Azobenzene-based colorimetric chemosensors for rapid naked-eye detection of mercury(II)

Article abstract of DOI:10.1002/chem.201003275

Two new highly selective colorimetric chemosensors for Hg²⁺, based on azobenzene and highly selective Hg²⁺-promoted deprotection of a dithioacetal have been designed and synthesized. In the presence of as little as 20 μM Hg²⁺

Full text of DOI:10.1002/chem.201003275

DOI: 10.1002/chem.201003275
Azobenzene-Based Colorimetric Chemosensors for Rapid Naked-Eye
Detection of Mercury(II)
Xiaohong Cheng,[a] Qianqian Li,*[a] Conggang Li,[b] Jingui Qin,[a] and Zhen Li*[a]
Abstract: Two new highly selective col-
orimetric chemosensors for Hg2+
which can easily be observed by the
naked eye. The underlying signaling
mechanism is intramolecular charge
transfer (ICT). The sensors have good
selectivity for Hg2+ with respect to sev-
eral common alkali, alkaline earth, and
transition metal ions. Furthermore,
they can be used for in-the-field meas-
urements by virtue of a dipstick ap-
proach without any additional equip-
ment.
,
based on azobenzene and highly selec-
tive Hg2+-promoted deprotection of a
dithioacetal have been designed and
synthesized. In the presence of as little
as 20 mm Hg2+, the sensors change their
color from light yellow to deep red,
Keywords: azo compounds · charge
transfer · colorimetry · mercury ·
sensors
Introduction
species should give a unique spectroscopic change.[8] Be-
cause of the strong thiophilicity of Hg2+, many fluorescent
chemodosimeters with high selectivity, long-range excitation
and emission wavelengths, and high quantum yield have
been designed on the basis of the mercury desulfurization
reaction.[9]
Here we report two colorimetric chemosensors (S1 and
S2) for Hg2+ based on azo derivatives and using intramolec-
ular charge transfer (ICT) as signaling mechanism, in which
the azobenzene moiety acts as electron donor and the alde-
hyde group as the electron acceptor. The design of these
two sensors took advantage of the well-known azobenzene
structure and the highly selective Hg2+-promoted deprotec-
tion of the dithioacetal, as we reported previously.[10] Precur-
sors 1 and 2 of S1 and S2, respectively, were synthesized ac-
cording to previous literature.[11] A straightforward protec-
tion reaction between the aldehyde group and ethyl mercap-
tan gave sensors S1 and S2 in high yields (Scheme 1). While
the formyl group is electron-withdrawing, after mercaptan
Mercury pollution is a global problem and has been receiv-
ing much attention.[1] Both human activities, including gold
mining, solid-waste incineration, and fossil-fuel combus-
tion,[2] and nonanthropogenic sources, such as volcanic and
oceanic emissions and forest fires,[3] have caused rapid in-
crease of Hg2+ levels in the environment. Ionic mercury is
converted to methylmercury by bacteria when it enters the
environment and subsequently bioaccumulated through the
food chain. Mercury accumulation in the body can cause
various neurological damage.[4] Thus, a convenient and fast
method to detect Hg2+ is in high demand. Recently, consid-
erable efforts have been made to develop straightforward
methods such as fluorescence[5] or electrochemical changes[6]
for rapid detection of mercury ions. Among these methods,
colorimetric sensors are especially promising because the
color change can easily be observed by the naked eyes, thus
requiring less labor and no equipment. A large number of
colorimetric sensors for Hg2+ have been developed.[7] As
potential sensing systems, specific chemical reactions with
high selectivity for toxic metal ions have drawn great atten-
tion. The reaction between the sensor molecule and target
[a] X. Cheng, Q. Li, Prof. J. Qin, Prof. Z. Li
Department of Chemistry
Wuhan University
Wuhan 430072 (P. R. China)
Fax : (+86)27-68756757
E-mail: lizhen@whu.edu.cn
[b] C. Li
State Key Laboratory of Magnetic Resonance
and Atomic and Molecular Physics
Wuhan Institute of Physics and Mathematics
Chinese Academy of Sciences
Wuhan, 430071 (P. R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003275.
Scheme 1. Hg2+-sensing process of S1 and S2.
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protection, the resulting dithioacetal moiety can be regarded
as a weak electron donor. Addition of Hg2+ converts the
protected aldehyde group back to the original aldehyde, and
the D–p–D’ structures in S1 and S2 are transformed into D–
p–A structures (Scheme 1). This increases the degree of ICT
and resulting in an obvious redshift of the absorption bands
of the compounds.[12] Addition of Hg2+ to dilute solutions of
S1 and S2 changes the color from light yellow to deep red in
a process visible to the naked eye (see Scheme 1). Here we
describe the synthesis and the spectroscopic evaluation of
both colorimetric chemosensors in detail.
Results and Discussion
Synthesis and structural characterization: Compounds S1
and S2 were prepared by the protection reaction between
ethyl mercaptan and aldehydes 1 and 2 (Scheme 1), in ac-
cordance with our previous report.[10] The synthetic route is
simple and purification easy.
Figure 1. UV/Vis spectra of S1 (10 mm, CH3CN) in the presence of in-
creasing concentration of Hg2+. Inset: Photograph of S1 (A) and S1+
Hg2+ (B, 50 mm).
Compounds S1 and S2 exhibit good solubility in common
organic solvents such as DMF, CH3Cl3, CH3CN, and THF.
They were characterized by spectroscopic methods, and
both gave satisfactory spectral data (see Experimental Sec-
tion). The signal of the aldehyde proton at d=10.04 ppm in
the naked eye (Scheme 1 and Figure 1), and this validates
our design of colorimetric chemosensors for Hg2+ ions
based on the ITC mechanism and the Hg2+-promoted de-
protection of dithioacetals.
1
the H NMR spectrum of 2 (see Figure S1 in the Supporting
Information) disappeared in the spectrum of S2. In the
FTIR spectrum of S2 (see Figure S2 in the Supporting Infor-
mation) the strong band at 1685 cmÀ1 ascribed to C=O
stretching of the aldehyde group disappeared owing to the
protection reaction between ethyl mercaptan and 2. The ab-
sorption maximum centered at about 460 nm in the UV/Vis
spectrum of 2 (see Figure S3 in the Supporting Information)
shifted to 420 nm in dithioacetal S2. The above results con-
firm conversion of aldehyde 2 to dithioacetal S2.
We studied the influence of traces of water in the Hg2+
solution on the sensing process (see Figure S5 in the Sup-
porting Information). The absorption decreased only to a
very limited degree, even when 100 mL of H2O was intro-
duced, which would not affect the results of the titration ex-
periment (Figure 2, inset V). Thus, the Hg2+-promoted de-
protection reaction of dithioacetal S1 occurred as expected,
and the color change was observed.
Hg2+-sensing properties: As shown in Scheme 1 and
Figure 1, compound 1 is deep red with maximum absorption
wavelength centered at about 455 nm in its UV/Vis spec-
trum, corresponding to the ICT character of the chromo-
phore due to the pull–push effect. After reaction with etha-
nethiol, the resulting dithioacetal S1 showed a light yellow
color with maximum absorption wavelength centered at
about 420 nm, due to a change in the electronic property
and hence different ICT efficiency. On adding Hg2+ ions to
a dilute solution of S1 in CH3CN, the absorption maximum
shifted from 420 to 455 nm, with an isosbestic point at about
440 nm, indicating formation of aldehyde 1. Surprisingly,
with increasing concentrations of Hg2+ ions, not only did
the absorption maximum further shift from 455 to 515 nm
with an isosbestic point at about 470 nm, but also the new
band at 515 nm showed a larger molar absorption coefficient
than the original one. The intensities at 420 and 515 nm are
compared in Figure S4 (see the Supporting Information),
which shows that intensity change almost linearly with the
concentration of Hg2+ ions. The different colors before and
after addition of Hg2+ ions could be easily distinguished by
Figure 2. A515/A420 change profile of S1 (10 mm, CH3CN) in the presence
of various metal ions (40 mm). Inset: Photograph of S1 in the presence of
various metal ions (50 mm). A: 1 (1 mM); B: S1 (100 mm); C–V: S1+Hg2+
,
Ba2+, Ni2+, Co2+, Ca2+, Cd2+, Mg2+, Zn2+, Pb2+, Mn2+, Cu2+, Fe2+, K+,
Na+, Ag+, Li+, Al3+, Fe3+, Cr3+, H2O (100 mL).
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Z. Li, Q. Li et al.
To assess the specificity of our sensor towards Hg2+, vari-
ous ions were examined in parallel under the same condi-
tions. As shown in Figure 2 and Figure S6 of the Supporting
Information, the reaction of S1 with Hg2+ resulted in strong
support this transformation (see Figure S7 in the Supporting
Information).
Like S1, S2 also shows good selectivity towards Hg2+
.
Traces of water and metal ions caused almost no apparent
absorption change (see Figures S8–S10 in the Supporting In-
formation). In Figure 3 and Figure S8 in the Supporting In-
formation, the A515/A420 ratio of S2 in CH3CN was almost
12-fold for Hg2+, while the values for the other metal ions
were less than 0.5-fold. The sensing process could also be
observed visually (see Charts S1 and S2 in the Supporting
Information). These results indicate that S2 is an Hg2+-spe-
cific colorimetric sensor.
absorbance, whereas Ba2+, Ni2+, Co2+, Ca2+, Cd2+, Mg2+
,
Zn2+, Pb2+, Mn2+, Cu2+, Fe2+, K+, Na+, and Ag+ showed
no change, and Fe3+, Al3+, and Cr3+ negligible changes.
Therefore, S1 has much higher selectivity for Hg2+ than the
other ions examined here. This unique property enabled
Hg2+ to be detected directly by the naked eye over other
species (Figure 2). In our previous report on fluorescent sen-
sors,[10] Ag+ could also promote conversion of dithioacetal
to aldehyde to some degree, similar to Hg2+. However, Ag+
did not induce any apparent absorption change even at a
concentration of 50 mm (Figures 2 and S6d in the Supporting
Information), that is, by adjusting the chemical structure,
better selectivity of this kind of chemosensors towards Hg2+
could be achieved.
The ultimate goal of this research is to develop naked-eye
colorimetric sensors. Compound S2 has better sensitivity
and lower detection limit and potentially it can be used for
direct naked-eye sensing of Hg2+. Figure 4 shows the results
of adding various amounts of Hg2+ to a 100 mm solution of
S2 in CH3CN. Although not as sensitive as measurements by
UV/Vis spectrometer, we could easily distinguish color
To improve the solubility of S1 in water, S2 bearing two
hydroxyl groups was synthesized (Scheme 1). Compound S2
exhibited a similar absorption profile to S1, owing to the
similar structure. Surprisingly, S2 displayed much better
Hg2+-sensing performance than S1. As shown in Figure 3, a
changes at concentrations as low as 20 mm of Hg2+
.
Figure 4. Photograph of S2 (10 mm, CH3CN) in the presence of increasing
concentrations of Hg2+. A–H): 0, 1, 1.5, 2, 2.5, 3, 4, and 5ꢁ10À5 molLÀ1
.
Taking advantage of the two hydroxyl group in S2, we at-
tempted to conduct the sensing process in in H2O/CH3CN
(1/9) at pH 7.0 maintained with HEPES buffer (20 mm). As
shown in Figure 5, the absorption maximum wavelength of
S2 shifted from 420 to 460 nm, with an isosbestic point at
about 440 nm, indicating formation of aldehyde 2. The trans-
formation was confirmed by UV/Vis spectra (see Figure S11
in the Supporting Information) with that of aldehyde 2 for
comparison. The inset of Figure 5 shows a good linear rela-
tionship between the A460/A420 ratio and the concentration of
Hg2+ in the range of 0–12 mm. A linear regression curve was
simulated, and the point at which it crossed the abscissa was
taken as the detection limit (ca. 0.4 mm).[13] As shown in Fig-
ure S12 of the Supporting Information, dithioacetal S2 can
also function in H2O/CH3CN (2/8), but the sensing behavior
is not as good as in organic solvents and solvent mixtures
with less H2O (H2O/CH3CN 1/9). One reason could be the
poor solubility of S2 in the mixed solvents. Further research
to develop water-soluble sensors by utilizing this idea is on-
going. The specificity of S2 towards Hg2+ in H2O/CH3CN
solution was also investigated, but the performance was not
as good as in pure CH3CN (see Figures S8 and S13 in the
Supporting Information).
Figure 3. Ratiometric calibration curve A515/A420 of S2 (10 mm, CH3CN) as
a function of the concentration of Hg2+. Inset: UV/Vis spectra of S2
(10 mm, CH3CN) in the presence of increasing concentration of Hg2+, and
photograph of S2 (A) and S2+Hg2+ (B, 50 mm).
change in absorption occurred even on addition of as little
as 2 mm Hg2+. With increasing concentrations of Hg2+, the
typical strong absorption band of S2 at 420 nm gradually de-
creased, and a new absorption band centered at 515 nm with
a larger molar absorption coefficient than S1 appeared. This
515 nm absorption was responsible for the color change and
can be observed by the naked eye. The UV/Vis spectra of
dithioacetal S2 before and after the addition of Hg2+ also
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Chem. Eur. J. 2011, 17, 7276 – 7281

Colorimetric Chemosensors for Mercury(II)
FULL PAPER
to aldehyde 2, indicating a mutual effect between the resul-
1
tant aldehyde and Hg2+. In addition, the H NMR spectra of
complex I was the same as that of complex II. An absorp-
tion peak centered at about 1605 cmÀ1 in the IR spectrum of
2 (see Figure S14 in the Supporting Information) indicated
2+
À
À
the presence of N=N groups. After addition of Hg ions,
a new broad peak at 1635 cmÀ1 appeared as the result of a
reaction between 2 and Hg2+ ions to form an N-bound Hg2+
complex (Scheme 2). ESI mass spectra (see Figure S15 in
Figure 5. UV/Vis spectra of S2 (10 mm) in the presence of increasing con-
centrations of Hg2+ in CH3CN/H2O (9/1, 20 mM HEPES, pH 7.0). Inset:
Ratiometric calibration curve A460/A420 as a function of the concentration
Scheme 2. Postulated response process of 2 towards Hg2+
.
of Hg2+
.
On addition of Hg2+ ions to a solution of S2 in pure
CH3CN, the absorption maximum of S2 changed by approxi-
mately 95 nm (from 420 to 515 nm), with an obvious color
change. However, after addition of Hg2+ ions to a solution
in CH3CN/H2O, the absorption profile only changed a little
(from 420 to 460 nm). To confirm the formation of the re-
sulting products and gain insight into the sensing mecha-
nism, control experiments were conducted. Dithioacetal S2
was treated with Hg2+ ions and the reaction product (com-
plex I) isolated, and aldehyde 2 was mixed with Hg2+ ions
the Supporting Information) revealed a main peak at 314.4
after Hg2+ ions were added to 2, corresponding to [2ÀH]+
(calcd m/z 314.1). A weak peak at about 515.9 was assigned
to traces of [2+HgÀH]2+ (calcd m/z 516.1), which leads to
changes in the absorption spectra and a color change. UV/
Vis titrations in CH3CN supported the formation of a com-
plex on treatment of 2 with Hg2+ (see Figures S16 and S17
in the Supporting Information), consistent with a previous
report.[14] In CH3CN/H2O, smaller changes in the absorption
spectra of the sensors may be due to interference of water
in the reaction between 2 and Hg2+. Although the above re-
sults were unexpected, they are beneficial to sensors due to
the much more apparent color changes before and after the
addition of the analytes, which led to a much larger red shift
(95 nm) in absorption and much more pronounced color
change.
1
to give complex II. Partial H NMR spectra of complexes I
and II are shown in Figure 6 with those of S2 and 2 for com-
parison. The resonance of the aldehyde proton appeared at
d=10.04 ppm. Both the aldehyde proton and the phenyl
protons of complex I displayed small upfield shift compared
Test strips, prepared by immersing filter paper into a solu-
tion of dithioacetal S2 in CH3CN (1ꢁ10À3 molLÀ1) and
drying in air, were used to determine the suitability of a dip-
stick method for detection of Hg2+, similar to that common-
ly used for pH measurements. When the test strips coated
with S2 were immersed into aqueous solutions of Hg2+ ions
with different concentrations, immediate color change from
yellow to pink was observed (Figure 7). The detection limit
of the test strips was relatively low (10À4 molLÀ). Develop-
ment of such an approach is attractive for in-the-field meas-
urements that would not require any additional equipment.
Conclusion
Figure 6. 1H NMR spectra of S2 (a), complex I (b), complex II (c), and
aldehyde 2 (d) in [D6]acetone.
New naked-eye colorimetric chemosensors S1 and S2 were
developed by coupling aldehyde protection/Hg2+-promoted
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Z. Li, Q. Li et al.
added to adjust the pH of the mixture to 8–9. The resultant mixture was
extracted with diethyl ether several times, the organic layers were com-
bined, and the product was purified by column chromatography with
ethyl acetate/methanol (11/1) as eluent to afford S2 as an orange solid
1
(110 mg, 94%). M.p. 87–888C; H NMR (300 MHz, CDCl3): d=7.79–
7.87 (m, 4H), 7.54–7.57 (d, J=9.0 Hz, 2H), 6.75–6.78 (d, J=9.0 Hz, 2H),
4.98 (s, 1H), 3.95 (s, 2H), 3.72 (s, 2H), 3.07 (s, 2H), 2.55–2.62 (m, 4H),
1.21–1.26 ppm (t, J=7.5 Hz, 6H); 13C NMR (75 MHz, CDCl3): d=152.75,
150.53, 144.15, 142.19, 128.68, 125.27, 122.63, 112.20, 60.66, 55.45, 52.35,
26.50 ppm; UV/Vis (in CH3CN): lmax =420 nm; MS (ESI) m/z 420.54
[M+], calcd 419.60.
Preparation of solutions of metal ions: 1 mmol of each inorganic salt
(NaNO3, KNO3, LiCl, Ba
Ca(NO3)2·4H2O, Pb(NO3)2, Ni
(NO3)2·3H2O, Al(NO3)3·9H2O, Fe
(SO4)·8H2O, (NH4)2Fe(SO4)2·6H2O, MgSO4, and HgACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
E
N
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
A
U
dissolved in distilled water (10 mL) to afford a 1ꢁ10À1 molLÀ aqueous
solution. The stock solutions were diluted to desired concentrations with
water when needed.
UV absorption changes of S1 due to Hg2+ ions: A solution of S1 (1.0ꢁ
10À5 molLÀ) was prepared in CH3CN.
A
solution of Hg2+ (1ꢁ
Figure 7. Photographs of test strips of S2 (A), S2+Hg2+ (10À5 molLÀ1
,
10À3 molLÀ) was prepared in distilled water. A solution of S1 (3.0 mL)
was placed in a quartz cell (10.0 mm width) and the absorption spectrum
recorded. The Hg2+ solution was introduced in portions and absorption
changes were recorded at room temperature each time.
B), S2+Hg2+ (10À4 molLÀ1, C), S2+Hg2+ (5ꢁ10À4 molLÀ1, D), S2+
Hg2+ (10À3 molLÀ1, E), and S2+Hg2+(5ꢁ10À3 molLÀ1, F).
UV absorption changes of S1 due to different metal ions: A solution of
S1 (1ꢁ10À5 molLÀ) was prepared in CH3CN. The solutions of metal ions
(1ꢁ10À1 molLÀ) were prepared in distilled water. A solution of S1
(3.0 mL) was placed in a quartz cell (10.0 mm width) and the absorption
spectrum was recorded. Different ion solutions were introduced and the
changes in absorption were recorded at room temperature each time.
UV absorption changes of S2 due to Hg2+ ions: Solutions of S2 (1.0ꢁ
10À5 mol/L) were prepared in CH3CN and CH3CN/H2O (9/1, 20 mm
HEPES, pH 7.0). A solution of Hg2+ (1ꢁ10À3 mol/L) was prepared in
distilled water. A solution of S2 (3.0 mL) was placed in a quartz cell
(10.0 mm width) and the absorption spectrum was recorded. The Hg2+
ion solution was introduced in portions and absorption changes were re-
corded at room temperature each time.
deprotection with the ICT mechanism, and the sensing
properties were studied. Both probes displayed high sensi-
tivity and selectivity for Hg2+ with respect to several
common alkali, alkaline earth, and transition metal ions. In
addition, S1 and S2 could serve as practical colorimetric sen-
sors for in-the-field measurements that would not require
any additional equipment by virtue of a dipstick approach.
Experimental Section
Materials and instrumentation: Dichloromethane was dried over and dis-
tilled from CaH2 under an atmosphere of dry nitrogen. All reagents were
of analytical-reagent grade and used without further purification. Doubly
UV absorption changes of S2 due to different metal ions: A solution of
S2 (1ꢁ10À5 molLÀ) was prepared in CH3CN. Solutions of metal ions (1ꢁ
10À1 molLÀ) were prepared in distilled water. The solution of S2
(3.0 mL) was placed in a quartz cell (10.0 mm width) and the absorption
spectrum was recorded. Different ion solutions were introduced and the
changes of the absorption changes were recorded at room temperature
each time.
1
distilled water was used in all experiments. The H and 13C NMR spectra
were measured on Varian Mercury300 spectrometer with tetramethylsi-
lane (TMS; d=0 ppm) as internal standard. FTIR spectra were recorded
on a PerkinElmer-2 spectrometer in the region of 3000–400 cmÀ1 on
NaCl pellets. The ESI mass spectra were measured on a Finnigan LCQ
advantage mass spectrometer. UV/Vis spectra were obtained with a Shi-
madzu UV-2550 spectrometer, and the pH values were determined by
using a DELTA 320 PH dollar. The thermometer used for measurement
of melting points was uncorrected.
Acknowledgements
Synthesis of compound S1: Under dry argon, 1 (71 mg, 0.28 mmol) and
ethanethiol (0.052 mL, 0.7 mmol) were dissolved in dry dichloromethane
(10 mL) with BF3·Et2O (0.1 mL, 0.84 mmol) as Lewis acid. After stirring
at 08C for 3 h, 0.1 molLÀ1 aqueous NaHCO3 was added to adjust the pH
value of the resultant mixture to 8–9. The resultant mixture was extracted
with diethyl ether several times, the organic layers were combined, and
the product was recrystallized from dichloromethane to afford S1 as a
We are grateful to the National Natural Science Foundation of China
(No. 20974084, 21075134), the Program of NCET (NCET-08-0411), and
the National Fundamental Key Research Program (2011CB932702) for
financial support.
1
yellow solid (90.5 mg, 90%). M.p. 98–998C; H NMR (300 MHz, CDCl3):
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Colorimetric Chemosensors for Mercury(II)
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Received: November 14, 2010
Revised: January 25, 2011
Published online: May 19, 2011
Chem. Eur. J. 2011, 17, 7276 – 7281
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