A novel rhodamine-benzimidazole conjugate as a highly selective turn-on fluorescent probe for Fe3+

Article abstract of DOI:10.1007/s10895-011-0901-8

In this manuscript, a novel probe RHBI based on the rhodamine-benzimidazole conjugate was designed and synthesized. RHBI showed an extreme selectivity for Fe³⁺ over other metal ions such as Pb²⁺, Ni²⁺, Co²⁺, Mn²⁺, Zn²⁺, Hg²⁺, Cd ²⁺, Ag⁺, Mg²⁺, Ca²⁺, Ba ²⁺, Na⁺ and K⁺ in acetonitrile. Upon the addition of 10 equiv. of Fe³⁺, a 1098-fold fluorescence intensity enhancement was observed at the maximum emission wavelength of 582 nm. Both the Job's plot and ESI-MS showed that RHBI coordinated with Fe³⁺ in a 1:1 stoichomitry and the calculated binding constant was 1. 01∈×∈10⁴ M^-1. The competition experiment for Fe³⁺ ions mixed with other metal ions exhibited no obvious change except Cu²⁺ that could induce a mild fluorescence quenching. Moreover, the fluorescence emission increased linearly with the Fe³⁺ concentration in the range of 6∈×∈10^-6- 4∈×∈10^-5 M and the detection limit was 1.5∈×∈10^-8 M.

Full text of DOI:10.1007/s10895-011-0901-8

J Fluoresc (2011) 21:2005–2013
DOI 10.1007/s10895-011-0901-8
ORIGINAL PAPER
A Novel Rhodamine-Benzimidazole Conjugate as a Highly
Selective Turn-on Fluorescent Probe for Fe3+
Junbo Li & Qihui Hu & Xianglin Yu & Yang Zeng &
Cancan Cao & Xiwen Liu & Jia Guo & Zhiquan Pan
Received: 20 March 2011 /Accepted: 20 May 2011 /Published online: 27 May 2011
#
Springer Science+Business Media, LLC 2011
Abstract In this manuscript, a novel probe RHBI based on
the rhodamine-benzimidazole conjugate was designed and
synthesized. RHBI showed an extreme selectivity for Fe3+
over other metal ions such as Pb2+, Ni2+, Co2+, Mn2+, Zn2+,
Hg2+, Cd2+, Ag+, Mg2+, Ca2+, Ba2+, Na+ and K+ in
acetonitrile. Upon the addition of 10 equiv. of Fe3+, a
1098-fold fluorescence intensity enhancement was ob-
served at the maximum emission wavelength of 582 nm.
Both the Job’s plot and ESI-MS showed that RHBI
coordinated with Fe3+ in a 1:1 stoichomitry and the
calculated binding constant was 1.01×104 M−1. The
competition experiment for Fe3+ ions mixed with other
metal ions exhibited no obvious change except Cu2+ that
could induce a mild fluorescence quenching. Moreover, the
fluorescence emission increased linearly with the Fe3+
concentration in the range of 6×10−6–4×10−5 M and the
detection limit was 1.5×10−8 M.
Introduction
Currently, considerable attention has been focused on the
development of the fluorescent probes for the metal ions due to
their useful application in environment, biology and chemistry
[1–5]. Iron is an essential element for biological system, both
its deficiency and excess can induce a variety of diseases [6].
For instance, trivalent iron ions provide the oxygen carrying
capacity of heme and act as a cofactor in many enzymatic
reactions. Therefore, seeking effective method for iron ions
assay is important. Several analytical methods such as atomic
absorption spectrometry [7, 8], inductively coupled plasma-
mass spectrometry [9, 10], and electrochemistry [11, 12] have
been used for iron assay. Most of them are complicated and
time consuming, not suitable for quick and on-line detection.
Therefore, a large number of fluorescent probes have been
developed for the determination of iron ions recently [13–17],
due to their advantages of simplicity, high sensitivity, and low
cost. However, due to the paramagnetic nature of Fe3+, most
of the reported Fe3+ probes undergo a fluorescence quenching
effect, which is not as sensitive as fluorescence enhancement.
Designing a new Fe3+ turn-on fluorescent probe with high
selectivity is still a challenge.
Rhodamine derivatives have been utilized as important
probes, owing to their excellent spectroscopic properties such
as large molar coefficient and high fluorescence quantum [18].
It is well known that rhodamine derivatives undergo an
equilibrium between spirolactam and ring-opened amide
form. The spirolactam form is no fluorescent (fluorescence
“off”), whereas ring-opening of the corresponding spirolac-
tam gives rise to strong fluorescence emission [19] (fluores-
cence “on”). Based on the fluorescence “off-on” mechanism,
.
.
Keywords Fluorescent probe Rhodamine
.
Benzimidazole Iron ion
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-011-0901-8) contains supplementary material,
which is available to authorized users.
:
:
:
:
:
:
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J. Li Q. Hu X. Yu (*) Y. Zeng C. Cao X. Liu J. Guo
Z. Pan (*)
Hubei Novel Reactor & Green Chemical Technology Key
Laboratory, Key Laboratory for Green Chemical Process
of Ministry of Education, Wuhan Institute of Technology,
693 Xiongchu Avenue,
Wuhan, Hubei Province 430074, People’s Republic of China
e-mail: yxlin2002@163.com
e-mail: zhiqp@163.com

2006
J Fluoresc (2011) 21:2005–2013
Scheme 1 Synthesis of
compound RHBI
many rhodamine-based turn-on fluorescent probes for metal
cations such as Cu2+ [20–26], Hg2+ [27–33], Pb2+ [34], Cr3+
[35] have been reported in past few years. Meanwhile,
several rhodamine derivatives have also been utilized as
fluorescent probes for Fe3+ [36–38].
for iron ion were the O atom from rhodamine B and the two sp2
N atoms: one from benzimidazole and the other from imine.
Fortunately, RHBI showed a reversible, selective and sensitive
fluorescence enhancement response to Fe3+ in acetonitrile.
Herein, we describe a new rhodamine derivative (RHBI)
which was obtained by the Schiff base condensation
between rhodamine B hydrazide and 2-formylbenzimidazole.
The design was based on the fact that the sp2 N atom of
benzimidazole had a strong affinity with Fe3+ [39–41] as well
as been presented as an effective binding unit in numerous
biomolecules [42]. The possible coordination sites in the probe
Experimental
Apparatus and Chemicals
The UV-vis absorption and fluorescence spectra were
recorded on Hitachi 1601 spectrophotometer and Hitachi
Fig. 1 Variation of fluorescence
intensity at 582 nm of RHBI
(10μM, CH3CN:H2O = 95:5) in
the presence and absence of
Fe3+ (100μM) as a function
of pH

J Fluoresc (2011) 21:2005–2013
2007
F-4500 spectrofluorometer, respectively. A 1.0 cm quartz
cuvette in a volume of 3.0 mL was used for all spectra
collection. Thin-layer chromatography (TLC) was per-
formed on glass plates coated with SiO2 F254. The plates
were inspected by UV light or in I2 vapor. Column
chromotography was performed on silica gel (160–200
Unless otherwise stated, all reagents were purchased from
commercial suppliers and used without further purification.
Solvents used were purified and dried by standard procedure
prior to use. Rhodamine B was purchased from TDI, 1,2-
phenylenediamine and glycolic acid were purchased from
Sigma-Aldrich. Silica gel (160–200 mesh) that used for
column chromatography was purchased from Qingdao Ocean
Chemicals (Qingdao, China). All metal salts used were
obtained from shanghai Chemical Reagent Corporation
(Shanghai, China). Acetonitrile in chromatographic grade
was used throughout the experiments as solvent. The stock
solutions (0.1 mM) of the perchlorate salts of Fe3+, Cu2+, Ba2+,
1
mesh). H and 13C NMR spectra were recorded on Bruker
AV 400 (400 and 100 MHz, respectively) using tetrame-
thylsilane (TMS) as an internal standard. MALDI-TOF
mass spectrum was performed on Bruker Biflex III MALDI-
TOF spectrometer. ESI mass spectrum was performed on
Bruker ApexII FT-ICRMS spectrometer.
Fig. 2 A Emission response of
RHBI (10μM) upon addition of
different metal ions (10 equiv)
in CH3CN; B fluorescence
enhancement factors (FEF) of
RHBI (10μM) upon addition of
different metal ions (10 equiv.).
Excitation and emission
wavelength at 520 nm and
582 nm, respectively

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J Fluoresc (2011) 21:2005–2013
Fig. 3 Fluorescence response of
RHBI (10μM) to Fe3+
(10 equiv.) in the presence of
other competing metal ions
(10 equiv.). Excitation and
emission wavelength were at
520 nm and 582 nm,
respectively
Ni2+, Mg2+, Na+, Ca2+, the nitrate salts of Pb2+, Mn2+, Ag+
and chloride salts of Co2+, Zn2+, Fe2+, Hg2+ in acetonitrile
were prepared, respectively. The stock solution (10 μM) of
compound RHBI was prepared by dissolving accurately
weighed RHBI in acetonitrile.
aliquot of Fe3+ was added. The resulting solution was
stirred thoroughly and allowed to stand at room temperature
for 2 min, and then, the fluorescence spectrum was
recorded. For fluorescence intensity measurements, the
excitation and emission wavelengths were at 520 nm and
582 nm, respectively. The slit width is 5 nm/5 nm.
General Procedure
Synthesis of Compound RHBI
Typically, 3.0 mL of the solution of RHBI (10 μM) was
placed in a quartz cell (10.0 mm width), and the appropriate
A solution of rhodamine B hydrazide (1.50 g, 3.29 mmol)
in methanol (30 mL) was stirred, and the solution of 2-
Fig. 4 Fluorescence titration of
RHBI (10μM) with Fe3+ in
CH3CN with the increasing of
Fe3+ concentration. Inset:
Variation of fluorescence
intensity at 582 nm with
increasing Fe3+ concentration

J Fluoresc (2011) 21:2005–2013
2009
Fig. 5 Job’s plots according to
the method for continuous
variations. The total
concentration of RHBI and
Fe3+ is 10μM
formylbenzimidazole (0.48 g, 3.29 mmol) in methanol
(30 mL) was added dropwise. The solution was refluxed for
24 h, and then the solvent was evaporated. The crude product
was purified by the silica gel column chromatography using
CH2Cl2/CH3OH = 10:1 as eluent, obtaining pure pale yellow
product (1.23 g, 64% yield). 1H NMR (CDCl3, 400 MHz) δ:
1.15 (t, J = 7.00 Hz, 12 H), 3.31 (q, J=7.00 Hz, 8 H), 6.23
(d, J=8.22 Hz, 2 H), 6.44 (s, 2 H), 6.54 (d, J=8.22 Hz, 2 H),
7.10 (d, J=7.44 Hz, 2 H), 7.18–7.23 (m, 2 H), 7.39 (d, J=
7.56 Hz, 1 H), 7.43–7.52 (m, 2 H), 7.66 (d, J=7.56 Hz, 1 H),
8.00–8.02 (m, 2 H), 10.46 (s, 1 H). 13C NMR (CDCl3,
100 MHz) δ: 12.6, 38.0, 65.8, 98.4, 102.0, 108.2, 123.6,
123.7, 126.6, 127.0, 127.3, 128.3, 128.4, 134.3, 148.9,
152.6, 153.3, 154.5, 166.2. MALDI-TOF: calcd: 585.3,
found: 585.5.
condensation of 1, 2-phenylenediamine with glycolic acid
at 100 °C in hydrochloric acid (6 mol/L) with a yield of
72% [43], and then the hydroxyl was oxidized with oxygen
in the presence of activated MnO2 to obtain compound 2
with a yield of 85% [44]. The probe molecule RHBI was
prepared by the classical Schiff base condensation reaction
of rhodamine B hydrazide [25] with compound 2 in
absolute methanol with a yield of 64%. The molecular
1
structure of RHBI was confirmed by H NMR, 13C NMR
and MALDI-TOF MS (seeing supporting information).
Moreover, the Schiff base was observed to be stable in
acetonitrile solution for at least 2 weeks.
A solution of compound RHBI in acetonitrile was
weakly fluorescent, indicating the spirolactam form exists
predominantly. Meanwhile, the characteristic peak near
65.8 ppm (9-carbon) in the 13C NMR spectrum also
supports the predominant existence of spirolactam form
[19]. Upon the addition of 10 equiv. of Fe3+ to the solution
of RHBI, a 1098-fold enhancement of fluorescence
intensity and a continuous red shift of the maximum emission
peak from 580 to 582 were observed. In other words, the free
RHBI was non-emissive, but its fluorescence can turn from
“off” to “on” when the Fe3+ was added, resulting in a 1098-
Results and Discussion
Synthesis and Spectral Characteristic of Probe RHBI
Probe RHBI was readily synthesized by the route as
outlined in Scheme 1. Compound 1 was prepared by the
Fig. 6 Possible binding mode
of Fe3+ with RHBI

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J Fluoresc (2011) 21:2005–2013
Fig. 7 Benesi-Hildebrand linear
analysis plot of RHBI at
different Fe3+ concentration
fold enhancement of fluorescent intensity. For a practical
application, the stability of the RHBI solution in different
pH was also investigated. As shown in Fig. 1, the solution of
probe RHBI (CH3CN:H2O = 95:5) was stable at a wide pH
span 4–14. The fluorescent enhancement factor (FEF) of the
solution was less than 20-fold at the pH span, indicating the
probe was stable at this condition. However, the fluorescence
enhancement was more than 250-fold between pH 1 and 3,
indicating the partial opening of rhodamine ring at a strong
acidic condition [27]. On the other hand, in the presence of
Fe3+, the fluorescence enhancement of RHBI solution was
nearly 826-fold between pH 4 and 11. The results showed
that probe RHBI could detect Fe3+ smoothly with a wide pH
span (4–11) because in this region RHBI with Fe3+ ion
could induce a remarkable fluorescence enhancement,
whereas RHBI without the Fe3+ ion did not lead to such
change.
Selectivity Studies
To investigate the selectivity, representative ions such as
Fe3+, Pb2+, Ni2+, Co2+, Mn2+, Zn2+, Hg2+, Cd2+, Ag+, Mg2+,
Fig. 8 Plot of fluorescence
intensity versus the
concentration of Fe3+

J Fluoresc (2011) 21:2005–2013
2011
Ca2+, Ba2+, Na+ and K+ were added to the solution of RHBI
in acetonitrile. The changes in fluorescence spectra of RHBI
(10μM) upon the addition of different metal ions (10 equiv.)
were shown in Fig. 2. The fluorescence spectrum of RHBI
(10μM) solution showed only a very weak fluorescence
above 500 nm, which was ascribed to the trace ring-opened
form of it [19]. Upon the addition of Fe3+ (10 equiv.), the
fluorescence intensity of solution was significantly enhanced
(1098-fold) at 582 nm (Fig. 2B), suggesting the clear
formation of the ring-opened amide form of RHBI [26].
However, upon the addition of other metal ions, the
fluorescence enhancement was lower than 50-fold, except
Ag+ (107-fold) and Hg2+ (95-fold), which was only 1/10 and
1/12 compared with the fluorescence enhancement of Fe3+.
Therefore, compound RHBI can detect Fe3+ with high
selectivity. Moreover, the competition experiment was also
investigated by adding Fe3+ (10 equiv.) to the solution of
probe RHBI (10μM) in the presence of other metal ions
(10 equiv.). As shown in Fig. 3, apparently, other metal
ions had no interference in the detection of Fe3+ (relative
error ≤ 5%), except that Cu2+ has very mild interference
(relative error = 17%), which made it feasible for the
practical applications.
On the basis of 1:1 stoichiometery, the binding constant of
the complex was calculated by the linear Benesi-Hildebrand
equation as the following expression: [45]
I0
I0
I0
¼
þ
I ꢀ I0 K½Lꢁ½Fe3þ
ꢁ
½Lꢁ
Where I is the fluorescence intensity at 582 nm; K is the
binding constant; [L] and [Fe3+] are the concentrations of
RHBI and Fe3+, respectively. I and I0 are the fluorescence
intensities of the solution of RHBI in the presence and
absence of Fe3+. On the basis of the plot 1/ΔI versus 1/[Fe3+],
the binding constant was calculated to be 1.01×104 M−1 (R=
0.99491) (Fig. 7). Meanwhile, the fluorescence intensity is
linear with Fe3+ concentration in the range of 6×10−6–4×
10−5 M, The linear regression equation was determined to be
F ¼ 93:77 ꢂ 106 ꢂ ½Fe3þꢁ ꢀ 56:95ðn ¼ 9; R ¼ 0:9976Þ (Fig. 8).
The detection limit was calculated to be 1.5×10−8 M with the
equation: detection limit = 3Sd/ρ, where Sd is the standard
deviation of blank measurement; ρ is the slope between the
fluorescence intensity versus Fe3+ concentration [46].
Conclusion
Stoichiometry and Binding Mode Studies
In conclusion, a novel fluorimetric probe RHBI was designed
and synthesized by the condensation of rhodamine B
hydrazide and 2-formylbenzimidazole. Studies showed that
RHBI exhibited highly selective and sensitive to Fe3+ over
other metal ions with a fluorescence turn on effect.
Meanwhile, common metal ions showed negligible detection
interference except Cu2+ with some extent. The future efforts
will be focused on the structure modification of the probe for
improving its water solubility so that it can also be operated
in aqueous solution for possible biological application.
To elicit the interactions between RHBI and Fe3+, the
fluorescence spectral variation of RHBI (10μM) in
acetonitrile was titrated with different concentration of Fe3+.
As shown in Fig. 4, upon the addition of Fe3+, the
fluorescent intensity gradually increased and the maxi-
mum emission wavelength red-shifted from 580 nm to
582 nm, indicating the coordination of Fe3+ and RHBI
[11]. The data of Job’s plot using a total concentration of
1×10−5 M RHBI and Fe3+ in acetonitrile exhibited a
maximum fluorescence signal when the mole fraction of
compound RHBI was closed to 50% (Fig. 5), indicating a
1:1 stoichiometric ratio between RHBI and Fe3+. Mean-
while, ESI-MS showed one peak at m/z = 425.2 for
Supporting Information
The original 1H NMR, 13C NMR and MALDI-TOF spectra
of RHBI and the related spectroscopic data are shown in
the supporting information.
–
[RHBI + ClO4 + Fe3++6H2O]2+ (calcd 425.1) also
supporting this 1:1 stoichiometery (Figure S1). On the
basis of the 1:1 stoichiometery, carbonyl O and the two sp2 N
atoms (one from benzimidazole and the other from imine)
are the most possible binding site for Fe3+. IR analysis of
RHBI showed that both amide carbonyl and C = N
absorption at 1715 cm−1 and 1634 cm−1 were shifted to
lower frequency (1589 cm−1) upon the addition of Fe3+
(Figure S2). This indicated that the carbonyl O and imine N
coordinated with Fe3+. Moreover, upon the addition of
EDTA to the solution of RHBI containing Fe3+ led to the
disappearance of the fluorescence of RHBI-Fe3+, indicating
that the chelation process is reversible (Figure S3). The
possible coordination mode is shown in Fig. 6.
Acknowledgements This work was supported by National Science
Foundation of China (No. 20901063), Wuhan Chenguang Scheme
(Grant 201050231049) established under Wuhan Science and Tech-
nology Bureau, the Open Research Fund of Key Laboratory for Green
Chemical Process of Ministry of Education, Wuhan Institute of
Technology under grant (No. GCP201003).
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