A highly selective and sensitive luminescent chemosensor for Zn 2+ ions based on cyclometalated platinum(ii) complexes

Article abstract of DOI:10.1039/c3dt32603a

A novel luminescent Zn²⁺ ions chemosensor, a cyclometalated platinum(ii) bipyridyl acetylide complex, was designed. Of particular significance is that it shows a high sensitivity towards Zn²⁺ ions without interference from other biologically important cations in acetonitrile. The tautomerization of amide favors detecting Zn²⁺ ions among other HTM (heavy and transition metal) ions in aqueous systems.

Full text of DOI:10.1039/c3dt32603a

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A highly selective and sensitive luminescent
chemosensor for Zn2+ ions based on cyclometalated
Cite this: Dalton Trans., 2013, 42, 4244
Received 31st October 2012,
Accepted 24th January 2013
platinum(II) complexes†
Li-Kun Zhang,a Ling-Bao Xing,b Bin Chen,b Qing-Zheng Yang,b Qing-Xiao Tong,*a
Li-Zhu Wu*b and Chen-Ho Tungb
DOI: 10.1039/c3dt32603a
www.rsc.org/dalton
A novel luminescent Zn2+ ions chemosensor, a cyclometalated anions,25,26 metal ions,27–34 and some small molecules.35
platinum(II) bipyridyl acetylide complex, was designed. Of particu- Cyclometalated platinum(II) complexes have been developed
lar significance is that it shows a high sensitivity towards Zn2+ targeting metal ions including Mg2+ 29
,
K+,30 Pb2+,31,32
ions without interference from other biologically important Hg2+,33,34 however, the detection of Zn2+ by luminescence
cations in acetonitrile. The tautomerization of amide favors signaling is less explored.36
detecting Zn2+ ions among other HTM (heavy and transition
metal) ions in aqueous systems.
In zinc-specific sensors, 2,2-dipicolylamine (DPA) was used
as one of the most popular receptors.9–11 Guerchais et al.37
recently reported a cyclometalated platinum(II) complex in-
corporating DPA receptor on the phenylacetylide ligand. Unfor-
tunately, this complex is not an ideal Zn2+ sensor because of
its low sensitivity and selectivity toward Zn2+ ions. Herein,
we report a luminescent Zn2+ sensor 1, a cyclometalated
platinum(II) bipyridyl acetylide complex containing an amide-
DPA moiety in the acetylide ligand. Complex 1 displayed high
affinity towards Zn2+ ions with Kd = 16.3 nM in acetonitrile.
Furthermore, it could distinguish Zn2+ from other heavy and
transition metal ions with a ca. 50 nm red-shift of lumine-
scence wavelength in aqueous systems (Fig. 1).
Zinc ions, the second most abundant transition metal ions,
play a critical role in biological processes such as gene
expression, apoptosis, and neurotransmission.1–4 It is also
believed that the disorder of zinc metabolism is closely associ-
ated with many severe neurological diseases, including Alzhei-
mer’s disease (AD), epilepsy and ischemic stroke.5–8 To gain an
insight into the vital role of zinc ions in biological processes,
a variety of fluorescent zinc sensors have been documented
and many of them have been successfully applied in vivo.9–12
Despite their fascinating features, fluorescent zinc sensors
have some intrinsic drawbacks, especially their short emission
lifetimes, resulting in signal contamination by autofluores-
cence and scattered light, and hence greatly reducing the
signal credibility.
The synthesis and characterization of complex
1 is
described in the ESI.† The electronic absorption spectrum of
complex 1 in acetonitrile exhibited intense vibronic-structured
absorption bands at wavelengths below 380 nm (ε ∼ 104 dm3
mol−1 cm−1), and a weak intensity absorption at 430 nm (ε =
8778 dm3 mol−1 cm−1) (Table S1, see ESI†). With reference
to previous spectroscopic work on cyclometalated platinum(II)
6-phenyl-2,2′-bipyridyl complexes,29,38 the high-energy intense
absorption bands were ascribed to intraligand (IL) π–π* tran-
sitions of the phenylbipyridine and alkynyl ligands. However,
In recent years, utilization of luminescent transition metal
complexes as chemosensors has received considerable atten-
tion due to their intriguing photophysical properties including
evident Stokes shifts, excitation in the visible range and rela-
tively long lifetimes.13–22 As one of the representative families
of luminescent metal complexes, square-planar platinum(II)
complexes have been designed to be highly sensitive chemo-
sensors23,24 for the detection of various analytes such as
aDepartment of Chemistry, Shantou University, Guangdong 515063, P. R. China.
E-mail: qxtong@stu.edu.cn; Fax: +86 754829 02 508; Tel: +86 754829 02 508
bKey Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technical Institute of Physics and Chemistry, The Chinese Academy of Sciences,
Beijing 100190, P. R. China. E-mail: lzwu@mail.ipc.ac.cn; Fax: +86 108254 3580;
Tel: +86 108254 3580
†Electronic supplementary information (ESI) available: Detailed experimental
procedures, synthesis and characterization data for complex 1, additional
spectroscopic data. See DOI: 10.1039/c3dt32603a
Fig. 1 Chemical structure of complex 1.
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Fig. 3 Luminescence spectra of complex 1 (1.58 × 10−5 M) in CH3CN in the
presence of Zn2+ ions (0 to 3 equiv.). Inset: curve of luminescence intensity at
584 nm of complex 1 versus [Zn2+]/[1]. Excitation at λ = 460 nm (isosbestic
point).
Fig. 2 The absorption spectral changes of complex 1 (1.58 × 10−5 M) in
CH3CN upon addition of Zn2+ (0 to 3 equiv.). Inset: solution color changes of
complex 1 upon addition of Zn2+ ions (3 equiv.).
the broad low-energy bands in the visible region were assigned
as [dπ(Pt)→π*(C^N^N)] MLCT transitions, probably mixed with
some [π(CuCR)→π*(C^N^N)] LLCT character. In contrast to
the previously reported platinum(II) complexes with an amine-
substituted acetylide ligand,29,37 there were no two distinctly
separated peaks in the low-energy bands, which might be
attributed to the weaker electron-donating ability of the amide
nitrogen and thus decreasing the contribution of the LLCT
state.
The metal ions binding properties of complex 1 were inves-
tigated by using a series of cation perchlorate salts in aceto-
nitrile solutions. The heavy and transition metal ions (HTM
such as Zn2+, Cd2+, Cu2+) produced detectable UV-vis spectral
changes (Fig. S1–S3, see ESI†). Fig. 2 shows the UV-vis spectro-
scopic changes of complex 1 with titration of Zn2+ ions. The
addition of Zn2+ ions to the solution of complex 1 resulted in a
blue shift in the low-energy bands and a concomitant enhance-
ment in the absorption maximal intensity. A well-defined iso-
sbestic point at 460 nm indicated that only one new absorbing
species was generated during the titration. And the solution
color changed from orange to bright yellow (inset of Fig. 2). In
order to determine the stoichiometric ratio of the formed
species during titration of complex 1 with Zn2+ ions, Job’s
method was employed to estimate the absorbance versus
XM ([Zn2+]/[Zn2+] + [1]). As shown in Fig. S4 (see ESI†), the
absorbance goes through a maximum at a molar fraction of
ca. 0.5, suggesting a 1 : 1 binding mode between Zn2+ ions and
complex 1.
Fig. 4 Luminescence responses (I/I0 with error bars) of complex 1 (1.58 × 10−5
M) in the presence of various metal ions in CH3CN. Bars represent the final lumi-
nescence intensity at 584 nm (I) over the original luminescence at 584 nm (I0).
Black bars represent the addition of perchlorate salts of competing metal ions
(1 equiv. of Ag+, Ni2+, Pb2+, Cu2+, Fe2+, Hg2+, Fe3+, Mn2+ and Cd2+, and 10 equiv.
of Ca2+, Li+, Na+ and K+) to the solution of complex 1. Red bars represent the
addition of 1 equiv. of Zn2+ ions to the solution of complex 1.
mode. The apparent dissociation constant (Kd) was deter-
mined from the change of luminescence intensity at 584 nm
as a function of the Zn2+/[1] mole ratio. From the titration
curve, a Kd of 16.3 0.5 nM was derived using a nonlinear
least-squares fit (Fig. S5, see ESI†).
For an ideal chemosensor, high selectivity is a matter of
necessity. To verify specific recognition to Zn2+ ions, the lumi-
nescence spectra of complex 1 responsive to other heavy, tran-
sition, and main group metal ions were recorded. The
competitive luminescence responses of complex 1 to Zn2+ ions
and other metal cations are shown in Fig. 4. Addition of Zn2+
ions caused pronounced luminescence changes, whereas other
metal ions Ag+, Ni2+, Pb2+, Cu2+, Fe2+, Mn2+, Ca2+, Li+, Na+ and
K+ only changed slightly. Cd2+ and Hg2+, the same (IIB) group
ions as the Zn2+ ions, induced luminescence enhancements
but to a much lesser extent than that of Zn2+ ions. As indicated
by the red bars in Fig. 4, a significant level of luminescence
enhancement was observed for Zn2+ ions as compared with
The luminescence quantum yield of complex 1 in aceto-
nitrile is 4.5 × 10−3 39
The weak luminescence is probably due
.
to either the photoinduced electron transfer (PET) from
the electron-rich amino group to the platinum(II) unit, or the
3
photoinduced energy transfer from the MLCT excited state to
3
the low-lying LLCT state. Both of the processes contribute to
3
the quenching of the emissive MLCT excited state. A titration
of complex 1 with Zn(ClO4)2 (0–3 equiv.) in acetonitrile led to a
significant enhancement in luminescence intensity with a
turn-on ratio of 26 (Fig. 3), which agreed with the 1 : 1 binding
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Fig. 6 1H NMR spectra of complex 1 in the absence and presence of Zn2+ in
DMSO-d6.
Fig. 5 Luminescence spectra of complex 1 in the presence of different HTM
ions in CH3CN–H2O (7 : 3 v/v). [1] = 1.11 × 10−5 M, excitation at λ = 460 nm.
other metal ions. Moreover, a competition experiment was also
carried out to further explore the utility of complex 1 as an
ion-selective probe for Zn2+ ions. Except for Cu2+ ions, the co-
existent metal ions had a negligible influence on the lumines-
cence intensity upon addition of Zn2+ ions that may bind
stronger to complex 1.40
The Zn2+-sensing ability was further explored in aqueous
systems. The luminescence spectrum of complex 1 displayed a
broad peak, similar to that taken in acetonitrile. Addition of
Zn(ClO4)2 resulted in an evident luminescence turn-on, but
with a smaller ratio. As can be seen in Fig. 5, all the other
screened metal ions were blue-shifted by ca. 50 nm compared
with the Zn2+-induced luminescence change. This observation
may be interpreted by the alternative amide tautomeric forms
when complexing with various metal ions as described by Xu
et al.11,40,41 The distinct responses could be used to dis-
tinguish Zn2+ ions from other HTM ions.
Fig. 7 Possible binding modes of complex 1 with different HTM ions.
absorption further confirmed the amidic acid binding pattern.
The possible binding modes are given in Fig. 7.
Time-resolved luminescence spectroscopy was studied to
understand the tautomerization of amide in its excited state.
In the decay profiles of complex 1, two lifetime decay constants
could be obtained, one major component with lifetime τ =
53 ns contributed to 71.1%, the other one with lifetime τ = 285
ns occupied 28.9%. Upon addition of one equivalent of Zn2+
ions, however, only a slow component with a lifetime of 302 ns
was detected (single-exponential decay) (Fig. S6, see ESI†). As
we know, amide has an amide form and an imidic acid form
in solution. The two lifetimes in complex 1 agreed with the
two forms. The binding of Zn2+ ions promoted the transform-
ation of the two forms, finally reaching only one form existing
in solution. The time-resolved luminescence spectroscopy in
CH3CN–H2O supported the transformation. Upon addition of
Zn2+ ions, one lifetime of 100 ns was solely observed. The
addition of Cd2+ ions gave two lifetimes, 4.5 ns with 8.46%
and 53 ns with 91.54%. Additionally, Ni2+ ions facilitated the
transformations, with 53.36% molecules having a lifetime of
7.3 ns and 46.64% possessing a lifetime of 62 ns. The differ-
ences in excited lifetime make it a promising way to realize the
recognition of Zn2+ ions via time-resolved techniques.
To shed more light on its binding modes, 1H NMR titration
was performed. Given that the luminescence titration curve in
DMSO was similar to that in acetonitrile, although with a
1
much less dynamic range,42 the H NMR titration was carried
out in DMSO-d6 (Fig. 6). After addition of 1 equiv. of Zn2+ ions,
the reactive hydrogen H(5) was downfield shifted from 10.59 to
10.72 ppm. A similar shift of the reactive hydrogen was also
found in the titration of Cd2+ in DMSO-d6 (Fig. S6†). The result
was consistent with the reported molecules ZTRS40 and CTS,41
where the change was ascribed to the transformation of NH to
OH. On the basis of the reported literature40,41 and the blue-
shifted luminescence of complex 1 with other HTM ions, we
tentatively assumed that Zn2+ ions were bound to imidic acid
nitrogen in acetonitrile and aqueous systems, whereas the
other HTM ions were through the carbonyl oxygen that might
block the resonance structure resulting in the large blue shifts
in luminescence spectra. IR spectra also provide valuable
information on the imidic acid binding mode. As shown in
Fig. S7,† complex 1 displays an N–H stretch (3429 cm−1) and a
CvO amide band (1605 cm−1). But in its Zn2+-complex, the
typical O–H (3402 cm−1) and C–O (1086 cm−1) stretching
In conclusion, a cyclometalated platinum(II) bipyridyl acety-
lide complex featuring an amide-DPA receptor has been devel-
oped as a specific luminescent Zn2+ ions chemosensor. It
displays a high affinity towards Zn2+ ions with a Kd of 16.3 nM
and possesses a high selectivity for Zn2+ ions with a significant
enhanced luminescence in acetonitrile. In aqueous solution,
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the selectivity among other HTM ions could be achieved by 19 S. M. Ji, H. M. Guo, X. L. Yuan, X. H. Li, H. D. Ding, P. Gao,
large red-shifts upon complexation of Zn2+ ions. The difference
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a
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Acknowledgements
This work was financially supported by the National Natural 23 K. M. C. Wong and V. W. W. Yam, Coord. Chem. Rev., 2007,
Science Foundation of China (No. 91027041) and the Chinese
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