A "turn-on" fluorescent Hg2+ chemosensor based on ferrier carbocyclization

Article abstract of DOI:10.1021/ol2033477

A "turn-on" fluorescent chemosensor with excellent selectivity and satisfactory sensitivity on Hg²⁺ detection in 100% water media has been established employing a carbohydrate based Ferrier carbocyclization reaction. The probe has also presente

Full text of DOI:10.1021/ol2033477

ORGANIC
LETTERS
A “Turn-on” Fluorescent Hg2þ
Chemosensor Based on Ferrier
Carbocyclization
2012
Vol. 14, No. 3
820–823
Xing Ma,†,‡ Jing Wang,† Qiuli Shan,† Zhuowei Tan,† Guohua Wei,*,† Dongbin Wei,*,†
and Yuguo Du*,†,‡
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center
for Eco-Environmental Sciences (RCEES), Chinese Academy of Sciences, Beijing
100085, China, and College of Chemistry and Chemical Engineering, Graduate
University of Chinese Academy of Sciences (GUCAS), Beijing 100049, China
wgh@rcees.ac.cn; weidb@rcees.ac.cn; duyuguo@rcees.ac.cn
Received December 15, 2011
ABSTRACT
A “turn-on” fluorescent chemosensor with excellent selectivity and satisfactory sensitivity on Hg2þ detection in 100% water media has been
established employing a carbohydrate based Ferrier carbocyclization reaction. The probe has also presented satisfactory results for the imaging
of Hg2þ ions in cells and organisms.
Mercury is one of the most hazardous species in nature,
which may cause prenatal brain damage, cognitive and
motion disorders, vision and hearing loss, and even death.1
Development of a facile and selective method for Hg2þ
detection has long been demanded in the area of food
safety and environmental protection. A number of fluor-
escent chemosensors for the selective detection of Hg2þ
ions have been exploited.2 Most early sensors showed a
fluorescence quenching (turn-off) response due to the
spinꢀorbit coupling effect of the Hg2þ ion,3 while exam-
ples of “turn-on” Hg2þ sensors have also been reported in
recent years.4 Coordination of Hg2þ to S-atom-based
receptors5 and utilizing diverse mercury-mediated reac-
tions such as mercury-deselenation reactions,6 mercury-
catalyzed hydrolysis,7 and the mercuration reaction8 are
major ideas in the development of Hg2þ fluorescent probes
so far. However, only a few successful examples of fluor-
escent probes for detecting Hg2þ ions in aqueous solutions
or pure water have been established,2n,9 thus hampering
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2006, 128, 9988–9989. (f) Coskun, A.; Akkaya, E. U. J. Am. Chem. Soc.
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Z.; Zhu, D. B. Chem. Commun. 2005, 2161–2163. (k) Liu, B.; Tian, H.
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† Chinese Academy of Sciences.
‡ Graduate University of Chinese Academy of Sciences.
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r
10.1021/ol2033477
Published on Web 01/20/2012
2012 American Chemical Society

analytical application in real water samples. Herein, we
designed a sugar functionalized fluorescent probe 1
(Scheme 1). Our strategy for the fluorescent detection of
mercury ion relies on the highly selective mercury-
mediated Ferrier carbocyclization, resulting in the forma-
tionof a fluorescent intermediateand leading toa dramatic
increase in fluorescence intensity. The introduction of a
sugar residue in fluorescent probe 1 can greatly improve its
water solubility and biocompatibility, as well as the colori-
metric and fluorescent “turn-on” selectivity and sensitivity
toward the mercury ion in 100% water.
probe 1 was obtained as a pale yellow solid in 28% overall
1
yield (Scheme 2). Probe 1 was characterized by H NMR
(Figure S1), 13C NMR (Figure S2), and mass spectra.
Scheme 2. Synthesis of Probe 1
As expected, synthetic probe 1 has good water solubility
and can completely dissolve in 100% pure water at 1 mg/mL
without adding any toxic organic cosolvent. The pH value
variation from 3.4 to 9.3 does not cause any significant
changes in fluorescence intensity (Figure S3), implying the
stability and potential application of probe 1 in an aqueous
and biological system. The aqueous solution of probe 1
(20 μM in pure water) itself is colorless and nonfluorescent
at a wavelength range of 450 to 700 nm (Figure S4). After
Hg2þ (1.0 μM in pure water) and probe 1 (20 μM) in pure
water (pH 6.0) were mixed, a weak fluorescence signal at
λex/λem = 570/594 nm was detected due to the in situ
generation of resorufin 5. The reaction proceeds relatively
fast and can be finished within 15 min. Interestingly, when
the pH value of the reaction mixture was adjusted to 6.96
(or greater) by a PBS buffer, a strong and stable fluores-
cence signal (>25 times) would suddenly appear due to the
formation of O-anionic compound 2 from resorufin 5
(Figures S5, S6). This is in accordance with the physico-
chemical property (pKa) of resorufin 5.10 Moreover, a
simultaneously purple color could be observed by the
“naked eye” under these conditions (Figure 1).
Considering the potential biological application, we set
the pH at 7.4 in the following studies to fit the natural
physiological requirement. The generation of O-anionic
compound 2 was confirmed by mass spectrometry analysis
(ESIꢀ mode, m/z 212.0) on the crude reaction solution
(namely, 1 þ HgCl2) (Figure S7), as well as the reaction
marker, carbo-sugar 3 (m/z 160.7, MꢀHþ). The fluores-
cence titration of probe 1 with an increasing amount of
Hg2þ (from 0 to 2.0 equiv) showed saturation behavior at
0.5 equiv of Hg2þ (Figure S8) in PBS buffer at pH 7.4.
Scheme 1. Mercury-Mediated Ferrier Carbocyclization of 1
Phase-transfer-catalyzed (PTC) coupling reaction of
4 and resorufin 5 was conducted in a 0.4 M aqueous
K2CO3ꢀCHCl3ꢀBu4NBr (tetrabutylammonium bromide)
system at 50 °C, and compound 6 was obtained in one pot
through consecutive glycosylation and HI elimination.
After removal of the acetyl groups with MeONa/MeOH
(keeping pH at 9), the desired water-soluble fluorescent
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Org. Lett., Vol. 14, No. 3, 2012
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Figure 1. “Naked-eye” color changes of probe 1 (20 μM in pure
water) before and after addition of Hg2þ (1.0 μM) at room
temperature (reacted 15 min and adjusted pH to 7.4 with PBS
buffer before measurement).
The result suggested that the mercury ion ran 2 cycles in
this Ferrier carbocyclization reaction (Scheme 1).7a,11
Figure 2a shows the Hg2þ concentration dependent emis-
sion fluorescence spectra of probe 1(20μM). When excited at
570 nm, the emission fluorescence intensity at 594 nm in-
creased about 25-fold upon increasing the concentration
of Hg2þ from 0 to 1.0 μM. A satisfactory linear relation-
ship between fluorescence intensity and Hg2þ concentra-
tion was observed with a correlation coefficient as high as
0.9935 (Figure 2b). According to fluorometric method, the
detection limit of probe 1 for Hg2þ was found to be 0.15 μM
(ca. 30 μg/L) at a signal-to-noise (S/N) ratio of 3. This
result is comparable to those reported previously and
shows potential application in emerging environmental
monitoring.2o,5f,5h
Figure 2. (a) Fluorescence titration spectra (λex = 570 nm, with
slit widths of 2.5 nm) of probe 1 (20 μM in pure water) with Hg2þ
from 0 to 1.0 μM at room temperature (reacted 15 min and
adjusted pH to 7.4 with PBS before determination). (b) The plot
of fluorescence intensity changes (λex/λem = 570/594 nm, slit
widths of 2.5 nm) of probe 1 (20 μM in pure water) against varied
concentration of Hg2þ from 0.1 to 1.0 μM at room temperature
(reacted 15 min and adjusted pH to 7.4 with PBS before
determination).
To evaluate the selectivity of probe 1, 12 typical cations
(Ca2þ, Cd2þ, Co2þ, Cu2þ, Mg2þ, Mn2þ, Zn2þ, Agþ, Pb2þ
,
Fe3þ, Fe2þ, Ni2þ) were investigated in parallel under the
same testing conditions. As shown in Figure 3, the reaction
of probe 1 with Hg2þ produces a strong fluorescence
response, while the other cation ions do not show this be-
havior. Adding Hg2þ into other cation ions solutions, re-
were incubated withprobe 1 (20, 20, and 100 μM at 24 h for
cells incubation, 30, 30, and 60 μM at 2 h for zebrafish
incubation, respectively), washed with PBS, and then
treated with a HgCl2 solution (10, 50 μM at 1 h for cells
incubation, 15, 30μM at0.5 h for zebrafish incubation). As
shown in Figure 4, significant fluorescence signals were
detected when mercury ions enter the cells and organisms,
and the fluorescence intensity showed in a concentration-
dependent way. The current experiment suggested that
probe 1 can be applied to detect Hg2þ ions in cellular
systems with satisfactory sensitivity and high selectivity,
which is valuable for studying the uptake, bioaccumula-
tion, and bioavailability of Hg2þ in living organisms.
In conclusion, an excellently selective and satisfactorily
sensitive “turn-on” fluorescent probe for the detection of
Hg2þ in 100% water media has been designed and synthe-
sized. The probe, presenting a rapid (less than 15 min)
“turn-on” fluorescence response and good water solubility
spectively, presented similar emission spectra at λex/λem
570/594 nm (Figure 3a) and fluorescence intensity at λem
=
=
594 nm (Figure 3b) as those of the Hg2þ ion alone. This
selectivity indicated that probe 1 has satisfactory specificity
toward Hg2þ species.
Carbohydrate-modified probe 1 was expected to have
good cell permeability and a rapid fluorescence response
and, thus, is applicable to Hg2þ monitoring in cells and
organisms. For this purpose, absorption of mercury ions
into cells and organisms was determined by using probe 1
preincubated in living things according to a literature
reported method.12 The A549 cell line (human lung ade-
nocarcinoma epithelial cell line) and 5-day-old zebrafish
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2011, 40, 2120–2130.
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Org. Lett., Vol. 14, No. 3, 2012

Figure 4. Fluorescence images of Hg2þ ions in A549 cells and
zebrafish with probe 1. Bright-field transmission image (AꢀC)
and fluorescence image (DꢀF) of A549 cells pretreated with 20,
20, and 100 μM of probe 1 for 24 h followed by incubation with
0, 10, and 50 μM of Hg2þ ions for 1 h, respectively (excited with
green light). Bright-field transmission image (GꢀI) and florescence
image (JꢀL) of zebrafish pretreated with 30, 30, and 60 μM of
probe 1 for 2 h followed by incubation with 0, 15, and 30 μM of
Hg2þ ions for 0.5 h, respectively (excited with green light).
is valuable for studying Hg2þ toxicity in living organisms.
The current work demonstrates a new type of carbohy-
drate assisted fluorescence sensor.
Figure 3. (a) Fluorescence spectra (λex = 570 nm, with slit
widths of 2.5 nm) of probe 1 (20 μM in pure water) upon addition
of various metal ions (1.0 mM in pure water). Dot line: probe 1 þ
metal cation, solid line: probe 1 þ metal cation þ Hg2þ (reacted
15 min and adjusted pH to 7.4 with PBS before determination). (b)
Fluorescence response (λex/λem = 570/594 nm, with slit widths of
2.5 nm) of probe 1 (20 μM in pure water) upon addition of various
metal cations (1.0 mM in pure water). White column: probe 1 þ
metal cation, black column: probe 1þ metal cation þ Hg2þ (reacted
15 min and adjusted pH to 7.4 with PBS before determination).
Acknowledgment. This work was supported by the
National Basic Research Program of China and the
National Natural Science Foundation of China.
Supporting Information Available. Experimental pro-
cedures for the synthesis and copies of 1HNMR and 13
C
NMR of probe 1, data for fluorescence of probe 1, and
other data.This material is available free of charge via the
Internet at http://pubs.acs.org.
and biocompatibility, is cleverly designed based on a
unique Hg2þ-mediated carbohydrate Ferrier carbocycliza-
tion reaction. The probe was also successfully applied to
theimaging of Hg2þ ionsinA549cells and zebrafish, which
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 3, 2012
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