One-dimensional Europium-coordination polymer as luminescent sensor for highly selective and sensitive detection of 2,4,6-trinitrophenol

Article abstract of DOI:10.1016/j.saa.2021.120303

Three isostructural lanthanide coordination polymers (LnCPs), [Ln(L)₆(DMF)]_n {HL = 2-(2-formylphenoxy) acetic acid, Ln = Sm (1); Eu (2); Tb (3)} have been synthesized by solvothermal reaction and characterized. Single crystal analyse

Full text of DOI:10.1016/j.saa.2021.120303

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 264 (2022) 120303
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
One-dimensional Europium-coordination polymer as luminescent
sensor for highly selective and sensitive detection of 2,4,6-trinitrophenol
⇑
Yanzhu Liu, Qingyan Sun, Hongbo Zhou, Hongyan Gao, Dongping Li , Yongxiu Li
Department of Chemistry, Nanchang University, Nanchang 330031, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
A Eu-based fluorescent probe was
synthesized based on a flexible
carboxylic acid.
ꢀ
ꢀ
It has high selectivity and sensitivity
to TNP in acetonitrile.
The quenching constant and low
detection limit is 2.6 ꢁ 104 Mꢂ1 and
3.39 lM, respectively.
ꢀ
The first example of function-
oriented lanthanide luminescent
sensor with formyl group as
hydrogen-bonding site in the
detection of TNP.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 May 2021
Received in revised form 11 August 2021
Accepted 16 August 2021
Available online 21 August 2021
Three isostructural lanthanide coordination polymers (LnCPs), [Ln(L)6(DMF)]n {HL = 2-(2-formylphenoxy)
acetic acid, Ln = Sm (1); Eu (2); Tb (3)} have been synthesized by solvothermal reaction and characterized.
Single crystal analyses revealed that the architectures of these LnCPs own one dimensional chain which
can be further packed into two-dimensional architectures by hydrogen bonds. Moreover, these LnCPs can
offer strategically placed uncoordinated formyl groups, which may act as hydrogen-bond acceptor in the
sensing of nitro explosives. Luminescence measurements reveal that LnCPs 2 and 3 exhibit strong lumi-
nescence in solid states. LnCP 2 shows quick, highly selective and sensitive detection of 2,4,6-
trinitrophenol (TNP) with the high quenching constant (2.6 ꢁ 104 Mꢂ1) and low detection limit
Keywords:
Lanthanide coordination polymers
Luminescence quenching
Fluorescent sensor
TNP
(3.39
lM), which indicates that LnCP 2 is more efficient than most of Eu-based coordination polymers
for the sensing of TNP. Furthermore, LnCP 2 represents the first example of one-dimensional Eu-based
sensors with formyl group as hydrogen-bonding site in the detection of TNP.
Ó 2021 Elsevier B.V. All rights reserved.
1. Introduction
compounds are highly toxic and strongly explosive. Among these
compounds, TNP is used as raw material or synthetic intermediate
It is well known that nitroaromatic compounds, such as 2,4,6-
trinitrophenol or picric acid (TNP), 1,3-dinitrobenzene (1,3-DNB),
2,4-dinitrotoluene (2,4-DNT), 2-nitroluene (2-NT), 2,4,6-
trinitrotoluene (TNT), nitrobenzene (NB), play a vital role in mili-
tary, national defense and industrial production [1,2], but these
for a series of industrial and commercial products, such as dyes
industries, pesticide industries, leather processing industries and
organic synthesis [3–6]. However, its widespread utilization brings
serious and permanent pollution and damages to the environment
and health of organism because of the good solubility and
unbiodegradable property of TNP [7]. It is reported that TNP has
acute toxicity, and its minimum lethal dose for humans is 5 mg/
kg and the maximum allowable level of TNP exposure is 9.3 mg/
⇑
Corresponding author.
E-mail address: nculdp@126.com (D. Li).
https://doi.org/10.1016/j.saa.2021.120303
1386-1425/Ó 2021 Elsevier B.V. All rights reserved.

Y. Liu, Q. Sun, H. Zhou et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 264 (2022) 120303
m3 in the air [8,9]. Therefore, it has been a compelling task to
develop a rapid and sensitive technique for detection of TNP in
both homeland security and environmental safety.
on Edinburgh FLS980 Instrument [27b]. Luminescence lifetimes
were measured by Hamamatsu compact fluorescence lifetime
spectrometer Quantaurus-Tau model C11367-11 using LED light
source [27c]. The excitation wavelength of Eu(III) and Tb(III) com-
plexes were 280 nm, while 618 nm and 543 nm as the monitoring
wavelength, respectively.
Recently, several spectroscopic techniques, for instance surface
enhanced Raman spectroscopy, fluorescence spectroscopy and ion
mobility spectrometry are used for detection of TNP and metal ions
[10–14]. However, most of spectroscopic techniques are costly,
non-portable, and need long analysis time or complicated sample
treatment. In the past few years, lanthanide coordination polymers
(LnCPs) have been used widely as chemical sensors for detecting
ions, organic solvent molecules and nitro explosives owing to their
simplicity, portability and high selectivity [15–18]. Furthermore,
LnCPs have unique luminescent properties, such as large Stokes
shift, long luminescence lifetimes, high luminescence quantum
yields, sharp line emissions [19,20], which make them distin-
guished as luminescent probes for potential applications. For
instance, LnCPs with open metal sites, exposed Lewis basic sites
and hydrogen bonding sites have been demonstrated to be efficient
luminescent sensors for recognition of nitro explosive and cations
[21–25]. However, those LnCPs are mostly constructed by rigid car-
boxylic acid ligands, and one-dimensional luminescent LnCPs with
hydrogen bonding sites for sensing of TNP are rarely reported up to
now.
2.2. Synthesis of LnCPs
Synthesis of LnCP 1: The complex was synthesized under
solvothermal condition. 2-(2-formylphenoxy) acetic acid
(54.0 mg, 0.3 mmol) and Sm(NO3)3ꢃ6H2O (44.4 mg, 0.1 mmol) were
dissolved in 6.2 mL mixture solution (1.5 mL dichloromethane,
4.5 mL toluene, and 0.2 mL DMF) firstly. And then, 0.1 mL
(0.5 mol/L) NaOH solution was added to the former mixture solu-
tion. The whole mixture solution was transferred in a Pyrex glass
bottle (20 mL), and then heated at 353 K for 72 h. After the reaction
mixture was slowly cooled to room temperature, pale yellow crys-
tals were received after washing with CH3CN for three times and
drying in an vacuum at 343 K for 24 h. Yield: 28.5%, based on Sm
(NO3)3ꢃ6H2O. Anal. Calcd for LnCP 1 (%): C, 47.22; H, 3.68; N,
1.83; O, 27.31. Found (%): C, 47.00; H, 3.58; N, 1.83; O, 27.2. FT-
IR (KBr pellet, cmꢂ1): 3684(w), 3073(w), 1689(s), 1665(s), 1596
(s), 1484(m), 1426(s), 1335(w), 1054(w), 864(s), 759(s), 697(w),
597(m), 521(m).
Synthesis of LnCP 2: The synthesis of CP 2 was similar to that of
LnCP 1, except that Eu(NO3)3ꢃ6H2O (44.6 mg, 0.1 mmol) was used
instead of Sm(NO3)3ꢃ6H2O. Pale yellow crystals of LnCP 2 were
obtained. Yield: 28.2%, based on Eu(NO3)3ꢃ6H2O. Anal. Calcd for
LnCP 2 (%): C, 47.19; H, 3.65; N, 1.82; O, 27.26. Found (%): C,
47.60; H, 3.79; N, 1.80; O, 27.4. FT-IR (KBr pellet, cmꢂ1): 3685
(w), 3075(w), 1692(s), 1666(s), 1595(s), 1483(m), 1428(s), 1334
(w), 1055(w), 865(s), 757(s), 696(w), 596(m), 523(m).
In this paper, the flexible carboxylic acid, namely {2-(2-
formylphenoxy) acetic acid}, was chosen as ligand to synthesize
lanthanide coordination polymer through solvothermal reaction
based on the following considerations: (1) Flexible carboxylic acid
ligands have advantages to construct LnCPs with various coordina-
tion models, charming architectures and excellent fluorescence. (2)
The O atoms from formyl group and methoxy group in this ligand
can act as hydrogen-bond acceptor to form intermolecular interac-
tions, which can not only stabilize the structure of LnCPs but also
promote the ability of sensing nitro explosives through the possi-
ble weak hydrogen-bonding interactions between the LnCPs and
analytes. (3) Up to now, one-dimensional LnCPs constructed by
flexible carboxylic acid are rarely reported as nitro explosives sen-
sors [26]. Herein, we present the design, solvothermal synthesis
Synthesis of LnCP 3: The synthesis of LnCP 3 was similar to that
of LnCP 1, except that Tb(NO3)3ꢃ6H2O (45.3 mg, 0.1 mmol) was
used instead of Sm(NO3)3ꢃ6H2O. Pale yellow crystals of LnCP 3
were obtained. Yield: 25.5%, based on Tb(NO3)3ꢃ6H2O. Anal. Calcd
for LnCP 3 (%): C, 47.21; H, 3.67; N, 1.8. FT-IR (KBr pellet, cmꢂ1):
3684(w), 3047(w), 1689(s), 1665(s), 1596(s), 1484(m), 1426(s),
1335(w), 1054(w), 864(s), 759(s), 697(w), 597(m), 521(m).
, ,
and characterization of Sm3+ Eu3+ and Tb3+ based one-
dimensional LnCPs, which can be used as potential luminescent
sensors. As we expected, LnCP 2 shows strong red luminescence
and ability of TNP detection by luminescence quenching effect.
2. Material and methods
2.3. Preparation of solid samples and nitro explosives solutions
2.1. Reagents and physical measurements
The solid samples (6 mg) were ground for 30 min and immersed
in CH3CN (10 mL), sonicated for 2 h to prepare the suspension. The
homogeneous resultant samples were used for luminescence stud-
ies. Stock of CH3CN solution (5 ꢁ 10-3 mol/L) with various nitro
explosives, including NB, 1,3-DNB, TNT, 2,4-DNT, 2-NT, TNP, were
also prepared for luminescence sensing experiments.
All reagents and solvents were obtained from commercial
sources and used without further purification. HL ligand was pre-
pared following the literature procedures [27a]. FT-IR spectra were
recorded by Bruker Alpha Fourier Infrared spectrometer with KBr
pellets in the range of 4000 ~ 400 cmꢂ1 at room temperature. Pow-
der X-ray diffraction patterns (PXRD) were recorded on SmartLab 9
KW powder diffraction instrument with Cu-K
a radiation (k = 1.5
4056 Å) at room temperature within the 2h range of 5 ~ 50° with
a scan speed of 0.1 s per step and a step size of 0.02°. Elemental
analysis (C, H, O, N) were recorded on a Perkin-Elmer 2400C ele-
mental analyzer. Thermogravimetric analysis (TGA) was per-
formed on Netzsch TG-209 thermal analyzer using a ceramic
crucible with alumina lids, and the rate of heating is 10 °Cꢃminꢂ1
in the range 30–800 °C under flowing nitrogen protection. UV–
Vis absorption spectra were recorded on an UV-2550 spectrometer.
Luminescence excitation and emission spectra in visible range
were determined at room temperature by a SENS-9000 steady-
state luminescence spectrometer. Luminescence quantum yields
were tested by an absolute method using an integrating sphere
2.4. Single crystal X-ray crystallography
X-ray crystal structures were determined with a Bruker SMART
1000 CCD diffractometer with Mo-K
a radiation (wavelength
0.71073 Å) at room temperature. Empirical absorption corrections
based on equivalent reflections were applied. The structures were
solved by direct methods and refined by full-matrix least-squares
methods on F2 using the SHELXS-97 crystallographic software
package. The detailed crystallographic data and structure refine-
ment parameters for LnCPs 1–3 were shown in Table. S2. Selected
bond distances and angles are listed in Table S3.
2

Y. Liu, Q. Sun, H. Zhou et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 264 (2022) 120303
in detail. As shown in Fig. 1a, the asymmetric unit consists of
one crystallographically independent Eu3+ cation, three L- ligands
and one coordinated DMF molecule. Each Eu3+ cation is nine-
coordinated by eight carboxylate oxygen atoms from six distinct
L- ligands and one oxygen atom from DMF molecule forming a
muffin-like shape geometry [27d] (Table S1). The adjacent Eu3+
cations are linked by the bridging L- ligands to form a one-
dimensional chain along the c-axis (Fig. 1b). Interestingly, the car-
3. Results and discussion
3.1. Crystal structure descriptions
Single crystals X-ray diffraction analyses revealed that the
isostructrual LnCPs 1–3 with the general formula [Ln(L)3(DMF)]n
crystallize in triclinic P-1 space group. Herein, we select [Eu(L)3-
(DMF)]n (LnCP 2) as an example to describe the crystal structure
(a)
(b)
(c)
(d)
Fig. 1. (a) A symmetric unit of LnCP 2. Hydrogen atoms have been deleted for clarity, symmetric codes: a) 2-x, 1-y, 2-z; b) 1-x, 1-y, 2-z; (b) The 1D chain structure connected
by the bridging L- ligands along the c-axis; (c) A closer view of two bridging coordination modes of carboxylate ligand in LnCP 2; (d) A 2D framework constructed by 1D chains
by intermolecular hydrogen-bonding interactions.
3

Y. Liu, Q. Sun, H. Zhou et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 264 (2022) 120303
boxyl group in ligand adopts l2
–
g1
:
g
1-bridging and l2
–
g1
:
g2
-
ground state 7F0 to the different excited states, such as 7F0 5G2,
–
bridging coordination modes, while the formyl group remains
uncoordinated in complex (Fig. 1b and 1c). It is worthy to note that
such uncoordinated Lewis basic formyl sites might interact with
the Lewis acidic analytes. The angle between three ions in the 1D
chain is ca. 169.52°, suggesting that the metals are nearing a lin-
early arranged. The Eu-O bond distances are in the range of
2.345(2) Å – 2.601(2) Å, while the adjacent Eu-Eu distances are
4.000(1) Å and 4.160(7) Å, respectively. The 1D chains are further
interconnected by nonclassical hydrogen bonds to form a 2D stack-
ing structure (Fig. 1d and S1, Table S4).
7F0 – 5L6, 7F0 – 5D3, 7F0 – 5D2, 7F0 – 5D1, respectively. It indicates that
the direct f ? f excitation is more dominant rather than ligand sen-
sitization comparing with the relatively weaker bands of ligand.
The strong direct f-f excitation absorptions of Eu3+ ions also reveals
that the asymmetry coordination environment and strong spin–or-
bit coupling interactions of Eu3+ ions in complex [27e–27f,40a].
LnCP 2 displays an excitation band at 394 nm and characteristic
emission bands of Eu3+ at 590 nm (5D0?7F1), 618 nm (5D0?7F2),
650 nm (5D0?7F3), and 696 nm (5D0?7F4), respectively. Among
them, the strongest emission peak is 618 nm (5D0?7F2), which
indicates that electric dipole is allowed and without an inversion
center for Eu3+. The excitation spectrum of LnCP 3 is dominated
by ligand-centered broad bands in the range of 250–450 nm, indi-
cating that the Tb3+ are sensitized by ligand via the antenna effect.
LnCP 3 shows characteristic green luminescence with typical line
emission of Tb3+ at 489 nm, 543 nm, 583 nm and 620 nm
(5D4?7F6-3) excited at 380 nm. The most intense emission transi-
tions, namely, 5D0?7F2 (618 nm) for Eu3+ and 5D4?7F5 (543 nm)
for Tb3+ compounds, were used to detect the luminescence life-
times. The decay lifetimes and luminescent quantum yields are
detected for three times, and the average values for lifetime and
quantum yields are 1.27 ms and 31% for LnCP 2, and 1.09 ms and
3.4% for LnCP 3, respectively.
3.2. Powder X-ray diffraction (PXRD) and thermogravimetric (TGA)
analysis
The purities of LnCPs were determined by powder XRD at room
temperature (Fig. 2). The powder XRD patterns correspond well
with the simulated results of single crystal X-ray diffractions, indi-
cating that the synthesized samples are relatively pure, and can be
directly used in subsequent experiments without further
purification.
In order to investigate the thermal stability of LnCPs, samples
1–3 were heated from 30 °C to 800 °C under nitrogen atmosphere
with a heating rate of 10 oCꢃminꢂ1. As illustrated in Figure S2, the
samples show no weight loss from 30 °C to 130 °C, suggesting that
the framework of LnCPs keep unchanged before 130 °C. And then,
they show a weight loss of about 9.53 % in the first weight loss
stage, which is due to the loss of coordinated DMF molecule (calcu-
lated value: 9.6 %). There is a temperature platform between 180 °C
and 240 °C. After that, the samples begin to decompose into lan-
thanide oxide, with a loss of about 43.77 % for LnCP 1 (calculated
value: 44.53 %), 45.17 % for LnCP 2 (calculated value: 44.26 %)
and 46.34 % for LnCP 3 (calculated value: 42.96 %).
Considering the exposed Lewis basic sites, long lifetime and rel-
atively higher luminescent quantum yields of compound, LnCP 2
may be a potential sensor for the recognition of nitro explosives.
Therefore, various solvents including acetonitrile (CH3CN), metha-
nol (CH3OH), ethanol (C2H5OH), N, N-dimethylformamide (DMF)
and H2O were explored as potential media for the sensing studies.
For that purpose, 6 mg of solid samples were dispersed in 10 mL of
various solvents. It seems that the luminescence emission intensi-
ties of LnCP 2 suspensions are dependent on the solvents. As
shown in Fig. 4, LnCP 2 exhibits the highest emission intensity in
CH3CN, while the lowest emission intensity was noted in H2O.
Actually, the luminescence emission of LnCP 2 was basically
quenched in H2O solvent, which may be caused by the coordina-
tion of water and metal ion [28]. Furthermore, the excitation spec-
trum of LnCP 2 transfers from the direct f ? f excitation in solid
state to the ligand sensitization in CH3CN solvents owing to the
solvent effect of compound [29], while the emission spectrum
are almost the same both in solid state and in solvents (Figure S4).
Here, the stability of LnCP 2 in CH3CN solution was also inves-
tigated by powder XRD method. After the sample was immersed
in CH3CN solution for 2 h, the solid was filtered and dried to record
the powder XRD (Figure. S5). The result revealed that the powder
XRD pattern after immersing in CH3CN solution was consistent
with the simulated data, which indicates that LnCP 2 is quite stable
in the CH3CN solution. Therefore, CH3CN was ultimately selected as
a sensing medium due to the highest luminescence intensity and
stability of the compound upon dispersion.
3.3. Luminescence properties of LnCPs
The luminescence properties of LnCPs were studied in solid
state under room temperature. However, the ligand and LnCP 1
show no luminescence when excited at 365 nm. The excitation
and emission spectra of LnCPs 2 and 3 are illustrated in Fig. 3. In
the excitation spectrum of LnCP 2, the excitation peaks near 385,
394, 416, 465 and 473 nm are attributed to transitions from the
3.4. Sensing of nitro explosives
LnCP 3
LnCP 2
In order to explore the sensing ability of LnCP 2, the lumines-
cence properties towards a series of nitro explosives, including
TNP, 1,3-DNB, 2,4-DNT, 2-NT, TNT, NB, were investigated. In the
experiments, equal concentration of nitro explosives solutions
LnCP 1
(200 lL) were added to the LnCP 2 suspension (2.5 mL) and mixed
well, and then the luminescence intensities of mixtures were
tested immediately. As shown in Fig. 5, the luminescence intensi-
ties of LnCP 2 are weakened by nitro explosives. Upon the addition
of TNP, the most evident fluorescence quenching phenomenon can
be surveyed. For instance, the luminescence quenching percentage
of LnCP 2 reaches up to 95.5% towards TNP, but only 14.5%, 15.1%.
7.3%, 6.9%, 2% towards other nitro explosives (2-NT > 2,4-DNT > T
Simulation
10
20
30
40
50
2 Theta ( )
Fig. 2. Powder XRD patterns of LnCPs 1–3.
4

Y. Liu, Q. Sun, H. Zhou et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 264 (2022) 120303
Excitation spectra of LnCP 3
Excitation spectra of LnCP 2
Emission spectra of LnCP2
5D0 7F2
(a)
(b)
Emission spectra of LnCP 3
5D4 7F5
7F0 5L6
7F0 5D2
5D4 7F6
5D0 7F1
7F0 5G2
5D4 7F4
5D4 7F3
5D0 7F4
7F0 5D3
7F0 5D1
5D0 7F3
300
400
500
600
300
400
600
Wavelength (nm)
700
Wavelength (nm)
Fig. 3. Solid state excitation and emission spectra of LnCP 2 (a, kex = 394 nm) and LnCP 3 (b, kex = 380 nm).
CH3CN
C2H5OH
CH3OH
(b)
(a)
DMF
H2O
600
650
700
CH3CN C2H5OH CH3OH
DMF
H2O
Wavelength (nm)
Fig. 4. Luminescence emission spectra of LnCP 2 obtained in different solvents as their suspensions (kex = 350 nm). Quenching efficiency based on the 618 nm peak.
(a)
(b)
OH
CH3CN
NB
120
100
80
60
40
20
0
O2N
NO2
1,3-DNB
TNT
NO2
2,4-DNT
2-NT
TNP
CH3
NO2
NO2
NO2
CH3
NO2
CH3
O2
N
NO2
TNP
NO2
NO2
NO2
TNP
2-NT 2,4-DNT TNT 1,3-DNB NB
Analytes
600
650
Wavelength (nm)
700
Fig. 5. (a) The luminescence emission intensities at 618 nm of LnCP 2 in various nitro explosives (kex = 350 nm); (b) The luminescence quenching efficiencies of different Nitro
explosives towards the CH3CN dispersion of LnCP 2.
NT > 1,3-DNB > NB). This quenching potential order does not
directly follow the electro-deficient trend of each analyte. In addi-
tion, the bright red luminescence of LnCP 2 almost disappeared and
the color of solution changed from colorless to yellow after adding
of the TNP solution. The experiments indicate that LnCP 2 has high
selectivity towards TNP, which prompts us to do further quantita-
tive fluorescence titration and competition experiments and to
obtain a more insight into the sensing performance.
3.5. Sensing of TNP
Considering the high quenching effect of TNP, luminescence
titration experiments were carried out by gradual addition of ana-
lyte into the LnCP 2 suspensions. Such a study revealed that LnCP 2
is highly sensitive towards TNP even at very low concentrations.
Fig. 6a shows the emission spectra of LnCP 2 suspension with
increasing TNP concentrations. The fluorescence intensity gradu-
5

Y. Liu, Q. Sun, H. Zhou et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 264 (2022) 120303
(b)
4
(a)
40
35
y=26.9422x+0.6534
3
2
R
=0.9900
30
25
20
15
10
5
2
1
10
30
50
0.02 0.04 0.06 0.08 0.10 0.12
[TNP]/mM
70
90
120
160
200
L
/
P
TN
V
0
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
[TNP]/mM
600
650
Wavelength (nm)
700
Fig. 6. (a) Concentration-dependent fluorescence quenching efficiencies of LnCP 2 suspension based on the 618 nm peak. Gradually addition of TNP solution to the LnCP 2
suspension (from 0 to 200 L, kex = 350 nm); (b) The Stern-Volmer plot for the response to TNP.
l
ally decreases with increase of the TNP concentrations. At around
400 mΜ concentrations of TNP, the fluorescence intensity is signif-
icantly quenched with a quenching yield of ca. 95.5%. The quench-
ing efficiency can be quantitatively analyzed by the Stern-Volmer
(S-V) equation: I0/I = KSV[A] + 1 [30]. I0 and I represent the suspen-
sion luminescence intensities before and after addition of analyte,
respectively. KSV is the quenching constant, and [A] is the concen-
tration of analyte. The Stern-Volmer plot for TNP is in good linear
proportion at low concentrations, but subsequently bends upwards
at higher concentrations, perhaps due to the presence of simulta-
neous static and dynamic quenching [31] (Fig. 6b). From the linear
fitting of the I0/I plot at low concentration, the quenching constant
(Ksv) was found to be 2.69 ꢁ 104 Mꢂ1 (R2 = 0.9900), which was lar-
ger than most of the previously reported Eu-based TNP sensors
(Table 1). In addition, the lowest detection limit (LOD) of LnCP 2
200 lL stock of various nitro explosives solution was added into
2.5 mL LnCP 2 suspension and then the same concentration of
TNP was added and mixed well. For these mixtures, the lumines-
cence emission spectra were investigated. As exhibited in Fig. 7,
the interference of the emission intensities of LnCP 2 by other com-
peting nitro explosives were negligible. High quenching effect of
LnCP 2 was observed only after the addition of TNP to the mixtures.
Hence, all the above-mentioned investigations indicate that LnCP 2
can be a highly sensitive, selective and efficient sensor for TNP
even in the presence of other interfering substances.
3.6. Possible mechanism for TNP sensing
To get a better understanding of the origin of the high selectiv-
ity of LnCP 2 towards TNP, the sensing mechanism was investi-
gated using the following experimental evidences. At first, after
immersed in TNP solution for 2 h, the PXRD pattern of LnCP 2
remained consistent with simulated curve, indicating that the
framework was preserved (Figure S5). Hence, the luminescence
quenching of LnCP 2 can’t attribute to the collapse of the frame-
work. Furthermore, there is no overlap between the emission spec-
trum of LnCP 2 and the absorption spectrum of TNP in CH3CN
solution, implying the fluorescence resonance energy transfer
(FRET) quenching mechanism can be excluded (Figure S4 and
Fig. 8). As shown in Fig. 8, good spectral overlap between the exci-
tation profile of LnCP 2 and the absorption spectrum of TNP was
observed, while partly overlap for 2-NT, 2,4-DNT and NB were also
observed. In addition, the UV–Vis absorption spectrum of TNP
exhibits absorption of considerable intensity at 350 nm, while
the other nitro explosives have negligible absorption at this wave-
with TNP was 3.39
lM as calculated with the equation of equation
3r
/k [32] ( : standard, k: slope), which is smaller than those of the
r
previous reported sensitive TNP sensors, such as ([Eu(L)2(H2O)]
BrꢃH2O)nb, {[Eu2(TDC)3(CH3OH)2]ꢃCH3OH}e and Eu2L3 etc (Table 1),
f
suggesting that LnCP 2 is a more efficient sensor in detection of
TNP[33–40]. The cyclic repeatability experiments of the quenching
ability of LnCP 2 in CH3CN solution can be reused at least three
times (Figure S6). These results indicate that LnCP 2 is an ultrasen-
sitive promising luminescence sensor, which may be used as fluo-
rescent ‘‘turn-off” sensor for detection of TNP.
An effective fluorescent sensor requires both high sensitivity
and selectivity. Obviously, competition experiments are very
important characteristics for evaluating the selectivity of sensors.
Therefore, the selectivity of LnCP 2 towards TNP in the presence
of other interfering analogs has been studied. In this regard,
Table 1
The comparison of various fluorescent sensors for detection TNP.
Entry
LnCPs
Solvent
Ksv (Mꢂ1
)
LOD (M)
Ref.
1
2
3
4
5
6
7
8
{[Eu(TFTA)1.5(H2O)2]∙H2O}na
CH3OH
H2O
DMF
DMF
CH3OH
H2O
6.2219 ꢁ 103
2.68 ꢁ 10-5
1 ꢁ 10-5
1 ꢁ 10-5
1 ꢁ 10-5
9.5 ꢁ 10-6
5 ꢁ 10-6
–
[35a]
[35b]
[25b]
[36]
[37]
[38]
1.7 ꢁ 104
b
([Eu(L)2(H2O)]BrꢃH2O)n
[Eu2L2(dmf)(H2O)3(
l
3-O)]ꢃH2O}nc
1.359 ꢁ 103
2.001 ꢁ 103
1.6 ꢁ 104
d
Eu4L3
{[Eu2(TDC)3(CH3OH)2]ꢃCH3OH}e
f
Eu2L3
2.912 ꢁ 103
1.07 ꢁ 104
2.1 ꢁ 104
g
[Eu(H2O)(DCPA)3]n
H2O
[39]
[Eu(2,5-FDC)0.5(Adip)(H2O)2]h
Eu(L)6(DMF)
DMF
CH3CN
DMF
H2O
–
[40a]
This work
[40b]
[40c]
[40d]
9
2.69 ꢁ 104
6.24 ꢁ 104
6.76 ꢁ 104
7.28 ꢁ 104
3.39 ꢁ 10-6
–
k
10
11
12
[Eu(L)1.5(DEF)]n
{[Eu2(HL)2(H2O)4]∙3H2O}nj
[Eu2(L)3(H2O)3]∙DMFi
2.5 ꢁ 10ꢂ6
H2O
1.95 ꢁ 10-6
TFTAa is tetrafluoroterephthalic acid; Lb is 1,3-bis(4-carboxyphenyl)imidazolium bromide; Lc is 4,4-[carbonylbis(azanediyl)]di-benzoic acid; Ld is 5,5-(carbonylbis(azanediyl))
diisophethaic acid; TDCe is thiophene-2,5-dicarboxylate; Lf is 4,40-(carbonylbis(azanediyl))bis(2-methoxybenzoic acid); DCPAg is 4,5-dichlorophthalaten; Adiph is adipic acid;
2,5-FDCh is furan-2,5-dicarboxylic; H2Li is 9,9-diethylfluorene-2,7-dicarboxylic acid; H4Lj is 3,30,5,50-azoxybenzenetetracarboxylic acid; Lk is 3-(3,5-dicarboxylphenoxy)
pyridine.
6

Y. Liu, Q. Sun, H. Zhou et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 264 (2022) 120303
cess since they can act as an efficient electron-transfer bridge [42].
It is worthy to note that there might be hydrogen-bonding interac-
tions between the hydroxyl groups of nitrophenols and the car-
boxylate groups of ligands as reported in literature [34]. In LnCP
2, there are three uncoordinated oxygen atoms (O4, O8, O12) of
formyl group, while O8, O12 have contributed to forming
hydrogen-bond. Therefore, the unoccupied O4 atoms can form
hydrogen-bond with phenolic hydroxyl groups from TNP, which
may also promote intermolecular electron transfer contributing
to luminescence quenching [33,43]. To further elucidate the
quenching ability of TNP on the emission of LnCP 2, the lumines-
cence lifetimes of the LnCP 2 in the absence and presence of TNP
were also measured. As illustrated in Figure S3, the luminescence
lifetimes were almost unchanged, which also indicates there are
no interaction between TNP and Eu3+ ion, and the quenching of
luminescence may related to hydrogen-bond interaction between
TNP and the flexible ligand [44,45]. Overall, the good detection
capability of LnCP 2 towards TNP over other nitro explosives may
ascribe to the possible quenching mechanisms of competitive
absorption, photoinduced electron transfer and hydrogen-bond
interaction.
LnCP 2
120
100
80
60
40
20
0
LnCP 2+other nitro explosives
LnCP 2+other nitro explosives+TNP
1,3-DNB 2-NT 2,4-DNT NB
TNT
TNP
Fig. 7. Competition experiments (kex = 350 nm), blue bars represent the existence
of TNP, red bars represent in the presence of other nitro explosives and LnCP 2 in
CH3CN solution, and black bars represent LnCP
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
2 in CH3CN solution. (For
4. Conclusions
length. These results indicate that competitive absorption exists
between excitation of LnCP 2 and TNP. Namely, TNP will strongly
absorb the energy of the excitation light and reduce the effective
energy transfer from ligand to the Eu3+ ion resulting in a significant
decrease in the luminescence. Next, photoinduced electron trans-
fer (PET) may account for the detection of electron-deficient nitro
explosives molecules. According to the molecular orbital theory,
explosive molecules with strongly electron-withdrawing nitro
group may capture electron from the ligand [41]. Upon excitation,
electrons may transfer from the conduction band of sensors to the
lowest unoccupied molecular orbitals of the analyst and therefore
quench the luminescence. Moreover, some intermolecular interac-
In summary, three one-dimensional lanthanide coordination
polymers were successfully synthesized using a flexible carboxylic
acid and characterized. Among them, LnCP 2 was selected as the
luminescent sensor for nitro explosives. It shows high sensitivity,
good selectivity and fast response for sensing of TNP with the
detection limit of 3.39
lM in CH3CN solution. The excellent
quenching performance for TNP can be attributed to the competi-
tive absorption, photoinduced election transfer and hydrogen-
bond interactions. These results indicated that LnCP 2 can be used
as a high selective and efficient fluorescent ‘‘turn-off” sensor for
detection of TNP. This work also provides a new example for syn-
thesis and design of function-oriented lanthanide luminescent sen-
sors with hydrogen bonding sites.
tions, such as
p-p stacking and hydrogen-bonding interactions,
have found to be important in intermolecular electron transfer pro-
Excitation spectra of LnCP 2
2,4-DNT
2-NT
1.0
0.8
0.6
0.4
0.2
0.0
NB
TNP
1,3-DNB
TNT
250
300
350
400
450
500
Wavelength (nm)
Fig. 8. The UV–Vis absorption spectra of various nitro explosives and the excitation spectrum (kex = 350 nm) of LnCP 2 in CH3CN solution.
7

Y. Liu, Q. Sun, H. Zhou et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 264 (2022) 120303
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CRediT authorship contribution statement
Yanzhu Liu: Conceptualization, Methodology, Software. Qing-
yan Sun: Data curation. Hongbo Zhou: Formal analysis, Investiga-
tion. Hongyan Gao:
. Dongping Li: Project administration,
Validation, Writing – review & editing. Yongxiu Li: Supervision.
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lanthanide coordination polymers for temperature sensing and enhanced Nd
III NIR luminescence, J. Lumin. 201 (2018) 410–418.
Declaration of Competing Interest
[20] Y. Liu, Y. Lu, X. Yang, X. Zheng, S. Wen, F. Wang, X. Vidal, J. Zhao, D. Liu, Z. Zhou,
Amplified stimulated emission in upconversion nanoparticles for super-
resolution nanoscopy, Nature 543 (2017) 229–233.
[21] E.L. Zhou, C. Qin, D. Tian, X.L. Wang, B.X. Yang, L. Huang, K.Z. Shao, Z.M. Su, A
difunctional metal-organic framework with Lewis basic sites demonstrating
turn-off sensing of Cu2+ and sensitization of Ln3+, J. Mater. Chem. C 6 (2018)
7874–7879.
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgments
[22] H.B. Tai, S.N. Lu, X. Zhang, Q.W. Liu, Z.G. Sun, Differently luminescent sensing
abilities for Cu2+ ion of two metal phosphonates with or without the free Lewis
basic pyridyl sites, J. Mol. Struct. 1234 (2021) 130175.
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sensitive anionic metal-organic framework with nitrogen-rich sites
fluorescent chemosensor for nitro explosives detection, J. Hazard. Mater. 344
(2018) 283–290.
The authors acknowledge the financial support of the National
Natural Science Foundation of China (52062034 and 21561021).
Appendix A. Supplementary data
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uncoordinated Lewis basic triazolyl sites as multi-responsive sensor for
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