A new sensitive and selective chromogenic and fluorescent chemodosimeter for Hg(II) in aqueous media and its application in live cell imaging

Article abstract of DOI:10.1016/j.dyepig.2013.06.021

A new rhodamine-based chemodosimeter(L2) was synthesized and characterized, which contained a tosyl group acted as a strong electron acceptor. As expected, L2 exhibited high selectivity and excellent sensitivity in both absorbance and fluorescence detecti

Full text of DOI:10.1016/j.dyepig.2013.06.021

Dyes and Pigments 99 (2013) 607e612
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A new sensitive and selective chromogenic and fluorescent
chemodosimeter for Hg(Ⅱ) in aqueous media and its application
in live cell imaging
,
,c,
Di Zhang a, Man Li a 1, Yuhua Jiang b, Cuicui Wang a, Zhenji Wang b, Yong Ye a
,
*
,c,
Yufen Zhao a
*
a Phosphorus Chemical Engineering Research Center of Henan Province, The College of Chemistry and Molecular Engineering, Zhengzhou University,
Zhengzhou 450052, PR China
b School of Pharmaceutical Science, Zhengzhou University, Zhengzhou, PR China
c Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry of Education), Department of Chemistry, Tsinghua University,
Beijing 100084, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new rhodamine-based chemodosimeter(L2) was synthesized and characterized, which contained a
tosyl group acted as a strong electron acceptor. As expected, L2 exhibited high selectivity and excellent
sensitivity in both absorbance and fluorescence detection of Hg2þ in aqueous solution. The coordination
of L2 with Hg2þ was chemically nonreversible and it was based on the fact that Hg2þ exhibited a strong
thiophilic affinity. Addition of Hg2þ to an ethanol aqueous solution of L2 resulted in a color change from
color-less to obvious pink color, these significant changes in color could be used for naked-eye detection.
Furthermore, fluorescence imaging experiments of Hg2þ ions in living MGC803 cells demonstrated its
value of practical applications in biological systems.
Received 8 April 2013
Received in revised form
8 June 2013
Accepted 11 June 2013
Available online 27 June 2013
Keywords:
Rhodamine
Fluorescent chemodosimeter
Tosyl group
Ó 2013 Elsevier Ltd. All rights reserved.
Hg(Ⅱ)
Aqueous media
Biological systems
1. Introduction
biochemical agents for determination of the Hg2þ ion in both the
environment and biological systems.
Recently, the design and development of fluorescent chemo-
dosimeters for the detection of heavy- and transition- metals ions
have received considerable attentions because fluorescent che-
modosimeters have several advantages over other methods such as
high sensitivity, selectivity, and real-time monitoring [1]. Mercury
is one of the most prevalent toxic metals in both the environment
and biological system [2]. Mercury can cause serious and irrevers-
ible DNA damage, mitosis impairment and nervous system defects
[3]. Therefore, there is a high demand for developing artificial re-
ceptors with high selectivity and sensitivity for chemical and
As is well known, rhodamine derivatives having a spirolactam
structure is non-fluorescent and colorless, whereas ring-opening
of the spirolactam give rise to a strong fluorescence emission
and a pink color [4e9]. As their excellent photochemical proper-
ties, such as high fluorescence quantum yields, large molar
extinction coefficient, long excitation and emission wavelengths,
rhodamine derivatives have been actively utilized as fluorescent or
colorimetric chemodosimeters for detection of Hg2þ [10e14]. Ac-
cording to the strong thiophilic affinity of Hg2þ, many chemo-
dosimeters based on rhodamine contained an “S” group [11e15].
As the reported mechanism, with the leaving of HgS, the intra-
molecular nucleophilic reaction is accomplished, which triggered
a domino reaction to bring about the opened-ring form of the
rhodamine spirolactam. However, there were only a few chemo-
dosimeters that can be used in biological systems [12], there is still
an intense demand for new efficient Hg2þ optical chemo-
dosimeters, especially those that can work in biological systems
with high selectivity and sensitivity.
* Corresponding authors. Phosphorus Chemical Engineering Research Center of
Henan Province, The College of Chemistry and Molecular Engineering, Zhengzhou
University, Zhengzhou 450052, PR China. Tel.: þ86 371 67767050.
E-mail addresses: yeyong@zzu.edu.cn, yeyong03@tsinghua.org.cn (Y. Ye).
Man Li contributed with Di Zhang, and they are both the first author.
1
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.06.021

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D. Zhang et al. / Dyes and Pigments 99 (2013) 607e612
Scheme 1. Synthetic route of L2.
Herein, we reported a novel chemodosimeter L2 which bearing
in a reported method [16]. The concrete way was described as
follows:
a rhodamine and sulfonamide group (Scheme 1). Chemodosimeter
L2 contained a “C]S” group, and as expected, it exhibited a
nonreversible, highly selective and sensitive recognition toward
Hg2þ over other examined metal ions in CH3CH2OHeH2O (1/1, v/v).
Additionally, the significant changes in the fluorescence color could
be used for naked-eye detection, and according to the fluorescence
imaging experiments of Hg2þ ions in living MGC803 cells, L2 could
be used for detecting Hg2þ in biological samples.
To a stirred solution of compound L1 (500 mg, 1 mmol) and Et3N
(204 mg, 2 mmol) in dry ClCH2CH2Cl (20 ml), a solution of p-
toluene sulfonyl chloride (190 mg,1 mmol) in 10 ml dry ClCH2CH2Cl
was added in dropwise and the mixture was stirred at 0 ꢀC for 5 h,
and then stirred at room temperature for 5 h. After completion of
reaction, monitored by TLC, solvent was evaporated off and water
was added to the residue. The aqueous layer was extracted with
CHCl3(25 ml ꢁ 3) and dried over anhydrous MgSO4. Purification of
the crude product by silica gel column chromatography with
CH3OH/CH2Cl2(1/25, v/v), gave 356 mg of white solid in a yield of
2. Experimental
54.4%. 1H NMR (400 MHz, CDCl3, ppm):
d 1.19e1.22 (t, 12H,
2.1. Apparatus
J ¼ 7.0 Hz), 2.34(s, 3H), 2.84e2.88 (q, 2H, J ¼ 5.3 Hz), 3.35e3.40(q,
8H, J ¼ 6.7 Hz), 3.56e3.63(t, 2H, J ¼ 6.0 Hz), 5.15e5.18(t, 1H,
J ¼ 6.0 Hz), 6.20e6.27(m, 4H), 6.43e6.33(d, 2H, J ¼ 2.0 Hz), 7.08e
7.10(q, 1H, J ¼ 2.7 Hz), 7.21e7.23(d, 2H, J ¼ 8.0 Hz), 7.49e7.51(t, 2H,
J ¼ 4.0 Hz), 7.60e7.62(d, 2H, J ¼ 8.0 Hz), 8.11e8.14(t, 1H, J ¼ 4.0 Hz).
13C NMR (100 MHz, CDCl3, ppm): 12.56, 14.15, 18.43, 21.45, 29.71,
41.89, 43.92, 44.46, 50.86, 58.46, 73.74, 97.93, 99.98, 103.07, 108.31,
123.29, 125.05, 127.03, 128.47, 128.60, 129.51, 129.61, 132.73, 137.14,
137.59, 142.93, 149.23, 150.99, 153.20, 191.92. ESI-MS: m/z ¼ 655.4
[M þ H]þ; HR-MS: Calcd for C37H43N4O3Sþ2 , Exact Mass: 655.2777
[M þ H]þ, Found: 655.2784[M þ H]þ, 677.2595[M þ Na]þ, 693.2341
[M þ K]þ, M.p: 178e180 ꢀC.
A Lambda 35 UV/VIS spectrometer (Perkin Elmer) was used for
absorption spectra measurements. Fluorescence spectra measure-
ments were performed on a HITACHI F-4500 fluorescence spec-
trophotometer, and the excitation and emission wavelength band
passes were both set at 3.0 nm. The melting points were deter-
mined by an X-4 microscopic melting point apparatus with a digital
thermometer (Shanghai, China). The pH was measured with a
Model pHs-3C meter (Shanghai, China). 1H and 13C NMR spectra
were recorded using a Bruker DTX-400 spectrometer. Samples
were dissolved in CDCl3 and placed in 5 mm NMR tubes. TMS was
used as internal reference. ESI mass spectra were carried out on an
HPLC Q-Tof HR-MS spectrometer (Waters Micromass) by using
methanol as mobile phase. Fluorescence images experiments were
carried out with a Nikon-80i inverted fluorescence microscope.
3. Results and discussion
The UVevis and Fluorescence studies were performed using a
10 mM CH3CH2OH solution of L2 in CH3CH2OHeH2O solution with
2.2. Materials
appropriate amounts of metal ions. Solutions were shaken for
about 25 min before measuring the absorption and fluorescent
intensity in order to make the metal ions chelate with the sensors
sufficiently. The solution of L2 was colorless and found to be very
stable in the above-mentioned solution system for more than one
week. In addition, a very weak fluorescence signal was observed at
590 nm (Fig. 1) upon excitation at 520 nm, confirming the presence
of ring-closed spirolactone.
All chemicals and reagents were used as received from com-
mercial sources without further purification. Solvents for chemical
synthesis and analysis were purified according to standard pro-
cedures. Chloride salts of metal ions (Kþ, Naþ, Ca2þ, Mg2þ, Ba2þ
,
)
Zn2þ, Fe2þ, Fe3þ, Mn2þ, Pb2þ, Cu2þ, Co2þ, Ni2þ, Cd2þ, Cr3þ, Hg2þ
and the nitrate salt of Agþ ions were used to evaluate the metal ion
binding properties by synthesized compounds. The metal ions were
prepared as 10.00 mmol/L in water solution. Double distilled water
was used throughout the experiment.
3.1. Fluorescence spectral responses of L2
Considering the strong thiophilic affinity of Hg2þ, we investi-
gated the color and fluorescence response of L2 to Hg2þ. As shown
in Fig. 1, the fluorescence spectra (lex ¼ 520 nm) of L2 measured in
CH3CH2OHeH2O (1/1, v/v) with the addition of respective metal
cations. There was very weak fluorescence signal (at 590 nm) in the
2.3. Synthesis
Compound L and L1 were synthesized by reported methods
[13,14]. Compound L2 was synthesized by a similar way described

D. Zhang et al. / Dyes and Pigments 99 (2013) 607e612
609
Fig. 2. Fluorescence intensity (at 590 nm) of L2 (10
Hg2þ in the presence of 100
M background metal ions in CH3CH2OHeH2O(1/1, v/v).
lex ¼ 520 nm, slit ¼ 3 nm).
mM) upon the addition of 100 mM
Fig. 1. Fluorescence spectra of L2 (10
mM) in CH3CH2OHeH2O(1/1, v/v) with the
m
presence of 10 equivalents of various species(lex ¼ 520 nm, slit ¼ 3 nm). Inset: Fluo-
(
rogenic color response of L2 (10
m
M) in CH3CH2OHeH2O(1/1, v/v) to Hg2þ (10 eq.)
under UV illumination (365 nm).
For practical application, the appropriate pH conditions for
successful operation of the chemodosimeter were evaluated. The
effects of pH on the fluorescence response of L2 obtained in the
absence and presence Hg2þ(10 eq.) in CH3CH2OH-Tris-HCl buffer
(1/1, v/v) were evaluated (Fig. S8). A best response toward Hg2þ ion
could be achieved within the pH range of about 6e10. This result
indicated that L2 can be used as a good fluorescent probe for Hg2þ
determination in nearly neutral conditions typical of many bio-
logical systems.
absence of metal ions indicating the rhodamine moiety of L2
adopting a spirocyclic form. Addition of 10 equiv. Hg2þ into solution
immediately resulted in a significant enhancement of fluorescent
intensity at about 590 nm. And the color of the solution changed
from a colorless state to yellow color under UV illumination
(365 nm)(Fig. 1, inset). However, under the identical condition, no
obvious response could be observed upon the addition of other ions
including Agþ, Kþ, Naþ, Ca2þ, Mg2þ, Ba2þ, Zn2þ, Fe2þ, Fe3þ, Mn2þ
,
Pb2þ, Cu2þ, Co2þ, Ni2þ, Cd2þ, Cr3þ. The results demonstrated that L2
was characteristic of high selectivity toward Hg2þ over other
competitive metal ions.
3.2. UVeVis spectral responses of L2
As shown in Fig. 3, UVevis spectrum of L2(10 mM) in CH3CH2OHe
The effect of ethanol content for the binding of L2 with Hg2þ was
studied, and the results were shown in Fig. S5. As the results
revealed, the fluorescence signal reached its maximum value at
about 50% aqueous ethanol. Hence, 50% aqueous ethanol media was
selected for the fluorometric and colorimetric method.
H2O(1/1, v/v) exhibited only very weak bands over 450 nm, indi-
cating the ring-closed spirolactone of L2 is predominant. In the
presence of 10 equiv. of Hg2þ, the absorbance was enhanced obvi-
ously and a new peak at about 568 nm was observed, accompanied
by a clear color change from colorless to pink (Fig. 3, inset). Other
metal ions, including Agþ, Kþ, Naþ, Ca2þ, Mg2þ, Ba2þ, Zn2þ, Fe2þ
,
The time dependence of the response of L2 to Hg2þ ions was
investigated [17], and the results were shown in Fig. S6. In this set
of experiments, several concentrations of Hg2þ ions were tested
with a fixed concentration of L2. The results showed that the
fluorescence signal of the L2 with Hg2þ ion increased for a few
minutes, and leveled off as the time continues, while the fluo-
Fe3þ, Mn2þ, Pb2þ, Cu2þ, Co2þ, Ni2þ, Cd2þ, Cr3þ did not show any
significant spectral change under identical conditions. These results
rescence intensity of blank solution (only L2, 10
mM) showed
almost unchanged at the same conditions. The fluorescence in-
tensity of L2 with Hg2þ reached its maximum value at about
25 min, after which the fluorescence intensity remained almost
constant. Therefore, a 25 min reaction time was selected in sub-
sequent experiments in order to make the metal ions chelate with
the sensors sufficiently.
Moreover, the competitive experiments of the background
metal ions with Hg2þ were also studied. As shown in Fig. 2, the
results demonstrated that background metal ions showed very low
interference with the detection of Hg2þ in CH3CH2OHeH2O(1/1, v/
v). Only Cr3þ and Fe3þ have posed a negligible effect on the fluo-
rescence response of L2 for Hg2þ, it may due to the quenching effect
of the paramagnetic Cr3þ and Fe3þ [18]. However, only slight
fluorescence decline were observed, and L2 displayed distinct
switch-on fluorescence emission for Hg2þ in contrast with that for
Cr3þ and Fe3þ. Moreover, it was investigated that the fluorescence
response of L2 toward Hg2þ in the presence of various coexistent
anions such as Clꢂ, Fꢂ, NO3ꢂ, SO42ꢂ, ClO4ꢂ and PO34ꢂ. It is gratifying to
note that all the tested anions have no interference (Fig. S7).
Fig. 3. Absorption spectra of L2 (10 mM) in CH3CH2OHeH2O (1/1, v/v) with the pres-
ence of 10 equiv. of various species. Insert shows the photo of L2 with different metal
ions in CH3CH2OH-H2O (1/1, v/v).

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D. Zhang et al. / Dyes and Pigments 99 (2013) 607e612
Fig. 4. UVeVis absorption spectra of L2 (10
mM) in CH3CH2OHeH2O(1/1, v/v) upon
Fig. 6. Fluorescence emission changes of L2-Hg2þ in CH3CH2OHeH2O(1/1, v/v) with
excess amount of KI (20 equiv.) (lex ¼ 520 nm, slit ¼ 3 nm).
addition of Hg2þ in concentrations of 0e5 equiv. Insert: Job’s plot for the complex of L2
and Hg2þ in CH3CH2OH-H2O(1/1, v/v) solution. [L2] þ [Hg2þ] ¼ 50
mM.
Fig. 5. Free L2 showed very weak fluorescence signal at the range
500e700 nm, upon addition gradual increase of Hg2þ to the solu-
tion of L2, a significant enhancement in fluorescence intensity at
590 nm was observed following excitation at 520 nm and gradual
increased with Hg2þ concentrations. The linear response of
the fluorescence intensity toward Hg2þ was obtained in Hg2þ
suggested that L2 could serve as a “naked-eye” chemodosimeter
selective for Hg2þ in CH3CH2OHeH2O (1/1, v/v) media.
To further investigate the interaction of Hg2þ and L2, the ab-
sorption spectra of L2 with varying Hg2þ concentrations in
CH3CH2OHeH2O(1/1, v/v) were recorded (Fig. 4). Upon addition of
increasing concentrations of Hg2þ ions to the solution, a new ab-
sorption band centered at 568 nm appeared with increasing in-
tensity, which can be ascribed to the formation of the ring-opened
amide form of L2 upon Hg2þ ions binding. The color of L2 also
changed from colorless to pink (Fig. S9). A plot of [Hg2þ]/{[Hg2þ]þ
[L2]} versus the molar fraction of Hg2þ was provided in Fig. 4-inset,
the analysis corresponding to 0.50 the concentration ratio indicates
the 1:1 stoichiometric ratio between Hg2þ and L2.
concentration range of 0e10
of Hg2þ is at the parts per million level. The detection limit of L2
was at least 0.067 M. Therefore, the proposed probe L2 was sen-
sitive enough to detect Hg2þ in industrial wastewater, which has a
discharge limit of 0.25 M defined by the Standardization Admin-
mM, demonstrated that the detection
m
m
istration (SA) of the People’s Republic of China [19]. The chemo-
dosimeter L2 displayed increased fluorescence intensity upon
addition of the metal ions and obvious color changes from colorless
to pink, which can be used for direct visual detection for
Hg2þ(Fig. S9).
3.3. The detection of Hg2þ
To investigate the practical applicability of L2, the detection
limit of this new chemodosimeter system was evaluated. The
fluorescence titration profile of L2 (10 m
M) with Hg2þ was shown in
Fig. 5. Fluorescence emission spectra of L2 (10
addition of Hg2þ in concentrations of 0e10 eq. Insert: The plot of fluorescence
intensity(at 590 nm) of compound L2(10
M) upon additions of Hg2þ from 1 to 10
in CH3CH2OHeH2O(1/1, v/v) (lex ¼ 520 nm, slit ¼ 3 nm).
mM) in CH3CH2OHeH2O(1/1, v/v) upon
m
mM
Scheme 2. The proposed reaction mechanism of L2 with Hg2þ
.

D. Zhang et al. / Dyes and Pigments 99 (2013) 607e612
611
Fig. 7. Fluorescence images of Hg2þ in MGC-803 cells with 1
fluorescence images (b, d) of MGC-803 cells incubated with 0
m
m
M solution of L2 in CH3CH2OHe H2O(1/1, v/v) for 30 min at 37 ꢀC, Bright-field transmission images (a, c) and
M, 10
M of Hg2þ for 30 min, respectively.
m
3.4. Mechanism
Hg2þ were also investigated and the results were shown in Fig. S15.
These results indicated the tosyl group displayed a beneficial effect to
the sensing effect of L2 with Hg2þ in CH3CH2OHeH2O(1/1, v/v).
In addition, the KI-adding experiments were conducted to
examine the reversibility of this reaction as shown in Fig. 6. When
excess KI (2 equiv. of Hg2þ) was added to the L2-Hg2þ water solu-
tion, the fluorescence emission and the pink color almost un-
changed, indicating that the coordination of L2 with Hg2þ is
chemically nonreversible.
3.5. Bioimaging applications of compound L2 in MGC-803 cells
[26,27]
To further assess the potential applications of the probe as an
Hg2þ probe in living cells, fluorescent imaging inside MGC-803 cells
was monitored by fluorescence microscopy. Incubation of MGC-803
Both the fluorescence and the UVevis experiments lead to a
significant OFF-ON signal. Thus, based on the 1:1 binding mode and
the nonreversible behavior between L2 and Hg2þ, and according to
our knowledge [13,14,20e23], we broached a conceivable mecha-
nism of Hg2þ complex with L2 (Scheme 2). In order to confirm the
reaction mechanisms, the ESI-MS spectrum of the mixtures of L2
with Hg2þ was obtained (Figs. S10 and 11). In the ESI-MS and HR-
MS spectra, a major ion peak was detected at m/z 621.2894,
which was characterized to be the ring opening products of
rhodamine, indicating the generation of Rhb-Ts (calculated
621.2894) as a final product. Another evidence was obtained by
comparing the 1H NMR of L2 and L2-Hg2þ in CDCl3 [24]. When 1 eq.
Hg2þ was added to solution of L2 in CDCl3, it got a mixture of L2 and
Rhb-Ts. From the 1H NMR spectra (Fig. S12) of L2 and Rhb-Ts, the
protons on the rhodamine moiety shifted downfield, such as H1, H2,
cells with 1
about 30 min at 37 ꢀC gave almost no intracellular fluorescence.
After washing with water two times, 10
M of Hg2þ were then
mM of the probe L2 in CH3CH2OHeH2O(1/1, v/v) for
m
supplemented to the cells, and all the reaction mixtures were
incubated at 37 ꢀC for another 30 min. After that, the fluorescence
from the intracellular area was observed (Fig. 7d), providing visual
evidence of the probe L2 entering cells and information on the
intracellular existence of Hg2þ. Furthermore, a bright-field trans-
mission image of cells treated with L2 and Hg2þ confirmed that the
cells were viable throughout the imaging experiments (Fig. 7a and
c). These preliminary experimental results demonstrated that L2
could be used for detecting Hg2þ in biological samples.
H
15 and H16, upon addition of Hg2þ. Based on the above findings, we
4. Conclusions
proposed that the reaction mechanisms of L2 with Hg2þ probably
proceed through the route depicted in Scheme 2.
In summary, we reported an efficient rhodamine-based fluo-
rescent chemodosimeter L2 for Hg2þ detection. Chemodosimeter
L2 exhibited nonreversible and highly selective binding with Hg2þ
over other metal ions. An Hg2þ-catalyzed desulfurization reaction
together with a nucleophilic reaction was adopted to realize both
target recognition and signal report, respectively. Obvious increases
in fluorescence and colorimetric changes were observed upon the
addition of Hg2þ into the ethanol/water (1/1, v/v) solution of che-
modosimeter L2. The colorimetric and fluorescent response to Hg2þ
As Jong Seung Kim et al. reported [25], the tosyl group which was a
strong electron acceptor, can introduced to strengthen the acidity of
the amide and further produce a stronger binding ability with cat-
ions. Comparing to compound L1, L2 displayed a more fluorescence
enhancement (shown in Figs. S13 and S14) with Hg2þ in CH3CH2Oe
H2O(1/1, v/v). Under the identical condition, the fluorescence
enhance- ment of Hg2þ to L1 and L2 is as high as 20 and 297-fold,
respectively. Moreover, the UVevis spectrum of L1, L1-Hg2þ, L2, L2-

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D. Zhang et al. / Dyes and Pigments 99 (2013) 607e612
[9] Zhang D, Wang M, Chai MM, Chen XP, Ye Y, Zhao YF. Three highly sensitive
and selective colorimetric and offeon fluorescent chemo- sensors for Cu2þ in
aqueous solution. Sens Actuator B 2012;168:200e6.
can be conveniently detected even by the naked eye, which pro-
vided a facile method for visual detection of Hg2þ. Moreover, the
preliminary experimental results demonstrated that L2 could be
used for detecting Hg2þ in biological samples.
[10] Shi W, Ma HM. Rhodamine B thiolactone: a simple chemosensor for Hg2þ in
aqueous media. Chem Commun 2008;16:1856e8.
[11] Yang YK, Ko SK, Shin IJ, Tae JS. Fluorescent detection of methylmercury by
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Acknowledgments
[12] Du JJ, Hu MM, Fan JL, Peng XJ. Fluorescent chemodosimeters using ‘‘mild’’
chemical events for the detection of small anions and cations in biological and
environmental media. Chem Soc Rev 2012;41:4511e35.
This work was financially supported by the National Science
Foundation of China (nos. 20972143, 20972130) and Program for
New Century Excellent Talents in University (NCET-11-0950).
[13] Zhang D, Li M, Wang M, Wang JH, Yang X, Ye Y, et al. A rhodamine- phos-
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tion in live cell imaging. Sens Actuator B 2013;177:997e1002.
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Appendix A. Supplementary data
[15] Huang JH, Xu YF, Qian XH. A rhodamine-based Hg2þ sensor with high selec-
tivity and sensitivity in aqueous solution: a NS2-containing receptor. J Org
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Supplementary data related to this article can be found at
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[16] Ghosh K, Sarkar T, Samadder A, Khuda-Bukhsh AR. Rhodamine-based bis-
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