A highly reactive (<1 min) ratiometric chemodosimeter for selective "naked eye" and fluorogenic detection of hydrazine

Article abstract of DOI:10.1039/c3ra42771d

Hydrazine is an important industrial chemical but also very toxic. Thus rapid detection of hydrazine is very important. We have judiciously designed and constructed a novel ICT-based ratiometric "naked eye" and fluorescence smart probe, carbazole based malononitrile (CBM), that rapidly (<1 min) and selectively detects hydrazine in the presence of different metal ions, anions and other amines in aqueous medium. As a possible application of the probe, hydrazine sensing in tap water was tested. The probe also shows an excellent performance in the "dip stick" method. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra42771d

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A highly reactive (,1 min) ratiometric chemodosimeter
for selective ‘‘naked eye’’ and fluorogenic detection of
hydrazine3
Cite this: RSC Advances, 2013, 3,
18872
Shyamaprosad Goswami,* Sima Paul and Abhishek Manna
Hydrazine is an important industrial chemical but also very toxic. Thus rapid detection of hydrazine is very
important. We have judiciously designed and constructed a novel ICT-based ratiometric ‘‘naked eye’’ and
fluorescence smart probe, carbazole based malononitrile (CBM), that rapidly (,1 min) and selectively
detects hydrazine in the presence of different metal ions, anions and other amines in aqueous medium. As
a possible application of the probe, hydrazine sensing in tap water was tested. The probe also shows an
excellent performance in the ‘‘dip stick’’ method.
Received 5th June 2013,
Accepted 8th August 2013
DOI: 10.1039/c3ra42771d
www.rsc.org/advances
Many different methods have been developed for the
detection of hydrazine. Traditional methods, including elec-
Introduction
trochemical analysis,10 chromatography,11 gas chromatogra-
phy,12 HPLC,13 and capillary electrophoresis,14 require long
procedure times and the involvement of intelligent instru-
ments, whereas chemical sensors provide another approach
which is simple, inexpensive, and rapid allowing real-time
monitoring. Spectrophotometry using colored derivatives,
such as p-dimethylaminobenzaldehyde15 and chlorosalicylal-
dehyde,16 and other colorimetric systems17 have also been
reported as analytical methods to detect hydrazine. The
‘‘naked eye’’ colorimetric sensors are especially promising
because the color change can easily be observed with the
naked-eye, thus requiring less labor and no equipment. On the
other hand, ratiometric colorimetric probes can enable the
measurement of absorption intensities at two different
wavelengths, providing a built-in correction for environmental
effects and increasing the dynamic range of absorption
measurements. This was considered as a good approach to
overcome the major limitation of intensity based probes, in
which variations in the environmental sample and probe
distribution were problematic for quantitative estimation.
Therefore, the design of ratiometric probes is currently of
great interest. In general, ratiometric probes can be designed
to function via two mechanisms: intramolecular charge
transfer (ICT) and fluorescence resonance energy transfer
(FRET). Although good ratiometric responses could be
frequently achieved in some FRET-based probes, generally
they need long pathways for their synthesis as well as strong
spectral overlap between the emission of the donor and the
absorption of the acceptor. On the other hand, ICT-based
ratiometric probes are structurally simple and have the
advantages that they are easy-to-make and have large emission
The design and synthesis of new molecular sensors towards
biologically and environmentally important species are always
essential for practical research in various fields of science.1
Fluorescent chemosensors are used as powerful tools to detect
neutral and ionic species owing to high selectivity, sensibility,
versatility and relatively simple handling.2 In these regards,
the design and synthesis of chemosensors is currently of great
interest.3 Hydrazine is a highly reactive base and reducing
agent4 and is used as an important reactant in the preparation
of pharmaceuticals, pesticides, corrosion inhibitors, photo-
graphy chemicals, emulsifiers and textile dyes in various
chemical industries.5 It is famous as a high-energy fuel in
rocket propulsion systems due to its flammable and detonable
characteristics.6 In industry it is often applied as a chemical
blowing agent. Hydrazine and its water solutions, however, are
highly toxic to humans and animals and can potentially lead to
serious environmental contamination during their manufac-
ture, use, transport and disposal.7 It is readily absorbed by
oral, dermal, or inhalation routes of exposure. Indeed, it has
been classified as a probable human carcinogen by the U.S.
Environmental Protection Agency (EPA) and has a low thresh-
old limit value (TLV) of 10 ppb.8 In contrast to its usefulness,
the carcinogenic and toxic effects of hydrazine potentially lead
to serious environmental contamination.9 In this regard, the
design and synthesis of hydrazine probes have attracted much
attention in recent years and are still in high demand.
Bengal Engineering and Science University, Shibpur, Howrah 711103, India.
E-mail: spgoswamical@yahoo.com; Fax: +91-3326682916
Electronic supplementary information (ESI) available: details of the synthetic
3
procedure with characterisation and spectral data are available. See DOI:
10.1039/c3ra42771d
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Experimental section
General
The chemicals and solvents were purchased from Sigma-
Aldrich Chemicals Private Limited and were used without
further purification. 1H NMR and 13C NMR spectra were
recorded on Brucker 400 MHz instruments. For NMR spectra,
d6-DMSO was used as the solvent with TMS as an internal
Scheme 1 Schematic approach in this work for the detection of hydrazine.
1
standard. Chemical shifts are expressed in d units and H–1H
Hz. UV-vis titration experiments were performed on a JASCO
UV-V530 spectrophotometer and fluorescence experiments
were done using a PTI (Photon Technology International)
fluorescence spectrophotometer with a fluorescence cell of 10
mm path.
shifts etc. However, to date, small-molecule fluorescent probes
for hydrazine detection are still very limited.18
Recently, Fan et al. reported interesting results of hydrazine
detection using a ratiometric turn-on type coumarin-based
fluorescent sensor.19 Now, considering all the above facts, we
report a ‘‘naked eye’’ ratiometric, colorimetric and fluorescent
probe based on a carbazole fluorescence moiety with a
dicyano-vinyl group as the reaction site containing multiple
binding groups. We introduced a dicyano vinyl group (electron
acceptor) into carbazole dialdehyde to get the carbazole based
malononitrile (CBM) probe, which can construct an ICT
system with charge from a carbazole moiety (electron donor)
from the donor proceeding to the acceptor upon excitation.
The specific reaction between arylidenemalononitrile and
hydrazine yielded the product hydrazone, which has also been
reported.20 Due to formation of hydrazone, the ICT process in
the CBM stopped and thus the molecule becomes fluorescent,
which helped us to detect hydrazine by using the CBM probe
(Scheme 1).
Methods for the preparation of the receptor
Synthesis of the receptor (CBM). Under an atmosphere of
dry nitrogen, carbazoledialdehyde (compound B, 300 mg, 1.07
mmol) and malononitrile (140 mg, 2.12 mmol) were refluxed
in absolute ethanol (15 mL) overnight with a trace of Et3N as a
catalyst. Then, the resultant mixture was cooled to room
temperature and the solvent was removed under reduced
pressure. The resultant residue was purified by silica gel
column chromatography using petroleum ether–dichloro-
methane (1 : 7, v/v) as eluent to afford carbazole based
malononitrile (CBM) as a deep yellow solid (299 mg, 80%).
1H NMR (CDCl3, 400 MHz): d (ppm): 8.032 (s, 1H), 7.852 (s,
1H), 7.527 (m, 2H), 7.376 (m, 2H), 4.364 (t, 2H, J = 9.6 Hz),
1.882 (m, 2H), 1.395 (m, 2H), 0.978 (t, 3H, J = 4.8 Hz).
13C NMR (CDCl3, 100 MHz): d (ppm): 156.55, 137.05, 130.57,
119.42, 118.15, 113.05, 111.51, 110.17, 69.09, 60.60, 32.14,
20.52, 13.82.
A convenient synthetic route to chemodosimeter CBM was
developed as shown in Scheme 2. Intermediate compounds
A21 and B22 were synthesized according to the reported
procedures and condensation of compound B with excess
malononitrile under refluxing conditions in ethanol using
Et3N as a catalyst afforded the desired designed target receptor
CBM. The detailed procedure of its preparation is discussed in
MS (ESI MS): (m/z, %): 375.24 [M+, 100%].
Elemental analysis: C = 76.74%, H = 4.55%, N = 18.71%
(calculated values: C = 76.78%, H = 4.56%, N = 18.65%).
Synthesis of hydrazine adducted CBM. CBM is mixed with
two equivalents hydrazine in acetonitrile at room temperature
to give a colorless solution. On removing the solvent, a solid
product was obtained, which was used for 1H NMR, elemental
analysis and mass spectroscopy.
the ESI . The structure of the CBM probe was confirmed by 1H
3
NMR, 13C NMR, ESI MS and elemental analysis (ESI ).
3
1H NMR (CDCl3, 400 MHz): d (ppm): 8.622 (s, 1H), 8.150 (s,
1H), 7.540 (m, 2H), 7.376 (m, 2H), 4.364 (t, 2H, J = 9.6 Hz),
2.052 (s, 4H), 1.882 (m, 2H), 1.395 (m, 2H), 0.979 (t, 3H, J = 4.8
Hz).
MS (ESI MS): (m/z, %): 307.52 [M+, 100%].
Elemental analysis: C = 70.35%, H = 6.87%, N = 22.78%
(calculated values: C = 70.33%, H = 6.89%, N = 22.78%).
Determination of fluorescence quantum yield
Here, the quantum yield Q was measured by using the
following equation,
2
2
Qx = Qs (Fx/Fs)(As/Ax)(nx /ns )
where, X & S indicate the unknown and standard solution
respectively, Q = quantum yield, F = area under the emission curve,
A = absorbance at the excitation wavelength and n = index of
Scheme 2 Synthetic route of the receptor (CBM). Reagents and conditions: (a)
butylbromide, K2CO3, KI, CH3CN, heated to 80 uC, 4 h; (b) POCl3, DMF, 100 uC,
20 h; (c) excess malononitrile, ethanol, Et3N (catalyst), reflux, N2 atmosphere,
overnight.
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refraction of the solvent. Here Q measurements were performed
using anthracene in ethanol as standard [Q = 0.27] (error y 10%).
Calculation of the detection limit
The detection limit (DL) of CBM for hydrazine was determined
from the following equation:23
DL = K 6 Sb1/S
where K = 2 or 3 (we take 2 in this case), Sb1 is the standard
deviation of the blank solution and S is the slope of the calibration
curve. Using the formula we get DL = 1.02 mM and details of
calculation of the detection limit is given in the supporting
information (Fig. S4, ESI ).
3
Method for the preparation of TLC plate sticks
Thin layer chromatography (TLC) plate sticks were easily
prepared by immersing a TLC plate into a solution of CBM (2
6 1024 M) in CH3CN (1 mM) and then exposing it to air to
evaporate the solvent. The detection of hydrazine was carried
out by inserting the TLC plate into different concentrations of
hydrazine solution (Fig. 7a and Fig. S5, ESI ) and evaporating
3
the solvent to dryness.
General method of UV-vis and fluorescence titrations
UV-vis and fluorescence method. For UV-vis and fluores-
cence titrations, a stock solution of the sensor was prepared (2
6 1025 M) in CH3CN–H2O (8 : 2, v/v). The solution of the
guest cation was prepared (2 6 1024 M) in CH3CN–H2O (8 : 2,
v/v) at pH 7.4 by using 10 mM HEPES buffer. The original
volume of the receptor solution was 2 ml. Solutions of the
sensor of various concentrations and increasing concentra-
tions of cations, anions and amine containing compounds
were prepared separately. The spectra of these solutions were
recorded by means of UV-vis and fluorescence methods.
Theoretical and computational methods. All the calcula-
tions were performed using the Gaussian 0324 program using
the B3LYP25 exchange correlation functional with a cc-pvTZ
basis set (triple f quality for diffused orbital electrons and lone
pairs).
Fig. 1 (a) UV-vis absorption titration spectra of CBM (2.0 6 1025 M) in the
presence of hydrazine (2.0 6 1024 M) at pH 7.4 in CH3CN–H2O (8 : 2, v/v)
[gradual addition of 0, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 20, 20, 20, 20 and
20 mL hydrazine, respectively]. The inset shows the naked eye colour change of
CBM with addition of hydrazine. (b) Time-dependent absorption intensity of
CBM at 405 nm (in red) and 258 nm (in black) in the presence of hydrazine (2
equiv.).
Br2, I2, ClO42, SO322, SO4
and HPO4
resulted in a
22
22
negligible response. These results suggest that CBM can serve
as a highly selective chemodosimeter for hydrazine (Fig. S2a,
Results and discussion
ESI ). Under the same conditions, the ratio change of the two
3
UV-vis and fluorescence study
absorption peaks (A258/A405) produced an excellent linear
function with the concentration of hydrazine between 0.995
The chromogenic and fluorogenic signaling behavior of CBM
was investigated in a CH3CN–H2O solution (8 : 2, v/v, 10 mM
HEPES, pH 7.4). As shown in Fig. 1, without hydrazine the
CBM probe showed absorption bands centered at 405, 324 and
258 nm. Upon addition of hydrazine, the absorption bands at
405 and 324 nm diminished and the band at 258 nm
increased. The presence of well-defined isosbestic points at
313 nm indicates the formation of the hydrazone product. The
color of the solution changes from yellow to colorless upon
addition of hydrazine, as shown in Fig. 1a (inset) which allows
the detection of hydrazine with the naked eye.
and 20.224 mM (R2 = 0.989, Fig. S1a, ESI ).
3
The detection limit for hydrazine was 1.02 mM based on the
equation23 DL = K 6 Sb1/S, where Sb1 is the standard
deviation of blank measurements and S is the slope of the
calibration curve (ESI ), which is much lower than the TLV (10
3
ppb) set by the EPA. This indicated that the CBM smart probe
could be used to quantitatively detect hydrazine concentration
in a relatively wide range. Because the optical signal changes
relied on the chemical reaction between the CBM probe and
hydrazine, the reaction rate might affect the experimental
results. We therefore investigated the influence of the reaction
time on the probing results (Fig. 1b). From the reaction curve
In contrast, the addition of other metal ions and anions,
namely Ag+, Cd2+, Co2+, Cu2+, Fe3+, Mn2+, Na+, Pd2+, Zn2+, Cl2,
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Fig. 2 Fluorescence emission spectra of CBM (2.0 6 1025 M) with hydrazine
(2.0 6 1024 M) at pH 7.4 in CH3CN–H2O (8 : 2, v/v) [gradual addition of 0, 10,
10, 20, 20, 20, 20, 20, 30 and 50 ml hydrazine, respectively]. The inset shows the
naked eye fluorescence change of CBM with addition of hydrazine.
Fig. 3 Fluorescence responses of CBM (2.0 6 1025 M) to hydrazine (2 equiv.)
containing 10 equiv. of various metal ions and anions at 413 nm (lex = 313 nm).
we can figure out that a plateau of ratiometric absorption
intensity changes is achieved within 1 min, indicating that the
sensor can achieve real-time detection of hydrazine.
(in CBM + Hyd), respectively, and a new resonance signal at d =
2.052 ppm appeared corresponding to amine hydrogen
(Fig. 5). Comparing the 13C NMR spectra of CBM and
corresponding product, we found that two peaks at 110.17
and 69.09 corresponding to malononitrilic carbon disappeared
and the peaks at 156.55, 137.05 and 130.57 shifted to 143.35,
132.05 and 124.42, respectively. The CBM sensor displayed a
characteristic m/z peak at 375.24. However, after the addition
of an excess amount of hydrazine this peak totally disap-
peared. At the same time, a new m/z peak at 307.52 assigned to
the corresponding hydrazone compound appeared. All these
results strongly demonstrated that the probe is highly effective
for hydrazine.
As shown in Fig. 2a, the CBM chemodosimeter exhibited
very weak fluorescence bands centered at 356 and 458 nm (Q =
0.123) upon excitation at 313 nm in the absence of hydrazine
or in the presence of other metal ions and anions, namely Ag+,
Cd2+, Co2+, Cu2+, Fe3+, Mn2+, Na+, Pd2+, Zn2+, Cl2, Br2, I2,
22
ClO42, SO322, SO4 and HPO422. Only the solution contain-
ing hydrazine showed remarkable change. The peak at 458 nm
shifted to 413 nm (Q = 0.466) accompanied by a color change
from light yellow to sky blue. This peak gradually increased in
intensity with gradual addition of hydrazine (2 6 1024 M). A
linear relationship was observed between the fluorescence
intensity and concentration of hydrazine in the range of 1.98–
DFT calculation. Meanwhile, the calculation based on
density functional theory (DFT) using the cc-pvTZ basis set
(triple f quality for diffused orbital electrons and lone pairs)
and the B3LYP hybrid functional method for CBM and the
corresponding hydrazone product was performed to get
18.18 mM (Fig. S1b, ESI ).
3
Next, ion interference experiments were carried out both in
buffered aqueous solution for evaluation of the sensing ability
of the CBM chemosensor (1 mM) towards hydrazine and in the
presence of 10 equiv. of a series of background metal ions and
anions by means of fluorescence spectra (Fig. 3). Clearly, the
presence of background metal ions and anions did not cause
any significant fluorescence change of CBM. This showed that
the CBM probe could selectively detect hydrazine in the
presence of other metal ions and anions.
Spectral study
The CBM chemosensor was also treated with some amine
containing compounds. It was found that these compounds
caused no obvious interference of probe CBM in aqueous
CH CN solution (Fig. S3, ESI ). In the time dependent
3
3
absorption spectra, we see the reaction is complete within 60
s with a rate constant of 5.2 6 1022 21, which strongly
s
supports a high reactivity of the probe (Fig. 4).
The ability of CBM to detect hydrazine was studied by 1H
NMR, 13C NMR and ESI TOF mass spectra. The 1H NMR
titration experiment confirms the corresponding hydrazone
product formation by the reaction between CBM and
hydrazine, by following the progressive shift of the protons
at 8.032 and 7.852 ppm (in CBM) towards 8.622 and 8.150 ppm
Fig. 4 (a) The time vs. fluorescence spectra of CBM (2.0 6 1025 M) in the
presence of hydrazine (2.0 6 1024 M) at pH 7.4 in CH3CN–H2O (8 : 2, v/v) at
different times: (a) 0, (b) 10, (c) 20, (d) 30, (e) 40, (f) 50, (g) 60 s. (b) The
fluorescence intensity vs. time at a fixed wavelength (413 nm) plotted using the
first order rate equation.
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Fig. 7 (a) Color changes of CBM on test paper in the absence and presence of
hydrazine under ambient and UV light. (b) Fluorescence detection of hydrazine
in distilled water and tap water by CBM. [CBM] = 2.0 6 1026 M, [hydrazine] =
from 0 to 2.0 6 1025 M at pH 7.4 in CH3CN–H2O (8 : 2, v/v). lex = 313 nm.
Fig. 5 Partial 1H NMR spectra of CBM and CBM + Hyd in CDCl3.
insight into the optical response. Comparing the level changes
of the HOMO (the highest occupied molecular orbitals) and
LUMO (the lowest unoccupied molecular orbitals) in CBM and
the corresponding hydrazone product, respectively, due to the
ICT effect, after reaction with hydrazine both of them
increased (Fig. 6). The LUMOs increased much more. The
HOMO–LUMO energy gaps were calculated as 2.9617 eV and
3.016 eV for CBM and the corresponding hydrazone product,
respectively; i.e. the energy gap between the HOMO and LUMO
of the corresponding product was larger than that of CBM.
This is in good agreement with the remarkable blue shift in
the absorption spectra.
widely used in various industrial processes and thus hydrazine
detection in aqueous samples is of interest. We analyzed
hydrazine in tap water and distilled water. An aliquot of
hydrazine was added to tap and distilled water and the pH was
adjusted to 7.4. The results of the recovery of hydrazine by
CBM from these two water samples were compared (Fig. 7b).
The analysis of hydrazine in both solutions agreed well at
hydrazine concentrations up to 2.0 6 1025 M. We also
prepared the TLC plates of CBM to determine the suitability of
a ‘‘dip-stick’’ method for the detection of hydrazine (Fig. 7a).
Conclusions
Applications
We also explored opportunities for CBM in practical applica-
tions because hydrazine is a suspected carcinogen and is
A new ‘‘naked-eye’’ ratiometric colorimetric and fluorescence
chemodosimeter (CBM) was constructed by taking advantage
of the formation of the specific product hydrazone by the
reaction between arylidenemalononitrile and hydrazine. CBM
displayed high sensitivity and selectivity for hydrazine with
respect to other metal ions and anions. The probe demon-
strated some advantages such as good sensitivity and high
selectivity for hydrazine, relatively good solubility in aqueous
media and good ratiometric response, as well as the successful
application in real water samples. The sensing event is
explained by spectroscopy along with the DFT calculation. In
addition, the probe could serve as a practical colorimetric
sensor for ‘‘in-the-field’’ measurement, which does not require
any additional equipment, just using a ‘‘dip-stick’’ approach.
Acknowledgements
Authors thank the DST and CSIR (Govt. of India) for financial
support. S.P. and A.M. acknowledge the UGC and CSIR
respectively for providing fellowships. Authors also specially
thank Aniruddha Ganguly for the DFT calculation.
Fig. 6 HOMO–LUMO energy levels and interfacial plots of the orbitals for CBM
and the corresponding hydrazone product (CBM + Hyd).
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