Multi-signaling detection of Hg2+ and Cu2+ by a ferrocene-pyrazole dyad associated with molecular-scale arithmetic

Article abstract of DOI:10.1016/j.inoche.2010.06.012

Ferrocene-based sensor R was synthesized and characterized by spectroscopic methods. The optical, electrochemical and cation-sensing properties of receptor R are presented. Receptor R behaves as a selective multi-channel signaling chemosensor molecule for Hg²⁺ and Cu²⁺ ions in CH ₃CN. The binding mode of Hg²⁺ with R was determined by ¹³C NMR titration. Both optical and electrochemical methods were applied for the detection of cations in water sample. The characteristic fluorescence of R with the Cu²⁺ selective ON-OFF and the Hg ²⁺ selective fluorescence bathochromic shift can be observed and the concept has been used to construct a combinational logic circuit at the molecular level.

Full text of DOI:10.1016/j.inoche.2010.06.012

Inorganic Chemistry Communications 13 (2010) 1109–1113
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Multi-signaling detection of Hg2+ and Cu2+ by a ferrocene–pyrazole dyad associated
with molecular-scale arithmetic
⁎
Veeramuthu Stalin Elanchezhian, Muthusamy Kandaswamy
Department of Inorganic Chemistry, School of Chemical Sciences, University of Madras, Guindy Campus, Chennai 600 025, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Ferrocene-based sensor R was synthesized and characterized by spectroscopic methods. The optical,
electrochemical and cation-sensing properties of receptor R are presented. Receptor R behaves as a selective
multi-channel signaling chemosensor molecule for Hg2+ and Cu2+ ions in CH3CN. The binding mode of Hg2+
with R was determined by 13C NMR titration. Both optical and electrochemical methods were applied for the
detection of cations in water sample. The characteristic fluorescence of R with the Cu2+ selective ON–OFF
and the Hg2+ selective fluorescence bathochromic shift can be observed and the concept has been used to
construct a combinational logic circuit at the molecular level.
Received 26 March 2010
Accepted 2 June 2010
Available online 9 June 2010
Keywords:
Chemosensor
Multi-channel signaling
Molecular-scale arithmetic and cation sensor
© 2010 Elsevier B.V. All rights reserved.
Cation–π interactions play a crucial role in many important
biological process of molecular recognition [1]. The non-covalent
bonding ability of the aromatic compounds has been already used in
the development of a variety of supramolecular hosts [2]. Benzene
rings are known to be effective donors for metal cations in a variety of
complexes [3]. The ferrocene and ferrocenophane compounds can be
used for design and synthesis of receptor for the molecular
recognition of cations [4]. A host capable of multi-center interaction
with the guest is often required to ensure maximum selectivity/
sensitivity. Bridged diarenes form very strong (1:1) complexes with
cations in which the cation moiety is optimally sandwiched in the cleft
between a pair of co-facial aromatic rings which act as a molecular
“venus fly trap” [5]. Considering these facts, the molecule R was
constructed with phenyl as one of the π-face and cyclopentadienyl
(Cp−) ring of ferrocene as another π-face which are bridged by 1H-
pyrazole ring. The binding ability of the pyrazole ring with cations has
been described extensively in the literature, several of them being
reviews [6]. Copper(II) has a specific high thermodynamic affinity for
typical N-donor ligands and fast metal-to-ligand binding kinetics [7].
Copper is one of the heavy metals which is an essential element not
only for life in mammals but also plants and play an important role in
carbohydrate and lipid metabolism [8]. The developmental methods
for detection of mercury are important for environment and human
health [9]. A number of Hg2+ selective chromogenic [10], fluorogenic
[11] and electrochemical [12] chemosensors have been reported.
Recently, considerable efforts have been made to develop multi-
signaling molecular probe for mercury ions [13]. In recent years there
has been a growing need or desire for constructing chemosensors for
fast and economical monitoring of our environmental samples,
especially for heavy metal ions in real time [14].
Besides this sensory role, such molecular devices also have the
potential for information processing, since their signaling can be
switched between two distinguishable states by environmental
stimuli [15]. Specific sequences of basic logic gates can be
programmed in a single molecular switch [16]. The INHIBIT gate
involves a particular combination of the logic functions AND and NOT.
Gunnlaugsson et al. [17] produced the first two-input INHIBIT gate.
Several nice examples of INHIBIT logic have appeared more recently
[18]. By combining the logic functions NOT and OR, it is possible to
develop a NOR gate [19] without physically joining the two
elementary gates.
As depicted in Scheme 1, receptor R was synthesized in good yield
from the reaction of ferrocenoylacetone [20] with phenylhydrazine.
The ferrocene derivative R was characterized by 1H and 13C NMR and
ESI mass spectrometry [21]. The metal recognition properties of
receptor R in CH3CN were evaluated by optical, electrochemical and
13C NMR titration studies. Both optical and electrochemical methods
were applied for the detection of cations in water sample also.
The UV–vis absorbance spectrum of R in CH3CN (1×10−4 M) were
obtained, which is consistent with most ferrocenyl chromophores in
that they exhibit two charge transfer bands in the UV–vis region [22].
The spectra contain a prominent absorption band with a maximum at
346 nm, which can safely be ascribed to a high energy ligand centered
π–π* and L–π* electronic transition. In addition to this band, another
weaker absorption in the visible region appears at λmax =447 nm
(ε=600 M− 1 cm− 1), which was assigned to another localized
excitation with a lower energy (LE) produced by two nearly
degenerated transitions, and Fe(II) d–d transition [23] or by metal–
ligand charge transfer (MLCT) process. The UV–vis spectra of R is
identical in acetonitrile and acetonitrile/water (9:1) solution.
⁎
Corresponding author. Tel./fax: +91 44 22300488.
E-mail address: mkands@yahoo.com (M. Kandaswamy).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.06.012

1110
V. Stalin Elanchezhian, M. Kandaswamy / Inorganic Chemistry Communications 13 (2010) 1109–1113
Scheme 1. Synthesis of receptor R.
The preliminary UV–vis binding interaction studies of R in CH3CN
(1×10−4 M) against cations of environmental relevance, such as Ca2+
neutral ligand. The new LE band at 783 nm is responsible for the
change of color from yellow to dark green which can be used for the
‘naked eye’ detection of Cu2+ ion. The UV–vis spectral change suggests
that the ferrocene moiety is oxidized upon complexation with Cu2+
ion and the change of color to deep green is characteristic of the
ferrocenium ion formation [24]. The dependence of absorbance at
783 nm on the Cu2+ ion concentration (Job's plot) strongly suggests
1:1 host–guest complex formation. This result has also been confirmed
by the ESI-MS, which shows two peaks with m/z=202 and 504,
corresponding to the species [R–Cu]2+ (m/zcalcd.=202.51) and [R–Cu
(ClO4)]+ (m/zcalcd.=503.96) respectively. The resulting titration
isotherm fitted exactly in a 1:1 binding model. The binding constant
determined [25] from the increasing absorption intensity at 783 nm
,
Mg2+, Cd2+, Zn2+, Ni2+, Cu2+, Hg2+ and Pb2+ as percholate salts,
shows selective response to Cu2+ and Hg2+. The change in the UV–vis
absorbance spectra of receptor R in CH3CN/water (9:1) (1×10−4 M)
due to the stepwise addition of Cu2+ ion in water is shown in the Fig. 1a.
The LE band at 447 nm disappeared along with progressive appearance
of new band at 783 nm observed up to one equivalent addition of Cu2+
ion. Two isobestic points at 276 nm and 600 nm were found to be
indicating the presence of unique complex in equilibrium with the
is K11=6.33 0.32×103 M−1
.
The receptor R successfully detects the Cu2+ ion in aqueous
solution. But in the presence of water, the Hg2+ ion tend to form the
hydrate ion instead of forming a complex with receptor R. Titration
experiments for receptor R in CH3CN (1×10−4 M) were performed. In
contrast, addition of Hg2+ ion showed intensity enhancement at LE
and HE band (Fig. 1b). Four well defined isobestic points at 237, 256,
272 and 311 nm were found indicating the presence of a unique
complex in equilibrium with the neutral ligand. The resulting titration
isotherm fitted nicely in a 1:1 binding model. This result is further
confirmed by ESI-MS, which shows two peaks with m/z=272 and
643, corresponding to the species [R–Hg]2+ (m/zcalcd =272.03) and
[R–Hg(ClO4)]+ (m/zcalcd =643.00) respectively. The binding constant
determined from the increasing absorption intensity at 346 nm is
K
11 =1.83 0.09×103 M−1
The competition experiments were carried out for complexation of
.
R between Cu2+ and Hg2+. It shows that the absorption of the R–Hg2+
system is influenced by the Cu2+ ion, but Hg2+ did not interfere with
the absorption of R–Cu2+. Therefore R could be used for the detection
of Cu2+ in the presence of other competing metal ions, including Hg2+
ion. In addition, R can be utilized for the detection of Hg2+ ion in the
presence of other competing ions except Cu2+ ion.
The binding of metal ion by a host molecule is accompanied by
conformational and electronic changes which in turn are reflected by
variations in the 1H and 13C NMR chemical shifts of those centers close
to the site of complexation [26]. The 13C chemical shift is known to be
sensitive to electronic changes and conformational changes but is
relatively less sensitive to solvent effect. The 13C NMR titrations were
performed by monitoring variations in the 13C NMR chemical shifts
in the presence of varying quantities of Hg2+ in CD3CN solution of
R which is shown in Fig. 2. Each signal present in 13C NMR spectrum of
R were identified and assigned by two dimensional NMR experiments
HMQC and HMBC. The chemical shifts and peak assignments are
shown in Table 1. Appreciable changes in chemical shifts were
observed for phenyl carbons and resonance associated with carbons
of substituted cyclopentadienyl ring. These signals were shifted
to downfield with maximum changes observed for Cd (Δδ=
+2.25 ppm) and C4,3 (Δδ≈2.91 ppm). This observation suggests
Fig. 1. (a) UV–visible spectral changes of R in CH3CN/H2O (9:1) (1×10−4 M) upon
addition of aqueous Cu2+ (5×10−3 M) ion from 0 to 1 equivalent, (b) UV–visible
spectral changes R in CH3CN (1×10−4 M) upon addition of Hg2+ (5×10−3 M) ion from
0 to 1 equivalent.

V. Stalin Elanchezhian, M. Kandaswamy / Inorganic Chemistry Communications 13 (2010) 1109–1113
1111
Fig. 2. Evolution of 13C NMR spectra of R in CD3CN upon addition of 0, 0.33, 0.66 and 1.00 equivalents of Hg2+ ion.
that in addition to the Hg–N binding interaction, Hg–π interaction
also involved in the binding mechanism, which enhance the binding
and signaling ability of R towards the Hg2+ ion. Ample experi-
mental and computational evidences have been reported for Hg–π
interaction with aromatic donors such as benzene and the cyclopen-
tadienyl (Cp−) anion [27].
the peak corresponding to the uncomplexed receptor R was diminished.
The DPV response of R in CH3CN/H2O (9:1) (1×10−4 M with 0.1 M
TBAP) upon addition of aqueous Cu2+ ion was investigated. The
oxidation peak at 0.480 V was anodically shifted to 1.116 V that is
ΔE=636 mV. The current intensity of the new peak increases until one
equivalent of the Cu2+ ion is added.
The electrochemical properties of compound R were investi-
gated by cyclic voltammetry (CV) in CH3CN (5×10−3 M) and dif-
ferential pulse voltammetry (DPV) in CH3CN/water (9:1) solution
(1×10−4 M) containing 0.1 M [n-Bu4N]ClO4 (TBAP) as supporting
electrolyte. The receptor R has shown a reversible one-electron
redox wave at the half-wave potential value of E1/2 =497 mV in CV.
The Electrochemical sensing properties of R towards metal ions
The results obtained on the stepwise addition of substoichiometric
amounts of Hg2+ to R in CH3CN (5×10−3 M with 0.1 M TBAP) shows
new reversible wave at E1/2 =0.639 V in CV (Fig. 3b). The current
intensity of this new peak increases until one equivalent of the Hg2+
ion is added and the peak corresponding to the uncomplexed receptor
R was completely disappeared. The DPV titration experiment of R in
CH3CN with Hg2+ ion shows the oxidation peak at 0.472 V, was
anodically shifted to 0.636 V that is ΔE=164 mV. The peak
corresponding to uncomplexed species completely disappeared
upon one equivalent addition. The electrochemical responses
obtained for R with Hg2+ ion was consistent with such cation–π
interaction with ferrocene moiety reported by Grossel et al. [28]
The fluorescence spectrum of R in CH3CN (5×10−5 M) were
obtained and the emission spectrum is characterized by the emission
maximum observed at 381 nm upon excitation at 315 nm. The change
in the fluorescence properties of R caused by different metal ions were
measured in CH3CN. The fluorescence of R shows selective response to
Cu2+ and Hg2+ ion. The fluorescence titration of R (5×10−5 M) with
aqueous Cu2+ ion was carried out in CH3CN/H2O (9:1) solution, which
is shown in Fig. 4a. The fluorescence intensity of R at 381 nm was
linearly reduced with the increasing concentration of Cu2+ ion. The
fluorescence intensity was quenched more than 90% when the
concentration of Cu2+ ion reached one equivalent. The possible
reason for the fluorescence quenching is the formation of a ground-
state non-fluorescent complex, which is supported by the change in
the absorption spectra. The energy or electron transfer [29] in R–Cu2+
is presumed resulting in the fluorescence quenching.
were also evaluated by CV and DPV. Titration studies with Ca2+
,
Mg2+,Cd2+, Zn2+, Ni2+, Cu2+, Hg2+ and Pb2+ as percholate salts,
to an electrochemical solution of receptor R in CH3CN containing
TBAP (0.1 M) as supporting electrolyte shows selective response
to Cu2+ and Hg2+ ion. The cyclic voltammetric response of R in CH3CN
(5×10−3 M with 0.1 TBAP) upon addition of Cu2+ ion is investigated at
the scan rate of 100 mV s−1. Formal potential of ferrocene/ferrocenium
couple of R was E1/2=0.497 V (Fig. 3a), which was perturbed by the
complexation of Cu2+ ion. The results were obtained on stepwise
addition of substoichiometric amounts of Cu2+ ion. A new wave
appeared at a more positive potential E1/2=0.919 V associated with the
formation of a complex species of Cu2+ion. The current intensity of this
new peak increases until one equivalent of the guest cation is added and
Table 1
Carbon-13 NMR shifts (δ) of R in CD3CN upon addition of Hg2+ ion.
[Hg2+] eq.
Cf,b
Cd
Ce,c
C4,3
C5,2
C6
CH3
0.00
0.33
0.66
1.00
126.53
127.79
128.71
128.78
128.27
130.98
132.91
133.04
129.25
130.38
131.14
131.21
68.95
69.36
71.86
–
69.07
69.71
70.18
–
70.19
70.51
–
13.12
12.47
11.99
–
The fluorescence titration of R with Hg2+ ion was carried out in
CH3CN solution (Fig. 4b). The results obtained from addition of 0.25,
0.50 and 0.75 equivalents of Hg2+ ion to receptor R show dual
–

1112
V. Stalin Elanchezhian, M. Kandaswamy / Inorganic Chemistry Communications 13 (2010) 1109–1113
Fig. 3. (a) Evolution of the CV of R (5×10−3 M with 0.1 TBAP) in CH3CN upon addition
of Cu2+ ion from 0 to 1 equiv and (b) upon addition of Hg2+ ion from 0 to 1 equiv.
Potential measured Vs Ag/AgCl with a glassy carbon working electrode was used
together with a platinum gauze counter electrode and scanned at 100 mV S–1
.
Fig. 4. (a) Fluorescence spectral change of R (5×10−5 M) in CH3CN/H2O (9:1) upon
addition of aqueous Cu2+ ion from 0 to 1 equiv, (b) Fluorescence spectral change of R in
CH3CN (5×10−5 M) upon addition of Hg2+ ion from 0 to 1 equivalent (Inset: Titration
profile up to 0–5 equiv).
emission at 381 nm for free receptor and weak emission at 424 nm for
complexed species. The emission of rigid R–Hg2+ complex at 424 nm
may be quenched by uncomplexed receptor present in the solution.
The one equivalent addition shows the intense red-shift emission at
424 nm alone. It indicates complete interconversion of ligand to 1:1
complex. Addition of Hg2+ ion above one equivalent did not show any
considerable change in emission. The unusual red-shift is attributed to
the changes in the conformation [30] of the receptor R induced by
coordination of Hg2+ ion.
The changes in the absorption spectra associated with color changes,
allow the potential “naked eye” detection. Both optical and electro-
chemical methods were applied for detection of Cu2+ in water
The characteristic fluorescence of R with Cu2+ selective ON–OFF
and the Hg2+ selective fluorescence red-shift can be observed and the
concept has been used to construct a combinational logic circuit at the
molecular level. The addition of Cu2+ ion results in quenching the
emission of R at 381 nm. The addition of Hg2+ ion shows the red-shift
emission of R–Hg2+ at 424 nm (Fig. 5). These behaviors can be
analyzed with combinational logic circuit. The two input signals are I1
(Hg2+) and I2 (Cu2+). The output signals are output 1 (the emission
band at 381 nm) and output 2(the emission band at 424 nm). As seen
from the truth table (Table 2), monitoring the fluorescence at 424 nm
(output 2) upon addition of Hg2+, Cu2+ and equimolar mix of Cu2+
and Hg2+ leads to an INHIBIT logic gate. INHIBIT is neither
communicative nor associative. However, output 1 results in a NOR
logic gate. So the strength of the INHIBIT gate is acknowledged in
combination with a NOR gate leading to the construction of a
combinational logic circuit.
In conclusion, the multi-signaling chemosensor R were synthe-
sized and characterized. The receptor R selectively binds with Cu2+
and Hg2+ ion which associates with efficient multi-channel signaling.
Fig. 5. Fluorescence spectra of R at different complexation situations with Hg2+ and
Cu2+ ions.

V. Stalin Elanchezhian, M. Kandaswamy / Inorganic Chemistry Communications 13 (2010) 1109–1113
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1113
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Truth table.
Input 1(Hg)
Input 2(Cu)
Output 1(381 nm)
Output 2(424 nm)
0
1
0
1
0
0
1
1
1 (high)
0 (low)
0 (low)
0 (low)
NOR
0 (low)
1 (high)
0 (low)
0 (low)
INHIBIT
Logic function
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samples and the results were successful. The important switching
function of the INHIBIT and NOR molecular logic gate R is output 1 at
381 nm and output 2 at 424 nm can be quenched (OFF) by Cu2+ ion.
These results will be useful for further molecular design to mimic the
function of the complex logic gates on controlling.
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Acknowledgments
The authors gratefully acknowledge the Council of Scientific and
Industrial Research, (Government of India) for the project grant.
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Appendix A. Supplementary data
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[21] (a) Synthesis of sensor R: A solution of ferrocenoylacetone (1.35 g, 5 mmol) and
phenylhydrazine (0.54 g, 5 mmol) in 20 mL of absolute ethanol was refluxed for
3 h. The solvent was evaporated under vacuum and the residue was dissolved in
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.inoche.2010.06.012.
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