C. P. R. acknowledges the financial support from DST,
CSIR, DAE-BRNS. A. M. acknowledges the senior research
fellowship of CSIR.
Notes and references
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¨
Fig.
5
(a) Fluorescence intensity observed in the titration of
Hg2+
against incremental amount of added (where
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presence of any of these biological molecules or medium (Fig. 5).
Similar titrations could not be carried out with the control
molecules, since there is a precipitation in the medium owing
to the absence of the carbohydrate moiety. This indicates the
superiority of L over C2 or C3 as a sensor molecule for Hg2+
,
wherein the carbohydrate moiety indeed assists the recognition.
The detection limit for Hg2+ has been determined using
fluorescence titration by keeping the L to Hg2+ ratio at 1 : 2
and was found to be 110 Æ 16 nM (50 Æ 10 ppb) (ESI 07w). This
is certainly lower than 300 to 10 000 nM reported in the
literature.7 Since the EPA limit for Hg2+ has been 2 ppb in
drinking water, the same has been checked by fluorescence
spectra and found to be 25 Æ 2% of enhancement in the intensity
(ESI 08w) at this concentration. The enhancement observed in the
present case is higher than that reported in the literature.7d Owing
to its water solubility and biocompatibility, L would find a wide
range of applications including those of environmental concern.
A new and simple glucose based receptor molecular system L
linked to anthracene through an imine moiety has been
synthesized and characterized. L shows selective sensing of the
Hg2+ ion by exhibiting a 13 Æ 1 fold fluorescence enhancement
among the thirteen ions of biological and ecological importance
studied in aqueous solutions of methanol and ethanol. None of
these ions was found to compete with Hg2+. The binding of
Hg2+ to L has been shown by absorption, ESI MS and 1H NMR
titrations and found that the Hg2+ interacts both through the
imine moiety (by binding) as well as the aromatic moiety
(through cationÁÁÁp) and forms a 2 : 1 complex between L and
Hg2+ and the molecular ion peak observed in the ESI MS
supports the presence of mercury based on the isotope peak
pattern. The interactions present between Hg2+ and L have been
modeled by DFT computations (Fig. 5b, ESI 09w).13 The
reversibility of the Hg2+ binding to L has been demonstrated
by titrating the solution with 50 mM solution of Na2EDTA
(ESI 10w). Hg2+ sensing by L has been found to be intact even in
the presence of human blood serum and milk and also in the
presence of albumin proteins. All these qualify the water soluble
and biocompatible L to be a selective sensor for Hg2+ that can
perhaps be used even in biological fluids, up to a minimum
detection limit of 110 Æ 16 nM (50 Æ 10 ppb), in the pH range
5 to 10. At the 2 ppb limit of Hg2+ in drinking water, the
fluorescence enhancement has been found to be 25 Æ 2%. The
possible application of L in sensing Hg2+ is further amplified
because of the colour change that it exhibits under visible as well
as UV light and hence the same may be extendable to even
biological cells and tissues.
13 J. A. Pople and coworkers, Gaussian 03, revision C.02, Gaussian,
Inc., Wallingford, CT, 2004 (total reference is given in ESI 08w).
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 2565–2567 2567