examples of receptor molecules are not very common in the
literature, and importantly, receptor molecules that show
enhancement on binding to the Hg2+ ion are even more scarce.3
Being one of the heavier metal ions, the Hg2+ ion is commonly
known to quench the fluorescence effectively due to an efficient
spin-orbit coupling.6 However, any optical feedback that
translates the metal ion-receptor binding phenomena into a
fluorescence turn-on response is preferred over the fluorescence
turn-off response for obvious ease in the detection process(es).
Thus, any sensor molecule that works on the basis of fluores-
cence turn-on response, as well as can be used for colorimetric
detection of Hg2+ in aqueous or mixed aqueous-organic solvent
medium, is important from the viewpoint of its application
potential as a staining or imaging reagent for biological
applications.7
The absorption spectra of L in THF/H2O (6:4, v/v) showed
two main absorption bands with λmax of 265 and 330 nm.
1
1
1
These transitions could be accounted for La r A and Lb
r 1A, respectively. Addition of a perchlorate salt of alkali,
alkaline earth, and common transition metal ions (Li+, Na+,
K+, Cs+,Ca2+, Mg2+, Sr2+, Ba2+, Cr3+, Fe2+, Co2+, Ni2+,
Cu2+, Zn2+, Cd2+) did not show any change in spectra of L.
However, an appreciable change in the electronic spectra for
L was observed in the presence of Hg2+; a new absorption
band with maximum at 370 nm was observed (Figure 1).
In this article, we report a quinoline-based sensor (L) having
an azine-type receptor functionality for specific recognition of
the Hg2+ ion in mixed aqueous-organic solvent medium. An
earlier report3e,10b on the possibility of the analogous bisazine-
type ligand to act as a bischelating ligand toward transition
metal ions has led us to synthesize the receptor L for specific
recognition of Hg2+. More importantly, the possibility of
using this reagent for detection of Hg2+ accumulated in
Epithelial cell line (KB 31 cells, preexposed to Hg2+ solution)
using confocal laser microscopy is also reported.
Figure 1
.
Changes in UV-vis spectra of L (8.0 × 10-5 M) in
THF/H2O (6:4, v/v) with varying [Hg2+]. Insets: (i) UV-vis
scanning spectra of L (8.0 × 10-5 M) in the presence of various
metal ions. (ii) Job’s plot for evaluation of the binding stoichiometry
between Hg(ClO4)2 and L in THF/water (4:6, v/v) medium. (iii)
Benesi-Hildebrand plot for evaluation of the binding constant and
linearity of this plot with R2 ) 0.99, which also confirms the 2:1
binding stoichiometry.
The reagent L was synthesized following a one-step
reaction between quinoline-2-carbaldehyde and hydrazine
hydrate in 85% yield (Supporting Information). Various
analytical and spectroscopic data agree well with the structure
proposed for this reagent and the desired purity for further
use (Supporting Information).
Use of organic solvent was unavoidable due to limited
solubility of L in water. Systematic spectrophotometric
titration with increasing [Hg2+] revealed a gradual decrease
of absorption band at 330 nm with the concomitant increase
in the new charge transfer transition band at 370 nm (Figure
1). The formation of this new charge transfer band with a
clear isosbestic point at 359 nm suggests that the L and L
bound to Hg2+ ions exist in equilibrium. The stoichiometry
for the binding of L to Hg2+ was determined using Job’s
plot method and was found to be 2:1ssuggesting the
formation of a complex such as [(Hg2+)2L]4+. Formation of
[(Hg2+)2L]4+ seems logical, as bisazine ligands analogous
to L are expected to act as a bischelating ligands.8 Further,
such a binding stoichiometry was also confirmed from the
ESI-MS spectral data, the signal for [L + 2Hg + Na]+ at
732.57 and [L + 2Hg + 2ClO4]+ at 908 with the anticipated
isotope distribution signifying the [(Hg2+)2L]4+ (Supporting
Information).
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The equilibrium constant for the formation of [(Hg2+)2L]4+
from the interaction of L and Hg2+ was calculated from the
plot 1/[A - A0] vs 1/[Hg2+]2 using the Benesi-Hildebrand
equation9 (Supporting Information) and was found to be 2.37
× 107 M-2 L2. Linearity of the plot for 1:2 binding also
confirms this binding stoichiometry.
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The absence of any other equilibrium process reveals that
either the binding of two Hg2+ ions to L is a simultaneous
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