Novel ratiometric fluorescent sensor for silver ions

Article abstract of DOI:10.1021/ol902624h

(Figure presented) Incorporating azacrown[N,S,O] Into furoquinoline fluorophore yields a novel ratiometric fluorescent sensor FQ-crown for silver Ions. UV-vis absorption and fluorescence emission Investigations Indicate that FQ-crown bears the features of a large Stokes shift, about 173 nm, and red-shift up to 50 nm In the emission spectra and high affinity for silver Ions (log K= 7.21) In ethanol In comparison with other competitive d¹⁰ metal Ions.

Full text of DOI:10.1021/ol902624h

ORGANIC
LETTERS
2010
Vol. 12, No. 2
292-295
Novel Ratiometric Fluorescent Sensor
for Silver Ions
Huan-Huan Wang,†,§ Lin Xue,†,§ Yuan-Yu Qian,*,‡ and Hua Jiang*,†
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of
Photochemistry, Institute of Chemistry, Chinese Academy of Sciences,
Beijing, 100190 P. R. China, Emergency Department of Chinese PLA General
Hospital, Beijing, 100853, P. R. China, and Graduate School of Chinese Academy of
Sciences, Beijing, P. R. China
hjiang@iccas.ac.cn; qyy301@sina.com
Received November 13, 2009
ABSTRACT
Incorporating azacrown[N,S,O] into furoquinoline fluorophore yields a novel ratiometric fluorescent sensor FQ-crown for silver ions. UV-vis
absorption and fluorescence emission investigations indicate that FQ-crown bears the features of a large Stokes shift, about 173 nm, and
red-shift up to 50 nm in the emission spectra and high affinity for silver ions (log K ) 7.21) in ethanol in comparison with other competitive
d10 metal ions.
Due to the widespread applications and the broad prospects
of sliver in the electronic industry and photographic and
imaging industry, much attention has been paid to the
negative impact of silver ions on the environment, especially
on organisms. It is believed that silver ions can bind to
various metabolites and enzymes such as inactivate sulphy-
dryl enzymes.1 Many approaches2 such as fluorescence,
UV-vis absorption, atomic absorption, and ICP atomic
emission spectroscopy have been employed to measure trace
amounts of silver ions. Among these methodologies, fluo-
rescence spectroscopy is widely used because of its high
sensitivity and facile operation. More importantly, most
fluorescent sensors are ready for in vivo and in vitro cellular
imaging to make the fluorescence approach superior to other
analytical methods. Therefore, much effort has been devoted
† Chinese Academy of Sciences.
‡ Emergency Department of Chinese PLA General Hospital.
§ Graduate School of Chinese Academy of Sciences.
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10.1021/ol902624h  2010 American Chemical Society
Published on Web 12/09/2009

to design various fluorescent sensors for versatile objectives.
Thus, a number of fluorescence sensors have been success-
fully applied to monitor metal ions such as calcium,3a-c
zinc,3d,e mercury,3f,g and copper,3h,i both in solution and in
cells. However, fluorescent sensors, in particular, ratiometric
sensors for silver ions,4 have remained rare up to now
because silver ions usually quench fluorescence emission via
the electron transfer and intersystem crossing (isc)
processes.4i In addition, a quantitative determination of
analyte can be realized by ratiometric measurement that rules
out extraneous influence factors including membrane perme-
ability, incubation time, and temperature for biological
systems. Thus, it is highly desirable to propose a novel sensor
with ratiometric module for silver ions. Nevertheless,
development of a ratiometric fluorescent sensor for silver
still faces tremendous challenges. In this context, we present
a novel ratiometric fluorescent sensor on the basis of the
internal charge transfer (ICT) mechanism.
The spectroscopic properties of FQ-crown were evaluated
in ethanol solution. As shown in Figure 1, FQ-crown
Figure 1. UV-vis spectra of FQ-crown (12.5 µM) upon titration
of Ag+ (0-1.2 equiv) in ethanol. Inset: UV-vis absorption bands
of FQ-crown at 365-390 nm.
Recently, our group reported a ratiometric fluorescent
sensor (FQ1) for zinc,5 which possesses a large Stokes shift
and red-shift in the emission spectra due to ICT. The findings
encouraged us to prepare a new ratiometric sensor FQ-crown
comprised of furoquinoline and azacrown[N,S,O] for silver
ions on the basis of a similar mechanism to that of FQ1
(Scheme 1). We envisaged that the crown moiety will display
exhibits a maximal absorption at 346 nm. Upon addition of
silver ions (0-1.2 equiv), the absorbance at 346 nm decreases
gradually, and simultaneously, a significant absorption band
like a shoulder at 405 nm increases, accompanying several
isosbestic points at 380, 322, 307, 267, and 258 nm.
Moreover, the absorbance at 405 nm remains constant in
the presence of more than 1 equiv of silver ions, indicating
the formation of a 1:1 complex between the FQ-crown and
silver ion, which is in good agreement with 1:1 stoichiometry
for the silver complex determined by the Job’s plot yielded
from UV-vis absorption (Figure S1, Supporting Informa-
tion).
Scheme 1. Synthesis of FQ-crown
As depicted in Figure 2, the fluorescence emission spectra
of FQ-crown display an emission band centered at 519 nm
high affinity for thio- or aminophilic metal ions such as silver
ions, and the furoquinoline moiety acts not only as a
fluorophore but as an auxiliary ligand for silver ions as
observed for FQ1, where the crystal structure reveals the
furoquinoline group participating in the coordination to
the zinc ion. It turns out that FQ-crown is an effective
ratiometric fluorescent sensor for silver ions and bears the
features of a large Stokes shift, about 173 nm, red-shift up
to 50 nm in the emission spectra, and high affinity for silver
ions (log K ) 7.21) in ethanol in comparison with other
competitive d10 metal ions.
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Figure 2. Fluorescence emission spectra (λex ) 380 nm) of FQ-
crown (5 µM) upon addition of Ag+ (0-1 equiv) in ethanol.
when excited at the isosbestic point 380 nm. Upon addition
of various concentrations of silver ions, the emission band
centered at 519 nm decreases, and a new emission peak
gradually appears at longer wavelength (568 nm) with a
(5) Xue, L.; Liu, C.; Jiang, H. Chem. Commun. 2009, 1061.
Org. Lett., Vol. 12, No. 2, 2010
293

bathochromic shift of about 49 nm. The quantum yield of
the complex in ethanol was calculated to be 0.18, which is
slightly higher than that of the free ligand (Φ ) 0.14). To
achieve a reliable binding constant (K) via fluorescence
titration,6 the titrations were conducted in a very dilute
solution (1.0 × 10-6 M). The binding constant was found to
be log K ) 7.21 ( 0.07 via a nonlinear least-squares
treatment of the fluorescence titration profiles (Figure S3,
Supporting Information).
Subsequently, we evaluated the response of FQ-crown
to other metal ions. The fluorescence emission spectrum of
FQ-crown demonstrates that only silver ions cause a
bathochromic shift (∆λem ) 49 nm) with slight fluorescence
intensity enhancements as depicted in Figure 3. In sharp
guished apparently from other metal ions. Furthermore, only
Mn2+ and Co2+ have a little impact on the I568nm/I519nm value
of the silver complex. The results imply the sensor bears
high selectivity for silver ions in the presence of other
competitive metal ions.
The survey of emission responses of FQ-crown in the
presence of various ions indicates that the nitrogen atom in
the furoquinoline moiety got involved in the coordination
with silver ions, which induces an ICT process similar to
that of FQ1,5 that is, the interaction between the silver ion
and the nitrogen atom in the furoquinoline moiety enhances
the electron-withdrawing ability of furoquinoline that leads
to an ICT process and consequently results in a large red
shift. We therefore conclude that the silver ion is bound to
the crown ether moiety and the furoquinoline fluorophore.
To seek further detailed information on the coordination,
we also carried out 1H NMR titration experiments in CD3OD.
As shown in Figure 4, upon increasing the amount of silver
Figure 3. Fluorescence emission spectra of FQ-crown (5 µM) in
the presence of various metal ions in ethanol (Top). Black bars
represent the ratio I568nm/I519nm of FQ-crown (5 µM) in the presence
of 1 equiv of Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Cd2+, Hg2+, and Pb2+
and 100 equiv of Mg2+, Ca2+, and K+. Gray bars represent the
ratio of I568nm/I519nm of FQ-crown (5 µM) in the presence of the
indicated metal ions, followed by 1 equiv of silver ion (Bottom).
1
Figure 4
.
Partial H NMR (400 MHz) spectral changes of FQ-
crown (20 mM) in CD3OD upon addition of (a) 0.2 equiv, (b) 0.4
equiv, (c) 0.6 equiv, (d) 0.8 equiv, and (e) 1.0 equiv of AgNO3 at
298 K.
contrast, other metal ions quench the emission in various
extent upon addition of 1 equiv of Cr3+, Fe3+, Cu2+, Hg2+,
Zn2+, Cd2+, and Pb2+, whereas Ca2+, Mg2+, Co2+, Ni2+, K+,
and Mn2+ slightly depress the fluorescence intensity.
To further gauge selectivity for silver ions over other
transition metal ions, we also examined transition metal/silver
coexisted systems. By monitoring the ratio of fluorescence
intensities at 568 and 519 nm, silver ions could be distin-
ions dissolved in D2O, considerable changes are exhibited
in the chemical shifts of protons in FQ-crown especially in
the crown moiety. In details, the proton Ha displays a
dramatic downshift from δ ) 2.64 to 3.09 ppm up to ∆δ )
0.45 ppm. Hb and Hc at δ ) 2.90 ppm also demonstrate a
similar but weak downshift to 3.09 ppm with ∆δ ) 0.19
ppm, while Hd and He show a slight change. On the other
hand, protons Hf, Hi, and Hj in furoquinoline, aggregating
(6) Boiocchi, M.; Boca, D. L.; Go´mez, E. D.; Fabbrizzi, L.; Licchelli,
M.; Monzani, E. J. Am. Chem. Soc. 2004, 126, 16507.
294
Org. Lett., Vol. 12, No. 2, 2010

in the absence of silver ions, spread over a large chemical
shift of ∆δ ) 0.32 ppm, while 1 equiv of silver ions was
introduced. The other aromatic protons such as Hg and Hk
also experienced downfield shifts. These data further confirm
the conclusion that both crown ether moiety and furoquino-
line fluorophore are involved in the coordination with silver
ions as envisaged. Additionally, all proton chemical shifts
remain constant when the amount of silver ions is beyond 1
equiv, suggesting high affinity for the silver ion.
shows high affinity and selectivity for silver ions over other
competitive metal ions. We expect that FQ-crown may have
wide applications for quantitative measurement of silver
through its ratiometric module.
Acknowledgment. We thank the Chinese Academy of
Sciences “Hundred talents program” and the National Natural
Science Foundation of China (90813002) for financial
support.
Supporting Information Available: Synthetic procedures,
characterization of FQ-crown, and additional spectroscopic
data. This material is available free of charge via the Internet
at http://pubs.acs.org.
In summary, we have prepared a ratiometric fluorescence
sensor FQ-crown for silver ions in ethanol, which demon-
strates a large Stokes shift of 173 nm and gives rise to
bathochromic shift of about 50 nm in fluorescence emission
spectra upon addition of silver ions. In addition, FQ-crown
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