2
Journal of Chemical Research 00(0)
Scheme 1. Synthetic protocol for sensor LS.
liquid chromatography,10 and infrared spectroscopy.11,12
Unfortunately, most of these methods are not appropriate for
real-time and on-site assays and require expensive devices
and complicated sample preparation. In recent years, chem-
osensors for detecting mercury ions have attracted great
attention due to their high selectivity and excellent sensitiv-
ity. In addition, fluorescent chemosensors which can directly
measure mercury ions in water samples are advantageous in
Figure 1. Absorption spectra of sensor LS (100μM) in ethanol
and water (1:1, v/v) solution obtained by adding aliquots of
2+
[
Hg ] (0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50,
.55, 0.60, 0.65, 0.70, and 0.75equiv.).
0
13
terms of time savings, lower costs, and simplicity.
There are two types of fluorescent chemosensors for Hg
detection that have been reported: coordination-based chem-
2+
1
4–16
17–26
osensors
and reaction-based chemosensors.
For reac-
2+
tion-based sensors, the determination of Hg is achieved by
specific chemical reactions between receptors and Hg . This
type of chemosensor usually exhibits excellent selectivity and
2+
2+
high sensitivity toward Hg due to their specific selective
2+ 13
reactivity to Hg . Therefore, research of reaction-based
chemosensors has gained more attention in recent years.
With large molar extinction coefficients, relatively long
excitation and emission wavelengths and high quantum
efficiencies, coumarin derivatives have been widely used
as fluorescent chemosensors. It has been reported that the
thioacetal group can be selectively desulfurized by Hg ,
resulting in the formation of the corresponding aldehyde
2
7
2
+
Figure 2. Fluorescent spectra of sensor LS (2μM) with
and changes of the fluorescent intensity. However, most of 1.0equiv. of various metal ions in ethanol and water (1:1, v/v).
these chemosensors have drawbacks of relatively high fluo- Inset: The colors of sensor LS upon addition of various metal
2
8
ions as viewed by the naked eye under a 365-nm UV lamp.
rescent background signals, limits of detection, and poor
2
9
solubility in aqueous solution. Herein, we have designed
and synthesized a novel “on-off” fluorescent chemosensor,
-(1,3-dithian-2-yl)-7-hydroxy-4-methylcoumarin
Scheme 1), in which a thioacetal group has been added to
a coumarin derivative. The sensor LS showed highly selec-
tive fluorescent sensing for Hg with a low detection limit
of 0.81nM in the pH range from 6.15 to 9.96 in ethanol/
water (1:1, v/v) solution. The recognition mechanism of
The fluorescence emissions of sensor LS (2μM) on add-
ing different metal ions (1.0equiv. of LS) including Ag ,
8
(LS)
+
(
3+
2+
2+
2+
2+
2+
3+
3+
2+
Al , Ba , Ca , Cd , Co , Cu , Cr , Fe , Hg ,
2+
2+
2+
2+
2+
Mg , Mn , Ni , Pb , and Zn were determined in etha-
nol/water (1:1, v/v). After excitation at 369nm, the sensor
LS exhibited very strong fluorescence emission at 451nm.
2
+
2+
Upon the addition of Hg (2μM), the fluorescence intensity
2
+
1
sensor LS toward Hg is proposed and confirmed by H
nuclear magnetic resonance spectroscopy (NMR) studies.
was quenched immediately (Figure 2). In contrast, no sig-
nificant fluorescence change was detected after addition of
other metal ions under the same conditions, except for with
2
+
3+
3+
Cu , Fe , and Cr . These common fluorescence-quench-
Results and discussion
2+
3+
3+
ing ions (Cu , Fe , and Cr ) quenched about 23%–36%
The ultraviolet–visible (UV–Vis) titration spectra of sensor of the fluorescence compared with LS alone. Under the 365-
2
+
LS upon addition of various concentrations of Hg were nm UV lamp, the remarkable light-blue fluorescence emis-
2+
carried out in ethanol and water (1:1, v/v) at room tempera- sion of sensor LS disappeared after adding Hg , but no
ture. As shown in Figure 1, the absorption band of receptor significant fluorescence change was detected for LS solution
LS in the UV–Vis spectrum appears at 272nm. Upon addi- with other metal ions. These results indicate that sensor LS
2
+
2+
tion of increasing concentrations of Hg (0–0.75equiv.) to exhibited high selectivity for Hg over other metal ions.
the solution of sensor LS (100μM), the absorption band at
The quantitative sensing abilities of sensor LS (2μM)
72nm was enhanced gradually, which clearly suggests toward Hg were studied and a working curve was obtained
2+
2
2
+
that LS participates in a reaction with Hg .
(Figure 3). With addition of increasing concentrations of