A new “off-on-off” sensor for sequential detection of Al3+ and Cu2+ with excellent sensitivity and selectivity based on different sensing mechanisms

Article abstract of DOI:10.1016/j.jphotochem.2020.112372

A new simple Schiff base, (E)-N'-(2-hydroxybenzylidene)imidazo[2,1-b]thiazole-6-carbohydrazide (X), was designed and synthesized based on salicylaldehyde and imidazo[2, 1-b]thiazole. X could be used as a sensor to identify Al³⁺ through a significant fluorescence enhancement because of CHEF and inhibition of PET process and then to detect Cu²⁺ through a highly efficient quenching behavior due to the paramagnetic quenching. The sensor showed a high selectivity for Al³⁺ and Cu²⁺ in methanol solution at pH = 5 and was not disturbed by other competing metal ions. Furthermore, based on the equation 3σ/slope, the detection limits of sensor for Al³⁺ and Cu²⁺ were calculated to be 3.1 × 10^?10 M and 9.8 × 10^-11 M, respectively. Additionally, the association constants of X[Al³⁺] and X[Cu²⁺] were also determined to be 2.3 × 10⁴ M^-1 and 6.1 × 10⁴ M^-1 on basis of Benesi-Hildebrand equation. In addition, the titration experiment of fluorescence and mass spectrometry showed that sensor was combined with Al³⁺ or Cu²⁺ both in 1:1 ratio. Moreover, the optimized structure and energy calculations of X, X[Al³⁺] and X[Cu²⁺] were obtained by Gaussian software based on the basis set of B3LYP/6?31 G(d) and B3LYP/LANL2DZ. And, the sensor successfully detected Al³⁺ and Cu²⁺ in the real water sample with a satisfactory recovery (91.4 % - 107.1 %) and RSD values (0.66 % - 1.91 %).

Full text of DOI:10.1016/j.jphotochem.2020.112372

Journal Pre-proof
A new “off-on-off” sensor for sequential detection of Al3+ and Cu2+ with
excellent sensitivity and selectivity based on different sensing
mechanisms
Yuankang Xu, Lei Yang, Hanyu Wang, Yanxia Zhang, Xiaofeng
Yang, Meishan Pei, Guangyou Zhang
PII:
S1010-6030(19)31933-1
DOI:
https://doi.org/10.1016/j.jphotochem.2020.112372
JPC 112372
Reference:
To appear in:
Journal of Photochemistry & Photobiology, A: Chemistry
Received Date:
Revised Date:
Accepted Date:
12 November 2019
24 December 2019
11 January 2020
Please cite this article as: Xu Y, Yang L, Wang H, Zhang Y, Yang X, Pei M, Zhang G, A new
“off-on-off” sensor for sequential detection of Al3+ and Cu2+ with excellent sensitivity and
selectivity based on different sensing mechanisms, Journal of Photochemistry and amp;
Photobiology, A: Chemistry (2020), doi: https://doi.org/10.1016/j.jphotochem.2020.112372
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A new “off-on-off” sensor for sequential detection of Al3+ and Cu2+ with excellent sensitivity
and selectivity based on different sensing mechanisms
Yuankang Xua, Lei Yangb,*, Hanyu Wanga, Yanxia Zhanga, Xiaofeng Yanga, Meishan Peia and
Guangyou Zhanga,*
a School of chemistry and chemical engineering, University of Jinan, Jinan 250022, China, E-mail
address: chm_zhanggy@ujn.edu.cn.
b Henan Sanmenxia Aoke Chemical Industry Co. Ltd., Sanmenxia 472000, China. E-mail address:
275665616@qq.com.
*Corresponding author: Guangyou Zhang, E-mail address: chm_zhanggy@ujn.edu.cn. Lei Yang,
E-mail address: 275665616@qq.com.
Graphical abstract
Highlights
- 1 -






A new simple fluorescence sensor (X) was designed and synthesized based on
salicylaldehyde and imidazo[2, 1-b]thiazole.
X could be used as “off-on-off” sensor for sequential detection of Al3+ and Cu2+ with
excellent sensitivity and selectivity based on different sensing mechanisms
The detection limits of sensor for Al3+ and Cu2+ were calculated to be 3.1 × 10-10 M and 9.8
× 10-11 M, respectively.
The optimized structure and energy calculations of X, X[Al3+] and X[Cu2+] were obtained by Gaussian
09 to prove the binding mode and sensing mechanism.
The sensor successfully detected Al3+ and Cu2+ in the real water sample with a satisfactory recovery and
RSD values
Abstract
A new simple Schiff base, (E)-N'-(2-hydroxybenzylidene)imidazo[2,1-b]thiazole-6-
carbohydrazide (X), was designed and synthesized based on salicylaldehyde and imidazo[2, 1-
b]thiazole. X could be used as a sensor to identify Al3+ through a significant fluorescence
enhancement because of CHEF and inhibition of PET process and then to detect Cu2+ through a
highly efficient quenching behavior due to the paramagnetic quenching. The sensor showed a high
selectivity for Al3+ and Cu2+ in methanol solution at pH = 5 and was not disturbed by other
competing metal ions. Furthermore, based on the equation 3σ/slope, the detection limits of sensor
for Al3+ and Cu2+ were calculated to be 3.1 × 10-10 M and 9.8 × 10-11 M, respectively. Additionally,
the association constants of X[Al3+] and X[Cu2+] were also determined to be 2.3 × 104 M-1 and 6.1
× 104 M-1 on basis of Benesi-Hildebrand equation. In addition, the titration experiment of
fluorescence and mass spectrometry showed that sensor was combined with Al3+ or Cu2+ both in 1:1
ratio. Moreover, the optimized structure and energy calculations of X, X[Al3+] and X[Cu2+] were
obtained by Gaussian software based on the basis set of B3LYP/6-31G(d) and B3LYP/LANL2DZ.
And, the sensor successfully detected Al3+ and Cu2+ in the real water sample with a satisfactory
recovery (91.4 % - 107.1%) and RSD values (0.66 % - 1.91 %).
- 2 -

Keywords: Schiff base; Excellent detection limit; Different sensing mechanisms; Theoretical
calculation; real water sample.
1. Introduction
In recent years, one of the hot areas is the detection and determination of metal ions like
aluminum and copper with high sensitivity and selectivity due to their importance in biological,
medical, environmental and industrial fields [1-3]. Aluminum, after oxygen and silicon, is the third
most abundant element in the earth’crust (about 8%) and is characterized by good ductility and high
melting point which has many applications in daily life, such as packaging materials, refractory
materials and building materials [4-7]. According to the World Health Organization (WHO), the
average daily human intake of aluminum is approximately 3-10 mg while the permissible limit is
7.4 ×10-6 M in drinking water [8]. However, aluminum is not one of the essential metal ions for the
human body, and its excessive absorption could lead to a various of diseases including myopathy,
Alzheimer’s disease and Parkinson’s disease [9-12]. Furthermore, influenced by acid rain and
industrial acid wastewater discharge, the content of aluminum in soil was up surged with the
increase of acidity, which restricts the growth and development of plants [13-15]. Copper is third
abundant and essential trace element in human body, next to iron and zinc, which could participate
in the metabolism of all sorts of enzymes and maintain normal hemopoietic function and the health
of central nervous system [16-19]. In the human body, the normal concentration of copper is about
15.6 - 23.6 µM [20]. The imbalances in capper concentration could lead to various diseases, such
as Alzheimer’s disease, Huntington disease and Wilson’s disease [21-23]. Meanwhile, plants could
experience symptoms of victimization including physiological arrest, stunted development and even
death when copper accumulate in a certain amount [24-26]. Thus, the detection of aluminum and
copper with effectivity and selectivity is of great significance for bioscience and environmental
sciences.
Till now, some high-efficiency fluorescent sensors with excellent optical response for
aluminum or copper have been developed and gradually replaced the traditional monitoring methods
- 3 -

(atomic absorption spectrometry, atomic emission spectrometry and inductively coupled plasma
mass spectrometry), which become a new monitoring method with simple operation, high sensitivity
and selectivity [27-30]. However, based on respective sensing mechanisms like photoinduced
electron transfer (PET), chelation enhanced fluorescence (CHEF), paramagnetic quenching and
others, some of fluorescent sensors could only detect a single metal ion, aluminum or copper, which
greatly limits their application [31-34]. In contrast, the multifunction sensor which could detect
multiple metal ions based on different mechanism exhibited many advantages, including
multiplicity, simple operation and so on [35, 36]. Nowadays, some sensors have been designed and
synthesized, which could detect both aluminum and copper [37]. Unfortunately, the sensors that
have been reported still have many shortcomings, including poor sensitivity and susceptibility to
interference from other ions. Moreover, there are few articles that could give a comprehensive
description and exploration of the sensing mechanism of multi-function sensor for aluminum and
copper [38]. As a result, there is still an urgent need to design and synthesize an ultra-sensitive
sensor to accurately detect aluminum and copper based on the coordinated control of multiple
sensing mechanisms.
Recently, a series of fluorescent sensors based on imidazo[2, 1-b]thiazole were designed and
synthesized by our group for different metal ions [39, 40]. As one of heterocyclic compounds,
imidazo[2, 1-b]thiazole, has shown excellent optical properties and good identification ability like
other commonly used fluorescent nuclei (rhodamine, naphthalimide and coumarin), suggesting that
it could be used as a potential fluorophore [41-43]. In addition, salicylaldehyde was the most
commonly used aldehydes for the synthesis of Schiff base, which provide hydroxyl groups to
increase the ability of binding metal ions [44, 45]. In this report, a new Schiff base, (E)-N'-(2-
hydroxybenzylidene)imidazo[2,1-b]thiazole-6-carbohydrazide (X), was prepared by linking
salicylaldehyde and imidazo[2, 1-b]thiazole with a C=N bond. As expected, the sensor X exhibited
a weak fluorescence intensity based on PET process. After binding with Al3+, a new complex,
X[Al3+], was formed and displayed a strong fluorescence intensity due to the inhibition of PET
process. Furthermore, the complex X[Al3+] could be further used as a new sensor for Cu2+ based on
the paramagnetic quenching, showing efficient fluorescence quenching behavior. In a word, X could
be used as a sensor for Al3+ and Cu2+ with excellent sensitivity and selectivity in methanol buffer
- 4 -

solution (methanol/H2O = 9/1, 10 mM tris, pH = 5.0) based on the coordinated control of multiple
sensing mechanisms.
2. Experimental section
2.1. Materials and sample preparation
All reagents and solvents were commercially available AR and CP and were used without
any treatment. All metal ionic solution are corresponding chloride and sulfate solutions, including
AlCl3, ZnCl2, Li2SO4, CdCl2, CoCl2, FeCl3, MgCl2, CrCl3, MnCl2, AgCl, KCl, CuCl2, NiCl2, HgCl2.
Stock solutions of the metal ions mentioned above were prepared with a concentration of 0.03 M
by distilled water. Stock solutions of Al3+ and Cu2+ were further prepared with a concentration of
0.03 M by tap water. The X was dissolved in methanol/H2O (v/v = 9 : 1) buffer solution (10 mM
tris, pH = 5.0) at room temperature with the concentration of 1 × 10-5 M.
2.2. Measurements
UV-vis spectra were obtained on a Shimadzu 3100 spectrometer. Fluorescence spectral data
was recorded on an Edinburgh Instruments Ltd-FLS920 Fluorescence Spectrophotometer.
Fluorescence measurements were recorded using excitation at 365 nm. The slits of excitation and
1
emission were 10 nm and 5 nm, respectively. H NMR measurement was performed on a Bruker
AV III 400 MHz NMR spectrometer with tatramethysilane (TMS) as internal standard and DMSO
as solvent. 13C NMR spectra data was taken on a Bruker AV III 100 MHz NMR spectrometer with
tatramethysilane (TMS) as internal standard and DMSO as solvent. Infrared spectral data was
obtained on a Bruker Vertex 70 FT-IR spectrometer using samples as KBr pellets. Thin layer
chromatography (TLC) analyses were performed to monitor all the reactions.
2.3. Calculation of quantum yield, detection limit and association constant
Quantum yield was calculated according to the following formula (1):
퐹 퐴푠푛푢2
푢
Ф푢 = Ф푠
퐹 퐴푢푛푠2
푠
Ф, F, A, and n represent the quantum yield, the integrated area under the corrected emission
spectra, the absorbance intensity at the excitation wavelength and the refractive index of solvent,
respectively. In addition, s refers to rhodamine B as the standard, and u refers to the target. The
quantum yield (Ф) of rhodamine B dissolved in anhydrous ethanol is 0.97.
The detection limit of sensor for metal ions was calculated by the following formula (2):
- 5 -

3휎
푆
퐷퐿 =
σ is the standard deviation of 10 times blank measurements and s is the slope between
fluorescence intensity versus metal concentration.
The association constant between X and metal ions was calculated by the Benesi-Hildebrand
eqn (3):
1
1
1
1
=
×
+
퐹 − 퐹0 푀푛+ 퐾푎[퐹푚푎푥 − 퐹0] 퐹푚푎푥 − 퐹0
where F is the fluorescence intensity of the X[Mn+] complex, which is in accordance with the
concentration of Mn+. F0 is the fluorescence intensity of free X. Fmax is the fluorescence intensity
of X[Mn+] complex in the presence of the maximum concentration of Mn+.
2.4. Theoretical calculations
Density functional theory (DFT) structural optimizations were performed with the Gaussian
09 program. In all cases, the structures were optimized using the B3LYP functional and the mixed
basis set 6-31+G (d) and LANL2DZ. Each structure was subsequently subjected to TD-DFT
calculation using the B3LYP functional [39]. For all optimized structures, frequency calculations
were performed to confirm the absence of imaginary frequencies. The molecular orbitals were
visualized and plotted with the GaussView 5.0 program.
2.5. Synthesis and characterization of X
Compound 1 (ethyl imidazo[2,1-b]thiazole-6-carboxylate) and compound 2 (imidazo[2,1-
b]thiazole-6-carbohydrazide) were prepared according to a previous report [40].
Synthesis of (E)-N'-(2-hydroxybenzylidene)imidazo[2,1-b]thiazole-6-carbohydrazide (X).
imidazo[2,1-b]thiazole-6-carbohydrazide (210 mg, 1.15 mmol) and 2-hydroxybenzaldehyde (166
mg, 1.36 mmol) were added to 50 ml round bottom flask containing 15ml of ethanol. Then the
mixture was stirred for 12 hours at room temperature until pale-yellow precipitate appeared. After
the reaction, the pale-yellow precipitate was collected by filtration and washed with cold ethanol to
obtain the pure solid X. Yield: 224 mg, 71.2 %. Ms (ESI): m/z = 287.07 [M + H]+, 309.03 [M +
Na]+. FTIR (KBr, cm-1): 3322 (N-H), 1670 (C=O). 1H NMR (400 MHz, DMSO) δ 12.18 (s, 1H),
11.48 (s, 1H), 8.71 (s, 1H), 8.40 (s, 1H), 8.00 (d, J = 4.5 Hz, 1H), 7.44 (t, J = 5.6 Hz, 2H), 7.32 –
7.25 (m, 1H), 6.92 (t, J = 7.6 Hz, 2H). 13C NMR (101 MHz, DMSO) δ 158.49, 158.01, 149.45,
149.13, 140.48, 131.66, 130.37, 120.71, 119.77, 119.09, 117.23, 116.92, 116.13.
- 6 -

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.
3. Results and discussion
As shown in scheme 1, X was designed and synthesized in medium yield according to the
synthetic route. Compound 1 (ethyl imidazo[2,1-b]thiazole-6-carboxylate) and compound 2
(imidazo[2,1-b]thiazole-6-carbohydrazide) were synthesized according to a previous report [40].
Then, X was synthesized by the reaction of Compound 1 and 2 with 71.2 % yield in ethanol and
characterized by 1H NMR (Fig. S1), 13C NMR (Fig. S2), FTIR (Fig. S3), ESI-MS (Fig. S4). All of
the data in the spectra were in good accordance with the structure.
3.1. The response time studies of X to Al3+
First of all, the effect of response time on the fluorescence intensity of X was investigated in
the presence of Al3+. As shown in Fig. S5, upon addition of Al3+ (50 µM), the fluorescence intensity
of X (1 × 10-5 M) at 455 nm gradually increased with time in methanol/H2O buffer solution (v/v =
9/1, tris = 10 mM, pH = 5.0) under excitation wavelength of 365 nm. Obviously, the response time
of X for Al3+ was less than 30 minutes. The fluorescence intensity reached the maximum and
remained stable in the next 10 minutes, indicating the high reactivity of X for Al3+. So, for the
stability of the experiment and to get a good experimental result, 1 hour of complexation time was
used in all experiments about Al3+, which means that all samples should be stationary for 1 hour
before testing.
3.2. The optical properties of X towards various metal ions
The sensing behavior of X toward various metal ions, including Al3+, Cu2+, K+, Ag+, Hg2+,
Cd2+, Mn2+, Li+, Ni2+, Fe3+, Zn2+, Cr3+, Mg2+ and Co2+, were investigated by fluorescence
- 7 -

spectroscopy in methanol/H2O buffer solution (v/v = 9/1, tris = 10 mM, pH = 5.0). As shown in Fig.
1, as expected, the free X exhibited weak fluorescence intensity with a quantum yield of 0.035 under
the excitation wavelength of 365 nm. When various metal ions (10 equiv.) were added to the system
above, the fluorescence intensity at 445 nm increased sharply with Al3+, showing a strong blue
fluorescence (Ф = 0.64) which can be seen in the Fig. 1 insert. However, the fluorescence intensity
still remained closed behavior with the addition of other metal ions. Meanwhile, the UV-vis
spectrum of X for various metal ions (Al3+, Cu2+, K+, Ag+, Hg2+, Cd2+, Mn2+, Li+, Ni2+, Fe3+, Zn2+,
Cr3+, Mg2+ and Co2+) were also explored under the same condition. As shown in Fig. S6, the free X
showed three absorbance bands at 288 nm, 299nm and 325nm, respectively. When various metal
ions were added, some metal ions could also cause obvious change in absorption spectrum besides
Al3+, including Fe3+, Ni3+, Cu2+ and Co2+. However, no noteworthy change in absorption bands could
be observed upon addition of other metal ions. That is, X could be used as a sensor to detect of Al3+
by emission color changed.
Fig. 1. Fluorescence spectra of X (10 µM) upon addition of various metal ions (10 equiv.) in
methanol/H2O buffer solution (v/v = 9/1, tris = 10 mM, pH = 5.0). Insert: the fluorescence intensity
of X in the absence and presence of Al3+ under UV light of 365 nm.
3.3. The sensing of X for Al3+
In order to further explore the sensing properties of X on Al3+, the related fluorescence and
UV-vis titration experiments were carried out in methanol/H2O buffer solution (v/v = 9/1, tris = 10
mM, pH = 5.0). As shown in Fig. 2, the fluorescence intensity of X was weak at 445 nm, which
could increase gradually as the concentration of Al3+ changed from 0 to 5 equiv. and tend to stabilize
- 8 -

when the concentration of Al3+ was greater than 5 × 10-5 M. As shown in Fig. S7, the fluorescence
intensity of X had a good linear relationship (R2 = 0.9833) with low level of Al3+. Based on the
fluorescence titration data, the detection limit of X for Al3+ was calculated to be 3.1 × 10-10 M by
the equation 3σ/slope. Otherwise, the association constants of X[Al3+] were also determined to be
2.3 × 104 M-1 on basis of Benesi-Hildebrand equation (Fig. S8). The UV-vis titration experiments
were also performed in Fig. 3, with the gradual addition of Al3+ (0 to 10 equiv.), the absorbance
bands at 288 nm, 299nm and 325nm displayed a lessening accompanied by appearance of new bands
at 308nm, 322nm and 374nm, respectively. Distinctive isosbestic points were found at 262 nm, 304
nm, 326 nm and 347 nm, indicating that a stable complex (X[Al3+]) was formed between X and Al3+.
As shown in Fig. 4, the job’s plot showed 1:1 stoichiometry between the X and Al3+ because the
maximum fluorescent intensity was centered at 0.5 M fraction. The ESI mass spectra of X[Al3+]
was also performed in Fig. S9, the peak at m/z 312.03 was attributed to [X + Al3+ - H+]2+ (calcd m/z
312.02). The titration results showed that X could be used as a sensor with excellent performance
(including high sensitivity and stable binding ability) toward Al3+.
Fig. 2. Fluorescence emission spectra of X (10 µM) upon gradually addition of Al3+ (0 to 20 equiv.)
in methanol/H2O buffer solution (v/v = 9/1, tris = 10 mM, pH = 5.0).
- 9 -

Fig. 3. Absorbance spectra of X (10 µM) upon gradually addition of Al3+ (0 to 10 equiv.) in
methanol/H2O buffer solution (v/v = 9/1, tris = 10 mM, pH = 5.0).
Fig. 4. Job’s plot of X in methanol/H2O buffer solution ([X] + [Al3+] = 1.0 × 10-4 M).
3.4. Interference experiment with other metal ions
In order to explore the anti-interference ability of X as a sensor for Al3+, the competition
experiment was carried out in Fig. 5. For this purpose, X was mixed with Al3+ (10 equiv.) and then
different interfering ions (Cu2+, K+, Ag+, Hg2+, Cd2+, Mn2+, Li+, Ni2+, Fe3+, Zn2+, Cr3+, Mg2+ and
Co2+) were added separately. The results showed that the response of X to Al3+ were relatively low
but still clearly detectable in the presence of Hg2+, Ni2+, Fe3+ and Co2+. However, when addition of
Cu2+, the fluorescence intensity of X[Al3+] system was completely quenched, displaying an efficient
“turn-off” behavior [46, 47]. Consequently, X could be used as a sensor to selectively recognize
Al3+ through obvious fluorescence enhancement without interference from most metal ions. And
- 10 -

the new complex X[Al3+] form by X and Al3+ could be further used as an efficient quenching sensor
for Cu2+.
Fig. 5. Fluorescence response of X (10 µM) to different metal ions (10 equiv.) in the
absence/presence of Al3+ (10 equiv.) in methanol/H2O buffer solution (v/v = 9/1, tris = 10 mM, pH
= 5.0). Black bar: X with Cu2+, K+, Ag+, Hg2+, Cd2+, Mn2+, Li+, Ni2+, Fe3+, Zn2+, Cr3+, Mg2+ and
Co2+. Red: X with different metal ions in the presence of Al3+.
3.5. The sensing of X[Al3+] for Cu2+
To gain insight into the sensing properties of X[Al3+] for Cu2+, the related fluorescence
titration experiments were conducted by gradual addition of various concentrations of Cu2+ in
methanol/H2O buffer solution (v/v = 9/1, tris = 10 mM, pH = 5.0). Based on the above experiments,
the fluorescence intensity of X was significantly enhanced when 10 equiv. of Al3+ were added. As
shown in Fig. 6, the fluorescence intensity at 445 nm decreased gradually with the increased of Cu2+
(0 to 4 equiv.) under an excitation wavelength of 365 nm. Moreover, upon addition of lower than 2
equiv. of Cu2+, the fluorescence intensity decreased drastically. However, the subsequent addition
of 2 equiv. of Cu2+ had little effect on fluorescence intensity. This indicated that X[Al3+] had
excellent sensing behavior for Cu2+. Therefore, as shown in Fig. S10, the change of fluorescence
intensity (I0 - I) at 445 nm was linearly proportional (R2 = 0.9967) to the low concentration of Cu2+
(0 - 1 × 10-8 M). And the detection limit was calculated to be 9.8 × 10-11 M based on the equation
3σ/s, which was lower than many reported Cu2+ sensor [48-50]. Otherwise, the association constants
of X[Cu2+] were also determined to be 6.1 × 104 M-1 on basis of Benesi-Hildebrand equation (Fig.
S11). The results of mass spectrometry titration in the presence of Cu2+ were shown in Fig. S12, the
- 11 -

peak at m/z 347.96 could be assigned to the complex [X + Cu2+ - H+]+ (calcd m/z 347.97), which
proves that the complexation ratio between X and Cu2+ was 1:1. The competition experiment was
shown in Fig. 7, the results indicated that X[Al3+] could maintain the quenching sensing behavior
for Cu2+ regardless of the presence or absence of other interfering ions (including K+, Ag+, Hg2+,
Cd2+, Mn2+, Li+, Ni2+, Fe3+, Zn2+, Cr3+, Mg2+ and Co2+). These results primarily indicated that the
complex X[Al3+] could be considered as a potential sensor for Cu2+ with excellent sensitivity and
anti-interfere capability.
Fig. 6. Fluorescence emission spectra of X[Al3+] upon gradually addition of Cu2+ (0 to 10 equiv.) in
methanol/H2O buffer solution (v/v = 9/1, tris = 10 mM, pH = 5.0).
Fig. 7. Fluorescence intensity of X[Al3+] in the presence of Cu2+ (3 equiv.) and followed by 3 equiv.
of various other metal ions (K+, Ag+, Hg2+, Cd2+, Mn2+, Li+, Ni2+, Fe3+, Zn2+, Cr3+, Mg2+ and Co2+)
in methanol/H2O buffer solution (v/v = 9/1, tris = 10 mM, pH = 5.0).
3.6. The effect of pH
The pH effect on the optical behavior of X, X[Al3+] and X[Cu2+] was investigated at different
- 12 -

pH varying from 2 to 12. As shown in Fig. 8, The fluorescence intensity of X remained constant at
different pH values. Upon addition of Al3+ (10 equiv.), no change in fluorescence intensity was
witnessed in basic conditions (pH＞8) which may be attributed to the hydrolysis behavior of Al3+
to form Al(OH)3 and reduced the concentration of X[Al3+]. Under acidic conditions (pH＜5), X
exhibited an efficient sensing capability for Al3+ because the hydrolysis behavior of Al3+ was
inhibited under acidic conditions resulting in the X being able to bind Al3+ more easily. When
addition of Cu2+ (5 equiv.) to the system in the presence of Al3+, the fluorescence intensity was
completely quenched at different pH values. The investigated results suggested that under acidic
conditions, X could be used as a sensor for detection of Al3+ and the complex X[Al3+] further could
act as a new sensor for Cu2+, which provides the possibility to detect Al3+ and Cu2+ under extreme
conditions [13, 46].
Fig. 8. The influence of pH value of solution on the fluorescence of X, X[Al3+] and X[Cu2+] in
methanol/H2O buffer solution.
3.7. The mechanism of binding mode and theoretical calculation
In order to clearly understand the X sensing mechanism for Al3+ and Cu2+, the possible binding
mechanisms were proposed based on based on fluorescence and mass spectrometry titration
experiments [39, 40]. As shown in scheme 2, X exhibited weak fluorescence based on the PET
mechanism and apparently formed a cavity to provide a complexation site for metal ions. After
binding with Al3+, the fluorescence intensity was significantly enhanced because the synergistic
effect of PET and CHEF. Obviously, X binding to Al3+ through three coordination sites N
(imidazole), N (C=N bond) and O (phenolic hydroxyl). When addition of Cu2+ to the system X[Al3+],
- 13 -

Cu2+ could replace Al3+ in the original position and form a stable complex with X because Cu2+ had
a stronger coordination ability than Al3+. Cu2+ was a paramagnetic metal ion and could strongly
quench the fluorescence intensity [35, 50]. So, the fluorescence disappeared again when Cu2+ were
added.
Scheme 2. The proposed binding mode of X with Al3+ and Cu2+.
To better understand the sensing mechanism of X on Al3+ and Cu2+, The optimized geometry
and energy calculations of X, X[Al3+] and X[Cu2+] were investigated using ab initio density
functional theory (DFT) combined with time-dependent density functional theory (TD-DFT), as
implemented in the Gaussian 09 package based on the B3LYP/LANL2DZ basis set (for transition
metal ion Cu2+) and B3LYP/6-31G(d) basis set (for all the other elements) [30]. The optimized
geometry of X, X[Al3+] and X[Cu2+] were presented in Fig. 9. For X, the imidazo[2, 1-b]thiazole
and phenolic hydroxyl of X were in the same plane, which made the whole sensor molecule showed
good coplanarity and stability. Moreover, it is highly consistent with the optimal structure proposed
previously, which was conducive to coordination with metal ions. For X[Al3+] and X[Cu2+], a three-
coordinate complex were formed between X and Al3+ or Cu2+, including Al3+-N (imidazole, 1.82
Å), Al3+-N (C=N bond, 1.84 Å), Al3+-O (phenolic hydroxyl, 1.68 Å) and Cu2+-N (imidazole, 1.92
Å), Cu2+-N (C=N bond, 2.04 Å), Cu2+-O (phenolic hydroxyl, 1.84 Å), respectively. Obviously, the
overall structure of sensor did not change significantly before and after binding metal ions.
- 14 -

Fig. 9. The optimized geometry of X, X[Al3+] and X[Cu2+].
Moreover, the spatial distributions and orbital energies of the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were shown in Fig. 10. In
the molecular orbital structure of X, the LUMO electron densities were mostly distributed in the
phenol group and C=N bond while that of the HOMO electron densities spread on the whole X. The
electrons were partially transferred under photon excitation, which implied a photoinduced electron
transfer (PET) process. As a result, X displayed a weak fluorescence intensity. As for X[Al3+], the
HOMO and LUMO were mainly located near the Al3+ and on the benzene ring, indicating that the
electron basically did not transfer in the excited state. So, the X[Al3+] showed strong fluorescence
due to the inhibition of PET process along with the CHEF mechanism [41]. As for X[Cu2+], the
HOMO and LUMO of X[Cu2+] were similar to that of the X, but the electron transfer was not
obvious. Moreover, the energy gap between HOMO and LUMO of X, X[Al3+] and X[Cu2+] were
calculated to be 4.15 eV, 3.35 eV and 3.91 eV. After binding with metal ions (Al3+ and Cu2+), the
HOMO-LUMO energy gap became reduced, indicating the formation of a stable complex.
Therefore, the calculated results were highly consistent with the experimental results and
phenomena, proving that the binding mode and response mechanism proposed above were feasible
and acceptable.
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Fig. 10. The HOMO and LUMO of X, X[Al3+] and X[Cu2+].
3.8. Application in real water samples
To further investigate practical application of X, the amounts of Al3+ and Cu2+ in tap water
were analyzed by proposed fluorescent method. The samples of Al3+ and Cu2+ with different
concentration were prepared with tap water. In order to obtain a good experimental with a small
deviation, all the measurements were performed three times. As shown in Table 1, a good agreement
was obtained between the added and recovered ion amounts. The recovery value for Al3+ and Cu2+
were found in the range of 91.4% - 106.4% and 105.5% - 107.1%, respectively. And the relative
standard deviation (RSD) of three measurements for Al3+ and Cu2+ was less than 1.91%, indicating
that the measured data were reliable. In a word, a satisfactory recovery and RSD values were
obtained, indicating that X could be used as a practical sensor to quantitatively detect Al3+ and Cu2+
in real water samples.
Table 1. Determination of Al3+ and Cu2+ in tap samples
Conclusions
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In summary, A new simple Schiff base, (E)-N'-(2-hydroxybenzylidene)imidazo[2,1-
b]thiazole-6-carbohydrazide (X), was designed and synthesized based on salicylaldehyde and
imidazo[2, 1-b]thiazole. The structure of X was characterized by 1H NMR, 13C NMR, FTIR, ESI-
MS. In the methanol solution, X could be used as a sensor to identify Al3+ through a significant
fluorescence enhancement at 445 nm with distinct color change from colorless to bright blue under
UV light. And the complex X[Al3+] could be used as a new sensor for Cu2+ through a highly efficient
quenching behavior. Furthermore, the detection limits of sensor for Al3+ and Cu2+ were calculated
to be 3.1 × 10-10 M and 9.8 × 10-11 M, respectively. Additionally, the association constants of X[Al3+]
and X[Cu2+] were also determined to be 2.3 × 104 M-1 and 6.1 × 104 M-1. In addition, the job’s plot
showed 1:1 stoichiometry between X and metal ions (Al3+ or Cu2+). Otherwise, the optimized
structure and energy calculations of X, X[Al3+] and X[Cu2+] were obtained by Gaussian 09 to prove
the binding mode and sensing mechanism. And, the sensor successfully detected Al3+ and Cu2+ in
the real water sample with a satisfactory recovery and RSD values, which provide a possibility for
practical application.
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.
Yuankang Xu is studying for master degree in Department of Chemistry at University of Jinan. She received her
BS in chemistry at University of Jinan in 2016.
Lei Yang is an employee of Henan Sanmenxia Aoke Chemical Industry Co. Ltd.
Hanyu Wang is studying for master degree in Department of Chemistry at University of Jinan.
- 17 -

Yanxia Zhang is a teacher in Department of Chemical Engineering, University of Jinan, China.
Xiaofeng Yang is an associate professor in Department of Chemical Engineering, University of Jinan, China.
She received his Ph.D. degree in 2009 from Saitama University, her research focus is chemosensor based on small
molecules.
Meishan Pei received his PhD degree in 2000 from Chinese Academy of Sciences. He is a professor in
Department of Chemistry and Physics of Polymers. His current research focuses on functional polymer science.
Guangyou Zhang received his PhD degree in 1993 from Nanjing Forestry University. He is a professor in
Department of Chemical Engineering, University of Jinan, China. His current research interests are mainly in the
development of sensors for surfactants, metal ions, pharmaceutical anions, proteins and DNA based on fluorescent
molecules.
All authors contribute equally.
Acknowledgements
The authors thank the Henan Sanmenxia Aoke Chemical Industry Co., Ltd. w0920 for financial
support. Financial support by the National Natural Science Foundation of China (Grants 21708013)
and the China Postdoctoral Science Foundation (Grants 2017M620288)
- 18 -

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