AIEgens-Functionalized Porous Materials for Explosives Detection

Chapter 5

AIEgens-Functionalized Porous Materials for Explosives Detection Downloaded by CORNELL UNIV on October 19, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1227.ch005

Dongdong Li and Jihong Yu* State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, P. R. China *E-mail: [email protected]

Explosives detection has become one of the current pressing concerns in global security. In the past decades, many chromophore-functionalized materials have been developed for the detection of explosives because they are more simple, sensitive, and cost-effective compared to other real time analytical methods. Aggregation-induced emission luminogens (AIEgens), a novel class of luminophores, show amplified sensing performance for the detection of explosives. In recent years, many AIEgens have been introduced into porous materials, including metal organic frameworks (MOFs), porous organic polymers (POPs) and mesoporous materials to constitute a new type of chromophore-functionalized porous materials. These materials with excellent photoluminescence emission properties and porous structure, exhibit a high sensitive detection performance to explosives. This chapter summarizes a wide range of AIEgens-functionalized porous materials and their sensing performance to explosives.

© 2016 American Chemical Society Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Downloaded by CORNELL UNIV on October 19, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1227.ch005

Introduction The reliable and accurate detection of explosives in groundwater or seawater has become increasingly important and urgent issue in modern society. So far, many real time analytical methods have been used for the detection and quantification of explosives, such as trained canine teams, gas chromatography, ion mobility spectrometry (IMS), surface-enhanced Raman spectroscopy, and so on (1). However, none of these methods is ideal for the detection of explosives due to certain features such as more complicated, lack of selectivity, high cost, and time-consuming. Fluorescence-based detection of explosives by harnessing organic dyes has drawn much more attention because they offer many benefits over other common detection techniques, such as good portability, high sensitivity, and selectivity (2). To date, various fluorescent chemosensors, including conjugated polymers, nanomaterials, and metal organic frameworks have been developed for the detection of nitroderivatives, especially, trinitrotoluene (TNT) and 2,4-dinitrotoluene (DNT), etc (3, 4). While traditional fluorescent dyes often suffer from the aggregation-caused quenching (ACQ) effect when dispersed in poor solvent or incorporated into solid matrix materials, resulting in drastically negative effects on the efficiency and sensitivity of the sensors. Aggregation-induced emission luminogens (AIEgens), which are nonemissive in their dilute solution, but luminesce intensively upon molecular aggregation, have drawn increasing research interest because of their striking turn-on fluorescence phenomenon (5). Restriction of intramolecular motion (RIM), including restricted intramolecular rotation (RIR) and restricted intramolecular vibration (RIV), is proposed as the main cause for their turn-on fluorescence phenomenon (6). Since AIE-active materials were found by Tang and coworkers, they have been widely used as efficient electroluminescent materials, sensitive chemosenors, and bioprobes, etc (7–10). Particularly, many AIEgens have been employed in the sensitive detection of nitroaromatics (11, 12). This is possibly because AIEgens are electron-rich molecules, which have Lewis acid-base interactions with electron acceptors nitroaromatic compounds. Furthermore, the AIE aggregate based sensors contain many cavities, which are suitable for the explosive molecules to enter and interact with the chromophores, making them quickly response to the explosives. Tang and coworkers synthesized a series of AIE-active 3D hyperbranched conjugated polymers, significantly increased the sensitivity and selectivity to picric acid (PA) detection (13–15). These results demonstrate that the pore structure plays an important role in explosives detection. Porous materials can be classified into inorganic open frameworks, such as zeolites and mesoporous materials, metal organic frameworks (MOFs), and porous organic polymers (POPs) according to the skeleton component (16). Their well-defined pore size, large surface area, high pore volume, and easily modifiable structures have drawn great interests of scientists. So far, many functional groups have been introduced into the porous materials and used in various fields such as catalysis, separation, gas storage, adsorption, and so on (17–19). In previous work, our group introduced AIEgens tetraphenylethene (TPE) into mesoporous materials and POPs via covalent bonds. The synthesized materials combine the 130 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


unique properties of the AIEgens and porous materials, demonstrating excellent performance in biomedicine and explosives detection. Many of these applications are built on the unique character that porous materials can rapidly associate analytes inside the pores via physical diffusion and/or chemical interaction. Other groups also designed and synthesized many AIEgens-functionalized porous materials, showing excellent performance in optical device and chemical sensing. In this chapter, we summarize the synthesis of AIEgens-functionalized porous materials (including MOFs, POPs, and mesoporous materials) to make the readers understand their structure easily, and further put forward their efficient sensing performance to explosives on the basis of structure. Figure 1 presents some representative explosive molecules that are detected by AIEgens-functionalized porous materials.

Figure 1. Chemical structures of the explosives and explosive-like substances. NB (nitrobenzene), NT (4-nitrotoluene), NP (4-nitrophenol), NBD (4-nitrobenzaldehyde), TNT (2,4,6-trinitrotoluene), PA (picric acid), DNCB (2,4-dinitrochlorobenzene), DNT (2,4-dinitrotolunene), 5-ATZ (5H-tetrazol-5-amine), NTO (5-nitro-2,4-dihydro-3H-1,2,4-triazole-3-one), HAT (3-hydrazinyl-4H-1,2,4-triazol-4-amine dihydrochloride), AT (4H-1,2,4-triazol-4-amine), DAT (4H-1,2,4-triazole-3,4-diamine hydrochloride).

131 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


AIEgens-Functionalized MOFs for Explosives Detection Metal organic frameworks (MOFs), highly crystalline hybrid materials that combine metal ions with rigid organic ligands, have emerged as an important class of porous materials. Since they were discovered, MOFs have attracted considerable attention due to their facile preparation, tunable pore size, and easy functionalization of surfaces. So far, many MOFs materials have been widely used in gas storage and separation, heterogeneous catalysis, chemical sensing, optoelectronics, biomedicine, and so on (17, 20). Luminescent MOFs combine the merits of large internal surface areas and readable fluorescence signals, providing for sensing certain species, such as volatile organic compounds (VOCs), explosive compounds, and toxic metal ions (21, 22). However, the low fluorescence efficiency of many luminescent MOF-based sensors undermines the sensitivity greatly, which make them incapable for sensing applications. That’s because the commonly used organic luminescent ligands in MOFs often subject to emission quenching when assembled into coordination complexes.

Figure 2. Molecular structure of the organic luminescent ligands with AIE properties used in MOFs. AIEgens are exactly opposite to the commonly used organic luminescent ligands, showing high fluorescence quantum yields in the solid state. As a star molecule in AIE systems, tetraphenylethene, an intriguing chromophore featuring AIE characteristics, has emerged as a popular building block to construct luminescent MOFs. That’s because TPE can be easily decorated with carboxylate 132 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


and pyridine coordination groups, which are thermally stable and water-stable. Up to now, many organic ligands based on TPE have been synthesized, as shown in Figure 2. These organic ligands can couple with metal ions to form a variety of luminescent MOFs, including layer-structured compounds and three-dimensional compounds. Dincã and coworkers synthesized the first MOFs materials based on TPE by the coordination of AIEgen 1 to Zn2+ and Cd2+ ions (Figure 3) (23). After anchoring TPE to metal ions within a rigid matrix, the rotation of the phenyl rings can be restricted, making the materials emit blue light centered at 480 and 455 nm. The Brunauer Emmett Teller (BET) surface areas of the materials are 317 and 244 m2/g, respectively, confirming that the porous structure can accommodate small guest molecules. Although the fluorescence quantum yields are only 1.0 and 1.8% for both compounds, they demonstrate the possibility by using AIE type chromophores to construct coordination assemblies with sustainable porosity. Further works will focus on improving the fluorescence quantum yields and extending their applications.

Figure 3. Portions of the X-ray crystal structures of Zn based MOF depicting (a) side and (b) top views of the two-dimensional sheets, and of Cd based MOF depicting (c) the Cd4 secondary building unit and (d) the truncated three dimensional structure. Reproduced with permission from Reference (23). Copyright 2011 American Chemical Society.



On the basis of previous work, Zhou and coworkers employed new extended TPE-based AIEgen 2 and crystallized it with Zr(IV) to form a robust tetravalent zirconium MOF (Figure 4) (24). The twisted AIEgen 2 conformation of the crystals induced bright blue fluorescence emission at 470 nm. More importantly, the quantum yield of crystals is as high as unity (99.9 ± 0.5%) in the solid state, which is primarily attributed to the immobilization of the AIEgen 2 as it is strongly coordinated to Zr(IV), preventing the torsional relaxation. These rigidifying methodology and high fluorescence quantum yield of crystals making them suitable for potential applications in molecular electronics and/or sensor technologies.

Figure 4. The synthesis of MOF from TPE-based AIEgen 2 and Zr(IV). Insert show the photos of AIEgen 2 and MOF under UV light. Reproduced with permission from Reference (24). Copyright 2011 American Chemical Society. Besides these two MOFs materials, other compounds with similar organic ligands were also synthesized (25). Du and coworkers chose AIEgen 1 and crystallized it with Co(II) to form a compound with a (4,8)-connected scu framework (26). Magnetic studies reveal that this compound is paramagnetic with weaker anti-ferromagnetic coupling compared with normal syn-anti carboxylate magnetic pathway, which is due to the non-planarity of the Co–O–C–O–Co unit. Meanwhile, the same AIEgen 1 can also be incorporated with Zn(II) to form a zinc-based MOF, which maintains its fluorescence up to 350 °C, close to the decomposition temperature of AIEgen 1 (400 °C) (27). Notably, the high temperature fluorescence of this compound enables the selective detection of gaseous ammonia at 100 °C by causing fluorescence shifts in its emission maximum without the interference of ethylenediamine, N,N-diethylformamide, and water vapors. Many other AIEgens 3-6 based on TPE were also synthesized and incorporated with metal ions to form AIE-active MOFs (28, 29). To construct highly porous MOFs for applications, especially for gas storage, Zhou and coworkers designed dendritic AIEgen 3 (30). Then they synthesized robust and porous MOF containing AIEgen 3 and a Cu2-paddlewheel structural motif by using a Zn2-paddlewheel based MOF as a template to prearrange the linkers for the Cu2-based MOF target. The obtained materials show a type I isotherm for N2 134 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


sorption at 77 K and 1 bar, revealing the microporous nature of the framework. The Langmuir surface area and total pore volume of the compound are 2615 m2 g−1 and 0.94 cm3 g−1, respectively, showing excellent gas (H2, CO2) adsorption capacity. Tang and coworkers also reported 2D layered MOFs composed of Zn4O-like secondary building units and AIEgen 6 (31). Compared with the previously reported ligands, AIEgen 6 has two dangling phenyl rings without carboxylate groups that will remain unrestricted even after the formation of MOFs. So the synthesized compound exhibits responsive turn-on fluorescence to various VOCs, such as benzene, toluene, o-xylene, m-xylene, p-xylene, and mesitylene. That’s because the motion of phenyl rings can be restricted through molecular interactions with analytes, leading to responsive turn on emission. Although so many TPE-based luminescent MOFs have been reported, and some of them exhibit sensing abilities toward ammonia or VOCs, their applications in explosives detection are still rarely explored. As is known, many luminescent MOFs are excellent candidates for explosives detection. The pores in luminescent MOFs can promote efficient mass transport, thus enhancing the interactions of explosive molecules with chromophore. In addition, the combination of effective electron and energy transfers can significantly improve the sensitive and selective detection of explosives. As AIEgens are electron-rich molecules, the AIE-based luminescent MOFs can also show excellent performance in explosives detection. Zhao and coworkers prepared TPE-based porous MOF 1 comprised of the AIEgen 2 and Zr6 metal cluster SBUs (Figure 5) (32). Each AIEgen 2 links four parallel arranged zinc carboxylate bridge chain SBUs through shared carboxylate groups, forming a 3D framework with 1D rhombus pores. After undergoing a solvent exchange process with dichloromethane, followed by drying under vacuum at 50 °C, they obtained the activated 1 sample with micro-porosity. It is worth noting that the emission wavelength of the compound varies apparently when exposed to different VOCs, such as benzene, m-xylene, and mesitylene. More importantly, the photoluminescence (PL) of the compound can be quenched greatly when exposed to the vapors of NB and DNT for 1 week. Although most of the AIE-MOFs materials were not used for explosives detection and the SBUs performed not so efficiently in sensing nitroaromatic explosives, they demonstrate their great potential of sensing nitroaromatic explosives, which may inspire more researchers to do these research areas. We introduce the preparation of AIE-MOFs is to make the readers further understand the sensing performance from the structure aspect. Unlike the detection of some well known carbon-based explosives, the demand for a facile and sensitive detection method for five-membered-ring energetic heterocyclic compounds is also urgent. Because five-membered-ring heterocyclic compounds such as NTO is one of the new explosive compounds used in insensitive munitions (IM) developed to replace traditional explosives, TNT. IMs are designed to detonate when fired but not in response to unplanned stimuli, such as mechanical shock and high temperatures (33, 34). Wang and coworkers synthesized AIEgen 7 and made it coordinate with Mg2+, Ni2+, and Co2+ to produce three MOFs compounds (TABD-MOF-1, -2, and -3) with the fluorescent quantum yields of 38.5%, 1.12%, 0.15%, respectively (35). The obvious hypsochromic shifts can be attributed to the stronger ligand-to-metal charge 135 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


transfer (LMCT) effect by rational alteration of the metal ions. Remarkably, the TABD-MOF-3 can selectively sense the powerful explosive NTO with turn-on fluorescence and the minimum amount of NTO detectable by the naked eye is as low as 10 μL of a 1 × 10−6 M solution, corresponding to a visible detection limit of ca. 6.5 ng/cm2 (Figure 6). Furthermore, AT, DAT, HAT, and 5-ATZ, which are source materials for the synthesis of high energy-density materials, can cause remarkable emission turn on. The present new AIE-MOF sensing method shows advantages of universality, high sensitivity, and eases of visualization and may shed light on the development of new probes for turn-on chemo/biosensing applications.

Figure 5. (a) X-ray crystal structure of the MOF 1 with 1D rhombus channels; right: the 1D zinc carboxylate bridge chain. (b) PL spectra of 1, activated 1 and activated 1 with selected guest molecules, excited at 365 nm. (c) PL spectra of activated 1 before and after exposure to the vapors of NB and DNT. Reproduced with permission from Reference (32). Copyright 2015 Royal Society of Chemistry.



Figure 6. (a) Photographs of TABD-MOF-3 deposited paper strips upon addition of THF solution of NTO at different concentrations under UV light. (b) Fluorescence spectra of TABD-MOF-3 in THF upon addition of NTO solution at different concentrations followed by addition of hexane. (c) Fluorescence enhancement efficiencies ((I−I0)/I0) obtained from different analytes by TABD-MOF-3. Excitation wavelength: 360 nm. Reproduced with permission from Reference (35). Copyright 2014 American Chemical Society.

AIEgens-Functionalized POPs for Explosives Detection Porous organic polymers (POPs) are a new generation of porous materials constructed from light elements, such as H, B, C, N, and O, which are linked by strong covalent bonds. So far, many POPs including covalent organic frameworks (COFs), polymers of intrinsic microporosity (PIMs), hyper-cross-linked polymers (HCPs), conjugated microporous polymers (CMPs), and porous aromatic frameworks (PAFs) have been developed (36). The excellent physical and chemical stability, low framework density, and various structural features have made them good candidates for gas storage and separation, catalysis, sensors, and so on (37). It is worth noting that the luminescent POPs have attracted much attention recently due to their promising applications in light harvesting, photocatalysis, sensing, and photovoltaic devices (38, 39). However, manipulation of strong light emission into POPs remains difficult due to the ACQ effect. So far, several POPs based on AIE building blocks have attracted much attention due to their potential applications in chemical sensing and bio-probes, owing to their enhanced emission in the aggregate form or solid state.



Figure 7. Molecular structure of the building blocks with AIE properties used in POPs. Thanks to the efficient preparation, the propeller-like structure of TPE molecule is expected to be a novel building block for the design of porous materials with special properties (Figure 7). In 2011, Han and coworkers synthesized TPE-based POPs through a Suzuki coupling polycondensation and oxidative coupling polymerization (Figure 8) (40). All the POPs exhibit strong photoluminescence properties with the maximum peaks ranged from 530 to 610 nm. Their BET surface areas vary between 472 and 810 m2 g–1 and the pore widths are mainly centered at 0.58 or 0.67 nm, proving the existence of porous structure. This work demonstrates that AIEgens can be promising building blocks for designing porous polymers with special properties.

Figure 8. Preparation of POPs by Suzuki coupling polymerization and oxidative coupling polymerization. Reproduced with permission from Reference (40). Copyright 2011 Royal Society of Chemistry. 138 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 9. Schematic representation of the synthesis of conjugated polymers CMPs with core–shell architecture. Reproduced with permission from Reference (42). Copyright 2011 Royal Society of Chemistry. Since then, Jiang and coworkers developed highly luminescent CMPs with the same AIEgen 8. Due to the crosslinking nature of CMPs, it can suppress the rotation of TPE units, thus allowing for high luminescence in both solution and solid states (41). The absolute fluorescence quantum yield of the CMPs was as high as 40% when the Yamamoto reactions were carried out at 2 h. The microporous structure can be confirmed by high resolution tunneling electron microscopy (HR-TEM) and N2 sorption isotherm measurements. This work suggests that the CMP architecture provides a new platform for the design of highly luminescent materials. In addition, they also developed a core-shell strategy for achieving color-tunable and -controllable light emission while retaining high luminescence efficiency (Figure 9) (42). By fixing the core size and changing the shell thickness, they successfully tuned the light emission from deep blue to sky blue, near white, and green, and the fluorescence quantum yield reaches up to 32%. Many other AIEgens 10-12 based on TPE were also used to synthesize AIE-active POPs (43). Zheng and coworkers developed a straightforward, cost-effective, and environmentally-friendly method to prepare stable organic molecular cages based on AIEgen 10 (44). The compound exhibits a good CO2 uptake capacity of 12.5 wt% and a high selectivity for CO2 over N2 adsorption of 80 with the BET surface area of 432 m2/g. This study opens new opportunities for the development of efficient cage-based porous materials in gas storage, catalysis, and chemosensors. Jiang and coworkers synthesized a hyper-cross-linked CMP via Friedel–Crafts alkylation of AIEgen 11 and/or 1,1,2,2-tetraphenylethane-1,2-diol (TPD) using a formaldehyde dimethyl acetal crosslinker promoted by anhydrous FeCl3 (45). The materials also show high CO2/N2 selectivity with the increased TPD content, suggesting an efficient 139 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


strategy for the design of organic microporous polymers for post-combustion carbon capture. Besides the capability of gas storage, the TPE-based luminescent POPs also show excellent performance for the detection of explosive molecules. In 2010, Tang and coworkers developed a polymer with 3D globular structure, which offered more diffusion channels for the excitons to migrate, allowing them to be quickly annihilated by the PA quenchers (46). When the PA concentration is increased to 0.12 mM, virtually no light is emitted from the polymer nanoaggregates in the 90% aqueous mixture and the static quenching constants (K) can be up to 1.45×105 M–1. The superamplification effect in the explosive detection process can be attributed to the pore structure and the electron donor ability of the fluorescent polymers. As the POPs materials based on the AIEgen possess a 3D topological structure with molecular cavities or voids, allowing the efficient diffusion of explosives and very large band-gap energy with a deep-blue emission. The POPs are thus expected to serve as an excellent fluorescent sensor for the detection of electron-deficient quenchers such as nitroaromatic explosives.

Figure 10. Representative ideal molecular structures for luminescent POPs synthesized by reacted AIEgen 13 with potassium vinyltrifluoroborate. Reproduced with permission from Reference (47). Copyright 2014 Royal Society of Chemistry. Recently, our group reported a novel one-pot synthetic strategy for the straightforward preparation of luminescent POPs based on the palladium catalyzed tandem Suzuki–Heck C–C coupling reactions of AIEgen 13 with potassium vinyltrifluoroborate (Figure 10) (47). Their luminescence can be adjusted from blue to green by selecting the aromatic halides and alternating the ratio of monomers and their BET surface areas change from 318 to 693 cm2 140 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


g–1. It is found that the luminescences of POPs are quenched upon the addition of different analytes such as DNCB, DNT, NT, and PA. Particularly, the POPs exhibit relatively sensitive sensing ability towards PA than other compounds, and the emissions could be quenched quickly upon the addition of PA. The faster rate of fluorescence quenching can be explained by the photo-induced energy transfer quenching mechanism. Other luminescent microporous organic polymers were also developed by our group via the palladium catalyzed Suzuki-Heck cascade coupling reactions of 4-vinylphenylboronic acid with AIEgen 8 (48). The synthesized polymer exhibits strong yellow emission, which shows selective quenching toward PA compared with other nitroaromatic analytes. These excellent performances suggest that the AIEgens functionalized POPs can be used as potential chemical sensor for explosives. Apart from these porous polymers containing only C, H elements, many other kinds of POPs have also been synthesized and are used for the detection of explosives. Octavinylsilsesquioxane was also used as building unit to react with AIEgen 8 to construct a class of luminescent porous inorganic-organic hybrid polymers though the Heck coupling reaction (Figure 11) (49). The BET surface areas can be up to 685 m2 g−1. The polymer shows poor abilities in sensing DNCB, DNT, NT, and NP, but has sensitive quenching behavior toward PA. The overlap of the PA absorption spectrum with the emission spectra of polymer indicates that there exists energy transfer process between them, which can be used to explain the reason of the rapid fluorescence quenching.

Figure 11. Syntheses of luminescent porous inorganic-organic hybrid polymers by the Heck coupling reaction of AIEgen 8 with octavinylsilsesquioxane, and their luminescence in ethanol (0.05 mg mL−1) in the presence of various amounts of PA. Reproduced with permission from Reference (49). Copyright 2015 Royal Society of Chemistry. 141 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Fluorescent cross-linked polymer with strong blue emission was prepared efficiently through a one-step polycondensation AIEgen 14 with cyclophosphazene by Han and coworkers (Figure 12) (50). Its fluorescence can be quenched by both the nitroaromatics TNT and PA significantly. The fluorescence quenching detection to PA is more sensitive than to TNT in the suspension or solid-state. That is because a spectral overlap occurs between the emission of polymer and PA absorption in wavelength ranging from 350 to 480 nm which prompts the energy transfer from the excited state of the polymer to the ground state of PA, resulting in the efficient fluorescence quenching.

Figure 12. Representative conjugated microporous organic polymer constructed from AIEgen 14, and their photos in the absence (a), in the presence of TNT (100 ppm) (b) and PA (50 ppm) (c) under UV light (365 nm) illumination. Reproduced with permission from Reference (50). Copyright 2011 Royal Society of Chemistry.

Hydrogen-bondings have great importance in life, such as in the double helix of DNA and stabilization of secondary structure of protein. Chen and coworkers designed and fabricated a fluorescent hydrogen-bonded organic framework HOF-1111 by using AIEgen 15 as building block (Figure 13) (51). The obtained materials showed high thermal stability and 3D structure, which can be used for sensing of aromatic compounds via a fluorescence quenching and enhancement mechanism. NB showed the highest quenching efficiency of 73%, while the quenching efficiency values of other analytes such as NP, NBO and DNCB were less than 60%. The highest quenching efficiency of NB can be attributed to not only the electron with drawing ability but also the high vapor pressure. Furthermore, fluorescence enhancement was observed when using benzene (BE), toluene (TO), para-xylene (PX), and trimethylbenzene (TM) as the detected objects due to their good electron-donating ability and relatively high vapors pressure. 142 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 13. (a) Local hydrogen bonding environments of AIEgen 15. (b) Quenching efficiency of HOF-1111 exposed to explosives. (c) Fluorescence enhancement of HOF-1111 exposed to aromatic compounds containing electron-donating groups. Reproduced with permission from Reference (51). Copyright 2015 American Chemical Society.

AIEgens-Functionalized Mesoporous Materials for Explosives Detection Mesoporous materials are important porous materials with pore diameters in the range of 2-50 nm. Since they were discovered in 1990s, synthesis and applications of mesoporous materials have received intensive attention because of their highly ordered structures, larger pore size, and high surface areas (52–54). So far, many mesoporous materials have been widely used in various fields such as separation, catalysis, sensors, and devices (18). Chromophore-functionalized mesoporous materials can potentially serve as a new generation of fluorescent chemo-/bio-sensor with remarkable sensitivity (55). That is because the pore size of mesoporous materials is large enough, which can rapidly associate analytes and drugs inside the pores via physical diffusion and/or chemical interaction. Thus the adsorbed molecules in the pores can interact efficiently with fluorophores in/on the pore wall, leading to dramatically enhanced sensing performance. So far, many AIEgens-functionalized mesoporous materials combining the unique properties of mesoporous materials and AIEgens have been developed. Our group developed the first AIEgen-functionalized mesoporous silica by post grafting AIEgen 16 on SBA-15 (56). The synthesized materials combine the unique properties of the AIEgen and porous materials, proving to be an excellent fluorescence probe for potential applications in drug delivery. In addition, we also introduced AIEgen 17 into bioactive hydroxyapatite (HAp) via co-condensation approach to form AIEgen-functionalized HAp (MHAp-FL) (57) (Figure 14). The PL intensity of the materials varies greatly with the loading and release of drugs ibuprofen (IBU), suggesting that the drug release process can be tracked 143 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


in terms of the change of PL intensity (Figure 15). These results suggest that this multifunctional material may be used as an excellent drug carrier in future bioapplications.

Figure 14. Molecular structure of the building blocks with AIE properties used in mesoporous materials.

Figure 15. (a) Fluorescence spectra of MHAp-FL with IBU released at different percentages. (b) The plot of PL intensity as a function of cumulative release amount of IBU. Reproduced with permission from Reference (57). Copyright 2013 Royal Society of Chemistry. AIEgens-functionalized mesoporous materials with other emission colors have also been synthesized for biomedicine. Shi and coworkers synthesized a novel type of green fluorescent mesoporous silica nanoparticles by hybridizing mesoporous silica nanospheres with AIEgen 18 (58); Tian and coworkers fabricated a novel type of folate-functionalized yellow fluorescent mesoporous silica nanoparticles with AIEgen 19 as core (59); Wei and coworkers used the AIEgen similar to 19 and cationic surfactant cetyltrimethyl ammonium as structure-directed template to fabricate uniform yellow luminescent mesoporous silica nanoparticles (60, 61). All these mesoporous materials show excellent performance in cell imaging or drug delivery, suggesting their potential applications in biomedicine. In addition, our group also demonstrated a strategy to integrate AIE and ACQ chromophores in periodic mesoporous organosilicas (PMOs) for high-efficiency multicolor emission (62). The high-quality white light can be obtained by fine tuning of ACQ dyes and AIE-PMOs and the quantum 144 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


yield is up to 49.6%. These high-performance multicolor luminescent materials can be applied in solid-state lighting, biomedicine, and other areas. It is expected that the explosive molecules can transfer into the mesopores more easily and interact with the chromophores, which may lead to dramatically enhanced sensing performance. Our group attempted to apply AIEgen-functionalized mesoporous SBA-15 for the detection of explosives (Figure 16) (63). The PL intensity of materials decreases significantly with the increasing loading amount of PA. The PL quenching can be clearly discerned at a level as low as 1.7 µM or 0.4 ppm and the quenching constants can be up to 2.5 × 105 M–1, much higher than that of TPE itself in the THF/water mixture (1: 9 v v–1) of 3.4 × 104 M–1, as well as those of other linear conjugated polysiloles reported in the literature (2 × 104 M–1) (64). The rapid fluorescence quenching response can be explained by the photo-induced electron transfer and/or energy transfer quenching mechanism. Importantly, this probe is recyclable after washing with proper solvents, thus proving to be a promising candidate for practical explosive detection in an environmentally friendly manner.

Figure 16. Reversible fluorescence quenching mechanism of TPE-functionalized mesoporous materials with PA based on photo-induced electron transfer and/or energy transfer. Reproduced with permission from Reference (63). Copyright 2012 Royal Society of Chemistry. However, the bulk solid materials such as SBA-15 cannot guarantee better dispersibility in water as a result of large size, which may limit their further applications in chemical detection. Mesoporous silica nanoparticles show excellent dispersibility in water solution. So we further investigated the detection capacity of AIEgens-functionalized mesoporous silica nanoparticles with pore diameters of 2.4 nm for explosives in water (Figure 17) (65). The PL intensity decreases significantly with the increasing loading amount of NB, NT and PA, and the detection limits are 0.43, 0.77, and 1.11 ppm, respectively. It is noting that the sensitivity of materials to the explosives show the sequence of NB > NT > PA. That’s because the addition of an electron-donating –CH3 group will decrease the electron-withdrawing ability of the nitro group and the larger molecular size of PA is 7.8 Å, which may limit its quick diffusion into the pores and adsorption around the TPE fluorophores. Whereas the smaller molecules NB (4.6 Å) and NT (4.6 Å) can access to the pores more easily, leading to higher quenching constants. These results suggest that the large pores of the materials play an important role in the explosives detection. We further prepared mesoporous silica nanoparticles with the pore size of 5.7 nm and used them for the sensing of explosive vapors. 145 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 17. Molecular modeling of PA, NT and NB using Materials Studio program. Reproduced with permission from Reference (65). Copyright 2014 Elsevier Ltd. We modified AIEgen 20 to mesoporous silica nanoparticles via carbonnitrogen double bond and then fabricated them into a film by dip-coating method (66). The fluorescence of the film is quenched significantly by nearly 40% in 2 s and 62% in 10 s, respectively, after exposing to DNT vapors (ca. 100 ppb), as shown in Figure 18. The fluorescence quenching almost reach a balance after 30 s with the fluorescence quenching efficiency up to 74%. The large pore size of the nanoparticles is favored for the DNT molecules to be transported into the pores quickly and effectively adsorbed around fluorophores 20 by the formation of a DNT-amine complex via acid-base interaction. The close vicinity between 20 and DNT greatly facilitates the electron transfer process and increases the chemosensory efficiency, thus enhancing the fluorescence quenching efficiency.

Figure 18. (a) Time-dependent fluorescence spectra of AIEgen-functionalized mesoporous silica nanoparticles film in DNT saturated vapor. (b) Time-course of fluorescence quenching efficiency of AIEgen-functionalized mesoporous silica nanoparticles film exposed to DNT vapor; the intensity is monitored at 485 nm. Reproduced with permission from Reference (66). Copyright 2015 Royal Society of Chemistry. 146 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Summary As a class of nonconventional fluorescent materials, AIEgens have aroused great attention because of their emission turn-on nature, instead of quenching. A number of AIE molecules have been used for the fabrication of AIEgens-based porous materials including MOFs, POPs, and mesoporous materials. The obtained materials combined the strong luminescence of the AIEgens and high pore volume and large surface areas of porous materials, providing potential applications in solid-state lighting, biomedicine, gas storage, and chemosensing. Particularly, these AIEgens-functionalized porous materials show sensitive and selective detection of explosives compared to other probing methods. The reasons can be explained that the fluorescent materials have proven to be excellent candidate for the rapid detection of explosives in virtue of the high sensibility, simplicity, short response time, and the ability to be applied in both solution and solid phase. Particularly, the AIEgens are electron-rich molecules, which have Lewis acid-base interactions with electron acceptors of nitroaromatic compounds, improving the capacity of photo-induced electron and/or energy transfer. AIEgens-functionalized porous materials show highly efficient fluorescence in the solid state, avoiding the interference of noise. Furthermore, the large pores and ordered structure of porous materials allow the explosive molecules to enter and interact with the chromophores, leading to high rapid fluorescence quenching response to the explosives. Thus, AIEgens-functionalized porous materials usually have superamplification effect in the detection process, showing a promising application in sensoring of explosives. However, so far most of the AIEgens-functionalized porous materials have been focused on the detection of PA in solution. The highly efficient detection of TNT or other explosives vapors from packed bombs or landmines under real circumstances is of great significance in national defence security. In addition, the fluorescence quenching is still the main method in fluorescence-based explosives detection. Other phenomena, such as the fluorescence enhancement or change of luminescence color in the presence of analytes, would be a more desirable method to improve the sensitivity of detection. It is noting that the fluorescence of the materials may be quenched by other electron-deficient compounds except nitrated explosives. Developing a fluorescent sensor that can selectively detect and differentiate different explosives is also highly desirable. As a kind of novel material, AIEgens-functionalized porous materials provide a good choice to solve these challenges, showing a promising and bright future.

Acknowledgments This work is supported by the State Basic Research Project of China (Grant No: 2014CB931802) and the National Natural Science Foundation of China (Grant Nos.: 21320102001 and 21501063).

References 1.

Toal, S. J.; Trogler, W. C. J. Mater. Chem. 2006, 16, 2871–2883. 147 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

2. 3. 4. 5. 6.


7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

25. 26. 27.

Sun, X. C.; Wang, Y.; Lei, Y. Chem. Soc. Rev. 2015, 44, 8019–8061. Nie, H. R.; Sun, G. N.; Zhang, M.; Baumgarten, M.; Müllen, K. J. Mater. Chem. 2012, 22, 2129–2132. Salinas, Y.; Martínez-Máñez, R.; Marcos, M. D.; Sancenón, F.; Costero, A. M.; Parra, M.; Gil, S. Chem. Soc. Rev. 2012, 41, 1261–1296. Mei, J.; Leung, N. L. C.; Kwok, R. T. K.; Lam, J. W. Y.; Tang, B. Z. Chem. Rev. 2015, 115, 11718–11940. Mei, J.; Hong, Y. N.; Lam, J. W. Y.; Qin, A. J.; Tang, Y. H.; Tang, B. Z. Adv. Mater. 2014, 26, 5429–5479. Chen, J. W.; Cao, Y. Macromol. Rapid Commun. 2007, 28, 1714–1742. Mahtab, F.; Lam, J. W. Y.; Yu, Y.; Liu, J. Z.; Yuan, W. Z.; Lu, P.; Tang, B. Z. Small 2011, 7, 1448–1455. Wang, M.; Zhang, G. X.; Zhang, D. Q.; Zhu, D. B.; Tang, B. Z. J. Mater. Chem. 2010, 20, 1858–1867. Zhang, Y. P.; Li, D. D.; Li, Y.; Yu, J. H. Chem. Sci. 2014, 5, 2710–2716. Li, H. M.; Zhu, Y. X.; Zhang, J. Y.; Chi, Z. G.; Chen, L. P.; Su, C. Y. RSC Adv. 2013, 3, 16340–16344. Liu, Y. J.; Gao, M.; Lam, J. W. Y.; Hu, R. R.; Tang, B. Z. Macromolecules 2014, 47, 4908–4919. Hu, R. R.; Leung, N. L. C.; Tang, B. Z. Chem. Soc. Rev. 2014, 43, 4494–4562. Wang, H.; Zhao, E. G.; Lam, J. W. Y.; Tang, B. Z. Mater. Today 2015, 18, 365–377. Liu, Y. J.; Lam, J. W. Y.; Tang, B. Z. Natl. Sci. Rev. 2015, 2, 493–509. Xu, R. R.; Pang, W. Q.; Yu, J. H.; Huo, Q. S.; Chen, J. S. Chemistry of Zeolites and Related Porous Materials; Wiley-VCH: Weinheim, Germany, 2007. Cui, Y.; Yue, Y.; Qian, G.; Chen, B. Chem. Rev. 2012, 112, 1126–1162. Hoffmann, F.; Cornelius, M.; Morell, J.; Froba, M. Angew. Chem., Int. Ed. 2006, 45, 3216–3251. Wu, D.; Xu, F.; Sun, B.; Fu, R.; He, H.; Matyjaszewski, K. Chem. Rev. 2012, 112, 3959–4015. Long, J. R.; Yaghi, O. M. Chem. Soc. Rev. 2009, 38, 1213–1214. Hu, Z.; Deibert, B. J.; Li, J. Chem. Soc. Rev. 2014, 43, 5815–5840. Zhao, D.; Timmons, D. J.; Yuan, D. Q.; Zhou, H. C. Acc. Chem. Res. 2011, 44, 123–133. Shustova, N. B.; McCarthy, B. D.; Dincă, M. J. Am. Chem. Soc. 2011, 133, 20126–20129. Wei, Z. W.; Gu, Z. Y.; Arvapally, R. K.; Chen, Y. P.; McDougald, R. N., Jr.; Ivy, J. F.; Yakovenko, A. A.; Feng, D. W.; Omary, M. A.; Zhou, H. C. J. Am. Chem. Soc. 2014, 136, 8269–8276. Shustova, N. B.; Ong, T. C.; Cozzolino, A. F.; Michaelis, V. K.; Griffin, R. G.; Dincă, M. J. Am. Chem. Soc. 2012, 134, 15061–15070. Sun, H. L.; Jiang, R.; Li, Z. S.; Dong, Y. Q.; Du, M. CrystEngComm 2013, 15, 1669–1672. Shustova, N. B.; Cozzolino, A. F.; Reineke, S.; Baldo, M.; Dincă, M. J. Am. Chem. Soc. 2013, 135, 13326–13329. 148 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


28. Wang, M.; Zheng, Y. R.; Ghosh, K.; Stang, P. J. J. Am. Chem. Soc. 2010, 132, 6282–6283. 29. Shustova, N. B.; Cozzolino, A. F.; Dincă, M. J. Am. Chem. Soc. 2012, 134, 19596–19599. 30. Wei, Z. W.; Lu, W. G.; Jiang, H. L.; Zhou, H. C. Inorg. Chem. 2013, 52, 1164–1166. 31. Zhang, M.; Feng, G. X.; Song, Z. G.; Zhou, Y. P.; Chao, H. Y.; Yuan, D. Q.; Tan, T. T. Y.; Guo, Z. G.; Hu, Z. G.; Tang, B. Z.; Liu, B.; Zhao, D. J. Am. Chem. Soc. 2014, 136, 7241–7244. 32. Liu, X. G.; Wang, H.; Chen, B.; Zou, Y.; Gu, Z. G.; Zhao, Z. J.; Shen, L. Chem. Commun. 2015, 51, 1677–1680. 33. Deshmukh, M. B.; Wagh, N. D.; Sikder, A. K.; Borse, A. U.; Dalal, D. S. Ind. Eng. Chem. Res. 2014, 53, 19375–19379. 34. Xu, Z. H.; Meng, X. G. Vib. Spectrosc. 2012, 63, 390–395. 35. Guo, Y. X.; Feng, X.; Han, T. Y.; Wang, S.; Lin, Z. G.; Dong, Y. P.; Wang, B. J. Am. Chem. Soc. 2014, 136, 15485–15488. 36. Xu, Y. H.; Jin, S. B.; Xu, H.; Nagai, A.; Jiang, D. L. Chem. Soc. Rev. 2013, 42, 8012–8031. 37. Feng, X.; Ding, X. S.; Jiang, D. L. Chem. Soc. Rev. 2012, 41, 6010–6022. 38. Das, G.; Biswal, B. P.; Kandambeth, S.; Venkatesh, V.; Kaur, G.; Addicoat, M.; Heine, T.; Verma, S.; Banerjee, R. Chem. Sci. 2015, 6, 3931–3939. 39. Zhang, W.; Qiu, L. G.; Yuan, Y. P.; Xie, A. J.; Shen, Y. H.; Zhu, J. F. J. Hazard. Mater. 2012, 221–222, 147–154. 40. Chen, Q.; Wang, J. X.; Yang, F.; Zhou, D.; Bian, N.; Zhang, X. J.; Yan, C. G.; Han, B. H. J. Mater. Chem. 2011, 21, 13554–13560. 41. Xu, Y. H.; Chen, L.; Guo, Z. Q.; Nagai, A.; Jiang, D. L. J. Am. Chem. Soc. 2011, 133, 17622–17625. 42. Xu, Y. H.; Nagai, A.; Jiang, D. L. Chem. Commun. 2013, 49, 1591–1593. 43. Xu, S. Q.; Zhang, X.; Nie, C. B.; Pang, Z. F.; Xu, X. N.; Zhao, X. Chem. Commun. 2015, 51, 16417–16420. 44. Zhang, C.; Wang, Z.; Tan, L. X.; Zhai, T. L.; Wang, S.; Tan, B.; Zheng, Y. S.; Yang, X. L.; Xu, H. B. Angew. Chem., Int. Ed. 2015, 54, 9244–9248. 45. Yao, S. W.; Yang, X.; Yu, M.; Zhang, Y. H.; Jiang, J. X. J. Mater. Chem. A 2014, 2, 8054–8059. 46. Liu, J. Z.; Zhong, Y. C.; Lu, P.; Hong, Y. N.; Lam, J. W. Y.; Faisal, M.; Yu, Y.; Wong, K. S.; Tang, B. Z. Polym. Chem. 2010, 1, 426–429. 47. Sun, L. B.; Zou, Y. C.; Liang, Z. Q.; Yu, J. H.; Xu, R. R. Polym. Chem. 2014, 5, 471–478. 48. Sun, L. B.; Liang, Z. Q.; Yu, J. H. Acta Chim. Sin. 2015, 73, 611–616. 49. Sun, L. B.; Liang, Z. Q.; Yu, J. H. Polym. Chem. 2015, 6, 917–924. 50. Hu, X. M.; Chen, Q.; Zhou, D.; Cao, J.; He, Y. J.; Han, B. H. Polym. Chem. 2011, 2, 1124–1128. 51. Sun, Z. Y.; Li, Y. X.; Chen, L.; Jing, X. B.; Xie, Z. G. Cryst. Growth Des. 2015, 15, 542–545. 52. Wight, A. P.; Davis, M. E. Chem. Rev. 2002, 102, 3589–3614. 149 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


53. Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710–712. 54. Zhao, D. Y.; Feng, J. L.; Huo, Q. S.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548–552. 55. Bae, S. W.; Tan, W.; Hong, J. I. Chem. Commun. 2012, 48, 2270–2282. 56. Li, D. D.; Yu, J. H.; Xu, R. R. Chem. Commun. 2011, 47, 11077–11079. 57. Li, D. D.; Liang, Z. Q.; Chen, J.; Yu, J. H.; Xu, R. R. Dalton Trans. 2013, 42, 9877–9883. 58. Yao, S.; Shao, A. D.; Zhao, W. R.; Zhu, S. J.; Shi, P.; Guo, Z. Q.; Zhu, W. H.; Shi, J. L. RSC Adv. 2014, 4, 58976–58981. 59. Wang, Z. L.; Xu, B.; Zhang, L.; Zhang, J. B.; Ma, T. H.; Zhang, J. B.; Fu, X.; Tian, W. J. Nanoscale 2013, 5, 2065–2072. 60. Zhang, X. Y.; Wang, K.; Liu, M. Y.; Zhang, X. Q.; Tao, L.; Chen, Y. W.; Wei, Y. Nanoscale 2015, 7, 11486–11508. 61. Zhang, X. Y.; Zhang, X. Q.; Wang, S. Q.; Liu, M. Y.; Zhang, Y.; Tao, L.; Wei, Y. ACS Appl. Mater. Inter. 2013, 5, 1943–1947. 62. Li, D. D.; Zhang, Y. P.; Li, Y.; Yu, J. H. Chem. Sci. 2015, 6, 6097–6101. 63. Li, D. D.; Liu, J. Z.; Kwok, R. T. K.; Liang, Z. Q.; Tang, B. Z.; Yu, J. H. Chem. Commun. 2012, 48, 7167–7169. 64. Sanchez, J. C.; DiPasquale, A. G.; Rheingold, A. L.; Trogler, W. C. Chem. Mater. 2007, 19, 6459–6470. 65. Miao, C. L.; Li, D. D.; Zhang, Y. P.; Yu, J. H.; Xu, R. R. Microporous Mesoporous Mater. 2014, 196, 46–50. 66. Li, D. D.; Zhang, Y. P.; Fan, Z. Y.; Yu, J. H. Chem. Commun. 2015, 51, 13830–13833.


AIEgens-Functionalized Porous Materials for Explosives Detection

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