A rhodamine-based fluorescent and colorimetric chemodosimeter for the rapid detection of Hg2+ ions in aqueous media

Article abstract of DOI:10.1021/ja054855t

A rhodamine-based fluorescent and colorimetric chemodosimeter for the rapid detection of Hg²⁺ ions in aqueous media was developed. The system, which utilizes an irreversible Hg²⁺-promoted oxadiazole forming reaction, responds instant

Full text of DOI:10.1021/ja054855t

Published on Web 11/09/2005
A Rhodamine-Based Fluorescent and Colorimetric Chemodosimeter for the
Rapid Detection of Hg2+ Ions in Aqueous Media
Young-Keun Yang, Keun-Jeong Yook, and Jinsung Tae*
Department of Chemistry and Center for BioactiVe Molecular Hybrids, Yonsei UniVersity, Seoul 120-749, Korea
Received July 20, 2005; E-mail: jstae@yonsei.ac.kr
Scheme 1. Hg2+-Induced Ring Opening and Cyclization of 1
The demand for chemosensors that are selective for specific target
anions or cations is continuously increasing. Especially important
in this regard are sensors that monitor toxic heavy metal ions. A
great effort has gone into the development of selective fluorescent
chemosensors1,2 due to their high sensitivity. Recently, colorimetric
sensing3 of metal ions has been shown to be a less labor-intensive
alternative to techniques based on fluorescence. Other fascinating
devices that utilize irreversible chemical events associated with
host-guest recognition have been employed as chemodosimeters
for ratiometric determination of analyte concentrations. Several
fluorescent chemodosimeters for metal ions, such as Cu2+ and Hg2+
have been described in the literature.4
,
Because of the strong thiophilic affinity of Hg2+, fluorescent
changes associated with mercury-promoted desulfurization reac-
tions, including hydrolysis,4a,d cyclizations,4e and eliminations,4f have
been used in the design of chemodosimeters for Hg2+. However,
several significant challenges still exist in this field. For example,
to drive these desulfurization reactions to completion, it is often
necessary to use either elevated temperatures or excess quantities
of Hg2+. Although being less thiophilic than Hg2+, other metal ions,
such as Ag+ and Pb2+, can also promote the desulfurization
reactions. Therefore, optimal ratiometric chemodosimeters for Hg2+
must possess fast response times at ambient temperature and the
ability to selectively and stoichiometrically detect Hg2+. Finally,
the dosimeters should operate in aqueous media and be sensitive
enough to detect Hg2+ at parts per billion levels.
of a pink color and a strong yellow fluorescence. This observation
shows that the mercury-induced ring-opening reaction takes place
rapidly at room temperature.
A fluorescence titration of Hg2+ was conducted using 1 µM
solution of 1 in water-methanol (80/20 v/v) at pH 7. Upon addition
of 1 equiv of Hg2+, the fluorescence intensity of the solution of 1
undergoes a ca. 26-fold increase and the emission maximum shifts
from 553 to 557 nm (Figure 1). The reaction (Scheme 1) responsible
for these changes reaches completion well within the time frame
(<1 min) of these measurements.10 The observed fluorescence
intensity was nearly proportional to the Hg2+ concentration. The
saturation behavior of the fluorescence intensity after 1 equiv of
Hg2+ reveals that the Hg2+ chemodosimeter has a 1:1 stoichiometry.
Further evidence for this stoichiometry comes from the independent
synthesis of 4 from 1 by using 1 equiv of Hg(ClO4)2 in acetonitrile
at room temperature (98% yield). The compound 4 has a large molar
absorptivity (logꢀ ) 4.67) and a high fluorescence quantum yield
(Φf ) 0.52).11
Only few fluorescence chemosensors for heavy metal ions have
been designed utilizing the well-known5 spirolactam (nonfluores-
cent) to ring opened amide (fluorescent) equilibrium of rhodamine
derivatives.4b,6 We envisioned that coupling this process with the
stoichiometric and irreversible Hg2+-promoted reaction of thiosemi-
carbazides to form 1,3,4-oxadiazoles7 would serve as the foundation
for a novel chemodosimeter for Hg2+
.
Similar results are obtained when different mercury salts, such
as HgCl2 and Hg(ClO4)2, are used. Therefore, it appears that
counteranions have a negligible effect on this chemodosimeter.
Moreover, various sodium salts, including NaCN, NaI, NaF, and
NaOAc, do not promote any fluorescence changes under the same
conditions used for the Hg2+ titrations.
The rhodamine derivative 1 appeared to be ideal for this
application since we anticipated that it would undergo oxadiazole
formation when the thiosemicarbazide moiety is liberated by Hg2+
-
facilitated ring opening of the spirocycle grouping (Scheme 1).
Below, we describe the results of studies that have led to the
development of a colorimetric and fluorescent chemodosimeter
system based on 1, which is highly sensitive and selective for
mercuric ions in aqueous solution.
The rhodamine derivative 1 was prepared in high yield from
Rhodamine 6G by using a two-step procedure (1, NH2NH2‚H2O,
MeOH, 95%; 2, PhNCS, DMF, 90%).8 This substance forms a
colorless water-methanol (80/20 v/v) solution at pH 7, indicating
that it exists in the spirocyclic form predominantly.9 Addition of
mercuric ion to the solution of 1 causes instantaneous development
Figure 1. Fluorescence response of 1 (1 µM) upon addition of Hg2+ in
water-methanol (80/20 v/v) at pH 7 (excitation at 500 nm).
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10.1021/ja054855t CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
stoichiometric manner to the amount of Hg2+. The selectivity of
this system for Hg2+ over other metal ions is remarkably high, and
its sensitivity is below 2 ppb in aqueous solutions. The findings
suggest that this method will serve as the foundation of practical
chemodosimeters for rapidly determining Hg2+ concentrations in
aqueous environments.
Acknowledgment. This work was supported by the Center for
Bioactive Molecular Hybrids (CBMH) at Yonsei University.
Supporting Information Available: Experimental procedures for
the synthesis, spectral data, and copies of 1H NMR and 13C NMR of 1
and 4, data for UV-vis and fluorescence titrations of 1. This material
is available free of charge via the Internet at http://pubs.acs.org.
Figure 2. Fluorescence spectra (excitation at 500 nm) of 1 (1 µM) in
water-methanol (80/20 v/v) at pH 7 in the presence of 1 equiv of Hg2+
,
,
Ag+, Zn2+, Cu2+, Pb2+, Cd2+, Ni2+, Co2+, Fe2+, Mn2+, Mg2+, Ca2+, Ba2+
Li+, K+, Na+, Rh3+, Cr2+
.
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Figure 3. Color changes of 1 (10 µM) upon addition of Hg2+, Ag+, Cu2+
Cd2+, Pb2+, Zn2+ (1.0 equiv) in water-methanol (80/20 v/v).
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Figure 4. (a) Fluorescence emission changes of 1 (10-7 M) upon additions
of Hg2+ (by 1 ppb) in water-methanol (80/20 v/v) at 25 °C. (b) The
fluorescence intensities at 556 nm.
Changes in the fluorescence properties of 1 caused by other metal
ions, including Ag+, Zn2+, Cu2+, Pb2+, Cd2+, Ni2+, Co2+, Fe2+
,
Mn2+, Mg2+, Ca2+, Ba2+, Li+, K+, Na+, Rh3+, Cr2+, were also
measured. Fluorescence spectra of solutions of 1 (1 µM), recorded
within 5 min after the addition 1 equiv of each of these metal ions,
are displayed in Figure 2. Only Ag+ and Zn2+ ions promote small
fluorescence intensity changes,12 while other metal ions did not
cause any significant changes under identical conditions. The
selectivity observed for Hg2+ over other ions is remarkably high.
In addition, the enhancement in fluorescence intensity resulting from
addition of Hg2+ is not influenced by subsequent addition of other
metal ions. Finally, while the colorless to pink color change
associated with the reaction of 1 with Hg2+ is readily detectable
visually, no significant color changes are promoted by other metal
ions (Figure 3).
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(9) The spirocyclic form was confirmed by 13C NMR spectra. The spiro carbon
peak appeared at 67.0 ppm. According to the pH titration, compound 1
retained the spirocyclic form in the pH range of 4-14. Below pH 4, the
fluorescence intensity increased, which implies that ring opening is
occurring. Compound 1 is stable as a solid and a solution for several
days.
To see practical applicability, the detection limit of this new
chemodosimeter system was evaluated. The fluorescence titration
profile of 1 (10-7 M) with Hg2+, shown in Figure 4, demonstrates
that detection of Hg2+ is at the parts per billion level.13 Under these
conditions, the fluorescence intensity of the solution of 1 was nearly
proportional to the amount of Hg2+ added (Figure 4b).
In summary, the investigation described above has resulted in
the development of a highly selective and sensitive chemodosimeter
for Hg2+ ion in aqueous solution. The system, which utilizes an
irreversible Hg2+-promoted oxadiazole forming reaction of rhodamine
derivative 1, is monitored by colorimetric and fluorescence intensity
changes that respond instantaneously at room temperature in a 1:1
(10) Typical desulfurization reactions used in known Hg2+ chemodosimeters
require elevated temperatures and/or relatively long reaction times.
(11) The fluorescence quantum yield was calculated using Rhodamine 6G (Φf
) 0.94 in EtOH) as a reference (Fischer, M.; Georges, J. Chem. Phys.
Lett. 1996, 260, 115-116).
(12) Fluorescent intensities promoted by Ag+ increased gradually over an 8 h
period.
(13) The EPA standard for the maximum allowable amount of Hg2+ in drinking
water is 2 ppb.
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