Pyrenyl-Functionalized Polysiloxane Based on Synergistic Effect for

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Pyrenyl-Functionalized Polysiloxane Based on Synergistic Effect for Highly Selective and Sensitive Detection of 4-Nitrotoluene Zhiming Gou, Xiaomei Zhang, Yujing Zuo, Minggang Tian, Baoli Dong, and Weiying Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08254 • Publication Date (Web): 29 Jul 2019 Downloaded from pubs.acs.org on August 1, 2019

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ACS Applied Materials & Interfaces

Pyrenyl-Functionalized Polysiloxane Based on Synergistic Effect for Highly Selective and Sensitive Detection of 4-Nitrotoluene Zhiming Gou, Xiaomei Zhang, Yujing Zuo, Minggang Tian, Baoli Dong, and Weiying Lin* Institute of Fluorescent Probes for Biological Imaging, School of Chemistry and Chemical Engineering, School of Materials Science and Engineering, University of Jinan, Shandong 250022, P. R. China.

*Corresponding author Email: [email protected] Fax: (+) 86-531-82769031

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Abstract 4-Nitrotoluene (NT) is an important intermediate for the manufacture of dyes, agricultural chemicals, medicaments, and synthetic fibers. But its wide use has resulted in a series of ecological problems and health issues due to high toxicity, mutagenicity, and carcinogenicity. However, no fluorescent probes with high selective and sensitive to NT has been reported yet, and the current probes usually prefer to detect multinitrosubstituted NCAs including TNT and TNP. Herein, we report a series of pyrene functionalized polysiloxanes with high selectivity and sensitivity to NT. Pyrene was introduced into polysiloxane through carbon-carbon double bond, and the formed rigid side chain fluorophore and flexible backbone structure of polysiloxane provide an ideal platform for highly selective detection of NT. We further explored the possible response mechanism and speculated that the high selectivity and sensitivity were derived from the synergistic effect between steric hindrance and dipolar interaction. In addition, paper sensors based on the obtained fluorescent materials were fabricated, their high sensitivity and visible fluorescence change indicated that the paper sensor is a simple testing tool for portable and visual detection of NT. Furthermore, the design strategy in this work provides a novel synthetic route to synthesize other fluorescent probes with unique selectivity to NACs detection. Keywords: polysiloxane, NT detection, high selectivity and sensitivity, NACs detection, fluorescence quenching


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1. Introduction As one of the most important chemical intermediates of nitroaromatic compounds (NACs), 4-nitrotoluene (NT) is widely applied to the manufacture of dyes, agricultural chemicals, medicaments, explosives, and synthetic fibers. However, NT has been classified as priority pollutant due to wide distribution, high toxicity, mutagenicity, and carcinogenicity.1 Thus the detection of NT has become an important and urgent issue. A variety of analytical technique have been adopted to detect NACs,2-3 such as highperformance liquid chromatography (HPLC), gas chromatography coupled with mass spectrometer (GC-MS), thermal neutron analysis (TNA), surface-enhanced Raman spectroscopy (SERS), cyclic voltammetry (CV), energy-dispersive X-ray diffraction (EDXRD), etc. These commercially techniques provide high selectivity, but some problems still exhibited, including high cost, high-power devices. Therefore, fluorescence probes for the detection of NACs have attracted considerable attention owing to its simplicity, short response time, high sensitivity and selectivity. Currently reported fluorescence probes were highly selective to 2,4,6-trinitrotoluene (TNT) and 2,4,6-trinitrophenol (TNP).4-6 Zhang et al. synthesized a series of NBNdoped conjugated polycyclic aromatic hydrocarbons and exhibit excellent sensitivity to TNT.7 Liu et al. fabricated a fluorescent paper sensor with high selectivity for detecting TNT and TNP.8 Lee and his co-worker develop a polymeric thin-film probe that exhibits reversible detection and separation of TNP with high sensitivity in aqueous solution.9 However, there is no fluorescence probe with high selectivity and sensitivity



to NT. Therefore, new design strategy needs to be explored to synthesize novel fluorescent probe for the high selective and sensitive detection of NT. To fill in the blank, we synthesized a series of pyrenyl-functionalized polysiloxanes (PyP) via Heck detection. As a typical polycyclic aromatic hydrocarbon, pyrene can endow polysiloxane with excellent fluorescence properties. The Si-O-Si bond of polysiloxane is flexible enough to bend and intertwine, and the functionalized polysiloxane is very easily to form a complex steric structure with rigid fluorophore and small gaps. The unique steric effect of polysiloxane and rigid side chain fluorophore may provide a special platform to selective detection of NT. As expected, PyP only exhibit high selectivity and sensitivity to NT, rather than multi nitrotoluene (2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (DNT)). Moreover, although 4nitrophenol (NP) contains similar one nitro substituted benzene ring structure compared to NT, there still exist significant difference between their selectivity. The high selectivity and sensitivity have proved the new perspective is rational. Furthermore, the cause of formation for the high selectivity was explored via theoretical simulation using Material Studio program, and the possible quenching mechanism was speculated. We further fabricated a series of paper sensors based on obtained fluorescent materials, which still exhibited high sensitivity and visible fluorescence change toward NT at micromolar level within 10 seconds. The paper sensor demonstrates a portable, visualized, and highly sensitive analytical detection method toward NT detection. The described design strategy could be extended to synthesize other fluorescent probes with high selectivity to NT detection, and possible quenching mechanism will provide a new


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perspective to novel fluorescent probes with unique selectivity to NACs detection. 2. Experimental section Materials All solvents were bought from chemical company and directly used without further purification and all reactants were supplied by Chemical technology (shanghai) Co., Ltd. Characterization and measurements NMR (1H and

13C)

spectra were recorded by Bruker AVANCE 400 MHz using

CDCl3 as solvent, and without tetramethylsilane as internal standard. The data of molecular weights was measured by gel permeation chromatography (GPC) using a Waters 515 liquid chromatograph with refractive index detector 2414 and using THF as the elution solvent. Fluorescent emission spectra were measured with Hitachi F 4600 spectrometer. Synthesis of polymethylvinylsiloxane. According to the classical synthetic procedure,10 vinyl-functional polysiloxane with 25% content of vinyl was synthesized through ring-opening polymerization between octamethylcyclotetrasiloxane (D4) and 2,4,6,8-tetramethyltetravinylcyclotetra-siloxane (D4vi). Polymethylvinylsiloxanes with two different theoretical molecular weight (500 g/mol (P500), and 1000 g/mol (P1000)) were synthesized and their yields were more



than 90%. Detailed data of molecular weights as shown in Table S1. Synthesis of pyrenyl-functionalized siloxane and polysiloxane. The synthetic route and structure of pyrenyl-functionalized siloxane and polysiloxane were shown in Scheme 1. The detailed procedure for PyP1 was as follows: The mixture of vinyl-functional polysiloxane P500 (1.0 g) and 1-bromopyrene (0.07 g, 0.25 mmol) were dissolved in toluene (30 mL) in 100 mL round-bottom flask. Then tris(2-methylphenyl)phosphine (30 mg, 9.8×10-4 mol), palladium acetate (15 mg, 6.7×10-5 mol) and triethylamine (5 mL) were added and the mixture was refluxed under N2 atmosphere for 48 h. The reaction mixture was cool down to room temperature and the crude product obtained after filtration and reduced pressure distillation. The obtained solid was dissolved completely in THF (3 mL) and a precipitate was formed after dump into methanol (40 mL), the precipitate was washed three time with methanol and then was dried at 50 oC in vacuum over for 24 h. The obtained light-yellow solid was PyP1. Yield: 97%. A series of pyrenyl-functionalized polysiloxanes (PyP2, PyP3, PyP4, PyP5, and PyP6) were synthesized following the above-mentioned procedure with different reactant ratio, and more detailed data as shown in Table S2. The synthesis of pyrenyl-functionalized siloxane (PyM and PyD) following the same procedure. Divinyltetramethyldisiloxane (MMvi) and D4vi were used to synthesized PyM and PyD, with siloxane to 1-bromopyrene molar ratios of 1:2 and 1:4, respectively. Yield: PyM (91%), and PyD (95%). Preparation of paper sensor.


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A series of paper sensors based on pyrenyl-functionalized siloxane and polysiloxane were synthesized according to former similar articles.11 The schematic diagram of the fabrication of the paper sensor were shown in Figure S1. The detailed procedure was as follow: Pyrenyl-functionalized siloxane or polysiloxane (20 mg) was dissolved in ethanol (50 mL). The solution of polymer was poured into Buchner funnel with commercial filter paper (7 cm) and filtrated under vacuum. The polymer easily absorbed on the surface and in pores of the filter paper. Then the resulting paper was dried at room temperature and cut into small test strips (5 cm × 1 cm) for further NT detection. Fluorescence property and NT detection Fluorescent emission spectra of pyrenyl-functionalized siloxane and polysiloxane in solution (0.2 mg/mL, dichloromethane) were excited at 330 nm. Nitroaromatic compounds (NACs) were detected by adding selected analytes to siloxane and polysiloxane solution (0.2 mg/mL, dichloromethane). 3. Results and discussion Synthesis and characterization of pyrene-functional siloxane and polysiloxane To construct a novel fluorescent probe with high selectivity and sensitivity to NT, pyrene was introduced into the siloxane and polysiloxane backbone through carboncarbon double bond as bridge structure. As typical polycyclic aromatic hydrocarbon, pyrene can form conjugate system with vinyl and endow the materials with excellent



fluorescence properties. The Si-O-Si bond of siloxane and polysiloxane is flexible enough to bend and intertwine, and finally form a complex steric structure with sidesubstituted fluorophore and small gaps. The small gaps serve as a doorkeeper to make sure that only NT could come in and out, which endow their high selectivity. The rigid bridge structure and intramolecular free rotation of carbon-carbon double bond can form a rigid side chain, which can act as dispatcher and provide shortcuts to enhance the interaction of NT to one or more fluorophore and finally ensure high sensitivity and responsiveness. We expected that the synergistic effect of the steric structure of fluorescent materials and electron-withdrawing property of NT will endow them with high selectivity and sensitivity to NT. PyP were synthesized by the Heck reaction of vinyl polysiloxane with 1bromopyrene (Scheme 1). For comparison, two types of pyrene-functionalized siloxanes were synthesized according to the same reaction condition and treating process. In view of the similar structure and same reaction condition, PyP1 was selected as the sample and its structure was confirmed by 1H NMR and 13C NMR (Figure S2). The peaks appeared between 8.0 and 8.5 ppm are attributed to pyrenyl hydrogen atom and the signals of vinyl still appeared at 5.8 to 6.0 ppm (Figure S2 (a)). The signals from pyrenyl carbon atom and double bonds linked to pyrene appeared from 120 to 133 ppm, and the peak at 137 ppm was assigned to unreacted double bonds from vinyl group (Figure S2 (b)). The NMR date indicated that pyrene was success modified on polysiloxane.


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Scheme 1. (a) Synthetic route of pyrenyl-functionalized siloxane and polysiloxane, and (b) the application of PyP3 in selective detection of NT. Fluorescence properties The fluorescent spectra of PyP in solution were shown in Figure 1. PyP1, PyP2 and PyP3 were synthesized from polysiloxane with Mn = 750 g.mol-1, and their intensity of emission peaks suggest that PyP3 with highest pyrene content exhibit highest luminescent intensity. While PyP4, PyP5 and PyP6 based on polysiloxane with Mn = 1300 g.mol-1, and their intensity indicate that low pyrene content resulted in better fluorescence property. Form the above results, we can find that the fluorescent intensity



of PyP is closely related with molecular weight of polysiloxane and pyrene content. To get excellent fluorescence, low molecular weight polysiloxane should be modified with high pyrene content, and high molecular weight polysiloxane should be modified with low pyrene content. To get more information about the fluorescence properties of PyP, the relation between fluorescent intensity and concentration was measured (Figure S3 and Figure S4). With increasing concentrations of PyP, the changes of their intensity showed a strong similar parabolic trend. The concentration of PyP3 at maximum fluorescence intensity is lower than that of PyP1 and PyP2. The concentration of PyP4 at maximum fluorescence intensity is lower than that of PyP5 and PyP6. The results clearly confirm the correlation between fluorescent intensity and the combination of molecular weight of polysiloxane and pyrene content. Therefore, the best combination to get better PyP is as follow: low molecular weight polysiloxane with high pyrene content, or high molecular weight polysiloxane with pyrene content.

Figure 1. (a) Fluorescence emission spectra and (b) intensity at 400 nm of PyP at 0.2 mg/mL in dichloromethane.


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Pyrene connected to various polymer moieties or short-chain molecules through different chemical bonds have been reported in former articles. To gain additional insight into the difference between reported pyrene based fluorescent probes and the obtained polysiloxane, their structure was simplified and optimized and then frontier molecular orbitals were calculated using Gaussian 03 program at the B3LYP/6-31 G(D) level. Frontier molecular energy of pyrene based fluorescent materials were depicted in Figure 2. The energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the first three pyrene based fluorescent materials are obvious higher than that of PyP, and their electron densities are mainly distribute around within the structure of pyrene ring. Whereas the electron densities (HOMO and LUMO) of PyP extends from pyrene ring to entire the whole of side chains, which existing smaller band gap (ΔE) than that of the former three fluorescent probes. The change could be attribute to the unique empty 3d orbital of silicon atom. A part of the electron cloud of the conjugated structure between pyrene and carbon-carbon double bond enters the 3d orbital of silicon atom. Silicon atoms can provide electron holes and contribute to electron transfer in an excited state, which is resulting in the excellent fluorescence property. The smaller band gap means a low exciton binding energy and high internal conversion rate, expecting that it will exhibits an easier response process and highly sensitive fluorescence response behaviors.



Figure 2. Frontier molecular energy (HOMO (blue line) and LUMO (pink line)) and simplified structure of pyrene and pyrene based fluorescent materials. Detection of Nitroaromatic Compounds (NACs). Nitroaromatic compounds (NACs) are important intermediates for the preparation of chemical products, but is harmful to personal safety and ecological environment due to their high toxicity, mutagenicity, and carcinogenicity. Therefore, the detection of NACs has attracted more attention. 4-Nitrotoluene (NT), 2,4-dinitrotoluene (DNT), and 2,4,6trinitrotoluene (TNT) were selected as samples for NACs detection. Fluorescence intension of PyP (PyP3 and PyP4) changed obviously within 10 s after adding various NACs, and the changes were observed and recorded (Figure 3). PyP3 and PyP4 both exhibit strong fluorescence and show high sensitivity to NT (Figure 3 (a) to Figure 3 (b)). To get a deeper comprehension about the high selectivity, we synthesized two


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kinds of pyrene-functionalized siloxanes as control experiments (Scheme 1). PyM exhibits weak fluorescence, and shows unobvious intensity change after adding NACs. The result indicates low responsiveness and poor selectivity to NACs (Figure 3 (c)). Notable, PyD show strong fluorescence and high sensitivity to NT (Figure 3 (d)). To visualize the selectivity and responsiveness, quenching percentage (QP) was calculated through the formula: QP = (I0-I)/I0×100%, where I0 and I are the fluorescence intensity of fluorescent materials before and after adding NACs, respectively. We can observe that the introduction of siloxane structure can significantly influence the responsiveness and selectivity of pyrene based fluorescent materials to NT, and the sequence of their selectivity to NT as follow: PyP4 ≈ PyP3 > PyD >> PyM (Figure 3(e)). With increase of the chain length of siloxane, PyP4, PyP3, and PyD all exhibit higher responsiveness and selectivity to NT, and their QP exceed 99.6 %. PyM contains short linear Si-O-Si bond structure and shows unobvious responsiveness and low selectivity to NT. The introduction of cyclotetra-siloxane structure can enhance the responsiveness and selectivity of PyD to NT, and the selectivity exhibit an inverse relationship with the number of nitro group. The high selectivity indicated that the introduction of cyclic chain structure is helpful to increase its responsiveness to NT and simultaneously decrease its responsiveness to DNT and TNT, thereby enhancing selectivity to NT. We can further observe the influence in PyP with two different molecular weight. The backbone structure of polysiloxane endow PyP3 and PyP4 with higher selectivity to NT than PyD, and simultaneously reduced their responsiveness with DNT and TNT. In addition, the length of polysiloxane chain resulted in slightly difference between the



selectivity of DNT and TNT. The above results indicate that circular chain structure and long linear chain could provide a strong hand for selective response of NT, and short linear chain does not provide any help for the detection.

Figure 3. Fluorescence emission spectra of (a) PyP3, (b) PyP4, (c) PyM, and (d) PyD at 0.2 mg/mL in dichloromethane with various NACs (NT, DNT, and TNT), and


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(e) their quenching percentage. To gain more details about NT detection, we record the luminescence quenching of pyrene functionalized siloxane and polysiloxane (PyD, PyP3, and PyP4) by successively adding NT solutions. As shown in Figure 4, the fluorescence intensity of three fluorescent materials significantly decreased after adding NT solutions and was completely quenched at 5.84×10-5 mol/L. To further study their response capability, the fluorescence quench percentage (QP) was calculated. Three pyrene-based materials all exhibit low half-quenching concentration (QC1/2) for NT detecting at micromolar level without exception (QC1/2 (PyD) ≈ 7.3×10-7 mol/L, QC1/2 (PyP3) ≈ 5.8×10-6 mol/L, and QC1/2 (PyP4) ≈ 8.8×10-6 mol/L). Since 5% quenching of the fluorescence is sufficient to detect an analyte,12 the low QC1/2 indicate that the minimum detectable mass concentration could achieve to nanomolar level. Their sequence of QC1/2 was as follow: QC1/2 (PyD) < QC1/2 (PyP3) < QC1/2 (PyP4). The sequence of QC1/2 suggest that long linear chain backbone structure lead to slightly higher QC1/2 than that of circular chain backbone structure, and simultaneously reducing sensitivity to NT at a relatively low concentration. Meanwhile, longer linear structure was conductive to increase selectivity to NT and reduced their responsiveness to DNT and TNT. The relationship between sensitivity and selectivity indicate that longer backbone structure could enhance selectivity and slightly decrease the minimum detectable concentration. The luminescence quenching efficiency was also evaluated by curve linear fitting. The nonlinear and upward curve suggested a combine quenching process between static and dynamic quenching mechanism. The quenching behavior can be attributed to the



collisional encounter’s mechanism and the donor-acceptor electron transfer mechanism between the excited fluorophores and NT. In addition, paper sensor represents a portable, visualized, and sensitive analytical method.13-14 To further enhance convenience and visuality for NT detection, we constructed a series of paper sensors based on PyP and their preparation route according to our former article (Figure S1).11 As expected, obvious fluorescence quenching phenomenon was observed under UV light (365 mm) after dropping micromolar NT samples with 10 seconds, and with increasing concentration of sample fluorescence intensity of paper sensor further decreased until completely quenched. The markable fluorescence quenching at 10-6 mol/L and naked-eye change indicated their high sensitivity to NT detection. The shelf life of the samples at least 7 months can provide a reliable foundation for their application. Furthermore, the paper sensors after detecting NT can be reproduced by washing with methanol. There is no obvious difference for NT detection at same NT concentration between the sensors and the reproduced sensors (Figure S5). Because NT is easily soluble in methanol, while the polymer is slightly soluble in methanol. According to the difference of the solubility in methanol between NT and polymer, methanol can be used to remove NT from paper sensor. Despite a small loss of polymers exists during the washing process, the recycled paper sensor still demonstrates high responsiveness to NT detection. The results demonstrate a great promising sensor for NT detection at micromolar level in a portable, visual and ultrafast way.


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Figure 4. Fluorescence intensity changes, quench percentage and its fitted curve of PyD ((a), (d), and (g)), PyP3 ((b), (e), and (h)), and PyP4 ((c), (f), and (i)) by adding successive concentration of NT solution, and paper sensor based on (j) PyD, (k) PyP3, and (l) PyP4 for NT detection. To further study the selectivity, 4-nitrophenol (NP) with extremely similar structure and property to NT was selected as sample for contrast experiments. The response of pyrene base fluorescent materials (PyD, PyP3, and PyP4) to NP was record and was depicted in Figure 5. The fluorescence intensity of pyrene base fluorescent materials slightly decreased after adding NP solutions. Their quenching efficiency indicated that



three fluorescent materials exhibited a proximate responsiveness to NP. Though NP own similar structure to NT, the obtained fluorescent materials only show high selectivity to NT. The above control experiment has demonstrated the unique selectivity to NT.

Figure 5. Fluorescence intensity changes of pyrene derivatives ((a) PyD, (b) PyP3, and (c) PyP4) to NP, and (d) their quenching efficiency. According to typical pyrene-based fluorescence probes, their responsiveness toward nitrotoluene derivatives usually positive correlate with the number of nitro group of NACs. Take nitrotoluene derivatives as example, the order of selectivity as follows: TNT >> DNT > NT. However, PyD and PyP demonstrated an inverse order of selectivity as follows: NT >> DNT > TNT. The high selectivity indicated that the introduction of cyclic structure and long-chain structure brings remarkable effect on


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responsiveness and eventually changes the order of selectivity. What is the reason to cause this abnormal phenomenon? To gain insight to this abnormal phenomenon and get better understand the forming reasons, a series of theoretical study was carried out using Materials Studio program. Frontier molecular energy of pyrene based fluorescent sensors as shown in Figure 2, the smaller band gap means a low exciton binding energy and high internal conversion rate, which is expecting an easier response process and highly sensitive fluorescence quenching behavior. The high selectivity and responsiveness of PyD and PyP to NT have proved the expectation. At present, the mechanism of analyte-induced emission intensity reduction is mainly ascribed to the forming of donor-acceptor complex between fluorophore moiety and analytes.15-16 The strong π-π interaction and charge-transfer between electron acceptor and electron donor result in fluorescence quenching behaviors. The responsiveness and sensitivity of fluorophores to different NACs is primarily determined by electron transfer process, and their driving force was provided by energy of electron transfer, lowest unoccupied molecular orbital (LUMO) and electron accepting ability of NACs. To gain more insight into the high selectivity, the molecular orbitals of nitroaromatics were calculated. As shown in Figure 6, the energy level difference between HOMO (pyrene derivatives) and LUMO (NACs) is smaller than the relative energy of HOMO and LUMO of PyP, which indicating the charge-transfer from PyP to NACs.17 The energy difference between HOMO (fluorophore) and LUMO (analytes) is closely related to the difficulty level of the charge-transfer from pyrene moiety to analytes. The smaller

energy difference, the easier charge transfer between pyrene and analytes. The lower



LUMO level of TNT endows it with lower reduction potential and stronger electron accepting ability. Therefore, the selectivity of traditional fluorescence sensors was consistent with their order of electron-withdrawing property (TNT > DNT > NT).

Figure 6. Molecule orbitals of nitroaromatics (NT, DNT, and TNT) and simplified structure of pyrene functionalized siloxane and polysiloxane. Pyrene based fluorescent sensors previously reported, their selectivity toward NACs were mostly consistent with the number of nitro group in solution media. The sensors mostly include one or more pyrene moieties via covalent linking and mainly divided into three types: mono-pyrene substituted molecular, bis-pyrene molecular, and multisubstituted polymer. Schematic diagram of their possible quenching mechanism in solution as shown in Figure 7 (a), Figure 7 (b), and Figure 7 (c), respectively.17-20 Pyrene group as strong π-electron acceptor, can form donor-acceptor complex with electron deficient NACs through strong π-π stacking interactions. Subsequent fluorescence


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quenching of the rich π-electron planar compounds result from the excited-state electron-transfer between donor and acceptor. In this work, pyrene groups connected with silicon atom through carbon-carbon double bond, and the empty 3d orbital of silicon atoms can interact with pyrene group by sp3 hybrid and form d-p conjugation. The conjugate system between silicon atom and vinylpyrene structure can stabilizes side chain and contribute to electron transfer in an excited state. The schematic diagram of possible quenching mechanism of PyD and PyP was depicted in Figure 7 (d) and Figure 7 (e). Molecular structure model of PyD and a moiety of PyP was shown in Figure 8 (a) and Figure 8 (b), carbon-carbon double bond is conducive to forming a rigid side chain. Stereoscopic cyclic structure of PyD is very helpful to aggregate and form a complex and fluffy steric structure with appropriate gaps, which providing high selectivity to NT. Rigid side chain and intramolecular rotation could provide a suitable internal environment for the capture of NT. Therefore, the special structure of PyD provides an ideal platform for the high detection of NT. Polymers can be made less flexible or more rigid by adding more bulky side groups.20-21 The sparse side chain structure can help maintain the flexibility of polysiloxane backbone and restrain the aggregate of pyrene structure. The long chain of PyP provides an excellent chance to bend and intertwine, and finally form a complex steric structure with rigid fluorophore and small gaps. Intramolecular free rotation of double bond act as dispatcher to provide shortcuts to enhance the interaction between NT and fluorophore and to make sure one NT could interact with one or more fluorophores, which ensure high sensitivity and responsiveness. Small gaps serve as a doorkeeper to make sure that only mono-nitro



substituted benzene derivatives could come in and out, which endow the high selectivity to NT. The steric hindrance effect based on PyD and PyP provides an ideal platform for the selective detection of NT, rather than TNT with stronger electron-withdrawing ability than NT.

Figure 7. The proposed mechanism of (a) mono-pyrene substituted molecular, (b) bis-pyrene molecular, and (c) multi-substituted polymer for TNT detection in former researches. The predicted mechanism of (d) PyD and (e) PyP for NT detection.

Figure 8. Molecular structure model and intramolecular rotation of (a) PyD and (b)


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the moiety of PyP. To further explore the high selectivity of PyP to NT, we optimized the molecular structure of PyP and calculated the binding energy of PyP to NACs (NP, NT, DNT, and TNT) using Materials Studio program. As shown in Figure 9, the sequence of the binding energy was as follow: TNT > DNT > NT > NP, which is consistent with the order of electron-withdrawing property. The more energy released, meaning the lower systematic energy after π-π stacking interactions, the more stable donor-acceptor complex. Theoretically speaking, DNT and TNT can easily interact with all phenyl groups due to their stronger electron-withdrawing property than NT. But because of the complex steric structure of polysiloxanes, their π-π stacking can only react with very few surrounding or peripheral exposed pyrene and then represent an extremely low responsiveness. Therefore, although the binding energy of polysiloxanes to DNT and TNT is so high, small gaps in the steric structure of polysiloxanes could still prevent them from entering the inside of the polymer. In addition, similar structure of NP to NT makes NP easier to enter the polymer, while lower binding energy make it very easy to exit and finally exhibit a low responsiveness. The huge difference of selectivity and responsiveness among four analytes was probably result from the different electronwithdrawing property of NACs and unique steric structure of PyP, and the high sensitivity to NT could be attributed to the good matching between steric structure of PyP and electron-withdrawing of NT. The above results suggest that the high selectivity and responsiveness not only due to thermodynamic factor but also due to kinetic factor, it is a result of the synergistic effect of two factors. PyP with rigid side chains and steric



structure allowed them to detect NT with high selectivity. To verify the speculation, the fluorescence of PyP solutions was firstly quenched by adding NT, and then the solution was diluted with dichloromethane. As shown in Figure 10, the solution could be observed first with marked fluorescence enhancement and then with fluorescence quenching during the diluting process. The fluorescence enhancement can be attributed to the change of steric structure of PyP. The steric structure gradually stretches and loosen, the composite structure between one NT to more pyrene was destroyed, and a part of pyrene groups broke away from NT and recovered fluorescence. Subsequent fluorescence quenching resulted from the decrease of PyP concentration, which was consistent with the trend change of fluorescent intension of PyP without adding NT during the diluting process (Figure S2 (f)). This situation was consistent with the above speculation in NT detection. Therefore, PyP showed a significant application prospect in selective detecting NACs.

Figure 9. The binding energy of PyP to NACs (NP, NT, DNT, and TNT) calculated using Materials Studio program.


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Figure 10. Fluorescence intensity changes of PyP3 was gradually diluted with dichloromethane after fluorescence quenching. 4. Conclusion A series of pyrene functionalized polysiloxanes with superior luminescent properties was synthesized via the Heck reaction. The obtained material exhibits high selectivity and excellent sensitivity to NT, which are different from the former reported fluorescence probe. We further explored the possible response process by calculation and speculated that the high selectivity is derived from the synergistic effect between steric hindrance and dipolar interaction. In addition, paper sensors based on the obtained fluorescent materials were fabricated. The markable fluorescence quenching at 10-6 mol/L and naked-eye change indicated their high sensitivity to NT. This work provides an easy and effective way to visible detection for NT. We expect that the described design strategy could be extended to synthesize other fluorescent probes with high selectivity to NT detection, and possible quenching mechanism will provide a new



perspective to novel fluorescent probes with unique selectivity to NACs detection.

Associated content Characterization of the polymers and supporting table and figures. Corresponding Author Weiying Lin, E-mail: [email protected]. Conflicts of interest There are no conflicts to declare. Acknowledgements We would like to thank Xiaowen Tang (School of Pharmaceutical Science, Sun Yatsen University) for providing Gaussian and Materials studio calculations. This work was financially supported by NSFC (21877048, 21472067, 21672083), Taishan Scholar Foundation (TS201511041), the startup fund of University of Jinan (309-10004, 160100332, 140200321), and Natural Science Foundation of Shandong Province (ZR2018BB022). References (1). Kovacic, P.; Somanathan, R., Nitroaromatic Compounds: Environmental Toxicity, Carcinogenicity, Mutagenicity, Therapy and Mechanism. J. Appl. Toxicol. 2014, 34 (8), 810-824.


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Table of Contens (TOC)


Pyrenyl-Functionalized Polysiloxane Based on Synergistic Effect for

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