Visible colourimetric and ratiometric fluorescent chemosensors for Cu(II) and Ni(II) ions

Article abstract of DOI:10.14233/ajchem.2016.19578

New fluorescent and colourimetric chemosensors based on copper(II) and nickel(II) coordination of bis-O,O-bidentate Schiff base ligands are reported. Its high selectivity towards the complexation with copper(II) and nickel(II) over other metals were demon

Full text of DOI:10.14233/ajchem.2016.19578

Asian Journal of Chemistry; Vol. 28, No. 5 (2016), 1035-1038
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2016.19578
Visible Colourimetric and Ratiometric Fluorescent Chemosensors for Cu(II) and Ni(II) Ions
*
SULAGNA BRAHMA, PRIYANKA RAY, RAJU SINGHA and JAYANTA K. RAY
Department of Chemistry, Indian Institute of Technology, Kharagpur-721 302, India
*Corresponding author: Fax: +91 3222 282252; Tel: +91 3222 283326; E-mail: jkray@chem.iitkgp.ernet.in
Received: 24 August 2015;
Accepted: 10 October 2015;
Published online: 30 January 2016;
AJC-17742
New fluorescent and colourimetric chemosensors based on copper(II) and nickel(II) coordination of bis-O,O-bidentate Schiff base ligands
are reported. Its high selectivity towards the complexation with copper(II) and nickel(II) over other metals were demonstrated by UV,
fluorescence, colour change in visible range and 1H NMR spectra.
Keywords: Schiff base, Metal ions, Complexation, UV, Fluorescence, Chemosensor.
compartments [8-10]. Ligands derived from 3-hydroxysalicyl-
dehyde and several simple diamines have been used to prepare
some N2O2 mononuclear and N2O2 + O4 binuclear complexes.
The latter mostly contain Cu(II), Ni(II) or Fe(III) in the inner
chamber and Mn(II), Co(II) or Fe(III) in outer compartment.
A few binuclear complexes of Cu(II) and Zn(II) are with Zn
ion coordinated in the outer O4 compartment [11].
In this paper, we have reported that, Cu(II) and Ni(II) can
also coordinate in the outer O4 compartment without forming
any binuclear complexes. Our system can act as flurescent
chemosensor as it is able to bind metal in organic medium and
change its emission in ratiometric fashion. This system displays
dual emission, which makes it ‘internally calibrated’ for the
study of the association with metal by means of steady state
fluorescence spectroscopy. Schiff bases “Salen” coordinates
metal ions through imine nitrogen and phenolic oxygen atoms.
But our Schiff bases, H4L1 and H4L2, contain two oxygen and
one nitrogen donor in each half of the molecule for bonding,
so there will be a competition between these two sites. The
disappearance of two peaks for two -OH groups in proton NMR
spectra of the ligands after the addition of metal ions proves
that –OH groups are taking part in binding.
INTRODUCTION
Fluorescent and ratiometric sensors for the detection and
measurement of copper and nickel ions has been actively
investigated, as these metal ions are significant environmental
pollutant and are essentioal trace element in biological system
[1]. A colourimetric and ratiometric fluorescent chemosensor
combines the sensitivity of fluorescence with the convenience
and aesthetic appeal of a colourimetric assay [2]. Colour
changes, as signaling an event detected by the naked eye, are
widely used owing to the low cost or lack of equipment required.
A number of chemosensors for metal ions have been reported
to be capable of correlating metal ions concentration with
changes in spectroscopic characteristics [3]. Many of these
chemosensors are based on the interactions of metal ion with
the π-electron system of a chromophore or fluorophore in the
host-guest chemistry, resulting upon complexation.
Although many interesting results have been reported in
recent years regarding colourimetric and ratiometric fluore-
scent chemosensors for Cu(II) and Ni(II) [4], there are still
abundant aspects that need to be conveniently addressed in
this field. Especially reports of highly selective chemosensor
for target metal ions are still limited. The perceived colour
change is useful not only for the ratiometric method of detection
but also for rapid visual sensing.
EXPERIMENTAL
The Schiff bases H4L1 and H4L2 were synthesized by the
usual condensation of bis-(4-aminophenyl) ether and bis-(4-
aminophenyl) sulphone (0.5 mol) with 2,3-dihydroxy benzal-
dehyde (1 mol) in methanol. Red (H4L1) and yellow (H4L2)
coloured solid precipitate obtained were collected by filtration.
Here we have shown that these two ligand can bind Cu(II) and
Ni(II) ions in bidentate fashion to each metal ions showing
change in colour in the visible range.
Method for the construction of copper(II) coordinated
supra molecular architectures of bis-N,O-bidentate Schiff base
ligands has also been reported earlier [5-7].
It is reported that bis-3-hydroxy derivatives of Salen can
bind d- and f-metal ions to their inner N2O2 and outer O4 sites,
respectively. It was also reported that some of these hexadentate
Schiff bases can simultaneously coordinate d-ions in both

1036 Brahma et al.
Asian J. Chem.
O O
s
2:1 molar ratio, stirred for 0.5 h at room temperature. After
that a reddish brown coloured precipitate was formed and
collected by filtration and washed thoroughly with water to
remove excess Ni(II) acetate. IR (KBr, νmax, cm-1): 1625, 1575,
1460 cm-1; 1H NMR (200 MHz, CDCl3): δ 9.60 (s, 2H), 6.15-
7.01 (m, 7H), 7.21-8.01(m, 7H); Mass (MALDI): m/z 624 (M+
+ Na); Elemental analysis calcd. for C26H16N2O6SNi2·8H2O:
C 41.86, H 4.29, N 3.75 %, found C 41.69, H 4.11, N 3.81 %.
O
N
N
N
N
OH
HO
HO
OH
OH
HO
HO
OH
H4L1
H4L2
Colour change from yellow to deep green and yellow to
red was observed when Cu(II) and Ni(II) ion solutions were
added to H4L1 respectively. Similarly the change from pale
yellow to green and pale yellow to orange were observed when
Cu(II) and Ni(II) ion were added to H4L2 in acetonitrile solution.
Addition of other metal ions such as Li(I), Na(I), K(I), Mg(II),
Ca(II), Fe(III), Cd(II),Ag(I) and Pb(II) produced insignificant
changes in both absorption and fluorescent spectra of both
H4L1 and H4L2. This means that H4L1 and H4L2 have an outstan-
dingly high selectivity for Cu(II) and Ni(II). So H4L1 and H4L2
could be contemplated as “naked eye” chemosensor for Cu(II)
and Ni(II) ion.
Bis{4-[(2,3-dihydroxy)methylideneamino]phenyl}-
ether (H4L1): Bis-(4-aminophenyl)ether (1 g, 2.27 mmol) was
added gradually to a methanol (100 mL) solution of 2,3-
dihydroxy benzaldehyde (0.63 g, 4.54 mmol) and the solution
was stirred for 0.5 h. The red shining precipitate was collected
by filtration. m.p. 175 °C; IR (KBr, νmax, cm-1): 3430, 1623,
1463, 1360, 1270, 1231; 1H NMR (200 MHz, CDCl3): 13.18
(2H, br s), 9.21 (2H, s), 8.93 (2H, s), 7.5 (4H, d, J = 8.8),
7.06-7.16 (6H, m), 6.91-7.0 (2H, m), 6.78-6.81 (2H, m); Mass
(ES+): m/z 441 (M++1); Elemental analysis calcd. for
C26H20N2O5: C 70.90, H 4.54, N 6.36 %, found C 70.54, H
4.75, N 6.25 %.
Bis{4-[(2,3-dihydroxy)methylideneamino]phenyl}-
sulfone (H4L2): Bis-(4-aminophenyl)sulfone (1 g, 2.04 mmol)
was added gradually to a methanol (100 mL) solution of 2,3-
dihydroxy benzaldehyde (0.13 g, 4.09 mmol) and the solution
was stirred for 1 h at 50 °C and the yellow precipitate was
collected by filtration. m.p. 251 °C; IR (KBr, νmax, cm-1): 3434,
1620, 1570, 1460; 1H NMR (200 MHz, DMSO-d6): 10.69 (2H,
br s), 9.96 (2H, s), 8.37 (2H, s), 7.16-7.71 (6H, m), 6.98-7.05
(6H, m), 6.81 (t, 2H, J = 7.5); Mass (ES+): m/z 489 (M++1);
Elemental analysis calcd. for C26H20N2O6S: C 63.93, H 4.13,
N 5.73 %, found C 63.88, H 4.23, N 5.55 %.
RESULTS AND DISCUSSION
Upon addition of Cu(II) acetate solution to the H4L1
solution, the ligand π-π* band at 339 nm decreases and a new
band at 427 nm (Fig. 1a) due to bond-pair coordination of
oxygen lone pair with Cu(II), which induces a colour change
from yellow to green due to d-d transition. A mole ratio plot
using the change in absorbance at 343 and 422 nm clearly
demonstrated the formation of 1:2 complex (Fig. 2a).Addition
of Ni(II) ion to the H4L1 suloution the ligand π-π* band at 346
nm decreases and a new band at 409 nm (Fig. 1b) due to the
same reason, induce a colour change from yellow to red. A
mole ratio plot using the change in absorbance at 340 and 408
nm clearly demonstrated the formation of 1:2 complex (Fig. 2b).
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(a)
H4L1 + Cu(II)
250
300
350
400
450
500
550
Wavelength (nm)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(b)
H4L1 + Ni(II)
Cu(II) complex of H4L1: Aqueous solution (50 mL) of
Cu(II) acetate hydrate (0.34 g, 1.7 mmol) was added to the
acetonotrile solution (50 mL) of H4L1 (0.37 g, 0.85 mmol) in
2:1 molar ratio, stirred for 0.5 h at room temperature. After
that a brown coloured precipitate was formed and collected
by filtration and washed thoroughly with water to remove
excess Cu(II) acetate. IR (KBr, νmax, cm-1): 1620, 1570, 1460;
1H NMR (200 MHz, CDCl3): 8.93 (2H, s), 7.49 (4H, d, J =
8.2), 7.06-7.15 (4H, m), 6.90-6.99 (4H, m), 6.78-6.90 (2H,
m); Mass (ES+): m/z 437.19, 564.44, 565.31 (M+); Elemental
analysis calcd. for C26H16N2O5Cu2·4H2O: C 49.13, H 3.78, N
4.40 %, found C 49.02, H 3.69, N 4.54 %.
250
300
350
400
450
500
550
Wavelength (nm)
Ni(II) complex of H4L2: Aqueous solution (50 mL) of
Ni(II) acetate hydrate (0.34 g, 1.8 mmol) was added to the
acetonotrile solution (50 mL) of H4L1 (0.44 g, 0.9 mmol) in
Fig. 1. Uncorrected UV absorption spectra of H4L1 (1.5 × 10-4) in aceto-
nitrile solution in the presence of Cu(II) and Ni(II) at concentration
ranging from 0-50 µM. a) H4L1 with Cu(II), b) H4L1 with Ni(II)

Vol. 28, No. 5 (2016)
Visible Colourimetric and Ratiometric Fluorescent Chemosensors for Cu(II) and Ni(II) Ions 1037
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
1.0
(a)
(a)
H4L2 + Cu(II)
0.8
422
0.6
0.4
343
0.2
0.0
0.0
0.5
1.0
1.5
2.0
2.5
300
350
400
450
500
550
[Cu(II)/H4L1]
Wavelength (nm)
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
1.0
0.8
0.6
0.4
0.2
0.0
(b)
(b)
H4L2 + Ni(II)
408
340
0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50
[Ni(II)/H4L1]
250
300
350
400
450
500
550
Fig. 2. (a) Molar ratio plot of H4L1 and Cu(II) using the change in
absorbance at 340 and 408 nm, clearly demonstrate the formation
of the Cu(II):H4L1= 2:1 molar ratio as judged by observing the clear
inflection point at [Cu(II)/H4L1] = 2; (b) Molar ratio plot of H4L1
and Ni(II) using the change in absorbance at 340 and 408 nm, clearly
demonstrate the formation of the Ni(II):H4L1 = 2:1 molar ratio as
judged by observing the clear inflection point at [Ni(II)/H4L1] = 2
Wavelength (nm)
Fig. 3. Uncorrected UV absorption spectra of H4L1 (1.5 × 10-4) in aceto-
nitrile solution in the presence of Cu(II) and Ni(II) at concentration
ranging from 0-50 µM; (a) H4L2 with Cu(II), (b) H4L2 with Ni(II)
Upon addition of Cu(II)acetate solution to the H4L2 solu-
tion, the ligand π-π* band at 342 nm decreases and a new
band at 422 nm due to bond-pair coordination of oxygen with
Cu(II), inducing a colour change from pale yellow to green
(Fig. 3a). Upon addition of Ni(II) ion to the H4L2 solution, the
ligand π-π* band at 339 nm decreases and a new band at 413
nm due to the same reason (Fig. 3b).
O
O
OH2
Cu
OH2
Cu
O
O
OH2
OH2
H4L1
So proposed mode of complexation of H4L1 and H4L2 with
Cu(II) and Ni(II) is as follows (Fig. 4). This is also supported
by both mass spectroscopy and CHN analysis.
The fluorescence of H4L1 and H4L2 were recorded in freshly
distilled acetonitrile as solvent. The addition of Cu(II) solution
to the H4L1 shows decrease in intensity at 393 nm (Fig. 5a)
after addition of 1 equiv of Cu(II). As expected, when more
than 1 equiv of Cu(II) was added a red-shifted fluorescence
emission band was found centered at 286 nm. Similarly on
addition of Ni(II) solution there was a decrease in flourosence
intensity at 394 nm and an increase at 287 nm (Fig. 5b). Same
phenomenon was repeated in case of H4L2 when Cu2+ and Ni2+
added individually to it (Fig. 5c and 5d).
O
O
OH2
Ni
OH2
Ni
O
O
H2O
H2O
OH2
OH2
OH2
OH2
H4L2
Fig. 4

1038 Brahma et al.
Asian J. Chem.
1800000
1800000
1600000
1400000
1200000
1000000
800000
600000
400000
200000
0
H4L1 + Ni(II)
H4L1 + Cu(II)
(b)
(a)
1600000
1400000
1200000
1000000
800000
600000
400000
200000
0
300
350
400
450
500
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
H4L2 + Cu(II)
1800000
1600000
1400000
1200000
1000000
800000
600000
400000
200000
0
1800000
1600000
1400000
1200000
1000000
800000
600000
400000
200000
0
H4L2 + Ni(II)
(c)
(d)
300
350
400
450
500
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
Fig. 5. Uncorrected emission spectra of H4L1 and H4L1 (1.5 × 10-4 M) in acetonitrile in the presence of Cu(II) and Ni(II) solutions at concen-
tration ranging from 0-50 µM. Excitation wavelength: 250 nm. (a) H4L1 with Cu(II), (b) H4L1 with Ni(II), (c) H4L2 with Cu(II), (d)
H4L2 with Ni(II)
3. (a) de Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P.
Conclusion
McCoy, J.T. Rademacher and T.E. Rice, Chem. Rev., 97, 1515 (1997);
A new colourimetric chemosensors for Cu(II) and Ni(II)
ions are developed, which give rise to large changes in UV,
fluorescence and show change in colour in the visible range,
providing a great advantage for the detection of those metal
ions. The solution colour change and the change in fluore-
scence spectra attributed to the deprotonation of phenolic -OH
conjugated to the imine attached to the phenyl ring. The design
strategy and characteristics physical properties of those probes
would help to extend the development of fluorescent and
colourimetric probes for metal ions.
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