Rhodamine-labelled new architecture for dual sensing of Co2+ and Hg2+ ions

Article abstract of DOI:10.1016/j.tetlet.2013.09.062

A new rhodamine-based chemosensor 1 has been designed and synthesized. The receptor selectively recognizes Co²⁺ and Hg²⁺ ions in CH₃CN/water (4:1, v/v; 10 μM tris HCl buffer, pH 6.8) by showing different extents of change in emission. The disappearance of colour of mercury-ensemble of 1 followed by appearance of distinct bluish colour under UV illumination upon addition of l-cysteine distinguishes Hg²⁺ from Co²⁺ ions. The receptor shows in vitro detection of both the ions in human cervical cancer (HeLa) cells.

Full text of DOI:10.1016/j.tetlet.2013.09.062

Tetrahedron Letters 54 (2013) 6464–6468
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Rhodamine-labelled new architecture for dual sensing of Co2+
and Hg2+ ions
⇑
Kumaresh Ghosh , Tanmay Sarkar, Anupam Majumdar
Department of Chemistry, University of Kalyani, Kalyani, Nadia 741235, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 July 2013
Revised 12 September 2013
Accepted 14 September 2013
Available online 21 September 2013
A new rhodamine-based chemosensor 1 has been designed and synthesized. The receptor selectively rec-
ognizes Co2+ and Hg2+ ions in CH3CN/water (4:1, v/v; 10
l
M tris HCl buffer, pH 6.8) by showing different
extents of change in emission. The disappearance of colour of mercury-ensemble of 1 followed by appear-
ance of distinct bluish colour under UV illumination upon addition of L
-cysteine distinguishes Hg2+ from
Co2+ ions. The receptor shows in vitro detection of both the ions in human cervical cancer (HeLa) cells.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Rhodamine-labelled receptor
Dual sensing of Co2+ and Hg2+ ions
In vitro detection of Co2+ and Hg2+ ions
Colorimetric detection
Fluorometric detection
The design and synthesis of optical chemosensors for the selec-
tive recognition of metal ions are of interest. In particular the
development of fluorescent sensor for transition metal ions such
as Hg2+ and Co2+, due to their biological and environmental impor-
tance is challenging. Among the different heavy metal ions, mer-
cury ion (Hg2+) is dangerous as it can accumulate in the human
body and causes a wide variety of diseases even in a low concen-
tration, such as prenatal brain damage, serious cognitive disorders
Co2+ and Hg2+ ions are simultaneously detected in semiaqueous
system both colorimetrically and fluorometrically. The disappear-
ance of the colour of the mercury-ensemble of 1 in the presence
of L
-cysteine distinguishes it from Co2+ ions. It is established that
rhodamine B and its derivatives (RhB) show good photo stability
and high fluorescence quantum yield. Due to their interesting phe-
nomenon they act as productive chemosensors towards metal ions
by switching in between the spirocyclic form (which is colourless
and non-fluorescent) and the ring-opened amide form which is
pink and strongly fluorescent.10,11 In the literature, although a
number of rhodamine-labelled receptors are known for sensing
of different metal ions, the simultaneous detection of both Co2+
and Hg2+ ions by rhodamine-labelled receptor module is indeed
absent. However, in the present case, receptor 1 performs as a dual
probe for sensing of Co2+ and Hg2+ ions exhibiting both colorimet-
and Minamata disease.1 Similarly, Co2+ ions play an important role.
2
It is well known that Co2+ is the main composition of Vitamin B12
.
Animals deprived of cobalt show retarded growth; anaemia, loss of
appetite and decreased lactation.3 In large doses cobalt and its salts
can be toxic. Occupational exposure (>0.05 mg/m3) causes irritant
and allergic effects.3b In excess it is associated with acute pneumo-
nitis, dermatitis, asthma, cancer of the lung and sinus, adverse ef-
fects on blood and kidneys along with other disorders of the
respiratory and central nervous systems.3c Selective monitoring
of Co2+ in industrial, environmental and food samples is, therefore,
needed. The chemosensors without rhodamine-labelled for sensing
of this particular metal ion are known in the literature.4–7
ric and fluorometric responses in CH3CN–water (4:1, v/v; 10
tris HCl buffer; pH 6.8).
lM
O
O
In continuation of our work on the sensing of cations8 and
anions9 of biological significance, we report in this communication
a new rhodamine—based compound 1, which recognizes both Co2+
and Hg2+ ions by exhibiting emission characteristics in semi-aque-
O
O
H
N
N
ous system [CH3CN/water (4:1, v/v; 10 lM tris HCl buffer; pH 6.8)].
The present design in this account represents an example where
N
O
1
N
⇑
Corresponding author. Fax: +91 33 2582 8282.
E-mail address: ghosh_k2003@yahoo.co.in (K. Ghosh).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.09.062

K. Ghosh et al. / Tetrahedron Letters 54 (2013) 6464–6468
6465
O
O
O
(ii)
(i)
Cl
HO
O
O
O
O
CHO
2
3
O
NH2
N
(iii)
COOH
N
(iv)
N
NH2
1
H2N
N
O
N
O
4
Scheme 1. Reagents and conditions: (i) SOCl2, pyridine, dry CHCl3, reflux, 10 h, 95%; (ii) salicylaldehyde, K2CO3, EtOH, reflux, 6 h, 62%; (iii) EtOH, reflux, 9 h, 87%; (iv) a. 3, dry
MeOH, reflux, 10 h; b. NaBH4, dry MeOH, reflux, 12 h, 68%.
The receptor 1 was synthesized according to Scheme 1. Initially
the diethylene glycol monobutyl ether was converted to chloride 2
on reaction with SOCl2 in CHCl3. Salicylaldehyde was next reacted
with 2 in the presence of K2CO3 under refluxing condition to give
the intermediate 3. The aldehyde 3 was next refluxed with the
amine 412 in dry CH3OH to give the Schiff’s base which on reduc-
tion with NaBH4 introduced the desired compound 1 in apprecia-
ble yield. All the compounds were characterized by 1H NMR, 13C,
FTIR and mass analysis.
The pronounced OFF–ON type of Co2+—selectivity was further
established from the fluorescence at 580 nm. The Co2+ ion binding
induced change in emission of 1 in the presence and absence of
15 equiv amounts of other metal ions was evaluated (Supplemen-
tary data, Fig. 3S) and the interference of the metal ions considered
in the present study, is established to be negligible. The stoichiom-
etries13 of the complexes of 1 with both Co2+ and Hg2+ ions were
established to be 1:1 and the binding constant values (Ka)14 were
found to be (8.95 0.89) Â 104 MÀ1 and (6.02 1.9) Â 104 MÀ1
for Co2+ and Hg2+, respectively (Supplementary data). The values
are close in magnitude and a small increase in Ka for Co2+ is attrib-
uted to its better fitting at the binding core of 1 (Fig. 3a). Due to a
minor change in emission we were unable to determine the bind-
ing constant values for other metal ions.
The metal ion binding properties of 1 towards the metal ions
such as Hg2+, Co2+, Cd2+, Fe3+, Mg2+, Co2+, Ni2+, Zn2+, Ag+, Pb2+
,
,
Al3+, Cr3+ and Sn4+ (taken as their perchlorate salts except Al3+
Cr3+ and Sn4+
(CH3CN:H2O = 4:1, v/v; 10
)
were investigated in CH3CN/H2O solution
M tris HCl buffer, pH 6.8). The solution
l
of 1 without cations, is nearly non fluorescent. However, on excita-
tion at 490 nm, a nonstructured emission at 580 nm underwent an
insignificant change upon contact with all the metal ions except
Hg2+ and Co2+ ions (Supplementary data). Figure 1 shows the
change in fluorescence ratio [(I À I0)/I0] of 1 at 580 nm in the pres-
ence of 20 equiv amounts of the different metal ions.
To support the binding structure, we recorded the FTIR and 1H
NMR of 1 in the presence and absence of the equiv. amount of
the mercury and cobalt salts. The amide carbonyl stretching of
the spirolactam part in FTIR appeared at 1678 cmÀ1 and reduced
to a lower wave number 1671 and 1675 cmÀ1 in the presence of
Hg2+ and Co2+ ions, respectively. In 1H NMR, the signals of 1 both
in the aromatic and aliphatic regions became broad in the presence
of Co2+ and Hg2+ ions (Fig. 3b). In the presence of Co2+ and Hg2+ ions
the signals for the different types of assigned protons underwent a
chemical shift change (see the caption of Fig. 3b). It is clear from
the spectral change that both Hg2+ and Co2+ are complexed in the
cavity involving the amide ion and poly ether chain.
It is evident from Figure 1 that the receptor is much selective to
Co2+ ions. Other metal ions except Hg2+ weakly perturbed the
emission of 1 at this wavelength. On progression of titration of 1
with the metal ions, it is observed that only in the presence of
Co2+ and Hg2+ ions a new peak at 580 nm appears with a significant
intensity. Figure 2a shows the emission titration spectra with Co2+
ions and also the associated change in colour under illumination of
UV light. Figure 2b, under identical condition, is the emission titra-
tion spectra obtained from the gradual addition of Hg(ClO4)2
solution to the solution of 1 (c = 2.25 Â 10À4 M) in CH3CN/H2O
The ring opening in 1 to form the metal chelated species of
type 1A was confirmed by 13C NMR (Supplementary data). The
gradual disappearance of the signal at 67.6 ppm for the tertiary
carbon of the spirolactam ring of 1 (labelled as ‘l’; Fig. 3a) inti-
mated the opening of the spirolactam ring. Careful analysis of
13C NMR reveals that the open form is in equilibrium with the
cyclic structure. As shown in Fig. 4a, without Co2+ ion, 1 scarcely
shows absorption at 555 nm, indicating that 1 exists in spirolac-
tam form. Addition of Co2+ (Fig. 4a) and also Hg2+ (Fig. 4b) sepa-
rately to the solution of 1 (c = 2.25 Â 10À4 M) in CH3CN/H2O (4:1,
(4:1, v/v; 10 lM tris HCl buffer; pH 6.8). Other ions failed to devel-
op the peak at 580 nm (Supplementary data). Thus these two ions
(Co2+ and Hg2+) are easily distinguishable from the rest of the ions
by examining the peak at 580 nm (Supplementary data).
v/v; 10 lM tris HCl buffer; pH 6.8) brought about a strong
absorption at 555 nm along with clear colour change from colour-
less to pink, as is normally noticed for rhodamine—based probes.
The appearance of pink colour is attributed to the opening of the
spirolactam rings and creation of the delocalized xanthene moie-
ties. This was not observed when the titrations were conducted
with other metal ions (Supplementary data). In case of Fe3+ weak
absorption at 555 nm along with faint pink colour of the solution
was noticed (Supplementary data). The stoichiometries13 of both
Co- and Hg complexes in the ground state and in excited state,
are observed to be 1:1 (supporting information). To check the
reversibility in the complexation, fluorescence and absorption
spectra of cobalt and mercury complexes of 1 in CH3CN/H2O
(4:1, v/v; 10
lM tris HCl buffer; pH 6.8) were observed upon
Figure 1. Change in fluorescence ratio of 1 (c = 2.25 Â 10À4 M) at 580 nm upon
addition of 20 equiv amounts of cations (taken as perchlorate salt; Al2(SO4)3,
Cr(NO3)3 and SnCl4 were considered for Al3+, Cr3+ and Sn4+).
addition of KI and Na2EDTA solution. Addition of KI and Na2EDTA
solution reduced both emission and absorption. The pink colour

6466
K. Ghosh et al. / Tetrahedron Letters 54 (2013) 6464–6468
(a)
(b)
60
50
40
30
20
10
0
Hg2+
50
Co2+
40
30
20
10
0
500
550
600
650
700
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Figure 2. Fluorescence titration spectra of 1 (c = 2.25 Â 10À4 M) in CH3CN/water (4:1, v/v; 10
l
M tris HCl buffer, pH 6.8) upon addition of (a) Co2+, (b) Hg2+; inset: colour
change of the receptor solution under illumination of UV light.
(a)
(b)
c
d
O
b
O
O
e
O
a
O
O-
N
O
H
g
O
N
f
N
h
N
H
i
j
N
l
N
O
N
N
O
k
= Co2+/ Hg2+
1
1A
Figure 3. (a) Suggested modes of interaction of 1 with Co2+/Hg2+ ions in solution; (b) partial 1H NMR (400 MHz) of (i) 1 (6.2 Â 10À3 M); with 1 equiv amount of (ii) Co(ClO4)2
[D
d for a = À0.02, b = À0.02, c, d = À0.02 to À0.03, f = 0.15, g = 0.12, h = 0.16] and (iii) Hg(ClO4)2
[Dd for a = 0.07, b = 0.25, c, d = 0.12 to 0.14, f = 0.12, g = 0.13, h = 0.22] in CDCl3.
(a)
(b)
2.5
2.0
1.5
1.0
0.5
0.0
2.5
2.0
1.5
1.0
0.5
0.0
Co2+
Hg2+
350
400
450
500
550
600
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
M tris HCl buffer; pH 6.8) upon addition of (a) Co2+, (b) Hg2+
.
Figure 4. Absorption titration spectra of 1 (c = 2.25 Â 10À4 M) in CH3CN/H2O (4:1, v/v; 10
l
of both the solutions vanished, indicating the reversibility in the
complex formation (Supplementary data).
the deep pink colour of 1-Hg2+ complex became faint and under
illumination of UV light gave blue fluorescence (Supplementary
data, Fig. 8S). In case of 1-Co2+ complex, the pink colour changed
Further, to differentiate the Hg2+ and Co2+ complexes of
receptor 1 from each other, we carried out an experiment adding
to slight wine colour on gradual addition of
L-cysteine. It is men-
L-cysteine. Addition of
L
-cysteine to the solution of Hg2+ and Co2+
tionable that homocysteine, glutathione and non thiol-containing
complexes of 1 brought different results in colour as well as in fluo-
a
-amino acid (e.g., L
-valine) were unable to distinguish the Hg2+
rescence. On addition of
L
-cysteine, fluorescence intensities of
and Co2+ complexes (Supplementary data, Fig. 9S).
1-Hg2+ and 1-Co2+ (Supplementary data) complexes were de-
creased almost to the same extent. This was also true in UV–vis
spectra (Fig. 5). But in case of Co2+-1 complex, the colour of the
In an effort to understand the detection limit for Co2+ and Hg2+
ions by sensor 1, fluorescence spectra of 1 were recorded in the
presence of respective ions of different concentrations (Supple-
mentary data, Fig. 10S). Analysis of the results gives the detection
limit 10À4 to 10À5 M.15 Visually both Co2+ and Hg2+ ions can be
detected successfully up to 1 Â 10À3 M.
solution in the presence of
this regard, addition of -cysteine diminished entirely the absorp-
tion intensity of 1-Hg2+ complex at 555 nm (Fig. 5a) whereas in
case of 1-Co2+ complex
-cysteine reduced the intensity at
555 nm to a small extent (Fig. 5b). In the presence of -cysteine,
L-cysteine was not fully discharged. In
L
L
The potential biological application of the sensor 1 was evalu-
ated for in vitro detection of Co2+ and Hg2+ ions in human cervical
L

K. Ghosh et al. / Tetrahedron Letters 54 (2013) 6464–6468
6467
(a)
(b)
2.00
2.5
2.0
1.5
1.0
0.5
0.0
1.75
1.50
1.25
(--)(Receptor + Hg2+)= A
(--)A+ Cysteine
(--) (Receptor + Co2+)= A
(--)A+ Cysteine
1.00
0.75
0.50
0.25
0.00
300
350
400
450
500
550
600
300
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Figure 5. Change in absorbance of (a) 1-Hg2+; (b) 1-Co2+ complex in CH3CN–H2O (4:1, v/v; 10
colour change upon addition of -cysteine.
lM tris HCl buffer; pH 6.8) upon addition of
L
-cysteine (c = 1.5 Â 10À3 M); Inset:
L
cancer (HeLa) cells. The HeLa cells were incubated with 5
l
l of
of the receptor inside the cells and the binding of Hg2+ and Co2+
ions with the receptor.
sensor 1 (10 M in CH3CN/H2O (4:1, v/v)) in DMEM (Dulbecco
l
modified eagles medium) medium (without FBS) for 30 min at
37 °C and washed with phosphate buffered saline (PBS) buffer
(pH 7.4) to remove excess of sensor 1. DMEM medium (without
FBS) was again added to the cells. The cells were next treated
The KI-adding experiments as experimental evidence to support
the reversibility in structural change was also applied in human
cervical cancer (Hela) cells. Red coloured cells obtained from the
incubation of the receptor followed by treatment with Co2+/Hg2+
with 5
l
l of Co(ClO4)2 (30
l
M) and incubated again for 30 min
became invisible in fluorescence upon addition of KI (30 lM) (Sup-
at 37 °C. A control set of cells which was devoid of Co2+ ion was
kept. Similar experiment was done for Hg(ClO4)2. The addition
of sensor 1 to the cells did not show any cytotoxicity (Supple-
mentary data) as evident from the morphology of the cells
(Fig. 6). In this context, Figure 6a and b represents the bright field
images of the cells before and after treatment of the cells with 1,
respectively. Cells incubated with 1 without Hg2+ and Co2+
(Fig. 6c) did not show any fluorescence property. It was also true
when cells were incubated with Co2+ and Hg2+ ions without
receptor 1 (Fig. 6d and e). On the contrary, cells incubated with
the receptor 1 and then with Hg2+ ions showed the occurrence
of red fluorescence (Fig. 6f). Again cells incubated with the
receptor 1 and then with Co2+ ions showed the occurrence of
red fluorescence (Fig. 6g). These facts indicate the permeability
plementary data). A similar result was also obtained in the case of
Hg2+ complex.
In conclusion, we have shown that the rhodamine-labelled
receptor 1 is capable of detecting Co2+ and Hg2+ ions simulta-
neously in aq. CH3CN by exhibiting different extents of change in
emission. The present example will be a new addendum in the
literature on rhodamine labelled receptors for simultaneous detec-
tion of Co2+ and Hg2+ ions. Simultaneous involvement of amide
parts of the rhodamines with the polyether chain favour the strong
chelation of Co2+ and Hg2+ ions over the other cations examined.
Inspite of almost identical behaviour in fluorescence of receptor
1 towards Hg2+ and Co2+, the disappearance of the colour of the
mercury- ensemble in the presence of
L-cysteine distinguishes it
from Co2+ ions. Moreover, the chemosensor 1 is found to be
Figure 6. Fluorescence and bright field images of HeLa cells: (a) bright field image of normal cells; (b) bright field image of cells treated with receptor 1 (10
37 °C; (c) fluorescence image of cells treated with 1 (10 M) for 1 h at 37 °C; (d) fluorescence image of cells treated with Co(ClO4)2 (30.0 M) for 1 h at 37 °C; (e) fluorescence
image of cells treated with Hg(ClO4)2 (30 M) for 1 h at 37 °C; (f) red fluorescence images of cells upon treatment with receptor 1 (10 M) and then Hg(ClO4)2 (30 M) for 1 h
at 37 °C; (g) red fluorescence images of cells upon treatment with 1 (10 M) and then Co(ClO4)2 (30.0 M) for 1 h at 37 °C. kex = 510 nm.
lM) for 1 h at
l
l
l
l
l
l
l

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K. Ghosh et al. / Tetrahedron Letters 54 (2013) 6464–6468
efficient in reporting the presence of both Co2+ and Hg2+ ions inside
the cell.
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We thank D.S.T. and U.G.C., New Delhi, India for providing facil-
ities in the department. T.S. thanks CSIR, New Delhi, India for a
fellowship. We also thank Prof. A. R. Khuda-Bukhsh, department
of Zoology, University of Kalyani for his generous help in cell study.
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Supplementary data
Supplementary data (Figures showing the change in fluores-
cence and UV–vis titrations of receptor 1 with the metal ions, bind-
ing constant curves, Job plots, sensitivity of 1 towards Hg2+ and
Co2+ ions, reversibility in complexation, detection limit, MTT assay,
13C NMR spectral changes of 1 with the addition of Co2+, experi-
mental procedure, 1H, 13C NMR spectra and mass spectra are avail-
able.) associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.tetlet.2013.09.062.
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