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A Pillararene-Based AIE-Active Supramolecular System for Simultaneous Detection and Removal of Mercury(II) in Water Hong-Bo Cheng, Ziyan Li, Yao-Dong Huang, Lei Liu, and Hai-Chen Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00363 • Publication Date (Web): 20 Mar 2017 Downloaded from http://pubs.acs.org on March 20, 2017

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ACS Applied Materials & Interfaces

A Pillararene-Based AIE-Active Supramolecular System for Simultaneous Detection and Removal of Mercury(II) in Water Hong-Bo Cheng,† Ziyan Li,§ Yao-Dong Huang,§ Lei Liu† and Hai-Chen Wu*,†,‡ †

Key Laboratory for Biomedical Effects of Nanomaterials & Nanosafety, Institute of

High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ‡

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Analytical

Chemistry for Living Biosystems, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China §

Key Laboratory of Systems Bioengineering, School of Chemical Engineering and

Technology, Tianjin University, Tianjin 300072, China

*

To whom correspondence should be addressed. Email: [email protected]

KEYWORDS: supramolecular polymer, pillararene, self-assembly, aggregation-induced emission, mercury(II)

1

ACS Paragon Plus Environment


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ABSTRACT.

Supramolecular polymers are polymers based on monomeric units held together with directional and reversible noncovalent interactions. Compared with traditional polymers, they possess better processability and better recycling properties, owing to their reversible monomer-to-polymer transition. Herein, we report the construction of a new supramolecular system through self-assembly of a thymine-substituted copillar[5]arene 1 and a tetraphenylethylene (TPE) derivative 2 in the presence of Hg2+. Copillar[5]arene 1 can coordinate with Hg2+ tightly through T-Hg2+-T pairings. On the other hand, 1 can bind with guest molecule 2 through host-guest interactions between the pillararene cavity and the nitrile moiety of 2. These joint interactions generate crisscrossed networks composed of 1, 2 and Hg2+, which eventually wrap into spherical nanoparticles. Due to the aggregation induced emission (AIE) properties of 2, the formed supramolecular polymer exhibits strong fluorescence which renders convenient the detection of the Hg2+-containing nanoparticles and the subsequent removal procedure. Furthermore, the polymer precipitate can be readily isolated by simple treatment and the pseudorotaxane 2⊂ ⊂1 can be recycled and reused. Our study has demonstrated a practical strategy for the sensing and removal of heavy metal ions in water by the construction of supramolecular polymers.

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INTRODUCTION

Supramolecular assembly allows the construction of a well-defined complex of molecules held together by weak and noncovalent interactions.1 Those forces include metal coordination, hydrogen bonding, van der Waals forces, π-π stacking, and electrostatic effects. Supramolecular complexes can be prepared from small molecules using a bottom-up approach in very few steps with dimensions ranging from nanometers to micrometers.2 The size and structure of the complexes are determined by the choice of the building blocks. In particular, macrocycles are playing a critical role in supramolecular chemistry, as they provide effective cavities that can tightly bind guest molecules. Conventional macrocycles such as crown ethers,3,4 cyclodextrins,5,6 calixarenes,7-9 cucurbituril,10,11 and cavitands12-14 are readily synthesized in large quantities and have been employed in various kinds of applications.15,16 Recently, pillararenes appeared as a new class of macrocyclic hosts, composed

of

hydroquinone

units

linked

by

methylene

bridges

at

the

para-positions.17-19 The main advantages of pillararenes include their highly symmetrical and rigid framework which affords the selective binding to guest molecules and their easy functionalization with different substituents on the benzene rings which enables tuning of their host-guest binding properties.18,19 Consequently, pillararenes have been applied in the fabrication of many interesting suparmolecular systems, such as chemosensors,20-23 nanomaterials,24-31 artificial ion channels,32-35 and supramolecular polymers.36-38 During the course of our research on the sensing and removal of heavy metal ions,39-42 we envisioned that pillararene-based supramolecular

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polymers might be used as the sensor and chelator for removing heavy metal ions owing to their great capacity for modification and recycling. The reversible monomer-to-polymer transitions should be the key to the successful construction and recycling of the supramolecular chelator. In this report, we focus on the sensing and removal of mercuric ions (Hg2+). Hg2+ is arguably one of the most toxic heavy metal ions in the environment and aquatically derived food. It is a neurotoxin and can cause severe adverse effects on human health through high dose exposure or food-chain accumulation.43 Intensive efforts have been made during the past decades to develop new methodologies for effective detection and removal of Hg2+ ions.44-47 However, the vast majority of the research focused on the analytical detection of Hg2+,48-50 with only a few studies dealing with the removal task. So far, several absorbents, such as hydrogel,51-53 porous silicas,54,55 nanoparticles,56 metal-organic frameworks (MOFs)57-59 and covalent organic frameworks (COFs),60 have shown potentials for the possible removal of Hg2+ in water. However, very few of the developed methods could perform the selective detection and efficient removal simultaneously. To the best of our knowledge, supramolecular polymers have not yet been used in the toxic heavy metal ions removal studies. Herein, we construct a new supramolecular system via self-assembly of a thymine-substituted copillar[5]arene 1 (catcher) and a tetraphenylethylene (TPE) derivative 2 (indicator) together with Hg2+. It is aimed to accomplish the goal of Hg2+ sensing and removal in one pot. Catcher 1 can coordinate with Hg2+ tightly through a T-Hg2+-T pairing to produce linear wires

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and at the same time it can bind indicator 2 through host-guest interactions (Figure 1). The growth of the 1-Hg2+ coordination chain crosslinked by compound 2 produces crisscrossed networks which eventually wrap into spherical nanoparticles. When free in solution, compound 2 is non-fluorescent; while intertwined in the supramolecular complex 2⊂ ⊂1@Hg2+, it exhibits strong aggregation induced emission (AIE) fluorescence. This enables convenient the detection of the aggregation process and facile monitoring of the removal procedure. More importantly, the host-guest complex 2⊂ ⊂1

can

be

recycled

by

a

simple

treatment

with

sodium sulfide

in

acetone/dichloromethane. This feature paves the way for practical applications of the supramolecular polymers in the sensing and removal of heavy metal ions.

Figure 1. The proposed strategy for the sensing and removal of Hg2+ based on the formation of supramolecular polymers. RESULTS AND DISCUSSIONS

Synthesis of the supramolecular complex 2⊂ ⊂1@Hg2+ 5



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The syntheses of thymine-functionalized copillar[5]arene 1, and TPE derivative 2 are illustrated in Figure S1 in the Supporting Information. In brief, bromo-functionalized copillar[5]arene 3 reacted with sodium azide in dimethyl formamide

followed

amino-functionalized

by

reduction

copillar[5]arene

with 4

in

triphenylphosphine 78%

yield.

Next,

to

afford

the

target

copillar[5]arene 1 was obtained in 75% yield by amidation between 4 and a thymine derivative 5. Compound 2 was prepared via a copper-catalyzed click reaction between a terminal alkyne-modified TPE 6 and a nitrile-containing azide 7. All the new compounds have been characterized by 1H NMR, 13C NMR, and high-resolution mass spectroscopy (HRMS) (Figure S2-S10). With the building blocks ready, we set out to investigate the construction of supramolecular polymers. First, we examined the binding of Hg2+ with the thymine moieties of copillar[5]arene 1. The 1H NMR titration experiments were carried out in deuterated acetone. As shown in Figure S11, the signal of the imide proton gradually disappeared as the concentration of Hg2+ was increased. This result confirmed the formation of the T-Hg2+-T pairs61 between Hg2+ and 1. In addition, the UV-vis spectral changes in the presence of Hg2+ also supported the conclusion. Upon gradual addition of 1.0 equiv. of Hg2+, the absorption intensity of copillar[5]arene 1 in acetone underwent a significant enhancement and the absorption maximum also slightly shifted from 326 nm to 329 nm (Figure S12). One more direct proof for the formation of the supramolecular polymer is the transmission electron microscopy (TEM) image showing the morphological structure of the 1@Hg2+ assembly (Figure S13). The

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micrometer long nanofibers must result from the formation of the T-Hg2+-T pairs which hold the copillar[5]arene molecules together. Moreover, in the control experiment, the morphologie of the mixture of 2 and Hg2+ existed as small nanoparticles under the same concentration (Figure S14). Next, we studied the host-guest interactions between copillar[5]arene 1 and indicator 2. It is well known that pillararenes possess high affinities toward neutral nitrile moieties.22 The binding between 1 and 2 was examined by 1H NMR spectroscopy (Figure S15). As shown in Figure S15, the proton NMR spectra of 1, 2, and the mixture of 1 and 2 indicated that the supramolecular complexation was a fast exchanging process in acetone-d6 on the proton NMR time scale. When two equiv. of 1 was mixed with 2 in solution, the phenyl protons Hg, Hh from 1 shifted downfield very slightly (Figure S15b), while the peaks of the alkyl protons signals H1, H2, H3 and the triazole proton H4 in 2 shifted upfield (∆δ = -0.08, -0.06, -0.07, and -0.04 ppm, respectively) due to the shielding effect of the electron-rich cavities of the copillar[5]arene. Besides, the resonance peaks related to protons H1-4 in 2 were accompanied with a remarkable broadening effect. These observations confirmed the interactions between the nitrile moieties in 2 and the cavities of 1, which lead to the formation of pseudorotaxane 2⊂ ⊂1. To investigate the association constant (Ka) of supramolecular system formed by 1 and 2, DMpillar[5]arene (Figure S16) was chosen as the model compound. 1H NMR titrations experiments were performed with a constant concentration of DMpillar[5]arene (4.00 mM) and varying concentrations of 2 in the range of 1.0-40.0 mM (Figure S17). By a non-linear curve-fitting method, the

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Ka of 2⊂DMpillar[5]arene was estimated to be (2.10 ± 0.36) × 103 M-1 (Figure S18).

Figure 2. Characterization of the supramolecular complex 2⊂1@Hg2+. (a) TEM and (b) SEM images of 2⊂1@Hg2+. (c) DLS study of 2⊂1@Hg2+ ([1] = [Hg2+] = 0.50 mM; [2] = 0.25 mM).

Subsequently, we investigated the construction of the proposed supramolecular complex 2⊂ ⊂1@Hg2+. For the sake of recycling the copillar[5]arene and fluorescent indicator, we focused on the synthetic route that started with the preparation of the pseudorotaxane 2⊂ ⊂1 followed by the Hg2+-complexation process. The UV-vis absorption spectra of 2⊂ ⊂1 showed a remarkable increase upon the addition of Hg2+ (Figure S19). TEM and scanning electron microscopy (SEM) were also used to characterize the morphology of the supramolecular complex 2⊂ ⊂1@Hg2+ (Figure 2a and 2b). Spherical aggregates with a diameter of about 250 nm were observed by TEM. Dynamic light scattering (DLS) measurements also confirmed the size of the aggregates was within the range of 200-300 nm (Figure 2c). In this case, copillar[5]arene 1 first binds with guest 2 to afford the pseudorotaxane 2⊂ ⊂1. This host-guest complex further coordinated with Hg2+ through T-Hg2+-T pairings which would line up the pseudorotaxanes. The T-Hg2+-T pairing has a direction almost perpendicular to that of the host-guest interactions in 2⊂ ⊂1 on average. Those forces jointly created crisscrossed networks which eventually wrapped into nanoscale spheres. These aggregates could be readily isolated from the solution after a simple 8


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centrifugation step. The above results laid a solid foundation for the practical applications of the supramolecular polymers in heavy metal ions treatment. Fluorescent emission of the supramolecular complex 2⊂ ⊂1@Hg2+ TPE derivatives display no fluorescence or weak fluorescence in a highly dispersed state, but become strongly fluorescent after aggregation.62 The indicator 2 we used in this study exhibits almost no fluorescence in solution. Even after 2 binds with 1 to form 2⊂ ⊂1, the fluorescence of 2 in the complex remains very weak (Figure 3). However, upon addition of Hg2+, the supramolecular polymer started to assemble. After embedded inside the supramolecular aggregates, the intramolecular rotations of

Figure 3. Fluorescence spectral responses of 2⊂1 (300 µM in acetone) upon titration with Hg2+ (from 0 to 350 µM) (λex = 350 nm). The inset picture shows the emission of (1) 2⊂1@Hg2+; (2) 2⊂1; (3) 1@Hg2+; (4) 2@Hg2+; (5) 2; and (6) 1 under 365 nm UV light illumination.

2 would be severely restricted and it started to fluoresce. The emission of 2⊂ ⊂1 at 460 nm increased sharply along with the addition of Hg2+ (Figure 3). When the equivalent of Hg2+ reached 1.0, the fluorescence of the supramolecular polymer 2⊂ ⊂1@Hg2+ was over 20 times higher than that of the pseudorotaxane 2⊂ ⊂1. Moreover, the emission in 9



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the visible light range allows convenient observation with naked eyes when the solution is excited with a handheld 365 nm UV light. This “instrument-free” feature is crucial when the Hg2+ sensing and removal tests are performed onsite. To investigate the detection limit and linear detection range, we further performed fluorescence measurements of 2⊂ ⊂1 in the presence of increasing amounts of Hg(ClO4)2. As shown in Figure S20 and S21, the fluorescence intensity increases linearly with an increase in the concentration of Hg(ClO4)2 within the range of 0-180 µM. Moreover, the detection limit of Hg2+ based on 3δ/s is 2.3 µM, where δ is the standard deviation of 10 blank measurements and s is the slope of the fluorescence intensity as a function of Hg2+ concentration. We further performed a competition experiment of the supramolecular complex 2⊂ ⊂1@Hg2+. As shown in Figure S22, significant quenching of the fluorescence of 2⊂ ⊂1@Hg2+ was observed upon gradual addition of ethylenediaminetetraacetic acid (EDTA), which showed that T-Hg2+-T pairings could be destroyed by the presence of EDTA. Selectivity of the pseudorotaxane 2⊂ ⊂1 toward Hg2+ Following the fluorescence measurement experiments, we investigated the selectivity of 2⊂ ⊂1 toward Hg2+. The fluorescence spectra of 2⊂ ⊂1 were recorded in the presence of various metal ions (1.0 equiv.) including Ba2+, Ca2+, Cd2+, Cs+, Co2+, Cu2+, Fe2+, Fe3+, K+, Mg2+, Mn2+, Zn2+, Ni2+, Pb2+ and Hg2+, respectively, under identical conditions. As shown in Figure 4, the fluorescence intensity of 2⊂ ⊂1 at 460 nm was used to assess the abilities of metal ions to induce the formation of the

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Figure 4. Fluorescence intensity I460 nm of 2⊂1 (300 µM in acetone) in the presence of 1.0 equiv. of various metal ions including Ba2+, Ca2+, Cd2+, Cs+, Co2+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Mg2+, Mn2+, Zn2+, Ni2+, and Pb2+, respectively (concentration: 300 µM).

supramolecular polymers. It is clear that all the metal ions tested except Hg2+ failed to link 2⊂ ⊂1 together to generate the fluorescent network structure and thus did not interfere with the detection and removal of Hg2+, most likely due to the unique coordination capacity of Hg2+ toward the thymine moieties in 1 (Figure 1). Practical application of Hg2+ removing with 2⊂ ⊂1 To exemplify the practicality of the proposed methodology, we performed the Hg2+ removal and probe recycling experiments in Hg2+-spiked water solution (Figure 5). Pseudorotaxane 2⊂ ⊂1 (10 mg, 3.87 µmol in 0.20 mL acetone) was suspended in a dilute aqueous solution of Hg(ClO4)2 (20 ppm in 10 mL) and stirred for 12 h. Then, the suspension was centrifuged at 1000 g for 40 min and the precipitate was filtered off. The solution was taken for inductively coupled plasma mass spectrometry (ICP-MS) analysis of the residual Hg2+ (Figure S23). The result showed that the concentration of the residual Hg2+ was less than 0.8 ppm, meaning that over 96% of

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Hg2+ could be effectively removed in a single treatment. Furthermore, the precipitate

Figure 5. Illustration of the application of 2⊂1 in removing Hg2+ in water. Step 1: pseudorotaxane 2⊂1 was added to the Hg2+ solution to form 2⊂1@Hg2+, which could be isolated by centrifugation (shown in picture). Step 2: the precipitate 2⊂1@Hg2+ was transferred from water to acetone/dichloromethane (v/v = 15:1) and redispersed in organic solution. Step 3: Na2S was added to the suspension of 2⊂1@Hg2+ and black HgS precipitated from the solution (shown in picture). Pseudorotaxane 2⊂1 in solution was recycled and reused.

was redispersed in acetone/dichloromethane (v/v = 15:1) to which was added Na2S. Black HgS precipitated after a quick stirring (30 min) and the pseudorotaxane 2⊂ ⊂1 in solution could be fully recycled after centrifugation. Reuse of the recycled pseudorotaxane complex showed almost no loss of activities in the sensing and capture of Hg2+ (Figure 6). These features are of utmost importance for the practical applications of supramolecular polymers in the sensing and removal of heavy metal ions in water.

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Figure 6. Hg2+ removal and probe recycling experiments. Pseudorotaxane 2⊂1 (10 mg in 0.2 mL acetone) was suspended in a dilute aqueous solution of Hg(ClO4)2 (20 ppm in 10 mL) and stirred for 12 h. The concentration of the residual Hg2+ after centrifugation was identified by ICP-MS. The recycled 2⊂1 was used in a new Hg2+ removal experiment. CONCLUSION

In summary, we have constructed a supramolecular polymer by the combination of T-Hg2+-T pairings and host-guest interactions. Copillar[5]arene 1 has two thymine groups, one on each side of the ring that can coordinate with Hg2+ tightly through T-Hg2+-T pairings. At the same time, 1 can bind with indicator 2 through host-guest interactions between the pillararene cavity and the nitrile moieties of 2. These binding forces work jointly together to create crisscrossed network structures which eventually wrap into spherical nanoparticles. Also, due to the AIE feature of indicator 2, the formed supramolecular polymer exhibits strong fluorescence which renders convenient the detection of the Hg2+-containing nanoparticles and the monitoring of the removal procedure. Moreover, the assembled polymer precipitate can be readily isolated by centrifugation, and the pseudorotaxane 2⊂ ⊂1 can be redispersed in organic solvents and recycled by the addition of Na2S without loss of activities. Our study has 13



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shown a practical strategy for the sensing and removal of heavy metal ions in water by the construction of metal ions-participated supramolecular polymers. This strategy may also be applied for the sensing and removal of other environmental pollutants in the future. EXPERIMENTAL SECTION General procedure for the preparation of the supramolecular aggregation 2⊂ ⊂1@Hg2+ Copillar[5]arene 1 (11.8 mg, 9.7 µmol) and TPE derivative 2 (3.3 mg, 4.8 µmol) were dissolved in 3.0 mL acetone. Then Hg(ClO4)2 (2.0 mg, 5.0 µmol) was slowly added into this solution and the reaction mixture was stirred for 10 minutes at room temperature to generate the supramolecular aggregation 2⊂ ⊂1@Hg2+. General procedure for the removal of Hg2+ with the pseudorotaxane 2⊂ ⊂1 and recycling of 2⊂ ⊂1 Pseudorotaxane 2⊂ ⊂1 (10.0 mg, 3.9 µmol in 0.2 mL acetone) was suspended in a dilute aqueous solution of Hg(ClO4)2 (20 ppm in 10 mL water) and stirred for 12 h. Next, the precipitate 2⊂ ⊂1@Hg2+ was separated by centrifugation (1000 g for 40 min) and then transferred to a mixture of acetone and dichloromethane (v/v = 15:1). Finally, Na2S (3.0 mM, 0.2 mL) was added to the organic solution and stirred for 30 min. Black HgS precipitated from the solution and the pseudorotaxane 2⊂ ⊂1 in solution could be fully recycled after centrifugation (1000 g for 40 min). ASSOCIATED CONTENT

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Supporting Information. Materials and Methods; Supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected] ACKNOWLEDGMENT This project was funded by National Key Research and Development Plan (2016YFA0203200) and the National Natural Science Foundation of China (numbers 21375130, 31571010, 21502195).

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Pillararene-Based Aggregation-Induced-Emission ... - ACS Publications

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