A rhodamine-based off-on fluorescent chemosensor for selectively sensing Cu(II) in aqueous solution

Article abstract of DOI:10.1007/s10895-010-0698-x

A novel chromogenic and fluorogenic chemosensor RhB-pMOSal comprising a rhodamine fluorophore and a salicylaldehyde receptor being connected by an iminohydrazine link was synthesized and fully characterized. Its sensing behavior toward various metal ions

Full text of DOI:10.1007/s10895-010-0698-x

J Fluoresc (2011) 21:141–148
DOI 10.1007/s10895-010-0698-x
ORIGINAL PAPER
A Rhodamine-Based Off–On Fluorescent Chemosensor
for Selectively Sensing Cu(II) in Aqueous Solution
Ruiren Tang & Kang Lei & Ke Chen & Hua Zhao &
Jingwen Chen
Received: 7 April 2010 /Accepted: 22 June 2010 /Published online: 20 July 2010
#
Springer Science+Business Media, LLC 2010
.
.
.
Abstract A novel chromogenic and fluorogenic chemo-
sensor RhB-pMOSal comprising a rhodamine fluorophore
and a salicylaldehyde receptor being connected by an
iminohydrazine link was synthesized and fully character-
ized. Its sensing behavior toward various metal ions in
neutral aqueous solution was investigated by absorption
and fluorescence spectroscopy. RhB-pMOSal exhibited a
reversible and sensitive “turn-on” response of absorption
Keywords Aqueous solution Copper(II) Chemosensor
.
Fluorescence sensing Rhodamine derivative
Introduction
The design and construction of chemosensory reagents for
probing specific analytes, especially the biologically and
environmentally-important species, with high selectivity
and sensitivity are still of a hot research subject in the
communities of chemistry, biochemistry, biology, material
science and others [1–9]. Fluorescent sensing, which
translates molecular binding events into tangible fluores-
cence signals, has received much attention in this field [1–
9]. Rhodamine-based derivatives are excellent candidates
for construction of fluorescent chemosensors due to their
well-known spectroscopic properties of large molar extinc-
tion coefficient, high fluorescence quantum yield, longer
wavelength excitation, and good photostability [6, 10]. The
rhodamine moiety in a free molecule takes a spirolactam
ring-closed form, which usually shows little absorption and
nonfluorescence. However, it transforms to ring-opened
amide form upon binding with a specific species, the
chelating or reaction of metal ions, which usually becomes
highly absorbent and fluorescent [11–17]. Based on such
structural character, many off–on-type chromogenic or
fluorogenic rhodamine-based chemosensors and reagents
for metal ions were fabricated in the past few years [11, 12,
2
+
and fluorescence toward Cu in aqueous acetonitrile
solution. Approximate 65 and 6-fold enhancement in the
absorbance at 556 nm and fluorescence intensity at 573 nm
2
+
were estimated when equivalent Cu was added to the
RhB-pMOSal solution. Under the same conditions, RhB-
pMOSal displayed more sensitive than a reported analogue
2
+
RhB-Sal to Cu ion. The competition experiments for
2
+
Cu mixed with common metal ions exhibited no obvious
3
+
change in absorption and emission except Cr ion that can
induce the fluorescence quenching of RhB-pMOSal to
some extent.
Electronic supplementary material The online version of this article
(
doi:10.1007/s10895-010-0698-x) contains supplementary material,
which is available to authorized users.
:
:
R. Tang K. Lei K. Chen
School of Chemistry and Chemical Engineering,
Central South University,
Changsha 410083, People’s Republic of China
H. Zhao
1
4–16, 18–31]. Among them, considerable attention has
been paid to the colorimetric/fluorescent sensing of Cu
Product Quality Supervision and Inspection of Yancheng,
Yancheng 224005, People’s Republic of China
2
+
ion [15, 19, 20, 23–25, 28, 29] due to its known important
role in the biological, environmental, and chemical systems
J. Chen (*)
School of Chemical and Biological Engineering,
Yancheng Institute of Technology,
Yancheng 224051, People’s Republic of China
e-mail: jwchen@ycit.edu.cn
[
32, 33]. Tong et al. [21] once reported a salicylaldehyde
rhodamine B hydrazone (RhB-Sal) that can selectively
2
+
recognize Cu ion in neutral aqueous solution. In this

1
42
J Fluoresc (2011) 21:141–148
paper, an analogue of RhB-Sal, in which an electron-
and column chromatography were purchased from Qingdao
Ocean Chemicals (Qingdao, China). All metal salts used in
the spectroscopic experiments were obtained from Shang-
hai Chemical Reagent Corporation (Shanghai, China) and
used without further purification. Acetonitrile in chromato-
graphic grade and newly double-distilled water were used
throughout the experiments as solvents. Aqueous Tris-HCl
(50 mmol L )-NaCl (0.10 mol L ) solution was used as
buffer to keep pH value (pH 7. 0), and to maintain the ionic
strength of all solutions in experiments. The stock solution
donating group −OCH was introduced to the 4-position of
3
2
-hydroxyphenyl moiety, was synthesized (RhB-pMOSal,
Scheme 1). Its sensing behavior toward various metal ions
in aqueous solution was investigated by means of absorp-
tion and fluorescence spectroscopy. The results exhibited
that RhB-pMOSal displayed more sensitive and selective
2
+
−1
−1
response than RhB-Sal to Cu ion under the same
conditions.
2
+
−3
−1
of Cu and other metal cations (1.0×10 mol L ) were
prepared from chloride salts using acetonitrile, respectively.
Experimental
−
4
−1
The stock solution of RhB-pMOSal (1.0×10 mol L )
was prepared by dissolving the accurately weighed RhB-
pMOSal in acetonitrile. The working solution of RhB-
pMOSal (10 μmol L ) was prepared by diluting stepwise
the stock solution with Tris buffer/acetonitrile (1/1, v/v).
Apparatus and chemicals
−1
The absorption and the fluorescence spectra were measured
on a Shimadzu UV-2450 spectrophotometer and a JASCO
FP-6500 spectrofluorometer equipped with a thermostated
cell compartment, respectively. A 1.0 cm quartz cuvette in a
volume of 3.0 mL was used for all spectra collection. The
pH measurements were carried out on a PHS-3C Exact
Digital pH meter equipped with Phonix Ag-AgCl reference
electrode (Cole-Paemer Instrument Co.), which was cali-
brated with standard pH buffer solutions. The mass spectra
General methods
An aliquot of CuCl2 stock solution was continuously
syringed into 3 mL of RhB-pMOSal working solution in
quartz cuvette. After full mixture and 5 min equilibration,
the absorption spectra were collected by scanning the
solution within 400–650 nm. The fluorescence spectra were
recorded by the same procedures with excitation light of
520 nm. The slit width of the excitation and the emission
were both set at 5 nm. The volume change for all
spectroscopic experiments did not exceed 1%. All experi-
ments were run in triplicate, and the average values were
were obtained on an LCQ electron spray mass spectrometer
1
(
ESMS, Finnigan). The H NMR spectra were recorded on
a Bruker DRX-500 spectrometer with tetramethylsilane
TMS) as an internal standard. All of the measurements
(
were performed at about 298.0±0.2 K. Elementary analysis
for C, H, N was performed on a Perkin-Elmer 240C
analytic instrument.
2
+
adopted. The stoichiometry of RhB-pMOSal and Cu was
determined by Job’s method from the obtained absorption
spectroscopic data, which was verified by ESI-MS results
and Benesi-Hildebrand method [34]. In the determination,
Rhodamine B was purchased from TDI, and 2-hydroxy-
4
(
-methoxybenzaldehyde, 2-hydroxybenzaldehyde and Tris
hydroxymethyl)aminomethane (Tris) were from Sigma-
Aldrich. Silica gel (300–400 mesh) that used for thin layer
2
+
the sum of concentration of Cu and RhB-pMOSal was
−
1
2+
kept at 40 μmol L and the molar ratio of Cu was
changed from 0 to 1.0. Each solution of RhB-pMOSal/Cu
(II) was prepared by adding appropriate volume of RhB-
O
O
.
HCl
O
.
N NH2
H2N NH2 H2O
2
+
pMOSal and Cu solution into a 10 mL volumetric flask,
and diluting to the scale mark with the Tris buffer and then
mixing fully.
N
O
N
N
O
N
Rhodamine B
Rhodamine B hydrazide
OCH3
Syntheses
HO
OH
OHC
CHO
Compound RhB-pMOSal was prepared by coupling
rhodamine B hydrazide [35] with 2-hydroxy-4-methoxy-
benzaldehyde referring the reported procedures for RhB-
Sal [21] with some modifications (Scheme 1). To 20 mL
anhydrous ethanol containing rhodamine B hydrazide
(0.23 g, 0.5 mmol), an excessive 2-hydroxy-4-methoxy-
benzaldehyde (0.6 mmol) was added and the mixture was
stirred vigorously at room temperature for 24 h. The
reaction progress was monitored by thin-layer chromatog-
HO
HO
O
O
OCH3
N
N
N N
N
O
N
N
O
N
RhB-pMOSal
RhB-Sal
Scheme 1 Synthesis of RhB-pMOSal and RhB-Sal

J Fluoresc (2011) 21:141–148
143
raphy (TLC). After completion of the reaction, the formed
precipitate was filtered, washed with cold ethanol (3×
H), 10.85 (1H, d, Phen-OH) (Supporting Information,
Figure S4). Elemental analysis. found (calcd.) (%): C, 74.93
(74.98); H, 6.43 (6.47); N, 9.95 (9.99).
1
0 mL) and then dried in vacuum, affording 0.17 g crude
product, which was further purified by silica gel column
chromatography using petroleum ether/ethyl acetate (2/1, v/
v) containing 1% (v/v) triethylamine as eluent. 0.16 g of
The intermediate rhodamine B hydrazide was prepared
referring to the literature [34] with some modifications as
follows. To a vigorously stirred solution of rhodamine B
(1.20 g, 2.6 mmol) in 100 mL anhydrous ethanol at room
temperature, 5.0 mL of hydrazine hydrate (80%) was added
dropwise. After the addition, the mixture was heated to
reflux in an oil bath. When the reaction finished, the
mixture was cooled to room temperature and the solvent
was removed under reduced pressure. Dichloromethane
(30 mL) and water (30 mL) were added to dissolve the
resultant solid. The organic layer was collected, washed
twice with water and dried over anhydrous Na SO .
RhB-pMOSal as white powder solid was obtained, yield:
+
6
1.0%. ESI-MS for C H N O : m/z, calcd. (M + H )
3
6 38 4 4
5
91.29, found 591.42 (100%) (Supporting Information,
1
Figure S1). H NMR (CDCl ): δ (ppm) 1.18 (12H, t,
3
NCH CH ), 3.34 (8H, q, NCH CH ), 3.76 (3H, s,
2
3
2
3
−
OCH ), 6.27 (2H, d, Xanthene-H), 6.38 (2H, s,
3
Xanthene-H), 6.48–6.51 (4H, t, Phen-H), 7.02 (1H, d,
Phen-H), 7.18 (1H, d, Phen-H), 7.54 (2H, t, Ar-H), 7.99
(
1H, d, Ar-H), 9.23 (1H, s, N=C-H), 11.084 (1H, s, Phen-
2
4
OH) (Supporting Information, Figure S2). Elemental
analysis. Found (calcd.) (%): C, 73.23 (73.20); H, 6.43
Filtration of sodium sulfate and evaporation of the solvent
gave crude solid product, which was further purified by
silica gel column chromatography using petroleum ether/
ethyl acetate (1/1, v/v) containing 1% (v/v) triethylamine
as eluent. White solid was obtained in 74.0% yield. ESI-
(
6.48); N, 9.41 (9.48).
RhB-Sal that used as control was synthesized according
to the above method using rhodamine B hydrazide and 2-
hydroxybenzaldehyde as raw materials. After further
purification of the crude product by silica gel chromatog-
raphy using the same eluent, a white powder was obtained
in 71.4% yield. ESI-MS for C H N O : m/z, calcd. (M+
+
MS for C H N O : m/z, calcd. (M+H ) 457.25, found
2
8 32 4 2
+
457.58 (4 %); (2 M+Na ) 935.17, found 935.58 (100%)
(Supporting Information, Figure S5). H NMR (CDCl ): δ
1
3
(ppm) 1.18 (12H, t, NCH CH , J=7.0 Hz), 3.36 (8H, q,
3
5
36
4
3
2
3
+
+
H ) 561.28, found 561.40 (90%); (2 M+Na ) 1143.38,
found 1143.25 (100%) (Supporting Information, Figure
S3). H NMR (CDCl ): δ (ppm) 1.18 (12H, t, NCH CH ,
NCH CH , J=7.0 Hz), 3.63 (2H, bs, NH ), 6.30 (2H, dd,
2 3 2
Xanthene-H, J =9.0 Hz, J =2.4 Hz), 6.44(2H, d,
1
2
1
Xanthene-H, J=2.4 Hz), 6.48 (2H, d, Xanthene-H, J=
9.0 Hz), 7.13 (1H, dd, Ar-H, J =5.4 Hz, J =3.3 Hz), 7.46
3
2
3
J=7.1 Hz), 3.33 (8H, q, NCH CH , J=7.1 Hz), 6.28 (2H,
2
3
1
2
dd, Xanthene-H, J =8.9 Hz, J =2.2 Hz), 6.48 (2H, d,
(1H, d, Ar-H, J=3.3 Hz), 7.47 (1H, d, Ar-H, J=3.3 Hz),
7.96 (1H, dd, Ar-H, J =5.4 Hz, J =3.3 Hz). (Supporting
Information, Figure S6). Elemental analysis. found
(calcd.) (%): C, 73.63 (73.66); H, 7.03 (7.06); N, 12.24
(12.27).
1
2
Xanthene-H, J=2.2 Hz), 6.50 (2H, d, Xanthene-H, J=
.9 Hz), 6.78 (1H, dd, Phen-H), 6.85 (1H, d, Phen-H), 7.10
1H, d, Phen-H), 7.17 (1H, s, Phen-H), 7.19 (1H, d, Ar-H),
1
2
8
(
7
.51 (2H, m, Ar-H), 7.98 (1H, d, Ar-H), 9.26 (1H, d, N=C-
Fig. 1 Cu +-induced variations of the absorption (a) and the fluorescence spectra (b) of RhB-pMOSal in 50 mmol L−1 Tris-buffer/acetonitrile (1/
2
1
, v/v, pH 7.0) solution

1
44
J Fluoresc (2011) 21:141–148
Fig. 2 Spectral response of RhB-pMOSal and RhB-Sal to Cu in 50 mmol L−1 Tris-buffer/acetonitrile (1/1, v/v, pH 7.0) solution
2
+
–
1
Results and discussion
RhB-pMOSal (10 μmol L ) shows only a very weak
absorption and emission (excitation at 520 nm), which is
ascribed to its spirolactam form dominating in the solution
Fluorescence response of RhB-pMOSal to solution pH
[
6]. The characteristic absorption band appears in the range
Rhodamine-based compounds are usually sensitive to
solvent character and solution pH, etc. [17]. A suitable pH
scope in which RhB-pMOSal is capable of acting as a
chemosensor for metal ions was firstly investigated. As
shown in Figure S7, with the solution acidity increasing,
the fluorescence of RhB-pMOSal (excited at 520 nm) in
of 450–600 nm with a l of 556 nm (ε=3.37×
max
2
−1
−1
2+
10 L mol cm ). Upon addition of Cu to the RhB-
pMOSal solution, the absorption enhanced dramatically
accompanied by an appearance of a new shoulder peak at
ca 521 nm. The absorbance increased gradually with
2
+
increasing Cu concentration and the solution color turned
instantaneously from colorless to clear pink (Fig. 1a, inset).
Similar to the absorption response, a significant en-
hancement of fluorescence corresponding to the delocaliza-
tion in the xanthenes moiety of rhodamine [37] occurred
(Fig. 1b). Meanwhile, the maximum emission wavelength
underwent a slight red shift from 573 to 576 nm. When the
5
0% aqueous Tris-buffered acetonitrile gradually enhanced
along with a clear color changes from colorless to pink.
Within the pH values of 10.0 to 7.0, the fluorescence and
the solution color could hardly be observed, which suggests
that the rhodamine moiety of RhB-pMOSal adopted a
spirocyclic form and this tautomer was insensitive to such
pH span. When pH was lower than 7.0, a remarkable
fluorescence enhancement accompanied by appearance of
pink was observed, which implies that the spirolactam ring
of RhB-pMOSal was opened due to protonation [36]
(
Supporting Information, Figure S7). Considering the
higher pH range could lead to hydrolysis for transition
metal ions, the proper pH span for RhB-pMOSal to sense
metal ions in aqueous solution was selected to be ca 7.0.
An aqueous Tris buffer with pH value of 7.0 was used
throughout the experiments. The ESI-MS result for RhB-
pMOSal stayed in neutral Tris-buffered acetonitrile solu-
tion for 6 days indicates this compound is highly stable
(
Supporting Information, Figure S8).
Absorption and fluorescence variation of RhB-pMOSal
2
+
in the presence of Cu
The electronic absorption and fluorescence spectra of RhB-
pMOSal in Tris-buffer/acetonitrile (1/1, v/v, pH 7.0)
solution are presented in Fig. 1. As shown in the figure,
Fig. 3 Job’s plot evaluated from the absorption spectra of RhB-
pMOSal and Cu at 556 nm in 50 mmol L Tris-buffer/acetonitrile
(1/1, v/v, pH 7.0) solution
2+
−1

J Fluoresc (2011) 21:141–148
Fig. 4 ESI-MS spectrum of
145
−
1
RhB-pMOSal (10 μmol L
)
2+
−1
and Cu (10 μmol L ) in
5
0 mmol L− Tris-buffer/aceto-
1
nitrile (1/1, v/v, pH=7.0)
solution
2
+
−1
2+
2+
2+
concentration of Cu reached to 10 μmol L , approximate
5 and 6-fold enhancements in the absorption (at 556 nm)
excess amount of Cu ; [Cu ] is the concentration of Cu
−1
6
ion added (mol L ).
2
+
and fluorescence (at 576 nm) were observed, respectively,
which were much more sensitive than the known com-
pound RhB-Sal for Cu (ca 30 and 2-fold, respectively)
Plotting of 1/(A–A ) versus 1/[Cu ] showed a liner
0
relationship (Fig. 5), which suggests that RhB-pMOSal
bound with Cu in a 1:1 stoichiometry. The association
2
+
2+
under the same conditions (Fig. 2). The spectroscopic
constant (K ) was determined from the slope to be 3.09×
10 L mol .
a
−1
2
+
4
variations suggest that Cu ion coordinated to RhB-
pMOSal, resulting in the spirolactam ring open and
concomitant formation of RhB-pMOSal-Cu(II) complex.
Similar to the previously reported [21], copper(II) could
chelate with the carbonyl oxygen, imino nitrogen, and
phenol oxygen of RhB-pMOSal. The sensitivity difference
between RhB-pMOSal and its analogue RhB-Sal for Cu
Moreover, when an appropriate amount of EDTA was
added to the RhB-pMOSal-Cu(II) solution, an instant color
change from pink to almost colorless and a simultaneous
fluorescence quenching were observed. The ESI-MS results
provided a direct evidence that RhB-pMOSal molecule
was liberated from the RhB-pMOSal-Cu(II) complex
(Supporting Information, Figure S9). In Fig. 4, there are
(
II) may be ascribed to the additional methoxy group at
para-position of benzene ring of RhB-pMOSal, which
could enhance the conjugation effect, and thus enhance the
binding capacity of RhB-pMOSal to Cu(II). Fitting of the
Job’s plot evaluated from the absorption spectra of RhB-
2
+
pMOSal and Cu
at 556 nm gave rise to a 1:1
stoichiometry for the complex (Fig. 3), which was
confirmed by the ESI-MS result (Fig. 4) and the Benesi-
Hildebrand method [34].
When assuming a stoichiometry of 1:1 for RhB-pMOSal-
Cu(II), the association constant (K ) of RhB-pMOSal with
a
2
+
Cu was determined using the Benesi-Hildebrand equation
as follow.
1
1
1
¼
þ
ð1Þ
A ꢀ A0
KaðAmax ꢀ A0Þ½Cu2þꢁ Amax ꢀ A0
A and A is the absorbance of RhB-pMOSal solution in
0
Fig. 5 Benesi-Hildebrand plot (absorbance at 556 nm) of RhB-
pMOSal using eq. 1, assuming 1:1 stoichiometry for association
between RhB-pMOSal and Cu
2
+
the presence and absence of Cu , respectively; A
is the
max
2
+
saturated absorbance of RhB-pMOSal in the presence of

1
46
J Fluoresc (2011) 21:141–148
2
+
Fig. 6 Absorption spectra of RhB-pMOSal in the presence of different metal ions (a) and the effect of the mixed metal ions on the Cu -induced
absorption of RhB-pMOSal (b)
2
+
three peaks at m/z of 591.30, 652.20 and 773.30
sensing Cu , common metal ions including alkali, alkaline
earth, and transition-metal ions were investigated under the
+
+
+
corresponding to [M + H ] (calcd. 591.29), [M − H +
2
+ +
+
2+
+
−1
Cu ] (calcd. 652.21), and [M − H + Cu + Tris] (calcd.
73.28), respectively. While in Figure S9, the intensity of
same conditions (50 mmol L Tris-buffer/acetonitrile
solution, 1/1, v/v, pH 7.0). When each of the tested metal
ions such as Li , K , Mg , Ca , Sr , Ba , Al , Fe ,
Fe , Co , Ni , Zn , Hg , Cd , Cr , etc (30 μmol L
for each) was added to the RhB-pMOSal solution,
respectively, its absorbance and color hardly changed. Only
ferrous ion (Fe ) led to a faint pink and a weak absorption
enhancement. However, a strong absorption around 556 nm
together with a shoulder peak around 521 nm was observed
when 10 μmol L Cu was syringed into the RhB-
pMOSal solution containing above mixed metal ions
7
+
+
2+
2+
2+
2+
3+
3+
peaks at m/z=652.20 and 773.30 decreased dramatically
whereas the peak at m/z=591.30 increased. These results
indicated clearly that the binding of RhB-pMOSal with
2
+
2+
2+
2+
2+
2+
3+
−1
2
+
Cu and the subsequent spirolactam ring opening are a
2
+
reversible process, and furthermore, compound RhB-
2
+
pMOSal may act as a turn-on chemosensor for Cu in
aqueous solution.
−1
2+
2
+
Selective sensing of Cu ion in aqueous solution
(Fig. 6a). When each of the above metal ions was mixed
The selectivity is one of the essential requirements for a
chemosensor to signal a specific species in a complex
system. To validate the selectivity of RhB-pMOSal for
and reversely added to the RhB-pMOSal solution contain-
−
1
2+
ing 10 μmol L
Cu , only slight enhancement in
absorption but no further color changes was observed
Fig. 7 Effect of various metal ions on the fluorescence of RhB-pMOSal in Tris-buffered acetonitrile (1/1, v/v, pH=7.0) solution

J Fluoresc (2011) 21:141–148
147
Acknowledgments Financial supports from the Natural Science
Foundation of Jiangsu Province (Grant BK2007069) and the Qing
Lan Project sponsored by Jiangsu Province (2008) are gratefully
acknowledged.
(
Fig. 6b), which indicates a good selectivity of RhB-
2
+
pMOSal for Cu ion in aqueous solution. Such distinct
effect was so substantial that it could act as a naked-eye
chemosensor for Cu .
2
+
The results obtained from the fluorimetric response of
RhB-pMOSal to all above metal ions corroborated its
2
+
selectivity for Cu . Figure 7 presents the fluorescence
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