A simple fluorescent schiff base for sequential detection of Zn2+ and PPi based on imidazo[2,1-b]thiazole

Article abstract of DOI:10.1016/j.jphotochem.2019.112026

A simple fluorescent schiff base, X, based on imidazo[2,1-b]thiazole-6-carboxylic acid and 3-ethoxy-2-hydroxybenzaldehyde, was designed and synthesized for detection of Zn²⁺ and PPi. X showed excellent sensitivity and selectivity toward Zn

Full text of DOI:10.1016/j.jphotochem.2019.112026

JournalofPhotochemistry&PhotobiologyA:Chemistry383(2019)112026
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Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
A simple fluorescent schiff base for sequential detection of Zn2+ and PPi
based on imidazo[2,1-b]thiazole
Yuankang Xua, Hanyu Wanga, Jinyan Zhaob, Xiaofeng Yanga, Meishan Peia, Guangyou Zhanga,
⁎
⁎
Yanxia Zhanga, , Li Linc,
a School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China
b Jinan Technician College, China
c College of Science, Sichuan Agricultural University, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A simple fluorescent schiff base, X, based on imidazo[2,1-b]thiazole-6-carboxylic acid and 3-ethoxy-2-hydro-
xybenzaldehyde, was designed and synthesized for detection of Zn2+ and PPi. X showed excellent sensitivity and
selectivity toward Zn2+ by exhibiting a large fluorescence enhancement (about 42 times). The binding ratio of
the X and Zn2+ was 1:1 confirmed by mass spectral analysis and Job’s plot and the association constant between
X and Zn2+ was calculated to be 2.2 × 105 M−1. The detection limit was as low as 1.2 × 10-9 M. Moreover, the
X[Zn2+] complex could be used as a sensor to detect PPi sensitively and the detection limit was 1.9 × 10-9 M.
Furthermore, the fluorescent signals of X were utilized to construct an INHIBIT logic gate at the molecular level.
The reason of fluorescence enhancement of X toward Zn2+ was verified by density functional theory (DFT) and
time-dependent density functional theory (TDDFT) calculations using Gaussian 09.
Schiff base
Zinc
PPi
Detection limit
Theoretical calculations
Logic circuit
1. Introduction
selectivity, high sensitivity, simple operation and other characteristics
[15–17]. Among many reported fluorescent probes for zinc, schiff bases
At the present, detection of transition metals in environmental and
biological has attracted increasing attention due to their important role
in the system [1,2]. Among the various transition metals, zinc, as one of
the essential trace elements in human body, is the second most abun-
dant transition element in the body and is involved in a variety of
metabolic and physiological processes, such as gene transcription/ex-
pression, cell growth and division, neurotransmission and mammalian
reproduction [3–6]. Minute quantity of zinc is beneficial to human
body, on the other hand, the excessive zinc will damage the human
body’s function, such as Alzheimer's disease and Parkinson's disease
[7–10]. Among the various anions, pyrophosphate (P2O74−, PPi) is an
important anion in many physiological processes, such as cellular ATP
hydrolysis, protein synthesis, DNA and RNA polymerizations, enzy-
matic reactions and other metabolic processes [11–13]. However, an
excess of PPi can cause calcium pyrophosphate deposition disease
(CPPD) and other diseases [14]. Hence, efficient recognition of zinc and
pyrophosphate is of great significance to biology, clinical medicine and
other fields.
have attracted much attention due to their simple synthesis, good so-
lubility and strong complexing capacity [18–21]. Detection of zinc are
often affected by other metallic ions, such as chromium, which have
similar properties [22,23]. In addition, the content of zinc in human
body is around 1.96 ˜ 5.34 × 10−6 mol L-1 [17]. Therefore, the design
and synthesis of a schiff base with simple structure as a fluorescent
probe for highly selective and sensitive of zinc is continuously to be a
challengeable work.
In this report, a new simple schiff base (X) was designed and syn-
thesized based on imidazo[2,1-b]thiazole-6-carboxylic acid and 3-
ethoxy-2-hydroxybenzaldehyde. imidazo[2,1-b]thiazole, which con-
tains two conjugated rings in its molecule, contains eight central atoms
at the heterocyclic rings with ten delocalized π electrons, was a po-
tential fluorescent nucleus [24]. Moreover, imidazo[2,1-b]thiazole ring
has two nitrogen atoms and one sulfur atom, which provides a possi-
bility for coordination of metal ions [25]. On the other hand, 3-ethoxy-
2-hydroxybenzaldehyde, as fluorescence receptor, are connected with
fluorescence nuclei by C]N bond in the probe molecule. In addition,
the hydroxyl and ethoxy groups on the benzene ring can not only in-
crease the ability of the probe to recognize ions, but also greatly
Nowadays, more and more attention has been paid on fluorescence
analysis rather than traditional detection methods because of its good
⁎ Corresponding authors.
E-mail addresses: chm_zhangyx@ujn.edu.cn (Y. Zhang), 14211@sicau.edu.cn (L. Lin).
https://doi.org/10.1016/j.jphotochem.2019.112026
Received 22 May 2019; Received in revised form 16 July 2019; Accepted 7 August 2019
Availableonline07August2019
1010-6030/©2019ElsevierB.V.Allrightsreserved.

Y. Xu, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry383(2019)112026
improve the solubility of the probe. As expected, compound X could be
used as a highly selective and sensitive fluorescent probe to detect zinc
ions in ethanol-water buffer system. Furthermore, the combination of
probe X and zinc ions can serve as a sensor for PPi detection in the same
system.
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 association constant between X and Zn2+ was calculated by the
Benesi–Hildebrand Eq. (2):
2. Experimental section
2.1. Materials and sample preparation
1
1
1
max
1
=
×
+
F
F0
Zn2+
Ka [F
F0]
Fmax F0
(2)
All reagents and solvents were commercially available AR and CP
and were used without any treatment. All metal ionic solution are
corresponding chloride or sulfate solutions, including ZnCl2, Li2SO4,
where F is the fluorescence intensity of the X-Zn2+ complex, which is in
accordance with the concentration of Zn2+ at 511 nm. F0 is the fluor-
escence intensity of free X. Fmax is the fluorescence intensity of X-Zn2+
AlCl3·6H2O,
CdCl2·2.5H2O,
CoCl2·6H2O,
FeCl3,
MgCl2·6H2O,
CrCl3·6H2O, MnCl2·4H2O, NaCl, FeSO4, KCl, CuCl2·2H2O, NiCl2·6H2O,
CaCl2 and HgCl2. All anionic solutions are corresponding sodium or
potassium solutions, including Na4P2O7, NaCl, NaF, NaBr, KI, NaNO2,
NaHCO3, Na2S, NaS2O3, Na2SO4, NaNO3, NaHSO3 and NaBF3. Stock
solutions of the ions mentioned above were prepared with a con-
centration of 0.03 M by distilled water. The stock solution of ZnCl2 was
diluted to different concentration as 1 × 10−6 M, 1 × 10-7 M, 1 × 10-8
M and was used for the titration test. Similar, the stock solution of PPi
was diluted to different concentration as 1 × 10−6 M, 1 × 10-7 M. The
probe X was dissolved in ethanol/H2O (v/v = 9 : 1) buffer solution
(10 mM tris, pH = 7.4) at room temperature with the concentration of
1 × 10-5 M.
complex in the presence of the maximum concentration of Zn2+
.
2.4. Theoretical calculations
Density functional theory (DFT) structural optimizations were per-
formed 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). Each structure was subsequently subjected to TD-DFT calculation
using the B3LYP functional [26]. For all optimized structures, fre-
quency calculations were performed to confirm the absence of ima-
ginary frequencies. The molecular orbitals were visualized and plotted
with the GaussView 5.0 program.
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 measure-
ments were recorded using excitation at 330 nm. The slits of excitation
and emission were 10 nm and 20 nm, respectively. 1H NMR measure-
ment was performed on a Bruker AV III 400 MHz NMR spectrometer.
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 re-
actions.
2.5. Synthesis of X
Compound
1 (ethyl imidazo[2,1-b]thiazole-6-carboxylate) and
compound 2 (imidazo[2,1-b]thiazole-6-carbohydrazide) were synthe-
sized according to reported procedure [25].
Synthesis of (Z)-N'-(3-ethoxy-2-hydroxybenzylidene)imidazo[2,1-
b]thiazole-6- carbohydrazide (X). 2 (90 mg, 495 mmol) and 3-ethoxy-
2-hydroxybenzaldehyde (83 mg, 500 mmol) were mixed in 15 ml
ethanol and stirred at room temperature for 12 h and a pale-yellow
precipitate appeared. The precipitate was filtered and then washed with
ethanol (1 ml × 2) to obtain the pure pale-yellow solid X. Yield: 77 mg,
46%. Ms (ESI): m/z = 331.09 [M + H]+, 353.07 [M + Na]+. FTIR
(KBr, cm−1): 3314 (N-H), 1667 (C = O), 1541 (C = N). 1H NMR
(400 MHz, DMSO) δ 12.18 (s, 1 H), 11.30 (s, 1 H), 8.70 (s, 1 H), 8.40 (s,
1 H), 8.00 (d, J =4.4 Hz, 1 H), 7.45 (d, J =4.4 Hz, 1 H), 7.01 (dd,
J = 7.8, 2.7 Hz, 2 H), 6.83 (t, J =7.9 Hz, 1 H), 4.05 (q, J =6.9 Hz, 2 H),
1.34 (t, J =6.9 Hz, 3 H). 13C NMR (101 MHz, DMSO) δ 158.46, 149.44,
149.20, 148.15, 147.52, 140.51, 122.04, 120.71, 119.41, 119.33,
117.22, 116.12, 115.81, 64.62, 15.25.
2.3. Calculation of quantum yield and association constant
Quantum yield was calculated according to the following formula
(1):
F As n 2
u
u
=
u
s F Au n2
(1)
s
s
Scheme 1. Synthesis routes of X. Conditions: (a) THF/ethanol, 25℃/r.f., 20 h/4 h; (b) ethanol, r.t., overnight; (c) ethanol, r.t., 12 h.
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Y. Xu, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry383(2019)112026
Fig. 1. (a) UV–vis spectra of X (1 × 10−5 M) in the presence of 6 equiv. various metal ions (Na+, Ca2+, K+, Li+, Zn2+, Cu2+, Mn2+, Ni2+, Cr3+, Co2+, Fe3+, Fe2+
,
Cd2+, Mg2+, Al3+ and Hg2+) in ethanol/H2O buffer solution (v/v = 9:1, tris = 10 mM, pH = 7.4). (b) UV–vis spectra for X (1 × 10−5 M) upon gradual addition of
Zn2+ (0–6 equiv.) in ethanol/H2O buffer solution. Inset: UV–vis spectrum of X and X[Zn2+].
3. Results and discussion
while negligible response of X was observed towards other metal ions.
In Fig. 2c, the signal with significant fluorescence enhancement can be
easily seen under the UV lamp (from colorless to yellow-green) only
when Zn2+ ions were added, indicating that the probe had good se-
lectivity to Zn2+ ions in ethanol/H2O solution. Moreover, the fluores-
cence response of X towards Zn2+ ions were examined in ethanol/H2O
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 prepared according to reported procedure [25].
Compound 1 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 as white solid. The
structure of X was 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.
solution in the presence of other competitive metal ions, such as Li+
,
,
Al3+, Cd2+, Co2+, Fe3+, Mg2+, Cr3+, Mn2+, Na+, Fe2+, K+, Cu2+
Ni2+, Ca2+ and Hg2+. As shown in Fig. S5, except for two transition
metal ions (Co2+ and Cu2+), most of the competitive cations had little
interference in the detection of Zn2+. Co2+ and Cu2+ could completely
quenched fluorescence of X[Zn2+]. This may be attributed to that these
two ions and probe formed fluorescent complexes with low fluores-
cence [27–31]. The above results showed that the sensor, X, could be
used as a selective fluorescent sensor for Zn2+ even in the presence of
other competitive metal ions, while it was less effective in the presence
of Co2+ and Cu2+ in ethanol/H2O buffer solution.
3.1. The UV–vis spectral studies of X
In Fig. 1, the UV–vis spectral selective properties of the X for various
In order to further explore the sensing capability of X toward Zn2+
ions, the detailed fluorescence titration experiments were conducted
and the experimental data were shown in Fig. 3. The free probe X
displayed maximum fluorescence intensity at 461 nm. In Fig. 3a, with
the increased of Zn2+ ions concentration, the fluorescence intensity of
X was gradually enhanced. That is, in Fig. 3b, the fluorescence intensity
of X gradually increased with the increase of Zn2+ ions concentration
from 0 to 6 × 10−5 M and reached a plateau when the concentration of
Zn2+ was up to 6 × 10−5 M. The fluorescence intensity had a good
linear relationship (R2 = 0.96) with the Zn2+ ions concentration from 0
to 7 × 10-8 M (Fig. 3b inset). Based on fluorescence titration data, the
detection limit was determined to be 1.2 × 10-9 M according to the 3σ/
s method and the association constant between X and Zn2+ was cal-
culated to be 2.2 × 105 M-1 (Fig. S6) by the Benesi-Hildebrand Eq. (2).
In addition, the binding ratio of X to Zn2+ were shown in Fig. S7. As a
result, when the molar fraction of [Zn2+] / ([X] + [Zn2+]) was around
0.5, the fluorescence intensity of [X] + [Zn2+] reached the maximum,
indicating that the stoichiometry between X and Zn2+ was 1:1. The
composition of complex had been proved by mass spectrometry ana-
lysis. The mass spectrometry results indicated that a peak at m/z
331.0926 corresponding to [X + H]+ and a peak at m/z 353.0745
corresponding to [X + Na]+ were observed. After addition of Zn2+, a
peak at m/z 428.9789 corresponding to [X + Zn2+ + Cl- - 2H+]- was
observed (Fig. S8), indicating X, Zn2+ and Cl- formed a stable ternary
complex. In a word, the titration results indicated that X had an ex-
cellent sensitivity towards Zn2+ based on the fluorescence turn-on re-
sponse in ethanol/H2O buffer solution.
metal ions (including Na+, Ca2+, K+, Li+, Zn2+, Cu2+, Mn2+, Ni2+
,
Cr3+, Co2+, Fe3+, Fe2+, Cd2+, Mg2+, Al3+ and Hg2+) and the UV–vis
spectral titration properties of the X for Zn2+ ions were carried out in
ethanol/H2O (v/v = 9:1) buffer solution (0.01 M tris, pH = 7.4). The
experiments were carried out at a concentration of X was maintained at
1 × 10−5 M. As shown in Fig. 1a, the absorption band of the X was
found at 302 nm and is of the n-π* electron transitions type. When 6
equiv. of different metal ions were added, most of them could induce
significant changed in the UV–vis spectrum, that is, the absorption band
at 302 nm gradually decreased and a new absorption band appeared
near 400 nm. As shown in Fig. 1b, upon gradually addition of Zn2+ to X
from 0 to 6 equiv., the absorption band at 302 nm gradually decreased
and two absorption bands appeared at 320 nm and 400 nm. At the same
time, in Fig. 1b inset, it can be clearly seen that there were four obvious
isosbestic points at 356 nm, 344 nm, 318 nm and 264 nm, indicating
that the X formed a stable complex with Zn2+ ions.
3.2. The fluorescence spectral studies of X
The fluorescence emission response of X (1 × 10−5 M) with dif-
ferent metal ions (6 × 10−5 M) was carried out in ethanol/H2O (v/
v = 9:1) buffer solution (0.01 M tris, pH = 7.4). As shown in Fig. 2a, X
exhibited a low fluorescence intensity (Ф = 0.016) under excitation
wavelength of 330 nm. Upon addition of various metal ions (Zn2+, Li+
Al3+, Cd2+, Co2+, Fe3+, Mg2+, Cr3+, Mn2+, Na+, Fe2+, K+, Cu2+
,
,
Ni2+, Ca2+ and Hg2+), only Zn2+ could induce obvious changed in the
fluorescence intensity (Ф = 0.3) at 511 nm. In Fig. 2b, after the addi-
tion of Zn2+ ions, the fluorescence intensity increased by 42 times,
The effect of pH on the fluorescence of X in the absence and
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Y. Xu, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry383(2019)112026
Fig. 2. (a) and (b) Fluorescence spectra of X (1 × 10−5 M) in the presence of 6 equiv. various metal ions (Zn2+, Li+, Al3+, Cd2+, Co2+, Fe3+, Mg2+, Cr3+, Mn2+
,
Na+, Fe2+, K+, Cu2+, Ni2+, Ca2+ and Hg2+) in ethanol/H2O buffer solution (v/v = 9:1, tris = 10 mM, pH = 7.4). (c) Photograph of a solution of X in the presence
of various metal ions under UV light.
presence of Zn2+ was evaluated. As shown in Fig. 4, the fluorescence
intensity of X was stable in the range of pH 2 to 7.4 and slightly in-
creased with the change of pH values from 7.4 to 12. In addition, when
addition of 6 equiv. Zn2+ to the tris buffer solution of probe X at dif-
ferent pH value, the fluorescence intensity of X[Zn2+] was found to be
the highest at pH 7.4. Under acidic conditions (pH＜6), X had no
fluorescence response to Zn2+ ions due to the protonation of amino
lead to the decrease of the coordination ability of X to Zn2+ ions. As the
pH increased (6–7.4), the fluorescence intensity of X[Zn2+] increased
gradually and reached its maximum at pH 7.4. Under alkaline condi-
tions (pH＞7.4), the fluorescence intensity of X[Zn2+] gradually de-
creased with the change of pH from 7.4 to 12, which was attributed to
the formation of Zn(OH)2 and reduced the concentration of X[Zn2+].
The results of pH titration indicated that the sensor could be used de-
tection of Zn2+ in physiological conditions for biological samples.
3.3. The fluorescence spectral studies of X[Zn2+
]
In addition, the reversibility of probe was an important performance
of probe. The fluorescence emission response of X[Zn2+] with different
anions (4 × 10−5 M) was carried out in ethanol/H2O (v/v = 9:1) buffer
solution (0.01 M tris, pH = 7.4). As mentioned above, the solution ex-
hibited moderate fluorescence intensity when Zn2+ ions were added
under excitation wavelength of 330 nm. As shown in Fig. 5a, upon
addition of various anions (Na4P2O7, NaCl, NaF, NaBr, KI, NaNO2,
NaHCO3, Na2S, NaS2O3, Na2SO4, NaNO3, NaHSO3 and NaBF3), only
Na4P2O7 (PPi) could induce obvious changes in the fluorescence in-
tensity at 511 nm. In Fig. 5b, after the addition of PPi, the fluorescence
intensity was quenched by 95%, while the addition of other metal ions
was quenched slightly in fluorescence intensity (NaF, NaBr, KI, NaS2O3,
NaNO3, NaHSO3 and NaBF3 could quenched about 10% of the
Fig. 3. (a) and (b) Fluorescence spectra of X (1 × 10−5 M) upon titration with Zn2+ (0–6 equiv.) in ethanol/H2O buffer solution (v/v = 9:1, tris = 10 mM,
pH = 7.4).
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Y. Xu, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry383(2019)112026
Fig. 4. Fluorescence spectra of X (1 × 10−5 M) at various pH values (in the absence and presence of Zn2+) in ethanol/H2O buffer solution (v/v = 9:1, tris = 10 mM).
fluorescence intensity). Interestingly, the fluorescence intensity in-
creased slightly (15%) after the addition of NO2-, which is attributed to
the fact that NO2- could replace the Cl- in the ternary complex to form a
more stable structure and thus enhanced the fluorescence [23]. In
Fig. 5c, under UV lamp, the fluorescence signal changes could be easily
observed from yellow-green to colorless when PPi was added, showing
good turn-off fluorescence behavior. Moreover, competitive fluores-
cence experiment was shown in Fig. S9. PPi was added to the solution
of X[Zn2+] in the presence of various anions, including NaCl, NaF,
NaBr, KI, NaNO2, NaHCO3, Na2S, NaS2O3, Na2SO4, NaNO3, NaHSO3
and NaBF3. all anions had no effect on the fluorescence detection of PPi.
These results indicated that the X[Zn2+] had good selectivity to PPi in
ethanol/H2O solution
Moreover, the fluorescence titration experiment of X[Zn2+] to PPi
were also conducted. As shown in Fig. 6a, with the increased of PPi
concentration, the fluorescence intensity of X[Zn2+] was gradually
decreased. In Fig. 6b, the fluorescence intensity of X gradually de-
creased with the increase of PPi concentration from 0 to 1.5 × 10−5
M
Fig. 5. (a) and (b) Fluorescence spectra of X[Zn2+] in the presence of 4 equiv. various anions (1. Na4P2O7, 2. NaCl, 3. NaF, 4. NaBr, 5. KI, 6. NaNO2, 7. NaHCO3, 8.
Na2S, 9. NaS2O3, 10. Na2SO4, 11. NaNO3, 12. NaHSO3, 13. NaBF3) in ethanol/H2O buffer solution (v/v = 9:1, tris = 10 mM, pH = 7.4). (c) Photograph of a solution
of X[Zn2+] in the presence of various anions under UV light.
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Y. Xu, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry383(2019)112026
Fig. 6. (a) and (b) Fluorescence spectra of X[Zn2+] upon titration with PPi (0–1.5 equiv.) in ethanol/H2O buffer solution (v/v = 9:1, tris = 10 mM, pH = 7.4).
and reached a plateau when the concentration of Zn2+ was up to
1.5 × 10−5 M. The fluorescence intensity had a good linear relation-
Zn2+ ions was also explored. Firstly, the stoichiometric ratio of X and
Zn2+ was obtained as 1:1 based on B-H equation (Fig. S6) and Job’s plot
experiment (Fig. S7). And then, this was further exploration through
mass spectrometry data (Fig. S8). The mass spectrometry results in-
ship (R2 = 0.98) with the PPi concentration from 0 to 6 × 10-7
(Fig. 6b inset). The detection limit was determined to be 1.9 × 10-9
M
M
according to the 3σ/s method, demonstrating that X[Zn2+] was a
highly sensitive sensor for PPi in ethanol/H2O solution.
dicated that a peak at m/z 428.9789 corresponding to [X + Zn2+
+
Cl− - 2H+
]
− was observed when addition of ZnCl2, indicating X, Zn2+
The reversibility experiments of X are shown in Fig. 7. Initially, X
showed very low fluorescence intensity. When 10 equiv. Zn2+ ions
were added, the fluorescence signal was enhanced. And then, Pyr-
ophosphate (PPi, 6 equiv.) was added to the solution of X[Zn2+] and
then the fluorescence signal at 511 nm was immediately disappeared.
Treated with Zn2+ again, the fluorescence of the system was recovered
again. This “off-on-off” fluorescence behavior was observed by the al-
ternative addition of Zn2+ and PPi to the system. What’s more, after
two cycles X[Zn2+] still showed a good response toward PPi. These
results indicated that the X[Zn2+] complex with high selectivity and
sensitivity could be used as a new sensor to detect PPi in ethanol/H2O
solution.
and Cl− formed a stable ternary complex. All of the experimental data
indicated that the binding ratio between X and Zn2+ ions was 1:1.
Moreover, as shown in Fig. 8, oxygen (carbonyl and hydroxy) and ni-
trogen (NeH and CN]) were electronegative so they have strong
complexation with metal ions (Fig. S10). In addition, the possible
binding mechanism between X and Zn2+ ions were proposed according
to experiments data and relevant literatures [32–37], as shown in
Scheme 2. The hydroxy group at 3-ethoxy-2-hydroxybenzaldehyde was
involved in the complexation of Zn2+ ions, while the ethoxy group was
not involved. The nitrogen atoms on C]N bond participate in the
complexation [23,32].
In order to prove the binding mechanism and fluorescence response
mechanism of X to Zn2+ ions, structure optimization and energy cal-
culation of X and X[Zn2+] were investigated using ab initio density
functional theory (DFT) combined with time-dependent density func-
tional theory (TDDFT) calculations, as implemented in the Gaussian 09
3.4. Binding mode and theoretical calculations
According to the above experimental data, the binding model of X to
Fig. 7. Reversible switching of the emission of X by repeated addition of Zn2+ and PPi in ethanol/H2O buffer solution (v/v = 9:1, tris = 10 mM, pH = 7.4).
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JournalofPhotochemistry&PhotobiologyA:Chemistry383(2019)112026
3.5. Logic gate behavior of X
The fluorescence behavior of X at 511 nm mimics the INHIBIT logic
gate, the circuit and the truth table for the INHIBIT logic gate were
shown in Fig. 10 [42]. In Fig. 10a, the INHIBIT logic gate was an “AND”
gate with an inverter in one of its input and had two input signals and
an output signal [43]. The emission intensity of X at 511 nm was re-
garded as an initial value. The fluorescent intensity output value (If 511
nm) below the threshold level were assigned as logic “0”, while the
output values above the threshold were assigned as logic “1” [44]. In
Fig. 10b, the output was only logic “1” when the input signal was
(IN1 = 1 and IN2 = 0). While in other input forms (IN1 = 0 and
IN2 = 0, IN1 = 0 and IN2 = 1, IN1 = 1 and IN2 = 1), the output sig-
nals were all logic “0” [45]. That is, the fluorescence intensity of X was
weak. After addition of Zn2+ ions, the fluorescence enhancement.
However, the fluorescence intensity was low in the presence of Cu2+ or
Co2+. When Zn2+ and Cu2+ or Co2+ coexist, the fluorescence intensity
of the system remained low. In Fig. 10c, the algorithm was similar to
the one above. The system fluorescence was observed in the presence of
Zn2+ ions alone and disappeared in the presence of PPi. Thus, this
behavior was very consistent with INHIBITORY logic gates [46].
Fig. 8. Electrostatic potential coloring of the molecular surface of the X. where
the light-gray, red, blue, white, yellow atoms denote C, O, N, H, S atoms, re-
spectively.
package based on B3LYP/6-31 G(d) basis [38]. As shown in Fig. S11,
the optimal structure of X and X[Zn2+] were obtained by gauss cal-
culations. For X, the imidazo[2,1-b]thiazole (fluorescence nuclei) and
3-ethoxy-2-hydroxybenzaldehyde (fluorescence receptor) part main-
tained a good planar property, respectively, and their dihedral angle
was about 68.18 degrees. After binding with Zn2+ ions, the dihedral
angle between the fluorescence nuclei and receptor becomes 7.1 de-
grees, indicating that the planar nature of the whole X[Zn2+] molecule
increased.
4. Conclusions
In summary, A novel simple schiff base fluorescent sensor, X, was
designed and synthesized based on 3-ethoxy-2-hydroxybenzaldehyde as
the receptor and imidazo[2,1-b]thiazole-6-carboxylic acid as the
fluorophore. X showed excellent sensitivity and selectivity for Zn2+ by
exhibiting a large fluorescence enhancement (about 42 times) with
distinct color changed from colorless to yellow-green under UV light,
whereas other competitive metal ions did not show any noticeable
change. Conversely, the fluorescence of the X[Zn2+] complex was re-
duced by the addition of PPi, confirming that the recognition process
was reversible. In general, X could act as a fluorescence probe for the
sequential detection of Zn2+ and PPi and the detection limit was
1.2 × 10−9 M and 1.9 × 10−9 M, respectively. The binding ratio of the
X and Zn2+ was 1:1 confirmed by mass spectral analysis and Job’s plot.
The binding mode and sensing mechanism of X with Zn2+ was verified
by DFT/TDDFT calculations using Gaussian 09. Additionally, an
INHIBIT logic gates had been successfully designed and constructed. In
addition, a series of other schiff base fluorescent sensors had already
designed and synthesized in our lab and their optical properties and
practical applications will be studied.
In addition, the spatial distributions and orbital energies of the
highest occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) of X and X[Zn2+] complexes were also ex-
hibited in Fig. 9. For free X, the highest occupied molecular orbital
(HOMO) was mostly distributed over 3-ethoxy-2-hydroxybenzaldehyde
(fluorescence receptor) and imine (C]N) groups whereas the lowest
unoccupied molecular orbital (LUMO) charge density was uniformly
distributed over the whole probe molecule [39]. There was partial
electron transfer in the excited, so X showed very weak fluorescence.
After binding with Zn2+ ions, the HOMO was still around the receptor
and C]N groups, while electrons in LUMO are mostly localized on the
fluorophore [40]. Moreover, the energy gap of X and X[Zn2+] were also
calculated to be 4.55 and 3.22, respectively. The results showed that the
role of Zn2+ ions was to increased the degree of intramolecular charge
transfer, enhanced the planarity and rigidity of the whole molecule.
This observation additionally supported the role of inhibition of photo-
induced electron transfer (PET) along with the chelation enhanced
fluorescence mechanism (CHEF) effect for the fluorescence enhance-
ment [5,18,33,40,41]. The calculated results and experimental phe-
nomena were highly consistent with the theoretical basis.
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)
Scheme 2. The proposed binding mode of X with Zn2+
.
7

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JournalofPhotochemistry&PhotobiologyA:Chemistry383(2019)112026
Fig. 9. Energy diagram of HOMO and LUMO orbital X and X[Zn2+].
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