A Sensitive Fluorescent Sensor for Highly Sensitive Detection of Water in Organic Solvents Based on Substituted Imidazole

Article abstract of DOI:10.1007/s10895-021-02729-w

A novel substituted imidazole derivative 1a with carboxyl and quinoline structure has been designed and synthesized. And our initial discovery is that this compound can effectively detect water in ethanol. And the metal in solution couldn’t affect the absorption and fluorescence Spectra of 1a. With the addition of water, the energy band appears a red-shift from 330nm to 355nm in the absorption spectra. And the emission spectrum undergoes an important change in its fluorescent effect in the presence of water. Furthermore, absorption peak of 1a displays a red-shift with increasing pH from 2.31 to 10.72. All available data (absorption and emission) strongly support the possible mechanism. Due to the pronounced fluorescence changing property, the substituted imidazole derivative 1a could be utilized as fluorescent probes for detecting water in ethanol. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10895-021-02729-w

Journal of Fluorescence
https://doi.org/10.1007/s10895-021-02729-w
SHORT COMMUNICATION
A Sensitive Fluorescent Sensor for Highly Sensitive Detection
of Water in Organic Solvents Based on Substituted Imidazole
Huiqian Li1 & Weijian Xue1 & Jinping Wang1 & Jie Ma1 & Bing Zhao1 & Xiangfeng Guo1
Received: 11 February 2021 /Accepted: 23 March 2021
#
The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
A novel substituted imidazole derivative 1a with carboxyl and quinoline structure has been designed and synthesized. And our
initial discovery is that this compound can effectively detect water in ethanol. And the metal in solution couldn’t affect the
absorption and fluorescence Spectra of 1a. With the addition of water, the energy band appears a red-shift from 330nm to 355nm
in the absorption spectra. And the emission spectrum undergoes an important change in its fluorescent effect in the presence of
water. Furthermore, absorption peak of 1a displays a red-shift with increasing pH from 2.31 to 10.72. All available data
(absorption and emission) strongly support the possible mechanism. Due to the pronounced fluorescence changing property,
the substituted imidazole derivative 1a could be utilized as fluorescent probes for detecting water in ethanol.
.
.
.
.
Keywords Substituted imidazole Carboxyl and quinoline A little amount of water No affected by metal ion Aggregation
Introduction
Imidazole, especially multi-substituted imidazole, has been
found in the core fragment of many kinds of anti-inflammatory
and analgesic drugs. [13–15] In addition, due to the optical ac-
tivity of imidazole derivatives, they are also used in many fields
such as fluorescent labeling agents, photoluminescence mate-
rials, biological imaging reagents, etc. [16, 17] In recent years,
due to the wide application of imidazole derivatives in biology,
industry and synthesis, several synthesis methods of imidazole
compounds have been reported, such as heterocyclic rearrange-
ment, four component condensation, phosphoric acid, acetic ac-
id, sulfuric acid or dimethyl sulfoxide (DMSO) catalytic reaction.
[18–25] Besides this, the π-π interaction can cause the organic
compounds possessing such structures to form molecular aggre-
gates with close intermolecular spacing and strong coupling. And
H-aggregation denotes the aggregation showing a blue-shifted
band the molecular absorption band. [26, 27] In this paper, a
novel multiaryl-substituted imidazole compound has been syn-
thesized for detecting water in ethanol by using disaggregation.
Although water is the most important liquid for human beings
and the living being in the world. As the most common contam-
inant in organic solvents, water is often adverse in many organic
synthetic reactions and industrial production processes, which
negatively affected not only the yield of chemicals but also their
activities. [1–5] Therefore the detection of trace water in organic
solvents is very important for organic synthetic reactions and
industrial productions. Normally, Karl-Fisher titration has been
widely employed to detect trace water in organic solvents. [6–8]
However, strict sample manipulation, the use of toxic reagents
and the need for special equipment are unavoidable defects of
this method. Nowadays, fluorescent sensors have been widely
employed to detect trace water in organic solvents caused by
their high sensitivity, inexpensive, quick and easy. [9–12]
Although several fluorescence water probes based on dyes,
which have disadvantages such as difficulty in synthesis, long
preparation period, certain toxicity of the dye itself and environ-
mental pollution, have been reported, it is still an urgent need to
develop a novel sensor which is easily synthesized, fast-response
and highly sensitive.
Experiment
Reagents
* Weijian Xue
hehexueweijian@163.com
Benzoin, 2-quinolinecarboxaldehyde, ammonium acetate, 6-
aminocaproic acid and Iodine were obtained from Shanghai
Aladdin Bio-Chem Technology Co., Ltd. All of the reagents
are analytical grade, which can be used without further
1
College of Chemistry and Chemical Engineering, Qiqihar University,
161006 Qiqihar, People’s Republic of China

J Fluoresc
purification. All solvents were obtained commercially and
used as received without further purification. Double-
distilled water was used throughout the experiments. The met-
al ion solutions were prepared from NaCl, KCl, Mg(ClO4)2,
Ca(NO3)2, Cr(NO3)3, FeSO4, CoSO4, NiSO4, Cu(NO3)2,
Zn(NO3)2, AgNO3, CdSO4, HgCl2, Pb(NO3)2, AlCl3 in dis-
tilled water with a concentration of 0.05 M. All spectroscopic
experiments were carried out at room temperature.
tetramethylsilane. Infrared spectral data were measured with
Nicolet Avatar-370. Mass spectra were obtained on a Waters
Xevo G2-S QT. Melting points were measured using an X-6
microscopic melting point apparatus (Beijing, China). The
UV–vis spectra were measured on a Puxi TU-1901 (Beijing,
China) spectrophotometer. Fluorescence measurements were
made on a Hitachi F-7000 (Tokyo, Japan). The samples were
excited by the light with wavelength of 350 nm, and the emis-
sion spectrum was recorded in the range of 360 to 690 nm.
The excitation and emission slit widths were kept at 1.0 and
2.5 nm, respectively.
Apparatus
1 H NMR and 13 C NMR spectra were recorded on a Bruker
AVANCE-600 spectrometer and referenced to internal
Preparation of the Substituted Imidazole
purified by chromatography on a silica gel column (Hexane/
EtOAc = 1:1) to give the desired product 1b as a light yellow
solid (138.8 mg, 40 % yield).
6-(4,5-diphenyl-2-(quinolin-2-yl)-1 H-imidazol-1-
yl)hexanoic acid was synthesized by a modified
procedure, benzoin 0.212 g (1.00 mmol), 2-
quinolinecarboxaldehyde 0.157 g (1.00 mmol), ammoni-
um acetate 0.077 g (1.00 mmol), 6-aminocaproic acid
0.131 g (1.00 mmol) and Iodine 0.025 g (0.10 mmol) were
mixed in DMSO (5 mL), then the resulting mixture was
allowed to stir 4 h at 70 . After the reactant was consumed,
as monitored by TLC, water (10 mL) was added to the
mixture, which was then extracted with EtOAc (3 × 10
mL). The extract was washed with 10 % Na2S2O3 solution
(w/w), dried with anhydrous Na2SO4, and evaporated. The
residue was purified by chromatography on a silica gel
column (Hexane/EtOAc = 1:1) to give the desired product
1a as a light yellow solid (96.8 mg, 21 % yield).
2-(4,5-diphenyl-1 H-imidazol-2-yl)quinoline was synthe-
sized by a modified procedure, benzoin 0.212 g (1.00 mmol),
2-quinolinecarboxaldehyde 0.157 g (1.00 mmol), ammonium
acetate 0.231 g (3.00 mmol) and Iodine 0.025 g (0.10 mmol)
were mixed in DMSO (5 mL), then the resulting mixture was
allowed to stir 4 h at 70 . After the reactant was consumed, as
monitored by TLC, water (10 mL) was added to the mixture,
which was then extracted with EtOAc (3 × 10 mL). The ex-
tract was washed with 10 % Na2S2O3 solution (w/w), dried
with anhydrous Na2SO4, and evaporated. The residue was
Results and Discussion
Absorption and Fluorescence Spectra of the 1a in
Different Solvents
Initially, UV and fluorescence spectra of sensor 1a
were obtained in different solvents. As shown in Fig. 1a,
the maximum absorption wavelengths of the compound
1a were at 340–360 nm in common organic solvent, such
as acetone, dimethylsulfoxide, acetonitrile, tetrahydrofu-
ran, dichloromethane, ethyl acetate, methanol, ethanol,
glycol, isopropanol. But nearly no absorption could be
found in water. The same phenomenon could be seen
from Fig. 1b, nearly no fluorescence could be found in
water. But maximum emission wavelength around 455 nm
could be found in common organic solvent. All these
suggested that 1a could well be dissolved in most solvents
except water.

J Fluoresc
Fig. 1 a Absorption spectra of 1a (10 µM) in different solvents. b Fluorescence spectra of 1a (10 µM) in different solvents. Acetone, dimethylsulfoxide,
acetonitrile, tetrahydrofuran, dichloromethane, ethyl acetate, methanol, ethanol, glycol, isopropanol, water
Fig. 2 a Fluorescence spectra of 1a (10 µM) in ethanol with different
concentration of H2O (0, 10, 20, 30, 50, 60, 70, 80, 90 v/v, %).
b Fluorescence intensity of 1a (10 µM) at 455 nm in ethanol with
different concentration of H2O (0, 10, 20, 30, 50, 60, 70, 80, 90 v/v,
%). c Fluorescence spectra of 1a (10 µM) in ethanol with different
concentration of H2O (0, 1, 2, 3, 4, 5, 6, 7, 8, 9 v/v, %). d Fluorescence
intensity of 1a (10 µM) at 455 nm in ethanol with different concentration
of H2O (0, 1, 2, 3, 4, 5, 6, 7, 8, 9 v/v, %)

J Fluoresc
Fig. 3 a Absorption spectra of 1a (10 µM) with different metal ions (50
µM) including Na+, K+, Mg2+, Ca2+, Cr3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+,
Cd2+, Hg2+, Pb2+, Al3+ in EtOH:H2O = 94:6. b Absorption intensity of 1a
(10 µM) at 350 nm with different metal ions (50 µM) in EtOH:H2O =
94:6. c Fluorescence spectra of 1a (10 µM) with different metal ions (50
µM) in EtOH:H2O = 94:6. d Fluorescence intensity of 1a (10 µM) at
455 nm with different metal ions (50 µM) in EtOH:H2O = 94:6
Fluorescence Determination of Water Content in
Ethanol
Figure 2a and b shows the fluorescence intensity of 1a
changed in ethanol with different concentration of H2O (0,
10, 20, 30, 50, 60, 70, 80, 90 v/v, %). It can be seen that with
the increase of water content, the fluorescence intensity de-
creases gradually and the emission peak get a red shift. In
order to get more details, Fig. 2c and d shows the fluorescence
intensity of 1a changed in ethanol with different concentration
of H2O (0, 1, 2, 3, 4, 5, 6, 7, 8, 9 v/v, %). It can be seen that
with the increase of water content, the fluorescence intensity
increased firstly then decreased. The maximum fluorescence
intensity could be detected in 6 % water in ethanol. The limit
of detection was as low as 0.22 % based on five times the
standard deviation rule.
Water exists in all kinds of organic solvents and is the most
common impurity in organic solvents. As the most common
contaminant in organic solvents, water is often adverse in many
organic synthetic reactions and industrial production processes,
which negatively affected not only the yield of chemicals but also
their activities. Therefore, the determination of the water content
in the organic solvent is one of the most important and most
commonly encountered analytical problems.
In order to explore the possibility for application of the
synthesized sensor in the detection of water content in organic
solvents, the fluorescence emission spectra excited by the light
of 350 nm of 1a dispersed in ethanol were further examined.
As shown in Fig. 2a, when the ethanol solution of 1a changed
to 10 % water in ethanol, the fluorescent intensity increased
significantly. This indicates that 1a can be used as fluorescent
probes to quickly detect water content in organic solvents.
Absorption and Fluorescence Spectra of the Sensor 1a
with Metal Ion
The absorption and fluorescence spectra of sensor 1a were
examined following the treatment with different metal ions

J Fluoresc
Fig. 4 a Absorption spectra of 1a (10 µM) with different pH from2.31 to 10.72 in EtOH:H2O = 1:1. c Fluorescence spectra of 1a (10 µM) with different
pH from 2.31 to 10.72 in EtOH:H2O = 1:1
(5.0 eq.) in 6 % water in ethanol solutions. As shown in Fig. 3,
after adding Na+, K+, Mg2+, Ca2+, Cr3+, Co2+, Ni2+, Cu2+,
Zn2+, Ag+, Cd2+, Hg2+, Pb2+, Al3+, the absorption intensity
(I350 nm) and fluorescence intensity (I455 nm) of 1a basically
no changed. All these suggested that 1a couldn’t recognize
any metal ion, and the metal in solution couldn’t affect the
interaction of 1a with water.
The Possible Mechanism for Water Detection in
Organic Solvents
In order to explore the mechanism for the detection of water
content in organic solvents, compound 1b was synthesized.
The absorption and fluorescence spectra of 1b dispersed in
different organic solvents (dichloromethane, H2O, acetoni-
trile, tetrahydrofuran, ethanol) were further examined. As
shown in Fig. S5 and S6, the maximum emission wavelength
of 1b dispersed in dichloromethane, tetrahydrofuran and ace-
tonitrile was around 455 nm. But nearly no fluorescence could
be found in water and ethanol. All these suggested that the
long carboxyl chain in 1a could increase the solubility in eth-
anol and denote to increase the fluorescent intensity with the
addition of water.
Figure 5a shows the absorption spectra of 1a changed in
ethanol with different concentration of H2O (0, 10, 20, 30, 40,
50, 60, 70 v/v, %). It can be seen that with the increase of
water content, the absorption intensity decreases gradually
and the absorption peak get a red shift. Figure 5b shows the
absorption spectra of 1a at 350 nm in ethanol with different
concentration of H2O (0, 1, 2, 3, 4, 5, 6, 7, 8, 9 v/v, %). It can
be seen that with the increase of water content, the absorption
intensity first increase and then decrease.
All these indicate that the presence of hydroxyl in water
will lead to the red-shift effect for the absorption peak and
change of absorption intensity. Combined with the results of
Fig. 4a, H-aggregation of 1a could disaggregate with addition
of water, and 1a could precipitate with the increase of water.
Because the electron density around N is higher, this atom
serves as a stronger acceptor of hydrogen bonds, which de-
creases the possibility of molecular aggregation caused by π-π
interaction in the presence of water. In a word, the change of
absorption and fluorescence of compound 1a is attributed to
the aggregation and disaggregate caused by addition of water
Absorption and Fluorescence Spectra of 1a in Acidic
and Basic Solutions
Since the compound 1a comprise both acidic (-COOH)
and basic (Ar-N) groups, their properties can be very dif-
ferent under acidic or basic conditions, and this was next
evaluated. Aqueous/ethanol (1:1 v/v) solutions were used,
ensuring complete dissolution of the fluorophores, and the
observed pH of the solutions adjusted by aqueous/ethanol
(1:1 v/v) NaOH/HCl solutions. As shown in Fig. 4a,
fluorophore 1a showed highly varied absorption peaks in
different acidic and basic solutions. Absorption peak of 1a
displayed a red-shift with increasing pH from 2.31 to
10.72. Then fluorescence spectra of 1a in acidic and basic
solutions could be observed. As shown in Fig. 4b,
fluorophore 1a showed highly varied fluorescence in dif-
ferent acidic and basic solutions. The fluorophore 1a
showed strong emission under basic conditions while be-
ing weakly emissive in acidic solutions, similar to azo
chromophores. [28] In general, azo chromophores can take
H-aggregation as the typical aggregation manner under
different conditions. [29–32] And H-aggregation denotes
the aggregation showing a blue-shifted band to the molec-
ular absorption band. All these suggested that compound
1a could take H-aggregation with pH decreasing and dis-
aggregate with pH increasing.

J Fluoresc
Fig. 5 a Absorption spectra of 1a (10 µM) in ethanol with different concentration of H2O (0, 10, 20, 30, 40, 50, 60, 70 v/v, %). b Absorption spectra of 1a
(10 µM) at 350 nm in ethanol with different concentration of H2O (0, 1, 2, 3, 4, 5, 6, 7, 8, 9 v/v, %)
and partially to the increase in polarity of the solvent caused
by the addition of water (Fig. 6).
Supplementary Information The online version contains supplementary
material available at https://doi.org/10.1007/s10895-021-02729-w.
Acknowledgements The authors would like to acknowledge Qiqihar
University.
Conclusions
Author Contributions Weijian Xue contributed to the conception of the
study and wrote the manuscript;
and quinoline group has been successfully developed. The
Huiqian Li performed the experiment;
Novel substituted imidazole derivative 1a containing carboxyl
emission of 1a was highly sensitive to solvents.
Significantly, the change of 1a in fluorescence intensity and
wavelength was suitable for the qualitative detection of water
in ethanol. And the metal in solution couldn’t affect the inter-
action of 1a with water. Furthermore, this is the first time that
substituted imidazole was adopted as the water sensing fluo-
rescent probe. This work makes the 1a a promising material
used without any modification in the detection of water in
organic solvents and with immense potential for applications
in quality management of the solvent products. The possible
mechanism for water detection in organic solvents is attribut-
ed to the aggregation and disaggregate of 1a caused by addi-
tion of water. Further work for developing substituted
imidazole-based advanced functional materials utilizing this
molecular design concept for potential applications is
ongoing.
Jinping Wang contributed significantly to analysis;
Jie Ma performed the data analyses and manuscript preparation;
Bing Zhao helped perform the analysis with constructive discussions;
Xiangfeng Guo helped perform the analysis with constructive
discussions.
Funding This research was supported by National Natural Science
Foundation (21801146), Natural Science Foundation of Heilongjiang
Province (No. QC2016016), University Nursing Program for Young
Scholars with Creative Talents in Heilongjiang Province (No.
UNPYSCT-2017159), The Fundamental Research Funds in
Heilongjiang Provincial Universities (No. 135209304), postdoctoral sci-
entific research development funding of Heilongjiang Province (No.
LBH-Q20187).
Data Availability All data generated or analyzed during this study are
included in this published article.
Declarations
H2O
1a
1a
1a
Confict of Interest Huiqian Li declares that she has no confict of inter-
est. Weijian Xue declares that he has no confict of interest. Jinping Wang
declares that she has no confict of interest. Jie Ma declares that she has no
confict of interest. Bing Zhao declares that she has no confict of interest.
Xiangfeng Guo declares that he has no confict of interest.
H2O
H2O
H2O
H2O
1a
H2O
H2O
H2O
H2O
1a
1a
H2O
H2O
H2O
H2O
Ethical Approval This article does not contain any studies with human
or animal subjects.
1a
1a
H2O
1a
H2O
H2O
H2O
1a
Informed Consent A statement regarding informed consent is not
H2O
applicable.
Fig. 6 The possible mechanism

J Fluoresc
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Kuroda N, Akiyama S, Irgum K (1998) Fluorescence and chemilu-
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Pigm 38:127
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