J Fluoresc
concentration of Hg2+. The detection limit of RBS for Hg2+
was 1.83×10−9 M (based on S/N=3). Thus, RBS was capable
of detecting both qualitatively and quantitatively of Hg2+
utilizing fluorospectrophotometry.
Recovery Study and Determination of Environment
Standard Sample
To develop practical applicability of RBS toward Hg2+, it
was applied to the determination of Hg2+ in tap water, river
water and soil samples. Tap water, river water and solid
samples were obtained from laboratory tab water, Nai
River and campus soil respectively. As results shown in
Table 1, it can be confirmed that the results for recoveries
study of spiked Hg2+ detected by the RBS probe were
satisfying and the relative standard deviation (RSD) was
1.91 %, 1.43 % and 2.94 % with 10 determinations, respec-
tively. To prove the reliability of the method, it was applied
to the determination of Hg2+in the environment standard
sample (Water-mercury (Numbers: GSBZ 50016–90, batch
number: 202030)). As results shown in Table 2, the relative
standard deviation (RSD) was 3.25 % and the average value
was 6.15 μg/L consistent with the standard value 6.14 μg/L.
So the method was high reliability.
A series of solutions containing RBS and Hg2+ were
prepared as the sum of the total concentration remained
constant at 1.00×10−6 M and then the 1:1 stoichiometry
between RBS and Hg2+ was confirmed by the Job’s plot
(the method of continuous variations) shown in Fig. 7. The
association constant of the complex was estimated on the
basis of 1:1 stoichiometry by the linear Benesi–Hildebrand
expression [24]:
1
1
1
1
¼
þ
ΔF
½RBSꢁΔΦ Ka½RBSꢁΔΦ ½Hg2þ
ꢁ
Where ΔF is the change in the fluorescence intensity at
585 nm; Ka is the association constant; ΔΦ is the difference
of fluorescence quantum yields between the complex and
RBS; and [RBS] and [Hg2+] are the concentrations of RBS
and Hg2+, respectively. On the basis of the plot of 1/ΔF and
1/[Hg2+] as shown in the inset (b) of Fig. 6, the association
constant was calculated to be 5.96×104 M−1 from the ratio
of the slope and intercept.
Conclusions
In summary, a new fluorogenic probe for detection of Hg2+
was synthesized. As a turn-on fluorescent probe, it responds
to mercury ions rapidly with an excellent selectivity and
competitiveness. The linear response range and the detec-
Reversibility and the Proposed Binding Mechanism
The reversibility and regeneration is an important factor for
a fluorescence chemosensor to detect its specific analyte in
practical application. In light of strong binding ability of the
iodide anion (I−) toward Hg2+, it was imagined that addition
of I− will take away Hg2+ from the RBS-Hg2+ complex,
liberating the spirolactone RBS and decreasing the fluores-
cence intensity at 585 nm. To colorless solution of RBS, 1
equiv of Hg2+ was added, then the resulting pink solution
was immediately treated with KI (2, 4 and 6 equiv of Hg2+),
which led to light-color solution and decreased fluorescence
intensity as shown in Fig. 8. Further addition of Hg2+ to
colourless solution of I− mediated Hg2+-decomplexed RBS
led to reappearance of pink colour and fluorescence inten-
sity enhancement at 585 nm which exceeded the extent of
those upon first Hg2+ addition to RBS. Therefor the re-
sponse of RBS to Hg2+ is reversible rather than a cation-
catalyzed reaction.
tion limit of this method were 5.00×10−9 to 1.00×10−6
M
and 1.83×10−9 M, respectively. In addition, the fluores-
cence can be switched on and off by the addition of
Hg2+ and KI alternately. It was successfully applied to detect
Hg2+ levels in tap water, river water and soil samples. We
anticipate that the probe would be more useful for detection
of Hg2+ in future application.
Acknowledgments This work was supported by the National Nature
Science Foundation of China (21175083).
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Due to the 1:1 stoichiometry and the result of revers-
ibility experiment, it could be concluded to the Hg2+-
binding complexation species RBS-Hg2+. The binding
event was proposed to involve the thione-S atom, the
imino-N atom and the methoxy-O atom, forming a stable
metal complex which required to open the spiro ring to
establish the delocalized xanthene moiety. The proposed
binding mechanism of RBS with Hg2+ was shown in
Scheme 2.