Fluorescent and colorimetric probes for mercury(II): Tunable structures of electron donor and π-conjugated bridge

Article abstract of DOI:10.1002/chem.201102376

A new series of intramolecular-charge-transfer (ICT) molecules (compounds 1, 2, and 3) were synthesized by attaching various electron-donating thiophenes groups to a triphenylamine backbone with an aldehyde group as the electron acceptor. Based on the protection reaction between ethanethiol and aldehyde, the corresponding dithioacetals (compounds S1, S2, and S3) were prepared to serve as novel colorimetric and fluorescent chemosensors for Hg²⁺ ions. Also, compound S1 was further utilized to construct the chemical-reaction-based conjugated polymer probe (PS1) towards Hg²⁺ ions. In the presence of as little as 10 nM Hg²⁺, compound PS1 displayed an apparent change in the fluorescent intensity. The sensing processes were revealed to be mediated by ICT, as confirmed by time-dependent DFT calculations. Furthermore, compound S1 was successfully applied to microscopic imaging for the detection of Hg ²⁺ in HeLa cells with ratiometric fluorescent methods. Copyright

Full text of DOI:10.1002/chem.201102376

FULL PAPER
DOI: 10.1002/chem.201102376
Fluorescent and Colorimetric Probes for Mercury(II): Tunable Structures of
Electron Donor and p-Conjugated Bridge
Xiaohong Cheng, Shuang Li, Huizhen Jia, Aoshu Zhong, Cheng Zhong, Jun Feng,
Jingui Qin, and Zhen Li*[a]
Abstract: A new series of intramolecu-
lar-charge-transfer (ICT) molecules
(compounds 1, 2, and 3) were synthe-
sized by attaching various electron-do-
nating thiophenes groups to a triphe-
nylamine backbone with an aldehyde
group as the electron acceptor. Based
on the protection reaction between
ethanethiol and aldehyde, the corre-
sponding dithioacetals (compounds S1,
S2, and S3) were prepared to serve as
novel colorimetric and fluorescent che-
mosensors for Hg2+ ions. Also, com-
pound S1 was further utilized to con-
struct the chemical-reaction-based con-
jugated polymer probe (PS1) towards
Hg2+ ions. In the presence of as little
as 10 nm Hg2+, compound PS1 dis-
played an apparent change in the fluo-
rescent intensity. The sensing processes
were revealed to be mediated by ICT,
as confirmed by time-dependent DFT
calculations. Furthermore, compound
S1 was successfully applied to micro-
scopic imaging for the detection of
Hg2+ in HeLa cells with ratiometric
fluorescent methods.
Keywords: charge transfer · color-
imetry · fluorescence · mercury ·
sensors
Introduction
several significant challenges still exist in this field. For ex-
ample, to drive these desulfurization reactions to comple-
tion, it is often necessary to use either elevated tempera-
tures or excess quantities of Hg2+. Herein, based on our pre-
vious work,[11] we report a series of new probes of Hg2+ ions
with high sensitivity and selectivity based on the Hg2+-pro-
moted deprotection reaction of the dithioacetal; all of the
probes displayed efficient chromo- and fluorogenic signaling
towards Hg2+ ions at room temperature.
As we know, thiophenes are ideal building blocks to pro-
vide the basis for the synthesis of conjugated p systems for
the following reasons: 1) the enormous and attractive poten-
tial of structural variation, which allows tuning of the elec-
tronic properties over a wide range; 2) their outstanding
chemical and physical properties; and 3) the high polariza-
bility of sulfur atoms in thiophene rings leads to a stabiliza-
tion of the conjugated chain and to excellent charge-trans-
port properties, which are one of the most crucial assets for
applications in molecular design.[12]
With such considerations at the forefront, a new series of
intramolecular-charge-transfer (ICT) molecules (compounds
1, 2 and 3; Scheme 1) were synthesized by attaching various
electron-donating thiophenes groups to a triphenylamine
backbone (T1) with an aldehyde group as the fixed electron
acceptor. Based on the triphenylamine backbone, the ethyle-
nedioxythiophene and thiophene moieties were introduced
to enhance the efficiency of the donor to benefit the deloc-
alization of the whole chromophore in compounds 1 and 2,
respectively (Scheme 1a and b). Then, we linked bithio-
phene moieties to the backbone to extend the length of p
conjugation to prepare compound 3 (Scheme 1c). Based on
the protection reaction between ethanethiol and aldehyde,
the corresponding dithioacetals (compounds S1, S2, and S3)
In recent years, the development of artificial detectors for
the sensing and recognition of environmentally and biologi-
cally toxic metal ions has been actively investigated.[1] In
this regard, the detection of mercury ions are especially im-
portant because both elemental and ionic mercury can be
converted by bacteria in the environment into methyl mer-
cury, which subsequently bioaccumulates through the food
chain.[2] Thus, methods for selective and sensitive detection
of mercury species have received a great deal of attention
lately.[3] Thanks to the great efforts of scientists, a number of
Hg2+ sensors have been developed with good performance,
for example, redox, colorimetric, and fluorescent Hg2+ sen-
sors by using proteins,[4] nucleic acids,[5] DNAs,[6] nanoparti-
cles,[7] and several types of small molecules[8] as Hg2+ accept-
ors. In particular, the detection of mercury ions based on a
specific chemical reaction between the sensor molecule and
target species is a rapidly growing area owing to its high se-
lectivity.[9] Indeed, many fluorescent chemodosimeters for
Hg2+ ion selective detection have been designed on the
basis of the mercury desulfurization reaction.[10] However,
[a] X. Cheng, S. Li, H. Jia, A. Zhong, Dr. C. Zhong, Prof. J. Feng,
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
lichemlab@163.com
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102376.
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The
UV/Vis
absorption
maxima (labs) and the maxi-
mum wavelength of fluorescent
emission (lem) for the new ICT
molecules in THF are summar-
ized in Table 1. Different ab-
sorption and emission behavior
of the aldehydes and resulting
dithioacetals
demonstrated
changes in the electronic prop-
erties before and after the pro-
tection reaction between mer-
captan and aldehydes, leading
to different ICT efficiencies.
Hg2+ sensing properties: As
demonstrated by the results
given in Table 1, the maximum
emission wavelength of 1 cen-
tered at about 525 nm. After
the protection reaction, the
maximum emission wavelength
Scheme 1. Structures of the new series of ICT molecules: a) and b) to enhance the efficiency of the donor;
c) to extend the length of p-conjugation; d) to construct the conjugated polymer probe.
were prepared to serve as novel dual-channel chemosensors
for Hg2+ ions. Also, compound 1 was introduced into the
conjugated polymer backbone (P1; Scheme 1d) and further
utilized to construct the chemical-reaction-based conjugated
polymer probe (PS1) towards Hg2+ ions. Taking advantage
of the distinct Hg2+-promoted deprotection reaction of the
dithioacetal, all of the probes displayed efficient chromo-
and fluorogenic signaling towards Hg2+ ions at room tem-
perature. Herein, we describe the synthesis and spectroscop-
ic evaluation of all of the dual-channel chemsensors in
detail.
of the resulting dithioacetal S1 shifted to 440 nm; the UV/
Vis absorption maximum wavelength shifted from 422 to
385 nm. This indicated that, after the protection reaction of
Results and Discussion
Synthesis and structural characterization: The general syn-
thetic procedure is given in Scheme 2. The Wittig reaction
of 2-formylthiophene and derivatives (5, 6, or 7) with diethyl
{[4-(diphenylamino)benzyl]triphenyl}phosphonate (4) gave
the corresponding triphenylaminevinyl thiophenes (8, 9, or
10). The followed Vilsmeier reaction of 8, 9, or 10 yielded
the corresponding aldehyde 1, 2, or 3, which was readily
converted into dithioacetals S1, S2, or S3 through a straight-
forward protection reaction between ethanethiol and alde-
hyde. By the Suzuki coupling reactions between 1 and 11,
compound P1 was prepared and converted into dithioacetal
PS1 by the protection reaction of ethanethiol and aldehyde
P1.
Compounds S1–S3 exhibited good solubility in common
organic solvents, such as chloroform, acetone, dichlorome-
thane, and THF. Compound PS1 exhibited good solubility in
THF. They were characterized by spectroscopic methods
and all gave satisfactory spectral data (see the Experimental
Section).
Scheme 2. General procedure for the synthesis of S1–S3 and PS1.
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Fluorescent and Colorimetric Probes for Mercury(II)
FULL PAPER
Table 1. Optical data for the sensor molecules.
were studied carefully to test the reproducibility of the sens-
ing behavior of dithioacetal S1 for Hg2+ ions, as demon-
strated in Figures S2 and S3 in the Supporting Information.
At a concentration of 1 mm, the fluorescent intensity of S1
changed even at concentrations of Hg2+ ions as low as 2ꢁ
10À6 molLÀ1 (Figure S3 in the Supporting Information); this
makes dithioacetal S1 a highly sensitive ratiometric fluores-
labs [nm]
lem [nm]
1
2
3
P1
S1
S2
S3
PS1
422
353
354
438
385
310
310
393
525
538
580
564
440
448
497
462
cent probe for the detection of Hg2+
.
To evaluate the specific nature of dithioacetal S1 towards
Hg2+ ions, the influence of added trace water and other
metal ions were investigated. As shown in Figure S4 in the
Supporting Information, the intensity was nearly unchanged
even upon the addition of relatively large amounts of water
(50 mL), indicating this would not affect the results of the ti-
tration experiment. Figure 2 shows that, except slight influ-
the aldehyde, the electronic property of the aldehyde group
changed, resulting in different fluorescent behavior due to
different ICT efficiency. Then, we added Hg2+ ions to a
dilute solution of dithioacetal S1 and investigated its re-
sponse to Hg2+ ions in THF in detail.
As shown in Figure 1, upon treatment with Hg2+ ions, the
emission intensity of dithioacetal S1 at 440 nm decreased
immediately with the fluorescent intensity at 525 nm in-
Figure 2. Fluorescence responses of S1 (10 mm) to various metal ions
([Hg2+]: 12 mm; other ions: 50 mm) in THF. Inset: Fluorescence photo-
graph of S1 (10 mm; a), S1+Hg2+ (2 mm; b), S1+Hg2+ (5 mm; c), S1+
Hg2+ (10 mm; d), S1+Hg2+ (20 mm; e), and S1+a mixture of other metal
ions (10 mm; f).
ence from Fe3+, for all other metal ions, including Mg2+
Mn2+, Zn2+, Ni2+, Pb2+, Ba2+, Ca2+, Co2+, Al3+, Cu2+
,
,
Cd2+, Cr3+, Fe2+, Li+, Na+, and K+, there were nearly no
changes in the fluorescence spectra observed upon the addi-
tion of metal ions with concentrations of 50 mm, indicating
that our probe had a selective response towards Hg2+ over
other metal ions. As demonstrated by the images given in
the inset of Figure 2, different sensing behavior could be
easily seen by the naked eye under a normal UV lamp, real-
izing our thought of the design of new chemosensors to-
wards Hg2+ ions based on the ITC mechanism and the Hg2+
-promoted deprotection reaction of dithioacetals.
Recently, colorimetric sensors have been especially prom-
ising because the color change can be easily observed by the
naked eye, thus less work and no equipment are required.
Therefore, the colorimetric behavior of S1 was also investi-
gated under the same conditions. As shown in Figure 3, the
apparent change in the absorption could be induced only
upon the addition of Hg2+ ions, even at concentrations as
low as 15 mm. With increasing Hg2+ concentrations, the typi-
Figure 1. Fluorescent emission spectra of S1 (10 mm) in the presence of in-
creasing concentrations of Hg2+. Inset: Fluorescence photograph of S1
(10 mm; a), S1+Hg2+ (20 mm; b).
creasing. When the concentration of Hg2+ ions became
higher, the emission spectra were more close to that of alde-
hyde 1 (emission maximum centered at about 525 nm). To
see the sensing process more visually, we compared the in-
tensities at different wavelengths, 525 and 440 nm; the ob-
tained experimental results are summarized in Figure S1 in
the Supporting Information. In fact, the fluorescence change
for the solution of dithioacetal S1 before and after the addi-
tion of Hg2+ ions could be easily distinguished by the naked
eye (from blue to green), as shown in the inset of Figure 1.
Furthermore, the fluorescent properties of dithioacetal S1 at
different concentrations (100 and 1 mm) towards Hg2+ ions
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Z. Li et al.
dehyde groups at 1665 cmÀ1 appeared in the IR spectrum as
a result of the reaction between S1 and Hg2+ or Ag+ ions,
and this spectrum was nearly the same as that of aldehyde 1,
proving the successful deprotection reaction of dithioacetal
S1. All of the above experimental results suggested that our
probe displayed both efficient fluoro- and chromogenic sig-
naling towards Hg2+ ions and could serve as novel dual-
channel chemosensors for Hg2+ ions.
To check the above experimental results from another
aspect, we further designed and synthesized two other di-
thioacetals, compounds S2 and S3, with similar molecular
structures as dithioacetal S1 (Scheme 2), in which the previ-
ous ethylenedioxythiophene moieties were replaced by thio-
phene and bithiophene ones, respectively. Figures S8–S24 in
the Supporting Information demonstrated the fluorescent
and colorimetric behavior of dithioacetals S2 and S3 in the
presence and absence of Hg2+ and Ag+ ions. Similar to S1,
dithioacetals S2 and S3 could give response to Hg2+ ions
with relatively good performances. The IR spectra in Figur-
es S14 and S24 in the Supporting Information also give
some evidence about this transformation. Thus, the obtained
experimental results for dithioacetals S2 and S3, on one
hand, further proved our idea for the development of good
chemosensors towards Hg2+ ions, on the other hand, the re-
sults indicated that, after optimizing the structural design,
more Hg2+ ions chemosensors with better performances
could be obtained by using the Hg2+-promoted deprotection
reaction coupled with the ICT mechanism.
To date, conjugated polymers (CPs) have often been used
as the optical platforms in sensitive chemical and biological
sensors[14] because they exhibited signal amplification along
the whole polymer backbone through the molecular wire
effect. Due to this fact, interest in the development of conju-
gated polymeric sensors for the detection of Hg2+ ions in-
creased and the reported examples demonstrated good per-
formance.[15] Thus, we wondered that if it was possible to
further increase the detecting sensitivity towards Hg2+ ions
by introducing compound 1 to the backbone of CPs (P1).
From this point, we designed and prepared a CP probe,
PS1, through a similar synthetic procedure to those for di-
thioacetals S1–S3 (Scheme 2).
Figure 3. UV/Vis spectra of S1 (10 mm) in the presence of increasing con-
centrations of Hg2+. Inset: Photograph of S1 (100 mm; a), S1+Hg2+
(5 mm; b), S1+Hg2+ (10 mm; c), S1+Hg2+ (50 mm; d), and S1+Hg2+
(100 mm; e).
cal strong absorption band of S1 at 385 nm gradually de-
creased along with the appearance of a new absorption
band centered at 422 nm, which was responsible for the
color change (from colorless to yellow) and perceptible to
the naked eye (the inset of Figure 3). Usually, the response
time of the chemical-reaction-based sensor system was rela-
tively slow; therefore, the reaction rate might have affected
the experimental results. As a result, some time was needed
to complete the reaction. However, in our case, both the flu-
orescent and colorimetric changes for the solution of di-
thioacetal S1 occurred immediately upon the addition of
Hg2+ ions by the virtue of the thiophilic properties of mer-
cury.
As we reported previously,[11] although being less thiophil-
ic than Hg2+ ions, Ag+ could also promote the deprotection
reactions. Herein, we also investigated the sensing behavior
of dithioacetal S1 towards silver ions. As shown in Figur-
es S5 and S6 in the Supporting Information, upon the addi-
tion of silver ions, the fluorescent and UV/Vis absorption
spectra underwent the same changes as those observed in
the case of Hg2+ titration experiments. The obtained results
were unexpected, but important because dithioacetal S1
could also be regarded as a good probe towards Ag+ ions,
since it is also necessary to detect Ag+ ions due to strong
toxicity towards humans and our environment.[13] The IR
spectra of dithioacetal S1 before and after the addition of
Hg2+ and Ag+ ions could give some proof of this transfor-
mation. As shown in Figure S7 in the Supporting Informa-
tion, no absorption peak centered at about 1665 cmÀ1 was
observed in the IR spectrum of dithioacetal S1, in accord-
ance with the absence of aldehyde groups. But after the ad-
dition of Hg2+ or Ag+ ions, a typical absorption peak of al-
Then, we tried to add Hg2+ to a dilute solution of PS1
(Figures S25–S33 in the Supporting Information). Similarly
to S1, the dilute solution of PS1 also showed a ratiometric
fluorescence response (Figure 4). With increasing concentra-
tions of Hg2+, the typical strong emission band of PS1 at
462 nm gradually decreased along with the appearance of a
new emission band centered at 564 nm, which was responsi-
ble for the fluorescent color change and was perceptible to
the naked eye under a normal UV lamp. By comparing the
fluorescent intensity of PS1 (1 mm) and S1 (1 mm) in the
presence of different concentrations of Hg2+, it was clear
that PS1 had a much higher sensitivity towards Hg2+ than
S1. As shown in Figure 4 and Figure S3a in the Supporting
Information, with the same concentrations of Hg2+ ions
(1.6 mm), the fluorescent intensity of PS1 at 462 nm de-
creased by 52% of the original one, whereas that of S1 at
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Fluorescent and Colorimetric Probes for Mercury(II)
FULL PAPER
Figure 5. Frontier molecular orbitals optimized at the B3LYP/6-31G*
level of theory.
1–3 primarily resided on the electron-donating triphenyla-
mine and the conjugated thiophene rings with low coeffi-
cients on the electron-withdrawing aldehyde group. The
LUMOs were mainly located on the electron-withdrawing
aldehyde groups due to the contribution of the p* orbitals of
the aldehyde group to the LUMOs of compounds 1–3. Thus,
the relatively well-separated distribution between HOMO
and LUMO indicated substantial charge transfer from the
donor moiety to the acceptor one when the molecules were
excited. In terms of compound T1, the separation between
HOMO and LUMO was not so clear, relative to 1–3, result-
ing in lower ICT efficiency. Therefore, the extent of ICT can
be enhanced by extending the length of p conjugation or by
using a stronger donor. This result supported our prior hy-
pothesis that the introduction of ethylenedioxythiophene
and thiophene moieties enhanced the efficiency of the
donor and benefitted charge transfer of the whole chromo-
phore.
Figure 4. Fluorescent emission spectra of PS1 (1 mm) in the presence of
increasing concentrations of Hg2+. Inset: Fluorescence photograph of
PS1 (10 mm; a) and PS1+Hg2+ (2 mm; b).
440 nm decreased by only 15%. Furthermore, compound
PS1 still responded to Hg2+ ions with a concentration of
0.1 mm, whereas S1 did not. From Figure S27 in the Support-
ing Information, the detection limit of PS1 towards Hg2+
could be determined as 10 nm, which was much lower than
that of S1 (2 mm; Figure S3 in the Supporting Information).
All of the results indicated that the molecular wire effect of
the CPs did work and offered the CPs excellent sensing per-
formances relative to the corresponding monomer, S1. From
the UV/Vis spectra (Figures S31 and S32 in the Supporting
Information), it was confirmed that the Hg2+-promoted de-
protection reaction of dithioacetal PS1 occurred as expect-
ed. The possible influence of water and the Hg2+ ion selec-
tive nature of PS1 were also investigated carefully. Similar
to dithioacetal S1, additional water did not cause big
changes in the fluorescent intensity (Figure S32 in the Sup-
porting Information), which would be negligible. However,
compound PS1 could still give a response to Ag+ ions (Fig-
ures S28 and S31 in the Supporting Information), although
other metal ions had little effect on its sensing behavior to-
wards Hg2+ ions (Figure S33 in the Supporting Information).
These results further confirmed the phenomena observed
above, and demonstrated that the probes reported herein
were dual ones for both Hg2+ and Ag+ ions.
Cyclic voltammetry (CV) was employed to estimate the
HOMO and LUMO energy levels of the sensor molecules.
The CV curves are shown in Figure S34 in the Supporting
Information, with the corresponding data summarized in
Table 2. All reported potentials were calibrated against the
ferrocene/ferrocenium (Fc/Fc+) couple, which was used as
Table 2. Electrochemical properties of sensor molecules.[a]
opt
HOMO[b] [eV]
LUMO [eV]
Eg
1
2
3
T1
S1
S2
S3
À5.20 (À5.20)
À5.27 (À5.35)
À5.18 (À5.21)
À5.44 (À5.34)
À5.04 (À4.92)
À5.04 (À5.11)
À5.06 (À5.00)
À2.62 (À2.21)
À2.73 (À2.45)
À2.73 (À2.59)
À1.79 (À1.47)
À2.14 (À1.67)
À2.11 (À1.83)
À2.34 (À2.11)
2.58 (2.99)
2.53 (2.90)
2.45 (2.62)
3.65 (3.87)
2.90 (3.25)
2.93 (3.28)
2.72 (2.89)
Theoretical calculations: To get an insight into the molecu-
lar structure and the different fluorescence and absorption
behavior before and after the protection/deprotection reac-
tion, DFT calculations were carried out at the B3LYP/6-
31G(d) level by using the Gaussian 09 program.[16] The cal-
culated HOMOs and LUMOs of compounds 1–3, S1–S3 and
T1 are illustrated in Figure 5. The HOMOs of compounds
[a] The corresponding calculated values are given in parentheses for com-
parison. [b] HOMO=À(Eoxonset+4.71) eV, LUMO=Egopt +HOMO. The
optical band gap (Egopt) was obtained from the empirical formula Eg =
1240/ledge
.
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Z. Li et al.
the internal standard. The HOMO levels were obtained cor-
responding to the initial onset oxidation potential of the
chromophores, according to the empirical equation
(E)HOMO =À(Eoxonset+4.71) eV, and their LUMO energies
were estimated from the optical band gaps and HOMO en-
ergies. Furthermore, we could still obtain some information
from Table 2, although there were some discrepancies exist-
ed between the calculation and experimental results. Taking
1 and S1, for example, the corresponding energy gaps be-
tween HOMO and LUMO were calculated to be 2.99 and
3.25 eV for 1 and S1, respectively, which were in agreement
with the remarkable redshift in both luminescence and ab-
sorption spectra of 1 compared with that of S1, leading to
the different optical behaviors before and after the addition
of Hg2+ ions. Similar results could be obtained from the ex-
perimental values. This was reasonable. From the LUMOs,
it was clear that ICT took place through a conjugated
bridge between the electron donor moieties and aldehyde
groups in compounds 1–3, whereas this was clearly prohibit-
detect the Hg2+ ions in cells with ratiometric fluorescent
methods.
By taking advantage of the distinct Hg2+-promoted de-
protection reaction of the dithioacetal, all of the probes dis-
played efficient chromo- and fluorogenic signaling towards
Hg2+ ions at room temperature. Moreover, more Hg2+ ion
chemosensors with better performances could be obtained
by simply adjusting and optimizing the chemical structure.
Conclusion
A series of ICT molecules (compounds 1, 2, 3, and P1) were
synthesized by attaching various electron-donating thio-
phenes groups to a triphenylamine backbone with an alde-
hyde group as the electron acceptor. Based on the protec-
tion reaction between ethanethiol and aldehyde, the corre-
sponding dithioacetals (compound S1, S2, S3, and PS1) were
prepared to serve as novel dual-channel chemosensors for
Hg2+ ions. All probes displayed high sensitivity and selectiv-
ity for Hg2+ with respect to several common alkali, alkaline
earth, and transition-metal ions. In addition, DFT calcula-
tions were carried out at the B3LYP/6-31G(d) level to inves-
tigate different fluorescence and absorption behavior before
and after the addition of Hg2+ ions. Probe S1 was successful-
ly applied to the microscopic imaging for detection of Hg2+
in HeLa cells with ratiometric fluorescent methods.
À À
ed in dithioacetals S1–S3 due to the saturated bridge ( C
À
S ) after the reaction with ethanethiol.
Cell imaging: The application of probe S1 to track intracel-
lular Hg2+ levels was also investigated by scanning micros-
copy. As shown in Figure 6, the S1-loaded cells showed in-
Experimental Section
1
Materials and instrumentations: The H and 13C NMR spectra were mea-
sured on
a Varian Mercury300 spectrometer with tetramethylsilane
(TMS; d=0 ppm) as an internal standard. The FTIR spectra were re-
corded on a PerkinElmer-2 spectrometer in the region of 3000–400 cmÀ1
on NaCl pellets. UV/Vis spectra were obtained by using a Shimadzu UV-
2550 spectrometer. Photoluminescence spectra were performed on a Hi-
tachi F-4500 fluorescence spectrophotometer. Gel permeation chroma-
tography (GPC) was used to determine the molecular weights of the
polymers. GPC analysis was performed on an Agilent 1100 series HPLC
system and a G1362A refractive index detector. Polystyrene standards
were used as calibration standards for GPC. THF was used as an eluent
and the flow rate was 1.0 mLminÀ1. HR-ESI-TOF mass spectra were re-
corded on a Waters Micromass LCT Premier XE. CV was carried out on
a CHI voltammetric analyzer in a three-electrode cell with a Pt counter
electrode, a Ag/AgCl reference electrode, and a glassy carbon working
electrode at a scan rate of 10 mVsÀ1 with 0.1m tetrabutylammonium per-
chlorate (purchased from Alfa Aesar) as the supporting electrolyte in an-
hydrous dichloromethane purged with nitrogen. The potential values ob-
tained in reference to the Ag/Ag+ electrode were converted to values
versus the saturated calomel electrode (SCE) by means of an internal
Fc+/Fc standard. Fluorescence cell imaging was performed with an
OLYMPUS IX71 scanning microscope. Emission of S1-loaded cells was
collected at 400–410 nm and 460–490 nm for blue and green channels, re-
spectively. The data for ratio fluorescence imaging were analyzed by
using software package provided by OLYMPUS instruments. Immediate-
ly before the experiments, cells were washed with PBS and then incubat-
ed with 100 mm S1 in PBS/DMSO (10/1, v/v) for 30 min. Cell imaging was
then carried out after washing the cells with PBS. Dichloromethane was
dried over and distilled from CaH2 under an atmosphere of dry nitrogen.
THF was dried over and distilled from K–Na alloy under an atmosphere
Figure 6. Scanning microscopic images of HeLa cells incubated with S1
and S1 upon treatment of Hg2+
.
tracellular fluorescence at an emission channel collected at
wavelengths longer than 455 nm, indicating that probe S1
could penetrate the cell membrane and potentially be used
for imaging of Hg2+ in living cells and in vivo. When the
cells were treated with a 200 mm aqueous solution of Hg-
ACHTUNGTRENNUNG(ClO)2 for 30 min, followed by subsequent staining with S1
(100 mm, 0.5 h), and then washing three times with phos-
phate-buffered saline (PBS; 10 mm, pH 7.12), the intensity
of the emission collected at the wavelength longer than
455 nm decreased, whereas the intensity of the emission col-
lected at wavelengths longer than 520 nm increased. Accord-
ingly, the green and blue fluorescence images of S1 and S1+
Hg2+ were monitored and the mean green to blue intensities
(FG/FB) were found in ratios of 0.36 and 6.09, respectively.
These were consistent with the observations in titration ex-
periments and demonstrated that probe S1 could readily
1696
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 1691 – 1699

Fluorescent and Colorimetric Probes for Mercury(II)
FULL PAPER
of dry nitrogen. All other reagents were used as purchased. Compounds
(300 MHz, CDCl3): d=7.39–7.38 (m, 4H; Ar-H), 7.38–7.36 (m, 2H; Ar-
H), 7.34–7.30 (m, 4H; Ar-H), 7.08–6.98 (m, 8H; Ar-H), 4.92 (s, 1H;
-CH-), 2.76–2.56 (m, 4H; -CH2-), 1.36–1.26 (m, 6H; -CH3); 13C NMR
(CDCl3): d=145.7, 144.2, 137.3, 135.6, 132.0, 130.6, 126.7, 126.0, 125.6,
124.1, 121.0, 120.5, 119.7, 117.6, 113.7, 109.5, 60.3, 40.6, 21.1 ppm; HRMS
(ESI-TOF): m/z calcd for C33H29Br2NS4 [M+H]+: 725.9628; found:
725.9655.
4, 5–7, and 11 were synthesized according to literature procedures.[17]
General procedure for the synthesis of compounds 1–3: Compound 8, 9,
or 10 (1 equiv) was dissolved in ClCH2CH2Cl then POCl3 (2.7 equiv) in
DMF was added dropwise to the mixture at 08C. After the temperature
rose to room temperature, the reaction was heated to 458C. After cool-
ing, the mixture was poured in an ice bath with stirring and later neutral-
ized with sodium carbonate. The solution was filtered and the crude
product was purified by column chromatography on silica gel to afford
the resulting powder.
Synthesis of P1: Compound 1 (179.2 mg, 0.3 mmol) and 11 (175.93 mg,
0.3 mmol) were placed in a round-bottomed flask. A mixture of a 1.5m
aqueous solution of K2CO3 (2 mL) and toluene (11 mL) were added to
the flask and the reaction mixture was degassed. The mixture was heated
at reflux with vigorous stirring for 3 d under a nitrogen atmosphere.
After the mixture was cooled to room temperature, it was poured into
methanol (200 mL). The precipitated material was recovered by filtration
through a funnel. The resulting solid material was washed with acetone
to remove oligomers and catalyst residues. Compound P1 was obtained
as a yellow solid (0.151 g, 65.2%). Mn =5000, Mw =7400, Mw/Mn =1.48
(GPC, polystyrene calibration); 1H NMR (300 MHz, CDCl3): d=9.89 (s,
1H; -CHO), 7.76 (brs, 4H, Ar-H), 7.65 (brs, 8H, Ar-H), 7.41 (brs, 4H,
Ar-H), 7.19 (brs, 2H, Ar-H), 7.10 (brs, 2H, Ar-H), 4.38 (brs, 4H, -CH2-),
2.05 (brs, 4H, -CH2-), 1.25 (brs, 4H, -CH2-), 1.07 (brs, 12H, -CH3),
0.76 ppm (brs, 6H, -CH3).
Synthesis of 1: Compound 8 (1.14 g, 2.0 mmol), DMF (1 mL, 13.0 mmol),
POCl3 (0.5 mL, 5.35 mmol), and ClCH2CH2Cl (50 mL) were used in the
general procedure. Petroleum ether/CHCl3 (1:1, v/v) was used as the
eluent to obtain 1 as a salmon pink powder (1.00 g, 83.7%). 1H NMR
(300 MHz, CDCl3): d=9.89 (s, 1H; -CHO), 7.38–7.35 (m, 8H; Ar-H),
7.05–7.01 (m, 2H; -CH=CH-), 6.98–6.95 (m, 4H; Ar-H), 4.38–4.37 ppm
(m, 4H; -CH2-CH2-); 13C NMR (CDCl3): d=179.5, 148.9, 147.3, 146.2,
138.7, 132.7, 131.4, 131.0, 128.6, 128.2, 126.2, 123.7, 116.5, 116.3, 115.5,
65.6, 64.8 ppm.
Synthesis of 2: Compound
9 (0.51 g, 1.0 mmol), DMF (0.5 mL,
6.5 mmol), POCl3 (0.25 mL, 2.7 mmol), and ClCH2CH2Cl (25 mL) were
used in the general procedure. Petroleum ether/CHCl3 (1:1, v/v) was used
as the eluent to obtain 2 as an orange–red powder (0.30 g, 56%).
1H NMR (300 MHz, CDCl3): d=9.87 (s, 1H; -CHO), 7.67 (d, 1H; Ar-
H), 7.40–7.38 (m, 6H; Ar-H), 7.13 (d, 1H; Ar-H), 7.11 (d, 2H; -CH=CH-
), 7.03 (d, 2H; Ar-H), 6.98–7.01 ppm (m, 4H; Ar-H); 13C NMR (CDCl3):
d=186.5, 156. 8, 151.4, 149.9, 145.3, 141.4, 136.6, 136.2, 134.7, 132.1,
130.2, 130.1, 127.5, 123.6, 120.4 ppm.
Synthesis of PS1: Under an atmosphere of argon, compound P1 (0.04 g,
0.05 mmol) and ethanethiol (0.02 mL, 0.25 mmol) were dissolved in dry
THF (3 mL) with BF3·Et2O (0.036 mL, 0.3 mmol) as the Lewis acid.
After being stirred at 08C overnight, the resulting mixture was then
poured into methanol (200 mL). The precipitate was collected by filtra-
tion and then washed with methanol and acetone three times to remove
oligomers. Compound PS1 was obtained as a yellow solid (0.035 g,
79.5%). Mn =6400, Mw =10000, Mw/Mn =1.56 (GPC, polystyrene calibra-
tion); 1H NMR (300 MHz, CDCl3): d=7.74 (brs, 6H, Ar-H), 7.58 (brs,
4H, Ar-H), 7.25 (brs, 2H, Ar-H), 7.05 (brs, 4H, Ar-H), 6.78 (brs, 2H, Ar-
H), 6.66 (brs, 2H, Ar-H), 5.31 (s, 1H, -CH-),4.38 (brs, 8H, -CH2-), 2.68
(brs, 4H, -CH2-), 2.15 (brs, 8H, -CH2-), 1.61 (brs, 6H, -CH2-), 1.24 (brs,
6H, -CH3), 0.86 ppm (brs, 6H, -CH3).
Synthesis of 3: Compound 10 (0.59 g, 1.0 mmol), DMF (1 mL,
13.0 mmol), POCl3 (0.37 mL, 4.04 mmol), and ClCH2CH2Cl (50 mL) were
used in the general procedure. Petroleum ether/CHCl3 (1:2, v/v) was used
as the eluent to obtain 3 as a salmon pink powder (0.36 g, 58%).1H NMR
(300 MHz, CDCl3): d=9.86 (s, 1H; -CHO), 7.68–7.67 (m, 2H; Ar-H),
7.38–7.35 (m, 6H; Ar-H), 7.25–7.23 (m, 2H; -CH2-),7.06–6.92 ppm (m,
8H; Ar-H); 13C NMR (CDCl3): d=182.7, 146.3, 137.7, 132.7, 129.3, 127.8,
127.3, 126.1, 124.0, 120.2, 116.3 ppm.
Preparation of solutions of metal ions: Each inorganic salt (1 mmol;
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, or HgACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
General procedure for the synthesis of compounds S1–S3: Under an at-
mosphere of argon, compound 1, 2, or 3 (1 equiv) and ethanethiol
(2.5 equiv) were dissolved in dry dichloromethane with BF3·Et2O
(3 equiv) as the Lewis acid. After being stirred at 08C overnight,
0.1 molLÀ1 aqueous NaHCO3 was added to adjust the pH value of the so-
lution to 8–9. The final product was extracted with diethyl ether and puri-
fied by column chromatography to afford the target compounds.
A
E
N
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
A
U
dissolved in distilled water (10 mL) to afford a 1ꢁ10À1 molLÀ1 aqueous
solution. The stock solutions were diluted to desired concentrations with
water when required.
Fluorescence titration of S1–S3 or PS1 with Hg2+ ions: A solution of S1–
S3 or PS1 (1ꢁ10À5 molLÀ1) was prepared in THF. The solution of Hg2+
(1ꢁ10À3 molLÀ1) was prepared in distilled water. A solution of S1–S3 or
PS1 (3.0 mL) was placed in a quartz cell (10.0 mm width) and the fluores-
cence spectrum was recorded. The Hg2+ ion solution was introduced in
portions and fluorescence intensity changes were recorded at room tem-
perature each time.
Synthesis of S1: Compound 1 (0.122 g, 0.2 mmol), ethanethiol (0.037 mL,
0.5 mmol), BF3·Et2O (0.072 mL, 0.6 mmol), and CH2Cl2 (10 mL) were
used in the general procedure. Petroleum ether/CHCl3 (1:1, v/v) was used
as the eluent to obtain S1 as a yellow solid (0.12 g, 86.7%).1H NMR
(300 MHz, CDCl3): d=7.29–7.19 (m, 8H; Ar-H), 6.92–6.87 (m, 6H; Ar-
H), 5.25 (s, 1H; -CH-), 4.20 (brs, 4H; -CH2-CH2-), 2.64–2.59 (m, 4H;
-CH2-), 1.21 ppm (t, 6H; -CH3); 13C NMR (CDCl3): d=146.5, 138.4,
132.6, 127.4, 125.8, 124.4, 117.1, 115.9, 65.1, 43.0, 26.9, 14.5 ppm: HRMS
(ESI-TOF): m/z calcd for C31H29Br2NO2S3 [M+H]+: 701.9805; found:
701.9826.
Fluorescence intensity changes of S1–S3 or PS1 with different metal
ions: A solution of S1–S3 or PS1 (1ꢁ10À5 molLÀ1) was prepared in THF.
The solutions of metal ions (1ꢁ10À1 molLÀ1) were prepared in distilled
water. A solution of S1–S3 or PS1 (3.0 mL) was placed in a quartz cell
(10.0 mm width) and the fluorescence spectrum was recorded. Different
ion solutions were introduced and the changes of the fluorescence inten-
sity were recorded at room temperature each time.
UV absorption changes of S1–S3 or PS1 by Hg2+ ions: A solution of S1–
S3 or PS1 (1.0 ꢁ10À5 molLÀ1) was prepared in THF. The solution of Hg2+
(1ꢁ10À3 molLÀ1) was prepared in distilled water. A solution of S1–S3 or
PS1 (3.0 mL) was placed in a quartz cell (10.0 mm width) and the absorp-
tion spectrum was recorded. The Hg2+ ion solution was introduced in
portions and absorption changes were recorded at room temperature
each time.
Synthesis of S2: Compound 2 (0.18 g, 0.2 mmol), ethanethiol (0.037 mL,
0.5 mmol), BF3·Et2O (0.072 mL, 0.6 mmol), and CH2Cl2 (10 mL) were
used in the general procedure. Petroleum ether/CHCl3 (1:1, v/v) was used
as the eluent to obtain S1 as a yellow solid (0.05 g, 40.7%). 1H NMR
(300 MHz, CDCl3): d=7.47–7.41 (m, 4H; Ar-H), 7.31–7.28 (m, 4H; Ar-
H), 6.95–6.87 (m, 8H; Ar-H), 4.84 (s, 1H; -CH-), 2.63–2.60 (m, 4H;
-CH2-), 1.25–1.18 ppm (m, 6H; -CH3); 13C NMR (CDCl3): d=146.7,
146.2, 139.3, 138.8, 133.2, 132.7, 127.6, 126.0, 125.3, 124.6, 117.2, 115.5,
65.3, 42.6, 26.5; HRMS (ESI-TOF): m/z calcd for C29H27Br2NS3 [M+H]+
: 643.9751; found: 643.9739.
Synthesis of S3: Compound 3 (0.062 g, 0.1 mmol), ethanethiol (0.02 mL,
0.25 mmol), BF3·Et2O (0.036 mL, 0.3 mmol), and CH2Cl2 (5 mL) were
used in the general procedure. Petroleum ether/CHCl3 (1:1, v/v) was used
as the eluent to obtain S1 as a yellow solid (0.025 g, 34.7%).1H NMR
UV absorption changes of S1–S3 or PS1 by different metal ions: A solu-
tion of S1–S3 or PS1 (1ꢁ10À5 molLÀ1) was prepared in THF. The solu-
tions of metal ions (1ꢁ10À1 molLÀ1) were prepared in distilled water. A
solution of S1–S3 or PS1 (3.0 mL) was placed in a quartz cell (10.0 mm
Chem. Eur. J. 2012, 18, 1691 – 1699
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1697

Z. Li et al.
width) and the absorption spectrum was recorded. Different ion solutions
were introduced and the changes of the absorption changes were record-
ed at room temperature each time.
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(no.20974084), the Program for Changjiang Scholars and Innovative Re-
search Team in University (IRT1030), the Program of NCET, and the
National Fundamental Key Research Program (2011CB932702) for finan-
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Received: July 30, 2011
Published online: January 5, 2012
Chem. Eur. J. 2012, 18, 1691 – 1699
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www.chemeurj.org
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