Selectively chemodosimetric detection of Hg(II) in aqueous media

Article abstract of DOI:10.1021/ol7020115

(Figure Presented) A new series of benzoylthiourea derivatives of 1, 3, and 5 were prepared, and their chemodosimetric behaviors toward metal cations were investigated in aqueous media at room temperature. Among various metal cations tested, exclusively Hg²⁺ ion responses to irreversible color changes of receptors, along with distinctive blue shifts in UV/vis spectra. The receptors can be applicable for the monitoring of Hg²⁺ ion in aqueous solution with a pH span 4-9.

Full text of DOI:10.1021/ol7020115

ORGANIC
LETTERS
2007
Vol. 9, No. 22
4515-4518
Selectively Chemodosimetric Detection
of Hg(II) in Aqueous Media
Min Hee Lee,† Byoung-Ki Cho,† Juyoung Yoon,‡ and Jong Seung Kim*,§
Department of Chemistry and Institute of Nanosensor and Biotechnology, Dankook
UniVersity, Seoul 140-714, Korea, DiVision of Nano Science and Department of
Chemistry, Ewha Womans UniVersity, Seoul 120-750, Korea, and Department of
Chemistry, Korea UniVersity, Seoul 136-701, Korea
jongskim@korea.ac.kr
Received August 16, 2007
ABSTRACT
A new series of benzoylthiourea derivatives of 1, 3, and 5 were prepared, and their chemodosimetric behaviors toward metal cations were
+
investigated in aqueous media at room temperature. Among various metal cations tested, exclusively Hg2 ion responses to irreversible color
+
changes of receptors, along with distinctive blue shifts in UV/vis spectra. The receptors can be applicable for the monitoring of Hg2 ion in
aqueous solution with a pH span 4−9.
Development of chemosensors for heavy-transition metal
ions, such as Hg2+, Cu2+, Pb2+, and Cd2+ ions, has received
considerable attention because the metals have an extremely
toxic impact on the environment.1 Among them, Hg2+ ion
is considered to be highly dangerous to human beings when
it is ingested or inhaled. Elemental mercury is converted into
methyl mercury by microbial action on the sediments or soil.
Once formed, methyl mercury can be taken up in the food
chain of aquatic organisms.2 Although, of course, all forms
of mercury can bioaccumulate, methyl mercury particularly
accumulates to a greater extent than other forms. Thus, its
accumulation in the human body affects a wide variety of
diseases even with a low concentration, such as prenatal brain
damage, serious cognitive and motion disorders, and Mi-
namata disease.3 Therefore, in recent years, many efforts have
been made to develop various chemosensors specifically for
Hg2+ ion detection.4-9 The most attractive approach in this
field involves highly selective reactions induced by a specific
metal cation. Some selective chemodosimeters for the Hg2+
ion have been designed to adopt mercury-promoted desulfu-
rization, leading to an irreversible chemical event between
† Dankook University.
‡ Ewha Womans University.
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Fitzgerald, W. F.; Damman, A. W. EnViron. Res. 1998, 78, 118. (c) Queen,
H. L. Chronic Mercury Toxicity: New Hope Against An Endemic Disease;
Queen and Company Health Communications, Inc.: Colorado Springs, 1998.
(d) Renzoni, A.; Zino, F.; Franchi, E. EnViron. Res. 1998, 77, 68.
(3) (a) Grandjean, P.; Weihe, P.; White, R. F.; Debes, F. Environ. Res.
1998, 77, 165. (b) Takeuchi, T.; Morikawa, N.; Matsumoto, H.; Shiraishi,
Y. Acta Neuropathol. 1962, 2, 40. (c) Harada, M. Crit. ReV. Toxicol. 1995,
25, 1.
§ Korea University.
(1) (a) Czarnik, A. W. Fluorescent Chemosensors for Ion and Molecule
Recognition; American Chemical Society: Washington, D.C., 1993. (b) de
Silva, A. P.; Fox, D. B.; Huxley, A. J. M. Coord. Chem. ReV. 2000, 205,
41. (c) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A.
J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97,
1515. (d) Desvergne, J. P.; Czarnik, A. W. Chemosensors of Ion and
Molecule Recognition; Kluwer: Dordrecht, 1997.
10.1021/ol7020115 CCC: $37.00
© 2007 American Chemical Society
Published on Web 10/02/2007

thioamide derivatives and Hg2+ ion.5-9 For example, Czarnik
et al. reported anthracene-containing thioamide derivative.6
Sonnenschein et al. developed a fluorescent 1,2,4-thiadiazole
derivative (oxidized form) and an iminoyl thiourea (its
reduced form).7 Chang et al. also devised an 8-hydroxy-
quinoline scaffold bearing benzothiazole group.8 Interest-
ingly, Tian et al. expoited mercury-promoted intramolecular
cyclic guanylation of thiourea connected on 1,8-naphthal-
imide inducing UV/vis and fluorescence spectral changes,
but the mostly they paid attention to the fluorescence
ratiometric chemodosimeter.9a
Scheme 1. Synthetic Routes to 1-9
Most chemodosimeters developed so far have been known
to be related to their fluorescence changes upon metal ion
introduction. However, chemodosimeters inducing color
changes, accompanied with their UV band shifts, are very
rare.9b-e
In this context, we have designed azo-component contain-
ing chemodosimeters responsible for the Hg2+ ion-induced
desulfurization which involves a cyclization leading to
irreversible and unique color changes. The azo unit herein
is very useful because its color change is remarkably
responsive to an electronic effect of the substrate species in
the event of the Hg2+ ion-induced chemodosimetric cycliza-
tion. In this paper, we report the syntheses of chemodosim-
eters (1, 3, and 5) and their photochemical elucidation of
their selective color changes toward Hg2+ ion.
Scheme 1 shows reaction pathways for 1-9. In order to
investigate an electronic effect on the chemodosimetric be-
havior, various substituents (1, -NO2; 3, -CO2Et; 5, -OMe)
were introduced to the para position of the azobenzene unit.
To make sure the mercury-induced desulfurization happened
in this event, 2 (90% yield), 4 (93% yield), and 9 (95% yield)
were isolated from the reaction of 1, 3, and 5 with 1 equiv
(1:1 stoichiometry) of Hg(ClO4)2 in DMSO at room tem-
perature, respectively. To obtain insight into a role of the
benzoylthiourea on desulfurization reaction, reference 8 was
also prepared. In the Supporting Information, there are syn-
thetic details and all spectroscopic data for compounds 1-9.
In the UV/vis spectra (see Figure S1, Supporting Informa-
tion) of the receptors, we observed a solvatochroism depend-
ing on solvents such as DMSO (λmax ) 490 nm, red) and
CH3CN (λmax ) 448 nm, orange). For the studies on chemo-
dosimetric response to metal cations, we here chose DMSO
because it provides more distinctive color changes of
receptors than CH3CN does. For further application necessary
to be carried out in water media, DMSO/H2O (4:1, volume
ratio) as a solvent was employed in all experiments.
Figure 1 shows UV/vis spectral and visual color changes
of 1 in aqueous solutions upon addition of 1 equiv of various
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Figure 1. UV/vis spectra and photographs of 1 (20.0 µM) upon
addition of ClO4- salts of X ) Al3+, Pb2+, Hg2+, Zn2+, Cu2+, Cd2+
,
Co2+, Ca2+, Ba2+, Sr2+, Mg2+, Na+, K+, and Cs+ (1 equiv) in
aqueous solution (DMSO/H2O ) 4:1, v/v).
4516
Org. Lett., Vol. 9, No. 22, 2007

metal ions, Al3+, Pb2+, Hg2+, Zn2+, Cu2+, Cd2+, Co2+, Ca2+,
Ba2+, Sr2+, Mg2+, Na+, K+, and Cs+. Remarkably, we
observed a selective blue shift from 486 to 406 nm upon
the addition of only Hg2+ ion, but no spectral change was
detected in other metal ions. For the chemodosimetric
mechanism of the blue-shift in UV/vis spectra, we envision
that the addition of the thiophilic Hg2+ ion induces a
cyclization of 1 followed by desulfurization into an imida-
zoline moiety which can decrease the intramolecular charge-
transfer property of the azo chromophore.
Colorimetric detection limit of 1 for Hg2+ ion was also
tested. In the solvent system of DMSO/H2O (4:1, v/v), the
detection limit within visual color changes is allowable to
1.0 µM level of Hg2+ ion in 1.0 µM solution of 1 as presented
in Figure 2c. In addition, the reaction responsible for these
changes reaches completion within the time frame (<1 min)
of these measurements.
In addition, we investigated a substituent effect on the
Hg2+ ion-induced chemodosimetric cyclization in 1, 3 and
5 (Figures S3 and S6, Supporting Information). In fact, 3
and 5 show similar photochemical changes to 1 upon Hg2+
ion introduction, but do different wavelengths from 1.
Compound 3 shows absorption spectral changes from 444
to 384 nm with one isosbestic point at 401 nm. The titration
profile and Job’s plot analysis indicate 1:1 stoichiometry
(Figure S4, Supporting Information) between 3 and Hg2+
ion. The visual color changes from deep yellow to colorless
in the presence of Hg2+ ion are also shown in Figure S3 and
its detection limit in Figure S5 (Supporting Information).
Table 1. Summarized Photochemical Properties of 1, 3, and 5
for Hg(II) Ion, Such as Maximum Absorption Wavelength (λabs
)
and Absorption Coefficients (ꢀ)
compd
λabs (nm)
ꢀ (M-1 cm-1
)
1
3
5
486
444
412
406
384
396
3.3 × 104
2.9 × 104
2.9 × 104
2.7 × 104
2.0 × 104
2.5 × 104
1‚Hg(II)
3‚Hg(II)
5‚Hg(II)
Figure 2. UV/vis titration spectra (a) of 1 (20.0 µM) and titration
profile (b) at 406 nm upon addition of various amounts of Hg-
(ClO4)2 (0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0, 3.0, and 5.0 equiv). Color
changes (c) of 1 in the presence of 1 equiv of Hg2+ in aqueous
solution (DMSO/H2O ) 4:1, v/v).
Table 1 summarizes photochemical features of 1, 3, and
5 for Hg2+ ion, respectively. Regarding the Hg2+ detection
by a naked eye, 1 seems to be the best among three receptors.
To clarify a role of the benzoyl isothiocyanate unit in the
receptors in the color changes, reference 8 was tested. It does
not give any spectral changes upon addition of even 100
equiv of metal ions (Figure S7, Supporting Information),
suggesting that the benzoylthiourea unit plays a critical role
in mercury-induced desulfurization.
From the Hg2+ titration to a solution of 1 (Figure 2a), we
noticed a definite ratiometry with one isosbestic point at 433
nm and an 1:1 reaction stoichiometry. To corroborate 1:1
ratio between 1 and Hg2+ ion, Job’s plot analysis was also
executed. The measured absorbance variation (∆ ) Aobs
-
Ai) reaches to a maximum when the molar fraction of ([1]/
[Hg2+] + [1]) is 0.5, confirming the 1:1 stoichiometry (Figure
S2, Supporting Information).
With respect to counteranion effects on the UV/vis spectral
changes of the chemodosimeters, reactions of receptors with
HgCl2 were also implemented. They gave the same changes
as the cases of 1-Hg(ClO4)2 and 3-Hg(ClO4)2, demonstrat-
ing that counteranion effect in the chemodosimeteric response
is negligible (Figures S9 and S10, Supporting Information).
Figure 4 indicates competitive chemodosimetric reactions
performed by using 20.0 µM of receptors and Hg2+ in the
presence of miscellaneous cations such as Al3+, Pb2+, Zn2+,
Cu2+, Cd2+, Co2+, Ca2+, Ba2+, Sr2+, Mg2+, Na+, K+, and
Cs+ (1 equiv). The selectivity of 1 or 3 toward mercury ion
is not interfered even in the presence of other various cations.
This observation is quite interesting by considering that Cu2+
and Pb2+ ions can interfere with the Hg2+ ion selectivity to
cause a serious problem in sensory application.10 As shown
in Figure 4, colorimetric chemodosimeters 1 and 3 in this
Figure 3. UV/vis titration spectra (a) of 3 (20.0 µM) and titration
profile (b) at 390 nm upon addition of various amounts of Hg-
(ClO4)2 in aqueous solution (DMSO/H2O ) 4:1, v/v).
Org. Lett., Vol. 9, No. 22, 2007
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Figure 5. Variation of absorbance of 1 (20.0 µM) and 3 (20.0
µM) in aqueous solution (DMSO/H2O ) 4:1, v/v) with and without
Figure 4. UV/vis spectra of (a) 1 (20.0 µM) and (b) 3 (20.0 µM)
in aqueous solution (DMSO/H2O ) 4:1, v/v) in the presence of
the Hg2+ ion (1 equiv) and miscellaneous cations (X) including
Al3+, Pb2+, Zn2+, Cu2+, Cd2+, Co2+, Ca2+, Ba2+, Sr2+, Mg2+, Na+,
K+, and Cs+ (1 equiv, respectively).
Hg2+ (1 equiv) ion as a function of pH: (a) λabs ) 406, (b) λabs
)
384 nm.
In conclusion, we here report azo-component containing
chemodosimeters (1, 3, and 5) responsible for the Hg2+ ion-
induced cyclization and desulfurization. Addition of only 1
equiv. of Hg2+ ion to an aqueous solution of 1, 3, or 5 gives
an instantaneous color change along with the blue-shifted
absorption band. The colorimetric chemodosimeters we have
developed can be applicable for the practical usage for the
monitoring of Hg2+ ion in aqueous solution with a pH span
4-9.
report still show a remarkable selectivity toward Hg2+ ion
in a solution containing Cu2+ and Pb2+ ions.
Absorption intensity changes of the receptors as a function
of pH in the presence and absence of Hg2+ ion are also
noticeable (Figure 5). We found that in a solution of DMSO/
water (4:1) the suitable pH span for Hg2+ determination is
between pH 4-9. In this region, the free 1 and 3 have no
response, while addition of Hg2+ ion can lead to a remarkable
response, suggestive of efficient complexation between
chemodosimeters and Hg2+ ion. As a result, our Hg2+-
selective receptors would be an ideal colorimetric chemo-
dosimeter for monitoring Hg2+ in aqueous solution in the
pH range of 4-9.
Acknowledgment. This work was supported by the SRC
program (R11-2005-008-02001-0(2007)) and Basic Science
Research of KOSEF (R01-2006-000-10001-0).
Supporting Information Available: Synthetic details,
NMR copies, and additional spectra data. This material is
available free of charge via the Internet at http://pubs.acs.org.
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