Siloxane-Based Nanoporous Polymers with Narrow Pore-size

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Siloxane-Based Nanoporous Polymers with Narrow Poresize Distribution for Cell Imaging and Explosive Detection Zhiming Gou, Yujing Zuo, Minggang Tian, and Weiying Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08582 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 11, 2018

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ACS Applied Materials & Interfaces

Siloxane-Based Nanoporous Polymers with Narrow Pore-size Distribution for Cell Imaging and Explosive Detection Zhiming Gou, Yujing Zuo, Minggang Tian, 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 Porous polymers are among the most promising porous materials for various application because they show the combined advantages of fluorescent porous materials and polymers. This study developed a cell imaging technique based on luminescent porous organosilicon polymers (LPOPs) that were synthesized via Friedel–Crafts reaction of octaphenylcyclotetrasiloxane with octavinylsilsesquioxanes. The porous organosilicon polymers possessed narrow pore-size distribution, high surface area, and monomodal nanopores centered at approximately 0.59 nm. The excellent properties to the porous polymers can be attributed to the fine structures of LPOPs. LPOP-2 owned the highest fluorescence intensity and micropore volume ratio in LPOPs and showed high selectivity for Fe3+ detection and excellent sensitivity to nitroaromatic compound detection. Interestingly, these porous polymers still exhibited excellent responsiveness to Fe3+ ion even when inside of living cells. We also fabricated a paper-based sensor using LPOP-2 to develop a simple method for visual detection of explosives. This rapid and visual paper sensor demonstrates promising application for explosive detection and can be expanded for the detection of other analytes.

Keywords: nanoporous polymers, cell imaging, Fe3+ ion detection, explosive detection, fluorescence quenching


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1. Introduction

Porous materials have received increasing interest since the discovery of zeolites and have been successfully applied in industrial catalysis.1-3 All kinds of microporous or mesoporous materials have been designed and constructed and have been widely used in catalysts; gas separation, storage, and sensing; and drug release.4-6 Luminescent porous polymers is one of the most promising porous materials for new applications, such as photocatalysis,7 luminescent sensing,8 energy transformation, and bioimaging,910

because they may perfectly consolidate the advantages of porous materials and

polymers.

However, some problems, such as non-uniform pore structure and wide pore-size distribution, still exist. The properties of porous materials depend largely on pore shape, pore-size distribution, and material composition. High specific surface areas and welldefined microporous structures can usually endow porous materials with excellent performance. Therefore, methods to construct such materials have attracted increasing attention. Considering the construction of a well-defined net structure and the reaction characteristics of polymerization, we expected that by using two types of organosilicon monomers with nuclear-type structure and multiple reaction sites to construct porous polymers, nuclear-type structure can provide a stable supporting structure, and multiple reaction sites can form fine structure to endow suitable pore size and uniform pore structures. The nontoxic nature and good biocompatibility of siloxane make porous organosilicon polymers suitable for application in biological systems.11-13



Polyhedral oligomeric silsesquioxanes (POSS) containing a rigidly cage-like core and a shell-like peripherally substituent group is suitable for the construction of hybrid porous material.14-15 Organic and inorganic components can endow hybrid materials with high thermal and chemical resistance and biocompatibility. Therefore, several novel inorganic-organic hybrid porous materials based on octavinylsilsesquioxane (OVS) have been synthesized for different requirements. Sun et al. reported a class of porous polymers derived from OVS with aromatic bromide monomers and formed a light-harvesting system with coumarin 6, which show excellent energy transfer efficiency.16 Wang et al. synthesized two sets of conjugated porous polymers with tunable fluorescence from dinginess to bright, and tunability built from different conjugated monomers, and coupling between electronic structures.17 Liu et al. prepared a series of tetraphenylethene-based porous polymers, which show good capacity for CO2 storage and high selective adsorption for dye molecules.18 However, some problems, such as wide pore-size distribution and non-uniform pore structure, still exist in the above porous polymers. Therefore, controlling the morphology of polymers and fabricating uniform porous structure is still a considerable challenge.

Octaphenylcyclotetrasiloxane (OPCTS) is a type of commercial chemicals and contains similar inorganic-organic structure to OVS but with a flexible ring-like core. Considering the structural features of two organosilicon monomers, OVS with a rigid core can serve as frameworks and OPCTS with a flexible core can act as rivets that fix these frameworks together to form a uniform pore structure. The reaction between two monomers with nuclear-type structure and multiple reaction sites can allow the


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fabrication of hybrid porous materials with uniform porous morphology, narrow pore size distribution, and high specific surface areas.

Based on the above precondition, we provided a simple route and synthesized a series of luminescent porous organosilicon polymers (LPOPs). The synthesized LPOPs exhibit narrow pore size distribution, monomodal micropore structure, excellent fluorescence properties, high surface area, and high thermodynamic stability. As expected, the fine structure of LPOPs endowed excellent properties to these porous polymers. The porosity above findings revels that the nuclear-type structure with multiple reaction sites can fabricate hybrid porous materials with a well-defined structure. Unlike traditional organosilicon luminescent material, no traditional rigid πconjugated plane structure in LPOPs was present. Pore properties and fluorescence properties of LPOPs could be synchronously adjusted by changing the molar ratio of reactants. Given the high specific area and the highest fluorescence intensity of LPOP2, it was chosen for fluorescence detection and image. The detection of various cations and anions were carried out, and we found that LPOP-2 exhibited high selectivity for Fe3+ detection, and their fluorescence was quenched almost completely by low Fe3+ concentration (1 mM). The high detection performance was not observed in the presence of other metal ions or anions. In addition, we discovered that fluorescence quench percentage (QP) was closely related with ionic radius of cations, only Fe3+ ion exhibit high fluorescence QP (>85%), whereas other larger or smaller metal cations showed limited QP. The comparison between quench percentage and ionic radius indicates that only the proper diameter Fe3+ ion could be entirely caught in ring structure



of cyclosiloxane for coordination interaction. What’s more, these porous polymers could easily stain HeLa cells and enter the cytoplasm, and show obvious blue and green fluorescence in dark field under confocal microscopy. It is the first time that LPOPs were applied to the cell image. Interestingly, these porous polymers still exhibited excellent responsiveness to Fe3+ ion even in a living cell. Furthermore, LPOP-2 displayed excellent sensitivity to nitroaromatic compound (NAC) detection. To develop a simple and visible assay for explosive detection, we made a novel type of test paper based on LPOP-2. This rapid and visual paper sensor is promising for application for explosive detection and can be expanded for the detection of other substances.

2. Experimental section

2.1 Materials

All reactants and solvents were purchased from commercial suppliers and directly used without any treatment. OVS was synthesized using the classic procedure.19

2.2 Characterization and measurements

Fourier transform infrared spectroscopy (FT-IR) was measured with a Bruker Tensor-27 using KBr disc from 4000 cm−1 to 400 cm−1. Solid-state C13 and 29Si NMR spectra were recorded by Bruker 600M. Field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM) images were characterized on ZEISS Supra 55 and JEOL-2100F, respectively. Nitrogen adsorption-desorption isotherm was obtained on ASAP 2020 from Micromeritics


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Instrument Corporation at 77 K. Thermogravimetric analysis (TGA) was performed on SDT-Q600 in N2 (100 mL min−1) at a heating rate of 10 °C min−1 from 800 °C. Fluorescent emission spectra were measured with Hitachi F 4600 spectrometer. Cell imaging was recorded with Nikon A1MP confocal microscope.

2.3 Synthesis of LPOPs

The synthesis procedure of LPOP-2 was as follows. OVS (0.63 g, 1.0 mmol), OPCTS (0.32 g, 0.4 mmol), anhydrous aluminum chloride (0.27 g, 2.0 mmol), and dichloromethane (30 mL) were placed in a 50 mL oven-dried flask. The mixture was stirred at room temperature for 0.5 h and then refluxed for 24 h. After cooling to room temperature, the mixture was filtered and sequentially washed with methanol, water, THF, and chloroform. The product was further purified under the Soxhlet extractor with THF and methanol successively for 24 h. Pale-yellow power (0.94 g, 98.9%) was obtained after dried in a vacuum at 70 °C for 48 h. LPOP-1 and LPOP-3 to LPOP-6 were synthesized as LPOP-2 following the above procedure. Detailed information on LPOPs with various molecular ratios is shown in Table S1. 2.4 Fluorescence property and analyte detection

Fluorescent emission spectra of LPOPs in the solid-state and suspension-state (0.3 mg/mL, water to ethanol at 1:9 v/v) were excited at 330 nm and 310 nm, respectively.

Ions and nitroaromatic compounds (NACs) were detected by adding corresponding analytes to LPOP-2 suspension liquid (0.3 mg/mL, water-to-ethanol ratio at 1:9 v/v).



2.5 Cytotoxicity assays and cell imaging The cytotoxicity of LPOP-2 was performed via the standard MTT assays. HeLa cells were first inoculated in culture plate until they adhered to the walls. Subsequently, LPOP2 suspension with different concentrations were added to the medium and cultured in a incubator (5% CO2 and 95% air, 37 °C) for 6 h. Cholecystokinin octopeptide-8 (10 μL) was added and the cells were continue cultured for 4 h. Finally, the absorbance of cells was measured by ELISA at 450 nm, and the cell survival rate was calculated to evaluate cytotoxicity.

The cell images of LPOPs in HeLa cells were measured by a confocal microscope. The HeLa cells were incubated with 4.0 μg of LPOP-2 for 1 h at 37 °C, and then washed with PBS to remove the excess LPOP-2. The new culture medium was added, and the sample was used to observe fluorescence images.

The responsiveness in living cells were measured by successively adding Fe3+ to HeLa cells stained with LPOP-2. The LPOP-2 was suspended in water/ethanol mixture and Fe3+ come from chloride salts in water, and their preparation is same as in other experiments. The response process was obtained by in situ observation using confocal microscopy.

3. Results and discussion

Synthesis and characterization of LPOPs

The build of network structure is very important for porous materials and significantly influences pore structure and property. These compounds with multiple


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reactive sites are often used to synthesize dendritic monomers or polymers, which allows easy construction of regular dendritic structure. Inspired by the synthesis of dendritic polymers, we designed and synthesized a series of porous polymers with fine structure via two kinds of core-shell-like siloxanes with nuclear-type structure and multiple reaction sites. LPOPs were synthesized by the Friedel-Crafts reaction of OVS and OPCTS (Scheme 1). The structures of the LPOPS were further confirmed by FTIR and solid-state NMR (13C NMR and 29Si NMR).

Scheme 1. (a) Synthetic route of luminescent porous organosilicon polymers (LPOPs), and (b) their application in cell imaging and explosive detection. The C=C stretching vibration peaks of vinyl and phenyl appeared from 1362 cm−1 to 1689 cm−1 (Figure S1 (a)), and this characteristic peak of LPOPs slightly shifted to long wave number relative to reactants. The decrease in unsaturated C–H at 3024 cm−1 and the increase in methylene at 2929 cm−1 both indicate the occurrence of cross-linking



reaction. The absorption peak at approximately 1114 cm−1 undoubtedly belong to Si– O–Si stretching vibration, and the peaks of LPOPs are broader than those of OVS and OPTCS, which suggest the different cross-linking degree of porous polymers.

In view of the similar structure, LPOP-2 was selected as the sample to represent LPOP series to measure the solid-state 13C and 29Si CP/MS NMR of LPOP-2 (Figure S1 (b) and Figure S1 (c)). The signals from phenyl carbon atom and unreacted double bonds appeared from 128 ppm to 140 ppm, and the peaks at 14.9 and 25.9 ppm were mainly attributed to methylene generated from vinyl group in OVS after Friedel-Craft reaction (Figure S1 (b)). The signals at approximately 69.9 ppm might be attributed to the carbon atom from residual THF. Aromatic rings become increasingly active after the introduction of alkyls, facilitating the formation of a network of porous polymer. The elimination or rearrangement of alkyl group would produce by-products, and the peaks at 86.3 ppm and 191 ppm belonging to carbonyl group might have resulted from the side reaction of Friedel-Crafts reaction. Two main characteristic peaks were observed at 26.1 and 68.2 ppm, and adjacent short weak peaks could be ascribed to the silicon atom of unreacted OVS and OPCTS and formed structural unit (Figure S1 (c)). The above results suggest the success of cross-linking reaction and no destruction to the core-like structure during reaction.

Morphology and porosity The multiple reaction species of OVS and OPCTS built together an ideal environment for constructing relatively uniform porous structure at nanometer scale.


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To observe the morphology and structure property of LPOPs, we selected LPOP-2 as the sample, which was measured by FE-SEM, HR-TEM, and nitrogen adsorptiondesorption at 77K, respectively. The FE-SEM image (Figures 1 (a) and (b)) indicates that LPOP-2 exhibits an irregular interconnected spongy structure based on particles and with a narrow range of pore-size distribution under 5 nm. The HR-TME images (Figures 1(c) and (d)) further affirmed that the polymer demonstrated an approximate pore diameter and relatively uniform pore structure, which were consistent with those of other OVS-based porous polymers.18 The above findings revels that the nuclear-type structure with multiple reaction sites can fabricate hybrid porous materials with a welldefined structure.

Figure 1. FE-SEM ((a) and (b)) and HR-TEM ((c) and (d)) images of LPOP-2.



The pore size distribution and specific surface area were calculated by nitrogen adsorption-desorption curves. LPOP-2 shows a sharp uptake at low pressure and a gradually increasing uptake at high pressures with hysteresis (Figure 2 (b)), suggesting that micropores and mesopores coexist in the porous structures.20 Only a unimodal distribution at approximately 0.49 nm occurred in the pore-size distribution curve, indicating that the porous structure exhibited a narrow pore-sized distribution and micropore structure. All of the adsorption-desorption isotherms of LPOPs showed similar variation trend to LPOP-2 and presented narrow pore-size distribution under 2 nm (Figure 2).


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Figure 2. Nitrogen adsorption-desorption isotherm and pore-size distribution curve (inset) of (a) LPOP-1, (b) LPOP-2, (c) LPOP-3, (d) LPOP-4, (e) LPOP-5, (f) LPOP-6.

As expected, porosity was significantly affected by the molar ratio of reactants. The SBET of LPOPs (LPOP-1 to LPOP-5) from 554 m2·g−1 to 1003 m2·g−1 was calculated from nitrogen adsorption-desorption isotherm. The corresponding micropore volume



ratio (Vmicro to Vtotal) was shown as follows: 0.88, 0.94, 0.84, 0.78, 0.85, and 0.85 (Table S2). Therefore, this series of polymers could be regarded as microporous materials according to IUPAC classification. LPOP-2 exhibited high specific area and the highest micropore volume ratio in LPOPs, indicating that LPOP-2 possessed the highest adsorption capacity in this series of luminescent porous organosilicon polymer.

Thermal stability To evaluate the thermal stability of the LPOPs, we performed TGA of LPOP-2 under N2 from room temperature to 800 °C (Figure S2). Mass loss of OVS between 150– 250 °C was attributed to the removal of the vinyl groups. Siloxane exhibited a ‘melting’ phase between 270 °C to 300 °C, and the largest mass occurred at approximately 300 °C, which resulted from the destruction of the POSS cage integrity and vinyl groups. The resulting hydrogen POSS and alkyl POSS was very low and demonstrated slow degradation behavior. The good thermal stability and high thermal decomposition temperature, Td at 5%, 426 °C, of LPOP-2 could be explained by the high cross-linked networks based on the Friedel-Crafts reaction. The initial loss that occurred at approximately 450 °C could be attributed to the cleavage of unreacted vinyl and phenyl groups. The decomposition that occurred between 480 °C and 620 °C may be attributed to the partial fragmentation of Si–O–Si skeleton structure. The TGA results showed that these porous polymers featured high thermostability.

Fluorescent properties

The fluorescent spectra of LPOPs in solid- and suspension-states are shown in


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Figure 4. At the maximum emissions (λem) of LPOPs, place redshift from LPOP-1 to LPOP-6 (435 nm to 580 nm) occurred in the solid state (Figure 3 (a)), and two adjoined emission peaks clearly separated along with the increase in OPCTS molar fraction. This phenomenon may be associated with vinyl benzene-like conjugated structure between unreacted vinyl of OVS and the aromatic groups of OPCTS. The conjugation behavior between Si and POSS cage can be recognized as a donor-acceptor system, which caused noticeable red shift among the different cross-linking degree of LPOPs.21-22 The color contrast of LPOPs under daylight and UV light (365 nm) is shown in Figure 3 (d). The results matched well with the corresponding chromaticity coordinates calculated by the CIE chromaticity diagram (Figure 3 (c)), changing from blue (x = 0.179, y=0.1188) to yellow (x = 0.3844, y= 0.4209). The detailed color parameters are shown in Table S3. The distance from the chromaticity coordinates center was inversely proportional to the color saturation,16 and the color purities of LPOPs generally increased with the rising cross-linked densities.17 The color purities of LPOPs initially decreased and then increased with the increasing mole ratio of OPCTS. This variation trend closely connected with the mole ratios because the reaction could not react well to forming high cross-linked network structure at relative high ratio of OVS or OPCTS due to the stereo-hindrance effect of multi-vinyl and aromatic groups of reactants and rigid cagelike structure of OVS. High color purity of LPOPs with high crosslinking density were generated at appropriate proportion between OVS and OPCTS. The redshifted phenomenon also occurred in the suspension state but was not as obvious as that in the solid state, which may have been derived from solvent effect and intermolecular force.21,



23

Therefore, the redshift that occurred both in the solid and suspension states were

mainly consistent with the increasing mole ratio of OPCTS. The results explained that the porous structure and fluorescence properties of LPOPs could be synchronously adjusted by changing the molar ratio of reactants.

Figure 3. Fluorescence emission spectra of LPOPs in (a) the solid state (λex= 330 nm) and (b) suspension state (0.3 mg/mL, water to ethanol at 1:9 v/v) (λex= 310 nm); (c) CIE parameters of LPOPs in the solid-state; (c) photographs of sample (sunlight) and luminescence images (365 nm UV light) of LPOPs; (d) photographs of LPOPs and corresponding luminescence images were excited by 365 nm UV light.


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The LPOPs demonstrated strong fluorescence in solid and suspension states. However, no traditional rigid π-conjugated plane structure in LPOPs was present.24 This phenomenon belongs to atypical fluorescence, which is caused by the unique electron configuration of silicon atom. Silicon atom exhibits five of the empty 3d orbital and can provide holes and efficient transport path of electrons, which interact with conjugated molecules by forming sp3 hybrid orbitals. Silicon atoms donate their lone of pair of electrons to benzene ring and form d-p conjugation. A part of the electron cloud of benzene ring enters the 3d orbital of a silicon atom, which stabilizes the bond between silicon atom and benzene ring and contributes to electron transfer in an excited state. To gain additional insight into this phenomenon, we simplified and optimized the molecular structure of LPOPs and calculated their molecular orbitals using Gaussian 03 program at the B3LYP/6-31 G(D) level. As shown in Figure 4, the calculated molecular orbitals exhibited electron distribution in the phenyl around silicon atom, with the alkyl group serving as an electron donating group and the substituent effect of alkyl group contributing to the increase in the electron density of benzene ring. Therefore, silicon atom served as bridge, and independent benzene rings were connected through this bridge to form large π bond-like structure. Finally, porous materials with strong fluorescence emission were generated.



Figure 4. Simplified and optimized molecular structure of LPOPs and their frontier molecular orbital and electron-density map. Detection of various ions

Fe3+ ion is one of most crucial elements in biological systems but causes disease or even death when their content in organisms is low or over a critical concentration.25-27 High micropore volume rate and excellent luminescent property will endow LPOP-2 with high sensitivity and responses to analytes. To study the sensibility of LPOPs to various cations, we added different chloride salts (Al3+, Ba2+, Ca2+, Co2+, Cu2+, Fe3+, Mg2+, Ni2+, and Sn2+) in the LPOP-2 suspension (water to ethanol at 1:9 v/v). The fluorescence color of these salts under 365 nm UV lamp was visible to the naked eye (Figure S3). After adding metal cations (1 mM), the fluorescence intensity of the suspension by adding Fe3+ ion significantly decreased and nearly quenched completely, whereas the other metal ions exhibited limited impacts on the fluorescence of LPOP-2 (Figure 5 (a)). Fluorescence quench percentage (QP) was calculated through the (I0 -


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I)/I0 × 100% formula, where I0 and I are the fluorescence intensity of LPOP-2 before and after adding cations or anions, respectively. We can observe Fe3+ exhibit high fluorescence QP of >85%, whereas other metal ions showed limited QP at Ca2+ (99 pm) > Sn2+ (93 pm) > Fe2+ (76 pm) > Co2+ (74 pm) > Cu2+ (72 pm) = Ni2+ (72 pm) > Fe3+ (64 pm) > Al3+ (50 pm). The sequence of the absolute value of QP was as follows: |QPFe(Ⅲ)| >> |QPAl( Ⅲ )| > |QPCu( Ⅱ )| ≈ |QPNi( Ⅱ )| > |QPFe( Ⅱ )| > |QPCo( Ⅱ )| > |QPSn( Ⅱ )| > |QPMg( Ⅱ )| > |QPCa(Ⅱ)| >|QPBa(Ⅱ)|. It is clearly observed that ionic radius of Al3+ ion is the smallest in the above cations and with lower |QP| under Fe3+ ion, while ionic radius of Ba2+ ion is the biggest cations and with lowest |QP| among various ions. The quench percentage of iron ions with different valences suggest that the charge of cation have huge influence on quenching behavior. Although Mg2+ ion with approximate ionic radius, the formation of precipitation between LPOP-2 and Mg2+ ion result in a low quench


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percentage. The above results illustrated that only the cation with proper diameter could be suitable to the pore size of LPOP-2. The mismatched host-guest in the complex would bring unfavorable factors and was disadvantageous to the coordination interaction between LPOP-2 and metal ions. The schematic diagram of coordination interaction is shown in Figure 6 (b). The cavity was almost impossible to enter for the large ions but was very easy to exit for the small ions. However, only Fe3+ ion could be entirely caught in ring structure. The great change of UV absorption of Fe3+ ion after interaction with LPOP-2 indicated that this interaction results in huge variation in adsorbing ability, this is directly confirming the ion binding induced emission change (Figure S6). The micropore structure with uniform networks and narrow pore-size distribution allowed good selectivity and high sensitivity of LPOP-2 for Fe3+ detection.

Figure 5. Fluorescent emission spectra of LPOP-2 in the suspension (0.3 mg/mL,



water–to-ethanol ratio at 1:9 v/v) with various (a) cations and (c) anions. Fluorescence quench percentage by adding various (b) cations and (d) anions. (ion concentration: 1 mM)

Figure 6. (a) Fluorescence emission spectra of the LPOP-2 suspension (0.3 mg/mL, water-to-ethanol ratio at 1:9 v/v) by adding successive concentration of Fe3+ ion (λex= 310 nm). (b) The sketch map of coordination interaction of LPOP-2 with various metal cations. According to the theory of frontier molecular orbital, the process of chemical reaction is first reflected in the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Therefore, to gain additional insight into the process of coordination interaction between LPOP-2 and Fe3+, the HOMO and LOMO of LPOPs before and after coordination with Fe3+ ion were calculated by Materials Studio program using DMol3 tools. To ensure the accuracy, we performed reasonable simplification and geometry optimization of the LPOPs+[Fe3+]. We discovered that the conformation of central cyclotetrasiloxane structure in LPOPs changed from twist-boat conformation to boat conformation after complexation. Steric effect of multi-phenyl


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groups led to the formation of twist-boat conformation, and the interaction between the LOPOs and Fe3+ ion resulted in a conformational change. As shown in Figure 7 (a), Fe3+ ion was in the bottom of the boat conformation and was located on the same layer with oxygen atom. This coplanarity shown in the process improves the stability of the complex compound. Therefore, the combined structure between chair conformation and coplanarity was kinetically and thermodynamically the most stable conformation. The molecular orbitals maps are shown in Figure 7 (b), and the black line and red line represent HOMO and LOMO, respectively. The electron cloud distribution showed that the frontier orbitals of LPOPs were optionally distributed around benzene rings, whereas the frontier orbitals exhibited coplanar-structure after coordination with Fe3+ ion. The band gap (△E) sharply decreased after coordination, and the small band gap indicated low exciton binding energy and high internal conversion rate. Thus, the LPOPs could not emit fluorescence as same intensity as before, which resulted in the fluorescence quenching behavior.



Figure 7. (a) Conformations and (b) molecular orbitals maps (HOMO and LUMO) of LPOPs before and after coordination with Fe3+ ion.

Cellular imaging

The good selectivity and high sensitivity of LPOP-2 to Fe3+ inspired us to explore additional actual applications. Fe3+ in living cells was detected, and their interaction process was observed under laser scanning confocal microscopy. First, cytotoxicity assays of LPOP-2 were performed in HeLa cell via standard MTT assays, and the cell


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survival rate was >85% after incubated in long time and high concentration, suggesting that LPOP-2 exhibits low cytotoxicity against living cells (Figure S7). Subsequently, the HeLa cells were stained with the LPOP-2 (4 μg/mL) for 1 h, and their confocal fluorescence images were observed under confocal microscopy, which was excited by 405 and 488 nm light source. The images show evident blue and green fluorescence in dark field (Figure 8 (a)), suggesting that the LPOPs could easily enter living cells. We have observed that LPOP-2 was mainly distributed in cytoplasm, and some absorbed on the surface of cells in the bright field, which perfectly overlapped with the fluorescent distribution in merged field. The interaction process between LPOP-2 and metal ions in living cells was investigated by in situ observation by using confocal microscopy (Figure 8 (c)). The fluorescence intensity ratio of green field to blue field gradually decreased with increasing of Fe3+ ion concentration, and the change tendency maintained good linear correlation. Encouraged by the good biocompatibility and imaging capability of LPOP-2 and to obtain additional insight into the application of LPOPs in living cell imaging, we investigated the cell imaging capability of the LPOPs. As shown in Figure 9, this series of LPOPs all exhibited evident blue and green fluorescence in dark field, suggesting that the change of reaction ratio does not affect their cell imaging capability. The above results indicated that live cells could be efficiently stained by LPOPs via simple incubation, suggesting additional possibility of LPOPs as medicine carrier for clinical therapy.



Figure 8. (a) Confocal fluorescence images of HeLa cells only stained with LPOP-2 (4 mg/mL), bar = 50 μm; (b) images of interaction process by adding different concentrations of Fe3+ ions, (c) linear relationship between fluorescence intensity ratio of green field to blue field and the Fe3+ concentrations, bar = 10 μm.


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Figure 9. Confocal fluorescence images of HeLa cells only stained with the LPOPs (4 mg/mL) for 1h, bar = 50 μm.



Detection of nitroaromatic compounds (NACs) Inspired by the good selectivity and high sensitivity of LPOP-2 to Fe3+, we also investigated its responsiveness for NACs. NACs are closely related to agricultural chemicals, medicament, and military applications and is harmful to personal health, public security, and ecological environment if misapplied.33-34 Therefore, monitoring and detecting NACs is important to decrease and prevent the occurrence of related accidents.35 The retrievability of the fluorescent porous polymers making them different from traditional fluorescent compounds and polymers.36 High specific surface area and the highest fluorescence intensity and micropore volume ratio in LPOPs encouraged us to select LPOP-2 for exploration on NAC detection strategies. 4-Nitrotoluene (NT), 2,4dinitrotoluene (DNT), and 2,4,6-trinitrotoluene (TNT) were selected to investigate luminescence response by adding various concentrations of NACs to LOPO-2 suspension. As shown in Figure 10, the fluorescence intensity markedly weakened with gradual increasing in analyte concentration. Notably, LPOP-2 exhibited high sensitivity towards three NACs at ppm concentration without exception. The luminescence quenching efficiency was also calculated by the Stern–Volmer equation, with a nearly linear curve for NT suggested a dynamic quenching process, whereas nonlinear and upward Stern-Volmer plots for DNT and TNT indicated that the quenching behavior was a combined action of the dynamic and static quenching mechanisms (Figure S8).37-39 The fluorescent emission quenching phenomenon can be elaborated by the interaction between porous structure and analytes and the donoracceptor electron transfer. The fluorescence quenching efficiency depends on the


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ability of electron transfer between the conduction band (CB) of LOPOs and the LUMOs of the NACs.40-41 Generally, the LUMOs of the analytes is lower than the CB energy-level of LPOPs upon excitation, and the electron-withdrawing property of NACS is enhanced with the increase of nitro group.42-43 In the nearly linear SternVolmer plots of the LPOP-2 for the three NACs at low concentration, the adsorption and collision behavior between the nanopores and NACs was predominant and was a dynamic quenching process at this point. With the increase of the number of nitro groups, the electron-withdrawing property of the analytes occurred in the order TNT>DNT>NT, and the electron-transfer efficiency from LPOP-2 to NACs demonstrated the same order. The fluorescence quenching efficiency of DNT and TNT dramatically enhanced that of NT in relatively high concentration. The Stern-Volmer constants of DNT and TNT markedly increased, and the corresponding plots dramatically bended upwards with an increase in the number of nitro groups. The Stern–Volmer plot of DT still maintained a nearly linear plot. This result revealed that the quenching efficiency of LPOP-2 towards NACs was the synergetic result of dynamic adsorption, electron transfer, and electrostatic interaction between the nanoporous materials and analytes. All of the Stern-Volmer constants were >16000 M−1, indicating the high sensitivity of LPOP-2 for NAC detection.



Figure 10. Luminescence quenching of FPOP-2 (0.3 mg/mL) in water and ethanol (1:9 v/v) with successive concentrations of (a) NT (0–3.6×10−4 M), (b) DNT (0–2.7×10-4 M), and (c) TNT (0–2.2×10−4 M) (λex= 330 nm). (d) Stem-Volmer constants (average value) measured by adding NT, DNT, and TNT to LPOP-2.

To develop a simple and visual detection method for explosives, we constructed a novel type of test paper based on LPOP-2 according to former similar articles.44-45 The schematic diagram of the preparation is shown in Figure 11. LPOP-2 steadily adsorbed on the surface of the commercial common filter paper through filtration of the LPOP-2 suspension liquid. To evaluate the responsiveness of test paper to explosives, we selected TNT and sprinkled it on the test paper at different concentrations. After natural drying, the test paper could be observed with marked fluorescence quenching under UV light (365 nm).


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The response level for TNT was very low at 10−5 mol/L, which indicated high sensitivity to TNT detection. This situation was consistent with the above test result in the solution. Therefore, the LPOPs showed a significant application prospect in monitoring and detecting explosives.

Figure 11. Schematic diagram of the preparation of test paper based on LPOP-2 and detection of TNT.

4. Conclusion A series of LPOPs with tunable porosities and luminescent properties was synthesized via the Friedel-Crafts reaction and was first applied to cell imaging. These microporous materials possessed high specific surface area, narrow pore distribution, high thermal stability, and excellent biocompatibility. The LPOP-2 showed high selectivity for the detection to Fe3+ and good high sensitivity to NAC detection. Interestingly, these



porous polymers still exhibited excellent responsiveness to Fe3+ ion even in living cells. We also developed a novel type of test paper based on LPOP-2 to achieve simple and visible detection of explosive. This rapid and visual paper sensor shows promising application for explosive detection and can be expanded for the detection of other analytes. This work suggests that LPOPs can be rationally designed and simply synthesized, and their application will inspire the development of organosilicon polymers and expand their application.

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 (21472067, 21672083), Taishan Scholar Foundation (TS201511041), the startup fund of University of Jinan (309-10004, 1009428), and Natural Science Foundation of Shandong Province (ZR2018BB022). References (1) Deville, H. d. S. C., Compt. Rend. Séances Acad. Sci. 1862, 54, 324.


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Table of Contens (TOC)

The first case of cell imaging based on luminescent porous organosilicon polymers (LPOPs) with narrow pore-size distribution, and LPOP-2 exhibits high selectivity to the detection of Fe3+ and excellent sensitivity to nitroaromatic compounds detection.


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Siloxane-Based Nanoporous Polymers with Narrow Pore-size

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