Inverse opal hydrogel sensor for the detection of pH and mercury ions

Abstract

An inverse opal hydrogel (IOH) sensor was constructed through colloidal templating photopolymerization of acrylic acid and pemaerythritol-triacrylate. Its dual responsive behaviours to pH and mercury ions (Hg²⁺) were demonstrated by detecting the shift of the diffraction wavelength. The diffraction wavelength of the IOH sensor was dramatically red-shifted when the pH was increased from 11 to 13, due to the swelling of the hydrogel. The shift of the diffraction wavelength can be directly observed by the naked eye through the colour change of the IOH. A fast response behaviour of the IOH sensor to pH was approximately completed within 3 s. Furthermore, carboxyl groups were used to detect Hg²⁺ as recognition groups. A low detection limit of 10 nM for Hg²⁺ was achieved in the optimized IOH sensor. The present work indicates the prospect of constructing multi-responsive IOH sensors using a single recognition group through the facile colloidal templating route.

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Introduction

Responsive photonic crystals (PhCs) are dielectrically periodical materials with a tunable photonic band gap. Their diffraction wavelength can be adjusted by the use of various external stimuli.^1–5 These responsive PhCs have prospective applications in numerous fields, such as photonic paper,^6,7 chemical and biological sensors,^8–12 and optically active components.¹³ Generally, there are two ways to construct responsive PhCs: one way is to synthesize particles with recognition groups, which are self-assembled to form the responsive PhCs. The other way is to fill responsive materials (mainly hydrogels) into the interstitial space of a defined template, and then remove the template to obtain the inverse opal hydrogels (IOHs). Colloidal crystal templating route provides a simple and inexpensive method to prepare IOHs.

IOHs with tunable diffraction wavelength have been reported to show response to pH^14,15 and metal ions.^16,17 In the case of pH recognition, IOHs generally consist of carboxyl, acrylamide or amine functional group. Asher and co-workers reported a polyacrylamide IOH, which showed a redshift of the diffraction wavelength as pH varied from 2 to 9.6, while a blueshift in pH 9.6–11 due to the influence of ionic strength.¹⁸ The pH and ionic strength exhibited contrary influences on the shift of the diffraction wavelength.¹ Consequently, the responsive line of diffraction wavelength possessed a turning point as the pH increased. Since then many efforts had been focused on responsive speed⁹ and mechanical strength of the IOH.¹⁹ However, few studies have been reported concerning the responsive behaviour of the IOH to high pH solution. High pH sensors are promising for the application in the chemical, environmental and biological fields,^20–25 such as corrosion monitoring in concrete structures,²⁰ waste water treatment,^21,22 nuclear fuel processing,²³ and microbial processes.^24,25 Thus, it is highly desirable and significant to design an IOH sensor that responds to high pH without a turning point. On the other hand, metal ion recognition using IOH is widely spread in the environmental protection, especially for heavy metal ions. Mercury ion (Hg²⁺) is one of the most toxic metal ions and it is harmful to health. Many methods have been developed to detect mercury ion.^26,27 Generally, these methods are using complex instruments, including atomic absorption spectrometry,²⁶ inductively coupled plasma atomic emission spectroscopy.²⁷ However, most of these methods are complicated, time consuming and expensive, which limit their practical applications. Recently, much more efforts have been devoted based on fluorescent measurements^28–31 due to the high sensitivity. Li and co-workers used carbon nanoparticles as the fluorescent sensing platform for the selective Hg²⁺ detection.²⁸ Tan et al. achieved highly sensitive detection of Hg²⁺ based on the fluorescence quenching of a terbium chelate probe.³¹ We are aimed to develop a simple and inexpensive method to detect Hg²⁺ by using UV-Vis spectrophotometer to measure the diffraction wavelength shift of the IOH sensor.

In this work, IOH sensors were constructed by utilizing acrylic acid (AA) as recognizing material, and their dually responsive behaviours to high pH and Hg²⁺ were investigated. The diffraction wavelength of the IOH sensor changed from 531 nm to 750 nm when pH was increased from 6 to 13. The responsive behaviour of the IOH sensor to pH can be almost completed within 3 s. In addition, the carboxyl groups were used as recognition group to detect Hg²⁺. The detection limit was determined to be as low as 10 nM. The maximum blueshift of the diffraction wavelength reached 25 nm due to the shrinkage of the IOH. The dually responsive IOH sensor would open up good prospects for the application in detecting high pH and recognizing Hg²⁺.

Experimental details

Materials

Benzil (Aldrich), pemaerythritol-triacrylate (PE-3A, Kyoeisha Chemical Co. Ltd., Japan), and 2-benzyl-2-(dimethylamino)-4′-morpholinobutyrophenone (BDMAMBP, Aldrich) were used. γ-Methacryloxypropyltrimethoxysilane (KH-570) was purchased from Nanjing Chuangshi Chemical Co., Ltd. Styrene (AR), potassium peroxysulfate (AR), sodium hydroxide (AR), potassium chloride (AR), mercury nitrate (AR), cadmium chloride (AR), copper chloride (AR), zinc chloride (AR), chromium trichloride (AR), sodium dodecyl sulfonate (AR), ethanol (AR), hydrogen peroxide (AR), sulfuric acid (AR), and acrylic acid (AR) were purchased from Sinopharm chemical reagent Beijing Co., Ltd. Styrene was purified by distillation under reduced pressure and stored in refrigerator. Potassium peroxysulfate was recrystallized twice, dried overnight, and kept in desiccator for use. All other chemical reagents were used without further purification.

Synthesis of monodispersed polystyrene nanospheres

Monodispersed polystyrene (PS) nanospheres were synthesized by emulsion polymerization according to the previously reported procedure.³² Briefly, sodium dodecyl sulfonate (from 70.8 to 117.1 mg) was dissolved in 100 mL deionized water in a 250 mL three-necked round-bottomed flask. When the solution was heated to 70 °C, 15 mL styrene was added to the flask and the system was stirred under the atmosphere of nitrogen. After stirring for 30 min, potassium peroxydisulfate (from 21.6 to 40.5 mg) was dissolved in 10 mL deionized water and added to the flask at 85 °C. The polymerization was carried out at 85 °C for 12 h with continuous stirring under nitrogen atmosphere. The diameter of the PS colloidal nanospheres was measured by using field emission scanning electron microscopy (FE-SEM).

Fabrication of polystyrene colloidal crystal templates

PS colloidal crystal templates were fabricated through vertical deposition method on glass slides.³³ The glass slides were treated with piranha solution, washed with deionized water, and dried in an oven. Caution: piranha solution is strong oxidant and should be handled with care. The treated glass slides were positioned vertically in a vial containing 0.18 wt% PS colloidal suspension at 65 °C. After 60 h, PS colloidal crystal templates were deposited on the slides.

Preparation of inverse opal hydrogels

Before constructing IOH, glass slides were treated and photoresists were prepared. Glass slides were immersed in a toluene solution of 5 wt% KH-570 overnight. Then the slides were washed with anhydrous ethanol and dried for use. During the preparation of the photoresists, 10 mg benzil (1 wt%, photoinitiator) and 10 mg BDMAMBP (1 wt%, photosensitizer) were dissolved in AA. Then, PE-3A was added to the above solution, and the mixture was stirred overnight. The whole preparation of the photoresist was carried out in the dark room at room temperature. The components of three photoresists were shown in Table 1.

Table 1 Components of the photoresists for constructing IOHs

No.	AA (g)	PE-3A (g)	Benzil (g)	BDMAMBP (g)
1	0.343	0.670	0.010	0.010
2	0.578	0.511	0.010	0.010
3	0.668	0.335	0.010	0.010

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The procedure for preparing IOH is illustrated in Fig. 1. First, PS colloidal crystal template was covered with the KH-570 treated glass slides, and photoresist was added to the interstitial space of the PS colloidal crystal template based on capillary effect. Second, the photoresist was solidified by the photopolymerization under the illumination of UV lamp (320 W high-pressure Hg Arc lamp) for 10 min. Third, the sandwiched structure was split. A polymer film containing PS colloidal crystal template was inclined to adhere to the KH-570 treated cover slide due to the polymerization between the photoresist and the unsaturated double bond of KH-570.^34,35 Finally, IOH was obtained by removing the PS colloidal crystal template through keeping the film in toluene for overnight. The resultant IOHs were dipped in water for 12 h before the study of their responsive behaviours to external stimuli.

Fig. 1 Schematic illustration of the procedures for preparing IOH through colloidal templating method.

Characterization

The transmission spectra were obtained using a UV-Vis spectrophotometer (Shimadzu UV-2550). The microstructures of the PS colloidal crystal templates and IOHs were obtained by a FE-SEM (Hitachi S-4300). The pH in the solution was adjusted by preparing NaOH solution at different concentrations, which were measured using a pH meter (Mettler Toledo FG2-ELK).

Results and discussion

Characterization of PS colloidal crystal templates and inverse opal hydrogels

IOHs were produced by using PS colloidal crystal templates, which were fabricated by the vertical deposition method. Monodispersed PS nanospheres were synthesized with the diameters of 250 nm and 263 nm, respectively. As shown in Fig. 2a and b, PS nanospheres were self-assembled into a face-centered cubic lattice with the (111) planes paralleling to the substrate. The colloidal crystal template exhibited brilliant colour due to the Bragg diffraction of visible light. Fig. 2c and d show the SEM images of the IOHs constructed by the PS colloidal crystal templates containing 250 nm and 263 nm PS nanospheres. The insets of Fig. 2c and d are photographs of the corresponding IOHs. The blueshift of structural colour between colloidal crystal template and IOH is attributed to a decreased refractive index due to inversion of polymer-filled region.^36,37 SEM images and photographs verify the high-quality of the prepared IOHs, which would be promising for the further application in characterizing the responsive properties.

Fig. 2 (a and b) SEM images of PS colloidal crystal templates constructed by 250 nm and 263 nm PS nanospheres, respectively. (c and d) SEM images of the IOHs formed by using 250 nm and 263 nm PS colloidal crystal templates, respectively. Insets are the corresponding photographs. The scale bar is 1 μm.

pH responsive behaviours of inverse opal hydrogels

Responsive behaviours of the IOH to pH were investigated by measuring the diffraction wavelength of the IOH in aqueous solution with different pH. Fig. 3a shows that the diffraction wavelength of the IOH, which has been fabricated by using 250 nm PS colloidal crystal template and photoresist 2, performed red-shift with the increase of pH. The diffraction wavelength of the IOH was located at 530 nm in deionized water at pH 6.3. Increasing of the pH resulted in a redshift of the diffraction wavelength. As the pH was increased from 6.3 to 11, the diffraction wavelength of the IOH almost kept the same. Nevertheless, a dramatic redshift was observed when the pH was changed from 11 to 13. The diffraction wavelength of the IOH arrived at 750 nm when the pH of the solution reached 13. The maximum redshift of diffraction wavelength is about 220 nm. Meanwhile, the variation of the IOH can be observed by the naked eyes, which changes from green, orange to red. In the previous study,¹⁸ the PhC sensor began to blueshift and showed a turning point due to the increasing of ionic strength as the pH was higher than 9.6. In contrast, a turning point was not observed when the pH was varied from 6.3 to 13 in this study. As a result, our IOH sensor can be expected to detect high pH aqueous solution.

Fig. 3 (a) Diffraction wavelength shifts as a function of the pH variation. Insets are the photographs of the IOH in the aqueous solution with different pH. (b) Responsive time of the IOH in the pH 13 aqueous solution. Inset is the transmission spectra of the IOH in pH 13 aqueous solution at different time.

Furthermore, we have studied the responsive rate of the IOH to pH. Fig. 3b shows the pH responsive time of the IOH fabricated by using 263 nm PS colloidal crystal template and photoresist 2. In deionized water, diffraction wavelength of the IOH was 589 nm. As the IOH was immersed in a pH 13 aqueous solution for 3 s, the diffraction wavelength of the IOH moved to 742 nm immediately. After 105 s, the diffraction wavelength further red-shifted 18 nm to reach the equilibrium.

The pH responsive behaviour of the IOH is attributed to the response of polyacrylic acid to the hydroxyl, which is illustrated in Fig. 4. The balance between the inner hydrogel and the outer aqueous solution is similar to the Donnan equilibria.^38,39 As shown in Fig. 4, the osmotic pressure comes to a balance between the inner hydrogel and the surrounding aqueous solution when the IOH is immerged in the deionized water. When the pH of the aqueous solution increases, the balance of osmotic pressure is broken. Hydroxyl groups permeate into the hydrogel to build a new osmotic pressure balance as follows. Neutralized with the hydroxyl groups, carboxyl groups are transferred to carboxylate anions. The carboxylate anions on the polymer chains form an electrostatic repulsion which tends to expand the polymer network. Meanwhile, as the polymer network is swollen, the chains between network junctions are required to assume elongated configurations, and an elastic restoring force consequently develops in opposition to the swelling process.^33,39 As a result, the hydrogel network stops to swell due to the new balance between the electrostatic repulsion force and the elastic restoring force.⁸

Fig. 4 Schematic diagram of sensing mechanism of IOHs to pH and Hg²⁺.

It is worth noting that the diffraction wavelength of IOH began to show obvious redshift at pH 11. The interesting phenomenon was attributed to the decomposition of PE-3A in the hydrogel. The PE-3A was reported to decompose in a NaOH solution in previous study.⁴⁰ When the pH was varied from 6.3 to 11, the red-shift of the IOH was small since the highly crosslinked IOH would prohibit the chain to elongate the configurations. Hence, the framework was slightly swelled. As the pH was higher than 11, the PE-3A decomposed gradually and the degree of crosslink decreased, hence many network junctions were broken and the chains were easy to elongate configuration. As a result, an obvious redshift was observed. In the previously reported study,^14,15 the polymer was lowly crosslinked, an obvious redshift can be observed at a pH range of 6 to 10 due to high activity of the longer chains between the network junctions. The distinctive responsive behaviour between previous work and our study is attributed to the different content of crosslinker in the hydrogel.

Responsive behaviour of inverse opal hydrogels to Hg²⁺

Carboxyl groups in the IOHs were also applied as the recognition groups to detect Hg²⁺. The responsive behaviours of the IOHs to Hg²⁺ were studied by measuring their diffraction wavelength shift in Hg²⁺ aqueous solution. Three IOHs are denoted as IOH1 (photoresist 1), IOH2 (photoresist 2) and IOH3 (photoresist 3), depending on the components of the photoresists shown in Table 1. As shown in Fig. 5a, the increase of concentration of Hg²⁺ results in continuous blueshift of the diffraction wavelength for all of these three IOHs, especially for IOH3. The blueshift of the diffraction wavelength is caused by the shrinkage of the IOHs.

Fig. 5 (a) The blueshift of diffraction wavelength as a function of the Hg²⁺ concentration, IOHs are denoted as IOH1 (photoresist 1), IOH2 (photoresist 2) and IOH3 (photoresist 3). (b) Responsive time of the IOH3 to 10 nM Hg²⁺ aqueous solution. Inset is the transmission spectra of IOH3 in 10 nM Hg²⁺ aqueous solution at different time.

The shrinkage of the IOHs is attributed to the formation of metal complex as well as the decrease of the electrostatic repulsion of the carboxylate anions.^38,39 In the present work, carboxyl group is attached to the framework of the IOHs intending to sense Hg²⁺, which can form complexes with carboxylate anions.³¹ As illustrated in Fig. 4, the formation of the complexes is attributed to the strong affinity of Hg²⁺ to carboxylate anions in the hydrogel, which results in an increase cross-linking in the hydrogel.⁴¹ Thus, binding Hg²⁺ causes shrinkage of the hydrogel in volume and blueshift of the diffraction peak of the IOHs. Besides, the formation of the complex reduces the electrostatic repulsion from the carboxylate anions. As the concentration of Hg²⁺ increases, more carboxylate anions are screened. Consequently, the hydrogels shrink in volume and show blueshift of the diffraction wavelength due to both crosslinking and screening.

Despite of continuous blueshift, three IOHs exhibit different amount of blueshift on diffraction wavelength. The phenomenon is attributed to two reasons: one factor is the different content of functional monomer in the photoresist. More carboxylate anions are crosslinked and screened by the Hg²⁺ as AA content increases, resulting in the stronger shrinkage of the IOHs. The other one is the change of the crosslinking density. Highly crosslinked IOH is difficult to shrink due to its strong mechanical strength. Thus, IOH3 shows the highest degree of shrinkage due to the highest amount of AA and the lowest amount of PE-3A. The phenomenon agrees well with the previously reported results in the similar IOH system.⁴² We further investigated the responsive time of IOH3 to Hg²⁺. Fig. 5b indicates that the equilibrium time to a 10 nM Hg²⁺ aqueous solution is about 11 min. Our result is faster than the recently reported result of 15 min for Cd²⁺ detection using the IOH.⁴¹

Selectivity is of significant importance for the design of sensors. To investigate the sensing selectivity of IOHs, the responsive behaviours of the IOHs to K⁺, Hg²⁺, Cd²⁺, Cu²⁺, Zn²⁺ and Cr³⁺ are studied. Fig. 6a shows the diffraction wavelength shifts as a function of the ion concentration in the aqueous solution. At the same concentration of aqueous solution, Hg²⁺ displays the highest blueshift in the diffraction wavelength among the six kinds of metal ions. Especially when the ion concentration is 1 mM (Fig. 6b), the diffraction wavelength shift of the IOH sensor in response to Hg²⁺ is 25 nm, much larger than that of K⁺, Cd²⁺, Cu²⁺, Zn²⁺ and Cr³⁺.

Fig. 6 (a) The diffraction wavelength shifts as a function of the ion concentration of K⁺, Hg²⁺, Cd²⁺, Cu²⁺, Zn²⁺, Cr³⁺ in the aqueous solution. (b) The diffraction wavelength shift of the IOH in the K⁺, Hg²⁺, Cd²⁺, Cu²⁺, Zn²⁺, Cr³⁺ aqueous solution at the concentration of 1 mM.

The selectivity of the IOH sensor was attributed to the affinity of recognition groups incorporated into the polymer network to the metal ions.^3,31,41 The formation of the complexes, with two ligands incorporated one metal ion centre, increases both crosslinking and screening. Hence, the cumulative formation constant log K₂ is used to justify the affinity of ligands to metal ion. log K₂ of Hg²⁺ to carboxylate anion (8.43) is the highest over the other metal ions (2.3 for Cd²⁺, 3.2 for Cu²⁺, 4.72 for Cr³⁺, K⁺ and Zn²⁺ is negligible).⁴³ Thus, the diffraction wavelength shift of IOH sensor for Hg²⁺ is much stronger than other metal ions. The sensing selectivity means the responsive behaviour of the IOH sensor to Hg²⁺ has little interferences from K⁺, Cd²⁺, Cu²⁺, Zn²⁺ and Cr³⁺ as coexisting metal ions in the aqueous solution.

Conclusions

In this study, we have fabricated dually responsive IOH sensor and demonstrated the responsive behaviours to pH and Hg²⁺. In the pH range from 11 to 13, the IOH exhibit monotonous redshift of the diffraction wavelength with a broad range of 219 nm, which was observed by the naked eyes directly. The fast responsive behaviour of the IOH sensor to pH was almost completed within 3 s. Furthermore, we demonstrated the responsive behaviour of the IOH to Hg²⁺ by utilizing carboxyl as recognition group. The IOH exhibited the Hg²⁺ detection limitation of 10 nM. This system could convert chemical signals into optical signals. The present work could provide high potential for developing multi-responsive sensors through the facile colloidal crystal templating route.

Acknowledgements

The authors gratefully acknowledge support from the National Natural Science Foundation of China (Grant no. 51003113, 61205194 and 91123032), National Basic Research Program of China (2010CB934103), and CAS-JSPS Joint Research Project (GJHZ1411) for financial support.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and general characterizations. See DOI: 10.1039/c4ra03013c

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