A simple sensor based on imidazo[2,1-b]thiazole for recognition and differentiation of Al3+, F? and PPi

Article abstract of DOI:10.1016/j.jphotochem.2020.112578

Based on imidazo[2,1-b]thiazole and 2,3-dihydroxybenzaldehyde, a simple sensor, (E)-N'-(2,3-dihydroxybenzylidene)imidazo[2,1-b]thiazole-6-carbohydrazide (X), was designed and synthesized and its fluorescence responses to various ions were explored in DMF/

Full text of DOI:10.1016/j.jphotochem.2020.112578

Journal Pre-proof
A simple sensor based on imidazo[2,1-b]thiazole for recognition and
differentiation of Al3+, F- and PPi
Zoualou Arvel Consty, Yanxia Zhang, Yuankang Xu
PII:
S1010-6030(20)30377-4
DOI:
https://doi.org/10.1016/j.jphotochem.2020.112578
JPC 112578
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To appear in:
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Accepted Date:
20 February 2020
15 April 2020
19 April 2020
Please cite this article as: Consty ZA, Zhang Y, Xu Y, A simple sensor based on
imidazo[2,1-b]thiazole for recognition and differentiation of Al3+, F- and PPi, Journal of
Photochemistry and amp; Photobiology, A: Chemistry (2020),
doi: https://doi.org/10.1016/j.jphotochem.2020.112578
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A simple sensor based on imidazo[2,1-b]thiazole for recognition and differentiation of Al3+, F-
and PPi
Zoualou Arvel Consty, Yanxia Zhang* and Yuankang Xu*
School of chemistry and chemical engineering, University of Jinan, Jinan 250022, China.
*Corresponding author: Yanxia Zhang, E-mail address: chm_zhangyx@ujn.edu.cn. Yuankang Xu,
E-mail address: chem_xuyk@hotmail.com.
Graphical abstract
Highlights

A new simple fluorescence sensor (X) was designed and synthesized based on 2,3-
dihydroxybenzaldehyde.


X could be used as multifunctional sensor for sequential detection of Al3+, F- and PPi.
The detection limits for Al3+, F- and PPi were calculated to be 3.57 × 10-8 M, 9.24 ×
10-9 M and 3.14 × 10-8 M, respectively.


The binding details and sensing mechanism for Al3+, F- and PPi were proposed and
proved by theoretical calculation.
X was proved to have practical application potential by real water sample experiment
and test paper strip experiment.

Abstract
Based on imidazo[2,1-b]thiazole and 2,3-dihydroxybenzaldehyde, a simple sensor, (E)-N'-(2,3-
dihydroxybenzylidene)imidazo[2,1-b]thiazole-6-carbohydrazide (X), was designed and synthesized
and its fluorescence responses to various ions were explored in DMF/H2O buffer solution (v:v = 1:1,
tris = 10 mM, pH = 7.4). The X could be used as “turn on” sensor to detect Al3+. The resultant
complex X[Al3+] could be used as a new multifunctional sensor to recognition of F- through the
further enhancement of fluorescence intensity and to detection of PPi by the fluorescence quenching.
The detection limits for Al3+, F- and PPi were calculated to be 3.57 × 10-8 M, 9.49 × 10-9 M and 3.24
× 10-8 M, respectively. According to ESI-MS studies, the binding ratio for Al3+ and F- were
determined both as 1:1. Otherwise, the binding details and sensing mechanism for Al3+, F- and PPi
were proposed and proved by theoretical calculation. The application experiments proved that X
could be made into test paper strips for Al3+, F- and PPi and could be used as a sensor to recognition
of Al3+, F- and PPi in tap water.
Keywords: Schiff base, Al3+, F-, PPi, Gaussian calculations, Practical application.
1. Introduction
As is known to all, metal ions are important to both human health and plant growth, which also
play an essential role in medicine, chemistry, industry and other fields [1]. For instance, aluminum
is one of the most abundant metal elements in the earth’s, accounting for about 8 percent, and is
extensively used in various aspects, such as daily necessities, manufacturing industry, medicine and
so on [2, 3]. Sometimes, aluminum could be inevitable ingested by the body even in excess of the
normal daily intake of 3-10 milligrams because it is widely used in food flavoring agents, aluminum-
based pharmaceuticals and cooking utensils [4]. However, excessive accumulation of aluminum in
the body could cause damage to the nervous system and increase the risk of various diseases,
including Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis, etc [5-7].

Moreover, the increase of aluminum concentration in soil will also restrict the growth and
development of plants [8-10]. So, it is imperative to quantitatively and efficiently detect aluminum
in organisms and environments.
In addition, anions also play a fundamental role in human physiological processes, especially
fluoride and pyrophosphate [11, 12]. Fluoride is the smallest anions with the strongest
electronegativity, which mainly distributed in natural water and the tissues of human body [13].
Proper levels of fluoride are necessary for human body and could enhance the structural stability of
bones and teeth, helping to prevent tooth decay and osteoporosis [14, 15]. However, excessive intake
of fluoride could lead to various serious diseases, including dental and skeletal fluorosis and immune
system disruption, while the deficiency of aluminum is also a major factor to stomach illness, tooth
decay, osteoporosis, etc [16-18]. As another important anion in the human body, pyrophosphate
4-
(P2O7 , PPi), is high in body fluids and is involved in many vital activities, such as RNA and DNA
replication, metabolism processes, ATP hydrolysis, the synthesis of protein and other physiological
processes [19-21]. Moreover, pyrophosphate is an important index in the diagnosis of arthritis,
chondrocalcinosis, calcium pyrophosphate deposition, rheumatism, etc [22-24]. For these reasons,
the detection of fluoride and pyrophosphate is particularly important and valuable.
Recently, fluorescence sensor for metal ions and anions has become one of the hot research
fields, which mainly due its simple operation, high selectivity and sensitivity, real-time detection
and functional diversity [25, 26]. Nowadays, a number of sensors that could identify metal ions and
subsequently anions have been reported, such as Al3+/F- and Al3+/PPi [27, 28]. However, aluminum
is a kind of metal ion which is difficult to detect because of its strong hydration ability and poor
coordination ability [29]. Therefore, most sensors for aluminum are designed and synthesized with
N and O complexation sites to improve the ability of recognition [30]. After binding with aluminum,
the fluorescence signal of sensor is changed under the excited state, thus realizing the specific
recognition of metal ions [31-35]. In addition, fluoride and pyrophosphate could restore the sensor’
original optical properties, realizing the cycle of optical properties, because they could extract
aluminum from the complex of sensor-aluminum to free the sensor [28, 36]. Moreover, some sensors
containing -OH and -NH groups have been developed to detection of fluoride by forming hydrogen
bonds because fluoride is the most electronegative anion [37-40]. However, these reported sensors
could only identify a single ion or two ions in sequence. As a result, it is still full of challenge and

charm to design new sensors that have multiple recognition capabilities for aluminum, fluoride and
pyrophosphate.
Up to now, Schiff base synthesized based on a variety of aldehydes could be used as a kind of
fluorescent sensor with simple synthesis and strong recognition ability [41, 42]. Recently, in our
group, some Schiff base were designed and synthesized by different aldehydes, including
salicylaldehyde, 2-hydroxy-1-naphthaldehyde and 3-ethoxy-2-hydroxybenzaldehyde, which could
be used as a sensor for detection of various ions [43-46]. Similarly, these sensors could form a cavity
near three sites that the oxygen atom on the hydroxy group and the nitrogen atoms on the C=N bond
and the imidazo group for complexing metal ions. However, these sensors contain only one hydroxyl
group in their structure, which results in their single performance that could only achieve the
selective recognition of one ion. In this study, a new fluorescent sensor (X) based on imidazo[2,1-
b]thiazole and 2,3-dihydroxybenzaldehyde was designed and synthesized. imidazo[2,1-b]thiazole
with two conjugated rings was a potential fluorescent compound and had been used in the synthesis
of fluorescent sensors for many times [45-47]. 2,3-dihydroxybenzaldehyde differed from other
aldehydes in that it contains two hydroxyl groups, making it more soluble and able to recognize ions
than aldehydes with one hydroxyl group.
As expected, X could be used as a multifunctional sensor for sequential detection of aluminum,
fluoride and pyrophosphate in DMF/H2O (v/v = 9/1, tris = 10 mM, pH = 7.4) buffer solution. The
fluorescence intensity of the sensor was very weak and increased after binding with aluminum due
to the formation of the complex X[Al3+]. Moreover, fluoride could further enhance the fluorescence
intensity of the above system, while pyrophosphate could make the optical properties of system
above disappear, displaying different fluorescence behaviors.
2. Results and discussion
As shown in scheme 1, X was designed and synthesized by the reaction of imidazo[2,1-
b]thiazole-6-carbohydrazide and 2,3-dihydroxybenzaldehyde with 53.9 % yield according to the
synthetic route [46]. Moreover, the structure of X was characterized by 1H NMR (Fig. S1), 13C NMR
(Fig. S2), FTIR (Fig. S3), ESI-MS (Fig. S4).

Scheme 1. Synthesis routes of X. Conditions: (a) THF/ethanol, r.t./refluxed, 20 h/4 h; (b) ethanol,
r.t., overnight; (c) ethanol, r.t., 12 h.
2.1. The sensing behavior of X to Al3+
Primarily, the time effect on the fluorescence intensity of X (1 × 10-5 M) in the presence of
Al3+ (10 equiv.) were explored in DMF/H2O buffer solution (v:v = 1:1, tris 10 mM, pH = 7.4). As
illustrated in Fig. S5, in the presence of Al3+, the fluorescence intensity of X was gradually increased
with the time from 0 to 12 minutes and remained unchanged for the next 4 minutes. Obviously, the
recognition time of X for Al3+ was about 12 minutes, which was lower than many reported sensors.
So, in order to ensure the accuracy of test results, 15 minutes complexation time should be applied
in all subsequent tests.
The sensing behavior and recognition ability of X toward various metal ions (Al3+, Zn2+, Ga3+,
Cd2+, Ag+, Hg2+, Mg2+, K+, Mn2+, In3+, Li+, Cu2+, Fe3+, Cr3+, Ni2+ and Co2+) were roundly explored
by fluorescence spectral and UV-vis spectral in DMF/H2O buffer solution (v:v = 1:1, tris 10 mM,
pH = 7.4). As shown in Fig. 1a, X with a concentration of 1 ×10-5 M displayed no fluorescence,
which may be caused by the PET mechanism. When various metal ions (10 equiv.) were respective
added to X, there was a significant fluorescence enhancement at 497 nm caused only by the addition
of Al3+, about 24 times (Fig. 1b). Meanwhile, the other metal ions could lead to a little or no change
in fluorescence intensity, such as Zn2+ and Co2+. Moreover, as shown in Fig. S6, the fluorescence
response of X to Al3+ was explored in the presence of other metal ions. As a result, the fluorescence
intensity of X in the presence of other metal ions displayed varying degrees of enhancement after
addition of Al3+. So, X could be used as a selective sensor for Al3+ even in the presence of some
other competitive metal ions. The UV-vis spectral of X was also investigated in the absence and

presence of various metal ions (10 equiv.). As shown in Fig. S7, the free X exhibited an absorbance
band at 305 nm. Obviously, there was many metal ions could cause change in the UV absorption
spectrum, including Al3+, Cu2+, Co2+, etc, indicating that X could not only form fluorescent
complexes (X[Al3+]) with Al3+ but also form complexes without fluorescence properties with other
metal ions. Evidently, X could be used as a specific sensor for Al3+.
Fig. 1. (a) fluorescence spectra of X in the presence of various metal ions (10 equiv.); (b) the changes
(I / I0) in fluorescence intensity caused by the addition of various metal ions.
Next, the fluorescence titration and ultraviolet titration were further carried out in DMF/H2O
buffer solution to investigate the sensing behavior of X toward Al3+ in details. As shown in Fig. 2a,
the fluorescence intensity at 497 nm increased gradually with the accumulation of Al3+ from 0 to 8
equivalents and reached a plateau with no enhancement upon further addition of Al3+, accompanying
with color change from colorless to yellow-green. The fluorescence quantum yield of X increased
from 0.001 to 0.105 when addition of Al3+. displaying an obvious fluorescence turn-on behavior.
Meanwhile, as shown in Fig. S8, as the gradual addition of Al3+ from 0 to 10 equiv., the UV
absorption intensity of X decreased gradually at 305 nm and increased gradually at 323 nm and 384
nm accompanied by a distinct isosbestic point at 320 nm, indicating that a stable complex X[Al3+]
was formed between X and Al3+. In addition, based on the fluorescence titration, the detection limit
and complexation constant of X for Al3+ were calculated to be 3.57 × 10-8 M (Formula 2, Y = 2.77X
+ 18.95, R2 = 0.9987, Fig. 2b) and 5.09 × 104 M-1 (Equation 3, Y = 4.14×10-8X - 0.00151, R2 =
0.9929, Fig. S9), respectively.

Fig. 2. (a) Fluorescence spectra of X with the addition of Al3+ (0 - 11 equiv.); (b) Fluorescence
intensity of X at 497 nm with the addition of Al3+ (0 - 6 × 10-7 M).
2.2. The sensing behavior of X[Al3+] to F- and PPi
Furthermore, the reversibility was another important index to measure the properties of
sensor. So, the fluorescence intensity of X[Al3+] was measured after treatment with various anions,
2-
-
-
-
2-
-
-
-
including F-, PPi, Cl-, Br-, I-, CrO4 , NO2 , HCO3 , S2-, S2O3 , SO4 , NO3 , HSO3 and BF4 . As
shown in Fig. 3, the fluorescence intensity of X[Al3+] exhibited medium fluorescence in DMF/H2O
buffer solution (1:1, tris 10 mM, pH = 7.4) under excitation wavelength of 340 nm. Upon addition
of various anions, the fluorescence intensity of X[Al3+] was further enhanced by the addition of F-
and completely disappeared in the presence of PPi, display different fluorescence emission
behaviors. However, the further change in the fluorescence intensity of X[Al3+] caused by other
anions were not significant, whether it was increased or decreased. Therefore, the complex X[Al3+]
could be further used as a new sensor for the multiple detection of F- and PPi, which was different
from the traditional sensors that sequential detection Al3+ and F-/PPi.

Fig. 3. Fluorescence spectra of X[Al3+] with the addition of 10 equiv. various anions in DMF/H2O
buffer solution (1:1, tris 10 mM, pH = 7.4).
Analogously, the time effect on the fluorescence intensity of X[Al3+] in the presence of F- (10
equiv.) were also explored in Fig. S5. In the presence of Al3+, the fluorescence intensity of X was
gradually increased with the time from 0 to 5 minutes and reached a platform after 5 minutes. So,
10 minutes complexation time was used in all subsequent experiments related to F-.
As shown in Fig. 4a, F- could cause the fluorescence intensity of X[Al3+] to be further enhanced
regardless of the presence or absence of other anions, indicating that the complex X[Al3+] showed
excellent selectivity for F- even in the presence of other interference anions. As shown in Fig. 4b,
the fluorescence intensity at 497 nm gradually increased with the incremental addition of F- from 0
to 15 equiv., after that the intensity remained stable. At the same time, the fluorescence quantum
yield was increased to 0.173 after addition of F-, corresponding to the fluorescence became bright
and visible. Moreover, on the basic of the fluorescence titration experiment, the detection limit and
complexation constant of X[Al3+] for F- were calculated to be 9.49 × 10-9 M (Formula 2, Y = 10.43X
+ 495.58, R2 = 0.9898, Fig. 4c) and 1.76 × 104 M-1 (Equation 3, Y = 0.02548X + 8.89E-5, R2 =
0.9871, Fig. 4d), respectively.

Fig. 4. (a) Competitive selectivity of X[Al3+] toward F- in the presence of other anions ions (10
equiv.); (b) Fluorescence spectra titration of X[Al3+] with the gradual addition of F- (0 - 15 equiv.);
(c) Fluorescence intensity of X at 497 nm with the addition of Al3+ (1 × 10-7 M - 8 × 10-7 M); (d)
Benesi-Hildebrand plot for determination of binding constant of X with F-.
As shown in Fig. 5a, it could be found that the decrease in fluorescence intensity caused by
PPi in the presence of other anions was similar to that caused by PPi alone, indicating that the
complex X[Al3+] had a good selectivity to PPi even in the presence of other co-existing anions. As
shown in Fig. 5b, the fluorescence intensity at 497 nm gradually decreased with the incremental
addition of PPi from 0 to 3 equiv. and almost completely disappeared when the concentration of PPi
was added to 5 × 10-5 M. Moreover, As shown in Fig. 5c, a good linear relationship (R2 = 0.9893)
was obtained between the I0 - I and low concentrations of PPi (1 × 10-7 M - 1 × 10-6 M), where I and
I0 were the fluorescence intensity of X[Al3+] with and without PPi. The detection limit of X[Al3+]
for PPi were calculated to be 3.24 × 10-8 M according to the formula 2. And, the complexation
constant of X[Al3+] for PPi was also calculated to be 1.06 × 105 M-1 (Equation 3, Y = 0.01988X +
0.00556, R2 = 0.9891, Fig. 5d).
Fig. 5. (a) Competitive selectivity of X[Al3+] toward PPi in the presence of other anions ions (10

equiv.); (b) Fluorescence spectra titration of X[Al3+] with the gradual addition of PPi (0 - 5 equiv.);
(c) Fluorescence intensity of X[Al3+] at 497 nm with the addition of PPi (1 × 10-7 M - 1 × 10-6 M);
(d) Benesi-Hildebrand plot for determination of binding constant of X[Al3+] with PPi.
Some multifunctional sensors for sequential detection of Al3+ and F- or Al3+ and PPi were listed
in Table 1, which could only identify one metal ion and an anion sequentially. In this work, X could
be used as a primary sensor for detection of Al3+ and the resultant X[Al3+] could also be used as a
secondary sensor for F- and PPi, which was one of its advantages. Moreover, the detection limits for
Al3+, F- and PPi were calculated to be 3.57 × 10-8 M, 9.49 × 10-9 M and 3.24 × 10-8 M, respectively,
which was obviously better than the reported sensors. Obviously, X had some advantages in
functional diversity and sensitivity over other sensors.
Table 1. Comparison to some published sensors.
Solvent system
MeOH
Primary sensor
Secondary sensor
Ref.
[36]
[27]
[49]
[50]
[51]
[48]
[28]
[33]
[52]
[53]
This
work
Al3+ (turn on, 9.24 × 10-7 M)
Al3+ (turn on, 1.1 × 10-7 M)
Al3+ (turn on, 5.36 × 10-6 M)
Al3+ (turn on, 3.38 × 10-8 M)
Al3+ (turn on, 6.2 × 10-8 M)
Al3+ (turn on, 2.94 × 10-8 M)
Al3+ (turn on, 4.96 × 10-7 M)
Al3+ (turn on, 3.9 × 10-7 M)
Al3+ (turn on, 8.6 × 10-7 M)
Al3+ (turn on, 7.7 × 10-8 M)
F- (turn off, 1.4 × 10-5 M)
F- (turn off, 1.47 × 10-6 M)
F- (turn off, 8.05 × 10-6 M)
F- (turn off, 2.62 × 10-7 M)
F- (turn off, 3.26 × 10-7 M)
PPi (turn off, 2.74 × 10-7 M)
PPi (turn off, 5.70 × 10-7 M)
PPi (turn off, 7.5 × 10-7 M)
PPi (turn off, 2.19 × 10-6 M)
PPi (turn off, 1.71 × 10-6 M)
F- (turn on, 9.49 × 10-9 M)
PPi (turn off, 3.24 × 10-8 M)
DMF/H2O (1:1)
EtOH/H2O (3:7)
DMSO
H2O
EtOH/H2O (1:99)
DMF/H2O (5:1)
DMF/H2O (4:1)
MeOH/H2O (7:3)
MeOH/H2O (1:1)
DMF/H2O (1:1)
Al3+ (turn on, 3.57 × 10-8 M)
2.3. The effect of pH
In addition, it is important for recognition of ions by the sensor to determine the pH response
range. As shown in Fig. S14, the fluorescence intensity of X, X[Al3+], X[Al3+ + PPi] and X[Al3+
+
F-] were monitored at various pH values (2 - 12) in DMF/H2O buffer solution (v/v = 1:1, tris = 10
mM). X exhibited weak and stable fluorescence in the pH range of 2 to 12 under the excitation

wavelength of 340 nm. The fluorescence intensity of X[Al3+] decreased gradually with the increased
of pH from 2 to 12. At the high pH (＞5.6), the formation of the complex X[Al3+] was affected by
the high concentration of OH-. Moreover, PPi could quench the fluorescence of the X[Al3+] in a pH
range of 2 to 10. In addition, the resultant X[Al3+] could efficiently detection of F- at a pH of 7.4.
The results of pH experiments indicated that X could be used as a multifunctional sensor for
sequential detection of Al3+, F- and PPi at a pH of 7.4, indicating that X had potential for the practical
application.
2.4. The binding mode and theoretical calculations
The complexation ratio between sensor and ions was investigated by mass spectrometric
titration. As shown in Fig. S11, a molecular ion peak m/z at 362.97 corresponded to the molecular
weight of [X – H+ + Al3+ + Cl-]+ (m/z calculated 362.99), which proved that X could form stable
complex with Al3+ at a stoichiometric ratio of 1:1. Furthermore, the Fig. S12 showed the ESI mass
spectra of X in the presence of Al3+ After addition of F-. Obviously, a new molecular ion peak was
appeared at 347.02 m/z, which could be attributed to the species of [X – H+ + Al3+ + F-]+ (m/z
calculated 362.99), indicating that X[Al3+] and F- complexed with a 1:1 stoichiometry. Meanwhile,
the 1:1 stoichiometry between X[Al3+] and PPi could also be demonstrated by the Benesi-
Hildebrand equation with a good linear relationship (R2 = 0.9891) between 1 / (I0 - I) and 1 / PPi.
Therefore, the complexation ratio of sensor to Al3+, F- and PPi were all 1:1.
Based on all of the above experiments, the possible binding mode of X with Al3+, F- and PPi
were illustrated in Scheme 2. X could form a moderate fluorescence complex with Al3+ and Cl-.
When addition of PPi to the X[Al3+], PPi could extract Al3+ from the complex to free the sensor,
resulting in fluorescence quenching of the system. As shown in Fig. S13, after addition of F-, the
absorbance peaks of X[Al3+] at 325 nm and 384 nm were increased without wavelength shift. In
addition, the isosbestic point at 320 nm was not affected by the addition of F- at all. The results
showed that, different from PPi, F- could replace Cl- to form more stable ternary compounds with
X and Al3+, which corresponded to the result of ESI-mass. Moreover, F- with highly electronegative
could also form hydrogen bond with the free hydroxyl next to it.

Scheme 2. The possible binding mode for X, X[Al3+] and X[Al3+][F-].
The optimal structures and energy levels of X, X[Al3+] and X[Al3+][F-] were investigated
using the Gaussian 09 program with the basis set of B3LYP/6-31 G(d) to verify the feasibility of the
structures of X, X[Al3+] and X[Al3+][F-] and explore the sensing mechanism for ions. The optimal
structures of X, X[Al3+] and X[Al3+][F-] were shown in Fig. 6. For free X, the whole molecule was
in the same plane, showing good planarity. After binding with Al3+ and Cl-, a 144.4-degree angle
was maintained between the imidazo[2,1-b]thiazole part and 2,3-dihydroxybenzaldehyde part of
molecular. After further addition of F-, a ternary complex with good planarity and stability was form
between X, Al3+ and F-. Obviously, the structure of X[Al3+][F-] was more stable and more similar to
the X’s than X[Al3+]’s. The results showed that the proposed structures of complex X[Al3+] and
X[Al3+][F-] were feasible, which proved that the binding mode of the sensor to Al3+ and F- was
possible.
Fig. 6. The optimized molecular structure of X, X[Al3+] and X[Al3+][F-], where light-gray, blue,
white, pink, cyan and green spheres denote C, O, N, H, Al, F and Cl atom, respectively.

Moreover, the highest occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) were also shown in Fig. 7. For free X, most of the electrons in HOMO
orbital were localized on the benzene ring, while the electrons in the LUMO orbital were distributed
throughout the whole molecule. Based on the PET mechanism, X exhibited very weak fluorescence
due to the partial electron transfer when excited. For X[Al3+] and X[Al3+][F-], the HOMO was still
concentrated near the benzene ring, while the LUMO was distributed near the benzene ring and
metal ions, indicating that electron transfer was partially inhibited under excitation. So after binding
ions, X[Al3+] and X[Al3+][F-] exhibited the appropriate fluorescence. In contrast, X[Al3+][F-]
showed stronger fluorescence than X[Al3+] because of its better planarity and conjugation.
Meanwhile, the energy gaps between HOMO and LUMO of X, X[Al3+] and X[Al3+][F-] were
calculated to be 3.98 eV, 3.24eV and 3.06eV, respectively. The results showed that the sensor could
identify Al3+ and F- based on photoinduced electron transfer (PET) and the chelation enhanced
fluorescence mechanism (CHEF). The calculation results were highly consistent with the
experimental results.
Fig. 7. HOMO and LUMO of X, X[Al3+] and X[Al3+][F-].
2.5. Application of X in real water sample
In order to explore the practical application ability of sensor, the sensor was applied to detect
Al3+, F- and PPi in real water samples, and satisfactory results were obtained. The sample solutions
of Al3+, F- and PPi were prepared from laboratory tap water. The recover experiments were carried

out by adding different concentrations of Al3+, F- and PPi to the sensor. The calculated results were
shown in Table 2, the recoveries for Al3+, F- and PPi were within the range of 99.8% - 101.1%, 93.7%
- 107.2 and 91.6% - 106.7, respectively. Moreover, the relative standard deviation (RSD) of three
measurements was less than 3.47%, which proved that the experimental data was credible and
usable. These results indicated that X had good selectivity to Al3+, and that X[Al3+] could also
efficiently detect F- and PPi. Therefore, X had potential for practical application in real water system.
Table 2. Determination of Al3+, F- and PPi in tap samples
Ion
Al3+
Sample
Ion added (M)
1.0 × 10-5
2.0 × 10-5
1.0 × 10-5
2.0 × 10-5
1.0 × 10-5
2.0 × 10-5
Ion recovered (M)
9.98 × 10-6
2.02 × 10-5
1.07 × 10-5
1.87 × 10-5
9.16 × 10-6
2.13 × 10-5
Recovery (%)
99.8
RSD (%)
1.61
1
2
1
2
1
2
101.1
2.18
F-
107.2
3.47
93.7
1.48
PPi
91.6
2.25
106.7
3.32
2.6. Practical application on test strips
According to the reported literature [38], the paper test was conducted to check the
practicality of the sensor and was shown in Fig. 8. Firstly, filter paper strips were immersed in
DMF/H2O solution of the X for 5minutes and then dried in the air. Then the water solution of Al3+,
F- and PPi was sequentially added dropwise to the above test paper. In this process, the significant
changes in fluorescence could be observed under the UV lamp. Therefore, X could be made into
test paper strips for qualitative and convenient detection of Al3+, F- and PPi.
Fig. 8. The changes in the color of test paper under the UV lamp (a) X; (b) X + Al3+; (c) X + Al3+
+ F-; (d) X + Al3+ + PPi.
Conclusions
In summary, a new simple sensor, X, was synthesized based on imidazo[2,1-b]thiazole and 2,3-

1
dihydroxybenzaldehyde and was characterized by H NMR, 13C NMR, FTIR, ESI-MS. In
DMF/H2O (v/v = 1/1, tris = 10 mM, pH = 7.4) buffer solution, X could be used as a “turn-on” sensor
for Al3+ with good selectivity and sensitivity. Furthermore, F- could further enhance the fluorescence
intensity of the above system, while PPi could make the optical properties of system above disappear,
displaying different fluorescence behaviors. In addition, the binding model and sensing mechanism
for Al3+, F- and PPi were proposed by experimental method and proved by theoretical calculation.
Otherwise, the application experiments proved that X had the potential of practical application.
Meanwhile, X had been successfully developed as a multifunctional sensor for detecting Al3+, F-
and PPi, which provided reference and guidance for the design and research of multi-target sensor
in the future.
Author statement
Zoualou Arvel Consty is studying for master degree in Department of Chemistry at University of Jinan.
Yanxia Zhang is a teacher in Department of Chemical Engineering, University of Jinan, China.
Yuankang Xu is studying for master degree in Department of Chemistry at University of Jinan.
All authors contribute equally.
Declaration of interests
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence the
work reported in this paper.
Acknowledgements
The authors thank the Henan Sanmenxia Aoke Chemical Industry Co., Ltd. w0920 for financial

support.
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