A selective "turn-on" sensor for recognizing In3+ and Zn2+ in respective systems based on imidazo[2,1-: B] thiazole

Article abstract of DOI:10.1039/c9pp00408d

A new simple and easily synthesized multitarget sensor, (E)-N′-(4-(diethylamino)-2-hydroxybenzylidene)imidazo[2,1-b]thiazole-6-carbohydrazide (X), was designed and synthesized using imidazo[2,1-b]thiazole-6-carboxylic acid and 4-(diethylamino)-2-hydroxybenzaldehyde. X could be used as a sensor to detect In³⁺ in DMF-H₂O buffer solution and detect Zn²⁺ in EtOH-H₂O buffer solution through fluorescence enhancement with detection limits of 1.02 × 10^-9 M and 5.5 × 10^-9 M, respectively. X exhibited an efficient "off-on-off" fluorescence behavior by cyclic addition of metal ions (In³⁺ and Zn²⁺) and EDTA. The stoichiometry between X and metal ions (In³⁺ and Zn²⁺) was 1:1. The binding mode and sensing mechanism of X with metal ions (In³⁺ and Zn²⁺) was verified by theoretical calculations using Gaussian 09 based on B3LYP/6-31G(d) and B3LYP/LANL2DZ basis, respectively. Moreover, X could be applied as a potential sensor for the quantitative detection of In³⁺ and Zn²⁺ with a satisfactory recovery and relative standard deviation (RSD) in real water samples.

Full text of DOI:10.1039/c9pp00408d

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DOI: 10.1039/C9PP00408D
ARTICLE
A selective “turn-on” sensor for recognizing of In3+ and Zn2+ in
respective system based on imidazo[2,1-b]thiazole
Yuankang Xua, Songfang Zhaob,*, Yanxia Zhanga, Hanyu Wanga, Xiaofeng Yanga, Meishan Peia,
Received 00th January 20xx,
Accepted 00th January 20xx
Guangyou Zhanga,*
DOI: 10.1039/x0xx00000x
A new simple and easily synthesized multitarget sensor, (E)-N'-(4-(diethylamino)-2-hydroxybenzylidene)imidazo[2,1-
b]thiazole-6-carbohydrazide (X), was designed and synthesized based on imidazo[2,1-b]thiazole-6-carboxylic acid and 4-
(diethylamino)-2-hydroxybenzaldehyde. X could be used as a sensor to detect In3+ in DMF-H2O buffer solution and detect
Zn2+ in EtOH-H2O buffer solution through fluorescence enhancement with detection limits of 1.02 × 10-9 M and 5.5 × 10-9 M,
respectively. X exhibited an efficient “off-on-off” fluorescence behavior by cyclic addition of metal ions (In3+ and Zn2+) and
EDTA. The stoichiometry between X and metal ions (In3+ and Zn2+) was 1:1. The binding mode and sensing mechanism of X
with metal ions (In3+ and Zn2+) was verified by theoretical calculations using Gaussian 09 based on B3LYP/6-31G(d) and
B3LYP/LANL2DZ basis, respectively. Moreover, X could be applied as a potential sensor for the quantitative detection of In3+
and Zn2+ with a satisfactory recovery and the relative standard deviation (RSD) in real water samples.
damage.20-23 Therefore, for the above reasons, the detection of
zinc and indium is crucial and urgent.
1. Introduction
Recently, various analytical technique for detection of Zinc
Metal ions are widely found in nature and closely related to the
physiological activities of living bodies.1-3 Among diverse metal
ions, zinc is the second most abundant trace element in the
human body and plays an important role in various
physiological activities, such as gene expression, cellular
metabolism, immune function, neural signal transmission and
DNA recombination or recognition.4-8 Thus, zinc is vital for
human health.9, 10 The permissible concentration of zinc is 2-2.5
g throughout the human body, whereas lack and excessive
intake of zinc can cause a variety of diseases, such as Alzheimer,
Parkinson, epilepsy, amyotrophic lateral sclerosis (ALS) and
ischemia.11-15 Likewise, indium is an important industrial metal
with good toughness, plasticity and ductility, and has significant
applications in many fields, such as manufacture of low fusion
gold, bearing alloys, semiconductor devices and transparent
electrically conductive films.16-18 Moreover, it is generally
believed that indium in pure metal form is a non-toxic and safe
metal.19 However, some of studies recently suggested that
indium and its compounds (indium phosphide and indium tin
oxide) not only interfere with iron metabolism during cell
absorption, transport and storage, but also cause severe lung
and indium have been reported, such as graphite furnace
atomic absorption spectrometry, molecular imprinted polymer
sensor and ion-selective electrodes, which are characterized by
complicated operation, skilled experts and involve expensive
equipment.19, 20, 24-26 However, compared with these methods,
fluorescence methods has attracted more and more attention
due to its quick response, simple operation, high sensitivity and
real application.27-30 In the past, there have been many reports
on the detection of zinc, but few on the detection of indium.18,
31-34
Detection of zinc and indium by fluorescent sensor in
organic solvent or aqueous solution is often interfered by other
metal ions.35-37 For example, cadmium, which has similar
properties and coordination modes to zinc, will affect the
detection of zinc.4 Analogously, the detection of indium is often
disturbed by the same group metal ions, aluminum and
gallium.21 In addition, the detection limit of sensor for metal
ions is usually between 10-5 M and 10-8 M.30 So, it is necessary
to design the sensor with high selectivity and sensitivity for
detection of In3+ and Zn2+. At the present, the research of multi-
target sensor is much less than that of single target sensor.20, 38,
39 Multi-target sensors with highly sensitivity and selectivity are
valuable and should receive close attention because they could
detect different metal ions in respective system.
a. School of chemistry and chemical engineering, University of Jinan, Jinan 250022,
China, E-mail address: chm_zhanggy@ujn.edu.cn.
In past reports, Schiff base, derived from 4-(diethylamino)-
2-hydroxybenzaldehyde, is often used as a “off-on” sensor to
detect single target in a particular system because they have
simple synthesis method, ideal solubility and strong complexing
b. Henan Sanmenxia Aoke Chemical Industry Co. Ltd., Sanmenxia 472000, China. E-
mail address: songfang2008@163.com.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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퐹 퐴푠푛푢2
ability of metal ions.40-45 Moreover, imidazo[2,1-b]thiazole,
which contains two conjugated rings and eight central atoms in
its molecule, was a potential fluorescent nucleus.46 In this study,
View Article Online
DOI: 10.1039/C9PP00408D
푢
Ф푢 = Ф푠
퐹 퐴푢푛푠2
푠
a new simple schiff base (
structure was designed and synthesized based on imidazo[2,1-
b]thiazole-6-carboxylic
hydroxybenzaldehyde.47-49 The sensor showed strong ability to
recognize metal ions because of the oxygen atom of -OH and
X) with a distinct “acceptor-donor”
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.
acid
and
4-(diethylamino)-2-
the nitrogen atoms of -CH=N- and -N=. As expected, X could
be used as a highly selective and sensitive fluorescent sensor to
detect zinc and indium by fluorescence enhancement in
ethanol-water and DMF-water buffer system and could
quantitatively assess zinc and indium in tap water samples. As
shown in Table S1, the comparison of some different reported
sensors for the detection of In3+ and Zn2+ were summarized.
Compared with these reported sensors, comprehensively
speaking, our sensor had a lower LOD for In3+ and Zn2+. On the
The association constant between
X and metal ions was
calculated by the Benesi-Hildebrand eqn (2):
1
1
1
1
=
×
+
퐹 − 퐹0 푀푛+ 퐾푎[퐹푚푎푥 − 퐹0] 퐹푚푎푥 − 퐹0
where F is the fluorescence intensity of the
which is in accordance with the concentration of Mn+. F0 is the
fluorescence intensity of free . Fmax is the fluorescence
[Mn+] complex in the presence of the maximum
X
[Mn+] complex,
whole, the
X
could serve as a sensor for In3+ and Zn2+.
X
intensity of
X
2. Experimental section
2.1. Materials and sample preparation
concentration of Mn+.
2.4. Theoretical calculations
All reagents and solvents were commercially available AR and
CP and were used without further treatment. All metal ionic
solution are corresponding chloride, sulfate and nitrate
solutions, including InCl3, Ga(NO3)3, ZnCl2, Li2SO4, AlCl3·6H2O,
CdCl2·2.5H2O, CoCl2·6H2O, FeCl3, MgCl2·6H2O, CrCl3·6H2O,
MnCl2·4H2O, AgCl, KCl, CuCl2·2H2O, NiCl2·6H2O, HgCl2. The anion
solution is the corresponding sodium salt solutions. Stock
solutions of the ions mentioned above were prepared with a
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.50 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.
concentration of 0.03 M in distilled water. The probe
X was
dissolved in ethanol/H2O (v/v = 9 : 1) buffer solution (10 mM tris,
pH = 7.4) and DMF/H2O (v/v = 9 : 1) buffer solution (10 mM tris,
pH = 7.4) at room temperature with the concentration of 1 × 10-
5 M.
2.5. Synthesis of X
Compound
compound
1
2
(ethyl imidazo[2,1-b]thiazole-6-carboxylate) and
(imidazo[2,1-b]thiazole-6-carbohydrazide) were
2.2. Measurements
prepared according to a previous report.46 Firstly, compound
1
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 emission were 10 nm and 10
was obtained by the reaction of thiazol-2-amine with ethyl 3-
bromo-2-oxopropanoate (80%) in THF solution. Then,
compound
1 and hydrazide hydrate (80%) were stirred
overnight in ethanol solution at room temperature to produce
compound 2 (white solid).
Synthesis of (E)-N'-(4-(diethylamino)-2-hydroxybenzylidene)
1
nm, respectively. H NMR measurement was performed on a
Bruker AV III 400 MHz NMR spectrometer with
tatramethysilane (TMS) as internal standard and CDCl3 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.
imidazo[2,1-b]thiazole-6-carbohydrazide
(X). imidazo[2,1-
2.3. Calculation of quantum yield and association constant
Quantum yield was calculated according to the following
formula (1):
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b]thiazole -6-carbohydrazide (compound
and 4-(diethylamino)-2-hydroxybenzaldehyde (101 mg, 0.523 were added, the fluorescence intensity of
mmol) were mixed and stirred in 10 ml ethanol at room increased (about 9 times), displaying an efficient “turn-on”
2
, 84 mg, 0.461 mmol) 0.11) to bright blue (Ф = 0.42). As shown in Fig. 1(b), when In3+
was significantly
X
temperature for 6 hours. Then the mixture was refluxed for 2 behavior. In addition, the fluorescence intensity of X was almost
hours. The solvent was removed by rotary evaporation. The unchanged in the presence of other metal ions, except for Ni2+
residue was washed with methanol to obtain the pure yellow and Co2+. Both Ni2+ and Co2+ could quench the fluorescence of
solid
X
. Yield: 102 mg, 62.1%. Ms (ESI): m/z = 358.13 [M + H]+.
X.
1
FTIR (KBr, cm-1): 3312 (N
-
H), 1670 (C=O), 1547 (C=N). H NMR
The absorbance sensing behavior of X
(1 × 10-5 M) to
(400 MHz, CDCl3) δ 11.12 (s, 1H), 9.90 (s, 1H), 8.21 (s, 1H), 8.15 various metal ions (In3+, Ag+, Zn2+, Ni2+, Mg2+, Cd2+, Cu2+, K+, Cr3+,
(s, 1H), 7.51 (d, J = 4.5 Hz, 1H), 7.01 (d, J = 8.6 Hz, 1H), 6.97 (d, J Fe3+, Mn2+, Ga3+, Al3+, Hg2+, Li+ and Co2+) were also explored in
= 4.5 Hz, 1H), 6.26 – 6.20 (m, 2H), 3.37 (q, J = 7.0 Hz, 4H), 1.18 DMF/H2O buffer solution (v/v = 9:1, tris = 10 mM, pH = 7.4). As
(t, J = 7.0 Hz, 6H). 13C NMR (101 MHz, DMSO) δ 160.20, 157.87, shown in Fig. S5, the absorption spectrum of free
150.49, 149.30, 140.90, 132.18, 120.69, 116.75, 115.86, 107.05, maximum peak centred at 366 nm. When 7.5 equiv. of various
X exhibited a
104.05, 98.05, 44.25, 13.01.
metal ions were added, the absorption of
unchanged, except for In3+, Ni2+, Co2+ and Cu2+. All four metal
ions could from stable complexes with . In order to explore the
anti-interference ability of , the competitive experiments were
carried out in presence of various metal ions. As shown in Fig.
S6, the fluorescence response of
to In3+ was disturbed to
X was basically
X
X
3. Results and discussion
X
As shown in scheme 1,
X was designed and synthesized in
varying degrees and maintained significant enhancement in the
presence of Ni2+, Cu2+, Fe3+, Hg2+ and Co2+. These results
medium yield according to the synthetic route. Compound
(ethyl imidazo[2,1-b]thiazole-6-carboxylate) and compound
(imidazo[2,1-b]thiazole-6-carbohydrazide) were synthesized
1
2
indicated that
X
could be used as a sensor to detect In3+ in
according to a previous report. The structure of
X was
characterized by H NMR (Fig. S1), 13C NMR (Fig. S2), FTIR (Fig.
S3), ESI-MS (Fig. S4). All the data in the spectra were in good
accordance with the structure.
1
3.1. The sensing behavior of X to In3+ in DMF buffer solution
The sensing behavior of
fluorescence emission spectra. Primitively, the fluorescence
X
to In3+ was studied by UV-vis and
sensing behavior of X
(1 × 10-5 M) to various metal ions (In3+, Ag+,
Zn2+, Ni2+, Mg2+, Cd2+, Cu2+, K+, Cr3+, Fe3+, Mn2+, Ga3+, Al3+, Hg2+,
Li+ and Co2+) were explored in DMF/H2O buffer solution (v/v =
9:1, tris = 10 mM, pH = 7.4). As shown in Fig. 1(a),
relatively weak fluorescence intensity with excitation at 365 nm.
When various metal ions (7.5 equiv.) were added into , only
In3+ could lead to a significant increase in fluorescence at 463
nm. From Fig. 1 insert, the color of was easy observed in the
absence and presence of In3+ under UV lamp. After addition of
In3+, the fluorescence intensity of
changed from dim blue (Ф =
X exhibited a
X
X
X
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DMF/H2O buffer solution with good selectivity and anti- for In3+ was calculated to be 1.02 × 10-9 M based on 3σ/s
interference ability.
equation, which is lower than that had been reported.17, 21 The
In order to further investigate the chemosensing properties association constant (Ka) of
with In3+ was calculated to be 1.01
, relevant titration experiments of
with In3+ were carried × 105 M-1 (Fig. S7) based on eqn (2). These results indicated that
out in DMF/H2O buffer solution (v/v = 9:1, tris = 10 mM, pH =
could be used as a sensitive sensor to detect In3+ in DMF/H2O
7.4). As shown in Fig. 2, the free
(1 × 10-5 M) exhibited an buffer solution.
absorption peak at 366 nm. Upon gradual addition of In3+ (0 - 8
Furthermore, the favorable reversibility was an important
× 10-5 M) to
solution, the absorption peak at 366 nm characterization of sensor. EDTA was an ideal metal chelating
X
of
X
X
X
X
X
decreased obviously, whereas new prominent absorption peaks agent, which could extract metal ions from the complex (sensor-
at 401 nm and 420 nm were appeared with an isosbestic point metal ions) and restore the fluorescence of sensor. The cycle
at 385 nm. The results indicated that
complex (
[In3+]) with In3+. In addition, the fluorescence As shown in Fig. 4, free
titration experiments were also shown in Fig. 3. As mentioned In the presence of In3+ (7.5 equiv.), the fluorescence intensity of
above, the free increased significantly at 463 nm. When the same
maintained a concentration of 1 × 10-5 M and
exhibited weak fluorescence. With the increased gradually of concentration (7.5 equiv.) of EDTA were added to
[In3+] system,
In3+ concentrations from 0 to 8 × 10-5 M, the fluorescence the fluorescence was completely quenched, indicated the
intensity of increased steadily (Fig. 3(a)) and reached a plateau regeneration of free
. Then continue to add In3+ (10 equiv.) to
X
could form a stable test was carried out by successively adding In3+ and EDTA to
exhibited weak fluorescence intensity.
X.
X
X
X
X
X
X
X
(Fig. 3(b)) at 463 nm, indicating that the fluorescence intensity the system above, the fluorescence intensity was significantly
reached the maximum. As shown in Fig. 3(b) insert, a good increased again. Moreover, keep adding EDTA, the fluorescence
linear relationship between the fluorescence intensity of
X and disappeared completely. In addition, X still maintains a good
the low concentration of In3+ (0 - 1 × 10-7 M) with R2 = 0.9864 response to In3+ after two cycles with a negligible fluorescence
and Y = 14.63X + 377.54 was obtained. The detection limit of
X
loss at 463 nm and exhibited efficient “on-off-on” behavior
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506.8572 corresponded to [
472.8330 corresponded to [
X
+ In3+ + Cl- - H+]+ and a peak at m/z
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DOI: 10.1039/C9PP00408D
3+ 3+
X + In ] , respectively. All of the
experimental data indicated that the binding ratio between
X
and In3+ ions was 1:1. Thus, the possible binding mechanism
between
X
and In3+ ions were proposed in scheme 2 according
20, 23, 38
to experiments data and relevant literatures.17,
The
oxygen atom on -OH, the nitrogen atoms on C=N bond were
involved in the complexation of In3+ ions.
Moreover, in order to prove the feasibility of the proposed
binding mechanism between
and energy calculation of and X
[In3+] were investigated using
X
and In3+, structure optimization
X
density functional theory (DFT) combined with time-dependent
density functional theory (TDDFT) calculations, as implemented
in the Gaussian 09 package based on B3LYP/LANL2DZ basis.50
The optimal structure of
imidazo[2,1-b]thiazole
hydroxybenzaldehyde maintained good planar property,
respectively. After binding with In3+, [In3+] had good planarity
X
and
X
[In3+] were shown in Fig. S9. For
and 4-(diethylamino)-2-
X,
X
and structural rigidity. Besides, the spatial distributions and
orbital energies of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO)
indicated that
detection of In3+.
Moreover, the fluorescence response of
different pH value (from 3 to 11) were shown in Fig. 5. The free
(1 × 10-5 M) exhibited a stable and weak fluorescence between
X could be used as a reversible sensor to
of X and X X, the HOMO and
[In3+] were shown in Fig. 6. For free
X
to In3+ at
LUMO were distributed around 4-(diethylamino)-2-
hydroxybenzaldehyde and C=N group. There was no obvious
electron transfer. For X
[In3+], the HOMO was still around 4-
X
pH value from 3 to 11. In the presence of In3+ (7.5 equiv.), the
fluorescence intensity did not change in the range of 3 to 5 and
gradually increased with the pH changed from 5 to 7.4. When
pH
decreased over the pH range of 7.4 to 10 and reached a stable
(diethylamino)-2-hydroxybenzaldehyde and C=N group, while
electrons in LUMO were mostly concentrated around the 4-
(diethylamino)-2-hydroxybenzaldehyde
and
marginally
＞ 7.4, the fluorescence intensity of X
[In3+] gradually
distributed over the C=N group and the imidazole ring.
Obviously, after binding with In3+, the degree of intramolecular
charge transfer was increased when excited. Moreover, the
value (pH
＞10). These results indicated that sensor responded
energy gap of
3.35 eV. Thus, theoretical calculation results proved the binding
model between
and In3+ was feasible and indicated that the
response of
to In3+ may be the joint result of photo-induced
X and X
[In3+] were calculated to be 4.31 eV and
best to In3+ at pH = 7.4.
Otherwise, the possible binding mode between
X
and In3+ was
X
also proposed and demonstrated. As shown in Fig. S7, an ideal
linear relationship (R2 = 0.9952) between 1 / (I - I0) and 1 / In3+
X
electron transfer (PET) and chelation enhanced fluorescence
mechanism (CHEF).11, 31, 38, 51, 52
suggested that the stoichiometry between
1:1. In addition, the binding model was further demonstrated
by mass spectrometry. As shown in Fig. S8, a peak at m/z
X and In3+ may be
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intensity at 463 nm while other metal ions lead to either no or
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DOI: 10.1039/C9PP00408D
slight change in the emission intensity relative to free X. From
Fig. 7 insert, the color of
X was direct observed to be dim blue
(Ф = 0.11) and medium blue (Ф = 0.26) in the absence and
presence of Zn2+ under UV lamp. As shown in Fig. 7(b), in the
presence of Zn2+, the fluorescence intensity of
X was increased
by about 6.5 times, showing an obvious fluorescence “off - on”
behavior. Moreover, some metal ions could cause a slight
increase in the fluorescence intensity of X
, such as In3+ and Cd2+.
On the contrary, Ni2+, Cu2+ and Co2+ could quench the emission
intensity due to intrinsic quenching nature.53-55 These results
indicated that
X could be used as a fluorescence enhancement
sensor for recognizing of Zn2+.
The UV absorption spectra of
X
(1 × 10-5 M) in EtOH/H2O
buffer solution (v/v = 9:1, tris = 10 mM, pH = 7.4) was also
investigated in the absence and presence of various metal ions,
including Zn2+, Li+, K+, Mg2+, Al3+, Mn2+, In3+, Ga3+, Hg2+, Ag+, Cd2+,
Fe3+, Cr3+, Ni2+, Cu2+ and Co2+. As shown in Fig. S10, the
In order to explore the practical application of X, X was
used as a sensor to detect In3+ in tap water. The various
concentrations of In3+ were prepared in tap water and
measured by fluorescence assay method. As shown in Table S2,
In3+ in tap water samples could be accurately measured with a
satisfactory recovery (91.2%, 92.3%, 92.8% and 94.1%) and the
relative standard deviation (RSD) of three measurements was
absorption spectrum of free
X exhibited a maximum peak
centered at 371 nm. When 5 equiv. of various metal ions were
added, both Cu2+ and Co2+ could induce a distinct change in the
ultraviolet absorption spectrum except for Zn2+, indicating that
they could also form complexes with
X.
In order to further explore the selectivity of
X
for Zn2+, the
less than 3.51%. Ultimately, these results indicated that X could
be applied as a potential sensor for the quantitative detection
of In3+ in real water samples.
competitive experiments were carried out in presence of
various metal ions (Li+, K+, Mg2+, Al3+, Mn2+, In3+, Ga3+, Hg2+, Ag+,
Cd2+, Fe3+, Cr3+, Ni2+, Cu2+ and Co2+). As shown in Fig. S11, most
metal ions, except for Cu2+ and Co2+, did not have much effect
3.2. The sensing behavior of X to Zn2+ in EtOH buffer solution
on the fluorescence response of
efficient in detecting Zn2+ in the presence of Cu2+ and Co2+.
X X was less
to Zn2+. Thus, the
Furthermore, the properties of
X in EtOH buffer solution were
These results indicated that
X could be used as a sensor to
also explored by fluorescence and ultraviolet absorption.
detect Zn2+ in EtOH/H2O buffer solution without interference
from most metal ions.
Primarily, the fluorescence response of
X to various metal ions
(Zn2+, Li+, K+, Mg2+, Al3+, Mn2+, In3+, Ga3+, Hg2+, Ag+, Cd2+, Fe3+,
Cr3+, Ni2+, Cu2+ and Co2+) was shown in Fig. 7. In Fig. 7(a), the free
To further explore the binding property of
X
with Zn2+,
relevant titration experiments were carried out in EtOH/H2O
buffer solution (v/v = 9:1, tris = 10 mM, pH = 7.4). Primarily, the
UV-vis titration was shown in Fig. 8. The free
showed an absorption peak at 371 nm. It can be obviously
observed that the absorption peak gradually decreased at 371
X
displayed a weak fluorescence intensity in EtOH buffer
solution (v/v = 9:1, tris = 10 mM, pH = 7.4) with excitation at 365
nm. Upon addition of various metal ions (5 equiv.), only Zn2+
could immediately cause a significant increase in fluorescence
X
(1 × 10-5 M)
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9PP00408D
nm and increased at 399 nm with the addition of Zn2+ from 0 to
5 × 10-5 M. In addition, a clear isosbestic point was found at 386
a stable value (pH
＞9.5). These results indicated that X could be
used as a sensor to detect Zn2+ in physiological conditions for
biological samples.
nm, indicating that
Zn2+. Moreover, the fluorescence titration experiments for the
binding of
with Zn2+ were also shown in Fig. 9. As mentioned
above, the free
maintained a concentration of 1 × 10-5 M and
X could form a stable complex (X
[Zn2+]) with
The possible binding mode between
proposed and demonstrated. The stoichiometry between
Zn2+ may be 1:1, which was proved by B-H equation (Fig. S12)
X
and Zn2+ was also
and
X
X
X
and mass spectrometry (a peak at m/z 422.9017 corresponded
exhibited weak fluorescence. With the increased gradually of
Zn2+ concentrations from 0 to 5 equiv., the fluorescence
+
to [
between
to experiments data and relevant literatures.38, 47-49 The oxygen
X
+ Zn2+]2 , Fig. S13). Thus, the possible binding model
X
and Zn2+ ions was proposed in scheme 3 according
intensity of
X increased gradually (Fig. 9(a)) and reached a
plateau at 463 nm (Fig. 9(b)) when the concentration was 5 ×
10-5 M, indicating that the fluorescence intensity reached the
maximum. As shown in Fig. 9(b) insert, a good linear
atom on -OH and the nitrogen atoms on C=N bond were
involved in the complexation of Zn2+ ions.
Moreover, in order to prove the feasibility of the proposed
relationship between the fluorescence intensity of
X and the
X
and Zn2+, structure optimization
low concentration of Zn2+ (1 × 10-7 M - 6 × 10-7 M) with R2 =
0.9858 and Y = 26X + 235.5 was obtained. The detection limit of
binding mechanism between
and energy calculation of and X
[Zn2+] were also investigated
X
X
for Zn2+ was calculated to be 5.5 × 10-9 M based on 3σ/s
using ab initio density functional theory (DFT) combined with
time-dependent density functional theory (TDDFT) calculations,
as implemented in the Gaussian 09 package based on B3LYP/6-
equation, which is lower than detection of Zn2+ that had been
reported.13, 15 The association constant (Ka) of with Zn2+ was
calculated to be 2.65 × 105 M-1 (Fig. S12) based on eqn (2).
Furthermore, the reversible binding of
to Zn2+ was also
verified by adding EDTA. As shown in Fig. 10, free exhibited
weak fluorescence intensity. When Zn2+ (5 equiv.) were added,
the fluorescence intensity of increased significantly at 463 nm.
When EDTA (5 equiv.) were added to
[Zn2+] system, the
X
31G(d) basis.50 The optimal structure of
X and X
[Zn2+] were
X
X
X
X
fluorescence signal was completely disappeared. Then continue
to add Zn2+ (10 equiv.) to the system above, the fluorescence
intensity was significantly increased again. Moreover, keep
adding EDTA, the fluorescence disappeared completely.
X still
maintained well response to Zn2+ after two cycles with a small
fluorescence loss at 463 nm and exhibited efficient “on-off-on”
behavior.
Moreover, the fluorescence response of
different pH value (from 2 to 11) were shown in Fig. 11. The free
(1
10-5 M) exhibited a stable and weak fluorescence
X
to Zn2+ at
X
×
between pH value from 2 to 11. In the presence of Zn2+ (5
equiv.), the fluorescence intensity did not change in the range
of 2 to 4.5 and gradually increased with the pH changed from
4.5 to 7.4. When pH
＞7.4, the fluorescence intensity of X
[Zn2+]
gradually decreased over the pH range of 7.4 to 9.5 and reached
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Journal Name
shown in Fig. S14. For X
[Zn2+], from vertical view, the whole
1
2
3
4
5
Z. Wang, J. Yang, P. Liu, Y. Yang, H. Fang, X. Xu, J. Rui, H. Xu, S.
Wang, Isolongifolanone-based molecular VfielwuoArrteicslecOennlcinee
molecule was basically in the same plane after binding with Zn2+.
Otherwise, the spatial distributions and orbital energies of
DOI: 10.1039/C9PP00408D
marker for imaging endogenous Zn2+ in vivo, Tetrahedron,
2017, 73, 5912-5919.
HOMO and LUMO of
[Zn2+], the HOMO was still around 4-(diethylamino)-2-
X and X
[Zn2+] were shown in Fig. 12. For
J. Yang, H. Fang, X. Fang, X. Xu, Y. Yang, J. Rui, C. Wu, S. Wang,
H. Xu, A novel tetrahydroquinazolin-2-amine-based high
selective fluorescent sensor for Zn2+ from nopinone,
Tetrahedron, 2016, 72, 4503-4509.
X
hydroxybenzaldehyde and C=N group, while electrons in LUMO
were uniformly distributed throughout the whole molecule,
M. Z. Jonaghani, H. Zali-Boeini, H. Moradi, A coumarin based
highly sensitive fluorescent chemosensor for selective
detection of zinc ion, Spectrochim. Acta, Part A, 2019, 207, 16-
22.
including imidazo[2,1-b]thiazole
and 4-(diethylamino)-2-
hydroxybenzaldehyde parts. The role of Zn2+ ions was to
increase the degree of intramolecular charge transfer.38 The
X
[Zn2+] were calculated to be 3.24 eV. Thus,
Y. Liu, Y. Li, Q. Feng, N. Li, K. Li, H. Hou, B. Zhang, Turn‐on’
‘
energy gap of
theoretical calculation results proved the binding model
between
and Zn2+ was feasible and indicated that the
response of
to Zn2+ may also be the joint result of PET and
fluorescent chemosensors based on naphthaldehyde‐2‐
pyridinehydrazone compounds for the detection of zinc ion in
water at neutral pH, Luminesce., 2017, 33, 29-33.
X
X
H. L. Chen, Z. F. Guo, Z. l. Lu, Controlling Ion-Sensing Specificity
of N-Amidothioureas: From Anion-Selective Sensors to Highly
Zn2+-Selective Sensors by Tuning Electronic Effects, Org. Lett.,
2012, 14, 5070-5073.
CHEF. The calculated results and experimental phenomena
were highly consistent with the theoretical basis.
Furthermore, in order to explore the practical application
of X, X
was used as a sensor to detect Zn2+ in tap water. The
various concentrations of Zn2+ were prepared in tap water and
were measured by fluorescence assay method. As shown in
Table S3, Zn2+ could also be accurately measured in tap water
samples. Suitable recoveries (97.3%, 104.1% and 106.2%) and
RSD (1.50%, 2.39% and 3.01%) were observed. Ultimately, these
6
7
D. K. Das, P. Goswami, B. Medhi, N-benzoate-N′
salicylaldehyde ethynelediamine: A new fluorescent sensor
for Zn2+ ion by “off-on” mode, J. Fluoresc., 2014, 24, 689-693.
L. Tang, X. Dai, K. Zhong, X. Wen, D. Wu,
A
Phenylbenzothiazole Derived Fluorescent Sensor for Zn(II)
Recognition in Aqueous Solution Through “Turn-On” Excited-
State Intramolecular Proton Transfer Emission, J. Fluoresc.,
2014, 24, 1487-1493.
results indicated that
X could be applied as a potential sensor
8
9
Y. W. Choi, G. J. Park, Y. J. Na, H. Y. Jo, S. A. Lee, G. R. You, C.
Kim, A single schiff base molecule for recognizing multiple
metal ions: A fluorescence sensor for Zn(II) and Al(III) and
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for the quantitative detection of Zn2+ in real water samples.
Conclusions
In summary, A new simple and easily synthesized multitarget
sensor(
b]thiazole-6-carboxylic
hydroxybenzaldehyde. The structure of
NMR, FTIR and ESI-MS spectroscopy.
X), was designed and synthesized based on imidazo[2,1-
acid
and
4-(diethylamino)-2-
was characterized by
X
10 J. H. Kang, J. Han, H. Lee, M. H. Lim, K. T. Kim, C. Kim, A water-
soluble fluorescence chemosensor for the sequential
detection of Zn2+ and pyrophosphate in living cells and
zebrafish, Dyes Pigm., 2018, 152, 131-138.
11 H. Liu, T. Liu, J. Li, Y. Zhang, J. Li, J. Song, J. Qu, W. Y. Wong, A
simple Schiff base as dual-responsive fluorescent sensor for
bioimaging recognition of Zn2+ and Al3+ in living cells, J. Mater.
X
could be used as a sensor
to detect In3+ in DMF-H2O buffer solution and detect Zn2+ in
EtOH-H2O buffer solution through fluorescence enhancement.
X
exhibited an efficient “off-on-off” fluorescence behavior by
cyclic addition of metal ions (In3+ and Zn2+) and EDTA. The
binding mode and sensing mechanism of
with metal ions (In3+
X
Chem. B., 2018,
6, 5435-5442.
and Zn2+) was verified by theoretical calculations using Gaussian 12 Y. Tang, Y. Huang, L. Lu, C. Wang, T. Sun, J. Zhu, G. Zhu, J. Pan,
Y. Jin, A. Liu, M. Wang, Synthesis of a new pyrene-devived
fluorescent probe for the detection of Zn2+, Tetrahedron Lett.,
2018, 59, 3916-3922.
09. Moreover, X could be applied as a potential sensor to detect
In3+ and Zn2+ in real water samples.
13 X. Feng, Y. Fu, J. Jin, J. Wu, A highly selective and sensitive
-
fluorescent sensor for relay recognition of Zn2+ and HSO4
-
Conflicts of interest
There are no conflicts to declare.
/H2PO4 with “on-off” fluorescent responses, Analytical
Biochemistry, 2018, 563, 20-24.
14 N. Behera, V. Manivannan, A Probe for Multi Detection of Al3+,
Zn2+ and Cd2+ Ions via Turn-On Fluorescence Responses, J.
Photochem. Photobiol., A, 2018, 353, 77-85.
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DOI: 10.1039/C9PP00408D
Graphical abstract
Short statement:
An imidazo[2,1-b]thiazole based compound (X) was
designed and synthesized as an “off-on-off” sensor for the
multiple recognition of In3+ and Zn2+ in different system.