A benzimidazole-based ratiometric fluorescent sensor for Cr3+ and Fe3+ in aqueous solution

Article abstract of DOI:10.1016/j.dyepig.2013.02.005

A novel ratiometric fluorescent sensor bearing two benzimidazole groups has been synthesized. The sensor showed a ratiometric fluorescence response with an enhancement of the ratios of emission intensities at 443 and 378 nm from 0.17 to 3.21 for Cr³⁺. The ratios of emission intensities at 443 and 380 nm were enhanced from 0.40 to 2.09 for Fe³⁺. Fe³⁺ revealed significant adsorption in the UV-vis region, which can be used to discriminate Cr³⁺ and Fe³⁺. Detection limits of the method for Cr ³⁺ and Fe³⁺ were 25 μM and 2 μM, respectively. In addition, the sensor showed good selectivity to Cr³⁺ and Fe ³⁺ over other metal ions.

Full text of DOI:10.1016/j.dyepig.2013.02.005

Dyes and Pigments 97 (2013) 475e480
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Short communication
A benzimidazole-based ratiometric fluorescent sensor for Cr3þ and Fe3þ
in aqueous solution
*
Meng Wang, Jungang Wang, Weijian Xue, Anxin Wu
Key Laboratory of Pesticide & Chemical Biology, Ministry of Education, Central China Normal University, 152 Luoyu Road, Wuhan 430079, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel ratiometric fluorescent sensor bearing two benzimidazole groups has been synthesized. The
sensor showed a ratiometric fluorescence response with an enhancement of the ratios of emission in-
tensities at 443 and 378 nm from 0.17 to 3.21 for Cr3þ. The ratios of emission intensities at 443 and
380 nm were enhanced from 0.40 to 2.09 for Fe3þ. Fe3þ revealed significant adsorption in the UVevis
region, which can be used to discriminate Cr3þ and Fe3þ. Detection limits of the method for Cr3þ and
Received 24 November 2012
Received in revised form
2 February 2013
Accepted 5 February 2013
Available online 16 February 2013
Fe3þ were 25
over other metal ions.
m
M and 2
m
M, respectively. In addition, the sensor showed good selectivity to Cr3þ and Fe3þ
Keywords:
Ó 2013 Elsevier Ltd. All rights reserved.
Fluorescent sensor
Benzimidazole
Ratiometric
Cr3þ
Fe3þ
Aqueous solution
1. Introduction
known to interfere with signal output [16]. Ratiometric fluorescent
sensors allow for the measurement of emission intensities at two
different wavelengths. Additionally, most of the foregoing sensors
have revealed the dual emission system can eliminate most or all
ambiguities in the detection through the self-calibration of two
emission bands. Thus, the ratiometric fluorescent sensor has
become one of the most promising sensing strategies for accurate
detection [17]. In spite of this, both Fe3þ and Cr3þ ratiometric
fluorescent sensors remain extremely scarce [18e20].
In addition, most of these sensors have revealed that the fluo-
rescence intensity decreases sharply with an increase in water
content; thus demonstrating their inability to coexist with large
amounts of water [15,21,22]. Here, we report a new ratiometric
fluorescent sensor for Fe3þ and Cr3þ based on a derivative of
benzimidazole, which subsequently demonstrates a blue-shift and
fluorescence-enhanced response toward Fe3þ and Cr3þ in aqueous
solution DMSO/H2O (1:99, v/v).
Trivalent chromium (Cr3þ) and iron (Fe3þ) are essential nutri-
ents for humans, playing a key role in many biochemical processes
at a cellular level [1,2]. Cr3þ also improves insulin sensitivity, while
also acting as a glucose tolerance factor through the metabolism of
carbohydrates, fats, and proteins [3]. The deficiency of chromium
can cause disturbances in glucose levels and lipid metabolism; on
the other hand, exposure to high levels of Cr3þ can negatively affect
cellular structures [4]. Fe3þ is indispensable for most organisms and
both its deficiency and overload can induce various disorders such
as tightly regulated iron trafficking, storage, and balance in an or-
ganism [5,6]. Nevertheless, Cr3þ and Fe3þ are well-known as fluo-
rescence quenchers due to their paramagnetic nature [7e9] and
sensors with a fluorescence enhancement signal when interacting
with guests are much more efficient. In recent years, a number of
fluorescent receptors for Fe3þ have been reported [7,8,10e13]; yet
few sensors for Cr3þ have been described in the literature [14,15].
However, whether fluorescent sensing systems for metal ions
function by fluorescence enhancement or quenching, as the change
in fluorescence intensity is the only detection signal; other factors
such as instrumental efficiency and environmental conditions are
2. Experiment
2.1. Reagents
All starting materials and catalysts were obtained commercially
and used without further purification. Most of the solvents were
distilled under N2 over the appropriate drying reagents (sodium or
* Corresponding author. Tel./fax: þ86 27 6786 7773.
E-mail address: chwuax@mail.ccnu.edu.cn (A. Wu).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.02.005

476
M. Wang et al. / Dyes and Pigments 97 (2013) 475e480
calcium hydride). Deionized water was used throughout the ex-
periments. Column chromatography: silica gel 200e300 mesh.
2.2. Apparatus
Absorption spectra were taken at room temperature using a
UV-2501 PC spectrophotometer (SHIMADZU CORPORATION) with
a variable wavelength between 200 and 900 nm and with the use of
a glass cuvette with a 0.5 cm optical path. Fluorescence spectra
measurements were performed on a Cary Eclipse spectrofluorom-
eter. Slits were set to provide widths of 5 nm for the emission
monochromators in all cases. NMR spectra were measured on a
Varian Mercury 400 spectrometer operating at frequencies of
400 MHz for 1H and 100 MHz for 13C, while the Varian NMR System
600 MHz spectrometer operated at frequencies of 600 MHz for 1H
and 150 MHz for 13C relative to tetramethylsilane as an internal
standard. HRMS were obtained on an FT-ICR MS equipped with an
electrospray source. IR spectra were recorded on a Tensor 27
infrared spectrometer and KBr pellets with absorption were re-
ported in cmꢀ1. The X-ray crystal structure determination of 1a was
obtained on a Bruker SMART APEX CCD system. Melting points
were determined using XT-4 apparatus and not corrected.
Scheme 2. Structures of the target compounds 1ae1d.
1227, 1002, 737, 691, 536; 1H NMR (600 MHz, CDCl3):
(d, J ¼ 6.0 Hz, 2H), 7.79 (s, 2H), 7.64 (s, 2H), 7.327.29 (m, 8H), 7.26e
7.24 (m, 9H), 5.50 (s, 4H); 13C NMR (100 MHz, CDCl3):
(ppm)
d (ppm) 7.83
d
143.71, 141.84, 135.48, 133.83, 133.54, 131.58, 129.17, 128.37, 126.32,
123.37, 122.63, 121.56, 120.57, 121.07, 109.62, 97.36, 85.78,
44.45 cmꢀ1; HRMS (ESI): m/z [M þ H]þ calcd for C38H27N4:
539.22350; found: 539.22302.
2.3.2. 1,10-((3,6-Bis(phenylethynyl)-1,2-phenylene)Bis(methylene))
Bis(5,6-dimethyl-1H-benzo[d]imidazole) (1b)
2.3. Synthesis of compounds 1
White solid; yield: 0.129 g; 80%; M.p.: 158e162 ꢁC; IR spectrum
The synthetic route to access the compounds is shown in
Scheme 1. 1,4-Dibromo-2,3-bis(bromomethyl)benzene was pre-
pared according to the procedures set out in the literature [23].
Compound 2 was prepared by a modification of the procedure
outlined by Murray [24]. 1,4-Dibromo-2,3-bis(bromomethyl)ben-
zene (3) was added to a stirred solution of benzimidazole (benzo-
triazole, 5,6-dimethyl-1H-benzo[d]imidazole, 2-chloro-1H-benzo
[d]imidazole) and powdered NaOH in DMF and H2O. The mixture
was stirred at 60 ꢁC for two days to give 2 in 50e92%. To a solution
of bis(triphenylphosphine) palladium(II) dichloride, copper(I) io-
dide and compounds 2 in DMF under argon was added 4-
phenylacetylene and Et3N. After heating under reflux for 12 h, 1
was obtained at a yield of 74e80% (Scheme 2) [25e27].
(KBr cmꢀ1): 2924, 2853, 2204, 1591, 1491, 1444, 1090, 996, 831, 758,
690; 1H NMR (400 MHz, CDCl3):
d
(ppm) 8.04 (dd, J ¼ 4.0, 4.0 Hz,
2H), 7.75 (dd, J ¼ 4.0, 4.0 Hz, 2H), 7.64 (s, 2H), 7.37e7.31 (m, 14H),
6.41 (s, 4H); 13C NMR (100 MHz, CDCl3):
(ppm) 145.71, 136.25,
d
133.22, 131.50, 128.38, 127.44, 125.76, 123.89, 121.92, 119.81, 110.11,
96.98, 86.83, 47.87 cmꢀ1; HRMS (ESI): m/z [M þ H]þ calcd for
C36H25N6: 541.2136; found: 541.2135.
2.3.3. 1,10-((3,6-Bis(phenylethynyl)-1,2-phenylene)Bis(methylene))
Bis(2-chloro-1H-benzo[d]imidazole) (1c)
Yellow solid; yield: 0.139 g; 78%; M.p. > 300 ꢁC; IR spectrum
(KBr cmꢀ1): 2923, 2362, 2206, 1591, 1494, 1218, 836, 754, 687, 430;
1H NMR (600 MHz, CDCl3):
d
(ppm) 7.77 (s, 2H), 7.56 (s, 4H), 7.26
(m, 10H), 7.03 (s, 2H), 5.43 (s, 4H), 2.37 (s, 6H), 2.29 (s, 6H); 13C NMR
(150 MHz, CDCl3): (ppm) 140.84, 135.74, 133.43, 132.65, 131.60,
2.3.1. 1,10-((3,6-Bis(phenylethynyl)-1,2-phenylene)Bis(methylene))
Bis(1H-benzo[d]imidazole) (1a)
d
129.10, 128.34, 126.29, 121.69, 120.18, 109.82, 97.09, 85.91, 44.36,
20.56, 20.24 cmꢀ1; HRMS (ESI): m/z [M þ H]þ calcd for C42H35N4:
595.28637; found: 595.28562.
Yellow solid; yield: 0.215 g; 76%; M.p.: 114e117 ꢁC; IR spectrum
(KBr cmꢀ1): 3052, 2208, 1665, 1612, 1492, 1457, 1390, 1327, 1282,
Scheme 1. Synthesis of compound 1.

M. Wang et al. / Dyes and Pigments 97 (2013) 475e480
477
2.3.4. 1,10-((3,6-Bis(phenylethynyl)-1,2-phenylene)Bis(methylene))
Bis(1H-benzo[d] [1-3]triazole) (1d)
Yellow solid; yield: 0.134 g; 74%; M.p.: 218e222 ꢁC; IR spectrum
(KBr cmꢀ1): 3059, 2203,1612,1476,1338,1265, 836, 734, 689, 526.1H
NMR (400 MHz, CDCl3):
10H), 7.19 (t, J ¼ 8.0 Hz, 2H), 7.04 (t, J ¼ 8.0 Hz, 2H), 6.80 (d, J ¼ 8.0 Hz,
2H), 5.67 (s, 4H); 13C NMR (100 MHz, CDCl3):
(ppm) 141.69, 140.76,
d (ppm) 7.68e7.64 (m, 4H), 7.30e7.25 (m,
d
135.07, 134.94, 133.86, 131.55, 129.09, 128.34, 125.58, 123.49, 122.80,
121.75, 119.64, 109.99, 98.13, 85.96, 45.49 cmꢀ1. HRMS (ESI): m/z
[M þ H]þ calcd for C38H25Cl2N4: 607.14524; found: 607.14508.
2.4. X-ray diffraction analysis of compound 1a
A crystal of 1a was grown by the slow evaporation of solutions of
the compound in CHCl3/CH3OH (8:2, v/v) and found to be suitable
for X-ray crystal structure analysis. Details of the crystal data have
been deposited with Cambridge Crystallographic Data Centre and
are available in Supplementary Publication No. CCDC 900488.
2.5. Measurement procedures
Fig. 1. Crystal structure of 1a$CH3OH.
Compound 1aed were weighed accurately, transferred to a
10 mL volumetric flask and dissolved and made up to 10 mL with
DMSO. Then 1 mL of the DMSO solution was transferred into
100 mL volumetric flask and made up to the volume with deionized
fluorescence and UVevis absorption spectroscopy. The selectivity
of 1a for Cr3þ and Fe3þ over other common metal ions at a bio-
logical concentration level was examined in DMSO/H2O (1:99, v/v,
water to obtain the stock solutions of 1aed (10 mM) in DMSO/H2O
10 m ,
M). Fig. 3 indicates that the addition of Kþ, Ba2þ, Sr2þ, Mg2þ
(1:99, v/v). The stock solutions of cations and anions were prepared
in DMSO/H2O (1:99, v/v) with a concentration of 3.0 ꢂ 10ꢀ3 M for
fluorescence spectral analysis and UVevis adsorption spectrum.
Each time a 3 mL solution of 1 was filled to a quartz cell of 1 cm
optical path length, the concentrations of cations and anions were
increased by the addition of different equivalents using a micro-
syringe. All the experiments used an excitation wavelength of
320 nm and room temperature.
Zn2þ, Co2þ, Cd2þ, Pb2þ, Hg2þ, Mn2þ, Ni2þ, and La3þ caused virtually
no change to the fluorescence spectrum of receptor 1a. The addi-
tion of Cu2þ and Agþ caused a minor decrease in the emission in-
tensity, which can be attributed to the fact that these transition and
heavy metal ions have an intrinsic quenching nature [28e31]. In
contrast, the addition of Cr3þ/Fe3þ to the solution of receptor 1a
resulted in a drastic fluorescence emission change. A steady
decrease in emission intensity at 443 nm was observed, along with
a concomitant increase in the intensity of the new fluorescence
band at 378/380 nm (up to 6.0/4.9-fold enhancement when
compared to the fluorescence intensity of free sensor 1a at 378 nm,
see Fig. 3B). The selectivity of Cr3þ/Fe3þ was calculated via a
3. Results and discussion
3.1. Structural characteristics of 1a
selectivity coefficient ðKCr3þ
¼ SCr3þ
=S0Þ, where S
3þ
3þ
=Fe3þ
=Fe3þ
was the response to Cr3þ/Fe3þ and S0 was the response tCor o=tFheer
A single crystal of 1a$CH3OH suitable for crystallography was
obtained from the slow evaporation of a concentrated CHCl3/
CH3OH (8:2, v/v) solution of 1a at ambient temperature. The
structure of 1a is depicted in Fig. 1. Two independent molecules in
the unit cell were visible. The two benzimidazole rings of one
molecule formed a dihedral angle of 22ꢁ. The distance between
cations [32]. The selectivity coefficients of Cr3þ/Fe3þ measured
ꢀ
C23B and C24B of the benzimidazole group was 3.184 A. The CH3OH
molecule played a key role in stabilizing the crystal structure by
providing an electron donor (oxygen atom).
3.2. Spectral characteristics
As shown in Fig. 2, compound 1a exhibited a strong absorption
band centered at 328 and 230 nm in DMSO/H2O (1:99, v/v); the
compound 1be1d demonstrated a similar characteristic absorption
peak (Figs. S1eS3). The high-energy peak at around 328 and
230 nm was due to the
displayed a well-defined maximum at 443 nm in the fluorescence
spectrum and recorded a concentration of 10 M in DMSO/H2O
p / p* electronic transition. Receptor 1a
m
(1:99, v/v) when excited at 320 nm. A similar peak was found in
1be1d, which held the same luminescent fragments (Figs. S4eS6).
3.3. Selectivity
The cation binding affinity of sensor 1 toward different alkali,
Fig. 2. UVevis adsorption spectrum of 1a (10 mM) on addition of 50 equiv of different
alkaline earth, and transition metal ions was investigated with
metal ions in DMSO/H2O (1:99, v/v).

478
M. Wang et al. / Dyes and Pigments 97 (2013) 475e480
Fig. 4. Fluorescence spectra of sensor 1a (10
mM) in DMSO/H2O (1:99, v/v) upon
addition of different Cr3þ/Fe3þ salts with excitation at 320 nm.
Cr3þ and Fe3þ to a solution of 1 in DMSO/H2O (1:99, v/v). This led to
the same type of profile as observed in Fig. 3A (Figs. 7 and 8). The
fluorescence peak quenching in the fluorescence intensity of 1a
was seen at 443 nm, while the enhancement was seen at 378/
380 nm. Such changes at two wavelengths offered an interesting
opportunity for the ratiometric fluorescence determination of
analytes. A linear relationship was obtained within a concentration
range of 0e310 m m
M for Cr3þ and 0e240 M for Fe3þ (inset of Figs. 7
and 8, respectively), and the linear regression parameters were
0.99372 and 0.95669 respectivly. The ratios of emission intensities
at 378 and 443 nm (I378/I443) enhanced from 0.17 to 3.21 for Cr3þ
,
while the ratios of emission intensities at 380 and 443 nm varied
from 0.40 to 2.09 for Fe3þ. The association constants (Ka) of receptor
1a for Cr3þ and Fe3þ were calculated on the basis of a Hill plot
(Figs. S7 and S8) [34,35]. It was found to be 1.75 ꢂ 103 Mꢀ1 and
8.51 ꢂ104 Mꢀ1 (error limits ꢃ 8%), respectively. The detection limit
Fig. 3. (A) Changes in the fluorescence spectrum of 1a (10 mM) upon addition of metal
salts in DMSO/H2O (1:99, v/v) with excitation at 320 nm (30 equiv). (B) A bar diagram
showing fluorescence intensity in the presence of different metals at 378 nm.
emerged to be 25 m m
M for Cr3þ and 2 M for Fe3þ from 10 blank
solutions based on the definition by IUPAC (CDL ¼ 3 Sb/m) [36]. In
both cases, the stoichiometries of the complex were determined to
be 1:1 by Job’s plot (Figs. S9 and S10).
against other tested cations are listed in Tables S1 and S2. The co-
efficients clearly indicated that sensor 1a had better selectivity for
Cr3þ and Fe3þ than other cations. The experiments of the counter
ion effect on the selective properties of Cr3þ/Fe3þ were also
measured (Fig. 4). The addition of CrCl3, FeCl3, and Fe(NO3)3 all had
a similar influence as that of Cr(NO3)3 and Fe(ClO4)3, which implied
that a different counter ion did not affect the binding of Cr3þ/Fe3þ
with receptor 1a.
3.5. Quantum yields
The fluorescent quantum yield of the compound 1a before and
after the addition of Cr3þ and Fe3þ ions was measured by
The absorption spectra of compound 1a (10 mM) in DMSO/H2O
(1:99, v/v) were also investigated (Fig. 2). The addition of 50 equiv
of Fe(ClO4)3 led to the great increase of the original absorbance.
Other metal ions including Cr3þ caused no change to the absorption
intensity, which can be used to discriminate between Cr3þ and
Fe3þ. Such adsorption change may be ascribed to the paramagnetic
effect of Fe3þ [33]. Fig. 4 showed that Fe3þ has a substantial
absorbance in the UV and near visible range. Fig. 5 showed the
absorbance spectrum of receptor 1a in the presence and absence of
Fe(ClO4)3 in DMSO/H2O (1:99, v/v). We found that the absorbance
intensity of receptor 1a in the presence of Fe(ClO4)3 was equal to
the absorbance intensity of receptor 1a and Fe(ClO4)3 (Fig. 6).
3.4. Ratiometric response to Cr3þ and Fe3þ
In order to evaluate sensor 1a as a fluorescent sensor for Cr3þ
Fig. 5. The absorption spectra of Fe(ClO4)3 in DMSO/H2O (1:99, v/v) with increasing
concentrations.
and Fe3þ, titrations were performed with successive additions of

M. Wang et al. / Dyes and Pigments 97 (2013) 475e480
479
Fig. 6. The absorption spectra of Fe(ClO4)3 (500
1a þ Fe(ClO4)3 (500 M) in DMSO/H2O (1:99, v/v).
mM), sensor 1a (10 mM), sensor
Fig. 8. Fluorescence spectra changes (lex ¼ 320 nm) of receptor 1a (10
addition of Fe3þ (0e30 equiv) in DMSO/H2O (1:99, v/v).
m
M) upon
m
comparison with quinine sulfate as the standard compound in
sulfuric acid according to the following equation [37,38]:
S6). While in 1d, the hydrogen atom was substituted by chlorine
atom. Interestingly, compound 1d also showed no selection for any
metal ions except Fe3þ. We doubt that the CeH group of the
benzimidazole was meant to complex with Cr3þ/Fe3þ. In order
to confirm our hypothesis further, we synthesized 1c retaining the
CeH site to evaluate the metal ion recognition behavior. Compound
Fs ꢂ As ꢂ Fu ꢂ n2
Fu
¼
Au ꢂ Fs ꢂ n20
where Fu and Fs are quantum yield for the sample and reference, Fu
and Fs are the integrated area under the corrected fluorescence
spectra for the sample and reference, Au and As are the absorbance
for the sample and reference, n and n0 are the refractive indexes of
the solvents used for samples and reference. The quantum yield of
compound 1a was 0.92. After the addition of Cr3þ ions, the quan-
tum yield increased to 1.43. While the quantum yield decreased to
0.59 after the addition of Fe3þ ions because the intrinsic quenching
phenomenon was so evident.
1c had the same effect with sensor 1a when binding with Cr3þ
/
Fe3þ, along with a sharp decrease in the fluorescence intensity as
compared with 1a especially for Fe3þ (Figs. S4); a feature which was
due to the intrinsic fluorescence quenching of Fe3þ. The blue-shift
in the fluorescence of 1a and 1c can be attributed to an intra-
molecular charge transfer (ICT), while the enhancement of fluo-
rescence intensity upon Cr3þ/Fe3þ complexation was most likely
due to photoinduced electron transfer (PET) processes [15,18]. Ac-
cording to the above analysis, we considered that the CeH group of
the benzimidazole may have played a key role in binding with Cr3þ
and Fe3þ. It was already known that Cr3þ/Fe3þ both have a strong
binding affinity with receptors possessing sp2 nitrogen of imidazole
[39e41]. The Cr3þ/Fe3þ ions were assumed to have a hexahedral
geometry in the complex, in which receptor 1a acted as a tetra-
dentate ligand while the other two coordination sites were occu-
pied by anions.
3.6. Possible mechanism
Compound 1b and 1d were synthesized to have a comparison
with 1a. In 1b, the CeH group was replaced by N atom. 1b was
found to be non-selective for any metal ions except Fe3þ, where
intrinsic fluorescence quenching was demonstrated (Figs. S5 and
4. Conclusion
In conclusion, we have synthesized a novel ratiometric fluo-
rescent sensor capable of recognizing and estimating the con-
centrations of Cr3þ and Fe3þ in aqueous solution. It is worthy to
note that examples of ratiometric fluorescent sensors for Cr3þ and
Fe3þ have been very sparse. Fe3þ ions showed significant ab-
sorption in the UV/Visible region and thus can be discriminated
from Cr3þ through this method. Significantly, treatment with Cr3þ
and Fe3þ caused the sensor to display a ratiometric fluorescent
response with a blue-shift and fluorescent enhancement. The
sensor also exhibited selectivity for Cr3þ and Fe3þ above other
metal ions.
Acknowledgments
We thank the National Natural Science Foundation of China
(Grant 21272085). We are also grateful to Xianggao Meng for his
help with X-ray diffraction analysis.
Fig. 7. Changes in the fluorescence intensity of spectrum of sensor 1a (10
increasing concentrations of Cr3þ (0e35 equiv) in DMSO/H2O (1:99, v/v).
mM) with

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M. Wang et al. / Dyes and Pigments 97 (2013) 475e480
[21] Andrea BB, Ana MC, Gil S. A new selective fluorogenic probe for trivalent
Appendix A. Supplementary data
cations. Chem Commun 2012;48:3000e2.
[22] Subarna G, Sisir L, Arnab B. Thiophene anchored coumarin derivative as a
turn-on fluorescent probe for Cr3þ
Talanta 2012;91:18e25.
: cell imaging and speciation studies.
Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.dyepig.2013.02.005.
[23] Lai YH, Yap AH. Synthesis and rigid conformers of 14,15-dimethyl-2,11-dithia
[3.3](1,3)(l,4)cyclophane and 12,13-dimethyl[2.2](1,3)(1,4)cyclophane.
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