A rhodamine-based "off-on" colorimetric and fluorescent chemosensor for Cu(II) in aqueous and non-aqueous media

Article abstract of DOI:10.1007/s10895-014-1393-0

A new rhodamine derivative (RhB-NSal) bearing an electron-withdrawing group -NO₂ at the 5-position of 2-hydroxyphenyl moiety was synthesized and its sensing behaviors for Cu²⁺ in acetonitrile and aqueous acetate-buffer/acetonitrile (

Full text of DOI:10.1007/s10895-014-1393-0

J Fluoresc
DOI 10.1007/s10895-014-1393-0
ORIGINAL PAPER
A Rhodamine-Based “Off-On” Colorimetric and Fluorescent
Chemosensor for Cu(II) in Aqueous and Non-aqueous Media
Kun Dai & Baolian Xu & Jingwen Chen
Received: 11 February 2014 /Accepted: 7 April 2014
#
Springer Science+Business Media New York 2014
Abstract A new rhodamine derivative (RhB-NSal) bearing
an electron-withdrawing group –NO2 at the 5-position of 2-
hydroxyphenyl moiety was synthesized and its sensing be-
haviors for Cu2+ in acetonitrile and aqueous acetate-buffer/
acetonitrile (2/3, v/v, pH 4.8) media were investigated. In each
medium, significant absorption and fluorescence enhance-
ments accompanied by an instant color change from colorless
to pink were observed for RhB-NSal upon addition of Cu2+.
RhB-NSal binds with Cu2+ forming a 1:1 stoichiometric com-
plex with an association constant of 6.72 ( 0.03) × 104 M−1
and 4.23 ( 0.03) × 104 M−1, respectively. RhB-NSal
displayed high selectivity for Cu2+ over possibly competitive
metal ions except that Fe3+ and Bi3+ ion can respectively bring
about a little interference in absorption and fluorescence with
its sensing for Cu2+. In dry acetonitrile, pronounced enhance-
ments in the absorbance and emission of RhB-NSal were
induced by Cu2+, with a detection limit of 0.49 μM, exhibiting
higher sensitivity than that of a known analogue bearing no
substituent on its phenol ring, RhB-Sal. In aqueous solution,
RhB-NSal displayed likewise a high selectivity but a lower
sensitivity for Cu2+ than that in acetonitrile, with a detection
limit of 14.98 μM, still more sensitive than RhB-Sal in ab-
sorption. By virtue of these properties, RhB-NSal could be
used as an “Off-On” fluorescent and colorimetric
chemosensor for Cu2+ in acetonitrile medium, and be devel-
oped to be a promising candidate of “Off-On” eye-naked
chemosensor for Cu2+ in a weakly acidic aqueous medium.
.
.
.
Keywords Chemosensor Copper (II) Electronic effect
.
Rhodamine derivative Spectroscopic response
Introduction
The rhodamine-based framework has attracted considerable
attention in the fabrication of chemosensors or
chemodosimeters owing to its known excellent spectroscopic
properties [1–4] and ease of modification [5]. The spirolactam
ring of a rhodamine molecule may be opened due to a metal
ion chelation, protonation or a chemical reaction, which ac-
cordingly results in the remarkable variation of the molecule
in its absorption and fluorescence. Based on this mechanism,
many rhodamine derivatives have been developed as colori-
metric and fluorescent chemosensors or chemodosimeters for
a wide variety of metal ions and other interested species
during these years [5–15]. Among the metal ions, copper (II)
is a typical environmentally- and biologically-relevant metal
ion which plays a pivotal role in the field of biology, chemis-
try, environment and clinical diagnosis, etc. In this regard, it is
of great significance to construct more sensitive chemosensors
or chemodosimeters for detecting the trace amount of Cu2+
which possibly exists in different media. Hereunto, a number
of rhodamine-based Cu2+ chemosensors have been designed
and constructed [16–23].
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-014-1393-0) contains supplementary material,
which is available to authorized users.
J. Chen (*)
School of Chemical & Biological Engineering, Yancheng Institute of
Technology, Yingbin Avenue 9, Yancheng 224051, People’s
Republic of China
A salicylaldehyde rhodamine derivative which bears no
substituent on its phenol ring, RhB-Sal, has been proven to
be able to sense Cu2+ in neutral aqueous media by virtue of its
selective and sensitive fluorescence enhancement [16]. In our
previous works, rhodamine-based analogues, RhB-pMOSal
[24] and RhB-Im [25], were developed as “off-on” fluorescent
e-mail: jwchen@ycit.edu.cn
:
K. Dai B. Xu
School of Petrochemical Engineering, Changzhou University,
Changzhou 213164, People’s Republic of China

J Fluoresc
chemosensors for selectively sensing Cu2+ in aqueous solu-
tion and acidic aqueous solution respectively, but their sensi-
tivity are not satisfactory. In this paper, the performance of a
newly synthesized rhodamine compound, RhB-NSal, to sense
Cu2+ in weakly acidic aqueous solution and dry acetonitrile
media was comparatively examined. In the molecule of RhB-
NSal, an electron-withdrawing group –NO2 was introduced to
the 5-position of the 2-hydroxyphenyl moiety (Scheme 1),
which allowed us to further examine its electronic effect on
the sensing performance. It was found that RhB-NSal
responded to Cu2+ selectively, sensitively and reversibly in
its absorption and fluorescence in acetonitrile medium over
other metal ions. In weakly acidic aqueous solution, RhB-
NSal displayed a high selectivity but lower sensitivity.
Compared with RhB-Sal, the introduction of 5-NO2 group
favored the sensing sensitivity of RhB-NSal for Cu2+ in both
media.
NSal. All chemicals involved in the experiments were of
analytical grade and used without further purification.
Apparatus
The electronic absorption spectra were obtained using a
Shimadzu UV-2450 spectrophotometer and the steady-state
fluorescence emission spectra were recorded on a JASCO FP-
6500 spectrophotometer equipped with a thermostated cell
compartment. A quartz cuvette of 1.0 cm optical path length
was used throughout the whole spectroscopic measurements.
The pH measurements were carried out on a PHS-3C Exact
Digital pH meter. The FTIR spectra were recorded on a
NEXUS 670 spectrometer using KBr pellets. The mass spec-
tra were obtained on an LCQ electron spray mass spectrom-
eter (ESMS, Finnigan). NMR spectra were measured on a
Bruker DRX-500 spectrometer with tetramethylsilane (TMS)
as an internal standard and CDCl3 as a solvent. All of the
measurements were carried out at 298.0 0.2 K. Elementary
analysis for C, H, N was performed on a Vario EL cube
elementar.
Experimental
Reagents and Chemicals
General Procedures
Rhodamine B was purchased from TDI, and 5-
nitrosalicylaldehyde was from Sigma-Aldrich. Silica gel
(300–400 mesh) used for thin-layer and column chromatog-
raphy was obtained commercially from Qingdao Ocean
Chemicals (Qingdao, China). Acetonitrile was distilled over
CaH2, and then stored over 4 Å molecular sieves prior to use.
All metal salts used in the spectroscopic experiments were
obtained from Shanghai Chemical Reagent Corporation
(Shanghai, China). Two sets of stock solutions of single metal
ion (1 mM) including Li+, K+, Mg2+, Ca2+, Sr2+, Ba2+, Zn2+,
Cd2+, Co2+, Fe2+, Fe3+, Bi3+ and Cr3+ were prepared from
their chloride salts except for Pb2+ and Ag+ from nitrate salt,
and Al3+, Cu2+, Mn2+, Ni2+ from sulfate salt using acetonitrile
and aqueous acetate-buffer/acetonitrile (2/3, v/v, pH 4.8) so-
lution as solvent, respectively. The stock solution of RhB-
NSal (100 μM) was prepared in dry acetonitrile, and two sets
of the working solutions were prepared by diluting respective-
ly the stock solution to 10 μM with dry acetonitrile and the
aqueous acetate buffer/acetonitrile (2/3, v/v, pH 4.80, acetate,
20 mM) solution as well. The component acetonitrile in the
buffer medium was used as a solubilizing reagent for RhB-
The sensing behaviors of RhB-NSal for various metal ions
were examined in both aqueous and non-aqueous media by
electronic absorption and fluorescence spectroscopy. The
UV–vis and fluorescence titrations were performed by trans-
ferring aliquots of each metal stock solution into 3 mL corre-
sponding RhB-NSal (10 μM) working solution in a quartz
cuvette. After each addition, the solution was fully mixed and
equilibrated for ca. 2 min. The UV–vis absorption spectra
were recorded within 450–600 nm, and the fluorescence spec-
tra were taken in the scope of 540–650 nm with excitation
light of 520 nm. The slit width of the excitation and the
emission was both set at 5 nm. All experiments were run in
triplicate, and the average values were adopted.
The binding ratio between RhB-NSal and Cu2+ ion was
determined by Job’s plot using the data of UV–vis absorption
spectra, and was verified by Benesi-Hildebrand method [26].
In the determination, the total concentration of Cu2+ and RhB-
NSal was kept at 10 μM and the molar ratio of Cu2+ was
changed from 0 to 1.0. Each solution was prepared by mixing
appropriate aliquots of the corresponding RhB-NSal and Cu2+
HO
O
O
O
O2N
OH
HCl
O
N
NH2
N
N
NH2NH2
H2O
CHO
Absolute ethanol
NO2
Absolute ethanol
N
O
N
N
O
N
N
O
N
Rhodamine B
Rhodamine B hydrazide
RhB-NSal
Scheme 1 Synthesis of RhB-NSal

J Fluoresc
Fig. 1 Variations of the
absorption (left) and fluorescence
(right) spectra of RhB-NSal
(10 μM) in dry acetonitrile in
the presence of different amount
of Cu2+
working solution, and diluting to 5 mL with dry acetonitrile
and aqueous acetate buffer, respectively.
Results and Discussion
Spectroscopic Response of RhB-NSal to Cu2+ in Dry
Acetonitrile and Aqueous Acetate Media
Synthesis
To evaluate the sensing potential of RhB-NSal for Cu2+ ion
possibly existing in complex media, the spectroscopic re-
sponse of RhB-NSal to free Cu2+ ion in dry acetonitrile and
aqueous acetate buffer/acetonitrile (2/3, v/v, pH 4.80) was
firstly investigated by absorption and fluorescence spectros-
copy. As shown in Fig. 1, the free RhB-NSal in dry acetoni-
trile exhibited weak absorbance and fluorescence emission
(λex=520 nm) due to the unique spirolactam form [2, 3].
The characteristic absorption band lies in the range of 450–
600 nm with a λmax of 555 nm (ε=3.71 × 104 L mol−1 cm−1).
Upon addition of Cu2+ ion into the solution, the absorption
enhanced instantly and a new peak with the maximum wave-
length at ca. 517 nm occurred. Concomitantly, the solution
color became pink from colorless instantaneously (Fig. 1 in-
set). The fluorescence intensity of the solution similarly en-
hanced with the increasing amount of Cu2+. When the con-
centration of Cu2+ approached 10 μM, approximate 45 and
15-fold enhancements in the absorbance (at 555 nm) and
emission intensity (at 575 nm) were measured respectively,
accompanied by a red shift of the maximum emission wave-
length from 568 to 575 nm. The detection limit was deter-
mined from the fluorescence titration data according to a
classical method [27] and was found to be 0.49 μM, which
was lower than those of many reported Cu2+ chemosensors
[25, 28–30]. As for a known analogue bearing no substituent
RhB-NSal was synthesized according to the reported proce-
dures [24, 25] using rhodamine B and 5-nitrosalicylaldehyde
as raw materials (Scheme 1). An excessive 5-
nitrosalicylaldehyde (0.1 g, 0.6 mmol) was added into
20 mL absolute ethanol containing rhodamine B-hydrazide
(0.23 g, 0.5 mmol), and the resultant mixture was stirred
vigorously for 24 h at room temperature. After the reaction
completed, the yellow precipitate was collected by centrifu-
gation and then dried in vacuum. Further purification by silica
gel column chromatography (eluent: petroleum ether/ethyl
acetate (4/1, v/v) containing 1 % (v/v) triethylamine) afforded
0.27 g yellow powder (yield: 73.3 %) with a melting point of
232-234 °C. ESI-MS for C35H35N5O5 in the acetate-buffered
acetonitrile solution (pH 4.80): m/z, [M + H+]+, 606.68, found
606.58; [2 M + H+ + Na+]2+, 1,235.36, found 1,235.50.
1
(Supporting Information Fig. S1). H NMR (CDCl3): δ
(ppm) 1.18 (12H, t, NCH2CH3), 3.36 (8H, q, NCH2CH3),
6.28 (2H, d, Xanthene-H), 8.08 (1H, d, Ar-H), 8.9 (1H, s,
N = C-H), 12.06 (1H, s, Phen-OH) (Supporting
Information Fig. S2). Elemental analysis. Found (calcd.)
(%): C, 74.34 (74.28); H, 7.43 (7.42); N, 11.55 (11.56).
FT-IR spectra (v, cm−1): 3,477 (s), 2,970 (m), 2,929 (m),
1,695 (m), 1,614 (s), 1,515 (s), 1,372 (m) (Supporting
Information Fig. S3).
Fig. 2 Spectral comparison
between RhB-NSal (10 μM) and
RhB-Sal (10 μM) in absorption
(left) and fluorescence emission
(right) in acetonitrile upon
addition of equivalent Cu2+

J Fluoresc
Fig. 3 Variations of the
absorption (left) and fluorescence
(right) spectra of RhB-NSal
(10 μM) in aqueous acetate-
buffer/acetonitrile (2/3, v/v,
pH 4.8) solution in the presence
of different amount of Cu2+
on its phenol ring, RhB-Sal, only ca 23 and 9-fold enhance-
ments at its maximum absorbance and emission were estimat-
ed, respectively, which indicates that RhB-NSal is more sen-
sitive than RhB-Sal toward Cu2+ under the same conditions
(Fig. 2). Also, the results suggest that the additional 5-nitro
group favors the sensing capability of RhB-NSal for Cu2+ ion,
which could be ascribed to the enhancement of the binding
capacity of RhB-NSal to Cu2+ due to the electron-attracting
effect of nitro group.
In comparison to RhB-Sal, the same amount of Cu2+ in-
duced more remarkable absorption increase of RhB-NSal in
aqueous media, a case similar to that in dry acetonitrile.
However, the difference of fluorescence enhancement be-
tween RhB-NSal and RhB-Sal was hardly observed under
the same conditions (Fig. 4). The results suggest that the
Cu2+ ion-induced fluorescence enhancement of RhB-NSal
has been impaired by the higher background resulted by H+
ion in the solution [32]. This may be the reason why most of
rhodamine chemosensors reported in literature were applied in
neutral aqueous or non-aqueous media so as to circumvent
such spectroscopic interference.
The spectroscopic response of RhB-NSal to Cu2+ in aque-
ous media was further examined. A weakly acidic aqueous
solution with a pH value of 4.80 maintained by acetate buffer
was selected in case of the hydrolysis of possibly coexisting
highly-valent metal ions. As shown in Fig. 3, free RhB-NSal
in the aqueous acetonitrile solution exhibited weak absorption
but medium strong emission (λex=520 nm), which suggests
part intramolecular opening of the spirolactam ring due to the
effect of H+ ion [31]. The characteristic absorption band also
lies in the scope of 450–600 nm with a λmax of 556 nm (ε=
7.52 × 104 L mol−1 cm−1). A similar fluorogenic and chromo-
genic response toward Cu2+ in this medium was observed for
RhB-NSal. Approximately 12 and 1.7-fold enhancements in
the maximum absorbance (at 556 nm) and emission (at
578 nm) of RhB-NSal (10 μM) were respectively estimated
when equivalent Cu2+ was added into the solution. The de-
tection limit was calculated according to the same method to
be 14.98 μM. Moreover, the maximum emission wavelength
underwent a slight red shift with the increase of Cu2+ as shown
in Fig. 3, which could be ascribed to the rigidity enhancement
of RhB-NSal molecule due to Cu2+ coordination [6].
Obviously, RhB-NSal exhibits lower sensitivity for Cu2+ in
aqueous solution than that in dry acetonitrile.
Binding Interaction Between RhB-NSal and Cu2+ in Dry
Acetonitrile and Aqueous Acetate Media
All above spectroscopic results suggest that Cu2+ ion binds
with RhB-NSal, resulting in the spirolactam ring of RhB-NSal
open, and concomitantly forming RhB-NSal-Cu(II) complex.
Similar to the previously reported rhodamine analogues, RhB-
Sal [16] and RhB-pMOSal [25], RhB-NSal could provide
carbonyl oxygen, imino nitrogen, and phenol oxygen as co-
ordination atoms to bind Cu2+. Compared with RhB-Sal, the
electron-attracting 5-nitro group of RhB-NSal facilitated the
dissociation of hydroxyl proton on the phenol ring, thus
enhancing its complexation ability with Cu2+, which could
accordingly contribute to its fluorescence response to Cu2+
ion. As for the molecule of RhB-pMOSal, the electron-
donating 4-methoxy group on the phenol ring may enhance
the conjugation effect of imino group –C = N–, and likewise
improve the binding capacity of imino nitrogen to Cu2+. In
view of these results, it could be anticipated that the
Fig. 4 Spectral comparison
between RhB-NSal (10 μM) and
RhB-Sal (10 μM) in absorption
(left) and fluorescence emission
(right) in aqueous acetate solution
upon addition of 10 μM Cu2+

J Fluoresc
Fig. 5 Chemical reversibility of
Cu2+-mediated absorbance (left)
and fluorescence (right) changes
of RhB-NSal in dry acetonitrile (a)
free RhB-NSal (10 μM); (b) a +
Cu2+ (10 μM); (c) b + EDTA
(10 μM); (d) c + Cu2+ (10 μM)
substituents which could reinforce the binding capacity of the
coordination atoms on a rhodamine molecule may enhance its
spectroscopic response to a metal ion. However, the H+ ion in
acidic aqueous medium may in most cases act as the same role
as a metal ion resulting in the spirocycle of a rhodamine
molecule open, and thus increase its emission background
noise [32, 33], which lowers to some extent the sensing
sensitivity of the rhodamine chemosensor.
shown in Fig. 5, when equivalent chelator, EDTA, was intro-
duced into the mixture of RhB-NSal (10 μM) and Cu2+
(10 μM) in dry acetonitrile, the pink solution immediately
turned almost colorless, accompanied by an instant attenua-
tion of its emission intensity. To this solution was reversely
added Cu2+ ion, the spectroscopic responses gradually recov-
ered. The similar circumstances were also observed for RhB-
NSal and Cu2+ in the aqueous medium (Fig. S6). These results
indicate that the binding of RhB-NSal with Cu2+ is a chemi-
cally reversible process, and the compound RhB-NSal may be
served as a potential recyclable chemosensor for Cu2+ ion in
aqueous or non-aqueous media by means of fluorescent and
colorimetric responses.
The binding ratio between RhB-NSal and Cu2+ in their
complex in both dry acetonitrile and aqueous acetate media
was examined by Job’s method, respectively. Fitting of the
Job’s plot afforded a 1:1 stoichiometry for the complex
(Fig. S4), which was verified by the Benesi-Hildebrand meth-
od [26]. Based on this result, the association constant (Ka) of
RhB-NSal with Cu2+ was determined using the Benesi-
Hildebrand equation. As shown in Fig. S5, the plotting of
1/(A–A0) versus 1/[Cu2+] showed satisfactory linear curves in
both media, which confirmed the 1:1 binding ratio between
RhB-NSal and Cu2+ in the RhB-NSal-Cu(II) complex. The
association constants (Ka) were calculated from the curve
slope to be 6.72 ( 0.03) × 104 M−1 and 4.23 ( 0.03) ×
104 M−1 in dry acetonitrile and aqueous media, respectively.
The value of the former is some higher than the latter though
they are not significantly different, which is in agreement with
the higher sensitivity of RhB-NSal for Cu2+ in dry acetonitrile
than that in aqueous solution.
Sensing Selectivity of RhB-NSal for Cu2+ in Dry Acetonitrile
and Aqueous Acetate Media
Selectivity is a basic requirement for a compound to be used as
a chemosensor. To evaluate the sensing selectivity of RhB-
NSal for Cu2+ in a complex environment, the spectroscopic
interference of possibly competitive metal ions including Li+,
K+, Mg2+, Ca2+, Sr2+, Ba2+, Ag+, Zn2+, Cd2+, Co2+, Fe2+,
Fe3+, Al3+, Bi3+, Cr3+, Mn2+, Ni2+ and Pb2+ was investigated in
dry acetonitrile and aqueous acetate media, respectively. As
demonstrated in Fig. 6 and 7, an instant color change from
colorless to pink and ca 11-fold absorption increase were ob-
served for RhB-NSal (10 μM) solution upon addition of equiv-
alent Cu2+. The equivalent Fe3+ led to a faint pink color and ca
2.5-fold absorption enhancement of the solution. Other tested
Moreover, the chemical reversibility, one of the properties
of a chemosensor, for the Cu2+-mediated spectroscopic re-
sponses of RhB-NSal in each medium was examined. As
Fig. 6 Photographs of the color
change of RhB-NSal (10 μM) in
dry acetonitrile solution upon
addition of equivalent metal ions

J Fluoresc
Fig. 9 Effect of single competitive metal ion (Mn+) on the fluorescence
emission of RhB-NSal (10 μM) in dry acetonitrile solution. Cu2+, 10 μM;
Each of the tested metal ions, 20 μM
Fig. 7 Absorption changes of RhB-NSal (10 μM) in dry acetonitrile
upon addition of each tested ion in concentration of 20 μM. Mn+ repre-
sents the single metal ion tested
metal ions in concentration of 20 μM for each hardly impacted
on the absorption of the RhB-NSal solution. These results
indicate a high specificity of RhB-NSal for Cu2+ ion in dry
acetonitrile. In an independent experiment, a substantial
change in the absorption was similarly observed for 10 μM
RhB-NSal in dry acetonitrile solution containing the mixed
competitive metal ions when equivalent Cu2+ ion was added,
which further proved the high selectivity of RhB-NSal for
Cu2+ (Fig. 8).
The effect of various possibly competitive metal ions on
the fluorescence response of RhB-NSal toward Cu2+ in dry
acetonitrile solution was subsequently investigated. As shown
in Fig. 9 and Fig. S7, the outcome satisfactorily corroborated
the high selectivity of RhB-NSal for Cu2+ shown in the
electronic absorption spectroscopy. Among the possibly com-
petitive metal ions, Bi3+ ion was found to induce a tiny
fluorescence enhancement of RhB-NSal (10 μM). All other
metal ions had little effect on the fluorescence of RhB-NSal
under the same conditions. In contrast, the equivalent Cu2+
resulted in a remarkable emission of RhB-NSal centered at
575 nm (ca. 15-fold) in addition to an obvious absorption
enhancement shown above. An independent experimental
result shown in Fig. 10 exhibited that the mixed competitive
metal ions in concentration of 20 μM for each among trig-
gered only ca. 3-fold fluorescence enhancement of RhB-NSal
(10 μM) whereas the subsequent addition of 10 μM Cu2+
induced ca. 17-fold enhancement.
The high selectivity of RhB-NSal for sensing Cu2+ was
likewise observed in acetate-buffered aqueous acetonitrile
solution as shown in Fig. S8–S13, except that Fe3+ and Bi3+
ion, similar to the cases observed in acetonitrile medium, were
respectively found to bring about some interference with its
absorption and fluorescence. Such interference could be over-
come by pre-lowering the solution acidity of the tested sam-
ples and then centrifugal separation by virtue of the property
of facile hydrolysis of both highly-valent metal ions, which
was evidenced in our experiments.
Conclusion
In summary, the sensing behaviors of a newly-synthesized
rhodamine derivative RhB-NSal for Cu2+ ion in dry
Fig. 8 Effect of the possibly competitive metal ions on the absorption of
RhB-NSal in dry acetonitrile solution. RhB-NSal, 10 μM; Cu2+, 10 μM;
Fig. 10 Effect of the mixed competitive metal ions on the fluorescent
sensing of RhB-NSal (10 μM) for Cu2+ (10 μM) in dry acetonitrile
solution. M represents the mixed competitive metal ions except for
Cu2+, the concentration of each ion among was 20 μM
M represents the possibly competitive metal ions mixed except for Cu2+
,
the concentration of each competitive ion among was 20 μM

J Fluoresc
rhodamine-based biosensor and comparison with biophysical data.
Phys Chem Chem Phys 15:2177–2183
acetonitrile and weakly acidic aqueous acetonitrile solution
(pH 4.80) were comparatively investigated. In each medium,
the absorption and the fluorescence of RhB-NSal enhanced
significantly, accompanied by an instant color change from
colorless to pink upon addition of Cu2+ ion. RhB-NSal was
expected to reversibly bind to Cu2+ in each medium, forming a
1:1 stoichiometric RhB-NSal-Cu(II) complex with an associ-
ation constant of 6.72 ( 0.03) × 104 M−1 and 4.23 ( 0.03) ×
104 M−1, respectively.
5. Li C-Y, Zhou Y, Li Y-F, Kong X-F, Zou C-X, Weng C (2013)
Colorimetric and fluorescent chemosensor for citrate based on a
rhodamine and Pb2+ complex in aqueous solution. Anal Chim Acta
774:79–84
6. Wang C, Wong KM-C (2013) Selective Hg2+ sensing behaviors of
rhodamine derivatives with extended conjugation based on
two successive ring-opening processes. Inorg Chem 52:
13432–13441
7. Sasaki H, Hanaoka K, Urano Y, Terai T, Nagano T (2011) Design and
synthesis of a novel fluorescence probe for Zn2+ based on the
spirolactam ring-opening process of rhodamine derivatives. Bioorg
Med Chem 19:1072–1078
8. Sreenath K, Clark RJ, Zhu L (2012) Tricolor emission of a fluores-
cent heteroditopic ligand over a concentration gradient of Zinc(II)
ions. J Org Chem 77:8268–8279
9. Park S, Kim W, Swamy KMK, Lee HY, Jung JY, Kim G, Kim Y, Kim
SJ, Yoon J (2013) Rhodamine hydrazone derivatives bearing thio-
phene group as fluorescent chemosensors for Hg2+. Dyes Pigments
99:323–328
10. Ambikapathi G, Kempahanumakkagari SK, Lamani BR, Shivanna
DK, Maregowda HB, Gupta A, Malingappa P (2013) Bioimaging of
peroxynitrite in MCF-7 cells by a new fluorescent probe rhodamine B
phenyl hydrazide. J Fluoresc 23:705–712
11. Kamal A, Kumar N, Bhalla V, Kumar M, Mahajan RK (2014)
Rhodamine-dimethyliminocinnamyl based electrochemical sensors
for selective detection of iron (II). Sens Actuat B: Chem 190:127–133
12. Sahana A, Banerjee A, Lohar S, Sarkar B, Mukhopadhyay SK, Das D
(2013) Rhodamine-based fluorescent probe for Al3+ through time-
dependent PET-CHEF-FRET processes and its cell staining applica-
tion. Inorg Chem 52:3627–3633
13. Sun S, Qiao B, Jiang N, Wang J, Zhang S, Peng X (2014)
Naphthylamine–rhodamine-based ratiometric fluorescent probe for
the determination of Pd2+ ions. Org Lett 16:1132–1135
14. Emrullahoğlu M, Karakuş E, Üçüncü M (2013) A rhodamine based
“turn-on” chemodosimeter for monitoring gold ions in synthetic
samples and living cells. Analyst 138:3638–3641
15. Mahapatra AK, Manna SK, Mandal D, Mukhopadhyay CD (2013)
Highly sensitive and selective rhodamine-based “off-on” reversible
chemosensor for tin (Sn4+) and imaging in living cells. Inorg Chem
52:10825–10834
16. Xiang Y, Tong A, Jin P, Ju Y (2006) New fluorescent rhodamine
hydrazone chemosensor for Cu(II) with high selectivity and sensitiv-
ity. Org Lett 8:2863–2866
RhB-NSal displayed similar high selectivity for Cu2+ over
co-existing metal ions except that Fe3+ brought about some
absorption interference and Bi3+ led to a little fluorescence
interference in each medium. Approximate 45- and 15-fold
enhancements in dry acetonitrile whereas 12- and 1.7-fold
enhancements in aqueous solution were estimated for RhB-
NSal (10 μM) in its maximum absorbance and emission
intensity when equivalent Cu2+ was added into each solution.
The detection limits for Cu2+ were calculated respectively to
be 0.49 and 14.98 μM. Clearly, the sensitivity of RhB-NSal
for Cu2+ in acetonitrile is higher than that in aqueous solution.
The H+-resulted stronger spectral background may be respon-
sible for the lower sensitivity in the latter medium. Moreover,
RhB-NSal in acetonitrile exhibited higher sensitivity in ab-
sorption and fluorescence for Cu2+ than that of the analogue
bearing no substituent on its phenol ring, RhB-Sal. In the
aqueous solution, however, RhB-NSal displayed comparable
fluorescence but more remarkable absorption response to
Cu2+ under the same conditions. The electron-attracting 5-
nitro group of RhB-NSal facilitates the dissociation of hy-
droxyl proton on the phenol ring, and thus enhances its
binding capacity with Cu2+, which may accordingly favor its
spectroscopic response to Cu2+ ion. By virtue of the perfor-
mance, RhB-NSal could be developed to be a promising “Off-
On” fluorescent and colorimetric chemosensor for Cu2+ in
acetonitrile and a candidate of eye-naked “Off-On”
chemosensor for Cu2+ in a weakly acidic aqueous medium.
17. Chereddy NR, Thennarasu S, Mandal AB (2012) A new triazole
appended rhodamine chemosensor for selective detection of Cu2+
ions and live-cell imaging. Sens Actuat B: Chem 171–172:294–301
18. Tang L, Guo J, Cao Y, Zhao N (2012) New application of a known
molecule: Rhodamine B 8-hydroxy-2-quinolinecarboxaldehyde
Schiff base as a colorimetric and fluorescent “off-on” probe for
copper (II). J Fluoresc 22:1603–1608
Acknowledgments We thank the financial support from the Natural
Science Foundation of Jiangsu Province (No. BK2012674) and from the
research fund of Key Laboratory for Advanced Technology in Environ-
mental Protection of Jiangsu Province (No. AE201029).
19. Mi YS, Cao Z, Chen YT, Xie QF, Xu YY, Luo YF, Shi JJ, Xiang JN
(2013) Determination of trace amount of Cu2+ with a multi-
responsive colorimetric and reversible chemosensor. Analyst 138:
5274–5280
References
20. Huang L, Hou F, Xi P, Bai D, Xu M, Li Z, Xie G, Shi Y, Liu H, Zeng
Z (2011) A rhodamine-based “turn-on” fluorescent chemodosimeter
for Cu2+ and its application in living cell imaging. J Inorg Biochem
105:800–805
1. Lakowicz JR (2006) Principles of fluorescence spectroscopy, 3rd
edn. Springer, New York, pp 67–69
2. Beija M, Afonso CAM, Martinho JMG (2009) Synthesis and appli-
cations of rhodamine derivatives as fluorescent probes. Chem Soc
Rev 38:2410–2433
3. Kim HN, Lee MH, Kim HJ, Kim JS, Yoon J (2008) A new trend in
rhodamine-based chemosensors: application of spirolactam ring-
opening to sensing ions. Chem Soc Rev 37:1465–1472
4. Goncalves MB, Dreyer J, Lupieri P, Barrera-Patino C, Ippoliti E,
Webb MR, Corrie JET, Carloni P (2013) Structural prediction of a
21. Yu C, Zhang J, Li J, Liu P, Wei P, Chen L (2011) Fluorescent probe
for copper(II) ion based on a rhodamine spirolactame derivative, and
its application to fluorescent imaging in living cells. Microchim Acta
174:247–255
22. Wang J, Long L, Xie D, Song X (2013) Cu2+-selective “Off-On”
chemosensor based on the rhodamine derivative bearing 8-

J Fluoresc
hydroxyquinoline moiety and its application in live cell imaging.
Sens Actuat B: Chem 177:27–33
23. Sikdar A, Roy S, Haldar K, Sarkar S, Panja SS (2013) Rhodamine-
based Cu2+-selective fluorosensor: synthesis, mechanism, and appli-
cation in living cells. J Fluoresc 23:495–501
24. Tang RR, Lei K, Chen K, Zhao H, Chen JW (2011) A
rhodamine-based off-on fluorescent chemosensor for selec-
tively sensing Cu(II) in aqueous solution. J Fluoresc 21:
141–148
25. Ding JH, Yuan LD, Gao L, Chen JW (2012) Fluorescence quenching
of a rhodamine derivative: selectively sensing Cu2+ in acidic aqueous
media. J Lumin 132:1987–1993
28. Xu Z, Zhang L, Guo R, Xiang T, Wu C, Zheng Z, Yang F (2011) A
highly sensitive and selective colorimetric and off-on fluorescent
chemosensor for Cu2+ based on rhodamine B derivative. Sens
Actuat B: Chem 156:546–552
29. Zhang D, Wang M, Chai M, Chen X, Ye Y, Zhao Y (2012) Three
highly sensitive and selective colorimetric and off-on fluorescent
chemosensors for Cu2+ in aqueous solution. Sens Actuat B: Chem
168:200–206
30. Yang Z, She M, Zhang J, Chen X, Huang Y, Zhu H, Liu P, Li J, Shi Z
(2013) Highly sensitive and selective rhodamine Schiff base “off-on”
chemosensors for Cu2+ imaging in living cells. Sens Actuat B: Chem
176:482–487
26. Benesi HA, Hildebrand JH (1949) A spectrophotometric investiga-
tion of the interaction of iodine with aromatic hydrocarbons. J Am
Chem Soc 71:2703–2707
27. Caballero A, Martínez R, Lloveras V, Ratera I, Vidal-Gancedo J,
Wurst K, Tárraga A, Molina P, Veciana J (2005) Highly selective
chromogenic and redox or fluorescent sensors of Hg2+ in aqueous
environment based on 1,4-disubstituted azines. J Am Chem Soc 127:
15666–15667
31. Arbeloa IL, Ojeda PR (1981) Molecular forms of rhodamine B.
Chem Phys Lett 79:347–350
32. Valeur B (2001) Molecular fluorescence: principles and applications.
Whiley-VCH Verlag GmbH, New York, Chapter 10
33. Karakuş E, Üçüncü M, Eanes RC, Emrullahoğlu M (2013) The
utilization of pH sensitive spirocyclic rhodamine dyes for monitoring
D-fructose consumption during a fermentation process. New J Chem
37:2632–2635