FULL PAPER DOI: 10.1002/asia.201300892

Superhydrogels of Nanotubes Capable of Capturing Heavy-Metal Ions Shasha Song, Haiqiao Wang, Aixin Song, and Jingcheng Hao*[a] Abstract: Self-assembly regulated by hydrogen bonds was successfully achieved in the system of lithocholic acid (LCA) mixed with three organic amines, ethanolamine (EA), diethanolamine (DEA), and triethanolamine (TEA), in aqueous solutions. The mixtures of DEA/LCA exhibit supergelation capability and the hydrogels consist of plenty of network nanotubes with uniform diameters of about 60 nm determined by cryogenic TEM. Interestingly, the sample with the same concentration in a system of EA and LCA is a birefringent solution, in which

spherical vesicles and can be transformed into nanotubes as the amount of LCA increases. The formation of hydrogels could be driven by the delicate balance of diverse noncovalent interactions, including electrostatic interactions, hydrophobic interactions, steric effects, van der Waals forces, and mainly hydrogen bonds. The mechanism of self-assembly from spherical biKeywords: gels · heavy-metal ions · nanotubes · self-assembly · surfactants

Introduction

are a subject of great interest because of their novel properties and potential applications in nanoscience areas.[11] Most of the reports of low-molecular-weight organic nanotubes were formed by single amphiphilic compounds, such as lipids, peptides, and polymers, but seldom by amphiphilic mixtures.[12, 13] The design of novel amphiphilic compounds mixed with commercial additives for the construction of hydrogels with nanotubes was required to achieve functional applications. Nowadays, high quantities of heavy-metal ions are produced in industrial output processes, for example, electroplating, metal finishing, and mining. Heavy-metal ions, such as Cu2 + , Co2 + , Ni2 + , Hg2 + , and Pb2 + , are highly toxic environmental pollutants worldwide. Completely different from organic wastewater, they are recalcitrant and can be accumulated in the environment and in living tissues to cause various diseases and disorders of living organisms, even at trace levels.[14, 15] Many technologies, such as chemical precipitation, ion exchange, chemical oxidation/reduction, reverse osmosis, electrodialysis, and ultrafiltration, were used to remove heavy-metal ions from industrial wastewater.[16] However, these techniques have inherent disadvantages, such as being less efficient, higher costs, and further generation of other waste products.[17] Therefore, seeking a highly efficient, cheap, green, and convenient method is urgently required. Recently, polymer hydrogels with a 3D crosslinked network structure as adsorbents to remove heavymetal ions have attracted much attention because of the facility to incorporate heavy-metal ions into the polymeric network, porous structure, and high surface area.[18–20] However, owing to the poor biological degradation and bioaffin-

Self-assembly can produce order from disorder, provide an understanding of life through living cells, and of course be one of the few practice strategies for completing ensembles of nanostructured materials. For self-assembled nanostructured materials, one of the challenges is the design of component systems to organize the desired patterns and functions.[1] As important self-assembling materials, self-assembled hydrogels originating from amphiphilic systems have rapidly increased in popularity for their responsive properties and functions.[2] Although the low-molecular-weight hydrogels (LMWHs) have good biological degradability and bioaffinity, LMWHs as adsorbents have rarely been reported,[3, 4] and limited their application in the treatment of wastewater containing heavy-metal ions, for example. LMWHs are viscoelastic and solidlike soft materials that consist of cross-linked networks with water.[5–7] However, the LMWHs often consist of network fibers;[8] tubular structures were seldom reported, except for some LMWHs with nanotubes that gelate organic solvents or mixtures of water/ polar organic solvents.[9, 10] LMWHs with tubular structures [a] S. Song, H. Wang, Dr. A. Song, Prof. Dr. J. Hao Key Laboratory for Colloid and Interface Chemistry & Key Laboratory of Special Aggregated Materials Shandong University, Ministry of Education Jinan 250100 (P.R. China) Fax: (+ 86) 531-856-4464 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201300892.

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layer vesicles into nanotubes was proposed. The dried hydrogels with nanotubes were explored to exhibit the excellent capability for capturing heavymetal ions, for example, Cu2 + , Co2 + , Ni2 + , Pb2 + , and Hg2 + . The superhydrogels of nanotubes from the self-assembly of low-molecular-weight gelators mainly regulated by hydrogen bonds used for the removal of heavy-metal ions is simple, green, and high efficiency, and provide a strategic approach to removing heavy-metal ions from industrial sewage.

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ity of polymer hydrogels, further pollution can be caused when they are used for the adsorption of heavy-metal ions, which prohibit their practical application. Lithocholic acid (LCA), a biocompatible bile acid, is an end product of the metabolism of cholesterol with a unique facially amphiphilic structure that consists of several chiral centers and rigid hydrophobic steroid nucleus.[21] The interesting structure makes it and its derivatives exhibit novel and abundant self-assembly behavior in solution. In previous reports, the gelation behavior of different secondary amines and bile acids have been studied.[22] Among the various organic amines, diethanolamine (DEA) were barely considered to form hydrogels with bile acids. However, in our experiments, we found that DEA had excellent gelation capability with LCA. Herein, we studied the self-assembly and gelation capability of LCA with different hydramines. The supergelation capability in mixtures of DEA/LCA hydrogels consisting of an abundance of network nanotubes with uniform diameters of about 60 nm was obtained. Moreover, the hydrogels obtained were excellent adsorbents to remove heavy-metal ions from aqueous solutions. The heavy-metal ions, Cu2 + , Co2 + , Ni2 + , Pb2 + , and Hg2 + , can be efficiently absorbed by DEA/LCA hydrogels in an environmentally friendly way, owing to the bidentate chelation of the heavymetal ions to the hydrogels. Therefore, DEA/LCA hydrogels can be used as highly efficient adsorbents in practical applications to remove heavy-metal ions from industrial sewage.

Results and Discussion

Figure 1. Cryogenic (cryo)-TEM images of samples formed by 100 mmol L1 of EA with 7 (a), 10 (b and c), 15 (d), 20 (e), and 30 mmol L1 (f) of LCA.

Dastidar et al. studied the gelation behavior of different bile acids with amines.[22] In their reports, DEA was a poor gelator and could barely form hydrogels with bile acids. Herein, we studied the gelation behavior of LCA with ethanolamine (EA), DEA, and triethanolamine (TEA). Owing to the different number of hydrogen atoms connected to the nitrogen atom of hydramine, the gelation capability was completely different. The hydrogels can only form in mixtures of DEA and LCA. LCA cannot dissolve in solutions of TEA (Figure S1 in the Supporting Information). When LCA was added to the solution of EA, birefringent solutions were obtained with a microstructured transition.

was very low, below 4.5 mmol L1, transparent solutions were obtained. When the LCA concentration reached 7 mmol L1, polydisperse uni- and multilamellar vesicles were found with a diameter range from about 200 to 700 nm (Figure 1 a). Some vesicles were slightly deformed; this is an indication of the softness and flexibility of the vesicle bilayers.[23, 24] With an increase in the concentration of LCA, 10 mmol L1, some long nanotubes appeared and coexisted with the deformed vesicles (Figure 1 b and c). The deformed vesicles were very large, with diameters of about 1 mm. The diameter of the nanotubes was about 50 nm, which was smaller than the nanotubes formed in hydrogels of the LCA/DEA system (see below). With detailed observations, the transition process from vesicles to nanotubes can easily be captured, as shown in Figure 1 b and c (the magnified image in Figure 1 b and the black arrow in Figure 1 c). When the LCA concentration was higher than 15 mmol L1, only nanotubes with uniform diameters of 50 nm were found (Figure 1 d–f). However, hydrogels were still not formed, even if LCA concentration reached to 30 mmol L1. The nanotubes of EA/LCA mixtures did not form network structures, but arranged a uniform direction; this indicates that hydrogels cannot form in this system. In previous works,

Transition between Vesicles and Nanotubes of EA/LCA System LCA can dissolve in solutions of EA and DEA; this is driven by protonation of EA or DEA by LCA and hydrogen bonds between the NH group of DEA or EA and the C=O group of LCA. Owing to the different structures between DEA and EA, different aggregates were obtained when LCA was added to the solutions of EA and DEA. At a fixed EA concentration of 100 mmol L1, various microstructures were observed with the addition of different amounts of LCA (Figure 1). When the LCA concentration

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Ou-yang et al. proved that a critical transition concentration (CTC) for vesicles to nanotubes existed in the vesicle and nanotube transition system.[25] The vesicles could convert into nanotubes when the surfactant concentration was higher than that of the CTC. The CTC obtained according to the theory formula[26] matched with that obtained through cryo-TEM observations. The transition from vesicles to nanotubes can be achieved with an increasing LCA con- Scheme 1. Mechanism for the transition from bilayer vesicles to nanotubes with increasing concentration of LCA. A cryo-TEM image (from Figure 1) is inserted to show a vesicle connected to a tube. centration at a fixed EA concentration. At a low LCA concentration, bilayer vesicles form with LCA and EA, which Scheme 1, in the bilayers, one molecule of LCA combined can be driven by the balance of hydrogen bonds, electrostatwith one molecule of EA through electrostatic interactions ic interactions, van der Waals forces, and hydrophobic interand hydrogen bonds, and another reversed molecule of actions. In the bilayers, the EA molecules can be protonated LCA by hydrophobic interactions and hydrogen bonds. by the COOH groups of the LCA molecules to produce EAH +LC ion pairs to produce electrostatic interactions. Hydrogels Formed in Stimuli-Responsive DEA/LCA The electrostatic interactions between molecules of EA and Systems LCA in the bilayers can be reflected in the FTIR spectra shown in Figure 2. In the LCA and EA mixtures, the carWhen LCA was dissolved in aqueous solutions of DEA, hybonyl stretching peaks at n˜ = 1704 cm1 disappeared and drogels were obtained. The gelation ability of mixtures of DEA/LCA was investigated with inverted test tubes. The hydrogels can be obtained at a low concentration of about 0.17 wt %, that is, more than 12 000 water molecules can be immobilized by one molecule of LCA; this falls into the category of “superhydrogel”.[27, 28] The gelation times of the hydrogels are shown in Table S1 in the Supporting Information. At a fixed DEA concentration of 100 mmol L1, the gelation kinetics can be remarkably promoted with the addition of LCA, leading to the formation of hydrogels with a translucent appearance. The self-assembly structures of DAE/LCA hydrogels with Figure 2. FTIR spectra of a) LCA and b) dry hydrogels of mixtures of different compositions were detected by cryo-TEM observa1 1 EA (100 mmol L )/LCA (20 mmol L ). tions. As shown in Figure 3, stiff nanotubes with an extremely uniform diameter of about 60 nm can be seen clearly. The 3D network structures formed through the physical entansplit into two peaks at n˜ = 1646 and 1549 cm1; this clearly glement of 1D nanotubes, which immobilized water moleindicated that the COOH of LCA was converted into  cules through solvation and surface tension, leading to gelaCOO with EAH + as the counterions. In addition to election. The diameter of the nanotubes did not change clearly trostatic interaction, hydrogen bonds could be the main with the variation of LCA concentration. The length of the driving force between the C=O groups of LCA molecules nanotubes can be estimated from the TEM images; they and the NH groups of EA molecules. With increasing LCA were more than 1 mm. The end of the nanotubes can be obconcentration, the vesicles could fuse to form necklace-like served in the images (arrows in Figure 3 a). The density of structures because many more vesicles appear, then the vesithe nanotubes increases with the addition of LCA into aquecle-capped tubular structures, and finally nanotubes form. ous solutions of DEA. As a result, hydrogels were opaque From Figure 1 b, inset, and c, the nanotubes and vesicle biat high gelator concentrations. The cryo-TEM images of the layers should have the same composition of EAH +LC ion hydrogels at high concentrations were not shown because pairs. the networks formed by the nanotubes became too dense to Based on TEM observations and FTIR measurements, be observed. The most frequent morphology observed in the the mechanism of self-assembly from spherical bilayer vesiTEM images consisted of many nanotubes coexisting with cles to nanotubes was proposed (Scheme 1). As shown in

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Figure 3. Cryo-TEM images of DEA (100 mmol L1) with a) 4.5, b) 10, c) 15, and d) 20 mmol L1 of LCA.

Figure 5. Cryo-TEM images of a hydrogel of DEA (100 mmol L1)/LCA (10 mmol L1): a) after shaking, b) after heating, c) at rest for 5 days after shaking, and d) cooled to room temperature for 2 days.

rare twist ribbons (arrows in the magnified image in Figure 3 b). The helical pitch (the distance between two adjacent turns) of the twist ribbons was estimated to be about 500 nm. Further observations demonstrated that the nanotubes were formed with twist ribbons (Figure 3 c and d, indicated by the arrows), which should be driven by the steric effects of the LCA molecules. The self-assembly process of hydrogels usually undergoes a gel–sol phase transition when the temperature increases, which is accompanied by a reduction in the mechanical intensity. Herein, the reversible gel–sol transition of DEA/ LCA hydrogels can be triggered by the introduction of extra energy, such as temperature and shearing forces. As shown in Figure 4, when heated, the hydrogel sample can be converted into solution with low viscosity and the solution can be turned back into a hydrogel in five days when cooled to room temperature. Moreover, the hydrogel can also be destroyed by shaking the sample vigorously to give a solution, which can be recovered as a hydrogel after being rested for two days. The hydrogel recovery times after being triggered by shearing forces and temperature are shown in Table S1 in the Supporting Information.

The microstructure transition was accompanied with the sol–gel transition induced by temperature or shearing forces. As shown in Figure 5 a and b, when the hydrogels were shaken or heated, the network nanotubes were broken to form the fractured profile of long nanotubes (indicated by the arrows). A probable reason might be that, with the introduction of energy, hydrogen bonds between LC and the NH group of DEAH + , and between two reversed molecules of LCA would be destroyed, leading to the breakage of the nanotubes. When the solutions were equilibrated at room temperature for several days, the network nanotubes reformed and the hydrogels were obtained again owing to recovery of the hydrogen bonds (Figure 5 c and d). With detailed observations, we found that a few twist ribbons were mingled with the long nanotubes (arrows in Figure 5 c and d); this indicated the formation of nanotubes of twist ribbons during the coiling process or at rest. The proposed mechanism of the interaction between molecules of LCA and DEA for the formation of network nanotubes are shown in Scheme 2. In the gelation process, the 1D hydrogen bonds are essential for the formation of network structures.[22] When LCA was added to a solution of DEA, the carboxyl groups of LCA molecules can protonate DEA molecules to form DEAH + ···LC ion pairs. In addition to electrostatic interactions between DEAH + ···LC ion pairs, there are two kinds of hydrogen bonds: one between the NH group of the DEA molecule and the C=O group of LCA molecule, and the other between the OH group and the CO group of two reversed LCA molecules. Moreover, hydrophobic interactions and steric effects also play important roles in the formation of hydrogels. As shown in Scheme 2, each molecule of LCA connects with

Figure 4. The reversible stimuli-responsive transition of the hydrogel (100 mmol L1 of DEA/10 mmol L1 of LCA) triggered by shearing forces and temperature.

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ed to the side-by-side connection of two reversed molecules of LCA in each unit. Capturing Heavy-Metal Ions

Scheme 2. Mechanism of the formation of nanotubes in DEA/LCA hydrogels.

another reversed molecule of LCA through two hydrogen bonds and a DEA molecule through electrostatic interactions. Thus, each array of the arrangement is composed of enormous cycles of two connected molecules of LCA and two molecules of DEA. The arrays combined together through electrostatic interactions, hydrogen bonds, van der Waals forces, and hydrophobic interactions. Owing to the rigid steroid skeleton and the chiral structure of LCA, the arrays cannot completely extend linearly, but tilt to cause the formation of twist ribbons, which coil to form nanotubes. FTIR measurements were used to investigate the electrostatic interactions between molecules of DEA and LCA. In the solid state, LCA has a stretching frequency of the carbonyl group at n˜ = 1704 cm1 (Figure 2 a). When LCA was mixed with DEA, this peak disappeared and split into two peaks: an asymmetric carbonyl stretching frequency at n˜ = 1632 cm1 and a symmetric carbonyl stretching frequency at n˜ = 1562 cm1, which confirmed the formation of the salt (Figure 6 a). The FTIR results revealed that, in the gel state, all COOH groups of LCA could be converted into carboxylate anions with ammonium cations as counterions. Small-angle X-ray diffraction (SAXD) can be used to elucidate the molecular packing in the 1D nanostructure. The sharp diffraction peaks shown in Figure 6 b suggested a highly ordered molecular arrangement in the DEA/LCA hydrogel. According to Braggs law, the reflection peak at 5.088 was calculated to correspond to a d spacing of about 1.74 nm. The d value of 1.74 nm was just a little larger than a cholate backbone length (1.5 nm);[5] this might be attribut-

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Heavy-metal ions, especially Hg2 + and Pb2 + , are highly toxic environmental pollutants. Different from organic wastes, heavy-metal ions are recalcitrant and can be accumulated in the environment and living tissues, leading to various diseases.[14, 15] Thus, seeking a green, convenient, more efficient, and environmentally friendly method for capturing heavy-metal ions to remove them from wastewater is of great value. Hydrogels of good biological degradability and biocompatibility were reported to be used for the adsorption of heavy-metal ions to be removed

Figure 6. a) FTIR spectrum of a xerogel of DEA (100 mmol L1)/LCA (20 mmol L1), and b) XRD patterns of the xerogels at different concentrations.

from wastewater.[17, 18] Herein, hydrogels of DEA/LCA were used to remove the heavy-metal ions from industrial sewage. For the adsorption process, dried hydrogel (1.0 g) was immersed into aqueous solutions of Cu2 + ACHTUNGRE(50 mL), Co2 + (50 mL), Ni2 + (50 mL), Pb2 + (50 mL), and Hg2 + (60 mL) and then left without being undisturbed. The heavy-metal ions were efficiently absorbed into the xerogels within several hours, as shown in Figure S2 in the Supporting Information. The concentration variation of heavy-metal ions was monitored by UV/Vis spectra after the addition of xerogels. For the five types of selected heavy-metal ions, Co2 + and Ni2 + could be determined directly, whereas Cu2 + , Pb2 + , and Hg2 + were measured by forming ligand complexes by adding ammonia, xylenol orange, and dithizone, respective-

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Figure 8. The concentration variation of a) Co2 + , b) Cu2 + , c) Ni2 + , d) Pb2 + , and e) Hg2 + as a function of time.

Figure 7. UV/Vis spectra of solutions of a) Co2 + , b) Cu2 + , c) Ni2 + , d) Hg2 + , and e) Pb2 + with time after being immersed in the DEA/LCA xerogels (1.0 g). Within 0 to 6 h, the solutions of Cu2 + were diluted 8 times and the other solutions were diluted 4 times before measurement; the solutions of Pb2 + and Hg2 + were diluted 20 and 6 times, respectively, before measurement. The times for the immersion of the xerogels in solution for each UV spectrum from top to bottom was 0, 1, 2, 3, 4, 6, 8, 10, 12, 15, and 24 h.

Table 1. Concentration of heavy-metal ion in solution before and after treatment with the absorbent DEA/LCA xerogel and the partition coefficients.

ly. From Figure 7, it is clear seen that concentration of heavy-metal ions decreased sharply after the addition of xerogels and all reached stable values after 15 h. Combined with the calibration curves (Figure S3 in the Supporting Information), the changes in ion concentrations with adsorption time are shown in Figure 8. The maximum adsorption values of Cu2 + , Co2 + , Ni2 + , Pb2 + , and Hg2 + were 101.52, 276, 426, 612.5, and 87.5 mg g1, respectively. The capability of the adsorbent to remove metal ions from solution can be expressed in terms of a partition coefficient (K), which is defined as the ratio of the amount of metal ions adsorbed to the adsorbents to those in aqueous solutions [Eq. (1)]: K¼

Ci  Ce VðmLÞ  Ce MðgÞ

Ci ACHTUNGRE[mg mL1]

Ce ACHTUNGRE[mg mL1]

K ACHTUNGRE[mL g1]

Co2 + Cu2 + Hg2 + Ni2 + Pb2 +

5.86 2.088 1.92 10.3 15

0.33 0.0576 0.17 1.77 2.75

837 1762 617 241 222

efficient (K) of the DEA/LCA xerogels adsorbing the heavy-metal ions. From the partition coefficient and the adsorption value of the heavy-metal ions, we can conclude that the DEA/LCA xerogel has different adsorption capabilities for different metal ions. The adsorption capability reported in our studies was much higher than that reported previously.[4, 17, 18, 30–33] Moreover, the present method is highly efficient, convenient, and green. To interpret the mechanism of DEA/LCA hydrogels capturing heavy-metal ions, FTIR spectra of the xerogel samples before and after the adsorption of metal ions were recorded. For LCA, a single peak at n˜ = 1704 cm1 was ascribed to the C=O stretching mode of COOH (Figure 2 a). When COOH was converted into COO, the singlet peak disappeared and produced two peaks, which were related to the asymmetric and symmetric stretching modes, respectively. According to the reported results, the frequency separa-

ð1Þ

in which Ci and Ce are the initial and equilibrium concentrations, respectively, of the heavy-metal ion in solution; V is the volume of the solution of heavy-metal ions; and M is the mass of the adsorbent.[29] Table 1 gives the partition co-

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Heavy-metal ion

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tion of the two peaks, Dn (Dn = nas(COO)ns(COO)), can explain the interaction mechanism between metal ions and hydrogels formed by carboxylates.[34–36] The Dn value of the monodentate complexes was much higher than that of the corresponding ionic species. For bidentate chelating complexes, the Dn value was much smaller than that of the ionic species. For DEA/LCA xerogels, the Dn value of ionic carboxylates was 158 (Dn = 15611403; Figure 9 a). When the hydro-

lar structures was observed with an increase in the proportion of LCA. When LCA was mixed with DEA, hydrogels composed of a network of nanotubes was obtained. Only precipitates formed in the LCA/TEA system. Hydrogen bonds between the hydramines and the COOH group of LCA played a pivotal role in the formation of diverse aggregates in the LCA/EA and LCA/DEA systems. Owing to the differences in the structures of EA, DEA, and TEA; the subtle balance of hydrogen bonds, electrostatic interactions, hydrophobic interactions, van der Waals forces, and steric effects; and shifting of the equilibrium induced various aggregation behavior. The hydrogels obtained in mixtures of LCA/DEA exhibited excellent adsorption capabilities for heavy-metal ions in an efficient, convenient, and environmentally friendly way. The results in this work should provide information for future applications of constructing hydrogels that are suitable for the removal of heavy-metal ions in wastewater.

Experimental Section Chemicals LCA (> 98 %) and TEA (99 %) were purchased from Acros organics (USA). DEA (99 %) and xylenol orange were purchased from Tokyo Chemical Industry Company, Ltd. (TCI). EA (99.5 %) was purchased from J&K Chemical Company, Ltd. (P.R. China). Dithizone, mercuric nitrate, copper sulfate, lead nitrate, nickel chloride, and cobalt nitrate hexahydrate were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, P.R. China) and were of p. a. quality. Ultrapure water with a resistivity of 18.25 MW cm was obtained by using a UPH-IV ultrapure water purifier (P.R. China). Sample Preparation Different amounts of LCA were weighed accurately into test tubes and then solutions of EA, DEA, and TEA of different concentrations were added. The solutions were stirred at room temperature until LCA dissolved completely. The samples were equilibrated at (25.0  0.1) 8C for at least 4 weeks before the self-assembly behavior was inspected. Characterization

Figure 9. FTIR spectra of a) the DEA/LCA xerogel and hydrogel samples after the adsorption of b) Co2 + , c) Cu2 + , d) Ni2 + , e) Pb2 + , and f) Hg2 + .

The cryo-TEM samples were prepared in a controlled-environment vitrification system (CEVS) at 25 8C. A micropipette was used to load 5 mL of the sample onto a TEM copper grid coated with a carbon support film. The sample was blotted with two pieces of filter paper and resulted in the formation of thin films suspended on the mesh holes. After about 5 s, the samples were quickly plunged into a reservoir of liquid ethane (cooled by the liquid nitrogen) at 165 8C. The vitrified samples were then stored in liquid nitrogen until they were transferred to a cryogenic sample holder (Gatan 626) and examined on a JEOL JEM-1400 transmission electron microscope (120 kV) at about 174 8C. The phase contrast was enhanced by underfocus. The images were recorded on a Gatan multiscan charge-coupled device (CCD) and processed with Digital Micrograph.

gels adsorbed heavy-metal ions, the Dn values were 140, 98, 150, 148, and 139 for Co2 + , Cu2 + , Ni2 + , Pb2 + , and Hg2 + , respectively. All values were smaller than those of DEA/LCA xerogels. Thus, we concluded that Co2 + , Cu2 + , Ni2 + , Pb2 + , and Hg2 + combined with COO through a bidentate chelating mode (Figure S4 in the Supporting Information); this endowed the of DEA/LCA hydrogel with a high adsorption capability for heavy-metal ions.

The XRD patterns were recorded by using a DMAX-2500PC diffractometer with CuKa radiation (l = 0.15418 nm) and a graphite monochromator. The samples were measured at room temperature between 0.8 and 108 in 2q mode (18 min1).

Conclusion

The solutions of Co2 + , Cu2 + , Ni2 + , Hg2 + , and Pb2 + (1.8–18 mg mL1) were prepared by dissolving solid CoACHTUNGRE(NO3)2·6 H2O, CuSO4, NiCl2, HgACHTUNGRE(NO3)2, and PbACHTUNGRE(NO3)2 in ultrapure water, respectively. The DEA/LCA xerogels were added to the solutions of heavy-metal ions. The variation in the concentration of heavy-metal ions after adsorption was monitored

Mixtures of LCA with hydramine (EA, DEA, and TEA) exhibited different aggregation behavior in aqueous solutions. In the LCA/EA system, the transition from vesicles to tubu-

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by UV/Vis spectroscopy on a HITACHIU-4100 spectrophotometer. The scan rate for each measurement was 300 nm min1. For the determination of Cu2 + , Hg2 + and Pb2 + , ammonia, dithizone, and xylenol orange were added to form the ligand complexes, respectively.

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The FTIR spectra over the range n˜ = 4000–400 cm1 were obtained on a VERTEX-70/70v FTIR spectrometer (Bruker Optics, Germany) by taking 64 scans with a final resolution of 4 cm1. Spectral manipulation was performed by using the OPUS 6.5 software package (Bruker Optics, Germany). The sample was mixed with KBr and pressed into a plate for the measurements.

Acknowledgements This work was financially supported by the NSFC (21033005 & 21273134), the National Basic Research Program of China (973 Program, 2009CB930103).

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