A carbazole-functionalized metal-organic framework for efficient detection of antibiotics, pesticides and nitroaromatic compounds

Article abstract of DOI:10.1039/c8dt04558e

Organic pollutants, such as antibiotics, pesticides, and nitroaromatic compounds (NACs), have posed a great threat to human health and sustainable development. Therefore, the detection of these organic pollutants is of great importance but challenging. In

Full text of DOI:10.1039/c8dt04558e

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Efficient extraction of sulfate from water using
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DOI: 10.1039/C8DT04558E
A carbazole-functionalized metal−organic framework for
efficient detection of antibiotics, pesticides and nitroaromatic
compounds
Ning Xu, Qinghua Zhang, and Guoan Zhang*
Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education,
Hubei Key Laboratory of Materials Chemistry and Service Failure, School of Chemistry and
Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, P.R.
China
E-mail: zhangguoan@gmail.com (G. A. Zhang), Tel.: +86-27-87559068; Fax: +86-27-87543632
Organic pollutants, such as antibiotics, pesticides, nitroaromatic compounds (NACs),
have posed great threat to human health and sustainable development. Therefore,
detection for these organic pollutants is of great importance but challenging. In this
work, we synthesized a rigid conjugated
,4'-(9-(4'-carboxy-[1,1'-biphenyl]-4-yl)-9H-carbazole-3,6-diyl)dibenzoic
H CBCD), and employed this ligand to react with Cd(II) ions to construct a
tricarboxylic acid ligand
4
acid
(
3
Cd-LMOF, namely [Cd (CBCD) (DMA) (H O) ]·10DMA (Cd-CBCD). Cd-CBCD
3
2
4
2
2
features a three-dimensional (3D) supramolecular framework based on the
two-dimensional (2D) layer structures through the π⋯π stacking interactions. The
fluorescence sensing measurements demonstrate that Cd-CBCD can detect nitrofurans
(NFs), 4-Nitroaniline (4-NA) and 2,6-dichloro-4-nitroaniline (DCN) with high
selectivity and sensitivity. This work represents the first carbazole-functionalized
metal−organic framework as a fluorescent sensor for the highly efficient detection of
antibiotics, pesticides and nitroaromatic compounds.
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Introduction
Metal-organic frameworks (MOFs), as the organic−inorganic hybrid porous materials,
have attracted considerable attention owing to their outstanding performances in the
fields of gas capture and storage,1-4 drug delivery,5-7 sensing,8-11 and proton
conductivity.12-15 Among the countless well-designed MOFs, luminescent
metal−organic frameworks (LMOFs) equipped with inherent fluorescent properties
represent one of the most attractive types because of their potential applications in
1
6
17
18, 19
20
nonlinear optics, chemical sensors, photo-catalysis,
white-light emission, and
so on. Generally, the fluorescence emission of LMOFs is mainly produced from metal
ions, organic ligands or metal−ligand charge transfer processes.21, 22 Compared with
the relatively limited choices of lanthanide metal ions, the reasonable design and
orientated synthesis of photo-responsive organic ligands is a facile way to acquire the
powerful luminophore.23, 24 Furthermore, the elaborate modification of the ligand
molecule can further achieve tunable microporous architectures and enhanced
physical/chemical functionalities.25-28 As a matter of fact, the sensitivity of
fluorescence sensing is highly dependent on the ability of LMOFs to donate electrons
when they are used as fluorescent sensors to detect electron-deficient compounds
because most reported LMOF sensors work in the form of luminescence quenching,
which is mainly resulted from the electron transfer or energy transfer processes
between MOF host and guest analytes.29,
30
Therefore, the introduction of
electron-rich π-conjugated moieties to the organic linkers is of vital importance for the
construction of LMOFs with remarkable recognition and sensing capabilities.
Carbazole is commonly found in various organic fluorescent molecules and
widely used to synthesize versatile optoelectronic materials.31, 32 Considering its large
π-conjugated system, rigid planar molecular structure and strong electron-donating
ability, carbazole can be immobilized in the rigid frameworks to form conjugated
LMOFs with notable fluorescence emission abilities.33-35 Different from the
introduction of organic chromophores and fluorescent tags, embedding carbazole into
the backbone structure of LMOFs can form a central-launched fluorescence emission
2

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mechanism without sacrificing its porosity.36-38 As a consequence, it is of great
theoretical significance and practical feasibility to fabricate conjugate-induced rigid
LMOFs with carbazole structural units.
DOI: 10.1039/C8DT04558E
It is well known that pesticides and antibiotics have been extensively used in
agriculture and animal husbandry to raise the agricultural output and treat some
certain bacterial infections.39, 40 However, in recent decades, the long-term residual
accumulation of pesticides and antibiotics in the ecosystem has caused intense public
concern regarding food safety and environmental protection because of their intrinsic
high toxicity and non-degradability.41,
42
The latest researches have already
demonstrated that long-term ingestion of pesticides and antibiotics may increase the
risk of mutagenicity and carcinogenicity and cause many adverse drug reactions such
as allergy, asthma, immunity decline.43-45 On the other hand, 4-Nitroaniline (4-NA), as
a typical nitroaromatic compound, has attracted more attention for its wide use as an
important chemical intermediate of various chemical products, including pesticides,
photostabilizers, engineering plastics, and azo dyes. Due to its widespread use and
high toxicity, 4-NA has been regarded as the serious organic pollutant for surface
water and soil, and then causing serious health hazards to human beings through the
ecological recycling system. Therefore, it is quite urgent to develop efficient
methods/technologies to detect this specific environmental contaminant. Presently, the
traditional instrumental detection methods for organic pollutants include capillary
electrophoresis (CE), ion mobility spectrometry (IMS), Raman spectroscopy (RS),
and liquid chromatography-tandem mass spectrometry (LC-MS).46-48 These detection
methods are usually complicated and time-consuming. In contrast, fluorescence
sensing technique, which is based on the change in luminescent spectra induced by
the interactions between sensors and analytes, has been proven as a very promising
method for organic pollutants detection owing to its high sensitivity, fast response,
low cost, simple operation and non-destructive features.
Given the above consideration, herein, we designed and synthesized a rigid
conjugated tricarboxylic acid ligand 4,4'-(9-(4'-carboxy-[1,1'-biphenyl]-4-yl)-9H
-
carbazole-3,6-diyl)dibenzoic acid (H CBCD) (Scheme 1), and employed this ligand
3
3

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DOI: 10.1039/C8DT04558E
namely
to
react
with
Cd(II)
ions
to
construct
a
Cd-LMOF,
[
Cd (CBCD) (DMA) (H O) ]·10DMA (Cd-CBCD). The constructed Cd-LMOF is an
3
2
4
2
2
extended three-dimensional (3D) supramolecular framework built on its
two-dimensional (2D) layer structures through the π⋯π stacking interactions. Due to
the rational incorporation of carbazole building blocks into the backbone structure of
the ligand, Cd-CBCD exhibits an intense blue-light emission characteristic and can
function as a promising luminescent sensor for the detection of antibiotics, pesticides
and nitroaromatic compounds with high selectivity and sensitivity. To our best
knowledge, Cd-CBCD represents the first carbazole-functionalized LMOF as a highly
effective fluorescence sensor for pesticides and antibiotics detections.
COOH
N
HOOC
COOH
Scheme 1. The chemical structure of H CBCD.
3
Experimental section
Materials and measurements
All the chemicals were purchased commercially and used directly. The details of
H CBCD synthesis were described in the ESI†. FT-IR spectra were recorded on an
3
Equinox 55 FT-IR spectrophotometer. UV-spectra analysis were carried out on a
Shimadzu UV-2600 spectrophotometer. Elemental analyses of C, H, and N were
measured on a Perkin-Elmer 2400 elemental analyzer. Powder X-ray diffraction
(PXRD) data were collected over the 2θ range of 4-50° using a Philips X'Pert PRO
automated diffractometer (Cu Kα radiation, λ = 1.54178 Å). Thermogravimetric
analyses (TGA) were performed in a temperature range from ambient temperature to
4

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8
00 °C in a N atmosphere with a Pyris1 thermogravimetric analyzer. Luminescence
2
spectra of the samples were recorded on a FLS920 spectrophotometer with a xenon
arc lamp as the light source at room temperature.
Synthesis of [Cd (CBCD) (DMA) (H O) ]·10DMA (Cd-CBCD)
3
2
4
2
2
A mixture of Cd(NO ) ·4H O (12.3 mg, 0.04 mmol) and H CBCD (6.0 mg, 0.01
3
2
2
3
mmol) was dissolved in 2 mL DMA. After ultrasonic treatment for several minutes,
the mixture was sealed in a Teflon-line stainless steel vessel (25 mL), and heated at
1
20 °C for 3 days, and then cooled to room temperature over 24 h. Colorless crystals
of Cd-CBCD were obtained with a yield of 45% (based on H CBCD). Anal. calcd
3
(%): C, 57.60; H, 6.28; N, 8.02. Found (%): C, 57.56; H, 6.31; N, 8.04. IR (KBr,
−
1
cm ) (Fig. S5†): 3451 (m), 2935 (w), 1605 (s), 1527 (s), 1397 (s), 1406 (s), 1310
w), 1270 (m), 1229 (w), 1186 (m), 1013 (m), 858 (m), 782 (s), 729 (w), 596 (w).
(
Fluorescence measurements
All luminescence measurements were carried out in DMA solution media at room
temperature. Before fluorescence measurements, 3.0 mg of finely grounded
Cd-CBCD powder was dispersed in 3.6 mL DMA and treated by ultrasonication for
1
0 minutes to form a steady turbid suspension. Then, fluorescence measurements
were conducted with gradual addition of different analytes to the above obtained
suspension. The fluorescence emission spectra were recorded from 380 to 650 nm
with an excitation wavelength (λ ) at 370 nm.
ex
After luminescence measurements, the quenching efficiency was calculated by
(
I − I)/I × 100%, where I and I are the luminescence intensities before and after
0 0 0
adding analytes, respectively. The relationship between fluorescence quenching
efficiency and the concentration of analytes can be quantitatively analyzed by the
Stern−Volmer (SV) equation (I /I = 1 + K [M]). K is the quenching effect
0
sv
sv
−
1
coefficient (M ) and [M] is the molar concentration of analytes.
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X-ray crystallography analysis
Single crystal X-ray analysis of Cd-CBCD was performed on a Bruker Smart Apex II
CCD detector using Cu Kα radiation (λ = 1.54178 Å) at 100 K. The structure of
Cd-CBCD was solved with direct methods and refined by the full matrix least-squares
2
49
on F with SHELXL. Anisotropic displacement parameters were applied to all
non-hydrogen atoms. The hydrogenen atoms were included and generated
geometrically. The free solvent molecules in Cd-CBCD were highly disordered, and
no satisfactory disorder model could be achieved. Thus, the PLATON/SQUEEZE
routine was used to remove their diffraction contributions. CCDC-1866375
(Cd-CBCD) contains the supplementary crystallographic data for this paper. A
summary of crystallographic data and structure-refinement parameters is given in
Table 1, and the lengths and angles of the selected bonds are listed in Table S1†.
Table 1 The crystal data and structure refinement for Cd-CBCD
Compound
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
Cd-CBCD
94 3 6 18
C H84Cd N O
1922.87
Triclinic
P1
13.3640(2)
14.8268(3)
17.2128(3)
88.1000(10)
82.4580(10)
84.0750(10)
3362.40(10)
1
b (Å)
c (Å)
α (°)
β (°)
γ (°)
V (Å3)
Z
−
3
D
c
/(g cm )
0.950
−
1
μ/(mm )
F(000)
4.149
978
θ range (°)
Reflections collected
Parameters
T (K)
2.590-58.209
9041
553
100(2)
GOF on F
2
0.960
R indices [I > 2σ(I)]
R
1
= 0.0382
= 0.0983
= 0.0460
= 0.1019
wR
2
R indices (all data)
R
1
wR
2
6

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Results and discussion
Crystal structure of [Cd (CBCD) (DMA) (μ-H O) ]·10DMA (Cd-CBCD)
3
2
4
2
2
Single crystal X-ray diffraction indicates that Cd-CBCD crystallizes in the triclinic P1
space group. The asymmetric unit consists of one and a half crystallographically
independent Cd(II) atoms, one CBCD ligand, one coordinated DMA molecule, one
μ-H O molecule, and six free DMA molecules. As revealed in Fig. 1a, the Cd(1) atom
2
adopts a regular octahedral coordination geometry surrounded by six oxygen atoms,
four of which from carboxylate groups constituting the equatorial plane and two of
which from two μ-H O molecules located at the axial sites. The Cd(2) atom is
2
coordinated by seven oxygen atoms in triangular prism coordination geometry: two
oxygen atoms from one chelating bidentate carboxylate group, two oxygen atoms
from one chelating-bridging tridentate carboxylate group, one oxygen atom from one
bridging bidentate carboxylate group, one oxygen atom from one μ-H O molecule and
2
one oxygen atom from one coordinated DMA molecule. Subsequently, two Cd(2) and
one Cd(1) are connected through four carboxylate groups and two μ-H O molecules to
2
form a centrosymmetric linear trinuclear unit of [Cd (COO) (μ-H O) ] as the SBU for
3
6
2
2
Cd-CBCD. The range of Cd−O bond lengths is from 2.207(2) Å to 2.553(3) Å. Each
CBCD ligand utilizes its three carboxylate groups to connect five Cd(II) atoms
through three distinct coordination modes and each SBU connects to six neighboring
SBUs by the CBCD ligands to form a 2D layer structure (Fig. 1b and c). Finally, the
2
D layers can further form an extended 3D supramolecular architecture through the
π⋯π stacking interactions and the solvent accessible volume is calculated as 53.6% by
PLATON (Fig. 1d).
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Fig. 1 (a) Linear trinuclear unit as SBU in Cd-CBCD; (b) the bonding mode of CBCD
ligand; (c) the 2D layer structure in Cd-CBCD; (d) the 3D supramolecular
architecture.
Thermal stability analysis and PXRD analysis
The thermal stability of Cd-CBCD was investigated by TGA. The TGA curve shown
in Fig. S6† reveals a continuous mass loss of 42.99% from room temperature to
2
4
60 °C owing to the removal of the free and coordinated DMA molecules (calcd:
3.65%). When the temperature increases to 380 °C, a quick mass loss is observed,
which may correspond to the decomposition of the framework. To confirm the phase
structure of Cd-CBCD, PXRD analysis was conducted, as shown in Fig. S7†. It is
seen that the measured PXRD patterns match the simulated ones from the single
crystal X-ray diffraction data quite well, confirming the synthesized sample as
Cd-CBCD.
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Photoluminescence Properties
1
0
It is well known that the MOFs constructed from d metal ions and electron-rich
π-conjugated ligands are the promising candidates for photoactive materials. To
determine the fluorescence properties of the organic ligand (H CBCD) and
3
Cd-CBCD, fluorescence measurements were performed in the solid state at room
temperature. As indicated in Fig. S9†, H CBCD presents an emission peak at 466 nm
3
(
λ = 405 nm), while Cd-CBCD displays an emission peak at 467 nm (λ = 390 nm),
e
x
e
x
which suggests that the emission peak of Cd-CBCD is very close to that of H CBCD.
3
In light of the similarity in the fluorescence spectra of H CBCD and Cd-CBCD, the
3
fluorescence of Cd-CBCD should originate from the CBCD-centered emission.
Moreover, Cd-CBCD exhibits a higher fluorescence intensity, indicating that the
formation of the framework structure could enhance the fluorescence emission of the
ligand.
Sensing of Antibiotics
Fig. 2 Fluorescence intensities of Cd-CBCD dispersed in DMA solution with 0.10
mM of various antibiotics.
In consideration of the widespread use of antibiotics and their potential harm to
human health, we sought to examine the application of Cd-CBCD in sensing trace
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quantities of antibiotics. In this study, DMA was selected as the dispersed solvent for
fluorescence measurements because of the superior stability of Cd-CBCD in DMA.
Five classes of frequently used antibiotics, β-lactams (penicillin, PCL),
chloramphenicols (chloramphenicol, CAP; thiamphenicol, THI), sulfonamides
(sulfadiazine, SDZ; sulfamethazine, SMZ), nitroimidazoles (metronidazole, MDZ;
dimetridazole, DTZ), and nitrofurans (NFs) (nitrofurazone, NZF; nitrofurantoin, NFT)
were checked (Scheme S2†). For the fluorescence measurements, 3.0 mg finely
grinded powder of Cd-CBCD was evenly dispersed in 3.6 mL DMA solution
containing these antibiotics to form stable turbid suspensions, and then the responsive
fluorescence intensities were recorded. As can be seen from Fig. 2 and Fig. S10†,
NZF and NFT exhibit the high quenching efficiencies of 91.6% and 88.3%,
respectively, while the quenching efficiencies of the other antibiotics are negligible.
The high quenching efficiency for NFs suggests that Cd-CBCD can be used as a
promising fluorescent sensor for the highly selective recognition of NFs.
Subsequently, the fluorescence titration experiments were performed to check the
sensing sensitivity of Cd-CBCD towards NZF and NFT. As shown in Fig. 3a and 3c,
a progressive decrease in the fluorescent intensity of Cd-CBCD is observed with the
incremental addition of NFs. Fig. 3b and 3d show the nearly linear SV plots of NZF
and NFT at low concentrations. With the increase of NZF and NFT concentrations,
the SV plots deviate from the linearity and bend upwards. The nonlinear SV plots at
higher concentrations may be ascribed to an energy-transfer process or
self-absorption.50, 51 From the linear fitting of the SV plots, the calculated K values
sv
4
−1
4
−1
for NZF and NFT were 9.72 × 10 M and 6.39 × 10 M , respectively. The
calculated limits of detection for NZF and NFT are 85 and 128 ppb according to the
3
δ IUPAC criteria, where δ is the standard deviation for three times of luminescent
intensity in the blank solution. The detection limits are comparable to or better than
the well-designed LMOFs for the detection of NZF/NFT (Table S2).52-55 To the best
of our knowledge, this work represents the first carbazole-functionalized LMOF as an
efficient luminescent sensor for the detection of trace levels of antibiotics.
Practically, anti-interference ability is an important performance for chemical
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sensors. The superior fluorescence sensing performance of Cd-CBCD towards NFs
inspires us to check the anti-interference ability of Cd-CBCD for NFs detection. As
shown in Fig. 4a, in the presence of other interfering antibiotics, the luminescence
quenching effect of Cd-CBCD caused by NZF is almost not influenced, indicating
that Cd-CBCD can effectively detect NFs with the exceptional anti-interference
ability. As a sensor, the recyclability is also one of the important evaluation
indicators. To evaluate the recyclability of Cd-CBCD, cyclic luminescence tests were
performed. The Cd-CBCD sample was recycled by centrifuging the suspension and
further washing several times with fresh DMA after luminescence tests. As shown in
Fig. 4b, the quenching efficiencies of Cd-CBCD for NZF remain almost unchanged
after five cycles, which suggests the good recyclability of Cd-CBCD. Apparently, the
luminescence test results indicate that Cd-CBCD is an ideal fluorescence sensor for
detecting antibiotics with high sensitivity, good recyclability and superior
anti-interference capability.
Fig. 3 Variations of the emission spectra of Cd-CBCD with the incremental
1
1

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concentrations of NZF (a) and NFT (c). The linear relationship of the SV plots of
NZF (b) and NFT (d) in low concentration ranges. (The inset is the SV plots of
selected antibiotics in the whole tested concentration range.)
Fig. 4 (a) The fluorescence emissive response of a mixture of Cd-CBCD and 0.10
mM competing antibiotics upon addition of 0.08 mM NZF. (b) Recyclability test on
Cd-CBCD sensing of NZF. (Blue bars: the initial fluorescence intensity without
adding NZF. Rose red bars: the fluorescence intensity after adding NZF.)
Detection of NACs and pesticides
Fig. 5 Fluorescence intensities of Cd-CBCD dispersed in DMA solution with 0.10
mM of various NACs.
Nitroaromatic compounds (NACs) are usually recognized as environmental pollutants
and potential explosives. The rapid and sensitive detection of NACs is very important
to national homeland security and environmental protection. The intense blue-light
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emission characteristic of Cd-CBCD promotes us to examine its application in
detecting NACs. In this work, seven common NACs (shown in Scheme S3†),
including nitrobenzene (NB), 2-nitrotoluene (2-NT), 3-nitrophenol (3-NP),
4
1
-nitrophenol (4-NP), 3-nitroaniline (3-NA), 4-nitroaniline (4-NA) and
,3-dinitrobenzene (1,3-DNB), were investigated, and their quenching effects on the
luminescence of Cd-CBCD were presented in Fig. 5 and Fig. S13†. It can be seen that
a significant quenching effect of 4-NA is observed on the fluorescence of Cd-CBCD
while the quenching effects of other NACs can be negligible. The quenching
efficiencies of Cd-CBCD follow the order of 4-NA > 3-NA > 4-NP > 3-NP >
1
,3-DNB > 2-NT > NB.
Fig. 6 (a) Variation of the emission spectra of Cd-CBCD with the incremental
concentration of 4-NA. (b) The linear relationship of the SV plot of 4-NA in low
concentration range. (The inset is the SV plot of 4-NA in the whole tested
concentration range.)
Fig. 7 (a) The fluorescence emissive response of a mixture of Cd-CBCD and 0.10
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mM competing NACs upon addition of 0.09 mM 4-NA. (b) Recyclability test on
Cd-CBCD sensing of 4-NA. (Cyan bars: the initial fluorescence intensity without
adding 4-NA. Orange bars: the fluorescence intensity after adding 4-NA.)
To further explore the concentration-dependent luminescence response of
Cd-CBCD towards 4-NA, the fluorescence emission of Cd-CBCD was monitored at
different concentrations of 4-NA. During the gradual addition of 4-NA to the
suspension of Cd-CBCD, the fluorescence emission peak decreases gradually. The
relationship between fluorescence quenching efficiency and 4-NA concentration
could be further analyzed by the SV equation. As shown in Fig. 6b, the SV plot of
4
-NA exhibits a good linear correlation between the emission intensity and the
concentration of 4-NA in the range of 0 to 6 mM, which indicates that Cd-CBCD can
also be employed as a potential chemosensor for the quantitative detection of 4-NA.
4
−1
From the SV plot, the calculated K value of Cd-CBCD for 4-NA is 5.70 × 10 M ,
sv
which is superior to the most reported MOF-based luminescent sensors using for
4
3
-NA detection (Table S4).56-60 The corresponding detection limit, as the ratio of
δ/K , is 116 ppb. Furthermore, the addition of 4-NA to a mixture of Cd-CBCD and
sv
high concentrations of other NACs immediately causes the almost equal quenching
effects of the fluorescence to the blank solution (Fig. 7a), which suggests the superior
anti-interference ability of Cd-CBCD. The cyclic luminescence tests indicate that
Cd-CBCD could be reused after simple centrifugation and washing, without
degradation in the detection performance (Fig. 7b). The high quenching efficiency
and superior anti-interference performance of Cd-CBCD towards 4-NA enable it to
effectively identify the existence of trace amounts of 4-NA.
The high quenching effect coefficient and low detection limit of Cd-CBCD
towards 4-NA inspire us to evaluate the possibility of the luminescence detection of
Cd-CBCD towards 2,6-dichloro-4-nitroaniline (DCN), which functions as a
broad-spectrum pesticide to control many kinds of plant diseases. To determine the
detection limit of Cd-CBCD towards DCN, fluorescence measurements were
conducted with gradually increasing the concentration of DCN, and a linear SV plot
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was obtained at low DCN concentrations, as show in Fig. 8. On the basis of the SV
4
−1
plot, the calculated K value is 4.47 × 10 M , and then the calcualted detection limit
sv
by the 3δ IUPAC criteria is 145 ppb, which is comparable to the reported LMOFs
6
1
using for the detection of DCN. In addition, Cd-CBCD could be fully recovered
after five detetion cycles and the quenching efficiencies were basically unchanged
(Fig. 9). These facts suggest that Cd-CBCD possesses high sensitivity and
recyclability for pesticide sensing applications.
Fig. 8 Variation of the emission spectra of Cd-CBCD with the incremental
concentration of DCN. (b) The linear relationship of the SV plot of DCN in low
concentration range. (The inset is the SV plot of DCN in the whole tested
concentration range.)
Fig. 9 Recyclability test on Cd-CBCD sensing of DCN. (Red bars: the initial
fluorescence intensity without adding DCN. Green bars: the fluorescence intensity
after adding DCN.)
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Mechanism of Luminescent Sensing
To determine the fluorescence quenching mechanism of Cd-CBCD towards NFs,
-NA and DCN, characterizations (including PXRD, UV-vis absorption spectra) and
4
theoretical calculations on the highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energy levels were performed. First, the
PXRD patterns of Cd-CBCD after detecting these analytes are similar to that of the
original sample, which confirms that the luminescence quenching effect is not caused
by the collapse of the framework (Fig. S8†). Simultaneously, on account of the
electron withdrawing properties of nitro-compounds, the electron may transfer from
the electron-rich framework to these analytes. Therefore, the HOMO and LUMO
energy levels of the ligand in Cd-CBCD and these analytes were calculated based on
density functional theory (DFT) method. It can be seen that under photon excitation,
the excited-state electron in the higher LUMO of H CBCD may transfer to the lower
3
LUMO of analytes, and then leading to different degrees of fluorescence quenching
effects (Fig. S11 and S14†). As for the checked antibiotics, the lowest LUMO
energies of NFs account for the maximum quenching effect for NFs. However, for
NACs, the order of quenching effect doesn’t show consistency with the LUMO
energies of NACs, especially for 4-NA, which has the highest LUMO energy but
displays the highest luminescence quenching effect. This phenomenon indicates that
the photo-induced electron transfer (PET) is not the only mechanism for luminescence
quenching. The resonance energy transfer (RET) process can also strongly promote
the fluorescence quenching process, which depends on the extent of spectral overlap
between the emission spectrum of the framework structure and the absorption
spectrum of the analytes. As revealed in Fig. S12 and S15†, there are the effective
spectral overlaps between the fluorescent emission spectra of Cd-CBCD and the
UV-vis absorption spectra of NFs, 4-NA and DCN, while no effective spectral
overlaps are observed between Cd-CBCD and the other analytes. Therefore, the
combination of PET and RET should be responsible for the high fluorescence
quenching effects of Cd-CBCD towards NFs, 4-NA and DCN. Additionally, the
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carbazole possesses a condensed ring structure and rich π-electrons, while
nitro-compounds have strong electron-withdrawing ability. The presence of carbazole
fragment could increase the electron cloud density of Cd-CBCD, thereby leading to
strong host-guest interaction by the electron-transfer or energy-transfer processes, and
then high selectivity and sensitivity for luminescent detection. Some researches have
also demonstrated that the incorporation of carbazole structural unit is a feasible way
to acquire the efficient luminophore for fluorescence sensing.62-64
Conclusions
In summary, we successfully synthesized a Cd-based luminescent MOF (Cd-CBCD)
based on a new carbazole-functionalized ligand (H CBCD) and Cd(II) ions.
3
Cd-CBCD is a 3D supramolecular framework built on its stacked 2D layers.
Fluorescence measurements indicate that Cd-CBCD exhibits high selectivity and
sensitivity for the detection of NFs, 4-NA and DCN. The high fluorescence quenching
efficiencies of Cd-CBCD towards NFs, 4-NA and DCN could be ascribed to the
coexistence of electron and energy transfers. The rational incorporation of
photon-responsive carbazole structural units provides a new idea for the design and
synthesis of highly efficient MOF-based chemosensors. This work represents the first
carbazole-functionalized LMOF as the fluorescent sensor for highly selective and
sensitive detection of antibiotics, pesticides and nitroaromatic compounds.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors thank the support of National Natural Science Foundation of China (No.
5
1571097). The authors also thank the support of analytical and testing center of
Huazhong University of Science and Technology.
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Table of contents entry
A carbazole-functionalized Cd-MOF exhibits highly selective and sensitive
fluorescence detection towards antibiotics, pesticides and nitroaromatic compounds.