A highly selective dual-channel Hg2+ chemosensor based on an easy to prepare double naphthalene Schiff base

Article abstract of DOI:10.1007/s11426-012-4798-0

A non-sulfur chemosensor based on an easy to prepare double naphthalene Schiff base is reported for the colorimetric and fluorometric dual-channel sensing of Hg²⁺ ions by taking advantage of the twisted intramolecular charge transfer (TICT) mechanism. This work provides a novel approach for the selective recognition of mercury ions. Moreover, this sensor serves as a potential recyclable component in sensing materials and the complex L-Hg²⁺ (L = 1-[(2-naphthalenylimino)methyl]-2-naphthalenol) can therefore be used as a fluorescent sensor for iodine anions. Notably, the color changes are very significant and all the recognition and recycling processes can be observed by the naked eye.

Full text of DOI:10.1007/s11426-012-4798-0

SCIENCE CHINA
Chemistry
•
ARTICLES •
May 2013 Vol.56 No.5: 612–618
doi: 10.1007/s11426-012-4798-0
2
+
A highly selective dual-channel Hg chemosensor based on an easy
to prepare double naphthalene Schiff base
ZHANG YouMing, SHI BingBing, ZHANG Peng, HUO JianQiang, CHEN Pei, LIN Qi,
*
LIU Jun & WEI TaiBao
Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China; Key Laboratory of Polymer Materials of
Gansu Province; College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China
Received August 6, 2012; accepted September 17, 2012; published online December 24, 2012
A non-sulfur chemosensor based on an easy to prepare double naphthalene Schiff base is reported for the colorimetric and flu-
2
+
orometric dual-channel sensing of Hg ions by taking advantage of the twisted intramolecular charge transfer (TICT) mecha-
nism. This work provides a novel approach for the selective recognition of mercury ions. Moreover, this sensor serves as a po-
2
+
tential recyclable component in sensing materials and the complex L–Hg (L = 1-[(2-naphthalenylimino)methyl]-2-naphtha-
lenol) can therefore be used as a fluorescent sensor for iodineanions. Notably, the color changes are very significant and all the
recognition and recycling processes can be observed by the naked eye.
Non-sulfur, mercury ions, dual-channel, Schiff base, naked eye, iodine anion
1
Introduction
sulfur-rich environments where mercury is abundant [9],
and undesired oxidation of the probes containing a sulfur
atom by air or oxidizing reagents also inevitably occurs.
Other drawbacks may be the requirement for elevated tem-
Mercury is a toxin which poses great threats to our envi-
ronment [1]. The pollution caused by mercury has sparked
2+
2
+
perature or excess quantities of Hg to drive these desulfu-
rization reactions to completion. Recently, various pho-
toinduced electron transfer (PET) and intramolecular charge
interest in the design of new tactics to monitor Hg ions in
biological and environmental samples [2]. Numerous
chemosensors with good properties have been reported [3].
Most of these traditional chemosensors contain a sulfur
moiety and the sensing process involves an irreversible co-
ordination of Hg to the S atom [4]. The mechanism for
traditional sensors includes an affinity irreversible organic
reaction driven by the extremely strong Hg–S affinity: i)
ring opening of spirocyclic systems (rhodamine and fluo-
rescein, etc.) [5]; ii) intramolecular cyclic guanylation of
thiourea derivatives [6]; iii) conversion of thiocarbonyl
compounds into their carbonyl analogues [7] or a sequential
desulfurization-lactonization reaction [8] (Scheme 1). This
thiophilic approach will suffer from potential interference in
2+
transfer (ICT) based fluorescent sensors for Hg ion detec-
tion have been reported to have high selectivity and low
background disturbance [10]. In contrast to the frequently
used fluorescence signaling mechanisms of PET and ICT,
the concept of twisted intramolecular charge transfer
2+
(
TICT), proposed by Lippert and co-workers [11], has sel-
dom been utilized, possibly due to the difficulty of control-
ling two crucial factors, the degree of electron transfer and
the change of molecular geometry. Correspondingly,
TICT-based fluorescent chemosensors are still very scarce,
although in principle, well-designed ones should show very
good performance. Schiff base derivatives have been widely
used in ion detection for a long time [12] by virtue of their
simple structures and good recognition performance. How
*
Corresponding author (email: weitaibao@126.com)
©
Science China Press and Springer-Verlag Berlin Heidelberg 2012
chem.scichina.com www.springerlink.com

Zhang YM, et al. Sci China Chem May (2013) Vol.56 No.5
613
2.2 Synthesis of sensor molecule L
Compound L can be readily prepared by a simple and
low-cost Schiff base reaction of 2-hydroxy-1-naphthalde-
hyde with β-naphthylamine (Scheme 2). 2-hydroxy-1-
naphthaldehyde (0.346 g, 2 mmol), β-naphthylamine (0.358
g, 2.5 mmol) and catalytic amount of acetic acid (AcOH)
were combined in hot absolute ethanol (30 mL). The solu-
tion was stirred under reflux for 4 hours. After cooling to
room temperature, the yellow precipitate was filtered,
washed three times with hot absolute ethanol, then recrys-
2
+
Scheme 1 Tactics for the sensing of Hg .
2
+
ever, the detection of Hg ions based on Schiff base deriva-
tives has rarely been reported.
2
O to give a yellow powder product L
tallized with EtOH/H
1
(
1.7 mmol) in 85% yield (m.p.149 ℃), H NMR (DMSO-d ,
6
Our research group has a longstanding interest in molec-
ular recognition [13]. Herein, we report a non-sulfur [14],
dual-channel (colorimetric and fluorescent), simple and
easy to prepare double-naphthalene Schiff base chemosen-
4
7
00 MHz, ppm) δ 15.99 (s, 1H, OH), 9.83 (s, 1H, N=CH),
13
.038.55 (m, 13H, ArH). C NMR (DMSO-d , 100 MHz,
6
ppm) δ 171.10, 155.14, 141.07, 137.04, 136.98, 133.52,
31.51, 129.40, 129.01, 128.94, 127.77, 127.63, 126.86,
126.56, 125.84, 123.48, 122.31, 120.30, 119.59, 117.78,
1
2
+
sor (L, Scheme 2) for Hg ions, based on the TICT mecha-
nism. The two naphthalene groups act as fluorophores,
while the presence of hydroxyl and imine groups in the
same sensor molecule confers the coordination capacity
1
08.56. IR (KBr) v: 1614 (CH＝N), 1544 (C=C), 3030
1
(
ArH), 3444 (O–H) cm . Anal. Calcd. for C H NO: C,
21
15
8
4.84; H, 5.05; N, 4.71. Found C, 84.81, H, 5.05, N, 4.71.
2
+
required to coordinate Hg ions. Sensor L showed both
2
+
colorimetric and fluorescent selectivity for Hg in dimethyl
sulfoxide (DMSO) over other common physiologically im-
portant metal ions. L was almost non-luminescent in solu-
tion (the “off” state) (Figure 5), as a result of the presence
2
.3 General procedure for UV-Vis spectroscopy ex-
periments
All UV–Vis spectra were recorded on a Shimadzu UV–2550
spectrometer after the addition of perchlorate metal salts in
DMSO, while keeping the ligand concentration constant
(2.0 × 10 M). Solutions of metal ions were prepared from
the perchlorate salts of Fe , Hg , Ag , Ca , Cu , Co ,
Ni , Cd , Pb , Zn , Cr , and Mg .
2
+
of the TICT state. Upon the addition of Hg , the coordina-
2+
tion between Hg and the naphthalene groups in L restrains
the formation of the TICT state, and this leads directly to a
strong blue emission (the “on” state) (Figure 5). The detec-
5
3+
2+
+
2+
2+
2+
2
+
6
2+
2+
2+
2+
3+
2+
tion limit of the sensor towards Hg is 3.2 × 10 M, which
indicates that this sensor could potentially be used as a
2+
probe for Hg monitoring. To the best of our knowledge,
2.4 General procedure for fluorescence spectroscopy
2+
this is a novel approach for selective recognition of Hg
ions. Furthermore, the sensor is easy to produce.
experiments
All fluorescence spectra were recorded on a Shimadzu
RF-5301 fluorescence spectrometer after the addition of
perchlorate metal salts in DMSO, while keeping the ligand
concentration constant (5.0 × 10 M). The excitation
wavelength was 356 nm. Solutions of metal ions were pre-
2
Experimental
6
2
.1 Materials and physical methods
3+
2+
+
2+
pared from the perchlorate salts of Fe , Hg , Ag , Ca ,
Fresh double distilled water was used throughout the ex-
periments. All other reagents and solvents were commer-
cially available at analytical grade and were used without
2+
2+
2+
2+
2+
2+
3+
2+
Cu , Co , Ni , Cd , Pb , Zn , Cr , and Mg .
1
13
1
further purification. H NMR and C NMR spectra were
recorded on a Mercury-400BB spectrometer at 400 MHz for
2.5 General procedure for H NMR experiments
1
1
For H NMR titrations, two stock solutions were prepared in
H. Chemical shifts are reported in ppm downfield from
6
DMSO-d , one containing the sensor L and the second con-
tetramethylsilane (TMS, δ scale with solvent resonances as
internal standards). UV-Vis spectra were recorded on a
Shimadzu UV-2550 spectrometer. Photoluminescence spec-
tra were recorded on a Shimadzu RF-5301 fluorescence
spectrophotometer. Melting points (uncorrected) were
measured on an X-4 digital melting-point apparatus. Infra-
red spectra were recorded on a Digilab FTS-3000 FT-IR
spectrophotometer.
taining an appropriate concentration of the metal ion. Ali-
Scheme 2 Synthesis of the sensor molecule L.

614
Zhang YM, et al. Sci China Chem May (2013) Vol.56 No.5
quots of the two solutions were mixed directly in NMR
tubes.
3
Results and discussion
The recognition profiles of the chemosensor L toward vari-
3
+
2+
+
2+
2+
2+
2+
ous metal cations, Fe , Hg , Ag , Ca , Cu , Co , Ni ,
2
+
2+
2+
3+
2+
Cd , Pb , Zn , Cr , and Mg , were primarily investigat-
ed by UV-Vis spectroscopy and fluorescence spectroscopy
2
+
in DMSO. When 20 equiv. of Hg were added to a DMSO
solution of sensor L (0.1 µmol), there was a significant color
change, from yellow to colorless, which was visible to the
naked eye (Figure 1). In the UV-Vis spectrum of a solution
of L in DMSO (0.1 µmol), a strong and broad absorption
from 405 to 502 nm was disappeared when 20 equiv. of
2
+
Hg were added (Figure 1). To validate the selectivity of
3
+
+
sensor L, the same tests were carried out using Fe , Ag ,
Figure 2 (a) Absorbance emission data for a 1:20 mixture of L (0.1 μmol)
3+ 2+ + 2+
2
+
2+
2+
2+
2+
2+
2+
3+
2+
and different metal ions. 1) only L, 2) Fe , 3) Hg , 4) Ag , 5) Ca , 6)
Ca , Cu , Co , Ni , Cd , Pb , Zn , Cr and Mg met-
al ions. None of these ions induced any significant changes
in the UV-Vis spectrum of the sensor L (Figure 2).
The fluorescence measurements for sensor L were per-
formed in DMSO solutions. As shown in Figure 3, sensor L
emitted very weakly (λem = 413 nm; λex = 356 nm), indicat-
ing that the expected fluorescence of the two naphthalene
units was quenched by the formation of the TICT state [15].
2
+
2+
2+
2+
2+
2+
3+
2+
Cu , 7) Co , 8) Ni , 9) Cd , 10) Pb , 11) Zn , 12) Cr and 13) Mg ; as
their perchlorate salts, in DMSO solution, acquired after 180 min. (b) color
3+
2+
+
changes observed for L (0.1 µmol) upon the addition of Fe , Hg , Ag ,
2
+
2+
2+
2+
2+
2+
2+
3+
2+
Ca , Cu , Co , Ni , Cd , Pb , Zn , Cr and Mg (20 equiv.) in
DMSO solution.
2
+
On the addition of 10 equiv. of Hg ions, the fluorescence
intensity of sensor L (0.025 µmol) increased rapidly. The
color change from colorless to blue could be distinguished
by the naked eye as shown in Figure 3. To validate the se-
lectivity of sensor L, the same tests were carried out using
3
+
+
2+
2+
2+
2+
2+
2+
2+
3+
Fe , Ag , Ca , Cu , Co , Ni , Cd , Pb , Zn , Cr and
2
+
Mg metal ions. None of these ions induced any significant
changes in the fluorescence spectrum of the sensor (Figure
4
).
As shown in Figure 5, the two naphthalene units in free L
are nearly perpendicular. Thus, the TICT state of the rotors
leads to essentially complete quenching of the fluorescence
Figure 3 Fluorescence spectra upon excitation at 356 nm in DMSO of L
2
+
(
0.025 μmol) before and after additon of Hg (10 equiv.). Inset: photo-
2+
graphs showing the change in the fluorescence of L after addition of Hg
(10 equiv.) in DMSO.
2+
emission of L. Upon the addition of Hg , the interaction
2+
between Hg and L decreased the twisted intramolecular
charge transfer ability, as shown by the disappearance of the
2+
absorption peak, and the formation of the L-Hg complex
decreased the dihedral angle between the two naphthalene
units [16], thus resulting in decrease in electron transfer.
2+
Thus, the binding of Hg prohibits the TICT formation and
enhances the coplanarity of the conjugated system of L [17],
significantly promoting the strong blue fluorescence emis-
sion. Concurrently, the binding of Hg can interrupt the
conjugation of L according to the tautomeric equilibrium
Figure 1 Absorbance spectra of target compound L (0.1 μmol) in DMSO
2+
2
+
in the presence of Hg (20 equiv.). Inset: photograph showing the change
2
+
in color of the solution of L in DMSO after addition of Hg (20 equiv.).

Zhang YM, et al. Sci China Chem May (2013) Vol.56 No.5
615
1
Figure 6 Partial H NMR spectra (400 MHz, DMSO-d
6
) of free L and in
2
+
the presence of varying amounts of Hg .
Figure 4 Fluorescence emission data for a 1:10 mixture of L (0.025 μmol)
3
+
2+
+
2+
and different metal ions. 1) only L, 2) Fe , 3) Hg , 4) Ag , 5) Ca , 6)
2
+
2+
2+
2+
2+
2+
3+
2+
Cu , 7) Co , 8) Ni , 9) Cd , 10) Pb , 11) Zn , 12) Cr and 13) Mg ; as
their perchlorate salts, in DMSO solution, acquired after 180 min. (λem
56 nm). Inset: Visual fluorescence emissions of probe L (0.1 μmol) after
=
3
3
+
2+
+
2+
2+
2+
2+
2+
2+
2+
the addition of Fe , Hg , Ag , Ca , Cu , Co , Ni , Cd , Pb , Zn ,
3
+
2+
Cr and Mg (10 equiv.) in DMSO on excitation at 365 nm using UV
lamp at room temperature.
2
+
Figure 5 Hg ions suppress the TICT process for L facilitating the de-
2
+
tection of Hg ions.
occurring during the ion recognition process. Correspond-
ingly, the color of the solution containing the sensor L
2+
changed from yellow to colorless after addition of Hg
ions.
The interaction and binding behavior between L and
2
+
1
Hg ions were investigated by H NMR spectroscopy (Fig-
ure 6). There is one intramolecular hydrogen bond in the
molecular structure of L: OH···N=C [18]. The formation of
1
Figure 7 (a) Absorbance spectra of L (0.1 μmol) in the presence of dif-
ferent concentrations of Hg (0.008.0 equiv.) in DMSO, acquired after
this strong hydrogen bond leads to the H NMR chemical
2+
shift of the OH group appearing at very low-field, 16.0 ppm.
1
80 min; (b) a plot of the variation in absorbance as a function of the con-
2
+
2+
After adding 0.5 equiv. of Hg , the OH peak at 16.0 ppm
disappeared and two new peaks appeared at 12.0 and 10.8
centration of Hg in the range from 0 to 8.0 equiv. Each measurement was
performed after mixing L and Hg in DMSO for 180 min. The detection
wavelength was 446 nm.
2+
a b
ppm, which can be attributed to the NH and NH protons,
respectively. Meanwhile, the signal of the hydrogen atoms
in the benzene rings showed a significant downfield shift,
indicating a charge transfer from the benzene groups to the
2
+
with the continuous addition of Hg (1.0 and 2.0 equiv.),
the resonance at 16.0 ppm for OH disappeared completely,
2
+
Hg ions [19]. There is also one intramolecular hydrogen
a b
and at the same time, the signals of NH and NH protons
2
+
1
bond in the molecular structure of L-Hg : NH
formation of this hydrogen bond leads to the H NMR
chemical shift of NH appearing at low-field. Additionally,
a
···O=C. The
appeared at 12.0 and 10.8 ppm. Thus, the results of the H
NMR titration suggested that the tautomeric equilibrium
was attained during the ion recognition process.
1
a

616
Zhang YM, et al. Sci China Chem May (2013) Vol.56 No.5
2
+
Figure 8 A job plot for the reaction between Hg and L, clearly indicat-
2
+
ing the 2 : 1 stoichiometry for L–Hg .
Figure 10 Time-dependent absorbance emission spectra of L (0.1 µmol)
2
+
upon addition of Hg (20 equiv.) in DMSO. (a) Absorbance emission
spectra: from top to bottom spectra were recorded after 0, 10, 20, 35, 50,
6
5, 80, 95, 110, 125, 140, 155, 170, and 185 min; (b) a plot of absorbance
as estimated by the peak height at 449 nm.
Figure 9 Time-dependent fluorescence emission spectra of L (0.025
2
+
µmol) upon addition of Hg (10 equiv.) in DMSO. (a) Fluorescence emis-
sion spectra: from bottom to top the spectra was recorded after 0, 10, 20,
Figure 11 Fluorescence emission spectra of L (0.025 µmol) in the pres-
ence of Hg (10 equiv.) or KI (1.8 mmol) in DMSO solution. The excita-
3
0, 40, 60, 70, 90, 115, 135, 150, 170, 190, and 220 min. λem = 356 nm; (b)
2+
a plot of fluorescence intensity as estimated by the peak height at 413 nm.
tion wavelength was 356 nm. Inset: photograph from left to right showing
2
+
2+
the change in the fluorescence of only L, L–Hg (10 equiv.) and L–Hg
10 equiv.) plus KI (1.8 mmol) in DMSO on excitation at 365 nm.
(
To further investigate the interaction between sensor L
2+
and Hg , the variation in the UV-Vis absorption spectral of
5
μmol. As shown in Figure 7, an isosbestic point at 353 nm
sensor L (2.0 × 10 M) in DMSO was recorded during the
2+
2
+
was clearly observed with increasing concentrations of Hg .
titrations with different concentrations of Hg from 0 to 0.2

Zhang YM, et al. Sci China Chem May (2013) Vol.56 No.5
617
2
+
Innovative Research Teams in Universities of the Ministry of Education of
China (IRT1177).
The association constant K
a
of the sensor L toward Hg
4
1
was 7.64 × 10 M , indicating that sensor L reacts with
Hg to form a stable complex. At the same time, fluores-
cence emission spectral variation of sensor L (5.0 × 10 M)
in DMSO was monitored during titrations with different
concentrations of Hg from 0 to 0.25 μmol (0, 0.6, 2.5, 3.2,
.0, 4.8, 5.6, 6.4, 7.2 and 9.6 equiv., respectively). The se-
lectivity of L for Hg over other metal ions was examined.
The results revealed that all potentially competitive metal
cations exerted no or little influence on the absorbance and
fluorescence detection of Hg in DMSO. A job plot was
implemented, demonstrating a 2/1 stoichiometry for the
2
+
−
6
1
2
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mixture of L and Hg(ClO ) in DMSO showed that the flu-
4
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orescence intensity (λmax = 413 nm) increased gradually
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(
reached > 90% of the maximum after 150 min. Meanwhile,
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by titration of the mercury-sensor complex with KI [17].
4
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The I ion is well-known to bind strongly to Hg ions [20].
2
+
2+
Addition of KI to a solution of the L–Hg complex induced
the opposite trend in the fluorescent spectra to that observed
on titration with Hg . Upon addition of 1.8 mmol of KI, the
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4
Conclusions
2
+
By taking advantage of the TICT mechanism, we have de-
veloped a highly selective dual-channel chemosensor for
mercury(II) ions based on a non-sulfur, simple, double
naphthalene Schiff base compound. This work highlights a
new approach to mercury ion detection. Notably, the chem-
ically reversible binding of L to Hg showed that L could
serve as a potential recyclable component in sensing mate-
rials. The complex L–Hg can thus be considered as a good
candidate for use in an iodine anion sensor. Moreover, this
work may stimulate the further development of new
chemosensors possessing excellent performance.
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