Imprinted photonic hydrogel pillar for self-reporting water treatment of heavy metal ions

Abstract

We demonstrate here a general protocol to prepare a self-reporting water treatment to remove heavy metal ions via an imprinted photonic hydrogel. The interconnected porous photonic layer with a photonic crystal structure on the surface of the hydrogel pillars can self-indicate the variation of the treatment’s adsorbing capacity by changing colour and increase the adsorption efficiency of the hydrogel pillars. Ion-imprinting technology was applied to improve the adsorption capacity of the hydrogel pillars. The Pb(II) ion was chosen as the preliminary case to verify the properties of the imprinted photonic hydrogel pillars. Studies were then performed utilizing Cu(II) and Zn(II) ions to corroborate the initial findings. The Ag(I) ion was a special sample of an oxidizing metal ion which can be reduced by the hydrogel. To investigate the universality of the adsorbent, we conducted similar experiments on Ni(II), Mn(II), Co(II), and Sn(II) ions. Furthermore, the used hydrogel pillars were proved to be recovered by eluting the ions. This protocol provides a promising contaminated water treatment with properties of self-reporting and high efficiency for the removal of heavy metal ions.

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Introduction

Heavy metal ions are highly toxic and exist in the aqueous waste streams of many industries. Toxic heavy metal ions in water cause many detrimental effects on the environment and human health.^1–4 Investigating how to remove the ions from waste water is considered one of the most meritorious research subjects. Many methods have been devised such as chemical precipitation,⁵ membrane filtration,⁶ hydroxyapatite filtration⁷ and adsorption.⁸ Among them, adsorption has been found to be a promising technique to remove heavy metal ions from effluents. Adsorption agents usually consist of chitosan-based sorbents,⁹ biosorbents^10,11 and hydrogels.^12,13 Among these agents, hydrogel has the advantage of low operational and maintenance costs and high efficiency, especially for heavy metal ions at low concentrations.¹⁴ Imprinted hydrogels, which are highly sensitive in targeting heavy metal ions, have been continually reported to be used as adsorbents in the past few decades.¹⁵ These materials usually incorporate various metal-complexing ligands such as carboxyl, amino acid, amide, amidoxime, imidazole, thiol, pyridine and triazole functional groups which could build imprinting cavities that target heavy metal ions.¹⁶ The previous studies demonstrated that imprinted hydrogels have high capacities and sensitivities in adsorbing heavy metal ions.¹⁷ However, this result cannot detect and observe the adsorption process of the adsorbent like when allochroic silicagel adsorbs water.¹⁸ Recently researchers have been focusing on the detective sensors of heavy metal ions and mainly on colorimetric metal ions sensing based on dyes¹⁹ and gold nanoparticles.²⁰ Nonetheless, most of these sensors are irreversible as reagents and the analyte species are also limited.

Our team has previously reported a highly sensitive colorimetric-sensing platform comprising photonic hydrogel sensors for the detection of heavy metal ions such as Cu²⁺, Pb²⁺ and Ag⁺.^21–23 The quick, exact, specific and visible response of the photonic hydrogel sensors assembled with metal-complexing ligands involve the expansion and contraction of the material when adsorbing and desorbing heavy metal ions. The change in volume leads to periodic variation of the refractive index of the photonic crystal.²⁴ Nevertheless it is impossible for a film to obtain a considerable adsorption capacity.

As an extension of our work, herein we further demonstrate that the photonic hydrogel not only serves as a visible sensor which can expose the adsorption process, but also acts as a sorbent. In order to achieve this purpose, we put forward a protocol to construct an inverse opal structure on the surface layer of the imprinted hydrogel pillars shown in Scheme 1. Ion-imprinting technology has been applied to increase the adsorption efficiency of the hydrogels for heavy metal ions.²⁵ The aim of the interconnected porous PC structure in the surface is to expedite the diffusion of heavy metal ions in the hydrogel blocks and have a high efficiency with fast kinetics.^26,27 If the PC hydrogel is combined with appropriate ligands, a simple, sensitive, colorimetric and high capacity water treatment could be obtained. The process of the experiments and theoretical evidence have been discussed in detail as follows.

Scheme 1 Schematic illustration of the protocol: preparation of photonic imprinted hydrogel pillars and the ligands used in this work. The scale bar of the SEM image is 1.0 μm.

Experimental section

Materials

Tetraethoxysilane (TEOS), ammonium hydroxide (28%), acrylamide, ethanol, N,N,N′,N′-tetramethylethylenediamine (TEMED), 3-sulfopropyl methacrylate potassium salt (SPM), bis-acrylamide (BIS), ammonium persulfate (APS), glycidyl methacrylate (GMA) and dimethylglyoxime (DMGO) were obtained from Sigma; pentaethylenehexamine (PEHA) was purchased from Aladdin; ethanol, lead nitrate (Pb²⁺), cupric nitrate (Cu²⁺), zinc acetate (Zn²⁺), silver nitrate (Ag⁺), cobalt nitrate (Co²⁺), manganese sulphate (Mn²⁺), nickel nitrate (Ni²⁺) stannous chloride (Sn²⁺), hydrochloric acid (37 wt%), glacial acetic acid, sodium diethyldithiocarbamate (DDTC), Xylenol Orange disodium salt (XO), zincon monosodium salt (ZCN), dithizone (DTZ), hydrofluoric acid (40 wt%), glacial acetic acid (HAc), natrium acetate, potassium chloride, boric acid and sodium hydroxide were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai China). All these chemicals were analytical grade and used as received without further purification. DI water was used throughout the experiments.

Instrumentation

The SEM image of the inverse opal hydrogel (photonic hydrogel) was obtained by using a FEISirion200 SEM. The FTIR pictures of the hydrogel were obtained by using an EQUINOX 55 Fourier transform infrared spectrometer. The analysis of the FTIR data is discussed in detail in the ESI.† The concentration of the different heavy metal ions was measured by a Quawell ultraviolet spectrophotometer.

Preparation of the hydrogel pillars

The silica colloidal crystal vials used in this study were synthesized by the improved method of Stöber.^28,29 The mixture of SPM, acrylamide, BIS, TEMED and GMA with different molar ratios dissolved in DI water was added into a 10 mL centrifuge tube. After injecting the mixture into a glass vial assembled with the PC layer inside, the initiator APS solution was added. The polymerization reaction was conducted at room temperature. After 60 min, when the polymerization reaction finished, heavy metal ions coordinated with PEHA were added into the vial. The aim was to form the IPN structure with polyacrylamide as well as building the imprinted cavities. This closed vial was put into the oven at 323 K for 4 hours. This step was to induce the formation of the interpenetrating polymer network. Finally, the vial was removed by scraping softly, and the photonic crystal on the surface of the hydrogel was carved using hydrofluoric acid. Details are given in the ESI.†

Heavy metal ion adsorption and removal experiments

Heavy metal ion sorption experiments were conducted in batches. As an example, wastewater containing lead ions was simulated by dilution of the stock Pb(NO₃)₂ standard solution with DI water to different Pb(II) ion concentrations (150, 300, 450, 600, 750, 900, 1050, 1200, 1350, 1500 mg L⁻¹). Then the Pb(II) ion imprinted photonic hydrogels with equal weights were put in the prepared Pb(II) solution for 6 hours at room temperature. After the adsorption processes, the solution at equilibrium was analysed using an ultraviolet spectrophotometer.

The adsorption capacity of the imprinted photonic hydrogel is expressed as follows:³⁰

where q_T is the amount of Pb(II) adsorbed onto a unit mass of the imprinted photonic hydrogel (mg g⁻¹), C₀ and C_T (mg L⁻¹) which can be calculated by Lambert–Beer’s law shown in the ESI† are the concentrations of Pb(II) in the initial solution and in the final solution after adsorption.

The adsorption experiments of Cu(II), Zn(II) and Ag(I) ions were similar to that of Pb(II). In addition, the removal experiments for all of these ions were conducted by soaking the post-reaction hydrogels in 0.1 M of HAc. The solution was then changed 4 times (every 30 min), and the hydrogel was then soaked in 0.5 M pH 7 EDTA solution to remove the remaining adsorbed heavy metal ions and residual hydrogen ions. Then the hydrogels were washed repeatedly with DI water to remove the redundant EDTA.

Results and discussion

Study on Pb(II) ions

Studies suggest that Pb(II) water pollution has become a severe problem and that plumbism harms the health of children more and more seriously.³¹ Pb(II) was chosen as the first sample to study the heavy metal ion removal behaviour of the imprinted photonic hydrogel pillars in detail. According to a report,²⁹ when amine groups are present, they would bind to Pb(II) ions to remove them from polluted water.

In these experiments, lead ions were added before polymerization of PEHA to form a coordinate bond with –NH₂. When polymerization had finished and the lead ions were eluted, there were imprinted cavities left with complementary shape and binding sites for Pb(II). This imprinted structure has proven to result in high capacity efficiency in previous works.^32,33 In this experiment we introduce a PC structure into the imprinted hydrogel and attempt to realize the self-reporting of the adsorption process.

An obvious colour-changing image and adsorption curves are shown in Fig. 1. Fig. 1a reveals that the PC structure on the surface layer of the hydrogel displays an increasing blue-shift with rising target ion concentration and is restored to its initial red colour after the target ion is eluted. Fig. 1b is the reflection spectrum of the PC layer and it shows the reflection peaks when the hydrogels have different adsorption capacities of the target ions. The previous studies have reported that a change in volume leads to periodic variation of the refractive index of the photonic crystal. In this experiment, the shrinkage of the hydrogel is caused by two factors as reported previously.²¹ First, when the concentration of Pb(II) ions increases, there will be a difference in ionic concentration between the inside and the outside of the hydrogel leading to dehydration and shrinking. Secondly, multiple amino ligands coordinate to Pb(II) ions constructing extra cross linkage of the polymer network, leading to shrinkage of the hydrogel. Consequently, with the adsorption capacity increasing, the hydrogel shrinks and the colour of the PC layer blue-shifts. The PC layer can expand and recover to the initial colour after eluting the target ion. This result verifies that the photonic hydrogel could detect the variation of Pb(II) adsorbing capacity automatically by changing the colour of the PC layer. This also demonstrates that the self-reporting hydrogel is reusable.

Fig. 1 (a) Colour images of the imprinted photonic hydrogel in the concentrations of (1) 0 mg L⁻¹, (2) 300 mg L⁻¹, (3) 600 mg L⁻¹, (4) 900 mg L⁻¹, (5) 1200 mg L⁻¹ and (6) the elution condition; (b) reflection spectrum of the PC layer in different concentrations of target ions; (c) equilibrium adsorption of Pb(II) with various target ion concentrations of imprinted photonic hydrogel, imprinted hydrogel, photonic hydrogel and hydrogel; (d) time profile of Pb(II) ion removal using imprinted photonic hydrogel, imprinted hydrogel, photonic hydrogel and hydrogel soaking in 1050 mg L⁻¹.

Fig. 1c shows the variation trend of the Pb(II) ion adsorption capacity of the hydrogels at different target ion concentrations with the same adsorption time. For all the hydrogels, the adsorption of ones with the PC structure could reach a higher capacity than others without the PC structure at a low Pb(II) target ion concentration. This is because the interconnected porous structure and large specific area of the PC layer is advantageous for diffusion of the target ions in the hydrogels.³⁴ However, the adsorptions can reach the same and invariable saturated adsorption with the Pb(II) concentration increasing. This shows that the PC structure can only increase the adsorption capacity at low target ion concentrations rather than raising the saturated adsorption. In addition, it is shown in Fig. 1c that the saturated adsorption capacity of the imprinted hydrogels is twice of that of the common hydrogels. The reason for this is that the imprinted cavities with amino ligands can easily capture lead ions compared with dispersive amino ligands in common hydrogels. As a result, imprinted hydrogels perform much better in removing target ions and possess a higher adsorption capacity than common ones. Moreover, Fig. 1d shows the time profile of Pb(II) removal by the hydrogels at a certain target ion concentration. Compared to hydrogels without the PC structure, the photonic hydrogels obtained a higher adsorption capacity in the same soaking time and reached equilibrium in a shorter time. This is because the interconnected porous structure of the PC layer accelerates the diffusion of ions and shortens the time of equilibrium adsorption of the hydrogels. What’s more, the data in Fig. 1d is further proof that the imprinting technology significantly raises the adsorption capacity of the hydrogels.

As a result, imprinted photonic hydrogel is demonstrated as a water treatment material which responds to the adsorption process of the target ion automatically by changing the colours of the PC layer. This water treatment not only has a large adsorption capacity but also possesses a high adsorption speed.

Study on Cu(II) and Zn(II) ions

Cu(II) and Zn(II) ions can also easily bind to amino ligands.^35,36 Experiments similar to the study of Pb(II) ions were conducted. Images in Fig. 2a and b display the blue-shift of the colour of the PC layer when the target ion concentration increases. Following elution, the colour returns to red. The data of the reflection spectra are shown in Fig. S6 in the ESI.† As in the case of Pb(II), this proves that the adsorption of Cu(II) and Zn(II) ions caused shrinkage of the hydrogel resulting in the colour change of the PC layer. This colour change was recovered by desorbing the target ions. It is concluded that the photonic hydrogels can respond to the adsorption of Cu(II) and Zn(II) ions voluntarily by changing colour and can be reused just like the hydrogel for removal of Pb(II). The adsorption data of the hydrogels after soaking in the Cu(II) and Zn(II) target ion solution for 6 h is shown in Fig. 2c. With the rise of target ion concentration, the adsorption of Cu-imprinted and Zn-imprinted hydrogels increases and finally reaches the saturated adsorption capacity. However, similar to the case of Pb(II), hydrogels with the PC layer had a higher adsorption capacity at low target ion concentrations and the same saturated adsorption compared to hydrogels without a PC layer. This result demonstrates again that the PC layer can only speed up the diffusion of target ions rather than increasing the saturated adsorption capacity.

Fig. 2 Colour images of (a) Cu and (b) Zn-imprinted photonic hydrogels in the concentrations of (1) 0 mg L⁻¹, (2) 300 mg L⁻¹, (3) 600 mg L⁻¹, (4) 900 mg L⁻¹, (5) 1200 mg L⁻¹ and (6) the elution condition; (c) equilibrium adsorption of Cu(II) and Zn(II) ions using various target ion concentrations of imprinted photonic hydrogel and imprinted hydrogel and (d) time profile of Cu(II) and Zn(II) ion removal with imprinted photonic hydrogel and imprinted hydrogel soaking in 1050 mg L⁻¹.

According to the time profile for the adsorption of Cu(II) and Zn(II) target ion hydrogels at a certain target ion concentration (Fig. 2d), the adsorption of these two kinds of hydrogels increased with the soaking time and finally reached the equilibrium adsorption capacity. However, photonic hydrogels showed larger adsorption capacities than non-photonic hydrogels in the same soaking time and they take shorter times to achieve the equilibrium adsorption. As a consequence, the PC structure improves the adsorption efficiency of the hydrogels in adsorbing Cu(II) and Zn(II) ions which matches the result of the Pb(II) ion.

Study on special Ag(I) ions

In this study, Ag(I) was chosen to represent metal ions with strong oxidizing properties. The image and SEM image of the hydrogel with the Ag(I) ion are shown in the ESI.† As shown in Fig. S4a† the hydrogel turned brown after being soaked in the Ag(I) target ion solution. The SEM image of the adsorbed hydrogel in Fig. S4b† demonstrates that there are many silver nanoparticles of about 20 nm in diameter in the hydrogel which are supposed to be reduced by reducing groups in the hydrogel. Because of the local surface plasmon resonance (LSPR) of silver nanoparticles, the hydrogel presents the colour brown. In addition, all the results in this case were obtained from the non-imprinted hydrogels because of the reduction phenomenon. Fig. 3a describes that after adsorbing for 6 h, the adsorption capacity of the photonic hydrogel is a little higher than that of non-photonic hydrogels at low target ion concentration. Also, as the concentration increases, the adsorption gap between the hydrogels decreases and they eventually reach the same saturated adsorption capacity. The comparison between photonic and non-photonic hydrogels in Fig. 3b reflects that at a certain target ion concentration, the photonic hydrogel removes more target ions than the non-photonic hydrogel during the same reaction time and the photonic hydrogel reaches the equilibrium adsorption earlier. It is reasonable to say that the photonic hydrogel retains the property of a high adsorption efficiency for target ions even though the oxidizing heavy metal ions can be reduced by the hydrogels.

Fig. 3 (a) Equilibrium adsorption of Ag(I) ions at various target ion concentrations using the photonic hydrogel and (b) time profile of Ag(I) ion removal by the photonic hydrogel soaking in 1050 mg L⁻¹.

Study on the universality of the hydrogels

The experiments detailed above have verified the self-reporting and high adsorbing efficiency properties of the imprinted photonic hydrogel for heavy metal ions. To learn the universality of this hydrogel, experiments on the adsorption of Ni(II), Mn(II), Co(II) and Sn(II) ions were conducted. Images in Fig. 4a show the colours of the surface layer of the hydrogels before putting them into a solution of target ions and Fig. 4b reveals the colour of the surface layer after it has been soaked in the solution for 6 hours. It is clear that all the PC layers of the hydrogels exhibit an obvious blue-shift. This result demonstrates that the self-reporting property of the imprinted photonic hydrogel is universal for heavy metal ions. The adsorption capacity and time profile experiments are similar to those with Pb(II), Cu(II) and Zn(II). We take Ni(II) for example and the results are shown in the ESI.†

Fig. 4 Colour variance images of imprinted photonic hydrogels (a) before and (b) after soaking in Sn(II), Mn(II), Co(II) and Ni(II) ions for 6 hours. The concentration of the solution was 900 mg L⁻¹.

Study on selection and competition

It has been demonstrated that the imprinted polymer is selective to the imprinted molecules or ions.^37,38 We conducted a series of experiments to study the selection and competition of the imprinted PC hydrogel. In the selection experiment, the Pb(II)-imprinted PC hydrogel pillars (Pb(II)-PP) were used to adsorb Cu(II) and Zn(II) ions and the Cu(II)-imprinted PC hydrogel pillars (Cu(II)-PP) were made to adsorb Pb(II) and Zn(II) ions. The adsorption capacity contrasted with the previous results of Cu(II) adsorbed by Cu(II)-PP, Zn(II) adsorbed by Zn(II)-PP and Pb(II) adsorbed by Pb(II)-PP. Fig. 5a and b describe the adsorption capacity of different imprinted PC hydrogel pillars. It is demonstrated that the Cu(II) ion adsorption capacity of Cu(II)-PP is much higher than that of Pb(II)-PP. The Zn(II)-PP adsorbs more Zn(II) compared with Pb(II)-PP and Cu(II)-PP. Pb(II)-PP is much more efficient in adsorbing Pb(II) than Cu(II)-PP and Zn(II)-PP. These results demonstrate that the imprinted ions are more likely to be adsorbed by the corresponding imprinted hydrogels. Consequently the imprinted PC hydrogels are relatively selective in adsorbing heavy metal ions.

Fig. 5 Equilibrium adsorption of (a) Pb(II)-PP adsorbing Cu(II) and Zn(II) target ions compared with Cu(II)-PP and Zn(II)-PP, (b) Cu(II)-PP adsorbing Pb(II) and Zn(II) target ions compared with Pb(II)-PP and Zn(II)-PP and (c) Pb(II)-PP adsorbing Pb(II) and Cu(II)-PP adsorbing Cu(II) in a mixed solution compared with those adsorbing in single ion solutions.

To study the competition of the imprinted PC hydrogels, experiments were conducted in mixed ion solutions instead of single ion solutions. The Pb(II)-PP hydrogel adsorbing Pb(II) ions was put into a mixed solution of 1050 mg L⁻¹ Cu(II), 1050 mg L⁻¹ Zn(II), 1050 mg L⁻¹ Ag(I) and the Cu(II)-PP hydrogel adsorbing Cu(II) ions was put into a solution of 1050 mg L⁻¹ Pb(II), 1050 mg L⁻¹ Zn(II), 1050 mg L⁻¹ Ag(I). The adsorption capacity was compared with the previous data of Pb(II)-PP and Cu(II)-PP in single target ion solutions. Fig. 5c shows that the adsorption capacity of Pb(II)-PP for adsorbing Pb(II) in a highly concentrated mixed solution possesses a similar trend and value as in a single ion solution. The Cu(II)-PP shows the same result. It can be concluded that the coexistence of multiple ions did not interfere much with the ability of the hydrogels to adsorb imprinted ions. Additionally, the imprinted PC hydrogels compete for the imprinted ions when adsorbing in mixed ion solutions. The reason why the adsorption capacity decreases slightly is because part of the unfixed –NH₂ ligands were captured by other ions in the solution.

Regeneration of the imprinted PC hydrogel

Reproducibility experiments were carried out to evaluate the possibility of regenerating the PC structure on the surface layer of the hydrogel as reported in previous methods.^39,40 To investigate the repeatability of the adsorption of the imprinted hydrogel part, we conducted adsorbing–desorbing experiments (shown in the ESI†) for three cycles. Fig. 6 displays the equilibrium adsorption capacity of the hydrogel pillars before and after eluting in all three experiments. The equilibrium adsorption capacity decreases by less than 10% of the original equilibrium adsorption after three cycles. This result confirms that the imprinted photonic hydrogel is regenerated but the equilibrium adsorption capacity has a small attenuation. At the same time, the previous work has demonstrated that the imprinted photonic hydrogel can recover to its original red colour after desorbing the heavy metal ions. As a result, this imprinted PC hydrogel pillar can maintain the PC structure and adsorption capacity during adsorption and desorption.

Fig. 6 Regeneration studies of imprinted photonic hydrogel adsorbing Pb(II) ions. The concentration of toxic ions was 1200 mg L⁻¹. The contact time was 12 h for each cycle. AD is short for adsorption and DE is short for desorption.

Conclusions

We synthesized an imprinted photonic crystal PAAM/PEHA hydrogel pillar material. The hydrogel changes colour when the adsorption capacity of the target Pb(II), Cu(II), Zn(II) and Ag(I) ion concentration is varied. The porous surface layer on the hydrogel and the ion-imprinted technology lead to higher adsorption capacities and shorter equilibrium times compared to common hydrogels. The hydrogel was used for many other heavy metal ions like Ni(II), Mn(II), Co(II), and Sn(II) and can be regenerated after eluting the target ions. These results show that the imprinted photonic crystal PAAM/PEHA hydrogel pillar material can be used as an efficient and distinctive water treatment for heavy metal ions which can reveal the adsorption process automatically.

Acknowledgements

We thank the support of the National Natural Science Foundation of China under Grant #51373097. This work was also supported by the Shanghai Jiaotong University Biomedical Engineering (PolyU) Cross Research Fund Project (#YG2012MS35).

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Footnote

† Electronic supplementary information (ESI) available: Preparation of silica colloidal crystals, the imprinted self-reporting photonic hydrogel pillar, FTIR analysis images, details of the hydrogel adsorbing Ag(I) ions and the method to measure the adsorption capacity of heavy metal ions. See DOI: 10.1039/c5ra16024c

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