A 'turn-on' fluorescent probe that selectively responds to inorganic mercury species

Article abstract of DOI:10.1016/j.tetlet.2010.05.081

A fluorescent probe selectively senses inorganic mercury in the turn-on mode through a mercury ionpromoted hydrolysis reaction that leads to a coumarin, among various other metal species except Au(III).

Full text of DOI:10.1016/j.tetlet.2010.05.081

Tetrahedron Letters 51 (2010) 3852–3854
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A ‘turn-on’ fluorescent probe that selectively responds to inorganic
mercury species
*
Yong-Suk Cho, Kyo Han Ahn
Department of Chemistry and Center for Electro-Photo Behaviors in Advance Molecular Systems, POSTECH, San 31 Hyoja-dong, Pohang 790-784, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A fluorescent probe selectively senses inorganic mercury in the turn-on mode through a mercury ion-
promoted hydrolysis reaction that leads to a coumarin, among various other metal species except Au(III).
Ó 2010 Elsevier Ltd. All rights reserved.
Received 24 April 2010
Revised 14 May 2010
Accepted 19 May 2010
Available online 23 May 2010
Mercury is a highly poisonous element and exists as elemental,
inorganic, and organic mercury forms. Contaminated living species,
soil, and water typically contain inorganic and organic mercury
forms.1
sensing. An additional feature of the sensing system is that the
fluorescent compound 3 is a two-photon absorbing material. A
similar sensing scheme, the deprotection–cyclization strategy,
was used to detect fluoride ions by Swager and co-workers.6
Vinyl ether 2 was readily synthesized in three steps (overall 14%
yield), starting from commercially available 4-(diethylamino)sali-
cylaldehyde 4 (Scheme 2). We have found that introduction of
the vinyl group, conversion of compounds 4 to 6, can be signifi-
Many fluorescent molecular probes for mercury ions have been
developed recently.2 Fluorescent probes provide easy and afford-
able detection methods, in addition to the typical instrumental
methods such as atomic absorption or emission spectroscopy and
ion-coupled plasma emission-mass spectrometry.3 Fluorescent
probes can be categorized to two types based on the sensing mech-
anism: one based on metal coordination and the other based on a
chemical reaction.2a The reaction-based fluorescent sensing ap-
proach for inorganic mercury species has attracted sudden re-
search interest recently, plausibly owing to the high selectivity
obtainable.2b,2h–m,2q–v,2y Recently, we devised a novel reaction-
based fluorescent probe that selectively detects methylmercury
in addition to inorganic mercury among various metal species.4
The sensing mechanism of the probe (vinyl ether 1 Scheme 1) is
the mercury ion-promoted hydrolysis of the vinyl ether group,
which is discriminated from those of most known probes that
are based on the coordination of hetero-atom-based ligands (par-
ticularly sulfur) to mercury ions.5
cantly improved by using
a conventional bromoethylation–
elimination process (44% yield), compared with the previous
copper-mediated coupling with toxic tetravinyltin (16% yield).4
O
O
O
Et2N
O
CO2Me
CO2Me
Cl
Cl
OH
Probe 2
1
O
O
OH
Et2N
Et2N
CO2Me
CO2Me
HgCl2
2
H2O
fast
CO2Me
3
We have investigated other vinyl ether systems that may give a
different selectivity pattern, such as the discrimination of methyl-
mercury from inorganic mercury species or vice versa. Herein, we
wish to report vinyl ether 2 that fluorescently senses only inor-
ganic mercury, not methylmercury along with other metal species,
in the turn-on mode.
Scheme 1. The reaction of probe 2 with HgCl2.
Et2N
OH
Et2N
O
BrCH2CH2Br
Br
Our rationale in the design of probe 2 is depicted in Scheme 1.
The mercury ion-promoted hydrolysis of the vinyl ether group in 2
would generate the hydroxy intermediate, which will readily form
coumarin 3. Because vinyl ether 2 is almost nonfluorescent but
coumarin 3 is highly fluorescent, we can realize a turn-on-type
K2CO3
CH3CN, Δ
CHO
Et2N
CHO
4
5
O
CH2(CO2Me)2
t-BuOK
DMSO, r.t.
Probe 2
piperidine
THF, Δ
CHO
6
* Corresponding author. Tel.: +82 54 279 2105; fax: +82 54 279 3399.
E-mail address: ahn@postech.ac.kr (K.H. Ahn).
Scheme 2. Synthesis of probe 2.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.05.081

Y.-S. Cho, K. H. Ahn / Tetrahedron Letters 51 (2010) 3852–3854
3853
A solution of vinyl ether 2 in a PBS buffer (pH 7.4) containing 5%
DMSO exhibited little fluorescence; however, upon treatment with
HgCl2 it became highly fluorescent as shown in Figure 1a. After the
reaction, we were able to isolate and characterize the product 3,
which is responsible for the high fluorescence. Also, the hydrolysis
process was confirmed by 1H NNR titration. A time-dependent
fluorescence measurement for a 1:1 mixture of vinyl ether 2 and
HgCl2 in the buffer solution at 25 °C showed that the fluorescence
intensity reaches ꢀ80% of the saturation after 50 min (Fig. 1a in-
set). Thus, the mercury ion-promoted hydrolysis of vinyl ether 2
is slower compared to the case of probe 1. However, it is not nec-
essary to wait the fluorescence saturation for the quantification
purpose, as we can obtain a linear relationship between the con-
centration of mercury ions and the fluorescence intensity at an
arbitrary time span.
ence between 1 and 2. The reactivity data suggest that, in the pres-
ent case, the ‘second’ oxymercuration–hydrolysis reaction by the
hydrolyzed alkylmercury species HgCl(CH2CHO) 7, which occurs
readily in the case of probe 1 (Scheme 3), seems to proceed slowly.
Fluorescence titrations of probe 2 with varying amounts of
HgCl2 (from 0 to 1.0 equiv) shows saturation in the emission inten-
sity at the equivalent point (Fig. 1c), which also supports the above
reasoning.
Probe 2 is specific toward mercury ions and shows essentially
no response toward other metal ions (as the chloride salts) such
OH
HgCl2
Ar
+ HCl
O
Ar
HgCl
H2O
O
Probe 2
I
Ar =
Et2N
Coumarin 3
Particularly, vinyl ether 2 does not respond to methylmercury
under the reaction conditions (Fig. 1b), whereas vinyl ether 1
senses both methylmercury and inorganic mercury species.4 This
selectivity difference may be originated from the reactivity differ-
CO2Me
CO2Me
ClHg(CH2CHO)
7
Scheme 3. Hydrolysis of probe 2 by HgCl2.
(a)
(b)
60
50
40
30
20
10
0
60
50
40
30
20
10
0
50
40
30
20
10
0
0
50 100 150 200
Time (min)
Probe 2 only
Probe 2 + MeHgCl
2 only
450
500
550
600
450
500
550
600
Wavelength (nm)
Wavelength (nm)
(c)
300
250
200
150
100
50
(d)
70
60
50
40
30
20
10
0
0
0
1
2
3
4
5
HgCl2 (equiv.)
Metal Species
(e)
60
50
40
30
20
10
0
(f)
Probe 2 + HgCl2
80
70
60
50
40
30
20
10
0
Probe 2 + all the other metal
ions except Au(III) + HgCl2
Probe 2, other metal ions
except Au(III)
450
500
550
600
0
2
4
6
8
10
Wavelength (nm)
[HgCl2] (μM)
Figure 1. (a) Time-dependent fluorescence change acquired for a 1:1 mixture of probe 2 and HgCl2. Inset: a plot of the fluorescence intensity change as a function of the
reaction time. (b) Fluorescence change for probe 2 only and for a 1:1 mixture of probe 2 and MeHgCl, acquired after 1 h. (c) Fluorescence intensity change of probe 2 as a
function of equiv of HgCl2, taken after 1 h for each addition. (d) Fluorescence change for a 1:1 mixture of probe 2 and each of the various metal ions (Cr2+, Ca2+, Mg2+, Co2+
,
Mn2+, Ni2+, Pb2+, Zn2+, Ba2+, Cd2+, Au3+, Fe3+, Cu2+, Ag+, and Hg2+; as the chloride salts), acquired after 1 h. (e) Fluorescence changes for probe 2 in the presence of HgCl2, all the
metal species and an equimolar amount of each metal species except Au3+, acquired after 1 h. (f) A plot of fluorescence intensity versus [HgCl2] (1–10
M) obtained for a 1:1
M of probe 2 in PBS buffer (pH 7.4) containing 5% DMSO (excitation at
l
mixture of probe 2 and HgCl2 after 1 h of each mixing. All the measurements were carried out with 3.0
405 nm; the intensity was estimated by the peak height at k = 475 nm).
l

3854
Y.-S. Cho, K. H. Ahn / Tetrahedron Letters 51 (2010) 3852–3854
as Cr2+, Ca2+, Mg2+, Co2+, Mn2+, Ni2+, Pb2+, Zn2+, Ba2+, Cd2+, Fe3+, Cu2+
,
Supplementary data
and Ag+, except Au(III) that is highly alkynophilic (Fig. 1d). This
specificity should result from the negligible hydrolysis of vinyl
ether 2 by the other metal species. Indeed, probe 2 fully responds
to the mercury ions even in the presence of all other metal ions ex-
cept Au(III) (Fig. 1e).
Supplementary data (experimental procedures for the synthesis
of compound 2 and the other fluorescent analysis data) associated
with this article can be found, in the online version, at doi:10.1016/
j.tetlet.2010.05.081.
Other mercury ions such as Hg(OAc)2 hydrolyzes probe 2, sim-
ilarly as HgCl2 does.
References and notes
Probe 2 responds to HgCl2 linearly in the micromolar level con-
centration range (Fig. 1f), and, from which, the detection limit of
the probe is estimated to be 0.2 ppm [Hg(II)].
1. (a) ATSDR, Toxicological Profile for Mercury, U.S. Department of Health and
Human Services, Atlanta, GA, 1999.; (b) ATSDR, ToxProfiles: Mercury, U.S.
Department of Health and Human Services, Atlanta, GA, 2005.
Fluorescence titrations of probe 2 with HgCl2 at different pH
conditions (pH 4.0–9.0) were carried out. At pH 6 or pH 9, the fluo-
rescence increased slowly and also the intensity was lower than
the case of pH 7 up to 1.5 h. At pH 4, the fluorescence increased
steadily as time went; the increase crossed over that of pH 7 after
50 min. In the absence of HgCl2, however, there was little change in
the fluorescence at pH 4, which indicates that the first step, oxym-
ercuration, is crucial for the vinyl ether hydrolysis up to this pH. As
we pointed out previously,4 hydrolysis of the oxymercurated inter-
mediate I is promoted by acid and thus becomes faster at lower pH.
In the absence of HgCl2, probe 2 itself underwent hydrolysis at a
lower pH such as pH 2, but this acid-promoted hydrolysis became
very slow at pH 3 and only a slight increase in the fluorescence
resulted.
In summary, we have developed a novel reaction-based fluores-
cent sensing system that shows specific response to inorganic mer-
cury species among methylmercury and various other metal
species, except Au(III). The probe shows a turn-on fluorescence
change as the vinyl ether undergoes mercury ion-promoted hydro-
lysis followed by cyclization to give a fluorescent coumarin. Stud-
ies are undergoing to detect mercury species by two-photon
fluorescence microscopy as well as to develop a new coumarin pre-
cursor for improved optical properties.
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Acknowledgment
This work was supported by Grants from the EPB center (R11-
2008-052-01001).
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