A highly selective, pH-tolerable and fast-response fluorescent probe for Fe3+ based on star-shape benzothiazole derivative

Article abstract of DOI:10.1016/j.jphotochem.2017.09.065

A star-shape benzothiazole-based molecule TBB, obtained through a simple condensation reaction of 2-aminothiophenol with 1,3,5-benzenetricarboxylic acid in the presence of available catalyst polyphosphoric acid (PPA), has been exploited as a highly select

Full text of DOI:10.1016/j.jphotochem.2017.09.065

Accepted Manuscript
Title: A highly selective, pH-tolerable and fast-response
3
+
fluorescent probe for Fe based on star-shape benzothiazole
derivative
Authors: Kai Jiang, Yan-Cheng Wu, Han-Qing Wu, Shu-Li Li,
Shi-He Luo, Zhao-Yang Wang
PII:
S1010-6030(17)31063-8
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2017.09.065
JPC 10912
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
18-7-2017
9-9-2017
25-9-2017
Please cite this article as: Kai Jiang, Yan-Cheng Wu, Han-Qing Wu, Shu-Li Li, Shi-He
Luo, Zhao-Yang Wang, A highly selective, pH-tolerable and fast-response fluorescent
probe for Fe3+ based on star-shape benzothiazole derivative, Journal of Photochemistry
and Photobiology A: Chemistry https://doi.org/10.1016/j.jphotochem.2017.09.065
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3+
A highly selective, pH-tolerable and fast-response fluorescent probe for Fe based
on star-shape benzothiazole derivative
a
ab
a
a
a
a
Kai Jiang , Yan-Cheng Wu *, Han-Qing Wu , Shu-Li Li , Shi-He Luo , and Zhao-Yang Wang*
a
School of Chemistry and Environment, South China Normal University; Key Laboratory of Theoretical Chemistry of Environment,
Ministry of Education, Guangzhou 510006, China
b
College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China.
*
Corresponding Author
E-mail: wangzy@scnu.edu.cn. Tel: 8620-39310258.
E-mail: wuycity@gmail.com.
Graphical abstract
Highlights
3
+





The star-shape probe TBB is highly selective towards Fe over other metal ions.
3+
TBB shows fast response (50 s) and good pH-tolerance (3-12) on detecting Fe .
3+
TBB is applied for the quantitative detection of Fe in real water samples.
3+
The 1:2 binding mode for TBB-Fe complex has been confirmed.
The dynamic quenching mechanism has been proved by TCSPC experiments.
1

Abstract
A star-shape benzothiazole-based molecule TBB, obtained through a simple condensation
reaction of 2-aminothiophenol with 1,3,5-benzenetricarboxylic acid in the presence of available
3
+
catalyst polyphosphoric acid (PPA), has been exploited as a highly selective fluorescent Fe probe
over other metal ions in aqueous media (H O/THF, v/v = 4/6). For this fast-response probe with a
2
response time of 50 s, its quenching mechanism involving in the combination of static and
dynamic quenching is confirmed by the test of time-correlated single photon counting (TCSPC)
3
+
experiments. Noticeably, the probe TBB is suitable for Fe detection in a wide pH range (3-12) of
3
+
aqueous solution, and it can be successfully applied for Fe quantitative measurement in real
water samples.
Keywords: Star-shape benzothiazole derivative; Fluorescent probe; Highly selective Fe3+
detection; pH-tolerable; Fast-response
2

1. Introduction
As the most abundant transition metal in the human body, iron is closely related to some
crucial physiological process, such as cellular metabolism, oxygen uptake and electron transfer, etc
1,2]. The deficiency of iron can induce the anaemia and neurodegenerative diseases, while
[
overload of iron might oxidize and finally damage cellular components [3-5]. In addition, excessive
iron in water brought about serious environmental problems. The U.S. Environmental Protection
Agency (EPA) has set a limit of iron in drinking water and food to 0.3 ppm (∼5.4 μM) [6,7].
Therefore, the detection of trace iron ion in real water samples or under wide-range pH
3
+
environment is highly desirable [3,8-18]. However, the reported Fe probes partially exhibited
some obvious drawbacks, such as synthetic difficulty [19,20], unsatisfactory selectivity [21-23],
interference from pH [24,25] and time-consuming [26,27], which negatively affected their
properties and application. Thus, it is meaningful to exploit more available, selective, time-saving
3
+
and practical probe for detection of Fe .
Usually, benzothiazole compounds are widely used in pharmaceutical field for their
pharmacological activities [28,29]. In recent years, benzothiazole derivatives with extensively
conjugate structures have been applied in the field of photoelectric materials [30-32]. At the same
time, due to the existence of binding sites from N and S atoms [33,34], some benzothiazole-based
fluorescent probes with high quantum yield and large Stokes shift have been designed also
3
+
[
35,36]. However, for the less reported benzothiazole-based Fe probes [37], it is still necessary to
improve selectivity [38], enhance pH tolerance [39], and reduce responding time [40]. In a word,
3
+
for the detection of Fe with high selectivity, broad pH-tolerability and fast response, the
3

development of benzothiazole-based fluorescent probe with better sensing performance is in
encouragement.
Herein, a star-shape molecule tri-benzothiazolyl benzene (TBB) is synthesized through a
3
+
simple reaction (Scheme 1), which can function as an excellent optical probe for Fe with dual
discriminating channels, i.e. detectable color change in room light and fluorescent ‘turn-off’
response. The probe TBB shows not only great selectivity, good sensitivity and rapid response
3
+
towards Fe , but also availability within a broad range of pH environment and real water samples.
3
+
Meanwhile, the 1:2 binding mode between TBB and Fe has been demonstrated by Job’s plots
and ESI-MS spectrum. What’ more, Stern-Volmer plots and time-correlated single photon counting
(TCSPC) experiments have been carried out to elucidate a static and dynamic hybrid quenching
process.
Scheme 1. Synthetic route for TBB.
2
. Experimental
2
.1. Chemicals and apparatus
All chemicals used within this work were of analytical grade purity. The salts, such as KNO ,
3
NaNO
3
, AgNO
3
, Cu(NO
3
)
2
, Fe(NO
3
)
3
, Zn(NO
3
)
2
, Ca(NO
3
)
2
, Mg(NO
3
)
2
, Al(NO
3
)
3
, HgCl
2
, Cd(NO
3
)
2
, CrCl ,
3
4

1
FeSO
4
, Co(NO
3
)
2
, BaCl
2
, NiSO
4
, Pb(NO
3
)
2
, were used as metal ions sources. H NMR spectrum was
collected on a BRUKER DRX-400 spectrometer in CDCl
3
using TMS as an internal standard. Positive
ions mass spectra (ESI-MS) were recorded on a Thermo LCQ DECA XP MAX mass spectrometer. IR
spectrum was performed on a BRUKER VECTOR33 and UV-vis absorption spectra were measured
by using a Shimazu UV-2700 ultraviolet absorption detector at room temperature. The
fluorescence spectra were recorded with a Hitachi F-4600 spectrophotometer at room
temperature. pH values were measured by a PHS-25C meter. Fluorescence lifetime was measured
by FLS 920 Fluorescence Spectrometer.
2
2
.2. Methods
.2.1. General procedure for spectra measurements
The probe TBB was dissolved in THF to acquire 1 mM stock solution. The measurements of
both UV-vis absorption and fluorescence spectra were conducted in 10 μM aqueous solution
under room temperature. For fluorescence measurements, both the excitation and emission slit
widths were set at 5 nm.
2
.2.2. Limit of detection
The limit of detection (LOD) was determined by using equation: LOD = 3δ/K. Herein, δ is the
standard deviation of the blank measurements (n = 10), and the K is the slope of the calibration
curve.
2
.2.3. Calculation of Stern-Volmer constant
5

The Stern-Volmer quenching constant (Ksv) was calculated according to the Stern-Volmer
equation: I /I = Ksv[Q] + 1, where I and I are the fluorescence intensities observed in the absence
and presence of Fe , respectively. [Q] is the quencher concentration.
0
0
3
+
2
.3. Synthesis of probe TBB
Referring to the literature [41], 1,3,5-benzenetricarboxylic acid 1 (5 mmol), 2-
aminothiophenol 2 (15.5 mmol) and 25 mL polyphosphoric acid (PPA) were added into a 50 mL
round bottom flask. After stirring for 24 h under 150 °C, the reaction was stopped by adding water
and then the mixture was regulated to the pH range of 9-10 with sodium hydroxide. The crude
product was obtained by filtration, and the further purification by recrystallization with
dichloromethane gave the target compound tri-benzothiazolyl benzene (TBB) as a white solid (481
1
mg, yield 33.6%), m.p. > 300 °C; H NMR (400 MHz, CDCl
3
, TMS), δ: 7.43-7.50 (m, 3H), 7.53-7.60
-
1
(
m, 3H), 7.99 (d, J = 8.0 Hz, 3H), 8.18 (d, J = 8.0 Hz, 3H), 8.93 (s, 3H); IR (film), ν, cm : 3055, 1632,
+
1
501, 1418, 1314, 1190, 766, 752, 719; ESI-MS, m/z (%): Calcd for C27
16
H N
3
S ([M+H] ): 478.04
3
(100). Found: 478.25 (100) [their corresponding characterization spectra as Figs. S1-S3 can be seen
in Supporting Information (SI)].
3
. Results and discussion
3
.1. Absorption and fluorescence studies of probe TBB in solution
In order to choose the solvent for probe solution, solvatochromism behavior of probe TBB (10
μM) was examined in nonpolar and polar solvents, such as PhMe, CH
2
Cl
2
, THF, EtOAc, CHCl , DMF
3
and EtOH. The photophysical data (see Figs. S4-S5 and Table S1 in SI) show that there are no
significant changes among different tested solvents. Due to the good miscibility with water and
6

relatively high fluorescence intensity, THF was finally chosen as constituent of the aqueous
solution.
It will be more valuable if probes can detect metal ion in aqueous solution. Therefore, the
emission property in different H O/THF systems was initially investigated. As shown in Fig. 1, the
2
main emission bands are located at nearly 380 nm and the fluorescence intensity exhibits no
obvious change within the range of water fraction between 0% and 40%. With the increase of
water fraction over 50%, there is a slight change. These results show that TBB has a certain
tolerance to water. However, the fluorescence intensity of TBB in H
decreases when the fraction of water achieves to 70%. This result is in accordance with the UV-vis
absorption spectra of TBB in the different ratio of H O/THF solution (Fig S6). And as the water
2
O/THF solution dramatically
2
fraction is equal to or over 70%, there are obvious level-off tails in longer wavelength region,
implying the fluorescence quenching caused by the aggregation of TBB in large amount of water
[
42]. Thus, the suitable solution system should be selected as a H
2
O/THF of 4/6 (v/v).
3
.2. Optical response towards Fe3+
From the UV-vis absorption spectra (Fig. 2), it is apparent that the prominent absorption
bands peak at 231 nm, 254 nm and 313 nm, which can be aligned with n-π*, π-π* and n-π*
electronic transition. Thereinto, the band at 313 nm is ascribed to the benzothiazole group [43].
3
+
Particularly, the addition of Fe causes linear growth of absorption peak at 313 nm along with
visible color change (Fig. S7). This can be assigned to the formation of metal-TBB ground-state
3
+
complexes [12,44,45], not resulting from the absorption of Fe itself (Fig. S8).
7

3
+
To further investigate the efficiency of probe TBB toward Fe , the fluorometric titration
experiment has been carried out. As shown in Fig. 3, the fluorescence spectrum of TBB in THF-H
2
O
displays a major emission peak at 387 nm and a shoulder band at 374 nm. A gradual decrease of
3
+
the fluorescence intensity of TBB at both peaks can be observed upon progressive addition of Fe ,
3
+
and the quenching efficiency achieves to 95.3% when the amount of Fe comes to 1 mM. The
3
+
fluorescence intensity at 387 nm is proportional to the concentration of Fe in the range of 0 - 0.5
-
6
mM (Fig. S9). Thus, the corresponding detection limit can be calculated to be 3.05 × 10 M (0.17
3
+
ppm) [10]. This value is lower than not only the suggested water quality standards for Fe (0.3
ppm) in drinking water based on EPA guideline [7], but also some data reported before [10,24,46-
4
9].
3
+
The selectivity of detecting Fe was subsequently studied by UV-vis absorption and
fluorescence spectra (Fig. 4). Each TBB solution with equimolar metal ion had been deposited in
3
+
room temperature for three days. Delightedly, only Fe gave rise to an apparent change in both
absorption and fluorescence spectra, while the other 16 metal ions showed no obvious spectral
3
+
change. This difference is probably attributed to the unique interaction between TBB and Fe ,
3
+
which can be explained by the matching of cavity size and the radii of the guest Fe [39].
3
+
Practically, the specific response to Fe can also be visually distinguished in room light (Fig. S10).
For interference study, the result is expressed through fluorescence quenching efficiency (Fig. 5),
and it can be learnt that the quenching behaviors are almost preserved in the presence of these
3
+
metal ions. Hence, TBB is able to selectively detect Fe via optical spectra and naked-eye.
3
.3. Investigation of binding mode and mechanism
8

The design of star-shape TBB was envisaged to provide two or more binding centers for
3
+
coordinating with Fe . To verify such a proposed binding behavior, the Job’s plot experiment has
3
+
been conducted to explore the binding stoichiometry between TBB and Fe on the basis of the
reported method [14]. As displayed in Fig. 6, the maximum fluorescence intensity emerges at a
molar fraction of 0.33, indicating that the stoichiometry 1:2 is most likely binding mode for TBB
3
+
and Fe .
Depending on the aforementioned fluorometric titration data, the Benesi-Hildebrand plot in
terms of 1:2 stoichiometry presented in good linearity (Fig. S11), and the Benesi-Hildebrand
3
+ 2
0
equation refers to: 1/(I -I) = 1/{K
(I
a 0
-Imin)[Fe ] } + 1/(I
0
-Imin), where I
0
is the initial fluorescence
3
+
intensity of probe in the absence of Fe , Imin is the fluorescence intensity at saturation point. I is
3
+
the fluorescence intensity measured in presence of Fe . The calculated association constant value
5
-2
3+
K is 1.62 × 10 M . This clearly implies the strong affinity between TBB and Fe .
a
Furthermore, positive-ion ESI-MS spectrum of TBB with 100 equiv. Fe(NO
)
3 3
in water/THF
3
+
-
solution (v/v = 4/6) shows a peak at m/z 1011.56 (Fig. S12). It is assignable to [TBB + 2Fe + 6NO
3
+
+
+
CH
3
OH + H O + H ] [calcd, 1011.88]. This also evidently confirms the 1:2 ratio of the complex.
2
3
+
Moreover, since N atom is more likely to coordinate with Fe compared to S atom [11,37], one
3
+
3+
Fe may bind with two N atoms on two benzothiazole moieties, while another Fe may bind with
the residual N atom and one S atom as relatively weak coordination. Based on the above analyses,
3
+
the conceivable binding mode with the similar binding sites to the reported Fe probes [11,16] is
proposed in Scheme 2.
9

3
+
Scheme 2. The proposed binding mode of probe TBB and Fe .
On the basis of the previous UV-vis absorption studies, the fluorescence quenching may
involve a static process due to the formation of ground-state complexes and metal to ligand
3
+
charge transfer (MLCT) reaction between TBB and paramagnetic Fe [50-52]. To gain insight of
the quenching mechanism, we converted the fluorometric titration results into the form of Stern-
Volmer plot as shown in Fig. 7. It can be found that the plots show linearity at lower concentration
3
+
of Fe (0-25 μM), which stands for the involvement of static quenching through the formation of a
non-fluorescent ground-state complex at lower concentration, and the quenching constant (Ksv) is
3
-1
therefore calculated to be 3.512 × 10 M . On the other hand, a steep curve at a higher
3
+
concentration of Fe is possibly due to dynamic quenching [53-55].
To further ascertain the quenching process, the lifetime measurement of TBB was then
performed by time-correlated single photon counting (TCSPC) in the absence and presence of the
3
+
Fe (Fig. S13). The lifetime data are summarized into Table 1. The decay of the TBB solution is
3
+
single-exponentially fitted, and its lifetime (τ) is 1.21 ns. Once adding Fe of 20 and 50 equiv.
respectively, τ is reduced to 1.11 ns and 0.99 ns correspondingly. These results indicate that the
factor of collisional dynamic quenching exists in this process [53,55,56].
1
0

3
.4. Response time
Time-response is a vital evaluation for the properties of probes [15,57]. Hence, the time
3
+
dependence on the fluorescence intensity of TBB towards Fe in H O/THF (v/v = 4/6) was also
2
investigated and recorded in Fig. 8. It can be observed that the coordination mainly happens in the
first 50 seconds and finally reaches to steady state in the remained measured time. This
3
+
phenomenon sufficiently demonstrates that there is a quick response of TBB towards Fe , and the
3
+
response time is shorter than that of some reported Fe probes as real time detection
4,7,25,26,57] (Table S2).
.5. pH effect on detection of Fe3+
[
3
3
+
To explore the stability of the fluorescent detection for Fe , we have studied the
fluorescence intensity of TBB in H O/THF (v/v = 4/6) varying different pH values from 1 to 12 (Fig.
). To our delight, the fluorescence intensity of TBB solution exhibits ignorable variation in the
2
9
wide pH range of 3-12, although there is a sharp decrease in the pH range of 1-2. Perhaps, TBB
undergoes a protonation in strong acidic environment, leading to fluorescent change caused by
charge transfer. Consequently, TBB possesses good tolerability in medium-strong acidic and
3
+
alkaline solution. This result is superior to that of other reported Fe probes [10,15,24,38,46], and
enables TBB's detection to be suitable over most of the surrounding environment.
3+
3
.6. Determination of Fe in water samples
In order to further develop the practical application of TBB, a calibration curve for the
3
+
determination of Fe was constructed (Fig. S14), showing a good linear relationship between the
3
+
fluorescence intensity of TBB and Fe concentration (0-0.5 mM). Later on, drink water, tap water
1
1

and lake water were prepared as different real water samples and each of them was investigated
3
+
for 5 groups of known Fe concentration. As shown in Table 2, the obtained recoveries and
corresponding Relative Standard Deviation (R.S.D) are satisfactory. Thus, this directly confirms the
3
+
quantitative detecting ability of TBB toward Fe in real water samples as the reported [58,59].
4
. Conclusions
In summary, a star-shape probe based on benzothiazole moieties has been synthesized via
3
+
simple method. It can selectively detect Fe through fluorescence spectra over other metal ions in
aqueous system (H O/THF, v/v = 4/6), and its fast response of 50 s meets the requirement for real
2
3
+
time detection. Noteworthily, this probe possesses not only favorable stability for Fe detection in
wide pH range of 3-12, but also good performance in terms of quantitative determination for Fe3+
in real water samples.
Acknowledgements
We are grateful to the Guangzhou Science and Technology Project Scientific Special (General
Items, No. 201607010251), Guangdong Provincial Science and Technology Project (No.
2
017A010103016) and NSF of Guangdong Province (No. 2014A030313429) for financial support.
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Fig. 1. The fluorescence spectra (λex = 300 nm) of probe TBB (10 μM) in H O/THF system with
2
different water fractions.
1
9

Fig. 2. UV-vis absorption spectra of TBB (10 μM in H
2
O/THF, v/v = 4/6) with the addition of
3
+
different concentrations of Fe (0-0.5 mM). Inset, bottom right: Visual color change of TBB before
3+
and after addition of Fe to 0.5 mM.
Fig. 3. The emission spectra of probe TBB (10 μM in H
2
O/THF, v/v = 4/6) with the addition of
3
+
different concentrations of Fe (0-1 mM), λex = 300 nm.
2
0

Fig. 4. Comparison of UV-vis absorption (a) and fluorescent spectra (b) of TBB (10 μM in H O/THF,
2
v/v = 4/6) after the addition of 0.5 mM of various metal ions.
Fig. 5. Comparison of fluorescence quenching efficiency of TBB (10 μM in H O/THF, v/v = 4/6)
2
3
+
upon addition of different metal ions (0.5 mM), and with Fe (0.5 mM) in the presence of various
metal ions (0.5 mM).
2
1

3
+
Fig. 6. Job’s plot for the evolution of binding stoichiometry between TBB and Fe (in H
2
O/THF, v/v
=
4/6, λex = 300 nm).
3
+
Fig. 7. Stern-Volmer plots in response to Fe . Inset: Stern-Volmer plot obtained at lower
3
+
concentration of Fe .
2
2

Fig. 8. Time-dependent emission intensity (at 387 nm) of probe TBB in H O/THF (v/v = 4/6).
2
Fig. 9. Fluorescence responses at 387 nm for free TBB (10 μM) and after the addition of 100 equiv.
3
+
Fe in H O/THF (v/v = 4/6) as a function of different pH values.
2
2
3

3
+
Table 1 Fluorescence lifetime data for TBB with different equivalents of Fe .
Sample
TBB
τ (ns)
1.21
1.11
0.99
χ2
1.077
1.065
1.015
TBB + 20 equiv. Fe3+
TBB + 50 equiv. Fe3+
2
4

3
+
Table 2 Determination of Fe in water samples.
3
+
3+
Fe added (mM)
Fe found (mM)
0.0000
0.0993
0.2037
0.3197
0.4247
0.4953
0.0000
0.0967
0.1976
0.3047
0.4342
0.5306
0.0000
Recovery (%)
—
R.S.D (n=3) (%)
0.21
0
0
0
0
0
0
.1
.2
.3
.4
.5
99.3
2.50
101.9
106.6
106.2
99.2
2.73
Drink water
Tap water
Lake water
4.27
3.10
2.75
0
—
0.18
0
0
0
0
0
.1
.2
.3
.4
.5
99.6
1.51
98.8
0.47
101.6
108.6
106.1
—
3.32
2.71
6.09
0
0.23
0
0
0
0
0
.1
.2
.3
.4
.5
0.0968
0.2021
0.3040
0.4098
0.5222
96.8
1.98
101.0
101.3
102.4
104.4
0.92
1.44
2.16
3.48
2
5