Novel molecularly imprinted polymers on metal-organic frameworks as sensors for the highly selective detection of zearalenone in wheat

Article abstract of DOI:10.1039/c9nj00589g

The work describes an opto-sensor for the highly sensitive detection of the zearalenone (ZON) in wheat. A molecularly imprinted polymer layer was coated onto the surface of a luminescent metal-organic framework with cyclodo-decanyl-2,4-dihydroxybenzoate (CDHB), an analog of ZON, chosen as the template and 3-aminopropyltriethoxysilane (APTES) as the functional monomer. The obtained MOF@MIP (MMIP) was fully characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), N₂ adsorption-desorption measurements, and Fourier transform infrared spectroscopy (FT-IR). Under optimal conditions, the relative fluorescence intensity of MMIP decreased linearly with a concentration of ZON ranging from 0.05-1.0 mg L^-1, with a detection limit of 0.018 mg L^-1 and a good recovery of 95.53-98.01% (RSD < 4.7%). The sensor was stable and selective. Ultimately, the rapid and accurate detection of zearalenone in wheat was successfully realized.

Full text of DOI:10.1039/c9nj00589g

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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A novel molecularly imprinted polymers on metal organic frameworks as sensor for
highly selective detection of zearalenone in wheat
Qiuzheng Du1, Pu Wu1, Fan Hu1, Geyuan Li1, Jianrong Shi*2 3 4, Hua He*1 5 6
Received 00th January 20xx,
Accepted 00th January 20xx
The work describes a opto-sensor sensor for highly sensitive detection of the zearalenone (ZON) in wheat. A layer of a
molecularly imprinted polymer layer was coated onto the surfaces of luminescent metal organic framework (MMIP) with
cyclodo-decanyl-2,4-dihydroxybenzoate (CDHB), an analog of ZON chosen as the template and the 3-
aminopropyltriethoxysilane (APTES) as the functional monomer. The obtained MMIP was fully characterized by scanning
electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), N2 adsorption-desorption
measurements and fourier transform infrared spectroscopy (FT-IR). Under optimal conditions, the relative fluorescence
intensity of MMIP decreased linearly with the concentration of ZON from 0.05-1.0 mg·L-1, with a detection limit of 0.018
mg·L-1 and good recovery from 95.53-98.01% (RSD<4.7%). The sensor was stable and selective. Eventually, rapid and
accurate detection of zearalenone in wheat was successfully realized.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Introduction
expensive instruments and professional operations, which are
inconvenient. Therefore, a simple, selective and sensitive analytical
ZON (also known as F-2 toxin) is produced by Fusarium fungi
method for detection of trace ZON is still required.
and widely exists in moldy cereal crops, such as wheat, corn and
1
Metal organic frameworks (MOFs) comprised of inorganic
(metal ions/metal clusters) and organic ligands, are referred to a
new family of crystalline porous hybrid. The richness of both
components can provide infinite numbers of MOF hybrid materials
other grains . A large number of researches have confirmed that
ZON can cause digestive system dysfunction, affect the
development of visceral organs, and may have carcinogenic effects
on the human body 2, 3. Thus ZON has been listed in the food safety
key monitoring content by World Health Organization and many
countries 4. The maximum limit of ZON is 350 and 100 μg·kg-1
respectively in unprocessed maize and unprocessed cereals in the
European Union 1. Numerous methods have been developed for the
ZON detection such as high-performance liquid chromatography
(HPLC), ultra high pressure liquid chromatography tandem mass
9,
in principle 10. The outstanding characteristics, such as infinite
tunability, high porosities, and excellent thermal stability increased
13
their applications in sensing 11, gas storage 12, nonlinear optics
,
14
catalysis and drug delivery 15. Amine functionalized MOFs have
excellent sensitivity to detect metal ions, small organic molecules,
and aromatic explosives as a luminescent probe 16-18, due to the
Lewis basic sites of amine. Lewis basic amine functionality of MOFs
can lead to the binding of analyte molecules through hydrogen
spectrometry
(UPLC-MS/MS),
gas
chromatography
assay (FLISA)
(GC),
and
bonding and induce electron transfer between MOFs and analytes 19
.
fluorophore-linked
immunosorbent
1, 5-8
electrochemical sensor
and so on. But these methods require
Molecularly imprinted polymers (MIPs) have been considered
as promising materials that are able to specifically rebind a target
molecule 20. MIPs have the advantages such as low cost, easy
preparation, specific selection and high stability. Recently,
developing advanced MIP-based chemosensors is becoming one of
1 Department of Analytical Chemistry, China Pharmaceutical University, Nanjing
211198, China
2 Key Laboratory of Food Quality and Safety of Jiangsu Province-State Key
Laboratory Breeding Base, Nanjing 210014, China;
24
the main recent focuses 21-23. Liu et al. prepared a dual-function
3 Key Laboratory of Control Technology and Standard for Agro-product Safety and
Quality, Ministry of Agriculture, Nanjing 210014, China
4 Key Laboratory of Agro-product Safety Risk Evaluation, Ministry of Agriculture,
Nanjing 210097, China;
Key Laboratory of Biomedical Functional Materials, China Pharmaceutical
University, Nanjing 211198, China
6 Key Laboratory of Drug Quality Control and Pharmacovigilance, Ministry of
Education, China Pharmaceutical University, Nanjing 210009, China
Corresponding Authors
* To whom correspondence should be addressed: China Pharmaceutical University,
639 Longmian Avenue, Nanjing 211198, Jiangsu Province, China. Tel.: +86 025
86185160; E-mail addresses: jcb_321@163.com, dochehua@163.com (H. He) and
QDs@MIP sensor for highly selective and sensitive detection of
ractopamine. Huy et al. 25 prepared MIPs on the surface of CdTe QDs
5
for selective optosensing of clenbuterol and melamine. In recent
26
years, Chen et al.
chose the modified silica nanoparticles as
27
support material to prepare the MIPs nanoparticles. Qian et al.
reported MIL-101 supported surface-imprinted nanoparticles for
the sensitive detection of metolcarb. Moreover, Fe3O4 nanoparticles
Institute of Food Safety and Nutrition, Jiangsu Academy of Agriculture Science, and quantum dots also can be used as solid supports for surface
Nanjing 210014, China, jianrong63@126.com (J.R. Shi).
Qiuzheng Du and Pu Wu contributed equally to this work and should be considered
co-first authors.
28, 29
imprinting
. Compared with traditional solid supported
materials, MOFs have porous structure and high specific surface
area, which can improve the adsorption capacity and rate of mass
b
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transfer.
the reaction, cyclododecanol (1.66 g) and DBU (1.6 mL) were added
View Article Online
DOI: 10.1039/C9NJ00589G
Based on the above considerations, we report a strategy to and stirred at 40℃ under N2 until the start material was
combine the high selectivity of MIPs with the fluorescence and high disappeared. Then 20 mL of ethyl acetate was added to the solution.
specific surface area of MOFs to prepare a new composite material The organic phase was adjusted to weak acidity with 2 mol·L-1 HCl
for direct sensing of ZON in wheat. The MOF was synthesized by and washed with saturated sodium chloride solution (15 mL×3), and
solvothermal reaction contained Al3+ as the metal center linked to once with deionized water. Then the mixture was separated and the
oxygen atoms from 2-aminoterephthalic acid (NH2-BDC) and then organic phase was dried over anhydrous Na2SO4 and vacuum
was modified by MIPs layer using CDHB, an analog of ZON as the evaporated to give a pale yellow crude product. The crude product
template and the 3-aminopropyltriethoxysilane (APTES) as the was purified by silica gel column (100-200 mesh silica gel, 10 cm
functional monomer via a sol-gel process to obtained MMIP. Thus, a column length), and with an ethyl acetate/petroleum ether = 0-10%
novel sensor with high selectivity as well as direct determining ZON gradient elution to give pure CDHB as a white solid with a yield of
in wheat was obtained. As far as we know, this is the first report of 51%.
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ZON detection in complex samples by fluorescence sensing MMIP.
Synthesis of MOF (NH2-MIL-53(Al))
Experimental section
MOF was synthesized using a modified hydrothermal method
Chemicals and materials
31. Specifically, AlCl3·6H2O (3.6 mmol) was mixed with NH2-BDC (3.6
2,4-dihydroxybenzoic acid (98.0%), N, N’-Carbonyldiimidazole mmol) in 30 mL deionized water. The mixture was stirred at room
(CDI, 98.0%), cyclododecanol (98.0%), 1,8-diazabicyclo undec-7-ene temperature to uniform dispersion, then transferred to a Teflon
(DBU, 98.0%), 3-Aminopropyltriethoxysilane (APTES, 98.0%), insert and placed in an autoclave to synthesis MOF. The autoclave
Tetraethyl orthosilicate (TEOS, 98.0%) were obtained from Aladdin was kept at 150°C for 5 h under static conditions. After the reaction
(Shanghai, P.R. China). N, N-dimethylformamide (DMF, 99.5%), was completed, the yellow product was centrifugal separation and
absolute ethanol (C2H5OH, AR), ammonia (AR), ethyl acetate (AR), washed with ethanol. Then the product was dispersed in 30 mL
aluminum chloride hexahydrate (AlCl3·6H2O, AR) and NH2-BDC DMF solution and treated at 150°C for 8 h to remove unreacted
(99.0%) were purchased from Shanghai Maclean Biochemical NH2-BDC in the pores. The product was centrifuged and dried in a
Technology Co., Ltd. Chrysin (CN, 98%), quercetin (QN, 98%) and vacuum oven at 130°C to remove trapped DMF.
acetaminophen (ACP, 98%) were purchased from China National Synthesis of MMIP
Institute for Food and Drug Control (Beijing, China). ZON (BR) was
The synthesis of fluorescent imprinted polymer (MMIP) was
adapted from the literature reported previously 27. Specifically,
purchased from Saan Chemical Technology (Shanghai) Co., Ltd.
Wheat was purchased from a supermarket (Nanjing, P.R. China).
template (CDHB, 0.125 mmol) was dissolved in 5 mL absolute
Instrumentation and Characterization
Fluorescence spectrometer
ethanol, then 0.5 mmol functional monomer APTES was added and
stirred for 2 h at room temperature to obtain prepolymerized
solution. Subsequently, MOF (50 mg) was dissolved in 25 mL
absolute ethanol and the prepolymerized solution was slowly added
dropwise to MOF solution, under a magnetic stirring for 10 min.
Then 0.75 mmol cross-linker TEOS and 2 mL 1 M catalyst (aqueous
ammonia solution) were added and stirred at room temperature for
24 h to obtain MMIP. Finally, MMIP was calcined in a vacuum oven
at 130°C for 2 h and washed by absolute ethanol and ammonia (9:1,
a/a) to remove the template until no CDHB was detected by the
ultraviolet-visible spectrophotometer. Then, the product was dried
in a vacuum oven at 60°C. Similarly, fluorescent non-imprinted
polymer (MNIP) was also prepared in the same process as MMIP in
the absence of template CDHB.
(RF-5301PC,
Shimadzu
Corporaꢀon, Japan)i Fourier transform infrared spectrometer (FT-
IR, Shimadzu 8400, Shimadzu Corporaꢀon, Japan) (400-4000 cm-1)i
Ua-visible spectrophotometer (Ua-1800, Shimadzu Corporaꢀon,
Japan)i scanning electron microscope (SEM, S-3400NII, Hitachi,
Japan)i X-ray powder diꢁractometer (XRD, X TRA-Switzerkand, JEOL
Lid, Japan)i NOaA4000e automaꢀc surface and porosity analyzer
(Quantachrome Instruments, USA)i ultrasonic generator (aH2200a,
40 kHz, aunshan, China)i analyꢀcal balance (BS124S, Sartorius,
Germany)i electrothermal constant temperature drying oven (101-
0B, Nanjing, China)i vacuum Drying box (DZF-6050, Shanghai,
China).
Synthesis of CDHB
Fluorescence measurements
The synthesis of CDHB was adapted from the literature
reported previously 30, the specific operation method was as follows:
2,4-dihydroxybenzoic acid (1.50 g) was mixed with CDI (1.79 g) in
anhydrous DMF (15 mL) and stirred at 26℃ for 2 h under at N2
atmosphere. Then thin layer chromatography plate (TLC) (ethyl
acetate:petroleum ether = 1:1) was used to detect if 2,4-
dihydroxybenzoic acid was completely reactive. After completion of
The fluorescence detection of MMIP and MNIP were carried
out under the same conditions. The excitation wavelength was 335
nm, and emission was recorded at 350-600 nm. The maximum
emission was at 425 nm. The excitation and emission slit widths are
2 nm unless otherwise specified. In addition, each spectrum was
obtained after three parallel scans. After removal of the template,
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an appropriate quantity of MMIP and a given concentration of (d, J=2.30 Hz, 1H), 5.24-5.15 (m, 1H), 1.81 (dd, J=13.6, 6.80 Hz, 2H),
View Article Online
DOI: 10.1039/C9NJ00589G
CDHB or ZON standard solution (4 mL) were added to EP tube. After 1.61 (dd, J=13.7, 4.90 Hz, 2H), 1.52-1.24 (m, 18H). ESI-MS (Fig. S3)
adsorption for a certain period of time, it was transferred into a m/z 319.1 ([M-H]-1) Calculate for C19H28O4: 320.2. FT-IR spectra (Fig.
fluorescent cuvette, then the fluorescence intensity before and S4) νmax/cm-1 3382 (O-H), 2940 (saturated C-H), 1657 (aromatic C=C),
after MMIP adsorption was measured by fluorescence 1519 (aromatic C=C), 1470 (aromatic C=C), 1277 (C-O), 850 and 787
spectrophotometer, and the change was observed. Three replicates (aromatic C-H). λmam abs (MeOH)/nm 259, 298 (Fig. S1C). The data
9
were performed for each measurement.
for all characterization were in agreement with the reported by
The control experiments of the MNIP were processed Urraca30.
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following the same procedure.
Characterization of MMIP
In the synthesis of MOF, we used the molar ratio Al3+ : H2N-BDC
Sample preparation
ZON is usually found in the cereal crops. In this study, wheat of 1:1, and water as the solvent to form the complex Al3+ : H2N-BDC :
was chosen as a complex sample to evaluate the application of the H2O at a high temperature of 150℃. Then we used DMF to
prepared MMIP sensor for detecting ZON. Acetonitrile (15 mL) with exchange free terephthalic acid to activate MOF. DMF was removed
aqueous NaCl (10 mL, 50 g·L-1) were added to the 80-mesh sieve at 130℃ in a vacuum oven. The synthesized MOF structure
wheat (5 g). The mixture was shaken in a shaker at high speed for 5 constitutes chains of µ-OH corner-sharing AlO6-octahedra, which
min and ultrasonically extracted for 15 min, then filtered. The are interconnected by H2N-BDC molecules to form one-dimensional
filtrate was rotary evaporated to near dryness at 50℃ and pores32. In this study, novel surface–imprinted nanoparticles using
redissolved in 4 mL chloroform for analysis. The recovery study was MOF as a support were prepared for the selectivity detection of
done by adding three concentrations (20, 60 and 120 μg·kg-1) of ZON. MOF has a large amount of -NH2 and residual -COOH on its
ZON to the wheat. The wheat-ZON mixture was allowed to stand for surface, which provided a possibility for many reactions to occur. A
12 h before extraction.
sol-gel polymerization method was used to prepare MMIP
fluorescently imprinted polymer. After adding the template
Analysis of wheat samples
After extraction, 4 mL of wheat samples and standard molecule CDHB, the functional monomer APTES and the crosslinker
samples were added to the centrifugal tube containing 0.6 mg of TEOS, CDHB was encapsulated in the fluorescently imprinted
MMIP fluorescent polymer. The fluorescence intensity before and polymer by interaction between them. When removing CDHB,
after adsorption of MMIP were determined by fluorescence MMIP with a specific size and recognition site was formed. The
spectrophotometer (Ex=335 nm, Em=425 nm) and the recovery rate synthesis procedure of the MMIP was illustrated in Scheme 1.
was calculated.
Results and discussion
Characterization techniques
Characterization of CDHB
CDHB was chosen as the template to prepare MMIP because
Scheme 1. Flowchart of the synthesis of MIPs.
ZON is highly toxic and expensive. CDHB is very similar to ZON in
The prepared MMIP was characterized by FT-IR, SEM, TEM,
many aspects, including structure, size, shape, hydrophobicity and
XRD and N2 adsorption-desorption measurements. The FT-IR
functional group. In addition, both CDHB and ZON have the same
spectra of MOF, MMIP, MNIP and MIP were compared in Fig. 1 to
phenolic hydroxyl group at the same position (Fig. S8). The specific
assess the formation of the MMIP. As shown in Fig. 1A, the peaks
steps for the separation and purification of CDHB crude products
located at the range of 3200~3700 cm-1 were attributed to the
were as follows: the eluent containing the target product was
stretching vibration of -NH2 and -COOH residues on the ligand. The
peaks at about 1160 and 1500-1700 cm-1 belonged to O-C-O and
concentrated by rotary evaporation at 45℃ to obtain colorless oily
products. The product was dissolved in 3 mL dichloromethane (DCM)
C=C, which originated from the ligand in MOF. In addition, the peak
solution, heated and dissolved, then slowly dripped into 30 mL
at 1400 cm-1 was attributed to the skeleton vibration absorption
petroleum ether under stirring condition, precipitated white solids,
peak of the polymer ligand. In Fig. 1C, the absorption peak of MIP at
and then placed for 30 min. White solids were collected by filtration,
3400 cm-1 was attributed to the residual silanol group and -NH2 in
and then dried to constant weight by rotary evaporation at 45℃.
The white solids (1.60 g) were obtained in yield of 51%. The
the MIP. There was almost no difference in the infrared spectrum
between MMIP (Fig. 1B) and MNIP (Fig. 1D), which indicated that
synthesis results of CDHB was shown in supporting information.
they had similar functional groups and distribution densitiesand and
CDHB was characterized by NMR spectra, mass spectra, Ua
the template molecular was knocked totally. Characteristic peaks in
the range of 3000~3800 cm-1 were attributed to functional groups
absorptions and IR absorptions. Specifically, 1H NMR spectra of
CDHB (Fig. S2) δH (500 MHzi DMSOi CDHB) δ 10.83 (s, 1H), 10.44
such as -NH2, -COOH and -OH residues, and bands around 1500-
(s,1H), 7.65 (d, J = 8.80 Hz,1H), 6.39 (dd, J=8.80, 2.30 Hz, 1H), 6.30
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1700 cm-1 indicated the presence of MOF. The absorption peak
around 1056 cm-1 was attributed to the tensile vibration N2 adsorption porosimetry in order to ensure the porosity of the
characteristics of Si-O-Si, and the peaks at 780 and 462ꢀcm-1 were MOF and MMIP prepared. N2 adsorption-desorption experiment
assigned to the vibration of the Si-O band, which confirmed that was performed at 77ꢀa to determine the surface area and porosity
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Porosity, as a mandatory parameter, should be measured by
View Article Online
DOI: 10.1039/C9NJ00589G
MMIP was successfully prepared 27, 31
.
of the sample (Fig. 4). The porous nature and pore structure of
MMIP could be clearly represented by the N2 adsorption isotherm
(Fig. 4B). The BET specific surface area and total pore volume of
MMIP were 306.76 m2 g-1 and 0.23 cm3 g-1, respectively. It was
proved that the formation of the MIP layer did not affect the
character of the MOF.
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Fig. 4 N2 adsorption-desorption isotherm of MOF(A) and MMIP(B).
Adsorption of MMIP
Fig. 1. FT-IR spectra of MOF(A),MMIP (B),MIP (C) and MNIP (D).
SEM images of MOF and MMIP were shown in Fig. 2. The
average size of the octahedral MOF particles ranged from 4 to 5 μm,
and the particles appeared to highly monodisperse (Fig. 2A). The
surface of MOF was smooth and homogeneous size. Fig. 2B showed
that MOF were fully encapsulated within the MIP coatings after
polymerization. However, after encapsulation, the morphology of
MMIP could still be seen as an octahedron, where the size was not
Optimization of the adsorption condition
Fig. S5 showed that before CDHB was removed, the fluorescence
intensity of MMIP was significantly lower than MNIP. After CDHB
was extracted, the fluorescence intensity of MMIP increased
much different from that of the MOF. The obtained rough surface significantly, almost reaching the strength of MNIP. After adding and
MMIP particles explained that the MIP shell was successfully
prepared. The MIP coating was thin and did not affect the shape of
MOF because the reaction only occurred on the surface of
MIL@MIP.
rebinding of CDHB, the fluorescence intensity of MMIP was
quenched again.
In order to obtain the best adsorption conditions, the
adsorption medium, material amount, pH value and adsorption
temperature were optimized respectively. The MMIP and MNIP was
dispersed in methanol, acetonitrile, chloroform and acetone
solutions of CDHB, respectively. It can be seen from the Fig. S6A that
chloroform was the most suitable adsorption solvent. The amount
of material selected was 0.2-2 mg. The results in Fig. S6B showed
the optimum material amount was 0.6 mg. The pH value was
selected 4-10, and the results were shown in Fig. S6C. It was found
that pH had little effect on adsorption. The adsorption temperature
was selected 20-40°C. In Fig. S6D, the optimum adsorption
temperature was 30°C. Therefore, the following experiments were
carried out under optimal conditions.
Fig. 2. SEM image of MOF(A) and MMIP (B).
MOF and MMIP were further characterized by XRD. The
diffractograms of MOF in Fig. 3A agreed with the theoretical XRD
patterns of reference MOF, of which the characteristic peaks were
observed mainly at 2θ 9.08° and 10.03° 31. It was worth noting that
after surface molecular imprinting (Fig. 3B), the intensity of MOF
XRD peaks decreased significantly, which further indicated that the
MIP coating procedure was successful.
Adsorption equilibrium and kinetic experiments
To evaluate the adsorption behaviors of the MMIP, adsorption
equilibrium and kinetic experiments were conducted. In the
adsorption equilibrium experiments, MMIP, MNIP or MOF was
exposed to different CDHB from 0.05 to 20 mg·L-1. As observed from
Fig. 5A, with the increasing concentration of CDHB, the fluorescence
intensity of MMIP gradually decreased. This phenomenon mainly
was owing to hydrogen bonding between the specific binding site of
MMIP and CDHB. Specifically, the F0/F of the MMIP was 1.39, which
was higher than that of MNIP (1.12) and MOF (1.15) at a given
CDHB solution concentration of 2.0 mg·L-1. The fluorescence
quenching of MMIP and MNIP was related to the concentration of
Fig. 3. Powder X-ray diffraction patterns for MOF (A) and
MMIP (B).
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the target, which was in accordance with the Stern-aolmer equation:
Fig. 6. (A) Selective adsorption of ZON, CN, QN and ACP by MMIP
View Article Online
DOI: 10.1039/C9NJ00589G
and MNIPi (B) Competitive adsorption of ZON, ZON+CN, ZON+ACP
F0 / F 1 KSV [Q]
and ZON+CN+ACP by MMIP and MNIP.
Among them, F0 and F were the fluorescence intensity before
and after the adsorption of the fluorescent polymer, respectively.
aSa was the coefficient of the Stern-aolmer equation and [Q] was
the adsorption target concentration. The imprinting factor (IF) was
Competitive adsorption of MMIP
The fluorescence of MMIP was analyzed in a mixed solution
of ZON and competitors to investigate the adsorption capacity of
MMIP in complex systems. Two kinds of competitive compounds
were used i.e., CN and ACP (Fig. S8). Fig. 6B and Fig. S9B showed
that the fluorescence quenching of the three mixtures was the most
pronounced, due to the similarity structure between ZON, CN and
template molecule. It was more compatible with the imprinted
cavity in MMIP and it was easy to combine with the recognition site
and occupy the cave. Compared the ZON+ACP mixture with the
separate ZON, it could be found that the F0/F of MMIP was relatively
closed and the change was not obvious, which showed that the
recognition of ZON by MMIP was less interfered by competitors.
According to the above analysis, MMIP can be used as a selective
sensing material, which can be used for the identification and
determination of ZON in complex systems.
determined by the ratio of aSa,
to aSa, MNIP, and was used to
MMIP
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evaluate the adsorption specificity. In this study, IF=aSa, MMIP/aSa, MNIP
= 3.17 were calculated. The result indicated that the affinity of
MMIP was much higher than that of MNIP.
Fig. 5. (A) Adsorption equilibrium of MMIP, MNIP and MOF
rebinding CDHB. (B) ainetic uptake of ZON onto MMIP, MNIP and
MOF.
To further study the adsorption property of the fluorescent
polymer, the kinetic experiments of ZON to MMIP, MNIP and MOF
were investigated. The results were shown in Fig. 5B. During the
adsorption process, the rate of adsorption was relatively fast and
then gradually decreased until equilibrium. The optimal adsorption
times of MMIP, MNIP and MOF were both 60 min. And the F0/F of
MOF was bigger than MNIP. The experimental results showed the
larger specific surface area of MOF increased the mass transfer rate
of MMIP, further indicating that MMIP had good adsorption
performance.
The mechanism of luminescence and fluorescence quenching
One of the synthesis methods of the MOF luminescent sensor
is to use organic molecules with rigid skeleton and Lewis base sites
to provide binding sites, and π-conjugated with Lewis base sites to
provide luminescence. The amino nitrogen forms a π-π conjugate
with the benzene ring. The electrons on the conjugated structure
are more easily excited and then returned to the ground state. The
energy is released in the form of photons. In this MOF, metal ions
(Al) are electron acceptors, and amino groups (NH2-BDC) are
electron donors. Thus, the luminescence of MOF is attributed to the
introduction of amino groups16.
Selectivity of MMIP
Selective adsorption experiments were performed to
determinate the selectivity of MMIP. The ZON, CN, QN and ACP
(structure was shown in Fig. S8) were used to evaluate the
selectivity of MMIP towards ZON. Fig. 6A and Fig. S9A showed that
only ZON resulted in significant fluorescence quenching of MMIP,
while CN and QN, ACP had little effect. Although both CN and QN
had hydroxyl functionality and the same position and type as ZON,
their size and shape were different from ZON. For ACP, the shape
and size were also different from ZON. These factors lead to poor
complementarity of CN, QN and ACP with the MMIP binding sites.
Therefore, the selectivity of MMIP towards CN, QN and ACP was
lower than ZON. In general, MMIP had a higher adsorption and
selectivity for ZON.
In order to analyze the mechanism of fluorescence quenching
upon MMIP with ZON, the effect of ZON on the fluorescence
intensity was investigated. MMIP and ZON were dispersed in
chloroform, respectively, and then both were scanned by Ua-ais. Fig.
S7A showed that Ua absorption spectra of MMIP and ZON were
similar, indicating that the absorption bands of them were similar.
When ZON and MMIP were combined by hydrogen bonding,
charges in the MMIP conduction bands would be transferred to the
lowest empty molecular orbital of ZON, which caused fluorescence
33
quenching of MMIP. Tu et al. had reported the similar charge-
transfer mechanisms. In addition, there was no spectral overlap
between ZON absorption and MMIP emission (425 nm), thus energy
transfer was excluded.
To further prove the fluorescence quenching mechanism, the
fluorescence lifetime of MMIP was checked in the absence and
presence of ZON. The emission lifetime of MMIP showed single
exponential function with a lifetime value of 10.18 ms and this
reduced in the presence of ZON with a lifetime value of 7.84 ms
(Figure S7B), which supported the previously indicated electron
transfer mechanism. In summary, the fluorescence quenching of
MMIP prepared in this study could be attributed to electron transfer.
This journal is © The Royal Society of Chemistry 20xx
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(%) (%)
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taken (μg·kg-1)
(μg·kg-1)
19.18
Detection range and limit of MMIP
View Article Online
20
60
95.53
96.52
98.01
DOI: 10.1039/C9NJ020.53809G
The detection linear range and limit of the MMIP were studied
to evaluate its analytical performance. In order to investigate the
accuracy of MMIP in the test, ZON chloroform solution with
concentration of 0.05, 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0 mg·L-1 were
selected as the adsorption solution. Fig. S10 showed that the
fluorescence spectra of MMIP at different concentrations of ZON
and the linear range of MMIP was 0.05-1.0 mg·L-1, with a
correlation of 0.9935 for ZON. The limit of detection was
determined as the corresponding amount of the three-fold value of
the background signal of the instrument generated by the matrix
blank, which was found to be 0.018 mg·L-1. The precision for nine
replicate detections of a 1.0 mg·L-1 ZON solution was 2.34%. As we
all know, different approaches have different superiorities and
disadvantages dealing. To further investigate the performance of
our method, we compared the results with some other methods for
the detection of ZON, which was reported previously in Tab. 1. It
could be seen that this approach provided a more simple,
affordable and fast analysis platform with a good recovery of ZON.
Therefore, the preparation of MMIP could provide an innovative
mentality for ZON detection in grains samples.
wheat
57.92
4.65
3.21
120
117.62
9
Conclusions
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A simple molecularly imprinted optical sensing material
based on luminescent MOF for detecting ZON in wheat sample was
developed. The synthesized MMIP exhibited a high adsorption
capacity and excellent binding affinity. The use of template CDHB to
prepare MMIP reduced the cost of synthesis and increased
operational safety. The quenching fluorescence intensity was
observed and allowed the determination of ZON concentration as
low as 0.018 mg·L-1. MMIP exhibited high selectivity, which could be
used for detecting ZON in complex matrices. So we provided a
convenient, simple and selective detection method for ZON analysis.
Conflicts of interest
The authors declare no competing financial interest.
Tab. 1. Comparison of different methods for the determination of
ZON in wheat.
Acknowledgements
Methods
HPLC
Materials
LOD
Recovery RSD (%)
(%)
Ref.
[34]
This work was financially supported by Guizhou Provincial Science
and Technology Department Joint Fund Project (Qian aehe LH word
[2016] No. 7076), Independent Innovation Fund Project of
Agricultural Science and Technology of Jiangsu Province in 2017
(NoCX (17) 1003.
MAA-4aP-
MMIP
240 ng
g-1
83.5-88.2
˂5.8
LC-MS
HPLC-
FLD
-
15 pg
-
79
8.5
[35]
[36]
Notes and references
MIP
87–97
2.3–5.6
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This work provides a rapid and simple method for the determination of trace substances in
complex systems.