Supramolecular Aggregation-Induced Emission Gels Based on Pillar[5

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 16597−16606

pubs.acs.org/journal/ascecg

Supramolecular Aggregation-Induced Emission Gels Based on Pillar[5]arene for Ultrasensitive Detection and Separation of Multianalytes You-Ming Zhang,*,† Wei Zhu,† Xiao-Juan Huang,‡ Wen-Juan Qu,† Jun-Xia He,† Hu Fang,† Hong Yao,† Tai-Bao Wei,† and Qi Lin*,† †

ACS Sustainable Chem. Eng. 2018.6:16597-16606. Downloaded from pubs.acs.org by YORK UNIV on 12/26/18. For personal use only.

Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, PR China ‡ State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China S Supporting Information *

ABSTRACT: A novel supramolecular AIE (aggregationinduced emission) gel (PMDP-G) based on pillar[5]arene which could ultrasensitively detect and separate multianalytes was efficiently constructed. First, a novel supramolecular gelator, PM (host), and a functionalized pillar[5]arene, DP (guest), were successfully designed and synthesized. Then, host PM and guest DP could construct a stable supramolecular AIE gel, PMDP-G, via “exo-wall” π−π interactions and host−guest interactions in cyclohexanol solution. Interestingly, PMDP-G shows bright blue-white AIE and the fluorescence quantum yield is about 60%. Moreover, PMDP-G could carry out ultrasensitive detection and separation of multianalytes such as Fe3+, Hg2+, Ag+, F−, and Br− by the competition between “exo-wall” π−π interactions and cation−π interactions. The LODs (limits of lowest detection) of these AIE gels for Fe3+, Hg2+, Ag+, F−, and Br− are in the range of 7.3 × 10−10−9.5 × 10−9 M, indicating high sensitivity. More importantly, the xerogel of PMDP-G could adsorb and separate Fe3+, Hg2+, and Ag+ from aqueous solution with nice adsorption property, and the adsorption rates are in the range of 90.12−99.95%. Meanwhile, supramolecular AIE gel PMDP-G could be used as a smart fluorescent display material. KEYWORDS: Ultrasensitive, Multianalytes, Detection and separation, Supramolecular AIE gel, Pillar[5]arene

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logical, and environmental areas.18,19 Up to now, many methods have been reported about ultrasensitive detection and separation of important analytes.20,21 However, some methods could only detect specific ions but not separate specific ions,22−24 and some methods could only separate specific ions but not detect specific ions.25,26 As a consequence, preparation of novel smart sensors that could be used for both ultrasensitive detection and separation of specific ions is a fascinating subject. As an important part of supramolecular polymers, stimuliresponsive supramolecular polymer gels concluded various specific noncovalent interactions,27−29 such as cation···π, C− H···π, π−π stacking, electrostatic interactions, and host−guest interactions, among others.30−32 In 2001, Tang et al.33 proposed AIE (aggregation-induced emission) and steers

INTRODUCTION Cations and anions play fundamental role in chemistry, environmental science, and other fields;1,2 however, ions pollution is also a serious issue.3−6 For instance, some cations such as Hg2+ shows strong toxicity for the human body and causing serious illness such as kidney failure, nervous system damage, and endocrine and central nervous system damage of humans and animals; 7−9 therefore, the detection and separation of these ions is vital important.10,11 Meanwhile, anions also play significant role in ensuring the good human health, besides biological and environmental systems.12−14 For example, fluoride ions (F−) are relevant to health care and play a beneficial role in preventing dental health and osteoporosis.15−17 Hence, ultrasensitive detect and separate different ions in the environment has become more and more important. Currently, ultrasensitive detection and separation of multianalytes has gained increasing attention for their fundamental investigation and intelligent applications in chemical, bio© 2018 American Chemical Society

Received: August 3, 2018 Revised: October 15, 2018 Published: October 19, 2018 16597

DOI: 10.1021/acssuschemeng.8b03824 ACS Sustainable Chem. Eng. 2018, 6, 16597−16606

Research Article

ACS Sustainable Chemistry & Engineering clear of the ACQ (notorious aggregation-caused quenching) problem. In this concept, the new class of organic fluorophore molecules are weakly emissive or nonemissive in solution but display strong fluorescence upon aggregation. Moreover, restriction of intramolecular motions (RIM), including restriction of intramolecular vibrations (RIV) and restriction of intramolecular rotations (RIR) in the aggregation state has been identified as the main cause for the AIE effect. This is because such motion is restricted in the aggregate state, which activates radiative decay and blocks the nonradiative pathway, resulting in activation of its AIE feature and a significant fluorescence enhancement.34 Since the first AIE paper was published, AIE-based materials have been widely studied.35,36 Self-assembly could render fluorophores exhibiting AIE features suitable for the monitoring and visualization of such aggregation processes, so it is in essence a special mode of aggregation. Meanwhile, the host−guest interactions will inhibit the intramolecular rotation. Thus, AIE-based supramolecular gels provide a new platform to fabricate ultrasensitive responsive materials. Pillar[n]arenes are a new family of star macrocyclic host molecules,37 present unique structures and easily functionalized properties.38 Importantly, based on previous reports,39−42 the pillar[n]arenes show strong AIE effects, which could also serve as a new idea to prepare supramolecular AIE gels. As far as we know, the reports on supramolecular AIE gels based on pillar[5]arene with strong AIE are very rare.43,44 On the basis of the excellent properties of pillar[n]arenes and our previous studies,45,46 we fabricated a novel supramolecular polymer (PMDP) based on pillar[5]arene. PMDP simultaneously contain the phthalimide moiety and pillar[5]arene moiety as π−π interactions site, multiassembly site, and nonemissive fluorogens. Importantly, PMDP could form stable 2D supramolecular AIE gel PMDP-G through “exo-wall” π−π interactions and host−guest interactions in cyclohexanol solution. Upon formation of the stable 2D structure, molecular rotation of phthalimide groups and pillar[5]arene groups were restricted, which gave rise to this system showing strong fluorescence intensity. Interestingly, PMDP-G could ultrasensitively detect Fe3+, Hg2+, and Ag+ in a gel state. In addition, by rationally introducing Fe3+ and Hg2+ into PMDP-G, the obtained metallogels (PMDP-GFe and PMDP-GHg) could ultrasensitive fluorescently sense F− and Br − by the competition between “exo-wall” π−π interactions and cation−π interactions, respectively (Figure 1). More importantly, the xerogel of PMDP-G could effectively adsorb Fe3+, Hg2+, and Ag+ from aqueous solution and as stimuli-responsive smart fluorescent display materials.

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Figure 1. (a) Chemical structures and cartoon representations of PM and DP; (b) formation of supramolecular AIE gel PMDP-G; (c) ultrasensitive detection properties of PMDP-G for multianalytes. Gelation Test. First, the gelators (PM and DP) and solvents were put in a reagent bottle (1 mL) and then heated (>58 °C) until the solid was dissolved completely. After the solution was cooled to room temperature, the supramolecular polymer PMDP will converted into a stable gel, PMDP-G, in cyclohexanol solution with strong fluorescence emission, and the corresponding critical gel concentration is about 5% (w/v, 10 mg mL−1 = 1%). Importantly, gelation was considered successful if no sample flow was observed upon inversion of the container at room temperature (the inverse flow method).47 Additionally, xerogels were obtained by drying under reduced pressure.48 Detection Method of Metal Ions. Supramolecular AIE gel PMDP-G was prepared by dissolving host PM (0.02 g) and guest DP (0.02 g) in cyclohexanol solution (0.2 mL), and the temperature is not lower than 58 °C followed by cooling to room temperature. Afterward, in order to investigate the recognition properties of PMDP-G, various metal ions aqueous solution (Cu2+, Co2+, Ni2+, Cd2+, Pb2+, Zn2+, Cr3+, Mg2+, Ba2+, Ca2+, Tb3+, Ag+, Hg2+, Fe3+, and La3+, c = 0.1 M) were added into PMDP-G (w/v, 10%), respectively. As a result, after addition of 2.0 equiv of Fe3+, Hg2+, or Ag+ into PMDP-G, the fluorescence emission of PMDP-G was quenched. Meanwhile, any changes in the fluorescence spectra of PMDP-G after addition of different metal ions were recorded on a Shimadzu RF5301PC fluorescence spectrometer. Synthesis of Compound PM. In our previous works, we have synthesized compound H, P, D, and DP.49,50 These synthesis procedures are depicted in Schemes 1 and S1. A mixture of bromofunctionalized pillar[5]arene (PM) (0.95 g, 1.0 mmol) and phthalimide potassium (0.37 g, 2.0 mmol) in DMF solution (30 mL) was stirred at 90 °C for 24 h under nitrogen atmosphere. The solution was evaporated under vacuum and the residue was purified by flash column chromatography (petroleum ether/ethyl acetate = 4/ 1, v/v) to afford PM as a yellow solid (0.82 g, 80.27%), mp: 60−62 °C. 1H NMR (CDCl3, 600 MHz), δ/ppm: 7.88−7.86 (m, 1H), 7.84− 7.83 (m, 1H), 7.76−7.75 (m, 1H), 7.70−7.68 (m, 1H), 6.78−6.75 (m, 10H), 3.83−3.81 (t, J = 6.5 Hz, 2H), 3.77−3.74 (m, 10H), 3.65− 3.63 (m, 27H), 3.58−3.56 (t, J = 6.1 Hz, 2H), 1.77−1.74 (m, 2H), 1.66−1.64 (m, 2H), 1.48−1.44 (m, 2H), 1.25−1.23 (m, 10H). 13C NMR (CDCl3, 151 MHz), δ/ppm: 168.41, 150.61, 150.49, 150.01, 133.79, 132.18, 128.26, 128.21, 128.18, 128.10, 123.10, 113.78, 68.40,

EXPERIMENTAL SECTION

Materials and Characterizations. Water used throughout was triply distilled. All reagents were commercially available and used as supplied without further purification. 1H NMR (600 MHz) and 13C NMR spectra (151 MHz) were carried out with a Mercury-600 BB spectrometer. High-resolution mass spectra were recorded with a Bruker Esquire 3000 Plus spectrometer. The fluorescence spectra were recorded on a Shimadzu RF-5301PC fluorescence spectrophotometer. The infrared spectra were performed on a Digilab FTS-3000 FT-IR spectrophotometer. Melting points were measured on an X-4 digital melting-point apparatus. The X-ray diffraction (XRD) pattern was generated using a Rigaku RINT2000 diffract meter equipped (copper target; λ = 0.154073 nm). Scanning electron microscopy (SEM) images of the xerogels were investigated using JSM-6701F instrument. 16598


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ACS Sustainable Chemistry & Engineering Scheme 1. Synthetic Route to Compound PM

Figure 2. (a) Temperature-dependent fluorescence spectra of DP (Sol), PM (Sol), PMDP-G (Sol), and PMDP-G (Gel) in cyclohexanol solution. (b) Fluorescent “Gel-Sol” cycles of PMDP-G, controlled by cooling and heating (λex = 300 nm). 55.63, 38.05, 31.76, 31.49, 29.78, 29.47, 29.39, 29.12, 28.58, 26.90, 26.84, 26.26. HR-MS m/z: calcd for C62H75N2O12 [PM + NH4]+: 1039.53; found: 1039.37.

various solvents are summarized in Table S1. The critical gel concentration (the lowest amounts of PMDP needed to sustain gel formation) of PMDP-G is determined to 5% (w/v, 10 mg mL−1 = 1%). Contemporary, the phase-transition temperature (Tgel) of PMDP-G is approximately 56−58 °C. Moreover, it is interesting to find that PMDP-G showed reversible transition (gel−sol) in response to temperature and performed many times with small fluorescence efficiency loss (Figure 2b). Assembly Mechanism. To study the assembly mechanism of PM and DP, fluorescence experiments, 1H NMR, 2D diffusion-ordered NMR spectroscopy (2D-NOESY NMR), FT-IR, X-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses were carried out. First, host PM has negligible fluorescence intensity; however, guest DP has strong fluorescence intensity in cyclohexane solution (Figure 2a). Interestingly, when adding guest DP into the host PM, the fluorescence intensity of the mixture increased a little, which indicated that the complex of PM and DP in cyclohexane solution. Next, as the concentration of DP increased (Figure

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RESULTS AND DISCUSSION Assembly Performance. First, both host PM and guest DP could not self-assemble into supramolecular polymer gel in commonly used solvents. However, after guest DP was added to cyclohexanol solution of host PM (Figure 2a), the fluorescence intensity of mixture will be slightly enhanced, which indicated that there are host−guest collaborations between DP and PM. Additionally, as shown in Figure S16, PMDP-G has weak fluorescence emission in hot cyclohexanol (T > Tgel); however, with the temperature dropping below (T < Tgel), the fluorescence intensity will suddenly increase and reached a steady state within 1 min, which indicated that the fluorescence emission of PMDP-G was AIE. Meanwhile, we have measured the fluorescence quantum yield of PMDP-G is about 60% according to the corresponding formula (Figure S17).51 The gelation properties of PMDP-G were explored in 16599


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Figure 3. Partial 1H NMR (600 MHz, DMSO-d6, 298 K) spectra of (a) 1.17 × 10−2 M PM; (b) 1.17 × 10−2 M PM and 2.35 × 10−2 M DP; (c) 1.15 × 10−2 M DP.

3), the signals of protons Ha and Hb on PM shifted upfield. Meanwhile, the signals of protons Hc, Hd and He on PM shifted downfield. What’s more, the signals of protons H2 and H3 on DP shifted downfield. These results confirmed that both the alkyl chains of DP were partially included in the cavity of PM through π···π and C−H···π interactions.52 At the same time, 2D-NOESY NMR experiments were implemented to further study the self-aggregation of PMDP. As shown in Figure S18, cross-peaks P and Q representing the correlations between the signals of protons Hd and He on PM and H2 on DP, also suggesting that the alkyl chains of DP were partially included in the cavity of PM. Moreover, as shown in Figure S19, there are no correlations between the signals of protons Ha, Hb and Hc, Hd, He, which imply that the phthalimide group was not threaded into the cavity of pillar[5]arene. Then, as the concentration of the PMDP increased (Figure 4), the signals of protons Ha and Hb on PM shifted upfield. Meanwhile, the signals of protons Hc, Hd, and He on PM

shifted downfield. Additionally, the signals of protons Hf, Hg, and Hh on DP shifted upfield. These results indicated that the “exo-wall” π−π interactions53,54 between the pillar[5]arene groups and phthalimide groups of PM, and π−π stacking interactions between the pillar[5]arene groups of DP were involved during assembly. From the 1H NMR titration and concentration-dependent 1H NMR spectra, all the signal peaks became broad at high concentration, which confirmed the formation of high-molecular-weight aggregates driven by host− guest interactions between PM and DP.55 According to the FT-IR spectra of PMDP-G (Figure S20a), the absorption band of the carbonyl group clearly red-shifted and became a broad peak. These results also suggested that π−π stacking interactions were involved in PMDP-G. Moreover, the assembly process of PM and DP was also investigated by the small-angle XRD patterns. As shown in Figure S21a, the small-angle XRD patterns of PM suggested that there was no long-range order in gelator PM. However, the xerogel of PMDP-G exhibited well-resolved XRD patterns that were characteristic of the long-range ordering of the molecules. These results verified that PM and DP could assemble into a stable supramolecular polymer. For the xerogel of PMDP-G, the d-spacings of 3.78 and 3.66 Å at 2θ = 23.52 and 24.29° also confirmed the π−π stacking consisted of the pillar[5]arene groups and phthalimide groups. In SEM, host PM showed a bulk structure (Figure S22a), and PMDP-G showed a regular microsphere structure (Figure S22b), which also supported the idea that PM and DP could assemble into 2D supramolecular polymer networks by π−π stacking interactions and host−guest interactions. According to the above research, we speculated about the possible self-assembly mechanism of supramolecular AIE gel PMDP-G. First, host PM and guest DP could assemble into a novel supramolecular polymer PMDP through C−H···π interactions and host−guest interactions; then, the supramolecular polymer could form stable supramolecular AIE gel PMDP-G with 2D networks through π−π stacking interactions and “exo-wall” π−π interactions in cyclohexanol solution (Figure 1a,b). Ion-Response Experiments. The photophysical properties of PMDP-G in the presence of various cations were

Figure 4. Partial concentration-dependent 1H NMR spectra (600 MHz, DMSO-d6, 298 K) of PMDP: (a) 9.78 × 10−3 M PM; (b) 1.94 × 10−2 M PM and 9.72 × 10−3 M DP; (c) 3.87 × 10−2 M PM and 1.94 × 10−2 M DP; (d) 9.53 × 10−3 M DP. 16600


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ACS Sustainable Chemistry & Engineering carefully investigated. First, a serious of metal ions (Cu2+, Co2+, Ni2+, Cd2+, Pb2+, Zn2+, Cr3+, Mg2+, Ba2+, Ca2+, Tb3+, Ag+, Hg2+, Fe3+, and La3+, c = 0.1 M) were diffused into PMDP-G. Meaningfully, PMDP-G could selectively sense Fe3+, Hg2+, and Ag+. Moreover, the supramolecular AIE gel PMDP-G could be tuned by Fe3+, Hg2+, or Ag+ to form different no fluorescence metallogels (PMDP-GFe, PMDP-GHg, or PMDP-GAg). The lowest fluorescence detection concentrations of PMDP-G for Fe3+, Hg2+, and Ag+ were determined by fluorescent titrations (Figure 6a, 6b and Figure S23). On the basis of the 3δ/S method,56 the LODs of PMDP-G for Fe3+, Hg2+, and Ag+ were 1.78, 9.5, and 3.17 nM, respectively (Figures 7a,b and S24). One of the values mentioned is that the lowest detection limits of this supramolecular AIE gel, PMDP-G, for Fe3+, Hg2+, and Ag+ are much better than reported fluorescent sensors for these cations (Table 1), which indicated that PMDP-G was capable of ultrasensitive detection of Fe3+, Hg2+, and Ag+ in the environment.

process could induce the AIE of PMDP-G quenching, so Fe3+, Hg2+, and Ag+ could be identified by PMDP-G through cation−π interactions. After that, the anions response properties of PMDP-GFe, PMDP-GHg, and PMDP-GAg were investigated by adding a series of anions such as AcO−, HSO4−, H2PO4−, F−, Cl−, Br−, I−, ClO4−, SCN−, CN−, S2−, and N3− (c = 0.1 M) into these metallogels. As shown in Figure 5b,c, metallogels PMDP-GFe and PMDP-GHg could carry out selective fluorescence detection of F− and Br−, respectively. The lowest detection limits of the changes in fluorescence spectra were 1.51 nM for F− (by PMDP-GFe; Figures 6c and 7c) and 0.73 nM for Br− (by PMDP-GHg; Figures 6d and 7d), indicating that PMDPGFe and PMDP-GHg were able to carry out ultrasensitive detection of F− and Br−, respectively. Due to the strong coordination ability of Fe3+ with F−,77 after addition of F− into the PMDP-GFe, the F− could competitively bind with Fe3+. Therefore, PMDP-GFe shows good selectivity to F−. Additionally, the coordination ability of Hg2+ is relatively strong with Br−;78 after addition of Br− into PMDP-HgG, the Br− competitively binds with Hg2+. Thus, PMDP-HgG shows good selectivity to Br−. More importantly, the lowest detection limits of these metallogels for F− and Br− are also much better than some reported fluorescent sensors for these anions (Table 1). Ion-Controlled Fluorescent Switch. In view of the above, metallogel PMDP-GHg could effectively sense Br− with specific selectivity; however, after addition of Hg2+ into Br−containing PMDP-GHg, the fluorescence intensity decreased again. Therefore, PMDP-G could act as an Hg2+ and Br− controlled “on−off−on” fluorescence response switch (Figure 8), and the switching behavior could be repeated at least four successive cycles with little fluorescent efficiency loss. Ion Detection and Separation. In order to investigate the adsorption property of PMDP-G for Fe3+, Hg2+, and Ag+, a xerogel sample of PMDP-G (0.001 g) was placed in aqueous solution of Fe3+, Hg2+, or Ag+ (5 mL, c = 1 × 10−4 M). Importantly, the xerogel of PMDP-G cannot dissolve in aqueous solution. After the xerogel of PMDP-G and Fe3+, Hg2+, or Ag+ were stirred at room temperature for 30 min, the precipitates were separated by centrifugation (1000r/min, 10 min) and the supernatant retained. The adsorption rates of PMDP-G for Fe3+, Hg2+, and Ag+ in aqueous solution were assessed by ICP (inductively coupled plasma) analysis. As a result, the adsorption rates for Fe3+, Hg2+, and Ag+ were about 99.95, 99.55, and 90.12% respectively. In contrast, a comprehensive comparison of the xerogel PMDP-G with reported sensors on adsorption rates for Fe3+, Hg2+, or Ag+ was studied (Table S2). This table indicated that the xerogel of PMDP-G has nice adsorption property of Fe3+, Hg2+, and Ag+ in aqueous solution. Fluorescent Display Materials. Finally, supramolecular AIE gel PMDP-G film could act as an ion-responsive smart fluorescent display material (Figure S25). For example, we prepared the gel film by pouring heated PMDP-G solution onto a clean glass surface and then drying by evaporation of solvent from the gel under reduced pressure. Upon addition of Hg2+ aqueous solution, the fluorescence intensity of AIE-based gel film was quenched. However, after aqueous solution of Br− was added, the fluorescence intensity could be effectively restored. Thus, the convenient smart fluorescent display material based on supramolecular AIE gel PMDP-G could be fabricated successfully.

Table 1. Comparison of the LODs and Adsorption Rates of Different Fluorescence Sensors for Ions ions

Fe3+

Hg2+

Ag+

refs 57 58 59 60 this work 61 62 63 64 this work 65 66 67 68 this work 69 70

F−

Br−

71 72 this work 73 74 75 76 this work

solvent

LOD (nM)

aqueous solution deionized water CH3CN-H2O (1:1, v/v) drinking water

900 450 261 240

aqueous solution

1.78

water solution HEPES buffer CH3CN-H2O (1:1) MeCN-H2O (8:2)

40 150 1770 2160

aqueous solution

9.5

aqueous solution simulated wastewater AgNO3 aqueous solution

3670 760 25.1 100

aqueous solution

3.17

micellar solution DMSO/ACN/H2/ triethylamine (47:47:5:0.5) aqueous solution CH3CN

adsorption rate (%)

99.95

99.55

90.12

478 25000 100000 93

aqueous solution

1.51

aqueous solution NH3 solution water solution food samples

490 1000 1670 8510

aqueous solution

0.73

Supramolecular AIE gel PMDP-G was constructed by the self-assembly of gelators (PM and DP) in cyclohexanol solution. It is worth noting that the self-assemble is a dynamic process, which could be effected by guest stimuli. When adding aqueous solution of cations (Fe3+, Hg2+, or Ag+) into PMDPG, these cations could diffuse in it. Importantly, Fe3+, Hg2+, and Ag+ have high ionic strength, which makes it easy to induce the π-electrons on phthalimide groups of PM to transfer to these cations and act as cation−π interactions. This 16601


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Figure 5. Fluorescent spectral changes (λex = 300 nm) of (a) PMDP-G with addition of different cations aqueous solution; (b) PMDP-GFe with addition of different anions aqueous solution; (c) PMDP-GHg with addition of different anions aqueous solution; (d) multianalytes response properties of PMDP-G and metal ions coordinated metallogels (PMDP-GFe and PMDP-GHg).

Figure 6. Emission spectra of (a) PMDP-G with increasing amounts of Fe3+; (b) PMDP-G with increasing amounts of Hg2+; (c) PMDP-GFe with increasing amounts of F−; (d) PMDP-GHg with increasing amounts of Br−.

that the Br− could complex with Hg2+. Additionally, although the carbonyl group absorption band of PMDP-G appeared at 1717 cm−1, with increasing Hg2+ to PMDP-G, the absorption band shifted to 1712 cm−1, which also indicated that PMDP-G could combine with Hg2+ by cation−π interactions. However, with increasing Br− into the PMDP-GHg, the carbonyl group absorption band shifted to 1714 cm−1 (Figure S20b). These results could be attributed to the competition between “exowall” π−π interactions and cation−π interactions.

Ultrasensitive Stimuli-Response Mechanism. The stimuli-response mechanism of PMDP-G was also investigated by 1H NMR, FT-IR, X-ray diffraction (XRD), and scanning electron microscopy (SEM) analysis. Taking PMDP-GHg as an example, as shown in Figure 9, after a serious of Hg2+ was added into the PMDP solution (DMSO-d6), the signal of proton Hb shifted downfield, which indicated that the PMDP combined with Hg2+ via cation−π interactions between the phthalimide groups and Hg2+. With the increasing amount of Br−, the signal of proton Hb shifted upfield, which indicated 16602


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Figure 7. Photographs of the linear ranges (a) PMDP-G for Fe3+; (b) PMDP-G for Hg2+; (c) PMDP-GFe for F−; (d) PMDP-GHg for Br−.

Figure 8. Fluorescent cycles of PMDP-G by the alternative addition of Hg2+ and Br− (λex = 300 nm). Figure 9. Partial 1H NMR spectra of PMDP (DMSO-d6) with various equivalents of Hg2+: (a) 0 equiv, (b) 1.0 equiv, (c) 2.0 equiv; and Br−: (d) 1.0 equiv.

Moreover, the XRD pattern of xerogel PMDP-G present dspacings of 3.78 and 3.66 Å by small-angle diffraction peaks at 2θ = 23.52 and 24.29°, respectively (Figure S21b), indicating the existence of 2D networks through π−π stacking interactions of the supramolecular polymers. However, for PMDP-GHg, only d-spacing of 3.34 Å by small-angle diffraction peak at 2θ = 24.52° appeared. As expected, after addition of Br− to PMDP-GHg, the strong peaks at 2θ = 23.27 and 26.83° appeared again. This result verified that Br− could competitively bind with Hg2+, and then PMDP-G recovered. In SEM, the images demonstrated that PMDP-G could selfassemble into microsphere structures (Figure S22b). However, metallogel PMDP-GHg showed amorphous structure (Figure S22f). As expected, after adding Br− to PMDP-GHg, the macromorphology of PMDP-G recovered (Figure S22g), owing to the competitive binding of Br− with Hg2+. According to aforementioned, the ultrasensitive stimuli-response mechanism of PMDP-G for Fe3+, Hg2+, Ag+, F−, and Br− was illustrated in Figure 1b,c.

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CONCLUSIONS In conclusion, a novel supramolecular polymer PMDP was successfully constructed by directly connecting pillar[5]arenebased supramolecular gelator PM and bis-bromohexanefunctionalized pillar[5]arene DP. Interestingly, supramolecular polymer PMDP could self-assemble into a stable supramolecular AIE gel, PMDP-G, by “exo-wall” π−π interactions and host−guest interactions in cyclohexanol solution with strong AIE. Importantly, PMDP-G shows multiple ultrasensitive stimuli-responsive to Fe3+, Hg2+, and Ag+. Meanwhile, these cations could be effectively adsorbed and separated by PMDP-G from aqueous solution. After addition of metal ions (Fe3+, Hg2+, or Ag+) into PMDP-G, metallogel PMDP-GFe, PMDP-GHg, or PMDP-GAg formed. Moreover, PMDP-GFe 16603


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ACS Sustainable Chemistry & Engineering and PMDP-GHg could ultrasensitively detect F− and Br− by the competition between “exo-wall” π−π interactions and cation−π interactions, respectively. It is an efficient method for the development supramolecular AIE gels, which provides a broad platform for design and construction of advanced and smart supramolecular materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b03824. 1 H NMR, 13C NMR, HR-MS, FT-IR spectra, SEM images, and fluorescent spectroscopic data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (You-Ming Zhang). *E-mail: [email protected] (Qi Lin). ORCID

Qi Lin: 0000-0002-3786-3593 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (Nos. 21661028; 21662031; 21574104), the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (IRT 15R56) and CAS instrument function development technology innovation project.

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Supramolecular Aggregation-Induced Emission Gels Based on Pillar[5

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