A bezoimidazole-based highly selective and low-background fluorescent sensor for Zn2+

Article abstract of DOI:10.1016/j.inoche.2011.10.018

In this article, a new benzoimidazole based Zn^{2 +} fluorescence sensor DABI has been prepared. The sensor displays a rapid and a linear response to Zn^{2 +} with a red-shifted 100-fold turn-on signal from the dark background. The presenc

Full text of DOI:10.1016/j.inoche.2011.10.018

Inorganic Chemistry Communications 15 (2012) 176–179
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
A bezoimidazole-based highly selective and low-background fluorescent
sensor for Zn2+
c
c,
Xiaoqing Wang a, Zhipeng Liu b,c, , Fang Qian , Weijiang He ⁎⁎
⁎
a
School of Material Science and Engineering, Liaocheng University, Liaocheng, 252000, PR China
School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, 252000, PR China
State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, 210093, PR China
b
c
a r t i c l e i n f o
a b s t r a c t
In this article, a new benzoimidazole based Zn2+ fluorescence sensor DABI has been prepared. The sensor
displays a rapid and a linear response to Zn2+ with a red-shifted 100-fold turn-on signal from the dark back-
ground. The presence of other metal ions especially Cd2+ does not interfere with its Zn2+ response. Molec-
ular modeling study suggests that the Zn2+-induced red shift of emission and fluorescence turn-on of DABI
could be correlated to the coplanation of two aryl planes of 2-aryl substituted benzoimidazole.
© 2011 Elsevier B.V. All rights reserved.
Article history:
Received 23 August 2011
Accepted 17 October 2011
Available online 25 October 2011
Keywords:
Benzoimidazole
Zn2+ fluorescence sensor
Dark background
Zinc is the second most abundant transition-metal ion in the
human body, it plays an essential role in biological processes such
as structural co-factors, catalytic centers, regulators of enzymes, reg-
ulators of gene expressions [1–3]. The spatiotemporal distribution
of biological zinc ions in organs or intracellular organelles is tightly
regulated by the homeostatic activity of zinc uptake/secretion and
buffering systems. The up-regulated levels of zinc are causative in
several neurodegenerative diseases, such as Alzheimer's, and Parkinson's
diseases [4–6]. Moreover, the widely use of zinc in industrial, make
of zinc contamination of natural waters are relatively frequent [7]. There-
fore, it is meaningful to detect Zn2+ both in environmental and bio-
logical systems. Because Zn2+ is spectroscopically silent due to its
d10 electron configuration, fluorescence techniques have become the
gold standard for Zn2+ sensing because of their high sensitivity and
spatiotemporal resolution [8–13].
Thus, the search for readily accessible fluorescence Zn2+ sensors
with high selectivity is still a challenging task.
2-aryl substituted benzoimidazole derivatives (2-ABIs) are com-
mon fluorophore widely exploited in the construction of metal ion
sensors [19]. The photo-induced energy transfer via the rotation of
the biaryl plane in the excited state make 2-ABIs has very weak fluo-
rescence. The metal ions coordination of 2-ABIs based metal ion sen-
sors can induce coplanation of two aryl planes of 2-ABIs and result
in the red shift and strong fluorescence enhancement of emission of
2-ABIs [19b]. The intrinsic characteristics such as dark-background
and metal ions induced emission enhancement make 2-ABIs are suit-
able fluorophore for the construction of turn-on florescent sensors
with low-background. In this work, we present the synthesis and
properties of DABI, new 2-ABIs based “turn-on” fluorescent sensor
for the selective detection of Zn2+. DABI features high Zn2+ selectivity
fluorescence response and a ca. 100-fold emission enhancement upon
Zn2+ binding.
DABI combines a benzoimidazole dye reporter (2-(4-hydroxyphenyl)-
benozimidazole (HPBI)) containing 2, 2′-bipicolylamine (BPA) as receptor
to satisfy the Zn2+ detection. The synthetic route of DABI is depicted in
Scheme 1. The reaction of 3-(chloromethyl)-4-hydroxybenzaldehyde
(1) [20] with BPA in CH3CN solution using K2CO3 as base afforded
compound 2 in 48% yield. DABI was obtained in 54% yield via con-
densation of compound 2 with o-phenylenediamine in ethanol
solution.
A number of fluorescence sensors for detection of Zn2+ in living
cells, tissues, and sample solutions have been reported to date
[8–13]. Examples using fluoresceins (such as ZPs) [14], rhodamines
(such as RhodZin-1) [15], naphthalimides (such as WZS) [16], NBDs
(such as NBD-TPEA) [17] and 8-hydroxyquinoline (such as sensor 3)
[18] have been documented. Such probes are among the current
state of the art, yet limitations exist. For example, most documented
fluorescence Zn2+ sensors cannot discriminate Zn2+ from Cd2+
owing to their closely related electronic and binding properties.
The photoresponsive properties of DABI were investigated by
UV–vis and fluorescence spectroscopy in acetonitrile solution. As
shown in Fig. 1, the UV–vis spectrum of DABI in acetonitrile exhibits
an absorption maximum in 309 nm (ε=4.13×104 M−1 cm−1), which
can be assigned into the π−π* transition band of benzoimidazole.
When titrated by Zn2+ (0−2.0 equiv), this band is decreased gradually,
⁎
Corresponding author at: School of Chemistry and Chemical Engineering, Liaocheng
University, Liaocheng, 252000, PR China. Tel.: +86 635 8230661; fax: +86 635
8239121.
⁎⁎ Corresponding author. Tel.: +86 25 83597066; fax: +86 25 83314502.
E-mail addresses: chliuzp@gmail.com (Z. Liu), heweij69@nju.edu.cn (W. He).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.10.018

X. Wang et al. / Inorganic Chemistry Communications 15 (2012) 176–179
177
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
0.0
0.4
0.8
1.2
1.6
2.0
[Zn2+]/[DABI]
[ Zn2+
]
360
380
400
420
440
460
480
Scheme 1. Synthesis of DABI.
Wavelength (nm)
Fig. 3. Fluorescence spectra of DABI (50 μM) in acetonitrile solution in the presence of
different concentrations of Zn2+ (0–2 equiv). (Inset:) The calibration curve I395 nm as a
function of Zn2+ concentration.
2.5
2.0
1.5
1.0
0.5
0.0
2.4
2.0
1.6
1.2
0.8
0.4
0.0
a
6
0.0 0.2 0.4 0.6 0.8 1.0 1.2
[ Zn2+] /[DABI]
5
4
3
2
1
0
165 mM
[ Zn2+]
0 mM
250
300
350
400
450
Wavelength (nm)
Fig. 1. Absorption spectra of DABI (50 μM) in acetonitrile solution in the presence of
different concentrations of Zn2+ (0–2 equiv). Inset: The calibration curves A309 nm
and A345 nm as a function of Zn2+ concentration.
accompanied by the red-shift to 345 nm (ε=3.8×104 M−1 cm−1). The
linear decrease of absorbance at 309 nm and increase of absorbance
at 345 nm with [Zn2+
]
up to a molar ratio ([Zn2+
]
total/[DABI]) of
total
b
1:1 and the unchangeable spectrum at even higher [Zn2+
]
imply
7
total
the 1:1 binding stoichiometry. Following a Benesi–Hildebrand-type
analysis, the binding constant of DABI was determined to be 7.4×
103 mol−1 dm3.
6
Zn2+
5
4
3
2
1
ions free + ions
different from Zn2+
0
360
380
400
420
440
460
480
500
Wavelength (nm)
Fig. 4. (a) F396 of DABI (50 μM) in CH3CN induced by different metal cations. Black bars
represent the F396 of free sensor or in the presence of 1 equiv of Ag+, Cd2+, Co2+, Cu2+
,
Fig. 2. The MS spectrum of Zn2+/DABI complex. The determined isotopic distribution
patterns of the peak with m/z of 484.25 are shown as inset (a). [M+Zn−H]+ is
shown as inset (b).
Fe2+, Hg2+, Mn2+, Ni2+, Na+ and Ca2+. Black bars, F396 in the presence of the indicated
metal ions, followed by addition of 1 equiv of Zn2+; (b) The emission spectra of DABI in
the presence of different metal ions in acetonitrile solution.

178
X. Wang et al. / Inorganic Chemistry Communications 15 (2012) 176–179
via the rotation of the biaryl plane and the photo-induced electron
transfer from 3°N atom of BPA group to the fluorophore in the excited
state should be response to the weak fluorescence of DABI in acetoni-
trile. On the other hand, the structure of the Zn2+/DABI complex
(b) was also optimized with an initial structure constructed with
direct Zn2+ coordination by three N atoms of BPA, and one oxygen
atom of HPBI. The optimized structure of the Zn2+/DABI complex dis-
plays a dihedral angle of 19.63° between two aryl planes. Based on
these data, the Zn2+ coordination induced coplanation of two aryl
planes of fluorophore results the red shift of both absorption and
emission band of DABI. The blockage of the photoinduced electron
transfer (PET) process from a BPA amine to a HPBI fluorophore in-
duced by Zn2+ coordination to a BPA amine should provide for Zn2+
induced emission enhancement.
-
In conclusion, we have developed a new benzoimidazole based
Zn2+ fluorescent sensor, DABI, which displays a rapid and a linear re-
sponse to Zn2+ with a red-shifted 100-fold turn-on signal from the
dark background. Excellent selectivity was observed against other
metal ions, especially Cd2+ that interferes with the existing Zn2+
Fig. 5. Photograph showing the fluorescence changes of DABI before (left) and after
(right) the addition of 1 equiv of Zn2+ in acetonitrile solution upon UV irradiation.
sensors. Molecular modeling study results suggest that the Zn2+
-
induced red shift of emission and fluorescence turn-on of DABI
could be correlated to the coplanation of two aryl planes of 2-HPBI.
The stoichiometry of the Zn2+/DABI complex has also been con-
firmed by mass spectroscopic determination. As shown in Fig. 2, the
electrospray ionization mass spectrum of this complex displays one sig-
Acknowledgments
nal of m/z 484.25, which can be assigned as the signal for [M+Zn−H]+
.
We thank the Natural Science Foundation of Shandong Province
(ZR2011BQ010) and the Liaocheng University Funds for Young Scientists
(31805) for financial support.
DABI exhibits very weak fluorescence in its apo form (Φb0.1%)
with one emission band centered at 345 nm in acetonitrile solution
(Fig. 3). Upon addition of Zn2+ into DABI solution, a new emission
band centered at 396 nm appears from dark background, and a re-
markable fluorescence enhancement of over 100-fold is observed.
The emission enhancement of DABI shows a linear enhancement
Appendix A. Supplementary data
with the increase of [Zn2+
]
when the ratio of [Zn2+
]total/[DABI]
Supplementary data to this article can be found online at doi:10.
1016/j.inoche.2011.10.018.
total
is below or equal to 1:1. When the ratio reached to 1:1, however,
higher [Zn2+
]
does lead to any further emission enhancement.
total
The quantum yield for the DABI/Zn2+ complex is about 0.5.
The Zn2+-specific turn-on response of DABI was further con-
firmed by screening other metal ions. As shown in Fig. 4, all tested
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For the full reference, see the Supporting Information