A dual-channel fluorescence-enhanced sensor for aluminum ions based on photoinduced electron transfer and excimer formation

Article abstract of DOI:10.1002/ejoc.200800327

Sensor 1 was developed as the first example of a fluorescence-enhanced Al3⁺ sensor with unique dual-channel emissions. The addition of Al3⁺ to 1 elicits a large fluorescence enhancement by inhibition of a quenching Photoinduced electron-transfer (PET) channel and also a dramatic fluorescence enhancement due to promotion of an emissive excimer channel formation. The dual-channel fluorescence-enhanced response of the sensor contributes to its high sensitivity and selectivity. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800327

FULL PAPER
DOI: 10.1002/ejoc.200800327
A Dual-Channel Fluorescence-Enhanced Sensor for Aluminum Ions Based on
Photoinduced Electron Transfer and Excimer Formation
Weiying Lin,*[a] Lin Yuan,[a] and Jianbo Feng[a]
Keywords: Sensors / Aluminum / Fluorescence / Coumarins
Sensor 1 was developed as the first example of a fluores-
cence-enhanced Al3+ sensor with unique dual-channel emis-
sions. The addition of Al3+ to 1 elicits a large fluorescence
enhancement by inhibition of a quenching Photoinduced
electron-transfer (PET) channel and also a dramatic fluores-
cence enhancement due to promotion of an emissive excimer
channel formation. The dual-channel fluorescence-enhanced
response of the sensor contributes to its high sensitivity and
selectivity.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
plays fluorescence enhancement at two emission channels
simultaneously upon binding with an analyte. Although
dual-channel fluorescence-enhanced sensors are exceedingly
sought because of their particularly favorable features such
as high sensitivity, selectivity, and convenient visible emis-
sion assays due to improved signal-to-noise ratio, they are
rarely developed with success.[5]
Introduction
Lately, fluorescent chemosensors for metal ions have be-
come particularly attractive due to their useful applications
in a wide variety of fields.
[1]
Aluminum is the third most
abundant element on earth. However, aluminum toxicity
can also elicit adverse environmental problems. It is be-
lieved that almost 40% of the world’s acid soils is due to
the effects of aluminum toxicity, which is the key factor
hampering plant (i.e., crop) performance on the acid soils.
Herein, we report coumarin 1 as the first example of a
fluorescence-enhanced Al3+ sensor with unique dual-chan-
nel emissions. Coumarin dyes have been extensively em-
ployed in fluorescent chemosensors primarily owing to their
[2]
In addition, aluminum is also deemed to be the primary
toxicant killing fish in acidified waters.[3] Therefore, the de-
velopment of fluorescent sensors for aluminum is critical in
environmental monitoring. A few cases of fluorescent Al3+
sensors that display limited sensitivity or selectivity have
been identified.[4] However, most of them exhibit changes
only in fluorescence intensity but not in fluorescence wave-
lengths. In practical applications, a change in fluorescence
intensity is often disturbed by many factors, such as concen-
trations of a sensor, temperature, sensitivity of an instru-
ment, etc. However, these issues can be addressed by em-
ploying ratiometric fluorescent sensors, which allow the
measurement of emission intensities at two different wave-
lengths to provide a built-in correction for environmental
effects.[1a]
[6]
excellent photochemical and photophysical properties.
Furthermore, the carbonyl oxygen atom in coumarins could
participate in coordinating with metal ions, which thus ef-
fectively modulates coumarin in the fluorescence character.
Therefore, coumarins could play a role both as a fluoro-
phore and a binding unit in sensors.[6d,6e] Compound 1 was
readily synthesized in a few steps. The binding of Al3+ to 1
concurrently switches on two distinct fluorescence emission
bands by combination of two signaling mechanisms: sup-
pression of a quenching PET channel and promotion of an
emissive excimer channel formation.
Results and Discussion
Most of the ratiometric sensors exhibit fluorescence sig-
nal amplification at one emission wavelength concomitantly
with signal reduction at another emission wavelength. By
contrast, a dual-channel fluorescence-enhanced sensor dis-
Compound 1 was readily synthesized by the route as out-
lined in Scheme 1. Condensation of 2,4-dihydroxybenzal-
dehyde with propanoic anhydride afforded hydroxycouma-
rin 2 in modest yield,[7] which was then treated with nBuBr
to give n-butoxycoumarin 3 in good yield. Reaction of 3
with NBS (1.2 equiv.) provided bromocoumarin 4 in excel-
lent yield.[8] Finally, 1 was prepared by treating 4 (two
equiv.) with ethylenediamine in anhydrous acetonitrile
(yield: 44%). The structures of the intermediates and the
final product were confirmed by NMR spectroscopy, ESI
MS, and elemental analysis.
[a] State Key Laboratory of Chemo/Biosensing and Chemometrics,
College of Chemistry and Chemical Engineering, Hunan Uni-
versity,
Changsha, Hunan 410082, P. R. China
Fax: +86-731-8821464
E-mail: weiyinglin@hnu.cn
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
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W. Lin, L. Yuan, J. Feng,
FULL PAPER
Figure 1. Solvent effect on the excimer fluorescence of 1. Normal-
ized at the monomer emission. The excitation wavelength was
340 nm: (᭿) CH2Cl2, (᭹) CH3OH/CH2Cl2/H2O (1:4:0.01), (᭝)
CH3OH/CH2Cl2/H2O (2:3:0.01).
Scheme 1. Synthetic route to sensor 1. Reagents and conditions: (a)
CH3CH2COONa, (CH3CH2CO)2O, triethylamine, reflux, 6 h; (b)
nBuBr, acetone, K2CO3, reflux, 5 h; (c) NBS, AIBN, CCl4, reflux,
8 h; (d) ethylenediamine, anhydrous acetonitrile, r.t., 4 h.
The metal recognition properties of 1 were initially evalu-
ated by UV/Vis analysis. Figure 2a shows the absorption
spectra of 1 in the absence and presence of different
amounts of Al3+. Free sensor 1 displays an absorption peak
with a maximum around 327 nm, which is slightly red-
Compound 5 containing one coumarin fluorophore was shifted (by about 7 nm) to 334 nm with a small decrease in
used as a monomer control. As shown in Figure S1 (Sup- the absorbance intensity upon addition of increasing con-
porting Information), monomer 5 only displays a weak flu- centrations of Al3+ ions. In addition, the spectra also exhi-
orescent emission band around 395 nm owing to a quench- bit a distinct isosbestic point at 340 nm, which is indicative
ing photoinduced electron-transfer (PET) process from the of complex formation. Fluorescence emission was further
amino group to the coumarin fluorophore.[1] By contrast, investigated to determine the fluorescence response of 1 to-
1, which possesses two coumarin moieties, shows not only ward Al3+ ions under excitation at 340 nm (Figure 2b). The
a monomer emission band at 395 nm, but also a broad and addition of Al3+ ions elicits not only a drastic increase in
redshifted emission peak around 500 nm, which indicates the monomer emission intensity around 395 nm owing to
that some excimer is already present in the apolar solvent inhibition of the quenching PET process,[1] but also a dra-
CH2Cl2.[9,10] Moreover, the excimer formation in 1 is de- matic fluorescence enhancement around 500 nm as a result
pendent on the polarity of solvents as indicated in Figure 1, of the promotion of an emissive excimer channel formation.
which is in good agreement with the characteristics of the The fluorescence enhancement factors (FEF) at 395 and
excimer as reported in the literature.[10] As 1 has minimum 500 nm were determined as 15.7- and 56.8-fold, respectively.
excimer fluorescence background around 500 nm in Thus, sensor 1 displays striking fluorescence amplification
CH3OH/CH2Cl2/H2O (2:3:0.01), this solvent system was at two emission bands. Furthermore, the excitation spectra
chosen in the subsequent examination of the fluorescence of the Al3+-bound sensor (Figure S2, Supporting Infor-
response of the sensor toward metal ions in order to max- mation) suggest that the two emission bands at 395 and
imize the detection sensitivity.
500 nm originate from the same ground state.[5a]
Figure 2. The absorption spectra (a) and emission spectra (b) of 1 (20 µ) with the addition of Al3+ ions (0, 0.1, 0.3, 0.5, 0.7, 1, 3, 5, 7,
9, 12 equiv.). Excitation at 340 nm.
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A Dual-Channel Fluorescence-Enhanced Sensor for Aluminum Ions
To determine the stoichiometry of sensor 1 and Al3+ in
the complex, Job’s method [11] was employed by using emis-
sion changes at 395 nm as a function of molar fraction of
Al3+. A maximum emission was observed when the molar
fraction of Al3+ reached 0.5 (Figure 3), which is indicative
of a 1:1 stoichiometry complexation between 1 and Al3+
(the same stoichiometry value was obtained by using the
emission changes at 500 nm, Figure S3). The stability con-
stant of the complex was then calculated to be
1.23ϫ105 –1 with a good linear relationship (R = 0.993)
by a 1:1 binding mode (Figure S4, Supporting Infor-
mation).[12] As expected, the binding of Al3+ to sensor 1
is reversible, as evidenced by the complete reversal of the
fluorescence signal to free sensor 1 when an excess amount
of ethylenediamine was added to Al3+-bound sensor 1 (Fig-
ure S5, Supporting Information). The proposed binding
mode of sensor 1 with Al3+ is shown in Figure 4. This pro-
posed mechanism is consistent with the dual-channel emis-
sions of the complex at 395 and 500 nm. Coordination of
Al3+ with the nitrogen atoms may effectively decrease their
electron-donating ability, so the quenching PET process
Figure 4. Proposed binding mode of 1 with Al3+
.
Figure 5. Partial 1H NMR (400 MHz) spectrum of 1 in CD3OD/
from the nitrogen atoms to the coumarin rings is inhibited, CDCl3 (2:3). (A) 1 only; (B) 1 + Al3+ (2 equiv.).
which results in a large increase in fluorescence intensity at
395 nm. In addition, the binding of Al3+ with two carbonyl
The sensor was treated with various metal ions to investi-
oxygen atoms of the coumarin rings (consistent with the
report that the carbonyl oxygen atom in the coumarin could
be involved in an interaction with metal ions [6d,6e]) and the
nitrogen atoms could elicit the folding of the sensor and the
π–π* stacking of the coumarin dyes for the intramolecular
excimer formation, consequently promoting the emission
channel at 500 nm. In line with the proposed mechanism,
the coordination of Al3+ with the nitrogen and two car-
bonyl oxygen atoms is further supported by the observation
that addition of Al3+ induces significant changes in the
NMR spectrum of sensor 1 (Figure 5). For example, upon
the addition of Al3+, the resonance signals corresponding
to Ha on the –NH–, Hb on the –NCH2–, and Hc on the
coumarin rings become slightly broad and shift downfield
to 0.81, 0.62, and 0.21 ppm, respectively.
gate the selectivity. Representative metal ions such as Na+,
K+, Ca2+, Mg2+, Co2+, Ni2+, Cu2+, Hg2+, Cd2+, Mn2+, and
Zn2+ only induce minimum perturbation in fluorescence
spectra of sensor 1, whereas Fe3+ gives a modest fluores-
cence enhancement (Figures S6 and S7, Supporting Infor-
mation). However, the addition of Al3+ to sensor 1 causes
the largest fluorescence enhancement, both around 395 and
500 nm. Thus, emissions from two channels are metal-ion
dependent. This contributes considerably to the high selec-
tivity of the sensor, supporting the benefit of dual-channel
fluorescence-enhanced emissions. Furthermore, the visible
emission of sensor 1 is also metal-ion dependent (Figure 6;
for a color version, see Figure S8 in the Supporting Infor-
mation); thus, the sensor could be employed for convenient
visual sensing of Al3+. To further explore the effective appli-
cations of the sensor, the fluorescence response of 1 to Al3+
in the presence of typical competing ions was studied. As
shown in Figure 7, when Al3+ (12 equiv.) was added to sen-
sor 1 in the presence of a wide variety of competing metal
ions (12 equiv.) most exhibited minimum interference in the
Figure 3. Job plot for the determination of the stoichiometry of
sensor 1 and Al3+. The total concentration of 1 and Al3+ was kept
constant (25 µ). Excitation and emission was at 340 and 395 nm,
respectively.
Figure 6. The visual fluorescence emissions of sensor 1 (20 µ) to
metal ions (12 equiv.) on excitation at 365 nm by using a UV lamp
at r.t. From left to right: free, Mg2+, Co2+, Al3+, Hg2+, Zn2+, and
Ni2+
.
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W. Lin, L. Yuan, J. Feng,
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detection of Al3+. The observation that Fe3+ has no marked
influence on Al3+ detection indicates that 1 has a higher
binding affinity to Al3+ than Fe3+. Thereby, sensor 1 is use-
silica-gel plates and column chromatography was conducted on sil-
ica gel (mesh 200–300), both of which were obtained from Qingdao
Ocean Chemicals.
ful for selectively sensing Al3+ even under competition from 4: To a solution of 3 (1.0 g, 4.3 mmol) in CCl4 (35 mL) was added
NBS (0.96 g, 5.4 mmol) and a trace amount of AIBN, and the mix-
ture was then heated to reflux. After reaction for 8 h, the solvent
was removed under reduced pressure, and the residue was then
purified by chromatography on silica gel (CH2Cl2/petroleum ether,
1:1) to afford 4 as a colorless powder. Yield: 1.08 g (81.1%). M.p.
92–94 °C. 1H NMR (400 MHz, CDCl3): δ = 1.00 (t, 3 H), 1.51 (m,
2 H), 1.81 (m, 2 H), 4.03 (t, 2 H), 4.43 (s, 2 H), 6.81 (d, J = 2.0 Hz,
1 H), 6.86 (dd, J = 8.4 Hz and J = 2.4 Hz, 1 H), 7.38 (d, J =
8.8 Hz, 1 H), 7.79 (s, 1 H) ppm. MS (ESI): m/z = 311.2 [M + H]+.
C14H15BrO3 (311.17): calcd. C 54.04, H 4.86; found C 53.89, H
4.92.
other related metal ions.
1: To a solution of 4 (73.8 mg, 0.30 mmol) in anhydrous CH3CN
(3 mL) was added ethylenediamine (0.01 mL, 0.15 mmol) in
CH3CN (1 mL). The reaction was kept at room temperature for
4 h, and the precipitate was collected and washed with Na2CO3
solution and CH3CN to afford 1 as a colorless powder. Yield:
49.6 mg (43.6%). M.p. 176–178 °C. 1H NMR (400 MHz; CDCl3/
CD3OD, 3:2): δ = 1.00 (m, 6 H), 1.53 (m, 4 H), 1.80 (t, 4 H), 2.80
(s, 2 H), 3.57 (s, 4 H), 3.99 (t, 4 H), 6.64 (d, J = 2.0 Hz, 2 H), 682
(d, J = 8.0 Hz, 2 H), 7.40 (d, J = 7.6 Hz, 2 H), 7.93 (s, 2 H) ppm.
MS (ESI): m/z = 520.2 [M + H]+. C30H36N2O6 (520.62): calcd. C
69.21, H 6.97, N 5.38; found C 69.57, H 6.62, N 5.78.
Figure 7. Fluorescence enhancement response of 1 (20 µ) to Al3+
(12 equiv.) in the presence of different competing metal ions
(12 equiv.). Excitation at 340 nm.
Conclusions
Sensor 1 was developed as the first example of fluores- General Procedures for the Metal-Ion Binding Studies: Metal chlo-
rate (Hg2+, Ni2+, Ca2+, Mg2+, Cd2+, Cu2+, Co2+, Zn2+, Na+, K+,
Al3+, Fe3+) or sulfate (Mn2+) stock solutions were prepared in
twice-distilled water. Sensor 1 was dissolved in CH3OH/CH2Cl2
(2:3) at room temperature to afford the sensor stock solution
(50 µ). Test solutions were prepared by placing 2 mL of the sensor
stock solution and an appropriate aliquot of each metal stock into
a 5-mL volumetric flask and diluting the solution to 5 mL with
methanol/CH2Cl2 (2:3). The resulting solution was shaken well and
left to stand at room temperature for 10 min before recording the
absorption and emission spectra of the metal-complexed sensor.
cence-enhanced Al3+ sensor with unique dual-channel emis-
sions. The addition of Al3+ to 1 results in not only a large
fluorescence enhancement at 395 nm because of inhibition
of a quenching PET channel, but also a dramatic fluores-
cence enhancement around 500 nm as a result of the pro-
motion of an emissive excimer channel formation. The
metal-ion-dependent dual-channel fluorescence-enhanced
response contributes substantially to the high sensitivity
and selectivity of 1. Although sensor 1 is potentially useful
in nonaqueous settings (environmental applications), it Unless otherwise noted, for all measurements, the excitation wave-
length was at 340 nm and both the excitation and emission slit
widths were 5 nm.
could also be extended to function in aqueous systems by
incorporating hydrophilic groups. The work toward this end
is in progress in our laboratory. In addition, we believe that
the dual-channel signaling mechanism of sensor 1 should
lead to the development of powerful sensors for other metal
ions with fluorescence-enhanced dual-channel emissions for
exciting applications in diverse fields.
Supporting Information (see footnote on the first page of this arti-
cle): Detailed experimental procedures and full characterization
data for all compounds synthesized; some spectra of the sensor.
Acknowledgments
Funding was partially provided by the Key Project of Chinese Min-
istry of Education (No:108167), the Scientific Research Foundation
for the Returned Overseas Chinese Scholars, State Education Min-
istry (2007–24), and the Hunan University research funds.
Experimental Section
General Information and Materials: Unless otherwise stated, all rea-
gents were purchased from commercial suppliers and used without
further purification. Solvents used were purified and dried by stan-
dard methods prior to use. Twice-distilled water was used through-
out all experiments. Melting points were determined with a Beijing
taike XT-4 microscopy and are uncorrected. ESI MS analyses were
performed by using a Waters Micromass ZQ-4000 spectrometer.
Electronic absorption spectra were recorded with a SHIMADZU
UV-2450 spectrometer. The emission spectra were measured with a
[1] For reviews and books, see: a) J. R. Lakowicz, Topics in Fluo-
rescence Spectroscopy, Volume 4: Probe Design and Chemical
Sensing, Kluwer Academic Publishers, New York, 2002; b)
A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. M.
Huxley, C. P. McCoy, J. T. Rademacher, T. E. Rice, Chem. Rev.
1997, 97, 1515–1566; c) B. Valeur, I. Leray, Coord. Chem. Rev.
2000, 205, 3–40.
[2] a) E. Álvarez, M. L. Fernández-Marcos, C. Monterroso, M. J.
Fernández-Sanjurjo, Forest. Ecol. Manag. 2005, 211, 227–239;
b) J. Barceló, C. Poschenrieder, Environ. Exp. Bot. 2002, 48,
75–92.
1
HITACHI F4500 fluorescence spectrophotometer. H NMR spec-
tra were measured with an INOVA-400 spectrometer by using TMS
as an internal standard. Elemental analyses were obtained with a
Vario El III Elemental Analyzer. TLC analyses were performed on
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A Dual-Channel Fluorescence-Enhanced Sensor for Aluminum Ions
[3]
[4]
[7] M. Catto, O. Nicolotti, F. Leonetti, A. Carotti, A. D. Favia, R.
Soto-Otero, E. Mendez-Alvarez, A. Carotti, J. Med. Chem.
2006, 49, 4912–4915.
[8] A. Horvath, P. Nussbaumer, B. Wolff, A. Billich, J. Med. Chem.
2004, 47, 4268–4276.
[9] J. F. Callan, A. P. de Silva, D. C. Magri, Tetrahedron 2005, 61,
8551–8588.
[10] a) Z. Zhong, Y. Zhao, Org. Lett. 2007, 9, 2891–2894; b) H.
Yuasa, N. Miyagawa, T. Izumi, M. Nakatani, M. Izumi, H.
Hashimoto, Org. Lett. 2004, 6, 1489–1492.
[11] a) W. C. Vosburgh, G. R. Cooper, J. Am. Chem. Soc. 1941, 63,
437–442; b) R. R. Avirah, K. Jyothish, Da. Ramaiah, Org.
Lett. 2007, 9, 121–124; c) G.-Y. Qing, Y.-B. He, Y. Zhao, C.-G.
Hu, S.-Y. Liu, X. Yang, Eur. J. Org. Chem. 2006, 1574–1580;
d) M. Nishijima, T. Wada, T. Mori, T. C. S. Pace, C. Bohne,
Y. Inoue, J. Am. Chem. Soc. 2007, 129, 3478–3479; e) S. A.
McFarland, N. S. Finney, J. Am. Chem. Soc. 2001, 123, 1260–
1261.
[12] a) H. A. Benesi, J. H. Hildebrand, J. Am. Chem. Soc. 1949, 71,
2703–2707; b) T.-L. Kao, C.-C. Wang, Y.-T. Pan, Y.-J. Shiao,
J.-Y. Yen, C.-M. Shu, G.-H. Lee, S.-M. Peng, W.-S. Chung, J.
Org. Chem. 2005, 70, 2912–2920; c) I.-T. Ho, G.-H. Lee, W.-S.
Chung, J. Org. Chem. 2007, 72, 2434–2442.
N. E. W. Alstad, B. M. Kjelsberg, L. A. Vøllestad, E. Lydersen,
A. B. S. Poléo, Environ. Pollut. 2005, 133, 333–342.
For selective examples of aluminum-responsive fluorescent sen-
sors, see: a) C. Jiang, B. Tang, R. Wang, J. Yen, Talanta 1997,
44, 197–202; b) M. Arduini, F. Felluga, F. Mancin, P. Rossi, P.
Tecilla, U. Tonellato, N. Valentinuzzi, Chem. Commun. 2003,
1606–1607; c) A. Jeanson, V. Bereau, Inorg. Chem. Commun.
2006, 9, 13–17; d) A. B. Othman, J. W. Lee, Y.-D. Huh, R. Ab-
idi, J. S. Kim, J. Vicens, Tetrahedron 2007, 63, 10793–10800; e)
S. M. Ng, R. Narayanaswamy, Anal. Bioanal. Chem. 2006, 386,
1235–1244; f) Y. Zhao, Zh. Lin, H. Liao, C. Duan, Q.-j. Meng,
Inorg. Chem. Commun. 2006, 9, 966–968; g) J. L. Ren, J. Zhang,
J. Q. Luo, X. K. Pei, Z. X. Jiang, Analyst 2001, 126, 698–702.
[5] For selected recent examples, see: a) K. Choi, A. D. Hamilton,
Angew. Chem. Int. Ed. 2001, 40, 3912–3915; b) J. V. Mello, N. S.
Finney, Angew. Chem. Int. Ed. 2001, 40, 1536–1538.
[6] For recent coumarin-based chemosensors for metal ions, see:
a) J. Wang, X. Qian, J. Cui, J. Org. Chem. 2006, 71, 4308–4311;
b) C. C. Woodroofe, S. J. Lippard, J. Am. Chem. Soc. 2003,
125, 11458–11459; c) S. Yamaguchi, I. Yoshimura, T. Kohira,
S. Tamaru, I. Hamach, J. Am. Chem. Soc. 2005, 127, 11835–
11841; d) C. T. Chen, W. P. Huang, J. Am. Chem. Soc. 2002,
124, 6246–6247; e) N. C. Lim, C. Brückner, Chem. Commun.
2004, 1094–1095.
Received: March 28, 2008
Published Online: June 18, 2008
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