A colorimetric and fluorometric dual-modal assay for mercury ion by a molecule

Article abstract of DOI:10.1021/ol0706399

The synthesis and sensing characteristics of a new class of colorimetric and fluorometric dual-modal probe for mercury ion are outlined. Judicious placement of two dithia-dioxa-aza macrocycles on the BODIPY chromophore generates this interesting molecule.

Full text of DOI:10.1021/ol0706399

ORGANIC
LETTERS
2
007
Vol. 9, No. 12
313-2316
A Colorimetric and Fluorometric
Dual-Modal Assay for Mercury Ion by a
Molecule
2
Mingjian Yuan, Yuliang Li,* Junbo Li, Cuihong Li, Xiaofeng Liu, Jing Lv,
Jialiang Xu, Huibiao Liu, Shu Wang, and Daoben Zhu
Beijing National Laboratory for Molecular Sciences (BNLMS),
CAS Key Laboratory of Organic Solids, Institute of Chemistry,
Chinese Academy of Sciences, and Graduate School of the Chinese
Academy of Sciences, Beijing 100080, People’s Republic of China
ylli@iccas.ac.cn
Received March 16, 2007
ABSTRACT
The synthesis and sensing characteristics of a new class of colorimetric and fluorometric dual-modal probe for mercury ion are outlined.
2+
Judicious placement of two dithia-dioxa-aza macrocycles on the BODIPY chromophore generates this interesting molecule. A highly Hg
selective fluorescence enhancing property (>7-fold) in conjunction with a visible colorimetric change from purple to red-pink can be observed,
-
2+
leading to potential fabrication of both “naked-eye” and ratiometric fluorescent detection of Hg
.
+
The design and construction of chemosensors with high
selectivity and sensitivity for heavy metal cations has
received considerable attention, as such metal ions can cause
sensitive and selective assays for Hg2 ions. In recent years
many efforts have been made to design various chemosensors
specific for Hg ion detection. Among the reported sensing
molecules, fluorophores with a fluorescent switching property
driven by photoinduced electron transfer (PET) are often
employed to signal metal ion binding. However, the sensitiv-
2+
4
1,2
severe risks for human health and the environment. Among
2+
2
them, mercury ion (Hg ) is considered as one of the most
dangerous cations for the environment because it is widely
distributed in air, water, and soil. Mercury can accumulate
in the human body and affects a wide variety of diseases
even in a low concentration, such as prenatal brain damage,
serious cognitive and motion disorders, and Minamata
2+
ity and selectivity of the Hg detection based on the PET
mechanism are still unsatisfactory because of their poor
(
3) (a) Grandjean, P.; Weihe, P.; White, R. F.; Debes, F. EnViron. Res.
3
disease. Therefore, it is very important to develop highly
1998, 77, 165-172. (b) Takeuchi, T.; Morikawa, N.; Matsumoto, H.;
Shiraishi, Y. Acta Neuropathol. 1962, 2, 40-57. (c) Harada, M. Crit. ReV.
Toxicol. 1995, 25, 1-24.
(
1) (a) Fluorescent Chemosensors for Ion and Molecule Recognition;
Czarnik, A. W., Ed.; American Chemical Society: Washington, D.C., 1992.
b) de Silva, A. P.; Fox, D. B.; Huxley, A. J. M. Coord. Chem. ReV. 2000,
05, 41-57. (c) He, X.; Liu, H.; Li, Y.; Wang, S.; Li, Y.; Wang, N.; Xiao,
J.; Xu, X.; Zhu, D. AdV. Mater. 2005, 17, 2811-2815.
2) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A.
J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97,
515.
(4) (a) Zhu, X.; Fu, S.; Wong, W.; Guo, J.; Wong, W. Angew. Chem.,
Int. Ed. 2006, 45, 3150-3154. (b) Yoon, S.; Albers, A. E.; Wong, A. P.;
Chang, C. J. J. Am. Chem. Soc. 2005, 127, 16030-16031. (c) Guo, X.;
Qian, X.; Jia, L. J. Am. Chem. Soc. 2004, 126, 2272-2273. (d) Nolan, E.
M.; Lippard, S. J. J. Am. Chem. Soc. 2003, 125, 14270-14271. (e) Song,
K. C.; Kim, J. S.; Park, S. M.; Chung, K. C.; Ahn, S.; Chang, S. K. Org.
Lett. 2006, 8, 3413-3416.
(
2
(
1
1
0.1021/ol0706399 CCC: $37.00
© 2007 American Chemical Society
Published on Web 05/03/2007

fluorescent switching and the interference with other cations,
such as Ag and Pb ions.5
+
2+
Boradiazaindacenes (BODIPY) are very attractive func-
tional groups for construction of molecular sensors because
of their advantageous characteristics, such as sharp absorption
and fluorescence bands, high extinction coefficients, high
fluorescence quantum yields, and high stability against light
7
and chemical reactions. Recently, many fluorescent chemosen-
sors have been reported based on modified boradiazaindacene
8
fluorophores. However, most of them display only fluores-
cence intensity changes, although efficient quenching or a
large fluorescence enhancement are observed due to the
2
efficient oxidative or reductive PET. Lately, a very interest-
ing modification converting a standard green emitting fluoro-
phore to a longer wavelength absorbing and red emitting
intramolecular charge transfer (ICT) fluorophore was re-
9
ported, where a proton-sensitive amino group (electron-
donating) was conjugated to the electron-withdrawing
BODIPY group. It demonstrated that the emission properties
can be switched on with acid addition, affording possibilities
for cation sensing applications with rationally designed
chemosensors. To date, there is still a demand for new
indicators with improved properties, especially colorimetric
Figure 1. (a) Absorbance spectra of compound DMS1 in the
presence of increasing Hg(II) concentrations (0, 2, 4, 6, 8, 10, 14,
1
0
2
0, 24, 30, 35, 40 µM) in a THF-water solution (30:70, v/v, 20
mM HEPES buffer, pH 7.2). The concentration of the DMS1 was
2 µM. (b) In absorption spectra I(564)/I(606) as a function of Hg
concentration.
2+
probes that can make “naked-eye” detection in the visible
wavelength region.6
This work is aimed at the design and construction of a
new class of colorimetric and fluorometric dual-channel assay
solution (30:70, v/v, 20 mM HEPES buffer, pH 7.2) at room
temperature. The UV/vis spectrum of DMS1 in aqueous
solution is characterized by a very intense band centered at
2+
to specifically detect the presence of Hg over a wide range
of other cations. To achieve this goal a modular molecular
system is synthesized that is comprised of BODIPY fluo-
rophore and two crown ether ligands with specific recogni-
tion ability to the Hg2 ion. Our strategy takes advantage of
-
1
-1
6
06 nm (ꢀ 85 000 M cm ), which is responsible for the
purple color of the solution. The absorption maximum of
DMS1 has about a 100 nm red shift in comparison to that
+
2
both PET and ICT processes on a single molecule.
7
a
of the standard BODIPY dye. This red shift was assigned
to an efficient ICT process from the donor nitrogen atom on
the dithia-dioxa-aza macrocyclesthe one that conjugated to
3
Compound 1 reacted with POCl in the presence of DMF
to afford compound 2 with 63% yield. Compound 3 was
obtained by reaction of compound 2 with 2,4-dimethylpyrrole
with added TFA as catalyst followed by oxidization with
2
,9,10
fluorophore to the acceptor BODIPY group.
Upon adding
Hg the intensity of the absorption maximum of DMS1 at
06 nm gradually decreased following the formation of a
2
+
DDQ and treatment with BF
3
2
-OEt with 31% yield. Target
6
molecule DMS1 was synthesized by the reaction of com-
pound 3 with compound 2 in toluene, using piperidine, glacial
-
1
-1
new band centered at 564 nm (ꢀ 95 000 M cm ), and an
isosbestic point at 578 nm was observed. The coordination
4 2
acetic acid, and Mg(ClO ) as catalyst to afford a purple solid.
2+
of the Hg to the ligand reduces the electron-donating ability
of the nitrogen atom at the dithia-dioxa-aza macrocycle,
which conjugated to BODIPY core, thus the ICT process is
not possible any more and the red shift in absorption spectra
is suppressed. In other words, the blue shift in absorption
Figure 1 showed the absorption spectral changes of DMS1
2+
as a function of the Hg concentration in a THF-water
(
5) Rurack, K.; Kollmannsberger, M.; Resch-Genger, U.; Daub, J. J. Am.
Chem. Soc. 2000, 122, 968-969.
6) (a) Sancen o´ n, F.; Mart ´ı nez-M a´ nˇ ez, R.; Soto, J. Chem. Commun. 2001,
2
+
(
spectra is observed upon Hg binding. More importantly,
2
262, 2263. (b) Gunnlaugsson, T.; Leonard, J. P.; Murray, N. S. Org. Lett.
004, 6, 1557-1560.
2+
the Hg sensing and the concomitant absorption changes
2
(
7) (a) Kollmannsberger, M.; Rurack, K.; Resch-Genger, U.; Daub, J. J.
were clearly visible to the naked eye, as can be seen in the
photograph, where the purple solution of DMS1 became red-
Phys. Chem. A 1998, 102, 10211-10220. (b) Pavlopoulos, T. G.; Shah,
M.; Boyer, J. H. Appl. Opt. 1988, 27, 4998-4999.
2
+
pink upon titration with Hg ions.
(8) (a) Gabe, Y.; Urano, Y.; Kikuchi, K.; Kojima, H.; Nagano, T. J.
Am. Chem. Soc. 2004, 126, 3357-3367. (b) Li, Z.; Miller, E. W.; Pralle,
A.; Isacoff, E. Y.; Chang, C. J. J. Am. Chem. Soc. 2006, 128, 10-11. (c)
Yamada, K.; Nomura, Y.; Citterio, D.; Iwasawa, N.; Suzuki, K. J. Am.
Chem. Soc. 2005, 127, 6956-6957. (d) Baruah, M.; Qin, W.; Vall e´ e, R. A.
L.; Beljonne, D.; Rohand, W.; Bonens, N. Org. Lett. 2005, 7, 4377-4380.
To verify specificity, experiments were conducted with
3+
2+
2+
2+
2+
other metal cations, such as Fe , Co , Mg , Ni , Zn ,
2
+
2+
2+
+
3+
Cd , Mn , Pb , Ag , and Al . As shown in Figure 2,
2+
under identical condition to Hg ion, no changes were
(
e) Qi, X.; Jun, E. J.; Xu, L.; Kim, S.-J.; Hong, J. S. J.; Yoon, Y. J.; Yoon,
J. J. Org. Chem. 2006, 71, 2881-2884.
9) Rurack, K.; Kollmannsberger, M.; Daub, J. Angew. Chem., Int. Ed.
001, 40, 385-387.
10) Coskun, A.; Akkaya, E. U. J. Am. Chem. Soc. 2005, 127, 10464-
0465.
observed in the UV/vis spectra of DMS1 upon addition of
2
+
2+
2+
2+
2+
2+
2+
+
(
10 equiv of Co , Mg , Ni , Zn , Cd , Mn , Pb , Ag ,
2
3
+
3+
Al , and Fe ions or mixed ions, this could probably be
(
attributed to their low affinity with the receptor DMS1.1
0,13
1
2314
Org. Lett., Vol. 9, No. 12, 2007

Figure 2. (a) Absorbance spectra change of DMS1 (2 µM) upon
addition of different metal cations (10 equiv) in a THF-water
solution (30:70, v/v, 20 mM HEPES buffer, pH 7.2). (b) Color
change of DMS1 in the presence of different metal cations. From
3
+
3+
2+
2+
2+
+
2+
left to right: control, Al , Fe , Co , Cu , Cd , Ag , Ni
,
2+
2+
2+
2+
Pb , Mn , Zn , and Hg . (c) Absorbance spectra change of
DMS1 (2 µM) upon addition of different metal cations (10 equiv)
2
+
Figure 3. (a) Emission spectra of compound DMS1 in the presence
of increasing Hg(II) concentration (0, 2, 4, 6, 8, 10, 14, 20, 24, 30,
35, 40 µM) in a THF-water solution (30:70, v/v, 20 mM HEPES
buffer, pH 7.2). Excitation wavelength was 540 nm with 5 nm slit
widths. The concentration of the chemosensor was 2 µM. (b) In
emission spectra I(578)/I(668) as a function of Hg2 concentration.
and subsequent addition of 30 µM (15 equiv) Hg in aqueous
solution. (d) Absorbance spectra change of DMS1 (2 µM) upon
3
+
3+
2+
2+
addition of mixed cations (mix ) Al + Fe + Co + Cu
+
2+
+
2+
2+
2+
2+
Cd + Ag + Ni + Pb + Mn + Zn ); each one is 10
equiv metal cations and subsequent addition of 100 µM (50 equiv)
+
2+
Hg in aqueous solution.
electron-donating ability of dithia-dioxa-aza macrocycle to
directly connect to the BODIPY fluorophores, thus the main
quenching process (PET) is partially suppressed and causes
an enhancement in the emission. Second, coordination of
the Hg to the ligand that is conjugated to the BODIPY
core suppressed the ICT process, and this process also
contributes to the fluorescence intensity enhancement. We
suggest that the suppressed PET process is the main reason
for >7-fold fluorescence intensity enhancement, but the
suppressing of the ICT process we mentioned above is the
only reason that gives rise to large changes in the absorption
and emission spectra.
The other cations also did not induce any significant color
change of DMS1. Therefore, DMS1 can be considered as
an effective colorimetric probe for Hg . The advantage of
this assay is that naked-eye detection of Hg2+ becomes
2+
2
+
possible.
Note that a fluorometric detection of Hg2+ is also possible
for DMS1. As expect DMS1 is virtually nonfluorescent in
apo state (φ ) 0.04), which resulted from the efficient
photoinduced electron transfer (PET) quenching of the
excited state of the BODIPY moiety by the long pair of
electrons on the nitrogen atoms in the dithia-dioxa-aza
macrocycle. As shown in Figure 3, upon adding the Hg2+
2,8
To explore further the utility of DMS1 as an ion-selective
ion, the fluorescence intensity of DMS increases by over
2+
fluorescence probe for Hg ion, the competition experiments
were conducted. Figure 4 depicts the fluorescence responses
7-fold (φ ) 0.33) accompanied by a large blue shift in the
emission spectrum. The emission spectrum of free DMS1
2
+
3+
2+
of DMS1 to the presence of 10 equiv of Hg , Fe , Co ,
displays a broad band with a maximum at 668 nm. When
2
+
2+
2+
2+
2+
2+
+
3+
2+
Mg , Ni , Zn , Cd , Mn , Pb , Ag , and Al . Due
to the low affinity with the receptor DMS1,
Hg was added to the solution of DMS1, an emission
intensity decrease at 668 nm and a significant increase at
1
0,13
the other
metal ions showed nearly no changes in emission spectra.
Importantly, the large cation-induced blue shift for Hg2
in emission spectra resulted in a color change from rose to
yellow. This visible emission allows DMS1/Hg2 to be
readily distinguished by the naked eye under UV light.
To uncover the PET and ICT process reference compound
5
78 nm were observed. How could we explain these
+
phenomena? We speculate that the fluorescence intensity
enhancement could contribute to two points. First, the capture
of Hg2 by the receptor resulted in the reduction of the
+
+
(11) Descalzo, A. B.; Mart ´ı nez-M a´ nˇ ez, R.; Radeglia, R.; Rurack, K.;
Soto, J. J. Am. Chem. Soc. 2003, 125, 3418-3419.
3
and compound 4 were used to investigate. We noticed that
(
(
12) Brannon, J. H.; Magde, D. J. Phys. Chem. 1978, 82, 705-709.
13) (a) Coskun, A.; Akkaya, E. U. J. Am. Chem. Soc. 2006, 128, 14474-
2
+
adding Hg to solutions of compound 3 did not induce any
1
4475. (b) Lee, S. J.; Jung, J. H.; Seo, J.; Yoon, I.; Park, K.-M.; Lindoy,
shifts in emission spectra, only fluorescence intensity
L. F.; Lee, S. S. Org. Lett. 2006, 8, 1641-1643. (c) Ros-Lis, J. V.; Mart ´ı nez-
M a´ nˇ ez, R.; Rurack, K.; Sancen o´ n, F.; Soto, J.; Spieles, M. Inorg. Chem.
2
+
enhancement can be observed. Adding Hg to solutions of
2
004, 43, 5183-5185.
compound 4 induced a large blue shift in both emission and
Org. Lett., Vol. 9, No. 12, 2007
2315

selectivity and sensitivity on the basis of the tuning of PET
2+
and ICT processes on a single molecule. A highly Hg -
selective fluorescence enhancing property (>7-fold) in
conjunction with a visible colorimetric change from purple
to red-pink can be observed. Significantly, our results make
2
+
“
naked-eye” and ratiometric fluorescent Hg detection in a
single molecule possible. Because the detection scheme is
entirely ligand based, the design strategy is general enough
to be readily extended to the development of chemosensors
for a variety of different metal ions.
Scheme 1. Synthesis of Dual-Modal Mercurysensor-1 (DMS1)
Figure 4. (a) Fluorescence spectra changes of DMS1 (2 µM) upon
addition of different metal cations (10 equiv) in a THF-water
solution (30:70, v/v, 20 mM HEPES buffer, pH 7.2). (b) Color
change of DMS1 in the presence of different metal cations. From
3
+
2+
2+
left to right and from top to bottom: control, Al , Cu , Co
,
3+
2+
+
2+
2+
2+
2+
2+
Fe , Cd , Ag , Ni , Pb , Mn , Zn , Hg . (c) Fluorescence
spectra changes of DMS1 (2 µM) upon addition of different metal
cations (10 equiv) and subsequent addition of 30 µM (15 equiv)
2+
Hg in aqueous solution (d) Fluorescence spectra changes of
3+
DMS1 (2 µM) upon addition of mixed metal cations (mix ) Al
3
+
2+
2+
2+
+
2+
2+
2+
+
+
(
Fe + Co + Cu + Cd + Ag + Ni + Pb + Mn
2
+
Zn ; each one is 10 equiv) and subsequent addition of 100 µM
2+
50 equiv) Hg in aqueous solution
absorbance spectra. These two phenomena proved that the
two binding sites underwent different photophysical pro-
cesses. One is the PET process and another is the ICT process
Acknowledgment. This work was supported by the
National Natural Science Foundation of China (20531060,
0474101, 20418001, 20473102, and 20571078) and the
Major State Basic Research development Program (2006-
CB806200 and 2005CB623602)
(see the Supporting Information).
1
The two binding sites one decoupled from the BODIPY
chromophore and the one conjugated to the BODIPY system
should have different basicities and so have different
stoichiometry and binding constants. To better understand
the stoichiometry and the binding constant of each ligand,
we investigate them respectively through reference com-
pound 3 and compound 4. Binding analysis by using the
method of continuous variation (Job’s plot) established that
Note Added after ASAP Publication. Dual-Model was
corrected to Dual-Modal in the title and throughout the paper
in the version published May 7, 2007; the incorrect version
was published May 3, 2007.
Supporting Information Available: Experimental details
including photophysical properties of 3, 4, and DMS1, details
of Job’s plots, and a Benesi-Hildebrand analysis. This
material is available free of charge via the Internet at
http://pubs.acs.org.
2+
both the receptors formed the 1:1 Hg -crown complex.
Following a Benesi-Hildebrand-type analysis, the binding
4
4
constants were determined to be 4.3 × 10 and 2.9 × 10
-
1
3
mol dm individually (see the Supporting Information).
In conclusion, we have developed a new class of colori-
metric and fluorometric probe for Hg2+ detection with high
OL0706399
2316
Org. Lett., Vol. 9, No. 12, 2007