A ratiometric fluorescent sensor for Ag1 with high selectivity and sensitivity

Article abstract of DOI:10.1021/ja029253d

A bicyclic cycloadduct 1 bearing a pyrenyl moiety has been synthesized and investigated as a ratiometric fluorescent sensor for Ag_I. In an aqueous ethanol solution of 1, the presence of silver ion induces the formation of a 1:2 metal-ligand complex, which exhibits a strong intensity enhancement of the pyrene excimer emission at the expense of the emission of monomeric pyrene. Copyright

Full text of DOI:10.1021/ja029253d

Published on Web 02/12/2003
A Ratiometric Fluorescent Sensor for AgI with High Selectivity and Sensitivity
Rong-Hua Yang,†,‡ Wing-Hong Chan,*,† Albert W. M. Lee,† Ping-Fang Xia,† Hong-Kui Zhang,† and
Ke’An Li*,‡
Department of Chemistry, Hong Kong Baptist UniVersity, Kowloon Tong, Kowloon, Hong Kong, and Department of
Chemical Biology, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing, 100871, China
Received November 7, 2002 ; E-mail: whchan@hkbu.edu.hk; likn@pku.edu.cn
The design and synthesis of fluorescent sensors with high
selectivity and sensitivity for heavy and transition metal ions
continues to grow at an unabated pace.1 Regardless of the signal
transducing function, signaling changes of a fluorescent molecule
at two bands that exhibit enhancement of one band at the expense
of the other upon cation binding, a ratiometric measurement can
be made which can increase the selectivity and sensitivity of a
Figure 1. Drawing of fluorescent sensor 1 and analogue compound 2.
measurement and can eliminate most or all of the possible variability
due to differences in instrumental efficiency and content of effective
dye.2 Many investigations have been conducted to make ratiometric
fluorescent sensors for metals;3 however, few reports have been
explored for AgI.4 As many heavy cations are known as fluorescence
quenchers, discrimination between AgI and chemically close ions
presents a challenge.
We are interested in studying macrocyclic ligands with a
fluorophore for use as selective metal ion chemosensors.5 Herein,
we report an effective ratiometric fluorescent sensor for AgI based
on a pyrene-functionalized heterocycle receptor. The structures of
fluorescent sensor 1 and analogue compound 2 are shown in Figure
1. Sensor 1 forms an intramolecular sandwich complex via silver
ion-induced self-assembly that results in a dramatic increase in
fluorescence intensity of the excimer and a dramatic decrease of
monomer fluorescence and provides excellent selectivity for AgI
over other heavy and transition metal ions.
Figure 2. Fluorescence emission spectra of 1 in the presence of different
concentrations of AgI (0, 4.0, 10, 15, 20, 40, 75, 150, 300 µM). Excitation
wavelength was 344 nm. The concentration of 1 was 0.8 mM.
Pyrene was selected as a signal-transducing unit. The emission
wavelength of pyrene has been proven to be extremely sensitive to
the polarity of the local environment. Formation of the self-
assembled complex results in a remarkable change in fluorescence
emission intensities of the pyrene excimer and monomer.6 Thus, it
is expected for the self-assembly of a pyrene-tethered receptor by
specific binding to offer a novel approach to the sensing of
substrates in solution.7 To obtain the specific binding to AgI, a N,O-
containing heterocycle was selected as the ion receptor unit. It was
found that soft coordination sites seem to generate great affinity
toward the d10 transition-metal ions such as AgI or HgII.4 Compound
1 was synthesized in 45% yield by reacting prop-1-ene-1,3-sultone5
with the corresponding nitrone in refluxing toluene for 24-48 h.
To address the role the hydroxyl group played in the metal binding,
compound 2 was also prepared by a similar procedure. The two
compounds were obtained as racemic mixtures.
The fluorescence behavior of 1 was dependent on solvent and
pH. The emission spectra of 1 and its fluorescence titration with
metal ions were recorded in ethanol-water solution (50:50, v/v)
at pH 7.0, the results of which are shown in Figure 2. The spectrum
of the free ligand 1 shows typical emission bands at 378 and 397
nm (excitation 344 nm), which are attributed to the pyrene
monomeric emission.8 When AgI was added to the solution of 1, a
significant decrease of the pyrene monomer emission and a red-
shifted, structureless maximum centered around 462 nm, typical
of pyrene excimer fluorescence,6 were observed with a clear
isoemission point at 412 nm. The emission intensity ratio, I462/I378
,
increases with the increase in AgI concentrations, which allows free
AgI concentration to be determined. It was noted that at the AgI
concentration over 0.4 equiv relative to 1 concentration, the pyrene
excimer emission had little deviation of I462/I378, which might be
closely related to the formation of intramolecular excimer.9 More
AgI concentration results in a decrease in both the monomeric and
the excimer emission intensities.
The fluorescence titration of 1 with various metal ions was
conducted to examine the selectivity. Reaction of 1 with AgI and
HgII leads to formation of pyrene excimer.10 Among other ions
investigated, alkali, alkaline earth ions, and anions have hardly an
effect on 1 fluorescence, while transition metal ions (CuI, CuII, PbII,
II
MnII, CoII, CdII, ZnII, FeIII, and Hg2 ) show strong quenching of
monomer fluorescence; however, there is not any enhancement of
the excimer emission of 1. The quenching of monomer pyrene
fluorescence results from the cation-π interactions between the
heavy atom and the electron-rich aromatic.11 Figure 3 shows the
dependence of the intensity ratios of pyrene at 462 nm to that at
378 nm (I462/I378) on the cations; it can be easily recognized that
high selectivity for AgI is obtained by compound 1.
† Hong Kong Baptist University.
‡ Peking University.
9
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J. AM. CHEM. SOC. 2003, 125, 2884-2885
10.1021/ja029253d CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Figure 4. A proposed structure for compound 1 + metal ion.
Figure 3. The responses of compounds 1 (black) and 2 (gray) to metal
solutions (150 µM, x-axis markers). Excitation was at 344 nm, and emission
was at 462 and 378 nm.
the structureless broad band observed at 462 nm can be assigned
to the emission of intramolecular excimer in the self-assembly of
1, where binding of the metal center occurs through the phenolate
hydroxyl group and the nitrogen atom of the heterocycle as
schematically illustrated in Figure 4, in which the molecular
modeling structure was energy-minimized using Chemoffice 7.0
MM2 utilities.
In summary, we have developed a new fluorescent sensor for
AgI with remarkably high selectivity and sensitivity. Moreover, this
molecule makes it possible to detect the AgI cation ratiometrically,
thereby eliminating most or all of the possible variability due to
differences in instrumental efficiency and content of effective dye.
From the changes observed in the fluorescence intensity ratio,
we were able to determine the stoichiometry and association
constant of 1 with metal ions by the following equations:12
1
1
1 - R
[Mn+]m )
‚
‚
(1)
(2)
[L]nT-1
Rn
n ‚ K
R - Rmin ΦλL
2
1 - R
R
) n ‚
‚
ΦλM2 L
Rmax - R
m
n
Acknowledgment. The work was supported by the Faculty
Research Grant of HKBU (FRG/01-02/II-41) and the China
Postdoctoral Science Foundation.
where R is the fluorescence ratio of the ligand at λ1 and λ2, and
min and Rmax are the limiting values of R at zero cation concentra-
R
tion and at saturating cation concentration, respectively. Nonlinear
fitting with R as a function of AgI concentration shows the formation
of a 1:2 metal-ligand complex and gives a corresponding associa-
Supporting Information Available: Synthesis and characteristics
of 1 and 2, sample preparation for fluorescence measurement, effects
of HgII concentration on 1 emission and those of AgI concentration on
2 emission, nonlinear fitting curves for the stoichiometry, and the effects
of coexistence on the AgI measurement (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
tion constant for AgI of 2.2 × 105 M-2
.
To explore the effects of ethanol on the sensing behavior of 1 to
metal ions, fluorescence responses of 1 to AgI were examined in
ethanol/water with different volume ratios. In the ethanol percent-
ages between 20 and 60, 1 exhibits strong and selective dual
fluorescence responses of monomer and excimer to AgI. As the
ethanol percentage decreases, 1 exhibits one emission band at 450
nm. On the other hand, when the ethanol percentage exceeded 70,
1 exhibited a strong monomer emission; however, there was no
excimer emission formation even at large AgI concentration. At
10% and 80% ethanol/water, the association constants of 1 with
AgI from the titration data were calculated to be 4.2 × 104 and 6.8
× 103 M-2, respectively.
References
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Subsequently, fluorescence decay measurements manifest the
interaction between 1 and AgI. Fluorescence decay curves for 1
and its AgI compound were recorded at 378 and 462 nm. In the
absence of AgI, the decay times of the pyrene monomer and excimer
were 11.6 and 14.4 ns, respectively. Moreover, the excimer
fluorescence showed a rising time of 3.4 ns due to the formation
of intramolecular excimer. By contrast, the excimer fluorescence
from the silver compound showed a longer decay time (16.2 ns)
and no rising time, which is in agreement with the observed binding-
enhancement fluorescence. Therefore, the fluorescence from the
silver compound is ascribed to the intramolecular excimer, which
can be formed rapidly by the association between adjacent pyrene
moieties in the molecule.
To gain insight into the role of the hydroxyl group of 1 in metal
binding, compound 2 was also investigated. The results showed
that upon reaction with AgI, 2 displayed a fluorescence decrease
both at 378 and at 398 nm with a little increase at 462 nm, indicating
there was no formation of pyrene excimer. On the basis of these
results, it is concluded that the fluorescence response of the sensor
results from the formation of 2:1 complex with the metal ion, and
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