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Communication
A Luminescent Hypercrosslinked Conjugated Microporous Polymer for Efficient Removal and Detection of Mercury Ions Lu Xiang, Yunlong Zhu, Shuai Gu, Dongyang Chen, Xian Fu, Yindong Zhang, Guipeng Yu,* Chunyue Pan,* Yuehua Hu
A hypercrosslinked conjugated microporous polymer (HCMP-1) with a robustly efficient absorption and highly specific sensitivity to mercury ions (Hg2+) is synthesized in a onestep Friedel–Crafts alkylation of cost-effective 2,4,6-trichloro-1,3,5-triazine and dibenzofuran in 1,2-dichloroethane. HCMP-1 has a moderate Brunauer– Emmett–Teller specific surface (432 m2 g−1), but it displays a high adsorption affinity (604 mg g−1) and excellent trace efficiency for Hg2+. The π–π* electronic transition among the aromatic heterocyclic rings endows HCMP-1 a strong fluorescent property and the fluorescence is obviously weakened after Hg2+ uptake, which makes the hypercrosslinked conjugated microporous polymer a promising fluorescent probe for Hg2+ detection, owning a super-high sensitivity (detection limit 5 × 10−8 mol L−1). 1. Introduction Hg, as one of the highly virulent materials commonly found in aquatic systems, can cause serious human illness, even an ultralow level (parts per billion; ppb).[1,2] Adsorption represents a powerful strategy for removing Hg, and the performance of some emerged porous solids such as metal organic frameworks (MOFs)[3–5] and nanoporous organic polymers (NOPs)[6–10] on Hg2+ adsorption have attracted ever-increasing attention due to their simplicity, L. Xiang, Prof. G. Yu, Prof. Y. Hu School of Minerals Processing and Bioengineering Central South University Changsha 410083, China E-mail: [email protected] L. Xiang, Y. Zhu, S. Gu, Dr. D. Chen, X. Fu, Y. Zhang, Prof. G. Yu, Prof. C. Pan College of Chemistry and Chemical Engineering Central South University Changsha 410083, China E-mail: [email protected]
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large adsorption capacity and high efficiency. However, a fatal drawback of the cation-containing MOFs may be their insufficient tolerance to acid/base situations that is usually encountered in the regeneration of the sorbents, significantly damaging their popularity. By comparison, NOPs express excellent physical-chemical stability, which offers a refreshable and cost-effective option for the decontamination of mercury ions. As an emerging class of NOPs, conjugated microporous polymers (CMPs) first developed by Cooper and co-workers[11] were featured by permanent nanopores, 3Dnetworks and strong photoluminescence properties resulting from the remarkable π-conjugation in sharp contrast to other kinds of organic porous materials that are not π-conjugated.[12] Based on such characteristics, CMPs have been reported as sensors for the detection of chemical hazards such as nitroaromatic compounds[13] and H2S.[14] Moreover, CMPs appear to satisfy the fluorescent response for detecting trace amounts of heavy metal ions due to their high quenching efficiency related to energy or electron transfer from the conjugated chain
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DOI: 10.1002/marc.201500159
A Luminescent Hypercrosslinked Conjugated Microporous Polymer for Efficient Removal and Detection of Mercury Ions
Macromolecular Rapid Communications www.mrc-journal.de
to the metal complexes. However, the preparation of starting blocks of CMPs requires multistep reactions in the catalysis of notable metals and the precious catalysts are difficult to eliminate during the purification, illustrating the tedious synthetic procedures and great difficulty for scale-up. Hypercrosslinked polymers (HCPs)[15,16] are currently drawing great attention due to their easy preparation, high chemical-physical stability and low cost. Combined with the low density and high surface areas, HCPs can be considered as promising materials for catalysis,[17] gas uptake, and separation.[18,19] Tan and co-workers[20] have reported a class of heterocyclic-functional hypercrosslinked microporous polymers as highly selective CO2 capturing materials, and Dai and co-workers[21,22] have prepared some task-specific porous organic polymers for effcient CO2 capture which were synthezised by using a one-step Friedel–Crafts reaction. Therefore, combined the photoluminescence property of CMPs with facile preparation technology of HCPs, a hypercrosslinked CMP with high fluorescence quenching sensitivity for mercury ions detection, which was built with a one-step reaction and catalysted without using notable catalysts, must be appealing and informative. Our strategy for preparing a CMP with high sensing ability toward mercury ions involves a one-step Friedel– Crafts alkylation[23] (Scheme 1). The polymerization of 2,4,6-trichloro-1,3,5-triazine (CC) and dibenzofuran (DBF) in the presence of AlCl3 readily afforded a furan-functional hyper-crosslinked conjugated microporous polymer (termed HCMP-1). Such construction chemistry for microporous polymers offers significant advantages for large-scale production and applications. Oxygen-binding benzene rings in electron-donating DBF feature lone pair electrons for interaction with Hg2+. Both N C linkages in 1,3,5-triazine (TZ) and O bridges in DBF tend to form stable chelates with heavy metal ions,[24] where coordination bonds form as the interaction between lone pair electrons and metal ions. These enable HCMP-1 a high affinity toward Hg2+ and result in outstanding adsorption efficiency, in spite of its moderate BET surface area. The
Scheme 1. Synthetic route to HCMP-1 network.
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major discovery here is a hypercrosslinked CMP system with strong fluorescent properties that exhibits a highly specific sensitivity toward Hg(II) species. This simple synthesis mechanism of luminescent hypercrosslinked CMPs will probably meet the actual application demands of metal ions detections in the near future.
2. Experimental Section 2.1. Synthesis of HCMP-1 2,4,6-Trichloro-1,3,5-triazine (CC, 1.84 g, 10 mmol) and AlCl3 (4.80 g, 36 mmol) were added into a dried 250 mL three-necked round-bottom flask fitted with constant pressure drop funnel and mechanical stirrer. After being degassed by dry N2 bubbling, the whole mixture was heated to 80 °C. A solution of DBF (2.52 g, 15 mmol) in 1,2-dichloroethane (100 mL) was slowly and dropwise added. The mixture was then stirred at reflux temperature for 24 h. The precipitated polymer was isolated by filtration, washed with ice water and ethanol. Further purification was done by soxhlet extraction with tetrahydrofuran and methanol for 48 h and dried in vacuum finally. The target product was obtained as a brown colored powder (HCMP-1, Yield: 85%). Anal. Calcd for (C42H18N6O3)n: C 77.06, H 2.77, N 12.84. Found: C 73.15, H 2.78, N 9.60.
2.2. Adsorption Experiments of Mercury Ions All adsorption experiments were carried out in a series of 25 mL stuffed colorimetric tubes containing HgCl2 aqueous solutions with different concentrations, and the tubes were shaken in an air bath at a fixed temperature of 30 °C (120 rpm) for a given period. During each batch, 20 mg HCMP-1 was added into the solution. After adsorption, the porous solids and liquid phase were separated by centrifugation. The porous solids were washed thoroughly with distilled water to neutral, and dried under vacuum. Hg2+ concentrations in the filtrate were analyzed by inductive coupled plasma atom emission spectrometer (ICP-AES).
3. Results and Discussion As shown in the FT-IR spectrum of the resultant HCMP-1 (Figure 1a), the intensity of C–Cl stretching at 850 cm−1 diminishes dramatically compared to 2,4,6-trichloro-1,3,5triazine, indicating almost complete conversion of C–Cl. Disappearance of characteristic peaks in the range of 700–680 cm−1 as well as the emergence of a new absorption at 800 cm−1 prove a para-substitution chemistry of oxygen atoms in dibenzofuran. The diagnostic peak shifting downfield at around 166.7 ppm in the 13C NMR spectrum (Figure 1b) could be attributed to the carbon atoms in TZ unit.[25] The signal of oxygen-bonded carbon in benzene appears at 156.2 ppm and that of triazine-substituted
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Figure 1. a) FT-IR spectra of DBF, CC, and HCMP-1. b) 13C CP/MAS NMR spectroscopy of HCMP-1.
carbon at 133.4 ppm, confirming the network structure. For clear illustration, field emission scanning electron microscope images (FE-SEM) were demonstrated in Figure 2a. Noted that the uniform sphere-shaped of HCMP-1 particles are agglomerated. Demonstrated in high-magnification images of high resolution transmission electron microscopy (HR-TEM, Figure 2b) are uniform alternately dark and bright worm-like structures, implying the rich porous characteristic. Powder X-ray diffraction spectrum in Figure S1, Supporting Information, is featureless, indicative of an amorphous nature of HCMP-1. Notably, TGA measurement indicates that HCMP-1 is stable up to 300 °C and shows a char yield of 60 wt% when heated to 800 °C under a nitrogen atmosphere (Figure S2, Supporting
Information), adding to the practical advantages. Meanwhile, the obtained product is efficiently recoverable in most common organic solvents such as dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N-methylpyrroli-done (NMP), tetrahydrofuran (THF), and even diluted HCl solution (10 wt%), implying a physical–chemical stable nature. The excellent thermal and chemical stabilities of HCMP-1 are most possibly resulted from the high charge transfer interactions between the 1,3,5-triazine rings and aromatic/heterocyclic units.[26,27] The immiscible network avoids unnecessary contamination to the detection system, being more convenient for extraction and separation of the probes relative to the small molecular ones.
Figure 2. a) SEM and b) TEM images for HCMP-1.
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A Luminescent Hypercrosslinked Conjugated Microporous Polymer for Efficient Removal and Detection of Mercury Ions
Macromolecular Rapid Communications www.mrc-journal.de
Figure 3. a) Nitrogen absorption/desorption isotherms of the HCMP-1 at 77 K; b) pore size distribution obtained by NLDFT calculation.
The porosity of HCMP-1 was investigated by the gas adsorption/desorption using nitrogen at 77 K (Figure 3a). As presented in Table S1, Supporting Information, Brunauer–Emmett–Teller (BET) surface area of HCMP-1 is up to 432 m2 g−1, and pore volume calculated at P/P0 = 0.99 is 0.31 cm3 g−1. Figure 1a shows a type I isotherm featured by a very sharp uptake at low-pressure ratio region (P/P0 < 0.05), which is typical for the microporous material. It exhibits an apparent hysteresis at high-pressure regions, which suggests that the obtained network is a microporous material with supererogatory mesoporosity alternatively, demonstrated by Antonietti and coworkers[28] and Banerjee[29] The pore size distribution curve calculated by NLDFT method (Figure 3b) reveals that HCMP-1 mainly displays micropores with pore size of 1.6 nm. The as-prepared sample (20 mg) was placed in dilute aqueous solution of Hg2+ with various pH. The pH effects of aqueous Hg2+ solution on adsorption efficiency and capability were highlighted for HCMP-1 (Figure S3, Supporting Information). The maximum adsorption capacity was demonstrated at a pH value of 4.5. In lower pH cases, the surface of HCMP-1 was protonated and hence H3O+ ions competed with free Hg2+, limiting the chelation between N, O atoms with Hg2+ and hence leading to a poor metal ions adsorption capacity.[30] With the increasing pH values, metal precipitation was observed and the adsorption capacity was deteriorated with the accumulation of metal ions onto adsorbent surfaces.[31] Figure 4 showed the Hg2+ adsorption capacity and adsorptivity of HCMP-1 depending on the adsorption time. It was clearly indicated that 75% of Hg2+ was adsorbed within a short contact time of 30 min, and more than 99.9% of Hg2+ (initial concentration of Hg2+: 10 mg L−1) can be removed in a prolonged time (20 h). At normal temperature, HCMP-1 attains its equilibrium
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with the adsorption capacity of 604 mg g−1. The uptake capability was abundant and attractive. The curves of log(Qe–Qt) versus contact time t and t/Qt versus t based on the experiment data were illustrated in Figure S4, Supporting Information. As summarized in Table S2, Supporting Information, the kinetic behavior of Hg2+ sorption onto HCMP-1 can be appropriately described by the pseudo-second-order model, suggesting that the Hg2+ adsorption may be a chemical process involving valence forces through the electron sharing or exchange between adsorbent and sorbate.[32] Concerning the sample after acid/base treatment or the regenerative sample, adsorption capacity for Hg2+ was maintained at the high levels (Figure S5, Supporting Information) which expounded that HCMP-1 not only exhibited good resistance to acid and alkali but also manifested outstanding regeneration capacity. This characteristic of HCMP-1 endowes its great potentials in the field of heavy metal ions treatment. Selectivity is another crucial issue for large-scale water decontamination. Displayed in Figure S6, Supporting Information, is the uptake capacities of HCMP-1 toward various kinds of heavy metal ions. Only Hg2+ was effectively removed by HCMP-1 (95.6%) while less than 20% of Ca2+, Cu2+, Cd2+, and Mg2+ were absorbed. Clearly, HCMP-1 can remove Hg species from wastewater selectively and effectively. UV–vis absorption spectra of HCMP-1 in the solid state and in ethanol were recorded in Figure S7, Supporting Information. HCMP-1 in ethanol suspension (Figure S8, Supporting Information) at room temperature harvested a strong fluorescence emission peak at 320 nm. This may be related to the generation of the long π-conjugated structure consisting of electron accepting 1,3,5-triazine and electron-donating furan units. The luminescent spectra of HCMP-1 showed a significant quenching effect with the prensence of a
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well fits the Stern–Volmer (SV) model: I0/I = 1 + KSV[C],[35] where I0 is the fluorescence intensity in the absence of the quencher, I is the fluorescence intensity after adding the quencher of concentration C, and the KSV is the quenching coefficient of the sensing material. The Stern–Volmer constant (KSV) of combined quenching was recorded up to 1.1 × 104 M−1. The strong fluorescence emission and super-high sensitivity (detection limit 5 × 10−8 mol L−1) of HCMP-1 inspire us to explore their potentials in the removal and monitoring of Hg2+.
4. Conclusions Figure 4. Effects of sorption time on the adsorption of Hg2+ (inset: Hg2+ adsorption capacity curves of HCMP-1 in 0–30 min).
small amount of Hg2+ (Figure 5L). As shown in the picture inset (Figure 5L), the suspension displays bright fluorescent in the absence of Hg2+(down), while it turned obvious darker in the presence of Hg2+ (40 × 10−5 mol L−1) (up). It was generally accepted that fluorescence quenching was resulted from energy and electron transfer which was from HCMP-1 to the metal complexes,[33] or the aggregation of the polymer chains induced by the metal ions.[34] The linear correlation coefficient (R) of PL quenching efficiency (I0/I) versus Hg2+ concentration almost reached 1 (Figure 5R), indicating that fluorescence quenching effect of Hg2+ on HCMP-1
In summary, a HCMP-1 constructed from commercially available materials by a one-step Friedel–Crafts reaction exhibits attractive luminescent characteristics and high efficient absorption of Hg2+. Due to the strong coordination interactions between electron-rich heteroatom and Hg2+, HCMP-1 can scavenge Hg2+ from wastewater effectively and selectively, and its Hg2+ uptake capacity is up to 604 mg g−1. Particularly, the strong luminescence of HCMP-1 could be quenched efficiently by Hg2+ (an ultralow detection limit of 5 × 10−8 mol L−1), indicating that it may serve as a sensitive sensor for the detection of Hg species. Further studies on these and other hypercrosslinked conjugated microporous polymer systems are ongoing and will be published in due course.
Figure 5. PL spectra of HCMP-1 solution in ethanol after adding Hg2+ with concentrations (0, 0.005, 0.05, 0.5, 5, 10, 15, 20, 30, 40) × 10−5 mol L−1 (inset: luminescent photograph of the HCMP-1 solution in the presence (up) and absence (down) of Hg2+ (40 × 10−5 mol L−1)) (L). Plot of PL quenching efficiencies (I0/I) versus Hg2+ concentrations (inset: PL intensity of HCMP-1 solution in ethanol vs Hg2+ concentrations) (R).
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A Luminescent Hypercrosslinked Conjugated Microporous Polymer for Efficient Removal and Detection of Mercury Ions
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: L.X. and Y.Z. contributed equally to this work. The authors acknowledge the financially support from the National Natural Science Foundation of China (CN) (Grant Nos. 21204103 and 21376272), China Postdoctoral Science (2012M521535 and 2014T70787), State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (2015-KF-8) and the Fundamental Research Funds for the Central Universities of Central South University(2015zzts168). Received: March 15, 2015; Revised: April 30, 2015; Published online: June 18, 2015; DOI: 10.1002/marc.201500159
Keywords: detection; hypercrosslinked conjugated microporous polymers; luminescence; mercury removal [1] X. G. Li, H. Feng, M. R. Huang, Chem. Eur. J. 2009, 15, 4573. [2] E. M. Nolan, S. J. Lippard, Chem. Rev. 2008, 108, 3443. [3] K.-K. Yee, N. Reimer, J. Liu, S.-Y. Cheng, S.-M. Yiu, J. Weber, N. Stock, Z. Xu, J. Am. Chem. Soc. 2013, 135, 7795. [4] O. M. Yaghi, G. Li, H. Li, Nature 1995, 378, 703. [5] K. M. Thomas, Dalton. Trans. 2009, 9, 1487. [6] Y. Zhang, S. N. Riduan, Chem. Soc. Rev. 2012, 41, 2083. [7] L. B. Sun, Y. C. Zou, Z. Q. Liang, J. H. Yu, R. R. Xu, Polym. Chem. 2014, 5, 471. [8] N. B. McKeown, P. M. Budd, Chem. Soc. Rev. 2006, 35, 675. [9] Y. Liu, S. Wu, G. Wang, G. Yu, J. Guan, C. Pan, Z. Wang, J. Mater. Chem. A 2014, 2, 7795. [10] S.-F. Wu, Y. Liu, G.-P. Yu, J.-G. Guan, C.-Y. Pan, Y. Du, X. Xiong, Z.-G. Wang, Macromolecules 2014, 47, 2875. [11] J. X. Jiang, Y. Y Li, X. F Wu, J. L. Xiao, D. J. Adams, A. I. Cooper, Macromolecules 2013, 46, 8779. [12] Y. H. Xu, S. B. Jin, H. Xu, A. Nagai, D. L. Jiang, Chem. Soc. Rev. 2013, 42, 8012.
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[13] M. Warzecha, J. Calvo-Castro, A. R. Kennedy, A. N. Macpherson, K. Shankland, N. Shankland, A. J. McLean, C. J. McHugh, Chem. Commun. 2015, 51, 1143. [14] J. Liu, K.-K. Yee, K. K. Lo, K. Y. Zhang, W.-P. To, C.-M. Che, Z. Xu, J. Am. Chem. Soc. 2014, 136, 2818. [15] M. Tsyurupa, V. Davankov, React. Funct. Polym. 2006, 66, 768. [16] J. Y. Lee, C. D. Wood, D. Bradshaw, M. J. Rosseinsky, A. I. Cooper, Chem. Commun. 2006, 25, 2670. [17] B. Li, Z. Guan, W. Wang, X. Yang, J. Hu, B. Tan, T. Li, Adv. Mater. 2012, 24, 3390. [18] S. Xiong, X. Fu, L. Xiang, G. Yu, J. Gua, Z. Wang, Y. Du, X. Xiong, C. Pan, Polym. Chem. 2014, 5, 3424. [19] M. Saleh, S. B. Baek, H. M. Lee, K. S. Kim, J. Phys. Chem. C 2015, 119, 5395. [20] Y. Luo, B. Li, W. Wang, K. Wu, B. Tan, Adv. Mater. 2012, 24, 5703. [21] X. Zhu, S. M. Mahurin, S.-H. An, C. Do-Thanh, C. Tian, Y. Li, L. W. Gill, E. W. Hagaman, Z. Bian, J.-H. Zhou, J. Hu, H. Liu, S. Dai, Chem. Commun. 2014, 50, 7933. [22] J. Zhang, Z. Qiao, S. M. Mahurin, X. Jiang, S. Chai, H. Lu, K. Nelson, S Dai, Angew. Chem. 2015, 127, 4665. [23] J. Zhu, Q. Chen, Z. Sui, L. Pan, J. Yu, B. Han, J. Mater. Chem. A 2014, 2, 16181. [24] S. Biniak, M. Pakula, G. Szymanski, A. Swiatkowski, Langmuir 1999, 15, 6117. [25] M. G. Schwab, B. Fassbender, H. W. Spiess, A. Thomas, X. Feng, K. Mullen, J. Am. Chem. Soc. 2009, 131, 7216. [26] G. Yu, C Liu, X Li, J Wang, X. Jian, C. Pan, Polym. Chem. 2012, 3, 1024. [27] S. Wu, S. Gu, A. Zhang, G. Yu, Z. Wang, X. Jian, J. Mater. Chem. A 2015, 3, 878. [28] P. Kuhn, A. I. Forget, D. Su, A. Thomas, M. Antonietti, J. Am. Chem. Soc. 2008, 130, 13333. [29] S. Kandambeth, A. Mallick, B. Lukose, M. V. Mane, T. Heine, R. Banerjee, J. Am. Chem. Soc. 2012, 134, 19524. [30] X. Jin, C. Yu, Y. Li, Y. Qi, L. Yang, G. Zhao, H. Hu, J. Hazard. Mater. 2011, 186, 1672. [31] Y. Xue, H. Hou, S. Zhu, J. Hazard. Mater. 2009, 162, 391. [32] M. R. Huang, Q. Y. Peng, X. G. Li, Chem. Eur. J. 2006, 12, 4341. [33] B. Wang, M. R. Wasielewski, J. Am. Chem. Soc. 1997, 119, 12. [34] I. B. Kim, U. H. Bunz, J. Am. Chem. Soc. 2006, 128, 2818. [35] Z. Xiang, D. Cao, Macromol. Rapid Commun. 2012, 33, 1184.
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Communication
A Luminescent Hypercrosslinked Conjugated Microporous Polymer for Efficient Removal and Detection of Mercury Ions Lu Xiang, Yunlong Zhu, Shuai Gu, Dongyang Chen, Xian Fu, Yindong Zhang, Guipeng Yu,* Chunyue Pan,* Yuehua Hu
A hypercrosslinked conjugated microporous polymer (HCMP-1) with a robustly efficient absorption and highly specific sensitivity to mercury ions (Hg2+) is synthesized in a onestep Friedel–Crafts alkylation of cost-effective 2,4,6-trichloro-1,3,5-triazine and dibenzofuran in 1,2-dichloroethane. HCMP-1 has a moderate Brunauer– Emmett–Teller specific surface (432 m2 g−1), but it displays a high adsorption affinity (604 mg g−1) and excellent trace efficiency for Hg2+. The π–π* electronic transition among the aromatic heterocyclic rings endows HCMP-1 a strong fluorescent property and the fluorescence is obviously weakened after Hg2+ uptake, which makes the hypercrosslinked conjugated microporous polymer a promising fluorescent probe for Hg2+ detection, owning a super-high sensitivity (detection limit 5 × 10−8 mol L−1). 1. Introduction Hg, as one of the highly virulent materials commonly found in aquatic systems, can cause serious human illness, even an ultralow level (parts per billion; ppb).[1,2] Adsorption represents a powerful strategy for removing Hg, and the performance of some emerged porous solids such as metal organic frameworks (MOFs)[3–5] and nanoporous organic polymers (NOPs)[6–10] on Hg2+ adsorption have attracted ever-increasing attention due to their simplicity, L. Xiang, Prof. G. Yu, Prof. Y. Hu School of Minerals Processing and Bioengineering Central South University Changsha 410083, China E-mail: [email protected] L. Xiang, Y. Zhu, S. Gu, Dr. D. Chen, X. Fu, Y. Zhang, Prof. G. Yu, Prof. C. Pan College of Chemistry and Chemical Engineering Central South University Changsha 410083, China E-mail: [email protected]
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large adsorption capacity and high efficiency. However, a fatal drawback of the cation-containing MOFs may be their insufficient tolerance to acid/base situations that is usually encountered in the regeneration of the sorbents, significantly damaging their popularity. By comparison, NOPs express excellent physical-chemical stability, which offers a refreshable and cost-effective option for the decontamination of mercury ions. As an emerging class of NOPs, conjugated microporous polymers (CMPs) first developed by Cooper and co-workers[11] were featured by permanent nanopores, 3Dnetworks and strong photoluminescence properties resulting from the remarkable π-conjugation in sharp contrast to other kinds of organic porous materials that are not π-conjugated.[12] Based on such characteristics, CMPs have been reported as sensors for the detection of chemical hazards such as nitroaromatic compounds[13] and H2S.[14] Moreover, CMPs appear to satisfy the fluorescent response for detecting trace amounts of heavy metal ions due to their high quenching efficiency related to energy or electron transfer from the conjugated chain
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DOI: 10.1002/marc.201500159
A Luminescent Hypercrosslinked Conjugated Microporous Polymer for Efficient Removal and Detection of Mercury Ions
Macromolecular Rapid Communications www.mrc-journal.de
to the metal complexes. However, the preparation of starting blocks of CMPs requires multistep reactions in the catalysis of notable metals and the precious catalysts are difficult to eliminate during the purification, illustrating the tedious synthetic procedures and great difficulty for scale-up. Hypercrosslinked polymers (HCPs)[15,16] are currently drawing great attention due to their easy preparation, high chemical-physical stability and low cost. Combined with the low density and high surface areas, HCPs can be considered as promising materials for catalysis,[17] gas uptake, and separation.[18,19] Tan and co-workers[20] have reported a class of heterocyclic-functional hypercrosslinked microporous polymers as highly selective CO2 capturing materials, and Dai and co-workers[21,22] have prepared some task-specific porous organic polymers for effcient CO2 capture which were synthezised by using a one-step Friedel–Crafts reaction. Therefore, combined the photoluminescence property of CMPs with facile preparation technology of HCPs, a hypercrosslinked CMP with high fluorescence quenching sensitivity for mercury ions detection, which was built with a one-step reaction and catalysted without using notable catalysts, must be appealing and informative. Our strategy for preparing a CMP with high sensing ability toward mercury ions involves a one-step Friedel– Crafts alkylation[23] (Scheme 1). The polymerization of 2,4,6-trichloro-1,3,5-triazine (CC) and dibenzofuran (DBF) in the presence of AlCl3 readily afforded a furan-functional hyper-crosslinked conjugated microporous polymer (termed HCMP-1). Such construction chemistry for microporous polymers offers significant advantages for large-scale production and applications. Oxygen-binding benzene rings in electron-donating DBF feature lone pair electrons for interaction with Hg2+. Both N C linkages in 1,3,5-triazine (TZ) and O bridges in DBF tend to form stable chelates with heavy metal ions,[24] where coordination bonds form as the interaction between lone pair electrons and metal ions. These enable HCMP-1 a high affinity toward Hg2+ and result in outstanding adsorption efficiency, in spite of its moderate BET surface area. The
Scheme 1. Synthetic route to HCMP-1 network.
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major discovery here is a hypercrosslinked CMP system with strong fluorescent properties that exhibits a highly specific sensitivity toward Hg(II) species. This simple synthesis mechanism of luminescent hypercrosslinked CMPs will probably meet the actual application demands of metal ions detections in the near future.
2. Experimental Section 2.1. Synthesis of HCMP-1 2,4,6-Trichloro-1,3,5-triazine (CC, 1.84 g, 10 mmol) and AlCl3 (4.80 g, 36 mmol) were added into a dried 250 mL three-necked round-bottom flask fitted with constant pressure drop funnel and mechanical stirrer. After being degassed by dry N2 bubbling, the whole mixture was heated to 80 °C. A solution of DBF (2.52 g, 15 mmol) in 1,2-dichloroethane (100 mL) was slowly and dropwise added. The mixture was then stirred at reflux temperature for 24 h. The precipitated polymer was isolated by filtration, washed with ice water and ethanol. Further purification was done by soxhlet extraction with tetrahydrofuran and methanol for 48 h and dried in vacuum finally. The target product was obtained as a brown colored powder (HCMP-1, Yield: 85%). Anal. Calcd for (C42H18N6O3)n: C 77.06, H 2.77, N 12.84. Found: C 73.15, H 2.78, N 9.60.
2.2. Adsorption Experiments of Mercury Ions All adsorption experiments were carried out in a series of 25 mL stuffed colorimetric tubes containing HgCl2 aqueous solutions with different concentrations, and the tubes were shaken in an air bath at a fixed temperature of 30 °C (120 rpm) for a given period. During each batch, 20 mg HCMP-1 was added into the solution. After adsorption, the porous solids and liquid phase were separated by centrifugation. The porous solids were washed thoroughly with distilled water to neutral, and dried under vacuum. Hg2+ concentrations in the filtrate were analyzed by inductive coupled plasma atom emission spectrometer (ICP-AES).
3. Results and Discussion As shown in the FT-IR spectrum of the resultant HCMP-1 (Figure 1a), the intensity of C–Cl stretching at 850 cm−1 diminishes dramatically compared to 2,4,6-trichloro-1,3,5triazine, indicating almost complete conversion of C–Cl. Disappearance of characteristic peaks in the range of 700–680 cm−1 as well as the emergence of a new absorption at 800 cm−1 prove a para-substitution chemistry of oxygen atoms in dibenzofuran. The diagnostic peak shifting downfield at around 166.7 ppm in the 13C NMR spectrum (Figure 1b) could be attributed to the carbon atoms in TZ unit.[25] The signal of oxygen-bonded carbon in benzene appears at 156.2 ppm and that of triazine-substituted
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Figure 1. a) FT-IR spectra of DBF, CC, and HCMP-1. b) 13C CP/MAS NMR spectroscopy of HCMP-1.
carbon at 133.4 ppm, confirming the network structure. For clear illustration, field emission scanning electron microscope images (FE-SEM) were demonstrated in Figure 2a. Noted that the uniform sphere-shaped of HCMP-1 particles are agglomerated. Demonstrated in high-magnification images of high resolution transmission electron microscopy (HR-TEM, Figure 2b) are uniform alternately dark and bright worm-like structures, implying the rich porous characteristic. Powder X-ray diffraction spectrum in Figure S1, Supporting Information, is featureless, indicative of an amorphous nature of HCMP-1. Notably, TGA measurement indicates that HCMP-1 is stable up to 300 °C and shows a char yield of 60 wt% when heated to 800 °C under a nitrogen atmosphere (Figure S2, Supporting
Information), adding to the practical advantages. Meanwhile, the obtained product is efficiently recoverable in most common organic solvents such as dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N-methylpyrroli-done (NMP), tetrahydrofuran (THF), and even diluted HCl solution (10 wt%), implying a physical–chemical stable nature. The excellent thermal and chemical stabilities of HCMP-1 are most possibly resulted from the high charge transfer interactions between the 1,3,5-triazine rings and aromatic/heterocyclic units.[26,27] The immiscible network avoids unnecessary contamination to the detection system, being more convenient for extraction and separation of the probes relative to the small molecular ones.
Figure 2. a) SEM and b) TEM images for HCMP-1.
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A Luminescent Hypercrosslinked Conjugated Microporous Polymer for Efficient Removal and Detection of Mercury Ions
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Figure 3. a) Nitrogen absorption/desorption isotherms of the HCMP-1 at 77 K; b) pore size distribution obtained by NLDFT calculation.
The porosity of HCMP-1 was investigated by the gas adsorption/desorption using nitrogen at 77 K (Figure 3a). As presented in Table S1, Supporting Information, Brunauer–Emmett–Teller (BET) surface area of HCMP-1 is up to 432 m2 g−1, and pore volume calculated at P/P0 = 0.99 is 0.31 cm3 g−1. Figure 1a shows a type I isotherm featured by a very sharp uptake at low-pressure ratio region (P/P0 < 0.05), which is typical for the microporous material. It exhibits an apparent hysteresis at high-pressure regions, which suggests that the obtained network is a microporous material with supererogatory mesoporosity alternatively, demonstrated by Antonietti and coworkers[28] and Banerjee[29] The pore size distribution curve calculated by NLDFT method (Figure 3b) reveals that HCMP-1 mainly displays micropores with pore size of 1.6 nm. The as-prepared sample (20 mg) was placed in dilute aqueous solution of Hg2+ with various pH. The pH effects of aqueous Hg2+ solution on adsorption efficiency and capability were highlighted for HCMP-1 (Figure S3, Supporting Information). The maximum adsorption capacity was demonstrated at a pH value of 4.5. In lower pH cases, the surface of HCMP-1 was protonated and hence H3O+ ions competed with free Hg2+, limiting the chelation between N, O atoms with Hg2+ and hence leading to a poor metal ions adsorption capacity.[30] With the increasing pH values, metal precipitation was observed and the adsorption capacity was deteriorated with the accumulation of metal ions onto adsorbent surfaces.[31] Figure 4 showed the Hg2+ adsorption capacity and adsorptivity of HCMP-1 depending on the adsorption time. It was clearly indicated that 75% of Hg2+ was adsorbed within a short contact time of 30 min, and more than 99.9% of Hg2+ (initial concentration of Hg2+: 10 mg L−1) can be removed in a prolonged time (20 h). At normal temperature, HCMP-1 attains its equilibrium
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with the adsorption capacity of 604 mg g−1. The uptake capability was abundant and attractive. The curves of log(Qe–Qt) versus contact time t and t/Qt versus t based on the experiment data were illustrated in Figure S4, Supporting Information. As summarized in Table S2, Supporting Information, the kinetic behavior of Hg2+ sorption onto HCMP-1 can be appropriately described by the pseudo-second-order model, suggesting that the Hg2+ adsorption may be a chemical process involving valence forces through the electron sharing or exchange between adsorbent and sorbate.[32] Concerning the sample after acid/base treatment or the regenerative sample, adsorption capacity for Hg2+ was maintained at the high levels (Figure S5, Supporting Information) which expounded that HCMP-1 not only exhibited good resistance to acid and alkali but also manifested outstanding regeneration capacity. This characteristic of HCMP-1 endowes its great potentials in the field of heavy metal ions treatment. Selectivity is another crucial issue for large-scale water decontamination. Displayed in Figure S6, Supporting Information, is the uptake capacities of HCMP-1 toward various kinds of heavy metal ions. Only Hg2+ was effectively removed by HCMP-1 (95.6%) while less than 20% of Ca2+, Cu2+, Cd2+, and Mg2+ were absorbed. Clearly, HCMP-1 can remove Hg species from wastewater selectively and effectively. UV–vis absorption spectra of HCMP-1 in the solid state and in ethanol were recorded in Figure S7, Supporting Information. HCMP-1 in ethanol suspension (Figure S8, Supporting Information) at room temperature harvested a strong fluorescence emission peak at 320 nm. This may be related to the generation of the long π-conjugated structure consisting of electron accepting 1,3,5-triazine and electron-donating furan units. The luminescent spectra of HCMP-1 showed a significant quenching effect with the prensence of a
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well fits the Stern–Volmer (SV) model: I0/I = 1 + KSV[C],[35] where I0 is the fluorescence intensity in the absence of the quencher, I is the fluorescence intensity after adding the quencher of concentration C, and the KSV is the quenching coefficient of the sensing material. The Stern–Volmer constant (KSV) of combined quenching was recorded up to 1.1 × 104 M−1. The strong fluorescence emission and super-high sensitivity (detection limit 5 × 10−8 mol L−1) of HCMP-1 inspire us to explore their potentials in the removal and monitoring of Hg2+.
4. Conclusions Figure 4. Effects of sorption time on the adsorption of Hg2+ (inset: Hg2+ adsorption capacity curves of HCMP-1 in 0–30 min).
small amount of Hg2+ (Figure 5L). As shown in the picture inset (Figure 5L), the suspension displays bright fluorescent in the absence of Hg2+(down), while it turned obvious darker in the presence of Hg2+ (40 × 10−5 mol L−1) (up). It was generally accepted that fluorescence quenching was resulted from energy and electron transfer which was from HCMP-1 to the metal complexes,[33] or the aggregation of the polymer chains induced by the metal ions.[34] The linear correlation coefficient (R) of PL quenching efficiency (I0/I) versus Hg2+ concentration almost reached 1 (Figure 5R), indicating that fluorescence quenching effect of Hg2+ on HCMP-1
In summary, a HCMP-1 constructed from commercially available materials by a one-step Friedel–Crafts reaction exhibits attractive luminescent characteristics and high efficient absorption of Hg2+. Due to the strong coordination interactions between electron-rich heteroatom and Hg2+, HCMP-1 can scavenge Hg2+ from wastewater effectively and selectively, and its Hg2+ uptake capacity is up to 604 mg g−1. Particularly, the strong luminescence of HCMP-1 could be quenched efficiently by Hg2+ (an ultralow detection limit of 5 × 10−8 mol L−1), indicating that it may serve as a sensitive sensor for the detection of Hg species. Further studies on these and other hypercrosslinked conjugated microporous polymer systems are ongoing and will be published in due course.
Figure 5. PL spectra of HCMP-1 solution in ethanol after adding Hg2+ with concentrations (0, 0.005, 0.05, 0.5, 5, 10, 15, 20, 30, 40) × 10−5 mol L−1 (inset: luminescent photograph of the HCMP-1 solution in the presence (up) and absence (down) of Hg2+ (40 × 10−5 mol L−1)) (L). Plot of PL quenching efficiencies (I0/I) versus Hg2+ concentrations (inset: PL intensity of HCMP-1 solution in ethanol vs Hg2+ concentrations) (R).
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A Luminescent Hypercrosslinked Conjugated Microporous Polymer for Efficient Removal and Detection of Mercury Ions
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: L.X. and Y.Z. contributed equally to this work. The authors acknowledge the financially support from the National Natural Science Foundation of China (CN) (Grant Nos. 21204103 and 21376272), China Postdoctoral Science (2012M521535 and 2014T70787), State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (2015-KF-8) and the Fundamental Research Funds for the Central Universities of Central South University(2015zzts168). Received: March 15, 2015; Revised: April 30, 2015; Published online: June 18, 2015; DOI: 10.1002/marc.201500159
Keywords: detection; hypercrosslinked conjugated microporous polymers; luminescence; mercury removal [1] X. G. Li, H. Feng, M. R. Huang, Chem. Eur. J. 2009, 15, 4573. [2] E. M. Nolan, S. J. Lippard, Chem. Rev. 2008, 108, 3443. [3] K.-K. Yee, N. Reimer, J. Liu, S.-Y. Cheng, S.-M. Yiu, J. Weber, N. Stock, Z. Xu, J. Am. Chem. Soc. 2013, 135, 7795. [4] O. M. Yaghi, G. Li, H. Li, Nature 1995, 378, 703. [5] K. M. Thomas, Dalton. Trans. 2009, 9, 1487. [6] Y. Zhang, S. N. Riduan, Chem. Soc. Rev. 2012, 41, 2083. [7] L. B. Sun, Y. C. Zou, Z. Q. Liang, J. H. Yu, R. R. Xu, Polym. Chem. 2014, 5, 471. [8] N. B. McKeown, P. M. Budd, Chem. Soc. Rev. 2006, 35, 675. [9] Y. Liu, S. Wu, G. Wang, G. Yu, J. Guan, C. Pan, Z. Wang, J. Mater. Chem. A 2014, 2, 7795. [10] S.-F. Wu, Y. Liu, G.-P. Yu, J.-G. Guan, C.-Y. Pan, Y. Du, X. Xiong, Z.-G. Wang, Macromolecules 2014, 47, 2875. [11] J. X. Jiang, Y. Y Li, X. F Wu, J. L. Xiao, D. J. Adams, A. I. Cooper, Macromolecules 2013, 46, 8779. [12] Y. H. Xu, S. B. Jin, H. Xu, A. Nagai, D. L. Jiang, Chem. Soc. Rev. 2013, 42, 8012.
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[13] M. Warzecha, J. Calvo-Castro, A. R. Kennedy, A. N. Macpherson, K. Shankland, N. Shankland, A. J. McLean, C. J. McHugh, Chem. Commun. 2015, 51, 1143. [14] J. Liu, K.-K. Yee, K. K. Lo, K. Y. Zhang, W.-P. To, C.-M. Che, Z. Xu, J. Am. Chem. Soc. 2014, 136, 2818. [15] M. Tsyurupa, V. Davankov, React. Funct. Polym. 2006, 66, 768. [16] J. Y. Lee, C. D. Wood, D. Bradshaw, M. J. Rosseinsky, A. I. Cooper, Chem. Commun. 2006, 25, 2670. [17] B. Li, Z. Guan, W. Wang, X. Yang, J. Hu, B. Tan, T. Li, Adv. Mater. 2012, 24, 3390. [18] S. Xiong, X. Fu, L. Xiang, G. Yu, J. Gua, Z. Wang, Y. Du, X. Xiong, C. Pan, Polym. Chem. 2014, 5, 3424. [19] M. Saleh, S. B. Baek, H. M. Lee, K. S. Kim, J. Phys. Chem. C 2015, 119, 5395. [20] Y. Luo, B. Li, W. Wang, K. Wu, B. Tan, Adv. Mater. 2012, 24, 5703. [21] X. Zhu, S. M. Mahurin, S.-H. An, C. Do-Thanh, C. Tian, Y. Li, L. W. Gill, E. W. Hagaman, Z. Bian, J.-H. Zhou, J. Hu, H. Liu, S. Dai, Chem. Commun. 2014, 50, 7933. [22] J. Zhang, Z. Qiao, S. M. Mahurin, X. Jiang, S. Chai, H. Lu, K. Nelson, S Dai, Angew. Chem. 2015, 127, 4665. [23] J. Zhu, Q. Chen, Z. Sui, L. Pan, J. Yu, B. Han, J. Mater. Chem. A 2014, 2, 16181. [24] S. Biniak, M. Pakula, G. Szymanski, A. Swiatkowski, Langmuir 1999, 15, 6117. [25] M. G. Schwab, B. Fassbender, H. W. Spiess, A. Thomas, X. Feng, K. Mullen, J. Am. Chem. Soc. 2009, 131, 7216. [26] G. Yu, C Liu, X Li, J Wang, X. Jian, C. Pan, Polym. Chem. 2012, 3, 1024. [27] S. Wu, S. Gu, A. Zhang, G. Yu, Z. Wang, X. Jian, J. Mater. Chem. A 2015, 3, 878. [28] P. Kuhn, A. I. Forget, D. Su, A. Thomas, M. Antonietti, J. Am. Chem. Soc. 2008, 130, 13333. [29] S. Kandambeth, A. Mallick, B. Lukose, M. V. Mane, T. Heine, R. Banerjee, J. Am. Chem. Soc. 2012, 134, 19524. [30] X. Jin, C. Yu, Y. Li, Y. Qi, L. Yang, G. Zhao, H. Hu, J. Hazard. Mater. 2011, 186, 1672. [31] Y. Xue, H. Hou, S. Zhu, J. Hazard. Mater. 2009, 162, 391. [32] M. R. Huang, Q. Y. Peng, X. G. Li, Chem. Eur. J. 2006, 12, 4341. [33] B. Wang, M. R. Wasielewski, J. Am. Chem. Soc. 1997, 119, 12. [34] I. B. Kim, U. H. Bunz, J. Am. Chem. Soc. 2006, 128, 2818. [35] Z. Xiang, D. Cao, Macromol. Rapid Commun. 2012, 33, 1184.
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