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A luminescent 3D Zn(II)-organic framework showing fast, selective and reversible detection of p-nitrophenol in aqueous media
Journal of Luminescence 180 (2016) 287–291
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
A luminescent 3D Zn(II)-organic framework showing fast, selective and reversible detection of p-nitrophenol in aqueous media Qingfu Zhang n, Mingyuan Lei, Jingjing Zhang, Yang Shi College of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, China
art ic l e i nf o
a b s t r a c t
Article history: Received 5 July 2015 Received in revised form 25 August 2016 Accepted 25 August 2016 Available online 31 August 2016
A luminescent Zn(II)-organic framework, [Zn(PIA)]n 2nDMA (1) [H2PIA ¼ 5-(pyridin-4-yl)isophthalic acid, DMA¼N,N-dimethylacetamide], was solvothermally synthesized and characterized by elemental analysis, IR, TGA, PXRD and single-crystal X-ray crystallography. The structure of 1 features a 3D framework with 1D channels along the a axis, which is constructed from electron-rich π-conjugated PIA2 ligands and paddlewheel [Zn2(CO2)4] secondary building units. The strong luminescent emission at 382 nm of activated 1a could be effectively quenched by trace amounts of p-nitrophenol (p-NP) in water with the limit of detection (LOD) as low as 35.0 ppb, suggesting that 1a is a potential luminescent sensory material for p-NP. & 2016 Elsevier B.V. All rights reserved.
Keywords: Metal-organic framework Crystal structure Luminescence Sensors p-Nitrophenol
1. Introduction Nitrophenols are a type of environmental contaminants used extensively in the production of pesticides, fungicides, dyes, and pharmaceuticals [1]. Most of them can remain in the environment for a long time with high toxicity and carcinogenicity, due to their stability and bioaccumulation. Among them, p-nitrophenol (p-NP) is particularly considered to be one of the priority toxic pollutants by the US Environmental Protection Agency (EPA) [2]. Nowadays, a series of detection methods including capillary electrophoresis, cyclic voltammetry, high performance liquid chromatography and gas chromatography-mass spectrometry have been developed for p-NP detection [3–6], but they all suffer from some obvious disadvantages like tedious sample pretreatment, long analysis time and expensive apparatus. Recently, luminescence-based detection has attracted great attention owing to its quick-response, high selectivity and reversibility [7]. A variety of luminescent materials containing nanoparticles, organics and biological molecules have been employed for detection of p-NP [8–10]. However, the widespread use of these materials is still limited due to their stability, toxicity and sensitivity, thus it is a challenging task to synthesize new materials for luminescence detection of p-NP. Metal-organic frameworks (MOFs), a new class of zeolite-like porous crystalline materials, have demonstrated widely potential applications in gas storage and separation, drug delivery, catalysis n
Corresponding author. E-mail address: [email protected] (Q. Zhang).
http://dx.doi.org/10.1016/j.jlumin.2016.08.055 0022-2313/& 2016 Elsevier B.V. All rights reserved.
and luminescence [11–14]. Recently, several luminescent MOFs have been developed for the detection of cations, anions, small inorganic and organic molecules, as well as biomolecules [15–18]. Most of these luminescence-based sensing reactions of MOFs conducted in organic solution, but less work about in aqueous solution has been reported [19]. To explore in this area, we prepared a 3D Zn (II)-MOF [Zn(PIA)]n 2nDMA (1) [H2PIA¼ 5-(pyridin-4-yl)isophthalic acid, DMA¼N,N-dimethylacetamide], which is built from electronrich π-conjugated PIA2 ligands and paddlewheel [Zn2(CO2)4] secondary building units (SBUs). The activated 1a dispersed in water displays a strong luminescence at 382 nm. Notably, the luminescence of 1a could be fast, selectively and reversibly quenched by pNP with the limit of detection (LOD) as low as 35.0 ppb, indicating 1a could be a promising luminescent sensing material for p-NP. To our knowledge, it is the first MOF-based luminescent sensor for detecting p-NP in aqueous media.
2. Experimental A mixture of Zn(NO3)2 6H2O (29.7 mg, 0.1 mmol), H2PIA (24.3 mg, 0.1 mmol) in 5 mL of DMA/EtOH (v/v ¼ 4:1) was sealed in a Teflon-lined stainless steel vessel and heated at 85 °C for 3 days, and then cooled to room temperature. Colorless block crystals were obtained. Yield: 34.8 mg, 72.4% (based on Zn(II)). Elemental Anal. Calcd. for C21H25N3O6Zn: C, 52.45; H, 5.24; N, 8.74%. Found: C, 52.52; H, 5.16; N, 8.75%. IR (KBr, cm 1): 3398, 1619, 1566, 1447, 1366, 1289, 1237, 1076, 1030, 840, 776, 734, 653, 580.
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Fig. 1. (a) Coordination environment of Zn(II) in 1. (b) The simplified [Zn2(CO2)4] SBU in 6-connected node and the simplified PIA2 ligand in 3-connected node. (c) The 3D framework showing 1D channels along the a axis. (d) Schematic representation of rtl topology.
Fig. 2. (a) The TGA curves of 1 and activated 1a under nitrogen atmosphere. (b) The PXRD patterns for 1, activated 1a, and the samples of 1a after exposure to various analytes.
3. Results and discussion Complex 1 crystallizes in the monoclinic space group P21/c. Each asymmetric unit of 1 consists of one Zn(II) ion, one PIA2 ligand and two lattice DMA molecules. The central Zn(II) ion is five-coordinated by four carboxyl oxygen atoms and one pyridyl nitrogen atom from five different PIA2 ligands in a squarepyramidal geometry (Fig. 1(a)). Both Zn–O/Zn–N bond lengths are in the normal range. A pair of symmetry-related Zn(II) ions are connected by four bidentate bridging carboxylate groups from four different PIA2 ligands to generate a paddlewheel [Zn2(CO2)4] SBU (Zn Zn separation of 2.99 Å), in which the axial positions are occupied by two pyridyl nitrogen atoms from another two
different PIA2 ligands (Fig. 1(b)). The adjacent 6-connected [Zn2(CO2)4] SBUs are further linked by 3-connected PIA2 ligands to form a non-interpenetrated 3D framework structure with rtl topology (Fig. 1(c) and (d)). The 1D nanotubular channels (9.4 9.2 Å2) were observed along the a axis, and the solventaccessible volume of 1 is about 54.6% per unit cell after removal of solvent molecules as calculated by PLATON [20]. The thermogravimetric analysis (TGA) curve of 1 shows an initial weight loss of 36.56% (calculated: 36.24%) in the range of 110–240 °C probably attributed to loss of two lattice DMA molecules. No further weight loss was observed below 390 °C, indicating a high thermal stability. Upon further heating, the framework began to decompose (Fig. 2(a)). The powder X-ray
Q. Zhang et al. / Journal of Luminescence 180 (2016) 287–291
289
Fig. 3. (a) Quenching efficiency of luminescence intensity for 1a dispersed in water upon addition of different analytes. (b) Fluorescence titration of 1a by gradual addition of p-NP in water. (c) The SV plot for p-NP. (d) Recyclability of 1a for p-NP detection.
Table 1 The comparison of different analytic methods for p-NP detection. Method
Linear range (μM)
LOD (ppb)
Reversibility Reference
Capillary electrophoresis Cyclic voltammetry High performance liquid chromatography Gas chromatography-mass spectrometry Luminescent method with MOFs’ sensory material
3.31–414 100–700 –
380 1391 35
– – –
[3] [4] [5]
–
2.4
–
[6]
5.04–47.5
35
47 cycles
This work
diffraction (PXRD) patterns of as-synthesized samples match well the simulated ones, indicating the good phase purity of 1 (Fig. 2 (b)). In addition, both TGA and PXRD data of the activated 1a (under vacuum at 80 °C overnight after soaking in methanol for 2 days) indicate the framework retains its integrality after removal of solvent molecules (Fig. 2). The luminescence spectrum of 1a dispersed in water exhibits strong emission at 382 nm upon excitation at 320 nm (see Fig. S1). The luminescence quantum yield (Φ) of 1a is 17%, measured by an integrating sphere, which is comparable to those reported for other Zn-MOFs [21–24]. Interestingly, different analytes have different effects on the luminescence intensity of the 1a dispersed in water. The non-nitroaromatics such as benzene, toluene, ethylbenzene (EB), xylene (o-, m- and p-), chlorobenzene (CB), 1,2-dichlorobenzene (1,2-diCB), bromobenzene (BB), and phenol basically do not affect the luminescence intensity, whereas the nitroaromatics, especially for p-NP, has a significant luminescence quenching effect
(Fig. 3(a)). The results demonstrate that 1a has high selectivity for p-NP compared to other analytes. The PXRD patterns of 1a show that the compound remains stable, even after exposure to different analytes (Fig. 2(b)). To further investigate the sensing capability for p-NP, fluorescence-quenching titrations were performed by gradual addition of p-NP into 1a dispersed in water. The luminescence quenching effect was close to 90% (Φ o 1%) when the concentration of p-NP was increased up to 200 mM (Fig. 3(b)). The quenching efficiency was further investigated by the Stern–Volmer equation (SV plot), F0/F¼ KSV[A] þ1, where F0 and F are the luminescence intensities of 1a without and with addition of analyte respectively, [A] is the molar concentration of analyte and KSV is the quenching constant of the sensing material. The calculated KSV (4.0 104 M 1) for p-NP is much higher than that of 1,3-DNB, 2,4DNT and TNT, demonstrating highly quenching ability of p-NP towards 1a (Fig. 3(c) and S2–S4). The luminescence intensity of 1a decreases linearly with increase of p-NP concentration in the range of 0.7–6.6 mg/mL with the limit of detection (LOD ¼ 3σ/K) of 35.0 ng/mL (35.0 ppb) (see Fig. S5), which falls below the permissible level in drinking water (60 ppb) established by the US EPA [25]. Moreover, it is noteworthy that 1a could be regenerated and reused by centrifuging the dispersed solution after sensing and washing several times with water. The almost regaining the initial luminescence intensity over seven cycles implied a high photostability of 1a for its long-time infield nitroaromatics detection application (Fig. 3(d)). The comparison of the method presented in this work with some other methodologies (Table 1), shows that the LOD of this method is not the lowest but the good reversibility make it a new and important approach in the determination of p-NP.
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Fig. 4. (a) Time-resolved fluorescence decay curves of 1a (λex ¼320 nm, λem ¼ 382 nm) in the absence (red) and presence (blue) of p-NP (200 μM), and the lamp profile is indicative with black dotted line. (b) Spectral overlap between absorption spectra of different analytes and emission spectrum of 1a. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
To explore the quenching mechanism, the luminescence lifetime at 382 nm of 1a were measured respectively in the absence and presence of p-NP. As shown in Fig. 4(a) and Table S3, the luminescence lifetime of 1a remains almost unchanged after addition of p-NP, suggesting that the static quenching plays an important role. According to the former reports [26–28], KSV ¼kq τ, where kq is the quenching rate constant and τ is the luminescence lifetime without an added quencher. The quenching rate constant calculated is 6.6 1012 M 1 s 1 (τ ¼ 6.06 ns, Table S3). This value is higher than the diffusion-controlled limit under these conditions, and it again confirms the static nature of luminescence quenching [28,29]. The spectral overlap between emission spectra of 1a and absorption spectra of p-NP confirms the energy transfer (Fig. 4(b)). The effectiveness of energy transfer greatly depends on the extent of spectral overlap between the emission band of fluorophore and the absorption band of analyte. The result shows the absorption spectrum of p-NP has a higher extent of overlap with emission spectrum of 1a than the other nitroaromatics, which is well consistent with the observed highest quenching efficiency for p-NP. Moreover, the hydroxyl group on para-position of nitro-group in p-NP offers the potential to make hydroxyl groups to interact with the framework through hydrogen bonding, which may promote the energy transfer between the pNP and the framework and result in a much higher quenching efficiency [30,31]. Additionally, it is speculated that the selective sensing of p-NP may be possibly related to the pore confinement of analyte inside the channel of 1a. The larger sizes of 1,3-DNB, 2,4-DNT and TNT (kinetic diameters 47 Å [32]) make them take a relatively long time to diffuse into the channels, but the p-NP with smaller size (kinetic diameter ca. 6.72 Å [33]) may more easily enter the channels so as to show the highest quenching effects.
4. Conclusions In conclusion, a luminescent 3D Zn(II)-MOF, [Zn(PIA)]n 2nDMA (1), has been synthesized by the solvothermal method with the electron-rich π-conjugated ligand. The activated 1a dispersed in water exhibits a strong luminescence emission, which can be efficiently quenched upon addition of p-NP with the LOD as low as 35.0 ppb. The quick response, high selectivity and good reversibility to p-NP show that 1a could be used as an efficient luminescence sensor for detecting p-NP. The quenching mechanism is considered to be attributed to static quenching and energy transfer mechanisms, hydrogen-bonding and pore confinement of the analyte inside the channel.
Acknowledgments This work was supported by the National Nature Science Foundation of China (21001061), the Natural Science Foundation of Shandong Province (ZR2014BQ034), and the National Training Programs of Innovation and Entrepreneurship for Undergraduates (201410447004).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2016.08.055.
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
A luminescent 3D Zn(II)-organic framework showing fast, selective and reversible detection of p-nitrophenol in aqueous media Qingfu Zhang n, Mingyuan Lei, Jingjing Zhang, Yang Shi College of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, China
art ic l e i nf o
a b s t r a c t
Article history: Received 5 July 2015 Received in revised form 25 August 2016 Accepted 25 August 2016 Available online 31 August 2016
A luminescent Zn(II)-organic framework, [Zn(PIA)]n 2nDMA (1) [H2PIA ¼ 5-(pyridin-4-yl)isophthalic acid, DMA¼N,N-dimethylacetamide], was solvothermally synthesized and characterized by elemental analysis, IR, TGA, PXRD and single-crystal X-ray crystallography. The structure of 1 features a 3D framework with 1D channels along the a axis, which is constructed from electron-rich π-conjugated PIA2 ligands and paddlewheel [Zn2(CO2)4] secondary building units. The strong luminescent emission at 382 nm of activated 1a could be effectively quenched by trace amounts of p-nitrophenol (p-NP) in water with the limit of detection (LOD) as low as 35.0 ppb, suggesting that 1a is a potential luminescent sensory material for p-NP. & 2016 Elsevier B.V. All rights reserved.
Keywords: Metal-organic framework Crystal structure Luminescence Sensors p-Nitrophenol
1. Introduction Nitrophenols are a type of environmental contaminants used extensively in the production of pesticides, fungicides, dyes, and pharmaceuticals [1]. Most of them can remain in the environment for a long time with high toxicity and carcinogenicity, due to their stability and bioaccumulation. Among them, p-nitrophenol (p-NP) is particularly considered to be one of the priority toxic pollutants by the US Environmental Protection Agency (EPA) [2]. Nowadays, a series of detection methods including capillary electrophoresis, cyclic voltammetry, high performance liquid chromatography and gas chromatography-mass spectrometry have been developed for p-NP detection [3–6], but they all suffer from some obvious disadvantages like tedious sample pretreatment, long analysis time and expensive apparatus. Recently, luminescence-based detection has attracted great attention owing to its quick-response, high selectivity and reversibility [7]. A variety of luminescent materials containing nanoparticles, organics and biological molecules have been employed for detection of p-NP [8–10]. However, the widespread use of these materials is still limited due to their stability, toxicity and sensitivity, thus it is a challenging task to synthesize new materials for luminescence detection of p-NP. Metal-organic frameworks (MOFs), a new class of zeolite-like porous crystalline materials, have demonstrated widely potential applications in gas storage and separation, drug delivery, catalysis n
Corresponding author. E-mail address: [email protected] (Q. Zhang).
http://dx.doi.org/10.1016/j.jlumin.2016.08.055 0022-2313/& 2016 Elsevier B.V. All rights reserved.
and luminescence [11–14]. Recently, several luminescent MOFs have been developed for the detection of cations, anions, small inorganic and organic molecules, as well as biomolecules [15–18]. Most of these luminescence-based sensing reactions of MOFs conducted in organic solution, but less work about in aqueous solution has been reported [19]. To explore in this area, we prepared a 3D Zn (II)-MOF [Zn(PIA)]n 2nDMA (1) [H2PIA¼ 5-(pyridin-4-yl)isophthalic acid, DMA¼N,N-dimethylacetamide], which is built from electronrich π-conjugated PIA2 ligands and paddlewheel [Zn2(CO2)4] secondary building units (SBUs). The activated 1a dispersed in water displays a strong luminescence at 382 nm. Notably, the luminescence of 1a could be fast, selectively and reversibly quenched by pNP with the limit of detection (LOD) as low as 35.0 ppb, indicating 1a could be a promising luminescent sensing material for p-NP. To our knowledge, it is the first MOF-based luminescent sensor for detecting p-NP in aqueous media.
2. Experimental A mixture of Zn(NO3)2 6H2O (29.7 mg, 0.1 mmol), H2PIA (24.3 mg, 0.1 mmol) in 5 mL of DMA/EtOH (v/v ¼ 4:1) was sealed in a Teflon-lined stainless steel vessel and heated at 85 °C for 3 days, and then cooled to room temperature. Colorless block crystals were obtained. Yield: 34.8 mg, 72.4% (based on Zn(II)). Elemental Anal. Calcd. for C21H25N3O6Zn: C, 52.45; H, 5.24; N, 8.74%. Found: C, 52.52; H, 5.16; N, 8.75%. IR (KBr, cm 1): 3398, 1619, 1566, 1447, 1366, 1289, 1237, 1076, 1030, 840, 776, 734, 653, 580.
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Fig. 1. (a) Coordination environment of Zn(II) in 1. (b) The simplified [Zn2(CO2)4] SBU in 6-connected node and the simplified PIA2 ligand in 3-connected node. (c) The 3D framework showing 1D channels along the a axis. (d) Schematic representation of rtl topology.
Fig. 2. (a) The TGA curves of 1 and activated 1a under nitrogen atmosphere. (b) The PXRD patterns for 1, activated 1a, and the samples of 1a after exposure to various analytes.
3. Results and discussion Complex 1 crystallizes in the monoclinic space group P21/c. Each asymmetric unit of 1 consists of one Zn(II) ion, one PIA2 ligand and two lattice DMA molecules. The central Zn(II) ion is five-coordinated by four carboxyl oxygen atoms and one pyridyl nitrogen atom from five different PIA2 ligands in a squarepyramidal geometry (Fig. 1(a)). Both Zn–O/Zn–N bond lengths are in the normal range. A pair of symmetry-related Zn(II) ions are connected by four bidentate bridging carboxylate groups from four different PIA2 ligands to generate a paddlewheel [Zn2(CO2)4] SBU (Zn Zn separation of 2.99 Å), in which the axial positions are occupied by two pyridyl nitrogen atoms from another two
different PIA2 ligands (Fig. 1(b)). The adjacent 6-connected [Zn2(CO2)4] SBUs are further linked by 3-connected PIA2 ligands to form a non-interpenetrated 3D framework structure with rtl topology (Fig. 1(c) and (d)). The 1D nanotubular channels (9.4 9.2 Å2) were observed along the a axis, and the solventaccessible volume of 1 is about 54.6% per unit cell after removal of solvent molecules as calculated by PLATON [20]. The thermogravimetric analysis (TGA) curve of 1 shows an initial weight loss of 36.56% (calculated: 36.24%) in the range of 110–240 °C probably attributed to loss of two lattice DMA molecules. No further weight loss was observed below 390 °C, indicating a high thermal stability. Upon further heating, the framework began to decompose (Fig. 2(a)). The powder X-ray
Q. Zhang et al. / Journal of Luminescence 180 (2016) 287–291
289
Fig. 3. (a) Quenching efficiency of luminescence intensity for 1a dispersed in water upon addition of different analytes. (b) Fluorescence titration of 1a by gradual addition of p-NP in water. (c) The SV plot for p-NP. (d) Recyclability of 1a for p-NP detection.
Table 1 The comparison of different analytic methods for p-NP detection. Method
Linear range (μM)
LOD (ppb)
Reversibility Reference
Capillary electrophoresis Cyclic voltammetry High performance liquid chromatography Gas chromatography-mass spectrometry Luminescent method with MOFs’ sensory material
3.31–414 100–700 –
380 1391 35
– – –
[3] [4] [5]
–
2.4
–
[6]
5.04–47.5
35
47 cycles
This work
diffraction (PXRD) patterns of as-synthesized samples match well the simulated ones, indicating the good phase purity of 1 (Fig. 2 (b)). In addition, both TGA and PXRD data of the activated 1a (under vacuum at 80 °C overnight after soaking in methanol for 2 days) indicate the framework retains its integrality after removal of solvent molecules (Fig. 2). The luminescence spectrum of 1a dispersed in water exhibits strong emission at 382 nm upon excitation at 320 nm (see Fig. S1). The luminescence quantum yield (Φ) of 1a is 17%, measured by an integrating sphere, which is comparable to those reported for other Zn-MOFs [21–24]. Interestingly, different analytes have different effects on the luminescence intensity of the 1a dispersed in water. The non-nitroaromatics such as benzene, toluene, ethylbenzene (EB), xylene (o-, m- and p-), chlorobenzene (CB), 1,2-dichlorobenzene (1,2-diCB), bromobenzene (BB), and phenol basically do not affect the luminescence intensity, whereas the nitroaromatics, especially for p-NP, has a significant luminescence quenching effect
(Fig. 3(a)). The results demonstrate that 1a has high selectivity for p-NP compared to other analytes. The PXRD patterns of 1a show that the compound remains stable, even after exposure to different analytes (Fig. 2(b)). To further investigate the sensing capability for p-NP, fluorescence-quenching titrations were performed by gradual addition of p-NP into 1a dispersed in water. The luminescence quenching effect was close to 90% (Φ o 1%) when the concentration of p-NP was increased up to 200 mM (Fig. 3(b)). The quenching efficiency was further investigated by the Stern–Volmer equation (SV plot), F0/F¼ KSV[A] þ1, where F0 and F are the luminescence intensities of 1a without and with addition of analyte respectively, [A] is the molar concentration of analyte and KSV is the quenching constant of the sensing material. The calculated KSV (4.0 104 M 1) for p-NP is much higher than that of 1,3-DNB, 2,4DNT and TNT, demonstrating highly quenching ability of p-NP towards 1a (Fig. 3(c) and S2–S4). The luminescence intensity of 1a decreases linearly with increase of p-NP concentration in the range of 0.7–6.6 mg/mL with the limit of detection (LOD ¼ 3σ/K) of 35.0 ng/mL (35.0 ppb) (see Fig. S5), which falls below the permissible level in drinking water (60 ppb) established by the US EPA [25]. Moreover, it is noteworthy that 1a could be regenerated and reused by centrifuging the dispersed solution after sensing and washing several times with water. The almost regaining the initial luminescence intensity over seven cycles implied a high photostability of 1a for its long-time infield nitroaromatics detection application (Fig. 3(d)). The comparison of the method presented in this work with some other methodologies (Table 1), shows that the LOD of this method is not the lowest but the good reversibility make it a new and important approach in the determination of p-NP.
290
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Fig. 4. (a) Time-resolved fluorescence decay curves of 1a (λex ¼320 nm, λem ¼ 382 nm) in the absence (red) and presence (blue) of p-NP (200 μM), and the lamp profile is indicative with black dotted line. (b) Spectral overlap between absorption spectra of different analytes and emission spectrum of 1a. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
To explore the quenching mechanism, the luminescence lifetime at 382 nm of 1a were measured respectively in the absence and presence of p-NP. As shown in Fig. 4(a) and Table S3, the luminescence lifetime of 1a remains almost unchanged after addition of p-NP, suggesting that the static quenching plays an important role. According to the former reports [26–28], KSV ¼kq τ, where kq is the quenching rate constant and τ is the luminescence lifetime without an added quencher. The quenching rate constant calculated is 6.6 1012 M 1 s 1 (τ ¼ 6.06 ns, Table S3). This value is higher than the diffusion-controlled limit under these conditions, and it again confirms the static nature of luminescence quenching [28,29]. The spectral overlap between emission spectra of 1a and absorption spectra of p-NP confirms the energy transfer (Fig. 4(b)). The effectiveness of energy transfer greatly depends on the extent of spectral overlap between the emission band of fluorophore and the absorption band of analyte. The result shows the absorption spectrum of p-NP has a higher extent of overlap with emission spectrum of 1a than the other nitroaromatics, which is well consistent with the observed highest quenching efficiency for p-NP. Moreover, the hydroxyl group on para-position of nitro-group in p-NP offers the potential to make hydroxyl groups to interact with the framework through hydrogen bonding, which may promote the energy transfer between the pNP and the framework and result in a much higher quenching efficiency [30,31]. Additionally, it is speculated that the selective sensing of p-NP may be possibly related to the pore confinement of analyte inside the channel of 1a. The larger sizes of 1,3-DNB, 2,4-DNT and TNT (kinetic diameters 47 Å [32]) make them take a relatively long time to diffuse into the channels, but the p-NP with smaller size (kinetic diameter ca. 6.72 Å [33]) may more easily enter the channels so as to show the highest quenching effects.
4. Conclusions In conclusion, a luminescent 3D Zn(II)-MOF, [Zn(PIA)]n 2nDMA (1), has been synthesized by the solvothermal method with the electron-rich π-conjugated ligand. The activated 1a dispersed in water exhibits a strong luminescence emission, which can be efficiently quenched upon addition of p-NP with the LOD as low as 35.0 ppb. The quick response, high selectivity and good reversibility to p-NP show that 1a could be used as an efficient luminescence sensor for detecting p-NP. The quenching mechanism is considered to be attributed to static quenching and energy transfer mechanisms, hydrogen-bonding and pore confinement of the analyte inside the channel.
Acknowledgments This work was supported by the National Nature Science Foundation of China (21001061), the Natural Science Foundation of Shandong Province (ZR2014BQ034), and the National Training Programs of Innovation and Entrepreneurship for Undergraduates (201410447004).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2016.08.055.
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