A highly selective fluorescent chemosensor for lead ions

Article abstract of DOI:10.1021/ja025710e

A simple fluorescent chemosensor 1 exhibits a 40-fold fluorescence enhancement upon addition of lead ion and yet is insensitive to most of univalent and divalent metal ions. The high affinity of 1 toward Pb²⁺ presumably results from the cooperative binding behaviors of two potential metal binding sites: the dicarbonyl and the 15-monoazacrown-5 ether. Copyright

Full text of DOI:10.1021/ja025710e

Published on Web 05/11/2002
A Highly Selective Fluorescent Chemosensor for Lead Ions
Chao-Tsen Chen* and Wan-Pei Huang
Department of Chemistry, National Taiwan UniVersity, Taipei, Taiwan 106, R.O.C.
Received January 25, 2002
Pb2+ is a detrimental metal ion causing adverse environmental
and health problems. A wide variety of symptoms (which include
memory loss, irritability, anemia, muscle paralysis, and mental
retardation) have been attributed to lead poisoning, suggesting that
Pb2+ affects multiple targets in vivo.1 Development of fluorescent
sensors for Pb2+ would help to clarify the cellular role of lead ions
in vivo as well as to monitor Pb2+ concentration temporally in the
Pb2+ contaminated sources. While many effective fluorescent
sensors2 have been successfully developed for alkali, alkaline earth,
and Zn2+,3 few have been explored for Pb2+. As many of heavy
metals are known as fluorescence quenchers via enhanced spin-
orbital coupling,4 energy, or electron transfer,5 development of
selective as well as sensitive fluorescent sensors for Pb2+ presents
a challenge.
Figure 1. A proposed 2:2 complex formed between 1 and Pb2+
.
is also prepared by the similar procedure to serve as a control.
Absorption and fluorescence emission titrations of 1 × 10-5 M of
1 and 2 with group I (Li+, Na+, and K+), group II (Mg2+, Ca2+
,
,
A selective, ratiometric fluorescent sensor based on polypeptide
scaffolds equipped with a microenvironment-sensitive fluorophore
for Pb2+ has been described.6 In addition, some of fluorophore-
appended macrocycles were shown to respond to Pb2+ via photo-
induced charge transfer,7a inhibition of photoinduced electron
transfer7b-d or self-assembling fluorescence enhancement.7e How-
ever, discrimination between chemically closely related metal ions
is not very high. And yet, there is still plenty of room for
improvements in terms of fluorescence intensity enhancement,
selectivity, as well as suitable fluorophores with visible light
excitation.
and Ba2+), and heavy metal ions (Fe2+, Mn2+, Ni2+, Cu2+, Zn2+
Cd2+, Hg2+, Ag+, and Pb2+ 10
)
are performed in acetonitrile at 25
°C.
The results show that the univalent metal ions do not cause
significant changes in either absorption or fluorescence emission
spectra of 1 or 2, whereas red-shift absorptions are observed upon
the addition of divalent metal ions. It is worth noting that the
complex 1‚Pb2+ exhibits only 15 nm red-shift and 2‚Pb2+ displays
63 nm shift, reflecting the binding modes are different in these
two complexes. The large bathochromic shift for 2‚Pb2+ can be
attributed to the strong stabilization of internal charge-transfer (ICT)
state by the complexation of metal ions with the two carbonyl
groups of the fluorophore via chelation.2c,11 In fluorescence titration
studies, 1 displays 40-, 12-, and 18-fold fluorescence enhancements
for Pb2+ (10 equiv), Ba2+ (100 equiv), and Cu2+ (100 equiv),
respectively, and 2 only responds to Cu2+ (50 equiv) with 26-fold
fluorescence enhancement among 15 metals investigated.
Herein we report a new chemosensor 1, which can signal Pb2+
selectively and display large fluorescence enhancement.
Disappearance of the characteristic carbonyl absorption of 1‚
Pb2+ complex in the infrared spectra and downfield shifts experi-
enced by the proton or carbon nuclei in the carbonyl proximity
support that the relatively strong binding and the high preference
for Pb2+ are due to the lariat carbonyl participation. While Job plot12
shows that the stoichiometry of 1‚Pb2+ complex is 1:1, the observed
sigmoidal binding curves and large Hill coefficient suggest that
the binding of Pb2+ and 1 might be cooperative. A 2:2 complex of
Pb2+ and 1 is thus proposed (Figure 1)7e,13,14 The structure is also
consistent with the small red-shift in the absorption since one of
the coordination sites is situated close to the negative, and the other,
to the positive end of molecular dipole, the opposite effects
counterbalancing each other to some extent.2c,11a The pronounced
fluorescence enhancement can be rationalized in terms of photo-
induced charge transfer (PCT)2c and metal binding-induced con-
formational restriction upon complexation.15 The magnitude of
enhancement in emission and selectivity from 1 are remarkable and
can be detected visually.
Ketoaminocoumarin is selected as a signal-transducing unit owing
to its high photostability and visible excitation wavelength.
Moreover, it has been established that the quantum yield (Φ) of
7-diethylamino-3-(4-dimethylaminobenzoyl)coumarin (2, Φ ) 0.001)
is approximately 350-fold lower than that of 3-benzoyl-7-diethyl-
aminocoumarin (3, Φ ) 0.35).8 Thus, judicious incorporation of 2
in the fluorescent chemosensor design as the signal transduction
unit would provide low basal fluorescence and give fluorescence
enhancement upon the substrate binding, only if the lone pair of
the nitrogen atom from benzoyl moiety is involved in the binding
event.
Chemosensor 1 is easily prepared by the condensation of 4-(N,N-
diethylamino)sailicylaldehyde with â-ketoester appended with 15-
monoazacrown-5 ether in the presence of piperidine.9 To address
the role of the monoazacrown ether played in the complexation, 2
To explore further the utility of 1 as an ion-selective fluorescent
chemosensor for Pb2+, the competition experiments are conducted
* To whom correspondence should be addressed. E-mail: ctchen@
mail.ch.ntu.edu.tw.
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J. AM. CHEM. SOC. 2002, 124, 6246-6247
10.1021/ja025710e CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
the aqueous solution of Pb(ClO4)2 is added to 1 in acetonitrile,
neither the binding strength nor the magnitude of fluorescence
enhancement is affected, whereas the stability of 1‚Pb2+ complex
is attenuated (2 orders of magnitude weaker) when 5% of water-
acetonitrile is used as the solvent for both of Pb(ClO4)2 and 1. It is
worth noting that the fluorescence enhancement is still quite
pronounced (I/I0 ) 6) in this organic/aqueous mixture.
In summary, the new fluorescent chemosensor 1 exhibits a high
affinity and selectivity for Pb2+. The remarkable fluorescent
response to Pb2+ binding is unprecedented. Our future efforts will
be focused on developing fluorescent chemosensors, which can
function in aqueous systems with high affinities for Pb2+. In
addition, a fluorescent chemosensor library17 for other metal ions
will be established by appending versatile molecular recognition
units, for example, different crown ethers, to the current molecular
design.
Acknowledgment. We thank National Science Council, Taiwan
for financial support.
Supporting Information Available: Selected synthetic procedures
and structural characterization for 1, metal binding profiles, binding
isotherms, as well as photospectroscopic data of titrations (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
Figure 2. Absorption and fluorescence spectra of 1 (1 × 10-5 M in
acetonitrile) in the presence of 1 mM of Mg2+ upon addition of increasing
amounts of Pb2+ (Dashed line corresponds to the 1‚Pb2+ complex without
Mg2+). Excitation is at 411 nm.
References
(1) Rifai, N.; Cohen, G.; Wolf, M.; Cohen, L.; Faser, C.; Savory, J.; DePalma,
L. Ther. Drug Monit. 1993, 15, 71 and references therein.
(2) (a) de Silva, A. P.; Fox, D. B.; Huxley, A. J. M.; Moody, T. S. Coord.
Chem. ReV. 2000, 205, 41-57. (b) Amendola, V.; Fabbrizzi, L.; Licchelli,
M.; Mangano, C.; Pallavicini, P.; Parodi, L.; Poggi, A. Coord. Chem.
ReV. 2000, 192, 649-669. (c) Valeur, B.; Leray, I. Coord. Chem. ReV.
2000, 205, 3-40. (d) Fluorescent Chemosensors for Ion and Molecule
Recognition; Czarnik, A. W., Ed.; American Chemical Society: Wash-
ington, DC, 1993.
(3) (a) Hirano, T.; Kikuchi, K.; Urano, Y.; Higuchi, T.; Nagano, T. Angew.
Chem., Int. Ed. 2000, 39, 1052-1054. (b) Burdette, S. C.; Walkup, G.
K.; Spingler, B.; Tsien, R. Y.; Lippard, S. J. J. Am. Chem. Soc. 2001,
123, 7831-7841.
Figure 3. The fluorescence intensity change profile of 1 (1 × 10-5 M in
acetonitrile) in the presence of selected metal ions. Excitation is at 411
nm, and emission is monitored at 491 nm.
in which 1 is first incubated with various metal ions at their
saturation concentrations, and Pb2+ is added until the total
concentration of Pb2+ reaches saturation concentration (100 µM).
Although absorption bands provide the most structurally relevant
information about the binding, the absorption as well as fluorescence
spectra are exploited to monitor the competition events. Figure 2
illustrates the changes of these spectra upon the addition of
increasing amounts of Pb2+ in the presence of 1 mM of Mg2+. The
absorption spectrum of 1 ‚ Mg2+ is consistent with the two carbonyl
groups of the fluorophore taking part in the binding, regardless of
the presence of the monoazacrown moiety. As the Pb2+ concentra-
tion increases, the characteristic absorption at 478 nm for 1‚Mg2+
complex gradually decreases, and a new characteristic absorption
at 428 nm, responsible for 1‚Pb2+ complex, starts to appear with a
concomitant fluorescence increase. Notably, Pb2+ ions do not simply
replace Mg2+ in the same binding sites but form a new fluorescent
complex.
(4) McClure, D. S. J. Chem. Phys. 1952, 20, 682-686.
(5) Varnes, A. W.; Dodson, R. B.; Wehry, E. L. J. Am. Chem. Soc. 1972, 94,
946-950.
(6) Deo, S.; Godwin, H. A. J. Am. Chem. Soc. 2000, 122, 174-175.
(7) (a) Addleman, R. S.; Bennett, J.; Tweedy, S. H.; Elshani, S.; Wai, C. M.
Talanta 1998, 46, 573-581. (b) Shen, Y.; Sullivan, B. P. J. Chem. Educ.
1997, 74, 685-689. (c) Beeby, A.; Parker, D.; Williams, J. A. G. J. Chem.
Soc., Perkin Trans. 2 1996, 1565-1579. (d) Padilla-Tosta, M. E.; Lloris,
J. M.; Mart´ınez-Ma´n˜ez, R.; Marcos, M. D.; Miranda, M. A.; Pardo, T.;
Sanceno´n, F.; Soto, J. Eur. J. Inorg. Chem. 2001, 1475-1482. (e) Xia,
W.-S.; Schmehl, R. H.; Li, C.-J.; Mague, J. T.; luo, C.-P. Guldi, D. M. J.
Phys. Chem. B 2002, 106, 833-843.
(8) Specht, D. P.; Martic, P. A.; Farid, S. Tetrahedron 1982, 38, 1203-1211.
(9) (a) Schuster, G. B.; Gompel, J. V. J. Org. Chem. 1987, 52, 1465-1468.
(b) Farid, S.; Specht, D. P.; Martic, P. A. Tetrahedron 1982, 38, 1203-
1211.
(10) Detailed photospectroscopic data are included in Supporting Information.
(11) (a) Roshal, A. D.; Grigorovich, A. V.; Doroshenko, A. O.; Pivovarenko,
V. G.; Demchenko, A. P. J. Phys. Chem. A 1998, 102, 5907-5914. (b)
Bourson, J.; Pouget, J.; Valeur, B. J. Phys. Chem. 1993, 97, 4552-4557.
(c) Brunet, E.; Alonso, M. T.; Juanes, O.; Sedano, R.; Rodr´ıguez-Ubis, J.
C. Tetrahedron Lett. 1997, 38, 4459-4462.
(12) Connors, K. A. Binding Constants; Wiley-Interscience: New York, 1987;
pp 24-28.
(13) Our recent studies reveal that monoaza-18-crown-6 ketocoumarin forms
a 2:2 complex with Pb2+ and two ligands are disposed in an antiparallel
fashion as determined by X-ray crystallography.
(14) 2:2 metal complexes have been observed in biscrown-distyrybenzene
systems by forming intermolecular sandwich between crown units and
metal ions. See: ref 7e.
(15) (a) Nizhegorodov, N. I.; Downey, W. S. J. Phys. Chem. 1994, 98, 5639-
5643 and references therein. (b) McFarland, S. A.; Finney, N. S. J. Am.
Chem. Soc. 2001, 123, 1260-1261. (c) McFarland S. A.; Finney, N. S.
J. Am. Chem. Soc. 2002, 124, 1178-1179.
The selectivity of 1 for Pb2+ over Ca2+, Zn2+, Cd2+, Fe2+, Mn2+
and Hg2+ is particularly important because Pb2+ targets both Ca2+
,
-
and Zn2+-binding sites in vivo16 and Cd2+, Hg2+, Fe2+, and Mn2+
are metal ions that frequently interfere with Pb2+ analysis. The
competition-based fluorescence intensity profiles for these metal
ions are shown in Figure 3. On the basis of the photospectroscopic
data, it seems that Zn2+ and Mn2+ behave like Mg2+, whereas Cd2+
,
Hg2+, and Fe2+ act differently. Cd2+, Hg2+, and Pb2+ are probably
bound in the same binding site with different affinities, and the
replacement occurs once the higher-affinity metal ion is added in
the solution. No interference is observed while performing titrations
with Pb2+ in a complex matrix containing 50-fold excess of
univalent metal ions.
(16) (a) Goldstein, G. W. Neurotoxicology 1993, 14, 97-102. (b) Simons, T.
J. B. Neurotoxicology 1993, 14, 77-86. (c) Goyer, R. A. EnViron. Health
Perspect. 1990, 86, 177-182.
(17) (a) Czarnik, A. W. Chem. Biol. 1995, 2, 423-428. (b) Chen, C.-T.;
Wagner, H.; Still, W. C. Science 1998, 279, 851-853. (c) Schneider, S.
N.; O’Neil, E. V.; Anslyn, E. V. J. Am. Chem. Soc. 2000, 122, 542-543.
(d) Szurdoki, F.; Ren, D.; Walt, D. R. Anal. Chem. 2000, 72, 5250-5257.
We also tested the possibility of using fluorescent chemosensor
1 for determining Pb2+ concentration in aqueous solution. When
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