A regenerable zinc(II) coordination polymer as a dual-luminescent sensor for detection of Cr2O72– and 2,4,6-trinitrophenol in aqueous phase

Article abstract of DOI:10.1080/00958972.2018.1536784

Developing a fast and sensitive method for the detection of 2,4,6-trinitrophenol (TNP) or Cr₂O₇^2– in aqueous solution has received great attention. In this work, a regenerable fluorescent coordination polymer [Zn(L)(H

Full text of DOI:10.1080/00958972.2018.1536784

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
A regenerable zinc(II) coordination polymer as a
2-
dual-luminescent sensor for detection of Cr2O7
and 2,4,6-trinitrophenol in aqueous phase
Yu Wu, Yun Huang, Yulan Wang, Xiang Zou, Jun Wang & Weiping Wu
To cite this article: Yu Wu, Yun Huang, Yulan Wang, Xiang Zou, Jun Wang & Weiping Wu
(2018): A regenerable zinc(II) coordination polymer as a dual-luminescent sensor for detection
of Cr2O72- and 2,4,6-trinitrophenol in aqueous phase, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2018.1536784
To link to this article: https://doi.org/10.1080/00958972.2018.1536784
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A regenerable zinc(II) coordination polymer as a dual-luminescent sensor for
2-
detection of Cr2O7 and 2,4,6-trinitrophenol in aqueous phase
YU WU*, YUN HUANG, YULAN WANG, XIANG ZOU, JUN WANG and WEIPING WU
College of Chemistry and Environmental Engineering, Sichuan University of Science & Engineering,
Zigong, 643000, P. R. China
2-
Developing a fast and sensitive method for the detection of 2,4,6-trinitrophenol (TNP) or Cr2O7
in aqueous solution has received great attention. In this work, a regenerable fluorescent
coordination polymer, [Zn(L)(H2L)]n (1), has been constructed using a novel Schiff base ligand,
N'-[(E)-(3,5-dibromo-2-hydroxyphenyl)methylene]isonicotinohydrazide (H2L), and exploited as
a dual detection probe for the selective sensing of Cr2O72- and TNP in water media. Competitive
fluorometric experiments involving mixtures of anions or nitro compounds demonstrated 1 as an
2-
2-
efficient and selective sensor for both Cr2O7 and TNP. The limits of detection for Cr2O7 and
2,4,6-trinitrophenol in aqueous solutions were 11.4 and 1.19 μM, respectively. Moreover, 1 can
2-
detect TNP and Cr2O7 in real water samples. All these features clearly demonstrate its great
potential of practical TNP and Cr2O72- detection in water media for public safety and security.
Keywords:
Fluorescence
sensing;
Coordination
polymer;
2,4,6-Trinitrophenol;
Nicotinoylhydrazone complex
1. Introduction
With the acceleration of industrialization, huge volumes of effluents containing high
concentrations of harmful ions and toxic organic pollutants have been discharged into bodies of
water. Therefore, water contamination and environmental protection have become a global
environmental issue [1-3]. Nitroaromatic compounds (NACs) not only as hazardous substances
pose serious threats to homeland security but also as pollutants have an impact on environmental
*Corresponding author. Email: wuyuhlj@126.com

protection; thus, rapid and sensitive detection of nitroaromatic compounds has been of intense
2-
interest over the past few years [4-6]. Detection of Cr2O7 ion is of vital importance because it
can accumulate in living organisms and cause serious damage, such as cancer, deformity, and
gene mutation to the human health [7]. The U.S. Environmental Protection Agency (EPA) has
2-
listed Cr2O7 as one kind of priority pollutants in the USA [8]. On the other hand, rapid and
selective detection of NACs such as 2,4,6-trinitrophenol (TNP) has also become a challenging
issue in recent times, considering their serious threat towards water pollution and homeland
security. TNP is not only a strong explosive but also used extensively in dyes, fireworks, and the
pharmaceutical and leather industries [9, 10].
Coordination polymers (CPs) are an emerging class of promising materials constructed
by the assembly of metal ions/clusters and organic ligands to generate multidimensional
frameworks [11]. CPs in association with other important fluorescent sensors have undergone
overwhelming development not only for their fascinating structures or topological networks but
also for diverse applications in the areas of heterogeneous catalysis, gas storage, gas separation,
drug delivery, and molecular sensing [12-17]. Currently, some sophisticated instrumental
techniques such as gas chromatography and spectrophotometry could be used to detect
2-
Cr2O7 /NACs accurately but unfortunately with a number of critical drawbacks such as time
consumption, high cost, and requiring complex equipment and trained personnel [18-20].
Comparatively, based on the electron-deficient characteristic of NACs and the photoinduced
electron transfer (PET) quenching mechanism, lanthanide and d10 metal node-based luminescent
coordination polymers (LCPs) have been designed as efficient fluorescent chemosensors for the
detection of NACs in the vapor/liquid phase with good sensitivity and ease of operation [21-28].
2-
A few studies have also involved the sensing and detection of Cr2O7 [29-31]. However,
2-
aqueous-phase detection of Cr2O7 /TNP by LCPs is relatively limited in the literature, perhaps
because of the chemical instability of LCPs in the respective analyte solutions [32-34]. In
furtherance of our successive quest to devise new LCP sensor materials [35, 36], we here report a
regenerable hydrazone-based Zn(II) coordination polymer as a dual detection fluorescent sensor

2-
for selective and sensitive detection of Cr2O7 as well as TNP. Thus, a novel one-dimensional
coordination polymer, [Zn(L)(H2L)]n (1), with excellent thermal and aqueous chemical stability
has been synthesized and fully characterized. The intrinsic luminescence properties of this
2-
hydrazone-based LCP have been exploited for selective sensing of Cr2O7 and TNP ignoring
interferences from among a variety of anions and NACs, respectively. Furthermore, LCP 1 has
been successfully applied to detect Cr2O72- and TNP in environmental water samples.
2. Experimental
2.1. Materials and methods
Most reagent-grade starting materials, salicylaldehyde (98%), hydrobromic acid (40%),
isonicotinic acid hydrazide (98%), zinc nitrate hexahydrate (99%), were purchased from
Shanghai Aladdin Bio-Chem Technology Co., LTD and used without purification. All solvents
were purchased from Chongqing Chuandong Chemical Co., LTD. Elemental analyses were
obtained with an Elementar Vario EL cube analyzer. Powder X-ray diffraction (PXRD)
measurements were collected on a Bruker AVANCE X-ray diffractometer with Cu-Kα radiation
(λ = 1.5418 Å) at 50 kV, 20 mA with a scanning rate of 5°/min and a step size of 0.02°. Fourier
transform infrared (FT-IR) spectra were measured on KBr pellets using a Nicolet Impact 750
FTIR spectrometer from 400−4000 cm-1. Thermogravimetric analysis was performed with an
STA 409PC Luxx Netzsch instrument under N2 atmosphere from room temperature to 950 °C at
a heating rate of 10 °C/min. Photoluminescence (PL) spectra were recorded at room temperature
utilizing an RF-5301PC spectrofluorophotometer. UV–vis spectra were obtained from a
Shimadzu UV-2501PC UV-Vis recording spectrophotometer.
2.2. X-ray crystallography
The single crystal X-ray diffraction data were collected on a Bruker SMART APEX
diffractometer that was equipped with graphite monochromated Mo radiation (λ = 0.71073 Å)
by using an -scan technique. Data reduction included absorption corrections by the multiscan

method using SADABS [37]. The structure was solved and refined with SHELXTL package [38].
All hydrogens were generated geometrically and refined isotropically using the riding model. All
non-hydrogen atoms were refined with anisotropic displacement parameters. Crystallographic
details for 1 are listed in table 1, while selected bond dimensions for 1 are listed in table S1.
CCDC number: 1835430.
2.3. Syntheses
2.3.1. Synthesis of N'-[(E)-(3,5-dibromo-2-hydroxyphenyl)methylene]isonicotinohydrazide
(H2L). An ethanol solution (30 mL) of 3,5-dibromosalicylaldehyde (10 mmol) and one drop of
acetic acid were added with stirring to an ethanol solution (30 mL) of isonicotinic acid hydrazide
(10 mmol). After stirring under reflux for 2 h, a yellow precipitate of H2L was isolated by
filtration and washed with ethanol and ether twice (scheme 1). 1H NMR (600 MHz, DMSO-d6):
12.78 (s, 1H, OH); 12.53 (s, 1H, NH, amide); 8.83 (s, 2H, aromatic); 8.57(s, 1H, –N=CH–,
azomethine); 7.87–7.80 (m, 4H, aromatic). 13C NMR (150 MHz, DMSO-d6): 165.4 (C=O), 160.3
(C-OH), 150.5 (C=N, pyridine), 146.7 (C=N, azomethine), 138.1, 131.2, 123.4, 122.6, 116.7,
113.5. IR (cm-1): 3425(m), 3231(m), 1650(vs), 1514(m), 1341(m), 1237(v), 1176(v), 922(m),
810(m). Elemental analysis: C, 39.13; H, 2.27; N, 10.53. Found: C, 39.48; H, 2.14; N, 10.37.
Scheme 1. Synthesis scheme of 1.
2.3.2. Synthesis of [Zn(L)(H2L)]n (1). A mixture of Zn(NO3)2·6H2O (0.0297 g, 0.1 mmol), H2L
(0.0399 g, 0.1 mmol), DMF (3 mL) and methanol (3 mL) was stirred for 30 min in air. The
resulting solution was placed in a Teflon-lined stainless steel vessel (25 ml) and heated to 105 °C

for 72 h. The solution was then cooled to room temperature at a rate of 5 °C h−1. Yellow block
crystals of 1, {N'-[(E)-(3,5-dibromo-2-hydroxyphenyl)methylene]isonicotinohydrazide-κN}
{N'-[(E)-(3,5-dibromo-2-hydroxyphenyl)methylene]isonicotinohydrazide-κ3O,N,O'}-zinc(II)
were obtained (yield 41% based on Zn). IR (cm-1): 3437(vs); 1662(vs); 1597(vs); 1512(v);
1440(v); 1217(m); 1152(v); 1053(m); 857(m); 699(m). Elemental analysis: C, 36.21; H, 1.86; N,
9.75. Found: C, 36.56; H, 1.89; N, 9.50.
2.4. Sensing method
For the detection of Cr2O72- and TNP, fluorimetric experiments were performed with 1 dispersed
in aqueous solutions of sodium salts (1×10−2 M) or NACs (1×10−3 M). For detection and sensing
2-
-
-
3-
2-
2-
studies, sixteen kinds of NaX (X = CO3 , NO3 , NO2 , F-, PO4 , Ac-, Cl-, Br-, C2O4 , SO3 , I-,
2-
2-
2-
-
2-
WO4 , SO4 , HCO3 , MnO4 and Cr2O7 ) and different NACs, 4-nitrotoluene (4-NT),
2-nitrotoluene (2-NT), 2,4-dinitrotoluene (2,4-DNT), 2,6-dinitrotoluene (2,6-DNT),
1,3-dinitrobenzene (1,3-DNB) and TNP, were chosen. Before detection, 3 mg of 1 was finely
ground and then added to a screw capped vial containing 3 mL of an aqueous solution of
anions/nitroaromatics. The suspension was then ultrasonically agitated for 30 min and used for
tests immediately. The luminescence quenching of each sample was recorded in the range of
400-650 nm upon excitation at 355 nm, and the fluorescence intensity at 476 nm was monitored.
All of the titrations were repeated 3 times to get an average value of the detection results. The
quenching efficiency was calculated as [(I0-I)/I0] × 100%, where I0 and I are the fluorescence
intensities before and after interaction with specific analyte.
3. Results and discussion
3.1. Crystal structure of [Zn(L)(H2L)]n (1)
Single-crystal X-ray diffraction analysis reveals that 1 crystallizes in the monoclinic crystal
system with the P21/c space group and features a one-dimensional chain structure. The results of
crystallographic analysis revealed that the asymmetric unit of 1 contains two independent Zn(II)

ions (Zn1, Zn2), two deprotonated L2- ligands, and two free base H2L ligands. As illustrated in
figure 1a and scheme S1, both Zn1 and Zn2 are five-coordinate; the specific geometries of the
Zn ions were measured according to the τ5 parameter proposed by Addison, for Zn1, τ5 = 0.353;
for Zn2, τ5 = 0.302. Since the τ5 parameter are both closer to 0 than 1, the geometries of Zn1 as
well as Zn2 are distorted square pyramidal. For Zn1, two phenoxo oxygen atoms, O(1) and O(2),
and one imine nitrogen atom N(3) of the deprotonated di-Schiff base with the help of one N(4)
nitrogen atom of the pyridyl ring from an adjacent bridging ligand constitute the equatorial plane,
whereas another nitrogen atom N(10) from the monodentate free base H2L occupied the axial
location. Interestingly, the hydrazone H2L acts as chelating, bridging and monodentate ligand in
the formation of one-dimensional polymeric chains, which is rarely seen in reported literatures.
Similar to Zn1, three ONO-donor pockets (O3, O4, N6) of ligand and N(1) form the equatorial
plane, with the axial position occupied by N(7). In the equatorial plane the Zn-O and Zn-N
distances are similar, being in the range of 2.000(4)-2.085(4) Å, while the axially coordinating
Zn-N bond distances are comparatively longer at 2.106(7) and 2.131(6) Å, respectively. The
deviations of the coordinating atoms, N(1) and N(4), from the mean plane are 1.055 Å and
1.138 Å. It is noteworthy that the salicyloyl-O of the H2L ligand enolizes during complexation
and deprotonates to bond through the carbonylate-O, which is also observed in other reports [39].
It is interesting to compare the structure of LCP 1 with the reported [Zn(o-phen)LCu(OAc)],
which differs in the structure of Schiff base ligand but also forms a five-coordinate square
pyramidal geometry with a N3O2 coordination sphere for the zinc ion [40]. While Zn-N bond
lengths of LCP 1 are a little longer than those of [Zn(o-phen)LCu(OAc)], Zn-O bond lengths and
bond angles [X-Zn-X (X = N, O)] are close to each other. In LCP 1, the Zn(II) ions are infinitely
connected by hydrazone ligands to generate a 1D chain (figure 1b). Furthermore, each hydrazone
ligand serves as a bridging ligand to connect two Zn(II) centers into supramolecular structures
and the two-dimensional supramolecular sheet structure of the complex is constructed by π–π
stacking interactions (3.4700 Å) with a slip-angle of 4.76°, as shown in figure 1c.

3.2. FT-IR and TGA analysis of 1
As the FT-IR spectrum shows (figure S1), υ(O-H) is observed at 3431 cm-1 and υ(N-H) is
observed at 3220 cm-1 in the IR spectrum of CP 1, confirming the existence of uncoordinated OH
and NH groups from the assistant Schiff base ligand. The appearance of a new υ(C-O) band in 1
in the range of 1370-1360 cm-1 definitely suggests bonding of the ligand to the zinc ion through
the phenolate-O. The newly-emerged bands in the 760-700 cm-1 and 570-510 cm-1 ranges have
been tentatively assigned to υ(Zn-O) and υ(Zn-N).
TGA was carried out to ascertain the thermal stability of LCP 1 (figure S2). The TGA
curve shows no obvious weight loss until 410 C, indicating its excellent thermal stability. The
weight loss of the monodentate coordinating H2L molecule is observed in the range of
410-440 C (observed, 44.2%; calculated, 46.3%). Subsequently, the decomposition of the
chelating Schiff base ligand starts and finally the coordination polymer is converted into metal
oxide, estimated to be ZnO (observed, 16.8%; calculated, 18.1%).
3.3. Fluorescence properties
The solid-state luminescence properties of 1 and the free Schiff base were first characterized at
room temperature. As figure S3 revealed, 1 exhibited a maximum emission peak at 485 nm upon
excitation at 355 nm, with a slight blue shift compared to the free Schiff base (λem = 493 nm).
The emission peaks of both can be attributed to π*-π or π*-n transitions. As shown in figure S4,
the luminescence performance of 1 dispersed in commonly used solvents were extensively
examined, including DMF, DMA, DCM, THF, DMSO, acetonitrile, methanol, ethanol, toluene,
1,4-dioxane, acetone and nitrobenzene. The PL spectra of emulsions of 1 in different solvents
exhibited broad peaks with emission maxima ranging from 465 nm to 486 nm, with the highest
luminescence intensity of 1 in the case of water dispersion and the lowest one in the case of
nitrobenzene, i.e., nitrobenzene showed the highest fluorescence quenching. The interactions
between the coordination polymer and organic solvent molecules may be the reason for the
observed fluorescence quenching effect. Upon excitation, energy transfer from the nearly

coplanar Schiff base ligand to the nitrobenzene might take place, resulting in the observed
fluorescence quenching [41-43]. Interestingly, 1 showed the strongest luminescence when
dispersed in water, which inspired us to check its aqueous stability. The aqueous chemical
stability of LCP 1 was investigated as follows: soaking LCP 1 in water at room temperature for
several days, after which the material was centrifuged and removed from the emulsion, washed
thoroughly with distilled water and diethyl ether, and dried under vacuum for PXRD test. As the
PXRD patterns demonstrated (figure S5), the crystalline structure integrity was perfectly
preserved. It is also found that the fluorescence emission from 1 (figure S6) is very stable in
aqueous emulsion as verified by a day-to-day fluorescence measurement, indicating its
wonderful aqueous stability, which in turn confirms the application of LCP 1 as a promising
candidate in the aqueous detection of analytes.
3.4. Fluorescence studies for Cr2O72- detection
Because of the strong luminescence property and excellent chemical stability of 1, we sought to
2-
explore its application in monitoring Cr2O7 ion in water through fluorescence sensing. As
depicted in figure 2a, among the various anions chosen for detection in water by 1, the existence
2−
−
of Cr2O7 leads to almost complete luminescence quenching, and MnO4 makes the
luminescence intensity slightly decrease, while other anions have no obvious effect on the
luminescence intensity. Considering the fact that mixed anions coexisted in the wastewater, the
2−
distinguishing quenching effect between Cr2O7 and other anions encouraged us to investigate
2−
the potential application of 1 to selective sensing Cr2O7 by fluorescence quenching. Thus,
competitive experiments were conducted by addition of 1.5 mL of Cr2O72− (2×10−2 M) to 1.5 mL
of aqueous solutions of other interfering anions (2×10−2 M). As shown in figure 2b, the
2−
fluorescence quenching effect of Cr2O7 on 1 was hardly influenced by other anions. These
results definitely indicate that 1 could be successfully used for selective probing Cr2O72− in water
from among the above-mentioned anionic series.

2−
Moreover, the fluorometric titration of varying concentrations of aqueous Cr2O7
(1−500×10-5 M) against 1 dispersed in water was carried out to test the sensitivity and quenching
2−
efficiency of 1 toward Cr2O7 . As shown in figure 2c, the emission intensity located at 476 nm
gradually decreased with incremental concentration of Cr2O72− aqueous solution. The quenching
efficiency was estimated by fitting the fluorescence intensity ratio versus concentration of
2−
Cr2O7 data to the Stern−Volmer (SV) equation, I0/I = 1 + KSV[Q], where, I0 and I are the
emission intensities of the aqueous suspension of 1 before and after interacting with the aqueous
2−
2−
solution of Cr2O7 , respectively, [Q] is the molar concentration of Cr2O7 , and KSV is the
quenching constant. As demonstrated in figure 2d, the SV plot was nearly linear over the added
Cr2O72− concentration range (0-100×10-5 M), and the calculated quenching constant value is KSV
= 1.897×103 M−1 with a linear fit coefficient value of 0.9907. The limit of detection (LOD) was
calculated to be 1.1417×10-5 M on the basis of 3σ/m [27]. It is noteworthy that aqueous-phase
2−
detection of Cr2O7 by LCPs has rarely been reported. Important reports on aqueous-phase
detection of Cr2O72− by LCPs are listed in table S2. Evidently, the KSV of LCP 1 is comparable to
2−
those of known MOF-based sensors for aqueous Cr2O7 ion detection and LOD value in the
present investigation is also comparable to previous reports.
3.5. Fluorescence studies for TNP detection
The strong luminescence of 1 in water suspension and its noticeable fluorescence quenching
upon exposing to nitrobenzene prompted us to investigate its sensing performance towards
nitroaromatic compounds in aqueous media, which has vital importance in environmental and
security issues. Several electron-deficient NACs were chosen as detection analytes and dispersed
in water with a constant concentration of 5×10-4 M, including 4-nitrotoluene (4-NT),
2-nitrotoluene (2-NT), 2,4-dinitrotoluene (2,4-DNT), 2,6-dinitrotoluene (2,6-DNT),
1,3-dinitrobenzene (1,3-DNB) and TNP. As revealed in figure 3a, 1 exhibited fast and high
luminescence quenching efficiency for TNP and significant quenching of up to 67.5% was
observed, whereas for the other NACs, it showed comparatively negligible quenching effects.

The quenching percentages were calculated using the formula [(I0−I)/I0] × 100% and found
roughly to be in the following order: TNP > 1, 3-DNB > 2,6-DNT ≈ 4-NT > 2,4-DNT > 2-NT. As
high selectivity is crucial for most probes to be applied in real sample detection, the fluorescence
quenching of 1 by TNP in the coexistence of other nitroaromatic compounds were systematically
studied. As figure 3b depicts, no significant changes of fluorescence intensity were observed in
the experiments in which other nitroaromatic compounds were simultaneously added into the
TNP-containing systems, demonstrating the excellent selectivity of 1 towards TNP.
Fluorescence quenching titration with varying concentrations of TNP were conducted
with an excitation wavelength of 355 nm at room temperature. Upon incremental addition of
TNP in the concentration range of 5-50×10-5 M to an aqueous suspension of 1, the observed
fluorescence quenching percentage was 24.4, 34.3, 41.9, 47.9, 52.9, 56.9, 60.4, 63.3, 65.8 and
67.9%, respectively (figure 3c). The photoluminescence quenching efficiency can be
quantitatively calculated according to the SV equation. As shown in figure 3d, TNP displays a
good linear SV relationship (R2 = 0.9976) in the concentration range of 5-50×10-5 M and the
calculated quenching constant value is KSV = 4.111×103 M−1. As seen in table S2, the KSV of 1 is
comparable to those of reported MOF-based sensors. The LOD was calculated to be as low as
1.19 μM on the basis of 3σ/m [27]. As is well known, fast and simple regeneration methods are
crucially important on the recyclable performance of luminescent sensor in terms of application
in real sample detection. Hence, we immersed 1 in an aqueous solution of 10−2 M TNP or
2−
Na2Cr2O7 for 30 min to form inclusions of 1@TNP or 1@Cr2O7 , after which the solid was
separated by centrifugation and washed several times with distilled water and diethyl ether. After
three cycles of repeated fluorescence measurements, the luminescence intensity of the recycled
sample was consistent with the original one, showing the satisfying recyclable performance of 1
2−
(figure 4). Next, the possible mechanism that the introduction of Cr2O7 /TNP decreased
luminescence intensity is discussed. Based on previous literature studies [44], energy loss caused
by the collision between luminescent coordination polymers and analytes during the energy
transferring process usually results in fluorescence quenching of luminescent sensors. The

2−
2−
UV-Vis spectra of Cr2O7 and TNP solutions show absorption at about 370 nm for Cr2O7 and
380 nm for TNP (figure S7). The strongest excitation wavelength of 1 locates at 355 nm; as a
2−
result, TNP and Cr2O7 in the solution can significantly absorb the energy of the excited light,
2−
resulting in luminescence decrease of 1. The collision between the TNP/Cr2O7 analytes and 1
2−
in association with the adsorption of TNP/Cr2O7 in the UV-Vis region reduce energy of the
transmission process and contribute to luminescence quenching.
3.6. Sensing of TNP/Cr2O72− in real water samples
2−
For the sake of investigating the TNP/Cr2O7 sensing behaviors of 1 in real water samples, a
total of eight water samples were collected, including four samples of the same type (tap or river
2−
water). The concentrations of TNP/Cr2O7 found and the recovery percentages in the
2−
environmental water samples are displayed in table 2. The recovery percentages of TNP/Cr2O7
were in the range of 90.85-99.12% for all the samples and the RSD values were in the range of
2.51-5.57 (RSD = (SD/mean value) × 100). The good recovery percentages and low RSD values
obviously imply that the probe can provide adequate recoveries and good analytical precisions
for real water samples.
4. Conclusion
Using
a
newly-synthesized
Schiff
base
ligand,
N'-[(E)-(3,5-dibromo-2-
hydroxyphenyl)methylene]isonicotinohydrazide, one novel luminescent CP 1 has been designed
2−
and synthesized which is well suitable for the selective and sensitive detection of both Cr2O7
and TNP in aqueous solution through fluorescence quenching, and is scarcely affected by other
possible coexistent interfering species. Most importantly, 1 exhibited exceptional thermal and
chemical stability in aqueous solutions, and its fluorescence could be recycled by a fast and
2−
simple method. Notably, the KSV value as well as LOD of 1 towards Cr2O7 and TNP are
comparable to those reported for MOF materials to date. The limits of detection of 1 towards
Cr2O72− and TNP in aqueous solutions can reach as low as 11.4 and 1.19 μM, respectively. Given

the satisfied performance of 1 toward the detection of Cr2O72− and TNP in real water samples, we
2−
believe that our work will trigger substantial research interest in the field of MOF based Cr2O7
and TNP sensors in water media.
Acknowledgements
The authors acknowledge financial assistance from National Natural Science Foundation of
China (Nos. 21501124 and 41603124), the program of Science and Technology Department of
Sichuan Province (Nos. 2016JY0048, 2016GZ0172 and 2017JY0194), the Education Committee
of Sichuan Province (Nos. 18ZB0425 and 18ZA0337), the Start-up Foundation of Sichuan
University of Science & Engineering (No. 2017RCL02), and Student’s Platform for Innovation
and Entrepreneurship Training Program in Sichuan Province (201710622066).
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Figure 1. (a) The coordination environment of Zn(II) in 1; thermal ellipsoids are drawn at the 30%
probability level and hydrogens are omitted for clarity; (b) view of the 1D chain along the a-axis;
(c) view of the 2D layer constructed by π-π interactions from adjacent chains.

Figure 2. (a) Fluorescence response of 1 to various anions in the water; (b) the selective
2-
detection of Cr2O7 on 1 in the presence of interfering anions in the water; (c) emission spectra
2-
of 1 in different concentrations of Cr2O7 anion in the water (excited at 355 nm); (d) the
Stern-Volmer plot of 1 in the presence of different Cr2O72- concentrations.

Figure 3. (a) Fluorescence response of 1 to various nitroaromatics in the water; (b) fluorescence
quenching of 1 in the water upon addition of TNP and other nitroaromatic compounds;
(c) emission spectra of 1 in different concentrations of TNP in the water (excited at 355 nm);
(d) the Stern-Volmer plot of 1 versus different TNP concentrations.

2-
Figure 4. The luminescence intensity of three recycles for 1 towards TNP (a) and Cr2O7 (b),
respectively.

Table 1. Crystal data and structure refinement information for 1.
Formula
C52H32Br8N12O8Zn2
1722.92
Formula weight
Crystal system
Space group
Crystal color
a [Å]
Monoclinic
P21/c
Yellow
15.1794(16)
12.6165(13)
40.257(4)
90.00
b [Å]
c [Å]
α [°]
β [°]
2.724(2)
90.00
γ [°]
V [Å3]
7700.9(14)
4
Z
Dc, g/cm3
µ, mm–1
1.486
4.822
F(000)
3328
θ Range, deg
Reflections collected
Goodness-of-fit on F2
R1, wR2 (I> 2σ(I))*
R1, wR2 (all data)**
2.30-20.23
13552/6157
1.069
0.0627, 0.1293
0.1377, 0.1483
2
2 2 1/2
*R1 = ∑(Fo – Fc)/∑(Fo); **wR2 = {∑[w(Fo – Fc2)2]/∑(Fo ) }

2−
Table 2. Detection performance of 1 for Cr2O7 /TNP in real water samples.
Tap water
Detected
River water
Added values
Samples
Recovery
(%)
Detected
Recovery
(%)
(μM)
valuesa (μM)
18.96±4.53
39.38±5.12
19.07±4.74
38.56±2.51
valuesa (μM)
18.74±5.57
36.34±4.69
19.82±3.42
37.03±5.02
20
40
20
40
94.80
98.46
95.34
96.40
93.68
90.85
99.12
92.58
2−
Cr2O7
TNP
aDetected values ± RSD (n=3)

Graphical abstract