Photonic hydrogels for the ultratrace sensing of divalent beryllium in seawater†

Junjie Qin ^ab, Bohua Dong ^a, Lixin Cao *^ab and Wei Wang *^ac
aSchool of Materials Science and Engineering, Ocean University of China, Qingdao, 266100, P. R. China. E-mail: caolixin@ouc.edu.cn; wangw@ouc.edu.cn
^bCollege of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, 266100, P. R. China
^cAramco Research Center-Boston, Aramco Services Company, Cambridge, MA 02139, USA. E-mail: wei.wang@aramcoservices.com

First published on 22nd March 2018

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Introduction

Energy sources and environmental pollution are among the biggest problems facing humanity nowadays. Nuclear power plants are one of the effective ways to solve the energy problem. Due to its special properties, beryllium (Be) is widely used in nuclear power plants as one of the best moderator reflector materials of nuclear reactors.^1–3 Also, beryllium has been found to have many applications in electronic instruments, aviation and aerospace engineering, and structural units in satellites.^4–6 However, virtually all Be compounds exhibit significant pulmonary toxicity.⁷ Epidemiological studies have also suggested that Be and its compounds may be carcinogenic,⁸ and may impair one's health since beryllium can be enriched in the organs and soft tissue through people's diet.⁹ Micromolar concentrations of Be²⁺ are known to inhibit certain enzymes. Research has shown that beryllium is likely to result in the lethal lung disease chronic beryllium disease (CBD)^10–12 and, therefore, it is ranked as a Class A Environmental Protection Agency (EPA-US) carcinogen.¹³ Since Be²⁺ ions might leak or be released into seawater from nearby nuclear power plants, the monitoring of Be²⁺ in the environment is important because of its harmful effects on aquatic biota.

The current methods for Be²⁺ detection have many drawbacks, such as space-consuming and expensive instruments, cumbersome sample pretreatment, complicated operation, ex situ detection, and sometimes low accuracy.^14–21 It is urgent to present an easy method to achieve in situ monitoring and a fast response toward Be²⁺, especially around nuclear power plants.

Colloidal photonic crystals (CPCs) have been developed as sensors for glucose,^22–24 vapor,^25,26 nanoparticles,²⁷ molecules,^28,29 biosensing,^30–34 temperature,^35–37 and ions^32,38–41 after Asher et al. first immobilized crystalline colloidal arrays (CCAs) into environmentally responsive hydrogels.^42,43 Most recently, based on the published methods,^43,44 this kind of hydrogel sensing materials has been applied to the detection of uranyl ions and achieved a limit of detection (LOD) of 10 × 10⁻⁹ M in water.⁴⁵ Because of the high interference of ionic impurities in the ordering of CCAs, i.e. non-close packed CPCs, only nonionic compounds can be used in the fabrication of these hydrogels, thus limiting the variety of the sensor systems. Unlike the hydrogels with embedded close packed CPCs, which respond to analytes by volume expansion only, hydrogels with non-close packed structures can efficiently respond to analytes via both volume swelling and shrinking. Cui et al. and Asher et al. have developed combined physical–chemical methods to fabricate the hydrogel system and extended the application range of the functionalized hydrogels.^46,47 Recently, we have fabricated a novel hydrogel sensor for mercury (Hg²⁺) detection and first demonstrated that polyacrylamide (PAM) photonic hydrogels have excellent stability at high ionic strength in seawater,⁴⁸ but so far there are no reports about this kind of hydrogel sensor applied to Be²⁺ detection, especially in seawater.

In this research, we have presented a new chemical route to introduce crown ether molecules as ionic recognition groups into a polymerized colloidal photonic crystal (PCPC) hydrogel system, and studied the stability, sensitivity, and selectivity of the functionalized hydrogel for Be²⁺ binding in seawater. In the fabrication, a 1,2-bis-(2,3-epoxypropyl)-benzene (BEPB) compound was first synthesized as the precursor for the crown ester molecules, and then covalently bonded into the polymer network of the acrylamide to form the benzo-9-crown-3 (B9C3) with the smallest cave structure through a cationic polymerization process.⁴⁹ The B9C3 can selectively chelate with Be²⁺ efficiently, and some studies have also tried to use this property for the detection of Be²⁺ in water samples by microcantilever deflection measurement or electrochemical analysis.^6,50 However, these methods are unable to analyze samples in seawater on site. With the developed photonic hydrogel sensor, we demonstrate a simple analytical platform for the rapid and sensitive detection of Be²⁺ ions in situ in seawater, as schematically illustrated in Fig. 1.

Experimental

Materials

Styrene (99%), poly(vinyl alcohol) (PVA, M_W ≈ 10800), acrylamide (99%), and beryllium sulfate (BeSO₄·4H₂O, 99.99%) were obtained from Aladdin. Catechol (99%) and epichlorohydrin (analytical grade) were purchased from Sinopharm of China. N,N′-Methylenebisacrylamide (analytical grade) was obtained from Macklin. Diethoxyacetophenone (DEAP, >95%) is a TCI product, and ion exchange resin (AG501-X8) came from Bio-Rad. Deionized water (18.2 MΩ cm⁻¹) was used throughout all experiments. Natural seawater was collected from Shilaoren in Qingdao, China, with coarse filtration.

Fabrication of physically cross-linked PVA–PCPC hydrogels

The polystyrene (PS) colloidal nanoparticles (NPs) were prepared using a method of emulsion polymerization,^51,52 following a process of dialyzing for two weeks to wash off the residual ions. The PS NPs purified by dialysis can self-assemble to form non-close packed CPCs in suspension (Fig. 1b(i)). In previous studies, PVA hydrogels have been prepared by a freeze–thaw process.^53–55 In our study, to form the PVA–PCPC hydrogel (Fig. 1b(ii)), firstly, a PVA solution (10 wt%) was made by dissolving 5 g PVA in 45 g DMSO with both heating and stirring. After the PVA was dissolved completely, 5 mL deionized water was added to mix with the PVA solution. Then, the mixed solution underwent gelation through cooling to 25 °C in a water bath. In a typical process, 0.5 g PVA gelation and 0.5 g PS CPC suspension (∼8 wt%) were mixed with 0.15 g ion-exchange resin in a glass bottle. The mixture was heated to 50 °C and vibrated with a vortex until a strong color, as a result of diffraction, appeared. After centrifuging for 3 minutes, the ion-exchange resin beads subsided to the bottom of the bottle and the bubbles in the suspension were also removed by this process. A cell of 125 μm thickness was prepared using two pieces of quartz glass with a parafilm as a spacer. After 0.5 mL of the CPC suspension was injected into the cell, it was placed into ultra-deep-freeze equipment (−24 °C). After a period of 5 h, the cell was taken out and put into a water bath at 25 °C for 16 h. This process allowed for the complete development of the crystalline structure of PVA. After that, the cell was opened, and the PVA–PCPC hydrogel was picked off, rinsed with deionized water, and kept in deionized water for further experiments.

Preparation of functionalized PAM–PCPC hydrogels with B9C3

In our approach, the benzo-9-crown-3 is formed by polymerizing BEPB, which was synthesized according to previously reported methods.^49,56 In a typical preparation process (Fig. 1b(iii)), a precursor solution, which contains acrylamide (110 mg) as the monomer, N,N′-methylenebisacrylamide (2 mg) as the crosslinker, and DEAP (10 μL, 10 wt% in DMSO) as the free radical photo-initiator, was first mixed, and then, BEPB (8 μL, 1 wt% of reaction solution) as the co-monomer was added into the solution with the protection of light. Subsequently, a piece of PVA–PCPC hydrogel (1 × 1 cm²) was immersed in the mixture. After softly shaking for a period of 2 h, the hydrogel was settled carefully in another identical quartz cell. 0.2 mL of the remaining precursor solution was injected into the cell to drive the air out of the quartz cell. Then, the cell was placed stably to let the monomer solution diffuse into the hydrogel. After equilibration for 1 h, the cell was placed in between two UV lamps (300 W) for photopolymerization for 50 min. After the photopolymerization reaction, the crown ether modified hydrogel (PVA–c-PAM–PCPC) was peeled off and washed with deionized water. To remove the PVA scaffold network, the PVA–PAM hydrogel was put into a water bath at 55 °C for 5 min, as shown in Fig. 1b(iv). Then, the B9C3 functionalized PAM–PCPC (c-PAM–PCPC) hydrogels were fabricated in thin film form. As a reference, unfunctionalized PAM–PCPC was also fabricated via photopolymerization under UV light under the same experimental conditions, but without the addition of BEPB.

Preparation of standard solutions

Standard seawater was prepared by dissolving components in deionized water according to a criterion of the ASTM.⁵⁷ A stock solution of Be²⁺ ions was prepared by dissolving BeSO₄·4H₂O (17.72 g, 0.1 mol) in 1 L of standard seawater. Other concentrations of Be²⁺ solution were diluted from the stock solution.

Characterization

A dynamic light scattering (DLS) method was used to measure the average size and zeta potential of the synthesized PS NPs, using a Brookhaven 90 Plus/PALS instrument. An Ocean Optics USB2000 optical fiber spectrometer was used to obtain the diffraction spectra. The angles between the optical fiber and the surface of the hydrogels were 90°, as shown in Fig. S1 (ESI†).

Results and discussion

Properties of PVA–PCPC and functional PAM–PCPC hydrogels

Highly monodisperse PS colloidal particles were prepared by emulsion polymerization.^51,52 The synthesized PS NPs have an average size of 98 nm in diameter, as shown by the TEM image in Fig. 2a. The polydispersity and zeta potential (ζ) of the PS NPs could reach 0.01 and −44 mV, respectively (Table S1, ESI†). The high monodispersity and high surface charge density of the PS NPs are the most crucial parameters for forming a robust non-close packed CPC structure in suspension for the PCPC fabrication. The SEM image in Fig. 2b shows the surface morphology of a dried CPC film on a glass substrate, indicating a regular face-centered cubic (fcc) lattice. To quantitatively introduce ion-recognition groups into the hydrogel, we proposed the use of BEPB as a co-monomer to polymerize with acrylamide via a photopolymerization process, and the structure of the synthesized BEPB molecules was confirmed by ¹H-NMR, as shown in Fig. S2 (ESI†). However, because the polymerization process of BEPB is cationic polymerization⁴⁹ and ionic compounds may disturb the periodic structure of the surface charge-stabilized CPCs, the non-close packed CPCs cannot be directly immobilized into hydrogels in the presence of precursor molecules with charged functional groups. To solve this problem, we first used PVA as a hydrogel scaffold to immobilize the CPCs, before further chemically grating ion-recognition groups into the polymer network in the hydrogel, since PVA–PCPC hydrogels⁵⁸ can be formed and then dissociated physically. Therefore, a combined cryotropic-chemical polymerization process⁴⁶ is adopted to fabricate functional PAM–PCPC hydrogels with B9C3. In this way, the hydrogels with covalently bonded functional groups are very stable and can be used in complex environments without the leakage of the functional groups.

Upon the formation of the PVA–PCPC and functional c-PAM–PCPC hydrogels (as confirmed by the FT-IR spectra in Fig. S3, ESI†) by polymerization reactions (Fig. 1b), the hydrogel was peeled from the quartz plate. As shown by the surface characteristics of the dried PVA–PCPC and c-PAM–PCPC hydrogels in Fig. 2c and d, respectively, the PVA and PAM hydrogels obviously filled in the interspaces between the PS NPs and the immobilization of the NPs does not destroy the regular fcc lattice of CPC, despite some distortion caused by the dryness of the hydrogel. We can see that the 3D photonic crystal structure in the functional c-PAM–PCPC hydrogel has a uniform distribution (Fig. 2e), and the thicknesses of the dried PVA–PCPC and c-PAM–PCPC hydrogels are 104.5 μm and 185 μm (Fig. 2e and f), respectively. The hydrogels with embedded 3D non-close packed CPCs are able to respond to analytes in media by volume changes both in terms of swelling or shrinking. To achieve reliable functionalized PCPC hydrogels for a sensing material in seawater, the diffraction properties and stability of the formed hydrogels have been studied. Fig. 3a shows the diffraction spectra and corresponding optical photographs of the nonfunctional PVA–PCPC, functionalized PVA–c-PAM–PCPC, and c-PAM–PCPC hydrogels from a typical fabrication. All three types of PCPC hydrogel exhibit vivid colors under white-light illumination, due to the diffraction of the immobilized 3D PS CPCs in the hydrogels. The original PVA–PCPC hydrogel shows a diffraction maximum at 457 nm, and its diffraction peak red-shifts to 545 nm when the second polymer network, from the acrylamide and BEPB, is inserted into the hydrogel. The swelling of the hydrogel is because the volume of the incorporated PAM, along with the free energy of the hydrogel, is increased.^59–61 After the dissolution of the PVA, the c-PAM–PCPC hydrogel shrinks and its diffraction peak blue-shifts to 517 nm. This is due to the volume loss of the PVA and the free energy decrease of the system. The c-PAM–PCPC hydrogel exhibits a stable and strong diffraction peak in the visible wavelength range, which forms the basis for using it as a sensing material for the detection of Be²⁺ ions through spectral measurement or even visual inspection by the naked eye.

The diffraction response of c-PAM–PCPC to seawater has also been investigated. When we take the c-PAM–PCPC hydrogel out of deionized water and put it into seawater, the volume of the hydrogel gradually swells until it reaches equilibrium after 3 minutes (Fig. 3b). The corresponding diffraction spectra undergo a red shift of ∼15 nm. The hydrogel swells because the ions at high concentrations in seawater incorporate into the hydrogel and the volume of the ions make the space between the PS NPs increase, with an increase in the free energy. We also tested the unfunctionalized PAM–PCPC hydrogel fabricated by photopolymerization without adding an ion-recognition agent, and found that the nonfunctional hydrogel also red-shifts its diffraction peak by ∼16 nm when it is transferred from pure water to seawater, as shown in the upper image of Fig. 3b. This result reveals that the diffraction red-shift is independent of the ion recognition groups in the hydrogels, implying that all the types of ion in seawater have no effect specific to the B9C3. Without a doubt, the addition of the functional agent will make the swell factor change. However, in our method, the content of the functional molecules (1 wt%) is too low to make an evident change to the swell factor of the hydrogels. Therefore, the swell of the hydrogels in seawater mainly relies on the swell factor of the PAM and PS particles, and so, it can be attributed to this reason why the Δλ of the unfunctionalized hydrogel and the functionalized hydrogel in deionized water and seawater were very close (16 nm and 15 nm). This property enables the functionalized hydrogel to only interact with specific ions, such as Be²⁺. As for the stability of the photonic hydrogels in seawater, we observed that the colors and diffraction spectra would reach an equilibrium in 3 minutes and no further change was seen with time. This phenomenon reveals the good stability of the hydrogels in seawater, which is necessary for the detection of ions.

Response of crown ether functionalized hydrogels to Be²⁺ in seawater

The kinetic interaction of hydrogels containing different amounts of B9C3 with Be²⁺ in seawater has been studied. Experiments were implemented with different hydrogels of the size 1 × 1 cm², and it was noticed that hydrogels with a higher content of B9C3 react with Be²⁺ more rapidly and Be²⁺ ions at lower concentration require a longer reaction time for their detection in seawater. At lower concentrations of Be²⁺, Be²⁺ ions need more time to diffuse uniformly into the hydrogels, which is caused by the osmotic pressure that is due to the difference in the mobile ion concentration inside and outside the gel (Donnan potential).⁶² As shown in Table 1, the reactions between Be²⁺ and B9C3 could reach equilibration within 28 min. Based on this observation, we then chose a reaction time of 35 min to ensure the completion of the reaction in all follow-on experiments for the detection of Be²⁺ in seawater.

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Functionalized hydrogels containing different amounts of B9C3 were used to investigate the diffraction response to different concentrations of Be²⁺ in seawater. Before the detection of Be²⁺ ions, the hydrogels were equilibrated in standard seawater for at least 10 minutes, and the diffraction wavelength at this time was recorded as the original position. It is clear in Fig. 4a and c that the existence of Be²⁺ in seawater makes the hydrogels swell and thus the diffraction wavelength shifts to red, but the sensitivity of the functionalized hydrogels with different amounts of B9C3 toward Be²⁺ in seawater is different. With two hydrogels having similar initial diffraction wavelengths, when the content of B9C3 is 2 wt%, the hydrogels are more sensitive to Be²⁺ in the concentration range of 10 μM to 10 mM, while at 1 wt% B9C3, the hydrogels are more sensitive to Be²⁺ in the concentration range of 0.01 nM to 10 μM. The shifted wavelengths are 36 nm (534 nm to 570 nm) and 22 nm (533 nm to 555 nm), respectively, as shown in Fig. 4a and c. The hydrogels with 1 wt% B9C3 content have better sensitivity to Be²⁺ than those with 2 wt%, and this may be caused by the lower elasticity of the B9C3 in the polymer network. With a continuous increase in Be²⁺ concentration, the diffracted wavelength no longer further shifts to red beyond a certain Be²⁺ concentration, depending on the content of B9C3 groups in the hydrogels. The threshold of concentration implies a saturation of chelated Be²⁺ ions by the B9C3 in the c-PAM–PCPCs. The diffracted wavelength and corresponding concentration of Be²⁺ have a linear relationship in certain concentration ranges (Fig. 4b and d). The linear detection scopes of the two hydrogels are 10⁻²–10⁻⁷ M and 10⁻⁵–10⁻¹¹ M, while the LODs are 8 × 10⁻⁸ M and 10⁻¹¹ M, respectively. As shown in Fig. 4b and d, the corresponding slopes of the fitting lines are 5.72 and 3.403, and the correlation factors (R) are greater than 0.997. Since the diffracted wavelengths of the hydrogels are proportional to the concentration of Be²⁺ in specific ranges, the functionalized photonic hydrogels with different amounts of B9C3 can be fabricated and used as sensing materials for the quantitative detection of Be²⁺ in seawater.

According to the analysis above, it is obvious that Be²⁺ can lead to a red shift in the diffracted wavelength by interacting with the B9C3 functionalized photonic hydrogels. The B9C3 molecule has the smallest crown among the crown ester family and the size of its cavity is very close to the small size of a Be²⁺ ion,^6,50,63,64 and thus the chelation effect is strong and selective between the crown ester and Be²⁺ ions. The swell of the hydrogel volume is due to the interposition of Be²⁺ in the ring of the B9C3 (Fig. 5).

The maximum diffraction peak of the functionalized photonic hydrogels obeys Bragg's law:

λ0 = 2neffdsinθ

where λ₀ represents the diffracted wavelength, d represents the space between the NPs in the fcc lattice, n_eff is the effective refractive index, and θ is the incident angle. In our experiment, the incident angle is vertical to the surface of the hydrogels. As we all know that d is directly proportional to the cube root of the volume,⁶² Bragg's law can be transformed as

λ0 = 2neffkV1/3

Therefore, the diffracted wavelength and its shift are proportional to the cube root of the hydrogel volume. This is the theoretical basis for the proposed sensors. Then, according to this theory, the binding between Be²⁺ ions and the B9C3 in the functionalized photonic hydrogels would lead to an increase in the hydrogel volume, following a red shift of the diffracted wavelength.

For comparison, we have also measured the response of the hydrogels to Be²⁺ in pure water. As shown in Fig. 6, the concentration range for detection of Be²⁺ in water is broader than that in seawater. For example, when the amount of B9C3 is 1 wt%, the linear scope of detection is 10⁻²–10⁻¹⁰ M. However, the linearity between the diffracted wavelength of the hydrogel and the concentration of Be²⁺ is poorer in water than in seawater, as reflected by the fitting correlation factor R, which is 0.977 for hydrogels containing 1 wt% B9C3, while R is 0.997 for the same hydrogel in a seawater system. However, when the amount of B9C3 is 0.5 wt%, the sensitivity of the photonic hydrogel shows no evident increase in pure water, in which the linear scope of detection remains in the same range of 10⁻²–10⁻¹⁰ M. Also, the fitting correlation factor R (0.93) is much poorer than that of the other hydrogels. In comparison, as shown in Fig. 4, the hydrogel with a lower content of B9C3 has a better sensitivity in seawater systems. This phenomenon can be understood as follows. Because of the low ion strength in pure water, some of the Be²⁺ ions that enter into the hydrogel first need to balance the osmotic pressure rather than chelate with B9C3, and this lowers the sensitivity of detection. However, seawater can function like a buffer solution and suppress the effects of ionic strength changes, and therefore, more Be²⁺ can bind to the crown ether and the resulting volume changes of the hydrogels are more effective and steadier in seawater. This advantage makes this kind of hydrogel sensing material more suitable to work in seawater systems.

To evaluate the selectivity, the effects of different kinds of ions on the responses of the photonic hydrogels in seawater were investigated. In the selectivity tests, the B9C3 content in the functionalized photonic hydrogels was 0.85 wt%, while the concentration of Be²⁺ was 10⁻⁵ M and the concentration of other added cationic ions was 10⁻⁴ M. As shown in Fig. 7, all other ions present in seawater show little response to the c-PAM–PCPC hydrogels and none of these ions can make the diffraction wavelength shift more significantly than Be²⁺, even if the concentration of the other ions is 10 times higher than that of Be²⁺. We also compared the detection sensitivities of other ions to Be²⁺ at the same concentration level (1 × 10⁻⁵ M) in seawater for the hydrogels, and the results showed that the diffraction wavelength was shifted by Be²⁺ ions by 12 nm and no diffraction wavelength shifts from the other ions were detectable, except for a 2 nm shift from Li⁺ ions. Because of the perfect match between the sizes of the small Be²⁺ ion and the small cavity of B9C3, the interaction between the crown ester and Be²⁺ is specific. Although some other ions, such as Li⁺, also have a small size, the monovalent ions have weak chelation with the crown ester molecules. The chelation constants of B9C3 with metal ions reported in literature⁶⁵ has revealed that the affinity of Na, K, Fe, Ca, Co, Zn, Mg, and Ag to the crown ether is very small (chelation constants <10² ± 10^0.85), while the chelation constants for the small sized cations of Be and Li are >10⁶ ± 10^2.1 and 10^2.2 ± 10^0.65, respectively. These data are consistent with the results observed in our hydrogel tests. Thus, it is certain that B9C3 could act as a highly selective ionophore in the construction of a Be²⁺ ion-selective sensor, i.e., the unique chelation between B9C3 and Be²⁺ makes the c-PAM–PCPC hydrogels highly selective to Be²⁺ ions in seawater. As c-PAM–PCPC was formed via the photopolymerization of acrylamide, N,N′-methylene bisacrylamide, and B9C3, in order to exclude the possibility that Be²⁺ will chelate with the amide groups in the PAM, we researched the effect of the amide groups on the diffraction spectra by employing PAM–PCPC hydrogels with and without B9C3. We have used the different kinds of hydrogels (unfunctionalized PVA–PCPC, unfunctionalized PAM–PCPC, and functionalized c-PAM–PCPC) to detect Be²⁺, and the results are shown in Fig. S8 (ESI†). We can see that the diffraction wavelength of both PVA–PCPC and PAM–PCPC show no evident change when interacting with Be²⁺ at concentrations of 10⁻⁵ M, while c-PAM–PCPC has a red-shift of 12 nm, confirming that the amide groups in PAM have no evident effect on Be²⁺.

For the c-PAM–PCPC hydrogel sensors, the Be²⁺ ions bonded during the test can be dissociated by washing the hydrogel multiple times in 0.01 M buffer solution (phosphate buffer, pH = 7), deionized water, and seawater, in turn. For example, the c-PAM–PCPC hydrogel with 2 wt% ion recognition molecules was used to detect Be²⁺ at 10⁻³ M, and after washing, the diffraction peak of the hydrogel can reverse to nearly its original position. When reusing the hydrogel sensor, the good stability and reproducibility of c-PAM–PCPC are demonstrated for the detection of Be²⁺ in seawater (Fig. 8).

Analysis of real seawater samples containing Be²⁺

To evaluate the applicability of the c-PAM–PCPC hydrogels in the detection of Be²⁺ in real samples, Be²⁺ was added to real seawater (Shazikou of Qingdao, China) to form simulating samples. The measured results were averaged from 3 tests for each sample and are listed in Table 2. It could be seen that the recoveries were from 103.3% to 115%, which indicates that the c-PAM–PCPC hydrogels are excellent sensing materials for the determination of Be²⁺ ions in seawater.

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Conclusion

In conclusion, we have demonstrated a new intelligent photonic hydrogel sensor for the analysis of ultratrace Be²⁺ in seawater. A new chemistry route was presented to introduce ion recognition molecules, B9C3, into the 3D photonic crystal embedded hydrogels. When used to detect the target, the B9C3 can chelate the Be²⁺ ions in seawater and cause a volume increase of the photonic hydrogel. According to Bragg's law, this was reflected by a red shift in the diffracted wavelength of the functionalized photonic hydrogels. Based on the good linearity, it can be used to quantitatively analyze the concentrations of targets. The LOD of this sensor for Be²⁺ in seawater can reach 10⁻¹¹ M. The proposed method is significant for the detection of beryllium leakage in seawater systems, especially around nuclear power plants. This research is anticipated to open new prospects in multiple areas, such as antibiotic analysis, environmental monitoring, and clinical diagnosis, especially in situ.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Lixin Cao and Bohua Dong thank the NSFC (51372234, 51172218 and 21301187). Bohua Dong thanks Fundamental Research Funds for the Central Universities (Grant 201564001), the China Postdoctoral Science Foundation (Grant 2015M582132 and 2016T90652), and the Qingdao Science and Technology Plan (Grant 15-9-1-60-jch).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8tc00242h

This journal is © The Royal Society of Chemistry 2018