Tb3+ Coordination Polymer Nanostructure: A

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Fluorescent Zn-PDC/Tb3+ Coordination Polymer Nanostructure: A Candidate for Highly Selective Detections of Cefixime Antibiotic and Acetone in Aqueous System Hong Pan, Sufan Wang, Xiaoyao Dao, and Yonghong Ni* College of Chemistry and Materials Science, Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecule-Based Materials, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal University, 1 Beijing Eastern Road, Wuhu, 241000, P. R. China S Supporting Information *

ABSTRACT: Tb3+-doped zinc-based coordination polymer nanospindle bundles (Zn-PDC/Tb3+, or [Zn(2,5-PDC)(H2O)2]·H2O/Tb3+) were synthesized by a simple solution precipitation route at room temperature, employing Zn(NO3)2, Tb(NO3)3, and 2,5-Na2PDC as the initial reactants, and a mixture of water and ethanol with the volume ratio of 10:10 as the solvent. The asobtained nanostructures presented strong fluorescent emission under the excitation of 298 nm light, which was attributed to the characteristic emission of the Tb3+ ion. It was found that the above-mentioned strong fluorescence of the nanostructures could be selectively quenched by cefixime (CFX) in aqueous solution. The other common antibiotics hardly interfered. Thus, asobtained Zn-PDC/Tb3+ nanostructures could be prepared as a highly sensitive fluorescence probe for selective detection of CFX in an aqueous system. The corresponding detection limit reached 72 ppb. The theoretic calculation and UV−vis absorption experiments confirmed that the fluorescence quenching of Zn-PDC/Tb3+ nanostructures toward CFX should be attributed to the electron transfer and the fluorescence inner filter effect between the fluorescent matter and the analyte. In addition, the strong fluorescence of the nanostructures could also be selectively quenched by acetone in the water system. sensing,8 drug delivery,9 and bioimaging.10 Among these CPs, lanthanide-based CPs (Ln-CPs) can generate strong fluorescent signals in the visible region, which endows the ability to chemical sensing through fluorescent quenching or enhancing. In fact, Ln-CPs have been used in the fluorescence sensors because of the virtues of high color purity, large Stokes shifts, high fluorescent quantum efficiencies, and long fluorescent lifetimes.11 To date, however, Ln-CPs used as sensing materials are mainly limited to the detection of metal ions,12 anions,13 organic explosives,14 small molecues,15 and indoor polluting gases.16 Few reports of using Ln-CPs nanostructures as sensing materials for detection of antibiotics are found in the literature.17 Markedly, it is an interesting work to rationally design and synthesize Ln-CPs for host−guest recognition and fluorescence detection. However, owing to high coordination numbers and complex coordination modes of Ln3+ ions, it is difficult to design and construct the desired Ln-CPs through traditional synthesis processes.18 Hence, an alternative strategy of integrating Ln3+ into CPs through the chemical modification is developed.19 Since Zn-MOFs composed of Zn and polycarboxylate ligands usually contain large cavities and

1. INTRODUCTION With the development of economy and the improvement of material conditions, the people have higher demand for their health. Owing to the fear of sickness, some antibiotic drugs are widely used. However, the abuse of antibiotics has brought serious ecological environmental pollution in recent decades. Antibiotic residue has become one of main pollutants in water systems. For example, the total antibiotics usage was 92 700 tons in China in 2013 and eventually 53 800 tons of them was found in the water environment.1 Therefore, the accurate detection of the antibiotics is very significant, but still retains a huge challenge. So far, the detection of antibiotics is mainly based on instrumental methods such as liquid chromatographytandem mass spectrometry (LC-MS),2 liquid chromatography with UV-diode array detection (LC-UV),3 capillary electrophoresis (CE),4 and capillary electrochromatography coupled to mass spectrometry (CEC-MS).5 However, the above methods have similar shortcomings including time-consuming, expensive, and complex operation. Thus, a fast, reliable, and inexpensive technology for the detection of antibiotics is needed urgently. Coordination polymers (CPs), a class of crystalline inorganic−organic materials composed of metal cations (or metal clusters) and organic ligands, have provoked increasing interest for various important applications, including gas adsorption and storage,6 heterogeneous catalysis,7 chemical © XXXX American Chemical Society

Received: November 6, 2017

A

DOI: 10.1021/acs.inorgchem.7b02827 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) The XRD patterns and (b) FTIR spectra of the ligand, Zn-PDC and Zn-PDC/Tb3+ coordination polymers. FTIR-8400S spectrometer by pressing a small amount of sample and KBr crystal into a plate. The TGA of the product was obtained on the differential thermal analyzer (Thermo Electron Company of America, SDTQ600) at a heating rate of 5 K min−1 under nitrogen protection. The ICP of the product was performed on the PerkinElmer Company Optima 7300 DV plasma atomic emission spectrometer. Nitrogen adsorption−desorption isotherms were measured at the liquid nitrogen temperature (77 K), using a Micromeritics ASAP 2460 analyzer. Surface areas were calculated by the Brunauer−Emmett− Teller (BET) method. The UV−vis spectra were monitored on a Metash 6100 UV−vis spectrophotometer. The solid UV−vis diffuse reflection was performed on a Shimadzu UV-2450 spectrophotometer. The cyclic voltammetry measurement was performed on a CHI660D by giving an applied voltage to the working electrode against a saturated calomel electrode in a three-electrode system at room temperature, using 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) solution as the supporting electrolyte. 2.3. PL Measurement. The PL of as-obtained Zn-PDC/Tb3+ nanostructures in various solvents was measured on an FLSP 920 spectrophotometer equipped with a 1 × 1 cm2 quartz cell, using the excitation wavelength of 298 nm, the excitation and emission slit widths of 5 nm, and a photomultiplier tube voltage of 700 V. To evaluate selectivity and sensitivity of as-obtained nanostructures to some common antibiotics, including β-lactams (penicillin, PCL; amoxicillin, ACL; cefixime, CFX; cefradine, CFD), aminoglycoside (gentamicin, GTM; kanamycin, KNM), macrolide (roxithromycin, ROX; azithromycin, AZM), nitrofurans (nitrofurazone, NZF; nitrofurantoin, NTF), and quinolones (ciprofloxacin, CPFX; norfloxacin, NFX), 2 mg mL−1 Zn-PDC/Tb3+ suspension, and 2 × 10−4 mol L−1 antibiotic solutions were separately prepared through dispersing proper amounts of fluorescent nanostructures or dissolving certain amounts of antibiotics into deionized water. During experiments, 1 mL of antibiotic solution was added into 1 mL of fluorescent suspension. The PL spectrum of the system was recorded. As a control, the PL spectrum of the 1 mg mL−1 Zn-PDC/Tb3+ suspension was also recorded. To investigate the influence of the solvent on the fluorescent intensity of Zn-PDC/Tb3+ nanostructures, furthermore, 5.0 mg of ZnPDC/Tb3+ was dispersed into 3 mL of various solvents, including ethanol, methanol, isopropanol, ethanediol, water, acetonitrile (CH3CN), chloroform (CHCl3), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), and acetone, respectively. Then, the fluorescence of the as-obtained system was measured.

provide a platform for postmodification, separation, or sensing, they are good hosts for the integration of Ln3+ ions.15,20 In this work, a simple room-temperature solution route was designed for successful preparation of Tb3+-doped Zn-PDC CPs nanostructures. Experiments showed that the integration of Tb3+ into Zn-PDC CPs hardly changed the morphology of the original CPs nanostructures and endowed strong fluorescence. Also, the fluorescence of as-prepared Zn-PDC/ Tb3+ nanostructures could be selectively quenched by cefixime antibiotic in aqueous solution. Under irradiation of 254 nm UV light, a marked color change from green to colorless could be observed by the naked eye. Hence, the as-obtained Zn-PDC/ Tb3+ nanostructures could be prepared as a highly selective and sensitive fluorescence probe for detection of cefixime antibiotic in a water system. The possible sensing mechanism to cefixime antibiotic was discussed in detail. Furthermore, the investigations also showed that the fluorescence of as-prepared ZnPDC/Tb3+ nanostructures could be affected by some solvents, including water, ethanediol, DMSO, acetone, and so on. Among them, acetone could fully quench the fluorescence, which provides a new approach for detection of acetone in aqueous solution, too.

2. EXPERIMENTAL SECTION All chemical reagents were analytically pure, purchased from Sinopharm Chemical Reagent Co. Ltd., and used without further purification. 2.1. Synthesis of Zn-PDC/Tb3+ CPs Nanostructures. To synthesize Zn-PDC/Tb3+ CPs nanostructures, 0.1 M Zn(NO3)2 and 0.1 M 2,5-Na2PDC aqueous solution were prepared in advance. In a typical experiment, 4 mL of 0.1 M 2,5-Na2PDC solution was added into a mixed solvent of deionized water and ethanol with the volume ratio of 10:10 at room temperature, and then, 3.8 mL of 0.1 M Zn(NO3)2 solution (95% molar ratio) and 6.9 mg of Tb(NO3)3 (5% molar ratio) were introduced under vigorous stirring. After stirring for 30 min, large amounts of white precipitates appeared. The white precipitates were collected by centrifugation, washed 3 times with distilled water and ethanol, respectively, and finally dried in vacuum at 60 °C for 10 h. As controls, Zn-PDC, Zn-PDC/Tb3+(10 atom %), Zn-PDC/ Tb3+(20 atom %), and Tb-PDC CPs nanostructures were also prepared through the above same process in the presence of Tb(NO3)3 with various amounts. 2.2. Characterization. Powder X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD-6000 X-ray diffractometer, employing a scanning rate of 0.02° s−1 and 2θ range from 10° to 50°. Scanning electron microscopy (SEM) images of the final product were taken on a Hitachi S-4800 field emission scanning electron microscope, employing the accelerating voltage of 5 kV. Fourier transform infrared (FT-IR) spectra were obtained on a Shimadzu

3. RESULTS AND DISCUSSION 3.1. Morphology and Structure Characterization. In the present work, white precipitates could be produced via mixing Zn(NO3)2 and 2,5-Na2PDC in the solvent of H2OEtOH with the volume ratio of 10:10 in the absence/presence of Tb(NO3)3. Figure 1a depicts XRD patterns of the products prepared from the systems with/without Tb(NO3)3. Two B

DOI: 10.1021/acs.inorgchem.7b02827 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

the XRD patterns and IR spectra of Zn-PDC/Tb3+ CPs with various amounts of Tb3+ and Tb-PDC CPs under the same conditions. From the XRD patterns shown in Figure S2a, one can clearly find that, with the increase of Tb3+ ion amounts, the peak intensities gradually decrease, implying that integrating more Tb3+ ion amounts can affect the structure of Zn-PDC. The IR spectra of Zn-PDC/Tb3+ with various Tb3+ ion amounts are displayed in Figure S2b. With the increase of Tb3+ ion amounts, two vibration peaks at ∼1588 and ∼3138 cm−1 that belonged to Tb-PDC gradually increase. The above facts prove the XRD result that the structure of Zn-PDC can be changed after more Tb3+ ions are integrated. Generally, when Tb3+ ions are only adsorbed by Zn-PDC CPs, the structure of Zn-PDC should not be varied. Also, Tb3+ ions can be easily removed through repeatedly washing due to the weak adsorption force. However, the present Zn-PDC/Tb3+ CPs always maintained strong PL emission after repeatedly washing under the assistance of ultrasound, implying the strong interaction existence between Tb3+ and Zn-PDC. Combining the XRD results shown in Figures S1 and S2a with IR spectra, thus, Tb3+ ion should be successfully integrated into Zn-PDC. The raised charge should be balanced by NO3− ions since nitrate salts were used in the current work. Further evidence of the successful integration of Tb3+ ion into Zn-PDC comes from the PL emission spectra of the final products. Since Zn-PDC CPs have no fluorescence in the region of visible light, the integration of Tb3+ ion will endow the fluorescent characteristics of Zn-PDC CPs. As shown in Figure 3a, under the excitation of 298 nm light, there are four strong emission peaks in the range from 450 to 650 nm, which come from the 5D4 → 7FJ (J = 6, 5, 4, and 3) characteristic emission of Tb3+.23,24 Among the four peaks, the strongest peak locates at 546 nm, which is attributed to the characteristic emission of 5D4 → 7F5. Figure S3 compares the emission spectra of the present Zn-PDC/Tb3+ and Tb-PDC. They exhibit the same peak situations and outlines except different peak intensities. Also, with the increase of Tb3+ ion amount in the product, the intensity of Zn-PDC/Tb 3+ increases. Furthermore, the above strong green emission can be easily observed by the naked eye under irradiation of 254 nm UV light (see the inset in Figure 3a). Simultaneously, the asprepared Zn-PDC/Tb3+ CPs also present excellent fluorescent stability. Figure 3b shows PL spectra of the Zn-PDC/Tb3+ suspension in water kept for different durations. After 8 days,

products and Zn-PDC CPs reported in ref 21 present the similar diffraction patterns, indicating the successful preparation of Zn-PDC ([Zn(2,5-PDC)(H2O)2·H2O]) CPs under the current experimental conditions. Also, the integration of Tb3+ ions does not vary the phase of Zn-PDC CPs. However, the peaks of the Zn-PDC/Tb3+ are slightly right-shifted about 0.1° compared with those of the pure Zn-PDC (see Figure S1), which should be ascribed to the different ionic radius of Tb3+ (0.92 Å) and Zn2+ (0.74 Å).22 To confirm the successful integration of Tb3+ ions, two products were analyzed by ICP technology. Table S1 lists the results of ICP analyses. Only Zn element is detected in the product prepared from the system without Tb(NO3)3, while Zn and Tb elements are found in the product obtained from the system with Tb(NO3)3. The content of Tb3+ is ∼4.7 atom % of the total content of metal ions, which is close to the feed ratio. Figure 2 exhibits the FESEM images of

Figure 2. FESEM images of Zn-PDC (a) and Zn-PDC/Tb3+ (b).

two products. Both products present spindle-bundle-like nanostructures. The sizes of nanospindles are 300−500 nm in diameter and 1.5−2.0 μm in length. Apparently, the above XRD and FESEM results showed that the integration of Tb3+ ion did not vary the phase and shape of Zn-PDC CPs. Figure 1b shows the FT-IR spectra of 2,5-PDC, Zn-PDC, and Zn-PDC/Tb3+ CPs. The peak at 1699 cm−1 in the 2,5-PDC ligand should be attributed to υCO of the −COOH group. However, this peak disappeared after forming Zn-PDC CPs, which should be caused by the full deprotonation of the −COOH group during the CPs formation. Two pairs of peaks located at 1650, 1606 cm−1 and 1407, 1352 cm−1 in Zn-PDC CPs separately belonged to the stretching vibrations υasCO and υsCO of the carboxylate groups of Zn-PDC.19 It is worthy to point out that Zn-PDC and Zn-PDC/Tb3+ display the similar IR spectra, implying that the encapsulation of 5% Tb3+ ions into Zn-PDC has little effect on the structure of Zn-PDC. Figure S2 displays

Figure 3. (a) Excitation (black line) and emission (green line) spectra of Zn-PDC/Tb3+. The inset is the corresponding photo under irradiation of 254 nm UV light. (b) The fluorescence stability of Zn-PDC/Tb3+ suspension in water under excitation at 298 nm. C

DOI: 10.1021/acs.inorgchem.7b02827 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) The TGA curves of Zn-PDC and Zn-PDC/Tb3+ and (b) N2 sorption−desorption curves of Zn-PDC and Zn-PDC/Tb3+ CPs.

Figure 5. (a) The fluorescence quenching efficiencies of the 5D4→ 7F5 transition of Zn-PDC/Tb3+ in various antibiotic solutions. (b) PL emission spectra of Zn-PDC/Tb3+ in the presence of various concentrations of CFX under excitation at 298 nm UV light. (c) The correlation curve between [(I0/I) − 1] and the concentration of CFX ([Q]) obtained by the Stern−Volmer equation. A linear relation is seen in the low concentration region (see the inset in c). (d) The cycle stability of Zn-PDC/Tb3+ fluorescent probe for the detection of CFX.

the BET surface area and pore volume of Zn-PDC (see Figure 4b). The BET surface area and pore volume decreased from 97.95 m2·g−1 and 0.28 cm3·g−1 of Zn-PDC CPs to 17.09 m2·g−1 and 0.083 cm3·g−1 of Zn-PDC/Tb3+ CPs, which should be ascribed to the substitution of Tb(III) with a bigger ion radius to Zn(II). 3.2. Detection of Antibiotics. It was found that some common antibiotics could cause different fluorescence quenching phenomena of Zn-PDC/Tb3+ CPs nanostructures. Figure 5a displays the fluorescent intensity ratios (I0/I) of ZnPDC/Tb3+ CPs nanostructures in an aqueous system before and after adding various antibiotics. Here, I0 and I separately stand for the fluorescent intensities of Zn-PDC/Tb 3+ nanostructures in the aqueous system before and after adding

the intensities of emission peaks have only slightly decreased, indicating excellent fluorescence stability of Zn-PDC/Tb3+ CPs. It was found that the integration of small amounts of Tb3+ ions into Zn-PDC hardly influenced the thermal stability of ZnPDC CPs. As shown in Figure 4a, the TGA curve of Zn-PDC shows a weight loss of 7% in the range from 20 to 150 °C, which is very close to the theoretic loss of uncoordinated H2O molecules of 6.7%. After 350 °C, a dramatic weight loss can be clearly seen. This fact implies that the present Zn-PDC framework is stable at the temperature below 350 °C. Since the TGA curve of as-obtained Zn-PDC/Tb3+ CPs almost overlaps with that of Zn-PDC CPs, they have the same stability. Nevertheless, the N2 sorption−desorption experiments showed that the integration of Tb3+ into Zn-PDC could strongly affect D

DOI: 10.1021/acs.inorgchem.7b02827 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Various Fluorescent Probes and Their Detection Limits for Certain Antibiotics fluorescent material

antibiotic

Zr-MOF (BUT-12 or BUT-13) Eu-BCA Cd-MOF Tb-AIP [Eu6Zn (μ3-OH)8(NDC)6(H2O)6]n [NaEu2(TATAB)2(DMF)3]OH Zn-PDC/Tb3+

nitrofurazone nitrofuran nitrofuran nitrofuran ronidazole ornidazole cefixime

detection limit/μM 0.17 0.21 0.30 0.35 0.32 0.80 0.14

or 0.46 and 0.16 and 0.55 and 0.30

ref 26 27 28 29 30 31 this work

Figure 6. HOMO(π) and LUMO(π*) energy levels of Zn-PDC/Tb3+ and 12 antibiotics.

certain antibiotics. One can readily find that GTM, KNM, ROX, ACL, AZM, and PCL cannot quench the fluorescence of Zn-PDC/Tb3+ nanostructures, that CFD, NZF, CPFX, NFX, and NTF can slightly quench the fluorescence of nanostructures, and that CFX strongly quenches the fluorescence of the as-obtained nanostructures. The above facts indicate that the present Zn-PDC/Tb3+ nanostructures have potential application as highly selective fluorescence probe for the detection of CFX antibiotic in aqueous solution, which has been confirmed by further investigations. As seen in Figure S4, when a certain interfering substance is added into the system containing ZnPDC/Tb3+, the emission intensity of Zn-PDC/Tb3+ slightly decreases. With the gradual addition of CFX, however, the fluorescence is significantly quenched. The above facts reveal that the interference from other antibiotics can be neglected. This is very favorable for the selective detection of CFX. Figure 5b exhibits the PL spectra of Zn-PDC/Tb3+ in the presence of various concentrations of CFX. With the increase of CFX in the system, the emission intensity of the Zn-PDC/ Tb3+ suspension gradually decreases. When the concentration of CFX reached 13.33 μM, the green fluorescence of the system could not be observed by the naked eye under the irradiation of 254 nm UV light (see the inset in Figure 5b). The fluorescent quenching efficiency can be quantitatively described with the Stern−Volmer (SV) equation

I0/I = 1 + K sv·[Q ]

CFX. On the basis of the value of quenching constant (Ksv) and the standard deviation (SD), the detection limit (LOD) was estimated to be 72 ppb (0.14 μM) using the following equation: LOD = 3SD/Ksv. Compared with some reported results, the present fluorescent probe exhibits a better or close detection limit (see Table 1). Importantly, our research uncovered that the as-obtained ZnPDC/Tb3+ fluorescent probe exhibited a rapid response to CFX. As shown in Figure S5, the emission intensity of ZnPDC/Tb3+ dramatically reduces after adding 1 mL of 2.0 × 10−4 mol L−1 CFX for merely 20 s, and subsequently, the peak intensity slowly decreases. After 4 min, the emission intensity of Zn-PDC/Tb3+ hardly changes. Undoubtedly, this is beneficial to the fast detection of CFX in actual application. Moreover, the present fluorescent probe also exhibited good cycle stability. After 5 cycles, the quenching efficiency only decreased 5% (see Figure 5d). Simultaneously, XRD analyses given in Figure S6 proved that the structure of Zn-PDC/Tb3+ hardly changed, implying the high stability of the as-prepared fluorescent matter. Generally, when the fluorescent matter bears the higher LUMO energy level than the analyte, the electron transfer from the fluorescent matter to the analyte occurs, which is considered to be a reason causing the fluorescence quenching.32 To ascertain the fluorescence quenching mechanism, the LUMO (−2.11 eV) and HOMO (−5.94 eV) energy levels of Zn-PDC/Tb3+ nanostructures were determined by cyclic voltammetry and solid UV−vis diffuse reflection in the present work (see Figure S7). When the LUMO energy level (−2.11 eV) of Zn-PDC/Tb3+ is higher than that of certain antibiotics, the electron transfer between the donor and the acceptor happens, which prevents the excitation electron back to the ground state. Thus, the fluorescence is quenched. Table S2 lists the HOMO and LUMO energy levels of Zn-PDC/Tb3+ and 12 antibiotics according to the ascending order of the energy gap. Compared with other antibiotics, CFX, NZF, and NTF have lower LUMO energy levels than Zn-PDC/Tb3+. The electrons can easily transfer from Zn-PDC/Tb3+ to them (see Figure 6). It is noteworthy that CFX, NZF, and NTF have close LUMO

(1)

where Ksv is the quenching constant (L mol−1), [Q] is the molar concentration of CFX (mol L−1), and I0 and I are the luminescence intensities of the 5D4 → 7F5 transition of Tb3+ at 546 nm before and after adding CFX, respectively. Figure 5c displays the correlation between [(I0/I) − 1] and [Q]. At low concentrations, a nearly linear correlation is visible (R2 = 0.99; see the inset in Figure 5c), and at higher concentrations, the correlation between [(I0/I) − 1] and [Q] is nonlinear. Such a nonlinear phenomenon should be attributed to the combination of the dynamic quenching and the static quenching.25 According to the linear relation, the Zn-PDC/Tb3+ nanostructures have the highest Ksv value of 1.1 × 105 L mol−1 toward E

DOI: 10.1021/acs.inorgchem.7b02827 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. (a) UV−vis absorption spectra of 12 antibiotics with the concentration of 20.0 μM and the normalized excitation spectrum of Zn-PDC/ Tb3+. (b) UV−vis absorption spectra of Zn-PDC/Tb3+ upon addition of different concentrations of CFX.

Table 2. Analytical Results (Mean ± σ, n = 3) for the Determination of CFX in Domestic Water

energy levels, but their quenching phenomena are quite different, suggesting that the electron transfer is not the sole mechanism quenching the luminescence of Zn-PDC/Tb3+. Recently, a fluorescence inner filter effect (IFE) between the fluorescent matter and the analyte is proposed to explain the fluorescence quenching of the fluorescent matter.33 When the excitation and/or emission lights of the fluorescent matter are absorbed by the analyte, the fluorescence IFE happens. Figure 7a displays the absorption spectra of tested antibiotics and the excitation spectrum of Zn-PDC/Tb3+ nanostructures. It is clearly found that the excitation peak of Zn-PDC/Tb3+ has the largest extent of overlap with the absorption peak of CFX against those of the other antibiotics. This indicates that CFX can strongly absorb the excitation light of Zn-PDC/Tb3+. As a result, the luminescence of Zn-PDC/Tb3+ is highly sensitively quenched. To further verify the IFE mechanism, UV−vis absorption titrations were performed with incremental addition of CFX to Zn-PDC/Tb3+. A rapid and significant absorption enhancement was observed upon gradually increasing the amount of CFX (20.0 μM) (Figure 7b). Owing to the absorption enhancement upon the addition of CFX, the excitation light of Zn-PDC/Tb3+ was weakened, leading to the fluorescence quenching. Thus, the ultrasensitive quenching response of Zn-PDC/Tb3+ nanostructures toward CFX should be attributed to the cooperation of the electron transfer and the fluorescence IFE. It is worthy to point out that the as-obtained fluorescent nanostructures can also be prepared into test paper for the qualitative detection of CFX antibiotic in an aqueous system (see Figure S8). 3.3. Detection of CFX in Real Samples. To explore the practical application of the as-prepared fluorescent probe, fresh Yangtze River water (collected in Wuhu, China) and tap water were selected as the cases. Table 2 displays the quantitative spike recoveries of CFX at the concentration range of 2−10 μM for two samples. The recoveries located at the range from 96.7% to 102.5%, and the relative standard deviation (RSD) was less than 6.0%. The above results indicated that the present fluorescent probe could be used for the detection of CFX in real samples. 3.4. Sensing for Acetone. Furthermore, different fluorescent phenomena could also be observed when Zn-PDC/ Tb3+ nanostructures were dispersed into various solvents. Figure 8a exhibits the fluorescent intensity change of the systems formed by dispersing 5.0 mg Zn-PDC/Tb3+ into 3 mL of various solvents, including ethanol, methanol, isopropanol, ethanediol, water, CH3CN, CHCl3, DMF, DMSO, THF, and

sample tap water

Yangtze River water

spiked/μM

measured/μM

0 2 4 6 8 10 0 2 4 6 8 10

not detected 2.05 ± 0.11 3.99 ± 0.14 5.98 ± 0.19 8.16 ± 0.24 9.81 ± 0.36 not detected 2.03 ± 0.12 3.87 ± 0.15 5.95 ± 0.13 8.10 ± 0.21 10.24 ± 0.25

recovery/%

RSD (n = 3, %)

102.5 99.7 99.7 102.0 98.1

5.3 3.5 3.2 2.9 3.7

101.5 96.7 99.2 101.3 102.4

5.9 1.3 2.2 2.6 2.4

acetone. The water system is selected as the reference. The fluorescence intensity of Zn-PDC/Tb3+ increases in the system including ethanol, methanol, isopropanol, CH3CN, CHCl3, DMF, and THF, respectively, and decreases in ethanediol, DMSO, and acetone, respectively. In particular, acetone strongly quenches the fluorescence, which makes it possible for the detection of acetone molecules. As shown in Figure 8b, the fluorescence intensity gradually decreases with increasing the content of acetone and almost disappears when 300 μL of acetone exists in the aqueous system. The correlation of the fluorescent intensity and the content of acetone can be well described with a linear decay (see the inset in Figure 8b). The LOD is estimated to be 55 ppm. The above fluorescence quenching can be explained by the fluorescent IFE between the fluorescent matter and acetone molecules. As shown in Figure S9, acetone has a wide absorption range from 200 to 325 nm, which highly overlaps with the excitation peak of Zn-PDC/ Tb3+ nanostructures. The above large overlap makes it easily compete with the fluorescent matter for the excitation light, which will lead to the fluorescent IFE. Furthermore, as a control, the absorption spectrum of CHCl3 is also exhibited in Figure S9. No absorption appears at the excitation situation of Zn-PDC/Tb3+ nanostructures. Thus, no IFE takes place between the fluorescent CPs and CHCl3 molecules. The fluorescence of Zn-PDC/Tb3+ nanostructures cannot be quenched by CHCl3.

4. CONCLUSION In summary, spindle-bundle-like Zn-PDC/Tb3+ nanostructures have been successfully prepared by a simple precipitation route F

DOI: 10.1021/acs.inorgchem.7b02827 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 8. (a) The emission intensity of the 5D4→ 7F5 transition of Zn-PDC/Tb3+ in different solvents. (b) Emission spectra and linear relationship (inset) of Zn-PDC/Tb3+ in water system with various amounts of acetone under excitation at 298 nm.

ORCID

at room temperature without the assistance of any surfactant or template. The as-obtained nanostructures presented bright green light under irradiation of 254 nm UV light. The above fluorescence could be selectively quenched by CFX antibiotic without interference of other antibiotics. The corresponding detection limit reached 72 ppb. UV−vis experiments showed that the absorption spectrum of CFX had the biggest overlap with the excitation spectrum of the as-obtained fluorescent matter, which is favorable to the absorption of excitation light. The theoretical calculation further confirmed that the fluorescent matter and CFX had the closest HOMO and LUMO energy levels, which is available to the donor−acceptor electron transfer. Moreover, the as-obtained Zn-PDC/Tb3+ nanostructures presented different PL emissions in various solvents. Acetone could dramatically quench the fluorescence, which could also be used for the selective detection of acetone in an aqueous system with the detection limit of 55 ppm. The present work provides a facile route for mild synthesis of luminescent Ln-doped CPs nanostructures with highly selective detection specific antibiotics and organic solvents simultaneously in water systems. This is of significance in monitoring water quality and treating wastewater.

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Yonghong Ni: 0000-0002-4979-9867 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (21571005), the High School Leading Talent Incubation Program of Anhui Province (gxbjZD2016010), the Innovation Foundation of Anhui Normal University (2017xjj104), and The Recruitment Program for Leading Talent Team of Anhui Province for the funding support.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02827. ICP analysis, XRD patterns, FTIR and PL spectra of ZnPDC/Tb 3+ CPs with various amounts of Tb 3+ , interference experiments, time-dependent PL emission spectra of Zn-PDC/Tb3+ after adding 1 mL of 2.0 × 10−4 mol L−1 CFX, XRD patterns of Zn-PDC/Tb3+ crystals before and after detecting cefixime, CVs of Zn-PDC/ Tb3+ in acetonitrile medium, the solid UV−vis spectrum of Zn-PDC/Tb3+, electrochemical data acquired at 100 mV s−1 and HOMO−LUMO gaps determined from spectroscopy of Zn-PDC/Tb3+, the color changes of fluorescent test papers for various concentrations of CFX, and UV−vis absorption spectra of Zn-PDC/Tb3+ and acetone and CHCl3 (PDF)

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DOI: 10.1021/acs.inorgchem.7b02827 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 319−323. (b) Jin, M.; Mou, Z. L.; Zhang, R. L.; Liang, S. S.; Zhang, Z. Q. An efficient ratiometric fluorescence sensor based on metal-organic frameworks and quantum dots for highly selective detection of 6mercaptopurine. Biosens. Bioelectron. 2017, 91, 162−168.

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DOI: 10.1021/acs.inorgchem.7b02827 Inorg. Chem. XXXX, XXX, XXX−XXX