A dual functional fluorescent sensor for the detection of Al3+ and Zn2+ in different solvents

Article abstract of DOI:10.1039/c9nj03298c

(E)-N′-((2-Hydroxynaphthalen-1-yl)methylene)imidazo[2,1-b]thiazole-6-carbohydrazide (X), a new fluorescent sensor, was designed and synthesized based on imidazo[2,1-b]thiazole and 2-hydroxy-1-naphthaldehyde, which could be used to detect Al³⁺ in methanol buffer solution and detect Zn²⁺ in ethanol buffer solution with detection limits of 1.45 × 10^-7 M and 1.29 × 10^-8 M, respectively. X showed high sensitivity and selectivity towards Al³⁺ and Zn²⁺ in the presence of other metal ions. The fluorescence intensity of X was reversible even after six cycles with the sequentially alternative addition of metal ions (Al³⁺ and Zn²⁺) and PPi, showing an efficient "off-on-off" fluorescence behavior. The complexation ratio of X to Al³⁺ and Zn²⁺ was 1:1, which was proved by mass spectrometry and the B-H equation. The binding mode and sensing mechanism of X with metal ions (Al³⁺ and Zn²⁺) were verified by DFT/TDDFT calculations using Gaussian 09 based on the B3LYP/6-31G(d) basis set.

Full text of DOI:10.1039/c9nj03298c

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A dual functional fluorescence sensor for detection of Al3+ and Zn2+
in different solvent
Yuankang Xu,a Hanyu Wang,a Jinyan Zhao,b Xiaofeng Yang,a Meishan Pei,a Guangyou Zhanga and
Received 00th January 20xx,
Accepted 00th January 20xx
Yanxia Zhanga,*
DOI: 10.1039/x0xx00000x
(E)-N'-((2-hydroxynaphthalen-1-yl)methylene)imidazo[2,1-b]thiazole-6-carbohydrazide (X), a new fluorescence sensor, was
designed and synthesized based on imidazo[2,1-b]thiazole and 2-hydroxy-1-naphthaldehyde, which could be used to detect
Al3+ in methanol buffer solution and detect Zn2+ in ethanol buffer solution with detection limits of 1.45 × 10-7 M and 1.29 ×
10-8 M, respectively. X showed high sensitivity and selectivity for Al3+ and Zn2+ in the presence of other metal ions. The
fluorescence intensity of X was reversible even after six cycles with the sequentially alternative addition of metal ions (Al3+
and Zn2+) and PPi, showing an efficient “off-on-off” fluorescence behavior. The complexation ratio of X to Al3+ and Zn2+ was
1:1, which was proved by mass spectrometry and B-H equation. The binding mode and sensing mechanism of X with metal
ions (Al3+ and Zn2+) was verified by DFT/TDDFT calculations using Gaussian 09 based on B3LYP/6-31G(d) basis.
Parkinson’s disease and so on.24-28 Therefore, the design of
efficient sensors for the detection of aluminum and zinc in
biological systems and the environment should be attracted
1. Introduction
The development of fluorescent sensors with high sensitivity
and selectivity toward various metal ions are particularly
attractive to current researchers because of its potential
applications in fields of medicinal, biological and environmental
applications.1-5 Most of the cations plays very crucial roles in
biological systems and affect human health and the balance of
the ecosystem.6, 7 Among the various metal ions, aluminum, is
the most abundant essential metallic element in the earth’s
crust and its content affects human health seriously.8-10
According to the World Health Organization (WHO) report, the
average daily intake of aluminum is about 3 to 10 milligrams.11-
12 However, aluminum is a potentially toxic metal ion and is not
a necessary trace element for the human body.13 Long-term
excess intake of aluminum can lead to a range of diseases,
including Alzheimer’s, osteoporosis and kidney failure.14-17
Another metallic element, zinc, is the second most abundant
trace element in the human body and plays vital roles in many
biological processes such as gene transcription, protein
metabolism, cellular metabolism, immune function and other
physiological processes.18-23 Even though zinc is responsible for
normal physiological functioning, and the imbalance of zinc
could result in organ damage and dysfunction, such as
Alzheimer’s disease, amyotrophic lateral sclerosis (ALS),
widespread attention.29, 30
Nowadays, simple Schiff base with the capability of
detecting target ion in environmental and biological samples
has gained increasing attention due to their simple synthesis,
ideal solubility, sensitivity and strong complexing ability of
metal ions.31-35 In previous reports, Schiff base, derived from 2-
hydroxy-1-naphthaldehyde, is often used as a “off-on” sensor
to detect zinc or aluminum in a particular system.1, 4, 5, 16, 36-39
However, most of the sensor are explored in single organic
solvent such as acetonitrile, methanol, ethanol, DMF and
DMSO.13 Till date, the effect of different solvents on the
properties of sensor are rarely reported.40 Hence, the design of
a sensor for detection of multiple metal ions in different solvent
is meaningful. In this report, a new simple Schiff base (X) was
designed and synthesized based on 2-hydroxy-1-
naphthaldehyde and imidazo[2,1-b]thiazole-6-carboxylic acid.
imidazo[2,1-b]thiazole, which contains two conjugated rings in
its molecule, was a potential fluorescent nucleus.41 In addition,
the hydroxyl groups on the naphthalene ring can not only
increase the ability of the probe to recognize ions, but also
greatly improve the solubility of the probe. As expected, X could
be used as a highly selective and sensitive fluorescent probe to
detect aluminum and zinc in methanol-water and ethanol-
water buffer system.
a.School of chemistry and chemical engineering, University of Jinan, Jinan 250022,
China, E-mail address: chm_zhangyx@ujn.edu.cn.
b.Jinan Technician college, China. E-mail address: zhao_jin_yan@126.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
2. Experimental section
2.1. Materials and sample preparation
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All reagents and solvents were commercially available AR and Density functional theory (DFT) structural optimiVziaewtiAornticslewOnelirnee
CP and were used without any treatment. All metal ionic performed with the Gaussian 09 program. In all cases, the
DOI: 10.1039/C9NJ03298C
solution are corresponding chloride, sulfate and nitrate structures were optimized using the B3LYP functional and the
solutions, including ZnCl2, Li2SO4, AlCl3·6H2O, CdCl2·2.5H2O, mixed basis set 6-31+G(d). Each structure was subsequently
CoCl2·6H2O, FeCl3, MgCl2·6H2O, CrCl3·6H2O, MnCl2·4H2O, NaCl, subjected to TD-DFT calculation using the B3LYP functional.42
AgCl, KCl, CuCl2·2H2O, NiCl2·6H2O, CaCl2 and HgCl2. The anion For all optimized structures, frequency calculations were
solution is the corresponding sodium salt solutions (Na4P2O7). performed to confirm the absence of imaginary frequencies.
Stock solutions of the ions mentioned above were prepared The molecular orbitals were visualized and plotted with the
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with a concentration of 0.03 M by distilled water. The probe
X
GaussView 5.0 program.
was dissolved in methanol/H2O (v/v = 9 : 1) buffer solution (10
mM tris, pH = 7.4) and ethanol/H2O (v/v = 9 : 1) buffer solution 2.5. Synthesis of X
(10 mM tris, pH = 7.4) at room temperature with the
Compound
compound
1
2
(ethyl imidazo[2,1-b]thiazole-6-carboxylate) and
(imidazo[2,1-b]thiazole-6-carbohydrazide) were
concentration of 1 × 10-5 M.
prepared according to a previous report.43 Firstly, compound
1
2.2. Measurements
was obtained by the reaction of thiazol-2-amine with ethyl 3-
UV-vis spectra were obtained on a Shimadzu 3100 spectrometer. bromo-2-oxopropanoate (80%) in THF solution. Then,
Fluorescence spectral data was recorded on an Edinburgh compound and hydrazide hydrate (80%) were stirred
1
Instruments Ltd-FLS920 Fluorescence Spectrophotometer. overnight in ethanol solution at room temperature to produce
Fluorescence measurements were recorded using excitation at compound 2 (white solid).
340 nm. The slits of excitation and emission were 10 nm and 20 Synthesis of (E)-N'-((2-hydroxynaphthalen-1-yl)methylene)
1
nm, respectively. H NMR measurement was performed on a imidazo[2,1-b]thiazole-6-carbohydrazide
(
X
). imidazo[2,1-
Bruker AV III 400 MHz NMR spectrometer. 13C NMR spectra data b]thiazole-6-carbohydrazide (compound
2, 127 mg, 697 mmol)
was taken on a Bruker AV III 100 MHz NMR spectrometer with and 2-hydroxy-1-naphthaldehyde (129 mg, 750 mmol) were
tatramethysilane (TMS) as internal standard and DMSO as mixed and stirred in 10 ml ethanol solution at room
solvent. Infrared spectral data was obtained on a Bruker Vertex temperature for 12 hour and a yellow precipitate formed. The
70 FT-IR spectrometer using samples as KBr pellets. Thin layer precipitate was filtered and then washed with ethanol (1 ml
×
chromatography (TLC) analyses were performed to monitor all 5) to obtain the pure yellow solid . Yield: 207 mg, 88.4 %. Ms
X
the reactions.
(ESI): m/z = 337.08 [M + H]+, 359.06 [M + Na]+. FTIR (KBr, cm-1):
3329 (N-H), 1679 (C=O), 1537 (C=N). 1H NMR (400 MHz, DMSO)
δ 12.96 (s, 1H), 12.23 (s, 1H), 9.71 (s, 1H), 8.43 (s, 1H), 8.14 (d, J
= 8.6 Hz, 1H), 8.01 (d, J = 4.5 Hz, 1H), 7.91 (dd, J = 11.8, 8.6 Hz,
2H), 7.61 (t, J = 7.3 Hz, 1H), 7.46 (d, J = 4.5 Hz, 1H), 7.41 (t, J =
7.4 Hz, 1H), 7.23 (d, J = 8.9 Hz, 1H). 13C NMR (101 MHz, DMSO)
δ 158.38, 158.18, 149.55, 147.40, 140.38, 133.00, 132.24,
129.43, 128.25, 128.15, 123.98, 120.80, 120.75, 119.43, 117.26,
116.18, 109.04.
2.3. Calculation of quantum yield 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.
3. Results and discussion
As shown in scheme 1,
X was designed and synthesized in
medium yield according to the synthetic route. Compound
1
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,
X
intensity of
X
concentration of Mn+.
2.4. Theoretical calculations
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The UV-Vis spectrum of
X
was explored in methanol/H2O
(ethyl imidazo[2,1-b]thiazole-6-carboxylate) and compound
(imidazo[2,1-b]thiazole-6-carbohydrazide) were synthesized
2
buffer solution (v/v = 9:1, tris = 10 mM, pH = 7.4). The
concentration of
was remained at 1 × 10-5 M. The UV-Vis
response of
to various metal ions (Al3+, Ag+, Co2+, Na+, Ca2+,
Cr3+, K+, Li+, Zn2+, Hg2+, Fe3+, Cu2+, Ni2+, Mg2+, Cd2+ and Mn2+) was
shown in Fig. S6. The exhibited four absorption peaks at 313
X
according to a previous report. The structure of
X was
X
characterized by H 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.
1
X
nm, 325 nm, 362 nm and 377 nm. When various metal ions were
added, most of them caused significant changes in the
absorption spectrum, such as Co2+, Zn2+, Fe3+, Cu2+, Ni2+ and
Mn2+, indicating that they could form stable complexes with
The absorption titration experiment of
toward Al3+ was shown
in Fig. S7, as the concentration of Al3+ increased from 0 to 1.5 ×
10-4 M, the absorption intensity of
at 362 nm and 377 nm were
gradually decreased, while that at 405 nm and 428 nm were
gradually increased. In addition, there was a distinct isosbestic
point at 383 nm, implying the formation of stable complex
3.1. The sensing of Al3+ in methanol buffer solution
Primarily, the fluorescence intensity of
X was gradually
X.
increased with time in the presence of Al3+. As shown in Fig. S5,
X
Time course for the fluorescence response of
X
(1 × 10-5 M) at
472 nm upon the addition of Al3+ (10 equiv.) in methanol/H2O
buffer solution (v/v = 9:1, tris = 10 mM, pH = 7.4). after addition
X
of Al3+ (10 equiv.), the fluorescence intensity of
X gradually
increased with the time and remained stable after 40 minutes.
Many sensors had a time effect on the response of Al3+.8, 13 In
this system, 1 hour of complexation time was used. So, all test
samples should be stationary for 1 hour before testing.
X
[Al3+] (Fig. S8).
Subsequently, fluorescence spectroscopy was also used to
investigated the sensing behavior of
toward Al3+ in
methanol/H2O buffer solution (v/v = 9:1, tris = 10 mM, pH = 7.4).
The concentration of
was remained at 1 × 10-5 M. As shown in
Fig. 1, the free showed a weak fluorescence intensity when
X
X
X
excited at 340 nm. After addition of various metal ions (Al3+, Ag+,
Mg2+, Cu2+, Li+, K+, Na+, Ca2+, Fe3+, Ni2+, Zn2+, Mn2+, Cr3+, Hg2+, Co2+
and Cd2+), only Al3+ could lead to a significant increase in the
fluorescence intensity of
X at 472 nm. While the fluorescence
intensity did not change when other metal ions were added.
The change in emission color could be clearly observed from
colorless to blue under UV lamp when Al3+ was added (Fig. 1
inset). To check the anti-interference ability of the
Al3+, competitive experiments were carried out. As shown in Fig.
S9, the
(1 × 10-5 M) was treated with Al3+ (10 equiv.) in the
presence of the same concentration of other common metal
ions. The
remained highly selective for Al3+ in the presence of
most metal ions (Ag+, Mg2+, Li+, K+, Na+, Ca2+, Zn2+, Mn2+, Hg2+
and Cd2+). Moreover, the responding of to Al3+ in the presence
of Cu2+, Fe3+, Ni2+, Cr3+ and Co2+ were relatively low but clearly
X towards
X
X
X
Fig. 2. Fluorescence titration spectra of
(0 - 15 equiv.) in methanol/H2O buffer solution (v/v = 9:1, tris = 10 mM, pH = 7.4).
Insert: the change of fluorescence intensity of
equiv.).
X
(1 × 10-5 M) upon the addition of Al3+
with the addition of Al3+ (0 - 15
detectable. Therefore, the
X
was a highly selective probe that
X
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could be used to detect Al3+ in the presence of most competing
metal ions.
In order to further explore the properties of
fluorescence titration experiment was performed in
methanol/H2O buffer solution (v/v = 9:1, tris = 10 mM, pH = 7.4).
View Article Online
DOI: 10.1039/C9NJ03298C
X
on Al3+,
During the test, the concentration of
X
was remained at 1 × 10-
5
M, while the concentration of Al3+ was gradually increased
from 0 to 1.5 × 10-4 M. As shown in Fig. 2, the free
X showed a
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weak fluorescence intensity (Φ = 0.019) when excited at 340 nm.
Upon incremental addition of Al3+, the fluorescence intensity at
472 nm gradually increased and reached a plateau (Φ = 0.28)
when the concentration of Al3+ was 15 equiv. (Fig. 2 inset). A
good linear relationship (R2 = 0.9928) between the fluorescence
intensity of
X
and Al3+ in the range from 1.0 × 10-6 M to 1.0 × 10-
5
M could be observed in Fig. S10. The detection limit was
calculated to be 1.45 × 10-7 M based on the formula LOD = 3σ/s.
Furthermore, the association constant Ka between
was calculated to be 1.94 × 104 M-1 according to the Benesi-
Hildebrand (eqn 2) method (Fig. S11)
X
and Al3+
The fluorescence intensity of
presence and absence of Al3+ was investigated. As shown in Fig.
S12, the had no fluorescence at pH values from 3 to 12 in
methanol solution. Add 10 equiv. Al3+ to the solution of
at
different pH values. The fluorescence intensity of
[Al3+] was
increased gradually with pH value from 12 to 7.4. However, the
fluorescence intensity of
[Al3+] was increased significantly in
the pH range from 7.4 to 5 and remained stable in the pH range
from 5 to 3. This indicated that the
[Al3+] was stable under
X at different pH values in the
3.2. The sensing of Zn2+ in ethanol buffer solution
In contrast, the
response to Zn2+ in ethanol buffer solution (v/v
= 9:1, tris = 10 mM, pH = 7.4) could be completed in less than
10 seconds, without requiring 1 hour of complexation time. So,
all samples could be tested directly without waiting.
X
X
X
X
X
The UV-Vis spectrum of
X was explored in ethanol/H2O
buffer solution (v/v = 9:1, tris = 10 mM, pH = 7.4). The
concentration remained at 1 × 10-5 M. The UV-Vis response of
X
X
X
acidic conditions, which is due to the strong alkalinity leading to
the excessive concentration of hydroxide affecting the binding
to various metal ions (Zn2+, Cd2+, Al3+, Ag+, Mg2+, Cu2+, Co2+, Fe3+,
Ni2+, Li+, K+, Ca2+, Na+, Cr3+, Hg2+ and Mn2+) was shown in Fig. S13.
of the probe with Al3+. The result of pH test indicates that the
X
The
X showed three absorption peaks at 326 nm, 361 nm and
can be used as a sensor to detection of Al3+ in acidic or neutral
medium.8
375 nm. When various metal ions were added, some metal ions
could cause significant changes in the absorption spectrum,
such as Co2+, Cu2+, Ni2+ and Mn2+, showing that they could form
The reversibility experiments of
X were shown in Fig.3.
Furthermore, the fluorescence changes were reversible even
after four cycles with the sequentially alternative addition of
Al3+ and PPi, displaying efficient ‘off-on-off’ behavior.
stable and non-fluorescence complexes with
titration experiment of
toward Zn2+ was shown in Fig. S14, as
the concentration of Zn2+ increased from 0 to 5 × 10-5 M, the
absorption intensity of at 361 nm and 375 nm were gradually
X. The absorption
X
X
Fig. 4. Fluorescence spectra of
ions (5 equiv.) in ethanol/H2O buffer solution (v/v = 9:1, tris = 10 mM, pH = 7.4).
Inset: the color of and
[Zn2+] system under UV lamp.
X
(1 × 10-5 M) upon the addition of various metal
Fig. 6. Reversible switching cycles of fluorescence intensity by addition of Zn2+ and
PPi.
X
X
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decreased, while that at 413 nm and 430 nm were gradually to be 1.19 × 105 M-1 according to the Benesi-Hildebrand (eqn 2)
increased. In addition, there was a distinct isosbestic point at method (Fig. S18)
383 nm, implying the formation of stable complex
S15). The UV properties of
on Zn2+ in ethanol buffer solution presence and absence of Zn2+ was investigated. As shown in Fig.
were similar to those on Al3+ in methanol buffer solution.
S19, the had no fluorescence at pH values from 2.5 to 10 in
Similarly, fluorescence spectroscopy was also used to ethanol solution. Add 5 equiv. Zn2+ to the solution of
at
explored the sensing behavior of
toward Zn2+ in ethanol/H2O different pH values. The fluorescence intensity of [Zn2+] was
buffer solution (v/v = 9:1, tris = 10 mM, pH = 7.4). The increased gradually with pH value from 2.5 to 7.4 and decreased
concentration of
was maintained at 1 × 10-5 M during the test. gradually when the pH value was greater than 7.4. Therefore,
As shown in Fig. 4, the free showed a weak fluorescence the results showed that the
had the best response to Zn2+
X
[Zn2+] (Fig.
The fluorescence intensity of X at different pH values in the
X
X
X
X
X
X
X
X
intensity when excited at 340 nm. After addition of various under neutral conditions. The reversibility experiments of
X
metal ions (Zn2+, Cd2+, Al3+, Ag+, Mg2+, Cu2+, Co2+, Fe3+, Ni2+, Li+, were also shown in Fig.6. Furthermore, the fluorescence
K+, Ca2+, Na+, Cr3+, Hg2+ and Mn2+), only Zn2+ could lead to a changes were nearly reversible even after six cycles with the
significant increase in the fluorescence intensity of X
at 484 nm. sequentially alternative addition of Zn2+ and PPi.
While the fluorescence intensity did not change when other
metal ions were added. The change in emission color could be 3.3. The binding model and theoretical calculation of X to Al3+ and
clearly observed from colorless to yellow-green under UV lamp Zn2+
when Zn2+ was added (Fig. 4 inset). Competitive experiments
Finally, the binding mechanism of the
X
to Al3+ and Zn2+ were
were also performed to check the response of
disturbed by other competing ions. As shown in Fig. S16, the
(1 × 10-5 M) was treated with Zn2+ (5 equiv.) in the presence of
X
to Zn2+ was not
explored. As can be seen from Fig. S11 and Fig. S18, a good
linear relationship (R2 = 0.9897 and R2 = 0.9975) between 1 / (I
- I0) and 1 / (Mn+) (Mn+ = Al3+ or Zn2+). And then, the job’s plot of
X
the same concentration of other common metal ions. The
X
X
with metal ions (Al3+ and Zn2+) complexation were performed
in Fig. S20 and Fig. S21. These results indicated that the
stoichiometric ratio of
and metal ions (Al3+ and Zn2+) was 1:1.44
remained highly selective for Zn2+ in the presence of most metal
ions, except for Cu2+ and Co2+. In most of the literature, Cu2+ and
X
Co2+ could form stable and non-fluorescent complexes with
X,
In addition, there was further exploration through mass
spectrometry data (Fig. S22 and Fig. S23). The mass
spectrometry results indicated that a peak at m/z = 397.0193
thus affecting the measurement of Zn2+.22, 23, 29 Therefore, the
X
was a highly selective probe that could detect Zn2+ in the
presence of most competing metal ions.
corresponding to [
addition of AlCl3 (Fig. S20) and a peak at m/z = 436.2311
corresponding to [
+ Zn2+ + Cl- - H+]- was observed when
addition of ZnCl2 (Fig. S21), indicating that the 1:1
stoichiometric between
and metal ions (Al3+ and Zn2+). In
addition, the possible binding mechanism of
to Al3+ and Zn2+
X
+ Al3+ + Cl- - H+]- was observed when
In order to further explore the properties of
X
on Zn2+,
fluorescence titration experiment was performed in
ethanol/H2O buffer solution (v/v = 9:1, tris = 10 mM, pH = 7.4).
X
During the test, the concentration of
X
was remained at 1 × 10-
X
5
M, while the concentration of Zn2+ was gradually increased
X
from 0 to 1.0 × 10-4 M. As shown in Fig. 5, the free
X showed a
were proposed according to experiments data and relevant
literatures.6, 13 As shown in Scheme 2, the hydroxy group at 2-
hydroxy-1-naphthaldehyde was involved in the complexation of
metal ions. The nitrogen atoms on C=N bond and imidazo
participate in the complexation.45-47 Just like the sensor formed
a cavity near these three sites for complexing metal ions.
In order to explore the binding mechanism and
weak fluorescence intensity when excited at 340 nm. Upon
incremental addition of Zn2+, the fluorescence intensity at 484
nm gradually increased and reached a plateau when the
concentration of Zn2+ was 5 equiv. (Fig. 5 inset). A good linear
relationship (R2 = 0.9976) between the fluorescence intensity of
X
and Zn2+ in the range from 1.0 × 10-7 M to 1.0 × 10-6 M could
be observed in Fig. S17. The detection limit was calculated to be
fluorescence response mechanism of
X
to Al3+ and Zn2+ ions,
[Al3+] and
[Zn2+] were investigated using ab initio density functional
1.29 × 10-8 M based on the formula LOD = 3σ/s. Furthermore,
structure optimization and energy calculation of
X
X, X
the association constant Ka between
X
and Zn2+ was calculated
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New Journal of Chemistry
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ARTICLE
Journal Name
had a good
1
2
3
4
5
6
7
8
respectively. Thus, these results indicated that
X
View Article Online
response to Al3+ and Zn2+ ions. Binding metal ions not only
increased the planarity and rigidity of molecules, but also
promoted intramolecular electron transfer. Theoretical
calculation results and experimental results indicated that the
response of the sensor to metal ions may be the joint result of
the mechanism of PET (photo-induced electron transfer) and
CHEF (chelation enhanced fluorescence mechanism).50-52
DOI: 10.1039/C9NJ03298C
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3.4. Application in water samples
The practical application of
X was also explored in
environmental system. The various concentrations of Al3+ and
Zn2+ were prepared in tap water and were measured by
fluorescence assay method. As shown in Table S1, Al3+ in tap
water samples could be accurately measured with a satisfactory
recovery (90.4% – 99.7%) and the relative standard deviation
(RSD) of three measurements was less than 1.7%. Furthermore,
as shown in Table S2, Zn2+ could also be accurately measured in
tap water samples. a good recovery (103.4% – 114.3%) and RSD
(0.46% – 1.38%) were obtained. Thus, these data indicated that
theory (DFT) combined with time-dependent density functional
theory (TDDFT) calculations, as implemented in the Gaussian 09
package based on B3LYP/6-31G(d) basis.42 As shown in Fig. 7,
the optimal structure of
X,
X
[Al3+] and
X
[Zn2+] were obtained by
gauss calculations. For
X,
the imidazo[2,1-b]thiazole
(fluorescence nuclei) and 2-hydroxy-1-naphthaldehyde
(fluorescence receptor) part maintained a good planar property,
respectively. After binding with metal ions (Al3+ and Zn2+), the
X
has a good potential for the quantitative detection of Al3+ and
molecules
(X X
[Al3+] and [Zn2+]) exhibited good coplanar
Zn2+ in real water samples.
properties. the dihedral angle of the fluorescence nucleus part
and fluorescence receptor part changed from 110.43 degrees to
[Al3+]) and 0.57 degrees ( [Zn2+]).
0.04 degrees (X X
Conclusions
In addition, the spatial distributions and orbital energies of
the highest occupied molecular orbital (HOMO) and the lowest
In summary, a simple fluorescence sensor (X) was designed and
synthesized based on 2-hydroxy-1-naphthaldehyde and
imidazo[2,1-b]thiazole. The structure of was characterized by
NMR and ESI-MS spectroscopy.
could detect Al3+ in
unoccupied molecular orbital (LUMO) of
X
,
X
[Al3+] and
X
[Zn2+]
X
complexes were also exhibited in Fig. 8. For free
X, the highest
X
occupied molecular orbital (HOMO) was mostly distributed over
2-hydroxy-1-naphthaldehyde and C=N groups. When excited, a
small number of electrons were transferred to the imidazo[2,1-
b]thiazole part, so that the sensor exhibited weak
methanol/H2O buffer solution (v/v = 9:1, tris = 10 mM, pH = 7.4)
and detect Zn2+ in ethanol/H2O buffer solution (v/v = 9:1, tris =
10 mM, pH = 7.4) with detection limits of 1.45 × 10-7 M and 1.29
× 10-8 M, respectively. The fluorescence intensity of
X was
fluorescence.48 The molecular orbital structure of
X
[Al3+]
reversible even after six cycles with the sequentially alternative
addition of metal ions (Al3+ and Zn2+) and PPi, showing an
efficient “off-on-off” fluorescence behavior. The complexation
complex was clearly similar to that of
X
[Zn2+]. After binding with
metal ions (Al3+ and Zn2+), the HOMO was still around the 2-
hydroxy-1-naphthaldehyde and C=N groups, while electrons in
LUMO were mostly concentrated around the metal ions and
ratio of
mechanism of
X
to Al3+ and Zn2+ was 1:1. The binding mode and sensing
with metal ions (Al3+ and Zn2+) was verified by
X
nearby groups.49 Moreover, the energy gap of
X
X
,
X
[Al3+] and
DFT/TDDFT calculations using Gaussian 09 based on B3LYP/6-
31G(d) basis.
[Zn2+] were also calculated to be 4.09 eV, 2.85 eV and 3.7 eV
,
Conflicts of interest
There are no conflicts to declare
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)
Notes and references
6 | J. Name., 2012, 00, 1-3
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