A rhodamine-based "turn-on" fluorescent probe for Fe3+ in aqueous solution and its application in bioimaging

Article abstract of DOI:10.1007/s00706-015-1521-9

A novel colorimetric and off-on fluorescent chemosensor which contained a strong electron-withdrawing group was synthesized. As expected, the chemosensor exhibited high selectivity and excellent sensitivity in both absorbance and fluorescence detection of Fe³⁺ (Θ (CH₃OH, H₂O) = 3/7, 1 mM Tris-HCl buffer, pH = 7.40). Upon addition of Fe³⁺, the solution of the chemosensor resulted in a color change from colorless to obvious pink. These significant changes in color could be used for naked-eye detection. Furthermore, fluorescence imaging experiments of Fe³⁺ ions in living MGC-803 cells demonstrated its value of practical applications in biological systems.

Full text of DOI:10.1007/s00706-015-1521-9

Monatsh Chem
DOI 10.1007/s00706-015-1521-9
ORIGINAL PAPER
A rhodamine-based ‘‘turn-on’’ fluorescent probe for Fe3+
in aqueous solution and its application in bioimaging
Yao Liu1 Yaqi Liu1 Zhanhui Xu1 Linchao Yang2 Zhenji Wang2 Di Zhang1,3 Yong Ye1
•
•
•
•
•
•
Received: 16 March 2015 / Accepted: 22 June 2015
Ó Springer-Verlag Wien 2015
Abstract A novel colorimetric and off–on fluorescent
chemosensor which contained a strong electron-with-
drawing group was synthesized. As expected, the
chemosensor exhibited high selectivity and excellent sen-
sitivity in both absorbance and fluorescence detection of
Fe3? (H (CH3OH, H2O) = 3/7, 1 mM Tris–HCl buffer,
pH = 7.40). Upon addition of Fe3?, the solution of the
chemosensor resulted in a color change from colorless to
obvious pink. These significant changes in color could be
used for naked-eye detection. Furthermore, fluorescence
imaging experiments of Fe3? ions in living MGC-803 cells
demonstrated its value of practical applications in biolog-
ical systems.
Graphical abstract
Keywords Chemosensor Á Fe3? ion Á Enhancement Á
Naked eye Á Fluorescence imaging
Introduction
The determination of metal ions has received tremendous
attention in the last two decades, because the metal ions
play a significant role in the environment, biology, and
chemistry [1–5]. Among the biologically important tran-
sition metal ions, iron is the most abundant essential trace
element and plays a crucial role in many biological pro-
cesses, such as in cellular metabolism, in oxygen transport
mechanism, in many enzymatic reactions as cofactor, and
in enzyme catalysis [6–9]. The increase and decrease in the
concentration of intracellular Fe3? leads to many diseases
such as Huntington, cancer, malfunctioning of organs, etc.
[10–12]. Therefore, the detection of iron content in the
physiological environment is very important.
Electronic supplementary material The online version of this
article (doi:10.1007/s00706-015-1521-9) contains supplementary
material, which is available to authorized users.
& Di Zhang
pandy811@163.com
& Yong Ye
yeyong03@tsinghua.org.cn
1
Phosphorus Chemical Engineering Research Center of Henan
Province, The College of Chemistry and Molecular
Engineering, Zhengzhou University, Zhengzhou, China
2
School of Pharmaceutical Science, Zhengzhou University,
Zhengzhou, China
The rhodamine framework has attracted great interest
from chemists because of its excellent photophysical
properties, such as excellent quantum yields, high extinc-
tion coefficients, great photostability, and relatively long
3
Institute of Agricultural Quality Standards and Testing
Technology, Henan Academy of Agricultural Sciences,
Zhengzhou, China
123

Y. Liu et al.
emission wavelengths [13–15]. As is well known, rho-
damine derivatives having a spirolactam structure are non-
fluorescent and colorless, whereas ring opening of the
spirolactam gives rise to a strong fluorescence emission
and a pink color [16, 17]. Fe3? is a well-known fluores-
cence quencher due to its paramagnetic nature, which
makes it difficult to develop a sensitive turn-on fluorescent
sensor [18]. Recently, some fluorescence chemosensors for
Fe3? have been reported [19–23]. However, only a few
sensors for Fe3? have been reported despite its importance
in many biochemical processes at the cellular level [24–
28]. Herein, we report a new rhodamine-based ‘‘turn-on’’
fluorescent chemosensor 2 (Scheme 1) for Fe3?. Che-
mosensor 2 is synthesized in high yield by reacting
5-bromopyridine-3-sulfonyl chloride with rhodamine B
ethylenediamine (chemosensor 1) in dichloromethane.
Compared with chemosensor 1 (60-fold), chemosensor 2
exhibited higher fluorescent intensity (230-fold) for Fe3?
(H (CH3OH, H2O) = 3/7, 1 mM Tris–HCl buffer,
pH = 7.40) solution. At the same time, chemosensor 2
exhibited high absorbance and fluorescent signal behavior
toward Fe3? over other common interfering metal ions and
anions in H (CH3OH, H2O) = 3/7 (1 mM Tris–HCl buffer,
pH = 7.40) solution. Moreover, fluorescence imaging
experiments of Fe3? ions in living MGC-803 cells
demonstrated its value of practical applications in biolog-
ical systems.
50 min before measuring the absorption and fluorescence
in order to make the metal ions chelate with the sensors
sufficiently. Compounds 1 and 2 were colorless and found
very stable in the above-mentioned solution system for
more than 2 weeks.
Fluorescence spectral responses of 2 with Fe31
As shown in Fig. 1, 2 exhibited a very weak fluorescence
in the absence of metal ions. When 10 equiv. Fe3? was
introduced to a 10 lM solution of 2 in solution, a
remarkable enhancement of fluorescence spectra was
observed. Al3? also caused an emission peak, but its
intensity was far lower than that caused by Fe3?. Under the
same condition, Al3? showed no obvious change on color,
at the same time, other ions such as Li?, K?, Na?, Ca2?
Mg2?, Ba2?, Zn2?, Fe2?, Ag?, Mn2?, Cu2?, Co2?, Ni2?
,
,
Cd2?, Cr3?, and Pb2? showed no obvious changes on
fluorescence intensity and color.
We also confirmed the competitive experiments that the
background metal ions showed very low interference with
the detection of Fe3? in H (CH3OH, H2O) = 3/7 (1 mM
Tris–HCl buffer, pH = 7.40) (Fig. 2). These results also
demonstrated that compound 2 was a selective sensor for
Fe3? over various other metal ions. We also investigated
the fluorescence response of compound 2 toward FeCl3 in
the presence of various coexisting anions such as Br-, I-,
NO3-, SO42-, OAc-, HCO3-, and H2PO4-. It was grati-
fying to note that all the tested anions have no interference
with the fluorescence response of compound 2 toward Fe3?
(Fig. 3).
Results and discussion
Fluorescence and UV–Vis studies were performed using a
solution of 10 lM 1 and 2 in H (CH3OH, H2O) = 3/7
(1 mM Tris–HCl buffer, pH = 7.40) with a r = 10 of
metal ions vs. 1 and 2, respectively. Solutions were kept for
In order to investigate the influence of the different acid
concentration on the spectra of 2 and find a suitable pH
span in which 2 can selectively detect Fe3? efficiently, the
acid titration experiments were performed. As shown in
Scheme 1
Cl
N
N
O
N
N
O
CH3CH2OH
Reflux
+
NH2
H2N
Cl
N
COOH
NH2
O
1
O
S
Br
N
N
O
O
Br
N
O
S
N
O
N
H
r.t.
,
2
CH Cl
TEA,
2
N
O
2
123

A rhodamine-based ‘‘turn-on’’ fluorescent probe for Fe3? in aqueous solution and its…
Fig. 3 Fluorescence intensity (at 579 nm) of 2 (10 lM) upon the
addition of 100 lM FeCl3 in the presence of 100 lM background
anions in (CH3OH, H2O) = 3/7 (1 mM Tris–HCl buffer,
Fig. 1 Fluorescence spectra of 2 (10 lM) in H (CH3OH, H2O) = 3/7
(1 mM Tris–HCl buffer, pH = 7.40) solution with the presence of 10
equiv. of various species (kex = 535 nm, slit = 5 nm)
H
pH = 7.40) solution (kex = 535 nm, kex = 579 nm, slit = 5 nm)
Fig. 2 Fluorescence intensity (at 579 nm) of 2 (10 lM) upon the
addition of 100 lM Fe3? in the presence of 100 lM background
metal ions in H (CH3OH, H2O) = 3/7 (1 mM Tris–HCl buffer,
pH = 7.40) solution (kex = 535 nm, kex = 579 nm, slit = 5 nm)
Fig. 4 Fluorescence intensity (at 579 nm) of free 2 (10 lM) and in
the presence of 10 equiv. Fe3? in H (CH3OH, H2O) = 3/7 (1 mM
Tris–HCl buffer) solution with different pH conditions (kex = 535 -
nm, kex = 579 nm, slit = 5 nm)
Fig. 4, the fluorescent titration curve of free sensor 2 in H
(CH3OH, H2O) = 3/7 (1 mM Tris–HCl buffer) solution
did not show obvious characteristic color of rhodamine
between pH 4.0 and 10.0, suggesting that spirolactam
tautomer of 2 was insensitive to the pH changes in this
range. However, the addition of Fe3? led to the fluores-
cence enhancement over a comparatively wide pH range
(4.0–8.0), which is attributed to opening of the spirolactam
ring. Consequently, 2 may be used to detect Fe3? in similar
physiological conditions.
To determine the stoichiometry of the iron-ligand
complex, Job’s method for fluorescence measurement was
applied. As shown in Fig. 6, the fluorescence reached a
maximum when the ratio of [Fe3?]/{[Fe3?] ? [2]} was
0.5, indicating a 1:1 stoichiometry of the Fe3? to 2 in the
complex.
UV–Vis spectral responses of 2
Generally, one of the most important and useful appli-
cations for a fluorescence sensor is the detection of metal
ions. Under optimal conditions, the linear response for the
fluorescence intensity response was between 10 and 70 lM
(Fig. 5), and the detection limit of Fe3? was measured to be
0.195 ppm [29, 30].
As shown in Fig. 7, UV–Vis spectrum of 2 (10 lM)
exhibited only very weak bands over 450 nm, indicating
that the ring-closed spironolactone of 2 is predominant.
After addition of 10 equiv. Fe3? into the solution, a sig-
nificant enhancement of absorbance at about 568 nm was
observed immediately, along with an obvious color change
123

Y. Liu et al.
Fig. 5 The fluorescence intensity (at 579 nm) of compound 2
(10 lM) as a function of the Fe3? concentration (10–70 lM) in H
(CH3OH, H2O) = 3/7 (1 mM Tris–HCl buffer, pH = 7.40) solution
(kex = 535 nm, kex = 579 nm, slit = 5 nm)
Fig. 7 UV–Vis absorbance spectra of 2 (10 lM) in the absence and
presence of 10 equiv. different metal ions in H (CH3OH, H2O) = 3/7
(1 mM Tris–HCl buffer, pH = 7.40) solution. Inset shows the photo 2
with different metal ions
Fig. 6 Job’s plots of the complexation between
[Fe3?] ? [2] = 100 lM
2 .
and Fe3?
Fig. 8 Fluorescence intensity (at 579 nm) of 2 (10 lM) to Fe3? in H
(CH3OH, H2O) = 3/7 (1 mM Tris–HCl buffer, pH = 7.40) solution
(kex = 535 nm, slit = 5 nm)
from colorless to purple (Fig. 7, inset). Under the same
condition, no obvious response could be observed upon the
addition of other ions including Li?, K?, Na?, Ca2?
Mg2?, Ba2?, Zn2?, Fe2?, Ag?, Al3?, Mn2?, Cu2?, Co2?
,
Mechanism
,
Ni2?, Cd2?, Cr3?, and Pb2?. The results demonstrated that
2 can be served as a ‘‘naked eye’’ probe with a character-
istic of high selectivity toward Fe3? over other competitive
metal ions.
To further investigate the interaction of Fe3? and 2, an
ultraviolet titration experiment was carried out (Supple-
mentary Material, Fig. S4). Upon addition of increasing
concentrations of Fe3? ions to the solution, a new
absorption band centered at 568 nm appeared with
increasing intensity, which can be ascribed to the formation
of the ring-opened amide form of 2 upon Fe3? ions
binding.
Further, it was of great interest to investigate the reversible
binding nature of the sensor (Fig. 8). Upon addition of 10
equiv. K3PO4 to the solution of 2 and Fe3? (10 equiv.), the
fluorescence intensity at 579 nm was quenched (blue line) due
to the competitive binding of Fe3? from 2 by K3PO4, and
further addition of 10 equiv. Fe3? could recover the strong
fluorescence again (green line). These indicated that the coor-
dination of compound 2 with Fe3? is chemically reversible.
The IR spectra showed that after Fe3? was added, the
amide carbonyl peak changed from 1670 to 1685 cm-1, but
the sulfur carbonyl peak did not change (1614 cm-1
,
123

A rhodamine-based ‘‘turn-on’’ fluorescent probe for Fe3? in aqueous solution and its…
Scheme 2
N
N
O
N
N
O
FeCl3
Br
Br
O
S
O
S
N
3-
H
N
N
PO4
N
H
N
N
O
O
O
O
Fe
Cl
Cl
2
H
2O
Supplementary Material, Fig. S5). This supported that the
amide carbonyl of 2 is actually involved in the coordination
with metal ions.
To gain a clear understanding of the structure of 2-Fe3?
1
complex, H NMR spectroscopy was employed (Supple-
mentary Material, Fig. S6). Fe3? is a paramagnetic metal ion
and can affect the proton signals that are close to the Fe3?
binding site. The proton signals Ha, Hb, Hc, Hd, He, Hf, Hg,
Hh, and Hi are also shifted upfieldupon addition of Fe3?. This
indicated the opening of the spirolactam ring upon coordi-
nation to Fe3? with associated charge transfer in the
xanthene moiety. These observations show that Fe3? binds to
probe 2 through one oxygen atom, two amine nitrogenatoms,
and one nitrogen atom at a piperazine moiety.
Fig. 9 Fluorescence intensity (at 579 nm) of 1 and 2 (10 lM) in H
(CH3OH, H2O) = 3/7 (1 mM Tris–HCl buffer, pH = 7.40) solution
with the presence of Fe3? (10 equiv.) (kex = 535 nm, kex = 579 nm,
slit = 5 nm)
To further investigate the binding stoichiometry of 2 and
Fe3? ions, the form of 2-Fe3? complex was also confirmed
by ESI–MS analysis (Supplementary Material, Fig. S7).
When FeCl3 was added to solution of 2, the peak at m/z =
850.9 (calcd 849.1) corresponding to [2 ? Fe3? ? 2-
Cl- ? H2O]?. This also indicated the formation of a 1:1
metal–ligand complex. Herein, according to our knowledge
[31–33], we broached a conceivable mechanism of Fe3?
complex with 2 (Scheme 2).
cells were used for this experiment. MGC-803 cells were
incubated with 2 (20 lM) in RPMI1640 medium (con-
taining 10 % fetal bovine serum) for about 30 min at
37 °C, and as shown in Fig. 10b, very weak fluorescence of
2 inside the living MGC-803 cells was observed. After
then, 50 lM of Fe3? was then supplemented to the cells,
and all the reaction mixtures were incubated at 37 °C for
2 h. After incubated at 37 °C for 2 h later, a significant
increase in the fluorescence from the intracellular area was
observed (Fig. 10d). A bright-field transmission image of
cells with Fe3? and 2 confirmed that the cells were viable
throughout the imaging experiments (Fig. 10a, c). These
results indicated that 2 might be useful for detecting Fe3?
in biological samples.
As is well known [34, 35], the tosyl group which was a
strong electron acceptor can be introduced to strengthen the
acidity of the amide and further produce a stronger binding
ability with cations. Comparing to compound 1, 2 dis-
played a more fluorescence enhancement (Fig. 9) with
Fe3? in H (CH3OH, H2O) = 3/7 (1 mM Tris–HCl buffer,
pH = 7.40) solution. Under the same condition, the fluo-
rescence enhancement of Fe3? to 1 and 2 is as high as 60-
and 230-fold, respectively. These results indicated the
electron drawing group in the compound 2 displayed a
In summary, we synthesized an efficient rhodamine-
based fluorescent reversible chemosensor 2 for Fe3?
beneficial effect to the recognition effect of 2 with Fe3?
.
.
Chemosensor 2 exhibited good selectivity in H (CH3OH,
H2O) = 3/7 (1 mM Tris–HCl buffer, pH = 7.40) with
dramatically enhanced fluorescence intensities. The sig-
nificant changes in the fluorescence color could be used for
naked-eye detection. Moreover, it was applied for imaging
in MGC-803 cells to confirm that it can be used as a
Bioimaging application of compound 2 in MGC-830
cells [36, 37]
Bioimaging applications of compound 2 for monitoring of
Fe3? ions in living cells were then carried out. MGC-803
123

Y. Liu et al.
Fig. 10 Fluorescence images of
Fe3? in MGC-803 cells with
20 lM solution of 2 in
RPMI1640 medium (containing
10 % fetal bovine serum) for
30 min at 37 °C, bright-field
transmission images (a, c) and
fluorescence images (b, d) of
MGC-803 cells incubated with
0 lM of Fe3? for 30 min and
50 lM of Fe3? for 2 h
fluorescent sensor for monitoring Fe3? in living cells. It
may therefore be applicable as a rhodamine-based turn-on
type fluorescent chemosensor.
by Job’s method from the obtained fluorescence spectro-
scopic data. In the determination, the sum of concentration
of Fe3? and each compound was kept at 100 lM and the
molar ratio of Fe3? was changed from 0 to 1.0. Compound
1 was synthesized by reported methods [34].
Experimental
N-[2-[30,60-Bis(diethylamino)-3-oxospiro[1H-isoindole-
1,90-[9H]xanthen]-2(3H)-yl]ethyl]-5-bromopyridine-3-
sulfonamide (2, C35H38BrN5O4S)
Fluorescence spectra measurements were performed on a
HITACHI F-7000 fluorescence spectrophotometer, and the
excitation and emission wavelength band passes were both
set at 5.0 nm. Absorption spectra were measured on a
Lambda 35 UV/VIS spectrometer, Perkin Elmer precisely.
NMR spectra were measured on a Bruker DTX-400 spec-
trometer in CDCl3 with TMS as internal standard. Mass
spectral determination was carried on a HPLC Q-Tof HR-
MS spectrometer (Waters Micromass) using methanol as
mobile phase. IR spectra in NaCl disks were conducted
using a PE-1710 instrument.
To a solution of 188 mg compound 1 (0.39 mmol) and
79 mg triethylamine (0.78 mmol) in 10 cm3 anhydrous
CH2Cl2, 100 mg 5-bromopyridine-3-sulfonyl chloride
(0.39 mmol) mixed with 10 cm3 anhydrous CH2Cl2 was
added dropwise at 0 °C, and the resulting mixture was
stirred at room temperature for 10 h. After removal of
solvent, the residue was purified by silica gel column
chromatography (H (EA,PE) = 1:2, Rf = 0.3) to afford
the pure product 190 mg (69 %) of 2 as a faint yellow
foamy solid. M.p.: 192–194 °C; 1H NMR (400 MHz,
CDCl3): d = 1.19 (t, 12H, J = 7.0 Hz), 2.82 (dd, 2H,
J = 9.8 Hz), 3.25 (t, 2H, J = 4.8 Hz), 3.36 (q, 8H,
J = 7.0 Hz), 6.21 (m, 4H), 6.38 (s, 2H), 6.84 (s, 1H,
NH), 7.07 (m, 1H), 7.49 (m, 2H), 7.94 (m, 1H), 8.24 (t, 1H,
J = 2.0 Hz), 8.78 (d, 1H, J = 2.1 Hz), 8.88 (d, 1H,
J = 1.9 Hz) ppm; 13C NMR (100 MHz, CDCl3):
d = 12.62, 39.89, 43.43, 44.40, 97.82, 104.28, 108.24,
120.74, 123.02, 123.89, 128.30, 128.38, 128.84, 130.29,
133.03, 137.03, 138.22, 146.03, 148.96, 153.33, 153.84,
169.46 ppm; HR-MS: m/z calcd for C35H38BrN5O4S
All chemicals and reagents were used as received from
commercial sources without further purification. Solvents
for chemical synthesis and analysis were purified according
to standard procedures. Double distilled water was used
throughout the experiment. Chloride salts of metal ions
(Li?, K?, Na?, Ca2?, Mg2?, Ba2?, Zn2?, Fe2?, Fe3?
,
Al3?, Mn2?, Cu2?, Co2?, Ni2?, Cd2?, Cr3?) and the nitrate
salt of Pb2? and Ag? were used to evaluate the metal ion-
binding properties by synthesized compounds. The metal
ions were prepared as 10.00 mM in water solution. The
stoichiometry of each compound and Fe3? was determined
123

A rhodamine-based ‘‘turn-on’’ fluorescent probe for Fe3? in aqueous solution and its…
[M?H]? 704.1906, 706.1886, found 704.1903, 706.1888
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Acknowledgements We are grateful to the National Science
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