High-efficiency fluorescent probe constructed by triazine polycarboxylic acid for detecting nitro compounds

Article abstract of DOI:10.1016/j.ica.2020.119591

By means of hydrothermal synthesis, a series of isomorphic lanthanide rare earth complexes [Ln₂(TTHA)(H₂O)₄]·7H₂O (Ln = Sm(1), Eu(2), Gd(3), Tb(4) and Dy(5)) were constructed by triazine polycarboxylic acid ligand (H₆TTHA). They were characterized by elemental analysis, IR, TG, PXRD, UV–vis, etc. In particular, the structure of complex 1 was analyzed as a representative, and the results showed that it was a porous 3D structure. Fluorescence sensing tests showed that complexes 2 and 4 can be used as novel fluorescent probes for the efficient detection of nitroaromatic compounds.

Full text of DOI:10.1016/j.ica.2020.119591

InorganicaChimicaActa507(2020)119591
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
High-efficiency fluorescent probe constructed by triazine polycarboxylic
acid for detecting nitro compounds
Nan Zhang1, Bing Li1, Xuemeng Wang, Dongqi Liu, Xue Han, Fengying Bai⁎, Yongheng Xing
College of Chemistry and Chemical Engineering, Liaoning Normal University, Huanghe Road 850#, Dalian 116029, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
By means of hydrothermal synthesis, a series of isomorphic lanthanide rare earth complexes [Ln2(TTHA)
(H2O)4]·7H2O (Ln = Sm(1), Eu(2), Gd(3), Tb(4) and Dy(5)) were constructed by triazine polycarboxylic acid
ligand (H6TTHA). They were characterized by elemental analysis, IR, TG, PXRD, UV–vis, etc. In particular, the
structure of complex 1 was analyzed as a representative, and the results showed that it was a porous 3D
structure. Fluorescence sensing tests showed that complexes 2 and 4 can be used as novel fluorescent probes for
the efficient detection of nitroaromatic compounds.
Lanthanide organic frameworks
Triazine polycarboxylic acid (H6TTHA)
Crystal structure
Fluorescence sensing
Nitroaromatic compounds
1. Introduction
been research hotspots [29–31]. The main reasons are as follows: (i)
their structure contains multiple carboxyl functional groups, which
Nitroaromatic compounds (NACs) act as a kind of all important
chemical raw materials are widely used in the fields of rocket fuels,
fireworks, dyes, medicines, pesticides and so on [1–5]. However, NACs
is also considered one of the most dangerous explosives due to their
high explosiveness, low security coefficient as well as the enormous
energy will be released during process of the explosion [6–10], and has
been used in war on a large scale by terrorists in wars, which not only
poses a serious threat to the lives and safety of people all over the
world, but also causes environmental pollution that is difficult to
eliminate [11–13]. Therefore, finding a fast, efficient and easy-to-op-
erate method to detect NACs has become a common concern worldwide
and urgently need to be solved [14–17]. Fortunately, among many
detection methods [18–20], fluorescent probes constructed by metal-
organic framework materials (MOFs) have stood out owing to their
high efficiency, sensitivity, recyclability and other advantages [21–24].
Lanthanide-based metal-organic framework materials are widely
used in the field of fluorescence sensing because of their flexible
structure, high surface area, permanent porosity, and high color purity.
As a new type of chemical sensor, MOFs have attracted great attention
in recent years [25–28]. It well known that the structure of complexes
determines their properties. The structure of MOFs is crucial to its lu-
minescence performance, particularly, ligands play an important part of
the structure of complexes. Therefore, the significance of choosing a
suitable ligand is self-evident. Among them, due to the particularity of
the structure of triazine polycarboxylic acid ligands, they have always
provides conditions for the coordination between metal ions and car-
boxyl oxygen; (ii) rare earth metal ions generally have more co-
ordination numbers when forming complexes, which further provides
possibility for building complexes with multiple configurations; (iii)
triazine ligands rifely have larger pores and have higher application
prospects in gas adsorption and luminescence and other aspects; (iv)
Triazine ligands have a number of unique properties, such as the pre-
sence of aromatic C]N bonds (triazine units) and the absence of weak
bonds (except for aromatic rings) [32–35]. This special structure makes
them highly chemically stable, and the presence of a large amount of
nitrogen atoms in the structure leads to its abundant nitrogen content,
which brings tremendous value to its practical application [36–38].
MOFs constructed by 1,3,5-triazine-2,4,6-triamine hexaacetic acid
(H6TTHA), a typical triazine polycarboxylic acid ligand with three
iminodiacetic acid groups, have been reported many times. For ex-
ample, Professor Wu group reported a series of lanthanide coordination
polymers [Ln2(TTHA)(H2O)4]·9H2O (Ln
= Eu, Tb, Gd, Dy) and
[Yb2(TTHA)(H2O)2] as well as{Na2[Co3(H2TTHA)2(H2O)12
]
(H2O)2}·4(H2O), {Na[Cu4(H2TTHA)(HTTHA)(H2O)8](H2O)3}·5(H2O),
[Cd3(TTHA)(H2O)4] and [Ca5(HTTHA)2(H2O)8] [39,40]; Complexes
([Na4(H3TTHA)Cl(H2O)],
[Mg2(H2TTHA)(H2O)],
[Cu2(H2TTHA)
(H2O)6], [Cu2(H2TTHA)(H2O)4]·4H2O, [Zn5Na2(TTHA)2(H2O)8] and
[Zn2(H2TTHA)(H2O)7]·2H2O) constructed by H6TTHA were also re-
ported by Arunachalam Ramanan’s group [41]. Prof. H. Xia research
team prepared a novel inorganic‐organic hybrid compound [Cu2(TTHA)
⁎ Corresponding author.
E-mail address: baifengying2003@163.com (F. Bai).
1 Nan Zhang and Bing Li contributed equally to this work.
https://doi.org/10.1016/j.ica.2020.119591
Received 9 January 2020; Received in revised form 12 March 2020; Accepted 12 March 2020
Availableonline13March2020
0020-1693/©2020ElsevierB.V.Allrightsreserved.

N. Zhang, et al.
InorganicaChimicaActa507(2020)119591
(bpy)2(H2O)]2·6H2O (bpy = 2,2′‐bipyridine) through coordination in-
(NO3)3·6H2O). Elemental Analysis (%) Calcd for C15H34N6O23Sm2: C,
19.73; H, 2.96; N, 9.20. Found: C, 19.85; H, 2.85; N, 9.32. IR data (KBr,
cm−1): 3452, 2931, 2848, 1619, 1551, 1496, 1434, 1386.
teraction of Cu2+ and H6TTHA [42]. Our research group also reported
two
complexes
[Cd4(μ3-O)(TTHA)(H2O)2]·3H2O
and
[Zn5Na2(TTHA)2(H2O)10], and introduced their photoelectric proper-
ties and potential nitro derivatives sensing [43]. Based on the above
research background, a lot of follow-up research works were also car-
ried out by our research group, a sequence of isomorphic Ln-H6TTHA
MOFs ([Ln2(TTHA)(H2O)4]·7H2O (Ln = Sm(1), Eu(2), Gd(3), Tb(4) and
Dy(5))) were synthesized by the hydrothermal methods. Base on lumi-
nescence properties of Eu3+, Tb3+, complexes 2 and 4 were used firstly
as fluorescent probes for fluorescent sensing of NACs.
[Eu2(TTHA)(H2O)4]·7H2O (2) Yield: 86% (based on Eu
(NO3)3·6H2O). Elemental Analysis (%) Calcd for C15H34N6O23Eu2: C,
18.85; H, 3.50; N, 8.66. Found: C, 18.90; H, 3.59; N, 8.59. IR data (KBr,
cm−1): 3431, 2937, 2848, 1612, 1552, 1496, 1448, 1406.
[Gd2(TTHA)(H2O)4]·7H2O (3) Yield: 87% (based on Gd
(NO3)3·6H2O). Elemental Analysis (%) Calcd for C15H34N6O23Gd2: C,
18.85; H, 3.47; N, 8.56. Found: C, 18.50; H, 3.50; N, 8.65. IR data (KBr,
cm−1): 3431, 2931, 2841, 1606, 1558, 1496, 1441, 1393.
[Tb2(TTHA)(H2O)4]·7H2O (4) Yield: 87% (based on Tb
(NO3)3·6H2O). Elemental Analysis (%) Calcd for C15H34N6O23Tb2: C,
20.33; H, 2.60; N, 9.49. Found: C, 20.35; H, 2.75; N, 9.54. IR data
(KBr,cm−1): 3438, 2931, 2848, 1619, 1564, 1503, 1448, 1393.
[Dy2(TTHA)(H2O)4]·7H2O (5) Yield: 76% (based on Dy
(NO3)3·6H2O). Elemental Analysis (%) Calcd for C15H34N6O23Dy2: C,
18.15; H, 3.43; N, 8.47. Found: C, 18.16; H, 3.46; N, 8.42. IR data
(KBr,cm−1): 3433, 294, 2848, 1619, 1558, 1489, 1448, 1400.
2. Experimental section
2.1. Materials and methods
All the chemicals purchased were of reagent grade or better and
were used without further purification. Ligand H6TTHA was success-
fully synthesized by referencing to the methods described in the lit-
erature (see Supporting information). IR spectra were performed on a
Bruker AXS TENSOR-27 FT-IR spectrometer to compress KBr pellets in
the range of 400–4000 cm−1. The elemental analyses of C, H, and N
were analyzed by a Perkin-Elmer 240C automatic analyzer. Thermo-
gravimetric analyses (TG) were performed under the condition of N2
atmosphere at a heating rate of 10 ℃ min−1 using a Perkin Elmer
Diamond TG/DTA. The photoluminescent spectra of the complexes
were measured on a HORIBA Fluoromax-4-TCSPC spectrofluorometer
equipped with Spectra LED Pulsed with LED source at room tempera-
ture (200–1000 nm) with 3.2-in Integrating Sphere that can be installed
in seconds replacing standard cuvette holder. UV–vis spectra were re-
corded with JASCO V-570 spectrophotometer with Φ = 60 mm In-
tegrating Sphere that can be installed in standard cuvette holder
(200–2500 nm, in the form of solid sample) and Lambda 35 UV–vis
Spectrometer (200–800 nm). PXRD patterns were obtained with a
Bruker Advance-D8 equipped with Cu-Kα radiation, in the range of
5° < 2θ < 60°, with a step size of 0.02° (2θ) and a count time of 2 s
per step.
2.3. X-ray crystal structure determination
Complex 1 was selected for testing single crystals of suitable di-
mensions for complex 1 was mounted on glass fibers for the X-ray
structure determinations. Reflection data were collected at room tem-
perature on a Bruker AXS SMART APEX II CCD diffractometer with
graphite monochromatized Mo-Kα radiation (λ = 0.71073 Å). A
semiempirical absorption correction was applied by the program
SADABS [46]. The program suite SHELXTL-97 was used for space-group
determination (XPREP), direct method structure solution (XS), and
least-squares refinement (XL) [47,48]. All non-hydrogen atoms were
refined with anisotropic displacement parameters. The positions of the
hydrogen atoms around the carbon atoms were included using a riding
model. Hydrogen atoms of coordination water molecules and lattice
water molecules were found in the difference Fourier map. Crystal data
and structure refinement parameters are given in Table S1.
3. Results and discussion
2.2. Preparation
3.1. Synthesis
2.2.1. Preparation of the H6TTHA
Ligand H6TTHA was synthesized on the basis of the method in re-
lated literature [44,45], see the Supplementary materials for specific
synthesis methods. The synthetic route is as follows (Scheme 1):
Complexes 1–5 as crystal or precipitates were obtained at the hy-
drothermal condition by heating to 160 °C for 72 h, pH was adjusted to
4, and molar ratio of metal salt to ligand H6TTHA of 2:1. First, in the
synthesis process, the products of the complexes 1–5 were first obtained
at 100 ℃, but there were many impurities in the crystals, and it was
difficult to eliminated impurities by washing so that we cannot obtain
pure crystals for X-ray diffraction. Furthermore, through the constant
exploration and improvement of the experimental environment, it was
found finally that crystals were obtained at both 80 °C and 120 °C, but it
is worth noting that when the reaction temperature was 160 °C, the
crystal of complex 1 obtained were the most transparent, and the yield
was higher with almost no impurities. Second, when the initial pH of
the reaction system was alkaline, only precipitation can be obtained.
Under neutral conditions, the crystallites began to appear. When we
adjusted the reaction system to an excessively acidic state, only clear
solution can be obtained. After many adjustments, we finally found that
when the pH of the system was 4, the crystals of the complex obtained
were the most perfect. Last, for the mol ratio of H6TTHA and rare earth
metal nitrates, considering that the rare earth metals generally adopt a
high coordination mode, we first selected a high ratio of metal to ligand
for experiments. However, as a ligand with six carboxyl groups si-
multaneously, H6TTHA itself can also be coordinated with a plurality of
metals. After a number of experimental trials, we finally found optimal
molar amount of the metal and ligand is 2:1.
2.2.2. Preparation of the complexes
0.0432 g of Sm(NO3)3·6H2O (0.1 mmol) and 0.0237 g of H6TTHA
(0.05 mmol) were dissolved in 3 mL of deionized water respectively.
After stirring at room temperature for 5 min, the obtained clarified
solution was mixed and stirred again for 0.5 h to make the solution fully
mixing, the pH of the mixture was adjusted to 4. Then, the mixture
solution was put into Teflon-lined stainless steel autoclave and heated
at 160 °C for 3 days, then cooled to room temperature, transparent
block crystals were obtained. The synthesis method of complexes 2–5 is
similar to that of complex 1, except that the rare earth nitrates used are
different. Detailed elemental analysis and infrared data are as follows:
[Sm2(TTHA)(H2O)4]·7H2O (1) Yield: 90% (based on Sm
Scheme 1. The synthetic route of the ligand H6TTHA.
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N. Zhang, et al.
InorganicaChimicaActa507(2020)119591
Fig. 1. (a) Coordination mode of metal ion in complex 1; (b) Building block of complex 1; (c) Connection fashions of H6TTHA ligand; (d) One-dimensional structure;
(e) Two-dimensional structure; (f) Sm1O9 single-capped square anti-prism geometry configuration; (g) Sm2O9 single-capped square anti-prism geometry config-
uration; (h) Three-dimensional structure.
Fig. 2. Sold-state luminescent spectrum of the complex 2: (a) excitation spectrum; (b) emission spectrum.
3.2. IR spectra
respectively. Red shift occurred compared to main position of the car-
boxyl peak at 1722 and 1397 cm−1 of the ligand H6TTHA, indicating
that the metal atom combinated with the ligand to form the complex.
Detailed infrared spectral data of complexes 1–5 are shown in Table S3.
The infrared spectra of all complexes are shown in Fig. S1.
Due to center metal atom of the complexes 1–5 has similar co-
ordination modes, indicate they are isomorphic rare earth complexes,
so the infrared spectra are similar to each other. Under this premise, we
take the infrared spectrum of complex 1 as an example for analysis. By
infrared spectroscopy analysis, the broader absorption band appeared
at 3452 cm−1 shown the presence of water molecules in the structure of
complex 1. The peak position at 1619 cm−1 and 1386 cm−1 can be
3.3. Solid UV–vis absorption spectra
The UV–vis absorption spectra as well as attribution of the com-
plexes 1–5 and the ligand H6TTHA were recorded (See Fig. S2 and
assigned to the asymmetric stretching vibration of the C]O bond
(νasCOO−) and the C]O symmetric stretching vibration (νsCOO
)
−
Table S4). Base on the luminescent characteristics of Eu3+ and Tb3+
,
3

N. Zhang, et al.
InorganicaChimicaActa507(2020)119591
Fig. 3. Solid-state luminescent spectrum of complex 4: (a) excitation spectrum; (b) emission.
Fig. 4. CIE chromaticity diagram of complex 2 (a) and complex 4 (b).
Fig. 5. Time resolved emission intensity of complex 2 (a) and complex 4 (b).
complexes 2 and 4 were selected for detailed UV–vis analysis. The two
absorption bands appeared at 217 nm and 250 nm of ligand H6TTHA
were attributed to its π-π* and n-π* transition. Similarly, complex 2
also presented two bands at 218 nm and 251 nm, which be assigned to
ligand-to-ligand transition (LLCT). Because of the f-f transition of Eu3+
is a spin-forbidden transition, lead to its UV absorption is too weak to
cannot be observed, the peak positions and shapes of complex 2 and
H6TTHA are almost identical. For complex 4, the absorption band
occurred at 264 nm also be ascribed to the ligand-to-ligand transition
(LLCT).
3.4. PXRD analyses
Fig. S3 shows the X-ray powder diffraction spectrum of complexes
1–5, where (a) is the diffraction of theoretical data obtained by per-
forming simulation calculation of the crystallographic data of the
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InorganicaChimicaActa507(2020)119591
3.6. Thermal properties
THF
CH2Cl2
In order to confirm the thermal stability of the complexes, ther-
mogravimetric analysis was carried out under a N2 atmosphere at a
temperature increase rate of 10 °C min−1, and the temperature range
was 30–1000 °C. Since the five complexes are isomorphic, TG curves are
also similar, so complex 1 was selected as the representative for dis-
cussion and analysis. The specific thermogravimetric loss is shown in
the Fig. S4. TG curve of complex 1 can be divided into two stages of
weight loss. The first weight loss occurred between 35 and 370 ℃,
which we attributed to the loss of seven free water molecules and four
coordinated water molecules (measured value: 21.0%: calculated value:
20.49%). The second weight loss, is about 42.0% (calculated value:
48.4%), during the temperature rise of 370–1000 ℃ should be due to
the collapes of ligand skeleton as deprotonated TTHA6- ligand. At this
point, the skeleton starts to collapse, eventually leaving the metal oxide
(measured value: 31.5%: calculated value: 36.10%).
DMA
DMF
EtOH
H2O
Toluene
MeOH
NB
500
550
600
650
Wavelength(nm)
Fig. 6. Luminescent spectra of complex 4 (1 mg/3 mL) in different solvents.
3.7. Solid-state photoluminescent spectra
complex 1, and (b) is the powder diffraction pattern measured by ex-
periments. By comparing a series of data, we found that the positions of
peaks in the five sets of data were one-to-one corresponding, indicating
that complexes 1–5 were pure phases, which provided a possibility for
us to further study the functions and properties of complexes. However,
through the careful comparison of the two groups of peaks, the simu-
lated and measured spectra still show different peak intensity, which
may be related to the low degree of crystallinity.
Based on the unique fluorescence properties of Eu3+and Tb3+, solid
samples of complexes 2 and 4 were selected for fluorescence determi-
nation at room temperature (See Figs. 2 and 3). For complex 2, under
the monitoring of 613 nm, the excitation spectra include a wide band at
360–385 nm attributed to π-π* electron transition of organic ligand. At
the same time, the narrow peaks at 390–460 nm, are belong to the
energy level transition of Eu3+ ion and the interaction with ligand.
These results suggest that the excitation spectra of the complex 2
contain not only from the organic ligands π-π* transitions, but also
from the strong energy level transitions of Eu3+ ions. In the emission
spectrum of complex 2 (λex = 394 nm), the characteristic peaks ap-
3.5. Structure descriptions
pearing at 590, 613, 638, and 696 nm are attributed to 5D0
→
7FJ
Complexes 1–5 have similar coordination environments, and all five
complexes are all belong to the Monoclinic crystal system, C2/c space
group. Here we take complex 1 as an example to analyze its structure.
In complex 1, there are two metal Sm atoms (Sm1 and Sm2), one
completely deprotonated TTHA6− ligand, four coordinated waters in
asymmetric unit structure. Fig. 1a shows the coordination mode of the
center metal Sm. Both Sm atoms adopt the same nine-coordination
mode to form a single-capped square anti-prism geometry (Fig. 1f and
g). Sm1 is coordinated with seven carboxyl oxygen atoms (O3, O4, O5,
O6, O7, O8 and O11) from the H6TTHA, and two oxygen atoms from
the coordinated water (O1, O2). Among it, O1, O2 adopt monodentate
coordination mode, O3, O4 in a bidentate chelation mode, O5, O6, O11
in a chelated monodentate mode, O7, O8 in bidentate mode respec-
tively coordinated with the central metal Sm1. Sm2 is also connected by
nine oxygen atoms, among O6, O9, O10, O11, O12, O13 and O14 from
the H6TTHA, O15 and O16 from the coordinated water. Among, O15,
O16 adopt monodentate coordination mode, O13, O14 adopt bidentate
chelation mode, O11, O12 and O6 in a chelated monodentate mode,
O9, O10 in bidentate mode respectively coordinated with the central
metal Sm2. Two Sm atoms are connected by O6 and O11, forming a
Sm2O4(COO)6 building block (Fig. 1d). The deprotonated ligand TTHA
is connected in the manner of μ6-η2η2η1η1η1η2η2η1η1η1 to link six Sm
atoms (Fig. 1c). Neighboring the building block were connected by
oxygen in two carboxyl groups from two arm from in the triazine ring
ligand to form one-dimensional chain (Fig. 1d), which extends to form
two-dimensional structure (Fig. 1e). In the building block, wherein each
ligand H6TTHA connects three building blocks, each building block
connects the four ligands, finally, forms a three-dimensional structure
by cross-linking (Fig. 1h). There are pores in the three-dimensional
structure, and the channel length and width are 16.1162 Å and
10.8367 Å, respectively. The crystallographic parameters of coordina-
tion complexes 1–5 are listed in Table S1, and some of the bond lengths
are listed in Table S2.
(J = 1–4), which are the characteristic emission peaks of Eu3+, re-
spectively. At 578 nm, owing to the symmetry-forbidden transition of
Eu3+, the emission peak of 5D0
→
7F0 is particularly weak. The extra
strong peak at 613 nm belongs to the 5D0–7F2 transition, which is an
electric dipole transition. Its emission intensity varies significantly with
the change of the coordination environment of Eu3+, while the 5D0–7F1
transition at 590 nm is a magnetic dipole transition. Without being
limited by symmetry, its emission intensity is hardly affected by the
Eu3+ coordination environment, and the ratio of the relative intensity
of the 5D0–7F2 transition to the 5D0–7F1 transition can indicate the
symmetry of the central ion lattice. From the emission spectrum of the
complex 2. It can be seen that the transition intensity of 5D0–7F2 is
much stronger than the 5D0–7F1, indicating that the central ion Eu3+ is
in a lower symmetrical environment, which is in line with the crystal
structure of the complex. For complex 4, at the emission wavelength of
542 nm, a wide peak band of appeared in complex 4 within the range of
330–380 nm, which should be ascribed to the π-π * transition of ligand
H6TTHA, and the peak at 490 nm could be attributed to the strong
energy level transition of Tb3+ ion. It can be known that in the complex
4, part of the energy is derived from the electronic transition of the
Tb3+ ion itself, and some is derived from the “antenna effect” of the
ligand H6TTHA. At the exciting wavelength of 370 nm, the character-
istic emission peaks at 543, 581, 619, 704 nm are respectively emitted
for 5D4–7FJ(J = 6–4) of Tb3+. Both CIE chromaticity diagrams of
complex 2 and 4 are present to Fig. 4.
3.8. Fluorescence lifetime
For luminescent materials, excited state lifetime is a key index to
evaluate their performance, therefore, we measured the excited state
lifetime of solid complexes 2 and 4, respectively (See Fig. 5). The lu-
minescence lifetimes of complexes 2 and 4 present a multi-exponential
decay process, in which the fluorescence lifetime of complex 2 consists
of three stages, by fitting a three-exponential decay curve the average
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N. Zhang, et al.
InorganicaChimicaActa507(2020)119591
Fig. 7. Quenching behavior of 4 (1 mg/3 mL) in EtOH: (a) nitrobenzene (NB); (b) 4-nitrotoluene (PNT); (c) 2,4,6-Trinitrophenol (TNP); (d) nitrosonaphthol (NNa);
(e) 4-(4-nitrophenylazo)resorcinol (AV).
parameters are listed Table S5, which are similar to the excited state
lifetime of some Eu3+ and Tb3+ organic framework materials. Such as a
MOF [EuL1.5(H2O)2]·1.75H2O (H2L = 5-methyl-1-(4-carboxylphenyl)-
1H-1,2,3-triazole-4-carboxylic acid) reported by prof. Xing group, the
excited state lifetime of this complex is 299 μs [49]; and the excited
state lifetime of complexes Eu(L)3(H2O)2](H2O) (HL: 4-(Dipyridin-2-yl)
aminobenzoic acid) and Tb(PBI)3(DPEPO) (HPBI: 3-phenyl-4-benzoyl-
5-isoxazolone, DPEPO: bis(2-(diphenylphosphino)phenyl)) are 428 μs
and 168 μs, respectively [50,51].
Table 1
Quenching constants (Ksv) and D of the complex 4 (1 mg/3 mL).
quenching agent
Ksv
LOD (M)
Nitrobenzene (NB)
4-Nitrotoluene (PNT)
2,4,6-Trinitrophenol (TNP)
Nitrosonaphthol (NNa)
4-(4-Nitrophenylazo) resorcinol (AV)
1.362 × 104
1.093 × 104
7.25 × 103
2.548 × 104
3.037 × 104
0.489
0.609
0.919
0.261
0.219
fluorescence lifetime value calculated by formula with τave = ∑Biτi is
843 μs and the lifetime of complex 4 consists of two stages, and average
fluorescence lifetime value is 129.4 ms, corresponding related
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InorganicaChimicaActa507(2020)119591
100
80
60
40
20
0
80
60
40
20
0
1
2
3
4
5
1
2
3
4
5
Fig. 8. Quenching behavior of the complex 2 and 4 for 4-(4-nitrophenylazo)resorcinol (AV) in repetitive experiments: (a) complex 2; (b) complex 4.
3.9. Fluorescence sensing detected for nitro compounds
detection of complex 2. The fluorescence intensity changes of complex
2 after addition of nitro compound were recorded in Fig. S6. The S-V
plots are shown in Fig. S7. The Ksv values and corresponding detection
limits of the five quenching agents are listed in Table S6.
3.9.1. Fluorescence sensing of complex 4 for nitro compounds
At an excitation wavelength of 370 nm, the potential sensing
properties of complex 4 on nitro compounds were investigated and
tested in nine different solvents (EtOH, Toluene, methanol (MeOH),
tetrahydrofuran (THF), dichloromethane (CH2Cl2), deionized water
(H2O), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA)
and nitrobenzene (NB). As shown as Fig. 6, in different solvents, al-
though there is some changes for the fluorescence intensity of the
complex, it is obvious that the characteristic peak position of Eu3+ does
not shift, which indicates that in the different systems, the first excited
state level change of the complex is not occurred, and it can draw a
conclusion that the polarity of the solvents does not affect the first
excited state level of the complex. However, in different solvents, the
fluorescent intensity luminescence intensity of complex 4 varies to
some extent, which is due to the different degree of electron transfer
transition between solvents and complex dispersed in different pola-
rities, so that the transfer rate of excited electrons in different solvents
is different. Among the above solvents, the fluorescence intensity of
complex 4 in ethanol is the highest, so ethanol is taken as the solvent for
post-detecting work. Meanwhile, nitrobenzene (NB) has obvious fluor-
escence quenching behavior, which further proves that it is wise for us
to choose nitro compounds to study fluorescence sensing performance.
Next, we investigated the fluorescence sensing of complex 4 aim at
five kinds of nitro compounds: nitrobenzene (NB); 4-nitrotoluene
(PNT); 2,4,6-trinitrophenol (TNP); nitrosonaphthol (NNa) and 4-(4-ni-
trobenzoazo) resorcinol (AV). The specific experimental process is as
follows: 1 mg of complex 4 was dissolved in 3 mL of ethanol, sonicated
to make a suspension, and then above five nitro compounds were
continuously added to the suspension respectively to record changes in
fluorescence intensity. After the addition of five nitro compounds to the
system, the decrease of fluorescence intensity can be clearly seen, and
the quenching behavior are recorded in Fig. 7.
3.9.3. Study on the cyclability and stability of sensor based on complexes 2
and 4
For fluorescent probes, good stability and recyclability are im-
portant indicators to judge whether they can be used in practical pro-
duction and life. Therefore, we investigated the difference in fluores-
cence intensity of complexes 2 and 4 by using five groups of parallel
experiments with 4-(4-nitrophenylazo) resorcinol (AV) as an example
(See Fig. 8). The test results show that the quenching efficiency of the
two complexes for the 4-(4-nitrophenylazo) resorcinol (AV) solution is
only slightly reduced and can be considered to be almost constant. It
can be seen that complexes 2 and 4 can be applied as a potential
fluorescent probe with repeatability.
3.9.4. Fluorescence respond mechanism
Nitroaromatic compounds (NACs) are powerful oxidants due to the
presence of electron-withdrawing groups. When these compounds are
exposed to electron-rich complexes, their unoccupied lower π* orbitals
are capable of absorbing electrons from the excited state of the com-
plexes, resulting in fluorescence attenuation. Generally, the conduction
band (CB) of an electron-rich MOF has an energy higher than the lowest
unoccupied orbital energy of a nitro compound. When the excited
electrons of the CB of MOFs are transferred to the LUMO, the nitro
compound will cause fluorescence quenching, and the fluorescence
quenching performance will be improved with the decrease of LUMO
orbital energy. That is to say, the photoinduced electron transfer me-
chanism (PET) leads to fluorescence quenching. In addition, by com-
paring the fluorescence lifetime value of complex 4 before and after the
addition of NACs, it was found that the lifetime value of complex 4
decreased after the addition of NACs, furthermore, indicating that the
photoinduced electron transfer mechanism (PET) leads to fluorescence
quenching.
In order to understand the quenching effect more accurately, the
quenching efficiency of five quenching agents was calculated by Stern-
Volmer equation: I0/I = KSV [Q] + 1 (Where I0 represents the initial
emission peak intensity, I shown the emission peak intensity after
quenching agent is added, and [Q] exhibits the molar concentration of
quenching agent, Ksv is the quenching constant [M−1]). Fig. S5 shows
the S-V plots of complex 4 after the addition of five quenchers. The Ksv
values and corresponding detection limits (LOD) (Calculated by for-
mula: 3σ/Ksv (σ is the standard deviation of the fluorescence intensity
of the blank solution) of the five quenching agents are listed in Table 1.
4. Conclusion
In this work, a series of new MOFs materials with 3D structure were
obtained through the interaction between rare earth nitrate and triazine
polycarboxylic acid ligand H6TTHA, and they were characterized by IR,
X-ray, PXRD, TG, UV–vis, etc.
Among them, complex 1 as perfect crystal was discussed in detail,
complexes 2 and 4 were used for the detection of aromatic nitro com-
pounds due to their superior fluorescence properties. Their high
quenching ability and high sensitivity make them promising as
3.9.2. Fluorescence sensing of complex 2 for nitro compounds
Similarly, we also carried out corresponding fluorescence sensor
7

N. Zhang, et al.
InorganicaChimicaActa507(2020)119591
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CRediT authorship contribution statement
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Nan Zhang: Conceptualization, Methodology, Writing - original
draft, Data curation, Software. Bing Li: Conceptualization,
Methodology, Writing
- original draft, Data curation, Software.
Xuemeng Wang: Data curation, Writing - review & editing, Software,
Investigation. Dongqi Liu: Data curation, Writing - review & editing,
Software, Investigation. Xue Han: Data curation, Writing - review &
editing, Software, Investigation. Fengying Bai: Supervision. Yongheng
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