Highly sensitive and selective fluorescent sensor for Zn 2+/Cu2+ and new approach for sensing Cu2+ by central metal displacement

Article abstract of DOI:10.1039/c1cc10696a

An "off-on" Zn²⁺ and "on-off" Cu ²⁺ fluorescent chemosensor C was designed and synthesized. The binding of C and Zn²⁺/Cu²⁺ is chemically reversible by the addition of EDTA disodium solution; moreover, the fluore

Full text of DOI:10.1039/c1cc10696a

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Highly sensitive and selective fluorescent sensor for Zn2+/Cu2+
and new approach for sensing Cu2+ by central metal displacementw
Zhanxian Li, Lifeng Zhang, Lina Wang, Yongkai Guo, Lianheng Cai, Mingming Yu* and
Liuhe Wei*
Received 5th February 2011, Accepted 16th March 2011
DOI: 10.1039/c1cc10696a
An ‘‘off-on’’ Zn2+ and ‘‘on-off’’ Cu2+ fluorescent chemosensor
C was designed and synthesized. The binding of C and Zn2+/Cu2+
is chemically reversible by the addition of EDTA disodium
solution; moreover, the fluorescence emission signal of ZnC
decreased with the addition of Cu2+, demonstrating that ZnC
could detect Cu2+ via metal displacement.
The development of fluorescent chemosensors with high
selectivity, sensitivity and low detection limit has been receiving
considerable attention because of their fundamental role in
medical, environmental and biological applications.1 Zinc is
the second most abundant transition metal ion in the human
Fig. 1 Process of C in sensing of Zn2+/Cu2+
.
body after iron, and is an essential cofactor in many biological
processes such as brain function and pathology, gene transcrip-
tion, immune function, and so on.2 Thus, a variety of fluores-
cent sensors for Zn2+ has been developed with some successful
applications to image Zn2+ in vitro and/or in vivo.3 On the
other hand, copper is a significant metal pollutant due to its
widespread use, but is also an essential trace element in bio-
logical systems.4 In this connection, considerable efforts have
been made to synthesize Cu2+-selective fluorescent chemosensors,
some of which have been used successfully in biological
applications.5 There have been great achievements in the
development of fluorescent sensors singly for zinc or copper,
but reports about fluorescent chemosensors for both zinc and
copper are rare.6 Therefore, it is still a challenge to develop
sensors that can selectively recognize multiple analytes.
signals of C can be restored by the addition of ethylenediamine-
tetraacetic acid (EDTA) disodium solution into ZnC and CuC
(Fig. 1) solutions respectively. Moreover, the fluorescence
emission intensity of ZnC is decreased with the addition of
Cu2+, demonstrating that ZnC could be a good ‘‘on-off’’
Cu2+ sensor candidate.
Using Diels–Alder reaction, compound C was easily synthesized
according to Scheme S1w and the reported method.7 The
synthetic procedure and detailed characterization of C are
shown in the Supporting Information.
The absorption spectrum of compound C (2.0 Â 10À5 M) in
ethanol reveals three bands with lmax at 205, 244, and 263 nm
(molar extinction coefficient e = 4.46 Â 104 MÀ1 cmÀ1, 3.46 Â
104 MÀ1 cmÀ1, and 2.82 Â 104 MÀ1 cmÀ1 respectively). The
addition of 4.0 equiv. of ZnCl2 (Zn2+, 8.0 Â 10À5 M) led to a
slight increase of the absorbance with no absorption wave-
length change (see ESIw, Fig. S5). The addition of 1.0 equiv. of
CuCl2 (Cu2+, 2.0 Â 10À5 M) (see ESIw, Fig. S6) was similar to
that of Zn2+ with no change of the lmax, but the absorbance
increased greatly (e205 = 8.46 Â 104 MÀ1 cmÀ1, e244 = 9.88 Â
104 MÀ1 cmÀ1, and e263 = 12.33 Â 104 MÀ1 cmÀ1). Such an
absorbance change indicates that reformed new excited electronic
states in ZnC and CuC are similar to those of compound C
except for the higher e.
Herein, we designed and synthesized a fluorescent chemo-
sensor C (see ESI,w Scheme S1) for Zn2+ and Cu2+, which
possesses multiple benzene and pyridine rings. The rotation of
C–C bonds between each two aromatic rings will be frozen if
the nitrogen atom of the pyridine ring coordinates metal ions,
bringing about an increase in the fluorescence emission intensity
of C. As expected, compound C is a fluorescent molecule
which displays a high selectivity for Zn2+ and Cu2+ ions
among relevant metal ions. Furthermore, the fluorescence
1H NMR analysis provides further evidence to support the
M–N bond formation (M = Zn2+/Cu2+, N = nitrogen atom
of the two adjacent pyridine rings). 1H NMR spectra of
compound C in DMSO-d6, C and Zn2+ in DMSO-d6, and C
with Cu2+ in DMSO-d6 were measured as shown in Fig. 2.
Department of Chemistry, Zhengzhou University, Zhengzhou, China.
E-mail: yumm@zzu.edu.cn, weiliuhe@zzu.edu.cn;
Fax: (+)86-371-67781205; Tel: (+)86-371-67781205
w Electronic supplementary information (ESI) available: Experimental
procedures, characterization data, UV-vis absorption and fluorescence
spectra. See DOI: 10.1039/c1cc10696a
c
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Experiments on the counterion effect on the selectivity of
Zn2+ (see ESIw, Fig. S10) and Cu2+ (see ESIw, Fig. S11) were
also performed. As for Zn2+, addition of sulfate, nitrate and
acetate anions enhanced the fluorescence of compound C very
little. While for Cu2+, those anions as above have little
influence on the selective effect of compound C, indicating
that anions didn’t coordinate Cu2+ when Cu2+ reacted with C.
The specificity of the chemosensor C toward Zn2+ and
Cu2+ was determined next, as shown in Fig. 4; upon addition
of the same amount of the various metal ions, respectively,
only Zn2+ enhanced the emission of C and Cu2+ decreased
that of C. However, under identical conditions, nearly no
fluorescence intensity changes were observed in emission spectra
with Na+, Mg2+, K+, Ca2+, Cr3+, Mn2+, Fe3+, Co2+, and
Ni2+, except that Cd2+ slightly increased and Hg2+ decreased the
emission intensity of C. That is, sensor C can readily distinguish
Zn2+ and Cu2+ from environmentally and biologically relevant
metal ions by fluorescent spectra (see ESIw, Fig. S12).
Fig. 2 1H NMR spectra of chemosensor C upon addition of 1.0 equiv.
of Zn2+ and Cu2+ in DMSO-d6.
The difference between C and ZnC is the shift to downfield of
signal intensity at around 8.17 (from 8.17 to 8.25) and the
decrease of signal intensity at 8.17, which was assigned to the
formation of a Zn2+–N bond. There is no obvious difference
between C and CuC except the slight changes of shift peaks,
Competition experiments were also conducted for
C
(Fig. 4A and B). When 4 equiv. of Zn2+ was added into the
solution of C in the presence of 16 equiv. of other metal ions, a
similar fluorescent spectra change was displayed to that with
Zn2+ ion only, except those of Cu2+ and Hg2+. Hg2+ induced
very slight fluorescence decrease, whereas Cu2+ greatly
quenched the fluorescence even if the amount of Zn2+ was
larger than that of Cu2+, showing the stronger binding ability
of Cu2+ than that of Zn2+ to C. As for the competition experi-
resulting from the paramagnetic property of Cu2+
.
Compound C exhibits relatively weak fluorescence emission
at about 375 nm in ethanol upon excitation at 246 nm (Fig. 3).
Upon titration of Zn2+, the emission intensity was dramatically
enhanced (I/I0 = 6.4) without change in emission wavelength
(Fig. 3), indicating a Zn2+-selective ‘‘off-on’’ fluorescent signaling
behavior. However, when Cu2+ was added, the fluorescence of
C was significantly quenched (I/I0 = 0.23) with no fluorescence
emission wavelength change (see ESIw, Fig. S7), indicating an
efficient Cu2+-selective ‘‘on-off’’ behavior. The increased emission
intensity is probably due to the complex of Zn2+ and C (ZnC),
in which the rotation of C–C bonds between each two aromatic
rings is inhibited. As for Cu2+, because of its paramagnetic
property and unfilled d shell, Cu2+ could strongly quench the
fluorescence of the fluorophore C through electron and/or energy
transfer processes.8 A Job’s plot indicated that C chelated each of
Zn2+ and Cu2+ with 1 : 1 stoichiometry (see ESIw, Fig. S8 and
S9). The detection limits on fluorescence responses of the sensor
ments on Cu2+, under the same conditions as those of Zn2+
,
to Zn2+ and Cu2+ are 1.7 Â 10À5 M and 8.0 Â 10À6
M
respectively. The process of C in sensing of Zn2+/Cu2+ is
depicted in Fig. 1.
Fig. 4 The light bars represent the fluorescence emission of a solution
of 4 equiv. of Zn2+ (Fig. 4A)/Cu2+ (Fig. 4B) and C. The dark bars
show the fluorescence change that occurs upon addition of Zn2+
(Fig. 4A)/Cu2+ (Fig. 4B) to the solution containing C and other cation
(the ratio of other cation to Zn2+/Cu2+ is 4 : 1). 1, C; 2, Zn2+/Cu2+; 3,
Na+; 4, Mg2+; 5, K+; 6, Ca2+; 7, Cr3+; 8, Cd2+; 9, Mn2+; 10, Fe3+
;
Fig. 3 Changes in fluorescence emission spectra of C (2.0 Â 10À5 M)
in ethanol upon addition of Zn2+. The final ratio of Zn2+ to C is
4 equiv.
11, Co2+; 12, Ni2+; 13, Cu2+/Zn2+; 14, Hg2+. Bars represent the
ratio of I À I0 to I0. I and I0 represent the emission intensity at 375 nm.
The overall emission spectra were measured at excitation of 246 nm.
c
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of ZnC being decreased, demonstrating that ZnC could be a
good ‘‘on-off’’ Cu2+ sensor candidate.
We thank the NSFC (50903075 and 50873093) for financial
support.
Notes and references
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Fig. 5 Fluorescence spectra of C (2.0 Â 10À5 M) + 4 equiv. of
Zn2+ upon the titration of Cu2+ (0–1 equiv.) in ethanol solution with
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the fluorescence emission spectra displayed a similar pattern at
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Based on the fluorimetric experiments (Fig. 5), we realized
that C would have higher binding affinity for Cu2+ than for
Zn2+. As expected, the addition of Cu2+ into ZnC solution
resulted in quick fluorescence changes (o2 min). The emission
intensity at about 375 nm of ZnC underwent a gradual decrease,
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‘‘on-off’’ fluorescent chemosensor was designed and synthesized,
which displays a high selectivity for the Zn2+ and Cu2+
among relevant metal ions. The mechanism of fluorescence
change may be explained as follows: the addition of Zn2+
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while the paramagnetic property and partially filled d shell of
Cu2+ led to the possibility of electron and/or energy transfer.
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5800 Chem. Commun., 2011, 47, 5798–5800
This journal is The Royal Society of Chemistry 2011