Fluorescence 'turn-on' detection of Cu2+ ions with aggregation-induced emission-active tetraphenylethene based on click chemistry

Article abstract of DOI:10.1016/j.tetlet.2011.04.065

A fluorometric sensor for detection of Cu²⁺ ions in aqueous solution with aggregation-induced emission-active tetraphenylethene based on click chemistry is reported. Upon addition of Cu²⁺, a reaction of azide-modified tetraphenylethe

Full text of DOI:10.1016/j.tetlet.2011.04.065

Tetrahedron Letters 52 (2011) 3283–3286
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Fluorescence ‘turn-on’ detection of Cu2+ ions with aggregation-induced
emission-active tetraphenylethene based on click chemistry
⇑
⇑
Takanobu Sanji , Mitsutaka Nakamura, Masato Tanaka
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259-R1-13 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
a r t i c l e i n f o
a b s t r a c t
A fluorometric sensor for detection of Cu2+ ions in aqueous solution with aggregation-induced emission-
active tetraphenylethene based on click chemistry is reported. Upon addition of Cu2+, a reaction of
azide-modified tetraphenylethene and diethylene glycol dipropiolate in the presence of sodium ascorbate
proceeded to yield covalently cross-linked networks, resulting in a dramatic enhancement of fluores-
cence. This assay showed high selectivity for Cu2+ ions even in the presence of other metal ions in a
mixture.
Article history:
Received 17 March 2011
Revised 12 April 2011
Accepted 13 April 2011
Available online 23 April 2011
Keywords:
Ó 2011 Elsevier Ltd. All rights reserved.
Aggregation-induced emission
Click chemistry
Copper
Fluorescence
Sensor
Copper is an essential transition metal for living systems and
plays a critical role in a variety of fundamental biological processes
in organisms.1 At certain high concentrations, however, it is highly
toxic to organisms. An elevated level of Cu2+, for example, associ-
ates with severe neurodegenerative diseases, such as Wilson’s
and Alzheimer’s diseases.2 In addition, copper, usually found in
the oxidized state, can be an environmental pollutant. Thus, the
sensitive and selective detection of Cu2+ is crucial. A variety of
designing strategies based on fluorescence techniques have been
developed.3 Among them, the reported Cu2+ ion fluorescent sen-
sors usually work in a fluorescence quenching mode because of
the paramagnetic nature of Cu(II), the binding of the metal ion
causes a quenching of the fluorescence emission and leads to a
turn-off signal.4 In recent years, a number of sensors for detection
of Cu2+ with fluorescence enhancement (‘turn-on’) have been re-
ported,5 but it is still a challenging task.
Recently, aggregation-induced emission (AIE)-active materials
are demonstrated to have a potential utility in the sensing fields
by us6 and other groups.7 Tang and co-workers first reported
AIE-active molecules.8 AIE-active molecules display no emission
in solution, but an intense emission when aggregated or in the so-
lid state because of suppression of nonradiative deactivation asso-
ciated with restriction of intramolecular rotations.9,10 Based on the
state (aggregation)-dependent fluorescence, AIE-active molecules
⇑
Corresponding authors.
E-mail addresses: sanji@res.titech.ac.jp (T. Sanji), m.tanaka@res.titech.ac.jp (M.
Figure 1. (a) Chemical structures of azide-modified TPE 1 and diyne 2. (b) A
schematic drawing of fluorescence ‘turn-on’ detection of Cu2+ ions based on click
chemistry with AIE.
Tanaka).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.04.065

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T. Sanji et al. / Tetrahedron Letters 52 (2011) 3283–3286
can be used as an alternative sensing element, where they aggre-
gate with analytes through multivalent noncovalent bonding inter-
actions, such as electrostatic, hydrogen-bonding, and metal
coordination interactions, etc., resulting in an intense emission.11
Here, we show alternative Cu2+ detection with fluorescence
enhancement using AIE-active tetraphenylethene (TPE) based on
click chemistry.
In the present work, we make use of the copper(I)-catalyzed
azide–alkyne cycloaddition (CuAAC), a typical example of ‘click
chemistry’,12–14 as a key step in the detection of Cu2+ 15,16
A sche-
.
matic illustration of our detection method is depicted in Figure 1.
We have designed azide-modified TPE 1 and a terminal diyne,
diethylene glycol dipropiolate 2. In the presence of trace amounts
of Cu+ ions (generated in situ by reduction of Cu2+ by sodium ascor-
bate), the reaction of 1 and 2 forms intermolecular cross-linked
products (like aggregates), which are envisioned to display an
emission, because of restriction of intramolecular rotations.17
The azide-modified TPE 1 was synthesized by the reaction of 3
and sodium azide (Scheme 1). The terminal diyne 2 was also pre-
pared by the reaction of propiolic acid and diethylene glycol. A
mixture of 1 and 2 in THF/H2O (1:1, v/v) was practically low lumi-
nescent, with a quantum yield (Uf) of 2.0 (relative to quinine sul-
fate as a standard).
Scheme 1. Synthesis of azide-modified TPE 1.
In a typical sensing experiment, a Cu2+ (copper(II) sulfate penta-
hydrate) solution (THF/H2O = 1/1, v/v) was added to an aqueous
THF solution (THF/H2O = 1/1, v/v) of 1 (20
lM), 2 (40 lM) and
sodium ascorbate (as reductant, 40 M) and the mixture was
l
stirred at room temperature. The fluorescence emission increased
gradually and then reached a constant value within 48 h. The
resulting mixture displayed an intense blue emission (Figs. 2 and
S1 , Supplementary data). The emission intensity at 490 nm in-
creased almost linearly as a function of Cu2+ concentration in the
range of 0–20 lM, at which it showed an approximately threefold
increase relative to the original value (Uf = 5.80). As can be seen in
Figure 2c, the intense emission of the solution after addition of
Cu2+ was easily confirmed by the naked eye. The detection limit
of Cu2+ was estimated to be ca. 1
l
M, which is substantially lower
than the action level of Cu in drinking water (1.3 ppm ꢀ 20 M) set
by the U.S. Environmental Protection Agency (EPA).18 Further addi-
tion of Cu2+ (>20
M) caused a fluorescence decrease (Figs. 2b and
l
l
c). This is because insoluble materials with highly cross-linking
(vide infra) were formed, which were easily confirmed by the
naked eye, under these conditions. However, when the mixture
was stirred at 50 °C to reduce the detection time, an intense emis-
Figure 2. (a) Fluorescence spectra and (b) changes of fluorescence intensity at
490 nm of a mixture of 1, 2, and sodium ascorbate upon addition of Cu2+ in THF/H2O
(1:1, v/v) ([1] = 20 lM, [2] = 40 lM, [sodium ascorbate] = 40 lM, kex = 350 nm). The
spectra were measured after stirring the solution with Cu2+ for 24 h at room
temperature. (c) A photograph of the sample under irradiation with UV light with a
Figure 3. Fluorescence spectra of 1/2/Cu2+/sodium ascorbate, 1/2/sodium ascor-
bate, 1/2/Cu2+
,
1/Cu2+
v/v; [1] = 20
M; kex = 350 nm).
/
sodium ascorbate, and 2/Cu2+/sodium ascorbate (THF/
wavelength of 365 nm ([Cu2+] = 0, 5, 10, 15, 20, 30, 40, and 50
right).
lM; from left to
H2O = 1:1,
lM; [2] = 40 lM; [sodium ascorbate] = 40 lM;
[Cu2+] = 20
l

T. Sanji et al. / Tetrahedron Letters 52 (2011) 3283–3286
3285
sion appeared within 12 h (Fig. S2, Supplementary data) and also
no decrease in the fluorescence intensity was found at high Cu2+
concentrations (>20 lM).
In control experiments for Cu2+ detection run in the absence of
1, 2, or the reductant, no fluorescence enhancement was observed
(Fig. 3).
The dramatic enhancement in the fluorescence intensity upon
addition of Cu2+ ions is rationalized in terms of cross-linking of 1
and 2 after CuAAC, as illustrated in Figure 1b. Indeed, electro-
spray ionization mass spectrometry detected the partially
cross-linked products in the reaction mixture (Fig. S3, Supple-
mentary data).
High selectivity of this assay for Cu2+ ions was confirmed by the
following experiments. Thus, fluorescence of the mixtures ob-
tained after the same treatment using sample solutions of various
other metal ions (Li+, Na+, K+, Mg2+, Ca2+, Mn2+, Fe2+, Fe3+, Co2+, Ag+,
and Zn2+) and also a mineral water (a mixture of Na+, Ca2+, Mg2+
,
and K+ ions) was examined. As shown in Figures 4a and b (gray
bar), only when Cu2+ was added, the fluorescence emission was
enhanced significantly, while the other metal ions did not. The
inset in Figure 4a clearly shows the different optical image in the
presence of Cu2+
.
Importantly, the fluorescence ‘turn-on’ responses to Cu2+ were
apparent even in the presence of other metal ions, such as Li+,
Na+, K+, Mg2+, Ca2+, Mn2+, Fe2+, Fe3+, Co2+, Ag+, and Zn2+, in a mix-
ture (white bar in Figs. 4b and c). It is noted that a high concentra-
tion of sodium ascorbate (ꢀ100
l
M) was required to detect Cu2+
M) in the presence of Fe3+ (20
(20 l lM). Addition of mineral water
did not affect the response to Cu2+ either.
We have demonstrated fluorometric detection of Cu2+ ions with
high selectivity in aqueous solution based on the click reaction of
the AIE-active TPE with azide groups and the diyne to form cova-
lently cross-linked emissive networks. The detection limit of this
assay is substantially lower than the action level of copper in
drinking water (20 lM), which facilitates ‘naked-eye’ detection
without the help of advanced instruments, although the sensitivity
is not high among the reported Cu2+ ion sensors for biological
applications. However, further experiments are required to dem-
onstrate the practical use of this fluorescence assay. In particular,
the assay must be tested in a more complex environment, for
example, other anions for Cu2+ ions and biologically relevant fluids.
Alternatively, it is necessary to test whether this assay has the abil-
ity to detect Cu+ ions because Cu+ rather than Cu2+ is the dominant
oxidation state in the biological reducing environment. This design
principle presented here could be applied for detection of other
analytes including metal ions using reactive AIE-active molecules.
Further study is in progress.
Acknowledgments
This work was supported by the Management Expenses Grants
for National Universities Corporations ‘Nano-Macro Materials, De-
vices and System Research Alliance’ from the Ministry of Educa-
tion, Culture, Sports, Science and Technology of Japan (MEXT).
Figure 4. (a) Fluorescence spectra of a mixture of 1, 2, and sodium ascorbate with
metal ions (Li+, Na+, K+, Mg2+, Ca2+, Mn2+, Fe2+, Fe3+, Co2+, Ag+, Zn2+, and mineral
water) in THF/water = 1/1, v/v ([1] = 20
lM; [2] = 40 lM; [sodium ascor-
bate] = 40 M; kex = 350 nm). The image on the top shows samples
l
M; [Mn+] = 20
l
Supplementary data
under irradiation with UV light with a wavelength of 365 nm (blank, Li+, Na+, K+,
Mg2+, Ca2+, Mn2+, Fe2+, Fe3+, Co2+, Cu2+, Ag+, Zn2+, and mineral water; from left to
right). (b) Gray bars represent the fluorescence response at 490 nm to metal ions.
White bars represent the fluorescence responses to Cu2+ detection in the presence
of other metal ions. The experimental conditions were the same noted in (a) and (c),
respectively. (c) Fluorescence spectra of a mixture of 1, 2, sodium ascorbate, and
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.04.065.
Cu2+ in the presence of other metal ions, including Li+, Na+, K+, Mg2+, Ca2+, Mn2+
Fe2+, Fe3+, Co2+, Ag+, Zn2+, and mineral water (THF/water = 1/1, v/v; [1] = 20
M;
[2] = 40 M; [sodium ascorbate] = 40 M, 100
M for Cu2+ detection in the presence
of Fe3+; [Cu2+] = 20 M; [Mn+] = 20
M; kex = 350 nm). The spectra shown in (a) and
(c) were measured after stirring for 24 h at room temperature.
,
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