A new fluorescence chemosensor based on benzothiazole derivative for Zn2+ and its logic gate behavior

Article abstract of DOI:10.1080/10426507.2016.1277527

A novel benzothiazole derivative J as a fluorescent chemosensor for Zn²⁺ has been designed and synthesized. The chemosensor J exhibits high selectivity and sensitivity to Zn²⁺ as fluorescence “turn-on” behavior, which the other cations do not make sense. The detection limit calculated by fluorescence titration method was 1.64 × 10^?8 M. Furthermore, the generated J-Zn²⁺ ensemble could recover the enhanced fluorescence upon the addition of EDTA generating an “on-off-on” recycle. In addition, the test strip based on J was fabricated, which could act as a convenient and efficient Zn²⁺ test kit.

Full text of DOI:10.1080/10426507.2016.1277527

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A new fluorescence chemosensor based on benzothiazole derivative for Zn2+ and its logic
gate behavior
Jing-Han Hu∗, Jing Qi, You Sun, Peng-Xiang Pei
College of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu,
730070, P. R. China
E-mail: hujinghan62@163.com
Abstract: A novel benzothiazole derivative J as a fluorescent chemosensor for Zn2+ has been
designed and synthesized. The chemosensor J exhibits high selectivity and sensitivity to Zn2+ as
fluorescence “turn-on” behavior, which the other cations do not make sense. The detection limit
calculated by fluorescence titration method was 1.64×10−8 M. Furthermore, the generated J-Zn2+
ensemble could recover the enhanced fluorescence upon the addition of EDTA generating an
“on-off-on” recycle. In addition, the test strip based on J was fabricated, which could act as a
convenient and efficient Zn2+ test kit.
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Keywords: Benzothiazole; Chemosensor; Zinc (Zn2+); Logic gate behavior；
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Introduction
Recently, the design and exploration of fluorescent sensors for detecting transition metal ions
have received more attention due to their advantages of high selectivity, sensitivity, and real-time
monitoring other methods.1-3 Zinc as the second most abundant and essential transition metal in
the human body influences many important physiological functions in biological activities such
as gene expression, neural signal transmission, DNA binding or recognition, cell apoptosis and
immune function.4-9 Lack or excess of zinc level in the human body may cause physiological or
pathological changes and produce varieties of diseases. The total concentration of zinc ion in
serum should be 10 μM.10-12 At present, a range of detection methods for zinc ion have been
developed such as inductive and coupled plasma mass spectrometry, atomic absorption
spectrometry, which may be time-consuming or need use some sophisticated
instrumentation.13-16 Accordingly, developing simple, selective and sensitive detection tools for
Zn2+ in living organisms is challenging.
Until now, a number of fluorescent sensors for Zn2+ based on various fluorophores such as
coumarin, quinoline, naphthalene, pyrene, and nanoparticles have been reported.17-22
Nevertheless, only a few related literatures about sensors for the zinc ion based on the thiazole
group could be found.23-26 Benzothiazole derivatives could provide an electron-rich environment
and good ligating ability.27-28
In view of the investigations and as a part of our research interests in molecular recognition,
29-32
we have designed and synthesized a new benzothiazole derivative J (2,
4-bis-[[(2-benzothiazolythio)-hydrazinylidene] methyl] phenol) as a selective fluorescent sensor
for Zn2+. It could achieve the recognition purpose by inhibition of photoinduced electron transfer
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(PET) and excited–state intramolecular proton transfer (ESIPT) effect. J showed a large
fluorescence enhancement at 498 nm in the present of Zn2+ in DMSO/H2O (pH=7.2, v/v=4:1)
HEPES buffer solutions. The detection process of Zn2+ usually was interfered by Mg2+,
especially Cd2+. Fortunately, the sensor J could detect and distinguish Zn2+ without their
interference. Lin et al.33 reported a similar sensor for Zn2+. Therefore we have compared the two
compounds listed in Table S 1 (Supplemental Materials). The results showed that J could detect
Zn2+ in higher water-content system. And J had a fluorescence response between pH 5 and 11.
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The mechanism of the detection process was verified by spectroscopic methods including H
NMR, UV-vis, and mass spectrometry.
Scheme 1 Synthesis of the sensor molecule J.
Results and discussion
In order to investigate the Zn2+ recognition abilities of the sensor J, we carried out a series of
host–guest recognition experiments in DMSO/H2O (pH = 7.2, v/v = 4:1) HEPES buffer solutions.
The chemosensor J was tested with aqueous solutions of the perchlorate salts of all common
cationic analytes such as Fe3+, Hg2+, Ag+, Ca2+, Cu2+, Co2+, Ni2+, Cd2+, Pb2+, Zn2+, Cr3+, and
Mg2+. As shown in Figure 1, in the fluorescence spectrum, the emission of J appeared at the
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maximum emission wavelength was 498 nm when excited at λex = 364 nm. Upon the addition of
Zn2+ ions (20 equiv.), the fluorescence intensity of sensor J increased, in which the color change
from colorless to bright green could be distinguished by the naked eye under a UV-vis lamp.
None of other ions (Fe3+, Hg2+, Ag+, Ca2+, Cu2+, Co2+, Ni2+, Cd2+, Pb2+, Cr3+, and Mg2+) induced
any significant changes in the fluorescence spectrum of the sensor J.
Further, to evaluate the selectivity of J, we measured the fluorescence intensity of J in the
presence of other cations such as Fe3+, Hg2+, Ag+, Ca2+, Cu2+, Co2+, Ni2+, Cd2+, Pb2+, Cr3+ and
Mg2+ ((4×10-4 M) including Zn2+ ion. (Figure 2) Therefore, it was clear that interference of other
ions was negligibly small during the detection of Zn2+.
Figure 1 (a) Fluorescence emission data for J and in the presence of various metal ions in
DMSO/H2O (pH = 7.2, v/v = 4:1) HEPES buffer solutions. (λex = 364 nm). (b) Color changes
observed upon the addition of various cations (20 equiv.) to solutions of sensor J (210-5 M) in
DMSO/H2O (pH = 7.2, v/v = 4:1) HEPES buffer solutions.
Figure 2 Fluorescence intensity changes of the J (20μM) to Zn2+ (20 equiv.) in the presence of
various test cations (20 equiv.) in DMSO/H2O (pH = 7.2, v/v = 4:1) HEPES buffer solutions.
The interaction between probe J and Zn2+ was also investigated in the fluorescence emission
spectra in DMSO/H2O (pH = 7.2, v/v=4:1) HEPES buffer solutions. It was recorded during the
titrations with different concentrations of Zn2+ 0 – 2 equivalent. As shown in Figure 3, when J
was treated with 1.6 equivalent of Zn2+, the fluorescence intensity at 498 nm would keep stable
and almost change no more.
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The detection limit is one of the most important parameters in ion sensing. The fluorimetric
detection limits of sensor J for Zn2+ were also determined. As shown in Figure S 4, the minimum
concentration of Zn2+ for fluorescence detection by the naked – eye was 1.00×10-7 M, by using a
UV – vis lamp at 365 nm. In the meantime, the detection limit of the fluorescence spectra
measurements was 1.64×10-8 M for Zn2+, (Figure S 5) as calculated on the basis of 3SB/S (where
SB is the standard deviation of the blank solution and S is the slope of the calibration curve).
Thus, J can be applied to detect Zn2+ at an extremely low concentration level of 10−7 M, which
may fully satisfy the requirements in biosensing, as labile Zn2+ in the human body was reported
to exist at a millimolar concentration level.34-35 These data indicated that probe J showed high
sensitivity toward Zn2+. Moreover, the association constant for Zn2+ was found to be 6.62×104
M-1 in the same media.36
Figure 3 Fluorescence spectra of J (20μM) in the presence of different concentration of Zn2+ (0
– 2 equiv.) in DMSO/H2O (pH = 7.2, v/v = 4:1) HEPES buffer solutions. Inset shows a plot of
fluorescence intensity depending on the concentration of Zn2+ in the range from 0 to 2
equivalents.
Generally, the reactive chemosensors have a long response time.37 In our case, the response of
J to Zn2+ was found to be very fast. After addition of the solution of Zn2+, the fluorescence
emission intensity of J in DMSO/H2O (pH = 7.2, v/v = 4:1) HEPES buffer solutions at 498 nm
increased rapidly below 3 s. This result suggested that the recognition profiles of sensor J with
Zn2+ was completed nearly instantaneously.
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The pH dependence of the fluorescence intensity of J – Zn2+ system in aqueous media was
examined by fluorescence emission spectroscopy. The solution of Zn2+ was added to a solution
of sensor J (210-5 M) in DMSO/H2O (pH = 7.2, v/v = 4:1) HEPES buffer solutions at pH values
ranging from 1 to 13 ranges (Figure S 6) The J – Zn2+ complex showed a significant
fluorescence response between pH 5 and 11, which included the physiologically relevant range
of pH 7.0–8.4. In conclusion, Zn2+ could be clearly detected by the fluorescence spectral
measurement using J within the physiological pH range (pH = 7.0–8.4).
In order to investigate the reversibility of the sensor J, the addition of Zn2+ (410-4 M) to
the solution of sensor J (210-5 M) leaded to fluorescence increasing, that was, “on”. When
adding EDTA into the solution of J – Zn2+ complex, J exhibited weak fluorescence, that was,
“off”. Hence, this “off–on–off” switching process could be repeated several times with little
fluorescent efficiency loss. The minimum decrease value of fluorescent intensity was only about
20 a. u. as shown in Figure 4
Figure 4 Reversible switching cycles of fluorescence intensity in DMSO/H2O (pH = 7.2, v/v =
4:1) HEPES buffer solutions. (λex = 364 nm)
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The interaction mechanism between J (110-2 M) and Zn2+ (1 M) was studied by H NMR
titration experiments. As shown in Figure 5 the sensor J showed three single peaks at 12.24 ppm,
10.87 ppm and 8.48 ppm in DMSO-d6, which were respectively confirmed to be the protons of
–OH, –NH=N and –CH=N. Upon the addition of Zn2+, the signal peak of the –OH at 12.24 ppm
disappeared. The job plot between J and Zn2+ was implemented. As shown in Figure S 7, the
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maximum point appeared at a mole fraction of 0.5, which indicated that it was a 1:1
stoichiometry of the binding mode of J (210-5 M) and Zn2+ (110-2 M). In addition, the
complex J–Zn2+ was subjected to mass spectral analyses. The ion peak was detected at m/z
508.54 (Figure S 8), which corresponded to [J+Zn2+].
Figure 5 Partial 1H NMR spectra (400 MHz, DMSO–d6) of free J and in the presence of varying
amounts of Zn2+.
In conclusion, because of the transition of the lone pair electrons (–C=N), free J only showed
weak fluorescence. Upon the addition of Zn2+, the deprotonation of phenolic hydroxyl group
resulted in inhibiting transition of electrons and blocking the PET process. And the cooperation
reaction occurred between heteroatom (N and O) of J and Zn2+ led to ESIPT process, destroyed
the intramolecular hydrogen bond. As a result, J showed strong green fluorescence. (Scheme 2)
Scheme 2 The proposed cartoon representation of J for Zn2+.
The selectivity and reversibility of this sensor prompted us to consider the present system as a
sequence dependent molecular keypad lock using the J, Zn2+ and EDTA as three different
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chemical inputs. Therefore, we made the chemical inputs designated respectively as “J”, “Z”,
and “E”. In the midst of the possible six input combinations, JZE, JEZ, EJZ, EZJ, ZEJ and ZJE,
the combination JZE gave minimum fluorescence output, whereas maximum output signaling
was displayed by EJZ (Figure S 9). The keypad contained various keys, which the correct
password order was JZE.
On the basis of the reversible fluorescent switch of J controlled by Zn2+, and EDTA, we used
it as a two-input molecular logic gate. As shown in Figure 6, when the output was zero, there
were three conditions: (a) both the Zn2+ and EDTA were absent, (b) only EDTA was present, (c)
both Zn2+ and EDTA were present. The output “0” corresponded to the gate being closed, which
meant quenched fluorescence emission. When only Zn2+ alone was present, the output was one,
which corresponded to the gate being open (strong fluorescence emission). Thus this repeated
behavior of J by fluorescence change could clearly indicate the sensor was reversible and
reusable for Zn2+.
Figure 6 Molecular logic gate table and the respective symbolic representation of the INHIBIT
logic gate function.
To investigate the practical application of chemosensor J, test strips were prepared by
immersing filter papers into DMSO/H2O (pH=7.2, v/v=4:1) HEPES buffer solutions of J (2×10−4
M) and then drying in air. The test strips containing J were conveniently utilized to detect Zn2+
(2×10−5 M) and other metal ions. As shown in Figure 7, when Zn2+ were added on the test kits,
the obvious color change from black to blue was observed under UV lamp (365 nm).
Unsurprisingly, potentially competitive ions exerted no influence on the detection of Zn2+ by the
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test strips. Therefore, the test strips could conveniently detect Zn2+ in DMSO/H2O (pH=7.2,
v/v=4:1) HEPES buffer solutions.
Figure 7 Photographs of J and J–Zn2+ on test papers after immersion in DMSO/H2O (pH=7.2,
v/v=4:1) HEPES buffer solutions. Left to right: (1) only J, (2) J with Zn2+, (3) J with other metal
ions, (2) J–Zn2+ with other metal ions under irradiation at 365 nm upon UV lamp.
Conclusions
In summary, we have developed a bi-hydrazone derivative based benzothiazole group for the
selective fluorescent detection of Zn2+ in aqueous medium. The detection limits were 1.00×10−7
M and 1.64×10−8 M of Zn2+ using the visual fluorescent color changes and fluorescence spectra
changes respectively, which may find potential applications for detecting micromole
concentrations of Zn2+ in both biological systems and the environment. In addition, the
fluorescence signals could be used to mimic the function of logical memory devices at the
molecular level.
Experimental section
Materials and physical methods
Reagents and solvents were commercially available at analytical grade and were used without
further purification. The fluorescence spectra were recorded with a Shimadzu RF-5301
spectrofluorimeter. The melting points were measured on an X-4 digital melting-point apparatus.
Electrospray ionization mass spectra (ESI-MS) were measured on an Agilent 1100
1
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LC-MSD-Trap-VL system. H NMR and C NMR spectra were recorded on a Mercury-400BB
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spectrometer at 400 MHz and 100 MHz. The chemical shifts are reported in ppm down field
from tetramethylsilane (TMS, δ scale with solvent resonances as internal standards).
Synthesis of sensor molecule J
The compound J can be readily prepared by a simple and low cost Schiff base reaction of
4-hydroxyisophthalaldehyde
and
2-hydrazinylbenzothiazole
(Scheme
1).
4-hydroxyisophthalaldehyde (0.7500 g, 5 mmol), 2-hydrazinylbenzothiazole (0.60 g, 5.5 mmol)
and a catalytic amount of acetic acid (AcOH) were combined in hot absolute EtOH (30 mL). The
solution was stirred under reflux for 4 h. After cooling to room temperature, the dark yellow
precipitate was filtered, washed three times with hot absolute ethanol, then recrystallized with
1
EtOH to obtain a yellow powdered product J (4.31 mmol) in 86.2% yield (m.p. >300oC). H
NMR (DMSO-d6, 400 MHz) δ: 12.24 (s, 2H), 10.87 (s, 1H), 8.48-7.01 (m, 13H,) (Figure S 1).
13C NMR (DMSO-d6, 100 MHz) δ/ppm: 167.4, 167.1, 162.8, 158.3, 129.2, 126.7, 126.5, 126.4,
122.3, 122.1, 121.9, 120.6, 117.3. (Figure S 2) ESI-MS calcd for [C18H16N6OS2+H+]+ 445.08.
Found 445.07. (Figure S 3)
Funding
This work was supported by the National Nature Science Foundation of China (No.
21467012).
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Figure 1 (a) Fluorescence emission data for J and in the presence of various metal ions in
DMSO/H2O (pH = 7.2, v/v = 4:1) HEPES buffer solutions. (λex = 364 nm). (b) Color changes
observed upon the addition of various cations (20 equiv.) to solutions of sensor J (210-5 M) in
DMSO/H2O (pH = 7.2, v/v = 4:1) HEPES buffer solutions.
Figure 2 Fluorescence intensity changes of the J (20μM) to Zn2+ (20 equiv.) in the presence of
various test cations (20 equiv.) in DMSO/H2O (pH = 7.2, v/v = 4:1) HEPES buffer solutions.
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Figure 3 Fluorescence spectra of J (20μM) in the presence of different concentration of Zn2+
(0.0 – 2.0 equiv.) in DMSO/H2O (pH = 7.2, v/v = 4:1) HEPES buffer solutions. Inset shows a
plot of fluorescence intensity depending on the concentration of Zn2+ in the range from 0 to 2
equivalents.
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Figure 4 Reversible switching cycles of fluorescence intensity in DMSO/H2O (pH = 7.2, v/v =
4:1) HEPES buffer solutions. (λex = 364 nm)
Figure 5 Partial 1H NMR spectra (400 MHz, DMSO–d6) of free J and in the presence of varying
amounts of Zn2+.
Input
EDTA
Outputa
Zn2+
λem=498nm
0
1
0
0
0
1
0
1
0
0
1
1
Figure 6 Molecular logic gate table and the respective symbolic representation of the INHIBIT
logic gate function.
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Figure 7 Photographs of J and J-Zn2+ on test papers after immersion in DMSO/H2O (pH = 7.2,
v/v = 4:1) HEPES buffer solutions. Left to right: (1) only J, (2) J with Zn2+, (3) J with other
metal ions, (2) J–Zn2+ with other metal ions under irradiation at 365 nm upon UV lamp.
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