Notes Bull. Korean Chem. Soc. 2014, Vol. 35, No. 7 2183
http://dx.doi.org/10.5012/bkcs.2014.35.7.2183
A New Coumarin-Based Colorimetric and Fluorometric Sensor for
Cu2+
Kyoung-Lyong An,†,‡ Koon Ha Park,‡,* and Kun Jun†,*
†Interface Materials & Process Research Group, Korea
Research Institute of Chemical Technology,
Daejeon 305-600, Korea. *E-mail: [email protected]‡Department of
Chemistry, Chungnam National University, Daejeon 305-764, Korea.
*E-mail: [email protected]
Received March 3, 2014, Accepted March 18, 2014
Key Words : Coumarin, Colorimetric, Fluorometric, Cu2+, Metal
sensor
Chemosensors, small chemical compounds that sense the
presence of analytes or energy, typically consist of two
components: a receptor moiety that interacts with the target
analytes and a read-out system that signals binding.1 And
one of the most utilized research tool for the study of
chemo-
sensors employs a colorimetric and fluorometric spectro-
scopic techniques.2 So far successful reports on metal ion
sensors have been documented including our recent result.3
Many different kinds of optical or fluorescent sensors have
several advantages (such as high sensitivity and
selectivity,
non-destructive analysis, low cost and real-time monitor-
ing),4 which allow naked-eye detection of color and fluore-
scent emission change upon metal ion binding without the
use of any expensive spectroscopic equipment.5
Copper, the third most abundant transition metal following
zinc and iron in human body, exerts essential functions in
many cellular enzymes and proteins such as Cu/Zn super-
oxide dismutase, dopamine monooxygenase, cytochrome C
oxidase and ceruloplasmin.6 However, it becomes toxic
when excessive amounts of copper ion were accumulated. It
is believed that the disruption of copper homeostasis is as-
sociated with neurodegenerative illnesses such as
Parkinson’s,
Wilson’s, Alzheimer’s and Menke’s diseases.7-10
Coumarin skeleton is often utilized as fluorescent sensor
due to its excellent photophysical properties like great
fluore-
scent intensity, high quantum yield, high photostability,
bio-
logical stability, nontoxicity and derivatizable backbone.11
There have been many excellent coumarin-based fluorescent
probes reported for not only anions but also cations such as
Fe3+, Ag+, Al3+, Ni2+ and Hg2+ during the last decade.12-16
Herein, we report a new colorimetric and fluorescent
“turn-off” sensor that responds to Cu2+ through coumarin
derivative. The binding properties of sensor A ward various
metal ions were investigated by UV-Vis absorption and
fluorescence spectroscopy.
Sensor A was synthesized as shown in Scheme 1. Compound
1 and 2 were prepared by the known procedures.17
Investigations on the photophysical properties revealed
that sensor A showed high selectivity to Cu2+ in 10 mM tris-
HCl buffer solution (acetonitrile/water = 9:1, pH = 7.01).
Sensor A showed an absorption band at 375 nm in 10 mM
tris-HCl buffer solution. In the presence of Cu2+ (20 μM),there
appeared a new red-shifted absorption band at 425 nm
at the expense of peak at 375 nm (Figure 1(a) in red). The
absorption bands at 375 and 425 nm linearly decreased and
increased, respectively, by the increasing concentration of
Cu2+ (Figure 1(b)). However, none of the other metal ions
(Ag+, K+, Li+, Na+, Ca2+, Cd2+, Co2+, Ni2+, Zn2+ and Fe3+
(20
μM, 2.0 equivalent, nitrate salt)) showed such a red-shift
intheir absorption spectra (Figure 1(a)).
The red-shift in the absorption spectra upon addition of the
Cu2+ can be rationalized by intramolecular charge transfer
(ICT). It is well known that an electron-donating group at
7-
position and an electron-withdrawing group at 3-position in
coumarin skeleton induce absorption band into visible
region as a result of effective ICT through the electron
push–
pull system.18 The complexation of a Cu2+ in sensor A would
increase electron-withdrawing character of 3-position,
result-
ing a stronger ICT.
To investigate the binding mode of sensor A to Cu2+, Job’s
method was carried out. A 1:2 stoichiometry was determin-
ed by Job’s plot. The maximum absorption change was
Scheme 1. Synthetic route to sensor A.
2184 Bull. Korean Chem. Soc. 2014, Vol. 35, No. 7 Notes
observed when the molar ratio of sensor A to Cu2+ was 0.33,
indicating a 1:2 stoichiometry for the complex (Figure 1(b).
inset (b)).
From Figure 2(a), sensor A showed strong fluorescence
emission at 485 nm in 10 mM tris-HCl buffer solution. Upon
the addition of metal ions up to 3 equivalent emission peaks
were slightly quenched by other metal ions, but remarkable
fluorescence quenching was observed by Cu2+ in 10 mM
tris-HCl buffer solution, peaking at λem = 485 nm. Upon
theaddition of Cu2+ up to 3 equivalent the emission peaks were
linearly decreased until complete quenching (Figure 2(b)).
The photophysical data (Figure 1 and 2) indicate that
sensor A has high selectivity and sensitivity to Cu2+. Com-
petitive recognition of Cu2+ in the presence of various
other
metal ions, even in equal concentration, was also studied
and
shown in Figure 3, where detection of Cu2+ was little
affect-
ed in the presence of other metal ions reflecting excellent
selectivity of sensor A for Cu2+.
Regarding to the possible complexation sites in sensor A
by two Cu2+ ions, we can imagine that oxygen in phenol
ring, nitrogen at C-3 and carbonyl oxygen at C-2 would
participate for the purpose.
In order to have clear knowledge on the site of complexa-
Figure 1. (a) Absorption spectra of sensor A (10 μM) in
thepresence of various metal ions (20 μM). Inset: The visible
imageof sensor A (100 μM) in the presence of Cu2+ (300 μM).
(b)Absorption spectra of sensor A (10 μM) upon the addition of
0-4equivalent of Cu2+ (0-40 μM). Inset (a): The absorption ratio
(AM/AA at 425 nm) as a function of [Cu
2+]. Inset (b): Job’s plot data forevaluating the stoichiometry
of sensor A + Cu2+ complex (at 425nm). The total concentration of
sensor A and Cu2+ was 10 μM.Conditions: 10 mM tris-HCl buffer
(acetonitrile/water = 9:1, pH =7.01).
Figure 2. (a) Emission spectra of sensor A (10 μM) in the
presenceof various metal ions (30 μM). Inset: The fluorescence
image ofsensor A (100 μM) in the presence of Cu2+ (300 μM) excited
byUV lamp (λex = 365 nm). (b) Titration curves by 0-5 equivalent
ofCu2+ ion (0-50 μM). Inset: The fluorescence intensity (at 485
nm)versus [Cu2+]. Conditions: 10 mM tris-HCl buffer
(acetonitrile/water = 9:1, pH = 7.01), λex = 385 nm.
Figure 3. Sensor A (10 μM) with various metal ions (20 μM) inthe
absence (blue) and presence (red) of Cu2+ (20 μM). (a)Absorption
ratio (AM/AA at 425 nm) and (b) fluorescence intensityratio (FM/FA,
at 485 nm). Conditions: 10 mM tris-HCl buffer(acetonitrile/water =
9:1, pH = 7.01), λex = 385 nm.
Notes Bull. Korean Chem. Soc. 2014, Vol. 35, No. 7 2185
tion in sensor A by Cu2+, more study would be required.
In conclusion, we have developed a new colorimetric and
fluorescent “turn-off” sensor for Cu2+ based on coumarin
Shiff base of hydroxycinnamaldehyde. It displays a 50 nm
red-shift of maximum absorption band with color change
from colorless to greenish-yellow upon addition of Cu2+ in
10 mM tris-HCl buffer solution (acetonitrile/water = 9:1, pH
= 7.01). And also remarkable fluorescence quenching was
observed upon the addition of Cu2+. The 1:2 stoichiometry
of sensor complex (sensor A + Cu2+) was confirmed by Job’s
plot based on absorption titration.
Experimental
Synthesis of Sensor A. To a solution of compound 2 (0.3
g, 1.3 mmol) in absolute ethanol (15 mL), was added 2-
hydroxycinnamaldehyde (0.22 g, 1.4 mmol). After the mix-
ture was stirred for 20 h at room temperature, a reddish
precipitate was filtered, washed with ethanol (10 mL) and
cold acetone (20 mL). It was dried under vacuum to afford a
reddish crystalline solid (0.24 g, 50%). HRMS (EI): 362.1627
(calcd. for C22H22N2O3, 362.1630); 1H-NMR (DMSO-d6,
500 MHz) δ 10.14 (s, 1H), 8.85 (d, 1H), 7.69 (s, 1H), 7.59(d,
1H), 7.46 (d, 1H), 7.43 (s, 1H), 7.18 (t, 1H), 7.10 (m,
1H), 6.89 (d, 1H), 6.83 (t, 1H), 6.72 (d, 1H), 6.55 (s, 1H),
3.43 (m, 4H), 1.12 (t, 6H); 13C NMR (DMSO-d6, 125 MHz)
δ 162.7, 158.0, 155.8, 154.2, 149.9, 139.1, 132.9, 130.4,129.1,
128.5, 127.8, 122.3, 119.3, 116.0, 109.3, 108.3, 96.3,
43.9, 12.2.
Acknowledgments. This work was supported by Institu-
tional Research Program of Korea Research Institute of
Chemical Technology (KRICT) and the R&D Program of
Small & Medium Business Administration (SMBA).
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