A fluorescence "turn-on" chemodosimeter for Cu2+ in aqueous solution based on the ion promoted oxidation

Article abstract of DOI:10.1039/c2dt12497a

We developed a novel method for Cu²⁺ detection based on the ion promoted oxidation reaction. Chemodosimeter L (weak fluorescence) can be oxidized into 3-benzothiazoly-7-N,N-diethylaminocoumarin (strong green fluorescence, coumarin 6) by Cu²⁺ with high selectivity and sensitivity in HEPES (10 mM, pH = 7.4) buffer containing 50% (v/v) water-CH ₃CN solution. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt12497a

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A fluorescence “turn-on” chemodosimeter for Cu2+ in aqueous solution based
on the ion promoted oxidation†
Junbo Li,* Yang Zeng, Qihui Hu, Xianglin Yu, Jia Guo and Zhiquan Pan*
Received 28th December 2011, Accepted 30th January 2012
DOI: 10.1039/c2dt12497a
We developed a novel method for Cu2+ detection based on
the ion promoted oxidation reaction. Chemodosimeter L
(weak fluorescence) can be oxidized into 3-benzothiazoly-7-
Scheme 1 Mechanism for Cu2+ catalyzed oxidative coupling reaction.
N,N-diethylaminocoumarin (strong green fluorescence, cou-
marin 6) by Cu2+ with high selectivity and sensitivity in
HEPES (10 mM, pH = 7.4) buffer containing 50% (v/v)
water–CH3CN solution.
Nowadays, much attention has been paid to the development of
new fluorescent chemosensors for metal ions because of their
potential applications in the fields of clinical biochemistry,
analytical chemistry and environmental chemistry.1–6 Among
these metal ions, Cu2+ is a widely used ion in industry and also
the third-most abundant transition metal ion in the body, which
Scheme 2 Synthesis and Cu2+ promoted oxidation of chemodosimeter
plays a critical role as a catalytic cofactor for a variety of metal-
L.
loenzymes, including superoxide dismutase, cytochrome c
oxidase, and tyrosinase.7,8 Despite its important role in organ-
isms, the accumulation of excess amounts of copper ions or their
misregulation can cause a series of severe diseases such as Alz-
heimer’s and Parkinson’s diseases.9 Therefore, the rapid detec-
tion of Cu2+ is very important in environmental and biological
systems. So far, many Cu2+ fluorescent probes have been
reported, however, most of them, especially the early reported
ones,10–12 were based on the fluorescence quenching response
upon binding with Cu2+, which is not as sensitive as fluor-
escence enhancement response. Currently, many signal processes
for fluorescence enhancement response for Cu2+ have been pro-
posed such as the ring opening of rhodamine derivatives,13,14 the
prohibition of the isomerization of CvN,15,16 the coordination
of N or O atoms,17,18 the hydrolysis of ligand,19,20 and the click
cycloaddition reaction.21 However, it should be noted that the
Cu2+ promoted oxidation reaction has not been used as a signal
process for chemosensors, although it has been used widely in
organic synthesis.22–24
the synthesis of 3-benzothiazoliny-7-N,N-diethylaminocoumarin
(L), which showed weak fluorescence due to the effective photo-
induced electron transfer (PET) process from N atom to cou-
marin. They demonstrated the enzyme sensing properties of L
based on the controllable oxidation of benzothiazoline.28 Our
group focuses our interests on the transition metal ions detection
using functional dyes such as rhodamine and coumarin.29,30 Cu-
catalyzed oxidative coupling reactions have been reported widely
(Scheme 1).22–24 The structure of benzothiazoline is similar to
the amines used in these catalyzed oxidative coupling reactions.
With this idea in mind, we proposed that the oxidation of 3-ben-
zothiazoliny-7-N,N-diethylaminocoumarin (L) (weak fluor-
escence)
to
3-benzothiazoly-7-N,N-diethylaminocoumarin
(strong green fluorescence) can be promoted by metal ions such
as Cu2+. Fortunately, the fluorescence of 3-benzothiazoliny-7-N,
N-diethylaminocoumarin (L) in HEPES (10 mM, pH = 7.4)
buffer containing 50% (v/v) water–CH3CN solution is weak
(“off” state). However, when 10 equiv. of Cu2+ was added into
the solution, a strong green fluorescence (“on” state) was
observed. Meanwhile, other common metal ions such as Fe3+,
Pb2+, Ni2+, Co2+, Mn2+, Zn2+, Hg2+, Cd2+, Ag+, Mg2+, Ca2+,
Ba2+, Na+ and K+ induced little fluorescence intensity
enhancement.
Chemodosimeter L was readily synthesized in three steps
(Scheme 2). The 7-diethylaminocoumarin was prepared accord-
ing to the literature31 and the 7-diethylaminocoumarin-3-alde-
hyde was obtained using the classical Vilsmeier reaction.32
Chemodosimeter L was obtained by condensation of 7-
In this manuscript, we demonstrated a new method for Cu2+
detection by metal ions induced oxidation, which was based on
the easy oxidation of benzothiazoline.25–27 Wang et al. reported
Hubei Novel Reactor & Green Chemical Technology Key Laboratory,
Key Laboratory for Green Chemical Process of Ministry of Education,
Wuhan Institute of Technology, 693 Xiongchu Avenue, Wuhan, Hubei
Province, 430074, PR China. E-mail: jbli@mail.wit.edu.cn,
zhiqpan@163.com
†Electronic supplementary information (ESI) available: Synthesis of
compound L, experimental details, NMR spectra, MS spectra, and
additional spectra. See DOI: 10.1039/c2dt12497a
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 3623–3626 | 3623

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Scheme 3 Possible mechanism for Cu2+ promoted oxidation of L.
Fig. 1 The UV-vis absorption spectra of L (10 μM) in HEPES
(10 mM, pH = 7.4) buffer containing 50% (v/v) water–CH3CN in the
presence of increasing concentrations of Cu2+ (0–10 equiv.). The inset
plots the absorbance at 391 nm and 467 nm versus the Cu2+
concentration.
diethylaminocoumarin-3-aldehyde with 2-aminobenzenethiol
with a yield of 60%.28
Fig. 1 shows the UV-vis absorption spectra of L in the pres-
ence of different concentrations of Cu2+ ions. When increasing
concentrations of Cu2+ were introduced in HEPES (10 mM, pH
= 7.4) buffer containing 50% (v/v) water–CH3CN solution, the
maximum absorption wavelength at 391 nm was decreased
accomplished with the increase of the absorption at 467 nm. Fur-
thermore, an isosbestic point at 413 nm was observed, which
indicated the formation of a new compound. Meanwhile, L
showed weak fluorescence in the absence of Cu2+ in HEPES
(10 mM, pH = 7.4) buffer containing 50% (v/v) water–CH3CN.
By contrast, upon addition of 10 equiv. of Cu2+, a remarkable
fluorescence intensity enhancement at 509 nm was observed.
The fluorescence intensity reached maximum within 1 min
(Fig. S1†). Furthermore, the addition of the chelating reagent
EDTA or diethylenetriamine to the solution did not reduce the
fluorescence intensity (Fig. S2†), which demonstrated that the
oxidation process is irreversible.33,34 Moreover, the absorption
and fluorescence of L in the presence of 10 equiv. of Cu2+ were
the same as those of the authentic sample of 3-benzothiazoly-7-
N,N-diethylaminocoumarin (coumarin 6), indicating the Cu2+
induced formation of it (Fig. S3, S4†). Besides, for a practical
application, the stability of L solution in different pH was also
investigated. As demonstrated in Fig. S5,† L solution (50% (v/v)
water–CH3CN) was stable at a wide pH span (4–11). Meanwhile,
L could also detect Cu2+ smoothly at the pH span from 4 to 11.
In order to investigate the necessity of the oxygen in the oxi-
dation of L, the solution of L was firstly purged with nitrogen
for 20 min to remove the oxygen, then the 10 equiv. of Cu2+ was
added and the fluorescence spectra were recorded (Fig. S6†).
The results showed that the fluorescence spectra were similar in
the absence and presence of oxygen, indicating that the ben-
zothiazoline was oxidized by Cu2+. It is well known that the oxi-
dizability of Fe3+ is stronger than Cu2+. However, when 10
equiv. Fe3+ was added into the solution of L, little fluorescence
intensity enhancement had been observed even at elevated temp-
eratures (Fig. S7†), which indicated that Fe3+ cannot oxidize
benzothiazoline to benzothiazole in the used solvent system. Fur-
thermore, the common oxidants such as H2O2, TBHP, NaClO
Fig. 2 (a) Fluorescence response of L (1 μM) upon addition of differ-
ent metal ions (10 equiv.) in HEPES (10 mM, pH = 7.4) buffer contain-
ing 50% (v/v) water–CH3CN. (b) Fluorescence response of L (1 μM)
upon addition of different concentration of Cu2+ (0–10 equiv.). The inset
plots the fluorescence intensity at 509 nm versus Cu2+ concentration. (c)
Photograph of L (1 μM) with different metal ions (10 equiv.).
(10 equiv.) could lead to a weak fluorescence intensity enhance-
ment (Fig. S8†). We proposed that the oxidation of benzothiazo-
line to benzothiazone by Cu2+ was the synergistic effect of
coordination and oxidization, and some substances with stronger
oxidizability such as Fe3+, H2O2, TBHP, NaClO showed little
oxidation. Moreover, as the ratio of the solvent (VCH3CN/VHEPES
)
changed from 9 : 1 to 1 : 9, the fluorescence intensity of the sol-
ution in the presence of 10 equiv. Cu2+ was decreased and the
maximum emission wavelength was also red shifted from
505 nm to 511 nm (Fig. S9†), the reason for which might be that
the water weakened the coordination between Cu2+ and ligand
L. The possible oxidative mechanism was shown in Scheme 3.
The oxidation of L using Cu2+ proceeds via initial one-electron
oxidation to give an aminium cation radical (1), and then the
radical (2) may also be formed by the proton loss from 1 fol-
lowed by a further one electron oxidation to give an iminium
cation (3).35,36 Finally, the one proton loss gives the oxidation
product coumarin 6. Moreover, the formed oxidation product
1
was separated and confirmed by the characterization such as H
NMR, 13C NMR and MS.
To investigate the selectivity, representative ions such as Cu2+,
Fe3+, Pb2+, Ni2+, Co2+, Mn2+, Zn2+, Hg2+, Cd2+, Ag+, Mg2+,
Ca2+, Ba2+, Na+ and K+ were added into the solution of chemo-
dosimeter L in HEPES (10 mM, pH = 7.4) buffer containing
50% (v/v) water–CH3CN. Changes of the fluorescence spectra of
L upon addition of different metal ions were shown in Fig. 2a.
Chemodosimeter L only showed very weak fluorescence due to
3624 | Dalton Trans., 2012, 41, 3623–3626
This journal is © The Royal Society of Chemistry 2012

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In summary, we described a fluorescence turn-on chemodosi-
meter for Cu2+ based on the ion promoted oxidation of ben-
zothiazoline to benzothiazole. Chemodosimeter L could detect
Cu2+ with high selectivity and sensitivity in aqueous solution.
Meanwhile, other common metal ions had little interference on
the detection of Cu2+. This method opens up a new way for the
detection of metal ions. Further efforts in our laboratory will
focus on the detection of other possible substances with this
easy oxidation process from benzothiazoline to benzothiazole.
Acknowledgements
Fig. 3 Fluorescence enhancement factors (FEF) of L (1 μM) in
HEPES (10 mM, pH = 7.4) buffer containing 50% (v/v) water–CH3CN.
Blue bars represent addition of different metal ions (10 equiv.) to the sol-
ution. Red bars represent addition of Cu2+ (10 equiv.) and different
metal ions (10 equiv.) to the solution. Excitation and emission were at
457 nm and 509 nm, respectively.
This work was supported by National Science Foundation of
China (No. 20901063), Wuhan Chenguang Scheme (Grant
201050231049) established under Wuhan Science and Technol-
ogy Bureau, the Open Research Fund of Key Laboratory for
Green Chemical Process of Ministry of Education, Wuhan Insti-
tute of Technology under grant (No. GCP201003).
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