A visible light excitable "on-off" and "green-red" fluorescent chemodosimeter for Ni2+/Pb2+

Article abstract of DOI:10.1039/c2nj40597k

A pyrene derivative has been synthesized as a visible light excitable fluorescent chemodosimeter which exhibits "on-off" and "green-red" fluorescence behavior for Ni²⁺ and Pb ²⁺ with different reaction speed, respectively, and its sensing ability toward metal cations and counter ions was investigated in detail. The high selectivity of the 'in situ' prepared Ni²⁺ and Pb²⁺ complexes toward EDA and the reaction speed difference provide two new effective methods for distinguishing between Ni²⁺ and Pb²⁺. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/c2nj40597k

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LETTER
A visible light excitable ‘‘on–off’’ and ‘‘green–red’’ fluorescent
chemodosimeter for Ni2+/Pb2+w
Lina Wang,a Mingming Yu,*a Zhenting Liu,a Wanying Zhao,a Zhanxian Li,*a
Zhonghai Ni,b Changchun Lia and Liuhe Wei*a
Received (in Gainesville, FL, USA) 12th July 2012, Accepted 22nd August 2012
DOI: 10.1039/c2nj40597k
A pyrene derivative has been synthesized as a visible light excitable
fluorescent chemodosimeter which exhibits ‘‘on–off’’ and ‘‘green–red’’
fluorescence behavior for Ni2+ and Pb2+ with different reaction
speed, respectively, and its sensing ability toward metal cations and
counter ions was investigated in detail. The high selectivity of the
‘in situ’ prepared Ni2+ and Pb2+ complexes toward EDA and the
reaction speed difference provide two new effective methods for
Besides, increasing conjugate structure is an effective method to
obtain a longer excitation wavelength. Thus, in this manuscript,
conjugate structure-increased pyrene was used as the fluorescent
unit. On the other hand, a nitrogen atom is widely used as a
coordination unit in the reported fluorescent chemosensors.
Therefore, compound L was selected. In this paper, a new visible
light excitable fluorescent chemodosimeter L was synthesized,
which exhibits ‘‘on–off’’ and ‘‘green–red’’ fluorescence behavior
upon addition of Ni2+ and Pb2+ respectively. The ‘in situ’
prepared Ni2+ and Pb2+ complexes of L showed remarkable
selectivity toward EDA (ethylene diamine). Furthermore, the
different reaction speed of L with Ni2+ and Pb2+ provides a new
distinguishing between Ni2+ and Pb2+
.
In recent years, the design and synthesis of sensors based on
the metal ion-induced changes in fluorescence appear to be
particularly attractive due to their simplicity, high sensitivity,
and instantaneous response.1 In this regard, metal ion-selective
fluorescent chemosensors have served as useful tools for the
detection of metal ions and thus have been widely exploited to
detect biologically or environmentally relevant metal cations, such
approach to distinguish between Ni2+ and Pb2+
.
Compound L was synthesized by the reaction of 1,3,6,8-
tetrakis(4-aminophenyl)ethynylpyrene and pyridine-2-carbaldehyde
in methanol (Scheme 1).
as Hg2+ 2 Cu2+ 3 and Zn2+ ions,4 etc. Compared with other
, ,
Compound L exhibits relative strong fluorescence emission at
509 and 545 nm (Ff = 0.438)11 in the mixture solution (DMF/
H2O = 50, v/v) upon excitation at 460 nm. Upon titration of
2.5 equiv. Ni2+ (left part of Fig. 1), the fluorescence of L was
significantly quenched (Ff = 0.019, I0/I = 37), showing
transition metals, Ni2+ is a significant environmental pollutant,
yet an essential trace element in biological systems.5 Several
examples of Ni2+-fluorescent probes have been reported.6 And
the design and synthesis of Ni2+-fluorescent chemosensors is an
appealing topic. Whilst Pb2+ is well known for its high toxicity even
at low concentration, sensitive and selective detection of aqueous
Pb2+ is of particular interest.7 On the other hand, a visible light
excited sensor is appealing because reduced UV-induced photo-
toxicity/autofluorescence and sensor-induced interference are essen-
tially demanded when the sensor is used for in vivo fluorescent
imaging of living cells.8 To our knowledge, there are very rare
reported examples of visible light excited sensors for Ni2+ and
formation of the LÁNi2+ complex and indicating efficient Ni2+
-
selective ‘‘on–off’’ behavior. An electron in the unfilled d shell of
Ni2+ absorbs light energy and electronic transition occurs
between de and dr orbits, which results in the decreased emission
intensity of L. On the other hand, addition of Pb2+ (the reaction
time of Pb2+ and L is long enough) not only decreased the
emission of L (Ff = 0.085) but also increased the ratio of
emission intensity at 545 nm to 509 nm (I545/I509 changed from
0.334 to 1.114) (Fig. 2), indicating efficient Pb2+-triggered
‘‘green–red’’ fluorescence behavior. The right part of Fig. 1
demonstrates that the reciprocal of the concentration of Ni2+
is linear to the emission intensity at 509 nm. The detection limit
on fluorescence response of the sensor to Ni2+ is 4.0 Â 10À6 M.
Job’s plot indicated that L chelated with Ni2+ at 1 : 1 stoichio-
metry (Fig. S1, ESIw) and the binding constant of L with Ni2+
was calculated to be 8 Â 103. Due to great rigidity and spatial
effects of L, part of nitrogen atoms of the Schiff base and
pyridine did not coordinate with Ni2+ (Fig. S2, ESIw).
Pb2+ 9
chemodosimeter for Ni2+ and Pb2+
. Our aim herein is to synthesize a visible light excited
.
Pyrene has often been used as an effective fluorescent probe
because its emission property is sensitive to its local environment.10
a The College of Chemistry and Molecular Engineering, Zhengzhou
University, Zhengzhou, China. E-mail: yumm@zzu.edu.cn,
lizx@zzu.edu.cn, weiliuhe@zzu.edu.cn; Fax: +86-371-67781205;
Tel: +86-371-67781205
b School of Chemical Engineering and Technology, China University of
Mining and Technology, Xuzhou 221006, China
The time-dependent fluorescence spectra of chemodosimeter
L in the presence of different amounts of Ni2+/Pb2+ were
obtained by measuring the fluorescence at 509 and 545 nm
w Electronic supplementary information (ESI) available: Experimental
procedures, spectroscopic characterization, UV-vis and fluorescence
spectra. See DOI: 10.1039/c2nj40597k
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Scheme 1 Synthetic route to compound L.
Fig. 3 Left: kinetics of fluorescence decrease rate at 509 nm by the
Fig. 1 Left: changes in fluorescence emission spectra of L (5.0 Â 10À6 M,
DMF/H2O = 50, v/v) upon addition of Ni2+ upon excitation at 460 nm.
Right: emission intensity of L at 509 nm (black dot) as a function of
reaction of L (5.0 Â 10À6 M, DMF/H2O = 50, v/v) and Ni2+ with
excitation at 460 nm. Right: kinetics of fluorescence change of I545/I509
by the reaction of L and Pb2+ with excitation at 460 nm.
addition of Ni2+
.
to 509 nm change from 0.3 to 0.9 (right part of Fig. 3). The
fluorescence change of the reaction of L and Pb2+ can be
monitored by a camera (Fig. 4). The different reaction speed of
L with Ni2+ and Pb2+ provides a new approach to distinguish
between Ni2+ and Pb2+
.
Experiments on the counterion effect on the selectivity of
Ni2+/Pb2+ (left part of Fig. 5) were also performed. Addition
of many kinds of nickel salts such as NiCl2, NiSO4, Ni(OAc)2,
and Ni(ClO4)2 can make the fluorescence of L decrease
Fig. 2 Left: the fluorescence emission spectra of L (5.0 Â 10À6 M,
DMF/H2O = 50, v/v) upon addition of Pb2+ with excitation at 460 nm.
Right: the emission intensity ratio at 545 nm to that at 509 nm of L
without and with Pb2+ (the ratio of Pb2+ to L is 18 equiv. respectively).
(Fig. 3). According to the fluorescence spectra obtained at the
time of Ni2+ addition, the coordination reaction of L and
Ni2+ is accomplished instantaneously and thus detection of
Ni2+ with L could be achieved within 5 seconds when the ratio
of Ni2+ to L is more or equal to 2.5 equiv. (left part of Fig. 3).
However, the reaction of L and Pb2+ is so slow that it needs at
least 160 min to make the ratio of emission intensity at 545 nm
Fig. 4 Kinetics of fluorescence pictures by the reaction of L (5.0 Â
10À6 M, DMF/H2O = 50, v/v) and Pb2+ (18 equiv.) with excitation at
365 nm. Time change (from left to right): L only, 40 min, 60 min,
70 min, 80 min, 90 min, 100 min, 120 min, 130 min, 160 min, 180 min
and 190 min.
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Fig. 5 Left: fluorescence emission spectra of L (5.0 Â 10À6 M, DMF/
H2O = 50, v/v) upon addition of different nickel salts (1.25 Â 10À5 M)
with excitation at 460 nm. Right: changes in fluorescence emission
spectra of L (5.0 Â 10À6 M, DMF/H2O = 50, v/v) and Ni2+ (1.25 Â
10À5 M) upon addition of EDA into ethanol. The excitation wave-
length was 460 nm.
Fig. 7 Process of L sensing of Ni2+/Pb2+
.
specificity of the chemodosimeter L toward Pb2+ is excellent
(right part of Fig. 1 and the light gray bars of Fig. S4, ESIw),
that is, sensor L can also detect Pb2+ if the amount of Pb2+ is
large enough and the reaction time is very long.
completely, indicating that anions do not coordinate with
Ni2+ when nickel salts react with L and counterion anions such
as acetate, sulfate, nitrate and perchlorate have no influence on the
detection of Ni2+. As for Pb2+, however, neither Pb(NO3)2 nor
Pb(OAc)2 made the fluorescence of L decrease, demonstrating that
the chloride ion has stronger binding ability compared with nitrate
and acetate ions (Fig. S3, ESIw).
Competition experiments were also conducted for L (dark
bars of Fig. 6). When 2.5 equiv. Ni2+ was added into the
solution of L in the presence of 10 equiv. of other metal ions
and the reaction time was within 5 min, a similar fluorescent
spectra change to that with Ni2+ ions only was observed,
indicating the stronger binding ability of Ni2+ than that of
other ions. As for the competition experiments of Pb2+ for L,
if the reaction time is long enough, a similar result to that of L
and Ni2+ was obtained (dark bars of Fig. S4, ESIw).
The specificity of the chemodosimeter L toward Ni2+ was
determined next, as shown in the light gray bars of Fig. 6.
Upon addition of 2.5 equiv. of the various metal ions,
respectively, only Ni2+ decreased the emission of L. However,
under identical conditions, nearly no fluorescence intensity
To examine the binding reversibility of chemodosimeter L to
Ni2+ and Pb2+, an ethanol solution of 10 equiv. EDA was added
to the LÁNi2+ and LÁPb2+ solutions respectively. Fluorescence
signals same as those for L are restored (right part of Fig. 5),
demonstrating that Ni2+ is being completely removed from the
LÁNi2+ complex by EDA. That is, the binding of L and Ni2+ is
really chemically reversible. However, no enhanced emission
intensity was observed when an ethanol solution of EDA was
added to the LÁPb2+ solution (Fig. S5, ESIw). This indicates that
EDA is not able to remove the Pb2+ ion from the LÁPb2+ complex
and hence prevents the emission of L. In other words, the ‘in situ’
prepared Ni2+ and Pb2+ complexes showed high selectivity toward
EDA, which can also be applied to distinguish between Ni2+ and
Pb2+. The process of L sensing of Ni2+/Pb2+ is shown in Fig. 7.
In summary, a visible light excitable fluorescent chemo-
dosimeter pyrene derivative has been synthesized, which exhibits
‘‘on–off’’ and ‘‘green–red’’ fluorescence behavior for Ni2+ and
Pb2+ respectively. The high selectivity of the ‘in situ’ prepared
Ni2+ and Pb2+ complexes toward EDA and the reaction speed
difference provided two new effective methods for distinguishing
changes were observed in emission spectra with Na+, Mg2+
K+, Ca2+, Cr3+, Mn2+, Fe3+, Cu2+, Zn2+, Cd2+, and Hg2+
,
,
except that Co2+ slightly decreased the emission intensity of L.
That is, chemodosimeter L can readily distinguish Ni2+ from
environmentally and biologically relevant metal ions by fluores-
cent spectra. If the amount of Pb2+ is equal to or more than
12 equiv. and the reaction time is long enough, Pb2+ can also
quench the emission of L to a large extent with an increase in the
ratio of emission intensity at 545 nm to 509 nm and the
between Ni2+ and Pb2+
.
Experimental
Materials and methods
All chemicals were purchased from commercial suppliers and
used without further purification. All reactions were per-
formed under an argon atmosphere with the solvents purified
with standard methods.
1H and 13C NMR spectra were recorded on a Bruker 400
spectrometer. Chemical shifts are reported in ppm using
tetramethylsilane (TMS) as the internal standard. Mass spectra
were obtained on a high resolution mass spectrometer (IonSpec4.7
Tesla FTMS-MALDI/DHB).
Fig. 6 Light gray bars represent fluorescence responses of L (5.0 Â
10À6 M, DMF/H2O = 50, v/v) upon addition of different metal salts
(2.5 equiv.); dark bars show the fluorescence change that occurs upon
addition of Ni2+ to the solution containing L and other cations (the
ratio of a background cation to Ni2+ is 4, lex = 460 nm). I and I0
represent the emission intensity at 509 nm. The metal salts represent
different chloride salts. The reaction time of L and metal ions is
controlled within 5 min.
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All spectral characterizations were carried out in HPLC-grade
solvents at 20 1C within a 10 mm quartz cell. UV-vis absorption
spectra were measured with a TU-1901 double-beam UV-vis
Spectrophotometer, and fluorescence spectra were determined on
a Hitachi F-4500 spectrometer. The fluorescence quantum yield
was measured at 20 1C with rhodamine B in water (F = 0.310)
selected as the reference.
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Acknowledgements
We thank the NSFC (50903075, 50873093 and 21176246) for
financial support.
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