Ratiometric fluorescent chemosensor for Hg2+ based on heptamethine cyanine containing a thymine moiety

Article abstract of DOI:10.1021/ol3004047

Based on a T-Hg²⁺-T binding mode, a sensitive ratiometric fluorescent chemosensor for aqueous Hg²⁺ was developed with a heptamethine cyanine chromophore containing a thymine moiety.

Full text of DOI:10.1021/ol3004047

ORGANIC
LETTERS
2
012
Vol. 14, No. 8
986–1989
Ratiometric Fluorescent Chemosensor for
Hg Based on Heptamethine Cyanine
Containing a Thymine Moiety
2
þ
1
Hong Zheng,* Xiao-Juan Zhang, Xin Cai, Qing-Na Bian, Min Yan, Ge-Hui Wu,
Xue-Wang Lai, and Yun-Bao Jiang
Department of Chemistry, College of Chemistry and Chemical Engineering, and the
MOE Key Laboratory of Analytical Sciences, Xiamen University, Xiamen 361005,
China
hzheng@xmu.edu.cn
Received February 18, 2012
ABSTRACT
Based on a TꢀHg2þꢀT binding mode, a sensitive ratiometric fluorescent chemosensor for aqueous Hg2þ was developed with a heptamethine
cyanine chromophore containing a thymine moiety.
The release of heavy metals into the environment origi-
nates from a variety of man-made and natural sources,
including industrial processes for agriculture, the paper
industry, pharmaceutical uses, fossil fuel combustion, the
intensities; however, in most practical applications,
changes in fluorescence intensity can also be caused by
other variable factors such as photobleaching, variation of
concentration of the probe molecule, different microenvi-
ronments around the probe molecule, or the stability of
the light source. Therefore, it is generally preferable for
a fluorescent sensory signal to involve new emission at a
different wavelength rather than modulation of an existing
signal. This new approach should take the ratiometric
signal measurements at two emissive wavelengths where
the intensity responses are generally reversed. Therefore,
this ratiometric method can overcome the limitations of
intensity-based measurements due to a built-in correction
for environmental effects. Up to now, only a few ratio-
1
electronics industry, and the burning of coal. Thus, the
detection of the toxic metal ions in various chemical
systems is an important topic. Among other heavy metal
2þ
ions, Hg has attracted great interest from chemists in
recent years because of its special chemical properties and
2
its lethal effects on the environment and living organisms.
As a consequence, a number of new chemosensors for
2þ
Hg with desirable properties have been designed and
synthesized and have recently been discussed in excellent
3
2þ
review articles. Monitoring of Hg by most of these
chemosensors is involved in only changes in the emissive
2þ
metric fluorescent probes for Hg have been reported,
and the ratiometric fluorescent signal of these probes,
however, are clearly based on mechanisms namely
(
1) (a) Shotyk, W.; Weiss, D.; Appleby, P. G.; Cheburkin, A. K.; Frei,
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(
Molecule Recognition; Kluwer: Dordrecht, 1997. (b) Fluorescent Chemo-
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1
0.1021/ol3004047 r 2012 American Chemical Society
Published on Web 04/09/2012

4
intramolecule charge transfer (ICT), excimerꢀmonomer
thymine-modified heptamethine cyanine probe 1. The syn-
thetic procedures of 1are given in Scheme 1, and its structural
identification was confirmed by H NMR, C NMR, and
ESI-MS spectroscopy (Supporting Information).
5
transfer, the excited-state intramolecular proton transfer
ESIPT), or energy transfer.
On the other hand, the self-association of dyes is a
6
,7
1
13
(
frequently encountered phenomenon, especially for many
classes of dyes when used in aqueous solution. The aggre-
gation effect of dyes also exerts a strong influence on their
spectroscopic characteristics, and these spectral changes
can be attributed to the aggregation of the dye molecules
in water to form dimers and higher order aggregates in the
a
Scheme 1. Synthetic Route of 1
“
J-” or “H-” type aggregation state. Though polymethine
cyanines are among the best known self-associating dyes in
aqueous solutions, and this self-association is clearly re-
8
flected by changes in the absorption spectra, chemosensor
studies based on this behavior have seldom been reported,
except for a colorimetric and ratiometric fluorescent che-
þ 9
mosensor for Ag . On the basis of these facts, we herein
report a new polymethine cyanine-based probe for the
efficient chromogenic and ratiometric fluorescent recogni-
2þ
tion of Hg as a new advance in this field.
Based on the fact that thymine (T) has proven to be one
2þ
of the most selective ligand binding to Hg in the form
10
a
2þ
Key: (a) CH OH, SOCl , reflux 3 h; (b) ethylenediamine, CH OH,
3
2
3
of TꢀHg ꢀT, we envisioned that this binding mode
reflux 3 h, N
CH OH, reflux 6 h, N
2
atmosphere; (c) 1,8-bis(dimethylamino)naphthalene,
atmosphere.
2þ
would contribute to fulfill the Hg -modulation of the
aggregation state of a heptamethine cyanine-based chro-
mophore in aqueous medium. Therefore, we introduced
a thymine group into the cyclohexene bridgehead of
the heptamethine cyanine chromophore, leading to a new
3
2
Next, the spectroscopic characteristics of probe 1 in
aqueous solution was studied. The absorption spectral
2þ
traces of 1 upon coordination with Hg in an optimized
buffer solution of 3,3-dimethylglutaric acidꢀNaOH(10mM
(
5) (a) Sasaki, D. Y.; Padill, B. E. Chem. Commun. 1998, 1581–1582.
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in MeOH/H O = 2/98, v/v) at pH 6.6 were monitored
2
(
1
first. As shown in Figure 1, the solution of 1 alone (2.0 ꢁ
S. K. Org. Lett. 2007, 9, 1129–1132. (d) Yang, M. H.; Thirupathi, P.; Lee,
K.-H. Org. Lett. 2011, 13, 5028–5031.
ꢀ5
10
M) exhibits an absorption maximum at 628 nm,
which is responsible for the blue color of the solution.
(
6) By ESIPT mechanism:Santra, M.; Roy, B.; Ahn, K. H. Org. Lett.
011, 13, 3422–3425.
7) (a) Coskun, A.; Akkaya, E. U. J. Am. Chem. Soc. 2006, 128,
4474–14475. (b) Shang, G. Q.; Gao, X.; Chen, M. X.; Zheng, H.; Xu,
2
2þ
With increasing Hg concentrations, the absorbance at
628 nm decreased, while the absorbance at 510 nm in-
(
1
J. G. J. Fluoresc. 2008, 18, 1187–1192. (c) Zhang, X. L.; Xiao, Y.; Qian,
X. H. Angew. Chem., Int. Ed. 2008, 47, 8025–8029. (d) Suresh, M.;
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Sun, L. N.; Li, F. Y. ACS Nano 2011, 5, 8040–8048.
creased accordingly. This pronounced hypsochromic shift
of the maximum absorption wavelength can be ascribed to
the H-aggregation state of the cyanine dye resulted from
2þ 8
the coordination of Hg . Meanwhile, an isosbestic point
was clearly observed around 538 nm, indicating the con-
version of the free molecules into aggregation molecules.
In addition, such a large blue-shift of 118 nm in the
absorption behavior changes the color of the resultant
solution from blue into pink, allowing “naked-eye” detec-
tion (Figure S3, Supporting Information). Furthermore,
(
8) Mishra, A.; Behera, R. K.; Behera, P. K.; Mishra, B. K.; Behera,
G. B. Chem. Rev. 2000, 100, 1973–2011.
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(
(
4
Kudo, M.; Tanaka, Y.; Kondo, Y.; Sawa, R.; Fujimoto, T.; Machinami,
T.; Ono, A. J. Am. Chem. Soc. 2006, 128, 2172–2173. (c) Tanaka, Y.;
Oda, S.; Yamaguchi, H.; Kondo, Y.; Kojima, C.; Ono, A. J. Am. Chem.
Soc. 2007, 129, 244–245. (d) Bagno, A.; Saielli, G. J. Am. Chem. Soc.
the experiments of ratiometric absorbance (A510/A628
)
þ
response of 1 toward other metal ions, including Ag ,
2þ
2007, 129, 11360–11361. (f) Lee, J.-S.; Han, M. S.; Mirkin, C. A. Angew.
2
þ
þ
2þ
2þ
3þ
2þ
2þ
2þ
þ
2þ
þ
2þ
3þ
3þ
Chem., Int. Ed. 2007, 46, 4093–4096. (g) Liu, J.; Lu, Y. Angew. Chem.,
Int. Ed. 2007, 46, 7587–7590. (h) Li, D.; Wieckowska, A.; Willner, I.
Angew. Chem., Int. Ed. 2008, 47, 3927–3931. (i) Xue, X.; Wang, F.; Liu,
X. J. Am. Chem. Soc. 2008, 130, 3244–3245. (j) Liu, C. W.; Hsieh, Y. T.;
Huang, C. C.; Lin, Z. H.; Chang, H. T. Chem. Commun. 2008, 2242–
Zn , Cd , Mn , Fe , Cu , Ni , Pb , Cr , Al ,
þ
2
Co , Na , Ba , Mg , Ca , and K , suggested the high
2
absorption selectivity to Hg (Figures S4 and S5, Sup-
porting Information).
We also noticed that the reaction of 1 with Hg pro-
duced obvious fluorescence emission changes (Figure 2).
2
1
244. (k) Che, Y. K.; Yang, X. M.; Zang, L. Chem. Commun. 2008, 1413–
415. (l) Wang, Z.; Lee, J. H.; Lu, Y. Chem. Commun. 2008, 6005–6007.
2þ
(m) Wang, H.; Wang, Y.; Jin, J.; Yang, R. Anal. Chem. 2008, 80, 9021–
9028. (n) Ye, B. C.; Yin, B. C. Angew. Chem., Int. Ed. 2008, 47, 8386–
A solution of 1 displayed two emission peaks, strong at
537 nm and weak at714 nm, whenexcited at 500 nm. When
8389. (o) He, S.; Li, D.; Zhu, C.; Song, S.; Wang, L.; Long, Y.; Fan, C.
Chem. Commun. 2008, 4885–4887. (p) Gao, X.; Xing, G.; Yang, Y.; Shi,
X.; Liu, R.; Chu, W.; Jing, L.; Zhao, F.; Ye, C.; Yuan, H.; Fang, X.;
Wang, C.; Zhao, Y. J. Am. Chem. Soc. 2008, 130, 9190–9191. (q) Ono,
A.; Torigoe, H.; Tanakac, Y.; Okamoto, I. Chem. Soc. Rev. 2011, 40,
2þ
Hg was added to the solution of 1, a distinct decrease in
the 714 nm emission and an increase in the fluorescence
at 537 nm were observed, with a clear isoemission point at
5855–5866.
Org. Lett., Vol. 14, No. 8, 2012
1987

1
for fluorimetry. The association constant between 1
1
662 nm. Both the absorbance ratio of A510/A628 and
emission ratio of F537/F714 increase with the increasing
in Hg concentrations, allowing the Hg concentration
to be determined ratiometrically (Figure S6, Supporting
Information).
2þ
10
ꢀ2 12
and Hg was estimated as 2.2 ꢁ 10 M . In addition,
the fluorescence quantum efficiency (Φ) of probe 1 in the
absence and presence of Hg were determined to be 0.012
2þ
2þ
2þ
1
3
and 0.042, respectively.
Then, the ratiometric fluorescent response of 1 was
analyzed to determine its selectivity in the presence
of metal ions under identical conditions. As shown in
Figure 3, the solution of 1 alone exhibits a low emission
ratio of F537/F714, but upon the addition of 1.0 equiv of
2þ
Hg there is a prominent enhancement of this value, and
the remainder of the tested metal ions did not greatly alter
the ratiometric value relative to 1 alone. In addition, a
competitive experiment revealed that all of the other tested
metal ions had minor or no interference with the ratio-
2þ
metric signal response to Hg ; therefore, these results
2þ
suggest that 1 has a high fluorescent selectivity for Hg in
the presence of these tested foreign metal ions (Figure S7,
Supporting Information).
2þ
Figure 1. Absorption spectra of 1 with Hg at pH 6.60 of
.01 M 3,3-dimethylglutaric acidꢀNaOH buffer solution
MeOH/H O = 2/98, v/v). [1] = 20.0 μmol/L, the concentration
of Hg values rangingfrom 0 to10.0μmol/L. λex/λem =500nm/
37 nm, 714 nm; slit: ex/em = 10.0/20.0 nm.
0
(
2
2þ
5
Figure 3. Fluorimetric responses of 1 to various cations at pH
6
tion (MeOH/H
.60 of 0.01 M 3,3-dimethylglutaric acidꢀNaOH buffer solu-
nþ
2
O = 2/98, v/v). [1] = 20.0 μmol/L, [M ] =
þ
2þ
1
Mn (4), Fe (5), Cu (6), Ni (7), Pb (8), Hg (9), Cr
0.0 μmol/L. From left to right: no cation (1), Ag (2), Zn (3),
2
þ
3þ
2þ
2þ
2þ
2þ
3þ
2þ
3
þ 2þ þ 2þ 2þ
(
(
10), Al (11), Co (12), Na (13), Ba (14), Mg (15), Ca
16), K (17), Cd (18). λ /λ = 500/537 nm, 714 nm; slit:
þ
2þ
ex em
ex/em = 10.0/20.0 nm.
2þ
In this work, with the aid of the TꢀHg ꢀT binding
2
þ
Figure 2. Fluorescence emission spectra of 1 with Hg at pH
.60 of 0.01 M 3,3-dimethylglutaric acidꢀNaOH buffer solution
MeOH/H O = 2/98, v/v). [1] = 20.0 μmol/L, the concentration
of Hg values rangingfrom 0 to 10.0 μmol/L. λex/λem = 500 nm/
mode, we carried out the ratiometric fluorescent sensing of
2þ
6
(
Hg in aqueous solution by modulating the aggregation
state of the heptamethine cyanine fluorophore. Therefore,
2
2þ
2þ
the binding stoichiometry of 1 to Hg was further deter-
1
5
37 nm,714 nm; slit: ex/em = 10.0/20.0 nm.
4
mined using a Job’s plot, and the results showed that the
ratiometric values of both the absorbance (A510/A628
)
Meanwhile, the limits of detection were calculated to
M
and the fluorescence (F537/F714) of the system (1 þ
be 1.6ꢁ 10ꢀ M for spectrophotometry and 4.8 ꢁ 10ꢀ9
8
2þ
Hg ) reached a maximum value when the molecular
2þ
fraction of Hg was close to 0.3 (Figure S8, Supporting
(
11) According to the definition by IUPAC (CDL = 3Sb/m). See:
IUPAC compendium of analytical nomenclature, Definitive Rules; Irving,
H. M. N. H., Freiser, H., West,T. S., Eds.; Pergamon Press, Oxford, 1981.
(13) In reference to rhodamine B (Φ) of 0.69 in methanol. See:
Velapoldi, R. A.; Tonnesen, H. H. J. Fluoresc. 2004, 14, 465–472.
(14) Vosburgh, W. C.; Copper, G. R. J. Am. Chem. Soc. 1941, 63,
437–442.
(12) (a) Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71,
703–2707. (b) Barra, M.; Bohne, C.; Scaiano, J. C. J. Am. Chem. Soc.
2
1990, 112, 8075–8079.
1
988
Org. Lett., Vol. 14, No. 8, 2012

2þ
that, in aqueous solutions, the binding of Hg with the
thymine moiety in probe 1 results in the forma-
2þ
tion of the 1ꢀHg ꢀ1 complex, bringing the two inner
heptamethine cyanine chromophores of this complex
into proximity, and this conformation is similar to that
of “H”-aggregates, leading to a change in the solution
color and to changes in the ratiometric fluorescence be-
havior. In addition, as the formation of a tightly bound
dimer aggregate may suppress the short-lived decay
1
5
channel and favor of the long-lived decay component,
fluorescent lifetimeinduced by a tightly bound aggregation
would be much (always several folds) longer than that by
1
5,16
monomer.
and after the addition of Hg , were determinated to be
.29 and 0.36 ns, respectively, and this moderate increasing
in lifetime may suggest the formation of loosely bound
Therefore, the lifetimes of probe 1 before
2þ
0
2þ
1
Figure 4. Partial spectra of H NMR titration of 1 with Hg
2þ
H-dimer aggregate of 1 in the presence of Hg
In summary, compound 1 was designed as new fluo-
rescent Hg sensor by making use of the binding of
thymine with Hg to induce cyanine aggregation in
aqueous solution. The fluorescent spectral results clearly
indicate that compound 1 can be used as a ratiometric
fluorescent sensor for Hg with excellent selectivity and
sensitivity. Further studies including design of more fluo-
rescent chemosensors toward various analytes based on
same strategy are underway in this laboratory.
.
.
2þ
Equivalents of Hg relative to 1 (from bottom to top): 0, 0.1,
.3, 0.5, 0.7.
2þ
0
2þ
Information). These data indicate the formation of a
2
2þ
:1 (1:Hg ) complex, and the binding stoichiometry
2þ
2þ
agrees well with the known TꢀHg ꢀT mode.
1
The H NMR titration in deuterated DMSO solution
was also carried out (Figure 4). The data revealed that the
proton signal of imide in thymine moiety at the lowest field
(
marked in red circle) in the NMR spectra disappeared in
2þ
2þ
Acknowledgment. This work was financially supported
by the MOST of China (No.2011CB910403), the National
Natural Science Foundation of China (No.20675067 and
No.20835005), the Natural Science Foundation of Fujian
Province of China 2010J01051, and the NFFTBS (No.
J1030415), which are gratefully acknowledged.
the presence of Hg , strongly suggesting that Hg bound
to the thymine moiety at the nitrogen atom of imide group.
In addition, the chemical shifts (δ 5.0ꢀ8.0) of the unsatu-
rated protons in the heptamethine cyanine moiety and the
chemical shifts (δ 8.6, overlap, s þ s, 2H) of two protons of
ꢀ
NH (one at the bridgehead of cyclohexenyl ring and one
at amide group) did not show any significant changes,
which can exclude the interaction of the heptamethine
Supporting Information Available. Synthetic details
2þ
1
13
cyanine chromophore with Hg . From the above experi-
mental evidence of both the binding stoichiometry and the
and H and C NMR characterizations of 3, 4, and 1,
the pH and time course effect, details of Job’s plots, and
some related absorption and fluorescence spectral re-
1
2þ
H NMR analysis of the Hg titrations, we can conclude
2
þ
sponses of 1 for Hg . This material is available free of
charge via the Internet at http://pubs.acs.org.
(
15) Rosch, U.; Yao, S.; Wortmann, R.; Wurthner, F. Angew. Chem.,
Int. Ed. 2006, 45, 7026–7030.
16) Zeena, S.; Thomas, K. G. J. Am. Chem. Soc. 2001, 123, 7859–
865.
(
7
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 8, 2012
1989