density of the MRPH₂₅₈-4 film is so low that the MRPH₂₅₈-4 film absorbs more water and the swelling ratio is above 2.0. Therefore, the maximum reflection wavelength of the MRPH₂₅₈-4 film is beyond the visible wavelength range and the structural color of the MRPH₂₅₈-4 film is not seen by naked eyes. When the amount of MBA increases from 0.02 to 0.05 g, the increasing crosslinking densities of the MRPH₂₅₈ films make the swelling ratio of the MRPH₂₅₈ films decrease, which causes the maximum reflection wavelengths of the MRPH₂₅₈ films to decrease and they are located in the visible wavelength range. Therefore, the MRPH₂₅₈-5, MRPH₂₅₈-6, MRPH₂₅₈-7 and MRPH₂₅₈-8 films show different bright structural colors, respectively. It is noteworthy that the structural color of the MRPH₂₅₈-5 film is almost the same before and after swelling when the amount of MBA is 0.02 g. Therefore, the amount of both xanthan gum and MBA are fixed at 0.02 g in the following multi-stimuli responsiveness study.

Fig. 9 (a) Optical photos of the MRPH₂₅₈-film with different amounts of MBA. (b) Swelling ratios of the MRPH₂₅₈ film under different amounts of MBA.

SEM images of the Au NR/PC₂₅₈ template and MRPH₂₅₈-5 film are shown in Fig. 10(a) and (b), respectively. Obviously, the voids of the PS₂₅₈ colloids are completely filled with PNIPAM hydrogel. The PS colloids show a regular face-centered cubic structure, indicating that the structure of the Au NRs/PC₂₅₈ template is completely preserved during the preparation process of the MRPH₂₅₈-5 film. According to the calculations from Fig. 10(a) and (b), the average face center distance of PS colloids slightly increases from 116 nm to 118 nm, after the PNIPAM hydrogel fills the Au NR/PC₂₅₈ template. As shown in Fig. 10(c), the maximum reflection wavelengths of the Au NR/PC₂₅₈ template and MRPH₂₅₈-5 film are at 570 nm and 600 nm, respectively, which is due to the fact that the air in the voids of the PS colloids are replaced by hydrogel with a higher refractive index than air.

Fig. 10 (a) SEM diagram of the Au NR/PC₂₅₈ template. (b) SEM image of the MRPH₂₅₈-5 film. (c) The reflection spectra of the Au NR/PC₂₅₈ template and MRPH₂₅₈-5 film.

3.4 Multi-stimuli responsiveness of the MRPH film

3.4.1 NIR-stimulus responsiveness of the MRPH film. The MRPH₂₅₈-5 film shows a bright orange-red structural color before irradiation, as shown in Fig. 11(a). After 10 s irradiation of 808 nm NIR light, the MRPH₂₅₈-5 film changes to an opaque white color and losses the structural color completely at the irradiation position, while the position of the MRPH₂₅₈-5 film without irradiation still maintains a bright orange-red structural color. It is also observed that a layer of water droplets appears on the surface of the MRPH₂₅₈-5 film after the irradiation. The volume and shape of the MRPH₂₅₈-5 film has little change. More interestingly, the structural color of the irradiation position recovers to an orange-red color within 10 s after the NIR light is removed. The change of the reflection spectrum during the recovery process of the structural color is monitored and shown in Fig. 11(b and c). It can be seen that the reflection spectrum of the MRPH₂₅₈-5 film begins to show a reflection peak when the surface temperature of the MRPH₂₅₈-5 film drops to 36.2 °C. Accompanying the continuing decline of the temperature (from 36 °C to 27 °C), the reflection of the reflection peak becomes stronger and stronger, and the reflection peak shifts to longer wavelengths (from 609 nm to 619 nm). All these results indicate that the MRPH₂₅₈-5 film can make a visual response to 808 nm NIR light. The NIR-stimulus responsiveness has two advantages of remote control and regioselectivity.

Fig. 11 (a) Optical photos of the MRPH₂₅₈-5 film before and after NIR light irradiation. (b) The reflection spectrum of the MRPH₂₅₈-5 films within 1 min after removing the illumination. (c) The relationship between the maximum reflection wavelength of the MRPH₂₅₈-5 film and the surface temperature within 10 s after removing the irradiation. (d) The reflection of the maximum reflection peak (619 nm) of the MRPH₂₅₈-5 film when NIR light switches on and off for 10 cycles.

Under the irradiation of NIR light, the Au NRs embedded in the MRPH₂₅₈-5 film at the irradiation position absorb NIR light and convert it into heat, so the surface temperature of the irradiation position increases rapidly and is higher than the LCST of the PNIPAM hydrogel in the MRPH₂₅₈-5 film, which leads to the phase change of part of the PNIPAM hydrogel network from the hydrophilic state to hydrophobic state. Therefore, the MRPH₂₅₈-5 film shows an opaque white color and the water in the film is “squeezed out”, just, in the irradiation position, but the positions without irradiation have no change. After the NIR light is removed, the surface temperature of the irradiation position drops, so the PNIPAM hydrogel network gradually turns from the hydrophobic state to the hydrophilic state. Squeezed-out water molecules are absorbed by the PNIPAM hydrogel again and the swelling ratio of the MRPH₂₅₈-5 film changes back. Therefore, the orange-red structural color appears again and the maximum reflection wavelength of the MRPH₂₅₈-5 film shifts to a longer wavelength during this recovery process.

Furthermore, we switch NIR light on or off 10 times to evaluate the recoverability of the NIR-stimulus responsiveness of the MRPH₂₅₈-5 film. It is found that the appearance of the MRPH₂₅₈-5 film has no change through observations with the naked eye. More importantly, the responsiveness of structural color remains quick as usual, and the reflection peak has no red or blue shift during 10 switching cycles. It is also found from Fig. 11(d) that the intensity of the maximum reflection peak at 619 nm has no obvious fluctuation. These results demonstrate that the NIR-stimulus responsiveness of the MRPH₂₅₈-5 film has good recoverability.

3.4.2 Temperature-stimulus responsiveness of the MRPH film. The appearance of the MRPH₂₅₈-5 film at different temperatures was recorded and the results are shown in Fig. 12(a). The MRPH₂₅₈-5 film shows a red structural color at 27 °C. When the ambient temperature increases to 34 °C, the MRPH₂₅₈-5 film appears partially blue. However, the structural color of the MRPH₂₅₈-5 film suddenly disappears at 36 °C. As the temperature continues to increase to 41 °C, the MRPH₂₅₈-5 film becomes completely opaque and shrinks significantly. Therefore, the MRPH₂₅₈-5 film has temperature-stimulus responsiveness and the LCST of the film is about 36 °C. Accordingly, the reflection peak of the MRPH₂₅₈-5 film decreases with the increasing temperature and even disappears when the temperature is over 36 °C, as shown in Fig. 12(b). When the MRPH₂₅₈-5 film experiences 10 temperature cycles between 27 °C and 41 °C, the film shrinks and losses the structural color at high temperature, but the volume and structural color of the MRPH₂₅₈-5 film can recover at low temperature. More importantly, the reflection of the maximum reflection peak (642 nm) for the MRPH₂₅₈-5 film has just slight fluctuation over 10 temperature cycles, as shown in Fig. 12(c). Therefore, the temperature-stimulus responsiveness of the MRPH₂₅₈-5 film shows good recoverability at an ambient temperature between 27 °C and 41 °C.

Fig. 12 (a) Optical photos of the MRPH₂₅₈-5 film at different temperatures. (b) The reflection spectra of the MRPH₂₅₈-5 film at different temperatures. (c) The intensity of the maximum reflection peak at 642 nm when the MRPH₂₅₈-5 film experiences 10 temperature cycles between 27 °C and 41 °C.

The difference in the temperature-stimulus responsiveness from NIR-stimulus responsiveness is that both the structural color and the film volume change with the increase of ambient temperature in the whole MRPH₂₅₈-5 film. Since the irradiation of NIR light just causes local temperature increase, the changes just happen in the irradiation position of the film. However, when the ambient temperature exceeds 36 °C, the whole network of the PNIPAM hydrogel in the MRPH₂₅₈-5 film changes from the hydrophilic state to hydrophobic state. A large amount of water in the film is squeezed out, so the volume of the whole film shrinks and the whole film exhibits a white color.

3.4.3 pH-responsive behavior of the MRPH film. Fig. 13(a) presents the structural color of the MRPH₂₅₈-5 film under different pH conditions. When the MRPH₂₅₈-5 film is immersed in aqueous solutions with pH values of 2.4, 5.6, 7.5, 10.3 and 12 respectively under room temperature, the structural color of the MRPH₂₅₈-5 film changes from green to pale yellow, yellow, orange and orange-red. By further testing the corresponding reflection spectrum, it can be found from Fig. 13(b) that the maximum reflection wavelengths of the MRPH₂₅₈-5 film in the corresponding pH values are 545 nm, 609 nm, 609 nm, 635 nm, and 643 nm, respectively. Fig. 13(c) exhibits the maximum reflection wavelength of the MRPH₂₅₈-5 film as a function of pH. Distinctly, the maximum reflection wavelength tends to be red-shifted with the increase of pH. According to the previous discussion in 3.3, the swelling ratio has great influence on the maximum reflection wavelength. In order to investigate whether the swelling ratio of the MRPH₂₅₈-5 film causes the change of the maximum reflection wavelength, the swelling ratios of the MRPH₂₅₈-5 film at different pH values are recorded in Fig. S3 (ESI†). It is found that the swelling ratio of the hydrogel film maintains 1.7–1.8 with the pH value varying from 2 to 12, indicating that the volume of the MRPH₂₅₈-5 film has no obvious change. Therefore, the shift of the reflection peak of the MRPH₂₅₈-5 film is not caused by the change of the swelling ratio. According to the discussion in section 3.2, the Au NR/PC₂₅₈ template can respond to pH due to the electrostatic repulsion after −COOH groups are deprotonated under high pH values. For the same reason, deprotonated −COOH groups of xanthan gum in the hydrogel cause the electrostatic repulsion between PS colloids and molecular chains of the hydrogel, which increases the face-center distance between PS colloids. These two reasons render the red shift of the reflection peak of the MRPH₂₅₈-5 film. In order to evaluate the recoverability of the pH-stimulus responsiveness of the MRPH₂₅₈-5 film, the reflection spectra alternating with pH between 2.4 and 12 for 10 cycles is displayed in Fig. 13(d). It is found that there is no distinct fluctuation of maximum reflection peak, demonstrating that the MRPH₂₅₈-5 film has good recoverability under various pH conditions.

Fig. 13 (a) Optical photos of the MRPH₂₅₈-5 film in five different pH environments. (b) Reflection spectrogram of the MRPH₂₅₈-5 film in five corresponding pH environments. (c) Variation curve of the maximum reflection peak wavelength of the MRPH₂₅₈-5 film with pH. (d) Recoverability of the pH-response behavior of MRPH₂₅₈-5 film after 10 cycles when the pH changes between 2.4 and 12.

4 Conclusions

In summary, we have proposed a simple and convenient vertical deposition method for the fabrication of Au NR/PC templates, which can respond to NIR light and pH. When the voids of PS colloids were larger than the average size of the Au NRs, the Au NRs can be held in the voids of PS colloids. The optimal formulation for preparing the Au NR/PC template with bright structural color and a flat surface was that the concentration of PS₂₅₈ was 0.1 wt% and the amount of Au NRs was 0.5 mg. Based on the Au NR/PC template, the MRPH film was prepared using the common method, infiltrating the pre-gel solution of PNIPAM into the Au NR/PC template and polymerizing under mild conditions (25 °C for 24 h). Typically, the introduction of xanthan gum was used to adjust the viscosity of the pre-gel solution of PNIPAM to prevent the face-centered cubic structure from being destroyed during the infiltrating process. The MRPH film was sensitive to various external stimuli including NIR light, temperature and pH. Under the irradiation of NIR light, the structural color was lost only at the irradiated position while the film volume had no obvious change. However, with the increase in ambient temperature, the whole MRPH₂₅₈-5 film completely lost structural color and shrank significantly. Furthermore, the structural color of the MRPH₂₅₈-5 film exhibited a red shift from green to orange-red as the pH increased. Therefore, both the Au NR/PC template and MRPH film have great potential application in visual detection materials. The simple and convenient fabrication method is beneficial to achieving industrial production of the Au NR/PC template and MRPH film.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China, China (Grant No. 51873167) and the National Innovation and Entrepreneurship Training Program for College Students (Grant No. CY202124).

References

1. R. K. Cersonsky, J. Antonaglia, B. D. Dice and S. C. Glotzer, Nat. Commun., 2021, 12, 2543.
2. P. Wu, J. Wang and L. Jiang, Mater. Horiz., 2020, 7, 338–365.
3. Z. Cai, Z. Li, S. Ravaine, M. He, Y. Song, Y. Yin, H. Zheng, J. Teng and A. Zhang, Chem. Soc. Rev., 2021, 50, 5898–5951.
4. S. Yoon, H. Park and W. Lee, Lab Chip, 2021, 21, 2997–3003.
5. S. Chen, Q. Ren, K. Zhang, W. E. I. Sha, T. Hao, H. Xu, J. Zhao and Y. Li, Sens. Actuators, B, 2022, 347, 131326.
6. J. Shin, S. G. Han and W. Lee, Sens. Actuators, B, 2012, 168, 20–26.
7. L. Z. Liu, X. Y. Sun, Z. Y. Yan and B. F. Ye, New J. Chem., 2021, 45, 7893–7899.
8. X. Shou, Y. Liu, D. Wu, H. Zhang, Y. Zhao, W. Sun and X. Shen, Chem. Eng. J., 2021, 408, 127349.
9. X. Sun, L. Liu, H. Zou, C. Yao, Z. Yan and B. Ye, Int. J. Nanomed., 2020, 15, 4959–4967.
10. D. Zhang, J. Liu, B. Chen, Y. Zhao, J. Wang, T. Ikeda and L. Jiang, ACS Nano, 2018, 12, 12149–12158.
11. Z. Zhang, Z. Chen, Y. Wang, J. Chi, Y. Wang and Y. Zhao, Small Methods, 2019, 1900519.
12. R. Yu, L. Zhu, Y. Xia, J. Liu, J. Liang, J. Xu, B. Wang and S. Wang, Adv. Mater. Interfaces, 2022, 9, 220041.
13. Y. Wang, Q. Zhao and X. Du, Mater. Horiz., 2020, 7, 1341–1347.
14. C. Zhao, H. Li, Y. Wang, K. Li, J. Hou, Y. Ma, M. Li and Y. Song, Adv. Opt. Mater., 2019, 1900127.
15. A. T. L. Tan, S. Nagelberg, E. Chang-Davidson, J. Tan, J. K. W. Yang, M. Kolle and A. J. Hart, Small, 2020, 16, 1905519.
16. J. Hu, S. Yang, Z. Chen, Y. Chen and J. Wei, ACS Appl. Polym. Mater., 2023, 5, 1002–1013.
17. S. Wu, J. Nan, Y. Wu, Z. Meng and S. Zhang, ACS Appl. Mater. Interfaces, 2022, 14, 27048–27055.
18. Y. Shang, Z. Chen, F. Fu, L. Sun, C. Shao, W. Jin, H. Liu and Y. Zhao, ACS Nano, 2019, 13, 796–802.
19. Y. Xia, S. Gao, R. Yu, Z. Zeng, H. He, X. Zhou, Y. Hu, M. Cao and S. Wang, ACS Appl. Polym. Mater., 2021, 3, 757–764.
20. X. Fei, T. Lu, J. Ma, S. Zhu and D. Zhang, Nanoscale, 2017, 9, 12969–12975.
21. H. Zhao, J. Gao, Z. Pan, G. Huang, X. Xu, Y. Song, R. Xue, W. Hong and H. Qiu, J. Phys. Chem. C, 2016, 120, 11938–11946.
22. M. Guo, X. Q. Yu, J. Zhao, J. W. Wang, R. K. Qing, J. D. Liu, X. Wu, L. Zhu and S. Chen, Sens. Actuators, B, 2021, 347, 130639.
23. Y. Zhang, J. Tang, K. Feng, H. Zhao, W. Yan, Z. Sun, F. Wu, Y. Sun and J. Gao, React. Funct. Polym., 2020, 148, 104504.
24. Y. Cao, G. Liu, B. Zheng, X. Wang, H. Li, G. Wang, L. Zhao and Y. Wang, Soft Matter, 2021, 17, 4969–4978.
25. Y.-L. Wang, X. Wang, H.-R. Yu, T. Liang, X.-B. Lv and C.-J. Cheng, Soft Matter, 2023, 19, 4880–4890.
26. W. Hong, X. Hu, B. Zhao, F. Zhang and D. Zhang, Adv. Mater., 2010, 22, 5043–5047.
27. C. Li, Q. Xue, Z. Ji, Y. Li, H. Zhang and D. Li, Soft Matter, 2020, 16, 3063–3068.
28. K. Ueno, K. Matsubara, M. Watanabe and Y. Takeoka, Adv. Mater., 2007, 19, 2807–2812.
29. L. Cai, Y. Wang, L. Sun, J. Guo and Y. Zhao, Adv. Opt. Mater., 2021, 2100831.
30. M. L. Zhang, F. Jin, M. L. Zheng and X. M. Duan, RSC Adv., 2014, 4, 20567–20572.
31. N. Griffete, H. Frederich, A. Maître, S. Ravaine, M. M. Chehimi and C. Mangeney, Langmuir, 2012, 28, 1005–1012.
32. J. Wang, Y. Hu, R. Deng, R. Liang, W. Li, S. Liu and J. Zhu, Langmuir, 2013, 29, 8825–8834.
33. W. Zhao, M. Quan, Z. Cao, Y. Zhang, J. Wen, D. Pan, Z. Dong, Z. Yang, D. Wang, H. Cao and W. He, Colloids Surf., A, 2018, 554, 93–99.
34. K. Matsubara, M. Watanabe and Y. Takeoka, Angew. Chem., Int. Ed., 2007, 46, 1688–1692.
35. X. Xiao, D. Shi, Z. Yang, Q. Yu, D. Kaneko and M. Chen, New J. Chem., 2021, 45, 4016–4023.
36. L. Wang, L. Sun, F. Bian, Y. Wang and Y. Zhao, ACS Nano, 2022, 16, 2640–2650.
37. A. Q. Xie, J. Guo, L. Zhu and S. Chen, Chem. Eng. J., 2021, 415, 128950.
38. H. Lim, D. Kim, G. Kwon, H. J. Kim, J. You, J. Kim, M. Eguchi, A. K. Nanjundan, J. Na and Y. Yamauchi, J. Phys. Chem. C, 2020, 124, 23730–23737.
39. H. Lim, D. Kim, Y. Kim, T. Nagaura, J. You, J. Kim, H. J. Kim, J. Na, J. Henzie and Y. Yamauchi, J. Mater. Chem. A, 2020, 8, 21016–21025.
40. H. Lim, T. Nagaura, M. Kim, K. Kani, J. Kim, Y. Bando, S. M. Alshehri, T. Ahamad, J. You, J. Na and Y. Yamauchi, RSC Adv., 2020, 10, 8309–8313.
41. C. Han, M. Y. Qi, Z. R. Tang, J. Gong and Y. J. Xu, Nano Today, 2019, 27, 48–72.
42. M. Hu, Y. Yang, X. Gu, Y. Hu, J. Huang and C. Wang, RSC Adv., 2014, 4, 62446–62452.
43. B. Nikoobakht and M. A. El-Sayed, Chem. Mater., 2003, 15, 1957–1962.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3sm01349a

‡ These authors contributed equally to this work.

---