In Situ Water-Compatible Polymer Entrapment: A Strategy forTransferring Superhydrophobic Microporous Organic Polymers toWaterDoo Hun Lee,†,∥ Kyoung Chul Ko,†,∥ Ju Hong Ko,† Shin Young Kang,† Sang Moon Lee,‡ Hae Jin Kim,‡

Yoon-Joo Ko,§ Jin Yong Lee,*,† and Seung Uk Son*,†

†Department of Chemistry, Sungkyunkwan University, Suwon 16419, Korea‡Korea Basic Science Institute, Daejeon 34133, Korea§Laboratory of Nuclear Magnetic Resonance, National Center for Inter-University Research Facilities (NCIRF), Seoul NationalUniversity, Seoul 08826, Korea

*S Supporting Information

ABSTRACT: Microporous organic polymer nanoparticles bearingtetraphenylethylene moieties (MOPTs) were prepared in the presenceof poly(vinylpyrrolidone) (PVP). The PVP was entrapped into themicroporous network of MOPT to form MOPT-P and played the rolesof size control, porosity enhancement, and surface property manage-ment. MOPT materials without PVP showed superhydrophobicity with awater contact angle of 151°. In comparison, the MOPT-P showedexcellent water compatibility. Moreover, due to the aggregation-inducedemission property of tetraphenylethylene moieties, the MOPT-P showedemission and excellent emission-based sensing of nitrophenols in water with Ksv values in the range of 1.26 × 104 ∼ 3.37 × 104

M−1. It is noteworthy that the MOPT-P used water only as a sensing medium and did not require additional organic solvents toenhance water dispersibility of materials. The MOPT-P could be recovered and reused for the sensing at least five times.

Recently, organic polymers with microporosity (pore sizes<2 nm) have been prepared through coupling reactions of

organic building blocks.1−4 For example, the Cooper researchgroup showed that the Sonogashira coupling is versatile for thesynthesis of microporous organic polymers (MOPs).5,6 Due totheir high surface areas and chemical stabilities, the MOPs havebeen used in various applications including adsorbents,catalysts, and sensing materials.1−4,7−9 Another commonfeature of MOPs prepared by the Sonogashira coupling istheir superhydrophobicity (Figure 1), enabling the fabricationof floating oleophilic adsorbent materials on water.10−12

However, the superhydrophobicity can be an obstacle inother applications of MOPs in aqueous media such as in thesensing of water pollutants.

Emissive MOPs have been applied for the sensing of waterpollutants.13−23 However, due to the unique organic nature ofMOPs, additional water-compatible organic solvents such astetrahydrofuran, acetonitrile, and ethanol have been added tooriginal aqueous systems. Thus, to avoid using the potentiallyharmful organic solvents, more synthetic efforts for water-compatible MOPs are required. The surfactants such ascetyltrimethylammonium bromide or sodium dodecyl sulfatewere introduced to enhance the dispersibility of MOP materialsin water.24−27 Recently, we applied MOPs as seed materials forthe synthesis of polyketones and discovered that linearpolymers can be entrapped into the microporous network ofMOPs.28,29 Thus, we thought that water-soluble polymers suchas poly(vinylpyrrolidone) (PVP) can be introduced into theMOPs to realize water-compatible MOPs.Aggregation-induced emission (AIE) has attracted consid-

erable attention of scientists as a useful phenomenon.30,31 For atypical example, tetraphenylethylene moieties have shown theAIE properties.30,31 Based on these AIE properties, variousfunctional organic materials including sensing materials havebeen developed.30−34 Recently, the emission properties of theMOPs including tetraphenylethylene-based ones have beenstudied.35−38 As shown in the control experiments in Figure 1,

Received: April 9, 2018Accepted: May 16, 2018Published: May 18, 2018

Figure 1. Superhydrophobicity of MOP materials (MOPT, 0.2 mg/mL in water) prepared by the Sonogashira coupling of tetrakis(4-ethynylphenyl)ethylene and 1,4-diiodobenzene.

Letter

pubs.acs.org/macrolettersCite This: ACS Macro Lett. 2018, 7, 651−655

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the emissive MOPs bearing tetraphenylethylene moieties weresuperhydrophobic due to their organic nature and couldtherefore not be applied for the sensing of water pollutants inwater. In this work, we report the synthesis of water-compatibleMOPs bearing both emissive tetraphenylethylene moieties andPVP (MOPT-P) and their sensing performance for nitro-phenols in water.Figure 2 shows synthetic schemes for MOPT and MOPT-P.

The MOPT control materials were prepared by the

Sonogashira coupling of tetrakis(4-ethynylphenyl)ethylene39

with 1,4-diiodobenzene. At the same synthetic conditions, theMOPT-Ps were prepared in the presence of PVP. Among thecommercial PVPs with various molecular weights, the PVP withan average molecular weight of 5.5 × 104 showed the bestresults in the synthesis of MOPT-P. The PVP with an averagemolecular weight of 1 × 104 showed an inefficient control ofparticle sizes to form irregular granules (Figure S1 in the SI). Incomparison, the PVP with an average molecular weight of 3.6 ×105 hindered significantly the growth of MOPT to result in gel-like materials (Figure S1 in the SI). As the molecular weight oramount of PVP increases, the hindering effect on the growth ofMOPT can be enhanced, resulting in smaller-sized materials.Similar trends were reported in the kinetic growth ofpolystyrene nanospheres with PVP.40 Both the MOPT andMOPT-P were obtained as yellow powder.The size and morphology of MOPT were investigated by

scanning electron microscopy (SEM). As shown in Figures 3aand 3d, the MOPT prepared without PVP showed irregular andrelatively large sizes in the wide range of 300−1000 nm. Incomparison, the MOPT-P showed nanoparticulate morpholo-gies with an average diameter of 136 ± 18 nm (Figures 3b−d).The reduced and controlled sizes of MOPT-P resulted from theefficient hindering of network growth by PVP. While theMOPT showed superhydrophobicity with a water contact angle

of 151° (Figure 1), the MOPT-P showed water compatibility,due to the nanoscale sizes and the existence of PVP on thesurface (vide inf ra). (Figure 3e).The porosity of MOPT and MOPT-P was characterized

through the analysis of N2 isotherm curves (Figure 4a).Interestingly, the MOPT showed a low surface area of 242 m2/g and a large hysteresis,41 possibly due to the facile π−πpacking of chemical moieties and a nonrigid pore structure41 inthe materials. Nonporous compact π−π packing of polymerchains with incomplete chemical networking can be one of thefactors lowering surface areas of MOP materials.42 Incomparison, the MOPT-P showed enhanced microporositywith a surface area of 578 m2/g. We think that the concomitantinsertion of PVP into networks might hinder the π−π attractionbetween networks during the growth of MOPT, resulting in theenhanced porosity. According to the analysis of pore sizedistribution, PVP can be inserted into mesopore parts ofMOPT, resulting in the enhanced microporosity of MOPT-P(Figure 4a). Powder X-ray diffraction studies (PXRD) revealedthat the MOPT and MOPT-P are amorphous, matching withconventional properties of MOPs in the literature5,6 (Figure S2in the SI). The roles of PVP in the formation of MOPT-P canbe summarized as follows: (1) size control to nanoscale, (2)surface property management to water compatibility, and (3)porosity enhancement through hindering the π−π packing ofnetworks.Infrared (IR) absorption spectroscopy indicated that some

portion of PVP remained in the MOPT-P with the strong CO vibration peak of PVP at 1671 cm−1 (Figure 4b). The PVPcould not be removed even after washing for an additional daywith refluxing ethanol, indicating the entrapment of PVP in thenetworks of MOPT. X-ray photoelectron spectroscopy (XPS)revealed N 1s orbital peaks of free PVP and PVP in MOPT-P at399.80 and 399.71 eV, respectively, indicating that the PVP inMOPT-P is chemically intact and entrapped physically (FigureS3 in the SI). Elemental analysis showed an N content of 0.86wt %, corresponding to 6.8 wt % of PVP in MOPT-P. Solid-state 13C nuclear magnetic resonance spectroscopy (NMR) ofMOPT and MOPT-P showed 13C peaks of alkyne and aromaticpeaks at 90 and 122−141 ppm, respectively. The 13C peaks ofPVP in MOPT-P appeared at 18−52 and 173 ppm (Figure 4c).

Figure 2. Synthetic schemes for MOPT and MOPT-P.

Figure 3. SEM images of MOPT (a) and MOPT-P (b−c). (d) Sizedistribution diagram of MOPT and MOPT-P. (e) Water contact angletests of MOPT-P and a photograph of MOPT-P in water.

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Figures 4d,e show the optical properties of MOPT andMOPT-P. While the MOPT showed maximum absorption at431 nm, the MOPT-P showed the maximum absorption at 396nm. The red-shifted absorption of MOPT, compared with thatof MOPT-P, resulted from the more extended conjugationlength of MOPT than that of MOPT-P. As shown in Figure 4d,while the MOPT-P was well dispersed in water and showedmaximum emission at 545 nm (excitation at 410 nm), theMOPT could not be dispersed even under sonication. Thus,the emission of MOPT could not be investigated in the water.Using [Ru(bpy)3](PF6)2 as a comparative compound,43 thequantum yield of MOPT-P was measured as 7.3%.Considering the excellent water compatibility and emission

properties of MOPT-P (Figure S4 in the SI), we studied theirsensing performance toward nitrophenols in water. It isnoteworthy that nitrophenols are one of the significantpollutants generated from industry.44 Figure 5 summarizesthe sensing results. First, the MOPT-P showed efficient andselective emission quenching behaviors through the photo-induced electron transfer to nitrophenols in water45,46 (Figure5a−d). Phenol, 4-chlorophenol, and 4-methylphenol could notquench the emission of MOPT-P (Figure S5 in the SI) because

electron transfer from excited MOPT-P to these substratesseems to be unfavorable due to the mismatch of LUMO energylevels between MOPT-P (the simulated LUMO energy levels at−2.58 ∼ −2.97 eV) and substrates (the simulated LUMOenergy levels at −0.47 ∼ −0.92 eV). (Refer to the simulationresults in Figures S6−8 in the SI.)The nitrophenol substrates studied in this work, 2,4,6-

trinitrophenol (TNP), 2,4-dinitrophenol (DNP), 4-nitrophenol(4NP), and 2-nitrophenol (2NP), showed their uniquequenching behaviors. The reproducibility of these behaviorswas confirmed by the repeated experiments.While TNP and DNP showed nearly complete emission

quenching of MOPT-P, the 2NP showed the most sensitivequenching behavior (Figure 5e and Table S1 in the SI). Theobserved quenching behaviors of these nitrophenols mightresult from combination of the size and electronic effects ofsubstrates. In the aspect of size effect, the 2NP may bepreferential in interaction with emissive sites in the MOPT-P,compared to DNP and TNP (Figure S9 in the SI). The DNPshowed slightly better sensitivity than the 4NP, possibly due tothe electronic effect. The simulated LUMO energy levels ofDNP, 4NP, and MOPT-P were located at −3.83, −2.77, and−2.58 ∼ −2.97 eV, respectively (Figure S7 in the SI).Interestingly, while the TNP usually showed the most sensitive

Figure 4. (a) N2 adsorption−desorption isotherm curves and pore sizedistribution diagrams (based on the DFT method) of MOPT andMOPT-P. (b) IR spectra and (c) solid-state 13C NMR spectra ofMOPT, PVP, and MOPT-P. (d) Photographs and (e) absorption(solid line) and emission (dotted line) spectra of MOPT and MOPT-P (0.050 mg/mL in water, excitation wavelength: 410 nm).

Figure 5. (a−e) Concentration-dependent emission quenchingbehaviors of MOPT-P in the presence of TNP, DNT, 4NP, and2NP in water (average results of three test sets in (e), excitationwavelength: 410 nm). (f) Recycling tests of MOPT-P toward TNPsensing (0.50 mM) in water.

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quenching behaviors in the emissive MOPs in the liter-ature13−23 due to the electronic factor, the 2NP was the mostsensitive in our study for the MOPT-P, indicating that theLUMO level of 2NP at −3.83 eV is sufficiently lower than thatof MOPT-P at −2.28 ∼ −2.97 eV (Figure S7 in the SI) and thesize effect of 2NP, DNP, and TNP is therefore more critical inthe sensing of MOPT-P (Figure S9 in the SI).The MOPT-P showed thermal stability up to 270 °C (Figure

S10 in the SI). Considering high stability, we studied therecyclability of MOPT-P for the TNP sensing in water. Asshown in Figure 5f, the MOPT-P maintained completely thesensing performance in the five successive TNP sensingprocesses in water. The MOPT-P materials recovered afterfive successive sensing processes showed maintenance of theemission property and the original morphologies (Figure S11 inthe SI).The Stern−Volmer constants (Ksv) of MOPT-P for nitro-

phenols were obtained through the plotting of emissionintensity changes and substrate concentrations (Table S1 andFigure S12 in the SI). The 2NP showed the best Ksv value of3.37 × 104 M−1 with a detection limit of 0.31 ppm. Accordingto the literature survey,13−23 the emissive sensing MOPmaterials were studied in tetrahydrofuran, acetonitrile, meth-anol, ethanol, or their mixtures with water, showing Ksv valuesfor nitrophenols in the range of 1.17 × 103 ∼ 2.40 × 104 M−1

(Table S2 in the SI). The sensing performance of MOPT-P inthis work is a rare example17 of heterogeneous polymermaterials in water only with promising Ksv values in the range of1.26 × 104 ∼ 3.37 × 104 M−1. We think that the good sensingperformance of MOPT-P resulted from its nanoscale size andexcellent water compatibility.In conclusion, the synthetic strategy for water-compatible

MOP nanomaterials was developed through the entrapment ofPVP into the microporous network. Through the incorporationof tetraphenylethylene moieties, water-compatible emissiveMOP materials were obtained. The resultant MOPT-P couldbe applied for the sensing of nitrophenols in water, showingexcellent sensitivities with Ksv values in the range of 1.26 × 104

∼ 3.37 × 104 M−1. We believe that the synthetic strategy in thiswork can be extended for applications of functional MOPmaterials to other aqueous systems.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsmacro-lett.8b00263.

Experimental procedure, SEM images of MOPTmaterials prepared by various PVP reagents, PXRDpatterns, XPS spectra, and TGA curves of MOPT andMOPT-P, emission quenching of MOPT-P by otherphenols, characterization of recovered MOPT-P, theStern−Volmer plot, and computational simulation ofsensing process (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: sson@skku.edu.*E-mail: jinylee@skku.edu.ORCIDKyoung Chul Ko: 0000-0003-4386-9112Jin Yong Lee: 0000-0003-0360-5059

Seung Uk Son: 0000-0002-4779-9302Author Contributions∥D.H.L. and K.C.K. contributed equallyNotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the Basic Science ResearchProgram (2016R1E1A1A01941074) through the NationalResearch Foundation of Korea (NRF) funded by the Ministryof Science, ICT, and Future Planning.

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