Coumarin-derivative-based off-on catalytic chemodosimeter for Cu 2+ ions

Article abstract of DOI:10.1039/b908638b

In this study, a new coumarin derivative is shown to be a highly effective turn-on fluorescent sensor that is catalytically hydrolyzed by Cu²⁺, and the catalytic process induces a large increase in the fluorescence intensity, due to amplificati

Full text of DOI:10.1039/b908638b

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Coumarin-derivative-based off–on catalytic chemodosimeter for Cu2+
ionsw
Mi Hee Kim, Hyun Hye Jang, Sujung Yi, Suk-Kyu Chang and Min Su Han*
Received (in Cambridge, UK) 30th April 2009, Accepted 11th June 2009
First published as an Advance Article on the web 3rd July 2009
DOI: 10.1039/b908638b
In this study, a new coumarin derivative is shown to be a highly
effective turn-on fluorescent sensor that is catalytically
hydrolyzed by Cu2+, and the catalytic process induces a large
increase in the fluorescence intensity, due to amplification of the
fluorescence signal.
can possibly be used to amplify the fluorescence signal induced
by a catalytic reaction.
In this communication, we present a new turn-on type
chemodosimeter that has nanomolar sensitivity and high
selectivity because of the catalytic hydrolysis of a hydrazone
derivative by Cu2+. Hydrazone derivatives have been used as
protecting groups for carbonyl compounds, and carbonyl
compounds are readily regenerated from hydrazones by
undergoing catalytic hydrolysis with Cu2+ ions.7 To develop
a selective chemodosimeter for Cu2+, we carried out the
catalytic regeneration of carbonyl compounds from hydrazone
derivatives by using Cu2+ ions. For this purpose, we selected
coumarin 334, which is a fluorophore with a high quantum
yield (y = 0.83), as a signaling moiety.8 When the catalytic
hydrolysis of the hydrazone occurs in the presence of Cu2+
ions, the regeneration of coumarin from the hydrazone
derivative induces a fluorescence change. The hydrazone
derivative used in this study (1) was synthesized from coumarin
334 and a hydrazine, as shown in Scheme 1.
Many transition-metal ions function as essential trace elements
in biological systems. Among such metal ions, Cu2+ is a
particularly important divalent cation that plays a vital role
in many cellular processes such as those occurring in the
human nervous system, gene expression, and the functioning
and structural enhancement of proteins; further, it functions as
a cofactor of many enzymes such as nuclease.1 However, if the
concentration level of copper exceeds that required in cellular
processes, copper can be toxic and can cause oxidative stress
and disorders associated with neurodegenerative diseases such
as Alzheimer’s disease.2 As a result, Cu2+ has attracted
considerable attention in recent years, and numerous fluorescent
Cu2+ sensors have been developed.3,5
Conventional Cu2+ sensors developed in the past are
fluorescent sensors that are based on chelating moieties.3
However, most of these sensors show decreased emission upon
Cu2+ binding, due to quenching of fluorescence by mechanisms
inherent to paramagnetic species; such decreased emission is
undesirable for analytical purposes.3,4 Therefore, many efforts
have been made to develop various fluorescent sensors speci-
fically for Cu2+ ion detection with fluorescence enhancement.5
Only a few sensors among such ‘‘off–on’’ Cu2+ sensors have
nanomolar sensitivity and high selectivity.5 These sensors
detect Cu2+ ions on the basis of highly selective catalytic
reactions induced by these ions.5a–c This approach is very
attractive for achieving high selectivity and sensitivity of
sensors.6 For example, Lu et al. developed a catalytic
chemosensor for Cu2+ by using DNAzyme;5a Czarnik
prepared a catalytic hydrolyzable rhodamine B hydrazide by
using Cu2+ ions;5b finally, Anslyn detected Cu2+ by carrying
out signal amplification on the basis of the Heck reaction.5c
These sensors had both nanomolar sensitivity and high
selectivity in the detection of Cu2+ ions; their fluorescence
was enhanced due to the detection. Moreover, these sensors
Fluorescence spectra of solutions of 1 were recorded 5 min
after the addition of Cu2+. Fluorescence emission spectra of 1
in the presence of Cu2+ in various concentrations are shown in
Fig. 1. The addition of Cu2+ ions induced the enhancement of
the fluorescence, and, as can be found from the inset of Fig. 1,
the observed fluorescence intensity was nearly proportional
to the Cu2+ concentration. From the titration results, the
detection limit of 1 for Cu2+ was estimated to be 8.7 Â 10À8 M
(see ESIw). This detection limit is acceptable within the US
EPA limit (B20 mM) for the detection of Cu2+ in
drinking water.
Another important property of the chemodosimeter is its
high selectivity for Cu2+ over other metal ions. To evaluate
the Cu2+ selectivity of 1, changes in the fluorescence properties
of 1 caused by other metal ions were also measured. Fluorescence
spectra of solutions of 1 (1 mM) recorded 5 min after the
addition of 10 equiv. of each of these metal ions are shown in
Fig. 2. Compound 1 showed remarkably high selectivity for
Department of Chemistry, Chung-Ang University, Seoul 156-756,
Republic of Korea. E-mail: mshan@cau.ac.kr; Fax: +82 2 825 4736;
Tel: +82 2 820 5198
w Electronic supplementary information (ESI) available: Synthesis of 1,
determination of the detection limit of 1 for Cu2+, the method for
quantification of Cu2+, UV-Vis and fluorescence spectra of both 1 and
1 in the presence of Cu2+, 1H-NMR spectrum of 1 in the presence of
Cu2+ and 1H-NMR spectrum of a mixture of coumarin 334 and
4-methoxyphenylhydrazine. See DOI: 10.1039/b908638b
Scheme 1 Synthetic route to 1.
ꢀc
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Fig. 1 Fluorescence emission spectra obtained by addition of Cu2+
(0–40 mM) to pH 5.0 buffer solution (acetonitrile : 0.01 M acetate
buffer = 5 : 5) containing 1 (1 mM). Inset: plot of normalized
fluorescence intensities of 1 at 502 nm versus Cu2+ concentration.
Fig. 3 Kinetics of fluorescence over background at varying Cu2+ levels.
Conditions: [1] = 1.0 Â 10À6 M, lex = 450 nm, buffer pH = 5.0.
catalytic sensing cycle. Finally, the catalytic cycle induces
and amplifies the fluorescence signal (Scheme 2).
To confirm the sensing mechanism of 1 for Cu2+, products
of 1 catalytically hydrolyzed by Cu2+ were investigated.
UV-Vis and fluorescence spectra of 1 incubated with 10 equiv.
of Cu2+ for 10 min were compared with those of coumarin
334, and the spectra of these two compounds were found to be
almost identical (see ESIw). Further, 1 was reacted with Cu2+
in deuterated acetonitrile-d3–D2O (1 : 1, v/v) for 10 min, and
the NMR spectrum of the solution was measured after
eliminating the paramagnetic metal ion (Cu2+) using Chelex
1
resin.9 The H-NMR spectrum of the resulting solution was
consistent with that of a 1 : 1 mixture of coumarin 334 and
4-methoxyphenylhydrazine (see ESIw). These data clearly
confirmed that the hydrolyzed product of 1 was coumarin
334. Further, the time-dependent fluorescence change in 1 was
evaluated in the presence of Cu2+ ions. In a typical experiment,
Cu2+ ions at fixed concentrations were added to a solution of
Fig. 2 Fluorescence emission spectra obtained by addition of various
metal ions to pH 5.0 buffer solution (acetonitrile : 0.01 M acetate
buffer = 5 : 5) containing 1 (1 mM). Inset: plot of normalized
fluorescence intensities of 1 at 502 nm versus metal ions.
Cu2+ over the other metal ions. No significant increase in the
fluorescence intensity was observed upon the addition of any
other metal ions.
1. Upon the hydrolysis of the hydrazone group in 1 by Cu2+
,
the fluorescence of the solution gradually increased and the
hydrolysis process was traced by measuring the fluorescence at
502 nm (Fig. 3). The entire quantity of 1 was hydrolyzed
within 10 min in the presence of 20 mM Cu2+ (20 equiv. to 1),
and the fluorescence was enhanced and became B18 times
(100% hydrolysis of 1) the initial fluorescence. However, no
To gain a better understanding of the properties of 1, having
high sensitivity and selectivity, we postulate its sensing
mechanism as follows: because Cu2+ ions show high affinity
for various amine ligands, Cu2+ is bound to 1, which in turn
promotes the hydrolysis of the hydrazone bond in 1.
The hydrolyzed products and the Cu2+ ion separate, and
the Cu2+ ion binds to another 1 and subsequently again
promotes the hydrolysis of the hydrazone bond via the
changes in fluorescence were detected in the absence of Cu2+
.
In particular, in the presence of 0.1 mM Cu2+ (0.1 equiv. to 1),
the fluorescence became more than four times (420% hydrolysis
of 1) the initial fluorescence at 1500 s. This observation implies
that 1 was catalytically hydrolyzed by Cu2+ and could
successfully detect a very low concentration of Cu2+ by signal
amplification, as shown in Scheme 2. Therefore, because of
signal amplification via the catalytic reaction, the developed
chemodosimeter is advantageous in that it has better sensitivity
than traditional chemodosimeters without any catalytic
properties.
The applicability of the developed chemodosimeter to the
analysis of Cu2+ ions in a practical sample was also investigated.
Possible interferences by other metal ions were assessed by
measuring Cu2+-induced fluorescence changes in 1 in the
presence of background metal ions. Fluorescence spectra of
solutions of 1 (1 mM) recorded 5 min after the addition of
Scheme 2 Cu2+-induced catalytic sensing cycle of 1.
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This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 4838–4840 | 4839

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This work was supported by the Korea Research
Foundation Grant funded by the Korean Government
(KRF-2007-331-C00162).
Notes and references
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Fig. 4 Fluorescence emission spectra obtained by addition of Cu2+
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In conclusion, we have proposed a new hydrazone derivative
that is easily prepared in a single step from coumarin 334,
and we have demonstrated that the hydrazone derivative is a
highly effective fluorescent sensor with strong fluorescence
enhancement in the presence of paramagnetic Cu2+ ions.
This sensor is catalytically hydrolyzed by Cu2+, and the
catalytic process induces a large increase in the fluorescence
intensity, due to signal amplification. Importantly, this
compound has high selectivity for Cu2+ over other anions;
moreover, this selectivity is retained even in the presence of an
excess of other metal ions. In addition, the detection limit
for Cu2+ is below an aqueous Cu2+ concentration of 100 nM
and this detection limit is acceptable within the US EPA
limit. Consequently, the chemodosimeter 1 can be employed
in a practical system for monitoring Cu2+ concentrations in
aqueous samples.
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ꢀc
This journal is The Royal Society of Chemistry 2009
4840 | Chem. Commun., 2009, 4838–4840