2,2′-(Hydrazine-1,2-diylidenedimethylylidene)bis(6-isopropyl-3-methylphenol) based selective dual-channel chemosensor for Cu2+in semi-aqueous media

Article abstract of DOI:10.1039/c4ra04656k

A novel receptor based on a substituted salicylaldehyde derivative, 2,2′-(hydrazine-1,2-diylidenedimethylylidene)bis(6-isopropyl-3-methylphenol) (1), was synthesized and characterized using various spectroscopic techniques. The UV-visible spectroscopic studies of the receptor showed selectivity for Cu²⁺in the presence of other metal ions by a change in colour from colourless to yellow. A red shift was observed with the appearance of a new absorption band at 450 nm in CH₃OH-H₂O (60 : 40 v/v). Fluorescence studies of the receptor showed turn-off recognition properties for the Cu²⁺ion with a 1 : 1 binding stoichiometry and the detection limit was 50 nM. This journal is

Full text of DOI:10.1039/c4ra04656k

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2,20-(Hydrazine-1,2-diylidenedimethylylidene)
bis(6-isopropyl-3-methylphenol) based selective
dual-channel chemosensor for Cu2+ in semi-
aqueous media†
Cite this: RSC Adv., 2014, 4, 39639
a
d
b
c
a
*
*
Umesh Fegade, Anu Saini, Suban K. Sahoo, Narinder Singh, Ratnamala Bendre
and Anil Kuwar
a
*
A novel receptor based on a substituted salicylaldehyde derivative, 2,20-(hydrazine-1,2-diylidenedimethyl-
ylidene)bis(6-isopropyl-3-methylphenol) (1), was synthesized and characterized using various
spectroscopic techniques. The UV-visible spectroscopic studies of the receptor showed selectivity for
Cu2+ in the presence of other metal ions by a change in colour from colourless to yellow. A red shift
was observed with the appearance of a new absorption band at 450 nm in CH3OH–H2O (60 : 40 v/v).
Fluorescence studies of the receptor showed “turn-off” recognition properties for the Cu2+ ion with a
1 : 1 binding stoichiometry and the detection limit was 50 nM.
Received 17th May 2014
Accepted 6th August 2014
DOI: 10.1039/c4ra04656k
www.rsc.org/advances
and Wilson's disease, and it has been observed that these
diseases are caused by excessive intracellular copper trans-
port.4–9 Cu2+ can also react with molecular oxygen to form
reactive oxygen species, which can damage lipids, nucleic acids
and proteins. The signicant physiological relevance of Cu2+
and its associated biomedical implications has resulted in
considerable interest in the design of highly selective and
sensitive copper chemosensors.
Non-cyclic compounds containing multiple coordination
sites have received considerable attention because of their
ability to complex various ionic and/or neutral molecules.10–16
Such non-cyclic derivatives of hydrazide show interesting
coordination properties towards transition metal ions due to
the presence of several potential coordination sites.17 The
proton of the –NH group becomes more labile when a
condensation reaction takes place between the terminal NH2
group and an aldehyde or ketone; the resulting acyl hydrazones
react with metal ions in the enol form.18–20 These receptors
coordinate strongly with metal ions, making them excellent
candidates for cation sensing. In the investigation reported
here, we detected Cu2+ ions via UV-visible and uorescence
spectroscopic techniques using a chemosensor based on a
derivatized salicyaldehyde and hydrazine hydrate. The
substituted salicyaldehyde was selected based on the natural
occurrence of the parent compound thymol (2-hydroxy-3-iso-
propyl-6-methyl benzene) and its broad spectrum of activity.21,22
Introduction
The design and synthesis of chemosensors with high selectivity
and sensitivity for transition metal ions such as copper are
receiving great interest in the eld of supramolecular chemistry
due to the importance of these metals in the material sciences
and biology.1–3 Copper is an essential trace nutrient in human
metabolism and plays a vital part in the physiological processes
of organisms, including connective tissue development and the
formation of bone and blood. However, free Cu2+ is potentially
both acutely and chronically toxic to aquatic life and micro-
organisms are affected at even micromolar concentrations. In
humans, the brain concentrates heavy metal ions such as
copper for use in metabolic processes. Copper is important for
the normal development and working of the brain and, as a
cofactor for many enzymes, it actively participates in many
physiological pathways in the brain. Any disturbance in the
uptake of copper may lead to neurodegenerative disorders.
Gene mutations are responsible for the two major genetic
disorders of copper metabolism in humans, Menkes' disease
aSchool of Chemical Sciences, North Maharashtra University, Jalgaon, 425001 MS,
India. E-mail: kuwaras@gmail.com; bendrers@rediffmail.com; Fax: +91-257-
2257403; Tel: +91-257-2257432
bDepartment of Applied Chemistry, SV National Institute of Technology, Surat,
Gujarat, India
cDepartment of Chemistry, Indian Institute of Technology, Ropar, Rupanagar, Punjab,
India. E-mail: nsingh@iitrpr.ac.in
dCentre for Nanoscience and Nanotechnology (UIEAST), Panjab University,
Experimental
Chandigarh, India
All laboratory reagents used for synthesis were of 99% pure
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra04656k
grade and were used without further purication. 1H- and
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13C-NMR spectra were recorded on a Varian NMR Mercury [G] is the Cu2+ concentration. The uorescence intensity at 485
System 300 spectrometer operating at 300 and 75 MHz, nm was used for the calculation. The concentration of [HG] was
respectively, in CDCl3 using Me4Si as an internal standard. The calculated by the equation [HG] ¼ DI/Io ꢁ [H].
uorescence and UV-visible spectra were recorded on a Fluo-
romax-4 spectrouorimeter and a Shimadzu UV-24500 spec-
trophotometer in the range 200–600 nm at 28 C using a 1 cm
cell. All the spectral experiments were performed in a mixed
solvent system of CH3OH–H2O (60 : 40 v/v). This solvent
Synthesis of receptor 1
ꢀ
Hydrazine hydrate (0.05 g, 1.0 mmol) was added to a solution
2-hydroxyl-3-isopropyl-6-methylbenzaldehyde (0.35 g, 2.0
mmol) in ethanol (50 mL) and the mixture was reuxed for 8 h
mixture was used because it resolved the solubility issues of the
at 80 ꢀC. The yellow solid obtained at room temperature was
ligand and the chosen receptor showed the best performance in
ltered, dried and further puried by recrystallization (83%
this solvent system.
yield).30 IR (KBr, cmꢂ1): 3420 (–OH), 1600 (–C]N–). 1H-NMR
(CDCl3, 300 MHz): d 1.26–1.29 (d, J ¼ 7.2 Hz, 12H, 4CH3), 2.37 (s,
Sample preparation
6H, Ar-CH3), 3.33 (heptet, J ¼ 7.2 Hz, 2H, 2CH–Me2), 6.68–7.11
(d, J ¼ 7.3 Hz, 4H, Ar-H), 9.10 (s, 2H, CH]N). 13C-NMR (CDCl3,
All stock and working solutions were prepared in ultrapure
water and spectroscopic grade methanol. A stock solution of
2,20-(hydrazine-1,2-diylidenedimethylylidene)bis(6-isopropyl-3-
methylphenol) (receptor 1) (c ¼ 5 ꢁ 10ꢂ3 M) was prepared in
CH3OH–H2O (60 : 40 v/v) solution and the corresponding
working solutions (c ¼ 5 ꢁ 10ꢂ6 M) were prepared simply by
diluting with CH3OH–H2O (60 : 40 v/v). Similarly, the stock
solutions of all the metal ions (c ¼ 5 ꢁ 10ꢂ3 M) were prepared in
CH3OH–H2O (60 : 40 v/v) and the corresponding working
solutions (c ¼ 5 ꢁ 10ꢂ5 M) were prepared by diluting with
CH3OH–H2O (60 : 40 v/v).
75 MHz): d 17.3, 24.6, 27.4,24.3, 117.4, 120.8, 129.3, 136.3, 137.9,
149.2, 156.3.
Results and discussion
Synthesis and characteristics of receptor 1
Receptor 1 was synthesized by the Schiff base condensation of
hydrazine hydrate with two moles of 2-hydroxyl-3-isopropyl-6-
methylbenzaldehyde in ethanol (Scheme 1). The compound was
characterized using various techniques.30
Photophysical studies
Recognition studies of receptor 1
The cation binding studies were performed on a UV-visible
The absorption behaviour of receptor 1 (c ¼ 5 ꢁ 10ꢂ6 M) with
various metal ions was studied in CH3OH–H2O (60 : 40 v/v). The
absorption spectrum of receptor 1 showed two maxima at 325
and 375 nm (Fig. 1). With the addition of Cu2+, the band at 375
nm disappeared and a new peak developed at 450 nm. The
reason behind the development of the new peak at 450 nm may
be the possible charge transfer between receptor 1 and Cu2+
(Fig. 1). The spectral shi also clearly delineated that the core
functionality provided by receptor 1 was suitable for the selec-
tive encapsulation of Cu2+. The effect of Cu2+ was so substantial
that it could also be detected by the naked eye as a distinct
colour change of the solution from colourless to yellow. In
general, cations such as Fe3+, Ni2+ and Co2+ are known to
interfere in Cu2+ ion detection. However, in this study, no
signicant change in the colour of the solution was observed
with the addition of these potentially interfering cations.23
For an in-depth study of the sensing ability of the Cu2+ ion,
titrations were performed by the addition of small amounts of
Cu2+ to a solution of receptor 1 (c ¼ 5 ꢁ 10ꢂ6 M). With
successive additions of Cu2+, there was a decrease in the
absorbance at 375 nm and an increase in the absorbance at 450
nm, with two isosbestic points at 295 and 405 nm (Fig. 2). To
spectrophotometer using different metal ions (Cr3+, Mn2+, Fe3+,
Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Pb2+, Na+, K+, Ba2+ and Al3+
)
with receptor 1 in CH3OH–H2O (60 : 40 v/v) at room tempera-
ture. The ability of receptor 1 to bind selectively to a particular
metal ion was investigated by performing titrations. The titra-
tions conrmed the linear relationship with the selective metal
ion and the change in absorbance intensity was used to calcu-
lation the linearity range and the correlation coefficient. These
titrations were carried out through the addition of a metal salt
solution in small aliquots (c ¼ 5 ꢁ 10ꢂ5 M) to a solution of
receptor 1 (c ¼ 5 ꢁ 10ꢂ6 M) in CH3OH–H2O (60 : 40 v/v) in a 10
mL volumetric ask. The absorbance intensity was recorded in
the range 200–600 nm together with a reagent blank.
The metal binding test was carried out on a Fluoromax-4
spectrouorimeter in CH3OH–H2O (60 : 40 v/v) at 28 ꢀC. The
uorescence intensity was recorded at lex/lem ¼ 405/585 nm
alongside a reagent blank. The excitation and emission slits
were both set to 5.0 nm. Titrations between receptor 1 and Cu2+
were used to evaluate the association constant (Ka) and the limit
of detection. These titrations were carried out by the successive
addition of metal salt solutions (c ¼ 5 ꢁ 10ꢂ5 M) to a solution of
receptor 1 (c ¼ 5 ꢁ 10ꢂ6 M) in a 10 mL volumetric ask.
The stoichiometry of the complex formed was determined by
preparing solutions of receptor 1 and Cu2+ in the ratios 1 : 9,
2 : 8, 3 : 7, 4 : 6, 5 : 5, 6 : 4, 7 : 3, 8 : 2 and 9 : 1. These solutions
were shaken and then the uorescence spectra were recorded.
The plot of [HG] versus [H]/([H] + [G]) was used to determine the
stoichiometry of the complex formed, where [HG] is the
concentration of the complex, [H] is the host concentration and Scheme 1 Synthesis of receptor 1 (a ¼ ethanol, 8 h reflux).
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with the calculated bond length for Cu–N and an average Cu–O
˚
˚
of 2.044 A and 1.876 A, respectively. The highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) of 1 were distributed uniformly over the entire
molecule (Fig. 3b and c). However, analysis of the frontier
molecular orbital plots of the 1:Cu2+ complex indicated that
intramolecular charge transfer occurred between receptor 1 and
Cu2+. The band gap between the HOMO and LUMO of 1 was also
lowered on complexation, which conrmed the observed red
shi in the absorption band.
The specicity of receptor 1 for the detection of Cu2+ in the
presence of other interfering metal ions was determined by the
addition of 1 equiv. of Cu2+ to receptor 1 in the presence of 2
equiv. of all other metal ions. As shown in Fig. 4, the results
showed that the miscellaneous competitive cations did not
lead to any signicant spectral change. The data clearly suggest
that there is no interference from other metal ions in the
Fig. 1 Changes in the absorbance spectrum of receptor 1 (c ¼ 5 ꢁ
10ꢂ6 M) in the presence of various metal ions (c ¼ 5 ꢁ 10ꢂ5 M) in
CH3OH–H2O (60 : 40 v/v).
sensing of Cu2+
.
conrm the relationship between the absorbance intensity and
the concentration of Cu2+, a graph was plotted of A450/A375 vs.
[Cu(II)] (Fig. 2 inset). The linear dependence of the concentra-
tion of Cu2+ conrmed that receptor 1 could be used for the
quantitative determination of Cu2+.
The uorescence properties of receptor 1 were studied with
addition of various metal nitrates (c ¼ 5 ꢁ 10ꢂ5 M) at an exci-
tation wavelength of 405 nm in CH3OH–H2O (60 : 40 v/v). No
signicant change was observed with the addition of different
metal nitrate salts, but on adding Cu2+ there was quenching in
the broad spectrum peak at 585 nm (Fig. S1†). To investigate the
sensing capability of the receptor for Cu2+, a titration was
carried out with incremental additions of Cu2+ to receptor 1.
Fig. 5 shows that the uorescence intensity is quenched with
the successive addition of Cu2+. Receptor 1 contains an intra-
molecular hydrogen bond between the phenolic –OH moiety
and the nitrogen of the imine group that undergoes excited
state intramolecular proton transfer and yields a normal emis-
sion at 585 nm from the proton transfer tautomer.25 Quenching
The charge transfer process during the encapsulation of Cu2+
by receptor 1 was investigated by density functional theory
calculations applying the B3LYP functional and the basis sets 6-
31G** (for C, H, N and O atoms) and LANL2DZ (for the Cu
atom), available in the computational code Gaussian 09W.24 The
optimized structure of receptor 1 and its complex with Cu2+ is
shown in Fig. 3a. With the complexation of 1 with Cu2+, a
lowering of the interaction energy by ꢂ106.67 kcal molꢂ1 was
observed, which indicated the formation of a stable complex
Fig. 2 Change in absorption profile of receptor 1 (c ¼ 5 ꢁ 10ꢂ6 M) upon gradual addition of Cu2+ (c ¼ 5 ꢁ 10ꢂ5 M) in CH3OH–H2O (60 : 40 v/v).
Inset shows the normalized response of absorbance signal with a regression of 0.9624.
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Fig. 3 (a) Density functional theory computed optimized structure of receptor 1 and its complex with Cu2+. (b) LUMO and (c) HOMO diagrams of
1 and the 1–Cu2+ complex.
between the deprotonated ligand (receptor 1) and a metal ion:
[1–Cu2+ + 2Na+], MW ¼ 458.046; calculated [C22H26N2O2 Cu2+
+
2Na], 458.20. The association constant (Ka) value of the 1–Cu2+
complex calculated from the uorescence titration by the Ben-
esi–Hildebrand27 (Fig. S3†) methodology was 666667 Mꢂ1. The
quenching can be expressed mathematically by the Stern–
Volmer equation (eqn (1)), which allows the calculation of
quenching constants:28
F0/F ¼ 1 + kqs0[Q] ¼ 1 + Ksv[Q]
(1)
where F0 and F are the uorescence intensities in the absence
and presence of the quencher, kq is the bimolecular quenching
constant, s0 is the lifetime of the uorescence in the absence of
the quencher, [Q] is the concentration of the quencher and Ksv is
the Stern–Volmer quenching constant. In the presence of a
quencher, the uorescence intensity is reduced from F0 to F.
The ratio (F0/F) is directly proportional to the quencher
concentration [Q]. Evidently:
Fig. 4 Absorbance ratio (A450/A375) of receptor 1 (c ¼ 5 ꢁ 10ꢂ6 M) with
1 equiv. of Cu2+ and 2 equiv. of the competitive metal ions.
of the emission at 585 nm was observed due to the involvement
of the phenolic –OH groups of receptor 1 in complex formation
with Cu2+, which inhibited the excited state intramolecular
proton transfer phenomenon.
Ksv ¼ kqs0
(2)
(3)
F0/F ¼ 1 + Ksv[Q]
The emission intensity of receptor 1 was linearly propor-
tional to the Cu2+ concentration (Fig. 5, inset). The detection
limit was calculated using 3*S/M following the IUPAC criterion,
where S is the standard deviation of a blank signal and M is the
slope of the regression line. The detection limit was 50 nM,
which is comparable with other reported Cu2+ sensors
(Table S1†).
The stoichiometry of the complexation of receptor 1 with
Cu2+ was studied using the continuous variation method (Job's
plot).26 Job's plot and the normalized plot obtained from uo-
rescence measurements showed the formation of the receptor 1
and Cu2+ complex in a 1 : 1 ligand to metal ratio (Fig. S2†).
These data were further conrmed by mass spectrometry anal-
ysis. The ESI MS data showed the formation of a 1 : 1 complex
According to eqn (3), a plot of F0/F versus [Q] shows a linear
graph (Fig. S4†) with an intercept of 1 and a slope of Ksv. The
linearity observed in Fig. S4† cannot conrm whether the
quenching is static or dynamic. This can be explained as a
dynamic process affects the excited uorophore, but not the
ground state, which results in no change in the absorption
spectra during dynamic quenching. To reveal static or dynamic
quenching without measurement of the uorescence lifetime,
the absorption spectrum was measured carefully to distinguish
between static and dynamic quenching. We observed the
change in the absorption spectrum that is the criterion for
static quenching29 and thus our quenching process is static in
nature.
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Fig. 5 Changes in fluorescent intensity of receptor 1 upon gradual addition of Cu2+ (lex/lem ¼ 405/585). Inset shows the response of fluo-
rescence signal with a regression of 0.96.
8 J. Chelly, Z. Tumer, T. Tonnesen, A. Petterson, Y. Ishikawa-
Brush, N. Tommerup, N. A. Horn and P. Monaco, Nat.
Conclusion
Genet., 1993, 3, 14.
We have designed and developed a selective and sensitive che-
mosensor 1 for the detection of Cu2+ in aqueous medium. The
detection of Cu2+ gave rise to signicant UV-visible absorption
and a colour change from colourless to yellow that could be
seen with the naked eye. Receptor 1 was not affected by the
presence of other interfering metal ions. The 1 : 1 stoichiometry
of the host–guest complex formation was conrmed from Job's
plot and a mass spectrometric method. Chemosensor 1 showed
a uorescence “turn-off” response for the selective detection of
Cu2+ with a detection limit as low as 50 nM.
9 J. F. Mercer, J. Livingston, B. Hall, J. A. Paynter, C. Begy,
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