A new fluorescent and colorimetric probe for trace hydrazine with a wide detection range in aqueous solution

Article abstract of DOI:10.1016/j.dyepig.2013.08.008

A new fluorescent and colorimetric probe based on the reaction and intramolecular charge transfer (ICT) effect is designed and synthesized. The probe responds rapidly toward hydrazine and exhibits distinct color changes from yellow to colorless, indicating its use as a color indicator for hydrazine. Moreover, the probe also shows a significant broad band fluorescence (410-700 nm) enhancement by ～120-fold after the addition of hydrazine. With a detection limit as low as 0.11 ppb, the probe can detect hydrazine in a wide concentration range because of the blunted sensing functional group. The contrast test shows almost no interruption from common elements in water, suggesting the high selectivity of this probe toward hydrazine. The theoretical calculation based on density functional theory (DFT) is also performed and two different ICT modes are found. This hydrazine probe would be a promising candidate applicable in environment protection, water treatment and safety inspection.

Full text of DOI:10.1016/j.dyepig.2013.08.008

Dyes and Pigments 99 (2013) 966e971
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A new fluorescent and colorimetric probe for trace hydrazine with
a wide detection range in aqueous solution
*
*
Yiqun Tan, Jiancan Yu, Junkuo Gao, Yuanjing Cui, Yu Yang , Guodong Qian
State Key Laboratory of Silicon Materials, Cyrus Tang Center for Sensor Materials and Applications, Department of Materials Science & Engineering, Zhejiang
University, Hangzhou 310027, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new fluorescent and colorimetric probe based on the reaction and intramolecular charge transfer (ICT)
effect is designed and synthesized. The probe responds rapidly toward hydrazine and exhibits distinct
color changes from yellow to colorless, indicating its use as a color indicator for hydrazine. Moreover, the
probe also shows a significant broad band fluorescence (410e700 nm) enhancement by w120-fold after
the addition of hydrazine. With a detection limit as low as 0.11 ppb, the probe can detect hydrazine in a
wide concentration range because of the blunted sensing functional group. The contrast test shows
almost no interruption from common elements in water, suggesting the high selectivity of this probe
toward hydrazine. The theoretical calculation based on density functional theory (DFT) is also performed
and two different ICT modes are found. This hydrazine probe would be a promising candidate applicable
in environment protection, water treatment and safety inspection.
Received 24 April 2013
Received in revised form
29 July 2013
Accepted 14 August 2013
Available online 22 August 2013
Keywords:
Chemosensor
Fluorescent probe
Hydrazine
Ó 2013 Elsevier Ltd. All rights reserved.
Fluorescence
Intramolecular charge transfer
Solvent effect
1. Introduction
chromatography and electrochemical detection [10,21,22], which
require either expensive experimental equipment or tedious
Hydrazine (N2H4), as a highly reactive base and strong reducing
agent, plays important roles in the synthesis of many chemicals,
such as pharmaceuticals, pesticides, photography chemicals and
various dyes [1e4]. In industry, it is often used as a chemical
blowing agent and corrosion inhibitor for heating system [5].
Because of its highly explosive nature, hydrazine is also known as
high energy fuel for propulsion systems in rocket and fuel cells
[6,7]. However, with such a wide application in industry, hydrazine
can easily cause environment pollution in the whole industrial
process like manufacture, use and disposal [8,9]. In fact, hydrazine
is toxic and readily absorbed by oral, dermal and inhalation uptake
which can potentially lead to gene mutation and cancer [10e12]. In
recent years, its damaging effect on lungs, kidneys, liver and central
nervous systems has been discovered [13e15].
operation processes. In recent years, fluorescence analysis system
has been widely used to detect various targets because of its ad-
vantages such as high efficiency, sensitivity, selectivity, economy,
simplicity and a large number of fluorescent sensors have been
reported for various analytes [23e27]. However, researches toward
developing small molecule fluorescent sensors for hydrazine have
been rarely reported [9,28]. Nevertheless, these current reports still
suffer from strong background signals, poor selectivity, high
detection limit and narrow measurement range. To solve this
problem, we develop a new fluorescent probe 2-(4-((4-(benzo[d]
thiazol-2-yl)phenyl)ethynyl)benzylidene)malononitrile (BP) for
hydrazine based on the ICT effect.
In this work, the photophysical properties, sensitivity and
selectivity of BP are reported. Studies on the sensing mechanism are
also carried out based on DFTand two different ICT modes are found.
Due to its environment toxicity, detonable characteristic and
widespread usage in industrial activities, development on reliable,
sensitive and selective methods for the detection of trace hydrazine
has attracted much attention [8,16e20]. Currently, a wide variety of
measurement techniques have been developed, such as
2. Experimental section
2.1. Materials and measurements
4-Bromobenzaldehyde, 2-aminothiophenol, 2-methyl-3-butyn-
2-ol and malononitrile were obtained from AladdinÒ with the
* Corresponding authors. Tel.: þ86 571 87952334; fax: þ86 571 87951234.
E-mail addresses: yuyang@zju.edu.cn (Y. Yang), gdqian@zju.edu.cn (G. Qian).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.08.008

Y. Tan et al. / Dyes and Pigments 99 (2013) 966e971
967
purity >98%. Cuprous iodide (CuI) and Bis(triphenylphosphine)
(500 MHz, CDCl3):
d
¼ 8.07 (d, 1H), 7.96 (m, 2H), 7.91 (d, 1H), 7.63
palladium(II) dichloride (Pd(PPh3)2Cl2) were purchased from En-
ergy Chemical with the purity of 98%. All the solvents are provided
by Sinopharm Chemical Reagent Co., Ltd with the purity >99.5%. All
the solvents and reagents were used as received without further
purification.
(m, 2H), 7.50 (s, 1H), 7.38 (m, 2H). Anal. Calcd for C13H8BrNS: C,
53.81; N, 4.83; H, 2.78. Found: C, 53.81; N, 4.82; H, 2.76. Mp:
132.7 ꢀC.
2.2.2. 4-(4-(Benzo[d]thiazol-2-yl)phenyl)-2-methylbut-3-yn-2-ol
(2)
1H NMR spectra were recorded in CDCl3 on a 500 MHz Bruker
Avance DMX500 spectrometer with tetramethylsilane (TMS) as an
internal standard. Elemental analysis was performed using a
Thermo Finnigan Flash EA1112 microelemental analyzer. Differen-
tial scanning calorimetry (DSC) was performed on a Netzsch In-
struments 200 F3 at a heating rate of 10 K/min under nitrogen
atmosphere. Fluorescence emission spectra and excitation spectra
were obtained on a Hitachi F4600 fluorescence spectrophotometer.
UVevis absorption spectra were obtained using a PerkineElmer
Lambda spectrophotometer. The Fluorescence decay curves were
measured by an Edinburgh Instrument F900. Fluorescence quan-
tum yield is measured with integrating sphere on Edinburgh In-
strument F900.
1 (1.44 g, 5 mmol), Pd(PPh3)2Cl2 (0.14 g, 0.2 mmol), 2-methyl-3-
butyn-2-ol (1.26 g, 15 mmol) and CuI (0.038 g, 0.2 mmol) were
added into a mixture of Et3N (2 mL) and toluene (8 mL) and
refluxed for 12 h. The solution was evaporated and purified by
chromatography using CH2Cl2 and EtOAC (20:1) as eluent, resulting
in white crystal 2 (0.64 g, 44%). 1H NMR (500 MHz, CDCl3):
d
¼ 8.06
(d, 1H), 8.03 (d, 2H), 7.92 (d, 1H), 7.54 (t, 3H), 7.40 (s, 1H), 2.06 (s,
1H), 1.65 (s, 6H).
2.2.3. 2-(4-Ethynylphenyl)benzo[d]thiazole (3)
A mixture of 2 (0.59 g, 2 mmol) and KOH (0.56 g, 10 mmol) in
toluene (10 mL) was refluxed for 2 h. The solution was then
extracted with CH2Cl2, and the organic phase was combined and
evaporated. The resulting black solid was purified by chromatog-
raphy, using CH2Cl2 and petroleum (1:1) as eluent, afforded light
All the theoretical calculations are performed based on DFT at
the B3LYP/6e31G(d) level [29,30]. The solvent effect on molecular
geometries is included by means of the polarizable continuum
model (PCM) [31,32]. Based on the optimized geometry, all the
molecular orbitals are calculated at the same level. All the calcu-
lations are performed in Gaussian 09 software [33].
yellow solid (0.42 g, 94%). 1H NMR (500 MHz, CDCl3):
d
¼ 8.06 (t,
3H), 7.92 (d, 1H), 7.62 (d, 2H), 7.52 (m, 1H), 7.41 (m, 1H), 3.23 (s, 1H).
Anal. Calcd for C15H9NS: C, 76.57; N, 5.95; H, 3.86. Found: C, 76.48;
N, 5.87; H, 4.14. Mp: 129.6 ꢀC.
2.2. Synthesis
2.2.4. 4-((4-(Benzo[d]thiazol-2-yl)phenyl)ethynyl)benzaldehyde
(4)
2.2.1. 2-(4-Bromophenyl)benzothiazole (1)
4-Bromobenzaldehyde (0.46 g, 2.5 mmol) was stirred into a
solution of 2-aminothiophenol (0.31 g, 2.5 mmol) in triethyl
phosphate (25 mL). After 10 min, acetic acid (2.5 mL) was added
into the solution and stirred for a further 10 min under temperature
60 ꢀC. Lead(IV) acetate (1.5 g, 3.4 mmol) was added with rapid
stirring. After stirred for a further 30 min, the solution was cooled to
room temperature and extracted with CH2Cl2. The organic phase
was collected and the solvent was evaporated. The resulting black
solid was purified by chromatography using CH2Cl2 and petroleum
(1:2) as eluent to obtain white solid 1 (0.46 g, 63%). 1H NMR
3 (0.94 g, 4 mmol) and 4-bromobenzaldehyde (0.74 g, 4 mmol)
were solved into a mixture of Et3N (4 mL) and toluene (16 mL). CuI
(0.038 g, 0.2 mmol) and Pd(PPh3)2Cl2 (0.14 g, 0.2 mmol) were
stirred into the solution and heated at 90 ꢀC for 12 h under argon
atmosphere. The resulting solution was evaporated and purified by
chromatography using CH2Cl2 and petroleum (1:1) as eluent to
yield yellow solid 4 (0.98 g, 72%). 1H NMR (500 MHz, CDCl3):
d
¼ 10.04 (s, 1H), 8.12 (t, 3H), 8.61 (m, 3H), 7.69 (m, 4H), 7.52 (t, 1H),
7.42 (t, 1H). Anal. Calcd for C22H13NOS: C, 77.85; N, 4.13; H, 3.86.
Found: C, 77.79; N, 4.18; H, 3.99. Mp: 138.6 ꢀC.
Scheme 1. Synthesis and proposed sensing mechanism of probe BP.

968
Y. Tan et al. / Dyes and Pigments 99 (2013) 966e971
2.2.5. 2-(4-((4-(Benzo[d]thiazol-2-yl)phenyl)ethynyl)benzylidene)-
malononitrile (BP)
Toluene
THF
Chloroform
Acetone
Acetonitrile
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
4 (1.36 g, 4 mmol) was solved into EtOH (20 mL) and the solution
was stirred at room temperature for 15 min. The mixture was then
added with malononitrile (0.30 g, 4.53 mmol) and refluxed for 2 h.
The resulting solution was evaporated and purified by chroma-
tography using CH2Cl2 as eluent to yield yellow solid BP (1.30 g,
84%). 1H NMR (500 MHz, CDCl3):
(t, 3H), 7.75 (s, 1H), 7.69 (d, 4H), 7.53 (t, 1H), 7.43 (t, 1H). Anal. Calcd
for C25H13N3S: C, 77.50; N, 10.85; H, 3.38. Found: C, 77.62; N, 11.01;
H, 3.52. Mp: 133.9 ꢀC.
d
¼ 8.13 (d, 2H), 8.10 (d, 1H), 7.93
350
400
450
500
550
600
650
700
3. Results and discussion
Wavelength (nm)
3.1. Design and synthesis
Fig. 1. Normalized absorption and relative fluorescence emission spectra of BP in
various solvents at 10 M.
m
ICT effect is a promising design strategy and has been widely
applied in the development of various fluorescent probes [34e36].
Probes baring ICT effect are often composed by conjugation of
donor and acceptor groups. Such a structure permits electrons
transfer from the donor to the acceptor part, leading to both red-
shift and intensity variation of fluorescence spectra [37e39].
Thereby, any changes in the donor or acceptor strength may cause
spectra alteration, which could be used to recognize the targeted
analytes. However, another interesting but easily neglected feature
of ICT mechanism is the conjugation length induced blunting. The
activity of the donor or acceptor would be reduced after combi-
nation with conjugated system, leading to the expansion of
detection range [24,40]. By taking the advantage of this charac-
teristic feature, a probe for hydrazine in a wide scope could be
realized.
Herein, as shown in Scheme 1, benzothiazole was chosen as the
electron acceptor because of its large conjugation system and
electron dislocation, which might enhance the ICT effect [36,41]. As
a strong electron withdrawing group and the sensing part for hy-
drazine, malononitrile is supposed to be replaced by hydrazine,
leading to different intramolecular electron redistribution of the
probe [28]. Thus both the absorption and fluorescence response can
be expected. Moreover, because of the large conjugated system of
the fluorophore, the reactivity of malononitrile, the sensing part,
will be significantly weakened, resulting in the broadening of the
detection range.
significant decrease with the increase of solvent polarity, while the
molar extinction coefficients remain almost unchanged.
3.3. Optical response
To test the practical utility of BP for the detection of hydrazine,
the absorption response of BP toward hydrazine was first studied in
the solution of THF. The probe showed almost intermediate color
change from light yellow to colorless. The UVevis spectroscopy
showed a slight shift from 395 nm to 368 nm, depending on the
gradual titration of hydrazine, as shown in Fig. 2. With the increase
of hydrazine concentration, the absorption peak of BP at 395 nm
decreased while that at 368 nm increased. The isosbestic point at
380 nm suggested the formation of a new substance, which might
be ascribed to inhibition of the ICT effect in the probe molecules
resulted from the substitution of malononitrile with hydrazine. The
titration measurement reached saturation with the addition of 12
equiv. hydrazine, indicating a much broader detection range. This
could be caused by the weakening of reaction activity resulting
from the large conjugated system.
Probe BH exhibited weak fluorescence emission at 460 nm upon
excitation at 395 nm in the solution of THF/water (1:1) at the
Table 1
Experimental photophysical properties of BP in different solvents.a
3.2. Photophysical properties
Sample
Solvent
3
b, ꢁ104
Fc
Stokes’
shift, nm
s d
, ns
L molꢂ1 cmꢂ1
3.6
As shown in Fig. 1, a significant increase on the Stokes’ shift with
the increase on the solvent polarity in the order of
toluene < THF < chloroform < acetone < acetonitrile can be seen
from the absorption and fluorescence spectra of BP in various sol-
BP
Toluene
CHCl3
44
42
41
29
27
71
s1 ¼ 0.38,
s2 ¼ 4.06
s1 ¼ 0.34,
s2 ¼ 3.96
s1 ¼ 0.31,
s2 ¼ 3.81
s1 ¼ 0.27,
s2 ¼ 2.15
s1 ¼ 0.27,
s2 ¼ 1.99
e
3.8
3.6
3.7
3.9
90
vents, which is attributed to the (pe
p*) transition of the conjugated
THF
90
system in the fluorescence probe [42], indicating a strong ICT effect
in the molecule during optical excitation [43e45]. The large Stokes’
shift and the correlation between solvent polarity and emission
color suggest the possible capability of BP as a fluorescent probe
potentially applicable in environmental detection.
The main photophysical parameters of BP are listed in Table 1.
With the increasing solvent polarity, the quantum yield of BP shows
a moderate decrease, from 0.44 in toluene to 0.27 in acetonitrile,
which can be ascribed to the relaxation of excited states caused by
solvent molecules [46]. This phenomenon is a common character of
fluorescent probes based on the ICT effect [47]. In the mixture of
THF/water (1:1), the fluorescence of BP could hardly be observed
for its poor quantum yield. Corresponding to the variations on
quantum yield, the fluorescent lifetime of BH also shows a
Acetone
Acetonitrile
145
168
THF/H2O (1:1)e
THF/H2O (1:1)
3.5
3.3
ef
e
g
BPeN2H4
52
55
s1 ¼ 0.57,
s2 ¼ 4.75
a
The concentrations of BP is 10
Molar distinction coefficient.
Quantum yield of the sample.
Fluorescence lifetime of BP and its reacted form of BPeN2H4.
The concentration of BP in THF/H2O is 10 nM.
The signal is too small to be observed.
mM in all the solvents except special stated.
b
c
d
e
f
g
The concentration of N2H4 is 120 nM.

Y. Tan et al. / Dyes and Pigments 99 (2013) 966e971
969
0 5
0 4
0 3
0 2
0 1
0 0
5000
0 equiv
2 equiv
10 equiv.
8 equiv.
4 equiv
6 equiv
8 equiv
10 equiv
12 equiv
4000
3000
2000
1000
0
6 equiv.
4 equiv.
2 equiv.
0
100
200
300
400
500
600
250
300
350
400
450
500
Time (s)
Wavelength (nm)
Fig. 4. Time related fluorescence intensity changes at 423 nm of BP in THF/water (1:1)
at 10 nM upon titration of hydrazine at 2, 4, 6, 8 and 10 equiv. The excitation wave-
length is 395 nm.
Fig. 2. Absorption spectra response of BP in THF at 10 nM toward hydrazine at
different equivalent.
concentration of 10 nM. However, after the addition of hydrazine,
drastic fluorescence enhancement (w100 times) at 423 nm could
be observed. Further titration experiments on BP were also per-
formed to test its fluorescence response. Upon the addition of hy-
drazine, gradual increase of fluorescence emission was exhibited.
The probe showed two fluorescence emission bands at 423 and
480 nm respectively, and get to saturation with the addition of 12
equiv. hydrazine. The dependence of fluorescence intensity on hy-
drazine amount in the range of 60e120 nM could be well fitted
could be as low as 3.4 nM (0.11 ppb), which was much lower than
the regulated threshold limit values (10 ppb) from U.S. Environ-
mental Protection Agency (EPA) [10].
To checkout its potential application in real-time detection, the
time dependent fluorescence changes of BP with different amount
of hydrazine were also studied, as shown in Fig. 4. The fluorescence
intensity of BP at 423 nm increased until saturated in 2e3 min.
To testify the selectivity of probe BP toward hydrazine, various
metal ions of environmental and biological interests such as Ni2þ
,
linearly, as shown in Fig. 3. Thus, the detection limit (3s/slope)
Ca2þ, Mg2þ, Al3þ, Zn2þ, Kþ, Pb2þ, Co2þ, Cd2þ, Cr3þ, Cu2þ and Naþ
were introduced to investigate their impact on the fluorescence
response of BP. As shown in Fig. 5, the relative fluorescence in-
tensity of BP remained almost unchanged after the addition of
metal ions. They did not exert interference with fluorescence
detection of hydrazine even at 10 equiv. either.
3.4. Theoretical calculations
To get insight into the sensing mechanism of BP toward hy-
drazine, computations on the probe before and after reacted with
hydrazine are performed based on density functional theory (DFT).
As shown in Fig. 6, electrons are mainly localized on the benzo-
thiazole part at the ground state and transfer to the malononitrile
accepter group after excitation, indicating stronger ICT effect. In this
process, the benzothiazole group plays a role of donor because its
Fig. 5. The fluorescence intensity at 423 nm of BP at 10 nM with various competing
Fig. 3. Fluorescence spectra of BP in THF/water (1:1) at 10 nM excited at 395 nm upon
ions upon excitation of 395 nm. The competing ions are at concentration of 1000 equiv.
titration of hydrazine.
(10 mM) and hydrazine is 12 equiv.

970
Y. Tan et al. / Dyes and Pigments 99 (2013) 966e971
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In summary, we have designed and synthesized a new fluores-
cent probe for hydrazine. The probe exhibits high sensitivity and
selectivity toward hydrazine with detection limit as low as 3.4 nM
(0.11 ppb). Meanwhile, based on ICT effect and large conjugated
system, detection in wide range of 6e10 equiv. can be realized. This
probe could also applicant in the real-time detection for its rapid
optical response to hydrazine, which could facilitate its application
on the environmental protection, emergency process.
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The authors gratefully acknowledge the financial support for
this work from the National Natural Science Foundation of China
(Nos. 51010002, 51272231 and 51229201), Natural Science Foun-
dation of Zhejiang Province (No. LY12E02004) and the Funda-
mental Research Funds for the Central Universities.
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