Fluorescence "turn on" chemosensors for Ag+ and Hg2+ based on tetraphenylethylene motif featuring adenine and thymine moieties

Article abstract of DOI:10.1021/ol801855s

(Chemical Equation Presented) Two new tetraphenylethylene (TPE) compounds 1 and 2 bearing adenine and thymine moieties, respectively, were found to be fluorescence "turn on" chemosensors for Ag⁺ and Hg ²⁺ by making use of the AIE fea

Full text of DOI:10.1021/ol801855s

ORGANIC
LETTERS
2008
Vol. 10, No. 20
4581-4584
Fluorescence “Turn On” Chemosensors
for Ag+ and Hg2+ Based on
Tetraphenylethylene Motif Featuring
Adenine and Thymine Moieties
Lei Liu, Guanxin Zhang, Junfeng Xiang, Deqing Zhang,* and Daoben Zhu
Beijing National Laboratory for Molecular Sciences, Organic Solids Laboratory,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China, and
Graduate School of Chinese Academy of Sciences, Beijing 100049, China
dqzhang@iccas.ac.cn
Received August 9, 2008
ABSTRACT
Two new tetraphenylethylene (TPE) compounds 1 and 2 bearing adenine and thymine moieties, respectively, were found to be fluorescence
“turn on” chemosensors for Ag+ and Hg2+ by making use of the AIE feature of TPE motif and the specific binding of adenine/thymine with
Ag+/Hg2+
.
Mercury ions act as severe environmental pollutants, and
several diseases are known to be associated with mercury
contamination.1 Silver ions also have adverse biological
effects, and there are are many reports on silver bioaccu-
mulation and toxicity.2 For instance, silver ions inactivate
sulfhydryl enzymes and combine with amine, imidazole, and
carboxyl groups of various metabolites. Thus, development
of sensitive and selective chemosensors for Ag+ and Hg2+
in various media is of considerable importance for environ-
mental protection and human health.
atomic absorption spectroscopy, inductively coupled plasma-
mass spectroscopy, and potentiometric methods based on ion-
selective electrodes have been described for the trace-quantity
(down to ppb range) determination of Ag+ and Hg2+.3,4a In
comparison, sensitive and selective optical sensors for Ag+
and Hg2+ with simple instrumental implementation and easy
operation have received a lot of attention. But, both Ag+
and Hg2+ are heavy transition-metal ions (HTM) which are
known as fluorescence quenchers. Most of the reported
fluorescent chemosensors for Ag+ and Hg2+ are based on a
Traditional quantitative approaches of Ag+ and Hg2+ are
expensive and time-consuming in practice. For instance,
(3) (a) Pu, Q. S.; Sun, Q. Y.; Hu, Z. D.; Su, Z. X. Analyst 1998, 123,
239–243. (b) Dadfarnia, S.; Haji Shabani, A. M.; Gohari, M. Talanta 2004,
64, 682–687. (c) Anderson, P.; Davidson, C. M.; Littlejohn, D.; Ure, M. A.;
Shand, C. A.; Cheshive, M. V. Anal. Chim. Acta 1996, 327, 53–60. (d)
Guo, Y.; Din, B. J.; Liu, Y. W.; Chang, X. J.; Meng, S. M.; Liu, J. H.
Talanta 2004, 62, 209–215. (e) Ceresa, A.; Radu, A.; Peper, S.; Bakker,
E.; Pretsch, E. Anal. Chem. 2002, 74, 4027–4036. (f) Javanbakht, M.;
Ganjali, M. R.; Norouzi, P.; Badiei, A.; Hasheminasab, A. Electroanalysis
(1) (a) Onyido, I.; Norris, A. R.; Buncel, E. Chem. ReV. 2004, 104, 5911–
5929. (b) Harris, H. H.; Pickering, I.; George, G. N. Science 2003, 301,
1203.
(2) (a) Ratte, H. T. EnViron. Toxicol. Chem. 1999, 18, 89–108. (b)
Kazuyuki, M.; Nobuo, H.; Takatoshi, K.; Yuriko, K.; Osamu, H.; Yashihisa,
I.; Kiyoko, S. Clin. Chem. 2001, 47, 763–766. (c) Wan, A. T.; Conyers,
R. A.; Coombs, C. J.; Masterton, J. P. Clin. Chem. 1991, 37, 1683–1687.
2007, 19, 1307–1314
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10.1021/ol801855s CCC: $40.75
Published on Web 09/12/2008
2008 American Chemical Society

fluorescence quenching mechanism.4 By now, only a few
fluorescence “turn on” chemosensors for Ag+ and Hg2+ have
been described.4a,5 Herein, we report fluorescence “turn on”
chemosensors for Ag+ and Hg2+ by making use of the unique
aggregation-induced emission (AIE) feature of tetraphenyl-
ethylene motif and the selective binding abilities of adenine
(with Ag+) and thymine (with Hg2+).6
TPE compounds 1 and 2 (Scheme 2), aggregation would
occur in the presence of Ag+ and Hg2+, respectively, leading
to fluorescence enhancement. Therefore, it is anticipated that
Scheme 2. Synthetic Approach for Compounds 1 and 2
The design rationale for these Ag+ and Hg2+ chemosensors
is schematically illustrated in Scheme 1 and explained as
Scheme 1
.
Design Rationale for the Ag+ and Hg2+
Chemosensors
selective fluorescence “turn on” chemosensors for Ag+ and
Hg2+ can be established with TPE compounds 1 and 2,
respectively. The intramolecular coordination may also occur
within 1 or 2 (see Scheme 1), and accordingly, the intramo-
lecular rotations would be also restricted, leading to fluo-
rescence enhancement.
In this paper, we will describe the synthesis and spectral
variation of 1 and 2 after addition of Ag+ and Hg2+. The
results show that the fluorescence intensities of 1 and 2 are
remarkably enhanced after introducing Ag+ and Hg2+; thus,
compounds 1 and 2 can function as fluorescence “turn on”
chemosensors for Ag+ and Hg2+.
The synthesis of compounds 1 and 2 started from 4,4′-
dihydroxybenzophenone which was converted to dibromo 3
by reaction with 1,3-dibromopropane. Compound 4 was
yielded through a McMurry reaction between compound 3
and diphenyl ketone. Further reactions of dibromotetraphe-
nylethylene 4 with adenine and thymine in the presence of
K2CO3 led to compounds 1 and 2, respectively, in acceptable
yields. The chemical structures of these new compounds were
established by spectroscopic and elemental analysis data.
Figure 1 shows the fluorescence spectrum of compound 1
and those in the presence of different amounts of AgClO4
in H2O/THF (5:1, v/v). Compound 1 shows rather weak
emission at this concentration (see Figure S1-1 of the
Supporting Information). However, after addition of AgClO4,
the emission band at 470 nm emerged and its intensity
increased gradually as displayed in Figure 1. Actually, the
fluorescence difference for the solution of 1 before and after
addition of Ag+ can be distinguished by the naked eye as
displayed in the inset of Figure 1. Moreover, the fluorescence
intensity of 1 at 470 nm increases linearly with the
follows: (1) tetraphenylethylene (TPE) derivatives show weak
fluorescence in solution, but they become strong emission
after aggregation. In fact, new detection methods based on
this AIE feature of TPE for protein and DNA have been
described recently;7 for example, a quaternized tetraphenyl-
ethylene salt has been successfully used as a fluorescent
probe for G-quadruplex formation and real-time monitoring
of DNA folding;7d (2) it is known that adenine and thymine
can selectively bind with Ag+ and Hg2+, respectively.6 For
(4) See, for examples: (a) Nolan, E. M.; Lippard, S. J Chem. ReV. 2008,
108, 3443–3480, and references therein. (b) Kim, J. S.; Choi, M. G.; Song,
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(6) Selective binding of Ag+: (a) Liu, L.; Zhang, D.; Zhang, G.; Xiang,
J.; Zhu, D Org. Lett. 2008, 10, 2271–2274. (b) Purohit, C. S.; Verma, S.
J. Am. Chem. Soc. 2007, 129, 3488–3489. (c) Purohit, C. S.; Mishra, A. K.;
Verma, S. Inorg. Chem. 2007, 46, 8493–8495. (d) Purohit, C. S.; Verma,
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Commun. 2008, 2242–2244. (f) Chiang, C. K.; Huang, C. C.; Liu, C. W.;
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Org. Lett., Vol. 10, No. 20, 2008

tion spectrum of 1 after addition of Ag+. The absorption band
around 256 nm became weak and the absorption intensity
of that around 317 nm increased gradually in the presence
of Ag+, leading to an isobestic point at 242 and 274 nm.
Furthermore, an absorption tail above 400 nm was detected.
These absorption spectral changes are in agreement with the
1
aggregation of 1 after addition of Ag+. (2) Variation of H
NMR spectra of 1 in the presence of Ag+ also indicates the
aggregation of 1. As shown in Figure S3 (Supporting
Information), the signals around 6.55-8.25 ppm due to the
aromatic protons of 1 were upfield shifted and became weak
1
after the gradual addition of Ag+. These H NMR spectral
changes are likely due to the “shielding effect” of neighbor-
ing molecules of 1 in the aggregation state. (3) TEM analysis
with the solution of 1 containing 1.0 equiv of Ag+ indicates
the formation of aggregates with fiber-like structure (see
Figure S8 of the Supporting Information). But, obvious
fluorescence enhancement was not oberserved upon addition
of Ag+ when the concentration of 1 was reduced to 1.0 ×
10-5 M. This result implies that the intramolecular coordina-
tion of 1 with Ag+ may not contribute to the fluorescence
enhancement of 1 after addition of Ag+.
Figure 1
.
Fluorescence spectra of compound 1 (5.60 × 10-5 M)
in H2O/THF (5:1, v/v) in the presence of increasing amounts of
AgClO4 (from 0 to 108 µM); λex. ) 319 nm. Inset: (1) the plot of
the fluorescence intensity (I470nm) vs the concentration of AgClO4;
(2) the photos of the solution of 1 before and after additon of 1.0
equiv of AgClO4 under UV light illumination (365 nm).
The fluorescence spectrum of 1 was also measured in the
presence of other metal ions including Ba2+, Ca2+, Cd2+,
Co2+, Cu2+, Fe3+, Fe2+, Hg2+, Mg2+, Mn2+, Ni2+, Pb2+ Zn2+,
Cs+, and K+ under identical conditions. As shown in Figure
3, variation of the fluorescence intensity of 1 at 470 nm was
concentration of AgClO4 as shown in the inset of Figure 1,
where a nearly linear plot of I470nm vs the concentration of
Ag+ in the range of 0-75 µM was displayed (I470nm ) 2.48
[Ag+] + 8.86, r ) 0.996, n ) 9). The detection limit of this
Ag+ assay can reach 0.34 µM (K ) 2). Similar fluorescence
enhancement was observed for 1 after the addition of Ag+
-
-
-
salts with different counteranions (NO3 , NO2 , PF6 ,
-
-
-
CF3SO3 , AsF6 , and BF4 ) (see Figure S5 of the Supporting
Information).
Such fluorescence enhancement observed for 1 in the
presence of Ag+ is attributed to the coordination of adenine
moieties of 1 with Ag+ ions leading to formation of
coordination complexes which may further aggregate due
to the low solubility (see Scheme 1); as a result, the
fluorescence due to the tetraphenylethylene unit of 1
increases. The following experimental facts support this
assumption: (1) Figure 2 shows the variation of the absorp-
Figure 3. Variation of the fluorescence intensity at 470 nm (I470nm)
of compound 1 (5.60 × 10-5 M) in H2O/THF (5:1, v/v) in the
presence of 1.0 equiv of the respective metal ions.
rather small (compared to that in the presence of Ag+) after
addition of 1.0 equiv of the respective metal ions, except
for that in the presence of Hg2+ which caused a little
interference. Cross-contamination tests were also performed
(see Figure S1-4 of the Supporting Information). These
results clearly indicate that compound 1 shows good
selectivity toward Ag+, and other competitive metal ions such
as Pb2+, Mn2+, Ni2+, and Co2+ induce a rather low interfer-
ence effect on this fluorescence assay for Ag+.
Figure 2
.
Absorption spectra of compound 1 (5.60 × 10-5 M) in
Figure 4 shows the fluorescence spectrum of compound 2
and those in the presence of different amounts of Hg(ClO4)2
in H2O/CH3CN (2:1, v/v). Compound 2 shows rather weak
H2O/THF (5:1, v/v) in the presence of increasing amounts of
AgClO4 (from 0 to 108 µM).
Org. Lett., Vol. 10, No. 20, 2008
4583

Co2+, Cu2+, Fe3+, Fe2+, Mg2+, Mn2+, Ni2+, Pb2+, Zn2+, Ag+,
Cs+ and K+ under identical conditions. As shown in Figure
5, variation of the fluorescence intensity at 467 nm (I467nm
)
Figure 4
.
Fluorescence spectra of compound 2 (1.34 × 10-4 M in
H2O/CH3CN (2:1, v/v) in the presence of increasing amounts of
Hg(ClO4)2 (from 0 to 214 µM); λex ) 319 nm. Inset: (1) the plot of
the fluorescence intensity (I467nm) vs the concentration of Hg(ClO4)2;
(2) the photos of the solution of 2 before and after additon of 1.0 equiv
of Hg(ClO4)2 under UV light illumination (365 nm).
Figure 5
.
Fluorescence intensity (I467nm) of compound 2 (1.34 ×
10-4 M in H2O/CH3CN, 2:1, v/v) in the presence of 1.0 equiv of
the respective metal ions.
emission at this concentration. But, after addition of Hg-
(ClO4)2, the emission band around 467 nm emerged and its
intensity increased gradually. Moreover, the fluorescence
intensity at 467 nm increased linearly with the concentration
was rather small (compared to that in the presence of Hg2+)
after addition of 1.0 equiv of the respective metal ions. Cross-
contamination tests were also performed (see Figure S2-4
of Supporting Information).The results clearly indicate that
compound 2 shows rather good selectivity toward Hg2+. This
is understandable by considering the specific binding of
thymine with Hg2+.
of Hg2+ in the range of 0-125 µM as displayed (I467nm
)
1.81 [Hg2+] - 2.99, r ) 0.995, n ) 8) in the inset of Figure
4. Thus, Hg2+ with concentration as low as 0.37 µM (K )
2) can be detected with compound 2. Similar fluorescence
enhancement was observed for 2 after addition of Hg2+ salts
with different counteranions (Cl-, Br-, CNS- and
CH3COO-) (see Figure S6 of the Supporting Information).
Similarly, such fluorescence enhancement is due to the
aggregation of 2 after addition of Hg2+. It is believed that
such aggregation was induced by coordination of thymine
units in 2 with Hg2+ as schematically shown in Scheme 1.
This assumption was supported by the corresponding absorp-
tion and 1H NMR spectral studies as well as TEM analysis:
(1) Figure S7 shows the variation of the absorption spectrum
of 2 after addition of Hg2+. The absorption band around 256
nm became weak and the absorption intensity in the range
of 312-600 nm increased gradually in presence of Hg2+.
These absorption spectral changes are in agreement with the
aggregation of 2 after addition of Hg2+. (2) Variation of 1H
NMR spectra of 2 in the presence of Hg2+ also indicates the
aggregation of 2. As shown in Figure S4, the signals around
7.00-7.80 ppm due to aromatic protons of 2 were upfield
shifted and became weak after the gradual addition of Hg2+.
These 1H NMR spectral changes are likely due to the
“shielding effect” of neighboring molecules of 2 in the
aggregation state. (3) TEM analysis with the solution of 2
containing 1.0 equiv of Hg2+ indicates the formation of
aggregates with the size up to 200 nm (see Figure S9 of the
Supporting Information).
In summary, compounds 1 and 2 were designed and
investigated with a view to developing new fluorescent
sensors for Ag+ and Hg2+ by making use of the specific
binding of adenine with Ag+ and thymine with Hg2+ as well
1
as the AIE feature of TPE motif. Absorption and H NMR
spectral studies indicated the aggregation of 1 and 2 in the
presence of Ag+ and Hg2+, respectively; the aggregation was
also directly confirmed by TEM analyses. Fluorescent
spectral results clearly demonstrate that compound 1 and 2
can be used as fluorescent sensor for Ag+ and Hg2+ with
good selectivity and sensitivity. Further studies include the
design of new analogues of 1 and 2 with good solubility in
water which will enable the practical application of these
types of Ag+ and Hg2+ sensors to be implemented.
Acknowledgment. The present research was financially
supported by NSFC, Chinese Academy of Sciences, and State
Key Basic Research Program.
Supporting Information Available: Synthesis and char-
acterization data; fluorescence and absorption spectra of
1
compound 1 and 2 under different conditions; H NMR
spectra and TEM images of 1 and 2 in the presence of Ag+
and Hg2+. This material is available free of charge via the
Internet at http://pubs.acs.org.
The fluorescence spectrum of 2 was recorded in the
presence of other metal ions including Ba2+, Ca2+, Cd2+,
OL801855S
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Org. Lett., Vol. 10, No. 20, 2008