Coordination Polymer Nanoarchitecture for Nitroaromatic Sensing

Article pubs.acs.org/JPCC

Coordination Polymer Nanoarchitecture for Nitroaromatic Sensing by Static Quenching Mechanism Yue Li, Kun Liu, Wen-Juan Li, An Guo, Fang-Yao Zhao, Huan Liu, and Wen-Juan Ruan* Department of Chemistry and Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: A nanoscale coordination-polymer (CP) architecture with a π-conjugated backbone (Zn-ata) was synthesized in this work. In aqueous media, this material exhibited a specific fluorescence response to nitroaromatics (detection limits of 0.2−1.3 ppm) but not to nitroaliphatics, despite their similar electron affinities. Interestingly, the order of sensitivities of Znata to different nitroaromatics was completely distinct from thermodynamic expectations. Through a Stern−Volmer analysis of time-resolved and steady-state emission data, as well as measurements of the adsorption amounts, we confirmed that the sensing process occurs through a static quenching pathway. In this mechanism, the preassociation process, instead of the subsequent electron transfer, determines the quenching efficiency. The selective adsorption of nitroaromatics on Zn-ata gives Zn-ata the ability to discriminate between nitroaromatics and nitroaliphatics and enhances its response sensitivity. To the best of our knowledge, this work is the first to demonstrate the dominant role of static quenching in a CP-based sensing system.

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unselective responses to all tested nitro compounds.12−18 Some studies have indicated that the quenching process occurs through a dynamic pathway,13,14 so the response sensitivity is determined by the feasibility of the electron-transfer process. Indeed, correlations between response sensitivity and electron affinity have been observed by different groups.14−16 On the other hand, a few CP sensors have been designed on a completely different energy-transfer mechanism.19−21,24 With this mechanism, the sensitivity of these sensors is determined by the spectral overlap between CP emission and analyte absorption; that is, the greater the spectral overlap, the higher the probability of energy transfer and the higher the efficiency of quenching. Because the absorption spectrum is characteristic for a chemical, these sensors usually exhibit a selective fluorescence response to a specific analyte. For example, Ghosh and co-workers have reported a series of energytransfer-based CP sensors that can discriminate 2,4,6trinitrophenol from other nitro compounds.19−21 As part of our study of CP-based chemical sensors,15,25−27 in this work, we used the fluorescent ligand 2,3,6,7-anthracenetetracarboxylic acid (H4ata) and d10-electron-configured Zn ion (to minimize the interference with ligand emission) to construct a CP architecture (Zn-ata) with a nanoscale particle size and strong fluorescence emission. The fluorescence response of this CP material is specific only to nitroaromatics

INTRODUCTION As an emerging class of materials with great chemical and structural diversity, coordination polymers (CPs) have potential applications in various fields.1−5 In particular, fluorescence sensing, which is based on the selective quenching of the luminescence of a CP by an analyte, shows great promise because of its advantages of sensitivity, operability, and reusability. Over the past few years, the CP sensors for the detection of a wide range of chemicals, including organic molecules, cations, anions, and biomolecules, have been reported.6−11 In particular, motivated by the pioneering work of Li and co-workers,12 a series of CP sensors have been developed for nitro explosives,13−21 which are extremely important in homeland security and environmental monitoring. Aside from composition and coordination structure, the particle size of a CP can also affect its sensing ability. Scaling down the particle size of a CP to the micro-/nanoscale would facilitate its dispersion in sample media and its contact with analytes, thus improve its sensing performance. For example, Xu et al. reported a nanoscale CP sensor that exhibited sensitive responses to nitro explosives with detection limits as low as 1.5 ppm in ethanol media.22 Most CP-based sensors work through an excited-state electron-transfer mechanism.13−18,23 In this mechanism, the excited electron of CP transfers to a quencher, instead of undergoing a transition to the ground state with fluorescence emission. Because of the strong electron-withdrawing ability of the nitro group, the energy demand of this electron-transfer process is easily satisfied, and these sensors usually exhibit © XXXX American Chemical Society

Received: August 24, 2015 Revised: December 1, 2015

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(Figure 1a). All of the diffraction peaks in the pattern can be readily indexed to the reported bulk phase of {[Zn-

but not to nitroaliphatics, and its order of sensitivity to different nitroaromatics is completely distinct from thermodynamic expectations. These phenomena can be well explained by a static quenching mechanism, which has not previously been observed in CP-based sensing systems. In this mechanism, the CP material also works as a concentrator and recognizer of the analyte, which largely enhances the response sensitivity and selectivity. This work also indicates that the preassociation of analyte on the sensor is important for the detection process.

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EXPERIMENTAL SECTION Materials and General Methods. All chemicals were obtained commercially and used as received unless stated otherwise. The 2,3,6,7-anthracenetetracarboxylic acid (H4ata) ligand was prepared according to the reported procedure.28 Scanning electron microscopy (SEM) images were recorded on a JEOL JSM-7500F scanning electron microscope. Transmission electron microscopy (TEM) images were obtained with a JEM-2010FEF transmission electron microscope operating at 200 kV. Powder X-ray diffraction (PXRD) was performed on a Rigaku D/max-2500 diffractometer with Cu Kα radiation (λ = 0.15406 nm) at 40 kV and 100 mA. Elemental analysis (C, H, and N) was carried out with a Perkin-Elmer 240C analyzer. Thermogravimetric analysis (TGA) was performed on a Rigaku standard thermogravimetry-differential thermal analysis (TG-DTA) instrument from ambient temperature to 700 °C at a heating rate of 10 °C min−1 in air, and an empty Al 2 O 3 crucible was used as the reference. N 2 adsorption−desorption isotherms were measured on a VSorb2800P surface area and pore size analyzer. Steady-state fluorescence experiments were performed on a Varian Cary Eclipse fluorescence spectrometer [λex = 290 nm, bandwidth (excitation) = 5 nm, bandwidth (emission) = 2.5 nm] equipped with a Varian Cary single-cell Peltier accessory to control the temperature at 25 °C. Fluorescence lifetimes were measured with an Edinburgh Analytical Instruments FLS920 spectrometer employing the time-correlated single-photon-counting technique with a time resolution of 0.5 ns, and 1000 counts were collected for each measurement. Lifetimes were determined by the deconvolution of the accumulated signals with a nonlinear least-squares fit. Synthesis of Zn-ata. In a typical procedure, 0.1 mmol of H4ata and 0.1 mL of pyridine were dissolved in 50 mL of a mixed solvent of dimethylformamide (DMF)/H2O (4:1, v/v). After the addition of 0.4 mmol of Zn(NO3)2·6H2O, this mixture was heated in an oil bath at 120 °C for 0.5 h. The obtained yellow precipitate was collected, washed with DMF and ethanol in sequence, and dried at 60 °C in air for 6 h. Fluorescence Measurements. The as-prepared sample of Zn-ata (10 mg) was immersed in 200 mL of a mixed solvent of H2O and ethanol (9:1, v/v), ultrasonicated for 1 h, and then stirred for another 2 h to form a stable suspension. Three milliliters of the suspension was used in each experiment and placed in an optical quartz cuvette. After the addition of analyte, the suspension was subjected to steady-state and time-resolved fluorescence measurements.

Figure 1. (a) PXRD pattern of Zn-ata. (b) Crystal structure of {[Zn(ata)0.5(DMF)(H2O)]·3DMF·3H2O}n. (c) SEM and (d) TEM images of an as-prepared Zn-ata sample.

(ata)0.5(DMF)(H2O)]·3DMF·3H2O}n (Figure 1b),29 indicating that our product exhibited the same framework structure. Further evidence supporting the structure of {[Zn(ata)0.5(DMF)(H2O)]·3DMF·3H2O}n was obtained from FTIR, elemental, and thermogravimetric analyses. Figure S1 (Supporting Information) shows the FT-IR spectrum of Znata. The broad band with the maximum at 3423 cm−1 is typical for H2O molecules with H bonds. The peak at 1635 cm−1 can be assigned to the stretching vibration of the CO bonds of DMF. The carboxyl group gives the peak at 1597 cm−1, indicating that the H4ata ligand was deprotonated to coordinate with metal ions. Elemental analysis gave the contents of C, H, and N as 42.47%, 4.33% and 6.14%, respectively, in good agreement with the theoretical values of 42.62% C, 5.00% H, and 6.63% N. As shown by the TGA curve (Figure S2, Supporting Information), the overall weight loss of Zn-ata from room temperature to 700 °C was 73.7%, close to the calculated value of 80.7%. Because this CP does not have a channel structure (Figures S3 and S4, Supporting Information) and only the external surface can come into contact with the analyte, the extension of the surface area is essential to its sensing application. The morphology of Zn-ata was observed by SEM and TEM. Figure 1c shows a typical SEM image of the as-prepared sample. Zn-ata has a flower-like structure that is made up of irregular nanosheets with thicknesses of 100−150 nm and lengths of 3−4 μm. The TEM image of an individual flower-like assembly (Figure 1d) also confirms the results of the SEM observations. This complicated morphology ensures the large surface area of the obtained sample. Brunauer−Emmett−Teller (BET) measurements gave the specific surface area as 52.8 m2 g−1, quite large for a CP material without a channel structure. Because the anthracene ring in the ligand of Zn-ata is a good fluorophore, this CP material is promising in terms of photoactivity. The photoluminescence properties of Zn-ata were examined with an aqueous suspension of the as-prepared sample (with no further activation process) (Figure 2). Upon excitation at 290 nm, Zn-ata gave a strong emission band with a

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RESULTS AND DISCUSSION Zn-ata was obtained as a yellow precipitate from the solvothermal reaction between H4ata and Zn(NO3)2 in the mixed solvent of DMF/H2O (4:1, v/v). The coordination structure of the prepared Zn-ata sample was analyzed by PXRD B

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(THF)], ketones (acetone), esters (ethyl acetate), amides (DMF), nitriles (acetonitrile), sulfoxides [dimethyl sulfoxide (DMSO)], alkanes (n-hexane, cyclohexane), chloroalkanes (CH2Cl2, CHCl3), and other aromatic compounds (benzene, toluene, chlorobenzene, anisole, and phenylmethanol), caused negligible change in the emission of the CP. The high selectivity of Zn-ata to nitroaromatics was further validated by competition experiments, in which its fluorescence response to NT was nearly unperturbed by the presence of other organics (Figure S5, Supporting Information). We also tested the emission change of Zn-ata upon the addition of nitroaliphatics, including nitromethane (NM), nitroethane (NE), and 2nitropropane (NP) (also shown in Figure 3), because most CPs and other material-based explosive sensors respond equally to nitroaromatic and nitroaliphatic compounds.12,17,18,31 Interestingly, in our sensing systems, these nitroaliphatic compounds could not effectively quench the photoluminescence of Zn-ata (intensity changes of NB > DNT > DNB. For example, a quenching percentage of 91% was obtained with 50 ppm of NT, whereas the same amounts of NB, DNT, and DNB could decrease the emission intensity by only 68%, 63%, and 35%, respectively. The distinction in the response sensitivity was further validated by fluorescence titration experiments, which were carried out with a batch of Zn-ata suspension with gradually increased analyte concentration (Figure 4). From the linear fitting of the fluorescence data in the low-concentration range (insets of Figure 4), the detection limits (3σ/k, where σ is the standard deviation of fluorescence measurements and k is the slope of the fitting line) for NT, NB, DNT, and DNB were calculated to be 0.2, 0.5, 0.6, and 1.3 ppm, respectively, showing that the sensitivity of Zn-ata is among the best of the reported CP-based explosive sensors (see Table S1 in the Supporting Information for a literature survey). The superior sensitivity of Zn-ata might be due to its static quenching mechanism, in which the preassociation process helps to enrich the analyte around the sensor (discussed below). Additionally, the measured detection limits are much lower than the reported lowest observed adverse effect levels (LOAELs) for long-period oral exposure to these nitroaromatics (8−625 ppm; Table S2, Supporting Information),32−36 supporting the potential application of Zn-ata in the security detection of nitroaromatics. As introduced above, both the electron and energy transfer of the excited state could cause the fluorescence quenching of CP. Comparison of the absorption spectra of nitroaromatic analytes with the emission position of Zn-ata excludes the energytransfer mechanism (Figure S6, Supporting Information). These nitroaromatics exhibit absorption only below 380 nm, whereas the emission band of Zn-ata starts at 390 nm, which means that the energy released from the relaxation of the excited state of Zn-ata is not sufficient to excite nitoaromatic molecules. Therefore, the fluorescence response of Zn-ata can only be ascribed to an electron-transfer mechanism. To validate this mechanism, we calculated the frontier orbitals of the tested nitroaromatics. As shown in Figure 5, the energies of the lowest unoccupied molecular orbitals (LUMOs) of these compounds are much lower than that of ata4−. Because the excitation of Znata is ligand-centered, this comparison indicates that electron injection from the excited CP to nitroaromatics is thermody-

Figure 2. Emission spectra of Zn-ata (0.05 g L−1) and free H4ata ligand (0.05 g L−1) in aqueous media with excitation at 290 nm.

maximum at 453 nm. For comparison, we also tested the emission of the free H4ata ligand. An emission band peaking at 458 nm was observed. The similarity between the emission wavelengths of the CP and the free ligand shows that the photoluminescence of Zn-ata is derived from ligand-centered electronic excitation and emission. It is noteworthy that the emission intensity of Zn-ata is much stronger than that of free H 4ata ligand (IZn‑ata /I H 4ata = 5.6). This enhanced CP luminescence is a typical example of aggregation-induced emission (AIE), which is caused by the restricted deformation of a ligand in a coordination framework.30 The strong emission of Zn-ata in aqueous media is highly desirable for a practicable fluorescence sensor. The fluorescence responses of Zn-ata to organics were measured directly in its aqueous suspension (10% ethanol was added to ensure the solubility of some of the analytes). As shown in Figure 3, a series of nitroaromatics, including nitrobenzene (NB), 4-nitrotoluene (NT), 1,3-dintrobenzene (DNB), and 2,4-dinitrotoluene (DNT), could markedly quench the photoluminescence of Zn-ata. In contrast, the addition of the same amount of other organics, including alcohols (methanol, ethanol), ethers [ethyl ether, tetrahydrofuran

Figure 3. Quenching percentages [(1 − I/I0) × 100%, where I0 and I are the emission intensities before and after analyte addition, respectively] of different organics (50 ppm) on the emission of a suspension of Zn-ata (0.05 g L−1) in H2O/ethanol (9:1, v/v) media. Inset: Emission spectra after addition of the organics. C

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Figure 4. Emission intensities of a suspension of Zn-ata (0.05 g L−1) upon the addition of (a) NT, (b) NB, (c) DNT, and (d) DNB (λex = 290 nm, λem = 453 nm). Insets: Linear fits of the emission intensities in the low-concentration range.

ΔG 0* = E D0 − EA 0 − ΔE0,0

(1)

where ED0 is the oxidation potential of the donor, EA0 is the reduction potential of the acceptor, and ΔE0,0 is the excitedstate energy. The negative values of the free energy changes confirm the energetic feasibility of excited-state electron transfer. Considering the short range of electron transfer and the nonporous structure of Zn-ata, we expect that, in the quenching process, the excitons generated in the bulk of the CP first migrate to the surface of the particle and are then captured by the analyte. The complete quenching (>95%) observed in fluorescence titration experiments at high analyte concentrations confirms that nearly all of the excitons can migrate to the CP surface in their lifetime. It is expected that the particle size of Zn-ata is already minimized to a scale shorter than the diffusion length of excitons in this work, which ensures the sensitivity of the fluorescence response. However, the calculated frontier orbitals and reduction potentials cannot explain the observed sensitivity order of different nitroaromatic analytes. These calculations indicate that the electron affinities of the tested nitroaromatics are in the order DNB > DNT > NB > NT, which is consistent with the strong electron-withdrawing ability of the nitro group and the weak electron-donating property of the methyl group. As a result, excited-state electron transfer from CP to dinitroaromatics is energetically more favorable than that to mononitrosubstituted aromatics. In contrast, in our fluorescence sensing experiments, NT and NB exhibited higher quenching efficiencies than DNT and DNB; that is, the response sensitivity order was completely different from the expectations based on electron affinity.

Figure 5. Frontier orbital levels of the tested nitroaromatic and nitroaliphatic compounds, calculated at the B3LYP/6-311++G(2df,2p)//B3LYP/6-31+G(d) level with the polarized continuum model (PCM) of solvation.

namically possible. This result was also corroborated by calculations of the potential according to Fu et al.’s method.37 As summarized in Table S3 (Supporting Information), the standard reduction potentials [E0 vs normal hydrogen electrode (NHE)] of nitroaromatics in the aqueous phase range from −0.33 to −0.52 V. By the same method, the oxidation potential of the ground state of ata4− is calculated to be 0.76 V. The excited-state energy of the ligand compared to the ground state is estimated to be 2.71 eV based on the emission wavelength of the CP (453 nm). With these results, the free energy changes (ΔG0* values) associated with the excited-state electron transfers from Zn-ata to NT, NB, DNT, and DNB were calculated to be −1.43, − 1.49, − 1.51, and −1.62 eV, respectively, according to the Rehm−Weller equation38 D

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Figure 6. Steady-state (I0c/Ic) and time-resolved (τ0/τ) Stern−Volmer plots for (a) NT, (b) NB, (c) DNT, and (d) DNB. The measured emission intensities were corrected for the competitive absorption of the analyte at the excitation wavelength. Insets: Linear fits of I0c/Ic in the lowconcentration range.

I0 c/I c = 1 + KSV[Q]

To explain this discrepancy, the quenching process was investigated by the transient fluorescence method (Figure S7, Supporting Information). The time-resolved and steady-state emission data were analyzed with Stern−Volmer plots (Figure 6). Before this analysis, a correction was applied to the measured emission intensities for the competitive absorption of the analyte at the excitation wavelength with the reported method39 Ic = I ×

where KSV is the Stern−Volmer constant. Although both static quenching and dynamic quenching give the same form of Stern−Volmer equation, the meanings of their KSV parameters are different. The KSV of static quenching is the binding constant between the analyte and sensor, whereas in the dynamic pathway, KSV is given by

KSV = kqτ0

A T(1 − 10−A 0) −AT

A 0(1 − 10

)

(3)

(2)

(4)

where kq is the quenching rate constant. In our sensing systems, the KSV values were fitted to be in the range of 0.010−0.053 ppm−1 [(1.7−7.3) × 103 M−1] for the tested nitroaromatics. If the quenching were by a dynamic pathway, given the measured τ0 value of 22.56 ns, the kq values of these analytes would be as high as (0.8−3.2) × 1011 M−1 s−1, far exceeding the value achievable at the diffusion limit (∼1010 M−1 s−1). This comparison excludes dynamic quenching and further corroborates that the fluorescence response is by a static pathway. At higher analyte concentrations, the I0c/Ic plots display marked upward curvature. Similar deviations have frequently been observed in conjugated-polymer-based detection systems in which static quenching is the dominant pathway.40 In these systems, the superlinear behavior was generally explained in terms of quencher-mediated aggregation41,42 or a sphere-ofaction mechanism (the analyte molecules within the distance of charge transfer can cause additional quenching).43,44 Considering the electrical neutrality of nitroaromatic analytes and the stable emission of Zn-ata suspensions observed even at high analyte concentrations, the superlinear response in our systems is not likely to be caused by the aggregation mechanism. On the other hand, the I0c/Ic plots in the whole concentration range can be well described (R2 = 0.9970−0.9995) by a modified Stern−Volmer equation with a sphere-of-action correction43,44

where A0 and AT are the absorbances of the sensor suspension before and after analyte addition, respectively. For all four tested nitroaromatics, the emission transients of Zn-ata at different quencher concentrations can be satisfactorily fitted to single-exponential kinetics. The fitting of these transients can give the emission lifetimes, which are informative for the kinetics of the quenching process: Static quenching would exhibit a constant emission lifetime at different quencher concentrations, whereas dynamic quenching (purely dynamic pathway or the combination of static and dynamic quenching) would be characterized by changes in the emission lifetime. In our sensing systems, as shown by the horizontal τ0/τ plots (where τ0 and τ are the fluorescence lifetimes in the absence and presence, respectively, of quencher), the emission lifetime of Zn-ata was hardly changed (less than ±2%) by the addition of nitroaromatics. These results indicate that the sensing process occurs through a static quenching pathway. The analysis of I0c/Ic plots also indicates a static quenching pathway. At low concentrations, the values of I0c/Ic display a linear correlation with quencher concentration (insets of Figure 6), showing that the quenching process follows the Stern− Volmer relationship of E

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to enhance its interaction with an analyte should be given primary attention in the design of new CP sensors.

(5)

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where α is the enhancement of the local quencher concentration and V is the volume constant. Based on the above discussion, we attribute the superlinear response observed at high analyte concentrations to a sphere-of-action mechanism. In the static quenching mechanism, the preassociation process, instead of the subsequent electron transfer, determines the quenching efficiency. To corroborate this assumption, we tested the adsorption of nitro compounds on Zn-ata. In these experiments, a given amount of Zn-ata was dispersed in the H2O/ethanol (9:1, v/v) solution of a nitro compound (50 ppm), and the suspension was stirred for 12 h to establish adsorption−desorption equilibrium. The analyte concentrations before and after the stirring process were measured by high-performance liquid chromatography (HPLC) to determine the amounts adsorbed. As shown in Figure 7, all of the

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications Web site at DOI: . The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b08259. FT-IR spectrum, TGA curve, BJH pore-size distribution, and space-filling representation of Zn-ata; results of competition experiments and transient fluorescence measurements; absorption spectra; calculated reduction potentials and reported LOAELs of nitroaromatics; detection limits of recently reported CP-based nitroaromatic sensors (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-22-2350-1717. Fax: +86-22-2350-2458. Author Contributions

Y.L. and K.L. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21307062, 20671053) and the Tianjin Research Program of Application Foundation and Advanced Technology (14JCQNJC08000).

Figure 7. Adsorption of nitroaromatics and nitroaliphatics on Zn-ata.

nitroaromatics exhibited high adsorption on Zn-ata, and the amounts adsorbed were in the order NT > NB > DNT > DNB, the same as the sensitivity order in fluorescence sensing experiments. In contrast, the adsorption amounts of NM, NE and NP were negligible, which is consistent with their low quenching efficiencies, although excited-state electron transfer from Zn-ata to these compounds is also thermodynamically possible (Figure 5 and Table S2, Supporting Information). These results confirm that the response sensitivity is determined by the preassociation process. It is expected that the π−π interaction between the π-conjugated backbone of Znata and the benzene rings of nitroaromatics ensures the selectivity to this type of compound.45 The large surface area of Zn-ata would enhance the adsorption of nitroaromatics and thus enhance the response sensitivity.

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REFERENCES

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CONCLUSIONS In summary, we have synthesized a luminescent nanoscale CP architecture and applied it to the detection of nitroaromatics in aqueous media. Investigations of the mechanism showed that this CP sensor works by a rare static quenching mechanism. To the best of our knowledge, this work is the first to demonstrate the dominant role of static quenching in a CP-based sensing system. Based on this mechanism, the fluorescence quenching process is determined by the preassociation of the analyte on the sensor, so the nitroaromatics exhibited unexpected relative sensitivities in our sensing system. Additionally, the preassociation also contributes to the recognition and enrichment of the analyte, which endows Zn-ata with the ability to discriminate between nitroaromatics and nitroaliphatics and to achieve a response sensitivity exceeding the diffusion limit. Our results also indicate that the reasonable selection of the CP backbone F

DOI: 10.1021/acs.jpcc.5b08259 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

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DOI: 10.1021/acs.jpcc.5b08259 J. Phys. Chem. C XXXX, XXX, XXX−XXX