A series of polyamide receptor based PET fluorescent sensor molecules: Positively cooperative Hg2+ ion binding with high sensitivity

Article abstract of DOI:10.1021/ol061297u

A series of PET fluorescent sensor molecules were designed and synthesized based on BODIPY fluorophore and polyamide receptors. Comparison of the photophysical properties of these sensor molecules, equipped with di-, tri-, and tetraamide receptor, provide

Full text of DOI:10.1021/ol061297u

ORGANIC
LETTERS
2006
Vol. 8, No. 17
3721-3724
A Series of Polyamide Receptor Based
PET Fluorescent Sensor Molecules:
+
Positively Cooperative Hg2 Ion Binding
with High Sensitivity
Jiaobing Wang† and Xuhong Qian*,‡,§
State Key Laboratory of Fine Chemicals, Dalian UniVersity of Technology,
Dalian 116012, China, Shanghai Key Laboratory of Chemical Biology,
East China UniVersity of Science and Technology, Shanghai 200237, China, and
State Key Laboratory of Elemento-organic Chemistry, Nankai UniVersity, China
xhqian@ecust.edu.cn
Received May 27, 2006
ABSTRACT
A series of PET fluorescent sensor molecules were designed and synthesized based on BODIPY fluorophore and polyamide receptors. Comparison
of the photophysical properties of these sensor molecules, equipped with di-, tri-, and tetraamide receptor, provided a deep insight into the
+
+
polyamide− −
Hg2 interactions, and an unusual positively cooperative tetraamide Hg2 complexation was disclosed. In addition, sensor S3
displayed several favorable sensing properties.
Mercury, widely distributed in the air, water, and soil,1 is
considered by the Environmental Protection Agency (EPA)
to be a highly dangerous element because of its severe
immunotoxic, genotoxic, and neurotoxic effects.2 As a
consequence, mercury-indicating methodologies, which are
developed to provide critical information for mercury hazard
assessment and mercury pollution management, are in high
demand. Among these techniques, fluorescent molecular
sensing, which translates molecular recognition into tangible
fluorescence signals, has received much attention.3
ions with either fluorescence off-on response or fluoresce-
nce color change.4 These sensors featured high water
solubility, unique Hg2+ ion selectivity, and significant signal
response upon Hg2+ ion complexation. However, deep
exploration is desired since some basic knowledge about the
(3) (a) Yoon, S.; Albers, A. E.; Wong, A. P.; Chang, C. J. J. Am. Chem.
Soc. 2005, 127, 16030. (b) Caballero, A.; Mart´ınez, R.; Lloveras, V.; Ratera,
I.; Vidal-Gancedo, J.; Wurst, K.; ta´rraga, A.; Molina, P.; Veciana, J. J.
Am. Chem. Soc. 2005, 127, 15666. (c) Coronado, E.; Gala´n-Mascaru¨s, J.
R.; Mart´ı-Gastaldo, C.; Palomares, E.; Durrant, J. R.; Vilar, R.; Gratzel,
M.; Nazeeruddin, M. K. J. Am. Chem. Soc. 2005, 127, 12351. (d) Ros-Lis,
J. V.; Marcos, M. D.; Mart´ınez-ma´n˜ez, R.; Rurack, K.; Soto, J. Angew.
Chem., Int. Ed. 2005, 44, 4405. (e) Guo, X.; Qian, X.; Jia, L. J. Am. Chem.
Soc. 2004, 126, 2272. (f) Zhang, Z.; Guo, X.; Qian, X.; Lv, Z.; Liu, F.
Kidney Int. 2004, 66, 2279. (g) Ono, A.; Togashi, H. Angew. Chem., Int.
Ed. 2004, 43, 4300. (h) Dickerson, T. J.; Reed, N. N.; LaClair, J. J.; Janda,
K. D. J. Am. Chem. Soc. 2004, 126, 16582. (i) Nolan, E. M.; Lippard, S.
J. J. Am. Chem. Soc. 2003, 125, 14270. (j) Descalzo, A. B.; Mart´ınez-
ma´n˜ez, R.; Radeglia, R.; Rurack, K.; Soto, J. J. Am. Chem. Soc. 2003, 125,
3418.
Recently, we disclosed that polyamide receptor based
fluorescent molecule sensors could be used to detect Hg2+
† Dalian University of Technology.
‡ East China University of Science and Technology.
§ Nankai University.
(1) Miller, J. R.; Rowland, J.; Lechler, P. J.; Desilets, M.; Hsu, L. C.
Water, Air, Soil Pollut. 1996, 86, 373.
(2) Tchounwou, P. B.; Ayensu, W. K.; Ninashvili, N.; Sutton, D. EnViron.
Toxicol. 2003, 18, 149.
(4) (a) Wang, J.; Qian, X. Chem. Commun. 2006, 109. (b) Wang, J.;
Qian, X.; Cui, J. J. Org. Chem. 2006, 71, 4308.
10.1021/ol061297u CCC: $33.50
© 2006 American Chemical Society
Published on Web 07/26/2006

actual polyamide-Hg2+ interactions remained unclear, which
hampered further utilization of these polyamide receptors,
such as in the development of functional materials for
mercury pollution treatment. In our previous work, we
reported that the o-phenylenediamine-based tri- and tetraa-
mide sensors, RS2 and RMS, chelated Hg2+ ion with a 1:1
stoichiometry, whereas the tetraamide receptor MR bound
Hg2+ ion in a 1:2 fashion.4 However, it was not clear (1)
whether it was necessary to incorporate all the amide arms
to achieve an efficient Hg2+ ion complexation, and (2) why
the same receptor showed different Hg2+ ion chelating
stoichiometry and in which way two Hg2+ ions were held
by one tetraamide receptor. In addition, the performance,
BODIPY fluorophore and polyamide receptor.5b-d Before
Hg2+ ions addition, S1, S2, and S3 all display very weak
fluorescence (Table 1), resulting from the efficient photo-
Table 1. Summarized Key Parameters of the Reported Sensors
entry
Φ0
Φ1
ꢀ (M-1 cm-1
)
Log Ks
RMS
RS2
S1
S2
S3
0.051
0.007
0.013
0.013
0.012
0.051
0.24
0.013
0.19
24000
11000
60000
60000
60000
4.41 ( 0.02
7.10 ( 0.05
aNA
4.72 ( 0.02
12.4 ( 0.25
0.61
a Not available.
induced electron transfer (PET) quenching process from the
electron-donating receptor moiety to the excited BODIPY
fluorophore.5d However, when 2 equiv of Hg2+ ions are
added, the quantum yield of sensor S3 increases immediately
(within a few seconds) and dramatically from 0.012 to 0.61
(Figure 2),6 and a maximum fluorescence enhancement factor
Figure 1. Chemical structures of the reported sensor molecules.
especially in terms of sensitivity, of the early reported sensor
molecules needs to be improved to enhance their practical
utility. To address these issues, we report herein three boron
dipyrromethene (BODIPY) based fluorescent Hg2+ ion sensor
molecules, S1, S2, and S3. We incorporated two, three, and
four amide arms into the aniline, o-hydroxyaniline, and
o-phenylenediamine based receptors to probe the effects of
the convergent amide arms on polyamide-Hg2+ interactions.
The BODIPY fluorophore was chosen because of its
outstanding photophysical properties such as high absorption
coefficient (ꢀ > 50000 M-1 cm-1), high fluorescence
quantum yield (Φ > 0.5), and high photostability.5
Figure 2. Excitation spectra (emission at 541 nm) and emission
spectra (excitation at 527 nm) of S3 (2 µM) in phosphate (0.1 M)
solution (pH ) 7.5) in the presence of different concentrations of
Hg2+ ions. The up-arrow indicates the increase of [Hg2+] from 0
to 4.2 µM. Inset: integrated fluorescence intensity from 533 to 700
nm as a function of Hg2+ ion concentration.
Absorption spectra of S1, S2, and S3, in the absence or
presence of Hg2+ ions, show identical shape and absorption
coefficient typical of a BODIPY fluorophore (λabs ) 523
nm, ꢀ ) 60000 M-1 cm-1), indicating that there are no
ground-state interactions between the virtually decoupled
(EF, I/I0) of 50 is accomplished, indicating that the PET
quenching pathway is efficiently blocked by Hg2+ ion
complexation. For sensor S2, more Hg2+ ions (40 equiv) are
needed to achieve a lower maximum EF of 15.7 In a sharp
contrast, sensor S1 does not exhibit any measurable EF even
in the presence of 70 equiv of Hg2+ ions; obviously, sensor
(5) (a) Haugland, R. P. Handbook of Fluorescent Probes and Research
Chemicals; Molecular Probes: Eugene, OR, 2002. (b) Kollmannsberger,
M.; Rurack, K.; Resch-Genger, U.; Daub, J. J. Phys. Chem. A 1998, 102,
10211. (c) Rurack, K.; Kollmannsberger, M.; Resch-Genger, U.; Daub, J.
J. Am. Chem. Soc. 2000, 122, 968. (d) Gabe, Y.; Urano, Y.; Kikuchi, K.;
Kojima, H.; Nagano, T. J. Am. Chem. Soc. 2004, 126, 3357.
(6) Casey, K. G.; Quitevis, E. L. J. Phys. Chem. 1988, 92, 6590. The
fluorescence quantum yields were determined by using Rhodamine B in
absolute ethanol (Φ ) 0.49) as a reference.
(7) See Supporting Information.
3722
Org. Lett., Vol. 8, No. 17, 2006

S1 does not form a complex with Hg2+ ion. This could be
attributed to the fewer chelating sites available as well as
the more conformational flexibility of the two appended
amide arms compared with S2 and S3.
A Job’s plot suggests that S2 forms a 1:1 complex with
Hg2+ ion. The association constant is determined to be 5.1
× 104 M-1 from the Hg2+ ions titration curve.7 In contrast,
a Job’s plot indicates that S3 chelates Hg2+ ions with an 1:2
stoichiometry (Figure 3), different from that of sensor S2,
Figure 3. Job’s plot of sensor S3 in 0.1 M phosphate buffered
water solution (pH ) 7.5). The total concentration of sensor and
Hg2+ ion is 4 µM.
Figure 4. 1H NMR spectra of MR (in D2O, 23 mM) in the presence
of different concentrations of Hg2+ ions. The up-arrow indicates
the increase of Hg2+ ion concentrations, and the final mole ratio of
[Hg2+] to [MR] is 0, 0.5, 1.0, 1.5, 2.0 and 4.0, respectively.
as well as the early reported RS2 and RMS. To give a clear
receptor-Hg2+ complexation structure, we recorded the 1H
NMR spectra (in D2O, Figure 4) of the tetraamide mercury
receptor (MR) in the presence of different concentrations of
Hg2+ ions. MR is chosen in the NMR studies for its higher
water solubility and more simplified NMR signals compared
with those of sensor S3. A distinct change occurs at the peak
centered at 6.94 ppm (assigned to the four aromatic protons
showing overlapped signals), which progressively shifts
downfield and splits into two peaks (7.54, 7.66 ppm) with
the stepwise addition of Hg2+ ions. This shift approximately
reaches its limit after the addition of 2 equiv of Hg2+ ions,
reminiscent of the 1:2 stoichiometry shown in the S3-Hg2+
complex and consistent with Job’s plot analysis, using NMR
data, which indicates a 1:2 MR-Hg2+ coordination mode.4b
In contrast, the single peak assigned for the eight Hc (Figure
1) protons experiences a slight net upfield shift from 4.11
to 3.98 ppm, which could be ascribed to the shielding effect
induced by the negatively charged deprotonated amide groups
arising from -N-Hg2+ complexation (-N, deprotonated
amide nitrogen, Figure 5).8
of reliable association constants, we carried out a titration
experiment with a distinctly more dilute (1 × 10-7 M)
Determination of the association constants K11 and K21,
in the two equations Hg2+ + S3 h S3‚Hg2+ and S3‚Hg2+
+
Hg2+ h S3‚Hg2+2, provides substantial information on S3-
Hg2+ interactions. Because the inserted Hg2+ ions titration
curve (Figure 2) is too steep to be used for the determination
Figure 5. Proposed tetramide-Hg2+ complex structures and the
energy-minimized conformations by using Hyperchem software
with the molecular mechanics subroutine.
(8) Consistent with our early work, an amide deprotonation process
occurred when Hg2+ ion was grasped by the polyamide receptor, as indicated
by the pH titration experiment. See ref 4 and Supporting Information.
Org. Lett., Vol. 8, No. 17, 2006
3723

solution and get a smoother titration curve.7 K11 and K21 are
determined by a nonlinear least-squares analysis of fluores-
cence intensity I versus Hg2+ ion concentration to be 1.1 ×
106 and 2.4 × 106 M-1, respectively. Notably, an unusual
positively cooperative S3-Hg2+ complexation effect is
observed because the ratio of K21 to K11, amounting to 2.2,
is significantly larger than the statistical value 0.25.9
nitrogen atoms are electronically conjugated with two
electron withdrawing groups, and thus, it could also grasp
only one Hg2+ ion.
Benefiting from so many favorable parameters (ꢀ, Φ, EF,
Ks), sensor S3 displays a super sensitivity unparalleled by
our early reported polyamide fluorescent sensor molecules
(Table 1). A clear emission turn-on response is observed
when as low as 2 ppb (content limit in drinking water set by
EPA) of Hg2+ ions are present.7 Importantly, the unique
selectivity of the polyamide receptors is still maintained
(Figure 6). These results endow sensor S3 an immediate
Thus, a full picture of the receptor-Hg2+ complexation
structure could be abstracted from the above-mentioned
evidence: as shown in Figure 5, the o-phenylenediamine-
derived tetraamide receptor catches the first Hg2+ ion by two
o-phenylenediamine nitrogens and two deprotonated amide
groups to form a favored tetrahedral Hg2+-ligand structure.10
Once the first Hg2+ ion is caught, the other two unbound
amide arms are further restricted and fixed into a more rigid
conformation, which facilitates the complexation of the
second Hg2+ ion. Consequently, we observed a positively
cooperative S3-Hg2+ complexation effect. When the tet-
raamide receptor accommodates two Hg2+ ions, each Hg2+
ion coordinates with one o-phenylenediamine nitrogen atom
and two negative deprotonated amide groups. In that way,
the electrostatic repulsion between the two divalent cations
is significantly weakened as a result of the electrostatic
complementary action. Other chelating sites may be occupied
by H2O to fulfill the usual tetrahedral Hg2+-ligand structure.
In sensors S2, S3, RS2, and RMS, both the steric and the
electronic reasons are speculated to affect the sensor-Hg2+
binding stoichiometry. Although RS2, like S3, could also
provide six coordinate sites (the 4-amino nitrogen atom of
the naphthalimide fluorophore is involved in Hg2+ ion
binding),4a steric repulsion, derived from the bulky fluoro-
phore, may prevent it from adopting a comformation that
could accommodate two Hg2+ ions. For sensor RMS, the
same tetramide receptor possesses a significantly lower Hg2+
ion binding strength since the two o-phenylenediamine
Figure 6. Visual fluorescence changes of S3 after the addition of
2 equiv of different metal ions.11 The photos were taken under a
handheld UV (365 nm) lamp 2 min after the addition of metal ions.
Conditions: 2 µM of S3 in 10 mM phosphate buffered water
solution (pH ) 7.5). From left: 0, control; 1, Cu2+; 2, Zn2+; 3,
Fe3+; 4, Hg2+; 5, Pb2+; 6, Ni2+; 7, Cd2+; 8, Co2+; 9, Ag+.
practical utilization as a highly sensitive Hg2+-ion “annuncia-
tor” for drinking water. Overall, comparison of these sensor
molecules highlights the potential and the versatility of this
polyamide receptor based fluorescence Hg2+ ion sensing
methodology. These sensor molecules cover a wide dynamic
window, which means that the requirement of monitoring
different Hg2+ ions concentrations, from millimole to sub-
nanomole range, could be met just by using an appropriately
designed polyamide fluorescent sensor molecule.
(9) Valeur, B. Molecular Fluorescence: Principles and Applications;
Wiley-VCH: Weinheim, Germany, 2002.
(10) Su, C.-Y.; Goforth, A. M.; Smith, M. D.; Loye, H.-C. Z. Inorg.
Chem. 2003, 42, 5685.
Acknowledgment. This work was supported by the
National Key Project for Basic Research (2003CB114400)
and the National Natural Science Foundation of China.
(11) In Figure 6, except for Hg2+ ion, the addition of other detected metal
ions did not induce any detectable spectral change. However, we noticed
that phosphate buffer might mask the real sensor-ion interactions because
some metal ions such as Pb2+ formed precipitate in phosphate solution.
Thus, to decisively rule out such possible interference, an analogous
experiment was also performed in neat water solution. Preserved was the
high selectivity of S3 for Hg2+ ion (EF ) 50) in addition to negligible
fluorescence enhancements by Pb2+ (EF ) 1.4) and Ag+ (EF ) 0.8),
respectively. See Supporting Information.
Supporting Information Available: Synthetic details,
characterization, and spectrascopic data of sensors S1-S3.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL061297U
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Org. Lett., Vol. 8, No. 17, 2006