A "Turn-on" Fluorescent Sensor for the Selective Detection of Mercuric Ion in Aqueous Media

Article abstract of DOI:10.1021/ja037995g

A water-soluble turn-on fluorescent sensor for Hg(II) is described. Incorporation of soft thioether donors into an aniline-derived ligand framework that can be linked to a fluorescein platform affords sensor MS1, which shows a ～5-fold increase in integrat

Full text of DOI:10.1021/ja037995g

Published on Web 10/30/2003
A “Turn-On” Fluorescent Sensor for the Selective Detection of Mercuric Ion
in Aqueous Media
Elizabeth M. Nolan and Stephen J. Lippard*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received August 18, 2003; E-mail: lippard@lippard.mit.edu
Scheme 1
Heavy metal ion pollution poses severe risks for human health
and the environment. Mercury contamination is widespread and
occurs through a variety of natural and anthropogenic sources
including oceanic and volcanic emission,1 gold mining,2 solid waste
incineration, and the combustion of fossil fuels.3 Once introduced
into the marine environment, bacteria convert inorganic mercury
into methylmercury, which enters the food chain and accumulates
in higher organisms, especially in large edible fish.1,3-7 Methyl-
mercury is neurotoxic and has been implicated as a cause of prenatal
brain damage,8-10 various cognitive and motion disorders,11,12 and
Minamata disease.13
Our increased understanding of the deleterious effects of mercury
exposure has sparked interest in the development of new tools for
detecting Hg(II) in the environment. One major challenge involves
creating Hg(II) sensors that function in water and are highly
selective for Hg(II) against a background of competing analytes.
Small synthetic molecules offer one approach to such probes. To
date, a number of small-molecule Hg(II) detection methods have
been examined and include colorimetric strategies,14-17 fluoroiono-
phores,18-27 and a dithioamide-functionalized lipid bilayer.28 Most
of these systems have limitations, which include interference from
other metal ions, delayed response to Hg(II), and/or a lack of water
solubility, requiring the use of organic or aqueous organic solvent
mixtures. Although a fluorescent probe based on the indoaniline
chromophore exhibiting selectivity for Hg(II) in water was recently
described,26 Hg(II) binding results in a decrease of quantum yield
(φ) and brightness (φꢀ).
Here, we report the synthesis and metal-binding properties of
MS1 (Mercury Sensor 1), a water-soluble, turn-on fluorescein-based
sensor that exhibits high selectivity and sensitivity for Hg(II). We
selected fluorescein as the reporting group due to its superior
brightness (φ ≈ 1, high ꢀ) and water solubility. Since Hg(II) ion
has a high affinity for soft donors such as sulfur, we incorporated
a 3,9-dithia-6-azaundecane moiety29 into an aniline-derived fluo-
rescein-based ligand framework previously developed in our
laboratory.30 N-(2-Aminobenzyl)-3,9-dithia-6-azaundecane (2) was
prepared in two steps starting from 3,9-dithia-6-azaundecane and
commercially available 2-nitrobenzyl bromide. Condensation of 7′-
chloro-4′-fluoresceincarboxaldehyde (1)31 with 2 in EtOAc, fol-
lowed by reduction of the resulting imine using NaB(OAc)3H in
1,2-dichloroethane and purification on silica gel (50:1 CHCl3:
MeOH), afforded MS1 as a magenta solid (Schemes 1 and S1,
Supporting Information).
allows efficient PET quenching at neutral pH. Upon disruption of
this quenching pathway by Hg(II) coordination, the emission max-
imum red-shifts slightly to 528 nm and the quantum yield increases
∼2.75-fold to 0.11. The absorption spectrum exhibits a blue shift
from 505 nm (ꢀ ) 61 300 M-1 cm-1) to 501 nm (ꢀ ) 73 200 M-1
cm-1) upon Hg(II) binding, resulting in a ∼3.3-fold increase in
brightness. A ∼5-fold increase in integrated emission is observed
upon addition of Hg(II) (Figure 1). The magnitude of this response
depends on the chloride ion concentration (Figure S2, Supporting
Information). Metal-binding titrations indicate that MS1 forms a
1:1 complex with Hg(II) in solution, which is responsible for the
33
fluorescence enhancement, with an EC50 of 410 nM.
The fluorescence response of MS1 to various cations and its
selectivity for Hg(II) are illustrated in Figure 2. The Hg(II) response
of MS1 is unaffected in a background of environmentally relevant34
alkali and alkaline earth metals including Li(I), Na(I), Rb(I),
Mg(II), Ca(II), Sr(II), and Ba(II). The Group 12 metals Zn(II) and
Cd(II), in addition to Cr(III) and Pb(II), do not inhibit the
fluorescence response of MS1 to Hg(II). Of the first-row transition
metal ions considered, only Cu(II) interferes with the Hg(II)-induced
At pH 7 and 100 mM ionic strength (50 mM PIPES buffer, 100
mM KCl), and in the presence of EDTA to scavenge any
adventitious metal ions, MS1 exhibits an emission maximum at
524 nm and a quantum yield of 0.04. The low quantum yield of
the unbound sensor results from photoinduced electron transfer
(PET) quenching of the fluorescein emission by the lone pair of
the aniline nitrogen atom.32 This nitrogen atom has a pKa of 7.1
(Figure S1), which indicates that the deprotonation equilibrium
Figure 1. Fluorescence response of MS1 to addition of Hg(II) in water at
pH 7 with 50 mM PIPES, 100 mM KCl buffer. [MS1] ) 1 µM. Aliquots
of 0.1 and 1 mM HgCl2 were added to yield final Hg(II) concentrations of
0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.3, 1.5, and 3 µM. Excitation
was at 500 nm.
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J. AM. CHEM. SOC. 2003, 125, 14270-14271
10.1021/ja037995g CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
CHE-9808061 and DBI-9729592. E.M.N. thanks NDSEG for a
graduate fellowship, Dr. Christopher J. Chang for insightful
discussions, and Evelyn M. Kim for assistance regarding EPA
guidelines.
Supporting Information Available: Figures S1 and S2, Scheme
S1, and synthetic and experimental details (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
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Figure 2. (a) Fluorescence response of MS1 to various cations in water at
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Acknowledgment. This work was supported by Grant GM65519
from the National Institute of General Medical Sciences. Spectro-
scopic equipment was maintained at the MIT DCIF with NSF grants
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