A fluorescent probe for both pH and Zn2+ based on 2-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenol

Article abstract of DOI:10.1016/j.saa.2011.05.079

A sensitive fluorescent probe 2-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenol (HBIZ) for pH and Zn²⁺ has been developed. Great changes have taken place in the UV-vis absorption and fluorescence spectra for HBIZ upon increasing pH of its aqueous sol

Full text of DOI:10.1016/j.saa.2011.05.079

Spectrochimica Acta Part A 79 (2011) 1876–1880
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
2
+
A fluorescent probe for both pH and Zn based on
2
-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenol
∗
Chuan-Chuan Ju, Hong-Ju Yin, Chui-Li Yuan, Ke-Zhi Wang
College of Chemistry, Beijing Normal University, Beijing 100875, China
a r t i c l e i n f o
a b s t r a c t
A sensitive fluorescent probe 2-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenol (HBIZ) for pH and Zn2+ has
been developed. Great changes have taken place in the UV–vis absorption and fluorescence spectra for
HBIZ upon increasing pH of its aqueous solution, acting as a pH-induced emission “off–on–off” switch
with large enhancement factors of ∼290 and ∼75 over the pH range of 1.00–5.40 and 5.20–10.40. A
over 100-fold fluorescence enhancement was also observed after complexation of HBIZ to Zn2+ in N,N-
dimethylformamide.
Article history:
Received 11 October 2010
Received in revised form 24 May 2011
Accepted 25 May 2011
Keywords:
Probe
Zinc
©
2011 Elsevier B.V. All rights reserved.
pH
Spectroscopy
1
. Introduction
2. Experimental
2.1. Synthesis
Preparation
The design and development of small molecules for sensing
applications are of great current interest [1–3]. Zinc(II) ion is an
abundant component of most living cells and plays an important
role in various biological processes such as gene transcription, reg-
ulation of metalloenzymes, neural signal transmission, and others
of
2-(1-phenyl-1H-benzoimidazol-2-yl)phenol
(HBIZ).
A mixture of N-phenylbenzene-1,2-diamine (0.92 g,
5 mmol) and 2-hydroxybenzaldehyde (0.61 g, 5 mmol) in 2-(2-
methoxyethoxy)ethanol (10 mL) was microwave heated at 160 C
2+
◦
[
4–8]. Consequently, the detection of Zn with low concentration
in vivo has attracted increasing interest. Many fluorescent sensors
that can selectively detect Zn2 have been reported in the past few
years [9–42], and most of them involve the principle of photo-
induced electron transfer in the signaling process with a moderate
degree of success [20–34], usually suffering from relatively low
signal levels.
for 3 h under N2 [24]. The reaction mixture was cooled to room
temperature, and poured into water (200 mL), then filtered. The
residue was redissolved in dichloromethane and purified by
column chromatography on silica gel with dichloromethane as
eluent. Further recrystallization from hexane–dichloromethane
(v/v = 1:1) afforded 0.35 g of white crystals (25% yield). 1H NMR
+
Herein, we report that an organic small molecule 2-(1-phenyl-
(500 MHz, DMSO-d , ıH, ppm): 6.56 (t, 1H), 6.88 (dd, 1H), 7.14 (t,
6
1
H-benzo[d]imidazol-2-yl)phenol (HBIZ) acts as a pH-induced
2H), 7.28 (m, 2H), 7.39 (t, 1H), 7.45 (m, 2H), 7.64 (m, 3H), 7.85 (d,
emission “off–on–off” switch with large enhancement factors of
1H). Anal. Calc. for HBIZ·1.3H O(C19H16.6N O2.3): C, 73.68; H, 5.36;
2
2
∼
290 and ∼75 over the pH range of 1.00–5.40 and 5.20–10.40, and
N, 9.04. Found: C, 73.24; H, 5.63; N, 9.14%.
2
+
also a Zn “turn on” fluorescent sensor with a over 100-fold flu-
orescence enhancement factor. To the best of our knowledge, the
dual fluorescent switching type sensors for pH and Zn2 have been
relatively few [43–47].
+
2.2. Instruments
UV–vis spectra were obtained on a GBC Cintra 10e UV-visible
spectrometer. 1H NMR spectra were collected on a Bruker DRX-
00 NMR spectrometer with Me SO-d as solvent. Emission spectra
3
2
6
(
ꢀex = 330 nm) were obtained on a Shimadzu RF-5301PC spectroflu-
∗ Corresponding author. Tel.: +86 10 58805476; fax: +86 10 58802075.
E-mail address: kzwang@bnu.edu.cn (K.-Z. Wang).
orimeter at room temperature.
1
386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.05.079

C.-C. Ju et al. / Spectrochimica Acta Part A 79 (2011) 1876–1880
1877
Fig. 1. pH effects on the UV–vis spectra of HBIZ (0.05 mM). (a) pH = 1.00–5.00 and
b) pH = 5.20–10.00. Arrows show spectral changes upon increasing pH.
(
Fig. 2. pH effects on the emission spectra (ꢀex = 340 nm) of HBIZ (0.05 mM). (a)
pH = 1.00–5.40 and (b) pH = 5.20–10.40. Arrows show spectral changes upon increas-
ing pH.
3
. Results and discussion
The UV–vis spectrophotometric pH titrations were carried out
over the pH range from 1.00 to 10.00 in Britten–Robertson buffer
40 mM H BO , 40 mM H PO , 40 mM CH COOH). The changes in
(
3
3
3
4
3
1
.00 to 10.40. At pHs lower than 1.5, HBIZ luminescenced very
the UV–vis absorption spectra for HBIZ as a function of pH are
shown in Fig. 1. The UV–vis absorption spectra showed that HBIZ
underwent two ground-state protonation/deprotonation processes
over the pH region studied. The first process occurred upon increas-
ing pH from 1.00 to 5.00, resulting in slight decline and a red-shift
of 5 nm in absorption peak at 280 nm, and appearance of two new
bands at 296 and 333 nm, and two obvious isosbestic points at 300
and 313 nm. The spectral changes observed here were assigned
to the dissociation of one proton on the protonated imidazole
ring of the HBIZ. As pHs further increased, the second deproto-
nation took place over pH from 5.20 to 10.00, and was assigned
to the phenolic hydroxyl proton dissociation, eliciting in remark-
able declines in all of absorption bands, due to the breakage of the
intramolecular hydrogen bond between phenolic hydroxyl proton
and imidazole nitrogen. Two ground-state acidity ionization con-
weakly. As pH increased from 1.5 to 4.5, the fluorescence intensities
increased sharply by a factor of 290. Conversely, further increases
in pH from 9.5 to 10.4 elicited 74-fold reduction in the emission
intensities. These two processes constitute a typical off–on–off
emission switching. The spectral changes observed here were also
due to the dissociation of one proton on the protonated imida-
zole ring of the HBIZ (see Fig. 3). Thus, over pH 1.5–10.5, HBIZ
acted as an “off–on–off” emission switch with impressive emission
intensity “on–off” ratios. The ground- and excited-state ioniza-
tion constants are related thermodynamically by a Förster cycle
[
48]. The Förster treatment results in Eq. (1), which describes the
*
relationship between the ground-state pKa and excited-state pKa
stants were derived to be pKa1 = 3.79 ± 0.009 and pK = 8.31 ± 0.09
a2
by nonlinear sigmoidal fit of the data of absorbance changes with
pH (see insets of Fig. 1).
The emission spectra of HBIZ were strongly pH dependent as
shown in Fig. 2. The emission intensities versus pH profiles (the
insets of Fig. 2) of HBIZ were composed of two sigmoidal curves
of opposite gradients which were due to two separate excited-
state protonation/deprotonation processes over the pH region from
Fig. 3. The protonation/deprotonation processes of HBIZ.

1
878
C.-C. Ju et al. / Spectrochimica Acta Part A 79 (2011) 1876–1880
Fig. 5. The relative fluorescent intensities of HBIZ (50 ␮M) in the presence of various
metal cations (∼10 mM) in DMF (ꢀex = 330 nm).
librium is almost certainly established between the ␲–␲* states.
Since no detected shift in the emission maxima for ꢁB and ꢁHB, two
excited-state ionization constants are close to their ground-state
ones for the protonation/deprotonation equilibria as summarized
in Fig. 3.
As can be seen from Figs. 4 and 5, when excessive or equivalent
metal cations were added to the DMF solution of HBIZ, different
2
+
2+
emission spectral changes were observed. Additions of Ba , Ca
,
Hg , K , Li , Mn , and Na+ did not give rise to any changes in the
2+
+
+
2+
fluorescence emission of HBIZ even at high concentration. While
additions of Fe2+, Ni , and Cu quenched the fluorescence to a
little extent, probably because there is electron or energy transfer
between metal cation and fluorophore [1]. Surprisingly, additions
of Zn2+ and Mg enhanced the fluorescence intensities of HBIZ
2+
2+
2+
2+
2+
significantly. Additions of Zn and Mg resulted in 120- and 25-
fold enhancements in fluorescence intensities with the emission
maximum blue-shifted by 70 and 80 nm, respectively. The results
were allowed to be readily distinguished by naked-eye, as shown
in Fig. S1 (Supplementary Material).
As can be seen in Fig. 6, the Job’s plot confirmed the formation
of 1:1 complex of Zn2+ with HBIZ. Quantitative investigations of
the binding behavior of HBIZ with Zn2+ were performed in DMF
by means of spectral titrations. As shown in Fig. 7 (top panel), the
absorption intensities of HBIZ at 283 and 310 nm slightly decreased,
Fig. 4. Fluorescence spectra of HBIZ (50 ␮M) in the presence of various metal cations
∼10 mM) in DMF (ꢀex = 330 nm).
(
based on pure 0–0 transitions in wavenumbers of ꢁB and ꢁHB for
the basic and acidic species, respectively [49,50]:
ꢀ
ꢁ
0
.625
T
∗
pK = pKa +
(ꢁB − ꢁHB)
(1)
a
In reality, the ꢁB and ꢁHB values are often difficult or even impos-
sible to obtain. A good approximation is to use the emission maxima
for ꢁB and ꢁHB, supposing that protonation/deprotonation equi-
Fig. 6. Job’s plots for the binding of Zn2+ with HBIZ.

C.-C. Ju et al. / Spectrochimica Acta Part A 79 (2011) 1876–1880
1879
as the concentrations of Zn2+ were successively increased. More-
over, successive additions of Zn2 were also companied by a new
+
3
−1
−1
absorption peak at 340 nm (ε = 3 × 10 M cm ) with an obvious
isosbestic point occurred at 320 nm, supporting the coordination
2
+
of HBIZ with Zn , most probably through the nitrogen and oxygen
atoms in HBIZ to form a six-membered chelate ring, resulting in
more conjugated system and the appearance of the new absorption
in the long wavelength region. The complex stability constant (Ks)
value of HBIZ with Zn2+ was calculated to be (1.36 ± 0.14) × 10 M
2
−1
by using the nonlinear least-squares method in the UV–vis spectral
titration experiment.
In the presence of Zn2+, the fluorescence intensities of HBIZ (see
bottom panel of Fig. 7) gradually increased to 100-fold original
intensity with an emission maximum appearing at 425 nm, which is
close to 438 nm previously reported for Zn(BIZ)2 [51]. The Ks value
of HBIZ with Zn2+ was calculated to be 1.55 (± 0.05) × 10 M by
using the nonlinear least-squares method in the fluorimetric spec-
tral titration experiment, which is close to the value obtained form
the UV–vis spectral titration experiment. Moreover, the effects of
coexisting ions on fluorescence intensities of HBIZ–Zn2 system
were also carried out, and the results are shown in Fig. 8. Clearly,
2
−1
Fig. 8. The relative fluorescence intensities of the binding of Zn2+ with HBIZ (50 ␮M)
in the presence of various metal cations (∼10 mM) in DMF (ꢀex = 330 nm).
+
2
+
2+
2+
2+
except for Cu , Fe and Ni that emission sensing of Zn was
interfered severely, the other ions tested had small effects.
In summary, 2-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenol
(
HBIZ) were evidenced to be a fluorescent switching probe for pH
2+
and Zn in water and N,N-dimethylformamide, respectively. The
pH-induced emission “off–on” and “on–off” switching ratios were
found to be ∼290 and ∼75 over the pH range of 1.00–5.40 and
5
.20–10.40, respectively. A over 100-fold fluorescence enhance-
2+
ment was produced after the complexation of HBIZ with Zn
.
These characteristics of HBIZ make it attractive for further molecu-
lar modifications and potential applications as fluorescence sensor
for pH and Zn2+
.
Acknowledgements
The authors thank the National Natural Science Foundation
(Nos. 20971016, 20771016, 90922004), Beijing Natural Science
Foundation (2072011), the Fundamental Research Funds for the
Central Universities, and Measurements Fund of Beijing Normal
University for financial supports.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2011.05.079.
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